Catalytic oxidative cracking of hexane as a route to olefins

Abstract

Catalytic oxidative cracking of naphtha is conceptually an alternative process to steam cracking. The performance of sol–gel synthesized Li/MgO in oxidative cracking of hexane as a model compound of naphtha, has been studied and compared to that of conventionally prepared catalyst. At a temperature as low as 575 °C, Li/MgO shows reasonable hexane conversions (28 mol%) and excellent selectivity to light olefins (60 mol%). It is proposed that hexane activation occurs on the catalyst surface via the Li +O − defect sites, where O − active sites abstract hydrogen from a secondary carbon atom. The formed hexyl radical in gas phase and in the presence of molecular oxygen will then undergo a complex radical chemistry resulting in a product mixture of C 1–C 5 hydrocarbons (paraffins and olefins) as well as combustion products. Presence of oxygen in the feed is crucial to prevent coking, and to regenerate the catalyst surface through reaction with adsorbed surface hydrogen atoms, thus maintaining catalyst activity. Oxygen also plays a significant role in accelerating radical chemistry in gas phase. Unlike steam cracking, catalytic oxidative cracking results in a relatively higher ratio of high olefins (butylenes + propylene) to ethylene. Thus presence of the catalyst provides a better control over product distribution. Promotion of Li/MgO with MoO 3 and Bi 2O 3 results in considerable improvements in catalyst activity and stability.