Nickel-tungsten-based catalyst and application thereof in hydroreforming process

Abstract

The invention belongs to the technical field of waste plastic and light alkane hydroreforming, and relates to a nickel-tungsten-based catalyst and application thereof in a hydroreforming process. The carrier is rod-shaped or nanosheet-shaped defective tungsten oxide WO2.72, the active component is metallic nickel, and the metallic nickel is loaded on the surface of the defective tungsten oxide WO2.72 in a monatomic form. The nickel-based catalyst prepared by the method is low in price, but has very high catalytic activity, and the catalytic activity is close to or even superior to that of some noble metal catalysts under the same condition. In the hydroreforming reaction of polyolefin plastics, the selectivity of the catalyst to C5-20 highly branched hydrocarbons is greater than 95%.

Description

Technical Field

[0001] The present invention belongs to the technical field of waste plastic and light alkane hydroreforming, and relates to a nickel-tungsten-based catalyst and its application in the hydroreforming process. Background Art

[0002] Currently, the consumption of plastic products is huge, but their average service life is relatively short, resulting in increasingly prominent problems of environmental pollution and a large amount of waste of resources. With the gradual depletion of petroleum resources and the increasing attention of people to environmental issues, the recycling and resource utilization of waste plastics have become particularly urgent. The treatment methods of waste plastics include landfill, incineration, biodegradation, and chemical recycling. The landfill method is to bury waste plastics underground, which is simple and economical but prone to secondary pollution; the incineration method converts plastics into energy through high-temperature incineration, but it may produce harmful gases and ash; the biodegradation method uses microorganisms or bioenzymes to degrade plastics, which is environmentally friendly but has a slow degradation rate. The hydrocracking method in chemical recycling not only solves the pollution problem but also converts plastics into liquid alkanes, which also alleviates the tight situation of petroleum resources to a certain extent. Since the 20th century, nickel-based metal catalysts have been applied to the hydroreforming process. Nickel-based metal catalysts do not belong to precious metals, so the cost is relatively low, but they can improve the efficiency of the reforming reaction, thereby increasing the production of gasoline and aromatics. Therefore, using nickel-based catalysts for processes such as alkane hydroreforming, light alkane isomerization, and alkane dehydrogenation can improve the octane number of hydrocarbon compounds and produce high-octane hydrocarbon compounds, covering a wide range from gasoline to aviation fuel. Light alkane isomerization not only produces high-octane isomerized oil, providing high-quality blending components for gasoline composition, but also the isomerized hydrocarbons generated can be dehydrogenated to obtain isomerized olefins, providing important raw materials for the manufacture of products such as MTBE, thus helping to solve the problem of imbalance between supply and demand of light isomerized olefins. However, nickel-based catalysts themselves have problems such as low stability and easy agglomeration. Summary of the Invention

[0003] The present invention provides a novel nickel-tungsten-based catalyst and its application in the hydroreforming process in view of the problems existing in traditional nickel-based catalysts in catalytic hydrogenation.

[0004] To achieve the above object, the present invention is realized by adopting the following technical solutions: A nickel-tungsten catalyst, the carrier is defective tungsten oxide WO

[0004] ,

[0003] , , , , x , x ,

[0005] , , , (2.5 < x < 3), the active component is metallic nickel, and the metallic nickel is supported on the surface of defective tungsten oxide WO x (2.5 < x < 3) in the form of single atoms.

[0005] The above nickel-tungsten catalyst is prepared by any one of the following methods: Add a nickel source and a tungsten source into a solvent. After mixing evenly, carry out a hydrothermal reaction. After the reaction ends, filter, wash, and dry to obtain a solid. Place the solid under high temperature and in a hydrogen atmosphere for a reduction reaction. After the reduction reaction ends, cool to room temperature to obtain a catalyst; or Add a tungsten source into a solvent. After mixing evenly, carry out a hydrothermal reaction. After the reaction ends, filter, wash, and dry to obtain a solid support. Use the impregnation method to add a nickel source into deionized water and disperse evenly, then dropwise add it onto the support. After mixing evenly, dry it, and place it under high temperature and in a hydrogen atmosphere for a reduction reaction to obtain a catalyst.

[0006] Preferably, the solvent is ethanol, isopropanol, or methanol. The hydrothermal reaction temperature is 140 - 220 °C, the hydrothermal reaction time is 24 - 72 h, the hydrogen reduction reaction temperature is 300 - 600 °C, and the reduction time is at least 3 h.

