Carbon-based solid acid pretreatment in corncob saccharification

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Carbon-based solid acid pretreatment in corncob saccharification: specific xylose production and efficient enzymatic hydrolysis Wei Qi, Chao He, Qiong Wang, Shuna Liu, Qiang Yu, Wen Wang, Noppol Leksawasdi, Chenguang Wang, and Zhenhong Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03959 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Sustainable Chemistry & Engineering

Carbon-based solid acid pretreatment in corncob saccharification: specific xylose production and efficient enzymatic hydrolysis Wei Qi1*,#, Chao He1,2, Qiong Wang1,#, Shuna Liu1, Qiang Yu1, Wen Wang1, Noppol Leksawasdi3, Chenguang Wang1, Zhenhong Yuan1,4 1

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, CAS Key

Laboratory of Renewable Energy, Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, No.2, Nengyuan Road, Tianhe District, Guangzhou 510640, China 2

University of Chinese Academic of Sciences, No.19(A), Yuquan Road, Shijingshan

District, Beijing 100049, China 3

Bioprocess Research Cluster, School of Agro-Industry, Faculty of Agro-Industry,

Chiang Mai University, No.239, Huay Kaew Road, Muang District, Chiang Mai, 50100, Thailand 4

Collaborative Innovation Centre of Biomass Energy, No.63, Nongye Road,

Zhengzhou 450002, China #

These authors contributed equally to this work

*Corresponding author: Wei Qi, [email protected]

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ABSTRACT: In this paper, we disclose a novel saccharification technology for lignocellulosic biomass. A new carbon-based solid (C-SO3H) acid catalyst was first synthesized by a simple, one-step hydrothermal carbonization method using microcrystalline cellulose and sulfuric acid. The functional group, chemical composition and structure of the catalyst were characterized. After five reuses, the solid acid catalyst still showed a high catalytic activity for corncob pretreatment. Under optimal pretreatment conditions (140 °C, 6 h, 0.25g corncob, 0.25g catalyst and 25ml water), xylose was directly released from corncob in a high yield (78.1%). Enzymatic hydrolysis of the pretreatment residue provided an enzymatic digestibility of up to 91.6% in 48 h. The structure, morphology, and components of the corncob and residues were analyzed. The high xylose and glucose yields confirmed the high catalytic activity of the synthetic carbon-based solid acid, providing green and effective lignocellulose utilization. KEYWORDS: Carbon-based acid catalyst; corncob hydrolysis; pretreatment; xylose; enzymatic hydrolysis

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INTRODUCTION Renewable resources for the production of energy and chemicals are attracting more and more attention to solve energy shortages and climate change.1,

2

The

conversion of lignocellulosic biomass, which contains polysaccharides such as hemicellulose and cellulose, into biofuels and platform chemicals has been widely investigated.3-7 Depolymerization of the polysaccharides in lignocellulosic biomass into monosaccharides is a key step in lignocellulose conversion. One of the challenges of lignocellulose saccharification is to disrupt the complicated structure of lignocellulose. Chemical, physical, physicochemical, and biological methods have been used to pretreat lignocellulose.8-13 Ball milling is a simple physical pretreatment method that was widely used to treat lignocellulose. Hideno et al. pretreated straw using different ball milling methods, and the glucose and xylose yields from enzymatic hydrolysis reached 80% and 50%, respectively; these values are significantly higher than those obtained for the feedstock.14 Ball milling could effectively improve the enzymatic digestibility by decreasing the particle size of the feedstock, but this method required a high energy consumption, which limited its large-scale application. Liquid hot water was widely used as a low-cost and non-polluting physicochemical method to pretreat lignocellulose. Garrote reported that 90% of hemicellulose and 10%~50% of lignin in corncob was removed, but some of the cellulose in the corncob was removed during the liquid hot water process.15 The liquid hot water pretreatment could effectively remove hemicellulose and significantly improve the enzymatic hydrolysis by increasing the accessibility between cellulose in the substrate and cellulase, but the process required a high temperature, and the 3



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oligosaccharide sugar in the hydrolysate after pretreatment could not be used directly. A biological method was also applied in the pretreatment of lignocellulose. Kurakake used two bacterial strains to pretreat office paper, and the sugar yield reached 94% under optimal conditions.16 Although the reaction conditions for the biological pretreatment process were mild and the energy consumption was low, the pretreatment process was time consuming. Chemical pretreatment mainly includes mineral acid and alkali pretreatment methods. The alkali pretreatment requires a large amount of the alkali material to destruct the lignin structure in lignocellulose and produce numerous black liquors; this limited its large-scale utilization, and a part of the hemicellulose was lost during the alkali pretreatment process.17 Mineral acid pretreatment is another common chemical pretreatment method. Chen used dilute phosphoric acid to pretreat corncob, and xylose and cellulose recoveries of 78.4% and 96.8%, respectively, were obtained; 70.4% of the hemicellulose and 89.7% of the cellulose from the corncob were fermented to ethanol.18 The fermentation indicated that mineral acid pretreatment was an effective pretreatment method, but mineral acids, such as sulfuric acid, cause environmental pollution, corrode equipment, and are toxic.19,

