WO2011111052A1 - Process for the preparation of heterogeneous fenton catalytic filter - Google Patents

Process for the preparation of heterogeneous fenton catalytic filter Download PDF

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Publication number
WO2011111052A1
WO2011111052A1 PCT/IL2011/000240 IL2011000240W WO2011111052A1 WO 2011111052 A1 WO2011111052 A1 WO 2011111052A1 IL 2011000240 W IL2011000240 W IL 2011000240W WO 2011111052 A1 WO2011111052 A1 WO 2011111052A1
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catalyst
iron
silica
oxide
iron ions
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PCT/IL2011/000240
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French (fr)
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Miron Landau
Mordechai Herskowitz
Geonker Satishkumar
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Ben-Gurion University Of The Negev Research And Development Authority
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/74—Iron group metals
    • B01J23/745—Iron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/002—Mixed oxides other than spinels, e.g. perovskite
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/70—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper
    • B01J23/76—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/83—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the iron group metals or copper combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36 with rare earths or actinides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/19—Catalysts containing parts with different compositions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • B01J37/0209—Impregnation involving a reaction between the support and a fluid
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/04—Mixing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/06—Washing
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/72—Treatment of water, waste water, or sewage by oxidation
    • C02F1/722—Oxidation by peroxides
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/72—Treatment of water, waste water, or sewage by oxidation
    • C02F1/725—Treatment of water, waste water, or sewage by oxidation by catalytic oxidation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
    • B01J21/08—Silica
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/66—Treatment of water, waste water, or sewage by neutralisation; pH adjustment
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00—Nature of the contaminant
    • C02F2101/30—Organic compounds
    • C02F2101/34—Organic compounds containing oxygen
    • C02F2101/345—Phenols
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2209/00—Controlling or monitoring parameters in water treatment
    • C02F2209/06—Controlling or monitoring parameters in water treatment pH
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2305/00—Use of specific compounds during water treatment
    • C02F2305/02—Specific form of oxidant
    • C02F2305/026—Fenton's reagent

Definitions

  • Fenton oxidation of organic impurities present in industrial wastewater using hydrogen peroxide is an efficient method for the purification of water at mild conditions, e.g., at atmospheric pressure and temperatures 50-80°C.
  • the shortcoming of this method is associated with the use of soluble homogeneous iron catalysts, which contaminate the water and require costly catalyst separation step.
  • the present invention allows the deposition of isolated metal (i.e., iron) ions onto a solid support, for example, on high-surface area silica, using a novel selective extraction deposition (SED) method.
  • isolated metal i.e., iron
  • SED selective extraction deposition
  • isolated is meant that the iron ions are essentially discretely located on the surface of the support, such that in the close vicinity of a given iron ion, no further iron ions are present. Such ions do not show reflections in XRD diffractograms characteristic for any iron-containing crystalline or amorphous phases.
  • iron in the state of isolated ions at silica surface is indicated by UV-Raman spectra (excited by 234 nm and 325 nm laser sources) exhibiting resonance bands at about 510, 1090 and 1140 cm -1 .
  • the discrete iron ions can be also detected by Mossbauer spectroscopy - isomer shift ca 0.45 mm/s at 60°C and average quadrupol splitting of 0.99-1.07 mm/s.
  • a suitable source of iron ions also designated herein "Fe-precursor” is combined with particles of a suitable solid support.
  • the resultant mixture is treated in a solution under suitable conditions (e.g., in an acidic environment), thereby releasing the iron ions from said Fe-precursor and allowing said iron ions to interact with reactive groups present on the surface of the support particles (such as hydroxyl groups (silanols) , in the case of silica-gel supports, or carboxylic groups) , before the hydrolysis and self- condensation of said iron ions can take place.
  • suitable conditions e.g., in an acidic environment
  • reactive groups present on the surface of the support particles such as hydroxyl groups (silanols) , in the case of silica-gel supports, or carboxylic groups
  • highly dispersed ferric iron (Fe 3+ ) ions are grafted on the surface of the solid support.
  • the invention is thus primarily directed to a process for the preparation of an iron-containing catalyst suitable for use in Fenton oxidation reactions, comprising preparing a mixture of a source of iron ions and solid support particles, said support particles having chemically reactive groups present on their surface, contacting said mixture with an acidic liquid medium thereby causing the release of iron ions from said iron source, to form a catalyst comprising said solid support particles and said iron ions located on the surface of said support particles.
  • a source of iron ions which is suitable for use as a starting material according to the process of the invention, is preferably a mixed oxide of iron and at least a second metal, said mixed metal oxide being decomposable in an acidic environment.
  • perovskite is intended herein to designate mixed metal oxides having the ideal and non-ideal perovskite-type crystalline structure.
  • the ideal perovskite crystalline structure is defined by the empirical formula AB0 3 in which A and B are cations of two different metals and in which the A cation is coordinated to twelve oxygen atoms while the B cation occupies octahedral sites and is coordinated to six oxygen atoms.
  • a description of the garnet-type structure is found in S. Geller; Crystal chemistry of the garnets, Zeitschrift fiir Kristallographie , 125, S. 1-47 (1967) .
  • the mixed oxide of iron and a second metal decomposes in an acidic liquid medium used in the process of the invention.
  • the second metal which forms part of the mixed oxide is selectively leached out under the conditions applied in the process, such that said second metal cannot significantly compete with the iron ions on the reactive sites at the surface of the support material.
  • LaFe0 3 perovskite can serve as a suitable iron source for the selective extraction- deposition of isolated iron ions on silica support in acidic aqueous solutions (namely, the lanthanum is preferentially leached out, whereas the iron selectively interacts with the silica support) .
  • the perovskite used in the process of the invention e.g., LaFe0 3
  • LaFe0 3 perovskite suitable for use according to the invention is prepared by dissolving lanthanum (La 3+ ) and iron (Fe 3+ ) salts (e.g., in their hydrated forms) in an aqueous solution at a temperature in the range between 30 and 50°C, followed by the addition of a complexant like glycine, citric or oxalic acid.
  • the complexant compound contains two functional groups (e.g., -COOH, or -COOH and -NH 2 , like in glycine) which can bind chemically to both the Fe and La ions in the solution. This prevents segregation of La and Fe components at the subsequent gelation step, thus yielding uniform distribution of La and Fe ions in the bulk of the obtained gel.
  • the solution is maintained at a temperature of not less than 60°C for a sufficient time to allow the formation of a gel-like material, which is subsequently dried and gradually calcined, to give the desired pure LaFe0 3 perovskite.
  • the preparation of a LaFe0 3 perovskite which is suitable as Fe-precursor in the process of the invention is illustrated below.
  • the support material is preferably a high surface-area solid material, for example, a material having surface area ranging from 100 to 1500 m 2 /g may be used, in particular silica, titania, zirconia or carbon.
  • the solid support contains chemically reactive groups capable of interacting with iron ions, such as hydroxyl and carboxylic acid groups.
  • the high surface- area support material is silica, which is selected from the group consisting of silica-gel, mesostructured silica and ⁇
  • the particle size of the high surface-area support is preferably in range between 20 and 150 ⁇ .
  • the solid support particles e.g., the silica particles
  • the solid iron source e.g., a mixed oxide of iron and a second metal, such as RFe0 3 perovskite, wherein R is most preferably lanthanum
  • a mixture which may be in the form of a powder blend prepared through the mechanical mixing of the two powders (dry blending) , or in a matrix form, prepared by embedding the mixed metal oxide RFe0 3 in a silica-gel matrix.
  • the weight ratio between the iron source and the support material which are combined together to form the mixture may be in the range of 1:10-1:1, preferably in the range of 1:5-1:2.
