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

Abstract

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 VNI) has been determined in the space group P4 2/ ncm ( a=9.8946, c=36.8715Å) from high resolution synchrotron powder diffraction data collected on an NH 4 +-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 33] polyhedra. There is a 2-dimensional 8-ring channel system between the [5 33] polyhedra layers. Rietveld refinement of the structure of VPI-9 in the as-synthesized form (Rb 44K 4[Si 96Zn 24O 240]·48H 2O) required a doubling of the c-axis ( a=9.8837, c=73.6505Å; V=7195 Å 3), and a reduction of the symmetry to P4 12 12. In addition to the 15 T-atoms and 30 oxygens in the framework, 9 Rb, 2 K and 3 H 2O 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 29Si 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.