Electron Transfers Induced by t-Stilbene Sorption in Acidic Aluminum

Jun 11, 2012 - This is in agreement with the trend observed in the ferrierite, ZSM-5, and mordenite zeolites: .... The pores are ∼7.3 Å in effectiv...
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Electron Transfers Induced by t-Stilbene Sorption in Acidic Aluminum, Gallium, and Boron Beta (BEA) Zeolites Raul F. Lobo,*,† Alain Moissette,*,‡ Matthieu Hureau,‡ Sonia Carré,‡ Hervé Vezin,‡ and Alexandre Legrand‡ †

Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡ Laboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Bât. C5 Université de Lille 1, 59655 Villeneuve d’Ascq cedex, France ABSTRACT: The adsorption of trans-stilbene (t-St) on the acidic aluminum-, gallium-, and boron-containing zeolite beta (H-AlBEA, H-GaBEA, and H-BBEA, respectively) is investigated using UV/vis and timeresolved UV/vis spectroscopy, electron paramagnetic resonance spectroscopy, and Raman spectroscopy. On H-AlBEA, the results show a fast and spontaneous one-electron oxidation of t-St, quickly followed by the recapture of an electron from the zeolite framework by the t-St•+ radical cation and the formation of a long-lived charge-transfer complex (t-St HAlBEA•−•+). This charge-transfer complex (I) evolves over a period of months into a spectroscopically distinct charge-transfer complex (II). Evidence for the (undetected) intermediate t-St•+ radical cation is obtained using time-resolved UV/vis spectroscopy. Similar electron-transfer processes have been observed in the acidic ferrierite, ZSM-5, and mordenite aluminosilicate zeolites. The key difference is that the rate of electron recapture by the t-St•+ radical cation is much faster than that in all of the other zeolites. This is in agreement with the trend observed in the ferrierite, ZSM-5, and mordenite zeolites: looser fit leads to a lower stability of the radical cation intermediate and leads to a rapid hole transfer to the zeolite framework to form a charge-transfer complex. In the case of boron-containing beta, the rate of formation of the radical cation is slow, and the yield is small. Gallium-containing zeolite beta shows intermediate behavior when compared with the Al and B forms of zeolite beta.

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INTRODUCTION The spontaneous ionization of molecules that fulfill the Lewis octet rule is contrarian to chemical intuition because, on the basis of elementary electronic structure principles, the most stable species is the diamagnetic molecule with eight electrons in the valence shell. This is strictly correct for isolated molecules; however, the interaction of a solvent with a reactant can drastically affect its reactivity. In particular, in electrophilic solvation,1 where there are interactions of reagents of electrondonating ability with electron acceptor solvents (such as protic acids), the spontaneous ionization of an otherwise stable reagent is not uncommon. An early example is the observation of radical cations of polycyclic arenes in solutions of SbF5/ SO2ClF by Forsyth and Olah.2 More recently, the one-electron oxidation of Janusene3 and of C-60 and C-70 fullerenes4 in superacid media has been described, and the formation of radical cations of polycyclic acenes in oleum has also been reported.5 Zeolites are crystalline tectosilicate framework oxides that contain voids and pores of molecular dimensions through which molecules can diffuse readily in and out of the crystals. The general formula6 to describe the composition of a zeolite is given by |M+n|[AlnSimO2(m+n)]n‑, where the negative charge © 2012 American Chemical Society

generated by the presence of Al (or another trivalent cation such as Ga or B) in the framework is balanced by extraframework cations M+. If the cation is a proton (H+), then the zeolite becomes a solid acid and can donate the proton to adsorbed species. It is often useful to think of zeolites as solid solvents7 because adsorbed species interact predominantly with framework and extra-framework atoms in ways that are akin to solvent−molecule interactions. The sum of the electrostatic and van der Waals interactions that stabilize the adsorbed species in the pores is also known as the confinement effect.8,9 In the case of acid zeolites such as H-ZSM-5 (H-AlZSM-5), the interaction of an adsorbed species is not qualitatively very different from the interaction of the same species in a protic solvent,1 and it is thus not surprising that the spontaneous ionization of adsorbed species in acid zeolites has been observed for polycyclic acenes and a number of other molecules with low ionization potentials. The spontaneous ionization of trans-stilbene,10−13 anthracene,14−16 phenothiazine,17−19 and para-terphenyl20,21 after adsorption on acid zeolites has been reported by us. These Received: April 26, 2012 Revised: June 11, 2012 Published: June 11, 2012 14480

