A method of treating a process waste stream contaminated with fluorinated hydrocarbons

Abstract

The method comprising adding a fluoride anion scavenger to the waste stream, contacting with a platinum group metal catalyst, the fluoride ions then being scavenged by the fluoride ion scavenger. The

Description

Field This specification relates to methods of treating process waste streams contaminated with fluorinated hydrocarbons. The methodology as described herein is particularly applicable to waste streams generated during recycling of waste ionomer components including waste catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers. The method is also applicable to waste streams generated during manufacture of ionomer components including catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers. Background Fuel cell and hydrogen producing water electrolyser production is set for rapid growth as investment is placed into the global hydrogen economy. Catalyst coated membranes (CCMs) are a major functional component of both fuel cells and electrolysers. Such CCMs generally comprise a conductive polymer membrane coated on either side by a catalyst containing layer. The CCMs are configured to drive oxidation and reduction reactions and support proton and electron transport, these processes been required for the fuel cell and electrolyser technologies to function. While variations in CCM component materials and configurations exist according to functional performance requirements in end use applications, they generally contain several components of value including one or more platinum group metal (PGM) catalysts and one or more proton conducting polymers. Typically, the membrane is formed of one or more ionomers such as perfluorosulfonic-acid (PFSA) ionomers. Ionomer may also be provided in one or both of the catalyst layers. The ionomer in the catalyst layers may be the same or different to the ionomer in the main membrane component and / or in the other catalyst layer(s). A CCM may comprise two different catalysts, one for driving an oxidation reaction on one side of the CCM and one for driving a reduction reaction on the other side of the CCM. A CCM may also comprise a recombination catalyst which is provided to catalyse the recombination of hydrogen and oxygen to form water, reducing the quantity of hydrogen crossing the membrane and mixing with oxygen to form a potentially explosive mixture. A CCM may also include a metal oxide (e.g., CeOj) as a peroxide scavenger. CCM catalysts can be based on platinum group metals such as platinum, ruthenium, iridium, palladium, or mixtures thereof. The platinum group metals may be provided in elemental (metallic) form, in compound form (e.g., an oxide, such as an iridium oxide catalyst), or as a PGM-base metal alloy (e.g., PtCo). Furthermore, the PGM catalyst materials may be supported on a substrate material (e.g., carbon, such as a platinum-on-carbon catalyst comprising particles of carbon on which platinum is disposed or PtCo-on-carbon). Catalyst coated membranes (CCMs) can also be provided in combination with additional functional layers to form multi-layer membrane electrode assemblies (MEAs). Such MEAs may have 3, 5, or 7 layers for example. With the increase in CCM manufacture for fuel cells and electrolysers, there is an associated increase in CCM waste materials, including a significant volume of scrap material created during CCM manufacture (e.g., due to failure at quality control) and also an increase in end-of-life (EoL) CCMs. Since CCMs contain several components which are rare and / or valuable, including platinum group metals (notably Pt, Pd, Ir and Ru) and ionomer (both in the membrane and catalyst layers), there is a growing demand for methods of recycling such components from waste CCM materials. One current method to recover PGMs from production scrap and end-of-life CCM material involves incineration. The incineration process yields a PGM rich (typically Pt and Ir) ash which is processed via conventional PGM refining routes. However, the incineration process releases harmful and toxic gases such as CO2 and HF from the polymers that are part of the membrane. Both these gases have negative impacts as they pollute the atmosphere, increase the greenhouse effect, and / or have harmful effects in the human body. As such, there is a need for a cleaner process which reduces or eliminates the emission of these gases. In addition to the above, the incineration method destroys the ionomer component which also has significant value. As such, it would also be desirable to provide a process which is capable of recovering both PGM and ionomer components as well as providing a process which is cleaner, safer, and more environmentally friendly. Processes for recovering perfluorosulphonic acid ionomer are known. See, for example, WO2016 / 156815 and US7255798. Furthermore, processes for recovering individual PGM catalyst components are known. See, for example, US7709135. However, to enable fuel cells and electrolysers to become more sustainable technologies, there is a need for commercially viable and environmentally friendly routes to manufacture CCM materials and also to recover, separate, and recycle both the PGMs and the ionomer components from waste CCM materials including production scrap and end-of-life material. It is an aim of the present specification to address this problem. Summary of Invention One potential problem when manufacturing or recycling CCM materials is the generation of process waste streams which are contaminated with fluorinated hydrocarbons such as per- and / or polyfluoroalkyl substances (PFAS). As such, a method of treating such waste streams to remove such contaminants is required. It has been noted that the paper "Single-Atom Pt Catalyst for Effective C-F Bond Activation via Hydrodefluorination" (ACS Catal. 