A novel highly ordered mesoporous carbon-based solid acid for synthesis of bisphenol-A

Abstract

A novel highly ordered mesoporous carbon-based solid acid was prepared through controlled sulfonation of F127 type mesoporous carbons prepared via a solvent evaporation induced self-assembly method. The influence of sulfonation temperature was investigated, suggesting an optimum temperature of 180 °C. The sulfonated samples were characterized by means of N₂ adsorption–desorption, XRD, TEM and FT-IR. The SO₃H group-functionalized mesoporous carbon showed a specific surface of 393 m² g⁻¹, pore volumes of 0.33 cm³ g⁻¹, an average pore size of 3.7 nm and a SO₃H group density of 1.71 mmol g⁻¹. The mesoporous carbon-based solid acid effectively catalyzed the condensation of phenol with acetone. The increased catalytic performance was attributed to a uniform mesoporous structure and hydrophobic surface properties.

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1. Introduction

Bisphenol-A (BPA) is an important raw material for the production of many polymers in the organic chemical industry, such as epoxy resins and polycarbonates.¹ Traditionally, it is produced by the acid-catalyzed condensation reaction of acetone and phenol. Because of equipment corrosion, environmental pollution and separation difficulty, homogeneous acid catalysts (e.g. H₂SO₄, HCl) have been eliminated. In order to solve these problems, the use of heterogeneous catalysts has been explored. Solid acids, which can be readily separated from the products, avoid the corrosion and pollution problems, thereby leading to lower production costs. Until now, a number of new kinds of catalysts have been developed, ranging from heteropoly acids,² mixed oxides,³ zeolites,⁴ acid-functionalized mesoporous silicas,^5,6 ionic liquid,⁷ silylated derivatives,⁸ etc. Most of these solid catalysts could be found only in the patent and literature, far from industrialized application. In industry, ion-exchange resins are most widely used for the synthesis of bisphenol-A due to their high catalytic performance, easy separation and less corrosion problems. It was reported that thiol-modified ion-exchange resins, such as Amberlyst, exhibit very good catalytic activity with around 90% selectivity.⁹ However, several drawbacks of the ion-exchange resins may directly reduce the catalyst lifespan, which increases the production costs to a certain extent. The disadvantages can be summed up in three aspects: (i) cannot be used at higher reaction temperatures because of the poor thermal stability. (ii) Easily poisoned by the reaction medium, side product water. (iii) Swelling and rupture of the resins are inevitable. Thus, efforts should be made to develop thermally stable, hydrophobic catalysts with longer lifespan for bisphenol-A synthesis.

Recently, much attention has been focused on carbon-based solid acid catalysts due to their chemical inertness, thermal stability and excellent catalytic activity.^10,11 Michikazu Hara et al.^12–16 first reported a series of carbon-based solid acid catalysts, which can be readily prepared by incomplete carbonization and sulfonation of polycyclic aromatic hydrocarbons or carbohydrates such as sugar, starch and cellulose, etc. Despite the small specific surface area (ca. 2 m² g⁻¹), the carbon material with sufficient amount of sulfonic acid groups showed excellent catalytic performance for liquid phase acid-catalyzed reactions. Wu and co-workers¹⁷ also reported a vapour-phase transfer method to prepare sulfonic acid group-functionalized mesoporous carbon. It catalyzed efficiently the liquid-phase Beckmann rearrangement of cyclohexanone oxime and condensation reactions involving bulky aromatic aldehydes. Moreover, graphene-based catalysis has also been prepared as an efficient solid catalyst.^18,19 Catalytic tests demonstrated the sulfated graphene was a good catalyst with high activity for the acid catalyzed liquid reactions such as the esterification of acetic acid. However, as for hydrophobic acid-catalyzed reactions, the hydrophilic functional groups inside the carbon restrict the incorporation of hydrophobic reactants into the active sites.²⁰ As a result, the carbon material exhibits poor or no catalytic activity.

