Patent Description:
The continuing increase in demand for energy which has occurred worldwide during the last few decades, together with concerns deriving from climate changes due to the emission of greenhouse gases, have resulted in an intensification of efforts to encourage both the development and use of new energy technologies based on alternative sources such as natural gas and hydrogen, and the capture of carbon dioxide for climatic purposes. There is therefore great interest in the development of new porous materials characterized by a suitable surface area and the presence of pores of micrometre dimensions, which can be used not only for the optimum storage of gases for energy and environmental applications, but also for applications in other sectors such as separation and catalysis.

Of the various classes of materials proposed for this purpose, porous carbons have received great attention in recent decades on account of their hydrophobic nature, high porosity and surface area, good thermal and mechanical stability and high adsorption capacity [<NPL>]. Porous carbons are generally synthesized from the carbonization of carbon-based precursors, mainly using two methods: i) physical activation, which implies pyrolysis of the precursor at high temperatures in the presence of gases such as carbon dioxide, steam, or others; or ii) chemical activation, in which the precursor is carbonized at lower temperatures in the presence of an activating agent such as an alkaline hydroxide or transition metal oxides [<NPL>.

Over the years porous carbons have been developed from various precursors, mainly: i) conventional carbons obtained from the carbonization and activation of renewable precursors based on biomass; and ii) carbons deriving from synthetic polymers. The latter class of carbons is very broad and has the advantage that the chemical compositions of the carbons produced, as well as pore morphology and distribution, can be better controlled [<NPL>]. In particular porous carbons replicated from porous materials (hard template porous carbons) such as zeolites [<NPL>], MOFs (Metal-Organic Frameworks) [<NPL>] and PAFs (Porous Aromatic Frameworks) [<NPL>], are systems in which the porous material acts as a template on a nanometric scale, so that the porous network of the initial material is maintained in the final carbon. Microporous carbons obtained from three-dimensional (3D) aromatic polymers are described in the prior art. For example, microporous carbons obtained from PAF have been described in <NPL>. In this article Y. have reported porous carbons obtained by chemical activation at various temperatures (<NUM>-<NUM>) of the porous polymer PAF-<NUM>, characterized by smaller surface area and pore volume than the starting material, but with a better capacity for the storage of CO<NUM>, CH<NUM> and H<NUM> in comparison with the starting material, particularly at low pressures. This improvement in adsorbent capacity has been attributed to the bimodal pore distribution which also includes <NUM>Å pores that have greater affinity for gases stored with the adsorbent.

<CIT> describes a method of forming microporous carbon materials from polymeric precursors such as phenolic resins.

According to the IUPAC classification, the term "micropore" is understood to mean pores having pore dimensions smaller than <NUM>Å. Micropores are in turn subdivided into two groups, that is ultramicropores (less than <NUM>Å) and supermicropores (from <NUM> to <NUM>Å).

This invention describes the preparation of microporous carbons from materials belonging to the class of 3D hyper-cross-linked polymers (known as HCP), in particular microporous carbons deriving from hyper-cross-linked polymers which can be obtained using a Friedel-Crafts reaction between a tetrahedral aromatic monomer and the cross-linking agent formaldehyde dimethyl acetal (FDA) in the presence of ferric chloride (FeCl<NUM>) as catalyst. The present inventors have described the synthesis and optimization of hyper-cross-linked polymers designated as UPO in International Patent <CIT> and in European Patent Application <CIT>. Also these materials have been published by the present inventors in the article with reference "<NPL>", in which the polymers have been referred to by the acronym mPAF.

Unlike carbonized PAF materials in which there is a decrease in surface area, the activation of UPO materials has resulted in microporous carbons characterized by an increase in both surface area and total pore volume, and in particular micropore volume. The presence of families of ultramicropores of dimensions below <NUM>Å has also been observed. It is known that a large number of ultramicropores and supermicropores of dimensions around <NUM>Å make a porous material particularly suitable for adsorbing and storing gases such as carbon dioxide, hydrogen and methane at low to intermediate pressures.

Other advantages in obtaining the new microporous carbon from UPO polymer relate to the economic and industrial aspect; in fact the process of polymer synthesis is very economical and easily scalable.

Thus one object of this invention is to provide a process for the preparation of a microporous carbon from a porous aromatic HCP polymer, the process being at the same time simple, cheap and easy to scale up.

Another object of this invention is that of providing a microporous carbon absorbing gases having improved gas adsorption properties and capable of being prepared through a simple, cheap and easy to scale up process.

