Direct Catalytic Conversion of Cellulose to 5 ... - MDPI

inorganics Article

Direct Catalytic Conversion of Cellulose to 5-Hydroxymethylfurfural Using Ionic Liquids Sanan Eminov 1,2 , Paraskevi Filippousi 2 , Agnieszka Brandt 2 , James D. E. T. Wilton-Ely 1, * and Jason P. Hallett 2, * 1 2

*

Department of Chemistry, Imperial College London, London SW7 2AZ, UK; [email protected] Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK; [email protected] (P.F.); [email protected] (A.B.) Correspondences: [email protected] (J.D.E.T.W.-E.); [email protected] (J.P.H.)

Academic Editor: Andreas Taubert Received: 21 June 2016; Accepted: 11 October 2016; Published: 20 October 2016

Abstract: Cellulose is the single largest component of lignocellulosic biomass and is an attractive feedstock for a wide variety of renewable platform chemicals and biofuels, providing an alternative to petrochemicals and petrofuels. This potential is currently limited by the existing methods of transforming this poorly soluble polymer into useful chemical building blocks, such as 5-hydroxymethylfurfural (HMF). Ionic liquids have been used successfully to separate cellulose from the other components of lignocellulosic biomass and so the use of the same medium for the challenging transformation of cellulose into HMF would be highly attractive for the development of the biorefinery concept. In this report, ionic liquids based on 1-butyl-3-methylimidazolium cations [C4 C1 im]+ with Lewis basic (X = Cl− ) and Brønsted acidic (X = HSO4 − ) anions were used to investigate the direct catalytic transformation of cellulose to HMF. Variables probed included the composition of the ionic liquid medium, the metal catalyst, and the reaction conditions (temperature, substrate concentration). Lowering the cellulose loading and optimising the temperature achieved a 58% HMF yield after only one hour at 150 ◦ C using a 7 mol % loading of the CrCl3 catalyst. This compares favourably with current literature procedures requiring much longer reactions times or approaches that are difficult to scale such as microwave irradiation. Keywords: biorenewables; platform chemicals; catalysis; biorefinery; lignocellulose

1. Introduction The biorefinery concept requires routes that are efficient in terms of both materials and energy for the conversion of biomass to useful platform chemicals [1,2]. Lignocellulosic biomass represents a promising renewable feedstock for commercial large-scale biorefining, as it is diverse, widely distributed, and can be grown on a billion ton scale [1]. Agricultural byproducts (e.g., wheat straw, sugarcane bagasse, corn stover) or bioenergy crops, which do not rely on the use of arable land (e.g., Miscanthus, switchgrass, tree wood), avoid competition with food production (the food vs. fuel debate) and result in a higher reduction of net CO2 emissions. Unfortunately, the processing of lignocellulosics requires energetically demanding pretreatment routes to separate lignin from cellulose [3], and subsequent cellulosic sugar and biofuel production relies on slow biocatalytic transformations (enzymatic saccharification and microbial fermentation) [4]. Cellulose and hemicellulose make up the largest part of lignocellulosic biomass and account for 50%–80% of the total mass [5]. Many recent papers have reviewed the use of ionic liquids for accessing the cellulose in lignocellulosic biomass; however, these articles focus mainly on the advantages of ionic liquids in the isolation of cellulose compared with traditional methods [6–12]. For example, it has been shown that ionic liquids can decrystallise the cellulose portion of biomass and disrupt linkages Inorganics 2016, 4, 32; doi:10.3390/inorganics4040032

