Textural, structural and acid properties of a catalytically active mesoporous aluminosilicate MCM-41

Abstract

As-prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterised by N2 adsorption/desorption at 77 K, 1H, 27Al and 29Si MAS NMR as well as by temperature-programmed ammonia desorption (TPAD) with in situ FTIR. 29Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, suggesting a broad range of Si—O—Si bond angles in this material. The obtained results from 1H MAS NMR and a combination of FTIR and TPAD investigations indicate that the washed, template-free MCM-41 and its protonic form behave like a solid-state acid with a broad acid strength distribution. With regard to the presence of bridging hydroxy groups in the investigated MCM-41 materials, the findings of these two methods are not consistent. Therefore, it is assumed that the broad absorption between 3650 and 3400 cm–1 in the FTIR spectra is connected with hydrogen-bonded vicinal silanol pairs. A signal in the 27Al MAS NMR spectra at 40 ppm is identified as a separate resonance, assigned preferentially to an isolated extranetwork tetrahedral aluminium species, whereby the assignment to a pentahedrally coordinated network aluminium species is also considered. When calcined, these species are reinserted into network positions. Subsequent catalytic testing leads to partial recovery of the former resonance. The results of the catalytic testing imply that the weak Bronsted acid sites interact with neighbouring coordinatively unsaturated aluminium species and that synergistically stronger acid sites form.