Composite catalyst for degrading CVOCs as well as preparation method and application of composite catalyst

Abstract

The invention belongs to the field of catalysts, and particularly relates to a composite catalyst for degrading CVOCs as well as a preparation method and application of the composite catalyst. The novel composite catalyst for CVOCs degradation comprises a molecular sieve and MOFs arranged on the surface of the molecular sieve. According to the technical scheme, the metal oxide obtained through pyrolysis of the MOFs material is combined with the molecular sieve, rich acid sites are reserved while the high-oxygen-deficiency oxidation active component is obtained, and the problem of acid site coverage caused by excessive loading is solved.

Description

Technical Field

[0001] The invention belongs to the field of catalysts, and in particular relates to a composite catalyst for CVOCs degradation and a preparation method and application thereof. Background Art

[0002] According to the definition of the World Health Organization (WHO), volatile organic compounds (VOCs) are organic compounds with a boiling point between 50 and 250°C, a saturated vapor pressure of more than 133.32 Pa at room temperature, and exist in the air in the form of steam. Most VOCs are highly toxic and volatile, and can remain in nature for a long time, causing persistent pollution to the environment. Some substances even have "three-hazard" effects, posing a serious threat to human and biological health. At the same time, there are also some chlorine-containing VOCs (CVOCs), such as CH3Cl, CCl4 and other chlorinated alkanes, which will be decomposed under the action of sunlight ultraviolet rays and release chlorine atoms, which react chemically with ozone molecules to cause ozone loss. Therefore, the research and development of efficient purification and removal technology for VOCs, especially CVOCs, is of great significance to human health and atmospheric environmental protection.

[0003] In view of the serious harm caused by CVOCs to human health, the natural environment and atmospheric ozone, its treatment technology has also become a hot issue in the environmental field at home and abroad.

[0004] Currently, there are two main types of CVOCs treatment technologies. One is enrichment and recovery technology, which commonly includes adsorption, absorption, membrane separation and condensation recovery; the other is decomposition and conversion technology, which mainly includes direct incineration, photocatalytic oxidation, biodegradation and catalytic combustion.

[0005] Catalytic oxidation is the preferred solution for treating CVOCs in current industrial applications, and its core is to develop catalysts with excellent performance. However, in practical applications, due to the presence of chlorine, catalytic oxidation technology still has two major problems that need to be solved. First, it is difficult for CVOCs to be completely converted into the final products CO2, H2O and HCl during the catalytic oxidation process, and it is often accompanied by the generation of CO, Cl 2 and other organic by-products. Polychlorinated by-products are important precursors for the formation of dioxins; secondly, the chlorine-containing by-products generated during the catalytic oxidation of CVOCs are easy to react with the active components of the catalyst, causing the properties of the catalyst to change and lose its activity. Therefore, how to suppress the amount of by-products generated during the catalytic oxidation of CVOCs, ensure that the reaction is stable and thorough, and avoid catalyst poisoning and deactivation, is the current technical barrier in the catalytic oxidation of CVOCs.

[0006] Due to the particularity of CVOCs, it is difficult for a single catalyst system to simultaneously meet the above requirements of low-temperature activity, high selectivity and anti-poisoning. Therefore, it is necessary to develop a multi-component composite catalyst, hoping to achieve efficient coupling of acidity and redox through multi-component functional coordination, so that the catalyst has acidity and redox suitable for CVOCs adsorption, C-Cl bond dissociation and deep oxidation of intermediates. Molecular sieve materials are often used as carrier materials for catalytic oxidation reactions of CVOCs due to their advantages such as adjustable pore size, good hydrothermal stability, and specific acid sites in the pores. Transition metal oxides show high catalytic oxidation activity in the catalytic degradation of CVOCs due to their variable valence state and strong redox ability, and their catalytic performance can be further improved by doping with other metal elements. However, transition metal oxides are very easy to combine with free chlorine elements in CVOCs to form metal chlorides, causing catalyst deactivation. Therefore, combining molecular sieves with abundant acid sites and transition metal oxides with abundant oxygen vacancies has become a feasible strategy for catalytic oxidation of CVOCs. The currently commonly used bonding strategy is the impregnation method. The above-mentioned transition metal oxide particles will inevitably aggregate in a high-temperature environment to form larger aggregates, which will lead to carbon deposition and sintering of the catalyst and a decrease in activity. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a composite catalyst for CVOCs degradation and a preparation method and application thereof.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A novel composite catalyst for CVOCs degradation comprises a molecular sieve and MOFs arranged on the surface of the molecular sieve.

[0010] The molecular sieve is an acidic molecular sieve; preferably one of HZSM-5 molecular sieve, SAPO-34 molecular sieve and β molecular sieve.

[0011] The MOFs is ZI F-67.

