Coking and activity of a reforming catalyst in near-critical and dense supercritical reaction mixtures

The Pt/γ-Al₂O₃ catalyzed isomerization of 1-hexene (P_c = 31.7 bar, T_c = 231°C) was investigated in a low volume tubular reactor at 1.1 T_c and at several pressures (0.7–11 P_c) that yielded reaction mixture densities ranging from gas-like to liquid-like values. At a space velocity of 135 g hexene/g cat/h, eight hour isomerization rates are roughly twofold higher and deactivation rates are threefold lower in near-critical (ρ_r = 0.85) when compared to subcritical (ρ_r = 0.42) reaction mixtures. The 2-hexenes÷3-hexenes selectivity ratio follows the 1-hexene isomerization rate trend. Coke laydown is more than twofold lower at near-critical relative to subcritical conditions. The in situ extraction of the coke compounds by near-critical and supercritical reaction mixtures prevents pore plugging that otherwise occurs at subcritical conditions during the eight hour runs. Although coke laydown decreases further at supercritical conditions, the isomerization rates are lower and deactivation rates are higher due to pore diffusion limitations in liquid-like reaction mixtures. It is therefore concluded that near-critical reaction mixtures provide an optimum combination of solvent and transport properties for maximizing the isomerization rates and for minimizing Catalyst Deactivation rates.