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The location of coke deposited on HZSM-5 zeolite catalyst during the supercritical catalytic cracking (400 °C, 4.0 MPa) of n -dodecane was systematically characterized by special temperature-programmed oxidation (TPO) and isothermal oxidation methods, and a steric hindrance model describing the interactions of the coke deposited on different locations was proposed to support the methods theoretically. Accord to the results, the sharp shoulder peak that appears on the TPO profile at high temperature range (> 700 °C) with low concentration of oxygen in the oxygen/nitrogen mixed carrier gas (≤ 2.0 Vol.%) can be ascribed to the characteristic peak representing the oxidation of the coke blocked in micropores, while the appearance of the CO peak during the isothermal oxidation at high temperature (≥ 550 °C) using air as carrier gas provides another method to detect the coke blocked in micropores. Through checking whether the characteristic sharp shoulder peak of the TPO profile appears or not as expected according to the steric hindrance model during the re-oxidation of several samples with different coke content and spatial distribution, which were prepared by partial oxidation of the spent catalyst using TPO and isothermal oxidation methods, the special TPO and isothermal oxidation methods combined with their theoretical support of the steric hindrance model were proved to be self-consistent in the characterization of the location of coke. The location information of coke was further verified by the typical method of nitrogen adsorption-desorption analyses of a series of partially oxidized samples prepared by TPO method. [ABSTRACT FROM AUTHOR]

[Display omitted] • Neutralization titration can reduce the acid density and strength of HZSM-5 zeolite. • The neutralization reaction just occurs in liquid phase rather than in solid phase. • Nature of neutralization titration is solid phase ion exchange between Na+ and H+. • Neutralization method facilitates the removal of extremely strong acid sites. • Both coke amount and harmfulness per unit mass of coke can be suppressed. Reducing the acid density and strength of HZSM-5 zeolite are typical strategies to inhibit coke deposition during catalytic cracking reactions. Herein, direct titration of HZSM-5/H 2 O suspension using dilute NaOH aqueous solution at room temperature was introduced to neutralize excessive Brønsted acid sites of HZSM-5 zeolite (Si/Al = 25). SEM, XRD, N 2 adsorption and n -propylamine temperature-programmed decomposition confirmed that simultaneous acid strength weakening and acid density reducing were achieved without obvious influence on crystalline or textural property. The evolution of the surface charge during the titration process tracked by Zeta-potential measurement reveals that the mechanism of such neutralization is: liquid phase neutralization between H+ and OH− & solid phase ion exchange between Na+ and H+. Supercritical catalytic cracking of n -dodecane over a series of partial neutralized HZSM-5 catalysts showed that moderate neutralization can enhance the conversion of n -dodecane by alleviating both the coke amount and harmfulness per unit mass of coke. [ABSTRACT FROM AUTHOR]