J. Chem. Soc., Faraday Trans. I, 1982, 78, 761-769 Carbon4 3 and Nitrogen- 15 Nuclear Magnetic Resonance and Infrared Spectroscopic Investigations of Pyridine Adsorbed on Silica-gel Surfaces BY THOMAS BERNSTEIN,* LEONID K I T A E V , ~ DIETER MICHEL AND HARRY PFEIFER Karl-Marx-Universitat, Sektion Physik, DDR-7010 Leipzig, Linnkstrasse 5, German Democratic Republic AND PETER FINK Friedrich-Schiller-Universitat, Sektion Chemie, DDR-6900 Jena, Lessingstrasse 10, German Democratic Republic Received 23rd March, 1980 Carbon-13 and nitrogen-1 5 high-resolution n.m.r. measurements have been combined with i.r. spectro- scopic investigations to study the interaction of pyridine molecules with OH groups on a partially dehydroxylated and phosphorus-modified silica-gel surface. On partially dehydroxylated silica gel strong hydrogen bonds are formed between the nitrogen atoms and the protons of the hydroxyl groups.The number of active sites is ca. 1.2 nm-2. The formation of pyridinium ions was observed in the case of adsorption on phosphorus-modified silica gel. The number of ions was estimated to be 0.05 nm-*. The adsorption of pyridine on silica surfaces has been investigated in numerous papers. 1-6 There is spectroscopic evidence that a strong hydrogen-bonding type interaction between the pyridine molecules and the surface hydroxyl groups occurs. The wavenumber of the i.r. stretching-vibration band of the OH groups is lowered by ca. 800 cm-l and the n.m.r. line of the free hydroxyl protons is shifted by 7 ppm downfield under the influence of the adsorbed pyridine molecule^.^ Since the formation of a pyridinium ion was not observed the structure of the adsorption complex is of special interest.Infrared spectroscopic investigations of pyridine molecules adsorbed on silica surfaces containing POH surface hydroxyl groups indicated that a stronger interaction occurs compared with the case of pure silica^.^*^ Pohle and Fink3 detected the formation of pyridinium ions at such a modified aerosil surface. In earlier work by other authors, however, no evidence for the protonation of the pyridine molecule was found.5 In this paper we discuss the results of high-resolution n.m.r. spectra and i.r. spectra of adsorbed pyridine molecules in order to derive more detailed conclusions about the nature of the interaction and the structure of the adsorption complex.EXPERIMENTAL MATERIALS For the n.m.r. investigations pyridine molecules were used which were enriched with 95% 15N nuclei (Isocommerz GmbH, Leipzig, G.D.R.). Before adsorption the pyridine was dried t Permanent address: Moscow State University, Faculty of Chemistry, SU-I 17234 Moscow, U.S.S.R. 76 1762 PYRIDINE ADSORBED O N SILICA GEL over zeolite 3A. For the i.r. measurements the pyridine was of spectroscopic grade and contained no water impurities. The unmodified silica gel was Kieselgel according to Stahl (HR 60, Merck, Darmstadt, F.R.G.) with a specific surface area (B.E.T., N,, 77 K) of 320 mz g-l. To calculate the amount of pyridine required for one statistical monolayer a molecular area of 0.4 nm2 was taken.Before adsorption the unmodified silica-gel samples were partially dehydroxylated at 673 K (pressure loW3 Pa). The samples used for modification with PC1, at 673 K [as described in ref. (2)] were pretreated by the same procedure. The specific surface area of the modified samples (SO,-P) was 226m2g-'. The number of POH groups was ca. 50% of the total. Before adsorption the Si0,-P samples were once more activated at 673 K (pressure Pa). METHODS 13C n.m.r. spectra and I5N n.m.r. spectra were taken at 22.63 MHz using a Bruker HX 90 Fourier-transform spectrometer and at 9.12 MHz using a Bruker HX 90R Fourier-transform spectrometer, respectively. The experimental error for the shifts is kO.5 ppm and that for the linewidths & 10 Hz, unless otherwise stated (cf.table 3). The chemical shifts were referred to the neat liquid. Negative shifts are to higher field. 