Aluminum Incorporation into MCM-48 toward the Creation of Brønsted Acidity

Abstract

The possibilities and limits of aluminum incorporation into the silica framework of a cubic MCM-48 have been thoroughly studied. Four different sources have been tested, namely aluminum sulfate (Al2(SO4)3), aluminum ethoxide (Al(Etox)3), aluminum isopropoxide (Al(iProx)3), and sodium aluminate (NaAlO2). Only sodium aluminate has been found suitable for the incorporation, as the aluminum sulfate negatively influences the synthesis pH and both alkoxides release alcohol upon their hydrolysis, which will dissolve in the micelles, interfering in the pore arrangements inside the cubic unit cell. The sodium aluminate source permits incorporation of aluminum up to ratios of Si/Al 20 without loss of the porosity; however, a broadening of the XRD signals indicate loss of long range ordering. High-resolution electron microscopy (HRTEM) confirms this loss of long-range pore ordering but also shows the maintenance of short-range pore ordering. Correlating this with the unaltered mesoporosity indicates the preservation of a 3D pore system. An extended 27Al MAS NMR study reveals that before calcination all aluminum atoms are tetrahedrally incorporated into the silicate framework. Those are situated at the surface of the framework to be compensated by surfactant cations, as no sodium was detected with 23Na-MAS NMR. After calcination, about 66% of the aluminum retains its tetrahedral geometry. Those aluminum atoms are compensated by a proton provided by the decomposing surfactant during the thermal removal of the template. Brønsted groups are identified in the 1H-MAS NMR and PAS-IR spectra at high aluminum concentration.