1026
Langmuir 1989,5, 1026-1030
whereas after standing for 15 min, the contact angle had decreased to 101O. A bilayer film formed on a silicon wafer showed initial contact angles of 111' for water, 74O for methylene iodide, and 45O for n-hexadecane and an OTS layer thickness of 27 A, all of which are in accord with literature values for this well-studied s y ~ t e m . ~ J ~ Two possible mechanisms may be suggested for the bilayer formation. The first involves a reaction of the sulfoxide group with the trichlorosilyl group to form a siloxysulfonium salt, similar to the intermediate in the reaction of sulfoxides with trimethylsilyl iodide.31 The second and more likely mechanism, in view of the lack of stability of the bilayer, is reaction of the trichlorosilyl group with water adsorbed on the hygroscopic sulfoxide surface to yield an OTS layer similar to that formed on a gold ~urface.~ A bilayer film adsorbed on a silicon ATR crystal had contact angles identical with the sample prepared on wafer silicon, and the spectrum of the OTS layer (obtained by spectral subtraction of the oxidized 1/Si monolayer spectrum) had features indicative of a good-quality OTS monolayer: absorbances a t 2960 (weak, u,(CH,)), 2918 (very strong, (u,(CHz)), 2851 (strong, (u,(CHz)),and 1468 (weak, 6 (CH,)) cm-'. Dichroic ratios for this OTS second
layer (run 2c, Table I) suggest that the alkyl chains are tilted by -17O, in line with previous results from this lab'2J7 and e l ~ e w h e r e . ~ J ~
Conclusion The results presented here demonstrate the feasibility of employing a chemical reaction to introduce a polar acceptor group into a phenyl ring containing monolayer assembly. The techniques of ellipsometry, contact angle measurements, FTIR, and XPS were employed and gave a consistent picture showing the oxidation by HzO2/HOAc of an aromatic sulfide to a sulfoxide in a monolayer. Approximately 80% of the sulfide groups was oxidized by the reagent at room temperature. Further studies are required to demonstrate the generality of this reaction and to answer the question of whether the reagent can "penetrate" more deeply into monolayers than is the case for the surface-localized sulfide group in l/Si. This study demonstrates that useful chemical methods can be employed on a self-assembled film to modify a property of interest. Acknowledgment. We acknowledge the contribution of Jon Littman, of the Corporate Research Laboratories, Eastman Kodak Co., for preparing the gold substrates.
Oxidation of Trimethylphosphine in Zeolite-Y: A Solid-state NMR Study David J. Zalewski,? Po-jen Chu,? Pierre N. Tutunjian,i and Jack H. Lunsford*it Department of Chemistry, Texas A&M University, College Station, Texas 77843, and Shell Development Company, Houston, Texas 77001 Received January 19, 1989. I n Final Form: April 5, 1989 The oxidation of trimeth lphosphine (TMP) to trimethylphosphine oxide (TMPO) has been followed in Y-type zeolites by using 7P MAS-NMR spectroscopy. In a Na-Y zeolite, TMP coordinates weakly with Na+ ions to give a resonance at -59.5 ppm. This form of TMP is readily oxidized at 25 O C to TMPO, which may be coordinated to Na+ (48.6 ppm) or may be physisorbed (41.4 ppm). Similarly, TMP which is weakly bound to a Lewis acid site (-61.5 ppm) in a dealuminated zeolite may be easily oxidized to Lewis-bound TMPO (56 ppm). Liquidlike TMP (-67.1 to -67.9 ppm) is somewhat less easily oxidized, but it also forms physisorbed TMPO. By contrast, the protonated adduct of TMP (-1.0 to -2.8 ppm) is more difficult to oxidize and requires a temperature in excess of 100 "C. TMPO is a weaker Brransted base than TMP, although the protonated adduct (65,74 ppm) is formed when TMPO is adsorbed in a H-Y zeolite or when TMP is oxidized in a dealuminated zeolite, which is more strongly acidic.
