Plastic depolymerization using silica-based catalysts

Abstract

Described herein is a method for depolymerizing plastic waste using a doped silica catalyst. The method provides high quality liquid depolymerization products that can be used as a cracker feedstock with high efficiency.

Description

Technical Field

[0001] This disclosure relates to methods for depolymerizing plastic raw materials, and more specifically to methods for depolymerizing plastic raw materials in the presence of a specific type of silica capable of producing valuable pyrolysis products. Background Technology

[0002] Plastics are inexpensive and durable materials used to manufacture a wide variety of products for a broad range of applications, leading to a significant increase in plastic production over the past few decades. Due to the durability of the polymers involved in plastic production, increasing amounts of plastic are filling landfills and occupying natural habitats worldwide, causing environmental problems. Even biodegradable and degradable plastics can last for decades, depending on local environmental factors such as UV exposure levels, temperature, the presence of suitable microorganisms, and other factors.

[0003] Current plastic recycling primarily includes mechanical recycling and chemical recycling. Generally speaking, mechanical recycling is the most common method for repurposing plastics. Through this method, plastics are mechanically transformed without altering their chemical structure, allowing them to be used in the production of new materials. Typical mechanical recycling steps include collecting plastic waste; sorting the waste into different types of plastics and colors; packaging the plastic by pressing or milling; washing and drying the plastic; reprocessing the plastic into pellets by bonding, extruding, and cooling; and finally obtaining the recycled raw materials. This is the most widely used technology for polyolefins such as polyethylene (PE) and polypropylene (PP).

[0004] On the other hand, chemical recycling reprocesses plastics and alters their structure, making them usable as raw materials for various industries or as basic inputs or feedstocks for manufacturing new plastic products. Chemical recycling typically involves the following steps: collecting the plastic and then heating it to temperatures that break down the polymer into smaller fragments. This process, also known as depolymerization, is a fundamental process in which plastic waste material is converted into liquid fuel through thermal degradation (cracking) in the absence of oxygen. Plastic waste is typically first melted in a stainless steel chamber under an inert purging gas such as nitrogen. The chamber then heats the molten material to a gaseous state, extracts it, and then condenses it in one or more condensers to produce a hydrocarbon distillate comprising straight-chain and branched aliphatic, cyclic aliphatic, and aromatic hydrocarbons. The resulting mixture can then be used as fuel or as feedstock for further thermocatalytic processes to obtain refined chemicals such as monomers that can be reintroduced into the plastics manufacturing cycle.

[0005] The step of converting molten plastic material into a gaseous stream can, in principle, be carried out solely by heat (thermal depolymerization). However, it has been shown that the presence of a catalyst in this stage allows depolymerization to occur at lower temperatures and more efficiently.

[0006] To this end, various catalysts, typically based on silica materials, have been proposed. JPH10249214 suggests that silica materials can be treated to generate mesoporous silica structures that can be used as depolymerization catalysts. According to CN110283617, after a first purely thermal depolymerization step, the pyrolysis gas stream can be subjected to a thermocatalytic stage with Fe-doped mesoporous silica to produce light hydrocarbons.

[0007] While the results obtained using the aforementioned mesoporous silica are interesting, they are still unsatisfactory. In fact, for use as a feedstock in the thermocatalytic cracking stage, the pyrolysis products from the first depolymerization stage should have a composition that is branched and has a higher C content. 28 The amount of aromatic compounds should be kept as low as possible. At the same time, efficiency should be maximized.

[0008] We were surprised to find that, with the addition of specific metals, mesoporous silica can yield highly efficient pyrolysis products with improved quality for use as cracker feedstock. Summary of the Invention

[0009] Therefore, one aspect of this disclosure is a process for depolymerizing plastics, comprising the following steps:

[0010] a) Provide raw materials from plastic waste;

[0011] b) Mix the plastic waste raw material with the catalyst to obtain a reactant mixture; and

[0012] c) Heating the reactant mixture to a temperature ranging from 280°C to 600°C to obtain the depolymerization product;

[0013] The process is characterized by the fact that the catalyst is selected from materials with a concentration ranging from 0.2 to 1.0 cm⁻¹. 3 Porosity per g (BET method), ranging from 100 to 500 m³ / g. 2 / g of surface area and average pore radius ranging from 0.5 to 50 nm and doped with metals or quasi-metals selected from the group consisting of: elements of Groups 3 to 15 of the Periodic Table (IUPAC version), excluding Fe and Mo.

