Synthetic Methodology for the Fabrication of Porous Porphyrin

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Synthetic Methodology for the Fabrication of Porous Porphyrin Materials with Metal−Organic−Polymer Aerogels Xin Zhao,† Lin Yuan,†,‡ Zeng-qi Zhang,† Yong-song Wang,† Qiong Yu,† and Jun Li*,† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry & Materials Science, Northwest University, Xi’an, Shaanxi 710069, China ‡ Xi’an Tieyi Binhe School, Xi’an, Shaanxi 710038, China S Supporting Information *

ABSTRACT: A promising fabrication strategy used for designing porous porphyrin materials and a group of rigid carboxyl porphyrins based metal−organic−polymer aerogels (MOPAs) has been proposed recently. These newly synthesized MOPAs were exemplarily characterized by FT-IR, UV−vis−DRS, EDS, PXRD, TGA, SEM, TEM, and gas sorption measurements. A gelation study has shown that solvents, molar ratio, temperature, and peripheral carboxyl number in porphyrins all affect gel generation. The MOPA series exhibit eminent thermal stability, high removal efficiency in dye adsorption, versatile morphologies, and permanent tunable porosity; also the BET surface areas fall within the range 249−779 m2 g−1. All of the mentioned properties are significantly superior to some other porous materials, which enable these compounds to be potential candidates for dye uptake, gas storage, and separation. gated35,36 as a versatile synthetic base for porous metal− organic frameworks37−39 due to their potential applications in catalysis, sensors, gas storage, and solar energy conversion.40−47 On account of their unique desirable properties, porphyrins have been ideal building blocks for the construction of functional MOPAs, working as prosperous functional porous materials for gas storage and adsorption/desorption. To our best knowledge, few papers have reported the utilization of porphyrin gelators to generate porous MOPAs to date.48−50 Therefore, for the purpose of synthesizing porous porphyrin materials, we report a potential synthetic route available based on rigid carboxyl porpyhrins via fabrication of MOPAs, as well as a series of carboxyl porpyhrin MOPAs (Figure 1). More importantly, properties originated from the porphyrins and MOPAs are likely to be maximally retained. Characterization of MOPAs’ spectral properties is easy to achieve, and their gas adsorption/desorption capacities for nitrogen and carbon dioxide have been studied as well. We believed it would be a promising method for the preparation of porous porphyrin MOPAs.

1. INTRODUCTION Metal−organic gels (MOGs) are among the remarkably multifunctional soft materials that are stabilized by noncovalent interactions, such as hydrogen bonds, π−π stacking, and van der Waals interaction.1,2 In the past few decades, the emergence of MOGs has motivated a huge interest in scientists3−6 due to their universal applications in tissue engineering, optics, catalysis, and controlled drug release and delivery.7−14 However, lack of thermal stability after desiccation restricts their use in commercial applications.15,16 Metal−organic− polymer aerogels (MOPAs), mainly based on the metal−ligand coordination interactions,17 as a family of MOGs have intriguing structural patterns and fascinating properties.18,19 Additionally, MOPAs cannot be redissolved and are thermally irreversible20,21 due to the strength of the metal−ligand coordination interaction between strong covalent bonds and other noncovalent effects.22 Further, these materials, extracted by supercritical carbon dioxide, possess versatile porosity, low density, and high internal surface area properties,23 which allows its broad use in fabrication of desirable monolithic solids,18,24 heterogeneous catalysis, adsorption/separation technology,25,26 and membrane/film processing.27,28 Porphyrins are promising components that can be employed in various applications due to their desirable structural and functional properties.29−33 Moreover, prophyrins are capable of being incorporated into metal−organic−polymer aerogels, which have both the physicochemical properties of porphyrins and the permanent porosity of MOPAs.34 Porphyrin-based metal−organic frameworks have been extensively investi© XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All starting materials and solvents were purchased from commercial sources and used without further purification unless otherwise stated. Elemental analyses (C, H, and N) were performed by Vario EL-III CHNOS instrument. ThermograviReceived: February 2, 2016

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DOI: 10.1021/acs.inorgchem.6b00274 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Preparation of MOPAs based on porphyrins.

