Accepted Manuscript Enhanced furfural production from raw corn stover employing a novel heterogeneous acid catalyst Wenzhi Li, Yuanshuai Zhu, Yijuan Lu, Qiyu Liu, Shennan Guan, Hou-min Chang, Hasan Jameel, Longlong Ma PII: DOI: Reference:

S0960-8524(17)31382-2 http://dx.doi.org/10.1016/j.biortech.2017.08.077 BITE 18686

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

15 June 2017 12 August 2017 14 August 2017

Please cite this article as: Li, W., Zhu, Y., Lu, Y., Liu, Q., Guan, S., Chang, H-m., Jameel, H., Ma, L., Enhanced furfural production from raw corn stover employing a novel heterogeneous acid catalyst, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.08.077

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Enhanced furfural production from raw corn stover employing a novel heterogeneous acid catalyst Wenzhi Lia, Yuanshuai Zhua,*, Yijuan Lua, Qiyu Liua, Shennan Guana, Hou-min Changb, Hasan Jameelb, Longlong Ma c,** a

Laboratory of Basic Research in Biomass Conversion and Utilization, University of

Science and Technology of China, Hefei 230026, PR China b

Department of Forest Biomaterials, North Carolina State University, Raleigh, NC

27695-8005, USA c

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] (Yuanshuai Zhu) **Co-Corresponding author E-mail address: [email protected] (Longlong Ma) Abstract: With the aim to enhance the direct conversion of raw corn stover into furfural, a promising approach was proposed employing a novel heterogeneous strong acid catalyst (SC-CaCt-700) in different solvents. The novel catalyst was characterized by elemental analysis, N2 adsorption-desorption, FT-IR, XPS, TEM and SEM. The developed catalytic system demonstrated superior efficacy for furfural production from raw corn stover. The effects of reaction temperature, residence time, catalyst loading, substrate concentration and solvent were investigated and optimized. 93% furfural yield was obtained from 150 mg corn stover at 200℃ in 100 min using

45 mg catalyst in γ-valerolactone (GVL). In comparison, 51.5% furfural yield was achieved in aqueous media under the same conditions (200℃, 5 h, and 45 mg catalyst), which is of great industrial interest. Furfural was obtained from both hemicelluloses and cellulose in corn stover, which demonstrated a promising routine to make the full use of biomass. Keywords: furfural, corn stover, heterogeneous acid catalyst, SC-CaCt-700 1. Introduction Lignocellulosic biomass typically contains over 50 wt% carbohydrates, which is regarded as a rich source of sustainable feedstock for diverse bio-based products (Zhang et al., 2016a; Kabir and Hameed., 2017). Among these products, furfural is deemed as an important platform chemical (Mariscal et al., 2016) due to its versatility to produce a broad range of useful chemicals and biofuels, such as furfuryl alcohol (Jiménez-Gómez et al., 2016; Halilu et al., 2016), tetrahydrofurfuryl alcohol (Yang et al., 2016), 2-methlfuran (Hutchings et al., 2017) and γ-valerolactone (GVL) (Bui et al., 2013; Hernández et al., 2016; Zhu et al., 2016). As a result the U.S Department of Energy has ranked furfural as one of the top 10 bio-based platform chemicals derived from sugars (Bozell and Petersen, 2010). In spite of many improvements in recent years, current commercial production of furfural still relies on traditional technology developed by Quaker Oats (Brownlee and Miner, 1948), employing H2SO4 as catalyst and hot steam as a stripping agent. The disadvantages associated with the process are obvious, such as high energy consumption, high equipment cost and corrosion and environmental issues caused by

the discharge of the acidic effluents, which hinders the sustainable development of the furfural industry (García-Sancho et al., 2014). Besides, furfural yield was unsatisfactory due to the high propensity of furan compounds to polymerize in this process (Hu et al., 2014). To overcome these obstacles, it’s necessary to explore greener and more efficient catalytic systems for the conversion of carbohydrates to furfural. To suppress the polymerization of furfural in acidic aqueous media, a wide range of solvent systems have been tested and optimized for t e con ersion o s gars into r ral incl an

