Solid superacid solvent-free synthesis method of pentaerythritol stearate

Abstract

The invention provides a solid superacid solvent-free synthesis method of pentaerythritol stearate, and belongs to the field of pentaerythritol stearate preparation. The method comprises the following steps: mixing molten stearic acid, pentaerythritol and a solid superacid catalyst in an inert atmosphere to obtain a mixture; the mixture is subjected to a stepped heating procedure reaction, the temperature is controlled to be 170-185 DEG C in the early stage of the reaction, and after the reaction is conducted for 3-5 h, the reaction temperature is increased to 190-200 DEG C to continue the reaction for 2-4 h till the esterification reaction is completed, and a reaction solution is obtained; filtering the reaction liquid to obtain a pentaerythritol stearate crude product; and carrying out reduced pressure distillation purification on the pentaerythritol stearate crude product to obtain a pentaerythritol stearate product. Through collaborative optimization of process design and a catalytic system, efficient esterification of pentaerythritol and stearic acid under a solvent-free condition is systematically realized, construction of a stable recyclable catalytic system is realized, side reaction is inhibited, and product purity is improved.

Description

Technical Field

[0001] This application relates to the field of pentaerythritol stearate preparation technology, and in particular to a solid superacid solvent-free synthesis method for pentaerythritol stearate. Background Technology

[0002] Pentaerythritol stearate, as an important polyol ester compound, has wide applications in lubricants, plasticizers, cosmetics, and other fields. Traditional synthesis methods mainly employ liquid acids (such as concentrated sulfuric acid) as catalysts, carrying out esterification reactions in the presence of organic solvents. While this method exhibits high catalytic activity, it suffers from the following significant drawbacks: First, liquid acid catalysts are highly corrosive, requiring sophisticated reaction equipment and generating large amounts of acidic wastewater, which is environmentally unfriendly. Second, separating the catalyst from the product is difficult, making recycling impossible and increasing production costs. Third, side reactions are easily triggered during the reaction, such as dehydration of pentaerythritol, oxidation and coloring of the product, affecting the final product's color and quality. Furthermore, the use of organic solvents not only increases the difficulty of separation and purification but may also leave residues in the product, limiting its application in high-end fields.

[0003] To overcome these problems, researchers began exploring the application of solid acid catalysts in esterification reactions. While traditional solid acid catalysts (such as zeolite molecular sieves and cation exchange resins) have solved the problems of catalyst separation and corrosion, their limited acid strength, small specific surface area, and susceptibility to deactivation at high temperatures result in low catalytic efficiency, making them unsuitable for the sterically hindered reaction system of pentaerythritol and long-chain fatty acids. Especially under solvent-free conditions, as the reaction proceeds, the system viscosity increases significantly, hindering the diffusion of reactant molecules. Traditional solid acid catalysts struggle to effectively overcome this mass transfer bottleneck, leading to a sharp decline in the reaction rate in the later stages and making further improvements in conversion difficult.

[0004] Therefore, developing a novel catalytic system that can maintain high catalytic activity, achieve stable operation and recycling of the catalyst under solvent-free conditions, and effectively control side reactions and ensure product purity has become a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This application provides a solventless method for the solid superacid synthesis of pentaerythritol stearate to solve the following technical problem: how to achieve efficient esterification reaction of pentaerythritol and stearic acid under solventless conditions, while constructing a stable and recyclable catalytic system to avoid side reactions and improve product purity.

[0006] This application provides a solvent-free method for the synthesis of pentaerythritol stearate as a solid superacid, the method comprising the following steps: S1. Under an inert atmosphere, molten stearic acid, pentaerythritol and solid superacid catalyst are mixed to obtain a mixture; the solid superacid catalyst is a sulfate and tungsten-phosphorus heteropolyacid modified solid acid supported by a ZrO2-SnO2-TiO2 ternary composite oxide. S2. The mixture is subjected to a step-heating reaction. In the early stage of the reaction, the temperature is controlled at 170-185℃. After the reaction has been going on for 3-5 hours, the reaction temperature is raised to 190-200℃ and the reaction continues for 2-4 hours until the esterification reaction is completed, and the reaction solution is obtained. S3. Filter the reaction solution to separate the solid catalyst and obtain the crude pentaerythritol stearate product. S4. The crude pentaerythritol stearate product is purified by vacuum distillation to obtain the pentaerythritol stearate product.

[0007] Optionally, in step S1, the molar ratio of pentaerythritol to stearic acid is 1:(4.0-4.5).

[0008] Optionally, in step S1, the mass of the solid superacid catalyst is 1.0 to 3.0% of the total mass of the pentaerythritol and the stearic acid.

[0009] Optionally, in step S1, the preparation method of the solid superacid catalyst includes the following steps: S101. Zirconium oxychloride, tin chloride and tetrabutyl titanate were prepared into a mixed solution, and ammonium carbonate was added under stirring until precipitation was complete to obtain a ternary composite hydroxide gel. S102. The ternary composite hydroxide gel is aged and then dried using supercritical carbon dioxide drying technology to obtain the precursor. S103, the precursor is impregnated in sulfate solution, filtered and dried after impregnation, and then calcined at 500-600℃ for 2-4h to obtain sulfate-modified ZrO2-SnO2-TiO2 support; S104. The sulfate-modified ZrO2-SnO2-TiO2 support is impregnated in a tungsten-phosphorus heteropoly acid solution. After impregnation, it is dried and activated at 300-400℃ for 1-3 hours to obtain the solid superacid catalyst.

[0010] Optionally, the conditions for supercritical carbon dioxide drying are: temperature of 40–60°C, pressure of 8–15 MPa, and time of 2–4 h.

[0011] Optionally, the amounts of zirconium oxychloride, tin chloride, and tetrabutyl titanate are (1-3):(0.5-1.5):1, based on the molar ratio of ZrO2, SnO2, and TiO2.

[0012] Optionally, the sulfate solution is an ammonium sulfate solution with a concentration of 0.5–1.5 mol / L; The solid-liquid ratio of the precursor to the sulfate solution is 1 g:(5-10) mL.

[0013] Optionally, the tungstic phosphotungstic acid solution is a phosphotungstic acid solution with a concentration of 0.05–0.2 g / mL; The solid-liquid ratio of the sulfate-modified ZrO2-SnO2-TiO2 support to the tungsten-phosphorus heteropolyacid solution is 1 g:(5-10) mL.

[0014] Optionally, in step S3, the separated solid catalyst is regenerated by calcination and then recycled.

