Density functional study on the thermal stabilities of phenolic bio-oil compounds

Shaw, Alexander and Zhang, Xiaolei (2019) Density functional study on the thermal stabilities of phenolic bio-oil compounds. Fuel, 255. 115732. ISSN 0016-2361 (https://doi.org/10.1016/j.fuel.2019.115732)

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Abstract

Pyrolysis of biomass provides a potential carbon-neutral route to fuels and precursor chemicals through the formation of bio-oil. Lignin accounts for up to 40% of the weight of biomass feedstocks and so the products of lignin deconstruction form a significant portion of the bio-oil. Understanding the thermal stability of bio-oil species is critical for predicting the relationship between product prevalence and pyrolysis temperature, which in turn will allow for greater control of the output of key products. In this work, density functional theory (DFT) was employed to assess the stabilities of key bio-oil compounds by calculating their bond dissociation enthalpies (BDEs). 140 individual bonds across twenty-seven common bio-oil compounds representing eight different bond types were assessed. It was found that the PW6B95 functional can be used as a reliable method for predicting pyrolysis product stability through calculation of functional group BDEs. This is mainly owing to its low mean unsigned error (MUE) (0.5 kcal mol−1 = 2.1 kJ mol−1) in predicting BDEs in a test set of six bonds, as well as correct treatment of aromatic substitution effects for phenolic derivatives. The assessment results reflected that the weakest bonds of phenolic bio-oil species were the O–Me and Ph–O bond of the methoxy groups and the O–H bond of hydroxy groups. The weak bond strength exhibited by methoxy group bonds correlates well with reduced presence of guaiacyl and syringyl type species following higher temperature pyrolysis. Conversely, the hydroxy Ph–O and propenyl Ph–C bonds exhibited high BDEs, which is in agreement with the persistent presence of phenol and styrene type species following high temperature pyrolysis.

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