Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity

Abstract

A simple donor-acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) was incorporated into nanosized mesoporous silica-alumina to form a composite, which in acetonitrile is highly dispersed. In this medium, upon visible light irradiation, the formation of an extremely long-lived electron-transfer state (Acr^•-Mes^•+) was confirmed by EPR and laser flash photolysis spectroscopic methods. The composite of Acr⁺-Mes-incorporated mesoporous silica-alumina with an added copper complex [(tmpa)Cu^II] (tmpa = tris(2-pyridylmethyl)amine) acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to produce p-tolualdehyde and hydrogen peroxide. Thus, incorporation of Acr⁺-Mes into nanosized mesoporous silica-alumina combined with an O₂-reduction catalyst ([(tmpa)Cu^II]²⁺) provides a promising method in the development of efficient and robust organic photocatalysts for substrate oxygenation by dioxygen, the ultimate environmentally benign oxidant.

Extensive efforts have been devoted to develop a variety of electron donor-acceptor linked molecules, which mimic charge-separation processes in the photosynthetic reaction center (1–11); however, long-lived charge separation has been attained only in frozen media at low temperature (12–18) because intermolecular back electron transfer in solution is predominant as compared to the corresponding intramolecular process. For example, photoexcitation of a charge-shift type donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) affords the electron-transfer (ET) state (Acr^•-Mes^•+) with an extremely long lifetime (e.g., 2 h at 203 K) in frozen media (18, 19). At room temperature in solution, however, Acr^•-Mes^•+ forms a π-dimer radical cation with Acr⁺-Mes, which decays by diffusion-limited bimolecular back electron transfer (18, 19). In order to avoid such bimolecular back electron transfer, electron donor-acceptor organic molecules must be isolated with respect to each other as in the natural photosynthetic centers where they are separated from each other in chloroplast thylakoid membrane protein environments. Another serious problem for electron donor-acceptor organic molecules, including Acr⁺-Mes, is instability under photoirradiation (20). The degradation of Acr⁺-Mes also results from intermolecular oxidation of Acr⁺-Mes by the ET state under photoirradiation (21).

We report herein incorporation of Acr⁺-Mes into nanosized mesoporous silica-alumina (AlMCM-41) by cation exchange to obtain the nanocomposite (Acr⁺-Mes @ AlMCM-41), which affords an extremely long-lived ET state (Acr⁺-Mes @ AlMCM-41) upon photoirradiation with visible light. Mesoporous silica and silica-alumina, which exhibit well-defined pore systems, tunable pore diameters (2–30 nm), narrow pore size distributions, and high surface areas, have frequently been used to introduce various chemical functionalities (22–28). The incorporation of Acr⁺-Mes into nanosized mesoporous silica-alumina can remarkably enhance the photostability of Acr⁺-Mes. The nanocomposite (Acr⁺-Mes @ AlMCM-41) has also allowed us to detect the ET state by spectroscopic methods in solution. This nanocomposite has been further combined with a copper complex (tmpa = tris(2-pyridylmethyl)amine) leading to the photocatalytic reactivity for selective oxygenation of p-xylene by dioxygen yielding p-tolualdehyde and hydrogen peroxide. The photocatalytic reactivity and stability of the nanocomposite were greatly improved as compared to the behavior of Acr⁺-Mes in solution.

Results and Discussion

Preparation of Tube-Shaped AlMCM-41 (tAlMCM-41).

Nanosized tAlMCM-41 was synthesized by the procedure shown in Scheme 1, which was slightly modified from a known synthetic method for a mesoporous silica (MCM-41) nanoparticle film (10). In order to decrease the particle size of the mesoporous silica-alumina, we increased the amount of water in the initial solution and decreased the reaction time to 1 h with stirring while elevating the reaction temperature to 353 K.

Scheme 1.

