Effects of temperature pretreatment on propane cracking over H-SSZ-13 zeolites

The effect of thermal treatment of SSZ-13 on catalytic activity has been investigated by using monomolecular propane conversion as a probe reaction. Two samples with Si/Al ratios of 12 and 6 were prepared and characterized by solid-state nuclear magnetic resonance (NMR), X-ray powder diffraction (XRD), N-2 adsorption, and temperature-programmed desorption (TPD). Samples were pretreated at different temperatures: mild activation at 773 K (treatment 1) and high-temperature activation at 823, 873, 973, and 1073 K (treatment 2). The catalysts showed similar catalytic propane reaction rates within the experimental error. On the other hand, after treatment 1 the catalysts showed higher cracking selectivity than dehydrogenation selectivity by a factor of 2, while the selectivity towards dehydrogenation was enhanced for the catalysts treated at high temperatures (cracking-to-dehydrogenation ratio of similar to 1). The change in selectivity suggests that the generation of different active sites-other than Bronsted acid sites (BAS), typically considered the active sites for hydrocarbon conversion. The SSZ-13-6 treated at 823 and 873 K showed different selectivity, without a notable change in reactivity and activation energies compared to the acid catalyst after treatment 1. After high-temperature treatments, SSZ-13-12 showed gradually decreasing activation energies and increasing selectivity toward dehydrogenation with increasing pretreatment temperature (from 823 to 1073 K). These results lead us to the hypothesis that different types of new active sites are generated in SSZ-13-12 and SSZ-13-6 samples.