Pyrolysis mechanisms of lignin: surface-immobilized model compound investigation of acid-catalyzed and free-radical reaction pathways

Abstract

The pyrolysis of surface-immobilized β-alkyl aryl ethers, m ≈ PhOCH 2 CH 2 Ph, ≈ PhCH 2 CH 2 OPh, and ≈ PhCH 2 CH 2 OPh- o- OCH 3, has been studied with a dispersed acid catalyst at 300–450 °C in order to gain an insight into the relative importance of ionic reactions vs. free-radical reactions in the thermal depolymerization of lignin. In the absence of acid catalyst, surface-immobilization does not hinder the free-radical reaction pathway to produce phenols, alkenes, alkanes, and aldehydes as the major products. Pyrolysis in the presence of small particle sized (15 nm) SiO 2-1% Al 2O 3 at 300 °C, where free-radical reactions are suppressed, produces an acceleration in the rate of decomposition and a substantial alteration in the product distribution as a consequence of the solid-solid interactions between the catalyst and substrate. Products from the silica-alumina catalyzed reactions are characteristic of acid-catalyzed cracking reactions which involve carbocation intermediates to produce products from ether cleavage and aromatic alkylation and dealkylation reactions resulting most notably in the near absence of alkenes. The surface-immobilized primary products continue to undergo acid-catalyzed reactions to form larger aromatics and coke. At temperatures in which the free-radical and acid-catalyzed reactions can compete (> 375 °C), products from the acid-catalyzed reaction dominate. A comparison of the products from the pyrolysis of lignin, biomass, and the surface-immobilized model compounds provides new evidence that the thermal degradation of lignin occurs principally by a free-radical reaction pathway.