Patent Application
20140291250
The present invention concerns nano-structured composite material based on compositions of manganese and cerium, in particular for use in processes for the removal of pollutants.
More in detail the invention concerns materials of the said kind, for the application as catalysts in processes for the removal of pollutants which are presents in liquid or gas phases, through adsorption and oxidation.
Still more in detail, the invention refers to the realisation of a new class of nano-structured catalytic compositions, based on sub-stoichiometric vitreous compounds (sub-oxides) of manganese (Mn) and ultrafine crystalline nanoparticles (i. e. with a diameter comprised in the range 1-6 nm) of compounds of cerium (Ce), which can be used in the environmental field as redox catalysts, adsorbing materials and heavy metals blasts.
As it is known, at present, liquid phase oxidation process, mainly concerning the treatment of wastewaters of a special type [Catalysis Today, vol. 53 (1999) p. 81], wherein by special are intended wastewaters having an industrial or civil origin and containing toxic compounds and/or compounds which are refractory to common processes of biological depuration, are at the center of a strong scientific and technologic activity worldwide.
To date, the oxidative treatments applied to the wastewaters of industrial origin, using oxygen/air as oxidizing agent, are conducted at generally high temperatures and pressures. Other kinds of treatments exist in presence of chemical oxidants (such as for example H2O2, O3, KMnO4) which allow to work in less critical conditions, but such oxidants appear to be very expensive. All these treatments are necessary for the mineralization of organic and inorganic compounds which are poorly biodegradable and which, therefore, would be toxic to flora and fauna, but have the drawback of being very costly in terms of investment, fixed and operative costs [Catalysis Today, vol. 53 (1999) p. 81].
To solve this problem, i. e. to obtain a significant mitigation of the operating conditions of the oxidative processes using air/oxygen and a overall improvement of their efficiency and costs the use of suitable solid catalysts was proposed [Ind. Eng. Chem. Res., vol. 27 (1988) p. 718]. The use of these catalysts promotes the kinetics of the oxidation reactions of the pollutants, thus enabling the conduction of the processes in milder conditions of temperature and pressure and considerably reduced residence times compared to the non-catalytic processes [Chem. Eng. Journal, vol. 73 (1999) p. 149].
In this context, the heterogeneous catalysts based on noble metals (Pt, Pd, Ru, Rh, ecc.) supported on refractory oxides such as alumina, silica, titania, ceria, more and mixtures thereof, exhibit greater activity in the oxidation of various types of pollutants, and in particular in relation to organic polymers (PEG), phenolic compounds, acetic acid and ammonia [Ind. Eng. Chem. Res., vol. 37 (1998) p. 309].
However, besides being very expensive, these materials are generally sensitive to deactivation phenomena such as fouling, sintering and leaching of the active phase. To these phenomena, it is also added the poisoning of active sites, caused by the adsorption on the catalyst of pollutants and decomposition products thereof, which irreversibly impair the efficiency and chemical stability thereof [Ind. Eng. Chem. Res., vol. 37 (1998) p. 309].
In parallel, several studies have shown that catalysts based on copper oxides or other non-noble metals (e.g. Co, Zn and Ti) were efficient and reactive in oxidation reactions of phenol in liquid phase with air/oxygen (CWAO) at temperatures comprised between 160 and 250° C.
Imamura et al. [Ind. Eng. Chem. Res., vol. 38 (1999) p. 1743] proposed catalytic systems based on the combination of manganese, cerium, cobalt and bismuth oxides specifically conceived to increase the efficiency of oxidation in liquid phase of several kinds of pollutants. Catalytic systems based on manganese and cerium oxides denote oxidising properties in respect of phenol at a temperature of 150° C.
Hocevar et al. [Journal of Catalysis, vol. 184 (1999) p. 39] described a further advancement in the development of catalysts that are active in the processes of oxidation of phenol in liquid phase with air/oxygen, proposing catalytic systems based on oxides of copper and cerium prepared by coprecipitation. These catalysts, which are stabilized by means of treatments of calcination at high temperature (900° C.), show a good resistance to the phenomena of poisoning (poisoning) by organic polymers.
Hocevar et al. [Applied Catalysis, B: Environmental, vol. 28 (2000) p. 113] have subsequently improved the catalytic systems based on oxides of copper and cerium using the sol-gel preparative methodology.
Arena et al. [Applied Catalysis, B: Environmental, vol. 45 (2003) p. 51] have subsequently shown that the activity of catalysts based on oxides of copper and cerium derived actually from the action in homogeneous phase of the Cu2+ ions released into solution by the same catalyst in the initial stages of the reaction.
Santos et al. [Environmental Science and Technology, vol. 38 (2004) p. 133] have shown that, in order to use the copper catalysts, obviating the phenomena of leaching, it was possible to conduct the reaction of oxidation of phenol in liquid phase with air/oxygen under basic pH (>8), through the use of carbonate/bicarbonate buffers. However, by adopting basic pH it was achieved only a partial reduction of the toxicity of the effluent due to the presence of phenols, but not the mineralization of the organic load.
Furthermore, it was demonstrated that the catalysts based on oxides of manganese and cerium operate the mineralization of phenol at temperatures below 250° C. and oxygen partial pressures lower than 50 atm, presenting anyway phenomena of deactivation due to fouling of the adsorbed species [Ind. Eng. Chem. Res., vol. 43 (2004) p. 5089.].
Imamura et al. [Ind. Eng. Chem. Prod. Res. Dev. 24, (1985) p. 75] described the preparation of a composite material based on manganese oxide and cerium oxide obtained by coprecipitation of manganese and cerium, in the form of their respective hydroxides and/or oxides, from a solution, using sodium hydroxide. The precipitate was removed from the solution of precipitation, and calcined at 350° C. to obtain the catalyst.
WO 2001/45833 describes various catalytic compositions based on cerium oxide.
US 2003/0166987 describes the preparation of catalysts based on oxides of manganese and cerium to be used in the destruction of organic compounds in liquid phase, obtained by coprecipitation and subsequent heat treatment at a temperature between 100 and 500° C. The catalyst was identified as a crystalline material characterized by a particle size of 30-7000 nm and total surface area of 60-120 m2/g.
US 2004/0192947 describes the preparation of a catalyst consisting of an array of minerals and, more specifically, a silica matrix containing in its interior some particles of metal oxides.
Larachi [Topics in Catalysis, vol. 33, n. 1-4, p. 109-134 (2005)] describes the application of the oxides of manganese in the oxidative catalysis in liquid phase (CWO).
WO2007/069238 claims a new methodology allowing for the preparation of catalysts consisting of a mixture of Mn2O3 and CeO2 with high degree of crystallinity and greater total surface area (i.e. 200-300 m2/g), prepared by alkaline co-precipitation of ions of cerium and manganese within a meso-porous matrix (type SBA), calcination of the system at high temperature (700° C.) and subsequent dissolution of the same matrix by etching in alkaline solution (nanocasting).
Arena et al. [Chem. Mater., Vol. 19, (2007) p. 2269] describe the preparation of catalysts based on highly dispersed oxides of manganese and cerium obtained by alkaline coprecipitation of tetravalent cerium and manganese (Ce4+ and Mn4+) generated in situ by oxidation of salts of trivalent cerium (Ce3+) and divalent manganese (Mn2+) with potassium permanganate (KMnO4), which are active in the reaction of oxidation of phenol in liquid phase [Ind. Eng. Chem. Res. Vol. 46 (2007) p. 6724].
From the foregoing reasoning it follows that, to date, the catalysts based on oxides of manganese and cerium are industrially prepared by conventional co-precipitation [US 2003/0166987]. The structure of the catalysts based on oxides of manganese and cerium prepared by coprecipitation is the result of a process of chaotic, therefore, not controllable, aggregation of matter. The technique of co-precipitation does not allow to structure the material at a nano-size level.
At present, therefore, the technique of co-precipitation can not check favorably the dispersion of the different elements and/or solid phases nor the extent of the total surface area and porosity of the material.
In light of the above, it is evident the need to achieve catalytic compositions with improved structural properties and new materials with improved chemical and physical properties that can be advantageously used in the removal of molecules highly polluting and difficult to remove.
In this context it is included the solution according to the present invention, which aims to provide nano-structured catalytic compositions comprised of sub-stoichiometric vitreous compounds of manganese and of ultrafine nanoparticles of compounds of cerium highly active in the removal of pollutants which are presents in liquid or gas phases, through adsorption and oxidation, as well as a process for their preparation.
