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Langmuir 1999, 15, 6132-6134
Two-dimensional
31
P Exchange NMR Study of Adsorbed Phosphine Layers
Ian D. Gay,* Bing Hu, and Tai-Cheng Sheng Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada Received March 15, 1999. In Final Form: June 24, 1999 Adsorption of organic phosphines on acidic surfaces produces phosphonium ions, which can be detected by 31P NMR, since their chemical shift is different from that of the free phosphine. In the presence of excess adsorbed phosphine, lines from both phosphine and phosphonium are observed. Two-dimensional exchange NMR is applied to trimethyl- and triphenylphosphines adsorbed on SiO2-Al2O3 and demonstrates that free phosphine and phosphonium ions are undergoing chemical exchange at ambient temperatures. There are two populations of PMe3H+ which exchange with different rate constants, 7 × 104 and 800 s-1, respectively. PPh3H+ exchanges much more slowly with PPh3, having a rate constant of 14 s-1. 31P NMR of adsorbed phosphines has recently become an important method of characterizing acidic surfaces.1-5 In the case of alkylphosphines, lines due to physisorbed, protonated, and Lewis-bound phosphines are readily identified and quantitated. In ref 4 it was found that on a silica-alumina cracking catalyst there is a rapid exchange process occurring between physisorbed PMe3 and a fraction of the PMe3H+ arising from the reaction of PMe3 with surface H+. This manifests itself in the NMR spectrum as a systematic change in chemical shift of the “exchanging” line from about -45 to -60 ppm as the concentration of physisorbed PMe3 is increased. However there is always an unshifted PMe3H+ line at -4 ppm, so some of this species is not exchanging, or is at most exchanging more slowly. There is also some evidence5 for exchange processes between protonated and physisorbed arylphosphines on this surface, although the situation is not so clear-cut in this case. We have now applied two-dimensional magic angle spinning (MAS) exchange NMR to elucidate these exchange processes more fully. We have used the basic twodimensional experiment described by Szeverenyi et al.6 except that usually 31P magnetization was generated by a 90° pulse, rather than by cross-polarization. The premixing pulse was phase-alternated in synchrony with the receiver, and TPPI was applied to the initial pulse or cross-polarization, to provide correct phasing in f1. Our experiments were done at a 31P resonance frequency of 60.5 MHz on a 3.5 T instrument, at an ambient probe temperature of 23 ( 1 °C. Because of the relatively low field and low chemical shift anisotropies of the species involved, it is practical to spin sealed samples7 at speeds where spinning sidebands are small and more elaborate experiments8 are not required.
* To whom correspondence may be addressed: telephone, (604) 291-4889; fax, (604) 291-3765; e-mail,
[email protected]. (1) Lunsford, J. H.; Rothwell, W. D.; Shen, W. J. Am. Chem. Soc. 1985, 107, 1540. (2) Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 40. (3) Lunsford, J. H.; Tutunjian, P. N.; Chu, P.; Yeh, E. B.; Zalewski, D. J. J. Phys. Chem. 1989, 93, 2590. (4) Sheng, T. C.; Gay, I. D. J. Catal. 1994, 145, 10. (5) Hu, B.; Gay, I. D. Langmuir 1999, 15, 477. (6) Szeverenyi, N. M.; Sullivan, M. J.; Maciel, G. E. J. Magn. Reson. 1982, 47, 462. (7) Gay, I. D. J. Magn. Reson. 1984, 58, 413. (8) Geen, H.; Bodenhausen, G. J. Chem. Phys. 1992, 97, 2928.
Figure 1. Two-dimensional exchange spectrum of 1.4 µmol/ m2 of PMe3 adsorbed on SiO2-Al2O3 cracking catalyst: spectrum excited by 90° pulses; 20 ms mixing time; MAS at 1.90 kHz. Contours are drawn at 3.5, 5, 7, 11, 15, 22, 32, 46, 66, and 95% of highest point in spectrum.
This experiment results in a two-dimensional spectrum where diagonal peaks appear for all resonances, and crosspeaks (off-diagonal) occur between those peaks which are undergoing exchange. The strength of the exchange peaks grows as the mixing time is increased, but spin-lattice relaxation leads to a practical upper limit of some 10s to 100s of milliseconds, in the present systems. Figure 1 shows the result of such an experiment, for PMe3 adsorbed on SiO2-Al2O3 cracking catalyst at a coverage of 1.4 µmol/m2. For this spectrum the magnetization was induced by an initial 90° pulse, and a mixing time of 20 ms was used. As can be seen, cross-peaks between the peak of “physisorbed” phosphine at -58 ppm and PMe3H+ at -4 ppm are clearly visible. The presence of these peaks shows that an exchange process is occurring between the species which give rise to these peaks. As shown in ref 4, the peak at -58 ppm is itself the result of a rapid exchange between PMe3H+ and physisorbed PMe3 which resonates at -62 ppm. We thus see that PMe3H+ and physisorbed PMe3 are undergoing two simultaneous exchange processes on this surface: a portion of the PMe3H+ undergoes a fast exchange leading to the coverage-dependent line between -45 and -60 ppm;
10.1021/la990302i CCC: $18.00 © 1999 American Chemical Society Published on Web 08/05/1999
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Figure 2. Same sample as Figure 1: spectrum excited by crosspolarization with a contact time of 2 ms; 10 ms mixing time; lowest contour at 5%.
