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

Abstract

Adsorption equilibria and NMR experiments were performed to study the adsorption and interactions of ammonia with metal-organic framework (MOF) 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.

Ammonia Adsorption and Co-adsorption with Water in HKUST-1: Spectroscopic Evidence for Cooperative Interactions

“…7 HKUST-1 (Cu 3 Btc 2 ) with the open Cu 2+ metal sites has been investigated for the removal of ammonia, nitric oxide, and CO 2 . [13][14][15][16] Ammonia uptake in HKUST-1 exceeds that of porous carbon. [13][14][15] However, the degradation associated with the presence of moisture renders HKUST-1 unsuitable for practical applications.…”

“…[13][14][15][16] Ammonia uptake in HKUST-1 exceeds that of porous carbon. [13][14][15] However, the degradation associated with the presence of moisture renders HKUST-1 unsuitable for practical applications. [13][14][15][16][17] Therefore, probing co-adsorption of gases in MOFs with open metal center such as HKUST-1 is important.…”

“…[13][14][15] However, the degradation associated with the presence of moisture renders HKUST-1 unsuitable for practical applications. [13][14][15][16][17] Therefore, probing co-adsorption of gases in MOFs with open metal center such as HKUST-1 is important. Previous work characterizing water and ammonia exposure of HKUST-1 were performed ex-situ after exposure to high concentrations of water and ammonia.…”

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

“…MOFs have an unprecedented ability to tailor functionality for targeted chemical interaction. Cu-BTC (also known as HKUST-1 or Cu 3 (BTC) 2 ) has been previously found to have an exceptionally high ammonia loading; however, this is at a cost of the material's structural stability 4 . Further studies on Cu-BTC have indicated that moisture itself is capable of degrading the MOF structure, rendering it ineffective for many potential applications 5,6,21 .…”

“…However, many MOFs with superior stability, such as UiO-66, have poor chemical removal capabilities, while many MOFs with open metal sites like MOF-74 and Cu-BTC have superior chemical removal capabilities 2,4,8,9 . The open metal sites in MOF-74 and Cu-BTC enhance the uptake of toxic gases such as ammonia, but these sites are also susceptible to binding water, poisoning the active sites and in many cases leading to structural breakdown.…”