Synthesis of Novel Heteroatom-Doped Porous-Organic Polymers as Environmentally E ﬃ cient Media for Carbon Dioxide Storage

: The high carbon dioxide emission levels due to the increased consumption of fossil fuels has led to various environmental problems. E ﬃ cient strategies for the capture and storage of greenhouse gases, such as carbon dioxide are crucial in reducing their concentrations in the environment. Considering this, herein, three novel heteroatom-doped porous-organic polymers (POPs) containing phosphate units were synthesized in high yields from the coupling reactions of phosphate esters and 1,4-diaminobenzene (three mole equivalents) in boiling ethanol using a simple, e ﬃ cient, and general procedure. The structures and physicochemical properties of the synthesized POPs were established using various techniques. Field emission scanning electron microscopy (FESEM) images showed that the surface morphologies of the synthesized POPs were similar to coral reefs. They had grooved networks, long range periodic macropores, amorphous surfaces, and a high surface area (S BET = 82.71–213.54 m 2 / g). Most importantly, they had considerable carbon dioxide storage capacity, particularly at high pressure. The carbon dioxide uptake at 323 K and 40 bar for one of the POPs was as high as 1.42 mmol / g (6.00 wt %). The high carbon dioxide uptake capacities of these materials were primarily governed by their geometries. The POP containing a meta -phosphate unit leads to the highest CO 2 uptake since such geometry provides a highly distorted and extended surface area network compared to other POPs.


Introduction
The high consumption of fossil fuels in power plants, automobiles, and various human activities contributes to the dramatically increasing level of carbon dioxide (CO 2 ) in the atmosphere [1]. Fossil fuel contributes to about 60% of greenhouse gas emission [2]. Most of the CO 2 emissions (70%) are produced from health production and electricity, agriculture, and industry sectors [3]. The emission of CO 2 , in turn, leads to serious environmental and economic problems globally [4][5][6]. The high CO 2 level is the main cause of global warming, climate changes, rise in sea and ocean levels, and increased acidity of CO 2 is the development of an efficient synthetic procedure that does not involve the use of metal catalysts, production of high surface area POPs that have 3D structures, the use of efficient and moderate reaction conditions within the post-synthetic procedures, and the use of POPs that have multiple adsorption sites to the capacity of CO 2 adsorption at very low pressures [30].
Polyphosphates are highly stable and have excellent mechanical and physical properties [45,46]. They have been used as catalysts, fire retardants, reagents for surface adhesion, and tooth preservers [47][48][49]. Recently, different polyphosphates [50] and organotin complexes [51] have been reported as efficient media for CO 2 storage. Polyphosphates containing benzidine are highly porous, have a high surface area, tunable pore structures, and showed excellent efficiency in the capture of CO 2 . Therefore, the aim of the current work was to synthesis novel POPs containing phosphate units using a simple and general procedure to be used as potential media for CO 2 storage. The polyphosphate-based POPs could be synthesized easily and could reduce off the damage caused to the environment due to the increased CO 2 emission.