2018, 8, 10, 9353-9358, September 10, 2018) describes a study in which a catalyst comprising platinum immobilised on a silicon carbide support is used in the hydrodefluorination of fluorinated hydrocarbons. In accordance with the present specification, the use of platinum group metal catalysts has been considered for treating process waste streams generated during the manufacture or recycling of CCM materials. However, one problem with this approach is that hydrodefluorination of fluorinated hydrocarbons liberates fluoride anions which can deactivate the catalyst. For example, if a platinum on silicon carbide catalyst is utilized as described in the aforementioned paper, the liberated fluoride anions can form Si-F bonds with the catalyst and deactivate the catalyst. In light of the above, the present specification provides a method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising: adding a fluoride anion scavenger to the process waste stream; and contacting the process waste stream with a platinum group metal catalyst to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. The fluoride scavenger prevents the liberated fluoride anions from deactivating the platinum group metal catalyst and thus enables process waste streams contaminated with fluorinated hydrocarbons to be more reliably treated for longer time periods. Various examples of suitable fluoride scavengers are provided in the detailed description. Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a flow diagram of a method for treating a process waste stream contaminated with fluorinated hydrocarbons according to the present specification; Figure 2 shows a flow diagram of a method for recycling a waste catalyst coated membrane component according to the present specification; and Figure 3 shows a schematic example of a configuration for treating a process waste stream contaminated with fluorinated hydrocarbons according to the present specification. Detailed Description As described in the summary section and shown in the flow diagram of Figure 1, the present specification provides a method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising: adding a fluoride anion scavenger to the process waste stream; and contacting the process waste stream with a platinum group metal catalyst to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. The fluorinated hydrocarbons contaminating such process waste stream may be per- and / or polyfluoroalkyl substances (PFAS). The platinum group metal catalyst may comprise platinum. Furthermore, the platinum group metal catalyst may comprise a platinum group metal such as platinum disposed on a substrate. The substrate may comprise or consist of an organic substrate, a carbon substrate, or an inorganic substrate, optionally a metal, a metal alloy, a ceramic, a metal oxide, a nitride, or a carbide. For example, the substrate can be an inorganic substrate such as silicon carbide. In the case of a silicon containing substrate material, liberated fluoride anions tend to form Si-F bonds which leads to deactivation of the catalyst if they are not scavenged in accordance with the present methodology. Various methods for scavenging the fluoride anions are possible. For example, the fluoride scavenger can form a soluble fluoride species or be a precipitant or an adsorbent which forms a solid material comprising the fluoride anions. Suitable soluble fluoride species are soluble fluoride salts (e.g., NaF, KF) which can be subsequently washed from ionomer dispersion and / or solid residues such as carbon support or PTFE reinforcement. Suitable precipitants include metal compounds which react with the fluoride anions in solution to form a solid metal fluoride salt which is precipitated from solution. Such metal compounds may be selected from one or more of: an alkaline earth metal compound; a calcium compound; a magnesium compound; a strontium compound; a transition metal compound; a titanium compound; a post-transition metal compound; an aluminium compound; a hydroxide. For example, a calcium compound such as calcium hydroxide can be used to mineralise the liberated fluoride anions as CaFj. The precipitated metal fluoride can then be separated from the process waste stream via filtration or centrifugation. In the case of using an adsorbent as the fluoride scavenger, the adsorbent can be a solid, insoluble metal compound which adsorbs fluoride anions. Such an adsorbent metal compound can be selected from one or more of: a metal oxide; magnesium oxide; calcium oxide; aluminium oxide; titanium oxide; a mixed metal oxide. The adsorbent can be separated from the process waste stream via filtration or centrifugation after adsorbing the fluoride anions. The process waste stream can be subjected to UV light when contacting the process waste stream with the platinum group metal catalyst to provide a heterogeneous photoinduced catalytic hydrodefluorination. Alternatively, or additionally, the process waste stream can be heated when contacting the process waste stream with the platinum group metal catalyst to produce a heterogeneous thermally induced catalytic hydrodefluorination. The above-described methodology can be used to treat process waste streams from a waste ionomer membrane recycling process and / or from an ionomer membrane manufacturing process (e.g., a catalyst coated ionomer membrane recycling process and / or a catalyst coated ionomer membrane manufacturing process). Figure 2 shows a flow diagram of a method for recycling a catalyst coated membrane according to the present specification. The catalyst coated membrane is subjected to acid leaching as described in WO2023 / 247913 to remove and recover platinum group metal components. The acid leaching may, for example, include an oxidative acid leach to recover platinum and a reductive acid leach to recover iridium. The solid ionomer membrane material is then treated with a base such as NaOH or LiOH to convert the ionomer to salt form without dispersing the solid ionomer membrane material. The ionomer salt is more readily dispersed when in such salt form. Furthermore, as the saltation is performed without undue heating to disperse the ionomer, excess base can readily be removed via solid-liquid separation to avoid contaminating