20

The required

neutralization process and washing eventually create numerous waste products. Green catalysis, such as solid acid catalysis, can address some of these problems by facilitating the use of mild operating conditions with high selectivity, easily separating products and allowing for catalyst reuse. Sagunuma used a SO3H-bearing amorphous carbon prepared from polyvinyl chloride (PVC) to hydrolyze cellobiose, it exhibited high catalytic performance.21 Pena synthesized a silicacoated cobalt spinel ferrite magnetic nanoparticle catalysts which functionalized with acid functional groups, it performed well in the hydrolysis of cellobiose and it could be easily 4


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separated by a outer magnetic field.22 However, there are only a few applications of lignocellulose pretreatment.23 In this aspect, Anasany used magnetic sulfonic acid solids from activated carbon and p-toluene sulfonic acid to pretreat switchgrass, miscanthus, and triticale hay for enzymatic hydrolysis. The enzymatic digestibility reached 60%, which is greater than that obtained for the raw materials but lower than that obtained using the other pretreatment methods.24 In this study, we report a simple, one-step hydrothermal carbonization method to synthesize a carbon-based solid acid using microcrystalline cellulose and sulfuric acid. This carbon-based solid acid showed a high catalytic activity in the pretreatment of corncob with a xylose yield of 78.1%; the carbon-based solid acid was easily separated from the residue after the reaction. Subsequent enzymatic hydrolysis of the residue revealed an enzymatic digestibility of 91.6% in just 48 h. The result indicated that the combined saccharification pretreatment catalyzed by the carbon-based solid acid synthesized using mild and green hydrothermal carbonization procedures and enzymatic hydrolysis for lignocellulose can achieve very high total yields of xylose and glucose. EXPERIMENTAL SECTION Materials Corncob powder (60~80 mesh) was obtained from Shandong Province, China. It was oven-dried to a constant weight at 80 °C for 12 h. The microcrystalline cellulose (Guaranteed Reagent (GR)) and 98% (v/v) sulfuric acid (GR) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Cellulase (190 FPU/g) was obtained from Imperial Jade Bio-Technology Co., Ltd. Synthesis of the carbon-based solid acid A mixture of microcrystalline cellulose and sulfuric acid was placed in a 100-mL 5



Teflon-lined stainless autoclave reactor (Dalian Tongda Science and Technology Development Co., Ltd.China) at 120-200 °C and the ratio of microcrystalline cellulose mass/sulfuric acid volume varied from 1:1 to 1:20; then, the reactor was heated in a muffle furnace (Jingda Electric Control Equipment Factory, China) for 4-24 h. The resulting products were filtered, repeatedly washed with ethanol and hot water (>80 °C) until no sulfate ions were detected using BaCl2, and then dried overnight in a vacuum oven at 105 °C. Finally, the carbon-based solid acid was ground to a powder and sieved using a 120-mesh screen. The reaction, which contained the carbon-based solid acid catalyst (0.5 g), corncob (0.25 g), and deionized water (25 mL), was carried out in a 100-mL thick-walled pressure bottle (Beijing Synthware Glass Co., Ltd. China) with a 2-cm magnetic bar on a magnetic stirrer in a 130 °C oil bath for 4 h. After the reaction, the supernatant was collected for further analyses. The product yields were monitored to later optimize the carbon-based solid acid preparation. Catalyst characterization Fourier transform infrared (FT-IR) spectra were collected on a TENSOR 27 spectrometer (Bruker, Karlsruhe, Germany) using the standard KBr pellet method from 400 cm-1 to 4000 cm-1. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250Xi spectrometer (Thermo, Waltham, USA). The binding energy was calibrated using the C 1s line at 284.8 eV. X-ray diffraction (XRD) was performed using an X’Pert Pro MPD (PANalytical, Netherlands) operating at 40 kV and 40 mA using a Cu Kα radiation source in the 2θ range from 5° to 80° with a scanning step length of 4°/min. The elemental composition of the catalysts was analyzed using an element analyzer test instrument (vario EL cube, Elementar, Frankfurt, Germany). 6


