Patent Application
20190092644
Layered double hydroxides (LDHs) are a class of compounds which comprise two or more metal cations and have a layered structure. A review of LDHs is provided in Structure and Bonding; Vol. 119, 2005 Layered Double Hydroxides ed. X Duan and D. G. Evans. The hydrotalcites, perhaps the most well-known examples of LDHs, have been studied for many years. LDHs can intercalate anions between the layers of the structure.
Core shell particles are described in the literature by “core @ shell” (for example by Teng et al, Nano Letters, 2003, 3, 261-264), or by “core/shell” (for example J. Am. Chem. Soc., 2001, 123, pages 7961-7962). We have adopted the “core @ shell” nomenclature as it is emerging as the more commonly accepted abbreviation.
SiO2/LDH core-shell microspheres are described by Shao et al, Chem. Mater. 2012, 24, pages 1192-1197. Prior to treatment with a metal precursor solution, the SiO2 microspheres are primed by dispersing them in an Al(OOH) primer sol for two hours with vigorous agitation followed by centrifuging, washing with ethanol and drying in air for 30 minutes. This priming treatment of the SiO2 microspheres was repeated 10 times before the SiO2 spheres thus coated with a thin Al(OOOH) film were autoclaved at 100° C. for 48 hours in a solution of Ni(NO3)2·6H2O and urea. Hollow SiO2—NiAl-LDH microspheres obtained by this process were reported as exhibiting excellent pseudocapacitance performance. Unfortunately, the requirement for the Al(OOOH) priming of the SiO2 surface, prior to LDH growth, makes this process unsuitable for use on an industrial scale.
Chen et al, J. Mater. Chem. A, 1, 3877-3880 describes the synthesis of SiO2 @ MgAl-LDHs having use in the removal of pharmaceutical pollutants from water. The synthesis described comprises coprecipitating LDH from a metal precursor solution containing the SiO2 microspheres followed by ultrasound assisted direct growth of LDH nanosheets on the surface of the SiO2 microspheres. Unfortunately, the reported method does not allow the morphology of the surface LDHs to be tuned and the surface area of the product SiO2 @ LDHs is not high.
Molecular sieves are materials typically having very small pores of precise and uniform size. According to IUPAC notation, microporous materials have pore diameters of less than 2 nm (20 Å) and macroporous materials have pore diameters of greater than 50 nm (500 Å). Mesoporous materials, which exist between the microporous and macroporous materials, have pore diameters in the range 2 to 50 nm (20-500 Å). Molecular sieves are typically composed of a porous framework structure which contains ring structures composed in particular of atoms in a tetrahedral arrangement. One representative of such framework structures composed of atoms in a tetrahedral arrangement is the group of the zeolites, in which such ring structures are formed. Medium pore size is understood as meaning that, in a molecular sieve having a framework structure that forms a ring structure, the ring is formed of at least ten atoms. Large pore size is understood as meaning ring structures formed of at least twelve atoms.
Generally when an inorganic, porous, framework, such as a zeolite or molecular sieve, is coated (or surface treated) with a precursor or a material such as LDH, the inherent porosity and surface area of the inorganic, porous, framework material is reduced. This typically arises due to the coating ‘filling in’ or covering the pores of the inorganic framework.
It is therefore an object of the present invention to provide inorganic porous framework @ LDH core-shell materials, wherein the thickness, size and morphology of the LDH layer grown on the surface of the inorganic porous framework material can be tuned easily for different applications. Furthermore, it is an objective of the present invention to provide inorganic porous frameworks coated with LDHs with porosities comparable to those of their constituent materials. It is also a further object of the present invention to provide inorganic porous framework @ LDH core-shell materials that have a high surface area.
According to a first aspect of the present invention, there is provided a core @ layered double hydroxide shell material having the formula:
Tp@{[Mz+(1−x)M′xy+(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent)}q
wherein T is a solid, porous, inorganic oxide-containing framework material; Mz+ is a metal cation of charge z or a mixture of two or more metal cations each independently having the charge z; M′y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having the charge y;
According to a second aspect of the present invention, there is provided a method of making a core @ layered double hydroxide shell material, as defined herein, which method comprises the steps;
According to a third aspect of the present invention, there is provided a core @ layered double hydroxide shell material obtainable by, obtained by or directly obtained by the process described here.
