Minireview: recent efforts toward upgrading lignin-derived phenols in continuous flow

Phenols are vital building blocks for manufactured goods, agrochemicals, and medicine. As the demand to circumvent fossil fuels intensifies, so does the need to establish alternate sources of chemical feedstock, with many researchers looking to biomass. Lignin, a complex biopolymer found in plants, represents a relatively untapped renewable source of phenolic compounds. Examples of aromatic compounds derived from lignin include phenol, guaiacol (2-methoxyphenol), and catechol (2-hydroxyphenol). The development of new technologies and chemistry to upgrade or modify these compounds is crucial to realizing the potential of biomass as chemical feedstock. Over the last two decades, continuous flow technology has become an established tool for chemists to expand the capabilities of reaction control and to provide a means to probe novel reactions that might otherwise be difficult to study or accomplish via conventional means. Flow also imparts many advantages, such as superior mixing capabilities for gas/liquid reactions, increased efficiency for photochemical transformations, and improved scalability. This mini review will highlight research efforts reported in the last half-decade (since 2016) toward upgrading lignin-derived phenolic substrates using continuous flow technology. Recent developments focus on reactions requiring oxygenation, oxidation, hydrogenation, and deoxygenation to name a few.

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