Chromium pillared montmorillonite

SCHEME 119. Selective oxidation of alcohols using a chromium-pillared montmorillonite catalyst and TBHP... 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

Chromia pillared montmorillonite with 1.0 -1.1 nm gallery height and a high specific surface area has been prepared successfully by using an elevated temperature (368 K) for the formation of hydroxy-chromium polycations [2]. This catalyst possesses significant activity in the hydrocracking of n-decane. Previously, we synthesized... 

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