JP2012187565A - Core-shell type catalyst and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a core-shell type catalyst in which the utilization efficiency of hydrogen peroxide is improved in a one pot reaction by controlling a positional relationship between a Pd site, which is active in synthesis of hydrogen peroxide, and a Ti site active in an oxidation reaction so that the produced hydrogen peroxide can be contacted with the Ti site before being diffused.SOLUTION: The core-shell type catalyst includes: a core comprising a granular substance; and a shell for covering the core. A surface layer portion of the core contains palladium and the shell contains titanium-containing silica having radial mesopores each extending to the surface of the shell from the core.

Description

  The present invention relates to a core-shell type catalyst that allows a one-pot reaction to proceed at a high rate and a method for producing the same.

  In recent years, hydrogen peroxide is widely used as an environmentally friendly oxidant because its only by-product is water. On the other hand, the current hydrogen peroxide synthesis method using anthraquinone has many problems such as requiring a large amount of energy and generating a large amount of organic waste. In view of this, research has been actively conducted to directly synthesize hydrogen peroxide from hydrogen gas and oxygen gas using a catalyst such as Pd. However, as shown below, since a side reaction in which hydrogen peroxide is decomposed into water also occurs with the generation of hydrogen peroxide, the efficiency is low and it has not been industrialized.

Therefore, a hydrogen peroxide synthesis catalyst such as Pd and an oxidation reaction catalyst such as TiO ₂ are added in one reaction vessel to simultaneously perform the synthesis of hydrogen peroxide and the oxidation reaction using the synthesized hydrogen peroxide. One-pot reactions have been attempted (Non-Patent Documents 1 to 3).

  One-pot reaction, in which two or more reactions are performed sequentially in one container, is an excellent synthesis method in that it does not require multiple separation / generation processes and can reduce the time required for the reaction. is there. In addition, in the one-pot reaction, even an unstable intermediate can be used for the reaction. Therefore, the one-pot reaction in which the generated hydrogen peroxide is immediately used for the oxidation reaction is interesting.

Green chem. 2008, 10, 934-938. Langmuir, 2009, 25, 11180-11187. Chem. Commun. 2010, 46, 6705-6707.

  In the reported examples so far, a catalyst in which Pd is supported on a titanium-containing zeolite which is an oxidation reaction catalyst, or a catalyst in which each is physically mixed has been used. However, when such a catalyst is used, the hydrogen peroxide produced on Pd diffuses into the solvent before coming into contact with the oxidation reaction site Ti, most of which is decomposed, and some peroxidation occurs. It is thought that only hydrogen is used for the oxidation reaction.

  In the one-pot reaction, the position of the Pd site active for the hydrogen peroxide synthesis and the Ti site active for the oxidation reaction so that the generated hydrogen peroxide can come into contact with the Ti site before diffusing into the solvent. The purpose is to control the relationship and improve the utilization efficiency of hydrogen peroxide.

  The present invention is a core-shell type catalyst in which the positional relationship between a Pd site active for hydrogen peroxide synthesis and a Ti site active for an oxidation reaction is controlled. Specifically, the present invention relates to a granular core, And a shell that covers the core, wherein a surface layer portion of the core contains palladium, and the shell contains titanium-containing silica having radial mesopores from the core toward the surface of the shell. Relates to the catalyst.

  The molar ratio of the titanium content to the silicon content contained in the shell: Ti / Si preferably satisfies 0.001 ≦ Ti / Si ≦ 0.05.

  The average particle diameter of the palladium is preferably 0.001 μm to 0.1 μm, and the palladium content is preferably 0.1 wt% to 5 wt%.

  The average pore diameter of the mesopores is preferably 2 nm to 50 nm.

  The average particle diameter of the core is preferably 0.1 μm to 1 μm.

  The present invention also includes (i) a step of supporting palladium on a surface layer portion of a granular core, and (ii) a silica precursor, a water-soluble titanium compound, and a mesopore template on the palladium-supporting core. A step of coating the core with titanium-containing silica having mesopores by causing the surfactant to act, (iii) by baking or refluxing the core coated with the titanium-containing silica in alcohol; And a step of removing the surfactant.

  The step (ii) includes, for example, a step of preparing a solution containing the silica precursor, the water-soluble titanium compound, and the surfactant, and a step of dispersing the core supporting the palladium in the solution. And including.

  According to the present invention, in the one-pot reaction, the utilization efficiency of the generated hydrogen peroxide for the oxidation reaction is increased.

