HZSM-5 molecular sieve-based hydrocracking catalyst, and preparation method and application thereof

Abstract

This invention belongs to the field of catalytic chemical technology, disclosing an HZSM-5 molecular sieve-based hydrocracking catalyst, its preparation method, and its applications. The catalyst prepared by this invention uses HZSM-5 molecular sieve as the support and non-noble metal Ni as the active phase. Different carbon sources are doped to prepare Ni / HZSM-5 catalysts with varying carbon doping, among which the CA-Ni / HZSM-5 catalyst prepared using citric acid as the carbon source exhibits the best performance. The catalyst prepared by this invention has low loading, high dispersion, and small Ni particle size. The catalyst maintains stable activity after five cycles. The catalyst described in this invention exhibits high catalytic activity in the LDPE hydrocracking reaction to produce C3 gas.

Description

Technical Field

[0001] This invention belongs to the field of catalytic chemical technology, and relates to an HZSM-5 molecular sieve-based hydrocracking catalyst, its preparation method, and its application. Background Technology

[0002] Plastics, with their lightweight, durability, versatility, and low cost, have been widely used in packaging, agriculture, healthcare, automotive manufacturing, and daily life since the mid-20th century. This has greatly promoted the development of modern industry and improved social convenience. However, plastic products, especially short-lived packaging materials such as bags, films, and bottles, quickly become waste after use. Their inherent chemical stability makes them difficult to degrade naturally, leading to a serious "white pollution" crisis that poses a long-term threat to ecosystems and human health. Traditional waste plastic management strategies, such as landfill and incineration, have significant drawbacks. Therefore, developing efficient and environmentally friendly waste plastic recycling and upgrading technologies is a key challenge and an important research direction in the global pursuit of sustainable development and the transition from a linear economy to a circular economy. Among numerous chemical recycling technologies for waste plastics, chemical conversion methods such as pyrolysis, catalytic cracking, and hydrocracking show great potential because they can convert polymer waste into valuable fuels or chemicals. Hydrocracking combines the breaking of carbon-carbon bonds with the hydrogenation reaction of unsaturated intermediates in a hydrogen atmosphere. Compared with simple pyrolysis or catalytic cracking, hydrocracking effectively inhibits coke formation, increases the yield of valuable products, and favors the formation of saturated alkanes, thus obtaining higher quality output. It is particularly suitable for producing clean light alkanes (such as ethane, propane, butane; C2–C4 fractions), which are important chemical feedstocks or clean fuels. Low-density polyethylene (LDPE), as a typical polyolefin waste, has become an ideal model compound for studying polyolefin hydrocracking due to its long molecular chain, predominantly linear structure with branches, high carbon-carbon bond energy, and chemical inertness.

[0003] An ideal catalyst must possess two different types of active sites: metallic sites for hydrogen dissociation activation and hydrogenation / dehydrogenation of intermediates, and acidic sites for protonation and selective cleavage of C-C bonds. In terms of metallic composition, noble metals (such as platinum and palladium) exhibit excellent hydrogenation activity and anti-coking properties, but their high cost limits large-scale industrial applications. Non-noble metal nickel (Ni) is considered a promising alternative due to its relatively low cost and good hydrogen activation capabilities. However, while the Ni / HZSM-5 bifunctional catalyst exhibits high activity in the hydrocracking of LDPE, it lacks selectivity for C3 chemicals. Summary of the Invention

[0004] In order to optimize the performance of the Ni / HZSM-5 bifunctional catalyst, the inventors of this application tried adding different carbon sources such as glucose, sucrose, urea, citric acid, etc., and found that these carbon sources can finely modulate the acidity, pore structure and metal dispersion of the catalyst through in-situ carbonization during the catalyst preparation process, thereby specifically solving the problem of insufficient selectivity of C3 chemicals.

[0005] The purpose of this invention is to provide an HZSM-5 molecular sieve-based hydrocracking catalyst, its preparation method, and its applications. This catalyst can convert LDPE plastics into C3 chemicals with high activity and high selectivity.

[0006] The technical solution of the present invention: An HZSM-5 molecular sieve-based hydrocracking catalyst, using HZSM-5 molecular sieve as a support, with added carbon source to disperse metallic Ni on the support surface, wherein the loading of metallic Ni is 5-15 wt.%, and the amount of carbon source added is 1-2 times that of metallic Ni; The average particle size of metallic Ni on the surface of the HZSM-5 molecular sieve-based hydrocracking catalyst is 7-10 nm.

