HYDROGEN FROM PLASTIC MATERIALS USING SAID CATALYTIC SYSTEM

FIELD OF THE INVENTION

The present invention belongs to the field of processes to produce clean hydrogen from plastic materials, basically from plastic waste materials, by using recyclable ionic liquid-based catalytic systems.

BACKGROUND

Plastic recycling is a sector in constant development due to the huge amount of waste generated as a result of the, generally excessive, use of these materials in different sectors.

The common procedures for recycling these materials are incineration, as well as various mechanical and chemical processes.

In this regard, products with great added value can be obtained from the decomposition of plastic waste materials such as hydrogen and decarbonized materials or hydrogen-depleted coproducts. In fact, hydrogen is currently in great demand due to its application in clean energy technologies. However, although some work has been done for producing clean hydrogen, no conclusive results have been obtained. The most currently available methods to produce hydrogen from waste plastic materials include the one step gasification process and the two step pyrolysis-steam reforming where hydrogen is produced in a gas steam containing CO and/or CO2, water vapour, etc.

Nevertheless, all these methods involve many disadvantages, such as, high energy consumption, very high temperatures, CO2 emission, catalyst recovery, the need to separate hydrogen from a gas mixture and consequently its low purity.

Other processes to produce hydrogen that have been investigated to contribute in reducing environmental impacts are the methane pyrolysis in molten metals and salts using concentrated solar energy (Energies, 2021, 14, 3107), as well as microwave-driven plasma-mediated methane decomposition (US 8,021,448 and J. Carbon Research, 2018, 4, 61). However, they do not use plastic waste as a source to produce hydrogen. An alternative method to produce hydrogen from plastic waste, in one single step, makes use of microwave technology and it is carried out at lower temperatures than those required in gasification processes under conventional heating methods (Nature Catalysis, 2020, 3, 902-912). EP3604211 also discloses a process for producing hydrogen from plastic waste by exposing them to microwave irradiation in the presence of elemental iron or an oxide thereof as catalyst. In fact, microwave irradiation has also been used to produce hydrogen from hydrocarbons and compounds having hydrocarbyl groups (EP3310472 and US 6,783,632) or from water by using a microwave absorbing susceptor and iron (US2007/295593).

On the other hand, ionic liquids have been used as solvents and co-solvents in many microwave- assisted chemical transformations, including alkylation reactions, resulted in high yields with reduced reaction times (US 6,596,130).

However, the main disadvantages of these microwave-assisted methods that use plastic waste to produce hydrogen are the handling of plastic in solid state which significantly difficult the procedure, the huge amount of catalyst employed (50 wt.%), the rapid deactivation of the catalyst due to the cogenerated carbon deposits on it (cocking phenomena) and the difficulty to recover/reuse the catalyst and the cogenerated carbon materials for their further valorization.

In view of all above, there is a need to develop processes to produce hydrogen from the recycling of plastic materials in order to overcome the problems arising from existing technologies, such as those mentioned above.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have developed an ionic liquid-based catalytic system to be used in plastic waste mitigation to produce clean hydrogen and hydrogen-depleted coproducts, which overcomes so far the main drawbacks of the state of the art by diminishing significantly the total amount of catalyst needed, avoiding catalyst deactivation by cooking, and facilitating the recovery of hydrogen-depleted co-products, as well as recovering and reusing the catalytic system.

The key factor of the catalytic system is the presence of ionic liquids as they have shown to be thermally stable, to have a good plastic solubility as well as chemical stability towards the metal catalyst and H2, and low carbon wettability. The catalytic system, which also include metal oxides, has enable a rapid and efficient production of clean hydrogen, along with a valuable carbon material, as the sole products, from plastic waste, mainly from plastic having a low recyclability rate, such as polyethylene, polypropylene and polystyrene.

The catalytic system has resulted to be particularly useful in decomposition reactions of plastic materials under electromagnetic irradiation, mainly under microwave irradiation.

Furthermore, as derivable from the example provided below, by using said catalytic system, hydrogen can be produced in high yields, particularly higher than 85%, also achieving hydrogen with a purity higher than 90% and with a selectivity of at least 95%.

Thus, a first aspect of the invention refers to the use of a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, and wherein the metal of the metal oxide is a single metal or a mixture of metals, in a process to produce hydrogen from the decomposition of plastic materials.

An additional aspect of the invention relates to a process for the production of hydrogen and hydrogen-depleted co-products, said process comprises: a) dissolving or dispersing a plastic material in a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, and wherein the metal of the metal oxide is a single metal or a mixture of metals, and b) exposing the resulting dissolution or dispersion obtained in step a) to an electromagnetic radiation.

A further aspect of the invention refers to a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, wherein the metal of the metal oxide is a single metal or a mixture of metals, and wherein the metal oxides are in the form of nanoparticles.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, a first aspect of the invention refers to the use of a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, and wherein the metal of the metal oxide is a single metal or a mixture of metals, in a process to produce hydrogen from the decomposition of plastic materials.

In the context of the present invention, the term "ionic liquid" refers to a salt in liquid state understanding as such any salt that melts without decomposing or vaporizing at least in a temperature range from -10 °C to 400 °C.

The ionic liquids useful within the present invention are typically fluid at room temperature and have a liquid range up to 400 °C at standard pressure.

The ionic liquid is preferably a compound of the general formula A ⁺ B ^_ wherein A ⁺ is a cation, particularly an organic cation, and B' is an organic or inorganic anion.

The term "organic cation" is intended to mean an organic molecule wherein a non-metal atom has donated one or more electrons to another atom or atoms so that the organic molecule has become a positively charged species. The positive charge could be either concentrated to one atom or distributed over the whole molecule.

In a preferred embodiment, the organic cation is a N-substituted cation where the cationic functionality is essentially associated with the nitrogen atom. More particularly, the organic cation is of the type where the N-substituted cation is an N- substituted heteroaromatic cation wherein the cationic functionality is associated with the nitrogen atom-containing heteroaromatic structure.

