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Dehydration of xylose to furfural in butanone catalyzed by Brønsted‐Lewis acidic ionic liquids

In this study, the most abundant C5 carbohydrate unit in biomass, namely xylose, was chosen as the feedstock, and its hydrothermal conversion for furfural production was carried out in a green and renewable solvent system composed of water and butanone, in order to study the role of FeCl3 and [bmim]Cl of ionic liquid catalyst in the conversion process of xylose. A key intermediate named xylulose from Lewis acid‐catalyzed xylose isomerization was quantified. It was concluded that appropriate content of FeCl3 and [bmim]Cl favored the isomerization‐dehydration reaction path along which xylose was converted into furfural, while excessive amount of either component would result in side reactions leading to furfural consumption at long reaction times. By comparison between ionic liquid catalysts that had different active metal sites, xylose conversion and furfural yields were found to increase when the Lewis acidity of the metal ions became stronger. Moreover, chlorometallate anions with better catalytic performance than the original neutral salts were formed during catalyst preparation, and in this way, the xylose conversion and furfural formation were further promoted. Finally, the optimized ionic liquid catalyst produced a highest furfural yield of 75% and xylose conversion of 99% at 140°C.

carbon numbers, and the precursors could further undergo hydrodeoxygenation reactions to give long-chain liquid alkanes that are able to be directly used as vehicle fuels and aviation fuels. 11,12 Conventional hydrothermal technologies for furfural production can achieve about 50% of theoretical furfural yield, but they pose problems like high energy consumption, high cost, equipment corrosion, and environmental pollution caused by acidic waste water and residues. 13 Therefore, researchers have developed plenty of innovative catalytic reaction systems in these years, aiming at the highefficiency production of furfural. The superiority over conventional systems mainly concentrated on the optimization for solvents and catalysts.
The catalysts used during the hydrothermal decomposition of biomass carbohydrates can be divided into two categories, namely homogeneous and heterogeneous catalysts. The latter has been the focus of much attention because of its characteristic of easy recycling. For example, Bruce et al 14 conducted the conversion of xylose in γ-valerolactone-water solvent mixture using micropore zeolite SAPO-34 as the catalyst, and they obtained 40% yield of furfural, while catalyst deactivation was not observed during 3-time recycling test. Homogeneous catalyst can spread in the reaction system adequately, and therefore, they can closely interact with reactants, leading to much better carbohydrates conversion results than heterogeneous catalysts. Zhao et al 15 chose pressurized CO 2 as the acidic catalyst and perform the catalytic conversion of xylose, arabinose and xylan in isopropanol-water solvent mixture for furfural production, and a maximum furfural yield of 69% was achieved from xylose. Le Guenic et al 16 carried out the xylose dehydration reaction catalyzed by FeCl 3 in the cyclopentyl methyl ether-water biphasic solvent with NaCl as an additive and achieved 75% yield of furfural.
Solvent configuration also has significant effects on furfural production, because the distribution and activities of sugar conformers differ in different kinds of solvent. Moreover, the solvent molecules may combine with reactants or intermediates, which could affect the stabilization of intermediates and transition states. 17,18 The most common solvent systems consist of three different types including single-phase solvent, biphasic solvent and ionic liquid. Recently, single-phase solvent system made up of water and organic solvent has been found to hinder side reactions to a certain degree and to raise furfural yields notably. The reason was that the organic solvent altered the distribution of sugar conformers and replaced the active water molecules near the target products to prevent the products from secondary reactions like hydration and fragmentation. 18,19 Ionic liquid has superior ability in biomass solvation, so it has been widely utilized as an advanced solvent in the hydrothermal conversion of biomass. 20 However, the potential of ionic liquid in practical application was restricted due to its much higher cost than traditional organic solvent.
