3.1 Catalyst characterization
The FT-IR spectra of PDVB and PDVB-SO3H-IS was shown in Fig. 1. The infrared absorption peaks of the two samples at 1449 cm− 1 were attributed to the stretching vibration of aromatic C = C in the polymer carrier [31]. The absorption peak of PDVB-SO3H-IS at 1043 cm− 1 was attributed to C-S stretching vibration and other absorption peaks at 1008 cm− 1, 1128cm− 1, 1176 cm− 1 were caused by the symmetrical and asymmetrical stretching vibrations of O = S = O indicating the presence of sulfonic acid groups [32, 33].
The XRD diagrams of PDVB and PDVB-SO3H-IS were shown in Fig. 2. The broad diffraction peak of the two samples between 10 and 30° was the C (002) diffraction peak, caused by the carbon on the disorderly connected benzene ring of the polymer. The weak diffraction peak at 35–50°C was the C (101) diffraction peak. The low angle diffraction peak of PDVB-SO3H-IS showed a slight shift compared to PDVB, indicating the successful grafting of sulfonic acid groups onto the polymer backbone [34]. In addition, due to the presence of a large number of sulfonic acid groups on PDVB-SO3H-IS, the diffraction peak intensity was significantly lower than that of PDVB.
N2 adsorption and desorption were carried out to obtain the texture properties of PDVB and PDVB-SO3H-IS. Figure 3 showed the N2 adsorption isotherm and pore size distribution of the two samples. Both PDVB and PDVB-SO3H-IS showed type IV isotherms with distinct hysteresis loops in the P/P0 range of 0.7–0.95, indicating the existence of mesoporous structure [35, 36].
Table 1
Texture parameters of PDVB and PDVB-SO3H-IS
Sample | SBET (m2/g) | Vp (cm3/g) | Dp (nm) |
PDVB | 793 | 1.58 | 15.9 |
PDVB-SO3H-IS | 650 | 1.50 | 10.7 |
Table 1 showed the specific surface area, pore volume, and average pore size of PDVB and PDVB-SO3H-IS samples. The specific surface area of PDVB could reach 793 m2/g, with a pore volume of 1.58 cm3/g and an average pore size of 15.9 nm. PDVB-SO3H-IS had the specific surface area of 650 m2/g, pore volume of 1.50 cm3/g, and average pore size of 10.7 nm. After sulfonation, the specific surface area of PDVB-SO3H-IS decreased compared to PDVB, but it was still high. A high specific surface area was conducive to increasing the effective acidic sites, and the mesoporous structure was conducive to the diffusion of reactants and products.
The microstructure of PDVB-SO3H-IS was characterized by SEM as shown in Fig. 4. PDVB-SO3H-IS exhibited the spongy structure, which corresponded to the results of N2 adsorption and desorption that the sample had mesoporous and macroporous pore structure. The microstructure of PDVB was similar to that of PDVB-SO3H-IS (Figs. 1), indicating that in situ sulfonation had little effect on the microstructure of the sample [37]. SEM-EDX analysis was performed on PDVB-SO3H-IS to determine the distribution of various elements in the sample. As shown in Fig. 4, the C element in PDVB-SO3H-IS was provided by the carrier, and the S element came from the sulfonic acid group. It can be seen that the SEM-EDX result corresponded to the FT-IR result which successful in-situ sulfonation.
Table 2
The elemental analysis and acid-base titration results of PDVB and PDVB-SO3H-IS
Sample | S content (mmol/g) | Acid density (mmol H+/g) |
PDVB | - | - |
PDVB-SO3H-IS | 1.55 | 1.58 |
Table 2 showed the elemental analysis and acid-base titration results of PDVB and PDVB-SO3H-IS. As shown in the table, the sulfur content of PDVB-SO3H-IS basically corresponded to the acid density, because both the acidic site and sulfur element of PDVB-SO3H-IS were provided by sulfonic acid groups, which also indicated that the acidic center of PDVB-SO3H-IS was mainly strong acidic sulfonic acid groups.
3.2. Catalytic performance of in situ sulfonated mesoporous polyvinylbenzene with different preparation conditions
Firstly, the effect of catalyst preparation conditions on the catalytic performance was investigated to optimize the catalyst preparation process. During the catalyst preparation process, the amount of sulfonation reagent directly affected the acid density of obtained PDVB-SO3H-IS. Therefore, the effect of sulfonation reagent sodium p-styrene sulfonate dosage on the catalytic performance of PDVB-SO3H-IS solid acid was studied. The sulfonation reagent dosage was calculated based on the molar ratio of sodium p-styrene sulfonate (SPSS) to divinylbenzene (DVB), and the results were shown in Table 3. As the molar ratio of SPSS to DVB increased from 0.1 to 0.3, the acid density and catalytic performance of PDVB-SO3H-IS increased simultaneously. When the molar ratio of SPSS to DVB was 0.3, the acid density of PDVB-SO3H-IS was 1.58 mmol H+/g. Using this as the catalyst, the phenol conversion rate of 28% and the Bisphenol-A selectivity of 91% were obtained at 90°C for 6 h. Continuing to increase the amount of sodium p-styrene sulfonate, the acid density of PDVB-SO3H-IS did not change much, and its catalytic performance remained unchanged. Therefore, the optimal molar ratio of SPSS to DVB was 0.3.
