J. Phys. Chem. 1991, 95, 3780-3790
Stepwlse Assembly of Charge-Transfer Complexes within Zeolite Supercages as Visual Probes for Shape Selectlvtty K. B. Yoon and J. K. Kochi* Chemistry Department, University of Houston, University Park, Houston, Texas 77204-5641 (Received: October 9, 1990) The shape-selective formation of various aromatic charge-transfer complexes occurs within zeolite supercages doped with methylviologen (MVz+) and other large mono- and dipyridinium acceptors. For example, ion exchange leads to a series of acceptor-doped zeolite Y such as MV(x)Y, typically with x = 1 and 2 MV2+per supercage. Mere exposure of a hexane slurry of the colorless MVY to various substituted benzenes, naphthalenes, and anthracenes leads immediately to brilliant, distinctively colored yellow, orange, and purple zeolites, while the supernatant solution remains singularly colorless. The diffuse reflectance spectra of the colored zeolites show characteristic charge-transfer bands that are the same as those obtained in solution with MVZ+and the corresponding arene. Thus, the successful isolation of the bright orange, single crystal of a naphthalene complex with MV2+allows X-ray crystallography to establish the relevant cofacial donoracceptor arrangement within the zeolite cavity. Steric restrictions of the supercage are indicated in these zeolite experiments by the size and shape of the arene donorsall the methylbenzenes including pentamethylbenzene rapidly forming brilliant yellow zeolites, the single striking exception being hexamethylbenzene. Shape selectivity is also indicated by the exclusion of 1,4-dimethoxynaphthaIene (but not the 2,bisomer) as well as 9-phenyl- and 9,lO-dimethylanthracene (but not 9-methylanthracene). A van der Waals "width" of roughly 8 A is sufficient to inhibit an arene from complex formation, and it represents the maximum value for the kinetic diameter u of arena in zeolite Y catalysis. Mass transport through zeolite Y is quantitatively evaluated by measuring the solvent effect on the rates of intercalation and extraction of various arene substrates.
Introduction The restricted (yet ready) intercalation of molecular guests into the intravoid space of zeolites represents an important facet of shape-selective catalysis.'J Despite the increasing interest in the use of zeolite catalysts in the liquid phase, h~wever,~"there is no general method available to assess the structural constraints for the passage of various organic substrates through the supercage, especially from nonaqueous slurries under mild conditions. Although the mass transport via the liquid phase avoids the vapor pressure limitation of the more conventional adsorption methods,' little is known about how the solvation of the zeolite framework affects the substrate mobility. Thus, a general method of easy applicability is highly desirable for testing the shape selectivity of a wide variety of substrates-particularly organic compounds of differing sizes, shapes, and properties. Especially germane to zeolites is their capacity as solid electrolytes to incorporate elevated levels of cationic (anionic) charge.'J Indeed organic salts can play a critical role as templating agents to initiate nucleation during synthesis, and they are often found entrapped within the zeolite framework.' In the ion exchange of zeolites, the very large cations are excluded to the exterior (1) (a) Breck. D. W. Zeolite Molecular Sieoes; Wiley: New York, 1974. (b) Csicsery, S.M. In Zeolite Chemistry and Catalysis; Rabo,J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 680 ff. (c) Weisz, P. B.; Frilette, V. Z . J . Phys. Chem. 1960, 64, 382. (2) (a) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic: London, 1982. (b) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic: London, 1978. (c) Jacobs, P. A. Carboniogenic Activity of Zeolite; Elsevier: Amsterdam, 1977. (d) Chen, N. y.; Garwood, W. E.; Dwyer, F. G. Shape Selective Catalysis in Industrial Applications; Marcel Dekker: New York, 1989. (3) (a) Corbin, D. R.; Eaton, D. F.;Ramamurthy, V. J . Am. Chem. SOC. 1988, 110,4848. (b) Corbin, D. R.; Seidel, W. C.; Abrams, L.; Herron, N.; Stucky, G. D.; Tolman, C. A. Inorg. Chem. 1985, 24, 1800. (4) (a) Turro, N. J.; Cheng, C.-C.; Abrams, L.; Corbin, D. R. J . Am. Chem. Soc. 1987,109,2449. (b) Turro, N. J.; Lei, X.;Cheng, C.-C.; Corbin, D. R.; Abrams, L. J . Am. Chem. Soc. 1985,107,5824. (c) Turro, N. J.; Wan, P. J. Am. Chem. Soc. 1985, 107, 678. ( 5 ) (a) Sohn, J. R.; Lunsford, J. H. J . Mol. Carol. 1985, 32, 325. (b) Yamaguchi, 1.; Joh, T.;Takahashi, S. J . Chem. Soc., Chem. Commun. 1986, 141. (c) Chiche, B.; Finiels, A.; Gauthier, C.; Geneste, P.; Graille, J.; Pioch, D. J. Org. Chem. 1986, 51, 2128. (6) (a) Rode, E.; Davis, M. E.; Hanson, B. E. J . Chem. Soc., Chem. Commun. 1985, 147. (b) Smith, K.; Butters, M.; Nay, B. Synth. Commun. 1985, 1157. (c) Suib, S.L.; Kostapapas, A.; Psaras, D. J . Am. Chem. SOC. 1984, 105, 1614. (7) (a) Occeilli, M.L.; Robson, H.E., Eds. Zeolite Synthesis; ACS Symp. Ser. 398; American Chemical Society: Washington, DC, 1989. (b) Szostak. R. Molecular Sicors, mnciples of Synthesis and Identification;Van Nastrand Reinhold: New York, 1989.
