Transcript CBE 40445 Lecture 15 Introduction to Catalysis Developed by Prof. Schneider1,2 Modified by Prof.

CBE 40445
Lecture 15
Introduction to Catalysis
Developed by Prof. Schneider1,2
Modified by Prof. Hicks1
1Department
of Chemical and Biomolecular Engineering
2Department of Chemistry and Biochemistry
University of Notre Dame
Fall 2011
W. F. Schneider
CBE 40445
Importance of Catalysts
Bartholomew and Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, 2006.
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W. F. Schneider
CBE 40445
What is a “Catalyst”
 A catalyst (Greek: καταλύτης, catalytēs) is a substance that
accelerates the rate of a chemical reaction without itself being
transformed or consumed by the reaction. (thank you Wikipedia)
k(T) = k0e-Ea/RT
Ea′ < Ea
k0′ > k0
k′ > k
Ea
Ea′
ΔG = ΔG
A+B
A+B+
catalyst
ΔG
C
ΔG
C + catalyst
uncatalyzed
catalyzed
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W. F. Schneider
CBE 40445
Catalysts Open Up New Reaction Pathways
‡
O
H
O
H2C
C
OH
CH3
C
CH3
C
CH3
CH2
‡
CH3
propenol
propanone
propenol
propanone
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W. F. Schneider
CBE 40445
Catalysts Open Up New Reaction Pathways
O−
C
CH2
OH−
+ H2O
CH3
−OH−
Base catalyzed
O
OH
rate = k[OH−][acetone]
C
CH3
C
CH2
CH3
propanone
‡
CH3
propenol
‡
propenol
intermediate
propanone
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W. F. Schneider
CBE 40445
Catalysts Open Up New Reaction Pathways
‡
‡
propenol
different
intermediate
propanone
propenol
O
propanone
C
CH3
OH
rate = k[H3O+][acetone]
CH3
C
Acid catalyzed
H3O+
CH3
CH3
−H3O+
OH
C
+
CH2
CH3
+ H2O
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W. F. Schneider
CBE 40445
Types of Catalysts - Enzymes
 The “Gold Standard” of
catalysts
 Highly specific
 Highly selective
 Highly efficient
 Catalyze very difficult
reactions
 N2  NH3
 CO2 + H2O  C6H12O6
Triosephosphateisomerase
“TIM”
Cytochrome C Oxidase
Highly tailored “active sites”
Often contain metal atoms
 Works better in a cell
than in a 100000 l
reactor
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W. F. Schneider
CBE 40445
Types of Catalysts – Organometallic Complexes
 Perhaps closest man has
come to mimicking
nature’s success
 2005 Noble Prize in
Chemistry
 Well-defined, metal-based
active sites
 Selective, efficient
manipulation of organic
functional groups
 Various forms, especially
for polymerization
catalysis
Polymerization:
 Difficult to generalize
beyond organic
transformations
Termination:
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W. F. Schneider
CBE 40445
Types of Catalysts – Homogeneous vs.
Heterogeneous
Zeolite catalyst
Catalyst powders
Homogeneous catalysis
Heterogeneous catalysis
Single phase
(Typically liquid)
Low temperature
Separations are tricky
Multiphase
(Mostly solid-liquid and solid-gas)
High temperature
Design and optimization tricky
Newer area of Research:
Tethered Catalysts (maintaining
selectivity of homogeneous catalysts
but tethered to a solid support)
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W. F. Schneider
CBE 40445
Types of Catalysts: Crystalline Microporous
Catalysts
 Regular crystalline structure
 Porous on the scale of molecular dimensions
 3 – 20 Å (microporous), 20-500 Å (mesoporous)
 Up to 1000’s m2/g surface area
 Catalysis through
 shape selection
 acidity/basicity
 incorporation of metal particles
Applied Catalysis A, 2009, 360, 59-65.
 Used as supports for other metal precursors
40 Å
10 Å
MCM-41 (mesoporous silica)
Zeolite (silica-aluminate)
Silico-titanate
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W. F. Schneider
CBE 40445
Types of Catalysts: Zeolites
What are zeolites ?
- Aluminosilicates
- microporous ( pores < 20Å)
- Crystalline
- Framework of AlO4 and
SiO4 Td-units (tetrahedral)
- Possess ordered pore
systems
- Acidity arises from
incorporation of Al
Morphology changes due to additives, quantities, pH, time,
etc. Shown below are SEM images of HZSM-5 (5.6 Å pores)
Al2O3 source
Neumann and Hicks, 2011.
