A Robust Two-dimensional Layered Metal-Organic Framework for Efficient Separation of Methane from Nitrogen

Fang Zheng^a, Lihang Chen^a, *, Rundao Chen^a, Zhiguo Zhang^{a, b}, Qiwei Yang^{a, b}, Yiwen Yang^{a, b}, Baogen Su^{a, b}, Qilong Ren^{a, b}, Zongbi Bao^{a, b,} *

^aKey Laboratory of Biomass Chemical Engineering of the Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China.

^bInstitute of Zhejiang University-Quzhou, Quzhou, 78 Jiuhua Boulevad North, Quzhou 32400, P. R. China.

Selective capture and separation of methane (CH₄) from nitrogen (N₂) is a feasible approach to mitigate the effects of global warming and to improve the heating value of low-quality natural gas. However, the efficient separation of CH₄/N₂ is a challenge issue since their very close kinetic diameters and thermodynamic properties. In this work, we successfully combine twisted organic ligand (4-DPDS = 4,4’-dipyridyldisulfide) and angular inorganic anion (CrO₄^2-) forming a new class of robust two-dimensional (2D) layered rhomboid grids with ionic polar environment. This a new type of 2D layered metal-organic framework (MOF) with permanent rhomboid pore channels, denoted as Ni(4-DPDS)₂CrO₄, was synthesized for the first time and used for CH₄/N₂ separation. Such a 2D layered material is rarely used for methane and nitrogen separation and shows high affinity towards CH₄ thereby affording admirable methane adsorption and separation performance. Specifically, it not only performs high stability as outstanding as or even better than previously reported 3D MOFs but also exhibits a relatively high CH₄ adsorption capacity of 0.95 mmol/g. Ni(4-DPDS)₂CrO₄ has a high affinity towards CH₄ with the highest reported Q_st value of 28.4 kJ/mol as well as a high CH₄/N₂ selectivity of 7.3, which is comparable to the state-of-the-art MOF materials reported so far.

By DFT-D calculations, it was found that twisted organic ligand plays a vital role in delivering a relatively rigid structure and the angular inorganic anions provides polar sites for strong guest-host interactions in close proximity. It revealed the energy favorable binding sites for methane molecules are located in the middle of the cavity decorated with CrO₄^2- anion, where inducing C-H···O hydrogen bond with the distance of 2.52 Å. Simultaneously, each adsorbed CH₄ molecule is surrounded by eight pyridyl rings, among which one forms dominant cooperative supramolecular interactions between C(δ^–) of CH₄ and H(δ⁺) from the pyridyl ring (C···H = 3.43 Å) and moderate-to-weak σ···π stacking interactions between the electrons in the π orbitals of the pyridyl ring and the electrons in H of CH₄ (H···pyridyl ring = 3.47 Å). Furthermore, the weak intermolecular electrostatic dipole interactions between the H(δ⁺) of CH₄ and S (δ^–) (H···S = 3.82 Å) are also observed. Such an angular inorganic anion provides a cooperative effect conferred by polar sites and multiple guest-host interactions in close proximity, affording tight binding affinity.

Dynamic breakthrough experiments were carried in a stainless-steel column manually packed with Ni(4-DPDS)₂CrO₄. The outlet gas passing through the column was analyzed using a Hiden HPR-20 EGA mass spectrometer for continuous sampling gas analysis. It certified N₂ was eluted almost immediately and then quickly approached a pure grade while no detectable CH₄, whereas CH₄ was retained in the packed column for a remarkable time (~15 min) until reaching the saturated uptake and breakthrough. After the concentration of eluting gas remained unchanged, the amount of CH₄ adsorbed into the column reached 0.94 mmol/g, with a selectivity of 4.7 to N₂ (0.2 mmo/g), which suggest it is a promising material in natural gas purification by further regenerability tests. It is a thriving exploration in two-dimensional MOFs that will emerge the development of various materials for CH₄ trap in the future.

Project summary by: Zongbi Bao, Key Laboratory of Biomass Chemical Engineering of the Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027 P.R. China.

Paper Reference: “A robust two–dimensional layered metal–organic framework for efficient separation of methane from nitrogen” Separation and Purification Technology (2022) 281, 119911 https://doi.org/10.1016/j.seppur.2021.119911

Hiden Product: HPR-20 EGA

Reference: AP-HPR-20-202218

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