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US 8,435,482B2 12 MIXED-METAL OXIDES PRECIPITATED WITH SUPERCRITICAL CO2 FIELD OF THE INVENTION This invention relates to mixed-metal oxides particularly, butnotexclusively,nanocrystallineoxides,moreparticularly, copper-manganese oxides, the precipitation of such oxides usingsupercriticalprecipitation,thepreparationofcatalysts using the oxides as precursors, the use of such catalysts in oxidationreactionsandtheoxidationproductsobtainedfrom such oxidation reactions. BACKGROUND TO THE INVENTION One of the most important catalysts in common usage today is hopcalite, a mixed oxide of copper and manganese that is used as the catalyst of choice for the oxidation of carbonmonoxideinbreathingapparatusinbothmilitaryand civil applications. Hopcalite catalysts are also used in other applications, for example removal of oZone from various off-gas emissions by conversion to oxygen and removal of volatileorganiccompoundsfrom,forexample,fuelcellfeed stocks. Itisgenerallyrecognizedthatactivehopcalitecatalystsare prepared by coprecipitation of a basic carbonate from a solu tionofthenitrates,andthatthebestcatalystsarederivedfrom amorphous precursors. HoWever, as With all catalysts that are incurrentcommercialusage,thereisaneedtoimprovethe formulationtoachieveimprovedactivitysothatthecatalyst lifetimecanbeextended,Whichisparticularlyimportantfor hopcalitesinceitisusedinbreathingapparatus,smokehoods andotherpersonalprotectionapplications. 2.contactingthesolutionWithasupercriticalantisolventto precipitatethemixed-metaloxideprecursor. A mixed-metaloxidemaythenbepreparedfromthepre cursorbyoxidation,forexamplebycalcination. Themethodmay furthercomprisethestepsof: 3. collecting the precipitated mixed-metal oxide and, optionaly, 4. activating the mixed-metal oxide for use as a catalyst. Preferably the mixture of metal precursor compounds comprises a mixture of salts of the metals With the same counter-ion,forexampleacetate,andthemixed-metaloxide is activated by heating in a gas mixture or by hydrothermal treatment.Themixed-metaloxideprecursorisgenerally,and unusually, a homogeneous mixture. Themethodofpreparingamixed-metaloxideaccordingto the invention provides an oxide Which brings the active com ponentstogetherWithoutrequiringthemtobepresentinthe same phase. Such an oxide can itselfbe used as a precursor to provideacatalystWithahighcatalyticactivity,thatis,about tWiceashighcomparedWithconventionallypreparedcata lysts. A supercritical?uidisacompoundatatemperatureand pressure above its thermodynamic critical point, Where the densitiesoftheliquidandgaseousphasesarethesame andthe compoundhastheabilitytodiffusethroughsolidsasthough itWas a gas and yet to dissolve materials as though itWas a liquid. Themethodofthepresentinventiontakesadvantageofthe poorsolubilityofcertainmetaloxideprecursorsinsupercriti cal?uidsinordertoprecipitatethosesamemetaloxidepre cursors from solution. The supercritical ?uids thereby act as an antisolvent. As describedherein,antisolventprecipitationcomprises precipitationfromasolventusingasupercriticalantisolvent. Thesupercriticalantisolventmaycompriseany?uidina supercriticalphasecapableofprecipitatingthemixed-metal oxide precursor from solution. In order to be capable of precipitatingthemixed-metaloxideprecursorfromsolution, the mixed-metal oxide precursor should be insoluble in the antisolvent,thesolventshouldbemiscibleWiththeantisol vent, and the antisolvent should be chemically inert toWards all other components of the reaction. At ambient conditions the antisolvent should not leave any residues in the precipi tate,becausethismay affectthestructureoftheprecursorand theresultingmixed-metaloxidefolloWingoxidation.Prefer ably,thesupercriticalantisolventcomprisesCO2,methane, ethane,propaneorn-butane;mostpreferablyCO2. The solvent preferably comprises a polar solvent such as DMF, DMSO oranalcohol,optionallyincludingWater. As usedherein,theterm“mixed-metaloxide”includesany compound (including covalent and ionic compounds) com prising at least tWo different metal atoms and at least one oxygen atom. This term includes compounds consisting exclusivelyofmetalandoxygenatoms,asWellascompounds additionally containing one or more atoms selected from hydrogen, sulphur, carbon and nitrogen. As used herein, the term “metal” includes alkali metals, alkaline earth metals, transition metals, lanthanides, actinides and Group I ele ments. Preferably, the mixed-metal oxide comprises iron, molybdenum,nickel,cobalt,copperandmanganese.More preferably, the mixed-metal oxide comprises copper and manganese. According to a preferred aspect of the present invention there is provided a method of preparing copper-manganese oxideprecursorcomprisingthesteps: a) preparing a solution of a copper and manganese precur sor in a solvent and 20 25 30 35 To date, there has been signi?cant interest in the use of supercriticalantisolventprecipitationforthepreparationof45 micro-andnano-particlesandthetechniquehasbeenusedto synthesiZe a range of materials including: polymers, explo sives,pharmaceuticalchemicals,superconductorsandsome catalysts.Withrespecttocatalyststhemethodhasbeenused to prepare a range of single oxides that can be used as either 50 the active phase or as a support material. HoWever, there have been no reports of the method being used to successfully prepare mixed-metal oxides for use as catalysts, as phase separation Was thought to be a problem. Mixed-metaloxidesmakeup,byfar,thegreatestnumber 55 ofcatalystsorcatalystprecursorscurrentlyusedtodayandit is, therefore, desirable to ?nd a method to prepare mixed metal oxide catalysts by supercritical antisolvent precipita tion. 60 DISCLOSURE OF THE INVENTION According to one aspect of the present invention there is provided a method of preparing a mixed-metal oxide precur sorcomprisingthesteps: 65 1. preparing a solution of a mixture of metal precursor compounds in a solvent and Inrecentyearstherehasbeenextensiveinterestintheuse of supercritical ?uids as antisolvents for precipitation. The particularadvantageofthistechniqueisthatitpermitsmuch higher diffusivities, up to tWo orders of magnitude greater, When compared With non-supercritical solvents. This fast diffusioncanproducesupersaturationimmediatelypriorto40 precipitation and thus leads to the formation of very small particles not accessible by standard catalyst preparation methods.PDF Image | MIXED METAL OXIDES PRECIPITATED WITH SUPERCRITICAL CO2
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