  • the preparation of the mixture in a matrix form it is carried out by suspending a suitable source of iron ions in a silica sol and solidifying said suspension, to form a silica matrix having said iron source embedded therein.
  • Suitable colloidal silica solutions are commercially available (Ludox LS) .
  • the preparation preferably involves the solidification of a suspension comprising the mixed metal oxide RFe0 3 particles and the solid support particles, to collect a dry solid, followed by calcination of the solid.
  • the mixed oxide RFe0 3 particles (e.g., LaFe0 3 perovskite), with particle diameter in the range from 30 to 200 ⁇ , are added to a colloidal solution of the solid support, for example, to a silica sol.
  • concentration of the silica sol is preferably between 10 and 40 wt.%, more preferably about 30 wt . % .
  • the average particle size of the silica particles is preferably less than 20 nm, preferably about 10 nm.
  • the aqueous suspension thus formed is mixed at room temperature, following which water is removed by evaporation under continuous mixing at a temperature in the range of 50-90 °C, and preferably at about 60°C, until complete dryness.
  • the dry solid is then calcined in air at a temperature in the range of 400 -600 °C, and preferably at 500°C, for a duration of 2-4 hours.
  • the solid thus obtained consists of RFe0 3 particles embedded in a silica matrix.
  • the resultant silica matrix having the iron source embedded therein which is preferably prepared by the procedure outlined above, has surface area in range of 180 to 320 m 2 /g (compared with a surface area of 30-50 m 2 /g for the. RFe0 3 starting material; the increase at the surface area is due to the incorporation of the RFe0 3 in the porous high-surface area silica matrix) .
  • the solid mixture which comprises the iron source and support particles (either as a powder blend or in a matrix form) is subsequently brought into contact with an acidic environment, to form the catalytically active composition of the invention through the selective extraction deposition procedure.
  • the last step namely, the activation of the mixture in an acidic environment, whereby iron ions are deposited onto the surface of the support particles, can be also carried out in-situ, upon contacting the mixture with an acidified stream of wastewater to be treated, as described in more detail below.
  • a preferred embodiment of the process of the invention comprises loading the mixture of a mixed oxide of iron and a second metal and solid support particles into a suitable reactor and generating an acidic environment in the reactor, for example, by passing an acidic aqueous solution through a continuous flow fixed bed reactor.
  • the iron source Fe-precursor
  • the aqueous solution which continuously flows through the reactor removes the second metal of the mixed oxide (e.g., the lanthanum) from the reactor.
  • the concentration of the lanthanum in the effluent water exiting the reactor reaches 30 ppmw, while the Fe concentration measured in the effluent water is preferably less than 0.05 ppmw, indicating that the iron selectively binds to the solid support, while the lanthanum is leached out.
  • the acidic conditions are generated in the continuous flow reactor using an acidic aqueous solution, e.g., an aqueous sulfuric acid, aqueous nitric acid, aqueous hydrochloric acid and other mineral acids, preferably having pH of about 3 to 4, which flows through the reactor with an LHSV ranging from 5 to 20 hr _1 .
  • an acidic aqueous solution e.g., an aqueous sulfuric acid, aqueous nitric acid, aqueous hydrochloric acid and other mineral acids, preferably having pH of about 3 to 4, which flows through the reactor with an LHSV ranging from 5 to 20 hr _1 .
  • LHSV refers to a "liquid hourly space velocity", a commonly used measure which equals the volumetric rate of a feed in the liquid state per volume of reactor.
  • Suitable continuous flow fixed-bed reactors allowing dense packing of the solid mixtures set forth above are, for example, acid-resistant tubes with diameters permitting the formation of a relatively thick, dense layer of said mixtures placed in the tube, e.g., a layer having thickness of not less than 3 cm.
  • a packed bed reactor is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can ' be randomly filled with small objects like catalysts particles. The purpose of a packed bed is typically to improve contact between two phases: the solid catalyst and liquid reagent (i.e. organic contaminants dissolved ion wastewater) in a chemical catalytic process.
  • the activation of the catalyst is generally conveniently carried out by passing the acidic liquid medium through the mixture (consisting of the solid support particles and the iron source) provided as a fixed bed in a continuous flow reactor, the activation can be alternatively carried in a batch reactor.
  • the selective extraction-deposition method set forth above (namely, the activation of the mixture to form the catalyst) is carried at a temperature ranging from 60 to 80 °C for a period ranging from 1 to several hours (for example, 10 hours) .
  • the dense packing of the mixture of the Fe-precursor e.g. the LaFe0 3 particles
  • the particles of the support material such as silica- gel
  • the mixture of the solid support particles and the source of iron. ions is exposed to the acidic environment to cause only partial decomposition of said iron source, such that the catalyst which is obtained contains, in addition to the solid support particles having iron ions located thereon, a further solid phase, consisting of said source of iron ions.
  • the additional solid phase present in the catalyst is preferably a mixed oxide of iron and a second metal having perovskite structure, most preferably LaFe0 3 .
  • the mixed oxide Fe-precursor e.g., the perovskite LaFeC>3 , reacts with the acid (e.g., H 2 S0 4 ) in water at pH of about 3 to 4, according to following reaction:
  • the decomposition of the perovskite LaFe0 3 takes place, with La 2 (S0 4 ) 3 being dissolved in the aqueous solution while the released Fe(3+) ions co-condense with surface hydroxyls (silanolos) , and are stabilized at the surface of the silica particles as OFe-OSi; 0H-Fe(0Si) 2 or Fe-(OSi) 3 species.
  • the weight ratio silica : Fe 3+ : residual perovskite in the catalyst of the invention is controlled by adjusting the silica : perovskite weight ratio in the initial mixture (which contains the iron source and the support particles, either as a powder blend or in a matrix form) within the range from 5:1 to 2:1, preferably from to 3.5:1 to 2:1.
  • the ratio silica : Fe 3+ : residual perovskite is subsequently determined by adjusting the amount of acid (e.g., H 2 S0 4 ) used for achieving the decomposition of the perovskite.
  • the acid should be used in deficiency: the amount of the acid used is between 20 and 50%, and preferably between 25 and 30%, of the amount needed for the complete decomposition of the iron precursor (i.e., LaFe0 3 perovskite) according to equation (1).
  • the preferred composition of the catalyst in terms silica : Fe 3+ : residual perovskite weight ratios is from 0.3 : 0.01 : 1.0 to 8.0 : 0.25 : 1.0, preferably from 3.5 : 0.1 : 1.0 to 4.5 : 0.2 : 1.0.
  • the resultant catalyst obtained after completion of the acid-activation step in a continuous fixed-bed reactor can be directly used in the purification of organic impurities through Fenton oxidation reactions carried out in the same reactor.
  • the catalyst upon completing the acid-activation step, can be removed from the reactor (either batch or tubular) and dried. It can be stored in air and then used in the purification of organic impurities through Fenton oxidation reactions.
  • the catalyst forms another aspect of the invention.
  • the catalyst comprises solid support particles (in particular- silica support particles) , having iron ions discretely located on the surface of said particles.
  • Suitable support particles in terms of material and surface area, are as set forth above.
  • the presence of discrete iron ions on the surface of the support particles is indicated by UV-Raman spectroscopy: the spectrum of the catalyst of the invention (excited by 234 and 325 nm lasers sources) exhibits resonance bands at one or more of the following: 510, 1090 and 1140 cm “1 ( + 5 cm -1 , more typically ⁇ 2 cm -1 ) .
  • the support particles are essentially devoid of iron oxide (Fe 2 0 3 ) or iron oxide- hydroxide crystalline phases, as indicated by their x-ray powder diffraction pattern.
