Boosting the guerbet reaction: A cooperative catalytic system for the efficient bio-ethanol refinery to second-generation biofuels

The catalytic activity of anionic ruthenium complexes toward the transformation of bio-ethanol to 1-butanol and higher alcohols is found to be dependent on the imidazolium counterion. After the identification of a parallel reaction involving the catalyst in hydrogen evolution, conversion and selectivity are impressively boosted by the addition of p-benzoquinones as co-catalysts. The catalytic system avoids the side reaction and led to highly competitive conversions up to 88% (0.2 % mol ruthenium catalyst loading, 1.5 % mol benzoquinone loading). Butanol and higher alcohols are produced in yields up to 85% (overall selectivity 97%) as a mixture of valuable alcohols for advanced biofuel and lubricants applications. The catalytic system can be recycled and the reaction shows comparable efficiency on a real matrix (alcohol from wine production chain wastes) even in the presence of significant amounts of water, thus closing a hypothetic economic circle. A reaction mechanism is proposed for the most promising ruthenium complex working in cooperation with the most efficient co-catalyst: p-benzoquinone.