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Fundamental Insights Into the Dissolution and Precipitation of Cellulosic Biomass from Ionic Liquid Mixtures

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Fundamental Insights Into the Dissolution and Precipitation of Cellulosic Biomass from Ionic Liquid Mixtures Fundamental Insights into the Dissolution and Precipitation of Cellulosic Biomass from Ionic Liquid Mixtures By David L. Minnick Submitted to the graduate degree program in Chemical and Petroleum Engineering and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. ________________________________ Chairperson: Aaron M. Scurto ________________________________ Bala Subramaniam ________________________________ Raghunath V. Chaudhari ________________________________ Laurence Weatherley ________________________________ Belinda Sturm Date Defended: July, 14th 2016 The Dissertation Committee for David L. Minnick certifies that this is the approved version of the following dissertation: Fundamental Insights into the Dissolution and Precipitation of Cellulosic Biomass from Ionic Liquid Mixtures ________________________________ Chairperson: Aaron M. Scurto Date approved: July, 22nd 2016 ii Abstract Ionic liquids (ILs) are a unique class of molecular salts that melt at temperatures below 100oC. The ionic functionality of ILs provide this class of molecules numerous advantages for applications in reactions, separations, and materials processing due to their molecular flexibility through cation/anion selection. Additionally, ionic liquids possess negligible vapor pressures and may lead to more sustainable or “green” processes by eliminating solvent-based air pollution. For these reasons ionic liquids are being targeted for implementation in a range of industrial processes as sustainable solvent technologies. The primary objective of this dissertation targets the application of ionic liquids to cellulosic biomass processing. For instance, chemical processing of biomass remains a challenge as the rigid inter- and intra- molecular hydrogen bonding network of cellulose renders it insoluble in nearly all aqueous and organic solvents. Alternatively, select ionic liquids (ILs) are capable of dissolving significant quantities. Through an ionic liquid mediated dissolution and precipitation process cellulose crystallinity is significantly reduced consequently enhancing subsequent chemical and biochemical reaction processes. Therefore, understanding the thermodynamics of ionic liquid – cellulose mixtures is imperative to developing an IL based biomass processing system. This dissertation illustrates the solid-liquid phase behavior for the dissolution and precipitation of cellulose in various IL/cosolvent, IL/antisolvent, and IL/mixed solvent systems with the ionic liquid 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIm][DEP]). The vast majority of molecular solvents dramatically decrease cellulose solubility in ILs and are therefore considered “antisolvents”. However, select quantities of polar aprotic solvents, when mixed with ILs, (despite having negligible solubility on their own) are capable of enhancing the iii thermodynamic solubility limit, even beyond the measured cellulose solubility limits of pure ILs. Furthermore, cosolvents enhance transport properties of IL/biomass mixtures and reduce ionic liquid moisture (H2O) sensitivity which is known to severely impact the dissolution capacity of cellulose. Spectroscopic techniques including Kamlet Taft solvatochromic analysis, FTIR, and NMR elucidate molecular interactions between the ionic liquid and solvent species and provide an understanding of the solvation sphere around the IL in relation to cellulose dissolution within the various IL-solvent mixtures. The phase equilibrium results indicate that mixed IL/cosolvent systems are even better solvents for cellulose dissolution than pure ILs from thermodynamic, transport, and economic perspectives. The majority of ionic liquid research targets biomass transformation and little consideration has been given to product extraction. Nonetheless, the high cost of the ionic liquid feedstock will require near quantitative recovery and recycle of the IL for the process to be economically viable. Therefore, efficient extraction of cellulose from ionic liquid mixtures is imperative for IL pretreatment processes. Conventional antisolvents for biomass precipitation include polar protic liquids which are highly effective at disrupting IL-cellulose interactions. For instance, liquid antisolvents are so effective that as little as 1 mass% residual water on an IL can reduce its cellulose capacity by as much as 55%. Therefore, ILs must be highly purified from liquid antisolvents prior to recycle. Preliminary analyses indicate that quantitative separation of an IL from a liquid antisolvent is highly energy intensive and could potentially impede large scale viability. Therefore, a