WO2025026689A1

WO2025026689A1 - Synthesis of hydrogen carrier compounds

Info

Publication number: WO2025026689A1
Application number: PCT/EP2024/069661
Authority: WO
Inventors: César Arturo MASSE DE LA HUERTA; Remy Benoit; Alexander Cruickshank; Vincent LOME
Original assignee: Hysilabs Sas
Priority date: 2023-07-28
Filing date: 2024-07-11
Publication date: 2025-02-06

Abstract

A method of synthesising a siloxane hydrogen carrier compound using a thermal plasma reactor. The method comprises introducing silica or a silicate compound, or a mixture thereof, into the reaction chamber of a plasma reactor, generating a thermal plasma in the reaction chamber, and contacting the silica or a silicate compound or mixture thereof with the thermal plasma in the presence of hydrogen or a hydrogen-containing gas mixture.

Description

Synthesis of hydrogen carrier compounds The present invention relates to a method of synthesising siloxane hydrogen carrier compounds. In particular, the present invention provides a method of synthesising siloxane hydrogen carrier compounds using a plasma reactor. The ability to store, transport and release hydrogen in a safe, convenient, and environmentally friendly manner and to produce and store hydrogen efficiently, economically and safely, are main challenges to be overcome in order to democratize the use of hydrogen as an energy vector. There are typically six routes for hydrogen delivery: it can be transported as a gas by pipeline, it can be produced on site, it can be transported as a compressed gas in tube trailers (for example as disclosed in WO2013/109918 (A1)), it can be transported as a condensed liquid in cryogenic trucks (for example as disclosed in WO2011/141287 (A1)), it can be stored in a solid-state hydrogen carrier material and released on-site (for example as disclosed in WO2009/080986 (A2)), and stored in a liquid-state hydrogen carrier material and released on-site. Hydrogen can be produced on-site by two means. It can be produced on site by one process and directly consumed in another process which is defined as captive hydrogen. The other means of on-site production is by water electrolysis, which produces hydrogen from water and electricity. This can be considered as an environmentally friendly process if the electricity used for electrolysis is derived from a renewable energy source. In addition to these delivery solutions, alternative solutions utilising hydrogen carrier materials are being developed. Hydrogen carriers are either solid-state or liquid-state materials that have the ability to store hydrogen and release it when needed. They bring advantages either for transport or storage, compared to incumbent solutions. Solid-state carriers include metallic hydrides. These enable the uptake of hydrogen by adsorption onto metal particles, resulting in metal hydrides. Among these, magnesium hydride is stable at low pressure and standard temperature, making it convenient to both transport and store. When needed, the material is heated to release the hydrogen gas. Solid-state solutions have been identified as best suited for same-site reversible processes of energy storage from renewable energies. However, handling solid materials is not as convenient as handling gas or liquid ones. Liquid hydrogen carrier materials can be any liquid-state material able to release hydrogen under specific conditions. The class of Liquid Organic Hydrogen Carriers (LOHC) is the most represented among the liquid hydrogen carriers. Hydrogenation, which is a catalytic reaction requiring energy in the form of heat, chemically bonds hydrogen to the liquid organic carrier. Typically, the carrier is an unsaturated and/or aromatic hydrocarbon such as toluene which is reacted with hydrogen to produce the corresponding saturated hydrocarbon. This may be transported in a liquid-sate at standard temperature and pressure, for example as described in WO2014/082801 (A1) or WO2015/146170 (A1). Although the amount of hydrogen to be stored in LOHC depends on the yield of the hydrogenation process, it can be up to 7.2% mass of hydrogen contained per mass of liquid carrier. Hydrogen is released from the saturated hydrocarbons by dehydrogenation. Dehydrogenation is an endothermic reaction and so additional energy in the form of heat (above 300°C typically) is needed to liberate hydrogen. In order to produce on-demand hydrogen, heat may be produced from grid electricity (without control on its origin and on its impact on the environment) or heat may be retrieved by burning a part of the organic carrier. Patent applications WO2010070001 (A1) and EP2206679 (A1) relate to a method for producing hydrogen comprising the steps consisting in a) reacting a compound (C) comprising one or more groups Si-H with a fluoride ions source, thereby forming hydrogen and a by-product (C1); and b) recovering the obtained hydrogen. All the examples use silane compounds as hydrogen carriers; except for polymethylhydrosiloxane (“PHMS”) in examples 1-2 and tetramethyldisiloxane in example 8. Patent application WO2011098614 (A1) relates to a method for producing hydrogen comprising the steps of i) contacting a compound (C) comprising one or more groups Si-H with a phosphorous based catalyst in the presence of a base in water as solvent, thereby forming hydrogen and a by-product (C1) without requiring any energy input (e.g. heat, electrical power,