In the past few years, studies have shown that some ionic liquids were able to catalyze the biomass dehydration reaction, while they were still responsible for biomass solvation. Consequently, researchers employed a little amount of ionic liquid as a homogeneous catalyst, and satisfying furfural yields as well as reduced reaction cost were achieved from biomass feedstocks. 21 For example, Serrano-Ruiz et al 22 used Brønsted acidic ionic liquid catalysts for furfural production from xylose in tetrahydrofuran-water and 85% yield of furfural was achieved. Wang et al 23 developed a Lewis acidic ionic liquid catalyst for the same reaction and 80% yield of furfural was obtained in γ-valerolactone-water solvent mixture. Nevertheless, the number of studies concerning furfural production from biomass carbohydrates catalyzed by ionic liquid catalysts was still limited, and the related catalytic reaction pathways remain unclear, especially regarding the function of different components in ionic liquid catalysts. In this work, we take advantage of both the high affinity toward biomass from the Brønsted acidic [bmim]Cl-containing ionic liquid and the outstanding catalytic performance from Lewis acidic metal chlorides for biomass dehydration reaction to produce furfural, and developed several ionic liquid catalysts including [bmim]Cl/FeCl 3 as a representative for furfural production from xylose. The main idea was to investigate the influence of metal chloride component and imidazole component in ionic liquid catalyst on the formation rules of furfural. Butanone-water solvent mixture was chosen as the reaction media. Butanone is commonly recognized as a green solvent that can be directly synthesized from biomass resources. 24 Cl (1-butyl-3-methylimidazolium chloride) and FeCl 3 with specified mole ratios to form 4 different ionic liquids labeled IL1, IL2, IL3, and IL4. The mole fractions of FeCl 3 (χ(FeCl 3 )) were set to 0.33, 0.5, 0.67, and 0.75, respectively. In other words, the number of moles of [bmim]Cl to that of FeCl 3 were 2:1, 1:1, 1:2, and 1:3 for IL1, IL2, IL3, and IL4, respectively. As for other ionic liquids, the mole fractions of the metal chlorides were 0.5.

| Ionic liquid catalyst characterization
Raman spectra were recorded using dispersive Raman spectrometer DXR SmartRaman (Thermo Scientific) equipped with 532 nm laser and CCD detector. Spectra were obtained with 5 cm −1 resolution, using 10 × 5 seconds time of exposures, and laser power was set at 5.8 mW.
The critical characteristic structures in ionic liquid were determined by ESI-MS spectra using an Agilent 6500 series accurate-mass quadrupole time-of-flight system (Agilent), and the fragmentor voltage was set to 180 V. The MS samples were prepared by diluting the ionic liquid in water with a concentration of about 5 mg/mL. MS data were acquired and analyzed by Agilent Mass Hunter software version B.09.00 (B9044.0).
Lewis/Brønsted acidity of the ionic liquid catalyst was verified by pyridine-adsorbed Fourier Transform Infrared Spectroscopy (Py-FTIR). The results were described in Supporting Information.

| Procedure of xylose conversion
In a typical experiment for furfural production, 1 mL water and 4 mL butanone were added to a glass pressure reactor (15 mL, Synthware), and then, 0.15 g xylose and 0.5 mmol ionic liquid catalyst were charged into the solvent mixture. The reactor was put into an oil bath preheated to 140°C, and the magnetic stirring rate was maintained at 600 rpm. When the reaction time reached preset values, the glass reactor was taken out from the oil bath and was cooled by air flow. The solution in the glass reactor was filtered through a 0.22-μm syringe filter and diluted tenfold with water prior to high-performance liquid chromatography (HPLC) analysis. Each experiment was repeated three times, and the resulting mean value and standard deviation are shown in the figures.

