Reaction Selectivity
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Transcript of Reaction Selectivity
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Catalysis Engineering - Selectivity
Reaction selectivity
1. Parallel reactions
2. Series reactions
BA
C
A B C
3. Independent reactions A BP Q
Irreversible !
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
Selectivity series reactions
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)
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Catalysis Engineering - Selectivity
Selectivity series reactions
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
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Catalysis Engineering - Selectivity
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?
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
Temperature effects
Yield depends on S or S
=
RT
EE
k
kS undesadesa
undes
des ,,exp
largest Ea dominates
undesadesa
undesadesa
EE
EE
,,
,,
favourableunfavourable
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Catalysis Engineering - Selectivity
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?
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - 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
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Catalysis Engineering - Selectivity
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)
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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 !
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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)
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Catalysis Engineering - Selectivity
Multicomponent adsorption / inhibition
Langmuir adsorption
( ) A AA i iK p
K p K p=
+ +1
11
Inhibitors / Competitors
Control adsorption/desorption !
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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
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Catalysis Engineering - Selectivity
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