Insight into the active site nature of zeolite H-BEA for liquid phase etherification of isobutylene with ethanol

摘要

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 ~(27) Al MAS NMR. The average values of the adsorption energy of NH _(3) 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 ( E _(NH _(3) ) = 61.6–68.9 kJ mol ~(?1) ) has been observed. It was concluded that the active sites of H-BEA for this reaction are Br?nsted hydroxyls representing internal silanol groups associated with octahedrally coordinated aluminum in the second coordination sphere.