Bisphenol F Synthesis from Formaldehyde and Phenol over Zeolite Y Extrudate Catalysts in a Catalyst Basket Reactor and a Fixed-Bed Reactor

Abstract

The objective of this study was to evaluate the applicability of zeolite Y as a catalyst for producing bisphenol F (BPF) from phenol and formaldehyde. Catalyst extrudates were prepared by extrusion after adding pseudoboehmite sol (PS) and Ludox (Lu) as alumina and silica binders, respectively. The compressive strength of the catalyst extrudates increased with the addition of Ludox. However, the formaldehyde conversion decreased as more Ludox was used as a binder, resulting in a decrease in the yield of BPF. This decrease is attributed to the reduction in the total amount of acid sites caused by the addition of Ludox. In this study, the Y_PS5_Lu5 catalyst was selected as the most suitable for BPF synthesis. In the BPF synthesis over the Y_PS5_Lu5 catalyst in a catalyst basket reactor, the optimum reaction temperature was determined to be 110 °C. The effect of stirring speed on the yield of BPF was found to be negligible in the range of 200 rpm to 350 rpm. The spent catalyst was able to recover a specific surface area and reaction activity similar to those of a fresh catalyst through regeneration in an air atmosphere at 500 °C. When the Y_PS5_Lu5 extruded catalyst was used in a continuous reaction in a fixed-bed reactor, there was no noticeable deactivation of the catalyst at low space velocities of the reactants. However, when the space velocity was increased to 18.0 h⁻¹, catalyst deactivation was clearly observed. This suggests that periodic regeneration of the catalyst is inevitable in a continuous reaction using the Y_PS5_Lu5 extruded catalyst.

1. Introduction

Bisphenol F (BPF) is an intermediate used in the manufacture of epoxide resins and polycarbonates in various industries, including molding, casting, sealing, coating, encapsulating, adhesives, laminating, reinforced plastics, and other chemical industries [1,2,3]. The demand and production of bisphenol F-type epoxy resins are expected to increase steadily in the future due to their lower viscosity and better resistance to solvents compared to bisphenol A-type epoxy resins [4]. The isomers of BPF include 4,4′-dihydroxydiphenylmethane (4,4-BPF), 2,4′-dihydroxydiphenylmethane (2,4-BPF), and 2,2′-dihydroxydiphenylmethane (2,2-BPF) [5].

BPF is produced via the hydroxyalkylation reaction of phenol (C₆H₅OH) with formaldehyde (HCHO) using an acidic catalyst (Scheme 1). In the conventional process, liquid protonated acids (oxalic acid, phosphoric acid, hydrochloric acid, etc.) are used as catalysts, which have disadvantages such as severe corrosion of equipment, difficulty in catalyst recovery, and environmental problems due to high toxicity [6,7,8]. Therefore, there is a need to develop heterogeneous catalysts with high activity and selectivity, as well as easy separation and high reusability in the industry. Various types of solid acid catalysts have been studied for BPF synthesis [3,4,5,6,7,8,9,10,11].

In particular, the application of mesoporous solid acid catalysts has attracted much attention in recent years. However, these catalysts are not highly reproducible in recovery and reuse, and their high cost limits their industrial applicability [4,7,10]. Zeolite is considered a promising catalyst for the synthesis of bisphenol F due to its stable crystal structure, clear morphology, and microporosity [2]. However, a major problem with these catalysts is the precipitation of bulky and high-molecular-weight products during the reaction, leading to catalyst deactivation.

Existing bisphenol F synthesis studies provide limited results on the performance of zeolite Y catalysts. Additionally, most existing studies focus on powder-type catalysts, and it is difficult to find results on the performance of molded catalysts suitable for the bisphenol F synthesis process [3,4,5,6,7,8,9,10,11]. To the best of our knowledge, this is the first study on the continuous reaction in a fixed-bed reactor using molded catalysts for bisphenol F synthesis.

