Magnetic easy-to-separate Fe3O4-SnO-SiO2 composite catalyst as well as preparation and application thereof

Abstract

The invention discloses a magnetic easy-to-separate Fe3O4-SnO-SiO2 composite catalyst and preparation and application thereof.The composite catalyst is of a core-middle layer-shell layer three-layer core-shell structure, a shell comprises a coating layer, the core comprises a magnetic core, the middle layer comprises an active layer, and the active layer is evenly adsorbed to the surface of the magnetic core; the magnetic core is Fe3O4; the active layer is any one or more than two of SnO, ZrO2 and TiO2; the coating layer is made of SiO2 and wraps the outer side of the active layer. The problems that efficient catalysis, convenient magnetic separation, long-acting circulation stability and precise regulation and control of a product structure cannot be synchronously solved in the prior art are solved. The conversion rate of the catalyst for catalytic synthesis of trimethylolpropane trioleate reaches 94.32%; the friction coefficient of the product is stabilized at 0.09-0.10, the wear volume is reduced by more than 68%, and the magnetic separation recovery rate is 98%.

Description

Technical Field

[0001] This invention relates to a method for preparing a catalyst for the synthesis of trimethylolpropane trioleate, specifically to a magnetically separable Fe3O4-SnO-SiO2 composite catalyst and its preparation and application. Background Technology

[0002] Trimethylolpropane trioleate (TMPTO), a core base oil for high-end lubricants and aviation hydraulic fluids, relies on the esterification reaction of trimethylolpropane (TMP) and oleic acid (OA) for its industrial synthesis. Existing literature 1 (Su HG, Zhao Q, Jiang C, et al. Preparation of highly dispersed SnO / TiO2 catalysts and their performances in catalyzing polyol ester[J]. RSC Advances,2023[J]. https: / / doi.org / 10.1039 / d2ra07334j) describes the preparation of SnO / TiO2 catalysts using an impregnation method. Stannous chloride or stannous oxalate is used as the tin source, and the tin loading and calcination conditions are optimized to catalyze the esterification reaction of trimethylolpropane and n-octanoic acid. The Lewis acid sites provided by Sn species enhance the esterification conversion rate. Reference 2 (Lu Feiyan. Construction of Fe3O4@SiO2@Sn-TiO2 composite photocatalyst and study on catalytic degradation of tetracycline hydrochloride [D]. Guangxi University, 2020. DOI:10.27034 / d.cnki.ggxiu.2020.001841) constructed a ternary core-shell structure with Fe3O4 as the magnetic core, SiO2 as the intermediate layer, and Sn-doped TiO2 as the outer shell for photocatalysis. The core deficiency of the existing technology lies in the failure to achieve an integrated design of multifunctional catalyst synergy. Although Reference 1 (SnO / TiO2) achieved a high esterification conversion rate of 99.6% (160 °C and 5 hours), its non-magnetic support necessitates time-consuming and labor-intensive centrifugation or filtration separation after the reaction, resulting in low efficiency and significant catalyst recovery losses; at the same time, the TiO2 support lacks an effective coating layer design, making its active Sn species prone to detachment during cycling, limiting stability (only verified after 6 cycles). While Reference 2 (Fe3O4@SiO2@Sn-TiO2) possesses a core-shell structure and magnetic separation potential, its active component, Sn-TiO2, is specifically designed for photocatalytic degradation reactions. The properties of its active sites do not match the functions of the Lewis acid centers required for esterification, making it unlikely to efficiently catalyze TMPTO synthesis. Furthermore, the long-term structural stability of this structure, designed to meet photocatalytic requirements, is questionable under high-temperature esterification conditions. Moreover, its research objectives do not involve the regulation of product molecular structure, thus failing to meet the requirements for high-quality TMPTO synthesis.

