Effect of heteroatom substituted mesoporous support on the selective hydrogenation of cinnamaldehyde in supercritical carbon dioxide

Selective hydrogenation of cinnamaldehyde (CAL) is being carried out in supercritical carbon dioxide over Pt nanoparticle, supported on Al-MCM-48 and Ti-MCM-48. To evaluate the influence of the support, results are compared with Pt-MCM-48 (Si only). Pt-Al-MCM-48 and Pt-Ti-MCM-48, (containing metal particles of similar sizes) exhibits remarkable selectivity for cinnamyl alcohol (COL). A strong influence of heteroatom (Al and Ti) substitution is found in the activity and selectivity of the reaction. The rate of formation of COL follows the order of Pt-Ti-MCM-48 > Pt-Al-MCM-48 > Pt-MCM-48 under the same reaction conditions. We therefore propose that due to the interaction of Ti with CO2, the support becomes activated, which increases the electron density of Pt and thereby interacts preferentially with the CO bond. Depending on the nature of the heteroatom, a significant change in the CO2 pressure dependent activity and selectivity is observed. For Pt-Al-MCM-48, the maximum selectivity for COL is observed in the higher pressure (single phase) region. On the contrary, as the interaction of Ti and CO2 is favorable in the lower pressure region, Pt-Ti-MCM-48 shows highest selectivity for COL at 7–8.5 MPa (biphasic region) and decreases in the single phase region. Conventional Arrhenius behavior is maintained for Pt-Al-MCM-48, whereas Pt-Ti-MCM-48 displays significant decrease in the reaction rate with increasing temperature.