Tetraphenyladamantane-Based Microporous Polybenzimidazoles for

Jun 3, 2015 - Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list of citations to th...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Tetraphenyladamantane-Based Microporous Polybenzimidazoles for Adsorption of Carbon Dioxide, Hydrogen, and Organic Vapors Biao Zhang, Guiyang Li, Jun Yan, 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: Tetraphenyladamantane-based microporous polybenzimidazoles were prepared through condensation polymerizations of 1,3,5,7-tetrakis(4-formyphenyl)adamantane with 3,3′-diaminobenzidine and 1,2,4,5-tetraaminobenzene tetrachloride, respectively, which exhibit large BET surface area up to 1023 m2 g−1 and narrow pore size distribution with pore size locating at about 0.6 nm. The sorption measurements show that they can uptake 17.3 wt % CO2 (273 K/1 bar), 1.6 wt % H2 (77 K/1 bar), 98.0 wt % benzene, and 53.6 wt % cyclohexane (298 K, P/P0 = 0.95). Moreover, according to the Henry’s law initial slope method and ideal adsorbed solution theory, the resultant polymers possess high CO2/N2 selectivity (71) and CO2/CH4 selectivity (12). The influences of porous structures, that is, porous size and distribution of the polymers, on the adsorption properties of gases and organic vapors are interpreted in terms of the pore-size-exclusive effect as well as variations of physicochemical parameters such as Henry’s constants, first virial coefficients, enthalpies of adsorption, and kinetic diameters, critical temperatures of adsorbates.

■

INTRODUCTION Microporous organic polymers (MOPs) with large specific surface area are becoming one of the most rapidly developing areas in functional polymer materials, primarily driven by their potential applications as adsorbents in capture and storage of small molecule gases such as carbon dioxide, hydrogen, methane and recovery of toxic organic vapors with respect to the increasing public attentions about clean energy and environmental pollution issues. Over the past decade, numerous MOPs have been successfully synthesized from various building blocks by means of diverse synthetic strategies, and some of them display either high gas adsorption capacity or excellent adsorption selectivity.1−5 However, from the viewpoint of actual applications, for example, in selective adsorption of CO2 from its gas mixture, it is desired that the microporous polymers possess both high gas adsorption capacity and high selectivity to efficiently capture CO2 from flux gas and landfill gas. Besides, excellent thermal and chemical stabilities should also be taken into account since the porous adsorbents are often utilized under harsh service condition. Gas adsorption capacity of one gas and its selectivity from other gases for a porous polymer are mainly dominated by its specific surface area, porosity and the interaction between gas molecule and polymer skeleton.6−9 Relative to surface area, the latter two factors may play a more important role in gas adsorption property, particularly for the adsorption selectivity. The studies have suggested that ultramicroporous structure (pore size less than 0.7 nm) with narrow pore size distribution © XXXX American Chemical Society

are advantageous for the adsorption of small gas molecules (H2, CO2)10−13 in comparison with the large gas molecules (N2, CH4) because of the pore-size-exclusive effect, while the introduction of nitrogen and oxygen heteroatoms preferentially enhances the affinity of porous surface toward quadrupolar CO2 molecule by virtue of the enthalpy effect.14−22 Based on the above findings, a rational molecular design and structural optimization become a feasible way to create new porous polymers with the combination of high adsorption capacity of H2 and CO2 gases as well as high selectivity of CO2/N2 and CO2/CH4 gas pairs for adsorption/separation applications. Recently, microporous polybenzimidazoles with rigid aromatic and heterocyclic skeleton have been synthesized through condensations polymerizations between aromatic aldehyde and o-diamine,23−25 aromatic aldehyde and odione,26 and aromatic carboxylic acid and o-diamine.27 Most of them display excellent gas adsorption/separation properties. Nevertheless, the reported polybenzimidazoles are generally constructed from triptycene, triphenylene, triphenylbezene, tetraphenylmethane and tetraphenylsilane building blocks, whereas tetraphenyladamantane-based polybenzimidazoles have not appeared up to now. In addition, the previous studies of adsorption properties are mainly focused on H2, CO2, N2, and CH4 gases, but the adsorption and recovery of toxic organic Received: March 23, 2015 Revised: May 19, 2015

A

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Scheme 1. Synthesis Routes to the Microporous Polybenzimidazole Networks

