Abstract
Zeolites are widely used as acid catalysts for the synthesis of fine chemicals in industrial processes. However, in most cases, diffusion limitations in the microporous network of zeolites prevents a good catalytic performance. In this sense, in the course of our research we have modified synthetic and natural mordenite zeolites by mild dealumination/desilication process to obtain mesoporosity. Commercial mordenite (CBV21A Si/Al = 10.5), and natural mordenite from Ethiopia (Si/Al = 6.2) have been treated under mild conditions (pH = 6) in order to maintain high crystallinity in the final zeolites. The impact of this mild treatment (based on ammonium acetate) is remarkable in the synthetic mordenite, creating good degree of mesoporosity while maintaining the bulk Si/Al ratio and crystallinity of the sample. In the case of the natural mordenite, the treatment leads to strong increase in the crystallinity of the sample, although only small mesoporosity is created. All samples were characterized by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), N2 adsorption-desorption isotherms and transmission electron microscopy (TEM). KEY WORDS: Zeolites, Mordenite, Natural mordenite, Modification, Mesoporous zeolites Bull. Chem. Soc. Ethiop. 2015, 29(1), 95-103DOI: http://dx.doi.org/10.4314/bcse.v29i1.8
Highlights
Zeolites are of great scientific and technological interest because of their unique intrinsic properties such as high surface area, excellent thermal stability, tunable surface properties and shape selectivity [1]
Catalytic studies involving zeolite crystals with precisely controlled compositions and crystal sizes are of extreme importance in elucidating the type of the shapeselective phenomena derived from the particular pore architecture of this class of microporous materials [2]
Mordenite consists of 12-membered ring (MR) pore channels of 0.67 × 0.70 nm interconnected by 8-MR pores of 0.34 × 0.48 nm [9]
Summary
Zeolites are of great scientific and technological interest because of their unique intrinsic properties such as high surface area, excellent thermal stability, tunable surface properties and shape selectivity [1]. Structures with 8-ring, 10-ring, or 12-ring channel apertures are the most common and these are usually known as small-, medium-, and large-pore zeolites, respectively [3] Large pore zeolites such as faujasite, mordenite, and beta zeolite play an important role in the design of hydrocarbon conversion catalysts [4]. Because the 8-MR channels are too small for most molecules to enter, it is generally considered a one-dimensional pore system, inducing singlefile diffusion [10] Such a pore size and shape limits the application of mordenite to large molecules. Mesoporosity in zeolites can be created through desilication, which consists of a controlled extraction of silicon from the framework, by treating the zeolite in an alkaline medium This treatment produced extended mesoporosity in zeolite crystals, with a minor effect on their acidity. The influence of the several modification parameters on the structural, chemical and textural properties of the mordenite samples was studied
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