ing single-p ase system single sol ent or mixt re o misci le sol ents

ip asic system mixt re o immisci le sol ents s c as al ylp enol

r

am et al

et al., 2012), alcohols (Iglesias et al., 2016), water/GVL

(Xu et al., 2015), water/toluene (Qing et al., 2017), water/dichloromethane (Deng et al., 2016), and water/cyclopentylmethyl ether (Le Guenic et al., 2016). Particularly, the interactions between xylose, furfural and solvents were also discussed for 20 solvents (Hu et al., 2014). More importantly, a green solvent γ-valerolactone (GVL) has attracted increasing attention since its superior efficiency in biomass utilization was reported (Gürbüz et al., 2013; Alonso et al., 2013; Luterbacher et al., 2014). γ-valerolactone (GVL) is a green solvent that can be produced from furfural (Bui et al., 2013; Hernández et al., 2016; Zhu et al., 2016). And it is also known to stabilize intermediates and products during conversion of sugars to furfural (Alonso et al., 2013). Besides solvent optimization, another furfural research focus involves the

development of novel catalysts. Yemis et al. studied furfural formation by microwave-assisted reaction using HCl as a catalyst, which resulted in a furfural yield of 48.4%, 45.7% and 72.1% from wheat straw, triticale straw and flax shives, respecti ely Yemiş an

a a

Kim et al

in estigate t e selecti e

furfural production from corn stover, switch grass and poplar in the presence of maleic acid with microwave heating, giving furfural yield of 61%, 57% and 56 respectively. Recently, several metal chlorides were found to be effective for the direct transformation of biomass to furfural using either a biphasic or a single-phase system, The above system showed a favorable furfural yield of nearly 80% from corncob (Zhang et al., 2013; Zhang et al., 2014; Wang et al., 2015). Despite these progresses, the further industrial application is still a challenge because of the toxicity, corrosion and environmental pollution caused by homogenous systems. In contrast with homogenous catalytic processes, heterogeneous catalysts are more desirable for potential furfural production since it can be reused repeatedly. Commonly reported solid acid catalysts, such as SO24-/TiO2-ZrO2/La3+ (Li et al., 2014), SO42-/SnO2- MMT (Qing et al., 2017), SAPO-44 (Bhaumik et al., 2015), commercial resins (Jeon et al., 2016; Le Guenic et al., 2016), and modified zeolites (Zhang et al., 2017a; Zhang et al., 2017b), sulfonated carbonaceous materials (Xu et al., 2015; Zhang et al., 2016b; Zhang et al., 2017c), afforded superior furfural yields and much easier recyclability over homogeneous catalysis. Among these heterogeneous catalysts, sulfonated mesoporous carbonaceous materials, typically prepared by sulfonation of carbon materials, exhibited encouraging selectivity for the

production of furan compounds. Usually, the commonly used mesoporous carbon material were prepared by hard template or soft template method. However, the remove of these templates via HF or calcinations always bring security risks and environmental issues. Besides, these carbon solid catalysts were traditionally sulfonated by concentrated H2SO4 or chlorosulfonic acid, which was inefficient and could cause collapse and deconstruction of mesoporous structure within carbon support. In order to overcome these disadvantages, it is necessary to develop a more environmental friendly and inexpensive method to prepare carbonaceous solid acid catalyst. Recently, Ferrero et al. reported the preparation of mesoporous carbon in an one-step process via the direct carbonization of organic salts, which was easy to prepare and showed promising applications as electrode materials in batteries or supercapacitors (Ferrero et al., 2015). Inspired by their work, we suppose that the frequently used electrode material also showed potential in catalysis applications in view of its simple preparation procedures and inexpensive cost as well as well-developed mesoporous framework. Thus, we prepared the mesoporous carbon material with slight modification under different carbonization temperature, followed by sulfonation with sulfanilic acid in water under atmospheric conditions based on our previous study (Zhu et al., 2017), which resulted in the novel carbon solid acid catalyst . Compared with similar catalysts, the sulfonation method was more efficient and avoided serious damage on the mesoporous structure, which enabled the catalyst to possess a larger specific surface area, average pore diameter and pore volume as well as higher SO3H density. In addition, it was supposed to be more environmental

