Framework-Dependent Adsorption Regimes of Linear and Cyclic Siloxanes in Metal-Organic Frameworks under Trace Concentrations.

Volatile siloxanes are persistent molecular contaminants in industrial gas streams and represent a significant concern in aerospace environments, where trace-level outgassing can lead to deposition on sensitive surfaces and performance degradation. While Metal-Organic Frameworks (MOFs) have emerged as promising adsorbents for siloxane capture, the adsorption behavior of structurally distinct linear and cyclic siloxanes under dilute conditions remains poorly understood, particularly in relation to pore architecture-dependent mechanisms. Here, we combine very low-pressure gravimetric adsorption measurements at 303 K with Grand Canonical Monte Carlo (GCMC) simulations to investigate the adsorption of hexamethyldisiloxane (L2) and octamethylcyclotetrasiloxane (D4) in three MOFs with contrasting pore structures: PCN-777(Zr), DUT-4(Al), and MIL-101(Cr). The results indicate that siloxane adsorption is governed by distinct mechanistic regimes depending on framework topology rather than a single adsorption pathway. DUT-4(Al) exhibits a confinement-driven regime characterized by ultralow-pressure uptake and strong host/guest interactions, associated with limited reversibility and enhanced retention, particularly for D4. PCN-777(Zr) follows a cooperative pore-filling regime at higher pressures, which limits its efficiency under trace conditions. MIL-101(Cr) displays intermediate adsorption onset with improved reversibility, resulting in a balanced uptake-regeneration behavior. Across all frameworks, D4 consistently adsorbs more strongly than L2, reflecting enhanced stabilization of cyclic siloxanes in confined environments, while remaining sensitive to pore architecture. Overall, this study provides a mechanistic description of siloxane adsorption in MOFs under trace conditions and highlights the role of pore topology in governing adsorption regimes and regenerability.