These and other objects are accomplished by the method according to independent claim <NUM> and microporous carbon according to independent claim <NUM>. The dependent claims define further characteristics of the invention and form an integral part of the description.

The scope of this invention also includes the use of gas-adsorbing microporous carbon which can be obtained by the method according to the invention to store a gas, in particular hydrogen, methane and/or carbon dioxide.

As will be illustrated in greater detail in the description below, the gas-adsorbing microporous carbon which can be obtained by the method according to the invention comprises an appreciable quantity of ultramicropores and supermicropores, which because of their dimensions are particularly suitable for adsorbing and storing the gases carbon dioxide, hydrogen and methane; in addition to this it has an improved surface area. Also this carbon can be obtained by a method of preparation which makes use of an HCP polymer obtained from widely available solvents and catalysts that are not therefore excessively expensive or particularly difficult to scale up. The advantageous and unexpected properties of the microporous carbon which can be obtained by the method according to this invention are demonstrated in particular in the experimental section of this description.

Further objects, characteristics and advantages will become obvious from the following detailed description. However although the detailed description and specific examples indicate preferred embodiments of the invention, these are provided purely by way of illustration, because various modifications and changes falling within the scope of the invention will become obvious to those skilled in the art from reading them.

The method of preparation according to the invention comprises a synthesis stage in which a Friedel-Crafts reaction is carried out using a Friedel-Crafts catalyst, a tetrahedral monomer of general formula (I) as defined below and formaldehyde dimethyl acetal (FDA) as a cross-linking agent. After the synthesis stage the porous aromatic HCP obtained is impregnated with a preferably basic activating agent solution and undergoes activating heat treatment at a temperature in the range from <NUM> to <NUM>.

The present inventors have in fact observed that in the presence of an activating agent, preferably with a ratio by weight with respect to the polymer of between <NUM>:<NUM> and <NUM>:<NUM>, the activating heat treatment at a temperature within the range from <NUM> to <NUM> results in a considerable increase in the quantity of micropores, especially those having pore sizes below <NUM>Å, together with an obvious increase in surface area (see Table <NUM> below).

The microporous carbon used in the method of preparation according to the invention is prepared from a hyper-cross-linked polymer synthesized from tetrahedral monomers of formula (I), whose structure is illustrated below:
<CHM>
in which A is selected from a C atom, a Si atom, a Ge atom, a Sn atom, an adamantane group, an ethane group and an ethene group,
<CHM>
in which each of B, C, D and E are ring structures selected from monovalent radicals of the compounds benzene, naphthalene, anthracene, phenanthrene, pyrene, optionally having one or more substituents selected from nitro, amino, hydroxyl, sulfonyl, halogen, phenyl, alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, aryl, alkenyl and alkynyl groups.

In formula (I) illustrated above a halogen preferred as optional substituent is fluorine; alkyls preferred as optional substituents are methyl, ethyl, propyl, butyl; haloalkyls preferred as optional substituents are chloromethyl, chloroethyl; hydroxyalkyls preferred as optional substituents are hydroxymethyl, hydroxyethyl; amino alkyls preferred as optional substituents are aminomethyl, aminoethyl; aryls preferred as optional substituents are benzene, toluene, phenol, aniline, acetophenone, benzaldehyde, benzoic acid, o-xylene, m-xylene, p-xylene, benzonitrile, biphenyl; alkenyls preferred as optional substituents are ethenyl, propenyl, <NUM>-methylethenyl, butenyl.

A preferred tetrahedral monomer for use in the method according to the invention is a compound of formula (I) in which each of the ring structures B, C, D and E is a monovalent radical of benzene having optionally one or more substituents as defined above. A particularly preferred tetrahedral monomer is tetraphenylmethane, in which A is a carbon atom and each of B, C, D and E is an unsubstituted monovalent radical of benzene, that is to say a phenyl radical.

Examples of suitable Friedel-Crafts catalysts for use in the method of preparation according to the invention of the HCP polymer used in the preparation of the microporous carbon are BF<NUM>, BeCl<NUM>, TiCl<NUM>, SbCl<NUM>, SnCl<NUM>, FeCl<NUM>, AlCl<NUM>, Sc(OTf)<NUM>, Mo(CO)<NUM>, SeCl<NUM>, TeCl<NUM>, sulfuric acid, hydrofluoric acid and super acids, such as for example HF·SbF or HSO<NUM>F·SbF<NUM>. The most preferred catalyst is FeCl<NUM>.