The tAlMCM-41 sample exhibits a well-defined X-ray powder pattern (Fig. S1A) with a well-resolved pattern with a prominent peak (100) observed at ca. 2θ = 2.56°. Such lines are characteristic of a highly ordered material with a hexagonal array. The hexagonal unit cell parameter (a₀) was calculated to be 11.9 nm using the formula , where the d₁₀₀ spacing (3.5 nm) was obtained from the peak in the XRD pattern by Bragg’s equation (2d sin θ = λ, where λ = 0.154 nm for Cu K_α radiation). Fig. 1A shows TEM images that reveal a tubular or rod-like morphology in the diameter of 50–100 nm with the length of 0.2–2 μm array. Uniform channels of ca. 4 nm in diameter exist in a tube, as shown in Fig. 1A, Inset. N₂ adsorption isotherms for tAlMCM-41 samples lead to a Brunauer, Emmett, and Teller (BET) surface area of 894 m² g^-1.

Fig. 1.

TEM images of (A) tAlMCM-41 and (B) sAlMCM-41. (The high resolution image of tAlMCM-41 is inserted in A).

Nanosized spherical AlMCM-41 (sAlMCM-41) was also synthesized using modifications of the reported method (see Experimental Section). In order to decrease the grain size, ethanol was added to prevent the growth of a grain by dissociation of the template cetyltrimethylammonium bromide (CTAB) assembly (Scheme 1). The short reaction time for initial stirring and decrease of the reaction temperature should lead to the small particle size. TEM images for sAlMCM-41 (Fig. 1B) revealed a spherical form of the particles. The N₂ adsorption isotherms for the synthesized sAlMCM-41 led to a BET surface area of 1.1 × 10³ m² g^-1.

Incorporation of Acr⁺-Mes into tAlMCM-41 and sAlMCM-41.

Na⁺-exchanged nanosized mesoporous silica-alumina (tAlMCM-41 and sAlMCM-41) was ion–exchanged with Acr⁺-Mes to prepare Acr⁺-Mes @ tAlMCM-41 and Acr⁺-Mes @ sAlMCM-41. Because the Acr⁺-Mes molecular size is small compared with the pore size of mesoporous silica with a diameter greater than 3 nm, cation exchange with Acr⁺-Mes occurs spontaneously upon mixing Na⁺-exchanged nanosized mesoporous silica-alumina with in MeCN (Fig. S2). This leads to a clear color change from white to yellow. Once Acr⁺-Mes is exchanged and incorporated into the mesoporous silica-alumina, it is stable and does not leach out even if suspended and stirred in MeCN at room temperature. Thus, the Acr⁺-Mes incorporated as described exists not on the mesoporous silica-alumina surface but rather inside the mesopore at the ion-exchange site (29). Table 1 lists the maximum amounts of Acr⁺-Mes incorporated, along with the nanocomposite’s texture parameters. The exchange ratios of cation sites from Na⁺ or H⁺ to Acr⁺-Mes of tAlMCM-41 and sAlMCM-41 were 16% and 18%, respectively, indicating that the effect of morphology on the cation exchange efficiency is negligible.

Table 1.

Texture parameters for the mesoporous silica-alumina and the Acr⁺-Mes loading capacity

Sample	ABET, m2 g-1	d, nm	Maximum amount of Acr+-Mes, mol g-1	Si/Al	Area occupied by each molecule, m-2
sAlMCM-41	1.1 × 103	3.3	7.3 × 10-5	45	3.9 × 1016
tAlMCM-41	8.9 × 102	3.5	1.2 × 10-4	23	8.1 × 1016

The absorption spectrum of Acr⁺-Mes @ tAlMCM-41 suspended in MeCN clearly shows the characteristic band at λ = 360 nm due to Acr⁺-Mes shown in Fig. 2 (red line), and this is similar to the sum of spectra due to the individual components tAlMCM-41 (black line) and Acr⁺-Mes (blue line). The diffuse reflectance absorption spectrum of solid Acr⁺-Mes @ tAlMCM-41 also resembles the absorption spectrum of Acr⁺-Mes in MeCN (Fig. S2C).

Fig. 2.

Absorption spectra of Acr⁺-Mes @ tAlMCM-41 (1.1 × 10^-4 mol g^-1, 3.0 mg) in an MeCN suspension at 298 K. (blue line: Acr⁺-Mes in MeCN solution, black line: tAlMCM-41 only, red line:Acr⁺-Mes @ tAlMCM-41 (suspended in MeCN).

Laser Flash Photolysis of Acr⁺-Mes @ tAlMCM-41.

