Physicochemical and catalytic properties of a silica-enriched zeolite ZSM-20

Abstract

Zeolite ZSM-20 with and enhanced Si : Al ratio after activation has been obtained by a modification of the usual synthesis route. Despite the increased silica content the crystallinity is high and the crystal structure is nearly identical to that of the ZSM-20 zeolites reported so far. The synthesis was found to be extremely sensitive to the conditions of gel preparation, especially to pH adjustment, the temperature–time regime for hydrolysis of the organic silica source, the evaporation of the released ethanol and the formation of the silica precursors. The well crystallized synthesis product was characterized by X-ray powder diffraction, infrared spectroscopy, thermal analysis, n-hexane adsorption and temperature-programmed desorption of ammonia. The acidity of this ZSM-20 zeolite is high compared with conventional preparations of ZSM-20 zeolites or with the structurally related zeolite Y.

The catalytic properties in the isomerization of m-xylene and the conversion of ethylbenzene have been studied. The isomerization of m-xylene is characterized by a high percentage of disproportionation (toluene formation) due to the high acidity. The distribution of the trimethylbenzene isomers corresponds to the thermodynamic equilibrium. Shape selectivity is obviously not operative owing to the open pore structure. Ethylbenzene is mainly dealkylated to benzene where the new ZSM-20 zeolite is capable of breaking the ethyl side chain to yield toluene.