Transcript Catalysis for Chemical Engineers

Catalysis for Chemical
Engineers
A Brief History and Fundamental
Catalytic Principles
What is Catalysis?
 The science of catalysts and catalytic
processes.
 A developing science which plays a
critically important role in the gas,
petroleum, chemical, and emerging
energy industries.
 Combines principles from somewhat
diverse disciplines of kinetics,
chemistry, materials science, surface
science, and chemical engineering.
What is Catalyst?
A catalyst is a material that enhances the rate and selectivity of a chemical
reactions and in the process is cyclically regenerated.
Fe2+ + Ce4+  Fe3+ + Ce3+
(Slow Reaction)
Homogeneous Catalysis
2Fe2+ + Mn4+  2Fe3+ + Mn2+ (Fast Reaction)
Mn2+ + 2Ce4+  Mn4+ + 2Ce3+
Fe2+ + Ce4+  Fe3+ + Ce3+
CO + H2O  CO2 + H2
S* + H2O  H2 + O-S*
O-S* + CO  CO2 + S*
CO + H2O  CO2 + H2
@ low temperature (Slow Reaction)
(Faster Reaction)
Heterogeneous Catalysis
What is Catalyst?
rD
N2
(Desired Reaction)
rU
NH3 (Undesired Reaction)
NO
SD/U =
From http://www.automotivecatalysts.umicore.com
rD
rU
Rate of formation of D
=
Rate of formation of U
Rh
SD/U
Pt
SD/U
How Important Is Catalysis?
Chemicals
Fibers, Plastics, Food,
Home Products,
Pharmaceuticals
Fuels
Heating,
Transportation, Power
Raw Materials
Four of the largest sectors of our world economy (i.e. the petroleum, power,
chemicals, and food industries), which account for more than 10 trillion dollars of
gross world product, are largely dependent on catalytic processes.
Development of Important
Industrial Catalytic Processes
Mittasch investigated over 2500
catalysts compositions!!!
Development of Important
Industrial Catalytic Processes
It played a vital role as a
feedstock for chemicals: 30
million tons per year in 2000
Development of Important
Industrial Catalytic Processes
Production of Liquid Fuels!!!
Development of Important
Industrial Catalytic Processes
NO
CO
CxHy
O2
N2
CO2
H2O
How to Define Reaction Rate??
Reaction Rate (r) =
1
dni
i * Q
dt
Q = V, W or S.A. of catalyst
i = Stoichiometric Coefficient
i iMi = 0 involving species Mi
(i is negative for reactants and
positive for products)
e.g. 2NH3 = N2 + 3H2
ni = # of moles of species Mi
2 x (NH3) -1 x (N2) -3 x (H2) =
2N + 6H – 2N – 6H = 0
Chemical Reactions
Four Basic Variables to Control Chemical Reactions:
Rate of Reaction = K(T) x F(Ci)
 i (Ci)i
K(T) = A exp(-E/RT)
H
H C
(1) Temperature
(2) Pressure
(3) Conc
(4) Contact time
I
H Cl
H
H C
I
H Cl
Energy Intensive &
damaging to equipment and
materials & non-selective
H
H C
H
Cl
I
Components of a Typical
Heterogeneous Catalyst
A. Active phase - metal that provides active sites where the
chemical reaction takes place
B. Support or Carrier - high surface area oxidewhich
disperses and stabilizes the active phase
(adds efficiency, physical strength, sometimes selectivity)
C. Promoter(s) - additive which improves catalyst properties,
e.g. activity, selectivity, catalyst life
Pt Nanoparticles on Al2O3
Supports
(a)
Heterogeneous Catalysis
A (g)  B (g)
Support
(Al2O3)
Active Metals
(Pt, Co, MoO2)
support
• Minimize P
• Minimize Mass Transport
Resistances
• Maximize Activity
• Minimize Poisoning and
Fouling
Components of a
Typical Heterogeneous Catalyst
Component
Active Phase:
Material T ypes
metals
Examples
noble metals: Pt, Pd; base metals: Ni, Fea
metal oxides
metal sulfides
t ransition met al oxides: MoO2, CuO
t ransition met al sulfides: MoS2, Ni3S2
Promote r:
textural
metal oxides
Al2O3, SiO2, MgO, BaO, T iO2, ZrO2
chemical
metal oxides