| Analytical methods
The filtered solution was analyzed on an UltiMate 3000 HPLC system (Thermo Scientific). A Bio-Rad Aminex HPX-87H column was used to quantify furfural whose retention time was 43.85 minutes, while a Bio-Rad Aminex HPX-87C column was used to quantify xylose and xylulose whose retention time was 10.97 and 13.28 minutes, respectively. An RI 2000 refractive index detector (Schambeck SFD GmbH) was used for signal collection with a sample cell temperature at 40°C. H 2 SO 4 solution (pH 2.5) was used as the mobile phase, and its flow rate and corresponding column temperature were kept at 0.6 mL/min and 60°C, respectively. The concentrations of xylose, xylulose, and furfural were determined by comparison against standard calibration curves. The conversion rate of xylose and the yield of xylulose and furfural were calculated as follows: Other liquid-phase products were analyzed by DSQ II gas chromatography-mass spectrometry (GC-MS) system (Thermo Scientific) equipped with a DB-wax capillary column (30 m × 0.25 mm × 0.25 mm). Helium (99.999%) at a flow rate of 1 mL/min was used as carrier gas. The GC oven temperature was programmed to increase from 40°C (1 minute) to 240°C (20 minute) at 8°C/min heating rate. The MS detector was operated in electron ionization (EI) mode (70 eV) with a scan range of m/z 35-450 and an ion source temperature set to 200°C. All detected chemicals were identified by comparison with the NIST (National Institute of Standards) MS library.
The distribution of xylose conformers in butanone-water and pure water was measured by 13 C-NMR. Details can be found in Supporting Information.

| Catalyst structure
During the synthesis process of [bmim]Cl/FeCl 3 ionic liquid catalysts, equivalent molar amount of FeCl 3 and [bmim]Cl reacted with each other to form a homogeneous and stable ionic liquid, in which [bmim]Cl + and FeCl 4 − existed as the main ion species. When excessive FeCl 3 was added, heavier anions like Fe 2 Cl 7 − and Fe 3 Cl 10 − may be formed. [26][27][28][29] However, previous studies selected pure ionic liquids as samples for characterization. In order to identify the structures of [bmim]Cl/FeCl 3 existing in the proposed butanone-water solvent mixture in our study, a small amount of ionic liquid was dissolved into the butanonewater solvent mixture to form a homogeneous solution, and the Raman spectra for the solution containing IL1, IL2, IL3, and IL4 were recorded, respectively. As shown in Figure 1 were not detected from Raman spectra. The reason could be the highly diluted ionic liquid in butanone-water solvent mixture, and therefore, it was less possible for free FeCl 3 and FeCl 4 − in solution to form dimeric anions or higher coordination. For further confirming the structure of ionic liquid in the proposed solvent system, ESI-MS spectra were recorded for the solution of IL4. As depicted in Figure 2, the existence of [bmim] + , FeCl 4 − , and FeCl 3 is strongly ascertained with molecular formula calculation scores higher than 94. Complex anions such as Fe 2 Cl 7 − and Fe 3 Cl 10 − still do not turn up in the spectrum, which is in accordance with the Raman spectra in Figure 1. It could be inferred that excessive FeCl 3 existed as free molecules in the solution, and most of them offered Lewis acidity, while some of them would undergo hydrolyzation reactions to provide Brønsted acidity. 32

| Effects of catalyst composition on xylose conversion and furfural yield
Xylose dehydration reactions were carried out using ionic liquid catalysts with different χ(FeCl 3 ), and the time-varying xylose conversion and furfural yield were shown in Figure  3. It is obvious that all the four catalysts have similar catalytic performance for the catalytic conversion of xylose, and when the reaction lasted for 90 minutes, all the xylose conversions reached over 90%. The catalytic activity ranking from high to low is as follows: IL3 > IL4>IL2 > IL1, which suggested that FeCl 3 might be more responsible for rapid xylose conversion than [bmim]Cl. On the contrary, considerable differences could be found in the time-varying furfural yield with different ionic liquids as catalysts. IL1 exhibited the worst catalytic performance, and only 38% yield of furfural was obtained even though the reaction lasted for 180 minutes. When catalyzed by IL2, IL3, and IL4, furfural yield could arrive at the maximum value (56%, 47%, and 41%, respectively) within the first 90 minutes. Afterward, furfural yields showed a steep drop at longer reaction times. One possible reason was that furfural underwent condensation reactions with xylose or xylulose to form byproducts like humins. 