Influence of internal diffusion on selective hydrogenation of 4-carboxybenzaldehyde over palladium catalysts supported on carbon nanofiber coated monolith

In this study, a macro-structured Pd catalyst supported on carbon nanofiber coated on cordierite monolith (Pd/CNF/TiO2/monolith) was employed for hydrogenation of 4-CBA. The effect of mass transfer on the catalyst performance was studied experimentally and the results are described using a simple kinetic model. The results were compared to Pd catalysts supported on activated carbon (Pd/AC) and on carbon nanofibers aggregates (Pd/CNF). Catalytic performance of the Pd/CNF/TiO2/monolith is similar to Pd/CNF and the Pd/AC with particles as small as 50 μm (Pd/AC50), whereas Pd/AC with larger support particles revealed a lower activity per Pd active surface site, due to internal mass transfer limitation. Also the selectivity to the intermediate hydrogenation product (4-HMBA), versus deep hydrogenation to p-TA, is clearly affected by internal mass transfer. Pd/AC with large particles (3000 μm) achieves a maximum yield to the intermediate product of only 35%, whereas all the other catalysts achieve typically 70%. Remarkably, the conversion level at which the maximum yield of the intermediate product is achieved is highest for the Pd/CNF/TiO2/monolith. This advantage is assigned to superior internal mass transfer properties, thanks to high porosity, low tortuosity and short diffusion length of the CNF layer. Clearly, the CNF/TiO2/monolith applied as a fixed bed outperforms slurry phase catalysts, abandoning the need of a filtration section