Figure 2. Synthesis of porphyrins: (a) tetrakis(4-carboxyphenyl)porphyrin (H2TeCPp); (b) 5,10,15-tris(4-carboxyphenyl)-20-phenylporphyrin (H2TrCPp), and 5-(4-carboxylphenyl)-10,15,20-triphenylporphyrin (H2CPp); (c) 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2DiCPp). metric analysis (TGA) was performed on a NETZSCH STA 449C thermal analyzer instrument in N2 at a heating rate of 10 °C min−1. UV−vis spectra were measured on a Shimadzu UV 1800 UV−vis− NIR spectrophotometer. Infrared spectra were measured on a Nicolet Avatar 330 FT-IR spectrometer with KBr pellets. Mass spectrometry (MS) analyses were carried out on a matrix-assisted laser desorption/ ionization time-of-flight mass spectrometer (MALDI-TOF MS, Krato Analytical Company of Shimadzu Biotech, Manchester, Britain). The powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 diffractometer using graphite monochromatic copper radiation (Cu Kα) at 40 kV and 30 mA over the 2θ range from 5° to 70°. The morphology of the aerogel was characterized by scanning electron microscopy (SEM) recorded on a FEI Quanta 400F thermal field

emission environmental scanning electron microscope. The samples for transmission electron microscope (TEM) observations were prepared by dispersing aerogel in EtOH by sonication and then immersing a carbon-coated copper grid. Nitrogen adsorption and desorption isotherms were measured at 77.3 K on a Quantachrome system. 2.2. Synthesis of Carboxyl Porpyhrins. The synthetic routes to the porphyrins are shown in Figure 2. The detailed procedures are presented as follows: Tetrakis(4-carboxyphenyl)porphyrin (H2TeCPp). The synthetic route of H2TeCPp used in our study was the well-known Adler− Longo method:51 propionic acid (150 mL) was refluxed together with methyl 4-formylbenzoate (6.0 g, 36 mmol) at 140 °C, and excess B


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Inorganic Chemistry

Figure 3. Typical procedure for the carboxyl porphyrin based MOGs and MOPAs. distilled pyrrole (2.5 mL, 36 mmol) in propionic acid (12.5 mL) was added dropwise in 30 min. Then the mixture was stirred with a magnetic stirrer bar for another 1 h. Evaporation of the solvent gave a crude product after cooling to room temperature and adding EtOH (20 mL). The mixture was cooled overnight and filtrated under vacuum before being purified by silica gel column chromatography. Elution of the column with 1% ethanol of dichloromethane gave the intermediate product H2TeCPp-OMe as a purple solid; the 1H NMR for H2TeCPp-OMe is shown in Figure S1a, Table S1 entry 1. H2TeCPp-OMe (0.2 mmol, 150 mg) was dissolved in tetrahydrofuran (10 mL) with 1 mol L−1 aqueous KOH (20 mL) added and heated to reflux at 66 °C for about 48 h under stirring. TLC was checked at the conclusion of the reaction. The pH was adjusted to 3 with HCl(aq) after the reaction finished. The precipitated solid was filtered and washed with distilled water and then vacuum-dried at 80 °C for another 12 h. The desired compound H2TeCPp was obtained finally. Yield: 37%. Mp: >250 °C. UV−vis (DMF): λmax/nm, 419 (Soret band), 515, 550, 593, 645 (Q bands). FT-IR: ν, cm−1, 3425, 2923, 1690, 1604, 964, 795. Anal. Calcd (found) for C48H30N4O8 (mol wt 790.20), %: C, 72.89 (72.89); H, 3.81 (3.80); N, 7.12 (7.09). MS: m/z 789.20 [M − 1]− amu. 5,10,15-Tris(4-carboxyphenyl)-20-phenylporphyrin (H2TrCPp). In a typical synthesis route as above, methyl 4-formylbenzoate (4.5 g, 27 mmol) and benzaldehyde (1.0 mL, 9 mmol) were dissolved in a certain ratio with 150 mL of propionic acid. Then the mixture was heated to reflux at 140 °C, and freshly distilled pyrrole (2.5 mL, 36 mmol) in propionic acid (12.5 mL) was added to the solution dropwise in 30 min. The crude product formed by vacuum distillation after 1.5 h of stirring; then the solution was cooled to room temperature and EtOH (20 mL) was added. After a night’s cooling, the precipitate was filtered, and further purification was carried out by silica gel column chromatography using dichloromethane as eluant, to afford the purple solid intermediate product H2TrCPp−OMe; the 1H NMR for H2TrCPp-OMe is shown in Figure S1b, Table S1 entry 2. H2TrCPp was prepared in the same procedure as the preparation of H2TeCPp with the replacement of H2TeCPp-OMe with H2TrCPpOMe. Yield: 34%. Mp: > 250 °C. UV−vis (DMF): λmax/nm, 418 (Soret band), 516, 550, 591, 643 (Q bands). FT-IR: ν, cm−1, 3419, 2922, 1704, 1606, 978, 804. Anal. Calcd (found) for C47H30N4O6 (mol wt 746.22), %: C, 75.74 (75.58); H, 3.99 (4.02); N, 7.44 (7.50). MS: m/z 745.20 [M − 1]− amu. 5,15-Bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2DiCPp). H2DiCPp was synthesized according to a literature procedure.52 Benzaldehyde (2.0 mL, 18 mmol) and freshly distilled pyrrole (30 mL, 43 mmol) were mixed with constant stirring under a nitrogen atmosphere. Then 2 or 3 drops of BF3·Et2O was added to the reaction system simultaneously, and the reaction continued in the dark for another 1 h. When the solution changed to a grayish-green, the reaction was finished by adding quantitative CH2Cl2 to the solution. The pH was adjusted to 8 with 0.1 mol L−1 NaOH and dried under

