Reverse-phase packaged palladium-based catalyst as well as preparation method and application thereof

Abstract

The invention discloses a preparation method and application of a reversed-phase packaging palladium-based catalyst, and the preparation method of the catalyst comprises the following steps: low-order phenolic resin and a template agent are co-assembled to form a mesoporous carbon nanosphere precursor solution, a carbon nanosphere substrate is formed through a low-concentration hydrothermal process, then a metal source precursor is loaded on the carbon nanosphere substrate, and the reversed-phase packaging palladium-based catalyst is obtained. Further epitaxially growing a tin dioxide layer in a palladium lattice confinement through an in-situ thermal reduction process to reversely package palladium nanoparticles; compared with an existing packaged metal catalyst, the preparation method of the reversed-phase packaged catalyst is not reported, the problem of dissolution of transition metal under severe conditions is effectively avoided, the catalytic activity and durability of the palladium-based catalyst are enhanced, meanwhile, the transition metal is coated inside, unstable metal can be removed without an acid pickling process, and the service life of the palladium-based catalyst is prolonged. And time and cost are saved. The catalyst shows excellent catalytic activity and stability in hydroxylation reaction of pyridine compounds.

Description

Technical Field

[0001] The invention relates to the field of catalysts, and in particular to a reverse-phase encapsulated metal catalyst and a preparation method and application thereof. Background Art

[0002] Phenolic structures are ubiquitous in compound molecules, and the hydroxylation of aromatic compounds is an important chemical reaction in various metabolic pathways and organic synthesis. The direct hydroxylation of electron-deficient (hetero)aromatics is a long-standing challenge because their non-nucleophilic nature and high oxidation potential usually exclude electrophilic and oxidative activation mechanisms. Currently, several mature methods for the hydroxylation of aromatic compounds rely on electrophilic oxygen intermediates or single-electron aromatic oxidation pathways, which are most suitable for neutral electron aromatics to electron-rich aromatics. Although traditional advanced oxidation processes based on transition metals or metal ions can generate active substances, sulfate radicals, to promote the formation of hydroxylation products of aromatic compounds, they inevitably lead to secondary contamination of metal ions and irreversible deactivation of catalysts, which greatly limits their practical application.

[0003] In heterogeneous catalytic reactions, supported metal catalysts are increasingly valued in the fields of environmental remediation and industrial manufacturing due to their inherent catalytic properties. The interfacial interactions between the active phase and the support and the effects of the interfacial density on the catalytic reaction mechanism and catalytic performance have always been of great concern. However, the leaching, agglomeration and poisoning of active metal components lead to catalyst deactivation, thus limiting their application. Currently, a novel and promising strategy for constructing encapsulated metal catalysts has shown unparalleled advantages in catalysis, especially under harsh conditions (e.g., strong acidity, alkalinity and high temperature), the active centers can remain unchanged.

[0004] At present, the common synthesis methods of encapsulated catalyst materials include confined growth method, in-situ precipitation method, ion exchange reverse loading method, etc. For example, CN117732498A provides an encapsulated catalyst for catalyzing glycerol carbonylation and its preparation method and application. The method uses metal organic framework material to load palladium nanoparticles. During the calcination process, the palladium nanoparticles are confined inside the framework, which improves the stability of the catalyst, and has high efficiency and high stability in the catalytic reaction of glycerol to prepare glycerol carbonate. CN115999537A provides a porous composite oxide encapsulated metal catalyst and its preparation method and application. The method adopts in-situ precipitation growth method to graft a hydrotalcite-based shell layer on the surface of a polymer ball, and after calcination in air and reduction in hydrogen, a high-activity specific surface area and high-stability oxide hollow ball encapsulated Pt nanoparticle catalyst is prepared, which has the characteristics of mild reaction conditions, high conversion rate and selectivity, fast reaction rate, good reusability, etc. for the transfer hydrogenation reaction of aldehydes and ketones. The team led by Xuefeng Guo of Nanjing University prepared the encapsulated catalyst Ni@Al2O3-IE by the ion exchange reverse loading method, which showed excellent low-temperature activity and high-temperature stability in the CO methanation reaction (Science Advances, 2020, 6, eaay7031). The reverse encapsulated metal catalyst can prevent the aggregation and sintering of the active nucleus and prevent the direct contact between the poison and the active nucleus while ensuring the electron transfer of the active components, thereby improving the stability of the catalyst. Compared with encapsulated metal catalysts, reverse encapsulated metal catalysts have higher interface density and mutually changing interface properties, and because the active phase is physically isolated by the oxide, it has high stability, which makes it show superior catalytic performance in the catalytic reaction. However, due to the encapsulation of precious metals by the oxide layer, there are certain technical requirements for the recovery of precious metals, and the cost of precious metal recovery is high. Summary of the invention

