Synthesis and Characterization of Magnesium-Substituted Aluminophosphate Molecular Sieves (MgAPO-11) and Their Kinetic Study of Catalytic Cracking of n-Hexane

Magnesium-substituted aluminophosphate molecular sieves (MgAPO-11) with various magnesium contents were synthesized by the static hydrothermal method using diisopropylamine as an organic template and characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance, and inductively coupled plasma atomic emission spectroscopy. When n(MgO)/n(Al2O3) was in the range of 0–0.30/0.70, the MgAPO-11 molecular sieves with AEL structure and high crystallinity could be obtained. The heteroatoms Mg(II) were confirmed to isomorphously substitute for the framework atoms Al(III) of AlPO4-11 molecular sieves. The catalytic activity of the MgAPO-11 for hydrocarbon catalytic cracking was evaluated using n-hexane as a model substrate. The MgAPO-11 showed a moderate activity for n-hexane catalytic cracking conversion in the reaction temperature range of 450–550 °C. The reaction was first order in n-hexane. The dependence of the rate constant on temperature can be expressed in terms of the Arrhenius formula in the whole temperature range. Considering both the kinetic data and product distribution, it was concluded that the monomolecular protolytic mechanism prevailed for this cracking reaction.