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Article

Metal-Organic Frameworks Decorated Cu2O Heterogeneous Catalysts for Selective Oxidation of Styrene

by
Mengyi Han
1,2,†,
Xue Tang
1,2,†,
Peng Wang
1,2,
Zhiyong Zhao
1,2,
Xiaohua Ba
1,
Yu Jiang
1 and
Xiaowei Zhang
1,*
1
Institute of Advanced Materials, Beijing Normal University, Beijing 100875, China
2
College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(5), 487; https://doi.org/10.3390/catal12050487
Submission received: 27 March 2022 / Revised: 19 April 2022 / Accepted: 25 April 2022 / Published: 26 April 2022
(This article belongs to the Special Issue Synthesis and Applications of Copper-Based Catalysts)
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Abstract

:
The selective oxidation of styrene with highly efficient, environmentally benign, and cost-effective catalysts are of great importance for sustainable chemical processes. Here, we develop an in situ self-assembly strategy to decorate Cu-based metal-organic framework (MOF) Cu-BDC-NH2 nanocrystals on Cu2O octahedra to construct a series of Cu2O@Cu-BDC-NH2 catalysts for selective oxidation of styrene. Using H2O2 as green oxidants, the optimized sample of Cu2O@Cu-BDC-NH2-8h could achieve 85% styrene conversion with 76% selectivity of benzaldehyde under a mild condition of 40 °C. The high performance of the as-prepared heterogeneous catalysts was attributed to the well-designed Cu+/Cu2+ interface between Cu2O and Cu-BDC-NH2 as well as the porous MOF shells composed of the uniformly dispersed Cu-BDC-NH2 nanocrystals. The alkaline properties of Cu2O and the –NH2 modification of MOFs enable the reaction to be carried out in a base-free condition, which simplifies the separation process and makes the catalytic system more environmentally friendly. Besides the Cu2O octahedra (od-Cu2O), the Cu2O cuboctahedrons (cod-Cu2O) were synthesized by adjusting the added polyvinyl pyrrolidone, and the obtained cod-Cu2O@Cu-BDC-NH2 composite also showed good catalytic performance. This work provides a useful strategy for developing highly efficient and environmentally benign heterogeneous catalysts for the selective oxidation of styrene.

1. Introduction

Benzaldehyde, as a vital intermediate and starting material, has been widely used in fine chemical industries such as pharmaceuticals, dyes, spices, and pesticides [1,2]. Conventional techniques for benzaldehyde production (e.g., oxidation of toluene) usually involve harsh conditions and complex synthesis processes, which leads to great energy consumption and serious environmental pollution [3,4,5]. Considering the sustainable development of the chemical industry, the selective oxidation of styrene to benzaldehyde by using environmentally benign catalysts and green oxidants becomes an ideal choice [6,7]. Hydrogen peroxide (H2O2) has been widely studied as one of the green oxidants due to its low price and environmentally-friendly property [8]. The H2O2 catalytic system usually generates benzaldehyde, accompanied with styrene oxide, and the acidity of H2O2 easily leads to the isomerization of styrene oxide into phenylacetaldehyde, which further decreases the selectivity of benzaldehyde [9,10,11]. Some basic additives (e.g., NaOH, NaHCO3) are often introduced into the reaction system to function as a buffer to improve the selectivity of the target product [10,11,12]. However, the added alkaline species have difficulties in separation from the reaction system, which is not beneficial for a sustainable chemical process. Thus, it is of great significance to develop highly efficient and environmentally benign heterogeneous catalysts with suitable alkalinity for the selective oxidation of styrene under base-free conditions.
Recently, transition metal-based materials such as Mg/CTAB [13], SO42−-Fe-V/ZrO2 [14], CuCr-MMO [12], NiCo2O4 [15], and CoFe2O4/TiO2 [3] have been explored as novel heterogeneous catalysts for styrene oxidation under a base-free catalytic system. However, the relatively low specific surface area, low porosity, and weak alkalinity of these catalysts is not conducive to the adsorption of oxidants as well as the diffusion of the substrates and products, thus limiting the improvement in their catalytic performance. As types of environmentally-friendly and Earth-abundant materials with easy availability and low costs, Cu2O and/or CuO with high alkalinity and easily controlled morphology have attracted great interest and show efficient catalytic activity in several chemical reaction processes [16,17,18]. However, the insufficient stability of Cu2O/CuO severely limits its practical application. In order to overcome this obstacle, many methods have been proposed to encapsulate Cu2O/CuO into porous materials or construct core-shell structures [19,20]. In particular, metal-organic frameworks (MOFs) have attracted immense attention due to their tunable structures, high porosity, and highly accessible active sites [21,22,23,24,25,26]. Among them, Cu-based MOFs such as Cu-BDC (BDC = 1,4-benzenedicarboxylate) have been widely investigated and shown excellent performance in a variety of catalytic reactions [27,28,29,30].
In this study, an in situ self-assembly strategy was developed to decorate Cu-based MOF (Cu-BDC-NH2) nanocrystals on Cu2O octahedra and cuboctahedrons for selective oxidation of styrene to benzaldehyde. The obtained Cu2O@Cu-BDC-NH2 catalysts exhibited a highly catalytic performance with H2O2 as green oxidants. The basicity of Cu2O and the introduction of the –NH2 group can effectively inhibit the excessive oxidation of reaction products and enable the reaction to be carried out under a base-free condition. The Cu-BDC-NH2 MOFs nanocrystals coated outside of Cu2O can effectively enrich the specific surface area and porosity of the catalyst, which is conducive to the adsorption of oxidants and accelerate the reaction process. The well-designed Cu+/Cu2+ interface and the synergistic effects between Cu2O and Cu-BDC-NH2 have contributed to the enhancement of styrene conversion and benzaldehyde selectivity. In addition, a series of Cu2O@Cu-BDC-NH2 with different MOF loadings and different Cu2O crystal phases were prepared, and the relationship between their morphology, composition, and structure and the catalytic performance was systematically investigated. Our work provides new perspectives for the development of a cost-effective, highly-efficient, and environmentally benign heterogeneous catalyst for the selective oxidation of styrene under mild conditions.

