Materials known as metal-organic frameworks (MOFs) have a rigid, cage-like structure that
lends itself to a variety of applications, from gas storage to drug delivery.
By changing the building blocks that go into the materials, or the way they are arranged,
researchers can design MOFs suited to different uses.
However, not all possible MOF structures are stable enough to be deployed for applications
such as catalyzing reactions or storing gases.
To help researchers figure out which MOF structures might work best for a given application, MIT
researchers have developed a computational approach that allows them to predict which
structures will be the most stable.
Using their computational model, the researchers have identified about 10,000 possible MOF
structures that they classify as “ultrastable,” making them good candidates for applications
such as converting methane gas to methanol.
The research paper appears in the journal Matter.
Scientists are interested in MOFs because they have a porous structure that makes them
well-suited to applications involving gases, such as gas storage, separating similar gases
from each other, or converting one gas to another.
Recently, scientists have also begun to explore using them to deliver drugs or imaging agents
within the body.
The two main components of MOFs are secondary building units — organic molecules that
incorporate metal atoms such as zinc or copper — and organic molecules called linkers,
which connect the secondary building units.
These parts can be combined together in many different ways, just like LEGO building blocks.
Currently, the most common way to design MOFs is through trial-and-error.
More recently, researchers have begun to try computational approaches to designing these
materials.
Most such studies have been based on predictions of how well the material will work for a particular
application, but they don’t always take into account the stability of the resulting
material.
In a 2021 study, the researcher Heather Kulik reported a new model that she created by mining
a few thousand papers on MOFs to find data on the temperature at which a given MOF would
break down and whether particular MOFs can withstand the conditions needed to remove
solvents used to synthesize them.
She trained the computer model to predict those two features — known as thermal stability
and activation stability — based on the molecules’ structure.
In the new study, Kulik and her students used that model to identify about 500 MOFs with
very high stability.
Then, they broke those MOFs down into their most common building blocks — 120 secondary
building units and 16 linkers.
By recombining these building blocks using about 750 different types of architectures,
including many that are not usually included in such models, the researchers generated
about 50,000 new MOF structures.
The researchers then used their computational models to predict how stable each of these
50,000 structures would be, and identified about 10,000 that they deemed ultrastable,
both for thermal stability and activation stability.
They also screened the structures for their “deliverable capacity” — a measure of
a material’s ability to store and release gases.
For this analysis, the researchers used methane gas, because capturing methane could be useful
for removing it from the atmosphere or converting it to methanol.
They found that the 10,000 ultrastable materials they identified had good deliverable capacities
for methane and they were also mechanically stable, as measured by their predicted elastic
modulus.
The researchers also identified certain building blocks that tend to produce more stable materials.
One of the secondary building units with the best stability was a molecule that contains
gadolinium, a rare-earth metal.
Another was a cobalt-containing porphyrin — a large organic molecule made of four
interconnected rings.
Students in Kulik’s lab are now working on synthesizing some of these MOF structures
and testing them in the lab for their stability and potential catalytic ability and gas separation
ability.
The researchers have also made their database of ultrastable materials available for researchers
interested in testing them for their own scientific applications.