[0005] The technical problem to be solved by the present invention is that the existing reverse encapsulated metal catalyst and its preparation method have the problem that the selection range of the encapsulation layer is small and the thickness of the encapsulation layer is uncontrollable. In view of the shortcomings of the prior art, the present invention provides a reverse encapsulated metal catalyst and its preparation method, which realizes the confined epitaxial growth of the tin dioxide layer on the palladium-based lattice through an in-situ reduction method on the surface of the carbon nanosphere substrate, thereby realizing the reverse encapsulation of palladium by tin dioxide, avoiding the alloying process between the shell and the core, effectively avoiding the problem of transition metal dissolution during long-term reaction, enhancing the catalytic activity and durability of the palladium-based catalyst, and at the same time, coating the transition metal inside, which can avoid the pickling process to remove unstable metals, saving time and cost.

[0006] To achieve the above-mentioned purpose, the present invention provides a method for preparing a reverse encapsulated palladium-based catalyst and its application, comprising first preparing a low-order phenolic resin, co-assembling the low-order phenolic resin with a template to form a mesoporous carbon nanosphere precursor solution, undergoing a low-concentration hydrothermal process to form a carbon nanosphere substrate, then loading a metal source precursor on the carbon nanosphere substrate, and further undergoing an in-situ thermal reduction process to form a reverse encapsulated metal catalyst.

[0007] Further, the phenolic resin is prepared, specifically comprising selecting a carbon source and an auxiliary agent and uniformly dispersing them in an aqueous solution; fully stirring at 50-80° C. to obtain a low molecular weight polymer of phenol and formaldehyde, and adjusting the pH value to obtain a low molecular weight phenolic resin polymer.

[0008] Furthermore, the carbon source includes one or more of phenol, resorcinol, catechol, hydroquinone and formaldehyde; and the mass content of the carbon source is 5-30%.

[0009] Furthermore, the auxiliary agent includes one or more of melamine, urea, mercaptosilane and triphenyl phosphite.

[0010] Further, the carbon nanosphere substrate is prepared, specifically comprising placing a phenolic resin and a template in an aqueous solution to adjust the concentration of the phenolic resin, and through hydrogen bonding forces, the phenolic resin and the template form a resin-template single bundle under low concentration conditions, and then hydrothermally heat under high temperature hydrothermal conditions of 100-130°C for 12-24h, and the resin-template single bundle is further cross-linked and spontaneously assembled into a low-energy cubic closed stacking mesostructure, whose space group After the hydrothermal treatment, the product is spherical and finally washed with deionized water and dried to form a carbon nanosphere substrate.

[0011] Furthermore, the metal source precursor is loaded, specifically including pre-mixing two metal solutions into a certain metal source precursor solution, then immersing the carbon nanosphere substrate in the metal source precursor solution, through ultrasound, the metal precursor is evenly dispersed on the carbon nanosphere substrate, and finally through hydrogen thermal reduction in situ to form a reverse encapsulated metal catalyst.

[0012] Furthermore, the metal source includes a palladium source selected from the group consisting of palladium chloride, palladium nitrate, palladium acetylacetonate, and tin tetrachloride.

[0013] Furthermore, the atomic ratio of the first metal to the second metal in the metal precursor solution is 1:(0.25-4), including one of 1:0.25, 1:0.3, 1:0.5, 1:1, 1:2, 1:3 or 1:4.

[0014] Furthermore, the hydrogen concentration during the thermal reduction process is maintained between 5% and 15%, and the thermal reduction temperature is maintained between 400° C. and 700° C.

