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

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