(633f) Fischer-Tropsch Synthesis Over Mo/Zeolite Catalysts

Conventional Fischer-Tropsch (FT) synthesis over Fe- or Co-based catalysts produces linear alkanes/alkenes as the dominant products, and the product distribution follows the Anderson-Schulz-Flory (ASF) law [1,2]. It is a challenge to maximize the product selectivity towards a given hydrocarbon fraction like gasoline or diesel. Composite catalysts combining the conventional FT catalysts with acidic zeolites were investigated to overcome the limitations in both product selectivity and quality through secondary reactions of the linear hydrocarbons on the zeolites to form aromatic compounds and branched paraffins, but encounter problems like catalyst deactivation and loss in CO conversion[3]. Another possible way to modify the ASF product distribution is to change the mechanism and kinetics of the chain-growing process. Incorporation of a FT active component into the zeolite cage/channel to limit the FT synthesis inside the micro-space of the zeolite may effectively modify the FT product distribution.

Our recent investigation revealed that Mo/HZSM-5 is an active catalyst for FT synthesis with a low H₂/CO (molar ratio ~1.0) syngas. Branched alkanes and aromatics are the dominant products over the Mo/HZSM-5. The FT reaction is supposed to proceed mainly inside the zeolite cage because of the very low formation of long-chain aliphatic hydrocarbons. A drawback of this catalyst is its high formation rate of CO₂ and lower hydrocarbons. This may relate to its high activity for the water-gas shift (WGS) reaction and hydrogenation reactions of the formed hydrocarbon fragments on Mo sites. Possible strategies to inhibit the formation of both CO₂ and lower alkanes may include regulating zeolite acidity, varying the micro-space environment of zeolite cage and modifying the metallic property of Mo species by introducing a third component. To confirm the effectiveness of these strategies in adjusting the FT product distribution on the Mo supported catalysts, several zeolites including Zeolite Y, Zeolite ß and ZSM-5 with different SiO₂/Al₂O₃ ratios were selected as the support for Mo catalysts. The Mo-supported zeolites were prepared by impregnation of the zeolite with an aqueous solution of (NH₄)₆Mo₇O₂₄. Their catalytic performance in FT synthesis has been evaluated using a packed-bed micro-reactor, equipped with on-line analytical capabilities. Preliminary results indicate that the zeolite structure and acidity (SiO₂/Al₂O₃ ratio) greatly affect the formation of liquid hydrocarbons. It also greatly modifies the product distribution. Zeolite Y was found to be a good support for Mo for FT synthesis reaction. High quality gasoline, consisting mainly of branched and cyclized alkanes, was produced with Mo/H-Y catalyst at 573 K, 1000 psi and GHSV of 3000 hr-1. Mo/HZSM-5 with SiO₂/Al₂O₃ ratio around 50 exhibited the best performance among the ZSM-5 zeolites examined. Higher alcohols and carboxylic acids (C1-C6) were produced with Mo/Zeolite catalysts, and may then be further converted to hydrocarbon products through dehydration and decarboxylation reactions.

REFERENCES

[1] Iglesia, E., Reyes, S.C., Madon, R.J., Soled, S.L., Adv. Catal., 39, 221-302, (1993).

[2] Gaubea, J., Klein, H.F., J. Mol. Catal. A: Chemical, 283, 60-68, (2008).

[3] Mertinez, A., Lopez, C., Appl. Catal. A: Gen., 294, 251-259, (2005).