[0015] Another preferred embodiment of the present invention provides a reverse-phase encapsulated palladium-based catalyst obtained by the above-mentioned preparation method.

[0016] Another preferred embodiment of the present invention provides an application of a reverse-phase encapsulated metal catalyst in the directional hydroxylation reaction of 2-phenylpyridine.

[0017] Furthermore, the pH value of the low concentration phenolic resin solution is 6-8; the structure directing agent used is an amphiphilic block copolymer, which is a triblock copolymer F127 (EO 106 PO 70 EO 106 ) and P123(EO 20 PO 70 EO 20 ) one or two; the reaction temperature is 40°C, and the reaction time is 0.5-4h.

[0018] Furthermore, the temperature of the hydrothermal process for forming the carbon nanosphere substrate is 80-120° C., and the hydrothermal time is 12-72 hours.

[0019] Furthermore, the in-situ thermal reduction stage includes a first high-temperature calcination stage and a second high-temperature calcination stage, wherein the first high-temperature calcination stage is to remove the block copolymer under an inert atmosphere to obtain a bimetallic alloy-carbon polymer, and the second high-temperature calcination stage is to obtain a reverse encapsulated metal catalyst by high-temperature calcination carbonization under an inert atmosphere.

[0020] In a preferred embodiment of the present invention, a reverse encapsulated metal catalyst is provided, which is obtained by the above-mentioned preparation method.

[0021] In another preferred embodiment of the present invention, there is provided an application of a reverse encapsulated metal catalyst in the hydroxylation reaction of 2-phenylpyridine.

[0022] Technical Effects

[0023] The preparation method of the catalyst of the present invention has simple synthesis steps, is applicable to the preparation of various reverse-phase encapsulated catalysts, has high catalytic activity and good stability, exhibits excellent catalytic performance in the hydroxylation reaction of 2-phenylpyridine, and achieves a 95% conversion rate of 2-phenylpyridine and a selectivity of 2-(2-hydroxyphenyl)pyridine of ≥80% under room temperature and alkali-free reaction conditions.

[0024] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1This is a dark field scanning electron microscope (SEM) image of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0026] Figure 2 is an X-ray diffraction (XRD) diagram of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0027] Figure 3 is a nitrogen isothermal absorption / desorption curve diagram of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0028] Figure 4 is a transmission electron microscope (TEM) image of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0029] Figure 5 This is a high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0030] Figure 6 1 is a Pd and Sn element distribution (mapping) diagram of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0031] Figure 7 This is a spherical aberration corrected high-angle annular dark field scanning transmission electron microscope (AC-HAADF-STEM) image of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention;

[0032] Figure 8 This is a 3d orbital electron spectrum (Pd3d XPS) diagram of the palladium element of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention.

[0033] Fig. 9 This is a 3d orbital electron spectrum (Sn3d XPS) diagram of the tin element of Pd@SnO2 / MCN of a preferred embodiment 1 of the present invention. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] The present invention provides a reverse encapsulated metal catalyst and its preparation method and application. Due to the electrostatic interaction between the surface of the polymer ball and the metal ions, Ni 2+ 、Co 2 、Zn 2+The metal ions are uniformly adsorbed on the surface of carbon nanospheres, and the hydrotalcite-like structured material formed on the surface of the polymer based on this is used as a catalyst precursor to achieve uniform dispersion of metal elements: since the hydrotalcite-like material is uniformly covered on the surface of the polymer sphere, after being calcined to remove the polymer and in-situ reduction of palladium nanoparticles, an oxide hollow sphere encapsulated palladium nanoparticle catalyst can be constructed. A reverse encapsulated metal catalyst involved includes: a carbon carrier and a nanocluster loaded on the surface of the carbon carrier, the nanocluster is formed by tin dioxide-coated palladium core-shell nanoparticles, and the tin dioxide-coated palladium core-shell nanocluster includes a palladium-based core as a core structure and a tin dioxide shell layer as a shell structure, and the tin dioxide shell layer can wrap the palladium-based nanoparticle core. The obtained catalyst has the characteristics of high active specific surface area, high dispersion, high mass transfer, low production cost, easy separation, good cycle stability, etc., and has broad application prospects in the fields of biomass conversion, medicine and fine chemical production.