Suitable N-substituted organic cations are, for example, ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium, piperazinium, guanidinium, tetramethylguanidinium, hexahydro-pyrimido-pyrimidine, octahydro-pyrimido[l,2- a]azepinium, 1,4-diaza-bicyclo[2,2,2] octane, 1,8-Diazabicyclo(5.4.0)undec-7-ene, where the nitrogen or at least one nitrogen in the ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium and piperazinium is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical. The substitution may be symmetric or asymmetric when more than one substituent is present.

In a preferred embodiment, the N-substituted organic cation is selected from imidazolium, pyridinium, piperidinium, hexahydro-pyrimido-pyrimidine, octahydro-pyrimido[l,2-a]azepine and tetramethyl guanidinium cations, where the nitrogen or at least one nitrogen in the imidazolium, pyridinium and piperidinium is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical. More preferably, the N-substituted organic cation is selected from pyridinium, piperidinium, hexahydro-pyrimido-pyrimidine, octahydro-pyrimido[l,2-a]azepine and tetramethyl guanidinium, where the nitrogen in the pyridinium and piperidinium is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical. Even more preferably, the organic cation is tetramethyl guanidinium o piperidinium, where the nitrogen in the piperidinium is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical, more preferably with at least one alkyl radical.

In a preferred embodiment, the organic cation is a P-substituted cation where the cationic functionality is essentially associated with the phosphorous atom. This includes phosphonium cations where the phosphorous is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical.

In a particular embodiment, the organic cations are selected from ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium, piperazinium, guanidinium, tetramethyl guanidinium, hexahydro-pyrimido-pyrimidine, octahydro-pyrimido[l,2-a]azepine, l,4-diaza-bicyclo[2,2,2] octane, l,8-Diazabicyclo(5.4.0)undec-7-ene and phosphonium, where the nitrogen or at least one nitrogen in the ammonium, imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium and piperazinium, or the phosphorous, is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical.

In a particular embodiment, the organic cations are selected from imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium, piperazinium, tetramethyl guanidinium, hexahydro-pyrimido-pyrimidine, octahydro-pyrimido[l,2-a]azepine and phosphonium, where the nitrogen or at least one nitrogen in the imidazolium, pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium and piperazinium, or the phosphorous in the phosphonium, is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical.

In a particular embodiment, the organic cations are selected from pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium, piperazinium, tetramethyl guanidinium, hexahydro- pyrimido-pyrimidine, octahydro-pyrimido[l,2-a]azepine and phosphonium, where the nitrogen or at least one nitrogen in the pyridinium, piperidinium, pyrazolium, pyrrolidinium, pyrimidinium and piperazinium, or the phosphorous in the phosphonium, is substituted with at least one alkyl, cycloalkyl, aryl or arylalkyl radical. In a particular embodiment, the organic cation of the ionic liquid is selected from: wherein R1, R2, R3, R4, R4 and R6 are independently selected from H, linear or branched C1-C18 alkyl; C ₅ -C 10 cycloalkyl; C6-C18 aryl and C7-C12 arylalkyl, provided that at least one of R1-R4 in the same cation is not H.

In a preferred embodiment, the organic cation of the ionic liquid is selected from: wherein R1, R2, R3, R4, R5 and R6 are independently selected from H, linear or branched C1-C18 alkyl; C ₅ -C 10 cycloalkyl; C6-C18 aryl and C7-C12 arylalkyl, provided that at least one of R1-R4 in the same cation is not H.

More preferably, the organic cation of the ionic liquid is selected from: wherein R1, R2, R3, R4, Rs and R6 are independently selected from H, linear or branched C1-C18 alkyl; C ₅ -C 10 cycloalkyl; C6-C18 aryl and C7-C12 arylalkyl, provided that at least one of R1-R4 in the same cation is not H.

Even more preferably, the organic cation of the ionic liquid is selected from: wherein R1, R2, Rs and Rs are independently selected from H, linear or branched C1-C18 alkyl; C ₅ - C10 cycloalkyl; C6-C18 aryl and C7-C12 arylalkyl, provided that at least one of R1-R2 is not H.

The proviso that at least one of R1-R4 in the same cation is not H means that in the imidazolium cation at least one of R1-R3 is not H, in the pyridinium cation R1 is not H, in the piperidinium cation R1 and R2 are not both H, in the ammonium cation at least of R1 to R ₄ is not H, and in the phosphonium cation at least one of R1-R4 is not H.

The term "alkyl" refers to a straight or branched hydrocarbon chain radical consisting of carbon and hydrogen atoms, containing no unsaturation, having 1 to 18 carbon atoms, more preferably 1 to 12 carbon atoms (C1-C12 alkyl), even more preferably 1 to 8 carbon atoms (C1-C8 alkyl), and which is attached to the rest of the molecule by a single bond, e. g., methyl, ethyl, n-propyl, /- propyl, n-butyl, t-butyl, n-pentyl, etc.

The term "aryl" refers to an aromatic group having between 6 to 18 carbon atoms, preferably between 6 and 12 carbon atoms, more preferably between 6 and 10 and even more preferable having 6 carbon atoms, comprising 1, 2 or 3 aromatic nuclei, including for example and in a nonlimiting sense, phenyl, biphenyl, naphthyl, indenyl, phenanthryl, anthracyl or terphenyl. Preferably, aryl refers to phenyl (Ph).

The term "cycloalkyl" refers to a saturated or partially saturated mono- or bicyclic aliphatic group having between 3 and 10, preferably between 3 and 6 carbon atoms ("C3-C6 cycloalkyl"), which is bound to the rest of the molecule by means of a single bond, including for example and in a non-limiting sense, cyclopropyl, cyclohexyl or cyclopentyl.

