J. Phys. Chem. 1984,88, 905-912
905
Bridging and Terminal Hydroxyls. A Structural Chemical and Quantum Chemical Discussion W. J. Mortier,* Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde. Katholieke Universiteit Leuven, Kard. Mercierlaan 92, B-3030 Leuven (Heverlee), Belgium
J. Sauer, Zentralinstitut fur physikalische Chemie, Akademie der Wissenschaften der DDR, DDR- 1 1 99 Berlin, Rudower Chaussee 5, German Democratic Republic
J. A. Lercher, and H. Noller Institut fur physikalische Chemie, Technische Universitat Wien, Getreidemarkt 9, A - 1060 Wien, Austria (Received: April 19, 1983; In Final Form: July 8, 1983)
Structural chemical arguments and results of nonempirical quantum chemical calculations are used to explain the difference in properties of bridging and terminal silanol groups. In this way, greater acidity and sensitivity toward adsorbed molecules or composition are predicted for bridging hydroxyls.
Introduction Proton-donor sites or electron-acceptor sites at solid surfaces, Le., Bronsted or Lewis sites, respectively, are at the origin of the acidity of heterogeneous catalysts. Hydroxyl groups are responsible for the Bronsted acidity of amorphous silicoalumina and of zeolites. Acid strength as well as the number of sites contribute to the total acidity. Although it is generally recognized that acidity is correlated with the catalytic proper tie^,'-^ little progress has been made in rationalizing these: merely because of the complex nature of the reactions, which involve structural, electronic, and collective properties of the solid and of the interacting molecules. The four-connected network formed by the corner-sharing of (Si04)&tetrahedra is stoichiometrically balanced, as in the quartz polymorphs. Silica hydrogel consists of incompletely polymerized silicic acid, Si(OH)4, and contains a considerable amount of free hydroxyls terminating the polymer network which are called terminal hydroxyls. This type of hydroxyl is also present in other forms of silica and in zeolites, at all sites where the aluminosilicate polymer is interrupted, Le., at the surface of the crystallites or at defect sites. The isomorphous substitution of the formally 4+ charged Si4+ by A13+ in the zeolite framework necessitates the presence of extraframework cations for charge neutralization. By acid exzeolites, these change or by deammoniation of ",-exchanged cations can be replaced by protons, attached to framework oxygens. The negative framework charge is then neutralized by the formation of bridging hydroxyls at the Si-O-A1 oxygen bridges as Si-OH-AI. Depending on the AI content, the sample type, and the pretreatment conditions, these are stable and dehydroxylation occurs mostly not below 400 OC. In amorphous silicoalumina, tetracoordinated A1 is also found, but the structural arrangement and the charge neutralization are different from the situation in zeolites. Cloos et aL5proposed that a central core with negative residual charge (due to tetrahedrally coordinated AI3+in the silicate network) is balanced by octahedrally coordinated hydroxyaluminum cations coating around this (1). Ward, J. W. In "Zeolite Chemistry and Catalysis"; Rabo, J. A,, Ed.; American Chemical Society: Washington, DC, 1976; ACS Monogr. No. 171, p 118. (2) Barthomeuf, D. ACS Symp. Ser. 1977, No. 40, 453. (3) Jacobs, P. A. 'Carboniogenic Activity of Zeolites"; Elsevier: Amsterdam, 1977. (4) Barthomeuf, D. In "Catalysis by Zeolites"; Elsevier: Amsterdam, 1980; p 55. (5) Cloos, P.; Leonard, A. J.; Moreau, J . P.; Herbillon, A,; Fripiat, J . J. Clays Clay Miner. 1969, 17, 279.
0022-3654/84/2088-0905$01.50/0
core. From the linear variation of the AIV'-O and AIIV-O vector multiplicities in the radial electron density distribution, Leonard et aL6 conclude that the atomic arrangement of AIV1is maintained at all compositions and that the relative amount of tetracoordinated A1 vs. sixfold-coordinated AI is constant. From NH3 adsorption experiments and the formation either of NH4+ at Bronsted sites or of chemisorbed N H 3 at Lewis sites, Fripiat et al.' concluded that the Bronsted acid sites disappear gradually with increasing dehydration temperature. It is therefore most probable that pretreated amorphous silicoalumina does not contain the bridging hydroxyls at its surface. The differences in catalytic activity of 1-4 orders of magnitude between zeolites and silicoaluminaS justify a more detailed examination of the nature of the hydroxyl groups. Two sources of information to attain this goal will be used: (i) structural-chemical arguments, as these were rationalized by Gutmann's rules for electron pair donor-acceptor interactions, and (ii) nonempirical quantum chemical model calculations which may provide more specific data on the intrinsic properties of terminal and bridging hydroxyl groups. After a preliminary outline of the structural chemical argumentation and of the quantum chemical results, a general discussion of the properties of the bridging vs. terminal hydroxyls will be made.
Bond Length Variations and Crystal Chemical Properties Bond length variations are associated with charge density shifts and will therefore influence the atomic properties. The rules, as established by G ~ t r n a n nto ~ ~rationalize '~ molecular interactions, may also contribute considerably to the understanding of the bonding in crystals. B a d ' came to similar conclusions to explain the bond length variation of coordination polyhedra in inorganic crystals using the bond strength received by the anions, but Gutmann's rules are more generally applicable. In inorganic crystals, we may describe the interactions between atoms of different electronegativities on the basis of electron-pair-acceptor (EPA)-electron-pair-donor (EPD) interactions, causing charge shifts and polarization of the bonds. (6) Leonard, A. J.; Ratnasamy, P.; Declerck, F. D.; Fripiat, J. J. Discuss. Faraday SOC.1971, 52, 98. (7) Fripiat, J. J.; Leonard, A.; Uytterhoeven, J. B. J . Phys. Chem. 1965, 69, 3274. (8) Fraissard, J . In 'Catalysis by Zeolites"; Elsevier: Amsterdam, 1980; p 343. (9) Gutmann, V. 'The Donor-Acceptor Approach to Molecular Interactions"; Plenum Press: New York, 1978. (10) Gutmann, V. Pure Appl. Chem. 1979, 51, 2197. (1 1) Bauer, W. H. Trans. A m . Crystallogr. Assoc. 1970, 6, 129.