[0007] A method for preparing an HZSM-5 molecular sieve-based hydrocracking catalyst, comprising the following steps: S1. Prepare a Ni(NO3)2·6H2O aqueous solution A with a concentration of 0.5-1.7 mol / L. Add a carbon source to the Ni(NO3)2·6H2O aqueous solution A to form solution B. Then add solution B dropwise into the HZSM-5 molecular sieve support and impregnate it evenly. After impregnation, age the HZSM-5 molecular sieve support at room temperature overnight and then dry it to obtain the catalyst precursor. The carbon source is selected from glucose, sucrose, urea, and citric acid; The mass ratio of carbon source to metallic Ni is 1:1 to 2:1. Drying temperature 110℃, drying time 6 h; S2, calcined, reduced and passivated catalyst precursor, to obtain HZSM-5 molecular sieve-based hydrocracking catalyst; The catalyst precursor obtained in step S1 was calcined under N2 atmosphere, then reduced in situ under H2 atmosphere, cooled after reduction, and then continuously passivated in an atmosphere with an O2 / Ar2 volume ratio of 1:99. After passivation, the temperature was lowered to room temperature, and the Ni loading was 5-15 wt.%.

[0008] The calcination temperature was 300 ℃, the heating rate was 2℃ / min, and the calcination time was 2 h. The in-situ reduction temperature is 450-650 ℃, the heating rate is 2℃ / min, and the in-situ reduction time is 1-4 h; The passivation temperature was 20 °C and the passivation time was 2 h.

[0009] The HZSM-5 molecular sieve-based hydrocracking catalyst obtained by the above preparation method is used for the hydrocracking reaction of low-density polyethylene (LDPE), a polyolefin plastic. The specific conditions are as follows: the HZSM-5 molecular sieve-based hydrocracking catalyst and the LDPE are mixed and reacted at a temperature of 280-310℃, a pressure of 2-4 MPa, and a stirring speed of 500 rpm for 2-4 h. The amount of hydrocracking catalyst is 6.25-12.5% ​​of the mass of LDPE.

[0010] The beneficial effects of this invention are: 1. The catalyst prepared in this invention uses HZSM-5 molecular sieve as a support and non-precious metal Ni as the active phase. In-situ carbonization is performed by introducing different carbon sources to adjust the metal-acid balance (MAB) of the catalyst. Citric acid is used as the carbon source, resulting in the CA-Ni / HZSM-5 catalyst exhibiting optimal performance. The raw materials for catalyst preparation are inexpensive and readily available, and the hydrogenation reaction conditions are mild.

[0011] 2. The catalyst prepared by this invention has low loading, high dispersion, and small Ni particle size. Even after five cycles, the catalyst maintains high selectivity for C3 gas. The catalyst described in this invention exhibits high catalytic activity in the hydrocracking reaction of LDPE (dimethyl olefin) plastics.

[0012] 3. The catalyst preparation method of this invention is simple and conducive to industrial production. Attached Figure Description

[0013] Figure 1 This is a TEM image of the catalyst prepared in Example 1, which uses citric acid as the carbon source, namely the CA-Ni / HZSM-5 catalyst.

[0014] Figure 2 This is a TEM image of the catalyst prepared in Example 1, which uses urea as a carbon source, namely the U-Ni / HZSM-5 catalyst.

[0015] Figure 3 This is a TEM image of the catalyst prepared in Example 1, which uses glucose as a carbon source, namely the G-Ni / HZSM-5 catalyst.

[0016] Figure 4 This is a TEM image of the catalyst prepared in Example 1, which uses sucrose as a carbon source, namely the S-Ni / HZSM-5 catalyst.

[0017] Figure 5 This is a TEM image of the Ni / HZSM-5 catalyst prepared in Example 2 without a carbon source. Detailed Implementation

[0018] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0019] Preparation Example 1 Ni / HZSM-5 catalysts with different carbon doping were prepared by impregnation method.

