WO2020161488A1

WO2020161488A1 - Process

Info

Publication number: WO2020161488A1
Application number: PCT/GB2020/050256
Authority: WO
Inventors: Tiancun Xiao; Peter P. Edwards; Xiangyu JIE
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2019-02-08
Filing date: 2020-02-05
Publication date: 2020-08-13
Anticipated expiration: 2021-08-08
Also published as: GB201901784D0

Abstract

The present invention provides a process for (i) producing monomers from a thermoplastic polymer; and/or (ii) producing olefins; and/or (iii) depolymerisation of a thermoplastic polymer; and/or (iv) recycling a thermoplastic polymer; comprising exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal. Also provided is a solid composition comprising a solid catalyst in admixture with one or more thermoplastic polymers, wherein the solid catalyst comprises a group 6 or group 4 transition metal. Also provided is the use of said solid composition to (i) produce monomers from a thermoplastic polymer; and/or (ii) produce olefins; and/or (iii) depolymerise a thermoplastic polymer; and/or (iv) recycle a thermoplastic polymer, and a microwave reactor comprising said solid composition.

Description

PROCESS

INTRODUCTION

[001] The present invention relates to a process for the depolymerisation of plastic. The process of the present invention provides a catalytic process sutiable for converting the thermoplastic polymers present in waste plastic into monomers.

BACKGROUND OF THE INVENTION

[002] Plastic materials are very widely used throughout the world and the trend is that many forms of plastics material will be more extensively used in the future. However it is well known that many plastic materials because of their stability in use are not bio-degradable and that significant problems exist in the disposal of such materials. Accordingly, if current production and waste management trends continue, roughly 12,000 Mt of plastic waste will be in landfills or in the natural environment by 2050 (R. Geyer, J. R. Jambeck, K. L. Law, Science Advances 2017, 3, 10.1126/sciadv.1700782).

[003] Between 1950 and 2015, cumulative waste generation of primary and secondary (recycled) plastic waste amounted to 6300 Mt, of this approximately 800 Mt (12%) of plastics have been incinerated and 600Mt (9%) have been recycled; only 10% of which have been recycled more than once. Around 4900 Mt— 60% of all plastics ever produced— have been discarded and are accumulating in landfills or in the natural environment.

[004] Processes for conversion of waste plastics and other solid hydrocarbon materials to other useful products are known. For instance, plastic depolymerisation techniques were developed actively during the 1980s and 1990s, but none was adopted commercially as mechanical recycling methods developed rapidly.

[005] Over the past 10 years there has been an increase in the amount of research into the conversion of plastic into useful products such as hydrocarbon fuels, particularly in the face of increasing oil prices. Furthermore, plastic recycling methods have increased due to improvement in waste collection and sorting methods. There are now several processes operating close to commercial viability in different parts of the world. Processes that have attracted significant commercial interest are pyrolysis processes to make synthetic crude oil mixtures and liquid-phase catalytic depolymerisation to make mixed distillates. [006] The present invention seeks to provide a simple and compact technology for conversion of waste plastic to useful monomeric materials by the rapid and efficient depolymerisation of a thermoplastic polymer.

SUMMARY OF THE INVENTION

[007] The present invention provides a process for the production of monomers of a thermoplastic polymer using the assistance of electromagnetic radiation. This allows the production of highly pure monomers with minimal by-products, such as CO2, CO and methane.

[008] This technology has particular application to the decomposition and/or recycling of waste plastics.

[009] Accordingly, in a first aspect the present invention provides a process for

(i) producing monomers from a thermoplastic polymer; and/or

(ii) producing olefins; and/or

(iii) depolymerisation of a thermoplastic polymer; and/or

(iv) recycling a thermoplastic polymer, comprising exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0010] In a second aspect, the present invention provides a solid composition comprising a solid catalyst in admixture with one or more thermoplastic polymers wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0011] In a third aspect, the present invention provides the use of a solid composition according to the second aspect for:

(i) producing monomers from a thermoplastic polymer; and/or

(ii) producing olefins; and/or

(iii) depolymerisation of a thermoplastic polymer; and/or

(iv) recycling a thermoplastic polymer. [0012] In a fourth aspect, the present invention relates to a process for the production of a solid composition comprising providing (i) a thermoplastic polymer and (ii) a solid catalyst comprising a group 6 or group 4 transition metal, and putting (i) and (ii) into admixture.

[0013] In a fifth aspect, the present invention relates to a solid composition obtainable or obtained by the process of the fourth aspect.

[0014] In a sixth aspect, the present invention provides a microwave reactor comprising a solid composition according to the second aspect.

[0015] Preferred, suitable, and optional features of any one particular aspect of the present invention are also preferred, suitable, and optional features of any other aspect.

BRIEF DESCRIPTION OF THE DRA WINGS

[0016] Figure 1 shows the evolved gas composition (vol. %) following microwave-assisted polyethylene depolymerisation over group 6 and group 4 transition metal carbide catalysts.

[0017] Figure 2 shows the gas yield and light olefin selectivity of polyethylene (PE) depolymerisation over transition metal carbides.

[0018] Figure 3 shows the evolved gas composition (vol.%) of polyethylene (PE) decomposition over group 6 transition metal oxides.

[0019] Figure 4 shows the evolved gas composition (vol.%) of successive cycles of test on PE depolymerisation under microwave irradiation over Cr3C2.

[0020] Figure 5 shows the gas yield and light olefin selectivity of PE depolymerisation over Cr3C2 in successive cycles of tests.

[0021] Figure 6 shows evolved gas composition (vol.%) of successive cycles of test on PP depolymerisation under microwave irradiation over Cr3C2.

[0022] Figure 7 shows the gas yield and light olefin selectivity of PP depolymerisation over Cr3C2 in successive cycles of tests.

[0023] Figure 8 shows thermogravimetric analysis (TGA) on Cr3C2 catalyst before and after microwave-assisted PE decomposition.

[0024] Figure 9 shows XRD patterns of fresh and spent Cr3C2 catalyst after successive cycles of microwave assisted PE depolymerisation. [0025] Figure 10 shows SEM images of fresh and spent O3C2 catalyst after successive cycles of microwave assisted PE depolymerisation.

[0026] Figure 11 shows waste plastics: milk bottles made of high density polyethylene (HDPE) and food storage containers made of polypropylene (PP).

[0027] Figure 12 shows the products distribution (Area%) during successive cycles of microwave-assisted polystyrene (PS) depolymerisation over Cr3C2.

[0028] Figure 13 shows waste polystyrene foam from a food container (A) and loose-fill packaging (B).

DETAILED DESCRIPTION OF THE INVENTION Definitions

[0029] As used herein the term“solid composition” refers to a composition which is solid at standard ambient temperature and pressure (SATP), i.e. at a temperature of 298.15 K (25 °C) and at 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm).

