Langmuir 1994,10, 1528-1531
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13C and 15N Cross-Polarization Magic Angle Spinning NMR Spectra of 15N-Enriched2-Phenethylamine Adsorbed on an Activated Clay Shelton Bank,**+Bin Yan,+John C. Edwards,i and Gabriel Ofori-Okait Department of Chemistry, State University of New York at Albany, Albany, New York 12222, and Texaco, Znc., P.O. Box 509, Beacon, New York 12508 Received November 19, 1993. In Final Form: February 15, 1994" The cross-polarization magic angle spinning (CPMAS) NMR spectra of I5N-enriched 2-phenethylamine as the ammonium salt and adsorbed on an aluminum-exchanged clay are compared with spectra for natural abundance 2-phenethylamine. An important gain in resolution is observed in the /3 carbon chemical shift region of the CPMAS of the 15N-enriched material relative to either the natural abundance material or the W-enriched amine. The CPMAS l5N NMR spectra of '5N-enriched 2-phenethylamine adsorbed on the aluminum-exchanged clay reveal the presence of multiple Bronsted sites for adsorption.
Introduction Clay catalysts have importance in industrial processes, highly stereospecific organic syntheses,and a great variety of theoretical and practical application^.'-^ Solid-state NMR spectroscopy has been used effectively in studying metal ions both bound and adsorbed on clay materials.P6 The changes in chemical shift and line widths that resulted from the different chemical environments of the ions provided information as to their identity and location. Recently we reported results from our studies of 2-phenethylamine 13C enriched in the 6 position adsorbed on various clays.4c Three different ammonium sites for adsorption were characterized by l3C cross-polarized magic angle spinning (CPMAS) spectroscopy. In some instances deconvolution methods were severely restricted due to the small chemical shift differences and considerable line widths. Moreover, differentiation between Bronsted and Lewis sites was impossible because of limiting differences in the chemical shifts of the LY and B carbon atoms and the large line widths. We have turned to 15N-enrichedmaterials to solve some of these problems. The chemical shift range for 15NNMR spectroscopy is much greater than that of the l3C chemical shift range and several studies of adsorbed species have taken advantage of this.7 Also eliminating the quadrupole effect of the 14N reduces line widths and thus improves resolution of components for l3C NMR spectroscopy. We studied 15N-enriched 2-phenethylamine on clay to take
* To whom correspondenceshould be addressed at the Department
of Chemistry,State University of New York at Albany, Albany, NY 12222. + State University of New York at Albany.
Texaco, Inc. Abstract published in Advance A C S Abstracts, April
1, 1994. (1)Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Hilger:
e
London, 1974.
(2) Pinnavaia, T. J. Science 1993, 220, 365. (3) Laszlo, P. Acc. Chem. Res. 1986, 19, 121. (4) (a) Bank,S.; Bank,J. F.; Ellis, P. D. J . Phys. Chem. 1989,93,4847. (b) Morris, H. D.; Bank, S.;Ellis, P. D. J . Phys. Chem. 1990, 94, 3121. (c) Bank, J. F.; Ofori-okai, G.; Bank, S. Clays Clay Miner. 1993, 41, 95. Kirkpatrick, R. J. Geochim. Cosmochim. (5) Weiss, C. A,; Altaner, S.; Acta 1990, 54, 1655. (6) Laperches,V.;Lambert,J.F.;Frost,R.;Fripiat,J. J. J.Phys.Chem. 1989, 90, 8821. (7) (a) Ripmeester, J. A. J . Am. Chem. SOC.1983,105,2925. (b) Maciel,
G. E.; Haw, J. F.; Chuang, I.-S.;Hawkins, B. L.; Early, T. A.; McKay, D. R.; Petrakis, L. J . Am. Chem. SOC.1983, 105, 5529. (c) Majors, P. D.; Ellis,P. D. J . Am. Chem. SOC.1987,109,1648. (d) Michel, D.; Germanus, A.; Pfeifer, H. J. Chem. Soc., Faraday Trans 1 1982, 78, 237.
