Microporous carbons derived from melamine and isophthalaldehyde: One-pot condensation and activation in a molten salt medium for efficient gas adsorption

In the present work, mixture of melamine and isophthalaldehyde undergo simultaneous polymerization, carbonization, and in situ activation in the presence of molten salt media through a single all-in-one route to design microporous carbons with high specific surface areas (~3000 m2/g). The effect of the activation temperature and molten salts on the polymerization process and final texture of the carbon was explored. Carbon materials prepared at 700 °C, in the presence of KOH (referred as MIK-700), exhibited a narrower pore-size distribution ~1.05 nm than those prepared in the presence of the eutectic KOH-NaOH mixture (MIKN). Additionally, MIK-700 possesses an optimum micropore volume (1.33 cm3/g) along with a high nitrogen content (2.66 wt%), resulting in the excellent CO2 adsorption capacity of 9.7 mmol/g at 273 K and 1 bar. Similarly, the high specific area and highest total pore volume play an important role in H2 storage at 77 K, with 4.0 wt% uptake by MIKN-800 (specific surface area and pore volume of 2984 m2/g and 1.98 cm3/g, respectively.) Thus, the facile one-step solvent-free synthesis and activation strategy is an economically favorable avenue for designing microporous carbons as an efficient gas adsorbents.

KOH is entirely consumed when carbon is heated at ~600 °C. At temperatures higher than 700 °C, the as-formed K 2 CO 3 starts to decompose into CO 2 and K 2 O and finally vanishes at 800 °C. At temperatures above 700 °C, potassium compounds including K 2 O and K 2 CO 3 are reduced to metallic potassium, which remains embedded in the carbon matrix.  2 According to the above discussions, the KOH activation mechanisms proposed are (a) Chemical activation by various potassium compounds involves the etching of carbon frameworks due to the redox reactions resulting in the generation of a porous network and (b) the intercalation of the metallic form of potassium into the carbon framework during the activation resulting in the expansion of this framework 12 . Activation followed by washing removes the intercalated metallic as well as other forms of potassium. As a result, the carbon framework remains expanded without reverting to its nonporous structure, and thus, high microporosity is generated. Sodium hydroxide is also widely used as activating agent. According to previous studies, anthracite NaOH activation significantly increases the surface area from 1017 to 2208 m 2 /g and the micropore volume from 0.05 to 0.4 cm 3 /g, with the NaOH to char ratio varying from 1 to 3 13 . However, at high ratios, the micropores enlarged into mesopores, owing to excessive NaOH etching. Moreover, these conventional methods of activation exhibit limitations in controlling the pore size and formation of bottlenecks and/or micropores, typically with a size below 0.7 nm. Contrary to this, one of the interesting synthetic protocol proposed recently involves the condensation and activation of polymer precursors in molten salt medium with these salts playing the role of an alternative solvent as well as porogen for designing porous carbons.
In this context, a direct synthetic strategy is reported for the one step polymerization, carbonization, and activation of the carbon precursors to produce microporous carbons. Two types of carbon materials were prepared at two different temperatures using KOH and a eutectic mixture of KOH-NaOH as the two molten salt media. The resulting carbon materials were evaluated in terms of their ability to serve as gas storage materials.

Results and Discussion
Porous materials are an emerging platform of scientific interest in the context of their applicability in gas storage, energy storage, and catalytic activities. The successful transformation of the monomers to microporous carbons ( Fig. 1) is evident from FTIR spectra. Attenuation of the peak corresponding to the aldehyde at 1690 cm −1 as well as the appearance of weak signals at ~3410 cm −1 corresponding to the stretching vibration of secondary amine (N−H), and methylene (CH) at 2917 cm −1 as the remnants of aminal linkages are confirmation of polymerization of organic precursors (Fig. 2). In addition, the peak of the triazine ring is evident at 1556 cm −1 . The presence of a broad band at 978-1210 cm −1 is ascribed to the vinyl group (=C-H) and (C-N) bonds. The decrease in the intensity of the bands between 1000-1600 cm −1 suggest the removal of the O/C-H/C atomic species during carbonization. However, the absence of sharp peaks of any characteristic functionality indicates the effective carbonization of the polymer structure at the targeted temperatures. Moreover, the results indicate the leftovers of nitrogen functionalities which plays a vital role for enhancing CO 2 adsorption 13 .
