Selective hydrogenation of chloronitrobenzene to chloroaniline in supercritical carbon dioxide over Ni/TiO2: Significance of molecular interactions

Abstract

The hydrogenation of chloronitrobenzene to chloroaniline was investigated over Ni/TiO 2 at 35°C in supercritical CO 2 (scCO 2 ), ethanol, and n-hexane. The reaction rate followed the order of scCO 2 > n-hexane > ethanol. In scCO 2 , the selectivity to chloroaniline and to aniline over Ni/TiO 2 were 97-99.5% and <1%, respectively, in the conversion range of 9-100%. The high chemoselectivity to chloroaniline cannot be achieved over Ni/TiO 2 in ethanol and n-hexane. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO 2 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 CO 2 with these reacting species is discussed. Possible reaction pathways for the hydrogenation of chloronitrobenzene in scCO 2 over Ni/TiO 2 are also proposed.

Selective hydrogenation of nitrostyrene to aminostyrene over Pt/TiO2 catalysts: Effects of pressurized carbon dioxide and catalyst preparation conditions

“…However, these catalysts sometimes show low selectivity to anilines because of the formation of several poisonous reaction intermediates such as nitrosobenzenes, phenylhydroxyamines, azoxybenzenes, and azobenzenes whose formation and accumulation should be avoided for the green production of anilines. The present authors reported that, when the hydrogenation of nitrobenzene to aniline was carried out in/under pressurized CO 2 , the selectivity to aniline was much higher than that obtained in conventional organic solvents [21][22][23][24][25]. The higher selectivity is ascribed to interactions of CO 2 with the substrate and the intermediates.…”

Molecular interactions with CO2 for controlling the regioselectivity of liquid phase hydrogenation of 2,4-dinitroaniline

“…[8][9][10] The cell was loaded with 2 cm 3 solvent, it was purged with CO2 three times, and then it was heated up to a temperature of 323 K while stirring. After the desired temperature was obtained, 4 MPa H2 was introduced, compressed CO2 was fed into the cell to the desired pressure, and then stirring was stopped after 2 min.…”

“…6 One of interesting actions of CO2 in CXLs is the interaction with functional groups of organic substrates and/or reaction intermediates, which would accelerate or retard the reaction rate and change the product distribution. The authors show the significance of molecular interactions of CO2 in the catalytic selective hydrogenation of -unsaturated aldehydes to saturated alcohols 7,8 and of nitro arenes to anilines 9,10 . In the latter, for example, 100% selectivity to anilines can be achieved at any conversion level up to 100% total conversion with supported nickel catalysts.…”

Multiphase reaction media including dense phase carbon dioxide and/or water: A case study for hydrogenation of phenol with a Pd/Al2O3 catalyst

“…The significance of these molecular interactions was demonstrated for several reactions in CXLs. One of noteworthy examples is selective hydrogenation of aromatic nitro compounds with a Ni/TiO 2 catalyst [9]. When a reaction mixture (H 2 , liquid substrate, solid catalyst) was pressurized by CO 2 , the total rate of conversion increased and the selectivity to the desired aniline products was almost 100% at any conversion level.…”