J. Phys. Chem. 1994, 98, 5622-5626
5622
IR Spectroscopic Studies and Quantum Chemical Calculations Concerning the 0-H Dissociation Energies in Zeolites NaHX and NaHY J. Datka,' E. Broclawik,+and B. Gil Faculty of Chemistry, Jagiellonian University, 30-060 Cracow, Ingardena 3, Poland Received: December 6, 1993; In Final Form: March 19, 1994"
Four kinds of bridging hydroxyls, (AlO)3Si-OH-Al(OSi)3, (AlO)2(SiO)Si-OH-Al(OSi)3,(AlO)(SiO)zSiOH-Al(OSi)3, and (SiO)3Si-OH-Al(OSi)3, exist in zeolites NaHX and NaHY. Thevalues of 0-H dissociation energies for all kinds of these hydroxyls have been derived from IR spectra (from the values of frequency shifts accompanying the hydrogen-bonding formation with aromatic hydrocarbons and their derivatives). They have also been calculated by the MNDO method by using a series of clusters with various numbers of A1 atoms. According to both experimental and theoretical results, the increase in the number of A1 atoms close to the bridge decreases the acid strength of bridging hydroxyls. The values of 0-H dissociation energies obtained from I R spectra and calculated by the MNDO method agree very well.
Introduction The IR spectra of zeolites NaHX and NaHY show two distinct bands at about 3650 and 3550 cm-l. The 3650-cm-l band is characteristic of Si-OIH-A1 groups (the protons of which are connected to 0 1 oxygens and project into supercages) and the 3550-cm-1 onesofSi-Oa-Al (the protons of which are connected to O3oxygens and project into hexagonal prisms). The Si-OIHAI groups which are easily accessible to reagent molecules are active sites for many reactions catalyzed by zeolites NaHX and NaHY. Their acid strength is an important parameter, characterizing the catalytic activity of zeolites. Our previous IR studies*-3 have shown that one kind of acidic hydroxyls exists in NaHX and three kinds in zeolites NaHY. They have been attributed to (AlO)3Si-OH-Al(OSi)3, (A10)2(SiO)Si-OHAl(OSi)3, (AlO)(SiO)2Si-OH-Al(OSi)3,and (Si0)jSi-OHAl(OSi)3, respectively. The results of IR studies agreed well with the 29Si MAS N M R data:4s5indeed, only one Si(4Al) signal in X and four signals, Si(3Al), Si(2Al), Si(lAl), and Si(OA1) (not forming bridging hydroxyls), in zeolite Y have been detected. The present study concerns 0-H dissociation energies of all four kinds of Si-OIH-AI hydroxyls appearing in zeolites NaHX and NaHY. The heterolitic 0-H dissociation energies (proton affinity = PA) have been obtained in two ways: in I R spectroscopic experiments and in MNDO quantum chemical calculations. Thus experimental and theoretical values can be compared and discussed in terms of modeling acidic centers and interpreting their properties. The experimental values of 0-H dissociation energies have been estimated from the values of frequency shifts (Au) accompanying the hydrogen bonding of the OH groups with aromatic hydrocarbons and their derivatives (benzene, toluene, p-xylene, fluorobenzene, and chlorobenzene) by means of the BellamyHallam-Williams relation.'j The theoretical values have been calculated as the energy differences between the clusters, modeling different kinds of bridging hydroxyl groups and their deprotonated forms. The calculations have been performed by means of the quantum chemical semiempirical MNDO Experimental Section Zeolite NaX (Si/Al = 1.06) was synthesized a t the FritzHaber-Institut (Berlin). It was transformed into an ammonium form by CH3COONHd solution (pH 6-6.7) treatment at room f Institute of Catalysis, Polish Academy of Sciences 30-239 Cracow, Niezapminajek 2. 0 Abstract published in Advance ACS Abstracts, April 15, 1994.
