Langmuir 1999, 15, 2761-2765
2761
Characterization of [(CH3)3P-H]+ Complexes in Normal H-Y, Dealuminated H-Y, and H-ZSM-5 Zeolites Using 31P Solid-State NMR Spectroscopy Baiyi Zhao, Hongjun Pan, and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received September 4, 1998. In Final Form: January 13, 1999 Trimethylphosphine reacts with protons in acidic zeolites to form [(CH3)3P-H]+ complexes within the cavities and channels. The mobility of the protonated adduct strongly depends on the available space, which is a function of the amount of (CH3)3P in the zeolite and the size of the guest molecule relative to the cavity or channel dimensions. In an H-Y zeolite containing one molecule per large cavity, the motion is sufficiently rapid to average out a large P-H dipolar interaction, such that even in a static NMR experiment the lines due to JP-H coupling are well resolved. The motional rate is greater than 100 kHz. By contrast, the geometric constraints imposed by the smaller channels in ZSM-5, which are comparable in size to trimethylphosphine, severely restrict the motion. The chemical shift and J coupling values were determined for H-ZSM-5 and dealuminated H-Y zeolites, which are known to be strongly acidic, and for a normal H-Y zeolite, which is less acidic. The 31P chemical shift values were the same within experimental error, but a smaller J coupling in H-ZSM-5 is opposite from what one might expect on the basis of the extent of proton donation. The latter observation suggests that other factors, such as the radius of curvature of the cavities and channels, may play a role in acid-base interactions.
Introduction Trimethylphosphine is a basic molecule (pKb ) 5.35 in aqueous media) that has been used to probe both Bro¨nsted and Lewis acidity in solid catalysts.1-10 Phosphorus-31 NMR is particularly useful in following the state of the molecule during its interaction with acid centers. When Bro¨nsted acidity is present, a protonated adduct, [(CH3)3PH]+, is formed, and this is characterized by a chemical shift of approximately -4 ppm and a JP-H coupling of approximately 500 Hz.1 Coordination with Lewis acids results in more negative chemical shifts (typically -40 to -50 ppm), and if aluminum is present, the JP-Al coupling may be resolved. From an analysis of the spinning sidebands, the dipolar coupling constant has been determined and the P-H or P-Al bond distances have been established.11,12 The catalysts that have been investigated include normal and dealuminated H-Y zeolites, chlorided alumina, silica-alumina, and sulfated zirconia. A review of this work, which includes a preliminary NMR study of (CH3)3P in beta (β), mordenite, and ZSM-5 zeolites, has been published.13 (1) Rothwell, W. P.; Shen, W.; Lunsford, J. H. J. Am. Chem. Soc. 1984, 106, 2452. Lunsford, J. H.; Rothwell, W. P.; Shen, W. J. Am. Chem. Soc. 1985, 107, 1540. (2) Lunsford, J. H.; Tutunjian, P. N.; Chu, P. J.; Yeh, E. B.; Zalewski, D. J. J. Phys. Chem. 1989, 93 (3), 2590. (3) Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. Soc. 1987, 109, 40. (4) Bendada, A.; DeRose, E. F.; Fripiat, J. J. J. Phys. Chem. 1994, 98, 3838. (5) Coster, D. J.; Bendada, A.; Chen, F. R.; Fripiat, J. J. J. Catal. 1993, 140, 497. (6) Sang, H.; Chu, H. Y.; Lunsford, J. H. Catal. Lett. 1994, 26, 235. (7) Lunsford, J. H.; Sang, H.; Campbell, S. M.; Liang, C.-H.; Anthony, R. G. Catal. Lett. 1994, 27, 305. (8) Riemer, T.; Kno¨zinger, H. J. Phys. Chem. 1996, 100, 6739. (9) Sheng, T.-C.; Gay, I. D. J. Catal. 1994, 145, 10. (10) Sato, S.; Toita, M.; Sodesawa, T.; Nozaki, F. Appl. Catal. 1990, 62, 73. (11) Chu, P.-J.; Carvajal, R. R.; Lunsford, J. H. Chem. Phys. Lett. 1990, 175, 407. (12) Chu, P.-J.; Lunsford, J. H.; Zalewski, D. L. J. Magn. Reson. 1990, 87, 68. (13) Lunsford, J. H. Top. Catal. 1997, 4, 91.
