A Type of MOF-Derived Porous Carbon with Low Cost as an Efficient Catalyst for Phenol Hydroxylation

Using MOF-5 as a template, the porous carbon (MDPC-600) possessing high specific surface area was obtained after carbonization and acid washing. After MDPC-600 was loaded with Cu ions, the catalyst Cu/MDPC-600 was acquired by heat treatment under nitrogen atmosphere. 0e catalyst was characterized by X-ray powder diffraction (XRD), N2 physical adsorption (BET), field emission electron microscope (SEM), energy spectrum, and transmission electron microscope (TEM). 0e results show that the Cu/MDPC-600 catalyst prepared by using MOF-5 as the template has a very high specific surface area, and Cu is uniformly supported on the carrier. 0e catalytic hydrogen peroxide oxidation reaction of phenol hydroxylation was investigated and exhibits better catalytic activity and stability in the phenol hydroxylation reaction. 0e catalytic effect was best when the reaction temperature was 80°C, the reaction time was 2 h, and the amount of catalyst was 0.05 g.0e conversion rate of phenol was 47.6%; the yield and selectivity of catechol were 37.8% and 79.4%, respectively. 0e activity of the catalyst changes little after three cycles of use.


Introduction
Dihydroxybenzene (catechol and hydroquinone) is an important organic intermediate with high value. Among them, catechol is an important organic intermediate for the synthesis of pesticides, medicines, and perfumes. It can also be used to produce dyes, photosensitive materials, electroplating materials, special inks, auxiliaries, and so on. Hydroquinone was mainly used in developer, anthraquinone dyes, azo dyes, synthetic ammonia cosolvents, rubber antioxidants, polymerization inhibitors, stabilizers for coatings and flavors, and antioxidants [1][2][3][4][5].
e production of dihydroxybenzene adopts the green process of direct oxidation of hydrogen peroxide. ese reaction conditions were mild, hydrogen peroxide was cheap and easy to obtain, and the oxidation by-product (water) was pollution-free [6]. However, the realization of the hydrogen peroxide oxidation phenol hydroxylation reaction process requires the participation of catalysts, so it is very important to develop an efficient and stable phenol hydroxylation catalyst.
Heterogeneous catalysis has attracted wide attention due to the advantages of easy separation in the process of catalytic reaction. e specific surface area will directly affect the activity of the heterogeneous catalyst. erefore, the catalytic performance can be improved by increasing the specific surface area and dispersing the catalytic active sites [7]. Porous carbon materials are often used as catalyst supports for heterogeneous catalysis because the surface of carbon materials has acidic, basic, or neutral oxygen-containing groups. ese groups interact with catalytic active sites and act as synergistic catalysis [8][9][10]. erefore, the catalytic activity of the catalyst can be greatly improved if the specific surface area and porosity of the porous carbon material can be increased.
Metal-organic framework materials (MOFs) are porous materials self-assembled by metal ions and organic ligands. ey have a periodic network structure. MOFs were widely used in gas separation, sensing, drug delivery, and heterogeneous catalysis [11][12][13][14][15][16]. In recent years, it was often used as a template for the preparation of some porous materials due to the designability of the structure of MOFs [17][18][19][20][21][22][23]. e carbonization of MOFs was particularly interesting. MOFs did not need the other carbons because of their organic ligands. e porous carbon obtained by carbonization of MOFs materials has a huge specific surface area due to the pore size, morphology, and volume of MOFs and can be determined by the selected metal ions and organic ligands. erefore, the porous carbon from MOFs was a very promising catalyst support material [24][25][26][27]. For example, Xiao Ma et al. [28] obtained N-doped porous carbon by one-step pyrolysis of ZIF-67 and then incorporated Co/CoO nanoparticles, which have excellent catalytic activity for the tandem dehydrogenation of ammonia borane and the hydrogenation of nitro compounds at room temperature. Li et al. [29] obtained the N-doped porous carbon (Cz-MOF-253) through rapid pyrolysis of the metal-organic framework material MOF-253, which exhibits excellent catalytic activity in the Knoevenagel condensation reaction. Khan et al. [30] carbonized MOF-5 at 900°C and then used the polyol reduction method to prepare PtM/PC900 catalyst, which was used in ethanol fuel cells with a good effect. Wu et al. [31] carbonized and chemically etched Cu-MOF to prepare a new three-dimensional matrix containing highly dispersed platinum nanoparticles and amorphous nickel for the first time and prepared two composite catalysts, both exhibition excellent reduction performance of nitrophenol. However, the application of MOFs materials in catalysts is currently limited, and it is still very important to expand new types of MOFs catalysts.
Transition metals were widely used as active sites of catalysts in the phenol hydroxylation reaction. Among them, copper-based catalysts have special attention. Many copper-based catalysts show good catalytic activity for phenol hydroxylation. For example, Wang et al. [32] reported that Cu-substituted mesoporous SBA-15 has excellent catalytic performance and high selectivity to catechol in the hydroxylation of phenol by hydrogen peroxide. At the same time, Cu 2+ loaded on zeolites such as HY, Hβ, USHY, NaY, and HZSM-5 also showed good catalytic activity for phenol hydroxylation. ese catalysts have higher activity than TS-based catalysts [33]. In addition, Cu/MCM-41 [5,34], Cu/ZSM-5 [35], Cu/HMS [36], and other copper-based catalysts also show good catalytic activity for phenol hydroxylation. erefore, Cu is a very potential catalytic active site. In this study, the porous carbon derived from MOFs was used to load copper ions and then subjected to high-temperature heat treatment. e catalyst is a new type of copper-containing porous carbon catalyst that has hardly been reported by previous researchers. e catalyst catalyzes the phenol hydroxylation reaction by hydrogen peroxide, which has good catalytic activity and stability. , sodium chloride (NaCl), and anhydrous sodium sulfate (Na 2 SO 4 ) were obtained from Aladdin Industrial Corporation (Shanghai, China). None of the chemicals selected in this work are purified further before use. .000 g Zn(NO 3 ) 2 ·6H 2 O (6.25 mM), terephthalic acid (2.8 mM), and PVP (150 mM) were poured into a 3-mouth flask; then, 105 mL DMF and 45 mL ethanol were added, and a clear solution was obtained after stirring and dissolved. Under stirring intensely, triethylamine (1 mM) was dripped into the solution which gradually became turbid; and then, keeping stirred for 1 h at 120°C before filtered, the filter cake was washed with DMF and ethanol. Finally, after being dried at 100°C, MOF-5 was prepared [37].