sodium sulfate (Na2SO4) for 20 min. After removing the solvent dichloromethane and unreacted pyrrole, the crude product was purified by silica gel column chromatography using dichloromethane as eluant to produce the yellowish-white powder dipyrrolylmethane (DPM). Samples of DPM and methyl 4-formylbenzoate were dissolved in 15 mL of propionic acid, respectively, and mixed with stirring in refluxing propionic acid (100 mL). The solvent was evaporated by vacuum distillation, and EtOH (20 mL) was added after 1 h of reaction. After cooling to room temperature, the solvent was removed by filtration. Purification was done by silica gel column chromatography with CH2Cl2, and a purple solid of compound H2DiCPp-OMe was obtained; the 1H NMR for H2DiCPp-OMe is shown in Figure S1c, Table S1 entry 3. H2DiCPp was synthesized with the same procedure as the preparation of H2 TeCPp, only changing H 2 TeCPp-OMe to H2DiCPp-OMe. Yield: 35%. Mp: >250 °C. UV−vis (DMF): λmax/ nm, 417 (Soret band), 515, 550, 592, 649 (Q bands). FT-IR: ν, cm−1, 3427, 2921, 1690, 1606, 964, 795. Anal. Calcd (found) for C46H30N4O4 (mol wt 702.20), %: C, 78.65 (78.61); H, 4.30 (4.27); N, 7.92 (7.98). MS: m/z 703.20 [M + 1]+ amu. 5-(4-Carboxylphenyl)-10,15,20-triphenylporphyrin (H2CPp). The synthesis route to H2CPp-OMe was similar to that for H2TrCPpOMe, only changing the proportion of methyl 4-formylbenzoate (1.5 g, 9 mmol) and benzaldehyde (2.8 mL, 27 mmol). Then purple solid compound H2CPp-OMe was obtained; the 1H NMR for H2CPp-OMe is shown in Figure S1d, Table S1 entry 4. H2CPp-OMe (0.2 mmol, 150 mg) was dissolved in THF (10 mL) with 1 mol L−1 aqueous KOH (20 mL) added and refluxed at 66 °C for about 72 h under stirring. The process of reaction was monitored by TLC. The pH was adjusted to 3 with HCl(aq) after the reaction finished. The precipitated solid was filtered and washed with distilled water and then vacuum-dried at 80 °C, to afford the final product H2CPp. Yield: 19%. Mp: >250 °C. UV−vis (DMF): λmax/nm, 417 (Soret band), 515, 549, 591, 646 (Q bands). FT-IR: ν, cm−1, 3446, 2921, 1732, 1608, 965, 798. Anal. Calcd (found) for C45H30N4O2 (mol wt 658.24), %: C, 82.10 (82.04); H, 4.55 (4.56); N, 8.54 (8.51). MS: m/z 659.30 [M + 1]+ amu. 2.3. Preparation of Carboxyl Porphyrin Based MOGs and MOPAs. The general routes to the formation of MOGs and MOPAs are shown in Figure 3. Typical Procedure for MOG1, MOG2, and MOG3. H2TeCPp (25.0 mg, 0.0316 mmol) and Cr(NO3)3·9H2O (76.0 mg, 0.190 mmol) were separately dissolved in 2 mL of DMF and 4 mL of EtOH. An aubergine solution was obtained after stirring, transferred to a 25 mL Teflon-lined autoclave, and kept at 90 °C in an oven for 24 h. The MOG1 was obtained. In the same way, when Cr(NO3)3·9H2O was replaced by Al(NO3)3·9H2O and Fe(NO3)3·9H2O, MOG2 and MOG3 were obtained. Typical Procedure for MOG4, MOG5, and MOG6. The reaction between H2TrCPp and Cr(NO3)3·9H2O was performed in a molar C


