**1. Introduction**

Fuel consumption has been increasing over the years to meet the high demand for energy required for various human activities. Therefore, CO2 concentration has increased drastically to unprecedented levels in the atmosphere [1]. Fossil fuel combustion is the main contributor (60%) to the increased CO2 concentration level in the environment [2]. Chemical, agro, power, and pharmaceutical industries contribute to approximately 70% greenhouse gas emission, which primarily causes climate changes and global warming [3,4]. The rise of sea and ocean levels, increased acidity of water, and drastic global weather changes are the main environmental problems associated with the increased CO2 emission, which will consequently lead to economic collapse [5,6]. Additionally, the CO2 levels in the environment cannot be lowered rapidly due to the large-scale and high consumption of fuels, which are difficult to be reduced. Novel strategies must be developed to not only resolve the environmental problems arising from global warming but also reduce carbon emission at the source. In addition, it is important to devise new technologies and design novel materials that can be used as a media to capture CO2 effectively [7–10].

Various technologies have been developed to capture and store CO2 that can efficiently reduce its atmospheric level [11–16]. Recently, researchers from both academia and industry directed their attention toward the capture and storage of CO2 [17–19]. Various chemical absorbents have been used as media for CO2 capture, in which amines (e.g., ethanolamine) are the most common ones [20]. The use of amines involves a simple process; however, it is limited because of high operational cost, energy requirement, and the use of very volatile chemicals [21]. Therefore, other techniques that involve the use of adsorbents were developed. Such materials reportedly exhibit adsorption capacity of >4.4% by weight, long life duration, recyclability, and reusability [22–24].

Chemical adsorption of CO2 is a simple as well as cost and energy effective process. Metal-based adsorbents such as metal oxides are known as common capture media for CO2 because of their basic and ionic nature [25]. For example, calcium and magnesium oxides can adsorb CO2 stoichiometrically to produce the corresponding metal carbonate through an exothermic reaction [26]. However, the adsorption capacity of materials varies on the basis of kinetic factors [25]. The adsorption capacity of calcium oxide is limited but sufficiently high to facilitate its use as an effective medium for CO2 capture. Several other materials such as ionic liquids in a solid matrix [27], zeolites [28], silica [29], and those containing activated carbons [30–32] have been evaluated as CO2 sorbents. Some of these materials possess unique thermal properties, high chemical stability, high surface area, tunable chemical structures, recyclability, and reusability. However, zeolites are not suitable for CO2 capture from flue gases because of their excellent hydrophilic properties [33]. In addition, materials containing activated carbon exhibit poor selectivity [34].

Activated carbon has been prepared from different materials such as polymers, resins, and biomass and can be used as an efficient adsorbent for CO2 [30]. Various chemical and physical processes have been conducted to activate and modify the surface area and pore volume of such adsorbents to increase their capacity for CO2 capture. The chemical process of activation requires the use of a base, while the physical one requires an appropriate carbonization gas [35,36]. The adsorption capacity of activated carbon depends on the distribution of the chemical activator within the matrix. Polyacrylonitrile in the presence of a base (e.g., potassium hydroxide; KOH) was used as an effective medium to capture CO2 and exhibited good CO2 uptake at 25 ◦C and under 1 bar [31]. The CO2 uptake was even higher for the resorcinol–formaldehyde resin at the same temperature and pressure in the presence of potassium carbonate as an activator [37].

Metal–organic frameworks (MOFs), synthesized from different molecular building units, have been investigated as adsorbents for CO2 because of their extended surface area [38–40]. The interaction between MOFs and CO2 is strong because it occurs through hydrogen bonding and requires a low heat of adsorption, similar to that observed for zeolites [25]. The CO2 storage capacity of MOFs can be enhanced through the addition of polar residues within their surfaces [41]. Porous-organic polymers (POPs) are highly stable chemically and thermally as well as have low density, tunable structure with a desirable surface area, and different functional groups; therefore, they act as good adsorbents for CO2 [33]. The presence of heteroatoms (e.g., nitrogen, oxygen, sulfur, phosphorus) within the skeleton of POPs enhances CO2 capture capacity [33]. The surface polarity of POPs can be increased by the addition of organic moieties containing polar groups or inorganic ions, which facilitates the strong interaction between CO2 and adsorbent materials [33]. Various POPs showed good CO2 capture capacity; however, the use of metals in the synthesis of POPs produces toxic pollutants. More research is still needed to optimize the synthetic procedures for POP production by employing simple and effective processes [42].

Nitrogen-rich heterocycles such as triazines have potential use in supramolecular applications because they interact with many chemicals through π–π interactions, hydrogen bond formation, and chelation [43]. Melamine has a high nitrogen content (66% by weight) and has been used in various applications such as the production of raw materials with high nitrogen content, plastic, medicinal products, metal-free catalysts, and CO2 adsorbents [44–46]. Melamine Schiff bases can be easily synthesized through the reaction of melamine and aromatic carbonyl compounds in the presence of a catalyst. Recently, we have synthesized various Schiff bases and investigated their use as additives to stabilize polymeric films against irradiation [47–53]. Melamine Schiff bases have all the qualities needed for their use as efficient adsorbents for CO2. In this study, we report the use of melamine Schiff bases, which are highly aromatic and porous, as an efficient media for the capture of CO2 at 40 bars and 323 K.

### **2. Materials and Methods**

### *2.1. Materials*

Chemicals, reagents, and solvents were purchased from Merck (Schnelldorf, Germany) and were used as received.

### *2.2. Physiochemical Measurements*

The surface morphology of Schiff bases was observed through field emission scanning electron microscopy (FESEM, TESCAN MIRA3, Kohoutovice, Czech Republic) at an accelerating voltage of 10 kV. The N2 adsorption–desorption isotherms of the Schiff bases were recorded on a Quantchrome chemisorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA) at 77 k. The Schiff bases **1–4** were degassed in a vacuum oven for a long period (6 h) at a high temperature (100 ◦C) under a flow of N2 gas (Cascade TEK, Cornelius, OR, USA) to ensure the removal of any residues or small molecules such as water from the pores of materials. The surface area of the Schiff bases was calculated using the Brunauer–Emmett–Teller (BET) equation at a relative pressure (*P*/*P*◦) of 0.98. The pore size of the Schiff bases was verified by the Barrett–Joyner–Halenda (BJH) method. The CO2 uptake was measured at 40 bars and 323 K using the H-sorb 2600 high-pressure volumetric adsorption analyzer (Gold APP Instruments Corp., Beijing, China), which has two degassing and analyzing ports that can be simultaneously operated. The experiment of CO2 storage was repeated for at least 10 times for pressure optimization. A known quantity of gas was injected into a measurement tube that contained the Schiff base sample until an equilibrium between the adsorbed gas and the Schiff base sample was established. The final equilibrium pressure was recorded automatically using a software program and the adsorbed quantity of gas was calculated from the obtained data.
