Computational Studies on the Catalyzed Conversion of Furans to Aromatics

摘要

Economic, environmental and political considerations have led to gradually replacement of natural gas for petroleum. One of the deficiencies of this major change of direction in the energy market is a decrease in petrochemical productions such as aromatics, which are important materials for a wide range of chemical processes. It in turn opens up a clear opportunity for renewable sources of energy and chemicals to play an essential role and provide a variety of important chemicals that are traditionally produced from petroleum.;This dissertation includes four projects on the catalyzed conversion of biomass-derived furans to aromatics. Various computational methods are employed to model different systems, investigate catalytic effects and rationalize them from an atomistic point of view.;In the first research project, density functional theory electronic structure calculations are used to explore the mechanism for the Diels-Alder reaction between 2,5-dimethylfuran (DMF) and maleic anhydride (MA). Reaction paths are reported for uncatalyzed, and Lewis and Bronsted acid-catalyzed reactions in vacuum and in a broad range of solvents. The calculations show that while the uncatalyzed Diels-Alder reaction is thermally feasible in vacuum, a Lewis acid (modeled as Na+) lowers the activation barrier by interacting with the dienophile (MA) and decreasing the HOMO-LUMO gap of the reactants. A Bronsted acid (modeled as a proton) can bind to a carbonyl oxygen in MA, changing the reaction mechanism from concerted to step-wise, and eliminating the activation barrier. Solvation effects are investigated with a continuum model of the solvent. This study shows that electrostatic effects play the largest role in determining the solvation energy of the transition state, which tracks the net dipole moment at the transition state. For the uncatalyzed reaction, the dipole moment is largely determined by charge transfer between the reactants, but in the reactions with ionic catalysts, there is no simple relationship between solvation of the transition state and charge transfer between the reactants. Non-electrostatic contributions to solvation of the reactants and transition state also make significant contributions to the activation energy.;Dehydration of the cycloadduct produced from the Diels-Alder reaction between 2,5-dimethylfuran and maleic anhydride to produce 3,6-dimethylphthalic anhydride, which is the topic of the second research project, exemplifies an important step in producing platform chemicals from biomass. The mechanisms of dehydration and catalytic effects of Lewis and Bronsted acids are investigated with density functional theory. The uncatalyzed reaction has a very high activation barrier (68.7 kcal/mol) in the gas phase and it is not significantly affected by solvation. With a Lewis acid catalyst, modeled as an alkali ion, the activation barriers are reduced, but intermediates are also stabilized. The net effect in vacuum is that the energetic span, or apparent activation energy of the catalytic cycle, is 77.9 kcal/mol, even higher than the barrier in the uncatalyzed case. In solution, however, the energetic span is reduced by as much as 20 kcal/mol, due to differences in the solvation energy of the transition states and intermediates. In the case of a Bronsted acid catalyst, modeled as a proton, the gas-phase transition state energies are reduced even more than in the Lewis acid case, and there is no strong stabilization of the intermediates. The energetic span in vacuum is only 13.8 kcal/mol and is reduced even further in solution. Bronsted acid catalysis appears to be the preferred mechanism for dehydration of this cycloadduct. Since the.;Diels-Alder reaction that produced the molecule has previously been shown to be catalyzed by Bronsted acids, this suggests that a single catalyst could be used to accelerate both steps. Diels-Alder reactions of furans yield oxanorbornene derivatives which can be converted to a variety of molecules, ranging from molecules of biological interest to naturally occurring organic compounds, and to aromatics via dehydration, a promising alternative for the synthesis of aromatics from renewables. With furan being one of the less reactive dienes, development of Lewis-acidic heterogeneous catalysts, without the shortcomings of the traditional homogeneous ones, is critically important. In the third project, we use computational chemistry to study the Diels-Alder reaction of furan and methyl acrylate in three zeotypic Lewis acids, Sn-, Zr- and Hf-BEA (BEA=zeolite beta). We find that all three exhibit the same ability to enhance the electrophilic character of the dienophile and promote modest charge transfer from the diene. Despite being moderately Lewis-acidic, they still achieve a reduction of about 12.5 kcal/mol in the activation energy relative to the reaction in the absence of catalyst...