Investigation of internal silanol groups as structural defects in ZSM-5-type zeolites

Abstract

A ¹H Magic-angle spinning nuclear magnetic resonance investigation of large and uniform crystals of HZSM-5 type zeolites, synthesized either by means of TPA or without organic templates, shows that using TPA leads to a high concentration of defect centres within the ZSM-5 structure. Owing to non-intact Si—O—Si bonds, up to 8% of the lattice Si were found to be present as Si—OH groups. A comparison of the ¹H—²⁹Si cross-polarization spectra with the variation of the ¹H n.m.r. chemical shift of the Si—OH groups as well as with the results of ion-exchange experiments suggests a vicinal arrangement of the silanol groups. High concentrations of such structural defects within the crystals lead to a slight decrease of crystal sorption capacity for n-hexane and significantly reduce crystal hydrophobicity. Both the intracrystalline self-diffusion and the molecular reorientation of sorbed molecules as measured by means of the ¹H n.m.r. pulsed field gradient technique and ¹³C n.m.r. lineshape analysis, respectively, show an enhanced molecular mobility of guest molecules within the defective ZSM-5 structure