The Nature of Fluorine Modified Oxide Surfaces: An NMR Study
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THE NATURE OF FLUORINE MODIFIED OXIDE SURFACES:
JOHN R. SCHLUP and ROBERT W. VAUGHAN Division of Chemistry and Chemical Engineering, Technology, Pasadena, California 91125
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AN NMR STUDY
California Institute of
ABSTRACT Fluorine modified oxide surfaces have received considerable attention both as research materials and as commercial catalysts. Pulsed NMR has been used to directly observe protons and fluorine on fluorine modified silica,llumina, and aluminosilicates. The center of mass of the F spectra is consistent with values reported for covalent siliconfluorine and aluminum-fluorine bonds. The proton and fluorine concentrations have been investigated as a function of sample preparation. INTRODUCTION The addition of fluorine to oxide catalysts has been shown to drastically change the properties of oxide catalysts. During the last twenty years, these properties have been demonstiryted with a wide variety of reactions, catalysts, and reactor conditions. Research on fluorine modified oxide catalysts has focused on improving product yields and selectivity and on obtaining a better understanding of the unmodified oxide catalysts as changes due to the addition of fluorine are observed. Fluorine modified oxide catalysts have been studied by measuring the rates of a variety of reactions and by measuring the surface acidity. Spectroscopic techniques have included infrared spectroscopy, nuclear magnetic resonance x-ray photoemission spectroscopy, x-ray diffraction, and inelastic electron tunneling spectroscopy. In general, the object has been to understand these catalysts by monitoring changes in their chemical behavior and in their adsorption of small molecules after the addition of fluorine. It is hoped the observed changes in behavior may then provide insight into changes at the catalytically active sites. Although this approach has provided much information about the catalysts, one would like to have a direct spectroscopic probe of the local environment of the hydroxyl groups and fluorine atoms. In the past infrared spectroscopy has been most widely used as a spectroscopic probe. However, since the fluorine atom vibrations are buried among the bands due to the lattice vibrations, one is limited to observing only the hydroxyl group stretching vibrations. Even then, past research has focused on the perturbations in these vibrations that result from modifying the surface or from the adsorption of small molecules. However, nuclear magnetic resonance provides a very specific probe of the local environments of hydrogen and fluorine atoms. The chemical shift interactions provides information on the electronic environment while the dipole-dipole interaction yields information about the location of neighboring atoms. NMR is also a useful tool for quantitative analysis. Several key questions arise as one investigates fluorine modified oxides. The type of bonding present must be understood. The 3 xrtence of fluorinehydroxyl group hydrogen bonds has also been proposed.' The focus of the Deceased.
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