Cracking Fundamentals

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Thermal Cracking Fundamentals

M. T. Klein Outline

1. Elementary Steps 2. Alkyl Aromatics 3. Alkyl Naphthenics 4. Hydroaromatics 5. Conclusions

Thermal Cracking Fundamentals 1 © Michael T. Klein et al.

Cracking Fundamentals

CATALYTIC: Carbenium Ion Stability Controls

THERMAL: Free Radical Stability Controls

Thermal Cracking Fundamentals 2 © Michael T. Klein et al. Thermal Cracking: Elementary Steps

1. Bond Fission R - R' R• + R'• Primary 2. Hydrogen Transfer Radical Allowed R• + R'H  RH + R'• R' • 3.  Scission R + • Can Continue  Bond 4. Radical Recombination/Disproportionation

R• + R'•  R - R' Recombination R + O' Disproportion

Kinetics

- SSA Standard - Long Chains Thermal Cracking Fundamentals 3 -  Scission to 1° Radical OK, Not Great © Michael T. Klein et al.

Pyrolysis Reaction Families

1. Bond Fission: R 1 - R 2  R1• + R 2•

log10 (A/s -1 ) = 16  1, E* = d° (bond strength)

Compound d°/kcal mol -1 t1/2 400°C t1/2 750°C

Ph-Ph 113.7 1.6 x 10 14 y 37y

PhCH 2Ph 89.6 2.4 x 10 6y 2.3h PhCH 2-CH 2Ph 61.4 14h 7.8ms 73.9 19y 3.6s PhCH 2-CH 2CH 2Ph

a half life for homolysis

ThermalPoutsma, Cracking Fundamentals M. L. Energy Fuels, Vol. 4, No. 2, 1990, p. 1. 4 © Michael T. Klein et al. Pyrolysis Reaction Families

2. Hydrogen Abstraction (Transfer): R 1• + RH  R1H + R•

log10 (A/l mol -1 s -1 ) = ~8 E*/kcal mol -1 = ~12-20

log10 k400 = 2-5

Polanyi Relation E* = E* 0 - q

11.5 0.25

Thermal Cracking Fundamentals 5 © Michael T. Klein et al.

Pyrolysis Reaction Families

3.  - scission:

 bond • • . . . +

-1) = ~ log10 (A/s 14 E*/kcal mol-1= 20-30

log10k400 = 2-5 Thermal Cracking Fundamentals 6 © Michael T. Klein et al. Pyrolysis Reaction Families

4. Radical Recombination: R 1• + R 2•  R1 - R 2

-1 -1 log10 (A/l mol s ) ~ 9.5  1

E*/kcal mol -1 ~ 0

log10 k400 ~ 9.5  1

Thermal Cracking Fundamentals 7 © Michael T. Klein et al.

Pyrolysis Reaction Families

5. Radical Disproportionation: R 1• + R 2• R1H + O 2

6. Radical Hydrogen Transfer:

H Ar CH 2 • + Ar CH Ar H 2  + • H

• Ar CH 2 + 7. Isomerization (1,5 Shift):

H • •

Thermal Cracking Fundamentals 8 © Michael T. Klein et al. Summary of Key Points

Alkyl Aromatics log10 AE* log10 k400 (s-1 orl/mols)

Fission 16  1 d° (68-69) -7 to -10 Hydrogen 8 12 2-5 Abstraction

 Scission 14 20-30 2-5 Term 9.5  1 08

Thermal Cracking Fundamentals 9 © Michael T. Klein et al.

Heteroatoms

1. Replacement of C with N and O leads to similar pathways, faster kinetics

2. Replacement with S leads to new paths, faster kinetics

Thermal Cracking Fundamentals 10 © Michael T. Klein et al. Thermal Cracking Compound Classes

Hydrocarbons

Alkyl Aromatics Alkyl Naphthenics Hydroaromatics

1: Tridecyl Cyclohexane (TDC) 1: Pentadecyl Benzene (PDB) 1: 2 Ethyl Tetralin (2 ET) 2: Phenyl Dodecane (PDD) 2: Tetralin 3: Butyl Benzene (BB) 3: Methyl Tetralin (MT) 4: 2-Ethyl Naphthalene (2-EN) 5: (2)-(3-phenylpropyl)-Naphthalene (PPN) 6: Dodecyl Pyrene (DDP) 7: 1,3-bis-(1-pyrene) propane (BPP)

Thermal Cracking Fundamentals 11 © Michael T. Klein et al.