[0030] As used herein the term“solid catalyst” refers to a catalyst which is solid at standard ambient temperature and pressure (SATP), i.e. at a temperature of 298.15 K (25 °C) and at 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm).

[0031] As used herein, the term“recycling” refers to the conversion of a material, generally a waste or unwanted material, into alternative materials which may either be more easily disposed of or have renewed application. Suitably, recycling converts a material into an alternative material having at least one new or renewed application. In the present invention, waste plastic/thermoplastic polymer may be recycled into a range of monomeric materials, including for example, ethylene, propylene and styrene.

[0032] As used herein, the term“depolymerise” refers to the decomposition of a polymeric material to yield a monomer or mixture of monomers.

[0033] As used herein the term“plastic” refers to a solid material which comprises one or more thermoplastic or thermosetting polymers. Suitably, the plastic (essentially) consists of one or more thermoplastic or thermosetting polymers. Suitably the plastic (essentially) consists of one or more thermoplastic polymers. Suitably, the plastic is waste plastic which may be a mixture of various plastics. Plastics may be referred to by the name of the polymer of which they consist. Examples of common plastics are polyethylene, polypropylene and polystyrene.

[0034] As used herein the term“thermoplastic polymer” refers to a polymer which becomes pliable or mouldable above a certain temperature and solidifies upon cooling, but can be remelted on heating. Typically thermoplastic polymers have a melting temperature from about 60°C. to about 300°C, from about 80°C. to about 250°C, or from about 100°C. to about 250°C. Suitably, the thermoplastic polymer is one which is commonly comprised in commercial plastic products. Suitable thermoplastic polymers generally include polyolefins, polyesters, polyamides, copolymers thereof, and combinations thereof. Examples of thermoplastic polymers include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylchloride (PVC), polyamideimide, polymethylmethacrylate (PMMA), polytetrafluoroethylene, polyethylene terephthalate (PET), natural rubber (NR), and polycarbonate (PC), polyvinylidene chloride (PVDC), acrylonitrile butadiene styrene (ABS), polyurethanes (PU).

[0035] As used herein the term “thermosetting polymer” refers to a polymer which is irreversibly cured and cannot be reworked upon reheating. Examples of thermosetting polymers are polyurethane and polyoxybenzylmethylenglycolanhydride (Bakelite™).

[0036] As used herein, the term“admixture” refers the physical combination of two or more substances in which the identities of each substance is retained. Admixture may suitably be prepared by mechanical mixing (e.g. blending) two substances together.

[0037] As used herein, the term “intimate admixture” means that substantially all of the individual particles of the admixture are composed of all of the component substances of the admixture. Reference is made to substantially all of the particles because it is possible from a purely statistical stand point that a small number of the particles might contain only one of the component substances or lack the full complement of component substances. Suitably, an intimate admixture is prepared by extrusion, dispersion or granulation of two of more substances which may optionally be further processed to a requisite particle size for instance by grinding, milling or blending.

Process

[0038] In one aspect, the present invention provides a process for

(i) producing monomers from a thermoplastic polymer; and/or

(ii) producing olefins; and/or (iii) depolymerisation of a thermoplastic polymer; and/or

[0039] In one embodiment, the present invention provides a process for producing monomers of a thermoplastic polymer, wherein the process comprises exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0040] Suitably the process provides monomers such as ethylene, propylene and styrene. Accordingly, in some embodiments when the solid composition comprises polystyrene exposure to electromagnetic radiation in the presence of a solid catalyst according to the present invention provides monomers comprising styrene. Similarly, when the solid composition comprises polyethylene exposure to electromagnetic radiation in the presence of a solid catalyst according to the present invention provides monomers comprising ethylene. Similarly, when the solid composition comprises polypropylene exposure to electromagnetic radiation in the presence of a solid catalyst according to the present invention provides monomers comprising propylene.

[0041] In another embodiment, exposure of a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst according to the invention provides a mixture of monomers. In one embodiment, exposure of a thermoplastic polymer comprising polyethylene to electromagnetic radiation in the presence of a solid catalyst according to the invention provides a mixture of monomers comprising ethylene and propylene. In another embodiment, exposure of a thermoplastic polymer comprising polypropylene to electromagnetic radiation in the presence of a solid catalyst according to the invention provides a mixture of monomers comprising propylene and ethylene.

[0042] In one embodiment, the solid composition comprises polyethylene and the monomer produced is ethylene. In another embodiment, the solid composition comprises polypropylene and the monomer produced is propylene. In another embodiment, the solid composition comprises polystyrene and the monomer produced is styrene. [0043] In one embodiment, the solid composition comprises a mixture of one or more of polyethylene, polypropylene and polystyrene and the monomers produce comprise at least one of ethylene, propylene and styrene.

[0044] In one embodiment, the present invention provides a process for producing olefins, wherein the process comprises exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0045] Suitably, the olefins are selected from ethylene, propylene and styrene. In one embodiment, the solid composition comprises one or more thermoplastic polymers selected from polyethylene and polypropylene and the olefin produced is ethylene and/or propylene. In another embodiment, the solid composition comprises polystyrene and the olefin produced is styrene.

[0046] In one embodiment, the present invention provides a process for depolymerisation of a thermoplastic polymer, wherein the process comprises exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0047] In one embodiment, the thermoplastic polymer is selected from one or more of polyethylene, polypropylene and polystyrene. In another embodiment, the thermoplastic polymer is polyethylene, polypropylene or polystyrene.

[0048] In one embodiment, the present invention provides a process for recycling a thermoplastic polymer, wherein the process comprises exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

[0049] In one embodiment, the process of each of the above embodiments is carried out in an atmosphere substantially free of oxygen. Suitably, an atmosphere free of oxygen. In another embodiment, process comprises exposing the composition to electromagnetic radiation in an atmosphere substantially free of oxygen, suitably free of oxygen.

[0050] In another embodiment, the process is carried out in an atmosphere substantially free of water. Suitably, an atmosphere free of water. In another embodiment, process comprises exposing the composition to electromagnetic radiation in an atmosphere substantially free of water, suitably free of water. [0051] In another embodiment, the process is carried out in an atmosphere substantially free of oxygen and water. Suitably, an atmosphere free of oxygen and water. In another embodiment, process comprises exposing the composition to electromagnetic radiation in an atmosphere substantially free of oxygen and water, suitably free of oxygen and water.

[0052] In another embodiment, the process is carried out in an inert atmosphere. In another embodiment, process comprises exposing the composition to electromagnetic radiation in an inert atmosphere.

[0053] The inert atmosphere may for instance be an inert gas or a mixture of inert gases. The inert gas or mixture of inert gases typically comprises a noble gas, for instance argon. In one embodiment the inert gas is argon. In another embodiment the inert gas is nitrogen.

[0054] The process may comprise purging the solid composition with an inert gas or mixture of inert gases prior to exposing the composition to the electromagnetic radiation.

