Figure 4b: Energy difference, with respect to separated molecules, for H7SiA1O7
+
H2O (orientation 1) and H7SiA1O7+
H3O+ (orientation 2).H
8Al
20
7INTER"0LECULAR SEPARATION, R CANGSTR0"SJ
Figure 4c: Energy difference, with respect to separated molecules, for H8Al201
+
H20 (orientation 1) and H8Al2O7+
H3O+ (orientation 2).There are three points to be observed from these plots. The first is that as the sum of the support the findings of Rimstidt and Dove, 40 who recently studied the hydrolysis of wollastonite
(5)
and obtained an activation energy of 79.2kJ mol-1• They suggest that the first step in the break-down of the silicate framework involves the chernisorption of hydronium ion to a bridgmg oxygen as shown in Figure 3d rather than an ion exchange reaction which substitutes the hydronium ion for the Cai+ in the lattice.
The second point concerns the behavior of the 830+
+
81SiA107 and the H30 ++
88Al207 reactions. The reaction energies go above zero for separation distances of about 3A. One possible reason for this is that SCF energies at large separation distances commonly fail to give meaningful results because the method poorly accounts for the the dispersive term in the energy at long distances. 41 Another reason may be basis set superposition error, i.e, failure to make the counterpoise correction. The extra computation time required to make these more accurate counterpoise corrections is relatively small because only the nuclear attraction integrals must be recalculated. During this study, however, the integrals had been discarded, so that the correction became infeasible. Nonetheless, one point was recalculated. This was for orientation 1 of the 830 ..+
H7SiA107 reaction at a distance of r=
1.5A. The uncorrected reaction energy was - 60.0 kJ mol-1; with the counterpoise correction it became - 49. 1 kJ mol-1 • The correction is clearly important, and corroborates the observation that SCF reaction energies are overestimated when the basis sets are not size consistent.41 This correction may remove the positive barrier in fourThe third point to be made is that these MO calculations accurately model the hydrophobic nature of the SiOSi linkage and the hydrophilic nature of aluminosilicates such as the zeolites. The orders of stability for the H20
+
dimer and the H30 ++
dimer reactions are SiOAl > AlOAl >SiOSi, and SiOSi > SiOAl > AlOAl, respectively. Combining them, the order is
H30 +
+
SiOSi > H30 ++
SiOAlTo our knowledge, there is no experimental work with which the above order may be compared.
Conclusions
'
In this study we have found that the bond lengths and angles in the hydroxyacid molecules H6Si207 and H7SiA107 matched experimental bond lengths to within 0.02A and angles to within
We find that we are unable to satisfactorily model the relative stability of SiOSi and AlOAl linkages vs. SiOAl linkages; our calculations indicate that AlOAl linkages are more stable than SiOAl linkages. Nevertheless, the magnitude of the energy for reaction (4) is relatively small (ca. 20 kJ mol-1) , and the result is a great improvement over similar calculations made by Sauer and Engelhardt6 and by Navrotsky et al. 1 using negatively charged molecules with minimal basis sets.
To provide a theoretical description of hydrolysis reactions we conducted calculations, using the supermolecule approach,41 in which H6Si2O7, H7SiA1O7 , and H8Al2O7 each react with H2O and H3O + . Our results suggest that the H3O +
+
SiOSi reaction is important in silicate surface re-actions. Also, the MO calculations reliably model the hydrophobic nature of silicalite and the hydrophilic nature of aluminosilicate zcolites. The stability scheme ( 6) may change somewhat if geometry variation is allowed and more orientations are permitted. Future studies should include such variation.,r
References
K. L. Geisinger, G. V. Gibbs, and A. Navrotsky, Phys. Chem. Minerals, 11, 266(1985).
G. V. Gibbs, Am. Miner., 67, 421,(1982).
M. O'Keeffe and G. V. Gibbs, J. Chem. Phys., 81, 876(1984).
4 M. O'Keeffe, B. Domenges, and G. V. Gibbs, J. Phys. Chem., 89, 2304(1985).
Z. G. Zhang, :vi. B. Boisen, Jr., L. W. Finger, and G. V. Gibbs, Am. Miner., 70, 1238(1985).
J. Sauer and G. Engelhardt, Z. Naturforsch., 37a, 277(1982). J. Phys. Chem., 85, 2590(1981).
A. Navrotsky, K. L. Geisinger, P. McMillan, and G. V. Gibbs, Phys. Chem. Minerals, 11, 284(1985).
J. Sauer, P. Hobza, and R. Zahradnik, J. Phys. Chem., 84, 3318(1980).
9
"A Handbook of Computational Chemistry: A Practical Guide to Chemical Structure and Energy Calculations," T. Clark, John Wiley and Sons, New York, 1985, p. 238.