In the study described here, NMR spectra of protonated and unprotonated trimethylphosphine in an H-Y zeolite have been obtained over a range of temperatures in both the static and MAS modes. Information is derived on the dynamics of the molecular species, the different sites in the zeolite, and the chemical shift anisotropy. These results, in part, are compared with those obtained in dealuminated Y and H-ZSM-5. Zeolite Y has a cavity dimension of 13 Å, which is considerably larger than the 5.5 Å diameter of the (CH3)3P molecule. By contrast, it is expected that molecular motion for this molecule would be much more restricted in the channels and channel intersections of ZSM-5, for which the channels are eliptical and have dimensions of approximately 5.2 Å × 5.5 Å. The channel intersections form cavities that are somewhat larger than the channels themselves. Even though the channel size and the molecular size are comparable, neither is static and the molecule is able to enter the zeolite. Chemical shifts and dipolar coupling parameters also were obtained for [(CH3)3P-H]+ in the three zeolites. It was of interest to determine whether these parameters could be related to the acid strength and hence the catalytic properties of the zeolites. In general, the Bro¨nsted acid strength of solids is an elusive property that is only qualitatively defined. The 13C chemical shifts of mesityl oxide derived from acetone, as reported by Haw and coworkers,8 appear to be related to acid strength, and other probe molecules may show similar effects. In principle, the JP-H coupling could be an even better indicator of acid strength. Experimental Section Zeolite Samples. The zeolites used in this study were normal H-Y (Linde LZ-Y62, Si/Al ) 2.5), dealuminated H-Y (Linde Y-84, Si/Al ) 3.4), and H-ZSM-5 (PQ Corp. CBV-5020, Si/Al ) 26). The zeolites were treated by slowly heating 1 g of the material under vacuum to 400 °C and then maintaining this temperature for 2 h. Trimethylphosphine (Strem) was adsorbed at 25 °C from the vapor phase (20 Torr) for a period of 2 h. Excess (CH3)3P was removed by heating some of the samples under vacuum for 1 h
10.1021/la981170g CCC: $18.00 © 1999 American Chemical Society Published on Web 03/19/1999
2762 Langmuir, Vol. 15, No. 8, 1999
Figure 1. 31P NMR spectra of (CH3)3P in H-Y(300) at 25 °C: (a) MAS proton-decoupled; (b) MAS proton-coupled; (c) static proton-decoupled; (d) static proton-coupled. at different temperatures (100, 200, or 300 °C). The corresponding samples are designated H-Y(300) and so forth. After addition of (CH3)3P and further treatment, if appropriate, the samples were rapidly transferred to an NMR rotor in a glovebox. NMR Experiments. The 13P NMR results were obtained with a Bruker MSL-300 spectrometer equipped with a double-airbearing drive probe. Phosphorus-31 resonates at 121.5 MHZ. Chemical shifts are reported relative to 85% H3PO4. For some of the samples, Zr(HPO4)‚H2O was included as an internal standard. The 90° pulse method was used in conjunction with magic-angle spinning and high-power proton decoupling. The pulse time was 5 µs. A spinning rate of 3.5 kHz was achieved by using dry nitrogen as the drive gas. Sample temperatures were maintained with a control unit to a precision of approximately (1 °C.
