(760a) Investigations of the Wax–Water Interface and Impacts of Water in the Micropores of Fischer-Tropsch Synthesis Catalysts
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 (PH2O/PSat.) to 60% to saturation. These promotional effects are observed both at wax-forming FTS conditions (483 K, 2.26â4.24 MPa total, H2:CO = 4, 0.44 MPa CO, 0â2.07 MPa H2O) and low-wax or near-methanation conditions (518 K, 0.8â1.33 MPa total, H2:CO = 14, 0.025 MPa CO, 0â0.95 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 âdipâ 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.
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