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GERT CLEMENT HELMUT KNOZINGER WALTER STAHLIN BENNO STÜBNER Institut für Physikalische Chimie Universitát Müchen Sophienstr. 11, 8 Müchen 2 WEST-GERMANY MOBILE PROTONS ON SILICA AND ALUMINA SURFACES PROTONES MOVILES SOBRE SUPERFICIES DE SILICA Y DE ALUMINA 1. INTRODUCTION The existence of mobile protons on oxide surfaces and in H-bonded adsorbates on oxide surfaces is well established for various systems (1). Physical techniques such as infrared spectroscopy (2-6), electrical conductivity measurements (7-9) and pulsed nuclear magnetic resonance (4, 10) have been applied for the detection of these mobile protons., The Brmnsted acidity of oxide surfaces and the existence of mobile protons are most probably related phenomena. Mobile protons may be provided by the surface hydroxyl groups of the oxide surface or they may alternatively be created only on chemisorption of suitable molecules, such as water, alcohols or certain hydrocarbons. The nature and surface properties of the oxide surface are therefore determining factors. It thus seems reasonable to compare such different surfaces as those of silica and alumina with respect to the behaviour of possible mobile protons in adsorbed layers on them. Using various physical and chemical techniques for the deter on of mobile protons, systems of adsorbed pyridines and alcohols on silica and alumina surfaces have been studied and the results are compared and discussed in the present paper. 2. EXPERIMENTAL 2.1. METHODS Gravimetric adsorption studies were carried out using a SARTORIUS micro balance model 4102 (sensivity 1 gig). A vacuum cell, which allowed the «in situ* heat treatment of the adsoçbent, was used for infrared transmission spectroscopy. Cell and procedures have been described in detail elsewhere (11, 12). The spectrometer was a PERKIN-ELMER apparatus type 225. Spectral parameters were adjusted so that the resolution was 2-3 cm -1 in the range between 1000 and 4000 cm -1 . Dielectric loss measurements were carried out with an ac bridge type VKB from RHODE & SCHWARZ in the frequency range 50 Hz to 300 kHz.The cell again allowed «in situ» heat and vacuum treatment. Details of the cell are described in (13). A teráohmmeter from KNICK was used for dc conductivity measurements. A cylindrical oxide pellet was used for these experiments, the fronts of which were contacted with platinum («Polierplatin» from DEGUSSA) and kept between platinum contacts at constant charge. Again the samples could be treated and kept under well defined atmosphere in the conductivity cell, which is described in (14). Some catalytic tests have been performed using the conventional microcatalytic pulse technique. 2.2. MATERIALS The adsorption of pyridines on silica and of alcohols on alumina leads to a continuous infrared absorption, whereas the phenomenon cannot be observed in the systems pyridine /alumina and alcohol /silica. Furthermore, the adsorption on alcohols and of water on alumina produces a protonic electrical conductivity and increases the catalytic activity for the cyclopropane isomerization. These observations are explained on the basis of the acid properties of the respective surfaces and the basicity of the adsorbates. The silica used was AEROSIL 200 from DEGUSSA with a nitrogen BET surface area of 190 m 2 /g. The aluminas were either a 5-phase (DEGUSSA P 110 C 1) with BET surface area of 100 m 2 /g or an 11-Al2 03 prepared from aluminium isopropoxide. This latter alumina had a surface area of 180 m 2 /g. The liquid adsorptives were generally dried over LINDE molecular sieve 3 A. Cyclopropane was 99,5 $ pure from BAKER Chemicals. 133 provided by the coordinately bonded alcohol molecules. Clearly, the coordination bond in species I strongly polarizes the alcoholic 0-H bond and thus facilitates the dissociation of the alcohol molecule. The number of mobile protons increases with decreasing hydroxyl content, so that the optical density of the continuous absorption is enhanced simultaneously. The surface hydroxyl groups are ruled out as possible excess proton sources because of their extremely low acidity, which is even too low for proton transfer to the strong base pyridine. ,111111111111l11111 IINII rimumm ^ ,,,^^1 ^^^^\^.