(476h) Competing Dehydration Reactions of Branched Alcohols on Solid Acid Catalysts

Etherification is useful for biomass upgrading because it is a dehydration reaction and can decrease the oxygen content of biomass-derived mixtures such as pyrolysis oils without hindering the carbon yield of the process. High oxygen content contributes to the poor stability and fuel properties, but etherifying alcohol groups, and hydrogenating unsaturated carbon-carbon bonds, is a potential pathway to create fuel additives that assist with complete combustion of fuels such as diesel. However, extensive branching, which is common for biomass-derived compounds, has been shown to hinder the acid-catalyzed etherification selectivity. While working with 5-hydroxymethylfurfural (HMF) as a model product from catalytic fast pyrolysis of woody biomass, we observed selectivity to etherification with ethanol to be as high as 95%, with similar results for etherification with phenol. However, if HMF is hydrogenated first, and tetrahydrofurfuryl alcohol is used as a model compound, selectivity to ether with 2-methylcyclohexanol was observed to be much lower, around 25%. A competing dehydration reaction results in unsaturated carbon-carbon bonds, which are not desirable for fuels applications. This side reaction is especially prevalent with increased branching of the reactants. As there is little conclusive comprehensive work on the impact of branching on the competing dehydration reaction mechanisms, this work steps through incremental levels of branching on the alpha- and beta-carbon using butanols as model compounds. 1-butanol, sec-butanol, iso-butanol, and tert-butanol represent all possible branching at the alpha carbon as well as branching at only the beta carbon. Using a gas phase, Carberry CSTR with online GC-FID, ZSM5 zeolite as a solid acid catalyst, and varying feed composition and temperature, initial reaction rates of etherification as well as the competing unimolecular dehydration reactions are studied to determine rate expressions that give information about how these reactions proceed and to predict conditions for preferential selectivity towards desired ethers.