The transient absorption spectra obtained by femtosecond laser excitation at λ = 388 nm for Acr⁺-Mes @ tAlMCM-41 and Acr⁺-Mes in deaerated MeCN are shown in Fig. 3 A and C, respectively. In both cases, the band at λ = 490 nm, which is due to the singlet excited state of the Acr⁺ moiety is shifted to λ = 500 nm, which is ascribed to the Acr^• moiety (18). The transient absorption band at λ = 560 nm due to the Mes^•+ moiety of the electron-transfer state of Acr⁺-Mes @ tAlMCM-41 (Fig. 3A) is somewhat red-shifted compared with the corresponding absorption band (λ = 540 nm) of the electron-transfer state of Acr⁺-Mes in deaerated MeCN (Fig. 3C) (18, 30). The rate constant for formation of the electron-transfer state of Acr⁺-Mes @ tAlMCM-41 was determined to be 3.4 × 10¹¹ s^-1 by monitoring the time profile of the absorbance change at λ = 500 nm due to the Acr^• moiety (Fig. 3B). This value is slightly smaller than the value observed for Acr⁺-Mes (Fig. 3D, 5.8 × 10¹¹ s^-1) in deaerated MeCN and similar to the value in benzonitrile (3.6 × 10¹¹ s^-1) (18). The red-shift of the absorption band due to the Mes^•+ moiety and the slight difference in the rate constant of formation of the electron-transfer state of Acr⁺-Mes @ tAlMCM-41 suggest that the Mes^•+ moiety may interact weakly with the cation exchange site in tAlMCM-41.

Fig. 3.

(A and C) Time-resolved absorption spectra of (A) Acr⁺-Mes @ tAlMCM-41 (1.0 mg, 1.0 × 10^-5 mol g^-1) and (C) Acr⁺-Mes (5.0 × 10^-4 M) in MeCN at 298 K taken at 0.7 ps (black), 2 ps (red) and 500 ps (brown) after femtosecond laser excitation at λ_ex = 388 nm. (B and D) Formation time profile at λ = 500 nm due to (B) Acr^•-Mes^•+ @ tAlMCM-41 and (D) Acr^•-Mes^•+ in deaerated MeCN.

The decay of the absorption at λ = 500 nm due to Acr^•-Mes^•+ incorporated into tAlMCM-41 occurs over more than 100 ms as shown in Fig. S3B, although the initial fast decay may result from bimolecular back electron transfer between two Acr^•-Mes^•+ molecules that are located relatively close to each other under the conditions of the laser excitation. This contrasts with the quick and complete decay within 0.1 ms (Fig. S3F) of Acr^•-Mes^•+ in an MeCN solution. Slow back electron transfer was also observed for Acr⁺-Mes @ sAlMCM-41 with the long-lived ET state (Acr^•-Mes^•+) (Fig. S3D). These results indicate that the intramolecular back electron transfer of the ET state (Acr^•-Mes^•+) in nanosized mesoporous silica-alumina (tAlMCM-41 and sAlMCM-41) occurs slowly even at room temperature. The decay time scale of the long-lived ET state (Acr^•-Mes^•+) in tAlMCM-41 was too slow to be completed within the time scale measurable by laser flash photolysis. In addition, the data at a time scale longer than 100 ms may be significantly affected by diffusion from the excitation spot during measurement. Thus, we adopted EPR spectroscopy for the measurement of ET lifetimes of more than 1 s (vide infra).

EPR Detection of the ET State of Acr⁺-Mes Incorporated into tAlMCM-41 upon Photoirradiation.

Photoirradiation of Acr⁺-Mes @ tAlMCM-41 with a 1,000-W high-pressure mercury lamp through a UV-light cutting filter (λ > 390 nm) at 298 K results in formation of the ET state (Acr^•-Mes^•+) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety (19). This was confirmed with an experiment where Acr⁺-Mes in tAlMCM-41 was photoirradiated as shown in Fig. 4A. The resulting spectrum consists of the superposition of the EPR signals of the acridinyl radical moiety (Fig. 4B) (19) and the radical cation of the mesitylene moiety (Fig. 4C), though line broadening was observed because of spin-spin interaction. The decrease in the EPR signal intensity obeyed first-order kinetics as shown in Fig. 4D, revealing the intramolecular back electron transfer of the ET state of Acr⁺-Mes. In order to determine the multiplicity of the ET state of Acr^•-Mes^•+ @ tAlMCM-41, an EPR spectrum was measured at 4 K in the dark after initial photoirradiation for a few seconds (Fig. S4A). The clear g = 2.0027 signal with fine structure together with a strong sharp signal at g = 4.0 clearly indicates the triplet multiplicity. The zero-field splitting parameters D and E values were determined from the fine structure at g = 2.0027, and they are 59.9 G and 11.4 G, respectively. By applying Eq. 1

