Synthesis of hydrocarbon via Fischer-Tropsch reaction on tungsten carbide catalyst

Student: Thanh Vinh Nguyen – Supervisor: Prof. Adesoji A. Adesina
School of Chemical Engineering and Industrial Chemistry

Introduction:

Fischer-Tropsch reactions have been recognized for years as a prospective method to produce gasoline-range hydrocarbons from syngas. Noble metals have been used as catalysts in Fischer-Tropsch reactions. However, catalyst cost is a major component of the industrial operating expenses. The electronic properties of early transition metal carbides (tungsten and molybdenum) are similar to those of more expensive Pt-group metals. Tungsten carbide on different oxide supports has therefore been examined in this work as a low cost alternative and in particular, has been prepared by a new method to improve its surface area, physicochemical properties as well as catalyst activities to enhance the development of new gas-to-liquid fuels technology.

Tungsten carbide conventionally prepared by the reaction between tungsten trioxide and methane is a low surface area product. The method applied in this project based on the carburization of tungsten sulphide prepared via precipitation from homogeneous solution, yielding nano-sized tungsten carbide particles (less than 75 nm). The catalysts were prepared on supports: Alumina, Silica, Titania, Zirconia, but only silica supported catalyst is presented in this manuscript due to space constraints. Thermogravimetry and other advanced analytical tools such as FTIR, SEM, TEM, TOC have been used to study kinetics of the reaction and properties of tungsten carbide product. Catalyst activity was obtained from an 8-port combinatorial reactor fitted with a computer-controlled GC station.

Experiment:

1. Method:
Calculated quantities of tungstic acid H₂WO₄ was dissolved in HNO₃, thioacetamide (CH₃CSNH₂), urea (CO(NH₂)₂) and distilled de-ionised water containing weighted support (SiO₂) to yield 12W:88Silica by weight. The mixture was kept in a 363K shaker-bath for 4 hours. The precipitation was then filtered and dried for 24 hours at 393K. The solid was subsequently ground to uniform powder and reacted with mixture of H₂ and C₃H₈ in a combinatorial reactor to obtain tungsten carbide on support silica. Mixtures of CO and H₂ were passed through the catalyst at temperatures 500 to 550 K to perform Fischer-Tropsch reaction.

2. Results and Discussion:
Kinetics of the reaction was investigated under both isothermal and variable heating rate conditions. The relationships between heating rate, temperature and rate of weight change is depicted below:

Excessive carbon deposition on the catalyst surface due to propane cracking at high temperature was an important variable to overcome. TOC (total organic carbon) analyses have shown that at temperatures below 873 K and heating rate 5 Kmin^-1, the direct carburization of the metal sulphide rather than carbon deposition was dominant.

FTIR spectra of products at different heating rate and isothermal temperatures were studied to find the best condition for tungsten carbide to be formed on the silica. The peak at 1110 cm^-1 is characteristic for Si-O bonding. The height of peak is small, suggesting that tungsten carbide is well dispersed on the silica. As can be seen, low heating rate between 2-5 Kmin^-1 appears to be preferable for the carburization process.

SEM and X-ray elemental mapping also confirmed the uniform distribution of fine tungsten carbide particles on the silica support.

SEM Silica		Atomic silicon map
Atomic oxygen map		Atomic tungsten map

Current results suggest that the pretreatment/activation of catalyst is an important rate determining step of the Fischer-Tropsch reaction. Treatment with syngas at 773 K for 10 hours gave promising results. A typical gas chromatograph spectra of the FT reaction is illustrated below, showing a wide range of hydrocarbons produced during FT reaction.

Acknowledgement:

The author is grateful to Faculty of Engineering, UNSW for financial support.

Publications:

The following publications are derived during the Taste of Research Summer Scholarship tenure:

• Tuan Huy Nguyen, Thanh Vinh Nguyen, Yong Joon Lee, Tomasz Safinski, Adesoji A. Adesina. (2005) Structural evolution of alumina supported Mo-W carbide nano-particles synthesized by precipitation from homogeneous solution, Materials Research Bulletin, 40(1), 149-157.
• Tuan Huy Nguyen, Thanh Vinh Nguyen, Adesoji A. Adesina, Carbothermal synthesis of nano-sized tungsten carbide catalyst (MS NO. NMS19): Proceedings at The 4th ASIA-PACIFIC Chemical Reaction Engineering Symposium, Korea, June 2005.

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