Baeyer-Villiger Oxidation And Acetalization Of Cyclic Ketones On Sulfated Tin Oxide Solid Superacid Catalysts

The concept of so-called superacid, which was firstly proposed by Conant in1923, has been widely used to describe acid systems that are stronger than theoretical100%weight concentration of H₂SO₄. The acid strength of these materials is lowerthan-11.93on the Hammett function scale. Supported solid acid catalysts, especiallysulfated metal oxides, are the good replacements for their conventional homogeneouscounterparts due to their several advantages such as strong acid site, large surface areaand environmental friendliness. Among them, sulfated tin oxide (SO₄^2-/SnO₂) is oneof the candidates with the strongest acidity on the surface. However, researchesconcerning modified SO₄^2-/SnO₂, not only in its structure but also in applications, arerelatively few.In this article, the basic knowledge of the development of the solid superacid（especially sulfated metal oxides） is introduced and reviewed. The major aspects ofthis study are as follows:（1） Preparation and characterization of the simple solid superacid of SO₄^2-/SnO₂,the modified solid superacid of SO₄^2-/SnO₂-Fe₂O₃and the supported solid superacidof SO₂-4/SnO₂/SBA-15. All of the listed solid superacids were prepared viaco-condensation method, followed by sulfation and calcination. The materials werecharacterized by Fourier-transform infrared spectroscopy （FT-IR）, powder X-raydiffraction （XRD）, thermogravimetric analysis （TG） and scaning electron microscopy（SEM）.（2） The obtained material (SO₄^2-/SnO₂) was used as catalysts in Baeyer-Villigeroxidation. Its catalytic activities were evaluated through Baeyer-Villiger oxidation ofcyclic ketones by varying reaction parameters. The reaction, which was conductedunder mild conditions, provided conversions above60%with selectivity above78%of several cyclic ketones only after6h with30%hydrogen peroxide as clean oxidant.The catalyst can be recycled for several times without any significant loss in catalyticactivity.（3） The Fe₂O₃-doped catalysts were used as catalysts in Baeyer-Villiger oxidationand acetalization reactions of cyclic ketones, separately. The prepared materials, especially0.5STF, were proved to be an efficient and reusable catalyst forBaeyer-Villiger oxidation and acetalization reactions of cyclic ketones.（4） The catalytic performance of SO₄^2-/SnO₂/SBA-15for the acetalization wereexplored. The results showed that the supporting of SO₄^2-/SnO₂with SBA-15improved the catalytic activity markedly, which caused by the increasing the surfacearea of sulfated tin oxide. The catalytic system bears the obvious advantages such assimply preparation and the high activity of the catalyst.