"Ablnitio Molecular Orbital Theory," W. J. Hehre, L. Radom, P. v. R. Schleyer, and J. A.
Z. Latajka and S. Scheiner, Chem. Phys. Lett., 105, 435(1984).
1-1. B. Schlegel, J. Chem. Phys., 77, 3676(1982).
H. B. Schlegel, J. Comput. Chem., 3, 214(1982).
J. S. Binkley, R. A. Whiteside, R. Krishnan, R. Seeger, D. J. De Frees, H. B. Schlegel, S.
Topiol, L. R. Kahn, and J. A. Pople, "GAUSSIAN80--An ab initio molecular orbital pro-gram." Department of Chemistry, Carnegie-Mellon University, Pittsburg, Pennsylvania
15213.
19
J.E. Wainwright and J. Starkey, Z. Kristalogr., 133, 75(1971).
W. Loewenstein, Am. Mineral., 39, 92(1954).
J. R. Goldsmith and F. Laves, Z. Kristallogr., 106, 213(1955).
J. Chem. Soc., Chem. Commun., 276(1981).
L. A. Bursill, E. A. Lodge, J. M. Thomas, and A.
J. Chem. Phys., 85, 2409(1981).
K. Cheetham,
J. Klinowski, J.M. Thomas, C. A. Fyfe, and J. S. Hartman, J. Phys. Chem., 85, 2590(1981).
J. J. Pluth and J. V. Smith, in "Intrazeolite Chemistry," G. D. Stucky and F. G. Dwyer, Eds., American Chemical Society, Washington, D. C., 1983, ch. 7.
30 A. K. Cheetham and M. M. Eddy, in "Intrazeolite Chemistry," G.D. Stucky and F. G. Dwyer, Eds., American Chemical Society, Washington, D. C., 1983, ch. 8.
31 J. M . Bennett, C. S. Blackwell, and D. E. Cox, in "Intrazeolite Chemistry," G. D. Stucky and F . G. Dwyer, Eds., American Chemical Society, Washington, D. C., 1983, ch. 9.
32 J. M. Thomas, J. Klinowski, S. Ramadas, M. W. Anderson, C. A. Fyfe, and G. Gobbi, in
"Intrazeolite Chemistry," G. D. Stucky and F. G. Dwyer, Eds., American Chemical Society, Washington, D. C., 1983, ch. 10.
33
34
R. H. Jannan, M. T. Melchior, and D. E. W. Vaughan, in "Intrazeolite Chemistry," G. D.
Stucky and F. G. Dwyer, Eds., American Chemical Society, Washington, D. C., 1983, ch. 16.
M. W. Phillips, G. V. Gibbs, and P. H. Ribbe, Am. Miner., 59, 79(1974).
3S A. Navrotsky, G. Perandeau, P. McMillan, and J.-P. Coutures,
36
37
38
Geochim. Cosmochim. Acta, 46, 2039(1982).
A. C. Lasaga, in "Kinetics of Geochemical Processes," A. C. Lasaga and R. J. Kirkpatrick, Eds., Mineralogical Society of America, Washington, D.C., 1981 , ch. 1, 4.
E. M. Flanigen, J. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Patton, R. M. Kirchner, and J. V. Smith, Nature, 211 , 512(1978).
W.R. Rodwell and L. Radom, J. Am. Chem. Soc., !03, 2865(1981).
39
40
41
42
G. A. Somorjai, in "Solid State Chemistry and Physics: An Introduction," P. F. Weller, Ed., Marcel Dekker, Inc., New York, 1974, pp. 706-714.
J. D. Rirnstidt and P. M. Dove, Geochim. Cosmochim. Acta, in press.
"Ab lnitio Calculations," P. Carsky and M. Urban, Springer-Verlag, Berlin, 1980, 247 p. \
S. F. Boys and F. Bernardi, Mo/. Phys., 19, 553(1970).