The electrochemical activation and incorporation of carbon dioxide into organic molecules
thesisposted on 03.04.2014, 12:48 by Anish P. Patel
The consumption of large quantities of fossil fuels on a yearly basis for energy purposes has led to the release of vast quantities of carbon dioxide into the Earth s atmosphere leading to global warming. In order to decrease the amount of carbon dioxide entering the atmosphere, procedures such as carbon dioxide capture and storage are currently being implemented, although this process is successful in decreasing atmospheric carbon dioxide concentration it results in the mindless storage of an otherwise synthetically useful reagent. The development of a viable method to capture carbon dioxide, followed by synthetic utilisation often referred to as CCU is currently gaining much attention. The main challenges in the development of a suitable utilisation reaction are energy and cost efficiency as well as the use of environmentally friendly conditions. Previous reports on utilisation include the catalytic incorporation of carbon dioxide into epoxides under electrochemical conditions, however this process had several drawbacks. In this project the shortcomings of this reported procedure have been addressed in order to develop a potentially viable utilisation process. The catalyst free electrocarboxylation of mono-substituted epoxides, using a magnesium anode/copper cathode electrode couple and tetrabutylammonium bromide supporting electrolyte in a sealed single compartment cell, was achieved under mild reaction conditions (1 atmosphere carbon dioxide pressure, 60 milliamperes constant current and 50 degree celsius heating), producing the corresponding 5-membered cyclic carbonate product in excellent yields (65-96%). Interestingly the use of sono-electrolysis allowed the reduction of para-substituted aromatic epoxides. The stoichiometric addition of tetrabutylammonium bromide was key for the effective activation of carbon dioxide and the epoxide. Similar carbon dioxide incorporation into analogous mono-substituted aziridines, synthesised using a modified Wenker synthesis allowed the formation of the corresponding cyclic carbamates in moderate to excellent yields (32-90%). The selective synthesis of 6-membered cyclic carbonates from oxetane substrates was also achieved in good yields (60-70%) as well the non-selective synthesis of polytrimethylene oxide. The electrochemical process also allowed the tandem formation of magnesium carbonate in quantitative yield (85%). Furthermore substrate free electrocarboxylation allowed the synthesis of alternative iron and zinc carbonates in excellent yield (83-85%) as well as the selective synthesis of aluminium oxalate (99%). Coupled with the high recovery of the supporting electrolyte (90%), this work has demonstrated the economical synthesis of industrially useful chemical feed-stocks under green conditions.