Introduction The surface chemistry of organophosphorus compounds has been the subject of several recent spectroscopic studies which have provided information both on the reactions of the adsorbed molecules and on the value of these compounds as probes of oxide surfaces. Lin and Klabunde' have used infrared spectroscopy to determine the mode of adsorption and the reactions which occur when organophosphates, organophosphites, and organophosphines are adsorbed on the basic oxides, MgO and CaO. Another type of vibrational spectroscopy, inelastic electron tunneling spectroscopy,has been employed by Templeton and Weinberg2 to study the reactions of dimethyl methylphosphonate and other phosphonate esters on alumina. Texas A&M University.
* Shell Development Co.
Schoonheydt et al.3were the first to demonstrate that basic phosphines, such as trimethylphosphine (TMP), could be used to probe acid sites in zeolites. The protonated adduct of TMP has a characteristic infrared band at 2485 cm-l in zeolites which have Brransted acidity. Because 31Pis particularly suitable for solid-state NMR studies (100% natural abundance, I = 1/2), this technique has been effectively used to examine basic phosphines and phosphine oxides adsorbed on acidic catalysts. Rothwell et al.4 demonstrated that TMP interacts with a H-Y zeolite (1) Lin, S.-T.; Klabunde, K. J. Langmuir 1985,1, 600. (2) Templeton, M. K.; Weinberg, W. H. J.Am. Chem. SOC.1985,107, 97; 1985, 107, 774. (3) Schoonheydt, R. A.; Van Wouwe, D.; Leeman, H. Zeolites 1982,2, 109. (4) Rothwell, W. P.; Shen, W.; Lunsford, J. H. J.Am. Chem. SOC.1984, 106. 2452.
0743-7463/89/2405-1026$01.50/0
0 1989 American Chemical Society
Oxidation of Trimethylphosphine in Zeolite- Y
to form [(CH,),P-HI+, which exhibits a chemical shift of -2 ppm and JP-H of ca. 550 Hz, as well as a liquidlike form of T M P which has a resonance at -67 ppm. Removal of aluminum from the framework results in Lewis acid sites which also interact with TMP.5 The resonances of these species are at -31 to -58 ppm, but in some cases the spectra reflect rapid chemical exchange between Lewis-bound and liquidlike (CH3)3P.6 Bein et aL7 have extended the study in acidic zeolites by using dimethylphosphine and trimethylphosphite. Phosphine oxides also are effective probes of acid sites, as demonstrated by the work of Baltusis et al.s on amorphous silica-alumina and y-alumina. Trimethylphosphine oxide (TMPO), for example, adsorbed on silica-alumina exhibits NMR resonances at 40, 53, and 65 ppm which are attributed to physisorbed, Lewis-bound, and protonated TMPO, respectively. The TMPO has an advantage over T M P in that it is not subject to rapid chemical exchange at ambient temperature. The disadvantage is that J p - H coupling has not been observed with TMPO; therefore the assignment of the protonated adduct is not definitive. The integrated spectra of protonated T M P and TMPO provide a reasonably reliable determination of the Bransted site concentration provided the concentration is 133/uc (ref 6; uc is unit cell). As a result of our studies on partially dealuminated zeolites, it became apparent that the susceptibility of the T M P to oxidation was qualitatively very different, depending on the state of coordination of the phosphine.6 These observations are reported here for Na-Y, H-Y, and partially dealuminated zeolites. The results also provide additional insight into the state of TMPO in zeolites and the competition between TMP and TMPO for acid sites.
