AU2014204736A1

AU2014204736A1 - Reactive medium comprising a porous substrate impregnated with an organic compound capable of forming gas clathrates, and use thereof for separating and storing CO2

Info

Publication number: AU2014204736A1
Application number: AU2014204736A
Authority: AU
Inventors: Christophe DICHARRY; Mehrdji Hemati; Harold MAFFRE; Eve Pere; Frederic Cedric Plantier; Jean-Philippe Stephane Torre
Original assignee: Centre National de la Recherche Scientifique CNRS; Institut National Polytechnique de Toulouse INPT; Universite de Pau et des Pays de lAdour
Current assignee: Centre National de la Recherche Scientifique CNRS; Institut National Polytechnique de Toulouse INPT; Universite de Pau et des Pays de lAdour
Priority date: 2013-01-14
Filing date: 2014-01-14
Publication date: 2015-07-23
Anticipated expiration: 2034-01-14
Also published as: RU2653821C2; CN105121004A; WO2014108568A1; FR3000907A1; FR3000907B1; RU2015134163A; AU2014204736B2

Abstract

The invention relates to a reactive medium comprising a porous substrate on which is deposited, in solid form, an organic compound capable of forming gas clathrates. The invention also relates to a process for separating CO

Description

1 Reactive medium comprising a porous substrate impregnated with an organic compound capable of forming gas clathrates and use thereof for separating and storing

CO

2 5 The present invention relates to a reactive medium comprising a porous substrate impregnated with an organic compound capable of forming gas clathrates, to the methods for preparing such a medium, as well as to its uses, notably in industrial gas separation methods. Natural gas is presently the third source of energy the most used worldwide, after 10 oil and coal. In the chain for treating natural gas, it is necessary to separate the pollutants which are present therein for economical or technical reasons. Among these pollutants, C02 is the one which is the most frequently encountered and which is generally the most concentrated in the gas flow to be treated. At the present time, there exist many methods for separating C02 in natural gas such as absorption by physical or hybrid chemical 15 solvents, adsorption on solid particles (adsorbents), the use of membranes and cryogenic methods, as notably described by Rufford et al., Journal of Petroleum Science and Engineering, 2012, 94-95, 123-154. These different techniques nevertheless have the drawback of generating high costs. In order to reduce these costs, alternative methods for separating C02 are 20 investigated, such as for example the use of gas clathrates. A gas clathrate is an inclusion complex consisting of several molecules, so called ((host-molecules forming a molecular cage around a gas molecule. In such structures, the host-molecules thereby form an open structure with cavities or channels in which atoms or molecules of gas having a suitable size are physically trapped, encapsulated. The molecules confined in 25 these cages are designated as ((invited-molecules >. As the molecular network forming the cages is stabilized by weak bonds of the hydrogen bond type, the thereby formed structures are also designated by the acronym of HOFs ("Hydrogen-bonded Organic Frameworks'). Molecules which may lead, in the presence of an invited-molecule of a suitable 30 size, to clathrates (or HOFs) should have particular characteristics in order to allow them to be associated with each other by forming cavities. These host-molecules making up clathrates should firstly be able to form between them weak bonds of the ((hydrogen bond type. As an example, mention may be made of water, which may form under certain conditions, clathrates commonly called 35 ((hydrates >, one of the most known being the one formed with methane (methane 2 hydrate). Organic host-molecules making up clathrates are also known to one skilled in the art, and have been the subject of many studies and scientific publications. As such and in a non-exhaustive way, mention may be made of: 1. Hydroquinone and its derivatives HO ~iOH 5 hydroquinone (A) Substitution of the hydrogen in the para-position of the phenol with an OH group leads to hydroquinone (A) (family of quinones), a model compound well known for forming clathrates (Mak et al., Hydroquinone, Encyclopedia of Supramolecular Chemistry, Vol.1, Editors J. L. Atwood and J. W. Steed, 2004, CRC Press, Taylor & Francis, pages 679 10 686) Among the derivatives of hydroquinone, mention may notably be made of hydroquinone molecules in which one or the two hydroxyl groups -OH are substituted with a thiol group -SH. 15 2. Phenol (B) and its derivatives Mention may notably be made of: OH phenol (B): As a derivative of phenol, mention may in particular be made of mono-substituted phenols such as p-cresol, p-bromophenol, ethyl phenol, t-butyl phenol, phenyl phenol, p 20 fluorophenol, m-fluorophenol and o-fluorophenol (MacNicol et al. Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45; J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London). 25 3. Urea (C) 0

