High temperature thermal expansion behavior of silicalite-1 molecular sieve: in situ HTXRD study

The measurement of the thermal expansion behavior of silica polymorph of zeolite MFI (Silicalite-1) was carried out in greater detail in the temperature range 373 K-673 K using high temperature X-ray diffraction (HTXRD) technique. The sample shows negative thermal expansion (NTE) in the temperature range studied, which is related to the transverse vibrations of the bridging oxygen atom between the two rigid polyhedrons. There is an anisotropy in the negative thermal expansion of this material that is contracting more along a axis than along the b and c axes. The bond angles and bond distances were determined from the Rietveld refinement data to look for any structural changes taking place as a function of temperature. The Si-O bond distances seem to play no role in the NTE, as there is no change in their distances as a function of temperature. It was observed that the Si-Si non-bonding distances for which the interatomic vectors are parallel to any of the crystallographic axes is responsible for the negative thermal expansion along the respective axes. The strong anisotropy in the expansion along a axis as compared to b and c directions is likely due to the stacking of the 2-dimensional layers in the bc direction. The observed decrease in Si-Si non-bonding distances with the constituent Si-O bond distances remaining constant, supports the correlation that the transverse vibrations of the bridging oxygen atoms in the structure are responsible for the NTE in this material. (c) 2007 Elsevier Inc. All rights reserved.