friendly and inexpensive because the mesoporous carbon material (C-CaCt) was synthesized without the use of hard templates or soft templates. To our knowledge, no publications have been reported for the application of this catalyst in biomass utilization. This study focuses on the enhanced furfural production from crude corn stover catalyzed by a novel strong heterogeneous acid catalyst. The novel catalyst was synthesized and characterized by various instruments. The efficacy and reusability of the catalyst was investigated in the process of co-converting cellulose and hemicelluloses in corn stover into furfural. In addition, we also compared the performance of this novel catalyst in various solvents including water with respect to furfural yield. 2．Materials and methods 2.1. Materials D-(+)-xylose (98%), furfural (99%), 5-HMF (99%), calcium citrate (AR, 99%), sulfanilic acid (AR, 99.5%), isoamyl nitrite (95%) were purchased from Aladdin ang ai C ina γ-valerolactone (95%) was bought from LangFang Hawk Technology and Development Co., Ltd (LangFang, Hebei, China). Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All reagents were used as received without further purification. Corn stover was collected from a farmland (Mengcheng, Anhui, China) and ground into particles to a size of 40 mesh, then dried in an oven to constant weight at 80℃. 2.2. Composition analysis of raw corn stover

The compositional analysis of the corn stover used in this work was conducted according to the Laboratory Analytical Procedures established by the National Renewable Energy Laboratory (Sluiter et al., 2008; Sluiter et al., 2005). The typical procedures are described in supporting information in details. The raw corn stover contains 31.6% glucan, 20.5% xylan, 2.1% other glycan, 19.4% acid-insoluble lignin, 3.4% acid-soluble lignin, 9.5% extractive,8.7% ash, and 4.8% other composition, which was close to reported values (Xu et al., 2016). 2.3. Synthesis of the catalyst The amorphous porous carbon was prepared according the reported method with minor modifications (Ferrero et al., 2015). In the modified procedure, calcium citrate (CaCt) was calcined at 600℃, 700℃ and 800℃ under N2 atmosphere for 1 h in a horizontal tube furnace (OTF-1200X, HEFEI KE JING MATERIALS TECHNOLOGY CO., LTD.) with a heating rate of 3℃/min. The resulting calcined calcium citrate was immersed in dilute hydrochloric acid overnight with stirring and then washed with excess distilled water, which was donated as C-CaCt-X (X=600, 700, 800). The solid carbon (C-CaCt-600, C-CaCt-700, C-CaCt-800) was collected by filtration and dried at 80℃ for subsequent use. The sulfonation of C-CaCt-X (X=600, 700, 800) was conducted based on the reported method (Price and Tour, 2006). Specifically, 1 g C-CaCt-X (X=600, 700, 800), 4 g sulfanilic acid and 2 g isoamyl nitrite were loaded into a 250 ml roundbottom flask with 150 ml distilled water. Then, the flask was heated to 80℃ in an oil bath and maintained for 13 h under magnetic stirring with a condenser. Ultimately, the

sulfonated carbonized calcium citrate (SC-CaCt-600, SC-CaCt-700, SC-CaCt-800) was separated by filtration and washed with water, ethanol, and acetone until the filtrate was clear. The filtrate residue was sonicated in 20 mL DMF for 10 min and rinsed with acetone, which was then collected and dried in an oven at 80℃ overnight. 2.4. Characterization of the catalyst Scanning electron microscopy (SEM) images were recorded on a SIRION 200 instrument (FEI Company, USA), whereas transmission electron microscopy (TEM) images were taken using a JEM 2011 instrument (JEOL, Japan). The functional groups of samples were measured by FT-IR spectra (Nicolet 8700 instrument, USA) using KBr disks. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific ESCALAB 250Xi device (Thermo-VG Scientific, UK). The elemental composition was measured by an Elementar model Vario EL III (Elementar Analysensysteme GmbH, Germany). The content of calcium of the catalyst was measured by ICP-AES (Optima 7300 DV, Perkinelmer, USA). The N2 sorption isotherms were measured using a Tristar II 3020 equipment at -196℃ (Micromeritics, USA). The specific surface area of the samples was calculated by the Brunauer–Emmett–Teller (BET) method and the pore diameter was calculated by the Barrett–Joyner–Halenda (BJH) method. 2.5. Procedure for the conversion of corn stover into furfural and 5-HMF Typically, the experiments were carried out in a 15 mL sealed thick-wall glass tube with 45 mg catalyst, 150 mg corn stover and 7 mL solvent under magnetic stirring. The tube was heated to a given temperature and maintained for different residence