H

2 N

NIH

2 (C) Cf. Harris et al., Urea inclusion compounds, Encyclopedia of Supramolecular Chemistry, Vol.2, Editors J. L. Atwood and J. W. Steed, 2004, CRC Press, Taylor & Francis, pages 1538-1549. 30 3 4. Dianin's compound (4-p-hydroxyphenyl-2,2,4-trimethylchroman) (D) O OH Dianin's compound (D) Cf. MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. 5 Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London. 5. Derivatives of Dianin's compound (D) wherein the positions R1, R2, R3 and R4 may be substituted, for example with a methyl group (Me = -CH 3 ) 10 wherein R 2 = R = R = H R= Me for example (MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London). 15 6. Derivatives of (D) obtained by substituting the carbons (C2) and (C4) wherein R 2 = H R = R = Me for example (MacNicol. et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and 20 related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D.D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London). 7. Derivatives of (D) obtained by changing the nature of the group responsible for the hydrogen bond 4 for example by replacing OH by SH: (MacNicol. Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., 5 London). 8. Quinazolinone (E) wherein Z =O 2-phenyl-3-p-(2,2,4-trimethylchroman-4-yl)phenylquinazolin-4(3H)-one 10 Cf. MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London). 15 9. Compounds derived from the molecules above wherein the oxygen is replaced with sulfur or selenium: Example 1: Derivatives of (C) S 11 Se C

H

2 N NH2 Thiourea H2N NH2 Selenourea Cf. Takemoto et al., Inclusion compounds of urea, thiourea and selenourea, pages 48-67 20 in J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London. . Example 2: Derivatives of (D): 4-p-hydroxyphenyl-2,2,4-trimethylthiachroman and 4-p hydroxyphenyl-2,2,4-trimethylselenachroman 5 4-p- hydroxyphenyl-2,2,4-trimethylthiachroman Cf. MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. 5 Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London. . Example 3: Derivatives of (E) by replacing the oxygen with sulfur wherein Z =S 10 Cf. MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D. D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London. 15 10. the compounds derived from the molecules above combining several modifications already described above: For example, a modification of (D) both of its heteroatom (replacement of oxygen with sulfur) and of the substituents of the ring (replacement of a hydrogen by a methyl) 2431 wherein R 2 = R = R= H; = Me 20 Cf. MacNicol et al., Structure and design of inclusion compounds: the clathrate of hydroquinone, phenol, Dianin's compound and related systems, pages 1-45 in J. L. Atwood, J. E. D. Davies, D.D. MacNicol., Inclusion compounds, Vol. 2, 1984, Academic Press Inc., London.

6 It is noted that gas hydrates, within which the host-molecules are water molecules, have already been studied for capturing C02. However, the use of gas hydrates does not give the possibility of obtaining high selectivity towards C02, for example in the case when the C02 is separated from certain gas mixtures, for example a mixture of C02 and of CH 4 . 5 Indeed, because of the proximity of the thermodynamic equilibrium curves of C02 hydrates and of CH 4 , the hydrate which is formed when water is in contact with a C0 2

