*3.3. N2 Adsorption–Desorption of Schi*ff *Bases 1–4*

The N2 adsorption–desorption measurements for the Schiff bases **1–4** were conducted at 77 K. The N2 isotherms and pore sizes and volumes of Schiff bases **1–4** are represented in Figures 6–9. The shape of the N2 isotherm for **1** was similar to the type IV isotherm. Schiff bases **2–4** showed N2 sorption isotherms that are almost identical to the type III isotherm, in which monolayer formation was not identified.

**Figure 6.** N2 isotherms and pore size and volume for Schiff base **1**.

**Figure 7.** N2 isotherms and pore size and volume for Schiff base **2**.

**Figure 9.** N2 isotherms and pore size and volume for Schiff base **4**.

The BET surface area (SBET), pore volumes, and average pore diameters of Schiff bases **1–4** were calculated (Table 1). Among the synthesized Schiff bases, **1** (containing a nitro group) exhibited the highest surface area (SBET = 11.6 m2/g) and total pore volume (0.036 cm3/gm), but the lowest pore diameter (1.69 nm). Schiff base **1** had a mesoporous structure, while, **2–4** (containing a hydroxy group at *ortho*-, *meta*- and *para*-position of the aryl ring) had microporous structures (pore diameter = 2.44–3.63 nm). Some POPs and tin complexes showed porous structures with similar pore diameters. For example, porous polyphosphates derived from either 1,4-diaminobenzene or benzidine exhibited a pore diameter of 1.96–2.43 nm [54] or 2.43–2.86 nm [55], respectively, compared to that of 2.43 nm for telmisartan tin complexes [56].



A gravimetric technique was used to detect the gas uptake quantity and, therefore, determine the gas adsorption isotherm [57]. In addition, the gas quantity that has been removed from the gas phase was used to estimate the physisorption isotherms of the gas. The desorption or adsorption branch of the isotherm can be used to calculate the pore size distribution. The CO2 sorption isotherms for Schiff bases **1–4** are shown in Figures 10–13 and their CO2 uptake are reported in Table 2.

**Figure 10.** Adsorption isotherm of CO2 for Schiff base **1**.

**Figure 11.** Adsorption isotherm of CO2 for Schiff base **2**.

**Figure 12.** Adsorption isotherm of CO2 for Schiff base **3**.

**Figure 13.** Adsorption isotherm of CO2 for Schiff base **4**.

**Table 2.** CO2 adsorption capacity of Schiff bases **1–4** at 323 K and 40 bars.


As seen in Figures 10–13, Schiff bases **1–4** do not have an apparent adsorption–desorption hysteresis, which indicates the reversible adsorption of CO2 within the Schiff base pores at the temperature and pressure used (323 K and 40 bars). The CO2 uptake for Schiff bases **1–4** was high (6.1–10.0 wt%), possibly because of the excellent pore diameter and the strong van der Waals interactions and hydrogen bonding between the Schiff bases and CO2. In addition, Schiff bases **1–4** contain strong Lewis base sites that aid the capture of CO2. Indeed, porous materials containing heteroatoms such as oxygen, nitrogen, and phosphorous can selectively capture CO2 over methane and nitrogen gases [54–56].

The surface area for the Schiff bases was relatively low (5.2–11.6 m2/g); however, they showed remarkable CO2 uptake (1.36–2.33 mmol/g; 6.1–10.0 wt%). Similar observations have been previously reported at similar temperature and pressure. For example, porous polyphosphates containing benzidine showed low surface area (27.5–30.0 m2/g) and high CO2 uptake (up to 14.0 wt%) [55]. On the other hand, polyphosphates containing 1,4-diaminobenzene exhibited high surface area (82.7–213.5 m2/g), but the CO2 uptake was limited to 0.6 wt% [54]. Telmisartan tin complexes showed surface area of 32.4–130.4 m2/g and up to 7.1 wt% CO2 uptake [56]. Materials with the highest surface area showed the most effective CO2 uptake. Polyacrylonitrile carbon fibers in the presence of a base provided a CO2 uptake of 2.74 mmol/g at room temperature and normal pressure [31]. In contrast, porous nanocarbons with a high surface area (1114 m2/g) in the presence of potassium oxalate and ethylenediamine provided a CO2 uptake as 4.60 mmol/g at a similar temperature and pressure [30]. Porous nanocarbons with a small surface area (439 m2/g) provided a low CO2 uptake (1.94 mmol/g [30]. Ionic liquids in a silica matrix led to materials having a very small surface area (1–9 m2/g) and relatively poor sorption capacity towards CO2 as 0.35 g of CO2 per g of adsorbent [27].

### **4. Conclusions**

Four melamine Schiff bases have been investigated as potential media for CO2 storage at 323 K and 40 bars. These Schiff bases have a relatively low surface area (SBET = 5.2–11.6 m2/g) and varied porous structures, showing pore volumes of 0.004–0.036 cm3/g and diameters of 1.69–2.63 nm. The Schiff bases showed remarkable CO2 uptake (6.1–10.0 wt%), possibly because of their high aromaticity and heteroatom contents. The Schiff base containing a nitro group showed the most effective CO2 uptake (10.0 wt%) owing to the high content of nitrogen (heteroatom) within the porous material. The Schiff bases containing a hydroxy group have a lower surface area and pore volume, but higher pore diameter compared to the one containing a nitro group. Such Schiff base is inexpensive and easily producible in high yield and, therefore, can be used at an industrial scale.

**Author Contributions:** Conceptualization and experimental design: G.A.E.-H., M.F.A., D.S.A., and E.Y.; Experimental work and data analysis: R.M.O. and E.T.B.A.-T.; writing: G.A.E.-H., D.S.A. and E.Y. All authors discussed the results and have approved the final version of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors are grateful to the Deanship of Scientific Research, King Saud University for funding through Vice Deanship of Scientific Research Chairs.

**Acknowledgments:** We thank Al-Nahrain and Al-Mansour Universities for the technical support.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
