Recent advances in heterogeneous catalysts for the synthesis of alkyl

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<p>Fuel 323 (2022) 124362</p><p>Available online 6 May 2022</p><p>0016-2361/© 2022 Elsevier Ltd. All rights reserved.</p><p>Review article</p><p>Recent advances in heterogeneous catalysts for the synthesis of alkyl</p><p>levulinate biofuel additives from renewable levulinic acid: A</p><p>comprehensive review</p><p>Jimmy Nelson Appaturi a,*, Jeyashelly Andas b, Yik-Ken Ma a, Bao Lee Phoon c,</p><p>Samaila Muazu Batagarawa d, Fitri Khoerunnisa e, M. Hazwan Hussin a, Eng-Poh Ng a,*</p><p>a School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia</p><p>b Faculty of Applied Sciences, Universiti Teknologi MARA, Cawangan Perlis, Kampus Arau, 02600, Arau, Perlis, Malaysia</p><p>c Nanotechnology & Catalysis Research Centre (NANOCAT), Level 3 Block A, Institute for Advanced Studies, Universiti Malaya, Kuala Lumpur 50603, Malaysia</p><p>d Department of Pure and Industrial Chemistry Umaru Musa Yaradua University, Katsina, Nigeria</p><p>e Chemistry Education Department, Universitas Pendidikan Indonesia, Jl. Setiabudhi 258, Bandung 40514, Indonesia</p><p>A R T I C L E I N F O</p><p>Keywords:</p><p>Levulinic acid</p><p>Alkyl levulinates</p><p>Fuel additive</p><p>Heterogeneous catalyst</p><p>Biomass</p><p>A B S T R A C T</p><p>Alkyl levulinates are one of the most desirable and promising compounds for biofuel production, since they have</p><p>been identified as one of the prospective fuel-blending chemicals. So far, numerous types of heterogeneous acid</p><p>catalysts have been used for the synthesis of alkyl levulinates. In this regard, designing and engineering new</p><p>catalysts for upgrading levulinic acid into biofuel additives have received remarkable attention by global re-</p><p>searchers and it has always been a challenging task. The aim of this article is to review the most six relevant</p><p>heterogeneous catalysts, namely zeolites, mesoporous silicas, carbonaceous materials, metal organic frameworks</p><p>(MOFs), ion exchange resins and enzymatic catalysts, in the esterification of levulinic acid. This review also</p><p>outlooks the catalyst preparation, structural and surface modification, aspects of esterification reaction and re-</p><p>action mechanisms involved. Subsequently, the challenges and future prospects in this field are also discussed.</p><p>1. Introduction</p><p>Biomass conversion into biofuel is an essential process since the fossil</p><p>fuel reserves are decreasing gradually due to the increasing worldwide</p><p>energy consumption and fast growing of global population (by 28% from</p><p>year 2015 to 2040) [1-3]. Apart from the exhaustion of non-renewable</p><p>fossil fuels, the consequences owing to the usage of fossil oil lead to</p><p>increasing emission of greenhouse gases and other environmental haz-</p><p>ards that have motivated many researchers in finding alternative</p><p>biomass platform chemicals and synthesis routes to produce high value-</p><p>added chemicals and biofuels [4-6]. Over the past decades, researchers</p><p>are also focusing on the development of renewable fuels such as bio-</p><p>diesel [7-9]. Nevertheless, the high selling price of biodiesel (as</p><p>compared to fossil diesel) owing to the high cost of vegetable price has</p><p>negatively impacted their sale price and this restriction has gravitated</p><p>researcher’s attention on other types of biofuels [10].</p><p>Although numerous works have been documented on biofuels, yet,</p><p>several concerns pertaining biofuels do exist. The First-Generation</p><p>Biofuels are generated from conventional food-crops (e.g. sugarcane,</p><p>wheat, corn, etc.) with renowned technologies, where this system has</p><p>been criticized by environmentalists as it creates detrimental impact on</p><p>the eco-system in the long run [11,12]. The Second-Generation Bio-</p><p>fuels–introduced from the non-edible lignocellulosic biomass (e.g.</p><p>agricultural waste)–are known as prominent stage of growing waste</p><p>biorefinery to obtain value added chemicals and liquid fuels [13]. At</p><p>present, the concept of biorefinery has largely been used for the pro-</p><p>duction of bioethanol from biomass derived carbohydrates. This bio-</p><p>refinery is also very useful to transform the lignocellulosic biomass into</p><p>important fine chemicals in current years [11].</p><p>Levulinic acid is a multipurpose platform chemical. It can be pro-</p><p>duced using low-cost Biofine® process through cellulose conversion</p><p>under weakly acidic and controlled hydrolysis conditions [14,15]. The</p><p>United States Department of Energy (DOE) has highlighted levulinic</p><p>acid as one of the promising building blocks for chemistry in 2004 and</p><p>2010, and ranked it among one of the Top 12 bio-based building blocks</p><p>for the synthesis of specialty chemicals, liquid fuels and fuel additives</p><p>* Corresponding authors.</p><p>E-mail addresses: jimmynelson@usm.my (J. Nelson Appaturi), epng@usm.my (E.-P. Ng).</p><p>Contents lists available at ScienceDirect</p><p>Fuel</p><p>journal homepage: www.elsevier.com/locate/fuel</p><p>https://doi.org/10.1016/j.fuel.2022.124362</p><p>Received 12 March 2022; Accepted 22 April 2022</p><p>Fuel 323 (2022) 124362</p><p>2</p><p>[16-19]. To date, the GF Biochemicals company (Caserta, Italy) has the</p><p>largest levulinic acid production plant in the world which targeted about</p><p>50,000 MT/annum in 2019 and by 2023, the company aims to expand</p><p>the facility of levulinic acid derivatives of 30 KT/annum [20].</p><p>Chemically, levulinic acid (also known as 4-oxo-pentanoic acid)</p><p>contains a carboxylic acid and a ketone groups which can be used as a</p><p>starting material for the production of numerous value-added chemicals</p><p>(e.g. γ-valerolactone, succinic acid, α-angelica lactone, alkyl levulinate,</p><p>1,4-pentanediol, 2-butanone, 2-methyl tetrahydrofuran and glycerol</p><p>ketal ester oligomer) through various synthesis pathways (e.g. hydro-</p><p>genation, reductive amination, esterification, halogenation, oxidation</p><p>and condensation) [21-25].</p><p>Among all these products, alkyl levulinates have attracted a great</p><p>deal of attention due to their physicochemical properties that are similar</p><p>to fatty acid methyl esters (FAME) in biodiesel. In addition, their fuel</p><p>blending and additives components could also partially replace fossil</p><p>fuels which can help in the secure of future energy demands targeted by</p><p>EU and EPCEU [3,13,26]. Furthermore, alkyl levulinates are also the</p><p>crucial chemicals for other industrially important applications, such as</p><p>green solvents in food industry, flavouring agents, plasticizers, fra-</p><p>grances and antifreeze agents (Fig. 1) [27,28]. Several synthesis routes</p><p>and advanced solid catalysts have been designed and developed to</p><p>synthesize alkyl levulinates using renewable substrates (e.g., cellulose,</p><p>furfural, furfuryl alcohol and levulinic acid) [29-35]. Amongst these</p><p>synthesis pathways, esterification of levulinic acid using short or long</p><p>chain alcohol catalyzed by solid acid catalysts is the simplest and</p><p>greener route where water is formed as a side reaction product</p><p>[17,19,36]. In the last decade, enormous attention has been given to the</p><p>heterogeneous catalytic esterification for synthesizing levulinate com-</p><p>pounds. Particularly, the recent six years witnesses a significant increase</p><p>in the number of publications for the synthesis of alkyl levulinate per</p><p>year after discovering its promising application as fuel additives (Fig. 2).</p><p>Esterification of levulinic acid with short-chain alcohols (such as</p><p>methanol and ethanol) has become an attractive reaction since the ethyl</p><p>levulinate product can be used as an additive in gasoline and biodiesel.</p><p>In addition, the solubility of ethyl levulinate in gasoline is better than</p><p>methyl levulinate and other bulkier alkyl levulinate. It is reported that</p><p>an addition of 5 wt% of ethyl levulinate into diesel successfully improves</p><p>the physical properties of fuel, e.g. lubricity, toxicity, sulfur content,</p><p>thermal stability, viscosity, flashpoint and fluid dynamic stability</p><p>[4,21,37]. Furthermore, ethyl levulinate is an excellent octane booster</p><p>for gasoline and it is used as a cold flow improver in biodiesel [38]. Ethyl</p><p>Fig. 1. The conversion of agricultural biomass into alkyl levulinates for potential industrial applications.</p><p>Fig. 2. Number of publications for the synthesis of alkyl levulinate</p><p>and thermal</p><p>activation on structural, acid, porous and catalytic properties of solids</p><p>are investigated in the esterification of levulinic acid with ethanol. The</p><p>work demonstrates that thermal activation of catalyst reduces esterifi-</p><p>cation performance as compared to the hydrated and dehydrated</p><p>counterparts due to the structural collapse. In turn, the hydrated cata-</p><p>lysts display better performance than the dehydrated and thermally</p><p>activated samples indicating the important role of coordinated water</p><p>molecules in UiO66 (Table 4, entries 12 and13). In addition, high</p><p>dependence between the quantity of functionalized sulfonic acid groups</p><p>and the catalytic activity is shown with the fully sulfonated (hydrated)</p><p>UiO66 shows the highest activity (87% yield of ethyl levulinate, 80 ◦C, 6</p><p>h) and is more efficient than the commercial Amberlyst 15 catalyst</p><p>(67%). Moreover, the catalyst shows insignificant loss in reusability test</p><p>and the catalytic structure is well preserved even after five reuse cycles.</p><p>The possible reaction mechanism pathways catalyzed by acidic and</p><p>basic MOF catalysts have been proposed by Badgujar et al. [13].</p><p>Compared with basic MOF catalyst, high alkyl levulinate yield is always</p><p>achieved by acidic MOF catalyst due to the fast and ease abstraction of</p><p>hydrogen by the SO3</p><p>− ligand from the alcohol. At the same time, it also</p><p>involves protonation of carbonyl oxygen atom of levulinic acid forming</p><p>sp2 carbocation, which subsequently undergoes nucleophilic attack by</p><p>the alcoholic oxygen atom. An intermediate sp3 carbon is formed which</p><p>is then converted to carbonyl group by eliminating water molecule,</p><p>producing alkyl levulinate as final product (Fig. 13a).</p><p>On the other side, the MOF basic catalysts can also give high alkyl</p><p>Table 4</p><p>Summary of the catalytic reaction systems catalyzed by metal organic framework (MOF) catalysts.</p><p>Entry Catalysta Alkyl levulinate</p><p>b</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 UiO-66-(COOH)2 EL 78 24 1:20 97.0 100 97.0 [116]</p><p>2 UiO-66 EL 78 24 1:20 25.6 100 25.6 [116]</p><p>3 [Cu-BTC][HPM] EL 120 6 0.5:10c 100 100 99.0 [25]</p><p>4 UiO-66 EL 78 8 1:15 94.0 100 94.0 [117]</p><p>5 UiO-66-NH2 BL 120 5 1:6 99.0 100 99.0 [117]</p><p>6 Blank C12L 80 8 1:1 NA NA 21 [117]</p><p>7 Blank C16L 80 8 1:1 NA NA 29 [117]</p><p>8 Blank C18L 80 8 1:1 NA NA 21 [117]</p><p>9 UiO-66 C12L 80 8 1:1 NA NA 72 [117]</p><p>10 UiO-66 C16L 80 8 1:1 NA NA 66 [117]</p><p>11 UiO-66 C18L 80 8 1:1 NA NA 60 [117]</p><p>12 UiO66-SO3H(100)</p><p>activated</p><p>EL 80 6 1:10 21.0 100 87.0 [120]</p><p>13 UiO66-SO3H(100)</p><p>hydrated</p><p>EL 80 6 1:10 87.0 100 87.0 [120]</p><p>14 UiO66-SO3H(100)</p><p>dehydrated</p><p>EL 80 6 1:10 80.0 100 80.0 [120]</p><p>15 UiO-66(Hf)-SO3H EL 120 2 1:20d 90.0 94.4 85.0 [119]</p><p>16 UiO-66(Hf)-SO3H ISPL 120 2 1:20e 88.0 90.9 80.0 [119]</p><p>17 UiO-66(Hf)-SO3H PL 120 2 1:20f 87.0 89.6 78.0 [119]</p><p>18 UiO-66(Hf)-SO3H ML 120 2.5 1:20 g 94.0 90.4 85.0 [119]</p><p>a [Cu-BTC][HPM]: metal-organic framework-supported phosphomolybdic acid; UiO-66(Hf)-SO3H: sulfonic acid-functionalized hafnium-based MOF.</p><p>bEL: Ethyl levulinate; BL: Butyl levulinate; ISPL: Isopropyl levulinate; C12L:Lauryl leulinate; C16L: Cetyl levulinate; C18L: Oleyl levulinate; PL: Propyl levulinate; ML:</p><p>Methyl levulinate.</p><p>c levulinic acid: 0.5 mmol, ethanol:1.7 x102 mmol.</p><p>d levulinic acid: 1 mmol, ethanol: 3.4 x102 mmol.</p><p>e levulinic acid: 1 mmol, isopropanol: 2.6 x102 mmol.</p><p>f levulinic acid: 1 mmol, 1-propanol: 2.7 x102 mmol.</p><p>g levulinic acid: 1 mmol, methanol: 4.9 x102 mmol.</p><p>Fig. 14. Simultaneous activation of both levulinic acid and ethanol by Zr acid</p><p>site and NH2 group. Adapted from Cirujano et al. [118].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>15</p><p>levulinate yield by offering fast abstraction of alcoholic proton through</p><p>nucleophilic attack by the amine functional group of the MOF. The</p><p>electron-rich alcoholic oxygen atom then attacks carbonyl carbon of</p><p>levulinic acid whereby the chemical interaction between the metal site</p><p>and carbonyl oxygen, which enhances the positivity of carbonyl oxygen,</p><p>will further promote carbonyl carbon nucleophilic attack. The water by-</p><p>product is then formed by breaking the metal–carbonyl oxygen coordi-</p><p>nation, giving alkyl levulinate as final product (Fig. 16b).</p><p>Table 4 summarizes the recent works concerning the esterification of</p><p>levulinic acid over MOF catalysts. As can be seen, most of the works</p><p>were focused on the UiO-66 so far while >90,000 MOFs have been</p><p>synthesized and over 500,000 MOFs have been predicted [121]. Unlike</p><p>other heterogeneous catalysts, MOF catalysts seem to give better cata-</p><p>lytic activity because of their very high surface area that allows high</p><p>functionalization of active groups besides also allowing reactant acti-</p><p>vation through high binding affinity to the metal sites. In addition, their</p><p>porous structure with specific pore size and shape always ensures high</p><p>product selectivity.</p><p>The enhanced esterification reaction of levulinic acid by MOF cata-</p><p>lysts is associated with the ease of abstraction of proton from the alco-</p><p>holic group by both acidic and basic ligands as evidenced in the</p><p>mechanism above (Fig. 16). Furthermore, the MOF materials provide</p><p>thermal and chemical stabilities besides having uniform nanoporous</p><p>structure. However, its major drawback is catalyst design in such a way</p><p>to avoid loss of active sites which consequently lead to severe toxicity</p><p>and pollution problems due to the metal present. Hence, more studies on</p><p>exploring and using new MOF catalysts on the esterification of levulinic</p><p>acid is highly appreciated.</p><p>2.5. Ion exchange resins</p><p>Ion exchange resin is a network organo-polymer compound that</p><p>contains active ion-exchange groups with vast surface areas, high ion-</p><p>exchange capacities and a wide range of functionalities [45,122].