Transition metal oxides as additives

Transition metal oxides have also been widely used due to their catalytic properties as they can be a good option in providing Lewis acids and Bronsted acids in the stripping column [27]. Bhatti el al. [27] studied the effects of five metal oxides, which were MoO₃, V₂O₅, Cr₂O₃, TiO₂ and WO₃, in a MEA-based solvent. It was reported that metal oxides with both types of acid (MoO₃ and V₂O₅) showed distinct effects and desorbed up to 94% and 84% of CO_2, respectively. The other three acids with only Lewis acid sites (Cr₂O₃, TiO₂ and WO₃) desorbed 44% of CO₂. The experiments were successfully run at much faster rates with 1.4–2 times greater amount of CO₂ desorbed.

This led to the introduction of five new metal oxide nanoparticles, which are Ag₂O, Nb₂O₅, NiO, CuO and MnO₂, on MEA solvents [75]. The experiment was run at a temperature as low as 80 °C, where Ag₂O and Nb₂O₅ displayed superior performance by desorbing up to 3.6 and 2.5 times greater CO₂ amounts with faster rates. The superior results from Ag₂O and Nb₂O₅ were due to the presence of a combination of acid sites: many Lewis acid sites as well as Bronsted acid sites. The remaining three nanoparticles required higher temperatures to better show their efficiencies, as only a few Lewis acid sites were present. In 2018, the researchers continued their study of ZrO₂ and ZnO metal oxide nanoparticles at a temperature range of 40–86 °C. The enhancement recorded up to 54% for the total amount of CO₂ desorbed.

Lately, in 2019, Bhatti et al. [89] continued to study the effect of Ag₂O and Ag₂CO₃ for amine solvent regeneration at a temperature of around 80 °C. The solvent used 30 wt% MEA with 5 wt% of each metal oxide nanoparticle. They reported an astounding increase in desorption rate of CO₂ up to approximately 1000%, which greatly reduced the energy consumption.

The above transition metal oxide nanoparticles are all prone to agglomeration, due to the adhesion of particles to each other by large van der Waals forces of attraction and sedimentation, in which the nanoparticles settle under the effect of gravity. Additionally, their thermal stability differs for each nanoparticle. For instance, the MoO₃ nanoparticle shows excellent thermal stability and it was reported that only 0.7% weight loss was recorded up to a temperature of 637 °C [118]. This is also almost the same as the results of another study, which had recorded a weight loss of ~1.54% up to 700 °C [119]. V₂O₅ showed weight loss in three stages, according to Xavier [120]; the first one at 250 °C that corresponded to water loss and the second was between 250 °C and 375 °C, due to the dehydroxylation of metal hydroxide and the removal of other ions. The third one shows weight loss up to 515 °C and was due to the loss of OH. The transition metal oxide nanoparticle with the highest thermal stability was Cr₂O₃ and it was reported that no considerable weight loss up to 1000 °C was observed [121]. This is similar to the Nb₂O₅, which showed a mass that remained unchanged throughout the heating process up to 1000 °C, revealing its excellent thermal stability [122]. WO₃ was reported to have a low thermal stability and showed up to 15% weight loss when the temperature rose to 250 °C [123]. The maximum weight loss, however, was reported to be at 150 °C. The next nanoparticle is Ag₂O, which showed a weight loss of about 9.85%, starting from 380 to 420 °C, due to the decomposition of this material [124]. NiO, CuO and MnO₂ reported weight loss at 207 °C [125], 190 °C [126] and 300 °C [127], respectively. Their thermal stability in descending order is Cr₂O₅, Nb₂O₅, MoO₃, Ag₂O, MnO₂, V₂O₅, NiO, CuO and WO, with the last having a poor thermal stability at 150 °C.

Among these nanoparticles, MoO₃ and V₂O₅ are reported to have superior performance since they react and dissolve into the MEA solvent [27]. For this reason, they are non-recyclable but can still be recovered by lowering the pH value of the solvent. The V₂O₅ nanoparticle is reported to be moderately hazardous in terms of its toxicological data yet is still environmentally friendly. The other nanoparticles show only slight toxicity and are relatively harmless to the aquatic environment.