/CH

4 mixture confines a non-negligible amount of CH 4 , which makes the application of this method for separating C02 not conceivable on an industrial scale, as this is described by van Denderen et al., Industrial & Engineering Chemistry Research, 2009, 48, 5802-5807 10 and Ricaurte et al., Industrial & Engineering Chemistry Research, 2013, 52, 899-910. On the other hand, organic clathrates, within which the host-molecules are organic molecules, are selective to certain gases such as C02, as shown by Lee and Yoon, The Journal of Physical Chemistry, C 2011, 115, 22647-22651, and Lee et al., ChemPhysChem 2011, 12, 1-4, in the case of an organic hydroquinone clathrate. 15 However, the hydroquinone applied in these studies is milled commercial solid hydroquinone and this type of solid conditioning proves to be impossible to use in an industrial method for treating gas. Indeed, it has several drawbacks, notably because this is a powder of crystals, the gas/solid exchange surface area is relatively small, there may be blocking of the contactors and its use leads to the formation of fine particles. Moreover, 20 the formation of organic gas clathrates requires particular pressure and temperature conditions, which may vary according to the medium in which they are formed. Therefore there exists a need for having a system within which organic gas clathrates may form, which may be used in an industrial process and having a satisfactory efficiency for capturing gases of interest, notably by means of a significant gas/solid 25 exchange surface area. Surprisingly, the inventors have identified that it was possible to form organic gas clathrates within a porous substrate on which is deposited an organic compound capable of forming clathrates. Moreover, unexpectedly, the inventors noticed that the efficiency for capturing gas, notably C02, obtained with such a substrate, was satisfactory. 30 In the subsequent text and generally, by < organic compound(s) > are meant in the sense of the present invention, organic compound(s) known from the state of the art, notably from the literature, as capable of acting as host-molecules for making up a clathrate, i.e. capable of assembling together in order to form gas clathrates. These organic compounds may notably be those aforementioned above. 35 Also, by the term of <clathrate is designated an inclusion complex consisting of one or several host-molecules forming a molecular cage (Fig. 1A). The term of <<gas 7 clathrate > then designates the assembly formed by a molecular cage and a gas molecule located within the cage (Fig. 1B). The reaction for capturing a gas molecule by a clathrate is described as <enclathration >, while the reaction for releasing a gas molecule by a gas clathrate is described as <declathration >. 5 Also, by the term of ((reactive medium is meant a medium or a system capable of reacting via a chemical reaction and/or a physical and/or physicochemical process when it is put in the presence of suitable chemical compounds. By the term of < porous substrate is meant a solid material comprising pores and on which may be deposited a given compound. According to the invention, an organic 10 compound capable of forming gas clathrates is deposited on the porous substrate. This organic compound gives the medium of the invention its reactivity, at least partly because of its capability of forming gas clathrates. The present invention therefore proposes a reactive medium comprising a porous substrate on which is deposited, at the surface and/or within the pores of the substrate, an 15 organic compound in solid form, acting as a host-molecule making up a clathrate. In said medium, the mass percentage of said deposited organic compound is from 5% to 60% by weight based on the total weight of said reactive medium. Said organic compound may notably be selected from hydroquinone and molecules of hydroquinone in which one or both -OH groups are substituted with -SH; 20 phenol, p-cresol, p-bromophenol, ethyl phenol, t-butyl phenol, phenyl phenol, p-fluorophenol, m-fluorophenol and o-fluorophenol; Dianin's compound and its derivatives in which the oxygen atoms are substituted with sulfur atoms; quinazolinone; urea, thio urea, seleno-urea, The suitable porous substrates within the scope of the invention have to be able to 25 be used in industrial processes, notably for separating gas, and in particular not degrade under temperature and/or pressure conditions applied within these processes. They should have great mechanical strength in order to avoid any phenomenon (attrition and fragmentation) which may modify the size of the substrate. Advantageously, the porous substrate is selected from the group consisting of 30 silica, alumina, active coal, molecular sieves and zeolites. Preferably, the porous substrate is silica or alumina. The alumina may be of the beta or gamma type. Gamma alumina is obtained by heating aluminium hydroxides at a low temperature, also called low temperature transition alumina; it crystallizes according to a 35 spinelle structure with defects.

8 Beta alumina is a sodium aluminate, produced on the basis of alumina A1 2 0 3 and containing a few percent of sodium oxide (Na 2 0), with very small amounts of silica and iron oxide. The particularity of this structure is its two-dimensional nature: the crystal is made with thin parallel layers of dense alumina separated from each other with sparsely 5 occupied planes in which the sodium ions are confined. Advantageously, the porous substrate comprises pores with a size comprised between 2 nm and 150 nm, preferably between 20 nm and 120 nm. The porous substrate for example comprises pores of 50 nm or 100 nm. According to an embodiment, the size of the pores of the porous substrate is such 10 that the organic compound may be deposited as a solid inside the pores and form therein gas clathrates when the reactive medium is put into the presence of gas. Generally, the porous substrate is in the form of porous particles, notably with an average size comprised between 20 pm and 5 mm, preferably between 40 pm and 700 pm for a fluidized bed method and between 1 mm and 5 mm for operations in a fixed 15 bed or in a movable bed. Typically, beads of porous material, for example silica beads are used. The selection of the size of the porous particles generally depends on the desired use and therefore varies according to the step of the industrial method in which it is desired to apply the reactive medium. Nevertheless, the performance of gas capture is 20 often improved when the size of the particles decreases. Therefore porous particles of small size, for example from 45 pm to 500 pm, will preferentially be selected. According to an embodiment, the porous substrate has a specific surface area from 30 m 2 /g to 1,000 m 2 /g. By using a porous substrate having a high specific surface area, the gas/solid 25 exchange surface area is considerably increased. The more the specific surface area increases, the more the amount of deposited organic compound increases and the more the number of capture sites of the gas molecules, i.e. the organic clathrates, increases thereby leading to better capture efficiency. Generally, the mass percentage of solid deposit is from 5% to 60%, 30 advantageously from 10% to 30% by weight based on the total weight of said reactive medium. The mass percentage of solid deposit may be determined by thermogravimetric analysis (ATG) techniques and differential calorimetric analysis (DSC) techniques. For a given mass percentage of solid deposit, the thickness of this deposit typically 35 depends on the average diameter of the pores of the porous substrate. This deposit should be sufficiently thick so that the gas clathrates may form and sufficiently thin so as 9 to avoid blocking of the pores, which will reduce access of the gas molecules to the clathrates. In the particular case wherein the pores would be totally filled with solid deposit, the gas molecules would only have access to the inlet surface area of the pores. Access to the clathrates would then be possible by gradual diffusion of the gas into the 5 blocked pores, which would make the clathration reaction extremely slow and unusable in an industrial process. According to the invention, the porous substrate comprises a solid deposit of an organic compound capable of forming gas clathrates. According to an embodiment, the organic compound is selected from the group 10 consisting of hydroquinone and its derivatives, of phenol and its derivatives, of Dianin's compounds and its derivatives, of quinazolinone and urea and of its derivatives. Preferably, the organic compound is hydroquinone and phenol. According to a particular embodiment, said organic compound is hydroquinone. The latter forms clathrates which are selective to certain gases, notably C02. Thus, its use 15 gives the possibility of obtaining a reactive medium having high selectivity towards C02, allowing easy separation of C02 from a gas mixture, for example a C0 2

