Phase selectivity in the syntheses of cage-based zeolite structures: An investigation of thermodynamic interactions between zeolite hosts and structure directing agents by molecular modeling

In this investigation we use molecular modeling to determine the van der Waals stabilizations of piperidinium derivatives and polycyclic quaternary ammonium compounds in zeolite frameworks with cage-based structures: NON, SGT, DDR, AEI, CHA, ITE, and STF. This effort follows earlier work by our group that studied the phase selectivities of these structure-directing agents (SDAs) under different conditions of synthesis [Y. Nakagawa, G.S. Lee, T.V. Harris, L.T. Yuen, S.I. Zones, Micropor. Mesopor. Mater. 22 (1998) 6985; P. Wagner, Y. Nakagawa, G.S. Lee, M.E. Davis, S. Elomari, R.C. Medrud, S.I. Zones, J. Am. Chem. Soc. 122 (2000) 263-271 G.S. Lee, S.I. Zones, J. Solid State Chem. 167 (2002) 289-298]. The stabilization energies are found to be a good indicator of selectivity for phases that often crystallize within the same inorganic conditions (e.g., AEI and CHA, ITE and STF). The calculated stabilization energies are especially good indicators of phase selectivity for the large polycyclic molecules. Although the predictions for the piperidinium derivatives are not as successful at distinguishing selectivity for ITE/STF pair, we again find that the larger molecules among this group are accurately predicted. We suggest that this may be due either to a greater number of configurations available in the ITE cage or to the greater rotational freedom permitted for the smaller molecules in the round ITE cage. We next compare the differences in stabilizations of the different SDA/framework pairs with the differences in their respective framework energies, which have been approximated from empirical relations derived from earlier calorimetry experiments by Piccione et al. In general, these differences are found to be of similar magnitude for different phases that crystallize with the same SDA. Finally, we discuss the effects that siloxy/silanol defects may have on the frequently observed trends in the framework density for series of phases that are crystallized with the same SDA. (c) 2005 Elsevier Inc. All rights reserved.