The Benzylphenylether thermolysis mechanism: Insights from phase behavior

Abstract

The thermolysis of benzylphenylether (BPE) was examined at 320°C in liquid, vapor, and supercritical methanol phases as a probe of the reaction mechanism. Toluene and phenol were the major products in all cases, but observed selectivities were dependent upon the reaction phase. Isotopic labeling experiments demonstrated that methanol participated in the reaction network as a hydrogen donor through a free-radical mechanism. These results are consistent with free-radical steps for neat BPE primary pyrolysis that include BPE fission followed by hydrogen abstraction and radical recombination reactions. A detailed set of 2 1 free-radical steps quantitatively explains the dependence of the selectivity and conversion on the phase behavior. Introduction Supercritical fluid (SCF) solvents represent interesting and potentially powerful tools for reaction engineering because of their extreme P-V-T behavior near their critical point. As reaction media, SCF solvent properties can potentially be optimized via adjustments of pressure to improve reaction rates and selectivities. As we show here, SCF solvents can also be useful in the interpretation of kinetic information in the probe of reaction mechanisms. BenjaminReported supercritical solvent effects include electrostatic interactions (Townsend et al., 1988;Johnston and Haynes, 1987), diffusional limitations (Lawson and Klein, 1985;Abraham, 1987;Helling and Tester, 1987;Yang and Eckert, 1988), and pressure effects (Johnston and Haynes, 1987;Simmons and Mason, 1972). However, the thermodynamic phase behavior can also affect global reaction kinetics and lead to interesting apparent solvent effects.The reaction of dibenzylether (DBE) in supercritical toluene provides an example of the influence of phase behavior on laboratory kinetics information. The conversion of DBE after 30 min at 375°C and constant initial DBE concentration, [DBE],, dropped from 90% at 5 atm to 50% at 48 atm, where the pressure increase was generated through the addition of toluene. Interpretation of the data by assuming a single-phase pressure effect would suggest an apparent activation volume of 2,000 cm3/mol, as much as three orders of magnitude greater Correspondence concerning this ppcr rhould he addrcsscd to M. T. Klein. than typical intrinsic activation volumes (le Noble, 1978).However, phase equilibrium calculations showed the existence of a two-phase region that explained the drop in conversion without recourse to pressure-dependent rate constants.Thus the adjustment of the pressure can also control the number and composition of phases in which a reaction occurs. This presents the opportunity to highlight the various elementary steps of a complex reaction mechanism since the concentrations of species can be varied easily by over two orders of magnitude by changing from the liquid to the vapor phase. This allows emphasis of, and discrimination between, unimolecular and bimolecular steps and their influence on global kinetics and selectivities. This is the object of the present paper, where we descr...

Thermolysis of Benzyl Phenyl Ether in Subcritical and Supercritical Water, and Supercritical Methanol.

“…This result is distinctly different from that of Wu et al 29), who reported that selectivities of phenol and toluene were almost the same (0.5).…”

Measuring biomass fast pyrolysis kinetics: State of the art

“…The first studies on lignin model compound pyrolysis were reported by Klein et al (Klein & Virk, 1983;Simmons & Klein, 1985;Wu FIGURE 21 Thermal decomposition of xylose unit via a combination of free-radical and concerted pathways yielding furfural and glycoldehyde, glyoxal, acetone, and furfural (Adapted from Shen et al, 2010. Copyright 2010 Benjamin, Klein Michael, & Sandler Stanley, 2004). Phenethyl phenyl ether (PPE) represents the β-O-4 linkage present in native lignin.…”

Reaction behavior of lignin in supercritical methanol as studied with lignin model compounds

“…Conversion of non-phenolic β-1 type model compound ( 7) into its α-methyl ether is a dominant reaction path due to the acidic character of the supercritical methanol. 16 Acid-labile hydroxyl groups such as α-hydroxyl group in the compound ( 7) can be eliminated to form quinonemethide-type intermediate under such an acidic condition, which is easily converted into α-methyl ether, and then further converted into tetramethoxystilbene (13), which is stable in supercritical methanol.…”