Structural and thermal characterization of wheat straw pretreated with aqueous ammonia soaking

Production of renewable fuels and chemicals from lignocellulosic feedstocks requires an efficient pretreatment technology to allow ready access of polysaccharides for cellulolytic enzymes during saccharification. The effect of pretreatment on wheat straw through a low-temperature and low-pressure soaking aqueous ammonia (SAA) process was investigated in this study using Fourier transform infrared (FTIR), pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS), solid and liquid state nuclear magnetic resonance (NMR), and thermogravimetry/differential thermogravimetry (TG/DTG) to demonstrate the changes in lignin, hemicellulose, and cellulose structure. After treatment of 60 mesh wheat straw particles for 60 h with 28-30% ammonium hydroxide (1:10 solid/liquid) at 50 °C, sugar recovery increased from 14% (untreated) to 67% (SAA treated). The FTIR study revealed a substantial decrease in absorbance of lignin peaks. Solid and liquid state NMR showed minimal lignin structural changes with significant compositional changes. Activation energy of control and pretreated wheat straw was calculated according to the Friedman and ASTM methods and found to be decreased for SAA-treated wheat straw, from 259 to 223 kJ/mol. The SAA treatment was shown to remove significant amounts of lignin without strongly affecting lignin functional groups or structure.