In this study, zeolite Y was used as a catalyst to produce bisphenol F from phenol and formaldehyde. Various binders were added to the powder-type catalyst and then molded into extrudates for use in a scaled-up process. The physicochemical properties of the catalyst extrudates were analyzed, and their effects on the performance of the bisphenol F synthesis were studied. The regenerability of the molded zeolite Y catalyst was confirmed in a batch reactor equipped with a catalyst basket. Additionally, studies on the optimization of space velocity and catalyst lifetime in a fixed-bed reactor loaded with molded zeolite Y catalysts were conducted.

2. Results and Discussion

2.1. Characteristics of Catalyst Extrudates

In this study, the content of pseudoboehmite sol, an alumina-based binder with the chemical formula Al₂O₃·nH₂O, was fixed at 5 wt%. The content of Ludox, a silica-based binder, was increased to 30 wt%. The SiO₂/Al₂O₃ ratio of the catalyst extrudates was analyzed using XRF, and the results are shown in

Table 1. Composition of binder, SiO₂/Al₂O₃ mole ratio, and compressive strength of catalyst extrudates.

Catalysts	Catalyst Shape	Binder		SiO2/Al2O3	Compressive Strength (N/mm2)
		PS (wt%) a	Lu (wt%) b
Y(80)	powder	-	-	80.0	-
Y_PS5_Lu5	extrudates	5	5	41.8	0.31
Y_PS5_Lu10	extrudates	5	10	43.7	0.32
Y_PS5_Lu20	extrudates	5	20	46.5	0.46
Y_PS5_Lu30	extrudates	5	30	49.4	0.57

^a PS: pseudoboehmite sol. ^b Lu: Ludox.

. Compared to the parent zeolite Y(80) powder, the SiO₂/Al₂O₃ ratio of the extrudates decreased significantly. The SiO₂/Al₂O₃ ratio of the extrudates, molded by adding 5 wt% each of pseudoboehmite sol and Ludox, was 41.8, which is about half of that of the parent zeolite Y. However, as the content of Ludox increases to 30 wt%, the SiO₂/Al₂O₃ ratio tends to increase to 49.4.

One of the most important physical properties of a molding catalyst is its compressive strength. In this study, the compressive strength of four types of extrudates ranged from 0.31 N/mm² to 0.57 N/mm². Notably, these values were consistent with the literature data [12,13]. Furthermore, the compressive strength of the extrudates increased with the addition of Ludox. According to the literature, Ludox, a colloidal silica, has a small particle size, small pore diameter, and low zeta potential, which can enhance the mechanical strength of extrudates [14]. Previous studies have shown that the strength of ZSM-5 extrudates increases with increasing silica content up to a certain level [15]. Similarly, it has been reported that for H-beta zeolite extrudates, the strength is highest when 30 wt% of Ludox is added, which aligns well with the findings of this study [14,15]. Generally, the strength of molded catalysts tends to increase with the amount of inorganic binders. However, excessive additions of inorganic binders may compromise the unique physicochemical properties of the parent zeolite.

The results of N₂ adsorption of catalyst extrudates are summarized in Figure 1 and

Table 2. Physical properties of catalyst extrudates.

Catalysts	SBET a (m2/g)	SLangmuir b (m2/g)	Average dp c (nm)	Vp d (cm3/g)
Y(80)	853	819	2.5	0.3
Y_PS5_Lu5	798	757	2.5	0.2
Y_PS5_Lu10	800	776	2.6	0.2
Y_PS5_Lu20	746	713	3.1	0.4
Y_PS5_Lu30	715	684	3.0	0.3

^a Surface area calculated by the BET method. ^b Surface area calculated by the Langmuir model. ^c Average pore size as determined by the BJH method. ^d Total pore volume estimated from the isotherm at P/P₀ = 0.99.

