High-Performance Na-CH3ONa/γ-Al2O3 Catalysts for High-Efficiency Conversion of Phenols to Ethers.

An efficient alkaline catalyst with a porous structure (Na-CH3ONa/γ-Al2O3) was prepared by the melting method. The wastewater from the semicoke plant (WWSCP) was extracted multiple times with isometric dimethyl carbonate (DMC)-cyclohexane mixed solvent at room temperature to obtain an organic phase (OP) with a high concentration of phenols. Ether (OPCP) was obtained by catalytic conversion of OP over catalyst Na-CH3ONa/γ-Al2O3 at 210 °C and with a reaction time of 2.5 h. Both OP and OPCP were analyzed with a gas chromatograph/mass spectrometer (GC/MS) and a quadrupole Exactive Orbitrap mass spectrometer (QPEOTMS). The results showed that only DMC, phenol, o-cresol, and other monohydric phenols were detected in OP, and only other saturated ethers such as anisole and O-methylanisole were detected in OPCP. Through the study of the catalytic conversion of the WWSCP-related model compound, it was found that Na-CH3ONa/γ-Al2O3 could effectively activate (deprotonate) phenol into phenate, and the strong nucleophilic oxyanion of phenate would attack the methyl carbon and carbonyl carbon on DMC to obtain methyl and methoxy groups. Thereby, phenate can be combined with methyl and methoxy groups to acquire the product anisole. In addition, the catalyst Na-CH3ONa/γ-Al2O3 was found to still have high catalytic activity after 10 repeated cycles. It was speculated that this was related to the abundant microporous and mesoporous structure of the catalyst Na-CH3ONa/γ-Al2O3.

Introduction

Anisole is an important organic synthesis intermediate, pharmaceutical intermediate, and fine chemical product. It can be used as antioxidants, plastic stabilizers, pesticides, dyes, and perfumes.[1,2] In industry, phenol used in the production of anisole is obtained through other organic synthesis reactions, which faces high production costs. However, the concentration of phenols in the wastewater from a semicoke plant (WWSCP) can reach 5000–15 000 mg L–1, which can be used as a good source of phenol for the production of anisole.[3,4] This not only reduces the concentration of phenols in the WWSCP but also reduces the production cost of anisole.

Catalysts that catalyze the conversion of phenols to ethers can be divided into homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts generally choose inorganic salts,[5,6] organic bases,[7] transition metal carbonyl compounds, and ionic liquids,[8,9] which have a high catalytic activity. Chen et al. studied the synthesis of anisole using nitrate as a catalyst, and the yield was stable at more than 90%.[6] It is pointed out that the smaller the ionization energy of nitrate cationic metal, the better the activity of nitrate, and the electron donating group can increase the reactivity of phenol. It is further speculated that its possible catalytic reaction is related to a Brønsted base (base B). Jiang et al. studied several ionic liquids (ILs) as catalysts, and the selectivity of anisole reached 100%.[9] The study found that the stronger the alkalinity of the anion, the higher the activity of the catalyst. Among them, the target product of chlorinated 1-normalylbutyl-3-methylimidazole chloride ([BMIm]Cl) was obtained in higher yield, proposing that its possible catalytic reaction was carried out at the Lewis base (L base) site. However, there are some problems such as a complex separation process and serious loss of homogeneous catalyst. Using some supported active species to prepare supported heterogeneous catalysts can overcome the difficulties in the separation of homogeneous catalysts. Subramanian et al. supported KF, NaOH, KOAc, amino acids on layered bimetallic hydroxide (LDHs), zeolite, Al2O3, and other carriers to achieve a simple separation of catalysts. Anisole has achieved high yields, but the yields are all lower than that of [BMIm]Cl.[10−12] At the same time, Subramanian et al. proposed that the catalytic activity is related to factors such as the loading of the active material, the alkalinity, and the structure of the support. Jyothi et al. showed that calcined Mgal-LDHs had a high catalytic activity, but there were problems such as harsh reaction conditions and low selectivity of target products; they proposed the reaction mechanism involving acid–base centers.[13] Lu et al. prepared an Fe2O3/γ-Al2O3 catalyst with γ-Al2O3 as the support to catalyze the degradation of phenol, and the removal rate of phenol was close to 100%.[14] However, such support has a certain acidity, and due to the adsorption or deposition of the generated intermediate products on the active site of the catalyst, the catalyst is prone to deactivation.[15]

Therefore, the study prepared a solid base catalyst Na-CH3ONa/γ-Al2O3 with a microporous and mesoporous structure for the reaction of phenol with dimethyl carbonate (DMC) and studied the effect of reaction temperature, reaction time, and reactant ratio on the catalyst activity. Concurrently, the details of catalyst Na-CH3ONa/γ-Al2O3 preparation, characterization, and catalytic activity for the selective O-methylation of phenol emphasizing the catalytic mechanism have been delineated. Finally, the catalytic conversion of the extracted organic phase (OP) in WWSCP on Na-CH3ONa/γ-Al2O3 was investigated.