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charge-transfer complex is highly stabilized by the zeolite (H-AlZSM-5), this stability can in fact reduce the rate of electron transfer to a point where this reaction is not observed for months (in H-AlZSM-5) but is observed in H-GaZSM-5. In this Article, we investigate the electron-transfer processes that occur upon adsorption of trans-stilbene (t-St) into zeolite beta with Al, Ga, and B in framework positions (H-AlBEA, HGaBEA, and H-BBEA, respectively). Zeolite beta is a high-silica zeolite containing 12-ring × 12-ring × 12-ring pores in its 3D pore network. The pores are ∼7.3 Å in effective diameter, and the pore intersections have an effective diameter in excess of 10 Å. Zeolite beta is a disordered structure formed of layers that stack with 1/3 translations of the unit cell dimension. This stacking can be + or − (that is, to the right or the left), leading to lack of long-range order. Locally, however, the pore geometry is only slightly affected by the presence of these stacking faults and for the purpose of this investigation is safe to assume that the adsorbed species encounters a uniform distribution of pore geometries. In contrast with the other zeolites investigated in the past with t-St, the fit of the molecule to the pore space is much looser, allowing for more freedom of motion of the molecule. Aluminum and gallium form structurally related Brønsted acid sites (Al−OH−Si and Ga−OH−Si) with the gallium sites being slightly less acidic than the aluminum sites.25−28 Boron, in contrast, by virtue of its small size, has a trigonal (rather than distorted tetrahedral) geometry in the zeolite framework, as depicted by Scheme 1.29−31

studies have included aluminum and gallium forms of zeolite ZSM-5,12,22 zeolites with different aluminum concentrations and with different pore structures. In addition to the 10-ring × 10-ring pore structure of zeolite ZSM-5, these processes have been investigated in zeolite ferrierite (AlFER) with a 10-ring × 8-ring 2D pore structure and on zeolite mordenite (AlMOR) containing 1D 12-ring pores with 8-ring pockets on the pore sides.10 The molecules investigated have a rod-like shape and a diameter of ∼5 Å fitting comfortably in the pores of these zeolites but without much freedom for rotation or librations of large amplitude. In all of these cases, ionization of the adsorbed species is observed (as has been reported for electrophilic solvation), but after the formation of the radical cation, additional electron-transfer processes and structural evolution of the charge-transfer complex is observed, as summarized by the equations below: H‐Zeo + R → R@H‐Zeo

(1)

R@H‐Zeo → R•+@H‐Zeo•−

(2)

R•+@H‐Zeo•− → R@H‐Zeo•−•+(I)

(3)

R@H‐Zeo•−•+(I) → R@H‐Zeo•−•+(II)

(4)