Preparation of MDPC-600.
Porous carbon was prepared by thermolysis of the previously obtained MOF-5. e white powder of MOF-5 was put into a tube furnace and heated to 600°C at a heating rate of 8°C/min with an N 2 flow of 150 mL/min. e MOF-5 powder was maintained at temperature of 600°C for 2 hours. Cooling the MOF-5 back down to room temperature, the samples were obtained (designed as sample A). Sample A was subjected to HCl (1.1 M) washing three times and purified water washing two times to dissolve and remove zinc oxide. e resulting carbon sample was dried in air at 120°C overnight (designed as MDPC-600) [38].

Preparation of Cu/MDPC-600.
Based on 0.5 g MDPC-600, the required Cu(NO 3 ) 2 ·3H 2 O (mass is calculated according to the Cu 2+ mass percentage of 1%, 2%, 3%, 4%, and 5%) was added to a small beaker containing absolute ethanol (the volume of absolute ethanol is the same as that of 0.5 g MDPC-600). After stirring and dissolving at room temperature, add 0.5 g of MDPC-600 powder. MDPC-600 powder is completely soaked in ethanol solvent. After being mixed thoroughly, they were dried at 90°C for 2 h to obtain Cu/MDPC-600 catalysts with Cu 2+ mass percentages of 1%, 2%, 3%, 4%, and 5%, respectively. en, the obtained Cu/MDPC-600 catalyst was heated to 600°C in a tube furnace under nitrogen atmosphere for heat treatment for 2 h to obtain the Cu/MDPC-600 catalyst.

