Synthesis optimization and structure analysis of the zincosilicate molecular sieve VPI-9

The synthesis of the zincosilicate molecular sieve VPI-9 has been optimized to produce a pure and highly crystalline material reproducibly. Its complex framework topology (structure type code VM) has been determined in the space group P4(2)/ncm (a=9.8946, c=36.8715 Angstrom) from high resolution synchrotron powder diffraction data collected on an NH4+-exchanged sample. Attempts to solve the structure by conventional methods or by exploiting anomalous scattering effects were unsuccessful. However, the application of a new approach to structure solution from powder data (FOCUS), which incorporates some of the principles used intuitively in model building into an automated structure determination procedure, generated the correct framework. With 7 T-sites, this framework topology is the most complex yet solved from powder diffraction data without manual intervention. The topology can be described in terms of two types of layers joined via isolated tetrahedra. One layer is a simple 4.8(2) net, and the other contains chains of pairs of [5(3)3] polyhedra. There is a 2-dimensional 8-ring channel system between the [5(3)3] polyhedra layers. Rietveld refinement of the structure of VPI-9 in the as-synthesized form (Rb44K4[Si96Zn24O240]. 48H(2)O) required a doubling of the c-axis (a = 9.8837, c = 73.6505 Angstrom; V = 7195 Angstrom(3)), and a reduction of the symmetry to P4(1)2(1)2. In addition to the 15 T-atoms and 30 oxygens in the framework, 9 Rb, 2 K and 3 H2O positions could be located within the 8-ring channels. Refinement of the 170 structural parameters using synchrotron data converged with R(wp) = 0.147 (R(exp) = 0.099) and R(F) = 0.069. The Zn atoms in the framework are completely ordered in layers. Each Zn is surrounded by four Si, and each Si by three Si and one Zn. The peaks in the Si-29 MAS NMR spectrum can be interpreted very convincingly in terms of the average Si-O-T angles found in the refinement. While the structure of VPI-9 has several features in common with those of the related zincosilicate molecular sieves VPI-7 and RUB-17, such as the framework density, 4.8(2) layers, 3-rings, and a subunit containing Zn, there are also some interesting differences, which might be related to the lower Zn concentration in VPI-9.