It is a conceptual diagram which shows the structure of the core-shell type catalyst of this invention. 2 is a diagram showing an outline of a procedure for preparing catalyst samples of Example 1, Comparative Example 1 and Comparative Example 2. FIG. It is a TEM image of the catalyst of Example 1 (Pd / SiO ₂ @TiMSS) and the catalyst of Comparative Example 2 (SiO ₂ @Pd (S) / TiMSS). It is a figure which shows the pore diameter distribution analyzed by the nitrogen adsorption / desorption isotherm of the catalyst sample of Example 1, the comparative example 1, and the comparative example 2, and the BJH method. It is a figure which shows the result of the one pot oxidation reaction of the thioanisole using the catalyst sample of Example 1, the comparative example 1, and the comparative example 2. FIG.

  The present invention relates to a core-shell type catalyst comprising a granular core and a shell covering the core. The material of the core is not particularly limited, and for example, at least one selected from the group consisting of ceramic particles, carbon particles, and polymer particles can be used. Examples of the ceramic particles include silica particles and titania particles. As the carbon particles, activated carbon or the like is preferable. The core preferably contains no impurities from the viewpoint of suppressing side reactions. For example, when the core contains silica particles, the silica particles may contain elements other than silicon and oxygen, but it is desirable that the purity is 98% by weight or more.

  The average particle diameter of the core is preferably 0.1 μm to 1 μm, and more preferably 0.3 μm to 0.5 μm, from the viewpoints of diffusibility and recoverability. The average particle diameter of the silica particles is, for example, the particle diameter at 50% cumulative deposition obtained by a laser diffraction particle size distribution measuring device.

  The surface layer portion of the core contains palladium. Such a form is formed by supporting palladium on the core by various methods. Next, by covering the core carrying palladium with the shell, palladium is unevenly distributed at the boundary between the core and the shell of the core-shell type catalyst. Palladium has a high activity for producing hydrogen peroxide using hydrogen and oxygen as raw materials. In addition to palladium, elements such as gold and platinum may be further included in the surface layer of the core.

  The active element such as palladium is preferably carried on the surface layer portion of the core as nanoparticles in a metallic state. The average particle diameter of such nanoparticles is 0.001 μm to 0.1 μm, and can be measured by microscopic observation with a TEM or the like. From the viewpoint of suppressing reaction efficiency and irreversible aggregation during the reaction, the palladium content in the entire core-shell type catalyst is preferably 0.1% by weight to 5% by weight, and 0.5% by weight to 2%. More preferably, it is% by weight.

  The shell includes titanium-containing silica having radial mesopores from the core toward the surface of the shell. Titanium has a high activity for the oxidation reaction of organic substances by hydrogen peroxide. In addition, the mesopores are advantageous in terms of material diffusion, such as selectively taking in organic substances. Therefore, the hydrogen peroxide generated in the surface layer portion of the core always passes through the shell before diffusing to the outside of the catalyst, thereby increasing the chance of contact with the organic matter and Ti sites taken into the mesopores. This increases the utilization efficiency of hydrogen peroxide.

  The shell has a plurality of regularly arranged cylindrical mesopores, and the mesopores are radially arranged so as to go from the core toward the surface of the shell. The mesopores have a substantially uniform size and usually have an inner diameter of about 2 to 50 nm. The mesopore diameter depends on the type of surfactant (structure-regulating agent) used when forming the shell. Moreover, a radial mesopore structure is formed by appropriately controlling the amount of the surfactant added. The thickness of the shell may be, for example, 10 to 100 nm.

  The core-shell type catalyst of the present invention comprises (i) a step of supporting palladium on the surface layer of a core containing silica particles, and (ii) a silica precursor, a water-soluble titanium compound and mesopores on the core supporting palladium. A step of coating the core with titanium-containing silica having mesopores by applying a surfactant that becomes a template of (iii), and firing the core coated with the titanium-containing silica to remove the surfactant It can synthesize | combine with the manufacturing method which has a process to do.

  In step (i), palladium can be supported on the surface layer of the core by a method such as an ion exchange method, an impregnation method, or an equilibrium adsorption method. As a raw material of palladium, palladium chloride or the like can be used.

  In step (ii), for example, a silica precursor, a water-soluble titanium compound, and a surfactant are allowed to act on the core supporting palladium by a method such as a sol-gel method. As a result, a titanium-containing mesoporous silica layer having radially arranged mesopores is formed. More specifically, the core supporting palladium is dispersed in water, and then ethanol, ammonia and a structure-regulating agent are added, and further a solution in which a silica precursor and a titanium compound are mixed is added.