Experimental Section

Materials

For the WWSCP pretreatment, mix DMC and cyclohexane to obtain the extractant that extracts the WWSCP three times to obtain an oil phase and water phase. The oil phase is distilled to obtain the OP. The OP is mainly composed of phenols and DMC.

WWSCP was collected from Shenmu city semicoke factory, Shaanxi Province, China. All of the experimental reagents, including phenol (>98%), γ-Al2O3 (>98%), sodium metal (>98%), sodium methoxide (>98%), phenol (>98%), DMC (>98%), and cyclohexane (>98%) are commercially purchased analytical reagents.

Catalyst Preparation

The catalysts were prepared by a one-pot method using γ-Al2O3 particles as the carrier. The γ-Al2O3 particles were first activated at 550 °C for 6 h in a muff furnace and cooled to room temperature. Then, the carrier, metallic Na, and CH3ONa were added to a high-pressure reactor with the molar ratio of 1:2:1, 1:3:1, and 1:4:1, respectively, and the mixture was reacted at 350 °C for 4 h to prepare Na-CH3ONa/γ-Al2O3. The catalysts were named YP-1, YP-2, and YP-3.

Catalyst Characterization

The catalysts were characterized with a Bruker Advance D8 X-ray diffractometer (XRD), Bruker TENSOR 27 Fourier transform infrared spectrometer (FTIR), V-Sorb 2800TP surface area and pore size analyzer (BET), ZEISS Sigma 300 scanning electron microscope (SEM), Thermo Fisher ESCALAB 250XI X-ray photoelectron spectrometer (XPS), TP-5080 multifunction adsorption instrument with CO2 temperature-programmed desorption (CO2-TPD), and DZ-TGA101 thermogravimetric analyzer (TGA).

Catalytic Activity Test

Taking phenol (analytical pure) and DMC to produce anisole as a model reaction, the reaction conditions were optimized to obtain the best reaction parameters. Finally, the OP was catalytically converted under optimal reaction parameters. In detail, the prepared catalyst (1% of the mass of phenol) was added into the mixture of phenol and DMC (molar ratio of 1:3), added to an autoclave, and reacted at 210 °C for 2.5 h. After cooling, the mixture was filtered to obtain the product.

The product of OP and ether (OPCP) were analyzed with an Agilent 7890/5973 gas chromatograph/mass spectrometer (GC/MS) equipped with an HP-5 MS capillary column a quadrupole analyzer, and the GC/MS was combined with an internal standard method to obtain the concentration of each substance before and after the reflection; then, the conversion rate and yield of each substance were calculated.

Results and Discussion

Catalyst Characterization Results

X-ray Diffraction Analysis

As shown in Figure , all catalysts show typical diffraction peaks of γ-Al2O3 at 2θ values of 37°, 46°, and 67°. In addition, characteristic diffraction peaks attributed to γ-NaAlO2 appear between 33° and 35.5°, which correspond to the (121) and (200) crystal planes, respectively.[16] With an increase in the added sodium, the intensity of the γ-NaAlO2 diffraction peak gradually increased, while the intensity of γ-Al2O3 diffraction peaks gradually decreased. This may be due to the fact that the addition of sodium increases the electron cloud density on the surface O2– on the catalyst, which makes the γ-NaAlO2 diffraction peak stronger. No characteristic diffraction peaks related to sodium metal were observed in the XRD pattern, so it was speculated that elemental Na might exist in an amorphous form.

Figure 1

X-ray diffraction patterns of the catalysts and carrier.