The term "arylalkyl" refers to an aryl group as defined above linked to an alkylene group which is attached to the rest of the molecule by a single bond. Preferred examples include benzyl (- CH ₂ -Ph) and phenethyl (-CH2-CH ₂ -Ph). In a preferred embodiment, R1, R2, R3 and R ₄ are independently selected from H, linear or branched C1-12 alkyl, C5-C10 cycloalkyl, C7-C10 arylalkyl and phenyl. More preferably, R1, R2, Ra and R ₄ are independently selected from H, linear or branched C1-C4 alkyl and C ₅ -Cs cycloalkyl, even more preferably R1, R2, Ra and R ₄ are independently selected from H and linear or branched C1-C4 alkyl, provided in all cases that at least one of R1-R4 in the same molecule is not H.

In a most preferred embodiment, at most only one of R1-R4 is H, and even most preferred is that none of R1 to R ₄ are H.

In another preferred embodiment, R ₅ is H. In another preferred embodiment, R ₅ and R6 are H.

In a more preferred embodiment, the organic cation is selected from N-methyl pyridinium; N- ethyl pyridinium; N-butyl pyridinium; 1-butyl pyrrolidinium; 1-ethyl pyrrolidinium; N-ethyl piperidinium; N-butyl piperidinium; 1,1-dimethyl piperidinium; 1,1-diethyl piperidinium; 1,1- dibutyl piperidinium; 1-butyl-l-methylpiperidinium; 1-methyl-1-propyl piperidinium; 1-butyl-l- methyl pyrrolidinium; 1-ethyl-l-methyl pyrrolidinium; 1-methyl-1-propyl pyrrolidinium; 1- methyl-l-octyl pyrrolidinium; 1-methyl-l-pentyl pyrrolidinium; tetrabutyl phosphonium; TMG (tetramethyl guanidinium), l,3,4,6,7,8-Hexahydro-2H-pyrimido[l,2-a]pyrimidine, 2,3,4,6,7,8,9,10-Octahydro-pyrimido[l,2-a]azepine, DABCO (1,4-etilenpiperazinium), tributylmethyl phosphonium; tributylethyl phosphonium; tributylhexylphosphonium; tributyl-n- octylphosphonium, tributylhexadecyl phosphonium and tetraphenyl phosphonium.

In a more preferred embodiment, the organic cation is selected from N-methyl pyridinium; N- ethyl pyridinium; N-butyl pyridinium; 1-butyl pyrrolidinium; 1-ethyl pyrrolidinium; N-ethyl piperidinium; N-butyl piperidinium; 1,1-dimethyl piperidinium; 1,1-diethyl piperidinium; 1,1- dibutyl piperidinium; 1-butyl-l-methylpiperidinium; 1-methyl-1-propyl piperidinium; 1-butyl-l- methyl pyrrolidinium; 1-ethyl-l-methyl pyrrolidinium; 1-methyl-1-propyl pyrrolidinium; 1- methyl-l-octyl pyrrolidinium; 1-methyl-l-pentyl pyrrolidinium; tetrabutyl phosphonium; TMG (tetramethyl guanidinium), 1,3,4,6,7,8-Hexahydro-2H-pyrimido[l,2-a]pyrimidine, 2,3,4,6,7,8,9,10-Octahydro-pyrimido[l,2-a]azepine, DABCO (1,4-etilenpiperazinium), tributylmethyl phosphonium; tributylethyl phosphonium; tributylhexylphosphonium; tributyl-n- octylphosphonium, tributylhexadecyl phosphonium and tetraphenyl phosphonium.

Even more preferably, the organic cation is selected from N-ethyl piperidinium; N-butyl piperidinium; 1,1-dimethyl piperidinium; 1,1-diethyl piperidinium; 1,1-dibutyl piperidinium; 1- butyl-l-methylpiperidinium; 1-methyl-1-propyl piperidinium; TMG (tetramethyl guanidinium); 1,3,4,6,7,8-Hexahydro-2H-pyrimido[l,2-a]pyrimidine; 2,3,4,6,7,8,9,10-Octahydro-pyrimido[1,2- a]azepine; tetrabutyl phosphonium; tributylmethyl phosphonium; tributylethyl phosphonium; tributylhexylphosphonium; tributyl-n-octylphosphonium, tributylhexadecyl phosphonium and tetraphenyl phosphonium.

In another particular embodiment, the anion of the ionic liquid is an organic or inorganic anion selected from: wherein: R1 is selected from H, linear or branched C1-12 alkyl and fluorinated or perfluorinated C1 alkyl;

R2 is selected from linear C1-12 alkyl, C5-C12 cycloalkyl and fluorinated or perfluorinated C1 alkyl;

R3 and R ₄ are independently selected from H and linear or branched C1-C12 alkyl, provided that R3 and R ₄ are not both H;

X' is selected from Ch, Br, I NO ₃ ’, BF ₄ ", PF ₆ ’, [FSO ₂ ] ₂ N and [(CF3SO2)N ]; and

MYn" is a halometallate anion, wherein M is a transition metal selected from Fe, Co, Mn, Ni, Cu, Zn, Pd, Pt and Au or a non-transition metal selected from Ga, Al and In, and wherein Y is a halogen and n is the value of the valence of the metal + 1.

In a particular embodiment, the anion of the ionic liquid is an organic or inorganic anion selected from: wherein:

R1 is selected from H, linear or branched C1-C12 alkyl and fluorinated or perfluorinated C1 alkyl;

R2 is selected from linear C1-C12 alkyl, C5-C12 cycloalkyl and fluorinated or perfluorinated C1 alkyl; R3 and R ₄ are independently selected from H and linear or branched C1-12 alkyl, provided that R ₃ and R ₄ are not both H;

X' is selected from Cl Br; I; NO ₃ ; BF ₄ ; PF ₆ ; [FSO ₂ ] ₂ N‘ and [(CF ₃ SO ₂ ) ₂ N ]; and

MYn" is a halometallate anion, wherein M is a transition metal selected from Fe, Mn, Co and Cu or a non-transition metal from Ga and In, and wherein Y is a halogen and n is the value of the valence of the metal + 1.

