Catalytic conversion of canola oil to fuels and chemicals: roles of catalyst acidity, basicity and shape selectivity on product distribution

摘要

Studies were performed at atmospheric pressure in a fixed-bed microreactor at temperatures of 400 and 500℃ over HZSM-5, silicalite, silica, silica-alumina, γ-alumina, calcium oxide and magnesium oxide catalysts to determine the various roles of catalyst acidity, basicity and shape selectivity on canola oil conversion and product distribution. Results showed that the initial decomposition of canola oil to long chain hydrocarbons and oxygenated hydrocarbons was independent of catalyst characteristics. However, subsequent decomposition (secondary cracking) of the resulting heavy molecules into light molecules (gas or liquid) appeared to be greatly enhanced by the amorphous and non-shape selective characteristics of the catalyst (as in silica-alumina, γ-alumina and silica). In contrast, a high shape selectivity in a catalyst (as in HZSM-5 and silicalite catalysts) permitted a mild secondary cracking resulting in a low gas yield and a high organic liquid product yield. On the other hand, it was interesting to observe that the presence of basic sites in a catalyst (as in calcium oxide and magnesium oxide) strongly inhibited secondary cracking. This resulted in the production of high yields of residual oil and low gas yields. The production of C_2-C_4 olefins, n-C_4 hydrocarbons and aromatic hydrocarbons of unconstrained sizes, which reflected thermal effects on the overall reaction scheme, were predominant in amorphous and non-shape selective catalysts. On the other hand, the formation of C_2-C_4 paraffins, branched chain and total C_4 hydrocarbons as well as aromatic hydrocarbons of constrained sizes (C_7—C_9) which were predominant in the shape selective catalysts showed that, apart from the products formed due to thermal effects, the type, structure and sizes of other products are determined principally by the shape selective characteristic of the catalyst.