[Thesis]: Study of the improvement of cellulose processing using ionic liquids

Dr. Cristina Jiménez (fourth by the right) with the tribunal members and supervisors. (From left to right: Dr. Mª Dolores Bermejo Roda, Dr. Natalia Plechknova, Dr. Jalel Labidi, Dr. Sona Raeissi, Dr. Cristina Jiménez, Dr. Rira Duarte, Prof. Jose Juan Segovia and Dr. Mª Jose Cocero).

Dr. Cristina Jiménez de la Parra defended past Thursday 24 th November her PhD thesis at the University of Valladolid. The thesis was directed by Mª Dolores Bermejo Roda and Mª José Cocero Alonso from the High Pressure Processes Group and it has the mention of International PhD because of her secondment in the QUILL Research Center in Queens University of Belfast (UK) under the supervision of Prof. Kenneth Seddon.

The thesis propose solutions to improve biomass processing using ionic liquids, that are salts liquid at room temperature and that are considered green solvents with great interest for biomass processing. Among the contributions proposed in the thesis it is a study of the influence of water in the properties of ILs in collaboration with TERMOCAL GROUP and the application of a new ionic liquid developed by the new doctor in collaboration with the QUILL research center from Belfast, that poses better properties for biomass processing. For example it would allow dissolving cellulose at only 40ºC instead of 80-100ºC with the subsequent energy saving.

In the last years there is a growing interest in the processing of cellulose and other biopolymers such as lignin, hemicellulose or chitin in order to use them as a raw material for obtaining materials, chemicals and fuels. Even when cellulose is the most abundant natural source of carbon, most of it is not currently processed due to the difficulty of dissolving it.

Traditionally, cellulose is mostly used for paper production using highly pollutant processes with strong acids and bases. In the last years there is a growing interest in using cellulose for material processing or as a source for producing chemicals and new process are under developments. Most of them are based on the fermentation of sugars from cellulose after an acid or enzymatic pre-treatment to obtain bioethanol, but this kind of processes present a number of limitations.

In 2002, the pioneer work of Swatloski and co-workers demonstrated that some ionic liquids were able to dissolve cellulose in high concentrations becoming a promising media for the processing of polymers, which created a large interest in industry.

In the chapter 1 the state of the art of cellulose processing in ionic liquids is analyzed. In first place an introduction about cellulose, ionic liquids and the mechanism of dissolving cellulose in these solvents is presented. In the second part, an analysis of the new technologies developed in the field of biomass processing using ionic liquids is presented, paying special attention to the patents published and the companies involved on its development. These technologies have been divided into six main groups: dissolution and precipitation of cellulose; biomass fractionation;  delignification or cellulose pretreatment for hydrolysis reactions or a fermentation process for the production of bioethanol; reactions to obtain bio-refining products and chemicals; preparation of cellulose composites and substitution reaction to obtain cellulose derivatives. Ionic liquids have generated a lot of interest as a clean alternative to the traditional polluting processes, both in industry and academia, owing to their capacity for dissolving biopolymers.

The processes proposed are simple and relatively easy to implement being somehow limited by the high viscosity of the mixture biopolymer/IL and in some cases with the recoverability and recyclability of the IL. Since 2005 to the present more than 70 patents related to the processing of biomaterials to form different chemicals and fuels as well as composite materials and substituted polymers, have been published. A great number of these patents are owned by companies such as BASF and Eastman Chemical Companies. Nevertheless, none of these processes is yet implemented at industrial scale.

In chapter 2 the densities and viscosities of aqueous mixtures of two cellulose dissolving ionic liquids, 1-allyl-3-methylimidazolium chloride [amim][Cl] and 1-ethyl-3-methyl imidazolium acetate [emim][Ac], have been experimentally determined for water mole fraction concentrations up to 35% at atmospheric pressure and temperature range from 298.15 to 373.15 K. Our viscosity and density data are plotted in figure 1.  These two ionic liquids were selected because they are two of the most widely used for cellulose processing. Using density data molar excess volumes were calculated, resulting in negative values.

Literature viscosity correlations were modified in order to describe the highly no ideal viscosity behaviour as well as the temperature dependence for both water concentrations lower than xH2O=0.4 and for all the water concentration range obtaining average deviations lower than 5 %.  These modified equations were also applied to correlate viscosity of water + ionic liquid viscosity data for other 1-alkyl-3-methylimidazolium chloride ionic liquids as well as for ethanol + 1-ethyl-3-methyl imidazolium acetate from literature obtaining an average deviations lower than 15% in the viscosity in most systems considered. This work was carried out at TERMOCAL Group at the University of Valladolid.