[0014] Preferably, the reactant mixture comprises 1 to 20% by weight of catalyst, more preferably 1 to 10% by weight, and particularly 1 to 5% by weight of catalyst, relative to the total reactant mixture.

[0015] Preferably, the plastic waste raw material is substantially made of polyolefins or mixtures of polyolefins. Specifically, when the polyolefin mixture is used as the plastic raw material, it includes a mixture of polyethylene and polypropylene. The polyethylene can be one or more of high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). The polypropylene (PP) can be a propylene homopolymer or a copolymer of propylene with a lower amount of ethylene and / or butene. Additionally, the raw material may include other polyolefins such as polybutene. In specific embodiments, the raw material may also include a polymer mixture that incorporates other materials such as polystyrene (PS), ethyl vinyl acetate copolymer (EVA), ethyl vinyl alcohol copolymer (EVOH), polyvinyl chloride (PVC), or mixtures thereof. In a preferred embodiment, the raw material consists of more than 80% polypropylene or a mixture between polyethylene and polypropylene, wherein polypropylene constitutes more than 50% of the polypropylene / polyethylene mixture.

[0016] When performing depolymerization processes, care should be taken to avoid introducing oxygen-containing atmospheres into the depolymerization system. Barriers to potentially oxygen-containing atmospheres can be achieved through a range of methods, such as nitrogen gagging and a vacuum system connected to the extruder barrel.

[0017] More specifically, the plastic raw material mixture can be fed into the feeding system of the depolymerization reactor through a hopper or two or more parallel hoppers, and the oxygen present in the plastic waste material atmosphere can be substantially eliminated inside the hopper.

[0018] The plastic raw material can be fed directly into the depolymerization reactor for small-scale testing. For larger-scale testing, it is preferable to feed the plastic raw material into the depolymerization reactor via an extruder that is also fed with the plastic raw material.

[0019] Preferably, the plastic waste is brought to a temperature at which substantially all substances melt, and then injected into the depolymerization reactor. The extruder receives the chopped plastic waste into the feed hopper, conveys the stream in the melting section, and heats the polymer through a combination of mixing energy and heat supplied by the barrel heater. Typically, the melting temperature ranges from 250 to 350°C.

[0020] Additives can be optionally incorporated into the melt to reduce the corrosiveness of plastic waste or improve depolymerization efficiency.

[0021] During the extrusion process, one or more degassing steps can be anticipated to remove residual moisture present in the product.

[0022] Before being fed into the reactor, the melt stream can be filtered to remove solid impurities present in the plastic waste.

[0023] Any extrusion system can be used, such as a single-screw extruder, a twin-screw extruder, a twin-screw extruder with a gear pump, or a combination thereof.

[0024] The mixing of plastic waste feedstock and catalyst can be carried out directly in the depolymerization reactor or pre-mixed outside the reactor. When mixing is carried out in the depolymerization reactor, several options for feeding the catalyst are available. The simplest, preferred for small-scale systems, is to directly pour the solid catalyst into the reactor under a nitrogen atmosphere. Alternatively, powdered catalyst can be fed into the reactor in the form of a liquid hydrocarbon slurry or a semi-solid paste using a specialized device.

[0025] Alternatively, mixing can be carried out outside the depolymerization reactor. Several options are possible in this case. According to one of them, the catalyst is mixed with the plastic waste in a homogenizer, and then the mixture is granulated. The resulting granules (which may also contain other additives) can then be loaded into an extruder hopper, which is used to feed the polymerization reactor. Alternatively, the plastic waste and catalyst can be loaded into separate hoppers. In this case, mixing can be carried out in the extruder while the plastic waste is melting, and then the plastic waste is fed into the depolymerization reactor.

[0026] The depolymerization reactor is preferably a stirred vessel operating at a temperature ranging from 300 to 550°C, more preferably from 350 to 500°C, and particularly from 350 to 450°C, having an inlet for the plastic raw material and catalyst and an outlet for the gaseous depolymerization products.

[0027] In fact, as a result of the depolymerization process, a gaseous flow is generated, which is sent to a condensation unit that liquefies all or part of the flow.

