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Title: Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers

Abstract

The dominant structural linkage in the abundant renewable energy resource, lignin, is the arylglycerol-{beta}-aryl ether commonly referred to as the {beta}-O-4 linkage. The simplest representation of this linkage, unadorned by substituents, is phenethyl phenyl ether (PhCH{sub 2}CH{sub 2}OPh; PPE). Our prior detailed kinetic studies of the pyrolysis of PPE at 330-425 C showed that a free-radical chain mechanism is involved that cycles through radicals formed at both the {alpha}- and {beta}-carbons. The previously unrecognized competing path involving rearrangement of the {beta}-carbon radical by an O,C-phenyl shift accounts for ca. 25% of the products. In this paper, we explore the effect of aromatic hydroxy and methoxy substituents on both the rate and product selectivity for the pyrolysis of these more complex lignin model compounds. The pyrolysis reactions are conducted in biphenyl solvent at 345 C and low conversions to minimize secondary reactions. The pyrolysis rates were found to vary substantially as a function of the substitution pattern. The rearrangement path involving the {beta}-carbon radical and O,C-phenyl shift was found to remain important in all the molecules investigated, and the selectivity for this path also showed a dependence on the substitution pattern.

Authors:
 [1];  [1];  [1]
  1. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
932084
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Energy & Fuels; Journal Volume: 21
Country of Publication:
United States
Language:
English
Subject:
09 BIOMASS FUELS; AROMATICS; BIPHENYL; CHAINS; ETHERS; KINETICS; LIGNIN; OXYGEN; PHENYL ETHER; PYROLYSIS; RADICALS; SECONDARY REACTIONS; SOLVENTS