[0020] (1) Precursor preparation: Prepare a Ni(NO3)2·6H2O aqueous solution A with a concentration of 0.5-1.7 mol / L. Add a certain mass of different carbon sources (glucose, sucrose, urea, citric acid) to solution A to form solution B, wherein the mass ratio of carbon to metallic Ni is 1:1-2:1. Then, add B dropwise to 1g of HZSM-5 molecular sieve support, impregnate evenly, age the impregnated catalyst overnight at room temperature, and then dry it at 110 ℃ for 6h to obtain the catalyst precursor.

[0021] (2) Precursor reduction: The catalyst precursor prepared in (1) was calcined in N2 atmosphere at a temperature of 300 °C and a heating rate of 2 °C / min. After calcination, it was reduced in situ in H2 atmosphere at a temperature of 450-650 °C and a heating rate of 2 °C / min for 1-4 h. After reduction, the temperature was lowered to 20 °C and then passivated for 2 h in an atmosphere with a volume ratio of O2 / Ar2 of 1:99.

[0022] Using different carbon sources, Ni catalysts supported on HZSM-5 supports were CA-Ni / HZSM-5 (citric acid), U-Ni / HZSM-5 (urea), G-Ni / HZSM-5 (glucose), and S-Ni / HZSM-5 (sucrose). TEM spectra are shown below. Figure 1-4 Ni particles are uniformly distributed on the surface of the support, with an average particle size concentrated in the range of 7-10 nm.

[0023] Example 1 The CA-Ni / HZSM-5 catalyst prepared in Example 1 was reacted at different temperatures.

[0024] Reaction Procedure: The hydrocracking reaction was carried out in a 50 ml stainless steel reactor. A specific amount of reactants and catalyst were added sequentially and quantitatively into the reactor. After sealing the reactor, gas purging was performed. First, nitrogen was used for purging three times, followed by exhaust of the tail gas. Then, hydrogen was used for purging three times. Afterward, hydrogen was introduced to reach the required initial pressure, and the tail gas was exhausted. The reactor was left to stand for 15 minutes, and the airtightness of the reactor was checked. The reaction was started after setting a specific reaction temperature and stirring rate. After the reaction was completed, the reactor was removed and cooled to room temperature in an ice-water bath below 10°C. The gas inside the reactor was collected and analyzed by gas chromatography. Afterward, the pressure inside the reactor was released, and the remaining solids in the reactor were processed, dried, and weighed. A US Agilent 6890N gas chromatograph was used for quantitative analysis of the liquid phase products. The specific conditions were as follows: the gas chromatography column was an HP-5 capillary column, the column temperature was 100℃, the temperature was increased to 300℃ at a rate of 20℃·min⁻¹, and maintained for 3 min. The vaporization chamber temperature was 300℃, the detector (flame hydrogen detector) temperature was 280℃, and the area normalization method was used for quantitative analysis.

[0025] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 4 MPa H2, reaction time 4 h. The reaction results are shown in Table 1.

[0026] Table 1. Experimental results of CA-Ni / HZSM-5 catalyst at different reaction temperatures.

[0027] Table 1 shows that the reaction temperature (280-310 ℃) has a significant impact on the hydrocracking performance of LDPE. As the reaction temperature increases from 280 ℃ to 310 ℃, the conversion rate increases from 23% to 100%, and the selectivity increases from 61.2% to 68.5%, demonstrating that the activity of CA-Ni / HZSM-5 catalyst in the hydrocracking of LDPE to C3 gas increases with increasing reaction temperature.

[0028] Example 2 The CA-Ni / HZSM-5 catalyst prepared in Example 1 was reacted under different pressures.

[0029] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 °C, reaction time 4 h. The reaction results are shown in Table 2.

[0030] Table 2. Experimental results of CA-Ni / HZSM-5 catalyst at different reaction pressures.

[0031] Table 2 shows that the reaction pressure (1-4 MPa) significantly affects the performance of LDPE hydrocracking to C3 gas. As the reaction pressure increases from 1 MPa to 2 MPa, the LDPE conversion rate rapidly increases from 41% to 74%. Furthermore, as the reaction pressure increases to 4 MPa, the activity of the CA-Ni / HZSM-5 catalyst for LDPE conversion also significantly improves, from 74% to 100%. Among the gas components, the C3 selectivity increases from 60.4% to 68.5%, indicating that at 310 °C, the reaction pressure has a significant impact on the catalytic activity, but a relatively small impact on the C3 gas selectivity.

