Method for distinguishing Brønsted-acid sites in mixtures of H-ZSM-5, H-Y and silica-alumina

We have examined the adsorption and reaction of ethylamine, isopropylamine, 2-ethylhexylamine, cyclooctylamine, and diphenylethylamine on H-ZSM-5, H-Y, and silica-alumina samples using temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA). Each of the amines decomposes at Brønsted-acid sites to alkene and ammonia products above 550 K by a reaction similar to the Hofmann elimination reaction, so that the number of molecules which decompose can be used to measure the number of acid sites in the sample. For H-ZSM-5, the concentration of sites determined using ethylamine and isopropylamine was approximately equal to the framework aluminum concentration; however, adsorption of cyclooctylamine and 2-ethylhexylamine in H-ZSM-5 was limited so that only a fraction of the sites were counted using these probe molecules. For the silica-alumina and H-Y samples, the concentrations of Brønsted-acid sites could be determined using ethylamine, isopropylamine, 2-ethylhexylamine, or cyclooctylamine; and the site concentrations determined in this way were essentially dependent of which amine was used. 2,2-Diphenylethylamine adsorption was limited on both silica-alumina and H-Y samples, but a much higher fraction of the Brønsted-acid sites on the silicaalumina sample could be counted. These results indicate that TPD-TGA measurements with various amines can be used to determine the concentration of Brønsted-acid sites in each of the components of a fluid-catalytic-cracking catalyst.