and ii) recovering the obtained hydrogen. All the examples use silane compounds as hydrogen carriers; tetramethyldisiloxane is the only siloxane containing compound recited in the lists of potential hydrogen carrier. WO2011098614 (A1) also discloses a step c) of recycling the obtained by-product (C1) with an acyl halide and contacting the obtained product with a metal hydride, thereby regenerating compound (C), wherein the acyl halide is CH3C(=O)Cl and the metal hydride is LiAlH4. Patent application WO2010094785 (A1) relates to a method for producing hydrogen comprising the steps of i) contacting a compound (C) comprising one or more groups Si-H with an amine based catalyst in a solvent selected from an alcohol or an aqueous solution, thereby forming hydrogen and a by-product (C1) without requiring any energy input (e.g. heat, electrical power etc…); and ii) recovering the obtained hydrogen. Most of the examples use silane compounds as hydrogen carriers; except for polymethylhydrosiloxane (“PHMS”) in example 12 and tetramethyldisiloxane in example 16. WO2010094785 (A1) also discloses a step c) of recycling the obtained by-product (C1) with an acyl halide and contacting the obtained product with a metal hydride, thereby regenerating compound (C), wherein the acyl halide is CH3C(=O)Cl and the metal hydride is LiAlH4. Whilst WO2010070001 (A1), EP2206679 (A1), WO2011098614 (A1) and WO2010094785 (A1) already represent a breakthrough in the field of hydrogen-based carrier system that releases hydrogen on-demand, said techniques would still benefit from improved efficiency, performance, and cost effectiveness. In addition, the overall regeneration method of the hydrogen-based carrier according to both WO2011098614 (A1) and WO2010094785 (A1) requires the use of the expensive LiAlH4 reducing agent leading to aluminium oxide by-products, and the retreatment process is energy-consuming as a lot of electricity is needed for the electrolysis step. Accordingly, the process is polluting, and releases carbon dioxide (CO₂), carbon monoxide (CO), fluorinated effluents and polycyclic aromatic hydrocarbons (PAH). There is thus still some progress to be made in order to develop a more environmentally friendly and carbon-free regeneration method applicable to hydrogen-based carrier materials. Further improvements in efficiency, performance, and cost effectiveness of such clean energy vectors are needed for a variety of applications, such as hydrogen delivery and hydrogen infrastructure building. There remains a need for improvements which allow greater amounts of hydrogen to be transported with enhanced efficiency, performance and that are cost effective. There remains a critical need for environmentally friendly liquid- state hydrogen carriers that are able to release hydrogen on-demand without the need for additional energy. In addition, there remains a need for an integrated and clean process wherein hydrogen carriers can not only be used as a valuable hydrogen source but also be produced without requiring carbon containing reactant and/or without carbon emissions. WO2021/084044 (A1) and WO2021/084046 (A1) describe hydrogen carrier compounds and a process for their manufacture which address some of the disadvantages discussed hereinabove. The compounds are liquid siloxane compounds produced from silica or a silicate compound in a process which requires only hydrogen and/or water and/or silicon as additional reagents. The compounds do not contain carbon and their production does not produce substantial carbon emissions. Although representing a significant advance in the field of hydrogen carrier compounds, the synthetic process described in WO2021/084046 (A1) is a multi-stage process requiring a number of sequential steps to be performed. It would be advantageous to be able to provide hydrogen carrier compounds such as those described in WO2021/084046 (A1) via an alternative, preferably simpler route. The use of plasmas in materials production, treatment and synthesis is known in the art. For example, plasmas are widely used in the semi-conductor industry for surface etching, chemical vapour deposition (CVD) and other surface modifications applications. They also find use in medical applications, such as for the decontamination of surfaces; surface modification and thin film deposition in polymer applications; as well as for waste treatment and analytical applications. Inductively coupled radio frequency (RF-ICP) plasma operations and microwave ICP (MW-ICP) have advantages over other plasma sources, for example, over direct-current (DC) plasmas, as they permit high temperatures and plasma densities and do not need to use consuming and potentially contaminating electrodes. RF-ICP and MW-ICP plasma reactors (or torches) are commercially available and have been used extensively in the manufacture of nano-particulate materials. Nano-particles of defined size and morphology can be produced by using the plasma to vapourise precursor materials which subsequently condense. High purity metal particles, non-metal particles such as silicon, and compounds such as tungsten carbide can be produced by the selection of suitable precursor materials. The present invention is based upon the discovery that a particularly configured plasma reactor and process can be used in the synthesis of useful materials, and in particular, in the synthesis of siloxane hydrogen carrier compounds such as those described in WO2021/084044 (A1) and WO2021/084046 (A1). Invention Accordingly in a first aspect, the present invention provides a method of synthesising a siloxane hydrogen carrier compound, the method comprising introducing silica or a silicate compound, or a mixture thereof, into a reaction chamber of a plasma reactor, generating a thermal plasma in the reaction chamber, and contacting the silica or a silicate compound or mixture thereof with the thermal plasma in the presence of hydrogen or a hydrogen- containing gas mixture. Brief description of the figures Figure 1 is a schematic representation of the temperature gradient in the plasma reactor. Figure 2 is a UV-visible emission spectrum of mixed Argon-Hydrogen gas plasma, where quartz (SiO₂) is introduced into the plasma. Figure 3 represents the Infrared spectroscopy of the synthesised product, and pure quartz powder (SiO2). Figure 4 is a graphic representation of the 29Si NMR spectroscopy of the reaction product. Preferably, the siloxane hydrogen carrier compounds produced using the method of the present invention comprise: (I) one or more linear siloxane hydrogen carrier compounds of formula (I):

wherein n is an integer of 1 or more, wherein radicals R and R’ do not contain carbon, and wherein R and R’ comprise Si and hydrogen and/or oxygen, (II) one or more cyclic siloxane compounds having the formula (II):

