Synthesis,characterization and catalytic performance of Al-MCM-41 mesoporous materials

A series of MCM-41 molecular sieves has been synthesized by using different Si/Al molar ratios and appropriate conditions for the synthesis. Silica fumed and sodium aluminate, both of inorganic origin, have been used in the synthesis, avoiding the use of organic sources. Isomorphous substitution of silicon in tetrahedral framework positions of structure by Al in the Al-MCM-41 material was observed by ²⁷Al NMR and ²⁹Si NMR spectra. The nitrogen adsorption isotherms of the samples showed higher specific surface area and more mesopore volume when there was an increase in the aluminium content. Simultaneously, a smaller thickness of the wall and diameter pore by XRD was observed. At higher degrees of incorporation of aluminium, the acidity of the samples increased, as is clearly shown by the results of temperature-programmed desorption of ammonia and pyridine adsorption by IR. As more aluminium was incorporated into the samples, the higher were the catalytic performances. The increase of the conversion, in hydroisomerization of n-heptane, shows that these materials acted as acid catalysts. This increase in the total conversion is related to the higher number of acid sites in the samples.