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For example, it has been shown that ionic liquids can decrystallise the cellulose portion of biomass

to the hemicellulose and lignin portion [6], while George et al. studied the possibility of removing and disrupt linkages to the hemicellulose and lignin portion [6], while George et al. studied the ligninpossibility from theof other biomass constituents using ionic liquids and selectively extracting the lignin and removing lignin from the other biomass constituents using ionic liquids and selectively hemicellulose two aspects could combine withaspects the present to enable extracting fractions the lignin[13]. and These hemicellulose fractions [13]. These two could research combine with the the directpresent synthesis of platform chemicals, such as 5-hydroxymethylfurfural (HMF), from biomass. research to enable the direct synthesis of platform chemicals, such as The hydrolysis of cellulose andfrom hemicellulose produces different sugar monomers, from which 5-hydroxymethylfurfural (HMF), biomass. The hydrolysis of cellulose andcould hemicellulose produces different sugarhave monomers, from which a wide range of important chemicals be produced. Several papers been published on the a wide range of important chemicals couldweight be produced. Several papers have beenwith published the chemical transformation of low-molecular carbohydrates to chemicals usefulon industrial chemical transformation weightand carbohydrates tostudied chemicals usefultransformation industrial application profiles [14–23]. of Forlow-molecular example, Dumesic co-workers thewith catalytic application profiles [14–23]. For example, Dumesic and co-workers studied the catalytic of biomass-derived oxygenated feedstocks into useful fuels and chemicals. These processes transformation of biomass-derived oxygenated feedstocks into useful fuels and chemicals. These included hydrolysis, dehydration, isomerisation, aldol condensation, reforming, hydrogenation, processes included hydrolysis, dehydration, isomerisation, aldol condensation, reforming, and oxidation [24]. Aand summary of the methods to catalytically transform lignocellulose hydrogenation, oxidation [24].alternative A summary of the used alternative methods used to catalytically biomass is shown in Figure 1 [25]. transform lignocellulose biomass is shown in Figure 1 [25].

Figure 1. Lignocellulosicpathways. pathways. A A scheme scheme for biorefinery [25].[25]. Figure 1. Lignocellulosic foraachemo-catalytic chemo-catalytic biorefinery

Various review articles have provided an overview of key areas, such as the synthesis and use

Various articles providedand an cellulose overview[26] of key such as thefor synthesis andofuse of of sugar review derivatives fromhave hemicellulose and areas, the methods used conversion sugarbiomass derivatives and cellulose[22,27]. [26] and the methods used for conversion of biomass wastefrom intohemicellulose useful platform chemicals Among the important bio-derived building wasteblocks, into useful platform chemicals [22,27]. Among the as important bio-derived building blocks, 5-hydroxymethylfurfural (HMF) has been reported one of the most promising [28,29], after the U.S. Department of Energy it as a major chemical that could be derived 5-hydroxymethylfurfural (HMF) has identified been reported as oneplatform