J. Phys. Chem. 1988, 92, 3005-3007 lyamic acid. With parameters AH = 26 000 cal/mol, A = 5 X lo6 s-l, and Tg = 524 K, the theoretical calculations of the e'-T dependence were carried out. The results are plotted in Figure 8 (broken lines) together with the experimental plots of Destruel and Yoon, for two frequencies: lo5 and lo6 Hz. The agreement is rather qualitative, but the difference between the results is easy to explain. The tail part in the low-temperature region of the experimental plots is due to the contribution of another process, visible on the d-T plot for lo4 H z (Figure 3, ref l), and due to the fact that the a relaxation in the polymer is most likely
3005
characterized by a very wide relaxation time distribution function.
Conclusions The model presented above, in spite of its rather simplifying assumptions, seems to explain qualitatively the origin of e'( T) maxima observed in recent experimental data. In addition, it describes the time dependence of e* and foresees the heating rate dependence of both positions and heights of the peaks. There are some possibilities to modify the model by changing the kinetic scheme and/or the temperature dependence of the relaxation time.
An ab Initio Quantum Chemical Investigation on the Effect of the Magnitude of the T-0-T Angle on the Bransted Acid Characteristics of Zeolites PPdraig J. O'Malley* and J. Dwyer* Department of Chemistry, The University of Manchester Institute of Science and Technology, Manchester M60 1 QD, United Kingdom (Received: June 15, 1987; In Final Form: December 2, 1987)
Ab initio molecular orbital calculations are used to monitor the effect of increasing SiOAl angle (LSiOAl) on the acidic properties of bridged hydroxyl groups in zeolites. A 3-21G basis set is employed in the calculations, and the bridged hydroxyl group is modeled via an aluminosiloxaneunit. Increasing LsiOAl is shown to give rise to a decrease in the stretching frequency of the OH group (pOH) with a corresponding decrease in the ionicity of the OH bond. The decrease in ionicity of the OH bond indicates that increasing CSiOAl gives rise to a decrease in acidity. The decreased strength of the OH bond as evidenced by the decreased poH value suggests that the OH bond is unstable when situated at a high LSiOAl link with the decreased polarity of the bond tending to favor homolytic dissociation over heterolytic dissociation. It is also shown that &OH exhibits decreased flexibility with increasing LSiOAl, suggesting that the SiOH bending frequency should be greater for zeolites with higher TOT angles.
Introduction Many of the catalytic properties of zeolites can be directly related to Br~rnstedacidity. Experimentally, it has been demonstrated that the acidity of zeolites varies with structure and composition.' Recently, quantum chemical calculations (both semiempirical and a b initio) have been shown to give a good account of the variations in acidity brought about by compositional properties such as the Si/Al ratio? increased electronegativity,' or isomorphous s~bstitution.~The investigation of the effect of structural characteristics, such as variation of the TOT angle on the acidic properties, has not received the same attention, h ~ w e v e r . ~ This is surprising as a striking difference between low-silica zeolites such as faujasite and the novel high-silica forms such as ZSM-5 is the larger T O T angle of the high-silica forms.6 In this report the effect of increasing TOT angles on the acidic properties of the bridging OH group is studied. Increasing the TOT angle is shown to give rise to a decreasing strength of the bridging OH bond with a concomitant decrease in the ionicity of the bond. The potential energy curve for the SiOH bond angle indicates that increasing the TOT angle also leads to an increase in the SiOH bending force constant.
Models and Methods Units used to model the zeolite framework are generally determined by balancing computer cost with the complexity of the (1) Dwyer, J. Chem. Znd. 1984, 258. (2) Kazansky, V. E. In Structure and Reactivity of Modified Zeolites;
Jacob, P. A., et al., Eds.; Elsevier: Amsterdam, 1984; p 61. (3) Datka, J.; Geerling, P.; Mortier, W.; Jacobs, P. J . Phys. Chem. 1985, 89, 3483. (4) O'Malley, P. J.; Dwyer, J. J . Chem. Soc., Chem. Commun. 1987, 72. ( 5 ) (a) Senchenya, I. N.; Kazansky, V. B.; Beran, S . J . Phys. Chem. 1986, 90,4857. (b) Zhidomirov, G. M.; Kazansky, V. B. Adv. Catal. 1986,34, 131. (6) Olson, D. H.;Kolcotailo, G. T.; Lawton, S.L.; Meier, W. N. J. Phys. Chem. 1981,85, 2238.
