Changes in the acidic properties of the zeolite H-ZSM-11 under the influence of hydrogen and water

Abstract

The nature of the effect of hydrogen and water on the spectrum of acidity of H-ZSM-11 zeolite has been studied. An explanation of the observed changes is proposed and a prognosis has been made on the relative effect of transformation of the functional groups of the surface on the catalytic properties of H-ZSM-11 in various reactions of acid type.Thanks to their characteristic physico-chemical properties zeolites are widely used in a variety of acid-base catalytic processes such as hydration and dehydration, esterification, cracking, hydrogenation, etc. Presentations on the nature of the active centers of the catalyst, their concentration, and their distribution with respect to strength are extremely important.The acid properties of zeolites are determined by two basic types of centers: protonic (Brönsted) and aprotonic (Lewis). Brönsted acidity is determined by the presence of different types of hydroxyl groups -structural or bridging hydroxyl groups, OH groups found on the outer surfaces of zeolite crystals, OH groups localized on polyvalent cations and OH groups localized on nonstructural particles of aluminum [1]. As a rule the catalytic properties of zeolites are connected to siloxy bridged hydroxyl groups. However in recent times activity has been noted of weakly acidic silanol OH groups in a series of catalytic reactions which occur under mild conditions [2].The distribution of acid centers by type, concentration, and strength forms a spectrum of acidity which may change under the influence of various factors. This is especially important for zeolite catalysts working at high temperatures and subjected to the effects of components of the reaction media. It is well known that the effect of high temperatures leads to a decrease in the Brönsted acidity and an increase Lewis acidity [3]. The presence of water molecules contained in the starting materials or evolved during the process reactions can facilitate the conversion Lewis acid centers of the zeolite into Brönsted centers which leads to a change in catalytic atcivity in a number of cases. Data show that hydrogen in the atmosphere over the zeolite surface may form protonic acid centers [4].Thus in the process of catalyst formation and the subsequent effects of the reaction media the spectra of acidity of zeolites may undergo important changes, understanding the nature of which may permit the favorable formation of functional characteristics of the surface of zeolites for the creation of effective catalysts for various processes proceeding by an acid-base mechanism.The objective of the present was to investigate the character of changes in the spectrum of acidity of the zeolite H-ZSM-11 under the influence of hydrogen and water vapor. 2660040-5760/06/4204-0266

Effect of acid–base characteristics of ZrO2–Y2O3 on catalytic properties in carboxylation of methanol

“…The weight was recorded with stepwise rise in temperature and evacuation. The total concentration of sites (C S ) was determined from the amount of test material adsorbed on the surface of the sample at 50°C [13].…”

Catalytic characteristics of massive and loaded sulfonic resins in the synthesis of ethyl tert-butyl ether at atmospheric and increased pressure

“…This method is expected to carry out thermodesorption in a stepwise (quasiequilibrium) regime starting from each step to a state of adsorption equilibrium. This increases the precision of determination of the concentrations and temperature intervals corresponding to acid centers of different types [12]. The total acidity of samples was calculated from the amount of ammonia adsorbed at 50°C.…”