Characterization of the location of coke deposited on spent HZSM-5 zeolite by special temperature-programmed oxidation and isothermal oxidation methods

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.