[0036] Embodiment 1:

[0037] (1) Preparation of carbon nanosphere substrate

[0038] 0.5 g of phenol and 6.5 mL of formaldehyde were evenly dispersed in a 0.1 M sodium hydroxide solution, and the mixture was stirred at 70 °C to obtain a low molecular weight polymer of phenol and formaldehyde. The block copolymer F127 was then added, and the concentration of the solution was further adjusted after stirring for 2 h to obtain a phenolic resin-F127 single micelle. The single micelle was further diluted with deionized water and placed in a hydrothermal kettle. The mixture was hydroheated in an oven at 100 °C for 12 h. After filtration and drying, a carbon nanosphere substrate was obtained.

[0039] (2) Introduction and growth of encapsulation metal

[0040] Weigh palladium chloride and tin chloride in an atomic ratio of Pd:Sn=1:1 to prepare a metal precursor solution, take 100 mg of the carbon nanosphere substrate obtained in step (1) and immerse it in the metal precursor solution, introduce metal and metal oxide precursors by wet chemical method, and then go through two high-temperature calcination stages in an inert atmosphere. The first high-temperature calcination stage is calcined at 300°C for 3h in an inert atmosphere to remove the block copolymer, and the second high-temperature calcination stage is calcined at 700°C for 3h in an inert atmosphere to carbonize in order to stabilize the ordered mesoporous structure, and finally obtain a metal catalyst Pd@SnO2 / MCN of palladium nanoparticles encapsulated with tin dioxide.

[0041] like Figure 1 As shown, the Pd1Sn1 / MCN catalyst of this embodiment is an ordered mesoporous carbon-supported metal nanoparticle with a highly uniform pore size distribution and a spherical diameter size of 100 to 140 nm; Figure 2 As shown, the carbon nanosphere substrate of the catalyst of this embodiment is a body-centered cubic Structure, the metal is highly dispersed on the surface of carbon nanospheres; Figure 3 As shown in Figure 2, the calculated specific surface area of ​​the catalyst is 441 m 2 / g, pore volume 0.19cm 3 / g; such as Figure 4-6 As shown, Pd and Sn particles are evenly distributed on the carbon nanosphere substrate; Figure 7-9 As shown, Pd nanoparticles are encapsulated in SnO2; Figure 8 As shown, palladium is represented by Pd 0 , Pd 2+ The coexistence state exists on the catalyst surface, such as Fig. 9 As shown, the tin element is Sn 4+ state exists on the catalyst surface.

[0042] 0.01 g of the above-obtained tin dioxide-encapsulated Pd metal catalyst Pd@SnO2 / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0043] Embodiment 2:

[0044] The difference from Example 1 is that the hydrothermal treatment is performed in a hydrothermal oven at 130° C. for 12 h.

[0045] 0.01 g of the above-obtained tin dioxide-encapsulated Pd metal catalyst Pd@SnO2 / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0046] Embodiment 3:

[0047] The difference from Example 1 is that the hydrothermal treatment is carried out in a hydrothermal oven at 130° C. for 24 h.

[0048] 0.01 g of the above-obtained tin oxide-encapsulated Pd metal catalyst Pd@SnO2 / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0049] Embodiment 4:

[0050] Different from Example 1, palladium chloride and cobalt chloride are weighed to prepare a metal precursor solution at an atomic ratio of Pd:Co=1:1, and finally a metal catalyst Pd@CoO / MCN in which cobalt oxide encapsulates Pd is obtained.

[0051] 0.01 g of the above-obtained tin oxide-encapsulated Pd metal catalyst Pd@CoO / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0052] Embodiment 5:

[0053] Different from Example 1, palladium chloride and nickel chloride are weighed to prepare a metal precursor solution at an atomic ratio of Pd:Ni=1:1, and finally a metal catalyst Pd@NiO / MCN in which nickel oxide encapsulates Pd is obtained.