Sulfonated ordered mesoporous materials as a promising replacement for sulfonic acids have been developed due to its defined pore structures, high specific surface area.^17,21–23 The ordered mesoporous structure provides more active sites for the reaction of large molecules.²⁴ Feng and co-workers²⁵ reported a method to prepare the sulfonated ordered mesoporous carbon and applied it to catalyze the formation of bisphenol-A. In this method, sulfonic acid-containing aryl radicals was grafted on the carbon surface by covalent attachment. The catalyst showed a phenol conversion of 8.9% (28.6% maximum) with high selectivity of 84.0% to p,p′-bisphenol-A. However, this method is time consuming and sacrifice the silica template. Wan²⁶ showed that a high-content sulfonic acid group functionalized ordered mesoporous polymer-based solid acid, prepared by a simple surfactant templating approach and oxidation treatment, was very selective and stable for bisphenol-A synthesis. The stability is attributed to the hydrophobic nature of the mesoporous polymer-based solids.

In this paper, we reported a novel highly ordered mesoporous carbon-based solid acid with high SO₃H density and specific surface area prepared through controlled sulfonation of F127 type mesoporous carbons prepared via a solvent evaporation induced self-assembly method. Then the catalysts were examined as solid acid catalysts in bisphenol-A synthesis. Its catalytic performance was also compared with other commercial acid catalysts.

2. Experiment section

2.1 Catalysts preparation

The synthesis method of FDU-type mesoporous carbon was reported previously.²⁷ In typical preparation procedure, 0.61 g phenol (6.50 mmol) was melted at 40–42 °C followed by the addition of 0.13 g 20 wt% NaOH (0.65 mmol) with magnetic stirring for 10 min. 1.05 g formalin (13.0 mmol, 37 wt%) aqueous solution was added drop-wise, and the mixture was stirred at 70 °C for 1 h. Then, the reaction mixture was cooled to room temperature and the pH was adjusted to neutral by adding appropriate quantities of 2 M HCl under stirring. Water was removed under vacuum below 50 °C. The mixture obtained was dissolved in ethanol. The ethanol solution of the above precursors was added to a second solution containing 1.00 g of triblock copolymer (PEO–PPO–PEO, Pluronic 127, Sigma-Aldrich) and 20.0 g of ethanol. The mixture was stirred to form a homogeneous solution with the molar ratio of F127 : phenol : formaldehyde : ethanol = 0.012 : 1 : 2 : 67. The solution was then transferred to a culture dish and ethanol was evaporated at room temperature over 5–8 h to form a transparent membrane. The membrane was heated in an oven at 100 °C for 24 h to obtain the phenolic resins/F127 composites. The composites were calcined at 350 °C under nitrogen for 5 h with a temperature increase rate of 1 °C min⁻¹ to remove the template F127. Further carbonization was carried out at 500 °C for 3 h at a rate of 2 °C min⁻¹ under nitrogen atmosphere. The sample obtained, FDU-15-500 (500 denotes the carbonization temperature), was sulfonated in concentrated H₂SO₄ (98 wt%) at 140–200 °C for 10 h in a Teflon-lined autoclave. The sulfonated sample, FDU-15-500 T_S (T_S denotes the sulfonation temperature), was then washed with hot deionized water to remove the physically adsorbed H₂SO₄ until the pH of filtrate became neutral and finally dried at 60 °C for 8 h under vacuum.

2.2 Catalysts characterization

The X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max 2500 type X-ray powder diffractometer with Cu Kα radiation at 40 kV and 30 mA. Nitrogen adsorption–desorption isotherms were recorded at 77 K on a Micromeritics Tristar 3000 instrument. All samples were degassed at 423 K under vacuum for at least 12 h prior to measurements. The Brunauer–Emmett–Teller (BET) method was applied to calculate the specific surface area. The total pore volume was estimated from the amount of N₂ adsorbed at P/P₀ = 0.98. The mesoporous volume, pore size distribution and pore diameter were determined from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. Transmission electron microscopy (TEM) images were obtained on FEI Tecnai G2 F20 microscope. FT-IR spectra were recorded on a Thermo Scientific Nicolet 560 FTIR spectrometer with 64 scans at 4 cm⁻¹ resolution. The amount of SO₃H group was quantified by an acid–base titration method, which involves an aqueous ion-exchange step of H⁺ ions (SO₃H group) of the catalyst with NaCl, followed by titration of the resulting filtrate with NaOH.²⁸ The total acid density was estimated by back-titration of excessive NaOH with HCl.