2. Results and Discussion

2.1. Synthesis and Structural Characterization

The synthetic process of Cu2O@Cu-BDC-NH2 is illustrated in Scheme 1. First, Cu2O was prepared via a facile wet chemical method, in which the ascorbic acid acts as a reducing agent for the reduction of Cu2+ to Cu+ in the presence of NaOH [31,32]. The added PVP in the formation process of Cu2O serves as the stabling and capping agent, which can enable the uniform distribution of Cu2+ ions and preferentially adsorb on the {111} plane of Cu2O through the interaction between the O atoms in PVP and the Cu ions on Cu2O [33,34]. The morphology of Cu2O can be precisely controlled by adjusting the amount and the molecular weight of the added PVP [33,34,35]. The Cu2O@Cu-BDC-NH2 composite was prepared by an in situ assembly method, in which the as-prepared Cu2O serves as the substrate and Cu resource to interact with the added ligands of H2BDC-NH2 for the in situ growth of Cu-BDC-NH2 on the surface of Cu2O. It is worth noting that only ligands, but no additional copper salts, are needed in the preparation process. In addition, the morphology and structure of the Cu2O@Cu-BDC-NH2 composites can be adjusted by controlling the reaction time of in situ assembly.
The morphological and structural properties of the as-prepared materials were demonstrated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). As shown in Figure 1a, when PVP with a molecular weight of 58,000 was used as the raw material, the as-prepared Cu2O showed an octahedral morphology with a very smooth surface and an average particle size of about 1~2 μm. The XRD patterns of the as-prepared octahedron microcrystals (Figure 2a, red curve) confirmed the Cu2O structure, and the (111) diffraction peak of Cu2O showed a much stronger intensity over other peaks, indicating that the od-Cu2O exclusively exposes {111} planes, agreeing with previous reports [16,17,18,33,34,35]. The in situ growth of Cu-BDC-NH2 on Cu2O was systematically investigated by SEM (Figure 1b–f) and XRD (Figure 2a) to reveal the morphology–structure relationship of the Cu2O@Cu-BDC-NH2 composites. As shown in Figure 1b, when Cu2O reacted with H2BDC-NH2 ligands for 4 h, the obtained Cu2O@Cu-BDC-NH2-4h sample exhibited a rougher surface covered with lots of nanoparticles, but no obvious change was observed in its XRD patterns (Figure 2a, blue curve). As the growth time was extended to 8 h, a much rougher surface of Cu2O@Cu-BDC-NH2-8h was observed (Figure 1c), indicating an increase in the number and size of nanoparticles covered on the surface of Cu2O. The XRD patterns of Cu2O@Cu-BDC-NH2-8h (Figure 2a, yellow curves) showed newly emerged characteristic peaks at 10°, 17°, and 25°, which were assigned to the diffraction peaks of (110), (20-1), and (131) planes of Cu-BDC-NH2 [33], indicating the gradual formation of the Cu-BDC-NH2. After the in situ growth of Cu-BDC-NH2 on Cu2O for 12 h, 20 h, and 32 h, as shown in Figure 1d–f, the Cu2O@Cu-BDC-NH2 composites exhibited a smaller Cu2O octahedron around with assembled nanosheets, and the corresponding XRD patterns presented enhanced peak intensity of Cu-BDC-NH2 (Figure 2a). The morphological and structural changes during the in situ growth process of Cu2O@Cu-BDC-NH2 indicate that Cu2O serves as the substrate and source of the Cu ions in the following growth of Cu-BDC-NH2 nanocrystals. The growth mechanism is similar to the previous reports [36]: initially, the Cu+ ions of Cu2O were gradually released into the solution, which was then oxidized to Cu2+ by dissolved O2, and the Cu2+ ions coordinated with the added H2BDC-NH2 to construct Cu-BDC-NH2. However, it should be noted that Cu-BDC-NH2 is mainly in the shape of nanoparticles during the initial growth stage (less than 8 h), which may be limited by the release rate of Cu+ from Cu2O. With the extension of in situ growth time, the internal Cu2O core was gradually consumed, and the external MOF components were gradually increased. When the reaction time increased over 12 h, the secondary growth of Cu-BDC-NH2 took place, which caused a structure with the assembled Cu-BDC-NH2 nanosheets on the surface of Cu2O.
