The synthesis of diaryl ethers by O-arylation of phenols with aryl halides is one of the most significant reactions in modern organic synthesis because diaryl ethers are important structural units in many biologically active compounds, industrial polymers, and natural products. Initially, the Ullmann C-O coupling of phenols with aryl halides was used to synthesize diaryl ethers in the presence of Cu powder [1]. However, this method requires harsh reaction conditions, such as a large excess of the phenol, long reaction times, high temperatures (>200 ℃) and stoichiometric amounts of Cu, which restricts its applications [2]. Much research has focused on overcoming these limitations and good progress has been achieved in this area. The development of Pd(0) - and Cu(I)-based catalytic systems enabled major breakthroughs in coupling reactions [3-5]. However, Pd-based approaches suffer from drawbacks such as the high cost of Pd and the use of expensive ligands, therefore large- scale industrial applications of this method are limited [3, 5]. Cu-based catalysts have been receiving increasing attention as cheap and readily available catalysts for Ullmann-type C-O coupling reactions.

In homogeneous reaction systems, Cu(I)-based catalysts show high activities and selectivities for O-arylation in the presence of an organic ligand, but separation of the products and reuse of the catalysts are difficult [6, 7]. Heterogeneous Cu-based catalysts for the C-O coupling reaction have therefore attracted attention [8]. Cu nanoparticles and supported Cu nanoparticles have been used as catalysts for O-arylation reactions and showed high catalytic activities [9-11]. Nano-CuO and CuO nanoparticles supported on carbon nanotubes modified with iron oxide showed to be highly efficient and reusable catalysts for the C-O cross-coupling of phenols with aryl halides [12, 13]. Park et al. [14] reported that Cu₂O nanocubes of size about 45 nm were highly active and recyclable catalyst for the cross-coupling of aryl halides and phenols. Cu₂O nanoparticles show high catalytic activity in C-O cross-coupling reactions, but they are unstable and easily oxidized in moist air. Recent results have indicated that graphene could stabilize Cu₂O nanoparticles and protect them from oxidation, and graphene-supported Cu₂O nanoparticles showed high catalytic activity in C-O cross-coupling reactions [15, 16].

Cubic SiC has excellent chemical stability, and thermal and electrical conductivities. It has been used as support in various catalytic and photocatalytic reactions [17-21]. Li et al. [22] modified SiC nanoparticles with Cu₂O by the reduction of a Cu(OH)₂ precipitate using N₂H₄·H₂O, and the obtained Cu₂O/SiC catalyst was used to photocatalytically reduce CO₂ to CH₃OH under visible-light irradiation. In this work, we used SiC as a support to stabilize Cu₂O nanoparticles because a p-n junction can be formed between Cu₂O and SiC. The Cu₂O/SiC catalyst was prepared using a two-step liquid-phase method and showed high activity in the O-arylation of phenols by aryl halides, with high yields and turnover frequencies (TOFs).

The detailed preparation of β-SiC with a specific surface area of 50 m²/g has been reported elsewhere [23]. A Cu₂O/SiC catalyst with a Cu loading of 5 wt% was prepared via a two-step liquid-phase route. In the first step, SiC (50 mg) and cupric acetate monohydrate (8.26 mg) were dispersed in absolute ethanol under sonication. The suspension was placed in a heated oven at 60 ℃ to obtain a mixture of SiC and copper acetate. In the second step, the resulting mixture was dispersed in diethylene glycol (80 mL) and reduced at 180 ℃ for 2 h to obtain the Cu₂O/SiC catalyst.

For comparison, Cu₂O/Al₂O₃ and Cu₂O/SiO₂ were prepared using the same method as was used for the Cu₂O/SiC catalyst. CuO/SiC was prepared by calcination of Cu₂O/SiC in a muffle furnace at 400 ℃ for 4 h in air. The Cu loading for each catalyst was 5 wt%.

The crystalline phases of the catalyst were identified by X-ray diffraction (XRD, Rigaku D-Max/RB X-ray diffractometer) using Cu K_α radiation. The structure and morphology of the catalyst were examined using field-emission scanning electron microscopy (SEM, S-4800, JEM-2100F) and transmission electron microscopy (TEM, JSM-7001F). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 3 MKII de VG spectrometer with a Mg K_α X-ray source. H₂ temperature-programmed reduction (H₂-TPR) was performed using an automatic adsorption instrument (TP-5080, Xianquan, China). The Cu loadings of the fresh and recycled Cu₂O/SiC catalysts were determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo iCAP6300, USA).

