Sol-gel methods for the preparation of catalysts

Sol-gel methods for the preparation of catalysts8th April 2013Daniel Casas Orozco

Preparation of CatalystsEnvironmental Catalysis GroupEngineering FacultyUniversidad de AntioquiaOutlineObjectivesIntroductionPreparation3.1 Fundamentals of Sol-gel process3.2 Drying of wet gels3.3 Stabilization and post-treatmentApplications4.1 Bulk catalytic materials4.2 Multimetallic composite catalytic materials

Sol-gel methodsfor the preparation of catalysts2OBJECTIVESShow the fundamentals of the sol-gel methodsKnow the characteristics of the different preparation methodsReview some applications of sol-gel processingSol-gel methodsfor the preparation of catalysts3INTRODUCTION1*1970sSynthesizing of metal-containing gels: disolving an adequate precursor of the metal in an alcoholic solution containing precursors of the support

Supercritical removal of solvent to mantain porosity of the wet gel (collapse of of the structure)Sol-gel methodsfor the preparation of catalysts4Regalbuto, J. (2007). Catalyst Preparation: Science and Engineering* Reference along all the presentation. Other sources are cited as well in the upcoming slidesSolventTc (K)Pc (MPa)Water64722,1Methanol5138,1Ethanol5146,11-propanol5375,2n-butanol5634,4Sol-gel methodsfor the preparation of catalysts5Ni/Al2O3, Ni/Al2O3-SiO2, Cu/Al2O3

Aerogels (supercritical drying)

Pd/Al2O3, Ru/SiO2, Pt/SiO2 , Pd/SiO2, Pt/Al2O3

Xerogels (evaporative drying)

Sol-gel methodsfor the preparation of catalysts61980s and 1990s

Functionalized ligands anchored to silica (nanometer-size distributed metal phase)

Aerogel-like morphology

Mono/bimetallic catalysts (cogelled catalysts)High activities and selectivitiesExcellent mass transferReasonable stabilityEasy regenerability

Sol-gel methodsfor the preparation of catalysts72000s

Metal-containing carbon xerogels and aerogels

Drying and pyrolisis oforganic gels obtained from the polycondensation of resorcinol and formaldehyde in aqueous mediaSol-gel methodsfor the preparation of catalysts8PREPARATIONSol: colloidal suspension of solid particles in a liquid

Colloid: suspension with very small dispersed phase (1nm 1 m)Sol-gel methodsfor the preparation of catalysts9

Fundamentals of sol-gel processIn a colloid, gravitational forces are negligible

Short-range effects: Van der Waals and electrostatic forcesSol-gel methodsfor the preparation of catalysts10Gel: continuous solid and liquid phases of colloidal dimensionsSol-gel methodsfor the preparation of catalysts11

Sol-gel process: a gel is formed from the particles of a sol. Attractive forces cause

Particle sticking

Network formingSol-gel methodsfor the preparation of catalysts12General sequence2:

Sol-gel methodsfor the preparation of catalysts13

2. De Jong, K. (2009). Synthesis of Solid Catalysts Sol-gel methodsfor the preparation of catalysts14

33. Simpson, R., Lee, R., Tillotson, T., Hrubesh, L., Swansinger, R., & Fox, G. (2003). Sol-gel methodsfor the preparation of catalysts15M: Si Zr, Al, TiR: -CH3, -C2H5x: Al (3), Ti, Si (4)Sol-gel methodsfor the preparation of catalysts16Sol-gel methodsfor the preparation of catalysts17The reaction proceeds linking increasing number of molecules Inorganic polymer lattice4Sol-gel methodsfor the preparation of catalysts18

4. Brinker, J. (1990). Sol-Gel Science: The physics and chemistry of sol gel processing Sol-gel methodsfor the preparation of catalysts19

4. Brinker, J. (1990). Sol-Gel Science: The physics and chemistry of sol gel processing 4Gel formation mechanisms

Aggregation: formation of particles by polycondensation followed by their aggregation

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Phase separation: separation of two immisible and interconected liquid phases: spinodal phase separation: oligomer-rich (solvent rich) and polymer rich (gel skeleton)

