J. Phys. Chem. B 1997, 101, 3043-3051
Intrazeolite Photochemistry. 17. Zeolites as Electron Donors: Photolysis of Methylviologen Incorporated within Zeolites Mercedes Alvaro,† Hermenegildo Garcı´a,*,†,‡ Sara Garcı´a,‡ Francisco Ma´ rquez,‡ and J. C. Scaiano*,§ Departamento de Quı´mica, UniVersidad Polite´ cnica, Apartado 22012, 46071 Valencia, Spain, Instituto de Tecnologı´a Quı´mica CSIC-UPV, UniVersidad Polite´ cnica, Apartado 22012, 46071 Valencia, Spain, and Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario, K1N 6N5, Canada ReceiVed: September 19, 1996; In Final Form: January 27, 1997X
Methylviologen (MV2+) has been adsorbed in a series of zeolites including alkaline ion faujasites (HNaY, LiNaY, NaY, KNaY, RbNaY, CsNaY, and NaX), Naβ, NaMor, and NaZSM-5 by ion exchange. Extra large pore aluminosilicate MCM-41 was also used. The resulting MV2+-doped zeolites have been characterized by an array of analytical and spectroscopic techniques such as chemical analyses, X-ray photoelectron spectroscopy, thermogravimetry-differential scanning calorimetry, diffuse reflectance spectroscopy, and fluorescence, FT-IR, FT-Raman, and MAS 13C NMR spectroscopies. Influence of the confinement and nature of the charge-balancing cation on the molecular properties of MV2+ resulted in shifts of the λmax of the absorption band in the diffuse reflectance spectra from 270 to 280 nm for MV-NaX and 290 nm for MVNaZSM-5 samples. In addition, changes of the relative intensities of the emission bands at 340 and 420 nm were also noted. The formation of the MV•+ cation radical as a persistent species by thermal treatment from the MV2+ ground state was not observed in any of the samples. In contrast, laser flash photolysis of these samples allowed in all cases the detection of MV•+ as a long-lived transient on the microsecond time scale. Formation of MV•+ was also observed by X-ray photoelectron spectroscopy. This constitutes the first firm experimental evidence that zeolites can behave as single electron donors. For MV-RbNaY and MV-CsNaY samples in outgassed sealed quartz cells, the photogenerated MV•+ is so long-lived that its diffuse reflectance spectrum and disappearance kinetics can be obtained by conventional diffuse reflectance spectroscopy. A relationship between the basic strength of the zeolite framework and the stability of photogenerated MV•+ has been established.
Introduction The ability of zeolites to generate radical cations spontaneously upon adsorption of electron-rich organic compounds by single electron abstraction is by now firmly established.1-12 Although the exact structure of the sites responsible for the electron acceptor ability of these microporous crystalline solids is still not completely established, a consensus is beginning to be reached that this property of zeolites is related to the presence of acid centers, the key question being the Bro¨nsted or Lewis nature of the sites involved. In contrast, the reverse situation in which zeolites act as single electron donors for electrondeficient organic species is far less documented. In a review covering zeolites in electron-transfer processes, no published references were given, although such possibility was anticipated.13 In this context, during the laser flash photolysis of the xanthylium ion embedded within ZSM-5, we have previously observed an absorption at 340 nm in the time-resolved diffuse reflectance spectrum attributable to the xanthyl radical that would be generated by single electron transfer from the zeolite to the excited state of the cation.14 However, this evidence was complicated by the concurrent T-T absorption of the triplet and by bleaching signals due to the ground state of the highly colored xanthylium ion. Thus, a spectrum corresponding exclusively to the xanthyl radical could not be obtained. †
Departamento de Quı´mica, Universidad Polite´cnica. Instituto de Tecnologı´a Quı´mica CSIC-UPV, Universidad Polite´cnica. § Department of Chemistry, University of Ottawa. X Abstract published in AdVance ACS Abstracts, March 1, 1997. ‡
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In the present work, we have prepared a wide and consistent series of zeolites containing methylviologen (MV2+). The behavior of MV2+ as a powerful single electron acceptor in its excited state has been firmly established in solution.15-24 In addition, the radical cation MV•+ generated after electron transfer has been characterized by several techniques in various media and a wealth of information is now available.24-37 In particular, MV•+ can be unambiguously identified by its characteristic UV-vis spectrum consisting of an intense absorption at 390 nm and a broad band of medium intensity with a partially resolved structure centered at ∼600 nm.38 Although our work was in progress, photoreduction of MV2+ exchanged in zeolites A, NaX, and NaY at 77 K has been reported.24 In this work, we have characterized an extensive series of MV2+-doped zeolites by a combination of experimental techniques including thermogravimetry, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DR), and fluorescence, Raman, IR, and 13C NMR spectroscopies. The samples were submitted to UV irradiation, and we have been able to establish unequivocally the formation of MV•+, both under reactive conditions in the submillisecond time domain for all the samples and in the absence of oxygen and moisture by conventional spectroscopic techniques in some cases. The aim of our work was to obtain conclusive experimental evidence that zeolites can also act as single electron donors and to establish a relationship between the physicochemical parameters of the zeolites and their ability as electron donors. Thus, we have established that in the same way that acidity of zeolites is © 1997 American Chemical Society
3044 J. Phys. Chem. B, Vol. 101, No. 16, 1997
Alvaro et al.
