FIELD OF THE DISCLOSURE

[001] This disclosure relates to a catalytic method for depolymerizing plastic feedstock, and to certain catalyst for the depolymerization. More particularly, it relates to methods for the depolymerizing plastic feedstock in the presence of metal organic framework (MOF) catalysts.

BACKGROUND OF THE DISCLOSURE

[002] Plastics are inexpensive and durable materials, which can be used to manufacture a variety of products that find use in a wide range of applications, so that the production of plastics has increased dramatically over the last decades. Due to the durability of the polymers involved in plastic production, an increasing amount of plastics are filling up landfill sites and occupying natural habitats worldwide, resulting in environmental problems. Even degradable and biodegradable plastics may persist for decades depending on local environmental factors, like levels of ultraviolet light exposure, temperature, presence of suitable microorganisms and other factors.

[003] Currently plastic recycling primarily includes mechanical recycling and chemical recycling. Globally speaking, mechanical recycling is the most used method for new uses of plastics, and through this method, plastics are mechanically transformed without changing their chemical structure, so they can be used to produce new materials. Typical mechanical recycling steps include collecting plastic wastes; sorting plastic wastes into different types of plastics and colors; packaging plastics by pressing or milling plastics; washing and drying the plastics; reprocessing the plastics into pellets by agglutinating, extruding and cooling the plastics; and finally recycled raw materials are obtained. This is the most widely used technology for the polyolefins like polyethylene (PE) and polypropylene (PP).

[004] Chemical recycling, on the other hand, reprocesses plastics and modify their structure so that they can be used as raw material for different industries or as a basic input or feedstock for manufacturing new plastic products. Chemical recycling typically includes the steps of collecting plastics, followed by heating the plastics to a temperature at which the polymers break down into small fragments. This process, also called depolymerization, is a basic process whereby plastic waste material is converted to liquid fuel by thermal degradation (cracking) in the absence of oxygen. Plastic waste is typically first melted within a stainless steel chamber under an inert purging gas, such as nitrogen. This chamber then heats the molten material to a gaseous state that is drawn and then condensed in one or more condensers to yield a hydrocarbon distillate comprising straight and branched chain aliphatic, cyclic aliphatic and aromatic hydrocarbons. The resulting mixture can then be used as a fuel or used as a feedstock for further thermocatalytic process in order to obtain refined chemicals such as monomers that can be reintroduced into the plastic manufacturing cycle.

[005] The step of converting the molten plastic mass into a gaseous stream can in principle take place only by the action of the heat (thermal depolymerization). However, it has been proved that the presence of a catalyst in this stage allow the depolymerization to take place at a lower temperature and more efficiently.

[006] To this end, various catalysts have been proposed often based on depolymerization tests carried out on virgin polymers or accurately presorted recycled plastics composed of a substantially single polymer. Under these “non real” conditions the catalysts may show a somewhat higher depolymerization activity with respect to thermal depolymerization only. However, these tests does not provide any information as to how the catalyst may perform under real conditions and in particular no information as to whether it will be affected, and to what extent, by the poisoning effect coming from the components of the real plastic waste.

[007] Zeolites based catalyst for example, show a good depolymerization activity with virgin or singled out recycled plastics but when used with more complex plastic waste feedstock suffer from a pronounced decay of catalyst activities. As a matter of fact, the performance of a catalyst in the depolymerization of complex plastic waste feedstock is unpredictable.

[008] We have surprisingly found that metal organic framework catalysts, particularly when the plastic waste contains polypropylene fraction, can give with high efficiency pyrolytic products with improved quality to be used as cracker feedstock.

SUMMARY OF THE DISCLOSURE

[009] It is therefore an aspect of the present disclosure a process for depolymerizing plastics, comprising the steps of: a) providing a melt plastic waste feedstock comprising at least recycled polypropylene and; b) subjecting the melt product obtained in (a) to a temperature ranging from 280°C to 600°C to obtain a depolymerization product; said process being characterized by the fact that either or both of the melt product and depolymerization product are contacted with a metal organic framework (MOF) catalyst.

[0010] Preferably, the amount of catalyst used ranges from 0.1 to 20 wt.%, more preferably 0.1-10 wt.% and especially from 0.1 to 5 wt.% with respect to the total weight of plastic waste feedstock and catalyst.