According to a fourth aspect of the present invention, there is provided a core @ mixed metal oxide material having the formula:
Tp@{[Mz+1−xM′y+xOw]pŸ]
wherein T is a solid, porous, inorganic oxide-containing framework material, Mz+1−x M′y+xOw is a mixed metal oxide, or mixture of mixed metal oxides, which may be crystalline or non-crystalline, wherein Mz+ and M′y+ are different charged metal cations; Mz+ is a metal cation of charge z or a mixture of two or more metal cations each independently having the charge z; M′y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having the charge y; z is 1 or 2; y is 3 or 4; 0<x<0.9; w>0; p>0 and q>0; Ÿ is the residue of an Xn− anion in which n>0.
According to a fifth aspect of the present invention, there is provided a method of making a core @ mixed metal oxide material, as defined herein, comprising subjecting a core @ layered double hydroxide shell material, as defined herein, to heat treatment.
The following terms “core @ layered double hydroxide shell”, “inorganic porous framework @ LDH core-shell material” and “core @ LDH” are be used synonymously throughout the application. All of these terms may be used interchangeably to refer to a central core material (e.g. an inorganic porous framework material) which is coated with a layer of layered double hydroxide. Similarly, the term “Tp @ {[Mz+(1−x)M′xy+(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent)}q” will be understood as referring to a solid, porous, inorganic oxide-containing framework material that is coated with one or more layers of layered double hydroxide of the given formula.
Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
The present invention provides a core @ layered double hydroxide shell material, as defined herein. The core @ layered double hydroxide shell materials are prepared by growing a LDH on to the surface of the solid, porous, inorganic oxide-containing framework material.
By growing the LDHs on the surface of the solid, porous, inorganic oxide-containing framework material the inventors surprising found that discrete particles of core @ layered double hydroxide material with high porosities, surface area and excellent absorption properties could be achieved. The treatment with and subsequent inclusion of an aqueous miscible organic (AMO) solvent in the core @ layered double hydroxide shell material was found to further increase the improvement in porosity, surface area and absorption demonstrated by the core @ layered double hydroxide shell materials.
Furthermore, by growing the LDHs on the surface of the solid, porous, inorganic oxide-containing framework material the thickness of the LDH layer is able to be controlled, which advantageously allows for uniform particles to be prepared.
Suitably, the core @ layered double hydroxide materials of the present invention comprise an LDH layer with an average thickness of between 5 nm and 300 nm. More suitably, the core @ layered double hydroxide materials of the present invention comprise an LDH layer with an average thickness of between 30 nm and 200 nm. Yet more suitably, the core @ layered double hydroxide materials of the present invention comprise an LDH layer with an average thickness of between 40 nm and 150 nm. Most suitably, the core @ layered double hydroxide materials of the present invention comprise an LDH layer with an average thickness of between 40 nm and 100 nm.
Furthermore, the core @ layered double hydroxide materials of the present invention allow for coated solid, porous, inorganic oxide-containing framework materials which retain the surface area and porosity characteristics of their component materials.
In a particular embodiment, the core @ layered double hydroxide materials have specific surface area (a Brunauer-Emmett Teller (BET) surface area) of at least 50 m2/g, preferably at least 100 m2/g, more preferably at least 250 m2/g, yet more preferably at least 350 m2/g, even more preferably at least 450 m2/g, still more preferably at least 550 m2/g, and most preferably at least 650 m2/g.
In another embodiment, the core @ layered double hydroxide materials have an external surface area of at least 50 m2/g, preferably at least 100 m2/g, more preferably at least 125 m2/g, even more preferably at least 150 m2/g, and most preferably at least 175 m2/g.
In a further embodiment, the core @ layered double hydroxide materials have a micropore surface area of at least 50 m2/g, preferably at least 100 m2/g, more preferably at least 150 m2/g, yet more preferably at least 200 m2/g, even more preferably at least 300 m2/g, and most preferably at least 400 m2/g.
A core-layered double hydroxide composite material according to the present invention comprises a solid core particle having solid LDH attached to its surface.
