Efficient and Stable Synthesis of Solketal on Mesoporous Aluminum Phosphate Catalyst

Abstract

Solketal is an important chemical product with widespread applications, and the raw materials glycerol and acetone are inexpensive, making it highly economically viable. The glycerol-acetone condensation reaction is a typical acid-catalyzed reaction. Traditional homogeneous acidic catalysts cause significant environmental pollution and are difficult to recover. Herein, PEG-800 was used as an additive, and a one-pot process was employed to prepare a series of aluminum phosphate catalysts (xP-Al-O) with different P/Al molar ratios. The physical and chemical properties of the prepared xP-Al-O catalysts were thoroughly investigated using XRD, FTIR, SEM, Py-FTIR, BET, and NH₃ (CO₂)-TPD methods. The results indicated that different P/Al molar ratios indeed affect the catalyst structure, and all prepared xP-Al-O samples exist in the form of amorphous aluminum phosphate, with weak acidic sites dominating the surface. The prepared catalysts were investigated for their catalytic behavior in the acetalization reaction of glycerol and acetone. The 1.1P-Al-O catalyst exhibited the highest acetone glycerol acetal yield and demonstrated good catalytic stability.

1. Introduction

Comprehensively, due to the impact of the global economic downturn, an increasing number of countries are focusing on the development of clean energy sources. The market for advanced biofuels was valued at USD 136.9 billion in 2024 and is projected to grow rapidly at a compound annual growth rate (CAGR) of 38.5% to USD 965.1 billion by 2030. The primary causes of this growth are technological advancements in feedstocks and decarbonization initiatives in the shipping and aviation sectors [1]. However, roughly 9–10% of the total is made up of crude glycerol, a byproduct of the manufacturing of biodiesel [2]. Thus, the glycerol market is weak, and supply outpaces demand, which poses a major risk to the long-term viability of the biodiesel sector.

The direct use of crude glycerol in the food and pharmaceutical industries is restricted by its high proportion of esters, inorganic salts, and other impurities [3], and its use as a fuel additive is hampered by its high boiling point and strong polarity [4]. One of the main strategic directions in the development of modern biorefinery technology is the conversion of glycerol into high-value-added compounds [5]. Nowadays, scholars all over the world are actively exploring more efficient and feasible ways to utilize the excess glycerol produced by the reaction. Currently, research hotspots mainly focus on the latter to develop high-value-added products such as 1,3 propanediol, propylene oxide, and acetal ketones [6]. The application of cyclic acetals and ketones obtained by condensation reactions of glycerol with aldehydes and ketones, respectively, in the field of fuels and organic chemical intermediates is usually considered to be one of the most promising methods of utilization [7,8]. Among them, the acetal reaction of glycerol with acetone produces the products, 2,2-Dimethyl-1,3-dioxolane-4-methanol and 2,2-Dimethyl-5-(hydroxymethyl)-1,3-dioxane. The major product of this reaction is solketal.

Solketal is a colorless and odorless liquid. It is widely miscible with water, alcohols, esters, ethers, and aromatics, and its boiling point is 189~191 °C under normal pressure. These characteristics make it an efficient green solvent for dissolving natural products and polar drug molecules [9]. Solketal can improve the low-temperature fluidity of biodiesel and increase the octane number; therefore, it can be used as a fuel additive [10]. This molecule is also used as a key organic synthesis intermediate in the synthesis of DL glycerin and α-cyanoacrylate medical adhesives, which are the active ingredients of aldehyde caries prevention drugs [11]. Therefore, solketal is a crucial chemical product. Green policies and the growth of the biodiesel sector are simultaneously driving the market at the same time, and demand is still rising. They also have very high economic benefits because the basic materials, glycerol and acetone, are inexpensive [12].

The acetal reaction between glycerol and acetone is shown in Figure 1, which produces two main isomers.