0 1984 American Chemical Society
906
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
The first rule relates the increased bond lengthening of bonds adjacent to the EPA-EPD interaction site with the increased strength of this interaction. The second rule states that charge adjustments are made throughout the molecule, whereby shifts from a more electronegative atom toward a more electropositive atom cause bond shortening, and bond lengthening for charge transfer from a more electropositive atom to a more electronegative. This parallels an increase in bond length for an increased ionicity in the bond. The third rule predicts an increase of the bond lengths originating at a coordination center for an increase in the coordination number. This is in full agreement with the variation of “effective” ionic radii with coordination number.12 The charge transfer at the EPA-EPD site is accompanied by an extra polarization of the bond, whereby the original decrease at the donor is overcompensated by a charge transfer from the adjacent atoms in the donor molecule (pileup effect) and the original decrease of positive charge of the acceptor atom is passed over to other parts of the molecule (spillover effect). We are now in a position to make a first, but fundamental, distinction between bridging and terminal hydroxyls. For a theoretical “transition” from a terminal to a bridging hydroxyl an extra EPA-EPD interaction at the oxygen exists whereby the adjacent bonds (0-H and Si-) are weakened (first bond length variation rule). If considered as a ”two-coordinated” or a “three-coordinated” oxygen, the bonds originating at the coordination center should be longer for the bridging hydroxyls (third rule). A weaker 0-H bond immediately suggests an increased probability for proton transfer to a proton-acceptor molecule, Le., an increased Bronsted acidity for the bridging hydroxyls.
Quantum Chemical calculation^'^ Quantum chemical calculations may provide supplementary structural information as well as insight into molecular properties, certainly when these are not accessible in any other experimental way. By an X-ray analysis, it is impossible to determine the structure of surface groups in general and of terminal hydroxyls in particular. IR and N M R spectroscopic studies on the other hand yield indirect structural information based on additional assumptions. Quantum-mechanical methods will prove to be particularly useful, especially since we will deal with vibrational properties and deprotonation energies. The material will be presented as follows. At first the choice of reasonable model compounds is outlined, followed by a justification of the chosen method, whereafter the results are presented. In order to make reliable predictions for the characteristics of interest, the results obtained in this paper are supplemented by and compared with those for related molecules (H20, H,COH). In this way the final data set for use in the general discussion is prepared. Previous theoretical work in this field was mainly based on A nonempirical study” dealt with semiempirical the acidity of the silanol group and predictions of the structure of hydroxyls can be found in the papers of Gibbs et al.; l8,l9 bridging hydroxyls were omitted in these studies. Models. In order to make nonempirical calculations feasible for solids, appropriate models are required. For nonmetallic solids small molecular cluster models (Figure 1) should be sufficient (12) Shannon, R. D. Acta Crysrallogr., Seer. B 1978, 34, 1751. (13) A preliminary report of some results has been given in: Sauer, J.; Zahradnik, R.; Schirmer, W. ‘Proceedings of the Workshop on Adsorption of Hydrocarbons in Microporous Adsorbents, Eberswalde, D.D.R., 1982”, preprints. (14) Dubsky, J.; Beran, S.; Bosacek, V. J . Mol. Caral. 1979, 6, 321. (15) Mortier, W. J.; Geerlings, P. J . Phys. Chem. 1980, 84, 1982. (16) Kazansky, V. B. In “Proceedings of the Fourth National Symposium on Catalysis (Catalysis Society of India), Dec 2-4, 1978”; Indian Institute of Technology: Bombay-400076; p 14. Mikheikin, I. D.; Abronin, I. A.; Zhidomirov, G. M.; Kazanski, V. B. J . Mol. Coral. 1977/78, 3, 435. (17) Heidrich, D.; Volkmann, D.; Zurawski, B. Chem. Phys. Lett. 1981, 80, 60. (18) Gibbs, G. V.; Meagher, E. P.; Newton, M. D.; Swanson, D. K. In %ructure and Bonding in Crystals”; OKeeffe, M., Navrotsky, A., Eds.; Academic Press: New York, 1981; Vol. I, p 195. (19) Newton, M. D.; Gibbs, G . V. Phys. Chem. Miner. 1980, 66, 221.
Mortier et al.
I
H 1 b
l a
H 2 b
2 0
Y n‘
I
H
3 a/b
A‘
Y n’
n
A’
n
I
H
4 o/b
Figure 1. Molecular models for which quantum chemical calculations were carried out. The prime marks those hydrogen atoms which were introduced in order to saturate the dangling bonds connecting the clusters with the bulk of the solid. a and b refer to the protonated and deprotonated forms, respectively.
for describing intrinsic properties of the hydroxyls. The hydrogen atoms marked by a prime must be added in order to minimize boundary effects. Hydrogen atoms substitute for “a quarter of a T atom (T = Si, Al)” in models 3 and 4 or for “a half of an 0 atom” in models 1 and 2. Silanol, la, orthosilicic acid, 3a, and to some extent trifluorosilanol, F,SiOH, serve as models for terminal OH groups whereas the systems 2a and 4a model the bridging O H groups. The use of such models is in accordance with the fundamental chemical concept of functional groups. Adopting these models, we completely ignore all effects due to long-range interactions such as the surface geometry and the cage dimensions of zeolites. E.g.,considering the general geometry of the framework rings in zeolite-type structures (see, e.g., ref 20), the 0-H vector will most often point in the direction of the center of the ring. The proximity of negatively charged framework oxygens in the same ring will therefore also influence the properties of hydroxyl groups (see General Discussion). The influence of the sample composition, explicitly discussed below, is not felt by our models either. This is not a drawback, but, in this way, the environmental effects may be separated from the intrinsic properties of the individual groups, provided that the calculations are reliable. Methods. For a general review of nonempirical molecular orbital calculations (also named “ab initio calculations”) and their applications in chemistry we refer to ref 21. The success of nonempirical calculations largely depends on the selection of the proper method from a broad variety. Today, the merits and limits of various methods are well understood (see, e.g., ref 21) providing a firm basis for qualified applications. Among the simplest methods are S C F calculations employing basis sets of so-called “split-valence” quality, which means that two orbitals of each (20) Mortier, W. J. ‘Compilation of Extraframework Sites in Zeolites”; Butterworth Scientific Ltd.: Guildford, U.K., 1982. (21) Carsky, P.; Urban, M . “Ab Initio Calculations. Methods and Applications in Chemistry”; Springer-Verlag: West Berlin, 1980; Lect. Notes Chem. No. 16. Schaefer, H. F., 111. “Modern Theoretical Chemistry”; Plenum Press: New York, 1977; Vol. 4.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 907
Bridging and Terminal Hydroxyls TABLE I: Molecular Geometry, Total Energy, and OH Stretching Force Constants for Models la, 2a, and 2b Obtained from 3-21G SCF Calculations'
r(SiH) r(SiH) LOSiH LOSiH iHSiH r(Si0) r(OH) L(S1OH) L(Si0AI) r(Al0) r(AIH) r(AIH) LOAlH LOAlH LHAIH E , au ~
f o ~N ,m-'
147.6 149.0 106.9 11 2.4 108.9 167.4 95.9 127.8 160.0 160.0 120.0 -364.18059 -242.28411 89 1.6
147.9 147.1 104.5 108.5 11 1.2 173.4 96.7 119.6 130.7 192.7 162.2 161.1 96.8 102.7 117.3 -6 06.5236 95
TABLE 11: Calculated Deprotonation Energies, AE(BSSE), and Gas-Phase Acidities, AHOoa AE(BSSE)~
150.5
114.2 161.7 (1 80.0)' 175.6 166.5 110.0 -606.00655
863.2
'
Bond distances in picometers. Angles in degrees. Atomic The linear Si-0-AI units, 1 au of energy = 2625.47 kJ/mol. structure is an artifact of this type of basis set (split-valence) (cf. ref 3 1).
