Selective gas adsorption and separation in metal–organic frameworks

Abstract

Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, or silica gels as adsorbents. With an ever increasing need for a more efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal-organic frameworks (MOFs), constructed by metal-containing nodes connected by organic bridges, are such a new type of porous materials. They are promising candidates as adsorbents for gas separations due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability. This critical review starts with a brief introduction to gas separation and purification based on selective adsorption, followed by a review of gas selective adsorption in rigid and flexible MOFs. Based on possible mechanisms, selective adsorptions observed in MOFs are classified, and primary relationships between adsorption properties and framework features are analyzed. As a specific example of tailor-made MOFs, mesh-adjustable molecular sieves are emphasized and the underlying working mechanism elucidated. In addition to the experimental aspect, theoretical investigations from adsorption equilibrium to diffusion dynamics via molecular simulations are also briefly reviewed. Furthermore, gas separations in MOFs, including the molecular sieving effect, kinetic separation, the quantum sieving effect for H2/D2 separation, and MOF-based membranes are also summarized (227 references).

Structurally simple complexes of CO2

“…These will not be discussed here as many other reviews are available on the topic. [33][34][35][36] Adsorption of CO2 into a framework generated by octahedral manganese(III) atoms bridged by formate ions has been reported. 37 The crystal structure reveals the formation of C-H···O hydrogen bonds between carbon dioxide and the hydrogen atom of the formate, a factor likely driving the adsorption.…”

Water adsorption in hydrophobic MOF channels

“…Among the proposed applications of MOFs, adsorptive storage and separation of strategic gases (H 2 , CO 2 , CH 4 ) are of particular importance and have gained a lot of attention in recent years. 1,2 Though a very large number of materials with varying structures and chemistry have been synthesised, and the adsorption of a handful of gases is routinely performed along with structure characterisation, little data is available on water adsorption in MOFs. [3][4][5][6][7][8][9] Some MOFs, such as MOF-5 (also known as IRMOF-1), HKUST-1 and DUT-4, are unstable in the presence of water vapour.…”

Hetero-bimetallic metal–organic polyhedra

“…The metal paddlewheel unit is a common structural building block in porous framework materials due to its facile synthesis and topologically versatile square planar geometry. 8 Furthermore, the intrinsic vacant coordination sites have been used as a structural connection, 9 explored for their potential to enhance gas adsorption, 10 and heterogeneous catalysis.…”