[0003] Besides the aforementioned closest existing technologies, traditional homogeneous acid catalytic systems (such as concentrated sulfuric acid and p-toluenesulfonic acid catalysis) can achieve esterification reactions, but they suffer from severe equipment corrosion, high-salt wastewater generation during product post-processing, and non-recoverable catalysts, leading to environmental and cost issues. Traditional heterogeneous catalytic systems (such as molecular sieves and single metal oxide catalysis) solve the equipment corrosion problem, but still rely on inefficient separation methods such as centrifugation and filtration, and active sites are prone to aggregation and loss. Conventional magnetic catalyst systems are mostly single Fe3O4-supported active components, exhibiting defects such as uneven dispersion of active components, poor synergy between magnetic properties and catalytic activity, and insufficient cycle stability. In summary, existing technologies have failed to achieve multi-functional synergy of "high-efficiency catalysis - convenient magnetic separation - long-term cycle stability - precise control of product structure," and cannot simultaneously solve the four key problems of catalytic efficiency, separation cost, cycle stability, and product quality in the TMPTO synthesis process, thus restricting the green industrial production of high-performance TMPTO. This is also the core technical pain point that this application urgently needs to address. Summary of the Invention

[0004] The purpose of this invention is to provide a magnetically separable Fe3O4-SnO-SiO2 composite catalyst and its preparation and application. This invention solves the problems that existing technologies cannot simultaneously address in terms of efficient catalysis, convenient magnetic separation, long-term cycling stability, and precise control of product structure. The prepared Fe3O4-SnO-SiO2 core-shell catalyst precisely catalyzes the synthesis of TMPTO through tetravalent tin ion sites, achieving an esterification conversion rate of 91.3%, which remains at 87.8% after 5 cycles. The product isomerization degree is significantly reduced to 1:0.08, the friction coefficient is stabilized at 0.09~0.10, the wear volume is reduced by more than 68%, and rapid separation can be achieved using an external magnetic field of 0.5 Tesla, with a recovery rate of 98%.

[0005] To achieve the technical goals of "highly efficient catalysis, easy magnetic separation, and long-term stable cycling," this invention provides a magnetically separable Fe3O4-SnO-SiO2 composite catalyst. The composite catalyst has a three-layer core-shell structure: the shell includes a coating layer, the core includes a magnetic core, and the intermediate layer includes an active layer, which is uniformly adsorbed on the surface of the magnetic core. The magnetic core is Fe3O4; the active layer is any one or more of SnO, ZrO2, and TiO2; and the coating layer is SiO2, which is wrapped around the outside of the active layer.

[0006] Preferably, the Fe3O4 magnetic core has a particle size of 50 nm; the SiO2 coating layer has a thickness of 15 nm to 50 nm, which can achieve a balance between "magnetic response efficiency" and "active site protection".

[0007] Preferably, the loading of the active layer is 50% to 70% of the mass of the Fe3O4 magnetic core; wherein, when the active layer is SnO, it can provide Lewis acid active centers for directional catalysis through the tetravalent tin ion sites on its surface.

[0008] Preferably, the active layer is anchored to the surface of the Fe3O4 magnetic core by an oleic acid surface modifier to enhance the bonding force between the active component and the core; the SiO2 coating layer is formed by hydrolysis and condensation of tetraethyl orthosilicate or silane coupling agent under alkaline conditions to ensure the compactness and stability of the coating layer.

[0009] This invention provides a method for preparing the magnetically separable Fe3O4-SnO-SiO2 composite catalyst as described above, the method comprising: (1) Add Fe3O4 powder to anhydrous ethanol, disperse by ultrasonication, add dispersant and stir to form a suspension; (2) Add SnO powder and trimethylolpropane to the suspension, heat to 40℃~60℃ and stir, slowly add oil acid to obtain the SnO-supported system; (3) In the supported SnO system, the SiO2 coating precursor is slowly added dropwise, the pH of the system is adjusted to 9~10, and the temperature is raised to 60℃~80℃ for reaction; (4) After the reaction is completed, the product is collected by magnetic separation, washed and freeze-dried to obtain the magnetically separable Fe3O4-SnO-SiO2 composite catalyst.

[0010] Preferably, in step (1), the dispersant is polyvinylpyrrolidone; the ultrasonic power is 280 W to 320 W and the time is 10 minutes to 20 minutes; the mass of the dispersant is 15% to 25% of the mass of Fe3O4.

[0011] Preferably, in step (2), the stirring time is 25 to 35 minutes; the mass ratio of SnO to Fe3O4 is (0.5 to 0.7): 1, and the amount ratio of SnO to oleic acid is (0.5 to 0.7) g: (0.4 to 0.6) mL; in step (3), the SiO2 coating layer precursor is tetraethyl orthosilicate or a silane coupling agent (such as KH550).

[0012] This invention provides a method for synthesizing trimethylolpropane trioleate using the magnetically separable Fe3O4-SnO-SiO2 composite catalyst. The method comprises: heating and melting trimethylolpropane, adding oleic acid and the magnetically separable Fe3O4-SnO-SiO2 composite catalyst, and reacting at 180℃~220℃; cooling after the reaction, recovering the catalyst by magnetic separation, and collecting the product trimethylolpropane trioleate.

