This is a scanning electron microscope image of a typical zeolite nanosheet. Image: University of Minnesota.
This is a scanning electron microscope image of a typical zeolite nanosheet. Image: University of Minnesota.

A team of researchers from the University of Minnesota and King Abdulaziz University in Saudi Arabia has developed a ground-breaking, one-step, crystal growth process for making ultra-thin layers of material with molecular-sized pores. The researchers demonstrated the applicability of the material, termed zeolite nanosheets, by using it to make ultra-selective membranes for chemical separations.

These new membranes can separate individual molecules based on their shape and size. As such, they could improve the energy-efficiency of chemical separation methods used to make everything from fuels to chemicals to pharmaceuticals. The researchers report the novel growth process in a paper in Nature, and have also filed a provisional patent on the technology.

"Overall, we've developed a process for zeolite nanosheet crystal growth that is faster, simpler and yields better quality nanosheets than ever before," said Michael Tsapatsis, professor of chemical engineering and materials science at the University of Minnesota and lead researcher of the study. "Our discovery is another step toward improved energy efficiency in the chemical and petrochemical industries."

Today, most chemical and petrochemical purification processes are based on heat-driven processes like distillation, which are very energy-intensive. For example, chemical separations based on distillation account for nearly 5% of the total energy consumption in the US. Several companies and researchers are developing more energy-efficient separation processes based on membranes that can separate molecules according to their size and shape. One class of these membranes is based on zeolites, which are silicate crystals that have pores of molecular dimensions. However, the multi-step processes required to fabricate these membranes are costly and difficult to scale up, and so commercial production remains a challenge.

In this new discovery, the researchers have developed the first bottom-up process for direct growth of zeolite nanosheets, which can be used to make high quality molecular sieve membranes. The nanosheets are only around 5nm thick but several micrometers wide (10 times wider than previous zeolite nanosheets). They also grow in a uniform shape, making it much easier to use them as the basis for producing membranes for chemical purification.

"With our new material, it’s like tiling a floor with large, uniform tiles compared to small, irregular chips of tile we used to have," said Mi Young Jeon, a chemical engineering and materials science PhD graduate at the University of Minnesota and first author of the study. "Uniform-shaped zeolite nanosheets make a much higher-quality membrane with surprisingly high separation values that can sieve-out impurities." The researchers' molecular dynamics calculations suggest that separation values in excess of 10,000 could be achieved with these nanosheets.

To grow the zeolite nanosheets, the researchers begin with seed nanocrystals that initially double in size and develop facets. These seed crystals then trigger the formation of a twin outgrowth that evolves to become the nanosheet. Nanosheets start to appear from one corner of the seed crystals and then continue to grow, completely encircling the seed to form a faceted nanosheet that is extremely thin and uniform in size and shape.

The uniform shape of the crystals came as quite a surprise when it was first observed four years ago. "In my 25 years of studying zeolite crystal growth, I'd never seen anything like this before," Tsapatsis said.

Other researchers were also surprised by early results. "It was exciting and rewarding to look at these thin crystals under the electron microscope and study their structure," said Andre Mkhoyan, a professor of chemical engineering and materials science at the University of Minnesota.

"After identifying the presence of a twin in the electron microscope, we knew we had found something that would be a big step forward in developing ultrathin porous crystals," added Prashant Kumar, a chemical engineering and materials science senior graduate student at the University of Minnesota, who performed electron microscopy experiments.

"The nanosheet's ability to grow in only two dimensions was initially unexpected but we were able to systematically unravel its structure and crystal growth mechanism" said Peng Bai, a postdoctoral researcher in both the Department of Chemistry and Department of Chemical Engineering and Materials Science at the University of Minnesota, who used quantum chemical methods to interpret the unique structure.

This story is adapted from material from the University of Minnesota, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.