reaction selectivity

Catalysis Engineering - Selectivity

Reaction selectivity

1. Parallel reactions

2. Series reactions

BA

C

A B C

3. Independent reactions A BP Q

Irreversible !


Selectivity parallel reactions

BA

C

Rate expressions

Pore diffusion limitations: Highest penalty on the highest order reaction diffusion limitations may or may not be preferred Equal order: no effect on selectivity

Kinetic selectivity:

r k c

r k c

B B An

C C Am

=

=

C

B

k

kS =

Rate selectivity:

LxC

Bapp

C

appB

J

J

r

r

=

=

fluxes out of particle


Shape selectivity: zeolites

Pore size of molecular dimensions: high penalty on branchingor bulkyness

Reactant selectivity

Transition state selectivity

Product selectivity

transition state does not fite.g. cis vs. trans products

in zeolite Beta

branched alkenes cannot enterZSM-5, linear ones can

p-xylene formation ZSM-5DMA formation ZSM-5


Toluene disproportionation

CH3H3C

CH3

H3C

CH3

CH3

CH3

CH3

+

De,rel.

>1000

>1000

>1000

1000

1

1

De,rel.

strong effect diffusion limitationtransition state selectivity?

ZSM-5


Selectivity series reactions

A B CRate expressions

Pore diffusion equations:

r k c

r k c k c

A B A

B B A C B

=

=

First order, equal diffusivities

Dd c

dxk c k c

Dd c

dxk c

eB

C B B A

eA

B A

2

2

2

2

=

=

Solution -> concentration profiles -> particle rates -> production selectivity



Solution concentrations A and B inside slab

Local selectivity equals fluxes through external slab surface:

)cosh(

)/cosh(

)cosh(

)/cosh(

)cosh(

)/cosh(

)cosh(

)/cosh(

C

CbB

B

B

C

C

CB

BbAB

B

BbAA

Lxc

LxLx

kk

kcc

Lxcc

+

=

=

)tanh(

)tanh(

)tanh(

)tanh(1

BBbA

CCbB

BB

CC

CB

B

A

B

LxA

B

c

c

kk

k

dc

dc

J

J

==

=

Integration yields concentration ratio (integrating factor)

local gas phase concentration(no external limitations)



Limiting cases (after integration)

1. Thiele moduli small, chemical control

2. Both Thiele moduli large, tanh -> 1

Diffusion limitation always lowers selectivity intermediateto be avoided !

0.0 0.2 0.4 0.6 0.8 1.0

Fraction A converted

0.0

0.2

0.4

0.6

0.8

F

r

a

c

t

i

o

n

A

c

o

n

v

e

r

t

e

d

t

o

B

1.

2.

=

11

1

0

S

S

bA

bA

bA

bB

c

c

S

S

c

c

=

11

1

0

S

S

bA

bA

bB

bB

c

c

S

S

c

c

at reactor entrance


Series reactions

0.0 0.2 0.4 0.6 0.8 1.0

Fraction A converted

0.0

0.2

0.4

0.6

0.8

F

r

a

c

t

i

o

n

A

c

o

n

v

e

r

t

e

d

t

o

B

1.

2.

maximum shifts to lowerconversion of A

Q: What is maximum yield B? At what conversion of A?


Selectivity independent reactions

Limiting cases:

1. i small -> kinetic control2. i large -> diffusion control

Rate expressionsA BP Q

r k c

r k cB B A

Q Q P

=

=

first order, equal diffusivities

Pore diffusion model for slab gives:

)cosh(

)/cosh(

)cosh(

)/cosh(

Q

QsPP

B

BsAA

Lxcc

Lxcc

=

=

ii

i

=

tanh( )So:

bP

bA

bPP

bAB

appQ

appB

c

cS

ck

ck

r

r==

bP

bA

bPP

bAB

appQ

appB

c

cS

ck

ck

r

r==

diffusion limitations always disfavours desired reaction


0.1 1 10 400.1

1

10

Independent reactions: distribution active phase

Becker & Wei J.Catal. 46(1977)372

Uniform

Eggwhite

Eggyolk

Eggshell

pore mouth poisoninguinform poisoning

pe

pp D

kR

,

=

me

mm D

kR

,

=

Criterion: longest catalyst life with effectiveness of 0.4 or higher


Temperature effects

Yield depends on S or S

=

RT

EE

k

kS undesadesa

undes

des ,,exp

largest Ea dominates

undesadesa

undesadesa

EE

EE

,,

,,

favourableunfavourable


Series reaction exampleVoge and Morgan I&EC PDD11 (1972)454

Butene dehydrogenation: ''21 44 cokeCCkk ===

0 2 4 6 8 10

particle diameter / mm

50

60

70

80

90

100

Selectivity butadieneat 35% conversion

Q:does this result agreewith theory?


Deactivation

Fouling may affect accessibility (decreases Deff )

Homogeneous lowers activityThiele moduli become smaller/larger

Pore mouthselectivity ?

lower selectivity

Poisoning no effect on accessibility

Homogeneous lowers activityThiele moduli become smaller

Pore mouth diffusion resistance increasefastest reaction most affected

equal or better selectivity

lower selectivity


Reaction selectivity

Selectivity

Kinetics

Diffusion limitations

Reaction conditionsIntrinsic selectivityAdsorption

Deactivation

Reactor type Modifier/solvent

By purpose orTo be avoided

HomogeneousPore mouth

Shape effect Zeolite type


Reaction modifiers

Selectivity control through selective adsorptionSeries reactions A --> B --> CAdd component I with:

KA > KI > KB

Reaction stops after conversion of A

Examples

Hydrogenation of ethyne in ethene feed for polymerization (CO, H2S)