of the most promising [28,29], after the from lignocellulosic biomass on a large [30]. HMF ischemical a versatile intermediate between from U.S. Department of Energy identified it as ascale major platform that could be derived biomass-based carbohydrate the industrial chemistry, with lignocellulosic biomass on a largechemistry scale [30].and HMF is apetroleum-based versatile intermediate between biomass-based widespread potential use in a variety of chemical manufacturing applications and industrial carbohydrate chemistry and the petroleum-based industrial chemistry, with widespread potential use products [31,32]. Most prominent among these is 2,5-furandicarboxylic acid (FDCA) [33], which can in a variety of chemical manufacturing applications and industrial products [31,32]. Most prominent be used to make renewable polymers [34] and liquid transport fuels, such as 2,5-dimethylfuran [35]. among these is 2,5-furandicarboxylic acid (FDCA) [33], which can be used to make renewable The potential of HMF as a synthetic building block is now widely recognised, which has led to a polymers and liquidoftransport fuels, such [35].HMF Theispotential of HMF surge [34] in the amount research utilising HMFasas2,5-dimethylfuran a feedstock [36]. While an attractive as a synthetic building block is now widely recognised, which has led to a surge in the amount of renewable building block, several byproducts from degradation reactions occur during or after HMF research utilising HMF as atofeedstock [36].ofWhile HMF is an attractive renewable building transformation, leading the formation levulinic acid, formic acid, and humins [37,38], andblock, reducing the overall yield; this is particularly prevalent acidic transformation, aqueous conditions, several byproducts fromHMF degradation reactions occur during orunder after HMF leading the HMF product combines with unreacted to produce humins [39].HMF to thewhere formation of levulinic acid, formic acid, andsugars humins [37,38], water-insoluble and reducing the overall HMF can be producedunder with sufficient selectivity and yieldwhere from food-grade fructose for it yield;However, this is particularly prevalent acidic aqueous conditions, the HMF product combines to become a nascent industrial process [40]. Production from more prevalent, lower cost sugars (such with unreacted sugars to produce water-insoluble humins [39]. However, HMF can be produced as glucose) requires a two-step catalytic mechanism involving isomerisation and dehydration, under with sufficient selectivity and yield from food-grade fructose for it to become a nascent industrial Lewis basic and Brønsted acidic conditions (Figure 2) [41]. The solubility of HMF can also be an issue process [40]. sugar Production from more prevalent, sugars (such as glucose) requires a two-step at high loadings in aqueous systems, lower leadingcost to byproducts formed from sugars with the catalytic mechanism involving isomerisation and dehydration, under Lewis basic and Brønsted acidic conditions (Figure 2) [41]. The solubility of HMF can also be an issue at high sugar loadings in aqueous systems, leading to byproducts formed from sugars with the HMF. This has led some researchers to explore removing HMF from the aqueous phase (which contains the sugars) using immiscible organic solvents, thereby reducing humin formation [42].