wherein n is an integer of 1 or more, or; any mixture of compounds having formulae (I) or (II). Preferably in formula (I), n is 2 or more, more preferably 3 or more, for example, 4 or more. Preferably n is less than or equal to 500, for example less than or equal to 50. Preferably in formula (II), n is 2 or more, more preferably 3 or more, for example, 4 or more Preferably, n is less than or equal to 500, for example less than or equal to 32, for example less than or equal to 17. In an embodiment, other reactants may be introduced into the reaction chamber and contacted with the plasma together with the silica or silicate compound. For example, elemental silicon may be used to promote the disproportionation of silica. The hydrogen-containing gas mixture may be a mixture of hydrogen and an inert gas such as a noble gas, preferably argon. In a preferred embodiment, the hydrogen-containing gas mixture may further comprise water. As detailed below, this humidified gas mixture may promote favourable reactions, such as the disproportionation of silica. Preferably, the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the magnitudes of one, two, three or the four properties amongst the temperature, the pressure, the density of ionized species and the particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone. In another embodiment, the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the lower value of the temperature, and/or the pressure, and/or the particle velocity, and/or the density of ionized species, decreases from the first zone, through the second zone, to the third zone; said decrease preferably occurs for at least two, or at least three, or for example the four properties, i.e. the temperature, the pressure, the density of ionized species and the particle velocity all together. Preferably the geometry of the plasma reactor is such that the second reaction zone is downstream of the first reaction zone and the third reaction zone is downstream of the second reaction zone. It is presently thought that the gradient which exists for the magnitudes of the temperature, pressure, density of ionized species and particle velocity in the chamber is important to the synthesis of the final hydrogen carrier compound products. In the first reaction zone, which is closest to the thermal plasma, reaction conditions are at the most severe. This ionizes the reactants thereby promoting certain reactions above others. Further away from the plasma, the magnitudes of the temperature, pressure, density of ionized species and particle velocity decay quickly so in the second reaction zone, other chemical reactions dominate. Further still away from the plasma, in the third reaction zone, reaction conditions are less severe and so promote yet further chemical reactions. It will be understood that the first, second and third reaction zones of the reaction chamber should not be thought of as distinct in the sense that there is a clear line of demarcation between them. Rather, there exists a gradient from the first reaction zone through the second reaction zone to the third reaction zone along which the above-mentioned properties all decrease as a function of distance from the plasma. As will be explained in more detail herein, it is presently thought that this gradient of physical properties allows the various chemical reactions to proceed in series thereby providing the siloxane hydrogen carrier compounds in a continuous and single-step manner. The first reaction zone corresponds to the hottest zone of the thermal plasma. This means that the magnitudes of the temperature, pressure, density of ionized species and particle velocity are at their highest. For example, temperatures of between 2000K and 20000K, preferably between 4500K and 16000K, for example, between 7000K and 12000K, may exist. Preferably in the first reaction zone, the silica or silicate compound and the hydrogen are substantially transformed into corresponding ions, radicals and electrons. Reaction conditions in the first reaction zone are severe so multiple and complex reactions are to be expected. Without wishing to be bound by theory, it is presently thought that at least some of the following transformations occur in the first reaction zone: SiO2 ↔ SiO + 2 + e- SiO + 2 ↔ Si+ + O2 SiO + ↔ Si + ▪ 2 O + O SiO + e- ↔ - - - 2 SiO2 SiO2 ↔ Si + O2 + e SiO₂- ↔ Si + O₂- SiO₂- ↔ SiO + O- SiO₂- ↔ SiO + O▪ + e- SiO ↔ Si ▪ - + 2- 2 O + O SiO + e ↔ Si + O SiO ↔ Si + O▪ SiO ↔ Si+ + O- SiO₂ ↔ SiO+ + e- + O▪ SiO+ + e- ↔ SiO SiO₂ ↔ SiO+ + O- SiO+ ↔ Si+ + O▪ SiO + e- - 2 ↔ SiO + O SiO - ▪ - 2 + e ↔ SiO + O + e SiO + e- ↔ SiO + O- 2 SiO₂ + e- ↔ SiO- + O▪ SiO- ↔ Si + O- SiO- ↔ Si + O▪ + e- SiO- ↔ Si+ + O2- SiO- ↔ Si+ + O- + e- SiO₂ ↔ Si + 2O▪ Si + e- ↔ Si- 2O▪ ↔ O₂ O▪ + e- ↔ O- 2O- ↔ O2- 2 SiO2 ↔ Si + O2 SiO₂ ↔ Si+ + O₂ + e- Si+ + e- ↔ Si SiO₂ ↔ Si+ + O₂- SiO₂ + e- ↔ Si + O₂ + e- SiO + e- - 2 ↔ Si + O2 SiO - + 2- 2 + e ↔ Si + O2 H ▪ 2 ↔ 2H H₂ ↔ H₂+ + e- H ↔ 2H+ - 2 + 2e H + e- ↔ H▪ - 2 + H H ▪ + - 2 ↔ H + H + e H▪ + e- ↔ H- H▪ ↔ H+ + e- H₂+ ↔ H▪ + H+ H ▪ ▪ ▪ ▪ ▪ 2O ↔ H + HO HO ↔ H + O H O + e- ↔ H- ▪ ▪ - - 2 + HO HO + e ↔ HO H O + e- ↔ H▪ + HO- ▪ + - 2 H ↔ H + e H₂O ↔ H+ + HO- The mixture of ions, radicals and electrons produced in the first reaction zone are provided to the second reaction zone where the reaction conditions are such as to promote the formation of an intermediate product mixture. Preferably the reaction chamber is orientated such that the transport of the products produced in the first zone to the second zone is facilitated or aided by the flow of the hydrogen or hydrogen-containing gas mixture within the reaction chamber. Due to its distance from the hottest zone of the thermal