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The concentration of acid sites on the catalysts was examined using an acid-base titration method.25 First, the total amount of acid of the catalyst was titrated as follows: 0.25 g catalyst was added to 30 mL of 0.05 M sodium hydroxide, which was subjected to ultrasonic vibration for 60 min at room temperature to react HO- and the acid group on the catalyst. After centrifugation at 10000 rpm for 5 min, 10 mL of the supernatant was titrated with 0.05 M hydrochloric acid to evaluate the amount of total acid in the catalyst. The catalyst (0.25 g) was also added to 30 mL of 0.05 M sodium chloride before subjecting the mixture to similar ultrasonic vibration under the same conditions to fully exchange Na+ and H+ on the SO3H group. After centrifugation at 10000 rpm for 5 min, 10 mL of the supernatant was titrated with 0.05 M sodium hydroxide to evaluate the concentration of SO3H groups on the catalyst. The acid strength of acid sites on the catalyst was measured by Hammett method.26 Optimization of the corncob pretreatment conditions A mixture of corncob, water, and catalyst was prepared at 160 °C for 6 h in an oil bath with a microcrystalline cellulose mass/sulfuric acid volume ratio of 1:10 in a 100-mL thick-walled pressure bottle with a magnetic stirrer. The following hydrolysis conditions were optimized: hydrolysis temperature (120–160 °C), hydrolysis time (4– 6 h), catalyst dosage (0–0.75 g), and amount of water (12.5–100 mL). After the reaction, the supernatant was collected to analyze the products. The mixture after hydrolysis under the optimum reaction conditions was dried in a vacuum oven to a constant weight at 50 °C before separating the corncob hydrolysis residue from the catalyst using a 120-mesh screen to analyze the morphology, structure, and content, and for enzymatic hydrolysis. Enzymatic hydrolysis of the corncob hydrolysis residue The hydrolysis process was carried out by subjecting 0.1 g of the corncob 7



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hydrolysis residue derived under the optimum reaction conditions or 0.1 g of raw corncob to enzymatic hydrolysis of the cellulase enzyme at loadings of 20 and 40 FPU/g for 1-96 h. The hydrolysate was subsequently diluted and collected for products analysis. Analytical procedures Xylose, glucose, arabinose, furfural, 5-hydroxymethylfurfural (5-HMF), acetic acid, formic acid, and D-glucuric acid in the hydrolysate were detected using high-performance liquid chromatography (Waters 2695, Milford, USA; Shodex SUGAR SH-1011 chromatographic column, mobile phase of 5 mM H2SO4, flow rate of 0.5 mL/min, column temperature of 50 °C). The sugar oligomers were measured according to a previously described method.2 The components of the corncob and hydrolysis residue were analyzed according to the National renewable Energy Laboratory standard analytical method.2 The analysis revealed that the corncob is composed of 35.87% glucan, 34.1% xylan, 3.49% arabinose, and 21.96% lignin (dried weight basis). The surface morphologies of the corncob and its hydrolysis residue were observed using scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan). The crystalline structure of the corncob and its hydrolysis residue were measured by XRD (described above). The crystallinity indices of the corncob and its hydrolysis residue were calculated according to the following equation:27 CrI % =

I crystalline (002) − I amporphous I crystalline (002)

× 100

(1)

where CrI is the crystallinity index, Icrystalline(002) is the strength of the crystalline zone (2θ=22.5°), and Iamorphous is the strength of the amorphous region. The yields of xylose, glucose, and arabinose were calculated according to the 8


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following equation: Sugar yield (% ) =

N × 100% M

(2)

where N is the quantity (mol) of each sugar in the hydrolysate after hydrolysis, and M is the quantity (mol) of each sugar in the corncob before hydrolysis. The enzymatic digestibility of the pretreated corncob was calculated according to the following equation: Enzymatic digestubility (%) =

n × 100% m

(3)

where n is the quantity of glucose in the hydrolysate after enzymatic hydrolysis (mol), and m is the quantity of glucose in the corncob hydrolysis residue before enzymatic hydrolysis (mol). The total sugar yield of the pretreatment and enzymatic process was calculated according to the following equation: Total sugar yield (%) =

a×b + c×d × 100% a+c

(4)

where a is the quantity of glucose in the corncob (mol), b is the total yield of glucose, c is the quantity of xylose in the corncob (mol), and d is the total yield of xylose. RESULTS AND DISCUSSION Optimization of the carbon-based solid acid preparation A series of carbon-based solid catalysts were prepared by a one-step hydrothermal carbonization method using microcrystalline cellulose and sulfuric acid. Their catalytic activities were tested by hydrolyzing corncob under conditions (140°C, 4h, 0.5g of catalyst, 0.25g of corncob and 25ml deionized water), under the conditions part of the hemicellulose in the corncob hydrolyzed. The influence of the preparation temperature (120~200 °C) on the catalyst activity and total amount of acid was shown in Fig. 1(a) under a fixed preparation time of 24 h and microcrystalline cellulose 9