Preparation of Tetraphenyladamantane-Based Polybenzimidazoles (PBI-Ads). The polymerization of PBI-Ad-1 was carried out in a dry Schlenk flask equipped with a stirrer. At −30 °C, the solution of TFPA (0.22 g, 0.41 mmol) in DMF (10 mL) was added dropwise into the solution of DAB (0.18 g, 0.82 mmol) in DMF (10 mL) and stirred under nitrogen protection for 12 h. The mixture was warmed to room temperature and reacted for additional 24 h. The system was aerated for 10 min and sealed. Then the mixture was heated gradually to 130 °C and kept at this temperature for 72 h. The isolated solid was extracted successively with THF in a Soxhlet apparatus for 24 h to remove the possibly unreacted monomers. Drying at 120 °C under vacuum gives yellowish brown product. Elem. Anal. Calcd for C62H44N8·5H2O: C, 75.13%; H, 5.49%; N, 11.31%. Found: C, 75.35%; H, 5.18%; N, 12.00%. The synthetic procedure of PBI-Ad-2 was similar to that of PBI-Ad-1 except that the tetra-amine monomer used was TAB instead of DAB. Elem. Anal. Calcd for C50H36N8·11H2O: C, 63.41%; H, 6.17%; N, 11.83%. Found: C, 62.72%; H, 5.33%; N, 11.61%. Instrumentation. Fourier transform infrared spectra (FTIR) were measured on a Nicolet 20XB FT-IR spectrophotometer in 400−4000 cm−1. Solid-state 13C CP/MAS spectra were recorded using a Varian Infinity-Plus 400 spectrometer at 100.61 MHz at MAS rate of 10.0 kHz using zirconia rotors 4 mm in diameter, a contact time of 4.0 ms and relaxation delay of 2.0 s. Elemental analyses were performed on an Elementar Vario EL III elemental analyzer. Wide-angle X-ray diffractions (WAXD) from 5° to 80° were determined with Rigku D/max2400 X-ray diffractometer (40 kV, 200 mA) with a copper

vapors such as benzene and cyclohexane still remain unexplored. The motivation of the present work, therefore, is to prepare tetraphenyladamantane-based polybenzimidazoles by the polycondensations from 1,3,5,7-tetrakis(4-formylphenyl)adamantane with commercially available 1,2,4,5-benzenetetramine (TAB) and 3,3′-diaminobenzidine (DAB), respectively. It is the first time to incorporate tetraphenyladamantane units in the three-dimensional microporous polybenzimidazoles. Moreover, the remarkably different length-diameter ratios of TAB and DAB monomers will definitely affect the topological structure and interconnecting behavior of struts in the network, which enables us to examine the influence of building blocks on porosity parameters of the resultant polymers, their adsorption of H2, CO2, N2, CH4, organic vapors, and separation capability of CO2/N2 and CO2/CH4 gas pairs in detail.

■

EXPERIMENTAL SECTION Materials. Adamantane, 3,3′-diaminobenzidine (DAB) and 1,2,4,5-benzenetetramine tetrahydrochloride (TAB) were purchased from J&K Chemical Co., Ltd. Dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and other common reagents were purchased from Shanghai Chemical Reagent Co. DMSO was purified by dehydration with 4 Å molecular sieves and distillation under reduced pressure just prior to use. Other reagents were used as received. 1,3,5,7-Tetrakis(4-formylphenyl)adamantane (TFPA) was prepared according to the previous procedures described in the literature.28 B

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C target at a scanning rate of 10°/min. Field-mission scanning electron microscopy (FE-SEM) experiments were carried on a Nova NanoSEM 450. Thermogravimetric analysis curves were recorded on a NETZSCH TG 209 thermal analyzer by heating the samples up to 800 °C at a rate of 10 °C min−1 under an air atmosphere. Sorption measurements for all the gases and vapors were conducted on an Autosorb iQ (Quantachrome) analyzer. The nitrogen adsorption and desorption were measured at 77 and 273 K up to 1 bar. The surface areas were calculated according to the Brunauer−Emmett−Teller (BET) model, while pore size distributions were obtained from nitrogen sorption isotherms at 77 K by the nonlocal density functional theory (NLDFT). Adsorption−desorption isotherms of H2 (77 K, 87 K), CO2 (273 K, 298 K), and CH4 (273 K) were measured up to 1 bar. The adsorptions of benzene and cyclohexane vapors were measured up to the saturated vapor pressure at 298 K.

Figure 2. FTIR spectra of the polybenzimidazole networks.

aldehyde group completely disappeared, indicating the sufficient reaction of monomers under the reaction condition, as described in the Experimental Section. Moreover, the elemental analyses show that, for either PBI-Ad-1 or PBI-Ad-2, the measured elemental composition is roughly consistent with the theoretical value. The products cannot dissolve in any common organic solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethlyacetamide (DMAc), Nmethylpyrrolidone (NMP) and tetrahydrofuran (THF), reflecting the typical characteristic of hyper-cross-linked polymers. The broad peaks in the WAXD patterns (Figure S1) show that the resultant polymers are amorphous in nature. In addition, the products observed under field emission scanning electron microscopy exhibit agglomerating morphology of small particles with rough surface and irregular shape (Figure 3), similar to other microporous polymers.31,32 The

■

RESULTS AND DISCUSSION Synthesis and Structural Characterization. The tetraaldehyde monomer 1,3,5,7-tetrakis(4-formyphenyl)adamantine (TFPA) was synthesized by the reaction of tetraphenyladamantane with titanium tetrachloride and α,α-dichloromethyl methyl ether according to the procedure reported in the previous papers.8 TFPA was then polymerized, respectively, with 3,3′-diaminobenzidine and 1,2,4,5-tetraaminobenzene tetrahydrochloride through one-pot condensation method to achieve two tetraphenyladmantane-based hyper-cross-linked polybenzimidazoles (PBI-Ad-1 and PBI-Ad-1), as shown in Scheme 1. The structures of PBI-Ads synthesized were confirmed by solid-state 13C CP/MAS NMR, FTIR spectra, and elemental analysis. As shown in Figure 1, the carbon signal of −N−C

Figure 3. FE-SEM images of PBI-Ad-1 (a) and PBI-Ad-2 (b).