[1]

where μ₀ is the permeability of a vacuum, g is 2.0027, R is the distance between two electron spins, and β_e is the Bohr magneton, R could be estimated to be 7.7 Å, which agrees with the expected distance of 7.2 Å between an sp² carbon atom at the 4 position of the mesityl moiety and sp² carbon atoms at the 3 and 6 positions of the acridinyl moiety (31). After completion of the decay of the EPR spectral signal intensity, renewed photoirradiation at 323 K led to the signal being instantaneously reproduced with almost complete recovery of intensity; this cycle could be repeated as shown in Fig. S4B.

Fig. 4.

(A, B, and C) EPR spectrum of (A) Acr⁺-Mes @ tAlMCM-41 in MeCN under photoirradiation (λ > 390 nm 120 s) using a high-pressure mercury lamp and UV cut-off filter (λ > 390 nm), (B) Acr^•-Mes produced by reduction of the ET state of Acr⁺-Mes (0.5 mM) by 1-benzyl-1,4-dihydronicotinamide (2.5 mM) in MeCN under photoirradiation and (C) Mes^•+ prepared by the photoinduced one-electron oxidation of mesitylene (75 mM) with mercury trifluoroacetate (20 mM) in trifluoroacetic acid under photoirradiation at 298 K. (D) Time profile of the EPR signal intensity for Acr^•-Mes^•+ @ tAlMCM-41 in MeCN at 298 K.

The rate constant for the decay of the EPR spectral signal intensity at high temperature significantly increased compared with rate constant observed at RT. Fig. S5A shows an Eyring plot for the rate constant of intramolecular back electron transfer in the ET state of Acr⁺-Mes @ tAlMCM-41 compared with the rate constant in solution. The activation enthalpy (ΔH^‡) of Acr⁺-Mes @ tAlMCM-41 was determined to be 20( ± 1) kcal mol^-1, which is slightly higher than the ΔH^‡ value of Acr⁺-Mes in solution (18( ± 1) kcal mol^-1). The larger ΔH^‡ value may be the result of a larger driving force for the back electron transfer in the Marcus inverted region (32) because of a decreased solvation in the nanosized mesoporous silica-alumina.

A Copper Complex Composite with Acr⁺-Mes @ AlMCM-41 for Photocatalytic Oxygenation of p-Xylene.

The effect of encapsulation of Acr⁺-Mes into nanosized mesoporous silica-alumina (tAlMCM-41 and sAlMCM-41) on the photocatalytic activity and catalyst stability was examined via examination of the photocatalytic oxygenation of p-xylene with molecular oxygen. It has been reported that no oxygenation of p-xylene occurs from singlet oxygen that is produced by the photosensitization with ZnTPP or C₆₀ (21); however, the electron-transfer oxidation of p-xylene with the electron-transfer state of Acr⁺-Mes is thermodynamically feasible because the free energy change for electron transfer from p-xylene (E_ox = 1.93 V vs SCE) to the Mes^•+ moiety (E_red = 2.06 V vs SCE) is negative (18). The visible light irradiation using a xenon lamp and UV cut-off filter (λ > 390 nm) of the absorption band (7.5 × 10^-5 M) in oxygen-saturated MeCN containing p-xylene (3.0 × 10^-2 M) results in the formation of p-tolualdehyde and hydrogen peroxide Eq. 2 (21). The photocatalytic activity was accelerated by the addition of Brønsted acid such as trifluoroacetic acid (TFA) to the photocatalytic oxygenation system (21). The conversion of p-xylene oxygenation was limited to a 44% yield by 80 min photoirradiation (380 nm < λ < 500 nm) of an O₂-saturated MeCN solution containing p-xylene (4.0 mM) and Acr⁺-Mes (0.20 mM). Fig. 5 shows the time courses of the turnover number based on Acr⁺-Mes used in the photocatalytic oxygenation of p-xylene with Acr⁺-Mes @ tAlMCM-41 dispersed in MeCN. The reaction was performed by photoirradiation (λ > 390 nm) in oxygen-saturated MeCN (2.0 mL) containing p-xylene (6.0 × 10^-5 mol) and Acr⁺-Mes @ tAlMCM-41 (15 mg, Acr⁺-Mes: 1.7 × 10^-7 mol) with TFA (6.7 mM, 1.3 × 10^-5 mol) or without TFA at room temperature. Such turnover numbers increased in the absence and presence of TFA for the first 4 h, as shown in Fig. 5. The turnover number reaches 350 at 100% conversion of p-xylene after 5 h in the reaction system of Acr⁺-Mes @ tAlMCM-41 with TFA (Fig. 5, red circles). The complete conversion of p-xylene was attained at the second cycle. When Acr⁺-Mes @ tAlMCM-41 was replaced by Acr⁺-Mes @ sAlMCM-41 under the same conditions, a similar photocatalytic activity and stability was observed as shown in Fig. 5 (red triangles). In the absence of a Brønsted acid, i.e., TFA, the turnover number is limited to 250 as shown in Fig. 5 (rectangular).

