The combination of well-defined acid sites, shape selective properties and outstanding stability place zeolites among the heterogeneous catalysts of the greatest practical relevance. The development of structure-performance descriptors for processes catalysed by these solids has been a matter of intense debate, both in industry and academia. The direct conversion of methanol-to-olefins is a prototype where various catalytic functions contribute to the overall performance. Propylene selectivity and resistance to coking are the two most important parameters in developing new MTO catalysts. Here, we present a systematic investigation on the effect of acidity on the performance of ZSM-5 for the production of propylene. Our results demonstrate that isolation of Brønsted acid sites is key to selectively form propylene. Simultaneously, the introduction of Lewis acid sites prevents the formation of coke, hence drastically increasing catalyst lifetime. Our insights provide conceptually new information into the tunability of active sites in zeolite acid-catalysed reactions.