thermal stabilities were evaluated by thermogravimetric analysis (TGA) under air atmosphere (Figure S2). The initial weightlosses occur at a temperature over 300 °C, and the decomposition of skeleton is over 500 °C. The small weightlosses at around 100 °C are possibly caused by the solvents remained in the porous networks. The porosity parameters of PBI-Ads were investigated by nitrogen sorption at 77 K (Figure 4a). The steep rises of nitrogen uptake at the very low relative suggest that the PBIAds belong to microporous materials. The data in Table 1 show that PBI-Ad-1 has the BET and Langmuir surface areas of 1023 and 1393 m2 g−1, respectively, which are similar or superior to the benzimidazole-linked polymers synthesized from other building blocks such as BILPs (SBET: 599−1306 m2 g−1)23−25 and TBIs (SBET: 582−609 m2 g−1).26 The BET surface area of

Figure 1. Solid-state 13C CP/MAS NMR spectra of polybenzimidazole networks. Asterisks (*) indicate peaks arising from spinning side bands.

N− in the imidazole ring appears at 152 ppm,23−27 the aliphatic carbons in adamantane moiety locate at about 40 ppm,29,30 while the aromatic carbons are corresponded to the overlapping peaks from 110 to 145 ppm. In FTIR spectra (Figure 2), the formation of benzimidazole linkage is indicated by the characteristic bands of C−N and CN vibrations at 1624, 1486, and 1445 cm−1, and N−H stretching vibration at 3428 cm−1.23−27 The bands at 2840 and 2936 cm−1 are assigned to the aliphatic C−H vibrations of adamantane moiety. Notably, after polymerization, the absorption at 1700 cm−1 due to C

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

interpenetration of polymeric segments in the network. In the case of PBI-Ad-2, the shorter and rigid TAB monomer restricts the interpenetration of networks to some extent. Moreover, the large steric hindrance because of the shorter linking struts may produce more topological defects.36 As a result, PBI-Ad-2 exhibits the broader pore size distribution than PBI-Ad-1, and compared with those of PBI-Ad-2, the values of ultramicroporous surface area and volume in PBI-Ad-1 increased by 39.9% and 33.3%, respectively. The larger ultramicroporous surface area and volume in PBI-Ad-1 are expected to significantly influence the adsorption properties of gases, which are discussed below. Adsorption of CO2 and Selectivity of CO2/N2 and CO2/ CH4. The sorption isotherms of CO2 gas at 273 and 298 K are presented in Figure 5. It is noteworthy that, for both samples,

Figure 4. (a) Adsorption (filled) and desorption (empty) isotherms of N2 at 77 K for PBI-Ad-1 (+400) and PBI-Ad-2. (b) Pore size distributions for PBI-Ad-1 (+0.08) and PBI-Ad-2.

PBI-Ad-2 is 926 m2 g−1, slightly smaller than that of PBI-Ad-1. Additionally, for PBI-Ad-1 and PBI-Ad-2, the total pore volumes calculated from the N2 sorption isotherms at P/P0 = 0.9 are 0.64 and 0.73 cm3 g−1, while micropore volumes using the t-plot method are 0.32 and 0.24 cm3 g−1, respectively. Furthermore, it is observed that the adsorption−desorption isotherms for PBI-Ad-1 and PBI-Ad-2 are nearly reversible, which are different from the significant adsorption−desorption hysteresis effect frequently reported for many other microporous polymers owing to the elastic deformation of the organic architecture under the measurement pressure,33−35 demonstrating that the tetraphenyladamantane-based polybenzimidazole skeletons are quite rigid. In addition, different from PBI-Ad-1, PBI-Ad-2 displays a large N2 uptake when the relative pressure exceeds 0.8, indicating the presence of macropores. The macropores are probably derived from the interparticulate voids in PBI-Ad-2 as the FE-SEM images in Figure 3 have revealed that the particles of PBI-Ad-2 are more loosely packed than PBI-Ad-1, which can also be a reason for the larger total pore volume in PBI-Ad-2 (0.76 cm3 g−1) compared to PBI-Ad-1 (0.64 cm3 g−1). The pore size distributions were calculated by the nonlocal density functional theory (NLDFT) from the adsorption isotherms of nitrogen at 77 K. DAB monomer has a larger length-diameter ratio than TAB. Therefore, after cross-linking reaction, it has been anticipated that the longer struts in PBIAd-1 would prop up the framework to generate the larger pores compared with PBI-Ad-2. However, Figure 4b shows that the two samples exhibit almost the same pore size of about 0.6 nm, locating in the ultramicroporous region. In other words, the remarkably different length-diameter ratios between DAB and TAB do not significantly alter the pore size of polymer networks. In addition, DAB monomer has one phenyl and C−C single bond more than TBA. The larger freedom and the longer strut make the mutual interpenetration of networks in PBI-Ad-1 much easier. Consequently, the pores in PBI-Ad-1 have been homogeneously subdivided into ultramicropores by the random

Figure 5. Adsorption (filled) and desorption (empty) isotherms of CO2 for PBI-Ad-1 and PBI-Ad-2.