  Surfactants are known to form micelle aggregates in solvents. In a solution containing a silica precursor and a water-soluble titanium compound, the surfactants form micelle aggregates in a regular arrangement, so that the growth of silica in the solvent is regulated, and the titanium-containing mesoporous silica layer is the core. To cover

  Molar ratio of titanium content to silicon content contained in shell: Ti / Si preferably satisfies 0.001 ≦ Ti / Si ≦ 0.05, and satisfies 0.005 ≦ Ti / Si ≦ 0.2. More preferably. By making the Ti / Si ratio within the above range, the efficiency of the oxidation reaction can be increased.

  As a solvent of a solution containing a silica precursor, a water-soluble titanium compound, and a surfactant, it is preferable to use water or a mixture of water and alcohol. As the alcohol, for example, a lower alcohol having about 1 to 5 carbon atoms (for example, methanol, ethanol, propanol, etc.) can be used. An acid, a base, or the like may be added to the solvent as a catalyst.

  A core carrying palladium is put into the obtained solution, reacted at 10 ° C. to 40 ° C. (preferably 20 ° C. to 30 ° C.), and dried as necessary. The obtained material is calcined or refluxed in alcohol to remove the surfactant and obtain a core-shell type catalyst.

  The amount of the surfactant contained in the solution is, for example, preferably 0.1 to 3 mol or 0.5 to 1.5 mol per 100 mol of water. The amount of the silica precursor contained in the solution is preferably 3 to 20 mol or 5 to 10 mol per 100 mol of water, for example. What is necessary is just to select the quantity of the water-soluble titanium compound contained in a solution according to the titanium content in a desired shell composition. The amount of alcohol contained in the solution is, for example, preferably 10 to 40 mol or 15 to 30 mol per 100 mol of water.

  The surfactant is not particularly limited, and for example, cetyltrimethylammonium bromide can be used. These may be used alone or in combination of two or more. The molecular weight of the surfactant is preferably 250 to 3000 or 300 to 1000.

  As the silica precursor, tetraethyl orthosilicate (tetraethoxysilane), tetramethoxysilane, or the like can be used. Moreover, silicon chloride, silicate, etc. can also be used as a silica precursor. These may be used alone or in combination of two or more.

  Although it does not specifically limit as a water-soluble titanium compound, For example, a tetraethoxy titanium and a triisopropoxy titanium can be used. A water-soluble titanium compound may be used individually by 1 type, and may be used in combination of multiple types.

  In step (iii), the core coated with titanium-containing silica is baked to remove the surfactant. The firing temperature is preferably 450 ° C. to 650 ° C., and the firing atmosphere is preferably in air or oxygen. When the surfactant is removed by refluxing with alcohol, organic alcohols such as methanol, ethanol, propanol, isopropanol, and butanol can be used alone or in admixture of two or more. Regarding the extraction temperature, it is desirable to reflux at about the boiling point from room temperature to about the boiling point, preferably from the viewpoint of extraction speed. At this time, an acid such as hydrochloric acid or a base such as ammonium nitrate may be added to the alcohol.

  At least a part of Ti contained in the shell preferably has a tetracoordinate structure. Tetracoordinate structure Ti is considered to have high activity for oxidation reaction.

  Next, the present invention will be specifically described based on examples. However, the present invention is not limited to the following examples.

径０．３μｍ）にパラジウム粒子（平均粒子径０．００３μｍ）を担持させた。具体的には、シリカ粒子０．５ｇを２５０ｍｌの蒸留水に、超音波の照射下で分散させ、これに塩酸水溶液（濃度０．０２ｍｏｌ／Ｌ）１００ｍｌに塩化スズ（ＳｎＣｌ_２）を０．５ｇ溶かした溶液を加え、１０分間撹拌した。その後、粒子の洗浄を数回行った。An outline of the sample preparation procedure is shown in FIG.

Palladium particles (average particle diameter of 0.003 μm) were supported on the 0.3 μm diameter). Specifically, 0.5 g of silica particles is dispersed in 250 ml of distilled water under ultrasonic irradiation, and 0.5 g of tin chloride (SnCl ₂ ) is added to 100 ml of an aqueous hydrochloric acid solution (concentration 0.02 mol / L). The dissolved solution was added and stirred for 10 minutes. Thereafter, the particles were washed several times.

