CN102527437A - Magnetically-separable noble metal catalyst and preparation method thereof - Google Patents

Abstract

The invention relates to a magnetically separable noble metal catalyst and a preparation method thereof. The catalyst is composed of magnetic carrier and noble metal nanoparticles. The magnetic carrier is a microsphere with a core-shell structure, ferric oxide is the core, and silicon dioxide is the shell. The noble metal nanoparticles are gold nanoparticles. The present invention uses the layer-by-layer self-assembly method to load gold nanoparticles on the magnetic microspheres, so that the catalyst can be effectively separated from the reacted solution by an external magnetic field and reused, which solves the problem of metal nanoparticles in the current noble metal catalytic reaction system. problem of separation.

Description

A magnetically separable noble metal catalyst and its preparation method

technical field

The invention belongs to the technical field of noble metal nano-catalysts, and in particular relates to a magnetically separable gold catalyst, its preparation method and its application in organic and inorganic reduction reactions.

Background technique

Noble metal nanoparticles are widely used as catalysts due to their small size and high surface activity. Noble metal nanoparticles can catalyze the breaking of H-H, C-H, C-C and C-O bonds under appropriate conditions. Because the particle has no pores, it can avoid some side reactions caused by the slow diffusion of reactants to the inner pores, so its activity and selectivity are higher than similar traditional catalysts. However, two disadvantages of noble metal nanoparticles limit their application in the field of catalysis. First of all, due to the small size and high dispersion of noble metal nanoparticles in the reaction system, it is difficult to completely separate them from the system; in addition, the high surface energy of noble metal nanoparticles tends to aggregate to form large particles, eventually losing their nanoparticle properties. One way to solve these problems is to immobilize them on solid supports. A large number of articles have reported carbon materials, metal oxides, and zeolites as supports for noble metal nanoparticles. However, their application also encountered the problem of difficult separation of catalysts. Since these carriers are all small particles of powder, traditional mechanical separation methods such as filtration and centrifugation are time-consuming, high energy consumption, and the separation efficiency is not ideal. Magnetic separation is a new type of green separation method. It can quickly and effectively recover the catalyst, reduce the pollution to the environment and reduce the energy consumption of the separation process. Therefore, the noble metal nanoparticles are loaded on the magnetic carrier and assembled into a magnetically loaded noble metal catalyst. The catalyst can be recovered by magnetic separation, so that it not only maintains good catalytic activity, but also can be reused.

Core-shell composites have unique properties and potential applications in many fields. Coating a layer of inert oxide shell such as silicon dioxide on the magnetic core can not only prevent the magnetic core from being corroded in the solution of strong acid and strong alkali, but also graft new functional groups on the outer surface of the inert oxide shell, thereby greatly Enrich its surface chemistry. Immobilizing noble metal nanoparticles on such a core-shell magnetic carrier can make the metal particles maintain high catalytic activity during recycling. At the same time, the noble metal nanoparticles are modified on the core-shell magnetic spheres with functional properties to obtain dual functions. Or multifunctional composites. The synergistic effect between the components in the composite material will promote the properties of each component in the material to be improved separately.

Layer-by-layer self-assembly is a simple and easy method to construct membrane materials. In the invention, polyethylene diamine is used as polycation, gold nanoparticles wrapped by citrate are used as anion, and a multilayer film is assembled on the surface of magnetic microspheres with a core-shell structure through electrostatic interaction. The gold nanoparticles are immobilized in the multilayer film of polyethylene diamine, and its porous structure ensures that the catalytic substrate can quickly reach the active sites of the gold nanoparticles, thereby ensuring the efficient catalytic performance of the gold nanoparticles.

Contents of the invention

The invention prepares a magnetically separable noble metal catalyst, which is composed of a magnetic carrier and noble metal nanoparticles. Wherein the magnetic carrier is a core-shell structure microsphere composed of iron ferric oxide and silicon dioxide, and the noble metal nanoparticles are gold nanoparticles.

The ferroferric oxide magnetic microspheres are prepared under high-temperature hydrothermal conditions, and the particle size is between 300-360 nanometers (see accompanying drawing 1); the thickness of the silica shell is from 20 nanometers to 25 nanometers Between; gold nanoparticles were prepared by reduction of citric acid in boiling state. The shape and size of the pre-prepared gold nanoparticles can be effectively controlled. The particle size of the gold nanoparticles in the present invention is about 12 nanometers, and the shape is spherical

The magnetically separable noble metal catalyst prepared by the present invention is a method of self-assembly layer by layer, wherein polyethylene diamine is used as a polycation, and gold nanoparticles wrapped by citrate are used as anions. Through electrostatic interaction, the gold nanoparticles are loaded on the The surface of magnetic microspheres with core-shell structure (see accompanying drawing 2).

