Synthesis of single-crystal UiO-67-(NH2)2 for effective SO2 adsorption and separation from flue gas

Abstract

The synthesis of stable and efficient porous adsorbents for adsorptive capture and recovery of SO₂ from flue gas is of great importance for minimizing air pollution and reducing costs. Herein, by choosing trifluoroacetic acid as modulator, the single-crystal UiO-67-(NH₂)₂ has been successfully synthesized benefitting crystal structure determination. The stability tests, gas adsorption measurements, and breakthrough experiments demonstrated that single-crystal UiO-67-(NH₂)₂ possesses moderate acid-base and hydrothermal stability, and exhibited high SO₂ uptake (19.72 mmol/g, 298 K and 1 bar), good SO₂/CO₂ separation selectivity (110.3～33.3, SO₂/CO₂ = 10/90), and excellent SO₂ recovery purity (93.3 %). However, the instability of the UiO-67-(NH₂)₂ framework, combined with the strong corrosive and reactive properties of SO₂, can lead to partial collapse of the framework during the adsorption and capture process. This ultimately results in a decreased SO₂ capture performance over time. In-situ DRIFT, GCMC simulation and DFT calculations revealed that the μ₃-OH of Zr₆O₄(OH)₄ clusters and the —NH₂ groups of the ligand can form multiple hydrogen bonds with SO₂ molecules. This ensures that SO₂ molecules are firmly grasped by UiO-67-(NH₂)₂ framework and facilitate the efficiently selective capture of SO₂ at low concentration.