Restricted Mass Transport Effects on the Pyrolysis Pathways for Lignin Model Compounds. Aryl Ether Linkages

Aryl ethers are important linking units in the structure of lignins and low rank coals that are thermally labile at 375°C. Mass transport limitations experienced in these cross-linked macromolecular solids can lead to alterations in thermochemical processing kinetics and product yields. The molecular level details of these alterations on reaction pathways are revealed through studies of model compounds that are constrained to an inert silica surface through covalent attachments. The thermolysis mechanisms for the silica-immobilized (s) 0-aryl ether model compounds, =PhCH,CH,OPh, =PhOCH2CH2Ph, =PhCH,CH,OPh-o-OCH,, and the a-aryl ether model, sPhOCH,Ph, have been investigated at 275-375°C. Free-radical rearrangement pathways involving 0,C-phenyl shifts are found to be significant for these aryl ethers under diffusional constraints leading to new, unpredicted pyrolysis products. The selectivity for these rearrangement pathways depends strongly on the structure of neighboring molecules, as determined from studies of two-component surfaces containing spacer molecules.