NMR evidence of different conformations of structure-directing cyclohexylamine in high-doped AlPO4-44 materials

A series of high-crystalline ZnAPSO-44 samples have been successfully synthesised using cyclohexylamine as structure-directing agent, keeping a constant (Si + Zn)/(Al + P) ratio of 0.25 and systematically varying the Si/Zn ratio. Chemical analyses by ICP shows a good correlation between the heteroatom (Si and/or Zn) content of the samples and that of the initial gels. A combination of single crystal and powder X-ray diffraction techniques evidences a high crystallinity and a total purity for all samples. In addition, single crystal X-ray diffraction studies of the samples from which large crystals could be isolated, allow us to certify that either the whole or the majority of the cyclohexylamine molecules adopt a NH2- or NH3+-equatorial conformation inside of the pores. Furthermore, for the first time, 1H MAS NMR spectroscopy was used as a proof of the protonation of SDA molecules within the microporous materials, unequivocally indicating that cyclohexylamine molecules were protonated. 13C MAS NMR studies indeed corroborated such protonation state of the SDA molecules and, more importantly, allowed to detect the presence of a mixture of NH3+-equatorial and NH3+-axial conformers of cyclohexylamine in the Si-richest samples, the former being always the dominant. Based on both 13C and 29Si cross-polarization MAS NMR measurements as well as on the calculated interaction energies of the two conformers as a function of the framework composition, it is proposed that this non-described NH3+-axial conformation of cyclohexylammonium within the microporous materials was associated to the presence of SiO2 islands into the AlPO4 framework, whereas the NH3+-equatorial conformer was the only one found in the case of isolated incorporation of Si4+ (and/or Zn2+) ions.