1.r. spectra were recorded with a UR-20 spectrometer (VEB Carl Zeiss, Jena, G.D.R.). Thin tablets (0.d. 2 cm, weight 7 mg ern-,) were placed in a quartz cell with CaF, windows in which dehydroxylation, modification, sample preparation and measurements were carried out. RESULTS AND DISCUSSION ADSORPTION ON PARTIALLY DEHYDROXYLATED SILICA GEL The coverage dependence of the observed 13C and 15N resonance shifts and of the widths of the 15N resonance lines are summarized in table 1. The spectra (cf. fig. 1) were recorded at room temperature. The influence of adsorption on the 13C n.m.r. frequencies is only slight. They do not change as a function of coverage and the shifts are of the same order of magnitude as in the case of non-specific interactions.On the other hand, the large and TABLE 1 .-CARBON- 13 AND NITROGEN- 1 5 RESONANCE SHIFTS S[C(i)] AND LINEWIDTHS A q FOR PYRIDINE MOLECULES ADSORBED ON PARTIALLY DEHYDROXYLATED SILICA GEL (AT ROOM TEMPERA- TURE; 6 IN ppm REFERRED TO THE LIQUID STATE, Avt IN Hz) 13C n.m.r.a I5N n.m.r. coverage (monolayers) S[C(2)] m(3)1 4c(4)1 6") A V; 3.79 1.30 0.79 0.62 0.41 0.16 0.08 0.04 gaseous state7 - 2.0 - 2.6 - 2.6 -2.5 - 2.6 - 2.9 - 2.6 - 2.9 - 0.7 - 0.9 0,3 - 0.3 - 0.3 - 0.2 - 0.6 - 1.7 -0.5 - 0.9 - 0.3 - 0.2 0.3 0.2 -0.1 b - - 6.2 - 15.6 -21.8 - 22.7 - 24.9 - 25.4 - 25.4 - 25.7 6.3 ~~ 20 30 35 35 30 30 30 45 C(4) / \ a Assignment of the carbon nuclei: yo i::: ; signal-to-noise ratio not sufficient.C(2) 'N'BERNSTEIN, KITAEV, MYCHEL, PFEIFER AND FINK 100 Hz H 763 FIG. 1 .-Nitrogen-15 n.m.r spectra of pyridine adsorbed on partially dehydroxylated silica gel [(a) and (b)] and on phosphorus-modified silica gel [(c) and (41; 0 is the coverage in monolayers and N is the number of scans: (a) I9 = 1.35, N = 200; (6) 19 = 0.04, N = 200000; (c) I9 = 1.69, N = 1000; ( d ) 6 = 0.33, N = 187000. coverage-dependent 15N resonance shifts to higher field (cf. also fig. 2) indicate appreciable influence by the adsorption sites on the electron density at the nitrogen atom. Moreover, the shifts are constant for coverages lower than ca. 0.5 statistical monolayers. This behaviour indicates the occurrence of a strong complex. The 15N n.m.r. shift remains constant as long as the number, N , of adsorbed molecules is smaller than the number, N,, of adsorption sites.For N > N,, a fast exchange between adsorbed and free molecules occurs giving rise to a decreasing shift.* From the bending point of the curve at ca. 0.5 statistical monolayers in fig. 2 the number of the adsorption sites is derived as ca. 1.2 nm-2. This value is in good agreement with the results obtained from the investigation of acetone adsorptions (1.4 nm-2). The results of these measurements and the order of magnitude of the 15N resonance shifts suggest that a strong hydrogen-bonding interaction between the lone-pair764 PYRIDINE ADSORBED ON SILICA GEL electrons of the nitrogen atom and the proton of the surface hydroxyl group occurs. The very low values for the resonance shifts of the 13C nuclei which are not in the vicinity of the nitrogen atom are consistent with this explanation because a specific interaction between the n-electron system of the molecule and the surface should give rise to considerable 13C resonance shifts of all nuclei in the ring.A strong interaction with Lewis-acid sites in unlikely to occur since such sites were 8 (monoiayers) FIG. 2.-Nitrogen-1 5 resonance shifts for pyridine molecules adsorbed on partially dehydroxylated ( x ) and phosphorus-modified (0) silica gel. Observed shift 6 (ppm) plotted against coverage B (monolayers). not observed for silicas pretreated at 673 K2 (cf. also the i.r. results described below). In the i.r. spectra of pyridine adsorbed on the partially dehydroxylated silica gel we observed bands which are typical of the hydrogen-bonded molecule in the region 1400-1700 cm-l (cf.fig. 3). The different complexes formed with adsorbed pyridine molecules [hydrogen bonds (HPy), interaction with Brarnsted-acid sites (pyridinium ion, BPy) and with Lewis-acid sites (LPy)] can be identified with the aid of the typical vibrations vSa, vlga and vlgb. Table 2 shows the values of these vibrations for the various complexes. The experimental results for adsorbed pyridine are listed in table 3. With increasing number of adsorbed pyridine molecules the extinction of the HPy bands also increases (bands at 1446, 1490 and 1595 cm-l, cf. fig. 3). Simultaneously the extinction of the stretching vibration band, vOH, of the free silanol groups decreases and a shifted and broadened stretching-vibration band appears (cf.fig. 3). In our measurements its shift is ca. 770 cm-l, which is in accordance with the values observed by other authors. This value is typical of a strong hydrogen-bonding interaction between the pyridine molecules and the surface hydroxyl groups. The species HPy was already desorbed at room temperature, and at 373 K the desorption was complete. The broad band at ca. 3680 cm-l is due to the vicinal silanol groups, which are hydrogen-bonded to each other. This band partially overlaps the intense stretching band of the free OH groups.BERNSTEIN, KITAEV, MICHEL, PFEIFER AND FINK J I 1 ~ 3747 6 5 3680 765 wavenumberlcm-' FIG. 3.