J. Phys. Chem. 1984, 88, 42-45
42
our red triplet absorption peaks (the low-energy shoulder and two intense peaks in the dashed curve, Figure 2). Thus, in this case it appears that the Wright and Balling analysis was performed on the red triplet. We do not attempt an explanation of the Wright and Balling observations. We do note that the potential surfaces in,\for example, the TI X (e + t2)JT case with spin-orbit coupling are complex, and this complexity may be reflected in the luminescence properties. We simply reiterate that interpretation of our Li/Xe triplet spectrum as arising from a single site seems clear-cut, and assignment of the red and blue triplets in Li/Kr each to a distinct Li site also seems secure. The situation in Li/Ar is less clear, but it seems likely that strongly overlapping red and blue triplets are present. Our work adds strong additional support to the growing body of evidence for a JT explanation of the triplet structure of S P transitions in matrix-isolated atoms. It would be very gratifying indeed if accurate JT calculations could reasonably explain both the absorption and MCD contours in the Li/NG systems. In that
-
event, the case for a J T explanation would be definitive, and the coupling parameters from such a treatment (along with the { values already obtained) would give detailed quantitative information about the interactions of a very simple probe (Li atoms) with noble gas host atoms. Note Added in Proof. Dr. M. C. M. O'Brien has now carried out accurate JT calculations in Li/Xe for the case, T X (e t2). Her resulting theoretical contours for both the absorption and MCD agree quite well with experiment.
+
Acknowledgment. We are much indebted to Professor Philip J. Stephens for a number of illuminating comments, especially for his suggestion, carried out in section IV.C.2, that the crystal field model be treated on an equal footing with the J T model of section 1V.A. This work was supported by the National Science Foundation under Grant CHE8025608. Registry No. Argon, 7440-37-1; krypton, 7439-90-9; xenon, 744063-3; lithium, 7439-93-2.
Conjugate Acid-Base Pairs in Zeolites D. Barthomeuft Corporate Research-Science Laboratories, Exxon Research and Engineering Company, Linden, New Jersey 07036 (Received: March 28, 1983) Basic site strength has been studied in various zeolites (X, Y, L, mordenite, ZSM-5) by using pyrrole adsorption. The NH shift of the IR band at -3200 cm-' and the presence of combination bands at 2940 and 2850 cm-' were used to characterize basicity strength for a series of alkaline cation forms. These two sets of results show a good correlation with the oxygen charges calculated by using the Sanderson equalization principle. The changes in basic properties with the AI/(AI + Si) content are also consistent with the calculated oxygen charge except for mordenite. A model of dipoles centered on the cations is proposed for the conjugate acid pairs formed between oxygen and cations in the walls of the cavities. Because of the importance of the geometric arrangement of cations in oxygen rings, the interaction of adsorbed molecules with these pairs should be dependent on the zeolite structure.
Introduction Acidity and basicity of oxides has been studied for a long time with a greater emphasis on acidic Concomitantly the acidic catalysis by solid acids has received much greater attention since base-catalyzed reactions appear to be of less practical importance. The existence of basic sites has been postulated for a long time in zeolites. Their behavior as catalytic centers has been reported in cationic zeolites for various In all cases the specific activity of basic sites increases in the order Li C N a C K C R b C Cs and for faujasite zeolites Y < X. It has also been reported that platinum particles dispersed in alkaline L zeolites bear a negative charge resulting from a synergism effect between the electron-donating properties of these basic catalysts and the L zeolite structure fielde8 An interesting way of characterizing basic sites has been reported recently using pyrrole as an acid.9 The shift in the wavenumber of the I R N H stretching vibration is used as an evaluatioon of the basic strength of sites interacting with the H atom. We applied this technique to the characterization of the basicity of a large variety of zeolites in an attempt to quantify the changes in basic strength with cation identity, aluminum content, and zeolite structure. Experimental Section Zeolites of the faujasite type X, Y , L and mordenite exchanged with various group 1A cations have been used. One Cs-ZSM-5 sample with a %/A1 ratio of 24 has also been tested. Samples have been prepared from the sodium forms contacted with the corresponding salt chloride solutions. Their chemical composition t Present address: Laboratoire de Chimie des Solides, UniversitE Paris VI, 75230 Paris Cedex 05, France.
