Method for preparing liquid fuel from waste plastics

Abstract

The invention discloses a method for preparing liquid fuel by utilizing waste plastics, which comprises the following steps: uniformly mixing the waste plastics and a catalyst, sealing to form a reaction system, reacting the reaction system, and separating out the catalyst to obtain the liquid fuel, the catalyst is a layered molecular sieve with mesopores and micropores and acidic sites on the outer surface, and particularly is a layered self-supported molecular sieve with an MFI / MEL topological structure. The method is mild in reaction condition and free of external hydrogen sources, and high-quality gasoline mainly containing isoparaffin is obtained through a self-hydrogen-supply strategy; solvent-free, noble-metal-free, cheap and stable molecular sieves are used as catalysts; the waste plastic is almost completely converted into gasoline, and only a small amount of C1-C3 gas is produced as a byproduct; a liquid product obtained by conventional filtration can be directly used as gasoline; the obtained gasoline is high-quality gasoline, mainly comprises a large amount of branched paraffin (C4-C12), contains a small amount of straight-chain paraffin, cycloparaffin and aromatic hydrocarbon, and is free of olefin; the reaction process is simple and environment-friendly.

Description

Technical Field

[0001] The invention relates to the field of catalytic conversion and utilization of waste plastics, and in particular to a method for preparing liquid fuel by catalyzing the degradation of waste plastics through solid acidic molecular sieves. Background Art

[0002] Plastics play an indispensable role in people's production and life. As of 2015, the total amount of plastics in the world was about 8.3 billion tons. It is estimated that by 2050, the total amount of plastics in the world will grow to 52 billion tons. After the use of plastics, 12 billion tons will be incinerated and 12 billion tons will be discarded or landfilled. This has caused serious environmental pollution and waste of resources. In recent years, people have begun to develop new recycling concepts, clean incineration methods, and fast degradable plastics in order to solve the environmental pollution caused by plastics. The application of chemical methods will play a core role in providing solutions. If waste plastics are degraded and converted into liquid fuels, it can solve environmental pollution while achieving "turning waste into treasure."

[0003] Polyethylene (PE, 29%), polypropylene (PP, 19%), polyvinyl chloride (PVC, 10%) and polystyrene (PS, 7%) are widely used, accounting for about 65% of plastics. Their common feature is that their skeletons are connected by saturated carbon-carbon bonds, which are chemically stable and difficult to break. Generally, they can only be broken under harsh conditions such as high temperature or hydrogenation and high pressure. Existing patents (CN1141331A, CN1236016C, CN1084546A, CN1077479A, CN1059454C, CN102977909A, CN101117585A, CN105536818A, WO2005 / 017069A1) report methods for producing gasoline, coal, diesel and other fuels from waste plastics. However, the reaction temperature generally needs to be increased to above 300°C to effectively degrade the plastics, and the degradation products generally contain more unsaturated hydrocarbons, which need further treatment before they can be used as liquid fuels. Patent WO2021257783A1 reported the use of a physical mixture of Pt / WO3 / ZrO2 and HY molecular sieve as a catalyst to convert waste plastics into gasoline and diesel at 200-250°C and 30 bar hydrogen. It uses precious metals and high-pressure hydrogen, which increases production costs and reduces operational safety. A Chinese patent (CN1141359C) reported a method for converting waste plastics into gasoline and diesel under milder conditions (50-320°C). However, its process is complicated and requires plastic thermal cracking-aluminum silicate catalytic thermal cracking-distillation-rare earth / iron oxide catalytic cracking to produce gasoline and diesel.

[0004] In summary, the existing technology for preparing liquid fuel from waste plastics has not yet reached the technical level of producing high-quality liquid fuel with high efficiency, high selectivity and low cost under mild conditions. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies in the prior art and to propose a method for preparing liquid fuel by catalyzing the degradation of waste plastics using a layered self-supporting solid acidic molecular sieve. The open framework tri-coordinated aluminum of the molecular sieve material is used as a strong Lewis acid site to efficiently activate the CH bonds of saturated alkanes and catalyze hydrogen transfer reactions. Through a self-hydrogen supply mode, waste plastics can be efficiently and selectively converted into high-quality liquid fuels under mild conditions (240° C.) without precious metals, external hydrogen sources, or solvents.

