Effect of the acidity of HZSM-5/MCM-41 hierarchical zeolite on its catalytic performance in supercritical catalytic cracking of n-dodecane: experiments and mechanism

The effect of the acidity of HZSM-5/MCM-41 hierarchical zeolite on its catalytic performance in the supercritical catalytic cracking of n-dodecane was investigated by controlled ion exchange with NaCl solution under ultrasonic conditions. XRD and nitrogen adsorption–desorption results showed that the crystal and porous properties were not affected, while NH3-TPD, the probe reaction of n-hexane cracking and temperature-programmed surface reaction (TPSR) of n-dodecane demonstrated that strong Brønsted acid sites were replaced by Na+ (weak Lewis acid sites) to different extents with varying concentrations of NaCl solution. Although the initial conversion of n-dodecane during the supercritical catalytic cracking was slightly reduced, its stability was greatly enhanced by ion exchange. An ultrastable catalyst with no obvious deactivation within 3 h of time on stream (TOS) was obtained by ion exchange with 0.8 M NaCl solution. A mechanism describing the role of the acid sites residing in both micropores and mesopores of the hierarchical catalyst was proposed based on coke analysis.