Heterogeneous catalysis (C9981)
0. Technical information
1. Basic principles, thermodynamics,
and a little bit of kinetics
2. Active sites and catalyst evaluation
Technical information
Syllabus
• Theory behind the
heterogeneous catalysis:
thermodynamics, kinetics,
diffusion.
• Synthesis of heterogeneous and
single-site catalysts
• Characterization
• Your presentations
• Examples of industrial catalytic
processes: Haber-Bosch
synthesis of ammonia; Threeway
catalysts; Zeolites in oil
refinement; Olefin metathesis.
Homeworks
• Not mandatory
• But useful for the exam
• We will go through the
homeworks in the lectures only
if there are some homeworks to
go through (uploaded in the
information system)
Definition
• Catalyst is…
– A substance that speeds up a chemical reaction
– Without being consumed or changed
+ CATA + CATA
• We get our products
– In shorter time; at lower rxn temperature; at lower pressure
– cheaper; economical; ecological
Types of catalyst (based on phase)
• Homogeneous
• Heterogeneous
• Enzymatic + CATA
Enzymatic catalysis
• Cytochrome P450
Catalytic cycle
Enzymatic catalysis
• Cytochrome P450
Active site
Homogeneous catalysis
• Gas phase
1/
Cl + O3 → ClO3
ClO3 → ClO + O2
ClO + O → Cl + O2
Total:
O3 + O → 2 O2
2/
Isomerization of
hydrocarbons with HF
(both in gaseous phase)
• Liquid phase
1/
Esterification of acids with
alcohols catalyzed by H+
(e.g. H2SO4, H3PW12O40)
2/
Olefin metathesis
over Grubbs catalyst
(organometallics dissolved
in liquid solvent along with
precursors
• A catalyst and rxn mixture in one unique phase
Heterogeneous catalysis
• A catalyst is present in a different, distinct phase in
contrary to the rxn mixture
• Gas phase rxn mixture over solid catalyst
– Olefin metathesis
Ethene
+
Butene
Propene
material???
Heterogeneous catalysis
• A catalyst is present in a different, distinct phase in
contrary to the rxn mixture
• Gas phase rxn mixture over solid catalyst
– Alcohol dehydration
Methanol Dimethylether
H3PW12O40; a molecular compound, not a material!
Heterogeneous catalysis
• A catalyst is present in a different, distinct phase in
contrary to the rxn mixture
• Liquid phase rxn mixture over solid catalyst
– Alcohol dehydration
Butanol
Dibutylether
+
Butenes
H3PW12O40; insoluble molecular compound
Heterogeneous catalysis
• A catalyst is present in a different, distinct phase in
contrary to the rxn mixture
• Liquid phase rxn mixture over solid catalyst
– Olefin metathesis
Longer olefins Longer olefins
Heterogeneous vs. Homogeneous
catalysis
• Well defined
= easier to characterize
the active site, describe
reactivity, and evaluate
catalytic performance
• Ill defined
= active site???
= more than one type
of active sites???
= number of active
sites???
Heterogeneous vs. Homogeneous
catalysis
• One phase with
reactants
= higher probability that
all necessary species
meet and react☺
• Distinct phase
= diffusion???
= adsorption???
= desorption???
Heterogeneous vs. Homogeneous
catalysis
• Separation???
• Reusability/Price???
• Toxicity???
• Batch rxns only???
• Distinct phase
= easy to separate
= reusable/recyclable
= clean products
= continuous processes
Heterogeneous vs. Homogeneous
catalysis
Homogeneous and Heterogeneous catalysis are
complementary to each other
We choose either hetero or homo based on the rxn we
want to catalyze (pros and cons)
Catalytic cycle
A + B → P
Ea >> Ea(cata); k = A∙e–Ea/RT
ΔG = ΔG(cata); ΔG = –RT∙lnK
Thermodynamics vs. kinetics
• Addition of catalyst into the rxn mixture
– does change „k“
– does not change „K“
A → P, speeded up by addition of a catalyst
K = 1; ΔG = 0 kJ mol–1; final rxn mixture: 1A + 1P
The catalyst is then equally efficient in catalysing
the opposite process
P → A; final rxn mixture: 1A + 1P
Thermodynamics vs. kinetics
• If a catalyst is efficient in dehydration
– then it can be possibly used in hydration
• If a catalyst is efficient in hydrogenation
– then it can be possibly used in dehydrogenation
I.e.: It depends on ΔG of studied reaction, whether
the catalyst hydrogenates or dehydrogenates.
