Deactivation

Catalysis and Catalysts - Activity, Selectivity and Stability

Activity, Selectivity, Stability

� Which property most important?� In process development, which one is easily measured?


Catalyst SelectivitySynthesis gas applications

CH4 CH3OH

CnH2n+2CnH2n

CnH2n+1OH(n = 1 - 6)

H2 / CO

Ni Cu

Cu + CoFe, Co


Examples of Catalyst Deactivation

5

00 1000 1800

Time (h)

k1.85

(g⋅c

m-3

⋅h-1

⋅%S-0

.85 )

S-344 (660 K)

S-324 (655 K)

aHDS

0 500 1000Time (h)

Met

hano

l Yie

ld (g

⋅cm

-3⋅h

-1) p

GHSV T

= 70 bar= 35000 h-1

= 515 K

bCO + 2 H2 CH3OH

Time (h)

0 3 6 9 12 15

1.0

0.8

0.6

0.4

0.2

0.0

r(re

l)

cFCC

Methanol Synthesis


Catalytic Reforming (Gasoline Production)

0 100 200

30

20

10

Con

vers

ion

(% o

lefin

s/in

itial

par

affin

s)

Time (h)

+ 0.17% W

+ 0.17% Re

+ 0.04% Ru

+ 0.04% Ir

Pt only

pHpHCLHSVT

= 1.35 bar= 0.10 bar= 1 h-1

= 745 K

2

C12H26 C12H24 + H2

Catalyst Pt (0.2%) / Al2O3

d

Deactivation due to coke deposition

Alloying quite successful


Time-Scale of Deactivation

10 -1 10 0 101 102 10 3 10 4 10 5 106 107 108

HydrocrackingHDS

Catalytic reformingEO

Hydrogenations AldehydesAcetylene

Oxychlorination

MAFormaldehyde

NH3 oxidationSCR

Fat hardening

Time / seconds TWC

10-1 100 101 102 103 104 105 106 107 108

1 year1 day1 hour

C3 dehydrogenation

FCCMost bulk processes0.1-10 year

Batch processeshrs-days


Deactivation of catalystsirreversible 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


Types of Deactivation

Sintering

Fouling

Non-selective poisoning

Selective poisoning

SS

S S

= active site= support= component in reaction mediumLeaching

Attrition

FineCatalystparticle


What are poisons?

Surface activemetal or ion

High M.W.product producer

Sinteringaccelerator

• Cu on Ni• Ni on Pt

• Pb or Ca on Co3O4• Pb on Fe3O4

• Fe on Cu• Fe on Si-Alfrom pipes

• acetylenes• dienes

• H2O (Al2O3)• Cl2 (Cu)

from feedor product

Strongchemisorber

H2S on NiNH3 on Si-Al


Time on Stream

Amou

nt o

f poi

soni

ng

activity

coke

metals

Cat

alyt

ic a

ctiv

ity

I IIIII

Typical Stability Profiles in Hydrotreating

Initially high rate of deactivation

• mainly due to coke deposition

Subsequently coke in equilibrium

• metal deposition continues


Influence of Pore Size on Vanadium DepositionHydrotreating of Heavy Feedstock

Radial position in catalyst pellet

Dep

osite

d va

nadi

um

Narrow-pore catalyst

Wide-pore catalyst

Outside Centre Outside


Carbon Filaments due to CH4 Decompostion873 K, Ni/CaO catalyst


Carbon Formation on Supported Metal Catalyst

CnHm

HH

H2

C

C

C

Metal crystallite SupportGrowingfillament


Sintering of Alumina upon Heating

Tcalc (K)

S BET

(m2 /g

)

Sintering

Reduction of surface area


Sintering of Supported Catalysts

particles migrate coalesce

monomer dispersion 2-D cluster 3-D particle

surface

vapour

interparticle transport

metastablemigrating

stable

Dependent on:

• carrier properties

• temperature

• composition of bulk fluid

• ….

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

Tmelting THüttig Ttamman

Pt 2028 608 1014

PtO 823 247 412

PtCl2 854 256 427

Rh 2258 677 1129

Rh2O3 1373 312 687

Purification (Cl removal) required in Water-Gas-Shift (supported Cu)??Sintering to be expected in three way-catalysis (supported Pt-Rh) ??


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


Corrosion / Leaching

� 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 often 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


Influence of Deactivation on Reaction Rate

conversionorkobs

process time

η⋅⋅= Tintrobs Nkk

‘constant’ ‘variable variable

• blocking of pores• loss of surface area

• loss of active sitesFouling

Sintering

Poisoning

initial level


Deactivation - depends on?

Sintering• loss of surface area• gradual or catastrophic• usually irreversible

process conditions

Fouling• physical blockage of surface by carbon or dust• usually regenerable

feed & process conditions

Poisoning• chemisorption on active sites• reversible or irreversible

feed conditions

Leaching• loss of active phase, e.g. by dissolution in reaction medium• most common in liquid phase• often reversible

process conditions

process conditionsMechanical deactivation• loss of catalytic material due to attrition/abrasion• loss of surface area due due to crushing• irreversible

kobs

feed & process conditions

Selective poisons: ‘Modifiers’• block side reactions• inhibit consecutive reactions (kinetics)

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


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


Tailored Reactor and Process Design

Time-scale of deactivation dominant

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


Different Engineering Solutions allowing for Regeneration

Adiabatic moving-bed reactors (Oleflex)

Tubular fixed-bed reactors in furnace (STAR)

Feed

Product

catalystReactors

Regenerator

Firedheaters Air

FuelAir

Steam

Flue gas

Feed

Product

Multiple tubularreactors

Furnace

Feed

Product

Reactors in operation

Air

Parallel adiabatic fixed-bed reactors (Catofin)

Fluidised-bed reactor and regenerator (FBD-4)

Spentcatalyst

Regen.catalyst

Product

Feed

Flue gas

Air

Fuel

RegeneratorReactor

Regeneration circuit

Propane dehydrogenation - deactivation by coke formation


Good Engineering Practice

� Feed purification for removal of poisons– upstream reactor– poison trap inside reactor on top of catalyst – 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


ExamplesProcess Catalyst Main deactivation

mechanismTime scale ofdeactivation

Consequences forcatalyst

Regeneration Consequences for process

FCC zeolite Coke s Regeneration on s scale Coke combustion Recirculation catalyst betweenreactor and regenerator

Oxidative dehydrogenation

various oxides Coke s idem Similar schemes as in FCC

Catalyticreforming

Pt/γ-Al2O3 Coke, Cl loss monthsdays

Alloying Coke combustionCl supplyredispersion

Fixed bed, swing operation, movingbed

Hydrotreating Co/Mo/S/Al2O3 Cokemetal sulphides

monthsdays

Once-through catalystAdapted 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 activecomponents, deposits(Zn, P from lubricants)

y Noble metals, stabilizedalumina (La, Ba)

Rejuvenation byleaching

Steam reforming Ni/Al2O3 Coke, whiskers K, Mg gasificationcatalysts

Coke combustion Excess steam

Dry reforming Ni coke S-doping Coke combustion Excess steam

Diesel soot Cu-Cl evaporation min , h Select other catalyst Add catalytic additives to fuel (Ce)

DeNO x V2O5/Al2O3 Formation surface salts months Select other carrier

Wacker oxidation Pd, Cu Catalyst deposit Low pH

Xylene oxidation Co, Mo, Br Mo,Co deposits Add new catalyst Deposits in reactor and downstream

Styrene Iron oxide Coke, sinteringmovement promoters

Structural promoters Coke gasificationin steam

Deactivation

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