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Tin-modified ionic liquid polymer: A novel and efficient catalyst for synthesis of 5-hydroxymethylfurfural from glucose
Fuel 268 (2020) 117136
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Full Length Article
Tin-modified ionic liquid polymer: A novel and efficient catalyst for synthesis of 5-hydroxymethylfurfural from glucose
T
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Guo Qiua, Biaohua Chenb, Chongpin Huanga, , Ning Liub, Xiuliang Suna a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: 5-Hydroxymethylfurfural Sn modified ionic liquid polymer Glucose Density functional theory calculation Isomerization
Methods for the conversion of carbohydrates into 5-hydroxymethylfurfural (5-HMF), which is an important platform chemical, have been attracted extensive attention. In present study, Sn modified ionic liquid polymer (PIL-Sn) was prepared by one-pot polymerization of imidazole and epichlorohydrin, and utilized as a novel and efficient catalyst for 5-HMF production. The optimal 5-HMF yield of 51.1% with 99% conversion was obtained from glucose catalyzed by PIL-Sn in dimethyl sulfoxide at 130 °C with reaction time of 1 h. This catalytic system can also be applied for the transformation of other carbohydrates with satisfactory yields. The mechanism of PILSn-catalyzed conversion of glucose to 5-HMF has been studied using density functional theory (DFT) calculations. The DFT results showed that the five-coordinated SnCl5- as a preferred species played an important role in the isomerization of glucose to fructose. This research provides a new strategy to design efficient catalysts for 5HMF synthesis.
1. Introduction Since excessive consumption of fossil fuels bring energy shortage
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and environmental pollution, it is a brook no delay task to search for alternative energy sources in order to achieve sustainable development [1,2]. With its abundance and non-pollution, biomass was regarded as
Corresponding author. E-mail address: [email protected] (C. Huang).
https://doi.org/10.1016/j.fuel.2020.117136 Received 6 September 2019; Received in revised form 11 January 2020; Accepted 16 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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temperature leads to more energy consumption, and does not meet the demands of energy conservation. Metal salts (CrCl2, CrCl3, FeCl3, SnCl4, GeCl4) as Lewis acids are effective catalysts for conversion of carbohydrates in various solvents [21,22]. CrCl2 and CrCl3, as the most frequently used and effective catalysts, are employed either alone as a catalyst or with ILs for converting carbohydrates into 5-HMF [23,24]. However, there are still a number of application bottlenecks, such as toxicity, high price and environmental pollution, which impel us to seek non-toxic and low-cost metal salts [20]. As is known to all, Lewis acid SnCl4 is a type of common, cheap, easy-handling, high-activity and low-toxicity catalyst in biomass conversion [25]. In this study, SnCl4 was introduced into polymerizate of imidazole and epichlorohydrin, a kind of functional ionic liquid polymer. Polymerized ionic liquids (PILs) have many potential applications with exceptional properties, such as high thermal stability, electrochemical activity and high ionic conductivity [26–28]. Sn modified ionic liquid polymer (PIL-Sn) with Brønsted and Lewis acidic sites was prepared by polymerization and following by altering the anions of PILs chain. The performance of PIL-Sn was excellent for glucose conversion to 5-HMF owing to its high catalytic activity, lower toxicity and lower reaction temperature. In addition, density functional theory (DFT) calculations were performed to verify PIL-Sn-catalyzed reaction mechanism for the isomerization of glucose to fructose, and the five-coordinated SnCl5- as a preferred species played an important role in reaction. These findings aim to offer a new idea to further design an efficient catalytic system for the conversion of renewable carbohydrates to 5-HMF.
one of the most promising sustainable energy sources in the future [3,4]. The transformation of renewable biomass to produce energy and chemicals has already become an intense topic in the studies [5]. 5Hydroxymethylfurfural (5-HMF), a top 10 potential, versatile and biomass-based platform chemical, has received close attention in recent years. It can be used to produce a wide spectrum of high value-added and non-petroleum chemicals, such as 2,5-furandicarboxylic acid, 5hydroxymethylfuroic acid, 2,5-dimethylfuran, 5-ethoxymethylfurfural and levulinic acid, etc [6,7]. This is because 5-HMF can be exploited as a precursor or a building block that plays a crucial role in industrial chemical manufactures. Despite the high potential and demand, the current production of 5-HMF at the industrial scale is marginal due to poor performance and low production volume [8,9]. Most of 5-HMF are sold as a specialty chemical for laboratory and research purposes. 5HMF derived from biomass source with high atom economy, and the current supply of 5-HMF is unsustainable in industry. Hence, developing a 5-HMF production system with high efficiency and sustainability will be an important milestone. At present, there were some reports for 5-HMF synthesis using various carbohydrates as the substrate such as fructose, glucose, sucrose, cellulose, lignin, inulin, etc. On the one hand, when lignocellulosic biomass were used as feedstock such as 27–49 wt% cellulose, 14–37 wt% hemicellulose and 16–33 wt% lignin, 5-HMF yield ranging from 2 to 60% were obtained in the conversion systems, accompanied by the complex pretreatment [10]. On the other hand, the facile synthetic method of 5-HMF is through three-step dehydration of hexose with an acid catalyst. Higher yield of 5-HMF is obtained with fructose as raw material, but fructose is not considered as an ideal feedstock due to its high cost and short supply in nature. On the contrary, glucose is a kind of abundant and low-cost carbohydrates making it a potential candidate for 5-HMF synthesis [11,12]. In general, glucose conversion and 5-HMF yield and selectivity are deeply affected by some reaction parameters such as the type of catalyst, reactant ratios, solvent, reaction time and temperature [13]. The prudent consideration of the reaction factors is required in order to direct the conversion to the favorable pathway and avoid these unwanted reactions [14]. For example, a higher temperature makes the reaction very violent that causes the presence of side reactions, while a lower temperature lead to an inefficient conversion. In addition, when water is used as solvent, it can result in the formation of hydration products levulinic acid and formic acid. The specific properties of the selected catalyst, such as acidic property and chemical stability, are critical in achieving the safe and efficient 5-HMF production [15]. Using glucose as feedstock, the conversion to 5-HMF involves two continuous steps: (1) isomerization of glucose into fructose; (2) tripled dehydration of the latter into 5-HMF. As a result of the difficulty in destroying stable structure of glucose, highly active catalysts are required to promote this two-step conversion. Thus, many researches have been recently focused on the development of promising catalysts to further exploit the efficient catalytic system for the production of 5HMF (Table S1) [16]. Among these catalysts, ionic liquids (ILs) are a kind of room temperature molten salts with advantages of negligible vapor pressure, non-volatility, non-flammability, good thermal stability and environmental friendliness [17]. Tong et al. found that N-methyl-2pyrrolidonium methylsulfonate ([NMP][CH3SO3]) could be an efficient catalyst with 72.3% yield of 5-HMF in dehydration of fructose at 90 °C for 2 h. Nevertheless, only 3.0% yield of 5-HMF was gained using glucose as a substrate [18,19]. 1-Sulfonic acid-3-methyl imidazolium tetrachloroferrate ([SMIM][FeCl4]) with both Brønsted and Lewis acidity has been utilized for converting glucose to 5-HMF with 18% yield after 4 h at 150 °C [20]. Qu et al. reported that 1-hydroxyethyl-3methylimidazolium tetrafluoroborate ([C2OHMIM]BF4) was used to catalyze glucose to 5-HMF with 67.3% yield at 180 °C for 1 h in dimethylsulfoxide (DMSO) [17]. However, due to its high cost, inefficient separation and recycling difficulty, the use of ILs is still restricted from large-scale applications. Moreover, the requirement of higher reaction
2. Materials and methods 2.1. Materials Glucose (99%), fructose (99%), sucrose (99%), cellobiose (99%), inulin (99%), cellulose (99%) and starch (99%) were used as substrates with analytical grade and obtained from Aladdin Chemical Co., Ltd (Shanghai, China). 5-HMF (98%, analytical grade), dimethylsulfoxide (DMSO, 99.5%), methylisobutylketone (MIBK, ≥99%), N-methyl pyrrolidone (NMP, 99%), dimethylformamide (DMF, 99.5%) and dimethylacetamide (DMA, 99.5%) were purchased from Beijing Chemical Reagent Company (Beijing, China). Imidazole (99%), epichlorohydrin (≥99.5%), methanol (99.5%) and ethanol (99%) were provided by J & K Scientific Ltd. (Beijing, China). All reagents were utilized as received without further purification. 2.2. Catalysts preparation Polyionic liquid precursor (PIL-pre) was prepared through a route which was analogous to previous reports [29]. In a typical preparation, 300 mL of ethanol was added in a three-necked round bottom flask and heated to a temperature at 30 °C. Then, 1 mol of imidazole was added under stirring vigorously. After imidazole dissolved in ethanol, 1 mol of epichlorohydrin was added dropwise to the stirred solution, and the resulting mixture was then stirred for 2 h. When the temperature reached up to 45 °C stepwise, the mixture was stirred for 12 h. After the mixture continued to be heated up stepwise to 85 °C and refluxed for 72 h. The resultant white precipitate was obtained after vacuum filtration, ethanol washing and drying. Finally, the white precipitate was ground into powder, labeled as PIL-pre. Various metal ion-containing PIL-based catalysts were obtained using an ion-exchange method. In a typical synthesis, 50 mL of ethanol containing 0.05 mol of PIL-pre was added to a conical flask, and ethanol solution containing 0.05 mol of of SnCl4 or FeCl3 was added dropwise to the stirred solution. Then, the mixture was stirred vigorously for 12 h at 40 °C. The precipitate was gained by centrifugation, at least three ethanol wash prior to drying at 80 °C overnight. The solids obtained were identified as PIL-Sn and PIL-Fe. 2
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2.3. Catalysts characterization
2.5. DFT calculations
Fourier transform infrared (FTIR) spectra of PIL samples were obtained from 400 to 4000 cm−1 (wavenumber) on a Bruker TENSOR 27 spectrometer using KBr pellet samples. The KBr pallets were arranged by grinding KBr (spectroscopic grade) with 0.1 wt% of PIL sample. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra were recorded on a Bruker AC300 spectrometer equipped with a 4 mm CP MAS probe and Standard cavity. X-ray photoelectron spectroscopy (XPS) data were recorded on ThermoFisher Scientific ESCALAB-250 instrument using Al Kα (hν = 1486.6 eV) with approximately 3 × 10−9 mbar base pressure. The PIL sample was outgassed at room temperature before the measurement. Meanwhile, the binding energy was calibrated with respect to C 1 s peak at 284.6 eV from carbon deposit. Scanning electron microscopy (SEM) micrographs of PIL catalysts were characterized using a FEI NOVA NanoSEM430 scanning microscope equipped with energy-dispersive X-ray spectrometer (EDX). The nominal resolution was 1.0 nm at 15 kV. The acidic sites type of PILs were determined by pyridine adsorption IR spectra, which were conducted using a Bruker TENSOR 27 spectrometer. The powder PILs catalysts (50 mg) were pressed into a thin sheet (13 mm diameter), and put into the sample cell. Then it was preheated at 200 °C for 2 h. After cooling the cell to room temperature (RT), FTIR spectrum was recorded as background. After that, pyridine vapor was brought in the sample cell at RT for 30 min, and the spectrum was recorded again. Subsequent evacuations were implemented at different temperatures, for 10 min followed by spectra collection. The spectra were received by deducting the spectra collected before and after the adsorption of pyridine.
