Selective Hydrogenation Of Aromatic Nitro Compounds And Cinnamaldehyde In Green Solvents

Many organic solvents used in chemical industry are volatile, flammable, poisonous, and harmful to the environment. Reducing the use of organic solvents or replacing them with environmentally benign solvents is becoming an important part of green chemistry and green chemical engineering.In this paper, the selective hydrogenation of aromatic nitro compounds such as p-nitroanisole and2-chloronitrobenzene were investigated in green solvents.In the hydrogenation of p-nitroanisole on5wt%Pd/C (as received), the conversion in H2O, ethanol or n-heptane was100%in20min of reaction. The selectivity to p-anisidine was in the following order:ethanol (>99%)> n-heptane (97%)> H2O (89%). In CO2-H2O, CO2-ethanol and CO2-n-heptane at4MPa CO2, the conversions were89%and55%in CO2-ethanol and CO2-n-heptane, respectively. In compressed CO2without liquid solvent, the conversion was the highest (ca.100%) at10-12MPa CO2, and the selectivity to p-anisidine was>99%.The hydrogenation of2-chloronitrobenzene over5wt%Pt/C (reduced at300℃for2h) was performed in polar solvents (ethanol and isopropyl alcohol), nonpolar solvents (n-hexane and n-heptane), and mixtures of these solvents with H2O (ethanol-H2O, n-hexane-H2O, n-heptane-H2O) to examine the effects of solvents on conversion and selectivity. Compressed CO2was also introduced into the above systems to investigate the influences of CO2on the reaction. In nonpolar solvents, e.g., n-hexane and n-heptane, addition of H2O lowered the accumulation of reaction intermediates. Introduction of compression CO2into n-heptane-H2O could further improve the rate of2-chloronitrobenzene hydrogenation.The catalytic hydrogenation of cinnamaldehyde (CAL) with different catalysts (5wt%Pd/C and5wt%Pt/C) was carried out in different reaction systems (typical organic solvents and CO2-expanded liquids). Pd/C was a better catalyst for the selective hydrogenation of CAL to hydrocinnamaldehyde (HCAL). The selectivity to HCAL increased significantly in H2O, isopropyl alcohol, ether, and n-hexane when compressed CO2(6MPa) was introduced. CO2-H2O and H2O were the greenest and most efficient reaction media for the selective hydrogenation of CAL to HCAL on Pd/C.