What is it about?

The nature of active acid sites of zeolite H-BEA with different Si/Al ratio (15–407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–180 °C have been explored. Described and discussed data concerning the strength and concentration of acid sites of H-BEA obtained by techniques of stepwise (Quasi-Equilibrium) thermal desorption of ammonia, X-ray diffraction, low-temperature adsorption of nitrogen, FTIR spectroscopy of adsorbed pyridine and solid-state 27Al MAS NMR. The average values of adsorption energy of NH3 on H-BEA was experimentally determined as 63.7; 91.3 and 121.9 mmol·g-1 (weak, medium, and strong, respectively). In agreement with this, the correlation between the rate of ethyl-tert-butyl ether synthesis and concentration of weak acid sites (ENH3 = 61.6–68.9 kJ·mol−1) has been observed. It was concluded that the active sites of the H-BEA for this reaction are Brönsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.

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Why is it important?

We determined the nature of the acid sites of the H-BEA zeolites responsible for the heterogeneous catalytic synthesis of ethyl tert-butyl ether from ethanol and isobutylene. QETD and catalytic studies emphasize that etherification of isobutylene with ethanol over zeolites occurs with the participation of weak Bronsted acid sites, which are characterized by an ammonia adsorption energy in the range of 59–69 kJ·mol-1. These weak acid sites are identified as the Brønsted hydroxyls representing internal silanol groups. Our correlations show an importance of the Brønsted weak acid sites in the catalytic performance of zeolites. The weak acid silanol groups may play an important role not only in the etherification reactions but also in the alkylation and acylation processes, which occur under soft conditions and do not require a break of chemical bonds in molecules.

Perspectives

It was a great pleasure to write this article, as it allowed to unite a group of researchers in different areas of physical chemistry. This article also led to increased cooperation in the study of the physical-chemical properties of zeolites, which contributed to a better understanding of the nature of their action in the processes of acid-base catalysis.

PhD Nina V Vlasenko
L.V.Pisarzhevskii Institute of Physical Chemistry NAS of Ukraine

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This page is a summary of: Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol, RSC Advances, January 2019, Royal Society of Chemistry, DOI: 10.1039/c9ra07721a.
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