1 / 29

CBE 40445 Lecture 15 Introduction to Catalysis

CBE 40445 Lecture 15 Introduction to Catalysis. Developed by Prof. Schneider 1,2 Modified by Prof. Hicks 1 1 Department of Chemical and Biomolecular Engineering 2 Department of Chemistry and Biochemistry University of Notre Dame. Fall 2011. Importance of Catalysts.

arissa
Télécharger la présentation

CBE 40445 Lecture 15 Introduction to Catalysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CBE 40445Lecture 15Introduction to Catalysis Developed by Prof. Schneider1,2 Modified by Prof. Hicks1 1Department of Chemical and Biomolecular Engineering 2Department of Chemistry and Biochemistry University of Notre Dame Fall 2011 CBE 40445

  2. Importance of Catalysts Bartholomew and Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, 2006. CBE 40445

  3. What is a “Catalyst” • A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction. (thank you Wikipedia) k(T) = k0e-Ea/RT Ea′ < Ea k0′ > k0 k′ > k ΔG = ΔG Ea Ea′ A + B A + B + catalyst ΔG ΔG C C + catalyst uncatalyzed catalyzed CBE 40445

  4. Catalysts Open Up New Reaction Pathways ‡ H O C H2C O OH CH3 C C CH3 CH2 CH3 CH3 ‡ propenol propanone propenol propanone CBE 40445

  5. Catalysts Open Up New Reaction Pathways O− + H2O C CH2 CH3 OH− −OH− Base catalyzed O OH rate = k[OH−][acetone] C C CH3 CH2 CH3 CH3 propenol propanone ‡ ‡ propenol intermediate propanone CBE 40445

  6. Catalysts Open Up New Reaction Pathways ‡ ‡ propenol different intermediate propanone propenol O OH propanone rate = k[H3O+][acetone] C C Acid catalyzed CH3 CH2 CH3 CH3 H3O+ −H3O+ OH C + CH3 CH3 + H2O CBE 40445

  7. The “Gold Standard” of catalysts Highly specific Highly selective Highly efficient Catalyze very difficult reactions N2 NH3 CO2 + H2O  C6H12O6 Works better in a cell than in a 100000 l reactor Types of Catalysts - Enzymes Triosephosphateisomerase “TIM” Cytochrome C Oxidase Highly tailored “active sites” Often contain metal atoms CBE 40445

  8. Types of Catalysts – Organometallic Complexes • Perhaps closest man has come to mimicking nature’s success • 2005 Noble Prize in Chemistry • Well-defined, metal-based active sites • Selective, efficient manipulation of organic functional groups • Various forms, especially for polymerization catalysis • Difficult to generalize beyond organic transformations Polymerization: Termination: CBE 40445

  9. Types of Catalysts – Homogeneous vs. Heterogeneous Zeolite catalyst Catalyst powders Homogeneous catalysis Single phase (Typically liquid) Low temperature Separations are tricky Heterogeneous catalysis Multiphase (Mostly solid-liquid and solid-gas) High temperature Design and optimization tricky Newer area of Research: Tethered Catalysts (maintaining selectivity of homogeneous catalysts but tethered to a solid support) CBE 40445

  10. Types of Catalysts: Crystalline Microporous Catalysts • Regular crystalline structure • Porous on the scale of molecular dimensions • 3 – 20 Å (microporous), 20-500 Å (mesoporous) • Up to 1000’s m2/g surface area • Catalysis through • shape selection • acidity/basicity • incorporation of metal particles • Used as supports for other metal precursors Applied Catalysis A, 2009, 360, 59-65. 40 Å 10 Å MCM-41 (mesoporous silica) Zeolite (silica-aluminate) Silico-titanate CBE 40445

