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 f ormation 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 crystallog raphy and forcefield-based computer simulations. A Monte Carlo docking algo rithm was used to develop trial structural models, which were used as the s tarting point for structure determination from synchrotron X-ray and neutro n powder diffraction data using Rietveld refinements. Our results demonstra te that the SDA cations site in a single location, but with two different o rientations. Experimental and computer simulation results indicate that fou r SDA cations can pack in the pores of CIT-1, invalidating the Monte Carlo docking result, which predicts that only three can be accommodated. Forcefi eld minimization studies demonstrated clear differences between two SDA cat ions 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.