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Inorganic Chemistry ratio of 1:6 in a component solvent of EtOH and DMF. Specifically, H2TrCPp (25.0 mg, 33.5 mmol) and Cr(NO3)3·9H2O (80.4 mg, 0.201 mmol) were dissolved in 2 mL of DMF and 2 mL of EtOH, respectively. The MOG4 was obtained at 150 °C after 36 h in a 25 mL Teflon-lined autoclave. MOG5 was synthesized successfully when the metal salt was changed to Al(NO3)3·9H2O. H2DiCPp reacted with Cr(NO3)3·9H2O to obtain MOG6 under the same conditions, and no gel was obtained when H2DiCPp was replaced by H2CPp. The MOGs (1−6) were subjected to solvent exchange, which was carried out by Soxhlet extraction with EtOH for 24 h to remove the excess metal salts, solvent, or ligand. The MOGs were extracted with supercritical CO2 at 40 °C (above the critical temperature for carbon dioxide)53,54 for 8 h. After the autoclave was depressurized slowly at room temperature for about 5 h, MOPAs (1−6) were obtained.

Table 3. Gelation Tests of Carboxyl Porphyrins and Cr(NO3)3·9H2O at Different Temperaturesa H2TeCPp H2TrCPp H2DiCPp H2CPp

60 °C

90 °C

120 °C

150 °C

S S S S

G PC PC S

G PG PC S

G G G S

c(Cr3+) = 0.0503 mol L−1. Solvent: DMF−EtOH. VDMF:VEtOH = 1:1. S: solvent; P: precipitate; PG: partial gel; PC: partial crystal; G: gel.

a

3. RESULTS AND DISCUSSION 3.1. Gelation Study. We have investigated the influential features of the gel formation and found several interesting points throughout the experimentation. Initially, solventdependent analysis revealed that the gel can be well-formed in DMF−MeOH (or EtOH) and partially formed in DMF− H2O, but no gel occurs in DMF (Table 1). These results were

elevated temperature except H2CPp. Besides, the mechanical strength of MOGs is evident with more carboxyls. Interestingly, laminar crystals were obtained in the reaction of H2TrCPp or H2DiCPp with Cr(III) salt at 90 and 120 °C, respectively, due in part to a competing process occurring between the formation of crystals and gels under certain conditions. Thus, many different types of morphology would be attainable by adjusting the temperature and gelators. Table 4 revealed the production of gels would be easier with a larger number of peripheral carboxyls in the porphyrin, since

Table 1. Gelation Tests of H2TeCPp and Metal Nitrates in Different Solventsa

Table 4. Gelation Tests of Gelators and Metal Nitrates at 150 °Ca

DMF

DMF−MeOH (1:1)

DMF−EtOH (1:1)