  • iron oxide Fe 2 0 3
  • iron oxide- hydroxide crystalline phases as indicated by their x-ray powder diffraction pattern.
  • the iron which forms oxide or oxide-hydroxide crystalline phases is not more than 10%, and preferably not more than 5%, and most preferably less than 1%, relative to the total amount of iron deposited on the solid support.
  • the catalyst comprises a further solid phase, which is crystalline perovskite phase (e.g. LaFe0 3 ) .
  • the support Fe: perovskite weight ratios are from 0.3 : 0.01 : 1.0 to 8.0 : 0.25 : 1.0, preferably from 3.5 : 0.1 : 1.0 to 4.5 : 0.2 : 1.0. It has been found that the presence of this additional perovskite phase extends the catalyst life and improves its performance.
  • the catalyst exhibits an X-ray powder diffraction pattern having wide reflection at about 23° 2 ⁇ indicative of amorphous silica support, at least one peak assigned to the perovskite crystalline phase, for example, at 31.9; 39.2; 45.8; 57.4; 67.9 and 76.7° 2 ⁇ ( ⁇ 0.1 2 ⁇ ) [which correspond to the (1 0 1), (1 2 1), (2 2 0), (2 0 2), (2 4 0), (2 4 2) and (2 0 4) diffraction planes for LaFe0 3 with an orthorhombic perovskite structure] , wherein said X-ray powder diffraction pattern is essentially devoid of peaks assignable to iron oxide or oxide-hydroxide crystalline phases.
  • a preferred catalyst of the invention comprises about 20-85% silica, 0.5-3.0% Fe and 10-75% LaFe0 3 perovskite (w/w) .
  • the catalyst can alternatively be activated in situ, in a wastewater treatment plant, where the selective extraction deposition method is carried out using acidified wastewater.
  • the acidic liquid medium used is an acidified wastewater stream, such that the catalyst is formed in-situ in a site of an industrial wastewater treatment.
  • the in-situ preparation of the catalyst comprises placing a mixture of the solid support particles and a mixed oxide of iron and a second metal and (either as a powder blend or in a matrix form) in a reactor, e.g., as described above, and passing acidified wastewater (e.g., wastewater to which an aqueous sulfuric acid was added, adjusting the pH to about 3 to 5) , wherein said wastewater further contains an oxidizer, through said reactor, preferably for a period ranging from 1 to several hundreds hours (for example, 100 hours) at a temperature in the range of 50 to 90°C.
  • acidified wastewater e.g., wastewater to which an aqueous sulfuric acid was added, adjusting the pH to about 3 to 5
  • said wastewater further contains an oxidizer
  • the catalyst is useful for the purification of water involving the Fenton oxidation of organic contaminants. It was found by the inventors that the passage of an acidified wastewater stream comprising an oxidant such as hydrogen peroxide through a layer of the heterogeneous Fenton catalyst of the present invention, allows an effective treatment of the wastewater, namely, an essentially complete destruction of the organic contaminants. In particular, it was demonstrated that carrying out the purification of the wastewater in a continuous flow reactor while the pH values measured at the inlet of said reactor are higher than 3.0 (e.g., 4.0-4.5), yielded appreciable and stable Total Organic Content (TOC) removal rates from contaminants-containing wastewater.
  • TOC Total Organic Content
  • the TOC conversion can vary from 50 to 97%, depending on the operation conditions and the composition of the catalyst.
  • the catalyst demonstrated stable operation at TOC conversions of more than 50% for more than 35 hours, preferably TOC conversions of at least 85% for not less than 90 hours.
  • BOD biological oxygen demand
  • the term "biological oxygen demand” (“BOD”) refers to the quantity of oxygen utilized in the bio/chemical oxidation of organic matter.
  • TOC shall be understood to mean total organic carbon.
  • the TOC value is independent of the oxidation state of the organic matter, and does not measure other organically-bound elements, such as nitrogen, hydrogen and inorganics which can contribute to the oxygen demand value measured by BOD.
  • a method for removing organic contaminants from wastewater comprising the oxidation of one or more organic contaminants in the presence of iron-containing catalyst, said catalyst . comprising solid particles having Fe ions discretely located on their surface, said catalyst being essentially free from iron oxide or oxide-hydroxide phases.
  • the method for treating wastewater using the catalyst of the invention is carried out in a "filtration mode", by passing the wastewater through a suitable continuous solid- liquid reactor, packed with the aforementioned heterogeneous catalyst (fixed bed reactor) .
  • the organic contaminants which may be oxidized and removed from wastewater, by the method of the present invention include a variety of organic compounds, including but not limited to, phenol, substituted including halogenated phenols, amines, nitro- compounds, esters, ethers and other organics typically present in industrial wastewater.
  • the conditions for the wastewater treatment are generally as follows.
  • the wastewater to be treated flows through a suitable continuous solid-liquid reactor, in which the catalyst is placed.
  • the wastewater stream may flow, at an LHSV ranging from 2 to 100 h "1 .
  • the oxidation of the contaminants is accomplished at a temperature in the range of 50-90°C, under acidic pH (the pH is adjusted to about 3- 5, preferably 4-4.5), with the wastewater stream containing an H 2 0 2 oxidizer ' in an amount corresponding to stoichiometric requirement for complete conversion of organic contaminants to CO 2 /H 2 O.
  • the oxidizer is used in a molar excess relative to the organic impurities.
  • the molar ratio between the oxidizer in the wastestrearn to the organic contaminant may be between 1 and 2, preferably about 1.3, relative to that required for complete oxidation of the organic contaminants to CO2/H 2 O.
  • the catalyst is packed in a continuous fixed-bed reactor either in a powder form or in the form of pellets having size and shape which provide minimum pressure drop in the catalysts bed.
  • the LHSV number of liters of wastewater treated by one liter of catalyst per hour
  • the LHSV is preferably in the range from 3 to 20 h "1 , e.g., 5 to 8 h _1 .
  • Figure 1 represents a flowchart of the experimental setup.
  • Figure 2 is the XRD pattern generated for a catalyst comprising silica particles having iron ions deposited thereon and a perovskite phase.
  • Figure 3 is Raman spectrum of the catalyst of the invention .
  • the scheme of the experimental setup is shown in Figure 1.
  • the feed (1) contained aqueous solution of phenol, sulfuric acid and hydrogen peroxide. This solution was fed to the tubular fixed bed reactor (catalytic filter) (3) with a HPLC pump (2) downstream.
  • the treated water was collected in the sample deposit flask (5) .
  • Numerals (4) and (6) indicate the cooler and thermal bath, respectively, used for cooling the water at the filter outlet and heating the filter during the test.
  • UV-Raman spectra were recorded at Jobin-Yvon LabRam 800 micro-Raman system, equipped with N 2 -cooled detector.
  • the catalyst was prepared in two steps. In the first step, nanocrystalline LaFe0 3 perovskite material was made and in the second step, the resultant perovskite was used as, Fe source for depositing iron onto a silica support by the selective extraction-deposition method of the invention.
  • a) Preparation of Fe precursor (LaFeQ 3 perovskite) La (N0 3 ) 3 -6H 2 0 (5.5 grams) and Fe (N0 3 ) 2 ' 6H 2 0 (3.6 grams) were dissolved in water (60 cm 3 ) at 40°C. To this solution was added glycine (H 2 NCH 2 C0 2 H, 0.95 grams) and it was slowly evaporated at 80°C.
  • the resulting gel was heated in air to 200 °C for removal of organic matter and the obtained solid was calcined in air at 600°C for 3 hours (heating rate 5°C/min).
  • the obtained material had surface area of 43 m 2 /g and represented a pure LaFe0 3 phase with perovskite structure of orthorhombic symmetry (XRD patterns according to JCPDS file no. 01-074-2203).
