Shortage of energy and environmental pollution is a growing problem nowadays. To get rid of pollution, attention has been given in the development of environmentally friendly and clean fuels such as H2, CH4 etc. So it is always a great challenge for society as well as scientists to store these fuels and use them efficiently with low cost and safety. Environment is getting polluted due to unreasonable use of fossil fuels and liberation of harmful and toxic gases (such as NOx, SOx, CO, H2S, NH3 etc.) from various industries into the environment. In this continuous developing age this has become an worldwide problem. Moreover these harmful gases are major threat to human health also. So effective removal of these gases are an urgent need to protect the environment as well as prevent the health problems.
Porous materials are widely used in the field of energy and environment as they contain adequate pore structures. Metal organic frameworks (MOFs) are these kind of porous materials. These are actually a kind of periodic porous materials and these are formed through self assembly of metal ions with organic ligands. These compounds posses the properties of both organic as well as inorganic materials. MOFs have special properties like large surface area, tunable pore sizes and high porosity and by virtue of these properties these materials can be used in gas storage and separation. All these qualities have led scientists to utilize MOFs in the field of energy and environmental application like energy storage, capture and separation of pollutants, sensing of harmful gases etc.
Energy storage: Microporous MOF materials with high surface area have the advantage in the storage of environment friendly fuels such as H2 and CH4. In these kind of compounds the metal centre remains coordinatively unsaturated i.e. one or more coordination site remains vacant and as a result the metal can bind hydrogen very easily. So they can be used in hydrogen storage. Hydrogen storage capacity can be enhanced by synthesizing MOF composites. MOF composites are synthesized by combination of MOFs with other materials such as metal nanoparticle, metal oxides, polyoxometallates etc. Such compounds optimizes the large void space that help in storage of hydrogen. These kinds of materials are also potential adsorbent for methane and depending on the pore size they can be utilized for storage of methane gas which is also an environment feiendly fuel.
Capture and separation of pollutants: All of us know that CO2 is a green house gas causing global warming. Moreover presence of CO2 gas with other natural gases prevent full combustion and leads to potential security problem. MOFs having high porosity, specific pore size and tunable chemical affinity have been extensively developed by chemists and these have been utilized in capture and separation of CO2. MOF composites enhance the CO2 uptake ability due to creation of new pores at the interface of two materials. Tunable pore size and modifiable functional groups help MOFs to show selectivity for a particular gas over others. That is if a MOF contains large pore size then it do not accommodate small molecule as in that case interaction becomes poor. Similarly if the pore size is small then it cannot accommodate large molecules as in that case the guest molecule do not get sufficient space to be seated in the cavity. Change in polarity of functional groups also changes the affinity of the framework for a particular molecule over others. Combination of all these properties of MOFs helped scientists to utilize these compounds in the field of separation of pollutant gas molecule from other gases in the environment.
Adsorption of harmful and toxic gases: Since MOFs contain large surface area so they can be utilized for adsorption of gases and thus many harmful gases like H2S, NH3, CO, NO, etc. can be removed from environment. Hydrogen sulfide (H2S) is a toxic chemical gas released from petroleum and natural gas industries. Humans are extremely sensitive to H2S due to its rotten egg smell. However removal of this kind of gas from environment is necessary to avoid pollution. Adsorption by MOFs is a highly effective technology for the removal of H2S gas from environment. Due to presence of sulphur atom this gas can bind with many metal centres especially with soft metal ions and this property has been utilized in the separation of H2S from the environment.
Ammonia (NH3) is a harmful and irritating gas. It gives rise to accidents from spill or explosions at chemical industries. So it is very necessary to remove NH3 from the environment. NH3 can interact with -OH and -NH2 group present in the organic ligand part of the framework through hydrogen bonding. Moreover if the MOF contain coordinatively unsaturated metal centre then NH3 can bind it by coordination and so use of MOFs containing such properties has been found highly effective in the removal of NH3 from the environment.
Carbon monoxide is a colourless, odourless gas that can bind with the iron (Fe) atom present in the hemoglobin resulting in decrease of oxygen carrying capacity of blood. Inhalation of a large amount of CO can cause poisoning and even life threatening to human being. So removal of this gas is very much necessary from the environment. Utilization of copper (Cu) containing MOF has been found effective in the removal of CO from environment due to electrostatic interaction of CO with the framework. Such kind of MOFs have also been found effective in the removal of nitric oxide gas (NO) which is also harmful for our respiratory system and hence should be removed from environment.
Chemical sensor: Identification of harmful and toxic gases from environment by sensing is the preliminary step to protect human’s health. It requires initially adsorption of the gas molecules on the surface producing a signal. Among the various sensing materials available, MOFs are very promising materials to build up such sensors due to their various properties. Due to high porosity they can act as good host for reversible adsorption and desorption of guest molecules and it ensures the repeatability of detection. The tunable pore size and large surface area help them in enhancing the selectivity and sensitivity. As a combination of all these factors we can utilize MOFs as sensors for detection of harmful and toxic gases in the environment and then separation of such gases by using suitable MOFs.
In conclusion, we can say that MOFs are a kind of porous materials that can be utilized in the application field of energy storage, separation of pollutant gases from atmosphere, sensing, adsorption and removal of harmful and toxic gases from the environment. That is they are very much useful materials for environment.
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