Objective of the present invention is therefore the realization of innovative nano-structured catalytic compositions, comprised of:
In particular, the process of preparation of the new nano-structured catalytic compositions according to the present invention allows to favorably control the size of the crystallites of the compounds of cerium, to the benefit of the activity of oxidation.
Moreover, the process of preparation of the new nano-structured catalytic compositions according to the present invention allows to favorably control the extension of the total surface area and the porosity of the compositions themselves, to the benefit of the property of adsorption.
Still, the process of preparation of the new nano-structured catalytic compositions according to the present invention allows to favorably control the dispersion of the various components that are present in the same compositions, to the benefit of the chemical reactivity and of the catalytic activity in the processes of oxidation in presence of air/oxygen.
Furthermore, according to a preferred aspect of the present invention, it is proposed the addition of some metals (such as for example V, Ti, Cr, Fe, Cu, Zn, Sn, Ga, Gd, Y, Zr, Al, Si, K, Li, Pb, Cs) within the nano-structured catalytic compositions comprised of sub-stoichiometric vitreous compounds of manganese and ultrafine nanoparticles of compounds of cerium according to the present invention, in order to improve their performance in the processes of adsorption and oxidation.
This and other results are obtained according to the present invention proposing a nano-structured composite material with catalytic properties with optimal characteristics with respect to: pore size, extension of the active surface, position and, therefore, accessibility, of the active sites, and selectivity of the active sites themselves.
The purpose of the present invention is therefore that of proposing a nano-structured composite material based on compositions of manganese and cerium, which allows to overcome the limits of the solutions according to the prior art and to achieve the previously described technical results.
Further object of the invention is that said material can be produced with substantially low costs, as far as both the production costs and the maintenance costs is concerned.
Not last object of the invention is that of realising a nano-structured composite material which is substantially simple, safe and reliable in its use as catalyst and/or adsorbing filter.
It is therefore a first specific object of the present invention a nano-structured composite material based on compositions of manganese and cerium, as defined in claim 1.
A second and a third specific object of the present invention consist of two processes of production of said material.
It is then a fourth specific object of the present invention the use of said material as catalyst and/or adsorbing filter in the processes of removal of highly polluting molecules which are difficult to remove.
Finally, it is a fifth specific object of the present invention a process for the removal of pollutants by means of said material.
Without wishing to be bound by theory, it is believed that the catalysts forming object of the present patent constitute a new class of nanostructured materials, since they are obtained by means of an original and controlled process of aggregation and structuring of basic nanoscopic unit having the function of building blocks.
Moreover, without wishing to be bound by theory, it is believed that the structure of the catalysts forming object of the present invention is original and completely innovative compared to that of the catalysts according to the prior art, since, to date, there are no examples of nanostructured catalysts based on sub-stoichiometric vitreous compounds (sub-oxides) of manganese (Mn) and ultrafine crystalline nanoparticles (i. e. with diameter comprised between 1 and 10 nm, preferably between 1 and 6 nm) of compounds of cerium (Ce).
It is an object of the present invention the realisation of innovative nano-structured catalytic compositions, comprised of:
Without wishing to be bound by theory, it is believed that the synergistic role shown by the two compounds (i. e. Mn and Ce) in the compositions forming the object of the present invention promotes the processes of electronic and ionic transfer (i.e. O2−) in redox processes, improving the catalytic performances of the compositions themselves.
In the context of the present invention, the expression “at least one compound of manganese” is intended to designate one or more than one compounds of manganese (Mn). Mixtures of more compounds of manganese (Mn) can be conveniently used for the purposes of the invention.
Herein below, the expression “compound of manganese” is intended, for the purposes of the present invention, in both plural and singular form, i. e. the inventive chemical formulation can comprise one or more than one compounds of manganese (Mn).
The term compound of manganese (Mn) is intended to designate any compound in which at least one atom of manganese is present, regardless of its valence state.
Therefore, the compound of manganese can comprise bivalent manganese (MnII), trivalent manganese (MnIII), tetravalent manganese (MnIV), exavalent manganese (MnVI) and eptavalent manganese (MnVII), and can also comprise mixtures thereof.
The use of the compound of manganese (Mn) in the catalytic composition according to the present invention can be considered convenient if this is present in an amount of at least 25%, more preferably at least 35%, still more preferably at least 45%, with respect to the total weight of the catalytic composition. The maximum amount of the compound of manganese (Mn) will not be limited with respect to the total weight of the catalytic composition; in general a maximum amount of this compound of 75%, more preferably of 65%, still more preferably of 55% will be considered convenient.
The compound of manganese (Mn) acts as a catalytically active phase in reactions of oxidation and reduction of both organic and inorganic compounds, whether they are present in gas or liquid phase or in both phases.
In accordance with a preferred embodiment of the present invention the compound of manganese (Mn) is a vitreous mixture of several sub-stoichiometric oxides (sub-oxides) of manganese.
In particular, said vitreous mixture di sub-stoichiometric oxides (sub-oxides) of manganese may also incorporate ions of other chemical elements different from manganese (hetero-ions), i. e. the cations of the elements Ce, V, Ti, Cr, Fe, Cu, Zn, Sn, Ga, Gd, Y, Zr, Al, Si, La, K, Li, Pb, Cs, or mixtures thereof.
Amongst said hetero-ions, the elements titanium (Ti), zirconium (Zr) and cerium (Ce) are more preferred, the elements Zr and Ce are still more preferred; Ce is still more preferred, both in the trivalent state (CeIII) and in the tetravalent state (CeIV) or even mixtures thereof.
In particular, then, the content of cations incorporated into the vitreous mixture of sub-oxides of manganese can be considered convenient if at least equal to 1%, more preferably at least 5%, still more preferably at least 10%, of the total weight of the compound of manganese (Mn). The maximum amount of cations incorporated into the compound of manganese (Mn) will not be limited with respect to the total weight of the same compound; in general an amount of incorporated hetero-elements at the maximum equal to 35%, more preferably at the maximum 25%, still more preferably at the maximum 15% will be considered convenient.
In the context of the present invention the expression “at least one compound of cerium” is intended to designate one or more than one compounds of cerium (Ce). Mixtures of more compounds of cerium (Ce) can be conveniently used for the purposes of the invention.
In the remaining part of the text, the expressions “compound of cerium” are intended, for the purposes of the present invention, in both plural and singular form, i. e. the inventive chemical formulation can comprise one or more than one compound of cerium (Ce).
The term compound of cerium is intended to designate any compound wherein at least one atom of cerium is present, regardless of its state of valence. Therefore, the compound of cerium can comprise both trivalent cerium (CeIII) tetravalent cerium (CeIV), or can even comprise mixtures thereof.
The compound of cerium can act in the catalytic composition at the same time as a co-catalyst (co-active phase) and support, being used in a greater amount, according to a first preferred embodiment, or can be used as electronic/structural promoter, in addition to other inorganic promoters, being used in a limited amount, according to a second preferred embodiment.
The amount of the compound of cerium in the catalytic composition will be conveniently chosen on the base of the function (first or second preferred embodiment) that this compound will have in the catalytic composition.
According to the first preferred embodiment of the invention, the compound of cerium will be used at the same time both as co-catalyst (co-active phase) and as inorganic support, being greater in the presence in the catalytic composition.
According to the first preferred embodiment of the invention, the use of the compound of cerium in the catalytic composition can be considered convenient if this is present in an amount of at least 20%, more preferably at least 30%, still more preferably at least 50%, with respect to the total weight of the catalytic composition. The maximum amount of the compound of cerium will not be limited with respect to the total weight of the catalytic composition; in general will be considered convenient an amount of this compound at the maximum equal to 85%, more preferably at the maximum 70%, still more preferably at the maximum 60%.
According to the first preferred embodiment, it is preferred that the compound of cerium is a nanocrystalline oxide, e.g. nanocrystalline CeO2 or nanocrystalline Ce2O3 or a mixture of the two nanocrystalline compounds.
According to the second preferred embodiment of the invention, the compound of cerium will be used as electronic/structural promoter, in combination with other inorganic promoters.
According to the second preferred embodiment of the invention, the use of the compound of cerium in the catalytic composition can be considered convenient if this is present in an amount of at least 2%, more preferably at least 5%, still more preferably at least 10%, with respect to the total weight of the catalytic composition. The maximum amount of the compound of cerium will not be limited with respect to the total weight of the catalytic composition; in general, an amount of at the maximum 40%, more preferably 35%, still more preferably 20% will be considered convenient.
