The effect of composition on the growth and morphology of zeolite A in solution
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Michael E. Mullins Chemical Engineering Department, Michigan Technological University, Houghton, Michigan 49931 (Received 20 May 2003; accepted 30 January 2004)
The zeolite LTA (Linde type A) framework is composed of an equal amount of silica and alumina arranged in a three-dimension tetrahedral structure. This high alumina content zeolite possesses a strong negative surface charge in a basic solution due to the substitution of aluminum atoms into a SiO4 tetrahedral structure making it difficult to form a continuous layer in solution. Synthesis parameters such as synthesis duration, chemical composition, and synthesis temperature were varied. The crystallization kinetics was carried out using the particle size measurement, percent crystallinity from x-ray diffraction analysis, infrared absorption of tetrahedra using attenuated total reflectance Fourier transform infrared spectroscopy, and the exponential growth model. The results showed that the zeolite A crystallization in a solution depends on the chemical kinetics of the zeolite formation, surface charge of the substrate, and the mass transfer in the solution.
I. INTRODUCTION
The crystallization process of a zeolite from the aluminosilicate synthesis solution depends on the chemical composition, types of chemical used, and the reaction temperature. The framework of zeolite A has an almost equal amount of silica and alumina; however, this composition ratio is also very close to zeolite X, which has a Si/Al ratio of 1–1.5. Therefore, a slightly variation in chemical composition or source of reagents can cause the formation of a different kind of zeolite. The aluminosilicate framework of LTA is composed of a simple cubic structure of eight tetrahedra (double four-rings, D4R) and an octahedron of 24 tetrahedra referred to as -cage; this arrangement produces truncated octahedral units centered at the corners of the cube. The opening of the  channel is about 6.6 Å. The large cavity in the middle of the unit cell, called an ␣ cage, has a diameter of 11.4 Å. The channel to this cavity has an opening of 4.2 Å in dehydrated form.1 Basically, a pure silica zeolite such as the silicalite-1 framework, constructed with SiO4 tetrahedra, is electronically neutral. However, by substituting a Si+4 atom with an Al+3, the electro-neutrality of the framework becomes imbalanced, requiring an additional cation to compensate the negative charge.2 Therefore, by having more numbers of Al+3 substituted for the silica atoms, more
DOI: 10.1557/JMR.2004.0238 J. Mater. Res., Vol. 19, No. 6, Jun 2004
http://journals.cambridge.org
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cations appear in the framework. These mobile cations directly affect the ionic strength of the zeolite particles in water, which can be seen in the zeta potential measurements. A previous study by the authors3 shows that the zeta potential of zeolite ZSM-5 at pH 12 with a Si/Al ratio of about 300 is about −38mV; however, the zeta potential zeolite LTA at the same alkalinity is about −64mV.4 The study by Boudreau et al.,5 which shows a
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