Ultramicroporous Carbons Derived from Semi-Cycloaliphatic

Oct 6, 2017 - Ultramicroporous carbons (UMC-Ts) have been successfully prepared using nitrogen- and oxygen-rich porous semicycloaliphatic polyimide as...
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Ultramicroporous Carbons Derived from Semi-Cycloaliphatic Polyimide with Outstanding Adsorption Properties for H2, CO2, and Organic Vapors Jun Yan, Biao Zhang, and Zhonggang Wang* State Key Laboratory of Fine Chemicals, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: Ultramicroporous carbons (UMC-Ts) have been successfully prepared using nitrogen- and oxygen-rich porous semicycloaliphatic polyimide as a precursor in the presence of KOH at different carbonization temperatures of 600, 700, and 800 °C, respectively. The evolution of porous and chemical structures of the resultant carbons in the course of carbonization as well as their effects on adsorption of H2, CO2, benzene, and cyclohexane are studied in detail. Compared with the porous polyimide precursor, after carbonization treatment, the products exhibit the significantly increased BET specific surface areas from 900 to 2406 m2 g−1 and create large amounts of ultramicropores with the pore size smaller than 0.5 nm, leading to outstanding adsorption capacities for CO2 (34.0 wt %, 273 K/1 bar) and H2 (3.7 wt %, 77 K/1 bar). Moreover, it is interesting to observe that UMC-Ts possess extraordinarily large uptake for benzene (74.4 wt %, 298 K) and cyclohexane (64.8 wt %, 298 K) at the very low relative pressure (P/P0 = 0.1), showing promising applications in CO2 capture, H2 storage, and removal of toxic organic vapors.

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catalysts.24,25 In addition, compared with the conventional activated carbons (CAs), the interaction between carbonized porous and CO2 molecule can be enhanced through the incorporation of heteroatoms by employing nitrogen- or oxygen-rich precursors. In the past several years, a number of hyper-cross-linked microporous polyimides have been prepared via polycondensation from various dianhydrides and multiamines.26−35 The network structure and extremely rigid heteroaromatic skeleton endow them with good resistance to high temperature and aggressive chemicals. Moreover, the presence of abundant electron-rich nitrogen and oxygen atoms in polyimides are favorable for the affinity porous wall toward CO2 gas. However, the review of literature reveals that the porous carbons based on microporous polyimides have been seldom reported up to now. Very recently, a semicycloaliphatic microporous polyimide (sPI) from bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride and tetrakis(4-aminophenyl)methane was synthesized in our group, which could uptake 23.26 wt % CO2 (273 K/1 bar) and 2.18 wt % H2 (77 K/1 bar).23 The thermogravimetric analysis shows that sPI possesses high residual carbon weight (over 50 wt %) at a high temperature of 800 °C, which is important as carbon resources for the preparation of porous

INTRODUCTION Over the past decade, microporous materials with large surface area have found fascinating applications in H2 storage, removal of toxic organic vapors, gas sensing, and heterogeneous catalysis.1−10 Particularly, with respect to the global warming caused by the increased CO2 concentration in atmosphere, CO2 capture by microporous materials are attractive owing to the merits such as energy savings, convenient recovery of CO2, recyclable use of adsorbent, and ease of operation, which are significantly advantageous over the conventional amineabsorbing process.11−16 Among porous materials, microporous organic polymers (MOPs) with building blocks linked entirely via robust covalent bonds receive increasing attention due to their excellent physicochemical stability under harsh service conditions.7,13,17,18 However, the adsorption properties of MOPs are still unsatisfied as the CO2 adsorption capacities in the reported organic porous polymers usually are only in the range from 10 to 20 wt % (273 K/1 bar) although a few polymers can reach 21.2−26.7 wt %.19−23 In this regard, seeking new porous materials with large gas adsorption capacities becomes a urgent task from the point of practical application of view. The studies show that the carbonized samples usually exhibit increased specific surface area compared with the parent microporous polymers and that the porous structure such as pore size and distribution can be optimized by changing the carbonization conditions like temperature, atmosphere, and © XXXX American Chemical Society

