catalysis for chemical engineers a brief history and fundamental catalytic principles
TRANSCRIPT
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)
2Fe2+ + Mn4+ 2Fe3+ + Mn2+
Mn2+ + 2Ce4+ Mn4+ + 2Ce3+
Fe2+ + Ce4+ Fe3+ + Ce3+
(Fast Reaction)Homogeneous Catalysis
CO + H2O CO2 + H2 @ low temperature (Slow Reaction)
S* + H2O H2 + O-S*O-S* + CO CO2 + S*
CO + H2O CO2 + H2
(Faster Reaction)Heterogeneous Catalysis
What is Catalyst?
From http://www.automotivecatalysts.umicore.com
NO
N2
NH3
(Desired Reaction)
(Undesired Reaction)
SD/U =rD
rU
rD
rU
Rate of formation of D
Rate of formation of U=
Rh SD/U
Pt SD/U
How Important Is Catalysis?
Raw Materials
Chemicals
Fuels
Fibers, Plastics, Food, Home Products, Pharmaceuticals
Heating, Transportation, Power
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
How to Define Reaction Rate??
Reaction Rate (r) =1
i * Q
dni
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 2 x (NH3) -1 x (N2) -3 x (H2) =2N + 6H – 2N – 6H = 0
ni = # of moles of species Mi
Chemical ReactionsFour Basic Variables to Control Chemical Reactions:
(1)Temperature(2)Pressure(3)Conc(4)Contact time
Rate of Reaction = K(T) x F(Ci)
K(T) = A exp(-E/RT)
C
H
H
H
I
Cl
C
H
H
H
Cl
I
C
H
H
HI
Cl
Energy Intensive & Energy Intensive & damaging to equipment and damaging to equipment and materials & non-selectivematerials & non-selective
i (Ci)i
A. Active phase - metal that provides active sites where thechemical reaction takes place
B. Support or Carrier - high surface area oxide whichdisperses 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
Components of a Typical Heterogeneous Catalyst
Pt Nanoparticles on Al2O3 Supports
macro-poresmeso-pores
Pt crystallites
high SA alumina
(10 nm)
(100-200 nm)
(1-5 nm)(a)
Heterogeneous Catalysis
A (g) B (g)
• Minimize P• Minimize Mass Transport
Resistances• Maximize Activity• Minimize Poisoning and
Fouling
Support(Al2O3)
Active Metals(Pt, Co, MoO2)
support
Active Catalytic Phases and Reactions They Typically Catalyze
Active Phase Elements/Compounds Reactions Catalyzed
metals Fe, Co, Ni, Cu, Ru, Pt,Pd, Ir, Rh, Au
hydrogenation, steam reforming, HCreforming, dehydrogenation, ammoniasynthesis, Fischer-Tropsch synthesis
oxides oxides of V, Mn, Fe,Cu, Mo, W, Al, Si,Sn, Pb, B
complete and partial oxidation ofhydrocarbons and CO, acid-catalyzedreactions (e.g. cracking, isomerization,alkylation), methanol synthesis
sulfides sulfides of Co, Mo,W, Ni
hydrotreating (hydrodesulfurization,hydrodenitrogenation, hydrodemetallation),hydrogenation
carbides carbides of Fe, Mo, W hydrogenation, FT synthesis
Support/Catalyst BET area (m2/g) Pore Vol. Pore Diam. (nm)
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
Kieselguhr ("Celite 296") 4.2 1.14 2,200
Typical Physical Properties of Common Carrier (Supports)
Heterogeneous Catalysis
A (g) B (g)
• Minimize P• Minimize Mass Transport
Resistances• Maximize Activity• Minimize Poisoning and
Fouling
Support(Al2O3)
Active Metals(Pt, Co, MoO2)
support
Heterogeneous CatalysisSteps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 3, 4, & 5 are chemical processes => Large dependences on temp
T2
T1
1.75
Phase
Order of Magnitude
cm2/s m2/s Temp and Pressure Dependences
From Elements of Chemical Reaction Engineering, S. Fogler
d
d
For Knudsen Diffusion
For Bulk, Molecular or Fick’s Diffusion
d <
d >
Heterogeneous CatalysisSteps 1, 2, 6, & 7 are diffusional processes => Small dependences on tempSteps 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
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
k
Internal Diffusion Effect on Reaction Rate
-rA = k η CAS
Where η = Effectiveness Factor
η = (CA)avg / CAS
CA
CAS
=coshcosh Φpore (1 - x/L)
( Φpore)cosh
η = (CA)avg / CAS = (tanh (Φpore) ) / Φpore
Φpore (Thiele Modulus) = L (k P / Deff)1/2
A Bk
L
x
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 ReactionIt 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 C
-rA = k [A]1 [B]1
Elementary ReactionIt 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 ReactionIt 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
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Proposed Reaction Mechanism
What If Adsorption Is Rate Limiting Step?
Adsorption of A
Surface RXN of A to B
Desorption of B
Length of Vector Is Proportional to RXN RateDirector of Vector Indicates Direction of RXN
Net RXN of AdsorptionNet RXN of Adsorption
Net RXN of Surface RXNNet RXN of Surface RXN
Net RXN of Desorption
Following Approximations Can Be Made:1. Adsorption of A is almost irreversible2. Both surface rxn and desoprtion steps are almost at equilibrium
Net RXN of Adsorption = Net RXN of Surface RXN = Net RXN of Desorption
What If Adsorption Is Rate Limiting Step?
Since it is an elementary reaction,
A + S A-Sk1
Where S is a free surface site and A-S is a chemisorbed complex
-rA = k1 CA CS v = CS / Ctotal
v = the fractional coverage of vacant site
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
B-S B + S
k2
k-2
k3
k-3
rnet = k2 CA-S –k-2 CB-S 0 k2 / k-2 = K2 = CB-S / CA-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:
Let us do the site balance,
Ctotal = CCSS + CCA-SA-S + CCB-SB-S = Const.
K2 = CCB-SB-S / CCA-SA-S
K3 = CB CCSS / CCB-SB-S
CS = Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
RT ln K = - G
What If Adsorption Is Rate Limiting Step?
CS = Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]
From the site balance and quasi-equilibrium approximation,
-rA = k1 CA CS
From the rate limiting step,
Ctotal
1 + [ (1 + K2) CB / (K2 K3) ]=
k1 Ctotal
1 + K’ CB
k1=
Where K’ = (1 + K2) / (K2 K3)
CA = PA / RT
If A and B behave according to the ideal gas law,
CB = PB / RT
CA CA
What If Surface Reaction Is Rate Limiting Step?
K1
1 + K1 PA
k2 PA-rA =
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Rate Limiting Step
Figure 1.16 from Fundamentals of Industrial Catalytic Processes
What If Desoprtion Is Rate Limiting Step?
K1
1 + (K1 + K1 K2) PA
k3 PA-rA =
A + S A-S
A-S B-S
B-S B + S
k1
k-1
k2
k-2
k3
k-3
Rate Limiting Step
K2
Fundamental Catalytic Phenomena and Principles
Catalyst Design
Catalytic Properties(Activity and Selectivity)
Chemical Properties(Oxidation State, Acidity,
Surface Composition)
Physical Properties(Surface Area, Pore
Structure, Pore Density)