CN108067294A - The catalyst and preparation method of package gold nano grain in molecular sieve crystal - Google Patents

Abstract

The invention discloses a catalyst and a preparation method for wrapping gold nanoparticles in molecular sieve crystals, and belongs to the field of metal molecular sieve catalyst preparation. The catalyst is prepared by a direct hydrothermal method and can be used in high-temperature carbon monoxide oxidation reactions. The preparation steps of the catalyst are as follows: hydrolyze silicon source with acid, then add aluminum source, sodium hydroxide and metal precursor, stir evenly, place it in a dynamic hydrothermal kettle for crystallization, wash and filter the crystallized product and drying to prepare the Au@ZSM-5 molecular sieve catalyst. In the method of the present invention, dynamic crystallization is adopted to promote the self-assembly of gold nanoparticles into the inside of the molecular sieve crystal, no organic template agent is used during the synthesis process, and no crystal seeds need to be added, and the synthesis time is greatly shortened. In the Au@ZSM-5 catalyst prepared by the present invention, the gold nanoparticles are wrapped inside the ZSM-5 molecular sieve crystal, and the initial size can be maintained without aggregation, growth and deactivation after continuous reaction for 8 days in the high-temperature carbon monoxide reaction.

Description

Catalyst and preparation method for encapsulating gold nanoparticles in molecular sieve crystals

Technical field:

The invention belongs to the field of metal molecular sieve catalyst preparation, and relates to a highly stable Au@ZSM-5 catalyst with gold nanoparticles wrapped in molecular sieve crystals, a preparation method and its application in high-temperature carbon monoxide oxidation reaction.

Background technique:

Since the Japanese scientist Haruta et al. (Chem.Lett.,1987,16,405; J.Catal.,1989,115,301) discovered that gold nanoparticles have amazing activity in the low-temperature oxidation reaction of carbon monoxide, the research on gold nanocatalysts has become a catalyst. The hot spot in the field has now been extended to important reactions such as propylene epoxidation, selective oxidation, selective hydrogenation and C-C coupling. A large number of studies have shown that the size of gold particles is an important factor affecting the catalytic activity of gold (Science, 2008, 321, 1331). The reduction of the size of gold particles can lead to great changes in its physical and chemical properties such as electronic structure, crystal structure, atomic ratio of surface and bulk phase, and then produce unique catalytic properties. However, fatal defects such as the extremely easy aggregation and growth of gold particles at high temperatures and poor reaction stability limit its application, which has become an urgent problem to be solved in the application of gold catalysts.

Molecular sieves have regular pore structures and cages, large specific surface area, high thermal stability, etc., so they have become widely recognized metal particle catalyst supports. The gold nanoparticles introduced into molecular sieves by conventional post-treatment methods (impregnation method, ion exchange method, etc.) are simply attached to the surface or channels of molecular sieves, so the high-temperature treatment steps required for the preparation of molecular sieves or the high-temperature conditions in the reaction are still It will cause the migration and aggregation of gold nanoparticles, which will greatly reduce the activity and reusability of the catalyst. In order to solve this problem, researchers have devoted themselves to assembling metal nanoparticles into molecular sieve crystals, so that the metal nanoparticles are restricted by the molecular sieve framework to improve the anti-sintering performance. However, it is very difficult to introduce metal nanoparticles into the synthesis of molecular sieve crystals. Recently, Egeblad et al. dispersed metals into silicon sources, and directly assembled gold into Silicalite-1 molecular sieve crystals under the guidance of organic templates (Angew.Chem.Int.Ed., 2010, 49, 3504); Xiao Fengshou et al. introduced palladium (Pd) nanoparticles into Beta and MOR molecular sieve crystals by seed crystal method (Angew.Chem.Int.Ed., 2017). The catalysts synthesized by these methods all have excellent properties, but organic templates or seeds are inevitably used in the synthesis process. However, the direct dynamic hydrothermal method without organic templates and seeds has not been reported so far to obtain molecular sieve catalysts coated with gold nanoparticles in crystals.