temperature. The Na/NH4 exchange degree was 28%. Zeolite Y (Si/Al = 2.56) was synthesized at the Institute of Industrial Chemistry (Warsaw). It was transformed into an ammonium form by NHdNO3 solution treatment a t 350 K. The exchange degree was 77%. For IR studies, zeolites N a N H a and N a N h Y were pressed into thin wafers and activated "in situ" in an IR cell a t 650 K (zeolite X) and 720 K (zeolite Y), for 1 h. Benzene, toluene, p-xylene, chlorobenzene (POCh Gliwice), and fluorobenzene (Fluka), all chemical grade, were used. IR spectra were recorded by using a Bruker IFS48 PC Fourier transform spectrometer, equipped with a MCT detector. Calculation Method The calculations were carried out by means of the quantum chemical semiempirical MNDO method. The recent version of the AMPAC package adapted for IBM PC/486 computers for semiempirical calculations with the MNDO Hamiltonian in standard parametrizationg has been used. S C F electronic structure calculations were performed for each of four clusters modeling various hydroxyls postulated in the lattice. 0-H dissociation energies were evaluated from the total energy difference between the cluster and its deprotonized form. The clusters describing different environments of Si-OH-AI groupings in faujasites were selected in such a way that the number of aluminum atoms in the vicinity of the bridge could be explicitly included in the second coordination sphere. In accordance with the postulated existence of AI,IS~~,,O~S~-OH-AI(OS~)~, where n = 1-4, types of hydroxyls in zeolites NaHX and NaHY (labeled as Si(nAl)), the following cluster models were considered here: [ (OH)2(H20)A10],1 [H3SiO]~,,Si-OH-Al[OSiH3]3. Each Si or A1 atom was tetrahedrallycoordinated. Si and A1 in the bridge were coordinated via oxygen to three other Si or A1 atoms in the second sphere. The third cationic coordination sphere could not be included into calculations and was already off the cluster boundary; thus the problem of saturating dangling bonds had to be solved. It has been the subject of separate studies and has already been discussed elsewhere10 and described by us for the particular clusters in the previous paper.11 We have found that the proper choice is the -SiH3 unit for the terminal silicon and the -Al(OH)*(H20) unit for the terminal aluminum. The basic cluster unit is shown schematically in Figure 8. Bond distances were optimized for each of four cluster models, keeping tetrahedral coordination of silicon and aluminum atoms, and the bridge angle was kept equal to its experimental value. Optimized geometrical parameters are marked also in Figure 8.
0022-3654/94/2098-5622%04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5623
0-H Dissociation Energies in Zeolites
ZEOLITE X
A
-1 .o
2800
2800
3200
3600 ~
ZEOLITEY
3200
2
3600
.
0
~
I -0.5 2800
3200
3600
C
P
%
D 2.0 TOLUENE
Figure 1. IR spectra of Si-OIH-AI groups in zeolite NaHX forming hydrogen bonds to chlorobenzene molecules. Part A: (a) spectrum of activated zeolite, (b) spectrum recorded after chlorobenzene sorption, (b - a) difference spectrum. Part B: spectfa of hydrogen-bonded OH groups recorded at various loadings (from about 10 to 80% hydroxyls
engaged). Part C: second-derivative diagrams of the spectra presented in part B.
