Hydrogenation of γ‑Butyrolactone to 1,4-Butanediol over CuCo/TiO₂ Bimetallic Catalysts

Titania-supported monometallic and bimetallic Cu–Co catalysts were prepared using (co)­impregnation and studied for the hydrogenation of γ-butyrolactone (GBL) to 1,4-butanediol (BDO) at temperatures from 100 to 180 °C and a hydrogen pressure of 3.4 MPa. The highest catalytic activity occurred at a Cu:Co atomic ratio of 1:9 (Cu_0.1Co_0.9/TiO₂), and a 95% yield of BDO was obtained. Characterization results showed mainly small nanoparticles (average size 2.6 nm) for pure Cu/TiO₂, large particles (∼19.8 nm) for pure Co/TiO₂, and a bimodal particle size distribution of both small (∼2.3 nm) and large (∼16.5 nm) particles for the bimetallic catalyst with a Cu:Co ratio of 1:1. The addition of ∼10 mol % Cu to Co/TiO₂ increased the reducibility of the Co and resulted in the formation of core–shell CuCo bimetallic nanoparticles with a Co-rich core and Cu-rich shell. GBL hydrogenation in liquid ethanol and water produced an ester (ethyl 4-hydroxybutanoate) and a carboxylic acid (4-hydroxybutanoic acid) as the major products, respectively. GBL hydrogenation in 1,4-dioxane likely went through a 2-hydroxytetrahydrofuran (2-HTHF) intermediate. The 2-HTHF underwent facile ring-opening tautomerization to 4-hydroxybutanal (4-HB), followed by rapid hydrogenation to BDO at a reaction rate up to 700 times faster than GBL hydrogenation. The Cu_0.1Co_0.9/TiO₂ catalyst maintained the BDO selectivity and about 80% of initial activity for GBL hydrogenation after 150 h time on stream in a continuous flow reactor