https://doi.org/10.3390/su14084750
“The γ-Al2O3 was first introduced as a suitable catalyst for the CO2 desorption process [76] in 2011, which eventually became the benchmark for the other studies on the utilization of nanoparticles. The utilization of γ-Al2O3 in 5M MEA was able to allow the reduction of the regeneration temperature from 120–140 °C to 90–95 °C and decrease the regeneration energy requirement by 27%. Table 5 depicts the summary of the past studies on the utilization of Al2O3 nanoparticles in solvent regeneration [58,59,60,100], for the nanoparticles ranging from 15–45 nm. It was reported that a larger nanoparticle has a higher number of regeneration sites of 19 at 45 nm, followed by 15 sites at 20 nm and finally just 8 sites for the base solvent methanol that was used in the study. Bubbles would detach faster on the surface of nanoparticles in comparison to the base solvent when the cross-section area was bigger and had a greater number of regeneration sites. Despite the nanoparticle size being larger than that of SiO2, as presented in the previous context, the cluster size of Al2O3 was reported to be smaller. Such studies reported that the cluster size of Al2O3/DI water nanoparticles was at 150–170 nm. Even comparing the 20 nm and 45 nm Al2O3, the average aggregation diameter was 306.5 nm and 169.7 nm, respectively, which shows that the 20 nm Al2O3 nanoparticle had a cluster size approximately 1.8 times larger than the 45 nm of the Al2O3 particle.”
Table 5. Summary of past studies using Al2O3 nanoparticles on the regeneration of solvent.
| Solvent | Size and Concentration | Temperature °C |
Remarks | Ref. |
|---|---|---|---|---|
| Deionized water | 45 nm 0–0.05 vol% |
100 |
|
[59] |
| Methanol | 20, 45 nm 0, 0.01 vol% |
60 |
|
[59] |
| Methanol | 45 nm 0.01 vol% |
<65 |
|
[60] |
| MEA | 15 nm 0.1 wt% |
103 |
|
[100] |
| MEA | 250 g | 90 |
|
[84] |
| Amine blend | 25 g | 96 |
|
[72] |
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On top of that, it is more effective to analyze the surface morphology of the nanoparticle for CO2 regeneration rather than the size or cluster size of nanoparticle. This is because, despite the fact that Al2O3 was reported to have a large nanoparticle size (45 nm) and smaller cluster size, the departure time of the CO2 bubble during solvent regeneration was faster in comparison to the smaller 15 nm SiO2 nanoparticle that had a larger cluster size. A 45 nm Al2O3 nanoparticle has a higher average surface roughness of 195.03 nm in comparison to a 20 nm Al2O3, which has an average surface roughness of 84.90 nm. The greater the surface roughness of the nanoparticle surface, the larger the number of sites of CO2 bubbles, which is beneficial for the separation of CO2 from the surface. The explanation of the surface morphology is much clearer than comparing the particle size and cluster size of the nanoparticles.
In terms of the concentration of the Al2O3 employed in previous studies, 0–0.05 vol% and 0–0.14 wt% of Al2O3 were blended into different solvent solutions, such as DI water, methanol and MEA. Al2O3 showed an interesting result, where the increase of the concentration resulted in a decrease in the regeneration performance of the nanoparticles. The reason proposed for this reduction in performance is (1) the formation of adsorbed bicarbonate and carbonate species upon the reaction of CO2 with this metal oxide nanoparticle and (2) the difference in characteristics, such as high surface potential, according to the variation pH results in the case of CO2 to be caught by the Al2O3 nanoparticle. This also indicates that it is not easy to desorb the CO2 from Al2O3 in comparison to the SiO2 nanoparticle.
It was reported that the enhancement is due to the physical enhancement mechanism, such as the activation effect, thermal conductivity effect and surface effect. The utilization of the nanoparticles leads to an increase in the dynamic motion of the molecules in the solvent. The collisions of the nanoparticles cause the gas dissolved in the solvent to be desorbed easily and in a larger volume, which enhances the regeneration performance. Next is the thermal conductivity effect, which is discussed through the increase in the thermal conductivity of γ– Al2O3 that can rapidly increase the temperature, which results in a rapid reduction of CO2 solubility. The temperature difference recorded for the γ– Al2O3 nanoparticle utilized in methanol solvent was higher in comparison to its blank test, which shows that the contribution of the nanoparticle enhances the heat transfer, which causes an increase in the regeneration performance [60]. The last physical enhancement mechanism discussed is the surface effect model. More bubbles are generated due to the utilization of nanoparticles. Consequently, the generated bubbles desorb easily, leading to enhanced regeneration.
Apart from that, it was also reported that the Al2O3 nanoparticle had a catalytic effect, as seen in several studies [72,74,76,77,81,84,93]. It was explained that although metal oxides are predominantly known as Lewis acid, they can indirectly have Bronsted acid sites on their surfaces, which can donate a proton to the base. The O atoms in the water molecules and the metal oxides have strong interactions, which leads to the water molecules on the surface of the metal oxides to split. This forms a hydroxyl group, which behaves like a Bronsted acid site. This nanoparticle was also studied to mimic the role of a bicarbonate ion in low CO2 loading, which led to the improvement in the regeneration energy of the amine solvent.
The Al2O3 showed a significant decrease in heat duty, such as 11.8852 Gj/ton, as seen in [84]. This is much lower than the blank test and other catalysts, apart from HZSM-5. This is supported by other examples in the literature [72,81,93]. These studies also have shown that Al2O3 has a higher MSA value, but a lower B/L ratio compared to other catalysts, including the HZSM-5 zeolite. The product of these two factors has been shown to be lower than HZSM-5, which also explains why HZSM-5 performs better. However, Al2O3 is still a good candidate, as it still performs better than other catalysts.
According to the nanoparticle selection criteria, the Al2O3 nanoparticle employed is reported to be unstable at high concentrations, allowing it to agglomerate. The nanoparticle is also thermally stable, as it degrades at a higher temperature than the required solvent regeneration temperature. A 4–6% weight loss was reported at 410 °C, which is the thermal degradation temperature, and a total of 23% weight loss was reported once the temperature reached 800 °C [115]. Apart from that, the Al2O3 nanoparticle is reusable in multiple absorption–desorption cycles. It also has shown benefits as it is a minor and reversible health hazard, making it a non-toxic chemical.
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