the adsorption−desorption curves are reversible. This property is quite favorable for the recovery of CO2 gas and reusability of the adsorbents since the adsorbed CO2 gas by the porous PBIAds can be readily released under the reduced pressure. The data in Table 2 show that, at 273 K and 1 bar, the adsorption capacities of CO2 for PBI-Ad-1 and PBI-Ad-2 are 17.3 and 13.7 wt %, respectively, which compete with other microporous polybenzimizoles under the same measurement condition, such as BILP-2,5,10,14,15 (11.8−17.6 wt %)s,23 TBIs (11.8−14.0 wt %),26 PBIs (4.5−13.2 wt %),27 but are inferior to BILP-1,3,4,6,7 (18.8−23.5 wt %).24,25 Moreover, the isotherms display that the CO2 uptakes for PBI-Ads have not reached saturation up to 1 bar, implying that much high capacity can be expected at the increased pressure. The two polymers have similar chemical structure and surface area but remarkably distinct CO2 adsorption capacities. PBI-Ad-1 exhibits the much higher CO2 uptake than PBI-Ad-2 even though PBI-Ad-2 has the larger total pore volume. As stated in the Introduction, ultramicroporous structure is advantageous for the preferential adsorption of small gas molecules like CO2 and H2. Relative to PBI-Ad-2, the more significant ultramicroporous characteristics in PBI-Ad-1, for example, the larger surface area and volume in the ultra-

Table 1. Porosity Parameters of the PBI-Ads Obtained by N2 Adsorption samples

SBET (m2 g−1)

SLangmuir (m2 g−1)

Smicro (m2 g−1)

Vtotal (cm3 g−1)

Vmicro (cm3 g−1)

pore size (nm)

PBI-Ad-1 PBI-Ad-2

1023 926

1393 1323

701 501

0.64 0.73

0.32 0.24

0.58 0.60

D

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Table 2. Uptakes of H2, CO2, organic vapors and selectivities of CO2 over N2 and CH4 CO2 selectivity (initial slope) sample

CO2/N2

PBI-Ad-1 PBI-Ad-2

71 70

CO2/CH4 9.9 9.5

CO2 selectivity (IAST)a

gas uptakeb (wt %)

vapor uptakec (wt %)

CO2/N2

CO2/CH4

CO2

H2

C6H6

c-C6H12

60 47

11 (12) 9 (9)

17.3 13.7

1.6 1.3

98.0 76.5

53.6 46.3

a

Calcd for the gas mixture of CO2/N2 with a ratio of 0.15/0.85, and the gas mixture of CO2/CH4 with a ratio of 0.05/0.95 (0.5/0.5) at 1 bar. Uptakes for CO2 at 273 K/1 bar and H2 at 77 K/1 bar. cUptakes for C6H6 (benzene vapor) and c-C6H12 (cyclohexane vapor) at P/P0 = 0.95 and 298 K. b

microporous region, may be a dominating reason for its higher adsorption capacity of CO2 gas. The effect of porous structure on CO2 adsorption is further studied by examining the physicochemical parameters such as first virial coefficient, Henry’s law constant and enthalpy of adsorption of CO2 gas. From the CO2 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 CO2 gas. As shown in Figure 6, the samples initially have the

Figure 7. Virial plots of CO2 and H2 for PBI-Ad-1 and PBI-Ad-2.

Table 3. KH, A0, and Q0 Values of CO2 Adsorption in the Microporous Polybenzimidazoles sample PBI-Ad-1 PBI-Ad-2

T (K) 273 298 273 298

KH mol g−1 pa−1 A0 Ln(mol g−1 pa−1) 1.43 4.74 1.04 3.85

× × × ×

−7

10 10−8 10−7 10−8

−15.758 −16.865 −16.082 −17.073

Q0 kJ mol−1 30.0 26.8

CH4 gas pairs, respectively (Figure S3 and Figure S4). For both samples, the selectivity factors of CO2/N2 are up to 71 at 273 K, competing with zeolitic imidazole frameworks (ZIFs, 20− 50),37 Bio-MOF-11 (81),38 and porous polymers such as APOPs (23.8−43.4) 14 and BILPs (59−113).23−25 The selectivities of CO2/CH4 for PBI-Ad-1 and PBI-Ad-2 are 9.9 and 9.5, respectively, which are superior or comparable to APOPs (5.3−6.7),14 MPIs (8−12),39 and BILPs (8−17).23−25 On the other hand, the CH4 molecule has the larger kinetic diameter (3.80 Å) than N2 (3.64 Å).40 Thus, according to the pore-size-exclusive effect of microporous polymers, relative to N2 gas, CH4 should have the smaller adsorption capacity, but the experimental measurements gave the contrary results. The reason can be attributed to the big difference in the critical temperature (Tc) of the two gases. The previous reports have demonstrated that the gas solubility coefficient in a polymer, which reflects the affinity of a gas toward polymer backbone, is positively correlated with its critical temperature.41 Relative to N2 (126 K), CH4 has a significantly higher Tc value (191 K).42 As a result, CH4 molecule is more preferentially adsorbed by the polymer skeleton than N2, leading to that the selectivity of CO2/CH4 is less than that of CO2/N2. In addition, the adsorption selectivities for the typical binary gas mixtures of flue gas (CO2/N2 = 0.15/0.85), natural gas (CO2/CH4 = 0.05/0.95), and landfill gas (CO2/CH4 = 0.50/