[0011] Preferably, the plastic waste feedstock comprises a mixture of polyethylene and polypropylene in a weight ratio 85:15 to 15:85 more preferably 80:20 to 20:80. The polyethylene can be one or more of high density polyethylene (HDPE), low-density polyethylene (LDPE), linear low density polyethylene (LLDPE). Polypropylene (PP) can be either propylene homopolymer or a propylene copolymer with lower amount of ethylene and/or butene. In addition, the feedstock may comprise other polyolefins like polybutene. In a particular embodiment, the feedstock may comprise also polymeric mixtures that incorporates other materials like polystyrene (PS), ethyl-vinyl acetate copolymer (EVA), ethyl-vinyl alcohol copolymer (EVOH), polyvinyl chloride (PVC), or mixtures thereof. In a preferred embodiment, the feedstock is constituted by more than 80% wt of a mixture between polyethylene and polypropylene in which polypropylene accounts for more than 50%wt of the polypropylene/polyethylene mixture.

[0012] When carrying out the depolymerization process, care should be taken for not introducing oxygen containing atmosphere into the depolymerization system. The barrier to the potentially oxygen-containing atmosphere can be obtained with a series of expedients such as nitrogen blanketing and vacuum system connected to a barrel of the extruder.

[0013] More specifically, the plastic feedstock mixture, can be charged into the feeding system of the depolymerization reactor by means of a hopper, or two or more hoppers in parallel, and the oxygen present in the atmosphere of the plastic waste material is substantially eliminated inside the hopper(s).

[0014] Plastic feedstock can be fed directly into the depolymerization reactor for small scale tests. For larger scale it is preferred to fed to the depolymerization reactor by means of an extruder which is turn fed with the plastic feedstock.

[0015] Preferably, plastic scrap is brought to a temperature at which substantially all the mass is melted and then injected into the depolymerization reactor. The extruder receives the plastic scrap cut in small pieces into the feed hopper, conveys the stream in the melting section and heat the polymer by combined action of mixing energy and heat supplied by barrel heaters. Usually, the melting temperature ranges from 250°C to 350°C.

[0016] Additives can optionally be incorporated in the melt aimed at reducing corrosivity of plastic scrap or improving depolymerization efficiency.

[0017] During the extrusion, one or more degassing steps can be foreseen to remove residual humidity present in the product.

[0018] Before being fed to the reactor, the melt stream can be filtered by in order to remove solid impurities present in the plastic waste.

[0019] Any extrusion systems can be applied, as single screw extruders, twin screw extruders, twin screw extruders with gear pump, or combination of the above.

[0020] The mixing of the plastic waste feedstock and catalyst can take place either directly into the depolymerization reactor or beforehand outside the reactor. When the mixing takes place in the depolymerization reactor, the catalyst can be fed according to several options. The simplest one, preferably used in small scale systems, is to directly pour the solid catalyst in the reactor under a nitrogen atmosphere. According to another option, the powdery catalyst may be fed to the reactor in a form of a liquid hydrocarbon slurry or a semisolid paste using dedicated devices.

[0021] As an alternative, the mixing can take place outside the depolymerization reactor. Also in this case several options are possible. According to one of them, catalyst is mixed with plastic scrap in a homogenizer apparatus and the mixture is then pelletized. The so obtained pellets, which can also contain other additives, may then be charged to the extruder hopper which is used to feed the polymerization reactor. It is also possible to charge into the hopper plastic scrap and catalysts separately. In this case, the mixing can take place into the extruder at the time of plastic scrap melting which is subsequently fed to the depolymerization reactor.

[0022] The depolymerization reactor is preferably an agitated vessel operated at temperature ranging from 300°C to 550°C, more preferably from 350°C to 500°C and especially from 350°C to 450°C with inlet for plastic feedstock and catalyst and outlet for the gaseous depolymerization product. [0023] In fact, as a result of the depolymerization process, a gaseous stream is generated that is sent to a condensation unit which totally or partially liquifies said stream.

[0024] The condensation section receives effluent gases from the depolymerization reactor and partially condense them in an oily depolymerized product substantially made up of hydrocarbons. A fraction of incondensable gases can be collected and stored separately. The condensation section can be composed by one or more stages, operated in pressure or not, at different temperatures in order to recover the maximum amount of products according to the volatility of the resulting formed compounds. The temperature range can vary of course depending on the operative pressure.

