J. Phys. Chem. 1981, 85, 1956-1958
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hydroxylated H forms of zeolite exhibit the weakest ability to accept electron density, and, moreover, the bearers of this ability are the skeletal Si atoms, which are sterically difficult to approach.
Conclusions The modeled AP+, A1(OHI2+,and Al(OH12+ions located in the cationic positions of faujasites can, on the basis of the above calculations, be considered as strong electronacceptor sites whose strength is greater than that of this type of site represented by Na+ or H+ ions, or by tricoordinated Si and A1 in the modeled dehydroxlyated H-Y zeolite.” Although the AlH and NaH forms of zeolite were modeled by limited clusters, it is encouraging that the calculated high electron-acceptor ability of the A1H forms of zeolite is in agreement with the experimentally determined high catalytic carbonogenic activity of A1H-Y zeolites compared with NaH-Y (cf. ref 2 and 4). On the other hand, it should be borne in mind that not only the A1 ions contribute to this activity as Lewis sites, but also acid skeletal hydroxyls. It is known that the 0-H groups bonded to cations (here Al(OH)2+and A1(OH)2+)are very weak proton-donor sites and do not participate in the catalytic activity of the Men+ zeo1ites.l Nonetheless, the modeled zeolite clusters dem(24) S. Beran, J. Phys. Chem., following paper in this issue.
onstrated that the 0-H groups of the A1(OH)2+and Al(OH)2+cations are much less acidic than the skeletal hydroxyls. In addition to the catalytic activity of zeolites, their structural stability is considered in great detail. As the catalytic processes and regeneration of zeolites on an industrial scale are carried out a t high temperatures and frequently in the presence of water vapor or other adsorbents, which can be highly destructive to the zeolite skeleton, it is necessary that a useful zeolite have not only high catalytic activity but also a stable structure. H-Y zeolites completely lack this structure stability, leading to development of the well-known stabilization processes. The thermal stability of Me”+ H-Y zeolites can also be considerably increased by introducing polyvalent cations into the cationic positions of H-Y zeolites (Re, Cr, Fe; cf. ref 1, 25, and 26). This study describes slight weakening of the T-0 skeleton bonds as a result of bonding of Al ions in the cationic positions to the skeletal oxygen atoms, but, on the other hand, coordination of these Al cations through the skeletal oxygens to the skeletal ring is accompanied by a much more important stabilizing effect, reflected in overall stabilization of the structure of the A1H zeolite form. (25) Z. Tvarliikovi and V. BosiEek, Collect. Czech. Chem. Commun., 45, 2499 (1980). (26) B. WichterlovB, submitted for publication.
Quantum-Chemical Study of the Lewis Sites in Dehydroxylated Faujasite Zeolites S. Beran The J. HeyrovsW Institute of Physical Chemktry and Electrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2, f i c h o v a 7, Czechoslovakia (Received: December 9, 1980)
The physical properties of the dehydroxylated forms of faujasite zeolites modeled by the Si5A106(OH)11 and Si4A120s(OH)1;clusters were studied by the CNDOI2 method. It was demonstrated that the tricoordinated AI produced by dehydroxylation is a very weak Lewis acid, in contrast to tricoordinated Si, whose electron-acceptor ability is much greater and comparable with that of the A1(OH)2+species coordinated in the cation position in the zeolite. The calculations further indicate the possibility of splitting of the six-membered zeolite ring containing tricoordinated Si.
Introduction Zeolitic aluminosilicates can be considered as solid acids capable of acting on the adsorbing molecule by donation of a proton (Bronsted site) or by accepting an electron pair from the molecule; i.e., they can act as Lewis Both of these zeolite functions play an important role in their use as acid-base catalysts in various catalytic processes. While the origin of Bronsted sites in zeolites is known in detail (they are skeletal and terminal hydroxyl groups), the structure and the nature of the Lewis sites are not completely clear. One way of generating Lewis sites is dehydroxylation of the H form of the zeolite, where it is assumed that tricoordinated A1 and Si atoms are formed according to the scheme:
Identical products are assumed in the formation of Lewis sites in other ways, such as dehydroxylation of various hydroxy cations4 or reduction of Cu2+ zeolite^.^ On the other hand, some a u t h o r ~ ’ -feel ~~~ that this arrangement produced by dehydroxylation of the H form of the zeolite is not the source of the Lewis activity of the zeolite but undergoes further changes to form extralattice A1 cations. This work was carried out to test the use of quantumchemical methods for elucidation of the nature of Lewis
(1) D. W. Breck, “Zeolite Molecular Sieves”, Wiley, New York, 1974. (2) P. A. Jacobs, “Carbogenic Activity of Zeolites”,Elsevier, New York, 1977. (3) H. W. Haynes, Jr., Catal. Rev.-Sci. Eng., 17, 273 (1978).
