[Li-Si-O]-MFI: A new microporous lithosilicate with the MFI topology
Chemistry of Materials 16(13): 2605-2614
We report the synthesis and structure solution of [Li-Si-O]-MFI (a = 19.793(4) Angstrom, b = 19.766(4) Angstrom, c = 13.266(3) Angstrom, space group P2(1)2(1)2(1)), a framework zeolite related to ZSM-5 (silicalite) with a Li/Si ratio of 4:92. Single-crystal synchrotron X-ray diffraction data, Si-29 NMR, and infrared spectroscopy indicate that the Li is randomly distributed over the framework sites. In addition, Li occupies ordered positions on extraframework sites that are occluded in voids outlined by the double five- or six-membered rings. The strong interaction between the extraframework Li cations and the negatively charged framework may be responsible for the distortion of the lattice from Pnma, the usual symmetry associated with as-synthesized members of the MFI-family, to its subgroup symmetry P2(1)2(1)2(1). Variable-temperature Li-7 and H-1 MAS NMR experiments show that the extraframework Li cations are not directly accessible to gas molecules, but that the protons formed on calcination are accessible.