Catalytic depolymerization of calcium lignosulfonate to high-value oxygenated aromatic compounds over the efficient Fe2O3-La0.8Sr0.2FeO3

Abstract

Calcium lignosulfonate (CLS), a by-product of papermaking, contains abundant oxygen-containing functional groups; it has great application value in the production of fine chemicals. Based on the excellent catalytic performance of the conventional hydrothermal depolymerization method, the synergistic depolymerization of calcium lignosulfonate was achieved by Fe₂O₃-La_0.8Sr_0.2FeO₃ composite oxides. Fe₂O₃ crystals with small size (30–50 nm) and high crystallinity (96.1%) were obtained via a simple one-step method at pH = 9 and 200 °C for 12 h. Fe₂O₃-La_0.8Sr_0.2FeO₃ was prepared via the combination of the hydrothermal method and the sol–gel method. The individual calcium lignosulfonate depolymerization exhibited a liquid-phase yield of 45.6% and the selectivity of aryl compounds containing oxygen of 40.3%. When using separate Fe₂O₃ or La_0.8Sr_0.2FeO₃, the liquid-phase yield reached 50.9 and 48.3%, respectively. The selectivity of aromatic compounds and syringin increased by 4.3 and 4.7% using Fe₂O₃, while the selectivity of guaiacol was enhanced by 7.0% with La_0.8Sr_0.2FeO₃. Through the synergistic effect of Fe₂O₃-La_0.8Sr_0.2FeO₃, the liquid-phase yield reached 78.3%, and the selectivity of aromatics, syringin, and guaiacol increased by 5.7, 11.6, and 12.0%. Moreover, the introduction of ethyl acetate in the ethanol–water system accelerated the transference of more phenolic compounds to the liquid phase with higher yield (78.3%) and selectivity of syringin (27.2%), aromatics (6.4%), guaiacol (20.8%), and oxygen-containing aryl compounds (67.1%) at V(ethyl alcohol) : V(water) : V(ethyl acetate) = 65 : 25 : 10. The employment of La_0.8Sr_0.2FeO₃ efficiently avoids the sintering of Fe₂O₃ and imparts excellent stability through four regeneration cycles; the yield of the liquid-phase product could be maintained at 70.6–76.8% under the coupling effect of adsorbed oxygen and oxygen vacancies.