On acid surfaces, trimethylphosphine (TMP) forms either Brbnsted or Lewis site adducts, TMPH+ or TMP:L, respectively. In TMPH+, the 31P magnetization decay is practically monoexponential and the 31P spin-lattice relaxation rate (1/T1) is not thermally activated. In TMP:L, the magnetization decay is biexponential and the relaxation rate is thermally activated. Using the known internuclear distances in TMPH or TMPH+, it is shown that the relaxation rates can be accounted for by using the homo-and heteronuclear (intramolecular) contribution rates. The only adjustable parameter is the preexponential factor in the correlation time (TO). TO is s for the PH vector reorientation and 10-13 s for the H-H (methyl) vector rotation in TMP:L. It is l t 7 s for the PH+ reorientation in TMPH+. It is observed that the resonance frequency in TMPH+ or TMP:L is temperature independent, ruling out chemical exchange. In one case only, (TMPH+ on dealuminated H-Y zeolite) a change in the isotropic chemical shift with temperature has been observed.
IntroductionThe use of trimethylphosphine (TMP) for measuring the number, the nature, and the strength of acid sites on the surface of catalysts has been advocated by the research groups of Lunsford's2-4 and Maciel.3 The downfield chemical shift of the 31P NMR resonance is a function of the deshielding of the nucleus when TMP interacts with an electron acceptor. For instance, in the proton adduct TMPH+ formed on a Brbnsted acid site, the 31P isotropic chemical shift observed under magic angle spinning (MAS) and no proton decoupling is ca. -4 ppm while physically adsorbed TMP shows a 31P MAS NMR line at ca. -60 ppm. The 31P resonance in the Lewis site adduct TMP:L is normally in between these limits. However, we have shown recently5 that on superacid solids such as ZrOrSO, or HfOrSO,, 31P MAS NMR lines are observed between 0 and 30 ppm and that these lines were not due to the oxidation of TMP. Lunsford et aL4 had observed that the oxidation products of TMP have resonance lines above 30 ppm. Thus, we suggested that the lines between 0 and 30 ppm were due to TMP interacting with strong Lewis sites.To some extent, however, the use of TMP for scaling the acid strength of acid sites has been disappointing. On aluminas and in acid zeolites where a broad distribution of Brbnsted and Lewis acidity exists69 the overall span of the most downfield shifted line assigned to TMPH+ is between -4.0 and -8 ppm (at room
Radiotracer [ 18F] and [ W l ] experiments demonstrate the deposition of oligomeric chlorocarbon species on fluorinated metal oxides and trapping of hydrochlorocarbon monomers, both without loss of surface fluoride lability, thus accounting for the unusual room-temperature heterogeneous catalytic fluorination of CH3CC13 and related species by anhydrous HF.
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