A method for preparing chain-length controllable alpha olefins from polypropylene plastics

Abstract

The present invention provides a method for preparing chain-length controllable alpha olefins from polypropylene plastics, comprising the following steps: (1) adding polypropylene waste to a pyrolyzer and heating to 200-500°C under a nitrogen environment, keeping the temperature constant; (2) under the action of the residual heat of the reaction, alpha olefins of different carbon chain lengths are distributed in the temperature range corresponding to the distillation device, and the distribution position of the alpha olefins of the target length is determined and collected; olefins with excessive carbon chain lengths are refluxed for pyrolysis; as the alpha olefins of the target length are collected and the reaction continues, more alpha olefins of the target length are produced and collected. The present invention adopts the reaction-distillation technology to establish a mechanism for identifying, separating and re-decomposing the chain length of the after-products after the cracking of polypropylene waste, and achieves the high yield and high purity of alpha olefins of a specific molecular weight.

Description

Technical Field

[0001] The invention belongs to the technical field of plastic recycling and relates to a method for preparing chain-length controllable alpha olefins from polypropylene plastics. Background Art

[0002] The methods for preparing alpha olefins from polypropylene plastics in the prior art include: (1) direct thermal decomposition method to degrade polypropylene waste to prepare high-purity alpha olefins. The reaction device adopts a continuous stirred tank reactor. After adding materials, nitrogen is purged to remove oxygen. After 5 hours of reaction at 360°C under mechanical stirring, heating is stopped. The product obtained by this method is a broadly distributed alpha olefin with an average molecular weight of about 420 and a purity of about 32%. (2) After the reaction, the product is distilled and placed in the same reactor with the reactants for a period of time. The obtained product is repeatedly polymerized and decomposed. After multiple decompositions, the product structure is more complex. It is easy to obtain a product with a higher content of internal olefins. In addition, due to the competitive reaction of polymerization and decomposition, the structure of the product is more complicated. For example, when the decomposition reaction is carried out in a batch reactor, it will automatically stop after reaching a certain level. (3) A reaction technology based on a temperature gradient reactor reported in Science uses a special temperature gradient cracking reactor. After 10 hours of reaction at 360°C under mechanical stirring, heating is stopped. The product is a broadly distributed alpha olefin with an average molecular weight of about 590 and a purity of 36%. It is impossible to produce alpha olefins with narrow carbon chain distribution and high purity. (4) The literature reports a catalytic decomposition method using a porous catalyst with a precious metal catalyst embedded in the mesopores. The mesoporous catalyst adopts the synthesis method of mSiO2 / Pt / MCM-48 published in the literature (for example: J. Am. Chem. Soc. 2023, 145, 17936−17944). A 5% mass fraction of catalyst was added to the cracking reaction, and the reaction temperature was 300 degrees Celsius. The other parts are the same as this scheme. The molecular weight of the obtained product is 415, which is close to the molecular weight reported in the literature. However, the purity of the α-olefin product is 42%. (5) CN 118085398 A discloses a method for converting plastics into polymer waxes, using reaction-separation coupling technology and similar Figure 1 The method converts plastic into olefin polymer wax by extending the distance from the reaction zone to the condenser in the device (3) so that the sublimated decomposition fragments return to the decomposition device, thereby reducing the content of high molecular weight (~1000) oligomers in the product.

[0003] It can be seen that the existing technology lacks selectivity for products with different chain lengths, resulting in a wide distribution of chain lengths obtained from the cracking reaction, low purity, and generally containing a large number of products with carbon numbers above 30, which cannot be directly used as industrial raw materials. Therefore, the method for preparing alpha olefins from polypropylene plastics needs to be further improved. Summary of the invention

[0004] The present invention adopts the reactive distillation technology, establishes the recognition, separation and re-decomposition mechanisms for the chain lengths of the products after the pyrolysis of polypropylene plastics, and realizes the preparation of alpha-olefins with high yield and high purity of specific molecular weights. Thereby, a method for preparing alpha-olefins with controllable chain lengths from polypropylene plastics is provided.

