Catalytic cracking of n-heptane over HZSM-5 catalysts with the activation of lattice oxygen

Abstract

Paraffin, one of the major components of straight-run naphtha, is relatively inert compared with olefin, which poses a difficulty for catalytic cracking of naphtha. The present study proposed introducing lattice oxygen into previous HZSM-5 catalyst system to improve conversion and light olefin selectivity. Vanadium oxide based catalyst was prepared separately as the lattice oxygen provider and then mixed with HZSM-5 catalyst for evaluation in a fixed-bed reactor using n-heptane as model compound. The results demonstrated that conversion of reactant increased from ∼51% to ∼59% by replacing 20% of HZSM-5 catalyst with V 2O 5/Al 2O 3; depending on different location of V 2O 5/Al 2O 3 relative to HZSM-5 catalyst in the reactor, conversion and product distribution varied significantly, while the addition of pure alumina did not have such an effect. Besides, water was formed in liquid products accompanied by reduction of V 5+ during the reaction, as revealed by FT-IR and XRD results, respectively; the oxidized V 5+ seemed to have greater capability in promoting conversion of n-heptane. Therefore, it was concluded that n-heptane could be activated by lattice oxygen provided by oxidized vanadia before further cracking over acidic sites in HZSM-5 catalyst.