Immobilization of Perylenetetracarboxylic Dianhydride on Al₂O₃ for Efficiently Photocatalytic Sulfide Oxidation

Abstract

Perylenetetracarboxylic dianhydride (PTCDA) derivatives have received significant attention as molecule photocatalysts. However, the poor recyclability of molecule-type photocatalysts hinders their widespread applications. Herein, immobilization of PTCDA on Al₂O₃ was achieved by simply physical mixing, which not only dramatically improved their recyclability, but also surprisingly improved the reactivity. A mechanism study suggested that the photo-exited state (PTCDA*) of PTCDA could promote the oxidation of thioanisole to generate PTCDA^•−, which sequentially reduces oxygen to furnish superoxide radicals to achieve the catalytic cycle. Herein, the immobilization support Al₂O₃ was able to facilitate the strong adsorption of thioanisole, thereby boosting the photocatalytic activity. This work provides a new insight that the immobilization of organic molecular photocatalysts on those supports with proper adsorption sites could furnish highly efficient, stable, and recyclable molecular-based heterogeneous photocatalysts.

1. Introduction

Photoredox catalytic organic reactions have attracted increasing attention [1]. Photoredox catalysts, mainly including metal complexes, dye molecules, and polymers, have been widely applied in many classic redox reactions (dehydrogenation, carbonyl reduction, and alcohol oxidation etc.) [2]. 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA), as one of the molecule-type photoredox catalysts, features a broad light absorption range and suitable redox property, which can achieve dehalogenation reactions, aerobic oxidation reactions, etc. [1,3]. A typical example is that PTCDA molecules can simultaneously achieve the oxidation of alcohols and the reduction of protons to release H₂ [3]. In addition, the imines derived from PTCDA can also serve as photocatalysts for dye degradation, wastewater treatment, etc. [4,5,6,7,8]. However, since PTCDA is a molecule photocatalyst, it is inevitable that issues such as photo-stability and difficulty will be encountered in catalyst separation and recovery. Therefore, improving its stability and recyclability by heterogenization, and still retaining and even improving its redox capabilities, is highly attractive. There are three main categories of heterogenization methods: the first one is to construct polymer/supermolecules semiconductors via pyrolysis etc. using PTCDA and its derivatives as precursors [9,10]; the second is to combine PTCDA-converted polymers/supermolecules with oxides [11,12,13,14]; and the third is to act as the functional monomers to modify the other polymers [15,16]. In the second strategy, the interaction between the PTCDA-based materials and the oxide is potentially highly important, but rarely explored. Thus, herein, we wish to immobilize PTCDA on oxides (e.g., Al₂O₃) and probe the catalytic effects of their interaction. Specifically, a simple grinding method was used to physically mix PTCDA with metal oxides, where Al₂O₃ was screened out as a proper support for PTCDA in Figure S1. As revealed by the XPS analysis and theoretical studies, this simple grinding procedure could induce strong interaction between the anhydride groups of PTCDA and Al₂O₃. FTIR analysis reflected that Al₂O₃ with Lewis acidity could provide sufficient molecular adsorption sites for reagents. Benefiting from the synergistic effects between PTCDA and Al₂O₃, the photocatalytic activity of PTCDA/Al₂O₃ was significantly higher than that of PTCDA. A mechanism study suggested that this immobilization strategy helps to enhance the absorption of reagents such as thioanisole, accelerate its conversion to the corresponding radicals, and thus promote the subsequent oxidation by the superoxide radicals from oxygen reduction.