To further identify the structural evolution during the formation of the Cu2O@Cu-BDC-NH2 composites, Fourier transform infrared (FTIR) spectra of the as-prepared Cu2O, Cu2O@Cu-BDC-NH2, and Cu-BDC-NH2 were obtained and the results are presented in Figure 2b. Compared to the FTIR spectrum of Cu2O, Cu2O@Cu-BDC-NH2 presented newly emerging peaks at 3480 and 3367 cm−1, which were attributed to the asymmetric stretching and symmetric stretching modes of the –NH2 group [21]. The bands at 1433 and 1617 cm−1 in Cu2O@Cu-BDC-NH2 were contributed by the symmetric stretching and asymmetric stretching modes of the COO- group, which originated from the H2BDC-NH2 ligand. The bands at 1105 and 630 cm−1 in Cu2O@Cu-BDC-NH2 were assigned to the bond of C–O–Cu and Cu–O, which is consistent with the band in Cu-BDC and Cu2O, respectively. The FTIR data confirmed the successful combination of Cu2O and Cu-BDC-NH2, which is consistent with the above XRD and SEM results.
The thermal stability of Cu2O@Cu-BDC-NH2 was investigated by thermogravimetric analysis (TGA) under a N2 atmosphere. As shown in Figure 2c, the weight loss in the temperature range of 150~300 °C was determined to be 16%, which can be attributed to the liberation of the coordinated DMF molecules, while the weight loss (14%) in the temperature of 300~480 °C corresponds to decomposition of the BDC2- ligand of Cu-BDC [36]. The results of the TGA indicate that the Cu2O@Cu-BDC-NH2-8h presented excellent thermal stability before 300 °C. Furthermore, the remaining solid with 68 wt.% was mainly attributed to Cu2O and other copper containing components. The N2 absorption–desorption isotherms of Cu2O@Cu-BDC-NH2-8h were identified as type II with a Brunauer–Emmett–Teller (BET) specific surface area of 16.6 m2 g−1 (Figure 2d), which was just a little larger than that of Cu2O (13.0 m2 g−1). This result indicates that the amount of the generated Cu-BDC-NH2 nanoparticles in Cu2O@Cu-BDC-NH2-8h was too small to contribute much to the higher specific surface area and porosity.
The XPS spectra were obtained to better understand the oxidation valence of Cu in Cu2O@Cu-BDC-NH2. The typical peaks of C, N, O, and Cu were identified in the full XPS spectrum (Figure 3a) of Cu in Cu2O@Cu-BDC-NH2-8h and the Cu 2p, C 1s, and O 1s spectra were investigated to trace the change in elemental states (Figure 3b–d). As shown in Figure 3b, the peaks at binding energy of 932.7 and 952.8 eV can be ascribed to Cu+ of Cu 2p3/2 and 2p1/2, indicating the existence of Cu2O in the surface of Cu2O@Cu-BDC-NH2-8h. The Cu 2p3/2 peak at the binding energy of 935.0 eV and the Cu 2p1/2 peak at binding energy of 955.2 eV in the Cu 2p spectra can be attributed to the Cu2+, which was contributed from the partial oxidation from Cu+ to Cu2+ during the in situ growth of the Cu-BDC-NH2 on the surface of Cu2O [36], suggesting the formation of the Cu+/Cu2+ interface in the Cu2O@Cu-BDC-NH2-8h composite. Besides, the comparison of the relative intensity of the Cu 2p spectrum indicate that the amount of Cu+ was more than that of Cu2+, and the relative elemental ratio of Cu2+/Cu+ was determined to be 0.699 with the area integration method. The C 1s spectra were also obtained and the groups of –C=O, C–O, and C–C were investigated at the binding energies of 288.3, 285.6, and 284.6 eV (Figure 3c). The O 1s of Cu2O@Cu-BDC-NH2-8h (Figure 3d) represents the peaks at 531.9 and 531.2 eV, which can be attributed to the existence of C=O and C–O/O–H in the absorbed carbonate and hydroxyl species, while the peak at the binding energy of 530.4 eV indicates the Cu–O bond in Cu2O or CuO. To further reveal the electronic change in the Cu ion in the formation of Cu-BDC-NH2 on Cu2O, the Cu 2p spectra of Cu2O, Cu2O@Cu-BDC-NH2-8h, and Cu2O@Cu-BDC-NH2-20h are compared in Figure 3e. The enhanced intensity of Cu2+ from Cu2O to Cu2O@Cu-BDC-NH2 indicates the increasing amount of Cu2+ with the formation of Cu2O@Cu-BDC-NH2 on the surface of the Cu2O, and the 0.15 eV positive shift in the Cu+ 2p3/2 spectra indicates the anti-oxidation of Cu2O and the stability of the Cu2O@Cu-BDC-NH2, which is consistent with the result in the Cu-LMM spectrum (Figure 3f). The XPS results confirmed the formation and evolution of the Cu+/Cu2+ interface between Cu2O and Cu-BDC-NH2 during the in situ growth of Cu2O@Cu-BDC-NH2 as well as the protective effect of the outer MOF shell on the Cu2O core. This is consistent with the above SEM, XRD, and FTIR results.

