Selective production of anhydrosugars and furfural from fast pyrolysis of corncobs using sulfuric acid as an inhibitor and catalyst
Introduction
Due to depleting fossil fuel reserves and growing concerns regarding the environment and climate change, biomass, which is commonly accepted as a clean and carbon-neutral renewable energy source, has great potential as an alternative to fossil fuels for the production of fuel precursors and chemical intermediates [1], [2], [3], [4], [5], [6], [7]. To date, a variety of processes has been extensively explored for converting biomass into fuel precursors and chemical intermediates, including thermochemical, biochemical and catalytic conversion [8], [9], [10], [11], [12], [13]. Among these processes, fast pyrolysis is one of the most promising thermochemical routes since it can achieve the highest yield of bio-oil and the lowest production cost [14], [15], [16]. Previous studies have mostly focused on the optimization of the reactor geometry and process parameters for maximizing the yield of bio-oil [17], [18], [19]. Fast pyrolysis reactors primarily include bubbling fluidized beds, circulating fluidized beds, and ablative reactors [20]. Currently, most fast pyrolysis reactors have been successfully scaled up to either pilot or demonstration plant levels [21]. However, the lack of markets for bio-oil due to its poor quality hinders the further commercialization of fast pyrolysis technology. In particular, the complex composition of bio-oil imposes an enormous challenge to its efficient utilization [22], [23], [24]. Bio-oil mainly contains water and hundreds of oxygenated organic compounds, including organic acids, alcohols, ethers, aldehydes, ketones, esters, pyrolytic sugars, monophenols and phenolic oligomers. The respective concentrations of these organic compounds are very low (usually less than 5 wt%) [25], [26]. Anhydrosugars and furfural, as the most representative and commercially valuable compounds in bio-oil, have been identified as versatile platform molecules for the synthesis of transportation fuels, commodity chemicals and functional materials [27], [28]. The very low concentrations of anhydrosugars and furfural (usually less than 5 wt%) in bio-oil tend to make their extraction and subsequent purification difficult and costly [29]. Simultaneous conversion of cellulose and hemicellulose into anhydrosugars and furfural during fast pyrolysis could be an effective approach for addressing the technical obstacles mentioned above and would facilitate the successful commercialization of fast pyrolysis technology.
Anhydrosugars from fast pyrolysis of biomass mainly include levoglucosan, xylosan and 1,6-anhydro-β-d-glucofuranose. Levoglucosan is mainly obtained by the thermal cleavage of glycosidic bonds in cellulose [30]. It is well known that fast pyrolysis of pure cellulose produces a high yield of levoglucosan (up to 70 wt%), whereas fast pyrolysis of raw biomass generates very low yield of levoglucosan (less than 3 wt% in this study) [31], [32]. This difference is attributed to the fact that even tiny amounts of alkali and alkaline earth metals (AAEM) in biomass can reduce the yield of levoglucosan by accelerating pyranose ring-opening reactions that favor the formation of low-molecular-weight oxygenates. Banks and coworkers showed that demineralization of biomass using deionized water, hydrochloric acid or Triton X-100 surfactant can promote the yield of levoglucosan and levoglucosenone [33]. Oudenhoven and coworkers found that organic acid leaching of biomass effectively decreased the amount of catalytically active AAEM, resulting in significant increases in the yields of organic pyrolysis oil and anhydrosugars [34]. Shafizadeh suggested that water washing combined with H2SO4 impregnation appeared to be most effective for improving the yield of pyrolytic anhydrosugars when lignin was present [35]. Brown [36] and Garcia-Perez [37] demonstrated that the yield of levoglucosan from fast pyrolysis of biomass can be significantly enhanced by the passivation of AAEM using mineral acid impregnation. Furfural is mainly derived from the dehydration reaction of hemicellulose. Blasi [38] and Lu [39] showed that acidic catalysts can improve the yield of furfural from fast pyrolysis of corncobs. H2SO4, Fe2(SO4)3 and ZnCl2 are the most effective catalysts for maximizing the yield of furfural.
Despite these advances, limited attempts have been made to simultaneously convert cellulose and hemicellulose into anhydrosugars and furfural by fast pyrolysis of biomass. Moreover, little information is provided on the role of H2SO4 during fast pyrolysis of acid-impregnated biomass and on the product distribution from fast pyrolysis of acid-impregnated hemicellulose. Hence, corncobs, one of the most abundant agricultural residues derived from corn processing, were selected as the feedstock in this study due to their high hemicellulose content. Fast pyrolysis of corncobs impregnated with different amounts of H2SO4 was systematically investigated for the selective production of anhydrosugars and furfural. For comparison, fast pyrolysis of H2SO4-impregnated cellulose, xylan and demineralized corncobs was also performed to identify the role of H2SO4 in the formation of anhydrosugars and furfural.
Section snippets
Materials
Corncobs were obtained from the Baodi feed mill in Tianjin, China. Before the experiments, the corncobs were pulverized, sieved to the desired particle size (0.25–0.40 mm) and then dried at 105 °C for 24 h. The C, H, N and O contents of the raw corncobs were 44.70%, 6.12%, 0.23% and 48.95%, respectively. The hemicellulose, cellulose and lignin contents of the raw corncobs were 31.60%, 37.63% and 20.77%, respectively, according to the method developed by Van Soest [40]. Sulfuric acid (analytical
Thermogravimetric analysis of raw and acid-impregnated corncobs
The slow pyrolysis behaviors of the raw and acid-impregnated corncobs were inspected by thermogravimetric analysis. Fig. 1A and B shows the weight loss (thermogravimetry, TG) and the weight loss rate (differential thermogravimetry, DTG) curves, respectively. There are two well-defined peaks in the DTG curves from the pyrolysis of the raw corncobs. The first peak centered at approximately 254 °C was mainly attributed to the degradation of the hemicellulose fraction in the corncobs, while the
Conclusions
Simultaneous conversion of cellulose and hemicellulose into anhydrosugars and furfural was successfully achieved by fast pyrolysis of H2SO4-impregnated corncobs. The optimal results were obtained from the fast pyrolysis of corncobs impregnated with 2.75 wt% H2SO4. The yields of levoglucosan, furfural and xylosan from the corncobs impregnated with 2.75 wt% H2SO4 were enhanced by 634%, 805% and 1406%, respectively, when compared with those from the raw corncobs. By comparing the yield of
Acknowledgements
This work was supported by the Major International (Regional) Joint Research Project of the National Science Foundation of China, China (grant number 51661145011), the National Natural Science Foundation of China, China (grant numbers 51776209, 21406227 and 51376186), the Science and Technology Planning Project of Guangdong Province, China (grant number 2015A020215024), the Natural Science Foundation of Guangdong Province, China (grant number 2014A030313672), and the Pearl River S&T Nova
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