Fig. 5.

Time courses of turnover number based on Acr⁺-Mes used in the photooxygenation of p-xylene (30 mM, 6.0 × 10^-5 mol) in O₂-saturated CD₃CN (2.0 mL) with Acr⁺-Mes @ tAlMCM-41 (15 mg, Acr⁺-Mes: 1.7 × 10^-7 mol) in the presence (•) and absence (□) of CF₃COOH (6.7 mM, 1.3 × 10^-5 mol) or Acr⁺-Mes @ sAlMCM-41 (12 mg, Acr⁺-Mes: 1.5 × 10^-7 mol) in the presence of CF₃COOH (6.7 mM, 1.3 × 10^-5 mol) (▴).

The photocatalytic activity and stability of Acr⁺-Mes incorporated into nanosized silica-alumina can be maintained without an acid by incorporation of a copper complex, (tmpa = tris(2-pyridylmethyl)amine), into nanosized mesoporous silica-alumina (vide infra). The amount of [(tmpa)Cu^II]²⁺ incorporated into sAlMCM-41 by cation exchange was determined to be 1.1 × 10^-5 mol g^-1, as determined by the reduction of the UV-vis absorption band around 800 nm assigned to the species [(tmpa)Cu^II]²⁺ in MeCN solution in the presence of sAlMCM-41 (Fig. S6B). The photocatalytic oxygenation of p-xylene was performed by photoirradiation (λ > 390 nm) of oxygen-saturated MeCN (2.0 mL) containing p-xylene (6.0 × 10^-5 mol), Acr⁺-Mes @ sAlMCM-41 (12 mg, Acr⁺-Mes: 1.5 × 10^-7 mol) and (4.0 × 10^-8 mol). Under these reaction conditions, all [(tmpa)Cu^II]²⁺ ions present are encapsulated into sAlMCM-41, because 4.0 × 10^-8 mol is much smaller than the maximum amount (1.3 × 10^-7 mol for 12 mg sAlMCM-41) that can be encapsulated. The oxidative conversion of p-xylene with [(tmpa)Cu^II]²⁺/Acr⁺-Mes @ sAlMCM-41 reaches nearly a 100% yield in 4 h photoirradiation as shown in Fig. 6A, where the conversion with Acr⁺-Mes in the presence of TFA is limited to 40%. The time course of each product yield in this photocatalytic process is shown in Fig. 6B, where p-xylene was converted to p-methylbenzyl alcohol, which was further oxidized after prolonged photoirradiation time to yield the final oxidation product: p-tolualdehyde accompanied by generation of H₂O₂. The final yield of H₂O₂ was determined to be 79% as shown in Fig. 6B. The photocatalytic activity of [(tmpa)Cu^II]²⁺/Acr⁺-Mes @ sAlMCM-41 was virtually the same as Acr⁺-Mes @ sAlMCM-41 with TFA (Fig. S7).

Fig. 6.