[0055] In one embodiment the solid composition is contacted with the catalyst prior to, during or both prior to and during exposure to the electromagnetic radiation. The composition may be contacted with the catalyst by any suitable method. For instance, the solid composition may be mixed with the catalyst. The solid composition may be mixed with the solid catalyst by methods known in the art such as milling, granulating, extruding or blending. Suitably the solid composition and the catalyst are in intimate admixture during exposure to the electromagnetic radiation.

[0056] In the process of the invention, the composition is exposed to electromagnetic radiation in the presence of the catalyst in order to effect, or activate, the decomposition of the polymers in the composition to produce monomers. Said decomposition may be catalytic decomposition. Exposing the composition to the electromagnetic radiation may cause the composition to heat up, but does not necessarily cause it to be heated. Other possible effects of the electromagnetic radiation to which the composition is exposed (which may be electric or magnetic field effects) include, but are not limited to, field emission, plasma generation and work function modification. For instance, the high fields involved can modify catalyst work functions and can lead to the production of plasmas at the catalyst surface, further shifting the character of the chemical processes involved. Any one or more of such effects of the electromagnetic radiation may be responsible for, or at least contribute to, effecting, or activating, the catalytic decomposition of the composition to produce monomers.

[0057] The process, and in particular the step of exposing the composition to the electromagnetic radiation, may alternatively be carried out at temperatures and/or pressures other than SATP. Indeed, both very low and very high temperatures can be employed, i.e. from far below ambient to far above ambient, as could very low and high pressures. Usually, however, the step of exposing the composition to the electromagnetic radiation is carried out at temperatures and pressures that are at or relatively close to SATP.

[0058] The process may for instance comprise exposing the composition to the electromagnetic radiation, at a temperature of from -20 °C to 400 °C, for instance from 0 °C to 200 °C, or at a temperature of from 5 °C to 100 °C, or for instance from 10 °C to 50 °C.

[0059] Additionally, the process may comprise exposing the composition to the electromagnetic radiation, at a pressure of from 0.01 bar to 100 bar, or for instance at a pressure of from 0.1 10 bar to 10 bar, for instance from 0.5 bar to 5 bar, or for example from 0.5 bar to 2 bar. In a more typical case, the process comprises exposing the composition to the electromagnetic radiation, at a temperature of from 0 °C to 200 °C and at a pressure of from 0.5 bar to 5 bar. For instance, it may comprise exposing the composition to the electromagnetic radiation, at a temperature of from 10 °C to 50 °C and at a pressure of from 0.5 bar to 2 bar.

[0060] Typically, the process is complete in about 1 second to about 3 hours, for instance in a batch-wise process. Suitably, the process is complete within about 1 second to about 1 hour, more suitably about 1 second to about 10 minutes, more suitably about 1 second to about 5 minutes.

[0061] In one embodiment, the process is complete in 10 seconds to about 3 hours, for instance in a batch-wise process. Suitably, the process is complete within about 10 seconds to about 1 hour, more suitably about 10 seconds to about 10 minutes, more suitably about 10 seconds to about 5 minutes.

[0062] In another embodiment, the process is complete in 30 seconds to about 3 hours, for instance in a batch-wise process. Suitably, the process is complete within about 30 seconds to about 1 hour, more suitably about 30 seconds to about 10 minutes, more suitably about 30 seconds to about 5 minutes.

Catalyst

[0063] The catalyst employed in the process of the present invention comprises a group 6 or group 4 transition metal. Suitably, the group 6 transition metal is selected from Cr, Mo and W, or Cr and W, or Cr and Mo. Suitably, the group 4 transition metal is selected from Ti and Zr. [0064] In one embodiment, the catalyst comprises at least one of Ti, Zr, Hf, Cr, Mo and W, or an oxide or carbide thereof.

[0065] In another embodiment, the catalyst comprises at least one of Ti, Zr, Cr, Mo and W, or an oxide or carbide thereof. In another embodiment, the catalyst comprises at least one of Ti, Cr, Mo and W, or an oxide or carbide thereof. In another embodiment, the catalyst comprises at least one of Ti, Cr and W, or an oxide or carbide thereof. In another embodiment, the catalyst comprises at least one of Ti, Cr, and Mo, or an oxide or carbide thereof. In another embodiment, the catalyst comprises at least one of Cr, Mo and W, or an oxide or carbide thereof.

[0066] In another embodiment, the catalyst comprises at least one of Cr and Mo, or an oxide or carbide thereof. In another embodiment, the catalyst comprises at least one of Cr and W, or an oxide or carbide thereof.

[0067] The group 6 or group 4 transition metal may be present in elemental form (i.e. zero oxidation state), as an oxide, as a carbide, or dispersed on a support. In one embodiment, the catalyst comprises/essentially consists of/consists of a group 6 or group 4 transition metal carbide or oxide. In another embodiment, the catalyst comprises/essentially consists of/consists of a group 6 or group 4 transition metal carbide. In one embodiment, the catalyst comprises/essentially consists of/consists of a group 6 transition metal carbide or oxide. In one embodiment, the catalyst comprises/essentially consists of/consists of a group 6 transition metal carbide.

[0068] In one embodiment, the catalyst comprises elemental chromium, chromium carbide (e.g, Cr3C2), chromium oxide (e.g. Cr2C>3) or chromium on a support. In one embodiment the support is selected from a carbon support (e.g. graphite or activated carbon), SiC and zeolite.

[0069] In another embodiment, the catalyst comprises elemental molybdenum, molybdenum carbide (e.g, M02C), molybdenum oxide (e.g. M0O₃) or molybdenum on a support. In one embodiment the support is selected from a carbon support (e.g. graphite or activated carbon), SiC and zeolite.

[0070] In another embodiment, the catalyst comprises elemental tungsten, tungsten carbide (e.g, WC), tungsten oxide (e.g. WO₃) or tungsten on a support. In one embodiment the support is selected from a carbon support (e.g. graphite or activated carbon), SiC and zeolite.

[0071] In another embodiment, the catalyst comprises elemental zirconium, zirconium carbide (e.g, ZrC), zirconium oxide (e.g. ZrC>2) or zirconium on a support. In one embodiment the support is selected from a carbon support (e.g. graphite or activated carbon), SiC and zeolite (e.g. graphite or activated carbon).

[0072] In another embodiment, the catalyst comprises elemental titanium, titanium carbide (e.g, TiC), titanium oxide (e.g. PO2) or titanium on a support (e.g. graphite or activated carbon). In one embodiment the support is selected from a carbon support (e.g. graphite or activated carbon), SiC and zeolite.

