Fabiola Vergara-Juarez

Objective: To identify the effect of the stabilizer volume/biomass quantity ratio on the extraction and stabilization of lignin contained in coconut husk using propionaldehyde-based thermal pretreatments via acetal formation in the phenolic OH groups of lignin. Design/methodology/approach: Lignin stabilization was studied using propionaldehyde thermal pretreatments, varying the propionaldehyde volume-to-coconut husk mass ratio (9.6, 14.4, and 19.2 mL per 4.5 g of coconut husk) and at an assay temperature of 90 °C. The stabilized lignin was characterized by Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet (UV) Spectroscopy, Thermogravimetric Analysis (TGA), Brunauer–Emmett–Teller (BET) Analysis, and Scanning Electron Microscopy (SEM). Results: Pretreatment with a propionaldehyde volume/coconut husk mass ratio of 19.2 mL/4.5 g resulted in the lowest concentration of total phenolic OH groups (0.330 ±0.088 mmoles g-1) and, consequently, the highest level of acetal groups. FTIR, TGA, BET, and SEM analyses provided additional evidence of lignin extraction and stabilization via the described pretreatments. Limitations on study/implications: The pretreatment employed in this study used propionaldehyde, reported as a stabilizer that prevents lignin β-O-4 bond degradation, thereby improving its chemical reactivity and preventing repolymerization. Findings/conclusions: The FTIR evidence is consistent with the appearance of the acetal group from the removal of hydroxyphenolic groups (Ar-OH), which is a characteristic effect of lignin stabilization by aldehydes. Lignin stabilization offers avenues for the valorization of this type of agro-industrial residue.

The C=C double bond is a typical feature in biodiesel components associated with their physicochemical behaviors, including metal degradation. In this work, copper was exposed to the gas-phase atmospheres of Methyl Hexanoate (MH) at 145 °C and 25% Methyl Trans-3-Hexenoate in Methyl Hexanoate (MT3H in MH) at 158 °C during 1000 h, representing saturated and unsaturated thermal degradation environments of biodiesel surrogates. FTIR, ¹H NMR, and GC-MS were used to characterize the chemical changes in the gas-phase atmospheres, whereas SEM allowed us to inspect the copper surfaces. Weight loss assays enabled the estimation of corrosion rates for copper exposed to HM and MT3H in MH atmospheres of 3.81 ± 1.27 and 5.08 ± 1.27 μm/year, respectively. Electrochemical measurements (linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS)) were used to evaluate the corrosion behavior of copper using aqueous extracts of condensed compounds from gas-phase degraded environments. Our electrochemical results indicate that similar corrosion processes occur in both extracts, remaining nearly unchanged with increasing exposure time. A porous layer of corrosion products on copper revealed that it is more active in the products generated with the MT3H in MH extract, suggesting the significant impact of the C=C bond on copper deterioration.

Biodiesel is a mixture of saturated and unsaturated Fatty Acid Methyl Esters (FAMEs) whose composition affects the corrosion behavior of metal containers during storage. This study examines the effect of the C=C bond present in selected FAMEs (Methyl Stearate, Methyl Oleate, and Methyl Linoleate) in aluminum corrosion in the absence of oxygen. First, mass loss assays were carried out at 100, 200, and 280 °C for 1000 h using pure Methyl Stearate (MS), 5% Methyl Oleate in Methyl Stearate (MS-5% MO), and 5% Methyl Linoleate in Methyl Stearate (MS-5% ML). Next, chemical changes in FAMEs were studied using FTIR, TGA, and GC/MS. SEM/EDS analysis allowed us to inspect the aluminum surfaces and their chemical characterization. We estimated higher corrosion rates for MS assays than those of unsaturated methyl ester mixtures. In a separate set of experiments, we used electrochemical techniques (potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy) to investigate aluminum corrosion induced by thermal-degraded products from FAMEs at 100, 200, and 280 °C for 300 h able to dissolve in aqueous extracts. These electrochemical experiments revealed that the products in the aqueous extracts from the unsaturated methyl ester mixture form a passive layer on the Al surface thicker than pure MS at the corresponding degradation temperatures.