Process for the preparation of rare earth-manganese composite amorphous catalysts for the catalysis of vocs and use thereof

Abstract

The application discloses a preparation method of a rare earth-manganese composite amorphous catalyst for VOCs catalysis and application thereof. The method comprises the following steps: uniformly mixing a rare earth element (Ce, La, Pr, Nd) precursor and manganese acetate, adjusting the pH of the solution to be in the range of 4.0-6.5 or 7.2-8.0; dropwise adding a potassium permanganate solution and moving the obtained mixed solution into a hydrothermal reaction kettle to react at 100 DEG C for 16 hours; and collecting black precipitates, washing, drying, calcining and grinding to obtain the rare earth-manganese composite amorphous catalyst. The application causes the tunnel structure of MnO2 to collapse by doping the rare earth element with a large ion radius to cause the lattice expansion of MnO2, and adjusts the pH and temperature to delay the crystal grain agglomeration and the crystal growth rate, so that the rare earth-manganese composite amorphous catalyst is obtained. The catalyst has a larger specific surface area, rich defect sites and uniform active centers, and exhibits excellent VOCs catalytic activity, and is simple to operate and low in cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of VOCs catalyst preparation, in particular to a rare earth doped high-performance amorphous manganese-based catalyst preparation and its application in VOCs catalytic purification. BACKGROUND

[0002] Currently, secondary pollutants (O3 and PM 2.5 ) generated by a series of chemical reactions involving volatile organic compounds (VOCs) as precursors are still the main atmospheric environmental problems faced by most areas in China, which are diverse in type, extensive in source, and urgent in treatment. Catalytic oxidation is one of the most effective methods for the end control of VOCs, and many scholars at home and abroad have invested a lot of research in the exploration of catalysts. Transition metal oxides, compared with noble metals, are low in price, simple in synthesis, and have good oxygen storage capacity, and are concerned in VOCs catalytic reaction, but there are still problems of poor low-temperature activity and low degree of deep oxidation. Therefore, it is crucial to develop new catalytic materials that can efficiently catalyze and purify VOCs under mild conditions.

[0003] There are many strategies for developing catalysts, such as adjusting the preparation method of the catalyst, constructing different metal interfaces,

[0004] designing the morphology and structure of the catalyst, etc. The oxygen vacancy concentration affects the escape energy of the surface lattice oxygen, which is an intrinsic factor that determines the catalytic activity, so oxygen vacancy engineering is often used to optimize materials and improve catalytic performance. Metal doping can adjust the oxygen vacancy concentration by introducing other elements, and if it can cause structural distortion to prepare amorphous materials, this amorphous structure with long-range disorder and short-range order has a high degree of unsaturation of surface atomic coordination, which can produce more oxygen defects. At the same time, amorphous materials have the characteristics of small grain size, high specific surface area and porosity, which can promote the diffusion and limited enrichment of VOCs on the surface, and can further improve the performance compared with high crystallinity catalysts.

[0005] MnO2 has a rich structure and electronic properties which are dynamically adjustable, and the connection mode of [MnO6] octahedron determines the crystal morphology of MnO2. Rare earth elements with a larger radius than Mn are prone to cause lattice expansion of MnO2, leading to the collapse of its tunnel structure, so by adjusting the catalyst ratio, preparation conditions, etc., stable amorphous materials can be obtained within a wide range of control, which is more convenient to operate.

[0006] CN104001502B provides a preparation method of a cerium-manganese catalyst for decomposing ozone, which is characterized by mixing a manganese source, a cerium source and potassium permanganate in a weak acid environment to obtain a catalyst with high crystallinity. CN108630457B uses potassium hydroxide as a precipitant to mix with a manganese source and potassium permanganate to prepare amorphous manganese oxide for use as a positive electrode material for a battery, but the calcination temperature needs to be controlled at 400°C. SUMMARY

[0007] One of the purposes of the present application is to solve the problem of poor low-temperature activity of current VOCs catalysts, and to provide a preparation method of a rare earth-manganese composite binary amorphous catalyst for efficiently degrading VOCs.

[0008] To achieve the above-mentioned purpose, the method is carried out in the following steps:

[0009] (1) Dissolve Mn(CH3COO)2·4H2O and a rare earth element precursor in deionized water, and magnetically stir at 400 r / min until completely dissolved to form a mixed solution, and adjust the pH to 4.0-6.5 or 7.2-8.0.

[0010] (2) Add potassium permanganate solution to the solution of step (1), stir for 30 min, and move the obtained brown liquid into a hydrothermal reaction kettle, and react at 80-110°C for 12-24 h.

[0011] (3) Wash, dry, calcine and grind the precipitate obtained in step (2) to obtain an amorphous rare earth-manganese composite binary catalyst material.

