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Conversion of corn stalk into furfural using a novel heterogeneous strong acid catalyst in γ-valerolactone
Accepted Manuscript Conversion of Corn Stalk into Furfural Using a Novel Heterogeneous Strong Acid Catalyst in γ-Valerolactone Zhiping Xu, Wenzhi Li, Zhijie Du, Hao Wu, Hasan Jameel, Hou-min Chang, Longlong Ma PII: DOI: Reference:
S0960-8524(15)01383-8 http://dx.doi.org/10.1016/j.biortech.2015.09.104 BITE 15613
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
2 August 2015 19 September 2015 21 September 2015
Please cite this article as: Xu, Z., Li, W., Du, Z., Wu, H., Jameel, H., Chang, H-m., Ma, L., Conversion of Corn Stalk into Furfural Using a Novel Heterogeneous Strong Acid Catalyst in γ-Valerolactone, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.09.104
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Conversion of Corn Stalk into Furfural Using a Novel Heterogeneous Strong Acid Catalyst in γ-Valerolactone Zhiping Xu a, Wenzhi Li b,*, Zhijie Du b, Hao Wu b, Hasan Jameel c, Hou-min Chang c, Longlong Ma d a
Department of Chemistry, University of Science and Technology of China, Hefei
230026, PR China b
Department of Thermal Science and Energy Engineering, University of Science and
Technology of China, Hefei 230026, PR China c
Department of Forest Biomaterials, North Carolina State University, Raleigh, NC
27695-8005, USA d
CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China *Corresponding author. Tel.: +86 0551 63600786 E-mail address: [email protected] (Wenzhi Li) ABSTRACT: A novel solid acid catalyst was prepared by the copolymerization of p-toluenesulfonic acid and paraformaldehyde and then characterized by FT-IR, TG/DTG, HRTEM and N2-BET. Furfural was successfully produced by the dehydration of xylose and xylan using the novel catalyst in γ-valerolactone. This investigation focused on effects of various reaction conditions including solvent, acid catalyst, reaction temperature, residence time, water concentration, xylose loading and catalyst dosage on the dehydration of xylose to furfural. It was found that the solid catalyst displayed extremely high activity for furfural production. 80.4% furfural yield
with 98.8% xylose conversion was achieved at 170 ℃ for 10 min. The catalyst could be recycled at least five times without significant loss of activity. Furthermore, 83.5% furfural yield and 19.5% HMF yield were obtained from raw corn stalk under more severe conditions (190 ℃ for 100 min). Keywords: furfural, xylose, corn stalk, PTSA-POM catalyst, γ-valerolactone 1. Introduction The problem of the greenhouse effect and global climate change due to the consumption of oil-based fuels, has attracted more and more attention all over the world (Zhang et al., 2012a). It has become an important research area for replacement of petroleum with non-fossil energy sources such as biomass, nuclear, solar and wind energy. Lignocellulosic biomass, the most abundant sustainable feedstock in the world, is the only non-fossil energy source which can be converted into liquid, solid and gaseous fuels, as well as other chemicals. It mainly consists of lignin (15-25%), cellulose (38-50%) and hemicellulose (23-32%) (Avci et al., 2013). The efficient production of biomass-derived fuels and high value-added chemicals requires the utilization of both hemicellulose and cellulose, which mainly contains of C5 and C6 sugars, respectively (Luterbacher et al., 2014; Gürbüz et al., 2013). Furfural is a versatile key derivative produced from pentosan-rich agricultural wastes such as wood chips, corn straws and bagasse. It can be used as an excellent solvent for many organic materials and as a building block chemical for the synthesis of many desired compounds such as furfuryl alcohol, 2-methylfuran (Yan et al., 2014a), succinic acid (Choudhary et al., 2013) and maleic acid (Guo et al., 2011).
Therefore, furfural has been identified as an attractive platform chemical for production of valuable bio-based chemicals and bio-fuels with huge market potential. Currently, furfural is produced in water using the most common homogeneous catalyst H2SO4 (Agirrezabal-Telleria et al., 2011). However, under these conditions, the furfural yield was quite low (only 45-55%) (Kim et al., 2012) due to the formation of humins in an aqueous solution of H2SO4. Hence, it is necessary to develop novel greener processes for producing furfural. In last decades, extensive studies on xylose and xylan (as model compound) dehydration have been performed with the objective of simplifying initial matrix (raw biomass) for furfural production. Mineral acids (Yemis et al., 2011) and metal chlorides (Zhang et al., 2014, 2013a) as homogeneous acidic catalysts for the production of furfural are efficient. However, these catalysts are corrosive, difficult to recover from reaction mixture and harmful to the environment. Furthermore, the post-processing is complicated and leads to large amounts of neutralization waste. In order to avoid the shortcomings of homogeneous catalysts, many heterogeneous solid catalysts have been developed for furfural production. Acidic zeolites and related modified zeolites such as H-mordenite, H-Beta zeolite, HUSY (Sahu et al., 2012), sulfonic acid functionalized mesoporous SBA-15 materials (Agirrezabal-Telleria et al., 2014) and MCM-41 (Zhang et al., 2012b), have been widely applied to catalyze the dehydration of xylose to furfural. Solid catalysts SO42-/TiO2-ZrO2/La+ (Li et al., 2014) and SO42-/ ZrO2-TiO2 (Zhang et al., 2012a) were also prepared for the conversion of xylose or corncob to furfural. But these catalysts can be deactivated easily due to the
carbon deposition over the active sites. Commercial resins Amberlyst-15 and Amberlyst-70 have also been applied to furfural production (Agirrezabal-Telleria et al., 2011, Zhang et al., 2013b). However, the both resins are limited to operating at low temperatures (Galletti et al., 2012). In recent years, a new type of solid Brønsted acid catalyst, biomass-based amorphous carbon which bears SO3H, COOH, and OH groups, has been extensively studied for biomass conversion processes (Hu et al., 2013; Yan et al., 2014b). The catalyst can be easily prepared from saccharides, lignin, cellulose and other biomass resources by incomplete carbonization and subsequent sulfonation. However, the sulfonation reaction is carried out at high temperature (＞ 160 ℃) with large amount of fuming sulfuric acid (15 wt% SO3) or concentrated sulfuric acid (98%) for the introduction of SO3H groups into the amorphous carbon surface. In addition, many other types of solid catalysts have also been tested such as mesoporous Nb 2O5 (García-Sancho et al., 2014), SAPOs (Bhaumik et al., 2014) and sulfonated graphene oxide (Lam et al., 2012). Besides catalysts, another special goal in furfural synthesis is to optimize the reaction system. One reaction system is a single-phase system such as ionic liquid (Zhang et al., 2013a; Zhang et al., 2013b), DMSO (Lam et al., 2011) and pure water (Lam et al., 2012) and the other is a mixture of water and an organic solvent creating a biphasic system, in which the organic solvent is used as the extraction phase. Organic solvents such as toluene, CPME, 1-butanol and MIBK (Bhaumik et al., 2014; Molina et al., 2012; Zhang et al., 2012b; Weingarten et al., 2010) have been used in this reaction system. In recent years, an environmentally friendly solvent γ-valerolactone
(GVL) has been extensively used for biomass conversion (Qi et al ., 2014; Alonso et al., 2013; Mellmer et al., 2014; Wettstein et al., 2012). Gürbüz et al. and Alonso et al. (2013) revealed that GVL could efficiently promote the conversion of xylose to furfural, and the yield was as high as 80%. Utilizing GVL as the reaction system significantly increases the conversion rate of xylose and decreases the degradation rate of furfural. Importantly, biomass-based GVL could be produced by the multistep processing of furfural (Bui et al., 2013). Recently, an easily prepared strong Brønsted acid catalyst has been prepared by copolymerization of p-toluenesulfonic acid (PTSA) and paraformaldehyde (POM) (Fan et al., 2010; Liang et al., 2008). However, the catalyst (denoted as PTSA-POM) can only be applied in organic solvents rather than aqueous solutions due to a poor water tolerance. It was observed that the calcination temperature has an important effect on the acidity and water resistance of the catalyst. In present work, the catalyst is modified through calcining at high temperatures. After finishing this treatment, water resistance of the material is improved remarkably. To the best of our knowledge, it is the first report that the catalyst can be used in aqueous solution and that this catalyst is the solid acid catalyst of choice for converting xylose, xylan and corn stalk into furfural. Interestingly, the novel catalyst displays high selectivity, strong acidity, high thermal stability. It also can be easily reused. This study focused on the acid-catalyzed conversion of xylose, xylan and corn stalk in GVL with the novel solid catalyst PTSA-POM. The effects of temperature, reaction time, and water concentration on the catalytic performance were evaluated.