Density functional theory (DFT) calculations were used for further verification of the catalytic mechanism of glucose conversion to 5-HMF on PIL-Sn. Three reasonable models of PIL-Sn monomer were built in Materials Studio 7.0, from which an appropriate cluster was selected through the optimal computation. Through the use of the LYP correlation function (B3LYP), the hybrid density functional methods were applied to associate with DFT algorithm in Gaussian 09 [30]. Specifically, the basis set of 6-31G(d) was used for calculation of C, H, O and N atoms, involving geometry optimization (GO), transition state (TS), and intrinsic reaction coordinate (IRC), when 6-311++G(d, p) as higher level basis set was used to calculate the singlet point energy [31]. For Sn atom, the basis set of LANL2DZ was utilized to calculate GO and TS. Moreover, by frequency calculations, there are zero imaginary frequencies in all GO clusters. All the TS models (one imaginary frequency) were verified via IRC path analyses. 3. Results and discussion 3.1. Characterization of PIL catalyst Fig. 1 shows the FTIR spectra of PIL. Obviously, a band at 3419 cm−1 was attributed to OeH stretching vibrations, indicating ring-opening reaction occurred to epichlorohydrin with imidazole [32,33]. A band at 1635 cm−1 was correspond to C]C stretching vibration in the imidazole ring, while the sharp band at 1560 cm−1 can be related to C]N stretching vibration [34,35]. The band at approximately 1446 cm−1 was associated with the deformation vibration of eCH2 on side-chain [36]. The sharp bands at 1170 cm−1 and 1095 cm−1 were assigned to CeO and CeN stretching vibration, respectively [37]. The determination of these groups indicates that the structure of the polyionic liquid is in accordance with the PIL design. The characteristic peaks of the modified PIL-Sn and PIL-Fe functional groups remained unchanged, indicating that the incorporation of metal ions did not destroy the skeletal structure of polyionic liquid. The 1 H/13C NMR spectra (Figs. S1 and S2) of PIL are as follow. PIL-pre. 1H NMR (D2O, 600 MHz) δ: 7.59 (s, 2H), 4.55 (d, J = 14.4 Hz, 2H), 4.38 (s, 1H), 4.26 (d, J = 12.2 Hz, 2H). 13C NMR (D2O, 150 MHz) δ: 122.49, 67.42, 51.42. PIL-Sn. 1H NMR (D2O, 600 MHz) δ: 8.71 (s, 0H), 7.37 (s, 2H), 4.34 (d, J = 14.2 Hz, 2H), 4.18 (s, 1H), 4.05 (d, J = 11.9 Hz, 2H). 13 C NMR (D2O, 150 MHz) δ: 123.12, 68.00, 52.17. The high resolution XPS spectra of PIL materials (Fig. 2) were measured to investigate the chemical bonding states of C, N and O. The C 1s spectrum (Fig. 2a) involves two characteristic peaks centered at 284.6 eV and 286.5 eV, which are attributable to CeC and C-O respectively [38]. Similarly, a peak located at 401.8 eV was observed in
2.4. General procedure for 5-HMF synthesis Carbohydrate was converted into 5-HMF in a 15 mL sealed tube equipped with certain amounts of carbohydrate, catalyst, and solvent. The temperature of solution was monitored by a temperature sensor probe. When the temperature reaches the set value, the sealed tube was placed in an oil bath. The mixture was stirred using an internal stirrer at a maximum constant rate of 500 rpm. When the reaction was finished, the tube was immediately cooled in an ice water bath. Afterwards, the mixture was filtered through a one-off syringe-driven filter involving a high-quality microporous filter membrane, and then 0.1 mL of solvent filtrate was decanted into a 25 mL volumetric flask using water or methanol as a diluent prior to analysis. For DMSO, NMP, DMA, DMF, sec-butanol and water as the solvent, the diluent was water. For MIBK as the solvent, methanol was used as a diluent. High-performance liquid chromatography (HPLC) equipped with a C18 column and a UV detector was employed to analyse the product, using a mixture of deionized water/methanol (60/40 v/v) as the mobile phase with a flow rate of 1 mL/min. The detection wavelength of 5HMF was 284 nm. HPLC equipped with a Hypersil APS-2 column and a Shodex IR detector was utilized to analyse the residual substrate. The temperature of Hypersil APS-2 column was set at 35 °C, and the mobile phase was deionized water with a flow rate of 0.2 mL/min. The glucose conversion (%), the 5-HMF yield (%), and the 5-HMF selectivity (%) were measured on a carbon basis according to the following equations.
Moles of glucose ⎞ Glucose conversion (mol%) = ⎜⎛1 − ⎟ × 100% Starting amount of glucose ⎠ ⎝
5 − HMF yield (mol%) =
Moles of 5 − HMF × 100% Starting amount of glucose
5 − HMF selectivity (mol%) =
Yield of 5 − HMF × 100% Glucose coversion
Fig. 1. FTIR spectra of PIL. 3
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Fig. 2. XPS spectra of C 1s (a), N 1s (b), O 1s (c) and Sn 3d (d) of PIL samples.
the N 1s spectrum (Fig. 2b); this is in line with the sp2-hybridized aromatic nitrogen (C]NeC) [39]. Fig. 2c showed the O1s spectra of PIL, where a peak centered at about 532.2 eV, arising from C-O bond in PIL monomer [38]. In addition, no evident change was found for the chemical bonding states and valence of C, N and O in the XPS spectra of Sn modified PIL, while the intensity of characteristic peak declined after the modification; this indicates that Sn modification has not destroyed the original skeleton structure of PIL-pre, and there are some interaction between metal cluster and PIL chain. The Sn 3d spectra of PIL-Sn were fitted with two peaks corresponding to Sn 3d5/2 and Sn 3d3/2 centered at around 487.8 eV and 496.2 eV, respectively; this is feature of tetravalent tin [40]. There are some aggregation structure of spherical particles in the SEM images of PIL (Fig. S3). The original aggregation state of PIL-pre was not destroyed in PIL-Sn, demonstrating that the modification process of tin made no difference in polymer structure. Surprisingly, some porous framework structure were observed in the SEM images of PIL-Sn (Fig. S4), indicating the formation of polymeric chain could generate a certain amount of pore structure. In addition, pyridine adsorption IR spectra were showed in Fig. 3, which determined the type of acid site on PIL catalysts. No characteristic band was observed in the pyridine-IR spectrum of PIL-pre, indicating neither Brønsted nor Lewis acid sites existed on the surface of PIL-pre. After tin or iron modification, some characteristic pyridine adsorbed bands at 1451, 1489 and 1540 cm−1 were appeared in the spectra of PIL-Sn and PIL-Fe. The bands at 1451 cm−1 correspond to Lewis acid sites, resulting from pyridine molecules coordinated with tin or iron ions [41]. The band centered at
Fig. 3. Pyridine-IR spectra of PIL.