(4) J. W. Ward, J. Catal., 10, 34 (1968). (5) G. M. Naccache and Y. BenTarit, J . Catal., 22, 171 (1971). (6) G. H. Kiihl, “Molecular Sieves”, J. B. Uytterhoeven, Ed., Leuwen University Press; Zurich, 1973, p 227.
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The Journal of Physical Chemistry, Vol. 85, No. 13, 1981 1957
Lewis Sites in Dehydroxylated Faujasite Zeolites
TABLE I: CNDO/2 Energy of the Highest Occupied Molecular Orbital, E,,,, Molecular Orbital, ELuMO (eV), for Six-Ring Models of Various Faujasites
(eV), and of the Lowest Unoccupied
type of ring (cation)
EHOMO ELUMo EHOMO ELUMo
\
charge of cluster
H'O
0 0 -1 -1
-11.08 - 0.47 -7.43 1.93
Na' - 11.70 - 1.77 -8.44 2.05
AI(OH),+~ -11.50 (-12.06) - 2.47 (- 2.03) -8.53 1.06
~ iOH ( lZ
~
+
- 1 2 . 0 8 (-12.29) - 4.07 (- 3.07) - 9.00 -0.48
1
3
+
~
-12.21 (-12.83) - 5.57 (- 3.94)
\
-Sic
-AIC
-11.13 - 2.05 -7.53 1.18
-11.97 -0.59 -7.53 2.50
/
I
Clusters of the T,O,(OH),, type with an additional proton located in its equilibrium position at an 0 , oxygen atom. Clusters of the T,O,(OH),, type with the corresponding cation located in SI'position based on the Na faujasite geometry m ) and, in parentheses, on the Li faujasite geometry (the A1-0, distance is 1.90 X lo-'' (the A1-0, distance is 2.44 X i. Clusters depicted in Figure 1. a
m ) . 1 8 3 * g
sites in zeolites by calculating the electronic structure and the physical characteristics of the assumed products of H-zeolite dehydroxylation.
Model and Method Modeling of zeolites in terms of clusters containing a finite number of atoms of the zeolite represents a considerable simplification (notably it does not include the electrostatic field of the zeolite). However, for reasons discussed in a number of works7-19and because of the good quality of the results given in these works, it can be assumed that quantum-chemical calculations on such a model can yield reasonable qualitative information on the electronic structure of zeolites. It was found that the model representing &fold windows with oxygen atoms terminated by protons, i.e., clusters of the T606(OH)12H(T stands for A1 or Si atom) type, can be used as a model of the H form (as well as of the cationic forms) of the zeolite with the greatest number of acceptable atoms for the CNDO/2 calculation of the electronic structure. The additional proton (or protons) is localized a t the skeletal oxygens in an equilibrium position and corresponds to a skeletal hydroxyl group-a Bronsted site. Zeolite dehydroxylation (formation of Lewis sites) can be modeled in the framework of this sort of zeolite model as dissociation of one of the terminal oxygen atoms (of one of the terminal hydroxyl groups) and the proton of the skeletal 0-H group. I t is assumed in actual zeolites that a proton of the skeletal hydroxyl group is localized on the dissociating oxygen atom, and thus this oxygen should be bonded to both the Si and A1 atoms.13-15 In this work, a 6-fold window leading from the sodalite cavity into the hexagonal prism was employed as a model of the zeolite, and dissociation of the O1 oxygen atom was assumed. This process thus leads to formation of tricoordinated Si and A1 atoms, which, for a Si:Al ratio of 5 and 2, can be modeled by the Si5A106(OH)lland Si4A1206(OH)ll-clusters, respectively (cf. Figure 1). In calculation of the electronic (7)I. D. Mikheikin, I. A. Abronin, G. M. Zhidomirov, and V. B. Kazanskii, J.Mol. Catal., 3, 435 (1978). (8)V. B. Kazanskii, A. M. Gritacov, V. M. Andreev, and G. M. Zhidomirov, J. Mol. Catal., 4, 135 (1978). (9)V. I. Lygin and V. V. Smolikov, Zh. Fiz. Khim., 49, 1526 (1975). (10) W. J. Mortier, P. Geerlings, C. VanAlsenoy, and H. P. Figeys, J. Phys. Chem., 83,855 (1979). (11)G. V. Gibbs, E. P. Meacher, J. V. Smith, and J. J. Pluth, ACS Symp. Ser., No.40, 19 (1977). (12)J. A. Tossel and G. V. Gibbs, Phys. Chem. Miner., 2 , 21 (1977). (13)J. Dubs@, S.Beran, and V. BosdEek, J. Mol. Catal., 6,321(1979). (14)S.Beran, J. Dubs@, V. BosdEek, and P. Jiru, React. Kinet. Catal.
issue.
