Fortunato, Michele Emanuele (2025) Bio-lubricants derived from waste vegetable oils. [Tesi di dottorato]

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Abstract

The increasing demand for sustainable and eco-friendly materials in the lubricant industry has led to intensified research on bio-based alternatives, particularly those derived from waste vegetable oils (WVO). As a renewable resource, WVO offers a valuable feedstock for the production of bio-based lubricants, thus addressing environmental concerns associated with conventional, petroleum-based lubricants. Additionally, utilizing waste oils as raw materials fosters a circular economy and, unlike edible and non-edible vegetable oils, does not compete with the food supply chain. A comprehensive overview of the fundamental properties, processing techniques, and environmental advantages of various types of lubricants is presented in Chapter I, with a focus on synthetic lubricants, such as esters, and those obtained from vegetable oils, establishing the framework for developing efficient bio-based lubricants with properties comparable to well-established mineral-based counterparts. The structure-property relationships of a large matrix of synthetized fatty acid alkyl esters, derived from various fatty acids (C9-C18) and alcohols, diols, and polyols are reported in Chapter II. By evaluating key properties for lubricants, such as dynamic viscosity, flow behavior, pour point, and oxidation stability, this chapter investigates how structural features, including chain length, saturation degree, and branching, impact esters performance. The properties of the investigated esters were compared with some commercial lubricants to assess their potential real-world application in the lubricant field. The kinetic and catalytic aspects of the esterification reaction of oleic acid with trimethylolpropane, used as a model system, are addressed in Chapter III. Specifically, batch tests were conducted to study the reaction kinetics in the absence of catalysts, revealing a strong autocatalytic behavior, particularly at high temperatures, due to the acidic nature of oleic acid. A kinetic model was developed, and parameters for the non-catalyzed reaction were derived. Two different types of catalysts, never tested before in this reaction, were evaluated: tungsten trioxide on silica and two sulfonated synthetic resins, also in batch mode. Both catalysts proved to be active in the studied reaction, though stability and mass-transfer limitations issue need to be addressed. The production of esters from used cooking oils (UCO), starting with hydrolysis of UCO, then moving to esterification, and finally, purification to obtain the desired product is described in Chapter IV. The esters were characterized to assess their physicochemical properties and potential as lubricants. Insights from previous chapters guided this study, and the batch process was scaled up from laboratory to pilot scale.

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