Turning abundant waste sulfur to polymers for manufacturing: Exploiting role of organic crosslinkers and benign catalysts

Abundant source of sulfur produced as byproduct of refineries demands sustainable valorization solutions. Use of organic crosslinkers and benign catalysts to polymerize sulfur for use in manufacturing offers a sustainable value-added application. This paper introduces MgO as an effective benign catalyst and oleic acid as an organic crosslinker for inverse vulcanization of sulfur. We further compare efficacy of MgO with other metal catalysts (ZnO and CaO) using molecular modeling and laboratory experiments. ZnO is commonly used in elastomeric formulations; however, industries try hard to keep the ZnO level low because of its environmental impact, particularly on aquatic organisms, and for economic reasons. Our rheological characterization shows the inverse vulcanization is enhanced when MgO is present as evidenced by the increase of viscosity of sulfur-oleic acid blends by 94 % relative to the blend without catalyst and 39 % relative to the blend with ZnO catalyst. Based on our quantum-based molecular modeling, using density functional theory (DFT), coordination complexes of Mg oleate (Mg (RCOO)₂), made from the reaction between oleic acid and MgO, are energetically better stabilized compared to Zn or Ca oleate. Stronger interactions in Mg oleate, leading to the more stabilized coordination complexes, could be a reason for higher viscosity for MgO/sulfur-oleic acid compared to ZnO/sulfur-oleic acid. In complexes of sulfur and metal oleate, regardless of the electronic nature of sulfur chains (ionic or radical form), coordination complexes containing MgO show binding energies that are higher than those of Zn complexes and notably higher than those of Ca complexes, indicating capability of Mg substitution in Zn oleate compounds as catalysts. Studying the addition reactions of sulfur radicals to oleic acid and the formation of C[sbnd]S bonds, using DFT modeling, shows the superiority of allylic ([sbnd]CH[dbnd]CH[sbnd]ĊH) sites of oleic acid over olefinic ([sbnd]C[dbnd]C[sbnd]) sites in reactions of sulfur radicals. Compared to sulfur radicals, sulfur chains in ionic character provide higher binding energies with metal oleates because of the electrostatic interactions between the positively charged metal and negative sulfur atoms. The study outcomes support sustainability and resource conservation in chemical manufacturing and petroleum refining industry.