Received: June 22, 2017 Revised: September 3, 2017

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DOI: 10.1021/acs.jpcc.7b06133 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C carbons with high yield. Besides, relative to wholly aromatic polyimides, the incorporation of cycloaliphatic structure in the network will facilitate the reduction of carbonization temperature so as to be helpful for the reconstruction of the network structure to create extra pores. Herein, the carbonization modification of semicycloaliphatic microporous polyimide (sPI) was undertaken with the motivation to further promote their gas adsorption capacity. The influence of the carbonization conditions on surface area and pore morphology as well as the adsorptions of gases and organic vapors for the carbonized products and the comparison with the precursor sPI were studied in detail.

Scheme 1. Chemical Structure of Semi-Cycloaliphatic Microporous Polyimide (sPI) and Its Carbonization to UMC-Ts

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EXPERIMENTAL SECTION Materials. Bcyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) was purchased from J&K Chemical Co. Tetrakis(4-aminophenyl) methane (TAPM) was prepared according to the previous procedures described in the literature.28 The semicycloaliphatic microporous polyimide (sPI) was synthesized from BCDA and TAPM using m-cresol as a solvent and isoquinoline as a catalyst.23 All the other reagents were purchased from Shanghai Chemical Reagent Co. and used as received. Instrumentation. Elemental analyses were determined with an Elementar Vario EL III elemental analyzer. X-ray photoelectron spectroscopy (XPS) analysis was performed on a ThermoVG ESCALAB 250 spectrometer employing an Al Kα (1486.6 eV). Transmission electron microscopy (TEM) images were obtained on a Hitachi HT-7700 operated at 100 kV. Adsorption and desorption measurements for all the gases and vapors were conducted on an Autosorb iQ (Quantachrome) analyzer. Prior to measurements, the samples were degassed at 150 °C under high vacuum overnight. Adsorption and desorption isotherms of nitrogen were measured at 77 K. H2 adsorption isotherms were measured at 77 and 87 K up to 1.0 bar. CO2 and N2 adsorption isotherms at 298 K up to 1 bar were measured in order to evaluate the adsorption selectivities of CO2/N2, which were calculated from the ratios of initial slope and ideal adsorbed solution theory (IAST) methods from the pure component sorption isotherms of gases. The adsorptions of benzene and cyclohexane vapors were measured with the pressure near to the saturated vapor pressure at 298 K. Synthesis of Ultramicroporous Carbons Based on Semicycloaliphatic Microporous Polyimide. sPI and KOH were thoroughly mixed with a weight ratio of 1:2 at room temperature. Then, the mixtures were transferred to a tube furnace, and the carbonizations were carried out under a flow of N2 at the temperature of 600, 700, and 800 °C for 1 h, respectively. After cooling to room temperature, the samples were washed with 2.0 M aqueous HCl three times to remove inorganic salts, followed by washing with a large amount of distilled water to neutrality. The resultant porous carbons were denoted as UMC-Ts, where T represents the activation temperature.

