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Hydrocracking under Fischer–Tropsch conditions; the effect of CO on the mass transfer resistance by metal clusters

Brosius, Roald, Fletcher, Jack C.Q.
Journal of catalysis 2014 v.317 pp. 318-325
catalysts, chromatography, cracking, cyclohexanes, diffusivity, hexadecane, mass transfer, micropores, models, synthesis gas, zeolites
Over-cracking of Fischer–Tropsch wax remains an obstacle toward successful integration of FT synthesis and upgrading with high middle-distillate yields. The effect of syngas CO on the hydrocracking of n-hexadecane over Pd/H-MFI zeolites with different metal loadings has been investigated. Secondary cracking is minimized with increasing metal loading in Pd/H-MFI zeolite catalysts, and surprisingly a commensurate n-hexadecane conversion loss is observed, whereas in Pd/H-Y zeolites activity increases with metal loading. By means of tracer chromatography with cyclohexane, and using the solutions of the bidisperse model, it is demonstrated that metal loaded on the H-MFI zeolite lowers the micro- and macropore diffusivities. The presence of CO results in increased activity and concomitant secondary cracking on Pd/H-MFI with high metal loading, reminiscent of a Pd/H-MFI with low metal loading, showing that Pd is only partially poisoned. Pd/H-Y with CO displays lower activity like a Pd/H-Y with less Pd and Pt/H-MFI loses all hydrocracking activity. Severe secondary cracking in the presence of CO attests to an increased distance between metal and acid sites due to Pd migration from the micropores.