Motions of Trimethylphosphine on the Surface of Acid Catalysts

Abstract

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

Characterization of the acid sites in dealuminated nanosized HZSM-5 zeolite with the probe molecule trimethylphosphine

“…Many authors observed the formation of Lewis acid site after dealumination of zeolites [10][11][12][13][14][15][16], but different opinions on the origin of Lewis acid site were reported in the literature. In general, there are two typical models.…”

“…However, the limitation of this technique lies in the difficulty to determine the strength of the acidity and the number of acid sites in a quantitative way for the different extinction coefficients of each adsorbed species. Trimethylphosphine has emerged as a promising approach to characterize the acid sites in zeolites [10][11][12] and other materials [13][14][15] using 31 P MAS NMR spectroscopy. Trimethylphosphine (TMP) is suitable for 31 P MAS NMR studies as it has isotope of 100% natural abundance, nuclear spin of 1/2, and large gyromagnetic ratio.…”

Motions of Trimethylphosphine Oxide in Carbon Nanotubes as Revealed by Solid‐state NMR

“…The reason may be that the TMPO is always adsorbed on the defects of the nanotubes, where the large conjugated system is broken. Furthermore, unlike the biexponential 31 P spinlattice relaxation behavior of TMP adsorbed on Lewis acid sites in zeolites, [28] the spin-lattice relaxation behaviors are monoexponential for TMPO in CNTs. Figure 3b shows a representative simulation of the relaxation curve from Figure 3a, which could be well fitted with the monoexponential function: [29] Figure 4b shows the variation of 31 P spin-lattice relaxation rate versus the inverse of the absolute temperature for TMPO in o-CNT.…”

“…Fourth, it has been found that CSA to the relaxation rate of adsorbed TMP is negligible even with CSA as large as 200 kHz. [28] Hence, the main relaxation contribution for 31 P nucleus is from the nine methyl protons in TMPO molecule. Spin-lattice relaxation rate dominated by proton dipolar interactions, and the modulation of these interactions by molecular motions, have been studied thoroughly.…”

Relation Between Acid and Catalytic Properties of Chlorinated Gamma-Alumina. a 31p Mas Nmr and Ftir Investigation

“…The 31 P nucleus is an attractive candidate for surface acidity characterization studies as it possesses a large chemical shift range and it is, from an NMR point of view, a sensitive nucleus (100% natural abundance). Among the possible phosphored bases, TMP has been employed to probe brönsted and Lewis acidity in zeolites [10][11][12][13] and other materials [14][15][16][17][18]. because of its high basic strength (TMP basicity is about 1000 times greater than that of pyridine) and its rather small size, this probe is able to be adsorbed on a large amount of the solid acid sites.…”