  • Perovskite powder (240 mg) , synthesized as described in part (a) above, was mixed with commercial silica-gel (720 mg) having surface area of 314 m 2 /g, a pore volume of 1.9 cm 3 /grams, an average pore diameter of 24 nm and a particle size of 30-100 ⁇ ; commercially available from PQ Co.
  • the activation was carried out in a tube ("continuous mode").
  • the solid mixture was packed in a stainless steel tube at a temperature of 80°C (inside the catalyst layer) .
  • the catalyst was dried under He flow of 20 cm 3 /min and was then discharged from the tube. Under these conditions, the LaFe0 3 perovskite underwent partial decomposition, as indicated by the analysis reported below.
  • the material obtained contained 19 wt . % of the residual (non-decomposed) perovskite phase. Neither iron oxide nor iron oxide-hydroxide phases were detected by XRD analysis.
  • the EDAX analysis indicated the presence of 5.9 weight % of iron.
  • the catalyst was prepared in two steps, according to the procedure set forth in Example 1.
  • nanocrystalline LaFe0 3 perovskite material was made as described in step a) of Example 1.
  • the resultant perovskite was used as Fe source for depositing iron onto a silica support by the selective extraction-deposition method of the invention, carried out in a flask, such that the LaFeC perovskite underwent complete decomposition.
  • Perovskite powder (280 mg) , synthesized as described in part (a) above, was mixed with commercial silica-gel (1440 mg) having surface area of 314 m 2 /g, a pore volume of 1.9 cm 3 /grams, an average pore diameter of 24 nm and a particle size of 30-100 pm; commercially available from PQ Co.
  • the activation was carried out in a flask ("batch mode").
  • the pH was kept at 3.0 through a continuous dropwise addition of 0.5M H 2 S0 4 solution for two hours.
  • the catalyst was separated by filtration and dried in air at 120°C for 4 hours. This catalyst, was treated again with H 2 S0 4 as described above, and then the treatment was repeated for the third time.
  • the obtained catalyst contained neither a perovskite phase nor any iron oxide or oxide-hydroxide phases.
  • a characteristic Raman spectrum is shown in Figure 3, where the Raman signals at 510, .1090 and 1140 cm -1 are due to the isolated Fe(3+) ions adsorbed at the silica surface and the other peaks at 800 and 980 cm -1 are assigned to the silica phase.
  • the material had surface area of 223 m 2 /g and pore volume 0.23 cm 3 /g.
  • the resultant composition consisting of the perovskite embedded in a silica matrix, was subjected to in-situ activation as described in the following example.
  • the mixture (i.e., in a matrix form) prepared in Example 5 (3 cm 3 ) was packed in a tubular stainless steel reactor at a temperature of 80 °C inside the catalyst layer.
  • the catalyst temperature was kept at 80°C.

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Abstract

The invention provides a process for the preparation of an iron-containing catalyst suitable for use in Fenton oxidation reactions, comprising preparing a mixture of a source of iron ions and solid support particles, said support particles having chemically reactive groups present on their surface, contacting said mixture with an acidic liquid medium thereby causing the release of iron ions from said iron source to form a catalyst comprising said solid support particles and said iron ions located on the surface of said support particles. The invention also provides the catalyst and a method for using the catalyst in purifying wastewater.

Description

Process for the preparation of heterogeneous Fenton catalytic filter
Fenton oxidation of organic impurities present in industrial wastewater using hydrogen peroxide is an efficient method for the purification of water at mild conditions, e.g., at atmospheric pressure and temperatures 50-80°C. The shortcoming of this method is associated with the use of soluble homogeneous iron catalysts, which contaminate the water and require costly catalyst separation step.
Attempts to prepare an efficient heterogeneous Fenton catalyst which is capable of working in continuous solid- liquid reactors at "filtration mode" by immobilization of iron ions on different solid supports or using solid, insoluble Fe-compounds met with considerable difficulties. The main reason for this is the requirement of acidic media (pH < 4, optimal value about 3.0) for efficient conversion of hydrogen peroxide to "OH radicals facilitated by redox transformations of iron ions, which oxidize the organic contaminants. Under the acidic conditions, iron compounds (including supported iron oxides nanoparticles , iron ions embedded in zeolites, pillared clays, mesostructured silica and in other matrices) leach to the water, resulting in catalyst deactivation.
Martinez et al. [Ind. Eng. Chem. Res. 46, 4396-4405 (2007)] describe the use of iron oxide supported over mesostructured SBA-15 material in a fixed-bed reactor for the catalytic wet hydrogen peroxide oxidation of phenolic aqueous solutions. US 2009/0286677 discloses an exhaust gas purifying catalyst comprising a perovskite-type composite metal oxide and a porous silica support.
The deposition of isolated iron ions on the surface of suitable supports constitutes a challenge due to the fast hydrolysis and condensation of iron ions in dissolved Fe- precursors (salts, alkoxides, etc.) used for catalyst preparation. So far, attempts to deposit iron ions on suitable supports, such as silica or activated carbon, yielded iron oxide nanoparticles or multimetallic clusters having relatively low catalytic activity, requiring low pH conditions to obtain reasonable reaction rates, thus resulting in substantial iron leaching and hence - catalyst deactivation .
The present invention allows the deposition of isolated metal (i.e., iron) ions onto a solid support, for example, on high-surface area silica, using a novel selective extraction deposition (SED) method. By the term "isolated" is meant that the iron ions are essentially discretely located on the surface of the support, such that in the close vicinity of a given iron ion, no further iron ions are present. Such ions do not show reflections in XRD diffractograms characteristic for any iron-containing crystalline or amorphous phases. The existence of iron in the state of isolated ions at silica surface is indicated by UV-Raman spectra (excited by 234 nm and 325 nm laser sources) exhibiting resonance bands at about 510, 1090 and 1140 cm-1. The discrete iron ions can be also detected by Mossbauer spectroscopy - isomer shift ca 0.45 mm/s at 60°C and average quadrupol splitting of 0.99-1.07 mm/s. According to the process of the invention, a suitable source of iron ions (also designated herein "Fe-precursor") is combined with particles of a suitable solid support. The resultant mixture, either in the form a powder blend, or in a matrix form in which the Fe-precursor is embedded in the support particles, is treated in a solution under suitable conditions (e.g., in an acidic environment), thereby releasing the iron ions from said Fe-precursor and allowing said iron ions to interact with reactive groups present on the surface of the support particles (such as hydroxyl groups (silanols) , in the case of silica-gel supports, or carboxylic groups) , before the hydrolysis and self- condensation of said iron ions can take place. In the resultant catalyst, highly dispersed ferric iron (Fe3+) ions are grafted on the surface of the solid support.
The invention is thus primarily directed to a process for the preparation of an iron-containing catalyst suitable for use in Fenton oxidation reactions, comprising preparing a mixture of a source of iron ions and solid support particles, said support particles having chemically reactive groups present on their surface, contacting said mixture with an acidic liquid medium thereby causing the release of iron ions from said iron source, to form a catalyst comprising said solid support particles and said iron ions located on the surface of said support particles.