For a better understanding the effects of the activating agents on the polymerization-carbonization-activation processes, we analyzed the thermal stability of the precursors. Figure 3 presents the thermogravimetric (TG) analysis curves for the three samples (melamine + isophthalaldehyde (MI), melamine + isophthalaldehyde + KOH (MIK), and melamine + isophthalaldehyde + KOH + NaOH (MIKN)). An overlap is observed in the TG curves of the samples with KOH and eutectic KOH-NaOH mixture, indicating that a similar mechanism is involved in the weight loss in the course of one-pot processes. Moreover, it reveals that any dissimilarity in the microstructure of the activated carbonaceous materials are rather due to the physical state of the activating agent  during polymerization. Furthermore, eutectic mixture is the physical mixture of NaOH and KOH. The mixture is used for lowering the melting point of individual salt and thus provides a molten medium for monomers to condense at desired polymerization temperature. As anticipated, the sample with this eutectic mixture shows a rapid mass loss at temperatures below 200 °C, which is attributed to its low melting point. However, it should be noted that a strong mass loss that appeared at 346 °C in the TG curve of MI has disappeared in the curves of the sample mixtures with the activating agents. This is justified by the notable evolution of unreacted melamine in MI. Thus, the activating agents act as an efficient reaction media for the in situ polymerization in comparison to the hydroxide-free process. Subsequently, a progressive weight loss observed at temperatures above 600 °C is attributed to the carbonization of the polymer. The major component left behind is the carbon with traces of heteroatoms, as proved by elemental analysis. Henceforth, TG curves are used to explore the polymerization-carbonization processes occurring in the eutectic mixtures, resulting in the excellent yield of carbons.
SEM images (Fig. 4) clearly show that the carbons fabricated in the presence of hydroxide activating agents exhibit a different morphology than those prepared without them. MI-700 and MI-800 show rough surfaces formed by irregularly interconnected networks with voids, in accordance with their low specific surface areas. However, MIK-700 and MIK-800 exhibit smooth surfaces with some spherical voids. Apparently, polymerization occurred in the liquid media, and the polymer framework has the ability to entrap the generated bubbles released as reaction products or drops of the eutectic mixture. Hence, spherical voids formed in the carbonized material, as observed in the SEM images. Furthermore, MIKN-700 and MIKN-800 exhibit hierarchical porosity with a large number of mesopores. This is attributed to excessive NaOH etching in the eutectic mixture. The diffraction patterns (XRD) of all the carbon materials are presented in Figure S1. From the figure, it is revealed that washing remove all the impurities of sodium and potassium, as no crystalline peak was observed. Consequently, the only peaks observed were attributed to carbon-based materials with the graphitic nature. The diffraction peak of the (002) plane attributed to the periodicity in the ABAB hexagonal close packing of graphitic structures along the z-axis is absent in almost all the diffraction patterns. Apart from this, a high intense scattering peak is observed at low 2θ values, indicating the disorderness with a low stacking order in graphene sheets, possibly owing to the extensive micropore generation during the activation process.
X-ray photoelectron spectroscopy reveals the surface characteristics of prepared carbons. The survey scans (Fig. 5) show the presence of a high percentage of carbon along with a moderately low oxygen content. The high-carbon content reveals that the prepared sample mainly composed of carbon. The moderate to low nitrogen contents are in accordance with the elemental analysis data. Elemental analysis data (Table 1) indicates that the material contains mainly carbon with a small amount of nitrogen. A careful analysis of the consequences of the carbonization process reveals a large loss of mass with the release of large volumes of volatile gases and some tarry substances 14,15 and as a result, H and N contents decrease with an increase in the carbon content 16 . All the samples are rich in carbon because the process combined carbonization and activation. The carbon content of MIKN increased gradually from 75.9 to 90.6% as the temperature was increased to 700 to 800 °C, whereas the content of nitrogen decreases from 1.65 to 0.77%. The increase in the carbon content during the carbonization is attributed to two factors: (i) Increased formation of carbon basal planes and (ii) aromatization 17 .
