[Thesis]: Study of the improvement of cellulose processing in ionic liquid by using carbon dioxide by Joana M.C. Lopes

Joana Lopes Thesis

Dr. Joana M. C. Lopes defended her thesis on Wednesday 5th October 2016 at the University of Valladolid.

The capacity of the ionic liquids (ILs) of dissolving high concentrations of cellulose and/or lignin at relatively low temperatures makes them promising solvents for the processing of lignocellulosic biomass waste. Many applications have been proposed over the years: pre-treatment for fermentation processes, chemical transformations for obtaining chemical or biofuels and substitution reaction for obtaining cellulose derived polymers among others. Ionic liquids are a group of salts that exist as liquids at relatively low temperatures (<100 °C). Their properties can be tuned by appropriate selection of cation and anion, and they have an immeasurably low vapour pressure that makes them to be considered as green solvents.

The main limitation of those processes is the high viscosity that is highly increased when ILs dissolve cellulose. It is known that when the ILs dissolves small amounts of molecular solvents their viscosity is drastically decreased. Carbon dioxide presents high solubilities in ILs even at low pressures thus, influencing their thermophysical properties and cellulose substitution reactions. Carbon dioxide presents as a promising co-solvent for the ionic liquid processing of lignocellulose as it is an inert gas without environmental limitation that, and can be easily separated of the mixture by depressurization. The aim of this work is study how CO2 can improve different aspects of biomass processing using ionic liquids. To accomplish the aim of this thesis several aspects have been taken into account: Effect of the CO2 in thermophysical properties of mixtures CO2 + ILs; Analysis of the influence of CO2 in reaction processes, specifically in substitution reactions in order to produce cellulose-derived polymers; Study the application of CO2 in recovering valuable materials derived from cellulose.

Physical properties of relevancy for biomass processing media CO2 + ionic liquids were determined and modelled: melting points depressions around 10K at moderated pressures of CO2 (around 10 MPa) were found for alkylimidazolium ILs. Parameters of the Group Contribution EoS (GC-EoS) developed by Skjold-Jørgensen were adjusted for the group 1-alkyl-3-metylimidazolium chloride. Thus now, using this EoS is possible to predict phase equilibrium of ionic liquids of the alkylimidazolium chloride family without additional experimental data. Among other data, solubilities of CO2 in these ionic liquids can be estimated (Figure 1). The melting point depression caused by CO2 has been calculated using the GC-EoS, and it can be qualitatively described. Better results are achieved after the correlation of the change in the pure IL volume during melting.

Figure 1 Comparison of the predicted solubility of CO2 in the alkylmethylimidazolium chloride based ILs used in this work at the vacuum melting point stated in Table 7. [C2mim][Cl] (solid line), [Amim][Cl] (dashed line), [C4mim][Cl] (dotted line) and [C2OHmim][Cl] (dashed dotted line)
Figure 1. Comparison of the predicted solubility of CO2 in the alkylmethylimidazolium chloride based ILs used in this work at the vacuum melting point stated in Table 7. [C2mim][Cl] (solid line), [Amim][Cl] (dashed line), [C4mim][Cl] (dotted line) and [C2OHmim][Cl] (dashed dotted line)

The viscosities and densities of mixture CO2 + [Amim][Cl] with molar fractions up to 0.25 and temperatures in the range 333-372 K were measured. Densities were used to calculate excess molar values that resulted strongly negative. This indicates that the CO2 + [Amim][Cl] mixtures present a highly packed structure and can confirm the generally accepted theory that CO2 is dissolved in the free spaces of ionic liquids and that the expansion of the ionic liquid inducted by the presence of CO2 is very small. Viscosities were correlated as a function of temperature and carbon dioxide molar fractions with an average relative error of 2.5% (Figure 2). The viscosities of other mixtures CO2 + ionic liquids were also correlated for ionic liquids of other families using literature data. In general [Amim][Cl] and the other ionic liquids present a linear decrease of viscosity with CO2 molar fractions up to around 0.5 mol that is more pronounced at lower temperatures, and can reach between 60-100% viscosity reduction with respect the viscosity of the pure ionic liquid, making the CO2 a promising co-solvent for viscosity reduction in process with ionic liquids.

