J. Chem. SOC., Faraduy Trans. I , 1982, 78, 231-254 Nitrogen- 1 5 Nuclear Magnetic Resonance Spectroscopy of Adsorbed Molecules BY DIETER MICHEL,* ANDREAS GERMANUS AND HARRY PFEIFER N.M.R.-Labor der Sektion Physik, Karl-Marx-Universitat Leipzig, DDR-70 10 Leipzig, German Democratic Republic Received 16th February, 198 I Nitrogen-15 n.m.r. spectroscopy has been applied to the study of the interactions between solid surfaces and molecules adsorbed on them. Nitrogen- 15 spectra of ammonia, trimethylamine, pyridine and acetonitrile molecules sorbed in various zeolites were measured at 9.12 MHz by means of the conventional Fourier-transform n.m.r. technique. In all measurements carried out, substances were employed which were enriched with nitrogen-1 5 nuclei (ca. 95%). The resonance shifts depend strongly on the nature of adsorption sites which may occur in the zeolites ( e g .Na+ cations, Bronsted- and Lewis-acid sites). The results clearly reveal the advantage of nitrogen- 15 n.m.r. investigations in characterizing acidic properties in comparison with the carbon- 13 n.m.r. measurements performed until now on adsorbate- adsorbent systems and emphasize that nitrogen- 15 spectroscopy may become a powerful tool for the study of surface phenomena. 1 . INTRODUCTION Until now resonance shifts for adsorbates have been measured predominantly for carbon- 13 nuclei since for protons highly resolved spectra could be obtained only in isolated cases. It is well-known1 that the peculiarities in the n.m.r. spectra of large nuclei like carbon-13 are due to (i) the greater shielding, (ii) their smaller magnetogyric ratio and (iii) their low natural abundance giving rise to smaller line-widths because of the lack of 13C-13C couplings and a reduced magnetic interaction with paramagnetic impurities of the adsorbent.An analogous situation holds for nitrogen-] 52 but while carbon-I3 n.m.r. spectra can be observed even in adsorbed molecules having only natural abundance, the much reduced sensitivity for nitrogen-1 5 nuclei requires in general the use of nitrogen- 15-labelled compounds. This follows from a comparison of the values of the n.m.r. sensitivity and of the natural abundance of carbon-I 3 and nitrogen-] 5 nuclei.' If we take the products of both factors as a measure of the relative signal intensity, a relative reduction by a factor of ca.50 results for nitrogen-15. A study of the nitrogen-15 resonance spectra of adsorbed molecules is of special interest because the nitrogen atoms of a variety of molecules possess lone-pair electrons which may participate in the formation of hydrogen bonds, giving rise to strong nitrogen- 1 5 resonance shifts. Moreover, molecules like pyridine and acetonitrile may be protonated directly at the nitrogen atom. Consequently, the difference between the nitrogen-1 5 resonance shifts of protonated and non-protonated species may be considerably larger than the resonance shifts for the adjacent carbon-13 spins and protons in the molecule. Hence nitrogen-1 5 measurements should be more favourable for a study of the acidic sites on a surface.In addition, through a study of the nitrogen-1 5 n.m.r. the electronic state of the ammonia molecule can be investigated in more detail than with proton resonance because the proton resonance shifts are small and often less than the line-widths, which prevents their accurate measurement. 237238 15N N.M.R. OF ADSORBED MOLECULES A brief description of the experimental conditions and of the zeolites used as adsorbents is given in section 2. In section 3 a procedure is developed to derive values for the resonance shifts of molecules bound in surface complexes and for the number of sites involved from the resonance shifts measured as a function of the total number of adsorbed molecules. The results for ammonia, pyridine and acetonitrile molecules adsorbed in various zeolite specimens are presented in section 4.2. EXPERIMENTAL Nitrogen-15 n.m.r. spectra were measured at 9.12 MHz by means of the Bruker Fourier- transform n.m.r. spectrometers WH 90 DS and HX 90 R. The repetition time varied between 0.2 and 0.8 s for the different samples. The pulse widths varied between 16 and 20 ps for ca. 40' pulses. In most cases spectra were taken with proton broad-band decoupling. Only for ammoniamolecules were the coupling constants measured. The number of transients was lo2-1 05. Resonance shifts were indirectly referred to liquid nitromethane. Positive shifts are to lower magnetic field. The correction of resonance shifts due to the adsorbent susceptibility was calculated to be < 0.6 ppm (for Nay, NaX and NaA) on the basis of bulk susceptibility data given in ref.(3) and hence were omitted for the other adsorbents. The deuterated lock substances ([2H6]DMS0, [2H6]acetone) were filled in 10 mm 0.d. glass tubes in which 8 mm 0.d. sealed tubes containing the adsorbate-adsorbent systems were inserted. For adsorbents we used zeolites of type NaX (ratio of the number of silicon to aluminium atoms Si/A1 = 1.3), NaY (Si/Al = 2.6), NaA and Na-mordenite (all substances supplied by VEB Chemiekombinat Bitterfeld, G.D.R.) and different decationated zeolites. Non-stabilized decationated Y-type zeolites, for which 88% of the total number of Na+ ions were replaced by NH: ions and which were then decomposed in ULICUO, are denoted by 88 HY. The stabilization process for the stabilized forms was carried out under self-steaming conditions. Zeolites of type 70 HY-St 840 supplied by Bosaeek were prepared from decationated zeolites of type 70 HY by controlled heating at a rate of 4 K min-l up to 840 K in the presence of ca.2.13 x lo3 Pa (16 Torr) of water vapour [cf. ref. (4) and ( 5 ) ] . The sample US-Ex represents a stabilized zeolite containing only a small amount of aluminium which was prepared by Lohse from NaY zeolite using a treatment described in ref. (6). The composition of the unit cell corresponds to Na,., o,, (AlO,), (SiO,),,,.,,,. Before adsorption by distillation in uucuo the adsorbents were activated at temperatures of 670 K (20 h, p = Pa) except for types 88 HY (620 K) and some samples with stabilized zeolites 70 HY-St 1040 (870 K). All adsorbates were labelled with 15N-nuclei [NH,, 99.9%; CH3CN, 95.2%; C,H,N, 96%, (CH,),NH, 94.4%; (CH,),N, 95.7%) The methylamines were prepared by decomposition of methylamine hydrochloride using concentrated solutions of NaOH.The amines were dried over freshly regenerated 3 A zeolite immediately before the preparation of the samples. 