Isomer-Selective Generation and Spectroscopic Characterization of Biofuel Intermediates

Biofuel combustion produces nitrogen-containing elusive species, which are precursors of soot. Threshold photoelectron spectra (white dotted lines) along with simulated spectra (red line) identifies those species isomer-selectively.

Imaging Photoelectron Photoion Coincidence Spectroscopy (iPEPICO) has evolved into a productive, isomer-specific detection tool in online analysis of flames, pyrolysis, catalysis and kinetics measurements, thanks to the ability to detect elusive but important species such as radicals and carbenes, at low concentrations. Clean and isomer-specific spectroscopic data are much-needed to detect these species and to understand the underlying chemical mechanisms in biofuel combustion, and, in turn, avoid e.g. soot- and nitrogen oxide (NOx) formation in Diesel engines. Because of the difficulty of selectively preparing pure samples of elusive species, such data are also rather scarce.

Researchers from the University of Würzburg (Germany) and the Swiss Light Source successfully prepared all three picolyl radical isomers (C5H4N-CH2*) purely via deamination of aminomethylpyridines in a microtubular reactor. Picolyl radicals are isosteric to benzyl and are, thus, soot precursors par excellence. In addition, they may lead to NOx emissions upon oxidation at elevated temperatures in Diesel engines, a process which needs to be avoided, too. Threshold photoelectron spectra were recorded using vacuum ultraviolet radiation and exhibit distinct ionization energies and vibrational fine-structure assigned to a totally symmetric ring deformation mode. These spectral features make the isomer-selective detection of picolyl radicals in biofuel combustion straightforward. The obtained data opens new avenues to obtain kinetic data, such as isomer-specific rate constants for oxidation and reaction with acetylene leading to polycyclic aromatic nitrogen containing hydrocarbons (PANHs).