Basic Zeolites: Structure, Preparation and Environmental, pp. 51-66
Authors: (Salvador Ordóñez, Eva Díaz - Department of Chemical Engineering and Environmental Technology, University of Oviedo, Oviedo, Spain)
Abstract: Zeolites are crystalline aluminosilicates widely used as catalysts, catalyst supports and adsorbents because of their tuneable acidic properties, high surface area, thermal stability and well-defined microporosity properties. However, these materials allow also the modification of its chemical properties, increasing their basicity. These materials could be attractive as adsorbents (acid gases as carbon dioxide or sulphur dioxide) or catalysts (aldolizations, transesterifications). Among the methods of tuning the basicity of zeolites, two possibilities can be found: the reduction of Si/Al ratio and the increase in electropositivity of the countercation in zeolites. Desilication could be reached by treatment of the zeolite in alkaline medium, in order to remove the Si atom from the structure. Add to reduce the Si/Al ratio, an increase in the mesoporosity of the zeolites is obtained, thus favouring the diffusion in the pores and the access to the basic sites generated. This treatment increases the acid site density of the material due to presence of relatively higher Al in the zeolite framework; however, if no extraframework Al species are formed during the treatment, the ratio between Lewis and Bronsted acid sites remains constant. In this way, a successive combination of desilication and dealumination leads to a mesoporosity development and to an increase in the acidity of the zeolite.