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. 2023 Nov 7;101:106689. doi: 10.1016/j.ultsonch.2023.106689

Effect and mechanism of ultrasound on acid loading in the preparation of silicon-based sulfonic solid acids

Wenlong Miao a,b, Tian Wang a,b, AV Ravindra c, Weichao Huang d, Jue Hu a,b, Haoran Xv a,b, Thiquynhxuan Le a,b,⁎, Libo Zhang a,b,⁎
PMCID: PMC10692911  PMID: 37976567

Highlights

  • •

    Micron sized sulfonic acid solid acid is prepared by using ultrasonic technology.

  • •

    A 66.6% increase in acid loading is achieved by using ultrasound.

  • •

    A 10.97% increase in specific surface area is achieved by using ultrasound.

  • •

    The strengthening mechanism of ultrasound in the preparation process is revealed.

Keywords: Ultrasound, Solid acid, Acid loading, Strengthening mechanism

Abstract

Silicon-based sulfonic solid acids have the advantages of high catalytic activity and selectivity, easy separation from products, low equipment corrosion, and environmental protection, and sulfuric acid loading is the key to their preparation. To overcome the shortcomings of low acid loading and uneven distribution in the existing preparation methods of micron-sized silicon-based sulfonic solid acids, a method was proposed to prepare micron-sized silicon-based sulfonic solid acids using ultrasonic enhanced technology. The effect of different reaction parameters, such as time, power, and temperature of ultrasonication, sulfonation temperature and time, and sulfuric acid concentration, on acid loading in solid acid was investigated in this work. The results showed that a micron-sized mesoporous silica-based solid acid was successfully synthesized with a high acid content of 0.8633 mmol/g, uniform acid distribution, high specific surface area of 269.332 m2/g, and large average particle size of 172.142 μm in this work. The introduction of ultrasound was found to expand the carrier's pore volume and increase the carrier's specific surface area and the number of hydroxyl groups, thereby increasing the acid loading capacity and the specific surface area of the solid acid sample by 66.6 % and 10.97 % respectively, compared with the case without ultrasound.

1. Introduction

Sulfuric acid catalysts are widely used in petrochemical, organic synthesis, and biomass conversion fields [1]. Using liquid sulfuric acid catalysts has several issues, such as transportation and storage, separation from products, corrosion of equipment, and production of large amounts of waste acid [2]. In contrast, solid acid catalysts have the advantages of easy transportation and storage, easy separation from liquid phase reaction systems, low equipment corrosion, low environmental pollution, good catalytic performance [3], and being more in line with modern industrial green development requirements.

Silicon-based sulfonic solid acids have attracted significant attention from researchers due to their high thermal stability, high mechanical strength, high reactivity, and good selectivity [4], [5], [6]. The existing preparation methodologies of silicon-based sulfonic solid acid mainly include the sol–gel method [7], the grafting method [8], [9], [10], the cocondensation method [11], [12], [13], and the impregnation method [14]. Among others, the impregnation method has the advantages of a simple operation process and mild reaction conditions. Its principle is to directly immerse the silica carrier into the solution of a sulfonic acid agent (such as sulfuric acid, chlorosulfonic acid, oleum), allowing the sulfonic acid agent to bond with the silicon hydroxyl group (Si-OH) on the silica carrier surface, thereby introducing the sulfonic acid group (-SO3H) onto the silica carrier surface and obtaining the silica-sulfonic acid (SiO2-SO3H) solid acid. As a crucial factor in the preparation of silicon-based sulfonic solid acids, the acid loading effect directly affects the catalytic performance[15]. However, the pore structure of the silica carrier and the number of hydroxyl groups on its surface are limited, and the covalent bond between silica and sulfonic groups is unstable through simple impregnation, leading to the easy detachment of the sulfonic group after being loaded on the silica surface. As a result, the acid loading capacity in the final solid acid is considerably low, and at the same time, the acid distribution is uneven.

Nanostructured catalytic materials have attracted the attention of researchers due to their high catalytic activity, large specific surface area, and high stability. At present, the preparation effect of nanostructured catalysts through different methods, such as sonochemistry [16], hydrothermal [17], combustion [18], and reverse microemulsion [19] has been reported, and their excellent catalytic performance has been confirmed in different reactions, such as the decomposition of nitropyran insecticides [20], the degradation of organic pollutants [21], [22], and electrochemical hydrogen storage [17]. Hosseinzadeh et al. [20] prepared a Z-scheme heterojunction nanocomposite material by the hydrothermal treatment of WO3 nanoplates and SrTiO3 nanoparticles, which exhibited excellent catalytic activity in the decomposition of nitropyran insecticides. Ajabshira et al. [21] prepared Nd2O3-SiO2 nanocomposites with enhanced photocatalytic activity through sonochemical methods, which have high decomposition efficiency for methyl violet pollutants. To improve the acid loading capacity, nanosized silica with larger pore structures and more hydroxyl groups was used as a carrier to prepare silicon-based sulfonic solid acid in most of the earlier studies. Lyu et al. [23] used tetraethyl orthosilicic acid (TEOS) as a silicon source to synthesize nanosized silicon-based sulfonic solid acid with an average particle size of 20 nm, a specific surface area of 380.2 m2/g, and an acid content of 1.33 mmol/g by one-step reverse microemulsion method. Liu et al. [24] successfully synthesized nanosized solid acid with high catalytic activity by one-pot co-polycondensation using nano PS-SiO2 microspheres as the carrier. Although nanoscale catalysts exhibit high acid loading and good catalytic activity, there are challenges in catalyst recovery and separation due to the small particle size. In addition, using the nanosized solid acid catalyst can block the outlet and pipeline of the reaction gas, increase the pressure inside the reactor, and even cause an explosion accident during reactions at high temperatures.