Suitable aprotic organic solvents for use in the method according to the invention for the HCP polymer used in the preparation of microporous carbon include both polar solvents and apolar solvents. Specific examples of solvents, provided purely by way of illustration, are dichloroethane, toluene, benzene, nitromethane (CH<NUM>NO<NUM>), carbon disulfide and chlorobenzene. The most preferred solvent is dichloroethane.

According to one embodiment of the method according to the invention the concentration of monomer of formula (I) in the aprotic organic solvent lies within the range from <NUM> to <NUM>.

According to one embodiment of the method according to the invention the molar ratio of monomer of formula (I) to FDA lies within the range from <NUM>:<NUM> to <NUM>:<NUM>.

The chemical structure of the porous polymer used as a precursor to obtain the microporous carbon obtained by the process described in this invention is illustrated by the following general formula (II):
<CHM>
in which A, B, C, D and E are as defined above in relation to formula (I), and in which n is an integer preferably between <NUM> and <NUM>. The thermal activation which makes it possible to obtain the microporous carbon used in the process of preparation according to the invention provides for impregnation of the HCP polymer of formula (II) with a solution of an activating agent in a solvent, in which the ratio of activating agent to polymer lies between <NUM>:<NUM> and <NUM>:<NUM>. Preferably the reaction mixture is kept stirred for a time of between <NUM> minutes and <NUM> hours. Preferably the ratio of activating agent to polymer lies between <NUM>:<NUM> and <NUM>:<NUM>.

According to a preferred embodiment of the method according to the invention the activating heat treatment is carried out at a temperature within the range from <NUM> to <NUM>, preferably at a temperature within the range from <NUM> to <NUM>, and even more preferably at a temperature within the range from <NUM> to <NUM>.

According to a preferred embodiment of the method according to the invention the activating agent is selected from potassium hydroxide (KOH), sodium hydroxide (NaOH), potassium carbonate (K<NUM>CO<NUM>), sodium carbonate (Na<NUM>CO<NUM>), zinc chloride (ZnCl<NUM>) or phosphoric acid (H<NUM>PO<NUM>). A particularly preferred activating agent is potassium hydroxide (KOH).

Examples of suitable solvents for mixing the activating agent for use in the method of preparation according to the invention are water, ethanol, methanol, acetonitrile, tetrahydrofuran and dimethylformamide, or mixtures of those solvents with water up to <NUM>% v/v. The most preferred solvents are water and ethanol, or a mixture of these up to <NUM>% v/v.

According to a preferred embodiment of the method according to the invention the concentration of activating agent in the solvent, expressed as molarity (mol/L), lies within the range between <NUM> and <NUM>.

According to another preferred embodiment of the method according to the invention the mixture is dewatered by distillation on a Rotavapor at a temperature of between <NUM> and <NUM>, or a flow of inert gas (nitrogen or argon) for a time of between <NUM> minutes and <NUM> hours.

According to yet another preferred embodiment of the method according to the invention the activating treatment is carried out under a flow of inert gas, in which the gas is preferably selected from N<NUM> and Ar.

According to yet another preferred embodiment of the method according to the invention, once the activation temperature has been reached, isothermal conditions are maintained for a time of between <NUM> minutes and <NUM> hours. The preferred time is <NUM> minutes. According to yet another preferred embodiment of the method according to the invention the heating gradient is between <NUM>/min and <NUM>/min.

According to yet another preferred embodiment of the method according to the invention the carbonaceous product is washed with water, hydrochloric acid (HCl) in a concentration of between <NUM> and <NUM>, and again with water to neutral pH.

A detailed description of the structural properties and porosity of the microporous carbon according to this invention, made with reference to its preferred embodiments, is provided in the following examples, with reference to the appended drawings, in which:.

The following examples are illustrative and do not restrict the scope of the invention as defined in the appended claims.

A number of specific examples of synthesis procedures and post-synthesis heat treatment are provided below.

Ferric chloride (<NUM>, <NUM> mol) and tetraphenylmethane (TPM <NUM>, <NUM> mol) were suspended in dichloromethane (DCE, <NUM>). The resulting mixture was vigorously stirred at ambient temperature to obtain a uniform solution and then formaldehyde dimethylacetal (FDA) (<NUM>, <NUM> mol) was added dropwise. The resulting dense gel was stirred at ambient temperature for <NUM> hours and then heated under reflux for <NUM> hours. After cooling to ambient temperature the gel was diluted with ethanol and washed several times with water until the pH became neutral, and finally dried in a stove overnight at <NUM>.

