Graphene-based Membrane For Gas Separation

Membrane separation was considered an emerging technology for gas separation due to the advantages of low energy consumption, less investment and simple equipment. However, gas separation membrane still faces the challenges of low mechanical strength, low permeation flux and low selectivity. And one efficient solution to overcome these challenges is to develop new membrane materials with high permeability, high selectivity, high thermal stability and chemical resistance. As a novel two-dimensional nano-carbon material with only single atom thickness, graphene is considered to be the ideal membrane material due to its excellent flexibility, high mechanical strength and chemical stability. Assembled graphene membrane composed by stacked graphene nano sheets has promising potential for gas separation application.In this thesis, graphene oxide （GO） was prepared through modified Hummers method with natural graphite with different particle sizes as raw material. The structure and surface properties of GO were characterized by FT-IR, XRD, UV-Vis, TEM and AFM technologies. Assembled graphene membrane were prepared using GO with different size distribution as precursor. First, GO composite membrane with microporous membrane filter as support has been prepared via vacuum filtration. And the effect of preparation conditions and the type of support on morphology and gas separation performance of GO composite membrane was investigated. Secondly, free-standing PAA/GO hybrid membrane was fabricated via solvent evaporation method with polyamic acids （PAAs） as addition component. And after carbonization, a flexible graphene carbon membrane was then obtained. The effect of raw materials and preparation conditions on gas separation performance was investigated. The structure and morphology of graphene carbon membrane was characterized by XRD and SEM technologies. The results show that:1. GO of single-atom thickness were prepared through modified Hummers method. The size of graphite has a great influence on the size of GO sheets:the size of GO sheets was decreased with particle size of raw graphite decreased.2. The uniform GO composite membrane was prepared via vacuum-assisted self-assembly approach. With GO amount reducing, the CO₂ flux of composite membrane increased. However, pinhole is inevitable, when membrane is too thin. To avoid the formation of pinhole, the GO dispersion was filtered under a relative low vacuum or sediment for a certain time before filtration. The effect of support on CO₂ flux of GO composite membrane was significant:flux decreased with pore size of the support decreasing. Gas separation performance of GO composite membrane was Knudsen diffusion.3. Graphene carbon membrane prepared via solvent evaporation induced sefl-assembly method was flexible and easy to scale up to a large area. Graphene carbon membrane with nacre-like hierarchical structure composed by graphene layers and carbon molecule sieve （CMS） exhibits excellent CO₂/CH4 separation performance:the CO₂ flux was up to 1493 Barrer with a CO₂/CH4 selectivity of 34.45. With the increasing of PAA content, both flux and selectivity of graphene carbon membrane increased. CO₂ flux and CO₂/CH4 selectivity of graphene carbon membrane varied with the type of the PAA. With the increasing of carbonization temperature, CO₂/CH4 selectivity increased gradually and CO₂ flux increased first and then decreased. As the size of GO sheets decreased, CO₂ flux of graphene carbon membrane increased and CO₂/CH4 selectivity decreased slightly.