plasma, reaction conditions in the second reaction zone are significantly less severe than those in the first reaction zone. For example, temperatures may be in the range from 9000K to 1000K, preferably between 8000K and 2000K. In an embodiment, the second reaction zone is provided with means capable of transporting oxygen ions at high temperatures. An example of such means is an oxygen permeable membrane. These are known in the art. Non-limiting examples include non-electrochemical means such as silver and silver alloys such as Ag0.05Zr; mixed metal oxide conductors, for example perovskite-type mixed metal oxides La0:6Ca0:4Co0:5Fe0:5O3δ and La0:6Ca0:4Co0:3Fe0:8O3δ (LCCF); and ionic conductors such as yttria-stabilised zirconia (YSZ). In this way, oxygen ions dissociated in the reactions above may be removed or captured. This has the effect of shifting the equilibrium to favour the formation of Si+ ions by preventing recombination (reoxidation) to re-form SiO₂. The mixture of ions, radicals and electrons produced in the first reaction zone is highly reactive and so in the second reaction zone, a complex sequence of reactions is expected to take place. Without wishing to be bound by theory, it is presently thought that at least some of the following transformations occur in the second reaction zone: 2H2 + O2 ↔ 2H2O H₂ + O▪ ↔ H₂O H▪ + O▪ ↔ HO▪ H+ + O- ↔ HO▪ H₂O ↔ H▪ + HO▪ Si + H₂O ↔ HSiOH Si + H₂O ↔ H₂SiO Si + H▪ ↔ SiH SiH + H▪ ↔ SiH2 Si + H2 ↔ SiH2 SiH2 + O▪ ↔ HSiOH SiH ▪ 2 + O ↔ H2SiO SiH₂ + H₂ ↔ H₃SiOH SiO + H2 ↔ H2SiO SiO + H2 ↔ HSiOH SiO + H ↔ HSiO HSiO + H▪↔ HSiO HSiO + H▪↔ H₂SiO SiO + H₂O ↔ H-Si(O)OH Si + HO▪ ↔ SiOH SiH + HO▪ ↔ HSiOH HSiOH ↔ H2SiO SiH + HO▪ ↔ ▪ 2 H2SiOH H2SiOH ↔ H2SiO + H SiO + HO▪ ↔ OSiOH OSiOH ↔ SiO₂ + H▪ Si + HO- ↔ HOSi- HOSi- + H₂O ↔ HOSiH + HO- SiH + HO- ↔ H(HO)Si- H(HO)Si- ↔ H₂SiO- H - 2SiO + H+ ↔ H2SiOH H SiO- + - 2 H2O ↔ H2SiOH +HO SiH + - - - - 2 HO ↔ H2(HO)Si H2(HO)Si ↔ H3SiO H₃SiO- + H+ ↔ H₃SiOH H3SiOH ↔ H2SiO + H2 It is presently thought that through the combination of reactions which take place in the first and second reaction zones, at least some of the following overall transformations take place to produce an intermediate product mixture. 1(a) Reduction of silica/silicate compound to silicon monoxide SiO₂ + H₂ → SiO + H₂O 1(b) Silicon mediated disproportionation of silica/silicate compound to silicon monoxide SiO₂ + Si → 2SiO Elemental silicon may be provided as an additional reactant introduced into the reaction chamber, for example metallurgical, photovoltaic or electronic grade silicon. The Si/SiO₂ ratio is preferably between 0.5 and 1.5, for example between 0.9 and 1.1. Preferably however, elemental silicon produced in situ through the full reduction of the silica or silicate compound via: SiO₂ + 2H₂ → Si, or via the transformations detailed above where elemental silicon is among the reaction products. 1(c) Hydrogenation and dehydrogenation of silica/silicate compound to silanone SiO₂ + 2H₂ → H₂SiO + H₂O The H2/SiO2 molar ratio is preferably between 0.1 and 1000, more preferably between 1 and 50, for example between 2 and 50. If desired, a catalyst may be used to aid the hydrogenation reaction, for example a metal, a metal immobilised on a support, an organic catalyst, or an organic complex of a metal or similar. The H₂SiO product may be formed in equilibrium with other species, for example the cis- and trans-isomers of HSi(OH). In addition to the majority product H2SiO, other compounds may also be produced, for example, SiO, HSi(O)(OH), H2Si(OH)2, H3SiOH, SiH₄ and elemental silicon. Elemental silicon, when produced, may take part in the silicon mediated disproportionation reaction 1(b). 1(d) Partial hydrogenation of silica/silicate compound to form formosilicic acid SiO₂ + H₂ → HSi(O)(OH) The H₂/SiO₂ molar ratio is preferably between 0.1 and 1000, more preferably between 1 and 50. If desired, a catalyst may be used to aid the hydrogenation reaction, for example a metal, a metal immobilised on a support, an organic catalyst, or an organic complex of a metal or similar. In addition to the majority product HSi(O)(OH), other compounds may also be produced, for example, SiO, H₂SiO, H₂Si(OH)₂, H₃SiOH, SiH₄ and elemental silicon. Elemental silicon, when produced, may take part in the silicon mediated disproportionation reaction 1(b). 1(e) Semi-hydrogenation of silica/silicate compound to form dihydroxysilane SiO2 + 2H2 → H2Si(OH)2 The H₂/SiO₂ molar ratio is preferably between 0.1 and 1000, more preferably between 1 and 50. In addition to the majority product H2Si(OH)2, other compounds may also be produced, for example, SiO, H₂SiO, HSi(O)(OH), H₃SiOH, SiH₄ and elemental silicon. Elemental silicon, when produced, may take part in the silicon mediated disproportionation reaction 1(b). 1(f) Full hydrogenation of silica/silicate compound to form silane SiO2 + 4H2 → SiH4 + 2H2O The H₂/SiO₂ molar ratio is preferably between 0.1 and 1000, more preferably between 4 and 50. If desired, a catalyst may be used to aid the hydrogenation reaction, for example a metal, a metal immobilised on a support, an organic catalyst or similar. In addition to the majority product SiH4, H2Si(OH)2, other compounds may also be produced, for example, SiO, H2SiO, HSi(O)(OH), H2Si(OH)2, H3SiOH, and elemental silicon. Elemental silicon, when produced, may take part in the silicon mediated disproportionation reaction 1(b). In the above manner, the reaction of silica and/or silicate compound with hydrogen in the first and second reaction zones of the reaction chamber produces an intermediate product mixture comprising some or all of SiO, H2SiO, HSi(O)(OH), H2Si(OH)2, H3SiOH, SiH, SiH₂ and SiH₄. The intermediate product mixture is provided