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
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
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 + H2
2CH4  C2H4 + 2H2
2CH4  C2H2 + 3H2
O2 = none
DH
DH
DH
DG = 8.5 kcal/mol
DG = 12.8 kcal/mol
DG = 22.2 kcal/mol
Stoichiometric Analysis-2
Reactant Cross-interactions
CH4 + 0.5 O2  CH3OH
 CH2O + H2
 CO + 2H2
CH4 + O2
 CH2O + H2O
 HCOOH + H2
 CO + H2 + H2O
 CO2 + 2H2
CH4 + 1.5O2  CH2O + H2O2
 HCOOH + H2O
 CO + 2H2O
 CO2 + H2 + H2O
OI
OI, DH
OI, DH
DG = -20.6 kcal/mol
DG = -20.0 kcal/mol
DG = -43.1 kcal/mol
OI, DH, O
OI, DH, O
OI, DH, O
OI, DH, O
DG = -70.9 kcal/mol
DG = -67.0 kcal/mol
DG = -87.3 kcal/mol
DG = -90.5 kcal/mol
OI, DH, O
OI, DH, O
OI, DH, O
OI, DH, O
DG = -31.0 kcal/mol
DG = -119.8 kcal/mol
DG = -136.5 kcal/mol
DG = -139.8 kcal/mol
Stoichiometric Analysis-3
Reactant Cross-interactions
CH4 + 2O2  HCOOH + H2O2
OI, DH, O
 CO + H2O2 + H2O OI, DH, O
 CO2 + 2H2O
OI, DH, O
DG = -98.6 kcal/mol
DG = -118.7 kcal/mol
DG = -189.5 kcal/mol
Reactant-Product interactions
CH4 + C2H6  C3H8 + H2
CH4 + C2H4  C3H8
CH4 + CH3OH  C2H5OH + H2
DG = 16.6 kcal/mol
DG = 4.5 kcal/mol
DG = 10.5 kcal/mol
etc.
DH, A
A
DH, A
Thermodynamic Analysis
(1) Assess thermodynamic feasibility (rank by DG)
(2) Rank and group reactions with common trend
CH4 + 2O2  CO2 + 2H2O
CH4 + O2
 CH2O + H2O
CH4 + O2
 HCOOH + H2
CH4 + 0.5 O2  CH2O + H2
CH4 + 0.5 O2  CH3OH
CH2O
 CO + H2
CH3OH
 CH2O + H2
OI, DH, O
OI, DH, O
OI, DH
OI, DH
OI
DH
DH
DG = -189.5 kcal/mol
DG = -70.9 kcal/mol
DG = -67.0 kcal/mol
DG = -20.0 kcal/mol
DG = -20.6 kcal/mol
DG = -17.0 kcal/mol
DG = 2.0 kcal/mol
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
Reaction Pathways
(1) Based on the analysis of surface mechanism establish the desired reaction
pathways for the reaction
CH4 + 0.5 O2  CH2O + H2
DG = -20.0 kcal/mol
OI, DH
(1) Must promote oxygen insertion (OI)
(2) Must be a mild dehydrogenation (DH)
(3) Must prevent strong dehydrogenation
(4) Must prevent oxidation
O2
CH4
H
CH3
CH2O
O O
CH3
H
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 CH3
(3) Adjacent sites to facilitate final dehydrogenation
O2
CH4
H
CH3
CH2O
O O
CH3
H
O
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)
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
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.
2.
3.
4.
5.
6.
7.
8.
Synthesis pH
Temperature
Reaction time
Reagent concentration
Nature and amount of catalyst
H2O/M ratio
Aging temperature and time
Drying conditions
Sol-gel Chemistry
Hydrolysis
1.
Synthesis pH
Nucleophilic attack
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]
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]1 or
2
Sol-gel Chemistry
Hydrolysis
3. H2O/Si Ratio
Acid Catalysts
Rate of Hydrolysis  [Water]1
Basic Catalysts
Rate of Hydrolysis  [Water]0
Sol-gel Chemistry
Condensation
1.
Synthesis pH
Sol-gel Chemistry
Condensation
2. Nature and Amount of Catalyst
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 m2g-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