Hydroxylation
Reaction of Phenol. Weigh 0.05 g catalysts of different Cu 2+ mass percentages of 1%, 2%, 3%, and 4%, 0.9 mL phenol (being heated in the waterbath and dissolved at 50°C), 30 mL deionized water, and 16 mL hydrogen peroxide solution of mass percentages 30% and were together added into a round-bottom flask. Under magnetic stirring at 80°C, the mixture reacted for 2 h. e used catalyst was filtered out, and ethyl acetate was used to extract the product and unreacted phenol from the filtrate three times [39]. e mixture of reactions was analyzed using a gas chromatograph-mass spectrometer (Shimadzu GC-MS-2010 Plus).

Characterization of
Catalyst. e Brunauer−Emmett−Teller (BET) specific surface areas were measured on the Belsorp-Mini II analyzer (Japan). e catalyst samples were measured using a Rigaku Ultima-IV XRD system with Cu-Kα radiation (λ � 0.1542 nm). e target voltage and current were 40 kV and 30 mA, respectively. e 2θ scan range and rate were 5-80°and 6°min -1 respectively. e catalyst samples were observed under a ZEISS Sigma HD field emission scanning electron microscopy (FESEM). e accelerating voltage was 8 kV. e morphology was observed using the secondary electron detectors in-lens. Meanwhile, the element contents in the samples were analyzed using an Oxford X-Max electric energy spectrum meter (X-MaxN). TEM of the samples was obtained using a JEOL JEM-2010 UHR transmission electron microscope with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out on an RBD upgraded PHI-5000C ESCA system (PerkinElmer) with MgK-α radiation (h] � 1253.6 eV). e contents of Cu and Fe in the catalyst were analyzed using inductively coupled plasma (ICP, novAA 300, Analytik Jena AG).

Sample Analysis by BET.
e specific surface area of the catalyst material is an important parameter to measure the catalytic performance. In order to evaluate the influence of the introduction of copper ions on the surface of the catalyst, specific surface area tests were done on MDPC-600 and Cu/MDPC-600. e N 2 adsorption/desorption isotherms of MDPC-600 (Figure 1(a)) and Cu/MDPC-600 ( Figure 1(b)) exhibit characteristic IV curves with hysteresis loops. e MDPC-600's specific surface area was 1029.5 m 2 / g, the total pore volume was 0.944 cm 3 /g, and the average pore diameter was 2.97 nm (Table 1). But the specific surface area of Cu/MDPC-600 becomes 824.2 m 2 /g ( Table 1). It can be seen that MDPC-600 has a huge specific surface area (1029.5 m 2 /g). While the specific surface area of Cu/MDPC-600 drops to 824.2 m 2 /g, the loading Cu occupies part of the pore volume and reduces the porosity. e total pore volume also decreased from 0.944 cm 3 /g to 0.609 cm 3 /g, and the average pore diameter decreased from 2.97 nm to 2.65 nm. However, the Cu/MDPC-600 loaded with Cu still has a huge specific surface area which can be beneficial to accelerate the reaction because it can increase the probability of contact between catalyst and reactant.

Sample Analysis by XRD.
In order to determine the filling of copper ions in the catalyst material, we performed XRD characterization on the material. Figure 2 shows the XRD patterns of Cu/MDPC-600. e XRD pattern of Cu/MDPC-600 matches the form of amorphous porous carbon. However, the characteristic peaks of Cu with low intensity can be observed on the spectrum at 43.3°, 50.5°, and 74.2°. e characteristic peaks of Cu become more obvious with the increase of Cu loading because a small number of copper ions are reduced to copper during the carbonization process. e characteristic peaks of other Cu ions are not obvious because they are dispersed in the porous carbon pores.