General
Chemicals and solvents were purchased from Merck (Schnelldorf, Germany). Melting points were recorded on an MPD Mitamura Riken Kogyo apparatus (Tokushima, Japan). Fourier-transform infrared (FT-IR) spectra in the range 400-4000 cm -1 were recorded on an 8300 Shimadzu FT-IR spectrophotometer (Tokyo, Japan). Proton nuclear magnetic resonance ( 1 H-NMR) spectra were recorded on a Bruker DRX300 NMR spectrometer (Zurich, Switzerland). The surface morphology was examined using TESCAN MIRA3 field emission-scanning electron microscope (FESEM, Kohoutovice, Czech Republic) at an accelerating voltage of 15 kV. The nitrogen adsorption-desorption isotherms (77 K) were recorded on a Quantchrome chemisorption analyzer. The samples were degassed in a vacuum oven at 70 • C for 6 h under nitrogen flow. Surface areas were calculated using the Brunauer-Emmett-Teller (BET) equation at a relative pressure (P/P • ) of 0.98. The Barrett-Joyner-Halenda (BJH) method was used to verify the pore sizes. The CO 2 uptake (at 40 bar and 323 K) was measured on an H-sorb 2600 high pressure volumetric adsorption analyzer (Beijing, China). The H-sorb 2600 analyzer has two analyzing and degassing ports that work simultaneously. A known quantity of gas was injected into the measurement tube containing the POP sample. When the equilibrium between the adsorbed gas and the POP sample was obtained, software was used to record the final equilibrium pressure automatically. The sample was degassed at a high temperature (200 • C) under vacuum for 5 h before the adsorption test. The adsorbed quantity of gas was measured from the data generated. Figure 1 represents the synthesized POPs. The images represented in Figures 2-4 were captured using the FESEM. The data represented in Figures 5-7 were calculated using the BJH method. The CO 2 uptake shown in Figure 8 was measured using the H-sorb 2600 analyzer.  Figure 1 shows the synthesized polyphosphates 1-3. Table 1 lists some of the physical properties of the synthesized MOFs. The structures of polyphosphates 1-3 were established from the FT-IR and 1 H NMR spectra. The bands observed in the ranges 1205-1233, 1135-1185, 1566-1594, and 1600-1620 cm -1 in the FT-IR spectra of 1-3 indicated the presence of P-O-P, P=O, C=C, and CH=N groups, respectively ( Table 2). The absence of any band corresponding to the carbonyl group confirmed the consumption of the phosphate ester. The singlets at 9.03-9.30 ppm in the 1 H-NMR spectra of 1-3 corresponded to the azomethine protons, while the multiplets at 6.77-7.82 ppm correspond to the aromatic protons (Table 2).

Morphologies of 1-3
The surface morphologies of polyphosphates 1-3 were investigated by FESEM. Figures 2-4 show the coral reef surfaces of 1-3. The surfaces are relatively uniform and amorphous, with grooved network structures and long-range periodic macropores. The particles have micro-sized irregular blocks with pore dimensions ranging from 49 to 981 nm. It can be seen that the grooves were parallel to each other and simultaneously perpendicular to the particle's outer surface cross the polymeric materials. Such a morphology improves both the porosity of the material and its efficiency for gas storage.

Morphologies of 1-3
The surface morphologies of polyphosphates 1-3 were investigated by FESEM. Figures 2-4 show the coral reef surfaces of 1-3. The surfaces are relatively uniform and amorphous, with grooved network structures and long-range periodic macropores. The particles have micro-sized irregular blocks with pore dimensions ranging from 49 to 981 nm. It can be seen that the grooves were parallel to each other and simultaneously perpendicular to the particle's outer surface cross the polymeric materials. Such a morphology improves both the porosity of the material and its efficiency for gas storage.      The CO2 adsorption isotherm can be predicted directly from the quantity of CO2 uptake using a gravimetric technique [52]. Also, the quantity of CO2 removed from the gas phase could be used to estimate the physisorption isotherms of the gas. The textural properties of the pores of polyphosphates 1-3 were determined from the N2 adsorption-desorption isotherms recorded at 77 K. The N2 isotherms and pore sizes of polyphosphates 1-3 are shown in Figures 5-7, respectively. Polyphosphates 1-3 have mesoporous structures and showed type-III nitrogen sorption isotherms, in which no monolayer formation was identified.
The Brunauer-Emmett-Teller surface areas (SBET), pore volumes, and average pore diameters of 1-3 are listed in Table 3. Among the synthesized polyphosphates, 2 exhibits the highest SBET (213.5 m 2 /g) and total pore volume (0.32 cm 3 /gm) and the lowest average pore diameter (1.96 mm).    The CO2 adsorption isotherm can be predicted directly from the quantity of CO2 uptake using a gravimetric technique [52]. Also, the quantity of CO2 removed from the gas phase could be used to estimate the physisorption isotherms of the gas. The textural properties of the pores of polyphosphates 1-3 were determined from the N2 adsorption-desorption isotherms recorded at 77 K. The N2 isotherms and pore sizes of polyphosphates 1-3 are shown in Figures 5-7, respectively. Polyphosphates 1-3 have mesoporous structures and showed type-III nitrogen sorption isotherms, in which no monolayer formation was identified.
The Brunauer-Emmett-Teller surface areas (SBET), pore volumes, and average pore diameters of 1-3 are listed in Table 3. Among the synthesized polyphosphates, 2 exhibits the highest SBET (213.5 m 2 /g) and total pore volume (0.32 cm 3 /gm) and the lowest average pore diameter (1.96 mm).    Polyphosphates 1-3 have a tetrahedral geometry with sp 3 hybridized phosphorus core [53]. The CO2 sorption isotherms for 1-3 ( Figure 8) showed no apparent adsorption-desorption hysteresis, indicating possible reversible adsorption of CO2 within the pores of 1-3 at 323 K and 40 bars. The CO2 uptake for polyphosphates 1, 2, and 3 was 2.04, 6.00, and 4.57 wt %, respectively ( Table 4). The high CO2 uptake could be due to the high SBET of the polyphosphates and strong van der Waals interaction and hydrogen bonding between CO2 and the polyphosphates. In addition, polyphosphates 1-3 Figure 7. N 2 isotherms and pore size for 3. uptake for polyphosphates 1, 2, and 3 was 2.04, 6.00, and 4.57 wt %, respectively ( Table 4). The high CO2 uptake could be due to the high SBET of the polyphosphates and strong van der Waals interaction and hydrogen bonding between CO2 and the polyphosphates. In addition, polyphosphates 1-3 contain strong Lewis base sites that help in capturing CO2. Indeed, POPs containing heteroatoms (O, N, S, or P) can capture CO2 selectively over nitrogen and methane [50,[54][55][56].  The CO2 uptake using carbon-containing materials such as porous nanocarbons in the presence of additives (e.g., ethylenediamine and potassium oxalate) as media for CO2 adsorption was 1.9-4.6 mmol/g at 25 °C [27]. While carbon fibers containing polyacrylonitrile in the presence of potassium hydroxide led to a CO2 uptake of 2.7 mmol/g at 25 °C and 1 atm [28]. The CO2 uptake using organotin

