Abstract
The crystallization of low-silica X zeolite (LSX) was studied in Na-K gel systems with different extents of replacement of Na by K while fixed content of other components. X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, infrared spectra, and nuclear magnetic resonance were used to characterize liquid and solid phase. In the synthesis of LSX, the molar ratio of K/(Na+K) affects the crystallization and the composition of final products. A higher mole fraction of K corresponded to a lower crystallization rate, higher concentration of Si in the liquid phase, and lower Si/Al ratio of the obtained LSX. The average size of LSX products steadily increased with the progressive replacement of Na by K in the initial gels, and crystal morphology of the LSX products gradually changed from round to octahedral. For alkylation of toluene with methanol over obtained LSX, the selectivity of ring alkylation product xylene decreased while the side chain alkylation products styrene and ethylbenzene increased with the increased x values except x = 0, which was due to its low crystallinity.
Highlights
Zeolite X is a kind of microporous material with a faujasite framework structure and a Si/Al molar ratio that varies from 1.0 to 1.5
This study investigated the effect of K/(Na+K) molar ratio in the synthesis of low-silica X zeolite (LSX)
The results suggests that the crystal size distribution becomes narrow (Figure 6) and the average size of the crystal products steadily increased with the increased replacement of Na by K in the initial gels
Summary
Zeolite X is a kind of microporous material with a faujasite framework structure and a Si/Al molar ratio that varies from 1.0 to 1.5. Cations remarkably affect the properties of the aluminosilicate gel and the forms of the zeolite framework because of their “structure-forming” or “structurebreaking” capacities.[11] Hydrothermal synthesis method in the Na-K system is a common way to synthesize zeolite X.12-14. Previous work studied the synthesis of LSX zeolite from kaolin clays heated with sodium carbonate in highly alkaline-mixed Na/K hydrothermal systems. They investigated the effect of the Na/K ratio on both the crystallization reaction and the characteristics of the products obtained.[14] they did not investigated the effects of different Na/K ratios on the chemical compositions of liquid and solid phases in the synthesis process, and not explained why K replacing Na could affect crystal products.
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