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 t he surface of (Al)MCM-41 channels and the distribution of these Al atoms am ong single Al atoms and various Al-Si-Al and Al-Si-Si-Al pairs was investig ated. The number of framework Al atoms on the wall surface was characterize d by the ion exchange capacity for Na- ions, number of Al pairs on the surf ace by exchange capacity for Co2+ ions. Two different geometrical arrangeme nts of Al pairs were characterized by VIS spectroscopy of bare Co2+ ions. A t 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 i nto the inner volume of the channel walls and at Si/Al similar to 11 only a bout 50% of Al atoms are on the channels surface. Al atoms on the wall surf ace preferentially form Al-Si-1.2-Al pairs, enabling siting of divalent cat ions. Number of single Al atoms increases with framework Al content and ove rcome 50% of surface Al atoms for Si/Al similar to 11. The Al-Si-1.2-Al pai rs 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 Sci ence B.V. All rights reserved.