1: Pyrolysis of Alkyl Aromatics: Experimental Results

COMPOUND PYROLYZED TEMPERATURE BATCH MAJOR PRODUCTS MINOR PRODUCTS REFS HOLDING TIME

Pentadecyl Benzene 375°C - 450°C 10 - 180 Min. Toluene, n-alkanes C 6 - C14 1 (PDB) Tetradecene, Styrene, o-olefins C 6 - C14 Tridecane 1-Phenyl alkanes

Phenyl Dodecane 400°C 30 - 240 Min. Toluene, 1-Undecene, n-alkanes C 6 - C12 1 (PDD) Styrene, n-Decane o-olefins C 6 - C11 1-Phenyl alkanes

Butyl Benzene 400°C 30 - 210 Min. Toluene, Styrene Ethyl Benzene 1 (BB) Propyl Benzene 2-Ethyl Naphthalene 400°C 15 - 120 Min. 2-Methyl Naphthalene, -- 1 (2-EN) Naphthalene, 2-Ethyl Tetralin

2-(3-Phenylpropyl) 350°C - 425°C 10 - 60 Min. Toluene, 2-isopropyl - 2 Naphthalene 2-Vinyl -Naphthalene, Naphthalene, 1-3 (PPN) Styrene, diphenyl propane 2-Methylnaphthalene

Dodecyl Pyrene 350°C - 425°C 60 - 180 Min. Pyrene, Dodecane, Alkanes C 6 - C12 2 (DDP) Methyl pyrene, -olefins, alkylpyrene Nonane Thermal Cracking Fundamentals 12 © Michael T. Klein et al. Elucidation of Pathways: A PDB Pyrolysis Example (Ref.1)

SELECTIVITY BEHAVIOR PRODUCT INITIAL SELECTIVITY WITH  IN CONVERSION REMARKS

Toluene 0.35  Constant - Primary Product - No Secondary Reactions

1- Tetradecene 0.35  With Conversion - Primary Product - Toluene and Tetradecene form in 1 step - Secondary decomposition

Styrene 0.12  With Conversion - Primary Product - Secondary Reactions

Tridecane 0.12 Constant - Primary Product - No Secondary RXN - Forms with Styrene in 1 step

Ethyl Benzene 0.02 With Conversion - Forms Mostly From Secondary Reactions

Thermal Cracking Fundamentals 13 © Michael T. Klein et al.

PDB Thermolysis at 375°C

Major Products Temporal Variations 0.08

0.07

0.06

TOL 0.05

0.04 Molar Yield 0.03 TET

0.02 TRI

0.01

STY + 0.00 + + + + +

0 20 40 60 80 100 120 140 160 180 Time (minutes) TOL+ STY 1-TET TRI Thermal Cracking Fundamentals 14 © Michael T. Klein et al. PDB Selectivity to Products

0.26 0.24 0.22 0.20

0.18 C13 0.16 0.14 0.12 0.10 0.08 0.06 STY Selectivity (mol yield/conv) 0.04 0.02 0.00

0.0 0.2 0.4 0.6

Conversion

Thermal Cracking Fundamentals 15 © Michael T. Klein et al.

PDB Selectivity to Products

0.60

0.50

0.40 TOL

0.30

0.20

1 TET Selectivity (mol yield/conv) Selectivity (mol 0.10

0.00

0.0 0.2 0.4 0.6 Conversion

Thermal Cracking Fundamentals 16 © Michael T. Klein et al. A Concerted Mechanism

CH2 CH2 CH H 2 CH2 CH H CH C12H 25 H C12H 25

CH 3 CH2 CH2 H + H CH C12H 25

Thermal Cracking Fundamentals 17 © Michael T. Klein et al.

