CN111215052B - Preparation of palladium monatomic catalyst and application thereof in methylene-containing aromatic oxidation reaction - Google Patents

Abstract

The invention relates to the preparation of a palladium single-atom catalyst and its application in the oxidation reaction of aromatic hydrocarbons containing methylene. The palladium single-atom catalyst uses manganese oxide carrier and palladium compound as raw materials, and is ball milled, stirred, evaporated to dryness, and treated at high temperature. produced by interaction. The characterization results show that the palladium element exists in the form of monoatomic palladium in the catalyst. And the catalyst exhibits excellent catalytic activity, selectivity and stability in the oxidation reaction of aromatic hydrocarbons containing methylene. The catalyst preparation method provided by the invention is simple and easy, and the catalyst is suitable for preparing aromatic ketone compounds, greatly improves the utilization rate of palladium, reduces the cost of the catalyst, and can promote the commercialization process of the single-atom catalyst.

Description

Preparation of a Palladium Single-Atom Catalyst and Its Application in the Oxidation of Methylene-Containing Arenes Applications

technical field

The present invention relates to the preparation of a palladium single-atom catalyst and its application in the oxidation reaction of aromatic hydrocarbons containing methylene, specifically, the single-atom catalyst with manganese dioxide as the carrier and metal palladium as the active component is used in a solvent-free condition Highly selective conversion of methylene-containing aromatics to aromatic ketones.

Background technique

Aromatic compounds are bulk chemical basic raw materials that can be obtained by simple processing of fossil energy such as petroleum and coal, and are used to produce a variety of functional chemicals, such as plastics, pharmaceuticals, dyes, oils, etc. The petrochemical industry is a very important basic pillar industry in the world industrial economy. In my country, the petrochemical industry accounts for about 20% of the total industrial economy and has a great impact on the national economy. In the conversion process of various aromatic hydrocarbons, selective catalytic oxidation plays an extremely important role in the chemical industry, and is one of the important means to increase the economic value of petroleum resources, that is, through the oxidation functionalization of CH bonds, hydrocarbons can be converted into corresponding alcohols , Ketones, aldehydes, acids, esters, phenols, ethers, alkyl peroxides, epoxides and other oxygen-containing compounds. In the catalytic conversion of aromatic hydrocarbons, the oxidation sites on aromatic hydrocarbon molecules can be divided into three categories: methyl (CH ₃ ), methylene (CH ₂ ), and methine (CH). This catalyst is specially used to catalyze the oxidation of methylene on aromatic hydrocarbon molecules.

Take acetophenone, an oxidation product of ethylbenzene, as an example. Acetophenone is a fine chemical product, which is used to make soap and cigarettes. It can also be used to make solvents such as cellulose ethers, cellulose esters and resins, and plasticizers for plastics. Also used in the synthesis of pharmaceutical intermediates. At present, there are two main ways for the industrial production of acetophenone. One is the Friedel-Crafts reaction production process of acetic anhydride and benzene. The disadvantage is that a large amount of acidic wastewater is associated. The other is the industrial production route of producing methyl ethyl ketone with co-production of phenol by-product acetophenone by isobutylbenzene method, the process is complicated, and the output of acetophenone is restricted by the market sales of methyl ethyl ketone. Therefore, it is undoubtedly a green and economical production route to develop a catalyst that can directly and efficiently synthesize acetophenone or the corresponding aromatic ketone by using common and cheap oxygen to catalyze the oxidized methylene of ethylbenzene or other aromatic hydrocarbons.

Many patents and literatures describe supported palladium-based nanoparticle catalysts that catalyze the methylene oxidation of arenes. Although the supported catalyst can be recycled conveniently, the biggest common problem of this type of palladium-based nanoparticle catalyst is that when the reaction occurs, the active sites of the reaction are distributed on the surface of the nanoparticle, and the catalytic performance of each active site is different, resulting in Side reactions occur, and in order to suppress side reactions, the conversion rate of raw materials is often chosen to reduce, resulting in low efficiency of the catalytic process and increased subsequent separation costs.