to the third reaction zone where the reaction conditions are such as to promote the formation of the siloxane hydrogen carrier compound. Preferably the reaction chamber is orientated such that the transport of the intermediate product mixture produced in the second zone to the third zone is facilitated or aided by the flow of the hydrogen or hydrogen-containing gas mixture within the reaction chamber. As discussed above, the intermediate product mixture produced in the first and second reaction zones is provided to the third reaction zone. Here, the temperature, pressure, density of ionised species and particle velocity are lower compared to the other zones such that further reactions are favoured. Due to its distance from the hottest zone of the thermal plasma, reaction conditions in the third reaction zone are significantly less severe than those in the first and second reaction zones. For example, temperatures may be in the range from 5000K to 300K, preferably between 3000K and 300K. Without wishing to be bound by theory, even if said reactions mostly occur in the third reaction zone, it is presently thought that through the combination of reactions which take place in the second and third reaction zones, at least some or all of the following overall transformations take place: 2(a) hydrogenation of SiO to form silanone SiO, produced primarily by reactions 1(a) and 1(b) reacts with hydrogen: SiO + H2 → H2SiO 2(b) hydration of SiO to form formosilicic acid SiO, produced primarily by reactions 1(a) and 1(b) reacts with water: SiO + H₂O → HSi(O)(OH) The required water may be provided from the by-product of reactions 1(a), 1(c), 1(f) and 2(c). 2(c) Hydrogenation and dehydration of formosilicic acid to form silanone Formosilicic acid, produced primarily from reactions 1(d) and 2(b) reacts with hydrogen: HSi(O)(OH) + H₂ → H₂SiO + H₂O The water produced in reaction 2(c) can be employed in reaction 2(b). 2(d) Hydrogenation of formosilicic acid to form dihydroxysilane Formosilicic acid, produced primarily from reactions 1(d) and 2(b) reacts with hydrogen: HSi(O)(OH) + H2 → H2Si(OH2) 2(e) Hydration of silanone to produce dihydroxysilane Silanone, produced primarily from reaction 1(c) reacts with water: H2SiO + H2O → H2Si(OH2) The required water may be provided from the by-product of reactions 1(a), 1(c), 1(f) and 2(c). 2(f) Partial hydrolysis of silane to produce dihydroxysilane Silane, produced primarily from reaction 1(f) reacts with water: SiH4 + 2H2O → H2Si(OH2) + 2H2 The required water may be provided from the by-product of reactions 1(a), 1(c), 1(f) and 2(c). It will be understood that some or all of reactions 2(a) to 2(f) may proceed to a greater or lesser extent in the third reaction zone of the reaction chamber. The exact composition of the product mixture will vary with factors such as temperature and pressure which are a function of distance from the plasma however, it is presently thought that the product mixture will comprise some or all of silanone (H₂SiO), dihydroxysilane H₂Si(OH)₂, and silane (SiH₄). It is expected that the final reaction steps to produce the hydrogen carrier compounds of formulae (I) and (II) will take place in the regions of the third reaction zone which are furthest from the plasma. Here, temperatures and pressures are lowest which promotes the formation of the final products. Compounds of formulae (I) and (II) may be formed by the polymerisation (polyaddition) of silanone: 3(a) nH2SiO → (H2SiO)n nH2SiO + H2O → HO-[SiOH2]n-H linearWater promoted polymerization nH₂SiO + H₂O → [H₂SiO]_n cyclicWater promoted polycyclization nH₂SiO → [H₂SiO]_n cyclicPolymeric cyclisation Compounds of formulae (I) and (II) may be formed by the polycondensation of dihydroxysilane: 3(b) nH2Si(OH)2 → (H2SiO)n + nH2O Compounds of formulae (I) and (II) may be formed by the oxygen-mediated partial oxidation of silane: 3(c) nSiH4 + nO2 → (H2SiO)n + nH2O Any silica compound or silicate compound may be used in the process of the present invention provided it is in, or can be made into, a form suitable to be introduced into the reaction chamber. Preferably, the silica compound is selected from: ^ a silica compound of generic formula SiO2,xH2O, ^ [SiO2]n with n superior or equal to 2, or ^ a mixture of two or more of said silica compounds. Suitable and readily available silica compounds include sand and minerals such as zircon, jade, mica, quartz, cristobalite and the like. Due to its low cost and abundance, sand is a preferred source of silica. The silicate compound can be defined as a silicate containing compound, and/or a mixture of two or more of said silicate containing compounds. Preferably, the silicate compound is selected from: ^ a sodium or potassium silicate compound of generic formula Na_2xSiO_2+x or K_2xSiO_2+x with x being an integer comprised between 0 and 2, or ^ a silicic acid compound of generic formula [SiOx(OH)4-x]x- with x being an integer comprised between 0 and 4 or of generic formula [SiOx(OH)4-2x]n with when n=1, x=0 or 1 and when n=2, x=1/2 or 3/2, or ^ a silicate compound with a polymeric structure such as a disilicate ion of structure (Si O )6- or a macroanio 2- 6- 2- 2 7 n of generic structure [SiO3 ]n, [Si4O11 ]n or [Si2O5 ]n with n superior or equal to 2, or ^ a mixture of two or more of said silicate compounds. Siloxane hydrogen carrier compounds The term “hydrogen carrier compound” can be understood as a chemical compound able to store hydrogen, transport hydrogen and release hydrogen on demand; the characteristic of the hydrogen carrier compounds produced according to the present invention is that they can store/transport/release hydrogen without requiring any energy input (e.g. heat, electrical power