Experimental Section Zeolite Samples. All of the zeolite samples used in this study were prepared from NaNH4Y (Union Carbide LZ-Y62, %/A1 = 2.55) or Na-Y (LZ-Y52, Si/Al = 2.38). The Na+ cations were exchanged with NH4+cations by treatment with either (NH4),S04 or NH4N03. A typical ion exchange consisted of stirring 5 g of zeolite with approximately 15 equiv of NH4+in 500 mL of distilled, deionized water. The solution was maintained a t 80 "C for 2 h. After the sample was washed, the procedure was repeated an additional 2 times. The last exchange was allowed to equilibrate for 1 2 h before collecting the zeolite. The crystallinity of the samples was verified by X-ray diffraction analysis. The final sodium concentration was determined by inductively coupled plasma (ICP) analysis. The steam-dealuminated zeolite was prepared by heating the NaNH4-Y zeolite under flowing N2/H20 to 600 "C for 3 h. The sample was then cooled under steam and ion exchanged with NH4+ (1M NH4N03, 60 "C, 12 h) to remove residual Na+ cations. This material is designated as SDY. X-ray diffraction analysis indicated that the material was crystalline and contained 28 framework Al/uc. This sample contained 53 Al/uc; thus, the majority of the A1 atoms removed by the steaming process remained inside the zeolite cavities as extraframework Al. Phosphine Adsorption. The zeolites were dehydrated by slowly heating the samples under vacuum to a temperature of 400 "C. The temperature was ramped at a rate of 2 "C/min. T o ensure uniform degassing the temperature was held a t each 100 (5)Lunsford, J. H.; Rothwell, W. P.; Shen, W. J. J.A m . Chem. SOC. 1985,107, 1540. (6)Lunsford, J. H.; Tuntunjian, P. N.; Chu, P.; Eshan, B. Y.; Zalewski, D. J. J. Phys. Chem. 1989,93,2590. (7)Bein, Th.; Chase, D. B.; Farler, R. D.; Stucky, G. D. In New Deuelopments in Zeolite Science Technology; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Kodansha Ltd.: Tokyo, 1985;pp 311-318. (8)Baltusis, L.; Frye, J. S.; Maciel, G. E. J. Am. Chem. SOC.1987,109, 40; 1986, 108, 7119. (9)Here uc means unit cell of the zeolite; a normal H-Y zeolite would have ca. 54 protons/uc.
Langmuir, Vol. 5, No. 4, 1989 1027 "C interval for 1 h before the temperature was raised to the next plateau. The samples were maintained a t 400 "C for 2 h before cooling and adding the phosphine. Trimethylphosphine was adsorbed onto the zeolite from the vapor phase (20 Torr), and excess vapor was removed by evacuation from the system. Trimethylphosphine oxide was added directly to the dehydrated zeolite sample by mixing crystalline OPMe3 with the zeolite sample in an argon-filled drybox. The mixture was heated to 80 "C under a vacuum, which allowed the TMPO to diffuse inside the zeolite cavities and the excess TMPO to sublime away from the zeolite. NMR Experiments. The NMR results reported in this paper were obtained on a Bruker MSL-300 spectrometer. Approximately 0.2 g of sample was loaded into the NMR rotor in an argon-filled drybox. Unless otherwise noted, the spectra were collected by using magic angle spinning (MAS) a t 3-4 kHz. Both protoncoupled and proton-decoupled spectra were collected. Low-temperature experiments were carried out by using dry nitrogen as the drive gas. The temperature was controlled to f 7 "C. Quantitative spectra were obtained by using a 90" pulse-andacquire sequence. A standard consisting of (NH4),HP04mixed with NH4N03was used to calculate the total number of 31Pspins. By varying the time between successive pulses, we found that all of the samples had T1values less than 5 s. Chemical shifts are reported relative to 85% H3P04,with negative shifts reflecting greater shielding (i.e., higher external field). The estimated error in the reported chemical shifts is f0.3 ppm.