Results and Discussion State of Trimethylphosphine in Zeolites. The proton-decoupled and coupled MAS spectra of (CH3)3P in H-Y(300) at room temperature are shown in spectra a and b of Figure 1, respectively. This sample contained about one (CH3)3P molecule per large cavity. The protondecoupled spectrum displays a strong resonance at -4.5 ppm which has been well characterized and is assigned to a [(CH3)3P-H]+ complex arising from interaction of (CH3)3P with Bro¨nsted acid sites in the zeolite.1 As will be shown below, the spectrum may be resolved into two components, which indicates the presence of two different types of protonic sites. The corresponding proton-coupled spectrum (Figure 1b) exhibits a doublet centered at - 4.5 ppm, with a JP-H coupling of 520 Hz. The doublet results from scalar coupling between protons derived from the zeolite and phosphorus in the (CH3)3P molecules. The value of 520 Hz indicates that there is nearly complete transfer of the proton to (CH3)3P. A value of approximately 515 Hz was observed for the [(CH3)3P-H]+ complex in an aqueous HCl solution.14 Since JP-H splitting appears only when both chemical exchange and spin diffusion rates are much
Zhao et al.
slower than 1/JP-H (i.e., ∼2 ms), the observable J coupling implies that the Bro¨nsted proton is not undergoing rapid exchange between the (CH3)3P molecules and the framework oxygen atoms of the zeolite. It also implies that the spin diffusion rate between the proton on the phosphorus atom and the hydrogens on the methyl groups, as well as the translational jump rate of the (CH3)3P base between Bro¨nsted acid sites, is below the J coupling limit. The absence of intense dipolar-derived spinning sidebands11 in the MAS spectrum establishes that the [(CH3)3P-H]+ complex is involved in rapid molecular reorientation. The proton-decoupled spectrum without MAS is shown in Figure 1c. The observed line width of approximately 380 Hz includes contributions from both chemical shift anisotropy (CSA) and isotropic chemical shift dispersion arising from slightly different local environments. Compared with a static CSA of 6.0 kHz (see below), the line width of 380 Hz further indicates that the complex is undergoing rapid molecular reorientation. It is significant that even without magic-angle spinning a well resolved J coupling can still be observed (Figure 1d). Resolution of the doublet is possible because of the rapid motion of the complex. In particular, it is necessary to average not only the CSA but also the much larger dipolar interaction between the H and the P nuclei (34 kHz).11 Thus, the observation of a resolved doublet sets a lower limit for the motional rate of about 100 kHz. On the basis of relaxation rate measurements, Fripiat and co-workers4 concluded that the reorientation frequency of the PH+ vector for [(CH3)3P-H]+ in a dealuminated Y zeolite at 0 °C was 4 MHz. One can speculate about the type of motion that might result in the required averaging of the dipolar interaction. Clearly, rotation of the complex about its C3 axis would have no effect. Although a large-amplitude libration could account for the averaging of the dipolar coupling, it seems to be more reasonable, in view of the absence of geometric constraints, that the [(CH3)3P-H]+ complex is undergoing rapid isotropic tumbling. However, due to the positive charge on the [(CH3)3P-H]+ complex, the tumbling motion must be localized in the region of the anionic lattice site. The J coupling of 520 Hz indicates that the phosphorus is characterized by sp3 hybridization.15 Since the electronegativities of the atoms are similar, the charge is expected to be uniformly distributed over the cation, which further supports the concept of tumbling motion in the zeolite. As a third alternative, the motion of the cation may result from site exchange in a β-cage. In doing so, the intact complex cation would move rapidly around the wall of the cavity. Because of the effect of temperature on molecular motion, the 31P proton-coupled MAS NMR spectra changed dramatically as the temperature of the zeolite was decreased, as shown in Figure 2. At -45 °C (spectrum b) spinning sidebands begin to appear; hence, the motionally averaged dipolar coupling must have a value similar to the magic-angle spinning speed (∼3.5 kHz) at this temperature. At even lower temperatures, viz., -80 and -100 °C, spectra d and e were obtained. The doublet due to J coupling becomes better resolved compared to those of the spectra at temperatures between - 20 and -65 °C. The two lines of the doublet are of unequal amplitude, both in the central component and in the spinning sidebands, because of the dipolar interaction.11,12 Surprisingly, the two lines of the doublet in spectrum b of Figure (14) Silver, B.; Luz, Z. J. Am. Chem. Soc. 1961, 83, 786. (15) Gorenstein, D. G. a Phosphorus-31 NMR: Principles and Applications; Academic Press: New York, 1984; Chapter 2.
