Structural characterization of Ti-silicalite-1: A synchrotron radiation X-ray powder diffraction study

We present high-resolution X-ray powder diffraction (XRPD) data collected a t the BM16 beamline at ESRF on a set of dehydrated Ti-silicalite-l (TS-1) s amples having Ti content (molar ratio) x = [Ti]/([Ti] + [Si]) in the range 0-0.022. The cell volume values resulting from Rietveld refinements of the powder diffraction data exhibit very good linear correlation (r = 0.99994) with Ti content: V(x) = 2093.0x + V(0) [V(0) = 5335.8 Angstrom(3)]. This wo rk represents a continuation of the important characterization work of the EniRicerche group [J. Catal. 137, 497 (1992)] performed on TS-1 previously treated with ammonium acetate, calcined, and measured in atmosphere (i.e., under partially hydrous conditions). Our results indicate that when XRPD me asurements are performed under carefully controlled vacuum conditions a hig hly linear correlation between V and x is obtained without any need for sam ple pretreatment, Finally, the very high quality of our experimental data a llows us to comment on the presence of some preferential framework T sites for Ti substitution. Such sites were recently proposed by two different gro ups on the basis of the results of molecular dynamics modeling. (C) 1999 Ac ademic Press.