Synthesis of Polyphosphates 1-3
Phosphate esters (tris(4-formylphenyl) phosphate, tris(3-formylphenyl) phosphate, and tris(4-formylphenyl) phosphate) were synthesized from the reaction of an appropriate hydroxybenzaldehyde and phosphoryl chloride in the presence of triethylamine in dry tetrahydrofuran (THF), as reported previously [50]. A mixture of the phosphate ester (8.21 g, 20 mmol) and 1,4-diaminobenzene (6.49 g, 60 mmol) in boiling dry ethanol (EtOH; 25 mL) containing glacial acetic acid (AcO 2 H; 0.5 mL) was stirred under reflux for 6 h. The mixture was allowed to cool to room temperature, and the solid obtained was collected by filtration, washed with EtOH (3 × 10 mL), and dried under vacuum for 4 h at 25 • C to give polyphosphates 1-3 ( Figure 1) in high yields. The structures of 1-3 were confirmed from the data obtained from the FT-IR, and 1 H NMR spectra, and their surface morphology was established by the use of FESEM. Figure 1 shows the synthesized polyphosphates 1-3. Table 1 lists some of the physical properties of the synthesized MOFs. The structures of polyphosphates 1-3 were established from the FT-IR and 1 H NMR spectra. The bands observed in the ranges 1205-1233, 1135-1185, 1566-1594, and 1600-1620 cm -1 in the FT-IR spectra of 1-3 indicated the presence of P-O-P, P=O, C=C, and CH=N groups, respectively ( Table 2). The absence of any band corresponding to the carbonyl group confirmed the consumption of the phosphate ester. The singlets at 9.03-9.30 ppm in the 1 H-NMR spectra of 1-3 corresponded to the azomethine protons, while the multiplets at 6.77-7.82 ppm correspond to the aromatic protons (Table 2).

Morphologies of 1-3
The surface morphologies of polyphosphates 1-3 were investigated by FESEM. Figures 2-4 show the coral reef surfaces of 1-3. The surfaces are relatively uniform and amorphous, with grooved network structures and long-range periodic macropores. The particles have micro-sized irregular blocks with pore dimensions ranging from 49 to 981 nm. It can be seen that the grooves were parallel to each other and simultaneously perpendicular to the particle's outer surface cross the polymeric materials. Such a morphology improves both the porosity of the material and its efficiency for gas storage.