[0028] The condensation section receives the effluent gases from the depolymerization reactor and partially condenses them into an oily product consisting primarily of hydrocarbons. A portion of the non-condensable gases can be collected and stored separately. The condensation section may consist of one or more stages operating at different temperatures under pressure or without pressure, in order to recover the maximum amount of product based on the volatility of the resulting formed compound. The temperature range can, of course, vary depending on the operating pressure.

[0029] Preferably, the condensation section has at least two condensation stages, and preferably operates at decreasing temperatures. As an example, in small-scale equipment, the first condensation stage operates in a temperature range of 100 to 120°C, and the second condensation stage operates in a temperature range of 2°C to -20°C.

[0030] At the end of the process, at least 80% by weight, and preferably 90% by weight, of the plastic raw material has been converted into liquid or gaseous depolymerization products.

[0031] As stated above, the primary use of the depolymerization product according to this disclosure is as a feedstock for crackers. In this regard, it is preferable to generate a high-yield liquid depolymerization product from the depolymerization process. In a preferred embodiment, the amount of liquid depolymerization product is higher than 65% by weight of the plastic waste feedstock, more preferably from 70 to 90% by weight.

[0032] Furthermore, it is preferable that the liquid depolymerization products have a composition as suitable as possible for cracker feedstock. This involves having very low amounts, or even the absence of fractions with C28 or higher. Preferably, the amount of fractions above C28 in the liquid depolymerization products is equal to or less than 4% relative to the total amount of liquid depolymerization products, preferably less than 3%, and more preferably less than 2%.

[0033] Furthermore, the quality of the cracker feedstock is higher when the amount of aromatic compounds is lower. Preferably, the amount of aromatic fraction in the liquid depolymerization product is equal to or less than 0.10% by weight, more preferably less than 0.07% by weight, and even more preferably less than 0.05% by weight.

[0034] Another indicator of the suitability of the liquid depolymerization product as a cracker feedstock is the branching index, defined as the molar ratio between internal double bonds and double bonds (α-olefins) at chain ends, as determined in the characterization section. Preferably, the branching index in the liquid depolymerization product is less than 0.15, more preferably less than 0.14 and 0.12.

[0035] As used in this article, “C6 to C8 aromatic compounds” refers to hydrocarbons with σ bonds and delocalized π electrons between the carbon atoms forming the ring, containing a total of 6 to 8 carbon atoms.

[0036] Preferably, in the catalyst according to this disclosure, the silica-based material is doped with a metal or metalloid selected from the group consisting of Ti, Sn, Bi, Al, Mn, W, and mixtures thereof. More preferably, the doping metal is selected from Ti, Sn, and Bi, and mixtures thereof. In a particularly preferred embodiment, the doping metal is selected from Ti, Sn, and Bi.

[0037] Preferably, the silica-based material of the catalyst has a particle size ranging from 0.20 to 0.50 cm⁻¹. 3 Porosity per g, ranging from 100 to 300 m³ / g. 2 The surface area per g and the preferred range of 0.5 to 20 nm, more preferably from 0.5 to 15 nm, and particularly from 1 to 10 nm, are the average pore radius.

[0038] The silica-based materials used as depolymerization catalysts according to this disclosure can be obtained by several methods. According to one preferred method, the novel mesoporous silica material is synthesized by silicate polycondensation in a citric acid / citric acid solution of Pluronic F127 [Chem. Mater. 2015, 27, 5161-5169]. Alternatively, a one-step aqueous process allows for the preparation of metal-doped silica by induced self-assembly using tetraethoxysilane, a suitable metal salt, and a triblock copolymer template [Studies in Surface Science and Catalysis 2006, 162, 369-376].

[0039] The catalyst according to this disclosure allows for high yields of liquid depolymerization products having a composition suitable for use as feedstock for crackers.

[0040] Characterization

[0041] The characteristics are determined using the following methods.

[0042] Porosity and surface area with nitrogen

[0043] Porosity was measured according to the BET method (using a Carlo Erba Sorptomatic 1900 instrument). The analyzed samples were preheated at 350°C under high vacuum for 7 hours. Data collection and refinement were performed using the instrument-associated Sorptomatic software.

[0044] Analytical methods

[0045] GC analysis of the liquid products from each experiment was performed using an Agilent 7890 GC (Agilent Technologies, Santa Clara, CA) equipped with a standard nonpolar column and a flame ionization detector. Based on their retention times, fractions were collected to total the following fractions as the total liquid product: S-RT = x <nC7,M-RT=nC7<x<nC11;L-RT=nC12<x<nC28,XL-RT=x> C28

[0046] The relative values ​​for the last three fractions are reported in Table 1.