Characterization of [(CH3)3P-H]+ Complexes
Langmuir, Vol. 15, No. 8, 1999 2763
Figure 3. Proton-coupled 31P MAS NMR spectra of (CH3)3P in H-Y zeolite recorded at 25 °C: (a) fully loaded; (b) after desorption for 1 h at 100 °C; (c) after desorption for 1 h at 200 °C.
Figure 2. Proton-coupled 31P MAS NMR spectra of (CH3)3P in H-Y(300): spectra recorded with the sample at (a) -20 °C, (b) -45 °C, (c) -65 °C, (d) -80 °C, and (e) -100 °C.
1 also are of unequal amplitude, even though the motion of the protonated adduct was sufficient to remove the spinning sidebands. The motion, however, must not be adequate to average out completely the dipolar interaction. As shown previously,1 the extent of filling of the large cavities in the zeolite by trimethylphosphine also influences the motion of the molecules. The proton-coupled, room temperature 31P NMR spectra acquired for samples that had been degassed at progressively higher temperatures are depicted in Figure 3. The fully loaded sample (spectrum a) contained approximately five (CH3)3P molecules per large cavity, which corresponds to the maximum amount allowed in the 13 Å cavity.2 In addition to the J coupled doublet, there is an intense resonance at -67 ppm that results from physisorbed (CH3)3P. Of the five molecules that reside in each large cavity, on average, two-thirds are protonated and one-third are not.2 The fact that the chemical shift and the J coupling remain largely unchanged, independent of loading, confirms that rapid proton or site exchange does not occur between the two molecular forms. Removal of the weakly bound (CH3)3P at 100 °C greatly diminished the amplitude of the resonance at -67 ppm (spectrum b), and the broader lines of the protonated adduct again reflect the increase in motion that is made possible by the free space in the cavities. Progressive thermal treatment to remove more (CH3)3P resulted in
the highly mobile form of the adduct that resulted in spectrum b of Figure 1. Following partial removal of (CH3)3P at 200 °C, the decoupled spectrum at 25 °C revealed a high-field shoulder (Figure 4, spectrum a) which may be deconvoluted into two partially resolved components. For the dealuminated H-Y sample, after removal of more weakly bound (CH3)3P at 300 °C, the better resolved spectrum b clearly shows the presence of two components. From spectrum c, which was acquired with the dealuminated Y sample at 50 °C, the presence of [(CH3)3P-H]+ in two different environments is more evident. These results are consistent with infrared1,16 and proton NMR17 evidence for different protonic sites in these zeolites. Indeed, the infrared spectrum of a normal H-Y zeolite is characterized by two O-H bands that are assigned to protons at two different sites. The infrared spectrum of the dealuminated zeolite is considerably more complex in that at least four bands may be related to acidic hydroxyls.16 Thus, there does not appear to be a simple correspondence between the NMR results shown in spectra b and c of Figure 4 and published IR results for the dealuminated Y zeolite. Another distinction between types of protonated adducts can be made if one considers the static, proton-decoupled spectrum of the fully loaded normal Y zeolite sample as shown in Figure 5. The solid line in the figure is the experimental 31P NMR spectrum, which can be deconvoluted into three components. Two types of [(CH3)3PH]+ complexes are characterized by components I and II in the deconvoluted spectrum. Component I reflects axial symmetry with σ| ) 47 ( 1.5 ppm, σ⊥ ) -27 ( 1.5 ppm, (16) Lonyi, F.; Lunsford, J. H. J. Catal. 1992, 136, 566. (17) Pfeifer, H.; Freude, D.; Hunger, M. Zeolites 1985, 5, 274.
2764 Langmuir, Vol. 15, No. 8, 1999
Zhao et al.
