Silicon-based heterogeneous palladium catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of catalysts, and particularly relates to a silicon-based heterogeneous palladium catalyst and a preparation method and application thereof. The preparation method comprises the following steps: mixing a block copolymer template agent, a pore diameter expanding agent and an acid solution for self-assembly to form block copolymer micelles, mixing the block copolymer micelles with a silicon source for hydrolytic condensation reaction to obtain silicon-oxygen cluster-block copolymer composite micelles, mixing the silicon-oxygen cluster-block copolymer composite micelles with an etching agent for etching and calcining to obtain a silicon-based porous foam matrix, and carrying out amino grafting modification on the silicon-based porous foam matrix to obtain the silicon-based porous foam material. And loading and reducing palladium to obtain the silicon-based heterogeneous palladium catalyst. The silicon-based heterogeneous palladium catalyst has high catalytic activity, selectivity and stability, is convenient to recycle and does not cause environmental pollution easily.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of catalysts, and particularly relates to a silicon-based heterogeneous palladium catalyst and a preparation method and application thereof. BACKGROUND

[0002] Palladium catalysts play a key role in organic synthesis. Traditional homogeneous palladium catalysts, including Pd(OAc)2, PdCl2, Pd(PPh3)4, [Pd(dba)2], etc., are commonly used in coupling, hydrogenation, oxidation and other reactions, especially in carbon-carbon coupling reactions such as Suzuki, Heck and Sonogashira reactions. However, the traditional homogeneous palladium catalysts have the following problems: 1. Difficult to recover: homogeneous catalysts are difficult to separate from the reaction system, increasing the production cost and environmental burden. 2. Poor stability: Palladium is easily lost during the reaction, resulting in reduced catalyst activity. 3. Poor catalytic performance: Due to the large particle size of the palladium clusters in the traditional palladium catalysts, the specific surface area is small, the utilization rate of palladium elements is low, and the catalytic performance is poor. 4. Environmental impact: Palladium loss may pollute the environment. Therefore, it is of great significance to develop a highly efficient, stable and easy-to-recover heterogeneous palladium catalyst. SUMMARY

[0003] Therefore, the purpose of the present application is to provide a silicon-based heterogeneous palladium catalyst and a preparation method and application thereof, which has high catalytic activity and selectivity, high stability, is easy to recover and reuse, and is not easy to cause environmental pollution.

[0004] In order to achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0005] The present application provides a preparation method of a silicon-based heterogeneous palladium catalyst, comprising the following steps:

[0006] The block copolymer template agent, the pore size expanding agent and the acidic solution are mixed to form block copolymer micelles by self-assembly;

[0007] The block copolymer micelles and the silicon source are mixed to carry out hydrolysis and condensation reaction to obtain silicon-oxygen cluster-block copolymer composite micelles;

[0008] The silicon-oxygen cluster-block copolymer composite micelles and the etching agent are mixed to carry out etching, and the obtained silicon-oxygen cluster-block copolymer composite micelles after etching are calcined to obtain a silicon-based porous foam matrix;

[0009] The silicon-based porous foam matrix, the organic aminosilane and the non-polar organic solvent are mixed to carry out amine group grafting modification to obtain an amine group functionalized silicon-based porous foam;

[0010] Mixing the basic water suspension of the amine group functionalized silicon-based porous foam and the basic complex solution of tetrachloropalladate, carrying out loading, mixing the obtained amine group functionalized silicon-based porous foam loaded with palladium and a reducing agent, carrying out reduction reaction, and obtaining a silicon-based heterogeneous palladium catalyst.

[0011] Preferably, the block copolymer template agent is a non-ionic triblock copolymer; the non-ionic triblock copolymer includes one or more of P123, F127 and P85.

[0012] Preferably, the silicon source includes one or more of tetraethoxysilane, tetramethoxysilane and tetrapropoxysilane.

[0013] Preferably, the pore size expansion agent includes one or more of 1,3,5-trimethylbenzene, 1,3,5-triisopropylbenzene and toluene.

[0014] Preferably, the self-assembly time is 1-4 h; the self-assembly temperature is 37-40℃; the self-assembly is carried out under stirring; the stirring speed is 300-800 rpm.

[0015] Preferably, the organic aminosilane includes 3-aminopropyltrimethoxysilane.

[0016] Preferably, the reducing agent is one or more of sodium borohydride, formaldehyde, ascorbic acid and hydrogen.

[0017] The application further provides a silicon-based heterogeneous palladium catalyst prepared by the preparation method described in the above technical solution, including: an amine group functionalized silicon-based porous foam and palladium nanoparticles loaded on the surface and pores of the amine group functionalized silicon-based porous foam; the silicon-based porous foam is mesoporous silica; the palladium nanoparticles include zero-valent palladium nanoparticles and divalent palladium nanoparticles.

[0018] The application further provides an application of the silicon-based heterogeneous palladium catalyst described in the above technical solution in an organic synthesis reaction.

[0019] The organic synthesis reaction includes one or more of a coupling reaction, a hydrogenation reaction and an oxidation reaction.

[0020] The application further provides a method for preparing a 4-methyl biphenyl derivative based on a Suzuki coupling reaction, including the following steps:

[0021] Mixing p-bromotoluene, phenylboronic acid, a basic substance, a catalyst and a polar mixed solvent, carrying out Suzuki coupling reaction, and obtaining a 4-methyl biphenyl derivative; the 4-methyl biphenyl derivative is 4-methyl-4'-pentyl-1,1'-biphenyl.

[0022] The catalyst is the silicon-based heterogeneous palladium catalyst described in the above technical solutions.

[0023] The present application provides a preparation method of a silicon-based heterogeneous palladium catalyst, comprising the following steps:

[0024] The block copolymer template agent, the pore size expansion agent and the acidic solution are mixed to form block copolymer micelles through self-assembly;

[0025] The block copolymer micelles and the silicon source are mixed to perform hydrolysis and condensation reaction, so as to obtain silicon-oxygen cluster-block copolymer composite micelles;

[0026] The silicon-oxygen cluster-block copolymer composite micelles and the etching agent are mixed to perform etching, and the obtained silicon-oxygen cluster-block copolymer composite micelles after etching are calcined to obtain a silicon-based porous foam matrix;

[0027] The silicon-based porous foam matrix, the organic aminosilane and the non-polar organic solvent are mixed to perform amine group grafting modification, so as to obtain an amine group functionalized silicon-based porous foam;

[0028] The basic aqueous suspension of the amine group functionalized silicon-based porous foam and the basic complex solution of tetrachloropalladate are mixed to perform loading, and the obtained palladium-loaded amine group functionalized silicon-based porous foam and the reducing agent are mixed to perform reduction reaction, so as to obtain a silicon-based heterogeneous palladium catalyst.

[0029] In the present application, the silicon-based porous foam matrix has a high-strength porous structure, high porosity and large specific surface area. The surface of the porous foam is modified and regulated by amine group functionalization to locally control the microenvironment. The loading amount of zero-valent palladium nanoparticles is high, the adsorption between the zero-valent palladium nanoparticles and the silicon-based porous foam is strong, and the zero-valent palladium nanoparticles are uniformly dispersed. The silicon-based heterogeneous palladium catalyst has high chemical stability and mechanical stability, maintains the structure and high catalytic activity during the reaction process, realizes high selectivity, is easy to recycle and reuse, and maintains stable performance (the catalytic performance of the catalyst is only reduced by 6% in the repeated catalyst recovery experiment for 7 times), can reduce production cost and reduce environmental impact, has a simple and controllable preparation method, and has excellent industrial production and application potential. BRIEF DESCRIPTION OF DRAWINGS

[0030] Fig. 1 TEM image of the silicon-based heterogeneous palladium catalyst prepared in Example 1 at 500 nm;

[0031] Fig. 2 TEM image of the silicon-based heterogeneous palladium catalyst prepared in Example 1 at 200 nm. DETAILED DESCRIPTION

[0032] The present application provides a preparation method of a silicon-based heterogeneous palladium catalyst, comprising the following steps:

[0033] mixing the block copolymer template agent, the pore-expanding agent and the acidic solution to form block copolymer micelles through self-assembly;

[0034] mixing the block copolymer micelles and a silicon source to perform a hydrolysis-condensation reaction to obtain a silicon-oxygen cluster-block copolymer composite micelle;

[0035] mixing the silicon-oxygen cluster-block copolymer composite micelle and an etching agent to perform etching, and calcining the obtained etched silicon-oxygen cluster-block copolymer composite micelle to obtain a silicon-based porous foam matrix;

[0036] mixing the silicon-based porous foam matrix, an organoaminosilane and a non-polar organic solvent to perform amine group grafting modification to obtain an amine group functionalized silicon-based porous foam;

[0037] mixing an alkaline aqueous suspension of the amine group functionalized silicon-based porous foam and an alkaline complex solution of tetrachloropalladate to perform loading, mixing the obtained palladium-loaded amine group functionalized silicon-based porous foam and a reducing agent to perform a reduction reaction to obtain a silicon-based heterogeneous palladium catalyst.

[0038] Unless otherwise specified, the source of the raw materials used in the present application is not particularly limited, and commercially available products known to those skilled in the art can be used.

[0039] The block copolymer template agent, the pore-expanding agent and the acidic solution are mixed to form block copolymer micelles through self-assembly.

[0040] As an embodiment, the block copolymer template agent is a non-ionic triblock copolymer; the non-ionic triblock copolymer includes one or more of P123, F127 and P85, and in a specific embodiment, P123; the P123 is a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer, and the molecular formula is EO20-PO70-EO20; the molecular formula of the F127 is EO106-PO70-EO106; and the molecular formula of the P85 is EO26-PO39-EO26.

[0041] The block copolymer in the application plays a role of a template agent in the synthesis of silicon-based porous foam, and has the following main functions: 1. structure directing agent, P123 forms a micellar structure in an acidic or neutral solution, drives the formation of a mesoporous / macroporous structure, can regulate the pore size, pore volume and specific surface area, and controls the pore arrangement mode (hexagonal, cubic, etc.); 2. stabilizing foam structure, in the carrier, P123 helps to form a fine and dense foam porous structure (containing mesopores / macropores); a three-dimensional interpenetrating honeycomb-like mesoporous network is formed; 3. adjusting the order and thermal stability of the pore channel, the structure (EO / PO ratio) of the copolymer affects the order of the pores; the ordered pore structure can enhance the stability and application performance (such as catalysis, adsorption, etc.) of the material; the pore size formed by F127 is larger and more uniform, the formation of macropores is better, and it can be better than P123 to construct a larger pore structure; the molecular weight of P85 is smaller, and it is easy to form a small pore structure, but the order of the pore channel formed is poor; single-block ether surfactants (polyethylene glycol hexadecyl ether (Brij-56), polyoxyethylene (10) oil ether (Brij-97)) are usually used to construct small pore materials such as SBA-15, and are not suitable for the carrier structure of the palladium-based catalyst; the cationic surfactant (cetyltrimethylammonium bromide (CTAB)) forms a smaller pore size hexagonal mesoporous structure (such as MCM-41), which cannot replace the formation of an amino carrier structure; the structure directing property of the flexible nonionic surfactant (Tween-80, Span-80) is weak, and the foam structure formed is not well supported. The main reasons for the poor effect of other copolymers (single-block ether surfactants, cationic surfactants, flexible nonionic surfactants) are as follows: 1. insufficient thermal stability, degradation under high-temperature hydrothermal conditions, resulting in weak micellar formation ability and difficulty in forming regular pore channels; 2. the ratio of hydrophobic segment / hydrophilic segment is not suitable, which also leads to unstable micelles; 3. poor compatibility with silica source, which cannot induce the formation of regular silica structure.

