Acid
modulation is among the most widely employed methods for preparing
metal–organic frameworks (MOFs) that are both stable and highly
crystalline, yet there exist few guiding principles for selecting
the optimal modulator for a given system. Using the Zr-based MOFs
UiO-66 and UiO-68-Me2 (UiO = Universitetet i Oslo) as representative
materials, here we present for the first time an in-depth structure–activity
study of acid modulators and identify key principles of modulation
for the synthesis of highly crystalline Zr-MOFs. By applying whole
pattern fitting of powder X-ray diffraction patterns as a technique
for evaluating modulator efficacy, complemented by scanning electron
microscopy, 1H NMR, and thermogravimetric analysis (TGA),
we demonstrate that the key to effective modulation is competition
between the linker and modulator for coordination to the Zr secondary
building units (SBUs). Specifically, we illustrate that a close match
in pK
a and structure between the linker
and modulator favors larger and more well-defined crystallites, particularly
with sterically unhindered aromatic acid modulators. Based on our
findings, we demonstrate that 5-membered heteroaromatic carboxylic
acids are among the most efficient acid modulators identified to date
for the synthesis of several representative Zr-MOFs with fcu net topologies. In addition, we find that coordination modulation
is superior to exogenous acid modulation at higher modulator concentrations.
Finally, we compare 1H NMR and TGA as data-driven methods
for quantifying linker deficiencies in modulated MOF syntheses. The
guiding principles established herein have critical implications for
the scalable and controllable synthesis of highly crystalline and
stable MOFs relevant to chemical separations, gas storage, and catalysis.
Metal–organic frameworks enable the delivery of hydrogen sulfide (H2S), an endogenous gasotransmitter with potential therapeutic value for treating disorders such as ischemia-reperfusion injury.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.