alkali or alkaline earth: K2O, PbO
st able, high surface area
metal oxides, carbons
Group IIIA, alkaline earth and t ransition
metal oxides, e.g. Al2O3, SiO2, TiO2,
MgO, zeolites, and Carbon
C arri er or
S upportb
Active Catalytic Phases and Reactions
They Typically Catalyze
Active P haseElements/Compounds
Reactions Cat alyzed
metals
Fe, Co, Ni, Cu,Ru, P t ,
P d, Ir, Rh, Au
hydrogenat ion, st eam reforming, HC
reforming, dehydrogenation, ammonia
synt hesis, FischerT ropsch synthesis
oxides
oxides of V,Mn, Fe,
Cu, M o, W , Al,
Si,
Sn, P b, B
com plete and partial oxidation of
hydrocarbons and CO, acid-cat alyzed
reactions (e.g. cracking,
isomerization,
alkylation), methanol synt hesis
sulfides
sulfides of Co, Mo,
W, Ni
hydrotreat inghydrodesulfurizat
(
ion,
hydrodenit rogenation,
hydrodemetallation),
hydrogenat ion
carbides
carbides of Fe, Mo, W hydrogenat ion, FT synthesis
Typical Physical Properties of
Common Carrier (Supports)
Support/Catalyst
BET area (m2/g)
Pore Vol.
Activated Carbon
500-1500
0.6-0.8
0.6-2
Zeolites (Molecular Sieves)
500-1000
0.5-0.8
0.4-1.8
Silica Gels
200-600
0.40
3-20
Activated Clays
150-225
0.4-0.52
20
Activated Al2O3
100-300
0.4-0.5
6-40
4.2
1.14
2,200
Kieselguhr ("Celite 296")
Pore Diam. (nm)
Heterogeneous Catalysis
A (g)  B (g)
Support
(Al2O3)
Active Metals
(Pt, Co, MoO2)
support
• Minimize P
• Minimize Mass Transport
Resistances
• Maximize Activity
• Minimize Poisoning and
Fouling
Heterogeneous Catalysis
Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on temp
Steps 3, 4, & 5 are chemical processes => Large dependences on temp
For Knudsen Diffusion
Order of Magnitude
Phase
cm2/s
m2/s
d
Temp and Pressure Dependences
T2
T1
1.75
d<

For Bulk, Molecular or
Fick’s Diffusion
d
From Elements of Chemical Reaction Engineering, S. Fogler

d>
Heterogeneous Catalysis
Steps 1, 2, 6, & 7 are diffusional processes => Small dependences on temp
Steps 3, 4, & 5 are chemical processes => Large dependences on temp
• Given that the rates of the chemical steps are
exponentially dependent on temperature and
have relatively large activation energies
compared to the diffusional process (20~200
kJ/mol Vs. 4-8 kJ/mol), they are generally the
slow or rate-limiting processes at low reaction
temperatures.
• As the temperature increases, the rates of
chemical steps with higher activation energies
increase enormously relative to diffusional
processes, and hence the rate limiting
process shifts from chemical to diffusional.
Kapp(T) = Aapp exp(-Eapp/RT)
Film Mass Transfer Effect on
Reaction Rate
If Boundary Layer is Too Thick,
Reaction Rate = Mass Transfer Rate
k
AB
-rA = kc (CAb – CAs)
where Kc = DAB / 
As the fluid velocity (U) increases and/or the
particle size (Dp) decreases, the boundary
layer thickness () decreases and the mass
transfer coefficient (Kc) increases
Internal Diffusion Effect on
Reaction Rate
k
AB
-rA = k η CAS
Where η = Effectiveness Factor
η = (CA)avg / CAS
L
x
cosh Φpore (1 - x/L)
CA
=
cosh( Φpore)
CAS
Φpore (Thiele Modulus) = L (k P / Deff)1/2
η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore
Internal Diffusion Effect on
Reaction Rate
While the equations above were derived for the simplified case of first-order
reaction and a single pore, they are in general approximately valid for other
reaction orders and geometry if L is defined as Vp/Sp (the volume to surface
ratio of the catalyst particle). Hence, L = z/2, rc/2 and rs/3, respectively, for a
flat plate of thickness z, a cylinder of radius rc, and a sphere of radius rs.
Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
A+BC
If it is an elementary reaction,
A
B
-rA = k [A]1 [B]1
C
Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
O3  O2 + O
Is this an elementary reaction?
If it is an elementary reaction,
-rO3 = k [O3]1
Elementary Reaction
It is one that proceeds on a molecular level exactly as written in the balanced
stoichiometric equation
O3  O2 + O
On molecular level, what really is really happening is:
O2 + O3  O2 +O2 + O
-rO3 = k [O3]1 [O2]1
We never really know for sure if we have an elementary reaction based on
the balanced stoichiometric equation!!!
Heterogeneous Catalysis
A (g)  B (g)
Active Metals
(Pt, Co, MoO2)
support
Proposed Reaction Mechanism
A + S
A-S
B-S
k1
k-1
k2
k-2
k3
k-3
A-S
B-S
B + S
What If Adsorption Is Rate
Limiting Step?
Adsorption
of A
Length of Vector Is Proportional to RXN Rate
Director of Vector Indicates Direction of RXN
Net RXN of Adsorption
Surface RXN
of A to B
Net RXN of Surface RXN
Desorption
of B
Net RXN of Desorption
Net RXN of Adsorption = Net RXN of Surface RXN = Net RXN of Desorption
Following Approximations Can Be Made:
1. Adsorption of A is almost irreversible
2. Both surface rxn and desoprtion steps are almost at equilibrium
What If Adsorption Is Rate
Limiting Step?
A + S
k1
A-S
Where S is a free surface site and A-S is a chemisorbed complex
Since it is an elementary reaction,
-rA = k1 CA CS
v = the fractional coverage of vacant site
v = CS / Ctotal
How can we experimentally measure Cs ???
Cs = functions of parameters that one can experimentally
measure or easily obtain
What If Adsorption Is Rate
Limiting Step?
Since both surface rxn and desorption steps are in near equilibrium,
A-S
B-S
k2
k-2
k3
k-3
B-S
rnet = k2 CA-S –k-2 CB-S  0
k2 / k-2 = K2 = CB-S / CA-S
B + S
rnet = k3 CB-S –k-3 CB CS  0
k3 / k-3 = K3 = CB CS / CB-S
Both K2 and K3 are equilibrium constants which one can obtain:
RT ln K = - G
Let us do the site balance,
Ctotal = CS + CA-S + CB-S = Const.
K2 = CB-S / CA-S
K3 = CB CS / CB-S
CS =
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
What If Adsorption Is Rate
Limiting Step?
From the site balance and quasi-equilibrium approximation,
CS =
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
From the rate limiting step,
-rA = k1 CA CS =
k1 Ctotal CA
1 + [ (1 + K2) CB / (K2 K3) ]
=
k1 Ctotal CA
1 + K’ CB
Where K’ = (1 + K2) / (K2 K3)
If A and B behave according to the ideal gas law,
CA = PA / RT
CB = PB / RT
What If Surface Reaction Is
Rate Limiting Step?
A + S
k1
k-1
A-S
Rate Limiting Step
A-S
B-S
-rA =
k2
k-2
k3
k-3
B-S
B + S
k2 K1 PA
1 + K1 PA
Figure 1.16 from Fundamentals of Industrial
Catalytic Processes
What If Desoprtion Is Rate
Limiting Step?
A + S
A-S
k1
k-1
k2
k-2
A-S
B-S
Rate Limiting Step
B-S
k3
k-3
B + S
-rA =
k3 K1 K2 PA
1 + (K1 + K1 K2) PA
Fundamental Catalytic Phenomena
and Principles
Chemical Properties
(Oxidation State, Acidity,
Surface Composition)
Physical Properties
(Surface Area, Pore
Structure, Pore Density)
Catalyst
Design
Catalytic Properties
(Activity and Selectivity)
Structure Sensitive Reactions
CO Oxidation over Au/TiO2:
Particle Size Effect
2 nm
2.5 nm
TiO2
6 nm
Au
Particle Size Vs. Electronic
Structure Change of Au