Unlike nanoscale solid acids, microscale solid acids can be separated from products by a simple filtration process, achieving the recycling of catalysts. However, the particle size of solid acid particles is directly related to their acid loading capacity. Generally, the larger the particle size of the solid acid is, the lower its acid loading capacity. Silicon-based solid acid catalysts with micron-sized particles were successfully prepared by using diatomaceous earth as the carrier with sulfonation technology, but the acid loading capacity was only 0.5126 mmol/g, and the sulfonation time was up to 6 h [25]. Cao et al. [26] prepared a sulfonic acid modified catalyst with an acid loading capacity of only 0.49 mmol/g using silica gel column chromatography with a size of 48–75 μm as the carrier. A silicon-based carrier with a higher pore structure and specific surface area can be obtained by recrystallizing silica under the action of inorganic acid, improving the acid-loading effect on silicon-based sulfonic solid acid particles [27]. Therefore, introducing more sulfonic groups onto the surface of silicon-based carriers more uniformly has become the key to preparing micron-sized silicon-based sulfonic solid acids.

Ultrasound is an elastic mechanical wave with special cavitation and mechanical effects. In solid–liquid reactions, ultrasound can vigorously agitate the liquid, reduce diffusion resistance, accelerate chemical reaction speed, strengthen the transfer process of substances, and promote the formation of new reaction interfaces; therefore, ultrasound is an effective strengthening method for solid–liquid reactions. In recent years, the strengthening effect of ultrasound on catalyst preparation has been confirmed. The introduction of ultrasound can improve the activity and stability of the catalyst, shorten the preparation time, and improve the selectivity of the catalyst [28], which has broad application prospects. Yang et al. [29] prepared the superstrong solid acid SO42-/ZrO2 by an ultrasound-assisted impregnation method and found that ultrasound can refine the ZrO2 carrier particles in the carrier's precipitation preparation and strengthen the carrier's binding and sulfonic acid in the impregnation process, thereby improving the acid loading effect. Zhang et al. [30] found that the SO42-/ZrO2-TiO2 solid superacid prepared by the ultrasonic coprecipitation method has a larger specific surface area, good thermal stability, and higher catalytic activity than that obtained by the mechanical agitation precipitation method. To our knowledge, ultrasound technology has never been reported for preparing micron sized sulfonic acid solid acids.

In this work, micron-sized sulfonic solid acid is prepared employing ultrasonic-enhanced technology using micron-sized chromatography silica gel (SG) as the carrier and sulfuric acid as the active component. The effect of different reaction parameters, such as ultrasonication time, power, temperature, sulfonation temperature and time, and sulfuric acid concentration, on the acid loading in solid acid is investigated. In addition, the acid-loading effect of solid acids under ultrasonic impregnation and conventional impregnation preparation conditions is compared. The particle size distribution, phase structure, specific surface area, microstructure, and thermal stability of solid acid samples obtained by both preparation methods are studied. The strengthening mechanism of ultrasound on acid loading during silicon-based sulfonic solid acid preparation is analyzed.

2. Experimental

2.1. Materials

Sulfuric acid (98 %, A.R.) was purchased from Chengdu Kelon Chemical Co., Ltd. (Chengdu, China). Sodium hydroxide standard titration solution (0.5000 mol/mL) was purchased from Guangzhou He Wei Pharmaceutical Technology Co., Ltd. (Guangzhou, China). Column chromatography silica gel SG (particle size of 75–320 μm, mainly 150–180 μm, specific surface area of 196.619 m2/g) was purchased from Qingdao Ocean Chemical Co., Ltd. (Qingdao, China). Glacial acetic acid (≥99.5 %, A.R.) was purchased from Tianjin Heng Xing Chemical Reagent Manufacturing Co., Ltd. (Tianjin, China). Anhydrous ethanol (≥99.7 %, A.R.) was purchased from Tianjin Heng Xing Chemical Reagent Manufacturing Co., Ltd. (Tianjin, China).