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All silica ~ weak acidity
SiO2/Al2O3 ~ Brønsted acidity
Types of Catalysts: Zeolites
Sodalite
(SOD)
pores ~3Å
[SiO4 ]4[AlO4]5-
LTA
-cages
Zeolite - A
(LTA)
pores ~ 4Å
FAU
A● large
cage (~ 12Å)
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formed
in A and X,Y
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Zeolite - X, Y
(FAU)
○pores
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Types of Catalysts: Amorphous Heterogeneous
Catalysts
 Amorphous, high surface area supports
 Alumina, silica, activated carbon, …
 Up to 100’s of m2/g of surface area
 Impregnated with catalytic transition metals
 Pt, Pd, Ni, Fe, Ru, Cu, Ru, …
 Typically pelletized or on monoliths
 Cheap, high stability, catalyze many types of reactions
 Most used, least well understood of all classes
SEM micrographs of alumina and Pt/alumina
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W. F. Schneider
CBE 40445
Types of Catalysts: Motivation for Tethered
Catalysts
Traditional Heterogeneous
(Insoluble)
 Easy to separate
 Multiple types of active
sites
 Less mobility / spatially
constricted
 Diffusion effects
Homogeneous (Soluble)
 High mobility - active
 Single type of active
site -selective
 Control of
stereochemistry
 Difficult to separate
Tethered
• Insoluble
• Single type of active
site-selective
• Easy to separate
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Types of Catalysts: Examples of Tethered
Catalysts
Zr
Me
H2
C
NH 2
Al
Al
OMe
O
Si
O S O
F3C F
F
F
O
O
O
Si
Si
Si
R. A. Shiels, K. Venkatasubbaiah and C.
W. Jones, Adv. Synth. Catal. (2008)
350, 2823-2834.
SiO2
Hicks, J. C.; Jones, C. W., Langmuir 2006, 22, 2676.
Hicks, J. C.; Dabestani, R.; Buchanan III, A. C.; Jones,
C. W., Chem. Mater. 2006, 18, 5022.
J. C. Hicks, B. A. Mullis and C. W. Jones,
J. Am. Chem. Soc. (2007) 129, 8426-8427.
(C6H5)2
P
(C6H5)2
P
Si
N
Zr
Cl
Cl
Q
Si
O
Hicks, J. C.; Dabestani, R.; Buchanan III, A.
C.; Jones, C. W., Inorg. Chim. Acta, 2008.
Ir (complex)
P(C6H5)
VS.
Si
O
F
Ir (complex)
P(C6H5)
OSiMe3
Si
O
OMe
O
O
OEt
O
SBA-15
Si
O
OEt
O
SBA-15
Collaboration between Hicks and Schneider Groups
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W. F. Schneider
CBE 40445
Important Heterogeneous Catalytic Processes
 Haber-Bosch process
 N2 + 3 H2 → 2 NH3
 Fe/Ru catalysts, high pressure and temperature
 Critical for fertilizer and nitric acid production
 Fischer-Tropsch chemistry
 n CO + 2n H2 → (CH2)n + n H2O , syn gas to liquid fuels
 Fe/Co catalysts
 Source of fuel for Axis in WWII
 Fluidized catalytic cracking
 High MW petroleum → low MW fuels, like gasoline
 Zeolite catalysts, high temperature combustor
 In your fuel tank!
 Automotive three-way catalysis
 NOx/CO/HC → H2O/CO2/H2O
 Pt/Rh/Pd supported on ceria/alumina
 Makes exhaust 99% cleaner
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W. F. Schneider
CBE 40445
Heterogeneous Catalytic Reactors
 Design goals
 rapid and intimate contact
between catalyst and
reactants
 ease of separation of
products from catalyst
Packed Bed
(single or multi-tube)
Fluidized
Bed
Slurry
Reactor
Catalyst
Recycle
Reactor
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W. F. Schneider
CBE 40445
FCC: Fluidized Catalytic Cracker
Gasoline Production
Gas oil enters the riser reactor
and is mixed with a zeolite
catalyst (Zeolite Y).
Acid-catalyzed cracking
reactions occur in reactor.
Coke formation occurs quickly
on the catalyst (carbon
deposition).
Catalyst residence time is ~ 1.5
seconds.
Catalyst is separated,
regenerated, and re-injected.
Bartholomew and Farrauto, Fundamentals of Industrial Catalytic Processes, Wiley, 2006.
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W. F. Schneider
CBE 40445
Automotive Emissions Control System
“Three-way” Catalyst
CO  CO2
HC  CO2 + H2O
NOx  N2
Monolith reactor
Most widely deployed
heterogeneous catalyst in
the world – you probably
own one!
Pt, Rh, Pd
Alumina, ceria, zirconia, …
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W. F. Schneider
CBE 40445
Length Scales in Heterogeneous Catalysis
Mass transport/diffusion
Chemical adsorption
and reaction
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W. F. Schneider
CBE 40445
Steps in a Heterogeneous Catalytic Reactor
Diffusion Steps: 1, 2, 6, 7.
Reaction Steps: 3, 4, 5.