[0073] In one embodiment, the catalyst is selected from CrsC₂, M0₂C, WC, ZrC, TiC, Cr₂C>₃, M0O₃, WO₃, T1O₂, ZrC>₂. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C, WC, ZrC, TiC, Cr₂C>₃, M0O₃ and WO₃. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C, WC, Cr₂C>₃, M0O₃ and WO₃. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C, WC, Cr₂C>₃ and M0O₃. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C, WC, Cr₂C>₃ and WO₃. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C, WC and Cr₂C>₃. In another embodiment, the catalyst is selected from Cr₃C₂, M0₂C and WC. In another embodiment, the catalyst is selected from Cr₃C₂ and WC. In another embodiment, the catalyst is selected from Cr₃C₂ and M0₂C.

[0074] In one embodiment, the catalyst comprises/essentially consists of/consists of chromium carbide. Suitably, the catalyst comprises/essentially consists of/consists of one or more of Cr3C2, CryC3 and Cr23C6. Suitably, the catalyst comprises/essentially consists of/consists of Cr₃C₂.

[0075] In one embodiment, the catalyst may comprise promotors. In one embodiment, the catalyst may further comprise one or more of Na, K, Li and Al. The promotors may be present at between 0.5 to 5 wt.% of the group 6 or group 4 metal, suitably present at between about 1 and 2 wt.% of the group 6 or group 4 metal.

[0076] In one embodiment, the catalyst may comprises additives, such as carbon, zeolite or SiC, suitably in admixture with the group 4 or group 6 transition metal catalyst.

[0077] In one embodiment, the catalyst comprises at least about 10 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise at least about 15 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, at least 20 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst, more suitably at least about 25 wt. % of group 6 or group 4 metal, more suitably at least about 30 wt. % of group 6 or group 4 metal, more suitably at least about 35 wt. % of group 6 or group 4 metal, more suitably at least about 40 wt. % of group 6 or group 4 metal, more suitably at least about 45 wt. % of group 6 or group 4 metal, more suitably at least about 50 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0078] In one embodiment, the catalyst comprises about 10 wt. % to about 95 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise about 20 wt. % to about 95 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, about 30 wt. % to about 95 wt. % of of group 6 or group 4 metal relative to the total weight of the catalyst, more suitably about 40 wt. % to about 95 wt. % of group 6 or group 4 metal, more suitably about 50 wt. % to about 95 wt. % of group 6 or group 4 metal, more suitably about 60 wt. % to about 95 wt. % of group 6 or group 4 metal, more suitably about 70 wt. % to about 95 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0079] In another embodiment, the catalyst comprises about 10 wt. % to about 90 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise about 10 wt. % to about 80 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, about 10 wt. % to about 70 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst, more suitably about 10 wt. % to about 60 wt. % of group 6 or group 4 metal, more suitably about 10 wt. % to about 55 wt. % of group 6 or group 4 metal, more suitably about 10 wt. % to about 50 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0080] In another embodiment, the catalyst comprises about 30 wt. % to about 75 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise about 30 wt. % to about 70 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, about 30 wt. % to about 65 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst, more suitably about 30 wt. % to about 60 wt. % of group 6 or group 4 metal, more suitably about 30 wt. % to about 55 wt. % of group 6 or group 4 metal, more suitably about 30 wt. % to about 50 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0081] In another embodiment, the catalyst comprises about 25 wt. % to about 75 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise about 30 wt. % to about 75 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, about 35 wt. % to about 75 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst, more suitably about 40 wt. % to about 75 wt. % of group 6 or group 4 metal, more suitably about 45 wt. % to about 75 wt. % of group 6 or group 4 metal, more suitably about 50 wt. % to about 75 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0082] In another embodiment, the catalyst comprises about 40 wt. % to about 60 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. For example, the catalyst may comprise about 45 wt. % to about 55 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst. Suitably, about 50 wt. % of group 6 or group 4 metal relative to the total weight of the catalyst.

[0083] Typically, in the above embodiments where carbon is present, it is present at between about 10 wt. % and 90 wt.% of the total weight of the catalyst, suitably about 40 wt.% to about 90 wt.%, more suitably about 50 wt.% to about 90 wt.%, more suitably about 50 wt. % to about 70 wt. %.

[0084] Alternatively, in the above embodiments where carbon is present, it is present at between about 5 wt. % and 40 wt.% of the total weight of the catalyst, suitably about 5 wt.% to about 30 wt.%, more suitably about 5 wt.% to about 20 wt.%, more suitably about 5 wt. % to about 15 wt. %.

[0085] Typically, the catalyst is in particulate form, wherein particle size is about 10 mm or less. In one embodiment, the particle size is about 5 mm or less, suitably, 1 mm or less. In another embodiment, the particle size is between about 20 nm and 1 mm.

[0086] Suitably, the mean particle size is about 50 nm to about 1 mm; or the mean particle size is about 50 nm to about 10pm; or the mean particle size is about 50 nm to about 500 nm; or the mean particle size is about 50 nm to about 100 nm.

[0087] The term“particle size” as used herein means the diameter of the particle if the particle is spherical or, if the particle is non-spherical, the volume-based particle size. The volume- based particle size is the diameter of the sphere that has the same volume as the non- spherical particle in question. Particle size as described herein can be determined by various conventional methods of analysis, such as SEM, TEM, Laser light scattering, laser diffraction, sieve analysis and optical microscopy (usually combined with image analysis).

[0088] In one embodiment, the catalyst has a nanoscale particle size. As used herein, a nanoscale particle size refers to populations of nanoparticles having d(0.9) values of 500 nm or less. For example, d(0.9) values of 400 nm or less. For example, d(0.9) values of 300 nm or less. For example, d(0.9) values of 200 nm or less. For example, d(0.9) values of 100 nm or less. [0089] As used herein, "d(0.9)" (which may also be written as "d(v, 0.9)" or volume median diameter) represents the particle size (diameter) for which the cumulative volume of all particles smaller than the d(0.9) value in a population is equal to 90% of the total volume of all particles within that population.

[0090] A particle size distribution as described herein (e.g. d(0.9)) can be determined by various conventional methods of analysis, such as Laser light scattering, laser diffraction, sedimentation methods, pulse methods, electrical zone sensing, sieve analysis and optical microscopy (usually combined with image analysis).

[0091] In one embodiment, the catalyst has a microscale particle size. As used herein, a nanoscale particle size refers to populations of nanoparticles having d(0.9) values of 500 pm or less. For example, d(0.9) values of 400 pm or less. For example, d(0.9) values of 300 pm or less. For example, d(0.9) values of 200 pm or less. For example, d(0.9) values of 100 pm or less.

[0092] In one embodiment, a population of the catalyst of the process have d(0.9) values of about 50nm to about 10 pm. For example, d(0.9) values of about 50 nm to about 1000 nm. For example, d(0.9) values of about 50 nm to about 900 nm. For example, d(0.9) values of about 50 nm to about 800 nm. For example, d(0.9) values of about 50 nm to about 700 nm. For example, d(0.9) values of about 50 nm to about 600 nm. For example, d(0.9) values of about 50 nm to about 500 nm. For example, d(0.9) values of about 50 nm to about 400 nm. For example, d(0.9) values of about 50 nm to about 300 nm. For example, d(0.9) values of about 50 nm to about 200 nm. For example, d(0.9) values of about 50 nm to about 100 nm.

