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 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
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
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
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.
Alkane cracking in zeolites JIN Janos Angyan
VASP calculations
◦ DFT with PW91 gradient corrections
◦ Ultrasoft pseudopotentials for C,H and O
Alkane cracking in zeolites JIN Janos Angyan
VASP calculations
◦ DFT with PW91 gradient corrections
◦ Ultrasoft pseudopotentials for C,H and O
◦ Cutoff energy 400 eV
Alkane cracking in zeolites JIN Janos Angyan
VASP calculations
◦ DFT with PW91 gradient corrections
◦ Ultrasoft pseudopotentials for C,H and O
◦ Cutoff energy 400 eV
◦ Structural optimizations (residual forces < 0.02 )
Alkane cracking in zeolites JIN Janos Angyan
VASP calculations
◦ DFT with PW91 gradient corrections
◦ Ultrasoft pseudopotentials for C,H and O
◦ Cutoff energy 400 eV
◦ Structural optimizations (residual forces < 0.02 )
◦ Transition states optimized by using QMPot (Sierka & Sauer) as externaloptimizer
Alkane cracking in zeolites JIN Janos Angyan
VASP calculations
◦ DFT with PW91 gradient corrections
◦ Ultrasoft pseudopotentials for C,H and O
◦ Cutoff energy 400 eV
◦ Structural optimizations (residual forces < 0.02 )
◦ Transition states optimized by using QMPot (Sierka & Sauer) as externaloptimizer
◦ Order of critical points verified by the calculation of Hessian
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94H1–C1 1.23 1.20 1.20 1.25 1.14H1–C2 1.25 1.32 1.33 1.27 1.58C1-C2 1.96 2.08 2.12 2.47 2.47
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94H1–C1 1.23 1.20 1.20 1.25 1.14H1–C2 1.25 1.32 1.33 1.27 1.58C1-C2 1.96 2.08 2.12 2.47 2.47H2–C2 1.09 1.11 1.10 1.10 1.10H2–O’ 2.23 2.14 3.44 2.31 2.79
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94H1–C1 1.23 1.20 1.20 1.25 1.14H1–C2 1.25 1.32 1.33 1.27 1.58C1-C2 1.96 2.08 2.12 2.47 2.47H2–C2 1.09 1.11 1.10 1.10 1.10H2–O’ 2.23 2.14 3.44 2.31 2.79Al–O 1.74 1.74 1.74 1.73 1.73Al–O’ 1.73 1.74 1.72 1.73 1.73
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94H1–C1 1.23 1.20 1.20 1.25 1.14H1–C2 1.25 1.32 1.33 1.27 1.58C1-C2 1.96 2.08 2.12 2.47 2.47H2–C2 1.09 1.11 1.10 1.10 1.10H2–O’ 2.23 2.14 3.44 2.31 2.79Al–O 1.74 1.74 1.74 1.73 1.73Al–O’ 1.73 1.74 1.72 1.73 1.73E‡true,theor 215 180 185 155 150∆Eads 30 40 50 50 48E‡app,theor 185 140 135 105 102
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Transition states in chabazite optimized by VASP
C
H
O
Al
O
H
C C
C
bond C2H6 C3H8 n-C4Ha10 n-C4Hb
10 i-C4H10
H1–O 2.97 2.62 3.01 3.25 2.94H1–C1 1.23 1.20 1.20 1.25 1.14H1–C2 1.25 1.32 1.33 1.27 1.58C1-C2 1.96 2.08 2.12 2.47 2.47H2–C2 1.09 1.11 1.10 1.10 1.10H2–O’ 2.23 2.14 3.44 2.31 2.79Al–O 1.74 1.74 1.74 1.73 1.73Al–O’ 1.73 1.74 1.72 1.73 1.73E‡true,theor 215 180 185 155 150∆Eads 30 40 50 50 48E‡app,theor 185 140 135 105 102E‡app,exp 160 130 150 135 120
(a) primary C–C bond(b) secondary C–C bond
Alkane cracking in zeolites JIN Janos Angyan
Ethane cracking
T5 cluster calculations at MP2/6-31G(d) and BLYP/6-31G(d) level
Barrier in kJ/molMP2/6-31G(d) 308.6ZPE -8.4thermal effects -4.6long range effects -60.7total 226.5experimental 190-200
Zygmunt, Curtiss, Zapol and Iton, J. Phys., Chem. B 104 (2000) 1944.
