The origin of the C-7-hydroconversion selectivities on Y, beta, ZSM-22, ZSM-23, and EU-1 zeolites

We carry out experimental hydroconversion tests of n-heptane paraffin on fi ve different zeolites: Y, beta, ZSM-22, ZSM-23, and EU-1, The experimental selectivities clearly depend on the catalyst pore structures. Through force field simulations, we investigate adsorption and diffusion properties as w ell as model cyclopropane intermediate stabilities inside each zeolite fram ework. In open structures such as Y and beta, no shape transition state sel ectivity or diffusion limitation are taking place. In the more restricted 1 0-membered ring (MR) channels of ZSM-22, ZSM-23, and 10-MR windows of EU-1, we show that the selectivities in branched isomers and cracked products ar e explained by the combined effects of transition state restriction and pro duct diffusion limitations. In particular, the selectivities in monomethyl- branched products are correlated to their relative diffusion barriers, in a greement with earlier results of E. B. Webb III et al. (1999, J. Phys. Chem . B 103, 4949). The explanation for this result has its foundation in the d egree of symmetry of the monomethyl-branched paraffin. In ZSM-22 and ZSM-23 , the 2,2- and 3,3-dMC(5) are not formed on account of transition-state sha pe restrictions, whereas a slight amount of 2,3- and 2,4-dMC(5) is formed a nd can diffuse out of the pores. In the EU-1 sieve, the large side pockets behave as the open Y or B structures, where all isomerization reactions can take place without restriction, but only the faster diffusing 2,3-dMC(5) a nd 2,4-dMC(5) products are released. The slower diffusing multibranched 2,2 -dMC(5) and 3,3-dMC(5) paraffins are transformed via methyl shift or cracke d inside the pockets producing a higher amount of cracked products in EU-1 than in ZSM-22 or ZSM-23. (C) 2001 Academic Press.