It can be seen that this reaction is a typical acid-catalyzed reaction, and its conversion efficiency and selectivity significantly depend on the acid content and acid strength of the catalyst. Since homogeneous acid catalysts have a high acid content and strong acidity, inorganic acids are commonly used as catalysts in traditional production [13]. Catalysts such as H₃PO₄ not only enhance the activity of the reaction and the selectivity of the products, but also eliminate the effect of mass transfer between the two phases of glycerol and acetone in the reactants on the results [14]. However, homogeneous acid catalysts are not easy to recycle and cause significant environmental pollution, which is not in line with the principle of sustainable use. Solid acid catalysts are characterized by controllable structures and properties, easy separation, and good stability, which can efficiently and selectively catalyze the production of desired products; therefore, they have obvious advantages in the acetal reaction [15]. Among them, phosphate catalysts have significant advantages in the acetal reaction, which have high catalytic activity and selectivity due to the tunable Brønsted/Lewis bis-acid-site synergistic mechanism, accelerating the nucleophilic addition by polarizing the carbonyl oxygen, and at the same time inhibiting the side reaction by the medium-strong acid property, and the selectivity of the acetal is generally greater than 95% [16]. Phosphate catalysts do not produce acidic wastewater after the acetal reaction and remain active after multiple cycles [17]. These properties make phosphate catalysts suitable for the green synthesis of solketals.

Shunsuke [18] synthesized a CePO₄ catalyst using a hydrothermal method and applied it to the acetalization of 5-hydroxymethylfurfural with methanol. The CePO₄ catalyst exhibited high catalytic activity and selectivity in the acetalization of 5-hydroxymethylfurfural with methanol, with yields of up to 78%. The catalyst maintained a high catalytic activity after five cycles of use, indicating excellent reproducibility. Li [19] synthesized a flower-like zirconium organophosphonate catalyst (ZrPP-x) using the hydrothermal method and applied it to the acetone condensation of glycerol. Glycerol conversion of 90.2% was achieved when the phenyl percentage was 20% in 6 h at 40 °C, and the conversion was only reduced from 90.2% to 87.5% after five times of recycling, and the structure was stable with the retention of an acid amount of 95.5%, as shown by XRD/SEM. Tang [20] prepared AlPO₄-Al₂O₃ composite catalysts by the sol-gel method, and the addition of Al₂O₃ enhanced the thermal stability of AlPO₄ and regulated the surface acidity. In the reaction of acetaldehyde with 1,2-propanediol, the best catalytic activity was obtained with 15% Al₂O₃ under 400 °C roasting, and the conversion of 1,2-propanediol reached 28.1%. The catalytic activity was positively correlated with the amount of acid in the weak acid center on the surface, and the catalyst was stable and reusable. Nanda [21] was the first to apply a continuous-flow reaction mode for the production of acetone glycidyl ether in 2014. By comparing six solid catalysts (Amberlyst 35 Dry, Amberlyst 36 Wet, zeolite, montmorillonite K-10, zirconium sulfate, and Polymax845), they concluded that Amberlyst 36 Wet exhibited the best catalytic performance. Under optimal reaction conditions of 40 °C, 4.1 MPa, and a reaction volume space velocity of 4 h, the yield of pentacyclic acetone glycidyl ether reached 88%. Shirani [22] used the cation exchange resin Purolite^® PD206 to catalyze the condensation of glycerol and acetone in a continuous flow system and investigated the effects of process conditions on the synthesis results. They found that at a temperature of 20 °C, pressure of 12 MPa, acetone to glycerol molar ratio of 5:1, feed rate of 0.1 mL min⁻¹, and catalyst loading of 0.77 g, the yield of acetone glycidylation glycerol reached 95%. This continuous catalytic reaction process is easily scalable, enabling increased production and achieving 100% selective synthesis of five-membered ring acetone glycidylation glycerol. Farooq [23] synthesized WO₃/SAPO-34 solid-acid catalysts using the impregnation method. Experimental results showed that the catalyst with a WO₃ loading of 20 wt% exhibited excellent aldol condensation performance. Under a glycerol/acetone ratio of 1:8, catalyst loading of 5 wt%, reaction temperature of 60 °C, and reaction time of 4 h, the ketal yield reached 96.7%.

Although mesoporous aluminum phosphate materials are well known, their potential for glycerol-acetone condensation has not yet been fully explored. Continuous flow processes, with their high mass-transfer efficiency and scalability, are ideal for the industrialization of aldol condensation. However, only a few studies have explored continuous-flow systems. This study is the first to combine mesoporous aluminum phosphate catalysts with a continuous-flow fixed-bed reactor, addressing the challenge of maintaining the activity of phosphorus-containing catalysts in flowing systems and providing a new strategy for the continuous and efficient synthesis of solketal. Currently, there are few studies on the application of mesoporous aluminum phosphate catalysts to the glycerol acetone acetal reaction. In the present work, a series of xP-Al-O (x = 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2) catalysts with different P/Al molar ratios were prepared by a one-pot synthesis process using PEG-800 as an additive. The effects of P/Al molar ratio on the catalyst structure, acid-base properties, and catalytic performance. Under the validation of the gas-solid phase continuous synthesis system, the 1.1P-Al-O sample showed high catalytic activity for the acetalization reaction of glycerol and acetone after roasting at 500 °C.

