Mechanism of Structure Direction in the Synthesis of Si-ZSM-5: An Investigation by Intermolecular ^1H-^(29)Si CP MAS NMR

Abstract

The role of tetrapropylammonium (TPA) ions as a structure-directing agent in the synthesis of pure-silica ZSM-5 (Si-ZSM-5) is investigated by solid-state NMR. It has previously been proposed that the mechanism of structure direction involves preorganization of silicate species around the TPA ions with subsequent assembly of these inorganic-organic composite structures to yield crystalline Si-ZSM-5 with occluded TPA molecules. However, such structures have not been directly observed. Here, ^1H-^(29)Si CP MAS NMR is performed between the protons of TPA and the silicon atoms of the zeolite framework precursors in a deuterated synthesis medium to probe the relationship between the organic and inorganic components. The ^1H-^(29)Si CP MAS NMR results indicate that short-range intermolecular interactions, i.e., on the order of van der Waals interactions (≈3.3 Å for H to Si), are established during the heating of the zeolite synthesis gel prior to the development of long-range order indicative of the ZSM-5 structure. ^1H-^(13)C CP MAS NMR spectra suggest that the TPA molecules within the composite inorganic-organic structures adopt a conformation similar to that which they have in the zeolite product. These results thus provide the first direct evidence for the existence of preorganized inorganic-organic composite structures during the synthesis of Si-ZSM-5. They also demonstrate the types of noncovalent intermolecular interactions that are relevant to the existence of an organic structure-directing effect. A modified mechanism of structure direction and crystal growth of Si-ZSM-5 is proposed.