0743-7463/94/2410-1528$04.50/0
advantage of using multiple spin probes7cto monitor the same interactions previously studied.4c
Experimental Section Clay Source and Preparation. Lithium taenolite was kindly provided by Dr. Jack Johnson (Exxon, Annandale, NJ). It is a syntheticfluormica with water swellingproperties. It has surface area of 10 m2/g and a cation exchange capacity of 120 mequiv/ 100 g. The layer spacings with and without the absorbed amine are 14.8and 12.7A, respectively. The aluminum-exchangedclay was prepared by heating 5 g of the clay with 50 mL of a 0.10 M Al(N03)3at 90 "C for 1 h, centrifuging, and washing the pellet with water until the pH value was neutral. The solid was dried at 80 "C for 24 h and then calcined at 200 "C for 2 h in air. Preparation of 'SN-Enriched 2-Phenethylamine. A solution was prepared from 0.16 g of BhNBr, 4.4 g (34.8 mmol) of benzyl chloride, 0.995 g (15.3 mmol) of KCN, and 2.1 g (31.8 mmol) of KCN-15 in 20 mL of acetonitrile and 3 mL of water. The reaction mixture was heated for 72 h at 85-90 "C. The product was obtained by extraction with chloroform. The yield of crude phenylacetonitrile was 3.86 g (32.8 mmol, 94% yield). The nitrile (3.86 g in 100 mL of ether) was reduced with lithium aluminum hydride (5.2 g, 136.5 mmol) in 150 mL of ether at 0 "C. After 1h, 3 mL of a concentrated Na2S04aqueous solution was added at 0 "C and the reaction mixture stirred for 1 h at room temperature. The reaction mixture was quenched with concentrated Na2S04 and filtered and the residual ether was removed in vacuo. The amine (2.5 g, 20.6 mmol, 59% yield) was obtained by fractional distillation in a Kugelrohr apparatus. The proton and carbon solution-state NMR spectra of the product were identical to those of the natural abundance phenethylamine except for the couplings due to 15N. The 15Ncontent of the amine was determined by l3C NMR spectroscopy to be 66%. Preparation of [*5N]2-Phenethylammonium Chloride. Anhydrous HCl was passed through a solution of 15N-enriched 2-phenethylamine (0.3 mL, 0.0024 mol) in dry diethyl ether (20 mL) at 0 "C for 10 min. The white precipitate was filtered and dried in vacuum; yield (0.31 g, 0.002 mol) 82%. Preparation of Amine on Clay. The amine adsorbed on the clay was prepared by adding 2-phenethylamine(0.1mL, 0.8 mmol) dissolved in 5 mL of dichloromethane to 0.5 g of the aluminumexchanged clay, stirring for 2 h, removing the solvent and unadsorbed amine in vacuo, and drying the adsorbed clay at 40 "C for 2 h. N M R Spectroscopy. The 13C NMR experiments were performed at 75.5 MHz on a Varian XL-300 spectrometer that has been modified for use with solid samples with a broad band Doty MAS probe (Doty Scientific, Inc.). Samples were packed in 5 mm 0.d. rotors and spun at 5-9 kHz. The CPMAS sequence used contact times of 1-5 ms (unoptimized) and a recycle time of 2.5 s; 50 Hz line broadening was applied. The 15N NMR spectroscopy experiments were performed on a Varian Unity0 1994 American Chemical Society
NMR Spectra of 2-Phenethylamine
Langmuir, Vol. 10, No. 5, 1994 1529
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Figure 1. l8C CPMAS of 2-phenethylammonium chloride at 75.5 MHz (7.05 T) with 1 me contact time, 4 s repetition time, and 50 Hz line broadening applied. (a) 16Nenriched with 2177 transients. The signal at 66 ppm (marked with an X) is a spinning sideband. (b) Natural abundance with 8OOO transients. The signal at 42 ppm has a contribution from a spinning side band. 300 spectrometer using a Doty Scientific 7-mm high-speed CPMAS probe at a resonance frequency of 30.4 MHz. The reference was an enriched sample of NH4(l6NOs)(-11 ppm). Contact times of 2 ms, recycle delays of 2 s, acquisition times of 40 ms, and MAS speeds of 3 kHz were used. In each case 4096 transients were collected using a 1H 90° pulse of 8 MS. A preacquisition delay of 35 M S was used to reduce ring-down artifacts. Deconvolutions of the data were performed using the deconvolution software provided in the Varian VNMR software. Also, the contact time array data were fitted using the Varian software in order to obtain values of T m and T1JH).
