Azo-bridged covalent porphyrinic polymers (Azo-CPPs): synthesis and CO₂ capture properties

Abstract

A series of azo-bridged, aromatic, nitrogen-rich, microporous covalent porphyrinic polymers (Azo-CPPs) were synthesized through a metal catalyst-free direct coupling of a kind of tetranitro building block, 4NO₂TPP, and several diamine or tetraamine compounds, under basic conditions. The Azo-CPPs were all amorphous and exhibited surface areas between 197 to 591 m² g⁻¹. The effect of building blocks on the pore properties was discussed. Meanwhile, the capacity of adsorption of CO₂ for all the Azo-CPPs was studied and the CO₂ uptake capacity was between 7.36% and 9.22% at 273 K and 1.10 bar. The CO₂/N₂ selectivities were measured between 27.4–53.9 at 273 K and 31.2–107.8 at 303 K, respectively, showing an increase in selectivity with increasing temperature.

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Introduction

Recently, microporous organic polymers (MOPs) have drawn great interest because of their intrinsic properties of a large specific surface area, high chemical and thermal stability, and low density.^1–4 They exhibited potential applications in heterogeneous catalysis,^5–11 gas storage and separation,^12–18 energy storage and conversion,^19–23 gas or chemicals sensors,^24–26 and optoelectronic devices,²⁷ and so on. Due to the urgent need for strategies to reduce global atmospheric concentrations of greenhouse gases, the carbon capture and storage (CCS), one of the very important applications of MOPs, has attracted significant interest.^28–30 Nowadays, tremendous efforts have been made to the design and synthesis of MOPs for CO₂ capture due to their high physicochemical stability and excellent adsorption capacity. Therefore, various CO₂-philic moieties mainly containing heteroatoms,^31,32 such as azos (–N=N–),³³ benzimidazole,³⁴ aminos (–NH₂),³⁵ benzothiazole,³⁶ dicarboximide,³⁷ carboxy and triazole,^38,39 and Tröger's base segments,^40–42 etc., have been incorporated into MOPs to enhance the interaction between the MOPs' surfaces and CO₂ molecules. Among them, the azo bridges, derived from coupling of nitro and amino groups under basic conditions^33,43 or Cu(I) catalyzed coupling of aminos⁴⁴ or zinc-induced reductive coupling of nitro compounds,⁴⁵ have revealed high CO₂/N₂ selectivity due to the N₂-phobicity. Other than the introduction of a kind of CO₂-philic sites, the azo groups also lead to nitrogen-rich segments at the same time, which afford active sites for binding metal ions and thus make the azo-containing MOPs a candidate as heterogeneous catalyst support. Till now, the multiple nitro containing monomers used for azo-bridged MOPs are limited to tetrakis(4-nitrophenyl)methane, 1,3,5,7-tetrakis(4-aminophenyl)adamantane, 2,6,12-triaminotriptycene, 1,3,5-tris(4-aminophenyl)benzene, and 1,1,2,2-tetrakis(4-nitrophenyl)ethane, namely the 3D and 2D linkages, respectively.^33,43–45 Therefore, designing other kind of building blocks with multifunctionalities is highly desirable. On the other hand, as a kind of rigid segment, porphyrin rings have been extensively used to build porous materials for CO₂ capture and conversion and heterocatalyst in preparation of chemically valuable products.^46,47 For example, porphyrin based materials, such as MOFs, COFs and porphyrin-linked porous polymer materials, have been developed to enhance the CO₂ capture profile of porphyrins as high as up to 33.4 wt% (MMPF-2).⁴⁸

Herein, we report a series of azo-bridged covalent porphyrinic polymers (Azo-CPPs) by using the metal catalyst-free direct coupling of nitro and amino groups under basic conditions. We utilized a porphyrin derivative with four nitro groups, 5,10,15,20-tetrakis(4′-nitrophenyl)porphyrin (4NO₂TPP), and several aromatic diamines or tetraamines with different length, steric hindrance, and rigidity as the building blocks to synthesize Azo-CPPs with moderate to high surface areas. We expect that the lone pair electrons of nitrogen atoms from the azo segments can enhance the binding affinity between the Azo-CPPs and CO₂. The synthesis and properties of the Azo-CPPs, the effect of diamines' structures on the surface areas, pore volumes and distribution, the CO₂ adsorption ability and selectivity had been studied in detail.