Fourier Infrared Analysis

The FTIR spectra of the catalysts show an obvious infrared absorption peak near 3450 cm–1 (Figure ), which is attributed to the stretching vibration characteristic peak of physically adsorbed water and the intrinsic surface O–H groups on the catalyst. The absorption peak at 1630 cm–1 is caused by the bending vibration of O–H bonds, and the absorption peaks at 1109 cm–1 belongs to methoxy groups.[17,18] Compared with the infrared absorption peak of the carrier γ-Al2O3, a new absorption peak belonging to γ-NaAlO2 appeared at 1450 cm–1, which may be caused by the reaction of the added Na-CH3ONa and γ-Al2O3. The intensity of the peak increases with the increased addition of metallic sodium because the loading of sodium causes sodium valence electrons to be taken away by oxygen, which increases the electron cloud density of oxygen atoms, thus causing the enhancement of the infrared absorption peak.

Figure 2

FTIR diagram of the catalysts and carrier.

Specific Surface Area and Pore Size Analysis

Figure shows the low-temperature N2 adsorption–desorption isotherm curve and Barrett–Joyner–Halenda (BJH) pore size distribution diagram of the carrier and catalysts. The YP-1, YP-2, and YP-3 catalysts show a type III curve between a P/P0 of 0.4 and 1.0 (Figure a), indicating that the pore structure of the catalysts is rich in mesopores. In addition, the catalysts showed an H3 hysteresis ring under higher pressures, indicating the presence of micropores and mesopores in the catalysts. According to the calculation and analysis by the density functional theory model, it can be seen that the pore diameter of the catalyst is mainly distributed in the mesoporous range 25–50 nm (Figure b).[19,20] The abundant mesoporous channels are conducive to the acquisition of reactants in the reaction, prevent carbon deposition, and extend the life of catalysts.

Figure 3

(a) N2 adsorption–desorption isotherms and (b) pore size distribution of catalysts and the carrier.

The BET specific surface area, average pore size, and pore volume data of the catalysts and carrier are listed in Table . The average specific surface area and pore volume of the catalyst are all smaller than those of the γ-Al2O3 carrier and gradually decrease with the increase of metallic sodium. The surface area was decreased from 170.27 to 100.24 m2 g–1, and the pore volume was decreased from 0.9 to 0.66 cm3 g–1. However, the average pore diameter increased from 24.61 to 43.23 nm. The possible reason is that the successful loading of Na-CH3ONa will react with γ-Al2O3 in the pore channels, thus increasing the average pore diameter.

Table 1

Structure Distribution Composition of the Catalysts and Carrier

sample	surface area (m2 g–1)	average pore diameter (nm)	pore volume (cm3 g–1)
γ-Al2O3	170.27	24.61	0.90
YP-1	153.01	28.01	0.81
YP-2	114.12	41.57	0.85
YP-3	100.24	43.23	0.66

CO2 Adsorption–Desorption Analysis

The CO2 adsorption–desorption curves show that the catalysts do not have desorption in the low-temperature region of 0–100 °C (Figure ), indicating that there are no weakly basic sites on the surface of the catalyst. The three catalysts showed obvious CO2 desorption peaks in the temperature ranges 200–300, 300–400, and 700–800 °C, which corresponded to the medium and strong basicity of the catalysts.[21,22] By integrating the peak areas of the CO2 desorption peaks of the catalysts and carrier in the three temperature regions, the corresponding change values of the peak areas were obtained. As shown in Table S1, the CO2 desorption peak areas in the three temperature regions increased with the increase of sodium content, revealing that the number of basic sites on the surface of the catalyst increased.

Figure 4

CO2 absorption and desorption curve of the catalysts and carrier.

X-ray Photoelectron Spectroscopy Analysis

The presence of Na and Al in the YP-3 catalyst was proved by an X-ray photoelectron analysis (Figure ). At the same time, through the quantitative analysis of the full spectrum of YP-3 (the results are shown in Table S2), it was found that the Na element deposited on the surface of YP-3 was about 4.74%. The analysis of Na 1s in catalyst YP-3 showed that there was no sodium or sodium oxide in the metallic state in YP-3, and it was further speculated that the Na element in the catalyst YP-3 existed in the ionic form. Meanwhile, an analysis of Al 2p in catalyst YP-3 found that Al exists in two states on the catalyst as γ-Al2O3 and AlO, and the characteristic peak at 74.4 eV belongs to meta-aluminate.[23,24]

Figure 5

X-ray photoelectron spectroscopy of catalyst YP-3 and fitting curve of Al 2p and Na 1s.