[0007] Preferably, the tungsten source is one or more of tungsten pentachloride, tungsten hexachloride, or tungsten trioxide; the nickel source is one or more of nickel nitrate, nickel sulfate, nickel chloride, or nickel organic salts. The molar ratio of the nickel source to the tungsten source is 1:(5 - 50).

[0008] The present invention provides the application of the above nickel - tungsten catalyst in the hydro - reforming process of waste plastics and light alkanes. This catalyst is applicable to the hydro - reforming processes of n - paraffins, polyethylene, polypropylene, polystyrene, and their mixtures. This bifunctional catalytic system combines the advantages of two metals, nickel and tungsten, and exhibits excellent catalytic performance. Nickel has good hydrogenation activity, can effectively activate hydrogen molecules, and has a low cost and is easy to obtain; while tungsten helps to improve the stability and sulfur - resistance performance of the catalyst, thereby prolonging the service life of the catalyst. And the present invention prepares a defective tungsten oxide WO 6-20 (2.5 < x < 3) as a support. The lack of lattice oxygen on the surface of this support endows it with excellent hydrogen adsorption and dissociation capabilities. This support is in a rod - shaped or nano - sheet - shaped structure. The metallic nickel in the catalyst exists in the form of single atoms, the nickel dispersion is extremely uniform, and the particle size is all < 1 nm, having very high activity. The type of solvent, hydrothermal temperature and time, and hydrogen reduction temperature, etc. all have significant effects on the catalyst performance. The present invention regulates the morphology of defective tungsten oxide and the loading state of metallic nickel by changing the hydrothermal time, hydrothermal solvent, and reduction temperature, making Ni / WO x (2.5 < x < 3) have high activity and high selectivity in polyolefin degradation.

[0009] Compared with the prior art, the advantages and positive effects of the present invention are as follows: 1. The nickel - based catalyst prepared in the present invention has a low price, but has very high catalytic activity, and under the same conditions, its catalytic activity is close to or even exceeds some noble metal catalysts.​​

[0010] 2. In the hydrogenation reforming reaction of polyolefin plastics, the catalyst of the present invention has a great effect on C 5-20 Liquid fuels showed the best selectivity for C 5-20 The selectivity of highly branched hydrocarbons is greater than 95%.

[0011] 3. The catalyst prepared by the present invention has a simple preparation process and readily available raw materials. Furthermore, there is a strong hydrogen overflow phenomenon between the metal and the support. Compared with other nickel-based catalysts, the temperature required during the reduction process is lower, significantly reducing energy consumption during the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 TEM characterization image of the catalyst prepared in Example 1.

[0013] Figure 2 The XRD characterization diagrams of the catalysts prepared in Example 1 and Comparative Examples 1 and 2 are shown.

[0014] Figure 3 This is the R space shell bond energy characterization diagram of the catalyst in Example 1 in XAFS.

[0015] Figure 4 This is the product selectivity distribution diagram of polyethylene catalyzed by nickel-defective tungsten oxide catalyst.

[0016] Figure 5 This is the product distribution diagram of short-chain alkane hydroreforming over nickel-defective tungsten oxide catalyst.

[0017] Figure 6 This is the product distribution diagram of the hydrogenation reforming of real plastics over nickel-defective tungsten oxide catalyst, including (Ⅰ) HDPE, (Ⅱ) HDPE, (Ⅲ) LDPE, (Ⅳ) LDPE, (Ⅴ) PP, and (Ⅵ) PP.

[0018] Figure 7 TEM characterization of the catalysts prepared in Example 3 and Example 4.

[0019] Figure 8 TEM characterization comparison of the catalysts prepared in Comparative Examples 1-2, where a is Comparative Example 1 and b is Comparative Example 2.

[0020] Figure 9 This is the synchrotron radiation R space shell bond energy characterization diagram of comparative example 1.

[0021] Figure 10 The electron paramagnetic resonance comparison between Example 1 and Comparative Examples 1 and 2 is shown. DETAILED DESCRIPTION

[0022] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention is further described below in conjunction with specific embodiments. It should be noted that, in the absence of conflict, the embodiments of the present application and the features therein can be combined with each other.

[0023] The following description sets forth many specific details to facilitate a thorough understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein, and therefore, the present invention is not limited to the specific embodiments disclosed below. In the following examples, percentages of substances not otherwise specified are by weight.