2.2. Synthesis process

The solid acid catalyst was prepared through two steps: ultrasonic impregnation and sulfonation. In the ultrasonic impregnation step, 8 g of silica carrier was placed in a 150 mL beaker and then immersed in a mixture of 15 mL deionized water and 15 mL 5 mol/L sulfuric acid solution (diluted with 98 % sulfuric acid) under ultrasound for some time. The impregnation step was carried out under different conditions of different temperatures (ambient temperature, 30, 50, 70, and 90 °C), ultrasound impregnation times (15, 30, 45, 60, and 75 min), ultrasound powers (36, 72, 108, and 144 W) and sulfuric acid concentrations (3, 5, 10, and 18.4 mol/L). In the sulfonation step, the impregnated mixture was heated in a hot-air oven at different sulfonation temperatures (100, 120, 140, 160, and 180 °C) and sulfonation times (1, 2, 3, 4, and 5 h). After sulfonation was completed, the sulfonated product was cooled to room temperature, and washed repeatedly with deionized water until the filtrate to a neutral pH, and dried at 80 ℃ for 12 h to obtain a solid acid product. The synthesis process is shown in Fig. 1.

Fig. 1.

Fig. 1

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Experimental flowchart.

2.3. Characterization

The phase structure of the sample was examined employing an X-ray diffractometer (XRD, Rigaku dX 2000) with Cu-Kα radiation at a voltage of 40 kV, a current of 25 mA, and a scanning range of 10 to 90°. The particle size distribution was measured using a Malvern laser particle size analyzer (Mastersizer 2000). The specific surface area and pore structure parameters of the samples were measured by N2 adsorption–desorption at 77 K on a Quadrasorb evo TM gas adsorption instrument. All samples were degassed at 150 °C for 5 h before the adsorption measurement. The specific surface area was calculated using the BET method, the pore size and volume were calculated using the BJH formula, and the pore size distribution was calculated using the density functional theory (DFT) method based on nitrogen adsorption. The infrared spectra of the samples were recorded in KBr medium on a NEXUS FT-IR spectrometer. The morphology was obtained by scanning electron microscopy (SEM, Czech TESCAN MIRA LMS). The thermal stability of samples was checked by a synchronous thermal analyzer (TG-DSC, TA) with a temperature range of 25-500℃ and a heating rate of 5℃ min−1 under a N2 atmosphere.

The acid loading of solid acid was analyzed by the acid-base titration method. Specifically, 0.1 g of catalyst was mixed with 25 mL of NaOH (0.04 mol/L), and then 2–3 drops of phenolphthalein reagent (0.5 wt%) were added. Subsequently, the obtained mixture was back-titrated with H2SO4 (0.04 mol/L) to calculate the acid load capacity.

3. Results and discussion

3.1. Optimization of preparation conditions for solid acids under ultrasound

In this experiment, the preparation process of silicon-based sulfonic solid acids is divided into impregnation and sulfonation steps. In the impregnation step, the silica carrier is directly immersed in the sulfuric acid solution, and the sulfonic acid group (-SO3H) is introduced onto the silica carrier surface by physical adsorption by forming chemical bonds with the hydroxyl group (–OH) on the silica carrier surface. However, the physically adsorbed acid can be easily removed after washing with water. Therefore, the purpose of the sulfonation step is to enable more adsorbed acid to bind with the hydroxyl groups (–OH) that are present on the silica carrier at a higher temperature, resulting in more stable covalent bonds between the silica group and the sulfonic group.

3.1.1. Effect of ultrasound impregnation time

The impregnation step was carried out at ambient temperature for different ultrasound impregnation times (15, 30, 45, 60, and 75 min) under an ultrasound power of 108 W, and the sulfonation step was carried out at a fixed temperature of 160 °C for 2 h. The effect of ultrasound impregnation time on the acid loading capacity is presented in Fig. 2 and Table 1.

Fig. 2.

Fig. 2

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Effect of ultrasound impregnation time on acid loading capacity.

Table 1.

BET parameters of samples obtained with different ultrasound times.

Sample specific surface area (m2/g) Pore volume (cm3/g) Average pore size (nm)
SSA-U15 227.468 1.242 12.396
SSA-U45 269.332 1.438 12.384
SSA-U75 242.348 1.284 12.390
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The loading capacity of solid acid first increases and then decreases with increasing ultrasonic immersion time. The acid loading capacity in solid acid rises from 0.4729 to 0.6838 mmol/g when the ultrasonic immersion time increases from 15 to 45 min and then decreases when the ultrasonic immersion time is extended to 60 or 75 min. The reason is that prolonging the ultrasound impregnation time can expand the carrier's pore volume and increase the carrier's specific surface area, thereby promoting active components' loading. It can be seen from Table 1 that, as the ultrasound impregnation time is extended from 15 to 45 min, the specific surface area increases from 227.468 to 269.332 m2/g, and the pore volume of solid acid samples increases from 1.242 to 1.438 cm3/g. The adsorption–desorption isotherms of samples obtained at different ultrasound times are shown in Fig. S1 in the supplementary materials. However, further extending the ultrasound impregnation time to 75 min reduces the specific surface area of solid acids by 10.04 % and pore volume by 10.71 % compared to that for 45 min. After the acid loading reaches the highest value, further extension of the ultrasound treatment time produces a useless micro-jet, which forms an impact force on the carrier, causing the collapse of the carrier bone structure, damaging the available specific surface area of the carrier and removing a portion of the loaded sulfonic acid groups, resulting in a decrease in the acid loading [31]. Therefore, 45 min is selected as the optimal ultrasound immersion time.

