COMBINED NEUTRON AND X-RAY-POWDER DIFFRACTION STUDY OF ZEOLITE CA LSXAND A H-2 NMR-STUDY OF ITS COMPLEX WITH BENZENE

The crystal structure of a low-silica zeolite with the faujasite struc ture, Ca LSX (Si/Al = 1), has been determined from powder X-ray and ne utron diffraction data. The crystal structure was refined by the Rietv eld least-squares method in the cubic space group Fd3 [a = 25.1080(1) Angstrom]. The final R(wp)-factor and the chi(2) for the combined refi nement were 10.1% and 1.81, respectively. The calcium ions are located preferentially in site I and site II, and distortions of the zeolite framework are ascribed to the interaction between these calcium ions a nd framework oxygen atoms. H-2 NMR studies of C6D6 adsorbed in Ca LSX (I molecule per supercage) confirm that deuterated benzene is bound st rongly at site II and interacts facially with calcium so that rapid ro tation about the six-fold axis of the sorbate is observed in the tempe rature range 260-330 K. The activation energy for this rotation is onl y 4.9 kJ mol(-1). By contrast, deuterated benzene in Na LSX exhibits r apid isotropic motion down to 260 K; the diffusion coefficient and act ivation energy for this system are calculated to be D-298 = 1.9 X 10(- 11), m(2) s(-1) and E(a) = 10.6 kJ mol(-1), respectively, from H-2 NMR spin-lattice relaxation measurements.