Influence of Gallium Isomorphous Substitution in the Acidic MFI Zeolite

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Influence of Gallium Isomorphous Substitution in the Acidic MFI Zeolite Framework on Hole Formation, Transfer, and Trapping upon Incorporation of Anthracene Alain Moissette,*,† Raul F. Lobo,*,‡ Herve Vezin,† and Claude Bremard† †

Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Bât. C5, Universite des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq cedex, France ‡ Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, United States ABSTRACT: The redox behavior of ACENE-3 upon sorption within the channels of Brønsted acidic H-GaZSM-5 is followed by using diffuse reflectance UVvisNIR spectrometry in parallel with continuous wave EPR techniques and with Raman spectrometry. Partial spontaneous ionization is clearly observed, whereas proton transfer does not occur. The intense internal proton electrostatic field, as well as the confinement effect on the occluded molecule, are put forward to explain charge separation and the long-lived ACENE-3•þ radical cation. The compartmentalization of ACENE-3•þ and trapped electron within H-GaZSM-5 hinders charge recombination but promotes hole transfer to the inner surface of the 10-membered ring (10-MR) channel. Pulsed X-band EPR experiments using the 2D-HYSCORE sequence were carried out to investigate the structural surrounding of unpaired electrons. The observation of electron couplings with 1H, 29Si, 69Ga, and 71Ga nuclei allows clear differentiation between the two successive charge separated states. The present results for H-GaZSM-5 are compared to previous analogous experiments conducted on H-AlZSM-5 for which hole transfer from the ACENE-3 organic molecule to the silicoaluminate framework was not observed at room temperature after the ACENE-3 spontaneous ionization in H-AlZSM-5. The hole transfer ΔG0 is the key parameter in the nonadiabatic electron-transfer theory that explains the hole transfer at room temperature within H-GaZSM-5 and not within H-AlZSM-5.

’ INTRODUCTION In most catalytic reactions involving microporous zeolite catalysts, the acidic sites of the zeolite play a pivotal role: hence, much attention has been paid to the nature of the acidic sites and the interactions of these sites with occluded organic molecules.1 However, electron transfer between acidic zeolites and molecules appears to be implied in several important catalytic chemical processes. One of the most intriguing properties of the acidic zeolites is their ability to generate spontaneously and to trap organic radical cations. One-electron oxidation (hole formation) is increasingly being exploited as a fundamental option for activating molecules toward chemistry mediated by cation radicals.2 The characterization of many radical cations trapped in zeolite pores is well documented.3 In contrast, convincing evidence of the fate of an ejected electron has been difficult to come by. Recent findings have shown that zeolites in their acid form function also as electron donors (hole acceptor).4,5 The nature and structure of the centers responsible for the redox ability of zeolites to induce catalytic chemical processes is a subject of current interest, but it has remained unclear if the redox behavior of acidic zeolite is important to hydrocarbon catalytic processes.47 A very large number of zeolite topologies are known today, but so far only a very limited number of such zeolites are actually used r 2011 American Chemical Society

in applications as sorbents, ion exchangers, catalysts, and catalyst supports. Medium pore zeolites such as MFI (ZSM-5) find several important industrial applications such as transformation of alcohols into hydrocarbons.8,9 Zeolites are crystalline aluminosilicate microporous minerals. Their structure is based on a three-dimensional network of SiO4 or AlO4 tetrahedra which are linked to each other via doubly bridging oxygen atoms. Materials with the framework-type MFI have the general formula Mn(AlO2)n(SiO2)96n, where M is the charge balancing extraframework cation, and consist of two intersecting channels with 10-membered ring (10-MR) pore apertures. The presence of Al(III) replacing Si(IV) requires charge balancing cationic species Mþ. Proton is one of the most frequently exchanged Mþ cations which gives rise to hydroxyl groups resulting in a bridging SiO(H)Al structure well-known as a strong Brønsted acid site (BAS). The isomorphous substitution of Si(IV) by other tetrahedrally coordinated heteroatoms such as B(III), Ga(III), and Fe(III) in small amounts provides new MFI-type materials showing distinct Received: December 16, 2010 Revised: March 2, 2011 Published: March 17, 2011 6635

