Selective hydrogenation of benzonitrile in multiphase reaction systems including compressed carbon dioxide over Ni/Al2O3 catalyst

•The addition of CO2 remarkably affected the reaction rate and product selectivity.•The decreased reaction rate is due to the formation of solid carbamate in the presence of CO2.•The yield of benzylamine was increased to >94.4% in the presence of CO2.•The presence of CO2 could prohibit the formation of dibenzylamine in organic solvents.•The acidic nature of H2O–CO2 benefited the elimination of NH3.

Selective hydrogenation of benzonitrile was studied with Ni/Al2O3 in compressed CO2, hexane–CO2, ethanol–CO2 and H2O–CO2 systems. The phase behavior and the effect of CO2 on the conversion of benzonitrile and the yield of benzylamine were discussed. The reaction rate was retarded in solventless, hexane and ethanol, but accelerated in water in the presence of compressed CO2. The decrease in reaction rate was mainly ascribed to the formation of carbamate from benzylamine and the intermediate 1-aminodibenzylamine reacting with CO2, and it precipitated out to coat on the surface of catalyst in solventless and hexane, and the dilution effect of compressed CO2 in ethanol. But the yield of benzylamine was increased in solventless, hexane and ethanol due to the following nucleophilic addition of benzylamine and benzylimine to N-benzylidenebenzylamine was inhibited. Although, the reaction rate increased in water for the enhanced solubility of H2 and benzonitrile in H2O and decreased mass-transfer resistance in the presence of compressed CO2, the yield of benzylamine decreased because of the acidic nature of H2O–CO2 favorites the elimination of NH3 and the formation of NH4HCO3. Accordingly, the possible reaction pathway of benzonitrile hydrogenation was proposed for all the studied systems.

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