[0006] In order to achieve the above purpose, the technical solution adopted by the present invention is: the layered acidic molecular sieve catalyst and waste plastic are mixed evenly and added into a closed reactor, reacted for a certain time under certain temperature conditions, and the catalyst and the product are separated by conventional filtering methods to obtain high-quality liquid fuel.

[0007] The method for preparing liquid fuel using waste plastics provided by the present invention comprises the following steps:

[0008] 1) Mix the waste plastic and the catalyst, seal them and form a reaction system.

[0009] 2) allowing the reaction system to react, separating the catalyst, and obtaining liquid fuel;

[0010] In step 1) of the above method, the waste plastic is a polymer having a skeleton composed of carbon-carbon saturated bonds, including but not limited to polyethylene, polypropylene, polystyrene, and polyvinyl chloride;

[0011] The catalyst is a layered molecular sieve having mesoporous micropores and acidic sites on the outer surface;

[0012] The outer surface of the layered molecular sieve has abundant acid sites, which can effectively contact with plastic macromolecules, and the super-strong Lewis acid sites on the outer surface can efficiently activate the CH bonds of waste plastics. The primary products obtained by activation and cracking can enter the micropores in the layer for further selective reaction, and prepare liquid fuels with high selectivity.

[0013] The layered molecular sieve may be a layered self-supporting MFI / MEL topological structure molecular sieve (named LSP-Z), wherein the molar ratio of Si:Al contained in the layered molecular sieve is 25 to 300, and may be 49 or 57;

[0014] The layered molecular sieve is prepared by the following method:

[0015] Taking the synthesis of a molecular sieve (LSP-Z57) with an MFI / MEL topological structure having a Si / Al ratio of 57 as an example, the synthesis process is as follows: aluminum isopropoxide is added to tetraethyl orthosilicate, and then tetrabutylammonium hydroxide (TBPOH) is added dropwise to the mixture while stirring, and then an aqueous sodium hydroxide solution is added to the mixture; the composition of the above mixture is 60SiO2:0.30Al2O3:18TBAOH:0.75NaOH:600H2O:240EtOH (the initial Si / Al ratio of the reaction feed is 100, and the Si / Al ratio of the synthesized molecular sieve becomes 57) ; After stirring for 12 hours, the mixture was sealed in a high-pressure hydrothermal reactor and heated at 120°C for 88 hours. The product was centrifuged and washed with water several times until the pH value was lower than 9; the product was then dried at 70°C for 12 hours and calcined at 550°C for 16 hours; then, the molecular sieve was washed with deionized water at 70°C for 12 hours, the suspension was centrifuged and the solid was collected, and then ion exchange was performed at 80°C using a 1 mol / L ammonium chloride aqueous solution. The process was repeated three times, and the solid was washed, dried at 70°C, and calcined at 550°C for 4 hours to convert the sodium-type molecular sieve into a hydrogen-type molecular sieve. High-resolution transmission electron microscopy and X-ray powder diffraction results show that the synthesized molecular sieve is a layered self-supporting structure ( Figure 1 ), with MFI and MEL topologies ( Figure 2 ).

[0016] The weight ratio of waste plastic to catalyst may be 20:1 to 0.1:1, specifically 5:1;

[0017] Before mixing, the waste plastics are crushed, rinsed and dried;

[0018] In step 2) of the above method, the reaction temperature may be 150-280° C., specifically 200-240° C.; the reaction time may be 0.5-24 hours, preferably 1.5-15.5 hours, more preferably 4 hours;

[0019] The reaction process generates a certain pressure, and the catalyst is separated by conventional filtering methods to obtain liquid fuel.