Results and Discussion Na-Y. The kinetic diameter of TMP is 5.5 A; therefore, the molecule cannot enter the sodalite units of Y-type zeolites, which have openings of only 2.2 A. Thus, T M P must reside inside the large cavities (diameter = 13 A). Dehydrated Na-Y zeolite absorbs three molecules of TMP per large cavity ( 2 4 / u ~ ) A . ~fully exchanged, dehydrated Na-Y zeolite has ca. 30 Na+ ions in the site I1 position, which is located in the six-membered rings but slightly in the large cavities (lc).'O This value corresponds to ca. 4 Na+ ions per large cavity. The 31PNMR spectrum of TMP physisorbed in Na-Y zeolite has a chemical shift value of -59.5 ppm with very weak spinning side bands.6 Neat TMP displays a chemical shift value of -62.5 ppm, and liquidlike TMP in zeolite-Y has a chemical shift value of -67 ppm. Depending on the solvent used, liquid TMP possesses a chemical shift value between -62 and -66 ppm." The downfield shift observed for (CH,),P in Na-Y indicated that the phosphorus is influenced by the presence of the Na+ ions. Further evidence for this interaction comes from the line widths of the observed resonance. The width at half-height of [ (CH3),P-Na]+ is 438 Hz, whereas the liquidlike T M P in a H-Y zeolite has a width at halfheight of 130 Hz. Part of the broadening in the Na-Y zeolite can be explained by the interaction of T M P with the sodium nuclear magnetic moment ( I = 3/2). The resonance at -59.5 ppm remains relatively sharp even when the spectrum is collected in the static mode, which provides evidence in support of the noncrowded environment existing inside the zeolite cages. At a loading level of three molecules per supercage, T M P is free to rotate around inside the cage structure of the zeolite. When oxygen was introduced into the Na-Y zeolite a t 25 "C, the TMP resonance at -59.5 ppm disappeared, and two new signals at 41.4 and 48.6 ppm appeared in the spectrum. Crystalline TMPO possesses a chemical shift value of 40.9 ppm. Treating dehydrated Na-Y with (10)Eulenberger, G.R.; Shoemaker, D. P.; Keil, J. G. J.Phys. Chem. 1967, 71, 1812. (11)Mavel, G. In Annual Reports on NMR Spectroscopy: NMR Studies of Phosphorlls Compounds; Mooney, E. F., Ed.; Academic Press: New York, 1973;Vol. 5B. (12)Turro, N. J. Pure Appl. Chem. 1986,58(9), 1219.
1028 Langmuir, Vol. 5, No. 4, 1989
Zalewski et al.
.
- 2;o a
I
I
I
I
1
1
1
1
I
-50
1
1
1
I
400
SHIFT (PPM) Figure 1. slP MAS-NMR spectra of TMPO in Na-Y zeolite. denotes spinning sidebands.
0
TMPO results in a spectrum (Figure 1)which is identical with that obtained through the oxidation of TMP. Therefore, the two resonances are assigned to an oxidized form of TMP, with the downfield resonance (48.6 ppm) assigned to the sodium complex, [(CH,),PO-Na]+, and the remaining resonance (41.4 ppm) assigned to physisorbed OPMe,. An analogous assignment has been made for TMP, where the [(CH3)3P-Na]+signal appears downfield from liquidlike (CH,),P. A spectrum obtained by using a 200-MHz spectrometer confirms that the two resonances result from chemical shift dependence rather than J coupling. NaH-Y. Previously, we reported the changes observed in the 31PMAS-NMR spectrum of TMP in a series of Y-zeolites as a function of proton concentration.6 As protons are incorporated into a Na-Y-zeolite, a new resonance at -2.8 ppm appears in the 31Pspectrum. This resonance was assigned to the trimethylphosphonium cation, [(CH,),P-H]+, on the basis of its JP+,coupling of 550 Hz and the observed signal enhancement in the cross-polarized spectrum. As the proton concentration increased, the amount of T M P that could be loaded into the zeolite also increased, to a maximum loading level of 4O/uc (5/lc), and the resonance shifted from -2.8 to -2.0 ppm. The increase in phosphine adsorption was attributed to the interaction between trimethylphosphine and the protons. A steady increase in the phosphonium cation concentration also was observed, until it reached a maximum value of 24/uc. Similar results were observed by Bein et al.7 Along with the increase in phosphonium ion concentration, there was a steady decrease in the signal assigned to [ (CH3)3P-Na]+. Since the internal surface area of the zeolite can only accommodate 24 T M P molecules per unit cell, the extra phosphine that is adsorbed must reside at the center of the large cavities. This position prevents any direct interaction with the cations. In Na,,H,,-Y zeolite, a resonance assigned to the sodium complex was observed at -61.5 ppm; however, in NaZ2H,-Y zeolite, this signal was absent. A large number of the Na+ cations are located in the supercage of the zeolite (site 11) and should be able to interact with the encapsulated TMP molecules,1° but steric hindrance must be preventing this interaction. TMP chemisorbed onto the walls of the zeolite cavity would prefer to coordinate with H+ rather than the Na+ cations. Figure 2 depicts the details of the oxidation process that occurs in H-Y zeolites. Trimethylphosphine was adsorbed onto a Na6H,,-Y zeolite. Integration of the 31PNMR spectrum indicates that the sample contained 32 TMP molecules per unit cell (4/lc). Before any oxidation oc-
I
,
,
,
I
,
,
,
,
I
1
-50
SHIFT (PPM) Figure 2. Oxidation of T M P encapsulated inside H-Y zeolite: (a) TMP in an H-Y zeolite before exposure to air; (b) 16 min after exposure to air; (c) 16 h after exposure to air. 0 denotes spinning sidebands.
