Studies on the acidity of mordenite and ZSM 5. 2. Loss of Broensted acidity by dehydroxylation and dealumination

Abstract

Quantitative determination of Bronsted acid site concentrations in calcined H-mordenite and H-ZSM-5 by means of conductometric titration and infrared spectroscopy reveals that the concentration of Bronsted acid sites present is lower than would be expected on the basis of the aluminum content. This discrepancy is attributed to dealumination and dehydroxylation processes occurring during calcination. A model is proposed according to which the degree of these processes strongly depends on the concentration of vicinal aluminum T sites in the zeolite: Bronsted acid sites associated with these T sites are lost, and the remaining Bronsted acidity is related to the concentration of the (remaining) isolated aluminum T sites. This model quantitatively correlates the experimentally determined concentration of Bronsted acid sites with the concentration of isolated aluminum T sites.

Efeito do glutaraldeído na adsorção de extrato proteico da Ipomoea Batatas (L) Lam na superficie de uma zeólita analcima

“…Esta diferença poderá ser compensada pelos contra-íons, sendo o sódio e o hidrogênio os mais utilizados [36][37][38].…”

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

“…4 Physical characterization of H-ZSM-5 following mild steaming shows evidence of a loss in the number of Brønsted acid sites due to dehydroxylation, but no indication of dealumination. 5 The only species observed by 1 H NMR, 27 Al NMR, and infrared spectroscopy are Si-OH-Al species associated with strong Brønsted acidity, nonacidic terminal Si-OH groups, and weakly acidic hydroxylated aluminum species. 6 No enhancement in the specific activity of Brønsted acid sties is observed for severe steaming (high temperature and water partial pressure), and such pretreatment results in significant delaumination of the zeolite framework and the deposition of extraframework alumina species in the pores.…”

DFT Cluster Modeling of Molecular and Dissociative Hydrogen Adsorption on Zn2+Ions with Distant Placing of Aluminum in the Framework of High-Silica Zeolites

“…Therefore, the fraction of alumium ions in the next nearest positions (NNA sites) is an important characteristic of aluminum distribution in the framework of high-silica zeolites [1][2][3].…”