Key Takeaways & Executive Findings
- •• • G-5,5'-BPyDC exhibits a N2O adsorption capacity of 2.32 mmol/g at 25°C and 4.0 MPa, which is 14.9% higher than that of G-4,4'-BPyDC (2.02 mmol/g), indicating enhanced uptake for industrial N2O capture processes. • • IAST selectivity of G-5,5'-BPyDC for N2O/N2 (50:50) is 29.26, and for (10:90) is 111.32, outperforming G-4,4'-BPyDC (6.61 and 17.03) by factors of 4.4 and 6.5, respectively, crucial for efficient separation in dilute N2O streams. • • Both HOFs exhibit thermal stability with decomposition temperatures ≥290°C, ensuring operational robustness in temperature-swing adsorption processes. • • Hirshfeld surface analysis shows that G-5,5'-BPyDC has a higher total hydrogen bond contribution (32.0%) compared to G-4,4'-BPyDC (29.7%), correlating with improved adsorption performance and providing a design metric for future HOF synthesis.
Abstract
Efficient capture of the greenhouse gas nitrous oxide (N2O) is critical for climate change mitigation and resource recovery. In this study, two guanidinium-based hydrogen-bonded organic frameworks (HOFs) with pyridyl nitrogen site isomerism, namely G-5,5'-BPyDC and G-4,4'-BPyDC, were constructed using 2,2'-bipyridine-5,5'-dicarboxylic acid and 2,2'-bipyridine-4,4'-dicarboxylic acid as ligands. The effects of ligand structure on hydrogen-bonded network, pore environment, and N2O/N2 adsorption separation performance were systematically investigated via single-crystal X-ray diffraction, thermogravimetric analysis, Hirshfeld surface analysis, and gas adsorption experiments. Both frameworks are built via N-H...O hydrogen bonds. The asymmetric unit of G-5,5'-BPyDC contains two methanol molecules, resulting in larger free volume and surface area compared to G-4,4'-BPyDC, which exhibits more compact packing. Both materials show decomposition temperatures above 290°C, indicating good thermal stability. Hirshfeld surface analysis reveals that the total contribution of O-H/H-O and N-H/H-N hydrogen bonds in G-5,5'-BPyDC (32.0%) is higher than that in G-4,4'-BPyDC (29.7%). At 25°C and 4.0 MPa, the N2O adsorption capacity of G-5,5'-BPyDC is 2.32 mmol/g, surpassing that of G-4,4'-BPyDC (2.02 mmol/g). IAST calculations show that the selectivities of G-5,5'-BPyDC for N2O/N2 (50:50 and 10:90) mixtures reach 29.26 and 111.32, respectively, significantly superior to those of G-4,4'-BPyDC (6.61 and 17.03). Pyridyl nitrogen site isomerism effectively optimizes N2O/N2 adsorption and separation by modulating pore polarity and hydrogen-bonded network, offering a new strategy for isomer design.
1. Introduction
Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential approximately 300 times that of CO2, and its emission from industrial processes such as adipic acid and nitric acid production poses significant environmental challenges. Conventional capture technologies, including amine scrubbing and cryogenic distillation, are energy-intensive and often inefficient for dilute N2O streams. Porous materials such as zeolites, metal-organic frameworks (MOFs), and activated carbons have been explored, but they frequently suffer from low selectivity, poor stability under humid conditions, or high regeneration costs. Hydrogen-bonded organic frameworks (HOFs) have emerged as promising alternatives due to their well-defined porosity, facile solution processing, and potential for structural tunability via non-covalent interactions. However, the design of HOFs with high N2O/N2 selectivity remains challenging, as the subtle differences in quadrupole moments and polarizabilities between N2O and N2 require precise control over pore chemistry and geometry.
This study addresses the bottleneck by employing pyridyl nitrogen site isomerism in guanidinium-based HOFs. By systematically varying the position of nitrogen atoms in the bipyridine dicarboxylate ligands, the authors modulate the hydrogen-bonded network and pore polarity, directly influencing adsorption capacity and selectivity. The experimental results demonstrate that G-5,5'-BPyDC, with nitrogen atoms at the 5,5' positions, achieves a remarkable N2O/N2 selectivity of 111.32 for a 10:90 mixture at 25°C and 4.0 MPa, significantly outperforming its isomer G-4,4'-BPyDC. This isomerism strategy offers a rational design route to enhance N2O capture performance, potentially enabling more energy-efficient separation processes.
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Lina JIA, Shiyao CHEN, Guoying ZHAO, Changyu SUN (2026). Hydrogen-bonded networks and N2O/N2 adsorption separation performance of pyridinecarboxylate guanidinium HOFs. The Chinese Journal of Process Engineering. https://doi.org/10.12034/j.issn.1009-606X.226111
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Frequently Asked Questions
What is the maximum N2O adsorption capacity and selectivity achieved by G-5,5'-BPyDC, and under what conditions?
At 25°C and 4.0 MPa, G-5,5'-BPyDC achieves an N2O adsorption capacity of 2.32 mmol/g. Its IAST selectivity for N2O/N2 mixtures is 29.26 for a 50:50 (v/v) mixture and 111.32 for a 10:90 (v/v) mixture, indicating high affinity for N2O even at low concentrations.
How does the thermal stability of these HOFs compare to conventional adsorbents, and what are the implications for industrial applications?
Both G-5,5'-BPyDC and G-4,4'-BPyDC exhibit decomposition temperatures above 290°C, which is comparable to many robust MOFs and zeolites. This thermal stability allows for regeneration via temperature swing without degradation, making them suitable for cyclic adsorption processes.
What is the role of pyridyl nitrogen site isomerism in enhancing N2O/N2 separation performance?
Pyridyl nitrogen site isomerism alters the position of nitrogen atoms in the ligand, which influences the hydrogen-bonded network and pore polarity. In G-5,5'-BPyDC, the nitrogen atoms are positioned to create a more polar pore environment and higher hydrogen-bond density (32.0% vs 29.7% for G-4,4'-BPyDC), leading to stronger interactions with N2O and higher selectivity.
What are the potential scalability and cost considerations for synthesizing these HOFs?
The synthesis involves commercially available bipyridine dicarboxylic acids and guanidinium salts, which are relatively inexpensive. The HOFs are formed via self-assembly under mild conditions, potentially allowing for large-scale production. However, further studies are needed to assess long-term stability and regeneration efficiency under industrial conditions.
How does the adsorption capacity of G-5,5'-BPyDC compare to other reported N2O adsorbents?
The N2O capacity of 2.32 mmol/g at 4.0 MPa is competitive with many MOFs and zeolites. For instance, some MOFs report capacities in the range of 1-3 mmol/g at similar pressures, but the selectivity of G-5,5'-BPyDC for N2O/N2 (111.32 for 10:90) is exceptionally high, making it a promising candidate for efficient separation.
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