Results The solid ammonium salt of '5N-enriched 2-phenethylamine was used as a model for the Bronsted reaction. The l3C CPMAS spectrum of the salt (Figure l a ) revealed significant differences compared to the 14Nanalog (Figure lb). Substantial narrowing of the a carbon peak (line width 150 vs 250 Hz) and an equally substantial narrowing of the j3 carbon peak (line width 75 vs 150 Hz) lead to baseline separation of these two signals. Of note the ipso carbon has the same line width (75 Hz) in both compounds. The l3C CPMAS spectrum of the 15N-enriched2-phenethylamine adsorbed on the aluminum-exchanged clay (Figure 2a) has two clearly separated signals in the chemical shift region of the j3 carbon centered a t 33.5, 30.5 ppm. Deconvolution indicates a third species a t 25.5 ppm. The line widths are respectively 150,200, and >300 Hz. The relative contributions are 0.44,0.24 and 0.33, respectively. The characterization of distinct species is considerably more straightforward compared to the spectrum of V4N12phenethylamine (Figure 2b). Deconvolution of the j3 carbon chemical shift region of 2-phenethylamine enriched with 13C in the j3 position leads to the same chemical shifts but the line widths are 300,400, and >600 Hz.lc Thus the effect of removing the quadrupolar l4N atom reduces the
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Figure 2. 13C CPMAS spectra of the aliphatic region for 2-phenethylamine adsorbed on aluminum exchanged taeniolite at 75.5 MHz (7.05 T) with 0.5 ma contact time, 4 s repetition time, and 50 Hz line broadening applied. (a) l6N enriched with 2177 transients. (b) Natural abundance with 8000 transients. The insets show the entire spectral regions. line width by a factor of approximately 2 and leads to clean separation of two of the signals. The spectrum (Figure 2a) of '5N-enriched 2-phenethylamine adsorbed on clay in the CY carbon chemical region is also markedly changed in compqison to the unenriched amine (Figure 2b). While the identification of discrete signals is difficult, there is considerable line width reduction and the chemical shift center of gravity is 43 ppm. The shape of the signal clearly indicates multiple components and a qualitative deconvolution indicates a very broad component a t 41 ppm and a narrower component at 42 ppm. Of greater importance perhaps is that the line width reduction simplifies the region around 35 ppm so that the assignments of the /3 carbon species are facilitated. 15N Spectra. The 15N CPMAS spectrum of 15Nenriched phenethylamine (Figure 3) clearly reveals two components and an additional one is seen by deconvolution. These Components are a t 6.5 (31 Hz), 5.0 (34 Hz), and 4.0 (26 Hz) ppm (line width), respectively. The relative conditions are 0.35,0.33 and 0.53, respectively. While the chemical shift differences of the several components are similar to those of the 13Cchemicalshifts, the considerably narrower lines makes individual identification much easier. Contact time variation (Figure 4) reveals that the component a t 6.5 ppm has different time constants from the components near 5 ppm. Of particular interest the more shielded signal has a shorter T N H(polarization transfer rate). The chemical shifts are related to the acid strengths of the sites and the TNHare related to the crystallinity. The spectrum taken after 2.5 months shows the essential features of the original spectrum with considerable line broadening. A deconvolution has three components with enhanced line widths of 47,42, and 108 Hz in similar proportions as the original with some increase in the most shielded signal.
