Internal Silanol Research Articles

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

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

Spectroscopic evidence for the presence of internal silanols in highly siliceous ZSM-5

FTi.r. and MAS n.m.r. evidence is presented which establishes the presence of internal silanols in ZSM-5. The concentration of internal silanols is shown to increase as aluminium content decreases and they are believed to be responsible for the phenomenon of excess ion exchange previously reported.