[0012] The Co content is 10%-40%; preferably >20%; preferably 21.9%-32.8%.

[0013] The present invention also includes a method for preparing the novel composite catalyst for CVOCs degradation, comprising the following steps: adding cobalt salt and ligand into deionized water to form a MOFs solution, adding molecular sieves into the MOFs solution, collecting and drying the precipitate after the reaction, and then calcining to obtain the composite catalyst for CVOCs degradation.

[0014] The ligand is 2-methylimidazole; the cobalt salt is cobalt nitrate hexahydrate; the molar ratio of the cobalt salt to the ligand is 1:50-80; preferably 1:65-75; more preferably 1:66.

[0015] The drying conditions are drying at 80-120°C for 12-24 hours; preferably drying at 110°C for 8 hours.

[0016] The calcination conditions are: calcination at 300-600°C for 4 hours in a tube furnace under N2 atmosphere; after cooling, calcination at 300-600°C for 4 hours in an air atmosphere; preferably, calcination at 500°C for 4 hours in a N2 atmosphere; after cooling, calcination at 350°C for 4 hours in an air atmosphere.

[0017] The present invention also includes an application of the novel composite catalyst for CVOCs degradation, which is used for CVOCs degradation. Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention develops a new type of CVOCs oxidation catalyst with high selectivity, high activity, high stability, low ignition temperature and long service life. Metal-organic framework (MOFs) is a new type of porous material with a three-dimensional pore structure. It can obtain derived metal oxides through pyrolysis. It has good dispersion and rich oxygen defects and is widely used in catalytic oxidation reactions. The technical solution of the present application combines the metal oxides pyrolyzed by MOFs materials with molecular sieves, retaining rich acidic sites while obtaining high oxygen defect oxidation active components, solving the problem of acid site coverage caused by excessive loading. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a graph of the dichloromethane conversion rate of the catalyst described in Examples 1-3 of the present invention;

[0020] Figure 2 This is a graph of carbon dioxide generation rate of the catalysts described in Examples 1-3 of the present invention;

[0021] Figure 3 Graph showing the by-product yields of the catalysts described in Examples 1-3 of the present invention;

[0022] Figure 4 The SEM images of the catalysts described in Examples 1-3 of the present invention are as follows;

[0023] Figure 5 This is a graph showing the dichloromethane conversion rate of the catalysts described in Comparative Examples 1-2 and Example 2 of the present invention;

[0024] Figure 6 This is a graph showing the dichloromethane conversion rate of the catalysts described in Example 4 and Example 2 of the present invention. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and the best embodiments.

[0026] Embodiment 1:

[0027] A novel composite catalyst for CVOCs degradation and a preparation method thereof, comprising the following steps;

[0028] 1) Dissolve 1.97 g of Co(NO3)2·6H2O in 13.54 mL of deionized water and 38.38 g of 2-methylimidazole in 135.4 mL of deionized water, then mix the two solutions to obtain a purple mixed solution, and stir magnetically at 25°C for 0.5 h;

[0029] 2) During the above-mentioned vigorous stirring process, 0.25 g of HZSM-5 molecular sieve powder was added;

[0030] 3) After stopping stirring, the mixture was centrifuged at 12000 rpm for 5 min, the purple precipitate was collected, and washed with deionized water for 3 times;

[0031] 4) drying the above precipitated material in an oven at 110°C for 8 hours;

[0032] 5) After cooling, the solid was placed in a tube furnace and calcined at 500°C for 4 h in a N2 atmosphere;

[0033] 6) After cooling, the solid was placed in a tube furnace and calcined at 350° C. for 4 h in an air atmosphere to obtain a catalyst for catalytic oxidation of VOCs;

[0034] The actual Co content is 32.8wt.%, denoted as 0.25HZSM@Co-N 500 A 350 ;

[0035] Embodiment 2:

[0036] The only difference between Example 2 and Example 1 is that 0.5 g of HZSM-5 molecular sieve powder is added in step 2), and the actual Co content of the obtained catalyst is 21.9 wt.%, which is recorded as 0.5HZSM@Co-N 500 A 350 ;

[0037] Embodiment 3:

[0038] The only difference between Example 3 and Example 1 is that 1 g of HZSM-5 molecular sieve powder is added in step 2), and the actual Co content of the obtained catalyst is 14.5 wt.%, which is recorded as 1HZSM@Co-N 500 A 350 ;

[0039] Embodiment 4:

[0040] The only difference between Comparative Example 4 and Example 1 is that 0.5 g of β molecular sieve powder is added in step 2), and the actual Co content of the obtained catalyst is 19.0 wt.%, which is recorded as 0.5β@Co-N 500 A 350 ;

[0041] Comparative Example 1:

[0042] The activity test was carried out using HZSM-5 as comparative example 1.

