MOF Zn-bzc-2CH3 for the Efficient Separation of Isomeric C4 Paraffins under Humid Conditions

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Abstract:
This article presents a novel approach to tuning the pore aperture of a cage-like metal-organic framework (MOF) to achieve efficient separation of isomeric C4 paraffins, specifically n-butane (n-C4H10) and isobutane (iso-C4H10). The authors introduce methyl groups to the narrow aperture of a Zn-bzc MOF to create a hydrophobic microenvironment that enhances stability and selectivity, achieving high n-C4H10 uptake and molecular-sieving separation under humid conditions. The study demonstrates that the modified MOF, Zn-bzc-2CH3, can sensitively capture n-C4H10 while excluding iso-C4H10, with an adsorption capacity of 54.3 cm³g⁻¹ and high-purity iso-C4H10 collection (99.99%).

Research Background:
1.Industry Problem: The separation of isomeric C4 paraffins, such as n-butane and isobutane, is crucial in the petrochemical industry. Traditional methods like distillation are energy-intensive and not sustainable. Adsorption separation using porous materials offers a more sustainable alternative but often suffers from a trade-off between selectivity and adsorption capacity.
2.Existing Solutions: Previous studies have explored various MOFs for isomeric C4 paraffin separation. These MOFs rely on thermodynamic or kinetic separation, but they often suffer from co-adsorption, leading to low separation efficiency. Molecular sieving, which offers high selectivity, typically results in low adsorption capacity due to the small pore sizes.
3.Innovative Approach: The authors propose a stepwise engineering approach to modify the pore aperture of a cage-like Zn-bzc MOF by introducing methyl groups. This method aims to achieve both molecular-sieving separation and high n-C4H10 adsorption capacity. The hydrophobic microenvironment created by the methyl groups enhances the stability of the MOF under humid conditions, addressing a significant challenge in practical applications.

Experimental Section:
1.Sample Preparation:
   -Zn-bzc MOF Synthesis: Zn(NO₃)₂·4H₂O and pyrazole-4-carboxylic acid (bzc) were dissolved in N,N-diethylformamide (DEF) and heated to 140°C for two days, followed by cooling and washing with methanol.
   -Modified MOFs: Zn-bzc-CH3 and Zn-bzc-2CH3 were synthesized by replacing the bzc ligand with 3-methylpyrazole-4-carboxylic acid (bzc-CH3) and 3,5-dimethylpyrazole-4-carboxylic acid (bzc-2CH3), respectively, under similar conditions.
2.Characterization:
   - Powder X-ray diffraction (PXRD) patterns were obtained using a BRUKER AXS D2 PHASER.
   - Scanning electron microscope (SEM) images were recorded using a Gemini SEM500.
   - Adsorption isotherms were measured using a MicrotracBEL Belsorp-max instrument at different temperatures.
3.Breakthrough Measurements:
   - Breakthrough experiments were conducted using a Multi-components Adsorption Breakthrough Curve Analyzer (BSD-MAB).
   - Samples were prepared into uniform particle sizes and filled into a cylindrical quartz tube.
   - Gas mixtures of n-C4H10/iso-C4H10 (v/v = 1/1) were flowed through the sample bed at 298 K, with humidity controlled by passing the gas through water or introducing wet carrier gas.
   - Outlet gas was detected by gas chromatography.
4.Stability Tests:
   - Zn-bzc and Zn-bzc-2CH3 MOFs were soaked in water for 72 hours or exposed to air for 30 days.
   - Stability was evaluated by PXRD and n-C4H10 adsorption isotherms.
   - Regeneration ability was tested by consecutive adsorption-desorption cycles.

Analysis and Testing:
1.PXRD Analysis:
   - The PXRD patterns confirmed the successful synthesis of Zn-bzc, Zn-bzc-CH3, and Zn-bzc-2CH3 MOFs.
   - The patterns showed high consistency with simulated data, indicating successful modification of the pore aperture.
2.Adsorption Isotherms:
   - The BET surface areas of Zn-bzc, Zn-bzc-CH3, and Zn-bzc-2CH3 were 472.9, 414.9, and 378.2 m²g⁻¹, respectively.
   - n-C4H10 and iso-C4H10 adsorption capacities were measured at 298 K: Zn-bzc (75.7 and 64.1 cm³g⁻¹), Zn-bzc-CH3 (55.17 and 50.85 cm³g⁻¹), and Zn-bzc-2CH3 (54.28 cm³g⁻¹ for n-C4H10 with no detectable iso-C4H10 adsorption).
3.Breakthrough Tests:
   - In dry conditions, Zn-bzc-2CH3 showed a breakthrough time of around 1200 s for n-C4H10, with high-purity iso-C4H10 collection (99.99%).
   - Under 100% humidity, the breakthrough time for n-C4H10 remained consistent, indicating minimal interference from water vapor.
4.Stability and Regeneration:
   - Zn-bzc-2CH3 maintained structural integrity after 72 hours in water and 30 days in air, as confirmed by PXRD.
   - The n-C4H10 adsorption capacity remained consistent after ten adsorption-desorption cycles, demonstrating excellent regeneration ability.

Conclusion:
The study presents a significant advancement in the separation of isomeric C4 paraffins by engineering the pore aperture of a cage-like MOF. The introduction of methyl groups not only enhances the molecular-sieving effect and n-C4H10 adsorption capacity but also improves the stability and humidity resistance of the MOF. Zn-bzc-2CH3 emerges as a benchmark adsorbent for efficient n-C4H10/iso-C4H10 separation under humid conditions, offering a sustainable and practical solution for the petrochemical industry.

Stepwise Engineering the Pore Aperture of a Cage-like MOF for the Efficient Separation of Isomeric C4 Paraffins under Humid Conditions
Authors: Lu Wang, Wenjuan Xue, Hejin Zhu, Xiangyu Guo, Hongliang Huang, Chongli Zhong
DOI: 10.1002/anie.202218596
Link: https://onlinelibrary.wiley.com/doi/10.1002/anie.202218596

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