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provide abundant nitrogen and oxygen heteroatoms for the final porous carbons (UMC-Ts). In the previous reports, the effect of polymer: KOH ratio on the porous structures of the obtained carbons and activation mechanism have been extensively studied. For example, Wang and Kaskel found that the increase of the charged KOH content evidently made the micropores broaden and formed larger micropores.2 In addition, Fuertes and co-workers prepared the N-doped porous carbons using KOH as activating agent and polypyrrole (PPy) as carbon precursor. They found that the mildly activated carbon (KOH/PPy = 2) exhibited more nitrogen content and narrower micropore size compared to the sample prepared under the severe condition (KOH/PPy = 4).4 It is known that the larger micropores and broader pore distribution are disadvantageous for adsorption properties of small gas molecules like CO2 and H2. On the basis of the above studies, in this work, we selected a mild activating condition with a lower polymer/KOH ratio of 1:2 for the preparation of polyimide-based porous carbons, with the emphasis on the study about the effect of carbonization temperature on the textural and gas adsorption properties. The chemical compositions of UMC-Ts and the polyimide precursor (sPI) were studied by means of elemental analysis (EA) and X-ray photoelectron spectroscopy (XPS). The XPS survey spectra are shown in Figure S1. The peaks at 532.6, 400.1, and 284.6 eV are attributed to the oxygen, nitrogen, and carbon signals, respectively.36,37 Their contents calculated from XPS spectra are given in Table 1, and the corresponding values measured by the elemental analysis results are also listed as a comparison. The results show that except for sPI the carbon contents of UMC-Ts measured by EA are lower than those measured by XPS. On the contrary, the oxygen obtained by EA displays apparently higher values. The reason lies in is that porous materials with large surface area are liable to adsorb moisture from the atmosphere, leading to the higher oxygen and hydrogen contents and thereby the lower nitrogen and carbon by EA measurement.38,39 Relatively, the values obtained by XPS method might be more reliable since the operation under the high vacuum condition can completely eliminate the effect of moist. It is seen from Table 1 that the hightemperature carbonization and KOH activation treatments bring about the reduction of nitrogen and oxygen, and the losses become more serious with the increase of temperature. The similar phenomena have been observed in previous reports for other porous materials.4,40 In order to further analyze the chemical evolution in the course of high-temperature carbonization of sPI, the high-

RESULTS AND DISCUSSION

Synthesis and Characterization. Chemical structure of the semicycloaliphatic microporous polyimide (sPI) is illustrated in Scheme 1. The network is composed of aliphatic and imide rings cross-linked with tetraphenylmethane nodes. After carbonization treatment, the large amounts of imide rings can B

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The Journal of Physical Chemistry C Table 1. Elemental Components of sPI and UMC-Ts by Elemental Analysis and XPS Methods elemental analysis (wt %)

a

XPS (wt %)

sample

C

N

H

Oa

C

N

O

sPI UMC-600 UMC-700 UMC-800

67.08 77.90 79.80 79.68

6.14 1.81 1.06 1.68

4.85 2.59 2.20 2.18

21.93 17.70 16.19 16.46

66.75 81.64 84.27 84.80

13.82 3.93 2.78 2.70

19.43 14.43 12.95 12.50

Calculated according to the contents of C, H, and N elements measured.

(D) is assigned to the carbonyl carbon of imide ring. After carbonization, UMC-Ts appear as a peak at 286.3 eV (C), belonging to the carbon atom of the newly generated ether linkage.36,41 The O 1s spectra of the four samples are shown in Figure 1 (right). The discrete peaks can be assigned to different oxygenbased functional groups. After carbonization treatment, the former signal at 532.73 eV (F) for oxygen in imide ring of sPI is absent. Alternatively, two new peaks at the binding energy of 532.28 eV (E) and 533.46 eV (G) emerge,36,41 which are attributed to the oxygen atoms of amide and ether linkages, respectively. Relative to amide, the peak intensity of ether linkages apparently enhances with the carbonization temperature. The assignments above are consistent with the previously reported values of polyimides42,43 and carbonized materials.24,36 The signals of nitrogen in N 1s spectra are much weaker than those of oxygen and carbon, but the comparison between UMC-Ts and sPI can also reveal important information (Figure S2). sPI only appears a single peak at 401.04 eV due to the imide ring, which is observed to shift toward the lower binding energy regions with the pyrolysis at high temperatures and appear majorly as two peaks at 400.17 and 399.56 eV, suggesting that new amide- and amine-type nitrogen species have formed.41 Together with the analysis of C 1s and O 1s spectra, it is rational to conclude that the high-temperature and KOH activation treatments have resulted in the cleavage of imide rings and formed new amide and ether linkages. Porous Properties. The porosity parameters of sPI and UMC-Ts were investigated by gas sorption using N2 probe at 77 K. Their adsorption−desorption isotherms are presented in Figure 2a. The uptakes of both sPI and UMC-Ts exhibit a steep rise of at the very low relative pressure (P/P0 < 0.01). This can

resolution C 1s, O 1s, and N 1s spectra were recorded. The C 1s and O 1s spectra are dissected by means of the spectra deconvolution software. As illustrated in Figure 1 (left), for sPI

