57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The main goals of this research project were to: 1) understand the properties of adsorbates in MOFs for a wide range of temperatures and pressures, 2) assess existing materials and design not-yet synthesized materials for separation of CO₂from mixtures and 3) establish a strong research program suitable for training of students and enriching the curriculum.

We proposed a simple model that permits the study of adsorption in MOFs with systematic variations in the charge distribution, size and force parameters. The model can be used to represent the series of the IRMOFs and may also be adapted to other MOFs with noncubic unit cells by modifying the geometry accordingly.

The model does not require a detailed description of the charge distribution of the material, which in many cases is not accurately known. We have reproduced the adsorption isotherms of CO₂ and CH₄in IRMOF-1 using an appropriate set of force parameters.

In order to study CO₂, we made key improvements in the simulation code, which we had developed to study the properties of spherical adsorbates. For example, because CO₂is a linear molecule that has a finite quadrupole moment, the interatomic potential includes a quadrupole-quadrupole term; also, we included additional steps in the monte carlo, to account for the rotation of the molecule.

We focus on the selectivity of CO₂ over CH₄. Our results suggest that the charge distribution of the MOF will be relevant only if it contains molecules with nonzero electric dipole. In particular, adding dipoles to the corners of the MOF’s unit cell would increase the selectivity. Surprisingly, components with nonzero electric quadrupole moment seem to be irrelevant.

We have also found that compressing the cell in only 10% significantly increased the selectivity. On the other, hand expanding the cell by 20% reduced it.

We conclude that a precise portrayal of the charge distribution of the MOFs is irrelevant in studies of adsorption of gases composed of molecules with zero dipolar or quadrupolar moments. In those cases, the force fields can accurately be described by Lennard Jones (LJ) interactions. Moreover, the results of this work suggest that even in the case of CO₂, the charge distribution is relevant only in the case where dipolar elements compose the MOF.

The model proposed in this work can be used to check existing or hypothetical substrates in order to determine which characteristics are essential for CO₂selectivity.

This research has been beneficial for the career of the PI and students participating. Receiving this funding helped the PI pursuing her career goals. For example, improving the codes to simulate linear molecules, enabling studies of CO₂; closely supervising the progress of three PhD students, one of whom is expected to defend his thesis by the end of 2013; designing and offering a new course on “Molecular Simulations” in the department of Physics at Howard University; advancing in the investigation of adsorption of CO₂in materials, aiming to help reduce the damaging impact of carbon dioxide on the planet.

Activities:

In 2011, Gatica served as co-chair of the organizing committee of a workshop on “Physical adsorption in nanostructured materials” at the University of Missouri.

Gatica designed a new course, Molecular Simulations. When the course was offered in the Fall 2011, five students attended and all completed the course. Students used Wolfram Mathematica and Fortran programming to solve assignments.

During the period of this grant the PI gave three invited talks, contributed five papers to conferences and published five articles. This grant partially supported three graduate students.