[0015] Optionally, in step S4, the pressure of the vacuum distillation purification is 1-500 Pa and the temperature is 160-280 °C.

[0016] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a solvent-free method for the solid superacid synthesis of pentaerythritol stearate. Through synergistic optimization of process design and catalytic system, the method systematically achieves efficient esterification of pentaerythritol and stearic acid under solvent-free conditions, construction of a stable and recyclable catalytic system, suppression of side reactions, and improvement of product purity. The specific logic is as follows: The efficient esterification under solvent-free conditions is achieved through the control of raw material state and the optimization of reaction process. In step S1, stearic acid is pre-melted to form a liquid reaction medium, which allows solid pentaerythritol and solid catalyst to be uniformly dispersed in it, creating a near-homogeneous reaction environment. This ensures sufficient contact and effective collision of reactant molecules without the need for solvent. Step S2 employs a stepped heating strategy. Initially, the esterification reaction is initiated at a suitable temperature to ensure a smooth reaction. Later, a moderate temperature increase reduces the system viscosity and promotes the removal of water generated during the reaction, shifting the reaction equilibrium towards the product side. This achieves efficient conversion of the raw materials in a solvent-free system. The core of constructing a stable and recyclable catalytic system lies in the catalyst's structural design and separation method. The solid superacid used is a ZrO2-SnO2-TiO2 ternary composite oxide as a support. Its stable framework structure can form strong chemical bonds with sulfate and tungsten-phosphorus heteropolyacids, preventing the loss of active components during the reaction and ensuring the stability of catalytic activity. Meanwhile, the catalyst is in solid form, and can be easily separated by filtration in step S3 after the reaction, providing a basis for subsequent regeneration and recycling, forming a stable and recyclable catalytic system. Side reactions are avoided through atmosphere control and temperature regulation. Step S1 is carried out under an inert atmosphere to isolate oxygen and prevent pentaerythritol and the product from deteriorating or losing color due to oxidation. Step S2 strictly controls the temperature range and reaction time of the step-by-step heating to avoid side reactions such as pentaerythritol dehydration and product coking caused by high temperatures, ensuring that the reaction proceeds in the direction of generating the target ester. The improvement of product purity relies on a fractional purification process. Step S3 removes impurities from the solid catalyst through filtration, yielding crude pentaerythritol stearate. Step S4 further employs vacuum distillation for purification, effectively removing unreacted raw materials and small molecule byproducts remaining in the crude product, ultimately yielding a high-purity pentaerythritol stearate product. The various steps throughout the process work in close coordination, balancing reaction efficiency, catalytic stability, product quality, and environmental friendliness in a solvent-free system. Attached Figure Description

[0017] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic flowchart of a solid superacid solvent-free synthesis method for pentaerythritol stearate provided in an embodiment of this application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0021] Figure 1 This is a schematic flowchart of a solid superacid solvent-free synthesis method for pentaerythritol stearate provided in an embodiment of this application.

[0022] like Figure 1As shown in the embodiments of this application, a solvent-free method for the synthesis of pentaerythritol stearate as a solid superacid is provided, the method comprising the following steps: S1. Under an inert atmosphere, molten stearic acid, pentaerythritol and solid superacid catalyst are mixed to obtain a mixture; the solid superacid catalyst is a sulfate and tungsten-phosphorus heteropolyacid modified solid acid supported by a ZrO2-SnO2-TiO2 ternary composite oxide. S2. The mixture is subjected to a step-heating reaction. In the early stage of the reaction, the temperature is controlled at 170-185℃. After the reaction has been going on for 3-5 hours, the reaction temperature is raised to 190-200℃ and the reaction continues for 2-4 hours until the esterification reaction is completed, and the reaction solution is obtained. S3. Filter the reaction solution to separate the solid catalyst and obtain the crude pentaerythritol stearate product. S4. The crude pentaerythritol stearate product is purified by vacuum distillation to obtain the pentaerythritol stearate product.

[0023] The solvent-free synthesis method for pentaerythritol stearate provided in this application aims at "high-efficiency conversion, green environmental protection, and high-quality product." The four steps are progressive and complementary, each fulfilling a key mission while forming a complete process loop through precise synergy, fully demonstrating the systematic and scientific nature of the process design. The specific roles of each step are as follows: Step S1: Raw material pretreatment and system construction – a prerequisite for efficient reaction The core function of S1 is to establish a stable, homogeneous, and pollution-free initial system for the esterification reaction. The introduction of an inert atmosphere isolates oxygen, preventing pentaerythritol from oxidizing and discoloring during subsequent high-temperature reactions, while also preventing product deterioration due to oxidation, thus ensuring product color and purity from the source. The pre-melting of stearic acid into a liquid state eliminates dispersion barriers of solid raw materials, allowing pentaerythritol powder and solid catalyst to be uniformly dispersed within it, forming a near-homogeneous reaction environment. This significantly increases the frequency of intermolecular collisions, laying the foundation for accelerated reaction. The specific ZrO2-SnO2-TiO2 ternary composite oxide-supported sulfate and tungsten-phosphorus heteropolyacid solid superacid catalyst is the core support for the "solvent-free" process. Its extremely strong acidic sites can efficiently catalyze the esterification reaction, and its solid form avoids the corrosion problems of traditional liquid acids. At the same time, the reaction can be carried out without solvents, reducing the burden of subsequent separation and environmental risks, which aligns with the concept of green chemistry.

[0024] Step S2: Stepped heating reaction – the core step to achieve high conversion rate S2 employs a staged temperature control strategy to precisely match the kinetic characteristics of different stages of the esterification reaction, which is key to overcoming reaction bottlenecks and achieving complete conversion. The initial reaction temperature is controlled at 170–185℃. This temperature ensures rapid initiation of the esterification reaction while avoiding side reactions such as pentaerythritol dehydration and product charring caused by high temperatures, allowing most raw materials to complete the reaction within 3–5 hours, achieving the main conversion. The temperature is raised to 190–200℃ in the later stages of the reaction to address the problems of increased system viscosity, difficulty in water removal, and decreased molecular diffusion rate at the end of the reaction. High temperature reduces system viscosity, increases molecular kinetic energy, and promotes rapid evaporation of residual water, continuously shifting the reaction equilibrium to the right and completely consuming unreacted raw materials, thus solving the common problem of stagnant conversion in the later stages of homogeneous esterification reactions.