mass/sulfuric acid volume ratio of 1:10. When the catalyst preparation temperature increased from 120 to 160 °C, the total acid amount of the catalyst increased and the total yields of xylose, arabinose, and glucose increased. These increases were attributed to the formation of incompletely carbonized amorphous structures; the -OH, -COOH, and -SO3H acidic functional groups were successfully grafted onto the carbon skeleton. This might be the increase of the temperature which contributed to the formation of carbon skeleton.25 When the catalyst preparation temperature further increased from 160 to 200 °C, the total acid amount of the catalyst decreased and the total yields of xylose, arabinose, and glucose decreased. The high temperature might lead to a full carbonization of the carbon material which was unfavorable for the SO3H grafted on the carbon skeleton and reduced the carboxyl and hydroxyl groups on the carbon skeleton.28 At a fixed preparation temperature of 160 °C and microcrystalline cellulose mass/sulfuric acid volume ratio of 1:10, the influence of preparation time (3~24 h) on the catalyst activity and total amount of acid was investigated. As shown in Fig. 1(b), when the catalyst preparation time increased from 3 to 6 h, the total acid amount of the catalyst increased and the total yields of xylose, arabinose, and glucose increased. The increased preparation time might increase the stability of the carbon skeleton, and the conjunction between functional groups and carbon skeleton become stable.29 When the catalyst preparation time exceeded 6 h, the total acid amount of the catalyst decreased and the total yields of xylose, arabinose, and glucose decreased. Longer catalyst preparation times might reduce the hydroxyl and carboxyl groups, which was not conducive to the hydrolysis reaction.30 The catalyst was prepared using various microcrystalline cellulose mass/sulfuric acid volume ratios at 160 °C for 6 h. Fig. 1(c) showed that when the ratio was varied 10


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from 1:2 to 1:10, the total acid amount of the catalyst increased and the total yields of xylose, arabinose, and glucose increased. The presence of a larger sulfuric acid portion in the reaction mixture might graft more acid groups onto the carbon skeleton.29 When the ratio was varied from 1:10 to 1:20, the total acid amount of the catalyst decreased and the total yields of xylose, arabinose, and glucose decreased. The superabundant sulfuric acid might lead to excessive dehydration of the carbon material which reduced the carboxyl and hydroxyl groups on the carbon material, and the excessive dehydration might lead to full carbonization of the carbon material which was unfavorable for the SO3H grafted on the carbon skeleton.28 The trends in the yields of monosaccharides, especially xylose and arabinose, were consistent with the total acid amount of the catalyst synthesized under different conditions. As shown in Fig. 1, the highest yields of xylose and arabinose were obtained when the total acid amount of the catalyst reached a maximum. The result indicated that more acid could promote the release of hemicellulose monosaccharides. The trend for the polysaccharide yields was opposite of that for the total acid trend, which indicated that the polysaccharides increased when the total amount acid of the catalyst decreased. Thus, lower amounts of acid produced more polysaccharides, but higher amounts of acid produced more monosaccharides. The concentrations of the by-products (furfural, formic acid, 5-HMF, acetic acid, and D-glucuric acid) in the hydrolysates were relatively low (~0.4 g/L), which indicated that some of the sugar was degraded to corresponding by-products, as shown in Fig. S1. From the above analysis, the optimal preparation conditions for the one-step synthesis of a carbon-based solid catalyst (C-SO3H) were 160 °C, 6 h, and a microcrystalline cellulose mass/sulfuric acid volume ratio of 1:10. Under these 11



conditions, the total amount acid was 5.50 mmol/g. Characterization of C-SO3H prepared under optimal conditions The XRD pattern of C-SO3H exhibits two broad but weak diffraction peaks at 2θ angles of 10°-30° and 30°-35°, which are attributed to amorphous carbon composed of aromatic carbon sheets oriented in a considerably random fashion,31 as shown in Fig. 2(a). The FT-IR spectrum of C-SO3H contains absorption peaks at around 1035 and 1205 cm-1, which are ascribed to the S=O and -SO3H stretching vibrations, respectively, and indicate successful introduction of -SO3H into the carbon material,32 as shown in Fig. 2(b). The absorption peak at around 1713 cm-1 is ascribed to the C=O bending vibration of -COOH.33 The absorption peak at around 3426 cm-1 is ascribed to the -OH stretching vibration.34 This observation reveals that the carbon material bears SO3H, COOH, and OH groups.35, 36 The COOH and OH groups on C-SO3H function synergistically with SO3H to hydrolyze lignocellulose by adsorbing the β-1,4-glucan in lignocellulose.34 The strength of the catalyst was -8.20 to -11.35. The XPS spectrum of C-SO3H indicates the existence of S on the catalyst surface (Fig. 2(c)). The peak at a binding energy of 168 eV is ascribed to S2p of -SO3H,37 which further indicates that -SO3H groups are introduced into the carbon skeleton. The peak at a binding energy of 164 eV is ascribed to S2p of -SH,38, 39 and the formation of the -SH group might be a result of the strong reducibility of glucose under hydrothermal carbonization, which restores -SO3H to -SH.29, 38 Reusability and characteristics of C-SO3H C-SO3H was prepared under optimum conditions and was reused five times for the hydrolysis of corncob under the same reaction conditions (140 °C, 6 h, 0.25 g of corncob, and 0.25 g of catalyst). After each reaction, C-SO3H was separated from the corncob hydrolysis residue using a 120-mesh screen, then washed, dried in an oven at 12


