Saponites containing divalent transition metal cations in octahedral positions — Exploration of synthesis possibilities using microwave radiation and NMR characterization

•Saponites containing Co2 +, Zn2 + or Cu2 + in octahedral positions have been prepared.•Microwave treatment favored the formation of ordered saponites.•Cu2 +-saponites were formed only if Mg2 + or Ni2 + were octahedral sheet co-cations.•MAS-NMR spectroscopy gave very precise information for Zn-saponite.

The synthesis using microwave radiation of saponites containing Co2+, Zn2+ or Cu2+ in octahedral positions has been explored (in the case of Cu2+, together with Mg2+ or Ni2+). Pure saponite phases were formed under some synthesis conditions, although in other cases some impurities were also formed. Microwave treatment at 180 °C favored the formation of ordered solids, but induced the crystallization of other phases in some cases. Cu2+-containing saponites were formed only if Mg2+ or Ni2+ were used as octahedral sheet co-cations. The solids were characterized by different techniques, including MAS-NMR spectroscopy, which gave very precise information in the case of Zn-saponite, allowing us to estimate the content of tetrahedral Al, but much more undefined information for samples with paramagnetic cations. Nevertheless, the presence of tetrahedral Fe3+ and octahedral Fe2+ species was evidenced. Cu2+ slightly perturbed the NMR signals, while Co2+ strongly perturbed 27Al, 23Na and especially 29Si NMR spectral signals.

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