the remainder exchanges at a slower rate, leading to the cross-peaks in Figure 1. Figure 2 shows the same sample, with the spectrum excited by cross-polarization, using a contact time of 2 ms and a mixing time of 10 ms. As can be seen, the crosspeaks show a strong asymmetry in their intensities. This is an extreme example of the phenomenon recently reported by Caldarelli and Emsley9 and arises from nonequilibrium excitation of the exchanging species when using cross-polarization. For the present sample, there is no value of contact time which will produce the equilibrium intensity ratio observed in a fully relaxed Bloch decay spectrum. In Figure 2, the small diagonal feature at -34 ppm is a spinning sideband, that at -42 to -45 ppm is the signal from PMe3 bound to Lewis sites.4 This signal is relatively stronger, compared to the other peaks, in a cross-polarized spectrum. As can be seen, there is no evidence of exchange between Lewis-bound PMe3 and either of the other species. If a sample is prepared with lower phosphine coverage,4 the physisorbed peak is absent, and the Lewis peak can be observed at higher sensitivity, without interference from the tail of the physisorbed peak. On such a sample the spin-lattice relaxation times are longer than with high coverage samples, and it is possible to use mixing times of up to 200 ms in the exchange experiment. At this mixing time, we observe no evidence for exchange between protonated and Lewis-bound species. It is possible to determine the rate constant of the exchange process by observing the intensity of the crosspeaks as a function of mixing time. Since exchange is only observed between a single pair of species, it is more economical of time to do this via a one-dimensional variant of the experiment10 which produces one-dimensional spectra containing one diagonal and one cross-peak. The results of such an experiment are shown in Figure 3. These data can be fitted to theoretical expressions11 to extract the exchange rate constant. The dependence of cross and diagonal peak intensities on mixing time is a function of the exchange rate constant k, the relaxation rates R1P and R1H of the “physisorbed” and protonated species, and the mole fraction x of the physisorbed species. The curves in Figure 3 show the (9) Caldarelli, S.; Emsley, L. J. Magn. Reson. 1998, 130, 233. (10) Connor, C.; Naito, A.; Takegoshi, K.; McDowell, C. A. Chem. Phys. Lett. 1985, 113, 123. (11) Jeener, J.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546.
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Figure 3. Peak intensity (arbitrary units) vs mixing time in one-dimensional exchange experiment: 0, diagonal peak from physisorbed PMe3; 4, exchange peak (×10). Solid lines are fits to eqs 20-25 of ref 11.
Figure 4. Two-dimensional exchange spectrum of 0.48 µmol/ m2 of PPh3 adsorbed on SiO2-Al2O3 cracking catalyst: spectrum excited by 90° pulses, 150 ms mixing time.
results of an unconstrained fit of the data to eqs 20-25 of ref 11. This produces values of 760 s-1 for k, 50 s-1 for R1P, 12 s-1 for R1H, and 0.92 for x. The quality of the fit is rather insensitive to the value of R1H. These values are in good accord with an x value of 0.89 measured in a fully relaxed Bloch decay spectrum, and R1 values of 45 and 17 s-1 for “physisorbed” and protonated peaks, respectively, estimated from the initial slope of an inversion-recovery experiment. If the relaxation rates are constrained to the values of 45 and 17 s-1, and the fitting is repeated, varying only x and k, a slightly poorer fit results, with x again equal to 0.92 and k ) 825 s-1. A satisfactory fit cannot be obtained by constraining x to the value of 0.89 observed in the Bloch decay spectrum. An attempt to do so causes the iterative fitting to converge to an untenably large value of R1H. This probably indicates a small systematic error in our intensity measurements. In view of the above, we believe that 800 ( 100 s-1 is a reasonable estimate for the rate constant of the slower exchange process. (Following the convention of ref 11, this value is the sum of the forward and reverse rate constants for the exchange equilibrium.) Since the faster exchange process leads to a single coverage-dependent peak, its rate must be fast compared to the frequency differences3.5 kHz in our fieldsbetween
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the resonances of PMe3H+ and physisorbed PMe3. The line of the rapidly exchanging species has an unapodized line width of 235 Hz, while physisorbed PMe3 at high coverage has a line width of 100 Hz. If the difference is attributed to exchange broadening, standard exchange NMR theory12 allows us to estimate a rate constant of 7 × 104 Hz for the fast processes. It thus appears that on this surface, there are two populations of PMe3H+ that exchange with physisorbed species at rates differing by a factor of nearly 100. The results of ref 4 show that the slowly exchanging population accounts for about 60% of the total. Figure 4 shows the result of a two-dimensional exchange experiment for triphenylphosphine at a coverage of 0.48 µmol/m2 on the same catalyst. Prominent exchange peaks are seen between the physisorbed phosphine resonance at -5.5 ppm and the PPh3H+ resonance at 6.1 ppm. The (12) Freeman, R. A Handbook of Nuclear Magnetic Resonance; Addison-Wesley Longman: Harlow, U.K., 1997; pp 31-35.
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small diagonal peak at 45 ppm probably arises from contamination with triphenylphosphine oxide. Onedimensional experiments, using mixing times ranging from 5 to 800 ms gave a fit of similar quality to that of Figure 3, leading to a derived k value of 14 s-1. Thus our proposal5 of an exchange process in the triphenylphosphine system is confirmed. For this large molecule, the rate of exchange is appreciably slower than that for the slow exchange of PMe3. The above results show that NMR is an effective tool for the study of exchange among adsorbed species on acidic catalysts. Future variable-temperature studies will permit the determination of activation parameters for the exchange processes and lead to an improved understanding of the behavior of surface acids. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for support via a research grant. LA990302I