about 1540 cm−1 assigned to Brønsted acid sites was weak; this were consistent with pyridine protonated by Brønsted acids [42]. Furthermore, there was a pyridine adsorbed band at 1489 cm−1 arising from the interaction of pyridine molecules with both Brønsted and Lewis acid sites [43]. Compared PIL-Sn samples, these corresponding bands intensity displayed a decrease in PIL-Fe, possibly owing to stronger Brønsted and Lewis acidity of PIL-Sn than PIL-Fe. To sum up, the 4
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yield firstly increased to the maximum value and afterwards reduced with the extension of reaction time, which may be due to the formation of black insoluble humins. However, there was a different trend in glucose conversion, which appeared a rapid increase firstly and then tend to be levelled off. Besides, the 5-HMF yield was comparatively low with less than 80% conversion of glucose at 110 °C. With the further increase of reaction temperature to 130 °C, the catalytic activity of PILSn enhanced significantly and an optimal 5-HMF yield as high as 51.1% with 99% glucose conversion was acquired for 1 h. At high temperatures, a vigorous increase of reaction rate was observed at its initial stage. On account of highly consuming of energy and hard control under higher temperatures, 130 °C was used as the optimal temperature hereafter. In general, various initial concentration of substrate could deeply affect glucose conversion, hence the effect of initial glucose concentration was further investigated (Table 1, entries 3, 6, 7 and 8). Remarkably, greater than 96% conversions of glucose were obtained at diverse initial concentrations, and a highest yield of 5-HMF was achieved under a relatively low initial glucose concentration, due to acceleration in a shift of chemical equilibrium under lower substrate concentration. A partial decrease in 5-HMF yield at high initial concentration was detected as a result of the generation of by-products from side reactions. In order to further study the effect of the amount of PIL-Sn catalyst, different amount of PIL-Sn was used to catalyze glucose conversion (Table 1, entries 3, 9 and 10). Apparently, 5-HMF yield and glucose conversion increased when increasing the amount of PIL-Sn. When PILSn dose decreased to 6.5 mg, the yield of 5-HMF and the conversion of glucose were greater than 18% and 90%, respectively, demonstrating that PIL-Sn was an effective catalyst in 5-HMF synthesis. Therefore, it was superior for higher 5-HMF yields when the amount of PIL-Sn was 25 mg.
Table 1 Overview of glucose conversion and 5-HMF yield catalyzed by PIL catalysts.a Entry
1 2 3 4 5 6b 7c 8d 9e 10f
Catalyst
No catalyst PIL-pre PIL-Sn PIL-Fe SnCl4 PIL-Sn PIL-Sn PIL-Sn PIL-Sn PIL-Sn
Experimental parameter Conversion/%
Yield/%
27.8 30.6 99.0 80.4 90.2 98.7 98.7 96.0 90.5 95.4
0 0.63 51.1 35.8 16.2 45.8 43.1 42.2 18.7 30.6
a Reaction conditions: 55.6 mM glucose, 25 mg catalyst, 130 °C, 5 mL DMSO, 1 h. b, c, dThe initial concentration of glucose was 83.3 mM, 111 mM and 166 mM, respectively. e, fThe initial mass of catalyst was 6.5 mg and 12.5 mg, respectively.
introduction of Sn or Fe ions facilitate the coexistence of Brønsted and Lewis acid sites on the surface of catalysts. 3.2. Catalytic effect of PIL catalysts on glucose conversion to 5-HMF The catalytic efficiencies of PIL catalysts on 5-HMF synthesis from glucose were probed at 130 °C for 1 h (Table 1). When no catalyst used in the system, there were zero 5-HMF yield and some black humins formed with less than 30% glucose conversion (Table 1, entry 1), suggesting that it was hard for 5-HMF synthesis without catalyst. Only traces amount of 5-HMF was formed using PIL-pre as a catalyst (Table 1, entry 2), demonstrating that PIL-pre did not work in glucose transformed to 5-HMF, which could be attributed to the absence of Brønsted and Lewis acid sites. It should be noted that practically full conversion of glucose and 51.1% yield of 5-HMF were achieved when employing PIL-Sn as the catalyst (Table 1, entry 3), which possessed high chemical catalytic activity in 5-HMF synthesis from glucose. The presence of Brønsted and Lewis acidic sites was responsible for high efficiency catalysis of PIL-Sn due to the incorporation of Sn species. The formations of a small amount of levulinic acid (LA) and formic acid (FA) were captured during experimental runs (Fig S5), indicating the effective suppression of hydration reaction. Nevertheless, 35.8% yields of 5HMF and 80.4% conversion of glucose could be provided (Table 1, entry 4) with PIL-Fe catalyst, indicating PIL-Fe had a lower number of catalytic active sites and relatively weaker acidity than PIL-Sn; these were in accordance with the results obtained from Pyridine-IR spectra. Notably, above 90% glucose conversion and less than 20% 5-HMF yield were provided when catalyzed by homogeneous SnCl4 (Table 1, entry 5), implying that homogeneous Sn4+ could promote glucose transformation and 5-HMF synthesis in the reaction, which were consistent with the results reported by Enslow et al [44]. The results of the spacetime yield of 5-HMF with different catalysts were summarized in Table S2. Relatively speaking, PIL-Sn had preferable catalytic performance compared with other catalysts, because of its more Brønsted and Lewis acidic sites. This further verified the catalytic efficiencies of PIL-Sn catalyst.
3.4. Catalytic activity of PIL-Sn in various solvents for 5-HMF synthesis Further investigation of the effect of solvents in 5-HMF synthesis was conducted to verify the significance of suitable solvent. Obviously, there was a great difference in glucose conversion and 5-HMF yield when diverse solvents were used in the reaction, confirming a crucial solvent effect (Fig. 5). The catalytic activity of PIL-Sn in DMSO for 5HMF synthesis was excellent, because 5-HMF rehydration to levulinic acid (LA) and formic acid (FA) was suppressed in some extent through coordinating DMSO molecules with 5-HMF molecules [45]. And higher yield of 5-HMF and conversion of glucose were obtained. In pure water, the conversion of glucose was above 50%, but 5-HMF yield were relatively lower, owing to the presence of side reaction in H2O. On account that polar aprotic solvent could act as an excellent medium for 5-HMF formation, the catalytic activity of PIL-Sn in NMP, DMA and DMF was superior to that in water. Remarkably, the yield of 5-HMF was comparable in sec-butanol, which could make it easily solubilize carbohydrates and effectively accelerate the process of dehydration reaction. Although a biphasic solvent MIBK/water could separate substrates and products efficiently to promote the shift of chemical equilibrium, a moderate yield of 5-HMF was gained, indicating PIL-Sn exhibited poor catalytic activity in MIBK. Taking into account these results, DMSO could be acted as a preferable solvent for attaining higher 5-HMF yield. 3.5. Comparison of catalytic conversion of various carbohydrates by PIL-Sn
3.3. Effect of process conditions for PIL-Sn-catalyzed conversion of glucose to 5-HMF
The performance of PIL-Sn for various carbohydrates during the synthesis of 5-HMF was further investigated when PIL-Sn catalyst worked well on glucose conversion (Table 2). Briefly, 5-HMF yield from fructose conversion was higher than 50%, indicating PIL-Sn catalyst with Brønsted acid sites has played a crucial role in the three-step dehydration of fructose (Table 2, entry 1). In addition, 87% yield of 5HMF was obtained from fructose when utilizing PIL-Fe as a catalyst,
Comparatively, the catalytic activity of PIL-Sn was superior to that of other PIL catalyst, owing to stronger Brønsted and Lewis acidity required for glucose conversion. Therefore, PIL-Sn was utilized for further exploring the catalytic conversion of glucose to 5-HMF. The reaction temperature deeply affected the 5-HMF yield and glucose conversion (Fig. 4a and b). At different temperatures from 110 °C to 150 °C, 5-HMF 5
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Fig. 4. Effect of reaction temperature on (a) conversion of glucose and (b) yield of 5-HMF. (Conditions: 55.6 mM glucose, 25 mg PIL-Sn, 5 mL DMSO.)