Table 3
The effect of SSPS dosage on the acid density and catalytic performance of PDVB-SO3H-IS
Molar ratio SPSS/DVB | Acid density (mmol H+/g) | Phenol Conv. (%) | p, p’-Bisphenol-A Sel. (%) |
0 | - | - | - |
0.1 | 0.56 | 13 | 85 |
0.2 | 1.10 | 22 | 90 |
0.3 | 1.58 | 28 | 91 |
0.4 | 1.60 | 28 | 91 |
0.5 | 1.63 | 28 | 91 |
Reaction conditions: phenol 120 mmol, acetone 24 mmol, catalyst 0.8 g, temperature 90 oC, time 6 h.
In addition, PDVB-SO3H-IS was prepared by copolymerizing sodium p-styrene sulfonate with divinylbenzene to obtain PDVB-SO3Na, which was then ion exchanged with sulfuric acid ethanol solution. Therefore, the influence of sulfuric acid concentration and the ion exchange time of PDVB-SO3Na and sulfuric acid ethanol solution on the catalytic performance of PDVB-SO3H-IS was investigated with the molar ratio of SPSS to DVB was 0.3. The results showed that it was suitable to develop PDVB-SO3H-IS with PDVB-SO3Na and 1 mol/L sulfuric acid ethanol solution ion exchange for 12 h. The catalytic performance of PDVB-SO3H-IS remained unchanged by continuing to increase sulfuric acid concentration and ion exchange time (Figs. 2).
3.3. Catalytic performance of in situ sulfonated mesoporous polyvinylbenzene with different reaction conditions
The effects of reaction conditions such as catalyst dosage, reaction temperature, and reaction time on the synthesis of Bisphenol-A catalyzed by PDVB-SO3H-IS were investigated with catalysts prepared under the optimal conditions. The catalytic performance of PDVB-SO3H-IS was closely related to the acid density, but the sulfonic groups on the sulfonated mesoporous polydivinylbenzene prepared by in situ sulfonation were limited. Therefore, this part first studied the influence of catalyst dosage on the synthesis reaction of Bisphenol-A. It was planned to increase the acidic centers in the reaction and improved the phenol conversion rate by increasing the catalyst dosage. The catalytic performance was shown in Fig. 5 (a).
The phenol conversion rate increased with the increase of PDVB-SO3H-IS dosage. When the dosage of PDVB-SO3H-IS was 0.8 g, the phenol conversion rate was the highest. Continuing to increase the dosage of PDVB-SO3H-IS, the phenol conversion rate actually decreased. This was because PDVB-SO3H-IS was a high molecular polymer, and an excessive amount of catalyst would cause the reaction liquid to appear in the slurry state. The reaction liquid could not be fully stirred, which affected mass transfer and led to the decrease of phenol conversion rate. Therefore, the appropriate dosage of PDVB-SO3H-IS was 0.8 g. The effect of reaction temperature was shown in Fig. 5 (b). With the reaction temperature increased from 70°C to 90°C, the phenol conversion rate rapidly increased, and the selectivity of Bisphenol-A did not change much. The optimal reaction temperature was 90°C, with the phenol conversion rate of 28% and Bisphenol-A selectivity of 91%. Continuing to increase the reaction temperature, the phenol conversion rate did not change much, but the selectivity of Bisphenol-A decreased because the high temperature led to the increase of triphenol and chromism in products [19, 38–40]. Subsequently, the effect of reaction time on the synthesis of Bisphenol-A catalyzed by PDVB-SO3H-IS was investigated with catalyst dosage of 0.8 g and reaction temperature of 90°C. The results were shown in Fig. 5 (c). As the reaction time increased, the conversion rate of phenol increased, and the selectivity of Bisphenol-A did not change much. After reacting 14 hours, the phenol conversion rate was 38%, which was close to the phenol theoretical conversion rate of 40% under the molar ratio of phenol to acetone of 5:1. Continuing to extend the reaction time, the phenol conversion rate and Bisphenol-A selectivity showed little change.
3.4. Recyclability of PDVB-SO3H-IS
The reusability of PDVB-SO3H-IS prepared under optimized conditions was investigated, and the results were shown in Fig. 6.
Reaction conditions: phenol 120 mmol, acetone 24 mmol, catalyst 0.8 g, temperature 90 oC, time 6 h.
PDVB-SO3H-IS was reused five times under reacting at 90°C for 6 hours. The conversion rate of phenol was maintained at around 28% and the selectivity of the target product Bisphenol-A was 91%, indicating that PDVB-SO3H-IS had good reusability. The excellent reusability of PDVB-SO3H-IS was attributed to the fact that the sulfonic acid group was attached to the support through covalent bonds.