Mechanism of F⁻ Elimination from Zeolitic D4R Units: A Periodic B3LYP Study on the Octadecasil Zeolite

Abstract

The mechanism of elimination of F -trapped inside the double four-membered ring (D4R) cages of octadecasil has been determined by means of periodic calculations at the B3LYP/VDZP//VTZP level of theory with the CRYSTAL06 program. Finite temperature and entropic effects are taken into account to study the efficiency of the elimination reaction, which is exothermic, requires acid media, and is catalyzed by cationic polar molecules with accessible protons, from which a hydronium ion has been considered in the present study. The mechanism consists of three steps that schematically correspond to hydrolysis of one SiOSi bridge in the D4R cage, extraction of the F -from the unit, and recondensation of the bridge with formation of slightly adsorbed HF · H 2 O in the large cavity that can be easily eliminated by diffusion. The limiting step is the first one and involves a quite low Helmholtz free energy barrier even at 100°C (51.6 kJ mol -1 ) that slowly decreases with increasing temperature.

Ab initio simulations on migration paths of interstitial oxygen in corundum

“…The energies of the transition states (O i migration barriers) have been obtained using the distinguished reaction coordinate technique following the method [30] as implemented in the CRYSTAL14 code. The optimized migration path corresponds to a trajectory of local minimum points on the potential energy surface (PES).…”

Graphene on clean (0001) α-quartz: Numerical determination of a minimum energy path from metal to semiconductor

“…The nature of the transition point as saddle point has been confirmed by the vibrational analysis of the system which has evidenced the existence of one negative eigenvalue of the numerical Hessian matrix .…”

“…2b, which lies about 1.3 eV below the van der Waals minimum, a barrier of ∼ 2.25 eV has to be overcome. The nature of the transition point as saddle point has been confirmed by the vibrational analysis of the system which has evidenced the existence of one negative eigenvalue of the numerical Hessian matrix [50,51].…”

Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests

“…Double four-ring cages (d4r units, face symbol 4 6 , t-cub tile in the nomenclature of natural tilings 1 ) are a prototypical building unit of zeolites and zeotypes. A recent statistical analysis of the zeolite frameworks included in the IZA (International Zeolite Association) Database of Zeolite Structures 2 showed that the t-cub tile is the second most frequent tile, occurring in 36 out of 239 zeolite frameworks.…”

“…Because the equilibrium Si-O-Si angle is closer to 150 degrees, 4,5 these building units are strained in all-silica zeolites. 6 The strain can be reduced through an incorporation of heteroatoms, especially germanium, at some corners of the cage, 4,7 or through encapsulation of fluoride anions inside the cages. 6 In fact, many all-silica zeolites containing d4r units have, so far, not been obtained in the absence of fluoride.…”