2. Results and Discussion

2.1. Characterization of xP-Al-O Catalysts

Figure 2 shows the wide-angle XRD diffraction pattern of xP-Al-O. As shown in the figure, within the 2θ range of 10–90°, the diffraction patterns of all samples exhibit similar trends. The XRD spectrum shows that the crystal structure of the xP-Al-O catalyst obtained by this preparation method changes with changes in the P/Al ratio. When x = 1.05, the XRD spectrum of the catalyst is approximately a straight line, indicating that the sample has no clear crystal structure at this point and is in an amorphous or non-crystalline state. When x < 1.05, a broad and weak diffraction peak appears near 2θ = 24° in the pattern, indicating the presence of amorphous aluminum phosphate in the sample. This suggests that when x < 1.05, phosphorus, aluminum, and oxygen elements combine to form a specific amorphous aluminum phosphate structure. When x > 1.05, the peak near 2θ = 24° becomes larger, indicating that the amorphous aluminum phosphate structure still exists in the catalyst. In summary, when x < 1.05, amorphous aluminum phosphate is present in the catalyst, and as the phosphorus content increases, the peak representing amorphous aluminum phosphate decreases. When x > 1.05, amorphous aluminum phosphate reappears in the catalyst, and as the phosphorus content increases, the peak representing amorphous aluminum phosphate increases. As the P content increases, no characteristic peaks of the P₂O₅ crystal structure appear in the XRD spectrum of the xP-Al-O catalyst, indicating that P₂O₅ is not formed. This also suggests that the composition of the catalyst is not simply a mixture of Al₂O₃ and P₂O₅, but rather a specific amorphous aluminum phosphate [24].

To determine the effect of phosphorus content on the morphology of the xP-Al-O catalyst, all samples were subjected to scanning electron microscopy at the same magnification. As shown in Figure 3, the morphological differences in the catalyst surfaces with varying phosphorus contents are not significant, with particles of different sizes aggregated, and no impurity crystals observed. When x = 0.9–1.1, as x increases, the particles in the catalyst become increasingly tightly packed, increasing the specific surface area. When x = 1.15–1.2, the catalyst particles become significantly larger but remain tightly packed. This corresponds to the BET results, which show an increase in pore size, with all samples exhibiting mesoporous structures.

Figure 4 shows the FT-IR spectra results of the xP-Al-O catalyst. All tested samples exhibited distinct infrared absorption peaks at 3500 cm⁻¹ and 1633 cm⁻¹, which are attributed to the stretching and bending vibrations of -OH groups. This may be related to the physical adsorption of water molecules on the catalyst surface and other -OH groups within the catalyst [25,26]. The vibrational signal appearing near 1106 cm⁻¹ corresponds to the asymmetric P-O-Al bond stretching vibration peak. The vibrational peak signal at 712 cm⁻¹ is relatively weak and is attributed to the symmetric bending peak of the P-O-P bond in the (PO₄)₃⁻ tetrahedron [27]. The vibrational signal appearing near 495 cm⁻¹ corresponds to the symmetric P-O-Al bond bending vibration peak [28]. This indicates that the catalyst contains an amorphous aluminum phosphate structure at this point, which is consistent with the XRD characterization results.

To investigate the effect of the introduced phosphorus content on the structural properties of the catalyst, particularly its pore structure, N₂ adsorption-desorption characterization was performed on xP-Al-O. The low-temperature nitrogen adsorption-desorption curves and BJH pore size distributions of all catalysts are shown in Figure 5. Figure 5a shows that all samples exhibit Type IV isotherms, which is typical adsorption behavior for mesoporous materials, and a distinct hysteresis loop is observed at P/P₀ = 0.6–1.0. Notably, all samples exhibit H1-type hysteresis loops, consistent with their N₂ isothermal desorption curves, indicating that the pore structure of the material is consistent and well-connected. As shown in Figure 5b, the pore sizes of the samples are primarily distributed between 10 and 20 nm. This indicates that the catalysts prepared with different phosphorus contents all exhibit typical mesoporous structures, consistent with the IUPAC standard definition for mesoporous materials (2.0–50 nm) [29]. As shown in Figure 5a, the change in the hysteresis loop position indicates that both the pore size and pore volume of the catalyst increase with increasing phosphorus content [30]. However, the catalyst still exhibits a Type IV curve and H1-type hysteresis loop, indicating that the pore structure of the catalyst remains unchanged, and the catalyst remains a mesoporous material [31].