Figure 6. Proton-coupled 31P MAS NMR spectrum at 25 °C of (CH3)3P in H-ZSM-5(300).
Figure 4. Proton-decoupled 31P MAS NMR spectra of the [(CH3)3P-H]+ complex: (a) in H-Y(200), recorded at 25 °C; (b) in dealuminated H-Y(300), recorded at 25 °C; (c) in dealuminated H-Y(300), recorded at 50 °C.
Figure 5. Static, proton-decoupled 31P NMR spectrum at 25 °C of fully loaded (CH3)3P in H-Y zeolite.
and an anisotropy δ ) 49.3 ppm (6 kHz).18 The motion of this type of [(CH3)3P-H]+ is partially restricted in the zeolite. The second type of protonated adduct has more available free volume; therefore, the molecule can undergo overall molecular reorientation. Such motion averages out the CSA components, giving a symmetric Lorentzian line pattern. The amplitude of component II relative to that of component I is too large to be attributed to a single species having orthorhombic symmetry. Within experimental error, the chemical shift of the isotropic component (18) δ ) σ| - σiso, σiso ) 1/3[σ| + 2σ⊥]. Maricq, M. M.; Waugh, J. S. J. Chem. Phys. 1979, 70, 3300.
is the same as the trace of the chemical shift tensor for the anisotropic component. The spectrum of physisorbed (CH3)3P also reflects an axially symmetric tensor pattern (component III) which is consistent with the C3 symmetry of the molecule. Overall molecular reorientation does not occur on the NMR time scale. The residual asymmetry component, however, is small and can be effectively removed by magic-angle spinning. It is surprising that the physisorbed (CH3)3P is more restricted in motion than one type of the protonated adduct. In H-ZSM-5 the motion of the [(CH3)3P-H]+ complex is much more restricted, even for samples that had been treated at 300 °C to remove part of the trimethylphosphine. The 31P MAS NMR spectrum of the H-ZSM-5(300) sample depicted in Figure 6 should be compared with that of the H-Y(300) sample (Figure 1, spectrum b). Clearly, the motion in the H-Y zeolite which eliminates the spinning sidebands does not occur in the H-ZSM-5 zeolite. The more restricted motion in the latter is consistent with the fact that the size of the complex is comparable to the dimensions of the channels and channel intersections, as noted above. With decreasing temperature, no significant change in the spectrum was observed, in contrast to the effect observed in Figure 2 for a Y-type zeolite. The static, decoupled spectrum (not shown) of [(CH3)3P-H]+ in H-ZSM-5 was characterized by a symmetric line having a width at half-height of 13 ppm, which is similar to that observed for component II in the fully loaded H-Y sample (Figure 5) but much larger than that observed in the partially loaded H-Y sample (Figure 1, spectrum c). Thus, the motion in H-ZSM-5 partially averages out the CSA but not the much larger dipolar coupling. Extraframework aluminum is probably present in the H-ZSM-5 zeolite, and one might have expected a resonance in Figure 6 at approximately -45 ppm due to Lewis-bound trimethylphosphine.4,12 In contrast to the protonated adduct, such complexes are relatively unstable and may be decomposed at moderate temperatures (e.g., 100 °C).6 Thus, treatment of the zeolite at 300 °C would have resulted in removal of (CH3)3 P from the Lewis acid sites. Comparison of Chemical Shift and J Coupling Values. In an attempt to evaluate the potential for using 31P chemical shift and J P-H values to probe acid strength in zeolites, these parameters were compared for the three materials reported here. There is considerable evidence, based on the heats of NH3 adsorption and catalytic activity, that the acid strength in H-ZSM-5 is much greater than that in a normal H-Y zeolite.16,19,20 For example, we have (19) Auroux, A. Top. Catal. 1997, 4, 71.