</p><p>These features are also depending on the type of resin used. Ion ex-</p><p>change resins are frequently employed as acid catalysts in alkylation,</p><p>addition, esterification, isomerization and other organic reactions due to</p><p>their high acidity density [122]. In the study of levulinic acid esterifi-</p><p>cation, Amberlyst-15, Amberlyst-46, Amberlyst-70, Purolite, Dowex,</p><p>and Aquivion have all been investigated as solid acid catalysts (Table 5).</p><p>In general, these resins contain sulfonic acid groups (-SO3H) that give</p><p>them acidic characteristics, which kick-start the reaction conversion</p><p>process.</p><p>In year 2012 Fernandes et al. [24] examined the catalytic perfor-</p><p>mance of sulfated oxides (SnO2, ZrO2, Nb2O5, TiO2), zeolites (H-USY, H-</p><p>BEA, H-MOR, H-ZSM-5, H-MCM-22), and sulfonic acid resin (Amberlyst-</p><p>15). They found that there is a link between the amount of solid acid</p><p>catalysts and the activity of sulfated oxides. However, this relation is not</p><p>applicable in the case for zeolite catalysts where the pore channels serve</p><p>a more essential role. Amberlyst-15 gives the highest yield of ethyl</p><p>levulinate (54% at 70 ◦C for 5 h, Table 5, entry 1) among the catalysts</p><p>evaluated, which is due to the strong acidity supplied by -SO3H func-</p><p>tional groups and SO4</p><p>2− species.</p><p>Ramli et al. [123] used Amberlyst-15 as a catalyst for synthesizing</p><p>Fig.15. Illustration of the structures of (a) UiO-66 and UiO-66-(COOH)2, (b) UiO-66 with Zr4+ coordination vacancies created when a linker molecule is missing are</p><p>evidenced by red arrows. Zr4+-pink; C-light blue; O-red, (c) UiO-66(Hf)-SO3H prepared using monosodium 2-sulfoterepthalate and HfCl4 via hydrothermal method.</p><p>Adapted from Wang et al. [116], Cirujano et al.[117] and Gupta and Kantam [119], respectively. (For interpretation of the references to colour in this figure legend,</p><p>the reader is referred to the web version of this article.)</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>16</p><p>alkyl levulinates. By using methanol, ethanol and 1-butanol, 82.2%,</p><p>70.9%, and 55.3% yields of methyl levulinate, ethyl levulinate and butyl</p><p>levulinate are achieved, respectively, at the optimum reflux conditions</p><p>(5 h at boiling point of respective alcohol) (Table 5, entry 2–4). In</p><p>addition, the Amberlyst-15 catalyst after reaction is easy to purify</p><p>through a simple washing step and is reusable even after five reaction</p><p>cycles.</p><p>Ion exchange resins have a large concentration of acid sites with a</p><p>relatively low acid strength. The acid strength of sulfonic groups is quite</p><p>similar in ion exchange resins, but there are some minor variances based</p><p>on their morphology. Tejero et al. [12] discovered that butyl levulinate</p><p>synthesis is tightly linked to the morphological resin characteristics that</p><p>improve acid centre accessibility. Eleven types of resins with different</p><p>morphologies (e.g., macroreticular and gel-type structures) are studied.</p><p>In general, very high butyl levulinate selectivity (>99.5%) is achieved</p><p>over all resins investigated with a levulinic acid conversion ranging from</p><p>64% to 94% (Table 5, entries 5–15). From the study, the swelling</p><p>property of polymeric resins affect the enlargement of pore size, thereby</p><p>enhancing the surface area and mass transfer that are both crucial de-</p><p>terminants in their catalytic activity. As a result, gel-type resins like</p><p>Dowex 50Wx2 (Table 5, entry 15) and Purolite (Table 5, entry 13) with</p><p>superior swelling ability produce higher yields than macro-porous resins</p><p>like Amberlyst 70 (Table 5, entry 11). In addition, gel-type resins con-</p><p>taining 2–4% DVB (particularly Dowex 50Wx2) (Table 5, entry 15) not</p><p>only are more selective but also are more active than zeolites and other</p><p>discussed catalysts in this review [12].</p><p>Trombettoni et al. [124] catalyzed the esterification of levulinic acid</p><p>and 1-Pentanol using newly developed acidic resins with different</p><p>morphologies, i.e., Amberlyst 15 (A-15, macroreticular structure),</p><p>polystyrene-supported p-toluensulfonic acid (PS-pTsOH, macroporous</p><p>micronized pellets) and perfluorosulfonic polymer Aquivion PFSA</p><p>(Aquivion mP98, non-porous micronized pellets). Pentyl levulinate is</p><p>generated with an excellent selectivity over this synthetic resin</p><p>(70–90 ◦C, 24 h). For instance, Aquivion mP98 (92.0%) is more selective</p><p>than PS-pTsOH (83%) and Amberlyst 15 (67%) in the synthesis of pentyl</p><p>levulinate (Table 5, entries 16, 20 and 24). Among these resins,</p><p>Aquivion-mp98 gives the highest yield with longer catalyst lifetime,</p><p>which is attributed to its hydrophobicity that hinders the adsorption of</p><p>water side product on its surface. In the case of Amberlyst 15, the water</p><p>produced during the esterification reaction tends to occupy the catalyst</p><p>surface, preventing the access of reactants to the acid sites. As a result, it</p><p>gives the lowest yield (58%) (Table 5, entry 26). In addition, the ester-</p><p>ification reactions using aliphatic alcohols with short, moderate and</p><p>branched chains are also studied. The results reveal that the conversion</p><p>of levulinic acid to the respective alkyl alcohol occurred in good-to-high</p><p>yields (50–92%) (Table 6). Furthermore, at the measured reaction</p><p>conditions, no by-products resulting from inter/intramolecular dehy-</p><p>dration of the various alcohols were identified.</p><p>Fig. 16. Reaction mechanisms using (a) acidic and (b) basic MOF catalysts for esterifying levulinic acid into alkyl levulinate. Adapted from Badgujar et al. [13].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>17</p><p>Using a low levulinic acid to butanol molar ratio of 1:4 and a reaction</p><p>temperature of 124 ◦C, Kokare et al. [125] optimized the response sur-</p><p>face for the preparation of butyl levulinate over Amberlyst-15, achieving</p><p>a maximum levulinic acid conversion of 97% (Table 5, entry 28). Russo</p><p>et al. [126,127] used intraparticle diffusion and kinetic models to</p><p>establish the intrinsic kinetic parameters for the production of ethyl</p><p>levulinate using Amberlyst-15 [126] and Smopex 101 resins [127].</p><p>According to Russo and colleagues [127], the Smopex 101 is a non-</p><p>porous ion exchange resin that avoids intraparticle diffusion con-</p><p>straints, making it a favourable choice for promoting the reaction.</p><p>However, this resin is no longer accessible on the market, so a substitute</p><p>has to be sought.</p><p>Russo et al. [128] recently investigated the catalytic reaction pa-</p><p>rameters onto the same esterification reaction using a kinetic model</p><p>where Amberlite IR120 was used as the catalyst. High yield of ethyl</p><p>levulinate (90.0%) was achieved under the optimized conditions (70 ◦C,</p><p>2.5 h, levulinic acid:ethanol molar ratio of 1:5) (Table 5, entry 29).</p><p>According to the kinetic study, the bulk density of Amberlite IR120 has a</p><p>direct impact on levulinic acid conversion. It is also found that the cation</p><p>exchange resins show the highest catalytic efficiency, and Amberlite</p><p>IR120 as a low-cost, eco-friendly esterification catalyst can be used</p><p>repeatedly without losing its activity.</p><p>In the esterification of levulinic acid with butanol, Iborra et al. [19]</p><p>compared ten types of acid ion exchange catalysts, which comprised of</p><p>four gel types (Amberlyst 121, Dowex 50Wx2, Dowex50Wx4 and</p><p>Dowex50Wx8), and six macroreticular types (Amberlyst 15, Amberlyst</p><p>16, Amberlyst 35, Amberlyst 36, Amberlyst 39 and Amberlyst 46). The</p><p>levulinic acid conversion is conducted within 4–6 h, and no organic by-</p><p>products are discovered. Gel-type resins have shown to be more active</p><p>over time than the macroreticular resins. The morphology of both types</p><p>of resins in the dry and swelled states is depicted in Fig. 17. In resins,</p><p>three types of pore sizes can be recognized, namely micropores, meso-</p><p>pores and macropores. Macroreticular resins have permanent specific</p><p>porosity. In contrast, gel-type resins have only micropores when dry, but</p><p>the mesopores are slowly developed when they swell in a polar media,</p><p>and these mesopores disappear when they shrink. Among ten resins</p><p>studied, Dowex 50Wx2 (gel-type resin) with low crosslinking degree is</p><p>the best selective resin catalyst (selectivity of butyl levulinate = 98.1%)</p><p>(Table 5, entry 37), which could be attributed to the increased accessi-</p><p>bility of active centres (due to swelling) when the reaction medium is</p><p>Table 5</p><p>Summary of the reaction conditions, results and types of alkyl levulinates formed over ion exchange resin catalysts.</p><p>Entry Catalyst Alkyl</p><p>levulinatea</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 2.5 wt% Amberlyst 15 EL 70 5 1:5 54.0 100 54.0 [24]</p><p>2 30 wt% Amberlyst 15 ML 64.7 5 1:20 NA NA 82.2 [123]</p><p>3 30 wt% Amberlyst 15 EL 78.4 5 1:20 NA NA 70.9 [123]</p><p>4 30 wt% Amberlyst 15 BL 117.7 5 1:20 NA NA 55.3 [123]</p><p>5 Amberlyst 46 BL 80 8 1:3 63.9 99.6 63.7 [12]</p><p>6 Amberlyst 15 BL 80 8 1:3 69.8 99.7 69.6 [12]</p><p>7 Amberlyst 35 BL 80 8 1:3 70.9 99.8 70.8 [12]</p><p>8 Amberlyst 16 BL 80 8 1:3 74.9 99.6 74.6 [12]</p><p>9 Amberlyst 36 BL 80 8 1:3 78.1 99.5 77.7 [12]</p><p>10 Amberlyst 39 BL 80 8 1:3 86.6 99.9 86.5 [12]</p><p>11 Amberlyst 70 BL 80 8 1:3 81.0 99.9 81.0 [12]</p><p>12 Dowex 50WX8 BL 80 8 1:3 81.3 99.7 81.1 [12]</p><p>13 Purolite CT224 BL 80 8 1:3 90.6 99.8 90.4 [12]</p><p>14 Dowex 50WX4 BL 80 8 1:3 92.4 99.9 92.3 [12]</p><p>15 Dowex 50WX2 BL 80 8 1:3 93.6 99.9 93.6 [12]</p><p>16 10 mol % Aquivion mP98 PENL 90 24 1:10 92.0 100 92.0 [124]</p><p>17 10 mol % Aquivion mP98 PENL 70 24 1:10 86.0 100 86.0 [124]</p><p>18 10 mol % Aquivion mP98 PENL 90 24 1:5 76.0 100 76.0 [124]</p><p>19 10 mol % Aquivion mP98 PENL 70 24 1:5 72.0 100 72.0 [124]</p><p>20 Polystyrene-supported p-</p><p>toluensulfonic acid</p><p>PENL 90 24 1:10 83 100 83 [124]</p><p>21 Polystyrene-supported p-</p><p>toluensulfonic acid</p><p>PENL 70 24 1:10</p><p>80 100 80 [124]</p><p>22 Polystyrene-supported p-</p><p>toluensulfonic acid</p><p>PENL 90 24 1:5 68 100 68 [124]</p><p>23 Polystyrene-supported p-</p><p>toluensulfonic acid</p><p>PENL 70 24 1:5 68 100 68 [124]</p><p>24 10 mol% Amberlyst 15 PENL 90 24 1:10 67.0 100 67.0 [124]</p><p>25 10 mol% Amberlyst 15 PENL 70 24 1:10 76.0 100 76.0 [124]</p><p>26 10 mol% Amberlyst 15 PENL 90 24 1:5 58.0 100 58.0 [124]</p><p>27 10 mol% Amberlyst 15 PENL 70 24 1:5 65.0 100 65.0 [124]</p><p>28 20 wt% Amberlyst 15 BL 124 1.25 1:4 97.0 NA NA [125]</p><p>29 Amberlite IR120 EL 70 2.5 1:5 NA NA 90.0 [128]</p><p>30 Amberlyst 46 BL 155–160 423b 1:3 84.7 97.3 82.4 [19]</p><p>31 Amberlyst 35 BL 155–160 393b 1:3 82.1 93.3 76.6 [19]</p><p>32 Amberlyst 15 BL 155–160 359b 1:3 82.1 94.3 77.4 [19]</p><p>33 Amberlyst 16 BL 155–160 386b 1:3 81.6 92.8 75.7 [19]</p><p>34 Amberlyst 36 BL 155–160 355b 1:3 80.2 90.1 72.3 [19]</p><p>35 Amberlyst 39 BL 155–160 328b 1:3 83.7 94.0 78.7 [19]</p><p>36 Amberlyst 121 BL 151–155 316b 1:3 82.7 95.0 78.6 [19]</p><p>37 Dowex 50Wx2 BL 151–155 232b 1:3 83.4 98.1 81.8 [19]</p><p>38 Dowex 50Wx4 BL 151–155 318b 1:3 83.3 96.8 80.6 [19]</p><p>39 Dowex 50Wx8 BL 151–155 373b 1:3 83.8 95.2 79.8 [19]</p><p>aML: Methyl levulinate; EL: Ethyl levulinate; BL: Butyl levulinate; PENL: Pentyl levulinate.</p><p>bContact time =</p><p>t × W</p><p>n</p><p>; t = time (min), W = Dried mass of catalyst (g), n = Initial moles of levulinic acid.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>18</p><p>polar.</p><p>A summary of the reaction conditions (types of resin, temperature,</p><p>time, reactants amount) and results (conversion, selectivity, yield, types</p><p>of alkyl levulinate produced) from previous works is depicted in Table 5.</p><p>It is shown that gel type resin catalysts (e.g., Purolite CT224, Dowex</p><p>50Wx2 and Dowex 50WX4) with good swelling ability are more pref-</p><p>erable since they provide enhanced porosity and excellent yield of alkyl</p><p>levulinates. The use of resin catalysts provides many benefits such as</p><p>simple recovery, high reusability and low corrosivity. Furthermore, it</p><p>preserves the reaction media’s flexibility and hydrophobicity. However,</p><p>the main disadvantages of using resins are their high cost, poor thermo-</p><p>stability, inability to handle high boiling solvents, and the breakdown of</p><p>functional groups especially when they are applied at high temperature.</p><p>Hence, these factors have to be taken into consideration when the resin</p><p>catalysts are used for commercial esterification application.</p><p>2.6. Biocatalysts</p><p>Biocatalysts have been proven as a promising alternative for esteri-</p><p>fication by offering mild reaction conditions and high product selectivity</p><p>[28]. Since years, countless efforts have been dedicated to maximize the</p><p>esters yield. In such, lipase as biocatalyst has played a signifying role in</p><p>the esterification. For example, in a recent work, Lipase from</p><p>C. antarctica was immobilized on the (3-aminopropyl)trimethoxysilane</p><p>(APTMS) and grafted on mesoporous silica nanoflowers via reverse</p><p>microemulsion method where tetraethoxysilane (TEOS) was used as the</p><p>silica source (Fig. 18) [129]. The prepared catalyst was used for the</p><p>conversion of levulinic acid into ethyl levulinate. Upon immobilization,</p><p>the so-called Lipase@silica nanoflowers-NH2 catalyst gave almost</p><p>complete conversion of levulinic acid (99.5%) in 8 h at 40 ◦C compared</p><p>to free lipase (67.9%) (Table 7, entries 3). The immobilized biocatalyst</p><p>revealed high catalyst stability and reusability of four times with above</p><p>80% yield. However, a decrease in the yield was observed after seventh</p><p>cycle, where only 68% yield of ethyl levulinate was produced. Never-</p><p>theless, this observation does not negate to overall performance of</p><p>biobased mesoporous silica catalyst where it clearly represents a novel</p><p>strategy to produce biofuels from biomass.