. The N₂ adsorption–desorption isotherms of the zeolite Y powder and the catalyst extrudates used in this study correspond to type I of the International Union of Pure and Applied Chemistry (IUPAC) classification (Figure 1). This means that the adsorption amount increased rapidly due to the micropores when the P/P₀ range was below 0.03. Moreover, the N₂ adsorption–desorption isotherms of the catalysts also exhibit a step-down of the desorption branch at P/P₀ near 0.5 and a hysteresis loop when the P/P₀ range is above 0.5, which is somewhat different from the typical type I isotherm and can be interpreted as being closer to the H4 loop of type IV [16]. H4 loops are often found with aggregated crystals of zeolites and some mesoporous zeolites. Since the adsorption curve of IUPAC type IV is typical of mesopore-bearing materials, we confirmed that the catalyst extrudates prepared in this study have some mesopores [16,17]. The development of mesopores in the catalyst extrudates is further confirmed by the pore size distribution in Figure 2. Consequently, it was found that the catalyst extrudates prepared in this study are mainly composed of microporous structures, with a partial generation of mesopores.

While the BET surface area of the parent zeolite Y was 852 m²/g, the BET surface area of the catalyst extrudates decreased to less than 800 m²/g. Additionally, the specific surface area gradually decreased with an increase in binder content, and the BET surface area of Y_PS5_Lu30 was 715 m²/g, resulting in a surface area loss of about 16% compared to the parent zeolite Y. For Y_PS5_Lu30, the increase in the amount of Al₂O₃ and SiO₂ in the binder is equivalent to approximately 14% of the mass of the parent zeolite Y. In other words, the amount of binder added and the percentage of surface area loss are not significantly different, indicating that no pore blockage or filling occurs during extrusion. On the other hand, the total pore volume of the catalyst extrudates is higher than that of the parent zeolite Y, which could be due to additional porosity at the zeolite–binder interface [18]. Based on the comprehensive evaluation of the N₂ adsorption isotherm, BET surface area, and pore size distribution results, it has been confirmed that the pore characteristics were not significantly lost during the extrusion process in the current study.

The NH₃-TPD profiles and the amount of acid sites of the catalyst extrudates are presented in Figure 3 and

Table 3. Amount of acid sites of catalyst extrudates determined by NH₃-TPD.

Catalysts	Weak Acid Sites (mmol/g)	Strong Acid Sites (mmol/g)	Total Acid (mmol/g)	Weak Acid Sites/Strong Acid Sites
Y(80)	7.6	26.5	34.1	0.29
Y_PS5_Lu5	16.3	25.4	41.7	0.64
Y_PS5_Lu10	14.3	22.2	36.5	0.64
Y_PS5_Lu20	12.1	18.3	30.4	0.66
Y_PS5_Lu30	10.7	14.9	25.6	0.72

, respectively. Generally, the NH₃-TPD profiles of HY zeolites exhibit two peaks: a low-temperature peak around 175 °C (indicating weak acid sites) and a high-temperature peak around 340 °C (indicating strong acid sites) [19,20]. Although the NH₃-TPD spectra cannot differentiate between Brønsted and Lewis acid site acidity, it is known that the low-temperature peak corresponds to the desorption of ammonia from the weakly acidic Lewis acid site, while the high-temperature peak is attributed to the desorption of bridged OH groups from the more acidic Brønsted site [12]. Firstly, in comparing the amount of acid sites in parent zeolite Y and Y_PS5_Lu5 extrudates, it is observed that the amount of weak acid sites in Y_PS5_Lu5 extrudates is significantly increased, whereas the amount of strong acid sites is slightly decreased. As a result, the total amount of acid sites in Y_PS5_Lu5 extrudates is higher than that of the parent zeolite Y. It is notable that the ratio of weak acid sites to strong acid sites increases by more than two times after the molding process. This can be attributed to the considerable increase in the amount of weak acid sites due to the addition of alumina. However, as the amount of Ludox is increased based on Y_PS5_Lu5 extrudates, both the amount of weak and strong acid sites decrease simultaneously, leading to a gradual decrease in the total amount of acid sites. Additionally, the ratio of weak acid sites to strong acid sites slightly increases with the increasing amount of Ludox. The decrease in the amount of acid sites in the catalyst with the addition of Ludox is due to the neutralization of a portion of the acid sites by the sodium cations present in Ludox (40 wt% SiO₂, 0.41 wt% Na₂O, and 59.5 wt% H₂O) [12].