times in a preheated oil bath. After the reaction, the reactor was cooled by running water. Finally, the products were collected by filtration and stored in a refrigerator for subsequent analysis. 2.6. Separation of catalyst from solid residue After each reaction, the mixture of solid residue and catalyst was collected by filtration. Then, the mixture was moved to a sieve of 400 mesh and rinsed with abundant acetone. The used catalyst (SC-CaCt-700) would pass through the sieve pore with the flow of acetone, whereas, the solid residue from corn stover was left on the sieve due to its bigger particle size, which achieved the separation of catalyst from solid residue. Ultimately, the catalyst was recovered by filtration from the mixture of catalyst and acetone, which was dried at 90℃ for several hours prior to reuse. 2.7. Quantification analysis for products The products were diluted and detected by HPLC (LC-2010AHT, SHIMADZU) equipped with a C18 column (XORBAX Eclipse XDB-C18, Agilent) and an UV detector (SPD-10A, SHIMADZU) at 280 nm. Methanol/water (2/3 v/v) was used as the mobile phase at a flow rate of 0.4 mL/min and column temperature was maintained at 30℃. Furfural and 5-HMF yield were calculated based on following equations: Furfural yield (from corn stover) = (moles of furfural produced / moles of starting xylan in corn stover) × 100% Furfural yield (from xylose) = (moles of furfural produced / moles of starting xylose) × 100%

Furfural yield (from fructose) = (moles of furfural produced / moles of starting fructose) × 100% Furfural yield (from glucose) = (moles of furfural produced / moles of starting glucose) × 100% Furfural yield (from cellulose) = (moles of furfural produced / moles of starting cellulose) × 100% 5-HMF yield = (moles of HMF produced/moles of starting glucan in corn stover) × 100%. 3. Results and discussion 3.1. Characterization of the catalyst Three kinds of catalysts were prepared under calcinations temperatures of 600℃, 700℃, 800℃ respectively. Table 1 shows the textural parameters and acid density of the different catalyst samples. All samples were observed to possess a well-developed porous structure and good SO3H density. Specifically, SC-CaCt-800 shows the strongest acid strength (2.62 mmol/g). SC-CaCt-700 possesses the most developed mesoporous structures with a specific surface area of 921 m2/g, pore volume of 2.57 cm3/g and an average diameter of 11.16 nm. Comprehensively considering the physical structures and acid density, 700℃ was selected as the optimal calcination temperature for the production of the catalyst. The empirical formula of C-CaCt-700 and SC-CaCt-700 was found to be C7.50H1.31O0.51N0.04 and C5.07H3.66O1.01N0.03 (SO3H) 0.23 based on elemental analysis. The content of calcium were measured by ICP-AES for both the novel catalyst and

its carbon support (before acid wash). The carbon support before acid wash contains abundant calcium while the SC-CaCt-700 only contains trace amount of calcium, which suggests that almost all calcium was removed during acid wash. Considering its trace amo nt it was elie e t at calci m i n’t play a catalytic role in t e reaction. The morphologies of C-CaCt-700 and SC-CaCt-700 were investigated by TEM and SEM, dense mesopores and interlaced channels were randomly distributed within C-CaCt-700 and SC-CaCt-700, which can be ascribed to irregular diffusion of gas during carbonization as well as the removal of CaCO3 and CaO via acid washing (Ferero et al., 2015). Besides, sulfur was distributed evenly within the catalyst by comparing the mappings of C, O, S, which is beneficial for inhibiting possible side reactions caused by excess acid sites in one pore. FT-IR spectrum showed the characteristic peaks at 1006 cm-1 and 1037 cm-1 together with bands at 1126 cm-1, 1182 cm-1 and 1221 cm-1, verifying the existence of SO3H within the catalyst (Zhang et al., 2016b). XPS spectra of SC-CaCt-700 showed single S2p peak, which verified that all S was confined to SO3H (Wang et al., 2016a). 3.2. Effects of reaction temperature and residence time on furfural and 5-HMF production The effects of reaction temperature and residence time on furfural and 5-HMF production from corn stover in GVL were investigated and the results are shown in Fig. 1. It is apparent that both temperature and time have a positive impact on furfural yield in the temperatures and times tested (Fig. 1A). A furfural yield of 93% was

achieved at 200℃ in 100 min, which is comparable to the best results reported in the literature for the direct conversion of corn stover to furfural, indicating that the novel catalyst was highly efficient and selective for such a conversion. Besides, yield loss reactions were e ecti ely s ppresse since