results, but also has satisfactory results for other carbohydrates. 3.6. DFT calculations for the catalytic conversion of glucose to 5-HMF When tin tetrachloride runs into the imidazole chain, it may form various tin species with different coordination with chloride ions based on the previous literature [46–48]. There are three different coordination modes for the ligand in Sn-containing compounds: tetracoordinated, pentacoordinated and six-coordinated center [49,50]. To investigate the possibility of the formation of various tin chloride complexes of PIL-Sn, DFT calculations were used to optimize hypothetical monomer model. The initial SnCl4 monomer has a regular tetrahedron configuration with a CleSneCl bond angle of 109.47° and SneCl distances of 2.31 Å (Fig. 6a and Table 3). The optimized structure of PIL-pre monomer is found that chloride ion and imidazole ring are bound together by hydrogen bonds, and the HeCl hydrogen bond length is 2.06 Å (Fig. 6b). The reciprocity of SnCl4 with a PIL-pre monomer is advantageous (ΔG = −8.59 kcal mol−1, in Table 3, entry 3) and results in forming a distorted trigonal bipyramidal SnCl5- configuration with five-coordination to a central tin atom (Fig. 6c). The repulsive force between the electropositive Sn4+ center and the chargecompensating imidazole cation gives rise to deformation of the equilateral trigonal bipyramidal configuration, so that the Cl ligands deviate from Sn4+ center to the imidazole ring in different extent. There are hydrogen bonds between SnCl5- species and imidazole cation with the average HeCl distance of 2.39 Å (Table 3, entry 3). In these optimize structure models, there were no change in the bond length of OeH before and after the incorporation of the Sn metal in the PIL-pre (Table 3). And the Cl ligand was far away from the H atom in the OH group owing to the attraction of the positive imidazole ring, indicating relatively weak interaction between Cl ligand and the H atom in the OH group. The six-coordinated SnCl62- anionic species employs an octahedral structure with an average SneCl distance of 2.50 Å (Table 3, entry 4). The interaction of SnCl62- anionic species with two PIL-pre monomer leads to the angular deflection of octahedral geometry (Fig. 6d). The binuclear six-coordinated Sn2Cl102− complex is predicted to connect two Sn center via two Cl− bridges, but it becomes two mononuclear five-coordinated SnCl5- complexes after optimization, indicating the binuclear Sn2Cl102− complex was unstable (Fig. 6e). The average SneCl bond length in optimized Sn2Cl102− structure is close to that found for mononuclear SnCl5− species. On the other hand, the SnCl62− and Sn2Cl102− complexes are thermodynamically unfavorable, because the free energy change values are positive, and it is not prone to form the complexes (Table 3). Obviously, the SnCl5− species is the preferred species with lower Gibbs free energy, more applicable average SneCl distance and stronger hydrogen bonding. Electrostatic potential calculation in SnCl5−-PIL shows that SnCl5−
Fig. 5. The effect of various solvents in glucose conversion catalyzed by PIL-Sn. (Conditions: 55.6 mM glucose, 25 mg PIL-Sn, 5 mL solvent, 130 °C, 1 h.) Table 2 The performance of PIL-Sn for converting various substrates to 5-HMF. Entry
Substrate
Conv. (%)
Yield (%)
1 2 3 4 5 6 7 8
Fructose Glucose Sucrose Maltose Cellobiose Inulin Cellulose Starch
99.8 99.0 94.6 74.6 72.1 90.6 10.3 52.9
53.9 51.1 35.6 26.2 24.2 25.7 1.67 10.0
(Conditions: 55.6 mM carbohydrate, 25 mg PIL-Sn, 130 °C, 5 mL DMSO, 1 h).
signifying that PIL-Fe was more preferable in fructose conversion than PIL-Sn. When using di-saccharides (sucrose, maltose and cellobiose) as substrates, 5-HMF yields and di-saccharides conversions were above 20% and 70%, respectively, (Table 2, entries 3–5) demonstrating PIL-Sn catalyst could promote hydrolysis of di-saccharides. With regard to polysaccharides, the optimum 5-HMF yields from inulin and starch were much higher than that from cellulose (Table 2, entries 6–8). Remarkably, 5-HMF yield reached as high as 10.0% at 130 °C after 1 h with starch as substrate, which was a kind of low-cost, plentiful and renewable resources. These results illustrate that hydrolysis and dehydration of carbohydrates were promoted by Brønsted and Lewis acid sites of PIL-Sn. Hence, PIL-Sn not only can catalyze glucose to get better 6
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Fig. 6. Optimized structures of (a) SnCl4, (b) PIL-pre monomer, (c) SnCl5−-PIL, (d) SnCl62−-PIL, (e) Sn2Cl102−-PIL monomer model; Electrostatic potential calculation in (f) PIL-Sn and (g) glucose. (Distances are in Å).
TS2 → M3). The subsequent intramolecular H* shift from C2 to C1 (M4 → TS3 → M5) brings about the aldose-ketose isomerization, involving the formation of an enediolate intermediate [48]. This is a critical process with an activation barrier of 28 kcal/mol, which is lower than reported in the literature [51]. The M5 → M6 is a conformation transition process from a bidentate coordinated complex to a monodentate complex, and both complexes involve O1-deprotonated acyclic fructose moiety coordinating with mononuclear Sn active site. Subsequently, the ring-closure reaction takes place to form O1-deprotonated cyclic fructose (M6 → TS4 → M7). A proton transfer from Clligand to terminal O1 facilitates the formation of D-fructopyranose and the recovery of SnCl5--PIL original structure (TS5 → M8 → F). From an experimental viewpoint, fructose as an isomerization product was detected at the beginning of the reaction (Fig S7), demonstrating the catalytic pathway for glucose isomerization to fructose in Scheme 1 was feasible and valid by DFT calculation [52]. On the one hand, the formation of complexes with quintuple chelate structure in the H-shift process was confirmed based on earlier findings [25]. By adding a small amount of ethanol, ethylene glycol and 1, 3-propanediol, respectively, into the reaction mixture, it was found that ethylene glycol substantially suppressed glucose conversion but ethanol and 1, 3-propanediol had no impact (Fig. 8). This illustrates there is the presence of quintuple chelate structure rather than hexatomic chelate or acyclic structure in the isomerization pathway. On the other hand, the presence of water can promote the ring-opening and ring-closure of hexose with a reduction in activation barrier (Fig. 7b). Water molecule serves as a proton donor and acceptor, mediating the hydrogen-transfer between the oxygen-containing groups of hexose [51]. The addition of water leads the increase of reaction rate and shortening of reaction time required to achieve the maximum yield [53], but the value of maximum yield remains unchanged; this suggests that adding H2O molecule can greatly reduce activation barrier, but do not give rise to the shift of chemical equilibrium (Fig S8). These experimental results correspond very well with DFT calculations.
Table 3 Gibbs free energies (ΔG, in kcal mol−1) and the respective average SneCl, SneSn and HeCl distances (r, in Å) of the possible formation of tin chloride complexes in PIL-Sn. Entry
Cluster
ΔG
r(Sn-Cl)
r(Sn-Sn)
r(H-Cl)
r(O-H)
1 2 3 4 5
SnCl4 PIL-pre monomer SnCl5−-PIL SnCl62−-PIL Sn2Cl102−-PIL
– – −8.59 4.41 11.52
2.31 – 2.42 2.50 2.42
– – – – 5.08
– 2.06 2.39 2.60 2.57
– 0.97 0.97 0.97 0.97
species acting as Lewis acid site on the right side displays a well defined negative region, while the imidazole ring on the left side is positive charged, and the whole imidazole chain is used to stabilize the PIL-Sn structure (Fig. 6f). In the individual electrostatic potential energy diagram of PIL-pre and SnCl4 (Fig. S6), it can be seen that PIL-pre is composed of the chlorine with strongly negative charges and the imidazole ring with positive charges, while the initial SnCl4 is electroneutral. The incorporation of Sn in the PIL-pre lead to the diffusion of the positive and negative charges. Larger negative potential diffuse quickly from Cl- to Sn4+ center, and the positive potential is transferred to the branch chain of PIL-pre. Hence, the whole SnCl5--PIL is in electrostatic potential equilibrium. In the electrostatic potential energy diagram of glucose (Fig. 6g), it can be seen that the hydrogen proton in the O1H group is positively charged and vulnerable to attack, leading to the preferential coordination of O1 with Sn center and the ring-opening of glucose. Scheme 1 shows the detailed isomerization mechanism of glucose to fructose catalyzed by SnCl5--PIL. The D-pyranoglucose (DGP) and D-fructopyranose (D-Fru) are designated as the starting (S) and final (F) state for the catalytic pathway. A transfer of hydrogen proton from O1 to O5 (R → TS1 → M1) is catalyzed by coordination with a single Sn center, leading to the opening of D-GP ring and the formation of aldehyde O1 moiety, which is endothermic by 30 kcal/mol (Fig. 7a). The O2-coordination to Lewis acidic site Sn facilitates a proton transfer from O2 to a basic Cl- ligand, resulting in deprotonation of O2H (M2 → 7
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Scheme 1. Catalytic pathway for glucose isomerization to fructose by SnCl5−-PIL.
Fig. 7. DFT-computed free energy diagram for the isomerization of glucose involving SnCl5−-PIL. (a) Reaction over SnCl5−-PIL monomer model, (b) The effect of H2O on glucopyranose ring-opening and fructofuranose ring-closure step. Fig. 8. Effect of various additives on conversion of glucose and yield of 5-HMF.
4. Conclusions CRediT authorship contribution statement An efficient conversion of glucose into 5-HMF has been achieved with PIL-Sn as a catalyst, and an optimal 5-HMF yield of 51.1% could be obtained at 130 °C after 1 h. The excellent catalytic activity of PIL-Sn was also exhibited when converting other carbohydrates. A possible reaction mechanism was confirmed by DFT calculations, and five-coordinated SnCl5- was a dominant species that catalyzed glucose isomerization with lower activation energy. The addition of water in DMSO could lead to a partial reduction of energy barriers. These findings provide a feasibility of ionic liquid polymers as effective catalysts for biomass conversion into 5-HMF.
Guo Qiu: Conceptualization, Investigation, Writing - original draft. Biaohua Chen: Supervision, Resources. Chongpin Huang: Conceptualization, Methodology, Resources. Ning Liu: Data curation, Formal analysis. Xiuliang Sun: Formal analysis. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 8
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influence the work reported in this paper. [25]
Acknowledgements
[26]
This work was supported by the National Natural Science Foundation of China under grant number 21476021.
