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

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 CH 4 + O 2  CH 2 O + H 2 O  H = -76.8 kcal/mol  G = -70.9 kcal/mol Current Technology CH 4 + H 2 O  CO + 3H 2 CO + 2H 2  CH 3 OH CH 3 OH + 0.5 O 2  CH 2 O + H 2 O Poor efficiency high energy and material cost

4. Stoichiometric Analysis-1 (1) List all possible stoichiometric chemical equations (2) Calculate the  H and  G of reaction (3) Chemical bond changes Primary Reactants CH 4 O 2 Reactant Self-interactions 2CH 4  C 2 H 6 + H 2 DH  G = 8.5 kcal/mol 2CH 4  C 2 H 4 + 2H 2 DH  G = 12.8 kcal/mol 2CH 4  C 2 H 2 + 3H 2 DH  G = 22.2 kcal/mol O 2 = none

5. Stoichiometric Analysis-2 Reactant Cross-interactions CH 4 + 0.5 O 2  CH 3 OHOI  G = -20.6 kcal/mol  CH 2 O + H 2 OI, DH  G = -20.0 kcal/mol  CO + 2H 2 OI, DH  G = -43.1 kcal/mol CH 4 + O 2  CH 2 O + H 2 OOI, DH, O  G = -70.9 kcal/mol  HCOOH + H 2 OI, DH, O  G = -67.0 kcal/mol  CO + H 2 + H 2 O OI, DH, O  G = -87.3 kcal/mol  CO 2 + 2H 2 OI, DH, O  G = -90.5 kcal/mol CH 4 + 1.5O 2  CH 2 O + H 2 O 2 OI, DH, O  G = -31.0 kcal/mol  HCOOH + H 2 OOI, DH, O  G = -119.8 kcal/mol  CO + 2H 2 O OI, DH, O  G = -136.5 kcal/mol  CO 2 + H 2 + H 2 O OI, DH, O  G = -139.8 kcal/mol

6. Stoichiometric Analysis-3 Reactant Cross-interactions CH 4 + 2O 2  HCOOH + H 2 O 2 OI, DH, O  G = -98.6 kcal/mol  CO + H 2 O 2 + H 2 O OI, DH, O  G = -118.7 kcal/mol  CO 2 + 2H 2 O OI, DH, O  G = -189.5 kcal/mol Reactant-Product interactions CH 4 + C 2 H 6  C 3 H 8 + H 2 DH, A  G = 16.6 kcal/mol CH 4 + C 2 H 4  C 3 H 8 A  G = 4.5 kcal/mol CH 4 + CH 3 OH  C 2 H 5 OH + H 2 DH, A  G = 10.5 kcal/mol etc.

7. Thermodynamic Analysis (1) Assess thermodynamic feasibility (rank by  G) (2) Rank and group reactions with common trend CH 4 + 2O 2  CO 2 + 2H 2 O OI, DH, O  G = -189.5 kcal/mol CH 4 + O 2  CH 2 O + H 2 OOI, DH, O  G = -70.9 kcal/mol CH 4 + O 2  HCOOH + H 2 OI, DH  G = -67.0 kcal/mol CH 4 + 0.5 O 2  CH 2 O + H 2 OI, DH  G = -20.0 kcal/mol CH 4 + 0.5 O 2  CH 3 OHOI  G = -20.6 kcal/mol CH 2 O  CO + H 2 DH  G = -17.0 kcal/mol CH 3 OH  CH 2 O + H 2 DH  G = 2.0 kcal/mol

8. Reaction Mechanism (1) Visualize molecular events leading to formation of desired product(s) (2) Eliminate non-plausible pathways CH 4 + 0.5 O 2  CH 2 O + H 2 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 CH 4  CH 3 -S  CH 2 -SO 2  2O-S CH 3 OH CH 2 O, H 2

9. Reaction Pathways (1) Based on the analysis of surface mechanism establish the desired reaction pathways for the reaction CH 4 + 0.5 O 2  CH 2 O + H 2 OI, DH  G = -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 CH 4 H CH 3 O2O2 OO O H CH 2 O

10. 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 CH 3 (3) Adjacent sites to facilitate final dehydrogenation CH 4 H CH 3 O2O2 OO O H CH 2 O

11. Catalyst Selection (1) Based on knowledge of catalyst materials (1) Mild dehydrogenating catalysts Usually oxide catalysts, metals are strong DH catalyst Cu 2+, Ni 2+, Fe 3+, Mn 2+, V 3+, V 5+, Ti 4+ (2) Mild oxidation catalysts Sc 3+, Ti 4+, V 3+, Cr 3+, Fe 2+, Zn 2+, Zr 3+, Nb 3+, Mo 6+ (3) Low mobility Co 3 O 4 > MnO 2 > NiO > CuO > Fe 2 O 3 > Cr 2 O 3 > V 2 O 5 > MoO 3 (4) Hard to reduce CoAl 2 O 4, NiAl 2 O 4, ZnTiO 4 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 Fe 3+ V 3+ V 5+ Ti 4+ Mild OI Sc 3+ V 3+ Ti 4+ Fe 2+ Zn 2+ Zr 3+ Nb 3+ Mo 6+ Possible Catalysts Single TiO 2, V 2 O 3 Mixed TiO 2 + MoO 3 V 2 O 3 + ZnO Complex Fe 2 O 3 Zn TiO 3

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 1.Synthesis pH 2.Temperature 3.Reaction time 4.Reagent concentration 5.Nature and amount of catalyst 6.H 2 O/M ratio 7.Aging temperature and time 8.Drying conditions

23. Sol-gel Chemistry Hydrolysis 1.Synthesis pH Nucleophilic attack

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

25. Hydrolysis 2. Nature and Amount of Catalyst Base Catalysts Strong Bases: Mineral Bases (NH 3 ) Weak Bases: Organic Bases (Amines) Rate of Hydrolysis  [Acid] 1 or 2 Sol-gel Chemistry

26. Hydrolysis 3. H 2 O/Si Ratio Acid Catalysts Rate of Hydrolysis  [Water] 1 Basic Catalysts Rate of Hydrolysis  [Water] 0 Sol-gel Chemistry

27. Condensation 1.Synthesis pH

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

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. Frame Pyrolysis (Fumed Silica) (a) 380 m 2 g -1 (b) 300 (c) 200 (d) 90

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 Supported phase-support interaction (transition layer attachment) Monolayer formation Bilayer formation

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

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

39. Precipitation Method