To get a better understanding of these silver compound ionic liquids, Eugene Mamontov, lead instrument scientist on BASIS, the Backscattering Spectrometer at ORNL’s Spallation Neutron Source, used quasielastic neutron scattering to probe the diffusion dynamics at the microscale that characterize these new solvents. Photo by Charlie Brooks |
The
separation of olefins and paraffin, two hydrocarbon compounds in
petroleum waste streams, is a heavy expense for the petrochemical
industry. The existing technology consumes a lot of energy because the
olefin-paraffin pairs have similar boiling and evaporation properties,
making it difficult and costly to separate them. Companies are looking
for techniques that reduce energy consumption and that economically
recycle such waste streams.
New
research at Oak Ridge National Laboratory shows that olefins can be
separated from paraffin efficiently and economically using silver
complex-based ionic liquids. Room temperature ionic liquids are a
promising class of novel materials—salts that are molten at or near room
temperature. They are made up of ions rather than molecules. Ionic
liquids do not easily evaporate, making them potentially recyclable and
environmentally friendly solvents.
There
are more than 400 ionic liquids that have been commercialized. The wide
variety of materials that can be used to make them enables scientists
to choose the ones best suited for a research project. These materials
have a wide range of potential applications in the chemical industry,
including in the separation of petroleum by-products in the oil
industry.
This
work builds on recent investigations into a membrane technology that
makes it easier to transport olefins out of a waste stream. In this
hybrid olefin-paraffin separation method, silver or copper ions are used
to bind to olefin to form an ionic liquid. The silver or copper ions
then act as carriers for the unwanted and transfer them through the
membranes. Two of this new class of silver compound ionic liquids are
currently under study.
To
get a better understanding of these silver compound ionic liquids,
scientists used quasielastic neutron scattering, or QENS, on BASIS, the
Backscattering Spectrometer at ORNL’s Spallation Neutron Source. QENS
can probe the diffusion dynamics at the microscale that characterize
these new solvents. Subsequent analysis of the scattering data points to
three distinct components in the mechanism that transports the olefins.
The
room-temperature ionic liquids for separating petroleum by-products
have low vapor pressure (they evaporate only at very high temperatures),
high thermal stability, and both positive and negative ions move freely
through them. The researchers investigated the usefulness of these
metal bearing solvents for the extraction and membrane separation of
inorganic, organic, and gaseous species; for sensing volatile organic
vapors; and for synthesizing novel materials.
A
key feature of these new solvents is that the silver ions are
incorporated as integral components of the ionic liquid at the molecular
level. They are very stable and the silver content is several orders of
magnitude higher than that of conventional liquid- or polymer-supported
membranes, which makes it easier to drive the olefins through the
membrane. In the current work, QENS neutron scattering provides
fundamental insight into the olefin transport that cannot be obtained
through other characterization techniques.
The
QENS analysis shows that in the temperature range of 300 to 340 K,
three dynamic components are present in the silver compound ionic liquid
solvents and can be described in the microscopic transport on the pico-
to nanosecond timescale. These occur in both the new solvents. The
findings are similar to those found in a study of an ionic liquid
developed earlier. This suggests that the three dynamic components found
by neutron scattering may be a common feature of the ionic liquids’
microscopic dynamics and are likely related to a fundamental lack of
order in these materials at the nanoscale.
