Structure-directing agents and stacking faults in the CON system: A combined crystallographic and computer simulation study

To better understand the role of structure-directing agents (SDAs) in the formation of molecular sieves and how SDA cations can inhibit the growth of stacking faults, we determined the location of SDA cations in the molecular sieve CIT-1, a member of the CON family, using a combination of crystallography and forcefield-based computer simulations. A Monte Carlo docking algorithm was used to develop trial structural models, which were used as the starting point for structure determination from synchrotron X-ray and neutron powder diffraction data using Rietveld refinements. Our results demonstrate that the SDA cations site in a single location, but with two different orientations. Experimental and computer simulation results indicate that four SDA cations can pack in the pores of CIT-1, invalidating the Monte Carlo docking result, which predicts that only three can be accommodated. Forcefield minimization studies demonstrated clear differences between two SDA cations that can be used to produce CON and a third that does not, but failed to provide information about how stacking faults are inhibited in CIT-1 by one particular SDA. (C) 2000 Elsevier Science B.V. All rights reserved.