Periodic B3-LYP calculations on H-Edingtonites, both alone and interacting with acetylene

Abstract

The EDI framework has been adopted, for the first time, to define a series of zeolites derived from the all-silica Edingtonite, featuring a tetragonal lattice with five SiO₂ groups per unit cell, by substitution of a single Si atom per unit cell by an Al atom and the addition of a charge-compensating acidic proton. Four different H-EDI structures have been considered, the geometries of which have been obtained by minimizing the mechanical energy defined in terms of a model ion potential force field. The relative stability and the anharmonic OH-stretching frequency have been computed for the resulting optimized geometries by an abinitio periodic method using the B3-LYP hamiltonian and polarized basis set of double zeta quality. The electrostatic potential and the electric field within the zeolite cavity have also been computed abinitio. The interaction of acetylene with a given H-EDI structure has then been studied at the abinitio B3-LYP level, by optimizing selected degrees of freedom. The binding energy and the OH frequency shift due to interaction with acetylene have been computed and compared with those from molecular cluster models containing the Brønsted site. From energetic and vibrational data of the interaction with acetylene, the periodic structure appears more acidic than the adopted model clusters. The H-EDI zeolite appears to be an attractive model system for its structural simplicity which allows one to study the interaction of small probe molecules using moderate computational resources.