Redox-responsive polymers for the reversible extraction of butanol from water

Abstract

Over the past few decades, increase in the demand for low molecular weight alcohols, like ethanol and butanol, for use as a biofuel has provided a new impetus to their production by the fermentation of polysaccharides, and the subsequent separation from the alcohols from the aqueous fermentation broth. The inhibitory nature of alcohols to their own production necessitates the continuous lowering of the concentration of the alcohol during the fermentation process. The technology for removing alcohol and other organics from aqueous solutions also finds application in industrial waste treatment facilities. The different techniques in use currently for in-situ removal of alcohol from fermentation broth, like distillation, suffer from drawbacks like high energy consumption. The goal of this project was to develop a redox-responsive polymer which has preferential selectivity for butanol causing the polymer to selectively extract butanol from aqueous fermentation broth. On application of electric potential, the redox moieties in the polymer were oxidized and charged resulting in an increase in the hydrophilicity of the chemical environment in the gel and the extracted butanol was released into a stripping medium. The switchable selectivity of the polymer for butanol allows its use for the development of continuous separation system for butanol extraction. In this project, novel co-polymer of hydroxybutyl methacrylate (HBMA) and vinylferrocene (VF) was synthesized by free radical polymerization. The HBMA backbone gave the polymer preferential selectivity for butanol, while the ferrocene (Fc) groups of VF made the polymer redox active. The thermodynamic parameters, equilibrium distribution coefficient and separation factor, which quantify the distribution of a species between two phases were experimentally determined for butanol and water distribution between the polymer and the aqueous phase when the redox moieties in the polymer were in the reduced and the oxidized states respectively. The values of these parameters confirmed that the oxidation and the consequent charging of the redox species resulted in a decrease in the polymer's affinity for hydrophobic molecule, butanol. The optimum composition of the co-polymer was arrived at by comparison of properties of polymers with different compositions. The redox active co-polymer of HBMA and VF was attached to electrically conducting substrates to prepare redox polymer electrodes (RPEs). The RPEs allowed the oxidation and reduction of the ferrocene groups in the polymer by the application of electric potential. Carbon black (CB) and carbon fiber mats, called carbon paper (CP) were used as the substrates. RPEs were prepared using five different techniques-three techniques were based on strategies reported in literature and involved the chemical modification of the functional groups on the surface of CB and CP to allow polymer grafting. In addition, an iCVD based technique was developed which functionalized the CP surface by deposition of a reactive polymer, poly(pentafluorophenyl methacrylate (PFM)-co-ethyleneglycol diacrylate (EGDA)). The polymer layer was chemically modified to allow redox polymer grafting.. Impregnation of porous CP with redox polymer gel resulted in electrodes with highest mass of polymer per unit mass of conducting substrate.

(cont.) The electrochemical activity and reversibility of the RPEs prepared using the different techniques were ascertained by cyclic voltammetry. The impregnated electrodes were used to demonstrate the successful use of the polymer gel to extract butanol from its aqueous solution, and release it into water upon oxidation. Conceptual scheme of a continuous separation system that can be built using these RPEs was proposed and the separation that can be achieved using such a system was determined by simulating the continuous separation process using finite element modeling. It was determined that the separation system integrated with a fermentation reactor can help maintain the concentration of butanol at a value 20% lower than the critical value beyond which fermentation is completely inhibited. The mechanism of electron transport in the polymer coated RPEs was investigated. Diffusion of electrons was found to be the rate controlling step. Further, it was found that diffusion of electrons due to the 'hopping' of electrons from one redox site to the next, and the electronic motion due to the motion of the polymer chains themselves played important roles in determining the apparent diffusivity of electrons. As part of the PhDCEP Capstone project, the potential of butanol produced through fermentation, commonly known as bio-butanol, was analyzed as a blend for gasoline was analyzed. It was found that although the market for gasoline blend is huge and growing, butanol suffers from higher cost of production with respect to its primary competitor, bio-ethanol. Chances of bio-butanol's potential success can be enhanced through a combination of technological breakthroughs including development of strains of high yield bacteria, use of inexpensive lignocellulosic biomass, and process design improvements.