As shown in Table 1, the specific surface area first increases and then decreases with increasing phosphorus content. The specific surface area (S_BET, 173 m²g⁻¹) of the 1.1P-Al-O catalyst is higher than that of other catalysts. Although its volume and pore size are not the largest among the samples, its comprehensive pore parameters are the most outstanding. Generally, larger S_BET and V_P values can enhance adsorption and catalytic activity by exposing more adsorption and catalytic active sites. As shown in Figure 5b, the pore size of the catalyst undergoes significant changes due to increased phosphorus loading. This may be attributed to the formation of amorphous aluminum-phosphate particles.

To further investigate the effect of phosphorus content on the acidic sites of the xP-Al-O surface, the samples were subjected to Py-IR characterization testing. As shown in Figure 6, all samples exhibited weak pyridine cation characteristic peaks at 1450 cm⁻¹, corresponding to Lewis acid sites. The spectra do not display any bands related to Brønsted acid sites (1540 cm⁻¹) [32,33]. This indicates that all samples primarily contain weak Lewis acid centers, consistent with the NH3-TPD results. The spectra predominantly display intense bands associated with physisorbed pyridine and hydrogen-bonded pyridine around positions of 1433 cm⁻¹ and 1572 cm⁻¹, which might be attributable to the weak acidity of the synthesized materials. The phosphorus content does not affect the type of acid on the catalyst surface. However, the intensities of all characteristic peaks exhibit a trend of first increasing and then decreasing with increasing phosphorus content, reaching a maximum at x = 1.1. This indicates that the amount of Lewis acid in all xP-Al-O samples follows a pattern of first increasing and then decreasing with the phosphorus content, with the highest acid content observed in the 1.1P-Al-O catalyst.

The acidic and basic surface properties of xP-Al-O catalyst samples were analyzed using TPD, as shown in Figure 7. The surface properties of the catalysts are listed in Table 2. The relative ammonia adsorption capacity is defined as the total desorption amount of each catalyst obtained by integrating the program-temperature desorption curve, divided by the total ammonia desorption amount of the 0.9P-Al-O catalyst (used as a reference, set to 100). The results indicate that all samples adsorbed NH₃ molecules at different concentrations, with a broad peak appearing between 100 and 500 °C, which is a typical desorption peak for amorphous materials [34]. According to the literature, the size of the peak area is related to the number of acidic sites, and the temperature range at which the peak appears is related to the strength of the acid. NH₃ desorption occurs in the temperature ranges of 100–250 °C and 250–400 °C, corresponding to the weak acidic sites and medium-strength sites on the catalyst surface [35,36]. The more acidic the sites, the larger the peak area.

For analytical convenience, the initial NH₃-TPD peak is resolved into smaller peaks at 100–150 °C, 100–250 °C, and 200–500 °C, with the two smaller peaks at 100–150 °C and 100–250 °C corresponding to weak acidic sites [37]. The acidity on the surface of the xP-Al-O catalyst primarily originates from the Al-OH and P-OH groups in the catalyst [25]. This reaction is an acid-catalyzed nucleophilic addition-dehydration process. A weak acid is sufficient to protonate the acetone carbonyl group, activating its nucleophilic attack on the glycerol hydroxyl group. However, a strong acid is prone to side reactions, such as glycerol dehydration to form acrolein, thereby reducing the selectivity of the target product and the stability of the catalyst. Therefore, the acetalization reaction is catalyzed by a weak acid, so only the first two peaks are analyzed here [23,38,39]. As shown in Figure 7a, as x gradually increases, the peak positions of all samples at 100–150 °C shift slightly toward higher temperatures, with peak areas first decreasing and then increasing. At 100–250 °C, the peak positions of all samples shift significantly toward higher temperatures, with peak areas first increasing and then stabilizing. Combining the data in Table 2, it can be seen that as x increases, the relative total acid content on the catalyst surface follows a pattern of first increasing, then decreasing, and finally stabilizing. When x = 1.1, the total acid content of the catalyst reaches its maximum value, which is because the P-OH groups are maximally formed in the amorphous structure, thereby significantly enhancing the weak acid performance of the catalyst [40]. This result is consistent with the BET and FTIR results.