According to the second preferred embodiment the compound of cerium can be chosen in particular amongst oxides (CeO2; Ce2O3), ceric peroxide (Ce—H2O—O2), ceric fluoride (CeF4), ceric sulphate [Ce(SO4)2], ceric ammonium nitrate [(NH4)2Ce(NO3)6], ceric ammonium sulphate [(NH4)2Ce(SO4)3], hexa-chloro ceric ammonium [(NH4)2CeCl6], cerous peroxide [Ce—H2O—H2O2], cerous basic carbonate [CeOHCO3], cerous hydroxide [Ce(OH)3], cerous carbonate [Ce2(CO3)3.nH2O], cerous oxycarbonate [Ce2O2(CO3)], cerous phosphate [CePO4.H2O], cerous fluoride [CeF3], cerous oxyfluoride [CeOF], cerous sulphate [Ce2(SO4)3], cerous sulfide [Ce2S3], cerium oxysulfide [Ce2O2S], cerous nitrate [Ce(NO3)3.6H2O], cerous chloride [CeCl3], cerous oxychloride [CeOCl], cerous bromide [CeBr3] and mixtures thereof.
The catalytic composition can comprise, in addition to the compound of manganese (Mn) or to the compound of cerium (Ce), some additional components.
For example, according to the realisation of the vitreous mixture of sub-oxides of manganese, the compound of manganese (Mn) can incorporate additional components such as alkali metals, alkaline earth, transition and rare earths. In particular, components containing Ce, V, Ti, Cr, Fe, Cu, Zn, Sn, Ga, Gd, Y, Zr, Al, Si, K, Li, Cs, or mixtures thereof can be added to the compound of manganese.
According to the second preferred embodiment of the invention, the compound of cerium (Ce) comprises at least one electronic/structural promoter chosen amongst the oxides of metallic or semi-metallic elements different from the compound of cerium.
For preferred but not limitative purposes, the so-called promotor (compound P) will be preferably chosen amongst alumina, silica, titania, germania, zirconia, aluminosilicates, zirconil-silicates, chromium oxides, copper oxides, molybdenum oxides, tantalum oxides, zinc oxides, yttrium oxides, vanadium oxides, iron oxides and mixtures thereof.
It is believed, without this limiting the scope of the invention, that the addition of the additional components to the compound of manganese (Mn) may favorably modify the properties of chemisorption of the surface, increasing the interfacial area of the catalytic phase, improve the acid-base properties of the catalytic element.
It is considered, without this limiting the scope of the invention, that the correct combination of the compound of cerium with one or more than one compounds of metallic or semi-metallic oxide can modify in a substantial way the structural properties of the catalyst, increasing in primis the extension of the total surface area, the resistance of the catalyst to sinterisation and to poisoning, as well as the properties of mechanical resistance.
Another object of the present invention is the process of production of the proposed catalytic materials.
The manufacture of the materials is realized by executing a precise method of preparation, which sees the preparation and the appropriate mixing of the following solutions:
i) reducing solution,
ii) oxidising solution,
iii) alkaline solution, and
iv) solution of promoters.
The method of preparation requires control of the following experimental parameters:
The term reducing solution is intended to designate a solution obtained by dissolution, in water or in a water/solvent mixture, of salts and soluble compounds of divalent manganese (MnII), trivalent cerium (CeIII) and other metallic elements (Me) capable of generating redox couples [Mex+→n·e− Me(x+n)+]. For exemplificative but non limitative purposes, amongst the chemical elements capable of generating redox couples the following may be cited:
The reducing solution always contains ions in the reduced state of the redox couple (Mex+).
For the preparation of the reducing solution various salts and soluble compounds (precursors) of manganese (Mn) can be used. For exemplificative but non limitative purposes as precursor agents of manganese the following can be cited: manganous sulphate (MnSO4), manganous nitrate (Mn(NO3)2), manganous fluoride (MnF2), manganous chloride (MnCl2), manganous bromide (MnBr2), manganous iodide (MnI2), manganous carbonate (MnCO3), manganese-ous formate (Mn(HCOO)2), manganese-ous acetate (Mn(CH3COO)2), manganese-ous acetylacetonate (Mn(C5H7O2)2), manganese-ous phtalocianina, and more.
As a precursor of manganese (Mn) for the preparation of the reducing solution it is preferable to use manganous chloride (MnCl2), more preferably manganous acetate (Mn(CH3COO)2), still more preferably manganous nitrate (Mn(NO3)2).
For the preparation of the reducing solution various salts and soluble compounds (precursors) of cerium III (Ce3+) can be used. For exemplificative but non limitative purposes as precursor agents of cerium (Ce) the following can be cited: cerous peroxide [Ce—H2OH2O2], cerous basic carbonate [CeOHCO3], cerous hydroxide [Ce(OH)3], cerous carbonate [Ce2(CO3)3.nH2O], cerous oxy-carbonate [Ce2O2(CO3)], cerous phosphate [CePO4.H2O], cerous fluoride [CeF3], cerous oxy-fluoride [CeOF], cerous sulphate [Ce2(SO4)3], cerous sulfide [Ce2S3], cerous nitrate [Ce(NO3)3.6H2O], cerous chloride [CeCl3], cerous oxy-chloride [CeOCl], cerous bromide [CeBr3] and mixtures thereof.
As a precursor of cerium (Ce) for the preparation of the reducing solution it is preferable to use cerous chloride [CeCl3], cerous sulphate [Ce2(SO4)3] or hydrate cerous nitrate [Ce(NO3)3.6H2O], more preferably cerous sulphate [Ce2(SO4)3] or hydrate cerous nitrate [Ce(NO3)3.6H2O], still more preferably hydrate cerous nitrate [Ce(NO3)3.6H2O].
The term oxidising solution is intended to designate a solution obtained by dissolving strong oxidizing chemical compounds in water. For exemplificative but non limitative purposes of strong oxidising chemical compounds the following can be cited:
As oxidizing agent for the preparation of the oxidising solution it is preferable to use hydrogen peroxide (H2O2), potassium permanganate (KMnO4) or a mixture of potassium permanganate (KMnO4) and ceric ammonium nitrate [(NH4)2Ce(NO3)6], more preferably potassium permanganate (KMnO4) or a mixture of potassium permanganate (KMnO4) and ceric ammonium nitrate [(NH4)2Ce(NO3)6], still more preferably a mixture of potassium permanganate (KMnO4) and ceric ammonium nitrate [(NH4)2Ce(NO3)6].
With the term alkaline solution it is intended to designate a solution obtained by dissolving an alkaline agent in water. For exemplificative but non limitative purposes as alkaline agents (inorganic and organic bases) the following can be cited: LiOH, NaOH, KOH, RbOH, CsOH, NH4OH, Ca(OH)2, Ba(OH)2, ammonium hydroxide (NH4OH) carbonates and bicarbonate (of Na, K, Li, Cs, Mg, Ca, etc.), amines, pyridine, urea and all the inorganic and organic compounds capable of generating OH− ions in solution.
As an alkaline agent it is preferable to use sodium hydroxide (NaOH), ammonium hydroxide (NH4OH) or potassium hydroxide (KOH), more preferably ammonium hydroxide (NH4OH) or potassium hydroxide (KOH), still more preferably potassium hydroxide (KOH).
With the term solution of promoters it is intended to designate a solution obtained by dissolution, in water or in a water/solvent mixture, of salts and soluble compounds (precursors) of electronic/structural promoting elements. The elements will be selected on a case by case basis in relation to their ability to give precipitates of stable hydroxides, oxides, carbonates and hydroxy-carbonates. For exemplificative but non limitative purposes, as possible promoting elements the following can be cited: Zr, Al, Si, Cu, Zn, Ga, Gd, Y, La, V, Mo and more. Ions of the promoting elements will have a stable state of valence, i. e. will not undergo oxidation or reduction by the reducing mixture or the oxidising mixture.
As precursors of the elements designated for the preparation of the solution of promoters it is preferable in general to use chlorides (Cl−), sulfates (SO42−) or nitrates (NO3−), more preferably sulfates (SO42−) or nitrates (NO3−), still more preferably nitrates (NO3−).
A first preferred embodiment of the method of preparation of the catalytic compositions forming object of the present invention provides for the simultaneous reduction/oxidation/precipitation of the designated metals and semi-metals, performed in a particular basic solution (pH>7) with high oxidizing potential (E>+0.80V).
