CN103007978A - Nanometer metal catalyst as well as preparation method and application thereof - Google Patents

Abstract

The invention provides a nanometer metal catalyst, comprising: 1) a composite carrier, which is a molecular sieve with one or more layers of _TiO2 attached to the surface; 2) a catalytic active center, which is one or more metals, and the Metal and TiO ₂ on the composite support form a metal-TiO ₂ structure. The invention also provides a preparation method of the catalyst and the application of the nanometer metal catalyst in catalyzing the oxidation reaction of cyclohexane to generate cyclohexanol and cyclohexanone. The preparation process of the catalyst of the present invention does not require high-temperature calcination, which can avoid agglomeration or sintering of gold particles under high-temperature conditions, and does not require reduction by a reducing agent; and the obtained nano-metal catalyst has uniform nano-noble metal particles, good dispersibility, and good catalytic performance; The loading of gold is low, and the catalytic activity is high; the metal particles are well dispersed, and the particles are only 2-10nm, and the combination is firm.

Description

A kind of nano metal catalyst and its preparation method and application

technical field

The present invention relates to the field of catalysts, in particular to a nano-precious metal catalyst and its preparation method and application, in particular to a nano-precious metal catalyst for the selective oxidation of cyclohexane to prepare cyclohexanol and cyclohexanone and its preparation method and application . the

Background technique

The selective oxidation reaction of cyclohexane to prepare cyclohexanol and cyclohexanone, commonly known as KA oil, is a very important reaction in industry. The oxidation product cyclohexanone is the key intermediate for the synthesis of monomer caprolactam and adipic acid of nylon-6 and nylon-66. Body, widely used in fiber, synthetic rubber, industrial coatings, medicine, pesticides, organic solvents and other industries. the

At present, there are three main industrial production processes of cyclohexanone and cyclohexanol in the world: phenol hydrogenation method, cyclohexane liquid phase oxidation method and cyclohexene hydration method, of which 90% of KA oil uses cyclohexane produced by oxidation. In this process, the conversion rate of cyclohexane is mostly controlled at about 4% to obtain a selectivity of 75-85% cyclohexanol and cyclohexanone. Due to the low conversion rate of cyclohexane and high energy consumption, and the waste lye generated in the production process has caused environmental pollution, it is urgent to develop a catalyst with high efficiency, high selectivity and environmental friendliness. the

Gold has always been considered as a chemically inert metal, but since Haruta et al. reported that the supported gold catalyst has excellent catalytic activity for the preferential oxidation of CO, the gold catalyst has its unique low-temperature catalytic activity, and its activity increases with the increase of humidity. The relatively low price and other characteristics have attracted the attention of scholars from various countries. Subsequent studies have found that gold catalysts have ideal catalytic activity in many reactions such as NO reduction, CO preferential oxidation, hydrocarbon oxidation, propylene epoxidation, and selective hydrogenation. In recent years, the catalytic performance of nano-gold catalysts in the selective oxidation of cyclohexane has also been studied. The Au/ZSM-5 and Au/MCM-41 catalysts synthesized by Chinese scholars such as Zhao Rui and others by the hydrothermal synthesis method catalyze the oxidation of cyclohexane at 150°C, and the selectivity of cyclohexanol and cyclohexanone is above 90%. , the conversion rate of cyclohexane can reach up to 16%; Zhou Jiji and Zhao Hong prepared a highly active Ag/MCM-41 catalyst by one-step hydrothermal synthesis, using molecular oxygen as the oxidant, at 140°C, 1.4MpaO ₂ Under the conditions of reaction for 3 hours, the conversion rate of cyclohexane reached 10.7%, and the total selectivity of cyclohexanol and cyclohexanone was 83.4%. MD Hughes et al. prepared the Au/C catalyst by in-situ reduction method, and found that Au/C had a good Cyclohexane oxidation is still catalytically active. These all show that the nano-gold catalyst has good catalytic performance for the selective oxidation of hydrocarbons, especially cyclohexane. Its disadvantages: the prepared gold catalyst tends to cause a large number of particles to gather during the reduction, so that the average particle size is above 20nm, the dispersion degree is low, and a highly active supported gold catalyst cannot be obtained.