0022-3654/84/2088-0042$0 1.50/0
TABLE I : Unit-Cell Composition mordenite Li L i 3 . 5Na3(A10 216. 2 141.5 Na Na6.5(A102)6.5~si02)~1,5 K K6.5(A10 2)6.5( 2 141.5 Rb Rb6, 5(A10 2 ) 6 , 5 ( si02)41, 5 y type Li Li40Na17(A102)57(Si02)135 Na Na57(A102)5 , ( s i 0 2 ) 1 3 j K K56Na1(A102)57(Si02)13~ Rb
b43Na
2)5,(si02)
135
TABLE 11: Pyrrole Adsorption on Mordenite in Various Cationic Forms intensity mordenite G( N H cationic stretch), 2940 2850 form cm-' cm-l cm" Li 3430 none none Na 3400 none none none none K 3380 none Rb 3320 none is given in Table I. Self-supported wafers (-20 mg, 4 1.8 cm) were evacuated at 500 OC overnight, contacted with O2 at 400
0 1984 American Chemical Society
Conjugate Acid-Base Pairs in Zeolites
The Journal of Physical Chemistry, Vol. 88, No. 1 , I984 43
TABLE 111: Pyrrole Adsorption on Zeolites in the K Form
uNH cm-1
U(NH AI/ stretch), (AI t Si) cm-'
mordenite L Y X
0.13
3380
0.24 0.29 0.455
3400 3330 3200
I
'
I
intensity 2940 2850 cm-' cm-' none none medium medium medium medium intense intense I
I
' I
1
3400
-
3300
-
I
I
I
1
A
1 3200
-
-0.25
-0.30
.0.35
\.
.0.40
OXYGEN CHARGE
3100
Figure 2. Pyrrole NH wavenumber change as a function of calculated oxygen charge for zeolites in various cationic forms: (A)Li, (0)Na, (A)
1
K, (O)Rb, ( 0 )Cs; (a) mordenite, (b) L, (c) Y , (d) X. "N H
Li 0.8
Na
K 1.2
Rb
Cs
1.6
IONIC R A D I U S ( A )
Figure 1. Pyrrole NH wavenumber change as a function of alkaline cation radius for various zeolites: (a) experimental limit for nonbasic oxides, (b) ZSM-5, (c) L, (d) mordenite, (e) Y,(f) X.
OC (200 torr-), and evacuated at 500 OC for 4 h. Pyrrole was adsorbed at its vapor pressure at room temperature for 10 min and the spectrum was recorded. According to the assignments of Scokart and Rouxhet? besides the band at -3400 cm-' ascribed to the N H stretching vibration, bands appear at 2940 and 2850 cm-' which are combination bands. The reference wavenumber of the -34OO-cm-' band on nonbasic oxides is close to 3420 cm-'?
Results and Discussion Table I1 reports, as an example, the bands characteristic of pyrrole adsorption on the various cationic forms of mordenite, and Table I11 gives the same information for various zeolites in the same cationic form (K). According to Scokart and Rouxhet? the shift from a limit reference value of -3410-3420 cm-' of the N H stretching band of adsorbed pyrrole to low wavenumbers is related to an increase in the basic strength of the adsorption sites, and the bands at ca. 2940 and 2850 cm-' are attributed to combination bands observed only in the presence of strong basic sites. In the light of such assignments, Table I1 shows that the basic strength of cationic mordenite increases in the order Li < Na < K < Rb. The value of 3430 cm-' obtained for the Li form is almost that of the reference value of adsorbed pyrrole, which indicates no detectable basicity of this zeolite. The absence of the combination bands at 2940 and 2850 cm-' suggests that, even for the other cation forms of mordenite, the basicity detected by the shift of the ca. 3400-cm-' band is not a very strong one. Zeolites X, Y , and L show trends similar to those observed in mordenite (Figure 1). Moreover the scale of basic strength is shifted with the Al/(AI + Si) ratio of the zeolite. For instance, the potassium forms give (1) Tanabe, K. 'Solid Acids and Bases"; Academic Press: New York, 1970. (2) Pines, H. "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press: New York, 198 1. (3) Sidorenko, Y. N; Galich, P. N.; Gutyrya, V. S.; Ilin, V. G.; Neimark, I. E. Dokl. Akad. Nauk SSSR 1967, 173, 132. (4) Yashima, T.; Sata, K.; Hara, N. J . Catal. 1972, 26, 303. (5) Unland, M. L.; Barker, G. E. U S . Patents 4 115 424 and 4 140 726. (6) Yashima, T.; Suzuki, H.; Hara, N. J . Catal. 1974, 33, 486. (7) Ono, Y. In "Catalysis by Zeolites"; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 19. (8) Besoukhanova, C.; Guidot, J.; Barthomeuf, D.; Breysse, M.; Bernard, J. R. J . Faraday Trans. 1 1981, 77, 1595. (9) Scokart, P. 0.;Rouxhet, P. G. Bull. SOC.Chim. Belg. 1981, 90, 983; J . Chem. Soc., Faraday Trans. I 1980, 76, 1476.
