ReviewPreparation of SnO2–Co3O4/C biochar catalyst as a Lewis acid for corncob hydrolysis into furfural in water medium
Graphical abstract
Introduction
In recent years, lignocellulosic biomass has been studied intensively as a renewable resource for bio-based fuels, chemicals, pharmaceuticals and materials [1]. Natural lignocelluloses include wood, agricultural residues, perennial herb and so on [2]. Corncob is believed to have the greatest potential of all agricultural residues due to its wide availability and low cost. The major compositions of the corncob are 35%wt cellulose, 38%wt hemicelluloses and 20%wt lignin which have very distinct function, as shown in Fig. 1 [3]. Hemicelluloses are polysaccharides in plant cell walls that include xyloglucans, xylans, mannans and β-1-3 or β-1-4 glucans and glucomannans. It has been reported that xylose can be produced from depolymerization of hemicelluloses and furfural can then be produced from xylose isomerization and dehydration [4].
Mesoporous carbon materials have been widely used as a carrier of MxOy/C catalysts due to their larger surface-to-volume value and high loading capacity of metal oxides. Gu have successfully synthesized Ce-doped SnO2/C by sol-gel method to enrich phosphopeptides for mass spectrometry analysis [5]. Jang et al. researched SiO2/C as an effective anodic material for lithium-ion batteries, which could provide high discharge capacity and coulombic efficiency [6]. Zhang et al. used activated carbon as carbon source to prepare Zn/Ni/C catalyst which was employed in hydrothermal degradation of cellulose to lactic acid meanwhile the metals were oxidized into metal oxides after these were used [7]. Kobayashi used Ru/C catalyst for the hydrolysis of cellulose to glucose with a high yield of 34% [8]. It was also reported that yields of 7.1% HMF and 0.7% furfural were obtained from cellulose degradation over carbon-supported ruthenium catalyst [9]. These reports show that the carbon-supported catalysts have attracted great interests in chemical industry.
It is known that the presence of both Lewis acid (electron receptor) and Brønsted acid (proton donors) is important for high yields of furfural from cellulose. However, in the past Brønsted acid had been widely used to degrade sugar into furfural, mostly with mineral acids such as H2SO4 [10]. Recently, Choudhary demonstrated that xylose could be converted to furfural by a series of reactions, including isomerization of xylose to xylulose followed by xylulose dehydration to furfural, in which the zeolite Sn-beta was used as Lewis acid for the xylose isomerization and HCl or Amberlyst-15 was used as Brønsted acid for the xylulose dehydration reaction [4]. Choudhary et al. also found that the xylose could be dehydrated to produce furfural in the presence of a single Brønsted acid catalyst, but this reaction required much more energy and the yield of furfural was lower than that using both Brønsted and Lewis acid catalysts [11]. In recent years, Lewis acid is regarded that it plays an important role on furfural and HMF production [12].
It is well known that some metal oxides can provide Lewis acid sites on their surface, such as ZrO2, Al2O3, SnO2 and SiO2. Additionally, Sn-SBA-15 and Sn-beta are also regarded as a strong Lewis acid due to the availability of Lewis acid sites [13]. Tin oxide was utilized in catalytic dehydration of xylose [14]. Jailma Barros reported that cellulose could be dehydrated to HMF with Sn (IV) as Lewis acid directly and achieved a yield of 6.6% [15]. In the separate study, SiO2–SnO2/C catalyst was examined for degradation of sugars and resulted in 83% yield of lactic acid [16]. However, the carbon-support catalyst tin oxide employed in degradation corncob for furfural has not been investigated yet.
This paper presents a hydrothermal degradation process of corncob saccharification and monosaccharide dehydration to produce furfural using SnO2–Co3O4–2/C biochar catalyst as Lewis acid and hydronium ion H3O+ as Brønsted acid. The SnO2–Co3O4–2/C biochar catalyst was prepared by sugar solution and lignocelluloses from corncob degradation, as well as mixed precipitate of Sn(OH)4 and Co(OH)2. Meanwhile, an environmentally-friendly hydrothermal process for corncob degradation over SnO2–Co3O4–2/C biochar catalyst which was made by recycling of waste residue and sugar solution from corncob degradation was illustrated in Scheme 1.
Section snippets
Material
Corncob was supplied from a local farm located in Hebei Province, China. Corncob was firstly chopped into small pieces and dried at 60 °C under vacuum for 24 h. Then the dried corncob particles were sieved through 20 and 80 meshes to collect particles sized between 0.9 mm and 0.2 mm for experiments. Xylose and xylulose were supplied from Bioreagent Company, Shanghai, China. The chemicals and organic solvents used in experiments were all of analytical grades and purchased from Tianjin Kermel
Corncob saccharification
The product yields of furfural, xylose and xylulose from corncob hydrolysis, respectively, in water medium alone, in water with the SnO2–Co3O4–2/C catalyst, and in water with H2SO4, which the concentration was equivalent to SnO2–Co3O4–2/C catalyst, were shown in Table 2. The experimental results proved that hemicellulose in the corncob was saccharified firstly and the yield of xylose was almost identical theoretically at first until xylose converts to xylulose. Before xylose conversion to
Conclusions
In this work, a new furfural production process was investigated with an effective catalyst SnO2–Co3O4–2/C for corncob degradation to produce furfural. Both of Lewis acid sties derived from bimetallic oxides SnO2–Co3O4 and Brønsted acid sties derived from hydronium ion of hot water were indispensable for the furfural production by corncob hydrolysis. The condition of SnO2–Co3O4–2/C catalyst was prepared with 1.5 g lignocellulose residue and 150 ml degradation solution from corncob degradation, as
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant no.21176055), Tianjin Key Research Program of Application Foundation and Advanced Technology (No. 11JCZDJC23600) and Application Bases and Key Research Program of Hebei Province (No. 11963924D). The authors would like to thank Professor Shusheng Pang, Department of Chemical and Process Engineering, University of Canterbury, for valuable discussions and linguistic revision.
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