Preferably, the siloxane hydrogen carrier compounds produced using the method of the present invention comprise: one or more linear siloxane hydrogen carrier compounds of formula (I):

wherein n is an integer of 1 or more, wherein radicals R and R’ do not contain carbon, and wherein R and R’ comprise Si and hydrogen and/or oxygen, one or more cyclic siloxane compounds having the formula (II) wherein n is an integer of 1 or more, or; any mixture of compounds having formulae (I) and (II). Preferably in formula (I), n is 2 or more, more preferably 3 or more, for example, 4 or more. Preferably n is less than or equal to 500, for example less than or equal to 50. Preferably in formula (II), n is 2 or more, more preferably 3 or more, for example, 4 or more Preferably, n is less than or equal to 500, for example less than or equal to 32, for example less than or equal to 17. Such compounds provide advantages compared to their poly(hydromethyl)siloxane analogs (ROMenH_nSi_nO_nR’). As an example poly(bis(hydro))siloxane can release more than twice the amount of hydrogen gas, weight for weight, when compared to poly(hydromethyl)siloxane. Also, poly(bis(hydro))siloxane compounds exhibit a full carbon-free recyclability compared to analogs containing carbon fragments in their backbone. In an embodiment of the present invention, the above carbon-free R and R’ radicals of formula (I) are selected from -SiH₃, -SiH₂OH, -SiH(OH)₂, and -SiOH₃; preferred radicals are -SiH₃, and -SiH₂OH. Illustrative examples of linear siloxane hydrogen carrier compounds which may be synthesised according to the present invention are: H3SiOH_2nSi_nO_nSiH3, H3SiOH_2nSi_nO_nSiH2OH, H3SiOH_2nSi_nO_nSiH(OH)2, H3SiOH_2nSi_nO_nSi(OH)3, (OH)3SiOH_2nSi_nO_nSi(OH)3, (OH)3SiOH_2nSi_nO_nSiH(OH)2, (OH)3SiOH_2nSi_nO_nSiH2OH, OHH2SiOH_2nSi_nO_nSiH2OH, OHH2SiOH_2nSi_nO_nSiH(OH)2, (OH)₂HSiOH_2nSi_nO_nSiH(OH)₂. or a mixture of two or more of these compounds, with n being an integer of 1 or more, preferably 2 or more, more preferably 3 or more, for example, 4 or more. Preferably, n is less than or equal to 500, for example less than or equal to 50. Illustrative examples of cyclic siloxane compounds of formula (II) include: n_{= 1} Tri(bis(hydro)cyclosiloxane) n = 2 Tetra(bis(hydro)cyclosiloxane) Penta(bis(hydro)cyclosiloxane) n = 3

