Catalytic conversion of ethane to valuable products through non-oxidative dehydrogenation and dehydroaromatization

Abstract

Chemical utilization of ethane to produce valuable chemicals has become especially attractive since the expanded utilization of shale gas in the United States and associated petroleum gas in the Middle East. Catalytic conversion to ethylene and aromatic hydrocarbons through non-oxidative dehydrogenation and dehydroaromatization of ethane (EDH and EDA) are potentially beneficial technologies because of their high selectivity to products. The former represents an attractive alternative to conventional thermal cracking of ethane. The latter can produce valuable aromatic hydrocarbons from a cheap feedstock. Nevertheless, further progress in catalytic science and technology is indispensable to implement these processes beneficially. This review summarizes progress that has been achieved with non-oxidative EDH and EDA in terms of the nature of active sites and reaction mechanisms. Briefly, platinum-, chromium- and gallium-based catalysts have been introduced mainly for EDH, including effects of carbon dioxide co-feeding. Efforts to use EDA have emphasized zinc-modified MFI zeolite catalysts. Finally, some avenues for development of catalytic science and technology for ethane conversion are summarized.

Fig. 1. History and projection of global demand for ethylene, benzene, toluene, and xylenes. Data originated from ref. 2.

Fig. 2. Temperature dependence of equilibrium conversion of EDH at various ethane pressures.

Fig. 3. Schematic image of reactivity of ethane on Pt particle. EDH proceeds on the Pt terrace sites, whereas coke formation and hydrogenolysis of ethane proceed on the “cus-Pt” sites circled in red.

Fig. 4. Typical geometric effects of Sn addition to Pt: (a) alloy formation and (b) surface modification. Face centered cubic and hexagonal close packing are denoted respectively as fcc and hcp.

Fig. 5. Schematic images of (a) re-dispersion of Pt–Sn alloy through oxidation and subsequent reduction (regeneration) and (b) segregation of Sn from Pt–Sn alloy during oxidation.

Fig. 6. Proposed effects of the promoters on the electronic state of Pt: (a) electron donation from the promoter (M) to Pt and (b) modification of the energy level of Pt 5d bands. Densities of states are denoted as DOS.

Fig. 7. Schematic image of the drain-off mechanism. Coke precursors such as ethylidyne move to the supports on which coke formation proceeds.

Fig. 8. Structure of (a) monochromate and (b) polychromate supported on a metal oxide.

Fig. 9. Schematic adsorption of ethane on reduced monochromate supported on a metal oxide: formation of (a) ethyl group and (b) ethoxy group.

Fig. 10. Structure of aluminum–chromium–chromate.

Scheme 1. Redox mechanism for EDH in the presence of CO₂ over Cr oxide catalysts.

Scheme 2. Mechanism of ethane activation via (a) the alkyl mechanism, (b) the carbenium mechanism, and (c) the concerted mechanism. Ga⁺ is a representative active site of Ga/H-ZSM-5.

Scheme 3. Reaction mechanism of EDH at the Ga⁺ site.

Scheme 4. Reaction mechanism of EDH in the presence of steam on a Ga oxide cluster.

Fig. 11. Ethylene selectivity vs. ethane conversion of each catalyst presented in Tables 1 and 2. Dashed lines represent ethylene yield. Also, EDH in the absence and presence of CO₂ are shown respectively with red circles and blue squares.

Fig. 12. Schematic images of (a) electrochemical EDH by a solid oxide electrolysis cell and (b) photocatalytic EDH over a particulate semiconductor with a cocatalyst.

Scheme 5. Structural change from [Zn(OH)]⁺ to isolated Zn²⁺ after the water gas shift reaction.

Scheme 6. Reaction pathway of EDA.

Scheme 7. Aromatization of oligomers over H-ZSM-5.

Scheme 8. Possible reaction pathway of EDA over Mo/H-ZSM-5.

Fig. 13. Twelve T-sites of MFI topology.

Hikaru Saito

Yasushi Sekine