Detection of reactive intermediates during laser pyrolysis of cellulose char by molecular beam mass spectroscopy, implications for the formation of polycyclic aromatic hydrocarbons

We report here the use of infrared laser pyrolysis as both an analytical tool, at low power, for the determination of pendant polycyclic aromatic hydrocarbons (PAH) in chars and as a means, at high power, for the generation of the reactive intermediate precursors of PAH. Cellulose, in the form of pelletized Avicel, was charred under argon for various times at 375 degreesC up to 3 h. The char formed in this way was pyrolyzed using a 65W carbon dioxide laser at powers corresponding to peak char surface temperatures, T-max, of up to approximately 1000 degreesC in a helium atmosphere and the products detected by molecular beam mass spectroscopy (MBMS). Relatively fast scanning of the mass spectrum allowed useful information to be gathered before the sample reached peak temperatures, < 1 s. Low laser power, T-max,: 850 degreesC, experiments produced PAH from pendant groups on the char surface corresponding to the molecular weight growth of the aromatic species with charring time. At long charring times the size of the PAH detected diminished as the chars became more intractable. In addition propargyl and formyl methyl radicals were observed as the PAH was ablated from the char surface. Additional evidence for these radicals comes from simulating the mass spectrum and low electron-energy ionization measurements. At high laser powers, Tmax 1700 degreesC, significant quantities of propargyl radical and acetylene were observed as the PAH was broken into fragments. By increasing the effective distance from hot char to the MBMS inlet we were able to observe moderate molecular weight growth of the propargyl radical to hexadiene and PAH. Only very small amounts of the cyclopentadienyl radical were observed and its condensation product, the naphthalene precursor, dihydrofulvalene. We, therefore, conclude that the important reactive intermediates in the fast pyrolysis of cellulose char are: acetylene, propargyl, formyl methyl and PAH fragment radicals. (C) 2002 Elsevier Science B.V. All rights reserved.