3. THEORY The most important problem to be solved in the interpretation of the resonance shifts, measured here as a function of the number of adsorbed molecules, is the determination of the resonance shifts for those molecules which are bound in complexes. In most cases this information cannot be drawn directly from experiment because the experimental resonance shifts extrapolated to zero pore-filling factors are not necessarily identical with the resonance shift for the molecules bound in complexes.One way to solve the problem is to consider the equilibrium between the physisorbed molecules (M), free adsorption sites (A) and the complexes formed (MA) and to derive the number of sites (NA) and the equilibrium constant k from the fit of the experimental resonance shifts as a function of the total number ( N ) of adsorbed molecule^.^D. MICHEL, A. GERMANUS AND H. PFEIFER 239 We consider here only one type of adsorption site. It is then possible to describe M+A$MA (1) (2) this equilibrium by the equation k from which the relation NC k = ( N - N C ) ( N A - N C ) can be derived. Nc denotes the number of molecules bound in complexes and NA the number of adsorption sites, i.e. the number of centres multiplied by the number of molecules which can be adsorbed on one centre.Hence, N - N , is the number of physisorbed molecules which are not bound in complexes and NA - N , is the number of free sites. Furthermore, we denote by 6 the observed resonance shift with respect to the physisorbed state (i.e. aM = 0 for physisorbed molecules) and by 6, the respective shift for a molecule bound in a complex; the latter has to be derived from the experimental data. If the condition for rapid exchange is fulfilled the two shifts are related by the equation d=6,-. NC N By the combination of eqn (2) and (3) a quadratic equation results: with I+kN+kNA NA N kN +-=o x 2 - x 6 X=-. 6, (3) (4) A suitable form of eqn (4) with respect to an analysis of experimental data is given 1 l + k N A N - + -(1 -x).X kNA NA From this equation it follows that in the general case of an arbitrary strength of complexes, i.e. for an arbitrary value of the quantity kN, a plot of l/x against N( 1 -x) should give a straight line with a slope 1 INA if the value for 6, has been chosen correctly. Hence, in order to analyse the experimental shift 6 as a function of the number N of adsorbed molecules the quantity 6, has to be chosen, by means of a trial-and-error procedure, in such a way that a straight line results. If we cannot exclude the possibility that very low ratios x 6 1 also occur, the problem is to find the smallest possible value for 6, by this trial-and-error procedure because for x < 1 a linear dependence always results. We should also consider the two special cases of a strong and a weak complex.A complex is said to be strong if the condition k N $ l (6) is fulfilled. Then it follows from eqn (4) that (NA/N)6, if N A < N 6, if N A b N . (7) This is the well-known result for the fast exchange if, due to the strong interaction,240 15N N.M.R. OF ADSORBED MOLECULES all molecules are adsorbed at adsorption sites, as long as their number, NA, is still higher than the total number, N , of molecules involved (cf. fig. 1). For a weak complex, i.e. if the condition is fulfilled, we obtain Since the number of sites, NA, is reasonably less than, or at least of a comparable magnitude to, the total number of molecules, N , we also have kNA Q 1. Thus the observed resonance shift, 6, is less than the shift, &, for the complex.A further characteristic feature is that the quantity 6 does not depend in this case on the total number, N , of molecules adsorbed (cf. fig. 1). k N + 1 (8) (9) 6 = [kNA/( 1 +kNA)] 6,. N A N FIG. 1 .-Schematic plot of resonance shifts, 6, observed as a function of the total number, N , of molecules in the case of (a) a weak and (b) a strong adsorption complex. 4. RESULTS AND DISCUSSION 4.1. SORPTION OF AMMONIA ON VARIOUS ZEOLITES NH, I N Nay, NaX, NaA, NA-MORDENITE AND DEALUMINATED Y-TYPE ZEOLITE The nitrogen-1 5 resonance shift for the adsorbed ammonia molecules referred to gaseous ammonia was measured as a function of the pore-filling factor and at different temperatures, viz. NH,/NaY (240-360 K), NH,/NaX and NHJNaA (240-300 K).Since for the resonance shifts in gaseous ammonia different values are reported in the literature, we also investigated gaseous samples both with and without proton decoupling. The results of our measurements are as follows: (i) The nitrogen-15 resonance shift for ammoniamolecules adsorbed in Nay, NaX, NaA and Na-mordenite type zeolites depends strongly on the pore-filling factor. The plot (cf. fig. 2) is characterized by the approach of the resonance shift for adsorbed ammonia molecules to values characteristic of liquid ammonia (1 8 ppma) if we have nearly complete pore filling (6 = 1) and gaseous ammonia (0 ppm) if we extrapolate to zero coverage (6 = 0). (ii) The resonance shifts for these adsorbate-adsorbent systems are of the sameD. MICHEL, A. GERMANUS AND H.PFEIFER 24 1 order of magnitude and remain constant within the temperature intervals chosen. (iii) For the dealuminated Y-type zeolite, the nitrogen- 15 resonance shift does not change as a function of the pore-filling factor (between B = 0.2 and 0.72, measurements at 300 K, fig. 2). Its value is about the same (16 ppm) as that reported for liquid - 0 - -0 0.5 e 1.0 FIG. 2.