J. Phys. Chem. 1993,97, 6492-6499
6492
Shape-Selective Assemblage of Charge-Transfer Complexes within Channel-Type Zeolites K. B. Y w d and T . J. Hub Department of Chemistry, Sogang University, Seoul 121 - 742, Korea
D. R. Corbin Central Research and Development Department, E.I. du Pont de Nemours & Co., Wilmington, Delaware 19880-0328
J. K. Kochi' Chemistry Department, University of Houston, Houston, Texas 77204-5641 Received: February 17, 1993; In Final Form: April 1. 1993
The channel-type zeolites (mordenite, zeolite-L and mazzite) with different-sized pores are uniformly ion exchanged with a series of pyridinium cations (A+) ofvarious sizes and shapes. Intercalation of these pyridiniumdoped (colorless) zeolites with aromatic donors (D) of differing sizes and shapes leads to brightly colored zeolites, in which the diffuse reflectance spectra reveal characteristic charge-transfer bands of the intermolecular complex [A+,D],encapsulated in the zeolite channel. The selective production of colored zeolites that depends on the size/shape of the pyridinium cation and the aromatic donor, as well as the pore size, thus defines the shape selectivity of the channel zeolites. The restricted (cylindrical) cavity of channel zeolites to act as shapeselective hosts to various charge-transfer complexes is compared to the large (spherical) supercage of the more common zeolite-Y.
CHART I
Introduction As crystallinealuminosilicates, zeolites consist of 3-dimensional arrays of Si04 and A104tetrahedra that enclose uniform cavities of uniquely differentiated shapes and sizes' into which only a few, relatively small guest molecules can be effectively accomm o d a b i 2 Especially germane to all zeolites is their capacity as solid electrolytes with negatively charged metal oxide surfaces to incorporate elevated levels of various cations to precisely tune the polar en~ironment.~ Thus, we recently reported4 that Na+ in zeolite-Y can be effectively ion-exchanged to very high degrees with organic cations (Afi) of different shapes and sizes-such as the methylviologen (MVz+) and diquat (DQ2+)dications, as well as the variously substituted pyridinium (P+),quinolinium (Q+), and acridinium (A&) monocations in Chart I. The mere exposure of these doped (colorless) zeolites to dilute solutions of aromatic donors (D) led to a series of distinctively colored zeolites, while the supernatant solution remained singularly colorless. The diffuse reflectance spectra of the colored zeolites revealed new absorption bands arising from the presence of thecorresponding 1:1charge-transfer complex within the zeolite cavity (Z), i.e.
Among the readily available structures, zeolite-Y is unique in that the intravoid space consists of a series of rather large quasispherical cavities (Le., supercages with 13 A diameter) that are tetrahedrally interconnectedvia apertures with 7.4-A openings.5 Most importantly,the stepwise assembly of intermolecular chargetransfer complexes according to eq 1 was found to be strongly dependent on the combination size/shape of A+ (Chart I) and the aromatic donor-in other words, shape-selective.6 We now inquire how the pore (cavity) configuration extant in other types of zeolites bear on this shape selectivity. In particular, we direct our attention to mordenite, zeolite-L, and mazzite (a)in which the confined voids consist of (quasi) cylindrical channels of various sizes, as illustrated in Chart 11. 0022-3654/93/2097-6492sO4.00/0
M e L N W 6 - M e
+W+ MV2+
DQ~+
CN
mCP+
Y,
he
OCP+
m
N
: Me
PCP+
he
Results
Incorporation of Pyridinium Acceptors into Chamel Zeolites. Ion exchange of mordenite (M),zeolite-L, and mazzite (a) in their sodium forms with the acceptors listed in Chart I was carried out in thoroughly mixed slurries with aqueous solutions of the pyridinium salts. The degree of acceptor incorporation was controlled by varying the (weight) ratio of A+ to the eol lite.^ The maximum levels of acceptor incorporation are listed in Table I for various (channel) zeolites, and the maximum exchange into the supercages of zeolite-Y and zeolite-A are also included for comparative purposes. For the channel zeolites (mordenite, zeolite-L, and mazzite), the extent of (maximum) ion exchange varied markedly with the Q 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6493
Channel-Type Zeolites
TABLE I: Ion Exchange of Pyridinium Acceptors into sodium zeoiites' acceptor
exc
PCP+
0.71 0.66 0.70 0.52 0.49 0.34 0.24 0.14
&CP+
m-CP+
MV2+ DQ2+
Q+
iQ+
ocd
n (7.4)b
L (7.1)b
M (6.5 X 7.0)b
ocd
exc
exc
A (4.2)b
