Effect of Zeolite Membrane Shell Thickness on Reactant Selectivity for Hydrocarbon Steam Reforming Using Layered Catalysts

A composite catalyst consisting of an outer layer of zeolite membrane encapsulating an inner reforming catalyst core was synthesized by a double physical coating method to investigate reactant selectivity (ratio of methane/toluene conversion rate) in steam reforming of methane (CH4) and toluene (C7H8). A double encapsulation (51 wt % H-beta zeolite) of a 1.6 wt % Ni-1.2 wt % Mg/Ce0.6Zr0.4O2 steam reforming catalyst was compared to a singly coated composite catalyst (34.3 wt % H-beta zeolite) to investigate zeolite thickness effects on the conversion of different sized hydrocarbons. Several characterization methods {scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), physisorption analysis [Brunauer-Emmett-Teller (BET) surface area, pore volumes, and pore size distributions], and X-ray diffraction (XRD)} were used to analyze pre- and post-reaction samples. SEM and EDS analyses showed that the H-beta zeolite was coated successfully on the steam reforming catalyst. The increase in the zeolite content from 34.3 to 51 wt % decreased both CH4 and C7H8 conversions (by up to 14% depending upon the temperature) as a result of the increase in diffusional limitations. Weisz-Prater criteria and Thiele moduli calculations confirmed that the reactions were performed under internal diffusion limitations. The C7H8 conversion of the 51 wt % composite (SR@beta 51%) catalyst was similar to the zeolite alone, indicating negligible contribution from the protected catalyst core. The reactant selectivity increased by up to 1.5 times on SR@beta 51% in comparison to the SR@beta 34.3% composite. Combined reforming at 800 degrees C on the SR@beta 51% catalyst indicated that the catalyst was stable during the 10 h time on stream.