n = 4 Hexa(bis(hydro)cyclosiloxane) n = 5 Hepta(bis(hydro)cyclosiloxane) n = 6 Octa(bis(hydro)cyclosiloxane) n = 7 Nona(bis(hydro)cyclosiloxane)

n = 8 Deca(bis(hydro)cyclosiloxane) n = 9 Undeca(bis(hydro)cyclosiloxane) _{n = 10} Duodeca(bis(hydro)cyclosiloxane) n = 11 Trideca(bis(hydro)cyclosiloxane)

n = 12 Tetradeca(bis(hydro)cyclosiloxane) n = 13 Pendeca(bis(hydro)cyclosiloxane) n = 14 Hexadeca(bis(hydro)cyclosiloxane) n = 15 Heptadeca(bis(hydro)cyclosiloxane) For the above linear siloxane hydrogen carrier compounds according to formula (I), a carbon-free radical (e.g. SiH₃) chain end is selected since it presents many advantages (compared to other carbon containing chain ends such as SiMe3 for instance): - lower molecular weight and better hydrogen content allowing a better weight gravimetric efficiency of the siloxane compound, meaning a higher ratio between the weight of hydrogen carried by the compound compared to its overall molecular weight. - Straightforward and without any carbon emissions recycling of the SiH₃ chain end, when compared to SiMe3 for instance, due to the hydrolysable nature of the -SiH3 fragment, which is not the case of Si-Me bonds. - In another embodiment of the present invention, the siloxane hydrogen carrier compounds comprise a mixture of two or more of any of the above defined siloxane hydrogen carrier compounds of formulae (I) and (II). In this “mixture” embodiment, when linear siloxane hydrogen carrier compounds of formula (I) represent the main species in substance amount (in mol) in the mixture (i.e. represent more than 50 mole percent), it is advantageous to restrict the amount of cyclic siloxane hydrogen carrier compounds of formula (II) to less than 20 mole percent, for example less than 10 mole percent in the mixture; in an embodiment, more than 0.01 mole percent, or even more than 0.1 mole percent of cyclic siloxane hydrogen carrier compounds of formula (I) can advantageously be present in said mixture. The molar ratio of the linear species to cyclic species in the “mixture” can be determined by 1H NMR analysis for example. In this “mixture” embodiment, when cyclic siloxane hydrogen carrier compounds of formula (II) represent the main species in substance amount (in mol) in the mixture (i.e. represent more than 50 mole percent), it is advantageous to restrict the amount of linear siloxane hydrogen carrier compounds of formula (I) to less than 45 mole percent, for example less than 20 mole percent in the mixture; in an embodiment, more than 1.0 mole percent, or even more than 5.0 mole percent of linear siloxane hydrogen carrier compounds of formula (I) can advantageously be present in said mixture. In an embodiment, the siloxane hydrogen carrier compounds of formula (I) and of formula (II) are liquids at normal temperature and pressure (20 °C and an absolute pressure of 1.01325 × 105 Pa) and have a dynamic viscosity between 0.1 and 10000 mPa.s, for example between 0.2 and 50 mPa.s, at a temperature of 20°C and a pressure of 1.01325 × 105 Pa. Dynamic viscosity can be measured according to any appropriate method; for example, it can be determined according to the ISO 1628-1 standard. The molecular weight of the cyclic siloxane hydrogen carrier compounds of formula (II) may range from 130 to 1850 g/mol. The molecular weight of the siloxane hydrogen carrier compounds of formula (I) can be measured according to any appropriate method; for example, it can be determined by GC-MS, e.g. a GC-MS analysis performed on an Agilent GC/MSD 5975C apparatus. The number average molecular weight (Mn) and/or the molecular weight distribution (Ð) of the linear siloxane hydrogen carrier compounds of formula (I) may range from 100 to 30000 g/mol and from 1.1 to 50, respectively. The number average molecular weight and the molecular weight distribution of the linear siloxane hydrogen carrier compounds of formula (I) can be measured according to any appropriate method; for example, it can be determined according to the ISO 16014 standard. The cyclic siloxane hydrogen carrier compounds of formula (II) may present a characteristic strong and sharp absorption band between 800 and 1000 cm-1 corresponding to the Si-H bonds, when analysed by FT-IR. Preferably, the cyclic siloxane hydrogen carrier compounds of formula (II) present a characteristic strong and sharp absorption band between 850 and 950 cm-1. The siloxane hydrogen carrier compounds of formula (I) and of formula (II) may present a characteristic resonance between 4.5 and 4.9 ppm corresponding to the SiH2O units, when analysed by solvent-free, solid state 1H NMR.1H NMR analyses can be performed on any appropriate spectrometer, e.g. a 400 MHz Bruker spectrometer. The siloxane hydrogen carrier compounds of formula (I) and of formula (II) may present a characteristic resonance between -45 and -50 ppm corresponding to the SiH₂O units, when analysed by solvent-free, solid state 29Si NMR.29Si NMR analyses can be performed on any appropriate spectrometer, e.g. a 400 MHz Bruker spectrometer. The siloxane hydrogen carrier compounds of formula (I) and of formula (II) may present a refractive index between 1 and 2, for example between 1.2 and 1.5, at a temperature of 20°C and at a wavelength of 589 nm. The refractive index of the siloxane hydrogen carrier compounds can be measured according to any appropriate method; for example, it can be determined according to the ASTM D1218 standard. The siloxane hydrogen carrier compounds of formula (I) and of formula (II) may have a boiling point between 30 and 500°C, preferably between 50 and 500°C, for example between 50 and 200°C, at a pressure of 1.01325 × 105 Pa. Boiling points can be measured according to any appropriate method; for example, the ISO 918 standard. The siloxane hydrogen carrier compounds of formula (I) and of formula (II) may have a flash point between 20 and 500°C, as measured for example by ISO 3679. In an embodiment according to the present invention, the siloxane hydrogen carrier compounds comprise any mixture of two or more of the said cyclic siloxane compounds of formula (II) ; said mixture preferably comprises at least 5 mole percent of ^H2SiO ^4 , at least 20 mole percent of ^H2SiO ^5 , at least 5 mole percent of ^H2SiO ^6 and at least 40 mole percent of ^H2SiO ^7+ species (i.e. formula (II) compounds with n respectively equal to 2, 3, 4 and equal or higher to 5) relative to the sum of the moles of siloxane hydrogen carrier compounds of formula (II) in the mixture. In an embodiment according to the present invention, the siloxane hydrogen carrier compounds consist in any mixture of two or more of the said linear siloxane compounds of formula (I) ; said mixture preferably comprises at least 50 mol% of compounds of formula (I) wherein n is between 5 and 30 (i.e. having between 5 and 30 repeating units of H₂SiO) relative to the sum of the moles of siloxane hydrogen carrier compounds of formula (I) in the mixture, for example more than 80 mol%. In a second aspect, the present invention provides the use of a plasma reactor in the synthesis of siloxane hydrogen carrier compounds, the use comprising introducing silica or a silicate compound, or a mixture thereof, into a reaction chamber of the plasma reactor, generating a thermal plasma in the reaction chamber, and contacting the silica or a silicate compound or mixture thereof with the thermal plasma in the presence of hydrogen or a hydrogen-containing gas mixture. Preferably, in this second aspect, the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the magnitudes of one, two, three or the four properties amongst the temperature, the pressure, the density of ionized species and the particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone. Preferably, the plasma reactor used in the second aspect is as described hereinabove in relation to the first aspect. Preferably, the hydrogen carrier compounds synthesised in the use of the second aspect are those which are described hereinabove in relation to the first aspect. In a third aspect, the present invention provides a plasma reactor for the synthesis of siloxane hydrogen carrier compounds from silica or a silicate compound, or a mixture thereof, said plasma reactor comprising a reaction chamber wherein a thermal plasma is generated and wherein the silica or a silicate compound or mixture thereof is contacted with the thermal plasma in the presence of hydrogen or a hydrogen-containing gas mixture, said reaction chamber comprising a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the second reaction zone is downstream of the first reaction zone and the third reaction zone is downstream of the second reaction zone, and such that the magnitudes of one, two, three or the four properties amongst the temperature, the pressure, the density of ionized species and the particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone. Preferably, the plasma reactor of this third aspect is as described hereinabove in relation to the first aspect. Preferably, the hydrogen carrier compounds synthesised in the plasma reactor of this third aspect are those which are described hereinabove in relation to the first aspect. For illustrative and non-limiting purposes, examples supporting the method of synthesising a siloxane hydrogen carrier compound of the present invention are detailed in figures 1 to 4. Figure 1 is a schematic representation of the temperature gradient in the plasma reactor. As the gas flow is directed upwards, reaction zones 1, 2 and 3 are described from the bottom to the upper part of the figure. The grey scale on the right gives an indication with its white/grey/black intensity of the temperatures in the respective reaction zones. Figure 2 is a UV-visible emission spectrum of mixed Argon-Hydrogen gas plasma, where quartz (SiO₂) is introduced into the plasma. The emission lines correspond to the following species: Si I : elemental silicon Si H-β : elemental hydrogen H· Si II : singly ionised silicon Si+ H-α : elemental hydrogen H· O I : elemental oxygen O, O^▪ , O^▪▪ This spectrum was obtained using a UV-visible spectrophotometer by OceanView. The receiving optical fibre was equipped with a collimating lens, and pointed at the plasma zone directly from a distance of roughly 20 cm. The plasma was initiated and the gas flows for argon and hydrogen were regulated before adding the quartz, and the emission spectrum was captured. This spectrum is used as qualitative evidence that the desired species, namely hydrogen, silicon, and oxygen are being ionised by the plasma. Thus, the SiO2 molecule of the quartz has been dissociated into its constituent elemental atoms. The dissociation of H₂ and SiO₂ allows for the Zone 1 reactions to occur. Figure 3 represents the Infrared spectroscopy of the synthesised product, and pure quartz powder (SiO₂). IR spectroscopy is an analytical technique that allows the identification of specific kinds of chemical bonds, via their absorption of specific wavelengths of light. Of interest, the peaks around 2200 cm-1 (circled), corresponding to the vibrational modes of the Si-H chemical bond. The absence of this absorption from the quartz shows that the process allowed for the hydrogenation of the SiO₂ starting material. These absorption bands are characteristic of hydrogen siloxanes, such as described in Inorg. Chem.1983, 22, 2163-2167. Spectra were obtained by IR absorption spectroscopy of the product, which had been ground into a fine powder, and analysed as a 10% mixture in potassium bromide. Potassium bromide was used as the background reference product. Figure 4 is a graphic representation of the 29Si NMR spectroscopy of the reaction product. NMR is an analytical technique that allows the identification of the chemical environment of a given element. In this case, the bonds surrounding the silicon atoms are studied. The peak at 49.5 ppm corresponds to a H2SiO group, where there are two silane Si-H bonds present. This analysis shows that the process created Si-H₂ groups, and therefore that the starting material has been hydrogenated. The process allowed for a reaction, permitting the synthesis of hydrosiloxanes. This peak is characteristic of hydrosiloxanes, as described in Inorg. Chem.1983, 22, 2163-2167. Figure 5 is a graph showing the evolution of partial pressures over the course of silica addition into the reaction chamber of the plasma reactor. A residual gas analyser mass spectrometer was installed downstream of the third zone of the reaction chamber of the plasma reactor. Considering that mass 30 corresponds to the SiH + 2 fragment, the base peak of silane (SiH4) in mass spectrometry, the mass 30 fragment is associated with the detection of silane. Silica was inserted in a periodic manner. Upon each addition, the partial pressure of H₂ decreased while the partial pressure of H2O and SiH4 species increased. These results indicate that the following reaction had taken place downstream of the reaction chamber of the plasma reactor: SiO2 + 4H2 → SiH4 + 2H2O