symmetry are used to describe the valence shell while one orbital is attributed to the inner shells. Recently, particularly effective 3-21G basis sets have been defined for all elements up to Ar.22 These were used for the majority of the calculations reported in this paper. Some are performed by employing the slightly better 4-31G basis which is unfortunately not defined for aluminum (for the silicon basis set, see ref 24). The split-valence basis sets are known to perform well for bond lengths (typical error 1 pm)22 and force constants (uniformly overestimated by 5-20%).25326 More specific information for XOH groups is included in Tables IV-VI. A limiting feature of these basis sets is their tendency to yield too large bond angles in structures as HzO and NH3. A correct description of the bonding situation of such atoms as oxygen or nitrogen is only possible if d-type functions are added to the basis sets. In the discussion we will refer to 6-31G* calculations (the asterisk denotes the presence of d functions on all atoms except H and He) for silano12' and orthosilicic acid.% The models 4 are too extended to be treated by the 3-21G SCF method. Therefore, in the discussion we will use the result^'*^^^ obtained by the simplest nonempirical method, the STO-3G method. The latter still provides valuable information on bond lengths (typical error 2 and 3 pm for AH and AB bond lengths) and bond angles (typical error 3-4°).21 Stretching force constants are overestimated by 20-30%.*l Results. In this paper we report the results of 3-21G and a few 4-31G calculations on models 1-3. For the models la, 2a, and 2b complete gradient optimizations of the molecular geometries were performed on the level of 3-21G S C F wave functions. The program system HOND0S3' was used, working with gradients of the energy with respect to Cartesian atomic coordinates. The optimization was repeated until the largest component of the gradient was less than 0.001 au. The bond distances and angles (22) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. SOC.1979, 102, 939. Gordon, M. S.; Binkley, J. S.; Pople, J. A,; Pietro, W. J.; Hehre, W. J. Ibid. 1982, 104, 2191. (23) Hehre, W. J.; Lathan, W. A.; Ditchfield, R.; Newton, M. D.; Pople, J. A. QCPE 1973, 12, 236. (24) Snyder, L. C.; Wassermann, 2.Chem. Phys. Lett. 1977, 51, 349. (25) Pople, J. A.; Schleger, H. B.; Krishnan, R.; Dcfrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R.A. In?. J. Quantum. Chem. 1981, S15, 269. (26) Blom, C. E.; Otto, L. P.; Altona, C. Mol. Phys. 1976, 32, 1137. (27) Sauer, J., unpublished. (28) Sauer, J. Chem. Phys. Lett. 1983, 97, 215. (29) SchrMer, K.-P.; Sauer, J., unpublished rcsults. (30) Dupuis, M.; Rys, J.; King, H. F. Nat Resour. Compuf. Chem. Software Car. 1980, 1 , Progr. no. Q H O ~ ( H O N D O ~ ) .
AH:,
estimatedC
molecule
3-21G
4-31G
3-21G
4-31G
1 3 F,SiOH
1640 1625
1663 1649 1516d
1480 1465
1560 1550 1415
2
1429
1265
'
Units: kJ/mol. The energy of the deprotonated system \vas calculated by using the same basis set as for the parent system. For the 4-31G optimum geometry (C,, LSiOF fixed a t the tetrahedral angle, R ( S i 0 ) = 164.9 p m , R ( 0 H ) = 93.9 p m , R(SiF) = 160.0 p m , LSiOH = 145.5"). See text f o r explanation.
obtained are presented in Table I together with the total energies and the force constants for the O H stretching vibration. The latter were calculated at the optimum geometry from a parabola fitting the energy at distances ro, ro 6 and ro - 6 (6 = 1 pm) for the O H bond. Table I1 contains the deprotonation energies. The optimum geometries of the parent systems for the specified basis sets were also taken for the deprotonated systems. The relaxation of the geometry parameters in the anionic systems H,SiO- and H3SiOAlH3- lowers the deprotonation energy by 41 (cf. ref 17) and 74 kJ/mol, respectively, as calculated from the energies of the optimized structures in Table I. The decrease in the Si0 bond length from 167 to 161 pm in H3SiO- (ref 17) and from 163 to 159 pm in (H0)3SiO- accounts for a mere 3 kJ/mol of the total energy decrease. The 4-31G total energies of (H0)3SiOH (Cs),3a, in the eclipsed
+
"
C,
(ecl i p s e d )
C, (staggered)
and staggered conformation of the hydroxyl proton are -590.101 81 and -590.101 77 au, respectively, Le., an energy difference between both conformations of 0.1 kJ/mol. Derivation of Reliable Estimates. We are now in a position to prepare the data for the General Discussion. Although the results reported in the preceding section have been obtained in a nonempirical way, they are subject to systematic errors. These errors can largely be eliminated by using the relative values for structural and energetic parameters (see, e.g., the collection of results in ref 22, 25, and 32). We furthermore intend to recommend reliable estimates of the properties of interest by calibration with accurate experimental and theoretical data for related molecules, in particular H 2 0 and H3COH. The limitations associated with small models as 1 and 2 will be estimated by comparison with results obtained by means of simpler (STO-3G) methods for larger models. T-O(H) Bond Lengths. The 4-31G calculations of models 3a and 3b predict that the Si-O(H) bond is shortened by 4 pm on deprotonation (Table 111). The comparison with the results for the models l a and l b (taken from ref 17) indicates that these simpler models overestimate the lengths of the protonated and deprotonated Si-0 bonds by 4.4 and 1.8 pm, respectively. Assuming that these increments apply to A1-0 bonds as well, the bond lengths of the models 2a and 2b (Table I) can be corrected. These results, also shown in Table 111, may be directly compared with the STO-3G results for the models 4a ( R ( S i 0 )= 167 pm, (31) Ernst, C. A.; Allred, A. L.; Ratner, M. A.; Newton, M. D.; Gibbs, G. V.; Moskowitz, J. W.; Topiol., Sid. Chem. Phys. Left. 1981, 81, 424. (32) Defrees, D. J.; Levi, B. A.; Pollack, S. K.;Hehre, W. J.; Binkley, J. S . ; Pople, J. A. J . Am. Chem. SOC.1979, 101, 4085.
908 The Journal of Physical Chemistry, Vol. 88, No. 5. 1984
Mortier et al.
TABLE 111: Influence of Protonation o n T - 0 Bond Lengths (T = Si, AI) length, pm method s c r / 4 - 3 1c SCI'/3-21 G
model
bond
protonated (a)
deprotonated (b)
difference (a - b )
la,b 3a,b increment (3 - 1 ) 2a,b t increment (3 - 1)
Si0 Si0
167.4a 163.0 -4.4 169.0 188.3 168i 1 184 t 4 . 5
160.8' 159.0 -1.8 159.9 173.8 159 5 1 171.5 T 2 . 5
6.6 4.0 -2.6 9.1 14 5 9 i 1 12.5 ? 2.0
Si0 A10 Si0 A10
estiina te recommended a
Reference 17.