Physical properties including the textural features (specific surface area, pore-size distributions, and pore volumes) of the prepared materials were evaluated to assess their role in gas (CO 2 and H 2 ) adsorption. For this purpose, adsorption-desorption isotherms were obtained at 77 K. The isotherms and pore size distributions are presented in Fig. 6(a,b). The textural properties are summarized in Table 2. The adsorption profiles (Fig. 6a) of all the prepared samples indicate rapid uptake of nitrogen gas at low pressures with Type I adsorption characteristic,  18 . This kind of isotherm indicates a high affinity of the adsorbate and adsorbent for each other, and also suggests that the adsorbent in question is comprised mainly of micropores. However, MIKN-700 and MIKN-800 exhibit both Type I and Type IV isotherms. The isotherms exhibit nitrogen uptake at high pressures, indicating the presence of medium-sized or large mesopores along with the micropores. Furthermore, upon comparison of specific surface areas it is revealed that the initial blank samples, MI-700 and MI-800, prepared without any hydroxide activating agent exhibit a moderately low surface area of 330 and 360 m 2 /g, respectively, which is still better than those of the samples prepared by the old conventional synthesis methods 8,9,19 . Samples synthesized in the presence of KOH only, MIK-700 and MIK-800, possess a high surface area of 2101 and 2090 m 2 /g, respectively. This could be because the KOH activation leads to the generation of micropores primarily, resulting in a higher micropore volume and hence a porous carbon with a higher surface area 20 . On the other hand, samples prepared with the KOH-NaOH mixture, MIKN-700 and MIKN-800, leads to the generation of a hierarchical porous structure with both meso-and micropores with the surface areas of 3246 and 2984 m 2 /g, respectively. It is worth noting that with an increase in the carbonization temperature, hierarchical   meso-and microporosity is induced with higher mesopore volume and wider pore size diameter. This can be explained by the decomposition mechanism of salts which upon high temperature reduces to metallic species (sodium and potassium). With the passage of time, a portion of metallic species tends to evaporate from the lattice. However, some proportion of it remains intercalated. Consequently, all these phenomena generate a hierarchical meso-and microporous structure. However, all the superactivated microporous structures were prepared with the mass ratio of 2/1 between the activating agents and monomers. The specific surface area and adsorption profiles of MIK and MIKN are better than those of the materials obtained by chemical activation at high activating agent/precursor ratios 21 . On comparison with the single salt activation method, MIKN-700 exhibit excellent textural features than UAK2-800 20 . Similarly, UANa-960p presents a method with use of 3:1 activating agent to carbon precursor ratio 13 . However, in our work we used lesser amount of activating agents (2:1) with excellent results. Present method generates microporous carbon structure with high specific surface area and narrow pore size diameters. This is accredited to the use of KOH-NaOH eutectic mixture rather than single salt activation. Hence, it is claimed that the present strategy provides a facile way to design highly microporous carbons. The micropore volumes of the prepared samples were estimated using the Dubinin-Radushkevich (D-R) equation shown below: where, W indicates the amount of the liquid adsorbed at P/P 0 , W 0 is the total amount of the adsorbate in the micropores, B is the adsorbent constant, and β represents the affinity coefficient (0.34 for N 2 ) 22 . The ratio of the micropore volume to total volume turns out to be 80, 1.5, 3.5, 80.7, 95.6, and 88.2% for MI-800, MIK-800, MIKN-800, MI-700, MIK-700, and MIKN-700, respectively. The samples prepared at 700 °C possesses a higher micropore volume compared to that prepared at 800 °C, suggesting that the activation at higher temperature is detrimental for the generation of micropores; instead, it stimulates the formation of mesopores. Similar outcomes were observed by Prahas et al. who reported that the elevation of the activation temperature drive the widening of the micropores into mesopores 23 .
