Chemistry, Chinese Academy of Sciences,

Effectiveness and potential of multiphase reaction systems using dense phase CO2 are demonstrated for several catalytic hydrogenation reactions of α, β-unsaturated aldehydes, nitro compounds, phenol, and other substrates. The reaction rate and the product selectivity depend on several factors, including phase behavior, molecular interactions of CO2 with reacting species, formation of CO from CO2, and so on. The molecular interactions are evidenced by in situ high pressure Fourier transform infrared spectroscopy and these are an important factor responsible for the positive effects of CO2 pressurization on the reactions. The interactions occur between CO2 and the carbonyl group of α, β-unsaturated aldehydes, for example, and make it more reactive to hydrogenation compared with C=C group, facilitating the selective transformation to the corresponding unsaturated alcohols. In the hydrogenation of nitrobenzene, the first step of hydrogenation to nitrosobenzene is slowed down but the second hydrogenation step to N-phenylhydroxylamine and the third transformation step to the desired product aniline are accelerated. As a result, the 100 % selective hydrogenation of nitrobenzene to aniline can be achieved at any conversion level up to 100 % with conventional supported Ni catalysts in the presence of dense phase CO2. For the hydrogenation of phenol with supported Rh catalysts under dense phase CO2, the selectivity to cyclohexanone i