The Potential of Metal Organic Frameworks in Petroleum Refining Processes: Separation, Diffusion, and the Ad Hoc Engineering of Efficient Membranes
Loredana Valenzano
Chemistry Department – Michigan Technological University – Houghton (MI) – USA
In petroleum refining industries, long-chain hydrocarbons are cracked into olefin and paraffin mixtures that are then separated via the energetically and monetarily demanding cryogenic distillation process. In the attempt of mitigating both energy and capital consumptions to provide for more efficient gas separation, selective sorption of light hydrocarbons by tunable sorbents such as metal-organic frameworks (MOF) appears to be the most promising alternative to traditional cryogenic distillation.
MOFs are novel porous materials assembled from inorganic bricks connected by organic ligands. From a crystal engineering stand point, MOFs are advantageous in creating a range of microporous (0.2–2.0 nm) to mesoporous (>50 nm) cavities, presenting easy functionalization of both the organic ligands, and the void. Of significant importance is the MOF-74-M family (M=metal), characterized by a high density of open metal sites; also known as CPO-27-M, these materials have demonstrated more separation potential than other known MOFs and zeolites.[1,2] Structurally, MOF-74-M consist of a 3D hexagonal packing of helicoidal MO5 chain connected by 2,5-dihydroxyterephthalte organic linkers.
In my research group, density function theory (DFT) as developed within a linear combination of atomic orbitals (LCAO) approach has been used as the computational tool to investigate the selective sorption of C2-C4 hydrocarbons by MOF-74-Mg/Zn first by adopting a molecular cluster approach, and later by applying periodic boundary conditions (PBC). While both methods agree in showing significant differences in binding energies between olefins and paraffins at the open metal sites of the MOF, results reported at molecular cluster level are significantly smaller than those obtained at PBC level, exemplifying the importance of fully accounting for the chemical environment experienced by the adsorbed hydrocarbons. In addition, the use of PBC models allows for correcting the binding energies for BSSE, molecular lateral interaction, zero-point energy, and thermal contributions. As such, results obtained at PBC level are directly comparable to experimental calorimetric values. Our work discusses, for the first time, the origin of the fictitious agreement between binding energies obtained with molecular clusters and experimental heats of adsorption, identifying its origin as due to compensation of errors.
In the attempt to provide a more comprehensive description of the behavior of small hydrocarbons in MOF-74-M, our research efforts are currently devoted to address the diffusion processes of C1-C3 species in the framework cavity, and in the exploration of the ad-hoc engineering of water-stable metal oxide surfaces able to reproduce the promising separation potential of MOF-74-Mg. By adopting a plane-wave approach, our investigations are based on the use of climbing-image nudge elastic band (CI-NEB) simulations coupled with Van der Waals functional (vdW-DF) and ultra-soft pseudopotentials.
References
[1] M. Gallo, D. Glossman-Mitnik, J. Phys. Chem. C 113 (2009) 6634.
[2] Y. He, R. Krishna, B. Chen, Energy Environ. Sci. 5 (2012) 9107.
[3] G.D. Degaga, L. Valenzano, Chem. Phys. Lett. 660 (2016) 313.
[4] G.D. Degaga, L. Valenzano, Chem. Phys. Lett. (2017) in press.