CO₂-TPD was used to determine the base strength of the surface of xP-Al-O samples, with the test results shown in Figure 7b. Generally, the weak base and medium-strong base sites of the catalyst correspond to CO₂ desorption temperatures in the ranges of 100–250 °C and 250–350 °C, respectively [39,41]. As shown in Figure 7b, all samples exhibit a broad desorption peak in the 250–350 °C range, corresponding to medium-strength base sites. The temperature range at which the peak appears is related to the base strength, while the peak area size is related to the number of base sites [31]. In Figure 7b, although the peak range of the xP-Al-O catalyst initially shifts slightly toward higher temperatures with increasing phosphorus content, it decreases overall, indicating a gradual weakening of the base strength of the catalyst. This is because the addition of P elements progressively weakens the base strength of the catalyst. Combining Figure 7b and Table 2, it can be seen that all catalyst samples only contain medium-strength base sites, and as the phosphorus content increases, the number of medium-strength base sites shows an overall decreasing trend, reaching a minimum value at x = 1.1. This indicates that the addition of phosphorus reduces the base strength and decreases the number of base sites.

2.2. Catalytic Performance

The formation of solketal from glycerol and acetone is a typical acid-catalyzed equilibrium reaction. Therefore, the catalytic activity of the catalyst and the reaction conditions play crucial roles in the acetalization reaction. On the one hand, the structure and performance of the catalyst significantly influence reaction activity. On the other hand, selecting optimal reaction conditions can maximize the yield of reactants. Therefore, it is essential to investigate the effects of these two aspects on the acetalization of glycerol and acetone.

The P/Al ratio of the catalyst directly influences its structure, thereby affecting the acetone-glycerol acetalization reaction kinetics. Previous experiments have shown that under the following conditions: 3 g of xP-Al-O catalyst (10–20 mesh), reaction temperature of 100 °C, n_Acetone/n_Glycerol = 3, reaction space velocity of 1 mL·g⁻¹·h⁻¹, and a reaction time of 6 h, the yield and selectivity were relatively high. Therefore, based on the above experimental conditions, the effect of the P/Al ratio on the glycerol-acetone synthesis reaction was studied in a continuous-flow fixed-bed reactor, and the optimal catalyst was determined. As shown in Figure 8, the yield first increases and then decreases with increasing P/Al ratio. When x = 1.1, the yield reaches its maximum at 70.2%. Based on the previous characterization results, this is related to the acidic sites on the surface of the amorphous aluminum phosphate structure of the catalyst. As shown in Table 2, the 1.1P-Al-O catalyst has the highest total acid content and the weakest basic strength, thereby exhibiting excellent catalytic performance in the aldol reaction between glycerol and acetone.

2.3. Stability of the Catalysts

The stability of catalysts is of great importance for their application in industrial catalysis. Therefore, the stability of the reaction between glycerol and acetone to produce acetone glycerol was investigated under the following conditions: a catalyst of 1.1P-Al-O with a particle size of 10–20 mesh and a mass of 3 g, a reaction temperature of 100 °C, n_Acetone/n_Glycerol = 3, and a reaction space velocity of 1 mL·g⁻¹·h⁻¹. Based on the above experimental conditions, experiments were conducted in a continuous flow fixed-bed reactor. The experimental results are shown in Figure 9. As can be seen from the figure, the overall trend shows an upward trend until 143 h of the reaction. By 408 h into the reaction, the yield decreased from the initial 66.8% to 60.3%, although it still maintained a relatively high activity. Selectivity remains > 99.9%. The experimental results indicate that the catalyst exhibits excellent stability and can be used for extended periods in a continuous flow system.

3. Materials and Reagents

The chemical reagents included pseudoboehmite (AlO(OH)·nH₂O (n = 0.08~0.62)), which was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Phosphoric acid (H₃PO₄, AR), ammonium hydroxide (NH₃·H₂O, wt% = 28%), glycerol (C₃H₈O₃, AR), acetone (C₃H₆O, AR), poly(ethylene glycol) (denoted as PEG-800, Mn = 800), and anhydrous ethanol (C₂H₅OH, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). N,N-Dimethylformamide (marked as DMF, wt% ≥ 99.5%) was purchased from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). All chemicals were used without purification. The deionized water (DI water, 18.2 MΩ) used was prepared in-house.