-0.25
I
I
I
-0.30
-0.35
-0.40
I
-0.45
OXYGEN CHARGE
Figure 3. Pyrrole NH wavenumber change as a function of calculated oxygen charges for various zeolites in the K form: (A)mordenite, (0)
L, ( 0 )y,(VI x.
the characteristic pyrrole bands reported in Table 111. This is in agreement with what is observed in Na, X, and Y . 9 Due to its too low A1 content, the Cs-ZSM-5 does not show any v(NH) shift or any combination band. Except for mordenite, as the aluminum content increases, the N H stretching vibration is shifted to lower wavenumbers and the two combination bands become more intense. The similar results obtained with all the cationic forms studied, Li, Na, Rb, Cs, show that for any given cation the basic strength of the zeolite increases with increasing aluminum content. The basic sites are very likely the oxygen atoms since the zeolites studied do not show significant O H groups. Since pyrrole does not enter the small cages, only the oxygens of the rings in the larger cages of these various structures are involved in the reaction. The cations are close to the center of these windows in sites like SII in the supercage of the faujasite structure, for instance. An increase in basic strength should correspond to an increase in the negative charge on the oxygen. Recently several methods have been used to calculate charges in atoms in zeolites.'OJ1 An average charge on the oxygen for the samples under study can be calculated by using the Sanderson electronegativity equalization principle.'* According to this principle, in a molecule with different atom electronegativities, the electrons will be redistributed such that they will be equally attracted to the nuclei in the bond. The molecule intermediate electronegativity is postulated to be the geometric mean of the compound atoms of the molecule under consideration. Using the Sanderson electronegativity scale one (10) Beran, S.; Dubsky, J. J . Phys. Chem. 1979, 83, 2538. (11) Jacobs, P. A,; Mortier, W. J.; Uytterhoeven, J. B. J . Inorg. Nucl. Chem. 1978, 40, 1919. (12) Sanderson, R. T. In "Chemical Bonds and Bond Energy"; Academic Press: New York, 1976.
44
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
TABLE IV: Acidity Measurements (mequiv/g) on Zeolitesa indicator (pKA) bromothymol bromocresol blue (6.8) green (4.6) 1.5 -0.05
zeolites Li-Y Na-Y 1.0 0.04 K-Y 0.1 0.01 Na-X 0. I 0.03 Rb-X 0 0 a Measurements performed on 550 “C pretreated zeolites. Titration with pyridine dissolved in dry isooctane. can calculate an average partial charge on the atoms of a chemical compound. The charge on the oxygen atom for the zeolites is plotted in Figure 2 along with the pyrrole N H wavenumber. For each of the zeolites the shift to low wavenumbers of the N H stretching band ihcreases with the negative oxygen charge. Zeolites X, Y, and L give results which fit the same curve while mordenite is quite different. For the same cation, i.e., K, the shift increases also with the same parameters for the zeolites: mordenite, L < Y < X (Figure 3). In fact, the results of Figure 3 indicate a slight inversion between the N H band shift and the oxygen charge for mordenite and L zeolite. By contrast the intensities of the combination bands at ca. 2940 and 2850 cm-’ are higher in L than in mordenite (Table 111), reflecting a stronger basicity for the former which is in line with the oxygen charge. Those results and the ones in Figure 2 suggest that the N H band shift in mordenite may be dependent on the zeolite structure. The pyrrole N H shift should be considered as a guide for basicity strength comparison only for similar structures. For different zeolites, both N H band shift and combination band intensities should be used for a qualitative comparison. These observations rely on a semiquantitative evaluation of the combination band intensities (no quantitative comparison between the various structures could be obtained) and on the validity of the Sanderson electronegativity principle for oxygen charge calculation. Some doubts exist on the physical meaning of Sanderson electronegativity calculation results at very low A1 contents in zeolite^.'