2.2. Catalytic Properties

The catalytic properties for the selective oxidation of styrene over the as-prepared catalysts under a base-free condition were investigated. The catalytic tests were carried out by using 10 mg of the catalysts at a mild temperature of 40 °C with 30 wt.% H2O2 as the green oxidant and acetonitrile as the solvent. First, the selective oxidation of styrene to benzaldehyde over the Cu2O@Cu-BDC-NH2-8h sample was chosen as a model reaction. The catalytic performance versus reaction time over Cu2O@Cu-BDC-NH2-8h was tested and the results are presented in Figure 4. As shown in Figure 4, initially, the yield of benzaldehyde gradually increased with the increase in reaction time, which began to decline after reaction for 10 h. During this process, a continuous decrease in benzaldehyde selectivity could be observed, which was mainly due to the formation of styrene oxide according to the GC-MS analysis. It is worth noting that an increase in other by-products such as benzoic acid and phenylacetaldehyde were tested with the extension of the reaction time to over 8 h, which was mainly due to the further oxidation of benzaldehyde and styrene oxide [12,37], resulting in a further decrease in the selectivity of benzaldehyde. The highest catalytic performance was archived at 10 h with 85% styrene conversion and 76% benzaldehyde selectivity. As shown in Table 1, the catalytic performance of the as-prepared Cu2O@Cu-BDC-NH2-8h sample was superior to most of the previously reported transition metal/metal oxide-based catalysts, and even comparable to some noble metal-based catalysts.
To explore the influence of the morphologies and structures of materials on the catalytic performance, a series of controlled experiments were conducted. First, the catalytic properties of the as-prepared Cu2O, Cu2O@Cu-BDC-NH2-xh, and Cu-BDC-NH2 were studied to investigate the active components of the Cu2O@Cu-BDC-NH2 composite, and the results are shown in Table 2. By comparing the catalytic performance of CuO (entry 2, Table 2), CuBDC-NH2 (entry 7, Table 2), and the blank experiment without any catalyst (entry 1, Table 2), it can be seen that both Cu2O and CuBDC-NH2 have active components for styrene oxidation, and Cu2O is beneficial to the high selectivity (82%) of benzaldehyde, while CuBDC-NH2 contributes to the high conversion rate of styrene (92%). The above result indicates that it is possible to obtain catalysts with excellent performance by appropriately adjusting the contents of Cu2O and CuBDC-NH2 in the Cu2O@Cu-BDC-NH2 composite. After the in situ growth of Cu-BDC-NH2 on Cu2O for 4 h, there was no obvious increase in the yield of benzaldehyde (entry 3, Table 2) than that of Cu2O, which may be attributed to the small amount of CuBDC-NH2. With the increase in Cu-BDC-NH2 loading, the sample of Cu2O@Cu-BDC-NH2-8h (entry 4, Table 2) showed a sharp increase in styrene conversion from 38% to 85% with a relatively high benzaldehyde selectivity (76%). However, the sample of Cu2O@Cu-BDC-NH2-12h (entry 5, Table 2), Cu2O@Cu-BDC-NH2-20h (entry 6, Table 2) with more Cu-BDC-NH2 loading did not exhibit higher benzaldehyde yield. These results indicate that the excellent performance of Cu2O@Cu-BDC-NH2-8h may be attributed to the Cu2+/Cu+ interface between Cu2O and Cu-BDC-NH2 nanoparticles, while excessive loading of CuBDC-NH2 nanosheets tends to obscure the active interface, limiting the performance of the Cu2O@Cu-BDC-NH2-12h and Cu2O@Cu-BDC-NH2-20h composite. Combined with the results of the XPS analysis, the sample of Cu2O@Cu-BDC-NH2-20h had a higher ratio of Cu2+/Cu+ than Cu2O@Cu-BDC-NH2-8h, therefore the construction of a well-designed Cu2+/Cu+ active interface with suitable ratio of Cu2+/Cu+ [38,39] is key to achieving a high-efficient catalyst of Cu2O@Cu-BDC-NH2.
Additional control experiments were conducted to investigate the effects of reaction conditions (temperature, the solvent, and the amount of H2O2) on catalytic performance. Besides catalysts, the reaction temperature also plays an important role in the reaction process. Therefore, we explored the effect of reaction temperature (30~70 °C) on the styrene oxidation reaction. As shown in Figure 5a, the conversion of styrene over the catalyst of Cu2O@Cu-BDC-NH2-20h at 30 °C was only about 20%, which may be attributed to the difficulty in the activation of H2O2 below 30 °C. A significant increase in the conversion of styrene was observed within 30~40 °C, while no more obvious increase could be found within 40~70 °C. This phenomenon indicates that Cu2O@Cu-BDC-NH2-8h became active at 40 °C, but the intermediate species were more easily decomposed into different products through the cleavage of C = C and CO bands at higher temperature [40,41,42], which resulted in the formation of benzoic acid and phenylacetaldehyde.