To make the properties of the palladium atoms at the catalytic active sites uniform, the palladium can be highly dispersed on the surface of the carrier, so that the palladium atoms are independent of each other and in a monodisperse state, that is, the palladium single-atom catalyst. At present, there is no literature report on the direct use of oxygen to catalyze the oxidation of arene methylene to aromatic ketones by the immobilized single-atom palladium catalyst.

According to the actual market supply and demand situation of the downstream products from oxidized aromatic ketones to aromatic ketones and the current situation of the development of single-atom catalysts, a new type of highly efficient supported single-atom palladium catalyst is developed, using cheap and easy-to-obtain oxygen as the oxygen source, without solvents Under these conditions, the high selectivity of aromatics to generate aromatic ketones can effectively simplify the production steps, reduce the energy consumption of subsequent separations, and reduce the discharge of harmful wastes, which has very important environmental protection significance and economic value.

Contents of the invention

The purpose of the present invention is to provide the preparation of a palladium single-atom catalyst and its application in the oxidation reaction of aromatic hydrocarbons containing methylene.

In order to achieve the above object, the technical solution of the present invention is as follows: manganese dioxide is used as the carrier, the active component is metal palladium, and the palladium content is 0.05-7wt% of the total mass of the catalyst.

The concrete preparation palladium single-atom catalyst process is as follows:

1) After the carrier used is milled in a ball mill for 2 hours, the carrier is dispersed in the aqueous solution and fully stirred, the concentration is 100-0.5g/L, and the stirring time is 10-120 minutes;

2) Adding an aqueous palladium metal precursor solution dropwise to the above dispersion liquid and fully stirring to make the metal precursor impregnated and adsorbed on the carrier. The concentration of the metal precursor aqueous solution is 1-10 ^-4 mgPd/mL, the mass ratio of the carrier to the metal precursor is 20000-50:1, the stirring temperature is 70-95 ° C, and the stirring time is 5-720 minutes; after that, Transfer to a vacuum drying oven at 100-150°C for 1-5 hours,

3) drying the catalyst at 25-100°C for 1-24 hours, and then calcining at 200-700°C for 10-400 minutes in an air atmosphere;

The Pd metal precursor is one or more of palladium chloride, palladium nitrate, palladium acetylacetonate and palladium acetate;

The crystal form of the carrier manganese dioxide is one or more of α-type, β-type, γ-type, δ-type and λ-type, preferably α-type.

The method for testing the activity of the catalyst of the present invention is as follows: a closed tank reactor is used to oxidize methylene-containing aromatic hydrocarbons to corresponding ketones under solvent-free conditions with oxygen as an oxygen source.

The methylene-containing aromatic hydrocarbons are one or more of ethylbenzene, methylethylbenzene, diethylbenzene, triethylbenzene, ethylchlorobenzene, ethylbromobenzene, propylbenzene, and diphenylmethane.

The oxygen pressure is 0.5-3MPa, and the oxygen pressure is preferably 0.5-3MPa; the reaction temperature is 60-200°C, and the reaction temperature is preferably 80-150°C; the reaction time is 2-100h, and the reaction time is preferably 8-25h; palladium in the catalyst The ratio of the molar weight of the aromatic compound to the molar weight of the aromatic compound is 10 ^-6 -10 ^-2 :1.

The invention provides a method for preparing an environmentally friendly and easy-to-operate novel palladium single-atom catalyst. The palladium is dispersed at the atomic level on the carrier. The catalyst has high activity for the methylene oxidation reaction of the side chain of aromatic hydrocarbon. As an oxygen source, it catalyzes the methylene C-H bond of the side chain of aromatic hydrocarbons under solvent-free conditions, and can still generate aromatic ketones with high selectivity under high conversion conditions, and the by-product is almost only water.