-Nitrogen-1 5 n.m.r. shifts, 6, of ammonia in various zeolites (in ppm referred to gaseous NH,) as a function of the pore-filling factor 0 at 300 K : 0, Nay; x , NaX; +, NaA; A, Na-mordenite; 0, US-Ex. [6 (liquid) = -382i-0.2 ppm, 6 (gas) = 399.9kO.l ppm on the CH,NO, These experimental data allow the unambiguous conclusion that at higher pore-filling factors the ammonia molecules are packed so closely that their resonance shifts become liquid-like.In the case of dealuminated samples (US-Ex) even for small pore-filling factors (3.8 molecules per large cavity) a strong association of ammonia molecules occurs leading to liquid-like resonance shifts and to the observed constancy of resonance frequencies over a wide range of coverage. Further proof of the strong resemblance between ammonia adsorbed at higher pore-filling factors in zeolites and liquid ammonia was found during a study of samples in which different portions of water were added to zeolites already containing a certain amount of ammonia. With an increasing number of adsorbed water molecules the nitrogen-1 5 resonance lines are shifted to lower fields in a manner similar to that observed for aqueous solutions of ammonia where this phenomenon was explained by hydrogen-bonding interaction^.^242 15N N.M.R.OF ADSORBED MOLECULES US-Ex-type zeolites and the Na-forms of X and Y zeolites have the same topology and their different behaviour is mainly due to the very small number of Na+ ions in the US-EX.~ Hence it is reasonable to attribute the linear change in resonance shifts with the number of adsorbed molecules for the sodium forms of X, Y, A and mordenite type zeolites (in contrast to the dealuminated zeolites) to an interaction with Na+ ions. Such an interaction is well-known from measurements of the heat of adsorption.1°-12 However, the nitrogen-I 5 resonance shifts reveal that even in the limit of zero pore filling, where the greatest adsorption effects are l1 only a small deviation of the resonance shift from the gas-phase value occurs.Moreover, for various zeolites (Nay, NaX, NaA, Na-mordenite) having different kinds and numbers of Na+ ions approximately the same plot of shifts against pore-filling factors is observed. Obviously the variation of the resonance shifts with decreasing coverage between the values measured for the liquid and the gas can simply be explained by the fact that the association of the ammonia molecules is increasingly prevented due to the interaction with the Na+ ions, which itself leads, however, to an almost negligible influence on the electron density at the nitrogen atom. Litchman and co-workersg* l4 showed that the nitrogen- 15 resonance shifts for various ammonia solutions and for the pure liquid can be explained satisfactorily by a superposition of different mechanisms which were determined empirically by comparing the behaviour of ammonia and of trimethylamine in various solutions.The contributions include hydrogen-bonding interactions between the nitrogen lone-pair electrons and the hydrogen atoms of neighbouring molecules as well as interactions of the hydrogen atoms of ammonia molecules with oxygen and nitrogen atoms in their surroundings. In the present study, the strong resemblance of the resonance shifts measured for high pore-filling factors with the values for the pure liquid clearly indicates the presence of hydrogen-bonding interactions between adsorbed ammonia molecules. This finding is in contrast to conclusions drawn by Basler et a1.12 On the basis of measurements of the apparent molar heat of adsorption of ammonia in the pores of zeolites NaX and NaY they concluded that at all coverages no considerable number of hydrogen bonds exists.The question now arises as to whether the remaining small shift with respect to the gaseous state at very low pore-filling factors (3-5 ppm to lower field for the different ammonia samples) is also due to a superposition of several contributions2 (such as the interaction between the lone-pair electrons and Na+ ions and a hydrogen-bonding interaction between the hydrogen atoms of ammonia and the oxygen atoms of the zeolite skeleton) which may compensate each other. For this purpose we measured the nitrogen-I5 shift for trimethylamine as a function of the pore-filling factor at 380 K.The difference between the shifts for zero and high pore-filling factors is of the same order of magnitude as the value for the liquid-phase shift referred to the gaseous state.13 As in the case of ammonia, for zero pore-filling only a small deviation (ca. 1-2 ppm) in the resonance shift for the gas results. In the case of trimethylamine, however, the resonance shift only reflects the interaction between the lone-pair electrons of the molecule and active sites in the zeolite because a contribution to the nitrogen- I 5 n.m.r. shift of the trimethylamine molecule arising from hydrogen-bonding interactions between the methyl protons and the oxygen atoms of the zeolite skeleton should be negligible. This conclusion is suggested by investigations of nitrogen-1 5 resonance shifts in various solution^.