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 08 9.5 1 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 k1 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 k2 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 k3 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) 5 4 dTOL dSTY = 1 2 3 2 1 0 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 I 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 kIkHk1/2 k = kT kIk'Hk1/2 k = C1/2 kT where C is the carbon number Alkylbenzene Pyrolysis at 400°C -1.0 -1.5 -2.0 k -2.5 g 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.