TABLE IV: 0 - H Bond Lengths (pm) Obtained by Different Methods calculation
a
TABLE VI: 0 - H Stretching Vibrational Frequencies (in em-') from 3-21G Calculations
system
STO-3C
3-21C
4-31C
obsd (estimated)
HOH HJOH la 3a l',SiOH 2a
99.0' 99.1' 98.6 98.1 97.8
96.7' 96.6' 95.9
95.0' 95.0'
95.7' 96.3'
94.2 93.8 93.9
96.7
(95) (96)
STO-3G
3-21G
4-31G
6-31G*
FOH H,NOH ClOH HOH H,COH 2a la 3a I ,SiOH
101.4' 101.4' 100.2' 100.0" 103.8"
99.0' 103.6' 104.2' 107.6' 110.3' 119.6 127.8
99.7' 106.0' 105.4' 111.2' 113.2'
99.F 1O4.lc
132.2' 140.0 144.5
117.1d 116.9e
a
27
107.3 108.0 110.5
'
Reference 22. Reference 17. e Reference 28.
1O5Sc 109.5'
Reference 32.
\vi thout scaline
f = 0.9102'
H,COH H,OC 2a la
3868' 3876'~c 3931 3995
3690' 3698b,C 3750 i 1 0 3811 i 1 0
'
'
TABLE V: X-0-H Bond Angles (Degrees) Obtained by Different Methods
system
molecule
obsd 3681a 3707'~~ 3665d 3745d
Reference 25. The scaling f a c t o r f f o r the force constants is the mean value derived from the experimental frequencies for HJOH cf= 0.9056) and H,O (f= 0.9147). T= '/,(vas + u s ) is given for easier comparison. d References 34 and 35 (see text).
Reference 22
calculation
calculation
obsd (estimated) 97.2" 101.4' 102.5' 104.5' 108.0" (111.5 t 2) (115 2 ) +
Reference
harmonic force constants for bond stretching vibrations are uniformly too large by 10-20%. Therefore, it has become a commonly accepted procedure to derive scaling factors for the force constants from known frequencies and to use them for predicting unknown frequencies for related molecule^.^^ The experimental gas-phase frequencies for water and methanol require very similar scaling factors of 0.9147 and 0.9056, suggesting their transferability. The average value (0.9102) was applied to models 2a and la. Deprotonation Energies and Gas-Phase Acidities. The gasphase acidity, AHoo,is defined as the standard heat for the following reaction at 0 K: XOH
R(A10) = 179.5 pm)" and 4b (R(Si0) = 158 pm, R(A1O) = 169 p ~ n ) . The ~ ~ only notable difference appears for the A1-0 distance of the protonated form (4a). As most reliable estimates, the averages of the STO-3G and corrected 3-21G results are given in the last two rows of Table 111. 0-H Bond Lengths. The results collected in Table IV indicate that the systematic deviations are the largest for the STO-3G method. Results for further molecules can be found in ref 22. The 3-21G and 4-31G results suggest that, compared to H 2 0 and H3COH (R(0H) about 96 pm), the O H bond is shorter by 1 pm in terminal hydroxyls and has approximately the same length in bridging hydroxyls. SiOH Bond Angles. The results are presented in Table V in the order of increasing values. It is seen that at all levels the trends are reproduced properly. While STO-3G yield bond angles for oxygen which are too small, the split-valence basis set (3-21G, 4-31G) results are too large. Only for the 6-31G* basis set does the error in the bond angle not exceed 1-3O. Our recommended estimate for the terminal hydroxyls is 115 2' based on the 6-31G* result (cf. ref 28). The recommended value for bridging hydroxyls of 111.5 f 2' is based on the finding (at the 3-21G level) that the bond angle for system 2a is in between the angles for H3COH and system la. 0 - H Stretching Vibrational Frequencies. It is feasible to separate this vibration from the framework vibrations because of its much higher frequency (cf. ref 29). The 3-21G results are shown in Table VI. It has already been mentioned that the
*
(33) Sauer, J.; Engelhardt, G. Z . Narurforsch. A 1982, 37, 277. (34) Kollmann, P.; Rothenberg, S . J . Am. Chem. SOC.1977, 99, 1333.
XO-
+ Ht
It is connected with the deprotonation energy calculated for a given basis set by the following relation: AHOo
=
+ AEcorr + AEbasis + AESCF
(1)
EZpE is the zero-point vibrational energy which related AHoowith the potential energy AE. If AE is calculated by means of the SCF method employing limited basis sets, the resulting deprotonation energy, AESCF, suffers from two shortcomings: (i) the correlation energy, AE,,,,, is missing in all SCF approaches and (ii) AEbasis is basis set dependent. From the calculations accumulated in the literature34so far, we concluded that fairly reliable estimates of the relative acidities can be obtained from S C F calculations, employing split-valence basis sets like 4-31G or 3-21G. Results obtained by small basis sets can be improved by using the same basis set for the deprotonated system, XO-, as for the parent system, XOH. The corresponding energies, AE(BSSE), differ from AE by the so-called basis set superposition error (BSSE).37 As the systems under study are structurally and chemically closely related with H 2 0 and H3COH, it is supposed that, for a particular basis set, the expression
should be constant. Table VI1 shows for H 2 0 and H3COH that relation 2 holds within about *lO kJ/mol. The average values (35) Jacobs, P. A.; Mortier, W. J. Zeolites 1982, 2, 226. (36) Bartmess, J. E.; McIver, R. T. In "Gas Phase Ion Chemistry"; Bowers, M . T., Ed.; Academic Press: New York, 1979; Vol. 2, Chapter 11. (37) Boys, S . F.; Bernardi, F. Mol. Phys. 1970, 19, 553.
