Kinetic Behaviour of Non-uniform Sites of ZSM-5 Catalysts

NH 3 -TPD studies are used to characterise zeolite acidity. Site behaviour (number of site ensembles, and energetics) differs between vacuum- and atmospheric-​NH 3 -TPD profiles. Accurate site behaviour is relevant to the construction of transient microkinetic models, pseudo steady-state kinetic models, and heterogeneity-based descriptions. Four transient microkinetic models of non-uniform sites give activation energies of NH 3 desorption between 132 and 137 kJ mol −1 for vacuum, and 87–157 kJ mol −1 for atmospheric studies. The pseudo steady-state kinetic model with re-adsorption gives heat of adsorption of 132 kJ mol −1 , and a standard entropy change of 140 J mol −1 K −1 for atmospheric studies. Although energy distribution varies, a novel heterogeneity model can unify the number of site ensembles observed across vacuum and atmospheric studies using the intrinsic desorption properties of the active sites. The implications for methanol-to-olefin conversion are discussed.