H
H
H
H
H
Flgure 1. Cluster models of dehydroxylated zeolies with depiction of Wiberg bond orders for clusters with the Si:AI ratio equaling 5 and, in brackets, 2, respectively.
structure of these models, the arrangement of the remaining atoms (i.e., the cluster geometry) was identical with the arrangement of the H form of the zeolite; 2o i.e., only the first step in the dehydroxylation of the zeolite waa described, not including the further rearrangement of the skeletal atoms after removal of the O1oxygen from the zeolite skeleton. The standard version of the CNDOI2 method2' with s,p basis set for the Si and A1 atornsl3J6was used in the calculations. The Wiberg bond orders22were used to characterize the bonding conditions in the clusters.
Results and Discussion The LUMO (lowest unoccupied molecular orbital) energy values calculated for molecules reflect the ability of these molecules to accept a lone electron pair. In the clusters produced by dehydroxylation, containing tricoordinated Si or Al, this LUMO is always largely localized on the corresponding trivalent Si or A1 atom. The LUMO energy values for the studied clusters, listed in Table I, indicate that tricoordinated Si and especially A1 exhibit a rather weak ability to accept a lone electron pair, compared with zeolites containing various forms of Al cations localized in the S i cation position, obtained by regular ion exchange of the A13+,Al(OH)2+,and Al(OH)2+cations. For tricoordinated Si the electron-acceptor ability of the cluster is comparable to that of the A1(OH)2+species localized in the S i positionlg but lower compared with the A1(OH)2+ and A13+ cations. Tricoordinated A1 exhibits very weak electron-acceptor ability compared with that of the hydroxylated H form of the zeolite. Consequently, it seems probable that the tricoordinated A1 centers produced in the first step of dehydroxylation are not bearers of the Lewis acidity of the dehydroxylated zeolites. From the point of view of the LUMO energy, the tricoordinated Si (20)D. N.Olson and E. Dempsey, J. Catal., 13,221 (1969). (21)J. A. Pople and D. L. Beweridge, "Approximate Molecular Orbital Theory", McGraw-Hill, New York, 1970. (22)K. B. Wiberg, J. Am. Chem. Soc., 90, 59 (1968).
J. Phys. Chem. 1981, 85, 1958-1960
1958
produced by dehydroxylation could probably act as the Lewis site, but the probability of existence of such an arrangement appears to be improbable (see further discussion). Further possible changes in the dehydroxylation products formed in the first step are reflected in the values of the Wiberg bond orders calculated for dehydroxylated clusters and depicted in Figure 1. For comparison, the Wiberg bond order of the Si-0 and A1-0 bonds of T606(OH)lz-type clusters, depending on the Si:A1 ratio, attain values of 0.80-1.00 and 0.55-0.65, respectively.l* It is apparent from the Wiberg bond orders for clusters containing tricoordinated Al that dehydroxylation leading to formation of tricoordinated A1 atoms results in strengthening of the Al-0 bond of this particular Al atom and insignificant changes in the values of the other bond orders. The calculations thus suggest that the cluster containing the tricoordinated A1 does not have a tendency
to split other bonds of the zeolite skeleton, although possible structural changes in bond lengths and valence angles are not excluded. Different conditions prevail in clusters containing tricoordinated Si. The unsaturated valences of the tricoordinated Si atom lead to strengthening of its Si-0 bonds, resulting in weakening of the remaining T-0 bonds of these three oxygen atoms. This effect is particularly noticeable for the corresponding Al-03 bond which becomes very weak, as is apparent from Figure 1. The calculations thus indicate the possibility of dissociation of this A1-0 bond, leading to formation of a >Si=O site and tricoordinated Al. The >Si-0 structure will not, however, probably be the final product of the dehydroxylation but will undergo further changes. For example, it could react with a water molecule to produce the >Si(OH), structure or interact with a skeletal Si atom leading to formation of five-membered ring and dicoordinated Al.