The present invention has the following advantages:

1. Without any protective agent, realize the uniform dispersion of palladium metal atoms on the carrier;

2. The catalyst prepared by this method has a uniform activity of the palladium metal atom in the active center, and the selectivity of the catalytic process is significantly higher than that of the palladium nanoparticle catalyst;

3. The reaction uses oxygen as the oxygen source, which is cheap and easy to obtain;

4. The by-product of the reaction is only water, which is environmentally friendly.

5. The reaction adopts solvent-free conditions, which can achieve a conversion rate as high as 70%-95% while maintaining high selectivity, reducing subsequent separation costs.

Description of drawings

Fig. 1 is prepared according to the method of Example 1 Pd/MnO ₂ Catalyst HAADF-STEM schematic diagram; From the HAADF-STEM schematic diagram, it can be seen that according to the Pd/MnO prepared by the method of Example ₁ Pd is dispersed in single atom form in the catalyst.

Figure 2 is a schematic diagram of XRD characterization results of Pd/MnO ₂ single-atom catalysts; the XRD characterization results of Pd/MnO ₂ single-atom catalysts prepared according to the method in Example 1 show that the carrier MnO ₂ is α-MnO ₂ .

Figure 3 Cyclic stability experiments of palladium single-atom catalysts.

Detailed ways

In order to further illustrate the present invention, the following examples are cited, but the scope of claims of the present invention is not limited by these examples. At the same time, the embodiment only provides some conditions for realizing this purpose, but it does not mean that these conditions must be met to achieve this purpose.

The preparation (0.7wt%) of embodiment 1 palladium single atom catalyst:

The α- _MnO2 carrier used was ball milled for 2 hours, weighed 1.00g, dispersed in 3ml of ultrapure water, stirred at room temperature for 1min, and then added 0.012mL of 59.5mgPd/mL palladium precursor solution (palladium chloride) at one time, 95 After stirring and evaporating to dryness in a water bath at ℃, continue heating for 0.5 hours, after that, transfer to a vacuum drying oven at 150 ℃ for 2 hours. After the air-drying treatment is completed, dry at 80°C for 12 hours, and then transfer to a muffle furnace for calcination at 600°C for 5 hours in an air atmosphere. A palladium single-atom catalyst with a loading of 0.7 wt% can be obtained. The obtained catalyst is the palladium single-atom catalyst supported by _MnO2 .

Fig. 1 is the Pd/ _MnO2 catalyst HAADF-STEM schematic diagram prepared according to the method of Example 1, from the HAADF-STEM schematic diagram, it can be seen that according to the Pd/ _MnO2 catalyst prepared by the method of Example 1, Pd is dispersed in the form of single atoms.

Figure 2 is a schematic diagram of the XRD characterization results of the Pd/MnO ₂ single-atom catalyst. The XRD characterization results of the Pd/MnO ₂ single-atom catalyst prepared according to the method in Example 1 show that the carrier MnO ₂ is α-MnO ₂ .

The preparation of comparative example 1 palladium nano-catalyst (0.7wt%):

Take 0.012mL of 59.5mgPd/mL palladium precursor solution (palladium chloride), add 15ml of ultrapure water, add 0.2ml of a 2% PVP (polyvinylpyrrolidone) aqueous solution by mass fraction, stir at room temperature for 1min, then add 0.1mmol/ mL NaBH ₄ aqueous solution 1.3mL, continue stirring at room temperature for 0.5min to obtain palladium nano-aqueous sol.

The _MnO2 carrier used was ball-milled for 2 hours, weighed 1.00g, dispersed in 3ml of ultrapure water, stirred at room temperature for 1 minute, then added all the aforementioned palladium nano-hydrosol at one time, stirred and evaporated to dryness in a water bath at 95°C, and then continued to heat for 0.5 hours . Transfer to a muffle furnace and bake in an air atmosphere at 600°C for 5h. A palladium single-atom catalyst with a loading of 0.7 wt% can be obtained. The obtained catalyst is the palladium nano-catalyst supported by _MnO2 .

Catalytic performance test of palladium single-atom catalyst and palladium nanocatalyst

Select a batch reactor, ethylbenzene is used as the reaction raw material (2ml), at 120°C, 2MPa O ₂ , 10mg catalyst, and the reaction time is 20h. After the reaction is stopped, the temperature is lowered to room temperature, the liquid product and the catalyst are separated by centrifugation, and the liquid product is separated by gas chromatography. analyze. The results are detailed in Table 1 below

Note: The reaction time of Comparative Example 1-2 is 8 hours.

