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 "B 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.

Predicted Effects of Site-Specific Aluminum Substitution on the Framework Geometry and Unit Cell Dimensions of Zeolite ZSM-5 Materials

“…Unit Cell Dimension Variability. The unit cell volume for H + and TPA + −ZSM-5 samples measured experimentally is approximately 5400 Å 3 , − substantially smaller than the values predicted by the present force field method, as expected on the basis of the force field parametrization route and the validation data. , The effect of Al insertion on unit cell parameters and volumes is discussed in terms of changes relative to an unsubstituted, siliceous MFI-framework model (silicalite) subjected to the same optimization procedure ( V = 5724 Å 3 ).…”

Non‐oxidative Coupling of Methane to Ethylene Using Mo2C/[B]ZSM‐5

“…stirring in water at room temperature), an in the presence of high temperatures and water vapor this process is accelerated . The amount of framework B can be estimated on the basis of the size of the unit cell because unit cell dimensions are linearly correlated with the boron content in the unit cell . According to the published correlation, the as‐made zeolite B‐MFI sample contained about 2.5 boron atoms per unit cell, this number decreased to 1.7 after calcination and decreased further to 1 per unit cell after carburization.…”

First Principles Studies on Boron Sites in Zeolites

“…16 Moreover, other authors reported bands at 1390 and 1150 cm -1 , ascribed to the simultaneous presence of trigonal and tetrahedral B-sites, respectively. 17 The latter band may be partially hidden by the crystalline silica matrix modes.…”

“…Acid boralites are also interesting for their change in the coordination geometry at the boron center, which occurs reversibly upon dehydration and rehydration . Specifically, both 11 B solid-state NMR − and IR studies − have shown that B is characterized by a trigonal geometry in dehydrated samples, whereas upon rehydration a tetrahedral structure of the boron site is recovered. However, upon being washed with water, trigonal boron can be readily hydrolyzed to a defective B-site and then completely extracted from the framework (de-boronation process). ,,, The trigonal-to-tetrahedral coordination change is also observed in the presence of other bases such as pyridine and ammonia. , Such a transformation, like in the case of water, is accompanied by a proton transfer from the SiOHB acid group to the probe molecule.…”