Preparation And Crystallization Mechanism Of Nanometer And Ultra - Fine A - Type Molecular Sieve

Zeolite A is largely used in gas separation filed because of its unique pore system. Zeolites A of small particle size have larger external surface, high surface energy and shorter and neater channels by comparing with the big particle size. So, reducing the particle size is one of the ways to improve the performance of zeolite A. Zeolites A of small particle size have been synthesized by the methods of template and free-template, respectively. The main results are as follows:1. Using TMAOH·5H2O as organic-template, nano-sized zeolite A was synthesized from a clear solution. The influences of the aging time, crystallization temperature and the concentration of Na+cation on crystallinity of as-synthesized products were studied. Then, we focused on the influence of silica sources (silica sol and TEOS) on the structure and morphology of as-synthesized products. Lastly, on the base of above researches, we investigated the influence of polymeric state of silicate precursors on the structure and morphology of as-synthesized productsResults indicated that:(1) The crystallinity of as-synthesized products become higher at first, then grow lower with the prolonging of aging time; became higher at first, then grew lower with the increasing of crystallization temperature; became bigger with the increasing concentration of Na+cation within a given range.(2) The products synthesized by silica sol have higher crystallinity and bigger Si/Al ratio. However, the products synthesized by TEOS have smaller particle size and more narrow particle size distribution.(3) The polymeric state of generated silicate precursors of TEOS in various pH media were different. Using the hydrolysis-polycondensation products of TEOS in various pH media as silica sources was help to research the crystallization mechanism of zeolite A. The as-synthesized products had the minimum Si/Al ratio and crystal size when the hydrolysis-polycondensation process of TEOS happened in natural medium.With the increase of acidity or basicity of the hydrolysis-polycondensation process, both the Si/Al ratio and crystal size of as-synthesized products also increased. Increasing acidity or basicity of the hydrolysis-polycondensation process was facilitated to higher crystallinity of as-synthesized products. However, the products had a trace amount of FAU phase, a part of amorphous phases and the lowest crystallinity when basicity of the hydrolysis-polycondensation process was too strong.2. Ultramicro zeolites A were synthesized by a two-step method (first step:aging at low temperature and with stirring; second step:crystallization with elevating temperature) from a organic template-free aluminosilicate gel system. This method open up a novel way for synthesize zeolites of other types. The results indicated that it was the nucleation period and the number of intermediate species of four-ring (4R) and double four-ring (D4R) increased significantly with the prolonging of the first step. The SiO2/Al2O3ratio and particle size of the solid phase of aluminosilicate gel decreased with increasing time of the first step. The crystal size of final products decreased as a consequence of the increase of nuclei number with the prolonging time of the first step. The minimum average size of as-synthesized products could reach to ca.500nm. However, the products transformed into zeolite A small particle aggregates and the morphology became irregular when the time of the first step was too long.