Porosity Measurements and Gas Storage Capacity of 1-3
The CO 2 adsorption isotherm can be predicted directly from the quantity of CO 2 uptake using a gravimetric technique [52]. Also, the quantity of CO 2 removed from the gas phase could be used to estimate the physisorption isotherms of the gas. The textural properties of the pores of polyphosphates 1-3 were determined from the N 2 adsorption-desorption isotherms recorded at 77 K. The N 2 isotherms and pore sizes of polyphosphates 1-3 are shown in Figures 5-7, respectively. Polyphosphates 1-3 have mesoporous structures and showed type-III nitrogen sorption isotherms, in which no monolayer formation was identified.
The Brunauer-Emmett-Teller surface areas (S BET ), pore volumes, and average pore diameters of 1-3 are listed in Table 3. Among the synthesized polyphosphates, 2 exhibits the highest S BET (213.5 m 2 /g) and total pore volume (0.32 cm 3 /gm) and the lowest average pore diameter (1.96 mm). Polyphosphates 1-3 have a tetrahedral geometry with sp 3 hybridized phosphorus core [53]. The CO 2 sorption isotherms for 1-3 ( Figure 8) showed no apparent adsorption-desorption hysteresis, indicating possible reversible adsorption of CO 2 within the pores of 1-3 at 323 K and 40 bars. The CO 2 uptake for polyphosphates 1, 2, and 3 was 2.04, 6.00, and 4.57 wt %, respectively ( Table 4). The high CO 2 uptake could be due to the high S BET of the polyphosphates and strong van der Waals interaction and hydrogen bonding between CO 2 and the polyphosphates. In addition, polyphosphates 1-3 contain strong Lewis base sites that help in capturing CO 2 . Indeed, POPs containing heteroatoms (O, N, S, or P) can capture CO 2 selectively over nitrogen and methane [50,[54][55][56].
The CO 2 uptake using carbon-containing materials such as porous nanocarbons in the presence of additives (e.g., ethylenediamine and potassium oxalate) as media for CO 2 adsorption was 1.9-4.6 mmol/g at 25 • C [27]. While carbon fibers containing polyacrylonitrile in the presence of potassium hydroxide led to a CO 2 uptake of 2.7 mmol/g at 25 • C and 1 atm [28]. The CO 2 uptake using organotin complexes containing telmisartan as adsorbent media was in the range of 3.6-7.1 wt % at 323 K and 50 bars [51]. Polyphosphates containing benzidine showed a remarkable CO 2 uptake (1.8-14.0 wt %) at 323 K and 50 bars [50]. Polyphosphate 2 (meta-phosphate) was more effective in CO 2 uptake as compared with 1 (para-phosphate) and 3 (ortho-phosphate). The meta-phosphate geometry of 2 imparts a highly distorted network to this POP as compared to 3 and 1. The extended surface area resulted in high CO 2 uptake. Polyphosphate 1 has the least distorted geometry and the lowest surface area, because of which the CO 2 uptake is lowest among the three polyphosphates. A similar observation has been previously made when tris(formylphenyl)phosphates containing benzidine were used as media for CO 2 capture [50].

Conclusions
The development materials for CO 2 storage may lower down the level of this gas to safe limits. With this viewpoint, three novel polyphosphates were synthesized in high yields, using a simple, efficient, and general procedure as potential media for CO 2 storage. The synthesized polyphosphates have a relatively high surface area (S BET = 82.7-213.5 m 2 /g), small pore size distribution in terms of pore volume (0.11-0.32 cm 3 /g), and small pore diameter (1.96-2.43 nm). The polyphosphates exhibit type III isotherm and have a high affinity for CO 2 uptake (up to 1.42 mmol/g; 6.00 wt %). The POP containing a meta-phosphate unit was the most effective material towards the CO 2 uptake since such geometry leads to a highly distorted network with an extended surface area. Thus, such material has potential to be used for reducing the environmental damage caused by high CO 2 levels.

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