Solar PV and wind energy conversion systems convert input energy directly to electricity, are techno-economically viable, and have already achieved grid parity. [3,4] Though renewables fulfill most modern energysource criteria, they still possess low energy density, are challenging to transport, less efficient, costly, and are intrinsically intermittent, thus requiring a storage media.Hydrogen, a less popular renewable, can be used as a fuel as apart from being abundant gas, possesses high energy density, [5] exhibits better combustion characteristics, [5] owes a nonpolluting nature, [6] and has several other favorable properties, listed in Table 1. Hydrogen is a promising alternative energy source [6,7] that overcomes most of the challenges faced by classical renewables.Figure 2 describes hydrogen energy transformation from primary energy sources to the end-users in an integrated way. The hydrogen production, storage, transportation and delivery, application, awareness and capacity building, safety codes and standards are all interrelated to form the hydrogen energy system. The two prime methods of producing hydrogen are i) from the conventional process, and ii) using renewable energy sources (such as biomass, solar, wind, and hydro). Hydrogen possesses significantly higher gravimetric energy density than other storage techniques (such as batteries, pumped hydropower, supercapacitors, Flywheels, pressurized air). The use of abundant renewable energy when available, hydrogen production, and storage is an optimal choice. Hydrogen can be stored as pressurized gas or as a cryogenic liquid or solid fuel. [27] Storage of hydrogen as a gas typically requires high-pressure tanks (350-700 bar tank pressure). Due to the low boiling point (−252.8 °C at atmospheric pressure), liquid hydrogen storage requires cryogenic temperature.The conventional conversion of fossil fuel feedstock to hydrogen includes steam-methane reforming, thermal cracking of natural gas, thermal decomposition (partial oxidation) of heavy oil, catalytic decomposition of natural gas, coal gasification, and steam-iron coal gasification. [20][21][22][23][24][25] When released into the atmosphere, the conventional methods generate CO 2 as a by-product, assisting global warming. The generation of renewable hydrogen makes it more economical and environmentally benign. Electrolysis uses electrical power supplied from renewables to produce hydrogen and oxygen. Biomass routes Hydrogen, a nonpolluting gas, is emerging as an ideal, suitable, and economical energy carrier. The current global non-carbon hydrogen production is 105.8 MW in 2020 and is expected to reach 218 MW in 2021. Hydrogen possesses low ignition energy of 0.017 mJ and reacts exothermically with air, posing severe safety challenges. Humanly undetectable gas needs accurate and sensitive sensors to prevent accidents. Amongst different hydrogen sensors currently developed, work function-based sensors are sensitive, selective, costeffective, smaller in size, less susceptible to environmental change, an...

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: The natural beauty and purity of our planet has been contaminated deeply due to human selfish activities such as pollution, improper waste management, and various industrial and commercial discharges of untreated toxic by-products into the lap of nature. The collective impact of this hazardous suspension into the natural habitat is very deadly. Challenges due to human activity on the environment have become ubiquitous. The chemical industry has a major role in human evolution and, predictably, opened gates of increased risk of pollution if the production is not done sustainably. In these circumstances, the notion of Green Chemistry has been identified as the efficient medium of synthesis of chemicals and procedures to eradicate the toxic production of harmful substances. Principles of Green Chemistry guide the scientist in their hunt towards chemical synthesis which requires the use of solvents. These solvents contaminate our air, water, land and surrounding due to its toxic properties. Even though sufficient precautions are taken for proper disposal of these solvents but it is difficult to be recycled. In order to preserve our future and coming generation from the adverse impacts associated with solvents it is very important to find alternative of this which will be easy to use, reusable and also eco-friendly. Solvents are used daily in various industrial processes as reaction medium, as diluters, and in separation procedures. As reaction medium, the role of solvent is to bring catalysts and reactants together and to release heat thus affecting activity and selectivity. The proper selection of the solvent considering its biological, physical and chemical properties is very necessary for product separation, environmental, safety handling and economic factors. Green solvents are the boon in this context. They are not only environmentally benign but also cost effective. The biggest challenge faced by the chemists is adaptation of methods and selection of solvents during chemical synthesis which will give negligible waste product and will remain human and nature friendly. During designing compounds for a particular reaction it is difficult to give assurance regarding the toxicity and biodegradability of the method. Chemists are still far away from predicting the various chemical and biological effects of the compounds on the back of the envelope. To achieve that point is formidable task but it will definitely act as inspiration for the coming generation chemists. The green solvents are undoubtedly a far better approach to eliminate the negative impacts and aftermath of any chemical synthesis on the environment. Our study in this review covers an overview of green solvents, their role in safer chemical synthesis with reference to some of the important green solvents and their detail summarization.

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Metal-Organic Frameworks (MOFs), an inorganic-organic hybrid material, have been at the centre stage of material science for three decades. MOFs are synthesized by metal ions and organic linker precursors and have become very potential materials for different applications ranging from sensing, separation, catalytic behaviour to biomedical applications and drug delivery. owing to its structural flexibility, porosity and functionality. It is also very promising in heterogeneous catalysis for various industrial applications. These catalysts can be easily synthesized with extremely high surface areas, tunable pore sizes, and incorporation of catalytic centres via post synthetic modification (PSM) or exchange of their components as compared to traditional heterogeneous catalysts which is the preliminary requirement of a better catalyst. Here, in this review, we have sketched the history of MOFs, different synthesis procedures, and MOF- catalysed reactions, for instance, coupling reactions, condensation reactions, Friedel-Crafts reactions, oxidation, etc. Special attention is given to MOFs containing different catalytic centres including open metal sites, incorporation of catalytic centres through PSM, and bifunctional acid-base sites. Discussion on the important role of catalytic centres present in MOFs and reaction mechanisms has been outlined with examples.