[0025] Preferably the condensation section has at least two condensation stages preferably operating at descending temperatures. As an example, in small scale equipment the first condensation stage is operated at a temperature range of 100-120°C and the second at a temperature range of from 2°C to -20°C:

[0026] It is also possible to subject the depolymerization product coming from the condensation stage to a second depolymerization stage carried out in the presence of the MOF already described. The second depolymerization stage can be carried out under similar conditions described for the previous depolymerization stage. When this set-up is issued, it is also preferred to recycle back the catalyst and part of the liquid or semiliquid mass to the first depolymerization reactor from which the solid residue is discharged. In analogy with the first depolymerization step, the gaseous effluent can be condensed in a subsequent condensation stage.

[0027] At the end of the process preferably at least 80% wt., and preferably at 90% wt., of the plastic feedstock has been converted in liquid or gaseous depolymerization product.

[0028] As mentioned above, the main use of the depolymerization product according to the present disclosure can be as a cracker feedstock. In this connection, it would be preferred to generate from the depolymerization process a high yield in liquid depolymerization product. In a preferred embodiment the amount of liquid depolymerization product is higher than 60%wt more preferably from 65 to 85% wt. of the plastic waste feedstock.

[0029] Moreover, it would also be preferable for the liquid depolymerization product to have a composition as much as possible suited for a cracker feedstock. This involves having a very low amount, or even absence, of fractions with C28 or higher. Preferably, in the liquid depolymerization product the amount of the higher than C28 fraction is equal to, or lower than, 4%, preferably lower than 3% and more preferably lower than 2% with respect to the total amount of liquid depolymerization product.

[0030] Also, the quality of cracker feedstock is higher when the depolymerization oil obtained from real plastic waste has low values of C6-C8 aromatics (Ar) and Branch Index (BI). This latter is defined as the molar ratio between internal double bonds with respect to double bond in chain end position (alfa-olefins) determined as described in the characterization section. Preferably, in the liquid depolymerization product the Branch Index is lower than lower than 0.5%wt and more preferably lower than 0.4%wt.

[0031] As used herein, “C6-C8 aromatics”(Ar) refer to a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle, wherein total of 6 to 8 carbon atoms are present. Preferably, their amount into the liquid depolymerization product is lower than 1.3%wt.

[0032] As already mentioned, the catalyst comprises a metal framework oxide (MOF).

[0033] MOFs are known to be highly porous crystalline materials, made of organic linkers and inorganic blocks joint together in a textured structure. Both MOF having either 3D (cage) or 2D (layered) framework type can be used as catalysts.

[0034] One of the MOF catalyst can be selected from the group consisting of MOF including topology e.g., MOFs with zeolitic topologies, such as zeolitic imidazolate frameworks (ZIFs), zeolite-like metal-organic frameworks (ZMOFs).

[0035] Another preferred group of MOF catalysts is that in which the organic linker is selected from the group consisting of carboxylates, phosphonates, N-based groups and N — O- containing groups. Among them the carboxylate group is preferred. Preferred carboxylate organic linkers are selected from fumaric carboxylate (FA), benzene- 1,4-dicarboxylate (BDC), 1,3,5-benzene tri carb oxy late (BTC), formic carboxylate.

Preferred MOF falling in this category are Zr-MOF-808 (Zr based BTC linker), Ce-MOF-808 (Ce based BTC+FA linker), MOF-199 (Cu based, BTC linker), MIL-88A (Fe2O ₃ based, fumaric carboxylate ligand). Exemplary formulas for MOF are the following Zr ₆ O4(OH) ₄ (-CO2)6(BTC)2(HCOO)6 for MOF-808; Zn ₄ O(BDC) ₃ for MOF-5, and CU3(C9H ₃ O6) ₂ for MOF-199. [0036] Additional exemplary MOFs can include, e.g., IRMOFs series (MOF-5), MOF- 74 series, Sandia Metal-Organic Frameworks (SMOFs) series, and zeolitic imidazolate framework (ZIF) series.

[0037] Exemplary ZIF series include but not limited to ZIF-4, ZIF-5, ZIF-6, ZIF-8, ZIF- 10, and/or ZIF-11.

