TEXTURAL AND CATALYTIC CHARACTERISTICS OF DECATIONIZED MORDENITE AND ZSM-5 ZEOLITE
The textural characteristics of acid forms of mordenite and ZSM-5 zeolite (HM and HZSM-5) were described based on the experiments of adsorption of nitrogen at 77 K in a specially constructed apparatus.
The specific surface (SP) of catalysts was determined by applying BET-equation (SPBET), Langmuir adsorption isotherm (SPLANG) and t-method. The „t-method” was used to determine the outer specific surface (SPextt) and the volume of micropores (Vμpt) of zeolite. The volume of micropores (VμpDR) was additionally determined also based on Dubinin-Radushkevich method. The reactions of diethyl ether dehydration in the temperature interval from 400 to 480 K got sped up in time on both zeolites (HM and HZSM-5) and did not fully follow the kinetics of the first order reaction. The initial period registered by the experiment (increase of reaction rate in time) is interpreted by the slow reaction of water occurrence in the reaction of dehydration, which is necessary for the occurrence of Brönsted acid centers, where diethyl ether dehydration takes place. Now the overall rate of the dehydration reaction is determined by the rate of water occurrence. When there is a sufficient number of acid centers, the rate of diethyl ether dehydration is determined by the surface reaction and the „initial period“ gradually disappears. In the reaction of diethyl ether dehydration on HZSM-5 catalyst, the reaction is slower, even water (which is a product of dehydration) occurs more slowly, and therefore in the observed temperature interval the reaction was not transferred to the first order reaction. By the adsorption of diethyl ether from gas phase on HM and HZSM-5 zeolite at 300 K, the insight was provided into hydrophilic-hydrophobic characteristics of the surface, as well as the insight into the character and strength of active centers of these zeolites. As the test reaction for determining acidity of the catalyst, the one selected was the reaction of isomerization of 3,3-dimethyl butene-1 (3,3-DMB-1), which takes place on Brönsted acid centers by the mechanism of carbonium ion. The results obtained by the experiments of adsorption of nitrogen and diethyl ether from gas phase are in accordance with the kinetic data of the diethyl ether dehydration reaction.
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