Development and optimization of metal-organic frameworks for ammonia management —— from molecular design to practical applications

Abstract

Ammonia (NH₃) is ubiquitous in many industrial processes and everyday applications. Despite its widespread use in fertilizers, carbon-free energy, and chemical synthesis, NH₃ poses significant challenges as it is both toxic and corrosive. Even at extremely low concentrations, NH₃ can adversely affect human health, damage delicate industrial processes, including semiconductor manufacturing, and harm the environment. Given these dual challenges, there is a pressing need for technologies capable of removing NH₃ at low pressures and efficiently separating and storing it under high-pressure conditions. Metal-organic frameworks (MOFs), with their highly tunable pore structures and vast surface areas, have emerged as promising candidates for tackling these challenges due to their exceptional gas adsorption properties. This thesis addresses these challenges by developing tailored MOFs optimized for NH₃ management across varying pressure regimes.

For trace NH₃ capture at ultralow pressures (< 0.01 bar), M-GA MOFs (M = Co, Mg, Mn; GA = gallic acid) were synthesized via hydrothermal methods using gallic acid and divalent metal ions. The pore dimensions (3.10-3.63 Å) of the frameworks were precisely tuned to match kinetic diameter (2.9 Å) of NH₃, enabling molecular confinement. Co-GA MOF achieved a record NH₃ adsorption capacity of 3 mmol g^-1 at 0.001 mbar (1 ppm), outperforming benchmarks like Mg₂(dobpdc) (1 mmol g^-1 at 0.01 mbar) and UiO-66-NH₂ (0.93 mmol g^-1 at 0.448 mbar). This exceptional performance stemmed from synergistic interactions of hydroxyl and carboxyl groups on the ligand formed strong O―H···N hydrogen bonds (binding energy: 85.17 kJ mol^-1 for Co-GA MOF), while van der Waals forces enhanced adsorption in the micropores. The materials exhibited ultrahigh selectivity, with ideal adsorbed solution theory (IAST) coefficients exceeding 10⁴ for NH₃/CO₂ at 0.1% NH₃ concentrations, critical for air purification in NH₃-sensitive environments like semiconductor cleanrooms. Dynamic breakthrough experiments further validated practicality. Under a 10 ppm NH₃/N₂ flow, Co-GA MOF maintained NH₃ levels below 1 ppm for 20,584 min g⁻¹ (∼14 days per gram), far exceeding NIOSH exposure limits (25 ppm over 8 hours). Stability tests confirmed resilience, with X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showing no structural degradation after cyclic NH₃ adsorption/desorption. These results, as evidenced by density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) analyses, establish M-GA MOFs as promising transformative materials for low-concentration NH₃ removal in industrial and environmental applications.

Beyond low-pressure removal, the separation of NH₃ at higher pressures is also critical for the industrial Haber-Bosch process. However, the conventional separation of NH₃ via cryogenic distillation remains energy-intensive and environmentally unsustainable. For NH₃ separation at moderate pressures (0.1-1 bar), flexible M-INA MOFs (M = Cu, Co, Mn; INA = isonicotinic acid) were engineered using the isonicotinic acid ligand and solvothermal synthesis. Characterization revealed Cu-INA MOF as microporous (142 m² g^-1 surface area), while Co- and Mn-INA MOFs exhibited negligible porosity. Remarkably, all three MOFs achieved high NH₃ uptake (~14 mmol g^-1) under moderate pressures, with Co- and Mn-INA MOFs relying on pressure-induced structural transitions to “open” pores via a gate-opening mechanism. Static isotherms and dynamic breakthrough experiments demonstrated exceptional NH₃ selectivity over N₂ and H₂. In-situ XRD and molecular dynamics (MD) simulations unveiled temperature-dependent lattice contractions and guest-induced framework expansion, with pyridine ring rotations reducing diffusion barriers. Adsorption thresholds for NH₃ in Cu-INA (0.01 bar, 5.58 kcal mol^-1), Co-INA (0.1 bar, 10.34 kcal mol^-1), and Mn-INA (0.4 bar, 16.78 kcal mol^-1) were found to correlate with the energy barriers for NH₃ entry, indicating that higher barriers correspond to higher thresholds. These findings emphasize the synergistic relationship between structural flexibility and selective adsorption observed in MOFs, thereby establishing M-INA MOFs as promising candidates for energy-efficient NH₃ separation. The insights into gate-opening mechanisms and host-guest interactions advance the design of adaptive adsorbents, offering a sustainable alternative to traditional cryogenic methods and aligning with global decarbonization goals.

A comprehensive design strategy for enhancing NH₃ adsorption in MOFs was presented in this wok. For low-pressure applications, ligand functionalization of terephthalic acid (TPA) derivatives with polar groups (-NH₂, -OH) was employed to strengthen binding via hydrogen bonding and dipole interactions, while open metal sites (Cu, Ni, Co, Zn, Mg) were introduced through controlled synthesis conditions to optimize coordinative NH₃ binding. Electrostatic potential (ESP) analysis revealed that -NH₂ group generated localized positive charges (+42.17 kcal·mol^-1), fostering strong N―H···N interactions with NH₃, while -OH groups created negative potentials (-24.32 kcal·mol^-1) for N―H···O bonding. Open metal sites in Zn-O-MOF (inspired by CPO-27 topology) further amplified low-pressure uptake, with Mg-O-MOF achieving surface area of1696.88 m² g^-1. For high-pressure storage, ligand extension via aromatic ring insertion expanded pore volumes from 0.80 cm³ g^-1 (Zn-MOF-B1) to 0.92 cm³ g^-1 (Zn-MOF-B3), enabling capillary condensation at 8 bar. Computational methods, including DFT, ESP analysis, and Grand Canonical Monte Carlo (GCMC) simulations, guided the rational design of MOFs by predicting ligand conformations, charge distributions, and adsorption isotherms. Experimental validation revealed that functionalized MOFs, particularly Zn-MOF-NH₂, exhibited superior low-pressure NH₃ uptake, aligning with simulated trends. Open metal site variants, such as Cu-O-MOF, demonstrated enhanced affinity at ultralow pressures, while ligand-extended MOFs achieved higher storage capacities at elevated pressures. Discrepancies between simulated and experimental Brunauer-Emmett-Teller (BET) surface areas indicated challenges including solvent retention and framework defects. Nonetheless, the integrated approach successfully bridged computational predictions with experimental outcomes, demonstrating the critical roles of ligand chemistry, metal coordination, and hierarchical porosity in tailoring MOFs for NH₃ capture. This strategy not only advanced NH₃-specific MOFs but also established a transferable workflow for tailoring materials to diverse gas adsorption challenges.

This research advances MOFs engineering for NH₃ management across various pressure regimes and applications. By integrating experimental synthesis, computational modeling, and mechanism analysis, the thesis provides a blueprint for advanced adsorbents that balance efficiency, selectivity, and industrial practicality, with applications in semiconductor manufacturing, environmental remediation and the emerging field of ammonia-based clean energy systems. Future work will focus on pilot-scale validation, energy consumption analysis, and expanding this framework to other hazardous gases, reinforcing the role of MOFs in achieving carbon neutrality and industrial sustainability.