A source of iron ions (Fe-precursor) , which is suitable for use as a starting material according to the process of the invention, is preferably a mixed oxide of iron and at least a second metal, said mixed metal oxide being decomposable in an acidic environment. Mixed oxides of the formulas RFe03 (R - rare-earth element selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) with perovskite structure, R3Fe50i2 with garnet structure as well as perovskites and garnets containing several rare earth metals having various compositions, e.g., Lai-xCexFe03, have been found to be especially useful. A description of the perovskite-type structure is found in Hans-Rudolf Wenk; Andrei Bulakh, (May 2004) Minerals: Their Constitution and Origin. New York, NY: Cambridge University Press. The term "perovskite" is intended herein to designate mixed metal oxides having the ideal and non-ideal perovskite-type crystalline structure. The ideal perovskite crystalline structure is defined by the empirical formula AB03 in which A and B are cations of two different metals and in which the A cation is coordinated to twelve oxygen atoms while the B cation occupies octahedral sites and is coordinated to six oxygen atoms. A description of the garnet-type structure is found in S. Geller; Crystal chemistry of the garnets, Zeitschrift fiir Kristallographie , 125, S. 1-47 (1967) .
The mixed oxide of iron and a second metal decomposes in an acidic liquid medium used in the process of the invention. However, the second metal which forms part of the mixed oxide is selectively leached out under the conditions applied in the process, such that said second metal cannot significantly compete with the iron ions on the reactive sites at the surface of the support material. It was found by the inventors that LaFe03 perovskite can serve as a suitable iron source for the selective extraction- deposition of isolated iron ions on silica support in acidic aqueous solutions (namely, the lanthanum is preferentially leached out, whereas the iron selectively interacts with the silica support) . The perovskite used in the process of the invention, e.g., LaFe03, is provided in the form of a powder having surface area between 15 and 60 m2/g and primary crystals size between 20 and 30 nm, while the size of the particles (aggregates of primary crystals) is in the range between 20 and 150 pm. In general, LaFe03 perovskite suitable for use according to the invention is prepared by dissolving lanthanum (La3+) and iron (Fe3+) salts (e.g., in their hydrated forms) in an aqueous solution at a temperature in the range between 30 and 50°C, followed by the addition of a complexant like glycine, citric or oxalic acid. The complexant compound contains two functional groups (e.g., -COOH, or -COOH and -NH2, like in glycine) which can bind chemically to both the Fe and La ions in the solution. This prevents segregation of La and Fe components at the subsequent gelation step, thus yielding uniform distribution of La and Fe ions in the bulk of the obtained gel. The solution is maintained at a temperature of not less than 60°C for a sufficient time to allow the formation of a gel-like material, which is subsequently dried and gradually calcined, to give the desired pure LaFe03 perovskite. The preparation of a LaFe03 perovskite which is suitable as Fe-precursor in the process of the invention is illustrated below.
The support material is preferably a high surface-area solid material, for example, a material having surface area ranging from 100 to 1500 m2/g may be used, in particular silica, titania, zirconia or carbon. As noted above, the solid support contains chemically reactive groups capable of interacting with iron ions, such as hydroxyl and carboxylic acid groups. More preferably, the high surface- area support material is silica, which is selected from the group consisting of silica-gel, mesostructured silica and β
silica-alumina. The particle size of the high surface-area support is preferably in range between 20 and 150 μιη.
As noted above, the solid support particles (e.g., the silica particles) and the solid iron source (e.g., a mixed oxide of iron and a second metal, such as RFe03 perovskite, wherein R is most preferably lanthanum) are combined together to form a mixture, which may be in the form of a powder blend prepared through the mechanical mixing of the two powders (dry blending) , or in a matrix form, prepared by embedding the mixed metal oxide RFe03 in a silica-gel matrix. The weight ratio between the iron source and the support material which are combined together to form the mixture may be in the range of 1:10-1:1, preferably in the range of 1:5-1:2.
Regarding the preparation of the mixture in a matrix form, it is carried out by suspending a suitable source of iron ions in a silica sol and solidifying said suspension, to form a silica matrix having said iron source embedded therein. Suitable colloidal silica solutions are commercially available (Ludox LS) . Thus, the preparation preferably involves the solidification of a suspension comprising the mixed metal oxide RFe03 particles and the solid support particles, to collect a dry solid, followed by calcination of the solid. To this end, the mixed oxide RFe03 particles (e.g., LaFe03 perovskite), with particle diameter in the range from 30 to 200 μπι, are added to a colloidal solution of the solid support, for example, to a silica sol. The concentration of the silica sol is preferably between 10 and 40 wt.%, more preferably about 30 wt . % . The average particle size of the silica particles is preferably less than 20 nm, preferably about 10 nm. The aqueous suspension thus formed is mixed at room temperature, following which water is removed by evaporation under continuous mixing at a temperature in the range of 50-90 °C, and preferably at about 60°C, until complete dryness. The dry solid is then calcined in air at a temperature in the range of 400 -600 °C, and preferably at 500°C, for a duration of 2-4 hours. The solid thus obtained consists of RFe03 particles embedded in a silica matrix. The resultant silica matrix having the iron source embedded therein, which is preferably prepared by the procedure outlined above, has surface area in range of 180 to 320 m2/g (compared with a surface area of 30-50 m2/g for the. RFe03 starting material; the increase at the surface area is due to the incorporation of the RFe03 in the porous high-surface area silica matrix) .
The solid mixture, which comprises the iron source and support particles (either as a powder blend or in a matrix form) is subsequently brought into contact with an acidic environment, to form the catalytically active composition of the invention through the selective extraction deposition procedure. It should be noted that the last step, namely, the activation of the mixture in an acidic environment, whereby iron ions are deposited onto the surface of the support particles, can be also carried out in-situ, upon contacting the mixture with an acidified stream of wastewater to be treated, as described in more detail below.
In practice, a preferred embodiment of the process of the invention comprises loading the mixture of a mixed oxide of iron and a second metal and solid support particles into a suitable reactor and generating an acidic environment in the reactor, for example, by passing an acidic aqueous solution through a continuous flow fixed bed reactor. Under the acidic conditions, the iron source ( Fe-precursor ) decomposes and releases the iron ions, which rapidly reacts with the chemically reactive groups of the solid support. The aqueous solution which continuously flows through the reactor removes the second metal of the mixed oxide (e.g., the lanthanum) from the reactor. The concentration of the lanthanum in the effluent water exiting the reactor reaches 30 ppmw, while the Fe concentration measured in the effluent water is preferably less than 0.05 ppmw, indicating that the iron selectively binds to the solid support, while the lanthanum is leached out.
The acidic conditions are generated in the continuous flow reactor using an acidic aqueous solution, e.g., an aqueous sulfuric acid, aqueous nitric acid, aqueous hydrochloric acid and other mineral acids, preferably having pH of about 3 to 4, which flows through the reactor with an LHSV ranging from 5 to 20 hr _1. The term "LHSV" refers to a "liquid hourly space velocity", a commonly used measure which equals the volumetric rate of a feed in the liquid state per volume of reactor. Suitable continuous flow fixed-bed reactors allowing dense packing of the solid mixtures set forth above are, for example, acid-resistant tubes with diameters permitting the formation of a relatively thick, dense layer of said mixtures placed in the tube, e.g., a layer having thickness of not less than 3 cm. A packed bed reactor is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can' be randomly filled with small objects like catalysts particles. The purpose of a packed bed is typically to improve contact between two phases: the solid catalyst and liquid reagent (i.e. organic contaminants dissolved ion wastewater) in a chemical catalytic process. It should be understood, however, that although the activation of the catalyst is generally conveniently carried out by passing the acidic liquid medium through the mixture (consisting of the solid support particles and the iron source) provided as a fixed bed in a continuous flow reactor, the activation can be alternatively carried in a batch reactor.
The selective extraction-deposition method set forth above (namely, the activation of the mixture to form the catalyst) is carried at a temperature ranging from 60 to 80 °C for a period ranging from 1 to several hours (for example, 10 hours) . It appears that the dense packing of the mixture of the Fe-precursor (e.g. the LaFe03 particles) with the particles of the support material (such as silica- gel) in a tube reactor increases the precursor-support contact interface. This minimizes the self-condensation of Fe3+ ions liberated from the precursor in the acidic solution due to a decrease in the distance the iron ions need to pass in the solution, from the precursor to the support surface.