ZSM-5 molecular sieve has unique three-dimensional straight channels, high silicon-aluminum ratio, and is widely used in industry. The gold nanoparticles are introduced into the ZSM-5 molecular sieve crystal, and the three-dimensional pores of the ZSM-5 molecular sieve form a restriction effect on the gold nanoparticles, which can effectively improve the anti-sintering performance and reaction stability of the gold nanoparticles. Therefore, it is of great significance to use a dynamic one-step hydrothermal green synthesis method without organic templates and seeds to prepare Au@ZSM-5 catalysts with highly stable molecular sieve crystals encapsulated with gold nanoparticles.

Invention content:

The object of the present invention is to provide a catalyst and a preparation method of gold nanoparticles encapsulated in molecular sieve crystals, which are directly prepared by a one-step dynamic hydrothermal synthesis method, and the synthesis method does not require the use of organic templates and seed crystals.

In the present invention, the gold source is mixed with the initial silica-alumina gel of the synthetic molecular sieve, and the gold precursor is combined with the primary structural unit in the initial silica-alumina gel, and then the primary structural unit containing gold is continuously self-assembled during the process of dynamic constant temperature heating and stirring Growth and dynamic stirring promote the uniform dispersion of gold in the synthetic gel, and finally generate Au@ZSM-5 molecular sieves with gold nanoparticles wrapped in crystals. The Au@ZSM-5 molecular sieve catalyst prepared by the present invention has ultra-high stability in the high-temperature oxidation reaction of carbon monoxide, and the particle size and catalytic performance remain unchanged for 8 days of continuous reaction, and has strong anti-sintering performance and reaction stability.

The present invention provides a catalyst in which gold nanoparticles are encapsulated in molecular sieve crystals. In the catalyst, the gold nanoparticles are encapsulated and embedded in the crystal of molecular sieve ZSM-5, and the gold nanoparticles are restricted by the pores of molecular sieves to improve their anti-sintering performance. , maintaining high stability in high temperature reactions; the catalyst is prepared using an acid hydrolyzed silicon source, neither an organic template nor a seed crystal, and a dynamic crystallization method is used in the preparation process.

Provided by the present invention is a method for preparing a catalyst that encapsulates gold nanoparticles in molecular sieve crystals. The specific steps of the method are as follows:

(1) hydrolyzing the mixture of silicon source and water in an acidic medium, stirring at a temperature of 20-95° C. for 2-12 hours to obtain a hydrolyzed mixture solution;

(2) Add the mixed solution of aluminum source and water into the mixture solution obtained in step (1), mix well and then add inorganic base to adjust the pH value of the mixed solution to alkaline, and stir the obtained mixture at a temperature of 20-60°C Aging 1 ~ 15h;

(3) adding a gold precursor to the aged mixture in step (2), stirring for 5-60 minutes to obtain a reaction mixture, the molar ratio of the total composition of the reaction mixture is: SiO ₂ /Al ₂ O ₃ =20-100, Au/SiO ₂ =0.05-0.4, H ₂ O/SiO ₂ =10-60;

(4) Transfer the reaction mixture obtained in step (3) to a stainless steel dynamic reaction kettle with a polytetrafluoroethylene lining, then heat the stainless steel dynamic reaction kettle to a temperature of 100 to 180° C., and use dynamic constant temperature crystallization, After crystallization for 8-72 hours, the product was taken out of the reactor, cooled to room temperature, washed, filtered, and dried in an oven at 80-120°C to obtain a solid product Au@ZSM-5 molecular sieve with gold nanoparticles encapsulated in the crystal.

The silicon source is any one of methyl orthosilicate, ethyl orthosilicate, silica sol and silicon dioxide; the acid medium is an aqueous solution of sulfuric acid, hydrochloric acid, phosphoric acid or nitric acid, and the pH value is 1.0-6.0.