It can be seen from Table 1 that when the reaction time is 20h, the selectivity and yield of Pd nanocatalysts to acetophenone are significantly lower than those of Pd single atom catalysts. When the conversion rate is similar (about 50%), the selectivity of Pd nano-catalyst to acetophenone is still significantly lower than that of Pd single-atom catalyst.

The catalytic performance comparison of different methylene-containing arene compounds of embodiment 2-9

Using 0.7wt% Pd/ _MnO2 single-atom catalyst to catalyze the methylene oxidation of different raw materials aromatics to the corresponding ketone products, the reaction results are shown in the following table 2

It can be seen from Table 2 that palladium single-atom catalysts exhibit excellent catalytic activity and selectivity in the oxidation reactions of different methylene-containing arene compounds.

Embodiment 10 Catalyst cycle stability test

To investigate the stability of the palladium single-atom catalyst involved in the present invention, under the reaction conditions of 120°C, 2MPa O ₂ , 10mg 0.7wt% Pd/MnO ₂ single-atom catalyst, and a reaction time of 20h, take ethylbenzene as the substrate (2ml) , the catalyst is recovered and recycled after each reaction is completed, and the reaction data column chart 3 is as follows: Figure 3 Palladium single-atom catalyst cycle stability experiment;

It can be seen from FIG. 3 that the palladium single-atom catalyst involved in the present invention has stable catalytic performance and good selectivity and activity in 5 cycles of catalyzing the oxidation of ethylbenzene to acetophenone.

Claims (5)

1. The application of a palladium single-atom catalyst in the oxidation reaction of aromatic hydrocarbons containing methylene, is characterized in that: in the catalyzer, manganese dioxide is used as a carrier, and the active component is metal palladium, and the palladium content is 0.05% of the total mass of the catalyst. -7 wt%; the palladium single-atom catalyst is used in the oxidation reaction of aromatic hydrocarbons containing methylene, using a closed tank reactor, under solvent-free conditions, with oxygen as the oxygen source; The preparation method of described catalyst comprises the following steps: 1) After the carrier used is milled in a ball mill for 0.5-5 hours, the carrier is dispersed in the aqueous solution and fully stirred to obtain a dispersion with a concentration of 100-0.5 g/L and a stirring time of 10-30 minutes; 2) Add the palladium metal precursor aqueous solution dropwise to the dispersion obtained in step 1) and stir fully to impregnate and adsorb the metal precursor on the carrier; the concentration of the metal precursor aqueous solution is 1-10 ^-4 mg Pd/mL, and the stirring temperature is 70-95°C, the stirring time is 5-30 minutes, after that, transfer to a vacuum drying oven at 100-150°C for 1-5 hours to obtain a catalyst; 3) After drying the catalyst obtained in step 2) at 25-100° C. for 8-24 hours, calcining at 200-700° C. in an air atmosphere for 100-400 minutes. 2. The application according to claim 1, characterized in that: The Pd metal precursor is one or more of palladium chloride, palladium nitrate, palladium acetylacetonate and palladium acetate; The crystal form of the carrier manganese dioxide is one or more of α-type, β-type, γ-type, δ-type and λ-type. 3. The application according to claim 1, characterized in that: the methylene-containing arene is one or more of ethylbenzene, diethylbenzene, triethylbenzene, propylbenzene and diphenylmethane. 4. The application according to claim 1, characterized in that: the oxygen pressure is 0.5-3MPa, the reaction temperature is 60-200°C, the reaction time is 2-100h, the ratio of the molar weight of palladium in the catalyst to the molar weight of the aromatic compound is 10 ^-6 -10 ^-2 :1. 5. The application according to claim 4, characterized in that the oxygen pressure is 0.5-3MPa, the reaction temperature is 80-150°C, and the reaction time is 8-25h.

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