A team of Australian and US scientists has made a breakthrough in the development of membranes that, paradoxically, allow large molecules through far more readily than smaller ones. The new nanocomposite membranes are more selective and faster acting than previous versions used for molecular separation, which could have implications for many applications of the technology.
Molecular separation is used in a diverse range of processes such as environmental remediation, seawater desalination, biological purification and other molecular separations, including in gas and petroleum production. Traditionally, these separations are done using technologies such as distillation, absorption and adsorption which are often costly in terms of equipment and energy use. More recently membranes have been used to separate desired compounds from mixtures by passing them through tiny holes.
Membranes are attractive as filters because they are a low-cost, energy-efficient, green technology, but their use for separating gases has so far been limited by the lack of the right sort of membranes to yield pure products with the desired combination of high speed and low operating cost, while remaining stable.
By combining organic polymers normally used to make membrane filters with inorganic substances, in this case silica nanoparticles, researchers from North Carolina State University, CSIRO, the University of Texas and MTR, California, USA, have developed a membrane with an extraordinary ability to separate large organic molecules from the gases in which they might be present. The research team of Dr Richard Spontak, Dr Timothy Merkel, Dr Benny Freeman, Zhenjie He, Dr Ingo Pinnau, Pavia Meakin and Dr Anita Hill achieved their results by embedding the very fine silica particles into high free-volume, glassy polymer membranes. The resulting membranes behave unlike similar membranes embedded with metal oxides, carbon black or other nanoscale parties. Instead of the reduced permeability typical of ‘filled’ membranes, the researchers found that this new class of membrane had increased permeability and enhanced selectivity, and a useful ability to filter gases and organic vapours at the molecular level.
While the work so far has only been demonstrated in the laboratory and used in pilot-scale trials, the researchers are confident that the new membranes will offer the prospect of greater efficiency to industries such as Australia’s A$2.6 billion natural gas export sector, and natural gas suppliers and petroleum processor now struggling with energy-intensive and expensive methods of separating gases.
The ‘ultrapermeable, reverse-selective, nanocomposite membranes’ are described in more detail in a paper published in the 19 April issue of Science.