Additionally, we compared different solvents and catalysts for xylose dehydration reaction. 2. Materials and Methods 2.1 Materials D-(+)-Xylose (≥99%), furfural (99%), 5-hydroxymethylfurfural (HMF, 99%), γ-valerolactone (98%), xylan (from corn cob, 90%), p-toluenesulfonic acid monohydrate (PTSA•H2O, 98.5%), paraformaldehyde (POM, AR), cyclopentyl methyl ether (CPME, 99%), NaOH aqueous solution (0.02M) and Amberlyst-15 (wet) were purchased from Aladdin Industrial Inc. (Shanghai, China). H2SO4 (96～98%, AR), HCl (36～38%, AR), AlCl3•6H2O (AR), 1,4-dioxane (AR), tetrahydrofuran (THF, AR), toluene (AR), dimethyl sulfoxide (DMSO, AR), 4-methyl-2-pentanone (MIBK, AR), n-butanol (AR), N,N-dimethylformamide (DMF, AR) and NaCl (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Zeolite catalyst HZSM-5 was purchased from The Catalyst Plant of Nankai University (Tianjin, China). All reagents were used without any purification. The corn stalk was collected from the north of Anhui province, China. The biomass was firstly ground to pass through a 40-mesh screen and then washed repeatedly with tap water to remove the clay from the surface. After washing process, the solid was dried in an oven at 80 ℃, until the weight remained constant. 2.2 Preparation of the catalyst The solid acid catalyst PTSA-POM was prepared following the procedures in the literatures (Fan et al., 2010; Liang et al., 2008) with slight modifications. Typically, a
mixture of PTSA•H2O (10.00 g) and H2SO4 (98%, 0.2 ml) was heated at 110 ℃ in a three-necked round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. When PTSA•H2O was completely melted, the required amounts of POM were added immediately. The reaction mixture was kept at 110 ℃ for 8 h, followed by 24 h at 130 ℃ to form a black solid. The black solid was filtered and washed repeatedly with deionized water until the filtrate reached pH 7.0. The resulting sample was dried at 120 ℃ for 12 h and then ground into a powder. The black powder obtained was then roasted at different temperatures for 6 h in a muffle to get the targeted catalyst. 2.3 Catalyst characterization The functional groups of fresh and used PTSA-POM were analyzed by Fourier transform infrared spectroscopy (FTIR) on a Nicolet 8700 (USA). Thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis was conducted from room temperature to 600 ℃ with a heating rate of 10 ℃/min under N2 flow using a TGA Q5000 (USA). High resolution transmission electron microscopy (HRTEM) images were taken on a JOEL-2010 microscope (Japan) at 200kV. Special surface area, pore volume and average pore width were estimated from N2 adsorption isotherm measured at -195.8 ℃ on Tristar Ⅱ 3020M, Micrometritics. Acidity of the material was determined through acid-base neutralization titration. The titration was carried out as follows: 0.100 g of the black solid and 20 ml of 2 M NaCl aqueous solution were stirred at room temperature for 24 h. The solids were filtered off using positive air pressure filtration and washed with 2 M NaCl aqueous solution (3×5 ml) and distilled water (3×5 ml). The combined filtrate was titrated with 0.02 M NaOH
standard solution using phenol red as an indicator (Margelefsky et al., 2008).
2.4 Catalytic tests The acid-catalyzed conversion of xylose, xylan and corn stalk to furfural by the novel solid catalyst was carried out in a magnetically stirred 25 ml autoclave, heated with an electronically controlled heating mantle. In a typical experiment, the required amounts of feedstock, catalyst, distilled water and GVL were loaded into the autoclave and sealed. The mixture was stirred at 500 rpm and heated from room temperature to the desired temperature over 30 minutes. Zero time was considered to be the instant the reaction temperature reached the target temperature. To end reactions, the reactor was immersed in cold water to reduce the reactor temperature. The liquid was filtered and analyzed using HPLC. 2.5 Analysis of furfural, HMF and xylose In monophasic systems, furfural and HMF were quantified by HPLC (Waters 515 pump) with Waters Symmetry®-C18 column (5μm, 4.6×150 mm) and an UV/Vis Detector (Waters 2489) at 280 nm. The temperature of column oven was maintained at 30 ℃. Mobile phase was methanol/distilled water (2/3, v/v) at a flow rate of 0.4 ml/min. In biphasic systems, the detector was transformed to a Refractive Index Detector (Waters 2414). Xylose was determined by means of the Waters XBridgeTM Amide column (5μm, 4.6×150 mm) and the Refractive Index Detector. Column oven and detector temperatures were maintained at 45 ℃. Mobile phase was acetonitrile/distilled water (3/2, v/v) with a flow rate of 0.4 ml/min. Authentic samples of xylose, furfural and HMF were used as standards, and calibration curves were used
for quantification. Xylose conversion, HMF yield, furfural yield and furfural selectivity were calculated as follows: Xylose conversion ＝ (1－moles of xylose in products / moles of initial xylose) × 100% Furfural yield (from xylose) ＝ (moles of furfural in products / moles of initial xylose) × 100% Furfural selectivity ＝ (furfural yield / xylose conversion) × 100% Furfural yield (from xylan or corn stalk) ＝ (moles of furfural in products / theoretical moles of xylose in xylan or corn stalk) × 100% HMF yield (from corn stalk) ＝ (moles of HMF in products / theoretical moles of glucose in corn stalk) × 100% 3. Results and discussion 3.1 Characterization of the solid acid catalysts A series of PTSA-POMs were synthesized under different conditions according to previous literatures (Fan et al., 2010; Liang et al., 2008). The present work found that the calcination temperature had an important effect on the acidity, water resistance and thermostability of PTSA-POM. Fig. 1A shows that the acid density values of the three kinds of catalyst (POM = 3.00, 4.00 and 5.00 g) were similar, decreasing slightly with higher calcination temperatures. Dehydration of xylose into furfural was carried out in GVL using PTSA-POMs (prepared at different calcination temperatures). After the reaction, the used catalyst was separated by filtration, washed
with distilled water, and then dried at 105 ℃ until the weight remained constant. Weighing the recovered catalyst shows the weight loss of PTSA-POM during the reaction. Weight loss of PTSA-POM calcined at low temperatures (120, 140, 155 ℃) was very high. An average weight reduction of 23% (36%, 24% and 9% weight loss) was achieved after the reaction. But at higher calcining temperatures (＞170 ℃), the weight loss was negligible. This was also indicated by the color of the reaction solution, which gradually faded as calcination temperature increased (Fig. S1). The filtrate color was black at 120 ℃ and 140 ℃, and it was brown at higher temperatures, such as 155 ℃, which indicated that an increase in calcination temperature resulted in a significant improvement of the water resistance of the catalyst. The weight loss and color changing might be related to the dissolution of black polymer solid (prepared at low temperature) with low degree of polymerization (DP). At high temperatures, the end groups of polymers reacted with each other, leading to the formation of larger long-chain molecules, resulting in a significant improvement of water resistance of the modified catalyst. Taking acidity and water resistance into account, 185 ℃ was chosen as the desired temperature for further studies. Furthermore, TG/DTG analysis could support that the thermal stability increased with increasing calcination temperature (Fig. 1B). A weight loss occurred at 100 ℃ was attributed to removal of water adsorbed on the catalysts. The materials were thermally stable below 250 ℃, and the weight decreased with a further increase in temperature. As depicted in Table 1, Fig. 1A and C, the effect of POM content on acidity and
thermostability of the material was studied by running neutralization titration and TG/DTG analysis. It was surprising that the POM content had almost no effect on the catalyst. On the one hand, H+ exchange values (Table 1), were between 2.2 mmol/g and 2.4 mmol/g, which were nearly constant taking into account experimental errors in the measurement. This was also demonstrated in Fig. 1A. When POM = 3.00, 4.00 and 5.00 g, the acid density values remained constant at the same calcination temperature. On the other hand, three kinds of catalyst (prepared using 20 wt%, 30 wt% and 50 wt% POM) were chosen for TG/DTG analysis. The TG/DTG curves of the three catalysts were difficult to separate from each other, which indicated that changing amount of POM did not significantly affect the thermostability of PTSA-POM (Fig. 1C). In further studies, the novel catalyst was synthesized using 3.00 g POM and 185 ℃ as the calcination temperature. 3.2 Solvent effects on the dehydration of xylose to furfural Although solvent dependence of the acid catalyzed dehydration of xylose to furfural has been observed, no comparative study has been reported for GVL under the same conditions. Thus, utilization of the novel solid catalyst PTSA-POM for xylose conversion in various solvents was investigated. The results were summarized in Table 2. GVL gave a furfural yield of 80.4% in 10 min at 170 ℃ with 98.8% xylose conversion (Table 2, Entry 1). High furfural yields were also obtained in dioxane (73.6%) and THF (67.9%). However, furfural yields were only 3.6% in DMF and 43.0% in DMSO with high xylose conversion under the same reaction conditions (Table 2, Entry 6, 9). This indicated that GVL, THF, and dioxane could increase the
conversion rate of xylose and decrease the degradation rate of furfural. DMSO and DMF accelerated the degradation of furfural and produced more humins as indicated by a deep brown color observed after the reactions in DMSO and DMF. Furfural yields in CPME (49.6%), toluene (46.8%) and MIBK (38.4%) (Table 2, Entry 4, 5, 7) were much lower than those in GVL, dioxane and THF. In the water-organic solvent two-phase system, organic solvent was only used in an extraction phase. In fact, when the dehydration reaction was carried out in pure water, the reaction rate was low and furfural was readily converted to humins. 3.3 Influence of various acid catalysts on furfural production Table 3 showed the dehydration results of xylose to furfural in four homogenous acids (H2SO4, HCl, PTSA, and AlCl3) and three heterogenous acids (HZSM-5, Amberlyst-15, and PTSA-POM) after reacting at 170 ℃ in GVL containing 10 wt% water for 10 min. A control experiment confirmed that only trace amount of furfural were obtained in the absence of a catalyst. Furfural yields obtained from xylose in the presence of HZSM-5, Amberlyst-15, H2SO4, HCl, PTSA, AlCl3, and PTSA-POM were 22.1%, 61.2%, 73.0%, 70.6%, 58.6%, 58.5%, and 80.4%, respectively. Over 85% conversion of the xylose was obtained in the presence of these acid catalysts except for HZSM-5. The results showed that PTSA-POM displayed the highest activity among all the tested catalysts, and the yield was as high as 80.4%. Other strong acid catalysts also showed activity toward the xylose dehydration with furfural yields ranging from 58% to 75%. In the case of HZSM-5, due to its low acidity, the observed furfural yield (22.1%) and xylose conversion (35.0%) were relatively