The first reaction depicts the adsorption of the organic species into the zeolite pores to form the R@H-Zeo moiety. The spontaneous ionization of this moiety is illustrated by reaction 2, a step that is analogous to the ionization observed in electrophilic solvents. In the zeolite, however, further reactions are observed such as the transfer of a hole h+ to the zeolite framework (reaction 3) to form a charge-transfer complex (I). This charge transfer complex is stabilized by the close physical proximity of the adsorbed species R to the positively charged hole on the zeolite structure. Over time, the spectroscopic evidence indicates that this complex (I) can change further into a spectroscopically distinct species (II) that otherwise appears to be similar to the initial charge transfer complex. The yield of radical cation formation can be high (in excess of 90%), and the species formed by these electron-transfer reactions can in fact be very long-lived (many months).16,21,23,24 The extent and rate of reactions 1−4 is highly dependent on the zeolite structure and its composition. For instance, in H-AlFER, the radical cation is highly stabilized by the tight fit between the organic species and the pore geometry. In the more spacious H-AlMOR, reaction 3 is fast enough that the radical cation R•+ is barely detected after the admolecule is occluded into the zeolite pores. From the early stages, the main species that is observed is the charge-transfer complex R@HZeo•−•+. This difference can be understood as a result of the poor stabilization of the radical cation by this larger pore zeolite.10 The composition of the zeolite also has an impact on the extent and rate of these reactions. We have reported, for example, that two samples of ZSM-5 (H-AlZSM-5 and H-GaZSM-5) with approximately the same heteroatom concentration display substantial differences in their ionization yield and the stability of the charge-transfer complex. For anthracene adsorbed on H-AlZSM-5, a rapid formation of the radical cation (reaction 2) is observed, but no formation of the charge-transfer complex is detected. On H-GaZSM-5,22 the formation of the radical cation is slower, but this species further reacts to form, in high-yield, the charge-transfer complex (reaction 3). This difference can be understood on the basis of nonadiabatic electron-transfer theory that shows that when the

Scheme 1

As a result of this trigonal geometry, the HO····B bond is very weak, and the OH unit is better described as a perturbed silanol group with a much reduced ability to protonate adsorbed species than the Al and Ga counterparts. We report the investigation of the adsorption of t-St on HAlBEA, H-GaBEA, and H-BBEA using UV/vis spectroscopy, electron paramagnetic resonance spectroscopy, and Raman spectroscopy. As was the case of H-AlFER, H-AlZSM-5, and HAlMOR, we find evidence of reactions 1−4 occurring in HAlBEA zeolites. The main difference is that the rate of hole transfer (reaction 3) is faster than that in all of the other three zeolites. No direct evidence of the radical cation t-St•+ is detected. This is in agreement with the trend observed in the FER, ZSM-5 and mordenite zeolites, that is, that a looser fit leads to a lower stability of the radical cation intermediate and leads to a rapid hole transfer to the zeolite framework to form a long-lived charge-transfer complex. In the case of H-BBEA, the rate of formation of the radical cation is slow, and the yield is small. H-GaBEA shows intermediate behavior when compared with the Al and B forms of zeolite beta.

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EXPERIMENTAL SECTION Materials. The zeolite synthesis compositions were selected such that the ratio of silicon to trivalent metal atom in the framework (Al, Ga, or B) was ∼15. The use of reagents known to contain iron impurities was avoided as well. The reagents used to prepare Al-zeolite beta (AlBEA) were tetraethyl orthosilicate (TEOS; Sigma-Aldrich 98% w/w), 35% w/w tetraethylammonium hydroxide (TEAOH) in water (Alfa 14481

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under argon in quartz cells through diffuse reflectance; the corresponding bare zeolite was used as the reference. The DRUVv spectra were plotted as the Kubelka−Munk function. Time-Resolved Diffuse Reflectance UV−Visible (TRDRUVv) Absorption Spectroscopy. The experimental setup of nanosecond diffuse reflectance spectroscopy, applicable to the detection of transient species (TDR) in light-scattering system, was described elsewhere.33,34 Excitation pulses at 300 and 495 nm (7 to 8 ns, 10 mJ) are provided by a 20 Hz Panther EX OPO tunable laser (Continuum, GSI group). The probe light is provided by a Xe lamp (XBO 150W/CR OFR, OSRAM). A UV filter was used to avoid photochemical reactions by the analyzing light. The reflected light is dispersed by a monochromator (Horiba Jobin-Yvon, iHR320) and analyzed with a photomultiplier (R1477-06, Hamamatsu) coupled to a digital oscilloscope (LeCroy 454, 500 MHz). For such powder samples, the transient absorption intensity is displayed as percentage absorption (% absorption), given by % absorption = 100 × (1 − R/R0), where R(λ,t) and R0(λ,t) represent the intensity of the diffuse reflected white-light probe with and without excitation, respectively. Kinetic traces were detected from 350 to 650 nm every 5 nm and from which the transient spectra can be reconstructed. To maintain sample integrity during the experiment, the sample was moved and/or shaken throughout the experience to ensure that a fresh region of the sample was available to each laser pulse. The concentration decay C(t) was accurately fitted using the Albery function to take into account the nonhomogeneity of the material.35 EPR. The CW and pulsed EPR spectroscopy experiments were carried out with a Bruker X-band ELEXYS E580E spectrometer. All experiments were performed at room temperature and at 4.2 K. Standard echo field sweep (EFS) experiments were performed using 16 steps phase cycling with π/2-τ-π-echo sequence recorded as a function of magnetic field. Hyperfine sublevel correlation spectroscopy (2D-HYSCORE) experiments were carried out using the four pulse sequence π/ 2-τ-π/2-t1-π-t2-π/2-echo at several τ values (108, 148, 168, and 208 ns) to avoid blind spot effect. The pulse length of the π/2 and π pulse was set to 18 and 32 ns, respectively. Raman Spectrometry. 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 excitation source. A laser power of 10− 100 mW was used. The spectra (4000−150 cm−1) were recorded with a resolution of 2 cm−1 using 600 scans.