[0032] Example 3 The CA-Ni / HZSM-5 catalyst prepared in Example 1 was reacted at different times.

[0033] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 ℃, 4 MPa H2. The reaction results are shown in Table 3.

[0034] Table 3. Experimental results of CA-Ni / HZSM-5 catalyst at different reaction times.

[0035] Table 3 shows that the reaction time has a significant impact on the performance of LDPE hydrocracking to C3 gas. When the reaction time increases from 1 h to 4 h, the LDPE conversion rate increases sharply from 40% to 100%, and the selectivity of C3 gas increases from 64.8% to 68.5%.

[0036] Example 4 The CA-Ni / HZSM-5, U-Ni / HZSM-5, G-Ni / HZSM-5 and S-Ni / HZSM-5 catalysts prepared in Example 1 were each cyclically reacted 5 times.

[0037] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 ℃, 4 MPa H2, reaction time 4 h. Reaction results are shown in Tables 4-7.

[0038] Table 4 shows the experimental results of five cycles of the CA-Ni / HZSM-5 catalyst.

[0039] Table 5 shows the experimental results of five cycles of the U-Ni / HZSM-5 catalyst.

[0040] Table 6 shows the experimental results of five cycles of the G-Ni / HZSM-5 catalyst.

[0041] Table 7 shows the experimental results of five cycles of the S-Ni / HZSM-5 catalyst.

[0042] As can be seen from Tables 4-7, compared with U-Ni / HZSM-5, G-Ni / HZSM-5 and S-Ni / HZSM-5 catalysts, the CA-Ni / HZSM-5 catalyst still maintains good stability after 5 cycles.

[0043] Example 5 The CA-Ni / HZSM-5, U-Ni / HZSM-5, G-Ni / HZSM-5 and S-Ni / HZSM-5 catalysts of Preparation Example 1 were used to catalyze the hydrocracking of LDPE.

[0044] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 ℃, 4 MPa H2, reaction time 4 h. The reaction results are shown in Table 8.

[0045] Table 8. Experimental results of LDPE hydrocracking reaction catalyzed by different carbon-doped Ni / HZSM-5 catalysts.

[0046] Table 8 shows that the addition of different carbon sources has a significant impact on the performance of LDPE hydrocracking to C3 gas. Among them, the catalyst CA-Ni / HZSM-5 with citric acid as the carbon source has the best catalytic activity for LDPE hydrocracking to C3 gas, with an LDPE conversion rate of 100% and a C3 selectivity of 68.5%.

[0047] Example 6 The CA-Ni / HZSM-5 catalyst of Preparation Example 1 was used to catalyze the hydrocracking of LDPE to produce C3 gas.

[0048] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 ℃, 4 MPa H2, reaction time 4 h. The reaction results are shown in Table 9.

[0049] Table 9. Catalysis of LDPE hydrocracking reaction by CA-Ni / HZSM-5 catalysts with different loadings.

[0050] The catalyst loadings for different amounts were 0.0826, 0.1239, 0.1652, 0.2065, and 0.2477 g, respectively. Table 9 shows that the conversion rate was 91% at a loading of 5%. As the Ni loading increased to 7.5%, the LDPE conversion and C3 selectivity increased to 100% and 68.5%, respectively. Further increasing the loading to 15% reduced the LDPE conversion to 68% and the selectivity to 64.5%, indicating that the CA-Ni / HZSM-5 catalyst requires a relatively low loading for the LDPE hydrocracking reaction.

[0051] Preparation Example 2 Ni / HZSM-5 catalysts with a loading of 5-15% were prepared by impregnation. The preparation method was the same as in Preparation Example 1, except that no carbon source was added during the catalyst preparation process.

[0052] Example 6 The catalyst used in Preparation Example 2 was used to catalyze the hydrocracking of LDPE to produce C3 gas.

[0053] Reaction conditions: 0.5 g catalyst, 4 g LDPE, 310 ℃, 4 MPa H2, reaction time 4 h. The reaction results are shown in Table 10.

[0054] Table 10. Catalysis of LDPE hydrocracking reaction by Ni / HZSM-5 catalysts with different loadings.

