Fischer-Tropsch synthesis on carbon nanotube-supported catalysts in water-in-oil emulsion
Abstract
Fischer-Tropsch synthesis (FTS) is an important gas-to-liquids technology in the modern energy industry. The current trend is to carry it out in liquid reaction media in order to take advantage of good heat transfer, high catalyst efficiency, convenience for catalyst regeneration/reloading and lower costs, due to either the use of liquid media or the associated reactor designs. Adding water vapor to FTS has been proven to promote the reaction in many catalyst systems, but there was no attempt prior to this point to add liquid water in organic media. The choice of catalyst supports would be critical from multiple perspectives. First of all, the co-existence of both water and oil necessitates a support that could maximize the liquid interface for better mass transfer as well as dispersion of the catalyst. The product-support interaction is another important issue, especially for FTS product in liquid phase where secondary reaction happens when primary products re-adsorb onto catalysts. And finally, as with any catalyst, the metal-support interaction greatly affects the catalyst particle sizes and reactivity.
We chose Ru catalyst supported on a multi-walled carbon nanotube/MgO-Al2O3 hybrid as the catalyst support for biphasic FTS, and carried out systematic studies that rationalized the choice of the support in terms of catalyst activity, deactivation, production selectivity as well as product separation. As it turned out, the amphiphilic properties of the nanohybrid-supported catalyst and the thus formed emulsion system were among the essential factors in the successful implement of the concept. A comprehensive interpretation of the results and a comparative investigation with some other catalyst supports have revealed the importance of choosing the right catalyst support, as well as given a better understanding of the FTS mechanism that incorporates recent progress reported in literature.
We then went on to study the synthesis of carbon nanotubes (CNTs) in the effort to find optimal conditions for their production. In addition, sharing similarities in mechanistic pathways in a broad sense, but with much heavier molecular weight, CNTs have stronger interaction with catalyst supports than FTS products and were therefore a more typical model to study and demonstrate the product-support interaction as well. On the planar substrate support, depending on the isotropy/anisotropy of the CNT-support interaction, either random networks or aligned arrays of CNT films were synthesized on the corresponding support. Here another important continuation from the study of FTS was the promoting effects of water. We’ve concluded that water facilitates CNT growth in the second step, i.e., the step where monomeric C units condense into CNTs, while in the case of FTS, water plays a role mostly in the first step of CO dissociating into the monomeric CH species. Finally, a novel method to fabricate devices of the CNT films were presented that tested the merits of using planar supports for CNT growth in actual applications.
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