Ultrasound-assisted low-temperature catalytic lignin-first depolymerization of pine wood biomass to selectively produce propyl guaiacol

Abstract

Lignin-first depolymerization of biomass using reductive catalytic fractionation (RCF) to valorize lignin while impeding the loss of the carbohydrate fraction in it is a potential resource recovery strategy for the economical operation of a biorefinery. This work is focused on achieving maximum retention of carbohydrates in addition to obtaining a high degree of delignification with selective production of monomers. Low-metal containing Ni, Pd, Pt and Ru catalysts supported on activated charcoal (ACC) catalysts are developed for pine wood delignification. The synthesized catalysts were characterized for their structure, texture, pore size distribution, electronic state and acidity using various analytical techniques. Pd/ACC demonstrated the highest degree of delignification (67.7%) at 200 °C. The RCF activity of the tested catalysts in terms of selectivity towards propyl guaiacol and trans-isoeugenol in lignin oil followed the trend: Pd/ACC (95%) > Ru/ACC (93.5%) ≈ Pt/ACC (92.8%) > Ni/ACC (48.0%). Employing an ultrasonic pre-treatment of the pine wood enhanced the degree of delignification and the selectivity towards lignin monomers significantly. A high degree of delignification (82.4%), high selectivity to propyl guaiacol (92.5%) and ∼95% retention of carbohydrates were achieved with Pd/ACC. The X-ray diffraction analysis of the carbohydrate pulp obtained after RCF showed a reduction in crystallinity, implying the possibility of subsequent enzymatic processing of the carbohydrate pulp. The superior activity of Pd/ACC is attributed to the better dispersion and hydrogenation activity of Pd as compared to other metals.