Kinetic Analysis of the Pyrolysis of Phenethyl Phenyl Ether:  Computational Prediction of α/β-Selectivities

Abstract

We calculated an overall alpha/beta-selectivity for the pyrolysis of phenethyl phenyl ether as a composite of the alpha/beta-selectivities in the hydrogen abstraction reactions by the phenoxyl and by the benzyl radical that is in excellent agreement with experiment. The difference between the individual selectivities for these radicals is explained by analyzing the electronic structure of the transition states. Spin delocalization of the single electron favors the alpha-pathways. An opposing effect occurs for polarized transition states, such as the transition states for the hydrogen abstraction by the electrophilic phenoxyl radical, where the adjacent ether oxygen in phenethyl phenyl ether stabilizes the beta-transition states. These results indicate that theory will be able to provide excellent predictions of alpha/beta-product selectivities for more complicated lignin model compounds bearing multiple substituents. We have developed a scheme to predict alpha/beta-product selectivities in the pyrolysis of model compounds for the beta-ether linkage in lignin. The approach is based on computation of the relative rate constant, which profits from error cancellation in the individual rate constants. The Arrhenius prefactors depend strongly on the description of the low-frequency modes for which anharmonic contributions are important. We use density functional theory in combination with transition-state theory in this analysis. Diagonal anharmonic effects for individual low-frequency modes are included by employing a second-order Wigner-Kirkwood expansion in a semiclassical expression for the vibrational partition function. The composite alpha/beta-product selectivity is obtained by applying quasi-steady-state kinetic analysis for the intermediate radicals.

Lignin pyrolysis reactions

“…1 Thermogravimetric analysis of milled wood lignin isolated from Japanese cedar (Cryptmeria japonica) [9] understanding the pyrolytic reactions of lignins. Hence, various model dimers including β-ether [20,27,28,34,[40][41][42][43][44][45][46][47][48][49][50][51][52][53], α-ether [20,30,34,45,47,49,51,54], β-aryl [20,31,32,47,54], β-β [33], and biphenyl [20,47,54] types, trimers [34,49,51] and oligomers/polymers such as dehydrogenative polymerization products of cinnamyl alcohols [35] and β-ether type polymers [55][56][57] have been used in pyrolysis studies. Phenolic (Ph) and non-phenolic (Non-Ph) model compounds are commonly used to represent the ends and repeating units of lignin macromolecules, respectively.…”

“…The Brit and Beste research group have extensively studied hydrogen abstraction reactions from the C α − H bond and subsequent cleavage of the β-ether bond using phenethyl phenyl ether and its derivatives [43,44,46].…”

Computational Study of Bond Dissociation Enthalpies for Substituted β‐O‐4 Lignin Model Compounds

“…The approach was nested to allow simultaneous rotation around several bonds. The conformational space for the radical products was searched similarly, but with diffuse functions added to the 6-31G* basis set at the location(s) of the unpaired electron (designated in our papers as the MIXED basis [26,28,29] ). Second, the lowest energy conformers, as determined at M06-2X/6-311 + +G**//MIXED, were optimized at M06-2X//6-31 + +G**.…”

“…In our previous work, the MIXED basis was sufficient for geometry optimization. [26,29] However, given the presence of hydrogen bonds in the systems studied, we included polarization and diffuse functions on all atoms. DFT calculations of OÀH BDEs in ortho-substituted phenols showed an increase in accuracy when such functions were included.…”

Effects of cross-linking methods for polyethylene-based carbon fibers: review

“…Such carbonization studies will be conducted in future investigations, as shown in Fig. 18a and b [96][97][98][99][100][101][102][103][104][105][106][107][108][109].…”