2. Results and Discussion

2.1. Synthesis, Activity Evaluation, and Interaction Study of PTCDA/Al₂O₃ Mixture

Al₂O₃, as an insulator with surface hydroxyl and Lewis acid sites [17,18], was screened out as a superior support for PTCDA (Figure S1), as it can provide synergistic sites for both O₂ and substrates. The preparation of PTCDA/Al₂O₃(physical mixture) is shown in Figure 1a. Through the TEM images of Al₂O₃ and PTCDA in Figure 1b, Al₂O₃ exhibited a layer morphology, while PTCDA had a rod-shaped morphology. And a crystal plane spacing of 0.162 nm was displayed, belonging to the (422) plane of Al₂O₃. After the physical mixture of Al₂O₃ and PTCDA was grounded, the rod-shaped PTCDA disappeared and was replaced by a morphology similar to Al₂O₃. It could be observed from HRTEM images (Figure 1b, iv) that there were certain amorphous substances on the surface of Al₂O₃, which could be classified as PTCDA molecules. EDS mapping also demonstrated the uniform distribution of PTCDA on the surface of Al₂O₃. The XRD patterns in Figure 1c confirmed that the diffraction peaks of PTCDA molecules gradually increased as the amount of PTCDA molecules increased, demonstrating the successful incorporation of PTCDA molecules into Al₂O₃. UV-Vis spectra reflected the light absorption properties of PTCDA and Al₂O₃ (Figure S2). Compared with Al₂O₃, PTCDA has significant absorption within the visible light absorption range, implying that this heterogenization strategy has the potential to improve the photocatalytic activity of PTCDA/Al₂O₃ (physical mixture). According to the reported literature [3], the reduction potential (−0.27 V vs. NHE, pH = 0) of PTCDA is more negativethan that (−0.13 V vs. NHE, pH = 0) of O₂/•O₂⁻, suggesting the possibility and feasibility of activating oxygen after mixing PTCDA and Al₂O₃. Subsequently, the photocatalytic activities toward selective oxidation of sulfide over the series of catalysts were evaluated (Figure 1d). Compared to independent Al₂O₃ or PTCDA, the heterogenized PTCDA showed a significant improvement in photocatalytic activity, where 5%PTCDA/Al₂O₃ gave the best performance.

To understand the reasons behind the unexpectedly boosted performance of the heterogenized PTCDA, the interaction between the PTCDA and Al₂O₃ was further investigated by FTIR (Figure 1d). Compared with pure PTCDA, the bands of C=O and C−O of heterogenized PTCDA showed significant red shifts, while there were no significant shifts in C−H or C=C, confirming that the interaction was likely to occur between the anhydride group of PTCDA and Al₂O₃. In great contrast to the minor shift of hydroxyl groups in calcinated Al₂O₃, the significant blue shift of hydroxyl groups induced by the calcination of the PTCDA/Al₂O₃ (physical mixture) and the apparent red shift in the characteristic infrared peaks of C=O and C−O, further confirmed that the anhydride group of PTCDA mainly interacted with the hydroxyl groups on the surface of Al₂O₃ (Figure S3). The interaction between the two was also studied by XPS (Figure 2a–c). Differently from the basically unchanged binding energy of C 1s in C−C or C=C, the binding energies of O 1s and C 1s in C−O−C and C=O decreased, and the binding energy of Al 2p increased, confirming that the mutual interaction would increase the electron density of PTCDA molecules and decrease the electron density on the surface of Al₂O₃ due to the electron-pulling effect of the anhydride group after the interaction. The calculated differential charge density (yellow) represents an increase in charge density, while cyan represents a decrease in charge density in Figure 2d. It was also demonstrated that the adsorption on the Al₂O₃ surface would increase the electron density of PTCDA molecules, as is consistent with the results of XPS.