TABLE 1: Frequency Shifts Au (cm-l) of Hydrogen-Bonded OH(lbOH(4) Groups zeolite Y zeolite X OH(1) OH(2) OH(3) OH(4) ., .. . . ., 165 228 246 273 fluorobenzene 181 233 267 313 chlorobenzene 21 1 290 325 356 benzene 228 283 322 367 toluene 242 329 392 434 p -x y 1ene
IR Spectroscopic Results. Zeolite NaHX. The spectrum of activated zeolite NaHX is presented in Figure 1A (spectrum a). Small portions of fluorobenzene, chlorobenzene, benzene, toluene, and p-xylene were sorbed a t room temperature, and IR spectra were recorded after each sorption. The spectra recorded after chlorobenzene sorption are presented in Figure 1, as an example. Hydrogen bonding of Si-OIH-AI results in a shift of IR band to lower frequencies (Figure lA, spectrum b). The bands of hydrogen-bonded Si-OIH-AI recorded at various chlorobenzene loadings (varying from about 10 to 80% of all hydroxyls engaged) are presented in Figure 1B. These bands are symmetrical and not split. It can best be seen in the second-derivatives diagram (Figure 1C). The same results were also obtained with other sorbates (fluorobenzene, benzene, toluene, p-xylene); in all cases the shifted bands were symmetrical and not split. This indicates that Si-OIH-AI groups in zeolite NaHX are homogeneous, which is in agreement with our earlier conclusions.~-3 The frequencies of shifted bands did not depend on the amount of sorbate, and the average values of frequency shifts Av are presented in Table 1. Zeolite NuHY. The spectra of activated zeolite NaHY are presented in Figure 2A-E (spectra a). As in the case of zeolite NaHX, small portions of fluorobenzene, chlorobenzene, benzene, toluene, and p-xylene were sorbed, and the spectra of Si-OlHA1 engaged in hydrogen bonding are presented in Figure 2A-E (spectra b) and Figures 3-5A,E. Contrary to zeolite NaHX, the
h
2800
3200
3600
2800
3200
3600
v [cm-'I
Figure 2. IR spectra of Si-OIH-AI groups in zeolite NaHY forming hydrogen bonds to fluorobenzene (A), chlorobenzne (B), benzene (C), toluene (D), andp-xylene (E): (a) spectra of activated zeolite; (b) spectra recorded after the sorption; (b - a) difference spectra.
shifted bands are asymmetric, and three submaxima are present in second-derivatives diagrams (Figures 3-5B,F). This indicates that three kinds of Si-OIH-AI groups of various acid strengths are present in zeolite NaHY, which agrees with our earlier c~nclusions.~-~ In order to find the position of submaxima in the band of Si-OIH-A1 engaged in hydrogen bonding, the band fit procedure according to Phita and Jones**J3has been performed (the frequencies of minima in the second-derivative diagrams were taken as input data). Before the band fit was performed, the spectra were smoothed by the spline function method.14 The results of band fit are presented in Figures 3-5C,G (low sorbate loadings) and Figures 3-5D,H (high sorbate loadings). As in the case of zeolite NaHX, the average values of frequency shift Av for all three kinds of Si-OlH-AI in zeolite NaHY; the results of band fit of the spectra recorded a t various loadings are also presented in Table 1. Thevalues of 0-H proton affinities (PA) were calculated from the Bellamy-Hallam-Williams (BHW) equation6 relating the values Av of two acids (with one of them considered as a "standard") engaged in hydrogen bonding with the same electron donors:
where X'-OH represents the "standard acid". The values of proton affinities (PA) were calculated from the values of BHW
~
Datka et al.
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994
5624
ZEOLITE Y FLUOROBENZENE
ZEOLITE Y
1::::m CHLOROBENZENE
TOLUENE
BENZENE
?------
0.10
$2 0.05
0.00
3300
p
O.O
3450
-0.4
3300
3
9
B -0.15
3450
0.00
3400
7 0.0
$ -0.05
B
D
3250
3300
G
k BAND FIT
-0.2
J- - - -$ 3450
1
3250
3400
3250
3400
3250
3400
O.OO -0.05 -0.10-
3400
3250
BAND FIT
0.0 3300
3450
3400
3250
H
n
H
I
k BAND FIT
I 0.10
f
005 0 00
3450 \I
010 005
e
Q
3300
0
e“ 51 2
0 00
3300
[cm-’I
3450 v [cm-’]
3250
3400 v
[cm-’]
3250
3400 v [cm-’]
Figure 3. IR spectra of Si-OIH-AI groups in zeolite NaHY forming hydrogen bonds to fluorobenzene (A-D) and chlorobenzene(E-H): (A,E) spectra of hydrogen-bonded OH groups recorded at various loadings (from about 20 to 80% hydroxyls engaged); (B,F)second-derivative diagramsof the spectra presented in parts A and E; (C,G)results of band fit for low loadings of fluorobenzene and chlorobenzene; (D,H) results of band fit for high loadings of fluorobenzene and chlorobenzene.
