Refinement of Olefins via the Addition of Ethylene to a Periodic Two-Dimensional Nickel-Based Material
Authors: Moncho Salvador 
Published: July 2019

Obtaining valuable materials from petrochemical resources is a critical area for Qatar industry. The separation and purification of light olefins from oil and natural gas is an essential but costly step in this process. The high energetic cost of cryogenic distillation, the current “state-of-the-art” separation technique, promotes the search for new technologies with alternative methods to extract olefins by, for example, the use of transition metal materials to reversibly complex olefins [1]. Few copper and silver compounds are known to improve the energetic efficiency, but are unusable by practical challenges, such as poisoning and deactivation [2]. Our group is interested in nickel bis(dithiolene) and its derivatives due to their ability to coordinate and release alkenes in a controlled, reversible, and selective way [3]. We have explored the use of nickel dithiolenes [4] and related molecular complexes [5] for olefin purification. However, homogeneous catalysts have practical issues that can be managed by developing related heterogeneous catalysts, with major industrial advantages associated to handling the catalyst, the separation process and the use of solvents [6]. A possibility would be the use of the active metal center (NiS4) as a motif in a non-soluble polymeric material. Kambe and coworkers recently synthesized a two-dimensional sheet with the appropriate metal centers [7] Among other studies of this new π-conjugated material [8], its ability to coordinate ethylene was explored by density functional theory (DFT) means [9]. In this presentation, we extend our work in molecular complexes to this analogous two-dimensional extended system using periodic boundary conditions (PBC) [10]. We use the screened-hybrid density functional HSE06 to include the exact exchange in periodic calculations, avoiding the known over-stabilization of barriers from pure DFT methods [11, 12]. PBC values were complemented with calculations on a series of cluster models. We analyzed two different cluster sizes: a small cluster formed by 5 nickel atoms and a large cluster containing 12 nickel atoms, mimicking one of the macro-hexagonal holes in the sheet. In addition to the cluster and periodic models for the extended material, we also included a molecular analog with CN groups. All methods provide a similar picture of the system, with the same preferred pathway. Our study includes binding motifs based on the molecular system but unnoticed in previous 2D extended studies (such as the coordination along the sulfur-nickel bond). Also, we include additional products that are only available in the periodic system, such as coordination between multiple nickel atoms. Pathways to form those coordination products were calculated both directly and through intermediates (such as a distorted geometry or coordination including Ni atom). Our calculations suggest that the interligand adduct is both the kinetic and thermodynamic product, formed through sulfur-nickel intermediate. These findings are different from those described by Zhou, because our study is more complete, and includes the nickel-sulfur coordination pathway. On the other hand, the nickel-mediated mechanism prediction agrees with the results for molecular catalysts. Evaluation of higher alkene coverages on the surface were used to estimate the molecular efficiency of the material. The attachment of a single ethylene by each unit cell (a reaction ratio of 1:3, Et:Ni) has been expanded with the simulation of different levels of coverage, including the efficiency of the molecular reaction (ratio 1:1) and improved efficiency (like ratio 2:1). The fast-growing number of combinations were simplified by using only the most stable coordination motif. Our results suggest that coordination is favored up to one molecule per Ni atom. Compared with the molecular complex, the 2D material shows similar reaction barriers and energies, as well as the same molecular efficiency. On the other side, weight efficiency is improved and the extended nature of the material eases practical issues. In conclusion, we believe that this material is a very promising potential catalyst for olefin purification.

Acknowledgements

This publication was made possible by NPRP grant No. 05-318-1-063 from the Qatar National Research Fund (a member of Qatar Foundation).

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