Corncob lignocellulose for the production of furfural by hydrothermal pretreatment and heterogeneous catalytic process

Abstract

Corncob with a high content of pentose-rich hemicelluloses shows great potential for furfural production. In this paper, an environmentally-friendly two-step process for furfural production was developed by the hydrothermal pretreatment of corncob and the heterogeneous catalysis of the hydrolysate using a solid acid catalyst. The hydrothermal pretreatment was applied to release hemicelluloses from the cell wall of the corncob, which were hydrolyzed into monosaccharide and oligosaccharide under the investigated conditions (160–190 °C, 0–60 min). Subsequently, heterogeneous catalysis of the hydrolysate obtained from the hydrothermal pretreatment was performed using SO₄²⁻/SiO₂–Al₂O₃/La³⁺ as a solid acid catalyst at 150 °C for 2.5 h. The maximum yield of furfural achieved was up to 21% under the investigated experimental conditions from the catalysis of hydrolysates obtained at 190 °C for 60 min in the hydrothermal process. Moreover, there was no obvious change in the catalytic activity of SO₄²⁻/SiO₂–Al₂O₃/La³⁺ after four runs, and only a decline of 5.28% yield of furfural was observed. This two-step process is an environmentally-friendly process for furfural production from pentose-rich biomass and serves as an effective approach for recovering the solid catalyst from the reaction medium.

---

1. Introduction

The imminent shortage of fossil fuels and their soaring prices have forced researchers to focus on the exploration of biomass as a kind of energy resource nowadays.^1,2 Lignocellulosic biomass is mainly composed of three major compounds (cellulose, hemicelluloses and lignin). Due to hemicelluloses, lignin and other compounds attaching onto the surface of the cellulose microfibrils, the primary challenge of biomass utilization is how to deconstruct the lignocellulosic raw materials into simple molecules.³ Thus, the pretreatment of biomass is considered to be a potential way to solve this problem by physical and chemical treatment.⁴ Hydrothermal pretreatment is widely used as an environmentally friendly method, due to the following reasons: (1) water is used only; (2) hemicelluloses can be converted into sugars and oligosaccharide; (3) corrosion of the equipment is limited.^5–7 During the hydrothermal pretreatment process, hemicelluloses in the cell wall of lignocellulosic biomass can be degraded into oligosaccharide or monosaccharide, which can further be converted into platform chemicals such as furfural, xylitol and ethanol etc.

Hemicelluloses, as the second most abundant plant material after cellulose in the lignocellulosic biomass, can undergo hydrolysis in acidic media to form sugars such as xylose, and then xylose is converted into furfural by a dehydration reaction.⁸ Furfural is a highly versatile and key feedstock used in the manufacture of a wide range of biofuel and important chemicals in different fields, such as oil refining, plastics, pharmaceutical, and agrochemical industries.⁹

Recently, the production of furfural and 5-hydroxymethylfurfural from biomass by a catalytic process has been extensively studied. In the industrial process, homogeneous acid catalysts, such as dilute sulphuric acid, were generally utilized for the furfural production.¹⁰ Some problems are presented in this process such as corrosion of the equipment and environmental risks. CO₂-assisted autohydrolysis treatment could generate in situ carbonic acid in water under severe conditions, and the carbonic acid formed was used to hydrolyze wheat straw to produce furfural as one of the main products under more severe conditions (above 210 °C) by a one-step process. This method is environmentally friendly, but the furfural yield is low and other main products were produced simultaneously.⁶ Recent studies have focused on ionic liquids (ILs) for the conversion of lignocellulosic biomass and cellulose into furfural and HMF.¹¹ However, the yields of products are relatively low. While acidic ILs can serve as both solvents and catalysts because they combine the advantages of mineral acids and ILs. Acidic 1-butyl-3-methylimidazolium hydrogen sulphate ionic liquid was employed to pretreat wheat straw.¹² Furfural was mostly formed under more severe conditions and 30.7% (w/w) was obtained at 161 °C/104.5 min. CrCl₂ in imidazoliun ionic liquids¹³ and GeCl₄ in 1-butyl-3-imidazolium chloride ([BMIM]Cl)¹⁴ have been reported for the conversion of carbohydrates into furfural. Although these homogeneous catalytic systems mentioned above achieved high furfural yield, product and catalyst recovery from ILs and the purity of ILs are the main challenges for the acceptance of this technology as a feasible alternative to conventional processes. Therefore, developing new strategies is deemed to be necessary for solving the problems mentioned above. Solid acid catalysts, which own the characteristics of good thermal stability and chemical stability, have been exploited for furfural production to overcome the disadvantages in the homogeneous process.¹⁵ There are many studies on the production of furan compounds using monosaccharides and polysaccharides as raw material^16–19 and solid acid catalysts such as microporous zeolites,²⁰ metal oxides¹⁷ and mesoporous silicas.²¹ The prominent advantage of these studies is that the solid acid can be separated easily after the reaction. But they are lacking economical efficiency because of the costly raw materials used in these studies. Research on the conversion of lignocellulosic biomass into furfural using solid acid catalysts has drawn more attention. Our group have achieved the one-step conversion of corncob into xylose and furfural using solid acids as the catalyst.²² This conversion process is environmentally friendly due to the absence of dilute acid and organic solvent. However, it is difficult to realize the recovery of the catalyst from the reaction system because solid catalysts are mixed with the solid residue, which also had a negative influence on the further conversion of the solid residue. Moreover, xylose and furfural are the main products in this process, and it is difficult to separate furfural from the mixed liquors.