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The Journal of Physical Chemistry C catalytic properties. In fact, the acidic strength of the protons in the bridged SiO(H)M0 (III) groups depends on the nature of the trivalent heteroatom (M0 = B, Ga, Al, Fe).1012 It was recently established that the ethanol transformation into hydrocarbons on H-MFI generates bulky aromatic radicals within the 10-MR channel.13 These radicals could play an important role in the catalytic process. To improve our knowledge of the catalytic mechanisms of this and other reactions, it is essential to provide further insight into electron transfer between aromatics and H-MFI zeolites. Rod-shape polyaromatics such as acene oligomers are suitable models to study the redox behavior of polyaromatics inside the 10-MR channel of H-MFI. The 10-MR channel of MFI-type materials allows controlling the access of aromatic hydrocarbons to the active sites. It has been shown that benzene, naphthalene (ACENE-2), anthracene (ACENE-3), and tetracene (ACENE-4) can be accommodated within the 10-MR channel of MFI zeolites. It was also reported that the incorporation of electron donor ACENEn into Brønsted acidic aluminum-rich H-AlZSM-5 zeolite generates spontaneously ACENE-n•þ (hole formation) and trapped electrons in high yield.14 The occurrence of positive hole formation, transfer, and trapping is of basic significance for understanding of the primary events in radical cation (hole) catalyzed reactions. Here we report the unique redox behavior of ACENE-3 upon incorporation within the 10-MR channel of Brønsted acidic H-GaZSM-5 zeolite using pulsed EPR techniques in combination with diffuse reflectance UVvisNIR absorption spectrometry and Raman spectroscopy. We have chosen ACENE-3 because it has been demonstrated to be an efficient electron donor, and it is one of the most characterized polyaromatic molecules in oxidation processes.15,16 The structural surroundings of unpaired electrons of two successive charge separated states were obtained through the proper assignment of unpaired electron couplings with 1H, 29Si, 69 Ga, and 71Ga nuclei using the two-dimensional hyperfinesublevel correlation experiment (2D-HYSCORE). The isomorphous substitution of Ga for Si in MFI as H-GaZSM-5 induces hole formation (ACENE-3•þ) and then promotes subsequently hole transfer at room temperature and trapping on the inner surface of the 10-MR channel. The present results for H-GaZSM5 are compared to analogous experiments conducted previously on H-AlZSM-5. All findings suggest a consistent interpretation of subtle influence of isomorphous exchange in the MFI zeolite framework on hole formation, transfer, and trapping upon ACENE-n incorporation.

’ EXPERIMENTAL SECTION Materials. NH4-GaZSM-5 with (NH4)2.2(GaO2)2.2 (SiO2)93.8 3 xH2O chemical formulas per unit cell was synthesized as reported in ref 5. NH4-GaZSM-5 is transformed into the acid form H2.2(GaO2)2.2(SiO2)93.8 (hereafter denoted H2.2GaZSM-5) by heating up to 723 K in argon flow. A sample of silicalite-1 (SiO2)96 synthesized as before but without the addition of gallium nitrate was prepared for comparison to H2.2GaZSM5. Anthracene (ACENE-3, C14H10, 99%, Merck-Schuchardt) was purified by sublimation and stored over molecular sieves. Pure and dry Ar gas was used. ACENE-3 Sorption in H2.2(GaO2)2.2(SiO2)93.8. Prior to sorption, weighed amounts (∼1.4 g) of hydrated H2.2-GaZSM-5 zeolite were freshly dehydrated by a new calcination procedure up to 723 K under argon. Then the dehydrated H2.2-GaZSM-5

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sample was cooled to room temperature under dry argon. Weighed amounts of dry ACENE-3 corresponding to one ACENE-3 per unit cell were introduced into the cell under dry argon, and the powder mixture was shaken. The powders were transferred under dry argon in a quartz glass Suprasil cell for Raman and DR experiments and in a cylindrical quartz tube for EPR experiments and sealed. Instrumentation. The zeolite products were characterized by X-ray powder diffraction (XRD) using a Philips X-pert powder diffractometer. The radiation source is Cu (KR), generated at 45 kV and 40 mA. The diffraction scans were measured from 5 to 50° 2Θ with a 0.02° step and 2 s count time per step. The scanning electron micrographs (SEM) were obtained using a Jeol JSM-7400 instrument. The N2 adsorption isotherms were measured using a Micromeritics ASAP 2010 analyzer. The pore volume is calculated using the t-plot method. Diffuse Reflectance UVVisibleNIR Absorption (DR). The UVvisibleNIR absorption spectra of the powdered samples were recorded between 200 and 1800 nm using a Cary 6000i spectrometer. The instrument was equipped with an external integrating sphere (DRA-1800) to study the powder zeolite samples through diffuse reflectance. The DR spectra were plotted as the KubelkaMunk function FðRÞ ¼ ð1 RÞ2 =2R ¼ K=Sc

ð1Þ

where R represents the ratio of the diffuse reflectance of the loaded zeolite to that of the dehydrated neat zeolite; K designates an absorption coefficient proportional to the concentration C of the chromophore; and Sc is the scattering coefficient of the powder. Spectral sets F(λ, t) were recorded as function of λ (wavelength) at several t (time) during the course of the ACENE-3 sorption. Raman Scattering Spectroscopy. A Bruker RFS 100/S instrument was used as a near-IR FT-Raman spectrometer with a CW Nd:YAG laser at 1064 nm as the excitation source. A laser power of 100200 mW was used. The spectra (3500150 cm1) were recorded with a resolution of 2 cm1 using 600 scans. X-Band EPR Spectroscopy. The CW EPR measurements were performed on a Bruker ELEXYS E580E spectrometer operating at X-band (∼9 GHz) with a microwave power of 0.1 mW, a modulation amplitude of 0.7 mT, and a modulation frequency of 100 kHz. All measurements were made at room temperature or at 4.2 K. The magnetic field was calibrated with a DPPH standard. The spin concentrations were determined at room temperature, relative to a reference standard by double integration of the CW spectra using Bruker software. These standard samples are calcined H3.4(AlO2)3.4(SiO2)92.6 ZSM-5 zeolite loaded with different nitroxide amounts (3-carbamoyl-2,2,5,5-tetramethyl3-pyrrolin-1-yloxy, free radical, 99%, Aldrich). Field-Sweep Echo Detected (FS-ED) were collected at 4.2 K using the two-pulse sequence (π/2τπτecho), where the echo intensity was recorded as a function of the magnetic field. A 12 ns π/2 and 28 ns π pulse lengths with a τ = 200 ns were employed. HYSCORE spectra were recorded at 4.2 K using the four-pulse sequence (π/ 2τπ/2τt1πt2π/2τecho) and a four-step phase cycle to remove the unwanted echoes.17 The microwave π/2 and π pulse durations used in the HYSCORE experiments were 12 and 28 ns, respectively. The echo intensity was measured as a function of t1 and t2. The times t1 and t2 were increased by a 12 ns step from a starting time of 40 ns, and a quantity of 256 data points were collected in each dimension. The HYSCORE spectra were recorded with different τ values (88, 128, 200, and 256 ns). Spectra were recorded with 6636