Figure 1. XPS high-resolution C 1s and O 1s spectra of sPI and UMCTs.

precursor, the aromatic and cycloaliphatic carbons locate at binding energies of 284.6 eV (A). The peak at 285.7 eV (B) shows the presence of C−N linkage, whereas that at 288.6 eV

Figure 2. (a) Adsorption (filled symbols) and desorption (empty symbols) isotherms of N2 for sPI and UMC-Ts. (b) Pore size distribution curves for sPI and UMC-Ts calculated by the NLDFT method. C

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The Journal of Physical Chemistry C be classified as type I sorption, indicative of the existence of substantial micropores.44 The surface areas of sPI and UMC-Ts were calculated from the adsorption isotherms using the Brunauer−Emmett−Teller (BET) model. The pressure ranges were determined according to the calculated plots from the adsorption isotherms (Figures S3 and S4).45 As shown in Table 2, the BET surface area (SBET) of sPI is only 900 m2 g−1. After Table 2. Porosity Parameters of sPI and UMC-Ts Obtained by N2 Adsorption sample

SBET (m2 g−1)

Smicro (m2 g−1)a

Vtotal (cm3 g−1)b

Vmicro (cm3 g−1)c

Vmicro/ Vtotal

sPI UMC-600 UMC-700 UMC-800

900 1980 2212 2406

559 1650 1937 2027

0.594 0.999 1.070 1.257

0.254 0.683 0.798 0.881

0.43 0.68 0.75 0.70

a

Microporous surface area calculated using the t-plot method. bTotal porous volume calculated at P/P0 = 0.90. cMicroporous volume calculated using the t-plot method. Figure 3. HR-TEM images of sPI and UMC-Ts.

high-temperature carbonization and KOH activation, the SBET values of UMC-Ts dramatically grow to 2406 m2 g−1, and the micropore surface area (SMicro), micropore volume (VMicro), and total pore volume (Vtotal) also increase from 559 m2 g−1, 0.254 cm3 g−1, and 0.594 cm3 g−1 to 2027 m2 g−1, 0.881 cm3 g−1, and 1.257 cm3 g−1, respectively. In contrast, before carbonization treatment, the microporous polyimide sPI displays apparent adsorption−desorption hysteresis because of the swelling effect and deformation of the pore structure in the course of measurements in liquid nitrogen due to the “softness” of organic polymer segments.7,17,18 Differently, UMC-Ts show completely reversible adsorption−desorption isotherms, indicating that that the carbonized porous architectures are more rigid than those of the organic sPI precursor. From the N2 isotherms at 77 K, the pore size distribution of sPI was obtained by applying the nonlocal density functional theory (NLDFT), whereas the pore size distributions of IUMC-Ts were obtained by applying the quench solid density functional theory (QSDFT) on the adsorption branch and assuming slit-like geometry on carbon material. As shown in Figure 2b, for UMC-600 and UMC-700, the former peak of the sPI precursor centering at 0.60 nm still can be found although the intensity becomes very weak, implying that partial microporous structure of sPI remains to some extent. In addition, two strong peaks at 0.49 and 0.44 nm in the ultramicropore region appear. With further increasing the temperature to 800 °C, the former peak of sPI disappears, and two new peaks appear corresponding to the ultrasmall pores at 0.44 nm and the larger pores with a broad pore size distribution from 0.60 to 1.06 nm. The internal pore morphologies of sPI and UMC-Ts were observed by high-resolution transmission electron microscopy (HR-TEM). Their images are illustrated in Figure 3. The pore channels can be clearly observed for sPI, UMC-600, and UMC700. The dark regions are attributed to the porous skeletons, whereas the white regions belong to the pore channels. In contrast, the porous structures in UMC-800 become blurred, and the pore sizes are heterogeneous. The above observations are consistent with the pore size analyses according to nonlocal density functional theory. At the slightly lower temperature, the pores of sPI can partially reserve. However, at a very high temperature, e.g., 800 °C, the microporous structures in sPI

