Novel zeolite catalysis to create value from surplus aromatics: preparation of C 2+- n-alkanes, a high-quality synthetic steamcracker feedstock

Abstract

With the advent of the second stage of the European Auto Oil Programme in the year 2005, the aromatics content of gasoline has to be reduced significantly which could lead to an oversupply of aromatics. The sources for aromatic hydrocarbons and their petrochemical use are briefly reviewed. Options for avoiding a surplus of aromatics in the years to come are discussed, i.e. diminishing their production and intensifying their conversion into valuable products. A novel catalytic route for hydrogenative ring opening of aromatics, which has been recently discovered in this laboratory, is described in some detail. It allows the conversion of pyrolysis gasoline from naphtha steamcrackers into a high-quality synthetic steamcracker feedstock composed of ethane, propane and n-butane, referred to as C 2+- n-alkanes. It is shown that there are two process variants, namely, (i) a direct route utilizing bifunctional zeolite catalysts and (ii) a two-stage route comprising ring hydrogenation on conventional catalysts followed by ring opening of the resulting cycloalkanes on monofunctional, i.e. acidic zeolite catalysts. It is demonstrated that, in the direct route, the nature of the aromatic feed hydrocarbon may have a significant influence on the yield of the desired light alkanes. The second, i.e. ring opening stage of the two-stage route is discussed in detail with emphasis on (i) the zeolite pore architecture, (ii) its aluminum content and (iii) the presence of metal promoters. With both process variants, excellent catalytic results can be achieved, viz. (i) a complete ring opening of the aromatic or naphthenic feed hydrocarbon, (ii) high yields of the desired C 2+- n-alkanes in the order of 70–80% or even higher, (iii) very low yields (<5%) of the undesired methane, and (iv) no measurable catalyst deactivation within 10 h. Key parameters to ensure such desirable results are (i) the use of a shape-selective zeolite catalyst, such as ZSM-5 or ZSM-11, (ii) the application of relatively high reaction temperatures in the vicinity of 400 °C, and (iii) elevated hydrogen pressures around 6 MPa.