Citation Formats

Britt, Phillip F, Kidder, Michelle, and Buchanan III, A C. Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers. United States: N. p., 2007. Web. doi:10.1021/ef700354y.
Britt, Phillip F, Kidder, Michelle, & Buchanan III, A C. Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers. United States. doi:10.1021/ef700354y.
Britt, Phillip F, Kidder, Michelle, and Buchanan III, A C. Mon . "Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers". United States. doi:10.1021/ef700354y.
@article{osti_932084,
title = {Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers},
author = {Britt, Phillip F and Kidder, Michelle and Buchanan III, A C},
abstractNote = {The dominant structural linkage in the abundant renewable energy resource, lignin, is the arylglycerol-{beta}-aryl ether commonly referred to as the {beta}-O-4 linkage. The simplest representation of this linkage, unadorned by substituents, is phenethyl phenyl ether (PhCH{sub 2}CH{sub 2}OPh; PPE). Our prior detailed kinetic studies of the pyrolysis of PPE at 330-425 C showed that a free-radical chain mechanism is involved that cycles through radicals formed at both the {alpha}- and {beta}-carbons. The previously unrecognized competing path involving rearrangement of the {beta}-carbon radical by an O,C-phenyl shift accounts for ca. 25% of the products. In this paper, we explore the effect of aromatic hydroxy and methoxy substituents on both the rate and product selectivity for the pyrolysis of these more complex lignin model compounds. The pyrolysis reactions are conducted in biphenyl solvent at 345 C and low conversions to minimize secondary reactions. The pyrolysis rates were found to vary substantially as a function of the substitution pattern. The rearrangement path involving the {beta}-carbon radical and O,C-phenyl shift was found to remain important in all the molecules investigated, and the selectivity for this path also showed a dependence on the substitution pattern.},
doi = {10.1021/ef700354y},
journal = {Energy & Fuels},
number = ,
volume = 21,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • We report reaction profiles and forward rate constants for hydrogen abstraction reactions occurring in the pyrolysis of methoxy-substituted derivatives of phenethyl phenyl ether (PhCH{sub 2}CH{sub 2}OPh, PPE), where the substituents are located on the aryl ether ring (PhCH{sub 2}CH{sub 2}OPh-X). We use density functional theory in combination with transition-state theory, and anharmonic corrections are included within the independent mode approximation. PPE is the simplest model of the abundant {beta}-O-4 linkage in lignin. The mechanism of PPE pyrolysis and overall product selectivities have been studied experimentally by one of us, which was followed by computational analysis of key individual hydrogen-transfer reactionmore » steps. In the previous work, we have been able to use a simplified kinetic model based on quasi-steady-state conditions to reproduce experimental {alpha}/{beta} selectivities for PPE and PPEs with substituents on the phenethyl ring (X-PhCH{sub 2}CH{sub 2}OPh). This model is not applicable to PPE derivatives where methoxy substituents are located on the phenyl ring adjacent to the ether oxygen because of the strongly endothermic character of the hydrogen abstraction by substituted phenoxy radicals as well as the decreased kinetic chain lengths resulting from enhanced rates of the initial C?O homolysis step. Substituents decelerate the hydrogen abstraction by the phenoxy radical, while the influence on the benzyl abstraction is less homogeneous. The calculations provide insight into the contributions of steric and polar effects in these important hydrogen-transfer steps. We emphasize the importance of an exhaustive conformational space search to calculate rate constants and product selectivities. The computed rate constants will be used in future work to numerically simulate the pyrolysis mechanism, pending the calculation of the rate constants of all participating reactions.« less
  • Using computational methods, we determine product selectivities for the pyrolysis of two model compounds for the beta-O-4 linkage in lignin: phenethyl phenyl ether (PPE) and alpha-hydroxy phenethyl phenyl ether (alpha-hydroxy PPE). We investigate the dependence of the product selectivities on the number of reactant conformers included. Utilizing density functional theory in combination with transition state theory, we obtain rate constants for hydrogen abstraction reactions by the key chain-carrying radicals, which determine the product selectivity within a steady-state kinetic model. The inclusion of the energetically second lowest reactant conformer of PPE and alpha-hydroxy PPE has a large effect on the productmore » selectivity. The final product selectivity computed for PPE agrees well with experiment. We find that the alpha-hydroxy substituent affects energetic as well as entropic contributions to the rate constant differently for alternative pathways of hydrogen abstraction and, thereby, significantly alters product distributions.« less
  • Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH2CH2OPh, PPE) is the simplest example representing the dominant -O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen-carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen-carbon bond dissociationmore » enthalpies for PPE and several oxygen substituted derivatives. Since carbon-carbon bond cleavage in PPE could be a competitive initial reaction under high temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen. On the other hand, the carbon-carbon bond dissociation enthalpy shows little variation with different substitution patterns on either phenyl ring.« less
  • Lignin is a potential renewable source of oxygenated chemicals and liquid fuels, its reaction pathways are often studied using model compounds. Phenethyl phenyl ether (PPE; PhCH2CH2OPh)) is the simplest model for the most common -O-4 linkage in lignin. Previously, we developed a computational scheme to calculate the / -product selectivity in the pyrolysis of PPE by systematically exploiting error cancellation in the computation of relative rate constants. The / -selectivity is defined as the selectivity between the competitive hydrogen abstraction reaction paths on the - and -carbon of PPE. We use density functional theory and employ transition state theory wheremore » we include diagonal anharmonic correction in the vibrational partition functions for low frequency modes for which a semi-classical expression is used. In this work we investigate the effect of oxygen substituents (hydroxy, methoxy) in the para-position on the phenethyl-ring of PPE on the / -selectivities. The total / -selectivity increases when substituents are introduced and is larger for the methoxy than the hydroxy substituent. The strongest effect of the substituents is observed for the -pathway of the hydrogen abstraction by the phenoxyl chain carrying radical for which the rate increases. For the -pathway and the abstraction by the R-benzyl radical (R=OH,OCH3) the rate decreases with the introduction of the substituents. These findings are compared with results from recent experimental studies.« less
  • We investigate phenyl shift and subsequent beta-scission reactions for PhCHXCHOPh [X = H, OH], which are part of the pyrolysis mechanism of phenethyl phenyl ether (PPE) and alpha-hydroxy PPE. PPE and its derivatives are model compounds for the most common linkage in lignin, the beta-O-4 linkage. We use density functional theory to locate transition states and equilibrium structures, and kinetic Monte Carlo in combination with transition state theory for kinetic simulations. Oxygen-carbon and carbon-carbon phenyl shift reactions proceed through cyclic intermediates with similar barriers. But, while subsequent beta-scission of the oxygen-carbon shift products proceeds with virtually no barrier, the activationmore » energy for beta-scission of the carbon-carbon shift products exceeds 15 kcal/mol. We found that about 15 % of beta-radical conversion can be attributed to carbon-carbon shift for PPE and alpha-hydroxy PPE at 618 K. Whereas the oxygen-carbon shift reaction has been established as an integral part of the pyrolysis mechanism of PPE and its derivatives, participation of the carbon-carbon shift reaction has not been shown previously.« less