Hae-Gu Park

Enhanced catalytic activity of cobalt catalysts for Fischer–Tropsch synthesis via carburization and hydrogenation and its application to regeneration

In Fischer–Tropsch synthesis (FTS), cobalt carbide (Co2C) is not a catalytically active material, but rather an undesired cobalt phase associated with low catalytic performance. It is known that Co2C can be easily transformed back to metal cobalt in a H2 environment at 220 °C. The transformed metal cobalt (hcp phase) even shows higher catalytic activity in low-temperature FTS, compared with the reduced cobalt metal from the cobalt oxide species. In this study, to obtain Co2C with high catalytic activity in FTS, we determined the optimum conditions for effective metal cobalt carburization and Co2C hydrogenation by monitoring the phase transformation of cobalt using X-ray absorption spectroscopy (XAS) and temperature-programmed hydrogenation (TPH). We also verified that the transitions effectively occur under the same conditions as those for FTS (2.0 MPa, 220 °C). Based on the conditions determined for the transitions, the deactivated cobalt catalyst can be completely regenerated in the FT reactor by simply altering the injected gases from syngas to CO and then H2. Moreover, the regenerated catalyst shows enhanced catalytic performance compared with the fresh catalyst. The selective formation of hcp cobalt metal via carburization and hydrogenation of the spent catalyst was found to be the key for both the improved catalytic activity and the effective regeneration in situ. As a result, the formation of Co2C, which is mainly considered a nuisance, could provide valuable applications in investigations into catalyst activation and regeneration in FTS.

Fischer–Tropsch Synthesis Over Cobalt Supported On Silica‐Incorporated Mesoporous Carbon

The influence of carbon–silica hybrid supports on cobalt catalysts in Fischer–Tropsch synthesishas been investigated. Mesoporous carbon–silica hybrid supports were synthesized through a hydrogen‐bonding‐assisted self‐assembly route by loading different amounts of silica (0 to 45 %) as a framework composition with mesoporous carbon. These carbon and hybrid supports were then impregnated with 15 wt. % cobalt and the resulting catalysts were characterized by various physicochemical, gravimetric, and spectroscopic methods. The dispersion of silica in the framework of mesoporous carbon modified the surface chemistry and enhanced the wetting properties of the support. Hence, the dispersion and reducibility of cobalt over the hybrid supports increased compared to those of the carbon‐supported catalyst. However, above the optimum silica incorporation of 34 %, the possibility of phase separation and structural transformation of silica at the high carbonization temperature decreased the dispersion and reducibility of the supported Co catalysts. The carbon‐supported catalyst showed a quick drop in the activity during the initial stages of the reaction due to the migration of cobalt particles over the hydrophobic and inert support surface. On the contrary, carbon–silica hybrid supports inhibited the mobility of cobalt particles and produced a higher density of active Co0 sites, leading to a higher initial activity. Carbon–silica‐supported catalysts with a silica content of up to 34 % were found to exhibit excellent steady state Fischer–Tropsch synthesis activity and C5+ selectivity compared to only carbon and carbon–silica‐supported catalyst with >34 % silica incorporation.

Maximum production of methanol in a pilot-scale process

Mathematical models for both bench- and pilot-scale methanol synthesis reactors were developed by estimating the overall heat transfer coefficients due to different heat transfer characteristics, while the effectiveness factor was fixed because the same catalysts were used in both reactors. The overall heat transfer coefficient of a pilot-scale reactor was approximately twice that of a bench-scale reactor, while the estimate from the correlation reported for the heat transfer coefficient was 1.8-times higher, indicating that the values determined in the present study are effective. The model showed that the maximum methanol production rate of approximately 16 tons per day was achievable with peak temperature maintained below 250 °C in the open-loop case. Meanwhile, when the recycle was used to prevent the loss of unreacted gas, peak temperature and production rate decreased due to low CO and CO2 fraction in the recycled stream at the same space velocity as the open-loop operation. Further analysis showed that, since the reaction was in the kinetic regime, the production rate could be maximized up to 18.7 tons per day by increasing the feed flowrate and inlet temperature despite thermodynamically exothermic reaction.