Mass Spectrometric Studies Aimed to Improve the Understanding of the Mechanisms of Fast Pyrolysis of Biomass and Solution-Processing of Thin-Film Solar Cell Devices

Priya Murria, Purdue University

Abstract

Fast pyrolysis of biomass involves heating it to high temperatures at high heating rates under inert conditions to produce bio-oil—a low-energy density liquid that requires further upgrading to produce hydrocarbon fuels. Optimization of this process requires identification of important intermediates and initial products as well as acquiring understanding of the pathways and mechanisms of the reactions that occur during fast pyrolysis of biomass. Since biomass itself is quite complex, model compounds are often employed as surrogates to improve the understanding of the reaction pathways and mechanisms of fast pyrolysis of the various components of biomass. In this work, a pyrolysis-mass spectrometry setup with very short residence time was used for identification of the initial products of cellulose and cellulose model compounds, xylan model compounds and lignin model compounds. Quantum chemical calculations were used to explore feasible reaction pathways and mechanisms for fast pyrolysis of biomass model compounds. Upon fast pyrolysis, cellulose and cellulose model compounds were found to depolymerize mainly via two competing pathways—hydroxymethylene-assisted glycosidic bond cleavage (HAGBC) pathway and a pathway involving unraveling of reducing ends mainly via loss of ethenediol or glycoladehyde molecules. Model compounds of xylan (the most abundant type of hemicellulose in plant material), xylobiose and xylotriose, were also found to undergo depolymerization via loss of ethenediol or glycolaldehyde molecules upon reducing end ring-openings. However, because of the absence of hydroxymethylene groups in xylose, the HAGBC mechanism is not feasible. Instead, glycosidic bond cleavage is proposed to occur via concerted Maccoll elimination and Pinacol ring-contraction reaction mechanisms. Concerted nonradical eliminations rather than radical reactions were found to be the major pathways for fast pyrolysis of cellulose, cellulose model compounds and xylan model compounds. Lignin model compounds with β-O-4 linkages (a trimer, a tetramer and a polymer—all with guaiacyl subunits) were used as model compounds to study fast pyrolysis reaction pathways and mechanisms of lignin. Dimeric and trimeric fast pyrolysis products were found to form directly from the pyrolyzed molecules rather than from initially formed monomers upon oligomerization, as opposed to literature reports. Both quantum chemical calculations and experimental data suggest that concerted nonradical Maccoll and retro-ene elimination mechanisms leading to cleavage of β-O-4 linkages are more favorable than radical reactions. These reactions lead to the formation of the key pyrolysis products. Solution-processing is a promising alternative to traditional vacuum-based methods for fabrication of thin-film electronic devices due to its potential for high-throughput and roll-to-roll fabrication at low cost. Thiol-amine mixtures have been found to dissolve a host of metal salts and precursors that are insoluble in either solvent by itself. However, little is known about the chemistry of this solvent system. Electrospray ionization mass spectrometry, synchrotron X-ray absorption spectroscopy, and Raman spectroscopy were used to identify the species formed via dissolution of CuCl and CuCl2 (salts used for the synthesis of many Cu-containing chalcogenides) in thiol-amine mixtures. In addition to this, films prepared from these solutions both with and without added sulfur were annealed and characterized via X-ray diffraction, Raman, and energy dispersive spectroscopy to understand how these species evolve upon heating and affect composition of the final films. The oxidation state of copper was found to be +1 in all the species observed in solutions of both CuCl and CuCl2 in the thiol-amine mixtures via ESI-MS and XAS studies. Raman spectroscopy revealed that dipropyl disulfide is an oxidation product formed from propanethiol when Cu(II) is reduced to Cu(I) upon dissolution of CuCl2 in thiol-amine mixtures. Interestingly, thin films prepared from solutions of CuCl and CuCl2 in thiol-amine mixtures were difficult to fully purge of Cl- and C-containing compounds and did not form copper sulfide films upon annealing. The addition of sulfur into the solutions greatly improved the removal of impurities and compositional uniformity of the films but did not result in copper sulfide films after annealing.

Degree

Ph.D.

Advisors

Kenttämaa, Purdue University.

Subject Area

Alternative Energy|Chemistry|Analytical chemistry|Energy

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