Preferably, the mixture of the solid support particles and the source of iron. ions is exposed to the acidic environment to cause only partial decomposition of said iron source, such that the catalyst which is obtained contains, in addition to the solid support particles having iron ions located thereon, a further solid phase, consisting of said source of iron ions. The additional solid phase present in the catalyst is preferably a mixed oxide of iron and a second metal having perovskite structure, most preferably LaFe03. The mixed oxide Fe-precursor, e.g., the perovskite LaFeC>3 , reacts with the acid (e.g., H2S04) in water at pH of about 3 to 4, according to following reaction:
2LaFe03 + 3H2S04 = La2 (S04) 3 + Fe203 + 3H20 (1)
In the presence of a support material such as silica in the acidic solution, the decomposition of the perovskite LaFe03 takes place, with La2(S04)3 being dissolved in the aqueous solution while the released Fe(3+) ions co-condense with surface hydroxyls (silanolos) , and are stabilized at the surface of the silica particles as OFe-OSi; 0H-Fe(0Si)2 or Fe-(OSi)3 species.
The weight ratio silica : Fe3+ : residual perovskite in the catalyst of the invention is controlled by adjusting the silica : perovskite weight ratio in the initial mixture (which contains the iron source and the support particles, either as a powder blend or in a matrix form) within the range from 5:1 to 2:1, preferably from to 3.5:1 to 2:1. The ratio silica : Fe3+ : residual perovskite is subsequently determined by adjusting the amount of acid (e.g., H2S04) used for achieving the decomposition of the perovskite. The acid should be used in deficiency: the amount of the acid used is between 20 and 50%, and preferably between 25 and 30%, of the amount needed for the complete decomposition of the iron precursor (i.e., LaFe03 perovskite) according to equation (1). As a result, the preferred composition of the catalyst in terms silica : Fe3+: residual perovskite weight ratios is from 0.3 : 0.01 : 1.0 to 8.0 : 0.25 : 1.0, preferably from 3.5 : 0.1 : 1.0 to 4.5 : 0.2 : 1.0. The resultant catalyst obtained after completion of the acid-activation step in a continuous fixed-bed reactor can be directly used in the purification of organic impurities through Fenton oxidation reactions carried out in the same reactor. Alternatively, upon completing the acid-activation step, the catalyst can be removed from the reactor (either batch or tubular) and dried. It can be stored in air and then used in the purification of organic impurities through Fenton oxidation reactions.
The catalyst forms another aspect of the invention. The catalyst comprises solid support particles (in particular- silica support particles) , having iron ions discretely located on the surface of said particles. Suitable support particles, in terms of material and surface area, are as set forth above. The presence of discrete iron ions on the surface of the support particles is indicated by UV-Raman spectroscopy: the spectrum of the catalyst of the invention (excited by 234 and 325 nm lasers sources) exhibits resonance bands at one or more of the following: 510, 1090 and 1140 cm"1 ( + 5 cm-1, more typically ± 2 cm-1) .
A key feature of the catalyst of the invention is that the support particles (e.g., the silica particles) are essentially devoid of iron oxide (Fe203) or iron oxide- hydroxide crystalline phases, as indicated by their x-ray powder diffraction pattern. By the term "essentially free" or "essentially devoid" is meant that the iron which forms oxide or oxide-hydroxide crystalline phases is not more than 10%, and preferably not more than 5%, and most preferably less than 1%, relative to the total amount of iron deposited on the solid support. Preferably, the catalyst comprises a further solid phase, which is crystalline perovskite phase (e.g. LaFe03) . Compositionally, the support: Fe: perovskite weight ratios are from 0.3 : 0.01 : 1.0 to 8.0 : 0.25 : 1.0, preferably from 3.5 : 0.1 : 1.0 to 4.5 : 0.2 : 1.0. It has been found that the presence of this additional perovskite phase extends the catalyst life and improves its performance. Thus, in a preferred embodiment of the invention, the catalyst exhibits an X-ray powder diffraction pattern having wide reflection at about 23° 2Θ indicative of amorphous silica support, at least one peak assigned to the perovskite crystalline phase, for example, at 31.9; 39.2; 45.8; 57.4; 67.9 and 76.7° 2Θ (±0.1 2Θ) [which correspond to the (1 0 1), (1 2 1), (2 2 0), (2 0 2), (2 4 0), (2 4 2) and (2 0 4) diffraction planes for LaFe03 with an orthorhombic perovskite structure] , wherein said X-ray powder diffraction pattern is essentially devoid of peaks assignable to iron oxide or oxide-hydroxide crystalline phases. For example, a preferred catalyst of the invention comprises about 20-85% silica, 0.5-3.0% Fe and 10-75% LaFe03 perovskite (w/w) .
As noted above, the catalyst can alternatively be activated in situ, in a wastewater treatment plant, where the selective extraction deposition method is carried out using acidified wastewater. Thus, according to one embodiment of the invention, the acidic liquid medium used is an acidified wastewater stream, such that the catalyst is formed in-situ in a site of an industrial wastewater treatment. For example, the in-situ preparation of the catalyst comprises placing a mixture of the solid support particles and a mixed oxide of iron and a second metal and (either as a powder blend or in a matrix form) in a reactor, e.g., as described above, and passing acidified wastewater (e.g., wastewater to which an aqueous sulfuric acid was added, adjusting the pH to about 3 to 5) , wherein said wastewater further contains an oxidizer, through said reactor, preferably for a period ranging from 1 to several hundreds hours (for example, 100 hours) at a temperature in the range of 50 to 90°C.
The catalyst, either prepared in advance or in situ by the process of the invention, is useful for the purification of water involving the Fenton oxidation of organic contaminants. It was found by the inventors that the passage of an acidified wastewater stream comprising an oxidant such as hydrogen peroxide through a layer of the heterogeneous Fenton catalyst of the present invention, allows an effective treatment of the wastewater, namely, an essentially complete destruction of the organic contaminants. In particular, it was demonstrated that carrying out the purification of the wastewater in a continuous flow reactor while the pH values measured at the inlet of said reactor are higher than 3.0 (e.g., 4.0-4.5), yielded appreciable and stable Total Organic Content (TOC) removal rates from contaminants-containing wastewater. The TOC conversion can vary from 50 to 97%, depending on the operation conditions and the composition of the catalyst. The catalyst demonstrated stable operation at TOC conversions of more than 50% for more than 35 hours, preferably TOC conversions of at least 85% for not less than 90 hours. Furthermore, it appears that the treated wastewater is not toxic and display high biodegradability and biological oxygen demand (BOD) relative to residual TOC. The term "biological oxygen demand" ("BOD") refers to the quantity of oxygen utilized in the bio/chemical oxidation of organic matter. The term "TOC" shall be understood to mean total organic carbon. The TOC value is independent of the oxidation state of the organic matter, and does not measure other organically-bound elements, such as nitrogen, hydrogen and inorganics which can contribute to the oxygen demand value measured by BOD.
Thus, according to another aspect of the invention, there is provided a method for removing organic contaminants from wastewater, comprising the oxidation of one or more organic contaminants in the presence of iron-containing catalyst, said catalyst . comprising solid particles having Fe ions discretely located on their surface, said catalyst being essentially free from iron oxide or oxide-hydroxide phases.
The method for treating wastewater using the catalyst of the invention is carried out in a "filtration mode", by passing the wastewater through a suitable continuous solid- liquid reactor, packed with the aforementioned heterogeneous catalyst (fixed bed reactor) .