inferior under the investigated conditions. However, strong mineral acids were also unfavorable for the formation of furfural on account of furfural degradation reactions in the strongly acidic systems. As for PTSA-POM, the absence of free hydrogen ions decreased the rates of furfural degradation reactions, leading to high furfural yield. 3.4 Influence of reaction temperature on furfural production Subsequently, PTSA-POM was selected to test the effect of different reaction temperatures on furfural production by running the experiments in GVL (containing 10 wt% water) at 140, 150, 160, 170, 180 ℃ (Fig. 2A, B and C). In Fig. 2A, it was observed that high temperature facilitated the dehydration of xylose to furfural, in comparison to the initial furfural yields at different temperatures. Meanwhile a much shorter reaction time was needed at a higher temperature to reach the maximum value of furfural yield. At low temperature (140 ℃), the furfural yields increased with extended reaction time, but at higher temperatures (150, 160 and 170 ℃), the furfural yields gradually reduced after reaching the optimal values. This was because elevated temperatures not only increased the rate of xylose dehydration (Fig. 2B), but also enhanced the rate of furfural degradation. According to the experimental results, the optimal reaction temperature and retention time for xylose conversion was 170 ℃ and 10 min, respectively. These conditions were chosen for further studies. 3.5 Influence of water concentration on furfural production In this part, given the low solubility of xylose in pure GVL and the high price of GVL, it is necessary to investigate the effect of additional water on furfural production using the novel solid catalyst. Experiments were performed with different
water weight ratios ranging from 5 wt% to 30 wt% (based on GVL) (Fig. 2D, E and F). Furfural yields at various water concentration showed a similar tendency with time, which showed a sharp increase initially and subsequently decreased (Fig. 2D). When compared with furfural yield and xylose conversion at each time point with different water concentration, it was found that water concentration had a major impact on the reaction. The higher water concentration in GVL , the lower the furfural yield and the time required to reach the optimum yield of furfural was longer. For example, 82.3% furfural yield was obtained in the presence of 5% H2O at 170 ℃ for 5 min, but only a 56.0% furfural yield was measured with 30% H2O. This could be attributed to the fact that water is not favorable for the xylose dehydration reaction and can promote the degradation of furfural to side products. 3.6 Influence of xylose and PTSA-POM loading on furfural production In acid catalytic dehydration of xylose, starting xylose concentration has a remarkable effect on furfural yield. Here, the influence of xylose concentration was studied. 91.4% furfural yield were obtained at a xylose loading of 1.0% (Fig. 3A, all the xylose loadings were based on GVL). With an increase of xylose loading to 10.0%, furfural yield and selectivity sharply decreased to 65.6% and 69.0%, respectively. With increasing initial xylose concentration, both the furfural yield, selectivity decreased. This could be due to the fact that higher xylose concentration leads to an increase in the molecular collisions occurring between the formed furfural molecules or the furfural and xylose molecules. Self-polymerization of furfural or cross-polymerization between xylose and furfural would bring about undesirable
reactions, resulting in poor furfural yield and selectivity. The influence of catalyst loading amounts was also investigated in GVL using PTSA-POM as catalyst at 170 ℃. The catalyst loadings were based on GVL. Xylose conversion increased with increasing catalyst dosages from 0.13% to 2.67% (Fig. 3B). Increasing catalyst mass ratio from 0.13% to 1.07% had a positive effect on furfural yield and selectivity. Furfural yield increased from 35.4% to 80.6%. Subsequently, further increasing of the catalyst charge resulted in a slight decrease in furfural yield and selectivity. This may be because that the excessive catalyst was beneficial for furfural degradation reaction (such as fragmentation, resinification, and condensation) (Gürbüz et al., 2013), leading to formation of humins or other products. 3.7 Investigation of the catalyst reusability Reutilization of solid catalyst is of great importance for industrial processes. To examine the stability of PTSA-POM for the conversion of xylose into furfrual in GVL, a five-cycle experiment was performed (Fig. 4). In order to minimize unavoidable losses from manipulation of the catalyst after each run, 0.30 g catalyst (2 wt.%) were used in these recycling experiments. The catalyst was recovered after each run by filtration, repeatedly washed with distilled water, dried in an oven at 100 ℃ and then used in subsequent reaction under identical reaction conditions. 81.6% of furfural yield was obtained, the first time the catalyst was used, and the yield gradually decreased to 68.3% after five successive recycles. The conversions of xylose maintained at around 90% till the fifth run. Thus, the catalyst recycles very well, although both of the furfural yield and xylose conversion had decreasing trends with
the consecutive catalytic runs, which could be ascribed to a mass loss of the catalyst during filtration process and a slight leaching of -SO3H during reactions. As depicted by Fig. S2, when compared to the fresh catalyst sample, the 2nd and 5th used catalysts separated from the reaction mixture exhibited attenuated signals, which could be assigned to hydroxyls (O-H stretching vibrations at 3410 cm-1 and O-H bending vibration at 1400 cm-1) and -SO3H groups (SO3-H stretching vibration at 1180 cm-1 and O=S=O stretching vibration at 1040 cm-1). This result indicated that the catalyst underwent partial leaching of -SO3H groups. 3.8 Production of furfural or 5-HMF from xylan and corn stalk in GVL To compare the results obtained with xylose as feedstock, the reaction was carried out with corn cob-derived xylan as feedstock. Reaction temperatures were 160, 170 and 180 ℃ (30 min heating up time). In a typical reaction, 0.40 g xylan was placed in an autoclave, 15 ml GVL, 1.5 ml water and 0.20 g catalyst were added. After reaching the desired reaction temperature, stirring was increased to 500 rpm. The formation of furfural and side products was highly dependent on the reaction temperatures (Fig. 5A). The maximum yield of furfural (69.2%) was achieved at 170 ℃ in only 10 min, which was lower than that from xylose (80.4%). After reaching the optimum value, furfural yield decreased sharply over time. It was clearly seen that the conversion of xylan into furfural was significantly more challenging than the dehydration of xylose. Because xylan, a polymer of xylose, was first transformed into xylose by hydrolysis and then changed into furfural by dehydration. It is apparent from the work described above that the solid catalyst and the
reaction system (GVL containing 10 wt% water as a solvent) were highly effective for converting xylose and isolated xylan (as model compounds) into furfural. The next step was to process raw corn stalk (containing cellulose, hemicellulose and lignin) using the catalyst in an one-pot reaction system. The xylan and glucan contents in the used corn stalk were 19.5 wt% and 39.6 wt%, respectively (Zu et al., 2014). In this part, all the furfural yields and HMF yields were based on this initial compositional data. Fig. 5B illustrated that from real raw biomass a high furfural yield of 83.8% was obtained at 170 ℃ after 100 min . Furfural yield of 85.4% was also achieved at 180 ℃ after 80 min. Moreover, 86.3% was achievable for furfural at 190 ℃ after 40 min. The results were higher than those obtained from xylose and xylan as substrates. According to previous studies (Zhang et al., 2014), this phenomenon could be explained by the formation of furfural from cellulose in raw corn stalk. However, the calculation method of furfural yield was based on xylan in corn stalk, so furfural formed from cellulose led to the increase of furfural yield. In addition, compared to xylose and xylan, when corn stalk was used as a feedstock, the optimal reaction temperature for furfural production was 190 ℃ rather than 170 ℃ because of a high temperature facilitating destruction of the network of raw biomass. Interestingly, in these reactions, the formation of a small amount of HMF was also seen at 160 ℃ and 170 ℃ (Fig. 5C). When the reaction temperature was increased to 190 ℃, a surprisingly high HMF yield of 19.5% was obtained. The origin of HMF was attributed to the fact that, under these reaction conditions cellulose in corn stalk also underwent conversion. The next project will mainly focus on developing a magnetic
catalyst for treatment of corn stalk with a high selectivity and ease of catalyst recyclability. 4. Conclusion A novel catalyst PTSA-POM was prepared by the copolymerization of PTSA and POM. After high-temperature calcining treatment, the catalyst showed improved water resistance and higher thermal stability. PTSA-POM has advantages over common dehydration catalysts in GVL and gave a furfural yield of 80.4%. More importantly, PTSA-POM also exhibited excellent catalytic activity for furfural production from corn stalk and the highest furfural yield of 86.3% was obtained. We believe that the use of PTSA-POM catalyst in GVL/water solvent offers a promising approach for the efficient conversion of renewable biomass feedstocks.