Corncob biomass, which is an abundant agricultural waste with a high content of pentose-rich hemicelluloses, presents great potential for furfural production.²³ In this work, a two-step process for the production of furfural from corncob lignocellulose was performed through hydrothermal pretreatment in combination with heterogeneous catalysis using a solid acid. In the first step, the aim of the hydrothermal pretreatment of corncob was to yield as many monosaccharides and oligosaccharides as possible in the hydrolysate. Subsequently, heterogeneous catalysis was applied to convert the hydrolysate into furfural using a solid acid catalyst (SO₄²⁻/SiO₂–Al₂O₃/La³⁺). This two-step process for producing furfural possessed the obvious features that the release and the further degradation of hemicelluloses from the cell wall of corncob and the production of furfural were carried out in different processes, so relatively high furfural yield could be realized and side reactions were limited. Furthermore, the solid acid catalyst could be easily recycled and reused, and the solid residue (mainly cellulose and lignin) obtained from the hydrothermal pretreatment could be used as the raw material to produce bioethanol, biomaterials or other chemicals.

2. Material and methods

2.1 Materials

Corncob used in this study was obtained from Shan Dong province, China. Corncob was smashed into particles with a size of 20–40 mesh, and oven-dried to constant weight. Na₂SiO₃, AlCl₃·6H₂O, La(NO₃)₃·6H₂O, NH₄OH and H₂SO₄, were purchased from Aladdin (China). The standard reagents of D-xylose, D-glucose, L-arabinose, furfural, 5-hydroxymethyfurfural (HMF), formic acid, acetic acid and levulinic acid were purchased from Sigma-Aldrich. All reagents were used without any purification.

2.2 Preparation of SO₄²⁻/SiO₂–Al₂O₃/La³⁺

The solid acid (SO₄²⁻/SiO₂–Al₂O₃/La³⁺) was prepared by coprecipitation and impregnation methods.²⁴ Na₂SiO₃ (0.1 mol) and La(NO₃)₃·6H₂O in a required stoichiometric ratio of lanthanum to the target product (1.0 wt%) were added to AlCl₃·6H₂O (0.2 mol) solution. Then the solution was adjusted to pH 10 with ammonia and kept for 24 h. The solid was separated by filtration, and then washed by water several times. The cleaned solid was dried at 110 °C for 24 h to form SiO₂–Al₂O₃/La³⁺, impregnated with 1.0 M H₂SO₄ at a proportion of 15 mL g⁻¹ for 6 h. The powder was dried at 110 °C for 12 h and calcined at 550 °C for 4 h to get the target catalyst.

2.3 Two-step process of corncobs biomass for furfural production

The typical furfural production was performed by a two-step process of hydrothermal pretreatment of the corncob biomass and followed by catalysis of the hydrolysate using the solid acid prepared above. Firstly, the experiment for the hydrothermal pretreatment of corncob was carried out in a 100 mL high-pressure batch reactor (SLM-100, Shenlang Co., Ltd, Beijing, China) at 170, 180, 190 °C in the time range of 0–60 min, respectively. Corncob (6 g) and deionized water (60 mL) were mixed under ultrasound for 30 min at room temperature to improve the dissolution rate of hemicelluloses from the corncob cell wall. The mixture above was transferred to a high-pressure batch reactor, and then the reactor was freed of oxygen by purging with nitrogen gas. After that, the reactor was heated up to the required temperature with magnetic agitation operating at 400 rpm min⁻¹. After the hydrothermal pretreatment, the reactor was cooled to room temperature. The hydrolysate and solid fractions were separated by centrifugal filtration. The solid residue was washed with hot water several times and dried for characterization analysis. The catalytic reaction of the hydrolysate was carried out in a 100 mL hydrothermal reactor at 150 °C for 2.5 h. The hydrolysate (10 mL) and the solid acid catalyst (0.1 g) were mixed and transferred to the reactor, and then the reactor was heated up to the desired temperature. After the catalytic reaction, the reactor was cooled with water. Products were filtrated to separate the liquid and solid fractions. The liquid obtained was kept for further analysis. The solid fractions were washed with ethanol and water several times, respectively, to remove adsorbed by-products. The solid residues obtained could be used for other applications, indirectly, after hydrolysis and fermentation for biofuel ethanol production and lignin. Each experiment was repeated twice under the same conditions to ensure the reproducibility of the results.

