Upgrading of Fischer-Tropsch synthesis bio-waxes via catalytic cracking: Effect of acidity, porosity and metal modification of zeolitic and mesoporous aluminosilicate catalysts

摘要

The development of novel catalytic processes for the upgrading of F-T waxes towards the production of clean transportation fuels as well as other valuable gaseous alkenes in the overall Biomass-to-Liquid (BTL) process would increase the sustainability of this growing technology significantly. Recent studies have shown that an alternative to the classical hydrocracking process for the upgrading of heavy waxy hydrocarbons could be the Fluid Catalytic Cracking (FCC) process, which aims mainly to the production of gasoline and gaseous (C3, C4) alkenes. In the present work, we have used "model" zeolitic and mesoporous aluminosilicate catalytic materials that exhibit different acidic and porosity characteristics, in order to elucidate the reaction mechanism of F-T wax cracking and to identify the effects of these catalyst properties on conversion and product yields. In addition, the effect of moderating the acidity of zeolites via steaming (in analogy to commercial FCC equilibrated catalysts) or by using alkaline metals that block the acid sites and the effect of using specific metals with dehydrogenation activity, have also been studied. It was shown that the Bronsted zeolitic acid sites, mainly the ones of higher strength, are responsible for the high cracking activity of the heavy paraffinic feed, while the Lewis acid sites contributed to a lesser extent. Furthermore, the different micropore structure/size of zeolites Y, ZSM-5 and Beta had a significant effect on conversion and product yields (C2-C4 gases, gasoline, LCO, coke), as well as on gasoline composition (PIONA) and octane number (RON/MON). The mildly acidic amorphous mesoporous aluminosilicates exhibited moderate conversion and different product yields and gasoline composition, compared to zeolites. The catalytic results of the present study were rationalized on the basis of well-established reaction mechanisms for n-paraffins cracking on zeolites, and can be utilized for the design of new cracking catalysts with tailored catalytic properties in F-T wax upgrading.