In Situ Single-Crystal X-ray Diffraction Determinations of Cooperative Ammonia Capture in a Metal-Organic Framework

Porous metal-organic frameworks (MOFs) are promising sorbents for ammonia (NH₃) storage and capture; however, precisely defining the roles of active sites and establishing atomic-level insight into NH₃ binding domains remain challenging. Here, we report a robust zinc-allopurinol-imidazole framework, ALP-MOF-3, exhibiting exceptional thermal stability and tolerance to corrosive NH₃. Using in situ single-crystal X-ray diffraction (SCXRD), complemented by Fourier-transform infrared (FT-IR) spectroscopy and computational analysis, we directly resolve the binding domains of adsorbed NH₃ molecules. The results reveal that open Zn(II) sites, in concert with Brønsted basic carbonyl and pyrimidine nitrogen functionalities, act cooperatively as strong anchoring sites for NH₃. This synergistic capture enables energy-efficient uptake of trace NH₃ (1000 ppm) under both dry and humid (30-80% RH) conditions. The excellent structural integrity and recyclability of ALP-MOF-3 highlight its potential as a durable NH₃ sorbent and provide design principles for next generation MOFs with tailored active-site environments for selective gas capture.