A Radical Mechanism

• • () + () 13 11

• () () + 13 + 13

• () 13 + () • 10 • • () 11 + () 10

• • () + 13 Products

Thermal Cracking Fundamentals 18 © Michael T. Klein et al. Comparison of Mechanisms

Observed Radical Concerted

First Order Yes Yes TOL = 1-TET Yes Yes

STY = TRI Yes ---

No Phenylbutene Yes ---

High Yield C 14 Yes --- High TOL Yield No Yes

Thermal Cracking Fundamentals 19 © Michael T. Klein et al.

PDB Thermolysis at 400°C

Deuterium Incorporation 1.0

0.9

0.8

0.7

0.6

0.5

0.4

Deuterium Incorporation 0.3

0.2

0.1

0.0

0.0 0.2 0.4 0.6 0.8 1.0 1/R (moles PDB/mole d12-tetralin)

Thermal Cracking Fundamentals 20 © Michael T. Klein et al. "Lumping" via Elementary Steps: A PDB Pyrolysis Example

• Three Parallel Chains: Facility of H-abstraction vs. -Scission

1: Highly Facile H-abstraction

•  13 + •

(Resonance Stabilized) Styrene tridecane - Radical 2: Highly Facile -Scission

• •  11 +

(Resonance tetradecene Stabilized) 3: Minor Pathways Thermal Cracking Fundamentals 21 © Michael T. Klein et al.

Chain Propagation Steps for PDB (R) Pyrolysis

k11 1 + R  1H + 1 k1 k12 1  1 + Q 1 k21 1 + R  1H + 2 2 + R 2H + 1 k' k' 12 k 21 1 + R  R + 2 22 2 + R  R + 1 2 + R 2H + 2 k2 2  + Q 2 k23 2 k32 2 + R  2H + 3 3+ R 3H + 2 k'23 k'32 2 + R  R + 3 k33 3 + R  R + 2 3 + R 3H + 3 k3 k 3 3 + Q 3 31 k13  + R   H +  3 3 1 1+ R 1H + 3 k' 31 k'13  + R  R +  3 1 1 + R  R + 3

Thermal Cracking Fundamentals 22 © Michael T. Klein et al. Reaction Model and Experimental Results: Points of Comparison

• Kinetics

- Pseudo-First-Order Rate Constant

• Selectivity

dTOL dSTY - dt = 1kR dt = 2kR - k 2STY

so for primary pyrolysis

dTOL 1 = dSTY 2

Thermal Cracking Fundamentals 23 © Michael T. Klein et al.

PDB Concentration (mol/l)

-4 10 Apparent Rate Constant (1/s)

-5 10

-3 0 1 10 10 -2 10 -1 10 10 PDB Concentration (mol/l) Thermal Cracking Fundamentals 24 © Michael T. Klein et al. PDB Concentration

7   1 / 2 (expt)

6 dTOL/dSTY (model)

4 dTOL dSTY =

1 2 3  

10-3 10-2 10-1 100 101

PDB Concentration (mol/l)

Thermal Cracking Fundamentals 25 © Michael T. Klein et al.

PDD Thermolysis Pathway

k1 () MINOR 10 1 + () +  2 + +  PRODUCTS 8 () 7 3

k 2 k3 k4

Thermal Cracking Fundamentals 26 © Michael T. Klein et al. Free-Radical Pyrolysis of PDD

1.0

0.9

0.8

0.7

0.6

D 0.5

0.4

0.3

0.2

0.1

0.0 0.0 0.2 0.4 0.6 0.8 1.0

12 Thermal Cracking Fundamentals1/R (Moles PDD/Mole 27 tetralin - d ) © Michael T. Klein et al.

Variation of k with Alkyl Chain Length

kIkHk1/2 k =    kT 

kIk'Hk1/2 k =   C1/2  kT 

where C is the carbon number

Alkylbenzene Pyrolysis at 400°C -1.0 -1.5 -2.0

-2.5

o -3.0

L -3.5 -4.0 -4.5 -5.0 048121620 24 Carbon Number in Alkyl Chain Thermal Cracking Fundamentals 28 © Michael T. Klein et al. Influence of Ring Size