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HMF. This Inorganics 2016,has 4, 32 led

some researchers to explore removing HMF from the aqueous phase (which 3 of 15 contains the sugars) using immiscible organic solvents, thereby reducing humin formation [42].

Figure to 5-hydroxymethylfurfural 5-hydroxymethylfurfural (HMF) (HMF) via via fructose. fructose. Figure 2. 2. Transformation Transformation of of glucose glucose to

While HMF production from sugar monomers is largely straightforward, the most desirable While HMF production from sugar monomers is largely straightforward, the most desirable and and challenging route to produce HMF involves widely available renewable sources such as challenging route to produce HMF involves widely available renewable sources such as cellulose [43]. cellulose [43]. However, a high-yield, low-cost, energy-efficient, and direct conversion of cellulose However, a high-yield, low-cost, energy-efficient, and direct conversion of cellulose into HMF is into HMF is still a challenge and the subject of substantial research. Many different routes and still a challenge and the subject of substantial research. Many different routes and methods have methods have been proposed but the routes published to date are low-yielding, complex, and hence been proposed but the routes published to date are low-yielding, complex, and hence high in cost or high in cost or energy inefficient. energy inefficient. The proposed synthetic route from cellulose to HMF is a multistep approach consisting of the The proposed synthetic route from cellulose to HMF is a multistep approach consisting of the production of glucose from cellulose, followed by glucose isomerisation to fructose, and finally HMF production of glucose from cellulose, followed by glucose isomerisation to fructose, and finally formation from the dehydration of fructose [44]. Interestingly (in the context of this work), the HMF formation from the dehydration of fructose [44]. Interestingly (in the context of this work), conversion of cellulose to HMF has also been shown to occur in the absence of a catalyst using ionic the conversion of cellulose to HMF has also been shown to occur in the absence of a catalyst using liquids, albeit in low yields [33]. ionic liquids, albeit in low yields [33]. The key step in the conversion of cellulose to HMF (Figure 3) is the hydrolysis of cellulose to The key step in the conversion of cellulose to HMF (Figure 3) is the hydrolysis of cellulose to sugar molecules, which is hampered by the poor solubility of polymeric cellulose in most solvents. sugar molecules, which is hampered by the poor solubility of polymeric cellulose in most solvents. Acid-catalysed hydrolysis of cellulose is one method that has been studied widely, in which the Acid-catalysed hydrolysis of cellulose is one method that has been studied widely, in which the hydroxonium cations interact with the cellulose network, leading to breakdown of the polymer. hydroxonium cations interact with the cellulose network, leading to breakdown of the polymer. The most common acids used in cellulose hydrolysis are mineral acids (H2SO4, HCl) and The most common acids used in cellulose hydrolysis are mineral acids (H SO4 , HCl) and organic organic acids, such as p-toluenesulfonic or carboxylic acids. Sulfuric acid2 has been used most acids, such as p-toluenesulfonic or carboxylic acids. Sulfuric acid has been used most frequently frequently (at elevated temperatures), which is unsurprising due to its low cost. While higher (at elevated temperatures), which is unsurprising due to its low cost. While higher temperatures and temperatures and longer reaction times are required with dilute acid [45], the treatment of cellulose longer reaction times are required with dilute acid [45], the treatment of cellulose with concentrated with concentrated acid accelerates the hydrolysis markedly, such that the polymer molecules acid accelerates the hydrolysis markedly, such that the polymer molecules depolymerise to monomers depolymerise to monomers and this substantially increases dehydration of the monomeric sugars to and this substantially increases dehydration of the monomeric sugars to HMF [46]. However, due to HMF [46]. However, due to low selectivity and corrosion concerns, dilute acid is preferable for the low selectivity and corrosion concerns, dilute acid is preferable for the controlled hydrolysis of cellulose. controlled hydrolysis of cellulose. Heterogeneous solid acid catalysts have also been used for the Heterogeneous solid acid catalysts have also been used for the depolymerisation of cellulose [47,48], depolymerisation of cellulose [47,48], though the catalytic activity and selectivity of solid acids are though the activity andcellulose selectivity of so solid acidsreaction are verytimes low for poorlyfor soluble very low forcatalytic the poorly soluble and longer arethe required thesecellulose systems and so longer reaction times are required for these systems compared with liquid acid catalysts. compared with liquid acid catalysts. Overall, while offering advantages for removal of the catalyst Overall, offering advantages for removal of the catalyst medium, the catalytic from thewhile medium, the catalytic activity and selectivity of suchfrom solidthe acids are very low and soactivity longer and selectivity of such solid acids are very low and so longer reaction times are required for these reaction times are required for these systems compared with liquid acid catalysts. systems compared with liquid acid catalysts. Besides the acid catalysed depolymerisation of cellulose, there exist other methods to achieve Besidessuch the as acid catalysed[49] depolymerisation of cellulose, there exist other to none achieve this effect, solvolysis and catalytic hydrothermal liquefaction [50].methods However, of this effect, such as solvolysis [49] and catalytic hydrothermal liquefaction [50]. However, none these methods has provided to be an effective and selective direct (“one-pot”) chemical of these methods has provided to be an effective selective (“one-pot”) chemical transformation of cellulose to useful chemicals such and as HMF and, direct as a result, they are either transformation of cellulose to useful chemicals such as HMF and, as a result, they are either uneconomical or environmentally unfriendly (or both). For this reason, the challenge remains to find uneconomical or environmentally unfriendly (or both). For this reason, the challenge remains to a potentially economically viable method for cellulose conversion to useful products without find a potentially economically viable method for cellulose conversion to useful products without significant environmental impact. significant environmental impact.

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Figure 3. Pathway to HMF from cellulose. Figure 3. Pathway to HMF from cellulose.

