17731: SO2-catalyzed steam pretreatment of wheat straw; changes in physiochemical properties and its effect on enzymatic hydrolysis

Agricultural crop residues such as wheat straw will likely be one of the first lignocellulosic feedstock utilized for ethanol production in Canada. Although AFEX, dilute acid and steam pretreatment have all been shown to be effective pretreatment methods for wheat straw, steam pretreatment is particularly attractive due to its simplicity and relative low capital and operating costs. We have shown that impregnation with SO₂ has many benefits such as, increasing the recovery of the hemicellulose sugars, reducing the severity of pretreatment and enhancing enzymatic hydrolysis of the cellulose. However, controlling the SO₂ absorption onto lignocellulosic materials with low moisture content is challenging and the loss of SO₂ after impregnation is not usually reported. Little has been reported on the actual mechanism of SO₂ catalysis and its effect on the physical and chemical properties of lignocellulosic residues as SO₂ concentration increases. In general, steam pretreatment with SO₂ results in dissolution of the hemicellulose fraction and causes ultrastructural rearrangement of the lignin, which increases accessibility to the cellulose. When SO₂ was applied at concentrations ranging from 0-10%, this resulted in substrates with xylan contents ranging from 0.3-6.0%. The hemicellulose content combined with substrate characteristics such as accessible surface area, degree of polymerization, surface properties and crystallinity were used to try to understand how changes at the fiber, fibril and microfibril level all affected enzymatic hydrolysis. These wheat straw results were also compared with corn stover, another potential feedstock for ethanol production.