[0054] 0.01 g of the above-obtained tin oxide-encapsulated Pd metal catalyst Pd@NiO / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0055] Embodiment 6:

[0056] Different from Example 1, palladium chloride and zinc chloride are weighed to prepare a metal precursor solution in an atomic ratio of Pd:Zn=1:1, and finally a metal catalyst Pd@ZnO / MCN in which zinc oxide encapsulates Pd is obtained.

[0057] 0.01 g of the above-obtained tin oxide-encapsulated Pd metal catalyst Pd@ZnO / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. After the reaction was completed, the solid catalyst was separated by centrifugation. The content of the product after the reaction was detected by gas chromatography, and the results are shown in Table 1.

[0058] Embodiment 7:

[0059] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:2).

[0060] Embodiment 8:

[0061] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:3).

[0062] Embodiment 9:

[0063] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:4).

[0064] Embodiment 10:

[0065] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:0.5).

[0066] Embodiment 11:

[0067] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:0.3).

[0068] Embodiment 12:

[0069] The difference from Example 3 is that the metal precursor solution (Pd:Sn=1:0.25)

[0070] The reaction process of Example 7-12 is as follows: add a quantitative amount of the above-obtained reverse encapsulated palladium-based catalyst (substrate: Pd = 1: 300 mol / mol) of the tin dioxide encapsulated Pd nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of n-butyraldehyde to a reaction kettle, and heat at 100° C. and 0.2 MPa O2 for 4 h. After the reaction is completed, the solid catalyst is separated by centrifugation. The content of the product after the reaction is detected by gas chromatography, and the results are shown in Table 1.

[0071] Embodiment 13:

[0072] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 4-methyl-2-phenylpyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0073] Embodiment 14:

[0074] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-(4-methoxyphenyl)pyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0075] Embodiment 15:

[0076] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-(4-fluorophenyl)pyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0077] Embodiment 16:

[0078] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-(4-chlorophenyl)pyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0079] Embodiment 17:

[0080] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenyl-3-aminopyridine, and 0.1 mmol of n-butyraldehyde were added to the reactor, and heated at 100° C. and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0081] Embodiment 18:

[0082] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of benzaldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0083] Embodiment 19:

[0084] The difference from Example 1 is that 0.01 g of the above-obtained tin dioxide reverse-encapsulated palladium nanoparticles catalyst Pd@SnO2 / MCN, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of phenylacetaldehyde were added to the reactor and heated at 100°C and 0.2 MPa O2 for 4 h.

[0085] Embodiment 20:

[0086] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of propionaldehyde were added to the reactor, and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0087] Embodiment 21:

[0088] Different from Example 1, 0.01 g of the above-obtained catalyst Pd@SnO2 / MCN of tin dioxide reverse encapsulated palladium nanoparticles, 5 mL of 1,2-dichloroethane, 1 mmol of 2-phenylpyridine, and 0.1 mmol of isobutyraldehyde were added to the reactor and heated at 100°C and 0.2 MPa O2 for 4 h. The results are shown in Table 1.

[0089] Table 1 Experimental results of Examples 1-26

[0090] Table 1

[0091]

[0092]

[0093] Comparative Example 1

[0094] Different from Example 1, the phenolic resin and the template agent were placed in an ethanol solution to prepare the carbon nanosphere substrate, and finally an ethanol-carbon nanosphere carrier was obtained with a specific surface area of ​​211 m 2 / g, the reaction results are shown in Table 2.

[0095] Comparative Example 2

[0096] The difference from Example 1 is that no Pd source is added to obtain a tin dioxide catalyst SnO2 / MCN with a specific surface area of ​​390 m 2 / g, the reaction results are shown in Table 2.

[0097] Comparative Example 3

[0098] The difference from Example 1 is that no Sn source is added to obtain a palladium catalyst Pd / MCN with a specific surface area of ​​417 m 2 / g, the reaction results are shown in Table 2.

[0099] Comparative Example 4

[0100] Different from Example 1, the high temperature calcination stage is calcined at 300°C for 3h in an inert atmosphere to remove the block copolymer, and obtain the reverse encapsulation catalyst Pd@SnO2 / MCN of tin dioxide encapsulated palladium nanoparticles with a specific surface area of ​​417m 2 / g, the reaction results are shown in Table 2.