[0005] A method for preparing alpha-olefins with controllable chain lengths from polypropylene plastics, comprising the following steps:

[0006] (1) Under an inert gas environment, add polypropylene waste to a pyrolyzer and heat it to 200 - 500 °C, and keep the temperature constant;

[0007] (2) Under the action of the waste heat of the reaction, alpha-olefins with different carbon chain lengths are distributed in the corresponding temperature ranges of the rectifier. Determine the distribution positions of the alpha-olefins with the target length and collect them; the olefins with too long carbon chain lengths are refluxed for pyrolysis; as the alpha-olefins with the target length are collected and the reaction continues, more alpha-olefins with the target length are produced and collected.

[0008] Further, the calculation formula for the collection temperature of the alpha-olefins with the target length is T = ab + C, where a is the coefficient of heat dissipation, and the value range is 0 < a < 1.1, b is the boiling point temperature of the alpha-olefins with the target length, and C is an empirical constant.

[0009] Further, a is the correction coefficient of heat dissipation, and the value range is 0 < a < 1.1. The value of a is related to the position of the temperature sensor on the tower and the material of the tower. When the sensor is inside the tower and measures the temperature on the tray, the value of a is approximately 1; when the measurement position is outside the tower, the coefficient is related to the material and thickness of the tower. When the tower material is not heat-conductive, it is impossible to achieve selective product collection by measuring the temperature. When the tower material is glass and the thickness is 2 mm, the value of a at this time is 0.5. When the tower material is a solid metal material with good heat conduction, the value of a is approximately 0.86.

[0010] Further, b is the boiling point temperature of the alpha-olefins with the target length, and C is the temperature of the gas components at the top of the tower, so it can be approximated as the ambient temperature, and the value is related to the climate and season.

[0011] Further, after the reaction and rectification reach equilibrium, stable distillation can be achieved. The sign of equilibrium is that the temperature of the rectification column is approximately constant, and the temperature change ΔT < 5 °C.

[0012] Further, the adjustment of the temperature gradient of the rectification column is achieved by adjusting the temperature of the pyrolyzer (200 - 500 °C) or applying external temperature control.

[0013] Furthermore, the rectifier is selected from a multi-plate distillation device, and the number of plates is further selected from 3-9.

[0014] Furthermore, the number of outlets of the rectifier can be set to 1-9, and one or more corresponding alpha olefins can be obtained as needed.

[0015] Further, a method for preparing high-purity alpha olefins from polypropylene waste is provided by using a three-plate reactive distillation device ( Figure 1 ), the three-plate reaction distillation device includes a pyrolysis part and a distillation part. The pyrolysis part is a pyrolyzer, and the distillation part is a distillation tower. The distillation tower is placed above the pyrolyzer. The distillation tower contains 3 plates, which are named plate one, plate two, and plate three from top to bottom. The plates are overflow plates, and plate one includes a gas collection bag. The specific steps are as follows: After adding polypropylene particles to the pyrolyzer, the inside of the three-plate reaction distillation device is purged with nitrogen to remove oxygen, and then the three-plate reaction distillation device is sealed. The top plate of the distillation tower is connected to a gas collection bag for collecting gas and maintaining stable gas pressure. After the reaction device is completed, the temperature of the pyrolyzer is raised to 360°C. The distillation tower part is not heated, and only the residual heat of the reaction is used for separation.

[0016] Furthermore, a method for preparing high-purity alpha olefins from polypropylene waste is provided by using a multi-plate, single-discharge reaction distillation device ( Fig.10 ), pyrolysis is a continuous stirring reactor, the reactor has a built-in stirrer and blades, and an external insulated thorn-shaped distillation column, the length of which is 52 cm, and a condenser is connected to the top for cooling the steam and sample extraction. Since the reaction device does not have a plate to store the distillate, the distillate will reflux and continue to pyrolyze until it is decomposed into volatile hydrocarbons that can escape from the top of the tower. The average molecular weight of the volatile hydrocarbons conforms to the empirical formula "T=ab+C".

[0017] Beneficial effects:

[0018] (1) The reaction-distillation technology of the present invention converts polyolefins into α-olefins of a certain molecular weight with a higher purity. The purity of the obtained α-olefins is increased to 90%, which is much higher than the purity obtained by traditional direct decomposition, reaction-separation, and catalytic decomposition (generally less than 40%).

