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Novel method to prepare nanoporous material compounds MFI/MCM-41 from Venezuelan kaolin. Effect of the NaOH concentration

Abstract:

A MFI/MCM-41 composite material, a zeolite type MFI and Al-MCM-41 were synthesized from kaolin, using TPA and CTAB as structure directing agents. The MFI/MCM-41 composite material was synthesized in two steps. In the first, MFI was obtained and then treated with solutions of different NaOH concentrations (1 - 4M), in the second step the reaction mixture was treated under MCM-41 synthesis conditions. The solids obtained were characterized by scanning electron microscopy (SEM), chemical analysis by energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), adsorption of N 2 at 77K, X-ray diffraction (XRD). In the absence of alkaline treatment two phases were observed - zeolite and mesoporous, whereas the zeolite crystals were larger than mesoporous particles. The alkaline treatment dissolved the zeolite structure creating mesoporosity, modifying the characteristic zeolite morphology and reducing its crystal domains. The treatment with 2 or 3M NaOH eases zeolite dissolution and leads to assemble of the zeolite building units around the surfactant and into the mesoporous walls. The incorporation of the zeolite building units into the mesoporous walls is governed by the relative quantities of crystalline and amorphous materials in the synthesis gel. The treatment with 4M NaOH led exclusively to the formation of Al-MCM-41.