Y (7.4)b
ocd
exc
ocd
ocd
exc
0.62 0.41 0.49 0.36 0.30 0.49 0.49 0.11
Ac+ From aqueous solutions of halide salts, respectively. Pore size in angstroms. Amount of acceptor incorporated into zeolite (mmol/g of hydrated). Number of acceptor per 7.5-A channel (M, L, and a) or per supercage (Yand A). a
TABLE II: Diffuse Reflectance Spectra of Charge-Transfer Complexes in Zeolite-La (Effect of Donor Size) arene donor (IPY MV2+ DQ2+ &CP+ (8.14) colorless 360 390 (8.05) (7.92) (7.85) (8.18) (7.90) (8.12) (7.96) (7.72) (7.50) (7.58) (7.43) (7.25) (7.11)
,a,X
370 375
colorless (415)c 380 405 380 -400 430
colorless 450 495 -510
410 400
colorless (425)c -420 435 430 430 465
colorless 480 560 530
PCPC
colorless
350 350
340 365
colorless (380)'
colorless (390)c
-360 390 370 395
colorless -420 -450 -480
365 395 365 -380 390
colorless 415 -460 -480
colorless colorless colorless colorlcss in nanometers. Ionization potential of Ar in electronvolts. From solid-state mixing after 5 days at room temperature. Stepwise Assembly of Charge-Transfer Complexes in Channel Zeolites.To ensure reproducibility, the pyridinium-dopedzeolites
CHART II
Mordenite
Zeolite-L
Mazzite
shape (size) of the acceptor-generally in the order: 0-,m-,p-CPe (small) > MVZ+(medium) > Ac+ (large). It is interesting to note that despite the larger entrance to the mazzite pore (7.4 A) relative to that of zeolite-L (7.1 A), higher loadmgsof most cations were achieved with zeolite-L, the interesting exception being Ac+ (thelargest acceptor). Thelatter isconsistent withmazzite having bigger channels that can incorporate an average of one cation into each 7.5 A length of channel (last entry, column 3) compared to only 0.3 Ac+ into zeolite-L (column 2). Further inspection of the results in Table I indicates that several cations can pack into the same linear (channel) space. For example, the fact that an average of one MV2+ can be exchanged into each 7.5 A length of zeolite-L (entry 4, column 2) must lead to overlappingacceptor units since each dication is approximately 12 A long!' The extremesof ion-exchangebehavior were shown by zeolitey and zeolite-A. On one hand, zeolite-Y exhibited little or no shape selectivityin the incorporationof the various pyriddum acceptors in Table I (column 4), owing to the rather wide aperture (7.4 A) to the large supercage. Contrastingly, the restricted aperture (4.2 A) of zeolite-A allowed little or no exchange to occur with the same acceptors (column 5 ) despite the reasonable size of its supercage with a 11.4-A diameter.
(A+)z consisting of mordenite, zeolite-L, and mazzite were uniformly controlled at an occupation of one acceptor for each 15-A length of channel for use in all the studies of charge-transfer complexes. In each case, the doped zeolite was washed with methanol prior to evacuation and heating at 100 OC for 10 h. The dried (colorless) zeolites were then transferred into a moisturefree glovebox ( zeolite-L mazzite > mordenite. The results
-
essentially invariant in the four zeolites. Moreover, the linear correlation indicated that K was not 1arge,12 since the color intensity of the charge-transfer complex of MV2+ and 1,4dimethoxybenzene in mordenite was significantly attenuated relative to that in the other zeolitesdespite the absolute amount (0.36 mmol g-l) of intercalated donor being in excess of that (0.30mmol a')of the cationic acceptor. Clurge-Traasfer Spectra of the Pyridinium/A"tic Complex- in Different hvhmeois. To ascertain how the intercalation of aromatic donors into the pyridinium-dopcd zeolites relates to the observation of chargetransfer bands, we focusad on the behavior of the MV2+/2,6-dimethoxynaphthalcnepair, for which X-ray crystallographe has established the pertinent acceptor-donor interaction in I (shown as the projection normal to their cofacial planes). The relevant spectral data are presented in Table VI11 for this charge-transfer complex (I) encapsulated in the channels of zeolite-L and mazzite (a) and the supercage of zeolite-Y in comparison with thoee'locked" in crystalline state or freely mobile in acetonitrile solution (Figure 6). Although the maximum
Channel-Type Zeolites
TABLE MI: Medium Effect 011 Charge-Transfer S~ectra 2,6-(MeO)zNaph-MV2+ HMB-MV" HMB-DQZ+ medium Am4 HPWb Am4 HPWb Amn HPWb CH&N 450 3348 385 3850 380 3805 crystal 460 3282 Y 450 4458 385 3947 405 5185 D 460 5312 380 5810 390 5328 L 450 4170 415 5128 425 5264 a Absorption maxima of the CT envelope in nanometers. Half-peak width was measured at half-height from the top of the peak, in cm-l.