In a particular embodiment, R1 is a fluorinated or perfluorinated methyl, more preferably is a perfluorinated methyl.

In another preferred embodiment, R ₂ is selected from a linear C1-C6 alkyl and fluorinated or perfluorinated methyl, preferably selected from a linear C ₁ -C ₄ alkyl and a perfluorinated methyl.

In another particular embodiment, R ₃ and R ₄ are independently a linear C1-C ₁₂ alkyl, preferably a linear C ₁ -C ₄ alkyl.

In another particular embodiment, X- is selected from [FSO ₂ ] ₂ N ^_ and [(CF ₃ SO ₂ ) ₂ N ].

In another particular embodiment, M is Fe(ll) or Fe(lll) and Y is chloride. Preferably, the anion MYn' is FeCI ₄ ;

In a preferred embodiment, the organic or inorganic anion of the ionic liquid is selected from dimethylphosphate, trifluoromethanesulfonate, bis(trifluoromethanesulfonyl)imide, chloride, bromide, iodide, tetrafluoroborate, hexafluorophosphate, trifluoracetate and FeCI ₄ -.

Preferably, the organic or inorganic anion of the ionic liquid is selected from bis(trifluoromethanesulfonyl)imide, chloride, hexafluorophosphate, trifluoracetate and FeCI ₄ -, even more preferably is selected from bis(trifluoromethanesulfonyl)imide, chloride, trifluoracetate and FeCI ₄ -.

In a preferred embodiment, the ionic liquid is selected form the following salts:

1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1,1,3,3-tetramethyl guanidinium trifluoromethanesulfonate, 1,1,3,3-tetramethyl guanidinium tetrafluroborate, 1,1,3,3- tetramethyl guanidinium bis(trifluoromethanesulfonyl)imide, 1,1,3,3-tetramethyl guanidinium methylsulfate, 1,1,3,3-tetramethyl guanidinium isobutyrate, 1,1,3,3-tetramethyl guanidinium hexafluorophosphate, 1,1,3,3-tetramethyl guanidinium FeCI ₄ ; N-methypyridinium trifluoroacetate, N-methypyridinium trifluoromethanesulfonate, N-methypyridinium tetrafluroborate, N-methypyridinium bis(trifluoromethanesulfonyl)imide, N-methypyridinium methylsulfate, N-methypyridinium isobutyrate, 1-methylimidazolium trifluoroacetate, 1- methylimidazolium trifluoromethanesulfonate, 1-methylimidazolium tetrafluroborate, 1- methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-methylimidazolium methylsulfate, 1- methylimidazolium isobutyrate, 1-ethyl pyrrolidinium trifluoroacetate, 1-ethyl pyrrolidinium trifluoromethanesulfonate, 1-ethyl pyrrolidinium tetrafluroborate, 1-ethyl pyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-ethyl pyrrolidinium methylsulfate, 1-ethyl pyrrolidinium isobutyrate, N-ethyl piperidinium trifluoroacetate, N-ethyl piperidinium trifluoromethanesulfonate, N-ethyl piperidinium tetrafluroborate, N-ethyl piperidinium bis(trifluoromethanesulfonyl)imide, N-ethyl piperidinium methylsulfate, N-ethyl piperidinium isobutyrate, 1-methyl-1-propyl piperidinium trifluoroacetate, 1-methyl-1-propyl piperidinium trifluoromethanesulfonate, 1-methyl-1-propyl piperidinium tetrafluroborate, 1-methyl-l- propyl piperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propyl piperidinium hexafluorophosphate, 1-methyl-1-propyl piperidinium FeCI4, tributylmethylphosphonium trifluoroacetate, tributylmethylphosphonium trifluoromethanesulfonate, tributylmethylphosphonium tetrafluroborate, tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide, tributylmethylphosphonium methylsulfate, tributylmethylphosphonium isobutyrate, tetraphenylphosphonium trifluoroacetate, tetraphenylphosphonium trifluoromethanesulfonate, tetraphenylphosphonium tetrafluroborate, tetraphenylphosphonium bis(trifluoromethanesulfonyl)imide, tetraphenylphosphonium methylsulfate, tetraphenylphosphonium isobutyrate, tetraphenylphosphonium hexafluorophosphate, tetraphenylphosphonium FeCI ₄ ', 1-methyl-l- propylpyrrolidinium trifluoroacetate, 1-methyl-1-propylpyrrolidinium trifluoromethanesulfonate, 1-methyl-1-propylpyrrolidinium tetrafluroborate, 1-methyl-l- propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propylpyrrolidinium methylsulfate, 1-methyl-1-propylpyrrolidinium isobutyrate.

Preferably, the ionic liquid is selected form the following salts:

1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1,1,3,3-tetramethyl guanidinium trifluoromethanesulfonate, 1,1,3,3-tetramethyl guanidinium tetrafluroborate, 1,1,3,3- tetramethyl guanidinium bis(trifluoromethanesulfonyl)imide, 1,1,3,3-tetramethyl guanidinium methylsulfate, 1,1,3,3-tetramethyl guanidinium isobutyrate, 1,1,3,3-tetramethyl guanidinium hexafluorophosphate, 1,1,3,3-tetramethyl guanidinium FeCI4, N-methypyridinium trifluoroacetate, N-methypyridinium trifluoromethanesulfonate, N-methypyridinium tetrafluroborate, N-methypyridinium bis(trifluoromethanesulfonyl)imide, N-methypyridinium isobutyrate, 1-ethyl pyrrolidinium trifluoroacetate, 1-ethyl pyrrolidinium trifluoromethanesulfonate, 1-ethyl pyrrolidinium tetrafluroborate, 1-ethyl pyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-ethyl pyrrolidinium isobutyrate, N-ethyl piperidinium trifluoroacetate, N-ethyl piperidinium trifluoromethanesulfonate, N-ethyl piperidinium tetrafluroborate, N-ethyl piperidinium bis(trifluoromethanesulfonyl)imide, N-ethyl piperidinium isobutyrate, 1-methyl-1-propyl piperidinium trifluoroacetate, 1-methyl-1-propyl piperidinium trifluoromethanesulfonate, 1-methyl-1-propyl piperidinium tetrafluroborate, 1-methyl-l- propyl piperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propyl piperidinium hexafluorophosphate, 1-methyl-1-propyl piperidinium FeCI ₄ ‘, tributylmethylphosphonium trifluoroacetate, tributylmethylphosphonium trifluoromethanesulfonate, tributylmethylphosphonium tetrafluroborate, tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide, tributylmethylphosphonium isobutyrate, tetraphenylphosphonium trifluoroacetate, tetraphenylphosphonium trifluoromethanesulfonate, tetraphenylphosphonium tetrafluroborate, tetraphenylphosphonium bis(trifluoromethanesulfonyl)imide, tetraphenylphosphonium isobutyrate, tetraphenylphosphonium hexafluorophosphate, tetraphenylphosphonium FeCI ₄ ‘, 1-methyl-1-propylpyrrolidinium trifluoroacetate, 1-methyl-1-propylpyrrolidinium trifluoromethanesulfonate, 1-methyl-1-propylpyrrolidinium tetrafluroborate, 1-methyl-l- propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propylpyrrolidinium isobutyrate.