TABLE 1: Structures and Chemical Compositions of the MV2+-Doped Aluminosilicates Prepared in This Work
unit cell composition
unit cell composition after exchange with MV2+
LiNaY NaY KNaY RbNaY CsNaY HNaY NaX Nass NaMor NaZSM5 HMCM41
Na27Li26Al53Si139O384‚50H2O Na53Al53Si139O384‚223H2O Na8K45Al53Si139O384‚175H2O Na14.5Rb38.5Al53Si139O384‚125H20 Na20Cs33Al53Si139O384‚136H2O Na31H22(AlO2)53(SiO2)139‚144H2O Na96Al96Si96O384‚178H2O Na4.6Al4.6Si60O129‚22H2O Na6.5Al6.5Si41.5O96‚23H2O Na4.6Al4.6Si92O193‚14H2O H7Al7Si95O204‚38H2O
Na27Li20.05(MV)2.8Al53Si139O384‚166H2O Na45.8(MV)3.6Al53Si139O384‚183H2O Na8K37.6(MV)3.7Al53Si139O384‚173H2O Na14.5Rb32.3(MV)3.1Al53Si139O384‚125H2O Na16.8Cs28.8(MV)3.8Al53Si139O384‚136H2O Na26H22(MV)2.5(AlO2)53(SiO2)139‚136H2O Na89.3(MV)3.3Al96Si96O384‚196H2O Na2.8(MV)0.9Al4.6Si60O129‚25H2O Na4.6(MV)1.9Al6.5Si41.5O96‚23H2O Na1.4(MV)1.6Al4.6Si92O193‚18H2O H3.8(MV)1.6Al7Si95O204‚38H2O
related to their single electron acceptor ability, the basicity of the framework strongly influences the electron donor behavior of zeolites. Results and Discussion Preparation and Characterization of the MV2+-Doped Zeolites. To encompass the widest range of chemical compositions and topologies, we have employed medium pore size zeolites (ZSM-5), as well as large pore zeolites of monodirectional (mordenite), bidirectional (β), and tridirectional (X and Y) topologies.39 We have also included extra-large pore aluminosilicate MCM-4140-42 to explore possible confinement effects when there is a tight fit of MV2+ on zeolites of smaller micropore dimensions. In addition, we have included a series of alkaline metal ion-exchanged Y zeolites to properly address the influence of basicity43-53 on the ability of these materials as single electron donors, as well as the stability of the resulting MV•+. Amorphous silica was not used, since two recent reports have studied the photolysis of MV2+ adsorbed on this nonmicroporous solid material.32,37 We note that the ion exchange capacity of silica should be ideally zero, while in practice, it is very low owing to some impurities and defects. In contrast, in the case of zeolites, the different valence between silicon and aluminum introduces negative charges in the lattice that require the presence of charge-balancing cations (one monovalent cation per each framework aluminum) to satisfy the electroneutrality of the solid.54-56 The ionic nature of the bond between the compensating cation and the lattice allows ion exchange without any alteration of the crystalline structure. Therefore, the amount of MV2+ incorporated in our zeolites (about 0.2 mmol of MV2+ per gram of zeolite; see Table 1) is 2 orders of magnitude higher than what can be adsorbed on silica by ion exchange (10-3 mmol of MV2+ per gram of silica).32 To obtain alkaline metal-exchanged Y zeolites, the commercial NaY sample was submitted to exhaustive ion exchange with alkaline ions before proceeding with the adsorption of MV2+. In these cases, chemical analyses were performed before and after the incorporation of MV2+. When there are two different cations in the supercage (Na+ and an alkaline ion), this procedure has allowed us to establish which cation is preferentially exchanged by MV2+. In general, we find that exchange of the second alkaline metal ion is preferred (see Table 1). Doping of zeolites with MV2+ was carried out by stirring at room-temperature aqueous solutions of MVCl2 in the presence of the corresponding alkaline ion form of the zeolite, except for MCM-41 for which an as-synthesized sample after calcination was used. Purposely, we also submitted a sample of HNaY to doping with MV2+.
MV2+ content (mmol g-1) 0.2103 0.2151 0.2237 0.1783 0.1984 0.1524 0.1981 0.2038 0.2784 0.2603 0.2147
loading of MV2+ per supercage 0.35 0.45 0.46 0.39 0.47 0.29 0.42 0.45
Table 1 summarizes the chemical composition of the MV2+doped zeolites prepared. The percentages of ion exchange were determined by atomic absorption spectroscopy of the remaining alkaline metal ion as well as combustion chemical analysis (C and N) of MV2+. In the case of zeolite Y, taking into account its high aluminum content, it would be possible to exchange up to a maximum of two MV2+ per supercage (corresponding to 1.76 mmol of MV2+ per gram of Y zeolite, approximately). Therefore, and also in view of the precedents in the literature,13,57-62 no problems were anticipated during the ion exchange. However, taking into account the high Si/Al ratio of NaZSM-5 and MCM-41, and hence, their low ion exchange capacity (ZSM-5, 3.7 × 10-1; MCM-41, 5 × 10-1 mmol of MV2+ × g-1), and the fact that the negative centers of the lattice associated with the random distribution of Al will be further apart, we were concerned with the possibility that some monopositive MVCl+ ion pairs could be introduced during the exchange. For polyvalent metal ions, it is known that ion exchange with their complexes with anionic ligands can occur, each cation thus compensating for only one negative charge of the lattice.63 Since the presence of chloride greatly influences the photochemical behavior of MV2+ on silica and other solids,32 it is necessary to determine the total absence of Cl- in our zeolitic samples. Otherwise, the possibility that upon excitation an electron transfer from Cl- to MV2+ would occur could invalidate our results. To determine the content of Cl-, we performed XPS analyses of the MV-NaZSM-5 and MV-MCM-41 samples. It is well established that this technique is highly sensitive to the presence of chloride on the outer surface of the particle that is readily characterized by its 2p electron band at 199 eV. No Cl- could be detected in any of these two samples. On the other hand, the samples turned blue, indicating the formation of MV•+ during X-ray excitation. Since the XPS sample chamber is under ultrahigh vacuum (