[0042] The type of block copolymer template agent used in the application mainly affects the size and arrangement mode of the micelles, which determines the final pore size, distribution uniformity, pore structure type (open / closed, spherical / vermicular), and affects the mechanical stability, specific surface area and channel connectivity of the final material.

[0043] The three-block copolymer P123 used in the application can self-assemble to form a micellar structure under acidic conditions, effectively inducing the formation of a three-dimensional open mesoporous foam structure, and is a key component for constructing a high specific surface area and porous carrier. Although copolymers such as F127 with similar structures can also partially replace them, there are obvious differences in pore size, pore wall thickness, pore channel permeability and catalytic performance. Therefore, the type and amount of block copolymer template agent must be accurately controlled to obtain a porous carrier with excellent structure and stability.

[0044] As an embodiment, the acidic solution comprises an acid and water; the acid is hydrochloric acid, nitric acid or methanesulfonic acid, and in particular embodiments, the acid is hydrochloric acid; the concentration of the acid in the acidic solution is 0.5-3 mol / L, and in particular embodiments, the concentration of the acid in the acidic solution is 0.7-3 mol / L.

[0045] The acidic condition plays the following key roles in the synthesis of mesoporous materials under the template induction: 1. Regulating the formation and stability of the copolymer (such as P123) micelles. The EO / PO block is more likely to self-assemble into micelles in the acidic medium, and the structure is maintained. The pH value of the system is adjusted by the acid, so that the template and the silicon source can better cooperate. 3. Promoting the hydrolysis and condensation of the silicon source (such as TEOS). TEOS (tetraethoxysilane) slowly hydrolyzes to form Si-OH under the catalysis of the acid, and further condenses to Si-O-Si. The acidity affects the reaction rate and the degree of cross-linking, and then determines the pore wall thickness and the stability of the pore structure. 3. Regulating the type and order of the mesoporous structure. The pH value affects the arrangement of the micelles and the cross-linking mode of the silicon network. Suitable acidity helps to form a three-dimensional foam structure rather than a two-dimensional hexagonal structure (such as SBA-15).

[0046] Nitric acid is a strong acid, halogen-free, and less likely to form a complex. The reaction rate is slightly faster, which may affect the structural order. Sulfuric acid is a strong acid and is inexpensive, but has high viscosity, which affects the assembly. It may react with the silicon source to form a sulfate (such as silicon sulfate) precipitate, affecting the formation of the pore channel, and the structure is prone to collapse. Methanesulfonic acid (CH3SO3H) is an organic strong acid, which is more gentle to the structure guide, but the cost is high. Acetic acid (CH3COOH) is mild, but the acidity is too low, and the micelles formed are unstable, resulting in disordered pore channels. Phosphoric acid (H3PO4) has good buffering properties, but the acidity is too weak, and the template cannot form stable micelles. The material is prone to form non-ordered porous or non-porous structures, and the product structure is chaotic. Hydrofluoric acid is a strong acid, but it reacts with silicon to cause corrosion, which is not suitable for synthesizing carriers. The coordination of organic acids (such as citric acid) is too strong, which interferes with the condensation reaction of the silicon source.

[0047] If the concentration of the acid in the acidic solution is too low (<0.5 mol / L), the micelles will be unstable, the silicon source will hydrolyze too slowly, and the pore structure will be loose or irregular. When the concentration of the acid in the acidic solution is in the medium concentration range (1-2 mol / L), it is the optimal concentration range, which can balance the hydrolysis rate of the silicon source and the order of the pore structure. If the concentration of the acid in the acidic solution is too high (>3 mol / L), the silicon source (such as TEOS) will condense rapidly, the pore wall of the porous structure formed will be thin and prone to collapse, and the product will be a non-regular lump.

[0048] In summary, the type of acid determines the pH condition, the reaction rate, and whether to introduce a side reaction (such as sulfate precipitation). The acid strength (pKa) affects the degree of hydrolysis and condensation of the silicon source (such as TEOS), and then affects the pore size / pore wall thickness. The concentration of the acid directly affects the self-assembly behavior and the order of the final structure, and too high / low concentration will lead to an out-of-control reaction.

[0049] The present application uses hydrochloric acid as the acidic medium, the concentration of which is controlled in the range of 0.7-3.0 mol / L, to ensure that the template P123 forms stable micelles and promotes uniform hydrolysis and condensation of the silicon source, thereby constructing a highly ordered foam-like mesoporous structure. Although other acidic media such as nitric acid, methanesulfonic acid can also be replaced under certain conditions, they generally affect the stability and uniformity of the pore structure, especially sulfuric acid and phosphoric acid, which cause the pore structure to collapse or the pore size distribution to be uneven due to side reactions or mismatched acidity, so the present application selects hydrochloric acid as the acidic template system.

[0050] As an embodiment, the pore-expanding agent includes one or more of 1,3,5-trimethylbenzene (TMB), 1,3,5-triisopropylbenzene (TIPB), and toluene, and in a specific embodiment, 1,3,5-trimethylbenzene; the mass ratio of the pore-expanding agent to the block copolymer template is 0.8-1.5:1, and in a specific embodiment, 1.0-1.2:1.

[0051] The type of pore-expanding agent affects: the molecular size, polarity, and affinity or repulsion of different pore-expanding agents to P123, which determines the efficiency of their entering the hydrophobic core of the micelles; thereby affecting the volume, arrangement, and pore size distribution of the micelles. When a low amount of pore-expanding agent is used (mass ratio of pore-expanding agent to block copolymer template <0.5), the pore expansion is not obvious; when a moderate amount of pore-expanding agent is used (mass ratio of pore-expanding agent to block copolymer template ≈1.0-1.5), the obtained porous structure has a large pore size and uniform structure, which is an ideal amino carrier structure; when a high amount of pore-expanding agent is used (mass ratio of pore-expanding agent to block copolymer template >2), the obtained micelle structure is unstable, the pore wall is too thin or even collapses, resulting in a decrease in specific surface area and poor mechanical stability.

[0052] Some pore-expanding agents have a mismatched polarity and are not easy to insert into the micelles; some molecules are too small (such as n-hexane), resulting in no obvious expansion effect; some molecules are too large (such as hexadecane), which is difficult to stably wrap, leading to aggregation or disintegration of the micelles; some aromatic hydrocarbons (such as o-xylene) have low solubility and poor compatibility in the reaction system, affecting the formation of ordered structures; and some pore-expanding agents may also affect the final structure due to side reactions or high volatility.

[0053] In the preparation of the silicon-based porous foam structure of the present application, 1,3,5-trimethylbenzene, as a hydrophobic swelling agent, can be effectively inserted into the micellar core of the block copolymer P123 to induce the formation of a three-dimensional porous foam structure. The type and amount of 1,3,5-trimethylbenzene have a significant impact on the order of the pore structure, the pore size, and the pore wall thickness. In addition to 1,3,5-trimethylbenzene, other organic solvents such as triisopropylbenzene (TIPB) and toluene can also be used as substitutes under certain conditions, but the swelling efficiency and structural stability are slightly different. Therefore, the present application preferably uses 1,3,5-trimethylbenzene, and controls the mass ratio of 1,3,5-trimethylbenzene to P123 in the range of 0.8 to 1.5 to obtain an ideal carrier structure.

[0054] As an embodiment, the block copolymer template agent, the pore size swelling agent, and the acidic solution are mixed by dissolving the block copolymer template agent in the acidic solution and adding the pore size swelling agent.

[0055] As an embodiment, the self-assembly is carried out under stirring; the stirring speed is 300-800 rpm, and in specific embodiments, it is 500-700 rpm; the self-assembly time is 1-4 h, and in specific embodiments, it is 2 h; the self-assembly temperature is 37-40℃, and in specific embodiments, it is 38-39℃.

[0056] The self-assembly temperature affects the formation speed and stability of the micelles. If the temperature is too low (<30℃), the micelles form slowly and the system is unstable. If the temperature is too high (>45℃), the micelles may be broken or deformed, affecting the integrity of the pore structure. The temperature range of 37-40℃ set in the present application is the best range for micelle formation and swelling.

[0057] The self-assembly time determines the full development and stability of the micelle structure. If the time is too short (<1h), the micelles have not fully formed and the pore size swelling agent (TMB) is not evenly distributed. If the time is moderate (2-3h), the micelles are evenly distributed and the pore size distribution is narrow. If the time is too long (>4h), the block copolymer template agent may cause structural instability due to spontaneous aggregation or phase separation.

[0058] The stirring rate affects the uniform dispersion of the pore-expanding agent (TMB) and the kinetics of micelle formation, directly affects the emulsification and uniform mixing of the block copolymer template and the pore-expanding agent, the uniformity of the micelle size and distribution, the order and repeatability of the pore structure, the efficiency of heat and mass transfer, the defects of the pore structure caused by bubble interference or bubble-induced formation (irregular bubbles are generated if the stirring is too fast). If the stirring rate is too low (< 300 rpm), the block copolymer template is not fully dissolved, the pore-expanding agent is not uniformly dispersed, and the template is uneven, resulting in uneven pore structure and wide pore size distribution. Within 300-500 rpm, the mixing is moderate, which is suitable for small batches and systems with sufficient micelle time. Within 500-700 rpm, the mixing is uniform and does not cause bubble entrainment. The reaction system is uniformly mixed, the template is uniform, the micelles are stable, the pore size distribution is concentrated, the pore size is consistent, and the structure is ordered. If the stirring rate is > 800 rpm, it is easy to cause violent stirring, generate bubbles, disturb and break the micelles, and affect the integrity of the pore structure, resulting in a disordered pore structure and easy formation of non-ideal structure or collapse. During the stirring process, the block copolymer template and the pore-expanding agent form a stable micelle structure and induce the formation of a porous foam skeleton. If the stirring rate is too low, the mixing may be uneven and the structure may be disordered. If the stirring rate is too high, it may cause bubble entrainment and interfere with the pore structure, so the stirring rate needs to be controlled within the range described in the present application.

