alkane cracking in zeolites -...

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


Outline

◦ Haag-Dassau cracking mechanismLessons to draw from experimental results


Outline


◦ Alkane physisorption on zeolites


Outline


◦ Alkane physisorption on zeolitesWhy is it important?


Outline



◦ Transition structures


Outline



◦ Transition structuresOverview of some ab initio results


Carbocations

alkaniumion

C

R

R

R

R

HH ++

R

C

R

R

R


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 species in zeolites

R–H

alkane



R–H

alkane

alkene

bifu

ncti

onal



R–H

alkane

alkene

bifu

ncti

onal

carbenium

+ HBrønsted acid

+ R



R–H

alkane

alkene

bifu

ncti

onal

carbenium

+ HBrønsted acid

+ R

RH2

alkanium

+ HBrønsted acid

+ +

+



R–H

alkane

alkene

bifu

ncti

onal

carbenium

+ HBrønsted acid

+ R

RH2

alkanium

+ HBrønsted acid

+ +

+

−R’H / H2



R–H

alkane

alkene

bifu

ncti

onal

carbenium

+ HBrønsted acid

+ R

RH2

alkanium

+ HBrønsted acid

+ +

+

−R’H / H2

−HLewis

acid


Cracking mechanisms


Cracking mechanisms

Bimolecular


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R

◦ in mono- and bifunctional catalysts


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R


◦ β-scission chain carrier


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R



◦ does not work in constrainedenvironment


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R




Monomolecular

R1H

alkene

H+

desorption

+

+R2

RH

RH2


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R




Monomolecular

R1H

alkene

H+

desorption

+

+R2

RH

RH2

◦ in monofunctional catalysts


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R




Monomolecular

R1H

alkene

H+

desorption

+

+R2

RH

RH2


◦ cracking or dehydrogenation


Cracking mechanisms

Bimolecular

R1H

R1

beta-scission

RH

alkene

++ R




Monomolecular

R1H

alkene

H+

desorption

+

+R2

RH

RH2


◦ cracking or dehydrogenation

◦ at high T, medium-pore zeolites(ZSM-5)


Monomolecular cracking (Haag-Dessau) mechanism

C

+H H

H3C CH3

CH3



C

+H H

H3C CH3

CH3

H-exchangeH3C

H

C

CH3

CH3 + H+



C

+H H

H3C CH3

CH3

H-exchangeH3C

H

C

CH3

CH3 + H+

CH3

dehydrogenationH3C

CH3

C+ + H2



C

+H H

H3C CH3

CH3

H-exchangeH3C

H

C

CH3

CH3 + H+

CH3

dehydrogenationH3C

CH3

C+ + H2

crackingC +H3C

H

CH3

+ CH4


Product distribution of propane on HZSM-5

Proton attacks on the central carbon atom:

Kwaak, Schachtler, Haag, J. Catal. 149 (1994) 465.




C

CH3

H H

CH3





C

CH3

H H

CH3H

+





C

CH3

H H

CH3H

+

H2 + C3H6

37%





C

CH3

H H

CH3H

+

H2 + C3H6

37%

CH4 + C2H4

63%





C

CH3

H H

CH3H

+

H2 + C3H6

37%

CH4 + C2H4

63%

Almost statistical cleavage of the alkanium ion.



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.




C

CH3

H H

CH3CH3

+

H2 + C4H8

33%





C

CH3

H H

CH3CH3

+

H2 + C4H8

33%

CH4 + C3H6

67%





C

CH3

H H

CH3CH3

+

H2 + C4H8

33%

CH4 + C3H6

67%

Propene and methane formation is more prevalent than isobutene production.



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.




C C CCH

H H H H

H

HHHH

C2H6 + C2H4

17% 15%





C C CCH

H H H H

H

HHHH

C2H6 + C2H4

17% 15%17%20%

CH4 + C3H2





C C CCH

H H H H

H

HHHH

C2H6 + C2H4

17% 15%17%20%

CH4 + C3H2 H2 + C4H8

15% 17%





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.





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.



Monomolecular cracking mechanism: open questions

◦ Activation energy?




◦ Nature of the transition structure(s)?





◦ Multiple reaction channels?






◦ Effect of zeolite framework?






◦ Effect of zeolite framework?

◦ Alternative mechanisms?


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.


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.


n-hexane cracking


Catalyst E‡app

H-ZSM-5 149±8

H-MOR 157±9

H-USY 177±9

CDHY 186±9



n-hexane cracking


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



n-hexane cracking


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



n-hexane cracking



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!



n-hexane cracking



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



n-hexane cracking



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



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 E‡app ∆Hads E‡

true

propane 155 −43 198

n-butane 135 −62 197

n-pentane 120 −74 194

n-hexane 105 −92 197





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...



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.


VASP calculations

◦ DFT with PW91 gradient corrections


VASP calculations


◦ Ultrasoft pseudopotentials for C,H and O


VASP calculations



◦ Cutoff energy 400 eV


VASP calculations




◦ Structural optimizations (residual forces < 0.02 )


VASP calculations





◦ Transition states optimized by using QMPot (Sierka & Sauer) as externaloptimizer


VASP calculations





◦ Transition states optimized by using QMPot (Sierka & Sauer) as externaloptimizer

◦ Order of critical points verified by the calculation of Hessian


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



C

H

O

Al

O

H

C C

C


10 i-C4H10

H1–O 2.97 2.62 3.01 3.25 2.94




C

H

O

Al

O

H

C C

C


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




C

H

O

Al

O

H

C C

C


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




C

H

O

Al

O

H

C C

C


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




C

H

O

Al

O

H

C C

C


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




C

H

O

Al

O

H

C C

C


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



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.


Ethane cracking

215kJ/mol

(195kJ/mol)75kJ/mol


Propane cracking


Propane cracking

180kJ/mol


Propane cracking

180kJ/mol

(190kJ/mol)


Propane cracking

180kJ/mol

(190kJ/mol) 65kJ/mol


Isobutane dehydrogenation

T5 cluster B3LYP/6-31G** and T3 cluster B3LYP/6-311** calculations

Milas and Nascimento Chem. Phys. Lett. 338 (2001) 67



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



∆E‡true(theor) = 190 kJ/mol ∆E‡

true(exp) = 172 kJ/mol


Isobutane dehydrogenation: carbenium intermediate


Isobutane cracking


n-butane: experimental activation energies

Eads= -62kJ/mol




Eads= -62kJ/mol

80kJ/mol

H/D exchange




Eads= -62kJ/mol

80kJ/mol

H/D exchange

115kJ/mol

dehydrogenation




Eads= -62kJ/mol

80kJ/mol

H/D exchange

115kJ/mol

dehydrogenation

135kJ/mol

cracking



Cracking of n-butane: attack on primary C-C bond


Cracking of n-butane: attack on secondary C-C bond




Conclusions

◦ reasonable agreement with available activation energy data


Conclusions


◦ reliable determination of adsorption energies would be needed (dispersionforces)


Conclusions



◦ complete mapping of multiple reaction pathways


Conclusions



◦ complete mapping of multiple reaction pathways

◦ future calculations on “true” catalysts (QM/MM methods)


alkane cracking in zeolites -...

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