2.4 Analysis of liquid products

Furfural was characterized by high performance liquid chromatography HPLC (Waters 2414) equipped with a refractive index detector and a Waters SunFire™ C18 (4.6 × 250 mm) column. The mobile phase consisted of acetonitrile and water (volume ratio of 15 : 85) with a 1.0 mL min⁻¹ flow rate at 30 °C.

Monosaccharides and organic acids (formic acid and acetic acid) were measured by the same HPLC and Bio-rad Aminex® HPX-87H (300 × 7.8 mm) column. 5 mM of H₂SO₄ was employed as the eluent with 0.5 mL min⁻¹ flow rate. The temperature of the column was 50 °C, and the detector’s temperature was 40 °C. In order to measure the oligomer concentrations, the hydrolysate from the hydrothermal pretreatment of corncob was subjected to quantitative post-hydrolysis (with 4% sulphuric acid at 121 °C during 60 min), and then analyzed by HPLC. Contrasted with the hydrolysate obtained in the first stage, the oligomer concentrations were measured by the increase in the monosaccharide concentration caused by posthydrolysis.²⁵

The acid soluble lignin concentration was determined using an ultraviolet spectrophotometer (Techcomp UV2300).

The concentration of furfural, monosaccharides and organic acids (g L⁻¹) were calculated by standard calibration curves. The furfural yield was defined as follows:

where g L⁻¹ furfural is the concentration of furfural obtained from each trial, % hemicelluloses is the percentage of hemicelluloses in corn cobs calculated by NREL standard and H is the solid :  liquid ratio.

2.5 Analysis of solid residues

The surface morphology analysis of the samples from the solid residue in the first step treatment was conducted with scanning electron microscopy (SEM) equipment (Prox, Phenom World, Netherlands). X-ray diffraction (XRD) patterns of samples were obtained on a Bruker diffractometer with Cu Kα radiation. The tube voltage was 40 kV and the current was 40 mA. The selected 2θ range was 10–40°, scanning at a step of 0.02°. According to the method of Segal et al.,²⁶ the crystallinity index (CrI) was calculated by:

CrI = [(I₀₀₂ − I_am)/I₀₀₂] × 100 (2)

where I₀₀₂ is the peak intensity on the 002 lattice plane, and I_am is the peak intensity of amorphous zone diffraction intensity of 2θ = 20.9°.

Carbohydrates and lignin in untreated and treated corncob were measured according to the NREL standard analytic method (NREL/TP-510-42618).

2.6 Catalyst recyclability

The reusability of the SO₄²⁻/SiO₂–Al₂O₃/La³⁺ catalyst was investigated. Experiments were carried out in a 100 mL hydrothermal reactor under the following reaction conditions: 10 mL of the hydrolysate (hydrothermal pretreatment conditions: 190 °C, 60 min) and 0.01 g of the solid acid catalyst at 150 °C for 2.5 h. After the reaction was complete, the liquid was collected for analysis. The solid catalyst was dried, impregnated with 1.0 M H₂SO₄ at the proportion of 15 mL g⁻¹ for 6 h, calcined at 550 °C for 4 h to burn up the humins. After that, the catalyst was reused for the next cycle by adding fresh hydrolysate under the same conditions. The experiments on the catalyst recyclability were repeated more than three times at the same conditions.

3. Results and discussion

3.1 Effects of reaction time and temperature on the chemical composition of hydrolysate in the first step process

Samples were reacted in a high-pressure batch reactor at different temperatures (170, 180 and 190 °C) over a range of 0–60 min with 6.0 g of dried corncob and 60 mL of deionized water.