Figure 4. IR spectra of Si-OIH-AI groups in zeolite NaHY forming hydrogen bonds to benzene (A-D) and toluene (E-H): (A,E) spectra of hydrogen-bonded OH groups recorded at various loadings (from about 20 to 80% hydroxyls engaged); (B,F)second-derivative diagrams of the spectra presented in parts A and E (C,G)results of band fit for low loadingsof benzene and toluene; (D,H) resultsof band fit for high loadings of benzene and toluene.
slopes ( b ) and PA,, values of standard acids using the following equation:isJ6
TABLE 2: PA Values (kJ/mol) for OH(l)-OH(4) Calculated According to Various Standard Acids stdacids PAst OH(1) OH(2) OH(3) phenol 1448 1402 1364 1292 1446 1447 1433 acetic 1358 benzoic 1413 1409 1367 1300 chloroacetic 1394 1402 1368 1299 trifluoroacetic 1345 1433 1397 1326 av values 1419 f 9 1386 f 13 1315 f 12
PA = PAst- 448 log b The BHW plots of OH(l)-OH(4) groups for benzoic acid taken as the “standard” are presented in Figure 6. The Au values for the standard acids were taken from ref 16. All the plots are linear (Figure 6). Linear plots were also obtained with other standard acids (acetic, chloroacetic, trifluoroacetic, and phenol). In practically all the cases the correlation coefficients were higher than0.9. The PAvalues for OH(l)-OH(4) obtained withvarious standard acids (PA values for these standard acids were taken from refs 17 and 18) are presented in Table 2. The average values are also presented in Figure 7. The highest PA value corresponding to the least acidic Si-OlH-AI in zeolite NaHX (OH(1)) was 1419 kJ/mol. Lowervaluesof 1386,1315, and 1300 kJ/mol were obtained for more acidic hydroxyls in NaHY (OH(2)-OH(4)), respectively. The experimental values of proton affinities were compared with the theoretical ones obtained in MNDO calculations. MNDO Calculations. Theoretical counterparts to the experimental PA values discussed above were obtained as the differences of the total S C F energies of the appropriate cluster and its deprotonated anionic form. Proton dissociation energies of the clusters [(OH)2(H20)AlO],i [H3SiO].&i-OH-AI[OSiH3] 3 with n = 1-4 correspond to PA values of OH(4)-OH( 1) hydroxyls. The geometry of each particular cluster was optimized with the restrictions mentioned above. The value of the bond angle equal to 136.8’ was kept constant as the only parameter taken from experimenti9to avoid the artificial repulsing between the groups introduced as boundary saturators. The rotation of tetrahedral Si(A1) groups rendered several almost equivalent minima; the final conformation has been chosen so as to emerge with the model in which the additional protons connected with aluminum
OH(4)
~
1278 1341 1289 1284 1310 1 3 0 0 k 11
atoms other than the central bridging unit would also protrude into the zeolite supercavity (see Figure 9). Figure 9 shows the sketch of the three-dimensional supercavity model for the OH(3) hydroxyl type. Optimal geometry parameters for the bridging units in four studied clusters change monotonously within narrow limits, which are shown in Figure 8. The same is true with the atomic charges also shown in the picture. The calculations for the deprotonated forms were carried out with the fixed geometry of the parent cluster. We have found that the automatic geometry optimization scheme was too crude to properly discriminate the OH bond distances in different clusters (see Figure 8). Thus we have decided to make additional calculations for a few pointson the one-dimensional O-H potential energy curve close to the minimum. The curve was then fitted to a Lennard-Jones-type potential from which the optimum ROH and proton dissociation energies were calculated. Table 3 shows the results of the calculation. Column 2 lists theoretical deprotonization energies of the model clusters labeled as Si(4Al)-Si(lAl). Column 3 shows the PA values for the appropriate OH( 1)-OH(4) hydroxyls measured by IR spectroscopy. The agreement between the two columns is strikingly good, which is also visualized in Figure 7 where both energy plots are almost parallel. The last column in Table 3 lists the fitted 0-H bond distances which also change monotonously in going from the shortest bond in the case of the Si(4Al) unit
0-H Dissociation Energies in Zeolites
The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5625
P-XYLENE
@
O.1°
0.05 a
0.00 3150
3400
B
0.03
22 -0.03 -0.08
I I I , I I I 1 . I l , , , , I I , . I I I
3150
3400
0.00 3150
3400
D
BANDF FIT a c
4
I
0.10 0.05
a
0.00 3150
3400 v [cm-'I
0 -OSiH3
Figure 5. IR spectra of Si-OlH-AI groups in zeolite NaHY forming hydrogen bonds top-xylene: (A) spectrqa of hydrogen-bonded O H groups recorded at various loadings (from about 20 to 80% hydroxyls engaged); (B) second-derivative diagrams of the spectra presented in part A, (C) results of band fit for low loadings of p-xylene; (D) results of band fit for high loadings of p-xylene.