[0025] Step S3: Catalyst Separation – Ensuring Green Process and Economic Efficiency The core function of S3 is to achieve efficient separation of the catalyst and product, enabling the greening and economic efficiency of the process. After the reaction, the reaction liquid remains liquid. Utilizing the difference in physical states between the solid catalyst and the liquid crude product, the two can be quickly separated by filtration. The separated solid superacid catalyst exhibits good stability and can be regenerated by simple calcination, allowing it to be recycled for the next batch of reactions. This not only reduces catalyst consumption costs but also reduces solid waste emissions. Furthermore, hot filtration avoids separation difficulties caused by solidification of the crude product after cooling, reducing operating energy consumption and providing feasibility for the industrial application of the process.

[0026] Step S4: Vacuum distillation purification – the ultimate guarantee of product quality. The core function of S4 is to remove trace impurities from the crude product, enabling precise control of product performance. Even after filtration, the crude product still retains small amounts of unreacted stearic acid, small-molecule byproducts, and other volatile impurities. These impurities can lead to increased acid value and decreased thermal stability, affecting its application in high-end fields such as lubricants and plasticizers. Vacuum distillation, by reducing system pressure, lowers the boiling point of impurities, thoroughly removing volatile impurities under gentle conditions. This effectively reduces the product's acid value, improves its color, thermal stability, and purity, ensuring the final product meets the requirements of high-end applications.

[0027] More importantly, the ZrO2-SnO2-TiO2 ternary composite oxide used in this application supports sulfate (SO4) ions. 2- The solid superacid catalyst of stearic acid and pentaerythritol works by synergistically constructing high-density, high-strength acidic sites through multiple components, thereby accelerating the esterification reaction of stearic acid and pentaerythritol through an acid catalytic mechanism. Simultaneously, the structural characteristics of the support and active components ensure catalytic stability and cyclicity. Its molecular-level mechanism can be specifically explained as follows: The ternary composite oxide support ZrO2-SnO2-TiO2 serves as a stable supporting framework for the active components, containing Zr... 4 + Sn 4+ Ti 4+ Transition metal ions possess empty d orbitals and high charge density, and on the one hand, they interact with SO4 through surface hydroxyl groups. 2- , Tungsten-phosphorus heteropolyacids form MO-SO3 - MO-PW 12 O 40 3- (M represents Zr) 4+ / Sn 4+ / Ti 4+ The ternary composite oxide contains stable chemical bonds, preventing the loss of active components during high-temperature reactions. Furthermore, its Lewis acidity regulates the electron cloud distribution of active components through an electron-induced effect, indirectly enhancing the proton dissociation ability of acidic sites. Simultaneously, the porous structure formed by the ternary composite oxide possesses a large specific surface area, enabling it to dissipate SO42-. 2- The reactants are uniformly dispersed with tungsten-phosphorus heteropolyacids to maximize the exposure of active sites and reduce diffusion resistance of reactant molecules. SO4 2- It is not a simple physical adsorption onto the carrier surface, but rather forms a chelate or bridged coordination structure with metal ions on the carrier surface. This coordination interaction triggers a significant electron transfer effect: SO4 2- The strong electronegativity of the H+ ion draws electrons from the metal ion, increasing the positive charge of the metal ion. This, in turn, polarizes the hydroxyl groups on the surface of the carrier attached to the ion, leading to a significant increase in the polarity of the OH bond in the hydroxyl group. + It readily dissociates to form strong Brønsted acid sites, which are far more acidic than traditional liquid acids and possess extremely strong proton-donating capabilities, making them one of the core active centers for catalyzing esterification reactions. Tungsten-phosphorus heteropolyacids (typically H3PW) 12 O 40 It has a unique Keggin molecular structure (PW) 12 O 40 3- It is itself a super-strong Brønsted acid, and the hydroxyl groups in the molecule that are linked to the PO bond can efficiently dissociate H. + Furthermore, the acidic sites are concentrated and uniform in intensity, allowing them to directly participate in protonation reactions; simultaneously, the tungsten-phosphorus heteropolyacid molecule coordinates with the carrier metal ions through surface oxygen atoms, and its negatively charged framework interacts with SO42-. 2- The coordination structure generates electronic interactions, further polarizing SO42-. 2- The associated hydroxyl group enhances its H + The dissociation efficiency is high, and the high stability of the Keggin structure allows it to maintain structural integrity during high-temperature reactions at 190–200 °C, avoiding the failure of acid sites.

[0028] Stearic acid (RCOOH, R is C) 17 H 35 The esterification reaction of pentaerythritol (C(CH2OH)4) with pentaerythritol is essentially an acid-catalyzed nucleophilic addition-elimination process. The catalyst participates in and accelerates the reaction through molecular-level steps: First, both the hydroxyl group in pentaerythritol and the carbonyl group in stearic acid contain lone pairs of electrons. Through intermolecular forces such as hydrogen bonding and coordination, they are adsorbed near the acidic sites on the catalyst surface. The large specific surface area and porous structure of the support allow reactant molecules to diffuse rapidly to the active center, while the strongly polar environment on the catalyst surface further polarizes the carbonyl group of stearic acid, laying the foundation for subsequent protonation. Subsequently, the strong Brønsted acid sites on the catalyst surface (from SO42-) 2- (and tungstic phosphatic acid) releases H + It combines with the oxygen atom of the carbonyl group in the stearic acid molecule to form a protonated carbonyl intermediate (RC(OH)). + =O), the molecular essence of this process is H. + Electron transfer, the lone pair electrons of the carbonyl oxygen atom accept H + Subsequently, the positive charge of the carbonyl carbon significantly increases, disrupting the original electron cloud balance and becoming an active center susceptible to nucleophilic attack. Next, the hydroxyl group in the pentaerythritol molecule acts as a nucleophile, and its lone pair of electrons on its oxygen atom attacks the protonated positively charged carbon atom of the carbonyl group, forming a tetrahedral transition state intermediate. The acidic sites of the catalyst stabilize the charge distribution of this intermediate through electrostatic interactions, dispersing the positive charge to the hydroxyl oxygen atom, lowering the energy barrier of the transition state, and significantly reducing the activation energy of this rate-controlling step, thus significantly increasing the reaction rate. Finally, under the continued catalysis of the acidic sites, the two hydroxyl groups in the tetrahedral intermediate undergo a dehydration reaction, with one hydroxyl group donating H+. + Another provides OH - The H atoms combine to form a water molecule and detach from the intermediate. Simultaneously, the intermediate undergoes electron rearrangement, restoring its carbonyl structure and forming a stable ester bond (R-COO-CH2-C(CH2OH)3), completing one esterification reaction. + It is released back into the acidic sites of the catalyst, thus regenerating the catalyst and entering the next catalytic cycle.

