Novel Nanosheet allows for efficient ‘Molecular Sieving’ – Zeolite Membranes have enormous potential in Energy and Chemical industries

Zeolites have played an important role in the chemical industry in past decades. These microporous, aluminosilicate materials are well-known catalysts and adsorbents for catalytic reforming and separation of petrochemicals. More recently, zeolites have also been used to remove radioactive cesium from seawater following the Fukushima Daiichi nuclear disaster. Now, recent work from the University of Cincinnati, has opened even more doors for the material by tweaking its geometry and surface chemistry.

Zishu Cao and her colleagues fabricated membranes by tiling with 6-nanometre-thick zeolite flat sheets, they synthesized by a modified hydrothermal crystallization procedure. The resultant membrane was much thinner than a conventional zeolite membrane, with a thickness of less than 500 nanometres versus a traditional membrane’s thickness of several micrometres.

Tiling enhancements

Cao’s adviser, Junhang Dong of the Department of Chemical and Environmental Engineering, says Cao’s two-dimensional zeolite sheets overcome the major transport issues the conventional thicker zeolite membranes typically experience when they are several microns thick.

Fukushima 1 Infographic jpg

“The potential for zeolite membranes in the energy and chemical industries is enormous,” says Dong, “but the practical realization of their use is hindered by two serious issues caused by intercrystalline spaces in the films and their randomly oriented polycrystalline structure.”

These intercrystalline spaces, or gaps between the randomly oriented crystals that comprise the films, undermine the separation selectivity by causing nonselective permeation of molecules and ions. In addition, the random orientation of the crystals in the films results in longer and un-preferred diffusion paths making the membrane permeation inefficient.

The two-dimensional, zeolite nanosheet tiled membranes synthesized by Cao, however, provide an oriented straight channel structure that provides both reduced intercrystalline spaces and shortened diffusion lengths for enhanced selectivity and membrane flux.

“Imagine you are using blocks to waterproof a roof. Now we are using tiles or shingles to construct the roof,” says Dong.


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Petrochemical inspiration

Their readily scalable membrane fabrication by zeolite nanosheet lamination was inspired by recent work from the University of Minnesota, where researchers synthesized organophilic pure-silica zeolite nanosheets suitable for petrochemical separations. In Cao’s work, they incorporated aluminium ions into the silica-based zeolite framework to make the surface ionic and strongly hydrophilic – both favorable properties for water and ion separations. To the group’s knowledge, the ionic zeolite nanosheet laminated membrane is the first of its kind.

In their recently published paper, Cao displayed its potential for water desalination. The group chose to study this application because of its relevance to a wide range of needs in treating high salinity wastewaters, from industrial activities such as oil and gas drilling and power plant desulphurization and cooling. They reported high water flux with high salt rejection rates for brines containing up to 24% dissolved sodium chloride by weight.

The group says many routes are possible – desalination was just an example of the membrane’s capabilities. From here, they are exploring high-performance battery ion separators, catalysts, adsorbents, and thin-film sensors.

More details can be found in Science Advances.


Army COE Creates New Energy Efficient ‘Graphene Oxide’ Water Filter at Commercial Scale

The Army Corps of Engineers have successfully created a usable prototype of a new type of water filter.

The membranes are made of a mixture of chitosan, a material commonly found in shrimp shells, and a new synthetic chemical known as “graphene oxide”. Graphene oxide is a highly researched chemical worldwide.

  According to the Army Corps, one problem encountered by scientists working with graphene oxide is not being able to synthesize the material on a scale that can be put to use.

“One of the major breakthroughs that we’ve had here is that with our casting process, we’re not limited by size,” explains Luke Gurtowski, a research chemical engineer working on the membranes.

These filters have been found to effectively remove a number of different contaminants commonly found in water.

Dr. Christopher Griggs is the research scientist in charge of overseeing development of the new membranes.

Dr. Griggs told us, “Anybody who’s experienced water shortages or has been concerned about their water quality, or any type of contaminants in the water, this type of technology certainly works to address that.”

Another challenged faced by conventional water filtering methods is maintaining high energy efficiency.

“It requires a lot of energy for the net driving pressure to force the water through the membrane,” Dr. Griggs explains. “…we’re going to have to look to new materials to try to get those efficiency gains, and so graphene oxide is a very promising candidate for that.”

The Engineer Research and Development Center currently has two patents associated with the new filters and hopes to apply them for both civil and military purposes in the near future.