'
10
20
t
'
30
J
I
'
'
'
'
40
'
I
'
50
dv/vo x 1000 standard
Figure 6. Bellamy-Hallam-Williams plots of O H ( 1)-OH(4) groups in zeolites NaHX and NaHY with benzoic acid as a "standard".
0 - OSiH, or OAI(OH),H,O
Figure 8. Optimal parameters for the bridging units of clusters used in the calculations.
si' Figure 9. Sketch of three-dimensional supercavity model for the OH(3) hydroxyl.
TABLE 3: Theoretical and Experimental Deprotonization Energies (kJ/mol) and Optimum Hydroxyl Bond Distances (A, for the Studied Clusters deprotonization energy
postulated for the least acidic hydroxyl in zeolite NaHX to gradually elongated OH bonds for more acidic hydroxyls in zeolites NaHY. These results strongly suggest that the proposed clusters are the proper models for the four distinct Bronsted acid centers in zeolites NaHX and NaHY. Discussion The results obtained in this study indicate that four kinds of Si-OIH-A1 bridging hydroxyls (OH( 1)-OH(4)) exist in zeolites NaHX and NaHY. They are assigned to (A10)3Si-OHAl( OSi)3, (AlO)z(SiO)Si-OH-AI( OSi)3, (A10) ( S i O ) & O H Al(OSi)3, and (SiO)3Si-OH-Al(OSi),, respectively. These four
cluster Si(4Al)/OH( 1) Si(3Al) /OH(2) Si(2Al)/OH(3) Si(lAI)/OH(I)
GPr 1393 1363 1282 1243
PAIR 1419 1386 1315 1300
pP' on 0.9607 0.9624 0.9626 0.9629
kinds of bridging hydroxyls correspond to Si(4AI), Si( 3A1), Si(ZAl), and Si( 1Al) species found in 29SiMAS NMR studies.495 The proportion between the amounts of OH( 1)-OH(4) hydroxyls was the same as between S i ( 4 ) S i ( 1Al) signals.20 The proton affinity values (PA) obtained from the IR spectroscopic experiments and MNDO quantum chemical calculations (presented in
Datka et al.
5626 The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 Figure 7) are practically the same. The agreement between experimental (IR spectroscopic) and theoretical data concerning PA of bridging O H groups is much better in the case of our MNDO calculations than in the case of CNDO calculations.21 According to CNDO results, the PA values for OH( 1)-OH(4) groups were 1533, 1505, 1478, and 1444 kJ/mol. They are considerably higher than experimental ones and our MNDO results (Table 3). We believe our theoretical results to be well grounded and reliable since both the quantum chemical method and the model were validated thoroughly, closely following modern standards. The cluster models are relatively big; thus the next cationic neighbors to the bridge could be represented by real atoms with no extra parameters. The choice of the MNDO method is a compromise between requirements imposed by a "chemical accuracy" level and technical demands stemming from largesystem calculations. It is the NDDO-type method which has claimed the first sufficient level of approximation requested for reliable geometry optimization and calculations of chemical energetics.22 The field of quantum chemical calculations for alumina-silica clusters is not new. Nevertheless, we feel that due to more rigorous treatment, compared to previously reported semiempirical calculations and bigger models than those feasible for ab initio methods, the results presented here are of stronger predictive, quantitative value. According to both MNDO results and IR data, the increase in the number of A1 