[27]
Appendix A. Supplementary data
[28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117136.
[29]
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Full Length Article
Tin-modified ionic liquid polymer: A novel and efficient catalyst for synthesis of 5-hydroxymethylfurfural from glucose
T
⁎
Guo Qiua, Biaohua Chenb, Chongpin Huanga, , Ning Liub, Xiuliang Suna a b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: 5-Hydroxymethylfurfural Sn modified ionic liquid polymer Glucose Density functional theory calculation Isomerization
Methods for the conversion of carbohydrates into 5-hydroxymethylfurfural (5-HMF), which is an important platform chemical, have been attracted extensive attention. In present study, Sn modified ionic liquid polymer (PIL-Sn) was prepared by one-pot polymerization of imidazole and epichlorohydrin, and utilized as a novel and efficient catalyst for 5-HMF production. The optimal 5-HMF yield of 51.1% with 99% conversion was obtained from glucose catalyzed by PIL-Sn in dimethyl sulfoxide at 130 °C with reaction time of 1 h. This catalytic system can also be applied for the transformation of other carbohydrates with satisfactory yields. The mechanism of PILSn-catalyzed conversion of glucose to 5-HMF has been studied using density functional theory (DFT) calculations. The DFT results showed that the five-coordinated SnCl5- as a preferred species played an important role in the isomerization of glucose to fructose. This research provides a new strategy to design efficient catalysts for 5HMF synthesis.
1. Introduction Since excessive consumption of fossil fuels bring energy shortage
⁎
and environmental pollution, it is a brook no delay task to search for alternative energy sources in order to achieve sustainable development [1,2]. With its abundance and non-pollution, biomass was regarded as
Corresponding author. E-mail address: [email protected] (C. Huang).
https://doi.org/10.1016/j.fuel.2020.117136 Received 6 September 2019; Received in revised form 11 January 2020; Accepted 16 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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temperature leads to more energy consumption, and does not meet the demands of energy conservation. Metal salts (CrCl2, CrCl3, FeCl3, SnCl4, GeCl4) as Lewis acids are effective catalysts for conversion of carbohydrates in various solvents [21,22]. CrCl2 and CrCl3, as the most frequently used and effective catalysts, are employed either alone as a catalyst or with ILs for converting carbohydrates into 5-HMF [23,24]. However, there are still a number of application bottlenecks, such as toxicity, high price and environmental pollution, which impel us to seek non-toxic and low-cost metal salts [20]. As is known to all, Lewis acid SnCl4 is a type of common, cheap, easy-handling, high-activity and low-toxicity catalyst in biomass conversion [25]. In this study, SnCl4 was introduced into polymerizate of imidazole and epichlorohydrin, a kind of functional ionic liquid polymer. Polymerized ionic liquids (PILs) have many potential applications with exceptional properties, such as high thermal stability, electrochemical activity and high ionic conductivity [26–28]. Sn modified ionic liquid polymer (PIL-Sn) with Brønsted and Lewis acidic sites was prepared by polymerization and following by altering the anions of PILs chain. The performance of PIL-Sn was excellent for glucose conversion to 5-HMF owing to its high catalytic activity, lower toxicity and lower reaction temperature. In addition, density functional theory (DFT) calculations were performed to verify PIL-Sn-catalyzed reaction mechanism for the isomerization of glucose to fructose, and the five-coordinated SnCl5- as a preferred species played an important role in reaction. These findings aim to offer a new idea to further design an efficient catalytic system for the conversion of renewable carbohydrates to 5-HMF.
one of the most promising sustainable energy sources in the future [3,4]. The transformation of renewable biomass to produce energy and chemicals has already become an intense topic in the studies [5]. 5Hydroxymethylfurfural (5-HMF), a top 10 potential, versatile and biomass-based platform chemical, has received close attention in recent years. It can be used to produce a wide spectrum of high value-added and non-petroleum chemicals, such as 2,5-furandicarboxylic acid, 5hydroxymethylfuroic acid, 2,5-dimethylfuran, 5-ethoxymethylfurfural and levulinic acid, etc [6,7]. This is because 5-HMF can be exploited as a precursor or a building block that plays a crucial role in industrial chemical manufactures. Despite the high potential and demand, the current production of 5-HMF at the industrial scale is marginal due to poor performance and low production volume [8,9]. Most of 5-HMF are sold as a specialty chemical for laboratory and research purposes. 5HMF derived from biomass source with high atom economy, and the current supply of 5-HMF is unsustainable in industry. Hence, developing a 5-HMF production system with high efficiency and sustainability will be an important milestone. At present, there were some reports for 5-HMF synthesis using various carbohydrates as the substrate such as fructose, glucose, sucrose, cellulose, lignin, inulin, etc. On the one hand, when lignocellulosic biomass were used as feedstock such as 27–49 wt% cellulose, 14–37 wt% hemicellulose and 16–33 wt% lignin, 5-HMF yield ranging from 2 to 60% were obtained in the conversion systems, accompanied by the complex pretreatment [10]. On the other hand, the facile synthetic method of 5-HMF is through three-step dehydration of hexose with an acid catalyst. Higher yield of 5-HMF is obtained with fructose as raw material, but fructose is not considered as an ideal feedstock due to its high cost and short supply in nature. On the contrary, glucose is a kind of abundant and low-cost carbohydrates making it a potential candidate for 5-HMF synthesis [11,12]. In general, glucose conversion and 5-HMF yield and selectivity are deeply affected by some reaction parameters such as the type of catalyst, reactant ratios, solvent, reaction time and temperature [13]. The prudent consideration of the reaction factors is required in order to direct the conversion to the favorable pathway and avoid these unwanted reactions [14]. For example, a higher temperature makes the reaction very violent that causes the presence of side reactions, while a lower temperature lead to an inefficient conversion. In addition, when water is used as solvent, it can result in the formation of hydration products levulinic acid and formic acid. The specific properties of the selected catalyst, such as acidic property and chemical stability, are critical in achieving the safe and efficient 5-HMF production [15]. Using glucose as feedstock, the conversion to 5-HMF involves two continuous steps: (1) isomerization of glucose into fructose; (2) tripled dehydration of the latter into 5-HMF. As a result of the difficulty in destroying stable structure of glucose, highly active catalysts are required to promote this two-step conversion. Thus, many researches have been recently focused on the development of promising catalysts to further exploit the efficient catalytic system for the production of 5HMF (Table S1) [16]. Among these catalysts, ionic liquids (ILs) are a kind of room temperature molten salts with advantages of negligible vapor pressure, non-volatility, non-flammability, good thermal stability and environmental friendliness [17]. Tong et al. found that N-methyl-2pyrrolidonium methylsulfonate ([NMP][CH3SO3]) could be an efficient catalyst with 72.3% yield of 5-HMF in dehydration of fructose at 90 °C for 2 h. Nevertheless, only 3.0% yield of 5-HMF was gained using glucose as a substrate [18,19]. 1-Sulfonic acid-3-methyl imidazolium tetrachloroferrate ([SMIM][FeCl4]) with both Brønsted and Lewis acidity has been utilized for converting glucose to 5-HMF with 18% yield after 4 h at 150 °C [20]. Qu et al. reported that 1-hydroxyethyl-3methylimidazolium tetrafluoroborate ([C2OHMIM]BF4) was used to catalyze glucose to 5-HMF with 67.3% yield at 180 °C for 1 h in dimethylsulfoxide (DMSO) [17]. However, due to its high cost, inefficient separation and recycling difficulty, the use of ILs is still restricted from large-scale applications. Moreover, the requirement of higher reaction
2. Materials and methods 2.1. Materials Glucose (99%), fructose (99%), sucrose (99%), cellobiose (99%), inulin (99%), cellulose (99%) and starch (99%) were used as substrates with analytical grade and obtained from Aladdin Chemical Co., Ltd (Shanghai, China). 5-HMF (98%, analytical grade), dimethylsulfoxide (DMSO, 99.5%), methylisobutylketone (MIBK, ≥99%), N-methyl pyrrolidone (NMP, 99%), dimethylformamide (DMF, 99.5%) and dimethylacetamide (DMA, 99.5%) were purchased from Beijing Chemical Reagent Company (Beijing, China). Imidazole (99%), epichlorohydrin (≥99.5%), methanol (99.5%) and ethanol (99%) were provided by J & K Scientific Ltd. (Beijing, China). All reagents were utilized as received without further purification. 2.2. Catalysts preparation Polyionic liquid precursor (PIL-pre) was prepared through a route which was analogous to previous reports [29]. In a typical preparation, 300 mL of ethanol was added in a three-necked round bottom flask and heated to a temperature at 30 °C. Then, 1 mol of imidazole was added under stirring vigorously. After imidazole dissolved in ethanol, 1 mol of epichlorohydrin was added dropwise to the stirred solution, and the resulting mixture was then stirred for 2 h. When the temperature reached up to 45 °C stepwise, the mixture was stirred for 12 h. After the mixture continued to be heated up stepwise to 85 °C and refluxed for 72 h. The resultant white precipitate was obtained after vacuum filtration, ethanol washing and drying. Finally, the white precipitate was ground into powder, labeled as PIL-pre. Various metal ion-containing PIL-based catalysts were obtained using an ion-exchange method. In a typical synthesis, 50 mL of ethanol containing 0.05 mol of PIL-pre was added to a conical flask, and ethanol solution containing 0.05 mol of of SnCl4 or FeCl3 was added dropwise to the stirred solution. Then, the mixture was stirred vigorously for 12 h at 40 °C. The precipitate was gained by centrifugation, at least three ethanol wash prior to drying at 80 °C overnight. The solids obtained were identified as PIL-Sn and PIL-Fe. 2
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2.3. Catalysts characterization
2.5. DFT calculations
Fourier transform infrared (FTIR) spectra of PIL samples were obtained from 400 to 4000 cm−1 (wavenumber) on a Bruker TENSOR 27 spectrometer using KBr pellet samples. The KBr pallets were arranged by grinding KBr (spectroscopic grade) with 0.1 wt% of PIL sample. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra were recorded on a Bruker AC300 spectrometer equipped with a 4 mm CP MAS probe and Standard cavity. X-ray photoelectron spectroscopy (XPS) data were recorded on ThermoFisher Scientific ESCALAB-250 instrument using Al Kα (hν = 1486.6 eV) with approximately 3 × 10−9 mbar base pressure. The PIL sample was outgassed at room temperature before the measurement. Meanwhile, the binding energy was calibrated with respect to C 1 s peak at 284.6 eV from carbon deposit. Scanning electron microscopy (SEM) micrographs of PIL catalysts were characterized using a FEI NOVA NanoSEM430 scanning microscope equipped with energy-dispersive X-ray spectrometer (EDX). The nominal resolution was 1.0 nm at 15 kV. The acidic sites type of PILs were determined by pyridine adsorption IR spectra, which were conducted using a Bruker TENSOR 27 spectrometer. The powder PILs catalysts (50 mg) were pressed into a thin sheet (13 mm diameter), and put into the sample cell. Then it was preheated at 200 °C for 2 h. After cooling the cell to room temperature (RT), FTIR spectrum was recorded as background. After that, pyridine vapor was brought in the sample cell at RT for 30 min, and the spectrum was recorded again. Subsequent evacuations were implemented at different temperatures, for 10 min followed by spectra collection. The spectra were received by deducting the spectra collected before and after the adsorption of pyridine.