Bridging and Terminal Hydroxyls
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 909
TABLE VII: Comparison of Observed Heats of Deprotonation at 0 K, AH,', and Calculated Energies of Deprotonation, aESCF molecule H,O H,COH difference
Binding Geometry of the Bridging Hydroxyls. Direct evidence for the location of protons in partially decationated Y-type zeolite was given by Jirak et al.47and by Bosacek et al.48 in neutrondiffraction studies. They were found to be located at about 100 AH,n(obsd)a 1635 1586 49 pm from the bridging oxygens, on an axis bisecting the T-0-T A F F angle in the T-0-T plane. Semiempirical quantum chemical 4-3 1G 1781 1713 68 calculations by Dubsky et al.14 have indeed confirmed that this 4-31G(BSSE) 1747 1675 62 configuration is the least energetic one. For Y-type ~ e o l i t t e s , ~ ~ ~ ~ ~ 3-21G(BSSL) 1805 1741 64 the O(3) and O(1) oxygens were the preferred proton sites, hereby average confirming also the kinetics of the formation of high-frequency ~ , -o ,&CF and low-frequency bands in the IR. However, all oxygens are 4-31G -146 -127 -136.5 t 9.5 probable sites for protons. The average T-0-T bonding angles 4-31G(BSSE) -112 -89 -100.5 t 11.5 (and therefore also the T-0-H angle) in dehydrated H zeolites 3-21G(BSSE) -170 -155 -162.5 t 7.5 vary with the structure type: d-H faujasite, 142.75' (ref 49); a Reference 36 d-H chabazite, 147.8' (ref 50); d-H mordenite, 153.45' (ref 51); and d-H stilbite, 147.1' (ref 52). From broad line ' H N M R for M: from this table are used for estimating the Moo spectra an Ale - H distance of 238 f 3 pm in d-H faujasite was values of the models included in Table 11. The uncertainty of the deduced.s3 Combining this distance with results of X-ray and estimated values is at least f 2 0 kJ/mol. The difference of about infrared studies a local structural model with R(OH) = 100-103 85 kJ/mol between the predictions from the 3-21G and 4-31G pm and an A1-0-H angle of about 116' was suggested.s3 calculations is unexpected. The comparison of the AHoo values This model can be substantially completed and modified by the for silanol with the result obtained by a better basis set including structural data derived from the present calculations. The Si-0 polarization functions on all atoms (1474 kJ/mol)17 suggests that and A1-0 bond overlap populations decrease significantly with the 3-21G results are more reliable. the addition of a proton on the bridging o ~ y g e n . ' ~From * ~ ~ our calculations, we predict an average bond lengthening of the T-0 General Discussion bond of 10 f 1.0 pm (Si/AI = 2), but with a different lengthening Binding Geometry of Terminal Hydroxyls. Experimental inof the Si-0 (9 1 pm) and A1-0 (12.5 f 2.0 pm) bonds upon formation on their geometry is rather scarce. Our prediction that protonation. Applications of the X-ray method for determining the S i 4 bond is lengthened by 4 pm on protonation (Table 111) the oxygen sites at which the protons are located (e.g., O(1) and may be compared with values of about 6 and 7-8 pm which are O(3) in faujasites) were based on the assumption that protonation derived from X-ray analysis of a partially hydroxylated t r i ~ i l i c a t e ~ ~ of an oxygen site will cause elongation of the T-0 bonds by 8 and some mono silicate^,^^^^^ respectively. However, the latter pm.49 The 0-H bond is predicted to be 1 pm longer than in results are affected by the presence of cations. Baur4' estimates terminal hydroxyls, i.e., 96 pm. The predicted bond angle of 111.5 the increase of the Si-0 distance when one extra positive charge 2' is smaller than in terminal hydroxyls. As a test of these at the oxygen is located to be 9.4 pm. There is nothing to compare predictions, we calculated the A1.s .H distance from the Al. * -0 with our prediction of 95 pm for the O H bond length. and 0-H distances and the S i U H angle. For the S i 4 4 angle The T-0-H binding angle is of importance for the electronic the value for the 0 ( 1 ) and O(3) sites (the sites of preferred properties. Indeed, for oxygen-containing compounds, where protonation) in d-H f a ~ j a s i t are e ~ ~adopted: hybridization of the s and p orbitals is likely, the electronegativity Si 139. l a A1 of the hybrid orbitals will depend on the percentage of s and p character, and therefore also on the bonding angle.42 The larger the bonding angle, the more pronounced the s character, and a higher electronegativity of the hybrid orbitals is to be expected. Experimental and theoretical evidence for the binding angle in I / X-0-H compounds is compiled in Table V. Considering the small 9 7 P 9 variation in the observed angles for compounds containing atoms H" which differ to a large extent in electronegativity and orbital structure, it is most likely that the T-0-H bonding angle is in The resulting Al. .H distance of 234 pm is in excellent agreement the range of 100-1 10'. More specific information is evident from with the experiment (238 f 3 pm). quantum chemical calculations. The first prediction of 108.8' Vibrational Properties. The most frequently applied method by Gibbs et al.43originates from the STO-3G optimized geometry for studying hydroxyl groups in zeolites or aluminosilicates is the for an isolated %(OH), molecule. The results for XOH angles IR stretching frequency. As discussed above, there is no doubt obtained with the better basis sets (cf. Table V, the results for about the nature of the hydroxyl groups in amorphous silica. The H2POHand HSOH are taken from ref 44) show a regular trend stretching frequency of these terminal hydroxyls is characterized in dependence on the position of the X atoms in the periodic table: by a sharp band around 3750 cm-' (see, e.g., ref 5 5 for a study H3Si-0-H, 117.1'; H2P-0-H, 110.4'; HS-O-H, 107.9'; H3Con aerosil). IR bands around 3650 and 3550 cm-' were une0-H, 109.5'; H 2 N U H , 104.1'; H0-0-H, 102.2'. Our final quivocally assigned to bridging hydroxyls vibrating in the large prediction of 1 15 f 2', including the correction for the systematic cage and the small cavities of faujasite-type zeolites, respectivelys6 error of the 6-31G* result, is the largest X-0-H bond angle reported so far (cf. Table V). Values of 113' and about 124' (as a lower limit) were derived from infrared4s and NMR46 (47) Jirak, 2.; Vratislav, S.; Zajicek, J.; Bosacek, V . J . Catal. 1977, 49, measurements, respectively. 112.
-
eF
*
*
-
(38) Wan, C.; Ghose, S.; Gibbs, G. V. Am. Mineral. 1977, 62, 503. (39) Megaw, M. D. Acta Crystallogr. 1952, 5, 477. (40) Jamieson, P. B.; Dent-Glasser, L. S. Acta Crystallog. 1966, 20, 688. (41) Baur, W. H. In "Structure and Bonding in Crystals"; OKeeffe, M., Navrotsky, A., Eds.;Academic Press: New York, 1981; Vol. 11, p 31. (42) Hinzc, J.; Jaffi, H. H. J . Chem. Phys. 1962, 84, 540. (43) Reference 18, Table I, and Figure 1. (44) Wallmeier, H.; Kutzelnigg, W. J . Am. Chem. SOC.1979, 101, 2804. (451 Peri. J. B. J . Phvs. Chem. 1966. 70. 2937. (46j Bernstein, T.; E&, H.; Freude; D.; Jiinger, I.; Sauer, J.; Staudte, B. Z . Phys. Chem. (Leiprig) 1981, 262, 1123.
(48) (49) (50) 2263. (51) 1319. (52)
Bosacek, V.; Beran, S.;Jirak, Z. J . Phys. Chem. 1981, 85, 3856. Olson, D. H.; Dempsey, E. J . Catal. 1969, 13, 221. Mortier, W. J.; King, G. S.D.; Sengier, L. J . Phys. Chem. 1979, 83,
Mortier, W. J.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1975, I O ,
Pearce, J. R.;Mortier, W. J.; King, G. S. D.; Pluth, J. J.; Steele, I . M.; Smith, J. V. "Proceedings of the 5th International Conference on Zeolites"; R e s , L. V. C., Ed.; Heyden: London, 1980; p 261. (53) Stevenson, R. L. J . Catal. 1971, 21, 113. (54) Dubsky, J.; Beran, S.; Bosacek, V. J . Mol. Catal. 1979, 6, 321. (55) Horill, P.; Noller, H. Z . Phys. Chem. (Frankfurr am Main) 1976, ZOO, 155.
