TEMPERATURE-PROGRAMMED DESORPTION AND CONVERSION OF ACETONE AND DIETHYL KETONE PREADSORBED ON HZSM-5

The formation of surface complexes during the temperature-programmed desorption, accompanied by conversion, of preadsorbed acetone, acetone-d6, diacetone alcohol, mesityl oxide, and diethyl ketone on HZSM-5 was studied using FTi.r. spectroscopy. The reaction products evolved were analyzed by mass spectrometry. It was shown that acetone is converted at low temperatures by zeolitic-bridging hydroxyls to mesityl oxide. Mesityl oxide undergoes further condensation and cracking between 150-degrees and 250-degrees-C, releasing mainly isobutene into the gaseous phase and forming unsaturated cyclic ketones that replace skeletal OH groups. These new surface ketones are thermally stable and act as catalytic poison. Diethyl ketone does not form analogous stable surface complexes and reacts predominantly via intramolecular dehydration to pentadiene that is released into the gaseous phase. Other hydrocarbons observed during the conversion of both acetone and diethyl ketone are most probably formed via the secondary reactions of isobutene and pentadiene, respectively.