https://doi.org/10.3390/su14084750
“Zeolites are crystalline silicates and aluminosilicates linked through the oxygen atom, which generates a three-dimensional network. They consist of channels and cavities of molecular dimensions [128]. H-type zeolite materials are capable of providing remarkable catalytic activity in the desorption of CO2. This zeolite material can be further classified into HZ, H-mordenite and H-Beta. HZ-type zeolite is part of the medium-pore zeolite family, which consists of strong Bronsted acid sites (BAS) [81]. Such example of HZ-type zeolites that possess excellent ability to catalyze the activity of CO2 desorption is Protonated Zeolite Socony Mobil-5 (HZSM-5 zeolite) which was reported in application by Idem et al. [76] and has also been demonstrated in other studies [72,74,77,93,129,130]. The other two types, HM and Hβ, belong to large-pore zeolite family [131]. According to Kim et al. [132] and Zhang et al. [81], HM and Hβ have superior BAS, a large-pore structure and high surface area, all of which contributes to the enhancement of the CO2 desorption performance. It was reported [106,129] that the contribution for the desorption enhancement effect of different solid acid catalysts is due to four critical characteristics, which can be seen in . The four critical properties are the total acid sites; the MSA, which determines the proportion of the acid sites that takes place in the catalytic reaction; the Bronsted acid to Lewis acid ratio, which determines the overriding mechanism; and finally, the number of Bronsted acid sites, which determine the number of protons provided by the catalyst for the desorption of CO2. However, it is vital to note that none of these characteristics work independently and are responsible for the enhancement effect. It is the combined properties that affect the CO2 desorption performance. For instance, when the Bronsted acid sites are more numerous than the Lewis Acid sites, along with a large mesoporous surface area, this would also lead to an increase in the rate of CO2 desorption rate and decrease in heat duty.”
“The summary of all zeolite materials used in past studies to enhance CO2 desorption performance is provided in . In this review, the zeolites employed in MEA solvent are taken as 30 wt% or 5 M MEA because it is considered to be the most commercially applied solvent in the CO2 post-combustion process [133]. The reason for this selection is due to the low cost and high absorption rate.”
Table 7. Summary of past studies using zeolites nanoparticles in the regeneration of solvent.
| Solvent |
Quantity |
Temperature
°C |
Remarks |
Ref. |
| MEA |
10, 30 and 60 g catalyst |
70–98 |
|
[74] |
| MEA |
25 g (catalyst was 3–4 mm in size) |
95 |
|
[77] |
| MEA |
12.5 g catalyst |
98 |
-
HZSM-5
-
Hβ
-
HM
-
AO
-
Hβ showed best performance and lowest heat duty followed by HZ, AO and HM.
-
Hβ has largest total acid sites and the highest BAS
|
[81] |
| MEA |
250 g |
90 |
-
HZSM-5
-
HY
-
HZSM-5 showed better desorption performance than HY
-
HZSM-5 had lower heat duty than HY
-
B/L of HZSM-5 and HY is 1.587 and 2.3
-
BET surface area of HZSM-5 and HY is 414.1020 and 615.4914 m2/g, respectively.
|
[84] |
| MEA |
10–70 g catalyst |
96 |
-
SAPO-34 compared with SO42−/TiO2
-
B/L ratio is 1.4607
-
MSA is 146.53 m2/g
-
30 g SAPO-34 displayed best CO2 desorption performance and lowest heat duty
-
Largest MSA(B/L value is by SAPO-34: 214.04)
|
[106] |
| MEA |
25 g catalyst |
98 |
-
HZSM-5
-
MCM-41
-
SO42−/ZrO2
-
MCM-41 has highest MSA followed by HZSM-5 and SO42−/ZrO2 (963.19, 151.56 and 72.53 m2/g)
-
HZSM-5 has largest B/L ratio, followed by MCM-41 and SO42−/ZrO2 (1.5116, 0.7505, 0.5834)
-
HZSM-5 showed best performance and lowest heat duty
|
[129] |
| DEAPA |
25 g catalyst |
90 |
-
SAPO-34
-
Catalyst was tested in DEAPA solvent in comparison to MEA
-
B/L ratio of SAPO-34 is 1.46
-
MSA of SAPO-34 is 146.53 m2/g
-
SAPO-34 5 showed best performance and lowest heat duty
|
[78] |
”
HZSM-5 was reported to reduce the regeneration temperature of the MEA solvent from 120–140 °C to 90–95 °C [
76]. This also allowed the heat duty to be drastically lowered from 3.53 MJ/kg CO
2 (based on non-catalytic performances in the pilot plant study [
11]) to 1.56 MJ/kg CO
2 (based on the lab-scale study [
76]). The HZSM-5 catalyst is a framework type aluminosilicate zeolite and is reported to have the best catalytic performance in 5 M MEA solution [
129]. The HZSM-5 zeolite is mainly a proton donor catalyst [
134] and its mechanism works by breaking down the carbamate in amine and reducing the activation enthalpy for the proton transfer [
129]. The performance of HZSM-5 was reported to be excellent, with 1.10 mol of CO
2 regenerated in the first 90 min at 371 K, which is higher than the other two catalysts that were compared (MCM-41 with 1.03 mol CO
2 regenerated and SO
42−/ZrO
2 with 0.94 mol CO
2 regenerated) [
129]. This is a 29.41% increase from the blank test of 5 M MEA solvent.
