Inhibition Study in Biofuels and Biochemicals Production from Lignocellulosic Biomass

Abstract

Developing biofuels and renewable chemicals from lignocellulosic biomass has great potential to reduce the dependence on fossil fuel and address the environmental issues. However, it encounters several techno-economic challenges. One of the major issues is the degradation compounds formed through pretreatment considerably inhibits the subsequent enzymatic hydrolysis and microbial fermentation and thus, severely limits the efficient utilization of lignocellulose. Therefore, the present study focuses on understanding the effects of those potential inhibitors on microbial fermentation and eliminating/reducing their toxicity in biochemical conversion, which is critical to the development of biorefinery. It is believed that the inhibitory activity of degradation compounds greatly depends on their chemical structures. The effects of carbonyl compounds were examined with lactic acid fermentation and it was observed that their inhibitory severity on cell growth rate and lactic acid yield followed the order: aromatic aldehydes > phenols > acids > furan aldehydes. Moreover, quantitative structure-inhibition relationship (QSIR) was attempted to associate inhibition activity with molecular descriptors. It was found that inhibition constant (KI) of aldehydes was well correlated with hydrophobicity (log P) and thiol reactivity (log KGSH). It revealed the carbonyl inhibition on lactic acid fermentation was governed by the hydrophobicity and electrophilic reactivity and the target of detoxification should be directed to remove or reduce the hydrophobicity and/or electrophilicity of carbonyl compounds. Since the aromatic aldehydes exhibit the strongest inhibition and they are frequently present in the prehydrolysates and thus, their inhibitory effects were further examined. The substituent effect of 13 aromatic aldehydes on acetone-butanol-ethanol (ABE) fermentation was assessed with the attempt to reveal the inhibition mechanism and develop effective detoxification method in biofuels production. It was observed that the inhibition activity was affected by the ortho substituents (OH > OCH3 > CHO) and strongly related to the position of hydroxyl group instead of the number of hydroxyl groups. The ortho- hydroxyl group significantly contributed to the aromatic aldehyde inhibition. The ortho-substituted 2-hydroxybenzaldehyde caused at least 20 fold stronger inhibition than meta- and para- substituted analogues of 3- and 4-hydroxybenzaldehydes. The presence of ortho- hydroxyl group can form an intramolecular hydrogen bond with carbonyl hydrogen and potentially increase the cell membrane permeability and electrophilicity. To develop effective detoxification method in butanol production from prehydrolysates is another research interest of this dissertation. Among the six detoxification strategies examined, anion exchange resin treatment was the most effective method but a lag phase of 72 h was observed in fermentation. To alleviate this problem, two-step detoxification strategy (Ca(OH)2+ anion resin) was developed, resulting in a satisfactory ABE fermentation of 11.11 g/L ABE produced within 48 h and a yield of 0.19 g/g sugar. In addition, the mineral salt was found to be toxic to the Clostridia and responsible for the long lag phase. The precipitation of salts by Ca(OH)2 could potentially shorten the lag phase. Finally, the butanol production from ethanol organosolv pretreated loblolly pine was investigated in both separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes. SSF was found to be a preferred process configuration compared to SHF in terms of butanol and ABE yield. Surprisingly, we found the addition of lignin into SHF process remarkable enhanced the ABE production. Furthermore, the supplementation with detoxified prehydrolysates into SSF improved the utilization of sugars present in lignocellulose, giving the butanol and ABE titer of 10.51 g/L and 18.29 g/L, respectively, which were 13% and 16% higher than that from solid only. This indicated the integration of prehydrolysates into the SSF process for butanol production was technically desirable. Our study suggested one tonne of dry wood could produce 46.6 gallons of ABE and 26.5 gallons of butanol in a SSF process, respectively.