Hydrogenation of Benzaldehyde and Cinnamaldehyde in Compressed CO₂ Medium with a Pt/C Catalyst:  A Study on Molecular Interactions and Pressure Effects

Abstract

The hydrogenation of benzaldehyde and cinnamaldehyde has been studied with a 5% Pt/C catalyst in compressed CO(2). The effect of CO(2) pressure on the total conversion was found to be different between the two aldehydes. The total conversion of benzaldehyde merely decreases with increasing CO(2) pressure, while that of cinnamaldehyde shows a maximum at a certain pressure. High-pressure FTIR measurements indicate the existence of interactions of CO(2) with the aldehydes. The absorption of nu(C=O) red-shifts at increasing CO(2) pressure, and this red-shift is more significant for cinnamaldehyde than for benzaldehyde, indicating that the C=O bond of the former becomes more reactive than the latter. The difference in the mode of interactions of CO(2) with these aldehydes has also been indicated by changes of nu(C=O) of CO(2). Thus, the conversion of benzaldehyde will decrease with increasing CO(2) pressure because of a simple dilution by introducing a larger quantity of CO(2). For cinnamaldehyde, the conversion will increase at low pressures because of increasing interactions with CO(2) molecules (increasing the reactivity of the C=O bond) but decrease at high pressures because of the simple dilution effect, similar to the case of benzaldehyde. The dense CO(2) molecules are not likely to change the catalytic activity of supported Pt particles, which was previously suggested from optical absorption of supported fine metal (Au) particles in a compressed CO(2) medium.

Hydrogenation of phenol with supported Rh catalysts in the presence of compressed CO2: Its effects on reaction rate, product selectivity and catalyst life

“…For the spectra measured in CO 2 , the ν(C=O) band was located at a position between those of liquid and gaseous cyclohexanone and red-shifted with CO 2 pressure. This red-shift would result from Lewis acid -base type interactions between CO 2 and its carbonyl group, as previously discussed [28,31]. Other absorption bands due to C-H and C-C bonds were not observed to be shifted with CO 2 pressure.…”

“…The expansion is caused from the dissolution of CO 2 molecules into the liquid phase [26,[32][33][34][35][36]. CO 2 -expanded liquid phase has recently been gaining considerable attention, because it will facilitate the dissolution of other coexisting gases of H 2 , O 2 , CO, and so on and may accelerate the reactions involved with these gaseous reactants although CO 2 is not a reactant but rather a diluent [26][27][28][32][33][34][35][36]. …”

“…The reaction rate and the product distribution should depend on several factors [26]. Our group has reported that interactions of carbonyl group with CO 2 molecules can cause the enhancement in the reactivity and/or the change in the reaction selectivity [27][28][29][30]. Conversion and selectivity of hydrogenation of α, β-unsaturated aldehydes (cinnamaldehyde and citral) to unsaturated alcohols were significantly enhanced by the presence of pressurized CO 2 [27][28][29][30].…”

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.…”