Melting the unmeltable

By Sebastian Hollstein

Glasses are an indispensable part of everyday life. One of the most important reasons for this is that glass objects can be manufactured almost universally and inexpensively in a wide variety of shapes and sizes using their corresponding melts. Processing in the (viscous) liquid phase offers a versatility that can hardly be achieved with other materials. However, this presupposes that the material from which the glass is made in terms of its chemical composition can be melted at all.

So-called metal-organic framework compounds—in short MOFs—have attracted a great deal of interest in recent years. Due to their special properties, they are considered to have great potential for future applications in energy and environmental technology, but also as sensor components and in the bio and life sciences. For example, MOFs can be used as starting materials for filter membranes for separating gases in technical combustion processes or for water treatment. The basis for the multitude of possible applications is above all one outstanding property of MOFs: their high and largely controllable porosity. MOF substances consist of inorganic particles that are connected by organic molecules to form a network of pores. As MOFs are predominantly in powder form, a primary challenge of the field is to produce bulk components. This is where glasses come into play.

Trade-off between properties and processability

But apart from a few exceptions, the porosity, of all things, prevents the materials from being meltable and, thus, processable into components of the desired shape. Chemists from the Friedrich Schiller University in Jena, Germany, and the University of Cambridge, United Kingdom, have now found a solution to this problem. They report on their research results in the journal »Nature Communications«.

In order to produce components for industrial applications from MOFs, they can be processed into so-called hybrid glasses, for example. To do this, however, you have to melt them down—a process that is not straightforward in this specific case. So far, only a handful of candidates of this class of substances have actually been demonstrated to be meltable. »In most known MOF materials, the high porosity is one of the reasons that—upon heating—they thermally decompose before reaching their melting point, that is, they burn,« explains Vahid Nozari, doctoral student at the Laboratory of Glass Science of the University of Jena. It is precisely the property that makes these materials so interesting that also prevents them from being processed using the glass route.