According to the first preferred embodiment of the method of preparation (protocol m1), the manufacture of the catalytic compositions forming object of the present invention provides for the following steps:
a) preparation, in accordance with precise weight ratios, of the reducing, oxidising, alkaline solutions and in case of the solution of promoters;
b) mixing the reducing solution with Ia solution of promoters until obtaining a new solution defined solution A;
c) mixing the oxidising solution with the alkaline solution, until obtaining a new solution defined solution B;
d) adding a little at a time, in a slow and continuous way, solution A to solution B, producing the precipitation of metals/semimetals under stirring, keeping constant pH, redox potential of the mixture B and rate of administration of the solution A;
e) ageing and thickening the precipitate and subsequent removal of the precipitate from the solution “titrant environment” by filtration;
f) washing filtrate and subsequent drying at 100° C.,
g) final consolidation of the structures of the precipitates and decomposition of any salts/hydroxides/carbonates/hydroxy-carbonates by thermal treatment, according to a precise protocol of calcination.
A second preferred embodiment of the method of preparation of the catalytic compositions forming object of the present invention provides for the simultaneous reduction/oxidation/precipitation of the designated metals and semi-metals, performed in a particular slightly acid solution (4≦pH≦5) with high reducing potential (E<0.70V).
According to the second preferred embodiment of the method of preparation (protocol m2), the manufacture of the catalytic compositions forming object of the present invention provides for:
a) preparation, in accordance with precise weight ratios, of the reducing and the oxidising solutions, and in case of the solution of promoters;
b) mixing the oxidising solution with the solution of promoters until obtaining a new solution defined A′;
c) adding a little at a time, in a slow and continuous way, solution A′ to the reducing solution, producing the precipitation of the metals/semi-metals under stirring, keeping constant pH, redox potential of the reducing solution and the rate of administration of the solution A′;
d) ageing and thickening the precipitate and subsequent removal of the precipitate from the solution “titrant environment” by filtration;
e) washing filtrate and subsequent drying a 100° C.;
f) final consolidation of the structures of the precipitates and decomposition of any salts/hydroxides/carbonates/hydroxy-carbonates by thermal treatment, according to a precise protocol of calcination.
Without wishing to be bound by theory, it is believed that the presence of an oxidizing agent during precipitation of the cerium ion favors the formation of ultrafine nanocrystalline ceria (CeO2), since the oxidation “out spheres” of the trivalent cerium (CeIII) ions in solution by the oxidizing agent leads to the formation of tetravalent cerium (CeIV) which is extremely reactive, therefore, able to extract more easily the oxygen of the hydroxide present in the first sphere of solvation, leading to the immediate formation of ultrafine crystalline nanoparticles (diameter 1-6 nm) of CeO2, as shown in the following reaction scheme:
wherein:
[oxy]aq is the oxidizing agent coordinated in water,
[rid]aq is the oxidizing agent that after transfer of an electron is in turn reduced.
Without wishing to be bound by theory, it is believed that the simultaneous presence of the divalent ion of manganese (MnII), of metal ions capable of generating redox couples [Mex+→ne−Me(x+n)+] and of a strong oxidizing agent promotes the formation of sub-oxides of manganese, which promotes the formation of oxides from the defective structures coordinatively unsaturated in which the manganese ion assumes not unitary formal valences.
Without wishing to be bound by theory, it is believed that the formation of sub-oxides of manganese improves the redox properties of the catalytic composition, improving the catalytic performance of total oxidation (mineralization) of organic molecules.
Without wishing to be bound by theory, it is believed that the formation of sub-oxides of manganese increases the mobility of oxygen ions present in the catalytic composition in the processes of oxidation-reduction.
Without wishing to be bound by theory, it is believed that the formation of sub-oxides of manganese improve the properties of conduction and electron transfer of the catalytic composition.
Without wishing to be bound by theory, it is believed that the formation of sub-oxides of manganese give the catalytic composition properties of photo-conduction and electro-conduction.
It was found that, using potassium permanganate (KMnO4) as oxidizing agent, the average oxidation value (Average Oxidation Number AON) of the sub-oxides of manganese is greater than that obtainable with the use of other oxidizing agents.
It was also found that, using potassium permanganate (KMnO4) as oxidizing agent, the content of sub-oxides of manganese present in the catalytic composition is greater than that obtainable with the use of other oxidizing agents.
The catalytic compositions according to the present invention are identifiable by the generic empirical formula MnaCebIcOx, wherein the coefficients a and b can assume values comprised between 0.001 and 0.999, whereas the coefficient c, which takes account of the presence of additional metal and semi-metal ions (I), can vary between 0.000 and 0.998. The value assumed by the coefficients a, b, c in the various catalytic compositions is such that their sum is always equal to 1. While the value of the coefficient x is obtainable from the following mathematical relationship:
x=(VMn·a+VCe·b+VI·c)/2
wherein the terms VMn, VCe and VI indicates the formal average valence in the catalytic composition respectively of the manganese (Mn) ion, of the cerium (Ce) ion and of the additional ion, such as to ensure the electroneutrality.
The total concentration of metals and semi-metals in the reagent solution is preferably between 0.1 and 3.0M, and more preferably is equal to 0.5M.
The atomic ratio manganese/cerium (Mnat/Ceat) in the catalytic composition will be preferably comprised between 0.75 and 2.0, and more preferably is equal to 1.0.
The pH of the solutions A and B and the temperature at which carry out the precipitation processes are chosen on the basis of the values of the constants of precipitation of the ions designated in the preparation of the catalytic composition.
According to the first method of manufacture described (protocol m1), the pH of the solution A is preferably comprised between 1.0 and 4.0, and more preferably is equal to 2.0.
According to the first method of manufacture described (protocol m1), the pH of the solution B is preferably comprised between 7.0 and 10.0, and more preferably is equal to 8.0.
According to the second method of manufacture described (protocol m2), the pH of the solution A′ is preferably comprised between 4.0 and 6.0, and more preferably is equal to 5.0.
According to the second method of manufacture described (protocol m2), the pH of the solution B′ is preferably lower than 3.0, more preferably lower than 2.0, still more preferably is equal to 1.0.
According to both the described methods of manufacture (protocols m1 and m2), it is preferable to operate the manufacture of the catalytic material in the temperature range of 30-100° C., more preferably 50-90° C., still more preferably 60-80° C.
Good results were obtained with the following catalytic compositions:
1. Mn0.5Ce0.5Ox, as obtained through the protocol described in Example 1, referred to in the description as sample CMm1, (see example 1);
2. Mn0.5Ce0.5Ox, as obtained through the protocol described in Example 2, referred to in the description as sample CMm2, (see example 2);
3. Mn0.5Ce0.4Zr0.1Ox referred to in the description as sample CMZm1, (see example 3);
4. Mn0.4Ce0.3Ti0.3Ox referred to in the description as sample CMTm1, (see example 4).
The catalytic compositions according to the present invention can optionally be dispersed on the surface of the supports, to improve the mechanical properties and to minimize the concentration. To this aim techniques can be used both of “dry coating” [S. Otles et al., Powder Technology 208 (2011) 378-382; Y. Ouabbas et al., Powder Technology 190 (2009) 200-209] and of “wash coating”, providing for the use of suitable binders.
For exemplificative but not limitative purposes of the present invention, as the support material can be cited alumina, silica, zirconia, titania, quartz, alumino-silicates, zirconyl-silicates, silicon carbides, silicon borates, nanotubes and carbon fibers, honeycomb ceramic structures, ceramic and metal membranes, and more.
The catalytic compositions according to the present invention can be in various forms. For example, they may be supplied in the form of powder, that is, of discrete solid particles of diameter less than 0.5 mm, or in other three-dimensional shapes (e.g., extruded, pellets, Rashig rings, etc.) of variable size between 5 and 50 mm.
If the composition is in the form of powder, in general the average particle size will be of 1-500 μm, preferably between 50 and 150 μm.
The selection of this range of particle size is particularly advantageous for the purposes of application of the present invention, the objective being to minimize the maximum surface area and at the same time to have a material easy to handle and which does not create difficulties of separation in gas/liquid/solid reactions (for example in fluid bed reactors).
A characteristic element of the present invention is in particular the innovative structure and morphology of the catalytic/adsorbing compositions described and claimed.
A characteristic element of the present invention is in particular the construction of innovative catalytic/adsorbing compositions based on compounds of manganese and cerium with a particular meso-porous structure with pores of extremely uniform size, having a diameter falling predominantly in the range of 6-20 nm (see examples 5-7, 9-11).