910
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
Mortier et al.
TABLE VIII: Observed Gas-Phase Acidities ( A H : ) and Values Derived from Calculated Deprotonation Energies (kJ/mol) system
H*O H,COH FH =SiOHd
obsdb
sy 5 t em
1645 1575
1635 1586
1 54OC 1550 (4-31G) 1465 (3-21G)
1555
phenol H ,CC OOH HI =Si(OH)Al=
calcda
calcda
ob?db
1470 1445 1315 1265 (3-21G)
a See Table 11, uncertainty at least t 2 0 kJ/mol. Reference 36. Calculated from UsCF given in ref 34. ing polarizatjon functjons. A value of 1475 40 kJ/mol u'aspredicted.''
From calculations includ-
_+
(for the characterization of these two types of bridging hydroxyls, see below). The lower frequency for the bridging hydroxyls could be indicative of a smaller force constant for bridging hydroxyls. In an excellent study, Kustov et aL5' recently used diffuse reflectance IR spectroscopy of hydroxyl groups in a wide spectral range to characterize the bridging and terminal O H groups in zeolites (X, Y , and mordenite), silica gel, and aluminosilicates. Terminal hydroxyls in silica, and silica-alumina, activated at 870 K and H zeolites activated at 650 K invariably had a stretching vibration at 3745 cm-I. The stretching frequency of the bridged hydroxyls was found between 3550 and 3660 cm-' and was considerably influenced by the sample composition. From the experimental correlation^,^^ it is possible to derive the frequency corresponding to our models (composition SiA104H with infinite ring size, Le., without interaction with the surrounding) as 3665 cm-I. Moreover, Mortier and GeerlingsI5 concluded that the influence of the bond angle on the 0-H overlap population and the 0-H stretching frequency could be neglected. This explains then the predictability of the O H stretching frequency of structurally unperturbed hydroxyls, irrespective of the framework type.35 Therefore, the frequencies which should be compared with the results of the quantum chemical calculations are 3665 and 3745 cm-', respectively (Table VI). The difference of 80 cm-' between both values is well explained by the calculated difference of 60 20 cm-I (Table VI). Very similar calculation^^^ referring to an empirically corrected theoretical geometry instead of the optimized geometry adopted here yield a difference of 75 f 5 cm-I. In absolute terms, the theoretical frequencies are overestimated by 65-85 cm-I. (The discrepancy is not removed when including d functions in the basis set.) Bending, overtone, and combination bands for both types of OH groups were more sensitive and were preferred by the authors of ref 57 as a better probe for the structural and acidic properties. The bending frequencies of bridging hydroxyls are 180-250 cm-' higher than those of terminal silanol groups, and this was explained by an increase of the bending force constant with the strength of the coordination. Hydroxyls of the bridged type were only reported for zeolites, but not for silica gel or amorphous aluminosilicate. N o calculations were made on this problem. 3a, indicate that there Our 4-31G calculations for is virtually no barrier of rotation around the Si-0 bond axis for the protons. Actually, a very small barrier of 0.1 1 kJ/mol is obtained in the C, conformation (cf. Figure 1). This is also confirmed by the more extended 6-31G* calculations.28 In contrast, this movement of the proton in bridging hydroxyls is restricted to an out-of-plane vibration due to the presence of the additional Ab -0 bond. Gengembre et aL5' came to the same conclusion in a comparative study of the hydroxyls in HX zeolites and in silica gel using thermogravimetric, IR, and dielectric measurements. From the contribution of the hydroxyls to the apparent dipole moment, it can be derived that their movement is restricted in zeolites (w < 0.8 D) but that, for silica, a free rotation of the OH around the S i 4 axis allows a better orientation
*
-
( 5 6 ) Jacobs, P. A.; Uytterhoeven, J. B. J.C.S. Chem. Soc.,Faraday Trans. 1 1973, 69, 359. ( 5 7 ) Kustov, L. M.; Borokov, V. Yu.; Kazansky, V. B. J. Catal.1981, 72,
Figure 2. Qualitative potential curves designed in accordance with the most reliable estimates for OH bond length, force constants, and deprotonation energies: (broken line) water and methanol; (solid line) terminal and bridging surface hydroxyls.
of the O H along the electric field vector ( K = 1.1 D). This illustrates the nature of the hydroxyls in zeolites (bridging) and in silica (terminal). The terminal hydroxyls also better resist dehydroxylation than the bridging hydroxyls. Acidity and IH N M R Chemical Shift. Fraissard8 reviewed the methods for studying Bronsted acidity by NMR. He does not consider the proton mobility as a reliable criterion but finds the chemical shift a better measure of the acid strength of the OH groups. He concludes that H-Y zeolites (chemical shift 7-8 ppm in comparison with gaseous Me,Si) are more acidic than silicaalumina (-5 ppm). Freude et report a single sharp line for silica gel and A1203with a chemical shift of 1.6 and 2.0 ppm, respectively, and attribute this signal to O H groups with small acidity. A signal with a resonance shift between 6.0 and 10 ppm with a large line width, indicative of a broad distribution of higher acidities, was observed for deep-bed pretreated and decationated zeolites of the typx Y and the mordenite type. In all these samples, the sharp line due to hydroxyls of low acidity was always present. Considering the chemical shifts (presented in parentheses) known for organic molecules in inert solvents the following acidity scale was suggested: 59 methanol (0.52) < low-acidity hydroxyls (1.5-1.8) < phenol (4.28) < high-acidity hydroxyls (8.6-9.9) < acetic acid (11.3). In Table VI11 the heats of deprotonation AHooderived from quantum chemical calculations are used to make a prediction on the place of terminal and bridged hydroxyls on the absolute acidity scale.36 When comparing the chemical shifts with the heats of deprotonation, we should keep in mind that the former are static properties which depend on the charge distribution of the unperturbed O H group but that a deprotonation involves the difference between the X-OH groups and its deprotonated counterpart. The gas-phase acidity values deduced from the calculations (Table VIII) make it very likely that the low-acidity hydroxyls found by N M R are the terminal ones whereas the high-acidity hydroxyls are just the bridging groups. According to the calculations, the place of the terminal hydroxyls on the acidity scale should be close to phenol, whereas the bridging hydroxyls should be clearly more acidic than acetic acid (cf. Table VIII). Similarly, from the shift of the IR hydroxyl bands for terminal groups on adsorption of different molecules, the hydroxyl groups of silica were found to be considerably more acidic than methanol and slightly more acidic than phenol.60
1A 0 I . ,l
( 5 8 ) Gengembre, L.; C a m , J.-C.; Chapoton, A.; Vandorpe, B. J. Chim. Phys. Phys.-Chim. Biol. 1979, 76, 959.
