Synthesis of Alkaline-earth Zeolites

Citation

Khodabandeh, Shervin (1997) Synthesis of Alkaline-earth Zeolites. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/35dq-qn66. https://resolver.caltech.edu/CaltechETD:etd-01102008-150302

Abstract

Zeolite molecular sieves have found extensive applications in ion-exchange, separation and catalytic processes, particularly in the chemical and petroleum industries. Currently, the state-of-the-art in synthesis of high-silica zeolites involves the use of complex organic molecules to direct the formation of zeolitic materials with novel pore structures. While efforts towards preparation of new zeolites using organic molecules as structure-directing agents continues, synthesis of calcium and other alkaline-earth zeolites has not received much attention since the inception of the systematic investigation of zeolite synthesis some 35 years ago. Of the approximate 50 natural zeolites discovered to date, over 20% have eluded synthesis and another 10% have proven exceedingly difficult to synthesize at typical hydrothermal conditions. The overwhelming majority of these zeolites are calcium-dominant. The difficulty encountered in the synthesis of these alkaline-earth zeolites is in direct contrast to their natural occurrence as alteration products of volcanic glasses. Thus, the objective of this work is developing practical methodolgies for the synthesis of alkaline-earth zeolites.

Hydrothermal transformation of perlite (a natural rhyolitic glass) to calcium zeolites is investigated as a first step towards developing synthesis procedures for the preparation of calcium and other alkaline-earth zeolites from pure starting reagents. In particular, synthetic analogues of the calcium zeolites gismondine, heulandite and epistilbite are obtained as alteration products of perlite glass reacting with calcium-containing solutions. The influence of the solution phase species and their concentrations, the pH and the temperature on the distribution of the zeolite products obtained are discussed. It is observed that the crystallization of heulandite from perlite is preceded by the transient formation of a gismondine-type zeolite most similar to the synthetic zeolite P1. This information is exploited to devise methodology for the preparation of zeolite P1 from pure starting reagents and its subsequent conversion to calcium and other alkaline-earth zeolites upon treatment with solutions containing alkaline-earth cations. Thus, a novel approach for the synthesis of alkaline-earth zeolites based on the hydrothermal conversion of zeolite P1 is developed. Details of the synthesis procedures are enumerated for the preparation of alkaline-earth zeolites CIT-3 (HEU), CIT-4 (BRE), epistilbite (EPI), harmotome (PHI), and yugawaralite (YUG). Transformation of zeolite P1 to alkaline-earth zeolites is governed by factors such as the Si/Al ratio of the starting P1 material, the composition of the solution phase and the presence or absence of seed crystals. The effects of these factors on the products obtained, i.e., phase selectivity, are discussed.

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