The aluminum source is any one of aluminum nitrate, sodium aluminate, aluminum sulfate and aluminum isopropoxide. The gold precursor is chloroauric acid or gold chloride. The method of dynamic crystallization is any one of dynamic magnetic stirring, dynamic electric propeller stirring and dynamic rotating stirring

The preparation method of the present invention does not use organic templates and seed crystals, avoiding the heat energy consumption and environmental pollution caused by roasting, and the preparation process is more environmentally friendly; the dynamic stirring crystallization method adopted in the present invention can effectively promote the self-growth of gold nanoparticles Assembled into the inside of the molecular sieve crystal, and the gold nanoparticles have small particle size and uniform distribution; in the prepared Au@ZSM-5 molecular sieve catalyst, the gold nanoparticles are well encapsulated inside the molecular sieve crystal, and the gold nanoparticles are well encapsulated in the high temperature oxidation reaction of carbon monoxide. High anti-sintering performance and reaction stability.

Description of drawings:

Fig. 1 is the XRD pattern of molecular sieve sample, wherein a is the XRD pattern of the molecular sieve sample obtained in Example 1, and b is the XRD pattern of the molecular sieve sample obtained in Example 3;

Fig. 2 is the HRTEM figure of the molecular sieve sample obtained in Example 1, wherein Fig. 2b is an enlargement of Fig. 2a;

Fig. 3 is the performance test of the molecular sieve sample in the embodiment example 1 in the oxidation reaction of carbon monoxide at 400°C;

Fig. 4 is the HRTEM of the molecular sieve sample in Example 1 after being oxidized with carbon monoxide for 8 days.

Detailed ways:

The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited thereby.

Example 1: Take 10 g of tetraethyl orthosilicate, add 32 ml of 0.12 M hydrochloric acid aqueous solution therein, adjust the hydrolysis pH value of the solution to 1.0, stir and hydrolyze at 20 ° C for 12 hours, then add 0.75 g of aluminum sulfate and 0.8 g of hydrogen Sodium oxide, after stirring and aging for 10h, add 0.1g of chloroauric acid to it, and stir for 0.5h, so that the molar ratio of the total composition of the mixture is SiO ₂ :0.05Al ₂ O ₃ :40H ₂ O:0.005Au, and then it is packaged Put it into a high-pressure dynamic hydrothermal reactor, crystallize at 180°C for 48 hours, wash the obtained product until neutral, centrifuge, dry, and roast to obtain Au@ZSM-5 molecular sieve. Its X-ray diffraction pattern (XRD) is shown in a in Fig. 1 .

Catalyst evaluation method: The catalytic oxidation reaction of carbon monoxide is carried out in a fixed-bed quartz reactor. The amount of catalyst used was 10mg, the feed gas was carbon monoxide and oxygen, the flow rate of the mixed gas was 33mL/min, and the reaction temperature was 400°C. According to the activity test, the activity of the Au@ZSM-5 catalyst prepared by this method remained unchanged after continuous reaction at 400 °C for 8 days. Its carbon monoxide reaction performance test is shown in Figure 3. After the catalyst was continuously reacted at 400° C. for 8 days, the size of the Au nanoparticles wrapped in the molecular sieve crystals remained unchanged, and its HRTEM diagram is shown in FIG. 4 .

Example 2: According to the operating steps and operating conditions of Example 1, the difference is that the addition of chloroauric acid is increased to 0.6g, the crystallization time is 72h, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 3: According to the operation steps and operating conditions of Example 1, the difference is that the silicon source is methyl orthosilicate, the acid solution for hydrolyzing the silicon source is hydrochloric acid, and the final product obtained is Au@ZSM-5 catalyst. The XRD pattern of the product is shown in b in Figure 1. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 4: According to the operation steps and operating conditions of Example 1, the difference is that the silicon source is silica sol, the acid solution for hydrolyzing the silicon source is nitric acid, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 5: According to the operation steps and operating conditions of Example 1, the difference is that the silicon source is silicon dioxide, the crystallization time is 50 h, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400°C for 8 days.