The core material, as stated above, is a solid, porous, inorganic oxide-containing framework material. Typically, this framework material is a molecular sieve which is composed of a porous framework structure which contains ring structures comprising atoms in a tetrahedral arrangement. The framework, as stated above, is porous and comprises pores having a diameter of up to 50 nm, suitably up to 40 nm, more suitably up to 30 nm and most suitably up to 20 nm. Accordingly, the framework material may be either microporous, containing pores with a diameter less than 2 nm, or mesoporous, containing pores with a diameter of between 2 and 50 nm.
In one embodiment, the framework material is microporous, i.e. having pores of diameter less than 2 nm, suitably less than 1.5 nm and more suitably less than 1 nm.
In another embodiment, the framework material is mesoporous, i.e. having pores of diameter of between 2 nm to 50 nm, suitably between 2 nm and 30 nm, more suitably between 2 nm and 20 nm and most suitably between 2 nm and 10 nm.
Preferably, the molecular sieve comprises, preferably is selected from, a silicate, for example aluminium silicate, vanadium silicate or iron silicate. Alternatively, the molecular sieve comprises or is silicon-aluminium phosphate (SAPO) or aluminium phosphate (AIPO).
According to an embodiment of the invention, the molecular sieve material is aluminium silicate. Typically, the silicon: aluminium molar ratio is from 1 to 100. Preferably, the aluminium silicate is one in which the silicon: aluminium ratio is 1 to 60, preferably 1 to 50, more preferably 1 to 40 and most preferably 1 to 30.
According to an embodiment, the solid, porous, inorganic oxide-containing framework material is a zeolite material. Zeolites are microporous crystalline solids with well-defined structures and, generally, they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Typically, the zeolite material will be composed of aluminium silicate. Preferably, the aluminium silicate zeolite has a framework structure selected from zeolite types LTA, FAU, BEA, MOR and MFI. In the case of the latter (BEA, MOR and MFI), this is the framework code according to the Structure Commission of the International Zeolite Association. Such three letter codes are assigned to particular zeolite structures to identify the type of material they are composed of and the structure they adopt. For example, LTA is the code for zeolite type Linde Type A and MFI is the code for zeolite type ZSM-5.
The aluminium silicate zeolite may have a framework structure containing non-framework cations. Such cations may be organic cations or inorganic cations. A framework structure may contain both inorganic and organic cations as non-framework cations. Such non-framework cations may, for example, be selected from NR4+, where R is an optionally-substituted alkyl group (e.g. R=Me, Et, Pr, Bu) Na+, K+, Cs+ or H+. Suitably, the non-framework cation is selected from Na+, H+ or NR4+, wherein R is methyl or ethyl.
The aluminium silicate zeolite may be a crystalline aluminosilicate zeolite having a composition, in terms of mole ratios of oxides, as follows:
αMn+2/nO:Al2O3:βSiO2:γH20
wherein Mn+ is at least one cation having a valence n, α=0.9±0.2; β is at least 2 and γ is between 0 and 40.
Each zeolite classification type (e.g. LTA, FAU etc) may have one or more further sub divisions associated with it. For example, FAU zeolites can be further sub divided into X or Y zeolites depending on the silica-to-alumina ratio of their framework; with X zeolites having a silica-to-alumina ratio of between 2 to 3 and Y zeolites having a silica-to-alumina ratio of greater than 3. It will be understood that all such sub-divisions are covered by the definitions recited above.
In a particular embodiment of the present invention, the solid, porous, inorganic oxide-containing framework material is selected from: i) an aluminium silicate with a framework structure selected from zeolite types LTA, FAU, BEA, MOR or MFI; ii) an aluminophosphate; iii) a silicoaluminophosphate; or iv) a mesoporous silicate. Suitably, the solid, porous, inorganic oxide-containing framework material is selected from: i) an aluminium silicate with a framework structure selected from zeolite types LTA, FAU or MFI; ii) a microporous aluminophosphate; iii) a microporous silicoaluminophosphate; or iv) a mesoporous silicate selected from MCM-41 (Mobil Composition of Matter No. 41) or SBA-15 (Santa Barbara Amorphous No. 15). More suitably, the solid, porous, inorganic oxide-containing framework material is selected from; i) an aluminium silicate with a framework structure selected from zeolite types FAU or MFI; ii) a microporous aluminophosphate; iii) a microporous silicoaluminophosphate; or iv) a mesoporous silicate selected from MCM-41 (Mobil Composition of Matter No. 41) or SBA-15 (Santa Barbara Amorphous No. 15). Most suitably, the solid, porous, inorganic oxide-containing framework material is selected from; i) an aluminium silicate with a framework structure selected from zeolite types FAU or MFI; ii) a microporous aluminophosphate; or iii) a microporous silicoaluminophosphate.