Then, the solvent effect was also explored at the reaction temperature of 40 °C. As shown in Figure 5b and Table 3, the conversion of styrene followed the order DMF > CH3CN > MeOH > EtOH > acetone, while the selectivity for benzaldehyde followed the order CH3CN > acetone > EtOH > DMF > MeOH. Among these solvents, CH3CN is more active in catalyzing styrene, with the highest selectivity (76%) for benzaldehyde and higher conversion of styrene (85%), which can be attributed to its high permittivity and lower boiling point [43,44]. Besides, CH3CN has been proven to be able to be miscible for styrene (oil phase) with H2O2 (aqueous phase), thus ensuring a large contact area for efficient styrene oxidation [12].
We also compared the catalytic activities with different amounts of H2O2: styrene (1:1, 3:1, 6:1, and 9:1) at 40 °C with CH3CN as the solvent. As shown in Figure 5c, it can be found that both insufficient and excessive H2O2 are not conducive to the high efficiency of the styrene oxidation to benzaldehyde. The low yield of benzaldehyde with the H2O2/styrene molar ratio of 1:1 can be attributed to the lack of oxidants. The decreased selectivity with higher H2O2/styrene molar ratio of 6:1 and 9:1 is due to the formation of by-products such as benzoic acid catalyzed by hydroxyl radicals generated by the decomposition of excess H2O2 [45,46]. Therefore, the H2O2/styrene molar ratio of 3:1 was selected as the optimum reaction condition.
The metal leaching tests were conducted to investigate the stability of the as-prepared Cu2O@Cu-BDC-NH2-8h catalysts. In a typical catalytic process, the solid catalyst of Cu2O@Cu-BDC-NH2-8h was removed from the reaction mixture by centrifugation after 4 h while the reaction continued for another 6 h. Results showed that the yield of benzaldehyde was 43.9% (styrene conversion: 50.9%, benzaldehyde selectivity 86.3%) in the first 4 h, and no increase in benzaldehyde yields (styrene conversion: 51.6%, benzaldehyde selectivity 84.7%) could be tested when the reaction continued for another 6 h without the solid catalyst, suggesting the superior stability and environmentally-benign properties of the as-prepared Cu2O@Cu-BDC-NH2-8h composite without metal leaching. The powder XRD patterns (Figure 6a) and SEM image (Figure 6b) of the Cu2O@Cu-BDC-NH2-8h catalyst after the catalytic reaction showed that the morphology and structure of the composite were well remained, which further confirmed the above results.
Aside from the Cu2O octahedra (od-Cu2O), the Cu2O cuboctahedrons (cod-Cu2O) were synthesized by adjusting the molecular weight of the added PVP. The morphology of od-Cu2O, cod-Cu2O, and their derived od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 was investigated by SEM. As shown in Figure 7a,c, when using PVP with the molecular weight of 130,000 instead of 58,000, a morphology change in Cu2O from octahedron to cuboctahedron was observed, and the size of cod-Cu2O was similar to that of od-Cu2O, ranging from 1 µm to 2 µm with no obvious agglomeration between the particles. The XRD patterns (Figure 7e) of the two types of Cu2O presented the same diffraction peaks but different relative intensities, indicating the different advantageous crystal planes. Specifically, the intensity of the diffraction peaks at 42.2° in cod-Cu2O was more enhanced than that in od-Cu2O, which corresponded to the shrinkage of {111} facets and the enlargement of {100} facets. The morphology variation can be attributed to the selected absorption of PVP on the {111} facets of Cu2O [34]. After the in situ growth of the Cu-BDC-NH2, both e od-Cu2O@ Cu-BDC-NH2 and cod-Cu2O Cu-BDC-NH2 (Figure 7b,d) showed a rougher surface, indicating the successful growth of Cu-BDC-NH2 nanocrystals on the Cu2O.
The catalytic performance of selective oxidation of styrene on od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 were conducted to investigate the effects of the morphology and structure of Cu2O on their catalytic properties. As shown in Table 4, od-Cu2O@Cu-BDC-NH2 exhibited higher selectivity for benzaldehyde (64%→76%) than that of cod-Cu2O@Cu-BDC-NH2, while no significant difference in the conversion of styrene was observed. This can be attributed to the fact that the one-coordinated copper sites on the exposed (111) crystal plane are favorable for double bond oxidation to aldehyde groups [35]. In contrast, a small amount of styrene oxide (11%) appeared in the reaction catalyzed by cod-Cu2O@Cu-BDC-NH2 that probably originated from the oxygen sites in the exposed (100) crystalline plane, which can promote the formation of olefin epoxide. This phenomenon suggests that the selectivity of different products can be further regulated by controlling the exposed crystal planes of Cu2O.

