A Simple, Inexpensive, and Reliable Method for Measuring Brønsted-Acid Site Densities in Solid Acids

A method based on amine decomposition has been developed for measuring the Brønsted-acid site densities using a standard, tubular reactor with an on-line gas chromatograph (GC). After exposing a sample to n-propylamine vapor and flushing it in flowing He at 473 K, the sample was ramped in flowing He to 773 K while trapping the products in the GC sample loop using liquid nitrogen. Site densities were determined from the quantity of propene formed by the decomposition of the n-propylammonium ions via the Hofmann-elimination reaction. The use of small amounts of sample was shown to be important for elimination of secondary products. The method was tested using H-ZSM-5, H-FER, H-MOR, and H-Y zeolites, as well as a γ-Al2O3 sample, and shown to give essentially exact agreement with temperature-programmed-desorption (TPD) and thermogravimetric analysis (TGA) measurements of n-propylamine performed in vacuum