To investigate the textural parameters deeply, pore size distribution curves were obtained by the nonlocal density functional theory (NLDFT). Figure 6(b) illustrates the pore size distributions of all the prepared samples. The pore size of the samples prepared in the presence of molten salts lies in the range of 0.5 and 3 nm. In contrast, MI possesses mesopores in combination with micropores, and therefore, the average pore size of these samples lies at ~6 nm. It is well known phenomenon that the pore size less than 1 nm is highly beneficial for gas adsorption and storage. Consequently, samples with micropores of an optimum pore size can be employed as efficient candidates for gas adsorption 24 .
At pressures up to 1 bar, the CO 2 adsorption isotherms obtained at 273, 283, and 298 K, respectively, are presented in Fig. 7 and the complete adsorption data is listed in Table 3. At 1 bar and 273 K, microporous carbons exhibit an excellent CO 2 uptake of 9.7 mmol/g (429.6 mg/g), 8.9 mmol/g (392.1 mg/g), 8.3 mmol/g (368.4 mg/g), 7.5 mmol/g (333.1 mg/g), 3.1 mmol/g (137.6 mg/g), and 2.9 mmol/g (130.1 mg/g), respectively, for MIK-700, MIK-800, MIKN-700, MIKN-800, MI-700, and MI-800, which are comparable to the CO 2 uptake of recently reported CO 2 adsorbents ( Table 4). The CO 2 adsorption isotherms of all the sorbents are nonlinear, indicating their microporous nature. All the prepared samples suffered a clear loss in the CO 2 uptake with an increase in temperature from 273 to 298 K. This trend is in accordance with the exothermic nature of the adsorption process, implying that the amount of adsorbed CO 2 decreases with an increase in the temperature 25 . One of the representative samples exhibits a CO 2 uptake of 9.7 mmol/g (429.6 mg/g) at 273 K which decreases to 7.0 mmol/g (312.2 mg/g) and 5.4 mmol/g (238.7 mg/g) at 283 and 298 K, respectively. Furthermore, the relationship between the CO 2 adsorption ability and the textural properties of the activated carbons materials was evaluated. From the textural studies it is revealed that the samples prepared in the presence of KOH exhibit well-defined micropores with a higher micropore volume and narrower pore size distribution. More specifically, MIK-700 and MIK-800 exhibit narrow pore sizes, suggesting that the contribution of narrow micropores to the adsorption capacity is greater than those of the wider micropores and mesopores. This is because, in low pressure studies, these narrow micropores are easily filled by the CO 2 molecules with a greater adsorption potential compared to wider pores. Furthermore, the higher activation temperature (MIKN-800) and presence of NaOH (MIKN-800 and MIKN-700) lead to wider micropores: 1.95 nm for MIKN-800 and 1.79 nm for MIKN-700. Consequently, there is an increase in the specific surface area and pore volume (2984 m 2 /g and Materials S BET a (m 2 /g) V total b (cm 3 /g) V micro c (cm 3 /g) V meso d (cm 3 /g) V micro /V total (%) D pore e (nm)    1.98 cm 3 /g for MIKN-800 and 3246 m 2 /g and 1.79 cm 3 /g for MIKN-700, respectively) but they contributed less to CO 2 adsorption at the atmospheric pressure because no pore filling occurred in these cases. It has been reported that narrow micropores play a crucial role in CO 2 adsorption 26,27 ; however, the correlation between the pore size and CO 2 adsorption has been rarely investigated. Presser et al. reported a perfectly linear relationship between the pore structure (with pore diameter of <0.8 nm) and CO 2 uptake at 1 bar for carbide-derived carbons 28 . Apart from a suitable texture, MIK-700 also possesses higher nitrogen content (2.66%) compared to other samples prepared in molten salt media. This can improve the Lewis acid-base interactions between CO 2 molecules and nitrogen moieties, leading to efficient adsorption. Consequently, the results confirm that the fabrication of efficient CO 2 carbon sorbents requires a precise control of the porosity, with a pore size of approximately 1 nm along with the hetero-atom doping. Easy regeneration is another critical feature that must be considered while designing CO 2 sorbents. In this respect, porous carbons offer certain advantages over other materials, such as zeolites. The isosteric heat of adsorption (∆H ads or Qst) was calculated on the basis of the Clausius−Clapeyron equation shown below.