A further characterizing element of the present invention is in particular a method of realization that, within wide limits, allows to impart specific properties of texture to the compositions based on compounds of manganese and cerium object of the invention (see examples 8-11).
In the context of the present invention the expression texture is intended to designate the set of physical characteristics of the catalytic/adsorbing compositions, such as surface area (inner and outer) and porosity (volume, shape and diameter of the pores).
The catalytic/adsorbing compositions of the present invention are formed by particles of nanoscopic spherical shape, hereafter referred to as nanospheres (see examples 1, 6, 7).
The catalytic/adsorbing compositions of the present invention are composed of nanospheres of uniform size (see examples 1, 6, 7).
The size of the nanospheres grow or shrink in view of the experimental conditions of implementation and the chemical formulation (see examples 1, 6, 7, 8).
Without wishing to be bound by theory, it is believed that the nanospheres are the discrete units at the base of the final structure of the catalytic/adsorbing compositions of the present invention (see examples 1, 6, 7, 8).
Without wishing to be bound by theory, it is believed that the catalytic compositions forming object of the present invention are materials structured at the nanoscopic level obtained through the aggregation/agglomeration of said nanospheres (nano-structured materials).
Without wishing to be bound by theory, it is believed that the nanospheres are the “building block” (brick) of the innovative structure of the catalysts of the invention (see examples 1, 6, 7, 8).
Without wishing to be bound by theory, it is believed that, in a manner similar to other nanostructured materials reported in the literature, the weaving of the catalytic/adsorbing compositions of the present invention can be properly carried out according to a precise pattern or a specific project (see Examples 1, 6, 7, 8).
Without wishing to be bound by theory, it is believed that the special structure of the catalytic compositions forming object of the present invention is, according to the technologies currently available, obtainable only and exclusively through the technique of the embodiment described in the present invention.
In recent years some manufacturing techniques have been developed that have improved the texture of the catalytic compositions based on cerium and manganese. In particular, in WO2007/069238 it is proposed a new method for the preparation of compositions based on crystalline ceria (CeO2) and trivalent manganese oxide (Mn2O3) with high surface area. However, the method described in this document, in addition to the application limits due to the complexity and large number of operating steps, does not allow control of the porosity of the compositions.
In addition, the method described in WO2007/069238 does not allow the control of other important structural properties such as crystal size of cerium and dispersion of the two compounds.
Without wishing to be bound by theory, it is believed that, by controlling the process of growth and aggregation of the nano-spheres, it is possible to obtain catalytic compositions based on cerium and manganese having specific and predetermined structural properties (see example 1, 6, 7, 8, 9).
It is believed in particular that, by controlling the size of the nanospheres, one can determine the porosimetry of the material and with it the extension of the total surface area (see examples 1, 6, 7, 8, 9).
The size of the nanospheres of the material according to the present invention are determined by the following parameters of the fabrication technique:
1. relative amount of manganese and cerium in the composition,
2. pH value of precipitation,
3. ageing temperature,
4. initial oxidation state of the metallic and semi-metallic ions,
5. strength of the alkaline agent,
6. oxidizing agent,
7. temperature and time of the calcination treatment.
In particular, the ratio manganese/cerium (MnCe) in the catalytic composition according to the present invention is determined in the control of the size of the nanospheres.
By varying the atomic ratio manganese/cerium (Mnat/Ceat) between 0.25 and 3.0 the diameter of the nanospheres ranges between 1 nm and and 32 nm (see example 8).
By varying the aforementioned parameters of the fabrication technique, catalytic compositions based on manganese cerium have been realised comprised of nanospheres with the average diameter varying between 0.5 nm and 40 nm, obtaining mesoporous materials having total pore volume variable between 0.2 and 1.0 cm3/g, average pore size varying between 5 nm and 50 nm, total surface area between 120 and 300 m2/g.
In particular, the sample CMm1 (composition Mn0.5Ce0.5Ox) is composed of nanospheres with average diameter of 20 nm which carry out a mesoporous material, having a total surface area of 160 m2/g and an average pore diameter of about 9 nm.
Without wishing to be bound by theory, it is believed that the texture of the catalytic compositions of the present invention is particularly advantageous for use in chemical processes where the reaction kinetics is negatively affected by phenomena of transfer of matter.
The manufacture of catalytic compositions with pores having regular structure and extremely uniform size is particularly advantageous for the purposes of application of the present invention. in fact, the objective is to provide catalytic compositions for use in catalytic processes suffering from problems of accessibility of the reactant species (MW>100 uma) within the porous structure of the catalytic system.
It is known that macro-porous (pore diameter>50 nm) and micro-porous (pore diameter<2 nm) materials have a limited importance from the catalytic standpoint than meso-porous (pore diameter 2-50 nm) material. In fact, the macro-porous catalysts generally have a lower catalytic activity due to the low surface area (circa 10 m2/g), while the catalytic systems with high internal surface area (>300 m2/g) have problems of diffusional nature, due to the fact that the dimensions of the micro-pores are not accessible to almost all of the reactant molecules (MW>30 uma). In contrast, the mesoporous materials combine the dual advantages of high surface area, usually ranging between 50 and 300 m2/g, with pore size suitable for the access of reactant molecules of medium to large size.
Without wishing to be bound by theory, it is believed that the manufacturing method described in the present invention allows to improve the chemical and physical properties of compositions based on manganese and cerium for use in the catalysis in liquid and gas phase.
It is believed, in particular, that the process of realization described in the present invention allows to improve the adsorbing properties and the properties of accumulation of the compositions based on manganese and cerium in respect of different classes of organic molecules and in a special way with respect to those known to belong to compounds highly polluting and difficult to biological removal. This aspect is particularly important in the processes of catalytic oxidation in liquid phase (CWO) of polluted compounds which are particularly toxic and resilient such as phenol and the carboxylic acids of low molecular weight (C1-C3).
In general, the materials with adsorbing properties are market both in the environmental field and in the industrial process of production. In particular, adsorbing materials are commercially used in the treatment of sanitizing, drying and purification of living and working environments, in removing molecules of pollutants, toxic, noxious or troublesome present and persistent in air and water, in the preparation of systems of chromatographic separation and in the cleaning of process stream.
The adsorbing properties of the catalytic compositions developed according to the present invention are particularly advantageous for the purposes of application, since they allow to maximize the ability to capture-remove-accumulate and retain certain highly polluting molecules such as pigments and dyes, polymers (PEG), phenols, alcohols, aldehydes and ketones, ethers, esters and carboxylic acids, contained in water from industrial processes.
The catalytic compositions forming object of the present invention show to be able to remove, by adsorption and accumulation, quantities of polluting compounds also equal to 100% of the weight of the catalytic composition.
Good results have been obtained in particular with the samples CMm1 and CMm2 (compositions Mn0.5Ce0.5Ox, respectively obtained through the protocols described in Examples 1 and 2) in the adsorption and oxidation of phenol, which have shown an ability to remove as much as ten times higher compared to the catalysts of CWAO known in the literature (see example 12).
Good results have been obtained in particular with the samples CMm1 and CMm2 in the adsorption and oxidation of acetic, formic and oxalic acid, which have shown an ability to remove as much as ten times higher compared to the catalysts of CWAO known in the literature.
Good results have been obtained in particular with the samples CMm1 and CMm2 in the accumulation of methyl-tertiary-butyl ether (MTBE) at low pressure in the gas phase and concentration in the liquid phase (see example 13).
Good results have been obtained in particular with the samples CMm1 and CMm2 in the adsorption at the same time of heavy metals and organic pollutants present in process water of the metalworking industries.
Good results were also obtained in particular with samples CMm1 and CMm2 in the purification of wastewater polluted with cyanides.
From the crystal-chemical point of view, the nano-spheres of the catalytic materials according to the present invention consist of two different types of solid phases, perfectly mixed, distinguishable only with observations of high resolution transmission electron microscopy in (HRTEM) and through analysis of X-ray diffraction (XRD). The first phase is constituted by a network of nanoscopic reticular structures, ie a phase composed of ultrafine nanocrystalites of oxides of cerium (CeOx; 1.5≦x≦2.0), with a diameter of 1-6 nm, aggregated/organised in “Nano Crystal Domains” (DNC).
The second phase is a solid “non-crosslinked” structure, defined as “Vitreous Solid Mixture” (MSV). The Vitreous Solid Mixture is a metastable phase consisting of oxides and sub-oxides of the metallic and semi-metallic elements forming object of the present invention (see Examples 14-16).
