Metal-organic frameworks (MOFs) built on calcium metal (Ca-MOFs) represent a unique subclass of MOFs featuring high stability, low toxicity, and relatively low density. Ca-MOFs show considerable potential for molecular separations, electronic, magnetic, and biomedical applications, although they are not investigated as extensively as transition metal-based MOFs. Compared to MOFs made of other groups of metals, Ca-MOFs may be particularly advantageous for certain applications such as adsorption and storage of light molecules because of their gravimetric benefit, and drug delivery due to their high biocompatibility. This review intends to provide an overview on the recent development of Ca-MOFs, including their synthesis, crystal structures, important properties, and related applications. Various synthetic methods and techniques, types of building blocks, structure and porosity features, selected physical properties, and potential uses will be discussed and summarized. Representative examples will be illustrated for each type of important applications with a focus on their structure-property relations.ethylene/acetylene, propane/propylene) through highly selective size exclusion mechanism, [6,11,24] which has not been achieved by conventional adsorbent materials. The potential uses of MOFs depend on their pore structure (pore size/pore shape), surface functionality, as well as the type of metal centers and ligands.Calcium-based metal-organic frameworks (Ca-MOFs) represent a subgroup of MOFs with calcium as metal centers. Unlike MOFs built on transition and post-transition metals (Zr, Fe, Co, Ni, Cu, Zn, etc.) which tend to form commonly observed SBUs and topology and, therefore allow for successful design and implementation of targeted structure and functionality, the prediction of coordination geometry and structural topology of Ca-MOFs are much more challenging. This could be attributed to the fact that the bonding interactions between calcium and organic ligands (commonly carboxylates or phosphates) are more ionic, and thus the coordination mode of calcium and the topology of Ca-MOFs largely rely on the nature of the organic ligands as well as the synthetic conditions. However, Ca-MOFs possess several advantages compared to those built on transition metals: 1) Ca-MOFs generally feature high thermal stability because of its high electropositivity which leads to strong, ioniclike bonds with organic ligands (e.g., carboxylates). 2) Calcium is earth-abundant (3.4% of Earth's crust) and nontoxic, making Ca-MOFs relatively inexpensive and environmentally safe, thus especially promising for biological related applications. 3) The lightweight of calcium metal offers gravimetric benefit for gas adsorption/storage related applications.To date, more than 150 Ca-MOFs have been reported (Table 1). These MOFs bear different structural features and have been evaluated for various applications, including molecular separation, drug delivery and controlled release, chemical sensing, and proton conductivity, to name a few. In this...