Experimental

Materials

5,10,15,20-Tetrakis(4′-nitrophenyl)porphyrin (4NO₂TPP) and 5,10,15,20-tetrakis(4′-aminophenyl)porphyrin (4NH₂TPP) were synthesized in our laboratory according to the literature.⁴⁹ Pd–4NO₂TPP was prepared by refluxing 4NO₂TPP powder and PdCl₂ in CHCl₃. The other diamine monomers including p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), benzidine (DABP), 2,2′-bis(trifluoromethyl)benzidine (TFDB), 4,4′-oxydianiline (ODA), and 4,4′-azodianiline (DAAzo), were all purchased from J&K Scientific Ltd. (Beijing) and used without further purification. N,N′-Dimethylformamide (DMF), palladium acetate, and KOH were all purchased from Beijing Chemical Reagents Co., China. DMF was purified by vacuum distillation over CaH₂ and stored over 4 Å molecular sieves prior to use.

Characterization

Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 782 Fourier transform spectrophotometer. ¹H NMR spectra was performed on a Bruker Avance 400 Spectrometer operating at 400 MHz, using CDCl₃ as the solvent. Solid-state ¹³C NMR were performed on a Bruker Avance III Spectrometer operating at a frequency of 100 MHz and using cross-polarization/magic angle spinning (CP/MAS). The magic angle spinning rate was set at 5.0 kHz to minimize spinning side band overlap. Atomic emission spectrum was recorded on Varian AA240. The powder wide-angle X-ray diffraction (PXRD) was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu/K-α1 radiation, operated at 40 kV and 200 mA. Thermogravimetric analysis (TGA) was recorded on a Netzsch thermal analysis system (STA 449C), in nitrogen, at a heating rate of 10 °C min⁻¹. TEM images were obtained with AJEM-2010 transmission electron microscopy (TEM), operating at an accelerating voltage of 200 KV. Nitrogen sorption experiments and micropore analysis were conducted at 77.3 K using Micromeritics ASAP 2020 HD88. Before sorption measurements, the samples were degassed in vacuum overnight at 120 °C. The surface areas were calculated from multipoint BET plot, and the pore volume was determined by nonlocal density functional theory (NLDFT). Carbon dioxide and nitrogen sorption isotherms at 273 K and 303 K were all obtained with a Micromeritics ASAP 2020 HD88 analyzer at the required temperature. The temperature during adsorption and desorption was kept constant using a circulator. Before the adsorption measurements, the samples were activated in situ by increasing the temperature at a heating rate of 10 °C min⁻¹ up to 423 K under vacuum and the temperature and vacuum was maintained for 5 hours before taking the sorption.

Synthesis of the Azo-CPPs

4NO₂TPP was used in the present work as a starting material and the chemical structure was characterized by FT-IR, ¹H and solid-state ¹³C NMR spectra as shown in Fig. S1 to S3 in the ESI.†

All of the Azo-CPPs were synthesized in the same procedure in Schlenk tube, according to the literature.³³ Azo-CPP-1 was given as an example. 4NO₂TPP (238.4 mg, 0.3 mmol), p-PDA (64.9 mg, 0.6 mmol), and KOH (168.3 mg, 3.0 mmol) were placed in a dry 25 mL Schlenk tube. The solids were dissolved in anhydrous DMF (15 mL). The content was degassed by the freeze–thaw method and backfilled with Ar gas three times before closing the tube. Then the tube was placed in oil bath at 150 °C and stirred for 24 hours, during which the purple Azo-CPP-1 precipitated from solution. The solid product was then collected by filtration and washed repeat with water, DMF, acetone, THF, and methanol, respectively. Then it was extracted by acetone, chloroform, and methanol in Soxhlet extractor for 24 hours respectively, in order to remove the residual monomers and/or low-molecular weight by-products. The powder was dried in a vacuum oven for 24 hours to afford the final Azo-CPP-1 (144 mg, yield: 55%). The other Azo-CPPs were synthesized similarly with final yields about 50%.