The realization of nanocrystalline particles of CeOx, with 1 with a diameter of the crystallites of less than 10 nm is very important for the purposes of the present invention, since the “ultrafine” size of the nanocrystallites improve the chemical reactivity of the same crystallite and increase the surface area [J. Phys. Chem. C 2010, 114, 12909-12919].
It is known that the ceria (CeO2) is highly stable in its crystalline form, rarely, in fact, the ceria exists in a form other than crystalline. Due to its great stability almost all the methods known for the preparation of catalysts based on cerium lead to the formation of materials with well developed crystals of ceria (for example with dimensions 20-50 nm) or with nanocrystals aggregated into particles in which the individual crystallites of ceria have average size greater than 10 nm or at least difficult less than 6 nm.
WO2007/069238 describes the manufacture of catalytic compositions based on crystalline oxides of trivalent manganese (Mn2O3) and tetravalent cerium (CeO2), the ultrafine size of which, diameter lower than 5 nm, are obtained through a method of preparing type “nanocasting” or “bottleship”, which provides for the confinement of the priming solution in a microporous matrix which is subsequently dissolved to allow the recovery of the catalytic preparation contained in it.
Unlike the method of preparation type “bottleship” described in WO2007/069238, according to the present invention catalytic compositions are realised wherein the ultrafine nanoparticular component is generated directly without the use of microporous matrices.
Unlike WO2007/069238, the method of preparation forming object of the present invention allows to modulate and control the size of the nanocrystallites of CeOx, with 1.5≦x≦2.
Without wishing to be bound by theory, it is believed that the simultaneous generation of ultrafine nanoparticles and vitreous suboxides is the main result of the manufacturing method described in the present invention, which leads to an improvement in catalytic performance.
Without wishing to be bound by theory, it is believed that the realisation of ultrafine nanocrystalline particles CeOx, with 1.5≦x≦2 (i. e. with diameter lower than 6 nm) will have more defects and lattice distortions in the crystal structure of the same oxides of cerium CeOx, with 1.5≦x≦2 (i. e. variations of cell parameters and vacancies of oxygen ions), improving the catalytic properties of the compositions forming object of the present invention.
Without wishing to be bound by theory, it is believed that the realisation of ultrafine nanocrystalline particles CeOx, with 1.5≦x≦2 (i. e. with diameter lower than 6 nm) improves the stability of the catalytic surface to the oxygen vacancies caused by the presence of trivalent cerium ions (Ce3+) and/or by other cations in the crystal lattice of CeOx, with 1.5≦x≦2.
Without wishing to be bound by theory, it is believed that the phase MSV is characterized by a strong heterogeneity in the valence and the degree of coordination of the Mn ions, from which follows the formation of sub-oxides. The areas of the material in greater oxygen deficiency may represent areas of higher reactivity, improving the catalytic performances of the compositions object of the present invention in oxidation reactions.
It is also believed that the phase DNC also acts as a supporting structure, able to confer greater mechanical stability and to favor the mutual dispersion of the different components, while the phase MSV assumes the role of adhesive/aggregant.
Without wishing to be bound by theory, it is believed that the manufacturing method described in the present invention allows to modulate the amounts of the two solid phases, improving the catalytic performance, the crystal-chemical characteristics and thus the structural and mechanical properties of the catalytic compositions object of the present invention.
Without wishing to be bound by theory, it is believed that the amount of DNC phase can be controlled by adjusting the parameters of the production process.
In particular, the amount of cerium is determinant in the control of the levels of DNC phase of the catalytic composition (see example 17).
According to the purposes of the present invention it is preferable an amount of DNC phase in the catalytic composition not lower than 40%, more preferably equal to 50%, still more preferably equal to 60%.
A specific aspect of the present invention is the realization of nano-structured catalytic compositions, comprised of suboxides of manganese and ultrafine crystalline nanoparticles of CeOx, with 1.5≦x≦2 (i. e. with diameter 1-6 nm), characterized by a high dispersion of its components and an increased amount of manganese in the surface layers of the same compositions, both in terms of concentration and in terms of active surface area exposed (see examples 18 and 19).
The manufacture of catalytic compositions with high content and dispersion of the active phase (Mn and Mn—Ce) in the surface layers of the same compositions is particularly advantageous for the purposes of this patent application. The aim is, in fact, to provide nano-structured catalytic compositions, comprised of sub-oxides of manganese and ultrafine crystalline nanoparticles di CeOx, with 1.5≦x≦2, highly active in the oxidation of organic molecules particularly refractory (resistant) to the normal processes of oxidation.
An important objective of the present invention is the realization of nanostructured catalytic compositions, consisting of sub-oxides of manganese and ultrafine crystalline nanoparticles of CeOx, with 1.5≦x≦2, characterized by a greater amount of the active phase in the surface layers (i. e. Mn and Mn—Ce>20% by weight) that leads to a more catalytically active surface area (>50 m2/g).
The surface composition of a heterogeneous catalyst is one of the aspects of greater importance for catalysis, as the reactant species after being chemisorbed on the surface of the catalytic material, are activated/transformed by the active sites present on the surface of the catalytically active phase (active phase). The number of these active sites increases with the catalytically active area and, therefore, with the concentration and dispersion of the active phase in the surface layers. At the same load of the active phase and total surface area, the more active catalytic compositions are known to be those characterized by a greater dispersion.
It is known that most of the techniques of production of catalysts currently in use in commercial and industrial use (ie, for example, co-precipitation, deposition-precipitation, impregnation, spray-emulsion, electro-spinning, combustion, etc.) does not allow the realization of catalytic compositions with high concentration of the active phase unless a marked reduction in the level of dispersion of the active phase of the surface area and catalytic activity.
Without wishing to be bound by theory, it is believed that the new process for manufacturing catalytic materials according to the present invention allows to provide nano-structured catalytic compositions, comprised of sub-oxides of manganese and ultrafine nanocrystallites of CeOx, with 1.5≦x≦2, wherein the manganese even at high concentrations (loads) is dispersed in almost ionic, without alterations in the properties of weaving or reductions in the extension of the surface area (see examples 18 and 19).
Without wishing to be bound by theory, it is believed that the new process for manufacturing catalytic materials according to the present invention allows to obtain the preferential accretion of a solid phase with respect to another (ie MSV and DNC) in the surface layers of the same nano-structured catalytic compositions, consisting of sub-oxides of manganese and ultrafine particles of nanocrystalline CeOx, with 1.5≦x≦2 (see example 17).
Without wishing to be bound by theory, it is believed that the particular structure of the catalytic compositions of the present invention exerts a stabilizer against the manganese atoms exposed on the surface of the same compositions, favoring, therefore, the development of the surface area catalytic activity (ie Mn and Mn—Ce).
The catalytic compositions of the present invention possess a higher concentration of manganese atoms in the surface layers than in deeper or bulk (see examples 18 and 19).
The atomic concentration of manganese in the surface layers of the catalytic compositions of the present invention can also be three times greater than that present in other catalytic compositions of a similar chemical formulation but made with a different method (see example 18).
The extension of the surface area due to the oxides of manganese present in the catalytic compositions of the present invention may also be four times higher than that reported by other compositions of a similar chemical formulation known in the literature (see example 19).
Without wishing to be bound by theory, it is believed that the high dispersion of manganese in the catalytic composition forming object of the present invention promotes and increases the amount of this element in the superficial layers of the same compositions.
Without wishing to be bound by theory, it is believed that the higher amount of manganese in the surface layers promotes the activity of oxidation in the processes of CWAO of organic molecules (phenols, carboxylic acids, hydrocarbons, etc.) and inorganic molecules (cyanide, ammonia, etc.); which are particularly toxic and refractory to the usual treatments of oxidation.
The surface of the catalytic compositions of the present invention can be complex from the point of view of the chemical formulation and the number of compounds and phases that compose it.
Without wishing to be bound by theory, it is believed that the production process described in the present invention allows to obtain a catalytic compositions in which sub-oxides of manganese and whose ultrafine nanoparticles of ceria are distributed in a substantially uniform manner over the entire surface.
Given the particular structure of the catalytic compositions of the present invention, constituted by at least two chemical elements (ie, Mn and Ce) and at least two different solid phases (ie DNC and MSV), it is possible to identify two different types of dispersion.
For the purposes of the present invention is defined as “phase dispersion” the level of dispersion/distribution of the phase DNC in the phase MSV.
For the purposes of the present invention is defined as “molecular dispersion” the level of dispersion/distribution of different chemical elements within the two phases.
