Stability of C3-C6 carbonium ions inside zeolites: A first principles study

Abstract Carbonium ions are crucial intermediates in catalysis as their nature can influence the reaction pathways. Understanding the stability of carbonium ions inside zeolites is very useful to understand the catalytic process. In this work, a series of C3-C6 carbonium ions were systematically studied inside zeolites SSZ-13, SAPO-34, Mordenite, SAPO-5, ZSM-5 and Faujasite, whose pores size are 3.72 A, 3.72 A, 4.70 A, 4.92 A, 7.35 A and 7.42 A, respectively. It was shown that the constituent of the zeolite as well as the confinement effect which is determined by the pore sizes are essential factors for the stability of cabonium ions. The stability of carbonium ions increases with the carbon number for silica based zeolites whereas it is not the case for aluminophosphate based zeolites. For small size carbonium ion C3H7+, the stability significantly increases with the pore size increases since the confinement effect from steric effect is little and the confinement effect from van der Waals, electrostatic interaction and electronic interaction etc. becomes dominant. Furthermore, C3H7+ is predicted to be a stable carbonium ion inside SAPO-5 due to the synergistic effect between the constituent of the zeolites and the confinement. In addition, it is found that the less hydrogen, the more stable for long chain carbonium ions due to the aromatic stabilization. This work provides useful information for zeolite catalyst designation and reaction mechanism discovery.