[0024] Example 1 Weigh 4.8g of tungsten hexachloride powder and 0.65g of nickel chloride hexahydrate powder, mix them, add 240ml of ethanol, ultrasonicate for 20min, stir at room temperature at 450 rpm for 40min after the ultrasonication, then pour the mixture into a polytetrafluoroethylene liner, assemble the hydrothermal kettle, and hydrothermally react at 140℃ for 24h. After the hydrothermal reaction, naturally cool to room temperature and then filter. The obtained solid is washed alternately with deionized water and anhydrous ethanol 5 times, then placed in a 120℃ oven and dried for 10h. The solid powder is added to a tube furnace, and a mixed gas with a volume ratio of hydrogen: nitrogen of 1:5 is introduced to replace the air for more than 5min. Then, the flow rate of the H2 / N2 mixed gas is constant at 100ml / min, and the heating switch is turned on. The temperature is increased to 400℃ at a rate of 2℃ / min and maintained for 3h for reduction reaction. After the reduction reaction, it is naturally cooled to room temperature to obtain the catalyst Ni / WO x (2.5 <x<3)。

[0025] The catalyst prepared in this example was dispersed in ethanol and characterized by TEM. The results are as follows: Figure 1 ,from Figure 1 It can be seen from the figure that the metallic nickel in the catalyst prepared in this example is very evenly loaded on the rod-shaped defective tungsten oxide carrier.

[0026] The catalyst prepared in this example was characterized by XRD. Figure 2 ,from Figure 2 It can be seen that the carrier successfully synthesized by this method is defective tungsten oxide with rich oxygen vacancies.

[0027] The catalyst prepared in this example was characterized by synchrotron radiation XAFS, and the XAFS characterization results of nickel metal and nickel oxide were used for comparison. Figure 3 ,from Figure 3 It can be seen that the nickel on the catalyst surface exists in the form of single atoms.

[0028] Add 0.2 g of catalyst and 4.0 g of polyethylene (average molecular weight 12k) into a batch reactor. Open the hydrogen valve and fill it into the reactor to displace the air in the reactor for 5 min. Then adjust the hydrogen pressure to 0.5 MPa. Raise the temperature to 260 °C at a heating rate of 20 - 30 °C / min. Then raise the hydrogen pressure to 3 MPa, and then keep the reaction at 260 °C for 5 h. After the reaction is completed, collect the gas and liquid products and conduct corresponding chromatographic analysis. After detection, the conversion rate of polyethylene reaches 100%, and the gas product in the product, that is, C 1-4 content accounts for 2.5%, and the gasoline product C 5-12 content accounts for 73.0%, and the diesel product C 13-20 content accounts for 21.7%, and the waxy product C 21-35 accounts for 2.8%.

[0029] Carry out a catalytic reaction with 0.2 g of catalyst and 4.0 g of n-heptane. After displacing the air with hydrogen for 5 min, raise the temperature of the reactor to 195 °C at a rate of 25 °C / min, then adjust the hydrogen pressure to 2 MPa, and set the reaction time to 10 hours. After detection, the conversion rate of n-heptane is 97.5%, among which the selectivity of cracking products is 19.3% and the selectivity of isomerization products is 80.7%.

[0030] The obtained product selectivity distributions are respectively as Figure 4 and Figure 5 shown. It can be seen from Figure 4 that the main product after the hydro-reforming of polyethylene in the batch reactor is gasoline C 5-12 , with more isomerization products and higher economic value; it can be seen from Figure 5 that the main product after the hydro-reforming of n-heptane is isomerized hexane, proving that the catalyst has good hydro-reforming ability.

[0031] Example 2 Add 4.8 g of tungsten pentachloride powder into 240 ml of ethanol, ultrasonic for 20 min. After the ultrasonic treatment, start stirring (450 rpm, the same below), stir for 40 min, transfer the mixed solution to a polytetrafluoroethylene inner liner, assemble the hydrothermal reactor, carry out hydrothermal reaction at 150 °C for 24 h, filter at room temperature, wash the obtained solid alternately with 1 L of deionized water and 1 L of absolute ethanol, and then place it in an oven at 120 °C to dry for 10 h to obtain WO x (2.5 < x < 3) support. Subsequently, dissolve 0.065 g of nickel nitrate hexahydrate in 0.3 g of ultrapure water. After dissolving evenly, dropwise add it into a beaker containing 1 g of support and manually stir with a glass rod for 20 min. Then dry it in an oven at 80 °C for 12 h. After drying, add it into a tube furnace and raise the temperature to 400 °C at a rate of 2 °C per minute, and reduce it in a hydrogen atmosphere for 3 h, and naturally cool to room temperature to obtain rod-shaped Ni / WOx (2.5 < x < 3) catalyst.

