Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Collaborative Specialization

Environment and Sustainability

Supervisor

Amarjeet S Bassi

2nd Supervisor

Charles Chunbao XU

Affiliation

School of Energy and Environment City University of Hong Kong

Joint Supervisor

Abstract

The promise of vegetable-based CEs in the lubrication industry lies in their ability to provide superior lubrication, thermal stability, and compatibility with various metals. The presence of unsaturated free fatty acids in the carbon chain of triglycerides and tertiary β-hydrogen group in the glycerol backbone causes low thermal and oxidative stability. Thus, this study investigated a cost-effective methodology that has been developed to produce biobased CEs with improved thermal and oxidative stability. This involves the conversion of vegetable oil triglycerides into trimethylolpropane (TMP) esters through a two-step transesterification-transesterification and a two-step hydrolysis-esterification process. To further increase the thermal and oxidative stability, an epoxidation reaction was performed in order to remove the C=C double bonds in the canola oil trimethylolpropane (COTMP) ester structure using tert-butyl hydroperoxide (TBHP) as an oxidizing agent in the presence of a heterogeneous catalyst. The conversion, selectivity, oxirane oxygen content (OOC), and iodine value of the final epoxidized canola oil trimethylolpropane (ECOTMP) ester was 95.2%, 98.62%, 4.12%, and 3.4 mg I2/g, respectively.

This work has focused on the conversion of vegetable oils to saturated complex esters through several chemical modifications (e.g., hydrolysis, esterification/transesterification, and epoxidation) and demonstrated the effective use of metal complexes as heterogeneous catalysts in the epoxidation reaction. The novelty of this study is the formulation of biolubricants with outstanding wear scar diameter (WSD) from vegetable oils and the development of a heterogeneous catalyst for epoxidation reaction to increasing the conversion, selectivity, and OOC of the final product.

Summary for Lay Audience

As concerns about sustainability grow and the bioeconomy gains traction, biobased complex esters (CEs) are regaining attention for their renewable, biodegradable, and environmentally friendly properties. These CEs show great promise in the lubrication industry, offering superior lubrication, thermal stability, and compatibility with various metals. They are poised to meet the rigorous demands of modern machining and forming processes.

However, traditional biobased CEs often suffer from low thermal and oxidative stability due to the presence of unsaturated fatty acids in triglycerides. To address this, a study explored a cost-effective method to produce biobased CEs with improved stability. The process involves converting vegetable oil triglycerides into trimethylolpropane (TMP) esters through two-step transesterification-transesterification or a novel two-step hydrolysis-esterification process. Additionally, an epoxidation reaction is used to remove double bonds in the ester structure, enhancing stability.

The resulting epoxidized canola oil trimethylolpropane (ECOTMP) ester showed high conversion, selectivity, oxirane oxygen content (OOC), and reduced iodine value, indicating improved stability. The study focused on transforming vegetable oils into saturated complex esters through chemical modifications, such as hydrolysis, esterification/transesterification, and epoxidation. It also demonstrated the effective use of metal complexes as heterogeneous catalysts in the epoxidation reaction.

This research not only develops biolubricants with exceptional wear scar diameter (WSD) from vegetable oils but also introduces a novel heterogeneous catalyst to enhance conversion, selectivity, and OOC in the final product. Overall, this work contributes to the advancement of sustainable lubrication solutions, leveraging abundant vegetable oil resources and innovative chemical processes to meet the evolving needs of the industry while minimizing environmental impact.

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