Experimental study of the effect of Si/Al composition on the aluminum distribution in (Al)MCM-41

The effect of framework Al content on the number of framework Al atoms on the surface of (Al)MCM-41 channels and the distribution of these Al atoms among single Al atoms and various Al-Si-Al and Al-Si-Si-Al pairs was investigated. The number of framework Al atoms on the wall surface was characterized by the ion exchange capacity for Na- ions, number of Al pairs on the surface by exchange capacity for Co2+ ions. Two different geometrical arrangements of Al pairs were characterized by VIS spectroscopy of bare Co2+ ions. At low framework Al content, Al atoms occupy only T sites on the surface of the channels. In aluminum rich (Al)MCM-41, Al atoms are incorporated also into the inner volume of the channel walls and at Si/Al similar to 11 only about 50% of Al atoms are on the channels surface. Al atoms on the wall surface preferentially form Al-Si-1.2-Al pairs, enabling siting of divalent cations. Number of single Al atoms increases with framework Al content and overcome 50% of surface Al atoms for Si/Al similar to 11. The Al-Si-1.2-Al pairs are present in two cationic sites and also the distribution of Al pairs into these sites depends on the framework Al content. (C) 2001 Elsevier Science B.V. All rights reserved.