3. Materials and Methods

3.1. Materials

N,N-dimethylformamide (DMF, AR), acetone (AR), benzaldehyde (AR), epoxy ethane (AR), and methanol (AR) was purchased from Beijing Tong Guang Fine Chemicals Company, Beijing, China. Ethanol (AR), sodium hydroxide (NaOH, AR), ascorbic acid (AR), acetonitrile (AR), sodium thiosulfate (AR), H2O2 (30 wt.%), polyvinylpyrrolidone (PVP, molecular weight: 58,000 and 130,000) were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ltd., Shanghai, China. Copper chloride (CuCl2, AR), copper nitrate trihydrate (Cu(NO3)2, AR), phenylacetaldehyde (>99.5%), and 2-aminoterephthalic acid (H2BDC-NH2, AR) were purchased from Shanghai Macklin Biochemical Co. Ltd., Shanghai, China.

3.2. Synthesis of Cu2O

The octahedron Cu2O (denoted as od-Cu2O) was prepared based on a previously reported method [31] with some modifications. In a typical process, 100 mL of 0.01 mol L−1 CuCl2 aqueous solution and 10 mL of 2 mol L−1 NaOH aqueous solution and 0.6 mol L−1 of ascorbic acid aqueous solution was first prepared. A total of 5 g of PVP (molecular weight: 58,000) was added into 100 mL of 0.01 mol L−1 CuCl2 aqueous solution under constant stirring over two hours. Then, the prepared 2M NaOH aqueous solution was slowly dropped into the above mixed solution until the color of the solution gradually changed from mild milk blue to sky-blue and then to dark brown. After the above mixed solution was immersed in a constant temperature bath for 30 min, the 0.6 mol L−1 ascorbic acid aqueous solution was slowly dropped into the above solution until the color of the solution was changed to brick-red. The turbid solution was obtained by continuing the bath at 55 °C for 2.5 h. The resulting product was washed with deionized water and ethanol several times, collected by centrifugation, and finally dried at room temperature under vacuum condition overnight.
The cuboctahedron Cu2O (denoted as cod-Cu2O): The preparation process of the cod-Cu2O was the same as that of the od-Cu2O, except that the PVP with molecular weight of 58,000 was replaced by the PVP with molecular weight of 130,000.

3.3. Synthesis of Cu2O@Cu-BDC-NH2

od-Cu2O@Cu-BDC-NH2: The od-Cu2O@Cu-BDC-NH2 was prepared by an in situ assembly method. First, the as-prepared od-Cu2O (80 mg) was uniformly dissolved in 20 mL ethanol, and 181.1 mg of 2-aminoterephthalic acid was added into 20 mL of a mixed solution of ethanol and DMF (1:1 vol) under continuous stirring. After stirring well, the two separate solutions were mixed at room temperature with magnetic stirring for different periods (4 h–32 h) to control the growth of MOFs on od-Cu2O. The resulting products of od-Cu2O@Cu-BDC-NH2-xh (x = 4, 8, 12, 20 and 32) were washed by deionized water and ethanol several times, collected by centrifugation, and finally dried at room temperature under vacuum condition overnight.
cod-Cu2O@Cu-BDC-NH2: The preparation process of the cod-Cu2O@Cu-BDC-NH2 was the same as that of the od-Cu2O@Cu-BDC-NH2, except that the precursor was changed from od-Cu2O to cod-Cu2O, and the resulting product of cod-Cu2O@Cu-BDC-NH2-8h was obtained after reaction for 8 h under magnetic stirring.

3.4. Synthesis of Cu-BDC-NH2 Nanosheets

The metal solution (named as M1) was prepared by dissolving 30 mg of Cu(NO3)2 into a mixed solution of 3 mL of DMF and 1 mL of CH3CN. The ligand solution (named as L1) was synthesized by dissolving 30 mg of H2BDC-NH2 into 4 mL of the mixed solution of DMF and CH3CN (1:3 vol). Then, the above solution M1 was added into L1, and the mixture was left to stand under an ambient environment for 24 h. Finally, the resulting product of Cu-BDC-NH2 was collected by centrifugation, washed with DMF three times, and then stored in DMF.

3.5. Characterization

The morphology of the as-prepared nanomaterials was characterized using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan) with an accelerating voltage of 5 kV. The crystallinity and structural details of materials were obtained through X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Germany) using a Cu Kα radiation (λ = 1.541 Å). Fourier-transform infrared (FT-IR) spectra were conducted on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) using the potassium bromide (KBr) pellet technique over a range of 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) data were analyzed using a Thermo ESCALAB 250Xi spectrometer (Thermo Scientific, USA). The nitrogen adsorption–desorption isotherm was collected by using a Micromeritics ASAP2460 instrument (Micromeritics, Norcross, GA, USA) at 77 K. The thermal property was measured using thermogravimetric analysis (TGA, METTLER TOLEDO TGA/DSC3+, Mettler Toledo, Switzerland) in the range of 25–800 °C at a heating–cooling rate of ±10 °C min−1 under a nitrogen gas flow rate of 20 mL min−1.

3.6. Catalytic Tests for Selective Oxidation of Styrene

The catalytic tests for the selective oxidation of styrene were carried out as follows: the as-prepared catalyst (10 mg), styrene (2 mmol), and acetonitrile (5 mL) were added into three-neck round-bottom flask (25 mL) equipped with a reflux condenser. The device was immersed into an oil bath and heated to the desired temperature under magnetic stirring. Then, 6 mmol of 30 wt.% H2O2 aqueous solution was added into the mixture to initiate the reaction. After a desired time, the reaction mixture was centrifuged, and the liquid layer was analyzed by a gas chromatography-mass spectrometer (GC-MS, Agilent 7890/5975C, Agilent, Santa Clara, CA, USA) using the external standard method for quantitative analysis.
The stability of the as-prepared catalyst was investigated by a mental leaching test. In a typical process, the catalytic reaction was stopped after 4 h and the solid catalyst of Cu2O@Cu-BDC-NH2 was separated and removed by centrifugation. The reaction solution without the Cu2O@Cu-BDC-NH2 catalyst was then stirred for a further 6 h, and the catalytic performance was analyzed by GC-MS.

