Catalytic cracking of n-octane on small-pore zeolites

The small-pore zeolites Alpha (LTA), chabazite (CHA), NU-3 (LEV), Rho (RHO), Sigma-1 (DDR), ZK-5 (KFI) and ZSM-58 (DDR) were hydrothermally synthesized and characterized by XRD, elemental analysis, 27Al MAS NMR spectroscopy and n-octane adsorption. The Brønsted acid forms of the zeolites were used as catalysts for cracking of n-octane in a continuously operated fixed-bed reactor. For comparison, n-octane cracking experiments were performed on the medium-pore zeolite H-ZSM-5 (MFI) and the large-pore zeolite H-Beta (∗BEA). In agreement with prior work on catalytic cracking of short-chain n-alkanes, it was found that the contribution of monomolecular Haag-Dessau cracking increases, as the pores are getting narrower. However, if a long-chain n-alkane, such as n-octane, is used as reactant, the typical products of both monomolecular and bimolecular cracking are observed on all 8-ring zeolites. For various small-pore zeolites, a pronounced influence of the pore dimensions was observed. Selectivities to C1 and C2 products were shown to increase with decreasing size of the pore windows and increasing spatial constraints of the pore systems.