Mechanistic studies of transformations over zeolite catalysts probed by novel solid-state NMR methods

Since their introduction as cracking catalysts in the sixties, zeolites have played a critical role in the American chemical industry. A primary consideration for the design of improved processes utilizing zeolites is the elucidation of the reaction mechanisms by which transformations occur over these materials. In situ solid state NMR has proven to be a most useful methodology for the study of zeolite chemistry which is largely responsible for our current understanding of zeolites as non-superacidic, shape selective, solid acid catalysts. While in situ methods are mature technologies, current state of the art in situ experimentation suffers from an overdependence on sealed microreactors where the chemistry is studied at long contact times. This does not well represent the conditions of industrial flow reactors, where short contact times and kinetic effects are commonplace. While some flow NMR probes have been developed, their static nature renders them unable to take advantage of magic angle spinning, an important line narrowing technique. The purpose of this research was to develop solid state NMR technologies allowing the study of heterogeneous chemical transformations under flowing conditions and at much shorter time scales then previously reported. The first novel technology presented is a solid state NMR probe which allows for the in situ activation and loading of a zeolite sample under flowing conditions while still allowing MAS. This probe was used to study the adsorption/desorption behavior of organics on HZSM-5. The second novel protocol is a pulse quench reactor: a microreactor which prepares samples for study under flowing conditions and makes provision for the rapid (hundreds of milliseconds) cooling of the sample from reaction temperature in order to probe short reaction time scales. Studies of the transformation of methanol to gasoline showed this process not to proceed consecutively from the initial olefins as commonly proposed, but rather through a hydrocarbon pool mechanism. Acetone condensation was also studied and shown to be a bimolecular process. Cyclopentenyl cations are shown to form in both systems, species which were never observed in the many previous in situ studies carried out in batch mode.