[0029] The molecular interactions of the catalyst components are not isolated, but rather form a synergistic effect that amplifies catalytic efficiency: the Lewis acidity of the ternary support and SO4 2-The Brønsted acid complementarity of tungsten-phosphorus heteropolyacids constructs a "high-density, high-strength" network of acidic sites, ensuring a continuous supply of protons. The stable structure of the support prevents the high-temperature loss of active components, enabling the catalyst to withstand the high temperature of 190–200°C in the S2 step. In a solvent-free system, the solid state of the catalyst concentrates the active sites on the surface, allowing reactant molecules to contact the active centers without crossing the solvent barrier, further improving proton transfer efficiency and reaction rate. This synergistic effect at the molecular level ultimately achieves highly efficient catalysis of the esterification reaction, while ensuring the cyclic stability of the catalyst and the greenness of the process.

[0030] In some embodiments, in step S1, the molar ratio of pentaerythritol to stearic acid is 1:(4.0 to 4.5).

[0031] The molar ratio of pentaerythritol to stearic acid was set at 1:(4.0–4.5) to ensure that the four hydroxyl groups in the pentaerythritol molecule could fully participate in the esterification reaction. A slight excess of stearic acid, as the carboxylic acid component, maximizes the coverage of the active hydroxyl sites of pentaerythritol, driving the esterification reaction towards the formation of the target ester and ensuring a high product yield.

[0032] In some embodiments, in step S1, the mass of the solid superacid catalyst is 1.0 to 3.0% of the total mass of the pentaerythritol and the stearic acid.

[0033] The feed mass of the solid superacid catalyst is 1.0 to 3.0% of the total mass of pentaerythritol and stearic acid. This ratio can provide sufficient strong acid sites to meet the catalytic requirements of the entire esterification reaction, while avoiding the increased separation burden and cost waste caused by excessive catalyst, thus achieving a balance between catalytic efficiency and process economy.

[0034] In some embodiments, step S1, the preparation method of the solid superacid catalyst includes the following steps: S101. Zirconium oxychloride, tin chloride and tetrabutyl titanate were prepared into a mixed solution, and ammonium carbonate was added under stirring until precipitation was complete to obtain a ternary composite hydroxide gel. S102. The ternary composite hydroxide gel is aged and then dried using supercritical carbon dioxide drying technology to obtain the precursor. S103, the precursor is impregnated in sulfate solution, filtered and dried after impregnation, and then calcined at 500-600℃ for 2-4h to obtain sulfate-modified ZrO2-SnO2-TiO2 support; S104. The sulfate-modified ZrO2-SnO2-TiO2 support is impregnated in a tungsten-phosphorus heteropoly acid solution. After impregnation, it is dried and activated at 300-400℃ for 1-3 hours to obtain the solid superacid catalyst.

[0035] In some embodiments, the conditions for supercritical carbon dioxide drying are: temperature of 40–60°C, pressure of 8–15 MPa, and time of 2–4 h.

[0036] In some embodiments, the amounts of zirconium oxychloride, tin chloride, and tetrabutyl titanate are (1-3):(0.5-1.5):1, calculated as the molar ratio of ZrO2, SnO2, and TiO2.

[0037] In some embodiments, the sulfate solution is an ammonium sulfate solution with a concentration of 0.5–1.5 mol / L; The solid-liquid ratio of the precursor to the sulfate solution is 1 g:(5-10) mL.

[0038] In some embodiments, the tungsten-phosphorus heteropolyacid solution is a phosphotungstic acid solution with a concentration of 0.05–0.2 g / mL; The solid-liquid ratio of the sulfate-modified ZrO2-SnO2-TiO2 support to the tungsten-phosphorus heteropolyacid solution is 1 g:(5-10) mL.

[0039] In catalyst preparation, zirconium oxychloride, tin chloride, and tetrabutyl titanate were prepared in a ZrO2, SnO2, TiO2 molar ratio of (1–3):(0.5–1.5):1. This aimed to construct a stable ternary composite support framework with suitable Lewis acidity through the synergistic effect of the three metal oxides, providing a foundation for the uniform loading and stable bonding of subsequent active components. Supercritical carbon dioxide drying was performed at 40–60℃, 8–15 MPa, and 2–4 h. This method completely removed moisture without damaging the porous structure of the precursor, avoiding pore collapse caused by traditional drying methods. The result was a support with a large specific surface area and well-developed pore structure, which facilitates the dispersion of active components and the diffusion of reactant molecules.

[0040] The sulfate solution used is an ammonium sulfate solution of 0.5–1.5 mol / L, with a precursor-to-solution solid-liquid ratio of 1 g:(5–10) mL. This combination of parameters ensures sufficient impregnation of the precursor surface, allowing sulfate ions to form a sufficient and stable coordination structure with the support metal ions, thus efficiently constructing strong Brønsted acid sites. Calcination at 500–600 °C for 2–4 h further consolidates the coordination between sulfate ions and the support, removes residual impurities, and optimizes the crystal structure and thermal stability of the support, providing reliable structural support for the catalytic reaction.

[0041] The tungstic phosphotungstic acid solution uses a 0.05–0.2 g / mL phosphotungstic acid solution, with a sulfate-modified support to solution solid-liquid ratio of 1 g:(5–10) mL. This ensures that the tungstic phosphotungstic acid molecules are uniformly loaded onto the support surface, forming a synergistic acidic network through electronic interactions with sulfate ions, thus supplementing and enhancing the strength and density of acidic sites on the catalyst. Activation at 300–400 °C for 1–3 h removes adsorbed water during loading, stabilizes the Keggin structure of the tungstic phosphotungstic acid, and ensures that it maintains structural integrity and acidic site activity during subsequent high-temperature esterification reactions.

[0042] In some embodiments, in step S3, the separated solid catalyst is regenerated by calcination and then recycled.

[0043] The separated solid catalyst is recycled after being regenerated by calcination. Calcination removes adsorbed reaction impurities and carbon deposits from the catalyst surface, restores the activity of acidic sites, and fully leverages the structural stability of the catalyst. This reduces both the cost of catalyst consumption and the emission of solid waste, aligning with the design concept of green processes.