[0093] The proportion of the catalyst relative to the total weight solid material (i.e. solid composition comprising thermoplastic polymer and catalyst) at the start of the process, is typically from 10 wt. % to 80 wt. %. Suitably, from 10 wt. % to 75 wt. %. It may for instance be from 10 wt. % to 70 wt. %, or for instance from 10 wt. % to 65 wt. %. In some embodiments, for instance, it is from 10 wt. % to 60 wt. %.

[0094] In another embodiment, the proportion of the catalyst relative to the total weight solid material (i.e. solid composition comprising thermoplastic polymer and catalyst) at the start of the process is from 10 wt. % to 80 wt. %. It may for instance be from 15 wt. % to 80 wt. %, or for instance from 20 wt. % to 80 wt. %. In some embodiments, for instance, it is from 25 wt. % to 80 wt. %, or for instance from 30 wt. % to 80 wt. %. Suitably, relative to the total weight solid material (i.e. solid composition comprising thermoplastic polymer and catalyst) at the start of the process is about 30 wt.% to about 70 wt.%. Solid Composition

[0095] The solid composition is in the solid state at standard ambient temperature and pressure (SATP), i.e. at a temperature of 298.15 K (25 °C) and at 100,000 Pa (1 bar, 14.5 psi, 0.9869 atm). In many cases, the composition is also in the solid state under the conditions (i.e. the temperature and pressure) under which the process is carried out.

[0096] In one embodiment, the solid composition comprises one or more thermoplastic polymers. In another embodiment, the solid composition comprises/essentially consists of/consists of one or more thermoplastic polymer.

[0097] In another embodiment, the solid composition comprises at least about 90 wt.% one or more thermoplastic polymer, suitably at least about 95 wt.%.

[0098] In another embodiment, the solid composition comprises at least one thermoplastic polymer selected from polyethylene (PE), polypropylene (PP) and polystyrene (PS).

[0099] In one embodiment, the solid composition comprises/essentially consists/consists of one or more of polyethylene, polypropylene and polystyrene. Suitably, the solid composition comprises at least 90 wt.% of one or more of polyethylene, polypropylene and polystyrene.

[00100] In one embodiment, the solid composition comprises/essentially consists/consists of polyethylene, polypropylene or polystyrene. In another embodiment, the solid composition comprises at least 90 wt.% of polyethylene, polypropylene or polystyrene.

[00101] In another embodiment, the solid composition comprises one or more thermoplastic polymers and a solid catalyst comprising a group 6 or group 4 transition metal. In this embodiment, the catalyst and the one or more thermoplastic polymers may be processed prior to exposure to electromagnetic radiation in order to provide the solid composition, which may optionally be in powdered or pelleted form.

[00102] Suitably, the solid composition may be prepared by mixing the catalyst and the one or more thermoplastic and thermosetting polymers, for instance by milling or extruding. Suitably, the catalyst is highly dispersed within the polymer.

[00103] The proportion of the catalyst relative to the total weight thermoplastic polymer(s) at the start of the process, is typically from 10 wt. % to 400 wt. %. Suitably, from 10 wt. % to 350 wt. %. It may for instance be from 10 wt. % to 300 wt. %, or for instance from 10 wt. % to 250 wt. %. In some embodiments, for instance, it is from 10 wt. % to 200 wt. %. [00104] In another embodiment, the percentage by weight of the catalyst relative to the total weight thermoplastic polymer(s) at the start of the process is from 15 wt. % to 200 wt. %. It may for instance be from 20 wt. % to 200 wt. %, or for instance from 30 wt. % to 200 wt. %. In some embodiments, for instance, it is from 35 wt. % to 200 wt. %, or for instance from 40 wt. % to 200 wt. %. Suitably, proportion of the catalyst relative to the total weight thermoplastic and thermosetting polymer(s) at the start of the process is from about 50% to 200 wt.%.

[00105] In one embodiment, the solid composition is substantially free of oxygen. In another embodiment, the solid composition is free of oxygen.

[00106] In one embodiment, the solid composition is substantially free of water. In another embodiment, the solid composition is free of water.

[00107] In one embodiment, the solid composition is substantially free of oxygenated species and water. In another embodiment, the solid composition is free of oxygenated species and water.

[00108] In another aspect, the present invention provides a solid composition comprising a solid catalyst in admixture with one or more thermoplastic polymers, wherein the solid catalyst a group 6 or group 4 transition metal.

[00109] Suitably, the catalyst is in intimate admixture with the one or more thermoplastic or thermosetting polymers.

[00110] In another aspect, the present invention relates to a process for the preparation of a solid composition comprising providing (i) a thermoplastic polymer and (ii) a solid catalyst comprising a group 6 or group 4 transition metal, and putting (i) and (ii) into admixture.

[00111] The solid composition may be put into admixture by methods known in the art, these include blending, compressing, milling and extruding. Suitably, the thermoplastic polymer and solid catalyst may be as defined in any of the embodiments described herein.

[00112] Suitably, the solid composition is in powdered or pelletized form.

[00113] With respect to the catalyst, thermoplastic polymer(s) and the features thereof, each of the above described embodiments are equally applicable to this aspect of the invention. [00114] The present invention further relates to the use of the above described solid composition to produce monomers of a thermoplastic polymer; and/or produce olefins; and/or depolymerise a thermoplastic polymer; and/or recycle a thermoplastic polymer.

[00115] This can be achieved by exposing the solid composition to electromagnetic radiation as described herein.

Electromagnetic Radiation

[00116] The electromagnetic radiation that is employed in the process of the invention, in order to produce monomers of a thermoplastic polymer; and/or produce olefins; and/or depolymerise a thermoplastic polymer; and/or recycle a thermoplastic polymer from the solid composition starting material may be radio frequency radiation, microwave frequency radiation, millimetre wave radiation, infrared radiation or UV radiation. A range of electromagnetic frequencies may be used independently, or in combination with one another, to irradiate the sample, including radio frequencies, microwave frequencies, millimetre waves, infrared and UV. Suitably, the radiation is radio or microwave frequency radiation.

[00117] Typically, however, the electromagnetic radiation that is employed in the process of the invention is microwave radiation. The term“microwave radiation”, as used herein, takes its normal meaning, typically referring to electromagnetic radiation having a wavelength of from one meter to one millimetre, and having a corresponding frequency of from 300 MHz (100 cm) to 300 GHz (0.1 cm).

[00118] In one embodiment, the electromagnetic radiation is microwave radiation.

