Application of Natural Zeolites As Support in Low-Temperature Fischer-Tropsch Synthesis

In the Fischer-Tropsch (FT) synthesis, CO and H2 (synthesis gas) are converted into plethora of hydrocarbons mainly paraffins and olefins and these can be further upgraded to high-quality fuels and chemicals. Different carbon sources such as natural gas, coal and biomass can be used as feed-stocks for the synthesis gas. In commercial applications, supported iron catalysts are commonly used in the Fischer-Tropsch synthesis, especially when the synthesis gas emanates from biomass where the CO/H2 needs adjustments via the WGS reaction and when the desired final products are mainly olefins. The activity and selectivity of an iron catalyst is dependent on several parameters, one of them being the support.
Zeolites and zeolite rocks are commonly used in different industrial applications. Natural zeolites present an attractive material as supports in FTS because of their high abundance, availability, low costs and their properties. Detailed mineralogical knowledge and profound characterization of natural zeolites are essential for fitting chemical composition to use. Si/Al ratios are very import as well as the other contaminates. A fundamental difference exists between commercial supports such as silica and alumina - with functional porous materials - and natural supports such as zeolites. In this study natural zeolite called clinoptilolite (a type of zeolite found in South Africa) was used as a catalyst support. This support proved to be promising for low-temperature Fischer–Tropsch synthesis (LTFTS) targeting liquid fuel production, as well as chemical feedstock. Synthesis of this highly active catalyst was by loading of iron on clinoptilolite through the wet impregnation method. The prepared catalyst was then characterized by XRF, BET surface area analyzer, XRD and SEM. The catalyst was then loaded into the reactor and reduced with hydrogen prior to FTS. The effects of its use as support in FTS were investigated in a fixed bed reactor.
From the XRF results the molecular ratio SiO2/ Al2O3 of the Clino-support was 5.86. The average crystal size of the particles from both HRTEM and XRD ranged 9.83 -11.649 nm and around 10.10 nm for used and fresh catalyst. It was found that the CO consumption rate of 1.02 x 10-4 mol/min.gcat of which 7.24 x 10-5 mol/min.gcat was the actual Fischer Tropsch rate with the remaining 2.93 x 10-5 mol/min.gcat consumed by the WGS reaction. The product distribution of the gaseous phase analysed were more olefinic than paraffinic. The product distribution for this condition follows a one alpha ASF distribution with an alpha value of 0.86. These findings may permit the development of new effective support materials, which are cost effective for clean fuel production via FTS process.