[0044] In some embodiments, in step S4, the pressure of the vacuum distillation purification is 1-500 Pa and the temperature is 160-280 °C.

[0045] In step S4, the pressure of vacuum distillation purification is controlled at 1–500 Pa and the temperature at 160–280 °C. The low-pressure environment can significantly reduce the boiling point of volatile impurities such as unreacted stearic acid and small molecule byproducts. Combined with a suitable temperature range, it can efficiently remove trace impurities without decomposing or deteriorating the target product, effectively reduce the acid value of the product, and improve the purity, color, and thermal stability of the product, thus meeting the requirements of high-end application scenarios.

[0046] In summary, the advantages of this application extend across multiple dimensions, including synthesis process, catalytic system, environmental performance, product quality, and industrial application, forming a complete technical solution that combines scientific rigor, practicality, and environmental friendliness.

[0047] In terms of process design, the core highlight lies in the construction of a solvent-free synthesis system, completely eliminating the use of solvents in traditional esterification reactions. This not only avoids the environmental pollution risks associated with solvents from the source but also eliminates the cumbersome steps of subsequent solvent separation and recovery, significantly reducing process energy consumption and production costs. Simultaneously, it minimizes the potential impact of solvent residues on product quality. Coupled with a precise step-by-step temperature increase strategy, most raw materials react smoothly at a gentle temperature in the early stages, avoiding side reactions. Later, a moderate temperature increase overcomes the reaction bottlenecks caused by increased system viscosity and difficulty in moisture removal, ensuring full conversion of the raw materials and effectively solving the common problem of stagnant conversion rates in the later stages of esterification reactions. Furthermore, the rational design of raw material ratios and catalyst dosage further ensures the stability and controllability of the reaction process, making the entire synthesis process more operable.

[0048] The innovative catalytic system is the core support for the high efficiency of this application. Using a ZrO2-SnO2-TiO2 ternary composite oxide as a support, a synergistic and complementary network of acidic sites is constructed through dual modification with sulfate and tungsten-phosphorus heteropolyacid. The ternary support not only possesses a stable framework structure and well-developed porous characteristics, providing ample loading sites for active components and promoting reactant molecule diffusion, but its own Lewis acidity also regulates the electron cloud distribution of active components, enhancing acid strength. Sulfate forms a stable coordination structure with the support, constructing strong Brønsted acidic sites, while the tungsten-phosphorus heteropolyacid, with its unique Keggin structure, supplements the acidic sites and strengthens the synergistic effect. Together, they endow the catalyst with high-density, high-strength acidic characteristics, and the catalyst exhibits excellent thermal stability, able to withstand high-temperature conditions during the reaction process, and continuously exert catalytic efficiency. More importantly, the solid catalyst can be regenerated through simple calcination, exhibiting good recyclability, reducing catalyst consumption costs and solid waste emissions, further aligning with the concept of green chemistry.

[0049] In terms of product quality assurance, each step of the process provides effective support: inert atmosphere protection prevents oxidation and deterioration of raw materials and products, ensuring the purity of the product color; the vacuum distillation purification step can accurately remove trace amounts of unreacted raw materials and small molecule byproducts, significantly reducing the product's acid value, improving its thermal stability and purity, and making the final product more competitive in performance. Meanwhile, the entire process is simple and efficient; catalyst separation can be completed through filtration, the purification step requires no complex equipment, the various steps have strong synergistic adaptability, no harsh reaction conditions are required, and it possesses good potential for industrial scale-up, balancing production efficiency and economic feasibility.

[0050] The final product of this application, pentaerythritol stearate, possesses excellent lubricity, thermal stability, compatibility, and emulsifying properties, making it suitable for a wide range of applications. In the industrial sector, it can be used as a high-end lubricant in machinery manufacturing and chemical equipment operation, reducing mechanical wear and energy consumption. It can also be used as a plasticizer in plastics and rubber processing, improving the flexibility and processing performance of products. In the cosmetics field, its excellent emulsifying and moisturizing properties make it suitable for creams, lotions, lipsticks, and other products, providing moisturizing, thickening, and system stabilization. In the food industry, it can be used as a food additive, serving as an emulsifier, stabilizer, or anti-caking agent to improve the texture and storage stability of food. Furthermore, it can be applied in coatings and inks as a dispersant or leveling agent, enhancing the application effect of products.

[0051] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0052] Example 1 (1) The preparation of solid superacid catalysts includes the following steps: S101. Weigh out zirconium oxychloride (ZrOCl2·8H2O), tin chloride (SnCl4·5H2O), and tetrabutyl titanate, respectively, so that the molar ratio of ZrO2, SnO2, and TiO2 is 2:1:1. Dissolve zirconium oxychloride and tin chloride in deionized water. Under vigorous stirring, slowly add tetrabutyl titanate dropwise to the above mixed solution to form a homogeneous mixed solution. Subsequently, under continuous stirring, slowly add 1.0 mol / L ammonium carbonate aqueous solution as a precipitant until the pH of the solution is approximately 9.0, and precipitation is complete, yielding a white ternary composite hydroxide gel.

[0053] S102. The obtained gel was allowed to stand at room temperature for 12 hours for aging. Then, the gel was transferred to a supercritical carbon dioxide drying apparatus and dried at 50°C and 10 MPa for 3 hours to obtain a loose and porous dried precursor.

[0054] S103. The precursor obtained above was impregnated in a 1.0 mol / L ammonium sulfate solution, with the solid-liquid ratio of precursor to ammonium sulfate solution controlled at 1 g: 8 mL. After impregnation for 4 h, the solution was filtered and dried overnight at 110 °C. Subsequently, the dried solid was placed in a muffle furnace and calcined at 550 °C for 3 h to obtain sulfate-modified ZrO2-SnO2-TiO2 support.

[0055] S104. The support obtained in step S103 is impregnated in a 0.1 g / mL phosphotungstic acid (i.e., tungstic phosphotungstic acid) ethanol solution, with the solid-liquid ratio of support to phosphotungstic acid solution controlled at 1 g: 8 mL. After impregnation for 6 h, it is dried at 80 °C for 12 h. Finally, the dried sample is activated at 350 °C for 2 h to obtain the desired solid superacid catalyst.