^^^^ 4 , n►^r^nn^ ^nnnn^`n_nn ^ ^^ri^+l^I!! n ^•y^^.Fi 2500 MOO3000 11100 r00no0 ,110o Fig. 1 1R-spectra of isobutanol adsorbed on 7 -1 -Al203 (pretreated at 500 °C) --- background, dotted, dash-dotted and solid line corresponding to increasing coverage __ __ ,-0 40 70)_ If this model applies one should also expect an increased protonic conductivity in the adsorbed phase on the alumina surface. In fact, dielectric loss measurements on an 77 -Al203 sample (thermally pretreated at 500 ° C) clearly gave strong conductivity losses in the frequency range between 0.1 and 5 kHz, which increased with increasing isobutanol coverage and with increasing temperature. The dc conductivity of an r7-Al 203 pellet pretreated under analogous conditions, also increased on isobutanol adsorption, although for detailed quantitative studies the overall resistance was still too high regarding the range and sensitivity of our measuring device. Detailed studies have therefore been carried out using H2 O and D 2 0 as adsorbates. Again, the conductivity of the alumina pellet strongly increased with increasing H2 0 coverage and it depends sensitively on the temperature (fig. 3). A pronounced deuterium isotope effect is observed, the conductivity being lower by a factor 3.2 for D 2 O than for H2 0 adsorbed on 77- Al e 03 at 80 ° C. This clearly indicates the protonic character of the surface conductivity and their is no doubt that the same is true for the alcohol system, the conductivity being lower because of the lower mobility of the respective anions and because of the slower rotational reorientation of the alcohol molecules in the adsorbed aggregates. Following a procedure described by Fripiat et al. (8), the degree of water dissociation on the alumina surface could be estimated to be roughly 6-7 `ó at 130 ° C. This value is higher than that obtained by Fripiat et al. (B) for water on glass surfaces at 25 ° C by a factor of appr. 6 and obviously the degree of dissociation is greatly enhanced for the adsorbed water as compared to bulk liquid water. Clearly, the coordination of water molecules onto Lewis acid sites must be responsible for this phenomenon. .— o • 34 a.,8 ^ „100° • 230°C \176°C b 14 k•^ 132°C o N 285° •Q 17. a, Ó cr 13 rn o 28 ° ^ \ ó P ^ u a, o 130° E 22 ^o 12 ^ o 11 16 0 2 3 f Torr1 5 7 9 11 mg H 2 O/g Al 20 3 Fig. 2 Fig. 3 Adsorption isotherms for isobutanol on 11 -Al203 (pretreated at 500 ° C1 dc-resistance of an 71-A/2 03 pellet as a function of temperature and water coverage The following model for the adsorbed phase may be put forward: chains or aggregates, the size of which is determined by the surface coverage, are bound to the surface via H-bonds. Coordinated alcohol molecules donate excess protons into these aggregates, i.e. strongly polarizable H-bonds are formed. 134 Finally, the possible importance of mobile protons which are created by the coordination of alcohol molecules onto incompletely coordinated Al 3+ sites, for the catalytic activity has been tested. The cyclopropane isomerization is a typical proton activated 3. RESULTS AND DISCUSSION 3.1. ADSORPTION OF PYRIDINES AND ALCOHOLS ON SILICA 3.1.1. PYRIDINES It has been shown in a previous publication (6) that pyridine and substituted pyridines are H-bonded to surface silanol groups. This bond type gives rise to typical perturbations in the OH stretching region of the infrared spectra. Additionally, a continuous absorption extends from the OH stretching band towards lower wavenumbers. The optical density of the continuous absorption increases with increasing basicity of the pyridine and decreases on substitution of bulky substituents in ortho position. This continuous absorption is strong evidence for the existence of mobile protons which may be formed by a transfer of protons from silanol groups to pyridine molecules which in turn may form Py—H+... Py dimers. The excess proton can thus move in a