It is widely accepted that it is imperative that emissions of carbon dioxide (CO2) and other acidic gases such as sulphur dioxide (SO2) and nitrogen dioxide (NO2) created by human activity is reduced in order to limit the negative effects of global climate change. One particular challenge is the reduction of CO2 emissions from flue gases produced by large industrial plant and coal-fired power stations. Current state-of-the-art technology uses aqueous solutions of organic amines for post-combustion CO2 capture, a so called “amine-scrubbing mechanism”. These amine functionalised capture systems dominate this area, due to potential formation of carbamates via H2N(δ−) . . . C(δ+)O2 electrostatic interactions, thereby trapping CO2 covalently. However, there are considerable costs associated with this process due to the substantial energy input required for the regeneration of the amine solutions, this is in addition to their highly corrosive and toxic nature. Thus there is a negative environmental penalty associated with the use of amines which significantly limits their long-term applications. There are, therefore, powerful drivers to develop efficient strategies to remove CO2 using alternative materials that simultaneously have high adsorption capacity, high CO2 selectivity and high rates of regeneration at an economically viable cost. Traditional microporous solid-state materials such as zeolites, porous membranes and activated carbon can effectively adsorb and remove CO2. However, the low separation efficiency and poor selectivity of these materials significantly limits their real-world applicability. Therefore there is a need to develop new materials with high CO2 storage capacity and selectivity that can be produced at an economically and environmentally viable cost.
Metal organic frameworks (MOF), a relatively new class of porous materials, are built up of metal cation nodes bridged by organic ligand linkers and they have huge potential to deliver significant breakthroughs in carbon capture. The advantages of MOFs over existing technologies include: (i) they can store greater amounts of CO2 than other classes of porous materials, including commercial materials such as zeolite 13X and activated carbon; (ii) their surface areas and pore volume can be adjusted via appropriate crystal engineering and topological connections in order to maximise the CO2 adsorption capacity; (iii) the pore surface and environment can be fine controlled and tuned via variation of organic and inorganic components that constitute the framework in order to enhance CO2 capacity and selectivity; (iv) the adsorbed CO2 molecules can be readily released via reduction of the pressure, i.e. the capture system can be regenerated without additional heating input; (v) the extended crystalline structure of MOF materials gives a unique opportunity to determine and study the mechanisms of carbon capture and storage (CCS) using advanced diffraction techniques.
US patent application US2007/0068389 describes the use of a number of Copper and Zinc based MOF materials to store carbon dioxide at room temperature. These materials show high uptakes of CO2 and have been shown to perform better than zeolites and activated carbons as carbon dioxide storage media inside gas canisters. However, there is a need for metal-organic frameworks with:                a) higher CO2 adsorption capacity;        b) selective adsorption of acidic gases & VOCs;        c) improved framework stability;        d) and improved ease of manufacture.        