^*'^ Such restrictions would not apply to mordenite. Moreover, C N D 0 / 2 calculations of atom charges in Na-X and Na-Y zeolites indicate only very small differences (positive or negative) for the oxygen charges between X and Y,l0 Such values do not show any correlation with pyrrole adsorption or catalytic results. If one accepts that pyrrole detects differences in basicity, the Sanderson calculation gives better trend. It might seem obvious that the oxygen charge increases with the decrease in the electronegativity of the cations (Li to Cs) or of the T atoms (Si to Al) in the ionic structure of zeolite. What is brought in addition to that information is a good correlation between these changes, the direct basicity measurements, and the catalytic properties described in ref 3-7. In order to have a better understanding of the basicity in these zeolites, it is necessary to consider also their acidity since the general theory of acids and bases associates both properties. It is well-known that cationic (group 1A) zeolites have weakly acidic properties.lS-” In faujasites the site number has been measured mainly for N a or K and their strength for the series of alkaline cations.15 A direct measurement of the acidity (strength and amount) of the present faujasite type samples is reported in Table IV. The titration has been performed with pyridine in the (13) Abbas, S. H.; AI-Dawood, T. K.; Dwyer, J.; Fitch, F. R; Georgopoulos, A,; Machado, F. J.; Smyth, S. M. In “Catalysis by Zeolites”; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 127. (14) Barthomeuf, D. “Advances in Catalytic Chemistry 11, Symposium, Salt Lake City, Utah, 1981”. (15) Ward, J. W. In “Zeolite Chemistry and Catalysis”; Rabo, J., Ed.; American Chemical Society: Washington, DC, 1976; ACS Monogr. No. 171, p 118 and references therein. (16) Kladnig, W. J. Phys. Chem. 1976, 80, 262. (17) Unger, K. K; Kittelmann, U . R.; Kreis, W. K. J. Chem. Technol. Biotechnol. 1981, 31, 435.
Barthomeuf TABLE V : Proposed Scale of Acid-Basc Properties for Group 1 A Metal Fauiasitesa acid base
:1
increase in acidity strength
NaX
KY lRbY NaX increase in KX basicity strength RbX csx a Using pyridine and pyrrole t o study respectively the acidity and the basicity.
+I
presence of colored indicators. The number of sites of each strength is lower for the larger cations (Rb < K < Na < Li) and slightly lower for the more aluminous zeolites (X IY). Then the acid strength decreases from Li to Rb and is slightly smaller for X than for Y cationic zeolites. Considering that simultaneously the basic strength increases, and also that some samples do not show basic properties (LiY, N a y ) , Table V can be drawn from the results for the cationic faujasites. It shows that X and Y are intercalated in the same scale and that faujasites in the Na or K form are at the boundary between acidic and basic zeolites. They have amphoteric properties. Of course, the boundary limit depends on the acid and the base used for the strength determination. The table is also in good agreement with the range of catalytic properties relevant to acidic or basic All the results indicate that the cationic faujasites constitute a family of solids with directly coupled acid-base sites, the strength of one increasing when that of the other one decreases. These sites can be visualized as acid-base pairs as proposed for or Zn0.’9920 In those oxides the strength of each site is determined mainly by the cation nature (Al, Zn ,...) and the crystalline structure. In this type of oxide the only way to change the acid-base properties is by changing the structure. What makes zeolite acid-base pairs different is that their strength is easily modified by changing the exchangeable cation or the aluminum content for the same zeolite structure in addition to moving from one structure to another. Since the effects of aluminum and cations can be combined, an almost continuous range of such conjugate acid-base pairs exists. A more detailed picture of those sites can be discussed by looking at their geometry. Among all the atoms bearing a positive charge, the cations are the most electropositive and are the only atoms accessible in the absence of A1 Lewis sites. Then, the acid-base pair sites exist in rings bearing cations in any zeolite cage. Besides those chemical properties, this creates dipoles on the walls of the cavities in addition to the well-described fields created by the cations themselves and the fields between charges across the cavities. In a perfectly symmetrical system where the cation lies in the center of the oxygen ring plane there is no resulting