The effects of reaction time and temperature on the xylose yield in the hydrolysates obtained from hydrothermal pretreatment of corncob biomass are shown in Fig. 1. At a relatively low temperature, the xylose yield was very low (0.28 g L⁻¹ at 170 °C, 60 min), indicating that a low temperature was not favorable for the release of xylan-type hemicelluloses from the cell wall of corncob. While at a higher temperature and longer time, the xylose yield increased quickly from 0.036 g L⁻¹ to 6.86 g L⁻¹ at 190 °C, which was because a higher temperature can accelerate the hydrolysis of xylan-type hemicelluloses into xylose. Noureddini and Byun²⁷ reported the same trend in their study. During the hydrothermal pretreatment process, some of the degraded hemicelluloses and cellulose were present in the form of oligosaccharide.²⁴ In order to measure the xylo-oligosaccharide concentration, the hydrolysate obtained from the hydrothermal pretreatment was subjected to quantitative post-hydrolysis. The yield of xylose from xylo-oligosaccharide increased to the highest value first (3.46 g L⁻¹ at 190 °C, 10 min) and then decreased to the minimum value (0.16 g L⁻¹ at 190 °C, 60 min) when prolonging the reaction time. The highest yield of xylose from xylo-oligosaccharide was 3.46 g L⁻¹ at 190 °C in 10 min under the investigated conditions. The decline of the xylo-oligosaccharide yield may be attributed to the fact that xylo-oligosaccharide can be easily hydrolyzed into xylose under the harsher conditions,²⁸ corresponding to the increment of the xylose content. As observed in Fig. 1, the xylose content after post-hydrolysis (the total content of xylose and xylooligosaccharide) increased with an increase of temperature and time (from 0.33 g L⁻¹ at 170 °C for 0 min to 7.01 g L⁻¹ at 190 °C for 60 min). While at 190 °C, a longer treatment time led to the little enhancement of the total xylose yield (6.9 g L⁻¹ at 190 °C for 40 min to 7.0 g L⁻¹ at 190 °C for 60 min), indicating that most hemicelluloses were released from the cell wall of corncob and further degraded under the harsher conditions.

Fig. 1 Effects of reaction time and temperature on the xylose yield obtained from the hydrothermal pretreatment. Reaction conditions: 6.0 g corncobs, 60 mL water.

The effects of reaction time and temperature on the compositions of the other hydrolysates are shown in Table 1. Trace amounts of glucose were detected in all experiments because the physicochemical characteristics of cellulose (the linear configuration, the high polymerization degree and the crystalline structure) made its hydrolysis more difficult than hemicelluloses.²⁹ At a relatively low temperature (170 °C), the yield of glucose increased to the maximum yield (0.14 g L⁻¹) within 30 min. However, when the pretreatment conditions became more severe (higher temperature and longer time), the yield of glucose declined rapidly and even disappeared completely at 190 °C. Compared with the xylose content, the glucose content was lower. This could be attributed to the fact that hemicelluloses are much easier to be released than cellulose from the cell wall of lignocellulosic biomass.³⁰ As the second most abundant component in corncob hemicelluloses, arabinose, with a branched chain, attaches to the backbone of xylan.³¹ As indicated in Table 1, the yield of arabinose was improved by prolonging the treatment time and an increase in temperature (from 0.25 g L⁻¹ at 170 °C for 0 min to 2.23 g L⁻¹ at 190 °C for 60 min). Moreover, during the hydrothermal process, sugar dehydration reactions in the acidic conditions also led to the formation of HMF and furfural from hexose and pentose mono sugars, respectively.³² In the initial stage of the hydrothermal pretreatment, hydronium ions were generated from water by autohydrolysis under the severe conditions.³³ In the further pretreatment stages, the hydronium ions originated from acetic acid, which is derived from the hydrolysis of acetyl groups on the backbone of hemicelluloses.^34,35 Then, acid hydronium ions were generated and resulted in the acidic conditions during this process. As seen in Table 1, the yield of furfural increased slowly (from 0 g L⁻¹ in 0 min to 0.33 g L⁻¹ in 60 min) at 170 °C due to the weakness of the catalytic ability of hydronium ions, which was also confirmed by the pH value of the hydrolysates in Table 1. When the pretreatment conditions became severe the furfural yield remarkably increased to 3.12 g L⁻¹ at 190 °C in 60 min. A higher temperature and longer time induced the formation of acidic hydronium ions, which could accelerate the dehydration of xylose into furfural, resulting in the increase of furfural yield.³⁶

Table 1 The effects of reaction time and temperature on the compositions of the other hydrolysates obtained from hydrothermal pretreatment of corncob biomass^a

Hydrothermal pretreatment conditions		pH value	Furfural (g L^−1)	Glucose (g L^−1)	Arabinose (g L^−1)	Formic acid (g L^−1)	Acetic acid (g L^−1)
Temperature (°C)	Time (min)
a Reaction conditions: 6.0 g of corncob, 60 mL of water.
170	0	4.78	0	0.05	0.25	0	0.11
170	10	4.50	0	0.06	0.49	0.03	0.35
170	20	4.17	0.05	0.08	0.67	0.04	0.47
170	30	4.15	0.11	0.14	0.73	0.04	0.68
170	40	4.14	0.16	0.07	0.73	0.03	0.91
170	50	4.10	0.26	0.03	0.94	0.05	1.05
170	60	4.01	0.33	0	1.04	0.10	1.37
180	0	4.39	0	0.14	0.42	0	0.38
180	10	4.29	0.06	0.10	0.59	0.03	0.67
180	20	4.19	0.18	0.06	0.71	0.04	0.91
180	30	4.14	0.41	0	0.91	0.08	1.24
180	40	4.00	0.73	0	1.07	0.10	1.60
180	50	3.90	1.18	0	1.39	0.15	1.87
180	60	3.80	1.45	0	1.57	0.16	2.07
190	0	4.16	0.03	0	0.76	0.03	0.70
190	10	4.01	0.36	0	1.06	0.06	1.09
190	20	3.87	0.97	0	1.41	0.14	1.65
190	30	3.74	1.96	0	1.69	0.17	1.92
190	40	3.60	2.35	0	1.87	0.22	2.25
190	50	3.57	2.68	0	1.99	0.29	2.66
190	60	3.46	3.12	0	2.23	0.32	2.93