For silicon alkoxides, reactions (nucleophylic atack on silicon atom) occur more readily when in acidic media (HCl, CH3COOH, HF) or basic media (NH3, KOH, amines)Different structures depending on reaction conditionsBetter than precipitation method for mixed oxides preparation: Al2O3-SiO2, SiO2-ZrO2, SiO2-TiO2 Higher composite acidity than individual metal oxidesSol-gel methodsfor the preparation of catalysts21Organic gels: Less used than inorganic onesUseful to synthesize three dimensional carbon structures in a solvent. Precursors of carbon gels by pyrolysis.Used materialsHydroxylated benzene compounds: cathecol, resorcinol, phenolAldehydes: formaldehyde, furfuralSol-gel methodsfor the preparation of catalysts22Resorcinol-formaldehyde polymers are the most known of their kind: water as a solvent and basic media provided by Na2CO3.Sol-gel methodsfor the preparation of catalysts23

Depending on pH interval, micro-macroporous, micro-mesoporous, only microporous or totally non-porous materials can be obtained

Same gel formation mechanisms: in this case, spinodal separation is more acepted.Sol-gel methodsfor the preparation of catalysts24Sol-gel methodsfor the preparation of catalysts25

2. De Jong, K. (2009). Synthesis of Solid Catalysts 2Sol-gel methodsfor the preparation of catalysts26

2. De Jong, K. (2009). Synthesis of Solid Catalysts 2Sol-gel methodsfor the preparation of catalysts27Applied with silica gels

Resorcinol formaldehdyde gels (acetone water)

Drying of wet gels

Supercritical drying (aerogels): avoids structure-damaging capillary forces. Autoclave the material, nitrogen atmosphere, hermetically closing and heating slightly above Tc. Give rise to extremely porous materials.Due to costs, supercritical drying in CO2 has been developed Solvent exchange by liquid CO2For aqueous resorcinol-formaldehyde Prior exchange of water with acetoneSol-gel methodsfor the preparation of catalysts28Drying of wet gels

Freeze drying (cryogels): pore liquids is frozen that subsequently sublimes. Applied for both inorganic and organic gelsSol-gel methodsfor the preparation of catalysts29Stabilization and post-treatment

Calcination of inorganic gelsElimination of pore residuals (alcohol,ligandsConvert organic precursors of the active metal into metal oxidesBurn off alkoxysilane-functionalized complexes in cogelled materialsT = 400 500 C, flowing air or O2.Care must be taken to avoid sinteringSol-gel methodsfor the preparation of catalysts30Stabilization and post-treatment

Pyrolysis of organic gelsConvert resorcinol-formaldehyde xerogels and aerogels into carbon xerogels or aerogelsT = 800 1000 C, inert atmosphere (N2)Steps:Water releaseFormation of carbon entitiesRearrangement of entities into graphitic semigraphitic structuresMetals can act as catalysts for those rearrangement reactionsSol-gel methodsfor the preparation of catalysts31Sol-gel methodsfor the preparation of catalysts32Stabilization and post-treatment

ReductionUsed when the metallic state is required for the catalyst to be activeH2 at T = 350 400 CFor bimetallic composites, allows the alloy formationNot always through H2 atmosphere: in aerogel environment, alcohol reducing properties do the workAPPLICATIONSBulk catalytic phase materials2

Sol-gel method used to prepare metal-oxide phases with controlled size, shape, packing mode, etc.Careful choice of molecular precursor and sol-gel chemical route: activation/condensation kinetics, drying and crystallization conditionsProduction of TiO2, ZrO2, Nb2O5, CrOx with surface area > 200 m/gSol-gel methodsfor the preparation of catalysts332. De Jong, K. (2009). Synthesis of Solid CatalystsMultimetallic composite catalytic materials

Traditional supported metal catalysts preparation:

Support preparationDistribution of active phase precursor over supportDrying, calcination or pyrolysis (activation)Sol-gel methodsfor the preparation of catalysts34Unified in sol-gel methodOne step method

Metal can be introduced during formation of the supportOften, more dispersion than impregnation-prepared materialsStronger anchoring of metal species in the porosity of the supportHigher temperature stabilityAttractive approach to multi-metal material on single or mixed oxides supports