/CH

4 or C0 2

/H

2 mixture. The present invention also relates to a method for preparing a reactive medium according to the invention comprising the following steps: 20 a) impregnating said porous substrate with said organic compound capable of forming gas clathrates in a liquid phase, and b) drying said thereby obtained impregnated porous substrate. Step a) consists of impregnating the porous substrate with a solution comprising the organic compound, thus leading to obtaining a porous substrate impregnated with the 25 organic compound in liquid form. Step b) then consists of drying the impregnated porous substrate in order to obtain a solid deposit of the organic compound at the surface and within the pores of the substrate. The drying step b) is for example applied in an oven. Typically, the porous substrate, for example silica beads, is immersed in a solution 30 comprising the organic compound, and then dried in the oven in order to obtain a thin layer of solid deposit covering the pores of the porous substrate. The steps a) and b) of the method as defined above may be carried out in several distinct apparatuses (impregnation via a liquid route) or in a single apparatus. According to an alternative, the impregnation method via a liquid route may further 35 comprise a step for filtering the porous substrate between the impregnation step a) and the drying step b).

10 This filtration step gives the possibility of suppressing the excess of solution comprising the organic compound in the porous substrate, giving the possibility, after drying, of obtaining a thin solid deposit layer of the organic compound on the porous substrate. 5 According to an embodiment, the solvent of the solution comprising the organic compound, which may be described as an ((impregnation solution >, is a solvent in which the organic compound is stable and soluble. The concentration of the organic compound in the impregnation solution and therefore the amount of organic compound which may be deposited on the porous substrate generally depends on the nature of the solvent. 10 According to an embodiment, the organic compound is hydroquinone applied as an alcohol solution saturated with hydroquinone. By ((alcohol solution is meant a solution in which the solvent is an alcohol, notably ethanol. The present invention also relates to a reactive medium which may be obtained by 15 the method described above. The object of the present invention is also the use of the reactive medium according to the invention for capturing C02. Thus, according to another of its objects, the present invention relates to a method for separating C02 from a gas mixture comprising C02 and at least one gas different from 20 C02; in said method, capture of C02 is achieved by enclathration by means of a reactive medium according to the invention. The method according to the invention may be discontinuous, semi-continuous or continuous. Capture of C02 by enclathration in the reactive medium according to the invention 25 has the advantage of being reversible. Indeed, if one proceeds with declathration of the gas in vacuo in order to release the gas of the clathrates, these clathrates may again capture C02, and this in an identical way with the previous capture reaction. In particular, the C02 capture reaction is reproducible in terms of reaction rate and of captured amount of gas. The reactive medium according to the invention may therefore be reused once it is 30 regenerated by declathration of the gas. The present invention is also directed to the use of the reactive medium according to the invention for separating the C02 in a mixture of gas comprising C02 and at least one gas different from C02. Typically, the reactive medium according to the invention may be used for 35 separating C02 from a mixture comprising C02 and CH 4 , or C02 and other saturated gas 11 hydrocarbons, or C02 and N 2 , or else further multi-constituent mixtures comprising C02,

H

2 , CH 4 , and other gas hydrocarbons. The separation is particularly efficient when this is a reactive medium comprising hydroquinone since the gas capture reaction by clathrates of hydroquinone is much more 5 selective towards C02 than towards CH 4 or H 2 (Lee et al., ChemPhysChem 2011, 12, 1-4). The present invention also relates to the use of the reactive medium according to the invention for treating natural gas, synthesis gas in pre-combustion or gas fumes in post-combustion. 10 By ((treatment of synthesis gas in pre-combustion >, is meant the treatment of combustible materials, such as oil and coal, upstream from their combustion. This treatment consists of separating the C02 from a C0 2