[0013] Preferably, the molar ratio of trimethylolpropane to oleic acid is 1:(2.5~3.5); the amount of the magnetically separable Fe3O4-SnO-SiO2 composite catalyst is 8%~12% of the mass of trimethylolpropane; the magnetic field strength for magnetic separation is 0.4 Tesla~0.6 Tesla, and the separation time is 3 minutes~8 minutes.

[0014] This invention provides an application of the magnetically separable Fe3O4-SnO-SiO2 composite catalyst as described above in the synthesis of trimethylolpropane trioleate.

[0015] This invention discloses a magnetically separable Fe3O4-SnO-SiO2 composite catalyst, its preparation, and its application. This invention solves the problems of existing technologies in simultaneously achieving high-efficiency catalysis, convenient magnetic separation, long-term cycling stability, and precise control of product structure, and has the following advantages: 1. The composite catalyst prepared in this invention has a core-shell structure, with Fe3O4 as the magnetic core, SnO as the active layer (providing Lewis acid sites for tetravalent tin ions), and SiO2 as the coating layer, achieving synergistic effects of magnetism, catalytic activity, and structural stability. The Lewis acid sites for tetravalent tin ions directionally activate the OA carboxyl group, promoting uniform ester bond formation and inhibiting molecular isomerization, thus achieving precise control of the TMPTO molecular structure. Utilizing the soft magnetic properties of Fe3O4, efficient separation of the catalyst and product can be achieved under an external magnetic field without residual magnetic interference.

[0016] 2. Compared to the esterification conversion rate of 28.6% in the uncatalyzed system, the catalyst of this invention can increase the TMPTO synthesis conversion rate to 94.32% with a single use, and the conversion rate is stably maintained between 91.86% and 94.1% after two to five uses, with a decay rate of only 1.04% and no loss of active sites. The catalyst can achieve rapid magnetic separation through an external magnetic field of 0.5 Tesla, which shortens the time by more than 80% compared to traditional centrifugation or filtration separation, and achieves a recovery rate of 98%, significantly reducing separation costs.

[0017] 3. The TMPTO synthesized by this invention exhibits a uniform ester bond distribution, with the degree of molecular isomerization decreasing from 1:0.37 in the uncatalyzed sample to 1:0.08, forming a regular molecular structure. Under a load of 100 N and a temperature of 30 °C, the friction coefficient of the TMPTO synthesized by this invention remains stable at 0.09–0.10, representing a reduction of over 40% compared to the uncatalyzed sample (initial friction coefficient > 0.15). The wear volume and wear rate are reduced by 68.4% and 62.5%, respectively, compared to the uncatalyzed sample. Under a load of 0–300 N and a temperature of 30 °C, the friction coefficient of the TMPTO synthesized by this invention remains stable at around 0.15 throughout the process, without significant fluctuations even when the temperature is increased to 150 °C, maintaining continuity and density, and significantly enhancing its extreme pressure resistance. Attached Figure Description

[0018] Figure 1 The images show scanning electron microscope (SEM) images and elemental distribution diagrams of the Fe3O4-SnO-SiO2 composite catalyst prepared in Example 1 of this invention.

[0019] Figure 2 The images show the esterification conversion rate and Fourier transform infrared (FT-IR) plots of the products of Example 1 and Comparative Example 1 of this invention.

[0020] Figure 3 The images show the nuclear magnetic resonance (NMR) spectra of TMPTO, the product of Example 1 and Comparative Example 1 of this invention.

[0021] Figure 4 The frictional properties of the oil synthesized by catalysis in Example 1 of this invention and the uncatalyzed sample in Comparative Example 1 are compared under different operating conditions.

[0022] Figure 5 The graph shows the performance of TMPTO prepared after 1 to 5 cycles of the products of Example 1 and Comparative Example 1 of this invention.

[0023] Figure 6 The images show the wear volume and wear rate of TMPTO prepared after tribological testing of the products of Example 1 and Comparative Example 1 after 1 to 5 cycles. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The pretreatment and source of the reagents in the following examples are as follows: The pretreatment involves placing 50 nm and 10 g of Fe3O4 powder and 10 g of commercially available SnO powder into a vacuum drying oven and drying at 60 °C for 4 hours to remove surface adsorbed water.