curred, the 31PNMR spectra displayed two resonances at -2.0 and -67.1 ppm (Figure 2a). As noted previously, these are assigned to the protonated and liquidlike forms of TMP. The origin of the weak resonance at 29 ppm is unknown. Integration of the spectrum indicates that there are three [(CH,),P-H]+ and one liquidlike TMP per large cavity. As oxygen was leaked into the sample, both forms of trimethylphosphine became oxidized, though not to an equivalent extent. Sixteen minutes after loading the rotor into the NMR cavity, 25% of the phosphine had been converted to trimethylphosphine oxide (Figure 2b). Two resonances at 38.6 and 41.7 ppm, assigned to TMPO, can be seen to grow into the spectrum. After 16 h, all of the liquidlike form of TMP had become oxidized, but much of the adsorbed phosphine remained as the trimethylphosphonium cation (Figure 2c); 50% of the TMP in the zeolite had been converted to the oxide. Even though part of the oxidized product originated at a Brcansted site, it is believed that only the liquidlike form can react with oxygen. An equilibrium between the TMP at weaker Brcansted acid sites and the liquidlike species would explain the above results. With the somewhat stronger acid sites, the equilibrium will favor the protonated adduct, [(CH,),P-H]+, which is not oxidized at room temperature. In a separate experiment, a H-Y zeolite was exposed to 21 (CH3),P/uc. This results in the appearance of only one resonance (-3 ppm) in the 31PNMR spectrum. A t this loading level, all of the encapsulated TMP coordinates with the strong Brcansted acid sites, forming exclusively the trimethylphosphonium cation. Prolonged exposure to oxygen at 25 "C did not result in any trimethylphosphine oxide. Moreover, another H-Y zeolite was equilibrated with TMP, and the excess TMP was removed by degassing the sample at 25 OC. The sample was then exposed to 150
Langmuir, Vol. 5, No. 4, 1989 1029
Oxidation of Trimethylphosphine in Zeolite- Y Torr of oxygen at 100 "C for approximately 16 h. The protonated adduct remained the dominant species in the zeolite; only 39% of the TMP had been converted to the oxide. There was no evidence for a protonated form of TMPO. These results clearly show that [(CH,),P-H]+ is much more difficult to oxidize than the other forms of TMP in the zeolite. As in the Na-Y case, oxidation of the liquidlike TMP in the H-Y zeolite yields two forms of OPMe3, but the chemical shift values of these two species were 38.6 and 41.7 ppm. The resonance of [(CH3)3PO-H]+was not observed (see below) because the Brernsted acid sites were coordinated with TMP. The two observed resonances are assigned to physisorbed forms of TMPO. The downfield resonance (41.7 ppm) is almost identical with the shift observed in Na-Y and is assigned to TMPO physisorbed on the walls of the large cavities. The remaining resonance (38.6 ppm) is located upfield from crystalline OPMe3 This is analogous to the upfield shift observed for liquidlike T M P in H-Y zeolite. This species is therefore assigned to TMPO located at the center of the large cavities. It is the closely packed phosphine environment that causes the upfield shift. The oxide resonance at 38.6 ppm only occurred when the corresponding resonance at ca. -67 ppm was observed in the unoxidized zeolite. The 31PNMR spectrum of TMPO adsorbed onto a H-Y zeolite displayed two resonances at 65 and 74 ppm. These are assigned to [ (CH3)3PO-H]+complexes. When TMP was subsequently adsorbed onto the zeolite, the oxide was displaced from the proton sites by the more basic phosphine. As this happened a resonance assigned to TMPO in the center of the supercage appeared at 37.4 ppm. Protonated (-3 ppm) and liquidlike (-67 ppm) TMP signals also were present in the spectrum (not shown). Variations in the position and side-band intensity of the