r ral yiel

i n’t ecrease e en w en

the time was increased to 180 min. Compared to furfural production, a lower 5-HMF yield (10% vs 93%) was obtained (Fig.1B), and a longer time (140 min vs 100 min) was required to reach the peak value, mainly because the crystalline structure of cellulose was less acid-labile than hemicellulose in corn stover. Further prolonging the time beyond the peak value, caused the 5-HMF yield to decrease at all temperatures possibly caused by rehydration of 5-HMF or condensation between furan compounds (Wang et al., 2016b). Controlled trials were conducted using no catalyst, C-CaCt-700 and SC-CaCt-X (X=600, 700, 800) and the results are shown in Table 2. Since only a trace amount of furfural was measured in the absence of the novel catalyst, the novel catalyst was considered to be of significance for the efficient production of furfural from corn stover. In addition, SC-CaCt-700 exhibited the best performance among the tested catalysts under the same reaction conditions, which verified that 700℃ was the optimal carbonization temperature. Although the acid density of SC-CaCt-700 was smaller than that of SC-CaCt-800, its more developed mesoporous structure could promote the adsorption and diffusion of reactants, which resulted in the better performance in controlled experiments. Also, shown in Table 2 is the comparison of this work with previous publications, where the performance of SC-CaCt-700 was

superior to similar previous results. Recently, converting glucose and fructose to furfural has been proposed and shown to be feasible (Cui et al., 2016; Gürbüz et al., 2013; Zhang et al., 2017a; Zhang et al., 2017b), which was illustrated as figure 4. To verify the co-conversion of hemicellulose and cellulose in this work, confirmation experiments were done employing model compounds as substrate under the same conditions. As shown in Table 3, fructose, glucose and cellulose resulted in moderate furfural yields of 17.7%, 18.6% and 21.8%, respectively, at 200℃ for 100 min, suggesting that furfural was derived partially from cellulose in the direct conversion of corn stover. No 5-HMF was detected probably because of the conversion and polymerization of 5-HMF to other products under such severe conditions (Wang et al., 2016b). However, at low temperature of 180℃, small amount of 5-HMF did form as shown in Table 3. Obviously, the co-conversion of hemicellulose and cellulose into furfural brings several advantages such as higher furfural yield and improved resource utilization, less solid residue and easier product separation. Therefore, the catalyst developed here is promising for furfural production, especially for the co-conversion of hemicelluloses and cellulose in corn stover. Further evidence for the co-conversion of cellulose and hemicellulose will be discussed later. 3.3. Production of furfural and 5-HMF from corn stover under various catalyst loading and substrate dosage The effect of catalyst loading was investigated at 200℃ for 100 min using 150 mg corn stover by varying the catalyst loading from 10-60 wt% (catalyst loading: mass

ratio of catalyst/corn stover). Usually, a higher furfural yield is expected with a higher catalyst dosage. However, excessive amounts of acid would also accelerate some yield-loss reactions (fragmentation, condensation and resinification) by the generation of humins or other by-products (Gürbüz et al., 2013; Xu et al., 2015), which limits the final yield of products. As shown in Fig. 2A, increasing catalyst loading from 10 to 40 wt% of increases the yield of furfural from 36% to 94%. Further increase in catalyst dosage results in the decrease of furfural yield. The yield of 5-HMF, on the other hand, increases rapidly from 10 to 30 wt% and decreases rapidly above the catalyst loading of 30 wt%. At the catalyst loading of 50 wt% of corn stover and above, no 5-HMF was detected, indicating that 5-HMF is not stable at high catalyst loading. Thus, pure furfural production can be achieved by using a catalyst loading of 50 wt%, as the yield of furfural at this loading is still high as shown in Figure 2A. However, the formation of humins and other insoluble by-products may hinder catalyst recycling. Experiments with various concentration of corn stover were also carried out as a higher loading of substrate definitely improves the economic efficacy in industrial production.

