Investigation of CO2 adsorption performance of spinel oxide & metal-organic structures

Abstract

Greenhouse gas emissions, especially CO2, have significantly contributes to global warming. As a result of this, clean energy and green chemical investments have getting more share in the whole portfolio of the relevant sectors. However, CO2 emissions of 2023 hit to the value of 37.4 Gt on annual basis. CO2 has reached in inexhaustible quantities and this necessitates its use as a synthetic fuel and chemical feedstock to lessen the carbon burden on energy and chemical industries. This significantly relies on the improvements in appropriate carbon capture and utilization technologies. On the other hand, the CO2 purity needs to be sufficiently high for the downstream CO2 utilization processes. Several CO2 capture approaches have been developed and these includes absorption, adsorption, cryogenic, membrane and natural carbonation based technologies. Their choice depends on the source of the gas for example direct CO2 capture from air, carbon capture from power plants, industrial sourced CO2 emissions capturing, chemical looping etc. These technologies may be coupled to reduce carbon emissions depending on the emission sources and in a sustainable way to control their adverse effect on climate change. Among these, absorption and adsorption are the two most applied approach ones with their pros and cons. The key difference between them comes from the CO2 capture mechanisms: Absorption occurs by dissolution or homogeneous reaction whereas adsorption proceeds the binding of CO2 onto the adsorbent surfaces. The high energy consumption for solvent recovery of absorptive systems makes the adsorption technologies be competitive. The main parameters for a feasible adsorption-based CO2 capture technology can be listed as (1) adsorption capacity ranging from range from 3 to 10 mmol/g, (2) efficiency of the adsorption process with a recovery rates of 90%, (3) selectivity for CO2 over competing gases at least 20:1 and lastly (4) holding the cost limits of 40–60 $ per ton of CO2. The design of the adsorbent's surface is highly important in order to achieve these targets. CO2 adsorption involves both physisorption and chemisorption. Since desorption of physisorbed CO2 requires less energy compared to chemisorption, highly porous materials are desirable alternatives for CO2 sequestration. Metal organic frameworks, activated carbon, zeolites or silicate materials all work for physisorption at high pressures. Zeolitic imidazolate frameworks (ZIFs) are a kind of metal-organic frameworks (MOFs) that are topologically similar to zeolites. They combine the advantages of both zeolite and MOF structures together such as high surface area and porosity, chemical stability and thermal resistance. While ZIFs are composed of the tetrahedraly coordinated transition metal (cobalt) cations connected by imidazole ligands whereas the zeolites are formed by Si–O–Si. In this regard, MOFs seem suitable for preparing composite adsorbents by hosting the secondary materials such as spinel oxides besides its common use alone as CO2 adsorbents. Spinel oxides are crystalline structures that have the general formula of AB2O4, where A represents a tetrahedral rare earth ion and B represents an octahedral alkaline earth transition metal ion. These spinel oxides exhibit excellent hydrothermal stability and catalytic activity despite not being composed of precious metals. Integrating spinel nanoparticles onto the MOFs through grafting can enhance the surface features of the resulting composites thereby improving its adsorption capacity. Hybridization of ZIFs with spinel magnetic nanoparticles may provide increased number of active sites on functional carbon material platforms so as to increase the ultimate adsorption efficacy. Nevertheless, there is a limited number of research on the synthesis of sophisticated composites based on ZIFs. To address this issue, new composite materials based on zeolitic imidazolate frameworks have been explored within the scope of the thesis. CO2 adsorption capacities of ZnFe2O4, ZIF-67 and their composite forms of ZnFe2O4@ZIF-67 and CoFe2O4@ZIF-67@ZIF-8 have been measured at different temperatures and pressures, namely by CO2 isotherms at four different temperatures of 0°, 10°, 15° and 20°C and High Pressure Gravimetric CO2 Sorption Analysis up to 20 bar. Complementary structural characterization measurements on these adsorbent materials were carried out by N2 Isotherms, Fourier Transform Infrared Spectroscopy (FTIR), Thermal Gravimetry, X-Ray Diffraction Spectroscopy and Scanning Electron Microscopy. In order to elucidate CO2 capture mechanism on these adsorbent materials, temperature-programmed desorption of CO2 by in-situ FT-IR has been applied. By evaluating all these characterization and measurement results, the new adsorbent compositions have been assessed in detail. It was shown that CO2 adsorption proceed via physisorption over ZnFe2O4 ZIF-67, ZnFe2O4@ZIF-67 and CoFe2O4@ZIF-67@ZIF-8 composites. This has been proven by the isosteric heats of adsorption values as calculated to be lower than 80 kJ/mol. From the anti-symmetric stretching FT-IR band splitted to yield the P and R branches of 2335 cm-1 and 2359 cm-1 due to rotational-vibrational coupling, the presence of gas phase CO2 in the open space of the adsorbent framework and adsorbed CO2 on multiple binding sites, respectively have been differentiated. The highest CO2 adsorption capacity value of 104.9 mg CO2/g for ZnFe2O4@ZIF-67, as calculated from the Langmuir Model from the CO2 isotherms at four different temperatures has shown the potential of this composite for CO2 capture. Gravimetric CO2 adsorption measurements at high pressure up to 20 bar gave a 3.26 mmol/g capacity for ZnFe2O4@ZIF-67. This exceeds the required target of 3 mmol/g and proved the potential of ZIF-spinel composite structures even the loss of surface area with spinel incorporation. The higher isosteric adsorption on the spinel structure of ZnFe2O4, that was 43.2 kJ/mol, may be indicative of the fact that CO2 may partly interact with oxygen sites of spinel through reversible bidendate or tridendate bonds due to the presence of unsaturated metal centers. As a plausible explanation, growing ZIF-67 on metal oxide core may tune its pore size and chemistry and also effect on generating co-ordinatively unsaturated metal sites. All these can give promise for the prospective CO2 capture applications by combining the advantages of both ZIF-67 and spinel structures resulting in better performance than pure ZIF-67.