| Alkylation of isobutane with 1-butene to produce isooctanes, a process which produces 13% of the U.S. gasoline pool, was investigated over several Nafion® and zeolite catalysts. These environmentally benign solid-acid catalysts have not yet proven to be commercially viable due to rapid deactivation by fouling. However, the rational use of carbon dioxide-based supercritical reaction mixtures was shown to minimize the deactivation of these solid acid catalysts, relative to liquid-phase operation. The advantage of using a supercritical reaction mixture is the ability to sensitively manipulate the fluid properties, such as density, diffusivity, and viscosity, from gas-like to liquid-like by changing the pressure. An optimum combination of these properties is needed to facilitate extraction of the compounds responsible for catalyst deactivation.; A systematic pressure-tuning investigation revealed that the optimum steady-state alkylate selectivity was observed at the lowest supercritical pressure (28% at 80 bar, 95°C, 5:1 isobutane/butene ratio, 0.05 h −1 olefin space velocity, 70 mol% CO2), and was enhanced four-fold over the selectivity observed at higher pressures (7% at 131 bar). The selectivity enhancement was due in part to an in situ product separation, where butene oligomers were selectively retained in the catalyst pores. At higher pressures (131–166 bar), the oligomers were solubilized by the CO2-based reaction mixture, effectively cleaning the catalyst. This finding demonstrates the potential for a multi-functional reactor design, where simultaneous reaction and product separation occur, and where the catalyst is periodically cleaned by high pressure CO2.; The ability to maintain steady butene conversion and alkylate selectivity on Nafion®/SiO2 catalysts provided a unique opportunity to model the reaction kinetics. Using a complementary lumped-parameter slurry reactor model, the effective rate constants reveal increasing pore-diffusion limitations with increasing pressure. The effective rate constant for butene dimerization, the dominant side reaction, was three orders of magnitude greater than the effective rate constant for alkylation. Based on model predictions, it was demonstrated experimentally that operating the reactor at lower space velocities could further enhance alkylation/dimerization ratio. The model predicts that to maximize the alkylate selectivity, one must operate the reactor at high Damköhler numbers, where nearly total butene conversion (99+%) occurs. This underscores the need to design more highly active alkylation catalysts to further enhance the alkylate selectivity. |
