MOLECULAR-DYNAMICS STUDIES ON THERMAL-BEHAVIOR OF AN MFI-TYPE ZEOLITE

Molecular dynamics (MD) simulations of an MFI-type zeolite have been p erformed at various temperatures. Crystal structures and vibrational s pectra obtained from the simulations were in good agreement with the e xperimental data. The temperature induced phase transition between ort horhombic and monoclinic phases, which was already known from experime ntal measurements, was also reproduced. The potential model used in th is study was thus sufficiently applicable to molecular simulations of this type of zeolites. Our MD simulations suggest Pn symmetry for the monoclinic phase instead of experimental P2(1)/n. A small change in th e thermal expansion coefficient was found on the phase transition. In the higher temperature range, another remarkable change was found: the thermal expansion coefficient became negative. This indicates the exi stence of a higher temperature orthorhombic phase.