The role of shape selectivity in catalytic fast pyrolysis of lignin with zeolite catalysts

Catalytic fast pyrolysis (CFP) of lignin with four different zeolite catalysts was investigated to determine the role of shape selectivity of zeolites in CFP. These zeolites included ZSM-5, mordenite, beta, and Y zeolites, which have various crystallographically determined static pore sizes between 5.6 and 7.6 Å. The molecular dimensions of pyrolysis products, including lignin-derived oxygenates and aromatics, were calculated using quantum chemical computations. The effective pore sizes of the four zeolites at 650 °C were then determined by analyzing the molecular size and conversion behavior of the pyrolysis products in CFP. Results suggest that thermal distortion of the zeolite pore structure under high-temperature conditions of CFP effectively enlarge the crystallographically determined pore sizes of the zeolites by 2.5–3.4 Å. Therefore, many lignin-derived oxygenates with a molecular size considerably larger than the static pore size were able to enter the pores of the zeolites and become effectively converted in our CFP tests. Bulkier monolignols derived from syringyl lignin, however, could not be effectively converted by ZSM-5 and mordenite zeolites due to size exclusion or pore blockage. Among the four zeolites, ZSM-5 produced the highest aromatic yield, followed in order by beta, mordenite, and Y zeolites. Beta and Y zeolites were the most effective catalysts for deoxygenating lignin-derived oxygenates. This analysis indicates that for CFP of softwood, ZSM-5 is the optimal catalyst because it can achieve satisfactory deoxygenation and aromatic production simultaneously, whereas for hardwood feedstock, beta zeolite may be used to convert bulky oxygenates derived from syringyl lignin.

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► The molecular dimensions of lignin pyrolysis products were calculated.
► The effective pore size of four zeolites at 650 °C were evaluated.
► The pore size of zeolite is substantially enlarged at high temperatures of CFP.
► Shape selectivity dictates the conversion of lignin-derived oxygenates in CFP.