We know how to shift equilibrium of chemical reaction☺
(#GenChem). This is not a job of the catalyst.
Thermodynamics vs. kinetics
• Reaction coordinate: A multidimensional problem
Thermodynamics vs. kinetics
• Reaction coordinate: A multidimensional problem
– e.g. selective oxidation reactions (Table 1)
• We are not shooting for the most stable product
(thermodynamically); Difficult!
Mountain
cottage
Spa
resort
Thermodynamics vs. kinetics
• Reaction coordinate: A multidimensional problem
– How is it possible that we sometimes end up in a mountain
cottage and not in the spa resort???
– We need a catalyst!
– The catalyst will lower the activation energy of our way to
the mountain cottage (beautiful views, crazy friends,…)
– The catalyst will increase the activation energy of our way
to the spa resort
– E.g. The catalyst has to increase the rate of hiking
(selective oxidation) and decrease the rate of hiking (total
oxidation). What a difficult job!
Thermodynamics vs. kinetics
• Reaction coordinate: A multidimensional problem
Thermodynamics vs. kinetics
• Reaction coordinate: From multidimensional to twodimensional
problem
Thermodynamics vs. kinetics
• Transitions state vs. Intermediate
Thermodynamics – heterogeneous
catalysis
A + B → P
1
1: Adsorption of reactants
on catalyst surface (hetero)
/ Formation of reaction
intermediate ABC (enzymes
and homo)
In heterogeneous catalysis
always thermodynamically
favorable
(coordinatively unsaturated
surface + adsorbate)
Let‘s imagine weak interaction of A, B, and catalyst surface…???
Let‘s imagine very strong interaction of catalyst surface, A, and/or B…??
2
3
1
Thermodynamics
A + B → P
1 2
3
3: Desorption of products
from catalyst surface
(hetero) / Rupture of
product intermediate PC
(enzymes and homo)
Let‘s imagine very strong interaction of P with catalyst surface…???
Thermodynamics
• Interactions of A,B, and P with catalyst not too weak,
not too strong
(= physi/chemisorption on catalyst surface)
Sabatier‘s principle; Volcano plot
Thermodynamics vs. kinetics
A + B → P
1 2
3
Before going any
further…
Steps 1 and 3 are
equally important to
step 2 in catalysis .
Adsorption and
desorption of A, B, and
P might have a
significant effect on Ea.
In kinetic studies in heterogeneous catalysis we estimate only apparent
activation energy (Eapp), where all these contributions are included.
Thermodynamics vs. kinetics
• Importance of reactant adsorption and product
desorption
– Example: Ethylbenzene dehydrogenation to styrene
The best
catalyst
Thermodynamics vs. kinetics
A + B → P
1 2
3
What is preceding the
step 1 and following
the step 3 ?
Diffusion!
Diffusion of A, B, and P
might have a significant
effect on Ea as well.
In kinetic studies in heterogeneous catalysis we estimate only apparent
activation energy (Eapp), where all these contributions are included.
Thermodynamics vs. kinetics
• Another representation of the complexity of heterogeneous
catalytic reaction
Adsorption, desorption,
and diffusion of A, B,
and P might have a
significant effect on Ea.
Thermodynamics vs. kinetics
• Another representation of the complexity of heterogeneous
catalytic reaction
Adsorption, desorption,
and diffusion of A, B,
and P might have a
significant effect on Ea.
