**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 [3–5]. 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 H2O<sup>2</sup> catalytic system usually generates benzaldehyde, accompanied with styrene oxide, and the acidity of H2O<sup>2</sup> easily leads to the isomerization of styrene oxide into phenylacetaldehyde, which further decreases the selectivity of benzaldehyde [9–11]. 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 [10–12]. 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.

**Citation:** 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

Academic Editors: Carolina Belver and Jorge Bedia

Received: 27 March 2022 Accepted: 25 April 2022 Published: 26 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Recently, transition metal-based materials such as Mg/CTAB [13], SO<sup>4</sup> <sup>2</sup>−-Fe-V/ZrO<sup>2</sup> [14], CuCr-MMO [12], NiCo2O<sup>4</sup> [15], and CoFe2O4/TiO<sup>2</sup> [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 [16–18]. 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 [21–26]. 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 [27–30].

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-NH<sup>2</sup> catalysts exhibited a highly catalytic performance with H2O<sup>2</sup> as green oxidants. The basicity of Cu2O and the introduction of the –NH<sup>2</sup> 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-NH<sup>2</sup> 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-NH<sup>2</sup> have contributed to the enhancement of styrene conversion and benzaldehyde selectivity. In addition, a series of Cu2O@Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> 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<sup>+</sup> 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 [33–35]. The Cu2O@Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> for the in situ growth of Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> composites can be adjusted by controlling the reaction time of in situ assembly.

**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-NH<sup>2</sup> for the selective oxidation of styrene with H2O<sup>2</sup> as green oxidants.

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 [16–18,33–35]. 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 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 [16–18,33–35]. The in situ growth of Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> composites. As shown in Figure 1b, when Cu2O reacted with H2BDC-NH<sup>2</sup> 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-NH<sup>2</sup> [33], indicating the gradual formation of the Cu-BDC-NH2. After the in situ growth of Cu-BDC-NH<sup>2</sup> on Cu2O for 12 h, 20 h, and 32 h, as shown in Figure 1d–f, the Cu2O@Cu-BDC-NH<sup>2</sup> composites exhibited a smaller Cu2O octahedron around with assembled nanosheets, and the corresponding XRD patterns presented enhanced peak intensity of Cu-BDC-NH<sup>2</sup> (Figure 2a). The morphological and structural changes during the in situ growth process of Cu2O@Cu-BDC-NH<sup>2</sup> indicate that Cu2O serves as the substrate and source of the Cu ions in the following growth of Cu-BDC-NH<sup>2</sup> nanocrystals. The growth mechanism is similar to the previous reports [36]: initially, the Cu<sup>+</sup> 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-NH<sup>2</sup> to construct Cu-BDC-NH2. However, it should be noted that Cu-BDC-NH<sup>2</sup> 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<sup>+</sup> 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-NH<sup>2</sup> took place, which caused a structure with the assembled Cu-BDC-NH<sup>2</sup> nanosheets on the surface of Cu2O.

Cu-BDC-NH2 nanosheets on the surface of Cu2O.

**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-NH<sup>2</sup> -4h, (**c**) Cu2O@Cu-BDC-NH<sup>2</sup> -8h, (**d**) Cu2O@Cu-BDC-NH<sup>2</sup> -12h, (**e**) Cu2O@Cu-BDC-NH<sup>2</sup> -20h, and (**f**) Cu2O@Cu-BDC-NH<sup>2</sup> -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.

**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. **Figure 2.** (**a**) XRD patterns of Cu2O, Cu2O@Cu-BDC-NH<sup>2</sup> -4h, Cu2O@Cu-BDC-NH<sup>2</sup> -8h, Cu2O@Cu-BDC-NH<sup>2</sup> -12h, and Cu2O@Cu-BDC-NH<sup>2</sup> -20h. (**b**) FTIR spectra of Cu2O, Cu2O@Cu-BDC-NH<sup>2</sup> -8h, and Cu-BDC-NH<sup>2</sup> . (**c**) The TGA curves and (**d**) N<sup>2</sup> adsorption–desorption isotherms of Cu2O@Cu-BDC-NH<sup>2</sup> -8h.

To further identify the structural evolution during the formation of the Cu2O@Cu-BDC-NH<sup>2</sup> composites, Fourier transform infrared (FTIR) spectra of the as-prepared Cu2O, Cu2O@Cu-BDC-NH2, and Cu-BDC-NH<sup>2</sup> were obtained and the results are presented in Figure 2b. Compared to the FTIR spectrum of Cu2O, Cu2O@Cu-BDC-NH<sup>2</sup> presented newly emerging peaks at 3480 and 3367 cm−<sup>1</sup> , which were attributed to the asymmetric stretching and symmetric stretching modes of the –NH<sup>2</sup> group [21]. The bands at 1433 and 1617 cm−<sup>1</sup> in Cu2O@Cu-BDC-NH<sup>2</sup> were contributed by the symmetric stretching and asymmetric stretching modes of the COO- group, which originated from the H2BDC-NH<sup>2</sup> ligand. The bands at 1105 and 630 cm−<sup>1</sup> in Cu2O@Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> was investigated by thermogravimetric analysis (TGA) under a N<sup>2</sup> 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 N<sup>2</sup> 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 m<sup>2</sup> g −1 (Figure 2d), which was just a little larger than that of Cu2O (13.0 m<sup>2</sup> g −1 ). This result indicates that the amount of the generated Cu-BDC-NH<sup>2</sup> 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<sup>+</sup> 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<sup>+</sup> to Cu2+ during the in situ growth of the Cu-BDC-NH<sup>2</sup> 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<sup>+</sup> was more than that of Cu2+, and the relative elemental ratio of Cu2+/Cu<sup>+</sup> 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-NH<sup>2</sup> 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-NH<sup>2</sup> indicates the increasing amount of Cu2+ with the formation of Cu2O@Cu-BDC-NH<sup>2</sup> on the surface of the Cu2O, and the 0.15 eV positive shift in the Cu<sup>+</sup> 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-NH<sup>2</sup> during the in situ growth of Cu2O@Cu-BDC-NH<sup>2</sup> 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.