[0059] Under acidic and heated conditions, the structure of the block copolymer template (EO-PO-EO of P123) spontaneously self-assembles to form stable micelles, with the hydrophobic segment (PO) forming the core of the micelles and the hydrophilic segment (EO) forming the shell, stabilizing the dispersion of the micelles in water. The pore-expanding agent enters the hydrophobic core of the micelles, expands the volume of the micelles, and forms a large-size template system. After the pore-expanding agent (TMB) is inserted into the micelles, the system forms a stable "soft template system" and a three-dimensional micelle network. The expanded template system of the pore-expanding agent can induce the formation of a large-pore foam structure. The system exhibits a stable emulsion, providing a "mold" for the coating of the silicon source and the solidification of the structure. At this stage, the entire solution exhibits an emulsion state, and this step is one of the decisive steps for the size and morphology of the final pore structure. Although the silicon source (e.g., TEOS) has not been added at this time, this step pre-constructs the structural basis of the template system, preparing for the next step of hydrolysis and condensation of the silicon source to "pore". The formation and regulation of the "template structure" in this step belong to the process of physical self-assembly + molecular interface regulation. During the reaction process, a highly ordered and size-controlled micelle / template structure is produced, which is the basis for the formation of a silicon-based porous foam carrier. No obvious chemical reaction occurs during the self-assembly process, but the formation and stability of the micelle structure are crucial for the subsequent hydrolysis-condensation of the silicon source to construct a silicon-based porous foam skeleton. Temperature, stirring rate, and reaction time have important effects on the uniformity of the template structure and the pore size regulation.

[0060] After obtaining the block copolymer micelles, the block copolymer micelles and a silicon source are mixed to perform a hydrolysis-condensation reaction to obtain a silicon-oxygen cluster-block copolymer composite micelle.

[0061] As an embodiment, the silicon source includes one or more of tetraethoxysilane (TEOS), tetramethoxysilane (TMOS) and tetrapropoxysilane (TPOS), and in a specific embodiment, the silicon source is tetraethoxysilane (TEOS); and the molar ratio of the block copolymer template and the silicon source is 1:4-6, and in a specific embodiment, the molar ratio is 1:5.

[0062] The amount of the block copolymer used in the present application mainly affects the ordered degree and the pore wall thickness of the pores controlled by the molar ratio of the template / silicon source. Too much block copolymer can cause the pore structure to collapse or be disordered, and too little block copolymer can cause the pore structure to be incomplete or uneven.

[0063] The functions of the silicon source in the present application include: 1. forming a porous framework (SiO2) structure, TEOS undergoes a hydrolysis-condensation reaction under acidic conditions: Si(OC2H5)4+H2O→Si(OH)4+C2H5OH, Si(OH)4→Si-O-Si network+H2O, and finally forms a covalently cross-linked Si-O-Si framework, which is solidified on the surface of the P123 micelles and replicates the morphology to form a mesoporous structure; 2. cooperating with the template to construct a "hard template+soft template" system, TEOS hydrolyzes and condenses outside the P123 micelles→builds a rigid mesoporous foam structure; and after the reaction is completed, the template is removed (such as calcination), leaving a three-dimensional silicon-based porous foam framework.

[0064] Tetramethoxysilane (TMOS) in the silicon source hydrolyzes faster and has a more compact structure, and the pore channels can be smaller, which can replace tetraethoxysilane (TEOS), but the conditions need to be adjusted; tetrapropoxysilane (TPOS) hydrolyzes slowly and needs strong acid to promote; TMOS is the most direct substitute for TEOS, but its hydrolysis is too fast, and the reaction time, temperature and acidity need to be re-optimized; other substitutes usually affect the final pore structure.

[0065] When the amount of TEOS is too small, the framework is insufficient, which can cause the porous structure to be incomplete and the pore wall to be thin and easy to collapse; when the amount of TEOS is moderate (such as P123:TEOS mass ratio≈1:3-4), the prepared pore structure is complete and uniform, and the framework strength is moderate; when the amount of TEOS is too large, it can cause rapid gelation, block the pore channels, and form non-porous particles or lumps; the hydrolysis speed of the silicon source (controlled by the type of silicon source) determines whether the pore wall thickness and structure are ordered, for example, TMOS hydrolyzes faster and is more compact, and it is difficult to form a large pore foam structure; the ratio of the silicon source to water and acid can affect the hydrolysis and condensation rate, which needs to be optimized to ensure that Si-OH can condense and solidify the micelle structure in time after being formed.

[0066] As an embodiment, the hydrolysis condensation reaction comprises stirring stage and standing aging stage in sequence; the stirring stage has a rotation speed of 500-800 rpm, and in particular embodiments, 600-700 rpm; the stirring stage has a time of 2-15 min, and in particular embodiments, 5 min; the stirring stage has a temperature of 10-40℃, and in particular embodiments, 23℃; the standing aging stage has a time of 10-40 h, and in particular embodiments, 20 h; the standing aging stage has a temperature of 20-70℃, and in particular embodiments, 40℃.

[0067] The silicon source is added to the micellar structure-stabilized emulsion, and under stirring, the silicon source is rapidly dispersed and hydrolyzed and preliminarily condensed under acidic conditions to uniformly coat the outer layer of the micelles, forming a precursor silica skeleton, thereby avoiding too fast local hydrolysis and formation of irregular skeletons leading to a wide pore size distribution; then, a standing aging stage (more than 24 h) is usually entered for structure solidification and self-assembly crystallization. The type and amount of the silicon source play a decisive role in structure formation, and insufficient amount of the silicon source can lead to incomplete skeleton and easy collapse of the structure; and excessive amount of the silicon source can cause blockage of the pores and reduce the specific surface area.

[0068] When the stirring rate is 500-600 rpm, the system is mild, suitable for small test systems (<500 mL) or slow addition of the silicon source (TEOS), so that the silicon source is uniformly hydrolyzed to form a regular pore structure; when the stirring rate is 600-700 rpm, it is a commonly recommended range, suitable for most cases, the solution is mixed sufficiently, and a uniform skeleton is formed; when the stirring rate is 700-800 rpm, it is suitable for cases where the system has high viscosity, the amount of the silicon source is large, or rapid dispersion is required, and the dispersion can be accelerated, but attention should be paid to avoid air bubbles from being entrained; it is not recommended to set the stirring rate below 400 rpm, otherwise the silicon source can not be fully dispersed, leading to uneven deposition of the skeleton; and it is also not recommended to set the stirring rate above 900 rpm, because high shear can cause the micelles to break and the structure to be disordered.

[0069] After obtaining the siloxane cluster-block copolymer composite micelles, the siloxane cluster-block copolymer composite micelles and an etchant are mixed to perform etching, and the obtained etched siloxane cluster-block copolymer composite micelles are calcined to obtain a silicon-based porous foam matrix.

[0070] As an embodiment, the etchant is an aqueous solution of ammonium fluoride; the concentration of ammonium fluoride in the aqueous solution of ammonium fluoride is 3-15 mg / mL, and in particular embodiments, 9.2 mg / mL; the mass ratio of the block copolymer template agent to ammonium fluoride is 30-200:1, and in particular embodiments, 4000:46; the etching temperature is 80-120℃, and in particular embodiments, 100℃; the etching time is 8-48 h, and in particular embodiments, 24 h; and the etching is performed in an autoclave.

[0071] As an implementation form, the etched system is further filtered, the obtained precipitate is sequentially washed and dried to obtain white powder; the filtering is vacuum filtration; the washing includes water washing and ethanol washing; the water washing is performed with deionized water; the number of times of the water washing is 2-4, and in a specific embodiment, the number of times is 3; the number of times of the ethanol washing is 1-2, and in a specific embodiment, the number of times is 2; the drying temperature is 60-80°C, and in a specific embodiment, the temperature is 70°C; the drying time is not particularly limited in the present application, and complete drying is sufficient.

[0072] The present application removes the residual block copolymer template by washing, cleans the impurity ions (such as F - , NH4 + ), removes the unreacted silicon source, hydrolysis product and small molecule byproduct (such as ethanol), cleans the surface adsorbed inorganic impurities, and avoids the generation of byproducts or pollution in the calcination process. In the present application, the washing is performed until the washing liquid is clear and free of foam, and the pH value is close to neutral. The water washing is mainly used to remove inorganic salts, NH4F byproducts (such as F - , NH4 + ), etc., and the pH value of the water is close to neutral. The water washing is performed until no turbidity or foam (such as P123 foam) is present. The ethanol washing helps to remove the block copolymer template and its degradation products, and the ethanol has good volatility, which is beneficial to subsequent drying. The ethanol washing has no obvious sticky feeling, and the sample color is white or off-white after drying. The washing process used in the present application can effectively remove impurities. Each washing is performed with an equal volume or slightly more than the sample volume of the washing agent (liquid-solid ratio is about 10:1), and each washing is separated by centrifugation or suction filtration.

[0073] As an implementation form, the calcination temperature is 450-650°C, and in a specific embodiment, the temperature is 550°C. The holding time is 3-12h, and in a specific embodiment, the holding time is 6h. The calcination is performed in air.

[0074] After obtaining the silicon-based porous foam matrix, the silicon-based porous foam matrix, organic aminosilane and non-polar organic solvent are mixed to perform amine group grafting modification, and an amine group functionalized silicon-based porous foam is obtained.

[0075] As an implementation form, the organic aminosilane includes 3-aminopropyltrimethoxysilane (APTMS). The mass of the silicon-based porous foam matrix and the volume of the organic aminosilane are (5-30) g:(10-60) mL, and in a specific embodiment, the mass and the volume are 15 g:30 mL.

[0076] As an embodiment, the non-polar organic solvent comprises a first non-polar organic solvent and a second non-polar organic solvent; the first non-polar organic solvent and the second non-polar organic solvent independently comprise one or more of toluene, petroleum ether, hexane and cyclohexane, and in particular embodiments, anhydrous toluene; the mass of the silicon-based porous foam substrate and the volume of the first non-polar organic solvent are in a ratio of (5-30) g:(100-500) mL, and in particular embodiments, 15 g:300 mL; the volume ratio of the organoaminosilane and the second non-polar organic solvent is 10-60:50-250, and in particular embodiments, 35:150. In the present application, the use amount of each raw material is within the above range, so that the reaction system is stable, the solid-liquid ratio is reasonable, the organoaminosilane is effectively diluted, local polymerization is avoided, the organoaminosilane reacts with the Si-OH on the surface of the silicon-based porous foam substrate to form a Si-O-Si bond, and the slow addition of the diluted organoaminosilane is conducive to uniform modification.

[0077] The non-polar organic solvent can well dissolve the organoaminosilane (such as 3-aminopropyltrimethoxysilane, APTMS), has relatively low polarity, can form uniform adsorption and reaction of APTMS molecules on the hydrophobic surface (SiO2), avoid rapid hydrolysis, and can also control the hydrolysis-condensation rate to avoid silane polymerization. Compared with an aqueous environment, toluene can significantly slow down the hydrolysis rate of APTMS; reduce the self-condensation of APTMS in the solution rather than being bonded to the porous silicon surface, which is conducive to improving the grafting efficiency and monolayer coverage.

