Download mobile protons on silica and alumina surfaces protones moviles

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.
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1R-spectra of isobutanol adsorbed on 7 -1 -Al203 (pretreated at
500 °C)
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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.
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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).
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(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).
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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