Figure 6. Variations of enthalpies of adsorption with the adsorbed amount of CO2 and H2 gases.

higher enthalpies of adsorption, but the Qst values rapidly drop with the increase of CO2 amount, implying that CO2 molecule has a stronger affinity toward polybenzimidazole skeleton than CO2 itself. The virial plots of CO2 for PBI-Ad-1 and PBI-Ad-2 exhibit quite good straight lines (Figure 7). The intercepts of the lines are the first virial coefficients (A0), which are related to interaction between CO2 molecule and pore surface. The Henry’s law constants (KH) are calculated from the equation KH = exp(A0), while the limiting enthalpy of adsorption (Q0), that is, Qst at zero surface CO2 coverage, are derived from the plot slope of ln KH versus 1/T. At both 273 and 298 K, the A0 and KH values of PBI-Ad-1 exceed those of PBI-Ad-2, respectively (Table 3), and the Q0 value of PBI-Ad-1 (30.0 kJ mol−1) is also higher than PBI-Ad-1 (26.8 kJ mol−1). The N2 and CH4 adsorption isotherms at 273 K for the two samples were measured in order to evaluate the adsorption selectivity of CO2 over other gases. As shown in Figure 8, under the same condition, CO2 has a considerably higher uptake than N2 and CH4 gases. The ratios of initial slope of CO2 to N2 and CH4 are used to measure the selectivities of CO2/N2 and CO2/ E

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 8. Adsorption isotherms of CO2, CH4 and N2 at 273 K for PBI-Ad-1 and PBI-Ad-2.

as PAF-1 (1.66 wt %, 5600 m2 g−1),7 PPN-3 (1.58 wt %, 4221 m2 g−1),44 and COP-4 (1.53 wt %, 2015 m2 g−1).45 As shown in Figure 6, the enthalpies of adsorption of H2 gas also display an apparently decreasing trend with the adsorbed amount of H2 molecules, implying that the H2-material interaction is stronger than the H2−H2 interaction. The limiting enthalpies of adsorption of H2 calculated from virial plots (Figure 7) are 7.7 and 7.4 kJ/mol for PBI-Ad-1 and PBIAd-2, respectively. Moreover, the first virial coefficient and Henry’s law constant of PBI-Ad-1 at both 273 and 298 K are higher than those of PBI-Ad-2 (Table 4), indicating that the

0.50) are investigated by the method of ideal adsorbed solution theory (IAST).43 The single-site Langmuir−Freundlich curves for CO2, CH4, and N2 well fit the corresponding experimental pure component isotherms (Figure S5 and Figure S6). The selectivities of CO2/CH4 and CO2/N2 are plotted as a function of pressure up to 1 bar. As shown in Figure S7 and Figure S8, for both CO2/N2 and CO2/CH4 gas pair, the selectivities are the highest at low coverage, and exhibit a decrease with the pressure to the lowest values at about 0.2 bar and then rise slowly again with the increase of pressure. The data in Table 2 show that, for PBI-Ad-1, the adsorption selectivity of CO2/N2 at the atmosphere pressure (1 bar) is 60, whereas those of CO2/CH4 for natural gas and landfill gas are 11 and 12, respectively. Moreover, it is found that, using the initial slope method, PBI-Ad-1 and PBI-Ad-2 exhibit the similar adsorption selectivities. But, according to IAST theory, PBI-Ad-1 displays the apparently higher gas selectivities than PBI-Ad-2. Hydrogen Adsorption of Microporous Polybenzimidazoles. H2 adsorption isotherms for both samples are presented in Figure 9, and the data are listed in Table 2. At

Table 4. KH, A0, and Q0 Values of H2 Adsorption in the Microporous Polybenzimidazoles sample

T (K)

PBI-Ad-1

77 87 77 87

PBI-Ad-2

KH mol g−1 pa−1 A0 Ln(mol g−1 pa−1) 1.51 3.77 1.38 3.68

× × × ×

10−6 10−7 10−6 10−7

−13.403 −14.791 −13.494 −14.816

Q0 kJ mol−1 7.7 7.4

large ultramicroprous volume and ultramicroprous surface area in PBI-Ad-1 also play a significant role in the enhancement of adsorption ability of hydrogen similar to CO2 gas. Adsorption of Benzene and Cyclohexane in Microporous Polybenzimidazoles. The sorption of benzene and cyclohexane for the two porous polybenzimidazoles were measured at 298 K, and the curves are presented in Figure 10. In the initial stage, the porous PBI-Ads exhibit a rapid rise of uptakes, suggesting that the polymer skeletons have a stronger affinity toward organic molecules. At P/P0 = 0.95 and 298 K, the sorption capacities of benzene for PBI-Ad-1 and PBI-Ad-2 are 98.0 and 76.5 wt %, respectively (Table 2). Besides surface area, the larger microporous volume of PBI-Ad-1 may be a major reason for its higher sorption capacity than PBI-Ad-2. Overall, the sorption capacities of benzene in both samples are superior to carbon material F42C (39.5 wt %),46 metal azolate framework MAF-2 (20.6 wt %),47 and compete with other porous polymers such as CEs (34.4−58.5 wt %),32 PANs (69.2−72.6 wt %),22 NPI-1 (90.5 wt %),48 PI-ADPM (99.2 wt %),30 but are lower than MPI-1 (119.8),39 PAF-1 (130.6 wt %),6 SMPI-0 (134.7 wt %),19 In addition to the high adsorption capacity of benzene vapor, PBI-Ad-1 also exhibit high uptake of cyclohexane (53.6 wt %), remarkably surpassing those of the wholly aromatic porous polymers such as PBIs (1.0−8.2 wt %),27 PAF-2 (0.7 wt %),49 and SMPI-0 (42.5 wt %).19 The

Figure 9. Adsorption (filled) and desorption (empty) isotherms of H2 for PBI-Ad-1 and PBI-Ad-2.

