ALKANE CRACKING IN ZEOLITES: AN OVERVIEW OF RECENT MODELING RESULTS ALKANE CRACKING IN ZEOLITES: AN OVERVIEW OF RECENT MODELING RESULTS J´ anos ´ Angy ´ an and Drew Parsons Institut f ¨ ur Materialphysik Universit ¨ at Wien Wien, Austria and Laboratoire de Chimie th ´ eorique Universit ´ e Henri Poincar´ e Vandoeuvre–l` es–Nancy Cedex, France Alkane cracking in zeolites J´ anos ´ Angy ´ an
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ALKANE CRACKING IN ZEOLITES:
AN OVERVIEW
OF RECENT MODELING RESULTS
ALKANE CRACKING IN ZEOLITES:
AN OVERVIEW
OF RECENT MODELING RESULTS
Janos Angyan and Drew Parsons
Institut fur MaterialphysikUniversitat Wien
Wien, Austria
and
Laboratoire de Chimie theoriqueUniversite Henri Poincare
Vandœuvre–les–Nancy Cedex, France
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanism
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
◦ Alkane physisorption on zeolites
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
◦ Alkane physisorption on zeolitesWhy is it important?
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
◦ Alkane physisorption on zeolitesWhy is it important?
◦ Transition structures
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
◦ Alkane physisorption on zeolitesWhy is it important?
◦ Transition structuresOverview of some ab initio results
Alkane cracking in zeolites JIN Janos Angyan
Outline
◦ Haag-Dassau cracking mechanismLessons to draw from experimental results
◦ Alkane physisorption on zeolitesWhy is it important?
◦ Transition structuresOverview of some ab initio results
Alkane cracking in zeolites JIN Janos Angyan
Carbocations
alkaniumion
C
R
R
R
R
HH ++
R
C
R
R
R
Alkane cracking in zeolites JIN Janos Angyan
Carbocations
alkaniumion
C
R
R
R
R
HH ++
R
C
R
R
R
carbeniumion
CC
C
R R
HR
R
H
+
+CC
C
R R
R R
Alkane cracking in zeolites JIN Janos Angyan
Carbocations
alkaniumion
C
R
R
R
R
HH ++
R
C
R
R
R
carbeniumion
CC
C
R R
HR
R
H
+
+CC
C
R R
R R
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
carbenium
+ HBrønsted acid
+ R
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
carbenium
+ HBrønsted acid
+ R
RH2
alkanium
+ HBrønsted acid
+ +
+
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
carbenium
+ HBrønsted acid
+ R
RH2
alkanium
+ HBrønsted acid
+ +
+
−R’H / H2
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
carbenium
+ HBrønsted acid
+ R
RH2
alkanium
+ HBrønsted acid
+ +
+
−R’H / H2
−HLewis
acid
Alkane cracking in zeolites JIN Janos Angyan
Alkane species in zeolites
R–H
alkane
alkene
bifu
ncti
onal
carbenium
+ HBrønsted acid
+ R
RH2
alkanium
+ HBrønsted acid
+ +
+
−R’H / H2
−HLewis
acid
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Monomolecular
R1H
alkene
H+
desorption
+
+R2
RH
RH2
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Monomolecular
R1H
alkene
H+
desorption
+
+R2
RH
RH2
◦ in monofunctional catalysts
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Monomolecular
R1H
alkene
H+
desorption
+
+R2
RH
RH2
◦ in monofunctional catalysts
◦ cracking or dehydrogenation
Alkane cracking in zeolites JIN Janos Angyan
Cracking mechanisms
Bimolecular
R1H
R1
beta-scission
RH
alkene
++ R
◦ in mono- and bifunctional catalysts
◦ β-scission chain carrier
◦ does not work in constrainedenvironment
Monomolecular
R1H
alkene
H+
desorption
+
+R2
RH
RH2
◦ in monofunctional catalysts
◦ cracking or dehydrogenation
◦ at high T, medium-pore zeolites(ZSM-5)
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking (Haag-Dessau) mechanism
C
+H H
H3C CH3
CH3
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking (Haag-Dessau) mechanism
C
+H H
H3C CH3
CH3
H-exchangeH3C
H
C
CH3
CH3 + H+
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking (Haag-Dessau) mechanism
C
+H H
H3C CH3
CH3
