Acidic properties of mixed tin+antimony oxide catalysts

Abstract

Mixed oxides of tin and antimony have been used as catalysts in a static system and outgassed both at room temperature and at 698 K in a study of the approximately zero order stages of the isomerization of 3,3-dimethylbut-1-ene (373 K), cyclopropane (411 K), but-1-ene (293 K) and cis-but-2-ene (293 K) and of the dehydration of isopropanol (343 and 408 K). With catalysts outgassed at room temperature, weakly acidic sites are present, and all the reactions probably occur by a carbonium ion type of mechanism with Brönsted acid sites as a source of protons. Rates increase to a maximum as the antimony content increases from zero to ≈ 50 atomic %, and then decline with further increase in the antimony content. Outgassing of the catalysts at 698 K increased the rate of isomerization of 3,3-dimethylbut-1-ene, but for cyclopropane and isopropanol decreased rates were observed due to poisoning by the propene product. For but-1-ene and cis-but-2-ene, the higher temperature outgassing procedure changed the rate against catalyst composition pattern considerably in that only catalysts with less than 50 % Sb were active, and a mechanism involving an allyl intermediate is proposed. Catalyst activity could be poisoned by treatment with bases or with sodium acetate. It is concluded on the basis of a proposed correlation between rates and acidity, that the catalyst composition corresponding to maximum acidity is different from that for maximum selective oxidation activity.