[0078] Petroleum ether is a non-polar organic solvent, has low cost, fast volatilization, and general solubility of APTMS, and can replace toluene; n-hexane is a non-polar organic solvent, has similar properties to toluene, and can dissolve APTMS, and can replace toluene; cyclohexane is a non-polar organic solvent, has good compatibility, but has slightly poor solubility of APTMS, and can be used; dichloromethane (DCM) is a weakly polar organic solvent, can be dissolved, but has corrosiveness and toxicity, and has a relatively fast reaction rate; ethanol / acetonitrile / methanol is a polar organic solvent, can accelerate the hydrolysis of APTMS, and forms polysiloxane agglomerates, and is not recommended for use; tetrahydrofuran (THF) / N,N-dimethylformamide (DMF) / dimethyl sulfoxide (DMSO) is a polar solvent, promotes condensation polymerization rather than grafting, and is not recommended; if toluene needs to be replaced, the present application recommends using petroleum ether, n-hexane or cyclohexane, but toluene has the most optimal comprehensive performance (good solubility, moderate hydrolysis rate, easy operation, and moderate cost). Toluene as a non-polar solvent can effectively control the hydrolysis-condensation rate of the organoaminosilane, and promote the ordered grafting of the amine group on the surface of the silicon-based porous foam substrate. Other solvents such as n-hexane and petroleum ether can also partially replace toluene, but toluene has the best performance in terms of solubility and reaction control.

[0079] As an embodiment, the mixing is mixing the silicon-based porous foam substrate and the first non-polar organic solvent, then adding the second non-polar organic solvent in which the organic aminosilane is previously dissolved, and stirring the obtained mixture in a protective gas; the protective gas is argon or nitrogen, and in particular embodiments, is argon; the stirring is performed at room temperature; the stirring rate is 300-600 rpm, and in particular embodiments, is 500-600 rpm; and the stirring time is 20-60 min, and in particular embodiments, is 30 min.

[0080] In the present application, when the stirring rate is 300-400 rpm, it is suitable for small scale (<200 mL) and low viscosity system, and the reactants can be mixed preliminarily; when the stirring rate is 500-600 rpm, it is the normal recommended range, and is suitable for most experimental conditions, and can effectively disperse the organic aminosilane and avoid local high concentration; when the stirring rate is >700 rpm, it is not recommended because it can introduce bubbles or cause local hydrolysis acceleration. In the present application, the stirring time is set in the above range, which can avoid uneven dispersion due to too short stirring time, and can also avoid accelerating the self-condensation of the organic aminosilane due to too long stirring time, thereby reducing the grafting efficiency.

[0081] As an embodiment, the temperature of the amine group grafting modification is 110-120°C, and in particular embodiments, is 110-115°C; the amine group grafting modification time is 12-24 h, and in particular embodiments, is 24 h; the amine group grafting modification is performed under reflux conditions; the amine group grafting modification process is performed under stirring conditions; and the stirring rate is 200-400 rpm, and in particular embodiments, is 200-300 rpm.

[0082] In the present application, during the amine group grafting modification process, the organic aminosilane (APTMS) is hydrolyzed and condenses with the silicon hydroxyl groups on the surface of the silicon-based porous foam carrier to form stable Si-O-Si bonds, thereby achieving effective grafting modification of the amine groups. In the present application, the system is uniformly heated during the grafting modification process, and the condensation of the organic aminosilane on the surface of SiO2 is promoted; however, the stirring speed should not be too fast to avoid the generation of bubbles or volatile disturbance; and the reflux time is set in the above range to ensure that the amine group grafting modification of the organic aminosilane on the surface of the porous silicon carrier is fully completed. When the reflux time is <10 h, the grafting is not sufficient and the reaction can be incomplete; when the reflux time is 12-24 h, the grafting is saturated and the organic aminosilane (APTMS) molecules completely react with the surface Si-OH; when the scale is enlarged or the concentration of the organic aminosilane (APTMS) is high, the reflux time can be extended to 30 h; however, when the reflux time exceeds 36 h, the self-polymerization between the APTMS molecules can be enhanced, thereby reducing the grafting efficiency.

[0083] In the amine group grafting modification process, the organic amino silane is hydrolyzed and condensed on the silicon hydroxyl (Si-OH) on the surface of the silicon-based porous foam substrate to form stable covalent bonds (Si-O-Si). The specific reactions include:

[0084] 1. Hydrolysis of the organic amino silane: the organic amino silane is partially hydrolyzed in the presence of trace amounts of water:

[0085] (CH3O)3Si-(CH3)3-NH2+H2O→HO-Si-(CH2)3-NH2+CH3OH

[0086] 2. Condensation of the hydrolysis product with Si-OH on the surface of the silicon-based porous foam substrate to form covalent bond grafting:

[0087] HO-Si-(CH2)3-NH2+Si-OH (carrier)→Si-O-Si-(CH2)3-NH2+H2O

[0088] 3. Self-condensation between the organic amino silane (APTMS) molecules can also occur:

[0089] A multilayer or aggregate is formed on the surface (not conducive to monolayer grafting); but under refluxing temperature and protective gas conditions, the self-condensation process is relatively controlled.

[0090] Finally, silane molecules with amine group (-NH2) functional groups are successfully grafted on the surface of the silicon-based porous foam substrate, forming stable Si-O-Si bond connections, and the material surface exhibits good nucleophilicity and coordination ability (such as subsequent loading of metal ions).

[0091] As an implementation, after the amine group grafting modification is completed, the product of the amine group grafting modification is cooled to room temperature, followed by first solid-liquid separation, the first solid obtained is washed first, and then suspended in an organic alcohol for heat treatment. The product of the heat treatment is cooled to room temperature, followed by second solid-liquid separation, and the second solid obtained is washed second and then dried.

[0092] As an implementation, the first solid-liquid separation and the second solid-liquid separation are filtration.

[0093] As an implementation, the first washing is sequentially performed 1-3 times with toluene, ethanol, acetone, and dichloromethane, and in specific embodiments, 2 times. The toluene washing is performed 2-3 times, and the washing is performed until the washing liquid gradually becomes clear, no visible oil film or turbidity is observed; the ethanol washing is performed 2-3 times, and the washing is performed until the pH value of the washing liquid tends to be neutral, and no sticky or ammonia odor residue is observed; the acetone washing is performed 1-2 times, and the washing is performed quickly to observe the clarity of the washing liquid, and the intermediate residue is removed; and the dichloromethane (DCM) washing is performed 1-2 times, and the washing is performed until the washing liquid is transparent and colorless, and no odor is observed.

[0094] Toluene and reaction solvent are both non-polar solvents, which can dissolve unreacted APTMS and its physical adsorption layer, and perform the first step of washing to remove free precursors and avoid their polymerization and deposition; ethanol is a polar solvent with hydrophilicity, which can remove hydrolysis products, oligomers, and part of APTMS, and help to dissolve the polar hydrolysis residues of organic amine silane; acetone is a polar solvent with strong solubility, which can dissolve small organic molecules and intermediates adsorbed on the surface, and has the advantages of fast evaporation and easy drying, and is suitable as a transition cleaning agent; dichloromethane (DCM) has moderate polarity and strong solubility, which can remove strongly adsorbed unbound APTMS residues and clean the surface of difficult-to-remove organic phase molecules, and has effective "desorption" effect.

[0095] As an embodiment, the reagent used in the second washing is dichloromethane; and the number of washing is 1 time. Dichloromethane washing includes preliminary washing and final rinsing, and the final rinsing stage is usually supplemented with one more washing after suspension ethanol drying, which helps to completely remove impurities. Dichloromethane (DCM) has moderate polarity, can dissolve most organic impurities, has strong volatility and is easy to remove from the material, and is particularly suitable for the last washing, which can effectively remove residual organic molecules (such as APTMS oligomers and impurities) on the surface of the pores; compared with other solvents, DCM does not hydrolyze or damage the amine group Si-O-Si structure, and the treatment is more gentle. The number of dichloromethane washing is 1-2 times, especially under the premise of previous multiple washing; 50-100 mL (or 10 times the volume of the material mass) of DCM is used each time, and the washing is performed until the washing liquid is clear, no visible turbidity or color, and no obvious irritating odor remains (APTMS has a slight amine smell), and the surface of the material is not greasy and sticky.

[0096] As an embodiment, the organic alcohol includes one or more of methanol, ethanol and isopropanol, and in a specific embodiment, it is ethanol; the temperature of the heat treatment is 50-70℃, and in a specific embodiment, it is 60℃; the time of the heat treatment is 8-16h, and in a specific embodiment, it is 8-12h; the ratio of the mass of the first solid after the first washing to the volume of the organic alcohol is 1g:(10-30)mL, and in a specific embodiment, it is 1g:(20-25)mL. If the volume of the organic alcohol is too small, the dispersion is uneven, which affects the elution efficiency; if the volume of the organic alcohol is too large, there is no obvious benefit, solvent is wasted, and handling is difficult; the present application selects an appropriate amount of organic alcohol for heat treatment, which can fully penetrate the inside of the pores and promote the diffusion and desorption of the residues.

[0097] In the amine group functionalized carrier modification process, the main role of the organic alcohol is as follows: 1. Solvent effect (suspension medium), the modified material is well dispersed in the organic alcohol, avoiding agglomeration; it helps to further dissolve the residual unreacted APTMS and its oligomers in the material. 2. Elute weakly adsorbed substances, heating to 60°C can enhance the solubility, promote the complete desorption of silane and its by-products weakly adsorbed in the pore, help to remove non-covalently bonded impurities, and improve the purity and surface availability of grafted amine groups. 3. Mild treatment environment, organic alcohol has low polarity and low volatilization temperature, and does not destroy the Si-O-Si structure formed; compared with strong polar solvents, organic alcohol treatment is more gentle to the modified structure and does not cause functional group hydrolysis or erosion. Organic alcohol is an excellent organic solvent for elution and heat treatment, which can effectively improve the surface purity and stability of the amine group modified carrier. The present application suspends the amine group grafted modified silicon-based porous foam matrix in organic alcohol for heat treatment, in order to promote the desorption of weakly adsorbed APTMS and remove the oligomeric silane that may be accumulated in the pore; under mild conditions (organic alcohol + heating), the elution of residual impurities is strengthened, while the grafted structure is avoided to be destroyed, ensuring the purity of amine groups on the surface of the silicon-based porous foam matrix.

[0098] Ethanol can be replaced by other types of organic alcohols with moderate polarity, good APTMS and organic residue dissolving capacity, no corrosion or hydrolysis of amine functional groups, easy to heat, easy to dry, no complex precipitation, such as methanol with medium-strong polarity, which can replace ethanol, and has stronger affinity with APTMS and slightly better solubility than ethanol; isopropyl alcohol (IPA) has moderate polarity and can replace ethanol, and isopropyl alcohol volatilizes more slowly and is more gentle to the pore, suitable for processing macroporous materials; acetone and acetonitrile have strong polarity and may destroy the amine bonding structure, and cannot replace ethanol; water (pure water / deionized water) has strong polarity and can easily hydrolyze Si-O-Si bonds, destroying the structural stability. Methanol or isopropyl alcohol is the most recommended choice to replace ethanol, but if the structural conservation requirement is higher, ethanol is still recommended as the first choice.