[0026] SnO powder was purchased from Qingdao Zhongke Runmei Lubricating Materials Technology Co., Ltd.

[0027] The abbreviations and key terms used in the following embodiments are defined as follows: TMPTO: Trimethylolpropane trioleate, a high-performance polyol ester base oil for lubricants; TMP: Trimethylolpropane, the core alcohol raw material for TMPTO synthesis; OA: Oleic acid, a fatty acid precursor for TMPTO synthesis; TEOS: Tetrabutyl orthosilicate, a precursor for SiO2 coating; Lewis acid sites of tetravalent tin ions: acidic sites with directional catalytic activity provided by SnO in composite catalysts; COF: Coefficient of friction, a core indicator characterizing the tribological properties of lubricating oil; TAN: Total acid number, a key parameter used to calculate the conversion rate of esterification reactions.

[0028] PVP: Polyvinylpyrrolidone.

[0029] Example 1 A method for preparing a magnetically separable Fe3O4-SnO-SiO2 composite catalyst, the method comprising: (1) Magnetic substrate dispersion: 1.0 g of pretreated Fe3O4 was added to 50 mL of anhydrous ethanol and placed in an ultrasonic cleaner. The mixture was ultrasonically dispersed at 300 W for 15 minutes. Then, 0.2 g of PVP dispersant was added and transferred to a magnetic stirrer. The mixture was magnetically stirred at 30 °C and 300 rpm for 30 minutes to form a uniform Fe3O4 suspension. (2) Loading of active components: Add 0.6 g of SnO powder and 10 mL of TMP to the Fe3O4 suspension, heat to 50°C, and stir at 300 rpm for 1 hour; slowly add 0.5 mL of OA with a pipette, and continue stirring for 30 minutes to make SnO uniformly adsorbed on the Fe3O4 surface. (3) SiO2 coating: 1.0 mL of TEOS was slowly added to the above system with a dropper (about 0.05-0.10 mL / min), and then ammonia was added to adjust the pH to 9.5. The temperature was raised to 70 °C and stirred for 2 hours to hydrolyze TEOS to form a SiO2 coating layer. (4) Magnetic separation and post-treatment: After the reaction is completed, the heating is turned off and cooled to room temperature. The solid is placed next to a 0.5 Tesla permanent magnet and left to stand for 5 minutes. The black solid at the bottom is collected by magnetic separation. The solid is washed three times with anhydrous ethanol. The magnetic separation operation is repeated after each washing. The washed solid is placed in a freeze dryer and dried at -50 ℃ for 48 h to obtain black Fe3O4-SnO-SiO2 composite catalyst powder.

[0030] Comparative Example 1 A catalyst-free method for synthesizing TMPTO, the method comprising: Raw material preparation and feeding: Accurately weigh 6.71 g (0.05 mol) TMP using an electronic analytical balance, add it to a 250 mL round-bottom flask, place a magnetic stir bar in the flask, and fix the flask on a heat-collecting constant-temperature magnetic stirrer. TMP melting: Turn on heating and stirring, set the temperature to 80 ℃, stir rate to 300 rpm, and maintain for 5 minutes until TMP is completely melted (in a transparent liquid state), ensuring that no solid particles remain; OA addition and reaction heating: After TMP is completely melted, accurately add 42.37 g (0.15 mol) of OA using a pipette, stir and mix for 1 minute to ensure uniform contact of the raw materials; then raise the heating temperature to 200 ℃, maintain the stirring rate at 300 rpm, and start timing the reaction. Isothermal reaction and product collection: The reaction was carried out at 200 °C for 4 h with continuous stirring to ensure uniform reaction. After the reaction was completed, the heating and stirring were turned off and the mixture was allowed to cool naturally to room temperature to obtain a pale yellow transparent liquid, which was labeled as "catalyst-free TMPTO sample" and used directly for subsequent performance testing (no catalyst separation step required).