Figure 1. Density and viscosity measurement at different temperatures from systems [emim][Ac] –water and [amim][Cl] – water.
Figure 1. Density and viscosity measurement at different temperatures from systems [emim][Ac] –water and [amim][Cl] – water.

In chapter 3, a new approach for improving cellulose processing using ionic liquids was undertook. The use of protic ionic liquid (PIL) was proposed. This kind of ILs has the particularities that can be easily recovered by distillation. After testing several acid and base combination, the optimum PIL among those investigated was the one synthetized by mixing the strong base 1,4-diazabicyclo[4.3.0]non-5-ene and methoxyacetic acid [dbnH][MeOAc]. This PIL present a melting point of 30ºC and viscosities lower than those of imidazolium chloride ILs (see figure 2). This PIL was able to dissolve 10% cellulose in weight at only 40ºC in 3 h stirring without cellulose degradation and it can be used to synthetize cellulose acetate with a degree of substitution of 1.9 at 40ºC in 24h . This work was performed in the laboratories of QUILL in the Queen University of Belfast (UK) and it was supervised by Prof. Kenneth R. Seddon.

Figure 2. Viscosity (v) and density (d)of the pure IL [dbnH][MeOAc] and viscosity (v) of solutions of differents concentrations of cellulose disolved in [dbnH][MeOAc].
Figure 2. Viscosity (v) and density (d)of the pure IL [dbnH][MeOAc] and viscosity (v) of solutions of differents concentrations of cellulose disolved in [dbnH][MeOAc].

In chapter 4, densities and viscosities of the ionic liquid 1-butyl-3-methylimidazolium chloride were determined (see figure 3) and other properties such as solubilities of CO2 and melting points were compiled from literature.

Figure 3. Comparison of experimental density and viscosity of [bmim][Cl] obtained in this work and bibliography data.
Figure 3. Comparison of experimental density and viscosity of [bmim][Cl] obtained in this work and bibliography data.

In chapter 5 the influence of CO2 in several reactions for cellulose processing is studied. The processes for cellulose processing considered in this chapter were hydrolysis and synthesis of cellulose acetate. In first place it was proved that CO2 did not cause cellulose precipitation, it was tested by keeping a cellulose solution of [bmim][Cl] under CO2 atmosphere for several days without observing cellulose precipitation. Similar results were obtained in hydrolysis with and without CO2 atmosphere (see table 1). This can be explained because in the hydrolysis process water was used as a reagent and the reduction of viscosity caused by water is higher than that caused by CO2 or because it is known that at moderate pH and in aqueous media CO2 can be found in aqueous media as HCO3+ or even as a CO32+. It may be hypothesized that the CO2 is causing a kind of buffer effect in hydrolysis medium, moderating the acidity and reducing the hydrolysis rate. Nevertheless at pressures of CO2 higher than 45 bar a reduction in the hydrolysis was observed.

The production of cellulose acetate was highly decreased when performed under CO2 atmosphere. It was hypothesised that part of the acetylating reagent (acetic anhydride) could be in the CO2 phase. Because under the P-T conditions used it present a solubility of 70% in mol in CO2 phase. Nevertheless there are not data available of the influence of [bmim][Cl] in this equilibrium. The preliminary experiments performed in this work are not sufficient to explain the reason why the CO2 is decreasing conversion in this reaction.

Table 1. Hydrolysis experiments at about 80ºC and different CO2 pressures at a reaction time of t=5 h
Table 1. Hydrolysis experiments at about 80ºC and different CO2 pressures at a reaction time of t=5 h

Thesis supervised by:

  • Dr. Mª Dolores Bermejo Roda, Universidad de Valladolid – Spain
  • Dr. Mª Jose Cocero, Universidad de Valladolid – Spain
  • Prof. Kenneth Seddon, QUILL Research Center in Queens University of Belfast – United Kingdom

Members of the Committee

  • Dr. Sona Raeissil, University of Shiraz – Iran
  • Dr. Jalel Labidi, Universidad del Pais Vasco – Spain
  • Dr. Rira Duarte, Universidade do Minho – Portugal
  • Dr. Natalia Plechknova, Queens University of Belfast – United Kingdom
  • Prof. José Juan Segovia Puras, research group TERMOCAL of the University of Valladolid – Spain


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