</p><p>In another effort on studying the effect of lipase immobilization on</p><p>levulinic acid conversion, Di et al. [28] discovered the excellent activity</p><p>of Candida antarctica lipase B immobilized on macroporous acrylic resin</p><p>(Novozym 435) in the synthesis of methyl levulinate. The biocatalyst</p><p>(93% yield, Table 7, entry 1) performed excellently as compared to other</p><p>lipases like Thermomyces lanuginosus lipase (TLL) immobilized on silica</p><p>gel (Lipozyme TL IM), Rhizomucor miehei lipase (RML) immobilized on</p><p>macroporous acrylic resin (Lipozyme RM IM), and Candida rugosa lipase</p><p>(CRL) immobilized on immobead 150 that only achieved 1–7% yield</p><p>after 24 h at 30 ◦C. In such, the production of methyl levulinate over</p><p>Novozym 435 was significantly promoted with the use of [bmim][PF6]</p><p>hydrophobic ionic liquid as co-solvent which hampers the hydrolysis of</p><p>lipases.</p><p>An interesting work by immobilizing lipase on SBA-15 (SBA-15-</p><p>APTS-GLU-Lip) for the production of isoamyl levulinate from levulinic</p><p>acid was recently reported by Salvi and Yadav [75]. The catalyst was</p><p>applied over various reaction media and apparently, methyl tert-butyl</p><p>ether (MTBE)—a hydrophobic solvent—was the best solvent. Interest-</p><p>ingly, under the optimized reaction condition, the catalyst recorded</p><p>upgraded yield of isoamyl levulinate (94%, Table 5, entry 4) and upon</p><p>reducing the chain length of acyl donor, a drop in the alkyl levulinate</p><p>yield was observed (Table 7, entries 5–6). These findings clearly</p><p>demonstrated the versality of SBA-15-APTS-GLU-Lip catalyst in</p><p>Table 6</p><p>Esterification of levulinic acid with various alcohols using different types of resin</p><p>catalyst.</p><p>Entry Alcohol Catalyst typeb Temperature (◦C) Conversion (%)</p><p>1</p><p>2</p><p>3</p><p>Methanol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>58</p><p>58</p><p>60</p><p>4</p><p>5</p><p>6</p><p>Ethanol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>55</p><p>50</p><p>60</p><p>7</p><p>8</p><p>9</p><p>Propanol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>75</p><p>26</p><p>70</p><p>10</p><p>11</p><p>12</p><p>Butanol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>76</p><p>58</p><p>58</p><p>13</p><p>14</p><p>15</p><p>Octanol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>84</p><p>62</p><p>80</p><p>16</p><p>17</p><p>18</p><p>Propan-2-ol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>50</p><p>32</p><p>12</p><p>19</p><p>20</p><p>21</p><p>Butan-2-ol Aquivion mP98</p><p>PS-p-TsOH</p><p>A-15</p><p>90</p><p>90</p><p>70</p><p>56</p><p>25</p><p>10</p><p>a. Adapted from Trombettoni et al. [124].</p><p>Reaction conditions: Molar ratio 1:10 (levulinic acid: alcohol), 10% mol of</p><p>catalyst, time 24 h.</p><p>bAquivion mP98: Perfluorosulfonic polymer Aquivion®; PS-pTsOH: poly-</p><p>styrene-supported p-toluensulfonic acid; A-15: Amberlyst 15®.</p><p>Fig. 17. Swelling of gel-type and macroreticular resin catalysts in polar medium (e.g., water) causes morphological changes. Adapted from Iborra et al. [19].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>19</p><p>catalyzing different alcohols.</p><p>For the first time, the combination effect of mesoporosity and mac-</p><p>roporosity were investigated by Zhou et al. [130]. The hierarchical</p><p>catalyst was prepared through immobilization of Candida antarctica</p><p>lipase B on organosilica material through adsorption approach (denoted</p><p>as 3DOM/m-OS) and nanoscale enzyme reactor method (designated as</p><p>NER@3DOM/m-OS). NER@3DOM/m-OS appears as an active biocata-</p><p>lyst in the conversion of levulinic acid into alkyl levulinate at an opti-</p><p>mized condition of 12 h and 1:10 M ratio of levulinic acid:alcohol. The</p><p>catalyst witnessed a growing trend in ester yield with an increase in</p><p>alcohol chain length. For example, dodecanol (91.14%) > octanol</p><p>(84.51%) > n-butanol (74.59%) (Table 7, entries 7–9). Such activity is</p><p>resulted from the inhibition of lipase activity when the alcohol chain</p><p>length decreases (hydrophobicity decreases). NER@3DOM/m-OS</p><p>retained its activity after nine reusability runs, recording alkyl</p><p>levulinate yields of 46.18% (butyl levulinate), 82.33% (octyl levulinate)</p><p>and 81.25% (DCL) that confirms the active</p><p>participation of</p><p>NER@3DOM/m-OS compared to Novozym 435 under the similar reac-</p><p>tion conditions.</p><p>On contrary, alkali-thermostable lipase LipR2 on Florisil nano-</p><p>particle showed a different trend in the esterification of butanol and</p><p>dodecanol, reaching the yields of 48.8% and 26.2% at 55 ℃ after 12 and</p><p>24 h, respectively (Table 7, entries 10–11) [131].</p><p>A solvent-free synthesis of amyl levulinate (AML) catalyzed by</p><p>immobilized Candida antarctica lipase B has recently been developed by</p><p>Jaiswal and Rathod [132]. The green production of 73.2% AML ester</p><p>was obtained at 50 ℃ using 3-fold molar excess of amyl alcohol. The</p><p>catalyst experienced minimum loss in the activity after five cycles. The</p><p>characterization of the reused catalyst by SEM analysis verified the</p><p>changes in the CALB morphology after cycling test, suggesting the drop</p><p>Fig. 18. Preparation flow of Lipase@silica nanoflower composite from silica nanoflowers via reverse microemulsion technique using TEOS silica source and</p><p>cetyltrimethylammonium bromide (CTAB) template. Adapted from Jia et al. [129].</p><p>Table 7</p><p>Summary of the reaction conditions, results, and the types of alkyl levulinates formed using biocatalyst.</p><p>Entry Catalysta Alkyl</p><p>levulinate b</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 Novozym® 435 ML 30 24 1:5 NA NA 93.0 [28]</p><p>2 Free lipase EL 40 8 1:10 NA NA 67.9 [129]</p><p>3 Lipase@silica nanoflowers-NH2 EL 40 8 1:10 NA NA 99.5 [129]</p><p>4 SBA-15-APTS-GLU-Lip ISAL 50 2 1:2 94.0 NA NA [75]</p><p>5 SBA-15-APTS-GLU-Lip OL 50 2 1:2 92.0 NA NA [75]</p><p>6 SBA-15-APTS-GLU-Lip HL 50 2 1:2 90.0 NA NA [75]</p><p>7 NER@3DOM/m-OS BL 40 12 1:10 NA NA 74.6 [130]</p><p>8 NER@3DOM/m-OS OL 40 12 1:10 NA NA 84.5 [130]</p><p>9 NER@3DOM/m-OS DODL 40 12 1:10 NA NA 91.1 [130]</p><p>10 LipR2 BL 55 12 1:10 NA NA 48.8 [131]</p><p>11 LipR2 DODL 55 24 1:10 NA NA 26.2 [131]</p><p>12 Candida antarctica lipase B PENL 50 10 1:3 NA NA 73.2 [132]</p><p>13 Candida antarctica lipase B HL 45 8 7:4 NA NA 96.0 [39]</p><p>14 Candida antarctica lipase B ISBL 45 8 7:4 NA NA 97.0 [39]</p><p>15 Candida antarctica lipase B ISAL 45 8 7:4 NA NA 98.0 [39]</p><p>16 CALB@nanodisperse BL 50 24 NA NA NA 85.5 [133]</p><p>17 CRL-ALG/NC/MMT EL 50 2 1:2 NA NA 92.9 [134]</p><p>18 Novozym 435 (Candida antarctica</p><p>lipase)</p><p>BL 50 2 1:2 85.0 NA NA [135]</p><p>a SBA-15-APTS-GLU-Lip: SBA-15 immobilized lipase and cross-linked with glutaraldehyde; NER@3DOM/m-OS: CALB immobilized on meso-molding 3DOM orga-</p><p>nosilica through anoscale enzyme reactor; LipR2: alkalithermostable lipase; CRL-ALG/NC/MMT: Candida rugosa lipase onto oil palm frond leaves derived nano-</p><p>cellulose and montmorillonite in alginate.</p><p>bML: Methyl levulinate; EL: Ethyl levulinate; BL: Butyl levulinate; ISBL: Isobutyl levulinate; HL: Hexyl levulinate; ISAL: Isoamyl levulinate; DODL: Dodecyl levulinate;</p><p>PENL: Pentyl levulinate.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>20</p><p>in the ester yield during regeneration activities. Nevertheless, mild re-</p><p>action condition and solvent-free catalytic system promise a great</p><p>breakthrough in green catalysis for the esterification of levulinic acid</p><p>and amyl alcohol.</p><p>A plausible reaction mechanism showing active participation of</p><p>lipase enzyme comprising of active amino acids, such as serine, aspartic</p><p>acid and the histidine, in the esterification reaction is shown (Fig. 19).</p><p>Briefly, the Michaelis-Menten complex is formed as an initial step when</p><p>the serine and glycine react with levulinic acid. Then the complex is</p><p>converted to the tetrahedral intermediate and further reacted with</p><p>serine hydroxyl group to release water as a by-product, forming an acyl-</p><p>enzymatic intermediate. Another tetrahedral intermediate forms when</p><p>Fig. 19. Amyl levulinate synthesis route over lipase enzyme. Adapted from Jaiswal and Rathod [132].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>21</p><p>the nucleophile (amyl alcohol) attacks the carbonyl carbon of acyl-</p><p>enzymatic intermediate. The tetrahedral intermediate then breaks</p><p>down into the amyl levulinate ester and lipase enzyme where the amino</p><p>acids of lipase enzyme help in proton abstraction and induce hydrogen</p><p>bonding between enzyme and incoming reactant species.</p><p>A detailed investigation on the effect of reaction parameters such as</p><p>biocatalyst loading, reaction medium, temperature and molar ratio of</p><p>reactants was also screened over Candida antarctica lipase B in the</p><p>esterification of hexanol [39]. Under the optimized condition, various</p><p>fuel additives of >96% yield (Table 7, entries 13–15) with oxygen</p><p>content ranging 24–28 wt% were prepared at lower reaction tempera-</p><p>ture of merely 45 ℃ as compared to the temperature reported in liter-</p><p>atures. This route hence produces a recyclable green catalyst that can</p><p>sustain its activity after five reuses.</p><p>Gao et al. [133] continued the effort on the exploitation of Candida</p><p>antarctica lipase B (CALB) catalyst in the esterification of levulinic acid.</p><p>In their work, nanoflowers with magnetic core having flower-like</p><p>organosilica with unique characteristics of perpendicular and wrinkle</p><p>channels were used as the support. These attributes were the reasoning</p><p>for the superior catalysis of the supported CALB (CALB@nanoflowers)</p><p>biocatalyst compared to the CALB supported monodisperse core-shell</p><p>magnetic organosilica (CALB-MMS) in the solvent-free synthesis of</p><p>butyl levulinate after 24 h (Table 7, entry 16). CALB@nanoflowers also</p><p>showed extended application in the esterification of long chain alcohols</p><p>such as n-caprylic alcohol and n-lauryl alcohol which proves its versa-</p><p>tility in catalyzing many alcohols into value-added fuel additives.</p><p>Hussin et al. [134] reported the preparation of Candida rugosa lipase</p><p>(CRL) supported nanocellulose (NC) and montmorillonite (MMT) in</p><p>alginate (ALG) biocatalyst. The catalyst designated as CRL-ALG/NC/</p><p>MMT showed a remarkable activity in the synthesis of ethyl levulinate</p><p>(92.9%, Table 7, entry 17). For comparison, the performance of CRL-</p><p>ALG/NC/MMT is equivalent to the reported work of Jia and co-</p><p>researchers [129]. However, CRL-ALG/NC/MMT requires very short</p><p>duration of 2 h and lower amount of ethanol to yield 92.9% of ethyl</p><p>levulinate compared to those required by Jia et al. [129]. In addition,</p><p>high reusability up to nine cycles was shown by CRL-ALG/NC/MMT and</p><p>this indisputably promises its use for the production of different AL.</p><p>In a study by Yadav and Borkar [135], a detailed investigation on the</p><p>influence of reaction parameters such as temperature, catalyst loading</p><p>and concentration of n-butanol in the presence of tert-butyl methyl ether</p><p>solvent was studied during the synthesis of butyl levulinate. Novozym</p><p>435 (Candida antarctica lipase) was chosen as the best catalyst under</p><p>optimal conditions, achieving over 85% conversion at 50 ◦C after 2 h of</p><p>reaction. The kinetic investigation was in accordance with ping-pong bi-</p><p>bi mechanism that relates with alcohol inhibition. In short, the catalyst</p><p>reported in this work is far superior than bio-glycerol derived carbon-</p><p>–SO3H catalyst [103] that gives only 50% conversion of BL after 3 h</p><p>reaction at a higher temperature of 80 ℃ (Table 3, entry 12). Hence,</p><p>higher reaction profile of lipase catalysts clearly evidences their excel-</p><p>lent catalytic activity than the other reported catalysts.</p><p>Table 7 summarizes the recent enzyme catalytic research on the</p><p>esterification of levulinic acid. As seen, high reaction performance can</p><p>be achieved over biocatalysts at low reaction temperature (<50 ℃) as</p><p>compared to those catalyzed</p><p>using other classes of catalyst. While it is</p><p>apparent that enzyme lipase has exceptional activity and selectivity, the</p><p>main drawbacks are poor reusability and stability. Immobilized bio-</p><p>catalysts, which are designed to give competitive activity, selectivity</p><p>and stability in organic medium, considerably overcome these re-</p><p>strictions [130].</p><p>3. Overview and future perspectives</p><p>Levulinic acid is considered as one of the most essential bio-derived</p><p>platform chemicals for extensive applications. Its derivative–alkyl lev-</p><p>ulinates–which are normally synthesized through acid-catalyzed ester-</p><p>ification are promising compounds used in broad industries including</p><p>pharmaceuticals, agriculture, cosmetics, polymers and plasticizer, resin</p><p>and coatings. Alkyl levulinates are also an excellent additive for gasoline</p><p>and diesel by improving the engine performance and facilitating low</p><p>harmful gas emission.</p><p>In this article, the most important heterogeneous catalysts, namely</p><p>zeolites, mesoporous silicas, carbonaceous materials, metal organic</p><p>frameworks, ion exchange resins and biocatalysts, for the trans-</p><p>formation of levulinic acid to alkyl levulinates via esterification have</p><p>been critically reviewed. A comprehensive understanding on the basic</p><p>aspects, including the nature of catalysts, reaction parameters effects</p><p>and reaction mechanisms of esterification, have been discussed. At</p><p>present, nearly 90% of research laboratory and industrial processes are</p><p>driven by heterogeneous catalysis. Thus, the discussed information in</p><p>this review is highly valuable for continuous development of heteroge-</p><p>neous catalysis, including research on new catalysts and modifications</p><p>for more selective, active and stable catalysts.</p><p>Each catalyst has its own set of benefits and drawbacks, and there is</p><p>always space for catalyst improvement. The development of modern</p><p>heterogeneous acid catalysts is quite challenging, and portraying a high</p><p>risk of loss of active site during preparation process. Thus, it requires an</p><p>appropriate technology, high laboratory skills and fundamental knowl-</p><p>edge. Zeolites (e.g. H-ZSM-5) offer strong acidity and accessible specific</p><p>pore sizes for the diffusion of reactant molecules thereby producing a</p><p>selective alkyl levulinate product. However, their small pore size only</p><p>allows esterification reactions involving small molecules. Such re-</p><p>strictions can be overcome by engineering the porosity of zeolite</p><p>through soft templating and desilication treatments. Furthermore, sur-</p><p>face modification via functionalization with active species (e.g., heter-</p><p>opolyacid) is also feasible for acidity enhancement, thereby improving</p><p>the reaction selectivity.