Fischer–Tropsch Synthesis: Effect of Water Over Iron-Based Catalysts

Abstract

The effect of water on the performance of potassium-promoted precipitated iron catalyst was investigated during Fischer-Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR) at two different reaction temperatures. Water was added in such a manner as to replace an equivalent amount of inert gas so that all other reaction conditions (e.g., reactant partial pressure, space velocity) remained the same before, during, and after water addition. The externally added water had a positive effect on CO conversion at 270°C whereas, for the reaction carried out at 230°C, the added water decreased CO conversion and deactivated the catalyst. From these findings, the addition of water at 230°C oxidized the catalyst, transforming the iron carbide to the Fe 3 O 4 phase. When the reaction was carried out at 270°C, severe oxidization did not take place and a carbide phase was retained. The loss in activity and the rate of deactivation were more pronounced at 230°C compared to the 270°C condition for the same catalyst. Mössbauer spectroscopic measurements revealed that for the reaction carried out at 230°C, the catalyst had 85% of the iron present as Fe 3 O 4 and the remaining as Hägg carbide (v-Fe 5 C 2 ), whereas at the higher temperature reaction condition the catalyst had about 66% of the iron present as e 9-Fe 2.2 C, with the remaining as Fe 3 O 4 . These findings were also supported by XANES analysis, where a high white line intensity was observed for the sealed used catalyst sample for the low temperature reaction condition, indicating a higher extent of oxidation. A low white line intensity was recorded for the used sample for the high temperature reaction condition, indicative of a higher extent of reduction.

The Effect of Copper Addition on the Activity and Stability of Iron-Based CO2 Hydrogenation Catalysts

“…Carbon monoxide selectivity drops below the detection limit at the lower temperature, but at a higher temperature there is only a minor increase in CO and methane. The loss in activity at low temperature is consistent with that reported by Pendyala et al [35] due to increased water oxidation of the iron catalyst during the Fischer Tropsch Synthesis (FTS) step.…”

Fischer–Tropsch Synthesis: Deactivation as a Function of Potassium Promoter Loading for Precipitated Iron Catalyst

“…The decrease in CO conversion (Fig. 2) for the unpromoted catalyst can [56] on the effect of water co-feeding to an iron-based FT catalyst (promoted with silica, potassium, and copper) showed that the effect of water was very sensitive to the reaction temperature. At high temperatures (270°C), the co-feeding of 10 % water at a CO conversion level of about 50 % resulted in increased WGS activity, leaving the FT activity virtually constant.…”

The application of synchrotron methods in characterizing iron and cobalt Fischer–Tropsch synthesis catalysts

“…CAER researchers recently investigated [97] the water effect over a K-promoted iron catalyst under a low H 2 /CO syngas ratio (i.e., H 2 /CO = 0.7), typical of coal or biomass-derived syngas -and where internal WGS is important for boosting the H 2 /CO stoichiometry for carrying out FT. At 270 • C, the catalyst displayed a reversible positive increase in CO conversion and the carbide phase of the catalyst was retained. However, at 230 • C the catalyst deactivated sharply with 10% H 2 O addition, and iron oxide was generated.…”

“…By removing catalysts from the reactor with increasing time on stream or simulating a CO hydrogenation environment using CO, H 2 , and H 2 O, ex situ and in situ studies have provided information on changes in oxidation state and coordination environment occurring during aging [63,[82][83][84][85][86][87][88][89][90][91][92][93][94], during water co-feeding [95][96][97], during mildly oxidizing conditions using H 2 O/He addition [98], as a function of conversion (i.e., water partial pressure, since water is a product of FT) [63,70], and as a function of H 2 /CO ratio [72]. The recent coupling of synchrotron Raman spectroscopy with XRD and XAS to investigate both the activation and reaction testing of Fe catalysts is also discussed [94].…”