0022-365419 1 /2095-3780$02.50/0
surface, but most interestingly, the interior cavities (e&, a-cages, supercages) are potentially accessible to many types of other cations.* In order to develop a general and convenient method for zeolite shape selectivity, we have successfully exchanged a series of relatively large organic cations of different shapes, sizes, and charges into the supercage of zeolite Y (hereafter referred to simply as Y).9 In particular, the cationic di- and monopyridinium acceptors methylviologen (MV2+), diquat (DQ2+), N-methylacridinium (AC+), N-methylquinolinium (Q+), and isoquinolinium (IQ+) are especially useful since they form a series of brilliantly colored, intermolecular complexes with various electron donors.*O The sharp distinction among different-sized substrates then constitutes the structural basis for shaping the donors in their selective access to the pyridinium acceptor lying within the restricted confines of the zeolite supercage.
Li
M P
he
'i '
Me AC'
IO+
O+
(8) (a) Rabo, J. A. Catal. Rev.-Sci. Eng. 1981, 23, 293. (b) Rabo, J. A. In ref 1b, p 332 ff. (c) Rabo, J. A.; Kasai, P. H.Prop. Solid Stare Chem. 1975, 9, 1. (d) Clearfield, A.; Troup, J. M. J . Phys. Chem. 1970, 74, 2578. (e) Dai, P. E.; Lunsford, J. H. J . Carol. 1980, 64, 173. (e) Beyer, H. K.; Karge, H.G.; BorMly. G. Zeolites 1988.8.79. (f) Liquornik. M.; Marcus, Y. J . Phys. Chem. 1968,72,2885. (g) Barrer, R. M.; Meier, W. M. Trans. Faraday Soc. 1959,55, 130. (h) Barrer, R. M.; Papadopoulos, R.; R e a , L. V. C. J. Inorg. Nucl. Chem. 1%7,29,2047. (i) Theng, B. K. G.; Vansant, E. F.;Uytterhoeven, J. B. Trans. Faraday Soc. 1968,64, 3370. (j) Chu, P.; Dwyer, F. G. Zeolites 1988, 8, 423. (9) For the preliminary report, see: Yoon, K. B.; Kochi, J. K. J . Am. Chem. Soc. 1989, I l l , 1128. (10) (a) Summers, L. A. The Eipyridinium Herbicides; Academic: New York, 1980. (b) Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1988,110, 1294. (c) Nakahara, A.; Wang, J. H. J. Am. Chem. Soc. 1%J, 67,496. (d) Hamity, M.; Lema, R. H.Can. J. Chem. 1988.66, 1552. (e) Nagamura, T.; Sakai. K.J . Chem. Soc., Chem. Commun.1986, 810. (f) Atherton, S.J.; Hubig, S.M.; Callan, T. J.; Duncanson, J. A,; Snowden. P. T.; Rodgers, M. A. J. J . Phys. Chem. 1987, 91, 3137. (g) Ebbesen, T. W.; Ferraudi, G. J. Phys. Chem. 1983,87,3717. (h) Deronzier, A. Tetrahedron Lett. 1984.25, 2867.
0 1991 American Chemical Society
Charge-Transfer Complexes within Zeolite Supercages TABLE I: Ion Exchange (Maximum) of Pyridinium Acceptors into sodlum zeolite@ MV2+ DQz+ zeolite exchangeb Nat exchange* Na+C NaY 0.85 (2.1) 52 0.86 (2.1) 53 NaX 0.61 (1.5) 26 0.49 (1.2) 21 NaA 0.10 (0.2) 3 0.06 (0.1) 2 From aqueous solutions of chloride and bromide salts, respectively. bAcceptor incorporated into zeolite (mmol per g of the hydrate); occupancy in parentheses (molecules per supercage). cPercent of total exchange (Na+ selective electrode).
In this study we have focused on aromatic donors (Ar) derived from substituted benzenes, naphthalenes, and anthracenes in the formation of highly colored charge-transfer (CT) complexes" with the pyridinium acceptors listed above, e.g.
