CHARACTERIZATION OF MG AND MN SUBSTITUTION IN ALPO4-5

We have examined the framework substitution of Mg and Mn into the A1PO4-5 structure by studying the adsorption of reactive probe molecules, including ethylamine, isopropylamine, 2-propanol, and propene. Lattice parameters from X-ray diffraction suggests that there is a maximum framework substitution of 5 mol% for both metals. However, samples with 5 mol% Mg or Mn appear to contain large concentrations of defects in the structure, based on the observation of slow adsorption of branched alkanes. In simultaneous temperature-programmed-desorption (TPD)/ thermogravimetric-analysis (TGA) measurements, both amines reacted to an olefin and ammonia in a well-defined decomposition feature between 575 and 650 K for isopropylamine and between 625 and 675 K for ethylamine. By comparison wth similar results on high-silica zeolites, the sites which decompose the amines are known to be Brønsted acid sites and decomposition occurs through an alkylammonium ion intermediate. For low concentrations of Mn and Mg (<1 mol%), the moles of isopropylamine and ethylamine which decompose in TPD-TGA are the same and equal to the moles of substituted metal atoms, indicating that the metals substitute for the A1 in the framework to form Brønsted-acid sites. These same sites also catalyze the dehydration of 2-propanol above 400 K and the oligomerization of propene at 300 K. © 1992.