Advanced Pore Structure Characterization Methods for Crystalline and Amorphous Materials
1. Characterization of Metal-Organic Frameworks (Shivam Parashar, Qing Zhu, Silvio Dantas and Alexander V. Neimark)
Unique adsorption and transport properties of MOF materials are determined by their complex 3D networks of pore compartments (cages, channels, windows) that differ in size, shape, and chemical functionalities. However, practical MOF samples are rarely the ideal crystals: they contain binders, various defects, and residual solvents. In this work, we propose a novel methodology for assessment from the experimental adsorption isotherms the degree of sample crystallinity, pore type distribution function, adsorption capacity and accessibility of individual pore compartments. Using Monte Carlo simulations, we construct the theoretical adsorption isotherm on the ideal MOF crystal and decompose this isotherm into the fingerprint isotherms corresponding to individual pore compartments. Information about the sample pore structure is obtained from matching the experimental isotherm to the theoretical fingerprint isotherms. This approach is demonstrated on four MOF samples: Cu-BTC, PCN-224, ZIF-412, and UiO-66 using Ar, N2 and CO2 at their normal boiling temperatures. The constructed fingerprint isotherms are verified against the experimental data obtained by in-situ adsorption crystallography. The method of pore level compartmentalization of adsorption isotherms provides a better understanding of the adsorption mechanisms and distribution of adsorbate molecule at the pore level that is instrumental for the selection and design of novel adsorbents with improved properties for gas separations, storage, and catalysis.
This work- https://pubs.acs.org/doi/full/10.1021/acsanm.1c00937
2. Characterization of Kerogen (Shivam Parashar, Peter I. Ravikovitch, Alexander V. Neimark)
Shale reservoirs are sedimentary porous rock made up of organic and inorganic parts with pore size distribution spanning from micropores to µm range. Kerogen is a key component of the organic part of shale where CH4 exists in adsorbed state. The aim of this work is to enhance the understanding of structure and adsorption properties of organic porosity in kerogen, as it is related to predicting storage capacities of oil and gas, and hydrocarbon recovery. First, we create atomistic 3D models of bulk kerogen, kerogen surface, and mesopores imbedded in the kerogen matrix. Using Grand Canonical Monte Carlo (GCMC) simulations, we calculate the reference adsorption isotherms on the bulk kerogen matrix of intrinsic microporosity, on the kerogen surface, as well as in a series of mesopores confined by rough kerogen walls. Next, we parameterized the Quenched Solid Density Functional Theory (QSDFT) to reproduce the structure of the kerogen surface heterogeneity and the adsorption isotherms of Ar, and N2. We approximated the reference kerogen surface isotherm by a simple exponentially decaying disjoining pressure isotherm, which is used in the Derjaguin-Broekhoff-de Boer (DBdB) model to predict adsorption isotherm in pores of larger sizes. We demonstrate that the reference GCMC isotherms on the surface and in the pores are reasonably approximated by the QSDFT and DBdB models. Based on GCMC, QSDFT and DBdB methods, we characterize an experimental sample and calculate micropore volume, surface area and pore size distribution. This approach provides a reliable characterization of the hierarchical micro-mesoporous kerogen matrix that is imperative for understanding the specifics of the hydrocarbon recovery and carbon sequestration in shale reservoirs.