Celeste A. Messina

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’.

Hydrogen Supply Options and Issues

Methods of hydrogen gas production for hydrogenation are discussed: steam reforming of natural gas, electrolysis, by-product clean-up by PSA (pressure swing absorption) or scrubbing, and others. Hydrogen can be obtained from a customer-owned hydrogen plant or as a merchant product supplied by an industrial gas supplier. The merchant product can be delivered in trailers as gas, liquid-high (up to 2,200 psig), low pressure (up to 130 psig), or supplied over the fence from a pipeline or hydrogen generating plant. Several factors must be considered in selecting an optimum supply, including availability and cost of capital and of operating labor, use patterns, average demand, instantaneous demand, use pressure, growth projections, and purity requirements. Recent field test data have shown that increased hydrogen purity can improve the hydrogenation processes.