How a crystalline sponge sheds water molecules

In a new study, scientists answer this question in detail for a porous, crystalline material made from metal and organic building blocks -- specifically, cobalt(II) sulfate heptahydrate, 5-aminoisophthalic acid and 4,4'-bipyridine.

Using advanced techniques, researchers studied how this crystalline sponge changed shape as it went from a hydrated state to a dehydrated state. The observations were elaborate, allowing the team to "see" when and how three individual water molecules left the material as it dried out.

Crystalline sponges of this kind belong to a class of materials called metal-organic frameworks (MOFs), which hold potential for applications such as trapping pollutants or storing fuel at low pressures.

"This was a really nice, detailed example of using dynamic in-situ x-ray diffraction to study the transformation of a MOF crystal," says Jason Benedict, PhD, associate professor of chemistry in the University at Buffalo College of Arts and Sciences. "We initiate a reaction -- a dehydration. Then we monitor it with x-rays, solving crystal structures, and we can actually watch how this material transforms from the fully hydrated phase to the fully dehydrated phase.

"In this case, the hydrated crystal holds three independent water molecules, and the question was basically, how do you go from three to zero? Do these water molecules leave one at time? Do they all leave at once?

"And we discovered that what happens is that one water molecule leaves really quickly, which causes the crystal lattice to compress and twist, and the other two molecules wind up leaving together. They leak out at the same time, and that causes the lattice to untwist but stay compressed. All of that motion that I'm describing -- you wouldn't have any insight into that kind of motion in the absence of these sort of experiments that we are performing."

The research was published online on June 23 in the journal Structural Dynamics. Benedict led the study with first authors Ian M. Walton and Jordan M. Cox, UB chemistry PhD graduates. Other scientists from UB and the University of Chicago also contributed to the project.

Understanding how the structures of MOFs morph -- step by step -- during processes like dehydration is interesting from the standpoint of basic science, Benedict says. But such knowledge could also aid efforts to design new crystalline sponges. As Benedict explains, the more researchers can learn about the properties of such materials, the easier it will be to tailor-make novel MOFs geared toward specific tasks.

The technique the team developed and employed to study the crystal's transformation provides scientists with a powerful tool to advance research of this kind.

"Scientists often study dynamic crystals in an environment that is static," says co-author Travis Mitchell, a chemistry PhD student in Benedict's lab. "This greatly limits the scope of their observations to before and after a particular process takes place. Our findings show that observing dynamic crystals in an environment that is also dynamic allows scientists to make observations while a particular process is taking place. Our group developed a device that allows us to control the environment relative to the crystal: We are able to continuously flow fluid around the crystal as we are collecting data, which provides us with information about how and why these dynamic crystals transform."

The study was supported by the National Science Foundation (NSF) and U.S. Department of Energy, including through the NSF's ChemMatCARS facility, where much of the experimental work took place.

"These types of experiments often take days to perform on a laboratory diffractometer," Mitchell says. "Fortunately, our group was able to perform these experiments using synchrotron radiation at NSF's ChemMatCARS. With synchrotron radiation, we were able to make measurements in a matter of hours."