Results and discussion

Synthesis and characterization of the Azo-CPPs

We synthesized the Azo-CPPs by the metal catalyst-free direct coupling of aromatic amines and nitro compounds under basic conditions, which had been reported recently.^33,43 Herein, a series of Azo-CPPs were prepared by the direct coupling of 4NO₂TPP with various aromatic diamines or tetraamines, including p-phenylenediamine (p-PDA, Azo-CPP-1), m-phenylenediamine (m-PDA, Azo-CPP-2), benzidine (DABP, Azo-CPP-3), 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB, Azo-CPP-4), 4,4′-oxydianiline (ODA, Azo-CPP-5), 4,4′-azodianiline (DAAzo, Azo-CPP-6), and 5,10,15,20-tetrakis(4′-aminophenyl)porphyrin (4NH₂TPP, Azo-CPP-7). The molar ratio of nitro to amine was set as 1 : 1, and the polymerization were all conducted in Schlenk tube under Ar atmosphere at 150 °C in oil bath. During the whole reaction course, the colour of the mixture turned to brown after the reaction began and there was no green colour appeared, indicating the deprotonation of inner NH did not occur obviously. All Azo-CPPs were readily obtained in moderate yields of about 50%. All the yields herein were comparable to the reported results, where the same reactions between nitro and amino groups were studied.⁴³ However, compared with the excellent yields (above 75%, as high as up to 92%) reported by H. M. El-Kaderi and J. Zhang et al.,^44,45 the yields here were relatively low. There may be some low molecular weight oligomers and even some unreacted monomers in the resultant mixture, which can be washed off by the subsequent washings. The formation of Azo-CPPs was characterized by FTIR, solid-state ¹³C NMR, PXRD, and thermogravimetric analysis (TGA).

The degree of polymerization was determined by comparing the FTIR spectra of the polymers and respective monomers. Representative FTIR spectra of a polymer (Azo-CPP-1) and its corresponding monomers were shown in Fig. 1a and b. The formation of azo (–N=N–) linkages was confirmed by the stretching bands at 1469 and 1398 cm⁻¹, of which the intensity was obviously enhanced.⁴⁵ The same bands can be also observed in other Azo-CPPs clearly (Fig. S4 to S6 in the ESI†). At the same time, the bands at about 3300–3400 cm⁻¹ can be attributed to the residual amino groups. The asymmetric and symmetric stretching band of unreacted and/or terminal nitro groups at 1520 and 1350 cm⁻¹ can still be observed, but their intensity attenuated obviously compared with 4NO₂TPP. The chemical shifts in the solid-state ¹³C NMR spectra of Azo-CPP-1 (Fig. 1c) located at 150.4, 143.0, 130.1, and 117.4 ppm confirmed the formation of the azo-linked aromatic polymers.^43–45 At the same time, the peak at 138.9 ppm in p-PDA almost disappeared totally in Azo-CPP-1, also indicating the high reaction extent.

Fig. 1 (a) and (b) The expand FTIR spectra between 3600–2800 cm⁻¹ and 400–1800 cm⁻¹; (c) solid-state ¹³C NMR spectra of Azo-CPP-1 and corresponding monomers (100 MHz); (d) PXRD patterns of Azo-CPPs; (e) and (f) HR-TEM of Azo-CPP-1. (f) is a zoom of the rectangular area in (e).

