Metal–organic frameworks and related materials for hydrogen purification: Interplay of pore size and pore wall polarity

Abstract

The separation of hydrogen from other weakly adsorbing gases is a topic of high industrial relevance. Microporous materials, such as zeolites, metal-organic frameworks (MOFs), and nanoporous molecular crystals, hold much promise as adsorbent materials for adsorption-based hydrogen separation units. However, most experimental and theoretical studies that have been reported so far have focused on relatively few gas mixtures (mainly CO 2 /H 2 and CH 4 /H 2 ). In this work, the suitability of five materials (zeolite: silicalite; MOFs: Mg-formate, Zn(dtp), Cu 3 (btc) 2 ; porous molecular crystal: cucurbit[6]uril) for the adsorption-based separation of carbon monoxide/hydrogen and oxygen/ hydrogen mixtures is assessed using force-field based grand-canonical Monte Carlo simulations. The simulations are employed to predict single-component and mixture isotherms, as well as adsorption selectivities. Moreover, a detailed analysis of the solid-fluid interactions is carried out on an atomistic level. The choice of materials is motivated by their structural properties: four systems contain relatively narrow channels (diameters , 6.5 A ˚), but differ in pore wall composition and polarity. The fifth system possesses coordinatively unsaturated metal sites, which can act as preferential adsorption sites for some guest molecules. The role of electrostatic interactions is fundamentally different for the two mixtures considered: for CO/H 2 separation, the employment of polar adsorbents is beneficial due to the enhanced electrostatic interaction with carbon monoxide. On the contrary, an increased polarity of the pore wall tends to reduce the O 2 /H 2 selectivity, because electrostatic interactions favour hydrogen over oxygen due to its larger quadrupole moment. In general, materials with narrow channels perform best in the separation of hydrogen from weakly adsorbing species, because the dispersive interactions are maximized in the channels. Moreover, they provide little space for the co-adsorption of hydrogen.

DFT-based evaluation of porous metal formates for the storage and separation of small molecules

“…[49,50] For the guest species, parameters tested in previous work were employed. [26,27] Simulation snapshots extracted from the Monte Carlo simulations were then optimised using the same force field parameters.…”

“…towards C 2 H 2 /CO 2 , CO/H 2 , and O 2 /H 2 mixtures, and analysed the spatial evolution of the interaction energy in the channels for these guest molecules. [26,27] Monte Carlo simulations of adsorption were also employed to aid the aforementenioned experimental investigation of Xe/Kr separation. [19] Electronic structure methods, such as density-functional theory (DFT), can be employed to study the interaction of porous materials with adsorbed guest molecules from first principles.…”

“…Force-field based calculations (GCMC, MD) could provide valuable insights into the adsorption and diffusion properties of metal formates. While previous works have used literature force fields,[24][25][26][27] the DFT-D results of this study could serve as a benchmark for the development of improved force fields to describe adsorption in porous metal formates.…”

Microporous metal-organic frameworks with suitable pore spaces for acetylene storage and purification

“…USTA-50 [31] has high C 2 H 2 /CH 4 and C 2 H 2 /CO 2 separation selectivity of 68 and 13.3 respectively, but the uptake amount of acetylene is only 91 cm 3 g -1 at 296 K and 1 atm. Recently optimizing pore spaces has been known as another efficient method to confine gas molecules in frameworks and increase adsorption selectivity [32][33][34][35]. Lin and co-workers [36] have demonstrated that by introducing methyl groups into ligand to create smaller pores can enhance the isosteric heat of adsorption and improve H 2 adsorption.…”

H2 purification by functionalized graphdiyne – role of nitrogen doping

“…6,[8][9][10][11] For all types of membranes, their permeances are inversely proportional to their thickness; as a result, a one-atom thin membrane is a promising candidate for separation processes. 6,[8][9][10][11] For all types of membranes, their permeances are inversely proportional to their thickness; as a result, a one-atom thin membrane is a promising candidate for separation processes.…”