[00119] In principle, microwave radiation having any frequency in the microwave range, i.e. any frequency of from 300 MHz to 300 GHz, may be employed in the present invention. Typically, however, microwave radiation having a frequency of from 900 MHz to 4 GHz, or for instance from 900 MHz to 3 GHz, is employed.

[00120] In one embodiment, the electromagnetic radiation is microwave radiation having a frequency of from about 1 GHz to about 4 GHz. Suitably, the microwave radiation has a frequency of about 2 GHz to about 4 GHz, suitably about 2 GHz to about 3 GHz, suitably about 2.45 GHz.

[00121] In one embodiment, the microwave radiation has a frequency selected from about 915 MHz and about 2.45 GHz. [00122] The power which the electromagnetic radiation needs to delivered to the composition, in order to effect the production of monomers of a thermoplastic polymer; and/or production of olefins; and/or the depolymerisation of a thermoplastic polymer; and/or the recycling a thermoplastic polymer, will vary, according to, for instance, the particular starting material employed in the composition, the particular catalyst employed in the composition, and the size, permittivity, particle packing density, shape and morphology of the composition. The skilled person, however, is readily able to determine a level of incident power which is suitable for effecting the decomposition of a particular composition.

[00123] The process of the invention may for example comprise exposing the composition to electromagnetic radiation which delivers a power, per cubic centimetre of the composition, of at least 1 Watt. It may however comprise exposing the composition to electromagnetic radiation which delivers a power, per cubic centimetre of the composition, of at least 5 Watts.

[00124] Often, for instance, the process comprises exposing the composition to electromagnetic radiation which delivers to the composition a power of at least 10 Watts, or for instance at least 20 Watts, per cubic centimetre of the composition. The process of the invention may for instance comprise exposing the composition to electromagnetic radiation which delivers to the composition at least 25 Watts per cubic centimetre of the composition.

[00125] Often, for instance, the process comprises exposing the composition to electromagnetic radiation which delivers a power of from about 0.1 Watt to about 5000 Watts per cubic centimetre of the composition. More typically, the process comprises exposing the composition to electromagnetic radiation which delivers a power of from about 0.5 Watts to 30 about 1000 Watts per cubic centimetre of the composition, or for instance a power of from about 1 Watt to about 500 Watts per cubic centimetre of the composition, such as, for instance, a power of from about 1.5 Watts to about 200 Watts, or say, from 2 Watts to 100 Watts, per cubic centimetre of the composition.

[00126] In some embodiments, for instance the process comprises exposing the composition to electromagnetic radiation which delivers to the composition from about 5 Watts to about 100 Watts per cubic centimetre of the composition, or for instance from about 10 Watts to about 100 Watts per cubic centimetre, or for instance from about 20 Watts, or from about 25 Watts, to about 80 Watts per cubic centimetre of the composition.

[00127] In some embodiments, for instance, the process comprises exposing the composition to electromagnetic radiation which delivers a power of from about 2.5 to about 60 Watts per cubic centimetre of the composition. Thus, for example, if the volume of the composition is 3.5 cm³, the process of the invention typically comprises exposing the composition to electromagnetic radiation which delivers about 10 W to about 200 W to the composition (i.e. the“absorbed power” is from about 10 W to about 200 W).

[00128] Often, the power delivered to the composition (or the“absorbed power”) is ramped up during the process of the invention. Thus, the process may comprise exposing the composition to electromagnetic radiation which delivers a first power to the composition, and then exposing the composition to electromagnetic radiation which delivers a second power to the composition, wherein the second power is greater than the first. The first power may for instance be from about 2.5 Watts to about 6 Watts per cubic centimetre of the composition. The second power may for instance be from about 25 Watts to about 60 Watts per cubic centimetre of the composition. Often in these embodiments, the electromagnetic radiation is microwave radiation, which may be as further defined hereinbefore.

[00129] The duration of exposure of the composition to the electromagnetic radiation may also vary in the process of the invention. Embodiments are, for instance, envisaged wherein a given composition is exposed to electromagnetic radiation over a relatively short period of time, to effect the production of monomers of a thermoplastic polymer; and/or production of olefins; and/or the depolymerisation of a thermoplastic polymer; and/or the recycling a thermoplastic polymer. For instance, the solid composition may be irradiated with microwaves for a period of about 1 second to about 3 hours.

[00130] In some embodiments, exposing the composition to the electromagnetic radiation causes the composition to be heated. Electromagnetic heating provides a method of fast, selective heating of dielectric and magnetic materials. Rapid and efficient heating using microwaves is an example in which inhomogeneous field distributions in dielectric mixtures and field-focussing effects can lead to dramatically different product distributions. The fundamentally different mechanisms involved in electromagnetic heating may cause enhanced reactions and new reaction pathways. Furthermore, the high fields involved can modify catalyst work functions and can lead to the production of plasmas at the catalyst surface, further shifting the character of the chemical processes involved.

In one embodiment, the process of the invention comprises heating said composition by exposing the composition to the electromagnetic radiation, suitably microwave radiation.

Microwave reactor [00131] In another aspect, the present invention relates to a microwave reactor comprising solid composition, said mixture comprising a solid catalyst in intimate mixture with one or more thermoplastic or thermosetting polymers, wherein the catalyst comprises a group 6 or group 4 transition metal.

[00132] With respect to the solid catalyst, solid composition and the features thereof, each of the above described embodiments are equally applicable to this aspect of the invention.

[00133] Typically, the reactor is configured to receive the composition to be exposed to radiation. The reactor typically therefore comprises at least one vessel configured to comprise the composition. The composition may have been provided via an inlet to the vessel. The vessel may be located in a reaction cavity, said cavity being the focus of the microwave radiation.

[00134] The reactor may be also configured to export reaction products such as liquids or gases. Thus, the reactor typically comprises an outlet through which gas and/or liquid, generated in accordance with the process of the invention, may be released or collected.

[00135] In one embodiment, the microwave reactor is configured to subject the composition to electric fields in the TM010 mode.

[00136] In one embodiment, the microwave reactor is has a power of between about 500 to 1000W.

EXAMPLES

Example 1. Microwave-initiated Polyethylene (PE) depolymerisation over group 4 and group 6 transition metal carbide catalysts

[00137] The solid catalysts used herein were obtained from commercial sources (Fisher Scientific) and used without further treatment.

[00138] Polyethylene (PE) powder was used as supplied from Sigma-Aldrich without further treatment.

[00139] The PE powder was physically mixed by mortar and pestle or by blender with a catalyst (TiC, ZrC, CT₃C2, M0₂C or WC) in a weight ratio of 2: 1 and then filled in a quartz tube. The sample was then exposed to 1000W microwave irradiation and the generated gases were collected and analysed by GC. The microwave system consisted of a microwave generation system, a microwave cavity and a control system. The system was computer controlled using the Labview software. The operating frequency was 2450 MHz (±25 MHz) from 10% to 100% of nominal power).