3.1.2. Effect of ultrasound power

The impregnation step was carried out at ambient temperature for 45 min under different ultrasound powers (36, 72, 108, and 144 W), and the sulfonation step was carried out at a fixed temperature of 160 °C for 2 h. The effect of ultrasound power on the acid loading capacity is shown in Fig. 3.

Fig. 3.

Fig. 3

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Effect of ultrasound power on the acid loading capacity.

Increasing the ultrasound power can increase the acid loading capacity in solid acids. This is because the higher the ultrasonic power is, the greater its cavitation and stirring effects, promoting the transfer of active components and strengthening the acid-loading effect. The loading capacity of solid acids reaches the highest value of 0.7004 mmol/g at an ultrasound power of 108 W.

However, it is not always the case that higher ultrasonic power is better for acid loading. When excessive ultrasonic power is used, the high temperature and pressure generated by its cavitation effect can break the Si-OH bond on the silica carrier, reduce the hydroxyl groups residing on the carrier, and destroy the existing active sites [32]. At the same time, when the acid load is saturated, further increasing the ultrasonic power promotes the elution of a portion of the acid load already loaded on the carrier, resulting in a decrease in the acid load. The acid loading capacity decreases to only 0.618 mmol/g when the ultrasound power is 144 W. Therefore, 108 W is selected as the optimal ultrasound power.

3.1.3. Effect of ultrasonic impregnation temperature

The impregnation step was carried out at different leaching temperatures (ambient temperature, 30, 50, 70, and 90 °C) for 45 min under an ultrasound power of 108 W, and the sulfonation step was carried out at a fixed temperature of 160 °C for 2 h. The effect of ultrasound impregnation temperature on the acid loading capacity is shown in Fig. 4.

Fig. 4.

Fig. 4

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Effect of ultrasound impregnation temperature on the acid loading capacity.

Appropriately increasing the ultrasonic impregnation temperature is beneficial to acid loading. The increase in temperature improves the mass transfer of acid-active components and promotes the condensation reaction between the acid-active components and the carrier, thus increasing the acid loading in solid acids. However, the acid load decreases at an excessively high temperature after the acid load reaches saturation. This phenomenon can be attributed to the fact that at a higher reaction temperature, the higher the vapor pressure is, the lower the difference between the internal and external pressure of cavitation bubbles generated by ultrasound. This weakens the release energy of cavitation bubble collapse, reducing the mass transfer rate during the loading process of acid-active components [33]. The acid loading reached the highest value at 50 °C; therefore, 50 °C is chosen as the optimal ultrasound impregnation temperature.

3.1.4. Effect of sulfonation temperature and sulfonation time

The impregnation step was fixed at 50 °C for 45 min under an ultrasound power of 108 W, and the sulfonation step was carried out at different sulfonation temperatures (100, 120, 140, 160, and 180 °C) and sulfonation times (1, 2, 3, 4, and 5 h). The effect of sulfonation temperature and sulfonation time on the acid loading of solid acids was studied, and the results are shown in Fig. 5 and Table 2.

Fig. 5.

Fig. 5

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Effect of sulfonation temperature (a) and sulfonation time (b) on acid loading.

Table 2.

BET parameters of samples obtained at different sulfonation temperatures.

Sample specific surface area (m2/g) Pore volume (cm3/g) Average pore size (nm)
SSA-U100 262.264 1.412 12.390
SSA-U160 269.332 1.438 12.384
SSA-U180 236.548 1.277 12.398
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As shown in Fig. 5 (a), appropriately increasing the sulfonation temperature benefits the acid loading effect. At a low sulfonation temperature (around 100℃), the bonding reaction between the sulfuric acid and the silicon hydroxyl group (Si-OH) on the silica carrier is slow. In addition, the loaded sulfonic acid group is unstable and undergoes hydrolysis by absorbing water, releasing sulfuric acid, thereby weakly bonded -SO3H is produced at about 100℃ [34]. therefore, the formation of sulfonic functional groups is relatively low. The acid loading capacity in the solid acid increases significantly as the temperature rises from 100 to 160 °C and reaches the maximum at 160 °C. The adsorption–desorption isotherms of samples obtained at different sulfonation temperatures are shown in Fig. S2 in the supplementary materials. The BET results of the solid acid samples obtained at different sulfonation temperatures are shown in Table 2.