/H

2 mixture, a so-called ((synthesis gas >. By ((treatment of gas fumes in post-combustion >, is meant the treatment of gas 15 effluents after combustion in air of combustible materials such as oil and coal. This treatment consists of separating C02 from a C0 2

/N

2 mixture, which may contain a small proportion of so-called ((annexed gases like oxygen, carbon monoxide (CO), sulfur oxides (SO,) or nitrogen oxides (NO.). The object of the present invention is also the various uses described above in a 20 discontinuous, semi-continuous or continuous industrial process. In a discontinuous operating mode, the method may be carried out by means of an installation comprising a reactor or column, having isolation valves, means for pressurizing this reactor as well as means for adjusting pressure such as for example an 25 expansion valve or an automatic valve, a pressure control device like a sensor or a manometer, a heating and cooling means such as for example a jacket or an internal heat exchanger, a device allowing measurement of the amount of gas entering the reactor such as for example a flowmeter, a device for recording and controlling the reactor in order to check the temperature and the pressure over time. The reactive medium is first 30 loaded into the reactor, and is then put into contact with the gas to be treated under adequate temperature and pressure conditions so that the organic molecules deposited in the reactive medium form clathrates with the gas present in the reactor. The reactive medium may be subject to a conditioning or regeneration step before the reaction, for example with a step for heating or applying vacuum. The reactive medium applied as 35 reactive particles may be motionless or in motion in the reactor (rotary basket for example). As the enclathration reaction occurs within the medium, it modifies the 12 composition of the initially introduced gas. Once the reaction is completed, the gas still present is purged through one of the valves of the reactor and the operating conditions of the reactor are modified so that the gas clathrate is no longer stable and may release the gas, for example by increasing the temperature of the reactor or by reducing its pressure. 5 The thereby recovered gas has a different composition from the initial gas, for example it has a higher C02 content than the initial gas. Another operating alternative consists of transporting after reaction the reactive medium into another apparatus, a so-called dissociation reactor or column, in order to carry out the phase for releasing the gas. This discontinuous method is schematically illustrated in Fig. 4. 10 In a semi-continuous operating mode, the method is most often carried out by means of several discontinuous separators either operating in a fixed bed or in a fluidized bed, at least one of which is being regenerated while the other ones operate in a reaction. According to an embodiment, the fixed bed is a reactor or column containing a fixed layer 15 of reactive particles, freely disposed, through which the gas may circulate, while inside a fluidized bed, the particles are set into motion with the gas flow entering the reactor. The fixed bed or fluidized bed reactor includes the same elements as for discontinuous operation, with in addition, a device for inflow and outflow of the gas. The particles have a narrow grain size but their average size may vary between 40 microns and 5 millimetres 20 according to the operations. The pressure drop through the bed is often a determining factor in the selection of the grain size, but the capture performance is often improved when the size of the particles is reduced. By an adequate layout of valves, it is possible to simulate a continuous method. For example, in Fig. 5, an example is illustrated with two fixed bed columns operating, one of which operates in a reaction phase while the other is 25 in a regeneration phase. Fig. 6 illustrates operation of a multi-staged fluidized bed in which the gas to be treated successively crosses several beds of fluidized particles. In a continuous operating mode, the solid and the fluid generally circulate as a counter current. The reactive medium is regenerated with an annexed gas stream before 30 being recycled (Fig. 7). Thus, a complete unit comprises two sections: one for capture and one for regeneration with recycling of the solid. The gas-solid contact is achieved either in movable beds or further in multi-staged fluidized beds with recirculation of solid between the stages (Fig. 8). 35 The present invention is also directed to the use of the reactive medium according to the invention for storing gas, such as CH 4 , C02 or further H 2

.

