Zeolite Titanium Beta: A Selective Catalyst for the Gas-Phase Meerwein–Ponndorf–Verley, and Oppenauer Reactions

Aluminium-free zeolite titanium beta was tested in the Meerwein–Ponndorf–Verley reduction of 4-methylcyclohexanone with various secondary and primary alcohols as hydrogen donors, using a fixed-bed continuous-flow gas-phase reactor. A high selectivity towards the thermodynamically unfavourablecis-4-methylcyclohexanol was observed, which is ascribed to transition-state selectivity in the straight channels of zeolite beta. However, in the gas-phase, the selectivity to thecis-alcohol is significantly lower than in the liquid-phase when using 2-propanol as the hydrogen donor. Based on kinetic and adsorption experiments, it is concluded that the concentration of alcohol reductant in the zeolite is an important parameter in determining selectivity. More hydrophobic alcohols give rise to an increased selectivity to thecis-alcohol product. From the sorption experiments it was concluded that this is due to a higher internal concentration of the alcohol reductant in the hydrophobic titanium beta zeolite. The observed selectivities and activities are consistent with a mechanism in which this alcohol is not only the hydrogen donor but is also required to remove thecis-alcohol product formed from the catalytic site by alcoholysis, before consecutive reactions can take place.