Claims

Claims 1. A method of synthesising a siloxane hydrogen carrier compound, the method comprising introducing silica or a silicate compound, or a mixture thereof, into a reaction chamber of a plasma reactor, generating a thermal plasma in the reaction chamber, and contacting the silica or a silicate compound or mixture thereof with the thermal plasma in the presence of hydrogen or a hydrogen-containing gas mixture; wherein the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the magnitudes of one, two, three or the four properties amongst the temperature, the pressure, the density of ionized species and the particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone.

2. A method according to claim 1 wherein other reactants are introduced into the reaction chamber and contacted with the plasma together with the silica or silicate compound, preferably elemental silicon.

3. A method according to claim 1 or claim 2 wherein the hydrogen-containing gas mixture comprises a mixture of hydrogen and an inert gas such as a noble gas, preferably argon.

4. A method according to any preceding claim wherein the hydrogen-containing gas mixture further comprises water.

5. A method according to any preceding claim wherein the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the magnitudes of the temperature, pressure, density of ionized species and particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone.

6. A method according to any one of claims 1 to 4 wherein the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the lower value of the temperature, and/or the pressure, and/or the particle velocity, and/or the density of ionized species, decreases from the first zone, through the second zone, to the third zone.

7. A method according to any one of claims 1 to 7 wherein the second reaction zone is downstream of the first reaction zone and the third reaction zone is downstream of the second reaction zone.

8. A method according to any preceding claim wherein in the first reaction zone, the silica or silicate compound and the hydrogen are substantially transformed into corresponding ions, radicals and electrons.

9. A method according to claim 8 wherein the ions, radicals and electrons produced in the first reaction zone are provided to the second reaction zone, and wherein the reaction conditions in the second reaction zone are such as to promote the formation of an intermediate product mixture.

10. A method according to claim 9, wherein the intermediate product mixture comprises SiO, HSi(O)(OH), H2Si(OH)2, H2SiO, SiH4, H3SiOH, SiH, SiH2 or any mixture of two or more of these compounds.

11. A method according to claim 9 or claim 10 wherein the intermediate product mixture is provided to the third reaction zone, and wherein the reaction conditions in the third reaction zone are such as to promote the formation of the siloxane hydrogen carrier compound.

12. A method according to any preceding claim wherein the second reaction zone is provided with means capable of transporting oxygen ions at high temperatures.

13. A method according to claim 12 wherein the means capable of transporting oxygen ions at high temperatures comprises an oxygen permeable membrane, preferably one or more of a silver or silver alloy, such as Ag0.05Zr; a perovskite-type mixed metal oxide such as La0:6Ca0:4Co0:5Fe0:5O3 δ and La0:6Ca0:4Co0:3Fe0:8O3δ (LCCF); or yttria- stabilised zirconia (YSZ).

14. A method according to any preceding claim wherein the siloxane hydrogen carrier compound comprises: (I) a linear siloxane compounds of formula (I):

wherein n is an integer (representing the number of repeating units) of 1 or more, wherein radicals R and R’ may be the same or different and comprise silicon and hydrogen and/or oxygen but do not contain carbon; and/or (II) a cyclic siloxane compounds having the formula (II)

wherein n is an integer of one or more (representing the number of repeating units H2SiO); or any mixture of compounds of formlae (I) and (II).

15. The use of a plasma reactor in the synthesis of siloxane hydrogen carrier compounds, the use comprising introducing silica or a silicate compound, or a mixture thereof, into a reaction chamber of the plasma reactor, generating a thermal plasma in the reaction chamber, and contacting the silica or a silicate compound or mixture thereof with the thermal plasma in the presence of hydrogen or a hydrogen-containing gas mixture; wherein the reaction chamber comprises a first reaction zone corresponding to the hottest zone of the thermal plasma, a second reaction zone adjacent to the first reaction zone, and a third reaction zone adjacent to the second reaction zone and away from the thermal plasma such that the magnitudes of the temperature, pressure, density of ionized species and particle velocity in the chamber exist on a decreasing gradient from the first zone, through the second zone, to the third zone.