(625f) Tunable Palladium Catalyst Using Organically Modified Silica As a Scaffold for Aqueous Phase Phenol Hydrogenation

Limited availability and harmful environmental impacts of fossil sources have created a need for development of sustainable technologies using renewable feedstocks. Manufacturing of nylon and esters require cyclohexanone and cyclohexanol as starting materials. These are also used as solvents in several processes. While conventional methods rely on fossil-derived crude oil to produce these chemicals, we are focusing on the production of cyclohexanone and cyclohexanol using bio-derived feedstock. We have developed palladium (Pd) catalysts using an organic-inorganic hybrid (OIH) silica support for aqueous phase hydrogenation of phenol to cyclohexanone and cyclohexanol. The organosilica support is synthesized using sol-gel method from an organometallic precursor, bis (trimethoxy)silyl ethylbenzene. Surface hydroxyl groups were capped using hexamethyl disilazane to modify surface polarity, hydrophilicity and textural properties. Palladium supported on scaffolds of different polarity showed significantly different catalytic activity and selectivity during aqueous phase phenol hydrogenation performed in batch reactor at 50 bar H₂ and 200 °C. SBA-15 supported catalyst was used for comparison due to its similar textural properties and hydrophilic nature. In acetone and water vapor adsorption experiments, the organosilica support with capped surface hydroxyl groups adsorbed 31 times more acetone than water while SBA-15 absorbed 1.7 times more water than acetone. This confirmed that organically modified silica support had higher affinity towards organics and was more hydrophobic in comparison with SBA-15. Rate constant for phenol vapor adsorption increased with increasing hydrophobicity of the support. Temperature programmed phenol desorption using DRIFTS suggested stronger phenol adsorption on organosilica supports as compared to SBA-15. Pd catalysts and the supports were further characterized by N₂ physisorption, Infrared Spectroscopy, Si NMR, pulse CO chemisorption, transmission Electron Microscopy (TEM), and liquid adsorption experiments. Thus, in this work we report a unique and facile approach of synthesizing a catalyst with tunable polarity/hydrophilicity and hence tunable aqueous phase phenol hydrogenation activity.