The Azo-CPPs were also characterized by thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). To investigate the thermal stability of Azo-CPPs, TGA was performed up to 800 °C with a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. As shown in Fig. S7 in the ESI,† all Azo-CPPs exhibited remarkable thermal stability up to 300 °C in N₂. Therefore, they had similar thermal stability compared with the networks obtained by C. T. Yavuz.⁴³ All the TGA curves showed two weight losses. The first weight loss between 300 and 400 °C can be attributed to the decomposition of low molecular weight residual impurities and the other weight loss after 450 °C meant the degradation of organic frameworks. All Azo-CPPs showed high char yields higher than 60%. Due to the nature of their rigid structures, there was no observable glass transition temperature within the range of 30–500 °C (Fig. S8 in the ESI†), just like most of reported Azo-CPPs. The PXRD patterns indicated no long-range structure for any of the Azo-CPPs. All the Azo-CPPs were amorphous with broad diffraction peaks at about 2θ = 20°, corresponding to an average distance of 4.4 Å showing a partial stacking of aromatic units, which were in well accordance with the results obtained by C. T. Yavuz and H. M. El-Kaderi,^33,43,44 respectively. The PXRD results did not show the characterized eclipsed or staggered structures like the reported covalent organic frameworks (COFs) which derived from 4NH₂TPP,^50–52 indicating that the reaction between 4NO₂TPP and diamines in the presence of excess KOH was irreversible. Therefore, the polymerization had not extended the coplanar structure in the 2-D directions to form regular layered structures. Factually, the Azo-CPPs extended in 3-D directions irregularly (Scheme 1) and an amorphous structure was obtained. The morphology was characterized by HR-TEM as shown in Fig. 1e and f, in which the alternating areas of light and dark contrast revealed their disordered porous structural natures. The SEM images of Azo-CPP-1 were shown in Fig. S9 in the ESI.† The obtained particles were aggregated together to form larger particles in size of several micrometers.

Scheme 1 Schematic representation of the synthesis of the Azo-CPPs.

Porosity properties and gas uptake capacities of the Azo-CPPs

The porous character of the Azo-CPPs was studied by nitrogen adsorption–desorption experiments at 77.3 K. All samples were degassed at 150 °C overnight under vacuum prior to the measurement. As shown in Fig. 2a and Fig. S10 in the ESI,† all the isotherms show a steep increase in adsorbed volume at low relative pressure, suggesting that the Azo-CPPs are microporous materials. All Azo-CPPs show type I N₂ sorption isotherms with Brunauer–Emmett–Teller (BET) surface areas ranging from 197 to 591 m² g⁻¹ (Table 1). The Langmuir surface areas of Azo-CPPs are varied from 265 to 798 m² g⁻¹ (Table 1). They all have remarkable hysteresis at the low pressure region in the N₂ isotherms, which indicate the existence of the mesopores, which were in well accordance to the pore size distribution as shown in Fig. 2b and S11 in the ESI.† These mesopores may be caused by the voids between submicrometer agglomerates in the Azo-CPPs.

Fig. 2 Porosity properties of Azo-CPP-1. (a) Nitrogen adsorption and desorption isotherms at 77.3 K; (b) pore size distributions (PSD) calculated by the NLDFT method; (c) CO₂ adsorption isotherms at 273 K of Azo-CPP-1.

Table 1 Porosity data for the Azo-CPPs from N₂ isotherms collected at 77.3 K

Polymer code	Amine	SABET^a [m^2 g^−1]	SALang^b [m^2 g^−1]	SAmicro^c [m^2 g^−1]	V0.1^d [m^3 g^−1]	Vtot^e [m^3 g^−1]	V0.1/Vtot
a BET surface area calculated over the pressure range 0–0.15 P/P0 at 77.3 K.b Langmuir specific surface area calculated from the nitrogen adsorption isotherm by application of the Langmuir equation.c Micropore surface area calculated from the nitrogen adsorption isotherm using the t-plot method.d V0.1, pore volume at P/P0 = 0.1 at 77.3 K.e Vtot, total pore volume calculated at P/P0 = 0.99 at 77.3 K.f Pd–4NO2TPP was used.
Azo-CPP-1	p-PDA	591	798	293	0.2438	0.5617	0.434
Azo-CPP-2	m-PDA	439	592	213	0.1793	0.3147	0.570
Azo-CPP-3	DABP	514	692	271	0.2127	0.3951	0.538
Azo-CPP-4	TFDB	518	696	274	0.2147	0.3803	0.565
Azo-CPP-5	ODA	563	759	274	0.2310	0.3925	0.589
Azo-CPP-6	DAAzo	433	588	164	0.1720	0.2980	0.577
Azo-CPP-7	4NH2TPP	197	265	118	0.0824	0.1338	0.616
Azo-CPP-1–Pd^f	p-PDA	432	623	216	0.2285	0.4802	0.476