4. Conclusions

In summary, a novel core-shell structured Cu2O@Cu-BDC-NH2 heterogeneous catalyst with tunable Cu+/Cu2+ interface, variable composition, and structure was synthesized by a facile in situ self-assembly method. With H2O2 as green oxidants, the resultant Cu2O@Cu-BDC-NH2 catalysts exhibited significant catalytic performance for the selective oxidation of styrene at 40 °C under base-free conditions, and the optimized conversion of styrene and selectivity of benzaldehyde could reach 85% and 76%, respectively. The delicate combination of Cu2O and Cu-BDC-NH2 not only provides a well-designed Cu+/Cu2+ active interface and porous MOF shells for mass transfer and protection of the active Cu2O component, but also provides appropriate acid–base regulation methods to improve the selectivity of the target product, thus suggesting a new perspective and simple strategies for the construction of a highly efficient and green catalytic system for the selective oxidation of styrene.

Author Contributions

Conceptualization, Methodology, X.Z.; Formal analysis, data curation, X.Z., M.H. and X.T.; Investigation, Data curation, P.W., Z.Z., X.B. and Y.J.; Writing-Original Draft Preparation, Review and Editing, X.Z., M.H. and X.T.; Writing-Review and Editing, X.T. and P.W.; Funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 52002029).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00487 sch001 550
Scheme 1. The illustration of the synthetic process from Cu2O toward Cu2O@Cu-BDC-NH2 for the selective oxidation of styrene with H2O2 as green oxidants.
Scheme 1. The illustration of the synthetic process from Cu2O toward Cu2O@Cu-BDC-NH2 for the selective oxidation of styrene with H2O2 as green oxidants.
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Catalysts 12 00487 g001 550
Figure 1. SEM images of (a) Cu2O, (b) Cu2O@Cu-BDC-NH2-4h, (c) Cu2O@Cu-BDC-NH2-8h, (d) Cu2O@Cu-BDC-NH2-12h, (e) Cu2O@Cu-BDC-NH2-20h, and (f) Cu2O@Cu-BDC-NH2-32h.
Figure 1. SEM images of (a) Cu2O, (b) Cu2O@Cu-BDC-NH2-4h, (c) Cu2O@Cu-BDC-NH2-8h, (d) Cu2O@Cu-BDC-NH2-12h, (e) Cu2O@Cu-BDC-NH2-20h, and (f) Cu2O@Cu-BDC-NH2-32h.
Catalysts 12 00487 g001
Catalysts 12 00487 g002 550
Figure 2. (a) XRD patterns of Cu2O, Cu2O@Cu-BDC-NH2-4h, Cu2O@Cu-BDC-NH2-8h, Cu2O@Cu-BDC-NH2-12h, and Cu2O@Cu-BDC-NH2-20h. (b) FTIR spectra of Cu2O, Cu2O@Cu-BDC-NH2-8h, and Cu-BDC-NH2. (c) The TGA curves and (d) N2 adsorption–desorption isotherms of Cu2O@Cu-BDC-NH2-8h.
Figure 2. (a) XRD patterns of Cu2O, Cu2O@Cu-BDC-NH2-4h, Cu2O@Cu-BDC-NH2-8h, Cu2O@Cu-BDC-NH2-12h, and Cu2O@Cu-BDC-NH2-20h. (b) FTIR spectra of Cu2O, Cu2O@Cu-BDC-NH2-8h, and Cu-BDC-NH2. (c) The TGA curves and (d) N2 adsorption–desorption isotherms of Cu2O@Cu-BDC-NH2-8h.
Catalysts 12 00487 g002
Catalysts 12 00487 g003 550
Figure 3. (a) XPS survey, (b) Cu 2p, (c) C 1s, (d) O 1s spectra of Cu2O@Cu-BDC-NH2-8h. (e) Cu 2p and (f) Cu-LMM XPS spectra of Cu2O, Cu2O@Cu-BDC-NH2-8h, and Cu2O@Cu-BDC-NH2-20h.
Figure 3. (a) XPS survey, (b) Cu 2p, (c) C 1s, (d) O 1s spectra of Cu2O@Cu-BDC-NH2-8h. (e) Cu 2p and (f) Cu-LMM XPS spectra of Cu2O, Cu2O@Cu-BDC-NH2-8h, and Cu2O@Cu-BDC-NH2-20h.
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Catalysts 12 00487 g004 550
Figure 4. The catalytic performance versus reaction time over Cu2O@Cu-BDC-NH2-8h.
Figure 4. The catalytic performance versus reaction time over Cu2O@Cu-BDC-NH2-8h.
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Catalysts 12 00487 g005 550
Figure 5. The effects of (a) temperature, (b) solvent, and (c) the amount of oxidant on the catalytic performance over Cu2O@Cu-BDC-NH2-8h for the selective oxidation of styrene.
Figure 5. The effects of (a) temperature, (b) solvent, and (c) the amount of oxidant on the catalytic performance over Cu2O@Cu-BDC-NH2-8h for the selective oxidation of styrene.
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Catalysts 12 00487 g006 550