3.1. Preparation of xP-Al-O

The proposed pseudoboehmite and phosphoric acid solution were used as sources of aluminum and phosphorus, and PEG-800 was chosen as the organic additive. Aluminum phosphate catalysts were prepared by a one-pot synthesis process, and the finished catalysts were noted as xP-Al-O, with x denoting the molar ratio of P/Al of the sample (x = 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2). First, 80 g of PEG-800 was weighed in 200 mL of deionized water and 200 mL of anhydrous ethanol, and the resulting solution was placed on a magnetic stirrer and stirred thoroughly. Subsequently, an appropriate amount of phosphoric acid solution was added. After the solution was evenly mixed, 78 g of pseudoboehmite was added to a beaker under constant stirring, and the mixed solution was placed in a water bath at 60 °C with continuous stirring until the pseudoboehmite was completely dissolved. Using a peristaltic pump, 28 wt% ammonia solution was added dropwise to the mixed solution at a rate of 2 mL min⁻¹, and the pH value of the solution was continuously tested until the pH of the mixed solution reached between 8.5–9, and then the dropwise addition was stopped immediately. The mixed solution was stirred in a magnetic thermostatic water bath at 60 °C until the anhydrous ethanol and deionized water evaporated. The resulting solid was then spread evenly in a porcelain disk and placed in an oven at 100 °C for 36 h. Subsequently, the sample was placed in a crucible and roasted in a muffle furnace at a rate of 5 °C min⁻¹ to 500 °C for 24 h. Finally, the resulting solid powder was milled, pressed, and sieved to obtain 10–20 mesh xP-Al-O particulate catalysts.

3.2. Characterization

The physical phase characterization of the catalyst samples was analyzed using X-ray diffraction (XRD) with a D8 Advance diffractometer from Bruker, Karlsruhe, Germany. Cu Kα rays were used as the radiation source (λ = 0.1542 nm), and the tube current and voltage of the diffractometer were set to 40 mA and 40 kV, respectively. The scanning range of the diffraction angle (2θ) was set to 10–90°, and the scanning speed was controlled at 8° min⁻¹, which was carried out according to the scanning work at a rate of 0.5° min⁻¹, to ensure that the resolution of the spectrum was taken into account in the quality of the data. The specific surface area and pore structure of the catalysts were characterized using a Micromeritics ASAP 2020 physical adsorption instrument (Norcross, GA, USA). To remove the surface-adsorbed impurities, the catalyst samples were subjected to a degassing pretreatment at 200 °C in a high-vacuum environment (pressure < 10⁻³ Pa) for 8 h. High-purity nitrogen (99.999%) was used as the adsorbent during the testing process, and static volumetric measurements were performed under standard temperature-pressure conditions (298 K, 1 atm) until the system reached adsorption equilibrium, thus completing the pretreatment phase of gas adsorption. After completing the adsorption, the adsorption data were processed using the Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda (BJH) methods to calculate the specific surface area of the material, pore size distribution, and other key parameters. The functional groups on the catalyst surface were analyzed using a Bruker TENSOR 27 Fourier transform infrared spectrometer (FTIR) from Germany. The samples were ground with spectroscopic grade KBr at a mass ratio of 1:100 in an onyx mortar until homogeneous and pressed into optically transparent flakes with a diameter of 13 mm using a tablet press for characterization and analysis. The samples were dried under an infrared lamp for 30 min before testing to remove moisture adsorbed on the surface. The infrared spectral scanning interval was set at 400–2000 cm⁻¹. The acidic sites on the catalyst surface were characterized using a Bruker TENSOR 27 Fourier Transform Infrared Spectroscopy system (Bruker, Germany) equipped with a QIRS-A02 quartz in-situ reaction cell (Xiamen, China) and a TOPS-HV02 high-vacuum module (Xiamen, China) to form an interconnected test platform (Py-FTIR). The test procedure was as follows: the catalyst powder was pressed into a 13 mm diameter self-supported sheet, and the temperature was increased to 300 °C at a rate of 10 °C min⁻¹ under vacuum. The activation was completed at a constant temperature for 60 min. After activation, the sample was cooled to 50 °C to collect the background spectra. The sample was then contacted with pyridine gas at 50 °C for 10 s to ensure that the pyridine was sufficiently adsorbed on the surface of the acidic sites. After adsorption was completed, the residual pyridine in the in-situ cell was removed. The sample was desorbed at 50 °C, and the infrared spectra were recorded in real time to analyze the interactions between pyridine and the acidic sites. The spectral range of the test was set at 1400–1700 cm⁻¹ to determine the type of acidic sites on the catalyst surface and the intensity of the interaction. The surface morphology of the catalyst was observed using a scanning electron microscope (SEM). Scanning electron microscope micrographs were obtained using a Nova Nano SEM 450 model manufactured by FEI (Hillsboro, OR, USA), which is a field emission-type device. A small amount of catalyst powder was uniformly applied to the conductive tape to ensure full contact between the powder and tape. The excess powder was removed, sprayed with gold, and placed in a scanning electron microscope for inspection. The acid–base properties of the catalysts were characterized using a Peotec PCA-1200 programmed temperature-raising chemisorption instrument (NH₃-TPD and CO₂-TPD) (Beijing, China). The test system comprised a high-precision mass flow controller, quartz micro-reactor, and thermal conductivity detector (TCD). Ar was used as the carrier gas, and NH₃ (CO₂) as the adsorbent atmosphere. Specific operation: The catalyst sample (100 mg) was weighed and loaded into a quartz sample tube, and argon gas was passed at a flow rate of 30 mL min⁻¹. The temperature of the system was increased to 200 °C at a rate of 10 °C min⁻¹, and the system was maintained at a constant temperature for 1 h. After the sample was cooled to 50 °C, NH₃ (CO₂) adsorption was carried out and maintained for 30 min to ensure that NH₃ (CO₂) was fully adsorbed on the acidic (basic) sites of the catalyst surface. At the end of the adsorption process, argon gas was switched to purge for 60 min to remove the physically adsorbed NH₃ (CO₂) on the surface of the sample to ensure the accuracy of the subsequent desorption signal. After blowing, the temperature was increased by 10 °C min⁻¹. The sample was heated up to 500 °C at a rate of 10 °C min⁻¹ to carry out the programmed temperature rise desorption experiment. During the entire test period, the adsorption amount of NH₃ (CO₂) and the desorption temperature were monitored online in real time using a TCD, and the desorption curves were recorded. The strength and number of acidic (basic) sites on the catalyst surface were evaluated by analyzing the positions and areas of the desorption peaks.