^^ l4 The similar behaviour for both ammonia and trimethylamine molecules therefore suggests that the explanation of the small resonance shift as a consequence of the superposition of several contributions including hydrogen-bonding interactions is not very probable.A more probable interpretation would be one given in terms of a specific adsorptionD. MICHEL, A. GERMANUS AND H. PFEIFER 243 but with a comparable electronic state relative to the gas phase. Moreover, the same value for the indirect spin-spin coupling constant (62 & 5 Hz) was measured at low pore-filling factors as that obtained for ammonia in the gas or liquid phase (61.6 Hz). Because it is well-known15 that the coupling constant of the ammonia molecule depends on the bonding angle, this fact also emphasizes that the geometrical structure of the ammonia molecule has not been changed in the course of adsorption in Na-forms.At higher pore-filling factors in NaY zeolites (0 2 0.3) the multiplet splitting disappears; this is probably due to an increasing proton exchange rate. For the NaX-type samples a multiplet splitting was not observed. In the case of NaA zeolites the splitting appeared up to the highest filling factor studied (0 x 0.8).13 An analysis of exchange phenomena was not undertaken due to a lack of relevant experimental data such as the line-shape function of the multiplet for various temperatures and pore-filling factors and the transverse spin-relaxation time [cf.the IH spin resonance study in ref. (16)J. NH, I N 88 HY TYPE ZEOLITE In contrast to the Na forms, where resonance shifts of the ammonia molecules were strongly influenced by molecule-molecule interactions, the nitrogen- 1 5 resonance shifts in the following systems clearly reflect the interactions with acidic sites. We first discuss the results obtained for ammonia molecules in decationated zeolites of type 88 HY. At room temperature spectra could only be taken if more than ca. 9 molecules were adsorbed per large cavity. For the samples with only 1.9-4.4 molecules per large cavity, measurements could be performed at temperatures above ca. 380 K where the line-widths are sufficiently small. From the values for the line-widths given in table 1, we can conclude that for HY-type zeolites the molecular mobility is reduced in comparison with the Na forms but still high enough at 380 K to enable the application of conventional Fourier-transform n.m.r.spectroscopy to the liquids. However, as mentioned above, a more detailed characterization of the thermal mobility will not be undertaken here because of the lack of relaxation data. TABLE 1 .--NITROGEN- 15 N.M.R. SHIFTS, 6, AND LINE-WIDTHS, Av;, OF AMMONIA MOLECULES ADSORBED IN 88 HY-TYPE ZEOLITES AS A FUNCTION OF THE NUMBER, N , OF MOLECULES PER 6 in ppm referred to liquid nitromethane, Av; in Hz. The line-widths for NaY at 300 K are ca. 20 Hz. LARGE CAVITY 20.5 - 369 265 300 17.1 - 364 195 300 14.6 - 364 125 300 9.0 - 362 42 300 4.4 -361 33 380 2.3 - 361 33 380 1.9 - 361 39 380 Typical spectra are shown in fig.3 and some results summarized in table 1 . With respect to the Na forms, the nitrogen-15 resonance lines for ammonia molecules adsorbed in 88 HY-type zeolites are shifted appreciably to lower fields. At low coverages (4 molecules per large cavity) the resonance shift remains constant at a value of - 360 ppm referred to liquid nitromethane, which is typical for NH,+ solutions not244 15N N.M.R. OF ADSORBED MOLECULES 15NH4N03- inaqueous solution - NH3 gas -350 -360 -370 -380 -390 - 400 -410 6 (PPm) FIG. 3.-Nitrogen-l5 n.m.r. spectra of adsorbed ammonia molecules in 88 HY [ N = 9 molecules per large cavity; number of transients N, = 500; temperature T = 380 K (a), T = 300 K (b)] and in Nay-type zeolites [(N = 3.4; T = 300 K (c)].All shifts are referred to liquid CH,NO, [a (NH, gas) = -399.9kO.l ppm,B d(NH:) = -359.55k0.17 ppm* in 12.3 rnol drn+ solution of NH,NO, in water]. containing C1- ions. From this result we can conclude that all molecules are converted into ammonium ions as a consequence of their interaction with structural hydroxyl groups in the HY-type zeolites. As discussed recently in more detail for pyridine molecules sorbed in the same zeolites,17 the interaction with structural hydroxyl groups leads to a large reduction in the molecular mobility. This fact may explain why the nitrogen-1 5 resonance lines are broadened and disappear if the number of molecules is equal to or less than the number of accessible hydroxyl groups. At elevated temperatures for medium pore-filling factors also the resonance shifts approach a value characteristic of pure ammonium ions.The change in the resonance shift as a function of temperature is reversible. This temperature dependence is probably due to a decrease in the relative amount of non-protonated molecules because these species are more mobile and will be desorbed at higher temperatures. This suggestion is supported by a decrease in the signal intensity which is stronger than would be expected according to the Curie law if the temperature increases. In conclusion, the formation of ammonium ions in HY-type zeolites (well-known also from i.r. spectroscopic investigations'** 19) can be quantitatively studied by means of nitrogen- 15 n.m.r.D. MICHEL, A. GERMANUS A N D H. PFEIFER 245 NH, IN STABILIZED ZEOLITES The extremely high sensitivity of the nitrogen- 15 resonance shifts to an interaction with acidic sites will also be demonstrated by the following results obtained for stabilized decationated zeolites.For these samples the spectra could only be measured at temperatures of 340 K and higher because of the strong line broadening. This is the reason why all systems discussed here were investigated at 380 K. In fig. 4 the 10 I 15 N 20 N FIG. Q.-Nitrogen-15 n.m.r. shift, 6, of ammonia in 70 HY-St 840 type zeolites (in ppm referred to gaseous NH,) as a function of the number, N , of molecules (for dC and NA see the text). 