trimethylphosphonium cation signal suggest that there are at least two forms of [(CH3)3P-H]+.4One form is bound to the surface and is immobilized, whereas the other displays considerable motion on the NMR time scale. Thus, the TMPO spectrum complements the TMP spectrum in that separate signals were observed for different Brernsted acid sites. In a related study using solid-state 15N NMR spectroscopy, two resonances were observed for NH4+in H-Y zeolite.', The chemical shift range of [(CH,),P-H]+ covers only ca. 1.8 ppm, whereas that of [(CH3)3PO-H]+ is 9 ppm. This suggests that either (a) the resolution is not sufficient to observe separate signals for the trimethylphosphonium cation or (b) TMP is more mobile than OPMe,, and the observed resonance reflects chemical exchange between two or more sites. An interesting result was obtained in a Na3,Hzo-Y zeolite, which contained 34 molecules of TMP per unit cell. The initial 31PNMR spectrum exhibited resonances at -2.8, -61.5, and -67.9 ppm, as shown in Figure 3a. These are assigned to [ (CH3)3P-H]+,[(CH,),P-Na]+, and liquidlike (CH3)3P,respectively. After brief exposure to air, the signal belonging to the sodium complex was greatly decreased, but the signals assigned to the protonated and liquidlike forms of TMP remain unchanged (Figure 3b). After prolonged exposure to air, the physisorbed form also becomes oxidized (Figure 3c). The oxide resonances appeared at 50.6 and 42.6 ppm, which is consistent with the formation of a sodium complex, [ (CH3),PO-Na]+, and a physisorbed form of TMPO. These results suggest that T M P associated with sodium is oxidized faster than the liquidlike TMP. (13) Earl,W. L.; Fritz, P. 0.; Gibson, A. A. V.; Lunsford, J. H. J. Phys. Chem. 1987,91, 2091.
a
- 2;8
d 11
b
-61.5
i
l-67.9
dL I\
C
..
1
1
1
1
1
0'
1
1
1
1
1
1
1
-50
SHIFT (PPM) Figure 3. Oxidation of TMP encapsulated in Na,,H,,-Y zeolite: (a) freshly prepared sample; (b) 15 min after exposure to air; (c) 24 h after exposure to air. 0 denotes spinning sidebands.
SDY. In the dealuminated zeolites, one has to contend with the presence of extraframework aluminum, which gives rise to Lewis acidity. Moreover, the Bransted acid strength, as determined by catalytic activity, is much greater in these materials than in the normal H-Y zeoli t e ~ .Previously, ~~ we reported the 31PNMR spectrum of a dealuminated zeolite which had been prepared by steaming a sample for 3 h.6 The appearance of the spectrum was dependent on the loading level of TMP. When the zeolite contained an amount of phosphine approximately equal to the framework aluminum content (28 u.c.), only one resonance was observed in the spectrum at an unusual shift of -19.4 ppm, which lies between that of a protonated and a Lewis-bound complex. This resonance was attributed to the rapid chemical exchange of T M P located at different sites. The assignment was verified by low-temperature studies, which froze out the exchange process. When the SDY zeolite was fully loaded with TMP, the rapid exchange was slowed down, and two resonances were observed in the spectrum at -2.3 and -61.5 ppm. These are similar to those observed in the low-temperaturestudy, and they are assigned to TMP located at the Brernsted and Lewis acid sites, respectively. The Lewis acid site probably is on the surface of aluminum oxyhydroxide (7-boehmite) located in the large cavities.15 Although the chemical exchange was slowed down, it was not completely stopped, as indicated by the fact that the JP-H coupling in the (14) DeCanio, S. J.; Sohn, J. R.; Fritz, P. 0.;Lunsford, J. H. J. Catal. 1986, 101, 132. Sohn, J. R.; DeCanio, S. J.; Fritz, P. 0.; Lunsford, J. H. J. Phys. Chem. 1986,90, 4847. (15) Shannon, R. D.; Gardner, K. H.; Staly, R. H.; Bergeret, G.; Gallezot, P.; Aurox, A. J. Phys. Chem. 1985,89, 4778.
Zalewski et al.
