Roles of Zn species in Zn-promoted HZSM-5 catalysts on the aromatization activity

Abstract

The roles of Zn species in Zn/HZSM-5 catalyst on the aromatization of palm fatty acid distillate (PFAD), glycerol, and {u1D45B}-pentane were investigated. The amounts of Zn loading and preparation methods strongly affect the Zn species on HZSM-5 support. The Zn species strongly depended on the method of introducing Zn. Compared with ZnO, the higher dehydrogenation activity of (ZnOH)+ showed the higher aromatization of glycerol. However, using PFAD as a feedstock, even the (ZnOH)+ species is essential for improving the aromatization. The ZnO also plays a key role in the deoxygenation of PFAD, shifting the deoxygenation pathway from decarbonylation on Bronsted acid site to decarboxylation on ZnO, thus preserving the Bronsted acid site for aromatization. The evolution of Zn species upon different thermal treatments was studied by XPS and IPA-TPD. Under H2 atmosphere, hydrodehydroxylation of (ZnOH)+ species generated (ZnH)+, further dissociated H2 molecule, forming (ZnH3)+species. The presence of (ZnH3)+ was found to improve the aromatics selectivity. Even the (ZnH3)+ species is not stable, it can be recovered upon H2 treatment. In order to improve the p-xylene selectivity, silicalite-1 was later coated on Zn/HZSM-5 catalyst. Even the selectivity of p-xylene was improved, the aromatics yield was found to be traded with p-xylene selectivity. To increase aromatics yield, Zn(II)ions was grafted on silicalite-1 layer. Due to its high dehydrogenation activity, {u1D45B}-pentane feedstock was firstly converted to olefins on Zn(II)ions/Sil-l then further aromatized in Zn/HZSM-5 catalyst, thus improving p- xylene yield.