RESUMO
Barocaloric effectsâsolid-state thermal changes induced by the application and removal of hydrostatic pressureâoffer the potential for energy-efficient heating and cooling without relying on volatile refrigerants. Here, we report that dialkylammonium halidesâorganic salts featuring bilayers of alkyl chains templated through hydrogen bonds to halide anionsâdisplay large, reversible, and tunable barocaloric effects near ambient temperature. The conformational flexibility and soft nature of the weakly confined hydrocarbons give rise to order-disorder phase transitions in the solid state that are associated with substantial entropy changes (>200 J kg-1 K-1) and high sensitivity to pressure (>24 K kbar-1), the combination of which drives strong barocaloric effects at relatively low pressures. Through high-pressure calorimetry, X-ray diffraction, and Raman spectroscopy, we investigate the structural factors that influence pressure-induced phase transitions of select dialkylammonium halides and evaluate the magnitude and reversibility of their barocaloric effects. Furthermore, we characterize the cyclability of thin-film samples under aggressive conditions (heating rate of 3500 K s-1 and over 11,000 cycles) using nanocalorimetry. Taken together, these results establish dialkylammonium halides as a promising class of pressure-responsive thermal materials.
RESUMO
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 (PXRD) patterns as a technique for evaluating modulator efficacy, complemented by scanning electron microscopy (SEM), 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.
RESUMO
Hydrogen sulfide (H2S) is an endogenous gasotransmitter with potential therapeutic value for treating a range of disorders, such as ischemia-reperfusion injury resulting from a myocardial infarction or stroke. However, the medicinal delivery of H2S is hindered by its corrosive and toxic nature. In addition, small molecule H2S donors often generate other reactive and sulfur-containing species upon H2S release, leading to unwanted side effects. Here, we demonstrate that H2S release from biocompatible porous solids, namely metal-organic frameworks (MOFs), is a promising alternative strategy for H2S delivery under physiologically relevant conditions. In particular, through gas adsorption measurements and density functional theory calculations we establish that H2S binds strongly and reversibly within the tetrahedral pockets of the fumaric acid-derived framework MOF-801 and the mesaconic acid-derived framework Zr-mes, as well as the new itaconic acid-derived framework CORN-MOF-2. These features make all three frameworks among the best materials identified to date for the capture, storage, and delivery of H2S. In addition, these frameworks are non-toxic to HeLa cells and capable of releasing H2S under aqueous conditions, as confirmed by fluorescence assays. Last, a cellular ischemia-reperfusion injury model using H9c2 rat cardiomyoblast cells corroborates that H2S-loaded MOF-801 is capable of mitigating hypoxia-reoxygenation injury, likely due to the release of H2S. Overall, our findings suggest that H2S-loaded MOFs represent a new family of easily-handled solid sources of H2S that merit further investigation as therapeutic agents. In addition, our findings add Zr-mes and CORN-MOF-2 to the growing lexicon of biocompatible MOFs suitable for drug delivery.