[0056] (2) The synthesis of pentaerythritol stearate includes the following steps: S1. To a four-necked flask equipped with a stirrer, thermometer, nitrogen inlet tube, and water separator, first add 106.4 g (0.40 mol) of molten stearic acid. Purge with nitrogen to displace the air in the flask and maintain an inert atmosphere. Under nitrogen protection, add 13.6 g (0.10 mol) of pentaerythritol (molar ratio of pentaerythritol to stearic acid is 1:4.0) and 2.4 g (2.0% of the total mass of pentaerythritol and stearic acid 120 g) of the prepared solid superacid catalyst, and mix thoroughly to obtain a mixture.

[0057] S2. The mixture is subjected to a stepped temperature-increasing reaction program: First, the reaction temperature is controlled at 180°C, and the reaction is carried out at this temperature for 4 hours. Then, the reaction temperature is increased to 195°C, and the reaction is continued for 3 hours. During the reaction, the generated water is separated by a water separator until no water is distilled off, at which point the esterification reaction is considered complete, and the reaction solution is obtained.

[0058] S3. After the reaction is complete, the hot reaction solution is filtered while hot to separate the solid catalyst, yielding a light yellow crude pentaerythritol stearate product. The separated solid catalyst can be recycled after being regenerated by calcination in a muffle furnace (450℃, 2h).

[0059] S4. The obtained crude product was transferred to a vacuum distillation apparatus and purified by vacuum distillation at a pressure of 200 Pa and a temperature of 220 °C to remove trace amounts of unreacted stearic acid and small molecule byproducts. After purification, 104.0 g of colorless and transparent pentaerythritol stearate product was obtained. The product was analyzed by 1H NMR spectroscopy (1H NMR spectroscopy). 1 The structure was characterized by HNMR, confirming that the product was correct.

[0060] Example 2 (1) The preparation of solid superacid catalysts includes the following steps: S101. Weigh out zirconium oxychloride (ZrOCl2·8H2O), tin chloride (SnCl4·5H2O), and tetrabutyl titanate, respectively, so that the molar ratio of ZrO2, SnO2, and TiO2 is 1:1.5:1. Dissolve zirconium oxychloride and tin chloride in deionized water. Under vigorous stirring, slowly add tetrabutyl titanate dropwise to the above mixed solution to form a homogeneous mixed solution. Subsequently, under continuous stirring, slowly add 0.8 mol / L ammonium carbonate aqueous solution as a precipitant until the pH of the solution is approximately 8.5, and precipitation is complete, yielding a white ternary composite hydroxide gel.

[0061] S102. The obtained gel was allowed to stand at room temperature for 10 hours to age. Then, the gel was transferred to a supercritical carbon dioxide drying apparatus and dried at 45°C and 12 MPa for 2.5 hours to obtain a loose and porous dried precursor.

[0062] S103. The precursor obtained above was immersed in a 0.8 mol / L ammonium sulfate solution, with the solid-liquid ratio of precursor to ammonium sulfate solution controlled at 1 g: 6 mL. After immersion for 5 h, the solution was filtered and dried at 105 °C for 12 h. Subsequently, the dried solid was placed in a muffle furnace and calcined at 520 °C for 3.5 h to obtain sulfate-modified ZrO2-SnO2-TiO2 support.

[0063] S104. The support obtained in step S103 is impregnated in a 0.08 g / mL phosphotungstic acid (i.e., tungstic phosphotungstic acid) ethanol solution, with the solid-liquid ratio of support to phosphotungstic acid solution controlled at 1 g:7 mL. After impregnation for 5 h, it is dried at 75 °C for 10 h. Finally, the dried sample is activated at 320 °C for 2.5 h to obtain the desired solid superacid catalyst.

[0064] (2) The synthesis of pentaerythritol stearate includes the following steps: S1. To a four-necked flask equipped with a stirrer, thermometer, nitrogen inlet tube, and water separator, first add 111.7 g (0.42 mol) of molten stearic acid. Purge with nitrogen to displace the air in the flask and maintain an inert atmosphere. Under nitrogen protection, add 13.6 g (0.10 mol) of pentaerythritol (molar ratio of pentaerythritol to stearic acid is 1:4.2) and 2.5 g (2.0% of the total mass of pentaerythritol and stearic acid, 125.3 g) of the prepared solid superacid catalyst, and mix thoroughly to obtain a mixture.

[0065] S2. The mixture is subjected to a stepped temperature-increasing reaction program: First, the reaction temperature is controlled at 175°C, and the reaction is carried out at this temperature for 3.5 hours. Then, the reaction temperature is increased to 192°C, and the reaction is continued for another 3.5 hours. During the reaction, the water generated is separated by a water separator until no water is distilled off, at which point the esterification reaction is considered complete, and the reaction solution is obtained.

[0066] S3. After the reaction is complete, the hot reaction solution is filtered while hot to separate the solid catalyst, yielding a light yellow crude pentaerythritol stearate product. The separated solid catalyst can be recycled after being regenerated by calcination in a muffle furnace (450℃, 2h).

[0067] S4. The obtained crude product is transferred to a vacuum distillation apparatus and purified by vacuum distillation at a pressure of 150 Pa and a temperature of 200 °C to remove trace amounts of unreacted stearic acid and small molecule byproducts. After purification, a colorless and transparent pentaerythritol stearate product is obtained.

[0068] Example 3 (1) The preparation of solid superacid catalysts includes the following steps: S101. Weigh out zirconium oxychloride (ZrOCl2·8H2O), tin chloride (SnCl4·5H2O), and tetrabutyl titanate, respectively, so that the molar ratio of ZrO2, SnO2, and TiO2 is 3:0.5:1. Dissolve zirconium oxychloride and tin chloride in deionized water. Under vigorous stirring, slowly add tetrabutyl titanate dropwise to the above mixed solution to form a homogeneous mixed solution. Subsequently, under continuous stirring, slowly add 1.2 mol / L ammonium carbonate aqueous solution as a precipitant until the pH of the solution is approximately 9.5, and precipitation is complete, yielding a white ternary composite hydroxide gel.

[0069] S102. The obtained gel was allowed to stand at room temperature for 14 hours to age. Then, the gel was transferred to a supercritical carbon dioxide drying apparatus and dried at 55°C and 9 MPa for 3.5 hours to obtain a loose and porous dried precursor.

[0070] S103. The precursor obtained above was impregnated in a 1.2 mol / L ammonium sulfate solution, with the solid-liquid ratio of precursor to ammonium sulfate solution controlled at 1 g:9 mL. After impregnation for 3.5 h, the solution was filtered and dried at 115 °C for 10 h. Subsequently, the dried solid was placed in a muffle furnace and calcined at 580 °C for 2.5 h to obtain a sulfate-modified ZrO2-SnO2-TiO2 support.