Acknowledgements This project was financially supported by Science and Technological Fund of Anhui Province for Outstanding Youth (1508085J01), the National Basic Research Program of China (No.2012CB215302) and the Key Programs of the Chinese Academy of Sciences (No. KGZD-EW-304-2).
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2011. Synthesis of furfural from xylose by heterogeneous and reusable nafion catalysts. ChemSusChem 4, 535-541. [16] Lam, E., Chong, J.H., Majid, E., Liu, Y., Hrapovic, S., Leung, A.C.W., Luong, J.H.T., 2012. Carbocatalytic dehydration of xylose to furfrual in water. Carbon 50, 1033-1043. [17] Li, H., Deng, A., Ren, J., Liu, C., Qi Lu, Zhong, L., Peng, F., Sun, R., 2014. Catalytic hydrothermal pretreatment of corncob into xylose and furfural via solid acid catalyst. Bioresour. Technol. 158, 313-320. [18] Liang, X., Gao, S., Gong, G., Wang, Y., Yang, J., 2008. Synthesis of a novel heterogeneous strong acid catalyst from p-toluenesulfonic acid (PTSA) and its catalytic activities. Catal. Lett. 124, 352-356. [19] Luterbacher, J.S., Rand, J.M., Alonso, D.M., Han, J., Youngquist, J.T., Maravelias, C.T., Pfleger, B.F., Dumesic, J.A., 2014. Nonenzymatic sugar production from biomass using biomass-derived γ-valerolactone. Science 343, 277-280. [20] Margelefsky, E.L., Bendjériou, A., Zeidan, R.K., Dufaud, V., Davis, M.E., 2008. Nanoscale organization of thiol and arylsulfonic acid on silica leads to a highly active and selective bifunctional, heterogeneous catalyst. J. Am. Chem. Soc. 130, 13442-13449. [21] Mellmer, M.A., Sener, C., Gallo, J.M.R., Luterbacher, J.S., Alonso, D.M., Dumesic, J.A., 2014. Solvent effects in acid-catalyzed biomass conversion reactions. Angew. Chem. Int. Ed. 53, 11872-11875.
[22] Molina, M.J.C., Mariscal, R., Ojeda, M., Granados, M.L., 2012. Cyclopentyl methyl ether: A green co-solvent for the selective dehydration of lignocellulosic pentoses to furfural. Bioresour. Technol. 126, 321-327. [23] Qi, L., Mui, Y.F., Lo, S.W., Lui, M.Y., Akien, G.R., Horváth, I.T., 2014. Catalytic conversion of fructose, glucose, and sucrose to 5-(Hydroxymethyl)furfural and levulinic and formic acids in γ-valerolactone as a green solvent. ACS Catal. 4, 1470-1477. [24] Sahu, R., Dhepe, P.L., 2012. A one-pot method for the selective conversion of hemicellulose from crop watse into C5 sugars and furfural by using solid acid catalysts. ChemSusChem 5, 751-761. [25] Weingarten, R., Cho, J., Conner, W.C., Huber, J.G.W., 2010. Kinetics of furfural production by dehydration of xylose in a biphasic reactor with microwave heating. Green Chem. 12, 1423-1429. [26] Wettstein, S.G., Alonso, D.M., Chong, Y., Dumesic, J.A., 2012. Production of levulinic acid and gamma-valerolactone(GVL) from cellulose using GVL as a solvent in biphasic systems. Energy Environ. Sci. 5, 8199-8203. [27] Yan, K., Wu, G., Lafleur, T., Jarvis, C., 2014a. Production, properties and catalytic hydrogenation of furfural to fuel additives and value-added chemicals. Renew. Sust. Energ. Rev. 38, 663-676. [28] Yan, L., Liu, N., Wang, Y., Machida, H., Qi, X., 2014b. Production of 5-hydroxymethylfurfural from corn stalk catalyzed by corn stalk-derived carbonaceous solid acid catalyst. Bioresour. Technol. 173, 462–466.
[29] Yemis, O., Mazza, G., 2011. Acid-catalyzed conversion of xylose, xylan and straw into furfural by microwave assisted reaction. Bioresour. Technol. 102, 7371-7378. [30] Zhang, J., Lin, L., Liu, S., 2012a. Effect production of furan derives from a sugar mixture by catalytic process. Energy Fuels 26,4560-4567. [31] Zhang, J., Zhuang, J., Lin, L., Liu, S., Zhang, Z., 2012b. Conversion of D-xylose into furfural with mesoporous molecular sieve MCM-41 as catalyst and butanol as the extraction phase. Biomass Bioenergy 39, 73-77. [32] Zhang, L., Yu, H., Wang, P., Dong, H., Peng, X., 2013a. Conversion of xylan, D-xylose and lignocellulosic biomass into furfural using AlCl3 as catalyst in ionic liquid. Bioresour. Technol. 130, 110-116. [33] Zhang, L., Yu, H., Wang, P., 2013b. Solid acids as catalysts for the conversion of D-xylose, xylan and lignocellulosics into furfural in ionic liquid. Bioresour. Technol. 136, 515-521. [34] Zhang, L., Yu, H., Wang, P., Li, Y., 2014. Production of furfural from xylose, xylan, corncob in gamma-valerolactone using FeCl3•6H2O as catalyst. Bioresour. Technol. 151, 355-360. [35] Zu, S., Li, W., Zhang, M., Li, Z., Wang, Z., Jameel, H., Chang, H., 2014. Pretreatment of corn stover for sugar production using dilutehydrochloric acid followed by lime. Bioresour. Technol. 152, 364–370.
Figure captions Fig. 1. Effect of calcination temperature and the amount of POM on acid density (A) or thermostability (B, C) of PTSA-POM catalyst. (B) TG/DTG curves of the catalysts prepared at different calcining temperatures; (C) prepared with different POM dosages. Fig. 2. Effect of temperature (A, B, C) and water concentration (D, E, F) on xylose (0.40 g) conversion to furfural in GVL (15.0 ml) catalyzed by PTSA-POM (0.20 g) at a stir speed of 500 rpm. 30 min heating-up time was required to achieve a desired temperature. For temperature treatments (A, B, C), H2O = 1.5 ml; for water concentration treatments (D, E, F), T = 170 ℃. Fig. 3. Effect of xylose loading (A) and PTSA-POM catalyst loading (B) on xylose conversion to furfural in GVL containing 10 wt% water (15.0 ml GVL, 1.5 ml H2O) at 170 ℃ for 10 min (30 min heating-up time) at 500 rpm. For xylose loading treatments (A), PTSA-POM catalyst = 0.20 g; for PTSA-POM catalyst loading treatments (B), xylose = 0.40 g. All the xylose and PTSA-POM weight ratios were based on GVL. Fig. 4. Reusability study of PTSA-POM. Reaction conditions: 0.40 g xylose, 0.30 g catalyst, 15.0 ml GVL, 1.5 ml H2O, 170 ℃, 10 min (30 min heating-up time), 500 rpm. After each run, the solid acid catalyst was separated by filtration and washed with distilled water, dried and then used in the subsequent reaction. Fig. 5. Effect of reaction temperature on xylan (A) and corn stalk (B, C) conversion to furfural and HMF in GVL catalyzed by PTSA-POM. Reaction conditions: 0.40 g
xylan (A) or corn stalk (B, C), 0.20 g catalyst, 15.0 ml GVL, 1.5 ml H2O, 30 min heating-up time, 500 rpm. Yields of furfural (B) and 5-HMF (C) were based on xylan content of 19.5% and cellulose content of 39.6%, respectively in the corn stalk used.
Tables and figures Table 1 Effect of the amount of POM on the acidity of PTSA-POM.
