Monitoring the formation of H-MCM-22 by a combined XRPD and computational study of the decomposition of the structure directing agent

MCM-22 is a siliceous microporous material with the MWW topology. The MCM-22 precursor is a partially disordered layered material that contains hexamethyleneimine (HMI) as the structural directing agent. The layered phase transforms into the ordered three-dimensional zeolite at 523 K, still retaining HMI molecules inside the cages and the channels. Four different samples were analyzed by high resolution synchrotron radiation powder diffraction, after thermal treatment at 373, 523, 623, and 823 K when the organic phase is completely removed. The localization of the HMI molecules was done by direct space Monte Carlo methods and using maximum entropy maps. The disordered molecules were localized in the 10-member ring (MR) sinusoidal channels, in the 12-MR inner part of the supercage and in the 10-MR crossing window. In the precursor, heated at 373 K, there is evidence that the different lamellar sheets are not completely connected one to the other. Molecular orbital (MO) theoretical calculations, applying both periodic and cluster approaches, were performed to shed light on the real symmetry of the calcined MCM-22 phase and on the features of the interlayer interactions in the MCM-22 precursor.