0
rn
e 4
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6497 mordenite, mazzite, and zeolite-Y) must occur within the internal pores (as opposed to the surfaces) since negligible doping occurs in zeolite-A with an aperture that is smaller than the kinetic diameters of the pyridinium acceptors ( u > 6 A). This conclusion also follows from the inherent limitationsof surface coveragtthe amounts of exchanged pyridinium ions simply being too large to be accommodated by the (limited) external surface.13 Thus the (partial) overlappingof pyridinium acceptors within the channels of mordenite, zeolite-L, and mazzite, as noted above, suggests that the planar pyridinium cations are stacked cofacially within the zeolite channels, not unlike the packing in crystals. Shape Selectivityin the StepwiseFormationof Charge-Transfer Complexes within Zeolites. The facile passage of aromatic substrates into the channel or supercage is readily detected by the bright coloration developed in the pyridinium-doped zeolites upon mixing as slurried solutions or finely ground powders. The absence of coloration in doped A-zeolites is consistent with the penetration of aromatic donors into the channel or supercage as a necessary prerequisite to the formation of chargetransfer colon in Tables I1 and IV-VI. Thus the very slow coloration of the doped mordenites only with para-substituted benzenes such as 4-methoxytolueneand 1,4-dimethoxybenzenein Table V defines the limits as u 6 A for access to this channel. Indeed the 6.5 x 7.0 A cross sectional area of the mordenite channel is just sufficient to allow cofacial interactions of pyridinium/aromatic pairs. The dramatic increase in the rate at which various aromatic donors are intercaIated into zeolite-L (relative to mordenite) underscores how the relatively slight change to a 7.1-A (circular) aperture can lead to fine tuning the shape selectivity of zeolites. Since a further increase to a 7.4-A aperture in mazzite leads to no significant change in shape selectivity,the excluded aromatic donors should have kinetic diameters sufficiently larger than 7.4 A. The kinetic diameters of pentamethylbenzene and hexamethylbenzene are estimated to be 7.15 and 7.95 A, respectively.41 We judge from the size exclusion of hexamethylbenzene, but not pentamethylbenzenelikewise, 9,lO-dimethyl- but not 9-methylanthracene and 1.4- but not 2,6-dimethoxynaphthalenefrom zeolite-L and mazzite that a van der Waals "width" of roughly 8 A is sufficient to restrict their access to these zeolite channels. The variable intensities of the colored zeolites as illustrated in Figure 4 may be used in conjunction with the linear correlation in Figure 5 as a supplementary (visual) methods for shape selectivity in the intercalation of aromatic substrates. Effect of Solventon ShapeSektivity. Intercalation of aromatic donors into the doped zeolites is most conveniently carried out from solutions by use of nonpolar solvents such as hexane. However, the slow intercalation of the excluded donor hexamethylbenzene ( a 8 A) into the pyridinium-doped zeolite-L (with a 7.1-A aperture), which is possible only in the absence of hexane, indicated that the solvent is not entirely innocent in the passage of aromatic donors into the zeolite channels. The latter suggests that the desorption of solvent is coincident with the adsorption of aromatic donors and that the kinetic diameter of hexamethylbenzene is at the threshold for such an observation. Indeed the enhanced rates of coloration observed at an elevated temperature (80 "C) probably reflects the increased rate of bond and latticevibrations'**sufficient to allow the passage of aromatic donors whose kinetic diameter may be up to an angstrom larger than the width of the zeolite aperture.14 The beneficial effects of elevated temperatures werealsoobserved in the slow interaction of hexamethylbenzene from hexane solution into the zeoliteY supercage.k Structuresof Charge-TransferComplexes in zeolite chrnaele and Supercage. Charge-transfer complexes of the planar MV2+ acceptor and planar aromatic donors exist in crystals as alternating cofacial pairs, as typified by the 2,6-dimethoxynaphthalene complex in I. In such complexes, the charge-transfer spectrum varies with the interplanar separation, the degree of (orbital) overlap, and the relative orientation of the acceptor/donor
-
Wavelength (nm) Figure 6. Comparison of the charge-transfer spectra of the complex of MV2+ and 2,6-dimethoxynaphthalenein various media: acetonitrile (CHICN),crystallinestate (X),zeolite-L(L),mazzite(D),and zeolite-Y (Y).