[0031] Example 2: Catalytic Cyclic Synthesis of TMPTO and Performance Testing The method for synthesizing TMPTO using the above-mentioned catalyst, the method comprising: Add 6.71 g TMP to a 250 mL round-bottom flask, install a magnetic stirrer, and heat to 80 °C until TMP is completely melted; add 42.37 g OA and stir to mix evenly; add 0.671 g of the composite catalyst prepared in Example 1, heat to 200 °C, stir at 300 rpm, and maintain stirring for 4 hours. The above-mentioned catalyst recovery method includes: After the reaction was completed, the mixture was cooled to room temperature and placed in a 0.5 Tesla magnetic field for 5 minutes. After the catalyst settled, the supernatant was collected by siphon absorption to obtain the TMPTO sample for one use. The catalyst at the bottom of the flask was washed three times with ethanol and dried at 80 °C for 2 hours to complete the recovery. The above-mentioned catalyst recycling catalytic method includes: The recovered catalyst was used to repeat the above-described method for the catalytic synthesis of TMPTO and the catalyst recovery method 2 to 5 times to obtain TMPTO samples of the corresponding number of times.

[0032] Performance verification: Structural characterization: The structural changes of the reaction system after 1 to 5 cycles were characterized using nuclear magnetic resonance and Fourier transform infrared spectroscopy.

[0033] Conversion rate test: calculated by TAN titration.

[0034] like Figure 1 The image shows a scanning electron microscope (SEM) image and elemental distribution map of the Fe3O4-SnO-SiO2 composite catalyst prepared in Example 1 of this invention. Figure 1 The scanning electron microscope image on the left shows that the Fe3O4-SnO-SiO2 composite catalyst of this invention forms uniform particle agglomerates with regular surface morphology, consistent with the coating characteristics of a core-shell structure; Figure 1 The elemental distribution diagram of O, Fe, Si, and Sn on the right further verifies that the Fe3O4-SnO-SiO2 composite catalyst of this invention exhibits a layered structure of "core-intermediate layer-shell layer". Specifically, Fe (yellow) corresponds to the Fe3O4 magnetic core, serving as the structural substrate to achieve magnetic response; Sn (purple) uniformly covers the surface of the Fe element region, corresponding to the SnO active layer, providing Lewis acid sites for the catalytic reaction; Si (green) is wrapped in the outermost layer, corresponding to the SiO2 coating layer, effectively protecting the active layer and enhancing structural stability; O (red), as a common element for all components, is distributed in a manner completely overlapping with the overall structure, confirming the complete coating of each layer.

[0035] The results show that the composite catalyst of the present invention successfully constructed a core-shell structure of "magnetic core-active layer-coating layer", with uniform distribution and tight binding of each component, which lays the structural foundation for its convenient magnetic separation and catalytic activity.

[0036] like Figure 2 As shown, the esterification conversion rates and Fourier transform infrared (FT-IR) spectra of the products of Example 1 and Comparative Example 1 are presented, where (a) is the esterification conversion rate of Comparative Example 1 under catalyst-free conditions and the Fe3O4-SnO-SiO2 composite catalyst of Example 1 after 5 cycles; (b) is the Fourier transform infrared (FT-IR) spectrum of the reaction system of Comparative Example 1 under catalyst-free conditions and Example 1 after 1 to 5 catalytic cycles. Figure 2 As shown in (a), compared to the esterification conversion rate of only 79.33% in the uncatalyzed system of Comparative Example 1, the conversion rate of the catalyst in Example 1 of this invention jumped directly to 94.32% after one use, and the conversion rate remained stable between 91.86% and 94.1% after two to five uses, with a decay of only 1.04%, fully demonstrating the excellent cycle stability of the catalyst, and no significant loss of active sites. Figure 2 (b) FT-IR spectrum of the uncatalyzed system at 3200 cm⁻¹ -1 There is a distinct hydroxyl (-OH) characteristic peak nearby (corresponding to the residual hydroxyl group from incomplete esterification by TMP), and at 2400 cm⁻¹ -1 Near the ester carbonyl group (C=O), the peak shape is broadened and the intensity is weaker; while in the system after 1-5 catalytic cycles, the peak at 3200 cm⁻¹ is stronger. -1The hydroxyl peak at 2400 cm⁻¹ has almost disappeared. -1 The sharp and stable peak of the ester carbonyl group indicates that the catalyst can effectively promote the complete esterification of TMP and OA, and the ester bond structure of the product did not deteriorate after multiple cycles. Furthermore, the FT-IR spectra corresponding to each cycle number showed almost no difference, further verifying that the catalyst is structurally stable during cycling and did not cause any additional impact on the functional groups of the product, thus ensuring the structural uniformity of TMPTO.