+ &
MV2+ Ar [MV2+,Ar] (1) Such C T complexes are commonly characterized by limited formation constants of K < 2 M-I, and they are to be classified in solution among the generally weak comple~es.'L'~Accordingly, our principal objective is the visual examination of these intermolecular complexes within the supercage of pyridinium-doped zeolites by the shape-selective intercalation of aromatic donors from organic solutions.
Results I. Ion Exchange of the Pyridinium Cations into Sodium Zeolites. When a colorless aqueous solution of methylviologen dichloride (MVC12 in excess) was slurried with sodium zeolite Y at room temperature, up to 0.85 mmol of MV2+ could be incorporated for each gram of NaY,I4 as determined spectrophotometrically by the loss of MV2+ (e = 20 700 M-' cm-' at A:, = 257 nm)I5 from solution. This ion exchange was accompanied by the leaching of 2 equiv of sodium ions into the aqueous solution (assayed by Na+ selective electrode) to correspond to an overall occupancy of each zeolite Y supercage by a pair of methylviologen dications. Such a doped zeolite with the maximum incorporation of the methylviologen acceptor was colorless, and it is hereafter designated as MV(2.O)Y for convenience. Similarly, the ion exchange of NaY was controlled with appropriate amounts of MVC12 (see Experimental Section) to yield a series of doped zeolites MV(x)Y with an average occupancy of x = 1.5, 1.0,0.8, and 0.4 molar equiv of MV2+ per supercage. Under the same conditions, sodium zeolite X incorporated somewhat smaller amounts of MV2+despite its greater exchange capacityI6 (Table I), and sodium zeolite A with the constricted pore opening of 4 A led to only minor exchange.l* (Note that sodium zeolite X, Y, and A all have comparable surface The results of Table I shows that the ion exchange of the tricyclic acceptor into zeolite Y was similar to that of MV2+. The diffuse reflectnce UV-vis spectra of both MVY and DQY showed absorption maxima at 275 and 310 nm, respectively, characteristic of the dication absorptions in solution.10 Moreover, the I3C N M R spectrum of MVY (measured with magic angle
w+
(1 I ) (a) White, B. G. Trans. Faraday Soc. 1969, 65, 2000. (b) Poulos, A. T.; Kelley, C. K.;Simone, R. J. Phys. Chem. 1981, 85, 823. (c) Jones, 11, G.; Malba, V. Chem. Phys. Lett. 1985, 119, 105. (12) (a) Mulliken, R. S.J. Am. Chem. Soc. 1950,72,601. (b) Mulliken, R. S.;Person, W. B. Molecular Complexes; Wiley: New York, 1969. (1 3) Foster, R. Organic Charge-Transfer Complexes; Academic: New York, 1969. (14) See also: (a) Krueger, J. S.;Mayer, J. E.; Mallouk, T. E. J . Am. Chem. Soc. 1988,110,8232. (b) Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91,643. (c) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. Interfacial Electrochem. 1986, 2108, 95. (15) Watanabe, T.; Honda, K. J. Phys. Chem. 1982,86, 2617. (16) The more limited ion-exchange capacity of NaX (comparable to that
of bentonite) might be related to the high - occuDancy .~ of Na+ on the channel walls. is., at site 111.~' (17) Breck, D. W. J. Chem. Educ. 1964, 41, 678. (18) The small amounts of MV2+associated with zeolite A are ascribed to external surface occupancy, as in kaolinite clays.19 (1 9) See: Worrall, W. E. Clays and Ceramic Raw Materials; Wiley: New York, 1975; Chapter 3. Compare also Barrer in ref 2b, Chapter 8.
