A gas-phase approach to the study of reaction mechanisms of biological and industrial sustainable processes

The quality of life was over time enhanced by a number of economic activities that require a massive consumption of energy. The population growth and the expansion of the global infrastructure have driven the increase of the world energetic needs and the improvement of the living standards. Actually, the demand for energy is predominantly satisfied by the use of non-renewable fossil fuels representing the main responsibles for a series of environmental problems, such as pollution and climate change. Considering also the progressive depletion of the petroleum supply, the employment of renewable resources is becoming extremely urgent. The search for alternative source of energy was first promoted by the serious increase of the petroleum price that followed the oil embargo proclaimed by the OPEC (Organization of the Petroleum Exporting Countries) in 1973. During the years of the energy crisis, natural gas (methane) played a significant role in solving the diminishing availability of crude oil still continuing to do so in combination with the use of renewable bio-resources, such as lignocellulosic biomass and solar energy. Lignocellulosic biomass represents a good candidate for supporting the energy request of the worldwide population. In particular, carbohydrates extracted from wood biomass can be converted into organic compounds and platform molecules, such as 5-hydroxymethylfuraldehyde (5-HMF), 2-furaldehyde (2-FA) and levulinic acid, that can be used directly as fuels or fuel precursors. These furan-type compounds are obtained from the thermal acid-catalysed dehydration of hexose (D-glucose) and pentose (D-xylose) sugars arising from the hydrolysis of the main components of biomass, cellulose and hemicellulose. Owing to the short lifetimes and low concentration of the intermediate species characterized by high reactivity, the achievement of a consistent reaction picture for the conversion of sugars to platform molecules is really challenging, but extremely difficult on the basis of the experimental data available in solution. Theoretical studies point to predict kinetics and thermodynamics of sugar dehydration, but the mechanistic details are still elusive and the proposed conversion pathways are the subject of a flourishing debate. For these reasons, the main focus of the present thesis has been the investigation of the reaction mechanisms concerning the processes that allow the utilization of lignocellulosic biomass and its conversion to green fuels. Taking into account the ionic or radical nature of most transient species, mass spectrometry represents an useful technique to reproduce the reactions in the gas-phase and to study structure and reactivity of ions in the absence of solvent molecules and counter-ions. Owing to the advantages of a gas-phase approach, this method was exploited in this work to investigate the reaction mechanisms of carbohydrate dehydration to platform chemicals. To this end, a crucial step was represented by the characterization of the ionic intermediates and products of the dehydration reactions involving D-hexose (glucose and fructose) and D-pentose (xylose, ribose and arabinose) sugars arising from biomass decomposition. Understanding reaction mechanisms on a molecular level is a pivotal step in order to effectively control the reaction outcome, optimize product yields and reduce the formation of side compounds. As a consequence, the deep knowledge of the sugar decomposition pathways aims at the design of new reagents and catalysts that can increase the selectivity towards the formation of 5-HMF and 2-FA. The same experimental method was also applied to the study of processes involving other sugar substrates or molecules structurally correlated to carbohydrates (L-ascorbic acid) and to the investigation of model reactions that lead to the activation of intrinsically inert bonds.