Polyolefin thermal cracking catalyst as well as preparation method and application thereof

Abstract

The invention discloses a polyolefin thermal cracking catalyst as well as a preparation method and application thereof, and relates to the technical field of catalysts. According to the polyolefin thermal cracking catalyst provided by the invention, the fluorine ions are firstly adopted to modify the catalyst carrier, and the fluorine ions are filled into the crystal lattice of the carrier to form crystal lattice distortion, so that the activity of the carrier is enhanced; and secondly, a second metal is loaded on the carrier to serve as an auxiliary active component, so that the catalytic activity and carbon deposition resistance of Ni are improved, and based on dual modification of the carrier and the active component, the catalyst provided by the invention has remarkably improved catalytic activity and long-range stability.

Description

Technical Field

[0001] This invention relates to the field of catalyst technology, specifically to a polyolefin thermal cracking catalyst, its preparation method, and its application. Background Technology

[0002] Polyolefins are among the most commonly used plastics, but current polyolefin products are typically single-use and discarded after use. Furthermore, polyolefins take over 500 years to degrade in nature. This has led to increasingly serious environmental pollution from discarded polyolefins, making the development of a polyolefin recycling technology an urgent priority.

[0003] Effective recycling of waste polyolefins can be achieved by degrading polyolefins. The main technologies for polyolefin degradation and recycling include pyrolysis, catalytic pyrolysis (one-step method), and pyrolysis-catalytic modification (two-step method). Pyrolysis typically involves heating polyolefins at 300–500°C in an oxygen-free environment to achieve recycling; however, this method is energy-intensive, and the resulting product components are relatively dispersed, limiting its application value. Catalytic pyrolysis introduces a catalyst into the conventional pyrolysis process, which can lower the pyrolysis temperature and increase the yield of liquid and solid phase products. However, the catalyst in this method is difficult to regenerate and recycle (it is prone to structural collapse and agglomeration of active components during use), and is discarded after a single use, resulting in waste. Furthermore, impurities can easily "poison" the catalyst, reducing the stability of the production process. Pyrolysis-catalytic modification involves modifying the pyrolysis products after plastic pyrolysis using a catalyst, enabling the resource utilization of waste plastics; however, the process is complex and costly. Therefore, it is of great significance to provide a thermal cracking catalyst that can combine long-term stability, high catalytic activity (high conversion rate and high liquid-solid phase yield) and regenerability. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a polyolefin thermal cracking catalyst, its preparation method, and its application.

[0005] The above-mentioned objective of this invention is achieved through the following technical solution: A method for preparing a polyolefin thermal decomposition catalyst includes the following steps: S1. The catalyst support is impregnated and modified with a fluoride ion solution, followed by calcination to obtain a fluoride-modified support; the molar ratio of fluoride ions to the catalyst support in the fluoride ion solution is (5-20):1; the catalyst support includes oxides; S2. Under solution conditions, the fluorine-modified support obtained in step S1 is mixed with Ni source and second metal source in a molar ratio of (5-30):1:(0.5-10), allowed to stand for aging, then dried, calcined at 300-500℃, and then roasted at 500-700℃ to obtain a polyolefin thermal decomposition catalyst; the electronegativity of the second metal is lower than that of Ni.

[0006] The catalyst provided by this invention includes an oxide support (in MO). x This indicates that M can be either a nonmetal or a metal; when M is a metal, MO x This refers to a metal oxide and an active component supported on a support, wherein the support is modified with fluorine, and the active component includes Ni and a second metal with lower electronegativity. In this invention, due to F... - With a small radius and high electronegativity, it possesses extremely strong gap-filling ability. The inventors discovered through experiments that a simple impregnation method in step S1 is sufficient to make F... - O on the lattice points of the oxide support cell is partially replaced. 2- Partially filling the voids within the crystal lattice, it locally creates lattice distortion, enhances the activity of the support, and generates strong Lewis acid sites (MF bonds). This significantly improves the catalytic activity of the support in promoting the breaking of C-C bonds in polyolefins, accelerating the pre-cracking of long polyethylene chains, while avoiding the introduction of... Acid sites lead to excessive cracking of polyolefins, thus reducing the generation of gas-phase cracking products such as methane; the addition of F also forms a solid solution inside the support, which can stabilize the crystal lattice, prevent the support from undergoing crystal transformation or structural collapse at high temperatures, and improve the stability of the catalyst.