[0019] (2) The reaction-distillation technology of the present invention converts polyolefins into α-olefins of a certain molecular weight with a relatively high purity. The distribution width of the obtained α-olefins, if approximated by normal distribution, has a variance of <3. The variance obtained by conventional technology is generally close to or greater than 8, and the distribution is very wide.

[0020] (3) The reaction-distillation technology of the present invention converts polyolefins into α-olefins of a certain molecular weight with relatively high purity. The obtained α-olefins have a high olefin concentration (10 mmol / g) and a wide adjustable range (0.5-10 mmol / g).

[0021] (4) The reaction-distillation technology of the present invention converts polyolefins into α-olefins of a certain molecular weight with higher purity. The molecular weight of the obtained α-olefins can be regulated by the reaction time (1-5h) and the number of plates.

[0022] (5) The present invention avoids the use of chemically unstable reactants or expensive and complex catalysts, and achieves the purpose of cracking under non-catalytic conditions. It avoids the dilemma of catalysts being non-reusable, having low activity, and having poor effects under the existing technology. Compared with the traditional continuous distillation method, it also avoids the loss of distillation process products and additional energy. The thermal decomposition products of polyolefins can be used as a chemical raw material for the production of fine chemicals, rather than just as fuels and lubricating oil products for extensive use. Therefore, this technology can pave the way for the application of polyolefin upgrading and recycling in the field of fine chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the structure of a three-plate reactive distillation device.

[0024] Figure 2 The yield of products on different plates in Example 1 changes with time.

[0025] Figure 3 The purity of the products on different plates in Example 1 changes with time.

[0026] Figure 4 This is the carbon number distribution of the product on three plates after 5 hours of reaction in Example 1.

[0027] Figure 5 This is the NMR spectrum of the product in tray 3 after reacting for 5 hours in Example 1.

[0028] Figure 6 The carbon number distribution of the product obtained in Comparative Example 1.

[0029] Figure 7 This is the carbon number distribution of the product obtained in Comparative Example 2.

[0030] Figure 8 Graph showing the relationship between the boiling point of the target product and the distillation plate temperature in Examples 2 and 4.

[0031] Fig. 9 The relationship between the molecular weight, purity, and olefin concentration of the product in Example 3 and the number of plates.

[0032] Fig.10This is a schematic diagram of the structure of the continuous stirred reactor in Example 5.

[0033] Fig.11 The carbon number distribution of the product in Example 5.

[0034] Fig.12 This is the carbon number distribution of the product obtained in Comparative Example 3.

[0035] Fig.13 This is an evaluation of the repeatability of the catalytic decomposition method used in Comparative Example 4.

[0036] Fig.14 This is an evaluation of the repeatability of the reactive distillation method used in Example 1. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in combination with specific embodiments and comparative examples.

[0038] The specific embodiments of the present invention are described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of the present invention. The experimental methods described in the embodiments of the present invention are conventional methods unless otherwise specified.

[0039] The following experimental methods and detection methods, unless otherwise specified, are conventional methods; the following reagents and raw materials, unless otherwise specified, are commercially available.

[0040] High-resolution mass spectrometry

[0041] Instrument model: thermo scientific Q Exactive MS

[0042] Spray voltage: 5000 V

[0043] Capillary temperature: 320.00 o C

[0044] Sheath gas: 45.00 Arb

[0045] Auxiliary gas: 15.00 / 20 Arb

[0046] Maximum injection current: 80.00 µA

[0047] Probe temperature: 300 o C

[0048] Ion source: APCI+MS

[0049] Instrument: Q Exactive Electrostatic Field Orbital Hydrazine Mass Spectrometer.

[0050] Mass spectrometry conditions: APCI ion source, nebulizer gas 45 arb; auxiliary gas 15 arb; spray voltage 5000 V, ion transfer capillary temperature 320 o C; auxiliary gas temperature 300Full MS, scanning range m / z; 50-750, 200-3000

[0051] 2D NMR

[0052] The HMBC two-dimensional nuclear magnetic resonance spectrum of the product was measured using a German Bruker Avance II 500 nuclear magnetic resonance instrument, with 16 scans and a relaxation time of 2 s.