atoms close to the bridge results in the decrease of the acid strength of bridging hydroxyls. This is evidenced by the increase of PA values, and shortening 0-H bond. Such an effect may be explained by lower electronegativity of A1 as compared to Si. The difference in PA values between the less acidic OH(1) in zeolite NaHX and the most acidic OH(4) in NaHY is 119 kJ/mol (according to IR data) and 150 kJ/mol (according to MNDO results). This difference is bigger than that between acetic acid and trifluoroacetic acid (101 kJ/mol). The fact that OH(l)-OH(4) groups differ so much in their acid strengths influences their catalytic properties. It is wellknown that zeolites NaHX containing mostly OH(1) groups are much less active in acid-catalyzed reactions than NaHY ones
(containing OH(2)-OH(4) groups). Our previous study concerning zeolite NaHYI has shown that the removal of 10% of all Si-01H-AI hydroxyls by pyridine reduced catalytic activity by 70% and elimination of 46% of Si-OIH-AI reduces catalytic activity by 95%. Similar results were also obtained by Lombardo et a1.,23 who observed that small doses of ammonia poisoned zeolite NaHY in a neopentane cracking reaction. The molecules of bases (ammonia and pyridine) react with the most acidic hydroxyls at first. The removal of such most acidic hydroxyls reduces strongly the catalytic activity.
Acknowledgment. The sample of zeolite X was kindly supplied by Dr. H. Karge from the Fritz-Haber-Institut (Berlin). References and Notes (1) (2) (3) (4)
Datka, J.; Gil, B. J . Caral. 1994, 145, 372. Datka, J.; Boczar, M.; Gil, B. Langmuir 1993, 9, 2496. Datka, J.; Gil, B.; Hobert, H.; Meyer, K. To be published. Klinowski, J. Chem. Rev. 1991, 91, 1459. (5) Engelhardt, G.; Michel, D. High-Resolution Solid State NMR of Silicates and Zeolites; John Wiley & Sons: Chichester, New York, Brisbane, Toronto, Singapore, 1987. (6) Hallam, H. E. Infrared Spectroscopy and Molecular Structure; Davies, M., Ed.; Elsevier: Amsterdam, 1963. (7) Thiel, W. Tetrahedron 1988, 44, 7398. (8) Dewar, M. J. S.;Thiel, W. J. Am. Chem. SOC.1977, 99,4899. (9) Dewar, M. J. S.; Zoebish, E. G.;Healy, E. F.; Steward, J. J. P. J . Am. Chem. SOC.1985, 107, 3902. (10) Sauer, J. Chem. Rev. 1989, 89, 199; J . Mol. Catal. 1989, 54, 319. (11) Broclawik, E.; Datka, J.; Gil, B. J. Mol. Caral. 1993, 82, 347. (12) Phita, J.; Jones, R. N. Can. J. Chem. 1967, 45, 2347. (13) Phita, J.; Jones, R. N. Can. J . Chem. 1966, 44, 3031. (14) Schulz, H. M. Spline Analysis; Prentice-Hall: Engelwood Cliffs, NJ, 1973. ( 1 5 ) Paukstis, E. A.; Yourchenko, E. N. React. Kinet. Catal. Lett. 1981, 6, 131. (16) Datka, J.; Boczar, M.; Rymarowicz, P. J . Catal. 1988, 114, 368. (17) McMahon, T. B.; Kaborle, P. J . Am. Chem. SOC.1977, 99, 2222. (18) Bartmess, J. E.; Scott, J. A.; McIver, R. E. J. Am. Chem. Soc. 1979, 101,6046. (19) Olson, D. H.; Dempsey, E. J . Catal. 1969, 13, 221. (20) Gil, B.; Broclawik, E.; Datka, J.; Klinowski, J. J . Phys. Chem. 1994, 98. 930. (21) Kazanski, V. B. Structure and Reactivity of Modified Zeolites; Elsevier: Amsterdam, 1984; p 61. (22) Sadlej, J. Semiempirical Methods of Quantum Chemistry; PWN, Ellis Hortwood: Warszawa, 1985. (23) Lombardo, E. A.; Sill, G.A.; Hall, W. K. J . Carol. 1989, 119, 426.