[0047] NMR data were used to characterize the percentage of aromatic, alkane, and olefin protons in the liquid products. All examples were analyzed by adding CDCl3 (0.6 g of depolymerized polymer / metal oxide mixture with 0.4 g of CDCl3). Data were collected at 25 °C with a 5 mm Prodigy probe on a Bruker AV500 MHz NMR spectrometer (Bruker Corporation, Billerica, MA, Massachusetts). One-dimensional 1 H NMR data use The software (Brook) uses an exponentially broadened window function for processing. Quantitative measurements are performed using a 15-second relaxation delay, a 30° flip-angle pulse, and 32 scans to facilitate accurate integration. Spectral integrals of protons in aromatic alkenes and alkanes are obtained and used to quantify the relative ratios of these protons.

[0048] Example

[0049] General de-galvanization procedure

[0050] 30 g of commercial grade polypropylene Moplen HP522H was loaded into a 500 ml circular glass reactor with three necks equipped with thermocouples and nitrogen inlets. An appropriate amount of solid catalyst (0.8 g) was then introduced into the glass reactor. Two glass condensers were connected in series and maintained at 110 °C and -8 °C respectively using oil baths (Cryostat Julabo). The reactor was placed in an electric heating system (jacketed bath), and the desired power was set to raise the temperature to 420-430 °C. The pyrolysis process occurred, and the following experimental parameters were recorded:

[0051] ·T 开始 The temperature of the reaction mixture during the first condensation of the liquid product was observed.

[0052] • L%, the yield of condensable liquid in an ice trap (relative to the amount of polymer loaded).

[0053] • S%, yield of solid / waxy residue in the reactor, excluding catalyst (relative to the charged polymer).

[0054] • G%, yield of gaseous products that did not condense in the appropriate section

[0055] Comparison Example 1

[0056] A solution of 2.60 g of Prönkel F127 (BioVision, an amphiphilic block copolymer) in 107.5 mL of deionized water was prepared at room temperature and poured into a 500 mL round-bottom flask equipped with a mechanical stirrer. 3.68 g of citrate monohydrate (Sigma-Aldrich, 17.53 mmol) and 2.54 g of sodium citrate dihydrate (MP Biomedicals, 8.64 mmol) were added to this solution. The mixture was stirred at room temperature for several hours until completely dissolved. Subsequently, a solution of 10.4 g of sodium silicate (Sigma-Aldrich, approximately 26.5 wt% SiO2, d = 1.39 g / mL), diluted in 30 mL of deionized water, was added to the buffered Prönkel F127 solution with stirring at room temperature. A white solid precipitated immediately; stirring was continued for 30 minutes, and the suspension was then allowed to stand at room temperature for 24 hours. The synthesized material was filtered and washed on G4 glass frit and dried under vacuum at 50°C. Finally, the powder was calcined in air at 350°C for 24 hours. 2.9 g of free-flowing powder was obtained, which showed a 533 m³ / s²·dBET characterization. 2 / g surface area, 0.327g / cm² 3 The catalyst exhibited a porosity of 12 nm and an average diameter of 12 nm. The resulting catalyst was used for depolymerization tests according to standard procedures. The results are reported in Table 1.

[0057] Example 1: Sn-doped catalysts

[0058] 0.75 g of F127 (BioVision, an amphiphilic block copolymer) and 0.59 g of NaCl (Sigma-Aldrich) were dissolved in 70 mL of deionized water at room temperature. 2.10 g of tetraethyl orthosilicate (TEOS, Thermo Fisher Scientific) was added to the solution at 35 °C with stirring, and the resulting mixture was maintained at this temperature for 4 hours. A latex solution was obtained. Then, a freshly prepared 0.2 M SnCl₄·5H₂O solution (Acros, 10 mL) was added at 35 °C with stirring for 24 hours. Subsequently, the reaction mixture was transferred to a PP bottle and hydrothermally treated under static conditions at 90 °C for 6 hours. The solid precipitate was recovered by filtration, washing, and drying overnight at 100 °C. Finally, the powder was calcined in air at 500 °C for 6 hours. 1.6 g of free-flowing powder was obtained. The catalyst thus obtained was used for depolymerization tests according to general procedures. The results are reported in Table 1.

