Shape-selective alkylation of benzene in aromatic mixtures over zeolites for benzene reduction in gasoline

It would be desirable to reduce benzene in gasoline by alkylation. This work examined three zeolite catalysts with different topologies including ZSM-5(50), BEA(38) and Y(30) where the figure indicates the Si/Al2 ratio.. Isopropylation of aromatics with propylene was studied using model feeds (5 wt.% benzene; 5 wt.% toluene; and 5 wt.% m-xylene in octane) with corresponding molar ratios of (benzene:toluene:m-xylene=1.0:0.84:0.73) in a down-flow fixed bed reactor at 250°C with WHSV 3.75 h-1. The structure type and acid nature of porous materials strongly affected their catalytic activity and stability for benzene alkylation. The BEA and ZSM-5 catalysts selectively promoted benzene conversion over toluene and m-xylene. On the other hand, Y zeolite gave higher m-xylene conversion than benzene conversion. The ZSM-5 catalyst showed higher benzene selectivity than the BEA catalyst. However, catalytic activity of BEA zeolite for benzene conversion was much higher (30%) than that of ZSM-5 (10%) even though the former shows lower acidity. The ZSM-5 catalyst was much less stable compared to BEA zeolite. It was observed that BEA zeolite was very stable up to 6 h time on stream among the three catalysts examined. The reactivity in reactions using ZSM-5 and BEA zeolites ranked in the order of Benzene> Toluene> m-Xylene. However, Y-zeolite showed the opposite trend which may be due to different features of the pore channel. In the case of ZSM-5 and BEA zeolites, the channel opening is only slightly larger than the size of a benzene molecule and does not permit the entrance of m-xylene. As a result, benzene is more selective than toluene and m-xylene over said catalysts. BEA zeolite showed the highest conversion of benzene and more stable catalytic performance under the conditions employed.