further process steps. The solid ionomer membrane material is then dispersed in a solvent such as water, an alcohol, or a mixture of water and an alcohol. This is typically done in an autoclave at elevated pressure. The resultant ionomer dispersion will typically include membrane reinforcement materials such as ePTFE and catalyst support materials such as carbon. These can be removed from the ionomer dispersion by, for example, centrifugation or filtration. The ionomer can then be subjected to an ion-exchange step to re-protonate the acid groups of the ionomer and freeze dried to yield a solid ionomer product ready for re-use. One or more of the aforementioned process steps can generate a process waste stream contaminated with PFAS and the present methodology can be used to ensure that the PFAS is stripped of fluorine by contacting with a PGM catalyst and that liberated fluoride anions are mineralized, e.g., using a fluoride scavenger such as a calcium compound to mineralise the liberated fluoride as CaFj. This ensures that the liberated fluoride anions do not deactivate the PGM catalyst. According to certain embodiments, a base is added to the process waste stream and the process waste stream is heated in the presence of the base while contacting the process waste stream with the platinum group metal catalyst to defluorinate the fluorinated hydrocarbons. The base may be a different component to the fluoride anion scavenger. The base can be a hydroxide such as sodium hydroxide or lithium hydroxide. The pH of the process waste stream after addition of the base can be: at least 8, 9, or 10; no more than 14, 13, or 12; or within a range defined by any combination of the aforementioned lower and upper limits. The process waste stream can be heated (after addition of the fluoride anion scavenger and optional base) to a temperature of: at least 200°C, 250°C, or 300°C; no more than 500°C, 400°C, 350°C; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, the process waste stream can be heated in a pressurized vessel at a pressure of: at least 1, 5, 10, or 15 MPa; no more than 30, 25, or 20 MPa; or within a range defined by any combination of the aforementioned lower and upper limits. Certain embodiments use platinum group metal catalyst at elevated temperature and pressure to break down fluoropolymer materials possessing an ether link group. Experimental evidence has been generated (e.g. in the form of elevated free fluoride concentration) showing that the presence of platinum group metal catalyst and perfluorinated polymers containing an ether linkage leads to breakdown of the polymer at the ether link. In certain examples, platinum group metal catalysts already present in waste ionomer membrane recycling streams can be used to catalyse the break-down of fluoropolymer materials. Since platinum group metal catalysts can already be present in recycling feeds from fuel cells and water electrolysers, this can be leveraged in the recycling process to modify and mineralise any undesirable small-chain perfluorinated compounds, without necessarily requiring the provision of an additional catalyst. By breaking down polymer into smaller fragments it can be easier to then process or separate these small fragments. Temperature and pressure conditions can be as described above. That is, the process waste stream can be heated to a temperature of: at least 200°C, 250°C, or 300°C; no more than 500°C, 400°C, 350°C; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, the process waste stream can be heated in a pressurized vessel at a pressure of: at least 1, 5, 10, or 15 MPa; no more than 30, 25, or 20 MPa; or within a range defined by any combination of the aforementioned lower and upper limits. Heating may be in water and optionally in the presence of a base as previously described. According to certain examples, such as illustrated in Figure 3, a vessel 30 is provided which contains PGM (e.g., palladium) coated onto a support (e.g., porous alumina beads or HF resistant non-porous plastic beads 32). A solution 33 containing PFAS (e.g., PFOA and / or PFOS) is introduced into the vessel 30 (ideally in a continuous process, but optionally in a batch process) along with a reducing gas 38 such as hydrogen (e.g., a mixture of nitrogen and hydrogen). The combination of metallic PGM and reducing gas can lead to the full hydrodefluorination of the PFAS, producing less harmful products. Optionally, the solution is heated during processing in this manner. Reducing gas 38 from the aforementioned vessel 30 can then be passed via a connecting tube 40 and valve 42 through an additional vessel 34 containing a basic solution 36 to neutralize any HF and yield an output gas 44 absent of HF. According to certain embodiments, additional process steps can be applied to the process waste stream after the step of contacting the process waste stream with a platinum group metal catalyst to defluorinated the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger. For example, once side chains have been cleaved from the ionomer in the presence of a platinum group metal catalyst (e.g., metallic Pt) and optionally a base, the resulting defluorinated hydrocarbons can be subject to a single-stage thermal depolymerisation process described in WO2010039820A2 to generate TFE building blocks for PTFE remanufacture. If not free from sulfate, a dual stage thermal decomposition would be necessary as described in WO2021130626A1, which is more energy intensive and yield is compromised. Combining the presently described methodology with a thermal depolymerisation process can result in a more efficient route to generating TFE building blocks from recycle streams for PTFE remanufacture. While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of treating a process waste stream contaminated with fluorinated hydrocarbons, the method comprising:adding a fluoride anion scavenger to the process waste stream; andcontacting the process waste stream with a platinum group metal catalyst to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger.