Figure 4 and

Table 4. Amount of Brønsted and Lewis acid sites determined by pyridine-FTIR.

Catalysts	Brønsted (μmol/g)	Lewis (μmol/g)	Brønsted/Lewis
Y(80)	115	142	0.81
Y_PS5_Lu5	55	116	0.48
Y_PS5_Lu10	41	219	0.18
Y_PS5_Lu20	29	122	0.23
Y_PS5_Lu30	21	137	0.15

provide the acid site concentrations in zeolite powders and extrudates, specifically focusing on the two bands that are characteristic of Brønsted (1540 cm⁻¹) and Lewis acid sites (1450 cm⁻¹). It is worth noting that the concentration of Brønsted acid sites in Y_PS5_Lu5 extrudates is significantly lower compared to its parent zeolite Y, while the amount of Lewis acid sites is slightly reduced. Consequently, the concentration ratio of Brønsted acid sites to Lewis acid sites in Y_PS5_Lu5 extrudates is smaller than that of the parent zeolite Y. Furthermore, as the amount of Ludox binder increases in the extrudates, the concentration ratio of Brønsted acid sites to Lewis acid sites tends to decrease. Based on the combined results of NH₃-TPD and pyridine-FTIR, it is observed that as the amount of Ludox increases, both the total amount and strength of acid sites decrease. This decrease is attributed to the decrease in the concentration ratio of Brønsted acid sites to Lewis acid sites.

2.2. BPF Synthesis in a Batch Reactor Equipped with Catalyst Basket

After loading the catalyst extrudates into the catalyst basket of a batch reactor, a BPF synthesis reaction was performed, and the results are compared in Figure 5. The typical reaction conditions were as follows: reaction temperature: 110 °C; catalyst/formaldehyde weight ratio: 90; phenol/formaldehyde mole ratio: 10; stirring speed: 250 rpm. After a 60 min reaction using the Y_PS5_Lu5 catalyst, the formaldehyde conversion, selectivity to BPF, and yield of BPF were 68.1%, 74.5%, and 50.8%, respectively. The amount of Ludox used as a binder was found to have an impact on the formaldehyde conversion. Increase in Ludox amount resulted in a decrease in the parent zeolite content in the catalyst extrudates, leading to a decrease in the BET surface area (Table 1). NH₃-TPD results also showed a decrease in the total amount of acid sites with increasing Ludox addition (Table 3 and Figure 3). However, the effect of Ludox addition on selectivity to BPF was negligible, suggesting that the shape selectivity was not affected by the zeolite Y pores, as evidenced by N₂ adsorption experiments that confirmed no pore blockage or filling caused by the binder. Nonetheless, the BPF yield decreased to 34.3% for the Y_PS5_Lu30 catalyst. Ultimately, the Y_PS5_Lu5 catalyst demonstrated superior performance in BPF synthesis. It should be noted, though, that decreasing the Ludox content does come with the disadvantage of reducing the compressive strength of the catalyst extrudates (Table 1). Nevertheless, the compressive strength of the Y_PS5_Lu5 catalyst is still comparable to that of zeolite molds reported in the literature, making it a potential candidate as the best catalyst for BPF synthesis.