have been completely destroyed due to the cleavages of imide bonds. The random fusing of the cracked fragments results in the generation of the pores with broad pore size distribution. H2 Adsorption Property. The H2 sorption isotherms at 77 and 87 K for both sPI and UMC-Ts are presented in Figure 4. In comparison with the sPI precursor, at 77 K and 1.0 bar, the H2 uptake of the carbonized UMC-Ts considerably increases from 2.2 to 3.7 wt % (Table 3), which value is superior to the previously reported values of porous organic polymers like PPF-1 (2.75 wt %)20 and ICOF-2 (3.11 wt %).46 Moreover, it is seen that the adsorbed H2 amount in IUMC-Ts continually grows with pressure and has not reached saturation up to 1 bar, implying that larger storage can be expected at a further increased pressure. In addition, despite the reduction of H2 uptake in UMC-Ts with the rise of temperature due to the physisorption nature, at 87 K and 1 bar, UMC-700 still possesses quite high hydrogen adsorption capacity (2.9 wt %). In addition to the large surface area, the newly created ultrasmall pores at around 0.44 nm are responsible for UMCTs’ extraordinary H2 adsorption capability as supported by the previous reports that ultrasmall pores (less than 0.5 nm) are advantageous for the preferential adsorption of small gas molecules like H2.10,47,48 The comparison of the H2 adsorption capacity in the three carbonized samples shows that at 87 K and 1 bar the H2 adsorption capacity in UMC-700 (2.9 wt %) exceeds that of UMC-800 (2.6 wt %) although the SBET and Vtotal of UMC-700 are smaller than those of UMC-800. The reason is that relative to those of UMC-700 the relatively larger pore size of UMC800 has a broad pore size distribution at 0.60−1.06 nm (Figure 2b), and the ratio of VMicro/Vtotal in UMC-800 is 0.70, which is lower than that of UMC-700 (0.75). The larger proportion of ultrasmall pores in UMC-700 plays a positive role in the H2 adsorption.10,47,48 From the H2 adsorption isotherms measured at different temperatures, the isosteric enthalpies of adsorption (Qst) were calculated by Clausius−Clapeyron equation and plotted as a function of the adsorbed amount of H2 molecules. As shown in Figure 5, all the samples initially have a higher enthalpy of adsorption, but the Qst values rapidly drop with the increase of D

DOI: 10.1021/acs.jpcc.7b06133 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Adsorption (filled symbols) and desorption (empty symbols) isotherms of H2 for sPI and UMC-Ts.