Entry
POM (wt%)
Acid density
Furfural
Xylose
H+b (mmol/g)
yield (%)c
conversion (%)c
a
1
20
2.4
77.9
98.1
2
25
2.4
78.0
97.6
3
30
2.2
77.4
96.7
4
35
2.2
76.5
95.0
5
40
2.3
76.2
96.3
6
45
2.4
76.0
96.0
7
50
2.3
75.8
95.1
8
60
2.3
74.5
96.3
a
POM weight ratios were based on PTSA ▪H2O (10.00 g), 185 ℃ calcination
temperature. b
Acid density values were determined through acid-base titration and calculated using
the amount of NaOH added to the corresponding material. c
Reaction conditions: 0.20 g catalyst, 0.40 g xylose, 15.0 ml GVL, 1.5 ml H2O,
160 ℃ (40 min heating-up time), 20 min, 500 rpm.
Tabe 2 Results of xylose dehydration experiments in diverse solvents by PTSA-POM.a
Entry
Furfural
Xylose
Furfural
yield %
conversion %
selectivity %
Solvent
1
GVLb
80.4
98.8
81.4
2
Dioxaneb
73.6
92.6
79.5
3
THFb
67.9
92.8
73.2
4
CPMEc
49.6
93.1
53.3
5
Toluenec
46.8
100
46.8
6
DMSOb
43.0
70.2
61.3
7
MIBKc
38.4
64.8
59.3
8
1-butanolc
11.3
46.9
24.1
9
DMFb
3.6
72.8
5.0
a
Reactions conditions: 0.20 g PTSA-POM, 0.40 g xylose, 170 ℃, 10 min (30 min
heating-up time), 500 rpm. b
Homogeneous system, organic solvent 15.0 ml containing 1.5 ml H2O.
c
Biphasic system, organic solvent 14.0 ml and 6.0 ml H2O.
Table 3 Conversion of xylose into furfural using various acid catalysts.a
Entry
Furfural
Xylose
yield (%)
conversion (%)
Catalyst
1
No catalyst
2.0
4.2
2
HZSM-5
22.1
35.0
3
H2SO4b
73.0
98.1
4
HClc
70.6
100
5
AlCl3 ▪6H2O
58.5
100
6
PTSA ▪H2Oc
58.6
100
7
Amberlyst-15
61.2
85.4
8
PTSA-POM
80.4
98.8
a
Reaction conditions: 0.20 g different acid catalysts, 0.40 g xylose, 15.0 ml GVL, 1.5
ml H2O, 170 ℃ (30 min heating-up time), 10 min, 500 rpm. b
0.24 mmol H2SO4 (corresponding to 0.20 g PTSA-POM).
c
0.48 mmol HCl and PTSA (corresponding to 0.20 g PTSA-POM).
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
HIGHLGHTS: ● Copolymerization of PTSA and POM produces a strong solid acid catalyst. ● Furfural yield of 80.4% from xylose was achieved. ● 83.5% furfural yield and 19.5% HMF yield were obtained from raw corn stalk. ● The catalyst can be reutilized at least for five cycles.
S0960-8524(15)01383-8 http://dx.doi.org/10.1016/j.biortech.2015.09.104 BITE 15613
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
2 August 2015 19 September 2015 21 September 2015
Please cite this article as: Xu, Z., Li, W., Du, Z., Wu, H., Jameel, H., Chang, H-m., Ma, L., Conversion of Corn Stalk into Furfural Using a Novel Heterogeneous Strong Acid Catalyst in γ-Valerolactone, Bioresource Technology (2015), doi: http://dx.doi.org/10.1016/j.biortech.2015.09.104
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Conversion of Corn Stalk into Furfural Using a Novel Heterogeneous Strong Acid Catalyst in γ-Valerolactone Zhiping Xu a, Wenzhi Li b,*, Zhijie Du b, Hao Wu b, Hasan Jameel c, Hou-min Chang c, Longlong Ma d a
Department of Chemistry, University of Science and Technology of China, Hefei
230026, PR China b
Department of Thermal Science and Energy Engineering, University of Science and
Technology of China, Hefei 230026, PR China c
Department of Forest Biomaterials, North Carolina State University, Raleigh, NC
27695-8005, USA d
CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China *Corresponding author. Tel.: +86 0551 63600786 E-mail address: [email protected] (Wenzhi Li) ABSTRACT: A novel solid acid catalyst was prepared by the copolymerization of p-toluenesulfonic acid and paraformaldehyde and then characterized by FT-IR, TG/DTG, HRTEM and N2-BET. Furfural was successfully produced by the dehydration of xylose and xylan using the novel catalyst in γ-valerolactone. This investigation focused on effects of various reaction conditions including solvent, acid catalyst, reaction temperature, residence time, water concentration, xylose loading and catalyst dosage on the dehydration of xylose to furfural. It was found that the solid catalyst displayed extremely high activity for furfural production. 80.4% furfural yield
with 98.8% xylose conversion was achieved at 170 ℃ for 10 min. The catalyst could be recycled at least five times without significant loss of activity. Furthermore, 83.5% furfural yield and 19.5% HMF yield were obtained from raw corn stalk under more severe conditions (190 ℃ for 100 min). Keywords: furfural, xylose, corn stalk, PTSA-POM catalyst, γ-valerolactone 1. Introduction The problem of the greenhouse effect and global climate change due to the consumption of oil-based fuels, has attracted more and more attention all over the world (Zhang et al., 2012a). It has become an important research area for replacement of petroleum with non-fossil energy sources such as biomass, nuclear, solar and wind energy. Lignocellulosic biomass, the most abundant sustainable feedstock in the world, is the only non-fossil energy source which can be converted into liquid, solid and gaseous fuels, as well as other chemicals. It mainly consists of lignin (15-25%), cellulose (38-50%) and hemicellulose (23-32%) (Avci et al., 2013). The efficient production of biomass-derived fuels and high value-added chemicals requires the utilization of both hemicellulose and cellulose, which mainly contains of C5 and C6 sugars, respectively (Luterbacher et al., 2014; Gürbüz et al., 2013). Furfural is a versatile key derivative produced from pentosan-rich agricultural wastes such as wood chips, corn straws and bagasse. It can be used as an excellent solvent for many organic materials and as a building block chemical for the synthesis of many desired compounds such as furfuryl alcohol, 2-methylfuran (Yan et al., 2014a), succinic acid (Choudhary et al., 2013) and maleic acid (Guo et al., 2011).
Therefore, furfural has been identified as an attractive platform chemical for production of valuable bio-based chemicals and bio-fuels with huge market potential. Currently, furfural is produced in water using the most common homogeneous catalyst H2SO4 (Agirrezabal-Telleria et al., 2011). However, under these conditions, the furfural yield was quite low (only 45-55%) (Kim et al., 2012) due to the formation of humins in an aqueous solution of H2SO4. Hence, it is necessary to develop novel greener processes for producing furfural. In last decades, extensive studies on xylose and xylan (as model compound) dehydration have been performed with the objective of simplifying initial matrix (raw biomass) for furfural production. Mineral acids (Yemis et al., 2011) and metal chlorides (Zhang et al., 2014, 2013a) as homogeneous acidic catalysts for the production of furfural are efficient. However, these catalysts are corrosive, difficult to recover from reaction mixture and harmful to the environment. Furthermore, the post-processing is complicated and leads to large amounts of neutralization waste. In order to avoid the shortcomings of homogeneous catalysts, many heterogeneous solid catalysts have been developed for furfural production. Acidic zeolites and related modified zeolites such as H-mordenite, H-Beta zeolite, HUSY (Sahu et al., 2012), sulfonic acid functionalized mesoporous SBA-15 materials (Agirrezabal-Telleria et al., 2014) and MCM-41 (Zhang et al., 2012b), have been widely applied to catalyze the dehydration of xylose to furfural. Solid catalysts SO42-/TiO2-ZrO2/La+ (Li et al., 2014) and SO42-/ ZrO2-TiO2 (Zhang et al., 2012a) were also prepared for the conversion of xylose or corncob to furfural. But these catalysts can be deactivated easily due to the
carbon deposition over the active sites. Commercial resins Amberlyst-15 and Amberlyst-70 have also been applied to furfural production (Agirrezabal-Telleria et al., 2011, Zhang et al., 2013b). However, the both resins are limited to operating at low temperatures (Galletti et al., 2012). In recent years, a new type of solid Brønsted acid catalyst, biomass-based amorphous carbon which bears SO3H, COOH, and OH groups, has been extensively studied for biomass conversion processes (Hu et al., 2013; Yan et al., 2014b). The catalyst can be easily prepared from saccharides, lignin, cellulose and other biomass resources by incomplete carbonization and subsequent sulfonation. However, the sulfonation reaction is carried out at high temperature (＞ 160 ℃) with large amount of fuming sulfuric acid (15 wt% SO3) or concentrated sulfuric acid (98%) for the introduction of SO3H groups into the amorphous carbon surface. In addition, many other types of solid catalysts have also been tested such as mesoporous Nb 2O5 (García-Sancho et al., 2014), SAPOs (Bhaumik et al., 2014) and sulfonated graphene oxide (Lam et al., 2012). Besides catalysts, another special goal in furfural synthesis is to optimize the reaction system. One reaction system is a single-phase system such as ionic liquid (Zhang et al., 2013a; Zhang et al., 2013b), DMSO (Lam et al., 2011) and pure water (Lam et al., 2012) and the other is a mixture of water and an organic solvent creating a biphasic system, in which the organic solvent is used as the extraction phase. Organic solvents such as toluene, CPME, 1-butanol and MIBK (Bhaumik et al., 2014; Molina et al., 2012; Zhang et al., 2012b; Weingarten et al., 2010) have been used in this reaction system. In recent years, an environmentally friendly solvent γ-valerolactone
(GVL) has been extensively used for biomass conversion (Qi et al ., 2014; Alonso et al., 2013; Mellmer et al., 2014; Wettstein et al., 2012). Gürbüz et al. and Alonso et al. (2013) revealed that GVL could efficiently promote the conversion of xylose to furfural, and the yield was as high as 80%. Utilizing GVL as the reaction system significantly increases the conversion rate of xylose and decreases the degradation rate of furfural. Importantly, biomass-based GVL could be produced by the multistep processing of furfural (Bui et al., 2013). Recently, an easily prepared strong Brønsted acid catalyst has been prepared by copolymerization of p-toluenesulfonic acid (PTSA) and paraformaldehyde (POM) (Fan et al., 2010; Liang et al., 2008). However, the catalyst (denoted as PTSA-POM) can only be applied in organic solvents rather than aqueous solutions due to a poor water tolerance. It was observed that the calcination temperature has an important effect on the acidity and water resistance of the catalyst. In present work, the catalyst is modified through calcining at high temperatures. After finishing this treatment, water resistance of the material is improved remarkably. To the best of our knowledge, it is the first report that the catalyst can be used in aqueous solution and that this catalyst is the solid acid catalyst of choice for converting xylose, xylan and corn stalk into furfural. Interestingly, the novel catalyst displays high selectivity, strong acidity, high thermal stability. It also can be easily reused. This study focused on the acid-catalyzed conversion of xylose, xylan and corn stalk in GVL with the novel solid catalyst PTSA-POM. The effects of temperature, reaction time, and water concentration on the catalytic performance were evaluated.
