Probing the Pyrolysis and Ion Chemistry of a Selection of Formates and Chloroformates
Creators: name: Lowe, Bethany
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2023
Subjects: Chemistry
Biofuels
Pyrolysis
Ions

description: This research explores the ion chemistry and pyrolysis of a selection of formates. Alternative fuels have become increasingly popular over the past decade, one of particular interest is biofuels. Biofuels contain a variety of oxygenated compounds that are not present in traditional crude-oil fuels such as formates. Therefore, knowing the pyrolysis and ion chemistry of a selection of formates is of particular interest in being able to determine the environmental implications of switching to biofuels. The pyrolysis in particular for these formates have largely been studied using shock-tube experiments where both unimolecular and bimolecular reactions occur. This study employs the use of a micro-reactor coupled to imaging photoelectron photoion coincidence spectroscopy (iPEPICO) using synchrotron vacuum ultra-violet (VUV) radiation at the Swiss Light Source (SLS). The sample is introduced in the dilute-gas phase to optimise for unimolecular reactions. This thesis therefore presents the unimolecular pyrolysis chemistry of methyl formate, ethyl formate and methyl chloroformate and the ion chemistry of methyl chloroformate, phenyl formate and phenyl chloroformate. In chapter 3, the thermal dissociation of the atmospheric constituent methyl formate was probed by coupling pyrolysis with iPEPICO. The pyrolysis products of dilute methyl formate, CH₃OC(O)H, were elucidated to be CH₃OH⁺, CO, 2 CH₂O and CH₄ + CO₂ as in part distinct from the dissociation of the radical cation (CH₃OH⁺˙ + CO and CH₂OH⁺ + HCO). Density functional theory, CCSD(T), and CBS-QB3 calculations were used to describe the experimentally observed reaction mechanisms, and the thermal decomposition kinetics and the competition between the reaction channels are addressed in a statistical model. One result of the theoretical model is that CH₂O formation was predicted to come directly from methyl formate at temperatures below 1200 K, while above 1800 K, it is formed primarily from the thermal decomposition of methanol. Chapter 4 utilises the same techniques but expands on it by taking advantage of threshold photoionization and ion imaging, parent ions of neutral pyrolysis products and dissociative photoionization products could be distinguished, and multiple spectral carriers could be identified in several ms-TPES. The TPES and mass-selected TPES for ethyl formate are reported for the first time and appear to correspond to ionization of the lowest energy conformation having a cis configuration of the O=C(H)-O-C(H₂)-CH₃ and trans configuration of the O=C(H)-O-C(H₂)-CH₃ dihedral angles. We observed the following ethyl formate pyrolysis products: CH₃CH₂OH, CH₃CHO, C₂H₆, C₂H₄, HC(O)OH, CH₂O, CO₂, and CO, with HC(O)OH and C₂H₄ pyrolyzing further, forming CO + H₂O and C₂H₂ + H₂. The reaction paths and energetics leading to these products, together with the products of two homolytic bond cleavage reactions, CH₃CH₂O˙ + ˙CHO and CH₃CH₂˙ + HC(O)O˙, were studied computationally at the M06-2X-GD3/aug-cc-pVTZ and SVECV-f12 levels of theory, complemented by further theoretical methods for comparison. The calculated reaction pathways were used to derive Arrhenius rate parameters for each reaction. The reaction rate constants and branching ratios are discussed in terms of the residence time and suggest carbon monoxide as a competitive primary fragmentation product at high temperatures. Chapter 5 explores the pyrolysis chemistry of methyl chloroformate (MCF) in a similar manner but also introduces the study of the ion chemistry. The TPES for MCF was acquired for the first time; the geometry change upon ionization of MCF results in a broad, poorly defined TPES. Franck-Condon simulations are consistent with an ionisation energy (IE) of 10.90 ± 0.05 eV. Ionized MCF dissociated by the expected loss of Cl with a measured appearance energy (AE) of 11.30 ± 0.01 eV. Together with the above IE, this AE suggests a reaction barrier of 0.40 eV, consistent with that found from SVECV-f12 calculations (0.41 eV). At higher internal energies, the loss of CH₃O˙ becomes competitive due to its more favourable entropy of activation. Pyrolysis of neutral MCF formed the anticipated major products of CH₃Cl + CO₂ (R1) and the minor products HCl + CO + CH₂O (R2), all species being confirmed by their mass-selected TPES. Several possible reactions were computationally explored but these two were confirmed to be the dominant reaction channels. R1 proceeds by a concerted Cl atom migration via a 4-membered transition state in agreement with that proposed in the literature. R2 is a two-step reaction proceeding first by loss of HCl to make 2-oxiranone which then decomposes to CH₂O and CO. Kinetic modelling of the neutral decomposition could be made to simulate the observed reactions only if the vibrational temperature of the MCF was assumed not to cool during the expansion. Chapter 6 further expands on the ion chemistry study by exploring the ion dissociation of phenyl formate (PF) and phenyl chloroformate (PCF). Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and tandem mass spectrometry were employed to explore the ionisation and dissociative ionisation of phenyl formate (PF) and phenyl chloroformate (PCF). The threshold photoelectron spectra of both compounds are featureless and lack a definitive origin transition, owing to the internal rotation of the formate functional group relative to the benzene ring, active upon ionisation. CBS-QB3 calculations yield ionisation energies of 8.88 and 9.03 eV for PF and PCF, respectively. Ionised PF dissociates by the loss of CO via a transition state composed of a phenoxy cation and a HCO moieties. The dissociation of PCF ions involves the competing losses of CO (m/z 128/130), Cl (m/z 121), and CO₂ (m/z 112/114), with Cl loss also shown to occur from the second excited state in a non-statistical process. The primary CO- and Cl-loss fragment ions undergo sequential reactions leading to fragment ions at m/z 98 and 77. The mass-analysed ion kinetic energy (MIKE) spectrum of PCF⁺ showed that the loss of CO₂ occurs with a large reverse energy barrier, which is consistent with the computationally derived minimum energy reaction pathway. Chapter 7 highlights the conclusions drawn from across the chapters 3-6 and brings chapter 1 back into focus with how these findings can help to inform future studies on pyrolysis. Furthermore, chapter 7 discusses how future technologies for biofuels can be shaped with the understanding of the pyrolysis products of these biofuel related compounds.