Sample Analysis by SEM and TEM.
e Figures 3(a) and 3(b) were the morphologies of MOF-5 observed by SEM. It can be seen that MOF-5 is clustered together like petals with a diameter of about 5 µm. Figures 3(c) and 3(d) were the morphologies of the porous carbon MDPC-600, and Figures 3(e) and 3(f ) were the morphologies of Cu/MDPC-600. It can be seen from the figure that the morphology of the porous carbon before and after loading Cu was unchanged. Both were irregular blocky morphology with a size between 50 nm and 500 nm, and honeycombshaped pores can be seen on the massive particles. e energy spectrum test of the catalyst with a Cu content of 3% is shown in Figure 4. ere are three main elements: C, O, and Cu. It also indirectly verifies that the MOF-5 is carbonized, and then, the ZnO hydrochloric acid is washed away to obtain porous carbon. It can be seen from Figure 4 that the supported Cu element distribution is relatively uniform, indicating that Cu is well dispersed in the catalyst. Figure 5 is a TEM image of the catalyst Cu/MDPC-600. From the figure, it can be seen that the catalyst has obvious amorphous characteristics, corresponding to the XRD characterization (Figure 2), and the morphology is irregular.

Sample Analysis by XPS.
e XPS spectrum of Cu/MDPC-600 in Figure 6 shows that Cu 2+ is doped into the pores of porous carbon. e XPS wide-scan spectrum in Figure 6(a) shows that the constituent elements of Cu/MDPC-600 include C, O, and Cu. Figure 6(b) is the high-resolution XPS spectrum of Cu 2p. It can be seen from Figure 6(b) that the peak at 934.7 eV comes from Cu 2p3/2, and there is a satellite peak at 943.0 eV, indicating the presence of Cu 2+ [40] in the material, and the valence state of copper supported in the catalyst is mainly divalent (Figure 6(b)), indicating that Cu exists in the form of copper oxide during the Cu loading process.

Hydroxylation of Phenol.
e obtained Cu/MDPC-600 was used to catalyze the phenol hydroxylation reaction, and the supported Cu mass fractions were 1%, 2%, 3%, 4%, and 5%, respectively. e obtained reaction results are given in Table 2. Table 2 provides that the yield of hydroquinone increases with the increase of Cu loading. e best conversion rate of phenol reaches 47.6% when the Cu loading was 3%. e yield and selectivity of catechol are 37.8% and 79.4%, respectively. It shows that Cu has a good catalytic activity after being heattreated on porous carbon. e yield of hydroquinone begins to decrease when Cu loading exceeds 3%. at is because the decomposition rate of hydrogen peroxide accelerates when there is increase in Cu loading. ere are other products in the reaction products in addition to hydroquinone, which mainly include tar substances produced by deep oxidation of hydroquinone. From Table 2, it can be seen that the content of other substances increases with the increase of Cu content. Because, the Cu content increase will accelerate the rate of deep oxidation of the product [41]. Based on the above facts, the optimal catalyst content is considered to be 3%. Table 3 lists the phenol conversion rate and hydroquinone yield of several other copper-containing catalysts catalyzed by the phenol hydroxylation reaction and compared with the catalyst Cu/MDPC-600. It is found that the catalyst Cu/MDPC-600 has a higher phenol conversion rate and hydroquinone selectivity than the catalyst CuO-TiO 2 .
e phenol conversion rate is slightly lower compared with catalyst Cu-SBA-15 and CuO/Al-SBA-15, but the selectivity of hydroquinone is much higher. Compared with the catalyst Cu-SBA-16, the selectivity of benzenediol is slightly lower, but the conversion rate of phenol is much higher. It can be seen that the catalyst Cu/MDPC-600 has relatively good catalytic performance. Compared with the catalyst Cu/TS-1-x, Cu/MDPC-600's phenol conversion rate and hydroquinone selectivity are slightly lower. However, the Cu/MDPC-600 possessed a simple preparation process and low cost.

