Metathesis of 1-hexene over a WO3/SiO2 catalyst in a gas-phase fixed bed reactor.

Abstract

The performance of a WO3/SiO2 catalyst has been investigated for the metathesis of 1-hexene in an isothermal, gas-phase fixed bed tubular reactor between 420-500°C and at latm. The reactor was fabricated from stainless steel and was placed in an electric tube furnace. The study focused on the effect of operating conditions (reaction temperature, feed gas composition and space time) on the yield of heavy olefins (C7 - C16) and the primary product, decene, using a full factorial experimental design. The objective was to determine the optimum operating conditions for the production of decene. Compositions of condensed liquid and vapour product streams were determined via gas-liquid chromatography. Oxidative and reductive pre-treatments of the catalysts were compared to a conventional pre-treatment under nitrogen in terms of their effect on conversion and product distribution. Experimental tests for the existence of external and internal mass transfer resistances were also conducted. An improved yield of decene was obtained after oxidative pre-treatment of the catalyst. The conversion of l-hexene, selectivity and yield of decene dropped after pre-treatment under hydrogen and nitrogen only. This may be due to over-reduction of the tungsten centres and the formation of an inactive non-stoichiometric surface phase. The best yield of decene obtained from the experimental design was 2.19% at 420°C, 51% feed composition and 661g.min.mo1-1. The yield of decene decreased by 0.11% when the reaction temperature was raised by 40°C, and by 0.17% when the feed gas composition was raised by 17.5% and increased by 0.21% when the space time was increased by 223g.min.mol-1. The large amount of ethylene and propylene produced as well as the formation of polynuclear aromatics suggest significant cracking of 1-hexene. For the size range and flow-rates that were employed, the reaction does not suffer from intraparticle and external mass transfer resistance effects. It was concluded that a combination of lower temperature, lower feed gas composition and higher space time should be used to maximize the yield of decene. A final experiment was conducted to test this conclusion. At 380°C, 55% feed composition and 2875g.min.mol-1, the yield of decene was 12.80%, which represented a significant improvement.