novel gas antisolvent method which precipitates cellulosic biomass by compressed carbon dioxide at low to moderate pressures is presented. The gas antisolvent separation process is especially unique as it is non-reactive and completely reversible. By simple depressurization of CO2 to just a few bar pressure below the separation point, ionic liquid iv solvation power for cellulose is completely regenerated. The second research objective of this dissertation highlights the solid-liquid phase equilibrium effects of both conventional and novel separation processes. Spectroscopic techniques identify key trends within the separation data. Finally, an energy analysis is presented to demonstrate the advantages of this novel CO2 based precipitation process relative to liquid antisolvent separations. The third thrust of this dissertation investigates biomass conversion in mixed IL/cosolvent systems with emphasis on the transformation of fructose to 5-hydroxymethylfurfural. Preliminary results indicate that polar aprotic cosolvents both enhance the 5-HMF reaction rate as well as stabilize the product. Therefore, mixed IL/cosolvent systems are promising for both dissolution and conversion of biomass. A second component of this objective targets the chemical transformation of platform chemicals from biomass that are highly soluble in ILs into less polar value-added products that exhibit low miscibility and spontaneously phase separate. For instance, hydrogenation of cellulose derived 5-hydroxymethylfurfural and hemicellulose based furfural yield furan products that are applicable as “drop-in” fuel replacements and industrial solvents. Significant emphasis is placed on tuning hydrogen solubility with compressed CO2 since previous studies demonstrate that liquid phase H2 concentration can be rate limiting for hydrogenation reactions in ILs. The majority ionic liquid synthesis methods utilize batch scale operations with limited knowledge of the chemical kinetics and purification techniques. Moreover, most IL synthesis methods use many of the hazardous solvents that ILs will purportedly replace. For ionic liquids to be part of a sustainable process they must also be synthesized in a likewise sustainable manner. Therefore, the fourth objective of this dissertation is to develop thermodynamic, v process, and life cycle assessment models for the continuous synthesis of an ionic liquid. Experimental vapor-liquid and liquid-liquid equilibrium involved in the production of the model ionic liquid 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) are presented and modeled using the Peng-Robinson equation of state and Gibbs excess activity coefficient models. In conjunction with previously measured kinetic reaction parameters, the thermodynamic modeling results are used to develop an Aspen Plus process model for the continuous production of [HMIm][Br]. Finally, a cradle-to-gate life cycle assessment is presented to compare the environmental impacts associated with the synthesis of [HMIm][Br] in a range of reaction solvents. The final dissertation objective investigates the application of ionic liquids to CO2 sequestration. Select ionic liquids have some of the largest CO2 absorption capabilities of any known solvent indicating a competitive advantage for utilization in carbon dioxide capture processes. However, the characteristically high viscosities and slow mass transport properties of ionic liquids create potential barriers to industrial implementation. Therefore, the performance of ionic liquids in packed absorption towers for CO2 capture are presented and discussed. The computational findings suggest that packed absorption towers designed to accommodate ionic liquids with even the highest known CO2 capacities yield capital costs in excess of towers designed for conventional amine based solvents due to the comparatively slow gas-liquid mass transfer rates of ILs. These results indicate that process intensification techniques should be considered to take advantage of the thermodynamic benefits of ionic liquids. vi Acknowledgements Six years ago after completing a bachelor’s degree in chemistry, making the transition into chemical engineering and pursuing a doctoral degree seemed but a pipe dream. Yet, here I stand today looking back on what a fun experience it has been thanks to the many great people who have accompanied and supported me along the journey. First and foremost, thank you to my advisor Aaron Scurto. Early on in my academic career you identified potential within me. It is because of your mentorship and guidance that I have developed into the Ph.D. scientist that I am today. Thank you for being a role model, teammate, and friend. I am deeply grateful for the investment you made in my education and look forward to our future collaborations as colleagues. I would also like to thank my committee members including Professors
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