(729f) Significance of Pre-Treatment on Surface Interactions and Performance of Eggshell Cobalt/SiO2 Catalyst for Fischer Tropsch Synthesis

The dispersion and performance of cobalt/SiO₂ catalyst used in Fischer-Tropsch Synthesis (FTS) are significantly affected by the catalyst preparation technique used. Numerous parameters impact the catalyst loading and dispersion; in this study we focus on three key parameters: effect of solvent during the precursor loading, effect of calcination atmosphere, and the inherent nature of support interactions with active metal. Based on our analysis a novel catalyst preparation technique is proposed and validated using a bench scale fixed bed reactor.

Silica and alumina are commonly used supports for FT catalysts due to cost, abundance and robust nature. Iglesia et al. [1] have indicated that for large metal particles (above 6 nm), FTS rate is proportional to cobalt surface sites (i.e. dispersion). Exploiting this, dispersion of silica supported catalyst can be improved by adding expensive noble metal (Rh, Ru, Pt and Pd) [2]. Surface wetting of silica with different polar solvents (ethanol,1 propanol and 1 butanol) is another method used to increase dispersion [3]. Synthesis of an active catalyst requires a balance of strong interaction between the active metal and support without formation of irreducible mixed metal support oxides. Ho et al. [3] have observed that when ethanol is used as a solvent for cobalt nitrate, there is an increase in dispersion while retaining high extent of reduction. Calcination atmosphere also impacts the final redistribution of active metal on support. Presence of water in the calcination environment enhances metal-support interaction [4,5]. On the other hand, byproduct water during FTS can also lead to the formation of irreducible silicates. Liyang et al. [4] have identified that metal dispersion increases if the environment is shifted from stagnant to flowing air. Sietsma et al. [5] state that a mixture of NO/He can act as a scavenger of oxygen and improves dispersion within mesoporous supports.

In this study we have analyzed physical interactions of a mesoporous silica support with different molecules in great detail. Silanol groups on silica can alter morphology and dispersion of active metals on the support. Solvents used for precursor such as water or alcohol attach to these silanol sites in specific configuration and compete with metal salts during ion exchange and adsorption. By fine tuning the solvent attachments on heat treated silica we have fabricated a cobalt/silica catalyst with high dispersion. Silica has affinity for both polar and non-polar molecule depending on the surface conditions. This property is exploited in preparing an egg shell profile. Simultaneous calcinations/reduction in dynamic hydrogen environment was used to further enhance the dispersion and reducibility.

BET analysis was used to identify the surface area and porosity of nascent and deposited silica gel. XPS has revealed cobalt phases and metal dispersion on the surface. XRD is indicative of variation in degree of crystallinity. H-chemisorption has helped in quantifying active metal dispersion, its extent of reduction and average crystal size. TPR gives an insight into metal-support interactions of different sample. FTIR was used to determine whether, if any mixed metal support oxides are present.The catalyst activity pattern parallels the dispersion i.e. high dispersion gives more conversion. Product selectivity has been controlled by egg shell thickness; current focus is in the production of diesel and aviation fuel. Gas chromatograph analyses of the resulting liquid fuel products show a very narrow hydrocarbon distribution.

References

1. E. Iglesia et al., Top. Catal. 26, 101 (2003).

2. N. Tsubaki, S. Sun, K. Fujimoto. J. Catal. 199, 236 (2001).

3. E. v. Steen et al., J. Catal. 162, 220 (1996).

4. B. Linyang et al., Catal. Commun. 10, 2013 (2009).

5. J. R. A. Sietsma et al., J. Catal. 260, 227 (2008).