Functionalized alkynes to alkenes (N-compounds)(Marieke Spee Utrecht)


Effect aging on three-way catalystsde Soete, IFP

Temperature (K)

~400 ~700

NO conversionto N2O

Aging

Additional catalytic component needed

Desired: 2NO + 2CO N2 + 2 CO2Undesired: 2NO + CO N2O + CO2

Selectivity declines by deactivation:

kinetics effect


1-Butene isomerization: FerrieriteMooiweer et al 1994

carbenium ion formation dimerization

(oligomerization)

skeletal isomerization

diffusion branched isomershighly hindered

cracking to isobutene uponescape from matrix

product shape selectivitymild cracking activity

C C C

C

C C C C C C C+ C

C C+ C C C C

C

C C

C

CH+

C C C+

CC C+ C

C

C

C C C

C


Competitive adsorptionSelective hydrogenation aromatics

S.Toppinen,Thesis 1996

0 2 4 6 8 10

space time / min.g.ml-1

0

5

10

15

20

25

30

c

o

n

c

e

n

t

r

a

t

i

o

n

/

w

t

.

%

CH3

CH3

CH3

CH2CH3

CH3

H3C CH3

Ni-alumina trilobe catalyst3 mm particles40 bar H2125oCsemi-batch reactor

Consecutive conversion behaviourrate constants ~ similaradsorption constants decrease

Propose a rate expression to account for this effect


Competitive series reactionseffect on selectivity

Acetylene Hydrogenation: butyne -> butene -> butane A1 A2 A3

butyne and butene compete for the same sites but: K1 >> K2 resulting high selectivity for butene (desired) possible even when k2 > k1

since: Sk K

k K121 1

2 2, =

Meyer and Burwell (JACS 85(1963)2877) mol%:2-butyne 22.0cis-2-butene 77.2trans-2-butene 0.71-butene 0.0butane 0.1

show this !


Zeolite Beta: Meerwein-Ponndorf-Verleyreduction and Oppenauer oxidation

Creighton, van Bekkum 1996

> 95% cis-isomer (fragrance intermediate) transition state selectivity molecular modelling

4-tert-butylcyclohexanone + isopropanol cis-4-tert-butylcyclohexanol

80 0C

BETAO

CH OH

CH3

CH3

C O

CH3

CH3

OH


Zeolite Beta: Aromatic acylationVan Bekkum

Classical route

Catalytic route

Yield >90%no corrosive by-products

OH

OH

CH3Cl2

CCl3FeCl3

OH

OH

CO

OH

OH

CH3O2

COOH OH

OH

CO

H2O

H-Beta

+cat.

+ 3 HClhv


Partial benzene hydrogenation

Ru-catalyst - clusters of crystallites Slurry reaction, elevated pressures Water-salt addition increases selectivity

+ + 2 H2 H2

RuSalt-water

Adsorption / Desorption properties affected


Reaction steps - Catalyst modification

Adsorption: reactants concentration / positioning /competition

Modifications by support (deep HDS)

catalyst surface (enantiomers)

use other support (CH4 reforming)

change fluid phase

add fluid phase (cyclohexene)

shape selectivity, pore architecture

Desorption:

Facilitate desorption (Ga for H2 )

solvent choice (alkylation)


Multicomponent adsorption / inhibition

Langmuir adsorption

( ) A AA i iK p

K p K p=

+ +1

11

Inhibitors / Competitors

Control adsorption/desorption !


Reaction steps - Catalyst modification

Surface reaction

Affect selectivity by transition state confinement (acylation zeolites)

shape selectivity (membrane coating)

deliberate diffusion limitations (p-xylene, isobutene)

reaction coupling

transient operation


MeOH + NH3 MMA (0.4 nm) MMA DMA (0.45 nm) DMA TMA (0.5 nm)

Catalyst and separationmethyl amines production

ordinary Si-Al catalyst

Carbon molecular sievelayer (~ 0.5 nm pores)

Classical a-selective catalystturned into a selective by a permselective coating

H.C.Foley et al. Chem. Eng. Sci. 49(1994)4771


Composite catalyst: FCCmultifunctional

macropores mesopores zeolitic pores

metal-porphyrins

low aciditymedium acidity

high acidity

Ni

residLCO/HCO

bulk

gasoline/LPG

catalyst designpore structurecatalytic functionsstabilizationstrengthmetal catchers


Alkylation of isobutane with 2-butenecatalyst: Beta

K.P.de Jong et al., ISCRE 1996

Reaction

Side reactions

Dimerization/oligomerizationProduct alkylation

Fouling, deactivation

70-120oC+CC

C

C

C

C

C

C

CC

C

C C

C

C

C

Low olefin concentrationCSTR operationSuitable solvent medium


Isomerization alkanes n=5,6,7

Bifunctional catalyst: H- Mordenite catalyst (500-550K) Pt doping

%

c

o

n

v

e

r

s

i

o

n

time (h)

0

100

H-Mor

H-Mor + Pt

zeolite catalysed isomerizationhydrogenation to prevent coking

Reaction Selectivityparallel reactionszeolitesToluene disproportionationseries reactionsgeneral solutionlimiting casesselectivity and conversion

independent reactionsdistribution active phase; poisoniongtemperature effectsseries reaction exampledeactivationselectivity and other catalyst aspectsreaction modifiersaging three way catalyst1-butene isomerizationcompetitive adsorptioncompetitive series reactionsZeolite BetaMPV reduction and Oppendauer oxidationAromatic acylation

partial benzene hydrogenationcatalyst modificationadsorption/desorptionmulticomponent adsorption/inhibitionsurface reactionmethyl amines productionFCC catalystAlkylation of isobutane with 2-buteneisomerization alkanes n=5,6,7

reaction selectivity

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