</p><p>Mesoporous silica catalysts require longer reaction time, higher</p><p>temperature and larger amount of alcohol for enhancing the esterifica-</p><p>tion reaction which violates the principle of green chemistry. Novel</p><p>hybrid mesoporous inorganic-organic silica catalysts with increased acid</p><p>sites are hence designed to boost up the reaction rate at lower temper-</p><p>ature, shorter reaction time and less amount of alcohol. Thus, the</p><p>principles of green chemistry and the green chemistry metrics parame-</p><p>ters, such as E-factor, atom economy, reaction and effective mass effi-</p><p>ciency, percentage yield and number of recycle, have to be considered</p><p>for the mesoporous silica catalysis system, so that the overall synthesis of</p><p>alkyl levulinates process is greener and more cost-effective with minimal</p><p>waste.</p><p>Carbonaceous catalyst is an active and robust material. Yet, the</p><p>synthesis procedure needs very high pyrolysis temperature (for biochar,</p><p>carbon cryogel and activated carbon) and high synthesis cost (for</p><p>SWCNT and MWCNT), which are the most important concern for scaling</p><p>up production. Furthermore, strong adsorption between levulinic acid</p><p>and sulfonic acid-carbon catalysts often lead to lose their catalytic ac-</p><p>tivity and reusability. Therefore, the more energy saving strategies and</p><p>modification of carbonaceous catalysts with high surface area and high</p><p>density of acidic sites which reflects real practical application are</p><p>needed.</p><p>MOFs are good catalysts for esterification of levulinic acid thanks to</p><p>their high porosity, uniform micropore size and high recyclability.</p><p>However, the catalysts are expensive and hard to scale up for industry</p><p>use. In addition, the active chemical sites have very high tendency to</p><p>lose during the esterification process. Thus, a good skill in designing of</p><p>stable MOF catalysts is needed. In addition, some metal centers present</p><p>in MOF are toxic, which restricts their practical application. Therefore,</p><p>green chemistry principles have to be followed when designing a MOF</p><p>catalyst so as can produce alkyl levulinates in a more eco-friendly and</p><p>cost-effective way.</p><p>The use of resins offers various advantages, including easy catalyst</p><p>separation, catalyst recyclability and reactor corrosion resistance. It also</p><p>maintains the correct level of hydrophobicity, flexibility and porosity in</p><p>the reaction media. High cost, poor thermal stability, inability to handle</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>22</p><p>high boiling solvents, stiffness, dissolution of basic functionality at se-</p><p>vere temperatures, formation of H-bonds, and destabilization of active</p><p>groups are the drawbacks of resin catalysts. When compared to the</p><p>limited literature survey data, it is obvious that using gel-type resins as</p><p>catalysts for levulinic acid esterification is a very promising green</p><p>alternative for further industrial application. However, polymeric resins</p><p>can only operate at temperatures lower than inorganic solid catalysts.</p><p>Lastly, enzymatic catalyst certainly makes some significant ad-</p><p>vancements in the esterification of levulinic acid especially with long</p><p>chain alcohols (e.g. n-caprylic alcohol, n-lauryl alcohol). In general,</p><p>enzyme gets denatured at high temperature which prevents its use in</p><p>extensive applications. However, with the devoted efforts, researchers</p><p>have designed enzymatic supported catalysts which garner a great</p><p>breakthrough in heterogeneous catalysis as they require only low reac-</p><p>tion temperature and solvent-free reaction environment compared to</p><p>other reported catalysts. These biological catalysts apparently are more</p><p>selective and active in the production of various alkyl levulinates.</p><p>The reaction mechanisms proposed in zeolites, mesoporous silicas,</p><p>carbonaceous materials, metal organic frameworks (MOFs) and ion ex-</p><p>change resins share several similarities. The reaction usually begins with</p><p>the diffusion of reactant molecules to the active sites, followed by the</p><p>formation of electron-deficient carbonyl carbon by protonation</p><p>(Brønsted acid sites) and electron-donating of carbonyl oxygen to the</p><p>metal (Lewis acid sites) of catalysts. Then, it is followed by nucleophilic</p><p>attack of alcohol molecules on the carbonyl carbon forming alkyl esters</p><p>and water by-product. The products are then desorbed from the active</p><p>sites and the catalyst is then regenerated. For some heterogeneous cat-</p><p>alysts with specific pores sizes like zeolites, MOFs and mesoporous sil-</p><p>icas, pore confinement effect is also a crucial factor in stabilizing product</p><p>intermediate and facilitating esterification reaction. On the other hand,</p><p>the biocatalyzed esterification undergoes different reaction pathway</p><p>where a lipase-levulinic acid complex is first formed through a confor-</p><p>mational change with an assistance of several active amino acids. The</p><p>reaction then proceeds through the nucleophilic attack after the acti-</p><p>vation of alcohol molecules by the lipase enzyme.</p><p>The high yield of alkyl levulinates is not only merely depending on</p><p>the active sites of the catalyst alone. Yet, other reaction engineering</p><p>aspects such as catalyst loading,</p><p>reactants molar ratio, reaction tem-</p><p>perature and time also play a crucial role in governing the catalytic</p><p>performance. Meanwhile, a complete understanding and detailed</p><p>investigation on the parameter studies could lead to plausible reaction</p><p>mechanism as well as kinetic model, which are lacked in the existing</p><p>publications. This valuable information and knowledge are essential for</p><p>designing more active and selective catalysts for the prospective in-</p><p>dustrial use. Prior transferring the knowledge to the industrial scale-up</p><p>process, several concerns about the type of catalyst, reaction conditions,</p><p>internal diffusion constrain, catalyst deactivation, heat and mass trans-</p><p>fers, chemical kinetics, activation energy, catalyst reusability and</p><p>product yield need to be considered in order to design the proper</p><p>reactors.</p><p>Furthermore, it is important to highlight that alkyl levulinates serve</p><p>as an excellent fuel blender and additive. Therefore, extensive research</p><p>on analyzing fuel blending properties for engine performance is</p><p>required. This effort helps to avoid the emission of polluting gases that</p><p>might cause drastic global warming. Hence, the engine performance</p><p>study by alkyl levulinate blends is an important study which should be</p><p>conducted since this alkyl levulinate blending additive improves the</p><p>physicochemical, combustion and emission performance of engine.</p><p>Additionally, the production of various alkyl levulinates ranging from</p><p>short, branched to long chain needs to be explored for producing the</p><p>most desirable and cost-effective fuel blending levulinate compounds.</p><p>Declaration of Competing Interest</p><p>The authors declare that they have no known competing financial</p><p>interests or personal relationships that could have appeared to influence</p><p>the work reported in this paper.</p><p>Acknowledgements</p><p>The financial support from RUI (1001/PKIMIA/8011128) grant is</p><p>gratefully acknowledged. J.N. 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Enzyme Microb Technol 2020;136:109506.</p><p>[135] Yadav GD, Borkar IV. Kinetic Modeling of Immobilized Lipase Catalysis in</p><p>Synthesis of n-Butyl Levulinate. Ind Eng Chem Res 2008;47:3358–63.</p><p>J. Nelson Appaturi et al.</p><p>per year</p><p>(before 2009–December 2021). Source: Scopus (keywords: levulinic acid, 4-</p><p>oxopentanoic acid, levulinate, esterification).</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>3</p><p>levulinate also possesses low energy density as compared to methyl</p><p>levulinate due to its high oxygen content: ~33 wt% for ethyl levulinate</p><p>and ~37 wt% for methyl levulinate [17]. Consequently, this charac-</p><p>teristic improves the hydrophilicity of fuel.</p><p>Deoxygenation (DO) and hydrodeoxygenation (HDO) processes are</p><p>strategies to reduce the oxygen content and improve the energy density</p><p>of fuel. However, DO and HDO of short-chain alkyl levulinates involve</p><p>complicated synthesis processes that require severe reaction conditions</p><p>[39]. In this regard, researchers are keen in studying long-chain alkyl</p><p>levulinates (C4–C10). The long-chain alkyl levulinates, such as hexyl</p><p>levulinate or octyl levulinate, have similar characteristics as biodiesel.</p><p>These fuel additives are capable to facilitate complete fuel combustion</p><p>thanks to its appropriate boiling point and high energy density [17].</p><p>Thus, it is envisaged that the use of long-chain alkyl levulinates in fuel</p><p>additive leads to cleaner combustion and reduces the emissions of haz-</p><p>ardous gas pollutants (such as NOx, SO2, particulates).</p><p>Classically, the esterification of levulinic acid with alcohols is carried</p><p>out using homogeneous Brønsted acids, such as H2SO4, H3PO4, HCl,</p><p>HNO3, HF and benzenesulfonic acid, to accelerate the reaction rate and</p><p>attain high alkyl levulinates yield [18,40,41]. These traditional homo-</p><p>geneous catalysts are selected due to their low cost, high availability and</p><p>high catalytic efficacy. However, they suffer from several substantial</p><p>shortcomings, such as deterioration of equipment due to their corrosive</p><p>and toxic nature, difficulty in recycling, environmental pollution due to</p><p>production of noxious liquid waste during extraction, laborious workup</p><p>for acid neutralization and high energy consumption [3,18,28,42]. In</p><p>respect to this, solid acid catalysts are becoming more preferable and</p><p>important nowadays as they promise green alternatives, and are</p><p>industrially benign with several typical good features such as high</p><p>reusability, robust, easy to separate and long term stability to reaction</p><p>media [37]. Nevertheless, some heterogeneous catalysts still suffer from</p><p>recovery issue, high-cost and tedious preparation, fast deactivation and</p><p>lengthier reaction time [25,43]. Therefore, research on discovering</p><p>active and selective heterogeneous catalyst is of paramount importance</p><p>to surmount these limitations.</p><p>Very recently, a review article published by Bhat et al. [44] specif-</p><p>ically portraying the use of homogeneous and heterogeneous hetero-</p><p>polyacid catalysts in the preparation of alkyl levulinates by using</p><p>biomass-derived furanic and levulinic chemical platforms (e.g. levu-</p><p>linic acid, furfuryl alcohol, 5-(hydroxymethyl)furfural and angelica</p><p>lactone). In the meantime, Liu et al. [45] wrote another review on</p><p>current approaches to synthesize alkyl levulinates via valorization of</p><p>biomass derivatives from feedstocks. Conversion of levulinic acid into</p><p>alkyl levulinates over zeolites, heteropolyacid, metal oxide and sulfonic</p><p>acid catalysts has been reviewed but the evaluations are made in a</p><p>broader context, and there is rarely a detailed description of the mech-</p><p>anisms involved. Similarly, Dutta et al. [46] also reviewed the esterifi-</p><p>cation of levulinic acid with various solid catalysts (zeolites, metal</p><p>oxides, silica, carbon nanotubes, etc.) as small portion of their review.</p><p>The discussion, however, is made in general and is not specifically</p><p>focused on details of the catalyst preparation, properties of catalysts and</p><p>reaction mechanisms. Another review article focuses on biomass feed-</p><p>stock pre-treatments, production routes from biomass precursors to</p><p>levulinic acid and alkyl levulinates, separation and purification of lev-</p><p>ulinic acid, and considerations of economic and environmental sus-</p><p>tainability of manufacture of platform chemicals [47]. Similarly, the</p><p>discussion on the conversion of levulinic acid into alkyl levulinates over</p><p>various types of catalysts is made in a broader perspective, and the</p><p>catalytic mechanism and properties of prepared catalysts leading to the</p><p>enhancement of reaction performance are rarely discussed.</p><p>This review article focuses on recent developments of several com-</p><p>mon types of heterogeneous catalysts (i.e., zeolites, mesoporous silicas,</p><p>carbonaceous materials, metal organic frameworks, ion exchange resins</p><p>and biocatalysts) on esterification of levulinic acid. Some specific as-</p><p>pects on catalyst preparation and properties of catalysts are also criti-</p><p>cally reviewed to provide a complete overview and insight on the</p><p>catalysts’ behaviour in the esterification of levulinic acid. In addition,</p><p>we also highlight an informative knowledge and inclusive foundation of</p><p>the catalysts to provide a useful insight for researchers to obtain high</p><p>alkyl levulinates yield from levulinic acid via esterification process. The</p><p>general aim of this review hence is to give a vivid look on i) the</p><p>importance of alkyl levulinates as a fuel blend, ii) the advantages and</p><p>performance of aforementioned heterogeneous catalysts, iii) the in-</p><p>fluences of reaction engineering aspects such as reaction temperature,</p><p>time duration, catalyst loading, reactants molar ratio, and reaction</p><p>medium, and iv) the roles of catalyst in the levulinic acid esterification</p><p>reaction mechanism. Moreover, the review also gives an overall outlook</p><p>with a short discussion on the future challenges related to the devel-</p><p>opment of alkyl levulinates from levulinic acid.</p><p>2. Recent progress on catalysts for preparing alkyl levulinates</p><p>2.1. Zeolites</p><p>Zeolites are microporous crystalline molecular sieves composed of</p><p>tetrahedral aluminate and silicate primary building units. The zeolites in</p><p>protonated form (prepared by ion-exchange) are powerful solid acids,</p><p>and they have widely been used in catalyzing numerous organic re-</p><p>actions, such as isomerization, alkylation, esterification, cracking, etc.</p><p>[48-50]. In addition, their unique pore structure and dimension also</p><p>make them as superior shape-selective catalysts in organic conversions</p><p>[51-53].