dipole moment. Very likely this is rarely the case. Even for the SI1faujasite site, small cations, Nazi and Li, located in the oxygen ring plane are in fact not at the same distance from all the oxygen since O4 oxygen atoms are displaced into the supercage (distance N a 0 2 = 2.33 A and N a 0 4 = 2.89 A). For the larger ions K,21,22Rb, and Cs which are moreover displaced into the supercage the resulting dipole moment is also different from zero. Moving to different site locations in various structures will, of course give rise to different resulting dipole moments. For hydrated L zeolite the accessible cations are located in an eight-membered ring in the channel (site D).23 No detailed (18) Peri, J. B. J . Phys. Chem. 1965, 69, 231. (19) Eischens, R. P.; Pliskin, W. A.; Low, M. J. D. J . Catal. 1962, I, 180. (20) Kokes, R. J. In “Catalysis-Progress in Research”; Basolo, R., Burwell. R. L.. Eds.: Plenum Press: London. 1973. (21) Eulenberger, G. R.; Shoemaker D. P.; Kid, J. G. J . Phys. Chem. 1967, 71, 1812. (22) Mortier, W. J.; Bosmans, H. J.; Uytterhoeven, J. B. J. Phys. Chem. 1972, 76, 650. (23) Barrer, R. M.; Villiger, H.Z. Kristallogr. 1969, 128, 352.
J. Phys. Chem. 1984,88, 45-49 information is available on the site configuration for dehydrated samples. For dehydrated mordenites two cationic sites SIvand SvIare occupied by monovalent cations in the channekz4 They are both in distorted elliptical rings, eight-membered for SIvand six-membered for SvI. Only the big Cs cations are centered in the middle of the rings. The SvIcations have a very one-sided coordination to oxygen. The systems appear to be much more asymmetrical than the SIIsites in faujasite. They should generate higher dipole moments. The contribution of these dipoles in any step dealing with adsorption of molecules in a cage appears not to be quantitatively related to the acid-base strength but to be strongly structure dependent. Even for the same acido-basic character (same chemical composition giving same zeolite elec(24) Smith. J. V. “Proceedings of the 5th International Conference on Zeolites”; Rees, L. V., Ed.; Heyden: London, 1980; p 194 and references therein.
45
tronegativity and same average oxygen and cation charges), two different cation-axygen ring configurations will polarize differently an adsorbed molecule. Those considerations may explain the high NH shift of pyrrole in mordenite, not in line with the absence of combination bands and with the average overall oxygen charges. In summary, the results presented allow a better understanding of the dual character of zeolites. For cation-containing zeolites it is relevant to think in terms of acido-basicity. In addition to chemical properties, dipoles are generated on the oxygen rings bearing cations. They are structure dependent. Both the acido-basic pairs and the resulting dipole have to be considered in interaction of a molecule with an adsorption and/or catalytic site.
Acknowledgment. We thank Dr. Roger Hulme and H. Suciu for discussions and for the gift of many samples and Mr. Frank Paciullo for the experimental work. Registry No. Pyrrole, 109-97-7.
Photoexcited Triplet State of Oxotitanium( I V ) Tetraphenylporphyrin Studied by ESR and Laser Photolysis. Proton-Induced Quenching of the Triplet State Mikio Hoshino,* Masashi Imamura, Solar Energy Research Group, The Institute of Physical and Chemical Research, Wako, Wako, Saitama 351, Japan
Shigeyuki Watanabe, and Yoshimasa Hama Science and Engineering Research Laboratory, Waseda University, Shinjuku- ku, Tokyo 160, Japan (Received: January 3, 1983)
Absorption spectroscopicstudies of TPPTi=O (oxotitanium(1V)tetraphenylporphyrin) in ethanol solution at room temperature revealed that TPPTi=O is in equilibrium with TPPTi=O(EtOH) in which an ethanol molecule, EtOH, is coordinated in the axial position of TPPTi=O. ESR spectra of the triplet TPPTi=O in ethanol, MTHF (2-methyltetrahydrofuran), and toluene solutions were measured at 77 K. The zero-field splitting parameter, D,of TPPTi=O(EtOH) was found to be smaller than that of TPPTi=O free from axial ligand. Laser photolysis studies of acid ethanol solution of T P P T i 4 at room temperature showed that the triplet TPPTi=O is efficiently quenched by proton with a rate constant of 3.4 X lo9 M-’ s-’. The result suggests that the Ti=O bond in the triplet TPPTi=O is polarized like Ti6+=0*-.
