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Catalyst Design and Preparation Dr. King Lun Yeung Department of Chemical Engineering

CENG 511 Lecture 3. Catalyst Design and Preparation Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology. Design of Catalyst. (1) Stoichiometric analysis of target reaction (2) Thermodynamic analysis (3) Molecular mechanism

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Catalyst Design and Preparation Dr. King Lun Yeung Department of Chemical Engineering

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  1. CENG 511 Lecture 3 Catalyst Design and Preparation Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology

  2. Design of Catalyst (1) Stoichiometric analysis of target reaction (2) Thermodynamic analysis (3) Molecular mechanism (4) Surface mechanism (5) Reaction pathway (6) Catalyst properties (7) Catalytic materials (8) Propose a catalyst

  3. Case Study Methane Partial Oxidation to Formaldehyde CH4 + O2 CH2O + H2O DH = -76.8 kcal/mol DG = -70.9 kcal/mol Current Technology CH4 + H2O  CO + 3H2 CO + 2H2  CH3OH CH3OH + 0.5 O2 CH2O + H2O Poor efficiency high energy and material cost

  4. Stoichiometric Analysis-1 (1) List all possible stoichiometric chemical equations (2) Calculate the DH and DG of reaction (3) Chemical bond changes Primary Reactants CH4 O2 Reactant Self-interactions 2CH4  C2H6 + H2DH DG = 8.5 kcal/mol 2CH4  C2H4 + 2H2DH DG = 12.8 kcal/mol 2CH4  C2H2 + 3H2DH DG = 22.2 kcal/mol O2 = none

  5. Stoichiometric Analysis-2 Reactant Cross-interactions CH4 + 0.5 O2  CH3OH OI DG = -20.6 kcal/mol  CH2O + H2OI, DH DG = -20.0 kcal/mol  CO + 2H2OI, DH DG = -43.1 kcal/mol CH4 + O2  CH2O + H2O OI, DH, O DG = -70.9 kcal/mol  HCOOH + H2OI, DH, O DG = -67.0 kcal/mol  CO + H2 + H2O OI, DH, O DG = -87.3 kcal/mol  CO2 + 2H2OI, DH, O DG = -90.5 kcal/mol CH4 + 1.5O2  CH2O + H2O2OI, DH, O DG = -31.0 kcal/mol  HCOOH + H2O OI, DH, O DG = -119.8 kcal/mol  CO + 2H2O OI, DH, O DG = -136.5 kcal/mol  CO2 + H2 + H2O OI, DH, O DG = -139.8 kcal/mol

  6. Stoichiometric Analysis-3 Reactant Cross-interactions CH4 + 2O2  HCOOH + H2O2OI, DH, O DG = -98.6 kcal/mol  CO + H2O2 + H2O OI, DH, O DG = -118.7 kcal/mol  CO2 + 2H2O OI, DH, O DG = -189.5 kcal/mol Reactant-Product interactions CH4 + C2H6  C3H8 + H2DH, A DG = 16.6 kcal/mol CH4 + C2H4  C3H8A DG = 4.5 kcal/mol CH4 + CH3OH  C2H5OH + H2DH, A DG = 10.5 kcal/mol etc.

  7. Thermodynamic Analysis (1) Assess thermodynamic feasibility (rank by DG) (2) Rank and group reactions with common trend CH4 + 2O2  CO2 + 2H2O OI, DH, O DG = -189.5 kcal/mol CH4 + O2  CH2O + H2O OI, DH, O DG = -70.9 kcal/mol CH4 + O2  HCOOH + H2OI, DH DG = -67.0 kcal/mol CH4 + 0.5 O2  CH2O + H2OI, DH DG = -20.0 kcal/mol CH4 + 0.5 O2  CH3OH OI DG = -20.6 kcal/mol CH2O  CO + H2 DH DG = -17.0 kcal/mol CH3OH  CH2O + H2DH DG = 2.0 kcal/mol

  8. Reaction Mechanism (1) Visualize molecular events leading to formation of desired product(s) (2) Eliminate non-plausible pathways CH4 + 0.5 O2  CH2O + H2 Surface Mechanism (1) Guess the most plausible surface mechanism that lead to the desired product(s) (2) Research known adsorption, molecular configurations of reactants and products CH4  CH3-S  CH2-S O2  2O-S CH3OH CH2O, H2

  9. CH3 CH3 H O O O H Reaction Pathways (1) Based on the analysis of surface mechanism establish the desired reaction pathways for the reaction CH4 + 0.5 O2  CH2O + H2OI, DH DG = -20.0 kcal/mol (1) Must promote oxygen insertion (OI) (2) Must be a mild dehydrogenation (DH) (3) Must prevent strong dehydrogenation (4) Must prevent oxidation O2 CH4 CH2O