[0038] In some non-limiting embodiments, the ZIF can have any useful topology defined by the metal cations that is identical to the zeolitic framework type SOD or RHO. SOD is a three letter framework type code for a sodalite structure type, and RHO is another three letter framework type code, as defined by the Structure Commission of the International Zeolite Association in the “Atlas of zeolite framework types,” Ch Baerlocher, L B McCusker, and D H Olson, Sixth Revised Edition, Elsevier Amsterdam, 2007. Examples are ZIF-4, ZIF-5, ZIF- 8 (also known as Basolite® Z1200), ZIF-9, ZIF-10, ZIF-11, ZIF-12, ZIF-65, ZIF-67, ZIF-68, ZIF-69, ZIF -70, ZIF-71, ZIF-76, ZIF-78, ZIF-90, ZIF-95, and ZIF- 100.

[0039] In another embodiment, the MOF (e.g., crystalline MOF) may be a highly porous coordination framework, HKUST-1. The framework of HKUST-1 is [Cu3(benzene- l,3,5-carboxylate)2] or [Cu3(BTC)2], also known as MOF-199, Cu-BTC, and Basolite™ C300 (Sigma Aldrich). It has interconnected [Cu2(O2CR)4] units (where R comprises an optionally substituted aromatic ring), which create a three-dimensional system of channels with a pore size of 1 nanometer and an accessible porosity of about 40 percent in the solid.

[0040] In yet another embodiment, the MOF is MOF-5, which is Zn4O(l,4- b enzodi carb oxy 1 ate)3.

[0041] A poison-suppressing agent can be used in association with the catalyst. Preferably, it can be selected from the group consisting of Ca(OH)2, Mg(OH)2, Ba(OH)2, Sr(OH)2, CaO, AI2O3, and Zr(HPO4)2. Among them, the use of Zr(HPO4)2 is preferred.

[0042] The data reported in the present disclosure show that the process according to the present disclosure allows conversion of virgin resins, and also complex plastic waste, in a liquid depolymerization product which is obtained in high yields and composition that makes it suitable for use as a cracker feedstock.

CHARACTERIZATION

The properties are determined according to the following methods. Analytical Methods

[0043] Characterization of liquid products: The liquid products from the two traps were characterized by Gas Chromatography (GC) and proton NMR ( ^J H NMR).

[0044] The GC analysis of the liquid product for each run was performed using an Agilent 7890 GC (Agilent Technologies, Santa Clara, CA) equipped with a standard non-polar column and a flame ionization detector. For the GC data, the weight percent for x < nC7, nC7 < x < nCl l, nC12 < x < nC28, x > C28 were used to characterize the liquid product.

[0045] NMR data were used to characterize the percent of aromatic protons, paraffinic protons and olefinic protons in the liquid product. The examples were analyzed with an addition of CDCh (0.6 g of depolymerize polymer/metal oxide mixture with 0.4 g of CDCI3). The data were collected on a Bruker AV500 MHz NMR spectrometer (Bruker Corporation, Billerica, MA) at 25°C with a 5mm Prodigy probe. One dimension ¹ H NMR data were processed using TOPSPIN® software (Bruker) with an exponential line broadening window function. Quantitative measurements were performed with a 15 second relaxation delay, a 30° flip angle pulse, and 32 scans to facilitate accurate integrals. The spectral integrations for aromatic olefinic, and paraffinic protons were obtained and used to quantify relative ratios of these protons.

[0046] Determination of Fe, Zr, Cu and Zn

[0047] The determination of Fe, Zr, Cu and Zn content in the solid catalyst component has been carried out via inductively coupled plasma emission spectroscopy on “I.C.P Spectrometer ARL Accuris” or, alternatively, on “I.C.P Spectrometer leap 7000”. The sample was prepared by analytically weighting, in a “Fluxy” platinum crucible”, 0.1H).3 grams of catalyst and 2 grams of lithium metaborate/tetraborate 1/1 mixture. After addition of some drops of KI solution, the crucible is inserted in a special apparatus "Claisse Fluxy” for the complete burning. The residue is collected with a 5% v/v HNO3 solution and then analyzed via ICP at the following wavelengths: iron, 259.94 nm / 261.187 nm; zirconium 349.62 nm/343.82 nm; copper 327.40 nm; zinc 213.86 nm.

EXAMPLES

General Depolymerization Procedure

[0048] General procedure for depolymerization test in a 500 ml round glass reactor [0049] 30 g of the polymer plastic were loaded in a 500 mL round glass reactor having three necks equipped with thermocouple and nitrogen inlet. Polymer plastic could be both a virgin resin obtained directly from the polyolefins production plants (examples 1-4 and comparative example 1) and samples of real plastic wastes (rpw) from municipal collection previously sorted (Examples 5-7 and comparative examples 2-3) .

