Mechanisms of Structure Direction in the Synthesis of Pure-Silica Zeolites. 1. Synthesis of TPA/Si-ZSM-5

Abstract

The role of tetrapropylammonium (TPA) as a structure-directing agent in the sodium-free synthesis of pure-silica ZSM-5 (Si-ZSM-5) is investigated by solid-state ^1H-^(29)Si CP MAS NMR. It has previously been proposed that the mechanism of structure direction in a sodium-containing system 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.l ^1H-^(29)Si CP MAS NMR results indicate that short-range intermolecular interactions, i.e., on the order of van der Waals contact distances, are established during heating of the zeolite synthesis gel prior to the development of long-range order indicative of the ZSM-5 structure. These interactions are independent of the presence of sodium in the gel or the silica source used. An attempt to isolate the composite species by trimethylsilylation of the synthesis mixture was partially successful. These results provide additional evidence for the existence of preorganized, inorganic-organic composite structures during the synthesis of Si-ZSM-5. A modified mechanism of structure direction and crystal growth of Si-ZSM-5 is proposed.