[0055] Combining Tables 9 and 10, it can be seen that when no carbon source is added during catalyst preparation, the optimal loading of the Ni / HZSM-5 catalyst for the hydrocracking of LDPE to C3 gas is 10%, with an LDPE conversion of 98% and a C3 selectivity of 58.2%. However, the catalyst with citric acid as a carbon source achieves 100% LDPE conversion and 68.5% C3 selectivity at a low loading of 7.5%. (See attached table for details.) Figure 1 and Figure 5 It can be seen that the addition of citric acid adjusted the dispersion of metallic Ni on the carrier surface and reduced the size of the metal particles. This is because the strong complexing ability conferred by the polycarboxyl structure of citric acid can effectively slow down the migration and aggregation rate of Ni species during the preparation process, forming metal particles with smaller and more uniform particle size distribution.

[0056] Table 11 Total acidity of different catalysts calculated by NH3-TPD characterization

[0057] As can be seen from Table 11, the addition of a carbon source reduced the total acidity of the catalyst. Among them, the CA-Ni / HZSM-5 catalyst, due to its moderate total acidity, formed the best metal-acid balance with the active center of metallic Ni, which not only ensured the effective breaking of the C-C bond, but also achieved timely hydrogenation saturation of the unsaturated intermediate. Thus, the catalyst was able to exert an effective metal-acid balance synergistic effect and improve the selectivity for the target product C3.

[0058] Compared with the traditional Ni / HZSM-5 catalyst, the different carbon-doped Ni / HZSM-5 catalysts involved in this invention show a selectivity of >10% for the hydrocracking of LDPE to produce C3 gas while maintaining a conversion rate of 98%.

[0059] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. An HZSM-5 molecular sieve-based hydrocracking catalyst, characterized in that, Using HZSM-5 molecular sieve as a support, a carbon source was added to disperse metallic Ni on the support surface. The loading of metallic Ni was 5-15 wt.%, and the amount of carbon source added was 1-2 times that of metallic Ni.

2. The HZSM-5 molecular sieve-based hydrocracking catalyst according to claim 1, characterized in that, The average particle size of metallic Ni on the surface of the HZSM-5 molecular sieve-based hydrocracking catalyst is 7-10 nm.

3. A method for preparing an HZSM-5 molecular sieve-based hydrocracking catalyst, characterized in that, The steps are as follows: S1. Prepare a Ni(NO3)2·6H2O aqueous solution A with a concentration of 0.5-1.7 mol / L. Add a carbon source to the Ni(NO3)2·6H2O aqueous solution A to form solution B. Then add solution B dropwise into the HZSM-5 molecular sieve support and impregnate it evenly. After impregnation, age the HZSM-5 molecular sieve support at room temperature overnight and then dry it to obtain the catalyst precursor. S2, calcined, reduced and passivated catalyst precursor, to obtain HZSM-5 molecular sieve-based hydrocracking catalyst; The catalyst precursor obtained in step S1 was calcined under N2 atmosphere, then reduced in situ under H2 atmosphere, cooled after reduction, and then continuously passivated in an atmosphere with an O2 / Ar2 volume ratio of 1:

99. After passivation, the temperature was lowered to room temperature, and the Ni loading was 5-15 wt.

4. The preparation method according to claim 3, characterized in that, In step S1, The carbon source is selected from glucose, sucrose, urea, and citric acid; The mass ratio of carbon source to metallic Ni is 1:1 to 2:

1. Drying temperature 110℃, drying time 6 h.

5. The preparation method according to claim 3, characterized in that, In step S1, The calcination temperature was 300 ℃, the heating rate was 2 ℃ / min, and the calcination time was 2 h; The in-situ reduction temperature is 450-650 ℃, the heating rate is 2℃ / min, and the in-situ reduction time is 1-4 h; The passivation temperature was 20 °C and the passivation time was 2 h.

6. The HZSM-5 molecular sieve-based hydrocracking catalyst obtained by any one of the preparation methods according to claims 3-5 is used in the hydrocracking reaction of low-density polyethylene (LDPE), a polyolefin plastic, characterized in that... The specific conditions are as follows: HZSM-5 molecular sieve-based hydrocracking catalyst and polyolefin plastic low-density polyethylene are mixed and reacted at a temperature of 280-310℃, a pressure of 2-4 MPa, and a stirring speed of 500 rpm for 2-4 h. The amount of hydrocracking catalyst is 6.25-12.5% ​​of the mass of LDPE.

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