A Multifaceted Study of Methane Adsorption in Metal–Organic Frameworks by Using Three Complementary Techniques

Abstract

Methane is a promising clean and inexpensive energy alternative to traditional fossil fuels, however, its low volumetric energy density at ambient conditions has made devising viable, efficient methane storage systems very challenging. Metal-organic frameworks (MOFs) are promising candidates for methane storage. In order to improve the methane storage capacity of MOFs, a better understanding of the methane adsorption, mobility, and host-guest interactions within MOFs must be realized. In this study, methane adsorption within α-Mg (HCO ) , α-Zn (HCO ) , SIFSIX-3-Zn, and M-MOF-74 (M=Mg, Zn, Ni, Co) has been comprehensively examined. Single-crystal X-ray diffraction (SCXRD) experiments and DFT calculations of the methane adsorption locations were performed for α-Mg (HCO ) , α-Zn (HCO ) , and SIFSIX-3-Zn. The SCXRD thermal ellipsoids indicate that methane possesses significant mobility at the adsorption sites in each system. H solid-state NMR (SSNMR) experiments targeting deuterated CH D guests in α-Mg (HCO ) , α-Zn (HCO ) , SIFSIX-3-Zn, and MOF-74 yield an interesting finding: the H SSNMR spectra of methane adsorbed in these MOFs are significantly influenced by the chemical shielding anisotropy in addition to the quadrupolar interaction. The chemical shielding anisotropy contribution is likely due mainly to the nuclear independent chemical shift effect on the MOF surfaces. In addition, the H SSNMR results and DFT calculations strongly indicate that the methane adsorption strength is linked to the MOF pore size and that dispersive forces are responsible for the methane adsorption in these systems. This work lays a very promising foundation for future studies of methane adsorption locations and dynamics within adsorbent MOF materials.

Solid‐State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal–Organic Frameworks

“…Light hydrocarbons have many practical applications, and tremendous effort has been put into researching suitable separation and storage procedures . MOF‐based methods have shown promising performance; consequently, several SSNMR studies have been conducted to characterize hydrocarbon‐MOF systems …”

“…Host‐guest interaction strengths can also be explored via VT experiments. Our group studied CH 4 adsorption in various MOFs (e.g., α‐Mg 3 (COOH) 6 , α‐Zn 3 (COOH) 6 , SIFSIX‐3‐Zn) using CH 3 D, CH 2 D 2 , and CD 4 guest molecules and static VT 2 H SSNMR spectroscopy . An increase in temperature resulted in a C Q ( 2 H) decrease, which was found to strongly correlate to the CH 4 binding strength.…”

“…[32] MOF-based methods have shown promising performance; [32] consequently,several SSNMR studies have been conducted to characterizeh ydrocarbon-MOF systems. [33][34][35][36][37] SSNMR spectroscopy can provide valuable insights into the adsorption mechanism of light hydrocarbonsi nM OFs. For instance,D eng andc o-workerss tudied CH 4, C 2 H 6 and C 3 H 8 adsorptioni nU iO-67 using spin diffusionS SNMR experiments.…”

“…Our group studied CH 4 adsorption in various MOFs (e.g., a-Mg 3 (COOH) 6 , a-Zn 3 (COOH) 6 ,S IFSIX-3-Zn)u sing CH 3 D, CH 2 D 2 ,a nd CD 4 guest molecules and static VT 2 HS SNMR Concept spectroscopy. [35] An increase in temperature resulted in a C Q ( 2 H) decrease,w hich was found to strongly correlate to the CH 4 binding strength.A ta ny given temperature, C Q ( 2 H) for CH 3 D and CH 2 D 2 in a-Mg 3 (COOH) 6 , a-Zn 3 (COOH) 6 ,a nd SIFSIX-3-Zn followed the trend of SIFSIX-3-Zn > a-Zn 3 (COOH) 6 > a-Mg 3 (COOH) 6 ,r eplicatingt he adsorption energy trends from DFT calculations and linking higher adsorption strength to smaller pore sizes.…”

Porous MOF‐205 with multiple modifications for efficiently storing hydrogen and methane as well as separating carbon dioxide from hydrogen and methane

“…Hydrogen (H 2 ) is considered one of the best energy carriers, because of its high energy density, its zero‐carbon emission, and its renewable nature . Methane (CH 4 ) has also been considered as another potential fuel for on‐board applications due to its substantial deposits and its low carbon emission if compared with petroleum . The lack of a cost‐effective and of a safe storage system is considered to be the major bottleneck in a wider use of H 2 and CH 4 for on‐board applications.…”