(591e) Supported Catalytically Active Liquid Metals Solutions (SCALMS) As Catalysts for Ethane Dehydrogenation

The increase in global ethylene demand combined with the availability of cheap shale gas in the U.S is attracting renewed research interest in catalytic ethane dehydrogenation to ethylene (EDH). Currently, in the U.S, ethylene is produced via steam cracking of ethane, where ethane is mixed with steam and heated to ~ 800 where it dehydrogenates to ethylene in a non-catalytic gas phase reaction. Although this process is widely used commercially, it suffers from multiple issues, including heavy coking (i.e. carbon deposition on the reactor walls) and high CO₂ emissions due to the required high reaction temperature. Use of a catalyst could help to improve selectivity toward ethylene and lower the reaction temperature, hence reducing the emissions associated with heating. However, all catalysts studied to-date for this reaction suffer from rapid deactivation due to coke deposition and often sinter at the high-temperature operation. There is hence a need for designing an active and stable catalyst that can endure high-temperature operation and coke formation.

Liquid metals, i.e. metals with a low melting point, have recently emerged as an interesting novel reaction medium. Currently, liquid metal catalysts can be categorized into two systems: first, liquid metal bubble column reactors (LMBR) that rely on bubbling gaseous reactants through a column of liquid metals. Second, supported catalytically active liquid metal solutions (SCALMS), which mimic conventional supported metal catalysts, i.e. which use conventional solid supports like alumina or silica to support small droplets of the liquid metal. LMBR system combine high heat capacity with excellent resistance to coking by separating the formed coke from the catalytically active molten metal due to density differences. While these properties make LMBR an attractive, robust reaction medium, LMBR require the use of large amounts of molten metals and are challenged by the lack of prior experience with molten metal reactors. Furthermore, LMBR are difficult to characterize using common catalyst characterization techniques since liquid metals are opaque to most regions of the electromagnetic spectrum that are used in conventional spectroscopic techniques.

In contrast, SCALMS offer an opportunity to use and study liquid metals under more easily accessible conditions as they bridge between conventional heterogeneous catalysts and LM systems. They hence enable integrating liquid metals into a conventional heterogeneous catalysis infrastructure, opening a window towards developing a fundamental understanding of how liquid metal catalyst interact with gaseous reactants using conventional solid catalysts characterization techniques. Furthermore, SCALMS provide more efficient use of metals by maximizing the available interfacial area. Most significantly, a few prior reports on the use of SCALMS, focused on Ga-based catalyst systems, reported unexpected resistance to coking in propane and butane dehydrogenation, but did not identify the mechanism by which SCALMS resist deactivation.

In the present work, we are exploring Bi-based SCALMS for ethane dehydrogenation in order to verify the presence of this unusual coking resistance and explore its origins. A particular focus of the studies is the synthesis of SCALMS with good control of particle size and composition in order to obtain the well-defined catalyst systems needed to understand the fundamental mechanism in which SCALMS maintain their stability in a coke forming environment and to extend this understanding towards designing a suitable catalyst for ethane dehydrogenation. Bismuth is used as solvent metal for the catalytically active metal, platinum, and the resulting SCALMS (PtBi_x/Al₂O₃ and PtBi_x /SiC) were evaluated against Pt/Al₂O₃ as an equivalent conventional heterogeneous catalyst at identical reaction conditions (600, GHSV: 397 hr^-1, ambient pressure). In agreement with literature, EDH over Pt/Al₂O₃ shows high initial conversion although at very high selectivity toward coke, followed by rapid deactivation. In contrast, PtBi_x/Al₂O₃ and PtBi_x /SiC show lower, but stable, conversion and stable selectivity for >10h time-on-stream. No deactivation is observed despite the fact that the cumulative amount of carbon produced on these SCALMS is comparable to that on the conventional Pt/Al₂O₃. Our results thus confirm that alloying a small amount of platinum in bismuth results in SCALMS that are both active for ethane dehydrogenation and resilient to catalytic deactivation via coking. Catalyst synthesis via various synthesis approaches (wetness impregnation, colloidal synthesis, and galvanic displacement), catalyst characterization, as well as results for catalytic performance and deactivation studies will be presented and discussed in detail in the presentation.