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105 °C until a constant weight was reached, and kept for the next use. The slight decrease of xylose yield from 70.4% to 63.0% indicated the catalysts still showed a high catalytic activity after five times, as shown in Fig. S2. The element content of the reused catalysts was analyzed (Table 1). The composition of C-SO3H is C0.443H0.321O0.259S0.066 after the first use and C0.443H0.342O0.261S0.050 after the last use, and the proportions of C, H, N, O, and S do not significantly change after each reuse, which indicates that the catalyst is stable for reuse. The total acid amount and -SO3H amount of the catalysts were titrated. The catalytic activity decreases after each use because the amounts of total acid and -SO3H decrease. Optimization of the corncob pretreatment conditions To obtain a high xylose yield for further enzymatic hydrolysis, the pretreatment conditions, including the hydrolysis temperature, hydrolysis time, catalyst loading, and water loading, were optimized. The reaction was initially conducted from 120 to 160 °C for 8 h with 0.25 g of the catalyst and 25 mL of water. When the temperature increases from 120 to 140 °C, the xylose yield increases from 67.6% to 77.5%; when the temperature further increases to 160 °C, the xylose yield decreases to 69.6%. The result indicates that a high temperature could speed the hydrolysis rate and the xylose degradation rate, as shown in Fig. 3(a). Below 140 °C, the rate of hemicellulose hydrolysis is faster than the xylose degradation, which increases the xylose yield; above 140 °C, the xylose degradation rate is faster than the hemicellulose hydrolysis rate, which decreases the xylose yield.40 The reaction was performed from 4 to 12 h at 140 °C with 0.25 g of the catalyst and 25 mL of water (Fig. 3(b)). When the reaction time increases from 4 to 6 h, the xylose yield increases from 67.6% to 77.3%. When the reaction time increases to 12 h, the xylose yield is only minimally affected. The xylan has been categorized into an 13



easy-to-hydrolyze fraction and a hard-to-hydrolyze fraction.41 Before 6 h, the xylose mainly generated from the easy-to-hydrolyze fraction of hemicellulose, and the xylose formation rate is faster than its degradation rate, which increases the xylose yield; after 6 h, the xylose mainly generated from the hard-to-hydrolyze fraction of hemicellulose, and the xylose formation rate and degradation rate are almost the same, which does not change the xylose yield.42 The catalyst loading was investigated from 0 to 0.75 g at 140 °C for 6 h with 25 mL of water, and 0.25 g of corncob. When the catalyst loading increases from 0 to 0.25 g, the xylose yield increases from 0.43% to 77.3%. The xylose yield is very low in the absence of the catalyst, which indicates the relatively negligible hemicellulose degradation under the reaction conditions. When the catalyst loading further increases to 0.75 g, the xylose yield decreases to 75.8%, which might be due to the presence of sufficient acid sites for undesired side reactions when the catalyst is overused. The optimal catalyst loading is 0.25 g, as shown in Fig. 3(c). The water content was investigated from 12.5 to 100 mL at 140 °C for 6 h using 0.25 g of the catalyst. When the water content increases from 12.5 to 25 mL, the xylose yield increases from 65.6% to 78.1% because the lower water content is detrimental to mass transfer. When the water content increases to 100 mL, the xylose yield decreases to 53.8%, which might be because the high water content increases the distance between the catalyst and corncob and hinders their contact. A water content of 25 mL is selected as the optimal value, as shown in Fig. 3(d). The catalytic activity of C-SO3H was compared with other catalysts, shown in Table 3. Compared with the results of Amberlyst-1529 and HZSM-529, a higher xylose yield was obtained in this study. The reaction time was shorter than Gp-SO3H-H2O229 to obtain a similar xylose yield. Compared with Fe3O4/C-SO3H30, the xylose yield was 14