Next, the silica particles adsorbed with tin are redispersed in 240 ml of distilled water, and 4.1 mL of an aqueous palladium chloride solution (concentration 0.0117 mol / L) is added thereto, and further 10 minutes later, an aqueous sodium formate solution (concentration 0). .15 mol / L) was added, followed by reaction at room temperature for 5 hours. The obtained suspension was centrifuged and washed several times to obtain a core (Pd / SiO ₂ ) supporting palladium.

Si (OC ₂ H ₅ ) as a silica precursor in Pd / SiO ₂ , Ti (O—iPr) ₄ as a water-soluble titanium compound, surfactant [C ₁₆ H ₃₃ N (CH ₃ ) as a structure regulator ₃ ] Br was allowed to act to form a titanium-containing mesoporous silica layer. Specifically, Pd / SiO ₂ was dispersed in 240 ml of distilled water, and 150 ml of ethanol, 0.75 g of a structure regulating agent and 2.8 ml of aqueous ammonia were added. Subsequently, a mixed solution of 0.75 ml of Si (OC ₂ H ₅ ), 0.02 ml of Ti (O-iPr) ₄ and 0.014 ml of acetylacetone was added and stirred overnight, and a shell of titanium-containing mesoporous silica (TiMSS) was added. Formed. The obtained core-shell type particles are washed with ethanol, dried at 100 ° C. overnight, and finally calcined at 550 ° C. for 6 hours to remove the surfactant, whereby the core-shell type catalyst (Pd / SiO 2) of the present invention is removed. ₂ @TiMSS).

<< Comparative Example 1 >>
In order to clarify the influence of the loading position of the palladium particles on the activity of the one-pot reaction, a titanium-containing mesoporous silica layer was formed and the palladium particles were loaded after removing the surfactant, as in Example 1. A core-shell type catalyst (SiO ₂ @Pd (R) / TiMSS) was synthesized. In SiO ₂ @Pd (R) / TiMSS, it is considered that palladium particles are present randomly in the titanium-containing mesoporous silica layer.

<< Comparative Example 2 >>
A core-shell type catalyst (SiO ₂ @Pd (S) / TiMSS) was synthesized in the same manner as in Example 1 except that the surfactant was removed after the titanium-containing mesoporous silica layer was formed and the palladium particles were supported. did. In SiO ₂ @Pd (S) / TiMSS, it is considered that palladium particles exist only on the outer surface of the titanium-containing mesoporous silica layer.

  In all catalyst samples, the palladium content was 0.7% by weight and the Ti / Si ratio was 0.006.

  The obtained catalyst sample was subjected to TEM observation, nitrogen adsorption / desorption measurement, XRD measurement, UV-Vis measurement, and XAFS measurement, and the catalytic activity was evaluated by a one-pot oxidation reaction of thioanisole. The reaction was carried out at 30 ° C. under bubbling of 50 mg of catalyst, 5 mL of acetonitrile, hydrogen (flow rate: 20 mL / min) and oxygen (flow rate: 20 mL / min).

[TEM observation]
FIGS. 3A and 3B show TEM images of Example 1 (Pd / SiO ₂ @TiMSS), and FIGS. 3C and 3D show TEM images of Comparative Example 2 (SiO ₂ @Pd (S) / TiMSS). Shown in FIG. 3 shows that in each catalyst sample, silica particles (core) having a particle diameter of about 280 nm are covered with a titanium-containing mesoporous silica layer (shell) having a thickness of about 30 nm. In Example 1, the palladium particles are present on the surface layer of the core, and in Comparative Example 2, it can be confirmed that the palladium particles are supported only on the outer surface of the shell. In any sample, the particle diameter of the palladium particles is about 3 to 4 nm, and no significant difference is observed. Further, as shown in FIGS. 3B and 3D, it can be confirmed that the mesopores are formed radially from the silica core.

[Nitrogen adsorption / desorption measurement]
FIG. 4 shows the nitrogen adsorption / desorption isotherm (A) of each catalyst sample and the pore size distribution (B) analyzed by the BJH method. The results of using the samples of Example 1, Comparative Example 1 and Comparative Example 2 are shown in plots (a), (b) and (c), respectively. Plot (d) shows the results when silica particles are used. All the samples other than silica (d) exhibited an isotherm of type IV (classification of adsorption / desorption isotherm by IUPAC) derived from the mesopore structure.

