Optimization of carbon nanotube growth via response surface methodology for Fischer-Tropsch synthesis over Fe/CNT catalyst

Light olefins such as ethylene, propylene, and butylene are key compounds in the chemical industry. Iron catalyst supported on carbon nanotubes (CNTs) is an appropriate candidate for light olefins production using synthesis gas in Fischer-Tropsch synthesis (FTS). First, catalytic chemical vapor deposition (CCVD) method was applied to synthesize CNTs using Fe/CaCO3 and acetylene as catalyst and hydrocarbon source, respectively. Applying response surface methodology, the optimum operating conditions were determined in CVD reactor for maximal yield and purity of CNTs. The effects of reaction time (30–60 min), reaction temperature (700–800 °C), and loading of the catalyst (10–30 wt% Fe) were investigated. The synthesized CNTs was characterized by Raman spectroscopy for its graphitic structure and purity. After acid-treatment of the synthesized CNTs, Fe/CNTs-synthesized catalyst was successfully fabricated by ultrasonic-assisted wet impregnation method. 20Fe/CNTs-synthesized, 20Fe/CNTs-commercial, and 20Fe/Al2O3 were analyzed in terms of physio-chemical properties and FTS catalytic performance. Comparison of transmission electron microscopy (TEM) and CO-chemisorption results showed that 20Fe/CNTs-synthesized reduces iron oxide agglomeration resulting in metal particles well-dispersed among the studied Fe-based catalysts. The catalytic performance of Fe-based catalysts was investigated using a fixed-bed reactor at 280 °C under 290 psi. 20Fe/CNTs-synthesized exhibited a lower rate of water-gas-shift (WGS) reaction compared with 20Fe/CNTs-commercial, with C2-C4 selectivity of 23.6% which is slightly less than that of its commercial counterpart. Analysis of FTS liquid products revealed that 20Fe/CNTs-synthesized provides liquid hydrocarbons in the range of C8-C18, while 20Fe/CNTs-commercial and 20Fe/Al2O3 obtained C9-C52 and C10-C20, respectively. After 120 h time on stream under steady state condition, higher activity had been maintained by the 20Fe/CNTs-synthesized catalyst compared to the 20Fe/CNTs-Commercial and 20Fe/Al2O3 catalysts.