The Journal of Physical Chemistry, Vol. 95, No.9, 1991 3781 spinning (MAS) and cross-polarization (CP) techniquts) showed three resonances at 49.6, 127, and 148 ppm that were essentially identical with those present in the solution spectrum of MV2+.m The treatment of zeolite Y and N-methylacridinium iodide afforded the bright yellow, highly fluorescent zeolite ACY, and the other monocationic acceptors from the N-methylquinolinium and isoquinolinium iodides were similarly exchanged into zeolite Y with ease to produce QY and IQY, respectively. In all cases, the pyridinium cations could be quantitatively recovered from the doped zeolites by several washings with concentrated (1 M) aqueous NaCl solution.22 ZI. Coloration of the Pyridinium-Doped Zeolites by Aromatic Donors. Qualitative tests of zeolite Y doped with MV2+ were carried out at ambient temperatures with the polymethylbenzene, naphthalene, and anthracene donors dissolved in either n-hexane, dichloromethane, benzene, or acetonitrile. Upon mixing the aromatic solution with MV(0.8)Y, a dramatic bright coloration of the zeolite was noted immediately, while the supernatant solution remained singularly colorless. All the methylbenzenes including mesitylene, durene, and pentamethylbenzene formed bright yellow zeolites-except hexamethylbenzene which left the colorless MVY intact. Naphthalene and its derivatives in Table I1 yielded bright yellow-orange zeolites with the exception of which had no 1,4-dimethoxy- and 2,6-di-tert-butylnaphthalene, visual effect. Anthracene together with its 9-methyl and formyl derivatives afforded orange to purple zeolites, but the 9-phenyl and 9,10-dimethyl derivatives led to no change in color. Tetracene and 2-tert-butylanthracene yielded brightly colored zeolites with most of the pyridinium acceptors in Table 111, except MV2+. Moreover, the higher acene homologue pentacene did not form colored zeolites with any of the acceptors in Table 111. Similar visual observations were obtained with zeolite X doped with either MV2+ or DQZ+,but no such coloration was noted with either zeolite A or amorphous silica-alumina treated in the same way. It is important to emphasize that despite the selective coloration of zeolites, as described above, all of these arene donors rapidly formed the highly colored C T complexes with both MV2+ and DQz+salts when they were unrestricted in solution (Tables I1 and 111, last columns) or when the pure donor-acceptor pairs were intimately mixed in the solid state by grinding with a neutral alumina support (Table 11, column 5). ZZZ. Charge- Transfer Absorption Bands in the Colored Zeolites. The brightly colored zeolites in Table I1 were spectroscopically characterized by comparison of their diffuse reflectance spectra with those of the corresponding charge-transfer complexes dissolved in acetonitrile (column 6) or suspended in the alumina matrix (column 5). For example, the yellow, orange, and purple powders isolated from MV(0.8)Y treated with durene, a-methoxynaphthalene, and anthracene, respectively, showed the = 380,430, well-resolved absorption bands in Figure 1 with ,A, and 490 nm. For comparison, the insets show the same absorption spectra of the corresponding MV2+ complexes in solution. The charge-transfer character of the absorption bands of the colored zeolites in Table I1 (columns 3 and 4) was indicated by the linear variation of the absorption maximum (pCT) with the ionization potential (IP) of the aromatic donor in accord with Mulliken theo1y.'2'~ Thus, the fit of the data from both the MV2+and the DQ2+-doped zeolite Y to the pair of parallel least-squares lines in Figure 2A is unmistakable, i.e.23 hvcr = IP - 5.3
(2)
(20) By comparison, the 13C NMR spectrum of MVz+ in acetonitrile showed resonances at 49.5, 127.5, 147.0, and 150.4 ppm. The low-field resonance was weak and not obscrved owing to the cross-polarization employed in the solid-state NMR spectrum.21 (21) (a) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites; Wiley: New York, 1987. (b) Yannoni, C. S.Acc. Chem. Res. 1982, 15, 201. (c) Maciel, G. E. Science (Washington, D.C.) 1984, 226, 282. (22) It is important to emphasize that the large amounts of MV2+ ex-
changed into zeolite Y (Table I) can only be accounted for by encapsulation within the supercage and not on the surface. (23) With a correlation coefficient of 1.06 and r = 0.993.
3782 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
Yoon and Kochi
TABLE II: Diffuse Refkctanee Spectra of Charge-Transfer Complexes in Zeolite Y; Effect of Aromatic Donors arene donor zeolite Yb A1203C substituents IP MV2+ DQ2+ MVZ+
solutiond MV2+
0
1,2,4,5-Me4 1-Me-4-Me0 1,4-(MeO), Me5 Me6 I-Me 1,4-Me2 1-Me0 1,4-(MeO), 2,6-(MeO)* 9-CHO 9-Me 9,10-Me2 2-1-BU 9-Ph
-
--
8.05 8.18 7.90 7.92 7.85 8.12 7.96 7.78 7.72 7.50 7.58 7.43 7.84 7.25 7.1 1 -7.3 7.18 6.97
380 370 400 395 none 380 400 425 430 none 450 490 405 518 none none none none
390 380 420 400 none 390 410 430 438 none 490 500 410 528 none 505 none 600
-360 370 410 370 400 -360 -380 -410 410 480 -440 475 420 515 520 490 -480 590
6.61
none
none
-
360 -365 406 357 385 -360 -380 -410 410 480 -430 472 390 504 526 484 500
-
-
600
X, (nm) as described in the text. cAlumina (10%) suspension. dAbsorption spectrum of arene C T comp-ex a Ionization wtential of Ar in eV. with 75 mM MV2+(PFO2in acetonitrile solution. TABLE III: Effect of Acceptors 011 the Charge-Transfer Spectra in Zeolite Y acceptor zeolite Yb EdE MeA BuA A DQ" 0.38 520 510 500 AC+ 0.43 500 500 480 MV2+ 0.47 500 480" 490 0.90 440 420 -410 Q+ 1.08 -400 -380 -380 IQ+
MeON 450 -420 430 -370 -350
solution' A 474 500 472 -380 360
DMB 415 -400 395 -350 -325
-
'Cathodic peak potential of acceptor in acetonitrile by cyclic voltammetry at u = 500 mV s-' (V vs SCE). b X ,(nm) from diffuse reflectance spectrum for MeA = 9methylanthracene, BuA = 2-tert-butylanthracene, A = anthracene, MeON = 1-methoxynaphthalene, DMB = 1,4-dimethoxybenzene donors. CAbsorptionspectrum of anthracene C T complex with the acceptor as either the CF3SO< or PFL salt in acetonitrile solution. dVery weak after 6 h.