5 NA = 12.5 resonance shifts for ammonia molecules in stabilized zeolites are plotted as a function of the pore-filling factor.In spite of the high enrichment with nitrogen-15 nuclei, the signal-to-noise ratio was not sufficiently high to study samples with < ca. 2 molecules per large cavity. However, the small change in the shifts below a pore-filling factor of 6 molecules per large cavity justifies an extrapolation of the values to zero coverage. The resonance shift thus obtained (1 16 ppm to lower field as referred to the gas) is considerably larger than the values reported in the literature20 for ammonium ions in different media and thus cannot be interpreted in terms of a dominant interaction of ammonia molecules with residual hydroxyl groups acting as Bronsted-acid sites. This conclusion is supported by analogous measurements using acetonitrile (see below). Moreover, BosaEek et aL21 showed recently that the Bronsted acidity of the (residual) OH groups of these stabilized forms is less than for the corresponding non-stabilized zeolites.Stabilized samples contain, besides the structural OH groups already mentioned, sites of the Lewis-acid type due to defects generated during the stabilization process. In order to derive the number of Lewis-acid sites which are involved in the interactions with adsorbed ammonia molecules we analyse the plot of the nitrogen-15 resonance shifts by means of eqn (5). The best fit results if 6, = 116 ppm is chosen, which can also be derived directly by means of a simple extrapolation of the experimental shifts to zero pore-filling factor (fig. 4). This finding is experimental proof of the existence of a strong adsorption complex.The number of sites obtained from the slope is NA = 12.5 per large cavity. In order to compare this value with the results of adsorption measurementsz1 we note that NA is given by the coordination number of the centres multiplied by their total number.246 15N N.M.R. OF ADSORBED MOLECULES For the following discussion we need an upper limit for the coordination number in order to determine a lower limit for the number of centres. It seems reasonable to take a coordination number of 4-6 in analogy with the arrangement of ammonia molecules in the vicinity of metal cations in solution. However, since the centres are located in front of the solid surfaces, the coordination number will be less than its value in solution. Thus the number of centres should be ca.2 per large cavity. In a recent paper by BosaEek et al.,l the number of irreversibly sorbed pyridine molecules was determined by sorption measurements to be ca. 0.17 mmol g-l or 0.25 pyridine molecules per large cavity for the same type of stabilized decationated zeolite. This value was taken in ref. (21) as a measure of the total number of acidic centres in the zeolite. However, this value is appreciably less than the number of Lewis-acid centres derived from the nitrogen-1 5 n.m.r. shifts for ammonia molecules. The discrepancy between both values goes beyond the uncertainties due to the choice of coordination number for the ammonia molecules. Clearly this comparison reveals the limitation of this method of determining the number of acidic centres.It may at least qualitatively account for the occurrence of certain sites but is not suitable for a more detailed characterization of their nature and order of magnitude. 4.2. SORPTION OF PYRIDINE I N NaY AND 88 HY TYPE ZEOLITES Since the lifetime of pyridinium ions (z = 5 x s at 313 K17) in 88 HY zeolites is milch smaller than the inverse value of the difference in Larmor frequencies of adsorbed pyridine molecules and pyridinium ions (u-l = 2 x s according to a resonance shift of 89 ppm, see below), the condition of a fast exchange is fulfilled. Moreover, in analogy to the interpretation of carbon- 13 resonance shifts,,, the pyridinium ion can be considered as a strong adsorption complex (see section 3). Hence, as long as the total number of molecules adsorbed is less than the number of Bronsted-acid sites the resonance shift, 6, observed is identical with the resonance shift, 6,, of the pyridinium ion [C = I, cf.eqn (7)) For higher pore-filling factors a mean resonance shift (10) is observed where 6, denotes the resonance shift for the adsorbed pyridine molecules andp, is the relative amount of pyridinium ions [cf. eqn (7) where 6, = 0 was chosen]. To analyse the plot of resonance shifts against the number, N , of molecules per large cavity in fig. 5 on the basis of eqn (1 l), the values for dM and 6, must be determined. It is reasonable to take for the quantity 6, the resonance shift (-90 ppm) observed in NaY zeolites because in NaY only non-protonated molecules occur, the resonance shifts of which do not depend on the pore-filling factor.Furthermore it was checked experimentally that the different numbers of Na+ ions in the sodium and decationated forms do not play a decisive role because resonance shifts of pyridine molecules on SiO, surfaces are of a comparable order of magnitude (cf. table 2). For the resonance shift 6, the value found for pyridinium ions in aqueous is taken (6, = 179.6 ppm). This assumption is supported by the following result: If the total number of adsorbed pyridine molecules ( N = 2.1 per large cavity) is only slightly in excess of the number of accessible OH groups [NOH x 2 per large cavity, cf. ref. (22)] then the observed resonance shift is about the same as that measured for the pyridinium ions in aqueous solution (cf. fig. 5 ) .Obviously at this pore-filling level almost all molecules are protonated. At a lower pore-filling level ( N = 1.7 per large cavity) no nitrogen-15 n.m.r. signal can be observed. An analogous situation occurred in the case of carbon-1 3 n.m.r. measurements. The disappearance of the spectra has been attributed here to the appreciable reduction in thermal mobility of the pyridinium ions which was = (l -PI) 6M +PI dlD. MICHEL, A. GERMANUS AND H. PFEIFER 247 FIG. 5.