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Studies of small basic probe molecules by NMR to identify surface acidic sites on oxides of aluminum and silicon have yielded information on the types of surface acid sites, on the numbers of these sites, and about the molecular dynamics of the adsorbed probe molecular. Initial work in this area employed amines as probes via /sup 13/C and /sup 15/N magic-angle spinning (MAS) NMR. More recent work has focused on the study of phosphines via /sup 31/P NMR. /sub 3//sup 1/P has the advantage over /sup 15/N of greater NMR sensitivity due to a higher magnetogyric ratio and 100% natural abundance of the spin-1/2 isotope, therefore permitting extensive sets of experiments over a wide range of surface coverage of phosphines. Although studies of silica-alumina with trialkyl-phosphines have been successful in the quantitative analysis of surface Bronsted sites, the quantitative analysis of surface Lewis sites has been hampered not only by small chemical shift differences between Lewis-bound and physisorbed phosphines but also by the similarity of surface binding equilibrium constants for these two types of sites; these factors result in the absence of a break point in the titration curves for Lewis-bound phosphines from which the concentration of Lewis sites couldmore » otherwise be calculated. In an effort to find a probe molecule that would give a good distinction between the Lewis-bound and physisorbed molecules on surfaces, the authors have investigated the use of trialkylphosphine oxides as probe molecules for surface acid sites of amorphous materials. In the present paper they report promising /sup 31/P NMR results contained on (CH/sub 3/)/sub 3/PO and (C/sub 2/H/sub 5/)/sub 3/PO on silica-alumina.« less |
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