Alkylation of phenol and substituted phenols with C1–C4 alcohols/olefins as an upgrading route for bio-oil oxygenates: A review

The global concern for depletion of fossil fuel and greenhouse gas emission demands a rigorous research on development of sustainable alternate fuels and important chemicals from the renewable second generation lignocellulosic biomass in the last decade. In this review, upgradation of pyrolysis bio-oil, derived from lignocellulosic biomass, using alkylation as an effective C–C coupling reaction between its phenolics and C1–C4 oxygenate constituents is discussed. There is a considerable loss of light oxygenates in existing bio-oil upgrading technologies employing hydrodeoxygenation process and this loss can be overcome by using them as alkylates in the alkylation reaction of the aromatic oxygenates, namely, phenolics, also present in it, before the subsequent hydrodeoxygenation step, which will avoid carbon loss and improve the yields of the valorization products. In this context, a review of research advances in the area of catalytic alkylation of phenolic compounds with different alkylating agents (C1–C4 alcohol/olefins) and with special attention to m-cresol alkylation with iso-propanol is discussed. This review classifies the reported literature into thermodynamic, catalytic activity, mechanistic, and kinetic analyses to give a clear picture of the research data on this reaction and an outlook for the process scale-up. The review of such reactions using this classification has not been reported so far.