Example 6: According to the operating steps and operating conditions of Example 1, the difference is that the aluminum source is sodium aluminate, the acid solution for hydrolyzing the silicon source is phosphoric acid, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 7: According to the operating steps and operating conditions of Example 1, the difference is that the aluminum source is aluminum isopropoxide, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 8: According to the operation steps and operating conditions of Example 1, the difference is that the aluminum source is aluminum nitrate, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 9: According to the operation steps and operating conditions of Example 1, the difference is that the hydrolysis medium of the silicon source is hydrochloric acid solution, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Example 10: According to the operation steps and operating conditions of Example 1, the difference is that the gold source is gold chloride, and the final product obtained is Au@ZSM-5 catalyst. The evaluation method of the catalyst is the same as in Example 1. The activity test shows that the activity of the Au@ZSM-5 catalyst prepared by this method remains unchanged in the carbon monoxide oxidation reaction at 400 °C for 8 days.

Claims (6)

1. A kind of 1. catalyst of package gold nano grain in molecular sieve crystal, it is characterised in that gold nano described in the catalyst Grain is wrapped the crystals for being embedded in molecular sieve ZSM-5, and gold nano grain is limited be subject to molecular sieve pore passage and improves it and resist Sintering character keeps high stability in pyroreaction；Using sour water solution silicon source in the catalyst preparation, both without using organic mould Plate agent, it is not required that using crystal seed, in preparation process by the way of dynamic crystallization.
2. 2. the preparation method of catalyst described in claim 1, it is characterised in that this method is as follows：

   (1) mixture of silicon source and water is placed in acid medium and hydrolyzed, when stirring 2~12 is small at 20~95 DEG C of temperature, obtained Mixture solution after hydrolysis；

   (2) mixed solution of silicon source and water is added in the mixture solution that step (1) obtains, added after mixing inorganic Alkali adjusts mixed liquor pH value to alkalescence, and obtained mixture, which is placed at 20~60 DEG C of temperature, stirs 1~15h of aging；

   (3) golden presoma is added in into the mixture after step (2) aging, obtains reaction mixture after stirring 5~60min, institute The molar ratio for stating reaction mixture total composition is：SiO₂/Al₂O₃=20-100, Au/SiO₂=0.05-0.4, H₂O/SiO₂=10- 60；

   (4) reaction mixture obtained by step (3) is transferred in the stainless steel dynamic response kettle equipped with polytetrafluoro liner, then will The stainless steel dynamic response kettle is heated at a temperature of 100~180 DEG C, will after 8~72h of crystallization using dynamic thermostatic crystallization Product takes out reaction kettle, washs product after being cooled to room temperature, filters, is dried to obtain in crystal and seals in 80~120 DEG C of baking ovens Fill the solid product Au@ZSM-5 molecular sieves of gold nano grain.
3. 3. preparation method according to claim 2, it is characterised in that the silicon source for methyl orthosilicate, ethyl orthosilicate, Any one in Ludox and silica；Acid medium is any one in the aqueous solution of sulfuric acid, hydrochloric acid, phosphoric acid or nitric acid Kind, pH value is 1.0~6.0.
4. 4. preparation method according to claim 2, it is characterised in that source of aluminium be aluminum nitrate, sodium aluminate, aluminum sulfate and Any one in aluminium isopropoxide.
5. 5. preparation method according to claim 2, it is characterised in that the gold presoma is gold chloride or chlorauride.
6. 6. preparation method according to claim 2, it is characterised in that the mode of the dynamic crystallization for dynamic magnetic agitation, Dynamically electronic propeller agitation and dynamic rotary stirring in any one.