As shown in Fig 2B (substrate loading: mass ratio of corn stover/GVL),

the highest yield of furfural and 5-HMF was obtained at the substrate loading of 2.14 wt%, which were 93% and 11%, respectively. Increasing the substrate concentration from 2.14 wt% to 3.0 wt%, the yield of furfural decreased from 93% to 84%. Similar decrease of 5-HMF was also observed with increasing corn stover loading. The phenomenon might be ascribed to the fact that higher concentration of substrate increased the possibility of molecular collision in reactions, which was beneficial for

the occurrence of self-condensation within products or cross-polymerization between products and reactants (Zhang et al., 2013; Danon et al., 2014). However, it is worth noting that furfural yield was still very high (84%) although the dosage of corn stover was raised to 3% only with 45 mg catalyst (Fig 2B). Thus, further investigation was conducted employing much higher substrate concentrations (5 wt% and 10 wt% of corn stover/GVL) and the results are shown in Table 4. Surprisingly, 72.9% furfural yield was obtained with 10 wt% corn stover at 200℃ for 100 min using 31.4 wt% catalyst. The results suggest the superior efficacy of SC-CaCt-700 as an acid catalyst even using 10 wt% corn stover concentration, which is beneficial for reducing cost in industrial production. 3.4. Furfural production from corn stover in diverse solvents Solvent plays a vital role on furfural production from corn stover because it not just affects the dispersion of corn stover, but also influences the distribution, degradation and isolation of products (Hu et al., 2014). To estimate comprehensively the performance of SC-CaCt-700 and reduce cost, different organic solvents and water were studied in the same reaction conditions and results are shown in Fig. 3. γ- alerolactone

V

an γ-butyrolactone (GBL) gave furfural yields of 93% and

89% respectively, which were superior than those in dioxane (18%), DMF (3%) and THF (17%). Despite of the high furfural yield in GBL, GVL is still preferred in biomass conversion applications mainly because of its good properties and sustainability (Bui et al., 2013; Hernández et al., 2016; Zhu et al., 2016). Aqueous system showed a moderate furfural yield of 25%, which prompted us to make a

further study about furfural production from corn stover in aqueous media and the results are shown in Table 5. Because of stronger polarity of water favoring the polymerization between furans (Maldonado et al., 2012), aqueous media gave a lower furfural yield compared to that in GVL. Even so, a good furfural yield of 51.5% was achieved at 200℃ for 300 min using 45 mg SC-CaCt-700 in aqueous media, which was comparable to other reported furfural formation in H2O (Morais et al., 2016; Danon et al., 2014; Wang et al., 2015; Bernal et al., 2014). Given that water as an inexpensive, green and abundant solvent, the results are promising and deserve further study for future industrial production opportunities. 3.5. Catalyst reusability In industry, the recycling of heterogeneous acid catalyst is of great significance to improve profitability and sustainability. To examine the reusability of SC-CaCt-700, a five-run consecutive recycling experiment was firstly studied using model compound as substrate and the results are shown in Table 6.After each run, catalysts were filtrated and washed with water, ethanol and acetone repeatedly. Then, the recycled SC-CaCt-700 was dried at 90℃ for several hours prior to reuse. Damped peaks of reused catalyst in FT-IR spectrum verified a slight leaching of SO3H after the 5th reuse. Although some acid sites were leached, the furfural yield remained stable with slight fluctuations in all recycling experiments, indicating satisfactory hydrothermal stability and strong acid density of SC-CaCt-700 during the conversion of xylose to furfural. It is also observed that only 75% furfural yield was obtained from pure xylose, as compared with 93% furfural yield from corn stover. These results give further

evidence for the co-conversion of cellulose and hemicellulose in corn stover to furfural. On the other hand, the catalyst reusability for corn stover conversion also needs to be tested, which is totally different from that of model compound conversion because more humins formed in the direct conversion of raw biomass. As depicted in Fig 1, the highest furfural yield was obtained at 200℃ for 100 min, while the furfural yield increased slowly after 40 min, which suggested that the reaction after 40 min was inefficient and uneconomical. Worse still, the prolonging reaction time could generate more humins and deposits into catalyst, which was the main cause for catalyst deactivation. Thus, further optimization of reaction conditions was necessary to ensure both reusability of catalyst and profitability of the process. After several experiments, a furfural yield of 95% was achieved at 200℃ for only 10 min with 150 mg catalyst and 50 mg corn stover in 7 ml GVL, which was as good as that of 93%. The optimized reaction condition brings two main advantages. Firstly, less energy was consumed due to the shorter reaction time, which lowered the cost of the process. Secondly, the catalyst reusability was effectively promoted as less humins and deposits was generated. Although more catalyst was employed, the dosage is still lower than similar conversion (Zhang et al., 2017a). The preparation of the catalyst, on the other hand, is easy and inexpensive, which won’t significantly increase the cost. Therefore, the reusability test for corn stover conversion was conducted under further optimized reaction conditions and the results are shown in Table 6. Obviously, the catalyst reusability for corn stover conversion is worse than that for xylose conversion.