[0020] The liquid fuel is mainly gasoline mainly composed of branched alkanes.

[0021] Compared with the prior art, the present invention has the following significant advantages:

[0022] (1) The reaction conditions are mild (200-240°C), no external hydrogen source, no solvent, no precious metals, and cheap and stable molecular sieves as catalysts;

[0023] Among them, the reaction has no external hydrogen source, and the system obtains high-quality gasoline mainly composed of isoparaffins through a self-hydrogen supply strategy ( Figure 3 )——PE further undergoes β-cracks under the activation of the catalyst to produce unsaturated long carbon chains and olefins. The unsaturated long carbon chains are dehydrogenated to provide hydrogen for the olefins, which are further converted into alkanes to obtain stable (olefin content <2%) and low-environmental-polluting (aromatic content <15%) high-quality gasoline.

[0024] (2) This strategy can efficiently convert polyethylene into gasoline under mild conditions, thanks to the unique acidic properties of LSP molecular sieves ( Figure 4 The infrared spectrum of LSP molecular sieve adsorbing pyridine shows that the Lewis acid of LSP molecular sieve (1454 cm -1 ) with high quantity and strong strength. Pyridine still cannot be completely desorbed from LSP molecular sieve at a high temperature of 450℃, while the number of Lewis acids in the infrared spectrum of pyridine of HZSM-5 is small and it is almost completely desorbed at 450℃. The infrared spectrum of LSP molecular sieve adsorbing 2,6-di-tert-butylpyridine shows that almost all of LSP molecular sieve The acid sites are accessible to the larger diameter 2,6-di-tert-butylpyridine molecules and protonated (3610 cm -1 , Acid hydroxyl vibration peaks: 3370, 1616, 1530 cm -1 , protonated 2,6-di-tert-butylpyridine vibration peak), while HZSM-5 The acid sites are largely inaccessible to the probe molecules.

[0025] The super strong Lewis acid sites of LSP-zeolite come from the unique active sites of layered self-supporting molecular sieves (LSP molecular sieves) - the open framework tri-coordinate aluminum sites as super strong Lewis acid sites, which can efficiently activate the CH bonds of saturated alkanes. Organophosphorus probe molecules are adsorbed on the molecular sieves and the open framework tri-coordinate aluminum sites of the LSP molecular sieves used are characterized. This site, as a super strong Lewis acid site, can activate the CH bonds of deuterated n-hexane and isopentane and realize hydrogen transfer reactions. It can be seen from the mass spectrum and chromatogram of the product that part of the isopentane activated by LSP is deuterated by the deuterium atoms provided by deuterated n-hexane ( Figure 5 E, m / z = 73-83), while deuterated n-hexane loses some deuterium atoms and is transferred with an equal amount of hydrogen atoms ( Figure 5 The model reaction in the figure shows that the LSP molecular sieve has a strong ability to activate the CH bonds of saturated alkanes, which enables it to activate polyethylene under mild conditions and convert it into high-quality gasoline.

[0026] (3) Waste plastics are almost completely converted into gasoline, with only a small amount of C1-C3 gas (<2%) as by-product. The liquid product obtained by conventional filtration does not require further distillation and can be used directly as gasoline;

[0027] (4) The obtained gasoline is high-quality gasoline, the main components of which are a large number of branched alkanes (C4-C12), a small amount of straight-chain alkanes, cycloalkanes and aromatic hydrocarbons, and no olefins;

[0028] (5) The catalyst is resistant to carbon deposition (<0.6%) and can complete four rounds of catalytic reaction without regenerating the catalyst. The gasoline yield does not decrease significantly and the atomic economy is higher than 90%;

[0029] (6) The reaction process is simple and environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a high-resolution transmission electron microscopy image of the LSP-Z57 molecular sieve prepared in the present invention.

