Mohd Azlan, Nadiah Syafiqah
(2025)
One-pot microwave conversion of oil palm mesocarp fibre to levulinic acid using lignin-derived cryogel incorporated with phosphotungstic acid-niobium (V) oxide.
PhD thesis, University of Nottingham, Malaysia.
Abstract
Levulinic acid is an important platform chemical with wide applications, and it can be produced from lignocellulosic biomass. In this study, a finely tuned method was developed to produce levulinic acid from oil palm mesocarp fibre using microwave-assisted synthesis in a lignin-derived carbon cryogel incorporated with phosphotungstic acid and niobium oxide, coupled with a gamma-valerolactone/water biphasic system. This catalytic system addresses challenges in producing levulinic acid from recalcitrant biomass, which can hinder the cellulose hydrolysis process due to the low selectivity of solid bifunctional acid catalysts in breaking down the tightly packed crystalline cellulose structure. These catalysts are often easily deactivated, difficult to recycle, and have poor thermal tolerance. The biphasic gamma-valerolactone/water system efficiently removed 95% lignin and 55% hemicellulose, making the cellulose substrate available for the conversion reaction. Meticulously tailored carbon cryogels, synthesised from lignin and xanthan gum crosslinked hydrogel under gentle sol-gel-hydrothermal conditions, offered desirable properties including a swelling ratio of 10.3 g g-1, compressive strength of 0.82 MPa at 90% strain, crosslinking yield of 71.7%, surface area of 57.41 m2 g-1, and porosity of 0.13 cm3 g-1. These cryogels, incorporated with phosphotungstic acid and niobium oxide, provided bifunctional Brønsted-Lowry and Lewis acidity at a ratio of 1.48 and acid sites of 12.50 mmol g-1, fostering a synergistic effect in catalysing tandem reactions with a synergy factor of 2.15. The cryogel catalyst exhibited a balanced distribution of micropores, mesopores, and macropores, enhancing thermal conductivity and allowing the diffusion of biomass particles. This resulted in an impressive conversion of oil palm mesocarp fibre to levulinic acid with 97.4% efficiency in a one-pot conversion. The final levulinic acid yield surged to 22.1% under optimal parameters for microwave power (388 W), catalyst loading (0.13 g), biomass loading (0.07 g), and the gamma-valerolactone to water ratio (8.41), approaching the theoretical maximum of 22.7%. The cryogel catalyst reduced energy requirements, resulting in low activation energies of 98.7 kJ mol-1 for cellulose conversion to glucose and 62.3 kJ mol-1 for glucose conversion to levulinic acid. The predominant conversion pathway presumably involves the direct conversion of glucose to levulinic acid, bypassing the HMF intermediate. The carbon cryogel catalyst exhibited robustness with an 85% efficiency after four cycles, declining to 52% by the eighth cycle, due to humin formation that may persist. The production cost of the cryogel catalyst is estimated at RM 5.23 per gram. This study proposes a technically and economically feasible, sustainable production of levulinic acid from biomass with potential for scale-up. Future work could focus on optimising the catalyst's cost reduction for scaling-up production, exploring alternative humin utilisation to upgrade by-products to value-added products in addition to levulinic acid production, and conducting computational chemistry to understand and optimise the microenvironments around active catalytic sites for better performance.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Yap, Chiew Lin
Gan, Suyin
Abdul Rahman, Mohd Basyaruddin |
| Keywords: |
lignin cryogel; phosphotungstic acid; niobium oxide; biomass conversion; levulinic acid; microwave-assisted |
| Subjects: |
T Technology > TP Chemical technology |
| Faculties/Schools: |
University of Nottingham, Malaysia > Faculty of Science and Engineering — Engineering > Department of Chemical and Environmental Engineering |
| Item ID: |
80326 |
| Depositing User: |
Mohd Azlan, Nadiah
|
| Date Deposited: |
05 Feb 2025 06:11 |
| Last Modified: |
13 Nov 2025 05:47 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/80326 |
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