B
A
C
A
d
1
1
xx)
Wavelength
I
I
SO0
,
)O
nm
Figure 1. Diffuse reflectance spectra of the colored zeolites from the exposure of MV(0.8)Y to (A) durene, (B) a-methoxynaphthalene, and (C) anthracene [untreated MV(O.8)Y before exposure to arene (---)I. The insets show the corresponding charge-transfer absorption spectra of MV2+and arene in acetonitrile solution.
Most notably, it follows from the Mulliken for~nulation~~ that the constant separation of Ahvm = 72 meV in Figure 2A corresponds as also to the difference in acceptor strengths of MV2+and W+, independently evaluated by the difference in their reduction potentials of fld = 70 mVS2' The latter is thus a corollary of (24) Bockman, T. M. To be published.
the linear variation of the charge-transfer bands (of a given arene with the series of doped zeolites in Table 111) with the reduction potentials of the pyridinium acceptors, Le. hvCT = Eorcd+ constant
(3)
Accordingly, it follows that the constant displacements among the three parallel correlations in Figure 2B of Ahvm = 300, 200,
Charge-Transfer Complexes within Zeolite Supercages
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3783 121
A. M V Y
I-
?"
r
B. NoY/DMB 115
I P , eV 51.5
C.
48.9
MVY/DMB
I
220
160
,
.
I
100
.
,
l
40
.
,
l
-20
ppm
Figure 4. I3C (MAS/CP) NMR spectra of (A) MV(0.8)Y, (B) pdi-
2.5
3.5
30
hS,,
,
eV
Figure 2. Mulliken correlations of the charge-transfer bands (hum) from and DQY ( 0 )to various arene donors and the exposure of (A) MVY (0) (B) anthracene (e), a-methoxynaphthalene (O), and p-dimethoxybenzene ( 0 )to the series of pyridinium-doped zeolite Y (as indicated).
Figure 3. ORTEP perspective showing the cofacial arrangement of MVz+ acceptor and 2,6-dimethoxynaphthalenedonor in the 1:1 charge-transfer complex. For clarity, PFc is not included.
and 500 mV correspond to the differences in donor strengths of the arenes, as also independently evaluated by the differences in the ionization potentials of AIP = 290, 180,and 470 meV, respectively.2s IV. Structures of Charge-Transfer Complexes within Zeolite Supercages. The charge-transfer structures responsible for the colored zeolites were established by the isolation of orange to purple crystals via the slow, deliberate evaporation of intensely colored solutions containing 1 :1 mixtures of MVZ+(PF6-)2and dimethoxynaphthalene, methylanthracene, or anthraldehyde in (25) (a) Masnovi, J. M.; Seddon, E. A.; Kochi, J. K.Con. J. Chem. 1984, 62, 2552. (b) Sankararaman, S. Unpublished results. (c) Kobayashi, T.; Nagakura, S. Bull. Chem. Soc. Jpn. 1974, 47, 2563. (d) Watanabe. K.; Nakayama, T.; Mottl. J. J. Quunr. Spectrmc. Rudiat. Transfer 1962, 2, 369. (e) Clark, P.A.; Brogli, F.; Heilbronner, E. Helu. Chim. Acta 1972.55, 1415. ( f ) Nounou, P.J. Chem. Phys. 1966,63.994. (8) Nagy, 0. B.; Dupire, S.; Nagy, J. B. Terruhedron 1975.31, 2453.