2.3 Catalytic activity measurement for the synthesis of bisphenol-A

All catalysts were tested in condensation of phenol with acetone under atmospheric pressure in a 50 mL glass reactor equipped with a reflux condenser and a magnetic stirrer. In a typical experiment, 0.58 g of acetone and 9.40 g of excess phenol was mixed under vigorous stirring in the presence of the mesoporous carbon-based solid acid catalyst (0.30 g). The reaction mixture was kept at 358 K in a water bath for 8 h. Samples were withdrawn from the reaction mixture at 2 h intervals and then separated using filtering syringe. The resulting solution was analyzed by a high performance liquid chromatograph (HPLC, Agilent 1100) equipped with a column of Zorbox C18, 0.5 μm, 250 × 0.46 mm. The mobile phase was a mixture of methanol–water (63 : 37, v/v) and the flow rate was 0.5 mL min⁻¹. The amount of bisphenol-A was determined by external standard method. Conversion and selectivity were calculated based on converted acetone.

3. Characterization of the catalyst

Carbonation temperature and different sulfonation procedures are extremely important, which have a great impact on the porous structure and the content of SO₃H group.^17,23,27,28 In our previous studies,²⁹ the effect of carbonization temperature was investigated. The optimal carbonization temperature was 500 °C and the mesoporous carbon-based solid acid catalyst carbonized under 500 °C contained highly ordered mesoporous channel structure. Here, the influence of sulfonation temperature on porous structure, strong acid density and catalytic performance were systematically investigated.

Fig. 1 shows the XRD patterns of FDU-15-500 and corresponding mesoporous carbon obtained after the sulfonation was performed at various temperatures (140–200 °C). FDU-15-500 clearly showed an intense diffraction peak together with two weak peaks which can be indexed as the 100, 110 and 200 planes of 2D hexagonal structure (P6mm) (Fig. 1), indicating a well-ordered structuring of the mesopores. The peak intensity of the 100 reflection decreased after sulfonation, indicating that the sulfonation treatment may partially destroy the mesostructure order of the material (see later from the BET surface areas in Table 1). However, the samples sulfonated at lower temperatures ranging between 140–180 °C still maintained the ordered mesostructure. By increasing the sulfonation temperature to 200 °C, the peak of 100 plane almost disappeared presumably as a result of framework collapse. It demonstrates that soft sulfonation is important to keep the mesoporous organization of the carbon material.

Fig. 1 Small-angle XRD patterns of mesoporous carbon-based solid acid catalysts under different sulfonation temperatures.

Table 1 Pore structure parameters of mesoporous carbon-based solid acid catalysts under different sulfonation temperatures

Sample	Total surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)	a0 (nm)
FDU-15-500	655	0.59	4.0	12.1
FDU-15-500-140	386	0.30	3.6	11.9
FDU-15-500-160	338	0.23	3.7	11.9
FDU-15-500-180	393	0.33	3.7	11.8
FDU-15-500-200	25	0.02	—	11.5

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Fig. 2 shows the N₂ adsorption–desorption isotherms and pore size distribution curves for FDU-15-500 and the corresponding sulfonated mesoporous carbon at various temperatures. FDU-15-500 and the samples sulfonated between 140–180 °C had typical type-IV N₂ adsorption isotherms with a well defined capillary condensation step at P/P₀ ≈ 0.4 and the H₁-type hysteresis loops in the relative pressure range of 0.45–0.7, and also showed narrow BJH pore size distribution. These indicated that the samples were characteristic of mesoporous carbon with cylindrical channels.

Fig. 2 N₂ adsorption–desorption isotherms (a) and BJH pore distribution (b) of mesoporous carbon-based solid acid catalysts under different sulfonation temperatures.

