Modeling of boron substitution in zeolites and implications on lattice parameters

Abstract

The effect of substituting a silicon center with boron in the framework of zeolites has been studied using quantum chemical (QM) calculations on model clusters with formula R₃SiOBMR₃ (M=H, Na; R=OSiH₃). Boron substitution has then been compared with that of Al. These calculations have shown that, when a proton is associated with boron, the tetrahedral BO₄ unit is replaced by a trigonal BO₃ entity whereas a silanol group is formed on the adjacent silicon. In contrast, the NaB model retains the BO₄ coordination. Comparison of NaB and NaAl models suggests an opposite behaviour of their silicate framework: the distance between Al and its adjacent Si neighbours is only slightly larger than Si–Si distances, due to the flexibility of the AlOSi bond angles; in contrast, the decrease of the BOSi angles, which accompanies the short B–O bond, necessitates the adjustment of the adjacent silicons, leading to a global decrease of the unit cell volume. These results are in agreement with available experimental data.