Figure 6. (a) XRD patterns and (b) SEM image of Cu2O@Cu-BDC-NH2-8h after the catalytic reaction.
Figure 6. (a) XRD patterns and (b) SEM image of Cu2O@Cu-BDC-NH2-8h after the catalytic reaction.
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Catalysts 12 00487 g007 550
Figure 7. SEM images of (a) od-Cu2O, (b) od-Cu2O@Cu-BDC-NH2, (c) cod-Cu2O, and (d) cod-Cu2O@Cu-BDC-NH2. (e) XRD patterns of od-Cu2O and cod-Cu2O. (f) The catalytic performance of od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 in the selective oxidation of styrene to benzaldehyde.
Figure 7. SEM images of (a) od-Cu2O, (b) od-Cu2O@Cu-BDC-NH2, (c) cod-Cu2O, and (d) cod-Cu2O@Cu-BDC-NH2. (e) XRD patterns of od-Cu2O and cod-Cu2O. (f) The catalytic performance of od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 in the selective oxidation of styrene to benzaldehyde.
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Table
Table 1. Compared catalytic performance of styrene oxidation with H2O2 as the oxidants and a base-free condition over different catalysts.
Table 1. Compared catalytic performance of styrene oxidation with H2O2 as the oxidants and a base-free condition over different catalysts.
YearCatalystsConversion (%)Selectivity (%)Temp. (°C)Time (h)Ref.
BenzaldehydeStyrene
Oxide
2022Cu2O@Cu-BDC-NH28576 4010This work
2022DCP-CTF@Pd-MC9595 658[5]
2021Al2O3-FeOx80-8060–856[37]
2020Ti-MCM-4878.985.2 6012[7]
2020CoFe2O4/TiO296.346.6 9012[3]
2020SO42−-Fe-V/ZrO262.374 804[14]
2019CuCr-MMO82.8-79.7605[12]
2019NiCo2O47830677010[15]
Table
Table 2. Catalytic performance of Cu2O and Cu2O@Cu-BDC-NH2 in selective oxidation of styrene to benzaldehyde a.
Table 2. Catalytic performance of Cu2O and Cu2O@Cu-BDC-NH2 in selective oxidation of styrene to benzaldehyde a.
EntryCatalystReaction Time (h)Conversion (%)Selectivity (%)
1—101972
2Cu2O103182
3Cu2O@Cu-BDC-NH2-4h103878
4Cu2O@Cu-BDC-NH2-8h108576
5Cu2O@Cu-BDC-NH2-12h108865
6Cu2O@Cu-BDC-NH2-20h109156
7Cu-BDC-NH2109243
a Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 °C.
Table
Table 3. Catalytic performance of Cu2O@Cu-BDC-NH2-8h for selective oxidation of styrene with different solvents a.
Table 3. Catalytic performance of Cu2O@Cu-BDC-NH2-8h for selective oxidation of styrene with different solvents a.
EntrySolventDielectric ConstantBoiling Point (°C)Conversion (%)Selectivity (%)
1CH3CN37.5828576
2DMF37.61539358
3Methanol33.6647246
4Ethanol24.3786975
5Acetone20.7563881
a Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 °C.
Table
Table 4. Catalytic performance of od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 in the selective oxidation of styrene a.
Table 4. Catalytic performance of od-Cu2O@Cu-BDC-NH2 and cod-Cu2O@Cu-BDC-NH2 in the selective oxidation of styrene a.
EntryCatalystConversion (%)Selectivity (%)
BenzaldehydeStyrene Oxide
1od-Cu2O@Cu-BDC-NH285767
2cod-Cu2O@Cu-BDC-NH2836411
a Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 °C.
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Han, M.; Tang, X.; Wang, P.; Zhao, Z.; Ba, X.; Jiang, Y.; Zhang, X. Metal-Organic Frameworks Decorated Cu2O Heterogeneous Catalysts for Selective Oxidation of Styrene. Catalysts 2022, 12, 487. https://doi.org/10.3390/catal12050487

AMA Style

Han M, Tang X, Wang P, Zhao Z, Ba X, Jiang Y, Zhang X. Metal-Organic Frameworks Decorated Cu2O Heterogeneous Catalysts for Selective Oxidation of Styrene. Catalysts. 2022; 12(5):487. https://doi.org/10.3390/catal12050487

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Han, Mengyi, Xue Tang, Peng Wang, Zhiyong Zhao, Xiaohua Ba, Yu Jiang, and Xiaowei Zhang. 2022. "Metal-Organic Frameworks Decorated Cu2O Heterogeneous Catalysts for Selective Oxidation of Styrene" Catalysts 12, no. 5: 487. https://doi.org/10.3390/catal12050487

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