Download - Catalyst Design and Preparation
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Catalyst Design and Preparation
Dr. King Lun Yeung
Department of Chemical Engineering
Hong Kong University of Science and Technology
CENG 511Lecture 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
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Case Study
Methane Partial Oxidation to Formaldehyde
CH4
+ O2
CH2
O + H2
O H = -76.8 kcal/mol
G = -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 H and G of reaction
(3) Chemical bond changes
Primary ReactantsCH4
O2
Reactant Self-interactions
2CH4 C2H6 + H2 DH G = 8.5 kcal/mol
2CH4 C2H4 + 2H2 DH G = 12.8 kcal/mol
2CH4 C2H2 + 3H2 DH G = 22.2 kcal/mol
O2 = none
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Stoichiometric Analysis-2
Reactant Cross-interactions
CH4 + 0.5 O2 CH3OH OI G = -20.6 kcal/mol
CH2
O + H2
OI, DH G = -20.0 kcal/mol
CO + 2H2 OI, DH G = -43.1 kcal/mol
CH4 + O2 CH2O + H2O OI, DH, O G = -70.9 kcal/mol
HCOOH + H2 OI, DH, O G = -67.0 kcal/mol
CO + H2 + H2O OI, DH, O G = -87.3 kcal/mol
CO2 + 2H2 OI, DH, O G = -90.5 kcal/mol
CH4 + 1.5O2 CH2O + H2O2 OI, DH, O G = -31.0 kcal/mol
HCOOH + H2O OI, DH, O G = -119.8 kcal/mol
CO + 2H2
O OI, DH, O G = -136.5 kcal/mol
CO2 + H2 + H2O OI, DH, O G = -139.8 kcal/mol
Stoichiometric Analysis-3
Reactant Cross-interactions
CH4 + 2O2 HCOOH + H2O2 OI, DH, O G = -98.6 kcal/mol
CO + H2O2 + H2O OI, DH, O G = -118.7 kcal/mol
CO2 + 2H2O OI, DH, O G = -189.5 kcal/mol
Reactant-Product interactions
CH4 + C2H6 C3H8 + H2 DH, A G = 16.6 kcal/mol
CH4 + C2H4 C3H8 A G = 4.5 kcal/mol
CH4 + CH3OH C2H5OH + H2 DH, A G = 10.5 kcal/mol
etc.
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Thermodynamic Analysis
(1) Assess thermodynamic feasibility (rank by G)
(2) Rank and group reactions with common trend
CH4 + 2O2 CO2 + 2H2O OI, DH, O G = -189.5 kcal/mol
CH4 + O2 CH2O + H2O OI, DH, O G = -70.9 kcal/mol
CH4 + O2 HCOOH + H2 OI, DH G = -67.0 kcal/mol
CH4 + 0.5 O2 CH2O + H2 OI, DH G = -20.0 kcal/mol
CH4 + 0.5 O2 CH3OH OI G = -20.6 kcal/mol
CH2O CO + H2 DH G = -17.0 kcal/mol
CH3OH CH2O + H2 DH G = 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 know adsorption, molecular configurations of reactants and products
CH4 CH3-S CH2-S O2 2O-S
CH3OH
CH2O, H2
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Reaction Pathways
(1) Based on the analysis of surface mechanism establish the desired reaction
pathways for the reaction
CH4 + 0.5 O2 CH2O + H2 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
CH4
H CH3
O2
O O
O
CH3
H
CH2O
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
CH4
H CH3
O2
O O
O
CH3
H
CH2O
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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
MixedTiO2 + MoO3V2O3 + ZnO
Complex
Fe2O3Zn TiO3
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Catalyst Preparation
(1) Unsupported Catalyst
are typically usually very active catalyst that do not require high
surface areae.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 Catalystrequires 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
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Unsupported Catalyst
Required preparation steps
Unsupported Catalyst
Typical preparation methods
(1) Fusion Method
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Unsupported Catalyst
Typical preparation methods
(2) Precipitation and Co-precipitation Methods
Unsupported Catalyst
(2) Precipitation and Co-precipitation Methods
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Unsupported Catalyst
(2) Precipitation and Co-precipitation Methods
Preparation of aluminum oxide
Unsupported Catalyst
Typical preparation methods
(3) Sol-gel synthesis
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Unsupported Catalyst
Typical preparation methods
(3) Sol-gel synthesis
Silica-alumina acid 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
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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
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Supported Catalyst
Typical support materials
Support Materials
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Metal Ion Distribution in Support Pellet
Supported Catalyst
Weak Interaction Interaction
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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
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Supported CatalystTypical preparation methods
(1) Precipitation method
Precipitation Method