Table 1 summarizes the textural parameters of mesoporous carbon before and after sulfonation. An obvious decrease for the sulfonated mesoporous carbon in specific surface area, pore volume and pore size should be due to the structural shrink or partial framework collapse.¹⁷ FDU-15-500-180 showed the highest BET surface area and the pore volume after sulfonation. The values were 393 m² g⁻¹ and 0.33 cm³ g⁻¹ respectively. When sulfonation temperature increased to 200 °C, the mesoporous carbon material totally lost its mesostructure under such hard sulfonation treatment. TEM images, presented in Fig. 3, prove that FDU-15-500 has ordered mesoporous morphology. After sulfonation, FDU-50-500-180 retains ordered mesoporous structure. The pore size is about 3.7 nm, well consistent with the result calculated by N₂ adsorption–desorption experiment (Table 1). In addition, the size of the carbon-filled channels is about 12.3 nm. The presence of elemental sulfur is clearly revealed by the EDX pattern (Fig. 4) and the content is estimated to be 1.74 mmol g⁻¹ from elemental analysis. The value is close to the result obtained from the acid–base titration method, which is 1.71 mmol g⁻¹ (Fig. 6).

Fig. 3 TEM images of FDU-15-500 (a) and FDU-15-500-180 (b).

Fig. 4 EDX spectra of FDU-15-500-180 catalyst.

Fig. 5 FT-IR spectra of mesoporous carbon-based solid acid catalysts under different sulfonation temperatures.

Fig. 6 Acid density of mesoporous carbon-based solid acid catalysts under different sulfonation temperatures.

The FT-IR spectra for FDU-15-500 and sulfonated samples are shown in Fig. 5. The vibration bands at 1032 cm⁻¹ was assigned to symmetric stretching of S=O bond caused by the introduction of SO₃H groups into the mesoporous carbon.^17,30 The presence of absorption at 1220 cm⁻¹ was due to Ar–OH stretching.³⁰ The bands at 1463 cm⁻¹, 1602 cm⁻¹ was due to C=C stretching mode remained after sulfonation.¹⁷ The band at 1711 cm⁻¹ was typical of the C=O stretching mode of the –COOH and –COO– groups. The broad band at 3426 cm⁻¹ was assigned to the O–H stretching modes of the –COOH and phenolic OH groups.²²

Fig. 6 showed the acid loadings obtained by acid–base titration method. The amounts of SO₃H and total acid increased with increasing sulfonation temperature, indicating that relatively high sulfonation temperature should be necessary. With respect to FDU-15-500-200, the values of SO₃H density and total acid density were 1.93 and 3.24 mmol g⁻¹ respectively.

4. Catalytic activity

The catalytic performance of FDU-15-500-SO₃H was investigated by condensation of phenol with acetone (Fig. 7). Acetone conversation increases with reaction time. FDU-15-500-180 clearly shows higher catalytic activity, reaching 42.7% conversation of acetone after 10 h of reaction. Bisphenol-A selectivity also increases slightly with reaction time. The maximum selectivity 68% was achieved at 8 h. It is worth noting that the selectivity decreases when the conversion goes on. The selectivity to bisphenol-A decreases after 8 h probably due to accumulation of byproducts or loss of active sites.

Fig. 7 Effect of sulfonation temperature on catalytic performance of mesoporous carbon-based solid acids. Conditions: 0.30 g catalyst; 358 K; 0.58 g acetone and 9.4 g phenol.

Among the mesoporous carbon obtained after the sulfonation was performed between 140–180 °C, FDU-15-500-180 exhibits the best catalytic performance due to the highest amount of SO₃H (Fig. 8). Nevertheless, the largest amount of active sites may not be the only reason for the best catalytic performance. With respect to FDU-15-500-200 with much low specific surface area (25 m² g⁻¹), a lower conversation was observed. In summary, FDU-15-500-180 shows the highest catalytic activity probably owing to the ordered mesoporous structure, high specific surface (393 m² g⁻¹) and large acid amount (SO₃H group, 1.71 mmol g⁻¹).

