Diphenyl carbonate (DPC) is a green engineering thermoplastic intermediate, which has low toxicity and is non- polluting. It is used to synthesize various organic compounds and polymers by halogenation, nitrification, hydrolysis and ammonolysis, and particularly for the preparation of polycarbonates by melt transesterification without the use of phosgene ^[1]. The transesterification of dimethyl carbonate (DMC) and phenol is the most promising method for the industrial production of DPC ^[2]. In view of the difficulties in separating and recycling homogeneous catalysts, increasing attention is being focused on highly active heterogeneous catalysts for the transesterification of DMC and phenol. The heterogeneous catalysts reported to date are single or composite oxides ^[3], zeolites ^[4], hydrotalcite-like compounds ^[5], and heteropoly compounds ^[6]. Although up to 40% yields have been obtained, these catalysts display poor selectivity or reusability. Among them, TiO₂/SiO₂ showed outstanding catalytic activity but was rapidly deactivated as a result of TiO₂ leaching ^[7]. The core-shell technique is the method most commonly used to obtain shell-protected TiO₂ materials to avoid instability of the active component in the core ^{[8, 9]}.

In this study, core-shell TiO₂@SiO₂ ^[10] was prepared using a combination of reverse microemulsion and precipitation methods and was used, for the first time, to catalyze the transesterification of DMC and phenol. The core-shell structure prevents leaching of the active component, i.e., TiO₂, and increases the reusability.

The preparation of TiO₂@SiO₂ is different from that of TiO₂@void@SiO₂ ^{[11, 12]}, which is used in photocatalysis. First, the TiO₂ core was precipitated from peroxo-titanium with ammonia water, and SiO₂ was coated on the TiO₂ by hydrolysis of tetraethoxysilane under alkaline conditions. The transesterification of DMC and phenol was carried out in a 100 mL three-necked round-bottomed flask, equipped with a N₂ inlet, magnetic stirring bar, dropping funnel, and fractionating column connected to a liquid-dividing head. Phenol (AR, Guangdong Guanghua Sci-tech Co., Ltd., China) and the catalyst were introduced into the flask under N₂. DMC (≥ 99%, Huasheng Co., Ltd., Shandong University of Petroleum, China) was added dropwise after the mixture was heated to 175 °C. Qualitative and quantitative analyses of the products were conducted using gas chromatography-mass spectrometry (Agilent 6890/ 5973) and gas chromatography (Agilent 7820A), respectively.

A transmission electron microscopy (TEM) image of TiO₂@SiO₂ is shown in Fig. 1. TiO₂@SiO₂ has a clear bilayer structure with a TiO₂ core of diameter 220–300 nm and a SiO₂ shell of thickness 40–60 nm. N₂ adsorption (BET) analysis showed that the SiO₂ shell has a mesoporous structure and an average pore diameter of 4.6 nm, which is favorable for the diffusion of reactants and products.

Fig. 1. TEM image of TiO2@SiO2.

The Fourier-transform infrared (FTIR) spectra of SiO₂ and TiO₂@SiO₂ are shown in Fig. 2. The peak at 1628 cm⁻¹ corresponds to the O–H stretching vibration of adsorbed water, and the peaks at 1096, 805, and 472 cm⁻¹ are attributed to the asymmetric stretching, symmetric stretching and bending vibrations, respectively, of Si–O–Si bonds. For SiO₂, the weak peak at 956 cm⁻¹ is assigned to the stretching vibration of Si–OH ^[13]. However, for TiO₂@SiO₂, the Si–O peak at 956 cm⁻¹ is blue-shifted to 964 cm⁻¹; this is considered to be the result of TiO₂ and SiO₂ interacting and forming Ti–O–Si bonds ^[14]. The peak at 964 cm⁻¹ is clearly observed for TiO₂@SiO₂ that has been used four times.

Fig. 2. Fourier-transform infrared spectra of SiO2, TiO2@SiO2, and recycled TiO2@SiO2.

The X-ray diffraction patterns of TiO₂@SiO₂ calcined at 200–500 °C are shown in Fig. 3. The calcination temperature greatly influences the crystalline structure of TiO₂@SiO₂. Anatase peaks are not detected for the TiO₂@SiO₂ sample calcined at 200 °C, which indicates that TiO₂ is amorphous or microcrystalline. When the calcination temperature is increased to 250 °C, the diffraction peak of anatase at about 25° (2θ) appears. When the calcination temperature is increased to 300 °C, the peak at 25° strengthens, and anatase peaks at 37.8°, 48.0°, 53.7°, and 62.5° appear. With further increases in the calcination temperature, the diffraction peaks of anatase TiO₂ are further enhanced, and the numbers of crystal defects and intercrystalline disordered structures decrease, indicating that the crystal structure of anatase TiO₂ tends to integrate with increasing calcination temperature. At calcination temperatures of 400 and 500 °C, TiO₂@SiO₂ has an integrated anatase crystal structure.

