Metal-organic frameworks become flexible

November 17, 2020

Materials consisting of inorganic and organic components can combine the best of two worlds: under certain circumstances, the so-called MOFs - short for metal-organic frameworks - are structured in the same order as crystals and are at the same time porous and deformable. This opens up the prospect of intelligent materials for energy-saving technical applications. However, so far only a few flexible MOFs have been identified.

A research team from Ruhr-Universität Bochum (RUB) and Technical University of Munich (TUM) has used experiments and simulations to find out by what means MOFs can be rendered flexible and why: they tricked the system by using clever chemical manipulations to enable a variety of energetically similar arrangements in the crystalline order.

The application potential of MOFs was first discovered around 20 years ago, and almost 100,000 such hybrid porous materials have since been identified. There are great hopes for technical applications, especially for flexible MOFs.

As shock absorbers, for example, they could react to sudden high pressure by closing their pores and losing volume, i.e. deforming plastically. Or they could separate chemical substances from each other like a sponge by absorbing them into their pores and releasing them again under pressure.

"This would require much less energy than the usual distillation process," explains Rochus Schmid. However, only a few such flexible MOFs have been identified to date.

MOFs under pressure

In order to get to the bottom of the underlying mechanisms within such materials, the Munich team has carried out a more detailed experimental analysis of an already widely known MOF. To this end, the researchers subjected it to uniform pressure from all sides, while observing what goes on inside using X-ray structure analysis.

"We wanted to know how the material behaves under pressure and which chemical factors are the driving force behind the phase transitions between the open-pored and closed-pore state," says Gregor Kieslich. The experiment showed that the closed-pore form is not stable; under pressure the system loses its crystalline order, in short: it breaks down.

This is not the case with a variant of the same basic structure: if the team attached flexible side chains of carbon atoms to the organic connecting pieces of the MOF that protrude into the pores, the material remained intact when compressed and resumed its original shape when the pressure decreased. The carbon arms turned the non-flexible material into a flexible MOF.

The secret of phase transformation

The Bochum team investigated the underlying principles using computer chemistry and molecular dynamics simulations. "We have shown that the secret lies in the degrees of freedom of the side chains, the so-called entropy," outlines Rochus Schmid. "Every system in nature strives for the greatest possible entropy, to put it simply, the greatest possible number of degrees of freedom to distribute the energy of the system."

"The large number of possible arrangements of the carbon arms in the pores ensures that the open-pored structure of the MOF is entropically stabilized," Schmid continues. This facilitates a phase transformation from the open-pored to the closed-pore structure and back again, instead of breaking down when the pores are squeezed together as would be the case without the carbon arms."

In order to calculate such a large system comprised of many atoms and to search for the many possible configurations of the arms in the pores, the team developed a precise and numerically efficient theoretical model for the simulation.

The key result of the study is the identification of another chemical option to control and modify the macroscopic response behaviour of an intelligent material by a thermodynamic factor. "Our findings open up novel ways to specifically achieve structural phase transformations in porous MOFs," concludes Gregor Kieslich.
In this research the working group Crystal Chemistry of Functional Materials at TUM under the direction of Dr. Gregor Kieslich cooperated with the Computational Materials Chemistry working group of the RUB under the direction of Prof. Rochus Schmid.

The work was funded by the Deutsche Forschungsgemeinschaft as part of the research group 2433 "Switchable MOFs" as well as by a start-up grant as part of the priority program 1928 "Coordination Networks: Building Blocks for Functional Systems", the Fonds der Chemischen Industrie and the University of Edinburgh. The high-pressure powder X-ray diffraction (HPPXRD) data were recorded at the Diamond Light Source in Oxfordshire, England.


Pia Vervoorts, Julian Keupp, Andreas Schneemann, Claire L. Hobday, Dominik Daisenberger, Roland A. Fischer, Rochus Schmid, Gregor Kieslich
Configurational Entropy Driven High?Pressure Behavior of a Flexible Metal?Organic Framework
Angewandte Chemie, 2020, DOI: 10.1002/anie.202011004

Technical University of Munich (TUM)

Related Pressure Articles from Brightsurf:

High blood pressure treatment linked to less risk for drop in blood pressure upon standing
Treatment to lower blood pressure did not increase and may decrease the risk of extreme drops in blood pressure upon standing from a sitting position.

Changes in blood pressure control over 2 decades among US adults with high blood pressure
National survey data were used to examine how blood pressure control changed overall among U.S. adults with high blood pressure between 1999-2000 and 2017-2018 and by age, race, insurance type and access to health care.

Effect of reducing blood pressure medications on blood pressure control in older adults
Whether the amount of blood pressure medications taken by older adults could be reduced safely and without a significant change in short-term blood pressure control was the objective of this randomized clinical trial that included 534 adults 80 and older.

Brain pressure controls eye pressure, revealing new avenues for glaucoma treatment
Neuroscientists have discovered that eye and brain pressure are physiologically connected.

A question of pressure
The Physikalisch-Technische Bundesanstalt (PTB) has implemented a novel pressure measurement method, as a byproduct of the work on the 'new' kelvin.

Volcanoes under pressure
When will the next eruption take place? Examination of samples from Indonesia's Mount Merapi show that the explosivity of stratovolcanoes rises when mineral-rich gases seal the pores and microcracks in the uppermost layers of stone.

Here's something that will raise your blood pressure
The apelin receptor (APJ) has been presumed to play an important role in the contraction of blood vessels involved in blood pressure regulation.

Under time pressure, people tell us what we want to hear
When asked to answer questions quickly and impulsively, people tend to respond with a socially desirable answer rather than an honest one, a set of experiments shows.

Arm cuff blood pressure measurements may fall short for predicting heart disease risk in some people with resistant high blood pressure
A measurement of central blood pressure in people with difficult-to-treat high blood pressure could help reduce risk of heart disease better than traditional arm cuff readings for some patients, according to preliminary research presented at the American Heart Association's Hypertension 2019 Scientific Sessions.

Heating pads may lower blood pressure in people with high blood pressure when lying down
In people with supine hypertension due to autonomic failure, a condition that increases blood pressure when lying down, overnight heat therapy significantly decreased systolic blood pressure compared to a placebo.

Read More: Pressure News and Pressure Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to