double minimum potential between the two nitrogen atoms. The origin of the continuous absorption in the infrared spectrum is related to such structures, and its appearance has also been reported for other systems (2-5). Details regarding its origin are discussed by Zundel 115, 16). Three main effects may contribute to the continuous absorption. Firstly, the protons in H-bonds may undertake correlated motions, a phenomenon which seems to predominate in uncharged bridges with double minimum potential well (16); secondly, a coupling of the 0—H vibration with other molecular vibrations may occur; and thirdly, H-bonds with symmetrical potential wells may develop extraordinarily high polarizability, so that they strongly interact with external electric fields. This latter effect appears to be predominantly determining the continuous infrared absorption in the case of charged bridges (16).Thus, H-bonded systems with excess protons adsorbed on oxide surfaces should fall under this category. The H-bonds will be polarized in the surface electric field, the degree of polarization being dependent on the distance from the surface and the orientation on the surface of the respective H-bridge. A continuous absorption in the infrared will result due to the variation of the shapes of the double minimum potentials. The optical density of the continuous absorption must increase with increasing excess proton density in the H-bonded structures, as observed for pyridines adsorbed on silica. 3.1.2. ALCOHOLS H-bonding of alcohols on silica surfaces leads to the formation of complex H-bonded structures (1, 17). The most important result of infrared spectroscopy which relates to the present context, is the fact that in these systems a continuous absorption does not appear (13). The silanol groups of silica surfaces are only slightly acidic. Proton transfer to a strong base molecule such as pyridine may thus oe possible but accursto only very low extent in the case of adsorbed alcohols, which are Weak bases. The excess proton density and the number of strongly polarizable H-bonds in the H-bonded alcohol aggregates on a silica surface must therefore be very low and a continuous absorption cannot be detected. 3.2. ADSORPTION OF PYRIDINES AND ALCOHOLS ON ALUMINA 3.2.1. PYRIDINES Previous studies have clearly shown that pyridine adsorption on alumina surfaces leads to H-bonded and coordinated species, but the pyridinium ion PyH + has never been detected (18). This is due to the low acidity of hydroxyl groups on alumina surfaces (18). The strong coordination interaction of pyridines occurs on Lewis acid sites (incompletely coordinated surface Al 3+ ions). Existence and strength of these sites critically influences the chemisorption of alcohols. 3.22. ALCOHOLS Alumina possesses catalytic dehydration activity. lsobutanol has therefore been chosen for adsorption studies because of its low reactivity as a primary alcohol. On adsorption of isobutanol on rl-Al20 3, a slow chemisorption process occurs which gives rise to the formation of surface carboxylate species as evidenced by the appearance of the intense symmetric and antisymmetric stretching vibrations of the C00 - -group at 1475 and 1565 cm -1 in the infrared spectra (see fig. 1). The carboxylate species contributes to the «irreversible» chemisorption (fig. 21. The extent of the «irreversible» chemisorption amounts to 3.15, 2.8 and 1.6 molecules/100 4 2 at 70, 100 and 130 ° C, respectively, on an x —Al 2 0 3 , which was dehydroxylated at 500 ° C (4.3 OH's/100 A 2 and 4.15 Lewis sites/100 A 2 1. Increasing OH density reduces the irreversibly chemisorbed amount, indicating that Lewis acid sites are involved in the irreversible chemisorption process. Besides the carboxylate species, coordinatively adsorbed alcohol molecules contribute to the irreversibly held quantity. This species may be schematically represented by R^ ,H and it was already postulated previously for alumina surfaces (1, 17). In the case of isobutanol, the use of the deuterium substituted molecule (CH 3) 2 CH CD2 OH readily indicated the formation