---

The pH values of the hydrolysates decreased from 4.78 to 4.01 at 170 °C (Table 1), which is ascribed to the formation of organic acids (formic acid and acetic acid). The highest yields of formic acid (0.32 g L⁻¹) and acetic acid (2.93 g L⁻¹) were obtained at 190 °C for 60 min. Formic acid was produced from the degradation of HMF and furfural,³⁷ and acetic acid was generated from acetyl group hydrolysis.³⁸ More formic acid and acetic acid resulted in a lower pH value.

3.2 The analysis of the difference of the chemical composition before and after the catalysis of the hydrolysates

Experiments were performed to investigate the change of the chemical composition (sugars, organic acids and furfural) in the catalytic treatment process. All hydrolysate samples (10 mL) obtained from the first step were reacted in a hydrothermal reactor at 150 °C for 2.5 h with 0.1 g of catalyst. As shown in Table 2, under the investigated conditions, the yield of glucose from hydrolysates obtained at 180 °C and 190 °C increased after the catalytic process. As illustrated in Table 2, the yield of xylose increased strikingly for the catalysis of hydrolysates obtained at higher temperatures within a shorter treatment time (for example, 180 °C in 10 min and 190 °C in 10 min). Xylo-oligosaccharide was hydrolyzed into xylose in the presence of solid acid catalysts. The xylose yield decreased in the catalyzed hydrolysate samples obtained within 50 min and 60 min at 190 °C, which was due to the formation of furfural by the dehydration of xylose via solid acid catalyst. Since arabinose could not be released completely from hemicelluloses during the hydrothermal pretreatment, little enhancement of its yield was observed after the catalytic process. The solid acid (SO₄²⁻/SiO₂–Al₂O₃/La³⁺) had no catalytic effect on the conversion of arabinose into furfural due to no significant decrease of the arabinose yield after the catalytic process, as shown in Table 2. It was also confirmed by the similar content of arabinose in the raw material and hydrolysates.

Table 2 The chemical composition of the liquid after catalysis^a

Hydrothermal pretreatment conditions		pH values	Xylose (g L^−1)	Glucose (g L^−1)	Arabinose (g L^−1)	Formic acid (g L^−1)	Acetic acid (g L^−1)
Temperature (°C)	Time (min)
a Reaction conditions: 10 mL of hydrolysates, 0.1 g of SO4^2−/SiO2–Al2O3/La^3+, 150 °C, 2.5 h.
170	0	3.01	0.15	0.05	0.26	0.02	0.17
170	10	2.79	0.56	0.09	0.79	0.32	1.02
170	20	2.81	0.84	0.07	0.86	0.64	1.81
170	30	2.77	0.71	0.03	0.88	0.77	2.32
170	40	2.93	0.71	0.03	0.87	0.47	1.98
170	50	2.90	0.69	0.04	0.96	0.63	2.29
170	60	2.80	0.67	0.06	1.00	1.14	3.14
180	0	2.89	0.70	0.09	0.27	0.35	0.99
180	10	2.75	4.33	0.06	0.47	0.65	2.19
180	20	2.79	4.77	0.08	0.54	1.25	2.91
180	30	2.72	3.89	0.18	0.30	1.19	3.33
180	40	2.98	4.64	0.14	2.60	0.57	2.48
180	50	2.82	3.64	0.17	2.94	0.31	3.32
180	60	2.82	4.01	0.16	3.70	1.08	3.39
190	0	2.57	2.16	0.09	1.87	0.09	1.24
190	10	2.59	4.87	0.07	2.64	0.70	2.13
190	20	2.68	5.20	0.09	3.35	0.85	2.29
190	30	2.87	5.16	0.09	2.91	1.07	3.52
190	40	2.68	2.75	0.10	3.18	0.98	2.88
190	50	2.69	1.73	0.14	2.60	1.47	2.91
190	60	2.68	1.39	0.17	2.48	1.52	2.85

---

The pH values of liquids after the catalysis process are displayed in Table 2. The pH values of all samples decreased after the catalytic process. This phenomenon was attributed to the presence of SO₄²⁻ in the solid acid, and the existence of more inorganic acid (formic acid and acetic acid) produced by further degradation of furfural. Therefore, higher yields of formic acid and acetic acid were obtained after catalytic process.

