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
thermal approaches.
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.