where, P 2 and P 1 are the values of p(CO 2 ), R denotes the universal gas constant, and T 2 and T 1 are 273 and 283 K, respectively. Figure S2 shows the Q st values as a function of adsorbed CO 2 ; the corresponding data is listed in the Table 3. The initial values of Q st are in the range of 31.6-40.5 kJ/mol at a low surface coverage. However, the values show a decreasing trend with an increase in the amount of adsorbed gas, indicating that the interactions between CO 2 and the pore walls are stronger than those between the CO 2 molecules. The Qst values for the prepared samples are comparable to those of the nitrogen-doped templated carbons from zeolite and mesoporous silica (31−36 kJ/mol) 29,30 , nitrogen-incorporated hierarchical porous carbons (33−37 kJ/mol) 31,32 , and triptycene-derived benzimidazole-linked polymers (29 kJ/mol) 33 . This implies that the adsorption is physical in nature, which strongly depends on the pore structure of porous carbons. If Qst is too large, the regeneration of the sorbents becomes too costly, and if it is too small, the CO 2 uptake and CO 2 selectivity over N 2 selectivity will be too small 34 . However, while choosing a suitable adsorbent material, the absolute uptake of CO 2 is not the only factor to be considered; selectivity over other competing gases and particulates is also an important feature. For a material showing excellent adsorption profile for pure CO 2 gas but simultaneously adsorbing a large amount of another gas in a more realistic gas mixture, the working capacity would be dramatically reduced. Ideally, CO 2 adsorbents must exhibit a selective adsorption behavior over all other sorbates. For this reason, CO 2 /N 2 selectivity studies were performed at 273 K. Selective adsorption behaviors of the adsorbents can be well-explained from the view point of molecular sieving, which demonstrates that kinetic diameters plays a dominant role during the gas adsorption. The kinetic diameters of CO 2 and N 2 are 3.30 and 3.64 Å, respectively, as reported earlier 35 . Hence, owing to the small diameter, CO 2 is adsorbed more than N 2 . Apart from this, another factor to be considered is the thermodynamics, emphasizing the role of the critical temperature of the two gases in the selective adsorption behavior. The critical temperatures (T c ) of gaseous CO 2 and N 2 are reported to be 304.2 and 126.2 K, respectively. As CO 2 has a higher critical temperature, CO 2 molecules easily condense and adsorb on the porous framework.
The selectivities for CO 2 over N 2 (Fig. 8) determined by initial slope method comes out to be 238, 141, 53, 230, The hydrogen adsorption isotherms obtained at 77 K and 1 bar are presented in Fig. 9 and the details are listed in Table 3. The adsorption capacity can be enhanced either by increasing the storage pressure or decreasing the adsorption temperature to 77 K. Here, we chose a low temperature adsorption phenomenon. All the isotherms are Type I (Langmuir-type) isotherms, demonstrating that the formation of a hydrogen monolayer results in a saturation condition which is a usual characteristic for microporous surfaces. However, with an increase in the pressure up to 1 bar, the gas uptake increases continuously but a clear plateau does not appear because of the low pressure measurements. Sample MIKN-800 shows the highest hydrogen storage capacity, reaching up to 4.0 wt% at 1 bar, which is attributed to its highest surface area as well as the maximum pore volume. The present work demonstrates a high performance than the previously reported data, 2.4 wt% for AX21 39 , 2.8 wt% for activated carbons 40 , 2.0 wt% for corncob 41 , 2.6 wt% for wood 42 , and 2.7 wt% for Quercus agrifolia-based ACs 43 . Additionally, MIKN-800 outperforms many MOFs and POPs in terms of its ability to store hydrogen gas at 77 K. This is due to high surface area with optimum pore structure, narrow pore-diameter with hierarchical meso-micro and ultra-micropores as well as nitrogen-doping. Heteroatom doping shows beneficial impact on gas storage property, as already reported in literature 44,45 . Moreover, samples MI-700 and MI-800 have the lowest surface areas and pore volumes and exhibit the lowest H 2 uptake of 0.95 wt%. Based on these results, we conclude that good correlation exists between the textural features (surface area and pore volume) and gas adsorption capacity of the carbons 46,47 .