[0031] Figure 2 The X-ray powder diffraction patterns of LSP-Z57 and LSP-Z49 molecular sieves prepared in the present invention are compared with those of HZSM-5 (MFI topology) and HZSM-11 (MEL topology).

[0032] Figure 3 It is a schematic diagram of the self-hydrogen supply mechanism of the present invention.

[0033] Figure 4 The infrared spectra of LSP-Z57 (A) and HZSM-5 (B) before and after the adsorption of pyridine; the infrared spectra of LSP-Z57 (C) and HZSM-5 (D) after the adsorption of 2,6-di-tert-butylpyridine.

[0034] Figure 5 Solid-state phosphorus NMR spectra and their corresponding structures (A, B) of the three-coordinated aluminum sites in the framework of the molecular sieve as an organic phosphorus probe; schematic diagram of the molecular sieve activation reaction of deuterated n-hexane and isopentane hydrogen transfer (C), product chromatogram (D) and product mass spectrum (E, F).

[0035] Figure 6 It is the chromatogram of the product obtained in Example 1 of the present invention.

[0036] Figure 7 This is a graph showing the changing trend of the yield of each product versus reaction time. DETAILED DESCRIPTION

[0037] The present invention is further described in detail below in conjunction with specific embodiments, and the examples provided are only for illustrating the present invention, rather than for limiting the scope of the present invention. The examples provided below can be used as a guide for further improvements by those of ordinary skill in the art, and do not constitute a limitation of the present invention in any way.

[0038] The experimental methods in the following examples, unless otherwise specified, are all conventional methods, and are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. The materials, reagents, etc. used in the following examples, unless otherwise specified, can all be obtained from commercial channels.

[0039] The present invention provides a method for preparing liquid fuel by using waste plastics, that is, a layered acidic molecular sieve catalyst and waste plastics are uniformly mixed and added into a closed reactor, reacted for a certain time under certain temperature conditions, and the catalyst and the product are separated by a conventional filtering method to obtain high-quality liquid fuel.

[0040] The liquid fuel is mainly gasoline mainly composed of branched alkanes.

[0041] Comparative Example 1

[0042] The reaction effect of conventional molecular sieve catalysts on the degradation of waste plastics is illustrated.

[0043] In the first step, the catalyst is HZSM-5 (Si / Al=60), and the reactant is high-density polyethylene. The two are mixed evenly (grinded and mixed), sealed, and a reaction system is formed. The weight ratio of reactant: catalyst is 5:1.

[0044] In the second step, the reaction system of the first step is reacted at 240° C. for 4 hours, and the catalyst and unreacted polyethylene are separated by conventional filtering methods, and the remaining liquid is the product liquid fuel.

[0045] The analysis results showed that the polyethylene conversion rate was 35.1% and the gasoline yield was 34.5%.

[0046] Comparative Examples 2 to 13

[0047] Except for the following differences, the rest are the same as Comparative Example 1:

[0048] In the first step, the catalyst is:

[0049] Comparative Example 2HY (Si / Al=5) molecular sieve

[0050] Comparative Example 3 Ultrastable HY (Si / Al=5.5) mesoporous molecular sieve

[0051] Comparative Example 4HY (Si / Al=5) mesoporous molecular sieve

[0052] Comparative Example 5 SBA-15 (hydrogen type, Si / Al=20) mesoporous molecular sieve

[0053] Comparative Example 6 MCM-41 (hydrogen type, Si / Al=12.5) mesoporous molecular sieve

[0054] Comparative Example 7 Fresh FCC Cracking Catalyst

[0055] Comparative Example 8 Balanced FCC Cracking Catalyst

[0056] Comparative Example 9 Short b-axis ZSM-5 catalyst

[0057] Comparative Example 10 [C4Py]Cl-AlCl3 butylpyridinium chloroaluminate ionic liquid

[0058] Comparative Example 11Ru / C

[0059] Comparative Example 12Pt / γ-Al2O3

[0060] Comparative Example 13Pt / WO3 / ZrO2+HY(30)

[0061] The analysis results show that

[0062] In Comparative Example 2, the polyethylene conversion rate was 3.8%, and the gasoline yield was 3.7%.