Contents of the invention

The purpose of the present invention is to provide a nanometer metal catalyst with good catalytic performance for the selective oxidation reaction of cyclohexane, so as to overcome the shortcomings of the existing catalysts. the

To achieve the above object, the present invention provides a nano-metal catalyst, comprising:

A composite carrier, which is a molecular sieve with one or more layers of _TiO attached to the surface;

The catalytic active center is one or more metal nanoparticles, and the metal nanoparticles and the TiO ₂ on the composite support form a metal-TiO ₂ structure.

In a preferred version of the nano-metal catalyst of the present invention, the metal is selected from at least one of gold, silver, copper, ruthenium, rhodium, palladium, osmium, iridium or platinum, preferably gold. the

In a preferred embodiment of the nanometer metal catalyst of the present invention, the molecular sieve has a pore structure with a pore diameter of 0.5-0.73 nm, a specific surface area of 300-550 m ² /g, and a silicon-aluminum ratio of 30-100. , the channel mechanism is a two-dimensional or three-dimensional micropore.

The particle diameter of the metal particles in the nanometer metal catalyst of the present invention is 2-5nm. the

In a preferred version of the nano metal catalyst of the present invention, the molecular sieve is ZSM-5 molecular sieve, ZSM-11 molecular sieve, ZSM-12 molecular sieve, ZSM-23 molecular sieve, β molecular sieve, MCM-22 molecular sieve, MCM-49 type molecular sieve or MCM-56 type molecular sieve; preferably MCM-22 type molecular sieve with a specific surface area of 450-520 m ² /g.

The MCM-22 molecular sieve is synthesized by a static hydrothermal method, using hexamethylimine as a template (HMI), and tetraethyl orthosilicate as a silicon source (TEOS). SiO ₂ :Al ₂ O ₃ :NaOH:HMI:H ₂ O 1:0.033:0.25:0.6:20 Stir evenly, put into a stainless steel reaction kettle with PTFE lining, 158°C hydrothermal crystallization for 7d After the crystallization is complete, filter, wash, dry, and calcinate in air at 550°C for 5 hours to remove HMI, and obtain molecular sieve MCM-22.

In a preferred embodiment of the nano-metal catalyst of the present invention, the particle diameter of the metal particles in the nano-metal catalyst is 2-10 nm. the

The present invention also provides a kind of preparation method of nanometer metal catalyst, comprises the following steps:

i) adding and mixing a solution as a titanium source on the molecular sieve, and using a sol-gel method to disperse and attach TiO ₂ to the molecular sieve in a single-layer or multi-layer manner to form a composite carrier;

ii) immersing the composite carrier obtained in step i) in a solution containing metal ions, and irradiating with light, using the photocatalytic reduction performance of _TiO2 to reduce the metal ion to simple metal and load it on the composite carrier, And form a metal-TiO ₂ structure with the TiO ₂ ;

iii) filtering, washing and drying the product obtained in step ii), so as to obtain the nanometer metal catalyst. the

In the preparation method of the present invention,

The titanium source in step i) is a solution containing butyl titanate, titanium sulfate, titanium chloride or titanium oxychloride. the

The light source for the light irradiation described in step ii) is an ultraviolet light source. the

In the above-mentioned preparation method, also include:

Before adding the solution as a titanium source on the molecular sieve, the molecular sieve is diluted with distilled water, ultrasonically oscillated and stirred in sequence, and then the solution as a titanium source is added to the molecular sieve obtained through the above treatment. the

An application of the above-mentioned nanometer metal catalyst in the reaction of catalyzing the oxidation of cyclohexane to generate cyclohexanol and cyclohexanone. By adopting the nanometer noble metal catalyst, the catalytic cyclohexane conversion rate is 9-12%, and the product selectivity is 91-95%. the

Compared with the prior art, the invention has the following advantages: the nanometer noble metal catalyst AuTiO ₂ /MCM-22 is prepared by photocatalytic reduction method.