--I.r. spectra of pyridine adsorbed on partially dehydroxylated silica gel: ( 1 ) silica gel pretreated at 673 K ; pyridine pressure: (2) 70, (3) 270 and (4) 660 Pa; ( 5 ) evacuated at room temperature; (6) evacuated at 373 K.The occurrence of complexes of types LPy and BPy could not be inferred from the spectra of unmodified specimens. ADSORPTION ON Si0,-P Resonance shifts and linewidths for the n.m.r. measurements are summarized in table 3. Owing to the decreasing intensity and the increasing linewidth, 13C resonance lines could not be detected for coverages c 1.2 statistical monolayers at room temperature. Therefore we recorded the 13C spectra at 373 K. However, 15N n.m.r. measurements could still be performed at room temperature. The 15N resonance lines (cf. fig. 1) are more strongly shifted to higher field (cf.also fig. 2) and the linewidths are larger (cf. tables 1 and 3) than for unmodified silica gel. The enlargement of the linewidths occurring, especially for low coverages, indicates the smaller mobility (by a factor > 2) of the pyridine molecules owing to the stronger interaction with the OH groups on the modified silica-gel surface. Infrared spectroscopic investigations of phosphorus-modified silicas1' 9 l8 indicated a larger acidity of surface POH groups compared with SiOH groups on unmodified silica. Pohle and Fink3 observed a stronger decrease in the wavenumber of the vOH band of the POH groups under the influence of pyridine molecules adsorbed on phosphorus-modified aerosil. The conversion of a proportion of pyridine molecules to protonated species leads to drastic changes in the n.m.r.spectrum (cf. the last row in table 3). We observe a strong 15N resonance shift in that direction where in general the resonance lines of the pyridinium ion appear, but the shift is still much less than the value for the ion (SI = - 115 pprnl6* 19). The changes in the 13C n.m.r. spectrum (recorded at a higher temperature) are small.766 PYRIDINE ADSORBED O N SILICA GEL TABLE 2.-cHARACTERISTIC I.R. VIBRATIONS OF PYRIDINE IN VARIOUS COMPLEXES” 3* ’-12 AND WAVENUMBERS OF ADSORBED PYRIDINE FOR 930 Pa PYRIDINE PRESSURE (a) AND AFTER 4 h DESORPTION AT ROOM TEMPERATURE (b) neat liquid hydrogen-bonding interaction (HPy) pyridinium ion (BPy) interaction with Lewis-acid sites (LPy) adsorbed on partially dehydroxylated silica gel adsorbed on Si0,P 1439 1440- 1448 1535-1 550 1445- 1460 (a) 1446s (b) 1448 w (a) 1448 w 1535 m 1552 m (b) 1535 m 1\552 m 1482 1580 1 48 5- 1 490 1 58 5- 1 600 1484- 1490 1638- 1640 1488-1500 1605-1635 1490 w 1595 s 1580 (shoulder) 1597 w 1493s 1598w 1630 w 1640 w 1493s 1630w - 1640 w - - - a Intensities: s, strong, m, medium, w, weak.Assignment of the characteristic vibrations follows ref. (1 3) and (14). TABLE CARBON-^^ AND NITROGEN-15 RESONANCE SHIFTS AND LINEWIDTHS FOR PYRIDINE MOLECULES ADSORBED ON Si0,-P (13C N.M.R: 373 K, 16N N.M.R.:ROOM TEMPERATURE; FOR DEFINITION OF SYMBOLS Cf. TABLE 1) coverage 13C n.m.r 15N n.m.r (mono- layers) 4C(2)1 m 3 ) 1 4c(4)1 W) A V+ 5.08 - 0.9 -0.2 0.1 - 2.7 29 1.68 - 2.2 0.4 0.5 - 18.8 47 1.19 - 2.2 0.5 0.4 - 26.6 40 0.8 1 - 3.7 - 0.6 0.5 - 26.6 40 0.61 - 3 k l 0+1 o+ 1 -28f1 55f 15 0.31 - 4 f 2 Of2 2 f 2 -31f2 140+20 pyridinium - 7.7 5.0 12.4 -115.1 ion in solution, ref.(15) and (16) These data show that the basis for the interpretation of the resonance shifts is a fast exchange between the adsorbed pyridine molecules (resonance shift SM) and their protonated form (pyridinium ion, resonance shift dl, relative number pI). Hence, the observed resonance shift dabs is given by Sobs = PI s* + (1 --PI) 6 M .BERNSTEIN, KITAEV, MICHEL, PFEIFER A N D FINK 36\45 l J ' 2 767 1 1 1 I 1400 1600 wavenumber/cm -' FIG. 4 . 