Conclusions
Sustainable microporous carbons were synthesized via a one-step condensation-activation strategy in the presence of molten salts. The hydroxide-containing molten salts are responsible for generating well-developed microporous structures with a high surface area. CO 2 and H 2 adsorption properties of the fabricated materials were investigated and the results reveal that the microporous carbons prepared using KOH at 700 °C exhibit an exceptionally high CO 2 adsorption (uptake = 9.7 mmol/g at 273 K and 1 bar). This outstanding CO 2 adsorption performance of the prepared carbons is principally attributed to the narrow micropores (1.05 nm) and ultimately the optimum micropore volume (1.33 cm 3 /g). Similarly, the high specific area and pore volume contribute significantly to the H 2 storage performance at 77 K, with 4.0 wt% uptake of H 2 by MIKN-800 (specific surface area and pore volume of 2984 m 2 /g and 1.98 cm 3 /g, respectively). Hence, present work contributes an effort to design solvent-free single step strategy for high surface area containing microporous carbons as novel materials for efficient CO 2 and H 2 adsorption with an excellent CO 2 /N 2 selective adsorption behavior.

Materials and synthesis.
All the reagents comprising melamine (99%), isophthalaldehyde (99%), sodium hydroxide (98%), potassium hydroxide (95%), and hydrochloric acid were obtained from Sigma-Aldrich. In this study, melamine and isophthalaldehyde were carbonized to design two different types of porous carbons. The first, labeled as MIK-X, was synthesized using 1.24 g of melamine (M) with 1.36 g of isophthalaldehyde (I), and 5.0 g of KOH (K). All the reagents were mechanically ground and heated directly at 250 °C for 180 min followed by carbonization at the targeted temperature for 60 min with the heating rate of 1 °C min −1 . The resulting product was neutralized to pH 7 by washing sequentially with 3 M HCl solution and distilled water. The obtained material was allowed to dry overnight at 120 °C. The other sample is denoted as MIKN-X. The preparation was carried out following the same procedure described for MIK-X, except that the composition of the activating agents was 2.5 g KOH and 2.5 g NaOH (KN). For evaluating the role of molten salts, blank samples containing a mixture of melamine and isophthalaldehyde (MI-X) were prepared in the absence of the hydroxides. In the names of the prepared samples, X denotes the targeted temperature of carbonization (700 or 800 °C).
Physicochemical characterization. The synthesized carbon materials were characterized by different analytical techniques. FTIR spectra of the fabricated samples were collected between 4000−400 cm −1 using a Fourier transform-infrared vacuum VERTEX 80 V spectrometer. A thermal gravimetric analyzer (TGA; TG209F3) determined the thermal stability of the prepared materials under nitrogen atmosphere. X-ray diffraction patterns were recorded on a D2 PHASER, BRUKER, X-ray diffractometer. All the recordings were made in the range of 2° to 80° (2θ). For exploring the morphological structures, field emission scanning electron microscopy (FESEM; Model SU8010, Hitachi Co., Ltd.) was used. Elemental analysis was performed using an EA1112 element analyzer. Further surface characterization was performed by X-ray photoelectron spectroscopy using XPS, VG Scientific Co., ESCA LAB MK-II. The textural features of the prepared materials were determined by obtaining N 2 adsorption−desorption isotherms from a Model Belsorp Max instrument (BEL Japan, Inc.). Carbon dioxide adsorption capacity at 273, 283, and 298 K and hydrogen storage at 77 K were determined by a Model Belsorp Max instrument (BEL Japan, Inc.). For the CO 2 /N 2 selectivity measurements, adsorption measurements for both the gases were carried out at 273 K using a Model Belsorp Max instrument (BEL Japan, Inc.). Prior to all the adsorption measurements, the fabricated materials were stored under vacuum conditions by heating at 120 °C for 8 h.