The size of the ultrafine nanocrystallites (1-6 nm) of CeO, with 1.5≦x≦2 and the remarkably intimate mixing of the phases DNC and MSV, obtained with the process described in the present invention, make the surfaces of the catalytic compositions of the present invention extremely homogeneous and uniform.
The catalytic compositions of the present invention do not show significant differences from the areas of surface chemistry, which are not present on the surface of the appreciable areas of segregation, characterized by a different concentration of the two solid phases.
The chemical analysis of the surface of the catalytic compositions of the present invention indicates that the chemical composition varies from point to point (spot diameter of 0.05 uM) to a maximum of ±6% compared to the average, this value falling to ±4% in the compositions with a high concentration of cerium and manganese (see example 20).
In the catalytic compositions of the present invention, the points of the surface with nanocrystalline domains (DNC) are characterized by the almost exclusive presence of cerium cations (see example 21).
In the catalytic compositions of the present invention, the points of the surface where there is the vitreous solid mixture (MSV) are characterized by the presence of cations of cerium, manganese and other elements, with each other in concentration ratios dependent on the chemical formulation (see Example 21).
In the phase MSV of the catalytic compositions of the present invention, the cerium and manganese cations are bound to each other in a non-coordinative way through oxygen bridges [—Mn—O—Ce-], which leads to dispersion of two ionic elements.
The contact between the phase and the DNC MSV is accomplished through the creation of an extremely reactive interface of contact.
Without wishing to be bound by theory, it is believed that at the interfacial area it is reached a high level of interaction between the different elements, which improves the catalytic activity due to the synergistic action of the same elements.
Without wishing to be bound by theory, it is considered that the catalytic activity of the compositions object of the present invention in the processes of oxidation and reduction is directly correlated with the interfacial area.
An important result of the present invention is the realization of catalytic compositions containing sub-stoichiometric oxides (sub-oxides) defective in terms of oxygen content.
Without wishing to be bound by theory, it is believed that the presence of the sub-oxides in the catalytic compositions of the present invention enhances their ability to chemically sorbing oxygen and oxygen molecules.
The elements cerium (Ce) and manganese (Mn) tend to form crystalline oxides and non-stoichiometric but no crystalline phases containing both cations (solid solutions). Thus, the catalytic compositions based on known compounds of manganese and cerium, contain various mixtures of the oxides blocks of the two elements (ie, MnO, MnO2, Mn2O3, Mn3O4 and Ce2O3, CeO2).
In particular, WO2007/069238 describes the exclusive presence of Mn2O3 and CeO2, while Chen et al. [Appl. Catalysis. B: Environmental, 32 (2001) 195] presents catalysts consisting of CeO2 and MnO2 mainly.
Otherwise, the catalytic compositions of the present invention contain highly defective nano-crystalline ceria in intimate contact with a phase consisting of a vitreous solid mixture of sub-stoichiometric oxides (sub-oxides) of manganese and cerium.
Without wishing to be bound by theory, it is believed that the high level of dispersion of the elements present in the catalytic compositions of the present invention confers greater defectivity in oxygen.
Without wishing to be bound by theory, it is believed that manganese binds to oxygen in a defective and non-coordinative way, forming sub-stoichiometric oxides (ie phase MSV), in which it assumes formally non-unitary values of oxidation.
Without wishing to be bound by theory, it is believed that the phase MSV increases ions vacancy (O2−) in the crystal lattice of ultrafine nanoparticles of CeOx, with promoting further lattice defects and strong distortion of the crystal lattice.
Without wishing to be bound by theory, it is believed that the oxygen vacancies and the presence of sub-oxides increases the number of sites of Lewis acids present on the surface, making the catalytic compositions of the present invention particularly reactive towards the electron-donors, which results in a better ability to chemically sorbing oxygenated organic species.
The presence of lattice defects in the ceria and areas of high unsaturation sites is particularly advantageous for the purposes of this patent application, as it improves the reactivity of the surface towards the oxygen, promoting the oxidation of organic molecules refractory (resistant) to the normal treatments of oxidation.
It is known that the performance of a catalyst depends on the number of active sites and that these are identifiable points with a higher surface energy as seats of lattice defects and electronic unsaturations.
In multi-component catalytic systems, the active sites are preferentially localized in the contact zones (interface) and, therefore, their concentration depends on the degree of dispersion.
In the catalytic compositions of the present invention the areas most affected by the presence of lattice defects and electronic unsaturations are those in which the ultrafine crystalline nanoparticles (DNC) are in direct contact with the phase MSV where the shortage of oxygen is higher.
Without wishing to be bound by theory, it is believed that the areas of interface between the solid phases DNC and MSV represent the areas with a higher concentration of active sites in the catalytic compositions of the present invention.
Without wishing to be bound by theory, it is believed that the high dispersion of the elements present in the catalytic compositions of the present invention and the extremely small size of the nanoparticles of CeOx, with 1.5≦x≦2, leading to a high concentration of lattice defects and electronic unsaturations in the same compositions.
Without wishing to be bound by theory, it is believed that the lattice defects and the points of electronic unsaturations promote both the chemisorption of organic species and oxygen at low temperature.
The materials of the present invention are particularly active in the processes of total oxidation in liquid phase and gas in the presence of air/oxygen. The materials developed are able to lead to complete oxidation of organic molecules of different nature, such as hydrocarbons, phenols, carboxylic acids, ethers, alcohols, organohalogen compounds and organo-nitrogen compounds, polymers, pigmenting substances particularly resilient and stable.
Good results have been obtained in particular with the samples CMm1 and CMm2 in the catalytic oxidation of phenol in the aqueous phase with air (CWOA) (see examples 22 and 23).
Good results have been obtained in particular with the samples CMm1 and CMm2 in the catalytic oxidation of acetic acid in the aqueous phase with air (CWOA) (see example 24).
Good results have been obtained in particular with the samples CMm1 and CMm2 in the removal of cyanides in the presence of heavy metals (see examples 25 and 26).
Without wishing to be bound by theory, it is considered that the catalytic compositions consisting of sub-oxides of manganese and ultrafine nanocrystallites of CeOx, with 1.5≦x≦2 forming object of the present invention do not undergo degradation/dissolution phenomena (leaching) due to the acidic environment in which the CWOA processes operate.
Without wishing to be bound by theory, it is considered that the catalytic compositions consisting of sub-oxides of manganese and ultrafine crystalline nanoparticles of CeOx, with 1.5≦x≦2 forming object of the present invention are not subjected to sintering during the operating CWOA processes.
Without wishing to be bound by theory, it is considered that the catalytic compositions consisting of sub-oxides of manganese and ultrafine crystalline nanoparticles of CeOx, with 1.5≦x≦2 forming object of the present invention are highly stable in an oxidizing atmosphere (air/oxygen) at a temperature of 600° C.
The invention will be described below for illustrative but not limitative purposes, with particular reference to some illustrative examples, with particular reference to the figures of the accompanying drawings, in which:
The chemical composition of the catalysts was obtained by measurements of X-ray fluorescence (XRF) using a spectrophotometer model BRUKER AXS-S4 Explorer X-ray Fluorescence spectrometer. The samples were analyzed in the state of transfer, without further manipulation.
The determination of the values of total surface area, of porosimetry and pore radius of the catalytic compositions was performed using measures of physical adsorption of nitrogen at its condensation temperature (−196° C.) and processing of the relative adsorption isotherm, in the range p/p0 from 0 to 0.3, by the BET method for determining the amount of N2 adsorbed monolayer in the “monolayer” (Vm). By the same isotherms porosimetric distribution curves were obtained, applying the methods “Horvath-Kavazoe” and “BJH” respectively for the evaluation and determination of the size and distribution of micro and meso pores. The measurements were made using an automated tool for the determination of adsorption isotherms (Micromeritics, mod. ASAP 20-10). Before the measures, all the samples were pre-treated at a temperature of 100-400° C. under vacuum until they reached values of residual pressure below 0.01 mbar.
The nature of the solid phases, understood as the degree of crystallinity, and the type of the various crystalline species that may be present have been evaluated through the use of a powder X-ray diffractometer, equipped with dual detector, model Philips X-Pert diffractometer. The instrument is equipped with a high voltage current generator (HT-generator PW1830), an X-ray monochromatic source (lamp of copper and nickel filter β) Cu—Kα (Λ=1.5406 Å). The measurements were carried out by applying to the source a voltage of 40 kV and a current of 30 mA. The average size of the crystallites was obtained by measures for widening of the band of diffraction of the att X as indicated by the equation of Sherrer:
where: λ=1.5406 Å, B is the width of the diffraction line calculated as FWHM in degrees 2θ and the position of the maximum of the line.