The organic contaminants which may be oxidized and removed from wastewater, by the method of the present invention, include a variety of organic compounds, including but not limited to, phenol, substituted including halogenated phenols, amines, nitro- compounds, esters, ethers and other organics typically present in industrial wastewater.
The conditions for the wastewater treatment are generally as follows. The wastewater to be treated flows through a suitable continuous solid-liquid reactor, in which the catalyst is placed. The wastewater stream may flow, at an LHSV ranging from 2 to 100 h"1. The oxidation of the contaminants is accomplished at a temperature in the range of 50-90°C, under acidic pH (the pH is adjusted to about 3- 5, preferably 4-4.5), with the wastewater stream containing an H202 oxidizer ' in an amount corresponding to stoichiometric requirement for complete conversion of organic contaminants to CO2/H2O. Preferably, the oxidizer is used in a molar excess relative to the organic impurities. For example, the molar ratio between the oxidizer in the wastestrearn to the organic contaminant may be between 1 and 2, preferably about 1.3, relative to that required for complete oxidation of the organic contaminants to CO2/H2O.
The catalyst is packed in a continuous fixed-bed reactor either in a powder form or in the form of pellets having size and shape which provide minimum pressure drop in the catalysts bed. The LHSV (number of liters of wastewater treated by one liter of catalyst per hour) is preferably in the range from 3 to 20 h"1, e.g., 5 to 8 h _1.
Brief description of the drawings
Figure 1 represents a flowchart of the experimental setup.
Figure 2 is the XRD pattern generated for a catalyst comprising silica particles having iron ions deposited thereon and a perovskite phase.
Figure 3 is Raman spectrum of the catalyst of the invention . Examples
In the Examples illustrated below, a stainless steel tube having a 8 mm inner diameter, filled with the composite catalyst powder prepared as described in examples 1, 3 and 5 and equipped with a jacket and thermostat, was used as a catalytic filter for purification of water containing 200 mppmw phenol. The scheme of the experimental setup is shown in Figure 1. The feed (1) contained aqueous solution of phenol, sulfuric acid and hydrogen peroxide. This solution was fed to the tubular fixed bed reactor (catalytic filter) (3) with a HPLC pump (2) downstream. The treated water was collected in the sample deposit flask (5) . Numerals (4) and (6) indicate the cooler and thermal bath, respectively, used for cooling the water at the filter outlet and heating the filter during the test.
XRD analysis was conducted using Philips 1050/70 powder diffTactometer (Bragg-Brentano geometry), fitted with a graphite monochromator providing a K diffracted beam (λ =1.541 A) and operating at V = 40 kV and I = 30 mA. The data were collected in the range of 2Θ = 10°- 80° with a step size of 0.2°.
EDAX analysis was conducted using microscope Quanta-2000, SEMEDAX Instrument, FEI Co.
The UV-Raman spectra were recorded at Jobin-Yvon LabRam 800 micro-Raman system, equipped with N2-cooled detector.
HPLC analysis was conducted using Cig reverse-phase column (Inertsil ODS-2):. TOC was measured using a TOC combustion analyzer Apollo 9000 HS model, Tekmar Dohmann Co.
Surface area measurements were carried out by the BET method based on N2-adsorption . Isotherms were obtained at liquid nitrogen temperature using NOVA-2000 (Quantachrome, Version 7.02) .
Example 1
Preparation of a catalyst comprising silica particles having iron ions deposited thereon
and perovskite phase
The catalyst was prepared in two steps. In the first step, nanocrystalline LaFe03 perovskite material was made and in the second step, the resultant perovskite was used as, Fe source for depositing iron onto a silica support by the selective extraction-deposition method of the invention. a) Preparation of Fe precursor (LaFeQ3 perovskite) : La (N03) 3-6H20 (5.5 grams) and Fe (N03) 2' 6H20 (3.6 grams) were dissolved in water (60 cm3) at 40°C. To this solution was added glycine (H2NCH2C02H, 0.95 grams) and it was slowly evaporated at 80°C. The resulting gel was heated in air to 200 °C for removal of organic matter and the obtained solid was calcined in air at 600°C for 3 hours (heating rate 5°C/min). The obtained material had surface area of 43 m2/g and represented a pure LaFe03 phase with perovskite structure of orthorhombic symmetry (XRD patterns according to JCPDS file no. 01-074-2203). b) Selective extraction-deposition of Fe-precursor on a silica support:
Perovskite powder (240 mg) , synthesized as described in part (a) above, was mixed with commercial silica-gel (720 mg) having surface area of 314 m2/g, a pore volume of 1.9 cm3/grams, an average pore diameter of 24 nm and a particle size of 30-100 μιτι; commercially available from PQ Co.
The activation was carried out in a tube ("continuous mode"). The solid mixture was packed in a stainless steel tube at a temperature of 80°C (inside the catalyst layer) . An aqueous solution of sulfuric acid with pH = 3 was pumped up through this tube at 80°C for 10 hours at a flow rate of 20 cnvVhour. The catalyst was dried under He flow of 20 cm3/min and was then discharged from the tube. Under these conditions, the LaFe03 perovskite underwent partial decomposition, as indicated by the analysis reported below.
The X-ray powder diffraction pattern of the resultant catalyst is presented in Figure 2, displaying wide reflection at 2Θ = 23°, corresponding to amorphous silica, and characteristic peaks of the perovskite at 31.9; 39.2; 45.8; 57.4; 67.9 and 76.7° 2Θ (+0.1 2θ) . According to XRD analysis, the material obtained contained 19 wt . % of the residual (non-decomposed) perovskite phase. Neither iron oxide nor iron oxide-hydroxide phases were detected by XRD analysis. The EDAX analysis indicated the presence of 5.9 weight % of iron. Example 2
Testing the activity of the catalyst of Example 1
A catalyst prepared as described in Example 1 was packed (3 cm3) in a tubular stainless steel reactor at a temperature of 80°C inside the catalyst layer. Water containing 200 ppmw phenol, H202 at concentration of 3% and sulfuric acid (pH = 4) were pumped through the reactor at an LHSV of 6 h" 1. The catalyst temperature was kept at 80°C. The water HPLC analysis at the reactor outlet did not detect phenol - its conversion was >99%. The removal of total organic carbon was 90%. The residual carbon was attributed to small amounts of oxalic and acetic acids produced by deep phenol oxidation. No visible deactivation was observed in a continuous run over 100 hours. The catalyst was stable against iron leaching - the iron content in the treated water was less than 0.05 ppmw.
Example 3
Preparation of a catalyst comprising silica particles having iron ions deposited thereon
The catalyst was prepared in two steps, according to the procedure set forth in Example 1. In the first step, nanocrystalline LaFe03 perovskite material was made as described in step a) of Example 1. In the second step, which is described in detail below, the resultant perovskite was used as Fe source for depositing iron onto a silica support by the selective extraction-deposition method of the invention, carried out in a flask, such that the LaFeC perovskite underwent complete decomposition. b) Selective extraction-deposition of Fe-precursor on a silica support:
Perovskite powder (280 mg) , synthesized as described in part (a) above, was mixed with commercial silica-gel (1440 mg) having surface area of 314 m2/g, a pore volume of 1.9 cm3/grams, an average pore diameter of 24 nm and a particle size of 30-100 pm; commercially available from PQ Co.