[0007] Based on the fluorine modification of the support, this application also introduces a second metal with lower electronegativity than Ni to synergistically act as an active component for catalysis. Ni, as the main catalytic active site, has a high electronegativity and exhibits strong binding strength with the products obtained from the degradation of polyolefins. This leads to the degradation products easily adsorbing onto Ni sites after catalysis, making them difficult to remove and thus masking the active sites and reducing catalytic activity. The second metal introduced in this invention (represented by Me) has even lower electronegativity and can donate electrons to Ni sites to form a Niδ-Meδ structure. This not only weakens the adsorption strength of Ni on cracking intermediates (olefins / aromatics), inhibiting polymerization and coking, resulting in a significant increase in the catalyst's resistance to carbon deposition and a longer service life, but also helps activate the C-C bonds in polyolefins, increasing the cracking rate and reducing energy consumption. Therefore, the bimetallic synergy can also improve catalytic activity, and the enhanced catalytic activity can significantly increase the likelihood of polyolefin molecular chains breaking from the middle, improving the selectivity of light aromatics or short-chain olefins, avoiding the generation of worthless heavy residues, and thus improving the economic value of the product. The inventors of this application have also discovered that fluorine-modified supports not only possess higher stability and more Lewis acidic sites, but the introduction of fluorine into the support can also improve the dispersion of metal elements through charge repulsion, enhance the polarization between positive and negative ions, transform bond properties, and increase bond strength, making the active metal component less prone to detachment, thereby further extending the catalyst lifetime. Based on the separate modification of the support and the active component, as well as the synergistic effect of the two modification methods, the catalyst provided by this invention exhibits significantly improved catalytic activity and long-range stability. Furthermore, due to the stable catalyst structure, the support is less prone to collapse, and the active component is less prone to aggregation. The catalyst of this invention can also be regenerated through a simple reduction method, significantly reducing catalytic costs.

[0008] It should be noted that the amount of F and metal elements introduced onto the support in this invention is controlled by the raw material ratio. Therefore, steps S1-S2 of this invention respectively define the ratio of the support to each preparation raw material. Insufficient F ion introduction will result in insufficient F modification to adequately improve catalyst performance; however, excessive F ion introduction can lead to excessive lattice distortion within the support, resulting in decreased catalyst support stability. Insufficient introduction of either metal element will decrease catalytic activity, while excessive introduction will lead to agglomeration and reduced long-term catalytic stability. The calcination and roasting temperatures affect the catalyst structure. Calcination and roasting at appropriate temperatures ensure that the active precursor is fully transformed into the target active phase while preventing catalyst structure collapse.

[0009] It should be noted that, in this invention, electronegativity is a physical quantity describing the ability of an atom or molecule to attract and share electrons. A higher electronegativity indicates a stronger affinity of the element for electrons, meaning the element more easily gains electrons from other atoms or ions to form negative ions or anions. In specific embodiments of this invention, the electronegativity of the element is obtained using the Pauling electronegativity data table.

[0010] Preferably, the molar ratio of fluoride ions to catalyst support in the fluoride ion solution in step S1 is (6-10):1.

[0011] Preferably, the particle size of the catalyst support in step S1 is 1-20 μm.

[0012] Preferably, the catalyst support in step S1 includes at least one of Al2O3, SiO2, TiO2, CeO2, and ZrO2.

[0013] More preferably, the catalyst support in step S1 includes Al2O3.

[0014] More preferably, the Al2O3 is γ-Al2O3.

[0015] γ-alumina is a transition phase with a huge specific surface area and high catalytic activity, while other phases, such as α-phase, have lower surface activity.

[0016] Preferably, the impregnation modification in step S1 is carried out at a temperature of 50-120°C for 1-3 hours.

[0017] Preferably, the calcination in step S1 is carried out under an inert atmosphere for 2-6 hours.

[0018] More preferably, the inert atmosphere includes nitrogen.

[0019] Preferably, the fluoride ion solution in step S1 comprises an aqueous solution of ammonium fluoride.

[0020] Preferably, the molar ratio of the fluorine-modified support to the Ni source and the second metal source in step S2 is (15-25):1:(2-5).

[0021] Preferably, the solution conditions in step S2 are provided by at least one of water, methanol, ethanol, isopropanol, and N,N-dimethylformamide. More preferably, the solution conditions in step S2 are provided by water.

[0022] Preferably, the Ni source in step S2 includes nickel nitrate.

[0023] Preferably, in step S2, the second metal source comprises a nitrate of a second metal.

[0024] Preferably, the static aging time in step S2 is 12-48 hours.

[0025] Preferably, the calcination in step S2 is carried out under an inert atmosphere for 1-5 hours.

[0026] More preferably, the inert atmosphere includes nitrogen.

[0027] Preferably, the calcination in step S2 is carried out in a reducing atmosphere for 3-6 hours.