( 5 9 ) Freude, D.; Hunger, M.; Pfeifer, H. Chem. Phys. Lett. 1982, 91, 307. (60) Hair, M. L.; Hertl, W. J. Phys. Chem. 1970, 7 4 , 9 1 .
Bridging and Terminal Hydroxyls
Figure 3. Charge transfer (direction of the arrows) from a donor molecule (EPD) to the proton (EPA) of a bridging and a terminal hydroxyl, respectively. A solid bent arrow denotes bond lengthening, and a dashed bent arrow a decrease in bond length.
With the size of models accessible for nonempirical methods it is impossible to simulate different sample compositions. However, employing the semiempirical CNDO method Kazanskii16succeeded in calculating relative values for the deprotonation energy of bridging hydroxyls in models being representative for samples of Si/AI ratios from 1 to 4. A decrease of the protonation energy with increasing %/AI ratio was found which parallels the findings of Jacobs and M ~ r t i e r . ~ ~ Interaction with Molecules. From the main characteristics of hydroxyl groups, the O H distance, the stretching frequency, and the heat of deprotonation (cf. Tables IV, VI, and VIII), we may try to develop a qualitative picture of the potential curves (Figure 2). Compared to water and methanol, for terminal hydroxyls the potential curve is less deep and has a smaller curvature near the minimum. The potential of the bridging hydroxyls is distinguished by both a long curvature near the minimum and a small heat of deprotonation which together lead to a shallower potential as a whole. This already indicates a higher sensitivity toward external perturbations such as the interactions with other molecules. The Bronsted acidity must be considered as a dynamic concept and a definition is meaningless if no interaction with proton-acceptor molecules is taken into account. The strength of this interaction will obviously vary with the electron-donor properties of the adsorbing molecules. An empirical scale for the donicity, the donor number, was established by Gutmann9 as -AH of the reaction of a donor molecule (EPD) with SbCIS acting as the acceptor molecule. Charge transfer and bond length variations are illustrated in Figure 3 for bridging and terminal hydroxyls in accordance with the bond length variation rules. From the induced bond lengthening, and the concomitant “weakening” of the 0-H bond, a bathochromic shift in the IR is predicted upon adsorption. Horill and Nollers5 followed the change in OH stretching frequency in aerosil after adsorption of molecules with a different donor number and found a strong correlation with the bathochromic shift. A decrease of the O H stretching frequency by about 100 cm-I for molecules with a very low donor number to about 700 cm-’ for pyridine was observed. Van Cauwelaert et aL6I report for the same 3750-cm-I band a decrease in frequency between 750 (adsorption of NH3) and 930 (adsorption of triethylamine) cm-I. The charge transfer from the acceptor site, i.e., the proton to the framework, will be easier in the case of a bridging hydroxyl because of the two T atoms coordinated to the oxygen, which necessarily caused a higher polarizability. Therefore, not only will the OH bond strength be smaller for bridging OH, the action of a donor molecule should also have a larger effect. The adsorption of alkanes on NaH-Y results in a bathochromic shift of the high-frequency band by 100 (n-butane) to 130 (n-dodecane) Depending on the sample composition (see further), (61) Van Cauwelaert, F. H.; Vermoortele, F.; Uytterhoeven, J. B.; Discuss. Faraday SOC.1971, 52, 66.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 911 Datka63also reports a bathochromic shift of the high-frequency band by 300-350 cm-’ after adsorption of benzene and toluene on NaH-Y zeolites. Following the relation of the shift with the donor number for the nonbridging hydroxyls in silica gel,5s the effect should only be about 100 cm-l. This illustrates the much higher acid strength of zeolites vs. silica. In a comparative study of the acidity in H-ZSM-5, H-ZSM-11, and dealuminated H-Y zeolites, Jacobs et al.64report extralattice O H (3720-3740 cm-I) and bridging O H (3600-3610 cm-’). The frequency shift upon interaction with benzene of the 3740-3720-cm-’ band is in the range of 40-80 cm-’ and that for the 3600-3610-cm-’ band is between 300 and 348 cm-I, indicating therefore again the bridging hydroxyls in the structures as the sites with the highest Bronsted acidity. Similar large differences in frequency shift of the O H groups in aerosil and ”a-Y zeolites upon adsorption of a variety of gases with largely differing proton affinities were found by Paukshtis and Y u r c h e n k ~ . Tops~le ~~ et used IR and temperature-programmed desorption techniques to distinguish “active” acidic sites (3600-cm-’ band) and weak acidic sites (37203740-cm-’ band) in ZSM-5 zeolites. N H 3 is desorbed at about 500 K from the weaker acidic terminal hydroxyls but the desorption from the stronger acidic bridged hydroxyls occurs only at 773 K. Influence of the Sample Composition. The O H stretching frequency of bridging hydroxyls is very sensitive to the sample composition. In a detailed and comprehensive study of the lattice OH vibrations in H zeolites of different structure and composition, Jacobs and M ~ r t i e were r ~ ~ able to rationalize the frequency of unperturbed OH; the stretching frequench of OH groups vibrating into cavities or channels with dimensions larger than eight-rings (such as the O1-H in faujasite-type structures) directly correlates with the average electronegativity of the sample, as evaluated by groups vibrating in using Sanderson’s m e t h ~ d . ~ ’ - Hydroxyl ~~ eight-rings or six-rings undergo a bathochromic shift caused by electrostatic interactions with the negatively charged framework oxygens of the same ring. The OH stretching frequency of the free hydroxyls will probably also depend on the sample composition. Jacobs et report for extralattice hydroxyls a stretching frequency 3720 (H-ZSM-5 and H-ZSM-I 1) and 3740 (dealuminated H-Y) cm-I. This band corresponds to the 3750-cm-’ band in amorphous silica gel. At present its dependency on the composition is not clear. The composition is easily evaluated for zeolites, but for amorphous silica the relative amount of protons in the gel, also of importance for the average electronegativity, is difficult to estimate. For X and Y zeolites with very different cation loadings, Angell and S ~ h a f f e r ~ ~ r e ponly o r t very minor shifts for a band between 3744 and 3748 cm-I. Kustov et aL5’ reported for terminal hydroxyls in silica gel, aluminosilicates, and zeolites a stretching frequency of 3745 cm-I, but a more sensitive bending frequency varying between 795-835 cm-]. If we consider the zeolite samples only, the bending-frequency range is smaller for the terminal hydroxyls (805-835 cm-I) than for the briding hydroxyls (990-1055 cm-I) for the same samples. These differences in sensitivity are most obviously also an effect of the differences in “coordination” of the oxygens in terminal vs. bridging hydroxyls. It was already observed that the gas-phase basicity of amines linearly correlated with the Sanderson electronegativity, the tertiary amines being much more sensitive than secondary amines, while the primary amines are the least sensitive.69 Similar differences in sensitivity of the 0 (62) Datka, J. J . Chem. Soc., Faraday Trans. 1 1980, 76, 705. (63) Datka, J. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 511. (64) Jacobs, P. A.; Martens, J. A.; Weitkamp, J.: Beyer, H. K. Faraday Discuss. 1981, 72, 353. (65) Paukshtis, E. A.; Yurchenko, E. N. React. Kinet. Catal. Lett. 1981, 16, 131. (66) Topsoe, N. Y.; Pedersen, K.; Derouane, E. G. J. Catal. 1981, 70,41.