[0063] In Comparative Example 3, the polyethylene conversion rate was 9.1%, and the gasoline yield was 9.0%.

[0064] In Comparative Example 4, the polyethylene conversion rate was 19.0%, and the gasoline yield was 18.7%.

[0065] In Comparative Example 5, the polyethylene conversion rate was <1% and the gasoline yield was 0%.

[0066] In Comparative Example 6, the polyethylene conversion rate was 9.6%, and the gasoline yield was 9.3%.

[0067] In Comparative Example 7, the polyethylene conversion rate was 5.8%, and the gasoline yield was 5.6%.

[0068] In Comparative Example 8, the polyethylene conversion rate was <1%, and the gasoline yield was <1%.

[0069] In Comparative Example 9, the polyethylene conversion rate was 12.0%, and the gasoline yield was 11.8%.

[0070] In Comparative Example 10, the polyethylene conversion rate was <1%, and the gasoline yield was <1%.

[0071] In Comparative Example 11, the polyethylene conversion rate was <1%, and the gasoline yield was <1%.

[0072] In Comparative Example 12, the polyethylene conversion rate was <1%, and the gasoline yield was <1%.

[0073] In Comparative Example 13, the polyethylene conversion rate was 12.5%, and the gasoline yield was 12.2%.

[0074] The above comparative examples show that in a hydrogen-free, solvent-free system, at 240°C for 4 hours, the effect of conventional catalysts on the conversion of waste plastics is relatively poor.

[0075] Example 1

[0076] In the first step, the catalyst is LSP-Z57 (Si / Al=57), and the reactant is high-density polyethylene, and the two are mixed evenly, and the weight ratio of reactant:catalyst is 5:1.

[0077] In the second step, the reaction system of the first step is reacted at 240° C. for 4 hours, and the catalyst and unreacted polyethylene are separated by conventional filtering methods, and the remaining liquid is the product liquid fuel.

[0078] The analysis results show that the polyethylene conversion rate is 81.7% and the gasoline yield is 81.1%. The specific composition of the product under this condition is shown in Table 1 and Figure 6 As shown:

[0079] Table 1: Product composition of this example

[0080]

[0081]

[0082]

[0083]

[0084] Example 2

[0085] Except for the following differences, the rest are the same as Example 1:

[0086] In the first step, the reaction time was changed from 4 hours to 24 hours.

[0087] The analysis results show that the polyethylene conversion rate is 90.3%, the gasoline yield is 87.2%, and the detailed product composition is shown in the following table:

[0088] Table 2: Product composition of this example

[0089]

[0090]

[0091]

[0092] In this example, the yield of each product was detected as the reaction time increased. It was found that after 4 hours of reaction, the yield of C7-C12 products decreased with the reaction time, while the yield of C4-C6 products increased with the reaction time. This is because the C7-C12 products underwent secondary cleavage ( Figure 7 Therefore, in order to obtain high-quality gasoline (with C8 alkanes having the highest value), it is necessary to avoid the situation where the secondary cracking of C7 and above alkanes due to excessively long reaction time is caused.

[0093] Table 3: Yield of each product over reaction time

[0094]

[0095]

[0096] Example 3

[0097] Except for the following differences, the rest are the same as Example 1:

[0098] In the first step, the catalyst is: LSP-Z49 (Si / Al=49) molecular sieve

[0099] The analysis results show that the polyethylene conversion rate is 78.8%, the gasoline yield is 77.2%, and the detailed product composition is shown in the following table:

[0100] Table 4: Product composition of this example

[0101]

[0102]

[0103] Compared with Example 1, the catalysts used have different Si / Al ratios and pore structures, which lead to different gasoline yields.

[0104] Table 5: The composition of the gasoline obtained in Example 1 is compared with the commercial gasoline.