[0071] S104. The support obtained in step S103 is impregnated in a 0.12 g / mL phosphotungstic acid (i.e., tungstic phosphotungstic acid) ethanol solution, with the solid-liquid ratio of support to phosphotungstic acid solution controlled at 1 g: 9 mL. After impregnation for 7 h, it is dried at 85 °C for 14 h. Finally, the dried sample is activated at 380 °C for 1.5 h to obtain the desired solid superacid catalyst.

[0072] (2) The synthesis of pentaerythritol stearate includes the following steps: S1. To a four-necked flask equipped with a stirrer, thermometer, nitrogen inlet tube, and water separator, first add 116.9 g (0.44 mol) of molten stearic acid. Purge with nitrogen to displace the air in the flask and maintain an inert atmosphere. Under nitrogen protection, add 13.6 g (0.10 mol) of pentaerythritol (molar ratio of pentaerythritol to stearic acid is 1:4.4) and 2.6 g (2.0% of the total mass of pentaerythritol and stearic acid, 130.5 g) of the prepared solid superacid catalyst, and mix thoroughly to obtain a mixture.

[0073] S2. The mixture is subjected to a stepped temperature-increasing reaction program: First, the reaction temperature is controlled at 185℃, and the reaction is carried out at this temperature for 4.5 hours. Then, the reaction temperature is increased to 198℃, and the reaction is continued for 2.5 hours. During the reaction, the generated water is separated by a water separator until no water is distilled off, at which point the esterification reaction is considered complete, and the reaction solution is obtained.

[0074] S3. After the reaction is complete, the hot reaction solution is filtered while hot to separate the solid catalyst, yielding a light yellow crude pentaerythritol stearate product. The separated solid catalyst can be recycled after being regenerated by calcination in a muffle furnace (450℃, 2h).

[0075] S4. The obtained crude product is transferred to a vacuum distillation apparatus and purified by vacuum distillation at a pressure of 80 Pa and a temperature of 250 °C to remove trace amounts of unreacted stearic acid and small molecule byproducts. After purification, a colorless and transparent pentaerythritol stearate product is obtained.

[0076] Comparative Example 1 This comparative example is modified from the one disclosed in Example 1 as follows: In the catalyst preparation steps, step S104 (tungsten-phosphorus heteropolyacid impregnation and activation) was removed, and only steps S101 to S103 were retained. Finally, a sulfate-modified ZrO2-SnO2-TiO2 solid acid catalyst was obtained, and tungsten-phosphorus heteropolyacid loading was no longer performed. The other steps, raw material amounts, and all process parameters of pentaerythritol stearate synthesis were completely consistent with those in Example 1.

[0077] Comparative Example 2 This comparative example is modified from the one disclosed in Example 1 as follows: In catalyst preparation step S101, only zirconium oxychloride is used as the support raw material, and tin chloride and tetrabutyl titanate are no longer added. That is, the support is a single ZrO2 oxide. The preparation steps, reagent dosages and process parameters of subsequent steps S102 to S104 are consistent with those of Example 1. Finally, a ZrO2 single support solid acid catalyst modified with sulfate and tungsten-phosphorus heteropolyacid is obtained. The other steps, raw material dosages and all process parameters of pentaerythritol stearate synthesis are completely consistent with those of Example 1.

[0078] Comparative Example 3 This comparative example is modified from the one disclosed in Example 1 as follows: In catalyst preparation step S102, instead of using supercritical carbon dioxide drying technology, the aged ternary composite hydroxide gel was dried in a forced-air drying oven at atmospheric pressure and 110°C for 12 hours to obtain a dried precursor. The subsequent catalyst preparation steps (S103, S104) and the synthesis steps of pentaerythritol stearate (S1-S4) were exactly the same as in Example 1.

[0079] The physicochemical properties of the solid superacids in Examples 1-3 and Comparative Examples 1-3 were determined, and the results are shown in Table 1. The methods for determining the physicochemical properties are as follows: Specific surface area and pore size distribution: determined by nitrogen adsorption-desorption (BET method) at -196℃ using a Micromeritics ASAP2020 physical adsorption instrument.

[0080] Acid strength and quantity: Ammonia-programmed temperature desorption (NH3-TPD) was used with a Micromeritics AutoChemII 2920 chemisorption analyzer.

[0081] Sulfur content: Elemental analysis was performed using an Elementar Vario ELcube elemental analyzer.

[0082] Table 1. Physicochemical properties of solid superacids in Examples 1-3 and Comparative Examples 1-3

[0083] As shown in Table 1, the solid superacid catalyst prepared in this invention exhibits excellent physicochemical properties. The catalysts in Examples 1-3, due to the synergistic strategy of using a ZrO2-SnO2-TiO2 ternary composite support, supercritical CO2 drying, and sulfate and phosphotungstic acid dual-acid site modification, all maintained a high specific surface area (165.92–172.18 m²). 2 It has a well-developed mesoporous structure (average pore size 7.96–8.41 nm). More importantly, its total acidity reaches 478.95–492.74 μmol NH3 / g, indicating that there are a large number of available strong acidic active sites on the catalyst surface.

[0084] In contrast, the performance of the comparative catalysts was significantly reduced due to the lack of key technologies. Comparative Example 1, lacking phosphotungstic acid support and relying solely on sulfate for acidity, had a significantly lower total acidity (352.68 μmol NH3 / g) than the examples, demonstrating the crucial role of phosphotungstic acid modification in enhancing acidity. Comparative Example 2 used a single ZrO2 support, with a specific surface area (142.36 m² / g). 2The concentrations of NH3 / g and total acid content (325.47 μmol NH3 / g) further decreased, highlighting the synergistic advantages of the ternary composite support in constructing a high specific surface area and abundant acid sites. In Comparative Example 3, the use of conventional drying resulted in severe collapse of the support pores, leading to a sharp decrease in specific surface area to 86.52 m² / g. 2 / g, and consequently the total acid content, which fully demonstrates that supercritical drying technology is indispensable for constructing and maintaining the ideal mesoporous structure of catalysts.

[0085] The properties of the pentaerythritol stearate products from Examples 1-3 and Comparative Examples 1-3 were determined, and the results are shown in Table 2. The performance testing methods are as follows: Product yield: Calculated by gravimetric method, yield (%) = (actual product mass / theoretical product mass) × 100%.

[0086] Acid value of the product: determined by potentiometric titration according to GB / T7304-2014 standard.

