Thermal expansion studies of silicalite-2 molecular sieves of MEL (ZSM-11) topology

Abstract

Thermal expansion behaviour of siliceous polymorph of MEL type molecular sieves i.e. silicalite-2 is investigated by high temperature X-ray diffraction technique in the temperature range 298–773 K. The negative lattice thermal expansion coefficients, αa = −3.252 × 10−6 and αc = −6.436 × 10−6 K−1 in the temperature range 298–773 K for silicalite-2 samples were observed. The thermal expansion behavior of silicalite-2 is anisotropic, with the relative strength of contraction along ‘c’ axis is more than that along ‘a’ and ‘b’ axes. NTE seen over a temperature range 298−773 K could be associated with transverse vibrations of bridging oxygen atoms in the structure, which results in an apparent shortening of the Si–Si nonbonding distances.