The stability of chromium in CrAPO-5, CrAPO-11, and CrS-1 during liquid phase oxidations

摘要

Chromium substituted aluminophosphate-5 (CrAPO-5), aluminophosphate-11 (CrAPO-11), and silicalite-1 (CrS-1) were synthesized, characterized, and investigated with regard to their stability under the conditions of liquid phase oxidations with tert-butyl hydroperoxide (TBHP). Experimental evidence is presented that these reactions did not take place in the micropores or at the outer surface of these catalysts but were homogeneously catalyzed by small amounts of leached chromium, The amount of chromium leached is related to the crystal size of the catalyst In the case of the large CrAPO-5 crystals only 0.3% of its chromium is leached during the reaction, which corresponds to a subskate/Cr ratio of 17.000. Nevertheless, this very small amount is responsible for the observed catalysis. The necessity for catalyst filtration at the reaction temperature in catalyst recycling experiments is emphasized. Moreover, previous results purported to be consisted with the notion that the reaction takes place in the micropores are reinterpreted in the light of the above results. (C) 1998 Academic Press. [References: 29]