[0012] The present application uses a simple hydrothermal synthesis method to obtain a series of rare earth-manganese binary catalysts with different compositions and different ratios, wherein the rare earth elements include Ce, La, Pr and Nd, and the molar ratio of the rare earth elements to manganese is 0.12-0.60. The prepared CeMn 0.24 The catalyst has a space velocity of 30000 ml·g -1 ·min -1 , a chlorobenzene concentration of 3000 mg / m 3 , and can reach a conversion rate of 100% at 200°C, while the CO2 yield is more than 90%. Compared with pure MnO2, the activity is significantly improved.

[0013] The concentration of Mn(CH3COO)2·4H2O in step (1) is 0.425 mol / L.

[0014] The rare earth elements in step (1) are Ce, La, Pr and Nd, and the precursors are selected from one of nitrate, acetate or chlorate. The mixed solution is, for example, a mixed solution of manganese acetate and cerium nitrate, a mixed solution of manganese acetate and cerium acetate, or a mixed solution of manganese acetate and cerium chloride.

[0015] The pH adjustment range in step (1) is 4.0-6.5 or 7.2-8.0, and the pH is adjusted in the range of zeta potential absolute value greater than 30 with the zero point charge (PZC) 6.8 as the boundary. The pH is adjusted by adding acid using acetic acid, hydrochloric acid, nitric acid, and adding base using 0.1 mol / L sodium hydroxide solution. The hydroxyl group in the weak acid and weak base environment interacts with the oxygen negative charge site to form a hydrogen bond, which can improve the solution stability, delay the crystal grain agglomeration and crystal growth rate in the hydrothermal reaction process, and help to form a highly dispersed amorphous structure. Over-acid or over-base will cause the deprotonation of hydroxyl group and reduce the solution stability, and the hydrolysis reaction of the precursor will be triggered in the strong base environment.

[0016] The concentration of the potassium permanganate solution in step (2) is 0.2-0.3 mol / L.

[0017] The hydrothermal reaction temperature in step (2) is 80-110℃, for example, 80℃, 90℃, 100℃, or 110℃. If the hydrothermal reaction temperature is too low, the binding force of the composite oxide is weak, and the product yield is not high. If the hydrothermal reaction temperature is too high, MnO2 is converted into Mn2O3, and the activity is reduced.

[0018] The hydrothermal reaction time in step (2) is 12-24h. For example, 12h, 16h, 20h, or 24h.

[0019] In step (3), the product is washed with anhydrous ethanol and deionized water for multiple times until it is neutral.

[0020] The drying temperature in step (3) is 90-110℃, and the drying time is 12-24h.

[0021] The calcination conditions in step (3) are as follows: in an air atmosphere, the calcination temperature is 300-600℃, and the calcination time is 3-5h. If the temperature is too high, the amorphous crystal phase is converted, the main active component is changed from MnO2 to Mn2O3, and the activity is reduced.

[0022] The second object of the present application is to provide a new method of oxygen defect engineering and catalyst morphology design. The amorphous catalyst material induced by the rare earth element-Mn composite metal oxide has smaller grain size and larger specific surface area. The unique twisted lattice and the coexistence of multiple valence states make it have rich surface defects and uniform active centers.

[0023] The rare earth-manganese composite amorphous catalyst synthesized by the above method has rich structural defects, which can be used as the adsorption and activation sites of reactant molecules, and the surface dangling bonds and local electrons can adjust the electronic state and oxygen species transmission process, which is the key to complete the entire chemical cycle.

[0024] Compared with the prior art, the present application has the beneficial effects that:

[0025] (1) The catalyst in the present application is amorphous structure, no obvious diffraction peak corresponding to MnO2 and other rare earth oxides is observed in the X-ray diffraction pattern, indicating that the catalyst has very low crystallinity, small micro size and good dispersibility. At the same time, the catalyst has large specific surface area and porosity, which is beneficial to the adsorption and diffusion of reaction molecules on the surface.

[0026] (2) The amorphous catalyst prepared by the present application has different rare earth elements and manganese, and under the conditions of space velocity 30000ml·g -1 ·min -1 , chlorobenzene concentration 3000mg / m 3 , 100% conversion rate can be reached at 200℃, and the CO2 yield is more than 90%. Compared with other methods for preparing manganese-based catalysts with clear crystal form, the present application has higher low-temperature activity, and the catalyst after reaction is still amorphous structure and has good stability.

[0027] (3) The amorphous catalyst prepared by the present application has simple operation, low cost and good performance, and has broad application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is the XRD result graph of the CeMn a composite catalyst.

[0029] Figure 2 is the SEM graph of MnO2 and CeMn 0.24 catalyst.