Preferably, the ionic liquid is selected form the following salts:

1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1,1,3,3-tetramethyl guanidinium trifluoromethanesulfonate, 1,1,3,3-tetramethyl guanidinium tetrafluroborate, 1,1,3,3- tetramethyl guanidinium bis(trifluoromethanesulfonyl)imide, 1,1,3,3-tetramethyl guanidinium methylsulfate, 1,1,3,3-tetramethyl guanidinium isobutyrate, 1,1,3,3-tetramethyl guanidinium hexafluorophosphate, 1,1,3,3-tetramethyl guanidinium FeCI ₄ ‘, N-ethyl piperidinium trifluoroacetate, N-ethyl piperidinium trifluoromethanesulfonate, N-ethyl piperidinium tetrafluroborate, N-ethyl piperidinium bis(trifluoromethanesulfonyl)imide, N-ethyl piperidinium isobutyrate, 1-methyl-1-propyl piperidinium trifluoroacetate, 1-methyl-1-propyl piperidinium trifluoromethanesulfonate, 1-methyl-1-propyl piperidinium tetrafluroborate, 1-methyl-l- propyl piperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propyl piperidinium hexafluorophosphate, 1-methyl-1-propyl piperidinium FeCI ₄ ^_ , tributylmethylphosphonium trifluoroacetate, tributylmethylphosphonium trifluoromethanesulfonate, tributylmethylphosphonium tetrafluroborate, tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide, tributylmethylphosphonium isobutyrate, tetraphenylphosphonium trifluoroacetate, tetraphenylphosphonium trifluoromethanesulfonate, tetraphenylphosphonium tetrafluroborate, tetraphenylphosphonium bis(trifluoromethanesulfonyl)imide, tetraphenylphosphonium isobutyrate, tetraphenylphosphonium hexafluorophosphate, tetraphenylphosphonium FeCI ₄ ‘.

Preferably, the ionic liquid is elected from 1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1,1,3,3-tetramethyl guanidinium trifluoromethanesulfonate, 1,1,3,3-tetramethyl guanidinium tetrafluroborate, 1,1,3,3-tetramethyl guanidinium bis(trifluoromethanesulfonyl)imide, 1,1,3,3- tetramethyl guanidinium methylsulfate, 1,1,3,3-tetramethyl guanidinium isobutyrate, 1,1,3,3- tetramethyl guanidinium hexafluorophosphate, 1,1,3,3-tetramethyl guanidinium FeCI ₄ ‘, 1- methyl-l-propyl piperidinium trifluoroacetate, 1-methyl-1-propyl piperidinium trifluoromethanesulfonate, 1-methyl-1-propyl piperidinium tetrafluroborate, 1-methyl-1- propyl piperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propyl piperidinium hexafluorophosphate, 1-methyl-1-propyl piperidinium FeCI ₄ ^_ , tetraphenylphosphonium trifluoroacetate, tetraphenylphosphonium trifluoromethanesulfonate, tetraphenylphosphonium tetrafluroborate, tetraphenylphosphonium bis(trifluoromethanesulfonyl)imide, tetraphenylphosphonium isobutyrate, tetraphenylphosphonium hexafluorophosphate, tetraphenylphosphonium FeCI ₄ ^_ .

Even more preferably, the ionic liquid is selected from 1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1,1,3,3-tetramethyl guanidinium bis(trifluoromethanesulfonyl)imide, 1,1,3,3- tetramethyl guanidinium FeCI ₄ ^_ , 1-methyl-1-propyl piperidinium trifluoroacetate, 1-methyl-1- propyl piperidinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propyl piperidinium FeCI ₄ ^_ , tetraphenylphosphonium bis(trifluoromethanesulfonyl)imide and tetraphenylphosphonium FeCI ₄ ‘.

Even most preferably, the ionic liquid is selected from 1,1,3,3-tetramethyl guanidinium trifluoroacetate, 1-methyl-1-propyl piperidinium bis(trifluoromethanesulfonyl)imide and tetraphenylphosphonium FeCI ₄ ‘, and even most preferably is 1,1,3,3-tetramethyl guanidinium trifluoroacetate or 1-methyl-1-propyl piperidinium bis(trifluoromethanesulfonyl)imide. The catalytic system used in the present invention also includes one or more metal oxides. Said metal oxides are oxides of one single metal or of two or more different metals. Each metal may be present in one single oxidation state or in different oxidation states.

In a particular embodiment, said metal or metals are transition metals selected from the first, second and third raw of the groups 7 to 11 of the periodic table.

The metal oxides may contain one transition metal as defined above and may as well contain two or more different transition metals, wherein each transition metal may be present in one single or in different oxidation states.

In a preferred embodiment, the metal or metals of the metal oxide are selected from Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au, more preferably from Mn, Fe, Co, Ni and Cu.

