Silver ionic liquids: Potential new solvents

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2011

The result could lead to a better way of separating olefins from paraffin-rich waste streams. 

KEY CONCEPTS
• Separation of olefins from paraffin-rich waste streams currently uses techniques that are energy intensive, expensive and not always successful.
• Silver complex-based ionic liquids show potential for being more effective because silver has a strong affinity for olefins.
• Work is underway to better understand how these ionic liquids can bind and transport olefins through the use of quasielastic neutron scattering.

Ionic liquids remain an area of strong interest because they have great potential for use in a number of applications, including lubricants. An ionic liquid, by definition is a salt with a melting point below 100 C.

There are at least one million combinations of anions and cations that can be used to prepare ionic liquids. Currently, more than 400 ionic liquids have been commercialized.

In a previous TLT article, research looking at the use of ionic liquids containing two cations and two anions as high-temperature lubricant basestocks was described (1). A series of dicationic liquids were prepared by synthesizing imidazole and triazole derivatives of polyethylene glycol. The anion used was lithium bis (trifluoromethanesulfonyl) amide. One of the ionic liquids displayed outstanding performance at 300 C in lubricant lab bench testing and may have potential for use in the next generation of high-temperature aircraft lubricants.

A potential new application for ionic liquids is the separation of olefins from paraffin-rich petroleum waste streams. Key examples include the need to separate ethylene and propylene from their respective paraffins, ethane and propane.

Dr. Eugene Mamontov, one of the lead instrument specialists at Oak Ridge National Laboratory’s Spallation Neutron Source in Oak Ridge, Tenn., says, “The currently accepted way to separate olefins from paraffins is through the use of low-temperature/ cryogenic distillation. This process is very difficult to do because a specific olefin and its corresponding paraffin are almost equally volatile. As a result, the process is very expensive and energy intensive.”

A second technology that has been looked at is to use membranes to separate olefins from paraffins using specific metal ions such as silver and copper as carriers to transport the olefins through the membranes. Attempts to use both immobilized liquid membranes and polymer membranes with fixed carrier sites have not been successful due to membrane instability in the former case and slow diffusion in the latter case.

A third approach that is looking attractive is using ionic liquids as solvents to extract olefins from the paraffin-rich petroleum waste stream. Prior to using ionic liquids in these waste streams, further work has been done to better understand their transport properties.

QUASIELASTIC NEUTRON SCATTERING
Mamontov worked with a group of researchers to better understand how silver complex-based ionic liquids can be used in this application. He says, “We decided to work with silver-based ionic liquids for two reasons. First, ionic liquids in general are very stable, exhibit low vapor pressures and do not decompose until 300 C. Ionic liquid derivatives containing silver are even more stable than other types. A second reason is that silver and copper can bind very effectively to other components including olefins because both metals have very effective binding sites. One functionality that these metals can readily attach to is olefins.”

Two ionic liquids studied are both prepared with the trifluoromethanesulfonyl imide anion. They are silver complexes with propylamine and 1-pentene. They are prepared by a complexation reaction of the neutral organic ligands with silver ions followed by anion metathesis with the lithium salt of bis (trifluoromethylsulfonyl) imide.

The transport properties of both silver complex-based ionic salts were evaluated using quasielastic neutron scattering (QENS). The experiments were conducted through the use of the Spallation Neutron Source’s Backscattering Spectrometer, BASIS. An image of the instrument is shown in Figure 2.


Figure 2. The Spallation Neutron Source’s Backscattering Spectrometer uses quasielastic neutron scattering to better understand the potential for silver ionic liquids to separate olefins from paraffin-rich petroleum waste streams. (Courtesy of Oak Ridge National Laboratory)

Mamontov says, “We sealed the ionic liquid samples in cylindrical aluminum containers and evaluated the results at temperatures between 300 K and 340 K. Upon contacting the mobile species of the ionic liquids, neutrons will either lose energy or gain energy. The change in energy is used to determine the movement of the molecules.”

The energy changes are extremely small, according to Mamontov. He adds, “Atoms in solid crystalline materials are very energetic as they vibrate very quickly as if they are on springs. Neutron energy transfers are relatively large and much easier to resolve. In liquids, molecular diffusion motions are not as energetic, which means that an instrument with better energy resolution and sensitivity such as the BASIS is required to study molecular motions.”

In the evaluation work, Mamontov indicates that ionic liquids go through several different types of motion, including jumping, rotating and quick bending of side groups. He says, “They diffuse quickly but only over very small distances in the range of a few nanometers. The process is much faster than regular diffusion.”

The cation has more of an influence on the degree of this fast molecular diffusion process. But the anion also indirectly influences this motion.

Mamontov says, “The motion of ionic liquids is very complicated. Once we have a better understanding, we will then include olefins in the experiments to determine how ionic liquids bind and then transport olefins.”

Additional information on this work can be obtained from a recent article (2) or by contacting Mamontov at mamontove@ornl.gov.

REFERENCES
1. Canter, N. (2007), “Using Dicationic Liquids as High Temperature Lubricants,” TLT, 63(5), pp. 12–13.
2. Mamontov, E., Baker, G.A., Luo, H. and Dai, S. (2011), “Microscopic Diffusion Dynamics of Silver Complex-Based Room-Temperature Ionic Liquids Probed by Quasielastic Neutron Scattering,” ChemPhysChem, 12(5), pp. 944–950.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.