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much higher under a milder reaction condition catalyzed by C-SO3H. The results indicated that the C-SO3H showed higher catalytic activity than other catalyst. Characterization of corncob and the pretreatment residues The XRD patterns of corncob and its hydrolysis residue were measured and their crystallization indices were calculated, as shown in Fig. S3. The diffraction intensity at 2θ=18° represents the amorphous area of the corncob. The diffraction intensity at 2θ=22.5° represents the crystalline part. The diffraction intensity of the hydrolysis residue at 2θ=18° is almost the same as that for the corncob, but the diffraction intensity at 2θ=22.5° increases rapidly after the hydrolysis reaction. The morphologies of the corncob and its hydrolysis residue are shown in Fig. S4. Before the hydrolysis reaction, the surface of the corncob is relatively smooth and dense, whereas the structure of its counterpart is destroyed after the hydrolysis reaction. The surface of the corncob becomes loose and rough with several pores with relative sizes in the micrometer range. The proportion of glucan increases from 35.87% to 67.94%, which indicates that most of the glucan is reserved. The proportion of xylan in the corncob residue is only 9.96%, as shown in Table 2. Enzymatic hydrolysis of the corncob hydrolysis residue Fig. 4 shows that the enzymatic digestibility of the pretreated corncob is higher than that of the raw material. Under an enzyme dosage of 20 FPU, the enzymatic digestibility of the raw material is 50.7% at 48 h and 59.2% at 96 h, whereas the enzymatic digestibility of the pretreated corncob is 78.4% at 48 h and 86.1% at 96 h. At an enzyme dosage of 40 FPU, the enzymatic digestibility of the pretreated corncob increases significantly to 91.6% at only 48 h, while the enzymatic digestibility of raw corncob is only 64.0% under the same conditions. At 96 h, 96.0% of the enzymatic 15



digestibility of the pretreated residue is obtained, whereas the digestibility of raw corncob is only 71%. The result of the enzymatic hydrolysis indicates that the carbon-based pretreatment process could effectively improve the enzymatic digestibility. The changes in the components of the corncob, shown in Table 2, indicate that almost all the hemicellulose and some of the lignin is removed. According to the crystallization index, the crystallinity of the corncob increases significantly after hydrolysis, which indicates that the amorphous hemicellulose and lignin in the corncob are successfully removed during the hydrolysis reaction; this result is in accordance with the component analysis of corncob and its hydrolysis residue. The changes in the morphology of the corncob, as shown in Fig. S4, indicate that the pretreatment process destroys its surface topography. Overall, a series of irregular holes and folds occurs on the corncob after pretreatment, which is beneficial to the enhancement of cellulose accessibility by the dissolution of hemicellulose and removal of lignin.43 The enzymatic digestibility of the residues after the solid acid pretreatment and the other pretreatment methods (liquid hot water, diluted acid, and alkali) are listed in Table 4. Using a liquid hot water pretreatment at 180 °C, the enzymatic digestibility of the corncob is only 82.4% at 48 h.44 Using the pressurized liquid hot water pretreatment, the enzymatic digestibility of sugarcane bagasse (SCW) achieved 88.0%.45 Enzymatic hydrolysis of the SCW residue from the 1.25% HCl pretreatment did not provide a satisfactory result: only 76.6% from the decomposition of cellulose during the pretreatment process.46 Pressurized 25% NH3 pretreatment and 1% NaOH and the liquid hot water pretreatment produced an enzymatic digestibility of the SCW residue of up to 95.0%.47 However, these pretreatment methods were relatively complicated and might cause environmental pollution. 16


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The yield of xylose and glucose from the optimal pretreatment (Eq. 2) and enzymatic hydrolysis (Eq. 3) of corncob, and the total sugar yield (Eq. 4) are shown in Table 5. The total sugar yield reaches 97.4%. CONCLUSION A novel biorefinery method was established for corncob saccharification. A new carbon-based solid was synthesized using a one-step carbonization of sulfuric acid and microcrystalline cellulose. This catalyst showed a high and specific catalytic activity for corncob hemicellulose hydrolysis with a 78.1% yield of xylose. The residue was easily separated from the solid catalyst through screening. The enzymatic digestibility of the residue achieved 91.6% and 96.0% at 48 h and 96 h, respectively. The combination of carbon-based solid acid hydrolysis of hemicellulose and enzymatic hydrolysis of cellulose could fully depolymerize corncob and provide a potential green and sustainable utilization of lignocellulose. ACKNOWLEDGEMENTS This work was supported financially by the National Natural Science Foundation of China (21376241, 51676193, 51506207, and 51561145015), the Youth Innovation Promotion Association, CAS (No. 2017401), the Key Project of the Natural Science Foundation of Guangdong Province (No. 2015A030311022), and Guangdong Key Laboratory of New and Renewable Energy Research and Development (Y709ji1001). N. Leksawasdi gratefully acknowledges financial support and in-kind assistance from the Sino-Thai National Research Council of Thailand (NRCT). Project Funding from the National Research University - Chiang Mai University (NRU-CMU) and National Research University - Office of Higher Education Commission (NRU-OHEC), Non-Food Agricultural Research Cluster, and CMU Mid-Career Research Fellowship program (Grant Number: W566_21022560) are acknowledged. 17



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Supporting Information Byproducts concentration in the pretreatment hydrolysate under different conditions, Reusability of the catalyst in corncob pretreatment, XRD patterns of corncob before and after pretreatment, SEM for corncob before and after pretreatment. REFERENCES 1.

