Study of the supercritical alkylation reaction of isobutane with 1-butene on a solid acid catalyst

Alkylation of isobutane with n-butenes to form highly branched isoparaffins is an important process in gasoline manufacture. This process provides high-octane, low volatility, very high heat of combustion gasoline blending component, virtually without aromatics, sulfur, nor nitrogen. These properties make alkylate an ideal gasoline component. This reaction is currently catalyzed using either liquid hydrofluoric acid (HF) or sulfuric acid (H₂SO₄). Though these liquid acids rapidly catalyze the alkylation reaction, there are a number of drawbacks associated with their use. Conjunct polymerization occurs when using H₂SO₄, producing large amounts of sludge by-product. This sludge requires regeneration to recover and recycle the acid. The primary problem associated with using HF pertains to safety. Due to its low volatility, HF leaks are particularly difficult to contain, they disperse rapidly and interact with atmospheric moisture forming aerosol clouds, posing a serious hazard to people and the environment. The cost of acid regeneration, environmental hazards, and worldwide regulations, have led investigators to search new technologies which rely on less corrosive, less toxic, easier to handle, cheaper, and environmentally safer catalysts. Zeolites have been studied extensively as possible candidates for this reaction. However, high catalytic activity for long periods of time on zeolites has not yet been realized. This deactivation makes the zeolite-catalyzed alkylation process economically unattractive. Supercritical alkylation on solid acid catalysts has been reported to slow down deactivation. However, the high reaction temperature employed leads to undesired products. In this study, the effects of using different operation conditions for alkylation on a USY zeolite were investigated. These experiments included gas, liquid, supercritical, and low-temperature carbon dioxide (CO₂) diluted supercritical reaction conditions. The use of diluent, to reduce the critical temperature of the reacting mixture favored both alkylate selectivity and coke precursors removal. Employing molar excess of CO₂ resulted in steady-state low conversion, in contrast to the observed in liquid phase (without CO₂) reactions at the same temperatures, where the conversions decreased continuously. Kinetic modeling including deactivation also was performed. Thus, supercritical alkylation using CO₂ as diluent shows to be a potential technology to be used in the search for a new alkylation process.