Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum Activation

Incorporating aluminum is the most widely applied and industrially relevant method to functionalize amorphous silica. However, established protocols yield predominately poorly distributed and inaccessible Al species, and as a result only similar to 10-15% of the present aluminum gives rise to the acid sites, hampering the overall catalytic potential. Herein, the influence of alkaline activations with aqueous NaOH and NH4OH on the porosity, acidity, and catalytic properties of Al-MCM-41 is studied. By performing room temperature activations in 0.01-0.1 M NaOH or 0.5 M NH4OH, the Ostwald ripening of silica in alkaline media is exploited, which results in high mass retention yields (100-74%) and a controlled transformation of the 3.6 nm mesopores of the parent material to a broad pore range from 3 to similar to 12 nm. Electron microscopy indicates the presence of additional interconnected intraparticle porosity, whereas no significant change in the shape and size of the original particles is observed. Elemental analysis reveals that the optimal alkaline activation with 0.05 M NaOH leads to a decrease in the Si/Al ratio at the surface, despite an increase in the bulk Si/Al ratio. Al-27 magic angle spinning nuclear magnetic resonance spectroscopy demonstrates a large conversion of octahedral Al into tetrahedral Al, doubling the purely tetrahedral fraction from 30 to 60%. Pyridine-probed Fourier transformed infrared spectroscopy shows a doubling of the Bronsted and Lewis acidity after activation. The compositional and spectroscopic results are ratified by monitoring the relative accessibility of the acid sites, i.e., effective acidity (mol acid sites per mol Al). The alkaline activation enhances the effective acidity by increasing access to the Al sites trapped inside the pore wall and by reincorporation of the octahedral Al as accessible tetrahedral sites. As a result, an unprecedented effective acidity is obtained after the Al incorporation, which is substantiated using a novel accessibility concept. The catalytic potential of the activation protocol is demonstrated by quadrupling the catalytic activity for the acid-catalyzed alkylation of toluene with benzyl alcohol, an over-50% activity gain, a slightly enhanced selectivity, and a strongly reduced coking in the acid-catalyzed coupling of furfural with sylvan.