1030 Langmuir, Vol. 5, No. 4, 1989 Table I. Chemical States and slP Chemical Shifts for Trimethylphosphine and Trimethylphosphine Oxide in Zeolite-Y chemical state
chemical shiftu
[ (CH3)3P-Hl+ [(CH3)3P-Nalt (CH3)3P-A1O(OH) (CH3),P, liquidlike [(CHa)sPO-HI+ [(CH&PO-Na]+ (CH,),PO-AlO(OH) (CH3)3P0,physisorbed (CH&PO, solid like
-1.0 to -2.8 -59.5 to -61.5 -61.f~~ -67.1 to -67.9 65, 74 48.6 to 50.6 56 41.4 to 42.6 37.4 to 38.6
"In ppm, relative to 85% H3P04. bPhosphine may be undergoing chemical exchange.
steamed zeolite was not well resolved. JP-H coupling will only be observed when the chemical exchange and spin diffusion are slower than about 30 P S . ~These conditions apparently were not met in the steamed samples. Moreover, the resonance at -61.5 ppm may actually result from the exchange between Lewis-bound and liquidlike TMP, as no resonance was observed at -67 ppm even though excess TMP was present. In addition to the rapid motion seen in these samples, the Lewis-bound TMP became oxidized much more readily than the liquidlike TMP in the normal H-Y zeolites. As oxidation occurred, the resonance at -62 ppm disappeared, and resonances at 65, 56, and 42 ppm appeared in the spectrum (Figure 3a, ref 6). These are assigned to TMPO located on Brernsted acid, Lewis acid, and physisorbed sites. In addition to these species, a resonance was observed for the trimethylphosphonium cation. Unlike the H-Y case, some of the protons in dealuminated zeolites are very strongly acidic; thus, the distinction between the basicity of TMP and TMPO is not a dominant factor. Therefore, the protonated forms of TMP and TMPO coexist in the dealuminated zeolite. TMPO coordinates with Lewis acids through an electron pair on the phosphoryl oxygen. An X-ray crystallographic study of (CH3),PO-SbC15 shows that the P-0-Sb bond angle is nonlinear (1390).16 On the other hand, PMe, is (16) Brandon, C.; Lindquist, A. Act. Chem. Scand. 1961, 15, 167.
bonded through the electron pair on the phosphorus and probably orients itself perpendicular to the surface. Another difference between TMP and TMPO is the binding energy. (CH3)3P0.A1(CH3)3 and (CH3)3P0.HOC6H5have heats of formation of 32 and 8 kcal/mol, respectively."J8 The corresponding (CH3)3Pderivatives have values of 21 and 32 kcal/mol. These values suggest that TMPO would preferentially bind to Lewis acid sites, while TMP prefers Bronsted acid sites.
Conclusions A number of different chemical states of trimethylphosphine and trimethylphosphine oxide have been identified by solid-state NMR in Y-type zeolites, and these are summarized in Table I. Even weakly coordinating cations such as Na+ introduce chemical shifts in TMP and TMPO which reflect deshielding of the phosphorus atom. The ease of oxidation of T M P depends on the state of coordination according to the following sequence: Lewis bound N Na+-coordinated > liquidlike Iprotonated. Presumably the protonated adduct [ (CH3),P-H]+ is not susceptible to attack by O2according to the reactiondg P:
+
02
-
P-0-0
- \7 P
P=O
+
0
0
The resulting TMPO does not compete favorably with TMP for weak Brernsted acid sites, but in the more strongly acidic dealuminated zeolites both types of protonated adducts are formed.
Acknowledgment. We gratefully acknowledge partial support of this research by the Reagents of Texas A&M University through the AUF-sponsored Materials Science and Engineering Program. Funds also were provided by the U.S. Army Research Office. Registry No. TMP, 594-09-2; TMPO, 676-96-0. (17) Nykerk, K. M.; Eyman,D. P. Inorg. Nucl. Chem. Lett. 1968, 4, 253. (18) Schindler, F.; Schmidbauer, H. Chem. Ber. 1968,101, 1656. (19) Hudson, R. F. Structure and Mechanism in Organo-Phosphom Chemistry; Academic Press: New York, 1965; pp 291-293.