narrow more rapidly to reach a optimal trapping size to capture H2 molecules, leading to the apparently higher Qst values for UMC-Ts than sPI at the high H2 adsorbed amount. CO2 Adsorption and CO2/N2 Selectivity. The CO2 adsorptions in sPI and UMC-Ts were studied through the measurement of CO2 uptakes at 273 and 298 K up to 1 bar. The sorption isotherms are shown in Figure 6, and the data are listed in Table 3. For all the four samples, the adsorption− desorption isotherms are completely reversible, suggesting that the CO 2 adsorptions in both sPI and UMC-Ts are physisorption in nature. Taking account of the recycling requirement for the adsorbent in the CO2-capture application, the ease of desorption of CO2 under the reduced pressure is significantly advantageous over the conventional liquid-amineabsorbing process owing to the high energy-consuming in regeneration of amine and recovery of CO2. Besides, it is surprising to see that the CO2 adsorption capacities in UMC-Ts considerably increase from 23.3 wt % for sPI to 34.0 wt % (7.7 mmol g−1) at 273 K and 1 bar, which can compete with the highest values for the porous carbons such as CPC-600 (33.1 wt %)24 and BIDC-1-700 (37.0 wt %)49 and are superior to other kinds of porous materials including zeolites, MOFs, and COFs measured under the same conditions such as ZIF-20 (12.4 wt %),50 SNU-4 (20.6 wt %),51 and COF-6 (16.7 wt %).52 Moreover, under the ambient condition (298 K/1 bar), the CO2 uptake of UMC-700 still approaches 17.9 wt %, higher than most porous organic polymers and N-doped porous carbons41 and comparable with those of MOFs such as SIFSIX2-Cu-i (23.8 wt %),53 Zn-MOF-74 (19.8 wt %),54 HKUST-1 (18.4 wt %),55 ZIF-70 (10.8 wt %),56 MOF-177(6.5 wt %),57 Zeolite 13X (21.1 wt %),58 SIFSIX-3-Cu (11.9 wt %),59 SIFSIX-3-Ni (12.7 wt %),60 and CCF-1-SE (8.6 wt %).61 The adsorption capacity of CO2 in a microporous material is mainly dominated by three major factors: specific surface area, binding ability with CO2 molecule, and microporous structure. The interaction between pore wall and CO2 molecule can be characterized by the isosteric enthalpies of adsorption (Qst). The variation of CO2 isosteric enthalpies with the adsorbed amount is illustrated in Figure S3. Different from H2 molecule, CO2 has the larger permanent dipole and quadrupole moment (1.52 × 10−26 esu cm2). Consequently, its isosteric adsorption enthalpy is greatly affected by the amount of heteroatoms in

Table 3. Uptakes of H2, CO2, and CO2/N2 Selectivity of sPI and UMC-Ts H2 (wt %)a

CO2 (wt %)a

sample

77 K

87 K

273 K

298 K

sPI UMC-600 UMC-700 UMC-800

2.2 3.4 3.7 3.6

1.7 2.6 2.9 2.6

23.3 32.8 34.0 30.1

13.3 17.3 17.9 15.2

CO2/N2 selectivityb 48.0 19.5 17.0 15.9

75.4 21.1 18.6 17.1

a Gas uptakes at 1.0 bar. bCO2/N2 selectivity at 298 K calculated by initial slope and IAST methods.

Figure 5. Variation of H2 isosteric enthalpies with the adsorbed amount.

H2 amount, implying that H2 molecule has a stronger affinity toward porous skeleton than does H2 itself. Besides, the comparison between sPI and UMC-Ts reveals that the Qst values of the carbonized UMC-Ts are higher than that of sPI particularly at the large H2 loading region. As mentioned earlier, after carbonation treatment, the UMC-Ts samples generate the ultrasmall pores (0.44−0.49 nm), leading to the more significant trapping effect for H2 molecules compared with that of sPI with the larger pores (0.60 nm). Moreover, considering that the kinetic diameter of H2 is only 0.29 nm, relative to sPI, when the pore walls have been covered with layers of gas molecules, the pore channels in UMC-Ts become E

DOI: 10.1021/acs.jpcc.7b06133 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 6. Adsorption (filled symbols) and desorption (empty symbols) isotherms of CO2 for sPI and UMC-Ts.