1 • 2-EN Pathway:

  +  + 

1 Savage and Klein Thermal Cracking Fundamentals 29 © Michael T. Klein et al.

Influence of Ring Size

• DDP 2 Pathway:

Methyl Vinyl Pyrene Pyrene

1-Undecene decane [1] + + + + minor products

Pyrene

[2] Dodecane +

2 Savage et al. Thermal Cracking Fundamentals 30 © Michael T. Klein et al. PPN Pyrolysis Pathway

2-VINYL 2-METHYL PPN NAPHTHALENE STYRENE NAPHTHALENE TOLUENE

 + + 2 +

Thermal Cracking Fundamentals 31 © Michael T. Klein et al.

BPP (1,3-bis-(1-pyrene) propane) Pyrolysis Pathway

BPP

Very Fast

Thermal Cracking Fundamentals 32 © Michael T. Klein et al. Pyrolysis of Alkyl Naphthenics : Experimental Results

Pyrolysis of Alkyl Naphthenics : Experimental Results

COMPOUND BATCH PYROLYZED TEMPERATURE HOLDING TIME MAJOR PRODUCTS MINOR PRODUCTS REFS

Tridecyl - 400°C 10-120 Min. Cyclohexane, n-alkanes, Cyclohexane n-dodecane, -olefins (C6 - C13), 1 Methylene - Alkyl Cyclohexanes, Cyclohexane, Cyclohexylalkenes 1-tridecene

Thermal Cracking Fundamentals 33 © Michael T. Klein et al.

Elucidation of Pathways: A TDC Pyrolysis Example (Ref.1)

SELECTIVITY VS PRODUCT INITIAL-SELECTIVITY CONVERSION REMARKS

Cyclohexane 0.08 ~ Constant - Primary Product - No Secondary RXNS

n-Tridecene ~ 0.08  With Conversion - Primary Product - Thermally Unstable - Forms in 1 step with Cyclohexane

Methylene Cyclohexane ~ 0.12  With Conversion - Primary Product - Undergoes Secondary RXN

Dodecane ~ 0.12 ~ Constant - Primary Product - Thermally Stable - Forms in 1 step with Meth-Cyclohexane

Substituted Cyclohexanes 0.038 - 0.015 -- - Minor Products

0.14

0.12

0.10 Cyclohexane

0.08

0.06 Selectivity Tridecene 0.04

0.02

0.00 0.00.1 0.2 0.3 0.4

0.14

0.12

0.10 Dodecane 0.08

0.06 Selectivity

0.04 Methylene Cyclohexane 0.02

TDC Thermolysis Pathway

() OTHER 11 1 + () +  10 2 + +  PRODUCTS () 9 3

• Pericyclic and Molecular Mechanisms Ruled Out: Saturated Hydrocarbon

• Free Radical Mechanism:

Lumping in 3-Parallel Chains

1. Facile H-Abstraction: • • C H + 10 21

(tertiary) 2. Facile -Scission

• • C H + 10 21 (-Radical) (Secondary) 3. Minor Pathways

Kinetics of Alkyl Naphthenics Pyrolysis

COMPOUND k PSEUDO (MIN -1) PYROLYZED TEMPERATURE (°C) REMARKS REF

Tridecyl Cyclohexane 400°C 0.0025 log10 [A] = 14.9 1 E = 59.4

Methyl Cyclohexane 425°C 2.3 x 10-4 3

Propyl Cyclohexane 425°C 0.0014 3

Butyl Cyclohexane 425°C 0.0021 3

COMPOUND BATCH PYROLYZED TEMPERATURE HOLDING TIME MAJOR PRODUCTS MINOR PRODUCTS REF.