Corn stover contains complicated components (hemicelluloses, cellulose, lignin, mineral salts…), which may result in complex products and promote the generation of humins. The generated humins as deposits would block some channels within the catalyst, which hindered the access between the reactants and acid sites and reduced the activity of catalyst. For higher temperatures (190℃ and 200℃), a trend of gradual decline for furfural yield appeared during five recycling runs. Actually, higher temperatures generated more humins, which resulted in increasing deposits within catalyst after each run. As a consequence, the catalytic activity of the catalyst gradually decreased instead of remaining stable. For low temperatures (170℃ and 180℃), a sharp decline of furfural yield was observed just after the 1 st run, while the furfural yield remained stable during the rest runs. It is difficult for reactants to break through the barrier of deposits within catalyst in lower temperature because of the weaker molecular movement. Thus, the catalytic activity would sharply decrease and only maintain a relatively low activity once the deposits were formed , which explained the sharp decrease of activity after 1st run and stable but low activity in the rest runs. Moreover, the furfural yield sharply decreased from 94.4% to 27.2% only after once reuse when higher substrate concentration (150 mg corn stover) was used, indicating its poor reusability for high substrate concentration. All in all, the reusability of the catalyst was acceptable and still need to be improved, especially for higher substrate concentration. 4. Conclusion The novel heterogeneous acid catalyst (SC-CaCt-700) with high specific surface

area and SO3H density was prepared under moderate conditions. SC-CaCt-700 was found to be highly capable of direct conversion of raw corn stover into furfural, giving enhanced furfural yield of 93% at 200℃ for 100 min in GVL. GVL and GBL gave superior performance among the tested solvents. In addition, 51.5% furfural yield was obtained from corn stover in water, which is of potential industrial interest. The developed strategy of co-converting hemicellulose and cellulose into furfural offers a promising approach for the sufficient utilization of raw biomass. E-supplementary data for this work can be found in e-version of this paper online. Acknowledgements This study was financially supported by the State Key Program of National Natural Science Foundation of China (51536009), Science and Technological Fund of Anhui Province for Outstanding Youth (1508085J01) and the National Key Technology R&D Program of China (NO. 2015BAD15B06).

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Figure captions Fig. 1. Effects of reaction temperature and residence time on furfural and 5-HMF production from corn stover in GVL. Reaction conditions: 45 mg catalyst, 150 mg corn stover, 7 ml GVL, 600 rpm. Fig. 2. Effects of catalyst loading (A) and substrate concentration (B) on furfural and 5-HMF production from corn stover. Reaction conditions: 200℃, 100 min, 7 ml GVL, 600 rpm, 150 mg corn stover in (A), 45 mg SC-CaCt-700 in (B). Catalyst loading: mass ratio of catalyst/corn stover; Substrate concentration: mass ratio of corn stover/GVL. Fig. 3. Solvent effect on furfural production from corn stover. Reaction conditions: 45 mg SC-CaCt-700, 150 mg corn stover, 7 ml solvent, 200 ℃, 100 min, 600 rpm. Fig. 4. Plausible reaction pathway for the conversion of hexoses to furfural.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Tables: Table 1. Textural parameters and strong acid density of the catalysts SBET(m2/g)

Va (cm3/g)

Db (nm)

SO3H densityc (mmol/g)

C-CaCt-600

1240

3.15

7.74

0

C-CaCt-700

1410

4.77

11.43

0

C-CaCt-800

1619

3.48

8.15

0

SC-CaCt-600

893

1.65

7.42

2.18

SC-CaCt-700

921

2.57

9.59

2.31

SC-CaCt-800

819

1.75

8.14

2.62

Entry

a

Pore volume

b

Average diameter

c

Calculated based on elemental analysis

Table 2. Furfural production from biomass using various catalysts Catalyst

Solvent

Reaction

Substrate

conditions

Furfural

References

yield (%)