Characterization of [(CH3)3P-H]+ Complexes
Langmuir, Vol. 15, No. 8, 1999 2765
Table 1. Chemical Shift Values and JP-H Coupling Constants for the [(CH3)3P-H]+ Adduct in Zeolites sample
chemical shift (ppm)
J coupling (Hz)
H-ZSM-5 dealuminated H-Y H-Y
-3.5 -4.9 -4.5
480 520 520
shown that the intrinsic rate constant for n-hexane cracking over H-ZSM-5 is more than a factor of 104 greater than that over a normal H-Y zeolite.20 The dealuminated H-Y zeolite is more problematic because it contains at least two sets of acidic protons, and the ratio of the two depends on the preparation of the sample. In all cases, however, the strongly acidic protons are a minority species and may constitute only about 20% of the total.16,21 Therefore, in evaluating heat of adsorption or spectroscopic results, one must consider whether a probe is specific for a particular type of acidity or whether one is measuring the average effect for all acidity. There are further complications in interpreting heat of adsorption results for NH3, since extraframework aluminum forms Lewis acid sites in dealuminated zeolites, and these are known to adsorb NH3 strongly. The distinction between Bro¨nsted and Lewis acid sites is not a problem when interpreting the NMR spectra of trimethylphosphine. The chemical shift and J coupling values for [(CH3)3PH]+ complexes in zeolites H-Y, dealuminated H-Y, and H-ZSM-5 are listed in Table 1. All of the samples had been heated under vacuum at 300 °C to remove the more weakly bound molecules. The chemical shift data, obtained using Zr(HPO4)2‚H2O as an internal standard, have an estimated error of (0.5 ppm. Although the chemical shift for the protonated adduct in H-ZSM-5 appears to be less than the values observed in the other two zeolites, the difference is only slightly larger than the experimental error and therefore may not be significant. In these experiments, no attempt was made to deconvolute the decoupled spectra into individual components. The error in the J coupling values is estimated to be (10 Hz; hence, the value for H-ZSM-5 is distinctly less than the values for H-Y and dealuminated H-Y. A direct comparison of the spectra confirms this difference. On the basis of the concept that the stronger acidity in the (20) Kotrel, S.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1999, 103, 818. (21) Fritz, P. O.; Lunsford, J. H. J. Catal. 1989, 118, 85.
H-ZSM-5 zeolite would result in more complete protonation of the base, it is difficult to understand why the JP-H coupling and acid strength are inversely related, unless the two factors are not correlated with each other. Instead of reflecting acid strength, the magnitude of the JP-H coupling may be directly associated with the conjugate acid-conjugate base interaction, which, in turn, is influenced by the radius of curvature of the cavities and channels. That is, in the Y-type zeolites, including both normal Y and dealuminated Y, the distance between the [(CH3)3P-H+] cation and the negatively charged (Si-OAl) moiety in the framework could be smaller than that in ZSM-5. A larger separation between the resulting cation and anion would make the proton transfer to (CH3)3P less energetically favorable. These charge separation effects may be particularly important in determining the extent of proton transfer, since zeolites have a low dielectric constant ( ≈ 1.6). Nevertheless, in all cases the proton transfer is extensive, and, as a consequence, the difference in J couplings is less than 10%. Conclusions The motion of [(CH3)3P-H]+ complexes in zeolites, which is clearly evident in their 31P NMR spectra, is strongly affected by the size of the cavities and/or channels, the loading of the material with trimethylphosphine, and the temperature of the sample. At one extreme, the motion of the protonated adduct at 25 °C in an H-Y zeolite with about one molecule per large cavity was adequate to nearly average out the chemical shift anisotropy and, more significantly, the P-H dipolar interaction. By contrast, under similar conditions in an H-ZSM-5 zeolite, the motion was much more restricted as a result of the similar size of the channels and the guest molecule. Well-resolved JP-H couplings in the range 480-520 Hz indicate that there is nearly complete transfer of the proton from the zeolite lattice to (CH3)3P. The variation in the J coupling magnitude may reflect the distance between the protonated adduct and the negatively charged zeolite framework. At the level of accuracy to which these experiments were carried out, the corresponding chemical shifts appear to be independent of the acid strength of the zeolites. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-9520806. LA981170G
