Efficient conversion of wheat straw into furan compounds, bio-oils, and phosphate fertilizers by a combination of hydrolysis and catalytic pyrolysis

Abstract

The development of sustainable techniques to convert lignocellulosic materials into value-added chemicals remains a significant challenge. Herein, we report a novel technique to directly convert wheat straw to furan compounds, bio-oils, and phosphate fertilizers. Untreated wheat straw was initially converted into 5-hydroxymethylfurfural (HMF) and furfural in a biphasic reaction system using FePO₄ and NaH₂PO₄ as co-catalyst. The remaining FePO₄ in the solid residues was used as the catalyst to pyrolyze the solid residues, producing bio-oils and bio-char-based phosphate fertilizers. This combination of FePO₄ and NaH₂PO₄ co-catalyst exhibited higher selectivity towards HMF and furfural production than using only FePO₄ as a catalyst in the conversion of wheat straw. The maximum HMF yield, 44%, was obtained when the reaction was carried out at 160 °C for 60 min, while the highest furfural yield, 92%, was achieved when the reaction occurred at 150 °C for 60 min. This reaction system is one of the most effective reaction systems to date for the conversion of wheat straw. Excessive Brønsted or Lewis acid sites (Fe ions) cannot give high yields of HMF and furfural due to the formation of by-products, indicating that a synergistic combination of Brønsted and Lewis acid sites is critical to obtain high yields of furan compounds. Interestingly, FePO₄ could effectively catalyze the pyrolysis of unconverted cellulose into new compounds, such as 5-methylfuran and 2,5-methylfuran, which is not observed in non-catalyzed pyrolysis.