The catalytic O-arylation reactions were conducted in a sealed stainless-steel reactor. A phenol (14 mmol), aryl halide (14 mmol), Cs₂CO₃ (14 mmol), tetrahydrofuran (THF, 10 mL), and the Cu₂O/SiCcatalyst (10 mg) were placed in the reactor. The reaction was performed under Ar at 150 ℃ for 3 h. After the reaction, the suspension was collected and centrifuged. The supernatant was analyzed using gas chromatography-mass spectrometry (Bruker SCION SQ 456GC-MS, Germany). The yields and TOFs were calculated based on the aryl halide using the following equations:

Yield = Conversion (%) x selectivity (%)

= (1 - n_a/n_b) x n_tp/(n_tp + n_bp) x 100%

TOF = Amount of aryl halides (mol) x conversion (%) x selectivity (%)/(mass of Cu₂O/SiC (g) x Cu₂O loading (%) x reaction time (h)/M[Cu₂O] (g/mol))

where n_ais the moles of aryl halide after reaction, n_b is the moles of aryl halide before reaction, n_tp is the moles of target product after reaction, and n_bp is the moles of byproduct after reaction.

Fig. 1 shows the XRD pattern of the Cu₂O/SiC catalyst. The diffraction peaks at 35.76°, 41.46°, 60.13°, 71.89°, and 75.72° correspond to β-SiC [18, 23]. The peak at 33.6° arises from stacking faults in SiC [24]. The diffraction peaks at 29.58°, 36.51°, 42.30°, 61.43°, and 73.57° can be indexed to the (110), (111), (200), (220), and (311) lattice planes of Cu₂O, respectively [25, 26], indicating that Cu is present on the SiC support primarily as the Cu₂O phase.

XPS was used to examine the chemical states and surface properties of the Cu₂O/SiC catalyst. As shown in Fig. 2(a), the peaks for Cu 2p_3/2 and Cu 2p_1/2 observed at around 932.5 and 952.4 eV are attributed to Cu⁺. This is in good agreement with the reported values for Cu₂O [27-29]. This confirms that the main species dispersed on the SiC support is Cu₂O. Fig. 2(b) shows a Cu LMM Auger peak at 569 eV, which is assigned to Cu₂O [26]. These results show that Cu₂O nanoparticles can be stabilized by using β-SiC as the support.

SEM and TEM were used to examine the morphology and crystal structure of the catalyst. Fig. 3(a) and (b) show that the Cu₂O nanoparticles are well dispersed on the β-SiC surface. Fig. 3(c) clearly shows that the Cu₂O nanoparticles are cubic, and based on statistical analysis, the mean diameter of the Cu₂O nanoparticles is about 61 nm. Fig. 3(d) shows that the Cu₂O nanoparticles have clear lattice fringes, and the interplanar spacing of the nanoparticles is 0.25 nm, corresponding to the (111) plane of Cu₂O [15], which matches the XRD results well. These results suggest that the Cu component in the Cu₂O/SiC catalyst is Cu₂O.

Fig. 4 shows the H₂-TPR results for Cu₂O/SiC and CuO/SiC; the results for Cu₂O/Al₂O₃ and Cu₂O/SiO₂ are also showed for comparison. The CuO/SiC reduction peak appears at around 287 ℃, corresponding to the reduction of Cu²⁺ to Cu⁰. Only one Cu₂O/SiC reduction peak is observed, at around 302 ℃, i.e., at a higher temperature than for CuO/SiC. Two reduction peaks are observed for the Cu₂O/Al₂O₃ catalyst because of partial oxidation of Cu₂O. According to a previous study, CuO is more easily reduced than Cu₂O [30]. Therefore, we can conclude that the reduction peaks for the Cu₂O/Al₂O₃ catalyst at 233 and 287 ℃ correspond to the reduction of Cu²⁺ to Cu⁰ and of Cu⁺ to Cu⁰, respectively. Similarly, the reduction peaks for Cu₂O/SiO₂ at 236 and 292 ℃ are associated with the reduction of Cu²⁺ to Cu⁰ and of Cu⁺ to Cu⁰, respectively. A comparison of the peaks for CuO/SiC, Cu₂O/Al₂O₃, and Cu₂O/SiO₂ shows that the reduction peak of Cu₂O/SiC at around 302 ℃ can attributed to the reduction of Cu⁺ to Cu⁰, suggesting that Cu is primarily dispersed on SiC as the Cu₂O phase.

The effects of the reaction conditions, namely temperature, reaction time, amount of base, and amount of catalyst, on the Ullmann-type C-O cross-coupling reaction were investigated; the results are summarized in Table 1. Entries 1-5 show that the yield of diphenyl ether increased with reaction time. When the reaction time was increased from 0.5 to 3 h, the diphenyl ether yield increased from 73% to 97%. However, lengthening the reaction time to 3.5 h did not further increase the yield. We therefore chose 3 h as the reaction time.