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Table Captions: Table 1 Catalyst composition and amount of SO3H after different numbers of reuses Table 2 Compositions of the corncob and pretreatment residue. Pretreatment conditions: 140 °C, 6 h, 0.25 g of corncob, 0.25 g of the catalyst, and 25 mL of water. Table 3 The catalytic activity comparison of C-SO3H and other catalysts. Table 4 Comparison of enzymatic digestibility after solid acid pretreatment and other pretreatment methods. All results were calculated based on an enzyme loading of 40 FPU. Table 5 Total sugar yield of the pretreatment and enzymatic processes. Pretreatment conditions: 140 °C, 6 h, 0.25 g of corncob, 0.25 g of the catalyst, and 25 mL of water; enzymatic conditions: 40 FPU, 50 °C, 96 h, and 150 rpm.

26


Page 26 of 40

Page 27 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Catalyst composition and amount of SO3H after different numbers of reuses

Amount Number N (%)

C (%)

H (%)

S (%)

O (%)

Catalyst composition

of SO3H

of reuses (mmol/g) 1

0.000

53.180

3.210

2.120

41.480

C0.443H0.321O0.259S0.066

0.600

2

0.000

54.100

3.570

1.980

40.330

C0.450H0.357O0.252S0.062

0.550

3

0.000

53.980

3.420

1.730

40.850

C0.450H0.342O0.255S0.054

0.490

4

0.000

53.780

3.390

1.680

41.140

C0.448H0.339O0.257S0.053

0.470

5

0.000

53.200

3.420

1.590

41.780

C0.443H0.342O0.261S0.050

0.410

27



Page 28 of 40

Table 2 Compositions of the corncob and pretreatment residue. Pretreatment conditions: 140 °C, 6 h, 0.25 g of corncob, 0.25 g of the catalyst, and 25 mL of water. Corncob

Lignin (%)

Glucan (%)

Xylan (%)

Before reaction

21.96

35.87

34.10

After reaction

14.01

67.94

9.96

28


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Table 3 The catalytic activity comparison of C-SO3H and other catalysts.

a

Catalyst

Xylose yield (%)

Glucose yield (%)

Amberlyst-1529

60.2

5.8

11.4

5.7

78.4

6.1

44.3

\

78.1

7.4

a

a

HZSM-529

Gp-SO3H-H2O229 b

Fe3O4/C-SO3H30 c

C-SO3H

a

Catalyst, 0.5g; corncob 0.25g; water, 25ml; 140°C, 12h.

b

Catalyst, 1g; corncob 0.5g; water, 50ml; 160°C, 16h.

c

Catalyst, 0.25g; corncob 0.5g; water, 25ml; 140°C, 6h.

29



Page 30 of 40

Table 4 Comparison of enzymatic digestibility after solid acid pretreatment and other pretreatment methods. All results were calculated based on an enzyme loading of 40 FPU. Enzymatic Enzymatic Raw material

Pretreatment method

time digestibility (%) (h)

corncob

a

Carbon-based solid acid

48

91.6

LHW44

48

82.4

c

HCl46

72

76.6

LHW45

48

88

e

NH345

48

95

NaOH47

72

85

NaOH + LHW47

72

95

b

corncob sugarcane bagasse

d

sugarcane bagasse sugarcane bagasse

f

sugarcane bagasse sugarcane bagasse

g

Optimal pretreatment conditions: a

140 °C, 6 h, 0.25 g catalyst, 0.25 g corncob, 25 ml water.

b

c

180 °C, 0.5 h, 12.5% (w/v) solid loading.

130 °C, 10 min, 1.25% HCl, 12.5% solid loading.

d

180 °C, 20min, 5% solid loading, 4MPa. 30


Page 31 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e

160 °C, 20min, 10% solid loading, 2MPa

f

80 °C, 3 h, 20 ml 0.25M NaOH, 1g sugarcane bagasse.

g

80 °C, 3 h, 20 ml 0.25M NaOH, 180 °C, 20min, 5% (w/v) solid loading.