(A) Time course of the conversion in photoinduced oxygenation of p-xylene (30 mM) catalyzed by Acr⁺-Mes (1.5 × 10^-7 mol, ▪) in the presence of CF₃COOH (1.3 × 10^-5 mol) and ([(tmpa)Cu^II]²⁺/Acr⁺-Mes) @ sAlMCM-41 (12 mg, Acr⁺-Mes: 1.5 × 10^-7 mol, [(tmpa)Cu^II]²⁺: 4.0 × 10^-8 mol, •) in O₂-saturated CD₃CN (2.0 mL). (B) Time profile of the photooxygenation of p-xylene (30 mM) in the presence of ([(tmpa)Cu^II]²⁺/Acr⁺-Mes) @ sAlMCM-41 (12 mg, Acr⁺-Mes: 1.5 × 10^-7 mol and [(tmpa)Cu^II]²⁺: 4.0 × 10^-8 mol) in O₂-saturated CD₃CN (2.0 mL) with visible light irradiation (λ > 390 nm) by a Xe lamp (500 W).

In order to clarify the effect of [(tmpa)Cu^II]²⁺ on the photocatalytic activity and stability of Acr⁺-Mes, we performed laser flash photolysis measurements. The addition of [(tmpa)Cu^II]²⁺ to an MeCN solution of Acr⁺-Mes accelerates the decay rate of Acr^•-Mes^•+ with increasing concentration of [(tmpa)Cu^II]²⁺ (Fig. S8A). The decay time profiles were recorded at λ = 500 nm due to Acr^•-Mes^•+ in the presence of various concentration of [(tmpa)Cu^II]²⁺ (0.25–1.0 mM) in deaerated MeCN containing Acr⁺-Mes (0.05 mM) after laser excitation (3.6 mJ/pulse) at λ_ex = 355 nm. The rate constant for electron transfer from the Acr^• moiety of Acr^•-Mes^•+ to [(tmpa)Cu^II]²⁺ was determined from the slope of a linear plot (Fig. S8B) as k_et = 4.7 × 10⁷ M^-1 s^-1. In the presence of O₂ in solution at low temperature, the absorption band at λ = 520 nm due to the known peroxo dicopper(II) complex [(tmpa)Cu^II(O₂)Cu^II(tmpa)]²⁺ together with an absorption at λ = 780 nm due to the superoxo species was observed (Fig. S6A) (33, 34). Thus, the complex [(tmpa)Cu^I]⁺ which is produced by the fast electron-transfer reduction from the Acr^• moiety of Acr^•-Mes^•+ to [(tmpa)Cu^II]²⁺ reacts with O₂ to produce the superoxo species, , which is further reduced (by more [(tmpa)Cu^I]⁺ that is present) to give [(tmpa)Cu^II(O₂)Cu^II(tmpa)]²⁺. Protonation of this species yields hydrogen peroxide, accompanied by regeneration of [(tmpa)Cu^II]²⁺.

By combining the results obtained above, the overall photocatalytic cycle with [(tmpa)Cu^II]²⁺/Acr⁺-Mes @ AlMCM-41 is summarized in Fig. 7, where tAlMCM-41 and sAlMCM-41 are shown as AlMCM-41. Upon photoirradiation of Acr⁺-Mes @ tAlMCM-41, the ET state of Acr⁺-Mes forms immediately followed by electron transfer from p-xylene to the mesitylene radical cation moiety of the ET state of Acr⁺-Mes. p-Xylene radical cation, which then undergoes deprotonation to produce the p-methylbenzyl radical, which reacts rapidly with O₂ to give the peroxyl radical. The disproportionation of the peroxyl radical yields p-methylbenzyl alcohol and p-tolualdehyde and O₂. p-Methylbenzyl alcohol is further oxidized to yield p-tolualdehyde as the final selectively oxygenated product of p-xylene (21). Enhanced catalytic reactivity and stability of [(tmpa)Cu^II]²⁺/Acr⁺-Mes @ AlMCM-41 compared with Acr⁺-Mes are attributed to the catalysis of [(tmpa)Cu^II]²⁺ for the two-electron reduction of O₂ to H₂O₂ via the metal complex superoxo and peroxo species (vide supra).

Fig. 7.

The proposed mechanism of photocatalytic oxygenation of p-xylene by O₂ with Acr⁺-Mes @ nanosized AlMCM-41 and [(tmpa)Cu^II]²⁺.