In further embodiment, the solid, porous, inorganic oxide-containing framework material is selected from a zeolite selected from HY 5.1 or ZSM5-23, the microporous aluminophosphate AIPO5, the microporous silicoaluminophosphate SAPO5, or a mesoporous silicate selected from MCM-41 (Mobil Composition of Matter No. 41) or SBA-15 (Santa Barbara Amorphous No. 15).
The LDH grown on the surface of the solid, porous, inorganic oxide-containing framework material, comprises, and preferably consists of, LDH represented by the general formula (I):
[Mz+1−xM′y+x(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent) (I),
wherein;
As stated above, Mz+ and M′y+ are different charged metal cations. Mz+ is a metal cation of charge z or a mixture of two or more metal cations each independently having the charge z; M′y+ is a metal cation of charge y or a mixture of two or more metal cations each independently having the charge y.
Having regard to the fact that z=1 or 2, M will be either a monovalent metal or a divalent metal. M′, in view of the fact that y=3 or 4, will be a trivalent metal or a tetravalent metal.
A preferred example of a monovalent metal, for M, is Li. Examples of divalent metals, for M, include Ca, Mg, Zn, Fe, Co, Cu and Ni and mixtures of two or more of these. Preferably, the divalent metal M, if present, is Ca, Ni or Mg. Examples of metals, for M′, include Al, Ga, In, Y and Fe. Preferably, M′ is Al. Preferably, the LDH will be a Li—Al, an Mg—Al, an Mg—Ni—Al or a Ca—Al LDH.
The anion Xn− in the LDH is any appropriate inorganic or organic anion. Examples of anions that may be used, as Xn−, in the LDH include carbonate, hydroxide, nitrate, borate, sulphate, phosphate and halide (F−, Cl−, Br−, I−) anions. Preferably, the anion Xn−, is selected from CO32−, NO3− and Cl−.
The AMO-solvent is a >90%, suitably >95%, more suitably >98% and most suitably 100%, aqueous miscible organic solvent. Examples of suitable water-miscible organic solvents for use in the present invention include lower (1-3C) alkanols, and acetone. Preferably, the AMO-solvent is methanol, ethanol, isopropanol or acetone, especially acetone or ethanol.
According to one preferred embodiment, the layered double hydroxides are those having the general formula I above in which:
Preferably, in the LDH of the above formula, M is Mg, Ni or Ca and M′ is Al. The counter anion Xn− is typically selected from CO32−, OH−, F−, Cl−, Br−, I−, SO42−, NO3−and PO43−. In a most preferred embodiment, the LDH will be one wherein M is Mg, M′ is Al and Xn− is CO32−.
The following represent particular embodiments of the core @ layered double hydroxide:
Tp@{[Mz+(1−x)M′y+x(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent)}q (I)
Tp@{[Mz+(1−x)M′y+x(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent)}q (la)
Tp@{[Mz+(1−x)M′y+x(OH)2]a+(Xn−−)a/n·bH2O·c(AMO-solvent)}q (lb)
Tp@{[Mz+(1−x)M′y+x(OH)2]a+(Xn−)a/n·bH2O·c(ethanol)}q (lc)
Tp@{[Mz+(1−x)M′y+x(OH)2]a+(Xn−)a/n·bH2O·c(ethanol)}q (ld)
Preferred, suitable, and optional features of any one particular aspect of the present invention are also preferred, suitable, and optional features of any other aspect.