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Figure 1. Diffuse reflectance UVvisible absorption spectra (200800 nm) recorded as a function of time after the mixing of solid ACENE-3 and H2.2-GaZSM-5 dehydrated at 723 K and under argon. (a) 1 week after the mixing. (b) 2 months after the mixing. (c) 6 months after the mixing. The spectra are vertically shifted for clarity.

different τ values to avoid the blind spot effect. All the results are presented for a τ value of 128 ns that minimizes these blind spots. The background echo decay in both t1 and t2 dimensions was removed by a second-order polynomial fit and apodized with a hamming function. Magnitude spectra calculation is performed after 2D Fourier transformation.

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Figure 2. Diffuse reflectance NIR absorption spectra (10001600 nm) recorded as a function of time after the mixing of solid ACENE-3 and H2.2-GaZSM-5 dehydrated at 723 K and under argon. (a) 1 h after the mixing. (b) 1 week after the mixing. (c) 1 month after the mixing. (d) 2 months after the mixing. (e) 6 months after the mixing. (e) 7 months after the mixing. The spectra are vertically shifted for clarity.

purely siliceous MFI (silicalite-1). These bands are close to those obtained after adsorption on silica gel18 and dissolution in cyclohexane.18,19 The intensity band increases as a function of time showed the gradual sorption process according to ACENE-3ðsolidÞ þ H2:2 -GaZSM-5 f ACENE-3@H2:2

’ RESULTS The NH4-GaZSM-5 and H2.2-GaZSM-5 samples were characterized extensively as reported in ref 5. It is shown that the samples are of excellent crystallinity and purity and that the Ga atoms are in the zeolite framework in tetrahedral coordination (within the detection limits of the X-ray absorption spectroscopy, FTIR, XRD, and N2 adsorption techniques). There are no extra-framework species in the sample, no evidence of Ga2O3 phases inside or outside the zeolite porous network, nor is there evidence of amorphous byproducts of the zeolite synthesis. Dehydrated H2.2-GaZSM-5 samples are typical of H2.2(GaO2)2.2 (SiO2)93.8 formulas with expected Brønsted acid and microporous properties. Diffuse Reflectance UVVisibleNIR Absorption Spectrometry. ACENE-3 and H2.2-GaZSM-5 solids are white powders.

The exposure under an argon atmosphere at room temperature of H2.2-GaZSM-5 freshly calcined under argon to dry solid ACENE-3 immediately turned the powder from white to light green. The green color intensified with time. The course of the reaction after mixing of powders with one ACENE-3 per zeolite unit cell loading was followed by UVvisibleNIR absorption spectroscopy using the diffuse reflectance (DR) technique. Figure 1 shows some of the spectra in the UVvisible region recorded as a function of time after the mixing. In agreement with the color change, the UVvisible part of the spectra exhibits a pattern that evolves with time. The spectra recorded just after the contact show prominent bands at 250, 342, 358, and 378 nm and a shoulder at ca. 326 nm. These bands are straightforwardly assigned to ACENE-3 occluded as an intact molecule in the pores of H2.2-GaZSM-5 in agreement with the spectrum observed for ACENE-3 sorbed on

ð2Þ

-GaZSM-5

In addition to these UV features, a new prominent band clearly develops at 710 nm with several minor bands. Note that other weak contributions including 313 and 353 nm bands are clearly detected after data processing using the multivariate curve resolution (MCR) method.20 All these spectral features correspond undoubtedly to the vibronic structure of the ACENE-3•þ radical cation. This assignment is based on previous studies relating to ACENE-3 spontaneous ionization and photoionization within MnZSM-520,21 (M = Liþ, Naþ, Kþ, etc.) as well as to investigations of ACENE-3•þ in Ar and Ne matrices,22,23 in silica materials,2426 and in solution.27,28 The intensity of these bands increased considerably with time following ACENE-3@H2:2 -GaZSM-5 f ACENE-3•þ @H2:2 -GaZSM-5• ð3Þ •þ

The bands assigned to ACENE-3 reached a maximum after about two months. Note besides that a very weak band centered at 410 nm tentatively assigned to the protonated form HACENE-3þ is observed in the early stages of the sorption process.26,29 This band is not detected any more after 2 days. Consequently, no proton transfer occurs during the ACENE-3 sorption in H2.2-GaZSM-5. Six months after the mixing, the green color turned brownish-pink concomitantly with complete disappearance of all the ACENE-3•þ spectral features following ACENE-3•þ @H2:2 -GaZSM-5• f ACENE-3@H2:2 -GaZSM-5• •þ

ð4Þ •þ

Reaction 4 is related to hole transfer from the ACENE-3 to the zeolite framework.21 The resulting spectrum, (c) curve in Figure 1, 6637



