A. Liepold et al., TEXTURAL, STRUCTURAL AND ACID PROPERTIES OF A CATALYTICALLY ACTIVE MESOPOROUS ALUMINOSILICATE MCM-41, Journal of the Chemical Society. Faraday transactions, 92(22), 1996, pp. 4623-4629
Citations number
34
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
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.