Structural characterization of as-synthesized B- and Ti-containing MFI-type molecular sieves

The influence of tetrapropylammonium (TPA) ions on the structural propertie s of as-synthesized Ti- (TS-1) and B-containing (BOR-C) MFI zeolites crysta llized from alkali-free gels is investigated. Thermogravimetric analysis sh ows that the negative charge of the BOR-C framework is compensated by the T PA ions, while only 'soft' interactions between TPA and Ti sites are observ ed. Rietveld analysis of the synchrotron X-ray powder diffraction patterns reveals that incorporation of Ti and B causes an expansion and a contractio n of the MFI framework, respectively. Regression curves describing the unit cell parameters variation can be used to determine the real framework comp osition. The presence of TPA ions inside the pores strongly influences the apparent d(Ti-O) and d(B-O) tetrahedral bond distances, which are shorter t han those in the calcined forms. The mobility of the TPA cations in silical ite-l, TS-l and BOR-C samples with the highest Ti and B content is studied by C-13 NMR and molecular dynamics simulations. As expected, the TPA cation mobility increases as the pore size increases (BOR-C < S1 <TS-1). (C) 2000 Elsevier Science B.V. All rights reserved.