Additionally, we compared different solvents and catalysts for xylose dehydration reaction. 2. Materials and Methods 2.1 Materials D-(+)-Xylose (≥99%), furfural (99%), 5-hydroxymethylfurfural (HMF, 99%), γ-valerolactone (98%), xylan (from corn cob, 90%), p-toluenesulfonic acid monohydrate (PTSA•H2O, 98.5%), paraformaldehyde (POM, AR), cyclopentyl methyl ether (CPME, 99%), NaOH aqueous solution (0.02M) and Amberlyst-15 (wet) were purchased from Aladdin Industrial Inc. (Shanghai, China). H2SO4 (96～98%, AR), HCl (36～38%, AR), AlCl3•6H2O (AR), 1,4-dioxane (AR), tetrahydrofuran (THF, AR), toluene (AR), dimethyl sulfoxide (DMSO, AR), 4-methyl-2-pentanone (MIBK, AR), n-butanol (AR), N,N-dimethylformamide (DMF, AR) and NaCl (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Zeolite catalyst HZSM-5 was purchased from The Catalyst Plant of Nankai University (Tianjin, China). All reagents were used without any purification. The corn stalk was collected from the north of Anhui province, China. The biomass was firstly ground to pass through a 40-mesh screen and then washed repeatedly with tap water to remove the clay from the surface. After washing process, the solid was dried in an oven at 80 ℃, until the weight remained constant. 2.2 Preparation of the catalyst The solid acid catalyst PTSA-POM was prepared following the procedures in the literatures (Fan et al., 2010; Liang et al., 2008) with slight modifications. Typically, a
mixture of PTSA•H2O (10.00 g) and H2SO4 (98%, 0.2 ml) was heated at 110 ℃ in a three-necked round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. When PTSA•H2O was completely melted, the required amounts of POM were added immediately. The reaction mixture was kept at 110 ℃ for 8 h, followed by 24 h at 130 ℃ to form a black solid. The black solid was filtered and washed repeatedly with deionized water until the filtrate reached pH 7.0. The resulting sample was dried at 120 ℃ for 12 h and then ground into a powder. The black powder obtained was then roasted at different temperatures for 6 h in a muffle to get the targeted catalyst. 2.3 Catalyst characterization The functional groups of fresh and used PTSA-POM were analyzed by Fourier transform infrared spectroscopy (FTIR) on a Nicolet 8700 (USA). Thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis was conducted from room temperature to 600 ℃ with a heating rate of 10 ℃/min under N2 flow using a TGA Q5000 (USA). High resolution transmission electron microscopy (HRTEM) images were taken on a JOEL-2010 microscope (Japan) at 200kV. Special surface area, pore volume and average pore width were estimated from N2 adsorption isotherm measured at -195.8 ℃ on Tristar Ⅱ 3020M, Micrometritics. Acidity of the material was determined through acid-base neutralization titration. The titration was carried out as follows: 0.100 g of the black solid and 20 ml of 2 M NaCl aqueous solution were stirred at room temperature for 24 h. The solids were filtered off using positive air pressure filtration and washed with 2 M NaCl aqueous solution (3×5 ml) and distilled water (3×5 ml). The combined filtrate was titrated with 0.02 M NaOH
standard solution using phenol red as an indicator (Margelefsky et al., 2008).
2.4 Catalytic tests The acid-catalyzed conversion of xylose, xylan and corn stalk to furfural by the novel solid catalyst was carried out in a magnetically stirred 25 ml autoclave, heated with an electronically controlled heating mantle. In a typical experiment, the required amounts of feedstock, catalyst, distilled water and GVL were loaded into the autoclave and sealed. The mixture was stirred at 500 rpm and heated from room temperature to the desired temperature over 30 minutes. Zero time was considered to be the instant the reaction temperature reached the target temperature. To end reactions, the reactor was immersed in cold water to reduce the reactor temperature. The liquid was filtered and analyzed using HPLC. 2.5 Analysis of furfural, HMF and xylose In monophasic systems, furfural and HMF were quantified by HPLC (Waters 515 pump) with Waters Symmetry®-C18 column (5μm, 4.6×150 mm) and an UV/Vis Detector (Waters 2489) at 280 nm. The temperature of column oven was maintained at 30 ℃. Mobile phase was methanol/distilled water (2/3, v/v) at a flow rate of 0.4 ml/min. In biphasic systems, the detector was transformed to a Refractive Index Detector (Waters 2414). Xylose was determined by means of the Waters XBridgeTM Amide column (5μm, 4.6×150 mm) and the Refractive Index Detector. Column oven and detector temperatures were maintained at 45 ℃. Mobile phase was acetonitrile/distilled water (3/2, v/v) with a flow rate of 0.4 ml/min. Authentic samples of xylose, furfural and HMF were used as standards, and calibration curves were used
for quantification. Xylose conversion, HMF yield, furfural yield and furfural selectivity were calculated as follows: Xylose conversion ＝ (1－moles of xylose in products / moles of initial xylose) × 100% Furfural yield (from xylose) ＝ (moles of furfural in products / moles of initial xylose) × 100% Furfural selectivity ＝ (furfural yield / xylose conversion) × 100% Furfural yield (from xylan or corn stalk) ＝ (moles of furfural in products / theoretical moles of xylose in xylan or corn stalk) × 100% HMF yield (from corn stalk) ＝ (moles of HMF in products / theoretical moles of glucose in corn stalk) × 100% 3. Results and discussion 3.1 Characterization of the solid acid catalysts A series of PTSA-POMs were synthesized under different conditions according to previous literatures (Fan et al., 2010; Liang et al., 2008). The present work found that the calcination temperature had an important effect on the acidity, water resistance and thermostability of PTSA-POM. Fig. 1A shows that the acid density values of the three kinds of catalyst (POM = 3.00, 4.00 and 5.00 g) were similar, decreasing slightly with higher calcination temperatures. Dehydration of xylose into furfural was carried out in GVL using PTSA-POMs (prepared at different calcination temperatures). After the reaction, the used catalyst was separated by filtration, washed
with distilled water, and then dried at 105 ℃ until the weight remained constant. Weighing the recovered catalyst shows the weight loss of PTSA-POM during the reaction. Weight loss of PTSA-POM calcined at low temperatures (120, 140, 155 ℃) was very high. An average weight reduction of 23% (36%, 24% and 9% weight loss) was achieved after the reaction. But at higher calcining temperatures (＞170 ℃), the weight loss was negligible. This was also indicated by the color of the reaction solution, which gradually faded as calcination temperature increased (Fig. S1). The filtrate color was black at 120 ℃ and 140 ℃, and it was brown at higher temperatures, such as 155 ℃, which indicated that an increase in calcination temperature resulted in a significant improvement of the water resistance of the catalyst. The weight loss and color changing might be related to the dissolution of black polymer solid (prepared at low temperature) with low degree of polymerization (DP). At high temperatures, the end groups of polymers reacted with each other, leading to the formation of larger long-chain molecules, resulting in a significant improvement of water resistance of the modified catalyst. Taking acidity and water resistance into account, 185 ℃ was chosen as the desired temperature for further studies. Furthermore, TG/DTG analysis could support that the thermal stability increased with increasing calcination temperature (Fig. 1B). A weight loss occurred at 100 ℃ was attributed to removal of water adsorbed on the catalysts. The materials were thermally stable below 250 ℃, and the weight decreased with a further increase in temperature. As depicted in Table 1, Fig. 1A and C, the effect of POM content on acidity and