33 It has been demonstrated that during furfural production from xylose dehydration, the reaction would choose to go along two different paths according to the acid type existing in the reaction system. 39,40 If the catalyst mainly possessed Lewis acidity, xylose would first undergo isomerization reaction to produce xylulose, followed by dehydration of xylulose for furfural formation. If the catalyst showed Brønsted acidity, xylose would lose three water molecules straightly to produce furfural. In order to further understand the dehydration mechanism of xylose, the time-varying xylulose yield in the presence of different ionic liquid catalysts was obtained by HPLC. As shown in Figure 4, with catalyst IL1 the highest xylulose yield during the whole reaction process was only about 5%. This indicated a relatively weak Lewis acidity of IL1, and most of xylose directly dehydrated to produce furfural. Comparatively, 11% yield of xylulose was achieved at 30 minutes when the reaction was catalyzed by IL2, and therefore, a stronger Lewis acidity of IL2 than IL1 was evidenced, leading to more xylose being selectively isomerized to xylulose. Within the first 30 minutes, the order of xylulose yield from high to low is as follows: IL2 > IL3 > IL4, suggesting that xylulose yield would decrease with relatively high χ(FeCl 3 ) of ionic liquid catalyst. This could be due to the strengthened Lewis acidity, because xylulose might undergo side reactions with furfural when catalyzed by Lewis acidic catalysts. 38 Through comprehensive consideration of both Figures 3  and 4, the low furfural yield catalyzed by IL1 might result from its weak Lewis acidity. Although a little xylose went through isomerization-dehydration reaction pathway with relatively low apparent activation energy than direct dehydration, the dominating route was still the direct dehydration that needs high apparent activation energy. 39 The order of furfural yield catalyzed by IL2, IL3, and IL4 from high to low was the same as the order of xylulose yield, and therefore, xylulose was proven to be a key intermediate during furfural production. Overall, IL2 showed the most satisfying catalytic performance. When [bmim]Cl held a predominant position (IL1), the dehydration of xylose was hard to occur because of high apparent activation energy. Besides, furfural tended to be involved in secondary reactions in the presence of Brønsted acid. 41 If FeCl 3 made up a large proportion in the catalyst, it would be easier for the occurrence of xylose conversion through isomerization-dehydration reaction due to low activation energy barriers. 39 Consequently, for the sake of achieving the best xylose conversion and furfural yield, the content of FeCl 3 and [bmim]Cl in ionic liquid should be synergistically optimized to obtain an appropriate acidity distribution.
In order to further verify the performance of the catalyst, control experiments were carried out in pure water and pure butanone solvent. The results showed that the dehydration reaction could hardly occur in pure butanone. The reason might be that the catalyst could not hydrolyze in butanone to provide necessary acidity. In water, the highest xylose conversion and furfural yield were 35% and 9% at 240 minutes, respectively. Thus, an appropriate amount of butanone had a positive organic solvent effect on xylose dehydration.

| Effects of catalyst composition on liquid-phase product distribution
In order to gain a deep insight into the effect of catalysts with different catalyst composition on the dehydration of furfural, the liquid-phase products were investigated by GC-MS and the product distribution corresponding to each catalyst was shown in Figure 5. The soluble products from xylose conversion were divided into three main categories, namely oxygenated aliphatics, furans, and cyclohexenones. Furfural (1) was a primary compound in the liquid-phase products, and it made up 38%, 94%, 90%, and 74% of the furans corresponding to IL1-IL4, respectively. Other furan products from xylose conversion mainly included 3-(2-furanyl)-3-penten-2one (2), 5-hydroxy-4,5-dimethyl-2,5-dihydrofuran-2-one (3), and 5-methyl-2(5h)-furanone (4), and all the furans account for over 73% of the soluble products (2) could come from the aldol condensation reaction between (1) and pentanone intermediates from retroaldolization of xylose. 42 Furanone (3) and (4) might be the hydration products of hydroxyl-rich acids from the ring opening of xylose. 