Thermal stability of the Mobil Five type metallosilicate molecular sieves—An in situ high temperature X-ray diffraction study

Abstract

We have carried out in situ high temperature X-ray diffraction (HTXRD) studies of silicalite-1 (S-1) and metallosilicate molecular sieves containing iron, titanium and zirconium having Mobil Five (MFI) structure (iron silicalite-1 (FeS-1), titanium silicalite-1 (TS-1) and zirconium silicalite-1 (ZrS-1), respectively) in order to study the thermal stability of these materials. Isomorphous substitution of Si 4+ by metal atoms is confirmed by the expansion of unit cell volume by X-ray diffraction (XRD) and the presence of Si–O–M stretching band at ∼960 cm −1 by Fourier transform infrared (FTIR) spectroscopy. Appearance of cristobalite phase is seen at 1023 and 1173 K in S-1 and FeS-1 samples. While the samples S-1 and FeS-1 decompose completely to cristobalite at 1173 and 1323 K, respectively, the other two samples are thermally stable upto 1623 K. This transformation is irreversible. Although all materials show a negative lattice thermal expansion, their lattice thermal expansion coefficients vary. The thermal expansion behavior in all samples is anisotropic with relative strength of contraction along ‘ a’ axes is more than along ‘ b’ and ‘ c’ axes in S-1, TS-1, ZrS-1 and vice versa in FeS-1. Lattice thermal expansion coefficients ( α v) in the temperature range 298–1023 K were −6.75 × 10 −6 K −1 for S-1, −12.91 × 10 −6 K −1 for FeS-1, −16.02 × 10 −6 K −1 for TS-1 and −17.92 × 10 −6 K −1 for ZrS-1. The highest lattice thermal expansion coefficients ( α v) obtained were −11.53 × 10 −6 K −1 for FeS-1 in temperature range 298–1173 K, −20.86 × 10 −6 K −1 for TS-1 and −25.54 × 10 −6 K −1 for ZrS-1, respectively, in the temperature range 298–1623 K. Tetravalent cation substitution for Si 4+ in the lattice leads to a high thermal stability as compared to substitution by trivalent cations.