Chemical Modification of Activated Carbon for Enhanced CO2 Adsorption
Adedeji Adebukola Adelodun
Prof. Young-Min Jo
CO2 Adsorption Selectivity Activated Carbon Surface Chemistry;
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Carbon dioxide is one of the important greenhouse gases responsible for 64% of the enhanced or otherwise referred to as anthropogenic greenhouse effect. Also, being a minor component of natural air and a major constituent of respired air, it is often used as an indicator for IAQ. Over the years, researchers have focused on the two aforementioned scenarios independently, however the need to prepare and device a material and mechanism respectively that could be efficiently applied to both CO2 levels and matrices became of research interest. The choice of activated carbon (AC), considering its inherent merits such as high surface area, low cost, readily availability, less susceptibility to moisture, ease of regeneration etc., became obvious. Although AC is capable of adsorbing pure CO2 with a capture capacity higher than that of zeolite in the order of 2.5:1, its selectivity is comparatively more inferior in the order of ca. 1:7.5. This prompted the need for chemical treatment. More than 95% of reviewed literatures reported lab-scale tests of their samples for only 100% CO2 capture efficiency. However, to an environmentalist, such adsorbents are of no relevance, except their reported efficiencies quantitatively indicate CO2 selectivity at either or both indoor and outdoor levels. Hence, the objective of this dissertation is to present scientific means by which AC could be modified in order to achieve highly enhanced CO2 selective capture at both low (0.3% for indoor) and high (10% for flue gas) scenarios. Amination has shown to be a promising method in achieving this, but the refractory nature and chemical inertness of the graphene structure of AC have rendered only few potion of its external surface to be available for this technique, hence, inclusion of some pre-treatment prior amination was experimented. Initially, H2O2 was used as a pre-oxidant for the incorporation of surface oxygen functionalities (SOFs) on AC in the preparation of a CO2 adsorbent. Experimental results showed that oxidation at 0oC improved both structural and chemical properties more than those at 25 oC, although oxidation at this latter temperature with 30% H2O2 incorporated the highest amount of useful SOFs for amination purpose. pHpzc value was most significantly reduced by altering the temperature either way while dilution seemed to have positive impact regarding this property. Pre-treatment notably enhanced the selectivity for CO2 adsorption while capacity capture was merely improved. Therefore, depending on the intended use, the concentration and oxidation temperature of H2O2 could be manipulated to enhance the amination of AC. The result obtained from this approach was not quite impressive. This prompted the introduction of highly energetic UV-C radiation to facilitate photo-oxidation by generating radicals, which was experimented along with dry phase oxidation using pure ozone. This obviously achieved significant improvement especially with the use of ozone where the intrinsic capture affinity of AC for low and high levels CO2 was ultimately enhanced from 0.016 and 0.46 to 0.36 and 0.9 mmol/g respectively. Integrated basic impregnation with the use of Ca(NO3)2 was the next novel technique deviced in this work in order to better the results achieved from previous attempts. Eventual CO2 results showed that Ca(NO3)2 pre-treated samples provide sites with highest affinity towards CO2 especially at trace level. Although AC samples with calcined CaO showed higher pure CO2 and high level capacities, for environmental use however, integrated treatment with Ca(NO3)2 gave better selectivity for both conditions considered. Yet, the set target was still elusive. Finally, the use of strong alkali solutions of KOH was experimented. Here, heat treatment of 1 M KOH-doped AC under inert (N2) and reducing (NH3) atmospheres was first tried. With N2 only, modified adsorbent became unstable to air upon retrieval at room temperature. Therefore, stabilization by amination was employed, which also enabled further use of increased KOH concentration. Coal-based AC (G2) provided more suitable physical properties for the impregnation of the basic chemical species over that of coconut shell based (G1). It was found that amine-stabilized 4 M KOH-doped coal-based AC (N-4K-G2) achieved the highest selectivity, as adsorption amount of 10% and 0.3% CO2 were significantly improved from 0.65 and 0.03 to 2.45 and 2.03 mmol/g respectively, with negligible reduction in the adsorption of pure CO2. With this result, surface modification of AC by amination to achieve both low and high level selectivity as high as the maximum adsorption capacity was achieved and reported for the first time. Also, estimated selectivities αs,g were respectively enhanced from 0.190 and 0.167 to 0.580 and 22.974 for both levels. For the first time, sintered KOH was stabilized on AC by amination to achieve high CO2 selectivity from environmental levels. Regeneration tests at 600 °C showed that thermal refreshment enabled excellent reusability of the prepared sorbent with an estimated efficiency of 90.25% after a three-cycle test. Adsorption properties such as adsorption equilibria, thermodynamics and kinetics were studied in details. By the conformity of all test samples with Freundlich isotherm, CO2 molecules were found to bind onto the heterogeneous surface of AC in a monolayer pattern. Also, Redlich-Peterson was the three-parameter model that provides the best fit for expressing the experimental data while Sips model gives a comparatively good fit as well. In general, three-parameter isotherms are much suitable than those of two due to the complex chemical nature of the substrate surfaces. The lowest degree of freedom (highest accuracy) of error estimation attributed to Chi-square analysis made it the most efficient of the four error functions used. Conclusively, it was inferred that amination could be improved significantly to enhance the CO2 selectivity to that of capacity capture (qmax) by KOH pretreatment and not by pre-oxidation. The interaction between the adsorbate and the modified sorbents is that between the available lone 2s2 lone-pair electron of the nitrogen center of the identified SNFs and the electrophilic carbon center of the weak Lewis acid CO2. For selectivity to be significantly enhanced, competent wet phase pre-treatment that could sufficiently activate the inner pore walls without increasing the acidity of sorbent in any depreciating way prior amination is required. In this light KOH pre-treatment offers such properties as integrated doping of sufficiently basic functionalities, in huge quantities was accomplished. Consequently, the original set goal of obtaining selective adsorption of CO2 for both indoor and flue gas levels (environmental applications) as high as the qmax (2.23 mmol/g) was achieved with surface stabilization by amination of pre-calcination of 4 M KOH on granular coal base activated carbon at a relatively lover temperature of 600 oC.