The activation was carried out in a flask ("batch mode"). The solid mixture was added to a glass vessel containing 50 cm3 of H2S04 solution at pH = 3.0 at a temperature of 80°C. The pH was kept at 3.0 through a continuous dropwise addition of 0.5M H2S04 solution for two hours. The catalyst was separated by filtration and dried in air at 120°C for 4 hours. This catalyst, was treated again with H2S04 as described above, and then the treatment was repeated for the third time. According to XRD analysis, the obtained catalyst contained neither a perovskite phase nor any iron oxide or oxide-hydroxide phases. The XRD patterns included only wide reflection at 2Θ = 23° corresponding to amorphous silica. The EDAX analysis detected in the catalyst 3.5 weight % Fe that existed in the form of highly dispersed iron ions. A characteristic Raman spectrum is shown in Figure 3, where the Raman signals at 510, .1090 and 1140 cm-1 are due to the isolated Fe(3+) ions adsorbed at the silica surface and the other peaks at 800 and 980 cm-1 are assigned to the silica phase.
Example 4
Testing the activity of the catalyst of Example 3
A catalyst prepared as described in Example 3 was packed (3 cm3) in a tubular stainless steel reactor at a temperature of 80°C inside the catalyst layer. Water containing 200 ppmw phenol, H2O2 at concentration of 3% and sulfuric acid at pH = 4 were pumped through the reactor at an LHSV of 6 h~ 1. The catalyst temperature was kept at 80 °C. The water HPLC analysis at the reactor outlet did not detect phenol - its conversion was >99%. The removal of total organic carbon was 60%. The residual carbon was attributed to oxalic and acetic acids produced by deep phenol oxidation. No visible deactivation was observed in a continuous run over 40 hours. The catalyst was reasonably stable against iron leaching - the iron content in the treated water was < 5 ppmw .
Example 5
Preparation of a mixture in the form of silica matrix and perovskite particles embedded therein
580 mg of perovskite powder prepared as described in step a) of Example 1 was immersed in 5.5 gram (4.55 mi) of colloidal silica solution (Ludox LS) containing 30.0 wt . % of Si02 in the form of colloidal 10 nm particles. The suspension was mixed at room temperature for 30 min and then water was slowly evaporated under continuous mixing at temperature of 60°C for a period of 1.5 hours. The dry solid was calcined in air at temperature 500°C for 2 hours. The fraction of particles with size ranging between 20 and 150 pm was collected by sieving the calcined material. The phase analysis showed that the obtained composition contained 22.5 wt . % perovskite phase and the rest was amorphous silica. The material had surface area of 223 m2/g and pore volume 0.23 cm3/g. The resultant composition, consisting of the perovskite embedded in a silica matrix, was subjected to in-situ activation as described in the following example.
Example 6
In-situ activation of the mixture of Example 5
The mixture (i.e., in a matrix form) prepared in Example 5 (3 cm3) was packed in a tubular stainless steel reactor at a temperature of 80 °C inside the catalyst layer. Water containing 200 ppmw phenol, H202 at concentration of 3% and sulfuric acid at pH = 4 were pumped through the reactor at an LHSV of 6 ~l for 10 hours, causing the deposition of iron ions on the surface of the silica support and the in situ formation of the catalyst of the imrention. The catalyst temperature was kept at 80°C. After this activation period a water stream containing 200 ppmw phenol, H202 at concentration of 3% and sulfuric acid at pH 4 was pumped continuously through the reactor at an LHSV of 6 h"1. HPLC analysis at the reactor outlet did not detect phenol - its conversion was >99%. The removal of total organic carbon was 95%. The residual carbon was attributed to small amounts of oxalic and acetic acids produced by deep phenol oxidation. No visible deactivation was observed in a continuous run over 50 hours. The catalyst was stable against iron leaching - the iron content in the treated water was less 0.05 ppmw.
Example 7 (comparative)
240 mg of LaFe03 perovskite prepared as described in step a) of Example 1 was mixed with 3 cm3 of a glass powder (fraction 20-150 μπ\) with surface area < 0.5 m /g that does not adsorb Fe (3+) ions . This mixture was packed in a tubular stainless steel reactor at a temperature of 80°C inside the catalyst layer. Water containing 200 ppmw phenol, H202 at concentration of 3% and sulfuric acid at pH = 4 were pumped through the reactor at an LHSV of 6 h"1. The catalyst temperature was kept at 80°C. The water HPLC analysis at the reactor outlet made after 10 h detected phenol, which conversion was 90%. The removal of total organic carbon at testing periods of 10, 15 and 20 hours was 32, 62 and 70%, respectively. However, after 25 hours of run the TOC conversion started to decrease gradually and dropped to 31% after 45 hours of run.

Claims

CLAIMS:
1) A process for the preparation of an iron-containing catalyst suitable for use in Fenton oxidation reactions, comprising preparing a mixture of a source of iron ions and solid support particles, said support particles having chemically reactive groups present on their surface, contacting said mixture with an acidic liquid medium thereby causing the release of iron ions from said iron source to form a catalyst comprising said solid support particles and said iron ions located on the surface of said support particles.
2) A process according to claim 1, wherein the iron source is a mixed oxide of iron and a second metal, said mixed metal oxide being decomposable in an acidic environment.
3) A process according to claim 2, wherein the mixed oxide of iron and a second metal is LaFe03 perovskite.
4) A process according to any one of the preceding claims, wherein the support particles comprise silica particles.
5) A process according to claim 4, wherein the mixture comprising a source of iron ions and silica support particles is prepared in the form of a powder blend.
6) A process according to claim 4, wherein the mixture comprising a source of iron ions and silica support particles is prepared by suspending said source of iron ions in a silica sol and solidifying said suspension, to form a silica matrix having said iron source embedded therein . 7) A process according to claim 2 or 3, wherein the iron ions are released through a partial decomposition of the mixed oxide of iron and a second metal, such that the catalyst formed contains a further solid phase consisting of said mixed metal oxide.
8) A process according to any one of the preceding claims, wherein the acidic liquid medium is an acidified wastewater stream, such that catalyst is formed in-situ in a site of an industrial wastewater treatment.
9) A process according to any one of the preceding claims, in which the catalyst obtained is characterized in that the iron ions are discretely located on the surface of the support particles, the catalyst being essentially free from iron oxide or oxide-hydroxide crystalline phases.
10) A catalyst comprising silica support particles having iron ions discretely located on the surface of said particles.
11) A catalyst according to claim 10, wherein the particles are essentially devoid of iron oxide or iron oxide- hydroxide crystalline phases.
12) A catalyst according to claims 10 or 11, which further comprises a crystalline perovskite phase.
13) A catalyst according to claim 12, which exhibits an X- ray powder, diffraction pattern having wide reflection at about 23° 2Θ indicative of amorphous silica, at least one peak assigned to the perovskite crystalline phase, wherein said X-ray powder diffraction pattern is devoid of peaks assignable to iron oxide or oxide-hydroxide crystalline phases, and wherein the catalyst is further characterized by UV-Raman spectrum having resonance bands at one or more of 510, 1090 and 1140 cm"1 (±5 cm"1) .
14) A catalyst according to claims 12 or 13, wherein the crystalline perovskite phase is LaFe03.
15) A catalyst according to claim 14, which comprises 20- 85% silica, 0.5-3.0% iron ions located on the surface of said silica and 10-75% LaFe03 perovskite (w/w) .
16) A catalyst according to claim 15, wherein the iron ions interact with hydroxyl groups (silanols) on the surface of the silica.
17) A method of removing organic contaminants from wastewater, comprising adding an oxidant to the wastewater and contacting said wastewater having acidic pH with the catalyst of any one of claims 10 to 16.
18) A method according to claim 17, wherein the wastewater stream flows through a continuous flow fixed bed reactor and passes through a layer of the catalyst packed in said reactor .
PCT/IL2011/000240 2010-03-12 2011-03-10 Process for the preparation of heterogeneous fenton catalytic filter WO2011111052A1 (en)

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