[0053] Determination of olefin concentration

[0054] The concentration of olefin bonds in the product was determined by NMR using the external standard method. The H1 spectrum of paraffin was measured using a nuclear magnetic resonance spectrometer. The scan relaxation time was 1 s and the number of scans was 32. The unsubstituted benzene (C6H6) and unsubstituted p-xylene (C8H 10 ) was calibrated and used as an external standard. The mass of the sample and the deuterated reagent was accurately measured using an analytical balance. The sample was then dissolved in a clean HPLC sample bottle and quickly transferred to an NMR sample tube after dissolution, and the tube was sealed with paraffin. Finally, the mass concentration of the alkenyl group was calculated using the following formula.

[0055] ;

[0056] in is the concentration of alkenyl groups; is the mass of the deuterated reagent; The molecular weight of the deuterated reagent; It is the unsubstituted benzene (C6H6) and unsubstituted p-xylene (C8H 10 ) concentration; Is C6H6 or C8H 10 The number of hydrogen atoms in the olefinic group; r is the number of hydrogen atoms in the olefinic group C6H6 or C8H 10 The ratio of hydrogen on is the number of hydrogen atoms on the olefinic group; m is the mass of the sample.

[0057] purity

[0058] Purity in the examples and comparative examples is defined as the mass fraction of the five carbon atoms around the center of the product distribution peak, that is, the distribution is approximated by a normal distribution, and the mass fraction within the range of ±2.5 signals of the peak position is obtained.

[0059] A method for preparing high-purity alpha olefins from polypropylene waste by reactive distillation, comprising the following steps:

[0060] (1) Under a nitrogen atmosphere, add polypropylene waste to a pyrolyzer and heat it to 200 - 500 °C, and keep the temperature constant;

[0061] (2) Under the action of the reaction waste heat, alpha olefins with different carbon chain lengths are distributed in the corresponding temperature ranges of the rectifier. Determine the distribution position of the alpha olefins with the target length and collect them; the olefins with too long carbon chain lengths are refluxed for pyrolysis; as the alpha olefins with the target length are collected and the reaction continues, more alpha olefins with the target length are produced and collected.

[0062] Further, the calculation formula for the collection temperature of the alpha olefins with the target length is T = ab + C, where a is the coefficient of heat dissipation, and the value range is 0 < a < 1.1, b is the boiling point temperature of the alpha olefins with the target length, and C is an empirical constant.

[0063] Further, a is the correction coefficient of heat dissipation, and the value range is 0 < a < 1.1. The value of a is related to the position of the temperature sensor on the tower and the material of the tower. When the sensor is inside the tower and measures the temperature on the tray, the value of a is approximately 1; when the measurement position is outside the tower, the coefficient is related to the material and thickness of the tower. When the tower material is not heat-conducting, it is impossible to achieve selective product collection by measuring the temperature. When the tower material is glass and the thickness is 2 mm, the value of a at this time is 0.5. When the tower material is a solid metal material with good heat conduction, the value of a is approximately 0.86.

[0064] Further, b is the boiling point temperature of the alpha olefins with the target length, and C is the temperature of the gas components at the top of the tower, so it can be approximated as the ambient temperature, and the value is related to the climate and season.

[0065] Further, after the reaction and rectification reach equilibrium, stable distillation can be achieved. The sign of equilibrium is that the temperature of the rectification column is approximately constant, and the temperature change ΔT < 5 °C.

[0066] Further, the adjustment of the temperature gradient of the rectification column is achieved by adjusting the reactor temperature ((200 - 500 °C)) or applying external temperature control.

[0067] Example 1: Preparation of high-purity alpha olefins by reactive distillation method