The specific surface area and pore volume of solid acid increase slightly but not significantly between 100 and 160 °C, indicating that a suitable increase in sulfonation temperature promotes the formation and stability of the covalent bonds between silicon and sulfonic groups, thereby enhancing the acid-loading effect with little effect on the surface area [35]. This is different from the reason for increasing the carrier specific surface in the ultrasonic impregnation step. However, as the sulfonation temperature further increases to 180 °C, the solid acid loading capacity decreases, and the specific surface area and pore volume decrease by 12.27 % and 11.2 %, respectively. In fact, the phenomenon of excessively high sulfonation temperature leading to a decrease in the specific surface area of solid acids has also been reported in other work. Wang et al [36]. reported that the specific surface area was 69 m2/g at 160℃, while it decreased to 29 m2/g at 180℃. This is because excessive sulfonation temperature causes the carrier structure to shrink and the pore structure to be destroyed [37], thereby reducing the specific surface area.

At the beginning of the reaction, the sulfonic acid groups are loaded onto the active site of the carrier at a certain speed. The loading of solid acid increases rapidly with the prolongation of the sulfonation time, as shown in Fig. 5b. This is because the prolongation of sulfonation time is beneficial to the formation of more chemical bonds between the carrier and sulfuric acid molecules adsorbed on the carrier through the impregnation step (that is, the formation of the -SO3H group) so that more active acid components are stable in the carrier [38]. When the sulfonation time is 2 h, the acid loading capacity reaches as high as 0.8633 mmol/g. A slight increase in the acid loading capacity is observed upon extending the sulfonation time to 3 h. After saturated acid loading, further prolonging the sulfonation time causes side reactions, decreasing the catalyst activity and solid acid loading [39]. Therefore, 160 ℃ and 2 h are selected as the optimal sulfonation temperature and time, respectively.

3.1.5. Effect of acid concentration

The impregnation step was carried out at an ultrasound impregnation time of 45 min, an ultrasound power of 108 W, and a soaking temperature of 50 ℃, and the sulfonation step was carried out at a sulfonation temperature of 160 ℃ and a sulfonation time of 2 h. The effect of different sulfuric acid concentrations (3, 5, 10, and 18.4 mol/L) on the loading capacity of solid acids was studied, and the results are shown in Fig. 6.

Fig. 6.

Fig. 6

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Effect of sulfuric acid concentration on acid loading capacity (a); SEM images of SG (b) and solid acid samples with sulfuric acid concentrations of 3 mol/L (c), 5 mol/L (d), 10 mol/L (e), and 18.4 mol/L (f).

As shown in Fig. 6 (a), the acid loading capacity in solid acids increases from 0.425 to 0.8633 mmol/g as the concentration of sulfuric acid increases from 3 to 5 mol/L, indicating that increasing the concentration of the active component sulfuric acid is beneficial for acid loading in solid acids. However, when the sulfuric acid concentration further increases to 10 or 18.4 mol/L, the acid loading of the solid acid decreases. The excessive acid concentration erodes the silica carrier surface, destroying the pore structure and even causing the pore structure to collapse severely, resulting in a decrease in the acid loading capacity [39]. The SEM image of the carrier is shown in Fig. 6 (b), and the morphology of the carrier exhibits a flat and irregular block-like structure. The morphology of the solid acid is smooth at sulfuric acid concentrations of 3 or 5 mol/L (Fig. 6 (b), (c)). However, when the sulfuric acid concentration increases to 10 or 18.4 mol/L (Fig. 6 (e), (f)), the morphology of the solid acid exhibits uneven deposits and cracks, indicating that the surface of the solid acid has been eroded by sulfuric acid, supporting the above analysis.

In summary, the optimal conditions for the ultrasonic preparation of silicon-based sulfonic solid acid are an ultrasound time of 45 min, ultrasound power of 108 W, ultrasound temperature of 50 °C, sulfonation temperature of 160 °C, sulfonation time of 2 h, and sulfuric acid concentration of 5 mol/L. Under these conditions, a micron-sized mesoporous silica-based solid acid with an acid content of 0.8633 mmol/g was synthesized. Ultrasonic preparation can prepare high acid loading in a relatively mild and short time compared to traditional impregnation methods.

3.2. Enhanced mechanism of ultrasound on acid loading

3.2.1. Comparison of acid loading effect under ultrasonic and conventional preparations

The reaction factors were fixed according to the optimal reaction conditions of ultrasonic preparation, and the acid loading capacity and esterification rate of solid acids prepared by ultrasonic and conventional impregnation with different sulfuric acid concentrations (3, 5, 10, and 18.4 mol/L) were compared. Ethyl acetate, one of the most important esters, is mainly used as a solvent for paints, coatings, nitrocellulose and resins. Ethyl acetate is generally obtained through the esterification reaction of acetic acid and ethanol under the catalysis of sulfuric acid. In this study, the prepared solid acids were used as catalysts for the synthesis of ethyl acetate. The operation of the esterification experiment is given in Text S1 in the Supplementary material. The acid loading capacity and esterification effect results are shown in Fig. 7(a) and 7(b), respectively.

