Time-Dependent CO₂ Sorption Hysteresis in a One-Dimensional Microporous Octahedral Molecular Sieve

Abstract

The development of sorbents for next-generation CO(2) mitigation technologies will require better understanding of CO(2)/sorbent interactions. Among the sorbents under consideration are shape-selective microporous molecular sieves with hierarchical pore morphologies of reduced dimensionality. We have characterized the non-equilibrium CO(2) sorption of OMS-2, a well-known one-dimensional microporous octahedral molecular sieve with manganese oxide framework. Remarkably, we find that the degree of CO(2) sorption hysteresis increases when the gas/sorbent system is allowed to equilibrate for longer times at each pressure step. Density functional theory calculations indicate a "gate-keeping" role of the cation in the tunnel, only allowing CO(2) molecules to enter fully into the tunnel via a highly unstable transient state when CO(2) loadings exceed 0.75 mmol/g. The energy barrier associated with the gate-keeping effect suggests an adsorption mechanism in which kinetic trapping of CO(2) is responsible for the observed hysteretic behavior.

Carbon dioxide sorption in a nanoporous octahedral molecular sieve

“…Such a value can be easily overcome, and is much smaller than both the activation energy barrier for CO2 to bypass the Na + cation as well as the diffusion energy barrier for the K + cation. Research on porous materials exhibited that the cation is movable in the porous tunnels [9,26,40,41]. Our calculations indicate that Scenario III is more energetically favorable, where CO2 pushes the Na + cation to further diffuse along the OMS-2 tunnel.…”

“…It is worth noting that during synthesis of these compounds, H2O molecules can be found to coordinate with the cation and hinder CO2 adsorption uptake [26,34]. However, in a previous experimental study of K-doped OMS-2, it has been shown that upon heating to 150°C, the H2O can be removed without affecting the K + dopant [34].…”

“…Due to a high activation energy barrier of ~6eV/CO2, CO2 remains in the equilibrium position without further diffusion (Scenario I, Fig 3(a)). As the concentration of CO2 increases, the activation energy barrier minimizes to 0.13 eV/CO2 [26], smaller than a diffusion energy barrier of 0.37 eV for a K + cation. These computational results suggest Scenario II (Fig 3(b)), where CO2 bypasses a K + cation to further diffuse along the OMS-2 tunnel when CO2 has a high concentration (averagely KMn24O48 with two or more than two CO2 molecules).…”

“…These computational results suggest Scenario II (Fig 3(b)), where CO2 bypasses a K + cation to further diffuse along the OMS-2 tunnel when CO2 has a high concentration (averagely KMn24O48 with two or more than two CO2 molecules). The reduction in activation energy barrier indicates that at elevated CO2 partial pressures, the transition state becomes more energetically favorable [26].…”

“…Previously, we found that adding K + ions to the (2×2) tunnels leads to hysteresis looping in the adsorption and desorption isotherms [26]. The onset of hysteresis only occurs at experimental partial pressures exceeding 7 bar (700 kPa).…”