1530 Langmuir, Vol. 10, No. 5,1994
Bank et al.
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Figure 3. 15N CPMAS spectrum (experimental(a) and deconvolution (b))of 15N-enriched2-Phenethylamineadsorbed on aluminum exchanged taeniolite at 30.41 MHz (7.05 T)with 2 ms contact time, 2 s repetition time, 32000 transients, and 5 Hz line broadening applied.
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Figure 4. Plot of 15N CPMAS amplitudes (relative scale) as a function of contact time. Boxes ( 0 )are data for resonance at 4.9 ppm and pluses (+) are for data at 6.8 ppm.
Discussion For solid-state CPMAS NMR spectroscopy, the splitting and broadening due to the dipolar interaction with 14Nexperiencing strong nuclear electric quadrupolar interactions that is not averaged out by magic angle spinningS affect the carbon atoms directly bonded to nitrogen and in some instances the more distant carbon atoms.9 Moreover the effect is variable. Comparison of Figures 1 and 2 illustrates these effects on the 13C NMR spectra. For 2-phenethylamine, both the ammonium salt and that adsorbed on aluminum-exchanged clay, the line widths for the /3 carbon signals are essentially halved by removing the effect of 14N. Thus the quadrupolar effect (8) Hexem, J. G.; Frey, M. H.; Opella, S.J. J. Chem. Phys. 1982, 77, 3847. (9)Frey, M. H.;Opella, S.J. J.Chem. Soc., Chem. Commun. 1980,474.
on the /3 carbon is substantial and is of the order of magnitude of 1-2 ppm. This effect was also observed for the /3 carbon of leucine in the solid state.1° For amines, the chemical shift of the /3 carbon atom often provides diagnostic value in assigning whether the amine is coordinated to Lewis or Bronsted sites.l0 In addition, multiple coordination sites have different chemical shifts and these can be related to the acid strength of the site. The multiple sites of varying acidity which were characterized previously were separated by 3 and 5 ppm, which a t 7.05 T is 150 and 375 Hz, respectively. The increased line widths for the 14N amine made facile identification and quantitation difficult. As shown in Figure 2, 15Nenrichment offers considerable advantage in identifying and quantifying distinct sites. The effect of 15N enrichment of the a carbon leads to substantial line width narrowing, but the distribution of sites on the clays makes the clear identification of individual sites not as straightforward as it is for the /3 carbon sites. This appears to be a function of the chemical shift differences of the different components. For the a! carbon the indicated difference is approximately 1-2 ppm which is about half that for the same components at the /3 position. The more dramatic and useful change in the a carbon region brought about by l6N enrichment is that the narrowing leaves the /3 carbon region as a very clear window. This is evident in the baseline separation of signals in the ammonium salt and allows peak assignments in the clay samples. The 16NCPMAS of the 1SN-enriched 2-phenethylamine also clearly demonstrates multiple amine coordinated components. Protonation of an amine results in a deshielding chemical shift change of the order of 6-20 ppm whereas coordination to a Lewis site brings about a deshielding chemical shift change of 50-70 ppm.7 From the chemical shift values of the three components compared with the amine and the protonated amine in solution, we can identify these sites as Bronsted sites of differing acidities. (10) Ofori-Okai, G.; Bank, S. Heteroat. Chem. 1992,3, 236.
