Brent M. T. Lok

The role of organic molecules in molecular sieve synthesis

Abstract During the last 15 years, organic quaternary species, ions and amines, have been extensively employed as ‘templating’ additives in both aluminosilicate and aluminophosphate molecular sieve syntheses. A ‘template theory’ was evolved to explain the structure-directing effect of these organic species. The charge distribution and the size and geometric shape of a template are believed to be the causes for structure-directing. Meanwhile, data available in the literature also strongly suggest that gel chemistry, i.e. hydroxyl ion concentration, Si0 2 /Al 2 O 3 ratio, temperature, etc., is extremely important. The basic questions to be answered by the ‘template theory’ are: (1) How can one template give rise to so many different structures? (2) How can so many templates, differing in size and shape, all direct the same structure? (3) On the other hand, why do certain structures not form in the absence of a specific template molecule? In this article we intend to carefully review much of the synthesis data available in the literature and to examine the importance of the basic gel chemistry as well as of the templating effect. Also, we want to explore the close relationship of these two effects in molecular sieve synthesis. Based on the data reviewed, we propose that during the hydrothermal synthesis of molecular sieves, both gel chemistry and template species can play important roles in the formation of a specific structure, but that templating becomes operative only in the environment of the right ‘gel chemistry’.

Aluminophosphate molecular sieves and the periodic table

New generations of crystalline microporous molecular sieve oxides have been discovered based on the novel aluminophosphate family by incorporating one or more of an additional thirteen elements from the Periodic Table into the AIPO 4 framework. Elements incorporated include Li, Be, B, Mg, Si, Ga, Ge, As, Ti, Hn, Fe, Co, and Zn, spanning monovalent through pentavalent framework cationic species. The new materials comprise more than two dozen structures and two hundred compositions, including multi-element frameworks containing combinations of up to six framework cations. Pore sizes range from 0.3nm to 0.8nm encompassing small, intermediate and large pore structures. The new molecular sieves are synthesized by hydrothermal crystallization of reactive aluminophosphate gels containing the additional framework elements and an organic template. Proof of framework incorportion includes the formation of novel structures, the enhancement of catalytic activity, elemental analysis, and various spectroscopic evidence. The Bronsted acidity observed ranges from weakly to strongly acidic. This landmark discovery of new generations of molecular sieve materials represents a remarkable diversity in crystal structure and crystal chemistry, and offers a nearly unlimited number of design parameters to tailor adsorptive and catalytic properties.

Characterization of zeolite acidity. II. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTi.r. spectroscopy techniques

Abstract The Ammonia Temperature Programmed Desorption (NH 3 — TPD ) technique has been used to study the acidity of six zeolite samples. The results indicate that, in general, three desorption peaks are observed. The three peaks fall in the temperature regions of; less than 473 K, 473–673 K, and higher than 673 K. The first desorption peak is believed to be associated with physically adsorbed or weakly chemically adsorbed ammonia molecules; the second peak is NH 3 molecules adsorbed on zeolite hydroxyl groups, and the third peak is associated mainly with dehydroxylation, strong Bronsted acid sites, and/or Lewis acid sites. The additional data provided by the FT i.r. enables interpretation of the NH 3 — TPD chromatogram obtained by the electronic microbalance.