[DMOF] Tuning the Pore Environment of MOFs toward Efficient CH 4 /N2 Separation under Humid Conditions

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Abstract:
The article presents a novel approach to enhancing the performance of metal-organic frameworks (MOFs) for the separation of methane (CH4) and nitrogen (N2) under humid conditions. The authors synthesized a series of isostructural MOFs by modifying the ligands and metal sites of the parent MOF (DMOF) to improve its stability and selectivity. The study demonstrates that the modified MOF (DMOF-A2) exhibits high CH4 uptake capacity and selectivity, along with excellent moisture resistance, making it a promising candidate for industrial CH4/N2 separation.

Research Background:
1.Industry Challenge: The separation of CH4 and N2 is crucial for the efficient utilization of natural gas. Traditional methods like cryogenic distillation are energy-intensive and costly. Adsorption-based separation using MOFs is a more energy-efficient alternative, but most MOFs suffer from reduced performance under humid conditions due to water vapor adsorption.
2.Existing Solutions: Previous studies have explored various MOFs for CH4/N2 separation, such as Ni-MA−BPY, Al-CDC, and Co3(C4O4)2(OH)2. These MOFs show promise but often lack sufficient moisture resistance, leading to decreased separation efficiency in humid environments.
3.Innovative Approach: The authors build on this foundation by introducing nonpolar aromatic rings into the MOF structure to enhance hydrophobicity and stability. They also fine-tune the pore size and metal composition to optimize CH4 adsorption and separation performance.

Experimental Section:
1.Synthesis of DMOF and Derivatives:
   -DMOF: Synthesized by solvothermal reaction of Zn(NO3)2·6H2O, BDC, DABCO, DMF, and HNO3.
   -DMOF-N: Similar synthesis with 1,4-NDC replacing BDC.
   -DMOF-A1: Synthesized using ADC instead of BDC.
   -DMOF-A2 and DMOF-A3: Prepared by substituting Zn with Ni and Cu, respectively, in DMOF-A1.
2.Characterization:
   - PXRD patterns confirmed the successful synthesis of the isostructural MOFs.
   - SEM images showed high purity and crystallinity of the samples.
   - XPS spectra confirmed the presence of Zn, Ni, and Cu in the respective MOFs.
   - TGA curves indicated thermal stability up to 350-400 °C for the functionalized MOFs.
3.Gas Adsorption Measurements:
   - CH4 and N2 adsorption isotherms were measured at 273 K and 298 K.
   - DMOF-A2 exhibited the highest CH4 uptake of 37 cm3/g at 298 K and 1 bar.
   - The selectivity of DMOF-A2 for CH4/N2 was 7.2 at 298 K and 1 bar.

Analysis and Testing:
1.PXRD Analysis:
   - Confirmed the structural integrity of the synthesized MOFs.
   - No degradation observed in DMOF-A1, DMOF-A2, and DMOF-A3 after exposure to 100% RH.
2.Gas Adsorption Isotherms:
   - DMOF-A2 showed a high CH4 uptake capacity and selectivity.
   - The initial heat of adsorption (Qst) values for CH4 were significantly higher than those for N2, indicating stronger binding affinity for CH4.
3.Water Vapor Sorption:
   - DMOF-A2 exhibited negligible water uptake (0.020 g/g) at 40% RH, demonstrating superior hydrophobicity.
4.Breakthrough Experiments:
   - DMOF-A2 maintained high CH4/N2 separation performance even at 40% RH, with minimal impact on separation capacity.

Summary:
The study successfully demonstrates that the fine-tuning of the pore environment in MOFs through ligand functionalization and metal substitution can significantly enhance their performance for CH4/N2 separation under humid conditions. DMOF-A2, with its high CH4 uptake capacity, selectivity, and moisture resistance, emerges as a robust candidate for industrial applications. The findings highlight the potential of engineered MOFs for efficient gas separation in challenging environments.

Tuning the Pore Environment of MOFs towards Efficient CH4/N2 Separation under Humid Conditions
Authors: Tong Li, Xiaoxia Jia, Hui Chen, Zeyu Chang, Libo Li, Yong Wang, and Jinping Li
DOI: 10.1021/acsami.2c01156
Link: https://pubs.acs.org/doi/10.1021/acsami.2c01156

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