Modeling Gas Adsorption in Flexible Metal–Organic Frameworks via Hybrid Monte Carlo/Molecular Dynamics Schemes

Abstract

Herein, we develop and validate a hybrid Monte Carlo (MC) / molecular dynamics (MD) simulation protocol that properly accounts for the extraordinary structural flexibility of metal-organic frameworks (MOFs). This is vital to accurately predict gas adsorption isotherms and guest-induced flexibility of these materials. First, the performance of three recent models to predict adsorption isotherms and flexibility in MOFs is critically investigated. While these methods succeed in providing qualitative insight in the gas adsorption process in MOFs, their accuracy remains limited as the intrinsic flexibility of these materials is very hard to account for. To overcome this challenge, we introduce a hybrid MC/MD simulation protocol that is specifically designed to handle the flexibility of the adsorbent, including the shape flexibility, thereby unifying the strengths of the previous models. It is demonstrated that the application of this new protocol to the adsorption of neon, argon, xenon, methane, and carbon dioxide in MIL-53(Al), a prototypical flexible MOF, substantially decreases the inaccuracy of the obtained adsorption isotherms and predicted guest-induced flexibility.As a result, this method is ideally suited to rationalize the adsorption performance of flexible nanoporous materials at the molecular level, paving the way for the conscious design of MOFs as industrial adsorbents.

Insight into the Role of Water on the Methylation of Hexamethylbenzene in H‐SAPO‐34 from First Principle Molecular Dynamics Simulations

“…In this sense the assumed loading here should merely be interpreted as a rough but realistic estimate and a limiting case. More advanced models, for instance relying on grand canonical Monte Carlo simulations to estimate the adsorption isotherms at operating conditions, are out of the scope of this study …”

“…More advanced models, for instance relying on grand canonical Monte Carlo simulations to estimate the adsorption isotherms at operating conditions, are out of the scope of this study. [81,82] The unit cell parameters used to describe the H-SAPO-34 framework depend on the calculation method and guest loading, as statically the unit cell parameters are calibrated for the empty-host structure at 0 K, while the unit cell parameters for the enhanced sampling simulations are obtained at 623 K and with realistic guest loadings. The equilibration procedures are described in Section 1.1.2 of the ESI and the resulting unit cell parameters are summarized in Table S1 (ESI).…”

Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties

“…In addition, several approaches combine these computational methods to generate the full energy landscape of guest–host systems of SPCs in a single simulation. The hybrid osmotic Monte Carlo (HOMC) method was first successfully applied in recreating the breathing behavior of MIL‐53(Cr), and later on other systems . The grand canonical ensemble is another approach for describing guest‐induced transitions in SPCs based on the free energy of the host, the adsorption isotherm of the phases, and the pressure‐dependent molar volume of the guests .…”

Design principles for the ultimate gas deliverable capacity material: nonporous to porous deformations without volume change

“…25 In similar systems, cooperative adsorption due to ring rotations in constant volume materials has been modelled and observed (although the minimum energy state was still porous). 29,30 It should be noted that such materials could only ever be computationally identified with a fully flexible treatment coupling framework dynamics and adsorption; 16,[31][32][33][34] hence, they will have remained hidden from almost any high-throughput adsorbent screening study to date. 35,36 Finally, we show that the isoreticular tunability 37,38 of metal-organic frameworks (MOFs) can be used to design an improved version of the M(NDC) adsorbent.…”

“…Rigorous atomistic simulations of adsorption in flexible nanoporous materials are complicated endeavors, not only because they often require enhanced sampling techniques, 32,34 but also because the predictions can be very sensitive to small fluctuations in structural dynamics. 35 In other words, quantitative isotherm predictions require an extremely accurate description of the potential energy surface and long simulations.…”