Which sites are the active sites in TiO2–SiO2 mixed oxides?

TiO2–SiO2 mixed oxides are very important industrial materials and catalysts. They have found several industrial applications and attracted great interest in academic and industrial research laboratories. While significant progress has been made in the understanding of their properties, the mechanism of their catalytic action is still debated. We summarize the past and recent literature on the subject. Tanabe (1974, 1981) has attributed the catalytic activity to the formation of Brønsted acidity, and his proposal has been supported by a number of reports even recently. On the other hand, our work has provided evidence that high purity TiO2–SiO2 mixed oxides DO NOT have Brønsted acidity. When used as catalysts for the isomerization of 1-butene, the reaction products consist of linear butenes only, with no skeletal isomerization nor other products typical of acid catalysis. Pure TiO2 has high catalytic activity and gives the same reaction products. The discrepancy with previous results is very likely due in some instances to the presence of acid-inducing impurities in materials used in early works and in others to the fact that methods used to detect Brønsted acidity, reliable with certain solids, give erroneous results with others. A reexamination of these methods is strongly recommended. Finally many studies relied on the low temperature catalytic activity in 1-butene isomerization to linear butenes, overlooking the fact that only skeletal isomerization is the real proof of Brønsted acidity.