Macro-structured carbon nanofibers catalysts on titania extrudate and cordierite monolith for selective hydrogenation

Abstract

Macro-structured carbon nanofibers materials on large Titania particles (CNF/TiO2) and cordierite monolith (CNF/TiO2/monolith) were synthesized in this thesis, for the purpose of avoiding or diminishing internal mass transfer limitations in catalysis. Their supported palladium catalysts (Pd/CNF/TiO2 and Pd/CNF/TiO2/monolith) were compared with some catalysts, including palladium catalysts supported on activated carbon (Pd/AC), meso-porous carbon (Pd/MC) and carbon nanofibers (Pd/CNF). Their catalytic properties were evaluated for selective hydrogenation of citral, cinnamaldehyde (CAL) and 4-carboxybenzaldehyde (4-CBA) to determine if as-prepared macro-structured carbon nanofibers catalysts offer advantages in enhancing the activity and the selectivity in these reactions. In catalyst preparation, the synthesized Pd/CNF/TiO2 and Pd/CNF/TiO2/monolith had the good textural and structural properties: the BET surface areas were 60 m2/g and 31 m2/g, respectively; the mesopore structure dominated the pore space of both catalysts, which is beneficial for eliminating mass transfer limitations. As-prepared Pd/CNF/TiO2 was used in selective hydrogenation of citral. Its catalytic property was estimated using Weisz-Prater criterion, and the result was compared to Pd/AC. The calculating results of Weisz-Prater numbers (less than 0.3 of each reactant) inferred the absence of internal diffusion limitations in Pd/CNF/TiO2. It is in accordance with the high citronellal selectivity in citral hydrogenation over it. Meso- and macro-pores, the dominant structures in Pd/CNF/TiO2, resulted in the elimination of internal diffusion limitation in the catalyst. In addition, CNF/TiO2/monolith was employed for two model reactions, selective hydrogenation of CAL and 4-CBA. The effects of mass transfer on catalyst performance were studied experimentally and the results are described using simple kinetic models. The results showed that, catalytic performance of Pd/CNF/TiO2/monolith is similar to Pd/CNF and Pd/AC with particles as small as 50 micron (Pd/AC50), whereas Pd/AC with larger support particles revealed lower activity due to internal mass transfer limitation. Remarkably, the conversion level at which the maximum yield of intermediate product is achieved is highest for 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, Pd/CNF/TiO2/monolith applied as a fixed bed outperforms slurry catalysts, abandoning the need of a filtration section.