Regularities of Lewis site formation upon the microwave irradiation of gibbsite-γ-Al(OH)₃

Abstract

Spatially separated Lewis acidic and basic sites can be formed upon the microwave irradiation of gibbsite. This is demonstrated by IR spectroscopy and indicates the possibility of the selective activation of chemical bonds of different nature within the initial crystals of Al³⁺ hydroxide. A scheme for the formation of spatially separated Lewis acidic and basic sites is proposed. It is shown that water as a product of the micro-wave activation of gibbsite is adsorbed on these Lewis acidic sites in molecular form (without subsequent dissociation) and is desorbed at calcination temperatures no higher than 100–110°C. During standard (contact) thermal treatment of Al³⁺ hydroxides and oxides, dehydroxylation and water removal take place at 350–550°C to form the acid-base pair-Al^δ+-O^δ−-. The microwave activation of gibbsite results in the formation of an amorphous component believed to consist of small -Al-O- complexes closely packed in the solid phase. It is established that the fraction of Al³⁺ atoms accessible for the low-temperature adsorption of CO in microwave-activated (for 10 min) gibbsite is 4.5 at % of the total number of Al³⁺ atoms present in the amorphous component of this material.