-Nitrogen- a function of the - 140 1 2 3 4 N .15 n.m.r. shift, 6, of pyridine in 88 HY-type zeolites (in ppm referred to CH,NO,) as number, N , of molecules ( T = 380 K). For pyridinium ions in solution (4.3 mol% C,H,NH+ in H,O) a value dI = - 179.6kO.l ppm was found.,, TABLE 2.-NITROGEN-15 RESONANCE SHIFTS 6 (IN ppm REFERRED TO NITROMETHANE) OF PYRIDINE MOLECULES ADSORBED ON SILICA GEL24 AND IN NaY TYPE ZEOLITES The coverage is given as number, N, of molecules per large cavity (Nay) or multiples, 0, of a statistical monolayer.adsorbent coverage 6 NaY N = 2.4 - 90.7 5.6 - 89.2 SiO, e = 0.08 -89.3 0.16 -89.9 0.79 -85.7 1.35 -78.9 studied independently by means of proton spin-re1axation.l’ Inserting the values for 6, and dI into eqn (1 l), the relative amount of pyridinium ions could be determined from a plot of nitrogen-15 resonance shifts. As can be seen from table 3 the number of pyridinium ions thus evaluated is about the same as that determined in ref. (22). In general the accuracy of the data derived from nitrogen-15 n.m.r. measurements should be higher because the differences between carbon- 13 resonance shifts for pyridine molecules and pyridinium ions in solution are much less (e.g.12.2 ppm for carbon C4) than the respective value for the nitrogen n.m.r. spectra (1 16 ppm, cf. fig. 6).248 lSN N.M.R. OF ADSORBED MOLECULES TABLE 3.-RELATIVE NUMBER, PI, OF PYRIDINIUM IONS IN 88 HY ZEOLITES CALCULATED WITH THE AID OF EQN (11) Values from analogous carbon- 13 n.m.r. investigations22 are given in parentheses (6 in ppm referred to liquid nitromethane). no. of pyridinium ions per large N 6 PI cavity 2.1 (2.6) -171.2 0.9 12 1.9 (1.9) 3.0 (3.0) - 165.4 0.847 2.5 (2.2) 4.6 (3.6) - 147.9 0.650 3.0 (2.5) I 1 1 I I I -50 -60 -70 -80 -90 -100 -110 -120 -no -uo -150 -m -170 -a -190 6 (PPm) FIG. 6.-Nitrogen- 15 n.m.r.spectra of pyridine molecules sorbed in NaY [(a) T = 300 K, N = 2.4 per large cavity; (6) T = 300 K, N = 5.6; (c) T = 380 K, N = 5.61 and in 88 HY-type zeolites [(d) T = 380 K, N = 4.61. 6 in ppm referred to CH,NO,, 6 = - 57.6 f 1.8 ppm for gaseous pyridine, 6 = - 63.9 f 0.1 ppm for liquid pyridine and 6 = - 179.6 f 0.1 ppm for pyridinium ions in solution (cf. legend to fig. 5). 4.3. SORPTION OF ACETONITRILE Nitrogen- 15 resonance shifts were measured for acetonitrile adsorbed in various zeolites and on SiO, surfaces. Except for CH,CN molecules adsorbed in the stabilized forms 70 HY-St, the line-widths are sufficiently small (ca. 20 Hz at 300 K for aceto- nitrile in Nay, NaX, US-Ex and on SO,, ca. 60-90 Hz for acetonitrile in HY zeolites) that the nitrogen-1 5 n.m.r.spectra could be measured at ambient temperature.D . MICHEL, A. GERMANUS A N D H. PFEIFER 249 , I I 10 -135 ' I ' ' ' ' 5 N FIG. 7.-Nitrogen-1 5 n.m.r. shifts, 6, of acetonitrile (in ppm referred to liquid CH,NO,) adsorbed in NaY (a), US-Ex (b), and 88 HY-type zeolites ( c ) at 300 K. For the shifts see also table 4. TABLE 4.-NITROGEN-15 N.M.R. SHIFTS, 6, OF ACETONITRILE MOLECULES IN DIFFERENT STATES 6 in ppm referred to liquid nitromethane. state/adsorbate 6 ref. zeolite 70 HY-St 840 (670 K, N = 7.8) (870 K, N = 8.1) 70 HY-St 1040 gas liquid protonated form (superacid medium) - - 109f3 -108+33 - - 126.5 25 - 1 3 6 . 4 25 - 239 20 9 FAR 1250 I5N N.M.R. OF ADSORBED MOLECULES For the system CH3CN/70 HY-St temperatures of ca. 400 K are necessary to obtain sufficiently small lines (ca.250 Hz). The resonance lines of the adsorbed acetonitrile molecules are in general shifted to higher fields relative to both the gaseous and liquid phases (cf. fig. 7). Only the resonance lines of acetonitrile molecules adsorbed in the stabilized forms are shifted to lower fields (cf. table 4). CH3CH I N NaX, Nay, US-EX TYPE ZEOLITES A N D ON sio, SURFACES As is shown in fig. 7 ( a ) the resonance shifts remain constant (- 156 ppm referred to liquid nitromethane) for CH,CN in NaY zeolites as long as the number of adsorbed molecules is less than ca. 5 per large cavity. This behaviour suggests that acetonitrile molecules interact with a relatively large number of adsorption sites. From the whole plot of 6 as a function of N , by means of eqn (5) a value for the number of active sites per large cavity (6k0.5) and a value for the shift of the complexed molecules (6, = - 156 ppm) can be calculated in agreement with the simple interpretation of the plateau (infinitely strong adsorption complex).For the physisorbed molecules a resonance shift of - 136.4 ppm (as in the liquid state) was taken. This assumption is supported by the following results: In order to check whether sodium ions can act as adsorption sites, similar measurements were performed using dealuminated zeolites of type US-Ex and silica gel. The different behaviour of acetonitrile molecules due to an appreciable reduction in the number of sodium ions can be clearly inferred from fig. 7(b). At nearly complete pore filling of the US-Ex-type zeolite the resonance lines appear in an interval which is typical of the liquid state of acetonitrile.Thus, in contrast to the pure Na forms the influence of molecule-molecule interactions is directly reflected by the measured resonance shifts at high pore-filling factors. The influence of an interaction with remaining adsorption sites clearly appears at lower pore-filling factors where the resonance lines are more and more shifted toward the value which is typical of the pure sodium Y-type zeolites. Obviously the number of sites is very small because a plateau cannot be observed within the limits of I t FIG. 8.-Nitrogen-15 n.m.r. shifts, 6, of acetonitrile (in ppm referred to liquid CH,NO,) adsorbed on silica gel. Value for 0 in monolayers, (-) fitted curve (see text), (0) experimental values.D.MICHEL, A. GERMANUS AND H. PFEIFER 25 1 a I I I I I I I \ \ \ 0 \ \ CH3CN, gas liquid / ,CH3CN adsorbed \ 0 5 10 N FIG. 9.