High catalytic cracking activity of Al-MCM-41 type materials prepared from ZSM-5 zeolite crystals and fumed silica

摘要

Two series of mesoporous aluminosilicate materials of the Al-MCM-41 type have been prepared from pre-formed ZSM-5 crystals.The synthesis involved the alkaline attack of the zeolite alone (M(-)/x samples) or in the presence of an external silicon source (e.g.fumed silica,M(S)/x samples) followed by a conventional 48 h hydrothermal treatment (100 °C) with cetyltrimethylammonium chloride as the structure directing agent.The effect of sodium hydroxide concentration (0.8 M ≤ x ≤ 3 M) on the properties of the final materials was investigated by powder XRD,nitrogen sorption,pyridine sorption (IRFT),SEM,TEM,MAS and 2D MQ-MAS ~(27)Al NMR.It was shown that the addition of fumed silica to the synthesis gel results in the improvement of the mesopore network structure.The stability of M(-)/x and M(S)/x materials towards boiling water (5 days) as well as their activity in the acid-catalysed transformation of methylcyclohexane at 350 °C were compared to those of ZSM-5,Al-MCM-41 and of a Al-MCM-41//ZSM-5/H mechanical mixture.Because of the existence of a 2-fold pore system with zeolite domains,M(S)/0.8 and M(S)/1.9 composites are more efficient catalysts than amorphous Al-MCM-41 and a mechanical mixture of Al-MCM-41 and ZSM-5.The best combination in terms of hydrothermal stability and catalytic activity was found for the M(S)/1.9 sample.