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displays bands at 326 (weak), 342, 358, and 378 nm analogous to those assigned to ACENE-3 occluded as an intact molecule plus a broad band between 460 and 600 nm. This broad band is of charge transfer type and is tentatively assigned to hole back transfer.21 Beyond six months after the mixing the charge transfer band decreases very slowly at room temperature according to charge recombination. ACENE-3@H2:2 -GaZSM-5• •þ f ACENE-3@H2:2 -GaZSM-5 ð5Þ The NIR part of the spectra exhibits a pattern that evolves with time and can be explained according to the reaction sequences 25. The spectrum of bare H2.2-GaZSM-5 dehydrated at 723 K under argon exhibits two prominent bands in the near-infrared domain at 1369 nm (7305 cm1) and 1410 nm (7092 cm1) (Figure 2, (a) spectrum). The 1369 nm (7305 cm1) peak is assigned to the first overtone of the ν(OH) mode of the silanol group with 3747 cm1 fundamental mode.3032 The band observed at 1410 nm (7092 cm1) is assigned to the first overtone of the ν(OH) mode of SiOHGa groups with 3615 cm1 fundamental mode.33,34 The fundamental frequencies are obtained by IR absorption spectrometry (see ref 5 Supporting Information). The wavelength and the intensity of the 1369 nm line corresponding to silanol groups remain constant with time, while a new broad band with 1390 nm maximum develops, superimposed to the 1410 nm band. This broad feature clearly increased in parallel to the ACENE-3•þ feature observed at 710 nm and is tentatively assigned to the spectral signature of the electron ejected during spontaneous ionization. This assignment is based on the analogous behavior observed after alkaline metal addition to zeolites.35 After 6 months, the intensities of the 1400 nm band had increased significantly with respect to the one at 1369 nm (Figure 2, e spectrum). The band with maximum at 1400 nm has increased in parallel to the disappearance of the ACENE-3•þ features and in parallel to the appearance of the charge transfer band at 500 nm. This additional intensity increase corresponds to additional trapped electron concentration. We are unable to assign with confidence a new broad NIR feature between 1000 and 1200 nm after complete disappearance of ACENE-3•þ features. Raman Spectrometry. The sorption and ionization processes of ACENE-3 into H2.2-GaZSM-5 were monitored as a function of time by FT-Raman spectrometry in a sealed cell under argon (Figure 5). Several days after the contact, the Raman features of ACENE-3•þ are observed at 1504, 1390, and 1260 cm1 in addition to the Raman bands of ACENE-3 (1557, 1480, 1401, 1260, 1186 cm1). During the course of the sorption, the Raman bands of ACENE-3 decrease gradually but remain apparent as weak bands in the spectrum recorded after 1.5 month. After 6 months, the Raman spectra had changed significantly. The Raman bands of ACENE-3•þ had disappeared, and new broad bands are observed at 1550, 1370, and 1260 cm1. To explain this unusual spectrum concerning ACENE-3 related species, we have to assume some resonance Raman enhancement of the vibrational modes at 1550, 1370, and 1260 cm1 of ACENE-3 surrounding the GaO4H•þ hole through the excitation of Raman scattering (λex = 1064 nm) within the contour of the hole back transfer band in the NIR region. These broad resonance Raman bands overlap the off-resonance Raman bands of remaining ACENE-3 occluded in the channels. EPR Spectroscopy. The above reaction set (eqs 35) induces electron transfers and paramagnetic species. EPR spectroscopy is a versatile method for studying the species with unpaired electrons.

Figure 3. Two pulse field sweep echo detected (FS-ED) spectra recorded at 4.2 K as a function of time after the mixing of solid ACENE-3 and dehydrated H2.2-GaZSM-5 (one ACENE-3 per UC loading). (a) 1 week after the mixing and (b) 6 months after the mixing.

First note that continuous wave X-band CW-EPR spectroscopic investigations on bare dehydrated H2.2-GaZSM-5 do not indicate any signal. However, several minutes after exposure of solid ACENE-3 to H2.2GaZSM-5 a weak quasi isotropic signal is detected at g = 2.007 with 10 G bandwidth. This spectrum is not shown because it is similar to the field sweep FS-ED spectrum exhibited below (Figure 3a). The appearance of an EPR signal is in agreement with the ionization reaction 2. However, comparison of the poorly resolved hyperfine structure of CW spectra with the 1H hyperfine splitting of a “pure” ACENE-3•þ in solution or in polycrystalline environments reveals only vague similarities.36,37 We believe that the isotropic signal with poor resolved hyperfine splitting is probably due to a pairing effect between ACENE-3•þ and the ejected electron within the zeolite framework. Unfortunately, it was not possible to undertake extensive investigation of the pairing by applying EPR as a function of temperature taking into account the weak ionization yield and the progress of the secondary reaction. We are unable to provide any evidence of the ferro (S = 1) or antiferromagnetic (S = 0) coupling between the two electronic spin (1/2) species in the radical cation electron moiety. In addition, it was not possible to characterize ACENE-3•þ and the trapped electron separately because no usable free-induction decay (FID) is observed in combination with echo generation. Note that previously the wellresolved hyperfine structure of ACENE-3•þ was clearly identified in aluminum-rich zeolite H3.4-AlZSM-5 using FID inversion recovery to suppress the echo process.21 The subsequent CW spectra recorded during the course of the sorption reveal that the intensity increases progressively in parallel with gradual broadening of the bandwidth and gradual disappearance of the weak features. Six months after the mixing, the CW-EPR spectrum with 15 G bandwidth shows an anisotropic distribution of g factor and reaches an intensity maximum. This signal is similar to the field sweep FS-ED spectrum exhibited below (Figure 3b) and is related to the [email protected]••þ moiety. Note that, as before, no extensive investigation of the pairing could be undertaken by applying EPR as a function of temperature, and consequently, no evidence of the ferro (S = 1) or antiferromagnetic (S = 0) coupling between the two electronic spin (1/2) species could be determined for the electronhole entity. The weak exchange coupling was assumed to be in the order of 0.1 cm1 with respect to previous work related to spontaneous ionization of biphenyl in H3.4-AlZSM-5.14 The spin quantity 6638