methoxybenzene intercalated into Nay, and (C) yellow zeolite from the intercalation of p-dimethoxybenzene into MV(0.8)Y. either acetonitrile or nitromethane. Of these, the bright orange crystals of the MV2+complex with dimethoxynaphthalene were particularly suitable for singlecrystal X-ray crystallography. The ORTEP diagram in Figure 3 reveals the relevant face-to-face interaction of the donor and acceptor planes with the interannular separation of 3.46 A that is characteristic of the van der Waals contact between aromatic rings.% Most importantly, the diffuse reflectance spectrum of the orange crystals showed the chargetransfer absorption band with X, r 440 nm to be essentially the same as that a t 450 nm of the orange zeolite obtained from the exposure of MVY to dimethoxynaphthalene. The constitution of the aromatic donor and pyridinium acceptor within the colored zeolites was also examined by solid state (MAS/CP) NMR spectroscopy. Thus, the 13CNMR spectrum of the methylviologen dication in MVY (vide supra) was unaltered in the colored zeolites derived from either dimethoxybenzene, .methylanthracene, methylnaphthalene, or methoxytoluene (see full details in the Experimental Section), and the intercalated aromatic donors were similarly found to be unshifted. For example, the solid-state 13CN M R spectrum of dimethoxybenzene adsorbed on sodium zeolite Y showed resonances a t 58.0,115, and 154 ppm in Figure 4B that were equivalent to those obtained in the solution I3CNMR spectrum. The corresponding spectrum of the orange zeolite obtained from MVY and dimethoxybenzene was a composite of the two, as shown in Figure 4C. The solid-state NMR studies further established that the constituents in the colored zeolites were the same as those present in the intact crystalline 1 :1 charge-transfer complexes. We thus conclude that the intercalation of aromatic donors into MVY leads to the stepwise assembly of the charge-transfer complex in the supercage with essentially the same face-to-face donor/acceptor interaction as that illustrated in Figure 3. V. Quantitative Effects of Aromatic Intercalation into Zeolite Y. The intracavity formation of charge-transfer complexes in the pyridinium-doped zeolite Y was further established by the measured uptake of the aromatic donor by gas chromatographic (26) Enkelmann, V. In Polynuclear Aromutic Compounds; Ebert, L. E., Ed.;Adv. Chem. Ser. 217; American Chemical Society: Washington, DC, 1988; pp 177-200.
Yoon and Kochi
3784 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 TABLE I V Intercalation of Aromatic Donors into Zeolite Y ' arene MV( 1.O)Y NaY benzene 1.03 (1.8) 1.59 (2.6) 1.02 (1.8) 1.35 (2.2) toluene 0.55 (1.0) 1.15 (1.8) p-xylene 0.99 (1.6) mesitylene 0.86 (1.5) 0.84 (1.3) durene 0.81 (1.4) 0.92 (1.5) pentamethylbenzene 0.40 (0.7) 0.58 (1.0)* 0.93 (1.5)b hexamethylbenzene 0.00 (0.0) 0.03 (0.0) 0.09 (0.2)b 0.16 (0.3)* 1.57 (2.5) p-methoxytoluene 1.17 (2.0) 1.67 (2.7) p-dimethoxybenzene 1.21 (2.1) 1.22 (1.9) naphthalene 1.16 (2.0) 1.10 (1.9) 1.30 (2.1) 1 -methyl1.34 (2.1) 1 -methoxy0.87 (1.5) 0.68 (1.0) 2,6-dimethoxy0.33 (0.6) 0.09 (0.2) 0.17 (0.3) 1,4-dimethoxy0.58 (1.0) 0.69 (1.1) anthracene 0.56 (1.0) 1.17 (1.9) 9-methyl9,lO-dimethyl0.05 (0.1) 0.01 (0.0) "Arene uptake from hexane solution at 25 O C (mmol per 9); occupancy in parentheses (molecules per supercage). sealed tubes at 80 OC for 15 h.
analysis under a standard set of conditions. The quantitative effects of (A) donor size, (B)acceptor shape, (C) acceptor loading, (D) solvent structure, and (E) time profile (rate) are described separately as follows. A. The effects of arene size (shape) were examined from n-hexane solutions at 25 OC with MV( l.O)Y, in which each zeolite supercage was occupied (on the average) by one MV2+acceptor. The results in Table IV indicate that roughly two benzene and toluene donors could be accommodated with each MV2+,but the number of intercalated donor molecules decreased with increasing methyl substitution to reach a limiting value of one pentamethylbenzene. Similar trends were also observed in the series of substituted naphthalenes and anthracenes. The shape control (by the position of the substituent) was manifested in the difference between the isomeric 2,6- and 1,4-dimethoxynaphthalenes(entries 13 and 14). Morever, the uptake of hexamethylbenzene and 9,lO-dimethylanthracene by MV( 1.O)Y was nil, a result which was wholly consistent with the absence of coloration in Table 11. Furthermore, the sealed tube experiments carried out at 80 OC resulted in only a barely perceptible uptake of hexamethylbenzene (entry 7b) and a slight yellow coloration of MV( 1.O)Y. Complex formation was not a prerequisite for arene intercalation into the zeolite, as shown by the comparative studies with undoped N a y . In fact, the uptake of aromatic donors by NaY was consistently greater than that of MV(l.O)Y, the largest discrepancy of almost one donor per supercage being observed with 9-methylanthracene. B. The effects of acceptor size were examined with the zeolite Y doped with the series of pyridinium acceptors in Table V (listed roughly according to decreasing size) by their successive treatment with the selected arene donors graded (left-to-right) according to increasing size. The synergistic effects of donor and acceptor sizes (shapes) are clearly seen by the global minimum and maximum observed in the upper right and lower left quadrants
01
0
I
05
I
I.5
1.0
MV2' Doping
,
2.0
molecules / supercage
Figure 5. Synergistic effects of the donor size (shape) and the level of MV2+exchange with zeolite Y for the intercalation of pmethoxytolucne (O), a-methylnaphthalene (a),and 9-methylanthracene (0)from hexane solution.