[0050] The real plastic waste used in the examples was analyzed and it resulted to be composed of about 97wt% of polyolefin in which the PP/PE ratio was about 30/70) with the residual containing traces of other common polymers (PET, PS, PA, PU) plus inorganic contaminants.

[0051] The solid catalyst (2.5wt% with respect to plastics) is then introduced in the proper amount into the glass reactor. Blank test without any catalyst can be also performed. Two glass condenser are connected in series and kept at 110°C and -8°C respectively using an oil bath (Cryostat Julabo). The reactor is placed in electrically heating system (mantle bath), and setting the desired power, the temperature was raised up to 450°C. The pyrolysis process takes place and the following experimental parameters are recorded:

• L%, sum of the yield of liquid condensable at 110°C + liquid condensable at -8°C (with respect the polymer charged)

• S%, yield of solid/waxy residue in the reactor, excluding catalyst (with respect to the polymer charged)

• G% yield in gaseous products not condensable in both condensers (with respect the polymer charged)

The results are reported in tables 1 and 2.

Comparative example 1

[0052] A depolymerization run was carried out according to the general depolymerization procedure disclosed above using virgin polypropylene as a depolymerization feedstock and without using a depolymerization catalyst. The results are reported in Table 1.

Example 1

Preparation of MIL-88 A

[0053] MIL-88A (Fe2O3 based, fumaric carboxylate ligand) was prepared according to the procedure reported in Applied Catalysis B: Environmental 221 (2018), 119-128 and literature references cited there. The resulting compound was characterized by X-Ray analysis (pattern in agreement with literature data) and elemental analysis (Fe = 21.3%wt.).

[0054] The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using virgin polypropylene as a depolymerization feedstock. The results are reported in Table 1.

Example 2

Preparation of MOF 808

[0055] MOF 808 (Zr based BTC linker) was prepared according to the procedure reported in Material Letters 160 (2015), 412-414. The resulting compound was characterized by X-Ray analysis (pattern in agreement with literature data) and elemental analysis (Zr = 19.0%wt.).

[0056] The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using virgin polypropylene as a depolymerization feedstock. The results are reported in Table 1.

Example 3

Preparation of MOF- 199

[0057] MOF- 199 (Cu based, BTC linker) was prepared according to the procedure reported in Tetrahedron 64 (2008), 8553-8557. The resulting compound was characterized by elemental analysis (Cu = 17.6%wt.).

[0058] The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using virgin polypropylene as a depolymerization feedstock. The results are reported in Table 1.

Example 4

Preparation of MOF-5

[0059] MOF-5 (Zn based, terephthalic acid linker) was prepared according to the procedure reported in Tetrahedron 64 (2008), 8553-8557. A slight modification of the preparation cited there provided in the work-up step for a solvent exchange of dimethyl formamide first with ethanol and subsequently with acetone in place of chloroform. The resulting compound was characterized by elemental analysis. [0060] The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using virgin polypropylene as a depolymerization feedstock. The results are reported in Table 1.

Comparative example 2

[0061] A depolymerization run was carried out according to the general depolymerization procedure disclosed above but using real plastic waste as a depolymerization feedstock and without using a depolymerization catalyst. The results are reported in Table 2.

Example 5

[0062] The same catalyst employed in example 2 was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using real plastic waste as a depolymerization feedstock. In addition to the catalyst, Zr(HPO4)2 as a poison suppressing agent (2.5%wt with respect to plastic waste) was used. The results are reported in Table 2.

Example 6

Preparation of MOF 808 - Cerium doped

[0063] Ce-MOF-808-1.5-20FA was prepared according to the procedure reported in

Microporous and Mesoporous Materials 324 (2021), 111303. The resulting compound was characterized by X-Ray analysis (pattern in agreement with literature data).

The so obtained catalyst was used in a depolymerization run carried out according to the general depolymerization procedure disclosed above using real plastic waste as a depolymerization feedstock. The results are reported in Table 2.

Example 7

[0064] The depolymerization run was carried out as in example 6 with the difference that, in addition to the catalyst, Zr(HPO4)2 as a poison suppressing agent (2.5%wt with respect to plastic waste) was used. The results are reported in Table 2.

Comparative Example 3

[0064] The depolymerization run was carried out as in example 5 with the difference that, Zeolite HY was used instead of MOF-808. The results are reported in Table 2. Table 1

Table 2

BI= Branching Index

Ar= Aromatics content