Density functional theory (DFT) calculations were used for further verification of the catalytic mechanism of glucose conversion to 5-HMF on PIL-Sn. Three reasonable models of PIL-Sn monomer were built in Materials Studio 7.0, from which an appropriate cluster was selected through the optimal computation. Through the use of the LYP correlation function (B3LYP), the hybrid density functional methods were applied to associate with DFT algorithm in Gaussian 09 [30]. Specifically, the basis set of 6-31G(d) was used for calculation of C, H, O and N atoms, involving geometry optimization (GO), transition state (TS), and intrinsic reaction coordinate (IRC), when 6-311++G(d, p) as higher level basis set was used to calculate the singlet point energy [31]. For Sn atom, the basis set of LANL2DZ was utilized to calculate GO and TS. Moreover, by frequency calculations, there are zero imaginary frequencies in all GO clusters. All the TS models (one imaginary frequency) were verified via IRC path analyses. 3. Results and discussion 3.1. Characterization of PIL catalyst Fig. 1 shows the FTIR spectra of PIL. Obviously, a band at 3419 cm−1 was attributed to OeH stretching vibrations, indicating ring-opening reaction occurred to epichlorohydrin with imidazole [32,33]. A band at 1635 cm−1 was correspond to C]C stretching vibration in the imidazole ring, while the sharp band at 1560 cm−1 can be related to C]N stretching vibration [34,35]. The band at approximately 1446 cm−1 was associated with the deformation vibration of eCH2 on side-chain [36]. The sharp bands at 1170 cm−1 and 1095 cm−1 were assigned to CeO and CeN stretching vibration, respectively [37]. The determination of these groups indicates that the structure of the polyionic liquid is in accordance with the PIL design. The characteristic peaks of the modified PIL-Sn and PIL-Fe functional groups remained unchanged, indicating that the incorporation of metal ions did not destroy the skeletal structure of polyionic liquid. The 1 H/13C NMR spectra (Figs. S1 and S2) of PIL are as follow. PIL-pre. 1H NMR (D2O, 600 MHz) δ: 7.59 (s, 2H), 4.55 (d, J = 14.4 Hz, 2H), 4.38 (s, 1H), 4.26 (d, J = 12.2 Hz, 2H). 13C NMR (D2O, 150 MHz) δ: 122.49, 67.42, 51.42. PIL-Sn. 1H NMR (D2O, 600 MHz) δ: 8.71 (s, 0H), 7.37 (s, 2H), 4.34 (d, J = 14.2 Hz, 2H), 4.18 (s, 1H), 4.05 (d, J = 11.9 Hz, 2H). 13 C NMR (D2O, 150 MHz) δ: 123.12, 68.00, 52.17. The high resolution XPS spectra of PIL materials (Fig. 2) were measured to investigate the chemical bonding states of C, N and O. The C 1s spectrum (Fig. 2a) involves two characteristic peaks centered at 284.6 eV and 286.5 eV, which are attributable to CeC and C-O respectively [38]. Similarly, a peak located at 401.8 eV was observed in
2.4. General procedure for 5-HMF synthesis Carbohydrate was converted into 5-HMF in a 15 mL sealed tube equipped with certain amounts of carbohydrate, catalyst, and solvent. The temperature of solution was monitored by a temperature sensor probe. When the temperature reaches the set value, the sealed tube was placed in an oil bath. The mixture was stirred using an internal stirrer at a maximum constant rate of 500 rpm. When the reaction was finished, the tube was immediately cooled in an ice water bath. Afterwards, the mixture was filtered through a one-off syringe-driven filter involving a high-quality microporous filter membrane, and then 0.1 mL of solvent filtrate was decanted into a 25 mL volumetric flask using water or methanol as a diluent prior to analysis. For DMSO, NMP, DMA, DMF, sec-butanol and water as the solvent, the diluent was water. For MIBK as the solvent, methanol was used as a diluent. High-performance liquid chromatography (HPLC) equipped with a C18 column and a UV detector was employed to analyse the product, using a mixture of deionized water/methanol (60/40 v/v) as the mobile phase with a flow rate of 1 mL/min. The detection wavelength of 5HMF was 284 nm. HPLC equipped with a Hypersil APS-2 column and a Shodex IR detector was utilized to analyse the residual substrate. The temperature of Hypersil APS-2 column was set at 35 °C, and the mobile phase was deionized water with a flow rate of 0.2 mL/min. The glucose conversion (%), the 5-HMF yield (%), and the 5-HMF selectivity (%) were measured on a carbon basis according to the following equations.
Moles of glucose ⎞ Glucose conversion (mol%) = ⎜⎛1 − ⎟ × 100% Starting amount of glucose ⎠ ⎝
5 − HMF yield (mol%) =
Moles of 5 − HMF × 100% Starting amount of glucose
5 − HMF selectivity (mol%) =
Yield of 5 − HMF × 100% Glucose coversion
Fig. 1. FTIR spectra of PIL. 3
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Fig. 2. XPS spectra of C 1s (a), N 1s (b), O 1s (c) and Sn 3d (d) of PIL samples.
the N 1s spectrum (Fig. 2b); this is in line with the sp2-hybridized aromatic nitrogen (C]NeC) [39]. Fig. 2c showed the O1s spectra of PIL, where a peak centered at about 532.2 eV, arising from C-O bond in PIL monomer [38]. In addition, no evident change was found for the chemical bonding states and valence of C, N and O in the XPS spectra of Sn modified PIL, while the intensity of characteristic peak declined after the modification; this indicates that Sn modification has not destroyed the original skeleton structure of PIL-pre, and there are some interaction between metal cluster and PIL chain. The Sn 3d spectra of PIL-Sn were fitted with two peaks corresponding to Sn 3d5/2 and Sn 3d3/2 centered at around 487.8 eV and 496.2 eV, respectively; this is feature of tetravalent tin [40]. There are some aggregation structure of spherical particles in the SEM images of PIL (Fig. S3). The original aggregation state of PIL-pre was not destroyed in PIL-Sn, demonstrating that the modification process of tin made no difference in polymer structure. Surprisingly, some porous framework structure were observed in the SEM images of PIL-Sn (Fig. S4), indicating the formation of polymeric chain could generate a certain amount of pore structure. In addition, pyridine adsorption IR spectra were showed in Fig. 3, which determined the type of acid site on PIL catalysts. No characteristic band was observed in the pyridine-IR spectrum of PIL-pre, indicating neither Brønsted nor Lewis acid sites existed on the surface of PIL-pre. After tin or iron modification, some characteristic pyridine adsorbed bands at 1451, 1489 and 1540 cm−1 were appeared in the spectra of PIL-Sn and PIL-Fe. The bands at 1451 cm−1 correspond to Lewis acid sites, resulting from pyridine molecules coordinated with tin or iron ions [41]. The band centered at
Fig. 3. Pyridine-IR spectra of PIL.