of the matrix. Especially prominent was the monotonic decrease in the amount of arene intercalated with increasing donor size (along each row). The corresponding increase in the amount of intercalated arene with decreasing acceptor size (down each column) was also evident, especially with the largest donors. (The less pronounced effect of acceptor size with the smaller arene donors was generally associated with the stoichiometric ratios of 1.5-2 for the occupancy of donor/acceptor pairs in the zeolite supercage.) C. The effects of acceptor loading were examined with a series of zeolites MV(x)Y doped with incrementally increasing amounts ( x ) of MV2+up to the limit of two per supercage. For example, the amount of p-methoxytoluene (-2.5 per supercage) intercalated into MVY was affected only slightly when the occupancy of the supercage by MV2+was increased from 0 to 0.4 and then 0.8, but it decreased sharply with further MV2+loading until it reached a limiting value of 0.5 in the maximally doped MV(2.O)Y. On the other hand, the significantly larger donor 9-methylanthracene (MeA) followed a more consistent trend, in which the intercalation decreased monotonically from two per supercage to nil attendant upon the increased MV2+ loading, as graphically illustrated in Figure 5 . The intermediate trend shown by 1methylnaphthalene in Figure 5 is thus in agreement with the synergistic effects of donor and acceptor sizes (shapes) listed in Table V. D. Pronounced and unusual effects of solvent structure on the efficiency of donor intercalation were consistently obtained with the pair of prototypical arenes in Table VI. Thus, n-hexane was singularly effective as the solvent of choice for the intercalation of durene and 1-methylnaphthaleneinto MV( 1.O)Y-the branched and higher homologues isooctane and n-dodecane being significantly less so. Alkanes were generally more effective then the chlorinated solvents, which showed markedly enhanced efficacy with polysubstitution in the order CH2C12 < CHC13 3a(I), 1662; total variables, 251; R = XllFol - IFcll/CIFol,0.053; R, = [&(lFol - IFc1)2/CwlFo12]i/2, 0.036; weights w = U ( F ) - ~ . Determination of Formation Constants of Charge-Transfer Complexes in Solution. The formation constants of the EDA complexes between the organic acceptors and various arena were
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J . Phys. Chem. 1991, 95. 3790-3796
determined by the Benesi-Hildebrand spectrophotometric met h ~ d . * For ~ the formation of the 1:l complex between the two components, the concentration dependence of the intensity of the C T band is given by [MV2+]/Acr = l/Kcm[Ar] + l/cct where Am and cm are the molar absorbance and extinction coefficient, respectively, of the EDA complex at the monitoring C T wavelength, under conditions in which [ArH] >> [MV2+]. For the colored arene donors such as 9-methyl- and 9,lO-dimethylanthracene and for anthracenes of poor solubility in acetonitrile, the conditions in which acceptor was present in excess, Le., [MV2+] >> [ArH], were used. In a typical experiment, 4.85 mg of MV(PF& was weighed into a quartz cuvette (1-cm path length) containing 2 mL of CH3CN. The solution (- 5 mM) was mixed with a small magnetic stirrer, and the UV-vis spectrum was measured with a sample of pure solvent as reference. A known amount of durene (ca. 13 mg) was then added with stirring, and the absorbance was measured at three wavelengths close to ,A, of the C T band. This procedure was repeated to give a total of at least seven data points. The change in volume that accompanied the incremental addition of arene was assumed to be negligible. For MV2+complexes in acetonitrile with the arene donor K (M-I), t (M-I cm-I) were as follows: anthracene, 2.0, 329; 9-methylanthracene, 0.6,976; naphthalene, 0.7,428; 1-methylnaphthalene, (57) (a) Benesi, H. A.; Hildebrand, J. H. J . Am. Chem. Soc. 1949. 71, 2703. (b) Person, W. B. J . Am. Chem. SOC.1%5,87, 167.