-(a) Nitrogen-15 n.m.r. shifts, 6, of acetonitrile adsorbed in NaX type zeolites at 300 K (in ppm referred to liquid CH,NO,). (6) Nitrogen-15 n.m.r. spectrum of acetonitrile adsorbed in NaX type zeolite for a coverage of 0 = 1.2 (complete pore filling, 0 = 1, corresponds to ca. 10 molecules per large cavity). experimental error. This is reasonable, too, since the number of sodium ions in the US-Ex-types is very Clearly these deviations from the system CH,CN/NaY zeolite underline the role of Na+ ions as adsorption sites for acetonitrile in the Na form of Y-type zeolites. However, it cannot be ruled out that adsorption sites other than Na+ may also be of importance for the shifts occurring at low pore-filling factors in US-Ex, since it was not possible to fit the experimental plot of the resonance shifts against pore-filling factor by means of eqn (5).This fact can be understood if the observed nitrogen-15 n.m.r. shift at low pore-filling factors is a result of a fast exchange in which more than one kind of active site is involved, e.g. residual Na+ ions and residual OH groups. To estimate the possible influence of the weakly acidic OH groups in US-Ex zeolites on the nitrogen-15 n.m.r. shift we investigated additionally the resonance shift of acetonitrile molecules adsorbed on silica gel. As is well-known, the surface OH groups of silica gel are only weakly acidic sites.Using eqn ( 5 ) we obtain from the plot in fig. 8 the value of 2.15 OH groups per nm2, which is in good agreement 9-2252 15N N.M.R. OF ADSORBED MOLECULES with the results of other methods (uiz. 2.2 per nm2 at a pretreatment temperature of 670 K26). The resonance shift, 6,, of the acetonitrile-OH-group complex is - 146 ppm (referred to liquid nitromethane) which is even smaller than the value for the acetonitrile-Na+ complex (- 156 ppm, see above). In conclusion, for the system CH,CN/US-Ex a fast exchange of the CH,CN molecules between the small number of OH groups and Na+ ions can be responsible for the experimental shifts observed at low pore-filling factors. Since in Nay-type zeolites the number of OH groups is much less than the number of Na+ ions these measurements confirm the result that sodium ions in the NaY zeolites are responsible for the resonance shifts of adsorbed acetonitrile molecules. The number of active sites (6 _+ 0.5, see above) is equal to about twice the number of accessible Na+ ions (ca.3.3 Na+ ions at S,, sites2’) so that the coordination number of the Na+ ions for CH,CN molecules in NaY zeolites is ca. 2. Measurements were also performed using NaX-type zeolites (cf. fig. 9). For higher pore-filling factors the nitrogen-15 resonance shifts are nearly the same as those measured for the Nay-type zeolites : they decrease monotonically with increasing pore-filling factors as long as the total number of molecules is larger than ca. 6.5 0.5 per large cavity and are independent of the number, N , of molecules for 4.0 6 N 6 6.5.In contrast to the NaY zeolites, for N < 4.0, an additional shift to still higher fields occurs. Presumably the CH,CN molecules adsorbed at low pore-filling factors interact preferentially with Na+ ions at S,,, sites (ca. 4 per large cavity2’) which practically does not occur in NaY zeolites. Only at higher pore-filling factors (4.0 < N < 6.5) can the influence of sodium ions at S,,-type sites be inferred from the resonance shift. The total number of interacting acetonitrile molecules derived from this plot of the resonance shifts is again ca. 6-7 per large cavity. However, for the NaX zeolites this number is less than the total number of sodium ions per large cavity. Obviously the fraction of the acetonitrile molecules which can interact simultaneously with the Na+ ions occurring in the large cavities is limited here for sterical reasons.At still higher numbers of adsorbed acetonitrile molecules (N > 10, i.e. pore-filling factors 8 > 1) the resonance shift for the molecules within the large zeolite cavities remains constant. In addition a resonance line appears at a frequency which is typical of liquid acetonitrile [cf. fig. 9(b)]. Obviously under these conditions a slow exchange occurs between species adsorbed in the zeolitic holes and a liquid-like phase at the outer surface of the crystallites. ACETONITRILE I N DECATIONATED ZEOLITES The nitrogen-1 5 n.m.r. shifts of acetonitrile molecules in a decationated 88 HY-type zeolite are plotted as a function of the number, N , of molecules per large cavity in fig.7(c). In contrast to the adsorption of ammonia and pyridine molecules in 88 HY-type zeolites, where the formation of the protonated forms can be followed in the spectra, the resonance shifts [ - 161 to - 147 ppm, fig. 7(c)] are of a comparable order of magnitude to those for unprotonated acetonitrile molecules adsorbed on silica-gel surfaces (- 146 to - 141 ppm, fig. 8). At higher pore-filling factors the resonance shifts [ - 147 ppm, fig. 7(c)] are about the same as the values measured for the pure sodium forms [ - 149 ppm in fig. 7(a), - 148 ppm in fig. 91. Furthermore, if we eliminate the influence of molecule-molecule interactions, i.e. if we take to a first approximation as reference the resonance frequencies measured for the liquid and the gaseous phases at complete and zero pore filling, respectively, or a weighted average value between both values at medium number N , then we obtain for the resonance shift 6, = - 180 ppm.This value differs appreciably from the value for protonated acetonitrile species in superacid solutions (- 239 ppm referred to liquid CH,N02).20 The number of sites (NA = 3.0f0.5 per large cavity), however, is of the sameD. MICHEL, A . GERMANUS A N D H. PFEIFER 253 magnitude as obtained from the nitrogen-1 5 n.m.r. shifts of pyridine molecules adsorbed in the same zeolites. This result reveals that even in decationated zeolites the CH,CN