Summary

A nanosized mesoporous silica-alumina (tAlMCM-41 and sAlMCM-41) has been used to stabilize the electron-transfer state of a simple electron donor-acceptor dyad cation (Acr⁺-Mes), which was readily incorporated into tAlMCM-41 and sAlMCM-41 by a cation-exchange reaction. The ET state of Acr⁺-Mes-incorporated into tAlMCM-41 has an extremely long lifetime that lasts more than 1 s at room temperature and decays via intramolecular back electron transfer in contrast to the diffusion-limited intermolecular back electron transfer observed in solution. Combination of a copper complex [(tmpa)Cu^II]²⁺ with Acr⁺-Mes-incorporated AlMCM-41 affords a nanocomposite [(tmpa)Cu^II]²⁺/Acr⁺-Mes @ AlMCM-41 that acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to yield p-tolualdehyde and hydrogen peroxide. Thus, the incorporation of an electron donor-acceptor cation into nanosized mesoporous silica-alumina combined with an O₂-reduction catalyst demonstrated in this study paves a promising way to develop efficient and robust photocatalysts in artificial photosynthesis to produce solar fuels.

Experimental Section

Syntheses of Silica-Alumina MCM-41 Nanotube (tAlMCM-41).

Nanosized tube-shaped mesoporous silica-alumina (tAlMCM-41) was synthesized by the following procedure. An aqueous micellar solution containing C₁₆H₃₃(CH₃)₃NBr (CTAB, 2.0 g), Al(Oⁱ Pr )₃ (0.236 g), NaOH (0.57 g), and deionized water (975 g) was prepared under stirring for 1 h. Then, tetraethylorthosilicate (TEOS) was slowly added to the solution at 353 K heated in an oil bath. The mixture was stirred for 1 h. The products were filtered and washed with deionized water, and the remaining solid residue was dried overnight at 333 K. Template removal was carried out by heating the samples in nitrogen up to 813 K at a heating rate of 2 K/ min followed by isothermal treatment at the same temperature for 2 h in O₂. The resulting white powder was dispersed in MeCN and sonicated with an ultrasonic sound wave (Branson, Sonifier250) for several minutes. After ultrasonication, the suspension was centrifuged for 5 min at 1,000 rpm (Hitachi himac CT4D), and the supernatant was successively centrifuged for 10 min at 4,000 rpm. After decantation, the remaining solid residue was dried in vacuo at room temperature.

Syntheses of Spherical Silica-Alumina MCM-41 (sAlMCM-41).

Nanosized spherical mesoporous silica-alumina (sAlMCM-41) was synthesized using the following typical synthetic procedure (35). CTAB (2.5 g) was dissolved in deionized water (46.3 g) and mixed with absolute ethanol (60 g). An aqueous ammonia solution (25%, 16.9 g) was added to this clear solution and stirred for 15 min. Sodium aluminate (NaAlO₂, 0.1 g) was added to the solution while stirring, then tetraethylorthosilicate (TEOS, 4.7 g) was added dropwise for a couple of minutes. Sol formation started immediately. The white precipitates were filtered with a membrane filter (ADVANTEC 47MM) and washed several times until pH 7.0 was reached. The samples then were dried for 12 h at 333 K in vacuo. The template was removed by heating the samples in nitrogen up to 813 K at a heating rate of 2 K/ min in N₂ followed by isothermal treatment at the same temperature for 2 h in O₂.

Cation-Exchange Procedure of Mesoporous Silica.

The following procedure was used to prepare Acr⁺-Mes @ tAlMCM-41 and Acr⁺-Mes @ sAlMCM-41 by exchanging sodium ion (Na⁺) with Acr⁺-Mes cation incorporated into nanosized mesoporous silica-alumina tAlMCM-41 and sAlMCM-41. A small amount of the mesoporous silica-alumina (tAlMCM-41 or sAlMCM-41) (0.25 g) powder was suspended by stirring in MeCN solution (25 mL) containing (41.2 mg, 1.0 × 10^-4 mol) for 12 h. The suspension was then centrifuged and the precipitate was dried at room temperature in vacuo. The content of Acr⁺-Mes ion incorporated into nanosized mesoporous silica (tAlMCM-41 and sAlMCM-41) by cation exchange was determined from the absorption spectral change of the supernatant measured before and after centrifugation. The amount of Acr⁺-Mes incorporated into nanosized mesoporous silica (tAlMCM-41 and sAlMCM-41) was determined to be 1.2 × 10^-4 mol g^-1 and 7.3 × 10^-5 mol g^-1, respectively, by measuring the absorbance due to Acr⁺-Mes in the supernatant at λ = 423 nm (ε = 5.6 × 10³ M^-1 cm^-1).

