Pressure-Tuning the Effective Diffusivity of Near-critical Reaction Mixtures in Mesoporous Catalysts

Abstract

The pressure-tunability of the diffusivity of a supercritical medium in a mesoporous catalyst, by approximately 2 orders of magnitude with moderate changes in pressure, is demonstrated. Effective diffusivities in porous Pt/γ-Al2O3 pellets at supercritical conditions were estimated by performing the geometric isomerization of 1-hexene (Tc = 231 °C, Pc = 31.7 bar) at supercritical conditions that encompass both the kinetic-controlled and the pore-diffusion-controlled regimes. Operation at supercritical conditions thwarts catalyst deactivation by coking and ensures steady catalyst activity. Effective rate constants (ηk) are calculated from steady-state 1-hexene conversion data obtained in a fixed-bed reactor at various supercritical pressures (37−73 bar) in the 235−310 °C range. The intrinsic kinetics parameters and the effective diffusivities are estimated from the rate constants (ηk) using the conventional theory of diffusion and reaction in catalyst pellets. The intrinsic activation energy was determined to be 184 ± 4.4 kJ/mol. The pore diffusivity of the hexene molecules was found to vary by nearly 2 orders of magnitude from 6.3 (10-6) cm2 s-1 at ρr (reduced density) = 2.7 (235 °C, 72.8 bar) to 1.4 (10-4) cm2 s-1 at ρr = 0.8 (311 °C, 36.7 bar), with relatively moderate changes in pressure and temperature of the near-critical reaction mixture.