Hydrocracking in Al-MCM-41: Diffusion effect

The effects of wall defects on the molecular diffusion and hydrocracking ability of mesoporous MCM-41 materials having particulate and hierarchical tubular morphologies with various extents of structural defects have been studied. The diffusion properties of the MCM-41 materials were investigated by ¹H pulsed-field-gradient NMR self-diffusion measurements of cyclohexane, whereas the catalytic performances of the aluminosilicate MCM-41 were evaluated using 1,3,5-triisopropylbenzene cracking as the test reaction. ³¹P MAS NMR of the adsorbed trimethylphosphine oxide probe molecules confirmed the existence of active sites in the Al-MCM-41 samples, whose acid distribution were evaluated in conjunction with elemental analyses. It was found that, the Al-MCM-41 sample with tubules-within-tubule (TWT) morphology showed superior catalytic activity and tolerance for deactivation during hydrocracking reaction compared to the particulate morphology sample having the same Al content. The TWT Al-MCM-41 sample, which has more extensive structural defects than the particulate Al-MCM-41, was also found to have higher cyclohexane diffusivity. It is conclusive that, the structural defects possessed by these unique Al-MCM-41 materials synthesized by the 'delayed neutralization method' effectively provoke interchannel connections, which facilitate transport of the adsorbate molecules, thus promote the catalytic activity of the catalysts.