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UD researchers have developed a new method for making iron-based
metal organic framework (MOF) materials. Pictured: Graduate student
Amanda Weaver (left) sets the UD-developed electrochemical process in
motion, while chemistry professors Eric Bloch (center) and Joel
Rosenthal (right) observe.
Metal organic frameworks (MOFs) are a promising class of materials
that have many applications as catalysts, sensors and for gas storage.
Widely studied over the past two decades, MOFs are typically produced
using chemical processes that require high heat and high pressure.
Now, University of Delaware chemists Joel Rosenthal and Eric Bloch
report that it is possible to produce iron-based MOF materials directly
using renewable electricity at room temperature.
The UD-developed method is 96% efficient in using electricity to form
the MOF materials quickly, reliably and inexpensively. The UD
researchers reported the advance in a new paper published in ACS Central Science, a journal of the American Chemical Society.
According to Rosenthal, professor of chemistry and biochemistry
in UD’s College of Arts and Sciences, an easy way to think about MOFs
is to imagine tinker toys, where clusters of metal atoms represent the
toy’s wooden wheels and small organic molecules represent the spindly
sticks that connect the clusters together.
In between are voids with tremendous potential for chemical storage
and separations. For example, a pile of MOF material the size of a pea
has an internal surface area the size of two football fields that can be
used to store gases like methane or hydrogen, separate gases and
catalyze reactions. They can even be used as sensors.
“The quality of the materials we can produce is as good as what you
could expect from the best thermal methods, but far more scalable and
sustainable,” said Rosenthal, an expert in electrochemistry. “Our
discovery is a major step forward in making MOFs a more practical option
for many different applications.”
Move this whole section up, swapping places with the section above it.
In a demonstration, the researchers used their unique method to
precisely pattern MOF materials on a surface by etching the conductive
material onto glass in the shape of the state of Delaware and growing
the MOF material on the conductive coating.
One challenge that has constrained MOFs to academic labs is that
making them on a large scale is difficult and not particularly
environmentally friendly. So, Rosenthal had the idea to start using
electricity to trigger the synthesis of MOFs. Using electricity allows
the amount of energy introduced to a synthetic process to be easily
adjusted at room temperature, creating a safer way to make MOFs without
the high temperatures, high pressures and sometimes toxic reagents that
are normally used.
Drive to the foot of the Delaware Memorial Bridge and on both the
Delaware and New Jersey sides you will see chemistry plants that are
each the size of a small arena or stadium. These plants house a few
reactors that do a handful of different chemical reactions to make
chemicals useful to society.
“To efficiently carry out many thermal chemical processes on
commercial or commodity scales generally requires these large footprints
and very expensive infrastructure, but electrochemistry provides a way
to break these rules,” said Rosenthal. “You don’t need to build a giant
electrochemical plant to efficiently scale up an electrochemical method.
Electrosynthesis is often much more versatile in terms of translation
from an academic lab to the commercial marketplace.”
The chemistry isn’t as simple as a child sitting in the living room
connecting wheels and sticks, though. Advances in MOF synthesis to date
have been limited by the combinations of metals that can be used and the
kinds of synthetic and organic materials that can be combined using
The paper specifically focuses on preparing MOF materials using
clusters of iron atoms. Rosenthal and Bloch aren’t the first to make
iron MOFs. Traditionally, Rosenthal explained, researchers make these
materials by taking an iron (3+) salt, an organic molecule and a
relatively expensive solvent that decomposes under certain reaction
conditions and heating it all up in a sealed container at high pressures
for at least a day, sometimes multiple days, then opening it up and see
what they get.
By contrast, he and Bloch begin with a solution containing solvent,
organic molecules and iron (2+) ions, which have an extra electron that
changes the way the iron behaves. The researchers use an electrode made
from either carbon or a type of conducting glass to pass electricity
through the solution and toggle the charge of the metal particles in the
solution from iron (2+) to iron (3+). It’s like a switch, making the
iron more highly charged so it can produce the MOF in a way that is
direct and efficient, without side reactions or effects typical of
traditional thermal chemistry methods.
“As the electrode is taking electrons from iron, that iron goes and
finds an organic linker and makes some MOF. It’s almost 100% efficient,
in that every electron we move results in MOF synthesis. There aren’t
any side reactions or undesired products,” said Bloch, an assistant
professor of chemistry and biochemistry who specializes in metal organic
frameworks and adsorptive materials.
Further, if the right kind of electrode is used, it is possible to
do more than create and collect the MOF product. The research team can
grow the material directly on the electrically conductive substrate, an
advantage that could enable MOFs to be used in various devices and
patterned supports, bringing advanced MOF sensors within reach.
Rosenthal explained that to make an MOF into a sensor you need a way
to interconnect it with an electrically conductive support to get a
readout. This isn’t something the research community had figured out how
to do well, until now, he said. Electrochemically synthesizing and
growing the MOF on the UD team’s electrode support provides a way to
hardwire the MOF for better communication between materials.
One way this technology might be used is in miniature sensors, maybe
in cell phones to measure air quality or to selectively detect particles
in the air as part of security measures at airports.
“Sensing gases and molecules now can be pretty straight forward,
similar to the way your smoke detector works to sense one type of gas
over another based on its reactivity,” said Bloch.
A cross section of the electrosynthesized MOF on the conducting
substrate is shown on the left in gray. The solid glass substrate
appears on the bottom of the image and the porous layers of the MOF
material are visible on top. On the right (multicolor), an X-ray
technique called EDX mapping identifies the elemental composition of the
MOF materials (red is iron, yellow is silicon, light blue is calcium,
and dark blue is indium), revealing that iron is what makes up the MOF
layers while the other elements are at the glass/MOF interface and
comprise the glass substrate. Each panel is about 10–20 microns wide,
over two times smaller than a speck of dust visible to the naked eye.
The electrosynthetic reaction is fast, too, causing MOF powder to
form in the solution within minutes. And while materials that sit too
long in solution often degrade with time or go on to become a different
material entirely due to side reactions, MOF materials created via
electrosynthesis are stable and simply settle onto the bottom of the
vial. Since the electrosynthetic process is carried out at room
temperature, material decomposition is much less of a concern.
The longer the electrolysis runs, the greater the amount of MOF
material that can be siphoned off as a product. The method’s simplicity
makes it versatile in terms of translating it from an academic lab bench
to the commercial marketplace, too, the researchers said.
Graduate student Anna Weaver, a co-author on the paper, only arrived
at UD this summer but Rosenthal said she played a key role in
demonstrating the effectiveness of the team’s method. Weaver ran several
late-stage experiments that provided additional data for the paper.
“Anna’s ability to make contributions so quickly speaks both to her
talents and the ease with which this chemistry can be carried out. It
doesn’t take learning a dark art to get this to work,” he said.
Electrically driven chemistry also opens the door to exploring
materials that have been predicted to have excellent properties for
MOFs, such as those based upon cobalt, but remain unknown because they
are incompatible with traditional chemistries that rely on heat to set
the reaction in motion.
“As catalysts, we know certain metals would be phenomenal as MOFs,
but the normal methods don’t work. We think this is a path for making
new MOFs that are stable and very reactive with totally different
properties than we have been able to access before,” said Bloch.
Other co-authors on the paper include current or former UD graduate
students in Rosenthal’s and Bloch’s labs, including Wenbo Wu, Gerald E.
Decker and Amanda Arnoff.
Article by Karen B. Roberts;
Photo by Kathy F. Atkinson
Published on September 07, 2021