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Figure 5. FT-Raman spectra (λex = 1064 nm) recorded as a function of time after the mixing of solid ACENE-3 and dehydrated H2.2-GaZSM-5: (a) solid ACENE-3; (b) 3 days after mixing; (c) 1.5 months after mixing; (d) 6 months after mixing.

Figure 4. 2D-HYSCORE patterns recorded as a function of time after the mixing of solid ACENE-3 and dehydrated H2.2GaZSM-5 (one ACENE-3 per UC loading). The HYSCORE spectra were recorded at 4.2 K, with τ = 128 ns and pulse lengths of 16 and 32 ns for the π/2 and π pulses, respectively. (a) ACENE-3•þ@H2.2-GaZSM-5• recorded 5 days after the mixing. (b) Intermediary state between radical cation and electronhole pair recorded 2 months after the mixing. (c) [email protected]••þ electronhole pair recorded 6 months after the mixing.

obtained by the double integration of the whole signal in the g = 2 region increases progressively to reach 0.7 ( 0.1 electrons

per ACENE-3 6 months after the mixing. The ionization through reactions 3 and 4 corresponds at most to 30% yield. After 6 months the spin quantity decreases very slowly at room temperature according to the charge recombination reaction 5. The two pulse field-sweep echo detected experiments (FSED) recorded at 4.2 K one week and six months after the mixing are presented in Figure 3. The X-band FS-ED spectra show similar changes as the CW EPR during the course of the reactions. The spectrum (curve a) obtained 1 week after the mixing shows an isotropic broad signal of 9 G and is related to the ACENE3•þ@H2.2-GaZSM-5• pair. FS-ED spectra depend on nuclear modulations and on the relaxation of the paramagnetic species, both of which affect the echo intensity. After 6 months, the signal (curve b) not only became broader (15 G) but also revealed anisotropy of the g tensor by displaying three apparent principal values of the g tensor (g1 = 2.015, g2 = 2.007, and g3 = 2.001). This spectrum is typical for a polycrystalline sample with g tensor with orthorhombic symmetry and is related to the [email protected]••þ moiety. This line shape results from the contribution of a large number of individual spin packets with different resonance positions. FS-ED EPR spectra were also recorded to determine the field position at which HYSCORE experiments would be performed. The hyperfine sublevel correlation (HYSCORE) technique is the 2D analog of electron spin echo envelope modulation (ESEEM) spectroscopy which is the method of choice to detect weak hyperfine interactions at low fields (usually X-band) when many nuclei contribute to the ESEEM spectrum. Several HYSCORE experiments with different τ values (τ = 88, 128, 200, and 256 ns) were recorded at the field position corresponding to g values shown in Figure 3 with nonselective π/2 pulse duration of 12 ns covering all the g region. Three selected HYSCORE patterns (þ, þ quadrant) recorded 5 days, 2 months, and 6 months after the mixing of solids corresponding to three reaction steps are displayed in Figure 4, respectively. Five days after the mixing, the HYSCORE spectrum in the (þ,þ) quadrant shows exclusively 1H nuclear frequency symmetric ridges with coordinates at (3, 27; 27, 3 MHz) centered at 14.5 MHz corresponding to the proton Larmor frequency with a width of 34.7 MHz perpendicular to the diagonal (Figure 4A). 6639