The BET specific surface area and BJH pore volume of each sample were as follows: Example 1 (473 m ² / g, 0.30 cm ³ / g), Comparative Example 1 (272 m ² / g, 0.15 cm ³ / g) Comparative Example 2 ( 240 m ² / g, 0.21 cm ³ / g), and all the samples had a high surface area. When the surface area is low, it is considered that the pores are blocked by palladium particles.

[XRD measurement]
When low angle XRD measurement was performed on each catalyst sample, a peak derived from a mesopore structure was observed, and it was revealed that mesopores having a diameter of about 2.2 nm were formed in the shell. .

[UV-Vis / XAFS measurement]
When the coordination structure of Ti was analyzed by UV-Vis spectrum and XAFS spectrum, it was found that it had an isolated four-coordinate structure suitable for the oxidation reaction using hydrogen peroxide.

[Hydrogen peroxide synthesis reaction]
Using each catalyst sample, hydrogen peroxide and hydrogen peroxide were synthesized (reaction conditions: catalyst 50 mg, 0.01 M HCl aqueous solution 20 ml, hydrogen (20 mL / min), oxygen (20 mL / min), 30 ° C).
The hydrogen peroxide concentration was measured with a hydrogen peroxide counter (HP-300, manufactured by Hiranuma). The hydrogen peroxide concentration after 3 hours of reaction is shown in Table 1.

  When the catalyst sample of Comparative Example 1 was used, the production amount was slightly lower than the others, but in all cases, it was found that the same amount of hydrogen peroxide was produced. From this, it was suggested that even when palladium particles that are catalytically active species were covered with titanium-containing mesoporous silica, as in Example 1, production and diffusion of hydrogen peroxide were not inhibited.

[Thioanisole oxidation reaction]
The result of the one-pot oxidation reaction of thioanisole is shown in FIG. Although the amount of hydrogen peroxide produced is similar, there is a large difference in activity in the one-pot reaction. When the production rates of the products (2) and (3) shown in FIG. 5 were compared, the Pd / SiO ₂ @TiMSS of Example 1 was 3 to 4 times more active than the other catalysts. This is because, in Pd / SiO ₂ @TiMSS, the generated hydrogen peroxide is efficiently supplied to the Ti site, whereas in SiO ₂ @Pd (S) / TiMSS of Comparative Example 2, the generated hydrogen peroxide is It is thought that it diffuses into the solvent and is decomposed by Pd present on the shell surface before coming into contact with the Ti sites in the pores. With the SiO ₂ @Pd (R) / TiMSS of Comparative Example 1, moderate activity was obtained. It was also revealed that the Pd / SiO ₂ @TiMSS of Example 1 produced a more useful product (2) with high selectivity.

  The core-shell type catalyst of the present invention exhibits high catalytic activity in a one-pot oxidation reaction. Use of the present invention is expected to improve the activity of various oxidation reactions.

Claims (8)

Comprising a granular core and a shell covering the core;
The surface layer portion of the core includes palladium,
The core-shell type catalyst, wherein the shell includes titanium-containing silica having radial mesopores from the core toward the surface of the shell.   The core-shell type catalyst according to claim 1, wherein the core is at least one selected from the group consisting of ceramic particles, carbon particles, and polymer particles.   The core-shell type catalyst according to claim 1 or 2, wherein a molar ratio of titanium content to silicon content contained in the shell: Ti / Si satisfies 0.001 ≦ Ti / Si ≦ 0.05. The palladium has an average particle diameter of 0.001 μm to 0.1 μm,
The core-shell type catalyst according to any one of claims 1 to 3, wherein the palladium content is 0.1 wt% to 5 wt%.   The core-shell type catalyst according to any one of claims 1 to 4, wherein an average pore diameter of the mesopores is 2 nm to 50 nm.   The core-shell type catalyst according to any one of claims 1 to 5, wherein an average particle size of the core is 0.1 µm to 1 µm. (I) a step of supporting palladium on the surface layer of the core of the granular material;
(Ii) Covering the core with titanium-containing silica having mesopores by acting a silica precursor, a water-soluble titanium compound and a surfactant serving as a template for mesopores on the core supporting palladium. The process of
(Iii) A method for producing a core-shell type catalyst, comprising a step of removing the surfactant by calcining or refluxing the core coated with the titanium-containing silica in alcohol. The step (ii)
Preparing a solution containing the silica precursor, the water-soluble titanium compound, and the surfactant;
The method for producing a core-shell type catalyst according to claim 7, comprising the step of dispersing the core supporting palladium in the solution.

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