No catalyst

GVL

200℃, 100 min

Corn stover

trace

This work

C-CaCt-700

GVL

200℃, 100 min

Corn stover

1.9

This work

SC-CaCt-600

GVL

200℃, 100 min

Corn stover

89.3

This work

SC-CaCt-700

GVL

200℃, 100 min

Corn stover

93

This work

SC-CaCt-800

GVL

200℃, 100 min

Corn stover

91.6

This work

PTSA-POM

GVL/

190℃, 100 min

Corn stalk

83.5

Xu et al.,

H2O SPTPA

GVL

2015 175℃, 30 min

Corn cob

73.9

Zhang

et

al., 2017c SO42/SnO2-M Toluene 190℃ 20 min MT/NaCl

/H2O

FeCl3

GVL

185℃, 100 min

Corn cob

66.1

Qing et al., 2017

Corn cob

79.6

Zhang al., 2014

et

Table 3. Furfural production from diverse substrates using SC-CaCt-700 in GVLa Substrate

Reaction conditions

Solvent

Furfural

5-HMF

yield (%)

yield (%)

150 mg Fructose

200℃, 100 min

7 mL GVL

17.7

nd

150 mg Glucose

200℃, 100 min

7 mL GVL

18.6

nd

150 mg Cellulose

200℃, 100 min

7 mL GVL

21.8

nd

150 mg Glucose

180℃, 30 min

7 mL GVL

19.1

2.7

150 mg Cellulose

180℃, 30 min

7 mL GVL

11.75

2.7

a

45 mg SC-CaCt-700 was used.

nd: not detected.

Table 4. Furfural production using higher corn stover concentration. a Corn stover

Catalyst

Furfural yield (%)

5-HMF (%)

Concentration (wt%)b

loading (wt%)c

5

12.8

45.0

0.9

5

17.1

66.0

2.3

10

8.5

23.7

1.1

10

14.2

52.5

1.9

10

20.0

64.7

3.6

10

25.7

69.4

7.5

10

31.4

72.9

7.3

10

37.1

63.5

3

10

42.8

46.0

0

a

Reaction conditions: 200℃, 100 min, 7 mL GVL.

b

Corn stover concentration: mass ratio of corn stover/GVL

c

Catalyst loading: mass ratio of catalyst/corn stover

Table 5. Furfural and 5-HMF production from corn stover in H2Oa Catalyst

Reaction conditions

Furfural yield (%) 5-HMF yield (%)

45 mg SC-CaCt-700

200℃, 140 min

38.3

1.6

45 mg SC-CaCt-700

200℃, 180 min

43.8

1.6

45 mg SC-CaCt-700

200℃, 240 min

46.4

2.9

45 mg SC-CaCt-700

200℃, 300 min

51.5

3.3

45 mg SC-CaCt-700

200℃, 360 min

50.7

4.4

45 mg SC-CaCt-700

200℃, 420 min

48.5

3.0

45 mg SC-CaCt-700

180℃, 300 min

18.0

-

45 mg SC-CaCt-700

190℃, 300 min

21.6

-

60 mg SC-CaCt-700

200℃, 300 min

34.5

1.9

75 mg SC-CaCt-700

200℃, 300 min

17.8

1.5

90 mg SC-CaCt-700

200℃, 300 min

12.9

1.0

a

For all experiments: 150 mg corn stover and 7 mL GVL were used.

Table 6. Reusability study of SC-CaCt-700 Reaction conditionsa

Furfural yield (%) 1st run

60 mg catalyst, 150 mg xylose, 74.9

2nd run

3rd run

4th run

5th run

75.4

76.6

77.3

76.5

88.1

86.1

70.2

61.0

78.0

74.3

60.6

58.3

53.3

52.0

51.4

49.5

45.3

43.7

43.5

42.1

27.2

-

-

-

170℃, 10 min 150 mg catalyst, 50 mg corn 95.0 stover, 200℃, 10 min 150 mg catalyst, 50 mg corn 89.5 stover, 190℃, 10 min 150 mg catalyst, 50 mg corn 85.0 stover, 180℃, 10 min 150 mg catalyst, 50 mg corn 68.3 stover, 170℃, 10 min 150 mg catalyst, 150 mg corn 94.4 stover, 190℃, 15 min a

Solvent: 7 ml GVL.

Highlights: ● A novel catalyst (SC-CaCt-700) was prepared and used to convert raw corn stover. ● F r ral was eri e

rom ot

emicell lose an cell lose in corn sto er

● Enhanced furfural yield of 93% was achieved in γ-valerolactone (GVL). 50. ● Furfural degradation was effectively suppressed even with increasing time.