Fig. 7.

Fig. 7

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Acid loading capacity (a) and esterification effect (b) of different solid acid samples prepared by ultrasonic and conventional impregnation.

Regardless of conventional or ultrasonic impregnation, the acid loading of solid acids shows an upward trend at first and then a downward trend with increasing sulfuric acid concentration (Fig. 9a). Within the concentration range of sulfuric acid investigated in our work, the maximum acid loading capacity of SSA-U samples is 0.8633 mmol/g. In comparison, the maximum acid loading capacity of SSA-C samples is only 0.5182 mmol/g, indicating that the introduction of ultrasound could increase the acid loading capacity of solid acids by 66.6 %. Fig. 9(b) shows the catalytic effect of different solid acid samples on the esterification of acetate. The esterification rate of acetic acid shows a good correlation with the acid loading capacity of solid acid, and the esterification effect of the SSA-U sample increased by 28.03 % compared with that of the SSA-C sample. In addition, the esterification effect of micron-scale solid acids is not as satisfactory as that of nanoscale solid acids, and some studies have reported this phenomenon [40], [41], [42]. However, micron-scale solid acids can solve the problem of separation and recovery during the use process.

Fig. 9.

Fig. 9

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SEM elemental mapping results of SSA-C (a) and SSA-U (b).

3.2.2. Enhancement mechanism of ultrasound on acid loading

Fig. 8 shows the XRD patterns of the silica carrier and the solid acid prepared by ultrasound. In the XRD patterns of the two samples, only one broad peak is observed in the range of 20°-30°, which corresponds to the diffraction peak of the natural amorphous structure of silica [43], [44], and no narrow peak is found. The solid acid prepared by ultrasound has a similar phase structure to the carrier, which shows that the ultrasonic preparation conditions used in this work do not damage the phase structure of the carrier.

Fig. 8.

Fig. 8

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XRD patterns of raw materials and prepared solid acids.

Fig. 9 shows the SEM elemental mapping results of O, Si, and S in the SSA-U and SSA-C samples. O and Si are derived from the silica support, and S is derived from sulfonic acid groups successfully bonded to the support. Compared with the SSA-C sample, the distribution of S in the SSA-U sample is more uniform, indicating that ultrasound can evenly disperse sulfonic acid groups on the surface of the carrier without aggregation, which is advantageous to the catalytic reaction.

TEM images of SSA-C are shown in Fig. 10a and 10b, while TEM images of SSA-U are shown in Fig. 10c and 10d. The results clearly confirm the SO3H shell on the surface of both solid acid samples. However, the solid acid surface prepared by ultrasound has more -SO3H loading and a more uniform distribution compared to conventional preparation methods. The particle size distributions of the SG and SSA catalysts prepared with 5 mol/L sulfuric acid are shown in Fig. 11(a-c), respectively. These particles were found to have size range of 75–320 μm; whereas, the average particle size of SG is 168.476 μm, SSA-C is 170.667 μm, and SSA-U is 172.142 μm. It demonstrates that the solid acid product prepared by ultrasound is at the micron level. This micron-sized solid acid can be separated from the product through a simple separation process for recovery, achieving the recycling of the catalyst. During the ultrasonic preparation process, the high shear effect and strong acoustic flow generated by the mechanical effect of ultrasound impact the surface of the silica carrier, which increases the pore size of the carrier, resulting in the average particle size of the solid acid product prepared by ultrasound being slightly larger than the average particle size of conventional methods. In addition, the solid acid prepared by ultrasound is loaded with a thicker SO3H shell, resulting in a slightly larger average particle size.

Fig. 10.

Fig. 10

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TEM images of SSA-C (a,b) and SSA-U (c,b).

Fig. 11.

Fig. 11

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Particle size distribution of SG (a), SSA-C (b) and SSA-U(c).

The pore structure parameters of the SG and SSA samples are studied by measuring the N2 adsorption–desorption isotherms (Fig. 12). The obtained adsorption–desorption isotherm curves correspond to the IV-type system as defined by the IUPAC for mesoporous materials [45]. From the pore size distribution curve, it can be seen that all three samples have a narrow pore size distribution, and their pore sizes are within the corresponding range of mesoporous materials. Therefore, the SG, SSA-C, and SSA-U samples are mesoporous materials. The BET specific surface area, pore volume, and average pore size of the SG, SSA-C, and SSA-U samples are shown in Table 3. The average pore size difference between SG, SSA-C, and SSA-U is insignificant, ranging from 12.384 to 12.405 nm. Compared with SG, the specific surface area of SSA-U increases from 196.619 to 269.332 m2/g, and the pore volume increases from 1.053 to 1.438 cm3/g, confirming that the introduction of ultrasound expands the pore volume of the carrier and increases the specific surface area of the carrier, thereby providing more active sites for the loading of sulfonic groups. The specific surface area of SSA-U increases by 10.97 %, and the pore volume increases by 11.65 % compared with that of SSA-C. This phenomenon is similar to the results reported by Yang et al. [29]. They found that the specific surface area and pore volume of ZrO2 were increased under ultrasonic treatment, resulting in more voids and providing more binding sites. This benefits the contact between the carrier and the acid-active component, increasing the acid-loading capacity. In other words, unlike conventional preparation, the local high-temperature, high-pressure environment, shock waves, and micro-jet flow generated by the cavitation effect and mechanical effect of ultrasound lead to a more significant improvement in the pore structure of the carrier in ultrasonic preparation. Obviously, the acid loading effect and the catalytic effect on the esterification reaction of solid acid are enhanced in the case of ultrasonic preparation.