thermostability of the material was studied by running neutralization titration and TG/DTG analysis. It was surprising that the POM content had almost no effect on the catalyst. On the one hand, H+ exchange values (Table 1), were between 2.2 mmol/g and 2.4 mmol/g, which were nearly constant taking into account experimental errors in the measurement. This was also demonstrated in Fig. 1A. When POM = 3.00, 4.00 and 5.00 g, the acid density values remained constant at the same calcination temperature. On the other hand, three kinds of catalyst (prepared using 20 wt%, 30 wt% and 50 wt% POM) were chosen for TG/DTG analysis. The TG/DTG curves of the three catalysts were difficult to separate from each other, which indicated that changing amount of POM did not significantly affect the thermostability of PTSA-POM (Fig. 1C). In further studies, the novel catalyst was synthesized using 3.00 g POM and 185 ℃ as the calcination temperature. 3.2 Solvent effects on the dehydration of xylose to furfural Although solvent dependence of the acid catalyzed dehydration of xylose to furfural has been observed, no comparative study has been reported for GVL under the same conditions. Thus, utilization of the novel solid catalyst PTSA-POM for xylose conversion in various solvents was investigated. The results were summarized in Table 2. GVL gave a furfural yield of 80.4% in 10 min at 170 ℃ with 98.8% xylose conversion (Table 2, Entry 1). High furfural yields were also obtained in dioxane (73.6%) and THF (67.9%). However, furfural yields were only 3.6% in DMF and 43.0% in DMSO with high xylose conversion under the same reaction conditions (Table 2, Entry 6, 9). This indicated that GVL, THF, and dioxane could increase the
conversion rate of xylose and decrease the degradation rate of furfural. DMSO and DMF accelerated the degradation of furfural and produced more humins as indicated by a deep brown color observed after the reactions in DMSO and DMF. Furfural yields in CPME (49.6%), toluene (46.8%) and MIBK (38.4%) (Table 2, Entry 4, 5, 7) were much lower than those in GVL, dioxane and THF. In the water-organic solvent two-phase system, organic solvent was only used in an extraction phase. In fact, when the dehydration reaction was carried out in pure water, the reaction rate was low and furfural was readily converted to humins. 3.3 Influence of various acid catalysts on furfural production Table 3 showed the dehydration results of xylose to furfural in four homogenous acids (H2SO4, HCl, PTSA, and AlCl3) and three heterogenous acids (HZSM-5, Amberlyst-15, and PTSA-POM) after reacting at 170 ℃ in GVL containing 10 wt% water for 10 min. A control experiment confirmed that only trace amount of furfural were obtained in the absence of a catalyst. Furfural yields obtained from xylose in the presence of HZSM-5, Amberlyst-15, H2SO4, HCl, PTSA, AlCl3, and PTSA-POM were 22.1%, 61.2%, 73.0%, 70.6%, 58.6%, 58.5%, and 80.4%, respectively. Over 85% conversion of the xylose was obtained in the presence of these acid catalysts except for HZSM-5. The results showed that PTSA-POM displayed the highest activity among all the tested catalysts, and the yield was as high as 80.4%. Other strong acid catalysts also showed activity toward the xylose dehydration with furfural yields ranging from 58% to 75%. In the case of HZSM-5, due to its low acidity, the observed furfural yield (22.1%) and xylose conversion (35.0%) were relatively
inferior under the investigated conditions. However, strong mineral acids were also unfavorable for the formation of furfural on account of furfural degradation reactions in the strongly acidic systems. As for PTSA-POM, the absence of free hydrogen ions decreased the rates of furfural degradation reactions, leading to high furfural yield. 3.4 Influence of reaction temperature on furfural production Subsequently, PTSA-POM was selected to test the effect of different reaction temperatures on furfural production by running the experiments in GVL (containing 10 wt% water) at 140, 150, 160, 170, 180 ℃ (Fig. 2A, B and C). In Fig. 2A, it was observed that high temperature facilitated the dehydration of xylose to furfural, in comparison to the initial furfural yields at different temperatures. Meanwhile a much shorter reaction time was needed at a higher temperature to reach the maximum value of furfural yield. At low temperature (140 ℃), the furfural yields increased with extended reaction time, but at higher temperatures (150, 160 and 170 ℃), the furfural yields gradually reduced after reaching the optimal values. This was because elevated temperatures not only increased the rate of xylose dehydration (Fig. 2B), but also enhanced the rate of furfural degradation. According to the experimental results, the optimal reaction temperature and retention time for xylose conversion was 170 ℃ and 10 min, respectively. These conditions were chosen for further studies. 3.5 Influence of water concentration on furfural production In this part, given the low solubility of xylose in pure GVL and the high price of GVL, it is necessary to investigate the effect of additional water on furfural production using the novel solid catalyst. Experiments were performed with different
water weight ratios ranging from 5 wt% to 30 wt% (based on GVL) (Fig. 2D, E and F). Furfural yields at various water concentration showed a similar tendency with time, which showed a sharp increase initially and subsequently decreased (Fig. 2D). When compared with furfural yield and xylose conversion at each time point with different water concentration, it was found that water concentration had a major impact on the reaction. The higher water concentration in GVL , the lower the furfural yield and the time required to reach the optimum yield of furfural was longer. For example, 82.3% furfural yield was obtained in the presence of 5% H2O at 170 ℃ for 5 min, but only a 56.0% furfural yield was measured with 30% H2O. This could be attributed to the fact that water is not favorable for the xylose dehydration reaction and can promote the degradation of furfural to side products. 3.6 Influence of xylose and PTSA-POM loading on furfural production In acid catalytic dehydration of xylose, starting xylose concentration has a remarkable effect on furfural yield. Here, the influence of xylose concentration was studied. 91.4% furfural yield were obtained at a xylose loading of 1.0% (Fig. 3A, all the xylose loadings were based on GVL). With an increase of xylose loading to 10.0%, furfural yield and selectivity sharply decreased to 65.6% and 69.0%, respectively. With increasing initial xylose concentration, both the furfural yield, selectivity decreased. This could be due to the fact that higher xylose concentration leads to an increase in the molecular collisions occurring between the formed furfural molecules or the furfural and xylose molecules. Self-polymerization of furfural or cross-polymerization between xylose and furfural would bring about undesirable
reactions, resulting in poor furfural yield and selectivity. The influence of catalyst loading amounts was also investigated in GVL using PTSA-POM as catalyst at 170 ℃. The catalyst loadings were based on GVL. Xylose conversion increased with increasing catalyst dosages from 0.13% to 2.67% (Fig. 3B). Increasing catalyst mass ratio from 0.13% to 1.07% had a positive effect on furfural yield and selectivity. Furfural yield increased from 35.4% to 80.6%. Subsequently, further increasing of the catalyst charge resulted in a slight decrease in furfural yield and selectivity. This may be because that the excessive catalyst was beneficial for furfural degradation reaction (such as fragmentation, resinification, and condensation) (Gürbüz et al., 2013), leading to formation of humins or other products. 3.7 Investigation of the catalyst reusability Reutilization of solid catalyst is of great importance for industrial processes. To examine the stability of PTSA-POM for the conversion of xylose into furfrual in GVL, a five-cycle experiment was performed (Fig. 4). In order to minimize unavoidable losses from manipulation of the catalyst after each run, 0.30 g catalyst (2 wt.%) were used in these recycling experiments. The catalyst was recovered after each run by filtration, repeatedly washed with distilled water, dried in an oven at 100 ℃ and then used in subsequent reaction under identical reaction conditions. 81.6% of furfural yield was obtained, the first time the catalyst was used, and the yield gradually decreased to 68.3% after five successive recycles. The conversions of xylose maintained at around 90% till the fifth run. Thus, the catalyst recycles very well, although both of the furfural yield and xylose conversion had decreasing trends with