2.2. Exploration of the Mechanism

Scavenger studies were conducted to determine the effects of various reactive species (Figure 3a). The phenomenon of almost no activity under an argon atmosphere proved the importance of oxygen. Furthermore, the quenching of ¹O₂ and •OH did not induce an obvious reduction in activity, indicating that these two oxygen reactive species were not involved in the oxidation reaction. Instead, the addition of •O₂⁻ sacrificial agents significantly reduced the reaction activity, confirming that •O₂⁻ was the main active species in the reaction system. Even though the •O₂⁻ radicals were important in the reaction system, the intensity of •O₂⁻ signals of PTCDA and heterogenized PTCDA under visible light irradiation (Figure 3b) were similar, indicating that the enhanced performances probably did not originate only from oxygen-reduction parts. We thus started to probe the influence of substrate oxidation using methanol as a probing reagent, and the Nash detection method was used to detect the presence of formaldehyde. It was found that the formaldehyde generated by the PTCDA/Al₂O₃ (physical mixture) system was 6 times higher than that of the PTCDA system, confirming that the oxidation parts were also accelerated. To further understand the remarkable enhancement, the methanol adsorption experiments were studied by FTIR (Figure S4). Classical and weak absorption peak of −CH_x of methanol was observed over Al₂O₃ and 5%PTCDA/Al₂O₃ (physical mixture), but not over PTCDA, indicating the preferential adsorption of methanol on the surface of Al₂O₃ compared to PTCDA. Similarly, the enhanced adsorption of thioanisole on 5%PTCDA/Al₂O₃ (physical mixture) was also observed (Figure 3d). Thus, it can be deduced that in the heterogenized mixture, Al₂O₃ would favor the adsorption of substrate, and PTCDA could activate O₂ to furnish •O₂⁻ radicals, then work synergistically to boost the oxidation activity of sulfide. Therefore, a reasonable mechanism was proposed (Figure 3e). Under visible light irradiation, PTCDA could be excited to the excited state, PTCDA*, which had the potential to oxidize the surface-adsorbed sulfide to corresponding intermediate radical 1, and itself was converted to PTCDA^•−. Then, O₂ could obtain electrons from PTCDA^•− to produce •O₂⁻ and further oxidize intermediate 1 to intermediate 2, then follow proton abstraction to produce sulfoxide.

Table 1. Universality study over 5%PTCDA/Al₂O₃ (physical mixture).

Entry	Substrates	Products	Time (h)	Conv. (%)	Sulfoxide Yield(%)
1			6	93	68
2			6	94	69
3			6	99	94
4			6	91	72
5			6	94	79
6			6	96	79
7			8	91	47
8			8	96	64

Reaction conditions: Photocatalyst, 10 mg; methanol, 2 mL; substrate, 0.1 mmol; white LED (≥420 nm), 500 mW cm⁻²; O₂, 1 atm. The conversion and yield of benzyl sulfoxide was determined by GC-MS with anisole as internal standard. The conversion and yield of other products was determined by ¹H NMR spectra with 1,3,5-trimethylbenzene as internal standard.

shows the reaction scope. The substrate-bearing electron donating or weak electron-withdrawing groups all gave high conversion rates and yields for production of the corresponding sulfoxides. And 1 g of thioanisole was used as a substrate to conduct the large-scale photocatalytic reaction under visible light irradiation. The conversion (92%) of thioanisole and yield (62%) of benzyl sulfoxide are given in Table S2, indicating that substrate could be scaled up to the gram level. There was no significant change in photocatalytic activity after multiple cycles (Figure S5a), and FIIR spectra (Figure S5b) confirmed its stability. Overall, the heterogenized PTCDA photocatalysts exhibited good universality and excellent recyclability.

3. Materials and Methods

3.1. Reagents and Solvents

Aluminum nitrate nonahydrate (Al(NO₃)₃•9H₂O), urea, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA, 98%), anisole, methanol, and thioanisole were all purchased from Energy Chemical and used without further purification.

3.2. Preparation of 2D Al₂O₃ Nanosheets as Precursor

Two-dimensional Al₂O₃ nanosheets were prepared using a hydrothermal method with Al(NO₃)₃•9H₂O as raw material [18,20]. A total of 4.5 g of Al(NO₃)₃•9H₂O and 5.1 g of urea were added to a 250 mL round bottom flask, and then 250 mL of ultrapure water was added and maintained at 150 °C for 24 h to obtain a white precipitate. After cooling, the obtained white precipitate was washed with ultrapure water to pH = 7.0 and then dried in an oven at 75 °C for 24 h to obtain the AlOOH precursor. The obtained AlOOH obtained was further calcined in an air atmosphere at 420 °C for 8 h to obtain Al₂O₃ nanosheets.

