A Study of Piperidinium Structure-Directing Agents in the Synthesis of Silica Molecular Sieves under Fluoride-Based Conditions

This study is a continuation of our efforts to understand the interplay in the self-assembly chemistry
for formation of molecular sieves from guest organocations and inorganic silicon oxide. In this particular
study we focus on the competitive interplay of the organocations and the synthesis cofactor fluoride anion.
The anions play a key role in structure determination, as a function of net solution concentration. They
compete with the role for the space-filling organocation in determining which molecular sieve host structure
will be specified. In this study we look at this competition in the synthesis for a series of 33 different
organocations derived from the piperidine ring system. Derivatives were prepared which both fixed
substituents on the carbon and nitrogen centers on the ring. Results were discussed in terms of product
selectivity from synthesis as a function of solution concentration for the reactants. A total of 17 different
host topologies were found in this series, and a correlation was seen for (a) open-framework lattices (low
framework densities) under the most concentrated reaction conditions and then (b) high framework density
products once the conditions were more dilute. Some surprising synthesis differences are seen in comparing
the performance of these structure directing agents (SDAs) in fluoride media vs hydroxide media (the more
conventional environment for zeolite/molecular sieve syntheses involving silicate chemistry). Finally molecular
modeling was used to understand some of the trends in product selectivity for closely related guest (SDA)
candidates.