The Journal of Physical Chemistry C Different shapes of ridges were observed according to the different τ values used to record the spectra. They are attributed to the τ suppression effect; the τ suppression effect is minimal with τ = 128 ns. This pattern is similar to the HYSCORE spectrum (τ = 128 ns) previously reported for ACENE-3•þ generated in concentrated H2SO4 and is also similar to the spectrum previously reported after the mixing of ACENE-3 and H3.4-AlZSM5: it is undoubtedly assigned to ACENE-3•þ.21 The observation of the ridge for the outer 1H signals in the map was reported to be due to high coupling value of the first proton of ACENE-3•þ. No 13 C signal was observed as previously reported for ACENE-3•þ in concentrated H2SO4.38 The reasons why 13C signals are hardly detected for linear polyacene radical cations are given in ref 38. Note that the HYSCORE pattern of Figure 4A does not provide any evidence of 29Si or 69Ga and 71Ga signals. We assumed that the low ionization yield in H2.2-GaZSM-5 explains the weakness of the signal/noise ratio because 27Al and 29Si signals are clearly detected in aluminum-rich H3.4-AlZSM-5 after ionization of ACENE-3 in high yield.21 In addition the 1H signal arising from the acidic zeolite proton is probably overlapped by the 1H pattern. The expected weak exchange and dipolar interactions between the two unpaired electrons have a very weak effect on the HYSCORE spectra typical of ACENE-3•þ. The structural situation of the ACENE-3•þ@H2.2-GaZSM-5• moiety cannot be extracted from the present HYSCORE investigation but is assumed to be analogous to the structure of ACENE-3•þ@Li3.5AlZSM-5• where ACENE-3•þ is facially coordinated to the Liþ cation in close proximity of the Al atom. In this case, the ejected electron appears delocalized in a restricted space around Liþ and ACENE-3•þ.39 The HYSCORE pattern (τ = 128 ns) recorded 2 months after the mixing also shows only a 1H pattern centered at 14.5 MHz (Figure 4B). However, this pattern exhibits two contributions. The first one shows weak 1H nuclear frequency symmetric ridge with coordinates at (3, 27; 27, 3 MHz). This contribution corresponds to the remaining amount of ACENE-3•þ. The other component gives a prominent 1H nuclear frequency symmetric ridge with coordinates at (9, 22; 22, 9 MHz). The HYSCORE spectra recorded six months after the mixing support this interpretation. The ridge with coordinates at (3, 27; 27, 3 MHz) has totally disappeared (Figure 4C), and a prominent ridge remains for all the τ values with coordinates at (9, 22; 22, 9 MHz) from which a maximum 1H hyperfine coupling, A, of ∼18 MHz was estimated. The weaker 1H couplings in the [email protected]••þ moiety than in ACENE-3•þ@H2.2GaZSM-5• arise from positive hole transfer from the radical cation to the nearest GaO4H site within the 10-MR channel of zeolite. The 2D-HYSCORE experiments show supplementary peaks centered at (νR þ νβ)/2 ∼ νL along the diagonal of the Larmor nuclear frequencies νL corresponding to 69Ga (3.58 MHz), 71Ga (4.5 MHz), and 29Si (2.9 MHz) nuclei. The 69Ga, 71Ga, and 29Si peaks show no τ dependence at τ = 88 and 128 ns. A coupling of νR ∼ νβ = 1.8 MHz with 71Ga and a coupling of 6 MHz with 69Ga were estimated from coordinates at (5; 5) and (1.3, 5.8; 5.8, 1.3), respectively. The 29Si signal (2.9; 2.9) along the diagonal (νR ∼ νβ) is probably a result of weak coupling of the 29Si nucleus. The structural situation of the [email protected]••þ moiety can be proposed from the present HYSCORE experiment. ACENE-3 is located close to the GaO4H•þ hole while the ejected electron is trapped as GaO4H• at a distance depending on the

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Ga distribution in the framework. The GaO4H•þ 3 3 3 GaO4H• distance is expected to be more than 0.4 nm and explains the very weak magnetic exchange.

’ DISCUSSION In this section, we report a detailed interpretation of the entire data set (see above) of the present work focused mainly on the hole formation and the hole transfer in H-GaZSM-5 upon ACENE-3 sorption based on a general mechanism previously reported for polyacene incorporation within acidic aluminosilicate MFI.14 We suggest a mutually consistent interpretation of the isomorphous substitution effect of Si(IV) by Ga(III) and Al(III) heteroatoms in MFI framework about the hole formation, trapping, and transfer processes upon ACENE-n incorporation. ACENE-3 Sorption and ACENE-3•þ Hole Formation. The porous structure of MFI including H-GaZSM-5 contains two types of intersecting 10-MR channels with circular opening (0.53 0.56 nm) and elliptical opening (0.51 0.55 nm). These openings are sufficiently wide to allow ACENE-3 without any bulky substituent to pass through and diffuse into the void space. Knowledge of GaOHSi bridges (BAS) of the inner channel surface pointing at the interior is therefore of great importance to understand the sorption and ionization phenomena. A reasonable structural model comes from previous Monte Carlo simulations that indicate the preferred sorption site of ACENE-3 in acidic H-AlZSM-5 is the straight channel near the channel intersection with the central aromatic ring facially coordinated to the proton of BAS.20 From all three techniques, UVvisNIR absorption, EPR spectroscopy, and Raman scattering spectrometry, the complete ACENE-3 incorporation into the 10-MR channel of Brønsted acidic gallium H-GaZSM-5 zeolite occurs following two parallel processes: sorption as an intact ACENE-3 molecule and spontaneous ionization. We explain the spontaneous charge separation using a mechanism previously reported for polyaromatic hydrocarbons and briefly summarized below.40 No proton transfer was detected. Analogous behavior with respect to protonation was reported previously during the course of ACENE-3 sorption in Hn-AlZSM-5.20 It is expected that the acid strength of accessible BAS within the 10-MR channel of H-GaZSM-5 and H-AlZSM-5 is not strong enough to transfer a proton to ACENE-3 in high yield.12,41 Using the gas phase proton affinity (PA) as a metric,42 the PA of anthracene (877 kJ/mol)43 is high enough that some spontaneous proton transfer should occur but is not high enough to expect complete proton transfer as is the case for a strong base such as pyridine (930 kJ/mol).43 In contrast, higher amounts of HACENE-3þ were reported during the course of ACENE-3 sorption in acidic mordenite and Y faujasite with many acid sites and large amounts of extra-framework species.26 The spontaneous ionization is a property of the inner surface of the channel open to sorbate, and the ionization efficiency depends both on the intrinsic ionization energy of the sorbate represented here by the gas-phase ionization potential (Ig= 7.44 eV; ACENE-3) and on the ionizing ability of the host represented here by the polarization energy at the sorption site and by the conduction band energy of the zeolite to capture the ejected electron.14 The polarization energy at the sorption site is closely linked to the local electrostatic field strength. The electrostatic field strength in aluminum-rich zeolites has been suggested to be extremely high and is due to the fact that the counterbalancing cations exposed in the channel are only partially shielded.44 The mean electrostatic field increases with the 6640