3.3. Catalytic Performance Test

The acetalization reaction of glycerol with acetone was carried out in a continuous flow fixed-bed reactor at atmospheric pressure, and the performance of the xP-Al-O catalyst for acetalization was tested by passing glycerol and acetone separately in a continuous flow fixed-bed reactor (Shanghai, China). The catalyst (3 g, 10–20 mesh) was accurately weighed and placed into a quartz tube with an outer diameter of 13 mm, an inner diameter of 10 mm, and a length of 800 mm, which was filled with quartz cotton at both ends and quartz sand packing at the upper part of the tube. Subsequently, the quartz tube was placed near the thermocouple of the reaction device to ensure that the sample section of the catalyst was in full contact with the thermocouple. The reaction was initiated using a vertical reactor and a programmed thermostat, and the temperature was increased to the set value at a rate of 10 °C min⁻¹. Nitrogen was passed through at a flow rate of 30 mL min⁻¹ for purging to avoid the influence of oxygen in the tube on the reaction. After reaching the heating endpoint, the nitrogen gas supply was stopped, and two peristaltic pumps were used to inject glycerol and glycerol separately from the upper end of the quartz tube at the set flow rate. The vaporized reaction solution reacted with the catalyst sample in a gas-solid phase, and the reaction products were condensed and converged into the flask under the device. After the reaction was stabilized, an appropriate amount of liquid was collected for product analysis. When conducting stability analysis, samples were taken from the flask immediately at each time point and analyzed for products. The reaction solution was analyzed using a gas chromatograph (GC9790, Zhejiang Fuli Analytical Instruments Co., Ltd., Taizhou, China) with a SE-54 capillary (30 m × 0.32 mm × 0.25 μm), detected by a hydrogen flame ionization detector (FID), and identified using a solketal standard sample and GC-MS mass spectrometer (Kyoto, Japan). The chromatographic analysis was performed at a column temperature of 250 °C, column pressure of 16.5 Psi, sampler temperature of 260 °C, and detector temperature of 280 °C. A micro-diluted reaction solution was obtained using the injector and injected into the gas chromatograph to complete the product analysis.