</p><p>Zeolites with different membered rings (MR) and Si/Al ratio (SAR)</p><p>have been used as acid catalysts in the synthesis of alkyl levulinates. For</p><p>instance, H-ZSM-5 (10-MR, SAR = 60), H-Y (12-MR, SAR = 10.2), H-</p><p>MOR (12-MR, SAR = 40) and H-BEA (12-MR, SAR = 50) zeolites were</p><p>tested in the synthesis of butyl levulinate via esterification of levulinic</p><p>acid with n-butanol (120 ◦C, 4 h) (Table 1, entry 5) [54]. The order of</p><p>catalytic activity in producing butyl levulinate was found to be H-BEA</p><p>(82.2% conversion) > H-Y (32.2%) > H-ZSM- 5 (30.6%) ≈ H-MOR</p><p>(29.5%), where the butyl levulinate selectivity was 100 % in all cases. H-</p><p>BEA is the most reactive catalyst, thanks to its 12-MR pore size with</p><p>three-dimensional pore channels that allow fast molecular diffusion. On</p><p>the other hand, H-MOR (10.7 μmol/m2) and H-ZSM-5 (10.5 μmol/m2)</p><p>contain larger amount of acid sites than H-BEA (2.3 μmol/m2) and H-Y</p><p>(4.8 μmol/m2), but the previous two candidates show lower catalytic</p><p>activity due to their smaller pore size and lower pore dimension channel</p><p>that obstruct molecular diffusion of the reactants.</p><p>In other work, the synthesis of ethyl levulinate via esterification of</p><p>levulinic acid with ethanol was studied using H-MOR (SAR = 7.1), H-</p><p>MCM-22 (SAR = 12.8), H-ZSM-5 (SAR = 13.2), H-BEA (SAR = 23.8) and</p><p>H-USY (SAR = 45.8) [55]. H-USY, H-BEA and H-MOR are large-pore</p><p>zeolites while H-ZSM-5 and H-MCM-22 exhibits medium-pores. It is</p><p>found that the catalytic performance of zeolites is depending</p><p>on their</p><p>pore structure (pore size, pore dimension and/or pore shape) that ac-</p><p>commodates the reaction intermediates and transition states of the</p><p>esterification of levulinic acid. No correlation between activity and</p><p>acidity was observed in the reaction. For instance, H-USY shows the</p><p>second-best performance although it appears less acidic zeolite tested</p><p>(H-MOR > H-ZSM-5 ≈ H-MCM-22 > H-BEA > H-USY). Instead, the</p><p>number of Brӧnsted acid sites located at the external surface of the ze-</p><p>olites is far more important than the total number of protons exhibited</p><p>by the zeolites.</p><p>Two main mechanisms for the esterification of levulinic acid with</p><p>alcohol on acidic zeolites, namely AAc2 and AAc1, have been identified</p><p>(Figs. 3 and 4) [55]. The AAc2 pathway requires protonation of carbonyl</p><p>oxygen atom followed by nucleophilic attack by the alcohol which leads</p><p>to the formation of a quaternary carbon intermediate. For AAc1 route</p><p>which is more energy demanding [56], the protonation occurs on the</p><p>hydroxyl group of carboxylic acid and later, water is eliminated to form</p><p>an acyl cation. The protonation of carboxylic acid and production of</p><p>respective intermediates via both mechanism pathways, however, can</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>4</p><p>be facilitated by the zeolite structure where the unstable carbocationic</p><p>species can be stabilized by the framework oxygen atoms [57].</p><p>H-ZSM-5 is a very important aluminosilicate zeolite catalyst used in</p><p>numerous catalytic reactions owing to its tuneable SAR ratio (1.3-∞),</p><p>high hydrothermal stability, medium pore size and strong acidity.</p><p>Nevertheless, its catalytic application is restricted by the reactions</p><p>involving bulk molecules where its acid sites are mostly located inside its</p><p>micropores. In respect to this, several approaches have been applied to</p><p>engineer the porosity of the zeolite. One of the established techniques is</p><p>using desilication process where the framework silicates are partially</p><p>extracted out from the zeolites using a specific concentration of alkaline</p><p>NaOH or acidic HF solution. The post-treatment produces zeolite solids</p><p>with hierarchical meso/macroporosity [58]. As compared with the</p><p>pristine H-ZSM-5 zeolite, the desilicated H-ZSM-5 shows higher surface</p><p>area and acidity due to considerable amount of surface defects (acid</p><p>sites) generated, which in turn enhances the catalysis in the synthesis of</p><p>butyl levulinate [59]. Similar observation was reported in the synthesis</p><p>of methyl, ethyl, butyl and octyl levulinates using the similar desilicated</p><p>H-ZSM-5 where the rate of esterification increases with increasing the</p><p>carbon chain length [60-62].</p><p>Desilication is also applied on H-Beta zeolite for preparing hierar-</p><p>chical micro/mesoporous H-Beta zeolite where different concentrations</p><p>of NaOH are used as desilication agents [63]. The results show that the</p><p>degree of mesoporosity is proportional to the NaOH concentration.</p><p>Furthermore, the H-Beta zeolite treated with higher concentration of</p><p>NaOH (containing higher degree of mesoporosity) is far more reactive</p><p>than the pristine sample in the liquid phase esterification of levulinic</p><p>acid with ethanol where 40% conversion of levulinic acid and 98% ethyl</p><p>levulinate selectivity are achieved by this hierarchical H-Beta zeolite</p><p>(levulinic acid: ethanol molar ratio = 1:6, 80 ◦C, 5 h, 20 wt% catalyst</p><p>loading) (Table 1, entry 4).</p><p>Meanwhile, hierarchical micro-/mesoporous Beta (BEA) zeolite</p><p>catalysts for the esterification of levulinic acid with butanol is also re-</p><p>ported [64,65]. The hierarchization is achieved by two-step treatments,</p><p>namely desilication followed by addition of cetyltrimethylammnonium</p><p>bromide (CTABr) [64] or tetradecylammonium bromide (TTABr) [65]</p><p>as a mesoporogen. Also, an addition of rice husk during the mixing of</p><p>surfactant with the desilicated mixture is found to be beneficial in</p><p>Table 1</p><p>Summary of the esterification reaction conditions and catalytic results over various zeolite catalysts.</p><p>Entry Catalyst Alkyl</p><p>levulinatea</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>mCatalyst (wt.</p><p>%)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 Desilicated H-</p><p>ZSM-5</p><p>EL 120 5 10 1:8 95.0 100 95.0 [61]</p><p>2 ZSM-12</p><p>nanolayers</p><p>EL 100 24 10 1:1 78.5 98.7 77.5 [66]</p><p>3 H-MCM-22 EL 70 5 2.5 1:5 12.0 NA NA [55]</p><p>4 Micro/meso-H-</p><p>BEA</p><p>EL 80 5 20 1:6 40.0 98.0 39.2 [63]</p><p>5 H-BEA BL 120 4 10 1:7 82.2 100 82.2 [54]</p><p>6 Micro/meso-H-</p><p>ZSM-5</p><p>BL 120 5 20 1:6 98.0 100 98.0 [59]</p><p>7 Meso-H-BEA BL 120 6 13 1:10 99.4 99.0 91.5 [65]</p><p>8 Meso-ZSM-5 BL 120 5 10 1:7 NA NA 91.4 [73]</p><p>9 Meso-H-BEA BL 120 4 10 1:7 95.6 91.0 82.6 [64]</p><p>10 Micro/meso-</p><p>HZSM-5</p><p>BL 120 5 20 1:8 98.0 100 98.0 [60]</p><p>11 Meso-HZSM-5 OL 120 4 25.4 1:7 99.0 100 99.0 [62]</p><p>12 ZSM-5 EL 90 3 25.6 1:1 92.0 72.0 66.2 [69]</p><p>aEL: Ethyl levulinate; BL: Butyl levulinate; OL: Octyl levulinate.</p><p>Fig. 3. Aac2 mechanism preceded by the protonation of the carbonyl oxygen and the subsequent nucleophilic attack of alcohol.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>5</p><p>improving the thermal stability and preserving the number of surface</p><p>acidity of the zeolite. The resulting modified solid catalyst shows higher</p><p>catalytic activity in the synthesis of butyl levulinate (95.6% conversion)</p><p>as compared to the parent H-BEA (82.2%), Al-MCM-41 (80.8%) under</p><p>the optimized reaction conditions (levulinic acid:butanol molar ratio =</p><p>1:7, 120 ◦C, 4 h, 10 wt% catalyst loading) (Table 1, entry 9).</p><p>The use of novel organosilane surfactant in tailoring the morphology</p><p>and porosity is reported in the synthesis of nanolayer-assembled ZSM-12</p><p>zeolite [66]. In the work, dimethyloctadecyl[3-(trimethoxysilyl)propyl]</p><p>ammonium chloride (TPOAC) is used as a secondary structure-directing</p><p>agent and also a mesoporogen [67]. A change in the ZSM-12</p><p>morphology is observed upon the addition of TPOAC where agglomer-</p><p>ated nanocrystals interconnect forming brick-like nanocrystals. The</p><p>zeolite morphology changes to nanolayers of ca. 20 nm with further</p><p>increasing the TPOAC amount, resulting in high external surface area</p><p>and pore volume that are conducive for the esterification reaction.</p><p>Hence, proper tuning of the TPOAC content and crystallization time are</p><p>essential in obtaining ZSM-12 with nanolayer structure. The addition of</p><p>TPOAC, however, deteriorates the acidity of ZSM-12 nanolayers due to</p><p>the introduction of more silicon atoms into the framework. Similar</p><p>approach using TPOAC is used for the synthesis of H-ZSM-5 and H-Y</p><p>zeolite nanosheets where both the mesoporosity and morphology are</p><p>successfully improved. The resulting ZSM-12 (78.5% conversion), ZSM-</p><p>5 (52.6%) and Y (58.0%) zeolite nanosheets are more reactive than the</p><p>parent ZSM-12 (50.6%) in the esterification of levulinic acid with</p><p>ethanol where the selectivity to ethyl levulinate is maintained at >95%</p><p>(levulinic acid: ethanol molar ratio = 1:1, 100 ℃, 24 h, catalyst loading</p><p>= 0.3 g) (Table 1, entry 2).</p><p>Surface functionalization of zeolites with active species is a well-</p><p>known method to enhance the surface acidity. For instance, dodeca-</p><p>tungstophosphoric acid (DTPA)—a heteropolyacid—has been func-</p><p>tionalized on desilicated H-ZSM-5 where solid zeolite served as both</p><p>catalyst support and primary catalyst [68]. In the study the authors</p><p>discovered that the levulinic acid conversion to ethyl levulinate is pro-</p><p>portional to the amount of DTPA grafted. The optimum levulinic acid</p><p>conversion of 94% was recorded by the DTPA-functionalized zeolite</p><p>catalyst under</p><p>optimum condition and its performance is far more su-</p><p>perior than that of desilicated H-ZSM-5 (28% levulinic acid conversion).</p><p>In another study, acetalization agent is found to be beneficial in</p><p>enhancing the esterification reactivity. Typically, the esterification of</p><p>levulinic acid catalyzed by unmodified ZSM-5 in the presence of ace-</p><p>talization agents (e.g. trialkylorthoformate, dialkoxypropane ketal and</p><p>benzaldehyde dialkyl acetal) in dimethyl carbonate solvent is reported</p><p>[69]. At 90 ℃, >50% of conversion into methyl levulinate and ethyl</p><p>levulinate are recorded using trimethyl orthoformate and triethyl</p><p>orthoformate as the acetalization agents, respectively. Besides, the uti-</p><p>lization of ketal dimethoxypropane and benzaldehyde dimethoxy acetal</p><p>as the acetalization reagents show >60% of conversion into methyl</p><p>levulinate at 75 ℃. The highest conversion (92%) with a 72% selectivity</p><p>of ethyl levulinate is achieved at 90 ℃ in a slight excess of the benzal-</p><p>dehyde diethyl acetal agents. The selectivity of alkyl levulinate increases</p><p>when the reaction temperature increases from 75 ℃ to 90 ℃. In general,</p><p>unmodified ZSM-5 zeolite shows poor performance in the esterification</p><p>of levulinic acid. The improved catalytic activity and selectivity in this</p><p>work are probably associated with different esterification mechanism</p><p>(Fig. 5). It is suggested that the acetalization reagents prompt the</p><p>intramolecular cyclization of levulinic acid through the A, B and C in-</p><p>termediates to produce α-angelica lactone. Subsequent ring-opening of</p><p>lactone assisted by the alcohol species is then facilitated by the acidic</p><p>ZSM-5 zeolite, producing alkyl levulinates.</p><p>A summary of the reaction conditions (types of zeolites, temperature,</p><p>time, mass of catalyst, reactants amount) and results (conversion,</p><p>selectivity, yield, types of alkyl levulinate produced) from previous</p><p>works is depicted in Table 1. It is shown that zeolites with small pore</p><p>sizes are unfavourable for esterification of bulkier reactants because the</p><p>organic guest molecules are completely hindered from entering the</p><p>pores. Instead, zeolites that have higher pore channel dimension and</p><p>larger pore sizes (e.g. H-BEA and H-USY), or with engineered hierar-</p><p>chical mesoporosity and layered structure are recommended as these</p><p>features shorten the diffusional path length and improves the mass</p><p>transport of reactants and products. Although hierarchical zeolites show</p><p>enhanced performance in the esterification reaction, their preparation</p><p>generally entails the use of individual or multiple organotemplates. The</p><p>use of templates possesses several drawbacks, including expensive and</p><p>eco-unfriendly. Furthermore, some organic templates may not be</p><p>commercially available and it is therefore labour intensive for preparing</p><p>them in laboratory [70].</p><p>On the other hand, mesoporous zeolites are easier to prepare via</p><p>desilication. Nevertheless, a decreased crystallinity, acidity and high</p><p>loss of zeolite materials are expected when the desilication is performed</p><p>at high pH for long time [71,72]. Also, desilication results in broad pore</p><p>size distribution [66]. Hence, proper use of treatment technique with</p><p>correct treatment conditions are of utmost importance in generating</p><p>hierarchical zeolite catalysts with high porosity and acidity without</p><p>structural deterioration.</p><p>2.2. Mesoporous silica</p><p>Since decades, global researchers have devoted their efforts in uti-</p><p>lizing mesoporous silica as one of the promising inorganic materials for</p><p>Fig. 4. Aac1 mechanism begins with the protonation of the hydroxyl group followed by the nucleophilic attack of alcohol to form alkyl levulinate.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>6</p><p>wide applications. In particular, silica supported materials have been</p><p>recognized for their good catalytic characteristics such as high surface</p><p>area, facile synthesis route, tuneable mesopore size and high thermal</p><p>stability [74]. Besides, the abundantly available –OH groups on the</p><p>silica surface provides easy functionalization which greatly enhance its</p><p>application in catalysis and other areas [75].</p><p>SBA-15 is a unique mesoporous material with enhanced porosity,</p><p>and hence paving the way for conducive selective production of esters</p><p>[76]. These desirable features allow esterification of levulinic acid with</p><p>ethanol over propylsulfonic acid functionalized SBA-15 mesosilica (Pr-</p><p>SO3H-SBA-15) to be performed at 117 ℃ for 2 h with 4.86-fold molar</p><p>excess of ethanol, giving 95.8% conversion (Table 2, entry 1) [77]. Ethyl</p><p>levulinate is a good anti-knock agent (anti-knocking index = 107.5) and</p><p>it is being used in automotive industry due to its sulfur-free content, low</p><p>CO2 emission, high lubricity and high flash point stability [78].