Perspectives on the effect of sulfur on the hydrocarbonaceous overlayer on iron Fischer-Tropsch catalysts

Abstract

Fischer-Tropsch synthesis (FTS) is commonly viewed as an alternative approach to the production of diesel fuels via sources independent of crude oil. The adaptability of the FTS process allows for the selective production of shorter chain C2 to C6 hydrocarbons and has the potential to be a legitimate source of useable chemical feedstocks with high value to the chemical manufacturing industry. Interestingly, although recognised as a poison in most catalytic systems, small amounts of sulfur in iron-based FTS catalysts has been demonstrated to promote catalyst reducibility and activity towards shorter chain hydrocarbons. However, it is not known what impact sulfur has on the formation of hydrocarbonaceous surface species that have been proposed to play a pivotal role in the mediation of reactants during iron FTS. Here we apply ambient pressure CO hydrogenation at 623 K on a selection of sulfur promoted iron FTS catalysts to investigate the effect of sulfur content on hydrocarbonaceous species formation. For the first time, we report the application of inelastic neutron scattering to quantify the presence of hydrocarbonaceous species under the presence of sulfur promotion. In combination with temperature programmed oxidation, X-ray diffraction, and Raman spectroscopy, we observe how low sulfur loadings (<700 ppm) perturb carbon and hydrogen retention levels. The results indicate that the presence and nature of the hydrocarbonaceous overlayer is sensitive to sulfur loading, with the reported loss in catalytic activity at high loadings correlating with the attenuation of hydrocarbonaceous surface species.

Carbon Pathways, Sodium‐Sulphur Promotion and Identification of Iron Carbides in Iron‐based Fischer‐Tropsch Synthesis

“…[84] This seems to be in contrast to the Fe-based, alkalicontaining catalyst materials for which the CH 4 selectivity increases with excessive addition of S. [22,83] In terms of the various C pathways operative, the effect of the added S might be simply to decrease the overall CO and H 2 dissociation, without altering the relation between the C pathway selectivities. Although the overall carbon deposition decreases with added S, [45,124,126,127] so does the CO conversion. [84] That is, whether S truly suppresses carbon deposition without decreasing the FTS reaction hydrocarbon production, as is sometimes discussed, needs to be compared in relative terms to the CO converted to FTS reaction hydrocarbon products.…”

An Inelastic Neutron Scattering Investigation of the Temporal Behaviour of the Hydrocarbonaceous Overlayer of a Prototype Fischer-Tropsch to Olefins Catalyst

“…The sample exposed to 3 h on stream displays two prominent bands at 591 and 941 cm −1 that are respectively assigned to (i) the A 1g phonon of Fe 3 O 4 and (ii) a combination band from alkenic δ(C-H) and a magnetic interaction associated with Fe 3 O 4 [16][17][18][19][20][21][22][23]30]. Thus, they are indicative of magnetite (Fe 3 O 4 ) and signify a degree of under reduction of the catalyst.…”

“…Spectra were recorded at 20 K at incident neutron energies of 650 meV and 250 meV using the A-chopper package [28]. Quantification of the ν(C-H) feature obtained by INS was achieved following a calibration protocol described elsewhere [17][18][19][20][21][22][23]29].…”

“…A body of work from the authors has used the technique of inelastic neutron scattering (INS) to investigate a range of iron-based FTS catalysts, with particular attention being paid to the catalyst conditioning phase [16][17][18][19][20][21][22][23]. The application of INS is beneficial, as it provides a means of obtaining the vibrational spectrum (50-4000 cm −1 in favourable circumstances) of the catalyst after a range of process operations [24,25].…”

“…These INS based investigations of FTS surface chemistry have been broadened to include an examination of a range of FTO catalyst formulations. In the first instance, this involved solely sulfur promoted hematite catalysts [22] but, recently, a range of doubly promoted Na and S modified hematite catalysts have been examined [23]. Following favourable FTO product profiles reported by Botes et al [2], a conventional hematite catalyst was promoted with ∼2000 ppm of sodium, whilst the sulfur promotion was examined over the range 0-250 ppm [23].…”