13 Indeed gases of interest may be captured by an enclathration reaction within the reactive medium according to the invention and the captured gases may be retained therein. If need be, the captured gases may be released by a declathration reaction. 5 According to another object, the present invention also relates to a method for treating natural gas, synthesis gas in pre-combustion or gas fumes in post-combustion, comprising the method for separating C02 according to the invention. According to another object, the present invention also relates to a reactor for separating C02 from a gas mixture consisting of C02 and of at least one gas different from 10 C02; said reactor comprising: - a chamber, - means for applying and maintaining said chamber at a defined operating pressure, - means for applying and maintaining said chamber at a defined operating temperature, 15 - means for circulating said gas mixture through said chamber; characterized in that said chamber comprises a reactive medium according to the invention as defined earlier. According to an embodiment, said reactor operates with a fluidized bed. According to another embodiment, said reactor operates with a fixed bed. 20 According to an embodiment, the operating pressure is comprised between 0 and 100 bars. According to an embodiment, said operating temperature is comprised between 0 and 150'C. Indeed, Lee and Yoon The Journal of Physical Chemistry, C 2011, 115, 22647-22651, and Lee et al., ChemPhysChem 2011, 12, 1-4 have reported operation at 25 temperatures as low as OC. The regeneration step may be carried out in vacuo (P = 0) or at a high temperature (generally less than or equal to 1500C, it being understood that this regeneration temperature is generally less than the decomposition temperature of the organic 30 compound, i.e. about 1200C for hydroquinone, about 400C for phenol). The invention will be better understood upon reading the description which follows only given as an example, and made with reference to the appended drawings, wherein: - Fig. 1 shows (A) a hexagonal arrangement example of an organic clathrate of type 1 formed by hydroquinone molecules and (B) a gas organic clathrate example in which 35 the sphere represents the gas molecule; 14 - Fig. 2 is a graph representing the mass in grams (g) of captured C02 by a reactive medium according to the invention versus time in minutes (min) during two consecutive enclathration cycles, the first cycle being illustrated by circles and the second cycle by squares; 5 - Fig. 3 is a graph representing the molar composition of the gas (in mol%) versus time (in min) during two successive formations when a reactive medium according to the invention is put into the presence of a C0 2

/N

2 mixture, with an initial composition equal to 49.9 mol% of C02. The circles and the triangles correspond to the concentration of C02 in the gas during the first and second formation, respectively. The squares and 10 the crosses correspond to the concentration of N 2 in the gas during the first and second formation, respectively. - Fig. 4 schematically illustrates a discontinuous method according to the invention. - Figs. 5, 6, 7 and 8 illustrate different types of methods according to the invention: (5) semi-continuous, (6) semi-continuous in multistaged fluidized bed, (7) continuous, (8) 15 continuous in a fluidized bed. - Figs. 9 and 10 illustrate Raman spectra of the reactive medium before and after the Experiment 3 of the Table 4 below, respectively. EXAMPLES 20 Example 1: Preparation of a reactive medium according to the invention. A reactive medium according to the invention was elaborated by impregnating porous particles with an alcohol solution saturated with organic compound. The porous substrate consists of silica, alumina or active coal. The impregnation 25 was achieved by immersing the porous substrate into the alcohol solution saturated with organic compound, and the substrate was then dried in the oven. ATG/DSC analyses were carried out. The tables below summarize the different tests conducted with different substrates and organic compounds. 30 The characteristics of the virgin substrates used as a porous substrate are summarized in Table 1: 15 Table 1 Nature Designation Diameter of the Average diameter of the particles pores (nm) (pm) Silica S1 20-45 100 Silica S2 200-500 100 Silica S3 20-45 30 Gamma AG 1700-2000 64 alumina Beta alumina AB 2400 45 Active coal CA ND ND ND = non determined 5 The characteristics of the organic compounds used as host-molecules are summarized in Table 2: Table 2 Molecule Designation Formula Molar mass (g/mol) Hydroquinone HQ C 6

H

6 0 2 110.1106 Phenol Ph C 6

H

6 O 94.1112 10 The characteristics of the reactive media made are summarized in Table 3: Table 3 Designation of Substrate Organic Impregnation level in Actual density the reactive used compound wt% of impregnated (in g/cm 3 ) medium used substrate P1 S1 HQ 21.2% 1.9981 P2 S1 HQ 23.9% ND P3 S1 HQ 7.0% ND P4 S1 HQ 27.0% 1.9244 P5 S2 HQ 26.1 % 1.8313 P6 S3 HQ 29.4% ND P7 AG HQ 23.8% 2.1450 16 P8 AB HQ 17.1 % 2.5015 P9 S1 Ph 26.2% 1.7867 P10 CA HQ 19.0% ND P11 CA Ph 21.2% 1.5519 ND = not determined Example 2: Use of a reactive medium according to the invention for capturing C02. The reactive media prepared in Example 1 were tested in the presence of pure 5 C02. The experiments were conducted by using scales with a magnetic suspension of the brand RUBOTHERM, in order to measure very accurately the mass of gas having reacted with the organic compound (to within 10 5 g). The relevant apparatus is described by Casas et al., 10 th European Symposium on Thermal Analysis and Calorimetry 10 (ESTAC), August 22-27, 2010, Rotterdam, The Netherlands. Independent experiments were conducted at different pressures of pure C02 and at different temperatures, where, for each experiment, two enclathration-declathration cycles were carried out. The experiments show that the reactive medium of the invention captures C02, that the capture method is reversible (after the declathration step of the gas 15 in vacuo, the following enclathration step of the gas takes place exactly like the previous one), and that the experiments are perfectly reproducible in terms of reaction rate and of amount of captured gas (Fig. 2). The operating conditions and obtained results are summarized in Table 4 (see infra). 20 Example 3: Use of a reactive medium according to the invention for separating C02 in a C0 2

