https://doi.org/10.3390/su14084750
“The catalytic effects of the nanoparticles are known to improve the energy efficiency of the desorption reaction, although the theoretical level of thermodynamic energy required remains unchanged. Figure 4 shows the conditions that are generally accepted to explain the high energy consumption in a CO2 desorption process [72,73].”
“Figure 4. Conditions leading to high energy consumption in a CO2 desorption process.”
“The main reasons for the large heat requirement for the regeneration of solvent is the high endothermic carbamate breakdown reaction as well as the difficulty of the deprotonation of pronated amine to water. Therefore, the function of this metal oxide catalyst is to donate the Lewis acid and Bronsted Acid to the N atoms in the carbamate structure. Overall, this weakens the N-C bond and less thermal energy would be required to break up the carbamate, thus resulting in an increase in the CO2 desorption rate [74]. This amine regeneration process can then be performed at lower temperatures which results in a decrease of the Qsen and Qvap [27].
The catalytic mechanism of metal oxides with MEA solvent has previously been proposed [24,75]. In several studies, the role of Lewis and Bronsted acid sites in the promotion mechanism of catalytic CO2 desorption process has also been reported [24,76,77,78]. The five commercially available nanocatalyst, which are Ag2O, Nb2O5, NiO, CuO and MnO2, demonstrated a catalytic mechanism, due to the presence of acid sites over the particles [75]. Lewis acid is one of the main acidic types and is typically provided by unsaturated metal atoms. This allows it to accept a lone pair of electrons [27]. Bronsted acid, on the other hand, can donate a proton to a base [79]. This is due to the hydroxyl groups that were converted from the surface oxide of the catalyst.
The catalytic role provided by Lewis acid can be seen in reaction (17) and (18), whereas the role Bronsted acid can be seen in reaction 19 and 20.
MEAH++L↔MEA+LH
LH+H2O↔L−+H3O+
MO+H2O→MO⋅H2O
MO⋅H2O→MOH⋅OH
According to reactions (19) and (20) [29], the oxygen atom in the water molecule and metal atoms (Al, Si, Mo, etc.) can chemically adsorb and split the water molecule on the metal oxides’ surfaces. This forms a hydroxyl group, which behaves as a Bronsted acid site [27,75,80]. Although metal oxides have the benefit of both Lewis and Bronsted acids, they are more commonly known as Lewis acid catalysts. Their Lewis acid sites can be converted to Bronsted acid sites [29].
The literature has previously reported the higher regeneration performance of Bronsted acid in comparison to Lewis acid in amine solvents. Bronsted acid can supply protons, take part in the catalytic desorption process and is released into the solution [29]. Because of this, the CO2 desorption rate is greatly enhanced, since the breaking down of carbamate can occur without waiting for the deprotonation reaction to occur [81]. In addition to that, the solvent regeneration process can be accelerated, since the Bronsted acid can generate protons in a tertiary amine solvent [82] and directly attack bicarbonate species [83]. An example is the study by Lai et al. [24], where the hydroxyl group on the TiO(OH)2 nanoparticle has the ability to accept or donate protons, which accelerate any proton driven reactions. Therefore, the protonation and deprotonation reactions occur faster and, therefore, benefit both the absorption and desorption processes. The reaction that occurs between MEA and CO2 in the presence of TiO(OH)2 can be catalyzed. The hydroxyl group of the TiO(OH)2 has the ability to donate or accept protons in any reactions involving protonation and deprotonation; therefore, the proton-involved reactions are accelerated.
An example of another catalyst in its catalytic regeneration mechanism is for HZSM-5 zeolite. It has been explained that HZSM-5 zeolite contains both the present of Lewis and Bronsted acids on its surfaces, which accounts for the superior catalytic performance of this zeolite [69,81]. In the mechanism, the Bronsted acid will supply a multitude of free protons for converting MEACOO− to MEACOOH even without the deprotonation step, as seen in the desorption mechanism of uncatalyzed amine. As for the Lewis Acid sites, the lone pair electrons in the O atom of the MEACOOH are attracted to the empty orbital of the Al atoms. Then the MEACOOH becomes zwitterion intermediate [77]. The NAC bond stretches and, eventually, breaks, which causes the zwitterion to become MEA and CO2. At high regeneration temperatures, the CO2 solubility in the aqueous solution is low and the CO2 will then transfer to the gas phase [81,84,85,86]. Therefore, this also explains why when more HZSM-5 catalysts are present, the concentrations of available protons would increase and, therefore, allow MEACOO- to react with the protons at an increased rate. This leads to a faster desorption rate and solvent regeneration performance.
It has been reported that the physiochemical properties of catalysts can affect the CO2 desorption performance during solvent regeneration process [29]. It is crucial to identify the parameters for future synthetization of nanoparticles. There are several factors that can impact the catalytic performance of the nanoparticles. According to past studies, there were three parameters that were reported to be essential physical properties of nanocatalysts [29]. These are: (a) mesopore surface area (MSA), (b) total surface area and (c) average pore diameter. The first property is the MSA, which represents the mesopore surface area. These are materials that contain pores between 2 and 50 nm. Smaller pores, such as those found in microporous materials (<2 nm), are inaccessible for large amine carbamate materials but is beneficial for ammonia solutions instead. The possible explanation for this is that ammonia carbamate are smaller in size than amine carbamates [87]; therefore, in this case, the total surface area is important. Besides, the average pore diameter is also crucial, as large pore size in a nanoparticle can give a better access to the interior surfaces and thus allow more available active acidic-basic sites as well as reduce the mass transfer resistance for the diffusion of the molecules through the pores of nanoparticles [88]. This is also an important property when evaluating the cyclic ability of the nanoparticle, as a larger pore size can avoid the pores being blocked with the large carbamate molecules and maintain the catalytic efficiency after several absorption-desorption cycles. The role of acidic sites (Lewis acid and Bronsted acid) of a nanoparticle is substantial and it has, in fact, been proven to be more influential than the MSA [27,75,89]. A study has shown that an increase in the Bronsted acid sites (BAS) would also have a larger positive impact to the CO2 desorption rate in comparison with the physical properties of the catalyst, which have less impact [87]. Other studies have mentioned that the total acid sites also have a good relationship with the CO2 desorption rate and the reduction in energy [27,75,90]. Apart from that, the acid strength of the catalyst is an important chemical property, despite limited studies regarding this aspect. An increase of the strong acid sites and acid strength of the nanoparticle would lead to an increase in the CO2 desorption rate [91]. Another study shows that weak acid can also affect the performance of the nanoparticle [80]. The role of basic sites are also important to consider, though the studies on this area have been limited since it was first reported in 2018 [29]. The relation between the basic sites and the kinetics of CO2 desorption has yet to be found, although even past studies have shown that an increase in the basic sites leads to an increase in the catalytic performance of the material [73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93].”