Reaction Mechanism of Phenol Hydroxylation.
e catalytic mechanism of phenol oxidation over Cu-Fe-SiO 2 /S composite was proposed and is shown in Figure 7, which is similar to a heterogeneous Fenton-like system [46]. In the first step, Cu 2+ captures an electron and a hydrogen proton of phenol to form phenoxy radicals. In the second step, phenoxy radicals are further oxidized by hydrogen peroxide to produce o-benzoquinone or p-benzoquinone and a proton and a water molecule. In the third step, o-benzoquinone or p-benzoquinone reacts with the proton to form catechol or  hydroquinone [47]. -OH is an ortho or parapositioning group. When the original positioning group on the benzene ring is -OH, the theoretical ratio of ortho to parasubstituent is 73%:27% [48]. e experimental results show that the catechol selectivity is 65.4-87.8%, not much different from the theoretical. e little difference may be due to the fact that the selection of catalytic active sites greatly influences the selectivity of catalysts in actual reactions [49].

Influence of Reaction
Temperature. e effect of reaction temperature on the hydroxylation of phenol is shown in Figure 8. Reaction conditions: 0.88 mL phenol, 16 mL hydrogen peroxide (30%), 30 mL deionized water, reaction time was 2 h, and catalyst dosage was 0.05 g.
It can be seen that no product was formed at 50°C and 60°C. With the temperature increase, the yield of catechol, phenol conversion, and catechol selectivity all increase above 60°C. When reaction temperature reached 80°C, the yield and the selectivity of catechol reached 37.8% and 79.4%, respectively. Considering the fast decomposition of H 2 O 2 over 80°C, the optimal temperature is 80°C.

Influence of Reaction Time.
e effect of reaction time on the hydroxylation of phenol is shown in Figure 9. Reaction conditions: 0.88 mL phenol, 16 mL hydrogen peroxide (30%), 30 mL deionized water, reaction time was 2 h, and catalyst dosage was 0.05 g.
It found that the phenol conversion rate and the catechol yield are 31.5% and 27.8%, respectively, when the reaction time was 0.5 h. e phenol conversion rate and the catechol yield increase slowly with the reaction time becoming longer. is may be because the reaction rate was decreased due to the decrease of H 2 O 2 concentration in reaction progresses. When the reaction time reach 2 h, the catechol yields were 47.6% and 37.8%, respectively. e selectivity of catechol decreases from 88.2% to 79.4% at the same time.
at was because the catechol was deeply   Journal of Chemistry oxidized leading to the production of other substances that increase, such as tar [50].

Influence of Catalyst Dosage and Recycling Experiment.
e effect of the amount of catalyst on the hydroxylation of phenol is shown in Figure 10. Reaction conditions: 0.88 mL phenol, 16 mL hydrogen peroxide (30%), 30 mL deionized water, reaction temperature was 80°C, and reaction time was 2 h.
When the amount of catalyst increases, the phenol conversion rate and catechol yield increase first and then decrease, and a slight increase finally. e highest point was the catalyst amount of 0.05 g. e selectivity of catechol decreases with the increase of the amount of catalyst because the increase of the amount of catalyst can promote the reaction. However, the excessive amount of catalyst will cause the excessive decomposition of H 2 O 2 and the excessive oxidation of products, which is not conducive to the reaction. e increase in the yield of other materials in Figure 9 also confirms this point. erefore, the optimal amount of the catalyst was 0.05 g.
In order to verify the recyclability of the catalyst, the catalyst was recovered, washed, and dried after the      experiment, and then, the phenol hydroxylation reaction was repeated. It was found that the phenol conversion rate and the catechol yield of Cu/MDPC-600 were 44.4% and 35.6%, respectively, after being repeated three times. It shows that Cu/MDPC-600 has higher catalytic activity and repeated use performance in the phenol hydroxylation reaction.

Conclusions
In this study, the catalyst Cu/MDPC-600 was obtained by carbonizing MOF-5 at 600°C and loading Cu ions after. e MDPC-600 obtained by carbonizing MOF-5 has a huge surface area of 1029.5 m 2 /g, and the specific surface area of the catalyst after loading copper ions was still 824.2 m 2 /g. e catalyst has a good catalytic effect in catalyzing the hydroxylation of phenol by hydrogen peroxide because of its huge specific surface area.
In addition, the stability of the combination of copper ions in the catalyst was relatively high due to the heat treatment under nitrogen atmosphere after loading, and the catalyst still has a good catalytic effect after repeated use of the catalyst 3 times.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.