skip to main content

Title: Identifying thermal phase transitions of lignin–solvent mixtures using electrochemical impedance spectroscopy
Lignin is unique among renewable biopolymers in having significant aromatic character, making it potentially attractive for a wide range of uses from coatings to carbon fibers. Recent research has shown that hot acetic acid (AcOH)–water mixtures can be used to recover “ultraclean” lignins of controlled molecular weight from Kraft lignins. A key feature of this discovery is the existence of a region of liquid–liquid equilibrium (LLE), with one phase being rich in the purified lignin and the other rich in solvent. Although visual methods can be used to determine the temperature at which solid lignin melts in the presence of AcOH–water mixtures to form LLE, the phase transition can be seen only at lower AcOH concentrations due to solvent opacity. Thus, an electrochemical impedance spectroscopy (EIS) technique was developed for measuring the phase-transition temperature of a softwood Kraft lignin in AcOH–water mixtures. In electrochemical cells, the resistance to double-layer charging ( i.e. , polarization resistance R p ) is related to the concentration and mobility of free ions in the electrolyte, both of which are affected by the phases present. When the lignin–AcOH–water mixture was heated through the phase transition, R P was found to be a strong function of more » temperature, with the maximum in R P corresponding to the transition temperature obtained from visual observation. As the system is heated, acetate ions associate with the solid lignin, forming a liquefied, lignin-rich phase. This association increases the overall impedance of the system, as mobile acetate ions are stripped from the solvent phase and thus are no longer available to adsorb on the polarizing electrode surfaces. The maximum in R P occurs once the new lignin-rich phase has completely formed, and no further association of the lignin polymer with AcOH is possible. Except at sub-ambient temperatures, the phase-transition temperature was a strong function of solvent composition, increasing linearly from 18 °C at 70/30 AcOH/water to 97 °C at 10/90 wt% AcOH/water. « less
; ; ;
Award ID(s):
Publication Date:
Journal Name:
Green Chemistry
Page Range or eLocation-ID:
1892 to 1897
Sponsoring Org:
National Science Foundation
More Like this
  1. Water + elastin-like polypeptides (ELPs) exhibit a transition temperature below which the chains transform from collapsed to expanded states, reminiscent of the cold denaturation of proteins. This conformational change coincides with liquid–liquid phase separation. A statistical-thermodynamics theory is used to model the fluid-phase behavior of ELPs in aqueous solution and to extrapolate the behavior at ambient conditions over a range of pressures. At low pressures, closed-loop liquid–liquid equilibrium phase behavior is found, which is consistent with that of other hydrogen-bonding solvent + polymer mixtures. At pressures evocative of deep-sea conditions, liquid–liquid immiscibility bounded by two lower critical solution temperatures (LCSTs) is predicted. As pressure is increased further, the system exhibits two separate regions of closed-loop of liquid–liquid equilibrium (LLE). The observation of bimodal LCSTs and two re-entrant LLE regions herald a new type of binary global phase diagram: Type XII. At high-ELP concentrations the predicted phase diagram resembles a protein pressure denaturation diagram; possible “molten-globule”-like states are observed at low concentration.
  2. This paper addresses the use of Ce 0.8 Gd 0.2 O 2−δ (GDC) infiltration into the Ni–(Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 (YSZ) fuel electrode of solid oxide cells (SOCs) for improving their electrochemical performance in fuel cell and electrolysis operation. Although doped ceria infiltration into Ni–YSZ has recently been shown to improve the electrode performance and stability, the mechanisms defining how GDC impacts electrochemical characteristics are not fully delineated. Furthermore, the electrochemical characteristics have not yet been determined over the full range of conditions normally encountered in fuel cell and electrolysis operation. Here we present a study of both symmetric and full cells aimed at understanding the electrochemical mechanisms of GDC-modified Ni–YSZ over a wide range of fuel compositions and temperatures. Single-step GDC infiltration at an appropriate loading substantially reduced the polarization resistance of Ni–YSZ electrodes in electrolyte-supported cells, as measured using electrochemical impedance spectroscopy (EIS) at various temperatures (600–800 °C) in a range of H 2 O–H 2 mixtures (3–90 vol% H 2 O). Fuel-electrode-supported cells had significant concentration polarization due to the thick Ni–YSZ supports. A distribution of relaxation times approach is used to develop a physically-based electrochemical model; the results show thatmore »GDC reduces the reaction resistance associated with three-phase boundaries, but also appears to improve oxygen transport in the electrode. Increasing the H 2 O fraction in the H 2 –H 2 O fuel mixture reduced both the three-phase boundary resistance and the gas diffusion resistance for Ni–YSZ; with GDC infiltration, the electrode resistance showed less variation with fuel composition. GDC infiltration improved the performance of fuel-electrode-supported full cells, which yielded a maximum power density of 2.28 W cm −2 in fuel cell mode and an electrolysis current density at 1.3 V of 2.22 A cm −2 , both at 800 °C.« less
  3. The wide spread use of hazardous and expensive solvents for the liquid–liquid extraction (LLE) of critical metals has been a growing source of waste in the metal refinement industry. We have developed and characterized room temperature liquid hydrophobic binary mixtures based on common pharmaceutical and food grade compounds as sustainable, cost effective alternatives to both ionic liquids and conventional solvents. Additionally, we introduce liquid mixtures with Proton Sponge® (1,8-bis(dimethylamino)naphthalene), one of the strongest known organic bases. These mixtures have been applied to the LLE of indium( iii ) ions from hydrochloric acid solutions, displaying an extraction efficiency greater than 99% in some systems. A systematic approach to identifying the underlying mechanism of extraction, in particular relating to the charge, solubility, and complexation of the indium species in the organic phase has been developed.
  4. Thermodynamic, structural, and transport properties of tetrabutylphosphonium hydroxide (TBPH) and tetrabutylphosphonium chloride (TBPCl)–water mixtures have been investigated using all-atom molecular dynamics simulations in response to recent experimental work showing the TBPH–water mixtures capability as a cellulose solvent. Multiple transitional states exist for the water—ionic liquid (IL) mixture between 70 and 100 mol% water, which corresponds to a significant increase in water hydrogen bonds. The key transitional region, from 85 to 92.5 mol% water, which coincides with the mixture’s maximum cellulose solubility, reveals small and distinct water veins with cage structures formed by the TBP+ ions, while the hydroxide and chloride ions have moved away from the P atom of TBP+ and are strongly hydrogen bonded to the water. The maximum cellulose solubility of the TBPH–water solution at approximately 91.1 mol% water, appears correlated with the destruction of the TBP’s interlocking structure in the simulations, allowing the formation of water veins and channeling structures throughout the system, as well as changing from a subdiffusive to a near-normal diffusive regime, increasing the probability of the IL’s interaction with the cellulose polymer. A comparison is made between the solution properties of TBPH and TBPCl with those of alkylimidazolium-based ILs, for which water appearsmore »to act as anti-solvent rather than a co-solvent.« less
  5. By operating in a region of liquid–liquid equilibrium, hot acetic acid–water mixtures can be used to simultaneously clean, fractionate, and solvate Kraft black-liquor lignins. Lignin-rich liquid phases of controlled molecular weight with key metals contents reduced to <50 ppm are obtained without a washing step.