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Solvent effects were investigated for selective hydrogenation of acetophenone (AP) with commercial 5% Pd/Al2O3 and 5% Pd/C catalysts using polar and nonpolar solvents. The rate of AP hydrogenation varied with the solvents used in different ways depending on the catalysts used. The highest AP conversion was achieved with water for the two catalysts and the AP conversion was correlated with hydrogen-bond-acceptance (HBA) capability (beta) for Pd/Al2O3 and hydrogen-bond-donation (HBD) capability (alpha) for Pd/C. These trends were the same as the corresponding solvent effects obtained previously with Rh/ Al2O3 and Rh/C catalysts, respectively (Green Chem., 2015, 17, 1877-1883). The influence of solvents on the rate of AP hydrogenation depends on the support materials and not on the metal species. Furthermore, AP hydrogenation was conducted with the four supported noble metal catalysts in different solvents pressurized by CO2 (CO2-dissolved expanded solvents). The influence of CO2 pressurization on the rate of reaction, the product selectivity, and the catalyst life was investigated to know whether or not CO2 molecules could function as a reaction promoter in heterogeneous AP hydrogenation reactions with supported Pd and Rh catalysts in different solvents.

Selective hydrogenation of acetophenone (AP) to 1-phenylethanol (PhE) was investigated over Rh/Al2O3 and Rh/C catalysts in 13 solvents including water and conventional organic solvents. Strong solvent effects on the overall rate of AP conversion were observed in different manners depending on the catalysts used. The conversion obtained is correlated with hydrogen-bond-donation (HBD) capability for Rh/C but with hydrogen-bond-acceptance (HBA) capacity for Rh/Al2O3. The solvent effects should result from interactions between the carbonyl group of AP and the solvent molecules through hydrogen bonding for Rh/C and from those between the solvent molecules and the catalyst surface for Rh/Al2O3 having HBD hydroxyl groups on its surface. Water is the most effective functional solvent in the selective hydrogenation of AP for C and Al2O3-supported Rh catalysts due to its high HBD capability (a) and low HBA capability (beta), respectively. For the hydrogenation with Rh/Al2O3 in water, its large polarity/polarizability index (pi*) may contribute to the high selectivity to PhE.

The selective hydrogenation of benzyl cyanide (BC) to the primary amine of 2-phenylethylamine (PEA) was investigated with a Pd/Al2O3 catalyst in different multiphase reaction media including n-hexane, water, and/or CO2. In neat n-hexane, the hydrogenation of BC occurs at a large rate but no PEA was formed and the secondary amine of N, N-bis(2-phenylethyl) amine (BPEA) was produced as the main product. The pressurization of n-hexane with CO2 decreased the rate of hydrogenation and the main product was BPEA. In this n-hexane-CO2 medium, the catalyst lost its activity during the reaction. When a similar volume of water was added to n-hexane, the rate of hydrogenation did not change so much but a small amount of the primary amine was detected to form in addition to the main product of BPEA. In contrast to these reaction media, a high PEA selectivity of > 90% was obtained and no catalyst deactivation occurred in an n-hexane-water-CO2 medium. The rate of hydrogenation was smaller than that in neat n-hexane but comparable to those in n-hexane-water and n-hexane-CO2 media. Those results show that synergistic effects appear on the selective hydrogenation of BC to PEA when both water and CO2 are included in the reaction medium. Possible roles and actions of water and CO2 have been discussed on the basis of the results of hydrogenation runs under different conditions and in situ ambient and high pressure Fourier transform infrared spectroscopy measurements that give insights into molecular interactions of CO2 with reacting species.

The catalytic hydrogenation of 2,4-dinitroaniline using a 0.5 wt% Pt/TiO2 catalyst was investigated in a multiphase medium of tetrahydrofuran (THF) pressurized by CO2 at different pressures and at 323 K. When CO2 pressure was increased, the overall rate of hydrogenation simply decreased but the selectivity to the desired product of 4-nitro-1,2-phenylenediamine increased. The noticeable enhancement of the selectivity to 4-nitro-1,2-phenylenediamine can be explained by chemical reactivities of CO2 molecules. In situ high-pressure FTIR and molecular simulations demonstrate that the dissolved CO2 molecules may interact with amino groups of the substrate and weaken the intra-hydrogen bonding between the amino and 2-nitro groups, which results in the change in the relative reactivity of the two nitro groups, yielding the desired product in a higher selectivity. The change in the intra-and inter-molecular interactions between the substrate and CO2 molecules was theoretically examined by DFT calculations.

The hydrogenation of chloronitrobenzene to chloroaniline was investigated over Ni/TiO2 at 35℃ in supercritical CO2 (scCO2), ethanol, and n-hexane. The reaction rate followed the order of scCO2 > n-hexane > ethanol. In scCO2, the selectivity to chloroaniline and to aniline over Ni/TiO2 were 97-99.5% and < 1%, respectively, in the conversion range of 9-100%. The high chemoselectivity to chloroaniline cannot be achieved over Ni/TiO2 in ethanol and n-hexane. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO2 with the following reactant and reaction intermediates: chloronitrobenzene, chloronitrosobenzene, and N-chlorophenylhydroxylamine. The molecular interaction modifies the reactivity of each species and accordingly the reaction rate and the selectivity. The influence of Cl substituent on the interaction modes of CO2 with these reacting species is discussed. Possible reaction pathways for the hydrogenation of chloronitrobenzene in scCO2 over Ni/TiO2 are also proposed.

The selective hydrogenation of nitrobenzene (NB) over Ni/γ-Al2O3 catalysts was investigated using different media of dense phase CO2, ethanol, and n-hexane. In dense phase CO2, the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0-100%. The phase behavior of the reactant mixture in/under dense phase CO2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO2 over Ni/γ-Al2O3 is also proposed.