(457d) Effect of Support Pretreatment on the Performance of Supported Iron Fischer-Tropsch Catalysts

Typical iron FT catalysts are unsupported, precipitated iron promoted with copper, potassium, and silicon oxides. While these unsupported iron catalysts have sometimes displayed high activity and stability [1], nevertheless, they are structurally too weak to be used in slurry bubble-column reactors (SBCRs), the most thermally efficient and economical reactors for FTS [2]. Supported iron catalysts, which have higher attrition resistance, have previously had much lower activity and productivity in comparison to unsupported iron catalysts. To overcome the low activity and attrition resistant problems, we explored the preparation of supported iron catalysts, and in our previous publications have reported the successful preparation of Fe supported on silica-stabilized alumina (AlSi).  This catalyst is the most active supported Fe FT catalyst reported to date and more importantly, it is extremely stable as demonstrated by the fact that its activity continues to increase over 700 h on stream [3, 4]. The key factor to this successful development was the silica stabilizer which enabled us to dehydroxylate the support at high temperature to remove acidic sites. In the current study, we investigate the effect of the calcination temperature of the AlSi on the final properties of the catalyst.

Experimental.  Starting from the same batch of silica-stabilized alumina support (AlSi) containing 5% silica, individual samples were calcined at either 700, 900, 1100, or 1200°C. Four catalysts were then prepared by co-impregnation of each sample of AlSi with aqueous solutions containing appropriate amounts of ferric nitrate, copper nitrate, and potassium bicarbonate in four steps to give catalysts with nominal compositions of 100Fe/7.5Cu/4K/150AlSi with overall iron loadings of 40%. The four catalysts are designated as Fe/700AlSi, Fe/900AlSi, Fe/1100AlSi, and Fe/1200AlSi. The samples were reduced at 320 °C in H₂.

The reduced and carbided samples were characterized by BET, XRD, H₂-TPR, syngas-TPR, and Mossbauer spectroscopy. Fischer-Tropsch synthesis (FTS) was conducted in a fixed-bed reactor (stainless steel, 3/8 inch OD) on samples (0.20 g, 125-177 mm) that were diluted with 2 g silicon carbide to improve the temperature distribution in the catalytic zone. Before FTS, the samples were re-reduced in situ in H₂ at 320°C, then cooled to 180°C. The system was then pressurized to 20 atm in syngas (H₂:CO = 1) and the catalysts were activated at 280°C. Rate data were obtained at reaction temperature of 260°C and H₂:CO = 1.

Results and Discussion.  The rate and productivity data for the four catalysts are reported in Table 1. There is a clear trend of increasing rate and productivity as the calcination temperature of the AlSi support increases. The rates for Fe/700AlSi and Fe/900AlSi are significantly lower than the rates of Fe/1100AlSi and Fe/1200AlSi, while Fe/1200AlSi has a slightly higher rate and productivity than Fe/1100 AlSi.  It is really quite remarkable that the rate of the catalyst increased by 260% (rate of 111 vs. 24 mmol (CO)/g_cat/h) when the only difference was that the support (AlSi) was calcined at 1200°C compared to 700°C.

Table 1.  Comparison of Iron Catalyst Performance

 Syngas-TPR provides useful information on the relative reducibility and carbiding behavior of the calcined form of the different catalysts. Usually syngas-TPR consists of two parts; (1) reduction of Fe₂O₃to lower iron oxides or iron metal and (2) carbiding of the iron oxides or iron metal to iron carbides. Syngas-TPR results show that reduction and carbiding of iron oxide is happening at lower temperatures with increasing calcination temperature of the support. In fact, the extent of carbiding increases from 3.3 to 9.3% when the calcination temperature of AlSi increases from 700 to 1200°C.

Mossbauer spectroscopy is a useful technique that can provide quantitative information on the amount of different iron phases in the samples. Table 2 summarizes those results for the carbided forms of the 4 catalysts used in this study. As is apparent, the amount of iron carbide (Fe_2.2C) increases with increasing calcination temperature of the support. It should be noted that the quantity of iron carbide is doubled as the calcination temperature of the support increases from 900 to 1100°C, while the carbide phase increase is marginal in raising the temperature from 700 to 900°C or 1100 to 1200°C.

It is clear that the higher calcination temperatures of the AlSi support resulted in greater removal of the hydroxyl groups, and thus weaker metal-support interaction, and more effective reduction/ carbiding (higher extent of carbiding) of the catalyst.  This then lead to higher activity.

Conclusions.  The results of the present work show that dehydroxylation of the support material has a huge effect on the final activity of the catalyst. A five-fold increase in activity was observed when the support was calcined at 1200°C compared to 700°C. This is mainly due to weaker interaction of the metal oxide with the alumina support which consequently leads to higher amount of iron carbide (active sites) formation.

References

(1)    Bukur, D.B.; and Lang, X. Ind. Eng. Chem. Res., 1999, 38(9), 3270.

(2)    Xu, J.; Bartholomew, C.H.; Sudweeks, J.; and Eggett, D.L. Topics in Catalysis, 2003, 26(1-4), 55.

(3)    K. Keyvanloo, W.C. Hecker, B.F. Woodfield, C.H. Bartholomew, , J. Cat., 2014, 319, 220.

(4)    K. Keyvanloo, M.K. Mardkhe, T.M. Alam, C.H. Bartholomew, B.F. Woodfield, W.C. Hecker, ACS Catal., 2014, 4, 1071