[0032] Replace the raw material with octadecane, the reaction temperature is 195 °C, the sampling time is 10 h, the reaction hydrogen pressure is 2 MPa, and the other conditions and operations are the same as those in the catalytic reaction of n - heptane in Example 1. The results of the product selectivity distribution map are as Figure 6 , from Figure 6 it can be seen that the products after hydro - reforming are mainly isomerized octadecane, proving that the catalyst has good hydro - reforming ability.

[0033] Example 3 Add 4.8 g of tungsten pentachloride powder and 0.65 g of nickel chloride hexahydrate powder into 240 ml of isopropanol, ultrasonic for 20 min, then stir at 450 rpm (the same below) for 40 min, then transfer the mixed solution to the inner lining of polytetrafluoroethylene, assemble the hydrothermal reactor, carry out hydrothermal reaction at 175 °C for 24 h, after the hydrothermal reaction, naturally cool to room temperature, and filter at room temperature. Wash the obtained solid with 1 L of deionized water and 1 L of absolute ethanol alternately, then place it in an oven at 120 °C and dry for 10 h to obtain the dried catalyst powder. Add the catalyst powder into a tubular furnace and heat it to 400 °C at a rate of 2 °C / min, reduce it in a hydrogen atmosphere for 3 h, and naturally cool to room temperature to obtain the nanosheet - shaped Ni / WO x (2.5 < x < 3) catalyst. Use the same raw materials and conditions in Example 2 for performance verification. After testing, the products of the catalyst prepared in this example for the hydro - reforming of octadecane are also mainly isomerized octadecane, and the conversion rate and selectivity of the catalyst are not much different from those in Example 2.

[0034] Example 4 Add 4.8 g of tungsten hexachloride powder and 0.45 g of nickel sulfate hexahydrate powder into 240 ml of methanol, ultrasonic for 20 min, then stir at 450 rpm (the same below) for 40 min, then transfer the mixed solution to the inner lining of polytetrafluoroethylene, assemble the hydrothermal reactor, carry out hydrothermal reaction at 185 °C for 24 h, after the hydrothermal reaction, naturally cool to room temperature, and filter at room temperature. Wash the obtained solid with 1 L of deionized water and 1 L of absolute ethanol alternately, then place it in an oven at 120 °C and dry for 10 h to obtain the dried catalyst powder. Add the catalyst powder into a tubular furnace and heat it to 400 °C at a rate of 2 °C / min, reduce it in a hydrogen atmosphere for 3 h, and naturally cool to room temperature to obtain the nano - short - rod - shaped Ni / WO x (2.5 < x < 3) catalyst. Use the same raw materials and conditions in Example 2 for performance verification. After testing, the products of the catalyst prepared in this example for the hydro - reforming of octadecane are also mainly isomerized octadecane, and the conversion rate and selectivity of the catalyst are not much different from those in Example 2.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that the hydrothermal time is extended to 80 h, and the rest of the preparation process remains unchanged.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that the reduction temperature is 650° C., and the rest of the preparation process remains unchanged.

[0037] The catalyst prepared in Comparative Example 1-2 was used to replace the catalyst used in the n-heptane catalytic reaction in Example 1. Other raw materials and conditions remained the same, and performance tests were conducted. The results are shown in Table 1 below.

[0038] Table 1 Performance test results of the catalysts prepared in Comparative Examples 1-2 The catalyst prepared in Comparative Example 1-2 was characterized by TEM. Figure 8 As shown, from Figure 8 It can be seen that different hydrothermal time and reduction temperature have a relatively large impact on the morphology of the support and the dispersion state of the metal nickel. Excessive hydrothermal time and reduction temperature destroy the rod-like structure on the support surface and reagglomerate into large spheres. The metal nickel on the surface is unevenly dispersed, and many large nickel particles appear. The active sites are reduced, and the performance of the catalyst is greatly reduced compared with Example 1. Comparative Example 1 was characterized by synchrotron radiation. The results are as follows Figure 9 As shown in FIG1 , the nickel metal in Comparative Example 1 is partially present as nickel particles, while the other part is present in the carrier in the form of nanoclusters. This makes the nickel dispersion on the carrier surface uneven, resulting in the catalyst requiring more energy for hydrogen dissociation and desorption than in Example 1, limiting the performance of the catalyst in polyolefin degradation. In order to verify the presence of oxygen vacancies in the carrier, electron paramagnetic resonance tests were performed on Example 1 and Comparative Examples 1-2. The results are shown in FIG1 . Figure 10 As shown, only Example 1 has a strong oxygen vacancy signal, which indicates that there is a large amount of missing lattice oxygen in the rod-shaped support. The abundant oxygen vacancies greatly enhance the catalyst's adsorption of hydrogen, resulting in more active sites on the support surface, which makes Example 1 exhibit excellent hydrocracking performance.

