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

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

Case Study Methane Partial Oxidation to Formaldehyde CH 4 + O 2  CH 2 O + H 2 O  H = kcal/mol  G = kcal/mol Current Technology CH 4 + H 2 O  CO + 3H 2 CO + 2H 2  CH 3 OH CH 3 OH O 2  CH 2 O + H 2 O Poor efficiency high energy and material cost

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

Stoichiometric Analysis-2 Reactant Cross-interactions CH O 2  CH 3 OHOI  G = kcal/mol  CH 2 O + H 2 OI, DH  G = kcal/mol  CO + 2H 2 OI, DH  G = kcal/mol CH 4 + O 2  CH 2 O + H 2 OOI, DH, O  G = kcal/mol  HCOOH + H 2 OI, DH, O  G = kcal/mol  CO + H 2 + H 2 O OI, DH, O  G = kcal/mol  CO 2 + 2H 2 OI, DH, O  G = kcal/mol CH O 2  CH 2 O + H 2 O 2 OI, DH, O  G = kcal/mol  HCOOH + H 2 OOI, DH, O  G = kcal/mol  CO + 2H 2 O OI, DH, O  G = kcal/mol  CO 2 + H 2 + H 2 O OI, DH, O  G = kcal/mol

Stoichiometric Analysis-3 Reactant Cross-interactions CH 4 + 2O 2  HCOOH + H 2 O 2 OI, DH, O  G = kcal/mol  CO + H 2 O 2 + H 2 O OI, DH, O  G = kcal/mol  CO 2 + 2H 2 O OI, DH, O  G = 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.

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 = kcal/mol CH 4 + O 2  CH 2 O + H 2 OOI, DH, O  G = kcal/mol CH 4 + O 2  HCOOH + H 2 OI, DH  G = kcal/mol CH O 2  CH 2 O + H 2 OI, DH  G = kcal/mol CH O 2  CH 3 OHOI  G = kcal/mol CH 2 O  CO + H 2 DH  G = kcal/mol CH 3 OH  CH 2 O + H 2 DH  G = 2.0 kcal/mol

Reaction Mechanism (1) Visualize molecular events leading to formation of desired product(s) (2) Eliminate non-plausible pathways CH 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

Reaction Pathways (1) Based on the analysis of surface mechanism establish the desired reaction pathways for the reaction CH O 2  CH 2 O + H 2 OI, DH  G = 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

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

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)

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

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

Unsupported Catalyst Typical preparation methods

Unsupported Catalyst Required preparation steps

Unsupported Catalyst Typical preparation methods (1) Fusion Method

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

Unsupported Catalyst (2) Precipitation and Co-precipitation Methods

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

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

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

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

Sol-gel Chemistry Hydrolysis 1.Synthesis pH Nucleophilic attack

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

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

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

Condensation 1.Synthesis pH

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

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

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

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

Supported Catalyst Typical support materials

Support Materials

Metal Ion Distribution in Support Pellet

Supported Catalyst Weak Interaction Interaction

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

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

Supported Catalyst Typical preparation methods (1) Precipitation method

Precipitation Method