Aesar), and aluminum triethoxide (Strem Chemicals, 99% w/w). Synthesis solutions were prepared using molar compositions 1 TEOS/20 H2O/0.5 TEAOH/0.07 Al(OCH2CH3)3. The synthesis solutions were prepared in two steps. In the first step, all components were mixed, excluding Al(OCH2CH3)3, and the solution was stirred until TEOS was hydrolyzed (∼2 h). Afterward, Al(OCH2CH3)3 was added, and the solution was stirred for ∼24 h. Synthesis solutions were heated under static conditions at 393 K to synthesize zeolite beta. Solids were separated from solution using a RC 6 plus centrifuge with a HB-6 rotor (Sorvall) and 50 mL FEP centrifuge tubes (Nalgene) operating at 12 000 rpm (RCF average 23 530 g-force) for 2 h. After separation, the zeolite products were redispersed in DI water using a sonicator bath (Fisher) and a vortex mixer (Fisher), and the centrifugation procedure was repeated until the supernatant (∼40 mL) had a conductivity of MOR > BEA. The relatively high oxidizing power value of t-St•+ (1.75 V versus SCE in solution) with respect to the AlO4H electron donor site might explain the hole transfer observed between t-St•+ and a closely associated AlO4H group.50 The low coverage of t-St in the pores of H-AlBEA, the absence of any solvent molecules stabilizing the charged species (other than Ar), and the high mobility of t-St in the spacious channels of zeolite beta are all factors that could affect the rate of electron transfer. However, the formation of long-lived charge-transfer complex in high yield might show that ejected electrons are probably trapped on the oxygen atoms of a AlO4H•‑ group far enough from the initial site of t-St ionization to prevent direct recombination. This result was already described for other polyaromatics and is assumed to be due to charge compartmentalization. Because of the relatively large pore diameter of zeolite beta, other reactions of t-St (dimerization and polymerization) have to be considered. So far, no resemblance could be found between the present spectral results and the data reported in the literature for such species: the presence of EPR active species in high yield is also not consistent with this hypothesis. The following mechanism can be proposed to describe the experimental results obtained after mixing solid t-St and H-AlBEA. • Sorption takes place in the gas phase by sublimation: t‐St(gas) + H‐AIBEA → t‐St@H‐AIBEA • t-St•+ formation occurs in parallel to sorption:

(5)

t‐St@H‐AIBEA → t‐St•+@H‐AIBEA•− (6) • Hole transfer occurs rapidly and a charge-transfer complex (electron/hole pair) is created:

t ‐St•+@H‐Zeo•− → t‐St@H‐AIBEA•−•+(I) (7) • A fraction of charge-transfer complexes of type 1 (t-St@ H-AlBEA•−•+ (I)) evolve by reorganization to chargetransfer complexes of type 2 (t-St@H-AlBEA•−•+ (II)): t‐St@H‐AIBEA•−•+(I) → t‐St@H‐AIBEA•−•+(II) (8)

