Introduction Recently the development of alternative sources to fuels and chemicals has received growing interest as political, economic and environmental factors drive the need to reduce society's reliance on petroleum based feed stocks. While batteries and fuel cells are being developed for short range transportation needs, long haul transportation, both ground and air, will continue to depend on infrastructure compatible hydrocarbon fuels. Biomass is one alternative renewable feedstock to such fuels. Processes currently being commercialized to convert biomass into fuels and chemicals rely on high energy reactions that require catalysts and/or hydrogen. These restraints require centralized processes and results in high conversion cost.In order for biomass based fuel and chemicals to have a significant market penetration they need to compete economically with petroleum based streams, thus finding ways to reduce the cost of the biomass conversion is essential. As part of a project to produce infrastructure compatible fuel and chemicals, Ceramatec is developing an electrochemical process, adapted from the Kolbe electrolysis. This process converts fatty acids from a biomass origin into long chain hydrocarbons, which can be further converted into lubricants and fuel using existing infrastructure.Background The conventional Kolbe electrolysis has been investigated since 1849 when Kolbe applied it to the synthesis of different hydrocarbons. The reaction pathway is generally considered to follow a mechanism in which a one electron oxidation at an electrode surface generates a radical and CO2. The radical can then undergo homo- or hetero- coupling with other radicals generated at the surface of the electrode.1While it has been studied for a long time, the Kolbe electrolysis has never been widely commercialized because the following issues limit the efficiency and yield: 1) it requires high over potentials, 2) the reactivity of the electrolyte components increases the number of side products, and 3) oxidation and reduction occur in same reactor increasing chances of side reactions.To circumvent these shortcomings, Ceramatec's decarboxylation process uses a two compartment electrochemical reactor, where the compartments are separated by a ceramic membrane. The separation afforded by the membrane allows the use of different anolyte and catholyte compositions. The ability to use different electrolytes allows: 1) the anolyte to be customized for the electrolysis of interest, and 2) the catholyte to be designed to have high conductivity and a low reduction potential. The latter helps reduce the overall cell potential, improving the energy efficiency of the process. The membrane used to separate the two compartments is a NaSelect� membrane (Ceramatec, Inc.) which only allows sodium ions to pass through it, thus maintaining the electroneutrality of the cell without cross contamination of the two electrolytes. The use of this membrane requires the anolyte to contain sodium salts of carboxylic acids, and the anodic reaction can be represented generically by the reaction below.2RCO2Na ? R-R + 2CO2 + 2e- + 2Na+The Na-ions that are produced in the anolyte are then transferred across the membrane with a 99% efficiency to the cathode compartment, where the corresponding reduction reaction occurs, as shown below using water as an example. 2H2O + 2e- ? 2NaOH + H2Separating the anolyte from the catholyte using a membrane which only permits the transfer of Na-ions, enables non-conventional electrolytes to be implemented as anolytes. Exploiting this feature, three different electrolyte systems are being developed to increase the efficiency and yield of the electrolysis of biomass derived carboxylic acids, and the results of this work will be presented.Experimental Overview Methanol is the conventional solvent used as an electrolyte for Kolbe electrolysis. Unfortunately, methanol oxidizes at low potentials, and thus during the decarboxylation process the formation of methyl esters negatively impacts the efficiency and yield of the reaction. We are investigating new polar electrolyte systems with solvents that will make the anolyte more stable to oxidation. The use of a two compartment reactor allows the electrolyte system to be chosen based on its oxidative stability, without being concerned with the reductive stability as it is physically isolated from the cathode. Also, the use of the two compartment reactor causes the distance between the anode and cathode to be separated into three regions: 1) the distance between the anode and the membrane, 2) the membrane thickness, and 3) the distance between the membrane and the cathode. This separation decreases the length of diffusion through the anolyte, permitting the anolyte conductivity to be sacrificed for improved oxidative stability.The other two electrolyte systems being investigated to increase the efficiency and yield are based on the use of anolytes comprised entirely of the species being converted by the decarboxylation process. In the first system this is achieved by using a molten salt electrolyte, at temperatures between 150-350 �C, comprised of the sodium carboxylate or mixture of sodium carboxylates that are being converted by the decarboxylation process. In this case the only anions present in the anolyte are the anions involved in the desired electrolysis and the only cation present is sodium which is selectively transferred to the catholyte by the membrane. For the second system, custom ionic liquids (ILs) are synthesized such that the anion of the IL is the carboxylate anion being converted by the decarboxylation process. Like the molten salt electrolyte, the only anions present in the IL are involved in the desired reaction at the anode, but in this case the anolyte also contains large polyatomic cations, which lower the melting point (0-150 �C) of the electrolyte, and hence the required operating temperature.Conclusion The use of these novel electrolytes has demonstrated that side product formation can be greatly reduced improving the efficiency and yield of the decarboxylation process. The progress and the status of this effort will be covered, highlighting the important differences between the different electrolyte systems.References (1) H. J. Schafer, Topics in Current Chemistry, Electrochemistry IV, 1990  , 152, 91.

Electrolysis of carboxylic acids for the conversion of biomass into hydrocarbons

247th National Meeting (2014)

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