about 1540 cm−1 assigned to Brønsted acid sites was weak; this were consistent with pyridine protonated by Brønsted acids [42]. Furthermore, there was a pyridine adsorbed band at 1489 cm−1 arising from the interaction of pyridine molecules with both Brønsted and Lewis acid sites [43]. Compared PIL-Sn samples, these corresponding bands intensity displayed a decrease in PIL-Fe, possibly owing to stronger Brønsted and Lewis acidity of PIL-Sn than PIL-Fe. To sum up, the 4
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yield firstly increased to the maximum value and afterwards reduced with the extension of reaction time, which may be due to the formation of black insoluble humins. However, there was a different trend in glucose conversion, which appeared a rapid increase firstly and then tend to be levelled off. Besides, the 5-HMF yield was comparatively low with less than 80% conversion of glucose at 110 °C. With the further increase of reaction temperature to 130 °C, the catalytic activity of PILSn enhanced significantly and an optimal 5-HMF yield as high as 51.1% with 99% glucose conversion was acquired for 1 h. At high temperatures, a vigorous increase of reaction rate was observed at its initial stage. On account of highly consuming of energy and hard control under higher temperatures, 130 °C was used as the optimal temperature hereafter. In general, various initial concentration of substrate could deeply affect glucose conversion, hence the effect of initial glucose concentration was further investigated (Table 1, entries 3, 6, 7 and 8). Remarkably, greater than 96% conversions of glucose were obtained at diverse initial concentrations, and a highest yield of 5-HMF was achieved under a relatively low initial glucose concentration, due to acceleration in a shift of chemical equilibrium under lower substrate concentration. A partial decrease in 5-HMF yield at high initial concentration was detected as a result of the generation of by-products from side reactions. In order to further study the effect of the amount of PIL-Sn catalyst, different amount of PIL-Sn was used to catalyze glucose conversion (Table 1, entries 3, 9 and 10). Apparently, 5-HMF yield and glucose conversion increased when increasing the amount of PIL-Sn. When PILSn dose decreased to 6.5 mg, the yield of 5-HMF and the conversion of glucose were greater than 18% and 90%, respectively, demonstrating that PIL-Sn was an effective catalyst in 5-HMF synthesis. Therefore, it was superior for higher 5-HMF yields when the amount of PIL-Sn was 25 mg.
Table 1 Overview of glucose conversion and 5-HMF yield catalyzed by PIL catalysts.a Entry
1 2 3 4 5 6b 7c 8d 9e 10f
Catalyst
No catalyst PIL-pre PIL-Sn PIL-Fe SnCl4 PIL-Sn PIL-Sn PIL-Sn PIL-Sn PIL-Sn
Experimental parameter Conversion/%
Yield/%
27.8 30.6 99.0 80.4 90.2 98.7 98.7 96.0 90.5 95.4
0 0.63 51.1 35.8 16.2 45.8 43.1 42.2 18.7 30.6
a Reaction conditions: 55.6 mM glucose, 25 mg catalyst, 130 °C, 5 mL DMSO, 1 h. b, c, dThe initial concentration of glucose was 83.3 mM, 111 mM and 166 mM, respectively. e, fThe initial mass of catalyst was 6.5 mg and 12.5 mg, respectively.
introduction of Sn or Fe ions facilitate the coexistence of Brønsted and Lewis acid sites on the surface of catalysts. 3.2. Catalytic effect of PIL catalysts on glucose conversion to 5-HMF The catalytic efficiencies of PIL catalysts on 5-HMF synthesis from glucose were probed at 130 °C for 1 h (Table 1). When no catalyst used in the system, there were zero 5-HMF yield and some black humins formed with less than 30% glucose conversion (Table 1, entry 1), suggesting that it was hard for 5-HMF synthesis without catalyst. Only traces amount of 5-HMF was formed using PIL-pre as a catalyst (Table 1, entry 2), demonstrating that PIL-pre did not work in glucose transformed to 5-HMF, which could be attributed to the absence of Brønsted and Lewis acid sites. It should be noted that practically full conversion of glucose and 51.1% yield of 5-HMF were achieved when employing PIL-Sn as the catalyst (Table 1, entry 3), which possessed high chemical catalytic activity in 5-HMF synthesis from glucose. The presence of Brønsted and Lewis acidic sites was responsible for high efficiency catalysis of PIL-Sn due to the incorporation of Sn species. The formations of a small amount of levulinic acid (LA) and formic acid (FA) were captured during experimental runs (Fig S5), indicating the effective suppression of hydration reaction. Nevertheless, 35.8% yields of 5HMF and 80.4% conversion of glucose could be provided (Table 1, entry 4) with PIL-Fe catalyst, indicating PIL-Fe had a lower number of catalytic active sites and relatively weaker acidity than PIL-Sn; these were in accordance with the results obtained from Pyridine-IR spectra. Notably, above 90% glucose conversion and less than 20% 5-HMF yield were provided when catalyzed by homogeneous SnCl4 (Table 1, entry 5), implying that homogeneous Sn4+ could promote glucose transformation and 5-HMF synthesis in the reaction, which were consistent with the results reported by Enslow et al [44]. The results of the spacetime yield of 5-HMF with different catalysts were summarized in Table S2. Relatively speaking, PIL-Sn had preferable catalytic performance compared with other catalysts, because of its more Brønsted and Lewis acidic sites. This further verified the catalytic efficiencies of PIL-Sn catalyst.
3.4. Catalytic activity of PIL-Sn in various solvents for 5-HMF synthesis Further investigation of the effect of solvents in 5-HMF synthesis was conducted to verify the significance of suitable solvent. Obviously, there was a great difference in glucose conversion and 5-HMF yield when diverse solvents were used in the reaction, confirming a crucial solvent effect (Fig. 5). The catalytic activity of PIL-Sn in DMSO for 5HMF synthesis was excellent, because 5-HMF rehydration to levulinic acid (LA) and formic acid (FA) was suppressed in some extent through coordinating DMSO molecules with 5-HMF molecules [45]. And higher yield of 5-HMF and conversion of glucose were obtained. In pure water, the conversion of glucose was above 50%, but 5-HMF yield were relatively lower, owing to the presence of side reaction in H2O. On account that polar aprotic solvent could act as an excellent medium for 5-HMF formation, the catalytic activity of PIL-Sn in NMP, DMA and DMF was superior to that in water. Remarkably, the yield of 5-HMF was comparable in sec-butanol, which could make it easily solubilize carbohydrates and effectively accelerate the process of dehydration reaction. Although a biphasic solvent MIBK/water could separate substrates and products efficiently to promote the shift of chemical equilibrium, a moderate yield of 5-HMF was gained, indicating PIL-Sn exhibited poor catalytic activity in MIBK. Taking into account these results, DMSO could be acted as a preferable solvent for attaining higher 5-HMF yield. 3.5. Comparison of catalytic conversion of various carbohydrates by PIL-Sn
3.3. Effect of process conditions for PIL-Sn-catalyzed conversion of glucose to 5-HMF
The performance of PIL-Sn for various carbohydrates during the synthesis of 5-HMF was further investigated when PIL-Sn catalyst worked well on glucose conversion (Table 2). Briefly, 5-HMF yield from fructose conversion was higher than 50%, indicating PIL-Sn catalyst with Brønsted acid sites has played a crucial role in the three-step dehydration of fructose (Table 2, entry 1). In addition, 87% yield of 5HMF was obtained from fructose when utilizing PIL-Fe as a catalyst,
Comparatively, the catalytic activity of PIL-Sn was superior to that of other PIL catalyst, owing to stronger Brønsted and Lewis acidity required for glucose conversion. Therefore, PIL-Sn was utilized for further exploring the catalytic conversion of glucose to 5-HMF. The reaction temperature deeply affected the 5-HMF yield and glucose conversion (Fig. 4a and b). At different temperatures from 110 °C to 150 °C, 5-HMF 5
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Fig. 4. Effect of reaction temperature on (a) conversion of glucose and (b) yield of 5-HMF. (Conditions: 55.6 mM glucose, 25 mg PIL-Sn, 5 mL DMSO.)
