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The depletion of fossil fuels and rising global warming challenges encourage to find safe and viable energy storage and delivery technologies. Hydrogen is a clean, efficient energy carrier in various mobile fuel-cell applications and owned no adverse effects on the environment and human health. However, hydrogen storage is considered a bottleneck problem for the progress of the hydrogen economy. Liquid-organic hydrogen carriers (LOHCs) are organic substances in liquid or semi-solid states that store hydrogen by catalytic hydrogenation and dehydrogenation processes over multiple cycles and may support a future hydrogen economy. Remarkably, hydrogen storage in LOHC systems has attracted dramatically more attention than conventional storage systems, such as high-pressure compression, liquefaction, and absorption/adsorption techniques. Potential LOHC media must provide fully reversible hydrogen storage via catalytic processes, thermal stability, low melting points, favorable hydrogenation thermodynamics and kinetics, large-scale availability, and compatibility with current fuel energy infrastructure to practically employ these molecules in various applications. In this review, we present various considerable aspects for the development of ideal LOHC systems. We highlight the recent progress of LOHC candidates and their catalytic approach, as well as briefly discuss the theoretical insights for understanding the reaction mechanism.

Urea-derived hydrogen-bond-donating( HBD) catalysts are good catalysts for organic transformations. Usually,urea derivatives undergos elf-recognition and aggregation during homogeneousc atalytic processes. To avoid this, an ew approach involvingt he incorporation of urea moietiesi nto metal-organic frameworks (MOFs) was recently developed. Herein,wesynthesized an ew porous, zinc-based MOF by using ureylenic acid that acts as ah ydrogen-bond-donatingc atalyst. According to our knowledge, the pores of this MOF are larger than those of other reported HBD-derivedM OFs and, as such, can accommodate large substrates that typicallyc annot access the pores of other HBD-based MOFs forthe Friedel-Crafts reaction.

A new three-dimensional europium-based metal−organic framework has been synthesized with the newly designed ligand L (6-[1-(4-carboxyphenyl)-1H-1,2,3-triazol-4-yl]nicotinic acid). This compound acts as a dual sensor for the phosphate anion and Fe 3+ ion in aqueous media. The mechanistic aspect of this selectivity and sensitivity was explored through several spectroscopic methods and then correlated with the corresponding structure.

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Crystalline solid materials are platforms for the development of effective catalysts and have shown vast benefits at the frontiers between homogeneous and heterogeneous catalysts. Typically, these crystalline solid catalysts outperformed their homogeneous analogs due to their high stability, selectivity, better catalytic activity, reusability and recyclability in catalysis applications. This point of view, comprising significant features of a new class of porous crystalline materials termed as metal‐organic frameworks (MOFs) engendered the attractive pathway to synthesize functionalized heterogeneous MOF catalysts. The present review includes the recent research progress in developing both hydrogen‐bond donating (HBD) MOF catalysts and MOF‐supported single‐site catalysts (MSSCs). The first part deals with the novel designs of urea‐, thiourea‐ and squaramide‐containing MOF catalysts and study of their crucial role in HBD catalysis. In the second part, we discuss the important classification of MSSCs with existing examples and their use in desired catalytic reactions. In addition, we describe the relative catalytic efficiency of these MSSCs with their homogeneous and similarly reported analogs. The precise knowledge of discussed heterogeneous MOF catalysts in this review may open the door for new research advances in the field of MOF catalysis.

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