The magnetically separable noble metal catalyst prepared by the invention has excellent catalytic activity in organic reduction reaction (reduction of p-nitrophenol to generate p-aminophenol) and inorganic reduction reaction (reduction of potassium ferricyanate to generate potassium ferrocyanate).

The magnetically separable noble metal catalyst prepared by the invention is convenient for separation and recovery. After the reaction, under the action of an external magnetic field, they will be separated rapidly within 30 seconds. The catalyst has good recyclability, and its catalytic activity remains above 90% after repeated use.

Description of drawings

Fig.1 Scanning electron micrographs of gold-loaded magnetic microspheres

Figure 2 Transmission electron micrographs of gold-loaded magnetic microspheres

Figure 3 The change of UV-visible spectrum before and after adding gold catalyst in the reaction of nitrophenol and sodium borohydride

Figure 4 The changes of ultraviolet-visible spectra before and after adding gold catalyst in the reaction of potassium ferricyanate and sodium thiosulfate

Figure 5, the curve of the reaction rate of the five cycles of the gold catalyst

Detailed ways

Example 1: Preparation of magnetic core with core-shell structure

First, ferric oxide microspheres were prepared. 2.7g FeCl ₃ 6H ₂ O and 7.2g sodium acetate were added to 100mL ethylene glycol solution, stirred magnetically until a uniform yellow clear solution was formed, then transferred to a polytetrafluoroethylene-lined high-pressure hydrothermal kettle at 200°C Under reaction 8h. The obtained iron tetroxide magnetic particles were repeatedly washed with water and ethanol, dried under vacuum at 50° C. and then used for future use. Before loading the silica shell layer on the surface of ferric oxide, 0.1 g of ferric oxide was ultrasonically treated with 15 mL of 2M HCl solution for 5 min. Magnetically separated, washed, and then added to a mixed solution of 400mL ethanol, 100mL ultrapure water and 15mL concentrated ammonia water. After mechanical stirring for 15 minutes, 3.5mL tetraethyl orthosilicate was added dropwise, and mechanical stirring was continued for 6 hours. The external magnetic field separates and removes the silica impurities in the solution to obtain magnetic microspheres with a core-shell structure. Washed repeatedly with ethanol and ultrapure water, and finally vacuum-dried before use.

Embodiment 2: Preparation of gold nanoparticles

250 mL of HAuCl ₄ ·3H ₂ O aqueous solution (1 mM) and 25 mL of sodium citrate aqueous solution (38.3 mM) were heated to boiling under magnetic stirring. Then the boiling sodium citrate solution was quickly added to the gold precursor solution, the mixed solution was stirred for 10 minutes, the heat source was removed, and the solution was stirred for 15 minutes to obtain a purple-red solution containing gold nanoparticles. After cooling, refrigerate and store for later use.

Example 3: Loading of gold nanoparticles on magnetic microspheres

The loading of gold nanoparticles on magnetic microspheres is carried out by the layer-by-layer self-assembly method according to the following steps: (i) 0.25g of magnetic microsphere carrier with core-shell structure is first added to 10mL concentration of 10mg/mL polyethylene diamine ( PEI) aqueous solution, stirred for 30 minutes, and a layer of PEI polycations was adsorbed on the surface of the microspheres. Then magnetic separation, repeated washing to remove excess PEI. The PEI solution contained 0.5M NaCl as a supporting electrolyte, and the pH of the solution was adjusted to 8.5. (ii) The magnetic microspheres adsorbed by PEI were added to 10mL of the pre-synthesized Au solution, stirred for 30min, and a layer of gold nanoparticles was adsorbed on the surface of the PEI polycation, and the above separation and washing process was repeated to obtain a PEI/ Au bilayer magnetic microspheres. Two steps (i) and (ii) were repeated to obtain magnetic microspheres assembled with different numbers of PEI/Au bilayers, and the loading of gold nanoparticles was continuously adjustable from 1.27% to 5.70%.

Example 4: Catalytic performance test of magnetic microspheres loaded with gold nanoparticles in the reduction reaction of p-nitrophenol

Magnetic microspheres containing 0.75×10 ^-3 mmol loaded gold nanoparticles were added to a mixed solution containing 1.5mL p-nitrophenol (0.01M), 100mL water and 1.5mL NaBH ₄ (1M), and the electrokinetics were continuously performed at 25°C. The reaction was stirred, samples were taken every 5 minutes, the catalyst was magnetically separated, and the ultraviolet-visible spectrum of the solution was tested. The reaction was completed within 20 min, and the color of the solution changed from bright yellow to colorless.