Figure 2 Viscosity predicted by the correlation as a function of CO2 molar fraction and CO2 pressure for different temperatures and initial water concentrations of the ionic liquid [Amim][Cl]
Figure 2 Viscosity predicted by the correlation as a function of CO2 molar fraction and CO2 pressure for different temperatures and initial water concentrations of the ionic liquid [Amim][Cl]

Carbon dioxide as been proved to be an alternative solvent for melting point and viscosity reduction with the new thermodynamic data on ILs field (Imidazolium chloride family) proposed in this work. Thus the synthesis of cellulose acetate in [Amim][Cl] was analyzed. The degree of substitution in general improves with temperature, reaction time and excess of acylating reagent. A mathematical model describing the reactions was developed (Figure 3). The model describes experimental data with an average deviation of 14%. To the best of our knowledge is the first kinetic model proposed to describe the process in literature. The parameters used to describe the reaction in [Amim][Cl] cannot describe the reaction in other ILs of the same family suggesting an important role of the IL in the reaction. Experimentally, new reactions were performed in the IL [Amim][Cl], at temperatures from 313 to 353 K covering temperatures lower than those usually studied in the system. It was found that at 313 K degrees of substitution lower than DS=2 are obtained after 24 h reaction while at 353 K reaction can be almost complete at 6 h being the last steps if substitution very slow. These results are consistent with other authors’ observations. The influence of catalyst scandium III triflate on degree of substitution, previously used for acetylating of other substances in IL was tested for first time for cellulose acetylation. Moreover, the acetylation reaction was studied using supercritical carbon dioxide.

Figure 3 Comparison of degree of substitution predicted by the model to experimental data from this work of the reaction at 353 K, 5 wt% cellulose concentration in [Amim][Cl] with 4.5 mol of acetic anhydride per mol of AGU
Figure 3. Comparison of degree of substitution predicted by the model to experimental data from this work of the reaction at 353 K, 5 wt% cellulose concentration in [Amim][Cl] with 4.5 mol of acetic anhydride per mol of AGU

The last part of the thesis is dedicated to the obtaining valuable materials from the cellulose processing where the study on the influence of the ILs, dissolution temperature and coagulation bath on surface area, volume size and pore size in cellulose aerogel production by supercritical CO2 drying is presented. The properties of cellulose aerogels prepared from a solution of microcrystalline cellulose in alkylmethylimidazolium ionic liquids can be tuned using different anions and cations and as well different dissolution temperature, concentration of cellulose and composition of coagulation bath. Imidazolium chloride ionic liquids present higher surface areas than other cellulose dissolving ILs with lower viscosities and melting points. An impregnation study was performed to determine the loading capacity of the cellulose aerogels with bioactive compounds (Figure 4). The drug loading capacity of the cellulose aerogels was positively influenced by their porosity properties. The aerogel was loaded with phytol as a model compound obtaining loads of around 50% w/w. Cellulose aerogels produced in this way absorbed more model compound (phytol) than other aerogels tested in literature, even when some of them presented better porosity properties. This indicates a positive effect of the material.

 

Figure 4 FTIR spectrum of non- and impregnated cellulose aerogels regenerated with phytol by supercritical CO2 at 10 MPa and 313 K
Figure 4. FTIR spectrum of non- and impregnated cellulose aerogels regenerated with phytol by supercritical CO2 at 10 MPa and 313 K

 

Thesis supervisors

  • Doctora María Dolores Bermejo Roda
  • Profesora Doctora María José Cocero Alonso

Members of the Committee

  • Dr. José Juan Segovia Puras, Chair Professor of Energy and Fluid Mechanics Engineering, University of Valladolid, Spain
  • Dr. Eduardo García-Versugo Cepeda, Senior Lecturer of Polymer Chemistry, University of Helsinki, Finland
  • Dr. Jason P. Hallett, Senior Lecturer of Chemical Engineering, Imperial College London, United Kingdom
  • Dr. Ana Vital Morgado Marques Nunes, Researcher of Chemical Engineering, New University of Lisbon, Portugal
  • Dr. Ángel Martín Martinez, Senior Lecturer, University of Valladolid, Spain

Joana Lopes

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