  10. CH3 CH3 H O O O H Catalyst Properties (1) Identify the desired catalyst properties based on surface mechanism/reaction pathway (1) Oxygen adsorption site leading to dissociated and immobile oxygen species (2) Mild dehydrogenation to produce CH3 (3) Adjacent sites to facilitate final dehydrogenation O2 CH4 CH2O

  11. Catalyst Selection (1) Based on knowledge of catalyst materials (1) Mild dehydrogenating catalysts Usually oxide catalysts, metals are strong DH catalyst Cu2+, Ni2+, Fe3+, Mn2+, V3+, V5+, Ti4+ (2) Mild oxidation catalysts Sc3+, Ti4+, V3+, Cr3+, Fe2+, Zn2+, Zr3+, Nb3+, Mo6+ (3) Low mobility Co3O4 > MnO2 > NiO > CuO > Fe2O3 > Cr2O3 > V2O5 > MoO3 (4) Hard to reduce CoAl2O4, NiAl2O4, ZnTiO4 Bond G.C. Catalysis by Metals, Academic Press (1962) Krylov O.V. Catalysis by Non-metals, Academic Press (1970)

  12. Propose a Catalyst Mild DH Fe3+ V3+ V5+ Ti4+ Mild OI Sc3+ V3+ Ti4+ Fe2+ Zn2+ Zr3+ Nb3+ Mo6+ Possible Catalysts Single TiO2, V2O3 Mixed TiO2 + MoO3 V2O3 + ZnO Complex Fe2O3 Zn TiO3

  13. Catalyst Preparation (1) Unsupported Catalyst are typically usually very active catalyst that do not require high surface area e.g., Iron catalyst for ammonia production are usually used for high temperature applications e.g., refractory aluminates for catalytic combustion intrinsically have a large surface area e.g., gamma alumina catalyst for isomerization clay catalyst for hydrogenation (2) Supported Catalyst requires a high surface area support to disperse the primary catalyst, the support may also act as a co-catalyst or secondary catalyst for the reaction

  14. Unsupported Catalyst Typical preparation methods

  15. Unsupported Catalyst Required preparation steps

  16. Unsupported Catalyst Typical preparation methods (1) Fusion Method

  17. Unsupported Catalyst Typical preparation methods (2) Precipitation and Co-precipitation Methods

  18. Unsupported Catalyst (2) Precipitation and Co-precipitation Methods

  19. Unsupported Catalyst (2) Precipitation and Co-precipitation Methods Preparation of aluminum oxide

  20. Unsupported Catalyst Typical preparation methods (3) Sol-gel synthesis

  21. Unsupported Catalyst Typical preparation methods (3) Sol-gel synthesis Silica-alumina acid catalyst

  22. Unsupported Catalyst • Sol-gel Chemistry • Synthesis pH • Temperature • Reaction time • Reagent concentration • Nature and amount of catalyst • H2O/M ratio • Aging temperature and time • Drying conditions

  23. Sol-gel Chemistry • Hydrolysis • Synthesis pH Nucleophilic attack

  24. Sol-gel Chemistry Hydrolysis 2. Nature and Amount of Catalyst Acid Catalysts Strong Acids: Mineral Acids (HCl) Weak Acids: Organic Acids (Acetic Acid) Rate of Hydrolysis  [Acid]

  25. Sol-gel Chemistry Hydrolysis 2. Nature and Amount of Catalyst Base Catalysts Strong Bases: Mineral Bases (NH3) Weak Bases: Organic Bases (Amines) Rate of Hydrolysis  [Acid]1or 2

  26. Sol-gel Chemistry Hydrolysis 3. H2O/Si Ratio Acid Catalysts Rate of Hydrolysis  [Water]1 Basic Catalysts Rate of Hydrolysis  [Water]0

  27. Sol-gel Chemistry • Condensation • Synthesis pH

  28. Sol-gel Chemistry Condensation 2. Nature and Amount of Catalyst

  29. Unsupported Catalyst Typical preparation methods (4) Frame Pyrolysis Fumed silica (a) vaporizer (b) mixing chamber (c) burner (d) cooling section (e) separation (f) deacidification (g) hopper (h) compactor

  30. (a) 380 m2g-1 (b) 300 (c) 200 (d) 90 Frame Pyrolysis (Fumed Silica)

  31. Supported Catalyst Maintains large catalyst surface area and prevents sintering during high temperature operation

  32. Supported Catalyst Typical support materials

  33. Support Materials

  34. Metal Ion Distribution in Support Pellet

  35. Supported Catalyst Weak Interaction Interaction

  36. Catalyst-Support Interactions Monolayer formation Supported phase-support interaction (transition layer attachment) Bilayer formation

  37. Catalyst-Support Interactions Formation of new compounds Formation of solid solution Grafted catalyst

  38. Supported Catalyst Typical preparation methods (1) Precipitation method

  39. Precipitation Method

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