Reports: ND1054802-ND10: Molecular Mechanisms Underlying the Adsorption of Alkanes and their Mixtures in Metal-Organic Frameworks

Jerome Delhommelle, University of North Dakota

The storage and separation of alkane compounds and isomers is a very important process in the petroleum industry. In practice, this can be achieved through e.g. cryogenic distillation, which is often used for the separation of hexane isomers to increase the octane ratings in gasoline. Nanoporous materials, including zeolitic materials or the recently synthesized materials known as Metal-Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs), have emerged as especially promising candidates for separation applications. The aim of this research is to develop new molecular simulation methods leading to a complete thermodynamic analysis of the adsorption of alkane mixtures in MOFs and COFs, as well as of their transport properties.

To carry out this thermodynamic analysis, we focus on developing a new method, the Expanded Wang-Landau (EWL) approach, which relies on determining the density of states, and thus the partition function, of fluids through the combination of state-of-the-art efficient sampling methods. In the first year of the grant, we carried out two methodological developments. First, together with an undergraduate student, Parker W. Anderson (now working towards a MS in Petroleum Engineering), we increased considerably the types of model that can used in conjunction with the EWL method, which now range from classical force fields to quantum tight-binding systems. This work has lead to the publication in 2017 of an article in J. Phys.: Condens. Matter. We also implemented a hybrid Monte Carlo approach, together with the Wang-Landau sampling, to carry out the determination of the thermodynamic properties of complex hydrocarbon molecular structures, leading to the publication in 2017 of a paper in Energy & Fuels. In the second year of the project, we applied the EWL method to compute the thermodynamic properties of adsorption of alkane mixtures and to screen a series of COFs for separation applications. The specific advantage of the EWL method, when applied to confined fluids, is the following. It yields an accurate estimate for the partition function of the adsorbed fluid, and thus, through the formalism of statistical mechanics, it leads to a direct evaluation of all thermodynamic properties of the adsorbed phase, including e.g. the entropy and the free energy of desorption. The latter is key, since it corresponds to the minimum isothermal work of regenerating the adsorbent, and it is especially useful when comparing the performance of adsorbents, since most of the operating costs are associated with degassing the adsorbent in preparation for the next cycle. This work was done with a postdoc, Dr. K. Gopalsamy, who joined the group last November and worked on determining the selectivity, as well as the desorption free energy, for the adsorption of alkane mixtures in these nanoporous materials. The results have been presented by Dr. K. Gopalsamy at the ACS Spring National Meeting in San Francisco, CA. They have also resulted in a manuscript current under review.

The second objective of the research supported by this grant consists of determining the transport properties of alkane mixtures in nanoconfined systems. This is achieved through the development of new simulation methods, based on the analysis of the nonequilibrium correlation functions to determine the relevant transport coefficients. This work is done with another postdoc, Dr. I. Valencia-Jaime, who joined the group in December. Dr. I. Valencia-Jaime is working on extending the transient-time correlation function (TTCF) approach to study the flow of alkane mixtures in nanoconfined, complex, geometries, such as those encountered in MOFs and COFs. This part of the research will continue on during the upcoming year, and transport coefficients for alkane mixtures in MOFs and COFs will be evaluated to analyze the suitability of these nanoporous materials to serve as membranes for gas separation.