77 K and 1.0 bar, PBI-Ad-1 and PBI-Ad-2 exhibit the H2 uptakes of 1.6 and 1.3 wt %, respectively. The adsorbed amount of 1.6 wt % can compete with previously reported benzimidazole-linked porous polymers BIIPs (1.4−2.3 wt %),23−25 TBIs (1.3−1.57 wt %),26 and other porous polymers even though some of them possess the larger surface area such F

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

■

ACKNOWLEDGMENTS We thank the National Science Foundation of China (Nos. 51473026, 51273031) and the Program for New Century Excellent Talents in University of China (No. NCET-06-0280) for financial support of this research.

■

Figure 10. Adsorption isotherms of benzene (+100) and cyclohexane vapors at 298 K for PBI-Ad-1 and PBI-Ad-2.

simultaneously high uptakes for aromatic and aliphatic vapors may be owing to the unique structure of tetraphenyladamantane unit. The large amount of phenyls and aliphatic rings in tetraphenyladamantane-based polybenzimidazoles can enhance affinity for both aromatic and aliphatic vapors, which are desirable as adsorbents for removal of organic pollutants in the environmental protection field.

■

CONCLUSIONS Two tetraphenyladamantane-based microporous polybenzimidazoles networks were prepared through one-pot condensation polymerization from 1,3,5,7-tetrakis(4-formyphenyl)adamantine with commercially available 3,3′-diaminobenzidine and 1,2,4,5-tetraaminobenzene tetrachloride, respectively. Their chemical structures were confirmed by means of solid-state 13C CP/MAS NMR, FTIR spectra and elementary analysis. The measurements of sorption of nitrogen at 77 K show that the polymers have large BET surface up to 1023 m2 g−1 with quite narrow pore size distribution centering at around 0.6 nm. Additionally, the polymers possess uptake of hydrogen (1.6 wt %, 77 K) and carbon dioxide (17.3 wt %, 273 K), and exhibit good selectivity of CO2/N2 (71, 273 K) and CO2/CH4 (12, 273 K). In addition, their uptakes for benzene and cyclohexane vapors reach 98.0 and 53.6 wt % at 298 K, respectively. The excellent gas adsorption capability for gas and organic vapors together with good thermal/chemical stability endow the porous polybenzimidazoles with promising applications in clean-energy and environmental protection fields.

■

ASSOCIATED CONTENT

S Supporting Information *

This section contains six figures, including the TGA curves, WAXD curves, and adsorption selectivities of CO2 over CH4 and N2 at 273 and 298 K of the polyimide networks. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02806.

