The mixture formed by NaP zeolite nanocrystals and 1-dodecyl-3-methylimidazolium chloride ([C
12mim][Cl]) ionic liquid was used for CO
2 removal in an isothermal high-pressure cell equipped with magnetic stirring
[35]. Under various experimental conditions, it was found that 0.02 wt% of zeolite nanoparticles, 0.4 wt% of [C
12mim][Cl] ionic liquid, and 0.05 wt% of sodium dodecyl benzene sulfonate in nanofluids resulted in the highest CO
2 removal compared to other conditions. This CO
2 removal increased by increasing ionic liquid and surfactant concentration up to a limiting value near the critical micelle concentration.
Using 2-methylimidazole zinc salt (ZIF-8) modified by tetraethylenepentamine (TEPA), which provided pores, and 1-ethyl-3-methylimidazolium bis(trifluoro-methanesulfonyl)-imide ([EMlm][NTf
2]) ionic liquid, used as a sterically hindered diluent, an amine-functionalized type III porous liquid was formed
[37]. It was found that with 30TEPA@ZIF-8 nanoparticles, the best CO
2 absorption capacity was obtained. Moreover, the CO
2 absorption loading of 0.124 mmol/g presented by 5-30TEPA@ZIF-8/[EMlm][NTf
2] was 4.43 times higher than the value obtained by the use of 5-0TEPA@ZIF-8/[EMlm][NTf
2], whereas a 745% increase of the absorption rate was reached.
3. Nanomaterials and CO2 Adsorption
Adsorption processing involves the use of a solid material on which CO2 (and other gases and solutes) is captured by means of physical or chemical processes or a combination of both. Key parameters to yield the best adsorptive properties of the materials are: porosity, pore size, operational stability, presence of reactive groups towards CO2 adsorption, etc., whereas the equipment used is usually described as a packed or fluidized bed.
The fabrication of carbon nanofibers (CNFs) via biaxial electrospinning was investigated
[40]. Polymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) were used as core and shell precursors, respectively. Further, Co
3O
4 nanoparticles were included in the PAN shell, increasing its roughness and surface area. The uniform distribution of Co
3O
4 resulted in a better flexibility of the hollow carbon nanofiber material (HCNF-Co), providing more vacant oxygen sites to increase CO
2 adsorption loading. HCNF-Co nanofibers exhibited CO
2 capture uptake of 3.28 mmol/g at 25 °C. Experimental results indicated that HCNF-Co had remarkable CO
2 selectivity (S = 26) over N
2.
Heterojunctions of Co
3O
4 with different morphologies and modified carbon nitride (CN) were investigated in order to optimize their properties to degrade CO
2 under UV–visible irradiation
[41]. A solvothermal synthesis was used to fabricate the cobalt oxide from metal–organic framework structures, yielding ultrathin 2D Co
3O
4 nanosheets (Co
3O
4-NS). These nanosheets presented improved photocatalytic properties compared to those of the bulk Co
3O
4/CN composites. CO
2 reduction was improved due to (i) the match of the planar surface of CN and the 2D structure of Co
3O
4-NS, which resulted in a larger interface, and (ii) improvement in charge carrier lifetime.
The authors of
[42] described the utilization of 2D nanomaterial MXenes and activated carbon (AC) to form sandwich-type materials and nanocomposites for CO
2 adsorption using a fixed-bed column. These investigations included CO
2 breakthrough measurements at a fixed 15% CO
2 concentration, with an inlet flow rate at 200 mL/min and temperatures in the 25–55 °C range. The highest CO
2 adsorption load (near 9 mg/g) was yielded with AC/MXene sandwich adsorbent at 25 °C, which was nearly a 37% improvement in CO
2 adsorption capacity over the use of pristine AC. AC/MXene sandwich-type nanomaterials can be used, with a small loss of their CO
2 adsorption uptake, under various cyclic experiments.
ZIF-8 hollow nanospheres, for selective CO
2 separation and storage, were developed
[43]. The optimum hollow ZIF-8 nanosphere material, with a uniform size distribution (
Figure 2), had a CO
2 adsorption uptake of 2.24 mmol/g at 0 °C and 1.75 bar, selective (12.15) CO
2/N
2 separation, 1.5–1.75 wt% CO
2 storage capacity, and a reasonable stability, up to four CO
2 adsorption/desorption cycles, at 25 °C.
Figure 2. Influence of synthesis time on the average diameter of soft template hollow ZIF-8 nanospheres. Surfactant/oil ratio: 75 g/L.
A heterogeneous catalyst comprising silver nanoparticles and a porous N-heterocyclic carbene polymer (Ag@POP-NL-3) was developed
[44]. This nanomaterial has a regular distribution of silver nanoparticles and nitrogen activation groups. The catalyst presented good properties for the selective adsorption and activation of CO
2, allowing the conversion, under mild conditions, of low CO
2 (30 vol%) concentrations, as presented in lime kiln waste gas, into cyclic carbonate. CO
2 was loaded onto the adsorbent by carboxylative cyclization of the gas with propargylic alcohols also present in the system.
The adsorption uptake of CO
2 on NaY@polyacrylate matrix was increased by 17.9% while H
2O adsorption uptake decreased by 36.6% compared to pristine NaY
[45]. In addition, H
2O adsorption was reduced by 54.8% after adding ZIF into composites.
