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Catalyst Selectivity
Synthesis gas applications
CH3OH
CH4
Ni
Cu
H2 / CO
Fe, Co
CnH2n+2
CnH2n
Catalysis and Catalysts - Activity, Selectivity and Stability
Cu + Co
CnH2n+1OH
(n = 1 - 6)
Examples of Catalyst Deactivation
CH3OH
1.0
b
p
= 70 bar
GHSV = 35000 h-1
T
= 515 K
c
FCC
0.8
r (rel)
Methanol Yield (gcm-3h-1)
CO + 2 H2
0.6
0.4
0.2
0.0
Methanol Synthesis
0
3
6
9
12
15
Time (h)
0
500
1000
Time (h)
a
k1.85 (gcm-3h-1%S-0.85)
HDS
S-344 (660 K)
5
S-324 (655 K)
0
0
Catalysis and Catalysts - Activity, Selectivity and Stability
1000
Time (h)
1800
Catalytic Reforming (Gasoline Production)
Conversion (% olefins/initial paraffins)
C12H26
pH
2
pHC
LHSV
T
30
C12H24 + H2
d
= 1.35 bar
= 0.10 bar
= 1 h-1
= 745 K
Deactivation due to
coke deposition
Catalyst Pt (0.2%) / Al2O3
20
+ 0.17% W
+ 0.17% Re
+ 0.04% Ru
10
+ 0.04% Ir
Pt only
0
100
200
Time (h)
Catalysis and Catalysts - Activity, Selectivity and Stability
Alloying quite successful
Time-Scale of Deactivation
10 -1
10 0
10 1
10 2
10 3
10 4
10 5
10 6
10 7
10 8
Hydrocracking
HDS
Catalytic reforming
FCC
Most bulk
processes
0.1-10 year
EO
MA
Formaldehyde
Aldehydes
Hydrogenations
Acetylene
Oxychlorination
C3 dehydrogenation
Batch
processes
hrs-days
Fat hardening
NH3 oxidation
SCR
Time / seconds
-1
10
0
10
TWC
1
10
2
10
3
10
4
10
5
10
1 hour 1 day
Catalysis and Catalysts - Activity, Selectivity and Stability
6
10
7
10
8
10
1 year
Deactivation of catalysts
irreversible loss of activity
Types of deactivation:
• Poisoning:strong chemisorption of impurity in feed
(Inhibition: competitive adsorption, reversible)
• Fouling: secondary reactions of reactants or products,
‘coke’ formation
• Thermal degradation: sintering (loss of surface area),
evaporation
• Mechanical damage
• Corrosion/leaching
Fouling or ‘self-poisoning’ often cause of deactivation
Catalysis and Catalysts - Activity, Selectivity and Stability
Types of Deactivation
S
S
Selective poisoning
Fouling
S
S
Non-selective poisoning
Catalyst
particle
Fine
Attrition
Sintering
= active site
= support
Leaching
Catalysis and Catalysts - Activity, Selectivity and Stability
= component in reaction medium
What are poisons?
Examples
Strong
chemisorber
Bases
• H2S on Ni
• NH3 on Si-Al
‘Toxic compounds’
(free electron pair)
Surface active
metal or ion
• Cu on Ni
• Ni on Pt
• Pb or Ca on Co3O4
• Pb on Fe3O4
High M.W.