TABLE I: Effect of Variation of the SiOAl Angle on the Si0 and AI0 Bond Length and the SiOH Angle
LSiOA1, deg 127 140 150 160 170
rAo/A 1.93 1.93 1.95 1.97 2.00
rsio/b, 1.73 1.73 1.73 1.74 1.74
LSiOH, deg 120.4 115.2 111.7 107.9 104.2
TABLE II: Effect of Variation in the SiOAl Angle on the Acidic Characteristics of the Bridging OH Group
LSiOA1, deg
rc,OH/A
vOH/cm-'
127 140 150 160 170
0.966 0.970 0.974 0.978 0.981
3931 3870 3828 3773 3717
qH 0.472 0.465 0.456 0.445 0.432
k H
401
0.442 0.431 0.418 0.404 0.386
calculation available. Many studies of zeolite properties have been performed using semiempirical methods such as CNDO, INDO, etc. Up to the present time most ab initio calculations have been performed with the STO-3G basis set. While the STO-3G basis set has been successful in describing the bonding characteristics of many molecules, it has been found' that splitting the valence shell region into a single and double Gaussian contraction (giving rise to a 3-21G basis set) improves the flexibility of the valence shell region and hence allows more adaptability to various bonding situations in molecules.' The 3-21G basis set was chosen therefore for the present study. The SiOAl fragment was modeled via the aluminosiloxane unit of Figure 1. This unit allows extensive geometry optimization to be performed at a reasonable computer cost, and the unit has been previously shown to give a good account ~
~
(7) Gordon, M. S.;Binkley, J. S.;Pople, J. A,; Pietro, W. J.; Hehre, W. J. J . Am. Chem. Soc. 1982, 104, 2797.
0 1988 American Chemical Society
3006 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 H
H H
H
I
H Figure 1. Model unit used for molecular orbital calculations.
40.98
0
11'8 L
-a -8
I
120
140
160 180 bmd angle /degees Gi OAl
Figure 2. Variation in AI0 bond length, OH bond length, charge on bridging proton, and SiOH angle as a function of TOT angle.
of the structural characteristics of zeolites when calculations are performed at the present level?,' All calculations were performed at the single determinant Hartree-Fock-Roothaan level, and a good account of the types of error expected by using the level of theory are outlined in ref 8. Calculations were carried out using the GAMESS quantum chemistry package as implemented on the CYBER 205 computer at UMRCC. Optimization of the geometry of the aluminosiloxane unit of Figure 1 was performed under the constraints given in ref 4. The stretching frequency of the OH bond was calculated as described in ref 8.
Results and Discussion The optimized SiOAl angle for the aluminosiloxane unit is 1 2 7 O . 4 This angle was then progressively fixed at a range of values between 127' and 17O0, and the resulting variations in the OH bond properties are outlined in Tables I and I1 and Figure 2. In Table I1 the optimized S i 0 and AlO bond lengths plus the SiOH angle for the various fSiOAl values are also presented. From Table I1 it is noted that increasing the SiOAl angle leads to a decrease in the OH bond strength as noted by an increase in re and a decrease in vOH. A corresponding decrease in ionicity of the OH bond is also predicted to occur (see qH and qoqH values of Table 11). It is of interest to examine what structural changes occur in the unit in the hope that some insight into the reasons for the above-mentioned OH bond properties may be obtained. Table I and Figure 2 show that, whereas the S i 0 bond length varies only slightly on increasing TOT angle, the AlO bond length shows a noticeable increase. This trend runs contrary to the findings of Gibbs et aL9 for the unprotonated form. For the (8) Mortier, W. J.; Saver, J.; Lercher, J. A.; Woller, H. J . Phys. Chem. 1984,. 88, 905. (9) (a) Gibbs, G. V.; Meagher, B. P.; Newton, M. D.; Swanson, D. K. In Strucfure and Bonding in Crystals; O'Keefe, M., Navrotsky, A,, Eds.; Academic: New York, 1981; Vol. 1 , p 195. (b) Newton, M. D. In ref 9a, p 175.