[0037] like Figure 3 The figures show the nuclear magnetic resonance (NMR) spectra of TMPTO products from Example 1 and Comparative Example 1, where (a) is the ¹H NMR spectrum of Comparative Example 1 under catalyst-free conditions, (b) is the ¹³C NMR spectrum of Comparative Example 1 under catalyst-free conditions, (c) is the ¹H NMR spectrum of Example 1 under catalyst-containing conditions, and (d) is the ¹³C NMR spectrum of Example 1 under catalyst-containing conditions. Figure 3 (a) and (c) 1 A comparison of the ¹H NMR spectra reveals that the uncatalyzed system exhibits a broadened double-bond hydrogen signal peak and multiple heterogeneous peaks (corresponding to cis-trans double bonds in molecular isomerization) in the 2–3 ppm chemical shift range. In contrast, the catalyzed system shows a sharper peak and a significantly reduced number of heterogeneous peaks in this range, indicating that the catalyst effectively inhibits the isomerization of TMPTO molecules, resulting in a more uniform double-bond configuration. Furthermore, the uncatalyzed system shows a higher intensity of the ester-adjacent hydrogen signal near the 4 ppm chemical shift (corresponding to incompletely esterified hydroxyl hydrogens), while this signal almost disappears in the catalyzed system, further confirming the catalyst's promoting effect on the complete esterification reaction. Figure 3 (b) and (d) 13 C10 NMR spectral analysis: In the uncatalyzed system, the carbon skeleton signal in the chemical shift range of 50–100 ppm was dispersed and the peak shape was broadened, reflecting the uneven distribution of ester bonds and disordered molecular structure. In contrast, the catalyzed system showed concentrated and stable peaks in this range, especially the signal at the ester carbonyl carbon (chemical shift ~150 ppm), which was sharper, proving that the catalytically synthesized TMPTO had a more uniform ester bond distribution and a more regular carbon skeleton structure. 1 H NMR and 13 The results of C NMR clearly show that the Fe3O4-SnO-SiO2 composite catalyst of the present invention achieves precise control of the TMPTO molecular structure through the directional catalytic effect of the Lewis acid sites of tetravalent tin ions, which greatly improves the structural uniformity of the product and lays the molecular basis for its excellent tribological properties.

[0038] Tribological testing: The SRV tribological testing machine was used, with a load of 0~300 N, a temperature of 30 ℃, a test stroke of 1 mm, and a frequency of 25 Hz (see details for corresponding results). Figure 4Friction properties were tested under different working conditions, including a load of 100N, a temperature range of 0~150℃, a test stroke of 1mm, and a frequency of 25Hz (see attached results). Figure 4 The performance of ester oils prepared after 1-5 cycles at 100 N load and 30 °C was tested (see details for corresponding results). Figure 5 and Figure 6 ).

[0039] like Figure 4 As shown, the frictional properties of TMPTO synthesized by catalyst in Example 1 of this invention and the uncatalyzed sample in Comparative Example 1 are compared under different operating conditions. (a) shows the friction coefficient curve under a load gradient at 30°C, with a test stroke of 1 mm and a frequency of 25 Hz; (b) shows the friction coefficient curve under a temperature gradient at a load of 100 N, with a test stroke of 1 mm and a frequency of 25 Hz. Figure 4 As shown in (a), at 30℃, the friction coefficient of the uncatalyst-free sample remained stable at around 0.15, while the friction coefficient of the catalyst-synthesized sample consistently remained in the range of 0.10~0.15, showing an overall lower coefficient. Furthermore, as the load increased stepwise from 0 to 300N, the friction coefficients of both samples did not show significant fluctuations, indicating that the catalytically synthesized TMPTO possesses excellent resistance to load impact and is suitable for high-load conditions. Figure 4 As shown in (b), under a load of 100N, the friction coefficient of the uncatalyzed sample fluctuated significantly after the temperature rose to 150 ℃ (reaching a maximum of close to 0.2); while the friction coefficient of the catalyzed sample remained stable at around 0.15 throughout the process, and there was no significant fluctuation even when the temperature was raised to 150 ℃. This fully demonstrates the high-temperature stability of the catalytically synthesized TMPTO - its lubricating film can still maintain continuity and density under high temperature conditions, avoiding the deterioration of lubrication performance caused by the temperature rise of the uncatalyzed sample.

[0040] In summary, the TMPTO synthesized by this invention exhibits superior tribological stability over a wide load and temperature range, providing performance assurance for its practical application under complex working conditions.