In the heterogeneous catalysis of hydrolysates using SO₄²⁻/SiO₂–Al₂O₃/La³⁺ as the solid acid catalyst, the oligosaccharides were further converted into mono sugars (xylose and arabinose) and sequentially into furfural and other chemicals under the function of solid acid catalysts. As illustrated in Fig. 2, the highest furfural yield of 21% was obtained from the hydrolysates obtained by the hydrothermal pretreatment process of corncob at 190 °C for 60 min. By the way, the side reaction of furfural unavoidably occurred, leading to the formation of humins and the degradation of furfural.³⁹

Fig. 2 The furfural yield of the liquid after catalysis. Reaction conditions: 10 mL of hydrolysates, 0.1 g of SO₄²⁻/SiO₂–Al₂O₃/La³⁺, 150 °C, 2.5 h.

3.3 Composition and property analysis of raw corncob and treated corncob

The influences of the hydrothermal reaction temperature and residence time on the chemical composition and solid residue yield of corncob are shown in Table 3. A minimum solid residue yield of 60.67% was achieved at 190 °C in 60 min. The corncob consists of 42.5% glucose, 31.3% xylose, 3.78% arabinose and 15.67% lignin (Table 3, entry 1). Enhancing the pretreatment temperature and time, the glucose content increased gradually from 42.50% to 64.35% (190 °C, 60 min). However, the xylose content decreased with increasing temperature and prolonged time and reached a minimum content of 3.15% (190 °C, 60 min). This result could also be confirmed by the small change in the xylose content of hydrolysates pretreated for a longer time (Fig. 1). A sharp decline of xylose content in the treated corncob verified the conclusion that the hydrolysis of hemicelluloses was easier than that of cellulose,⁴⁰ and a large amount of xylose was formed from the hydrolysis of hemicelluloses during the hydrothermal pretreatment process.⁴¹

Table 3 Composition of untreated and treated corncob under different hydrothermal pretreatment conditions

Entry	Sample	Solid residue yield (%)	Composition (%)
			Glucose	Xylose	Arabinose	Lignin
1	Raw material		42.50	31.31	3.78	15.67
2	170-0	96.50	41.82	30.43	2.96	15.53
3	170-10	90.00	43.37	29.73	2.35	18.88
4	170-20	87.17	45.82	28.36	1.55	19.19
5	170-30	88.17	46.22	25.81	0.65	21.91
6	170-40	84.00	50.15	21.87	0	21.15
7	170-50	80.00	51.57	22.12	0	22.69
8	170-60	79.17	52.19	21.54	0	21.42
9	180-0	89.00	43.63	30.13	1.34	16.18
10	180-10	81.33	48.78	19.27	0.58	20.96
11	180-20	76.50	48.04	16.05	0	23.55
12	180-30	76.67	52.38	15.08	0	26.82
13	180-40	74.83	60.16	8.85	0	25.08
14	180-50	73.00	60.73	5.07	0	28.19
15	180-60	69.67	61.21	6.84	0	28.71
16	190-0	81.50	45.14	28.62	0	18.37
17	190-10	74.50	50.54	20.09	0	24.92
18	190-20	71.50	51.21	13.63	0	27.84
19	190-30	67.00	55.42	9.57	0	28.62
20	190-40	64.50	59.96	10.78	0	28.96
21	190-50	64.00	63.91	3.61	0	29.19
22	190-60	60.67	64.35	3.15	0	30.60

---

Arabinose disappeared from the solid residue after pretreatment at 180 °C for 20 min, which was also confirmed in the discussion mentioned above in Table 1. Moreover, the lignin content increased from 15.67% to 30.60% under the investigated conditions. Increasing reaction severity led to an increment of lignin content, which was attributed to the release of hemicelluloses from the cell wall of the corncob and the formation of humins in the catalytic process,⁴² and it was difficult to remove humins from the solid residue easily.

3.4 XRD and SEM analysis of corncob before and after treatment

X-ray diffraction profiles of raw corncob and treated corncob are shown in Fig. 3a. Two typical diffraction peaks were detected in the range of 2θ = 15–23° for all samples. Reflections of (101) and (002) correspond to the transverse arrangement of the crystallites in cellulose I.⁴³ After the hydrothermal pretreatment process, reflection peaks near 16.0° and 21.7° of the treated corncob were similar to that of the raw material, indicating that the microcrystalline structure of cellulose in corncob was preserved during the process. Because of the thermal contraction behavior of the cellulose crystals, the diffraction peak at 21.7° shifted to higher angles with increasing pretreatment temperature.⁴⁴ The cellulose crystallinity of treated corncob increased from 17.1% (170 °C, 60 min) to 31.9% (190 °C, 60 min) with an increase of the pretreatment temperature. Under a specific temperature (190 °C), the crystallinity of cellulose increased from 17.3% in 0 min to 31.9% in 60 min with a prolonged reaction time.