[0059] Example 2: Ti-doped catalysts

[0060] The depolymerization catalyst was prepared according to the procedure reported in Example 1, but using a 0.2 M TiCl4 solution (Sigma-Aldrich, 10 mL) as the doped metal source. 1.2 g of free-flowing powder was obtained. The catalyst thus obtained was used for depolymerization tests according to a general procedure. The results are reported in Table 1.

[0061] Example 3: Bi-doped catalysts

[0062] The depolymerization catalyst was prepared according to the procedure reported in Example 1, but using a 0.2 M BiCl3 solution (Sigma-Aldrich) as the doped metal source. The reaction was initiated from 2.25 g of Prönkel F127: 3.0 g of free-flowing powder was obtained, which showed a 267 m²·s⁻¹ under BET characterization. 2 / g surface area, 0.229g / cm² 3 The catalyst exhibited a porosity of 17 nm and an average diameter of 17 nm. The resulting catalyst was used for depolymerization tests according to standard procedures. The results are reported in Table 1.

[0063] Comparative Example 2: Fe-doped catalysts

[0064] The depolymerization catalyst was prepared according to the procedure reported in Example 1, but using a 0.2 M FeCl3 solution (Sigma-Aldrich) as the doped metal source. The reaction was initiated from 2.25 g of Prönkel F127: 2.2 g of free-flowing powder was obtained, which showed a 325 m³ / h under BET characterization. 2 / g surface area, 0.577g / cm 3 The catalyst exhibited a porosity of 35 nm and an average diameter of 35 nm. The resulting catalyst was used for depolymerization tests according to standard procedures. The results are reported in Table 1.

[0065] Table 1

[0066]

[0067] The results shown in Table 1 illustrate the advantages of depolymerization in the presence of the catalyst disclosed herein. Examples of the invention provide a large quantity of liquid fractions while maintaining high quality, exhibiting lower amounts of aromatic compounds and branching indices compared to those obtained with comparative catalysts. This ensures better quality of the depolymerization products suitable as cracker feedstock.

Claims

1. A process for depolymerizing plastics, comprising the following steps: a) Provide raw materials from plastic waste; b) Mixing the plastic waste raw material with a catalyst to obtain a reactant mixture, wherein the plastic waste raw material is a polyolefin or a mixture of polyolefins; and c) Heating the reactant mixture to a temperature ranging from 280°C to 600°C to obtain the depolymerization product; The process is characterized by the fact that the catalyst is selected from a range of 0.2 to 1.0 cm⁻¹ as measured by the BET method. 3 Porosity per g, ranging from 100 to 500 m³. 2 Silica-based materials with a surface area of ​​ / g and an average pore radius ranging from 0.5 to 50 nm and doped with metals selected from the group consisting of: Ti, Sn, Bi, and mixtures thereof. The reactant mixture comprises 1 to 20% by weight of the catalyst relative to the total reactant mixture.

2. The process according to claim 1, wherein the silica-based material has a particle size ranging from 0.20 to 0.50 cm³. 3 / g porosity.

3. The process according to claim 1, wherein the silica-based material has a range from 100 to 300 μm. 2 / g of surface area.

4. The process of claim 1, wherein the silica-based material has an average pore radius ranging from 0.5 to 20 nm.

5. The process of claim 4, wherein the silica-based material has an average pore radius ranging from 0.5 to 15 nm.

6. The process according to claim 1, wherein the mixture of polyethylene and polypropylene is used as a plastic raw material.

7. The process according to claim 1, wherein at least 90% of the plastic waste raw material has been converted into liquid or gaseous depolymerization products.

8. The process according to claim 7, wherein the amount of liquid depolymerization product is higher than 70% by weight of the initial feed plastic waste raw material.

9. The process according to claim 8, wherein the amount of liquid depolymerization product is 75 to 85% by weight of the initial feed plastic waste raw material.

10. The process of claim 7, wherein the amount of the liquid depolymerization product higher than C28 fraction is equal to or less than 4% relative to the total amount of liquid depolymerization product.

11. The process according to claim 7, wherein the amount of aromatic fraction in the liquid depolymerization product is equal to or less than 0.10% relative to the total amount of liquid depolymerization product.

12. The process according to claim 7, wherein the branching index of the liquid depolymerization product is less than 0.15.

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