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W. F. Schneider
CBE 40445
Characteristics of Heterogeneous Supported
Catalysts
 Surface area:
 Amount of internal support surface accessible to a fluid
 Measured by gas adsorption isotherms
 Loading:
 Mass of transition metal per mass of support
 Dispersion:
 Percent of metal atoms accessible to a fluid
M
M
M
support
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W. F. Schneider
CBE 40445
Rates of Catalytic Reactions
 Pseudo-homogeneous reaction rate
 r = moles / volume · time
 Mass-based rate
 r′ = moles / masscat · time
 r′ = r / ρcat
 Heterogeneous reactions happen at surfaces
 Area-based rate
 r′′ = moles / areacat · time
 r′′ = r′ / SA,
SA = area / mass
 Heterogeneous reactions happen at active sites
 Active site-based rate
TOF (s−1)
Hetero. cats. ~101
Enzymes ~106
 Turn-over frequency TOF = moles / site · time
 TOF = r′′ / ρsite
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W. F. Schneider
CBE 40445
Adsorption and Reaction at Solid Surfaces
 Physisorption: weak van der Waals attraction of a fluid
(like N2 gas) for any surface
 Eads ~10 – 40 kJ/mol
 Low temperature phenomenon
 Exploited in measuring gross surface area
 Chemisorption: chemical bond formation between a fluid
molecule (like CO or ethylene) and a surface site
 Eads ~ 100 – 500 kJ/mol
 Essential element of catalytic activity
 Exploited in measuring catalytically active sites
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W. F. Schneider
CBE 40445
Comparing Physi- and Chemisorption on MgO(001)
1.25
Calculated from first-principles DFT
O
1.48
O
Physisorbed CO2
-2 kcal mol-1 GGA
: :
CO2
C
2-
:O:surf
1.51
SO2
O
O
O
:
Mg
2.10
1.77
Chemisorbed SO2
(“sulfite”)
-25 kcal mol-1 GGA
: :
S
2-
:O:surf
2.60
1.45
SO3
1.48
1.66
2.12
Chemisorbed SO3
(“sulfate”)
-50 kcal mol-1 GGA
O
O
MgO(001) supercell
O
: :
S
2-
:O:surf
Schneider, Li, and Hass, J. Phys. Chem. B 2001, 105, 6972
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2.58
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W. F. Schneider
CBE 40445
Measuring Concentrations in Heterogeneous
Reactions Kinetics
 Fluid concentrations
 Traditionally reported as pressures (torr, atm, bar)
 Ideal gas assumption: Pj = Cj RT
Rate = f(Pj,θj)
 Surface concentrations
Metal particle surface
 “Coverage” per unit area
 nj = molesj / area
 Maximum coverage called monolayer
 1 ML: nj,max = ~ 1015 molecules / cm2
 Fractional coverage
 θj = nj / nj,max
 0 ≤ θj ≤ 1
θj = 1/5
● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ●
○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○
W. F. Schneider
CBE 40445
Adsorption Isotherms
 Molecules in gas and surface are in dynamic equilibrium
A (g) + M (surface) ↔ M-A
 Isotherm describes pressure dependence of equilibrium
 Langmuir isotherm proposed by Irving Langmuir, GE, 1915




(1932 Noble Prize)
Adsorption saturates at 1 monolayer
All sites are equivalent
Adsorption is independent of coverage
rated  kd NA
ratea  ka PA N *
Site conservation
θA + θ* = 1
+
Equilibrium
rateads = ratedes
A 
KPA
, K  ka kd
1  KPA
● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ●
○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○
W. F. Schneider
CBE 40445
Using the Langmuir Isotherm
 Example: CO adsorption on 10% Ru/Al2O3 @ 100°C
PCO (torr)
100
150
200
COads (μmol/gcat)
1.28
1.63
1.77
1.94
2.06
2.21
CO adsorption on Ru/Al O at 100C
CO adsorption on Ru/Al 2O3 at 100C
Non-linear regression
250
300
2
400
3
Linearized model
2.6
200
nCO,∞ = 2.89 μmol/gcat
K = 0.0082
2.4
1.6
nCO 
1.4
nCO, KPCO
1  KPCO
P /n
1.8
n
CO
cat
(mol/g )
2
CO CO
(torr g /mol)
cat
2.2
150
100
PCO
P
1
 CO 
nCO nCO, KnCO,
1.2
1
0.8
50
200
300
400
100
200
300
400
Pressure
(torr)
Pressure
(torr)
● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ●
100
○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○
W. F. Schneider
CBE 40445
Brunauer-Emmett-Teller Isotherm (BET)
 Relaxes Langmuir restriction to single layer adsorption
 Monolayer adsorption; multilayer condensation
 Useful for total surface area measurement
 Adsorption of boiling N2 (78 K)
V
Vmono
ΔHads/ΔHcond
cz

(1  z )(1  (1  c) z )
z P
Pvap
, ce
( H ads H cond )
ΔHcond
RT
ΔHads
Solid Surface
● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ●
○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○
W. F. Schneider
CBE 40445