THE INFLUENCE OF STRUCTURE AND ACIDITY OF ZEOLITIC MATERIALS ON THE TRANSALKYLATION OF DIISOPROPYLBENZENE WITH BENZENE IN SUPERCRITICAL CARBON DIOXIDE

摘要

Monoalkylbenzenes (isopropylbenzene and ethylbenzene) are important intermediates for the synthesis of large-scale chemical commodities, i.e phenol (from isopropylbenzene) and styrene (from ethylbenzene). The synthesis of isopropylbenzene (cumene, CUM) by benzene (B) isopropylation over acidic catalysts is usually accompanied by the formation of polyalkylates, mainly isomers of diisopropylbenzene (DIPB). From an industrial point of view, transformation of these by-products into valuable products is interesting. Thus, dialkylates can be transformed to CUM by further reaction with B (transalkylation). The installed capacity of cumene through transalkylation is around 700.000-1.000.000 tons per year, depending on the zeolite technology adopted in the alkylation section, and it is expected to grow as the producing plants change into zeolite based technology. Due to the size of molecules, and due to the fact that the mechanism of the transalkylation reaction is known to involve a bulky bimolecular compound as the reaction intermediate, zeolitic materials belonging to the large-cavities class have been primarily investigated and claimed [1,2,3]. The hydrogen form of Beta (H-B), Y (H-Y) and ZMS zeolite types were specifically disclosed in several patents [4,5,6]. On the other hand, transalkylation reactions proceed on strong Bronsted acid sites [7], which are function of the framework Al content. The presence of extra-framework Al can decrease the catalytic activity of the zeolite. Theoretical models developed to explain relations between acidity and framework composition suggest that, as framework Al decreases, the fraction of "isolated" Al atoms increases resulting in a raise in the acid strength of the active sites [8]. Accordingly, the relation between the Si/Al ratio, responsible of the number and strength of the active sites, and the catalytic activity of the zeolites can be described by curves with a maximum, where the catalyst performance is the best.