2. A method according to claim 1,wherein the fluorinated hydrocarbons are per- and / or poly-fluoroalkyl substances (PFAS).

3. A method according to claim 1 or 2,wherein the platinum group metal catalyst comprises platinum.

4. A method according to any preceding claim,wherein the platinum group metal catalyst comprises a platinum group metal disposed on a substrate.

5. A method according to claim 4,wherein the substrate comprises an organic substrate, a carbon substrate, or an inorganic substrate, optionally a metal, a metal alloy, a ceramic, a metal oxide, a nitride, or a carbide.

6. A method according to any preceding claim,wherein the fluoride scavenger is a precipitant or adsorbent which forms a solid material comprising the fluoride anions.

7. A method according to claim 6,wherein the precipitant is a metal compound which reacts with the fluoride anions in solution to form a solid metal fluoride salt which is precipitated from solution.

8. A method according to claim 7,wherein the metal compound is selected from one or more of: an alkaline earth metal compound; a calcium compound; a magnesium compound; a strontium compound; a transition metal compound; a titanium compound; a post-transition metal compound; an aluminium compound; a hydroxide.

9. A method according to claim 7 or 8,wherein the precipitated metal fluoride is separated from the process waste stream via filtration or centrifugation.

10. A method according to claim 6,wherein the adsorbent is a solid, insoluble metal compound which adsorbs fluoride anions.

11. A method according to claim 10,wherein the solid metal compound is selected from one or more of: a metal oxide; magnesium oxide; calcium oxide; aluminium oxide; titanium oxide; a mixed metal oxide.

12. A method according to claim 10 or 11,wherein the adsorbent is separated from the process waste stream via filtration or centrifugation after adsorbing the fluoride anions.

13. A method according to any one of claims 1 to 5,wherein the fluoride scavenger forms a soluble fluoride species.

14. A method according to any preceding claim,wherein the process waste stream is a subjected to UV light when contacting the process waste stream with the platinum group metal catalyst to provide a heterogeneous photoinduced catalytic hydrodefluorination.

15. A method according to any preceding claims,wherein the process waste stream is heated when contacting the process waste stream with the platinum group metal catalyst to produce a heterogeneous thermally induced catalytic hydrodefluorination, optionally in the presence of a base.

16. A method according to any preceding claim,wherein the process waste stream is a waste stream from a waste ionomer membrane recycling process.

17. A method according to any one of claims 1 to 15,wherein the process waste stream is a waste stream from an ionomer membrane manufacturing process.

18. A method according to claim 16 or 17,wherein the ionomer membrane is a catalyst coated ionomer membrane.

19. A method according to any preceding claim,wherein after contacting the process waste stream with a platinum group metal catalyst to defluorinate the fluorinated hydrocarbons liberating fluoride anions which are scavenged by the fluoride scavenger, the remaining defluorinated hydrocarbons are subjected to thermal depolymerization.T +44(0)30 0300 2000A

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