/N

2 mixture. The reactive medium was prepared according to the same procedure as the one described in Example 1. The conducted analyses gave the possibility of measuring that 25 the amount of hydroquinone present in this reactive medium was equal to 27% by mass. A mass of 66 g of reactive medium, as a particle, was introduced into a reactor operated in a batch mode with reference to the operating modes already described above. The reactor is set to a temperature of 250C by using its jacket in which circulates a heat 30 transfer fluid. The temperature of the reactor is measured via a PT100 probe. The particles are then put into contact with a gas containing 49.9% by moles of C02, by loading the reactor at a pressure of 35 bars by using an expansion valve and by 17 controlling the pressure via a pressure sensor placed on the reactor. The pressure and the temperature are recorded over time with an acquisition system, connected to a computer, at a frequency of 1 Hz. The technical details of the installation are given in Torr6 et al., Chemical Engineering Science, 2012, 82, 1-13. The reactor is then isolated 5 by closing the supply valve and the time-dependent change in the composition of the gas present in the reactor is tracked in situ by gas chromatography. Two successive formations were achieved, with between both formations, a step for regenerating the particles produced for 48 h in vacuo. Fig. 3 shows that the composition of the gas is actually modified over time, with a molar fraction of C02 which decreases and a nitrogen 10 molar fraction which increases. At the end of the experiment, the C02 concentration was lowered from 9.8% and 9.5% during the first and the second formation respectively. These results, which depend on the amount of reactive medium initially introduced, are reproducible on the two formations produced. This shows that the enclathration reaction occurs, that it is selective towards C02, and that it is carried out with relatively rapid 15 kinetics (less than one hour for attaining the final concentration). * Spectroscopic characterization of the formation of the gas clathrates: The obtained results show that the organic molecule deposited in the substrate is 20 not modified after impregnation and that the compound formed after reaction between the deposited organic product within the porous particle and the gas is actually a gas clathrate. The characterization of the formed structure may be accomplished for example by using vibrational spectroscopic techniques such as Raman or infrared spectroscopy which 25 gives the possibility of obtaining characteristic spectra of the studied compounds. When the relevant organic molecule is reorganized in order to form a clathrate, easily identifiable modifications of its vibrational spectrum characteristic of the formed structure result from this. Also, the gas which is trapped within the clathrate has a characteristic signature and may easily be identified. 30 For example, mention may be made of the characterization of the reactive medium before and after Experiment #3 of Table 4, produced by putting the reactive medium P2 (silica/hydroquinone) in contact with a C0 2

/CH

4 gas mixture with 75 mol% of C02. The characterization was achieved by using a Raman spectrometer of the brand JOBIN YVON, model T64000. 35 Figs. 9 and 10 illustrate two Raman spectra of the reactive medium. The spectrum of Fig. 9 was obtained before reaction and the spectrum of Fig. 10 after reaction.

18 The peaks obtained on the spectrum of Fig. 9 indicate that the compound deposited in the reactive medium is actually hydroquinone, in a so-called ((alpha phase (no clathrate), with reference to the basic spectrum of this product (which may be found in Davies J. E. D. Clathrates and inclusion compounds. Part 1. Infrared and Raman studies 5 of several p-quinol (hydroquinone) clathrates. J. Chem. Soc., Dalton Trans., 1972, 11, 1182-1188). The spectrum of Fig. 10 of this same reactive medium after reaction with a C0 2

/CH

4 mixture shows without any ambiguity that: (i) The structure of the hydroquinone was modified by the reaction: the hydroquinone formed a clathrate (the corresponding 10 structure is said to be ((hydroquinone in a beta form), this being proven by the characteristic shape of the bulk of peaks P1 (Davies, 1972, supra) measured from 810 to 860 cm- 1 , (ii) that this hydroquinone clathrate is actually a gas clathrate containing C02, the latter being proven by the presence of the peak P2, measured at 1,378 cm- 1 , characteristic of C02 included in this structure (J. W. Lee, K. J. Choi, Y. Lee, J. H. Yoon. 15 Spectroscopic identification and conversion rate of gaseous guest-loaded hydroquinone clathrates. Chemical Physics Letters 2012, 528, 34-38) The obtained results are summarized in Table 4 (see infra). 20 Table 4: Results obtained in Examples 3 and 4 when the reactive particles are put into contact with pure C02 or a gas mixture containing C02. Exp. Apparatus Reactive Gas used Initial Initial Initial Final composition Captured gas mass # used medium T' pressu composition of the gas in CO 2 (g of C0 2 ) per g of (*C) re of the gas in (% by moles) particle (bars) CO 2 (% by moles) Form 1 ' Form 2 Form 1 Form 2 #1 Scales P1 C02 50.0 30.0 100.0 - - 0.0293 0.0293 #2 Pilot P1 C0 2