The material obtained is referred to as UPO-<NUM> (so described in <CIT>). <NUM> of UPO-<NUM> material was added to a <NUM> solution of KOH in ethanol (<NUM>% v/v in water) with a material : KOH ratio of <NUM>:<NUM> by weight, and left stirring overnight. Subsequently the mixture was dewatered under vacuum at <NUM> and the solid was placed in an alumina crucible, which was in turn placed inside a quartz tube. Carbonization was carried out under a flow of nitrogen with a temperature gradient of <NUM>/min up to the temperature of <NUM> and held under isothermal conditions at <NUM> for <NUM> minutes. After carbonization the material obtained was washed with deionized water, <NUM> HCl and with successive wash cycles with deionized water to neutral pH. The resulting material was dried in a stove at <NUM> for <NUM> hours (sample described as C-UPO-<NUM>).

Ferric chloride (<NUM>, <NUM> mol) and tetraphenyladamantane (TPA <NUM>, <NUM> mol) were suspended in dichloromethane (DCE, <NUM>). The resulting mixture was vigorously stirred at ambient temperature to obtain a uniform solution and then formaldehyde dimethylacetal (FDA, <NUM>, <NUM> mol) was added dropwise. The resulting dense gel was stirred at ambient temperature for <NUM> hours and then heated under reflux for <NUM> hours. After cooling to ambient temperature the gel was diluted with ethanol and washed several times with water until the pH became neutral, and finally dried in a stove overnight at <NUM>.

The material obtained is referred to as UPO-<NUM>. <NUM> of UPO-<NUM> material was added to a <NUM> solution of KOH in water with a material : KOH ratio of <NUM>:<NUM> by weight, and left stirring for <NUM> hours. Subsequently the mixture was placed in an alumina crucible, which was in turn placed inside a quartz tube. Carbonization was carried out under a flow of nitrogen with a temperature gradient of <NUM>/min up to the temperature of <NUM> and held under isothermal conditions at <NUM> for <NUM> minutes. After carbonization the material obtained was washed with deionized water, <NUM> HCl and with successive wash cycles with deionized water to neutral pH. The resulting material was dried in a stove at <NUM> for <NUM> hours (sample described as C-UPO-<NUM>).

Ferric chloride (<NUM>, <NUM> mol) and tetraphenylsilane (TPS <NUM>, <NUM> mol) were suspended in dichloromethane (DCE, <NUM>). The resulting mixture was vigorously stirred at ambient temperature to obtain a clear solution and then formaldehyde dimethylacetal (FDA, <NUM>, <NUM> mol) was added dropwise. The resulting dense gel was stirred at ambient temperature for <NUM> hours and then heated under reflux for <NUM> hours. After cooling to ambient temperature the gel was diluted with ethanol and washed several times with water until the pH became neutral, and finally dried in a stove overnight at <NUM>.

Ferric chloride (<NUM>, <NUM> mol) and <NUM>,<NUM>,<NUM>,<NUM>-tetraphenylethane (TPEa <NUM>, <NUM> mol) were suspended in dichloromethane (DCE, <NUM>). The resulting mixture was vigorously stirred at ambient temperature to obtain a clear solution and then formaldehyde dimethylacetal (FDA, <NUM>, <NUM> mol) was added dropwise. The resulting dense gel was stirred at ambient temperature for <NUM> hours and then heated under reflux for <NUM> hours. After cooling to ambient temperature the gel was diluted with ethanol and washed several times with water until the pH became neutral, and finally dried in a stove overnight at <NUM>.

The material obtained is referred to as UPO-<NUM>. <NUM> of UPO-<NUM> material was added to a <NUM> solution of KOH in ethanol (<NUM>% v/v in water) with a material : KOH ratio of <NUM>:<NUM> by weight, and left stirring overnight. Subsequently the mixture was dewatered under vacuum at <NUM> and the solid was placed in an alumina crucible, which was in turn placed inside a quartz tube. Carbonization was carried out under a flow of nitrogen with a temperature gradient of <NUM>/min up to the temperature of <NUM> and held under isothermal conditions at <NUM> for <NUM> minutes. After carbonization the material obtained was washed with deionized water, <NUM> HCl and with successive wash cycles with deionized water to neutral pH. The resulting material was dried in a stove at <NUM> for <NUM> hours (sample described as C-UPO-<NUM>).