of this species by the shift by 40 cm -1 toward lower wavenumbers of antisymmetric and symmetric—CD2 — methylene stretching vibrations (bands at 2100 and 2200 cm -1 shift to 2060 and 2160 cm -1 ). Reversible adsorption via H-bonds, thus occurs on surfaces which already contain the coordinated alcohol species. The formation of H-bonds during the reversible adsorption clearly shows up in the infrared spectra by strong broad absorptions which are centered at 3170 and 3330cm 1 (not recognizable in the representation of fig.1). Additionally, as shown in fig. 1, a strong continuous absorption extends over the entire accessible spectral range. The optical density of this continuous absorption increases with increasing alcohol coverage for a surface of fixed Lewis acid site density and increases for fixed alcohol coverage with increasing Lewis acid site density, i. e. with increasing number of coordinately adsorbed alcohol molecules. These observations obviously demonstrate, that the mobile protons which give rise to the continuous absorption must be 135 BIBLIOGRAPHY reaction and a non-classical carboniumion of structure II C H2 CH2 CH2 II ^ . \. .^ + i ^ H' has been proposed as an intermediate (19, 20). Pure and unmodified aluminas are known to only possess low activity for such reactions because of their low intrinsic BrOnsted acidity. The modification of a partially dehydroxylated n-AI2 03 surface with the irreversibly retained amount of isobutanol which has been shown to provide acidic protons, should then enhance the activity of the surface for cyclopropane isomerization. Some pulse experiments were carried out under well defined standard conditions at 130 ° C using fl-Al 2 0 3 (thermally pretreated at 500 ° C). The conversion of cyclopropane to propylene was significantly enhanced after modification of the surface by the irreversible isobutanol chemisorption at 130 ° C by a factor of 10 from only 2 % for the untreated surface to 20 % for the modified surface. The catalytic test again provides strong evidence for the creation of acidic (mobile) protons on coordination of the alcohol molecule onto Lewis acid sites. 4. CONCLUSIONS The low intrinsic acidity of the silanol groups on silica surfaces allows for a protonation of sufficiently strongly basic adsorbates, such as pyridines or other strong nitrogen bases. The protonated species may then form aggregates with excess protons. The charged H-bonds such as e. g. N-H+... N are strongly polarizable (15, 16) and give rise to a continuous infrared absorption due to their interaction with the surface electric field. The basicity of alcohols, on the other hand, is too low, so that an appreciable proton transfer from silanol groups to the adsorbate is not detectable by infrared spectroscopy. Alumina on the contrary does not provide sufficiently acidic protons to protonate pyridines. However, the strong Lewis acidity of the surface is responsible for the creation of acidic protons on adsorption of hydroxyl bearing adsorbates. These protons are in turn responsible for the infrared continuous absorption, the surface electrical conductivity and the enhanced catalytic activity for proton catalyzed reactions. 1. H. KNOZINGER, in «The Hydrogen Bonding - Recent Developments in Theory and Experiments», P. Schuster, G. Zundel and C. Sandorfy, Eds., North Holland Publ. Comp. p. 1263 (1976). 2. N. W. CANT, and L. H. LITTLE, Canad. J.. Chem.,42, 802 (1964). 3. M. J. D. LOW, and V. V. SUBBA RAO, Canad. J. Chem., 47, 1281 (1969). 4. J. J. FRIPIAT, C. VAN DER MEERSCHE, R. TOUILLAUX, and A. JELLI, J. Phys. Chem., 74, 382 (1970). 5. H. NOLLER, B. MAYERBOCK, and G.ZUNDEL,SurfaceSci., 33, 82 119721. 6. H. KNOZINGER, Surface Sci., 41, 339 (1974). 7. A. SOFFER, and M. FOLMAN, Trans. Faraday Soc., 62, 3559 (1966). 8. J. J. FRIPIAT, A. JELLI, G. PONCELET, and J. ANDRE, J. Phys. Chem., 69, 2185 119651. 9. J. H. ANDERSON, and G. A. PARKS, J. Phys. Chem., 72, 3662 (19681. 10. M. I. CRUZ, W. E. E. STONE, and J. J. FRIPIAT, J. Phys. Chem., 76, 3078 119721. 11. H. KNOZINGER, H. STOLZ, H. BÜHL, G. CLEMENT and W. MEYE, Chem.