The reaction temperature is also an important factor in the Ullmann-type C-O cross-coupling reaction. The yield of diphenyl ether increased from 10% to 97% with increasing reaction temperature from 80 to 150 ℃, therefore, subsequent reactions were performed at 150 ℃ (Table 1, entries 6-9). Increasing the amounts of the base and catalyst also led to the increase in the diphenyl ether yield. The yield of diphenyl ether reached 97% when 1.0 equiv. of Cs₂CO₃ and 10 mg of the Cu₂O/SiC catalyst were used (Table 1, entries 10-16). In conclusion, the optimum reaction conditions are as follows: iodobenzene 14 mmol, phenol 14 mmol, Cs₂CO₃ 1.0 equiv., Cu₂O/SiC 10 mg, and THF 10 mL, 150 ℃, 3 h, and an Ar atmosphere.

Table 2 shows the results for the O-arylation of substituted phenols with aryl halides using Cu₂O/SiC as the catalyst. The Cu₂O/SiC catalyst showed high activity in the O-arylation of phenol and iodobenzene, with a yield of 97%; the TOF was 1136 h^-1, which is much higher than those of most heterogeneous catalysts [13, 16] (Table 2, entry 1). The electronic effects of substituent groups on iodobenzene and phenol on the yield of the O-arylation reaction were studied.

For iodobenzenes with electron-donating substituents such as methyl and methoxy groups the yields were slightly lower (Table 2, entries 2-4). In contrast, the yields were slightly increased by the presence of electron-withdrawing groups such as acetyl and nitro groups on iodobenzene (Table 2, entries 5 and 6). However, for substituted phenols, electron- withdrawing and electron-donating groups both decreased the yields, because of an increase in the number of side reactions, but all the yields were still higher than 80% (Table 2, entries 7-11).

The effects of different aryl halides on the O-arylation yield were also investigated. The yields decreased significantly, in the order iodobenzene > bromobenzene >chlorobenzene, because of the low reactivities of bromobenzene and chlorobenzene (Table 2, entries 12-17).

We also investigated the C-S cross-coupling reactions of thiophenol and iodobenzene and substituted iodobenzenes. The results are shown in Table 3. The product yields were slightly higher because the reactivity of thiophenol is higher than that of phenol in the cross-coupling reaction.

All the above results suggest that Cu₂O/SiC is a highly active heterogeneous catalyst for the O-arylation of phenol, substituted phenols, and thiophenol with iodobenzene or substituted iodobenzenes, and the TOF values achieved with the Cu₂O/SiC catalyst for these reactions are much higher than those for most heterogeneous catalysts. The catalyst also shows some catalytic activity in the O-arylation of phenol and substituted phenols with bromobenzene and chlorobenzene.

The high activity of Cu₂O/SiC in the Ullmann-type C-O cross-coupling reaction probably originates from the p-n junction between SiC and Cu₂O. Cu₂O is a p-type semiconductor. The β-SiC we used was an n-type semiconductor, based on the positive slope in the Mott-Schottky curve (Fig. 5) [31]. When Cu₂O nanoparticles are dispersed on the β-SiC surface, the electrons in SiC diffuse to Cu₂O and the holes in Cu₂O diffuse to SiC until they reach a balance. A p-n junction is then formed [32], which enhances the O-arylation activity.

The recyclability of the Cu₂O/SiC catalyst was investigated by performing the O-arylation of phenol with iodobenzene under the optimum conditions. The data in Table 4 show that the yield in the first run was 97%; it decreased to 84% in the second run and to 64% after five runs. The decrease in the activity is caused by loss of the active component (Cu₂O). ICP-OES showed that the Cu content of fresh Cu₂O/SiC was 4.81 wt%, and it decreased to 2.53 wt% after use. Weak interactions between Cu₂O nanoparticles and SiC may result in the loss of active components. It is therefore necessary to develop methods for strengthening the forces between Cu₂O and SiC to improve the reusability of the Cu₂O/SiC catalyst. This could perhaps be achieved by adjusting the thermal treatment conditions or modifying the SiC surface by chemical oxidation with concentrated sulfuric acid or permanganate to increase the number of surface functional groups on SiC.

We prepared a Cu₂O/SiC catalyst via a simple two-step liquid-phase method. The XRD, XPS, SEM, TEM, and H₂-TPR results showed that Cu species were dispersed on the SiC primarily as the Cu₂O phase. The effects of the reaction conditions, i.e., temperature, reaction time, amount of base used, and amount of catalyst used, were investigated in the O-arylation of iodobenzene with phenol. The optimum conditions were iodobenzene (14 mmol), phenol (14 mmol), Cs₂CO₃ (1 equiv), and THF (10 mL), catalyzed by Cu₂O/SiC (10 mg) at 150 ℃ for 3 h under Ar. A high yield of diphenyl ether and a TOF of 1136 h^-1 were achieved under the optimum conditions. The high activity of the Cu₂O/SiC catalyst probably originates from the p-n junction between SiC and Cu₂O. The Cu₂O/SiC catalyst was used with various substrates to obtain a wide range of diaryl ethers. It also showed high activity in the Ullmann C-S coupling reaction, suggesting that this catalyst has general applicability to Ullmann-type cross-coupling reactions.