Light responsive adsorbents for carbon capture
2017-03-02T00:19:21Z (GMT) by
The discovery of novel light responsive adsorbents for carbon capture and release is reported here for the first time. These materials were found to exhibit dynamic or static photoswitchable properties in the broadband or visible light range during CO2 adsorption measurements. Prior to the publication of my work, no dynamic photoswitchable adsorbent or full utilisation of visible light to trigger carbon capture and release was known. Thus the pioneering work presented in this thesis provides a significant breakthrough for the carbon capture technologies that could potentially allow the process to be operated at a significantly lower cost and energy than conventional methods. Commonly, the separation of CO₂ from a stream of gases involves the chemisorption of CO₂ by amines, the release from which is triggered by pressure and temperature. Although it is an effective process, amine degradation and regeneration are the main problems in post-combustion capture technologies. The regeneration of amines requires a large energy input which can result in ~30 % reduction in power plant capacity. Such limitations prompted research on different sorbent materials that display reversible adsorption using low cost triggers. While temperature, pressure, pH and microwave stimuli responsive materials have potential applications in gas adsorption technologies, light, particularly in the visible light range is the least disruptive stimulus for triggering carbon capture and release efficiently and it is highly abundant in nature, in the form of sunlight. New classes of porous materials: Metal-organic Frameworks (MOFs) and porous aromatic frameworks (PAFs) were explored as potential CO₂ sorbent materials due to their excellent performance in CO₂ apture and tunable properties. To achieve highly efficient utilisation of sunlight in carbon capture processes, porous materials with light responsive properties were developed according to the energy levels in the solar spectrum. The solar spectrum consists of 7 % ultraviolet, 47 % visible and 46 % infrared light. The concept of using non-filtered light as a practical approach was ideal. However, the use of filtered light to eliminate UV light and obtain visible light irradiation was also attractive as it constitutes the major fraction of solar radiation intensity. Furthermore, it is more environmentally friendly and greater exposure due to deeper light penetration could be achieved. Thus porous adsorbents that could respond to light in the broadband range or specifically in the visible light range were developed. In the course of the research project, four novel light responsive materials were successfully developed; two of which respond in the broadband light range and the other respond in the visible light range. The first dynamically photoswitchable MOF (Zn(AzDC)(4,4’-BPE)0.5) for reversible carbon capture and release was reported. The MOF contained azobenzene and stilbene derivative linker molecules, in which photoirradiation resulted in the fast-bending motion of these molecules. Thus, up to 42 % and 64 % static and dynamic photoresponse were achieved respectively. This was the highest dynamic photoresponse obtained at the time of publication. The development of more broadband light responsive adsorbents was conducted further in the research. The work led to the discovery of photoresponsive PAF containing diarylethene (DArE) guest molecules (DArE@PAF-1). The dynamic carbon capture and release was triggered by the interaction between photoactive DArE molecules with the PAF. At high guest loading concentration, photoresponse can reach up to 24 %. As earlier work showed great promise for light triggered carbon capture applications, improvements were focused on using more penetrating light trigger, such as visible light. As a result, an adsorbent that could respond exclusively to visible light was developed. Furthermore, unique adsorption behaviour was obtained for a reversible carbon capture and release process. The introduction of visible light guest molecules (methyl red, MR) into the host (Mg-MOF-74) displayed a pressure-gate system, in which light could be used to tune the gate-opening pressure. Photoirradiation of the sample resulted in the photoswitching of the guest molecules, whereby lesser contact with the host allowed more adsorption sites to be accessible. Thus upon irradiation, a significant increase in CO₂ uptake adsorption of up to 84 % was obtained. This is the highest photoresponse ever reported to date under static condition using visible light trigger. Further development led to the discovery of a water stable adsorbent that is visible light responsive. Again, methyl red (MR) was used as the visible light responsive molecule. The incorporation of the guest molecule into water stable MIL-53(Al) resulted in a similar photoresponse trend, in which prolonged irradiation resulted in the increase in CO₂ uptake adsorption of up to 9 % at lower guest loading (15 wt %) and 46 % at higher guest loading (50 wt %). The light responsive nature and recyclability of these adsorbents represents a new strategy for lowering the cost and energy for sorbent regeneration, a main problem associated with conventional carbon capture technologies.