[0068] A method for preparing high-purity alpha olefins from polypropylene waste by reactive distillation, comprising the following steps: This example uses a three-tray reactive distillation device ( Figure 1), the three-plate reaction distillation device includes a pyrolysis part and a distillation part. In this embodiment, the pyrolysis part is a pyrolyzer 1, and the distillation part is a distillation tower 2. The distillation tower 2 is placed above the pyrolyzer 1. The distillation tower 2 contains 3 plates, which are named plate 1, plate 2, and plate 3 from top to bottom. The plates are overflow plates, and plate 1 includes a gas collection bag 3. The specific implementation is as follows: After adding polypropylene particles to the pyrolyzer 1, nitrogen is used to purge the inside of the three-plate reaction distillation device to remove oxygen, and then the three-plate reaction distillation device is sealed. The top plate 1 of the distillation tower 2 is connected to a gas collection bag 3 for collecting gas and maintaining stable gas pressure. After the reaction device is completed, the temperature of the pyrolyzer 1 is raised to 360°C, and the distillation tower 2 is not heated, and only the residual heat of the reaction is used for separation. Therefore, there is a temperature difference from the bottom to the top of the distillation tower 2, and the temperature difference is affected by the reaction temperature. Samples were taken 1, 2, 3, 4, and 5 hours after the start of the reaction to analyze the product yield, molecular weight, and purity. Figure 2 It shows that when the reaction time is greater than 2 hours, the pyrolysis residue is about 12wt.%, the yield of the first plate is about 2wt.%, the yield of the second plate increases with time, and the yield of the third plate decreases with time, indicating that the high molecular weight olefins gradually turn into low molecular weight. The reaction enters the equilibrium stage after 4 hours, and the components remain basically unchanged. The basis for judging the equilibrium stage is that the temperature change amplitude ΔT at the center of the tower is less than 5℃. Purity analysis shows that as the reaction proceeds, the purity of the products on each plate gradually stabilizes and increases ( Figure 3 The average molecular weight of the three-layer plate remained basically unchanged at different reaction times. Analysis of the carbon number distribution of the three-layer plate product after 5 hours of reaction showed that the average molecular weights of plates 1 to 3 were ~50, 210, and 335, respectively ( Figure 4 ); the corresponding a value is 0.49 and the C value is 23°C. In addition, the carbon number distribution shows that the carbon chain distribution of the products in the three-layer tower plate does not overlap, indicating that the reactive distillation method can obtain narrowly distributed olefin products. Representative material structure characterization is as follows Figure 5 As shown, after 5 hours of reaction, the third product on the tower plate is alpha olefin, indicating that polypropylene plastic is successfully converted into a high-value alpha olefin product.

[0069] Comparative Example 1

[0070] This comparative example is a direct pyrolysis method for pyrolyzing polypropylene without reactive distillation. This method uses a continuous stirred tank reactor, that is, only a pyrolysis part, no distillation part. The specific operation is as follows: add polypropylene material and a stirring magnet to the pyrolyzer, purge with nitrogen to remove oxygen and seal, react at 360°C for 5 hours under mechanical stirring, and then stop heating. The product distribution is as follows Figure 6As shown, the carbon number distribution of the obtained product is C6-C66, the average molecular weight is about 420, and the purity is 52%. The carbon chain distribution is much larger than the product obtained by the reactive distillation method in Example 1, and the purity is lower than the product in Example 1, indicating that the reactive distillation method proposed in Example 1 is more conducive to obtaining narrow distribution and high purity alpha olefins than the traditional direct pyrolysis method.

[0071] Comparative Example 2

[0072] This comparative example uses a reaction separation method reported in Science (Science, 2023, 381, 666-671), which uses a temperature gradient to separate the pyrolysis product from the unpyrolysis raw material in time. This method does not use a distillation process. The specific operation is as follows: add polypropylene material and stirring magnet to the pyrolyzer, purge with nitrogen to remove oxygen and then seal, pyrolyze at 360°C under mechanical stirring, and collect the pyrolysis product through the top of the pyrolyzer. Stop heating after 5 hours of reaction. Figure 7 As shown, the carbon number distribution of alpha olefins obtained by the reaction separation method is C16-C85, which is much wider than the result obtained by reaction distillation in Example 1; the average molecular weight of the product obtained by reaction separation is about 590, which is higher than the product obtained by reaction distillation; the purity of the product obtained by reaction separation is 36%, which is higher than the product obtained by reaction distillation. It can be seen that the reaction distillation method proposed in the patent application is more conducive to converting polypropylene into narrow distribution and high purity alpha olefins than the traditional reaction separation method.

