Investigation of dense carbon dioxide as a solvent medium for the catalytic oxidation of hydrocarbons.

摘要

Selective oxidation of hydrocarbons is one of the important chemical transformations to make value added products. This dissertation investigates oxidation of two reaction systems: 2,6 di-tert-butyl phenol (DTBP) to 2,6 di-tert-butyl quinone (DTBQ) and p-xylene to terephthalic acid (TPA) in CO2 expanded liquids (CXLs) as the solvent medium. Tunable solubilities of gases and catalysts, transport and solvent properties makes CXLs an ideal choice for this study.; The oxidation of DTBP serves as a proxy for the synthesis of Vitamin E, Vitamin K and antioxidants. Detailed study of the reaction using a Schiff base catalyst, Co(salen) in CXLs revealed subtle differences in the intrinsic catalytic activity. A calibration of the oxidizing potential of the catalyst on a variety of substrates was also obtained.; The synthesis of TPA from p-xylene gains significance as a commodity product, from its preferred use as a monomer for polyethylene terephthalate (PET) manufacture. PET is a widely used feedstock for plastics, films and fibers. Industrial TPA synthesis utilizes a cobalt-manganese-bromine catalytic system in acetic acid that affords product, in both high yield and purity. However, the use of air as the oxidant source limits oxygen availability, and halide catalyst-solvent system leads to corrosion, safety, and environmental issues. This reaction was studied in CXLs to address the aforesaid issues in terms of reaction, separation and safety aspects, with relevant phase behavior measurements. Reaction studies were performed in bench scale stirred vessels and performance parameters including substrate conversion, product selectivity and purity was quantified with appropriate analytical techniques. It was demonstrated that industrial quality TPA can be produced on a bench scale reactor and that presence of CO2 could reduce the induction time associated with the catalyst (if any) and increase the yield of TPA. Modeling of vapor phase flammability provided a quantification of the safety benefits, with the use of CO2 as an inert gas, instead of nitrogen. Using the antisolvent properties of CO2, the feasibility of selective crystallization of TPA for separation purposes was demonstrated. Mass transfer studies showed evidence of oxygen availability limitations in the reaction phase, based on which an alternate reactor design is proposed.; This work has led to a better understanding of the intrinsic catalytic behavior of Schiff base catalysts. It has also brought to light critical aspects of the chemistry and engineering in the synthesis of TPA in both neat organic and CXL media.