Sean M. Mercer

CO2-triggered switchable solvents, surfactants, and other materials

Waste CO2 at atmospheric pressure can be used to trigger dramatic changes in the properties of certain switchable materials. Compared to other triggers such as light, acids and oxidants, CO2 has the advantages that it is inexpensive, nonhazardous, non-accumulating in the system, easily removed, and it does not require the material to be transparent. Known CO2-triggered switchable materials now include solvents, surfactants, solutes, catalysts, particles, polymers, and gels. These have also been described as “smart” materials or, for some of the switchable solvents, “reversible ionic liquids”. The added flexibility of switchable materials represents a new strategy for minimizing energy and material consumption in process and product design.

Design, synthesis, and solution behaviour of small polyamines as switchable water additives

The practice of adding salt to water to induce salting out of contaminants or to break emulsions and suspensions is generally avoided industrially because of the expense of the necessary treatment of the salty water afterwards. However, the use of switchable water, an aqueous solvent with switchable ionic strength, allows for reversible generation and elimination of salts in aqueous solution, through the introduction and removal of CO2. In the effort to improve the efficiency of these switchable salts, a physical study modeling their reactivity and solution behaviour has been performed, resulting in a set of design principles for future switchable water additives. The resulting polyamines synthesized using this template show the highest efficiency recorded for a switchable water additive.

CO2-switchable drying agents

CO2-switchable desiccants have been prepared and evaluated for the drying of isobutanol. CO2 addition triggered the binding of water to the drying agent, while CO2 displacement triggered the waters facile release. The switchable desiccants were capable of absorbing more water and were able to regenerate at much milder conditions than traditional desiccants like molecular sieves.

Recycling of a homogeneous catalyst using switchable water

Aqueous/organic biphasic catalysis allows easy separation of a homogeneous catalyst from product, but is often inefficient when hydrophobic substrates are used. A system based on switchable water is monophasic in the absence of CO2 and biphasic in its presence. Catalysis can be performed in the monophasic solvent, and then switched to a biphasic system, separating catalyst from product. Removal of CO2 allows for easy recycling of the catalyst. Hydroformylations have been achieved using this solvent system. The catalyst was recycled several times with minimal loss of activity.

Nitrogen-containing polymers as potent ionogens for aqueous solutions of switchable ionic strength: application to separation of organic liquids and clay particles from water

Polymeric tertiary amines react reversibly with CO2 in the presence of water to form trialkylammonium-bicarbonate polyelectrolytes. These polyamines are useful as ionogens for “switchable water”, meaning an aqueous solution having switchable ionic strength. The performance of the polymeric ionogens in two different water purification applications was evaluated. Moderate results were obtained in the salting out of organic contaminants from aqueous solutions with up to 77% of CH3CN forced out from a 1 : 1 (w/w) mixture of CH3CN–H2O. However, very promising results were obtained in the sedimentation of clay particles. A notable enhancement in both the settling time required and the supernatant turbidity was observed even with low loadings of the polymers (>10 ppm). The addition of CO2 is required for the polymers to induce settling. Dimethylamino-functionalized polymethyl methacrylate proved to be the best polymer evaluated thus far in this process. In addition to enhancing the settling of both kaolinite and montmorillonite suspensions at different concentrations, it was also successfully shown to enhance the settling of oil sand tailings.

The Effect of Switchable Water Additives on Clay Settling

The recycling of process water from strip mining extractions is a very relevant task both industrially and environmentally. The sedimentation of fine tailings during such processes, however, can often require long periods of time and/or the addition of flocculants which make later water recycling difficult. We propose the use of switchable water additives as reversible flocculants for clay/water suspensions. Switchable water additives are compounds such as diamines that make it possible to reversibly control the ionic strength of an aqueous solution. Addition of CO(2) to such an aqueous solution causes the ionic strength to rise dramatically, and the change is reversed upon removal of the CO(2). These additives, while in the presence of CO(2), promote the aggregation of clay tailings, reduce settling times, and greatly increase the clarity of the liberated water. The removal of CO(2) from the liberated water regenerates a low ionic strength solution that does not promote clay aggregation and settling until CO(2) is added again. Such reversible behavior would be useful in applications such as oil sands separations in which the recycled water must not promote aggregation. When added to kaolinite and montmorillonite clay suspensions, switchable water provided process waters of lower turbidity than those additives from inorganic salts or by CO(2)-treatment alone. When recollected, the switchable water supernatant was shown to be recyclable over three cycles for enhanced settling of kaolinite.

Computational Investigation of the Hydration of Alkyl Diammonium Chlorides and Their Effect on THF/Water Phase Separation

The solvation behavior of alkyl diammonium chlorides of varying alkyl chain length and the molecular details of their effect on the salting-out of organic molecules in aqueous phase have been investigated by classical molecular dynamics simulations. More specifically, systems containing water, tetrahydrofuran (THF), and their mixtures with α,ω-alkyl diammonium chlorides [H3N(CH2)nNH3]Cl2 (n = 2, 4, 6, 8, and 10) were simulated at ambient temperature and pressure. Various force fields were tested and one was chosen based on its ability to reproduce the physical properties of the pure THF solution and its mixture with water. Structural and thermodynamic analyses of the simulated salt-solvent mixtures reveal different extents of hydration of the dications depending on the alkyl chain length and indicate that the hydrophobic interactions between the dication alkyl chain and organic molecules play a key role in the solvation of the latter species. In fact, shorter dications are shown to promote THF/water phase separation, in agreement with previous experimental findings.