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The effect of the average monomer strut length on the physical properties of Azo-CPPs, such as BET surface area, Langmuir surface area, micropore surface area, and micropore volume, had been studied in detail. We chose seven kind of aromatic amine monomers as the building blocks to investigate the effect of building blocks on the final porosity properties and gas uptake capacities of the Azo-CPPs, as shown in Scheme 1. As the length of linker increased gradually in the order of p-PDA < DABP < DAAzo < 4NH₂TPP, the BET surface areas decreased gradually in the order of Azo-CPP-1 > Azo-CPP-3 > Azo-CPP-6 > Azo-CPP-7, under otherwise same conditions. For example, when the linker changed from p-PDA to DABP, the BET surface area shows a loss of 13% (from 591 to 514 m² g⁻¹). The Langmuir specific surface areas, the micropore surface areas (t-plot method), and even the pore volumes (both V_0.1 and V_tot) showed the same tendency. This phenomenon was in well accordance to the ‘strut length’ effects as investigated by A. I. Cooper and coworkers in their study on CMPs.⁵³

To investigate the effect of steric hindrance on the surface areas, we introduce the bulky trifluoromethyl groups into the strut linker (the meta-position to the aminos, Azo-CPP-3 and Azo-CPP-4). Obviously different from a recently reported result,⁴³ it can be seen that the introduction of –CF₃ groups had a negligible effect on the surface areas (Azo-CPP-3, SA_BET = 514 m² g⁻¹; Azo-CPP-4, SA_BET = 518 m² g⁻¹). These phenomena can be attributed to the relatively far distance between the bulky substitute groups (in the meta-position) and the reactive aminos. In the reported results, the methyl substitutes were all in the ortho-position of aminos, which showed relatively greater steric hindrance effect when the amino were attacked in the polymerization. However, in the present study, the bulky –CF₃ substitutes were relatively far from the –N=N– groups; therefore the force between them were too small to cause a tilted geometry with respect to the aromatic phenyls.

We also investigated the effect of non-linear connectivity on the surface areas by using m-PDA as a linker (Azo-CPP-1 and Azo-CPP-2). When turned p-PDA to m-PDA, the surface area was reduced in expect (decreased from 591 to 439 m² g⁻¹). Surprisingly, the reduction in the surface area was as high as 25.7%, which was obviously higher than the results obtained by C. T. Yavuz et al. (7%).⁴³

To assess the effect of flexible bridges between biphenyl linkers, Azo-CPP-3 and Azo-CPP-5 were checked. When compared to Azo-CPP-3 with rigid biphenyl units (SA_BET = 514 m² g⁻¹), Azo-CPP-5 with flexible ether links showed even higher surface areas (SA_BET = 563 m² g⁻¹). This result was rather contrary to the common belief that flexible linkers yield considerable losses in overall porosity, but in well accordance to the reported results. As explained by C. T. Yavuz,⁴³ there were two competitive acts. Although the flexible ether links brought free rotation and consequent contraction in the construction of porous structure, it also eliminated the restriction on the in-plane alignment of biphenylic azo units. As a result, the more conjugated, semi-rigid oxygen bridge showed an enhanced surface area, compared with the biphenyl bridge.