H-exchangeH3C
H
C
CH3
CH3 + H+
CH3
dehydrogenationH3C
CH3
C+ + H2
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking (Haag-Dessau) mechanism
C
+H H
H3C CH3
CH3
H-exchangeH3C
H
C
CH3
CH3 + H+
CH3
dehydrogenationH3C
CH3
C+ + H2
crackingC +H3C
H
CH3
+ CH4
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking (Haag-Dessau) mechanism
C
+H H
H3C CH3
CH3
H-exchangeH3C
H
C
CH3
CH3 + H+
CH3
dehydrogenationH3C
CH3
C+ + H2
crackingC +H3C
H
CH3
+ CH4
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
C
CH3
H H
CH3
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
C
CH3
H H
CH3H
+
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
C
CH3
H H
CH3H
+
H2 + C3H6
37%
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
C
CH3
H H
CH3H
+
H2 + C3H6
37%
CH4 + C2H4
63%
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of propane on HZSM-5
Proton attacks on the central carbon atom:
C
CH3
H H
CH3H
+
H2 + C3H6
37%
CH4 + C2H4
63%
Almost statistical cleavage of the alkanium ion.
Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of i-butane on HZSM-5
Proton attacks the tertiary carbon atom:
C
CH3
H H
CH3CH3
+
Ono, Kanae, J. Chem. Soc. Faraday Trans. 87 (1991) 663.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of i-butane on HZSM-5
Proton attacks the tertiary carbon atom:
C
CH3
H H
CH3CH3
+
H2 + C4H8
33%
Ono, Kanae, J. Chem. Soc. Faraday Trans. 87 (1991) 663.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of i-butane on HZSM-5
Proton attacks the tertiary carbon atom:
C
CH3
H H
CH3CH3
+
H2 + C4H8
33%
CH4 + C3H6
67%
Ono, Kanae, J. Chem. Soc. Faraday Trans. 87 (1991) 663.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of i-butane on HZSM-5
Proton attacks the tertiary carbon atom:
C
CH3
H H
CH3CH3
+
H2 + C4H8
33%
CH4 + C3H6
67%
Propene and methane formation is more prevalent than isobutene production.
Ono, Kanae, J. Chem. Soc. Faraday Trans. 87 (1991) 663.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
C2H6 + C2H4
17% 15%
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
C2H6 + C2H4
17% 15%17%20%
CH4 + C3H2
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
C2H6 + C2H4
17% 15%17%20%
CH4 + C3H2 H2 + C4H8
15% 17%
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
C2H6 + C2H4
17% 15%17%20%
CH4 + C3H2 H2 + C4H8
15% 17%
In spite of different number of equivalent bonds, each product is formed withthe same probability.
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Product distribution of n-butane on HZSM-5
Proton can attack on three different types of bonds:
C C CCH
H H H H
H
HHHH
C2H6 + C2H4
17% 15%17%20%
CH4 + C3H2 H2 + C4H8
15% 17%
In spite of different number of equivalent bonds, each product is formed withthe same probability.Larger activation entropy for external bonds compensatesfor the smaller activation energy for internal bonds.
Kranilla, Haag, Gates, J. Catal. 135 (1992) 115.
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking mechanism: open questions
◦ Activation energy?
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking mechanism: open questions
◦ Activation energy?
◦ Nature of the transition structure(s)?
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking mechanism: open questions
◦ Activation energy?
◦ Nature of the transition structure(s)?
◦ Multiple reaction channels?
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking mechanism: open questions
◦ Activation energy?
◦ Nature of the transition structure(s)?
◦ Multiple reaction channels?
◦ Effect of zeolite framework?
Alkane cracking in zeolites JIN Janos Angyan
Monomolecular cracking mechanism: open questions
◦ Activation energy?
◦ Nature of the transition structure(s)?
◦ Multiple reaction channels?
◦ Effect of zeolite framework?
◦ Alternative mechanisms?