Alkane cracking in zeolites JIN Janos Angyan
Isobutane dehydrogenation
T5 cluster B3LYP/6-31G** and T3 cluster B3LYP/6-311** calculations
Milas and Nascimento Chem. Phys. Lett. 338 (2001) 67
Alkane cracking in zeolites JIN Janos Angyan
Isobutane dehydrogenation
T5 cluster B3LYP/6-31G** and T3 cluster B3LYP/6-311** calculations
Carbocation collapses directly, without alkoxide formation
Activation energy: 223.5 kJ/mol (exp.: 172 ±6 kJ/mol)
Milas and Nascimento Chem. Phys. Lett. 338 (2001) 67
Alkane cracking in zeolites JIN Janos Angyan
Isobutane dehydrogenation
∆E‡true(theor) = 190 kJ/mol ∆E‡
true(exp) = 172 kJ/mol
Alkane cracking in zeolites JIN Janos Angyan
Isobutane cracking
∆E‡true(theor) = 150 kJ/mol ∆E‡
true(exp) = 170 kJ/mol
Alkane cracking in zeolites JIN Janos Angyan
n-butane: experimental activation energies
Eads= -62kJ/mol
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-butane: experimental activation energies
Eads= -62kJ/mol
80kJ/mol
H/D exchange
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-butane: experimental activation energies
Eads= -62kJ/mol
80kJ/mol
H/D exchange
115kJ/mol
dehydrogenation
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
n-butane: experimental activation energies
Eads= -62kJ/mol
80kJ/mol
H/D exchange
115kJ/mol
dehydrogenation
135kJ/mol
cracking
Narbeshuber, Vinek, Lercher J. Catal. A 157 (1995) 338.
Alkane cracking in zeolites JIN Janos Angyan
Cracking of n-butane: attack on primary C-C bond
∆E‡true(theor) = 185 kJ/mol ∆E‡
true(exp) = 200 kJ/mol
Alkane cracking in zeolites JIN Janos Angyan
Cracking of n-butane: attack on secondary C-C bond
∆E‡true(theor) = 155 kJ/mol ∆E‡
true(exp) = 185 kJ/mol
Alkane cracking in zeolites JIN Janos Angyan
Conclusions
◦ reasonable agreement with available activation energy data
Alkane cracking in zeolites JIN Janos Angyan
Conclusions
◦ reasonable agreement with available activation energy data
◦ reliable determination of adsorption energies would be needed (dispersionforces)
Alkane cracking in zeolites JIN Janos Angyan
Conclusions
◦ reasonable agreement with available activation energy data
◦ reliable determination of adsorption energies would be needed (dispersionforces)
◦ complete mapping of multiple reaction pathways
Alkane cracking in zeolites JIN Janos Angyan
Conclusions
◦ reasonable agreement with available activation energy data
◦ reliable determination of adsorption energies would be needed (dispersionforces)
◦ complete mapping of multiple reaction pathways
◦ future calculations on “true” catalysts (QM/MM methods)
Alkane cracking in zeolites JIN Janos Angyan
Conclusions
◦ reasonable agreement with available activation energy data
◦ reliable determination of adsorption energies would be needed (dispersionforces)
◦ complete mapping of multiple reaction pathways
◦ future calculations on “true” catalysts (QM/MM methods)
Alkane cracking in zeolites JIN Janos Angyan
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