Table 5 Total sugar yield of the pretreatment and enzymatic processes. Pretreatment conditions: 140 °C, 6 h, 0.25 g of corncob, 0.25 g of the catalyst, and 25 mL of water; enzymatic conditions: 40 FPU, 50 °C, 96 h, and 150 rpm. Total sugar Process

Xylose yield (%)

Glucose yield (%) yield (%)

Pretreatment

78.1

7.4 97.4

Enzymatic hydrolysis

86.6

96.0

31



Figure Captions: Fig. 1. Effects of the carbonization temperature (a), carbonization time (b), and microcrystalline cellulose mass/sulfuric acid volume ratio (c) on the catalytic activity and total amount of acid of the catalyst. Fig. 2. XRD patterns (a), FT-IR spectra (b), and XPS (c) of C-SO3H synthesized under optimal conditions. Fig. 3. Effect of hydrolysis temperature (a), hydrolysis time (b), catalyst dosage (c), and water content (d) on the sugar yields from corncob pretreatment. Fig. 4. Enzymatic digestibility of corncob before and after pretreatment. Pretreatment condition: 140°C, 6h, 0.25g corncob, 0.25g catalyst and 25ml deionized water. Enzymatic condition: 5% substrate concentration, 50°C.

32


Page 32 of 40

100 90 80

5

Arabinose oligosaccharide Arabinose Xylose oligosaccharide Xylose Glucose oligosaccharide Glucose

Total amount of acid

4

Suger yield (%)

70 3

60 50

2

40 30 20

1

10 0

0

120

160 180 140 o Temperature ( C)

200

(a) Arabinose oligosaccharide Arabinose Xylose oligosaccharide Xylose Glucose oligosaccharide Glucose

To ta l a mount of a cid

90 80

7 6 5

70 60

4

50 3

40 30

2

20 1 10 0

0

3

6

12 Time (h)

18

(b)

33


24

Total amount of acid(mmol/g)

100

Sugar yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Total amount of acid(mmol/g)

Page 33 of 40


90 80

6

Arabinose oligosaccharide Arabinose Xylose oligosaccharide Xylose Glucose oligosaccharide Glucose

Total amount of acid

5

70

4

60 3

50 40

2

30 20

1

Total amount of acid (mmol/g)

100

Sugar yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

10 0

0

1:2 1:10 1:15 1:5 1:20 Ratio of microcrystalline cellulose mass and sulfuric acid volume

(c) Fig. 1. Effects of the carbonization temperature (a), carbonization time (b), and microcrystalline cellulose mass/sulfuric acid volume ratio (c) on the catalytic activity and total amount of acid of the catalyst.

34


Page 35 of 40

1800 1600

Intensity(a.u)

1400 1200 1000 800 600 400 200 0 10

20

30

40

50

60

70

80

2-Theta/degree

(a) 1.05 1.00 0.95

Trandmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.90 0.85

1035cm-1 S=O str.

0.80 0.75

1205cm

3426cm-1 OH str.

SO3H str. 1713cm

0.70 4000

-1

1613cm-1 C=C str.

C=O bend.

3500

-1

3000

2500

2000

1500 -1

Wavenumber (cm )

(b)

35


1000

500


1600 SO3H 1400

Counts (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

1200

1000

800 SH 600

400 175

170

165

160

Binding Energy (eV)

(c) Fig. 2. XRD patterns (a), FT-IR spectra (b), and XPS (c) of C-SO3H synthesized under optimal conditions.

36


Page 37 of 40

100 90

Arabinose oligosaccharide Arabinose

Xylose oligosaccharide Xylose

Glucose oligosaccharide Glucose

80

Sugar yield (%)

70 60 50 40 30 20 10 0

120

150 130 140 ? Hydrolysis temperature ( C)

160

(a) 100 90




80 70

Sugar yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20 10 0

4

10 6 8 Hydrolysis time (h)

(b)

37


12


100




90 80

Sugar yield (%)

70 60 50 40 30 20 10 0

0

0.5 0.1 0.25 Catalyst dosage (g)

0.75

(c) 100 90




80 70

Sugar yield(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

60 50 40 30 20 10 0

12.5

75 25 50 Water content (ml)

100

(d) Fig. 3. Effect of hydrolysis temperature (a), hydrolysis time (b), catalyst dosage (c), and water content (d) on the sugar yields from corncob pretreatment.

38


Page 39 of 40

100 90

Enzymatic digestibility (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50 40

Before pretreatment 20 FPU Before pretreatment 40 FPU After pretreatment 20 FPU After pretreatment 40 FPU

30 20 0

20

40

60

80

100

Time (h)

Fig. 4. Enzymatic digestibility of corncob before and after pretreatment. Pretreatment condition: 140°C, 6h, 0.25g corncob, 0.25g catalyst and 25ml deionized water. Enzymatic condition: 5% substrate concentration, 50°C.

39



For Table of Contents Use Only

Synopsis A green saccharification technology was established for corncob which was combined with reusable carbon-based solid acid and enzymatic hydrolysis.

40


Page 40 of 40

Carbon-based solid acid pretreatment in corncob saccharification

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