[0030] Figure 3 is the HRTEM graph of MnO2 and CeMn 0.24 catalyst.

[0031] Figure 4 is the XRD graph of XMn 0.36 composite catalyst.

[0032] Figure 5 is the XRD graph of CeMn 0.24 composite catalyst prepared by different cerium precursors.

[0033] Figure 6 is the XRD graph of CeMn 0.24 composite catalyst prepared under different pH conditions.

[0034] Figure 7 is the XRD graph of CeMn 0.24 composite catalyst prepared under different hydrothermal temperatures.

[0035] Figure 8CeMn prepared for different calcination temperatures 0.24 XRD patterns of the composite catalysts.

[0036] Figure 9 CeMn prepared for different calcination temperatures a VOCs catalytic activity patterns of the composite catalysts. DETAILED DESCRIPTION

[0037] The technical solutions of the present application are further illustrated below through a detailed description.

[0038] Example 1

[0039] A preparation method of a cerium-manganese amorphous catalyst: 4.16 g of Mn(CH3COO)2·4H2O and 0.89 g of Ce(NO3)3·6H2O were dissolved in deionized water, and magnetic stirring was performed until the solids were completely dissolved to form a mixed solution. 1.0 ml of acetic acid was added to adjust the pH to 4.5. A 0.26 mol / L KMnO4 solution was added to the above mixed solution and magnetic stirring was performed at room temperature for 30 min to mix uniformly. Subsequently, the mixed solution was transferred into a 100 ml hydrothermal reactor, and reaction was performed at 100℃ for 16 h. After cooling, the collected precipitate was washed with anhydrous ethanol and deionized water alternately for three times each until neutral, and was dried in a 100℃ oven for 24 h. The obtained solid was placed in a muffle furnace, and was heated to 500℃ at a heating rate of 5℃ / min under an air atmosphere, and was maintained for 4 h. After grinding, a CeMn 0.12 amorphous catalyst.

[0040] Example 2

[0041] In Example 1, the amount of Ce(NO3)3·6H2O added was changed to 1.77 g, and other operations were performed according to Example 1 to prepare a CeMn 0.24 amorphous catalyst.

[0042] Example 3

[0043] In Example 1, the amount of Ce(NO3)3·6H2O added was changed to 2.65 g, and other operations were performed according to Example 1 to prepare a CeMn 0.36 amorphous catalyst.

[0044] Example 4

[0045] In Example 1, the amount of Ce(NO3)3·6H2O added was changed to 3.54 g, and other operations were performed according to Example 1 to prepare a CeMn 0.48 amorphous catalyst.

[0046] Example 5

[0047] The amount of Ce(NO3)3·6H2O added in Example 1 was changed to 4.43g, and other operations were performed as in Example 1 to obtain CeMn. 0.60 Amorphous catalysts.

[0048] Example 6

[0049] In Example 1, Ce(NO3)3·6H2O was replaced with La(NO3)3·6H2O, and the amount added was 2.06g. Other operations were the same as in Example 1 to obtain LaMn. 0.36 Amorphous catalysts.

[0050] Example 7

[0051] In Example 1, Ce(NO3)3·6H2O was replaced with Pr(NO3)3·6H2O, and the amount added was 2.61g. Other operations were the same as in Example 1 to obtain PrMn. 0.36 Amorphous catalysts.

[0052] Example 8

[0053] In Example 1, Ce(NO3)3·6H2O was replaced with Nd(NO3)3·6H2O, and the amount added was 2.63g. Other operations were the same as in Example 1 to obtain NdMn. 0.36 Amorphous catalysts.

[0054] Example 9

[0055] In Example 1, Ce(NO3)3·6H2O was replaced with Ce(CH3COO)3·xH2O, and the amount added was 1.27g. Other operations were the same as in Example 1 to obtain CeMn. 0.24 -C amorphous catalyst.

[0056] Example 10

[0057] In Example 1, Ce(NO3)3·6H2O was replaced with Ce(Cl)3·7H2O, and the amount added was 1.49g. Other operations were the same as in Example 1 to obtain CeMn. 0.24 -L amorphous catalyst.

[0058] Example 11

[0059] The amount of acetic acid added in Example 2 was changed to 0.1 ml, the pH was adjusted to 6, and other operations were performed as in Example 2 to obtain CeMn. 0.24 -pH 6.0 amorphous catalyst.

[0060] Example 12

[0061] The amount of acetic acid added in Example 2 was changed to 3 ml, the pH was adjusted to 4, and other operations were performed as in Example 2 to obtain CeMn.0.24 -pH4.0 amorphous catalyst.