■

REFERENCES

(1) McKeown, N. B.; Budd, P. M. Polymers of Intrinsic Microporosity (PIMs): Organic Materials for Membrane Separations, Heterogeneous Catalysis and Hydrogen Storage. Chem. Soc. Rev. 2006, 35, 675−683. (2) Dawson, R.; Cooper, A. I.; Adams, D. J. Nanoporous Organic Polymer Networks. Prog. Polym. Sci. 2012, 37, 530−563. (3) Thomas, A. Functional Materials: From Hard to Soft Porous Frameworks. Angew. Chem., Int. Ed. 2010, 49, 8328−8344. (4) Wu, D.; Xu, F.; Sun, B.; Fu, R.; He, H.; Matyjaszewski, K. Design and Preparation of Porous Polymers. Chem. Rev. 2012, 112, 3959− 4015. (5) Chang, Z.; Zhang, D. S.; Chen, Q.; Bu, X. H. Microporous Organic Polymers For Gas Storage and Separation Applications. Phys. Chem. Chem. Phys. 2013, 15, 5430−5442. (6) Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M.; et al. Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area. Angew. Chem., Int. Ed. 2009, 48, 9457−9860. (7) Lu, W. G.; Yuan, D. Q.; Sculley, J.; Zhao, D.; Krishna, R.; Zhou, H. C. Sulfonate-Grafted Porous Polymer Networks for Preferential CO2 Adsorption at Low Pressure. J. Am. Chem. Soc. 2011, 133, 18126−18129. (8) Zhao, Y. F.; Yao, K. X.; Teng, B. Y.; Zhang, T.; Han, Y. A Perfluorinated Covalent Triazine-based Framework for Highly Selective and Water-Tolerant CO2 Capture. Energy Environ. Sci. 2013, 6, 3684−3692. (9) Lu, J.; Zhang, J. Facile Synthesis of Azo-Linked Porous Organic Frameworks via Reductive Homocoupling for Selective CO2 Capture. J. Mater. Chem. A 2014, 2, 13831−13834. (10) Saleh, M.; Lee, H. M.; Kemp, K. C.; Kim, K. S. Highly Stable CO2/N2 and CO2/CH4 Selectivity in Hyper-Cross-Linked Heterocyclic Porous Polymers. ACS Appl. Mater. Interfaces 2014, 6, 7325− 7333. (11) Germain, J.; Svec, F.; Fre′chet, J. M. J. Preparation of SizeSelective Nanoporous Polymer Networks of Aromatic Rings: Potential Adsorbents for Hydrogen Storage. Chem. Mater. 2008, 20, 7069− 7076. (12) Luo, Y.; Li, B.; Wang, W.; Wu, K.; Tan, B. Hypercrosslinked Aromatic Heterocyclic Microporous Polymers: A New Class of Highly Selective CO2 Capturing Materials. Adv. Mater. 2012, 24, 5104−5107. (13) Yuan, Y.; Sun, F.; Li, L.; Cui, P.; Zhu, G. Porous Aromatic Frameworks with Anion-Templated Pore Apertures Serving as Polymeric Sieves. G. Nat. Commun. 2014, 5, 4260−4267. (14) Song, W. C.; Xu, X. K.; Chen, Q.; Zhuang, Z. Z.; Bu, X. H. Nitrogen-Rich Diaminotriazine-Based Porous Organic Polymers for Small Gas Storage and Selective Uptake. Polym. Chem. 2013, 4, 4690− 4696. (15) Han, J.; Fan, X.; Zhuang, Z. Z.; Song, W. C.; Chang, Z.; Zhang, Y. H.; Bu, X. H. A Triphenylene-Based Conjugated Microporous Polymer: Construction, Gas Adsorption, and Fluorescence Detection Properties. RSC Adv. 2015, 5, 15350−15353. (16) Zhang, D. S.; Chang, Z.; Lv, Y. B.; Hu, T. L.; Bu, X. H. Construction and Adsorption Properties of Microporous TetrazineBased Organic Frameworks. RSC Adv. 2012, 2, 408−410. (17) Li, G. Y.; Zhang, B.; Yan, J.; Wang, Z. G. The Directing Effect of Linking Units on Building Microporous Architecture in Tetraphenyladmantane Based Poly(Schiff Base) Networks. Chem. Commun. 2014, 50, 1897−1899. (18) Lu, W. G.; Yuan, D. Q.; Sculley, J.; Zhao, D.; Krishna, R.; Zhou, H. C. Sulfonate-Grafted Porous Polymer Networks for Preferential

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C CO2 Adsorption at Low Pressure. J. Am. Chem. Soc. 2011, 133, 18126−18129. (19) Yang, Y.; Zhang, Q.; Zhang, Z.; Zhang, S. Functional Microporous Polyimides Based on Sulfonated Binaphthalene Dianhydride for Uptake and Separation of Carbon Dioxide and Vapors. J. Mater. Chem. A 2013, 1, 10368−10374. (20) Zhu, Y. L.; Long, H.; Zhang, W. Imine-Linked Porous Polymer Frameworks with High Small Gas (H2, CO2, CH4, C2H2) Uptake and CO2/N2 Selectivity. Chem. Mater. 2013, 25, 1630−1635. (21) Islamoglu, T.; Rabbani, M. G.; El-Kaderi, H. M. Impact of PostSynthesis Modification of Nanoporous Organic Frameworks on Small Gas Uptake and Selective CO2 Capture. J. Mater. Chem. A 2013, 1, 10259−10266. (22) Li, G. Y.; Zhang, B.; Yan, J.; Wang, Z. G. Tetraphenyladamantane-Based Polyaminals for Highly Efficient Captures of CO2 and Organic Vapors. Macromolecules 2014, 47, 6664−6670. (23) Rabbani, M. G.; El-Kaderi, H. M. Synthesis and Characterization of Porous Benzimidazole-Linked Polymers and Their Performance in Small Gas Storage and Selective Uptake. Chem. Mater. 2012, 24, 1511−1517. (24) Rabbani, M. G.; El-Kaderi, H. M. Template-Free Synthesis of a Highly Porous Benzimidazole-Linked Polymer for CO2 Capture and H2 Storage. Chem. Mater. 2011, 23, 1650−1653. (25) Rabbani, M. G.; Reich, T. E.; Kassab, R. M.; Jackson, K. T.; ElKaderi, H. M. High CO2 Uptake and Selectivity by Triptycene-Derived Benzimidazole-Linked Polymers. Chem. Commun. 2012, 48, 1141− 1143. (26) Zhao, Y. C.; Cheng, Q. Y.; Zhou, D.; Wang, T.; Han, B. H. Preparation and Characterization of Triptycene-Based Microporous Poly(benzimidazole) Networks. J. Mater. Chem. 2012, 22, 11509− 11514. (27) Yu, H.; Tian, M. Z.; Shen, C. J.; Wang, Z. G. Facile Preparation of Porous Polybenzimidazole Networks and Adsorption Behavior of CO2 Gas, Organic and Water Vapors. Polym. Chem. 2013, 4, 961−968. (28) Li, G. Y.; Zhang, B.; Wang, Z. G. Microporous Poly(Schiff Base) Constructed from Tetraphenyladamantane Units for Adsorption of Gases and Organic Vapors. Macromol. Rapid Commun. 2014, 35, 971− 975. (29) Shen, C. J.; Bao, Y. J.; Wang, Z. G. TetraphenyladamantaneBased Microporous Polyimide for Adsorption of Carbon Dioxide, Hydrogen, Organic and Water Vapors. Chem. Commun. 2013, 49, 3321−3323. (30) Shen, C. J.; Yu, H.; Wang, Z. G. Synthesis of 1,3,5,7-Tetrakis(4Cyanatophenyl)-Adamantane and Its Microporous Polycyanurate Network for Adsorption of Organic Vapors, Hydrogen and Carbon Dioxide. Chem.Commun. 2014, 50, 11238−11241. (31) Rose, M.; Klein, N.; Senkovska, I.; Schrage, C.; Wollmann, P.; Böhlmann, W.; Böhringer, B.; Fichtner, S.; Kaskel, S. A New Route to Porous Monolithic Organic Frameworks via Cyclotrimerization. J. Mater. Chem. 2011, 21, 711−716. (32) Yu, H.; Shen, C. J.; Tian, M. Z.; Qu, J.; Wang, Z. G. Microporous Cyanate Resins: Synthesis, Porous Structure, and Correlations with Gas and Vapor Adsorptions. Macromolecules 2012, 45, 5140−5150. (33) Weber, J.; Antonietti, M.; Thomas, A. Microporous Networks of High-Performance Polymers: Elastic Deformations and Gas Sorption Properties. Macromolecules 2008, 41, 2880−2885. (34) Jiang, J. X.; Trewin, A.; Su, F.; Wood, C. D.; Niu, H.; Jones, J. T. A.; Khimyak, Y. Z.; Cooper, A. I. Microporous Poly(tri(4ethynylphenyl)amine) Networks: Synthesis, Properties, and Atomistic Simulation. Macromolecules 2009, 42, 2658−2666. (35) Ghanem, B. S.; Msayib, K. J.; McKeown, N. B.; Harris, K. D. M.; Pan, Z.; Budd, P. M.; Butler, A.; Selbie, J.; Book, D.; Walton, A. A Triptycene-Based Polymer of Intrinsic Microposity That Displays Enhanced Surface Area and Hydrogen Adsorption. Chem. Commun. 2007, 31, 67−69. (36) Yu, H.; Shen, C. J.; Wang, Z. G. Micro- and Mesoporous Polycyanurate Networks Based on Triangular Units. ChemPlusChem 2013, 78, 498−505.

