Aluminum-Doped Zirconia-Supported Copper Nanocatalysts: Surface Synergistic Catalytic Effects in the Gas-Phase Hydrogenation of Esters

The efficient gas-phase selective hydrogenation of a series of esters to the corresponding alcohols was achieved over well-dispersed aluminum-doped zirconia-supported copper nanocatalysts (Cu/Al-ZrO2), which were prepared through a homogeneous coprecipitation route in the presence of cetyl trimethyl ammonium bromide. The characterization revealed that the structure and catalytic performance of Cu/Al-ZrO2 nanocatalysts were profoundly affected by the addition of Al. Compared with the Al-free catalyst, Al-doped materials had higher specific surface areas and smaller copper nanoparticles. In particular, the results confirmed that the incorporation of Al into the ZrO2 framework could form tetrahedrally coordinated Al3+ species, leading to the improvement of metal dispersion and the formation of more surface Lewis acid sites. In the gas-phase selective hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG), 100% DMO conversion, 97.1% EG selectivity, and a high turnover frequency of 16.9 h(-1) were achieved over Cu/Al-ZrO2 catalyst with a Al/(Cu + Zr + Al) mass ratio of 0.1. The high efficiency of Cu/Al-ZrO2 catalysts in DMO hydrogenation was attributed mainly to the surface synergistic catalytic effect between highly dispersed metallic copper species and strong Lewis acid sites, which promoted the hydrogenation reaction related to the ester groups, unlike the single case of Cu+-Cu-0 synergy reported previously that was found to control the extent of hydrogenation. The obtained catalysts displayed excellent catalytic performance in the gas-phase hydrogenation of other esters including dimethyl succinate, dimethyl maleate, dimethyl adipate, and 1,4-cycolhexane dicarboxylate.