Proton Inventory of the Resin-Catalyzed Hydrolysis of Ethyl Acetate‘ Julio F. Mata-Segreda’ School of Chemistry. Universtty of Costa R i a , Cludad universitaria “Rodrig0 Facio”, Sen Josd 2060, Costa Rica (Received: January 20, 198 1; In Final Form: March 26, 198 1)
The kinetics of the hydrolysis of ethyl acetate catalyzed by the strongly acidic ion-exchangeresin Dowex 50W-X2 was studied at 25 “C in mixtures of light and heavy waters. The second-order rate constants vary with the deuterium atom fraction ( n )as 106k,/(s-’ mequiv-’) = (1.82 f 0.06)(1 - n + 0.83r1)~/(1- n + 0.69r1)~.This result suggests that the observed overall solvent isotope effect is generated by three protons changing their binding state in going from the reactant to the transition state.
Introduction Many features of active sites in enzymes are best described in terms of the hydrophobic nature of those catalytic entities. Therefore, it was thought to be useful to study reactions of biological interest taking place inside a polymer matrix, in order to explore the effect of a hydrophobic environment on the stability of model transition-state complexes. There have been many reports on the catalytic effect of ion-exchange resins on hydrolytic reactions3-10 and of (1) Resin Catalysis, part 3. For part 2, see ref 10. A preliminary version of this work was presented at the 5th IUPAC Conference on Physical Organic Chemistry, Santa Cruz, CA, Aug 1980. (2) Research Fellow of the Consejo Nacional de Investigaciones C i e n t h a s y TecnolBgicas (Costa Rica). (3) (a) Haskell, V. C.; Hammett, L. P. J. Am. Chem. SOC. 1949, 71, 1284. (b) Bernhard, S. A.; Hammett, L. P. Ibid. 1953,75,1798. (c) Ibid. 1953, 75,5834. (d) Bernhard, S.A.; Garfield, E.; Hammett, L. P. Ibid. 1954, 76, 991. (e) Riesz, P.;Hammett, L. P. Ibid. 1954,76,992. (f) Samelson, H.; Hammett, L. P. Ibid. 1956, 78, 524. (9) Chen. C. H.; Hammett, L. P. Ibid. 1958,80, 1329. (4)Nieto, A. J. Chem. Educ. 1974,50,846. (5)Barral, M. A.; de Cabrera, A. M. T.; Castro, A. A.; Parera, J. M. Rev. Fac. Ing. Quim. Univ. Nac. Litoral 1970,39,275. (6)Bhatia, S.:Raiamani, K.: Raikhowa, P.: Rao, M. G. Ion Exch. Membr. 1973,1, 127.(7)(a) Gold, V.;Liddiard, C. J. J. Chem. Soc., Faraday Trans. 1 1977, 73,1119. (b) Gold, V.;Liddiard, C. J.; Martin, J. L. Ibid. 1977,73,1128. Shenoy, S. C.; Rao, M. S.; Rao, M. G. J. Appl. Chem. (8)Rajamani, K.; Biotechnol. 1978,28,699. 0022-3654/81/2085-1958$01.25/0
theoretical contributions to the formulation of the kinetics in such systems.” The most relevant features of resincatalyzed reactions are the following: (1)The kinetics are first order in substrate concentration and first order in the resin bulk c~ncentration.~.~,’ (2) Diffusion processes are faster than chemical stepss6Vs (3) The reactions usually show lower values than the corresponding homogeneously catalyzed reactions, but more negative AS*.’OJ2 (4) The magnitude of solvent effects on the rate of these heterogeneous reactions is greater than the effect observed for the homogeneous analogue^.^ This work presents results on the effect of solvent isotopic composition on the rate of hydrolysis of ethyl acetate catalyzed by the strongly acidic ion-exchange resin Dowex 50W-X2, in order to obtain a quantitative description of the structure of the transition-state complex for the resin-catalyzed hydrolysis of simple esters. We shall also compare our results with the homogeneous counterpart, based on data found in the literature.13J4 ~~~~~
~
(9)Mata-Segreda, J. F. Rev. Latinoam. Quim. 1979,10, 57. (10)Mata-Segreda, J. F. Rev. Latinoam. Quim., in press. (11)(a) Helfferich, F. J. Am. Chem. SOC.1954,76,5567.(b) Helfferich, F. “Ion Exchange”; McGraw-Hill: New York, 1962. (12)(a) Regen, S. L.; Besse, J. J.; McLick, J. J. Am. Chem. SOC. 1979, 101,116. (b) Regen, S. L.; Besse, J. J. Ibid. 1979,101,4059.(c) Regen, S.L.; Heh, J. C. K.; McLick, J. J . Org. Chem. 1979,44, 1961. (13)Nelson, W.E.;Butler, J. A. V. J. Chem. SOC. 1938,957.
0 1981 American Chemical Society
