Conversion of lignin-derived phenolic compounds to aromatic hydrocarbons over zeolite during catalytic pyrolysis

Abstract

The goal of this study was to investigate the chemistry of the catalytic conversion of model compounds over various zeolite catalysts and to compare the effect of different type of zeolite catalysts to convert diverse model compounds. Eleven phenolic compounds were selected as representative lignin model compounds and classified with their side chain e.g. N (having no side chain), S (having saturated side chain) and O (having oxygenated functional group in side chain). These model compounds were pyrolyzed for 20 sec at 600 °C over three kinds of zeolite catalysts having different pore size (ZSM-5: 5.6 Å, BETA: 6.4 Å, Y: 7.4 Å). As pyrolysis products of model compounds benzene, toluene, xylenes and naphthalenes were produced as main aromatic hydrocarbons and other oxygenates compounds such as dimethylphenols as well as trimethylphenols were also generated. The acidity of catalyst had different values ranged from 1.141 to 1.837 mmol/g of catalyst, where surface area of Y was the largest among the three types of zeolite catalysts (574.61 m2/g). In comparison with S (syringyl) unit, production of aromatic hydrocarbons from G (guaiacyl) unit was relative low because G unit has lower H/Ceff ratio than S type. That of aromatic hydrocarbons were the lowest at O type compounds which has the lowest H/Ceff ratio (vanillin: syringaldehyde: acetosyringone:). Among the three types of zeolite catalysts, Y was found to be the most effective catalyst for producing monocyclic aromatics like benzene, toluene and p-xylene except for O type compounds. BETA was the most effective catalyst for the formation of naphthalenes. In the case of ZSM-5, lignin model compounds could not be effectively converted to aromatic hydrocarbons over ZSM-5 compared with other zeolite catalysts due to its small pore dimensions. Especially, the yield of pyrolysis products from phenol was very low over ZSM-5 since phenol was adsorbed at acid site. The yield of catalytic pyrolysis products increased with increasing surface area of zeolite catalyst. These results revealed that product selectivity was clearly affected by pore size of zeolite catalysts.