(67) Sanderson, R. T.“Chemical Bonds and Bond Energy”, 2nd ed.; Academic Press: New York, 1976. (68) Mortier, W. J. J . Cats[. 1978, 55, 138. (69) Jacobs, P. A.; Mortier, W. J.; Uytterhoeven, J. B. J . Inorg. Nucl. Chem. 1978.40, 1919. (70) Angell, C . L.; Schaffer, P. C. J . Phys. Chem. 1965, 69, 3463.
J. Phys. Chem. 1984, 88, 912-918
912
1s photoelectron band between bridging and nonbridging oxygens in silicate glasses were observed: 7 1 the absolute shift of the 0 1s band of the bridging oxygen was greater than for the nonbridging oxygen in the Si02-Na20 system. The sensitivity of the physicochemical properties toward composition being different for different "coordinations", we may conclude that bridging hydroxyls will be more readily influenced by the average electronegativity than the terminal hydroxyls. The observations in the literature confirm this statement. Not only are the physical properties as "such" composition dependent, but also the sensitivity for external perturbations depends on the composition of the matrix. For terminal hydroxyls in mixed oxides S i 0 2 / M g 0 , SiO2/Al2O3,and AI2O3/Mg0 and their pure components, Lercher and N01ler~~ demonstrated that the shift of the hydroxyl bands after adsorption of acetone increases with increasing intermediate electronegativity of the oxide (electronegativity range in the Sanderson scale: 2.8-4.2; Av range: 260-340 cm-I). For bridged hydroxyls in H zeolites (electronegativity range: 3.8-4.2; Au range: 313-325 cm-'), similar effects were observed by Jacobs73for the adsorption of benzene. Because of the use of different donor molecules, it cannot be decided which type of hydroxyl is more compositional dependent. (71) Briickner, R.; Chun, H.-U.; Goretzki, H.; Sammet, M. J. Non-Crys?. Solids 1980,42, 49. (72) Lercher, J. A,; Noller, H. J . Catal. 1982,77, 152. (73) Jacobs, P. A. Catal. Rev.-Sci. Eng. 1982,24, 415.
Acknowledgment. W.J.M. thanks the "Belgisch Nationaal Fonds voor Wetenschappelijk Onderzoek" for a research position as "Onderzoeksleider", the Belgian Ministry of "Nationale Opvoeding and Nederlandse Cultuur" together with the T.U. Wien for financing a stay at the "Institut fur Physikalische Chemie", and Dr. D. Freude, Dr. A. Leonard, and Prof. H. Pfeiffer for stimulating discussions. The force constants were kindly calculated by K.-P. Schriider, J.S. thanks Prof. W. Schirmer for his promoting interest in these studies. Note Added in Proof: We are grateful to Prof. H. Pfeifer for careful reviewing of the manuscript and for drawing our attention to two recent in which evidence is produced for the presence of strongly acidic, bridging hydroxyls in amorphous aluminosilicates. In a more recent note76it was shown that this type of hydroxyl is thermally unstable and can be easily removed from the surface of amorphous aluminosilicates by outgassing at 670 K for 15 min. Registry No. la, 14475-38-8; 2a, 88337-13-7; 2b, 88337-14-8; 3a, 10193-36-9. (74) Kazansky, V. B. Kine?. Karol. 1982,23, 1334. (75) Hunger, M.; Freude, D.; Pfeifer, H.; Bremer, H.; Jank, M.; Wendlandt, K.-P. Chem. Phys. Let?., in press. (76) Borovkov, V. Yu.; Alexeev, A. A.; Kazansky, V. B. J . Catal. 1983, 80, 462.
Radiolytically Induced One-Electron Reduction of Methylvioiogen in Aqueous Solution. Platinum-Catalyzed Formation of Dihydrogen' Margherita Venturi, Istituto di Scienze Chimiche, Facoltci di Farmacia, Universitci di Bologna, 401 26 Bologna, Italy
Quinto G. Mulazzani,*2 Istituto di Fotochimica e Radiazioni D'Alta Energia, Consiglio Nazionale delle Ricerche, 401 26 Bologna, Italy
and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received: May 2, 1983)
The reaction of the methylviologen dication (MV2') with radiolytically generated (CH,),COH radicals in deaerated aqueous solution is rapid and quantitative, producing the methylviologen cation radical (MV'.). At pH 1, MV'. decays via [ MV2+]-dependentsecond-order kinetics in the course of minutes according to an H+-assisted disproportionation reaction that yields hydrogenated methylviologen as a final product; at pH 4.2, MV'. is more stable but does decay in the course of a day. Dihydrogen is a product of the radiolysis with G(H,) equaling the "background" yield of H2 from the primary radiolytic act and the scavenging of H atoms by 2-propanol. In the presence of poly(viny1 alcohol)-stabilized Pt sols (total [Pt] = 50 pM), MV+. decays in less than 1 s. G(H2) is at or near zero during the initial phases of the reaction when the Pt is adsorbing the radiolytically generated H2; at the same time, hydrogenated product is formed efficiently by Pt-catalyzed disporportionation. Continued exposure causes C(H2) to rise above "background" level and reach a plateau; the yield of hydrogenation product decreases concomitantly and reaches a minimum level that is not negligible. The maximum efficiency of H2production above "background", relative to that of the same system in the absence of MV'., is -79% at pH 1 and -43% at pH 4.2; the minimum efficiency of hydrogenation, relative to the zero-dose limit, is -10% at pH 1 and -25% at pH 4.2. The implications of these results to the photochemical generation of H2 in the presence of Ru(bpy)32', MV2*, and Pt are discussed.
Introduction The Pt-catalyzed formation of H2 in aqueous solutions in which a one-electron-reduced viologen radical has been photochemically generated serves as the basis of many model solar energy conversion schemes. In the most highly studied system, the me-
thylviologen dication (l,l'-dimethyl-4,4'-bipyridinium ion, MV2+) acts as the electron-transfer relay between the electronically excited state of R ~ ( b p y ) ~ (bpy ~ + = 2,2'-bipyridine) and the reduction of H 2 0 on the surface of the colloidal platinum aggregate^.^ The reactive species at the Pt is the methylviologen radical cation
(1) Research supported in part by Consiglio Nazionale delle Richerche and in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy.
(2) Visiting scholar, Boston University, Fall 1981. (3) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982,22, 117 and references therein.
0022-3654/84/2088-0912$01.50/0
0 1984 American Chemical Society