O'Malley and Dwyer protonated case, however, the unit of Figure 1 is best regarded as a combination of electron donor (H3SiOH) and electron acceptor (AlH3) units. The A10 bond distance can be regarded therefore as a pseudointermolecular parameter, and the S i 0 bond length variation must be considered in terms of the H3SiOH unit. With this in mind Table I1 and Figure 2 show that LSiOH is decreased with increase in LSiOA1. It has been shown that SiOX angles are intimately related to the percentage of s character in the oxygen hybrid ~ r b i t a l .A~ decrease in the SiOH angle can be associated with a decrease in the s character of the oxygen hybrid orbital. The decrease in SiOH angle brought about by an increasing SiOAl angle should therefore result in increased bond length plus a decrease in ionicity for both Si0 and O H bonds. It is clear that this effect is most pronounced for the O H bonds and provides a good explanation of the trends noted in Table I and Figure 2. By regarding the unit of Figure 1 as an electron donor (H,SiOH)-electron acceptor (AlH3) unit,l0 the polarized nature of the O H bond in the H3SiOHAlH3unit compared with H3SiOH can be attributed to the polarizing effect of the neighboring AlH3 unit (as predicted by Gutmann's rules).I0 This polarizing effect would be expected to decrease with increasing A10 bond length. From Figure 2 it is seen that both the SiOH angle and the O H bond length vary linearly over the entire LSiOAl range. The increased length of the OH bond can be directly correlated with the decrease in the SiOH angle. The charge on the bridging proton, however, varies in a nonlinear fashion with LSiOA1, exhibiting an enhanced rate of decrease above 140O. This variation in charge does however resemble quite closely the rate of increase in A10 bond length (see Figure 2). It is therefore likely that the charge distribution in the OH bond is affected by both the SiOH angle and the polarizing effect of the AlH3 unit. For small A10 bond lengths the polarizing ability of the AlH3 counteracts the decreased ionicity imposed by the smaller SiOH angle. The decreased polarizability on increasing A10 bond lengths would render greater dominance to the SiOH angle contribution and, as predicted, the q H value exhibits an enhanced rate of decrease for SiOAl values above 140'. The data given in Table I1 show that increasing the TOT angle leads to a decrease in vOH with a corresponding decrease in the ionicity of the OH bond as demonstrated by q H and qoqH. It has been generally accepted that the stretching frequency of the O H bond (iiOH) can be taken as an indicator of the Br~lnstedacidity; Le., the lower the frequency, the greater the acidity. However, a combination of theoretical and experimental studies" has shown that acidity (which is principally a measure of the ease at which an -OH bond dissociates heterolytically) is not well-characterized by the stretching frequency of the -OH bond. Acidity was shown to be better characterized by the charge on the proton. On the basis of these arguments, therefore, it can be stated that increasing the TOT angle leads to a decrease in the ability of the bridging O H group to dissociate heterolytically and hence should lead to a decrease in acidity. At the same time, however, the strength of the OH bond as determined by re and vOH is decreased. The weaker O H bond strength suggests that formation of O H groups near high TOT angle sites is unlikely to occur. Experimental information regarding the precise location of proton sites in zeolites is scarce, and currently neutron diffraction is used to locate such sites in zeolites Y and Rho.'* As both zeolite pairs have only one type of crystallographic T site, a preference for low or high TOT angle sites cannot be distinguished in their case. If O H groups do form near high TOT angle sites, the possibility of homolytic fission being favored compared with heterolytic dissociation is increased, hence leading to the possibility of SiO' and (10) (a) Geerlings, P.; Tarevil, W.; Botrel, A.; Mortier, W. J. J . Phys. Chem. 1984,88, 5512. (b) Guttman, V. In The Donor-Acceptor Approach to Molecular Interactions; Plenum: New York, 1978. (11) Kazansky, V. B.; Gritscov, A. M.; Andreeve, V. M.; Zhidomirov, G. M. J . Mol. Catal. 1978, 4, 135. (12) (a) Jirak, Z.; Vratislav, S.; Bosacek, V. J . Phys. Chem. Solids 1980, 41, 1089. (b) Fischer, R. X.; Baur, W. H.; Shannon, R. D.; Stanley, R. H. J . Phys. Chem. 1987, 91, 2227.