Once the porosity of Azo-CPPs had been established, we considered their applications in gas storage. There are two kinds of CO₂-philic groups in the Azo-CPPs obtained herein, e.g., the porphyrin rings which contain pyrrole segments (secondary amine, similar to that of benzimidazole) and the azo groups, either newly generated or originally existed (for Azo-CPP-6). Therefore, it can be predicted that the Azo-CPPs would have high CO₂ adsorption ability. In general, with increasing specific surface areas and micropore volumes, the adsorption capacity for CO₂ increases. We tested the CO₂ adsorption properties at 273 K and a pressure up to 1.10 bar and the data are summarized in Table 2, from which we can see that all Azo-CPPs showed moderate capacity of CO₂ adsorption, which was not so consistent with our original expecting. The CO₂ adsorption for all Azo-CPPs ranged from 7.36% to 9.22% (Fig. S12 in the ESI†), which were higher than or comparable to those of some reported microporous polymers, for example, some nitrogen containing furan-based porous organic frameworks (FOF-1, 7.70%, 273 K).⁵⁴ However, they were much lower than those of highly porous benzimidazole-linked polymer BILP-1 (SA_BET = 1172 m² g⁻¹, 18.8%, 273 K),⁵⁵ ALP-1 (SA_BET = 1235 m² g⁻¹, 23.6%, 273 K),⁴⁴ microporous polycarbazole (SA_BET = 2220 m² g⁻¹, 21.2%, 273 K),⁵⁶ Azo-COP-2 (SA_BET = 729 m² g⁻¹, 11.2%, 273 K).⁴³ When compared with the abovementioned excellent results, the present Azo-CPPs did not show any advantage in CO₂ adsorption, maybe mainly due to their relatively lower surface areas. For example, the BET surface area of Azo-CPP-1 is only 591 m² g⁻¹, much lower than that of microporous polycarbazole (SA_BET = 2220 m² g⁻¹, 21.2%, 273 K). However, it was worth to note that Azo-CPP-7 showed relatively high CO₂ adsorption of 7.49%, although the surface area of Azo-CPP-7 (SA_BET = 197 m² g⁻¹) was only one third of Azo-CPP-1 (SA_BET = 591 m² g⁻¹, 9.19%, 273 K). Otherwise, we used Pd in the inner core to see the CO₂ capture abilities compared to materials with free base porphyrins. Azo-CPP-1–Pd was prepared in a similar procedure. We found the BET surface area was a little smaller than that of Azo-CPP-1 (432 vs. 591 m² g⁻¹) (Fig. S13 to S15 in the ESI†). The CO₂ capture for Azo-CPP-1–Pd was almost in the same level as Azo-CPP-1 (9.17 wt% vs. 9.19 wt% at 273 K and 1.10 bar), although the surface area was relatively smaller.

Table 2 CO₂ and N₂ capture capacities, CO₂/N₂ selectivities for the Azo-CPPs

Polymer code	CO2 adsorption (wt%)		N2 adsorption (wt%)		CO2/N2^b
	273^a K	303 K	273 K	303 K	273 K	303 K
a CO2 absorbance of Azo-CPPs at 273 K and 1.10 bar.b Selectivity estimated using the ratios of the Henry law constant calculated from the initial slopes of the single-component gas adsorption isotherms at low pressure coverage (<0.15 bar).
Azo-CPP-1	9.19	4.80	0.61	0.07	53.9	82.0
Azo-CPP-2	9.11	5.54	0.82	0.30	27.4	31.2
Azo-CPP-3	9.22	3.99	0.54	0.10	45.3	66.5
Azo-CPP-4	8.52	5.12	0.41	0.05	40.6	107.8
Azo-CPP-5	8.52	5.08	0.60	0.09	37.9	78.6
Azo-CPP-6	7.36	4.02	0.36	0.07	47.2	72.2
Azo-CPP-7	7.49	4.17	0.52	0.16	34.3	45.3