Fig. 3. X-ray diffraction patterns of TiO2@SiO2 calcined at different temperatures. (1) 200 °C; (2) 250 °C; (3) 300 °C; (4) 400 °C; (5) 500 °C.

The catalytic activities of TiO₂@SiO₂ in the transesterification of DMC and phenol are listed in Table 1. The calcination temperature strongly influences the catalytic performance of TiO₂@SiO₂. The catalytic activity declines with increasing calcination temperature from 200 to 500 °C, and phenol conversion reaches 41.8% at 200 °C. The Ti in amorphous TiO₂ is tetracoordinated, and tetracoordinated Ti is highly active in the transesterification of DMC and phenol ^[15]. However, the Ti in anatase TiO₂ is six-coordinated, and six-coordinated Ti is inactive in this transesterification. The amount of tetracoordinated Ti in TiO₂@SiO₂ decreases, and that of six-coordinated Ti increases with increasing calcination temperature. Consequently, the catalytic activity of TiO₂@SiO₂ decreases with increasing calcination temperature. A comparison of the activities of amorphous TiO₂ and TiO₂@SiO₂ calcined at 200 °C shows that the phenol conversion with amorphous TiO₂ is just 23.3%, whereas that with TiO₂@SiO₂ is 41.8%. The FTIR spectra in Fig. 2 show that TiO₂ and SiO₂ interact and form Ti–O–Si bonds. The formation of Ti–O–Si bonds is considered to be an important factor for high catalytic activity of TiO₂@SiO₂.

Table 1 Effect of calcination temperature on catalytic activity of core-shell TiO2@SiO2. Calcination temperature (°C) Phenol conversion (%) Transesterification selectivity (%) Yield (%) MPC DPC TiO2 * 23.3 99.8 8.7 14.5 200 41.8 100 17.7 24.1 250 32.1 99.8 17.8 14.3 300 23.9 99.8 12.5 11.3 400 4.5 96.7 3.2 1.1 500 5.0 99.1 3.8 1.2 Reaction conditions: phenol 160 mmol, DMC 160 mmol, catalyst 0.20 g, reaction temperature 150–180 °C, reaction time 9 h. * Amorphous TiO2 prepared by the same method as TiO2@SiO2 without adding SiO2 with calcination temperature of 200 °C.

Table 1 Effect of calcination temperature on catalytic activity of core-shell TiO2@SiO2.

The effects of the amount of TiO₂@SiO₂ on the transesterification of DMC and phenol are shown in Fig. 4. The phenol conversion increases with increasing catalyst amount, and the phenol conversion and transesterification selectivity reach 41.8% and 100%, respectively, with 0.20 g of TiO₂@SiO₂. When the amount of TiO₂@SiO₂ is higher than 0.20 g, the phenol conversion changes slightly; 0.20 g is therefore the optimal amount of TiO₂@SiO₂.

Fig. 4. Effect of amount of TiO2@SiO2 on transesterification of DMC and phenol. Reaction conditions: phenol 160 mmol, DMC 160 mmol, calcination temperature 200 °C, reaction temperature 150–180 °C, reaction time 9 h.

The SiO₂ shell prevents leaching of TiO₂ from TiO₂@SiO₂ and increases the reusability. The used TiO₂@SiO₂ was recovered by filtration, washed with DMC, and dried at 200 °C. Figure 5 shows that the catalytic activity of recycled TiO₂@SiO₂ is similar to that of the fresh catalyst. The phenol conversion is above 40%, and the transesterification selectivity remains at 100% over four runs. These results demonstrate that TiO₂@SiO₂ has excellent reusability. Moreover, the Ti content is 51.9% for fresh TiO₂@SiO₂ and is 51.8% for TiO₂@SiO₂ after four cycles; this shows that the SiO₂ shell effectively prevents leaching of the active component, TiO₂.

Fig. 5. Reusability of TiO2@SiO2. Reaction conditions: phenol 160 mmol, DMC 160 mmol, catalyst 0.20 g, calcination temperature 200 °C, reaction temperature 150–180 °C, reaction time 9 h.

In summary, the TiO₂@SiO₂ prepared using a combination of reverse microemulsion and precipitation methods can be used as a heterogeneous catalyst for the transesterification of DMC and phenol. When TiO₂@SiO₂ was calcined at 200 °C and the catalyst amount was 0.20 g, the phenol conversion and transesterification selectivity were 41.8% and 100%, respectively. TiO₂ and SiO₂ interact and form Ti–O–Si bonds, and this improves the catalytic performance of TiO₂@SiO₂. The core-shell structure effectively prevents leaching of TiO₂ and increases the TiO₂@SiO₂ reusability. Core-shell TiO₂@SiO₂ is therefore a highly efficient heterogeneous catalyst for the transesterification of DMC and phenol and has promising applications.