Textural analysis relating to both specific surface area and pore size distribution in the samples was obtained by physisorption of N<NUM>. Measurement was carried out at low temperatures (<NUM>), and at increasing pressure values of between <NUM>-<NUM> and <NUM> bar.

Before carrying out the measurements the samples were pretreated at <NUM> for <NUM> hours under high vacuum. As a comparison the pore distributions of the materials before chemical activation were also reported. BET surface areas were calculated within the pressure range P/P<NUM> = <NUM>-<NUM>, and pore size distributions were calculated using the "Quenched Solid Density Functional Theory" (QSDFT) procedure on carbon surfaces with a slit/cylinder geometry applied to the adsorption branch.

The N<NUM> adsorption-desorption isotherms at <NUM> (Tables A) and the pore distributions (Tables B) for the microporous carbons before carbonization (circles) and after carbonization (triangles) are shown in <FIG>.

All the isotherms show a net increase in the quantity of gas adsorbed after the carbonization process, particularly at low pressures (<NUM>-<NUM><NUM>/g). The profiles of the adsorption isotherms show a pseudo-Langmuir adsorption isotherm for samples C-UPO-<NUM>, C-UPO-<NUM> and C-UPO-<NUM>; this shows that the resulting carbons are wholly microporous, unlike the starting polymers which also have mesoporosity as revealed by the presence of hysteresis peaks in the pressure range P/P<NUM> <NUM>-<NUM>. Carbons C-UPO-<NUM>, C-UPO-<NUM>, C-UPO-<NUM>, C-UPO-<NUM> and C-UPO-<NUM>, which also show mesoporosity, behave differently.

The surface areas after carbonization are all increased, as is the total pore volume, as shown in Table <NUM>.

The pore distributions obtained after activation are mostly characterized by the appearance of ultramicropores of dimensions <NUM>Å or below. The microporous carbons present in the literature obtained from 3D polymers comparable to the hyper-cross-linked polymers (like the UPO-n) show the opposite behaviour: one example is provided by carbonized PAFs (Porous Aromatic Frameworks) [<NPL>], which are characterized by a fall in surface area and total pore volume following activating treatment, while the pore distribution remains almost microporous with a family of pores in the range <NUM>-<NUM>Å.

It will be noted that in carbons C-UPO-<NUM>, C-UPO-<NUM> and C-UPO-<NUM> deriving from polymers prepared from the precursor TPM the microporous volume is more than <NUM>% of the total. These carbons are mainly characterized by families of ultramicropores smaller than <NUM>Å and families of micropores lying within the range <NUM>-<NUM>Å, which nevertheless have appreciable importance for the storage of gas. A particularly promising carbon is C-UPO-<NUM>, which is characterized by a microporous volume of <NUM>Å, of which <NUM>Å ultramicropores represent a percentage of more than <NUM>% of the total micropore volume. This material is greatly indicated for the storage of hydrogen, carbon dioxide and methane at low pressures.

The carbons obtained from polymers deriving from precursors other than TPM (C-UPO-<NUM>, C-UPO-<NUM> and C-UPO-<NUM>) are also characterized by families of ultramicropores smaller than <NUM>Å, and families of micropores lying within the range <NUM>-<NUM>Å.

Finally, carbons C-UPO-<NUM> and C-UPO-<NUM> do not have the family of ultramicropores and are characterized by small micropores of around <NUM>Å. However these are characterized by higher surface area and micropore volume in the C-UPO-<NUM>-<NUM> series of carbons.

Claim 1:
A method of preparing a microporous carbon, comprising the step of subjecting a hyper-cross-linked polymer of formula (II):
<CHM>
wherein A is selected from a C atom, a Si atom, a Ge atom, a Sn atom, an adamantane group, an ethane group and an ethene group, and in which each of B, C, D and E is a ring structure selected from the radicals of the compounds benzene, naphthalene, anthracene, phenanthrene, pyrene, optionally having one or more substituents selected from nitro, amino, hydroxyl, sulfonyl, halogen, phenyl, alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, aryl, alkenyl and alkynyl groups, and in which n is an integer comprised between <NUM> and <NUM>, to a heat carbonization treatment at a temperature comprised within the range of from <NUM> to <NUM>, in the presence of an activating agent in a solvent,
wherein the obtained microporous carbon is characterized by an increased BET surface area and an increased quantity of micropores having a pore size below 7Å as compared to the hyper-cross-linked polymer of Formula (II) before carbonization, wherein the BET surface area and quantity of micropores having a pore size below 7Å are measured as disclosed in the description.