position of the charge-transfer band was essentially invariant, the spectral envelopewas significantlybroader in the zeolite media (especially mazzite), as given in Table VIII, column 2, by the bandwidth at half-maximum ( A Y Gaussian-based ~ fwhm). Similar trends were also observed (Table VIII) in the chargetransfer spectra of the MV*+, D@+, 0-CPt, andp-CP+ complexes with the C6-symmetrical donor hexamethylbenzene (shown in Figure 3). The results in Table VIII, column 3 and 5 , also significantly quantify the unusual bathochromic shifts observed in zeolite-L (vide supra).
Discussion The shapeselective behavior of zeolites in the intracavity formation of charge-transfer complexes is manifested in two ways: fmt, in the ion-exchangewithvarious pyriddumacceptors, and second, in the intercalation of different aromatic donors as follows. Shape-Selective Ion Exchange of Pyridinium Acceptors into Zeolites. The shape-selective nature of pyridinium incorporation into the zeolites is best considered in light of the results in Table I, which show the ion-exchange capacity with (i) a particular cation to increase with pore size and (ii) a particular zeolite to decrease with cation size/shape. Furthermore, the pyridinium doping of the zeolites with apertures greater than 6.5 A (viz.,
-
Yoon et al.
6498 The Journal of Physical Chemistry, Vol. 97,No. 24, 1993
3.0 2.5
R=0.95
Location of Charge-Transfer Complexes in ZeoliteY. The intercalation of aromatic donors into pyridinium-doped zeolite-Y was relegated in the previous study to the supercage.4 However, the shape selectivity of the channel zeolites examined here is not distinguished from that in zeolite-Y. This comparison coupled with the 7.4-A aperture extant in zeolite-Y leads us to conclude that the relevant charge-transfer complexes can occupy the interconnecting windows of zeolite-Y supercages. However, we judge from the rather high R values in zeolite-Y (Figure 7) that the charge-transfer complexes are, for the most part, located within the supercages. Experimental Section
R=O.gl 2.0
3.5 3.0
2.5 2.0
76
7.0
0.0
0.2
0.4
0.6
0.0
(ev> Figure 7. Comparison of the correlation factors R (as indicated) in the IP-ERED
Mulliken relationships of the charge-transfer bands (hvm)of various arene complexes with pyridinium acceptorsin Chart I for various zeolites and in acetonitrile solution. pair.15.16 As such, the crossed orientation of the long axes of MV2+ and 2,6-dimethoxynaphthaleneclearly occupies too much space to "fit" into the channels of either zeolite-L or mazzite. Instead the colinear arrangement depicted in I1 appears to be a more reasonable structure for the complex within the channels.
If the acceptordonor orientation governs the spectral bandwidth of the charge-transfer complex, Avm should be comparable to that in the "locked" crystal. However, the results in Table VI11 show the significant increase of Aum for the MV2+/2,6dimethoxynaphthalene complex in all the zeolites, irrespective of pore size. Orientation is clearly not a factor that governs the spectral bandwidth, and this conclusion is supported by analogous increases in bandwidth for the encapsulated (Cs-symmetrical) hexamethylbenzene complex. Thus, we are left with the interannular separation and the degree of overlap (caused by sliding the ring systems past each other) as the important structural limitation in charge-transfer complexes that occupy zeolite channels. Accordingly we tentatively attribute the increased bandwidths to a multitude of colinear CT structures lying in the zeolite channel with differing degrees of overlap. The scattered data in the Mulliken plots for zeolite-L and mazzite with low correlation factors of R = 0.92 and 0.91 (Figure 7), compared to those observed in zeolite-Y or in acetonitrile solution, indicate that such collinear arrangements are less stable than the crossed orientation (see I). Finally, the unusual bathochromic shifts of the hexamethylbenzene complexes of MV2+ and DQ2+ in zeolite-L (compare Table I1 entry 4 with Table 111, entry 3) can be described to a decreased interannular distance between the donor and the dications. (Note the large kinetic diameter of HMB forces it to fully occupy the 7.1-A channels.)