The core @ LDH shell material of the invention, as described above, may be prepared by a method which comprises the steps:
In a particular embodiment, core @ LDH shell material of the invention, as described above, may be prepared by a method which comprises the steps:
In carrying out the method of the invention, preferably porous, inorganic framework material particles are dispersed in an aqueous solution containing the desired anion salt, for example, Na2CO3. This aqueous solution containing one or more anionic salts (e.g. Na2CO3) will be understood to be the anionic solution described herein. A metal precursor solution, i.e. a solution combining the required monovalent or divalent metal cations and the required trivalent cations may then be added, preferably drop-wise, into the dispersion of the core material particles. Preferably, the addition of the metal precursor solution is carried out under stirring. The pH of the reaction solution is preferably controlled within the pH range 8 to 12, typically 8 to 11, more preferably 9 to 10. Typically, NaOH may be used to adjust the pH of the solution.
During the reaction, the LDH produced from the metal precursor solution reaction is formed on the surfaces of the core material particles as nanosheets.
Without wishing to be bound by theory, it is believed that a small amount of aluminium leaching from the porous, inorganic framework material allows the seeded growth of the LDHs on to their surface.
It is preferred that the temperature of the metal ion containing solution in step (a) is within a range of from 20 to 150° C., preferably from 20 to 80° C., more preferably from 20 to 50° C. and most preferably from 20 to 40° C.
The obtained solid product is collected from the aqueous medium. Examples of methods of collecting the solid product include centrifugation and filtration. Typically, the collected solid may be re-dispersed in water and then collected again.
The finally-obtained solid material may then be subjected to drying, for instance, in an oven for several hours.
In the event that a product containing AMO-solvent is required, the material obtained after the collection/re-dispersion procedure described above may be washed with, and preferably also re-dispersed in, the desired aqueous miscible organic (AMO) solvent, for instance acetone. If re-dispersion is employed, the dispersion is preferably stirred. Stirring for more than 2 hours in the solvent is preferable. The final product may then be collected from the solvent and then dried, typically in an oven for several hours.
The growth of LDH nanosheets on the surface of the framework particles is “tuneable”. That is to say, by varying the chemistry of the precursor solution, the pH of the reaction medium and the rate of addition of the precursor solution to the dispersion of framework particles, the extent of, and the length and/or thickness of, the LDH nanosheets formed on the framework particle surfaces can be varied.
In another aspect of the present invention, there is provided a core @ layered double hydroxide shell material obtainable by, obtained by, or directly obtained by the process described hereinabove.
The core @ LDH shell materials of the invention may be used as catalysts and/or catalyst supports.
The inventors additionally found that when the core @ layered double hydroxide shell materials of the present invention are subjected to calcination, the layered double hydroxide undergoes water loss followed by decomposition to produce core @ mixed metal oxide materials which have use as catalyst supports and sorbents. These core @ mixed metal oxide materials are represented by the formula
Tp@{[Mz+1−xM′y+xOw]pŸ]
Accordingly, in a further aspect, the present invention provides the core @ mixed metal oxide materials represented by the formula given above. According to a yet further aspect, the present invention provides a method of making core @ mixed metal oxide materials having the formula
Tp@{[Mz+1−xM′y+xOw]pŸ]
Tp@{[Mz+(1−x)M′xy+(OH)2]a+(Xn−)a/n·bH2O·c(AMO-solvent)}q
The term heat treatment may be used interchangeably with the term calcined, and both refer to subjecting the core @ layered double hydroxide to heat, which results in a loss of moisture and/or a reduction and/or an oxidation and/or the decomposition of the core @ layered double hydroxide material.
Preferably, the core @ layered double hydroxide shell material is calcined at a temperature in the range of 100° to 1000° C., preferably in the range of 250° to 750° C. and more preferably in the range of 400° to 550° C. The heat treatment will typically be carried out in air or under nitrogen, oxygen, argon or hydrogen, suitably in air or under nitrogen or hydrogen.
Left—a comparison with starting material, where (a) is HY5.1, (b) is HY5.1 @ LDH-A and (c) is LDH-A.
Variation of Zeolite to LDH Ratio in HY15 @ AMO-LDH
While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1512458.9 | Jul 2015 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/052158 | 7/15/2016 | WO | 00 |