Fig. 12.

Fig. 12

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Adsorption-desorption isotherm curves and pore size distribution of SG (a), SSA-C (b) and SSA-U(c).

Table 3.

BET parameters of different samples.

Sample Specific surface area (m2/g) Pore volume (cm3/g) Average pore size (nm)
SG 196.619 1.053 12.400
SSA-C 242.711 1.288 12.405
SSA-U 269.332 1.438 12.384
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Fig. 13 presents the FT-IR spectra of the SSA-U and SSA-C samples. The FTIR spectra of SSA-U and SSA-C exhibit almost identical characteristic peaks but with differences in the intensity of these peaks. The broad peak at around 3470 cm−1 for both samples corresponds to the stretching vibration of silicon hydroxyl groups, indicating a large amount of silicon hydroxyl groups on the silica surface of SSA-C and SSA-U samples [46]. Compared to the SSA-C sample, the characteristic peak at 3470 cm−1 in the SSA-U sample is stronger, indicating more silicon hydroxyl groups on the surface of the SSA-U sample. This could be because the mechanical effect of ultrasound discourages particle agglomeration, thus favoring silica surface exposure to more active hydroxyl groups. The band located at 1630 cm−1 corresponds to the bending vibration of the physically adsorbed water hydroxyl –OH group [47], and the intensity of this peak for SSA-U is also greater than that of SSA-C. This is because the increase in hydroxyl groups on the surface of SSA-U can enhance water adsorption [46]. Also, the increased pores due to the higher acid loading enhance the ability to adsorb water on the surface for the SSA-U. The absorption peaks around 1110, 802, and 468 cm−1 correspond to the Si-O asymmetric stretching, Si-O symmetric stretching, and Si-O-Si bending vibration peaks in the Si-O-Si bond. The broadening of the absorption peak near 1110 cm−1 is due to the overlap of the Si-O-Si stretching vibration peak with the stretching vibration peak of O Created by potrace 1.16, written by Peter Selinger 2001-2019 S Created by potrace 1.16, written by Peter Selinger 2001-2019 O [48], [49], [50]. Compared with SSA-C, the SSA-U sample has a broader absorption peak near 1110 cm−1, indicating that the sulfonic acid groups have been successfully loaded onto the silica surface in both solid acids, and the SO3H density SSA-U > SSA-C.

Fig. 13.

Fig. 13

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FTIR spectra of SSA-U (a) and SSA-C (b).

The TGA and DSC curves of the SG, SSA-C, and SSA-U samples are shown in Fig. 14. The weight loss of all three samples can be divided into two stages over the testing temperature range. The weight loss in the first stage (from 30 to 100 °C) is mainly due to the removal of physically adsorbed water. The weight loss in the second stage (100–500 °C) is caused primarily by the condensation of hydroxyl groups on the surface of silica into water [46], [51] and the decomposition of sulfonic acid groups [52]. In the first stage, the weight loss order is SSA-U > SSA-C > SG; their weight losses are 4.22 %, 3.82 %, and 2.3 %, respectively. The SSA-U sample showed the highest weight loss in the first stage because it contains more adsorbed water due to its larger specific surface area, more voids, and higher acid loading. In the second stage, both the condensation of hydroxyl groups and the decomposition of sulfonic acid groups occurred in SSA-C and SSA-U samples in the range of 100–500 ℃, but only the condensation of hydroxyl groups to water occurred in SG, resulting in significantly more weight loss in SSA-C and SSA-U compared to SG. The weight loss of SSA-U in the second stage (3.4 %) is greater than that of SSA-C (1.6 %) due to more hydroxyl groups and successfully loaded sulfonic acid groups on the surface of SSA-U solid acids. This result is consistent with the FTIR results. The DSC measurement further confirms the above conclusions; the removal of physically adsorbed water causes the endothermic peak near 50 ℃, while the endothermic peak near 200 ℃ is caused by the condensation of hydroxyl groups and the decomposition of sulfonic groups.

Fig. 14.

Fig. 14

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TGA and DSC curves of SG, SSA-C, and SSA-U.