The HZSM-5 was also compared with
γ-Al
2O
3 in a amine solvent regeneration study [
77]. The study focused on understanding the reasons behind the drastic reduction in energy required for the CO
2 desorption process. The absolute heat duty reported for the HZSM-5 zeolite in 5 M MEA was 26.67 MJ/kg CO
2, followed by the
γ-Al
2O
3 at 29.55 MJ/kg CO
2. Both of which were lower than the heat duty for blank MEA at 42.53 MJ/kg CO
2. Therefore, there was a 37.3 reduction in the relative energy requirement for the employment of the HZSM-5 in comparison to the blank MEA. The study also reported on the amine degradation test where both catalysts were reported to not have any degradation effect on amine solvents. This is because the HZSM-5 does not have strong acid sites, which would mean it is not corrosive, but also that the operating temperature for amine regeneration is mild, which means that the HZSM-5 would not have negative degradation effects despite it potentially breaking C-H bonds at a relatively high temperature (450–500 °C).
Another study employed a different concentration of catalysts at 10 g, 30 g and 60 g of HZSM-5 in 5 M MEA [
74] at 378 K. The amount of mol that CO
2 desorbs was 1.75, 1.84 and 2.04 mol CO
2, respectively. This shows a 13.64, 19.48 and 32.47% increase from the blank test, respectively. It was also reported that the product of B/L ratio to mesoporous surface area was largest for the HZSM-5 catalyst, which is at 191.9 and shows a heat duty reduction of 47.54% compared to the blank test.
Another four different Bronsted acid catalysts, H-type zeolites, namely, HZSM-5, HM, Hβ and Al
2O
3, were studied in two different amine systems in order to improve the CO
2 desorption process [
81]. The Hβ catalyst was reported to have superior results in terms of the solvent regeneration and increasing the desorption performance up to 1360.8%. The relative energy requirement was also reduced by 66.1% in comparison to the blank run of MEA solvent. This increased performance by the Hβ catalyst can be explained by its large mesoporous surface area, a larger number of BAS and prominent total acid sites. Therefore, it can be deduced that the MSA, BAS and total sites all have an important effect in enhancing CO
2 desorption performance. This could explain the poor catalytic effect of the other catalysts. For instance, HZ was reported to have an 8.31% decrease from Hβ, while HZ has a larger MSA, but a smaller BAS and fewer total acid sites. This is similar to HM, which experienced a 12.61% decrease from Hβ. HM was reported to have a higher BAS and total acid sites, but the MSA was the smallest. In addition to that, the Hβ catalyst displayed remarkable stability as its cyclic capacity did not show severe reduction, the catalytic desorption performance did not show significant changes and the structure of the catalyst, analyzed through XRD, indicated that the crystalline structure was maintained throughout the four absorption–desorption cycles.
The H-Y type zeolite catalyst was also reported in a study by comparing it to the conventional HZSM-5 zeolite [
74]. The MSA was reported to be 24.3 m2/g, which is much smaller than the HZSM-5. The B/L ratio, however, was significantly larger than the other two catalysts, which makes this zeolite a suitable candidate for solvent regeneration. In this study, it was found that the catalytic performance of the H-Y zeolite was drastically inferior compared to the conventional HSZM-5 zeolite, and this was probably due to the low MSA and high microporous area of the H-Y structure. This provided a very low mass transfer coefficient, which then led to low catalytic efficiency. In regard to the other factors for the nanoparticle selection criteria, the H-Y zeolite has not been widely studied in the past; therefore, there is insufficient information regarding this catalyst.
SAPO-34 was also another type of catalyst tested for the regeneration of CO
2-loaded MEA solvent [
106]. The catalyst ranged from 30 g–70 g at 96 °C in a MEA solvent. The catalyst with the most amount of CO
2 desorbed in decreasing order was 30 g > 20 g > 50 g > 10 g > 60 g > 70 g at 0.91, 0.89, 0.88, 0.84, 0.82 and 0.81 mol CO
2 desorbed, respectively. This gives the 30 g employed SAPO-34 the lowest heat duty at 24.23 MJ/kg CO
2. Although the SO
42−/TiO
2 showed higher total acid sites and B/L ratio, it was the SAPO-34 catalyst that showed better specific surface area and MSA. This supports the claim that these characteristics are not individually responsible, and so, the combined effects of these factors should be evaluated. Due to the higher combined value of the Bronsted/Lewis acid site ratio (B/L) and a larger mesopore surface area (MSA), the CO
2 desorption rate of SAPO-34 was reported to be higher and the heat duty lower than the other tested catalyst. Cyclic ability was also tested as it is an important characteristic in determining a good catalyst. In this study, five desorption cycles were tested, and it was reported that the amount of desorbed CO
2 did not have a significant declination. depicts the heat duty calculated for zeolites used in past studies, according to the studies reported in .
”
“Figure 7. Heat Duty (MJ/kg CO2) for HZSM-5 [74,77,81,84,129], Hβ [81], H-mordenite [81], H-Y [74,84] SAPO-34 [78,106], and Blank [74,77,78,81,84,106,129].”