Fig. 3 XRD patterns (a) and SEM images (b) of corncob and the treated samples.

The morphological structure of raw corncob and treated corncob sample are illustrated in Fig. 3b. The color of the corncob changed from yellow to dark brown after hydrothermal pretreatment. The untreated corncob showed a continuous and flat smooth surface in the SEM images. Compared with raw corncob, the treated sample revealed enormous differences in the surface morphology. The smooth structure of the raw material was destroyed under harsh conditions and abundant holes of different sizes can be observed on the treated corncob surface, which was attributed to the dissolution of hemicelluloses from the cell wall of corncob. This phenomenon was in good agreement with the results reported by Zhang et al.⁴⁵

3.5 Catalyst recyclability

Experiments were performed to investigate the reusability of the solid acid (SO₄²⁻/SiO₂–Al₂O₃/La³⁺) and the results are illustrated in Fig. 4, There was no obvious change in the catalytic activity of SO₄²⁻/SiO₂–Al₂O₃/La³⁺ after four runs, and the furfural yield only declined 5.28%. The excellent reusability of SO₄²⁻/SiO₂–Al₂O₃/La³⁺ could be attributed to the thermal stability of SO₄²⁻/SiO₂–Al₂O₃/La³⁺and the supplement of fresh SO₄²⁻.

Fig. 4 The recyclability of SO₄²⁻/SiO₂–Al₂O₃/La³⁺. Reaction conditions: 10 mL of hydrolysate (hydrothermal pretreatment conditions: 190 °C, 60 min) and 0.01 g of solid acid catalyst at 150 °C for 2.5 h.

4. Conclusion

An environmentally friendly two-step process for furfural production from corncob lignocellulose was performed by a mineral acid-free approach for the realization of the recovery of the solid acid catalyst. During the hydrothermal pretreatment of corncob, monosaccharides and oligosaccharides were the two major components in the hydrolysates, and the highest total xylose yield (xylose and xylo-oligosaccharides) of 7.01 g L⁻¹ was obtained at 190 °C for 60 min. Subsequently the separated hydrolysate was further catalyzed by solid acid (SO₄²⁻/SiO₂–Al₂O₃/La³⁺) catalysts for furfural production, and a highest furfural yield of 21% was obtained from the catalysis of hydrolysates obtained from the hydrothermal pretreatment of corncob at 190 °C for 60 min. The next work will focus on improving the furfural yield in the second step process.

Acknowledgements

This work was supported by the grants from Guangdong Natural Science Funds for Distinguished Young Scholar (S20120011250), Program for New Century Excellent Talents in University (NECT-12-0194), Program for State Key Laboratory of Pulp and Paper Engineering (2014C14) and the Fundamental Research Funds for the Central Universities of SCUT (2014ZG0003, 2015PT011).