[0099] The effects of suspending the product of amine group grafting modification in organic alcohol for heat treatment include: 1. Complete desorption of weakly adsorbed or uncovalently bonded APTMS and its hydrolysis / condensation products, even if the former has been washed with multiple steps of organic solvent, a small amount of unreacted or weakly adsorbed APTMS, linear or oligomeric by-products thereof may still remain; warm organic alcohol treatment can accelerate molecular motion, enhance diffusion, and improve desorption efficiency; and avoiding the influence of these residues on surface active sites or catalytic reaction selectivity in subsequent use. 2. Enhancing the stability of the functional group layer, after heat treatment, the surface-grafted amine group silane layer may undergo further crosslinking, curing or structure arrangement; and helping to improve the thermal stability and mechanical stability of the material during subsequent metal loading or reaction process. 3. Mild environment protects the amine functional group from being destroyed, the organic alcohol is a neutral, volatile organic solvent; compared with water or strong polar solvents, it will not destroy the Si-O-Si bond formed; the temperature of heat treatment (60℃) belongs to the mild range, which can accelerate elution and avoid thermal decomposition or hydrolysis of functional groups.

[0100] The temperature of heat treatment is set in the range of 50-70℃, <50℃, the solubility is not strong, the diffusion speed is slow, at 60℃, the diffusion is fast, the solubility is strong, and the stability is good, >70℃, part of the organic functional groups may be damaged; the time range of heat treatment is 8-16h, <6h, desorption is not sufficient, 8-12h, desorption is sufficient, energy consumption is moderate, >16h, the effect is not obviously improved, time and energy consumption are wasted.

[0101] After the amine group grafting modification is completed, the reaction system is cooled to room temperature, and the solid is collected by filtration, the obtained solid is washed with toluene (2-3 times), ethanol (2-3 times), acetone (1-2 times) and dichloromethane (1-2 times) in sequence, to completely remove unreacted APTMS, its hydrolysis / condensation products and other organic residues, until the washing liquid is clear and colorless, the washed solid is suspended in ethanol, heated at 60℃ for 12h to further desorb the residues, cooled, filtered again and washed once with dichloromethane, and finally dried for use.

[0102] As an embodiment, the drying temperature is 50-80℃, and in a specific embodiment, it is 60-70℃; the drying time is 8-16h, and in a specific embodiment, it is 12h; and the drying equipment is a vacuum drying oven or a forced air drying oven. The above drying temperature is lower than the degradation temperature of APTMS, ensuring the stable existence of amine groups, and too low temperature makes it difficult to volatilize the solvent completely, and too high temperature (>90℃) may cause degradation of the surface organic layer or partial rupture of the Si-O-Si bond, affecting the stability of the material. The above drying time can ensure complete removal of DCM and residual ethanol and other organic solvents; vacuum drying can accelerate desorption and avoid oxidation.

[0103] After obtaining the amine group functionalized silicon-based porous foam, the present application mixes the basic aqueous suspension of the amine group functionalized silicon-based porous foam and the basic complexing solution of tetrachloropalladate to carry out loading, mixes the obtained palladium-loaded amine group functionalized silicon-based porous foam and a reducing agent to carry out reduction reaction, and obtains a silicon-based heterogeneous palladium catalyst.

[0104] As an embodiment, the preparation method of the basic aqueous suspension of the amine group functionalized silicon-based porous foam is as follows: the amine group functionalized silicon-based porous foam is suspended in an aqueous solution with an adjusted pH value being alkaline, and stirring is carried out; the mass ratio of the amine group functionalized silicon-based porous foam to the volume of the aqueous solution with an adjusted pH value being alkaline is (1-10) g:(30-300) mL, and in a specific embodiment, 5 g:150 mL; the alkalinity is pH value=7.9-8.1, and in a specific embodiment, 8.0; the reagent used for adjusting the pH value to be alkaline is LiOH solution; the concentration of the LiOH solution is 0.05-0.2 N (equivalent concentration), and in a specific embodiment, 0.1 N; the aqueous solution is deionized water solution; the stirring is carried out at room temperature, and in a specific embodiment, at 20-30℃; the stirring rate is 300-600 rpm, and in a specific embodiment, 500-600 rpm; and the stirring time is 10-30 min, and in a specific embodiment, 15-20 min.

[0105] The present application activates the surface amine group of the amine group functionalized silicon-based porous foam through the preparation process of the basic aqueous suspension of the amine group functionalized silicon-based porous foam and optimizes the subsequent palladium ion complexing and adsorbing conditions. The stirring step is a complexing preparation process, and does not need to be heated, but can be operated at room temperature; if the environmental temperature is low (for example, <18℃ in winter), the temperature can be appropriately increased to 25-30℃ to accelerate the surface activation and complexing pre-equilibrium. When the stirring rate is 300-400 rpm, it is suitable for a system with a small volume (150 mL) to avoid violent collision of particles, when the stirring rate is 500-600 rpm, it is suitable for a system that needs to enhance dispersion, has more particles or has slightly large viscosity, and when the stirring rate is >700 rpm, it is easy to cause bubbles and physical structure disturbance; when the stirring time is <10 min, the adsorption / complexing is insufficient, which affects the uniformity of the subsequent metal ions, when the stirring time is 15-20 min, it can ensure that the surface of the carrier is fully exposed and activated, and when the stirring time is >30 min, there is no obvious gain, and instead, oxygen may be introduced to affect the stability of the reduction reaction.

[0106] As an embodiment, the tetrachloropalladate includes one or more of lithium tetrachloropalladate, sodium tetrachloropalladate and potassium tetrachloropalladate, and in a specific embodiment, lithium tetrachloropalladate; PdCl2+2LiCl→Li2PdCl4, the complex is highly soluble in water, and can form a stable [PdCl4] 2-The ion system, as a stable complex precursor of palladium, can be complexed with the amine group -NH2 on the surface of the carrier to form a Pd-N bond, achieve efficient loading, and be easily reduced to palladium nanoparticles in the subsequent reduction process, and the distribution is uniform. Compared with PdCl2, the complex solution of lithium tetrachloropalladate has higher stability, ion dispersity, and complex efficiency with amine groups. The structure and complex behavior of sodium tetrachloropalladate are similar to those of lithium tetrachloropalladate, but it is more convenient to use.

[0107] As an embodiment, the preparation method of the alkaline complex solution of the tetrachloropalladate salt comprises the following steps: dissolving an alkali metal chloride and a palladium-containing substance in water, heating and stirring, cooling to room temperature, solid-liquid separation, adjusting the pH value of the obtained filtrate to alkaline, and obtaining the alkaline complex solution of lithium tetrachloropalladate.

[0108] As an embodiment, the alkali metal chloride includes one or more of LiCl, NaCl, and KCl, and in a specific embodiment, LiCl. NaCl can replace LiCl to generate Na2PdCl4, and NaCl has similar performance to LiCl, but the sodium ion slightly affects the reaction environment; KCl can replace LiCl to generate K2PdCl4, and KCl has similar performance to LiCl, but K + The solubility is slightly lower; the presence of ammonia ions in NH4Cl may affect the complexation.

[0109] As an embodiment, the palladium-containing substance is PdCl2 or sodium tetrachloropalladate. Sodium tetrachloropalladate (Na2PdCl4) is directly soluble in water and has been complexed, and can completely replace lithium tetrachloropalladate or PdCl2 and is commonly used as a catalyst precursor; tetrachloropalladate (H2PdCl4) has good water solubility, but is strongly acidic and cannot replace PdCl2; palladium nitrate (Pd(NO3)2) has good solubility, but has a different ligand structure, and the complexation mechanism changes greatly due to different ligands, and also cannot replace PdCl2.

[0110] As an embodiment, the mass ratio of the alkali metal chloride, the palladium-containing substance, and water is 0.3-1.5:0.5-3.5:50-250, and in a specific embodiment, 0.35-1.4:0.75-3:50-200. The amount of the alkali metal chloride is slightly excessive in terms of molar ratio to ensure sufficient complexation, the amount of the palladium-containing substance can control the target Pd content to 1-10 wt% (for 5 g of the carrier), and the amount of water can ensure that PdCl2 is fully dissolved and complexed.

[0111] As an embodiment, the temperature of the heating and stirring is 70-85°C, and in a specific embodiment, 70-80°C, the rate is 400-700 rpm, and in a specific embodiment, 600-700 rpm, and the time is 20-60 min, and in a specific embodiment, 20-40 min.

[0112] In the present application, the temperature of heating and stirring is <60℃, the solubility of PdCl2 is low, the complexation reaction is slow, and the reaction is incomplete. In the range of 70-80℃, the reaction of PdCl2 and LiCl is promoted to form Li2PdCl4. When the temperature is >85℃, it is not recommended to be selected, which is easy to cause Pd 2+ Hydrolysis forms colloids or precipitates, which is not conducive to the formation of stable complexes. When the stirring rate is 400-500 rpm, it is suitable for small volume (<100 mL) systems. When the stirring rate is 600-700 rpm, it is suitable for systems with more solutes and more water, which can accelerate the dissolution process. When the stirring rate is >800 rpm, it is not recommended to be selected, which may introduce bubbles and produce oxidative interference species. When the stirring time is <15 min, PdCl2 is not easy to completely dissolve, and the complexation is not sufficient. When the stirring time is 20-40 min, it is the best value interval, and the complex solution formed is clear and stable. When the stirring time is >1 h, there is generally no additional benefit, and there may be a slight risk of hydrolysis (depending on the pH value and oxygen conditions). Within the temperature, rate and time range of heating and stirring provided in the present application, a clear and uniform Li2PdCl4 complex solution can be generated for subsequent adsorption and loading.

[0113] As an embodiment, the solid-liquid separation is filtration; the reagent used to adjust the pH value of the obtained filtrate to alkaline is LiOH solution; the concentration of the LiOH solution is 0.05-0.2N (equivalent concentration), and in specific embodiments, it is 0.1N; the alkaline is pH=7.9-8.1, and in specific embodiments, it is 8.0.

[0114] As an embodiment, the loading is carried out under stirring; the temperature of the stirring is room temperature, and another embodiment is 20-30℃, and in specific embodiments, it is 20-25℃; the stirring time is 12-16 h, and in specific embodiments, it is 14 h; the stirring rate is 80-200 rpm, and in specific embodiments, it is 80-120 rpm.

[0115] In the present application, the stirring time is 12-16 h, which is suitable for the establishment of adsorption reaction equilibrium, and is conducive to sufficient complexation / distribution. Within 8-10 h, it is acceptable for rapid experiments, but the adsorption may not be complete. When the stirring time is >18 h, there is generally no obvious improvement, which may affect the stability of the solution due to system aging or bacterial contamination. When the stirring temperature is <15℃, the stirring efficiency is reduced, and the complexation rate is slow. When the stirring temperature is 20-25℃, it is most commonly used, which takes into account the reaction activity and Pd 2+ stability. When the stirring temperature is >30℃, it has a speed-up effect, but slightly increases the Pd 2+Hydrolysis / reduction risks exist, so caution is advised when selecting this method. A stirring rate of 80–120 rpm provides gentle stirring, suitable for adsorption without damaging the pore structure. A rate of 150–200 rpm is suitable for systems with fine particles or where enhanced mixing is required. Rates >250 rpm are not recommended as they may lead to bubble entrapment or particle damage. This invention promotes Pd adsorption by setting the stirring temperature, rate, and time within the above ranges. 2+ The complexation and adsorption between the metal precursor and the amine group completes the uniform distribution of the metal precursor on the support surface.