43 Oxygenated aliphatics mainly consisted of acetic acid (5), 2-ethyl-hexenal (6), 5-methyl-4-hepten-3-one (7), 3,4-dimethyl-3-hexen-2-one (8), and 4-hydroxy-4-methyl-2-pentanone (9), and they comprised 2%-14% of the soluble products. The byproduct (5) primarily resulted from the retroaldolization of xylose in the presence of Brønsted acid. 44,45 C6-C8 aldehydes and ketones (6) (7) (8) (9) were likely to be formed by the aldol condensation reactions between the solvent molecule (butanone) and other small molecules from the retroaldolization of xylose. [46][47][48] However, this reaction could not have profound effect on furfural production. Control experiments were carried out under the same conditions using tetrahydrofuran-water solvent mixture, which is a common solvent configuration that is much less possible for the occurrence of side reactions between furfural and solvent. A low furfural yield of 44% was achieved. Cyclohexenones were mainly made up of 6-methyl-3-(1-methylethyl)-2-cyclohexen-1-one (10), 2hydroxy-3-methyl-6-(1-methylethyl)-2-cyclohexen-1-one (11), and 4-hydroxy-2-methyl-5-(1-hydroxy-1-isopropyl)-2cyclohexen-1-one (12), and they constituted 5%-19% of the soluble products. The origin of cyclohexenones might be the cyclization of the chain intermediates. 49 Through comparison of liquid-phase products between different ionic liquid catalysts, it could be obviously found that IL2 and IL3 promoted the production of furans, while IL1 and IL4 had poor selectivity for furans. Furans accounted for 84%, 91%, 92%, and 73% of the soluble products corresponding to IL1-IL4, respectively, and furfural made up large proportions of the furans. It could be inferred that both Lewis acid and Brønsted acid favored the formation of furans concerning dehydration, ring opening, aldol condensation, hydration, etc IL4 showed the worst furan selectivity among all the four catalysts, which was consistent with the lowest furfural yield at 180 minutes in Figure 3, because strong Lewis acidity from excessive FeCl 3 facilitated the occurrence of side reactions involving furfural consumption at long reaction times. 37,38 When compared with IL3 and IL4 that had larger proportion of FeCl 3 , IL1 and IL2 had higher selectivity for oxygenated aliphatics. Therefore, [bmim]Cl could accelerate the retroaldolization reaction. The selectivity ranking of cyclohexenones corresponding to different catalysts from low to high is as follows: IL1 < IL2 ≈ IL3 < IL4, indicating that large content of FeCl 3 would be beneficial for the cyclization reaction, which agreed with the experimental results from our previous study. 50 According to the above discussion about liquid-phase product, we proposed a reaction network for xylose conversion catalyzed by bifunctional catalyst [bmim]Cl/FeCl 3 with different content of [bmim]Cl and FeCl 3 , as shown in Figure 6. The primary reaction path involved three stages, namely ring opening, isomerization, and dehydration, among which the dehydration reaction was catalyzed by Brønsted acid while ring opening and isomerization reaction were catalyzed by Lewis acid. 51 Xylose could also undergo direct dehydration reaction to lose three molecules of water leading to furfural formation when the reaction was only catalyzed by Brønsted acid, though it has been illustrated that this route F I G U R E 5 Liquid-phase product distribution catalyzed by different ionic liquids (30 mg/mL xylose, 0.1 mol/L catalyst, 140°C, butanone-water (4:1), 180 min) needed high activation energy and would hardly occur in the presence of Lewis acid. 39 Various chain intermediates would be formed during the reaction, and they further went through side reactions like cyclization, retroaldolization, condensation, and hydration to form certain byproducts. 52

| Catalytic performance of different metal chlorides and imidazolium ionic liquids
For the purpose of demonstrating the superiority of the ionic liquid catalysts developed in this work over conventional inorganic salt catalysts, four common Lewis acidic homogeneous catalysts, namely FeCl 3 , AlCl 3 , CuCl 2 , and CrCl 3 , as well as the ionic liquids prepared by mixing these four salts and [bmim]Cl at 1:1 mole ratio, were selected as the catalysts for the catalytic conversion of xylose. The corresponding xylose conversion and furfural yield were described in Figure 7. Apparently, the ionic liquid catalyst [bmim]Cl/ FeCl 3 had even better catalytic performance than FeCl 3 , and it could achieve higher xylose conversion and furfural yield in the same reaction condition.  35 Among all the tested metal chloride catalysts, AlCl 3 showed the best catalytic performance. Nevertheless, the cost of AlCl 3 was much higher than FeCl 3 , and the magnetic characteristic of [bmim]Cl/FeCl 3 enabled its potential in easy recovery and industrial application, which was not possible for [bmim]Cl/AlCl 3 . 