Exemplary transition metal oxides includes oxides of manganese, iron, cobalt, nickel, copper and silver, and mixed oxides of iron and another transition metal such as cobalt, copper, nickel, or manganese, mixed oxides of manganese and another transition metal such as cobalt, nickel, or copper, and mixed oxides containing nickel and cobalt.

As regards structural types, spinel type oxides, ilmenite type oxides, and perovskite type oxides may be used.

Exemplary metal oxides includes manganese (IV) oxide (MnO2), cobalt (II, III) oxide (CO3O4), chromium (VI) oxide (CrO3), silver (I) oxide (Ag2O), Fe (II, III) oxide (FeaO4), nickel (II) oxide (NiO) and copper (II) oxide (CuO), as well as spinel type mixed metal oxides like cobalt iron oxide (COxFea-xO4, with 0 less than or equal to x less than or equal to 3), such as CoFe ₂ O ₄ , Col.5Fe1.5O4, and Co2FeO ₄ , copper iron oxide (Cu _x Fe3-xO ₄ , with 0 less than or equal to x less than or equal to 3), such as CuFe2O ₄ , nickel iron oxides (Ni _x Fe3-xO ₄ , with 0 less than or equal to x less than or equal to 3), manganese iron oxides (Mn _x Fe3-xO ₄ , with 0 less than or equal to x less than or equal to 3), copper manganese oxides such as Cu1.5Mn1.5O4, cobalt manganese oxides such as Co2MnO ₄ , nickel cobalt oxides such as NiCo2O ₄ , as well as ilmenite type oxides like nickel manganese oxides such as NiMnO3.

In a preferred embodiment, the metal oxide is selected from Fe (II, III) oxide, such as Fe ₃ O ₄ , and nickel oxide NiO.

In a preferred embodiment of the present invention, the metal oxide or metal oxides comprised in the catalytic system used in the present invention are in the form of nanoparticles. Thus, in a particular embodiment, the catalytic system used in the present invention comprises one or more ionic liquids and metal oxide nanoparticles wherein the metal of the metal oxide is a single metal or a mixture of metals.

In a preferred embodiment, the metal or metals are those transition metal as defined above.

As used herein the term "metal oxide nanoparticles" refers to metal oxide nanoparticles having an average particle size in the nanometer scale, typically ranging between 1 nm to 100, preferably from 1 nm to 50 nm, and more preferably between 1 nm to 20 nm.

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 or, if a crystalline phase is present, X-ray diffraction (XRD) (usually combined with image analysis). The values of particle size obtained by the techniques known in the art are in agreement within the experimental error.

A particle size in nanometer scale refers to populations of nanoparticles having d(0.9) values of 100 nm or less.

In the present invention, the average particle size is determined by transmission electron microscopy (TEM), using a FEI Tecnai G2 F20 S-TWIN, which allows working with electron energies up to 200 kV, obtaining images with atomic resolution.

The catalytic system used in the present invention can be prepared by dispersing the metal oxide or the preformed metal oxide nanoparticles in the ionic liquid or by preparing the metal oxide nanoparticles in situ in the ionic liquids. For example, the metal oxide, preferably in the form of nanoparticles, is added to the ionic liquid with continuous stirring, either at room temperature or at an elevated temperature, such as from 50 °C to 70 °C achieved for example by heating the ionic liquid using a hot plate. A dispersion is thus formed in which the metal oxide, preferably in the form of nanoparticles, is evenly and homogeneously distributed throughout the ionic liquid. Alternatively, when the metal oxide is in the form of nanoparticles, these can also be prepared in situ by dissolving the metal precursor in the ionic liquid whose role is either as nanoparticle stabilizer and/or as reducing agent. The nanoparticles can be also prepared according to procedures known in the art, such as those described in J. Am. Chem. Soc. 2004, 126 (1), 273-279 and Micromachines (Basel) 2021, 12 (10), 1168.

The catalytic system may comprise up to 15 wt.% of metal oxide with respect to the total weight of ionic liquid, preferably up 10 wt.%, even more preferably up to 5 wt.%, and most preferably from 0.5 to 5 wt.% with respect to the total weight of ionic liquid.

In another preferred embodiment, the weight ratio between the ionic liquid and the metal oxide ranges from 200/1 to 50/1, more preferably from 150/1 to 75/1.

As mentioned above, the catalytic system used in the present invention allows a rapid and efficient production of clean hydrogen, along with a valuable carbon material, as the sole products, from plastic materials.

Furthermore, an additional aspect of the invention relates to a process for the production of hydrogen and hydrogen-depleted co-products, said process comprises: a) dissolving or dispersing a plastic material in a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, and wherein the metal of the metal oxide is a single metal or a mixture of metals, and b) exposing the resulting dissolution or dispersion obtained in step a) to an electromagnetic radiation.

The term hydrogen-depleted co-products refers to the reaction product, other than hydrogen, and that mainly contains carbon species as a result of the decomposition of the plastic material.

In the first step of the process of the invention, a plastic material is dissolved or dispersed in the catalytic system comprising the ionic liquid(s) and metal oxide(s). It is the ionic liquid present in the catalytic system that allows the dissolution or dispersion of the plastic material.

As used herein the term "plastic material" refers to a solid material which comprises one or more thermoplastic or thermosetting polymers. Suitably, the plastic material essentially consists of one or more thermoplastic or thermosetting polymers.

In an embodiment, said plastic material comprises/essentially consists of/consists of one or more thermoplastic polymers. Suitably, the plastic material essentially consists of one or more thermoplastic polymers. 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. Suitable thermoplastic polymers used herein have a melting temperature from about 60 °C to about 300 °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, polyalkylene oxides, 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).

As used herein the term "thermosetting polymer" refers to a polymer which is irreversibly cured and cannot be reworked upon reheating. Suitably, the thermosetting polymer is one commonly comprised in commercial plastic products. Examples of thermosetting polymers are phenolic resins, epoxi resins, unsaturated polyesters, ureas and melamines.