The Scherrer equation was applied to the XRD peak characteristic of the crystalline species, in the acquisition of a Gaussian distribution of the peak.
The identification of the diffraction peaks was performed on the basis of reference spectra contained in the Joint Committee Powder Diffraction Standards (JCPDS) database.
The morphology and structural characteristics at the microscopic level were analyzed by transmission electron microscopic observations using an instrument PHILIPS CM12 having a maximum resolution (point-to-point) of 2.4 Å. The samples were prepared by dispersing the catalyst in iso-propyl alcohol under ultrasound and then depositing a drop of the suspension on screens of gold.
The micrographs describing the texture of the various materials were obtained through the use of SEM microscope, Philips model XL EGF/SFEG Scanning Electron Microscopy, equipped with a field emission gun for the generation of electrons and a EDAX probe for analytical measurements. The images were obtained by working with an acceleration voltage of 5-20 kV, and a distance lens to sample of 20-10 mm. The measurements were performed after treatment of the sample under vacuum (ultra vacuum) and covering of gold (coating by gold sputtering).
Surface chemistry analysis data were obtained using a spectrometer-spectroscope by Physical Electronics GMBH PHI 5800-0 (XPSESCA), operating with monochromatic radiation Al—K α to a power of the incident ray of 350 W and with a pass energy of 10-60 eV.
The tests in the liquid phase in the removal of phenol, acetic acid, cyanide and heavy metals have been conducted in an autoclave made of AISI 316 steel completely covered with PTFE working at high pressure.
The concentration of phenol, acetic acid and cyanide was determined by measures of liquid chromatography using a HPLC system (Dionex Pump Series P680) provided with two pumps for solvent, a control module of the pneumatic system, a manual injector (Rheodyne model) and a detector (Dionex UV-Vis detector UVD 170U). The separation and identification of compounds was performed using a packed type chromatographic column operating at reverse phase (C16 Acclaim PolarAdvantage 3 μm, dim. 4.6×150 mm), while for the operational control of the instrument, the acquisition and the data processing was conducted using a suitable software. The concentration of total organic carbon (TOC) in solution was determined by means of an automated tool Shimazu (Mod 5000 A). The amount of carbon dioxide formed during the reaction CWO of phenol was determined gravimetrically, bubbling the gas stream leaving the reactor in a saturated solution of Ba(OH)2. The precipitate of BaCO3 was filtered, washed, dried at 100° C. and then weighed.
The materials used for the preparation of the sample CMm1 are:
In accordance with the first description of the method of manufacture (m1 protocol), for the preparation of 10 g of the catalytic composition Ce0.5Mn0.5Ox, the following solutions were prepared:
The oxidizing solution was added to the alkaline solution to obtain a new solution defined solution B. In a slow and continuous way, the reducing solution was added to the solution B, producing the precipitation of metals/semi-metals under strong stirring, while maintaining a constant pH, redox potential of the solution B and rate of administration of the reducing solution to the solution B.
The experimental conditions adopted in the preparation were the following:
After preparation, the sample was washed with distilled water at 90° C. until neutralization of the pH of the washing waters and, therefore, placed in an oven for drying at 100° C. for a time of 16 hours. Subsequently, the catalyst was subjected to a calcination treatment in air at 400° C. for 6 hours.
The materials used for the preparation of the sample CMm2 are:
In accordance with the second description of the method of manufacture (protocol m2), for the preparation of 10 g of the catalytic composition Ce0.5Mn0.5Ox were prepared by the following solutions:
The oxidizing solution was added to the solution of promoters obtaining a new solution defined A′. In a slow and continuous way, solution A′ was added to the reducing solution, producing the precipitation of the oxides under strong stirring, while maintaining a constant pH, by addition of potassium hydroxide (KOH) 0.2 M, and rate of administration of the solution A′.
The experimental conditions adopted in the preparation were the following:
After preparation, the sample was washed with distilled water at 90° C. until neutralization of the pH of the washing waters and, then, placed in an oven for drying at 100° C. for a time of 16 hours. Subsequently, the catalyst was subjected to a calcination treatment in air at 400° C. for 6 hours.
The materials used for the preparation of the sample Mn0.5Ce0.4Zr0.1Ox are:
In accordance with the first description of the method of manufacture (m1 Protocol), for the preparation of 10 g of the catalytic composition Mn0.5Ce0.4Zr0.1Ox were prepared by the following solutions:
The reducing solution was added to the solution of promoters obtaining a new solution defined solution A. The oxidizing solution was added to the alkaline solution to obtain a new solution defined solution B. In a slow and continuous way, solution A was added to the solution B, producing the precipitation of the oxides under strong stirring, while maintaining a constant pH, redox potential of the mixture B and rate of administration of the solution A to solution B.
The experimental conditions were adopted in the preparation:
After preparation, the sample was washed with distilled water at 90° C. until neutralization of the pH of the washing waters and, therefore, placed in an oven for drying at 100° C. for a time of 16 hours. Subsequently, the catalyst was subjected to a calcination treatment in air at 400° C. for 6 hours.
The materials used for the preparation of the sample Mn0.4Ce0.3Ti0.3Ox are:
In accordance with the first description of the method of manufacture (m1 Protocol), for the preparation of 10 g of the catalytic composition Mn0.4 Ce0.3Ti0.3Ox were prepared by the following solutions:
The reducing solution was added to the solution of promoters obtaining a new solution defined solution A. The oxidizing solution was added to the alkaline solution to obtain a new solution defined solution B. In a slow and continuous, solution A was added to the solution B, producing the precipitation of the oxides under strong stirring, while maintaining constant pH, redox potential of the mixture B and rate of administration of the solution A to solution B.
The experimental condizi oni taken in the preparation were:
After preparation, the sample was washed with distilled water at 90° C. until neutralization of the pH of the washing water and then placed in an oven for drying at 100° C. for a time of 16 hours. Subsequently, the catalyst was subjected to a calcination treatment in air at 400° C. for 6 hours.
Observation and scansion electron microscopy (SEM: Scanning Electron Microscopy) at low magnification, the powder of the catalytic compositions of the present invention appear as macro particle aggregates of square shape, the size on average between 50 and 150 μm completely different from the equivalent coprecipiatating powder, consisting of excessively fine particles (5-20 μm). At high magnifications, the catalytic compositions of the present invention appear to consist of particles nanoscopic h and spherical in shape, never previously observed for these compounds. As evidence of this, the SEM images of
The mesoporous structure of the catalytic composition object of the present invention has been demonstrated by physisorption of nitrogen (N2) at a temperature of 77K. As demonstrated by the graph of
This type of isotherm connotes the mesoporous materials according to the classification of Brunauer, Deminge, Deming and Teller. In particular, this material shows a process of adsorption of type “Langmuir” until the relative pressure (P/Po) of approximately 0.25, at which one observes the maximum overlap of the surface resulting in the formation of a monolayer (the point m), ie the formation of a single layer of nitrogen adsorbed on the entire surface. For successive increments of the pressure (P/P=0.25 to 0.55)°, on the surface of the catalytic composition are deposited several layers of nitrogen (the coating of type multilayer adsorption) of highest pressure that lead to the capillary condensation (liquefaction) gas inside the pores, as evidenced by the presence of the hysteresis loop.
The graph of
The graph of
The graph of
The graph of
The pattern correlates the development of the structure of the catalytic composition with the control of certain parameters related to the fabrication technique described in the present invention. In particular, the schema describes the growth of nano-spheres with increasing atomic ratio of manganese/cerium (Mnat/Ceat) in the catalytic composition.
The graph of
The image of
The image of
The bar graph of
The two graphs of
The adsorption isotherm of MTBE composition of Mn0.5Ce0.5Ox (Sample CMm1) shown in
Moreover, the intensity of the reflections crystalline <111> and <200> is greater in the catalytic composition to greater atomic fraction of cerium (CeX=0.75).
In the images of
By comparing the SEM image of the texture (
The graph of
The graph of
The graph of
The graph of
The present invention has been described at the chapter illustrative but not limitative purposes, according to its preferred embodiments, but it is to be understood that variations and/or modifications can be apport to you by those skilled in the art without departing from the scope of protection, as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| RM2011A000292 | Jun 2011 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IT2012/000172 | 6/8/2012 | WO | 00 | 4/21/2014 |
