Tungsten oxide catalyst on various supports for metathesis of 1-hexene

Abstract

In this work, supported tungsten-based catalysts with various supports and tungsten loading amounts were studied for their activities in metathesis of 1-hexene. Five supports were utilized in this work, i.e. HMS, A1-HMS, ZSM-5, silica-alumina and alumina. The supported catalysts were prepared by impregnation of the supports with sodium tungstate solution via incipient-wetness method. The tungsten loaded supports were calcined at the temperature of 500℃ for 5 hours. The supported catalysts as well as the unloaded synthesized supports were characterized using X-ray diffractometer, nitrogen adsorptometer, diffuse reflectance ultraviolet-visible, Raman scattering, aluminum nuclear magnetic resonance, inductively coupled plasma-atomic emission spectrometers. Owing to very high surface area of HMS, no WO₃ crystallite was found in its Raman scattering and DR-UV spectra. This is the evidence of high dispersion of oxide on the surface of HMS. Other supports at the same WO₃ loading gave the spectra of the crystallite phrase in different quantities. To seek for the optimal condition for only olefin metathesis without concurrent cracking, the catalysis over WO₃/HMS at various temperatures from 200 to 500℃ and WO₃ loading amounts from 1 to 9%wt were investigated using 30.5% 1-hexene vapor in nitrogen as feed. The result show that at high temperatures and low WO₃ loading, the cracking process corresponding to formation of alkanes and rather than light olefins, is predominant for the strongly acidic ZSM-5. For the rest of supports, metathesis is found to be competitive with cracking. As a result, for those less acidic supports, light olefins are major products but no alkane larger than ethane. For all supports except alumina, selectivity to propylene increases with increasing the WO₃ loading but conversion of 1-hexene is not significantly affected by an increase of the WO₃ loading.