Thermogravimetric Analysis

The TGA curve of YP-3 shows that the weight loss was mainly observed at approximately 100, 230, and 700 °C (Figure ). The weight loss at 100 °C is attributed to the elimination of physically adsorbed and interlayer water; the loss at 230 °C is due to the dehydroxylation between the layers and loss of anions, and the loss at 700 °C is related to the decomposition of Na-CH3ONa/γ-Al2O3.[25,26] The decomposition of the catalyst occurs until 700 °C, demonstrating its good stability, which plays an important role in the catalytic activity and stability of its phenol methylation.

Figure 6

Thermal weight loss curve and derivative diagram of catalyst YP-3.

Analysis of Scanning Electron Microscope Results

The SEM image (Figure ) shows that the pristine γ-Al2O3 is porous with an uneven particle size and relatively compact structure.[27,28] With the increase of sodium during synthesis, the surface structure of the catalyst was deformed, which may be due to the reaction between the added Na-CH3ONa and the γ-Al2O3 molecular framework support. Compared with γ-Al2O3, YP-3 shows a large number of flocs covering the surface of the crystal grains, which may be because the loaded Na+ partially blocked the mesoporous structure and adhered to the surface. The catalyst mainly contains C, O, Al, and Na elements, which is confirmed by the EDS spectrum. Among them, O and Al mainly come from the γ-Al2O3 framework, and Na mainly comes from the added Na-CH3ONa. In addition, the content of the Na element is about 4.63 wt %, which is consistent with the XPS analysis; both are close to the theoretical load value of 5.0 wt %.

Figure 7

SEM image of the catalyst and its corresponding EDS energy spectrum.

Performance of the Catalyst

Effect of Reaction Temperature on Catalyst Activity

As shown in Figure , under the conditions of reaction time of 2.5 h and molar ratio of reactants of 1:3, the effects of γ-Al2O3, YP-1, YP-2, and YP-3 on phenol conversion and anisole yield were investigated at the reaction temperature of 150–220 °C. The conversion of phenol and the yield of anisole increased rapidly with the increase of reaction temperature and tended to be stable within 210–220 °C. In addition, the conversion of phenol and yield of anisole both increased with the increase of the added amount of sodium. The phenol conversion at 210 °C was 50%, 80%, 89%, and 98% with the corresponding anisole yield of 30%, 58%, 84%, and 96%, respectively. It is worth noting that the content of phenylmethyl carbonate (MPC) as a byproduct was the highest when the reaction temperature was 150 °C, and its contents were 14%, 11%, 9%, and 10%, respectively. As the reaction temperature rose to 210 °C, the yield of MPC decreased to 9%, 2%, 1%, and 0.5%, respectively. This may be due to the fact that the phenol O-methylation and esterification reactions were parallel reactions under low-temperature conditions. The oxygen anions of phenol simultaneously attacked the methyl carbon and carbonyl carbon of DMC, resulting in the formation of anisole and MPC.[29,30] As the reaction temperature increases, the oxygen anions of phenol mainly attack the methyl carbon of DMC to produce anisole. The Na-CH3ONa/γ-Al2O3 catalyst can provide active sites for the cleavage of the O–H bond of phenol. With the increase in the amount of metallic sodium, the catalyst exhibits a stronger ability to deprotonate phenol, which makes the reaction more violent.

Figure 8

Effect of temperature on the performance of different catalysts and the carrier.

Effect of Reaction Time on Catalyst Activity

As shown in Figure a, the effect of the reaction time (0.5–3 h) on the conversion of phenol and the yield of anisole was investigated under the conditions of YP-3 as the catalyst at 210 °C and a reactant molar ratio of 1:3. When the reaction time was 0.5 h, the conversion of phenol was 50%, and the yields of anisole and MPC were 38% and 8%. As the reaction time increased to 2.5 h, the phenol conversion and anisole yield reached the maximum values of 98% and 96%, while the yield of MPC was only 3%. When the reaction time was further increased to 3 h, the conversion and yield remained stable, indicating that the reaction reached equilibrium at 2.5 h.

Figure 9

Effect of (a) time and (b) molar ratio on the catalytic reaction.

