Synthesis of yellow and green pigments of zeolite LTA structure: Identification of their chromophores

Abstract It is shown that sodium polysulfides can be inserted into zeolite LTA when a mixture of dried zeolite with anhydrous sodium sulfide and sulfur is heated under vacuum. A yellow pigment is obtained when this mixture is heated in the temperature range of 280–300 °C, while a green pigment is obtained after heating at ca . 450 °C. Structural modifications of the Na-A zeolite are observed by XRD and IR spectroscopy. The inserted chemical species are clearly identified by the use of Raman and diffuse reflectance UV–vis spectroscopies. It is shown that the yellow chromophore is S 6 2 - . It is also shown that the color of the green pigment must be assigned to the contributions of the yellow chromophore S 6 2 - and of the well known blue chromophore S 3 - . It is emphasized that the encapsulation inside zeolite LTA voids allows the stabilization of Na 2 S 6 , which is not observed as a solid polysulfide.