Alkane cracking in zeolites JIN Janos Angyan
Activation energies
E
ZeOH + C H
transition structure
ZeOH...C H
app
trueEadsE
E
ZeOH + C H
transition structure
ZeOH...C H
app
trueEadsE
n 2n+2
2n+2n
Experimental (apparent) activation energies should be corrected by adsorp-tion energies to obtain intrinsic (true) activation energies.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst
H-ZSM-5
H-MOR
H-USY
CDHY
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app
H-ZSM-5 149±8
H-MOR 157±9
H-USY 177±9
CDHY 186±9
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads
H-ZSM-5 149±8 −86±6
H-MOR 157±9 −69±3
H-USY 177±9 −50±3
CDHY 186±9 −50±3
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads E‡
true
H-ZSM-5 149±8 −86±6 235±14
H-MOR 157±9 −69±3 226±12
H-USY 177±9 −50±3 227±12
CDHY 186±9 −50±3 236±12
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads E‡
true
H-ZSM-5 149±8 −86±6 235±14
H-MOR 157±9 −69±3 226±12
H-USY 177±9 −50±3 227±12
CDHY 186±9 −50±3 236±12
Differences in apparent activation energies are due to adsorption energies!
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads E‡
true
H-ZSM-5 149±8 −86±6 235±14
H-MOR 157±9 −69±3 226±12
H-USY 177±9 −50±3 227±12
CDHY 186±9 −50±3 236±12
Differences in apparent activation energies are due to adsorption energies!◦ intrinsic activation energy insensitive to acid strength
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads E‡
true
H-ZSM-5 149±8 −86±6 235±14
H-MOR 157±9 −69±3 226±12
H-USY 177±9 −50±3 227±12
CDHY 186±9 −50±3 236±12
Differences in apparent activation energies are due to adsorption energies!◦ intrinsic activation energy insensitive to acid strength
◦ acid strengths of these zeolites are identical
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-hexane cracking
Apparent activation energies in different catalysts
Catalyst E‡app ∆Hads E‡
true
H-ZSM-5 149±8 −86±6 235±14
H-MOR 157±9 −69±3 226±12
H-USY 177±9 −50±3 227±12
CDHY 186±9 −50±3 236±12
Differences in apparent activation energies are due to adsorption energies!◦ intrinsic activation energy insensitive to acid strength
◦ acid strengths of these zeolites are identical
Babitz et al. Appl. Catal. A 179 (1999) 71.
Alkane cracking in zeolites JIN Janos Angyan
n-alkane cracking in H-ZSM-5
True activation energies seem to be independent of the chain length
alkane
propane
n-butane
n-pentane
n-hexane
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-alkane cracking in H-ZSM-5
True activation energies seem to be independent of the chain length
alkane E‡app ∆Hads E‡
true
propane 155 −43 198
n-butane 135 −62 197
n-pentane 120 −74 194
n-hexane 105 −92 197
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-alkane cracking in H-ZSM-5
True activation energies seem to be independent of the chain length
alkane E‡app ∆Hads E‡
true ∆Hads E‡true
propane 155 −43 198 -40 195
n-butane 135 −62 197 -50 185
n-pentane 120 −74 194 -60 180
n-hexane 105 −92 197 -71 176
unless one uses another set of adsorption energies...
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-alkane cracking in H-ZSM-5
True activation energies seem to be independent of the chain length
alkane E‡app ∆Hads E‡
true ∆Hads E‡true
propane 155 −43 198 -40 195
n-butane 135 −62 197 -50 185
n-pentane 120 −74 194 -60 180
n-hexane 105 −92 197 -71 176
unless one uses another set of adsorption energies...
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-alkane cracking in H-ZSM-5
True activation energies seem to be independent of the chain length
alkane E‡app ∆Hads E‡
true ∆Hads E‡true
propane 155 −43 198 -40 195
n-butane 135 −62 197 -50 185
n-pentane 120 −74 194 -60 180
n-hexane 105 −92 197 -71 176
unless one uses another set of adsorption energies...
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
Exprimental n-alkane adsorption energies
-140
-120
-100
-80
-60
-40
-20
0
0 2 4 6 8 10
Eads
(kJ/
mol
)
chain length
Vlugt, Krishna, Smit J. Phys. Chem. B 103 (1999) 1102.