4 . r . spectra of pyridine adsorbed on Si0,-P: (1) silica gel after modification and activation at 673 K; pyridine pressure: (2) 130, (3) 400, (4) 660 and (5) 930 Pa; spectra taken after evacuation at (6) room temperature, (7) 473 and (8) 573 K.The formation of pyridinium ions in decationated zeolites was investigated by means of 13C n.m.r. and 15N n.m.r. spectroscopy in ref. (6) and (19), respectively. With decreasing pore-filling factor the observed resonance shift, dabs, approaches a value which is typical for the pyridinium ion. However, in contrast to the measurements of Gay,20 who studied the formation of pyridinium ions on pure silicas by the addition of a known number of HCl molecules (up to a molar ratio of pyridine to HCl > l), in ref. (6) and (1 9) a very strong broadening of the observed carbon- 1 3 and nitrogen- 1 5 resonance lines occurred in the region of p I values where all molecules were protonated. This broadening is due to the much lower mobility of the protonated species.,' The broadening of the 15N resonance line of pyridine adsorbed on Si0,-P surfaces also indicates a reduction in the molecular mobility.Owing to the large widths and the smaller surface areas of the Si0,-P specimens, measurements at coverages 0.3 monolayers could not be performed in spite of the high enrichment with nitrogen-1 5 nuclei. Taking the resonance shift dobs = -31 ppm observed for a coverage of 0.3 statistical monolayers, together with the values dM = -26 ppm (for adsorbed pyridine molecules in unmodified silica-gel samples) and = - 115 ppm, we derive the number, N,, of pyridinium ions formed on Si0,-P surfaces as NI < 0.05 per nm2. In fig. 4 i.r. spectra for the adsorption on Si0,-P are shown.Besides the OH stretching vibration band of the free SiOH groups (qOH = 3747 cm-l), a band due to the free POH groups appears (FOH = 3668 em-l). The additional weak band at 3645 cm-l may be attributed either to the occurrence of another type of POH group [e.g.768 PYRIDINE ADSORBED ON SILICA GEL geminal groups, P(OH),, hydroxyl groups on clusters of phosphorus oxide] or to species of the type - formed after the oxidation of trivalent phosphorus. According to the i.r. spectra the concentration of POH surface groups is of the same order of magnitude as the concentration of free SiOH groups. If pyridine molecules are adsorbed on Si0,-P there are clear differences relative to pure silica. Even at the lowest adsorbate pressures we observed bands (at 1552 and 1490 cm-l) characteristic of the pyridinium ion (cf.fig. 4 and table 2). Their appearance is accompanied by a strong decrease in the extinction of vOH of the free POH groups but by only a small change in the wavenumber, FoH, for the free SiOH stretching vibration. With increasing pyridine pressure we obtain the band characteristic of LPy complexes (1630 cm-l) and for a pressure > 530 Pa the bands characteristic of the HPy species appear. The appearance of a further band at 1535 cm-l also points to protonated pyridine molecules. The difference in wavenumbers (1 535 and 1550 cm-l) could be due to the different types of POH groups already mentioned. Desorption at room temperature removed the HPy species completely. The BPy and LPy species could be removed completely only at a temperature of 573 K (cf.fig. 4). Simultaneously, the extinction of the vOH band for the free POH groups increases, which indicates the participation of the POH groups in the formation of the pyridinium ions. Low et aL5 did not observe protonated molecules for Cabosil pretreated at 873 K. Since pyridinium ions can be detected unambiguously under our conditions this result5 suggests that Brsnsted-acid sites are removed after pretreatment at 873 K. CONCLUSIONS The specific interaction between pyridine molecules and adsorption sites on partially dehydroxylated silica gel occurs via the formation of hydrogen bonds. These bonds are formed between a proton of the hydroxyl group and the lone-pair electrons of the nitrogen atom and causes a strong nitrogen-15 resonance shift to higher field.The only small changes observed in the carbon-13 n.m.r. spectra indicate that no specific interaction between the n-electron system and surface sites occurs. In the i.r. spectrum the characteristic bands of the hydrogen bond appear. The formation of pyridinium ions was observed in the case of phosphorus-modified silica gel. 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