**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-NH<sup>2</sup> -8h. (**e**) Cu 2p and (**f**) Cu-LMM XPS spectra of Cu2O, Cu2O@Cu-BDC-NH<sup>2</sup> -8h, and Cu2O@Cu-BDC-NH<sup>2</sup> -20h.

#### *2.2. Catalytic Properties 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 asprepared 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-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.% H2O<sup>2</sup> 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.

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-NH<sup>2</sup> were studied to investigate the active components of the Cu2O@Cu-BDC-NH<sup>2</sup> composite, and the results are shown in Table 2. By comparing the catalytic performance of CuO (entry 2, Table 2), CuBDC-NH<sup>2</sup> (entry 7, Table 2), and the blank experiment without any catalyst (entry 1, Table 2), it can be seen that both Cu2O and CuBDC-NH<sup>2</sup> have active components for styrene oxidation, and Cu2O is beneficial to the high selectivity (82%) of benzaldehyde, while CuBDC-NH<sup>2</sup> 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-NH<sup>2</sup> in the Cu2O@Cu-BDC-NH<sup>2</sup> composite. After the in situ growth of Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> 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-NH<sup>2</sup> 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<sup>+</sup> interface between Cu2O and Cu-BDC-NH<sup>2</sup> nanoparticles, while excessive loading of CuBDC-NH<sup>2</sup> 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<sup>+</sup> than Cu2O@Cu-BDC-NH2-8h, therefore the construction of a well-designed Cu2+/Cu<sup>+</sup> active interface with suitable ratio of Cu2+/Cu<sup>+</sup> [38,39] is key to achieving a high-efficient catalyst of Cu2O@Cu-BDC-NH2. *Catalysts* **2022**, *12*, 487 7 of 16

**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-NH<sup>2</sup> -8h.

**Table 1.** Compared catalytic performance of styrene oxidation with H2O2 as the oxidants and a basefree condition over different catalysts. **Table 1.** Compared catalytic performance of styrene oxidation with H2O<sup>2</sup> as the oxidants and a base-free condition over different catalysts.

2019 CuCr-MMO 82.8 - 79.7 60 5 [12] 2019 NiCo2O4 78 30 67 70 10 [15]

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,

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-


V/ZrO2


**Table 2.** Catalytic performance of Cu2O and Cu2O@Cu-BDC-NH<sup>2</sup> in selective oxidation of styrene to benzaldehyde <sup>a</sup> . **Entry Catalyst Reaction Time (h) Conversion (%) Selectivity (%)** 

**Table 2.** Catalytic performance of Cu2O and Cu2O@Cu-BDC-NH2 in selective oxidation of styrene to

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.

*Catalysts* **2022**, *12*, 487 8 of 16

benzaldehyde a.

<sup>a</sup> Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 ◦C. temperature 40 °C.

Additional control experiments were conducted to investigate the effects of reaction

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 H2O<sup>2</sup> 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 [40–42], which resulted in the formation of benzoic acid and phenylacetaldehyde. 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 [40–42], which resulted in the formation of benzoic acid and phenylacetaldehyde.

**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-NH<sup>2</sup> -8h for the selective oxidation of styrene.

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 H2O<sup>2</sup> (aqueous phase), thus ensuring a large contact area for efficient styrene oxidation [12].

1


**Table 3.** Catalytic performance of Cu2O@Cu-BDC-NH<sup>2</sup> -8h for selective oxidation of styrene with different solvents <sup>a</sup> .

<sup>a</sup> Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 ◦C.

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 H2O<sup>2</sup> 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 H2O<sup>2</sup> [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.

**Figure 6.** (**a**) XRD patterns and (**b**) SEM image of Cu2O@Cu-BDC-NH<sup>2</sup> -8h after the catalytic reaction.

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-NH<sup>2</sup> and cod-Cu2O@Cu-BDC-NH<sup>2</sup> was investigated by SEM. As shown in Figure 7a,c, when using PVP with

tion.

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-NH<sup>2</sup> and cod-Cu2O Cu-BDC-NH<sup>2</sup> (Figure 7b,d) showed a rougher surface, indicating the successful growth of Cu-BDC-NH<sup>2</sup> nanocrystals on the Cu2O. 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.

**Figure 6.** (**a**) XRD patterns and (**b**) SEM image of Cu2O@Cu-BDC-NH2-8h after the catalytic reac-

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

*Catalysts* **2022**, *12*, 487 10 of 16

The catalytic performance of selective oxidation of styrene on od-Cu2O@Cu-BDC-NH<sup>2</sup> and cod-Cu2O@Cu-BDC-NH<sup>2</sup> 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-NH<sup>2</sup> 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-NH<sup>2</sup> 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.


**Table 4.** Catalytic performance of od-Cu2O@Cu-BDC-NH<sup>2</sup> and cod-Cu2O@Cu-BDC-NH<sup>2</sup> in the selective oxidation of styrene <sup>a</sup> .

<sup>a</sup> Reaction conditions: 10 mg of catalysts, 2 mmol of styrene, 6 mmol of H2O2, 5 mL of acetonitrile, temperature 40 ◦C.

#### **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), H2O<sup>2</sup> (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.