/CH

4 50.0 40.0 25.7 21.1 - - #3 Pilot P2 C0 2

/CH

4 50.0 35.0 75.0 69.9 72.0 - #4 Scales P3 C02 50.0 30.0 100 - - 0.0192 #5 Pilot P4 C02/N 2 25.0 35.0 49.9 40.1 40.5 - #6 Pilot P7 C02/N 2 25.0 35.0 49.9 36.6 - - #7 Pilot P8 C02/N 2 25.0 50.0 25.0 16.3 - - #8 Scales P9 C02 27.0 30.0 100.0 - - 0.0269 0.0248 #9 Scales P11 C02 29.5 29.9 100.0 - - 0.109 (*) Form = formation 19 Conclusions These results illustrate the conducted tests, with two different apparatuses and: - the use of several substrates (silica, alumina, active coal) 5 - the use of several organic molecules which may form gas clathrates (hydroquinone, phenol) - the use of several gases and gas mixtures (C02, C0 2

/CH

4 , C0 2

/N

2 ) having compositions ranging from 25% to 100% of C02 - different reaction conditions: initial pressure from 29.9 to 50 bars, reaction 10 temperature from 25 to 500C - the possibility of achieving several successive reaction cycles.

Claims

1. A reactive medium comprising a porous substrate on which is deposited at the 5 surface and/or within the pores of said substrate, an organic compound in solid form, acting as a host-molecule making up a clathrate, characterized in that the mass percentage of the deposited organic compound is from 5% to 60% by weight based on the total weight of said reactive medium. 10

2. The reactive medium according to claim 1, wherein said organic compound is selected from hydroquinone and hydroquinone molecules in which one or both groups -OH are substituted with -SH; phenol, p-cresol, p-bromophenol, ethyl phenol, t-butyl phenol, phenyl phenol, p-fluorophenol, m-fluorophenol and o-fluorophenol; the Dianin's compound and its derivatives in which the oxygen atoms are substituted 15 with sulfur atoms; quinazolinone; urea, thio-urea, seleno-urea.

3. The reactive medium according to claim 1 or 2, wherein the porous substrate is selected from the group consisting of silica, alumina, active coal, molecular sieves and zeolites. 20

4. The reactive medium according to claim 1, 2 or 3, wherein the porous substrate is silica.

5. The reactive medium according to any of claims 1 to 4, wherein the porous 25 substrate comprises pores with a size comprised between 2 nm and 150 nm.

6. The reactive medium according to any of claims 1 to 5, wherein the porous substrate is in the form of porous particles with an average size comprised between 20 pm and 5 mm. 30

7. The reactive medium according to any of claims 1 to 6, wherein the organic compound is hydroquinone or phenol.

8. A method for separating C02 from a gas mixture comprising C02 and at least one 35 gas different from C02, wherein enclathration of C02 is achieved in a reactive medium as defined by any of claims 1 to 7. 21

9. The method according to claim 8, such that it is semi-continuous or continuous.

10. A method for treating natural gas, synthesis gas in pre-combustion or gas fumes in 5 post-combustion, comprising the method for separating C02 according to claim 8 or 9.

11. A reactor for separating the C02 from a gas mixture consisting of C02 and of at least one gas different from C02; said reactor comprising: 10 - a chamber, - means for applying and maintaining said chamber at a defined operating pressure, - means for applying and maintaining said chamber at a defined operating temperature, 15 - means for circulating said gas mixture through said chamber; characterized in that said chamber comprises a reactive medium, as defined by any of claims 1 to 7.

12. The reactor according to claim 11, characterized in that said reactor operates with a 20 fluidized bed.

13. The reactor according to claim 11 or 12, characterized in that said reactor operates with a fixed bed. 25

14. The reactor according to any of claims 11 to 13, characterized in that said operating pressure is comprised between 0 and 100 bars.

15. The reactor according to any of claims 11 to 14, characterized in that said operating temperature is comprised between 0 and 150'C. 30