Essay Example on Fischer Tropsch synthesis









1 Introduction Fischer Tropsch synthesis FTS is a catalytic process that produces a broad distribution of clean liquid fuels and chemicals from syngas obtained from natural gas coal and biomass 1 2 Cobalt based catalysts are preferred in FTS processes due to their high activity high selectivity to linear paraffins and low water gas shift activity However the challenge with cobalt based FT catalysts is that they deactivate over time 3 8 To make the FTS process economically effective a stable performance of the catalyst is required Therefore studying the catalyst deactivation is an important topic to develop better industrial catalysts The general reasons for catalyst deactivation are poisoning re oxidation deposition of carbonaceous and oxygenated compounds and sintering of active phase 3 4 The poisoning of cobalt can be avoided by purifying the feed gas properly before the reaction Oxidation of Co based catalysts has been studied and ruled out as deactivation mechanisms 9 10 Sintering is the loss of active metal surface area due to the crystalline growth of metal particles and takes place via Ostwald ripening and or coalescence 3 11 Kistamurty et al 12 proposed that Ostwald is the most dominating mechanism from transition electron microscopy TEM experiments on a Co SiO2 catalyst 

The deactivation by different carbonaceous species deposition on the catalyst surface has been studied earlier 4 6 Moodley et al 6 identified different types of carbon species atomic carbon residual wax in the pores of the catalyst and polymeric carbon on a spent Co Pt Al2O3 FTS catalyst using temperature programmed hydrogenation mass spectrometry TPH MS From their studies it was concluded that only polymeric carbon contributes to the long term deactivation Peña et al 13 proposed that strongly adsorbed hydrocarbons and amorphous polymeric carbon seem to be contributing to the catalyst deactivation Also Tan et al 14 found two types of resilient carbon species surface carbide and poly aromatic carbon after wax extraction when they studied the 20 wt Co γ Al2O3 FT catalyst in a fixed bed micro reactor FTS produces not only hydrocarbons but also small quantities of oxygenated compounds such as alcohols carboxylic acids ketones and esters 15 Kollar et al 16 reported the formation of several oxygenated compounds on the surface of 15 Co 0 5 Pt γ Al2O3 catalyst Paredes Nunez et al 17 18 found the presence of two types of formate species on the surface of the catalyst fast reacting and slow reacting formate species using infrared spectroscopy on a Co Siralox catalyst Jalama et al 19 investigated the effect of ethanol addition during FTS using a Co TiO2 catalyst They found that the ethanol addition increased the selectivity to light products increased the olefin to paraffin ratio and decreased the catalyst activity

These effects were reversible when ethanol was removed from the feed This is believed to be related to oxidation of Co to CoO by ethanol and re reduction to Co when ethanol was removed from the feed to recover its initial activity Scalbert et al 20 found deposition of oxygenated compounds most likely carboxylic acids and aldehydes and an increase in their amount with time on a Co Alumina FT catalyst using XRD DRIFT spectroscopy They proposed that these strongly adsorbed species are responsible for catalyst deactivation by covering the active sites Pinard et al 21 analysed the different types of carbon species present on the spent Co Ru Al2O3 FT catalyst Carbon extracted from the spent catalyst was found to be consisting of atomic carbon alcohols carboxylic acids and polymeric carbon Temperature programmed hydrogenation infrared TPH IR indicated that complete removal of carboxylate species required temperatures above 600 oC They found that only carboxylic acids and polymeric carbon were resistant to a rejuvenation treatment under hydrogen It was proposed that these high molecular weight carboxylic acids formed during FTS may also block the cobalt active sites and deactivate the catalyst 21 Fourier Transform infrared FTIR spectroscopy is the most widely used and the most effective spectroscopic technique for characterization of the surface chemistry of heterogeneous catalysts 22 To study the deposition of carboxylates on the catalyst surface effectively it is necessary to employ an in situ infrared reaction cell in which infrared spectra could be obtained while the samples are subjected to a wide range of temperatures and pressures 

These experiments not only make it possible to study the adsorbed molecules while FTS reaction is in progress but also open ways to understand the catalyst deactivation mechanisms and to develop better industrial catalysts The effect of pressure and temperature on the performance of the catalyst has been studied extensively 23 27 However the formation of oxygenated compounds on the catalyst surface and their role in catalyst deactivation during FTS at different operational conditions is not explored very well Therefore the aim of this study is to investigate the effect of pressure and temperature on the formation of oxygenated compounds on the surface of the Co TiO2 catalyst during FTS using diffuse reflectance infrared Fourier Transform DRIFT spectroscopy

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