</p><p>Following this, the authors reported insignificant amount of levulinic</p><p>acid by-products (e.g., ethers) arising from the intermolecular dehy-</p><p>dration of alcohols. High surface hydrophobicity despite of moderately</p><p>strong sulfonic acid sites has positively affected the activity of Pr-SO3H-</p><p>SBA-15 in the esterification reaction. During 3rd reusability cycles, Pr-</p><p>SO3H-SBA-15 experiences less pronounced loss in the activity (Table 2,</p><p>entry 2) which justifies its heterogeneity.</p><p>In a continuous effort on introducing propylsulfonic acid on SBA-15,</p><p>a group of researchers have synthesized hexyl levulinate for the first</p><p>time via esterification with hexanol [79]. In this context, long-chain</p><p>alkyl levulinates are superior as biodiesel additives owing to their</p><p>high solubility in fuel [79]. The reported catalyst requires prolonged</p><p>reaction time (8 h) and elevated temperature (140 ℃) to achieve 100%</p><p>conversion of levulinic acid into 94% of hexyl levulinate (Table 2, entry</p><p>3). The catalyst is also economically feasible and exhibited high oper-</p><p>ational stability even after four reruns without any significant change in</p><p>its original structure.</p><p>Meanwhile, sulfated hexagonal mesoporous zirconosilicates (P6mm)</p><p>containing various isomorphous substituted Zr contents (S-ZrSBA15) are</p><p>also prepared to catalyze levulinic acid and ethanol at 70 ℃ [37]. The S-</p><p>ZrSBA15 with moderate Zr loading (Si/Zr = 10.7) is the best catalyst</p><p>attaining a great yield of ethyl levulinate (79.0%) as compared to the</p><p>conventional sulfated zirconia without mesoporosity (31.8%) (Table 2,</p><p>entry 4). This undoubtedly highlights the necessity of ordered meso-</p><p>porous structure in affording good catalytic activity. A linear correlation</p><p>between the accessibility and well-dispersion of acid sites in S-ZrSBA15</p><p>(Si/Zr = 10.7) is also observed with its exceptional activity in the pro-</p><p>duction of ethyl levulinate from levulinic acid. Despite producing se-</p><p>lective ethyl levulinate, yet the study requires 10-fold excess of ethanol</p><p>and longer reaction time (24 h), which definitely leaves some space for</p><p>further improvement.</p><p>The activity of different heteropolyacids (tungstophosphoric acid</p><p>(TPA), molybdophosphoric acid (MPA) and silicotungstic acid (STA))</p><p>Fig. 5. (a) Activation of levulinic acid through its ketone moiety towards intramolecular cyclization forming pseudo lactone, and (b) ring-opening of pseudo lactone</p><p>by the alcohol species. Adapted from Chaffey et al. [69].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>7</p><p>supported on mesoporous AlSBA-15 is compared in the esterification of</p><p>levulinic acid with ethanol [80]. Upon investigation on the influence of</p><p>catalytic reaction parameters (e.g., levulinic acid to ethanol molar ratio,</p><p>reaction time and catalyst mass), STA/AlSBA-15 was the most reactive</p><p>catalyst, giving 87.4% conversion and 100% selectivity towards ethyl</p><p>levulinate (Table 2, entry 5). The observed high catalytic performance is</p><p>due to its high mesoporosity with improved surface characteristics that</p><p>facilitates the accessibility of reactants to the acidic sites of STA-AlSBA-</p><p>15.</p><p>Meanwhile, SBA-15 incorporated with niobium and grafted with</p><p>meso-tetra-(4-carboxyphenyl)-porphyrin (Nb-TCPP-SBA-15-AM)–a</p><p>metal complex heterogeneous catalyst– has recently been prepared and</p><p>tested in the production of methyl levulinate and propyl levulinate at 60</p><p>℃ using 5-fold excess of alcohols [81]. After 6 h reaction, the hetero-</p><p>geneous catalyst successfully converts 74% and 10% of levulinic acid,</p><p>into methyl levulinate and propyl levulinate, respectively (Table 2, en-</p><p>tries 6–7).</p><p>In another work, the advantages of vapor phase esterification of</p><p>levulinic acid over ZrO2 supported mesoporous SBA-15 catalysts are</p><p>explored by Siva Sankar et al. [82] in a fixed-bed quartz reactor. The</p><p>significant role of SBA-15 as an excellent support in stabilizing ZrO2</p><p>active species is clearly demonstrated by the 7 wt% ZrO2 loaded SBA-15</p><p>catalyst using 7-fold excess of methanol (Table 2, entry 8). However, the</p><p>production of methyl levulinate experiences notable reduction upon</p><p>excessive amount of methanol (9 and 11-fold excess) due to the</p><p>increasing formation of γ-valerolactone as the minor product (Table 2,</p><p>entries 9–10).</p><p>SBA-16 consisting of 3D porous channel is of particular interest and</p><p>often adopted as a great platform to host guest materials for improved</p><p>activity and selectivity [83]. In this regard, a series of tungsten oxide</p><p>(WO3) incorporated SBA-16 catalysts has been synthesized via one-pot</p><p>route and used as recyclable catalyst in the selective synthesis of alkyl</p><p>levulinates from different alcohols [84]. Under the optimized condition,</p><p>3 wt% WO3-SBA-16 catalyst gives complete conversion in the esterifi-</p><p>cation of levulinic acid with ethanol, yielding 95% ethyl levulinate</p><p>(Table 2, entry 11). Nonetheless, a sudden decline was observed in the</p><p>conversion and yield over secondary alcohols, namely isopropanol</p><p>(93%, 61%) and isobutanol (64%, 13%), respectively (Table 2, entries</p><p>12–13). 3 wt% WO3-SBA-16 catalyst shows very high catalyst reusability</p><p>(up to 10 times) by promoting steady conversion, however experiences a</p><p>dramatic loss in the ethyl levulinate yield (Table 2, entry 14). The</p><p>unique characteristics of both Lewis and Brønsted acidity originated</p><p>from O = W = O and W-OH sites, respectively, are the main highlight in</p><p>this work as compared to other reported mechanism which involves the</p><p>Table 2</p><p>Summary of the activity of mesoporous silica catalysts under different operational reaction conditions in the synthesis of various alkyl levulinates.</p><p>Entry Catalysta Alkyl levulinate</p><p>b</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 Pr-SO3H-SBA-15 EL 117 2 1:4.86 95.8 NA NA [77]</p><p>2 Pr-SO3H-SBA-15 (3rd reuse) EL 117 2 1:4.86 92.9 NA NA [77]</p><p>3 SBA-15-Pr-SO3H HL 140 8 1:3 100 95.0 95.0 [79]</p><p>4 S-ZrSBA15(10.7) EL 70 24 1:10 NA NA 79.0 [37]</p><p>5 STA-AlSBA-15 EL 75 5 1:10 87.4 100 87.4 [80]</p><p>6 Nb-TCPP-SBA-15-AM ML 60 6 1:5 74.0 59.0 43.6 [81]</p><p>7 Nb-TCPP-SBA-15-AM PL 60 6 1:5 10 38.0 3.8 [81]</p><p>8 7 wt% ZrO2-SBA-15 ML 250 1 1:7 100 96.0 96.0 [82]</p><p>9 7 wt% ZrO2-SBA-15 ML 250 1 1:9 100 97.0 97.0 [82]</p><p>10 7 wt% ZrO2-SBA-15 ML 250 1 1:11 100 93.0 93.0 [82]</p><p>11 3 wt% WO3-SBA-16 EL 250 1 1:7 100 95.0 95.0 [84]</p><p>12 3 wt% WO3-SBA-16 ISPL 250 1 1:7 93.0 61.0 65.6 [84]</p><p>13 3 wt% WO3-SBA-16 ISBL 250 1 1:7 64.0 13.0 8.3 [84]</p><p>14 3 wt% WO3-SBA-16 EL 250 10 1:7 100 77.0 77.0 [84]</p><p>15 40WD-S EL 78 10 1:64 76 100 76.0 [85]</p><p>16 20 wt% H4SiW12O40-SiO2 ML 65 6 1:28 79.0 92.4 73.0 [87]</p><p>17 20 wt% H4SiW12O40-SiO2 EL 75 6 1:20 75.0 89.3 67.0 [87]</p><p>18 25 wt% HPW/Al-MCM-41 HL 100 10 1:5 100 NA NA [88]</p><p>19 25 wt% HPW/Al-MCM-41 (6th</p><p>reuse)</p><p>HL 100 10 1:5 93.3 NA NA [88]</p><p>20 45 wt% HPW/MCM-41 EL 80 10 1:2.5 83.7 NA NA [89]</p><p>21 25 wt% HSiW/MCM-41 HL 70 10 1:10 100 NA NA [90]</p><p>22 H3PW12O40/ZrO2–Si(Ph)Si ML 65 3 1:7 NA NA 99.9 [91]</p><p>23 10Al-MS BL 120 4 1:5 NA NA 90.0 [92]</p><p>24 5Al-MS BL 110 4 1:5 NA NA 65.0 [92]</p><p>25 20Al-MS BL 120 4 1:5 NA NA 79.0 [92]</p><p>26 Sn T-4 BL 120 4 1:5 100 90.5 90.5 [93]</p><p>27 Sn T-4 EL 120 12 1:5 88.7 93.5 82.9 [93]</p><p>28 10ZrPM(DS-E) EL 70 5 1:5 69.2 100 69.2 [94]</p><p>29 10ZrPM(DS-E) OL 130 5 1:5 76.7 100 79.7 [94]</p><p>30 10ZrPM(DS-E) EL 70 5 1:5 66.5 100 66.5 [94]</p><p>31 10ZrPM(DS-E) OL 130 5 1:5 74.9 100 74.9 [94]</p><p>32 SO4</p><p>2− /25ZrKIL-2 EL 70 5 1:12.66 38.0 100 38.0 [95]</p><p>33 SO4</p><p>2− /15ZrKIL-2 EL 70 5 1:12.66 51.0 100 51.0 [95]</p><p>34 SO4</p><p>2− /8ZrKIL-2 EL 70 5 1:12.66 29.0 100 29.0 [95]</p><p>35 SO4</p><p>2− /4ZrKIL-2 EL 70 5 1:12.66 8.0 100 8.0 [95]</p><p>a Pr-SO3H-SBA-15 and SBA-15-Pr-SO3H: propylsulfonic acid functionalized SBA-15; S-ZrSBA15(10.7): sulfated mesoporous zirconosilicates with Si/Zr ratio of 10.7;</p><p>STA-AlSBA-15: Silicotungstic acid supported on Al-SBA-15; Nb-TCPP-SBA-AM: Niobium incorporated meso-tetra-(4-carboxyphenyl)-porphyrin on SBA-15; 7ZS: ZrO2</p><p>supported SBA-15; 3WS: WO3-SBA-16; 40WD-S: Well-dawson heteropolyacid on silica; 10 Al-MS: 10 wt% H3PW12O40 on Al-MCM-41; Sn T-4: mesoporous stanno-</p><p>silicates with Si/Sn molar ratio of 25; 10ZrPM(DS-E): Zr(IV) substituted Keggin polyoxometalate grafted on mesoporous silica; SO4</p><p>2− /15ZrKIL-2: 15 wt% sulfated</p><p>zirconia modified mesoporous KIL-2.</p><p>bML: Methyl levulinate; EL: Ethyl levulinate; PL: Propyl levulinate; BL: Butyl levulinate; ISPL: Isopropyl levulinate; ISBL: Isobutyl levulinate; HL: Hexyl levulinate; OL:</p><p>Octyl levulinate.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>8</p><p>adsorption of levulinic acid on Brønsted sites of catalyst [73,90]. In this</p><p>work, both Lewis and Brønsted sites play significant roles in catalyzing</p><p>the esterification reaction via the plausible reaction mechanism as</p><p>shown in Fig. 6 [84].</p><p>Wells-Dawson (WD) heteropolyacid supported on amorphous silica</p><p>catalyst (40WD-S containing 40 wt% WD) is prepared via sol-gel method</p><p>[85]. The catalytic esterification of levulinic acid with ethanol at 78 ℃</p><p>favours the production of ethyl levulinate (76% yield) with 100% con-</p><p>version where no side products (e.g., angelica lactone, 4-etoxy γ-valer-</p><p>olactone or aldehyde) are formed (Table 2, entry 15). However, this</p><p>catalyst requires longer reaction hour (10 h) and very large amount of</p><p>absolute ethanol (128-fold excess) to react with levulinic acid. Never-</p><p>theless, the 40WD-S catalyst remains intact even after third reaction</p><p>cycles, signifying better catalyst stability as compared to the non-</p><p>supported WD even though the bulk WD gives 93% yield of ethyl</p><p>levulinate.</p><p>The esterification reaction over Wells-Dawson (WD) heteropolyacid</p><p>supported on amorphous silica catalyst is initiated with the adsorption</p><p>of levulinic acid on the Brønsted sites of the catalyst as shown in Fig. 7,</p><p>followed by the formation of protonated intermediates and subsequent</p><p>attacked by oxygen atom of ethanol as a nucleophile. This leads to the</p><p>production of oxonium ion that forms into a new oxonium ion upon a</p><p>proton transfer. The removal of water molecule and proton then yields</p><p>ethyl levulinate as the final product. Similarly, in the work reported by</p><p>Liu et al. [86], the Brønsted sites are the active sites for the adsorption of</p><p>levulinic acid before converting into ethyl levulinate.</p><p>In year 2013, Yan and co-workers [87] synthesized Keggin incor-</p><p>porated mesoporous silica (H4SiW12O40-SiO2) using Brij30 surfactant as</p><p>a mesoporous template. The catalysts are prepared in different</p><p>H4SiW12O40 loadings (10–30 wt%) and employed in the synthesis of</p><p>methyl levulinate and ethyl levulinate under reflux conditions at 65 ℃</p><p>and 75 ℃ for 6 h, respectively. 20 wt% H4SiW12O40 supported SiO2</p><p>exhibits excellent catalytic activity, providing 73% yield of methyl</p><p>levulinate with 79% conversion while 75% of levulinic acid is converted</p><p>into 67% ethyl levulinate (Table 2, entries 16 and 17).</p><p>In other work, Al-containing MCM-41 supported with various</p><p>amounts (5–45 wt%) of H3PW12O40 (HPW) is also reported [88]. The</p><p>impregnated catalysts are then studied in the synthesis of hexyl levuli-</p><p>nate by varying the reaction parameters. The work shows that the HPW</p><p>clusters can uniformly be dispersed on the inorganic mesoporous sup-</p><p>port with HPW loading as high as 45 wt%. Complete conversion is</p><p>achieved over 25 wt% HPW/Al-MCM-41 catalyst with hexyl levulinate</p><p>as the only product (Table 2, entry 18). However, this present study</p><p>requires extended reaction time (10 h) and excess alcohol (5-fold molar</p><p>excess) compared to the work reported by Tabrizi et al. (3 h, 3-fold</p><p>molar excess) [79] (Table 2, entry 3). In addition, the synthesized</p><p>catalyst suffers from partial leaching of loosely bounded HPW species</p><p>that results in a drop in the recyclability performance study (Table 2,</p><p>entry 19).</p><p>The similar group [89] also impregnates and supports siliceous</p><p>MCM-41 with various loadings (5–45 wt%) of H3PW12O40 (HPW) for</p><p>synthesizing ethyl levulinate. The dependence of reaction parameters,</p><p>such as catalyst dosage, reaction temperature, reaction time and molar</p><p>ratio of reactants, on the conversion of levulinic acid into ethyl levuli-</p><p>nate is explored. By increasing the HPW loading, the meso-ordering</p><p>degree, surface area and pore volume are substantially reduced. It is</p><p>shown that the activity of the catalysts is affected by the structural</p><p>characteristics and HPW loading where the latter factor controls the</p><p>Fig 6. The proposed reaction pathway for the esterification of levulinic acid with alcohols over WO3-SBA-16 catalyst. Adapted from Enumula et al. [84].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>9</p><p>Keggin anion density and HPW dispersion. The highest conversion of</p><p>83.7% is recorded over 45 wt% HPW/MCM-41 (Table 2, entry 20),</p><p>whereas <5% of conversion is achieved by 5 wt% HPW/MCM-41.</p><p>Using the similar impregnation method, various amounts (5, 10, 15,</p><p>20, 25, 30, 35, 40, and 45 wt%) of 12-silicotungstic acid (H4SiW12O40,</p><p>HSiW) have successfully been anchored on MCM-41 by Chen et al. [90].</p><p>The efficiency of HSiW/MCM-41 catalysts are then investigated in the</p><p>esterification of various alcohols (C1-C6). Remarkably, 100% conver-</p><p>sion is achieved by 40 wt% HSiW/MCM-41 catalyst in the presence of</p><p>hexanol (Table 2, entry 21), which can be explained by the excellent</p><p>solubility of levulinic acid in hexanol as compared to C1-C5 alcohols</p><p>(<87%). However, the amount of hexanol used in this system is higher</p><p>as compared to that reported by Wu et al. [88] (Table 2, entry 18).