Green synthesis with Aloe Vera of MgAl2O4 substituted by Mn and without calcination treatment

Abstract

Magnesium aluminates (MgAl2O4) with Mn substituting sites A and B were synthesized by the microwave-assisted combustion method applying high power (900 W) using Aloe Vera as a green chelating agent. The study evaluated the effect of the presence of Aloe Vera and the subsequent heat treatment on the structural characteristics of spinel powders by X-ray diffraction (XRD). The effect of heat treatment was evaluated in two ways: with and without calcination. The results showed that the substitutes occurred forming the following phases: Mg0.21Mn2.36Al0.43O4, Mg0.13Mn2.63Al0.25O4, Mg0.31Mn2.06Al0.63O4 substituting site A; and MgMn1.88Al0.13O4 and MgMn1.75Al0.25O4 substituting site B. The presence of Mn and Aloe Vera ions affected the spinel crystal structure. The crystallinity degree is less intense when the magnesium aluminate was substituted by the Mn ion due to the amount of deformations in the crystal system induced by the substituent ion. However, the calcination of the powders increased the crystallinity degree in all scenarios. Furthermore, the substitution caused alteration in the lattice parameters due to the difference between the ionic radius of the substituent and the Mg2+ or Al3+ ion evidenced in the positions of 2θ. The spinel cubic phase was found in most materials, although the tetrahedral phase was observed in some Mn-substituted structures. The crystallite sizes of the powders were influenced by the presence of phytochemicals present in Aloe Vera. A decrease in crystallite size was observed when the materials were calcined as a result of the gasification of biomolecules from the plant extract. However, these biomolecules help in dissociating precursors, albeit slowly, which prevented crystallite growth. The use of Aloe Vera as a chelating agent proved to be efficient in the synthesis of magnesium aluminate spinels, pointing out the synthesized material as a promising route for the green synthesis field.