Fluoride Ion Supported on gamma-Alumina: Characterization and Catalytic Activity

[摘要] Caesium or potassium fluoride supported on gamma-alumina have been widely used as solid bases and catalysts in organic synthesis, in the presence of the solvent and under heterogeneous conditions. The nature of the active sites on the surface of these solids was not yet fully understood. The aim of the present work is to obtain information regarding the nature and the active sites of the supported metal fluoride, using a series of analytical techniques and chemical reactions. The chemical reactions involve the individual interactions of the Lewis acid sulphur tetrafluoride and its hydrolysis products, thionyl fluoride and sulphur dioxide, the Lewis acid carbonyl fluoride and its hydrolysis product carbon dioxide, and anhydrous hydrogen fluoride with the supported metal fluoride. The interactions of the probe molecules described above with gamma-alumina pretreated with sulphur tetra-fluoride, thionyl fluoride, carbonyl fluoride or anhydrous hydrogen fluoride are also studied. Terminal hydroxyl groups present on the surface of gamma-alumina supported metal fluoride are removed by treatment with sulphur tetra-fluoride, thionyl fluoride or sulphur dioxide then anhydrous hydrogen fluoride. The resulting materials caesium or potassium fluoride supported on gamma-alumina, are catalytically examined, using the chlorofluorination of sulphur tetrafluoride with chlorine monofluoride at room temperature to give sulphur chloride pentafluoride as a model reaction. The reactions described above are studied under heterogeneous conditions at room temperature, using the radiotracers [14C]-carbon [36Cl]-chlorine, [18F]-fluorine and [35S]-sulphur labelled compounds. The experiments carried out using [35S]-sulphur labelled sulphur tetrafluoride, thionyl fluoride or sulphur dioxide show that there is more than one adsorbed species present on the surface of the supported metal fluoride, these are weakly adsorbed molecules of sulphur tetra-fluoride, thionyl fluoride and sulphur dioxide, and permanently retained pentafluorosulphate and fluorosulphite. For each system studied the surface activity is dependent on the metal fluoride loading in the composition range 0.6 - 20.0 mmol g -1, and is at a maximum at 5.5 mmol g The overall uptake of the volatile sulphur containing probe molecules by the supported metal fluoride is in the order SF4> SO2 ~ SOF2. [18F]-Fluorine exchange is observed in the sulphur tetrafluoride/supported metal fluoride system but not in the thionyl fluoride/supported metal fluoride system. [18F]-Fluorine exchange is observed between [18F]-fluorine labelled carbonyl fluoride and the supported metal fluoride. The trifluoromethoxide anion is detected after each reaction. Experiments carried out using [14C]-carbon labelled carbonyl fluoride show that there are two species present on the surface of the supported metal fluoride, weakly adsorbed molecules of carbonyl fluoride and carbon dioxide. The surface activity is at a maximum at 5.5 mmol g -1. The reaction of carbon dioxide and the supported metal fluoride is barely detectable but experiments carried out using [14C]-carbon labelled carbon dioxide confirm that weakly adsorbed carbon dioxide occurs. The uptake of fluorine by calcined gamma-alumina during treatment with sulphur tetrafluoride, thionyl fluoride, carbonyl fluoride or anhydrous hydrogen fluoride, is in the order SF4 ~ SOF2 > COF2 ~ HF. It is found that treatment of gamma-alumina with anhydrous hydrogen fluoride or carbonyl fluoride results in labile surface fluorine. However treatment of calcined gamma-alumina with sulphur tetrafluoride or thionyl fluoride results in at least two forms of surface fluorine being produced one which is not labile and consisted of ca. 40% of the total fluorine present at the surface. Caesium or potassium fluoride supported on gamma-alumina prepared from aqueous or non-aqueous solution and treated with sulphur tetrafluoride, thionyl fluoride or sulphur dioxide then anhydrous hydrogen fluoride are efficient catalysts for the chlorofluorination of sulphur tetrafluoride by chlorine monofluoride. Catalytic activity is at a maximum at 5.5 mmol g -1 and poisoning by chlorine monofluoride appears to be less important than was the case for unsupported caesium fluoride. Catalysts prepared by mixing the two salts in the absence of the solvent or those treated with carbonyl fluoride or anhydrous hydrogen fluoride give very poor yields of sulphur chloride pentafluoride.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry