CN103908976A - Method for preparing selectivity-adjustable efficient supported precious-metal core-shell catalyst coated with metal-organic framework ZIF-8 membrane - Google Patents

Abstract

The invention discloses a method for preparing a metal-organic framework ZIF-8 film-coated loaded noble metal high-efficiency core-shell catalyst with adjustable selectivity, which belongs to the field of new materials. It is characterized in that the pre-prepared supported noble metal catalyst is coated with a ZIF-8 membrane selective separation layer, and the defects of the membrane layer are repaired by polyelectrolyte, and the number of times of film formation is adjusted to obtain continuous ZIF-8 membranes with different densities. High selectivity of regulation. Compared with similar catalysts, the preparation process is simple, the conditions are mild, and it is easier to scale up. The invention solves the problem that the heterogeneous supported noble metal catalyst has low selectivity to reactants and products, and at the same time helps to alleviate the loss and poisoning of the active components of the heterogeneous supported noble metal catalyst during the reaction.

Description

Preparation method of metal-organic framework ZIF-8 membrane-coated supported noble metal efficient core-shell catalyst with tunable selectivity

technical field

The invention prepares a selectivity-adjustable metal-organic framework ZIF-8 film-coated loaded noble metal high-efficiency core-shell catalyst. The main reason is to endow the loaded noble metal catalyst with high selectivity to reactants and products by coating a ZIF-8 membrane with molecular sieving function; at the same time, it also helps to solve the problem that the active components of heterogeneous catalysts are easy to lose and poison during the reaction , leading to a problem of reduced activity.

Background technique

Nanoscale noble metal (Pd, Pt, Au, etc.) catalysts have high catalytic activity in important industrial reactions such as catalytic hydrogenation, selective oxidation, carbon monoxide conversion, and carbon-carbon coupling. Early studies focused on homogeneous systems. Although homogeneous catalysts can provide ideal activity, their recovery and separation problems restrict their application in industrial production. In comparison, heterogeneous catalysts have high-efficiency separation and recovery performance while maintaining a relatively ideal activity. The supported nano-noble metal catalyst realizes the homogeneous catalyst by loading the active components on various support carriers (organic polymer carrier, carbon carrier, molecular sieve, SiO ₂ , Al ₂ O ₃ , TiO ₂ , Fe ₃ O ₄ , etc.). multiphase. However, the active components of this type of catalyst are easy to lose and poison during the reaction process, and are easy to sinter at high temperature, thereby affecting the stability and life of the catalyst. To solve this problem, the force between the particle and the carrier is strengthened by introducing special functional groups [Crudden C M, et al., J.Am.Chem.Soc., 2005,127:10045-10050]; Introduced into the pores of molecular sieves to fix nanoparticles and alleviate their loss [Wang H, et al., ChemCatChem, 2010, 2:1303-1311]. However, the above methods do not address the issue of catalyst selectivity for reactions and products.

Metal organic frameworks (MOFs) are porous crystalline materials that are self-assembled by metal ions or metal clusters and organic ligands through covalent or ion-covalent interactions. It has outstanding advantages such as high specific surface area, porosity, and adjustable skeleton structure, and has great application potential in gas storage and separation, heterogeneous catalysis, molecular recognition, and sensing. Zeolitic imidazolate frameworks (ZIFs), as a widely studied MOFs, have larger specific surface area and pore volume, higher thermal stability and chemical corrosion resistance than other types of metal-organic frameworks. The topology of ZIF-8 is similar to that of silica-alumina zeolite (SOD): the silica-alumina tetrahedron and bridging oxygen atoms are replaced by metal ions and imidazole, respectively, and its macropore size is The permissible dynamic diameter is less than Molecules come in and out freely, so it has the characteristics of molecular sieving. Therefore, using ZIF-8 as a matrix to load or coat noble metal nanoparticles can effectively improve the selectivity of the catalyst, and at the same time, reduce the loss of active components and poisoning.

At present, there are mainly two methods for preparing this catalyst. One is to obtain metalZIF-8 core-shell catalyst by embedding noble metal nanoparticles into ZIF-8 channels or coating ZIF-8 film on the outside of nanoparticles. As Jiang et al., by mixing and grinding ZIF-8 and (CH ₃ ) ₂ Au(acac) two solids, AuZIF-8 catalyst was obtained by H ₂ reduction method and used for CO oxidation [Jiang H L, et al ., J.Am.Chem.Soc.,2009,131:11302-11303]; Li et al loaded Au and Ag nanoparticles onto the surface and pores of ZIF-8 by continuous precipitation reduction method to obtain a bimetallic catalyst , to catalyze the reduction of p-nitrophenol by NaBH4 to produce 4-aminophenol [Li Z, et al., Chem. Mater., 2013, 25:1761-1768]. Lu et al put PVP-Pt into the synthesis of ZIF-8 In liquid, the coating is completed by adsorbing PVP-Pt during the synthesis of ZIF-8 [Lu G, et al., Nat. Chem. 2012, 4:310-316]; Kuo et al. Synthesize ZIF-8 film on the surface to obtain eggshell PdZIF-8 catalyst [Kuo CH, et al., J.Am.Chem.Soc., 2012, 134:14345-14348]; Yang et al. used 2-methylimidazole In order to stabilize the noble metal nanoparticles by two N atoms, first prepare 2-mIM-Pt, and then use 2-miM as an organic ligand to induce the synthesis of ZIF-8 single crystal [Yang Q H, et al., Chem Common, 2013 , 49:3330-3332]. The above catalyst types all belong to ZIF-8 particle-wrapped non-supported homogeneous noble metal nanoparticles. The selectivity of this catalyst is single and difficult to adjust.

Contents of the invention

The technical problem to be solved in the present invention is to propose a new type of core-shell catalyst obtained by coating heterogeneously loaded noble metal microspheres with a ZIF-8 film in view of the disadvantages of the current preparation of heterogeneous catalysts. That is, a continuous and uniform ZIF-8 film layer is grown on the surface of the pre-synthesized Pd/SiO ₂ supported catalyst, that is, a Pd/SiO ₂ ZIF- ₈ film core-shell catalyst is obtained. By controlling the growth of the outer surface of the Pd/SiO 2 supported catalyst The number of ZIF-8 membranes changes the thickness and density of ZIF-8 membranes to endow the catalyst with adjustable selectivity; it also helps to solve the problem of easy loss of catalyst active centers, and has been verified in hydrogenation reactions.

Technical scheme of the present invention is as follows:

A method for the preparation of a selectivity-tunable metal-organic framework ZIF-8 membrane-coated supported noble metal high-efficiency core-shell catalyst is to pre-synthesize SiO ₂ microsphere support and Pd nanoparticles, and then load the Pd nanoparticles on the A supported Pd/SiO ₂ catalyst is formed on SiO ₂ , and then the pre-synthesized supported noble metal catalyst is coated with a ZIF-8 film selection layer; when the secondary growth of ZIF-8 is achieved by repairing film defects and controlling the thickness The dense and continuous ZIF-8 film makes the catalyst selectivity adjustable.

In the pre-synthesized supported noble metal catalyst, the carrier used is activated solid SiO ₂ microspheres, the activation method is 95 ° C, hydrothermal treatment 8h. The noble metal used is Pd nanoparticles stabilized by PVP, and when the ZIF-8 film is grown for the second time, the ion-type high molecular polymer polystyrene sulfate sodium salt is used to repair the defects of the film layer.

Specific steps are as follows:

(1) Preparation of Pd nanoparticles

First prepare 2mM H ₂ PdCl ₄ aqueous solution, then mix it with PVP, H ₂ O, HCl, heat to boiling, add ethanol immediately, and reflux for 3h;

(2) Preparation of _SiO2 support

According to the Stober method, using TEOS as the silicon source and NH ₄ OH as the catalyst, under the mixed system of ethanol/water, hydrolyze at room temperature for 1 hour;

(3) Mix the Pd nanoparticles prepared by the method (1) with the _SiO2 obtained by the method (2) at room temperature for 24 hours to obtain a Pd/ _SiO2 catalyst;

(4) Mix the Pd/SiO ₂ aqueous solution obtained by the method (3) with the PDDA aqueous solution, let it stand at room temperature for 30 minutes, and then centrifugally wash it with deionized water several times; then mix it with the PSS aqueous solution and let it stand at room temperature for 30 minutes, then centrifugally wash it with deionized water and methanol repeatedly;

(5) Put the Pd/SiO ₂ modified with PDDA and PSS obtained in method (4) into the ZIF-8 synthesis solution, ice bath for 2 hours, and room temperature for 1 hour; the composition of the synthesis solution is zinc nitrate, 2-methylimidazole ( 2meIM), methanol as the raw material of the film-forming solution, and the molar ratio of the synthetic solution is about: 2-mIM:Zn(NO ₃ ) ₂ ·6H ₂ O:CH ₃ OH=10:1:810.

The invention solves the problems of easy loss of active centers, poisoning, and high temperature sintering faced by traditional heterogeneous catalysts, and obtains films with different thickness and compactness by adjusting the number of times of growing ZIF-8 films, and then for different sizes. Molecules are sieved to endow the catalyst with adjustable selectivity, which has a wide range of application prospects.

Description of drawings

Figure 1a is a TEM image of _SiO2 carrier.

Figure 1b is the TEM image of the Pd/SiO ₂ supported catalyst.

Figure 1c is a TEM image of the catalyst after one film formation.

Figure 1d is the TEM image of the catalyst after the second film formation.

Figure 2a is the SEM image of the catalyst after one film formation.

Figure 2b is the SEM image of the catalyst after the second film formation.

Figure 3 is the XRD spectra of different samples.

Fig. 4 is the activity curve of different catalysts with time.

Figure 5 is a bar graph showing the selectivity of catalysts with different film thicknesses to olefins of different sizes.

Detailed ways

The specific embodiments of the present invention will be described in detail below in conjunction with the technical solutions and accompanying drawings.

Example 1

(1) Preparation of Pd nanoparticles

First prepare 2mM H ₂ PdCl ₄ aqueous solution, then mix it with PVP, H ₂ O, HCl, heat to boiling, add ethanol immediately, and reflux for 3h;

(2) Preparation of _SiO2 support

According to the Stober method, using TEOS as the silicon source and NH ₄ OH as the catalyst, under the mixed system of ethanol/water, hydrolyze at room temperature for 1 hour;

(3) Mix the Pd nanoparticles prepared by the method (1) with the _SiO2 obtained by the method (2) at room temperature for 24 hours to obtain a Pd/ _SiO2 catalyst;

(4) Mix the Pd/SiO ₂ aqueous solution obtained by the method (3) with the PDDA aqueous solution, let it stand at room temperature for 30 minutes, and then centrifugally wash it with deionized water several times; then mix it with the PSS aqueous solution and let it stand at room temperature for 30 minutes, then centrifugally wash it with deionized water and methanol repeatedly;

(5) Put the Pd/SiO ₂ modified with PDDA and PSS obtained in method (4) into the ZIF-8 synthesis solution, ice bath for 2 hours, and room temperature for 1 hour; the composition of the synthesis solution is zinc nitrate, 2-methylimidazole ( 2meIM), methanol as the raw material of the film-forming solution, the molar ratio of the synthetic solution is about: 2-mIM:Zn(NO ₃ ) ₂ 6H ₂ O:CH ₃ OH=10:1:810;

(6) The catalyst obtained in method (5) was vacuum-dried overnight at 80°C and placed in a desiccator for testing.

Example 2

(1) Preparation of Pd nanoparticles

First prepare 2mM H ₂ PdCl ₄ aqueous solution, then mix it with PVP, H ₂ O, HCl, heat to boiling, add ethanol immediately, and reflux for 3h;

(2) Preparation of _SiO2 support

According to the Stober method, using TEOS as the silicon source and NH ₄ OH as the catalyst, under the mixed system of ethanol/water, hydrolyze at room temperature for 1 hour;

(3) Mix the Pd nanoparticles prepared by the method (1) with the _SiO2 obtained by the method (2) at room temperature for 24 hours to obtain a Pd/ _SiO2 catalyst;

(4) The catalyst obtained in method (3) was vacuum-dried overnight at 80°C and placed in a desiccator for testing.

Example 3

(1) Preparation of Pd nanoparticles

First prepare 2mM H ₂ PdCl ₄ aqueous solution, then mix it with PVP, H ₂ O, HCl, heat to boiling, add ethanol immediately, and reflux for 3h;

(2) Preparation of _SiO2 support

According to the Stober method, using TEOS as the silicon source and NH ₄ OH as the catalyst, under the mixed system of ethanol/water, hydrolyze at room temperature for 1 hour;

(3) Mix the Pd nanoparticles prepared by the method (1) with the _SiO2 obtained by the method (2) at room temperature for 24 hours to obtain a Pd/ _SiO2 catalyst;

(4) Mix the Pd/SiO ₂ aqueous solution obtained by the method (3) with the PDDA aqueous solution, let it stand at room temperature for 30 minutes, and then centrifugally wash it with deionized water several times; then mix it with the PSS aqueous solution and let it stand at room temperature for 30 minutes, then centrifugally wash it with deionized water and methanol repeatedly;

(5) Put the Pd/SiO ₂ modified with PDDA and PSS obtained in method (4) into the ZIF-8 synthesis solution, ice bath for 2 hours, and room temperature for 1 hour; the composition of the synthesis solution is zinc nitrate, 2-methylimidazole ( 2meIM), methanol as the raw material of the film-forming solution, the molar ratio of the synthetic solution is about: 2-mIM:Zn(NO ₃ ) ₂ 6H ₂ O:CH ₃ OH=10:1:810;

(6) Mix the catalyst obtained by the method (5) that has been formed into a film once with the PSS aqueous solution and let it stand at room temperature for 30 minutes, deionized water and methanol were centrifuged and washed several times, and then put into the ZIF-8 synthesis solution, ice bathed for 2 hours, and room temperature for 1 hour ; The composition of the synthetic solution is zinc nitrate, 2-methylimidazole (2meIM), and methanol as the raw materials of the film-forming solution. The molar ratio of the synthetic solution is about: 2-mIM:Zn(NO ₃ ) ₂ ·6H ₂ O:CH _3OH =10:1:810;

(7) The catalyst obtained by method (6) was vacuum-dried overnight at 80°C, and placed in a desiccator for testing.

Example 4

Take 0.1g of catalyst and put it in the reaction device, then add 15nl of ethyl acetate and 0.3ml of n-hexene, use hydrogen to purge the reaction device for 3 times, and react at 35°C for 24h. After the reaction, the catalyst and feed liquid were separated by centrifugation, and the results were detected by gas chromatography (chromatographic column: HP-5; FID).

Example 5

Take 0.1g catalyst and put it in the reaction device, then add 15nl ethyl acetate and 0.3ml cyclohexene, use hydrogen to purge the reaction device 3 times, and react at 35°C for 24h. After the reaction, the catalyst and feed liquid were separated by centrifugation, and the results were detected by gas chromatography (chromatographic column: HP-5; FID).

Example 6

Take 0.1g of catalyst and put it in the reaction device, then add 15nl of ethyl acetate and 0.3ml of cyclooctene, use hydrogen to purge the reaction device for 3 times, and react at 35°C for 24h. After the reaction, the catalyst and feed liquid were separated by centrifugation, and the results were detected by gas chromatography (chromatographic column: HP-5; FID).

Claims (7)

1. a preparation method for the coated efficient nucleocapsid catalyst of carried noble metal of selective adjustable metallic organic framework ZIF-8 film, is characterized in that, the coated ZIF-8 film of pre-synthesis loaded noble metal catalyst is selected to layer; In the time of diauxic growth ZIF-8, by repairing and the thickness regulation and control of rete defect, obtain fine and close continuous ZIF-8 film, catalyst is had selectively adjustable.

2. preparation method according to claim 1, is characterized in that, in the time of synthetic loaded noble metal catalyst, used carrier is the solid SiO after activation ₂microballoon; Activation method is 95 DEG C, hydrothermal treatment consists 8h.

3. preparation method according to claim 2, is characterized in that, described noble metal is Pd nano particle.

4. preparation method according to claim 3, is characterized in that, described Pd nano particle is the stable Pd nano particle of PVP.

5. according to the preparation method described in claim 1,2,3 or 4, it is characterized in that, in the time of diauxic growth ZIF-8 film, adopt ionic high molecular polymer to repair rete defect.

6. preparation method according to claim 5, is characterized in that, described ionic high molecular polymer is anionic high molecular polymer.

7. preparation method according to claim 6, is characterized in that, described anionic high molecular polymer is polystyrene sodium sulfate salt.