[0116] In one embodiment, after loading, the process further includes: centrifuging the loaded suspension, washing the resulting solid with water to obtain palladium-loaded amine-functionalized silicon-based porous foam; the centrifugation temperature is room temperature, specifically 20-25°C in this embodiment, the rotation speed is 3000-5000 rpm, specifically 4000 rpm in this embodiment, and the time is 5-15 min, specifically 10 min in this embodiment; the number of water washes is 3-4 times, specifically 3 times in this embodiment. This invention rapidly and efficiently separates adsorbed Pd through centrifugation. 2+ Silicon-based porous foam solids are used to remove unadsorbed Pd from the supernatant. 2+ Complexes, free Li + Cl - This process removes impurities, providing a clean carrier precursor for subsequent washing and reduction reactions. Low centrifugation temperatures (e.g., 4°C) are not recommended, as excessively low temperatures in aqueous systems may lead to ion precipitation or increased system viscosity, hindering separation. Heating above 30°C is also discouraged to prevent localized reduction or structural disturbance. Centrifugation speeds <3000 rpm are suitable for separating larger particles or porous frameworks; 4000–4500 rpm is the recommended range, suitable for separating nanocatalytic carriers; speeds >5000 rpm are prone to causing pore structure collapse and particle compaction and are not recommended. Centrifugation times <5 min result in insufficient sedimentation and easy loss; 10 min is the recommended value, suitable for recovering most silicon-based porous foam solids; centrifugation times >15 min are generally unnecessary and may lead to material compaction, hindering washing. This invention ensures the complete recovery of palladium-loaded silicon-based porous foam structures by controlling the temperature, speed, and time of centrifugation, preparing the material for subsequent washing and reduction steps.

[0117] In one embodiment, the reducing agent is one or more of sodium borohydride (NaBH4), formaldehyde (HCHO), ascorbic acid (Vc), and hydrogen (H2), with sodium borohydride being used in a specific embodiment. When the reducing agent is not hydrogen, the mass ratio of the palladium-loaded amine-functionalized silicon-based porous foam to the reducing agent is 5:1.5 to 6, with 5:2 to 5 in a specific embodiment.

[0118] NaBH4 (sodium borohydride) is a strong reducing agent, slowly releasing H - (active hydrogen): Pd 2+ + BH4 - + 3H2O→ Pd 0 ↓ + H3BO3 + 3.5H2↑, which can efficiently reduce Pd 2+ to Pd 0 ; at the same time, the operation is mild and easy to control, and is used for the reduction synthesis of nanometer metal precursors. The reducing agent that can replace sodium borohydride is as follows: formaldehyde (HCHO) has a moderate reducing ability, is commonly used, is suitable for alkaline conditions, is mild, and has a larger particle size; ascorbic acid (Vc) has a moderate to weak reducing ability, can be used, is relatively mild, and is biologically friendly; hydrogen has a strong reducing ability, is industrially available, has high-temperature (>200℃) and high-pressure reaction conditions, and sodium citrate (Na3C6H5O7) has a weak reducing ability, and is not recommended to replace sodium borohydride.

[0119] As an embodiment, when the reducing agent is not hydrogen, the mixing is as follows: the amine group functionalized silicon-based porous foam loaded with palladium is resuspended in water to obtain an aqueous suspension of the amine group functionalized silicon-based porous foam loaded with palladium; the reducing agent is dissolved in water to obtain an aqueous solution of the reducing agent; the aqueous solution of the reducing agent is added dropwise to the aqueous suspension of the amine group functionalized silicon-based porous foam loaded with palladium; the concentration of the amine group functionalized silicon-based porous foam loaded with palladium in the aqueous suspension of the amine group functionalized silicon-based porous foam loaded with palladium is 15-70 mg / mL, and in specific embodiments, is 16.7-66.7 mg / mL; the concentration of the reducing agent in the aqueous solution of the reducing agent is 15-240 mg / mL, and in specific embodiments, is 50-150 mg / mL; the volume ratio of the aqueous suspension of the amine group functionalized silicon-based porous foam loaded with palladium to the aqueous solution of the reducing agent is 15-70:15-240, and in specific embodiments, is 50:200. In the present application, the concentration of sodium borohydride in the aqueous solution of sodium borohydride is set in the above range, so as to facilitate the control of the reduction rate and the particle size. The present application does not have special limitations on the rate of the dropwise addition, and a dropwise addition rate known in the art can be used.

[0120] As an embodiment, the temperature of the reduction reaction is room temperature, and in specific embodiments, is 20-35℃, and the time is 15-60 min, and in specific embodiments, is 20-30 min; the reduction reaction is carried out under stirring; the stirring rate is 200-600 rpm, and in specific embodiments, is 300-500 rpm. The reduction reaction can be carried out at room temperature, and if the temperature is too high, the decomposition of NaBH4 is accelerated, and the reaction is out of control. The time of the reduction reaction can be adjusted according to the concentration of NaBH4 and the volume of the system, high-speed stirring promotes uniform reduction, and avoids the aggregation of Pd.

[0121] In the reduction reaction, the present application reduces the divalent palladium ion (Pd 2+ ) adsorbed on the surface of the carrier to zero-valent palladium (Pd 0 ) nanoparticles, thereby obtaining a nanometer palladium supported catalyst with catalytic activity. The reduction reaction of Pd 2+ → Pd 0 occurs on the surface of the carrier, avoiding palladium agglomeration, forming highly dispersed Pd 0 nanoparticles, the size and distribution of which affect the final catalytic performance; NaBH4 is a strong reducing agent, and the reduction rate can be adjusted by dropwise addition; the pH value, temperature, and concentration of the system all affect the uniformity and size control of the final particles.

[0122] As an embodiment, after the reduction reaction, the method further comprises: solid-liquid separation of the product of the reduction reaction, and washing and drying the obtained solid in sequence to obtain a silicon-based heterogeneous palladium catalyst.

[0123] As an embodiment, the solid-liquid separation is centrifugation; the temperature of the centrifugation is room temperature, and in specific embodiments, it is 20-25℃; the speed is 4000-6000 rpm, and in specific embodiments, it is 5000-6000 rpm; the time is 10-20 min, and in specific embodiments, it is 10-15 min. The temperature of the centrifugation in the present application is in the above range, and it is not recommended to be treated at a high or low temperature; if the temperature is too low (such as 4℃), the viscosity of the solution increases, affecting the sedimentation; if the temperature is too high (>30℃), slight rearrangement or agglomeration may occur on the surface of the Pd particles. When the speed of the centrifugation is 4000 rpm, it is commonly recommended and suitable for the sedimentation of porous structure carriers; when the speed is 5000-6000 rpm, it can be used for rapid sedimentation, small particles, or dispersed systems; >6000 rpm is not recommended, which may cause compaction of the pore structure or agglomeration of the nanoparticles. When the time of the centrifugation is 5-10 min, it is suitable for large particles and rapid separation; when the time is 10-15 min, it is most common, ensuring complete sedimentation and facilitating washing; >20 min is usually unnecessary, which may cause particle compaction and unfavorable redispersion.

[0124] The present application recovers the catalyst solid loaded with Pd 0 by centrifugation, removes the reduction byproducts (such as Na + , BO3 3- , and unreacted NaBH4) in the solution, and provides a clean and efficient separation basis for subsequent washing and drying.

[0125] In one embodiment, the washing process consists of sequential water washing and acetone washing; the reagent used for water washing is deionized water; the number of water washings is 3-5 times, specifically 3-4 times in this embodiment; the reagent used for acetone washing is anhydrous acetone; the number of acetone washings is 2-3 times, specifically 3 times in this embodiment. The amount of reagent used in each washing can be adjusted according to the sample volume, generally 300-500 mL. Water washing can remove unreacted NaBH4 and inorganic byproducts (NaBO2, Na...). + Li + Cl - (etc.) and water-soluble small molecule impurities (such as unadsorbed Pd) 2+ (Complex), the final water wash solution should be: clear, transparent, and colorless, with a pH value close to neutral (6.5-7.5); free of bubbles or foam residue (which may occur if NaBH4 is not completely reacted). Conductivity can be used for testing; a decrease in conductivity to near that of pure water indicates a clean wash. Acetone washing removes residual impurities and moisture (acetone can form an azeotrope with water), as well as surface-adsorbed organic residues. Acetone evaporates quickly, which helps subsequent vacuum drying to be more thorough and shortens drying time. Use anhydrous acetone to avoid reintroducing moisture. The final acetone wash solution should be colorless and clear, without gray or yellow hues (indicating the presence of Pd colloids / impurities). The filter cake should not be sticky, oily, or difficult to dry. Water washing continues until the wash solution is clear, colorless, and has a pH value close to neutral to remove water-soluble byproducts. Anhydrous acetone washing is then used to further remove residual moisture and organic impurities.

[0126] In one embodiment, the drying temperature is 40–60°C, specifically 50–55°C in this embodiment, and the drying time is 8–16 hours, specifically 8–12 hours in this embodiment; the drying is vacuum drying; the vacuum degree of the vacuum drying is <10 mmHg (1330 Pa), specifically ≥1 mmHg and <10 mmHg in this embodiment. This invention removes volatile solvents such as water and acetone through drying, preventing Pd nanoparticles from agglomerating or oxidizing during the drying process, maintaining the porous structure and high specific surface area of ​​the material, and preparing samples for subsequent characterization (such as TEM) or catalytic reaction testing. Setting the vacuum degree of vacuum drying within the above range effectively lowers the solvent boiling point, accelerates drying, and prevents oxidation. The drying temperature within the above range ensures gentle dehumidification, avoiding structural collapse or functional group damage. The drying time within the above range ensures thorough drying, especially removing residual solvent inside the pores, to completely remove residual solvent and maintain the dispersion of palladium nanoparticles. Excessively high drying temperatures (>70°C) may lead to: Pd… 0 Agglomeration leads to larger particles; pyrolysis of organic amine functional groups (-NH2); insufficient vacuum during drying (>20 mmHg) makes it difficult to completely remove organic solvents from the pores, and the powder may retain an odor or undergo adsorption side reactions.