Sol-gel methodsfor the preparation of catalysts35One step method

Dissolution of metal salts in precursory sol-gel solution

Dissolution of active metal precursor in homogeneous solution of support precursor.Metal precursor does not participate directly in sol-gel chemistryExamples: Pd/SiO2 xerogels, Pd/Al2O3 aerogels, Co/Al2O3 xerogels, Co-Mo/SiO2 xerogels

Sol-gel methodsfor the preparation of catalysts36Cogelation of metal chelates with support precursors

Used since 1980sNanometer-sized uniform metal particles in silicaThe active metal is anchored through a complex to the supportReactants:Metal precursor: acetates as Pd(OAc)2, Cu(Oac)2, Ag(Oac)Alkoxysilane-functionalized metal complex:

Sol-gel methodsfor the preparation of catalysts37Sol-gel methodsfor the preparation of catalysts38

Two steps methodSometimes used for enhance materials respect to their characteristics in one-step preparation methodsIn some cases, active phase remains entrapped or occluded in the bulk supportPd-Ag/SiO2 aerogelRu/SiO2 xerogelsNi/C aerogelsStepsPreparation of the xerogel or aerogel suppportSupporting of active metalMethods like impregnation or ion exchange are used for metal supportingSol-gel methodsfor the preparation of catalysts39Sol-gel methodsfor the preparation of catalysts40

Rojas-Cervantes et al. (2004)5

Support: polymerization of resorcinol with formaldehyde and calcination of xerogels by nitrogen at 1000 C.Metal precursor

Na and K carbonates, Mg nitrateZr propoxide copolymerizationMethod:

(Resorcinol metal precursor aqueous solution) + aqueous formaldehyde. 1 h geling, 85 C for seven days in oven(Resorcinol formaldehyde aqueous solution) + Zn propoxide. 30 min geling, 85 C for seven days in ovenResulting gels dried at 110 C for 10 h in N2. Xerogels obtainedCarbonised derivatives by pyrolysis of xerogels at 1000 C in N2.Sol-gel methodsfor the preparation of catalysts415. Rojas-Cervantes et al. (2004). Basic metalcarbons catalysts prepared by solgel methodSol-gel methodsfor the preparation of catalysts42

Sol-gel methodsfor the preparation of catalysts43

Condensation of benzaldehyde with ethyl cyanoacetate (Knoevanagel reaction)Sol-gel methodsfor the preparation of catalysts44

Hajek et al. (2002)6Active phase: Ru-SnRuCl3.xH2OSnCl2Support: SiO2TetraethoxysylaneMethodRu and Sn precursors in 1,2 ethanodiolSi precursor added dropwiseWater additionGeling at 343 KAged gels dried at decreasing pressureReduction with NaBH4 solutionDrying for 2h at 473K (N2 atmosphere)Sol-gel methodsfor the preparation of catalysts45Hajek et al. (2002). Preparation and properties influence of bimetallic Ru-Sn sol-gel catalysts: influence of catalyst reductionSol-gel methodsfor the preparation of catalysts46

Sol-gel methodsfor the preparation of catalysts47Hydrogenation of 3-pheynyl-2-propenal

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REFERENCESRegalbuto, J. (2007). Catalyst Preparation: Science and Engineering (p. 474). Boca Ratn, FL: CRC Press.De Jong, K. (2009). Synthesis of Solid Catalysts (p. 401). Weinheim: Wiley-VCH.Simpson, R., Lee, R., Tillotson, T., Hrubesh, L., Swansinger, R., & Fox, G. (2003). Sol-gel manufactured energetic materials.Brinker, J. (1990). Sol-Gel Science: The physics and chemistry of sol gel processing (p. 908). San Diego, CA: Academic Press.Rojas-Cervantes, M. ., Alonso, L., Daz-Tern, J., Lpez-Peinado, a. ., Martn-Aranda, R. ., & Gmez-Serrano, V. (2004). Basic metalcarbons catalysts prepared by solgel method. Carbon, 42(8-9), 15751582.Hajek, J., Kumar, N., Karhu, H., Cerveny, L., Vayrynen, J., Salmi, T. I., & Yu, D. (2002). Preparation and properties influence of bimetallic Ru-Sn sol-gel catalysts: influence of catalyst reduction. Studies in Surface Science and Catalyisis, 143, 757765.

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