The effect of reaction temperature on BPF synthesis over the Y_PS5_Lu5 catalyst is depicted in Figure 6. As the reaction temperature increased from 70 to 110 °C, the formaldehyde conversion increased from 44.3% to 68.1%, while the BPF selectivity slightly decreased from 83.3% to 74.5%. Consequently, the BPF yield increased from 36.9% to 50.8% with the rise in reaction temperature. To maintain the reaction in the liquid phase, as formalin and phenol are prone to vaporize above 110 °C under atmospheric pressure, the reaction is typically conducted at 110 °C or lower [1,2,5]. Therefore, 110 °C was chosen as the optimum reaction temperature in this study.

Figure 7 shows the impact of stirring speed on BPF synthesis using the Y_PS5_Lu5 catalyst in a batch reactor with a catalyst basket. Increasing the stirring speed to 350 rpm from 200 rpm did not significantly affect the selectivity to bisphenol F or the conversion of formaldehyde. Therefore, the stirring speed has a negligible effect on the yield of BPF within the range of 200 to 350 rpm. Generally, the influence of stirring speed on reaction rate in batch reactors is studied to understand the impact of mass transfer on heterogeneous catalysts [21]. If the reaction rate increases with higher stirring speed, it is advantageous to further increase the stirring speed, because external mass transfer plays a role in the reaction rate. However, in the experiments using Y_PS5_Lu5 catalyst extrudates, the reaction rate is not dependent on the stirring rate, indicating that the Y_PS5_Lu5 catalyst has little external diffusion limitation. Consequently, it is confirmed that the reaction rate for the synthesis of bisphenol F using the Y_PS5_Lu5 catalyst is more influenced by internal mass transfer or surface reactions rather than external mass transfer [21].

Unsurprisingly, increasing the catalyst amount simultaneously increased the formaldehyde conversion and selectivity to BPF (Figure 8). Raising the catalyst/formaldehyde ratio from 15 to 60 resulted in a BPF yield increase from 31.1% to 50.8%. However, further optimization work is required during the reactor design stage, because increasing the catalyst amount could complicate the operation and economy in the scale-up process.

2.3. Catalyst Regeneration and Reuse in Batch Reactor

The BPF synthesis reaction was carried out in a batch reactor equipped with a catalyst basket using Y_PS5_Lu5 catalyst for 1 h. The spent catalyst was then recovered and subjected to calcination under an air atmosphere at 300 °C for 3 h to regenerate the catalyst. The regenerated catalyst was loaded back into the batch reactor with a catalyst basket, and the BPF synthesis reaction was performed. This regeneration and reuse procedure was repeated to assess the regenerability of the spent catalyst. Figure 9 illustrates the effect of the regeneration temperature of the spent catalyst on the repeatability of the catalyst. It can be observed that for the catalyst regenerated at 300 °C, both formaldehyde conversion and selectivity to BPF decreased significantly as the cycle of regeneration and reuse increased (Figure 9a). In the cycle of regeneration and reuse of the spent catalyst described in the previous section, BPF synthesis experiments were conducted by increasing the calcination temperature to 400 and 500 °C, respectively. Repeated regeneration of the spent catalyst at 400 °C resulted in a slight decrease in formaldehyde conversion, while the selectivity to BPF did not show any significant decrease (Figure 9b). Moreover, when the regeneration temperature of the spent catalyst was increased to 500 °C, both formaldehyde conversion and selectivity to BPF remained constant, indicating successful regeneration.

The results of the BPF synthesis after five cycles of the regeneration and reuse procedure are summarized in Figure 10. Given that the regenerated catalyst exhibited the highest reaction activity at 500 °C, this temperature was selected for the regeneration of the spent catalyst. When comparing the images of the catalysts after five cycles, it is evident that the catalyst regenerated at 300 °C has a very dark color. The catalyst regenerated at 400 °C is lighter than the one regenerated at 300 °C, but still darker than the fresh Y_PS5_Lu5 catalyst (Figure 11). On the other hand, the catalyst regenerated at 500 °C has a bright white color, similar to the fresh catalyst, indicating sufficient regeneration based on exterior color alone.