[0127] The silicon-based heterogeneous palladium catalyst provided by the present application is an amino-modified heterogeneous palladium catalyst based on a multi-level pore mesoporous structure, which exhibits high catalytic activity and selectivity, mainly due to the following reasons: 1. The advantage of the carrier structure, the silicon-based porous foam is a highly ordered mesoporous material with: large pore volume, large specific surface area and connected three-dimensional pore structure; it is beneficial for the substrate molecules to enter and contact the catalytically active center, thereby improving the reaction rate; it helps the product to diffuse out quickly, reducing the probability of side reactions; the size of the pore can be adjusted, which helps to form size selectivity. 2. The electronic effect and coordination of the amino ligand, the amino ligand is a good σ-donor ligand, which, after coordination with palladium: stabilizes the palladium(0) / palladium(II) active center; adjusts the electron density of palladium, which helps to improve the activity and selectivity of palladium in the catalytic reaction; inhibits the agglomeration of metal clusters, thereby improving the life of the catalyst; improves the regioselectivity and stereoselectivity of the substrate. 3. The advantage of heterogeneous catalysis compared with homogeneous catalysis: heterogeneous catalysts are easy to separate and recover; can be reused, reducing metal pollution; the heterogeneous structure can limit the side reaction path, thereby improving the selectivity of the reaction. 4. Dispersion and stability of palladium species, palladium on the silicon-based porous foam carrier is in a highly dispersed state (even single molecule or small cluster); high dispersion means more accessible active sites; the synergistic effect of the ligand and the carrier can inhibit the sintering and loss of palladium. 5. Fine-tuning of the reaction environment (microenvironment effect), the porous structure + ligand regulation creates a favorable "catalytic microenvironment", for example: increasing the residence time of certain intermediates, promoting the reaction process; or inhibiting the formation of unwanted intermediates.

[0128] The reasons for the good stability of the silicon-based heterogeneous palladium catalyst provided by the present application are as follows: 1. High dispersion of palladium species, effectively preventing agglomeration and sintering. Palladium is highly dispersed on the surface of the carrier in the form of nanoscale or single atom; highly dispersed palladium species are more difficult to migrate or aggregate into large metal clusters, thereby avoiding palladium sintering, which is an important mechanism for catalyst deactivation; the amino ligand on the carrier forms a stable coordination bond with palladium, which also inhibits the migration of palladium. 2. The amino ligand provides electronic stabilization, the amino group has good electron donor ability, which forms a strong coordination with palladium; this strong coordination can stabilize the valence state change of the metal center The stability is improved by avoiding the free release of palladium species in the reaction, which leads to catalyst loss or uneven distribution. 3. The structure of the silicon-based porous foam carrier is stable, heat-resistant and chemical-resistant. The silicon-based porous foam is a highly cross-linked silicon-based mesoporous material: good thermal stability, can withstand high temperature reaction; high chemical stability, good resistance to acid, alkali and various organic solvents; helps to prevent the collapse of the catalyst structure during long-term operation or multiple cycles. 4. The porous structure limits the migration and loss of palladium. The three-dimensional interconnected porous network structure of the silicon-based porous foam carrier can "trap" palladium species in the pores; this not only prevents the mechanical shedding or elution of palladium by the solvent, but also reduces metal loss; therefore, the palladium loss is very small during recovery after the reaction, and the recycling performance is excellent. 5. The metal fixation effect is enhanced by the cooperation of the ligand and the carrier. The role of the amino ligand is to "soft anchor" the metal, while the silicon-based porous foam carrier is a "hard container". This is a synergistic effect: the ligand fixes the palladium through electronic interaction, and the pores prevent its loss through spatial restriction; this makes it difficult for palladium to be reduced to a metallic state and precipitated during use, and also makes it difficult for palladium to flow into the solution (avoiding homogenization). 6. The catalyst can maintain activity under various reaction conditions. Experiments show that the silicon-based heterogeneous palladium catalyst remains structurally and actively in various coupling reactions (such as Suzuki, Heck, etc.), even in the presence of water or polar solvents; this indicates that it has good chemical and reaction condition stability.

[0129] The heterogeneous catalyst has good stability because the palladium is strongly coordinated by the amino ligand and fixed on the structurally stable MCF (mesoporous silica) carrier, forming a synergistic protection of electronic stability + spatial limitation, which prevents the agglomeration, loss and structural damage of palladium, and is suitable for multiple cycles and harsh reaction conditions.

[0130] The present application also provides a silicon-based heterogeneous palladium catalyst prepared by the preparation method described in the above technical solution, which comprises: an amine group functionalized silicon-based porous foam and palladium nanoparticles loaded on the surface and pores of the amine group functionalized silicon-based porous foam; the silicon-based porous foam is mesoporous silica; and the palladium nanoparticles include zero-valent palladium nanoparticles and divalent palladium nanoparticles.

[0131] As an embodiment, the particle size of the palladium nanoparticles is 1.5-5 nm, and another embodiment is 1.8-3.6 nm, and a specific embodiment is 2.7 nm.

[0132] The standard deviation indicates that the palladium nanoparticles prepared by the present application have a narrow distribution, indicating good particle dispersion.

[0133] As an embodiment, the mass percentage content of the amine group functionalized silicon-based porous foam in the silicon-based heterogeneous palladium catalyst is 70-95%, and a specific embodiment is 93%, and the mass percentage content of the palladium nanoparticles is 5-30%, and a specific embodiment is 7%.

[0134] The application further provides application of the silicon-based heterogeneous palladium catalyst in organic synthesis reactions.

[0135] As an embodiment, the organic synthesis reaction comprises one or more of coupling reaction, hydrogenation reaction and oxidation reaction, and in particular embodiments, the coupling reaction.

[0136] The application further provides a method for preparing 4-methyl biphenyl derivatives based on Suzuki coupling reaction, comprising the following steps:

[0137] The p-bromotoluene, phenylboronic acid, alkaline substance, catalyst and polar mixed solvent are mixed to perform Suzuki coupling reaction to obtain 4-methyl biphenyl derivatives; the 4-methyl biphenyl derivatives are 4-methyl-4'-pentyl-1,1'-biphenyl.

[0138] The catalyst is the silicon-based heterogeneous palladium catalyst in the above technical solution or the silicon-based heterogeneous palladium catalyst prepared by the preparation method in the above technical solution.

[0139] As an embodiment, the molar ratio of the p-bromotoluene and phenylboronic acid is 1:1-1.5, and in particular embodiments, the molar ratio is 1:1.2; the alkaline substance is potassium carbonate; the molar ratio of the p-bromotoluene and alkaline substance is 1:1.8-3, and in particular embodiments, the molar ratio is 1:2-3; the polar mixed solvent is anhydrous ethanol and water in a volume ratio of 1:1, or N,N-dimethylformamide (DMF) and water in a volume ratio of 1:1; the ratio of the amount of substance of the p-bromotoluene to the volume of the polar mixed solvent is (0.5-1) mmol:(5-10) mL, and in particular embodiments, the ratio is 1 mmol:8 mL; the amount of substance of Pd in the catalyst is 3-7 mol% of the amount of substance of the p-bromotoluene, and in particular embodiments, the amount of substance is 5 mol%. The p-bromotoluene is a coupling reaction substrate, the phenylboronic acid provides aryl, and the alkaline substance promotes the reaction; the polar mixed solvent used is environmentally friendly and helps the substrate to dissolve and the Pd catalytic cycle.

[0140] As an embodiment, the mixing is: the p-bromotoluene, phenylboronic acid and alkaline substance are dissolved and dispersed in the polar mixed solvent, and the catalyst is added.

[0141] As an implementation form, before the Suzuki coupling reaction, argon or nitrogen is introduced to replace oxygen; in specific embodiments, nitrogen is used; the Suzuki coupling reaction is carried out in air or a protective gas; the protective gas is argon or nitrogen, and in specific embodiments, nitrogen is used; the Suzuki coupling reaction is carried out at a temperature of 70-80°C, and in specific embodiments, 80°C; the holding time is 6-12h, and in specific embodiments, 7-8h; the Suzuki coupling reaction is carried out under stirring; the stirring rate is 600-800rpm, and in specific embodiments, 600-700rpm; the reactor used in the Suzuki coupling reaction is a small three-neck flask or a closed reaction tube with a condenser reflux.

[0142] The catalyst prepared in the application has good stability and can be operated in air; under a medium temperature condition, the reaction rate and Pd stability can be considered, and sufficient reaction time is ensured; the stirring rate in the above range promotes uniform mixing, and the reactor used ensures that the solvent does not evaporate.

[0143] As an implementation form, after the Suzuki coupling reaction, the product obtained in the Suzuki coupling reaction is cooled to room temperature, followed by solid-liquid separation, washing and drying of the obtained solid catalyst, and then repeated use, and the obtained filtrate is extracted twice with ethyl acetate, followed by concentration of the combined organic phase, column chromatography separation and purification of the obtained crude product, to obtain a 4-methyl biphenyl derivative; the solid-liquid separation is filtration; the reagent used for washing is ethanol or water; the drying is anhydrous sodium sulfate drying; the number of times of repeated use is ≥6, and in specific embodiments, 8-10; the concentration is rotary evaporation, reduced pressure concentration or reduced pressure rotary evaporation, and in specific embodiments, rotary evaporation; the temperature of the rotary evaporation is 20-50°C, and in specific embodiments, 35°C; the time is 5-15min, and in specific embodiments, 10min; the pressure of the reduced pressure concentration is 80-160mbar, and in specific embodiments, 120mbar; the time of the reduced pressure concentration is 10-15min, and in specific embodiments, 12min; the column chromatography separation and purification is carried out using a silica gel chromatographic column. The amount of ethyl acetate used in the application is not particularly limited, and the amount well known in the art can be used.

[0144] The reason that the silicon-based heterogeneous palladium catalyst provided by the application is easy to recycle and reuse is that: 1. The heterogeneous structure itself is easy to separate. The catalyst involved in the application is a solid catalyst, which can be separated from the liquid phase in the reaction system by simple means: centrifugation, filtration, and even magnetic separation under the condition of allowing modification of magnetism (if Fe3O4 or the like is introduced into the carrier); compared with the homogeneous catalyst (which needs complex methods such as extraction and reprecipitation), the heterogeneous form is more convenient, fast and economical. 2. Palladium is firmly anchored to the carrier, and the loss is extremely low. The amino ligand is firmly loaded on the surface of the silicon-based porous foam carrier by forming a stable coordination bond with palladium; the space limitation in the porous structure further prevents the dissolution or agglomeration of palladium; 3. The structure is stable and can withstand multiple reaction conditions. The silicon-based porous foam carrier material has good thermal stability and solvent compatibility; it is not easy to swell, disintegrate due to acid, alkali or organic solvent; the amino ligand also shows good chemical stability and is not easy to degrade in multiple reaction cycles. 4. The number of times of reuse and the retention of catalytic efficiency.

[0145] The number of times of reuse: in various typical coupling reactions (such as Suzuki-Miyaura reaction), the silicon-based heterogeneous palladium catalyst can be reused for ≥8-10 times; after each use, the catalyst can be reused after simple filtration or centrifugal washing and drying; in most experiments, no significant decrease in reaction activity is observed.

[0146] Retention of catalytic efficiency: the conversion rate and yield are almost lost (≥95%) in the first few rounds (1-5 rounds); after the 6th-10th round, it still remains at 85-90% or even higher, depending on the reaction substrate; some catalysts are analyzed by TEM after being used for 10 times, and the results show that: the palladium is not obviously agglomerated; the palladium content is not obviously reduced; and the structure of the catalyst does not change significantly.

