Yang, Pengfei; (2025) Constructing Efficient Transport Pathways within Membranes Utilising Cucurbit[n]urils. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Advanced materials and structures are fundamental to membrane technology, determining the overall performance of membranes in various applications. The precise design and regulation of transport pathways within these membranes are critical for optimizing their structure, enhancing mass transfer processes, and achieving high performance. This study addresses the increasing demand in the fields of environmental sustainability and energy, focusing on applications such as wastewater treatment, carbon capture, and proton conduction. The goal is to enhance the separation efficiency of water and pollutants, CO2/CH4 separation, and proton conduction performance through the precise construction of transport pathways. This study introduces the use of macrocyclic molecules, specifically cucurbit[n]uril (CB[n], where n refers to the number of glycoluril building blocks), as key units for constructing these transport pathways within membranes. By employing these materials, this approach achieves precise construction and regulation of transport pathways at the sub-nanometre scale, thereby synergistically improving both physical and chemical mass transfer processes. Moreover, the study elucidates the relationship between membrane structure and performance, offering theoretical insights for the design and fabrication of high-performance CB[n]-based membranes. The main research contributions are as follows: The initial research focuses on enhancing water transport efficiency by constructing highly effective water transport pathways within graphene oxide (GO) membranes through the intercalation of cucurbit[6]uril (CB[6]). The incorporation of CB[6] molecules as structural "pillars" between GO nanosheets enables precise control over the interlayer spacing, facilitating water transport while maintaining selective permeability for dye molecules. This strategy effectively addresses the conventional trade-off between pure water flux (PWF) and dye rejection. Notably, the CB6GO-4 membrane exhibited a maximum PWF of 171.3 L m-2 h-1 bar-1, more than 5.9 times higher than that of pristine GO membranes, while maintaining a dye rejection efficiency exceeding 92%. Additionally, the optimized interlayer distance in the CB6GO-3 membrane achieved an ultrahigh dye/salt separation selectivity of up to 761.4. The second research task aims to construct highly efficient CO2 transport pathways to address the CO2/CH4 permeability-selectivity trade-off in gas separation. Cucurbit[7]uril (CB[7]), known for its high CO2 adsorption capacity and CO2/CH4 selectivity, is integrated into hydrogel membranes (HMs) to form efficient CO2 transport pathways. The intrinsic cavity of CB[7], along with the interface between the CB[7] and polymer chains, creates physical pathways for transport. Moreover, the selective CO2 adsorption properties of CB[7] and the increased water retention of the membrane significantly improve both CO2 permeability and CO2/CH4 selectivity. The hybrid HMs with the highest CB[7] content achieved a remarkable CO₂ permeability of 972.2 Barrer and a CO2/CH4 selectivity of 81.5, representing improvements by factors of 2.2 and 2.1, respectively, compared to unmodified HMs, surpassing the upper-bound trade-off limit. The third project presents, for the first time, CB[n]-based proton exchange membranes (PEMs) using a nature-inspired chemical engineering (NICE) approach. By incorporating cucurbit[6, 7, 8]urils (CB[6, 7, 8]) into sulfonated poly(ether ether ketone) (SPEEK), the study creates efficient proton transport pathways. The carbonyl groups of the CB[n] molecules act as proton-conducting sites, while their host-guest interactions with water molecules enhance proton conductivity. Proton exchange membrane with CB[8] molecules shows higher proton conductivity compared with the membranes with CB[6] and CB[7]. By optimizing the loading amount of CB[8], the CB[8]/SPEEK-1% hybrid membrane exhibits the highest proton conductivity of 198.0 mS cm-1 at 60°C and 100% relative humidity (RH), and a maximum power density of 214 mW cm-2. In summary, the CB[n]-based membranes developed in this thesis demonstrate significantly enhanced performance due to the highly efficient transport pathways. This work opens new avenues for the fabrication of CB[n]-based membranes for advanced separation technologies and proton conduction applications.

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