[0062] Example 13

[0063] In Example 2, acetic acid was replaced with sodium hydroxide solution, the pH was adjusted to 8.0, and other operations were performed as in Example 2 to obtain CeMn. 0.24 -pH 8.0 amorphous catalyst.

[0064] Example 14

[0065] By changing the hydrothermal temperature in Example 2 to 80°C, and following the same procedures as in Example 2, CeMn was obtained. 0.24 -HT80 amorphous catalyst.

[0066] Example 15

[0067] By changing the hydrothermal temperature in Example 2 to 110°C, and following the same procedures as in Example 2, CeMn was obtained. 0.24 -HT110 amorphous catalyst.

[0068] Example 16

[0069] The calcination temperature in Example 2 was changed to 300°C, and other operations were the same as in Example 1, to obtain CeMn. 0.24 -RT300 amorphous catalyst.

[0070] Example 17

[0071] The calcination temperature in Example 2 was changed to 400℃, and other operations were performed as in Example 2 to obtain CeMn. 0.24 -RT400 amorphous catalyst.

[0072] Example 18

[0073] The calcination temperature in Example 2 was changed to 600℃, and other operations were performed as in Example 2 to obtain CeMn. 0.24 -RT600 amorphous catalyst.

[0074] Comparative Example 1

[0075] In Example 1, without adding Ce(NO3)3·6H2O, and with other operations following the same procedure as in Example 1, a MnO2 catalyst was prepared.

[0076] The crystal phase structure of Examples 1-18 and Comparative Example 1 was characterized by XRD results as follows: Figure 1 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8As shown, by adjusting the catalyst preparation parameters within the controllable range, the examples all tend to have an amorphous structure.

[0077] The nitrogen adsorption-desorption test results of Examples 1-18 and Comparative Example 1 are shown in Table 1. All examples showed a large specific surface area, and the phase transformation was more conducive to the anchoring of reactant molecules, promoting their adsorption and diffusion on the catalyst surface.

[0078] Table 1. Results of nitrogen adsorption-desorption of catalyst.

[0079]

[0080]

[0081] The electron microscopy results of Example 2 and Comparative Example 1 are shown below. Figure 3 and Figure 4 MnO2 has a rod-like structure, while CeMn has an amorphous structure. 0.24 The grain size is significantly reduced, exhibiting uniform spherical particles and no regular lattice fringes were observed. This structure of long-range disorder and short-range order leads to more defects and more uniform active centers.

[0082] The VOCs catalytic activity of Examples 1-5 and Comparative Example 1 was evaluated, and the results are shown in the figure. Figure 9 At a total flow rate of 100 ml / min and a space velocity of 30000 ml·g, -1 ·min -1 Chlorobenzene concentration 3000 mg / m³ 3 Under the test conditions, CeMn 0.24 The catalyst can achieve 100% conversion at 200℃, which is lower than that of MnO2 catalyst at 60℃.

[0083] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a rare earth-manganese composite amorphous catalyst for the catalytic oxidation of VOCs, wherein the rare earth element is Ce, La, Pr, or Nd; characterized in that, Includes the following steps: (1) Dissolve the rare earth element precursor and Mn(CH3COO)2·4H2O in deionized water and stir magnetically until the solid is completely dissolved to form a mixed solution. Adjust the pH of the solution to the range of 4.0~6.5 or 7.2~8.

0. The molar ratio of the rare earth element precursor to Mn(CH3COO)2·4H2O is 0.12~0.

60. When adjusting the pH with acid, the acid is acetic acid, hydrochloric acid, or nitric acid. When adjusting the pH with alkali, the alkali is 0.1 mol / L NaOH solution. The concentration of Mn(CH3COO)2·4H2O in the mixed solution is 0.425 mol / L. (2) Add potassium permanganate solution to step (1), stir for 30 min, transfer the resulting brown liquid into a hydrothermal reactor, and perform hydrothermal reaction at 80~110 ℃ for 12~24 h; the concentration of potassium permanganate solution is 0.2~0.3 mol / L; (3) The black precipitate obtained in step (2) is washed, dried, calcined and ground to obtain a composite amorphous catalyst; the calcination time is 3~5 h and the calcination temperature is 300~600 ℃.

2. The method as described in claim 1, characterized in that, The rare earth element precursor is either a nitrate or an acetate.

3. The method as described in claim 1, characterized in that, The precipitate was washed repeatedly with anhydrous ethanol and deionized water until neutral. The drying temperature was 90-110 °C and the drying time was 12-24 h. After drying, the precipitate was calcined in air.

4. A rare earth-manganese composite amorphous catalyst prepared by the method described in any one of claims 1-3.

5. The application of the rare earth-manganese composite amorphous catalyst as described in claim 4 in the catalytic oxidation of VOCs.

Images