Extraction of Wheat Straw with Aqueous Tetra-n-Butylphosphonium Hydroxide

Abstract

The stability of tetra-n-butylphosphonium hydroxide ([P 4444 ][OH] (aq) ) solutions and their potential for wheat straw extraction are investigated. Under certain concentration ranges, aqueous [P 4444 ][OH] (aq) is known to rapidly dissolve up to 20 wt% of cellulose at 25 C. However, at elevated temperatures and at the high concentration ranges required for cellulose dissolution, [P 4444 ][OH] (aq) irreversibly decomposes. This was determined by following the kinetics of decomposition at different temperatures and concentrations, using 31 P NMR analysis of the solutions. A lower concentration range of 40 wt% [P 4444 ][OH] (aq) was observed for fractionation of wheat straw, avoiding significant decomposition of the expensive phosphonium component. Herein, the possibilities for producing cellulose-rich fractions with reduced lignin contents and hemicellulose-rich extracts are discussed. A proposal is given for a full process cycle using [P 4444 ][OH] (aq) , where the phosphonium salt is used in fractionation and recovered by anion metathesis as a chloride salt. Although not demonstrated in this article, the chloride salt may be converted back to the hydroxide by means of, e.g., ion exchange.