Materials. The sodium forms of zeolite-A (4A, Lot No. 94 1089060329), zeolite-L (ELZ-L, Lot No. 96 1687041001-S), zeolite-Y (LZY-52, Lot. No. 968087061020-S), and mordenite (LZM-5, Lot. No. 962487061003-S) were obtained from Union Carbide Co. Mazzite (zeolite-Q, E71948-036-1) was a developmental sample from Linde and had the following chemical composition (anhydrousbasis): A1203, 16.7%;5302, 75.7%; NazO, 6.5%. The zeolites were all treated with aqueous 1 M NaCl solution and then subsequently washed with distilled (deionized) water until the wash was pH 7 and the silver ion test for chloride was negative. Methylviologen dichloride and diquat dibromide from Hannong Chemical Co. were recrystallized repeatedly until colorless and pale yellow, respectively. The hexafluorophosphate salts of MV2+and DQz+were obtained by metathesiswith NH4PF6 in aqueous solution. MV(PF&: IH NMR" (CD&N), 6 8.84 (d, JHH = 4 Hz, 4 H), 8.36 (d, JHH = 4.8 Hz, 4H), 4.39 (5,6H). DQ(PF6)2: IH NMR" (CD3CN), 6 9.1 1-8.16 (m, 8H), 5.15 (s, 4H). The iodide and triflate salts of N-methylquinolinium (Q+) and N-methylisoquinolinium(iQ+), N-methylacridinium (A&), and the ortho (0-CP+), meta (m-CP+), and para (p-CP+) isomers of N-methylcyanopyridinium were prepared by quaternization of the corresponding (free) bases with methyl iodide and methyl trifluoromethanesulfonate, respectively. Q+OTf-: IH NMR (CD3CN), 6 9.12 (br s, lH), 9.04 (br s, 1H) 7.9-8.4 (m, 5H), 4.56 (s, 3H); iQ+OTf-: IH NMR (CDSCN),6 9.55 (br s, lH), 7.9-8.4 (m, 6H), 4.43 (s, 3H). oCP+I-: IH NMR (D20), 6 9.35 (d, JHH = 6 Hz, 2H), 8.96-8.50 (m, 2H), 4.73 (s, 3H). mCP+I-: 'H NMR (D2O) 6 9.42 (s, lH), 9.1 (d, JHH= 6 Hz, lH), 8.85 (s, lH), 8.23 (t, JHH= 6 Hz, lH), 4.46 (s, 3H). pCP+OTf-: IH NMR (D20), 6 9.06 (d, JHH = 7.2 Hz, 2H), 8.45 (d, JHH = 6.6 Hz, 2H), 4.46 (s, 3H). The aromatic donors (Aldrich) were purified by standard methods.'* n-Hexane was treated with concentrated sulfuric acid prior to distillation from sodium under an argon atmosphere. Acetonitrile was treated with KMn04 and distilled from P205. All the solvents were stored in Schlenk flasks under an argon atmosphere and kept in the dark. Ion Exchange of the Pyridinium Acceptors into Zeolites. The amounts of acceptors in the doped zeolites for the formation of arene charge-transfer complexes were controlled to one acceptor per every 15-A length of the channels in mordenite, zeolite-L, and mazzite, and 0.9 ion per every supercage of zeolite-Y. Typically, MV/M was prepared by shaking the 1 L flask containing LZM-5 (10 g), MVCl2 (0.74 g), and 900 mL of water for 15 hat ambient temperature. Other MVz+-exchangedzeolites were prepared by a similar procedure with the corresponding amounts of MVCl2 and the zeolite. The MVz+-exchangedzeolites were filtered through sintered glass filters, and they were washed with copiousamounts of distilled deionized water until thechloride test with AgNOs was negative. The amounts of unexchanged MV2+ ion in the wash were quantified by UV-vis spectrometry monitored at X = 257 nm (e = 20 417).19 The DQ2+-exchanged zeolites were prepared by the ion exchange of the corresponding zeolites with DQBr2. Thep-CP+- and o-CP+-exchangedzeolites were obtained from the corresponding iodide salts via a similar procedure. All the doped zeolites were first driedin air at ambient temperatures, and the air-dried zeolites were then briefly washed
Channel-Type Zeolites with methanol and subsequently evacuated at 25 OC for 2 h in vacuo ( < l e 5 Torr). The temperature was slowly increased to 100 "C over a period of 2 h, and they were evacuated further at this temperature for an additional 10 h. The dried zeolites were transferred to an argon-filledglovebox and kept in tightly capped glass containers. To achieve the maximum incorporation of acceptors into the zeolite, the added amounts of acceptors and zeolites in the slurries were controlledat a ratio of three acceptors per every 7.5-A length of the channels in mordenite, zeolite-L and mazzite, and to the supercage of zeolite-Y. The exchanges were then repeated to ensure the maximum incorporation. The amounts of unexchanged acceptor ions in the washes were quantified by UV-vis spectrophotometry. Formation of Intrazeolite