results, but also has satisfactory results for other carbohydrates. 3.6. DFT calculations for the catalytic conversion of glucose to 5-HMF When tin tetrachloride runs into the imidazole chain, it may form various tin species with different coordination with chloride ions based on the previous literature [46–48]. There are three different coordination modes for the ligand in Sn-containing compounds: tetracoordinated, pentacoordinated and six-coordinated center [49,50]. To investigate the possibility of the formation of various tin chloride complexes of PIL-Sn, DFT calculations were used to optimize hypothetical monomer model. The initial SnCl4 monomer has a regular tetrahedron configuration with a CleSneCl bond angle of 109.47° and SneCl distances of 2.31 Å (Fig. 6a and Table 3). The optimized structure of PIL-pre monomer is found that chloride ion and imidazole ring are bound together by hydrogen bonds, and the HeCl hydrogen bond length is 2.06 Å (Fig. 6b). The reciprocity of SnCl4 with a PIL-pre monomer is advantageous (ΔG = −8.59 kcal mol−1, in Table 3, entry 3) and results in forming a distorted trigonal bipyramidal SnCl5- configuration with five-coordination to a central tin atom (Fig. 6c). The repulsive force between the electropositive Sn4+ center and the chargecompensating imidazole cation gives rise to deformation of the equilateral trigonal bipyramidal configuration, so that the Cl ligands deviate from Sn4+ center to the imidazole ring in different extent. There are hydrogen bonds between SnCl5- species and imidazole cation with the average HeCl distance of 2.39 Å (Table 3, entry 3). In these optimize structure models, there were no change in the bond length of OeH before and after the incorporation of the Sn metal in the PIL-pre (Table 3). And the Cl ligand was far away from the H atom in the OH group owing to the attraction of the positive imidazole ring, indicating relatively weak interaction between Cl ligand and the H atom in the OH group. The six-coordinated SnCl62- anionic species employs an octahedral structure with an average SneCl distance of 2.50 Å (Table 3, entry 4). The interaction of SnCl62- anionic species with two PIL-pre monomer leads to the angular deflection of octahedral geometry (Fig. 6d). The binuclear six-coordinated Sn2Cl102− complex is predicted to connect two Sn center via two Cl− bridges, but it becomes two mononuclear five-coordinated SnCl5- complexes after optimization, indicating the binuclear Sn2Cl102− complex was unstable (Fig. 6e). The average SneCl bond length in optimized Sn2Cl102− structure is close to that found for mononuclear SnCl5− species. On the other hand, the SnCl62− and Sn2Cl102− complexes are thermodynamically unfavorable, because the free energy change values are positive, and it is not prone to form the complexes (Table 3). Obviously, the SnCl5− species is the preferred species with lower Gibbs free energy, more applicable average SneCl distance and stronger hydrogen bonding. Electrostatic potential calculation in SnCl5−-PIL shows that SnCl5−
Fig. 5. The effect of various solvents in glucose conversion catalyzed by PIL-Sn. (Conditions: 55.6 mM glucose, 25 mg PIL-Sn, 5 mL solvent, 130 °C, 1 h.) Table 2 The performance of PIL-Sn for converting various substrates to 5-HMF. Entry
Substrate
Conv. (%)
Yield (%)
1 2 3 4 5 6 7 8
Fructose Glucose Sucrose Maltose Cellobiose Inulin Cellulose Starch
99.8 99.0 94.6 74.6 72.1 90.6 10.3 52.9
53.9 51.1 35.6 26.2 24.2 25.7 1.67 10.0
(Conditions: 55.6 mM carbohydrate, 25 mg PIL-Sn, 130 °C, 5 mL DMSO, 1 h).
signifying that PIL-Fe was more preferable in fructose conversion than PIL-Sn. When using di-saccharides (sucrose, maltose and cellobiose) as substrates, 5-HMF yields and di-saccharides conversions were above 20% and 70%, respectively, (Table 2, entries 3–5) demonstrating PIL-Sn catalyst could promote hydrolysis of di-saccharides. With regard to polysaccharides, the optimum 5-HMF yields from inulin and starch were much higher than that from cellulose (Table 2, entries 6–8). Remarkably, 5-HMF yield reached as high as 10.0% at 130 °C after 1 h with starch as substrate, which was a kind of low-cost, plentiful and renewable resources. These results illustrate that hydrolysis and dehydration of carbohydrates were promoted by Brønsted and Lewis acid sites of PIL-Sn. Hence, PIL-Sn not only can catalyze glucose to get better 6
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Fig. 6. Optimized structures of (a) SnCl4, (b) PIL-pre monomer, (c) SnCl5−-PIL, (d) SnCl62−-PIL, (e) Sn2Cl102−-PIL monomer model; Electrostatic potential calculation in (f) PIL-Sn and (g) glucose. (Distances are in Å).
TS2 → M3). The subsequent intramolecular H* shift from C2 to C1 (M4 → TS3 → M5) brings about the aldose-ketose isomerization, involving the formation of an enediolate intermediate [48]. This is a critical process with an activation barrier of 28 kcal/mol, which is lower than reported in the literature [51]. The M5 → M6 is a conformation transition process from a bidentate coordinated complex to a monodentate complex, and both complexes involve O1-deprotonated acyclic fructose moiety coordinating with mononuclear Sn active site. Subsequently, the ring-closure reaction takes place to form O1-deprotonated cyclic fructose (M6 → TS4 → M7). A proton transfer from Clligand to terminal O1 facilitates the formation of D-fructopyranose and the recovery of SnCl5--PIL original structure (TS5 → M8 → F). From an experimental viewpoint, fructose as an isomerization product was detected at the beginning of the reaction (Fig S7), demonstrating the catalytic pathway for glucose isomerization to fructose in Scheme 1 was feasible and valid by DFT calculation [52]. On the one hand, the formation of complexes with quintuple chelate structure in the H-shift process was confirmed based on earlier findings [25]. By adding a small amount of ethanol, ethylene glycol and 1, 3-propanediol, respectively, into the reaction mixture, it was found that ethylene glycol substantially suppressed glucose conversion but ethanol and 1, 3-propanediol had no impact (Fig. 8). This illustrates there is the presence of quintuple chelate structure rather than hexatomic chelate or acyclic structure in the isomerization pathway. On the other hand, the presence of water can promote the ring-opening and ring-closure of hexose with a reduction in activation barrier (Fig. 7b). Water molecule serves as a proton donor and acceptor, mediating the hydrogen-transfer between the oxygen-containing groups of hexose [51]. The addition of water leads the increase of reaction rate and shortening of reaction time required to achieve the maximum yield [53], but the value of maximum yield remains unchanged; this suggests that adding H2O molecule can greatly reduce activation barrier, but do not give rise to the shift of chemical equilibrium (Fig S8). These experimental results correspond very well with DFT calculations.
Table 3 Gibbs free energies (ΔG, in kcal mol−1) and the respective average SneCl, SneSn and HeCl distances (r, in Å) of the possible formation of tin chloride complexes in PIL-Sn. Entry
Cluster
ΔG
r(Sn-Cl)
r(Sn-Sn)
r(H-Cl)
r(O-H)
1 2 3 4 5
SnCl4 PIL-pre monomer SnCl5−-PIL SnCl62−-PIL Sn2Cl102−-PIL
– – −8.59 4.41 11.52
2.31 – 2.42 2.50 2.42
– – – – 5.08
– 2.06 2.39 2.60 2.57
– 0.97 0.97 0.97 0.97
species acting as Lewis acid site on the right side displays a well defined negative region, while the imidazole ring on the left side is positive charged, and the whole imidazole chain is used to stabilize the PIL-Sn structure (Fig. 6f). In the individual electrostatic potential energy diagram of PIL-pre and SnCl4 (Fig. S6), it can be seen that PIL-pre is composed of the chlorine with strongly negative charges and the imidazole ring with positive charges, while the initial SnCl4 is electroneutral. The incorporation of Sn in the PIL-pre lead to the diffusion of the positive and negative charges. Larger negative potential diffuse quickly from Cl- to Sn4+ center, and the positive potential is transferred to the branch chain of PIL-pre. Hence, the whole SnCl5--PIL is in electrostatic potential equilibrium. In the electrostatic potential energy diagram of glucose (Fig. 6g), it can be seen that the hydrogen proton in the O1H group is positively charged and vulnerable to attack, leading to the preferential coordination of O1 with Sn center and the ring-opening of glucose. Scheme 1 shows the detailed isomerization mechanism of glucose to fructose catalyzed by SnCl5--PIL. The D-pyranoglucose (DGP) and D-fructopyranose (D-Fru) are designated as the starting (S) and final (F) state for the catalytic pathway. A transfer of hydrogen proton from O1 to O5 (R → TS1 → M1) is catalyzed by coordination with a single Sn center, leading to the opening of D-GP ring and the formation of aldehyde O1 moiety, which is endothermic by 30 kcal/mol (Fig. 7a). The O2-coordination to Lewis acidic site Sn facilitates a proton transfer from O2 to a basic Cl- ligand, resulting in deprotonation of O2H (M2 → 7
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Scheme 1. Catalytic pathway for glucose isomerization to fructose by SnCl5−-PIL.
Fig. 7. DFT-computed free energy diagram for the isomerization of glucose involving SnCl5−-PIL. (a) Reaction over SnCl5−-PIL monomer model, (b) The effect of H2O on glucopyranose ring-opening and fructofuranose ring-closure step. Fig. 8. Effect of various additives on conversion of glucose and yield of 5-HMF.
4. Conclusions CRediT authorship contribution statement An efficient conversion of glucose into 5-HMF has been achieved with PIL-Sn as a catalyst, and an optimal 5-HMF yield of 51.1% could be obtained at 130 °C after 1 h. The excellent catalytic activity of PIL-Sn was also exhibited when converting other carbohydrates. A possible reaction mechanism was confirmed by DFT calculations, and five-coordinated SnCl5- was a dominant species that catalyzed glucose isomerization with lower activation energy. The addition of water in DMSO could lead to a partial reduction of energy barriers. These findings provide a feasibility of ionic liquid polymers as effective catalysts for biomass conversion into 5-HMF.
Guo Qiu: Conceptualization, Investigation, Writing - original draft. Biaohua Chen: Supervision, Resources. Chongpin Huang: Conceptualization, Methodology, Resources. Ning Liu: Data curation, Formal analysis. Xiuliang Sun: Formal analysis. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to 8
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influence the work reported in this paper. [25]
Acknowledgements
[26]
This work was supported by the National Natural Science Foundation of China under grant number 21476021.
[27]
Appendix A. Supplementary data
[28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117136.
[29]
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