The authors of
[46] described a maximum CO
2 loading (0.75 mmol/g) on triethylamine-doped rice husk silica nanoparticles, with an average increase in CO
2 adsorption with the increase (1 to 5 wt%) in the amine loading on the surface modifiers. Amine loadings greater than 5 wt% produced agglomeration of the particles which is detrimental with respect to CO
2 capture. CO
2 uptake corresponded to the Langmuir isotherm model.
The use of Zn-N pillar MOFs resulted in: CO
2 capture of 3.82 mmol/g (25 °C and 101 kPa), a selectivity CO
2/N
2 factor of 132, and stable structure (no change after exposure to 1000% RH environment for seven days)
[47].
The performance of graphene oxide (GO)-coated zinc tetraphenylporphyrin (ZnTPP/GO) nanocomposites in the photocatalytic degradation of CO
2 was investigated
[48]. The encapsulation of GO in ZnTPP nanocrystals promotes CO
2 adsorption, interfacial reaction, and stability and accelerates the separation of photoinduced carriers on ZnTPP (0.1 ps vs. 425.9 ps), the transportation from ZnTPP to GO (2.3 ps vs. 83.6 ps), and their final enrichment on GO.
A porous ZIF-11@ZIF-8 core–shell composite structure metal–organic framework was fabricated using the solvent-assisted linker exchange (SALE) procedure
[49]. Adsorptions at 25 °C and equilibrium pressures up to 4 bar showed an increase (near 100%) in CO
2 adsorption uptake of ZIF-11@ZIF-8 nanoparticles (8.21 mmol/g) compared to the pristine ZIF-11 (4.35 mmol/g). Experimental results on gas uptake fitted well with the Langmuir isotherm equation. CO
2/N
2 and CO
2/CH
4 selectivities also increased by 131% and 92%, respectively.
Activated carbon (AC) was synthesized from date fruit seeds and chemically activated with KOH to improve CO
2 loading
[50]. From thermogravimetric analyses, 94% and 67% higher average CO
2 capture loads were measured for KOH-promoted ACs compared to the original adsorbents. The activated carbon improved its fluidization by the use of hydrophobic silica nanoparticles (NPs). The SiO
2-decorated (2.5 wt%) modified ACs had a 45% higher bed expansion ratio, which was associated with the absence of bubbles and a homogeneous fluidized regime.
Mesoporous CeO
2, ZrO
2, and Ce-Zr composite nanoparticles with a large surface area were fabricated using the hydrothermal template-assisted synthesis procedure, and CO
2 adsorption properties of these materials were investigated under equilibrium and dynamic operations
[51]. Better CO
2 adsorption was yielded for Ce-Zr nanomaterial due to the presence of strong O
2− base sites and many surface oxygen species. After five adsorption/desorption cycles, the composites presented a reasonable stability with a slight decrease in CO
2 adsorption uptakes in dry flow and in the presence of water vapor.
Treatment via surface N
2 plasma of zinc porphyrin (ZnTCPP) ultrathin nanosheets induced nitrogen vacancies (NVs) and resulted in a material with photocatalytic CO
2 reduction activity and selectivity
[52]. It was shown that the photocatalytic activity of NVs-ZnTCPP can be attributed to nitrogen-vacancy-induced spin polarization by reducing the reaction barriers and inhibiting the recombination of photoexcited carriers.
It was reported
[53] that CO
2 uptake (11.8 mmol/g (78% total adsorption)) after four cycles on a MOF-derived nano-CaO (average size of 100 nm) was due to high stability produced in the final material by the change in the original fiber-bundle-like MOF structure to nanosheets, and further to regular CaO spheres. CO
2 uptake onto the adsorbent corresponded to the following equation:
Lewis base and dual hydrogen bond donor (HBD) units were integrated into an organosilicon precursor, and triazine and hydrazo site co-modified periodic mesoporous organosilicas (THPMOs) were prepared via a hydrothermal self-assembly method
[54]. The THPMOs had BET surface areas in the 699–876 m
2/g range and low-pressure CO
2 adsorption loadings at 0 °C. If combined with tetrabutylammonium iodide (TBAI) ionic liquid, the mixture promoted the model cycloaddition of CO
2 in an effective form, with the gas fixed to epoxides.
In
[55], nano-TiO
2 was added to cement pastes to investigate its performance regarding the CO
2 uptake rate. Prismatic samples with dimensions of 16 × 4 × 4 cm of 0.5 water/binder cement paste with and without nano-TiO
2 particles were used. CO
2 uptakes showed that nano-TiO
2 addition improves the CO
2 uptake rate of cement pastes, changing the pore structure and allowing the removal of more CO
2 at lower gas concentrations. CO
2 loaded similarly to Equation (4), but Ca(OH)
2 reacted with the gas to form CaCO
3 and water.
MIL-101(Cr)-NH
2 has higher CO
2 adsorption capacity than MIL-101(Fe)-NH
2 [56], whereas the adsorption of methane and nitrogen by MIL-101(Cr)-NH
2 is lower than the adsorption of these gases by MIL-101(Fe)-NH
2, leading to a higher selectivity of CO
2 over the two gases for MIL-101(Cr)-NH
2. At elevated temperature and pressure, the chemisorption mechanism is predominant, which is attributable to the performance of amines, which adsorbed more CO
2 at these higher temperatures and pressure. Gas adsorption was explained by the use of a hybrid equation between Langmuir and Khan models:
where qs, Q, and n represent the parameters of the model, with P being the pressure of the vapor phase at the equilibrium.