The Journal of Physical Chemistry C Ga(III) or Al(III) content and the counterbalancing cation number, while the small size of proton induces a stronger field in its close proximity than the larger Liþ, Naþ ..., or Csþ ion despite the fact that the OH bond is partially covalent. The high polarization energy in close proximity of accessible BAS under the constraint of 10-MR channel topology induces the spontaneous ACENE-3 ionization in H2.2-GaZSM-5 and in HnAlZSM-5. The ACENE-3 ionization yield in H2.2-GaZSM-5 is relatively weak (∼30%), while it is higher in H3.4-AlZSM-5 (∼90%). The BAS density within the 10-MR channel is expected to increase from H2.2-GaZSM-5 to H3.4-AlZSM-5. In addition, the calculated atomic charges from natural bond orbital population analysis indicate that the H and O charges of BAS increase from H-GaZSM-5 (qH = þ0.585e; qO = 1.12e) to H-AlZSM-5 (qH = þ0.590e; qO = 1.14e).45 These two parameters are in agreement with polarization and acidity strength increases from H2.2-GaZSM-5 to H3.4-AlZSM-5 leading to a boost in the ionization yield under the confinement of the 10-MR channel of H3.4-AlZSM-5. Note that spontaneous ACENE-3 ionization is also effective in Li3.4-AlZSM-5 with ∼20% ionization yield, but the local polarizing effect of SiO(Mþ)Al with a larger cation (M = Naþ, Kþ, Rbþ, Csþ) was not strong enough to generate ACENE-3 ionization in the 10-MR channel.39 The compartmentalization of the trapped electron away from the initial site of ionization decreases dramatically the propensity for charge recombination. The charged centers give rise to deep potential wells that capture mobile ions and electrons and slow down their motion for some time.46 The evidence of ejected electrons in the ACENE-3•þ@H2.2GaZSM-5• charge separated state comes from the NIR broad absorption band centered at 1420 nm. The structural situation of the radical pair within the zeolite framework can be described through ACENE-3•þ occluded within the 10-MR straight channel in front of the SiO(Hþ)Ga moiety and the trapped electron delocalized in a restricted space around another SiO(Hþ)Ga group at a distance expected to be more than 0.4 nm. Alternatively, it should be assumed that the stabilization of ACENE-3•þ is achieved by electron consumption by unknown active sites of single electron oxidation. The extra-framework species, Lewis acid sites, and residual molecular oxygen were invoked previously to consume ejected electrons. No experimental feature indicates possible single electron oxidation sites to generate ACENE-3•þ. These explanations may be ruled out for the present ACENE-3 spontaneous ionization in H2.2-GaZSM-5 as well as in H3.4-AlZSM-5 and Li3.4-AlZSM-5 taking into account the experimental results and the experimental conditions.47 Hole Transfer and Trapping. UVvisNIR absorption, EPR, and Raman spectroscopy provide convergent arguments to characterize an additional reaction after charge separation as hole transfer according to reaction 4 to create the [email protected]••þ moiety. The compartmentalization of ACENE-3•þ and electron within ACENE-3•þ@H2.2-GaZSM5• can promote hole transfer inside the 10-MR channel. Surprisingly, no such hole transfer to the silicoaluminate framework was observed at room temperature following the ACENE-3 spontaneous ionization in H3.4-AlZSM-5 or Li3.4-AlZSM-5. The poor oxidizing strength of ACENE-3•þ represented by E0 = 1.09 V vs a standard calomel electrode as reference was invoked to explain the absence of hole transfer because other occluded radical cations such as biphenyl (E0 = 1.96 V) and ACENE-2 (E0 = 1.54 V) with higher oxidizing strength induce subsequent hole transfer after spontaneous ionization in H-AlZSM-5.14,48 The

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formation of a hole on an AlO4•þ center of the H-AlZSM-5 has been studied previously by hybrid quantum mechanics. Generation of a hole produces a substantial geometry relaxation of the AlO distance to the oxygen atom with the unpaired electron, and the zeolite framework stabilizes the positive charge by longrange effects.49 According to nonadiabatic electron-transfer theory, the driving force dependence on the hole transfer rate is given by the following equation50 hole transfer rate ¼ ½4π3 =ðh2 λkB TÞ1=2 jHj2 exp½ ðΔG0 þ λÞ2 =ð4λkB TÞ