1. This method uses the principle of photocatalytic reduction to reduce all gold ions to elemental gold, and as long as the light reaction time is long enough, the loading rate of gold can be close to or even reach 100%;

2. Eliminates the difficult-to-control operation process, does not require high-temperature calcination to avoid agglomeration or sintering of gold particles under high-temperature conditions, and does not require reducing agent reduction;

3. The nano-noble metal particles of the nano-noble metal catalyst are uniform, well-dispersed, and have good catalytic performance;

4. Low loading of gold and high catalytic activity;

5. The metal particles have good dispersion, the particles are only 2-10nm, and the combination is firm;

6. The preparation conditions are mild, easy to operate and control the particle size;

7. The preparation process is simple; the catalytic performance of the nano-noble metal catalyst AuTiO ₂ /MCM-22 prepared by using molecular oxygen as the oxidant to catalyze the selective oxidation of cyclohexane was investigated. The nano-gold catalyst prepared by this method has high catalytic activity and good stability.

Detailed ways

The implementation of the present invention will be described in detail below in conjunction with the examples, so as to fully understand and implement the implementation process of how to apply technical means to solve technical problems and achieve technical effects in the present invention. It should be noted that, as long as there is no conflict, each embodiment and each feature in each embodiment of the present invention can be combined with each other, and the formed technical solutions are all within the protection scope of the present invention. the

Nano metal catalyst of the present invention:

1) Composite carrier, which is a molecular sieve with one or more layers of _TiO2 attached to the surface;

2) The catalytic active center is one or more metal elements, and the metal elements and TiO ₂ on the composite support form a metal-TiO ₂ structure;

Wherein, in the above-mentioned nano-metal catalyst, by 100 parts by mass, it includes:

Molecular sieve 92.0-98.5 parts by mass;

TiO ₂ 1.0-5.5 parts by mass;

Elemental Metal Balance

Moreover, the particle diameter of the metal particles in the nanometer metal catalyst is 2-10 nm. the

The metal is at least one selected from gold, silver, copper, ruthenium, rhodium, palladium, osmium, iridium or platinum, preferably gold. the

The molecular sieve has a regular pore structure, two-dimensional or three-dimensional micropores, a pore diameter of 0.5-0.73nm, a specific surface area of 300-550m ² /g, and a silicon-aluminum ratio of 30-100.

The molecular sieve is ZSM-5 molecular sieve, ZSM-11 molecular sieve, ZSM-12 molecular sieve, ZSM-23 molecular sieve, β molecular sieve, MCM-22 molecular sieve, MCM-49 molecular sieve or MCM-56 molecular sieve; preferably MCM-22 molecular sieve with a specific surface area of 450-520m2/g. the

A preparation method of nano metal catalyst, comprising the following steps:

i) adding and mixing a solution as a titanium source on the molecular sieve, and using a sol-gel method to disperse and attach TiO ₂ to the molecular sieve in a single-layer or multi-layer manner to form a composite carrier;

ii) impregnating the composite support obtained in step i) in a solution containing metal ions, and irradiating with light, using the photocatalytic reduction performance of TiO ₂ to reduce the metal ions to simple metals loaded on the composite support, and with The TiO ₂ forms a metal-TiO ₂ structure;

iii) filtering, washing and drying the product obtained in step ii), so as to obtain the nanometer metal catalyst. the

in,

The titanium source in step i) is a solution containing butyl titanate, titanium sulfate, titanium chloride or titanium oxychloride. the

The light source for the light irradiation described in step ii) is an ultraviolet light source. the

Step ii) adding the metal ion solution into HAuCl ₄ solution.