J . Phys. Chem. 1988, 92, 3007-3011
likely to arise from O H groups centered on high T O T angle positions. In Figure 3 the potential energy curves obtained for variation of the SiOH angle at fixed SiOAl angles of 127O and 160" are presented. Increase in the SiOAl angle to 160° is shown to give rise to a much steeper potential energy curve for the SiOH angle variation. This indicates that the LSiOH is less flexible at high LSiOA1. Recently, Kustov et a1.I0 have measured the SiOH bending frequency for a variety of zeolites. In general, they find that the LSiOH bending frequency for mordenite is higher than that of faujasite. The decrease in flexibility of &OH demonstrated in Figure 3 for high TOT angles predicts such a trend in the bending frequency, indicating that the increased bending frequency arises from the larger TOT angles of mordenite.
&ion[
/
1600
k
127'
\
7
\
3007
\
-606.520-
/
\ \
Conclusion Increasing LSiOAl is predicted to lead to a decrease in the stretching frequency of the bridging OH bond. An increase in the SiOH bending frequency is also predicted to occur. Both predictions have been confirmed experimentally and suggest that the magnitude of the TOT angle is an important factor for consideration when attempting to explain the acidic characteristics of zeolites. Note Added in Proof. After submission of this article, a report by Pelmenshchikov et a1.I6 has appeared which examined the effect of TOT angle variation on acidity using ab initio molecular orbital calculations at the STO-3G level. The findings in that study are in good agreement with those reported here and confirm the influence of structural factors on acidic characteristics.
/
Acknowledgment. We thank BP Research Centre, Sunburyon-Thames, for the award of an EMRA for research into zeolite chemistry and catalysis.
(13) Shih, S. J . Catal. 1983, 79, 390. (14) Katchevov, A. V.; Slinkin, A. A. In Structure and Reactivity of Modified Zeolites; Jacobs, P. A., et al., Eds.; Elsevier: Amsterdam, 1984; D 77. (15) Kustov, L. M.; Borovkov, V.; Yu,Kazansky, V. B. J. Catul. 1981, 72, 149. Shih, S. J . Catal. 1983, 79, 390.
(16) Pelmenshchikov, A. G.; Pavlov, V. I.; J . Phys. Chem. 1987, 91, 3325.
Zhidomirov, G. M.; Beran, S .
HO, and 0,- Radicals at Elevated Temperatures Hilbert Christensen Studsvik Energiteknik AB, S-611 82 Nykoping, Sweden
and Knud Sehested* R i m National Laboratory, DK-4000 Roskilde, Denmark (Received: June 29, 1987; In Final Form: December 9, 1987)
The spectra of HO, and 0,- radicals were determined in the temperature range 20-300 "C in aqueous solutions with 0.2-MPa oxygen and about 10-MPa hydrogen pressure. In this temperature range the wavelengths at maximum absorption and the is 0.15 and 0.25% K-' for H 0 2 half-widths of the spectra change very little. The relative temperature coefficient of e,& and 02-,respectively. In the temperature range 20-100 "C the change in pK, is small, but above 100 O C pK, increases with an increasing rate up to a value of 6.15 (molal unit) at 285 "C. In acid solution, pH 52, the bimolecular decay rate constant for HOz has an activation energy of 20.6 kJ mol-' (4.9 kcal mol-'). At higher pH the decay is caused by the reaction between HOZand 02-.This reaction follows the Arrhenius parameters up to 100 "C with an activation energy of 7.6 kJ mol-I (1.8 kcal mol-'). Above 100 "C the decay rate increases drastically. At higher temperatures (1200 "C)a reaction with an activation energy of 80 12 W mol-' (19 3 kcal mol-I) becomes rate determining. This reaction is tentatively ascribed to the equilibrium reaction 0, + 0 2 - s 04,-followed by H+ + 04- H02- + 02.The mechanism may involve the unstable intermediate HO,. The high activation energy is ascribed to the forward reaction in the equilibrium which makes this reaction negligible at lower temperatures.
*
Introduction With the objective of studying the radiation chemistry of water at the operating temperatures in power reactors, we have continued a program to determine reaction rates and activation energies of radiolytic reactions by pulse radiolysis at elevated temperatures.'" 0022-3654/88/2092-3007$01.50/0
-
*
In the present investigation the radical species H 0 2 and 02and their bimolecular decay reactions in aqueous solutions at high (1)
Christensen, H.; Sehested, K. Radial. Phys. Chem. 1980, 16, 193.
(2) Christensen,
H.;Sehested, K. Radiut. Phys. Chem. 1981, 18, 723.
0 1988 American Chemical Society