[0073] Example 2: Effect of reaction zone temperature on distillation tower product distribution

[0074] The operation steps and implementation apparatus of this embodiment are the same as those of Example 1, but the reaction temperature of this embodiment is controlled to 200°C or 500°C. When the decomposition reaction temperature is 200°C, the polymer slowly decomposes into gaseous products, including hydrogen, methane and propylene. At this time, the temperature gradient difference of the distillation tower is small, and the bottom temperature is only 50°C. This method improves the selectivity of gaseous products. When the reaction temperature is 500°C, the polymer decomposes rapidly, and the distillation tower is balanced after 5 hours. The average molecular weights of plates one to three are ~52, 407 and 618 ( Figure 8 ), where the product with a molecular weight of 618 is solid at room temperature. The corresponding a value is approximately 0.49 and the C value is 26°C. This experiment shows that this method can also achieve control of the product molecular weight by controlling the reactor temperature.

[0075] Example 3: Preparation of high-purity alpha olefins by reactive distillation with 9 distillation plates

[0076] The difference between the reactor used in this example and that in Example 1 is that the number of distillation plates is 9 and the material is 304 stainless steel. The rest is the same as in Example 1. After 5 hours of reaction, the equilibrium stage is reached. The molecular weight, purity and olefin concentration of the product vary with the number of plates as shown in the figure. Fig. 9 As shown. The results show that from the bottom to the top of the distillation tower, as the number of plates decreases, the molecular weight of alpha olefins gradually decreases, and the purity and olefin concentration gradually increase. Compared with Example 1, the reactive distillation method with more plates can obtain high-purity alpha olefins of various molecular weights, indicating that the method technology has universal applicability; in addition, different plate temperatures correspond to a certain average molecular weight, and the curve a is 0.86 and C is 20 °C.

[0077] Example 4: Fixed-point material extraction from distillation tower

[0078] In this embodiment, the position of the target fraction on the tray is determined by the boiling point of the target fraction and the material of the reactor, thereby realizing fixed-point material extraction. Here, a glass distillation tower is used as an example to illustrate the feasibility, and the other parts are the same as in Example 1. When the distillation tower is made of glass and the glass thickness is 2 mm, in the empirical equation T=ab+C, the parameter a is 0.53 and C is 25°C. If you want to obtain C22 hydrocarbons with a target molecular weight of 335, you need to sample at a tray temperature of about 160°C after equilibrium, such as Figure 8 shown.

[0079] Example 5: Preparation of high yield of specific molecular weight alpha olefins by reactive distillation method for degradation of polypropylene

[0080] A multi-plate, single-discharge reaction distillation device is used to increase the yield of alpha olefins with a certain molecular weight. The high-temperature cracking part is a continuous stirring reactor 4, which has a built-in stirrer and blades 5, and an externally connected heat-insulating thorn-shaped distillation column 6. The distillation column 6 is 52 cm long and has a condenser at the top for cooling steam and sample extraction ( Fig.10 ). Since the reaction device does not have a tray for storing fractions, the fraction with a higher boiling point will enter the reflux and continue to be pyrolyzed until the molecular weight is low enough to escape from the distillation column. The difference from Example 1 is that Example 1 has three outlets, while Example 5 has only one outlet, the purpose of which is to increase the yield of a specific molecular weight. The reaction was terminated after 10 hours, and a narrow distribution of alpha olefins ( Fig.11 ), wherein the purity of four alpha olefins C22, C25, C28 and C31 near the main peak is about 76%, and the total yield is 91wt.%, which is higher than Example 1. The results show that the reactive distillation method can be expanded according to demand to improve the yield of a product with a certain molecular weight.

[0081] Comparative Example 3

[0082] This comparative example uses a catalytic pyrolysis method to pyrolyze polypropylene to prepare alpha olefins. A porous catalyst with a mesoporous embedded precious metal catalyst recently invented in the literature is used. The mesoporous catalyst adopts the synthesis method of mSiO2 / Pt / MCM-48 published in the literature (J. Am. Chem. Soc. 2023,145, 17936−17944). A 5% mass fraction of catalyst was added to the cracking reaction, the reaction temperature was 300°C, and the other parts were the same as in comparative example 1. The molecular weight of the product obtained in comparative example 3 is 415, which is close to the molecular weight reported in the literature, but the purity of the alpha olefin product is 37%, which is much lower than that of Examples 1, 2, and 3 of the reactive distillation method ( Fig.12 ). This result shows that the reactive distillation method is superior to the catalytic pyrolysis method of mesoporous catalysts in the literature, and can achieve a narrower distribution width and higher product purity.

