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

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 Si⁴⁺ 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 Si⁴⁺ in the lattice leads to a high thermal stability as compared to substitution by trivalent cations.