Figure 7. Adsorption isotherms of benzene (a) and cyclohexane (b) at 298 K for sPI and UMC-Ts.

play a more important role for the increase of CO2 uptake because of trapping effect for small gas molecules. Besides, similar to H2 adsorption, the CO2 adsorption capacity in UMC-800 is the lowest among the there carbonized samples. In addition to the slightly lower concentrations of CO2-philic nitrogen and oxygen elements compared to those of UMC-600 and UMC-700, the relatively larger pore size, wider pore size distribution and the smaller ratio of VMicro/Vtotal in UMC-800 are also disadvantageous factors. In order to assess the selectivity of CO2 over N2, the adsorption isotherm of nitrogen at 298 K for UMC-Ts and sPI were measured (Figure S5). The initial slopes of CO2 and N2 derived from the adsorption isotherms were calculated (Figure S6), and their ratios of CO2 to N2 are used as the CO2/N2 selectivity. In addition, the selectivities of CO2/N2 were also evaluated by the IAST method by specifying the flue gas composition as CO2/N2 = 0.15/0.85. The obtained selectivities of CO2/N2 are plotted as a function of pressure up to 1 bar (Figure S7), and the results are listed in Table 3. It is seen that the adsorption selectivities by the two methods give the same changing trend. The results in Table 3 show that compared with that of sPI the CO2/N2 selectivity of the UMC-600 drops

porous samples. As shown in Figure S5, among the four samples, sPI exhibits the highest adsorption enthalpy, followed by UMC-600, whereas the Qst values of UMC-700 and UMC800 are the lowest, which is consistent with their ranking order of the contents of CO2-philic nitrogen and oxygen elements in UMC-Ts and the sPI precursor. In addition, it is observed that the Qst values of sPI display apparent decrease with the increase of CO2 coverage on the porous surface, indicative of the strong affinity toward CO2 molecule. In contrast to sPI, after carbonization treatment, the drop trend of adsorption enthalpies in UMC-Ts becomes unobvious. Particularly, for UMC-800, the variation of Qst values with the increased adsorbed CO2 amount is flat, implying that the binding ability of CO2 molecule with pore wall is nearly equal to CO2 itself. However, in spite of the higher affinity of sPI for CO2 molecule, UMC-Ts still exhibits the extraordinarily higher adsorption capacities than sPI. Therefore, the significantly improved CO2 adsorption property of UMC-Ts should be caused by their larger specific surface area and optimized pore size rather than the interaction between CO2 molecule and pore surface. After carbonization, the newly created ultrasmall pores in UMC-Ts F

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The Journal of Physical Chemistry C from 75.4 to 21.1, and the values further decrease with the rise of carbonization temperature. The reason for the decreased CO2/N2 selectivity for UMC-Ts might be attributed to the losses of the contents of heteroatoms caused by hightemperature carbonization and KOH activation treatments, resulting in the lower affinity of CO2 toward UMC-Ts than sPI, consistent with the results that IUMC-Ts have a lower CO2 enthalpy of adsorption compared with sPI as illustrated in Figure S8. Despite this, the CO2/N2 selectivity of 21.1 is still far higher than that of activated carbons and can compete with many nitrogen-doped porous carbons such as CPC-650 (17),24 BIDC-2-700 (19.1),49 and NPC-650 (12.5).62 Adsorption of Organic Vapors. The high porosity and large surface areas of UMC-Ts inspire us to study their adsorption of toxic organic vapors taking account of the potential demands for applications in air-cleaning and recovery of organic chemicals. For example, benzene and cyclohexane are the widely used solvents and organic chemicals. Their emission from the chemical industry, paints, varnishes, and chemical cleaners are seriously harmful to human health even though at a low concentration. Nevertheless, a review of literature shows that relative to the studies on CO2 capture the adsorption of the organic vapors is less documented for both porous organic polymers and carbons. The adsorption isotherms of UMC-Ts and sPI at 298 K for benzene and cyclohexane vapors are presented in Figure 7. It is interesting to observe that the adsorption behaviors for organic vapors between UMCs and sPI are significantly different. The uptakes of benzene and cyclohexane in UMC-Ts rapidly rise at the initial stage and then level off with the increase of relative pressure, which is characteristic of type I sorption and shows that the carbonized pores have a stronger affinity toward organic molecules. In contrast to UMC-Ts, the adsorbed amounts of benzene and cyclohexane in sPI in the low-pressure region are low and continually increase with the loading pressure. After carbonization treatment, besides ultramicropores, the pores in UMC-Ts become broad and generates a large amount of micropores larger than 0.6 nm. The relatively larger pore sizes are disadvantageous for the adsorption of small gas molecules but can provide the accessibility of benzene and cyclohexane vapors into the porous carbons. In addition, upon exposing to benzene or cyclohexane vapors, the soft polymer segments in sPI are liable to be swollen by the organic molecules, leading to deformation and enlargement of pores, and the degree of swelling increases with the pressure of organic vapors. Different from sPI, the carbonized porous skeletons in UMC-Ts having treated at high temperature are more rigid and highly resistant to organic solvents so that the adsorbed amounts remain almost unchanged with the pressure after the initial rapid uptake at the low relative pressure. At the high relative pressure (P/P0 = 0.9), sPI exhibits high adsorption capacities of benzene and cyclohexane because of a swelling effect. However, the data in Table 4 show that at a very low vapor pressure (P/P0 = 0.1) the uptakes of benzene and cyclohexane in UMC-800 reach 74.4 and 64.8 wt %, respectively, which are much higher than those of sPI (39.2 wt %) and surpass other porous carbons and porous organic polymers. The excellent adsorption capacity of UMC-Ts at the very low vapor pressure is extremely important for the capture of toxic volatile organic compounds (TVOCs) since the indoor concentration of TVOCs is usually low.