[0083] Comparative Example 4

[0084] The purpose of this comparative example is to verify the repeatability of the catalytic pyrolysis method and compare it with Example 1. The operation of this comparative example is the same as that of Comparative Example 3. After 5 reactions, the molecular weight of alpha olefin increased by about 100% compared with the first time, and the purity dropped significantly to about 15%. It can be seen that the changes in the mSiO2 / Pt / MCM-48 catalyst after multiple reactions have significantly reduced the catalytic performance ( Fig.13 ), that is, the catalytic method currently faces the challenge of not being able to be recycled multiple times. Compared with the catalytic method, the reactive distillation method proposed in the present invention has a better stability catalyst. After repeating 10 times, the molecular weight and purity of alpha olefins remain unchanged ( Fig.14 ), indicating that the method has good stability. In summary, the method described in the present invention solves the problem of poor repeatability of the catalytic cracking method of mesoporous embedded precious metal catalysts and enhances practicality.

Claims

1. A method for preparing alpha olefins with controllable chain length from polypropylene plastics, comprising the following steps: (1) Under a nitrogen atmosphere, add polypropylene waste to a pyrolyzer and heat it to 200 - 500 °C, and keep the temperature constant; (2) Under the action of the residual heat of the reaction, alpha olefins of different carbon chain lengths are distributed in the temperature range corresponding to the distillation device, and the distribution position of the alpha olefins of the target length is determined and collected; the olefins with carbon chain lengths that are too long are refluxed for pyrolysis; as the alpha olefins of the target length are collected and the reaction continues, more alpha olefins of the target length are produced and collected; the calculation formula for the collection temperature of the alpha olefins of the target length is T=ab+C, where, a is the coefficient of heat dissipation, with a value range of 0 < a < 1.1, b is the boiling point temperature of the target length alpha olefin, and C is the temperature of the overhead gas components.

2. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: After the reaction and rectification reach equilibrium, stable distillation can occur. The sign of equilibrium is that the temperature of the rectification column is approximately constant, and the temperature change ΔT < 5 °C.

3. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: The adjustment of the temperature gradient of the rectification column is achieved by adjusting the temperature of the pyrolyzer to 200 - 500 °C.

4. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: The rectifier is selected from a multi-tray rectification device.

5. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: The number of outlets of the rectifier is set to 1 - 9.

6. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: A three-tray reactive distillation device is used. The three-tray reactive distillation device includes a pyrolysis section and a rectification section. The pyrolysis section is a pyrolyzer, and the rectification section is a rectification column. The rectification column is placed above the pyrolyzer. The rectification column contains 3 trays, which are named tray one, tray two, and tray three from top to bottom. The trays are overflow trays, and tray one includes a gas collection bag. The specific steps are as follows: After adding polypropylene particles to the pyrolyzer, purge the inside of the three-tray reactive distillation device with nitrogen to remove oxygen, and then seal the three-tray reactive distillation device. The gas collection bag is used to collect gas and maintain stable air pressure. After the reaction device is completed, raise the temperature of the pyrolyzer to 360 °C, and the rectification column part is not heated. Only the waste heat of the reaction is used for separation.

7. A method for preparing chain-length controlled alpha olefins from polypropylene plastics as claimed in claim 1, characterized in that: A reactive distillation device with multiple trays and a single outlet is used. Pyrolysis is carried out in a continuously stirred reaction kettle. The reaction kettle is equipped with a stirrer and blades, and is externally connected to a thermally insulated thimble-shaped rectification column with a length of 52 cm. The top of the rectification column is connected to a condenser for cooling the vapor and sample extraction. Since there are no trays in the reaction device that can store fractions, fractions with higher boiling points will enter the reflux and continue to pyrolyze until they decompose into volatile hydrocarbons that can escape from the top of the column. The average molecular weight of the volatile hydrocarbons conforms to the empirical formula "T = ab + C".

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