[0028] More preferably, the reducing atmosphere comprises a hydrogen-argon mixture.

[0029] Preferably, in step S2, the second metal is selected from any one of Cr, Ti, Mn, Fe, Zn, Mo, Cd, Ta, and La.

[0030] More preferably, in step S2, the second metal is selected from Cr.

[0031] The electronegativity of Cr (1.66) is lower than that of Ni (1.91), which can effectively weaken the adsorption strength of Ni for cracking intermediates (olefins / aromatics) and inhibit polymerization and coking; at the same time, Cr 3+ Than Ni 2+ With a high charge density, it is more easily oxidized by oxygen to form Cr2O3, thus preventing Ni from being deeply oxidized during the catalytic process and losing its catalytic activity.

[0032] More preferably, in step S2, the fluorine-modified support obtained in step S1 is first mixed with the second metal source under solution conditions, allowed to stand for aging, and then dried to obtain a fluorine-modified support loaded with the second metal; the fluorine-modified support loaded with the second metal is mixed with the Ni source under solution conditions, allowed to stand for aging, and then dried, and then calcined and roasted in sequence.

[0033] When the second metal is Cr, it is necessary to load the second metal first, followed by Ni. This is primarily because Cr... 3+ Than Ni 2+ High charge density makes it easier to bind to strong interaction sites on the carrier. If nickel is loaded first, chromium will cover the nickel particles, resulting in alloy inhomogeneity. Secondly, aluminum oxide reacts with nickel at high temperatures to form NiAl2O4, a spinel phase that is difficult to reduce and inactive. Chromium oxide can block this reaction like a "barrier coating".

[0034] In a specific embodiment of the present invention, the time for aging after mixing the fluorine-modified support obtained in step S1 with the second metal source under solution conditions is t1; the time for aging after mixing the fluorine-modified support loaded with the second metal with the Ni source under solution conditions is t2, t1 + t2 = t, t = the aging time described in step S2 = 12-48h. More specifically, t1 = t2.

[0035] This invention also protects the polyolefin thermal decomposition catalyst prepared by the above preparation method.

[0036] This invention also protects a method for catalytic degradation of polyolefins, comprising the following steps: The above catalyst is mixed with polyolefin at a mass ratio of 1:(2-30), and reacted at 200-400℃ to obtain polyolefin degradation products.

[0037] Preferably, the reaction time is 24-60 hours.

[0038] Controlling the reaction time within 24-60 hours ensures that the catalyst activity does not decrease excessively during the catalytic process. However, even if the catalyst activity decreases significantly, the present invention can still restore the catalyst activity through regeneration.

[0039] Preferably, the particle size of the polyolefin is <5 mm.

[0040] Preferably, the polyolefin contains 80-200 ppm of sulfur impurities and 40-100 ppm of chloride impurities.

[0041] The catalyst provided by this invention has a higher tolerance for unavoidable impurities in waste polyolefin raw materials.

[0042] Compared with the prior art, the present invention has the following beneficial effects: The catalyst obtained by this invention has excellent catalytic activity, with a reactant conversion rate of over 90.1% during PE degradation, a yield of over 70.9% for high-value C5-C20 fractions, and a continuous operating life of over 40 hours. Attached Figure Description

[0043] Figure 1 This is a comparison of XRD patterns of F-Al2O3 and γ-Al2O3 obtained in step S1 of Example 1 of the present invention.

[0044] Figure 2 This is a SEM image of γ-Al2O3.

[0045] Figure 3 This is a SEM image of the catalyst prepared in Example 1 of the present invention.

[0046] Figure 4This is an EDS surface scan diagram of the catalyst prepared in Example 1 of the present invention. Detailed Implementation

[0047] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents. The raw material information used in each embodiment and comparative example is as follows: γ-Al2O3: Commercially available, ground to a particle size of 5-6 μm.

[0048] TiO2: Commercially available anatase type, ground to a particle size of 5-6μm.

[0049] Polyethylene tubing: purchased from Sanais, model 0640 (outer diameter 6mm, inner diameter 4mm). Pre-treatment was performed after purchase, including the following steps: shredding and hammer grinding to a particle size <5mm, magnetic separation to remove iron, air separation to remove light impurities, followed by PVC removal under an inert atmosphere at 200℃, and residual chlorine removal via a Ca(OH)2 bed.