NMR Spectra
of 2-Phenethylamine
15N NMR spectroscopy has been used valuably for pyridine adsorbed on various surfaces7 but not for alkylamines which are stronger bases.ll Accordingly this study provides the 15NNMR spectroscopic comparison with the stronger bases on heterogeneous surfaces which have been studied by 13CNMR spectroscopy. Pyridine on alumina and silica-alumina is preferentially adsorbed at Lewis sites7 whereas 2-phenethylamine on activated clays is adsorbed a t Bronsted sites. With the differences in Lewis/Bronsted strength of the two bases and the differences in surfaces, it is premature to speculate on the generality or implications of the results. The observation of distinct acid sites, rather than a continuous distribution of sites giving rise to a broad signal, has been considered in detail for 15Nresonances a t Lewis acid sites on y - a l ~ m i n a .Heterogeneity ~~ of the surface and very slow or no exchange between these species are the important factors contributing to the phenomenon. Activated clays are clearly heterogeneous with a variety of possibilities of interlayer, ion-exchanged, defect, and edge sites. While lateral diffusion and movement about a specific axis of the amine bound to the clay are possible, we have earlier shown that a Bloch decay spectrum is very similar to the CPMAS spectrum. This rules out extensive motion.4c The deterioration of the spectrum with time reveals that two changes have occurred. First, there has been some redistribution of components and second the distribution of sites within each component has increased. Thus some motion on longer time scale may be involved, but more important contributions come from reaction with adventitious moisture and/or air. Changes in the I5NNMR spectra even with tight spinner caps are w e l l - k n o ~ n . ~ ~ Thus in agreement with the conclusions derived from the 13C CPMAS spectra, 2-phenethylamine is adsorbed on the clay a t multiple sites with different acid strength, and two sites are clearly observed in 15N NMR spectroscopy. Since the deshielding effect of coordination to a proton depends upon the electron density a t the nitrogen atom,12 we can assume that the chemical shift values provide a measure of the relative acid strengths of the sites. In the previous work with 13C-enriched 2-phenethylamine adsorbed on clays both edge sites and interlayer sites were observed. The interlayer sites had greater restriction to motion than the edge sites and this was evident in the spectra. Additionally, the most acidic sites (11) See, however, Earl,W. L.; Fritz, P.0.; Gibson,A. V.; Lunsford, J. H.J . Phys. Chem. 1987,91, 2091. (12) Buchanan,G.W. Tetrahedron 1989,45,1989.
Langmuir, Vol. 10, No. 5, 1994 1531 were observed on a clay with only edge sites. On this basis a relationship between greater mobility and greater acidity is expected. The TNHof the most acidic site is 1.8 ms, whereas the TNHof the least site has a TNHof 0.8 ms. With the assumption that the magnitude of TNHdepends upon mobility, the more acidic sites are therefore more mobile. A rationale that would account for these observations involves interlayer and edge acid sites. Thus it is possible that the shorter TNH is associated with the somewhat less acidic interlayer sites. Since the chemical shift change on the @ carbon in the l3C NMR brought about by coordination to a proton is shielding and the chemical shift change on the N atom in the I5N NMR brought about by coordination to a proton is deshielding, then if they monitor the same species in roughly the same distributions, the patterns of the resonances should be related in a mirror image manner. Inspection of Figures 2a and 3a reveals this pattern. Moreover the relative contributions of the separate components in each of the spectra is similar but not exactly the same. Given that these are CP spectra and the CP efficiencies of various resonances in both 13Cand l5N CP spectroscopy are involved, the agreement is quite reasonable.
Conclusions The advantages brought about by enriching an amine with '5N for adsorption studies are several. The sites of amine adsorption on clay are clearly Bronsted sites. For l5N CPMAS the ability to distinguish between Bronsted and Lewis sites is based on chemical shift differences which are so large that any ambiguity is resolved relative to 13C CPMAS chemical shift differences. Moreover the individual Bronsted sites are clearly denoted by '5N CPMAS. An equally important gain in resolution is observed in the @ carbon chemical shift region of the l3C CPMAS of the 15N-enriched material relative to either the natural abundance material or the 13C-enrichedamine. For studies a t extremely low concentrations of amine where both sensitivity and resolution are critical, enrichment of both the @ carbon with 13Cand the amine with '5N would allow for detection and quantitation of preferential adsorption. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund administered by the American Chemical Society for the support of this research. We thank Dr. Jack W. Johnson of the Exxon Research and Engineering Co. for providing the clay sample and for determining the interlayer spacings.