2-Ethyl Tetralin 375°C - 450°C 10 - 120 Min. Naphthalene, Toluene, Methylidan, 1 1,2-Dialin, 2-Ethyl- Substituted Naphthalene, tetralin, Benzenes 2 Ethyl Dialin

2-Methyl Tetralin 797°C - 890°C Naphthalene, 1,2- -- 4 Methyl Naphthalene, Indene, Styrene, Benzo cyclobutene

Tetralin 450°C 60 - 300 Min. Naphthalene, 1,2 Indene, Decalin, 5 dihydronaphthalene Toluene, 1-Methylindan, Ethylbenzene, n-Butyl Benzene Styrene, Propylbenzene

2ET Pyrolysis at 375°C: Temporal Variation of Major Products

2.6

2.4 2-Et-Dialins

2.2 2-Et-Naphthalene Z Z 2.0 Z 1.8 B

1.6 Z B Z

1.4 Z F B Naphthalene Z Molar Yield 1.2 F

1.0 B B 0.8 F F H 0.6 F H Dialin B F H H F H B 0.4 H H 1 0.2 Tetralin 1 1 1 1 1 0.0 1 0 20 40 60 80 100 120 Time (Minutes) Thermal Cracking Fundamentals 40 © Michael T. Klein et al. 2ET Pyrolysis at 450°C: Temporal Variation of Major Products

100

80 H H 2ET 60 H

H 40

Molar Yield H B B 20 Naphthalene B B B 0 0204060

8.0

Tetralin B 6.0 H Dialin B B 2-Et-Naphthalene 4.0 H H J J B Molar Yield J H 2.0 B H

0.0 02040 60

Elucidation of Pathways: 2-ET (Ref. 1)

Initial S.N. Product Selectivity Remarks

1: Naphthalene ~0.4 - Major Product - Forms via Secondary RXNs 2: Tetralin ~0-0.05 - Forms Primarily via Secondary RXN 3: Dialin ~ high - Primary Product (inaccurate) - Undergoes Secondary RXN Probably to Naphthalene 4: 2-Ethyl Naphthalene ~ high - Probably Rapid Secondary (inaccurate) Formation From Decomposition [Probably -Nonprimary] of 2-Ethylindans (forms in fast step) 5: 2-Ethyl Dialin high - Primary Product ~ 0.4 - Undergoes Rapid Decomposition Probably to 2-Ethyl Naphthalene

ROP

k 2 k5

k3 k6

k 8 k9

A Free-Radical Mechanism for 2ET Pyrolysis

1a • + • C25 H • 1b + H • • 1c • • R • + 2a + RH • 2b + RH R • = H • = • C2 H 5 2c + RH • • 3 + H • • 4a + • C25 H

4b + H •

• 5 + H •

Concerted = 2 H + H

Free Radical • 1 + 2 • 2 • + H • • 3 + H •

4 + + H • • H 2 5 • + H • + H 2

2-Methyl Tetralin Pyrolysis Pathways: (Ref. 4)

•

• •

•

1. Savage, P.E. "Chemical and Mathematical Modelling of Asphaltene Reaction Pathways," Ph.D. thesis, University of Delaware, December 1986.

2. Javanmardian, M.; Smith, P.J.; Savage, P.E. "Pyrolysis of Compounds Containing Polycyclic Aromatic Moieties," Am. Chem. Soc., Div. Fuel Chem., 1988, 33, 242.

3. Fabuss, B.M.; Kafesjian, R.; Smith, J.O.; Satterfield, C.N. "Thermal Decomposition Rates of Saturated Cyclic Hydrocarbons," Ind. Eng. Chem. Proc. Des. Dev., 3(3), 248, 1964.

4. Trahanovsky, W.S.; Swenson, K.E. "Flash Vacuum Pyrolysis of 2,3-Dialkytetralins," J. Org. Chem., 46, 2984, 1981.

5. Mushrush, G.W.; Stalick, W.M.; Lacy, G.D.; Yaghoubi, R. "Pyrolysis of Tetralin at 450°C," J. Anal. Appl. Pyrolysis, 14, 17-23, 1988.