Aojie Denga,
Junli Ren*a,
Huiling Lia,
Feng Pengb and
Runcang Sunab
aState Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510640, Guangdong, China. E-mail: renjunli@scut.edu.cn; Fax: +86-20-87111861; Tel: +86-20-87111861
bInstitute of Biomass Chemistry and Technology, College of Materials Science and Technology, Beijing Forestry University, Beijing, 100083, China
First published on 26th June 2015
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.
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 material16–19 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.
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:
![]() | (1) |
CrI = [(I002 − Iam)/I002] × 100 | (2) |
Carbohydrates and lignin in untreated and treated corncob were measured according to the NREL standard analytic method (NREL/TP-510-42618).
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
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.
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 SO42−/SiO2–Al2O3/La3+, 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. |
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.
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
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