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In order to investigate the gas adsorption selectivity of the Azo-CPPs, CO₂ and N₂ sorption properties were measured by volumetric methods at the same conditions of 273 K, and the selectivity was estimated according to a reported method.³³ The ratios of the Henry law constants, which were calculated from the initial slopes of the single-component gas adsorption isotherms at low pressure coverage (0–0.15 bar), were used to calculate the selectivity. For all Azo-CPPs, the calculated CO₂/N₂ adsorption selectivities were in the range of 27.4–53.9 at 273 K, and 31.2 to 107.8 at 303 K, respectively (Table 2 and Fig. S16 to S22 in the ESI†). These values at 273 K were comparable to or better than those of the nitrogen-containing nanoporous covalent triazine-based frameworks (CTFs, selectivity of CO₂/N₂ 14–41) and some nitrogen containing MOPs (selectivity of CO₂/N₂ at 273 K, 17.3–30.6) under the same conditions.^57–59 Compared with the reported Azo-COPs (selectivity of CO₂/N₂ at 273 K, from 63 to 124),^33,43 the selectivities obtained herein were relatively lower, although they containing the same functional groups (azo bridges). It was considered that the “sieving effect” caused by the small pore width (0.48–0.80 nm)³³ lead to the high CO₂/N₂ selectivity. The dominant pore width of all the Azo-CPPs obtained herein was larger than 1.14 nm, which was also larger than those of Azo-COPs.

As reported by C. T. Yavuz et al., the azo groups were “N₂-phobic” groups, therefore the selectivities of CO₂/N₂ increased with increasing temperature. While according to the results by H. M. El-Kaderi and J. Zhang,^44,45 the pore width of Azo-POF-2 (major pore width: 1.5 nm) and ALPs (major pore width: 1.0 nm to 1.3 nm) are larger than those of Azo-COPs, therefore the selectivity of CO₂/N₂ decreases as temperature increases. In fact, the selectivity of all Azo-CPPs herein showed a similar tendency to the Azo-COPs when temperature rose higher. For instance, when the test temperature went up from 273 K to 303 K, the selectivity of CO₂/N₂ increased to 31.2–107.8. The same tendency but with larger pore width was due to not only the azo bridges but also the porphyrin groups. In other words, the “N₂-phobicity” came from both the azo bridges and the porphyrin groups in the present work. For example, Azo-COP-1 (SA_BET = 608 m² g⁻¹)³³ and Azo-CPP-1 (SA_BET = 591 m² g⁻¹) had a similar surface area, but the N₂ adsorption of Azo-COP-1 (2.95% at 273 K and 1.0 bar) was larger than that of Azo-CPP-1 (0.61% at 273 K and 1.1 bar) under otherwise same conditions. At higher temperatures, this phenomenon was even serious (1.19% for Azo-COP-1 at 298 K and 1.0 bar vs. 0.07% for Azo-CPP-1 at 303 K and 1.10 bar). These Azo-CPPs have potential applications in post-combustion CO₂ capture and sequestration technology.

Conclusions

In conclusion, a series of azo-bridged, aromatic, nitrogen-rich, microporous covalent porphyrinic polymers (Azo-CPPs) were synthesized, based on 4NO₂TPP and several diamine or tetraamine compounds. The Azo-CPPs were all amorphous and exhibited middle to high surface areas between 197 to 591 m² g⁻¹. The effect of building blocks on the pore properties was discussed. Meanwhile, the capacity of adsorption of CO₂ for all the Azo-CPPs was studied and the CO₂ uptake capacity was measured between 7.36% and 9.22% at 273 K and 1.10 bar. The CO₂/N₂ selectivities were measured between 27.4–53.9 at 273 K and 31.2–107.8 at 303 K, respectively, showing an increase in selectivity with increasing temperature, which made the Azo-CPPs potential candidates for applications in post-combustion CO₂ capture and sequestration technology.

Acknowledgements

The authors thank National Natural Science Foundation of China (No. 51103168 and 21201093) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Structure characterization of the Azo-CPPs; the nitrogen adsorption and desorption isotherms and pore size distributions of all Azo-CPPs; the CO₂ adsorption isotherms, and the CO₂/N₂ selectivities of all Azo-CPPs. See DOI: 10.1039/c5ra17671a

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