Ammonia (NH₃) is a promising carbon‑free energy carrier due to its high hydrogen content (17.7 wt %) and established logistics. However, its toxicity drives the need for adsorbents with tunable uptake properties. Metal–organic frameworks (MOFs), especially the zirconium‑based UiO‑67, offer ultrahigh porosity and exceptional stability, making them ideal platforms for NH₃ capture.
In a study published in ENGINEERING Chemical Engineering, researchers systematically investigated two complementary strategies to tailor NH₃ adsorption in UiO‑67: defect engineering via modulator‑induced linker vacancies, and post‑synthetic metalation with copper to introduce open metal sites.
For defect engineering, three modulators of different acidity – acetic acid (pKa 4.76), difluoroacetic acid (pKa 1.24), and trifluoroacetic acid (pKa 0.23) – were used at varying molar equivalents. The more acidic fluorinated modulators produced much higher defect densities, ranging from 5.4 % (Acet8) to 50.1 % (Trif20) – a nearly ten‑fold control. These defects transform trigonal windows into larger lozenge windows, which profoundly changes the NH₃ adsorption isotherms. Low‑defect samples exhibit a clear first transition step around 20 kPa due to delayed pore filling through trigonal windows, while high‑defect samples lack this step but show a pronounced second transition step around 65 kPa from pore filling through lozenge windows. Fluorine atoms on the modulators act as additional adsorption sites for NH₃, boosting uptake at high pressure. As a result, the low‑defect Acet8 and high‑defect Trif20 display strikingly different profiles: Acet8 has 30.6 % higher capacity at 30 kPa, but Trif20 exceeds it by 21.1 % at 100 kPa.
The second strategy involved incorporating bipyridyl linkers (bpydc) into UiO‑67, followed by chelation of copper(II). The bpydc linkers alone enhance NH₃ uptake via hydrogen bonding. After copper loading, the chelated Cu sites act as strong Lewis acids, binding multiple NH₃ molecules more strongly. However, a trade‑off exists: while copper creates strong adsorption sites, it also reduces surface area and pore volume. The optimum was found at low bpydc content (< 5 %). Sample 5py‑Cu (5 % bpydc) achieved a 50.6 % increase in NH₃ uptake compared to its copper‑free counterpart. Higher bpydc contents (e.g., 20py‑Cu) led to pore blockage – BET surface area dropped from 2247 m²·g⁻¹ (5py‑Cu) to 1323 m²·g⁻¹ (20py‑Cu). NH₃‑TPD showed that 5py‑Cu retains 84 % more NH₃ after purging than 20py‑Cu.
Both defect‑engineered and copper‑loaded materials showed excellent regenerability over three cycles (less than 5 % capacity loss) and good hydrolytic stability after 24 h in water.
This work establishes defect control as a means to shape the adsorption isotherm profile, while metal loading directly governs uptake capacity. The synergistic combination provides a versatile platform for designing UiO‑67 adsorbents with application‑specific NH₃ storage, separation, and sensing performance.

DOI
10.1007/s11705-026-2653-7