[00140] The mass of each gas compound was estimated based on the volume content via GC analysis. The selectivity of light olefins is determined as the mass of produced light olefins divided by the mass of total gas products.

Eq. 1

'gas: could be the generated gas mixture or the specific gas compound.

Eq. 2

[00141] Figure 1 illustrates the evolved gas composition from microwave-assisted polyethylene (PE) depolymerisation over the group 6 and group 4 transition metal carbide catalysts. All of the tested metal carbides yielded light olefin (mainly ethylene and propylene) production under microwave irradiation. In each case about 50 vol.% of effluent gases were light olefins with ethylene accounting for 35 vol.% or more.

[00142] As shown in Figure 2, the chromium carbide catalyst had a selectivity to light olefins of about 70% with the total gas yield of about 35 wt.% and the ethylene yield of about 18 wt.%.

Example 2. Microwave-initiated Polyethylene (PE) depolymerisation over group 6 transition metal oxide catalysts

[00143] Polyethylene powder was mixed with a group 6 transition metal oxide catalyst (Cr2C>3, M0O3, WO3) and the composition exposed to 1000W microwave radiation in accordance with the procedure of Example. The generated gas were collected and analysed as described in Example 1. [00144] Figure 3 shows that similar high concentrations of light olefins were observed following depolymerisation over group 6 transition metal oxides.

[00145] Table 1 summaries the gas compositions (vol.%) obtained over both Examples 1 and 2.

Table 1

Example 3. Microwave-initiated Polyethylene (PE) depolymerisation over carbon supported group 6 transition metal catalysts

[00146] Two carbon supports were utilised in this study, activated carbon (AC) and graphite (G). Catalysts were prepared comprising chromium or molybdenum by an incipient wetness impregnation method.

[00147] Chromium nitride, Cr(NC>₃)₃-9H₂0 (Chromium (III) nitrate nonahydrate, Sigma- Aldrich), was used as Cr precursor whilst ammonium molybdenum tetrahydrate (NH₄)₆Mq₇q₂₄·4H₂0 (Ammonium molybdenum tetrahydrate, Sigma-Aldrich) was used as Mo precursor. The carbon (AC or G) was mixed with an aqueous solution of chromium nitrate or ammonium molybdenum tetrahydrate, the concentration of which was calculated to produce a desired metal content. The mixture was then dried overnight and then the resulting solid mixtures were calcined in a furnace under argon at 900 °C for 10 h. Typically, the Cr and Mo presented as oxides in the prepared catalysts after calcination.

[00148] Table 2 shows the evolved gas composition (vol. %) of the carbon supported catalysts, and Table 3 show the gas yields and selectivity for light olefins.

Table 2

Table 3

Example 4. Microwave-initiated Polyethylene (PE) depolymerisation over group 6 transition metal catalyst powder

[00149] Further experiments were carried out using Cr metal powder. Catalysts were prepared which consisted solely of Cr metal fine powder or a mixture of the metal powder and carbon (AC or G). The molar ratio of Cr to carbon was 3:2 in order to simulate Cr3C2. The catalysts were mixed with PE and exposed to 1000W microwave according to the procedure used in Example 1.

[00150] Table 4 shows the evolved gas composition (vol. %) of the metal powder catalysts, and Table 5 show the gas yields and selectivity for light olefins. The mixture of Cr powder and graphite showed a similar performance compared to Cr3C2. Table 4

Table 5

Example 5. Microwave-initiated polyethylene (PE) depolymerisation over promoted chromium carbide catalyst

[00151] Potassium, sodium and aluminium were selected as promoters to modify the Cr3C2. Potassium carbonate, sodium carbonate and aluminium nitrate (Sigma Aldrich) were used as the precursors of K, Na and Al, respectively. The concentration of K and Na were prepared as 1 wt.% and Al was 2 wt.%. The Cr3C2 was mixed with an aqueous solution of precursor and then calcinated at 350°C for 3 hours under argon. The resulting catalysts were used for PE depolymerisation following the same procedure as Example 1.

[00152] Table 6 shows the evolved gas composition (vol. %) of the promoted catalysts, and Table 7 shows the gas yields and selectivity for light olefins. It was found that the concentration of olefins in the exit gases stream was increased when K and Na were added, whilst the gas yield was significantly improved with the addition of Al.

Table 6

Table 7

Example 6. Re-usability of chromium carbide catalyst for polyethylene (PE) and polypropylene (PP) depolymerisation

[00153] Chromium carbide (Cr3C2) was selected as the group 6 transition metal carbide for the reusability test. The feed (either PE or PP powder) and Cr3C2 were physically mixed with weight ratio of 3: 10 (PP:cat). The resulting catalysts were used for PE or PP depolymerisation following the same procedure as Example 1. However, in subsequent cycles the spent catalysts from the previous cycle was retained and mixed with fresh feed (either PE or PP powder) as described above.

[00154] No obvious deactivation of catalyst was observed after 6 cycles, and both gas yield and light olefins selectivity remained at very high level. Figure 4 illustrates the evolved gas composition of the PE depolymerisation. An ethylene concentration of over 45 vol.% was observed in the effluent gases, and propylene accounted for ca. 6-8 vol.%. As shown in Figure 5, the gas yields were between 20 and 30 wt.% of feedstocks and the selectivity to light olefins was about 68% at each cycle of test.

[00155] Figure 6 illustrates the evolved gas composition of the PP depolymerisation. The propylene comprises more than 25 vol.% in the exit gas stream and the concentration of ethylene was about 20 vol.%. Similarly, both the gas yields and the C2-C3 olefin selectivity remained steady throughout the successive cycle of test, with gas yields of about 30 wt.% and the C2-C3 olefin selectivity ca. 70% (Figure 7).

[00156] For the PE depolymerisation, conversion during the 6 cycles was about 100% and less than 2 wt.% coke deposition was observed after 6 cycles of test. This is evident in our thermogravimetric analysis (TGA) (Figure 8).

[00157] XRD and SEM of the fresh and spent catalyst showed no obvious deactivation (Figures 9 and 10).

Example 7. Microwave-assisted depolymerisation of waste plastics over group 6 metal carbide catalyst [00158] Waste plastics were obtained from supermarket chains (Figure 11), and were subjected to microwave-assisted depolymerisation. Two types of waste plastic were used: milk bottles made of HDPE and food storage containers made of polypropylene. The plastics were first crushed using a blender and the small pieces were then mixed with CT₃C₂ catalysts in a weight ratio of 1 : 1. The mixtures were then exposed to 1000W microwave irradiation in order to produce olefins in accordance with the procedure described in Example 1.