the consecutive catalytic runs, which could be ascribed to a mass loss of the catalyst during filtration process and a slight leaching of -SO3H during reactions. As depicted by Fig. S2, when compared to the fresh catalyst sample, the 2nd and 5th used catalysts separated from the reaction mixture exhibited attenuated signals, which could be assigned to hydroxyls (O-H stretching vibrations at 3410 cm-1 and O-H bending vibration at 1400 cm-1) and -SO3H groups (SO3-H stretching vibration at 1180 cm-1 and O=S=O stretching vibration at 1040 cm-1). This result indicated that the catalyst underwent partial leaching of -SO3H groups. 3.8 Production of furfural or 5-HMF from xylan and corn stalk in GVL To compare the results obtained with xylose as feedstock, the reaction was carried out with corn cob-derived xylan as feedstock. Reaction temperatures were 160, 170 and 180 ℃ (30 min heating up time). In a typical reaction, 0.40 g xylan was placed in an autoclave, 15 ml GVL, 1.5 ml water and 0.20 g catalyst were added. After reaching the desired reaction temperature, stirring was increased to 500 rpm. The formation of furfural and side products was highly dependent on the reaction temperatures (Fig. 5A). The maximum yield of furfural (69.2%) was achieved at 170 ℃ in only 10 min, which was lower than that from xylose (80.4%). After reaching the optimum value, furfural yield decreased sharply over time. It was clearly seen that the conversion of xylan into furfural was significantly more challenging than the dehydration of xylose. Because xylan, a polymer of xylose, was first transformed into xylose by hydrolysis and then changed into furfural by dehydration. It is apparent from the work described above that the solid catalyst and the
reaction system (GVL containing 10 wt% water as a solvent) were highly effective for converting xylose and isolated xylan (as model compounds) into furfural. The next step was to process raw corn stalk (containing cellulose, hemicellulose and lignin) using the catalyst in an one-pot reaction system. The xylan and glucan contents in the used corn stalk were 19.5 wt% and 39.6 wt%, respectively (Zu et al., 2014). In this part, all the furfural yields and HMF yields were based on this initial compositional data. Fig. 5B illustrated that from real raw biomass a high furfural yield of 83.8% was obtained at 170 ℃ after 100 min . Furfural yield of 85.4% was also achieved at 180 ℃ after 80 min. Moreover, 86.3% was achievable for furfural at 190 ℃ after 40 min. The results were higher than those obtained from xylose and xylan as substrates. According to previous studies (Zhang et al., 2014), this phenomenon could be explained by the formation of furfural from cellulose in raw corn stalk. However, the calculation method of furfural yield was based on xylan in corn stalk, so furfural formed from cellulose led to the increase of furfural yield. In addition, compared to xylose and xylan, when corn stalk was used as a feedstock, the optimal reaction temperature for furfural production was 190 ℃ rather than 170 ℃ because of a high temperature facilitating destruction of the network of raw biomass. Interestingly, in these reactions, the formation of a small amount of HMF was also seen at 160 ℃ and 170 ℃ (Fig. 5C). When the reaction temperature was increased to 190 ℃, a surprisingly high HMF yield of 19.5% was obtained. The origin of HMF was attributed to the fact that, under these reaction conditions cellulose in corn stalk also underwent conversion. The next project will mainly focus on developing a magnetic
catalyst for treatment of corn stalk with a high selectivity and ease of catalyst recyclability. 4. Conclusion A novel catalyst PTSA-POM was prepared by the copolymerization of PTSA and POM. After high-temperature calcining treatment, the catalyst showed improved water resistance and higher thermal stability. PTSA-POM has advantages over common dehydration catalysts in GVL and gave a furfural yield of 80.4%. More importantly, PTSA-POM also exhibited excellent catalytic activity for furfural production from corn stalk and the highest furfural yield of 86.3% was obtained. We believe that the use of PTSA-POM catalyst in GVL/water solvent offers a promising approach for the efficient conversion of renewable biomass feedstocks.
Acknowledgements This project was financially supported by Science and Technological Fund of Anhui Province for Outstanding Youth (1508085J01), the National Basic Research Program of China (No.2012CB215302) and the Key Programs of the Chinese Academy of Sciences (No. KGZD-EW-304-2).
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Figure captions Fig. 1. Effect of calcination temperature and the amount of POM on acid density (A) or thermostability (B, C) of PTSA-POM catalyst. (B) TG/DTG curves of the catalysts prepared at different calcining temperatures; (C) prepared with different POM dosages. Fig. 2. Effect of temperature (A, B, C) and water concentration (D, E, F) on xylose (0.40 g) conversion to furfural in GVL (15.0 ml) catalyzed by PTSA-POM (0.20 g) at a stir speed of 500 rpm. 30 min heating-up time was required to achieve a desired temperature. For temperature treatments (A, B, C), H2O = 1.5 ml; for water concentration treatments (D, E, F), T = 170 ℃. Fig. 3. Effect of xylose loading (A) and PTSA-POM catalyst loading (B) on xylose conversion to furfural in GVL containing 10 wt% water (15.0 ml GVL, 1.5 ml H2O) at 170 ℃ for 10 min (30 min heating-up time) at 500 rpm. For xylose loading treatments (A), PTSA-POM catalyst = 0.20 g; for PTSA-POM catalyst loading treatments (B), xylose = 0.40 g. All the xylose and PTSA-POM weight ratios were based on GVL. Fig. 4. Reusability study of PTSA-POM. Reaction conditions: 0.40 g xylose, 0.30 g catalyst, 15.0 ml GVL, 1.5 ml H2O, 170 ℃, 10 min (30 min heating-up time), 500 rpm. After each run, the solid acid catalyst was separated by filtration and washed with distilled water, dried and then used in the subsequent reaction. Fig. 5. Effect of reaction temperature on xylan (A) and corn stalk (B, C) conversion to furfural and HMF in GVL catalyzed by PTSA-POM. Reaction conditions: 0.40 g
xylan (A) or corn stalk (B, C), 0.20 g catalyst, 15.0 ml GVL, 1.5 ml H2O, 30 min heating-up time, 500 rpm. Yields of furfural (B) and 5-HMF (C) were based on xylan content of 19.5% and cellulose content of 39.6%, respectively in the corn stalk used.
Tables and figures Table 1 Effect of the amount of POM on the acidity of PTSA-POM.
Entry
POM (wt%)
Acid density
Furfural
Xylose
H+b (mmol/g)
yield (%)c
conversion (%)c
a
1
20
2.4
77.9
98.1
2
25
2.4
78.0
97.6
3
30
2.2
77.4
96.7
4
35
2.2
76.5
95.0
5
40
2.3
76.2
96.3
6
45
2.4
76.0
96.0
7
50
2.3
75.8
95.1
8
60
2.3
74.5
96.3
a
POM weight ratios were based on PTSA ▪H2O (10.00 g), 185 ℃ calcination
temperature. b
Acid density values were determined through acid-base titration and calculated using
the amount of NaOH added to the corresponding material. c
Reaction conditions: 0.20 g catalyst, 0.40 g xylose, 15.0 ml GVL, 1.5 ml H2O,
160 ℃ (40 min heating-up time), 20 min, 500 rpm.
Tabe 2 Results of xylose dehydration experiments in diverse solvents by PTSA-POM.a
Entry
Furfural
Xylose
Furfural
yield %
conversion %
selectivity %
Solvent
1
GVLb
80.4
98.8
81.4
2
Dioxaneb
73.6
92.6
79.5
3
THFb
67.9
92.8
73.2
4
CPMEc
49.6
93.1
53.3
5
Toluenec
46.8
100
46.8
6
DMSOb
43.0
70.2
61.3
7
MIBKc
38.4
64.8
59.3
8
1-butanolc
11.3
46.9
24.1
9
DMFb
3.6
72.8
5.0
a
Reactions conditions: 0.20 g PTSA-POM, 0.40 g xylose, 170 ℃, 10 min (30 min
heating-up time), 500 rpm. b
Homogeneous system, organic solvent 15.0 ml containing 1.5 ml H2O.
c
Biphasic system, organic solvent 14.0 ml and 6.0 ml H2O.
Table 3 Conversion of xylose into furfural using various acid catalysts.a
Entry
Furfural
Xylose
yield (%)
conversion (%)
Catalyst
1
No catalyst
2.0
4.2
2
HZSM-5
22.1
35.0
3
H2SO4b
73.0
98.1
4
HClc
70.6
100
5
AlCl3 ▪6H2O
58.5
100
6
PTSA ▪H2Oc
58.6
100
7
Amberlyst-15
61.2
85.4
8
PTSA-POM
80.4
98.8
a
Reaction conditions: 0.20 g different acid catalysts, 0.40 g xylose, 15.0 ml GVL, 1.5
ml H2O, 170 ℃ (30 min heating-up time), 10 min, 500 rpm. b
0.24 mmol H2SO4 (corresponding to 0.20 g PTSA-POM).
c
0.48 mmol HCl and PTSA (corresponding to 0.20 g PTSA-POM).
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
HIGHLGHTS: ● Copolymerization of PTSA and POM produces a strong solid acid catalyst. ● Furfural yield of 80.4% from xylose was achieved. ● 83.5% furfural yield and 19.5% HMF yield were obtained from raw corn stalk. ● The catalyst can be reutilized at least for five cycles.