-Ing: Techn., 42, 548 (1970). 12. H. KNOZINGER, Acta Cient. Venez., 24, Supl. 2, 76 (19731. 13. W. STAHLIN, Thesis, University of Munich (1976). 14. G. CLEMENT, Thesis, University of Munich (1966). 15. G. ZUNDEL, «Hydration and Intermolecular Interaction», Academic Press (1969). 16. G. ZUNDEL,in «The Hydrogen Bonding-Recent Developments in Theory and Experiments», P. Schuster, G. Zundel and C. Sandorfy, Eds., North Holland Publ. Comp., (1976), in press. 17. H. JEZIOROWSKI, H. KNOZINGER, W. MEYE, and H. D. MOLLER, JCS Faraday, 169, 1744 (1973). 18. H. KNOZINGER,Adv. Catalysis, 25, 184 11976). 19. J. W. HIGHTOWER, and W. K. HALL, J. Amer. Chem. Soc., 90, 851 (1968). 20. Z. M. GEORGE, and H. W. HABGOOD, J. Phys. Chem., 74, 1502 (1970). RESUMEN ACKNOWLEDGEMENTS Financial support of this work by the Deutsche Forschungsgemeinschaft, the Stiftung Volkswagenwerk and the Fonds der Chemischen Industrie is gratefully acknowledged. The authors wish to thank Professor G. Zundel for his very valuable suggestions. 136 La adsorción de piridinas sobre s/lice y de alcoholes sobre alúmina produce una absorción continua en espectros infrarojos, mientras que este efecto no está observado en los sistemas piridina/alúrnina e alcohol/si/ice. Además, la adsorción de alcoholes y de agua sobre alúmina produ ce una conductividad eléctrica protónica y aumenta la actividad catalítica para la isomerisación de ciclopropano. Se explica estas observaciones por las propiedades ac/dicas de las respectivas superficies y por la basidez de los adsorbatos DISCUSSION J. M. HERRMANN : El doctor KNOZINGER puede decirnos algunos detalles sobre el proceso de la conductividad de la superficie de la alúmina en contacto de agua ? Tiene también una idea sobre la forma cargada del agua molecular, si ella existe, en la superficie de la aluminada ? Por fin, las variaciones de conductividad no pudieran ser debidas a efectos físicos de contactos eléctricos entre la muestra y los electrodos en presencia de agua ? H. KNOZINGER : No tenemos evidencia experimental sobre la estructura del proton solvado en la superficie, pero parece razonable que la especie H5 02 existe sobre la superficie tal como en soluciones aquosas. El mecanismo de la conductividad tambien debe ser comparable con la cúal que es operando en soluciones, es decir el protón en el puente de hidrógeno preferiblemente coge la posición cerca de la molécula en dirección del campo eléctrico, y después de una reorganización de Ias moléculas en la capa hidratante este proceso puede repetirse. No se observo ningunos efectos físicos de los contactos eléctricos entre muestra y eléctrodos. El contacto es perfecto debido a la preparación de los contactos de platino sobre Ias pastillas, como descrito en el parte experimental. J. CUNNINGHAM : I am very much interested in the model which Professor Knózinger describes and which envisages the possibility that adsorption and polarization of iso-butanol orlo coordinatively unsaturated metal ion sites can greatly enhance the acidity of the RO-H bond. My two questions concern the influence which surface concentration of adsorbed alcohol may have upon the fate of such acidic RO-H groups : firstly, does Professor Kni5zinger have data on the minimum surface alcohol concentration at which mobility of the proton between adjacent alcohols becomes possible ? , and secondly, is there not a real danger that such acidic protons will had to dehydrater between adjacent alcohol molecules ? H. KNOZINGER : 1) We cannot exactly determine the surface coverage at which protonic conductivity becomes possible. It seems, however, that appreciable conductivity is not observed, unless sufficiently large chains or clusters of alcohols and H2 0 have formed. Thus, the formation only of the coordinated species apparently does not lead to measurable surface conductivity. 2) Certainly dehydration may take place, but this depends on surface pretreatment, alcohol structure and adsorption temperature. Under our experimental conditions, using isobutanol at temperatures below 130 °C no dehydration could be detected. 137