[0041] like Figure 5 As shown, the performance graphs of TMPTO prepared after 1-5 cycles of the products of Example 1 and Comparative Example 1 are presented, where (a) is the tribological curve and (b) is the wear trajectory thermogram, the data of which are sourced from... Figure 4 (a); (c) 3D topographic images of the wear trajectory, the data of which are sourced from Figure 4 (a). From Figure 5As can be seen from the tribological curve of (a), the initial coefficient of friction (COF) of the uncatalyzed sample is as high as 0.16, and it continues to fluctuate with the extension of the test time (reaching a maximum of 0.18); while in the catalytic system of Example 1, the coefficient of friction of TMPTO is stably maintained in the range of 0.09~0.10 regardless of whether it is 1 cycle or 5 cycles, with a fluctuation range of less than 5%, which fully demonstrates the excellent tribological stability of catalytically synthesized TMPTO.

[0042] Combination Figure 5 (b) Wear trajectory thermogram: The wear trajectory depth of the uncatalyzed sample exceeds -7000 nm (the dark blue area accounts for a large proportion), and the depth distribution is uneven; while the wear depth of the sample with 1 to 5 catalytic cycles is concentrated in the range of -1000 to -3000 nm, which is shallower and more uniformly distributed, indicating that the catalytically synthesized TMPTO can form a more effective lubricating protective film and significantly reduce the interfacial damage of the friction pair.

[0043] Observe again Figure 5 (c) 3D morphology image: The wear trajectory of the uncatalyzed sample shows a rough morphology of "deep grooves + edge burrs", and the surface damage is severe; while the wear trajectory of the sample after catalytic cycle is smoother and the texture is more uniform, with no obvious deep grooves or burrs, which further confirms its low wear characteristics.

[0044] In summary, the TMPTO synthesized by this invention not only significantly reduces the coefficient of friction (by 43.75% compared to the uncatalyzed sample), but also greatly reduces the wear volume and surface damage. Even after five catalyst cycles, the lubrication performance of the product remains stable, fully demonstrating the outstanding effect of this catalytic system in improving the tribological properties of ester oils.

[0045] like Figure 6 As shown, the tribological test results of TMPTO prepared after 1-5 cycles of the products of Example 1 and Comparative Example 1 are presented as wear volume and wear rate graphs. The data are sourced from [source missing]. Figure 5 (a). By Figure 6 It can be seen that the wear volume of the uncatalyzed sample is as high as approximately 3.8 × 10⁻⁶. 6 μm 3 The wear rate is close to 8×10 -3 mm 3 / N‧m; while in the catalytic system of Example 1, even after the first use, the wear volume of the product had decreased to approximately 1.2 × 10⁻⁶ N‧m; 6 μm³, with a wear rate of only about 3×10. -3 mm 3 / N‧m, which was reduced by 68.4% and 62.5% respectively compared with the uncatalyzed sample. After 2-5 catalyst cycles, the wear volume of the product remained stable at 1.2×10 6 ~1.8×10 6μm 3 Within this range, the wear rate remained at 3×10. -3 ~4×10 -3 mm 3 Within the range of / N‧m, the fluctuation range is less than 30%. This result further verifies the long-term stability of the catalytic system of the present invention: not only can it significantly reduce the wear of ester oils on the first use, but after multiple cycles, the anti-wear performance of the product remains excellent, fully demonstrating the precise control of the TMPTO molecular structure by the catalyst, and providing performance assurance for its long-term application in high-end lubrication scenarios.

[0046] Comprehensive analysis reveals that the composite catalyst prepared in this invention has a core-shell structure, with Fe3O4 as the magnetic core, SnO as the active layer (providing Lewis acid sites for tetravalent tin ions), and SiO2 as the coating layer, achieving synergistic effects in magnetism, catalytic activity, and structural stability. Specifically, the Lewis acid sites for tetravalent tin ions directionally activate the OA carboxyl group, promoting uniform ester bond formation and inhibiting molecular isomerization, thus achieving precise control over the TMPTO molecular structure. Under a load of 100 N and a temperature of 30 °C, the friction coefficient of the TMPTO synthesized by this invention remains stable at 0.09–0.10, a reduction of over 40% compared to the uncatalyzed sample (initial friction coefficient > 0.15). The wear volume and wear rate are reduced by 68.4% and 62.5%, respectively, compared to the uncatalyzed sample. Under a load of 0–300 N and a temperature of 30 °C, the friction coefficient of the TMPTO synthesized by this invention remains stable at around 0.15 throughout the process, without significant fluctuations even when the temperature is increased to 150 °C, maintaining continuity and density, and significantly enhancing its extreme pressure resistance. This invention utilizes the soft magnetic properties of Fe3O4 to achieve efficient separation of catalyst and product within 5 minutes under an external magnetic field, with no residual magnetic interference. Rapid magnetic separation can be achieved with an external magnetic field of 0.5 Tesla. Compared with traditional centrifugation / filtration separation, the time is reduced by more than 80%, the recovery rate reaches 98%, and the separation cost is significantly reduced. Moreover, the catalyst can be recycled, there is no high-salt wastewater discharge, and the product TMPTO meets the requirements of high-end lubrication scenarios, providing a feasible technical route for green industrial production.

