Changes in the structure of ZSM-5 zeolites during boron and aluminium substitution

Abstract

XRD and chemical analysis reveal that the orthorhombic unit cell of zeolite ZSM-5 structure undergoes linear contraction or expansion with increasing substitution level of B or Al, respectively. Evidence for a full substitution of B or Al in the framework to ca. three heteroatoms per unit cell is also provided by the ¹¹B MASNMR spectrum and a linear decrease in the framework vibration frequency with the increasing Al per unit cell, as observed in the IR spectra. Only the boralite with an analytical content of 3.7 B atoms per unit cell contained ca. 24% of extraframework B. The similar electronegativities of B and Si explain the structural vibration frequency in the IR region, which is independent of the amount of B per unit cell.