[0101] Table 2 Comparative Examples 1-4 Experimental Results

[0102]

[0103] The reverse encapsulated metal catalyst of the present invention realizes the directional hydroxylation of 2-phenylpyridine in the absence of a strong oxidant, and the catalyst has the advantages of high activity, high selectivity and green reaction.

[0104] The catalyst of the present invention utilizes the strong electrostatic interaction between the surface of carbon nanospheres and metal ions to synthesize a series of oxide clusters. The overall synthesis steps are simple, green, and efficient, and the catalyst is suitable for the preparation of various reverse-phase encapsulated metal catalysts. The catalyst of the present invention is applied to the directional oxidation of 2-phenylpyridine to 2-(2-hydroxyphenyl)pyridine, and a substrate conversion rate of 95.6% and a 2-(2-hydroxyphenyl)pyridine reaction selectivity of 93.2% are obtained. The reaction process is green and efficient, the catalyst has high activity and good stability, and has good application prospects.

[0105] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for preparing a reverse-phase encapsulated palladium-based catalyst, characterized in that: The method includes preparing phenolic resin in advance, and then forming a carbon nanosphere substrate through a low-concentration hydrothermal process. Then, a metal source precursor is loaded on the carbon nanosphere substrate, and a reverse encapsulated metal catalyst is formed through an in-situ thermal reduction process.

2. The method for preparing a reverse-phase encapsulated palladium-based catalyst according to claim 1, characterized in that: The preparation of phenolic resin specifically includes selecting carbon source and auxiliary agent and uniformly dispersing them in an organic solution of alkaline catalyst; fully stirring at 50-80°C to obtain a low molecular weight polymer of phenol and formaldehyde, and adjusting pH value to obtain a low molecular weight phenolic resin polymer.

3. The method for preparing a reverse-phase encapsulated palladium-based catalyst as claimed in claim 2, characterized in that: The carbon source comprises one or more of phenol, resorcinol, catechol, hydroquinone and formaldehyde; the mass content of the carbon source is 5-30%.

4. The method for preparing a reverse-phase encapsulated palladium-based catalyst and its application as claimed in claim 2, characterized in that: The auxiliary agent includes one or more of melamine, urea, mercaptosilane and triphenyl phosphite.

5. The method for preparing a reverse-phase encapsulated palladium-based catalyst according to claim 1, characterized in that: The method comprises placing the phenolic resin and the template in an aqueous solution to adjust the concentration of the phenolic resin, forming a resin-template single bundle with the phenolic resin and the template through hydrogen bonding under low concentration conditions, and then hydrothermally heating the phenolic resin and the template under high temperature conditions of 100-130° C. for 12-24 hours. h The resin-template single bundle is further cross-linked and spontaneously assembled into a low-energy cubic closed stacking structure with a space group of Im3m; after the hydrothermal treatment, it is spherical and finally washed with deionized water and dried to form a carbon nanosphere substrate.

6. The method for preparing a reverse-phase encapsulated palladium-based catalyst according to claim 5, characterized in that: The metal source precursor is loaded, specifically comprising pre-mixing two metal solutions into a certain metal source precursor solution, then immersing the carbon nanosphere substrate in the metal source precursor solution, uniformly dispersing the metal precursor on the carbon nanosphere substrate through ultrasound, and finally thermally reducing it in situ with hydrogen to form a reverse encapsulated metal catalyst.

7. The method for preparing a reverse-phase encapsulated palladium-based catalyst according to claim 6, characterized in that: The metal source includes palladium chloride and one of tin tetrachloride and stannous chloride.

8. The method for preparing a reverse-phase encapsulated palladium-based catalyst according to claim 6, characterized in that: The hydrogen concentration during the thermal reduction process is maintained between 5-15%, and the thermal reduction temperature is maintained between 400-700°C.

9. A reverse-phase encapsulated palladium-based catalyst, characterized in that: The method is obtained by any one of claims 1 to 8.

10. Use of the reverse encapsulated metal catalyst as claimed in claim 9 in the directional hydroxylation reaction of 2-phenylpyridine.

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