Ionic liquids (ILs) [51] are being developed as alternative, potentially more commercially Ionic liquids (ILs) [51] aremedia being developed as alternative, potentially more commercially attractive attractive and low-emission for separating cellulose from lignin due to their unique chemical and low-emission media for separating cellulose from lignin due to here their combines unique chemical and solvent and solvent characteristics [6–12,52–57]. The research reported ionic liquids as characteristics The(relatively research easily reported here combines liquids as effective low with impact effective low[6–12,52–57]. impact solvents handled, negligibleionic vapour pressure, recyclable) solvents (relatively handled, vapourofpressure, recyclable) with low-toxicity metal low-toxicity metal easily catalysts for the negligible direct conversion cellulose to value-added chemicals. For this purpose, liquids (Figureof4)cellulose have been studied under conditions in catalysts fortwo theionic direct conversion tosystematically value-added chemicals. Fordifferent this purpose, two ionic order(Figure to arrive4)athave an effective system for cellulose The potential for selectively liquids been systematically studieddissolution. under different conditions in order tomaking arrive at fromsystem fructose glucose using ionic liquids (ILs) as solvents was firstmaking demonstrated by Zhao et anHMF effective fororcellulose dissolution. The potential for selectively HMF from fructose al. [57], who used catalytic amounts of metal salts to convert sugars to HMF in [C 4 C 1 im]Cl, with or glucose using ionic liquids (ILs) as solvents was first demonstrated by Zhao et al. [57], who used CrCl2 providing HMF salts yieldto at convert 100 °C insugars 3 h. The were thanwith thoseCrCl achievable in catalytic amounts 70% of metal to yields HMF in [C4higher C1 im]Cl, 2 providing ◦ most aqueous systems, due to the suppression of HMF hydrolysis to levulinic and formic acids. 70% HMF yield at 100 C in 3 h. The yields were higher than those achievable in most aqueous Chromium salts subsequently verified by other groups as theformic preferred [52–56]. systems, due to thewere suppression of HMF hydrolysis to levulinic and acids.catalyst Chromium salts However, the perceived toxicity of chromium is a key issue for commercial application, although were subsequently verified by other groups as the preferred catalyst [52–56]. However, the perceived this is largely based on Cr(VI) compounds, whereas the Cr(III) used here is an essential trace metal toxicity of chromium is a key issue for commercial application, although this is largely based on Cr(VI) required for the formation of glucose tolerance factor and for insulin metabolism, and is therefore compounds, whereas the Cr(III) used here is an essential trace metal required for the formation of


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glucose tolerance factor and for insulin metabolism, and is therefore widely used as a nutritional supplement and animals [58]. Additionally, risk of exposure to metal salts dissolved in widely usedfor ashumans a nutritional supplement for humansthe and animals [58]. Additionally, the risk of ionic liquids lowered they are retained the solvent medium extraction of exposure to is metal saltsasdissolved in ionic in liquids is lowered as during they are retainedorinseparation the solvent the product. medium during extraction or separation of the product.

Figure Figure 4. 4. Ionic Ionic liquids liquids used used for for the the dissolution dissolution and and breakdown breakdown of of biomass biomass in in this this work. work.

