Acetaldehyde as a probe for the chemical properties of aluminophosphate molecular sieves. An in situ FT-IR study

Loading of acetaldehyde from the gas phase into transition metal substitute d aluminophosphate sieves of AFI structure resulted in mainly physisorbed C H3CH=O in the case of FeAlPO4-5. By contrast, FT-IR spectroscopy revealed e xtensive chemical reaction upon loading into metal-free AlPO4-5 and CoAlPO4 -5 materials at temperatures as low as - 100 degrees C. The low temperature product is identified as acetaldol, which dehydrates to crotonaldehyde upo n warm-up to room temperature. The reactive sites are proposed to be Lewis acid sites in the aluminophosphate framework. Using FT-IR monitoring of ace taldehyde in the pores of aluminophosphate sieves as a sensitive method for the detection of Lewis acid sites, we find that their abundance varies sub stantially with metal substitution. Interestingly, isomorphous Fe substitut ion of the aluminophosphate framework seems to suppress the formation of su ch defect sites. (C) 2000 Elsevier Science B.V. All rights reserved.