[0039] The catalyst prepared in Example 1 can effectively realize the hydroreforming of light alkanes. Under the experimental conditions, the hexane conversion rate reaches 97.5% and the selectivity of the isomerized substrate reaches 80.7%. Figure 1 , Figure 3 , Figure 7 , Figure 8 , Figure 9It can be seen that the dispersion degree of metallic nickel and the morphology of defective tungsten oxide have a relatively significant impact on the performance of the catalyst. One is the dispersion degree of metallic nickel on the surface of the support. From Figure 1 and Figure 8 , we can see that there are significant differences in the dispersion degree of nickel on the surface of nickel-tungsten oxide catalysts prepared by different methods. When ethanol is used as the solvent and the Ni / WO x (2.5 < x < 3) catalyst is prepared by the in-situ synthesis method, metallic nickel exists in the form of single atoms on the surface of defective tungsten oxide, and the single-atom nickel provides abundant active sites for the reaction. The single-atom form of nickel and the particulate form of nickel result in a large difference in the energy required for the dissociation of hydrogen on the surface of the support. The second is the morphology of the support. From Figure 7 , it can be seen that when the solvent is isopropanol, the synthesized morphology is nanosheet-like, and when methanol is used as the solvent, the synthesized morphology is nanorod-like, rather than rod-shaped. This has a certain impact on the catalytic activity of the reaction. With different solvents, the catalyst morphology may be rod-shaped (ethanol), nanosheet-like (isopropanol), nanorod-like (methanol), etc., and supports with various morphologies. Defective tungsten oxides with various morphologies all have a certain degradation effect on polyolefins. Among them, WO x (2.5 < x < 3) with a rod-shaped structure exhibits the highest activity in the polyethylene hydrocracking reaction. From Figure 10 in the electron paramagnetic resonance, it can be seen that only Example 1 has abundant oxygen vacancies, and the existence of oxygen vacancies indicates that Example 1 has excellent hydrogen dissociation ability. The highly dispersed nickel on its surface makes it have excellent hydrocracking ability and exhibits characteristics such as high conversion rate and high selectivity in the polyethylene degradation reaction.

[0040] As described above, it is only a preferred embodiment of the present invention and not a limitation to the present invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or modifications into equivalent embodiments with equivalent changes and apply them to other fields. However, as long as it does not depart from the technical solution content of the present invention, any simple modification, equivalent change, and modification made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A nickel-tungsten catalyst, characterized in that The carrier is a rod-shaped or nanosheet-shaped defective tungsten oxide WO x The active component is nickel metal, which is loaded on defective tungsten oxide WO in the form of single atoms. x Surface where 2.5<x<3.

2. The method for preparing the nickel-tungsten catalyst according to claim 1, characterized in that: Prepared by any of the following methods: Adding a nickel source and a tungsten source to a solvent, mixing them evenly and then performing a hydrothermal reaction, filtering, washing and drying to obtain a solid after the reaction is completed, placing the solid under high temperature and a hydrogen atmosphere for a reduction reaction, and cooling to room temperature after the reduction reaction is completed to obtain a catalyst; or The tungsten source is added to the solvent, mixed evenly and then subjected to a hydrothermal reaction. After the reaction is completed, the solid carrier is filtered, washed and dried to obtain a solid carrier. The nickel source is added to deionized water by an impregnation method and dispersed evenly, and then added dropwise to the carrier. After mixing evenly, it is dried and placed under high temperature and hydrogen atmosphere for a reduction reaction to obtain a catalyst.

3. The method for preparing the nickel-tungsten catalyst according to claim 2, characterized in that: The solvent is ethanol, isopropanol or methanol, the hydrothermal reaction temperature is 140-220° C., and the hydrothermal reaction time is 24-72 hours; the reduction reaction temperature is 300-600° C., and the reduction time is at least 3 hours.

4. The method for preparing the nickel-tungsten catalyst according to claim 2, characterized in that: The tungsten source is one or more of tungsten pentachloride, tungsten hexachloride or tungsten trioxide; the nickel source is one or more of nickel nitrate, sulfate, chloride or organic salt, and the molar ratio of the nickel source to the tungsten source is 1: (5-50).

5. Use of the nickel-tungsten catalyst in claim 1 in the hydrogenation reforming process of waste plastics and light alkanes.

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