New strategies for the synthesis of hypercrosslinked polymers (HCPs) applied in separation proces
Creators: name: Ding, Liang
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: hypercrosslinked polymers
HCPs
CO2/N2 gas adsorption
deep eutectic solvents
adsorption capacity of HIPS HCPs monoliths
sustainability

description: Hypercrosslinked polymers (HCPs), known for high surface area and great thermochemical stability, have gained attention in various scientific and industrial fields since they were first reported by Davankov and Tsyuruna in 1971 via Friedel Crafts chemistry. However, their irregular pore structures, insoluble powder form, and reliance on hazardous solvents hinder their processibility and sustainability. Hence, this thesis explores new synthesis protocols for HCPs to improve their processibility and sustainability, and systematically evaluates their performance in various separation processes. In this thesis, well-developed 3D printing technology and the eco-friendly deep eutectic solvents (DESs) are innovatively combined with Friedel-Crafts chemistry to produce HCPs monoliths and a series of HCPs from various starting monomers respectively. In particular, it emphasised the relationship between the CO₂/N₂ gas selectivity and the textural properties of HCPs, providing deep insight on the molecular design of porous HCPs and control over their porosity. Necessary knowledge of the latest development of HCPs and sustainable DESs is introduced in the first two chapters. Methodology, materials and characterisation techniques are summarised in Chapter 3. The research work presented in this thesis starts with synthesising structure-retained high-impact polystyrene (HIPS) HCPs monoliths via the combination of 3D-printing technology and novel stepwise hypercrosslinking strategy. Both the mechanical strength and porosity of HIPS monoliths are maintained by adjusting the reaction time at each crosslinking step. These engineered porous HIPS monoliths (SA: 337 m² g⁻¹) have been employed for the adsorption of dyes, oils and even biomolecules such as taxadiene, which is an essential intermediate in the synthesis of anticancer medicine. Notably, the capacity of HIPS monoliths for taxadiene (20 mg L⁻¹) is comparable to that of a commercial adsorbent named HP-20. After validating the viability of producing configuration-preserved HCPs monoliths by 3D printing, this thesis focuses on improving the sustainability of HCPs production. In Chapter 5, eco-friendly DESs replace conventional halogenated solvents, with [ChCl][ZnCl₂]₂ and [ChCl][FeCl₃]₂ identified as feasible alternatives for synthesising HCPs while serving as the catalyst in Friedel-Crafts reactions simultaneously. This versatile protocol, utilising various monomers, yields HCPs with narrower pore size distributions, enhancing CO₂/N₂ gas selectivity despite the fact that the specific surface areas of HCPs synthesised in DESs were 20 – 60% lower than those produced in halogenated solvents. The highest CO₂/N₂ selectivity was achieved by poly-α,α’- dichloro-p-xylene that synthesised in [ChCl][ZnCl₂]₂, reaching a value of 105. Moreover, possible mechanisms for the DES-intermediate hypercrosslinking process are proposed. This work not only successfully expands the application of DES in polymerisation chemistry, but also highlight the advantage of DES in adjusting the pore size of HCPs. Based on the findings from Chapter 5, starting materials from bio-resources are preferred to further improve the sustainability. Hence, kraft lignin processed from corn stalks is used as the starting material to crosslink with different agents in DESs. Despite the deficient BET surface area, these lignin-based HCPs proved impressive CO₂ capture ability because of their abundant oxygen-containing groups and uniform narrow micropores. Moreover, they exhibited outstanding CO₂/N₂ selectivity in the practical gas mixture, reaching 835. This work successfully verified the feasibility of DESs for post-crosslinking type synthetic route in addition to internal and external hypercrosslinking. Significantly, the advantage of DESs in promoting narrow pores was further confirmed. To conclude, this thesis recognises the strong desire for processable HCPs and greener HCPs, developing novel synthetic protocols for HCPs and deepening the understanding of the relationship between textural properties of the HCPs and their separation performance.