Abstrakt
The relationship between the chemical composition and the activity of zeolites has been unambiguously established and used to optimize some reactions. However, the relationship between the micropore size and the activity of catalysts remains elusive.
Here, we report the combination of the ADOR approach and postsynthetic alumination for the preparation of isoreticular Al-IPC materials as model catalysts to assess the effect of micropore size on their catalytic activity. XRD, SEM, N-2 ad-/desorption, chemical analysis, Al-27 NMR, and FTIR spectroscopy of adsorbed acetonitrile-d(3) show that the prepared isoreticular Al-IPC zeolites have similar Al content and coordination states as well as similar types and total numbers of acid sites but differ in micropore size, decreasing in the following order: Al-IPC-7 (14, 12, and 10-ring channels) > Al-IPC-2 (12 and 10) > Al-IPC-6 (12, 10, and 8) > Al-IPC-4 (10 and 8).
Furthermore, the kinetic study of gas-phase ethanol-to diethyl ether transformation evidenced a lack of confinement in ethanol dimers of IPC-6 and IPC-4 (k(first) = 0.4-0.9 mol(DEE) (mol(H+).s)(-1)) compared with IPC-2 (k(first) = 3.8 mol(DEE) (mol(H+).s)(-1)) and IPC-7 (k(first) = 6.3 mol(DEE) (mol(H+).s)(-1)) zeolites. The catalytic activity of IPC-zeolites in liquid-phase tetrahydropyranylation was correlated with the relative concentration of acid sites on the external surface of Al-IPC-n crystals, both increasing in the following order: IPC-4 (35% of "external" acid sites) < IPC-6 (29%) < IPC-2 (25%) < IPC-7 (21%).
The use of Al-IPC-n as model catalysts in important acid-catalyzed reactions can shed a light on the fundamental structure-activity relationship and help to improve or optimize existing catalytic processes.