product
producer
Sintering
accelerator
• Fe on Cu
• Fe on Si-Al
from pipes
• H2O (Al2O3)
• Cl2 (Cu)
• acetylenes
• dienes
from feed
or product
Catalysis and Catalysts - Activity, Selectivity and Stability
Typical Stability Profiles in Hydrotreating
Initially high rate of deactivation
• mainly due to coke deposition
Subsequently coke in equilibrium
• metal deposition continues
II
activity
coke
metals
Catalysis and Catalysts - Activity, Selectivity and Stability
Time-on-Stream
III
Amount of poisoning
Catalytic activity
I
Influence of Pore Size on Vanadium Deposition
Hydrotreating of Heavy Feedstock
Deposited vanadium
Wide-pore catalyst
Narrow-pore catalyst
Outside
Centre
Outside
Radial position in catalyst pellet
Catalysis and Catalysts - Activity, Selectivity and Stability
Carbon Formation on Supported
Metal Catalyst
Catalysis and Catalysts - Activity, Selectivity and Stability
Carbon Filaments due to CH4 Decompostion
873 K, Ni/CaO catalyst
Catalysis and Catalysts - Activity, Selectivity and Stability
Sintering of Alumina upon Heating
SBET (m2/g)
Sintering
Reduction of surface area
Tcalc (K)
Catalysis and Catalysts - Activity, Selectivity and Stability
Sintering of Supported Catalysts
monomer dispersion
2-D cluster
3-D particle
vapour
particles migrate
surface
coalesce
interparticle transport
migrating
metastable
Dependent on:
• carrier properties
stable
• temperature
• composition of bulk fluid
• ….
Catalysis and Catalysts - Activity, Selectivity and Stability
Predictable?
THüttig and TTamman
Sintering is related to melting
THüttig : defects become mobile
Ttamman: bulk atoms become mobile
Tmelting
THüttig Ttamman
Al2O3
2318
695
1159
Cu
1356
407
678
CuO
1599
480
800
CuCl2
893
268
447
Catalysis and Catalysts - Activity, Selectivity and Stability
Deactivation due to Mechanical Damage
 during transport, storage, packing, use
– loading in barrels, unloading, packing of reactor
– in reactor: weight of column of particles
– attrition in moving systems (fluid beds, moving beds)
 during start-up, shut-down
– temperature variations (thermal shocks)
– chemical transformations
• sulphiding, reduction
• regeneration: high T, steam
Catalysis and Catalysts - Activity, Selectivity and Stability
Corrosion / Leaching - Examples
 Alumina
– dissolves at pH > 12 and pH < 3,
so close to these pH-values corrosion and leaching
– use carbon instead at very low or very high pH
 Sulphiding of oxides in the presence of H2S
 Liquid-phase catalysis
– in heterogenisation of homogeneous catalysts activity was
due to the leached compounds rather than the solid phase
– in solid-catalysed fat hydrogenation traces of the Ni catalyst
appear in the product; with Palladium this is not the case
Catalysis and Catalysts - Activity, Selectivity and Stability
Influence of Deactivation on Reaction Rate
conversion
or
kobs
initial level
process time
kobs  kintr  NT 
‘constant’
‘variable
variable
• blocking of pores
• loss of surface area
Fouling
• loss of active sites
Sintering
Catalysis and Catalysts - Activity, Selectivity and Stability
Poisoning
Deactivation - depends on?
Poisoning
• chemisorption on active sites
• reversible or irreversible
Selective poisons: ‘Modifiers’
• block side reactions
• inhibit consecutive reactions
(kinetics)
feed & process conditions
feed conditions
process conditions
Mechanical deactivation
• loss of catalytic material
due to attrition/abrasion
• loss of surface area due
due to crushing
• irreversible
kobs
Fouling
• physical blockage of surface
by carbon or dust
• usually regenerable
feed & process conditions
process conditions
process conditions
Leaching
• loss of active phase, e.g. by
dissolution in reaction medium
• most common in liquid phase
• often reversible
Catalysis and Catalysts - Activity, Selectivity and Stability
Sintering
• loss of surface area
• gradual or catastrophic
• usually irreversible
Heat
Stability too low; What to do?
