Viscosity and Rheology of Ionic Liquid Mixtures Containing Cellulose

Chapter 8

Downloaded by UNIV DE SHERBROOKE on October 11, 2017 | http://pubs.acs.org Publication Date (Web): September 29, 2017 | doi: 10.1021/bk-2017-1250.ch008

Viscosity and Rheology of Ionic Liquid Mixtures Containing Cellulose and Cosolvents for Advanced Processing David L. Minnick, Raul A. Flores,1 and Aaron M. Scurto* Department of Chemical & Petroleum Engineering and Center for Environmentally Beneficial Catalysis (CEBC), University of Kansas, Lawrence, Kansas 66045, United States *Phone: +1 (785) 864-4947; Fax: +1 (785) 864-4967; E-mail: [email protected] 1Current Address: Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

Select ionic liquids (ILs) are capable of dissolving significant quantities of biomass (cellulose, hemicellulose, and lignin) but are limited by prohibitively high viscosities even for relatively small biomass loadings. This may be rectified by using mixtures of ionic liquids with certain polar aprotic solvents, which can improve thermodynamic solubility, reduce transport properties, and decrease solvent cost compared to using pure ionic liquids. Here, the viscosity and rheology of microcrystalline cellulose dissolved in IL/cosolvent mixtures were measured at 40°C and 60°C using 1-ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] and dimethyl sulfoxide (DMSO) as a model ionic liquid (IL) and cosolvent. Increasing the DMSO cosolvent composition significantly reduced the apparent viscosity of IL/cellulose mixtures. The zero-shear viscosity and Power Law (Ostwald-de Waele) parameters are reported for the various mixtures. The cellulose solution exhibits increasingly Newtonian behavior with higher cosolvent loadings. The cost of a mixed solvent system for processing cellulose was analyzed. Optimal ratios of cosolvent to ionic liquid based on cost and solubility were found to be between 25-50 mass% of cosolvent, which corresponds to viscosities that were lower than the pure IL system by up to 80%.

© 2017 American Chemical Society Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Introduction Cellulose is an abundant and renewable biopolymer which may represent a future feedstock for the widespread production of chemicals, fuels, and materials. However, utilizing biomass remains a challenge as cellulose and hemicellulose are difficult to process using conventional techniques and solvents. Cellulose is insoluble in nearly all aqueous and organic solvents due to the complex hydrogen bonding network of cellulose hydroxyl groups. However, some ionic liquids (ILs) can dissolve large quantities of cellulose driven primarily by the anion’s ability to solvate cellulose and disrupt cellulose-cellulose interactions (1). The cation is believed to contribute, albeit to a lesser extent, to the dissolution mechanism (2). Many different ionic liquids have been screened for biomass solubility (1, 3) with the most widely studied ILs being 1-butyl-3-methylimidazolium chloride ([BMIm][Cl]) (4–6) and 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) (7–9). Ionic liquids with acetate ([OAc]) anions have been shown to react or degrade in the presence of cellulose, high temperatures, and CO2 (10–16). There is some indication that cellulose also degrades in [BMIm][Cl] above 95°C (17). Furthermore, many of the most promising ILs for biomass processing have relatively high viscosities in their pure form. Moreover, upon dissolution of even small cellulose quantities, ionic liquid (IL)-cellulose mixtures demonstrate significantly high viscosities and some ILs, such as [BMIm][Cl] form gels and undergo other solid-transitions upon dissolution of larger quantities of cellulose (18). Alternatively, we have recently identified the ionic liquid, 1-ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] (Figure 1) as a potential model IL for biomass dissolution as it has a relatively low pure-component viscosity (60 cP at 60°C), high cellulose solubility, and does not form gels with large amounts of dissolved cellulose (19, 20). In addition, [EMIm][DEP] is not known to react with cellulose and does not contain halides that can lead to corrosion issues with metals (21, 22).

Figure 1. 1-Ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] and DMSO. A significant disadvantage of using pure ILs for biomass processing is their high viscosities which can range from 20-2000+ cP depending on temperature. 190 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Table 1 displays the pure component viscosities for several ionic liquids capable of dissolving cellulosic biomass at 40°C and 60°C. Whereas [BMIm][Cl] has a significant biomass dissolution capacity, (estimated at 19+ mass% as it forms gels), its pure component viscosity (no cellulose) at 40°C is 3800 cP, which, from a processing standpoint, presents severe mass transfer limitations to biomass dissolution and chemical conversion. Furthermore, mixtures of [BMIm][Cl] containing high cellulose loadings are known to form gels thus reducing their processability. As will be shown here, in addition to previous literature discussions, the viscosity of an increases exponentially with cellulose composition; for instance the viscosity of [EMIm][DEP] containing 3 mass% cellulose at 40°C is greater than 3000 cP. These high mixture viscosities present significant transport limitations for industrial biomass and cellulose processing applications. The elevated viscosity will most directly affect pumping and mixing unit operations of IL-biomass processes. In addition, high viscosities will reduce the rate of mass transfer of biomass or reagent dissolution into the IL. Importantly, any chemical processing steps such as hydrolysis may be diffusionally, not kinetically, limited by the significant mixture viscosities of IL-cellulose solutions. In addition, the rate of heat transfer into or out of the mixture will be significantly diminished. A final disadvantage of using pure ILs is their relative expense, which poses an economic challenge to the implementation of IL technologies/processes at industrial scales, especially for commodity chemicals.

