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The CO2 sorption properties of sodium hafnium oxide (Na2HfO3) were investigated in this study. Na2HfO3 was synthesized by solid-state synthesis using Na2CO3 and HfO2 as starting materials. The solid-state synthesized Na2HfO3 appeared structurally similar to other mixed metal oxides such as Na2ZrO3, but stacking disorder appeared to be common in Na2HfO3. The synthesis conditions, including the Na : Hf ratio (between 0.5 and 1.5 : 1), synthesis temperature, time and heating rate, were investigated to optimize CO2 sorption properties of Na2HfO3. The Na2HfO3 sorbent showed comparable CO2 uptake capacity, reaction rate and excellent cycling stability compared to other metal oxide sorbents. Na2HfO3 with Na : Hf = 1 : 1 and 1.25 : 1 showed the highest CO2 uptake among all Na2HfO3 samples obtained, with a CO2 uptake capacity of around 15 wt% (at 650–800 °C). The CO2 uptake rate of NHO-1 and NHO-1.25 was fast with over 80% of the equilibrium uptake reached within 250 s. Na2HfO3 remained stable even after 100 cycles with less than 3% difference in the CO2 uptake capacity between the 1st and 100th cycles. We performed kinetic analysis on the CO2 sorption data and found that the Avrami–Erofeev model fitted the kinetic data best among the kinetic models used. Apart from sorbent optimization, we showed that 3D-printing of Na2HfO3 : HfO2 mixtures can be used to produce structured Na2HfO3 sorbents with a slightly improved CO2 uptake rate and the same CO2 uptake capacity as the powder-based solid-state synthesized Na2HfO3 sorbent.

For the first time, the use of stannate-based sorbents was investigated as high temperature CO2 sorption to evaluate their potential to contribute towards reducing carbon emissions. The sorption capacity and kinetics of commercial tin oxide, sodium, potassium and calcium stannates and lab synthesised potassium stannates were tested using thermogravimetric analysis. Commercial K2SnO3 was found to possess the largest CO2 uptake capacity (2.77 mmol CO2/g or 12.2 wt%) at 700 °C, which is among the highest for potassium sorbents, but the CO2 desorption was not successful. On the contrary, the in-house synthesised K-stannate (K-B) using facile solid-state synthesis outperformed the other sorbents, resulting in a CO2 uptake of 7.3 wt% after 5 min, an adsorption rate (0.016 mg/s) one order of magnitude higher than the other stannates, and stability after 40 cycles. The XRD and XPS analyses showed that K-B contains a mixture of K2SnO3 (76%) and K4SnO4 (21%), while the Scherrer crystal sizes confirmed good resistance to sintering for the potassium stannates. Among the apparent kinetic model tested, the pseudo-second order model was the most suitable to predict the CO2 sorption process of K-B, indicating that chemical adsorption is dominant, while film-diffusion resistance and intra-particle diffusion resistance governed the sorption process in K-B. In summary, this work shows that solid-state synthesised potassium stannate could be an effective sorbent for high temperature separation, and additional work is required to further elucidate its potential.

Efficient, facile, and energy-efficient desorption processes are highly sought after in industrial processes. Visible light exists all around us and can be considered a ubiquitous energy source that can be used to drive photothermal processes such as the release of guest molecules from porous sorbents. Herein, we present four sustainably synthesized porous coordination polymers, M(dhbq)(H2O)2 (where dhbq = 2,5-dihydroxy-1,4-benzoquinone, and M = Fe, Mg, Mn, or Zn) with visible light photoresponsive properties. Efficient desorption of CO2 corresponding to up to 47.6 % of the total uptake capacity was achieved upon visible light irradiation for 10 min. A negligible decrease (< 97 %) in CO2 uptake was observed for up to 10 light-swing adsorption cycles and working capacities of up to 37.5 g kg- were obtained in Fe(dhbq). M(dhbq) possess highly desirable properties that make them interesting for applications related to cost-effective and energy-efficient desorption processes.

One of the biggest drawbacks of solid physisorbents for CO2 is their poor performance at high temperatures. This is due to their relatively low heat of CO2 sorption, which drives the adsorption/desorption equilibrium to favor desorption at above ambient temperatures. We show here that Hybrid Ultramicroporous Materials (HUMs), comprised of metals coordinated to organic ligands, and inorganic pillars, are highly effective physisorbents for capture CO2 in particular at very low partial pressure and even at moderate temperatures. We fine-tuned the HUM structure using various inorganic pillars and metal centers and produced a HUM NbOFFIVE-1-Cu that can effectively physisorb CO2 at 100 °C with CO2 uptake capacity comparable to at that 30 °C. NbOFFIVE-1-Cu is also highly stable even after 45 sorption cycles with no change in its CO2 capture properties.

This thesis presents a comprehensive study on the design and optimization of CO2 sorbents, targeting two distinct applications: CO2 capture from point source emissions and Direct Air Capture (DAC). The research first introduces Na2HfO3 as a potential CO2 sorbent for point source emissions, using the molten salt effects of Na2CO3 and the thermal stability of HfO2. This combination results in a CO2 sorbent with impressive thermal and cyclic stability, through various optimization steps to enhance CO2 capture performance and efficiency. The study then shows into the structural disorder in Na2ZrO3, a chemically similar CO2 sorbent. This investigation fills a knowledge gap, offering new insights into the sorbent's behavior in CO2 capture. For DAC applications, the thesis explores the design of inorganic anion pillared metal-organic frameworks, focusing on the adjustment of M5+−F− bond lengths in inorganic anion pillars within M5+OFFIVE-1-Ni samples. These structural modifications impact the CO2 capture properties, particularly in terms of capacity and kinetics, demonstrating the potential of structural tuning in enhancing sorbent effectiveness. The synthesized samples exhibit good cyclic and water stability, suggesting their potential for practical DAC applications.

Carbon capture is an important and effective approach to control the emission of CO2 from point sources such as fossil fuel power plants, industrial furnaces and cement plants into the atmosphere. For an efficient CO2 capture operation, many aspects of the CO2 capture steps need to be carefully considered. Currently the most mature CO2 capture technology is liquid amine scrubbing. Alternatively, solid sorbents can be used to effectively capture CO2 while alleviating the disadvantages associated with liquid amine sorbents. In this review, we critically assess solid metal oxide CO2 sorbents, especially oxides of group 1 (Li, Na and K) and group 2 (Mg, Ca, Sr and Ba) metals, for capturing CO2 at moderate to high temperatures. In particular, we focus on the recent advances in developing synthetic metal oxide sorbents, and the correlation between the design, synthetic approaches and their cyclic CO2 capture performance, which are characterised by CO2 uptake capacity, rate of carbonation and cyclic stability. The state-of-the-art, challenges, opportunities and future research directions for these metal oxide sorbents are discussed. By devoting more research effort to address the issues identified, there can be great potential to utilise Group 1 and 2 metal oxides as cost-effective, highly efficient sorbents for CO2 capture in a variety of carbon capture applications.

Hybrid ultramicroprous materials (HUMs) with a number of different variations of the inorganic pillars as well as surface functional groups have been tested as CO2 sorbent for low-concentration CO2 capture. Specifically well-known HUM NbOFFIVE-1-Ni (NbOFFIVE=(NbOF5)2-, 1=pyz, Ni=nickel(II)) like HUMs were synthesized in this study by replaces Nb with V and Ta. Replacing the metal center from Nb to V or Ta showed that the CO2 adsorption isotherm, in particular at low partial pressures, adopted different shapes and gradient. This study shows that the CO2 adsorption properties at low partial pressures on HUMs can be affected by the metal present in the inorganic pillars.