</p><p>Designing heteropoly acid (HPA) supported mesoporous catalysts</p><p>using one-step preparation route is a challenging task. However, it was</p><p>made possible when Su et al. [91] successfully fabricated ZrO2 materials</p><p>functionalized by both Keggin type HPA and benzene-bridged organo-</p><p>silica, H3PW12O40/ZrO2–Si(Ph)Si via co-condensation-hydrothermal</p><p>process (Fig. 8). The performance of this highly ordered mesoporous</p><p>organic-inorganic hybrid catalyst is evaluated in the esterification of</p><p>levulinic acid into methyl levulinate. At 65 ℃, this catalyst exhibits</p><p>much better performance than the mesoporous H4SiW12O40-SiO2 cata-</p><p>lyst synthesized by Yan et al. [87] (Table 2, entry 16), by yielding 99.9%</p><p>methyl levulinate after a period of only 3 h (Table 2, entry 22). The</p><p>superior activity of H3PW12O40/ZrO2–Si(Ph)Si is solely dependent on</p><p>the strong Brønsted acidity, well-defined mesoporous and surface hy-</p><p>drophobicity characteristics.</p><p>Aluminum is an important primary building unit in the mesoporous</p><p>silica as it can generate and improve the acidity of the inorganic matrix.</p><p>In respect to this, Chermahini and Nazeri [92] incorporated different</p><p>amounts of aluminum onto MCM-41 support via post-grafting process</p><p>using aluminum isopropoxide as the aluminum source. All modified</p><p>samples (nAl-MS, n = 5, 10, 20) retain their mesoporous characteristics</p><p>after grafting process with pore size distribution is slightly shifted to</p><p>larger porosity. Among the catalysts prepared, 10Al-MS is the best</p><p>catalyst for the esterification of levulinic acid into butyl levulinate,</p><p>yielding 90% butyl levulinate at 120 ℃ after 8 h reaction time (Table 2,</p><p>entries 23–25). The catalytic study is also tested with isobutanol (IB) at</p><p>Fig. 7. Possible reaction mechanism for the esterification of levulinic acid with ethanol. Adapted from Pasquale et al. [85].</p><p>Fig. 8. Preparation route of H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst. Adapted from Su et al. [91].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>10</p><p>110 ℃ for 8 h using 10Al-MS and 89% yield of isobutanol levulinate is</p><p>recorded.</p><p>Tin-containing Technische Universiteit Delft Number 1 mesoporous</p><p>catalyst (SnTUD-1) has been synthesized by Pachamuthu et al. [93]</p><p>under hydrothermal conditions, and it has been tested in the esterifi-</p><p>cation of levulinic acid with n-butanol. SnTUD-1 catalyst prepared with</p><p>Si/Sn molar ratio of 25 (Sn T-4) is the most outstanding catalyst among</p><p>other Sn loaded catalysts (Si/Sn = 100, 50), accomplishing complete</p><p>conversion with 90.5% yield of butyl levulinate after 4 h of reaction</p><p>(Table 2, entry 26). Along with this, the authors also reported the</p><p>remarkable activity of Sn T-4 in the esterification of levulinic acid with</p><p>ethanol that yielded nearly 82.9% of ethyl levulinate, however, after</p><p>prolonged reaction time of 12 h (Table 2, entry 27). Furthermore, Sn T-4</p><p>is recyclable with insignificant changes in the butyl levulinate yield even</p><p>after four consecutive regeneration cycles.</p><p>Most recently, a newly designed highly selective catalyst containing</p><p>both Lewis and Brønsted acidities, viz. Zr(IV) substituted Keggin poly-</p><p>oxometalate (POM) grafted on MCM-41 silica (xZrPM(DS-E), x = 10 or</p><p>20 in wt%), has been synthesized via co-condensation of tetraethox-</p><p>ysilane (TEOS) in the presence of POM salt and CTAB mesoporous</p><p>template [94]. In this catalyst, Zr(IV) centre ions in POM can attract</p><p>electron pair (Lewis acid site) and concurrently, dissociate the coordi-</p><p>nated water molecules through polarization effect (in-situ Brønsted</p><p>acidity). Furthermore, the nucleophilic oxygen atoms located at outer</p><p>surface of POM provides simultaneous activation of reactants when it is</p><p>bound to H+ counter-cations. The prepared heterogeneous catalysts,</p><p>especially 20ZrPM(DS-E), experiences significant reduction in surface</p><p>area, pore volume, pore size and meso-ordering due to the functionali-</p><p>zation of POM. As a result, 10ZrPM(DS-E) catalyst performs outstand-</p><p>ingly than 20ZrPM(DS-E) in the esterification of ethanol (conversion</p><p>69.2%, 100% ethyl levulinate selectivity, 70 ℃) and octanol (conversion</p><p>76.7%, 100% octyl levulinate selectivity, 130 ℃) after 5 h reaction time</p><p>(Table 2, entries 28–29). In addition, 10ZrPM(DS-E) is considerably</p><p>stable and resistant to leaching which leads to good catalyst reusability</p><p>after third reaction cycles (Table 2, entries 30–31).</p><p>The preparation of sulfated zirconia supported on Kemijski Inštitut</p><p>Ljubljana Type 2 (KIL-2) mesoporous silica has been elaborated by</p><p>Popova et al. [95]. The catalysts are prepared by incipient wetness</p><p>impregnation with 4, 8, 15 and 25 wt% of ZrO2, and then subjected to</p><p>sulfation modification (Fig. 9). The acid</p><p>properties of the catalysts are</p><p>found to be directly correlated to the ZrO2 loading and nature of acid</p><p>sites (Brønsted and Lewis, strength, distribution). Upon sulfation, the</p><p>catalysts exhibit higher conversion than the non-sulfated counterparts in</p><p>the esterification of ethanol due to their stronger Brønsted acidity.</p><p>Furthermore, the sulfated catalysts also show different reactivity</p><p>depending on the chain length of alcohols. Nevertheless, excessive</p><p>loading of ZrO2 leads to ZrO2 dispersion problem and mesostructure</p><p>collapse that affect their catalytic activity. The optimum conversion of</p><p>51% is recorded after 5 h by 15 wt% ZrO2 loaded catalyst (SO4</p><p>2− /</p><p>15ZrKIL-2) with ethyl levulinate forms as the only product (Table 2,</p><p>entry 32). However, the catalysts suffer from minor leaching of sulfate</p><p>group where the leaching of this active group relies on the dispersion of</p><p>ZrO2 supported on the mesosilica support (Table 2, entry 33).</p><p>The functionalized mesoporous silica often takes advantage of its</p><p>high acidity and high mesoporosity. However, the preparation of these</p><p>catalysts can also be time-consuming, expensive and laborious due to</p><p>multi-stage preparation phase. Thus, the development of an active and</p><p>selective mesoporous silica catalyst with outstanding stability, strong</p><p>acidity and improved structural features is highly envisaged.</p><p>2.3. Carbonaceous materials</p><p>Mineral acids (e.g., HCl and H2SO4) are traditionally used for the</p><p>esterification of levulinic acid. Nonetheless, mineral acids cause many</p><p>environmental and operational problems during the catalytic reaction</p><p>process. Hence, researchers are trying to design less corrosive and</p><p>recyclable catalysts such as carbonaceous materials to replace the ho-</p><p>mogeneous acid catalysts. Carbonaceous materials such as graphene,</p><p>graphite and carbon nanotubes are outstanding materials for heteroge-</p><p>neous catalysis offering exceptional physical, chemical and thermal</p><p>properties.</p><p>In 2014, Oliveira and Silva [96] prepared sulfonated multiwall car-</p><p>bon nanotubes (MWCNT) catalyst for ethyl levulinate production. The</p><p>solids are obtained via concentrated sulfuric acid treatment at 150–280</p><p>℃ where the MWCNT sulfonated at 150 ℃ and 210 ℃ (CNT-150, CNT-</p><p>210) exhibits very high amount of acidity. Extreme sulfonation condi-</p><p>tion (>250 ℃) is not recommended as it causes structure collapse that</p><p>affects the surface area, pore size and acidity of catalyst as proven by the</p><p>temperature-programmed decomposition experiment (Fig. 11). The</p><p>prepared CNT-210 catalyst shows the highest conversion of 54% with</p><p>100% ethyl levulinate selectivity in the esterification of levulinic acid</p><p>(Table 3, entry 1). Nevertheless, the sulfonated catalyst experiences</p><p>significant loss in the catalytic activity and is not reusable due to strong</p><p>adsorption of levulinic acid onto the acid sites [96].</p><p>Apart from MWCNT, Zheng and co-workers [97] attempted to use</p><p>reduced graphene oxide (rGO) as high surface area support for incor-</p><p>porating various amounts of H3PW12O40 (HPW) heteropolyacid (5–45</p><p>wt%). With increasing the HPW loading, the acidity of catalyst is</p><p>enhanced through well dispersion of HPW on rGO surface, which further</p><p>improves the catalytic activity of xwt% HPW/rGO (x = 5–45 wt% HPW)</p><p>in esterification of levulinic acid and ethanol (96.9% conversion, 80 ◦C,</p><p>11 h). In addition, the high polarity of HPW also allows strong chemical</p><p>interaction with rGO support which in turn minimizing HPW leaching</p><p>[98].</p><p>Similar research team also incorporated 5–45 wt% HPW on 3D</p><p>graphene aerogel to study the effect of different carbonaceous support</p><p>[99]. It is shown that 45 wt% HPW/3D graphene aerogel (SBET = 123</p><p>m2/g) exhibits 2.2 times higher surface area than 45 wt% HPW rGO</p><p>(SBET = 56 m2/g), where high surface area offers high HPW dispersion</p><p>that is beneficial for acidity and ethyl levulinate catalytic enhancements</p><p>(Table 3, entry 8) [99].</p><p>Carbon-based catalysts are environment-friendly since they are</p><p>Fig. 9. The structure of (a) weak and (b) strong Brønsted acid sites of sulfated Zr/KIL-2. Adapted from Popova et al. [95].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>11</p><p>derived from renewable biomass. In this respect, Li and co-workers</p><p>[100] prepared sulfonated carbon catalyst from loofah sponge</p><p>biomass. Several catalyst preparation parameters, such as carbonization</p><p>temperature (400–900 ◦C), pyrolysis time (30–180 min), sulfonation</p><p>temperature (100–200 ◦C) and sulfonation time (9–24 h), are also</p><p>studied. It is found that these parameters affect not only the carbon</p><p>skeleton formation but also the surface acid density with carbonization</p><p>and sulfonation temperatures show the most profound effect. Hence, the</p><p>controlled treatment conditions are needed to allow the formation of</p><p>more polycyclic aromatic hydrocarbons that can promote the func-</p><p>tionalization of sulfonic acid groups. Among the samples prepared, the</p><p>sulfonated carbon catalyst prepared using 600 ◦C and 90 min for py-</p><p>rolysis, and 160 ◦C and 18 h for sulfonation records the levulinic acid</p><p>conversion as high as 91% under the optimized reaction conditions.</p><p>In the meantime, the similar group also reported the use of pine</p><p>needle leaves as carbon source for the preparation of carbonaceous solid</p><p>[101]. Comparing with the loofah sponge (12.5 m2/g), pine needle-</p><p>derived carbon shows higher surface area (29.0 m2/g). As a result, the</p><p>pine needle-derived sulfonated carbon exhibits better catalytic perfor-</p><p>mance (96.1% conversion, 8 h) than the loofah sponge derived-carbon</p><p>(91.0% conversion, 10 h) at 80 ◦C. Interestingly, the surface area of</p><p>used pine needle-derived sulfonated carbon increases (67.1 m2/g) but</p><p>this feature does not improve the levulinic acid esterification perfor-</p><p>mance due to leaching problem; a decrease of nearly 50% of acid density</p><p>was noticed [102].</p><p>In another work, sulfonated carbon catalyst from sugarcane bagasse</p><p>was prepared by Liu et al. [86] via incorporation with various amounts</p><p>of sulfuric acid (25–125 mL/g). They found that the optimum</p><p>carbonization-sulfonation conditions for the catalyst are 75 mL/g of</p><p>sulfonation ratio and 15 h of sulfonation time [86]. This resulting</p><p>catalyst (67.7%) gives a better ethyl levulinate yield than the purchased</p><p>sulfonated activated carbon catalyst (47.7%) at 120 ◦C for 9 h. In the</p><p>esterification reaction, sulfonic acid group is the active site for the ethyl</p><p>levulinate formation (Fig. 12). First, the adsorption of levulinic acid</p><p>occurs at the Brønsted acid sites (–SO3H), resulting in the formation of</p><p>carbocation. Then, a nucleophilic attack by ethanol takes place and</p><p>oxonium cation (H3O+) is formed. The oxonium cation then releases a</p><p>proton back to the catalyst, producing EL as the product and eliminating</p><p>a water molecule as the side product [85]. Basically, most of the sulfo-</p><p>nated carbon catalysts undergo similar esterification mechanism. For</p><p>example, Song et al. [41] report that arylsulfonic acid functionalized</p><p>hollow mesoporous carbon spheres has the similar mechanism as that of</p><p>Varkolu et al. [103] where H+ plays a crucial role in the reaction.</p><p>Furthermore, the interaction between C = O groups of levulinic acid</p><p>with the extra-surface functional groups (e.g. hydroxyl, carboxyl or</p><p>sulfonic groups) would increase the sticking coefficient of reacting</p><p>molecules, accelerating the esterification rate of levulinic acid (Fig. 13)</p><p>[104].</p><p>Besides activated carbon, sulfonated carbon cryogel derived from</p><p>furfural, lignin and ionic liquid have also been used in catalyzing the</p><p>levulinic acid esterification reaction [105,106]. The selection and</p><p>composition of starting material are found to be important for the</p><p>preparation of carbon cryogel with desired characteristics (e.g., high</p><p>porosity and high surface</p><p>area). In addition, long calcination time im-</p><p>proves the surface porosity but decreases the acidity. The study also</p><p>indicates that the carbonized and calcined cyrogels, which show more</p><p>hydrophobic characteristics, are beneficial for levulinic acid esterifica-</p><p>tion as high hydrophobicity promotes levulinic acid esterification by</p><p>inhibiting water adsorption on the catalyst surface [107]. In general,</p><p>calcined cryogel is more preferrable due to the use of low energy for</p><p>catalyst preparation and better catalytic performance as compared to</p><p>the other counterparts.</p><p>An attempt to use carbon cryogel to esterify levulinic acid has been</p><p>made by Zainol and co-workers [105]. As similar to other carbon ma-</p><p>terials, prolong the calcination time can improve the surface porosity</p><p>but the acidity will be decreased. In this work, three materials, namely</p><p>cryogel, calcined cryogel and carbonized cryogel which are prepared</p><p>under different gas atmospheres, are also compared. The carbonized and</p><p>calcined cyrogels have been shown to be more hydrophobic and this</p><p>feature is good for levulinic acid esterification as it suppresses water</p><p>competitive adsorption on the catalyst [107]. In general, the perfor-</p><p>mance of both carbonized and calcined cryogel is similar. However,</p><p>calcined cryogel is more favoured due to lower temperature preparation.</p><p>Carbon cryogel prepared from liquefaction of oil palm biomass with</p><p>ionic liquid is also reported [98]. The carbon cryogel liquefied oil palm</p><p>fronds exhibit high thermal stability and large surface area (578 m2/g)</p><p>which consists of mesopores and micropores, which are beneficial for</p><p>the ethyl levulinate conversion. This study reveals that the optimum</p><p>ethanol-to-levulinic acid molar ratio is 15:1. The reaction conversion</p><p>and ethyl levulinate yield slightly drop when the ratio increases to 30:1.