Powder X-ray Diffraction.

X-ray diffraction patterns of tAlMCM-41 and sAlMCM-41 were recorded on a Rigaku RINT-1100 X-ray diffractometer using Cu-K_α radiation (40 kV, 25 mA) of wavelength 0.154 nm at a 0.01 step size and 1-s step time over the range 1.5° < 2θ < 8.0°. The samples were prepared as thin layers on aluminum slides.

Transmission Electron Microscopy.

TEM images were collected on a JEOL JEM-2100 that has a cold field emission gun with an accelerating voltage of 200 keV. The observed samples were prepared on a copper microgrid covered with an amorphous carbon film by dropping a diluted suspension or by placement of mesoporous silica-alumina powder.

EPR Measurements.

The EPR spectra were taken on a JEOL X-band spectrometer (JES-RE1XE) with quartz EPR tubes (i.d.=4.5 mm and 2.0 mm) in the MeCN suspension. The EPR spectrum of the ET state of Acr⁺-Mes incorporated into sAlMCM-41 or tAlMCM-41 was measured under photoirradiation with a high-pressure mercury lamp (USH-1005D) through a UV-light (λ < 390 nm) cut glass filter and water filter (light path length: 3 cm) to cut off the IR irradiation for focusing on the sample cell in the EPR cavity at 4–373 K. The g values and the zero-field splitting parameters (D and E) were calibrated using a Mn²⁺ marker. Acr^•-Mes was prepared by 1 min photoirradiation of a degassed MeCN solution containing Acr⁺-Mes(ClO₄) (0.5 mM) and 1-benzyl-1,4-dihydronicotinamide (BNAH) (2.5 mM) in a 2.0 mm quartz EPR tube. The mesitylene radical cation was prepared by the photoirradiation (3 min) of a degassed trifluoroacetic acid solution containing mesitylene (75 mM) and mercury(II) trifluoroacetate (20 mM) in a 2.0 mm quartz EPR tube (36).

Reaction Procedure.

A CD₃CN solution (2.0 mL) containing (7.5 × 10^-5 M), Acr⁺-Mes @ tAlMCM-41, or Acr⁺-Mes @ sAlMCM-41 and substrate (3.0 × 10^-2 M) in an NMR tube sealed with a rubber septum was saturated with oxygen by bubbling oxygen gas (99.999%) through a stainless steel needle for 5 min. The solution then was irradiated with a xenon lamp (Ushio Optical Modulex SX-UID501XAMQ) through a colored glass filter (Asahi Techno Glass Y43) transmitting λ > 390 nm at room temperature. The irradiated solution was analyzed periodically by ¹H NMR spectroscopy to identify and quantify the products with a JEOL JNM-AL300 spectrometer. The amount of H₂O₂ produced was determined by iodide ion titration (37). The amount of formation of was determined by measuring visible absorption spectrum (λ_max = 361 nm, ε = 2.5 × 10³ M^-1 cm^-1).

Time-Resolved Absorption Spectral Measurements.

Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast Integra-C (Quantronix Corp.), a TOPAS optical parametric amplifier (Light Conversion Ltd.), and a commercially available Helios optical detection system provided by Ultrafast Systems LLC. For nanosecond laser flash photolysis experiments, a deaerated MeCN dispersion of the composites was excited by an Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at λ = 355 nm with the power of 3.6 mJ per pulse. The transient absorption measurements in the visible and near-IR region were performed using a continuous xenon lamp (150 W) as a probe light, a photomultiplier (Hamamatsu R2949; 350–800 nm), and a InGaAs-PIN photodiode (Hamamatsu G5125-10; 800–1200 nm) as a detector, respectively. The output from the photodiodes and a photomultiplier was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). The experiments at various temperatures were performed using a thermostated cell holder.