Table 1. Viscosity of select ionic liquids that dissolve cellulosic biomass. Ionic Liquid

Viscosity at 40°C [cP]

Viscosity at 60°C [cP]

[BMIm][Cl]a

3800

700

[BMIm][OAc]b

112

43

[EMIm][OAc]c

61

26

[EMIm][DEP]d

142

59

a

From Ref. (23) where [BMIm][Cl] is technically a subcooled liquid below its pure component melting point. b Ref. (24). c Ref. (25). d Experimental Data here.

One potential solution to overcome IL/cellulose mixture viscosity limitations is to use an IL/cosolvent mixture instead of a pure IL. However, most aqueous and organic solvents act as anti-solvents when added to IL/cellulose mixtures thereby decreasing the solubility of cellulose in IL mixtures (8). At first glance, using liquid antisolvents like water, alcohols, etc. provide what appears to be a relatively convenient separation method using benign (or relatively benign) reagents. However, the overall energy and cost of removing and recycling the antisolvent from the IL must be carefully considered. In addition, any IL/antisolvent system will inevitably absorb water from even “dried” biomass, which also must be removed prior to recycle. Alternatively, there are a few polar aprotic solvents which behave as cosolvents for cellulose in certain composition and temperature 191 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


regimes. We have recently reported that solvents including dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and 1,3-dimethyl-2-imidazolidinone (DMI) can actually increase the solubility of cellulose in ionic liquids versus pure ILs despite these solvents having no cellulose solubility of their own (20). Aprotic cosolvents do not seem to interfere with the hydrogen bonding between the cellulose and primarily the anion. Figure 2 illustrates the enhancement in equilibrium solubility of cellulose in IL mixtures with various cosolvents (20). As shown, there is a very wide composition range where the cosolvent does not negatively affect the solubility and may actually increase the solubility with maxima in the 70-80 mole% cosolvent range (approximately 50% cosolvent loading on a mass scale). However, beyond certain cosolvent compositions the solubility of cellulose in the mixture rapidly decreases. Thus, considerable quantities of ionic liquid may be replaced with a less expensive and lower viscosity cosolvent. Costa Gomes and coworkers have demonstrated that dissolution time, temperature, and mixture viscosity of cellulose in IL solutions can be reduced by inclusion of a cosolvent (26, 27). Therefore, ideal cosolvents should lower the mixture viscosity but not decrease the solubility of cellulose in the mixture. Additionally, the cosolvent species should be mutually soluble in the IL at a wide composition range so that a liquid-liquid phase split does not occur. Finally, chemical stability is also imperative: the cosolvent should not react with either the IL or cellulose. The use of comparatively cheaper cosolvents will have the additional effect of lowering the cost of the bulk IL/cosolvent mixture because less IL will be needed.

Figure 2. Cellulose solubility in [EMIm][DEP]-cosolvent mixtures at 40°C where cosolvent loading is represented on a molar percent basis. Lines are smoothed data. Data from Ref (20). 192 Shiflett and Scurto; Ionic Liquids: Current State and Future Directions ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

In this contribution, we present viscosity and rheology data for cellulose in mixtures of the model ionic liquid 1-ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] with DMSO as a model aprotic cosolvent. Temperatures of 40°C and 60°C were investigated for the pure IL and IL/cosolvent mixtures containing cellulose loadings from 1 to 5 mass%. A brief analysis discussing the solvent feedstock cost reduction resulting from utilizing ionic liquid/cosolvents mixtures for cellulose processing is also provided.


Materials and Methods Materials 1-Methylimidazole (99% purity) was obtained from Acros Organics. Triethyl phosphate (99.9% purity), dimethyl sulfoxide (99.9% purity, 63ppm H2O), and microcrystalline cellulose (MCC) were obtained from Sigma Aldrich, Inc. The microcrystalline cellulose sample used in this study was previously characterized by our group and had a measured crystallinity value of 61%, determined by a solid-state NMR technique (28). Furthermore, the MCC sample had an average MW of 152,789 g/mol ± 3,000 g/mol corresponding to a degree of polymerization of 937 anhydroglucose units (± 19 AGU) determined by a previously described technique using an Ubbelohde viscometer (20). Ionic Liquid Synthesis 1-Ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] (MW 264.26 g/mol) was synthesized from 1-methylimidazole and triethyl phosphate using a Biotage Initiator microwave reactor according to a previously described procedure (29). Unreacted precursors and impurities were removed from the IL by liquidliquid extraction with ethyl acetate. [EMIm][DEP] was dried on a high vacuum line (

Viscosity and Rheology of Ionic Liquid Mixtures Containing Cellulose

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