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The nature of fluorinated oxide catalysts : a nuclear magnetic resonance investigation

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  • Chemistry


NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Fluorination of oxide catalysts has been shown to drastically change the catalytic properties of these materials. The catalytic activity of these materials has been studied using a wide variety of reactions. Research on fluorinated oxides has focused upon improving product yields and product selectivity and upon obtaining a better understanding of the unmodified oxide catalyst as changes due to fluorination are observed. The purpose of this investigation has been to demonstarate the utility of pulsed nuclear magnetic resonance (NMR) spectroscopy as a direct spectroscopic probe of the local chemical environment of the hydroxyl groups and the fluorine atoms of these materials. Quantitative analysis of the hydroxyl group and fluorine atom concentrations of these materials is difficult. Most techniques used previously require temperatures in excess of 1200 K and result in destruction of the sample. NMR has been shown to be an effective non-destructive quantitative tool. Precision of 5% has been demonstrated with samples containing as few as 5 x 10(18) hydrogen or fluorine atoms. The chemical shift interaction has shown that the fluorine forms covalent bonds to the silicon and aluminum atoms of silica, alumina, and various aluminosilicates. If an aluminosilicate contains 11 wt% alumina, only SiF and SiOH species are observed. SiF, SiOH, AlF, and Al0H species are observed for aluminosilicates containing 48 wt% alumina. The SiF bond is identical for all fluorinated silicas and aluminosilicates calcined at 873 K. The Carr-Purcell-Meiboom-Gill data show that the hydroxyl groups are well isolated from one another for all samples. The same is true for the fluorine atoms. The decay constants of the hydrogen and fluorine 90°-[...]-180°and Carr-Purcell-Meiboom-Gill experiments differ by at least a factor of ten. Therefore, the nuclei are experiencing fluctuations in their local magnetic fields, probably as a result of anisotropic motion or diffusion. The behavior of the spectra observed at temperatures between 110 K and 290 K is very dependent upon the type of oxide, the degree of fluorination, and the calcining temperature.

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