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

Abstract

[EN] 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.