Kinetics
• We know kinetics☺ (#ChemKin)
• Rxns of zero order, first order, and second order
• But in catalysis also 0.39 order, –0.5 order… (many
elementary rxns, fight for active site between
reactants and products,…!)
Thermodynamics vs. kinetics
A + B → P
1 2
3
2: In order to decrease Ea
for step 2 we need to
design an active site of the
catalyst as good as possible
What is the active site???
How do we measure the
„goodness“ of the active
site???
Active sites
Active sites
• Active sites are often dynamic: They has to be created first!
Active site/catalyst evaluation
• Turn-over frequency (TOF; [s–1])
= number of catalytic cycles performed by 1 active
site per time unit
• Precise numbers for homogeneous and enzymatic
catalysis
• Numbers for heterogeneous catalysis???
TOF [s–1]
Hetero: ~1–100 s–1
Homo: ~10–1000 s–1
Enzymes: ~10000–1000000 s–1
Active site/catalyst evaluation
• Turn-over number (TON; [-])
= number of catalytic cycles performed by 1 active
site before deactivation (~lifetime)
• Precise numbers for homogeneous and enzymatic
catalysis
• Numbers for heterogeneous catalysis???
Active site/catalyst evaluation
• Selectivity is ability of catalyst to form one product
from a pool of products (possibly many)
• Selectivity (S; [%])
= number of D molecules produced / R molecules
converted
reactant R
Active site/catalyst evaluation
• Enantiomeric enrichment is ability of catalyst to
form selectively one enantiomer
• Enantiomeric enrichment (ee; [%])
= S(PR) – S(PS)
(e.g. Final mixture consists of 90 % PR and 10 % PS
ee = 80 %)
S [%] ee [%]
Hetero: 1–100 1–100
Homo: > 90 > 70
Enzymes: > 99.99 > 99.99
Active site/catalyst evaluation
• Selectivity is ability
of catalyst to form
one product from a
pool of products
(possibly many)
• Specificity is ability
of catalyst to react
with one reactant
among many
(similar) reactants
Active sites
Electronic and steric properties do play an important role! (TOF, TON,
Selectivity, ee, specificity).
Catalyst evaluation
• Conversion (C; [%])
= number of R molecules converted / number of R
molecules introduced
• Yield (Y; [%])
= number of P molecules produced / number of R
molecules introduced
• Selectivity (S; [%])
= number of P molecules converted / number of R
molecules converted
R → P + P‘
𝑆 =
𝑌
𝐶
Catalyst evaluation
• Productivity (Site Time Yield, STY; [g g–1 s–1])
= amount of product P produced per unit of time and
unit of catalyst mass
= high conversion, high selectivity, low catalyst mass,
high flow
= industrial viewpoint
R → P + P‘
Catalyst evaluation
• Ecological point of view
• Atom efficiency [%]
= mass of desired product / total mass of all products
• E factor [-]
= mass of all wastes (solvents, gases, unreacted
precursors, byproducts) / mass of desired products
• EQ factor [-]
= E factor, mass of each waste multiplied by Q
(environmental unfriendliness), e.g. Q = ~0 for water,
~ 1 for NaCl, and ~1000 for toluene…
Catalyst evaluation
• Ecological point of view
• Atom efficiency [%], E factor [-], EQ factor [-]
1/2 O2
Ag/Al2O3
SiO2.TiO2
Gas phase, flow mode
Liquid phase, toluene as
solvent, batch mode
O
NH2
+
HN
HO
HO
NH
+
III IV
HW1. Aminolysis of styrene oxide over Zr catalyst, in
toluene, batch, 40 °C
Rxn mixture, time 0
1 Zr : 250 styrene oxide : 3000 toluene; aniline in excess
Rxn mixture, time = 10 min, 1H NMR analysis – ratio of integrated
areas, protons in red/blue circle
1 Product III : 0.01 Product IV : 19.4 styrene oxide : 777 toluene (CH3
group integration)
Calculate conversion, TOF [h−1], selectivity to product IV.
What is the fraction of the (by-)products we did not analyze?