Mechanisms of aromatization of dilute ethylene on HZSM-5 and on Zn/HZSM-5 catalysts

[Display omitted] •The presence of zinc change completely changes the mechanism of the ZSM-5 catalyst.•Protonic sites activity depends on the concentration of next nearest neighbours pairs of Al.•Distribution of zinc species depends of the molar Si/Al ratio of the ZSM-5.•Dimerization of ethylene occurs onto zinc exchange species.•Ethylene aromatization involves cyclo-butyl-zinc species. Catalysts with a Zn/H+ molar ratio ranging from 0 (H-ZSM5) to 1.8 (Zn/H-ZSM5), were prepared by wet impregnation from three commercial zeolites with Si/Al ratio of 15, 40 and 75. The texture, acidity and reactivity of the bifunctional catalysts were characterized by: N2, IR followed by adsorption of pyridine and NH3-TPD, and tested at 500 °C with low partial pressure of ethylene (PC2H4 = 0.005 MPa) and a high GHSV (ca 13,300 h−1). The intrinsic activity of protonic acid sites in ethylene transformation depends on their density or rather on the concentration of next nearest neighbours pairs of Al atoms. Brønsted acid pair sites converts ethylene into aromatics while isolated protonic site are almost inactive. Two apparent primary products were distinguished: iso-butene and propene, resulting from the oligomerization-cracking catalytic pool, followed at high conversion by the concomitant formation of alkanes and aromatics through hydrogen transfer. The deposition of Zn was achieved by wet impregnation of the nitrate precursor. The thermal decomposition of the ZnOH+ cation by calcination is a function of the fraction of Al atoms in the nearest neighbours. Three main species of Zn could be identified: i) oxo-binuclear Zn2+ cations (O−-Zn2+-O-Zn2+-O−), ii) mononuclear Zn2+ cations (O−-Zn2+-O− and iii) ZnO particles located on the external surface. These species were quantified from the residual Brønsted acid sites probed by pyridine. A correlation has been established: one neutralized protonic site generates one Zn Lewis acid sites which coordinates two pyridine molecules. The ethylene conversion is proportional to the concentration of oxo-binuclear Zn cations and mononuclear Zn cations only correlate with hydrogenation and hydrogenolysis of olefins, while bulk ZnO is inactive. A metallacyclic mechanism of ethylene aromatization involving cyclo-butyl-zinc species has been proposed accordingly.