Since ILs can be tuned or designed to suit a certain application, and Brønsted acids have Since ILs can be tuned or designed to suit a certain application, and Brønsted acids proven effective catalysts for fructose dehydration [55,59], the mildly acidic ionic liquid have proven effective catalysts for fructose dehydration [55,59], the mildly acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulphate, [C4C1im][HSO4] (Figure 4), was chosen as a 1-butyl-3-methylimidazolium hydrogen sulphate, [C4 C1 im][HSO4 ] (Figure 4), was chosen as a solvent solvent system for the conversion of fructose to HMF [52] with a low loading of chromium(III) system for the conversion of fructose to HMF [52] with a low loading of chromium(III) chloride. chloride. An excellent HMF yield of 96% was reported using this system after 3 h at◦100 °C without An excellent HMF yield of 96% was reported using this system after 3 h at 100 C without any any detectable trace of the common byproducts, levulinic acid, formic acid, or humins detectable trace of the common byproducts, levulinic acid, formic acid, or humins (water-insoluble (water-insoluble polymers formed from the reaction of HMF with the monosaccharide or a polymers formed from the reaction of HMF with the monosaccharide or a reactive intermediate). reactive intermediate). This imidazolium-based ionic liquid shares the same anion ([HSO4]−) with a This imidazolium-based ionic liquid shares the same anion ([HSO4 ]− ) with a family of ionic liquids family of ionic liquids that have recently been identified as true low-cost options ([HNEt3][HSO4] that have recently been identified as true low-cost options ([HNEt3 ][HSO4 ] and [C1 Him][HSO4 ]), and [C1Him][HSO4]), with a similar cost to traditional organic solvents such as acetone [60]. with a similar cost to traditional organic solvents such as acetone [60]. However, when using cellulose However, when using cellulose as the feedstock for HMF formation, it appears that the key step is as the feedstock for HMF formation, it appears that the key step is the dissolution and depolymerisation the dissolution and depolymerisation of cellulose to glucose monomers rather than the glucose to of cellulose to glucose monomers rather than the glucose to fructose isomerisation and subsequent fructose isomerisation and subsequent fructose dehydration. Ionic liquids appear to overcome fructose dehydration. Ionic liquids appear to overcome these problems as they can play the role of both these problems as they can play the role of both solvent and catalyst. Therefore, our attention solvent and catalyst. Therefore, our attention turned principally to Lewis basic ILs, such as those with turned principally to Lewis basic ILs, such as those with halide anions. This approach is similar to halide anions. This approach is similar to the interplay between acidity and basicity during glucose the interplay between acidity and basicity during glucose to HMF transformation in all solvent to HMF transformation in all solvent systems, including ILs, with the coordinating ability of the IL systems, including ILs, with the coordinating ability of the IL anion used to promote glucose anion used to promote glucose isomerisation. For example, the Lewis acidic metal catalyst SnCl4 has isomerisation. For example, the+ Lewis acidic metal catalyst SnCl4 has been used in ILs with a been used in ILs with a [C4 C1 im] cation and a range of anions [53] where the− non-coordinating BF4 − + [C4C1im] cation and a range of anions [53] where the non-coordinating BF4 anion gave the highest anion gave the highest yield from glucose. This is in contrast to the acid-catalysed mechanism for yield from glucose. This is in contrast to the acid-catalysed mechanism for HMF production from HMF production from fructose reported previously. It was therefore decided that the solvent system fructose reported previously. It was therefore decided that the solvent system best suited to direct best suited to direct cellulose conversion to HMF would draw inspiration from those used to achieve cellulose conversion to HMF would draw inspiration from those used to achieve high yields from high yields from glucose and capable of some cellulose dissolution, such as chloride-based systems. glucose and capable of some cellulose dissolution, such as chloride-based systems. 2. Results and Discussion 2. Results and Discussion 2.1. Catalyst and Ionic Liquid Selection 2.1. Catalyst and Ionic Liquid Selection Our previous study on monosaccharides achieved a yield of 96% HMF from fructose and 90% Our previous study onsulphate-based monosaccharides a yieldILs, of 96% HMF from fructose and 90% from glucose by employing andachieved chloride-based respectively [52,61]. The next goal from glucose by employing sulphate-based and chloride-based ILs, respectively [52,61]. The next was to find suitable conditions and ILs for the conversion of cellulose, which is the most important goal to find suitable conditions ILs abundance for the conversion cellulose, of which is the most targetwas of renewable chemicals due to and its high [1]. The of conversion cellulose is more important target renewable chemicals due toto itsbe high abundance The conversion of cellulose is challenging as itof requires suitable conditions dissolved and [1]. hydrolysed to monosaccharides more challenging as it requires suitable conditions to be dissolved and hydrolysed to before it can be converted to HMF (Figure 5). monosaccharides before it can be converted to HMF (Figure The experiments were carried out in ionic liquids with 5). a low water content; however, rigorous The experiments were carried out in ionic liquids with a lowofwater content;used however, rigorous exclusion of moisture during the reactions was employed. Many the catalysts were added in exclusion of moisture during the reactions was employed. Many of the catalysts used were added in their hydrated form. their hydrated form.


