Selective Conversion of Lignin-Derivable 4-Alkylguaiacols to 4-Alkylcyclohexanols over Noble and Non-Noble-Metal Catalysts

Abstract

Recent lignin-first catalytic lignocellulosic biorefineries produce large quantities of two potential platform chemicals, 4-n-propylguaiacol (PG) and 4-n-propylsyringol. Because conversion into 4-n-propylcyclohexanol (PCol), a precursor for novel polymer building blocks, presents a promising valorization route, reductive demethoxylation of PG was examined here in the liquid-phase over three commercial hydrogenation catalysts, viz. 5 wt % Ru/C, 5 wt % Pd/C and 65 wt % Ni/SiO2–Al2O3, at elevated temperatures ranging from 200 to 300 °C under hydrogen atmosphere. Kinetic profiles suggest two parallel conversion pathways: Pathway I involves PG hydrogenation to 4-n-propyl-2-methoxycyclohexanol (PMCol), followed by its demethoxylation to PCol, whereas Pathway II constitutes PG hydrodemethoxylation to 4-n-propylphenol (PPh), followed by its hydrogenation into PCol. The slowest step in the catalytic formation of PCol is the reductive methoxy removal from PMCol. Moreover, under the applied reaction conditions, PCol...