0.9, 323; 1-methoxynaphthalene, 0.9, 322; durene, 0.8, 293; pentamethylbenzene, 0.8, 308; hexamethylbenzene, 1.O, 286; p-methoxytoluene, 0.4,247. For the AC+ complexes of anthracene: 2.7, 612; and p-methoxytoluene, 0.4, 172. Instrumentation. Infrared spectra were measured on a Nicolet lODX FTIR spectrometer. N M R spectra were recorded on a 90-MHz JEOL FX 934. 'H NMR chemical shifts are reported in ppm downfield from internal T M S standard. "C N M R chemical shifts are reported in ppm with the central resonance of the solvent peak as reference. The N M R spectra for solid samples were analyzed by solid-state I3C NMR on a Bruker CXP 200 spectrometer using magic angle spinning (MAS) cross-polarization (CP)and high-power decoupling. The proton resonance frequency was 200 MHz, and the corresponding 13Cresonance frequency was 50 MHz. A 150-mg sample was contained in a Macor rotor which was spun at 3 kHz. Gas chromatographic analyses were performed on a Hewlett-Packard 5890A with 12.5-m S E 30 capillary column. GC-MS measurements utilized a Hewlett-Packard 5890A chromatograph interfaced to a H P 5970 mass spectrometer (EI, 70 eV). Acknowledgment. We thank T. M. Bockman for helpful discussions, M. Narayana (Shell Development Co.) for the CPMAS N M R spectra, J. D. Korp for crystallographic assistance, and the National Science Foundation, Robert A. Welch Foundation, and Texas Advanced Research Program for financial support.
Broad-Line and High-Resolution NMR Studies Concerning the Hydroxonium Ion in HZSM-5 Zeolites Patrice Batamack, Claudine hrimieux-Morin,* Jacques Fraissard, and Dieter Freudet Laboratoire de Chimie des Surfaces, associi au CNRS (URA 1428), UniversitC Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received: October 18, 1990)
Bronsted sites must be able to react with adsorbed water molecules to form hydroxonium ions. This study shows by line-shape simulation of the IH NMR signal measured at 4 K that for water concentrations about equal to the concentration of bridging hydroxyl groups in defect-free HZSM-5 zeolites two configurations exist: water molecules hydrogen-bonded to the bridging OH group and the hydroxonium ion. The result is confirmed by IH MAS NMR investigations at room temperature. In addition, 27Aland 29SiMAS NMR were used to characterize HZSM-5 zeolites under study synthesized without template, with TPA+, with butylamine as template. A11 samples contain 4 bridging hydroxyl groups per unit cell. For the zeolite synthesized with butylamine and loaded with 2,4, and 8.4 water molecules per unit cell the concentration of hydroxonium ions per unit cell is 0.28, 0.64, and 1.56, respectively.
Introduction The hydroxonium ion is the classic Bronsted acid in aqueous solutions. Therefore, H zeolites known to be strong solid acids should also contain hydroxonium ions in the hydrated form. Jentys et a1.I attributed IR bands arising upon hydration of the HZSM-5 zeolite to hydroxonium ions. They excluded the possibility that the adsorbed water molecules are hydrogen-bonded to bridging hydroxyl groups, because this adsorbate structure should lead to only one band of perturbed hydroxyl groups as opposed to the various bands observed; but they did not discuss the coexistence of both the ionic and the hydrogen-bonded physisorption structure as well.' Vega and LuzZ4 studied H-RHO type zeolites by IH MAS NMR and found after addition of small amounts of water to activated samples a narrow peak at 5.9 ppm and a broad signal at ca. 4 ppm, while at the highest loadings the spectrum consists Author to whom correspondence should be addressed. Permanent address: Sektion Physik der Karl-Marx-Universitit Leipzig, LinnCtrasse 5, DDR-7010Leipzig, Germany.
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of two narrow peaks at 4.6 and 9.1 ppm. Only the signal at 4.6 ppm could be unambiguously identified as water physically adsorbed in the zeolite cavities. Scholle et aL5 studied water adsorption on HZSM-5 and assigned a broad line at 6 ppm to protons of Bronsted acid sites exchanging rapidly with protons of the water molecules adsorbed on these sites. Mastikhin et a1.6 explained a shift of the IH NMR line upon hydration of H-Y zeolite a fast proton exchange between bridging OH groups, water molecules, and hydroxonium ions. Hunger et ale7studied H-Y zeolites and amorphous silicaalumina and discussed two lines in the 'H MAS NMR spectra: a line at ca.6.5 ppm is caused by water adsorption (1) Jentys, A.; Warecka, G.; brewinski, M.; Lercher, J. A. J. Phys. Chcm. 1989, 93, 4837. (2) Vega, A. J.; Luz, Z. J . Phys. Chem. 1987, 91, 365. (3) Luz, Z.; Vega, A. J. J . Phys. Chem. 1987, 91, 374. (4) Vega, A. J. J . Am. Chem. Soc. 1988, 110, 1049. (5) Scholle, K. F.M. G. J.; Kentgens, A. P. M.; Veeman, W. S.; Frenken, P.; van der Velden, G. P. M. J . Phys. Chem. 1984, 88, 5 . (6) Mastikhin, V. M.; Lapina, 0. B. Reucf. Kinef. Lea. 1979, 11, 353.
0 1991 American Chemical Society
