Equilibrium Separation of Siloxanes in Metal–Organic Frameworks

We present an in silico assessment of metal–organic
frameworks (MOFs) for the equilibrium separation of linear and cyclic
siloxanes. Using a combination of configurational bias/continuous
fractional component Monte Carlo (CB/CFCMC) simulations and Ideal
Adsorbed Solution Theory (IAST), we investigated the adsorption of
both equimolar and nonequimolar mixtures of linear and cyclic siloxanes
in a selection of synthesizable MOFs with medium to large pore volumes.
We showed that configurational entropy effects drive the preferential
adsorption of linear siloxanes over cyclic siloxanes. Based on synthesizability
metrics we identified ZIF-70 as a promising adsorbent for the separation
of linear and cyclic siloxanes. Self-diffusivities of linear and cyclic
siloxanes in ZIF-70 calculated from molecular dynamics simulations
show that equilibrium can be reached on reasonable time scales. We
explored vacuum-temperature swing adsorption (VTSA) as a potential
method for the recovery of adsorbed linear siloxanes from ZIF-70,
achieving a 75% reduction in adsorbed phase concentration. Additionally,
we demonstrated that supercritical CO2 offers an alternative
desorption strategy by displacing adsorbed linear siloxanes in ZIF-70
at pressures above 200 bar driven by entropy effects.