[0147] The silicon-based heterogeneous palladium catalyst provided by the application can be efficiently recycled after the reaction by simple physical means due to its solid structure, strongly coordinated metal sites and stable mesoporous carrier structure, and the activity and selectivity remain good during the reuse process. In typical coupling reactions, it can be recycled for more than 8-10 times, and the conversion rate is always maintained at more than 85-95%, showing excellent industrial application prospect and green chemical value.

[0148] The technical solutions in the application will be described clearly and completely in combination with the embodiments in the application, but they cannot be understood as limitations on the protection scope of the application.

[0149] Example 1

[0150] Preparation of the silicon-based heterogeneous palladium catalyst:

[0151] (1) Preparation of the silicon-based porous foam matrix (preparation of the carrier): (2) Preparation of the silicon-based heterogeneous palladium catalyst:

[0152] A solution of 40.00 g of copolymer P123, Pluronic® PE Lutrol® P123 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (BASF Corporation) was dissolved in 100 mL of a mixture of 36.5 wt% hydrochloric acid and 650 mL of water to produce an acidic solution (1.3 mol / L concentration of hydrochloric acid in the acidic solution), then 34 mL of mesitylene was added, the solution was heated to 38 °C, after 2 h of reaction with stirring at 500 rpm, 92 mL of tetraethoxysilane was added, stirring was continued for 5 min, the mixture was aged at 40 °C for 20 h under static conditions, then NH4F (460 mg) dissolved in 50 mL of water was added, and the mixture was then transferred to an autoclave and aged at 100 °C for 24 h, the precipitate was filtered, washed with water and ethanol each once, and dried completely at 60 °C, the obtained white powder was calcined in air at 550 °C for 6 h to obtain a silicon-based porous foam matrix;

[0153] (2) Preparation of amine-functionalized silicon-based porous foam (support modification):

[0154] The above silicon-based porous foam matrix (10 g) was added to 200 mL of anhydrous toluene, then a solution of 3-aminopropyltrimethoxysilane (27 mL) dissolved in 100 mL of toluene was added, the mixture was stirred under argon for 30 min, then refluxed at 110 °C for 24 h at 200 rpm, after cooling to room temperature, the solid was collected by filtration and washed with toluene, ethanol, acetone, and dichloromethane each twice to remove unreacted precursors, the obtained solid was suspended in ethanol at a ratio of 1 g of the solid to 20 mL of ethanol, heated at 60 °C for 12 h, then the solid was collected by filtration and washed with dichloromethane, and dried at 60 °C for 12 h;

[0155] (3) Preparation of silicon-based heterogeneous palladium catalyst (supporting and reduction of palladium)

[0156] The amine-functionalized silicon-based porous foam (5 g) was suspended in a deionized water solution (150 mL) adjusted to pH = 8 with 0.1 N LiOH, stirred at 500 rpm for 15 min at 20 °C, Li2PdCl4was prepared by dissolving 695 mg LiCl and 1.454 g PdCl2in water (100.0 mL) and stirred at 600 rpm until the solution was homogeneous at 80 °C; the resulting deep red solution was cooled and filtered, the pH was adjusted to 8 and then added to the aqueous suspension of the amine-functionalized silicon-based porous foam, the resulting suspension was stirred at 100 rpm for 12 h at 20 °C, then the suspension was centrifuged at 4000 rpm for 10 min, the isolated brown solid was washed with water 3 times, the resulting brown solid was re-suspended in 150 mL of water, NaBH4(3.10 g) suspended in 50 mL of water was added slowly and stirred for 30 min, after centrifugation (6000 rpm for 10 min) and washing with water (3 x 400 mL) and acetone (3 x 400 mL), the solid was dried under vacuum (less than 10 mm Hg) at 50 °C for 12 h to give a dark brown silicon-based heterogeneous palladium catalyst.

[0157] Example 2

[0158] The difference from Example 1 is that sodium tetrachloropalladate was used, and the rest is the same as Example 1.

[0159] Example 3

[0160] The difference from Example 1 is that the obtained silicon-based porous foam matrix white powder was calcined in air at 600 °C for 6 h, and the rest is the same as Example 1.

[0161] Comparative Example 1

[0162] The traditional homogeneous catalyst palladium acetate Pd(OAc)2was used as a comparative example.

[0163] Comparative Example 2

[0164] The traditional heterogeneous catalyst palladium-carbon catalyst was used as a comparative example.

[0165] Application Example 1

[0166] The activity of the silicon-based heterogeneous palladium catalyst prepared in Example 1 was tested in a Suzuki coupling reaction, and the specific steps were as follows:

[0167] Dissolve and disperse p-bromotoluene (1.0 mmol), phenylboronic acid (1.2 mmol) and potassium carbonate (2.0 mmol) in 4 mL of alcohol-water mixed solvent (1:1 by volume of absolute ethanol and water), add the silicon-based heterogeneous palladium catalyst prepared in Example 1, the amount of Pd in the catalyst is 5 mol% of the amount of p-bromotoluene, and oxygen is removed by nitrogen blowing, the reaction system is heated and kept at 80°C, and the Suzuki coupling reaction is carried out at 600 rpm stirring for 8 h; after the reaction is completed, the obtained solid silicon-based heterogeneous palladium catalyst is separated by filtration, washed with ethanol, dried with anhydrous sodium sulfate, the filtrate is extracted with ethyl acetate (2 times, 10 mL each time), the organic phase is combined, and after 40°C rotary evaporation for 10 min, the crude product is separated and purified by silica gel chromatographic column to obtain the target product: 4-methyl biphenyl derivative (4-methyl-4'-pentyl-1,1'-biphenyl), the yield (based on the initial amount of p-bromotoluene) is 98%, and the amount of by-product is less than 1%.

[0168] The washed silicon-based heterogeneous palladium catalyst is repeatedly used, and after 6 cycles, the activity of the silicon-based heterogeneous palladium catalyst (the yield of the target product) is only reduced by 8%.

[0169] Comparative Application Example 1

[0170] The difference from Application Example 1 is that the silicon-based heterogeneous palladium catalyst prepared in Example 1 is replaced by the traditional homogeneous catalyst palladium acetate (Pd(OAc)2), and the rest is the same as Application Example 1.

[0171] The yield of the target product reaches 95% when the traditional homogeneous catalyst palladium acetate is used for the first time, and the amount of by-product is about 2%, but the traditional homogeneous catalyst palladium acetate cannot be reused.

[0172] Performance test

[0173] The silicon-based heterogeneous palladium catalyst prepared in Example 1 was analyzed by transmission electron microscopy (TEM). The results are shown in Figs. 1-2 .

[0174] From Figs. 1-2It can be seen that the three-dimensional foam-like mesoporous structure of the carrier in the silicon-based heterogeneous palladium catalyst is clear, the structure is complete, the channel is clear, and the pore size is uniform, which indicates that the carrier has good metal loading and space limiting capacity, and helps to improve the stability and catalytic activity of the zero-valent palladium nanoparticles. The zero-valent palladium nanoparticles in the silicon-based heterogeneous palladium catalyst are well dispersed, which indicates that the loading process does not damage the structure of the carrier. The palladium nanoparticles are uniformly dispersed on the surface and in the channels of the silicon-based porous foam carrier structure, and are mainly concentrated in the inside of the channels and on the surface near the channel wall; the zero-valent palladium nanoparticles are spherical or spherical-like in appearance, and no obvious agglomeration phenomenon is observed; the diameter of the zero-valent palladium nanoparticles is in the range of 1.5-5 nm, the size is small and concentrated, and is mainly distributed in the range of 2-3.5 nm, with an average particle size of 2.7±0.9 nm, which indicates that the present application can effectively control the size and dispersity of the zero-valent palladium nanoparticles and avoid the occurrence of agglomeration phenomenon, further confirming the advantages of the silicon-based heterogeneous palladium catalyst as a high-efficiency heterogeneous palladium catalyst in structure control and nanodispersion.

[0175] Although the above embodiments have made a detailed description of the present application, it is only a part of the embodiments of the present application but not all the embodiments, and other embodiments can be obtained according to the present embodiments without creativity, which all belong to the protection scope of the present application.

Claims

1. A process for the preparation of a silicon-based heterogeneous palladium catalyst, characterized in that, The method comprises the following steps: mixing a block copolymer template agent, a pore size expander and an acidic solution, self-assembling to form block copolymer micelles; mixing the block copolymer micelles and a silicon source, and performing a hydrolysis-condensation reaction to obtain a silicon-oxygen cluster-block copolymer composite micelle; mixing the silicon-oxygen cluster-block copolymer composite micelle and an etching agent, and performing etching, and calcining the obtained etched silicon-oxygen cluster-block copolymer composite micelle to obtain a silicon-based porous foam matrix; mixing the silicon-based porous foam matrix, an organic aminosilane and a non-polar organic solvent, and performing amine group grafting modification to obtain an amine group functionalized silicon-based porous foam; mixing an alkaline aqueous suspension of the amine group functionalized silicon-based porous foam and an alkaline complex solution of tetrachloropalladate, and performing loading, mixing the obtained palladium-loaded amine group functionalized silicon-based porous foam and a reducing agent, and performing a reduction reaction to obtain a silicon-based heterogeneous palladium catalyst.

2. The production method according to claim 1, characterized by, The block copolymer template agent is a non-ionic triblock copolymer; the non-ionic triblock copolymer comprises one or more of P123, F127 and P85.

3. The preparation method according to claim 1, characterized in that, The silicon source comprises one or more of tetraethoxysilane, tetramethoxysilane and tetrapropoxysilane.

4. The method of claim 1, wherein, The pore size expander comprises one or more of 1,3,5-trimethylbenzene, 1,3,5-triisopropylbenzene and toluene.

5. The preparation method according to claim 1, characterized in that, The self-assembly is performed under stirring at a rotation speed of 300-800 rpm.

6. The method of claim 1, wherein, The organic aminosilane comprises 3-aminopropyltrimethoxysilane.

7. The preparation method according to claim 1, characterized in that, The reducing agent comprises one or more of sodium borohydride, formaldehyde, ascorbic acid and hydrogen.

8. The silicon-based heterogeneous palladium catalyst prepared according to the process of any one of claims 1 to 7, characterized in that, The method comprises the following steps: an amine group functionalized silicon-based porous foam and palladium nanoparticles loaded on the surface and pores of the amine group functionalized silicon-based porous foam; The silicon-based porous foam is mesoporous silica; the palladium nanoparticles comprise zero-valent palladium nanoparticles and divalent palladium nanoparticles.

9. The silicon-based heterogeneous palladium catalyst of claim 8 is used in an organic synthesis reaction. The organic synthesis reaction comprises one or more of a coupling reaction, a hydrogenation reaction and an oxidation reaction.

10. A method for preparing a 4-methylbiphenyl derivative based on a Suzuki coupling reaction, characterized by, The method comprises the following steps: mixing p-bromotoluene, phenylboronic acid, an alkaline substance, a catalyst and a polar mixed solvent, and performing a Suzuki coupling reaction to obtain a 4-methyl biphenyl derivative; the 4-methyl biphenyl derivative is 4-methyl-4'-pentyl-1,1'-biphenyl; The catalyst is the silicon-based heterogeneous palladium catalyst of claim 8.

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