Solketal yield was calculated using the external standard method [42]. The external standard was DMF, and the specific formula was as follows:

$Yield=f_S\times {\displaystyle \frac{m_D\times A_S\times M_G}{m_G\times A_D\times M_S}}\times 100\%$

$Selectivity={\displaystyle \frac{A_S}{A_S+A_B}}\times 100\%$

Y_S was the yield of solketal, f_S was the relative correction factor of solketal versus DMF, m_D was the added mass of DMF, m_G was the mass of glycerol, A_S was the peak area of solketal, A_D was the peak area of DMF, A_B was the peak area of byproduct, M_G was the relative molecular mass of glycerol (92.094 g mol⁻¹), M_S was the relative molecular mass of solketal (132.160 g mol⁻¹).

4. Conclusions

In summary, a series of mesoporous aluminum phosphate catalysts was successfully synthesized by a one-pot process using pseudo-boehmite as the aluminum source and phosphoric acid as the phosphorus source. The prepared catalysts were characterized and analyzed to investigate the effect of the P/Al molar ratio on their structure. Weak acidic sites significantly influenced the activity of the aluminum phosphate catalysts. All aluminum phosphate catalysts exist in the form of amorphous aluminum phosphate and exhibit mesoporous structures. In the glycerol-acetone acetalization reaction, all catalysts exhibit a high selectivity of >99.9%. Among catalysts with different P/Al molar ratios, the catalyst with a molar ratio of 1.1 and a calcination temperature of 500 °C exhibited the best catalytic performance. Under reaction conditions of 100 °C, a space velocity of 1 mL·g⁻¹·h⁻¹, n_Acetone/n_Glycerol = 3, and a reaction time of 6 h, the 1.1P/Al-O catalyst achieved the highest yield of 70.2% and also exhibited high selectivity. Additionally, the 1.1P-Al-O catalyst demonstrated excellent stability, maintaining high solketal yield and selectivity even after 408 h of continuous flow reaction system operation.

Author Contributions

Conceptualization, J.W., X.Z. and X.W.; investigation, Y.Z. and J.Z.; writing—original draft preparation, J.W.; writing—review and editing, X.Z. and X.S.; supervision, X.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors thank Shanghai University for supporting this work.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1 Acetal reaction between glycerol and acetone.

Figure 2 XRD patterns of xP-Al-O catalysts with different P/Al ratios.

Figure 3 SEM images of xP-Al-O samples.

Figure 4 FT-IR spectra of xP-Al-O catalysts with different P/Al ratios.

Figure 5 (a) Low-temperature nitrogen adsorption-desorption isotherms, (b) BJH pore size distributions of xP-Al-O samples.

Figure 6 Py-IR spectra after pyridine adsorption on xP-Al-O catalysts with different P/Al ratios.

Figure 7 (a) NH₃-TPD and (b) CO₂-TPD profiles of xP-Al-O catalysts with different P/Al ratios.

Figure 8 Effect of P/Al molar ratio on solketal selectivity and over xP-Al-O catalyst.

Figure 9 Stability test of 1.1P-Al-O sample.

Table 1

Structural parameters of different samples.

Catalysts	SBET (m2g−1)	VP (cm3g−1)	Da (nm)
0.9P-Al-O	102	0.29	11.2
0.95P-Al-O	75	0.23	12.5
1.0P-Al-O	153	0.49	12.7
1.05P-Al-O	140	0.52	14.9
1.1P-Al-O	173	0.59	13.7
1.15P-Al-O	139	0.65	18.7
1.2P-Al-O	118	0.56	18.8

Table 2

Acid–base parameters of xP-Al-O catalysts *.

Catalysts	Relative Amount of NH3 Desorption	Relative Amount of CO2 Desorption
0.9P-Al-O	100	100
0.95P-Al-O	101	104
1.0P-Al-O	156	81
1.05P-Al-O	252	80
1.1P-Al-O	268	64
1.15P-Al-O	189	77
1.2P-Al-O	188	83

* Defined as the total amount of NH₃ and CO₂ desorbed per gram of catalyst divided by the total amount of NH₃ and CO₂ desorbed per gram of 0.9P-Al-O (based on 100), calculated by integrating the TPD curve.

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