Understanding Confined Fluids using Theory, Simulation, and Data-Driven Methods

Summary

Theory and simulation are used to advance our understanding of pure and multicomponent fluids confined in tight spaces, such as the pores of a zeolite, metal organic framework, shale, or sandstone. We develop and apply highly efficient simulations methods to predict thermodynamic properties of confined fluids (e.g., adsorption isotherms, selectivities, Henry’s constants, enthalpies of adsorption, etc.) and confined fluid phase behavior, in rigid and flexible frameworks. These tools are also employed to understand fundamentally the connection between dynamic properties and fluid structure. To this end, we explore the possibility of using equilibrium properties of confined fluids to predict their dynamic behavior over a broad range of pore loadings. In addition, we maintain the NIST/ARPA-e Database of Novel and Emerging Adsorbent Materials, a centralized resource for the scientific community to find and compare single- and multi-component adsorption isotherms reported in the literature.

Description

The most promising materials for capturing CO₂ (or other greenhouse gases like CH₄) from air or flue gas possess large internal surface area per unit mass because they are composed of pores with diameters in the range of tens-to-hundreds of nanometers. Collectively referred to as mass separating agents (MSAs), these porous materials include adsorbents (e.g., metal-organic frameworks), polymer membranes, and combinations of the two (e.g., mixed matrix membranes). Once captured, terrestrial CO₂ sequestration and mineralization processes involve fluid in contact with porous sedimentary rock, such as shale, for storage or subsequent chemical transformation. These rock formations are composed of complex and chemically heterogeneous pore networks and contain other chemical species like water and hydrocarbons. Clearly, understanding the behavior of pure CO₂ in confined environments, such as those afforded by these different materials is critical. Furthermore, when a fluid is confined to spaces comparable to molecule dimensions, its properties can be dramatically altered from the bulk due to the increased surface area exposed to the confining environment (e.g., pore wall); that is, existing measurements, data, and equations of state of bulk fluids are not applicable in these situations. In addition, CO₂ is rarely encountered as a pure fluid in practice. Thus, separation scientists and engineers actually seek properties of confined CO₂ mixtures when identifying candidate MSAs and developing processes for gas capture and sequestration. Unfortunately, given the diverse range of possible feed conditions (e.g., composition, temperature, and density) and material types (e.g., adsorbents, membranes, and porous rocks), comprehensive experimental characterization of this parameter space is a daunting task without a coordinated effort involving multiple entities; such an effort does not exist. Thus, there is a gaping knowledge gap in the development of carbon and greenhouse gas capture and sequestration processes.

Our research efforts employ theory and simulation to study and predict the behavior of pure and multicomponent fluids in confinement. The scope of our work extends to greenhouse gas capture and sequestration processes, as well as to the broader topic of MSA-based separations which require less energy than conventional chemical distillation. In addition to studying confined pure fluids, such as CH₄, CO₂, and H₂O, we develop advanced simulation methods to predict binary adsorption isotherms under a variety of feed conditions, which in turn provide selectivities and enthalpies of adsorption, key pieces of information separation scientists and engineers need to identify candidate MSAs. The fluid phase behavior of confined mixtures, which is difficult to determine experimentally, is also a topic we address with simulation, since this is an important issue in sequestration processes where residual water and hydrocarbons can be encountered, potentially leading to transport issues and pore blockage. Dynamic properties such as the confined self-diffusivity are important but require substantially greater computational and experimental effort. Here, we have developed scaling relations that allow one to predict the self-diffusivity as a function pore loading. Extending this to multicomponent fluids is the next step in this area of work.

Ultimately, we seek to use theory and simulation to help fill the substantial knowledge gap regarding the properties of confined fluid mixtures. In addition, this data can be fed into process simulation software for exploratory design, and can serve as the basis for advancing thermodynamic theories of confined fluids (i.e., beyond IAST) or for training AI models. Fundamentally and crucially, simulations provide insight into the molecular-level processes and mechanisms (e.g., gating) that determine selectivity, capacity, and transport in porous materials. Knowledge of this type can ultimately be used to rationally design MSAs with desired pore geometries and pore chemistry.

Other Related Projects

FEASST: Free Energy and Advanced Sampling Simulation Toolkit

NIST Data Resources for Adsorption Science and Technology

other related Publications

Molecular Simulation of Capillary Phase Transitions in Flexible Porous Materials