Figure 12 depicts the change in BET surface area of the regenerated catalyst. For the Y_PS5_Lu5 catalyst regenerated at 300 °C, the specific surface area decreased significantly with repeated regeneration and reuse. While the specific surface area of the fresh catalyst was 798 m²/g, the specific surface area of the catalyst regenerated five times decreased to 31 m²/g, indicating that the surface area of the spent catalyst was not effectively recovered. The specific surface area of the Y(80)_PS5_Lu5 catalyst regenerated at 400 °C five times slightly decreased compared to the fresh catalyst (Figure 12). Furthermore, the BET surface area of the catalyst regenerated at 500 °C is almost the same as that of the fresh Y_PS5_Lu5 catalyst. Therefore, 500 °C was selected as the regeneration temperature to fully refresh the spent catalyst.

2.4. Continuous BPF Synthesis in a Fixed-Bed Reactor

The continuous synthesis of bisphenol F was carried out using the Y_PS5_Lu5 catalyst in a fixed-bed reactor. At a space velocity (liquid hourly space velocity, LHSV) of 1.0 h⁻¹, the formaldehyde conversion remained above 95.5% until the time-on-stream exceeded 300 min, indicating that no deactivation was observed (Figure 13). The selectivity to BPF and the BPF yield at 300 min of time-on-stream were 72.3% and 69.0%, respectively, which were comparable to those at the start of the reaction (

Table 5. Effect of space velocity on conversion, selectivity, and yield over Y_PS5_Lu5 extrudates in a fixed-bed reactor (temperature: 110 °C, P/F = 10).

Liquid Hourly Space Velocity (h−1)	Time-on-Stream (min)	Formaldehyde Conversion (%)	Selectivity to BPF (%)	Yield of BPF (%)
1.0	60	97.4	75.8	73.8
	300	95.5	72.3	69.0
2.0	60	73.7	74.8	55.1
	300	63.4	76.6	48.6
18.0	60	37.0	73.7	27.3
	300	15.3	58.2	8.9

).

In another continuous reaction where the LHSV was increased to 2.0 h⁻¹, the formaldehyde conversion was 73.7% at the beginning of the reaction (time-on-stream 60 min) and gradually decreased to about 63.4% at time-on-stream 300 min (Figure 13 and Table 5). The selectivity to BPF at 300 min of time-on-stream was 76.6%, similar to the start of the reaction, but the BPF yield dropped to 48.6%. When the continuous reaction was performed under harsh conditions with a space velocity of 18.0 h⁻¹, the formaldehyde conversion significantly decreased from 37.0% at time-on-stream 60 min to 15.3% after time-on-stream 300 min. The selectivity to BPF also decreased from 73.7% to 58.2%. Therefore, the BPF yield decreased by one-third from 27.3% to 8.9%. A significant catalyst deactivation was observed when the space velocity was increased to 18.0 h⁻¹. The catalyst deactivation is believed to be caused by high-boiling-point compounds with bulky and high molecular weight blocking the pores of the zeolite catalyst or fouling the active site [4]. Therefore, periodic catalyst regeneration is unavoidable even in continuous reactions using Y_PS5_Lu5 catalysts.

3. Experimental Details

3.1. Preparation of Catalyst Extrudates

The zeolite Y (SiO/Al₂O₃ = 80, NH₄⁺ form) powder was purchased from A.H.A International Co., Ltd. (Hefei, China) and converted to the H⁺ form by calcination at 550 °C for 4 h in an air atmosphere. For catalyst molding, pseudoboehmite sol (PS) and Ludox HS-40 (Lu) were used as binders, and the contents of the binders are shown in Table 1. In the names of the catalyst extrudates, the numbers refer to the content of each binder added during the extrusion procedure. For example, Y_PS5_Lu5 catalyst refers to the catalyst extrudates formed by adding 5 wt% of pseudoboehmite sol (PS) and 5 wt% of Ludox HS-40 (Lu) to zeolite Y.

