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Reactive adsorption of ammonia and ammonia/water on CuBTC metal-organic framework: A ReaxFF molecular dynamics simulation

Evidence and attribution

Evidence

Prose below summarizes the peer-reviewed article identified by doi, title, and pdf_path in the front matter. For exact numbers and figures, use the published paper and normalized/extracts/2013huang-venue-paper-4_p1-2.txt.

Summary

ReaxFF molecular dynamics is used to study reactive adsorption of NH\(_3\) on dehydrated CuBTC (HKUST-1), including temperature- and concentration-dependent framework stability and NH\(_3\)/H\(_2\)O mixtures. The work emphasizes chemisorption at Cu sites, partial framework response to ammonia, and a critical NH\(_3\) loading (about one NH\(_3\) per Cu) beyond which collapse behavior changes, with water moderating ammonia uptake and stabilizing the framework under some conditions.

Methods

ReaxFF reactive MD (RMD) used a parameter set that merges prior Cu/O/H work with the published glycine ReaxFF (shared O/H functions; non-bonded C/Cu from combination rules), retrained so the combined field describes CuBTC and its interaction with NH\(_3\) and H\(_2\)O (parameter tables in supporting information of the article). The dehydrated CuBTC model starts from the hydrated crystal geometry with coordinated waters removed. Adsorbates were placed in channels and tetrahedral pockets with a 3.0 Å minimum separation to avoid overlaps.

Simulations used a 1×1×1 periodic cell of dehydrated CuBTC (Cu\(_{48}\)C\(_{288}\)O\(_{192}\)H\(_{96}\)), i.e. on the order of 10³ atoms in the framework supercell, plus the reported numbers of NH\(_3\) and/or H\(_2\)O adsorbate atoms. Calculations were run in the NPT ensemble with a Verlet integrator, time step 0.25 fs, Berendsen thermostat and barostat (damping 100 fs and 2500 fs, respectively), and trajectories stored every 50 fs; equilibration and production segment lengths in ps/ns are given in the article’s protocol tables (papers/Huang_Joshi_MOF_JCPSA6_138_3_034102_1.pdf).

Water stability scans covered 301–389 K with 12–196 H\(_2\)O molecules (0.25–4.0 equivalents per Cu site, following a prior NMR study cited in the paper). NH\(_3\) loading followed experimental uptake at 301 K, 318 K, and 348 K (120, 88, and 63 NH\(_3\), respectively, in the 1×1×1 cell). NH\(_3\)/H\(_2\)O mixtures included 24/120, 48/120, and 96/120 H\(_2\)O/NH\(_3\) ratios (among the cases reported).

Findings

At moderate temperature, dehydrated CuBTC tolerates water up to about 4 H\(_2\)O per Cu without loss of hydrostatic stability, whereas at 550 K collapse can occur even at 1 H\(_2\)O per Cu. NH\(_3\) in channels and micropores chemisorbs at Cu rather than pairing as NH\(_3\) dimers. Equimolar Cu\(_2\)(NH\(_2\))\(_4\) and (NH\(_4\))\(_3\)BTC-like motifs appear at 348 K, described as consistent with prior experiments. Partial framework collapse upon NH\(_3\) adsorption can occur while Cu–Cu dimer motifs remain stable under the conditions sampled. The work identifies a critical NH\(_3\) loading of about one NH\(_3\) per Cu: depending on whether the NH\(_3\) concentration is below or above this threshold, the dehydrated framework can remain stable to 378 K or collapse at 250 K, respectively. For H\(_2\)O/NH\(_3\) mixtures, water does not show strong direct interaction with Cu in the same sense as ammonia, but it suppresses chemisorbed NH\(_3\) and stabilizes the framework to some extent.

Limitations

ReaxFF accuracy depends on the Cu/MOF and amine parameterization; long-time equilibria and polydisperse experimental samples may require larger cells and longer runs than reported.

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