  11. Types of Catalysts: Zeolites Morphology changes due to additives, quantities, pH, time, etc. Shown below are SEM images of HZSM-5 (5.6 Å pores) • What are zeolites ? • Aluminosilicates • microporous ( pores < 20Å) • Crystalline • Framework of AlO4 and SiO4 Td-units (tetrahedral) • - Possess ordered pore systems • - Acidity arises from incorporation of Al Al2O3 source Neumann and Hicks, 2011. All silica ~ weak acidity SiO2/Al2O3 ~ Brønsted acidity

  12. Types of Catalysts: Zeolites Sodalite (SOD) pores ~3Å [SiO4 ]4- [AlO4]5- Zeolite - A (LTA) pores ~ 4Å -cages FAU LTA Zeolite - X, Y (FAU) pores ~ 7.4Å A large cage (~ 12Å) formed in A and X,Y

  13. Types of Catalysts: Amorphous Heterogeneous Catalysts • Amorphous, high surface area supports • Alumina, silica, activated carbon, … • Up to 100’s of m2/g of surface area • Impregnated with catalytic transition metals • Pt, Pd, Ni, Fe, Ru, Cu, Ru, … • Typically pelletized or on monoliths • Cheap, high stability, catalyze many types of reactions • Most used, least well understood of all classes SEM micrographs of alumina and Pt/alumina CBE 40445

  14. Traditional Heterogeneous (Insoluble) Easy to separate Multiple types of active sites Less mobility / spatially constricted Diffusion effects Homogeneous (Soluble) High mobility - active Single type of active site -selective Control of stereochemistry Difficult to separate Types of Catalysts: Motivation for Tethered Catalysts • Tethered • Insoluble • Single type of active site-selective • Easy to separate

  15. Types of Catalysts: Examples of Tethered Catalysts Hicks, J. C.; Jones, C. W., Langmuir 2006, 22, 2676. Hicks, J. C.; Dabestani, R.; Buchanan III, A. C.; Jones, C. W., Chem. Mater.2006, 18, 5022. R. A. Shiels, K. Venkatasubbaiah and C. W. Jones, Adv. Synth. Catal. (2008) 350, 2823-2834. J. C. Hicks, B. A. Mullis and C. W. Jones, J. Am. Chem. Soc. (2007) 129, 8426-8427. Hicks, J. C.; Dabestani, R.; Buchanan III, A. C.; Jones, C. W., Inorg. Chim. Acta, 2008. Collaboration between Hicks and Schneider Groups CBE 40445

  16. Important Heterogeneous Catalytic Processes • Haber-Bosch process • N2 + 3 H2→ 2 NH3 • Fe/Ru catalysts, high pressure and temperature • Critical for fertilizer and nitric acid production • Fischer-Tropsch chemistry • n CO + 2n H2→ (CH2)n + n H2O , syn gas to liquid fuels • Fe/Co catalysts • Source of fuel for Axis in WWII • Fluidized catalytic cracking • High MW petroleum → low MW fuels, like gasoline • Zeolite catalysts, high temperature combustor • In your fuel tank! • Automotive three-way catalysis • NOx/CO/HC → H2O/CO2/H2O • Pt/Rh/Pd supported on ceria/alumina • Makes exhaust 99% cleaner CBE 40445

  17. Design goals rapid and intimate contact between catalyst and reactants ease of separation of products from catalyst Heterogeneous Catalytic Reactors Packed Bed (single or multi-tube) Fluidized Bed Slurry Reactor Catalyst Recycle Reactor CBE 40445

  18. FCC: Fluidized Catalytic Cracker Gasoline Production Gas oil enters the riser reactor and is mixed with a zeolite catalyst (Zeolite Y). Acid-catalyzed cracking reactions occur in reactor. Coke formation occurs quickly on the catalyst (carbon deposition). Catalyst residence time is ~ 1.5 seconds. Catalyst is separated, regenerated, and re-injected. Bartholomew and Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, 2006. CBE 40445

  19. Automotive Emissions Control System “Three-way” Catalyst CO  CO2 HC  CO2 + H2O NOx  N2 Monolith reactor Most widely deployed heterogeneous catalyst in the world – you probably own one! Pt, Rh, Pd Alumina, ceria, zirconia, … CBE 40445

