Abstract
The nature of active acid sites of zeolite H-BEA with different Si/Al ratios (15–407) in liquid phase etherification of isobutylene with ethanol in a continuous flow reactor in the temperature range 80–180 °C has been explored. We describe and discuss 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 the adsorption energy of NH3 on H-BEA were experimentally determined as 63.7; 91.3 and 121.9 mmol g−1 (weak, medium, and strong, respectively). In agreement with this, a correlation between the rate of ethyl-tert-butyl ether synthesis and the concentration of weak acid sites (ENH3 = 61.6–68.9 kJ mol−1) has been observed. It was concluded that the active sites of H-BEA for this reaction are Brønsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.
Highlights
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
Quasi-Equilibrium ThermoDesorption (QETD) and catalytic studies emphasize that etheri cation of isobutylene with ethanol over zeolites occurs with the participation of weak Brønsted acid sites, which are characterized by an ammonia adsorption energy in the range of 59– 69 kJ molÀ1
The adsorbed pyridine FTIR studies and NMR data suggest that these weak acid sites are identi ed as the Brønsted hydroxyls representing internal silanol groups
Summary
Zeolites are the most important solid acids as highly active and selective catalysts in a number of commercial chemical processes.[1,2,3,4] The acidity allows protonic zeolites to have widespread application as solid catalysts in a large range of chemical processes, e.g., catalytic cracking,[5,6,7] isomerization,[8,9,10] and alkylation.[11,12,13] The catalytic activity of zeolites is mainly associated with the Brønsted acid and (or) Lewis acid sites, and a result of the replacement of Al for Si in the tetrahedral units, which generates a net negative charge that has to be balanced by extra framework cations or protons. Recently it has been shown that external silanols in zeolites have weak acidic nature.[76,77] The signi cance of weak acid surface groups in some catalytic reactions occurring in so conditions was reported.[78,79] The good performance of zeolite BEA in MTBE synthesis from methanol and isobutylene showed activity comparable to the commercial ion-exchange resin Amberlyst-15 that has been explained as a result of the adsorption of methanol on the silanols of the zeolite BEA.[80] Contrary, zeolites HY and HZSM-5 showed substantially lower activity in the MTBE synthesis due to the methanol is mainly adsorbed on the bridging OH groups Another example of studying role of weak acid sites is presented by the effect of zeolites acidity on their catalytic activity in etheri cation of 2-naphthol with ethanol over H-beta, HMOR and H-ZSM-5.81 t was expected that the etheri cation activity of zeolites may be affected by the weak acid sites. These data supported by FTIR spectroscopy of adsorbed pyridine and solid-state 27Al MAS NMR gives an understanding of the nature and structure of the weak acid sites which are active in the etheri cation of isobutylene with ethanol
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