Vermiculites from Brazil and Palabora: Structural changes upon heat treatment and influence on the depolymerization of polystyrene

Abstract

Although clay minerals have been widely used as catalysts and catalyst supports, the reports of natural clay minerals studied in their role as a catalyst are still rare. This work, therefore, for the first time reports the possible catalytic effects of the two raw vermiculites from Brazil (VerS) and Palabora (VerP) on the depolymerization of polystyrene (PS). The main experimental methods used to obtain a detailed structural characterization of vermiculites before and after the depolymerization experiment were X-ray powder diffraction (XRD), thermogravimetric DTA/TG analysis, infrared (FT-IR) and Raman spectroscopy. The detailed assignment of the vibration bands of vermiculites occurring in the near-IR and the analysis of carbon coatings on the surface of vermiculite using Raman spectra are performed for the first time. The thermal products of PS are styrene monomer (SM) about 54.5 mass% and pyrolytic volatile oligomers (dimers and trimers) about 32.1 mass%. The presence of VerS and VerP changes the relation between SM (about 50.7 and 53.6 mass%, respectively) and oligomers (about 37 and 33.3 mass%) due to the pyrolytic carbons deposited on the VerS as coke and on the VerP as glassy carbon. The variable yields of the products can be attributed to the natural vermiculite properties such as are different negative layer charge emerging from the cation substitution in tetrahedra, number of water molecules released during thermal treatment, and textural properties (S-BET and pores).