Table 4. Comparison of Uptakes of Benzene and Cyclohexane Vapors between sPI, UMC-Ts, and Other Porous Materials Reported in the Literature benzene (wt %)

cyclohexane (wt %)

P/P0

P/P0

sample

0.1

0.9

0.1

0.9

sPI UMC-600 UMC-700 UMC-800 PCN-AD PSN-DA PBI-Ad-2 SMPI-10 PI-ADPM MPI-BTA PAN-2 PSN-3 PAF-2

39.2 55.4 65.6 74.4 27.1 29.0 24.9 25.6 30.2 21.4 10.8 14.4 8.6

176.6 66.3 73.5 86.8 98.0 86.1 70.7 133.8 99.2 72.8a 69.2 80.5 13.6

22.1 45.7 55.5 64.8 13.0 23.2 18.7 11.6 20.9 11.1 7.2 11.2 0.2

78.1 52.7 60.5 73.4 57.4 77.9 42.1 42.1 59.7 40.4 38.3 63.7 0.6

ref this this this this

work work work work 8 12 13 20 31 34 49 63 64

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CONCLUSIONS This paper presents the first report on the preparation of unltramicroporous carbons (UMC-Ts) using microporous polyimide (sPI) as a carbon resource. Compared with sPI precursor, the carbonized products exhibit dramatically increased BET surface areas from 900 to 2406 m2 g−1. The synergistic effect of large surface area, the newly created ultramicropores (less than 0.5 nm), and the heteroatoms-doped porous skeletons gives rise to the excellent gas adsorption capability. For example, the uptakes of CO2 and H2 in UMC-Ts reach 34.0 wt % (273 K/1 bar) and 3.7 wt % (77 K/1 bar), respectively, which are among the highest values for porous carbons and porous organic polymers. Moreover, it is interesting to observe that UMC-Ts display considerably high uptake for organic vapors at a very low relative pressure (P/P0 = 0.1) and 298 K, e.g., 74.4 wt % for benzene and 64.8 wt % for cyclohexane. The outstanding adsorption properties for gases and organic vapors endow UMC-Ts with promising applications in CO2 capture, H2 storage, and removal of indoor toxic organic vapors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06133. XPS survey spectra XPS high-resolution N 1s spectra, adsorption isotherms of CO2 and N2 at 298 K, initial slopes for CO2/N2 adsorption selectivities, variation of CO2 isosteric enthalpies with the adsorbed amount (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhonggang Wang: 0000-0003-0451-1919 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.jpcc.7b06133 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (Nos. 51473026 and U1462125) for financial support of this research.

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