So -called metal -organic scaffolding or moped are materials that are holey like a Swiss cheese and, the chemical industry hopes, possibly as useful as a Swiss pocket knife. The chemical mini science structures can save, filter, separate them from each other as very selective filters, or concentrate on a catalyst. MoFs are often put on gases, but they could also serve to absorb water like a sponge and even get from dry air. This technique with which water in the desert could be gained from the air in theory is not yet mature-but now a German-American research team has been able to further optimize the process of water catch from desert air.
MOFs are made of metals and organic matter that form a porous structure of tiny cavities with a large inner surface area. As early as 2015, researchers had constructed special aluminum MOFs that trap water molecules even at a typical desert low humidity of 20 percent. Water can be squeezed out of the fully soaked MOF sponge with the help of the sun without additional energy: The water evaporates from the nanopores and condenses on cooling fins, which can then guide it into collection containers in drinking water quality. A prototype was finally able to extract almost three liters of water from desert air with one kilogram of MOF per day.
So far, it was insufficiently clear why water molecules got stuck in the aluminum mofs. The team of researchers around Joachim Sauer from Humboldt University in Berlin has now tried to find out with X-ray crystallography and quantum chemical calculations in the cavities of the moped. The observations made it clear how and where the water molecules get caught in the scaffolding at the atomic level. It shows that an interaction of the water molecules also plays an important role here: While the first water molecules combine with the organic fabrics of the mop, the subsequent water molecules first form chains within the cavities, then cluster and finally a network of clusters. The study was published in the specialist magazine "Science".
For the later function of the water collector, it is important that the first water molecules are not too tightly connected to the MOFs, otherwise the water cannot simply be squeezed out of the material. With the help of observations in experiments and computer models, the researchers were now able to design MOFs that form water molecule clusters but at the same time do not stick too tightly to the organic substances.
"The development of water-absorbing materials has so far been based on the principle of trial and error. Now that we understand how the molecular evolution of water structures in organometallic materials works, we can specifically optimize them at the atomic level," Sauer says in a press release. Depending on the outside temperature and humidity, water molecules behave differently – the study could now help to produce certain environmentally optimized MOFs that can possibly be used even at low temperatures in cooler regions.
