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

Joaquin Martinez-Ortigosa,Jose A Vidal-Moya,Fernando Rey,Jorge Simancas,Teresa Blasco,Susana Valencia

doi:10.3390/app11156850

Abstract

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 Q3-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.

Highlights

Accepted: 21 July 2021Zeolites are crystalline microporous materials typically in aluminosilicate composition whose structures are formed by silicon and aluminum tetrahedra linked by corner-sharing oxygen atoms
We extend the synthesis of silicalite-1 using tetrapropylammonium (TPA) and tetrapropylphosphonium (TPP) cations and mixtures thereof as organic structure directing agents (OSDAs), since TPA is the most used organic molecule for the synthesis of MFI
The shifts of the TPA peak towards higher temperatures and of the TPP peak towards lower temperatures suggests the coexistence of both TPA and TPP inside the ZSM-5 channels. These results allow concluding that the synthesis of silicalite-1 zeolites using the dual-template procedure with TPA and TPP cations is feasible and the crystal size of the zeolites decreases when phosphorous agents are used under the synthesis conditions studied

Summary

Introduction

Zeolites are crystalline microporous materials typically in aluminosilicate composition whose structures are formed by silicon and aluminum tetrahedra linked by corner-sharing oxygen atoms. These tridimensional structures generate channels and cavities of molecular dimensions leading to materials largely utilized in industrial applications, such as catalysis, adsorption, and ion exchange, among others [1,2,3]. Siliceous zeolites are neutral materials, but the presence of Al in the framework generates a negative charge that must be compensated by cations that can be inorganic, organic, or protons. Acidic sites are generated, and this is one of the main properties of zeolites that allows their application as catalysts in acid catalyzed processes [10,11,12,13,14]

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