Propene Production by Butene Cracking. Descriptors for Zeolite Catalysts

Among the possible on-purpose technologies for propene production, direct conversion of butene-rich fractions to propene represents an attractive alternative to conventional routes such as steam cracking or fluid catalytic cracking. Here, we present an approach for designing an efficient ZSM-5-based catalyst for the selective cracking of butenes to propene by properly balancing diffusional and compositional effects. Instead of the large coffin-shaped ZSM-5 crystallites with very high Si/Al ratios generally reported, the optimal catalyst in terms of propene selectivity and catalyst life was found to be a ZSM-5 zeolite with a squared morphology, submicron-sized crystals (0.8 x 0.3 x 1.0 mu m), and a Si/Al molar ratio of around 300. For this crystal conformation, the short dimensions of both sinusoidal and straight channels facilitate propene diffusion and reduce its consumption in consecutive reactions, limiting the formation of C5+ oligomers and aromatics and maximizing propene selectivity. Coffin-type ZSM-5 crystals, with higher diffusional restrictions than square-shaped crystals, show faster catalyst deactivation than the latter, independently of the crystal size and Al content. However, among the ZSM-5 zeolite crystallites with a coffin morphology, the one presenting intergrowths on the (010) face, with a larger proportion of sinusoidal channels, shows a lower aromatic selectivity and deactivation rate, whereas the other two, with straight channels open to the clean (010) faces, favor the formation of aromatics by direct cyclization-dehydrogenation of oligomeric intermediates.