Sulfur radicals embedded in various cages of ultramarine analogs prepared from zeolites

The colored (greenish) products analogous to ultramarine obtained from zeolite A, cancrinite and erionite by means of high-temperature (500–800 °C) treatment of zeolites mixed with elemental sulfur and alkalis can maintain the original structure of the parent zeolite under certain preparation conditions. The ESR spectra of samples recorded at room temperature always indicate the presence of radicals S3− (the blue chromophore). The spectra of samples prepared from zeolites recorded at 77 K exhibit additional signals that could be assigned to the yellow chromophore (the radical S2−) encapsulated inside the β or ε-cages. It is remarkable that in the case of products with preserved cancrinite structure, where the S2− radicals are encapsulated inside the ε-cages, the above signals are visible already at room temperature. It may result from retarded mobility of these species in small cancrinite cages.

Graphical AbstractStructures of SOD, LTA, cancrinite and erionite.Figure optionsDownload full-size imageDownload as PowerPoint slide