In an embodiment, the plastic material comprises at least about 90 wt.% one or more thermoplastic and thermosetting polymers. In another embodiment, the plastic material comprises at least 90 wt.% of one or more thermoplastic polymers as defined above.

In an embodiment, the plastic material comprises/essentially consists/consists of one or more of polyethylene, polypropylene and polystyrene. Suitably, the plastic material comprises at least 90 wt. % of one or more of polyethylene, polypropylene, polystyrene or mixtures thereof.

In a preferred embodiment, the plastic material is a thermoplastic polymer. More preferably, the plastic material is polyethylene, polypropylene, polystyrene or mixtures thereof.

In an embodiment, the plastic material is plastic waste which may be a mixture of various plastics. In a preferred embodiment, the plastic waste comprises a thermoplastic material as those defined above, and more preferably the waste plastic comprises polyethylene, polypropylene, polystyrene or mixtures thereof. In an embodiment, the plastic material consists essentially of shredded plastic waste.

In this particular case, the process of the invention can also be considered as a process for recycling plastic material, while producing hydrogen as well as a hydrogen-depleted co- products. Thus, the present invention also provides a process of recycling plastic waste to produce hydrogen and hydrogen-depleted co-products, said process comprises: a) dissolving or dispersing a plastic material in a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, and wherein the metal of the metal oxide is a single metal or a mixture of metals, and b) exposing the resulting dissolution or dispersion obtained in step a) to an electromagnetic radiation.

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, plastic waste may be recycled into a range of materials including hydrogen and hydrogen-depleted co-products.

Step a) of each of the above processes can be made by dissolving or dispersing the abovereferred plastic material in the previously prepared catalytic system as defined above. Alternatively, the plastic material is first dissolved or dispersed in the ionic liquid or ionic liquids and, subsequently, the metal oxide or metal oxides are dispersed in the formed dissolution or dispersion. Preferably, nanoparticles of metal oxide are dispersed in the formed dissolution or dispersion.

In an embodiment, the plastic material may be processed by any method known in the prior art prior to be dissolved or dispersed in powdered or pelleted form.

In a particular embodiment, the weight proportion of plastic material relative to the total weight of ionic liquid(s) ranges from 1 to 10 wt.%, preferably from 2 to 7 wt.%, more preferably is 5 wt.%.

In another particular embodiment, the weight proportion of plastic material relative to the total weight of catalytic system (including the ionic liquid(s) and the metal oxide(s)) ranges from 1 to 10 wt.%, preferably from 2 to 7 wt.%, more preferably is 5 wt.%.

In another particular embodiment, the weight proportion of metal oxide o metal oxides relative to the total weight of plastic material ranges from 10 to 25 wt.%, preferably from 10 to 20 wt.%. Step b) of each of the above processes implies exposing the dissolution or dispersion obtained in step a) to an electromagnetic radiation.

In an embodiment, said dissolution or dispersion is firstly purged under intert atmosphere in order to remove the oxygen.

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, nitrogen, helium or a mixture thereof. In one embodiment, the inert gas is argon. In another embodiment, the inert gas is nitrogen.

Thus, the process may comprise purging the dissolution or dispersion with an inert gas or a mixture of inert gases prior to exposing said dissolution or dispersion to the electromagnetic radiation.

In an embodiment, step b) of each the above processes is carried out in an atmosphere substantially free of oxygen, preferably in an atmosphere free of oxygen. In another embodiment, step b) comprises exposing the composition to electromagnetic radiation in an atmosphere substantially free of oxygen, preferably free of oxygen.

In step b) of each of the above processes, the dissolution or dispersion containing the plastic material is exposed to electromagnetic radiation in the presence of the catalytic system in order to effect, or activate, the decomposition of the polymers to produce hydrogen and hydrogen- depleted co-products. Said decomposition is a catalytic decomposition in which the electromagnetic radiation is responsible for, or at least contribute to, effecting or activating said decomposition to produce hydrogen and hydrogen-depleted co-products.

Said step b) may be carried out at temperature and pressure conditions which range from very low to very high temperatures and pressures, provided that the ionic liquid(s) is/are maintained in liquid state. However, in a preferred embodiment, the step of exposing the dissolution or dispersion to the electromagnetic radiation is carried out at temperatures and pressures that are at or relatively close to standard conditions.

The process may for instance comprise exposing the dissolution or dispersion to the electromagnetic radiation, at a temperature of from -10 °C to 350 °C, preferably from 1 °C to 300 °C, more preferably from 10°C to 250 °C. Additionally, the process may comprise exposing the dissolution or dispersion to the electromagnetic radiation, at a pressure of from 0.1 bar to 10 bar, preferably at a pressure of from 0.5 to 5 bar, more preferably from 0.5 bar to 2 bar. In an embodiment, it comprises exposing the dissolution or dispersion to the electromagnetic radiation, at a temperature of from 10 °C to 250 °C and at a pressure of from 0.5 bar to 2 bar.

Typically, the process is completed in about 10 seconds to about 3 hours, preferably in 30 seconds to about 1 hour, more preferably in 1 minute to about 10 minutes.

The electromagnetic radiation that is employed in step b) of the process of the invention, in order to recycle plastics or to produce hydrogen from the 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 dissolution or dispersion, including radio frequencies, microwave frequencies, millimetre waves, infrared and UV. However, in a preferred embodiment, the radiation is microwave frequency radiation.

The term "microwave" is intended to have is generally accepted meaning, namely covering electromagnetic radiation of a frequency in the range of 300 MHz (100 cm) to 300 GHz (0.1 cm). However, preferably, microwave radiation of a frequency in the range of 500 MHz to 100 GHz is used to assist the chemical transformation of the plastic material into hydrogen and hydrogen- depleted co-products.

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.