molecules are not protonated but interact with the structural OH groups via hydrogen bonds.Finally, measurements using stabilized decationated zeolites are discussed. With respect to the gas phase, the nitrogen- 15 resonance line is shifted here by ca. 17 ppm to lower fields. This result is of special interest because the resonance frequency is substantially different from both systems containing Bronsted-acid sites of various strengths (SiO,, 88 HY) and those having sodium ions. Clearly, protonation of acetonitrile cannot account for this shift. Also other weak interactions already described cannot be responsible for the shift because for all of them the nitrogen-15 resonance line appears at higher fields. Because the resonance shift to lower fields (relative to the gas phase) only occurs when a stabilized zeolite is investigated we attribute it to an interaction with Lewis-acid sites created during the stabilization process.This interpretation is supported by analogous experiments using ammonia, as described above. 5 . CONCLUSIONS (1) The first detailed nitrogen-1 5 n.m.r. study, presented here, has revealed that this method is a powerful tool for the study of surface phenomena. The resonance shifts depend very strongly on the nature of the adsorption sites, and the changes in the spectra are often much larger than in the case of analogous carbon-13 n.m.r. measurements (cf. e.g. section 4.2). (2) To derive the resonance shifts for the surface complexes, and hence to eliminate the influence of exchange processes on the nitrogen-15 n.m.r. shifts observed, the equilibrium of the reaction with surface sites is considered (section 3).This method could be checked directly in the case of adsorbate-adsorbent systems characterized by strong adsorption complexes (e.g. CH,CN adsorbed in Nay, pyridine adsorbed in 88 HY). (3) A very surprising result is that the nitrogen-15 resonance shifts for ammonia molecules adsorbed in the sodium forms of various zeolites (types A, X, Y and mordenite) are mainly due to molecule-molecule interactions although a strong influence of the Na+ ions can be clearly inferred from results of adsorption-heat measurements.10311 In the case of decationated zeolites (88 HY) the spectra reveal unambiguously, as expected, the formation of NH,' ions (strong shift to lower fields relative to the gaseous phase). Lewis-acid sites in stabilized decationated forms give rise to a still larger resonance shift to lower fields (section 4.1).(4) The formation of pyridinium ions in decationated zeolites can be clearly followed in the nitrogen- 15 n.m.r. spectra. From the resonance shifts the number of interacting OH groups is derived. Nitrogen-1 5 n.m.r. spectroscopy seems to be better suited to this analysis than analogous carbon-1 3 n.m.r measurements published recently22 (section 4.2). (5) Nitrogen-1 5 n.m.r. resonance shifts of acetonitrile molecules can be favourably used in the characterization of interactions with the exchangeable cations of the zeolite ( e g . Na+) and with Lewis-acid sites (section 4.3). In summary, suitable use of the adsorbate in nitrogen-1 5 n.m.r. measurements should enable a selective study of the different active sites present in an adsorbent.254 15N N.M.R. OF ADSORBED MOLECULES H.Pfeifer, Phys. Rep. C., 1976, 26, 293; D. Michel, Surf. Sci., 1974, 42, 453. D. Michel, A. Germanus, D. Scheller and B. Thomas, Z . Phys. Chem. (Leipzig), 1981, 262, 113. D. Michel, W. Meiler, A. Gutsze and A. Wronkowski, 2. Phys. Chem. (Leipzig), 1980, 261, 953. Z. Tvarbikova, V. Patzelova and V. BosaEek, React. Kinet. Catal. Lett., 1977, 6, 433. V. Patzelova, J . Chromatogr., 1980, 191, 175. U. Lohse, E. Alsdorf, and H. Stach, 2. Anorg. Allg. Chem., 1978, 447, 64. T. Bernstein, P. Fink, D. Michel and H. Pfeifer, J . Colloid Interface Sci., in press; V. 1. Borovkov, G. M. Zhidomirov and V. B. Kazanski, Zh. Struct. Chim., 1975, 16, 308. M. Witanowski, L. Stefaniak, S. Szymanski and H. Januszewski, J . Magn. Reson., 1977, 28, 217. W. M. Litchman, M. Alei Jr and A. E. Florin, J. Am. Chem. Soc., 1969, 91, 6574. lo R. M. Barrer and R. M. Gibbons, Trans. Faraday Soc., 1963, 59, 2569. l1 A. V. Kiselev, in Proc. 5th Int. Conf. on Zeolites, Naples, 1980, ed. L. V. C. Rees (Heyden & Sons, l2 W. Basler, D. Clausnitzer and H. Lechert, Ber. Bunsenges. Phys. Chem., 1975, 79, 527. l3 A. Germanus, Diplomarbeit (Karl-Marx-Universitat, Leipzig, 1980). l4 M. Alei Jr, A. E. Florin and W. M. Litchman, J . Am. Chem. SOC., 1970,92,4828. l5 R. E. Wasylishen and T. Schaefer, Can. J . Chem., 1973, 51, 3087. l6 E. B. Whipple, P. J. Green, M. Ruta and R. L. Bujalski, J . Phys. Chem., 1976, 80, 1350. H. J. Rauscher, D. Michel and H. Pfeifer, J . Mol. Catal., in press. A. V. Kiselev and V. I. Lygin, IR Spectra of Surface Compounds (in Russian) (Nauka, Moscow, 1973), p. 366ff. London, 1980), p. 400. l9 L. H. Little, IR Spectra of Adsorbed Species (Academic Press, New York, 1966), p. 365. 2o G. A. Webb and M. Witanowski, Nitrogen-NMR (Plenum Press, London, 1973), p. 204. 21 V. Bosakk, V. Patzelova, Z. Tvarbikova, U. Lohse, W. Schirmer and H. Stach, Proc. Workshop, AdForption of Hydrocarbons in Zeolites (Academy of Sciences, ZIPC, Berlin, 1979), vol. 1, p. 157. 22 H. J. Rauscher, D. Michel, D. Deininger and D. Geschke, J . Mol. Catal., 1980, 9, 369. 23 R. 0. Duthaler and J. D. Roberts, J . Am. Chem. SOC., 1978, 100, 4969. 24 L. E. Kitaev, T. Bernstein, P. Fink, D. Michel and H. Pfeifer, J. Chem. Soc., Faraday Trans. I , 25 M. Alei Jr, A. E. Florin, W. M. Litchman and J. F. O’Brien, J . Phys. Chem., 1971, 75, 932. 26 A. V. Kiselev and V. I. Lygin, IR Spectra of Surface Compounds (in Russian) (Nauka, Moscow, 1973), 27 W. J. Mortier, H. J. Bosmans and J. B. Uytterhoeven, J. Phys. Chem., 1972, 76, 650. in press. p. 99. (PAPER 1 /253)