[0043] Comparative Example 2:

[0044] The Co / HZSM-5 catalyst was prepared by the traditional impregnation method. The specific steps are as follows:

[0045] 1) Dissolve 1.97 g Co(NO3)2·6H2O in 5 ml deionized water, add to 2.0 g HZSM-5 support and stir;

[0046] 2) Leave at room temperature for 12 hours;

[0047] 3) Place the above mixture in an oven and dry at 110°C for 8 hours;

[0048] 4) The above mixture was placed in a muffle furnace and calcined at 350°C for 4 hours.

[0049] The actual Co content is 20wt.%, recorded as 20Co / HZSM.

[0050] The examples and comparative examples were tested, wherein the activity test conditions were: 500 ppm dichloromethane, O2 and N2 as balance gases, reaction temperature of 50-400°C, and space velocity of 20000 h -1 , Experimental equipment: VOCs catalytic oxidation evaluation device.

[0051] Figure 1-3 The catalytic performance of the catalysts described in Examples 1-3 on DCM is shown respectively. Figure 4The SEM images of the catalysts described in Examples 1-3 of the present invention are as follows; it can be seen from the figure that the catalysts described in Examples 1-3 have high catalytic activity for DCM, and the degradation of DCM at about 310°C is more than 90%. At the same time, the catalysts described in Examples 1-3 have high CO2 yields. The catalysts described in Examples 1 and 2 have extremely low CH3Cl selectivity, while a large amount of CH3Cl byproducts accumulate on the catalyst described in Example 3, which confirms that Co plays an important role in deep oxidation, and its mass fraction should be kept above 20%.

[0052] Figure 5 The catalytic performance of the catalysts described in Comparative Examples 1 and 2 and Example 2 for DCM is shown. It can be seen from the figure that the catalytic activity of the catalyst described in Example 2 for DCM is significantly higher than that of Comparative Examples 1 and 2. The T90 of the catalyst described in Example 2 for DCM is 40°C lower than that of Comparative Example 2, while the conversion rate of Comparative Example 1 for DCM at 390°C is only 68%.

[0053] Figure 6 The catalytic performance of the catalysts described in Example 4 and Example 2 for DCM is shown. From the figure, it can be seen that the T90 of the catalyst described in Example 4 for DCM is 308°C, which also shows good DCM catalytic performance.

[0054] In summary, the present invention has developed a new type of CVOCs oxidation catalyst with high selectivity, high activity, high stability, low ignition temperature and long service life. Metal-organic framework (MOFs) is a new type of porous material with a three-dimensional pore structure. It can obtain derived metal oxides through pyrolysis. It has good dispersion and rich oxygen defects and is widely used in catalytic oxidation reactions. The technical solution of the present application combines the metal oxides pyrolyzed by MOFs materials with molecular sieves, retaining rich acidic sites while obtaining high oxygen defect oxidation active components, solving the problem of acid site coverage caused by excessive loading.

[0055] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A composite catalyst for CVOCs degradation, characterized in that: It comprises a molecular sieve and MOFs arranged on the surface of the molecular sieve.

2. The composite catalyst for CVOCs degradation according to claim 1, characterized in that: The molecular sieve is an acidic molecular sieve; preferably one of HZSM-5 molecular sieve, SAPO-34 molecular sieve and β molecular sieve.

3. The composite catalyst for CVOCs degradation according to claim 1, characterized in that: The MOFs is ZIF-67.

4. The composite catalyst for CVOCs degradation according to claim 1, characterized in that: The Co content is 10%-40%; preferably >20%; preferably 21.9%-32.8%.

5. A method for preparing a composite catalyst for CVOCs degradation according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: adding cobalt salt and ligand into deionized water to form MOFs solution, adding molecular sieve into the MOFs solution, collecting precipitate after reaction, drying and then calcining to obtain composite catalyst for CVOCs degradation.

6. The preparation method according to claim 5, characterized in that: The ligand is 2-methylimidazole; the cobalt salt is cobalt nitrate hexahydrate; the molar ratio of the cobalt salt to the ligand is 1:50-80; preferably 1:65-75, more preferably 1:

66.

7. The preparation method according to claim 5, characterized in that: The drying conditions are drying at 80-120°C for 12-24 hours; preferably drying at 110°C for 8 hours.

8. The preparation method according to claim 5, characterized in that: The calcination conditions are: calcination at 300-600°C for 4 hours in a tube furnace under N2 atmosphere; after cooling, calcination at 300-600°C for 4 hours in an air atmosphere; preferably, calcination at 500°C for 4 hours in a N2 atmosphere; after cooling, calcination at 350°C for 4 hours in an air atmosphere.

9. Use of the composite catalyst for CVOCs degradation according to any one of claims 1 to 4, characterized in that: Used for CVOCs degradation.

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