References

1. G. W. Huber, S. Iborra and A. Corma, Chem. Rev., 2006, 106, 4044–4098.
2. A. J. Ragauskas, C. K. Williams, B. H. Davison, G. Britovsek, J. Cairney, C. A. Eckert, W. J. Frederick, J. P. Hallett, D. J. Leak, C. L. Liotta, J. R. Mielenz, R. Murphy, R. Templer and T. Tschaplinski, Science, 2006, 311, 484–489.
3. M. E. Himmel, S.-Y. Ding, D. K. Johnson, W. S. Adney, M. R. Nimlos, J. W. Brady and T. D. Foust, Science, 2007, 315, 804–807.
4. N. Mosier, C. Wyman, B. Dale, R. Elander, Y. Y. Lee, M. Holtzapple and M. Ladisch, Bioresour. Technol., 2005, 96, 673–686.
5. F. Carvalheiro, T. Silva-Fernandes, L. Duarte and F. Gírio, Appl. Biochem. Biotechnol., 2009, 153, 84–93.
6. S. P. Magalhaes da Silva, A. R. C. Morais and R. Bogel-Lukasik, Green Chem., 2014, 16, 238–246.
7. A. R. C. Morais, A. C. Mata and R. Bogel-Lukasik, Green Chem., 2014, 16, 4312–4322.
8. C. Rong, X. Ding, Y. Zhu, Y. Li, L. Wang, Y. Qu, X. Ma and Z. Wang, Carbohydr. Res., 2012, 350, 77–80.
9. A. S. Dias, S. Lima, D. Carriazo, V. Rives, M. Pillinger and A. A. Valente, J. Catal., 2006, 244, 230–237.
10. R. Karinen, K. Vilonen and M. Niemelä, ChemSusChem, 2011, 4, 1002–1016.
11. A. M. da Costa Lopes and R. Bogel-Łukasik, ChemSusChem, 2015, 8, 947–965.
12. A. V. Carvalho, A. M. da Costa Lopes and R. Bogel-Lukasik, RSC Adv., 2015, 5, 47153–47164.
13. H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science, 2007, 316, 1597–1600.
14. J. Y. G. Chan and Y. Zhang, ChemSusChem, 2009, 2, 731–734.
15. T. Suzuki, T. Yokoi, R. Otomo, J. N. Kondo and T. Tatsumi, Appl. Catal., A, 2011, 408, 117–124.
16. A. S. Dias, S. Lima, M. Pillinger and A. A. Valente, Carbohydr. Res., 2006, 341, 2946–2953.
17. H. Li, A. Deng, J. Ren, C. Liu, W. Wang, F. Peng and R. Sun, Catal. Today, 2014, 234, 251–256.
18. X. Shi, Y. Wu, P. Li, H. Yi, M. Yang and G. Wang, Carbohydr. Res., 2011, 346, 480–487.
19. L. Zhang, H. Yu and P. Wang, Bioresour. Technol., 2013, 136, 515–521.
20. P. L. Dhepe and R. Sahu, Green Chem., 2010, 12, 2153–2156.
21. A. J. Crisci, M. H. Tucker, M.-Y. Lee, S. G. Jang, J. A. Dumesic and S. L. Scott, ACS Catal., 2011, 1, 719–728.
22. H. Li, A. Deng, J. Ren, C. Liu, Q. Lu, L. Zhong, F. Peng and R. Sun, Bioresour. Technol., 2014, 158, 313–320.
23. B. Rivas, A. B. Moldes, J. M. Domínguez and J. C. Parajó, Enzyme Microb. Technol., 2004, 34, 627–634.
24. Y. Li, X.-D. Zhang, L. Sun, J. Zhang and H.-P. Xu, Appl. Energy, 2010, 87, 156–159.
25. G. Garrote, H. Domínguez and J. C. Parajó, Holz Roh- Werkst., 2001, 59, 53–59.
26. L. Segal, J. J. Creely, A. E. Martin and C. M. Conrad, Text. Res. J., 1959, 29, 786–794.
27. H. Noureddini and J. Byun, Bioresour. Technol., 2010, 101, 1060–1067.
28. D. Scordia, S. L. Cosentino, J.-W. Lee and T. W. Jeffries, Biomass Bioenergy, 2011, 35, 3018–3024.
29. G. Garrote, H. Domínguez and J. C. Parajó, Holz Roh- Werkst., 1999, 57, 191–202.
30. H.-Y. Mou, E. Orblin, K. Kruus and P. Fardim, Bioresour. Technol., 2013, 142, 540–545.
31. J. Xu, M. H. Thomsen and A. B. Thomsen, Biomass Bioenergy, 2009, 33, 1660–1663.
32. M. S. Tunc and A. R. P. van Heiningen, Ind. Eng. Chem. Res., 2008, 47, 7031–7037.
33. H. A. Ruiz, R. M. Rodríguez-Jasso, B. D. Fernandes, A. A. Vicente and J. A. Teixeira, Renewable Sustainable Energy Rev., 2013, 21, 35–51.
34. R. Kumar, G. Mago, V. Balan and C. E. Wyman, Bioresour. Technol., 2009, 100, 3948–3962.
35. A. Romaní, G. Garrote, J. L. Alonso and J. C. Parajó, Bioresour. Technol., 2010, 101, 8706–8712.
36. A. S. Mamman, J.-M. Lee, Y.-C. Kim, I. T. Hwang, N.-J. Park, Y. K. Hwang, J.-S. Chang and J.-S. Hwang, Biofuels, Bioprod. Biorefin., 2008, 2, 438–454.
37. J. Song, H. Fan, J. Ma and B. Han, Green Chem., 2013, 15, 2619–2635.
38. R. Weingarten, G. A. Tompsett, W. C. Conner Jr and G. W. Huber, J. Catal., 2011, 279, 174–182.
39. R. O’Neill, M. N. Ahmad, L. Vanoye and F. Aiouache, Ind. Eng. Chem. Res., 2009, 48, 4300–4306.
40. G. Garrote, R. Yáñez, J. L. Alonso and J. C. Parajó, Ind. Eng. Chem. Res., 2008, 47, 1336–1345.
41. W. Weiqi, W. Shubin and L. Liguo, Bioresour. Technol., 2013, 128, 725–730.
42. J.-W. Lee and T. W. Jeffries, Bioresour. Technol., 2011, 102, 5884–5890.
43. S. Park, J. Baker, M. Himmel, P. Parilla and D. Johnson, Biotechnol. Biofuels, 2010, 3, 10.
44. Y. Nishiyama, J. Wood Sci., 2009, 55, 241–249.
45. M. Zhang, W. Qi, R. Liu, R. Su, S. Wu and Z. He, Biomass Bioenergy, 2010, 34, 525–532.

---