 Understand the cause of deactivation
 Take logical measures
– at catalyst level
– sound reactor and process design
– good engineering practice
Catalysis and Catalysts - Activity, Selectivity and Stability
Catalyst Level
 improvement of active phase or support
– e.g. use titania instead of alumina in SCR
 optimisation of texture
– use wide-pore catalyst in HDM to prevent pore blocking
 profiling of active phase
– e.g. egg-yolk profile will protect active sites against
poisoning and fouling if these are diffusion-limited and the
reaction is not
 reduce sintering by structural promoters or stabilisers
 make catalyst more attrition resistant
– encapsulation of active material in porous silica shell
increases attrition resistance without influencing activity
Catalysis and Catalysts - Activity, Selectivity and Stability
Tailored Reactor and Process Design
Relation between time-scale of deactivation and reactor type
Time scale
Typical reactor/process type
years
fixed-bed reactor;
no regeneration
months
fixed-bed reactor;
regeneration while reactor is off-line
weeks
fixed-bed reactors in swing mode, moving-bed reactor
minutes - days
fluidised-bed reactor, slurry reactor;
continuous regeneration
seconds
entrained-flow reactor with continuous regeneration
Catalysis and Catalysts - Activity, Selectivity and Stability
Different Engineering Solutions
allowing for Regeneration
Propane dehydrogenation - deactivation by coke formation
Adiabatic moving-bed reactors (Oleflex)
Parallel adiabatic fixed-bed reactors (Catofin)
Feed
Feed
Regenerator
Fired
heaters
Air
Air
Product
catalyst
Reactors
Reactors in operation
Regeneration circuit
Product
Tubular fixed-bed reactors in furnace (STAR)
Fluidised-bed reactor and regenerator (FBD-4)
Product
Fuel
Air
Flue gas
Flue gas
Regen.
catalyst
Multiple tubular
reactors
Furnace
Steam
Feed
Catalysis and Catalysts - Activity, Selectivity and Stability
Product
Feed
Spent
catalyst
Reactor
Fuel
Air
Regenerator
Good Engineering Practice
 Feed purification for removal of poisons
– upstream reactor
– poison trap inside reactor on top of catalyst (if flow is
downward)
– overdesign of reactor if catalyst itself is poison trap
 Optimisation of reaction conditions
– use of excess steam in steam reforming decreases coke
deposition
– catalyst deactivation in selective hydrogenation of CCl2F2
strongly increases above 500 K  operate below 510 K
 Optimisation of conditions as function of time-on-stream
– compensate for activity loss by increasing T with time
Catalysis and Catalysts - Activity, Selectivity and Stability
Examples
Process
Catalyst
Main deactivation
mechanism
Time scale of
deactivation
Consequences for
catalyst
Regeneration
Consequences for process
FCC
zeolite
Coke
s
Regeneration on s scale
Coke combustion
Recirculation catalyst between
reactor and regenerator
Oxidative
dehydrogenation
various oxides
Coke
s
idem
Catalytic
eforming
Pt/-Al2O3
Coke, Cl loss
months
days
Alloying
Coke combustion
Cl supply
redispersion
Fixed bed, swing operation, moving
bed
Hydrotreating
Co/Mo/S/Al2O3
Coke
metal sulphides
months
days
Once-through catalyst
Adapted porosity
Coke combustion
Fixed bed, slurry, moving bed
Methanol
Cu/ZnO/Al2O3
Sintering (Cl)
y
Stabilization
Feed purification
Water-gas shift
Cu/ZnO/Al2O3
Poisoning (S, Cl)
y
Stabilisers (ZnO)
Feed purification
Three-way
catalyst
Pt, Pd
Sintering, loss of active
components, deposits
(Zn, P from lubricants)
y
Noble metals, stabilized
alumina (La, Ba)
Rejuvenation by
leaching
Steam reforming
Ni/Al2O3
Coke, whiskers
K, Mg gasification
catalysts
Coke combustion
Excess steam
Dry reforming
Ni
coke
S-doping
Coke combustion
Excess steam
Diesel soot
Cu-Cl
evaporation
min , h
Select other catalyst
DeNOx
V2O5/Al2O3
Formation surface salts
months
Select other carrier
Wacker oxidation
Pd, Cu
Catalyst deposit
Xylene oxidation
Co, Mo, Br
Mo,Co deposits
Add new catalyst
Styrene
Iron oxide
Coke, sintering
movement promoters
Structural promoters
Catalysis and Catalysts - Activity, Selectivity and Stability
Similar schemes as in FCC
Add catalytic additives to fuel (Ce)
Low pH
Deposits in reactor and downstream
Coke gasification
in steam