In the preparation process of solid acids, sulfuric acid molecules are loaded on the surface of silicon-based carriers generally through physical adsorption, chemical adsorption, or both at the same time. Generally, acid molecules loaded by physical adsorption are easily removed after washing with water and cannot be fixed on the carrier. The other part of the sulfuric acid molecules forms stable chemical covalent bonds with the hydroxyl groups (–OH) on the silica carrier through a reaction (-Si-OH + H2SO4 → Si-O-SO3H + H2O). Those acid molecules loaded through chemical adsorption can be stably loaded in the solid acid product. The chemical adsorption mechanism of this experiment is shown in Fig. 15.

Fig. 15.

Fig. 15

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Chemical adsorption mechanism of solid acids.

Due to its unique cavitation and mechanical effects, ultrasound not only plays a stirring role in the liquid phase but also forms many cavitation bubbles. During the preparation, the local high temperature and pressure generated when the ultrasonic cavitation bubbles burst and the high shear effect with strong acoustic flow generated by the mechanical effect of ultrasound improve the pore structure of the carrier and strengthen the reaction between sulfuric acid and the silica carrier [53]. Therefore, compared to traditional methods, employing ultrasound expands the carrier's pore volume, increases the carrier's specific surface area, avoids the agglomeration of particles, and strengthens the mass transfer of the sulfonic acid. Furthermore, it increases the number of hydroxyl groups on the carrier surface, exposing more binding sites and allowing more active component sulfuric acid to be fixed on the carrier surface through chemical adsorption, thereby improving the stability of sulfonic acid groups on the surface of the carrier. The strengthening mechanism of ultrasound on the preparation of silicon-based sulfonic solid acids is shown in Fig. 16. The results of this study show a higher acid loading capacity, more uniform acid distribution, and larger specific surface area for the solid acids prepared using ultrasound, which has good application potential in catalytic reactions.

Fig. 16.

Fig. 16

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Schematic diagram of the preparation of silicon-based sulfonic solid acids under ultrasound.

A comparison of the preparation reaction conditions and acid loading capacity between our work and other works is listed in Table 4. Micron-sized silicon-based solid acids with high acid loading capacity were successfully prepared through a mild, rapid, and efficient sonochemical route in our work. Compared with other works, the preparation process in our work can achieve higher loading of solid acids in a shorter time and under mild preparation conditions.

Table 4.

Comparison of the preparation reaction conditions and acid loading capacity between our work and other works.

Carrier Average particle size Preparation conditions Acid loading capacity (mmol/g) Reference
Diatomite 25 μm sulfonation at 140℃ for 6 h 0.5126 [29]
Mesoporous microspheres 5–8 μm sulfonation at 240℃ for 4 h 0.9 [35]
Mesoporous Silica 75–110 μm sulfonation at 500℃ for 4 h 0.236 [54]
Diatomite 30–40 μm Aging for 24 h, calcine, sulfonation at 160℃ for 4 h 0.24 [55]
Mesoporous silicon 5–10 μm Aging for 24 h, sulfonation at 60℃ for 12 h 1.037 [56]
Silica gel(SG) 172 μm ultrasound immersion for 45 min, sulfonation at 160 ℃ for 2 h 0.8633 This work
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4. Conclusion

Micron-sized solid acids have more advantages in use and recycling due to their better dispersion and separation effects than nano-sized solid acids, but they have the disadvantage of low acid loading capacity. In this work, micron-sized silicon-based solid acids with high acid loading capacity were successfully prepared through a mild, rapid, and efficient sonochemical route. Under the optimized conditions of ultrasonic treatment time, power and temperature, sulfonation temperature and time, and sulfuric acid concentration, micron-sized mesoporous silica-based solid acid was obtained with a high acid content of 0.8633 mmol/g, specific surface area of 269.332 m2/g, and a large average particle size of 172.142 μm. Experimental results showed that compared with conventional preparation, the introduction of ultrasound increased the loading capacity and catalytic performance of solid acid by increasing the specific surface area and the number of hydroxyl groups of the carrier. Specifically, the acid loading, specific surface area and pore volume of the solid acid product were increased by 66.6 %, 10.97 %, and 11.65 %, respectively and the acid distribution was more uniform, thereby enhancing the catalytic efficiency of the ethyl acetate synthesis process by 28.03 %. The results of this work provide a new idea for the preparation of micron-sized solid acids.

CRediT authorship contribution statement

Wenlong Miao: Formal analysis, Writing – original draft. Tian Wang: Formal analysis. A.V. Ravindra: Writing – review & editing. Weichao Huang: Writing – review & editing. Jue Hu: Writing – review & editing. Haoran Xv: Formal analysis. Thiquynhxuan Le: Conceptualization, Project administration, Writing – review & editing. Libo Zhang: Resources, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support of the Yunnan Major Scientific and Technological Project (202302AG050008) and Yunnan Fundamental Research Projects (202201AU070088 and 202101BE070001-023).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2023.106689.

Contributor Information

Thiquynhxuan Le, Email: quynhxuanlt@kust.edu.cn.

Libo Zhang, Email: zhanglibopaper@126.com.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Supplementary data 1
mmc1.docx (1.3MB, docx)

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