[0050] Example 1 A method for preparing a polyolefin thermal decomposition catalyst includes the following steps: S1. Weigh 0.4 mol of NH4F solid into a plastic beaker, add 200 mL of deionized water and stir until completely dissolved to obtain a 2 M NH4F solution; add 0.05 mol of γ-Al2O3 to the NH4F solution, stir at 80 °C for 2 h, centrifuge to collect the precipitate, wash three times with deionized water and ethanol respectively, place the obtained solid in an oven to dry at 80 °C for 12 h, and then place it in a tube furnace to calcine at 550 °C for 4 h under a nitrogen atmosphere to obtain a fluorine-modified support, denoted as F-Al2O3; the molar ratio of fluoride ions to catalyst support in the NH4F solution is 8:1; S2. Add 0.01 mol Cr(NO3)3·9H2O to 250 mL of deionized water and stir for 1 h to obtain a 0.04 M chromium nitrate solution. Pour the solution into F-Al2O3, stir, and let it stand for 12 h. Wash the solution three times with deionized water, centrifuge, and dry the precipitate for 12 h. Add 0.0025 mol Ni(NO3)2·6H2O to 250 mL of deionized water and stir for 1 h to obtain a 0.01 M nickel nitrate solution. Add the above precipitate, stir, and let it stand for 12 h. Wash the solution three times with deionized water, centrifuge, and dry the precipitate for 12 h. Calcine the precipitate at 450 °C under nitrogen for 3 h, and then calcine it at 550 °C under H2 / Ar for 4 h to obtain a polyolefin thermal decomposition catalyst. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:1:4.

[0051] Example 2 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Ni(NO3)2·6H2O added is 0.01 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 5:1:1.

[0052] Example 3 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Cr(NO3)3·9H2O added is 0.005 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:1:2.

[0053] Example 4 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Cr(NO3)3·9H2O added is 0.02 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:1:8.

[0054] Example 5 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S1, the amount of NH4F added is 0.6 mol. The molar ratio of fluoride ions to catalyst support in the NH4F solution is 12:1.

[0055] Example 6 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S1, γ-Al2O3 is replaced with an equimolar amount of TiO2.

[0056] Example 7 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, Cr(NO3)3·9H2O is replaced with an equimolar amount of Fe(NO3)3·9H2O.

[0057] Comparative Example 1 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, chromium is not introduced. Ni(NO3)2·6H2O is dissolved in water to obtain a nickel nitrate solution, and then F-Al2O3 is added for subsequent operations.

[0058] Comparative Example 2 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: Step S2 is omitted. The F-Al2O3 obtained in step S1 is the catalyst obtained in this comparative example.

[0059] Comparative Example 3 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: Step S1 is skipped. After dissolving Cr(NO3)3·9H2O in water to obtain a chromium nitrate solution, γ-Al2O3 is added for subsequent operations.

[0060] Comparative Example 4 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: Replace NH4F in step S1 with an equimolar amount of NH4Cl.

[0061] Comparative Example 5 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: Replace Cr(NO3)3·9H2O in step S2 with an equimolar amount of Pd(NO3)2·2H2O.

[0062] Comparative Example 6 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Ni(NO3)2·6H2O added is 0.025 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:10:4.

[0063] Comparative Example 7 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Ni(NO3)2·6H2O added is 0.0005 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:0.2:4.

[0064] Comparative Example 8 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S2, the amount of Cr(NO3)3·9H2O added is 0.0005 mol. The molar ratio of F-Al2O3 support to nickel nitrate and chromium nitrate is 20:1:0.2.

[0065] Comparative Example 9 A method for preparing a polyolefin thermal cracking catalyst, wherein the only difference from Example 1 is: In step S1, the amount of NH4F added is 1.5 mol. The molar ratio of fluoride ions to catalyst support in the NH4F solution is 30:1.

[0066] Performance testing Reactant conversion rate and product yield test: The catalysts obtained in the examples and comparative examples were mixed with polyethylene gas tubes at a mass ratio of 1:10 and reacted at 300°C for 2 hours to obtain polyolefin degradation products; the composition of the system after reaction was detected by thermogravimetric analysis and gas chromatography.

[0067] Continuous operating life test: The catalysts obtained in the examples and comparative examples were mixed with polyethylene at a mass ratio of 1:10 and reacted at 300°C. The composition of the system was monitored in real time using a gas chromatograph. Every 8 hours, 5 wt% of the catalyst was taken for reduction and regeneration (regeneration method: H2 reduction at 500°C for 1 hour) until the conversion rate of polyethylene remained below 80% for 1 hour. This was considered to have reached the maximum continuous operating life of the catalyst, and the reaction was terminated.

[0068] The test results are shown in Table 1 below: Table 1. As can be seen from Table 1 above, the catalyst obtained by the present invention has excellent catalytic activity, with a reactant conversion rate of over 90.1% during PE degradation, a yield of over 70.9% for high-value C5-C20 fractions, and a continuous operating life of over 40 hours.