Fig. 8 Effect of BET specific surface area and surface acid density on catalytic performance. Conditions: 0.30 g catalyst; 358 K; 8 h; 0.58 g acetone and 9.4 g phenol.

In order to evaluate the catalytic activity of FDU-15-500-180 in this reaction, a comparative study was made with three solid acid catalysts (commercial strongly acidic cation exchange resin 001 × 7, D072 and amorphous carbon-based solid acid). Fig. 9 shows time profiles of acetone conversion and bisphenol-A selectivity on these catalysts. With respect to 001 × 7 and amorphous carbon, the reaction gradually reached equilibrium after 8 h reaction and selectivity to bisphenol-A is lower than 45%. Among these catalysts, FDU-15-500-180 shows the highest activity.

Fig. 9 Effect of different catalysts on phenol–acetone condensation reaction. Conditions: 0.30 g catalyst; 358 K; 0.58 g acetone and 9.4 g phenol.

Although the acid amount of 001 × 7, D072 and amorphous carbon is obviously larger than FDU-15-500-180 (Fig. 10), FDU-15-500-180 shows the best catalytic performance with 39.4% conversation. As for 001 × 7, SO₃H group loading is 2.41 mmol g⁻¹, but it shows much less activity. This result may be related to the small pores which partially block access to the active surface sites. D072 with large pores and high density of SO₃H group still exhibits less activity than FDU-15-500-180, which is probably attributed to the weak hydrophilic property of the material. For this reason, byproduct water may poison the active sites. With respect to amorphous carbon, extremely low surface area (<2 m² g⁻¹) and nonporous structure are responsible for the low activity. In summary, the enhancement of catalytic performance of the FDU-15-500-180 is due to its ordered mesoporous structure and hydrophobic substrate of carbon-based catalyst, which ensure good accessibility to active sites.

Fig. 10 Effect of surface acid density on catalytic performance. Conditions: 0.30 g catalyst; 358 K; 8 h; 0.58 g acetone and 9.4 g phenol.

For the further improvement of the catalyst of condensation of phenol with acetone, mercapto acetic acid as promoters was introduced. Both of acetone conversation and bisphenol-A selectivity increase significantly with increasing mercapto acetic acid, until 0.06 g (Fig. 11a). 75.2% conversation and 85.0% selectivity was observed at the addition of 0.12 g. However, mercapto acetic acid exhibits no catalytic activity without the catalyst (Fig. 11b), demonstrating that ordered mesoporous carbon-based solid acid involves active sites required for the formation of bisphenol-A.

Fig. 11 Effect of mercapto acetic acid on phenol–acetone condensation reaction. (a) Conditions: 0.30 g catalyst; 358 K; 8 h; 0.58 g acetone and 9.4 g phenol. (b) Conditions: 0.30 g catalyst; 0.12 g mercapto acetic acid; 358 K; 0.58 g acetone and 9.4 g phenol.

The proposed mechanism for bisphenol-A formation is similar to that reported in ref. 6 and 31 as shown in Scheme 1. The mercapto group is supposed to form carbocation intermediates through nucleophilic attack on the protonated acetone. The positive charged sulfur intermediates may be more stable than the protonated acetone,³² leading to increases in reaction rate. Then, bisphenol-A is formed by nucleophilic attack of the positive charged sulfur intermediates by the second phenol. Selectivity to p,p′-bisphenol-A significantly increases owing to enhanced steric hindrance of the positive charged sulfur intermediates.

Scheme 1 Possible mechanism in the condensation reaction of phenol and acetone.

5. Conclusion

A novel highly ordered carbon-based solid acid catalyst has been prepared through controlled sulfonation of FDU-15-500 mesoporous carbons prepared by soft-template method. The final bisphenol-A selectivity was 68%, which was more excellent than the commercial strongly acidic cation exchange resin 001 × 7, D072 and amorphous carbon-based solid acid. The uniform mesoporous structure and hydrophobic property make it a highly active catalyst for the synthesis of bisphenol-A.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (Grants No. 21104035).

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