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

International audienceThe adsorption of the acid gas H2S 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 H2S 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 H2S molecules mainly form hydrogen bonded species, either as hydrogen donor (in MIL-47 V) or hydrogen-acceptor (in MIL-53Cr) with the mu(2)-O and mu(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 H2S 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 CH4/H2S separation by means of Pressure Swing Adsorption

Adsorbate Interactions in Metal Organic Frameworks Studied by Vibrational Spectroscopy

“…[39] Structural changes from a large pore to a narrow pore were only observed for MIL-53 (Cr), and identified by a red shift of the n18a aromatic C-H bending mode of the terephthalate linkers. [40] The authors showed that the hydrogen bonding between the hydroxyl groups in Vibrational Spectroscopy 87 MIL-53 and the H 2 S provokes closure of the pore (Figure 7).…”

Fundamental Aspects of H2S Adsorption on CPO-27-Ni

“…19 These dramatic shifts in the (S-H) stretch observed on CPO-27-Ni and the change from yellow to dark green of the activated sample upon H 2 S adsorption indicate the involvement of Ni 2+ sites into the adsorption mechanism. The mentioned change in colour is further investigated by Uv-Visible spectroscopy as described in the next section.…”

Characterization of adsorbed water in MIL-53(Al) by FTIR spectroscopy and ab-initio calculations

“…38,[44][45][46][47][48][49] Among these studies, those using FTIR spectroscopy are less numerous. [49][50][51][52] Assuredly, spectral band identification is a laborious and tedious task, nonetheless, useful to identify the a) Electronic mail: jmarcos@u-bourgogne.fr role of each component of the structure. Knowing which are the major vibrational modes of a given porous structure is a worth information for guiding the formulation of a theoretical model concerning the flexibility or elasticity of MOFs.…”