[0047] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A magnetically separable Fe3O4-SnO-SiO2 composite catalyst, characterized in that, The composite catalyst has a three-layer core-shell structure consisting of a core, an intermediate layer, and a shell. The shell includes a coating layer, the core includes a magnetic core, the intermediate layer includes an active layer, and the active layer is uniformly adsorbed on the surface of the magnetic core. The magnetic core is Fe3O4; The active layer is any one or more of SnO, ZrO2 and TiO2; The coating layer is made of SiO2 and is wrapped around the outside of the active layer to form a complete core-shell structure.

2. The composite catalyst according to claim 1, characterized in that, The magnetic core has a particle size of 50 nm; the coating layer has a thickness of 15 nm to 50 nm.

3. The composite catalyst according to claim 1, characterized in that, The loading of the active layer is 50% to 70% of the magnetic core mass; the active layer is SnO, providing tetravalent tin ions. 4+ Lewis acid site.

4. The composite catalyst according to claim 3, characterized in that, The active layer is anchored to the surface of the Fe3O4 magnetic core by an oleic acid surface modifier; the SiO2 is formed by hydrolysis and condensation of tetraethyl orthosilicate or a silane coupling agent under alkaline conditions.

5. A method for preparing the magnetically separable Fe3O4-SnO-SiO2 composite catalyst as described in any one of claims 1 to 3, characterized in that, The method includes: (1) Add Fe3O4 powder to anhydrous ethanol, disperse by ultrasonication, add dispersant and stir to form a suspension; (2) Add SnO powder and trimethylolpropane to the suspension, heat to 40 ℃~60 ℃ and stir, slowly add oil acid to obtain the SnO-supported system; (3) In the supported SnO system, the SiO2 coating precursor is slowly added dropwise, the pH of the system is adjusted to 9~10, and the temperature is raised to 60 ℃~80 ℃ for reaction; (4) After the reaction is completed, the product is collected by magnetic separation, washed and freeze-dried to obtain the magnetically separable Fe3O4-SnO-SiO2 composite catalyst.

6. The preparation method according to claim 5, characterized in that, In step (1), the dispersant is polyvinylpyrrolidone; the ultrasonic power is 280 W to 320 W and the time is 10 minutes to 20 minutes; the mass of the dispersant is 15% to 25% of the mass of Fe3O4.

7. The preparation method according to claim 5, characterized in that, In step (2), the stirring time is 25 to 35 minutes; the mass ratio of SnO to Fe3O4 is (0.5 to 0.7): 1; the amount ratio of SnO to oleic acid is (0.5 to 0.7) g: (0.4 to 0.6) mL; in step (3), the SiO2 coating precursor is tetraethyl orthosilicate or a silane coupling agent.

8. A method for synthesizing trimethylolpropane trioleate using the magnetically separable Fe3O4-SnO-SiO2 composite catalyst according to any one of claims 1 to 3, characterized in that, The method comprises: heating and melting trimethylolpropane, adding oleic acid and the magnetically separable Fe3O4-SnO-SiO2 composite catalyst, and reacting at 180 ℃~220 ℃; cooling after the reaction, recovering the catalyst by magnetic separation, and collecting the product trimethylolpropane trioleate.

9. The method according to claim 8, characterized in that, The molar ratio of trimethylolpropane to oleic acid is 1:(2.5~3.5); the amount of the magnetically separable Fe3O4-SnO-SiO2 composite catalyst is 8%~12% of the mass of trimethylolpropane; the magnetic field strength for magnetic separation is 0.4 Tesla~0.6 Tesla, and the separation time is 3~8 minutes.

10. The application of the magnetically separable Fe3O4-SnO-SiO2 composite catalyst as described in any one of claims 1 to 3 in the synthesis of trimethylolpropane trioleate.

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