ð6Þ

where H is the electronic coupling between the donor and acceptor states; ΔG0 is the reaction free energy; and λ is the reorganization energy. The reorganization energy (λ) was reported to be small in rigid frameworks and is largely controlled by the tight fit between sorbate size and pore dimensions. ACENE-3 sorption processes within H-AlZSM-5 and H-GaZSM-5 involve analogous confinement within the 10-MR channel and consequently analogous λ values. Therefore, the thermodynamics of the hole transfer, i.e., ΔG0, remains the main parameter controlling the hole transfer at room temperature. ΔG0 corresponds to the difference in free energy between the ACENE-3 oxidation midpoint potential (E0 = 1.09 V vs SCE as reference) and the energy of highest occupied valence orbital of the surrounding zeolite cluster. This level is expected to be dependent on Al or Ga content. Unfortunately, there is no direct information about this energy level. We assume that ΔG0(GaZSM-5) > ΔG0(AlZSM-5) to explain the hole transfer at room temperature within H-GaZSM-5 and not within H-AlZSM-5. Note that reversible hole transfer was observed at higher temperature (473 K) within H-AlZSM-5 without overall charge recombination. In summary, the hole transfer from ACENE-3•þ generates new paramagnetic species as the [email protected]••þ electronhole pair with the following spectral characteristics. The UV absorption bands are analogous to those of ACENE-3 occluded as an intact molecule with a supplementary broad band in the visible region assigned to back hole transfer. The Raman spectrum is similar to occluded ACENE-3 with some broadening of bands characteristic of the effect of chemical surrounding on occluded ACENE-3. The 2D-HYSCORE pattern demonstrates clearly the participation of the 1H nucleus to the electron spin distribution in ACENE-3 3 3 3 GaO4H•þ moiety and the participation of 69Ga and 71Ga to the electron spin distribution in GaO4H• and GaO4H•þ groups. These findings are not able to separate the surroundings of trapped hole from trapped electron. All the unpaired electrons are probably localized on the nearest oxygen atoms of the GaO3O(H)SiO3 bridges. The ACENE-3 3 3 3 GaO4H•þ moieties are probably localized on the inner surface of the 10-MR channel, while the ejected electrons are probably trapped away of the positive hole as GaO4H• entities. After six months, the spin quantity decreases very slowly in agreement with a very slow electron hole recombination rate. The compartmentalization of the positive hole away from the electron trapping site hinders efficiently the charge recombination. In the present case, the extremely long lifetime of these intrazeolite species prevents the observation of ACENE-3 as an isolated molecule after total recombination. The long distance between the hole and the trapped electron is assumed to dramatically slow down the final charge recombination. However, it 6641


The Journal of Physical Chemistry C should be noted that a similar study carried out with ACENE-4 occluded in H-AlZSM-5 exhibited an identical advancement process with high charge separation yield. In that case, a very slow recombination reaction was observed concomitantly with the appearance of ACENE-4@H-AlZSM-5 occluded as an intact molecule without any protonation.40 In addition, analogous behavior was also observed recently after sorption of para-terphenyl in the channels of medium pore Brønsted acidic zeolites.51 The examination of these experimental results and careful comparison with analogous spectroscopic results devoted to para-terphenyl radical cations and related oligomers in solution provide no evidence of oligomerization reaction in the channels of acidic medium pore zeolites. Indeed, the para-terphenyl radical cations are reported to be active species of oligomerization and polymerization in acidic solutions contrary to zeolite void space where the reaction does not occur due to the restricted mobility that hinders the oligomerization.5254 Note also that the encapsulation of molecules inside the rigid pores of zeolites can be achieved to isolate functional groups to block reactions between molecules.5557

’ CONCLUSIONS The unique redox behavior of ACENE-3 within the 10-MR channel of Brønsted acidic H-GaZSM-5 zeolite has been reported. During the course of the ACENE-3 incorporation, no proton transfer occurs, but spontaneous ionization with medium yield is effective. The long-lived ACENE-3•þ (hole formation) and ejected electron are induced by the intense proton electrostatic field of the inner zeolite surface. The compartmentalization of ACENE-3•þ and the trapped electron within H2.2-GaZSM-5 hinders charge recombination but promotes hole transfer to the inner surface of the 10-MR channel. The thermodynamics of the hole transfer, i.e., ΔG0, is the main parameter in the nonadiabatic electron-transfer theory that explains the hole transfer at room temperature within H-GaZSM-5 and not within H-AlZSM-5. The 1H nucleus of ACENE-3 participates in the electron spin distribution of the hole, while the hole and electron are probably trapped on the oxygen atoms of distant GaO3O(H)SiO3 bridges taking into account the observed couplings with 29Si, 69 Ga, and 71Ga nuclei. One may conclude that hole formation, hole transfer, and hole trapping involving aromatic hydrocarbons are quite sensitive to rather subtle effects under confinement of Brønsted acid zeolites. It may be possible that complex electronic processes between acidic zeolites and aromatic hydrocarbons are implicated in several important catalytic chemical processes previously attributed exclusively to classical acid catalysis. ’ ACKNOWLEDGMENT R.F.L acknowledges funding by the U.S. Department of Energy Basic Energy Sciences under Grant No. DE-FG0207ER15921 and DE-FG02-99ER14998. ’ REFERENCES (1) Busca, G. Chem. Rev. 2007, 107, 5366–5410. (2) Mirafzal, G. A.; Lozeva, A. M.; Olson, J. A. Tetrahedron Lett. 1998, 39, 9323–9326. Lahr, D. G.; Li, J. H.; Davis, R. J. J. Am. Chem. Soc. 2007, 129, 3420–3425. (3) Garcia, H.; Roth, H. D. Chem. Rev. 2002, 102, 3947–4007. (4) Li, L.; Zhou, X.-S.; Li, G.-D.; Pan, X.-L.; Chen, J.-S. Angew. Chem., Int. Ed. 2009, 48, 6678–6682.

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Influence of Gallium Isomorphous Substitution in the Acidic MFI Zeolite

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