Synthesis of highly acidic and well ordered MgAl-MCM-41 and its catalytic performance on the isopropylation of m-cresol

Abstract

Highly ordered mesoporous MgAl-MCM-41 materials with different n Si/ n (Mg+Al) ratios have been successfully prepared using magnesium acetate, sodium aluminate and sodium silicate as the source of magnesium, aluminum and silicon, respectively. The samples are labeled MgAl-MCM-41( x) where x denotes the molar n Si/( n Mg+ n Al) ratio in the synthesis gel. The obtained materials have been characterized by powder X-ray diffraction, nitrogen adsorption, 27Al MAS NMR spectroscopy and AAS analysis. XRD and nitrogen adsorption measurements show that the hexagonal structure and textural parameters of the MgAl-MCM-41 materials are similar to MCM-41 materials. The wall thickness and pore diameter of the MgAl-MCM-41 samples can be controlled by varying the magnesium content in the synthesis gel. 27Al MAS NMR analysis clearly confirms that most of the aluminum in the calcined product occupies the tetrahedral coordination state. Temperature programmed desorption (TPD) of pyridine indicates that strong and medium acid sites of MgAl-MCM-41 are much larger as compared to AlMCM-41 and increase upon increasing the Mg content. The catalytic activity of MgAl-MCM-41 materials with different n Si/ n (Mg+Al) ratios has been investigated using isopropylation of m-cresol with isopropyl alcohol. MgAl-MCM-41(20) shows more m-cresol conversion and higher selectivity towards 2,4-diisopropyl-5-methyl phenol (2,4-DI-5MP) as compared to the AlMCM-41(23) catalyst under optimized reaction conditions. Moreover, the above catalyst registers a very high m-cresol conversion and high selectivity of 2,4-DI-5MP at a very low reaction temperature of 200 °C, due to strong acid sites generated by divalent Mg 2+ ions incorporation in the silica walls.