(37) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties. J. Am. Chem. Soc. 2009, 131, 3875−3877. (38) An, J.; Geib, S. J.; Rosi, N. L. High and Selective CO2 Uptake in a Cobalt Adeninate Metal-Organic Framework Exhibiting Pyrimidineand Amino-Decorated Pores. J. Am. Chem. Soc. 2010, 132, 38−39. (39) Li, G. Y.; Wang, Z. G. Microporous Polyimides with Uniform Pores for Adsorption and Separation of CO2 Gas and Organic Vapors. Macromolecules 2013, 46, 3058−3066. (40) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1994. (41) Shah, V. M.; Hardy, B. J.; Stern, S. A. Solubility of Carbon Dioxide, Methane, and Propane in Silicone Polymers - Effect of Polymer Side-Chains. J. Polym. Sci., Polym. Phys. Ed. 1986, 24, 2033− 2047. (42) Jeans, J. An Introduction to the Kinetic Theory of Gases; Cambridge University Press: London, 1982. (43) Myers, A.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption. AlChE J. 1965, 11, 121−127. (44) Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch, O.; Mulle, T.; Bräse, G. J.; Blümel, J.; Krishna, R.; Li, Z.; et al. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/ Separation. Chem. Mater. 2010, 22, 5964−5972. (45) Xiang, Z.; Zhou, X.; Zhou, C.; Zhong, S.; He, X.; Qiu, C.; Cao, D. Covalent-Organic Polymers for Carbon Dioxide Capture. J. Mater. Chem. 2012, 22, 22663−22669. (46) Lillo-Ródenas, M. A.; Cazorla-Amorós, D.; Linares-Solano, A. Benzene and Toluene Adsorption at Low Concentration on Activated Carbon Fibres. Adsorption 2011, 17, 473−481. (47) Zhang, J. P.; Chen, X. M. Exceptional Framework Flexibility and Sorption Behavior of A Multifunctional Porous Cuprous Triazolate Framework. J. Am. Chem. Soc. 2008, 130, 6010−6017. (48) Li, G. Y.; Wang, Z. G. Naphthalene-Based Microporous Polyimides: Adsorption Behavior of CO2 and Toxic Organic Vapors and Their Separation from Other Gases. J. Phys. Chem. C 2013, 117, 24428−24437. (49) Ren, H.; Ben, T.; Wang, E.; Jing, X.; Xue, M.; Liu, B.; Cui, Y.; Qiu, S.; Zhu, G. Targeted Synthesis of A 3D Porous Aromatic Framework for Selective Sorption of Benzene. Chem. Commun. 2010, 46, 291−293.

H

DOI: 10.1021/acs.jpcc.5b02806 J. Phys. Chem. C XXXX, XXX, XXX−XXX