A Multi-Nuclear MAS-NMR Study on the Structural Properties of Silicalite-1 Zeolite Synthesized Using N- and P-Based Organic Structure Directing Agents

The nature of organic structure directing agents (OSDAs) is of paramount importance in the final properties of zeolites, particularly the framework and porosity. Recently, the use of P-containing OSDAs has been employed for new zeolites, but there is little discussion compared to their analogues N-OSDAs. The main objective of this work is the characterization of pure silica MFI zeolite (silicalite-1) prepared by the dual-template route with tetrapropylammonium (TPA), tetrapropylphosphonium (TPP) cations, and mixtures thereof aiming to understand by advanced NMR methods how the nature of the organic influences the physico-chemical properties of the zeolite. Silicalite-1 has been successfully synthesized using the dual-template procedure with TPA and TPP molecules. Both OSDAs are incorporated into the zeolite without any specific preference, differently to that observed before for the TEA/TEP system, and homogenously mixed inside of the zeolite voids. The presence of TPP leads to the incorporation of less F, raising the concentration of Q(3)-defective sites in the silicalite-1 zeolites. Detailed NMR results indicate that those structural defects are close to the -CH3 group of the entrapped OSDAs in the zeolite and these defects consist of at least two silanol groups stabilizing the Si-O- species, which is responsible for the charge balancing.

Automated summary

The nature of organic structure directing agents plays a crucial role in determining the final properties of zeolites. The incorporation of tetrapropylphosphonium (TPP) as an OSDA in silicalite-1 zeolites leads to the formation of more defective sites compared to tetrapropylammonium (TPA). Advanced NMR techniques reveal that the presence of TPP influences the physico-chemical properties of zeolites by causing the formation of specific structural defects near the entrapped OSDA molecules.