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Figure 5. Transformation of cellulose to HMF via glucose and fructose. Figure 5. Transformation of cellulose to HMF via glucose and fructose.

Initially, the conditions and IL used were kept exactly the same as in our previous studies on

Initially, the conditions and to ILcellulose used were exactly the same as in compare our previous studies on monosaccharides and applied as akept substrate in order to directly HMF yields from celluloseand to those from Two ionic liquids 4C1im][HSO 4] and [C4Cyields 1im]Cl from monosaccharides applied to monosaccharides. cellulose as a substrate in order to [C directly compare HMF were to examined with monosaccharides. the use of chromium,Two zinc,ionic and copper salts, have been reported good were cellulose those from liquids [C4which C1 im][HSO [C4 C1asim]Cl 4 ] and catalysts for the dehydration of sugars to HMF [52], and aliquots were collected after 1.5 and 3 h. examined with the use of chromium, zinc, and copper salts, which have been reported as good The catalysts samples were not collected before 1 h because the dissolution of cellulose was slower than expected for the dehydration of sugars to HMF [52], and aliquots were collected after 1.5 and 3 h. The samples at these conditions. After 1.5 and 3 h, HMF yields were very poor with the use of chromium catalysts were not collected before 1 h because the dissolution of cellulose was slower than expected at these and other catalyst mixtures in the [C4C1im][HSO4] ionic liquid (Table 1). Only a 5% HMF yield was conditions. After 1.5 and 3 h, HMF yields were very poor with the use of chromium catalysts and other observed using CrCl3·6H2O and CrCl3·6H2O–CuCl2 catalysts after 3 h at 120 °C (Table 1, Entries 1 and catalyst mixtures in the [C4 C liquid (Table 1). Only a 5% HMFafter yield observed 4 ] ionic 2). In the presence of this IL1 im][HSO without any catalyst, no HMF formation was detected 3 hwas (Table 1, ◦ C (Table 1, Entries 1 and 2). In the usingEntry CrCl33). ·6H O and CrCl · 6H O–CuCl catalysts after 3 h at 120 2 3 2 2 Unlike the sulphate-based ionic liquid, the yields were higher in the [C4C1im]Cl ionic presence ofwith this 54% IL without catalyst, no HMF formation was 3 h (Table 1, Entry 3). liquid, and 11%any HMF yields with the use of CrCl3·6H 2O detected and CrCl3after ·6H2O–CuCl 2 catalysts, respectively (Table 1, Entries and 5). the Only 3% yield observed in [C the4 C presence of [C4C 1im]Cl with Unlike the sulphate-based ionic4liquid, yields werewas higher in the liquid, 1 im]Cl ionic without catalyst (Table Entry After3the of the IL, ZnCl 2 and CrCl 3·6H2O–ZnCl 2 54% and 11%any HMF yields with1,the use 6). of CrCl ·6Hselection O and CrCl · 6H O–CuCl catalysts, respectively 2 3 2 2 were also examined as potential catalysts for the conversion of cellulose to HMF. The maximum (Table 1, Entries 4 and 5). Only 3% yield was observed in the presence of [C4 C1 im]Cl without any yields of 3%1,and 30% 6). wereAfter obtained ZnClof 2 and CrCl3·6H2O–ZnCl2 catalysts (Table 1, Entries 8 catalyst (Table Entry the using selection the IL, ZnCl2 and CrCl3 ·6H2 O–ZnCl2 were also and 9). examined as potential catalysts for the conversion of cellulose to HMF. The maximum yields of 3% and 30% were obtained ZnCl2 of and CrCl3to·6H catalysts (Table 1,°C. Entries 8 and 9). 2 O–ZnCl Tableusing 1. Conversion cellulose HMF (% yield)2 in ionic liquids at 120 EntryTable 1. Conversion Ionic Liquid Catalyst of cellulose to HMF (% yield) in ionic liquids at 1.5 120h◦ C. 3 h 1 [C4C1im][HSO4] CrCl3·6H2O