</p><p>This is because excess ethanol is required to shift the reaction forward as</p><p>proposed by the Le Chatelier’s principles. Nonetheless, excess of ethanol</p><p>can dilute the concentration of levulinic acid, which can affect the re-</p><p>action activity.</p><p>Reaction parameters also play significant roles in the levulinic acid</p><p>esterification reaction. In this regard, Varkolu and his team [103]</p><p>investigated the effect of various alcohols (i.e., methanol, ethanol, 1-</p><p>propanol, 2-propanol and 1-butanol) in the esterification of levulinic</p><p>acid using sulfonated carbon catalyst derived from the glycerol poly-</p><p>merization. The reaction was conducted at 80 ◦C for 3 h with ethanol,</p><p>and other respective reflux temperature for other alcohols. A decrease in</p><p>the esterification reactivity is observed by extending the carbon chain</p><p>length or increasing the carbocation stability of alcohol. Similarly, the</p><p>esterification reaction becomes less selective when the alcohol chain</p><p>length increases. Among the tested alcohols, methanol and ethanol show</p><p>complete conversion with 99% of selectivity towards desired products.</p><p>The catalyst remains usable up to 5 cycles without loss of catalytic</p><p>activity.</p><p>Yang et al. [108] used glucose-derived amorphous carbon and con-</p><p>ventional activated carbon as carbon sources for sulfonation with</p><p>concentrated sulfuric acid. As compared to commercial activated car-</p><p>bon, the glucose suffers from incomplete carbonization but the resulting</p><p>Fig. 11. Temperature-programmed decomposition experiment under helium</p><p>flow where the SO2 peak at 280 ◦C is getting weaker. Adapted from Oliveira and</p><p>Silva [96].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>12</p><p>amorphous carbon can be easily sulfonated, leading in higher sulfonic</p><p>acid density besides bearing carboxylic acid and phenolic functional-</p><p>ities. In addition, the glucose-derived amorphous carbon has high</p><p>swelling capability allowing high accessibility and diffusion of reactants</p><p>to the acid sites even though it has lower surface area than the sulfo-</p><p>nated commercial activated carbon. As a result, it gives better catalytic</p><p>performance (TOF = 95.8 h− 1) in butyl levulinate esterification syn-</p><p>thesis than sulfonated commercial activated carbon (23.3 h− 1), Nafion-</p><p>212 resin (59.0 h− 1), Amberlyst-15 (10.7 h− 1), Hβ zeolite (7.2 h− 1) and</p><p>HZSM-5 (4.3 h− 1). In addition, catalyst loading and reactants molar</p><p>ratio are also important for assuring high conversion of levulinic acid</p><p>with the optimized levulinic acid to alcohol molar ratio was 1:5 [99].</p><p>Meanwhile, the use of excessive catalyst might cause catalyst agglom-</p><p>eration which in turn causing inefficient contact between reactants and</p><p>the catalyst.</p><p>In 2021, Peixoto et al. [109] developed biochar catalysts for EL</p><p>synthesis. The biochar is made from vineyard pruning wastes and un-</p><p>dergoes CO2 activation at 800 ◦C. Three different reagents are used to</p><p>functionalize -SO3H group onto the biochar, namely H2SO4 (Bio-C1),</p><p>ClSO3H (Bio-C2), and 2-(4-chlorosulfonylphenyl)-ethyltrimethoxysilane</p><p>(CSPETS) (Bio-C3). Although the surface area of Bio-C2 (129 m2/g) and</p><p>Bio-C3 (113 m2/g) is smaller than that of Bio-C1 (559 m2/g), the</p><p>Brønsted acid density of Bio-C3 is found to be higher (0.983 mmol H+/</p><p>g). The arylsulfonic group in Bio-C3 provides stronger proton releasing</p><p>ability, which explains why it has a larger Brønsted acid density [110].</p><p>As a result, the Bio-C3 shows better catalytic activity with the ethyl</p><p>levulinate yield of 92% (Table 3, entries 18). The author makes no</p><p>mention of the activity of other counterparts in the conversion of levu-</p><p>linic acid.</p><p>Table 3 summarizes the recent works on the esterification of levu-</p><p>linic acid over carbon-based catalysts. In summary, pore dimension,</p><p>surface area, acidity/functional groups and hydrophobicity of the</p><p>catalyst can directly affect the esterification performance of levulinates.</p><p>Meanwhile, the reaction parameters, such as alcohol to levulinic acid</p><p>molar ratio, catalyst dosage and type of alcohol, show key influence on</p><p>the levulinates formation. To date, the esterification of levulinic acid</p><p>seems only focus on HPA and sulfonated carbon catalysts, and on the</p><p>esterification of ethanol. Hence, more studies on designing new carbon</p><p>catalysts on esterifying other alcohols (e.g., longer chain, secondary and</p><p>tertiary alcohols) are expected in near future.</p><p>Carbonaceous catalysts are advantageous for esterification of levu-</p><p>linic acid since most carbonaceous materials are metal-free and can be</p><p>obtained from biomass. Biomass is a fascinating catalyst precursor since</p><p>it is inexpensive, abundant and environmentally sustainable. Some in-</p><p>vestigations found that carbon derived from loofah sponges and carbon</p><p>derived from pine needles contributed less pollution to the synthesis</p><p>process. Nonetheless, it is found that most of the carbonaceous catalysts</p><p>require high temperature activation to achieve specific morphology and</p><p>Table 3</p><p>Esterification of levulinic acid catalyzed by carbonaceous materials.</p><p>Entry Catalysta Alkyl</p><p>levulinate b</p><p>Reaction conditions Reaction results Ref.</p><p>Temp.</p><p>(◦C)</p><p>Time</p><p>(h)</p><p>Mole ratio (Levulinic acid:</p><p>Alcohol)</p><p>Conversion</p><p>(%)</p><p>Selectivity</p><p>(%)</p><p>Yield</p><p>(%)</p><p>1 Blank EL 70 5 1:5 2.0 100 2.0 [96]</p><p>2 MWCNT EL 70 5 1:5 2.0 100 2.0 [96]</p><p>3 Sulfonated MWCNT (CNT-150) EL 70 5 1:5 54.0 100 54.0 [96]</p><p>4 Sulfonated MWCNT (CNT-210) EL 70 5 1:5 54.0 100 54.0 [96]</p><p>5 Sulfonated MWCNT (CNT-280) EL 70 5 1:5 20.0 100 20.0 [96]</p><p>6 Sulfonated hydrothermal carbons-</p><p>Glucose</p><p>EL 60 3 1:5 97.0 97.0 94.1 [111]</p><p>7 Sulfonic acid/hollow mesoporous</p><p>carbon spheres</p><p>EL 78 4 1:7 NA NA 99.9 [41]</p><p>8 Sulfonic acid/Bio-glycerol derived</p><p>carbon</p><p>ML 80 3 1:5 100.0 NA NA [103]</p><p>9 Sulfonic acid/Bio-glycerol derived</p><p>carbon</p><p>EL 80 3 1:5 100.0 99.0 99.9 [103]</p><p>10 Sulfonic acid/Bio-glycerol derived</p><p>carbon</p><p>PL 80 3 1:5 96.0 NA NA [103]</p><p>11 Sulfonic acid/Bio-glycerol derived</p><p>carbon</p><p>2PL 80 3</p><p>1:5 80.0 NA NA [103]</p><p>12 Sulfonic acid/Bio-glycerol derived</p><p>carbon</p><p>BL 80 3 1:5 50.0 NA NA [103]</p><p>13 Sulfonated carbonized sugarcane</p><p>bagasse</p><p>EL 120 9 1:5 92.5 95.4 88.2 [86]</p><p>14 Sulfonated Loofah sponge-derived</p><p>carbon</p><p>EL 80 10 1:5 91.0 NA NA [100]</p><p>15 Sulfonated pine needle-derived</p><p>carbon</p><p>EL 80 8 1:5 96.1 NA NA [102]</p><p>16 Sulfonated glucose-derived</p><p>amorphous carbon</p><p>BL 100 4 1:5 90.5 100.0 90.5 [108]</p><p>17 Sulfonated commercial activated</p><p>carbon</p><p>BL 100 4 1:5 18.0 NA NA [108]</p><p>18 CSPETS Sulfonic acid functionalized</p><p>biochar</p><p>EL 130 6 NA 100 NA 92 [109]</p><p>19 45 wt% HPW/3D graphene aerogel EL 80 9 1:10 89.1 NA NA [99]</p><p>20 45 wt% HPW/rGO EL 80 11 1:7.5 96.9 NA NA [97]</p><p>21 Carbon cryogel EL 150 4 1:15 87.2 99.2 86.5 [112]</p><p>22 Lignin-furfural carbon cryogel EL 150 4 1:15 87.2 100.0 87.2 [36]</p><p>23 Porous microspherical ionic liquid</p><p>carbon cryogel</p><p>EL 78 6 1:19 89.0 100 66.9 [106]</p><p>24 Liquefied oil palm frond-based carbon</p><p>cryogel</p><p>EL 78 5 1:15 70.9 100 71.7 [113]</p><p>25 Fe-doped sulfonated carbon cryogel EL 80 4 1:10 N/A NA 95.8 [114]</p><p>a CSPETS: 2-(4-chlorosulfonylphenyl)-ethyl-trimethoxysilane; HPW: Tungstophosphoric acid; RGO: Reduced graphene oxide.</p><p>bEL: Ethyl levulinate; BL: Butyl levulinate; ML: Methyl levulinate; PL: Propyl levulinate; 2PL:2-propyl levulinate.</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>13</p><p>high activity, which is a high-cost method for industry. For instance,</p><p>biochar activation requires 800 ◦C and carbon cryogel carbonization</p><p>requires 500 ◦C to achieve satisfactory morphology. Therefore, further</p><p>research and efforts are necessary in order to minimize the synthesis</p><p>temperature and manufacturing cost.</p><p>2.4. Metal organic frameworks</p><p>Metal Organic Frameworks (MOFs) are three dimensional and highly</p><p>porous materials formed by linkages between metal ion clusters and</p><p>multidentate hydrophobic linkers. The intrinsic high thermal and</p><p>chemical stability of the compounds are associated with the strong co-</p><p>ordination bonds existing between benzene dicarboxylate linkers [107].</p><p>MOFs have been used as catalysts in various hydrocarbons conversion</p><p>thanks to their high internal surface area and high density of active sites</p><p>[115]. Incorporation of different functional groups on MOFs allows</p><p>great opportunities for tailoring and modifying their pore structures,</p><p>permitting rapid diffusion and transport of reactant molecules of specific</p><p>shapes, thus achieving high shape selectivity. Besides, the improved</p><p>catalyst modification by stabilizing the encapsulated catalytic species</p><p>also provides avenue for efficient catalytic activity.</p><p>Several works concerning the application of MOFs in the conversion</p><p>of levulinic acid to alkyl levulinates have been reported. This includes</p><p>the work of Wang et al. [116] reporting the performance of Zr-</p><p>containing UiO-66 and its Brønsted acid functionalized derivatives,</p><p>UiO-66-(COOH)2, for levulinic acid esterification with ethanol</p><p>(Fig. 12a). Although UiO-66-(COOH)2 shows remarkable decrease in the</p><p>surface area due to partial filling of –COOH group, the presence of</p><p>Brønsted acidic − COOH groups and Lewis acidic Zr clusters in UiO-66-</p><p>(COOH)2 provoke synergistic effect, giving better catalytic performance</p><p>(23.9% ethyl levulinate yield) as compared to pristine UiO-66 solid</p><p>(4.2% ethyl levulinate yield) at 78 ◦C for 8 h. In addition, the superior</p><p>catalytic activity is also contributed by the chemical interactions be-</p><p>tween the catalyst and reactants. Their computational study reveals that</p><p>the Zr site favourably coordinates with the carbonyl group of levulinic</p><p>acid, whereas the –COOH group of UiO-66-(COOH)2 is hydrogen-</p><p>bonded with the substrate that promotes ease elimination of –OH</p><p>group in levulinic acid. At the optimum reaction condition, UiO-66-</p><p>(COOH)2 catalyst gives 97.0% ethyl levulinate yield whereas UiO-66</p><p>yields only 25.6% of ethyl levulinate (Table 4, entry 1 and 2). In addi-</p><p>tion, increasing the ethanol concentration is beneficial for ethyl levuli-</p><p>nate yield as esterification of levulinic acid is a reversible reaction, with</p><p>the forward reaction favored by excess ethanol. However, increasing the</p><p>catalyst amount reduces ethyl levulinate yield as excess of water pro-</p><p>duced from the catalyst reverses the esterification reaction. The catalyst</p><p>also shows excellent recyclability up to five cycles.</p><p>Cirujano et al. [117] reported acid catalyzed esterification of levu-</p><p>linic acid with various alcohols using Zr-terephthalate (UiO-66) and Zr-</p><p>2-aminoterephthalate (UiO-66-NH2) MOF catalysts (Fig. 12b). Their</p><p>work mainly focuses on the effects of substituted functional groups on</p><p>the benzene ring, alcohol chain length, particle size and structure defects</p><p>on the esterification reaction. It is shown that the existing of –NH2 group</p><p>gives rise to acid-base functionality to UiO-66-NH2 where the Zr site</p><p>activates levulinic acid and concurrently, the amino group of the ligand</p><p>activates the alcohol (Fig. 14). Nevertheless, both UiO-66-NH2 and</p><p>pristine UiO-66 catalysts show almost similar catalytic activity. Mean-</p><p>while, the reaction rate constant is linearly dependent on the linker</p><p>deficiency (defect site) by which the active sites are mostly located. On</p><p>the other hand, the esterification of levulinic acid is independent of the</p><p>UiO-66 particle size while this parameter only has minor catalytic effect</p><p>on UiO-66-NH2 due to the levulinic acid diffusion hindrance to the</p><p>Fig. 12. The proposed mechanism of ethyl levulinate formation catalyzed by sulfonated carbon catalyst. Adapted from Varkolu et al. [103].</p><p>Fig. 13. The interaction of C = O groups of levulinic acid with the extra-surface</p><p>functional groups of carbonaceous catalyst. Adapted from Ogino et al. [104].</p><p>J. Nelson Appaturi et al.</p><p>Fuel 323 (2022) 124362</p><p>14</p><p>active site by the linked amino groups. Both Zr-MOF catalysts are used to</p><p>esterify levulinic acid with methanol, ethanol, butanol and C12, C16 and</p><p>C18 alcohols. The study shows the catalytic activity increases with</p><p>extending the carbon chain of alcohols (Table 4, entries 4–11).</p><p>The Brønsted acidity in MOF can be introduced using ligand bearing</p><p>functional groups, which is demonstrated by Hf-MOF functionalized</p><p>with sulfonic acid (UiO-66(Hf)-SO3H) (Fig. 15c) [119]. For comparison,</p><p>UiO-66(Zr)-SO3H was also prepared. The UiO-66(Hf)-SO3H is shown</p><p>exhibiting higher catalytic activity than UiO-66(Zr)-SO3H due to its</p><p>larger amount of Brønsted acidity originating from the ligated hydroxyl</p><p>groups of Hafnium metal clusters (Hf-μ3-OH) besides the presence of</p><p>sulfonic acid groups at the ligands. Meanwhile, the activity of UiO-66</p><p>(Hf)-SO3H is also tested in the conversion of levulinic acid into alkyl</p><p>levulinates where various alkyl levulinates are produced in good yield</p><p>over UiO-66(Hf)-SO3H catalyst, thus indicating its promising potential</p><p>as active solid Brønsted acid catalyst (Table 4, entries 14–17).</p><p>Guo et al. [25] synthesized MOF-supported phosphomolybdic acid</p><p>[Cu-BTC][HPM] using 1,3,5-benzenetricarboxylic acid (BTC as ligand),</p><p>copper nitrate (Cu2+ as coordination metal) and phosphomolybdic acid</p><p>(HPM as acid site). An excellent ethyl levulinate yield close to 100% was</p><p>recorded at 120 ◦C at 6 h thanks to its ordered and appropriate pore</p><p>structure, and the acidity of HPM (Table 4, entry 3). The HPM species</p><p>were also stably functionalized on the MOF support resulting in good</p><p>catalyst reusability compared to the homogeneous catalysts.</p><p>Desidery et al. [120] synthesized partially and fully sulfonated</p><p>UiO66 catalysts where the effects of hydration, dehydration</p>