The power which the electromagnetic radiation needs to deliver to the composition, in order to effect the recycling of plastic/produce hydrogen, will vary, according to, for instance, the particular starting plastic material or the particular catalytic system employed. The skilled person, however, is readily able to determine a level of incident power which is suitable for effecting the decomposition of a particular plastic material.

The process of the invention may for example comprise exposing the dissolution or dispersion containing the plastic material and the catalytic system to electromagnetic radiation which delivers a power, per cubic centimetre of the dissolution or dispersion, of at least 1 Watt, preferably from about 1 Watt to about 5000 Watts per cubic centimetre of the dissolution or dispersion, more preferably from 1 Watts to about 1000 Watts per cubic centimetre of the dissolution or dispersion, more preferably from 100 Watts to about 1000 Watts.

In some embodiments, exposing the dissolution or dispersion to the electromagnetic radiation causes the composition to be heated. Different mechanisms involved in electromagnetic heating may cause enhanced reactions and new reaction pathways. In one embodiment, step b) of the process of the invention comprises heating said composition by exposing the composition to the electromagnetic radiation, preferably to microwave radiation.

In a particular embodiment, the electromagnetic radiation causes the dissolution or dispersion to reach a temperature higher than 80 °C, preferably higher than 100 °C, more preferably from 120 °C to 350 °C, even more preferably from 150 to 300 °C.

A further aspect of the invention refers to a catalytic system comprising one or more ionic liquids and one or more metal oxides, wherein the ionic liquid comprises an organic cation and an organic or inorganic anion, wherein the metal of the metal oxide is a single metal or a mixture of metals, and wherein the one or more metal oxides are in the form of nanoparticles.

In a particular embodiment, the particle size of the nanoparticles ranges between 1 and 100 nm, preferably from 1 nm to 50 nm, and more preferably between 1 nm to 20 nm, as measured by transmission electron microscopy (TEM) as described in the examples.

All the technical features of the catalytic system used in the process to produce hydrogen from decomposition of plastic materials hydrogen of the invention as described above apply equally to the catalytic system of the invention where the one or more metal oxides are in the form of nanoparticles.

Example 1.

14 wt.% of polypropylene (PP) was dissolved in a catalytic system consisting of 3 g of 1-methyl- 1-propylpiperidinium bis(trifluoromethanesulfonyl)imide ionic liquid (commercially available) and 0.025 g of Fe ₃ O ₄ . The Fe3O4 was in the form of nanoparticles with an average particle size of 4 nm.

The nanoparticles of Fe3O4 were prepared according to the following procedure: 2 mmol of iron(lll) acetylacetonate, 10 mmol of 1,2-hexadecanediol, 6 mmol of oleic acid, 6 mmol of oleylamine, were dispersed in 20 mL of phenyl ether magnetically stirred under a flow of nitrogen. The mixture was heated to 200 °C for 30 min and then, heated to reflux (265 °C) for another 30 min. The black-brown mixture was cooled to room temperature and under ambient conditions. 40 mL of ethanol was added to the mixture and the black material was precipitated and separated via centrifugation. Then, the sample was dissolved in hexane in the presence of 0.05 mL of oleic acid and 0.05 mL of oleylamine and centrifugated (6000 rpm, 10 min) to remove any undispersed residue. The product, 4 nm Fe3O4 nanoparticles, was then precipitated with ethanol and centrifuged (6000 rpm, 10 min) to remove the solvent.

The morphology and particle size were studied by scanning electron microscopy (SEM), using a QUANTA 200 FEG (FEI Company, Hillsboro, OR, USA) microscope operated in high vacuum mode at 5 and 10 kV equipped with a secondary electron detector (ETD). Complementary measurements were carried out by transmission electron microscopy (TEM), using a FEI Tecnai G2 F20 S-TWIN, which allows working with electron energies up to 200 kV, obtaining images with atomic resolution.

The mixture of polypropylene, the catalytic system and Fe3O4 nanoparticles was placed in a quartz tube and then purged under inert atmosphere at a flow rate of 50 ml/min for 15 minutes. Then, it was subjected to microwave irradiation at a maximum power of 1000W reaching a final temperature of 275 °C, during 15 minutes. The gases generated were analysed continuously by gas chromatography. Once the reaction was finished, the carbon material generated was separated and analysed by XPS and nitrogen physisorption.

Efficiencies: Hydrogen purity > 90 %; Yield > 85 %; Hydrogen selectivity > 95 %.

Example 2.

2 wt.% polypropylene was dissolved in a catalytic system consisting of 50 g tetramethylguanadinium trifluoroacetate ionic liquid (commercially available) and 0.5 g NiO. The NiO is in the form of nanoparticles with an average particle size of 40 nm.

The nanoparticles of NiO were prepared according to the following procedure: Nickel (II) acetate tetrahydrate (Ni(CH ₃ CO2)2-4H ₂ O) was dissolved into ethanol, and 2.0 M ammonia (NH ₄ OH) was added dropwise into the solution to precipitate a precursor of NiO nanoparticles. The precipitate was then centrifugated and dried at 70 °C. The resulting nickel hydroxide nanoparticles were treated at 400 °C to obtain the NiO nanoparticles. The morphology and particle size were studied by scanning electron microscopy (SEM), using a QUANTA 200 FEG (FEI Company, Hillsboro, OR, USA) microscope operated in high vacuum mode at 5 and 10 kV equipped with a secondary electron detector (ETD). Complementary measurements were carried out by transmission electron microscopy (TEM), using a FEI Tecnai G2 F20 S-TWIN, which allows working with electron energies up to 200 kV, obtaining images with atomic resolution.

The mixture was placed in a quartz tube and then purged under inert atmosphere at a flow rate of 50 ml/min for 15 minutes. It was then subjected to microwave irradiation at a power of 1000 W for 5 minutes reaching a temperature of 300 °C. The gases generated were analysed continuously by gas chromatography. Once the reaction was finished, the carbon material generated was separated and analysed by XPS and nitrogen physisorption.

Efficiencies: Hydrogen purity > 90 %; Yield > 85 %; Hydrogen selectivity > 95 %.