Effect of Al, Ga, and B Framework Substitution. The experimental results obtained after mixing t-St and H-GaBEA show evidence of t-St spontaneous ionization upon incorporation. The DRUVv spectra recorded for months after contact demonstrate the initial formation of t-St•+ and the occurrence of a fast hole transfer to form stable charge-transfer complexes. The spectral features of this moiety are nearly identical to those 14488

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previously reported after mixing t-St and H-AlBEA. The DRUVv spectra show the appearance of two new bands in the visible region centered at about 490 and 620 nm and assigned to charge-transfer complexes. The total disappearance of the 1639 cm−1 band of isolated t-St molecule on the Raman spectrum indicates high t-St conversion, and the observation of a relatively high CW-EPR signal is in agreement with significant t-St ionization. After mixing t-St with H-BBEA, similar behavior is observed, but the ionization yield is much lower. Therefore, the data clearly show the decrease in ionization from Al to Ga and to B, (B ≪ Ga < Al) and the ionization is very weak within H-BBEA. Therefore, the trends for the Al, Ga, and B do follow the relative acidity of the Brønsted acid sites formed by the different trivalent cations in the zeolite framework that decrease from Al to B. Moreover, note that time-resolved experiments demonstrate clearly the longer lifetime of t-St•+ photoinduced in H-GaBEA than in H-BBEA. The results are consistent with the stability that the framework provides to the radical cation. These observations do not explain the changes in the near-IR region of the UV−vis spectra of H-BBEA (Figure 1C) at long times (trace g). These changes indicate that all internal hydroxyl sites in H-BBEA are affected by the presence of the radical species in the sample, even though this sample contains fewer radical species than the other two. At this time, there is no clear explanation for these spectral changes. The boron coordination to the framework is certainly different from Al and Ga and can be put forward to explain the very low ionization observed here. In the proton form and in the dehydrated samples, B is trigonal, not tetrahedral, and there is a long distance between the B···O(H)−Si moiety, making the silanol behave almost like a surface Si−OH species. The coordination of framework boron is reversible, and upon ion exchange, B goes back to a tetrahedral coordination, similar to Al and Ga. It could be that the adsorption of t-St perturbs the B···O(H)-Si site to such an extent that the B becomes tetrahedral. We examined the diffuse-reflectance FTIR spectra of t-St@H-BBEA and of H-BBEA (not shown), and there was no detectable change to the 1400 cm−1 absorption band assigned to trigonal framework B.31 On the basis of this observation, a change in the B coordination upon adsorption of t-St is ruled out. In contrast, X-ray spectroscopy shows that Ga is essentially like Al but with somewhat longer T−O bonds; consequently, lower differences are expected with respect to B.

shows parallels to the behavior of molecules with electrondonating ability dissolved in electrophilic solvents.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.F.L. acknowledges the financial support of the National Science Foundation under grant number NSF-CBET-0931059.

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REFERENCES

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CONCLUSIONS We provide clear evidence of the importance of the geometric fit of adsorbed species in the pores of acidic zeolites for the rate and extent of spontaneous one-electron oxidation of the adsorbate (t-St). In zeolites beta and mordenite, where the fit of t-St in the pores is loose as compared with 10-ring pore zeolites like ferrierite and H-ZSM-5, the formation of the t-St•+ radical cation is very rapid, followed by the recapture of an electron from the zeolite framework by the electron-deficient t-St•+ radical cation and the formation of a long-lived charge-transfer complex (t-St H-AlBEA•−•+). In the weakly acidic H-BBEA, slow and low yield of radical cation species is observed. In H-GaBEA, the rates of electron-transfer and charge-transfer complex formation are slower than in H-AlBEA, but the maximum ionization yield is still comparable to the results observed with H-AlBEA. The slow ionization rate of t-St in H-GaBEA allows for the direct observation of the t-St•+ radical cation intermediate. The electron-acceptor nature of the inner space of acidic zeolites and its interaction with adsorbed species 14489

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