Introduction Photochemical reactions of synthetic metalloporphyrins have been the subject of numerous studies.l” Particular attention has been paid to the photoreaction of magnesium(I1) and zinc(I1) porphyrin^*^^,^ because of their importance as a model for chlorophylls which dominate the primary photochemical charge separation in photosynthesis. Recently, the photochemistry of various metalloporphyrins, efficient absorbers of sun light, has been extensively studied for the purpose of solar energy conversion and s t ~ r a g e . ~Therefore, J~ the understanding of the photochemical and photophysical properties of the porphyrins becomes important more than before. (1) Engelsma, G.; Yamamoto, A,; Markham, G.; Calvin, M. J . Phys. Chem. 1962,66, 25 17-3 1. (2) Harriman, A,; Porter, G.; Seale, N. J . Chem. SOC.,Faraday Trans. 2 1979, 75, 1515-21. (3) Whitten, D. G.; Wildes, P. D.; DeRosier, C. A. J . Am. Chem. SOC. 1972, 94, 78 11. ( 4 ) Mercer-Smith, J. A,; Sutcliffe, C. R.; Schmehl, R. H.; Whitten, D. G. J . Am. Chem. Soc. 1979, 101, 3995-7. (5) H a d , Y.; Manassen, J. J . Am. Chem. SOC.1977, 99, 5817-8. ( 6 ) Bizet, C . ; Morliere, P.; Brault, D.; Delgado, D.; Bazin, M.; Santus, R. Photochem. Photobiol. 1981, 34, 315-21. (7) Seely, G. R. Photochem. Photobiol. 1978, 27, 639-54. (8) Shakhverdov, P. A. Dokl. Akad. Nauk SSSR 1967, 174, 1141-3. (9) Lever, A. B. P.; Ramaswamy, B. S.; Licoccia, S. J . Photochem. 1982, 19, 173-82. (10) Borgarello, E. B.; Kalyancsundaram, K.; Okubo, Y.; Graetzel, M. Helu. Chim. Acta 1981, 64, 1937-42.
0022-3654/84/2088-0045$01.50/0
Metalloporphyrins are known to possess their own characteristic properties which depend on the central metal. It is recognized, for example, that the luminescence properties of metalloporphyrins are dependent on the magnetism, the atomic number, and the size of the incorporated metal.” In the present paper, we report studies on the absorption, the fluorescence, and the triplet ESR spectra as well as photoinduced triplet quenching in an ethanol solution studied by the laser-flash photolysis technique. Experimental Section Crude TPPTi=O prepared according to the literature’* was purified ten times on alumina columns by using 1:l mixture of benzene and chloroform as developing solvent. Toluene and MTHF were purified by fractional distillation and stored on Na-K alloy in vacuo in order to remove traces of water. Ethanol was used without further purification. Absorption and fluorescence spectra were recorded on a Cary 14 or a Hitachi 200-20 spectrophotometer and a Hitachi M P F 4 spectrofluorimeter, respectively. X-Band ESR spectra were measured by a Jeol JES-FE 3AX spectrometer. Magnetic fields and microwave frequencies were calibrated by using Mn(I1) in MgO powder and DPPH, and a Takeda Riken TR 5501 frequency (11) Allison, J. B.; Becker, R. S . J . Chem. Phys. 1960, 32, 1410-7. (12) Fournari, P.; Guilard, R.; Fontesse, M.; Latour, J. M.; Marchon, J. C. J . Organomet. Chem. 1976, 110, 205-17.
0 1984 American Chemical Society
