The Environmental Chemical Sciences (ECS) program of the Division of Chemistry will support the collaborative research program of Prof. Barney Ellison and Prof. Daily of the University of Colorado and Prof. John Stanton of the University of Texas at Austin. This interdisciplinary collaborative team of investigators and their students will develop a novel biomass spectrometer that combines tunable vacuum ultraviolet photo ionization mass spectrometry (VUV PIMS) detection with Fourier transform infrared (FTIR) spectroscopy to study the fundamental chemical processes of forest fires. The project will combine state-of-the art spectroscopic methods to characterize the organic chemistry of reactive carbon species formed during biomass burning (Ellison), advanced spectral analysis methods to analyze infrared and mass spectra (Stanton) and computational fluid dynamics calculations and Monte Carlo simulations (Daily) to model the fluid dynamics of the planned biomass spectrometer.

The biomass spectrometer will enable the group to study the mechanism of biomass cracking during forest fires by identifying chemical intermediates and by-products of biomass burning. The project will solve a very difficult and complex chemical problem and enable the interpretation of highly complex field data which is obtained during forest fires. The project will provide excellent interdisciplinary training opportunities for students who wish to pursue a professional scientific career in an area of great environmental importance.

Project Report

Forest fires is the large-scale burning of forests and grasslands. These fires naturally occur but many are set to clear farmland. These fires are massive and cover many km2 of land mass. Forest fires inject massive quantities of reactive materials and aerosols into the atmosphere. Our objectives are 1) to develop a micro-reactor to carry out the pyrolysis of complex organic molecules and b) to apply these micro-reactors to biomass monomers. We are using a novel biomass spectrometer to study the fundamental chemical steps in the anaerobic pyrolysis of biomass. A microtubular reactor has been fabricated to study the first steps in the thermal cracking and oxidation of polymers of sugars (cellulosic materials) and lignins (wood). A sketch of this apparatus is below. Lignins and carbohydrates are fundamental fuels in forest fires. One aspect of our work is to connect the molecular processes of forest fires to atmospheric chemistry. John Daily (mechanical engineering) and Barney Ellison (organic chemistry) work closely with John Stanton at Institute of Theoretical Chemistry, Univ. of Texas/Austin to model the fundamental molecular steps in the thermal cracking of biofuels. We have completed several projects. 1) Bond dissociation enthalpies (BDEs) can exhibit dramatic variations resulting from substituent effects. The remarkable range of experimental OH bond dissociation enthalpies have been reproduced using CBS-APNO calculations with very good accuracy, so we have employed these calculations to extend the available BDE data. The effect on these BDEs of lone pairs on the atom adjacent to oxygen shows that conjugation in the product radicals is the most important interaction leading to the wide range of values. The BDE’ s were found to be linearly related to both the spin density at the radical center and to the change in X− O bond order in going from X− O− H to X− 2) We have monitored changes in the infrared spectrum of gas-phase CH2=C=O in the presence of water vapor and deuterated water vapor. The products observed from ketene hydration are CH3COOH, (CH3COOH)2, and CH3CO-O-COCH3. The time-dependence of product formation supports a reaction mechanism in which ketene hydrates to form acetic acid, which then combines with either another acetic acid monomer to form a dimer, or with ketene to form acetic anhydride. These results show that ketene can undergo hydration under atmospherically-relevant temperatures and relative humidities. This reaction could help to reconcile under-predictions of atmospheric carboxylic acids, especially in biomass burning plumes. 3) Production of CH3COOH via gas-phase hydration of CH2=C=O by water (uncatalyzed and in the presence of an additional water molecule) was theoretically characterized using high-level coupled-cluster methods, followed by a two-dimensional master equation analysis to compute thermal reaction rate constants. The results show that the formation of acetic acid quite likely occurs in high-temperature combustion of biomass, but that the rate of formation should be negligible under ambient atmospheric conditions. 4) A micro-reactor system (approximately 0.5–1 mm inner diameter by 2–3 cm in length) coupled with photoionization mass spectrometry and matrix isolation/infrared spectroscopy diagnostics is described. Short residence time flow reactors (roughly ≤ 100 μs) combined with suitable diagnostic tools have the potential to allow observation of unimolecular decomposition processes with minimum interference from secondary reactions. However, achieving the short residence times desired requires very small micro-reactors that are difficult to characterise experimentally because of their size. In this article the benefits of using these micro-reactors are presented along with some details of the systems employed. This is followed by some general flow considerations and then some simple analyses to illustrate particular features of the flow. Finally, computational fluid dynamics simulations are used to explore the flow and chemical behaviour of the reactors in detail. Some findings include: (1) The reactor operates in the laminar domain. (2) Heating and large pressure differences across the reactor result in a compressible flow that chokes (meaning the velocity reaches the sonic condition) at the reactor exit. (3) When helium is the carrier gas, under some circumstances there is slip at the boundaries near the downstream end of the reactor that reduces the pressure drop and heat transfer rate; this effect must be accounted for in the simulations. (4) Because the initial reactant concentration is held to less than 0.1%, secondary reactions are minimised. (5) Although the fluid dynamical residence time from entrance to exit ranges from 25 to 150 μs, in practice the period over which reactions take place is much shorter. In essence, there is a ‘sweet spot’ within the reactor where most reactions take place. In summary, the micro-reactor, which has been used for many years to generate radicals or study unimolecular decomposition chemical mechanisms, can be used to extract kinetic information by comparing simulations and measurements of reactant and product concentrations at the reactor exit.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Zeev Rosenzweig
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University of Colorado at Boulder
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