Quantum Mechanical Studies and Molecular Modeling of Lignin Monolignols as Low Carbon Steel Inhibitors in Sulfuric Acid Media Yahmin Yahmin (a*), Siti Khoirunnisa (b), Indra Setiawan (b), Nadia Erlina Mayangsari (b), Nurul Ahsani Alfiah (b)
a) Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Negeri Malang
Jalan Semarang 5 Malang 15145, Indonesia
*yahmin2013[at]gmail.com
b) Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Negeri Malang
Jalan Semarang 5 Malang 15145, Indonesia
Abstract
Computational quantum mechanics research and organic molecular modeling can provide information about the characteristics of organic compounds and the adsorption mechanism of corrosion inhibitors. Lignin extracts from various plant parts were reported to act as corrosion inhibitors and have reduced the corrosion rate (2.51-15.26%) in steel. The aims of this study were (1) to predict the molecular structure of protonated lignin using density functional theory- (2) predict and study the reactivity of lignin species and the interfacial adsorption properties of low carbon steels- and (3) investigated the interaction of neutral and protonated lignin compounds with atoms in H2SO4. Research conducted using this type of computational research. The research design stages were carried out: (1) calculation of the molecular structure optimization of monolignol lignin monomers (p-caumaryl-, coniferyl-, and sinapyl alcohol) as neutral and protonated inhibitor species, and (2) modeling of corrosion inhibitor molecules adsorbed on the surface of low carbon steel in H2SO4 medium. The predicted result of the protonated structure shows the tendency of the H+ proton bounded to the O-phenolic and O-methoxy atom. The lignin monolignol molecule with the adsorbed sulfate ion results in physisorption interactions, while the chemisorption interaction of lignin species occurs through the transfer of heteroatom electrons and the heterocyclic π ring from the inhibitor to the empty d-orbital of Fe in sulfuric acid. Based on the quantum mechanics calculations results, p-coumaryl alcohol molecule has the highest inhibition efficiency value of 23.41%. The results of the MC calculation, enthalpy of inhibitor-Fe 19,673 kcal/mol. While acid-Fe -126,082 kcal/mol shows the function of acid as an adsorption medium is stronger than ionizing Fe. Based on calculation of DFT/B3LYP 6-31G(d,p) the adsorption of p-coumaryl alcohol in H2SO4 has the highest inhibition efficiency with a bond energy of 78,5 kJ/mol.