Catalytic activity evaluation: After adding NaBH ₄ to p-nitrophenol, the wavelength 400nm in the ultraviolet-visible spectrum corresponds to the absorption of p-nitrophenolate. The intensity of this absorption peak decreases as the reaction progresses. Since an excess of NaBH ₄ was added during the reaction, the reaction can be approximately regarded as a first-order reaction. Take the negative logarithm of the light absorption value measured at the wavelength of 400nm, and plot the graph with time as the abscissa. After curve fitting, the slope of the obtained straight line is the reaction rate of the reduction reaction. In the blank experiment, only the same volume and concentration of the reaction substrate was added without catalyst. Ultraviolet-visible spectroscopy was characterized by a Lambda-35 ultraviolet-visible spectrometer produced by PerkinElmer in the United States.

Without catalyst, the reaction proceeds extremely slowly. After adding a small amount of catalyst, the reaction proceeded rapidly, and the absorption peak at 400nm decreased sharply. The reaction rate was calculated according to the slope of the -lnA ₄₀₀ /t curve. After adding the catalyst, the reaction rate increased nearly 90 times.

Example 5: Catalytic Performance Test of Magnetic Microspheres Loaded with Gold Nanoparticles in the Reduction of Potassium Ferricyanate

Magnetic microspheres containing 1.0×10 ^-3 mmol gold nanoparticles were added to a solution containing 2mL K ₃ [Fe(CN) ₆ ] (0.01M), 30mL H ₂ O and 2mL Na ₂ S ₂ O ₃ (0.1M), At 40°C, the reaction was continuously electric stirring for 90 minutes, and samples were taken every 10 minutes, and the ultraviolet-visible spectrum of the solution was tested by magnetic separation of the catalyst. Catalytic activity evaluation was carried out according to the method of Example 4. Because excessive sodium thiosulfate is added in the reaction process, so this reaction can also be approximately regarded as a first-order reaction. The characteristic absorption peak of K ₃ [Fe(CN) ₆ ] is at 420nm. In the experiment, after taking the negative logarithm of the light absorption value at 420nm wavelength measured in the experiment, the abscissa is plotted with time. After curve fitting, the slope of the obtained straight line is the reaction rate of the reduction reaction. Without the addition of a catalyst, the reaction is almost impossible to proceed. After adding the catalyst, the reaction proceeded smoothly, and the absorption peak at 420 nm gradually decreased. The reaction rate was calculated from the slope of the -lnA ₄₂₀ /t curve. After adding the catalyst, the reaction rate increased by nearly 8 times.

Example 6: Recycling of magnetic microspheres loaded with gold nanoparticles in two reaction systems

After a catalytic experiment was completed according to Examples 4 and 5, the catalyst in the solution was separated by an external magnetic field, washed and dried repeatedly with ultrapure water. Repeat the above experimental steps to enter multiple cycles. In the present invention, the catalyst is recycled 5 times. After 5 times of use, the catalytic performance of the catalyst remained above 90%. Therefore, the magnetically separable gold nanocatalyst in the present invention has a stable structure and excellent recyclability.

Claims (8)

1. A magnetically separable noble metal catalyst, comprising a superparamagnetic magnetic carrier and catalytically active noble metal nanoparticles, characterized in that: the magnetic carrier is a microsphere with a core-shell structure, and the noble metal nanoparticles are gold nanoparticles. 2. The precious metal catalyst as claimed in claim 1, characterized in that: the microspheres of the core-shell structure, the core is ferric oxide particles with a particle diameter of 300-360nm, and the shell layer is a microsphere with a thickness of 20-25nm. silica. 3. The noble metal catalyst according to claim 1, characterized in that: the gold nanoparticles are spherical in shape and have a particle diameter of about 12 nm. 4. The preparation method of a magnetically separable noble metal catalyst as claimed in claim 1, characterized in that: a layer-by-layer self-assembly method is adopted. 5. The use of the magnetically separable noble metal catalyst according to claim 1, 2 or 3, characterized in that the catalyst is used to catalyze organic and inorganic reduction reactions. 6. purposes as claimed in claim 5, is characterized in that: described organic reduction reaction is the reaction of p-nitrophenol and sodium borohydride, and inorganic reduction reaction is the reaction of potassium ferricyanate and sodium thiosulfate. 7. A magnetically separable noble metal catalyst as claimed in claim 1, characterized in that: an external magnetic field is used to rapidly separate and recycle the catalyst. 8. A magnetically separable noble metal catalyst as claimed in claim 1, characterized in that: after five cycles of use, the catalytic activity remains above 90%. the

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