(22f) 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

Introduction

Fischer-Tropsch synthesis
(FTS) is proposed as an important step during the conversion of biomass into
hydrocarbons and oxygenated fuels and chemicals.[1]
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).[2] 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.[3] 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.[4] 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 590^oC for 4 hours, as shown in x-ray photoelectron spectroscopy experiments.[5] As the metal
is the active Co phase for FTS[6],
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: 270-300^oC, 25 bar, and H₂/CO ratio of 2.
Selectivities were measured at 290^oC. Error bars on rates correspond
to 95% confidence interval.

Summary

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.