TEXTURAL, STRUCTURAL AND ACID PROPERTIES OF A CATALYTICALLY ACTIVE MESOPOROUS ALUMINOSILICATE MCM-41

As-prepared, calcined and catalytically tested MCM-41 materials have b een comprehensively characterised by N-2 adsorption/ desorption at 77 K, H-1, Al-27 and Si-29 MAS NMR as well as by temperature-programmed a mmonia desorption (TPAD) with in situ FTIR. Si-29 MAS NMR spectra of M CM-41 closely resemble those of amorphous silica, suggesting a broad r ange of Si-O-Si bond angles in this material. The obtained results fro m H-1 MAS NMR and a combination of FTIR and TPAD investigations indica te 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 vici nal silanol pairs. A signal in the Al-27 MAS NMR spectra at 40 ppm is identified as a separate resonance, assigned preferentially to an isol ated extranetwork tetrahedral aluminium species, whereby the assignmen t to a pentahedrally coordinated network aluminium species is also con sidered. When calcined, these species are reinserted into network posi tions. Subsequent catalytic testing leads to partial recovery of the f ormer resonance. The results of the catalytic testing imply that the w eak Bronsted acid sites interact with neighbouring coordinatively unsa turated aluminium species and that synergistically stronger acid sites form.