[0105]

[0106] From the results of the above examples, it can be seen that, compared with the comparative example, the effect of the technology of the present invention is that, under mild conditions, the gasoline yield can be increased from <15% to >90%, and the prepared gasoline does not need to be post-treated or distilled and can be used directly.

[0107] Example 4

[0108] Except for the following differences, the rest are the same as Example 1:

[0109] In the first step, the reaction temperature was 220 °C and the catalyst / substrate ratio was 1:3.

[0110] The analysis results show that the polyethylene conversion rate is 90.0%, the gasoline yield is 87.9%, and the detailed product composition is shown in the following table:

[0111] Table 6: Product composition of this example

[0112]

[0113]

[0114]

[0115] Example 5

[0116] Except for the following differences, the rest are the same as Example 1:

[0117] In the first step, the reaction temperature is 260°C, and the reactants are high-density polyethylene, low-density polyethylene and their recycled materials. The analysis results are shown in the following table:

[0118] Table 7: Gasoline yields from different plastic substrates

[0119] Reactants Gasoline yield (%) High-density polyethylene 85.1 High density polyethylene recycled material 78.3 Low density polyethylene 65.1 Low density polyethylene recycled material 64.9

[0120] Taking the conversion of high-density polyethylene recycled material at 260°C as an example, the composition of the products obtained by the conversion reaction is shown in the following table. The composition of the products of the other three groups is similar to that in the following table:

[0121] Table 8: Product composition of this example

[0122]

[0123]

[0124]

[0125] Example 6

[0126] Except for the following differences, the rest are the same as Example 1:

[0127] In the second step, the solids after the reaction (including the catalyst and the unbroken long-chain polyethylene) were collected each time, mixed with the newly added polyethylene and directly subjected to the next round of reaction (heated to 240°C and maintained for 4 hours).

[0128] The analysis results show that the gasoline yield did not decrease significantly after four cycles. The reaction feed conditions and detailed product composition are shown in the following table:

[0129] Table 9: Feeding conditions of the reaction in four cycles

[0130]

[0131] Table 10: Gasoline yield and composition in four cycles

[0132]

[0133] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that the present invention can be further improved. In a word, according to the principles of the present invention, the application is intended to include any changes, uses or improvements to the present invention, including departure from the disclosed scope in the application, and changes made with conventional techniques known in the art.

[0134] The present invention has been described in detail above. For those skilled in the art, without departing from the purpose and scope of the present invention, and without the need to carry out unnecessary experimental conditions, the present invention can be implemented in a wide range under equivalent parameters, concentrations and conditions. Although the present invention provides specific embodiments, it should be understood that the present invention can be further improved. In a word, according to the principles of the present invention, the application is intended to include any changes, uses or improvements to the present invention, including departure from the disclosed scope in the application, and changes made with conventional techniques known in the art.

Claims

1. A method for preparing liquid fuel using waste plastics, comprising the following steps: 1) Mix the waste plastic and the catalyst, seal them and form a reaction system. 2) allowing the reaction system to react, separating the catalyst, and obtaining liquid fuel.

2. The method according to claim 1, characterized in that: The waste plastic is a polymer whose skeleton is composed of carbon-carbon saturated bonds; The catalyst is a layered molecular sieve with mesopores and micropores and acidic sites on the outer surface.

3. The method according to claim 2, characterized in that: The catalyst is a layered self-supporting molecular sieve with an MFI / MEL topological structure, wherein the molar ratio of Si to Al contained in the catalyst is 25-300.

4. The method according to any one of claims 1 to 3, characterized in that: The weight ratio of waste plastic to catalyst is 20:1 to 0.1:

1.

5. The method according to any one of claims 1 to 4, characterized in that: In step 2), the reaction temperature is 150-280° C.; the reaction time is 0.5-24 hours.

6. The method according to any one of claims 1 to 5, characterized in that: The obtained liquid fuel is mainly gasoline composed mainly of branched alkanes.

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