Molecular Insight into the Adsorption of H2S in the Flexible MIL-53(Cr) and Rigid MIL-47(V) MOFs: Infrared Spectroscopy Combined to Molecular Simulations

Abstract

The adsorption of the acid gas H 2 S has been explored in both MIL-47(V) and MIL-53(Cr) porous metal−organic frameworks (MOFs) by combining infrared measurements and molecular simulations. It is shown that while the MIL-47(V) structure remains rigid upon H 2 S adsorption up to a pressure of 1.8 MPa, the MIL-53(Cr) solid initially present in the large pore form (LP) switches to its narrow pore version (NP) at very low pressure before undergoing a second structural transition from the NP to the LP versions at higher pressure. Such structural transitions further explain the different shape of the adsorption isotherms for both MILs. A further step consists of providing some insights into the microscopic arrangements of the adsorbate molecules within the pores of the MILs. At the initial stage of adsorption, the H 2 S molecules mainly form hydrogen bonded species, either as hydrogen donor (in MIL-47 V) or hydrogen-acceptor (in MIL-53Cr) with the μ 2 -O and μ 2 -OH groups, respectively, present at the MOF surfaces. At higher pressure (1.8 MPa), the adsorbates are preferentially arranged within the channel in order to form dimers with a high orientational disorder. Both experimental and simulated adsorption enthalpies for H 2 S decrease in the following sequence: MIL-53(Cr) NP > MIL-47(V) > MIL-53(Cr) LP. The conclusions drawn from this work are then discussed considering the use of such materials for the CH 4 /H 2 S separation by means of Pressure Swing Adsorption.