  20. Length Scales in Heterogeneous Catalysis Chemical adsorption and reaction Mass transport/diffusion CBE 40445

  21. Steps in a Heterogeneous Catalytic Reactor Diffusion Steps: 1, 2, 6, 7. Reaction Steps: 3, 4, 5. CBE 40445

  22. Characteristics of Heterogeneous Supported Catalysts • Surface area: • Amount of internal support surface accessible to a fluid • Measured by gas adsorption isotherms • Loading: • Mass of transition metal per mass of support • Dispersion: • Percent of metal atoms accessible to a fluid M M M support CBE 40445

  23. Rates of Catalytic Reactions • Pseudo-homogeneous reaction rate • r = moles / volume · time • Mass-based rate • r′ = moles / masscat· time • r′ = r / ρcat • Heterogeneous reactions happen at surfaces • Area-based rate • r′′ = moles / areacat· time • r′′ = r′ / SA, SA = area / mass • Heterogeneous reactions happen at active sites • Active site-based rate • Turn-over frequency TOF = moles / site · time • TOF = r′′ / ρsite TOF (s−1) Hetero. cats. ~101 Enzymes ~106 CBE 40445

  24. Adsorption and Reaction at Solid Surfaces • Physisorption: weak van der Waals attraction of a fluid (like N2 gas) for any surface • Eads ~10 – 40 kJ/mol • Low temperature phenomenon • Exploited in measuring gross surface area • Chemisorption: chemical bond formation between a fluid molecule (like CO or ethylene) and a surface site • Eads ~ 100 – 500 kJ/mol • Essential element of catalytic activity • Exploited in measuring catalytically active sites CBE 40445

  25. Comparing Physi- and Chemisorption on MgO(001) Calculated from first-principles DFT 1.25 O O 1.48 Physisorbed CO2 -2 kcal mol-1 GGA C CO2 : 2- :O:surf : 1.51 2.10 1.77 Chemisorbed SO2 (“sulfite”) -25 kcal mol-1 GGA Mg SO2 O O O : S : 2- :O:surf : 2.60 1.45 1.48 Chemisorbed SO3 (“sulfate”) -50 kcal mol-1 GGA SO3 1.66 2.12 O O O MgO(001) supercell S : 2- :O:surf Schneider, Li, and Hass, J. Phys. Chem. B2001, 105, 6972 : 2.58 CBE 40445

  26. Measuring Concentrations in Heterogeneous Reactions Kinetics • Fluid concentrations • Traditionally reported as pressures (torr, atm, bar) • Ideal gas assumption: Pj = CjRT • Surface concentrations • “Coverage” per unit area • nj = molesj / area • Maximum coverage called monolayer • 1 ML: nj,max = ~ 1015 molecules / cm2 • Fractional coverage • θj = nj / nj,max • 0 ≤ θj ≤ 1 Rate = f(Pj,θj) Metal particle surface θj = 1/5 CBE 40445

  27. Adsorption Isotherms • Molecules in gas and surface are in dynamic equilibrium A (g) + M (surface) ↔ M-A • Isotherm describes pressure dependence of equilibrium • Langmuir isotherm proposed by Irving Langmuir, GE, 1915 • (1932 Noble Prize) • Adsorption saturates at 1 monolayer • All sites are equivalent • Adsorption is independent of coverage Equilibrium rateads = ratedes Site conservation θA + θ* = 1 + CBE 40445

  28. Using the Langmuir Isotherm • Example: CO adsorption on 10% Ru/Al2O3 @ 100°C nCO,∞ = 2.89 μmol/gcat K = 0.0082 CBE 40445

  29. Brunauer-Emmett-Teller Isotherm (BET) • Relaxes Langmuir restriction to single layer adsorption • Monolayer adsorption; multilayer condensation • Useful for total surface area measurement • Adsorption of boiling N2 (78 K) ΔHads/ΔHcond ΔHcond ΔHads Solid Surface CBE 40445

More Related