Thesis

A novel titania–silica catalyst for converting methyl lactate to lactide towards a circular economy for PLA

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Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T17979
Person Identifier (Local)
  • 202066857
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • In modern society, plastics have become indispensable owing to their durability, high strength, flexibility, and low production cost. Among all plastics, polylactic acid (PLA) is attractive for its biodegradable origin and favourable mechanical properties, yet the lack of efficient closed-loop recycling constrains its wider deployment. Chemical recycling of waste PLA into lactide (LD) accelerates degradation and conserves resources with its critical step is LD production from monomer methyl lactate (MLA) which is the product of PLA alcoholysis. LD production has relied on an energy-intensive one-step gas phase transesterification process, with Titania–silica catalysts identified as delivering the best performance. However, the yields remain relatively low (below 42%). Accordingly, this work focuses on developing an alternative one-step liquid-phase transesterification process of MLA to LD employing Titania–silica catalysts. This one-step LD production process was investigated from cross-material benchmarking, mechanistic understanding, experimental testing, and economic assessment aspects, and it advances a blueprint for circular PLA. Firstly, a decision-grade dataset spanning 17 widely used polymers was compared and analysed across mechanical, thermal, chemical, and environmental metrics. The findings further confirm that PLA serves as an excellent model polymer for circular-economy studies, as it uniquely combines bio-based and biodegradable features, market relevance, strong tensile properties, and tractable recycling pathways. A one-pot Ti-based catalytic system for the conversion of MLA to LD was experimentally investigated. It was concluded that TiOx/SiO₂ outperformed anatase TiO₂ across 170–220 °C, reaching 62 % conversion and 49 % LD selectivity at 220 °C, whereas anatase remained below 35 % conversion and 1 % selectivity. XPS detected ~6 % Ti(III) and STEM-EDS confirmed atomic dispersion. This supports the conclusion that Ti(III) sites and silica support govern MLA→LD efficiency, positioning TiOx/SiO₂ as the preferred catalyst. Subsequently, the complete reaction pathway for the transesterification of MLA to LD over the Titania–silica catalyst was explored, accompanied by transition state searching to identify the key intermediates and activation energies. The computed free-energy profile features a feasible barrier for dimerization (𝛥𝛥𝛥𝛥≠ = 0.63 eV) and a very high ring-closure barrier to LD (𝛥𝛥𝛥𝛥≠ = 1.82 eV), identifying cyclisation as the rate-limiting step and predicting release of two methanol equivalents. Based on the findings from the experiment work, the catalyst-support performance was further evaluated by investigating MLA adsorption on various catalyst surfaces adopting Density Functional Theory (DFT) modelling. The influence of catalyst metal valence states and support on the reaction was explored. It has been found that Ti(III)–O–Si sites on silica maximize MLA binding (adsorption energy around −1.69 eV) via synergistic hydrogen bonding, Ti–O σ-bonding, and π-backbonding, whereas TiO₂-based sites suffer electron leakage into subsurface layers and weaker adsorption (−0.77 eV). Finally, three feasible PLA recycling routes were evaluated at plant-scale using Techno-economic Analysis (TEA) in Aspen Plus. An optimized pathway leveraging TiO₂/SiO₂ catalysis increased annual PLA output from 5.96 × 10⁴ to 9.04 × 10⁴ tonne per year (+59.87 %) while reducing production cost by 22.87 % to US $ 1773 per tonne. Sensitivity analysis identified catalyst loading, feed rate, and raw-material prices as dominant cost drivers. Overall, this work provides a sustainable and economically viable solution for recycling waste PLA, aligning with growing demand of the plastics industry for environmentally friendly alternatives. The findings are crucial for advancing a circular plastics economy and highlight the importance of catalyst optimization in improving the efficiency and sustainability of PLA recycling processes.
Advisor / supervisor
  • Zhang, Xiaolei
  • Li, Jun
Resource Type
DOI
Date Created
  • 2025

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