The extrusion procedure is described in detail as follows. After adding binder and water to the zeolite Y powder, the dough was kneaded by hand. Additional kneading was performed using an in-house kneader to increase the density of the dough and remove air from it. The kneaded dough was fed into an in-house piston extruder and extruded into a noodle-shaped form with a diameter of 2 mm, which was then dried at room temperature. The dried extrudates were then cut into a length of 6 mm and calcined at 550 °C for 3 h, resulting in catalyst extrudates with a diameter of 2 mm and a length of 6 mm. A representative image of fresh Y_PS5_Lu5 extrudates is shown in Figure 11a.

3.2. Characterization of Catalyst Extrudates

BELSORP-miniII (BEL Japan Co., Tokyo, Japan) was used to measure the N₂ adsorption–desorption isotherm. The catalyst sample was pretreated at 200 °C for 4 h under vacuum to remove impurities from the pores. N₂ gas adsorption and desorption experiments were then conducted at −196 °C. The specific surface area was calculated using both the Brunauer–Emmett–Teller (BET) method and the Langmuir model. The size of the pores was determined using the Barrett, Joyner, and Halenda (BJH) method. The total pore volume was estimated from the isotherm at P/P₀ = 0.99.

NH₃ temperature programmed desorption (NH₃-TPD) analysis was performed to assess the strength and amount of acid sites on the catalyst. The NH₃-TPD analysis utilized the BEL-CAT-B from BEL JAPAN Co. To eliminate impurities and moisture from the catalyst surface, the catalyst was outgassed under a flow of He (50 mL/min) while being heated from room temperature to 550 °C at a rate of 10 °C/min. The outgassing procedure was then conducted at 550 °C for 1 h, followed by cooling to 100 °C. After adsorbing NH₃ under a flow of 50 mL/min of NH₃ (3% in helium balance) for 30 min, helium was flowed for 2 h to eliminate physisorbed NH₃ from the catalyst surface. The NH₃ desorbed from the catalyst was measured using a thermal conductivity detector while heating the sample from 100 to 700 °C at a rate of 10 °C/min to obtain the NH₃-TPD profile.

The kinds of acid sites on the catalyst surface were analyzed by the Fourier transform infrared spectroscopy of adsorbed pyridine (pyridine-FTIR) using a Spectrum GX (PerkinElmer, Shelton, CT, USA). After degassing of the disk-type sample (0.013 g) for 2 h in a vacuum below 1.0 × 10⁻² torr and at 350 °C, pyridine was allowed to adsorb onto the catalyst for 30 min at room temperature. The IR spectrum was collected in a temperature range of 150 °C under a vacuum below 1.0 × 10⁻² torr. The amount of pyridine adsorbed on the Brønsted (1540 cm⁻¹) and Lewis (1450 cm⁻¹) sites was determined by integrating their respective band areas using the following extinction coefficients: ε(Brønsted)1540 = 1.35 and ε(Lewis)1450 = 1.5 cm mol⁻¹ for pyridine [18].

X-ray fluorescence (XRF) measurements were performed using a ZSX Primus wavelength dispersive spectrometer from Rigaku Co. (Tokyo, Japan). In this study, XRF was used to determine the SiO₂/Al₂O₃ ratio. The strength of the molded catalysts was evaluated using an in-house compressive strength tester. Ten samples of each catalyst extrudate type were taken to measure the compressive strength, and the average value was used. The compressive strength of the catalyst extrudates was calculated using Equation (1).

$$\mathrm{C}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\left({\displaystyle \frac{\mathrm{N}}{{\mathrm{m}\mathrm{m}}^2}}\right)={\displaystyle \frac{\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\left(\mathrm{N}\right)}{\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{c}\mathrm{i}\mathrm{r}\mathrm{c}\mathrm{l}\mathrm{e}\left(\mathrm{m}\mathrm{m}\right)\times \mathrm{l}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{t}\mathrm{h}\left(\mathrm{m}\mathrm{m}\right)}}$$