Example 1

1) Preparation of composite carrier:

Prepared by existing technology: microporous molecular sieve MCM-22 is synthesized by static hydrothermal method, using hexamethylimine as template (HMI), and tetraethyl orthosilicate as silicon source (TEOS), at a certain temperature The molar ratio of reactants SiO ₂ :Al ₂ O ₃ :NaOH:HMI:H ₂ O is 1:0.033:0.25:0.6:20 and stirred evenly, put into a stainless steel reaction kettle lined with polytetrafluoroethylene, 158°C After hydrothermal crystallization for 7 days, after the crystallization is complete, filter, wash, dry, and calcinate in air at 550°C for 5 hours to remove HMI and obtain molecular sieve MCM-22. Take 1.0g of calcined MCM-22, 15mL of absolute ethanol, and a certain amount of butyl titanate and mix thoroughly to form a suspension A, then mix 5mL of absolute ethanol and a certain amount of deionized water to form a solution B, and then dissolve the solution B slowly Add dropwise to solution A to form solution C, adjust the pH of solution C to a certain value with 1mol/L nitric acid, continue to stir until a stable and transparent gel is formed, age at room temperature for 24 hours, dry at 90°C for 12 hours, and calcine at a constant temperature of 550°C for 3 hours , to obtain a white powder, which is TiO ₂ /MCM-22 composite carrier.

2) Catalyst preparation:

AuTiO ₂ /MCM-22 nano-gold catalyst containing 0.5% Au mass fraction was prepared.

Add 1.0g of the calcined TiO ₂ /MCM-2 obtained in step 1) into 100mL distilled water and ultrasonically shake for 10min; then continue to add distilled water to dilute to 900mL; and magnetically stir for 1h to make it evenly mixed; then add 0.5mL0.05moL /L of HAuCl ₄ solution was mixed for 1 hour; the mixture was placed under a 15W UV lamp for irradiated reaction for 4 hours (without light, deposition-precipitation reaction for 4 hours is the deposition-precipitation method); finally, the obtained product was filtered and washed for 90 °C to obtain a finished AuTiO ₂ /MCM-22 nano-gold catalyst with a gold mass fraction of 0.5%.

Parameters of the preparation method

Example 2

Same as Example 1, except that the addition amount of HAuCl ₄ is 0.1mL0.05moL/L.

Example 3

Same as Example 1, except that the addition amount of HAuCl ₄ is 0.2mL0.05moL/L.

Example 4

Same as Example 1, the difference is that the amount of HAuCl ₄ added is 1.0mL0.05moL/L.

Example 5

Same as Example 4, the difference is that the reaction time of irradiation under the ultraviolet lamp is 2h. the

Example 6

Same as Example 4, the difference is that the reaction time of irradiation under the ultraviolet lamp is 6h. the

Example 7

Same as Example 4, the difference is that the reaction time of irradiation under the ultraviolet lamp is 7h. the

Comparative example 1

Same as Example 1, the difference is that in step 2), the mixed solution is not exposed to light, but directly undergoes deposition-precipitation reaction for 4 hours (sedimentation-precipitation method). That is, metal ions are directly deposited on the surface of the support without reducing the metal ions to simple substances by the photocatalyst of _TiO2 . Comparative example 2

Directly use the deposition-precipitation method to deposit the chloroauric acid aqueous solution on the carrier MCM-22 molecular sieve to prepare the gold content of 0.5%Au/MCM-22;

According to Example 1-7 and Comparative Example 1-2, the nano-gold catalyst AuTiO ₂ /MCM-22 prepared with gold loading methods, different gold loading amounts, and different light exposure times and the loading rate of the gold are shown in Table 1, using the American IRIS Intrepid II The actual mass fraction of gold in the above nano-gold catalyst samples measured by XSP inductively coupled plasma atomic emission spectrometer (ICP-AES) is shown in Table 2.

