Effect of dealumination by acid treatment of a HMCM-22 zeolite on the acidity and activity of the pore systems

Abstract

Acid leaching of a HMCM-22 zeolite (Si/Al = 14.5) was carried out at 100 °C for different times (3, 30, 60 and 480 min) with a 4 M nitric acid solution. The resulting samples were characterised by elemental analysis, X-ray diffraction, pyridine adsorption followed by infrared spectroscopy (FTIR) and nitrogen adsorption and their catalytic properties were established with methylcyclohexane transformation at 350 °C. The role played in this reaction by the three micropore systems was established by coupling selective deactivation of supercages by coking and poisoning of the outer cup acid sites with 2,4-dimethylquinoline. The change in the acid and catalytic properties with the time of acid treatment shows that dealumination is dominated by the accessibility of the framework Al atoms. Thus, the outer cup acid sites are highly sensitive to the acid treatment. The acidity and activity of the supercages are also strongly affected whereas those of the narrow sinusoidal channels are hardly affected.