Tuning Ethanol Upgrading toward Primary or Secondary Alcohols by Homogeneous Catalysis

Ethanol is one of the most promising renewable resources for producing key industrial commodities. Herein, we present the direct conversion of ethanol to either primary or secondary alcohols, or to hydrocarbons, using ruthenium PNP pincer complexes [(RPNP)RuHXCO] (R = iPr, Ph, Cy, tBu; X = Cl, H–BH3) as catalysts. Using phenyl-substituted phosphines leads to the selective production of secondary alcohols over primary alcohols. Hence, employing [(PhPNP)RuH(Cl)CO] (Ru-1) as a catalyst in ethanol, containing 20 mol % of NaOtBu, at 115 °C leads to 89% selective production of the secondary alcohols. A yield of 12% of 2-butanol, and in total 22% of secondary alcohols, was achieved. In addition, minor amounts of 2-butenes/butane (≤5%) were observed. On the contrary, when using bulky phosphine substituents, such as t-butyl, the selectivity completely shifts toward primary alcohols. Thus, using [(tBuPNP)RuH(Cl)CO] (Ru-5) leads to >99% selectivity of 1-butanol (13% yield) over secondary alcohols at 115 °C. The catalytic system is highly competitive for producing 1-butanol with 22% yield obtained at 130 °C. Our methodology unveils the potential for developing methods to use bulk bio-alcohols to selectively produce primary or secondary alcohols and hydrocarbons under mild conditions.