[0087] Product color: conforms to GB / T3143-1982 standard, using the platinum-cobalt colorimetric method.

[0088] Table 2. Performance of pentaerythritol stearate products from Examples 1-3 and Comparative Examples 1-3

[0089] As shown in Table 2, the excellent catalyst performance directly translates into outstanding synthesis results. In Examples 1-3, under the described synthesis method, the yield of pentaerythritol stearate reached as high as 90.24%–92.81%, while the acid value was as low as 1.32–1.82 mgKOH / g, the color was light (20–25 Pt-Co), and the hydroxyl value was low (11.47–12.36 mgKOH / g). These data indicate that the method of this invention not only has extremely high reaction conversion rate but also effectively suppresses side reactions, resulting in products with high purity and excellent quality.

[0090] In contrast, the comparative examples all showed varying degrees of decline in product performance. Comparative Example 1, due to insufficient catalyst acidity, saw its yield drop to 85.36%, its acid value increase to 3.21 mg KOH / g, and its color deepen, demonstrating that the strong acid sites provided by phosphotungstic acid are crucial for achieving deep esterification and high product purity. Comparative Example 2, with further reductions in catalyst specific surface area and acidity, exhibited even worse performance indicators, indicating that the ternary support is essential for maintaining high efficiency throughout the reaction process. Comparative Example 3 showed the worst product performance, with a yield of only 78.52% and an acid value as high as 4.48 mg KOH / g. This directly confirms that the mesoporous structure of the catalyst is the physical basis for ensuring effective diffusion and contact of reactant molecules (especially in the later stages of the reaction) with active sites; without good mass transfer, even acidic sites cannot fully function, ultimately leading to incomplete reaction and increased byproducts.

[0091] After the product separation was completed in Example 1, the recycling performance of the recovered solid superacid catalyst was tested, and the results are as follows: The solid catalyst separated in step S3 of Example 1 was placed in a muffle furnace and regenerated by calcination at 450°C for 2 hours. This regenerated catalyst was then used in a new round of pentaerythritol stearate synthesis under the same reaction conditions as in Example 1. Five cycles showed that the catalyst exhibited excellent cycling stability: the product yield was 89.85% and the acid value was 1.91 mg KOH / g in the first cycle; the yield remained at 88.72% and the acid value was 2.03 mg KOH / g in the third cycle; and the yield was still maintained at 87.51% and the acid value was 2.15 mg KOH / g in the fifth cycle. The product color remained below 30 Pt-Co throughout the cycling process. Characterization of the catalyst after the fifth cycle showed that its specific surface area still reached 155.36 m². 2 The total acid content was 435.62 μmol NH3 / g, only about 9-10% lower than that of the fresh catalyst. This indicates that the solid superacid catalyst prepared in this application not only has high initial activity, but also excellent structural stability and recyclability, verifying its good economic efficiency and sustainability in industrial applications.

[0092] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0093] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A solvent-free method for the synthesis of pentaerythritol stearate as a solid superacid, characterized in that, The method includes the following steps: S1. Under an inert atmosphere, molten stearic acid, pentaerythritol and solid superacid catalyst are mixed to obtain a mixture; the solid superacid catalyst is a sulfate and tungsten-phosphorus heteropolyacid modified solid acid supported by a ZrO2-SnO2-TiO2 ternary composite oxide. S2. The mixture is subjected to a step-heating reaction. In the early stage of the reaction, the temperature is controlled at 170-185℃. After the reaction has been going on for 3-5 hours, the reaction temperature is raised to 190-200℃ and the reaction continues for 2-4 hours until the esterification reaction is completed, and the reaction solution is obtained. S3. Filter the reaction solution to separate the solid catalyst and obtain the crude pentaerythritol stearate product. S4. The crude pentaerythritol stearate product is purified by vacuum distillation to obtain the pentaerythritol stearate product.

2. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 1, characterized in that, In step S1, the molar ratio of pentaerythritol to stearic acid is 1:(4.0-4.5).

3. The method for the solvent-free synthesis of pentaerythritol stearate as a solid superacid according to claim 1, characterized in that, In step S1, the mass of the solid superacid catalyst is 1.0 to 3.0% of the total mass of the pentaerythritol and the stearic acid.

4. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 3, characterized in that, In step S1, the preparation method of the solid superacid catalyst includes the following steps: S101. Zirconium oxychloride, tin chloride and tetrabutyl titanate were prepared into a mixed solution, and ammonium carbonate was added under stirring until precipitation was complete to obtain a ternary composite hydroxide gel. S102. The ternary composite hydroxide gel is aged and then dried using supercritical carbon dioxide drying technology to obtain the precursor. S103, the precursor is impregnated in sulfate solution, filtered and dried after impregnation, and then calcined at 500-600℃ for 2-4h to obtain sulfate-modified ZrO2-SnO2-TiO2 support; S104. The sulfate-modified ZrO2-SnO2-TiO2 support is impregnated in a tungsten-phosphorus heteropoly acid solution. After impregnation, it is dried and activated at 300-400℃ for 1-3 hours to obtain the solid superacid catalyst.

5. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 4, characterized in that, The conditions for supercritical carbon dioxide drying are: temperature 40–60℃, pressure 8–15 MPa, and time 2–4 h.

6. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 4, characterized in that, The amounts of zirconium oxychloride, tin chloride, and tetrabutyl titanate are calculated as follows, based on the molar ratio of ZrO2, SnO2, and TiO2: (1–3):(0.5–1.5):

1.

7. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 4, characterized in that, The sulfate solution is an ammonium sulfate solution with a concentration of 0.5–1.5 mol / L; The solid-liquid ratio of the precursor to the sulfate solution is 1 g:(5-10) mL.

8. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 4, characterized in that, The tungsten-phosphorus heteropolyacid solution is a phosphotungstic acid solution with a concentration of 0.05–0.2 g / mL; The solid-liquid ratio of the sulfate-modified ZrO2-SnO2-TiO2 support to the tungsten-phosphorus heteropolyacid solution is 1 g:(5-10) mL.

9. The solvent-free method for synthesizing pentaerythritol stearate as a solid superacid according to claim 1, characterized in that, In step S3, the separated solid catalyst is regenerated by calcination and then recycled.

10. The method for the solvent-free synthesis of pentaerythritol stearate as a solid superacid according to claim 1, characterized in that, In step S4, the pressure of the vacuum distillation purification is 1-500 Pa and the temperature is 160-280 °C.

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