3.3. Preparation of PTCDA/Al₂O₃(Physical Mixture)

Typically, the synthesis process of 5%PTCDA/Al₂O₃ is as follows: 0.0105 g of PTCDA is mixed with 0.2 g of Al₂O₃, thoroughly grinding in a mortar until the material is evenly mixed, resulting in a composite material of PTCDA and Al₂O₃, labeled as 5%PTCDA/Al₂O₃ (physical mixture). A series of x%PTCDA/Al₂O₃ was also fabricated by changing the mass percentages of PTCDA, where x denotes 1, 3, 5, and 10, respectively.

3.4. General Procedure for the Photocatalytic Selective Oxidation of Sulfide

As is typical, 10 mg of photocatalyst, 1 mL of methanol, and 0.1 mmol of substrate were added to a 10 mL of Pyrex reactor. Then, the reactor was purged with O₂ for 1 min and maintained at 1 atmosphere. And the reactor was placed in an oil bath at 30 °C for 6 h. Afterwards, 0.01 mmol of 1,3,5-trimethoxybenzene was added as the internal standard, and then the photocatalyst was filtered. The conversion and yield of benzyl sulfoxide were determined using Agilent Technologies 7820 gas chromatography equipped with a WondaCap 5 column with anisole as the internal standard. The conversion and yield of other products ere determined by ¹H NMR spectra, with 1,3,5-trimethylbenzene as internal standard.

3.5. Characterizations

The XRD patterns were obtained using a Rigaku Ultima IV X-RAY diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 5 to 80° at a scanning rate of 8° min⁻¹. Transmission electron microscopy (TEM) measurements were carried out using a JEOL F200 microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted using a Escalab 250Xi spectrometer at (Thermo, Waltham, MA, USA) room temperature with an Al Kα X-ray source (hν = 1486.6 eV). The C 1s peak at 284.8 eV was used as the reference for the calibration of the binding energy. UV-vis diffuse reflectance spectra were measured on an Agilent Technologies Cary Series UV-vis-NIR spectrometer (Agilent, Santa Clara, CA, USA). Fourier Transform infrared spectroscopy (FTIR) measurements were made using a Bruker VERTEX 70v spectrometer (Bruker, Billerica, MA, USA). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) measurements were made using a Bruker Tensor II spectrometers (Bruker, Billerica, MA, USA).

3.6. Computational Details

All calculations were performed in the framework of the density functional theory (DFT), with the projector-augmented plane–wave method, as implemented in the Vienna ab initio simulation package (VASP) [21]. The (111) surface was chosen as the active surface to represent the as-prepared γ-Al₂O₃ in our calculation model. The vdW interactions were included using the DFT-D2 method of Grimme [22]. The cut-off energy was set to 400 eV. The energy criterion was set to 10⁻⁵ eV in the iterative solution of the Kohn–Sham equation. The Brillouin zone integration was performed using a 2 × 2 × 1 k-mesh. All the structures were relaxed until the residual forces on the atoms had declined to less than 0.02 eV/Å.

4. Conclusions

In summary, a quite simple immobilization strategy was applied to modify the PTCDA molecules, which furnished heterogenized PTCDA with remarkably enhanced reactivity and excellent recyclability. Our studies revealed that the simple grinding procedure could induce a strong interaction between the anhydride group of PTCDA and the hydroxyl group of Al₂O₃. Al₂O₃ with Lewis acidity provided anchoring sites for reagents to significantly facilitate the electron transfer from reagent to PTCDA*, thus greatly improving the overall performance. This work provides new insights for the low-cost production of superior heterogenized molecular photocatalysts, which lays a foundation for large-scale photosynthesis of fine chemicals and pharmaceuticals using molecular photocatalysts.