[00159] Both types of waste plastic were rapidly depolymerised to light olefins under microwave irradiation. It was found that about 50 vol.% of light olefins (C2-C3) were produced from these waste plastics decompositions with a selectivity for C2-C3 olefins of over 75% (T ables 8 and 9). More propylene was obtained from food storage container depolymerisation, whilst more ethylene was found in the exit gases stream from the milk bottle depolymerisation (Tables 8 and 9).

Table 8

Table 9

Example 8. Microwave-assisted depolymerisation of polystyrene (PS) over group 6 metal carbide catalyst

[00160] Polystyrene pellets were bought from Sigma-Aldrich and the pellets were ground into powder using a mortar. The resultant powder was mixed with a Cr3C2 catalyst in a weight ratio of 1 : 1 (PS:cat) and the composition exposed to 1000W microwave radiation, and the generated gas collected and analysed in accordance with the procedure of Example 1. [00161] PS was efficiently depolymerised to its monomer, styrene, after about 3-5 min under 1000W microwave irradiation. Again, the Cr3C2 catalyst showed good reusability after several cycles without any noticeable changes or deactivation.

[00162] Figure 12 presents the products distribution obtained through the successive cycles of PS depolymerisation. Fresh PS was mixed with spent catalyst from a previous cycle in a weight ratio of about 1 :1. No gas products were produced via PS depolymerisation, and only liquid products were observed.

[00163] The mass balance of these experiments was about 99.8% and the conversion was also 100%. Thus, the selectivity (Area %) of each product obtained via GCMS analysis is equivalent to its yield (wt.% of feed). In general, the catalytic process has a selectivity to styrene of ca. 80-90%. By-products such as benzene, toluene, ethylbenzene and alpha methylstyrene accounted for less than 10% of product yield.

[00164] Various ratios of Cr3C2 catalyst to PS were studied at varying microwave power

(Table 10). Styrene selectivity was maintained at at least 80% across all conditions.

Table 10

Example 9. Microwave-assisted depolymerisation of waste plastics over group 6 metal carbide catalyst

[00165] Two types of waste polystyrene foam were used a feed in a depolymerisation process which included a polystyrene sheet used as a food container (Figure 13A) and polystyrene foam chips used as loose-fill packaging (Figure 13B). Each PS foam sample was shredded using a blender into small pieces which were then mixed with Cr3C2 catalyst in a weight ratio of 1 :1. The mixtures were then exposed to microwave irradiation at a power of 500W or 750W in order to produce olefins in accordance with the procedure described in Example 1.

[00166] Table 11 shows that a styrene selectivity of about 80% (%area GCMS) was observed in each case with no gas formation.

Table 11

[00167] It has been demonstrated above that olefins can be readily recovered from thermoplastics polymers through microwave assisted catalytic decomposition using group 6 or group 4 transition metal catalysts. The catalytic process is rapid and provides monomeric olefins with high selectivity.

[00168] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference in their entirety and to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein (to the maximum extent permitted by law).

[00169] All headings and sub-headings are used herein for convenience only and should not be construed as limiting the invention in any way.

[00170] The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise paragraphed. No language in the specification should be construed as indicating any non-paragraphed element as essential to the practice of the invention.

[00171] The citation and incorporation of patent documents herein is done for convenience only and does not reflect any view of the validity, patentability, and/or enforceability of such patent documents.

[00172] This invention includes all modifications and equivalents of the subject matter recited in the paragraphs appended hereto as permitted by applicable law.

Claims

1. A process for

(i) producing monomers from a thermoplastic polymer; and/or

(ii) producing olefins; and/or

(iii) depolymerisation of a thermoplastic polymer; and/or

(iv) recycling a thermoplastic polymer comprising exposing a solid composition comprising a thermoplastic polymer to electromagnetic radiation in the presence of a solid catalyst, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

2. A process according to claim 1 wherein the solid catalyst comprises a transition metal selected from chromium, molybdenum, tungsten, titanium and zirconium.

3. A process according to any one of claims 1 and 2 wherein the solid catalyst comprises a transition metal selected from chromium, molybdenum and tungsten.

4. A process according to any one of the preceding claims wherein the transition metal is in the form of a transition metal carbide or oxide.

5. A process according to any one of the preceding claims wherein the transition metal is in the form of a transition metal carbide.

6. A process according to any one of the preceding claims where the solid catalyst comprises chromium carbide,

7. A process according to any one of the preceding claims where the solid catalyst comprises Cr₃C₂.

8. A process according to any one of the preceding claims wherein the solid catalyst further comprises a promotor, suitably selected from an alkali metal, carbon or aluminium.

9. A process according to any one of the preceding claims wherein the weight ratio of thermoplastic polymer to solid catalyst is between about 0.2:1 to about 3:1.

10. A process according to any one of the preceding claims wherein the weight ratio of thermoplastic polymer to solid catalyst is between about 0.5: 1 to about 2: 1.

11. A process according to any one of the preceding claims wherein the thermoplastic polymer and the solid catalyst are in admixture, suitably intimate admixture.

12. A process according to any one of the preceding claims wherein the solid composition comprises the thermoplastic polymer and the solid catalyst in admixture, suitably intimate admixture.

13. A process according to any one of the preceding claims wherein the thermoplastic polymer comprises one or more thermoplastic polymers selected from polyethylene, polypropylene and polystyrene.

14. A process according to any one of the preceding claims wherein the monomers produced are one or more of ethylene, propylene and styrene.

15. A process according to any one of the preceding claims wherein the electromagnetic radiation is microwave radiation.

16. A process according to any one of the preceding claims wherein said exposure to electromagnetic radiation is conducted in an inert atmosphere.

17. A solid composition comprising a solid catalyst in admixture with a thermoplastic polymer, wherein the solid catalyst comprises a group 6 or group 4 transition metal.

18. A solid composition according to claim 17 wherein the solid catalyst comprises a transition metal selected from chromium, molybdenum, tungsten, titanium and zirconium.

19. A solid composition according to any one of claims 17 and 18 wherein the solid catalyst comprises chromium carbide, suitably Cr3C2.

20. A solid composition according to any one of claims 17 to 19 wherein the solid catalyst further comprises a promotor, suitably selected from an alkali metal, carbon or aluminium.

21. A solid composition according to any one of claims 17 to 20 wherein the ratio of thermoplastic polymer to solid catalyst is from about 0.5:1 to about 2:1.

22. A solid composition according to any one of claims 17 to 21 in pelletized or extruded form.

23. Use of a solid composition according to any one of claims 17 to 22 for

(i) producing monomers from a thermoplastic polymer; and/or

(ii) producing olefins; and/or

(iii) depolymerisation of a thermoplastic polymer; and/or

(iv) recycling a thermoplastic polymer.

24. A process for the preparation of a solid composition comprising providing (i) a thermoplastic polymer and (ii) a solid catalyst comprising a group 6 or group 4 transition metal, and putting (i) and (ii) into admixture.

25. A solid composition obtainable by the process of claim 24.