Pretreatment of wheat straw with ozone and soaking aqueous ammonia

Allan Gao

Wheat straw is an abundant agricultural residue in the United States that contains cellulose and hemicellulose sugar polymers. The enzymatic hydrolysis of these sugar polymers creates monomeric sugars which can be used as a substrate to produce valuable fuels and chemicals. However, hydrolysis of the sugar polymers in wheat straw requires a pretreatment step to reduce the recalcitrance of the biomass against access of hydrolytic enzymes. In this work ozone and soaking aqueous ammonia (SAA), which are two methods of pretreatment that utilize low temperature and low pressure, were investigated for their effect on wheat straw. The changes in lignocellulosic structure that occur after ozone and SAA pretreatment were systematically characterized, and the results of these studies offered some insight as to how these pretreatments could be used in combination for greater effectiveness. Analysis such as FTIR, Py-GC/MS, solid/liquid state NMR, and TG/DTG were used to characterize the changes on wheat straw biopolymers (lignin, hemicellulose, and cellulose). Ozone pretreatment was found to act preferentially on S2/S6 and G2 lignin monomers, and modified a large portion of lignin in the wheat straw after 2 hr of pretreatment time. SAA pretreatment on the other hand did not significantly alter the lignin structure, but did remove lignin from the biomass and subsequently improved the yield of enzymatic hydrolysis. Further studies demonstrated that washing of the wheat straw before ozonation to remove part of the extractive and ash content with either water or dilute acid improved the yield of enzymatic hydrolysis, indicating that extractives in the wheat straw may have a negative influence on the ozone pretreatment process. A sequential combination of ozone pretreatment followed by SAA pretreatment resulted in greater sugar yields after enzymatic hydrolysis than either single pretreatment alone. Additionally, the combination pretreatment significantly reduced the chemical usage and reaction time required of both ozone and soaking aqueous ammonia pretreatments.

Pretreatment of wheat straw with ozone and soaking aqueous ammonia

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