284701 Potassium Promoted Mo2 C Supported Catalysts for Fischer-Tropsch Synthesis
Potassium Promoted Mo2C Supported Catalysts for Fischer-Tropsch Synthesis
Richard C. Ezike* and Levi T. Thompson*
*Department of Chemical Engineering and Hydrogen Energy Technology Laboratory,
University of Michigan, Ann Arbor, MI 48109
Fischer-Tropsch synthesis (FTS) is proposed as an important step during the conversion of biomass into hydrocarbons and oxygenated fuels and chemicals. Side reactions can include the water gas shift reaction. The reaction is typically carried out at pressures between 10-60 bar and in two temperature regimes: high temperature FTS (300-350°C) and low temperature FTS (200-250°C). Molybdenum carbides (Mo2C) have been reported to be active for FTS as well as a number of reactions including hydrodenitrogenation, hydrodesulfurization, and hydrocarbon isomerization. During FTS, Mo2C catalysts typically yield light hydrocarbons (C1-C4) at atmospheric pressure. However, higher hydrocarbons with chain lengths in the range of those for gasoline (C7-C11), diesel fuel (C10-C19), and waxes (C20+) are more desirable products. Adding metals that are known to be active for FTS such as Co, Ni, Fe, and Ru onto Mo2C could improve their selectivities to higher hydrocarbons. In addition, promoters such as potassium could help to modify the selectivity away from hydrocarbons and toward alcohols. The goal of work described in this paper was to investigate the effect of adding late transition metals including Pt and Co as well as potassium onto Mo2C on rates and selectivities for FTS.
Materials and Methods
The Mo2C catalysts were synthesized using a temperature programmed reaction procedure. Approximately 1.3 g of ammonia paramolybdate (AM) was loaded into a quartz tube reactor on top of a quartz wool plug and placed in a vertical furnace. The AM was sieved to 125-250 μm prior to carburization. The AM was reduced in H2 at 400 mL/min as the temperature was increased from room temperature (RT) to 350 °C at a rate of 278°C/h, and then held at this temperature for 12 h. The reactant gas was then switched from H2 to a 15% CH4/H2 mixture and the temperature was increased from 350 to 590 °C at a rate of 160 °C/h. The final temperature was maintained for 2 h prior to quenching the material to RT. The resulting material was passivated using a 1% O2/He mixture at 20 mL/min for at least 5 h. The Mo2C-supported metal catalysts were prepared via wet impregnation of the unpassivated Mo2C with a deaerated aqueous solution containing chloroplatinic acid or cobalt nitrate hexahydrate. After decanting the excess solution, the material was loaded into a quartz reactor and dried in H2 at 400 mL/min for 3 h at RT. Subsequently, the temperature was increased to 110 °C in ≈1 h and held there for 2 h. The temperature was then increased to 450 °C at a rate of 340 °C/h and held for 4 h. Finally, the material was quenched to room temperature and passivated in a 1% O2/He mixture at 20 mL/min for at least 5 h. Potassium as K2CO3 was added concurrently with the metal (termed concurrent, or CC) or sequentially (termed sequential, or SEQ) following deposition of the metal.
Results and Discussion
Figure 1 compares the product formation rates and selectivities for the tested catalysts. The addition of Pt to the Mo2C did not affect the overall rate while the addition of Co reduced the overall rate compared with Mo2C and Pt/Mo2C. This behavior in the Co/Mo2C may be due to the incomplete reduction of Co after treatment in 15% CH4/H2 at 590oC for 4 hours, as shown in x-ray photoelectron spectroscopy experiments. As the metal is the active Co phase for FTS, the incomplete reduction could cause the lower activities. The rates did not change significantly as potassium was added, regardless of the order of addition.
With regards to the selectivities, CO2 was the primary product. Without potassium, the selectivities to alcohols were very low (<5%). The addition of potassium significantly increased the alcohol selectivity, and the sequential method resulted in higher alcohol selectivities than the concurrent method for both the Mo2C-supported Pt and Co catalysts.
Figure 1: Rates and Selectivities for FTS catalysts. Reaction Conditions: 270-300oC, 25 bar, and H2/CO ratio of 2. Selectivities were measured at 290oC. Error bars on rates correspond to 95% confidence interval.
The FTS rates were similar for the Mo2C and Pt/Mo2C catalysts, while the Co/Mo2C catalyst was less active. The addition of K did not affect the rates. In terms of overall rate, the Mo2C and Pt/Mo2C catalysts performed similarly, with the Co/Mo2C catalysts was less active. When K was added, the alcohol selectivity increased. On both the Pt and Co/Mo2C, the addition of K using the sequential method resulted in improved production of alcohols compared with concurrent loading.
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