(760a) Investigations of the Wax–Water Interface and Impacts of Water in the Micropores of Fischer-Tropsch Synthesis Catalysts

Fischer-Tropsch synthesis (FTS) is a powerful gas-to-liquids process for converting syngas (CO + H₂) into CH₄, larger hydrocarbons, water, and other oxygenates. Rates of FTS are limited by activation of strongly-chemisorbed CO* and C–C bonds can be formed by insertion of CO* or CH_x* species into growing hydrocarbon chains. Large product distributions of hydrocarbons arise, ranging from methane to ~C₃₀ paraffin wax, because the rate is limited by CO activation and not chain growth. Large hydrocarbon waxes —and perhaps water—condense within the micropores of the catalyst. Studies that systematically varied the co-fed water and water formed in-situ demonstrated that water increases the rate of FTS and selectivity towards C_≥5 species on Co and Ru catalysts [1,2]. However, other studies have shown that water has no effect on or decreases FTS rates on Co catalysts [3,4]. These seemingly contradictory results may be explained by a relationship between the phase of water and the nature (composition, pore size) of the oxide supports caused by capillary condensation of water within the pores of the support. Thus, disparate behavior may be linked to the presence and amount of liquid water that may promote FTS rates and selectivity towards desired products. Additionally, wax formed in-situ can reduce the effective pore size of the support either by coating the surface of the support or by filling the central void space of the pores. Understanding the nature of the wax–water interface and the interactions of water and the FTS catalyst will give insights into the ubiquitous role of water in surface chemistry, capillary condensation in complex micropore environments, and provide strategies for improving catalyst supports.

Here, we synthesized catalysts on well-ordered SBA-15 materials with narrow pore size distributions ranging from 4-20 nm. These catalysts will allow for capillary condensation to occur at various pressures depending on the pore size. For Ru catalysts on wide-pore supports (13 nm), water nearly doubles FTS rates when increasing the water pressure from 5% to saturation (P_H2O/P_Sat.) to 60% to saturation. These promotional effects are observed both at wax-forming FTS conditions (483 K, 2.26–4.24 MPa total, H₂:CO = 4, 0.44 MPa CO, 0–2.07 MPa H₂O) and low-wax or near-methanation conditions (518 K, 0.8–1.33 MPa total, H₂:CO = 14, 0.025 MPa CO, 0–0.95 MPa H₂O). At low-wax conditions, the rate initially decreases as the water pressure increases, before the observed promotional effects. This initial reversible inhibition indicates that there is a phase change within the micropores of the catalyst. Additionally, the selectivity towards methane shifts from 32% at dry conditions to 15% with co-fed water. This dramatic shift in selectivity indicates that co-fed water directly modifies the chemistry either by inhibiting chain termination or increasing the rate of CO* activation. This initial ‘dip’ in reactivity is not observed to the same extent at FTS conditions, and the shift towards heavier products is not as dramatic, indicating that wax and the interplay between wax and water plays a key role in FTS rates and CH₄ selectivity.

References

[1] Iglesia, E. Design, Synthesis, and Use of Cobalt-Based Fischer-Tropsch Synthesis Catalysts. Appl. Catal. A Gen. 1997, 161 (1–2), 59–78.

[2] Hibbitts, D. D.; Loveless, B. T.; Neurock, M.; Iglesia, E. Mechanistic Role of Water on the Rate and Selectivity of Fischer-Tropsch Synthesis on Ruthenium Catalysts. Angew. Chemie Int. Ed. 2013, 52 (47), 12273–12278.

[3] Dalai, A. K.; Das, T. K.; Chaudhari, K. V.; Jacobs, G.; Davis, B. H. Fischer–Tropsch Synthesis: Water Effects on Co Supported on Narrow and Wide-Pore Silica. Appl. Catal. A Gen. 2005, 289 (2), 135–142.

[4] Dalai, A. K.; Davis, B. H. Fischer–Tropsch Synthesis: A Review of Water Effects on the Performances of Unsupported and Supported Co Catalysts. Appl. Catal. A Gen. 2008, 348 (1), 1–15.