(1)

3.3. Synthesis of Bisphenol F in a Catalyst Basket Reactor and a Fixed-Bed Reactor

Phenol (99.0%, Daejung Co., Busan, Republic of Korea) and formaldehyde (37.0%, Duksan Pure Chemicals Co., Ansan-si, Republic of Korea) were used as the raw materials for synthesizing bisphenol F. A three-necked round flask (500 mL) was used as a reactor, and a catalyst basket (internal volume 12 mL) was attached to the stirring rod (Figure 14). The catalyst extrudates were loaded into the catalyst basket of the reactor, and phenol and formalin were added to the reactor. The temperature of the reactant was measured using a thermocouple (K-type), and the stirring speed was adjusted using a mechanical overhead stirrer. After reaching a certain reaction temperature, the liquid product was collected at specific time intervals. Liquid chromatograph (1260 Infinity II LC, Agilent, Santa Clara, CA, USA) and gel permeation chromatograph (LC-20A, Shimadzu, San Jose, CA, USA) were used to determine the formaldehyde conversion and selectivity to BPF, respectively. Formaldehyde conversion, selectivity to BPF, and BPF yield were calculated using the following equations.

$$\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{R}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{e}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{e}}{\mathrm{I}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{l}\mathrm{d}\mathrm{e}\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{e}}}\times 100$$

(2)

$$\mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{B}\mathrm{i}\mathrm{s}\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{o}\mathrm{l}\mathrm{F}}{\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\left(\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{r}\right)}}\times 100$$

(3)

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\times \mathrm{S}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}}{100}}$$

(4)

3.4. Synthesis of Bisphenol F in a Fixed-Bed Reactor

A schematic diagram of a fixed-bed reactor for bisphenol F synthesis is shown in Figure 15. An SUS reactor with an inner diameter of 1.3 cm was filled with 6 mL of catalyst extrudates. Glass beads were placed above and below the catalyst to prevent channeling in the catalyst bed. The reactant supply tube entered the reactor, and the product discharge tube from the reactor to the gas chromatography was heated with a heating band to maintain a temperature of 60 °C. Once the reactor reached the desired reaction temperature and stabilized, the feed (phenol and formalin) was introduced. The flow rate of the feed was adjusted using a metering pump. After the product began to flow out of the reactor outlet, the product was collected at specific time intervals to measure the formaldehyde conversion, selectivity to BPF, and BPF yield.

4. Conclusions

The addition of Ludox increased the compressive strength of the catalyst extrudates. As the binder content increased, the specific surface area gradually decreased. However, it was found that the extrusion process did not significantly affect the pore properties of the parent zeolite Y. The total amount of acid sites in the catalyst extrudates was reduced by the addition of Ludox.

In the BPF synthesis, the yield of BPF decreased as the amount of Ludox used as a binder increased. Among the catalysts tested, Y_PS5_Lu5 catalyst was found to be the most suitable for BPF synthesis. The reaction temperature selected as optimum in the catalyst basket reactor was 110 °C. The stirring speed had a negligible effect on the yield of BPF in the range of 200 rpm to 350 rpm, indicating that the reaction rate of bisphenol F synthesis using Y_PS5_Lu5 catalyst extrudates depends more on internal mass transfer or surface reaction than external mass transfer. The spent catalyst regained a specific surface area and reaction activity similar to those of a fresh catalyst after regeneration in an air atmosphere at 500 °C.

When the Y_PS5_Lu5 extruded catalyst was used in a continuous reaction in a fixed-bed reactor, catalyst deactivation was not observed at low space velocities of the reactants. However, when the space velocity was increased to 18.0 h⁻¹, catalyst deactivation became evident, indicating that periodic regeneration of the catalyst is necessary in continuous reactions using the Y_PS5_Lu5 extruded catalyst.