30 The catalytic activity ranking for the four metal chlorides from good to bad is as follows: AlCl 3 > CrCl 3 > FeCl 3 > CuCl 2 . Pearson listed the absolute electronegativity of a large number of cations, among which Al 3+ > Fe 3+ ≈ Cr 3+ > Cu 2+ , and generally, high electronegativity represented strong Lewis acidity. 53 Thus, it could be ascertained that metal chlorides with strong Lewis acidity would favor furfural production from xylose dehydration considerably. Once the metal chlorides were mixed with [bmim]Cl to form ionic liquid catalysts, the conversion from xylose to furfural would also be greatly facilitated. including high cost and environmental pollution. The catalytic reaction system proposed in this work is composed of butanone-water mixture as a green solvent system and ionic liquids as catalysts, and therefore, it introduces some excellent aspects such as low ionic liquid dosage and environmentally friendly solvent. From a practical point of view, the reusability of the ionic liquid catalyst would be a challenge because of its special physicochemical characteristics compared with traditional homogeneous and heterogeneous catalysts. For example, ionic liquids usually had low volatility and high viscosity. 58 Researchers have developed some technologies for the recycle of ionic liquid, such as induced phase separation, distillation, adsorption, and extraction. 58,59 Hayashi et al 30 found that [bmim]Cl/FeCl 3 showed a large magnetic susceptibility that offered opportunities for easy recycling, though a large amount of the ionic liquid might be required. However, the large-scale utilization of these technologies was still restricted due to some unresolved disadvantages. 59,60 Besides, a general criterion for ionic liquid recycling is still absent, and therefore, comprehensive study will be carried out in the future to develop an efficient method for evaluating the ionic liquid catalysts used in this work.
We previously employed [bmim]Cl/AlCl 3 as a catalyst in the production of furfural from xylose in γ-valerolactonewater solvent mixture, and 80% yield of furfural was achieved from xylose. 23 When γ-valerolactone was replaced with butanone, the furfural yield slightly decreased to 75%. Therefore, the two solvents had similar performance. However, the market price of γ-valerolactone was much higher than that of butanone. Although butanone may have some toxicity issues, it has been evaluated and listed as a green and recommended solvent. 24,25 Besides, butanone-water mixture is biphasic at room temperature, which enables the low-cost extraction and separation of furfural. Hence, the proposed reaction system had great potential for the production of furfural from biomass carbohydrates.

| CONCLUSIONS
In this research, several ionic liquid catalysts made up of [bmim]Cl and different metal chlorides were developed for the catalytic conversion of xylose in butanone-water solvent mixture to produce furfural. After catalyst optimization, 75% yield of furfural was achieved at 140°C. [Bmim]Cl/FeCl 3 catalysts with content of [bmim]Cl and FeCl 3 were prepared by altering χ(FeCl 3 ), and the effects of catalyst composition on the distribution of liquid-phase products were investigated. The results indicated that FeCl 3 promoted the isomerization reaction from xylose to xylulose, while excessive amount of FeCl 3 or [bmim]Cl would result in the occurrence of side reactions. These reactions could include the cyclization of chain intermediates to form cyclohexenones and the condensation reactions between furfural and intermediates, resulting in decreased furfural yield. Comparison between various catalysts suggested that the ionic liquid catalysts developed in this work gained substantial improvement in the catalytic performance when compared with crude metal chlorides and [bmim]Cl. The main reason was that new types of anions with strong catalytic activities were formed during ionic liquid catalyst preparation. Efficient catalytic conversion of xylose with relatively low apparent activation energy was achieved by employing the catalyst with both Lewis acidity and Brønsted acidity, and the primary reaction pathway followed ring opening, isomerization, and dehydration reactions, leading to high-efficiency production of furfural. Nevertheless, in order to optimize the xylose conversion process, the catalyst composition was supposed to be further adjusted by applying an appropriate mole ratio of metal chloride to [bmim]Cl.