Ammonia Vapor Removal by Cu₃(BTC)₂ and Its Characterization by MAS NMR

Adsorption equilibria and NMR experiments were performed to study the adsorption and interactions of ammonia with metal−organic framework HKUST-1, or Cu₃(BTC)₂ (BTC = 1,3,5-benzenetricarboxylate). Ammonia capacities determined from chemical breakthrough measurements show significantly higher uptake capacities than from adsorption alone, suggesting a stronger interaction involving a potential reaction with the Cu₃(BTC)₂ framework. Indeed, ¹H MAS NMR reveals that a major disruption of the relatively simple spectrum of Cu₃(BTC)₂ occurs to generate a composite spectrum consistent with Cu(OH)₂ and (NH₄)₃BTC species under humid conditionsthe anticipated products of a copper(II) carboxylate reacted with limited ammonia. These species are not detected under dry conditions; however, reaction stoichiometry combined with XRD results suggests the partial formation of an indeterminate diammine copper(II) complex with some residual Cu₃(BTC)₂ structure retained. Cu(II)-induced paramagnetic shifts exhibited by various species in ¹H and ¹³C MAS NMR spectra are consistent with model compounds and previous literature. Although results show extensive ammonia capacity of Cu₃(BTC)₂, much of the capacity is due to reaction with the structure itself, causing a permanent loss in porosity and structural integrity.