[0069] As can be seen from the data in Examples 1-4, the catalytic activity is better when the molar ratio of the fluorine-modified support to the Ni source and the second metal source in step S2 is the preferred ratio of (15-25):1:(2-5) (Examples 1 and 3). Specifically, in Example 2, with a fixed amount of support and second metal added, increasing the Ni loading actually decreased the catalytic activity; in Example 4, with a fixed amount of support and Ni added, increasing the second metal loading also negatively impacted the catalytic effect, indicating that the ratio of the three components needs to be limited within a certain range for them to fully synergize.

[0070] According to Examples 1 and 5, when the molar ratio of fluoride ions to catalyst support in the fluoride ion solution in step S1 is the preferred ratio of (6-10):1 (Example 1) of the present invention, the catalytic activity is better. Increasing the amount of fluoride ions introduced does not necessarily improve the catalytic activity.

[0071] According to Comparative Example 1, without the introduction of a second metal, it is impossible to provide electrons to Ni sites to form a Niδ-Meδ structure, which weakens the adsorption strength of Ni for cracking intermediates (olefins / aromatics), thereby reducing both the activity and lifespan of the catalyst.

[0072] According to Comparative Examples 2-3, both F modification and the introduction of second metals have a key impact on the catalyst's performance.

[0073] According to Comparative Examples 4-5, inappropriate material selection also fails to improve catalytic performance. For example, replacing F with Cl (Comparative Example 4) results in Cl having a lower interstitial filling capacity than F and a lower electronegativity, leading to insufficient modification of the support. Replacing the second metal with a lower electronegativity with Pd (Comparative Example 5) improves catalytic activity but fails to enhance the catalyst's resistance to carbon deposition, resulting in a reduced service life.

[0074] According to Comparative Examples 6-9, inappropriate amounts of F ions and metal active components can lead to insufficient synergy among the components, resulting in a decrease in catalytic effect.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a polyolefin thermal decomposition catalyst, characterized in that, Includes the following steps: S1. The catalyst support is impregnated and modified with a fluoride ion solution, and then calcined at 500-700℃ to obtain a fluoride-modified support; the molar ratio of fluoride ions to the catalyst support in the fluoride ion solution is (5-20):1; the catalyst support includes oxides; S2. Under solution conditions, the fluorine-modified support obtained in step S1 is mixed with Ni source and second metal source in a molar ratio of (5-30):1:(0.5-10), allowed to stand for aging, then dried, calcined at 300-500℃, and then roasted at 500-700℃ to obtain a polyolefin thermal decomposition catalyst; the electronegativity of the second metal is lower than that of Ni.

2. The preparation method according to claim 1, characterized in that, The particle size of the catalyst support in step S1 is 1-20 μm; and / or the catalyst support in step S1 includes at least one of Al2O3, SiO2, TiO2, CeO2, ZrO2, and MgO.

3. The preparation method according to claim 1, characterized in that, The impregnation modification in step S1 is carried out at a temperature of 50-120℃ for 1-3 hours. And / or, the calcination described in step S1 is carried out under an inert atmosphere for 2-6 hours.

4. The preparation method according to claim 1, characterized in that, The static aging time described in step S2 is 12-48 hours; And / or, the calcination described in step S2 is carried out under an inert atmosphere for 1-5 hours; And / or, the calcination described in step S2 is carried out in a reducing atmosphere for 3-6 hours.

5. The preparation method according to claim 1, characterized in that, Step S2: The second metal is selected from any one of Cr, Ti, Mn, Fe, Zn, Mo, Cd, Ta, and La.

6. The preparation method according to claim 5, characterized in that, In step S2, the second metal is selected from Cr.

7. The preparation method according to claim 6, characterized in that, In step S2, the fluorine-modified support obtained in step S1 is mixed with the second metal source under solution conditions, allowed to stand for aging, and then dried to obtain the fluorine-modified support loaded with the second metal. Under solution conditions, the fluorine-modified support loaded with the second metal is mixed with the Ni source, allowed to stand for aging, and then dried, and then calcined and roasted in sequence.

8. A polyolefin thermal decomposition catalyst prepared by any one of claims 1-7.

9. A method for catalytic degradation of polyolefins, characterized in that, Includes the following steps: The catalyst described in claim 8 is mixed with polyolefin at a mass ratio of 1:(2-30), and reacted at 200-400°C to obtain the polyolefin degradation product.

10. The method for catalytic degradation of polyolefins as described in claim 9, characterized in that, The particle size of the polyolefin is <5mm.

Images