Effect of Reactant Ratio on Catalyst Activity

As shown in Figure b, the effect of the reactant molar ratio (1:1.5–1:3.5) on the conversion of phenol and the yield of anisole was investigated under the conditions of YP-3 as the catalyst at 210 °C for 2.5 h. When the molar ratio was changed from 1:1.5 to 1:3, the yield of anisole increased with the increase of phenol conversion, and the yield of MPC first increased to a certain extent and then decreased to 3%. When the reactant molar ratio was 1:3, the phenol conversion and the anisole yield reached maximum values of 98% and 96%. When the molar ratio of reactants was set to 1:3.5, the phenol conversion rate and the yield of anisole changed little.

Stability of Catalysts

The stability of catalysts was studied by a recycling test for multiple cycles under the optimal reaction conditions (temperature was 210 °C, reactant molar ratio 1:3, and time 2.5 h, and YP-3 was the catalyst). It can be seen that the phenol conversion and the anisole yield gradually decreased as the number of repeated cycles increases (Figure S1). After the catalyst was repeatedly used 10 times, the phenol conversion and the anisole yield decreased to 92% and 90%. The slight decrease in yield may be due to the blockage of some pore structures on the surface of the catalyst after the cyclic reactions, resulting in a decrease of active sites on the catalyst surface.[31,32]

Catalytic Results of OP

The optimal reaction conditions are obtained through the above model compound reaction. Under these conditions, the catalyst CH3ONa/γ-Al2O3 is used for the catalytic conversion of OP to obtain OPCP. The composition and content of OP and OPCP were analyzed by GC/MS, and the results are shown in Figure .

Figure 10

Main components and relative contents of OP and OPCP.

As depicted in Figures S2 and S3 and Tables S3 and S4, in total, 27 organic compounds were detected in OP and OPCP with GC/MS. As shown in Figure , the OP mainly contained DMC, phenol, o-cresol, p-cresol, and other substitute phenols. Only DMC and various phenols were present in OP, which indicates that the extractant has high selectivity for phenols. The OPCP mainly contains anisole, 2-methylanisole, 4-methylanisole, and other substitute ethers and does not contain any phenols. The results show that the phenols in OP have been completely converted by the catalyst CH3ONa/γ-Al2O3.

The catalytic mechanism is speculated by correlation between the structure and performance of the catalyst, as illustrated in Scheme . First, phenols are adsorbed on the surface of the Na-CH3ONa/γ-Al2O3 catalyst. Due to the presence of strong basic sites, the phenols are deprotonated (O–H cleavage) to form phenate (I). Then, the −OCH3 groups on the catalyst can combine with the hydrogen ions of phenol to form methanol, which further promotes the cleavage of O–H bonds of phenol, while the −CH3 groups on the catalyst can combine with the phenol oxygen anions to form anisole. The oxygen anions on the phenate exhibit strong nucleophilicity, which can simultaneously attack the methyl carbon(II) and carbonyl carbon(III) of DMC at low temperatures (0–150 °C), leading to splitting the DMC into methyl, methoxy, and remaining groups (M). Under high temperatures (150–220 °C), the strong oxygen anions in the phenate mainly attack the methyl carbon of DMC, resulting in the methyl group becoming separated from DMC. Subsequently, the −CH3 and −OCH3 groups enter the pores of the catalyst to supplement. Due to the relatively large structure of group M, it cannot enter the pores of the catalyst and can only attach to the surface of the catalyst. Besides, a small amount of group M may combine with the oxygen anions of phenate to form MPC, which further decomposes into anisole and CO2. A large amount of group M can only combine with free hydrogen ions to generate monomethyl carbonate, which is then decomposed into methanol and CO2.

Scheme 1

Reaction Mechanism Diagram of Phenol with DMC

Conclusion

A variety of characterization results show that Na-CH3ONa is evenly distributed in the γ-Al2O3 surface and interacts with γ-Al2O3, forming highly active catalyst Na-CH3ONa/γ-Al2O3 with a large amount of micropores and mesh structures. Among them, the mesh can meet the transportation of reactants and products and avoid clogging the catalyst aperture, and the active substance contained in the micropore is responsible for participating in the catalytic reaction. The optimal reaction conditions are reactive at 1:3, 210 °C for 2.5 h, and phenols in OP are almost completely converted to ethers. Through the analysis of the catalytic mechanism, at low temperatures (0–150 °C), the oxygen anions on the phenol will attack the methyl carbon and carbonyl carbon on the DMC at the same time, and only a small amount of anisole is formed at this time. At high temperatures (150–220 °C), the oxygen anions mainly attack the methyl carbon on DMC, resulting in the production of a large amount of anisole.