Table 1

Table 2

Example 8

The nano-gold catalysts prepared in Examples 1-7 and Comparative Examples 1-2 above were used to catalyze the selective oxidation reaction of cyclohexane. The reaction was carried out in a 300mL high-pressure reactor, and the catalyst (see Table 3 for the specific addition amount), 100mL cyclohexane and 0.4g65%(ω)TBHP were added in sequence, and stirred at a high speed. When the temperature in the kettle rose to the temperature required for the experiment, _O2 was introduced to maintain the pressure at 1.0MPa, and the reaction was performed for 1h. After the reaction was over, the reaction solution was cooled to room temperature with ice water, and the contents of cyclohexanol and cyclohexanone in the product were analyzed by the Agilent 6890N gas chromatograph using the internal standard method with chlorobenzene as the internal standard, and analyzed by the iodometric method. Cyclohexyl hydroperoxide, acid and ester analysis by acid-base titration. The results are shown in Table 3.

The nanometer noble metal catalyst is used in the reaction of catalyzing the oxidation of cyclohexane to generate cyclohexanol and cyclohexanone, and the conversion rate of the catalytic cyclohexane is 9-12%, and the product selectivity is 90-95%. The results of the oxidation reaction of cyclohexane with different catalysts are shown in Table 3. the

table 3

Note: K is cyclohexanone, A is cyclohexanol, CHHP is cyclohexyl hydroperoxide.

Claims (10)

1. A nanometer metal catalyst, comprising: A composite carrier, which is a molecular sieve with one or more layers of _TiO attached to the surface; The catalytic active center is one or more metal nanoparticles, and the metal nanoparticles and the TiO ₂ on the composite support form a metal-TiO ₂ structure. 2. The catalyst according to claim 1, wherein the metal is selected from at least one of gold, silver, copper, ruthenium, rhodium, palladium, osmium, iridium or platinum, preferably gold. 3 . The catalyst according to claim 1 , wherein the molecular sieve has a pore structure with a pore diameter of 0.5-0.73 nm, a specific surface area of 300-550 m ² /g, and a silicon-aluminum ratio of 30-100. 4. The catalyst according to claim 3, wherein the molecular sieve is ZSM-5 molecular sieve, ZSM-11 molecular sieve, ZSM-12 molecular sieve, ZSM-23 molecular sieve, beta molecular sieve, MCM-22 type Molecular sieves, MCM-49 molecular sieves or MCM-56 molecular sieves; preferably MCM-22 molecular sieves with a specific surface area of 450-520 m ² /g. 5. The catalyst according to any one of claims 1-4, characterized in that, the particle diameter of the metal particles in the nano-metal catalyst is 2-10 nm. 6. A preparation method of the nano-metal catalyst according to any one of claims 1-5, comprising the following steps: i) adding and mixing a solution as a titanium source on the molecular sieve, and using a sol-gel method to disperse and attach TiO ₂ to the molecular sieve in a single-layer or multi-layer manner to form a composite carrier; ii) immersing the composite carrier obtained in step i) in a solution containing metal ions, and irradiating with light, using the photocatalytic reduction performance of _TiO2 to reduce the metal ion to simple metal and load it on the composite carrier, And form a metal-TiO ₂ structure with the TiO ₂ ; iii) filtering, washing and drying the product obtained in step ii), so as to obtain the nanometer metal catalyst. 7. The method according to claim 6, characterized in that the titanium source in step i) is a solution containing butyl titanate, titanium sulfate, titanium chloride or titanium oxychloride. 8. The method according to claim 6, characterized in that, the light source for the light irradiation in step ii) is an ultraviolet light source. 9. according to the method described in any one in claim 6-8, it is characterized in that, described preparation method also comprises: Before adding the solution as a titanium source on the molecular sieve, the molecular sieve is diluted with distilled water, ultrasonically oscillated and stirred in sequence, and then the solution as a titanium source is added to the molecular sieve obtained through the above treatment. 10. The application of a nanometer metal catalyst according to any one of claims 1-5 in the reaction of catalyzing the oxidation of cyclohexane to generate cyclohexanol and cyclohexanone.