DMF−H2O (1:1)

S

G

G

PG

S

G

G

PG

S

G

G

P

Cr(NO3)3· 9H2O Al(NO3)3· 9H2O Fe(NO3)3· 9H2O

H2TeCPp H2TrCPp H2DiCPp H2CPp a

c(M3+) = 0.0317 mol L−1; M = Cr, Fe, Al; temperature 90 °C. S: solvent; P: precipitate; PG: partial gel; G: gel

Cr(NO3)3·9H2O

Al(NO3)3·9H2O

Fe(NO3)3·9H2O

G G G S

G G S S

G S S S

Solvent: DMF−EtOH. VDMF:VEtOH = 1:1. S: solvent; G: gel.

a

this process is driven by the coordination effect between the substituent and the metal ions. Overall, the formation of MOGs in the gelation process is closely associated with the number of carboxyls. Another point extracted from Table 4 is that the Cr(III) ion is the easiest one to form a gel with porphyrin; then the Al(III) ions, and Fe(III) ions can form gels only with H2TeCPp. 3.2. Spectroscopic Characterization of MOPAs. IR spectra of carboxyl porphyrins and MOPAs are shown in Figure S2; the corresponding aerogels (MOPA1−3) of H2TeCPp in the carbonyl stretching vibration are red-shifted from 1690 cm−1 to 1560 cm−1 and 1420 cm−1, respectively, both of which are assigned to antisymmetric νas(COO−) and symmetric νs(COO−) stretching vibrations of carboxylate ions, suggesting that coordination bonds possibly have a crucial effect on the formation of MOGs. The shoulder peaks at 1690 cm−1 in the MOPAs evinced the presence of uncoordinated carboxyl and the disappearance with the reduction of the external carbonyl. Besides, the porphyrin ring near 965 cm−1 in the N−H bending vibration peak still exists, indicating that carbonyl (−COOH) was involved in the coordination in the gels formation, while N atoms inside the porphyrin were not. The IR spectra of MOPA4−MOPA6 gelated by H2TrCPp and H2DiCPp were analogous to those of MOPA1−MOPA3, implying that the peripheral coordination bond of porphyrin is vital to the formation of the porphyrin-based MOGs. The UV−vis−DRS spectra of MOPA1−MOPA3 gelated by H2TeCPp were characterized by a Soret band and four Q bands (Table S2), which also confirmed the uncoordinated N atoms in the porphyrin rings. This may be because M(III) (M = Cr, Fe, Al) have greater affinity for O atoms than N atoms. Blue-

possibly due to the relatively large solubilities of porphyrin and metal salts in DMF, which is not conducive to gel formation. The solubility of porphyrin decreases (since porphyrin has a low solubility in MeOH) and the hydrogen bond interaction of solvent molecules increases when adding MeOH (or EtOH) to DMF, resulting in the formation of gels. When replacing MeOH with H2O, the lower solubility of porphyrin in DMF− H2O leads to partial gel formation or precipitation. The gels can be formed in a wide range from 1:1 to 1:9 molar ratio of H2TeCPp and the metal salts. The mechanical strength of the gel increases with increasing concentration of M(III) (M = Cr, Fe, Al) ions (Table 2). It is therefore believed that a higher metal ion concentration is more favorable for the formation of the three-dimensional network structure, followed by the formation of the gel ultimately. Temperature-dependent study results are presented in Table 3; almost all carboxyl porphyrins are likely to form gels at Table 2. Gelation Tests of H2TeCPp and Metal Nitrates in Different Molar Ratiosa 3+

−1

c(M )/mol L Cr(NO3)3·9H2O Al(NO3)3·9H2O Fe(NO3)3·9H2O

1:1

1:3

1:5

1:6

1:9

0.0053 PG PG PG

0.0106 PG PG PG

0.0265 PG PG G

0.0317 G G G

0.0477 G G G

Temperature: 90 °C. Solvent: DMF−EtOH. VDMF:VEtOH = 1:1. PG: partial gel; G: gel.

a

D


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Inorganic Chemistry

Figure 4. SEM (a) and TEM (b) images of MOPAs.

observation, which are applied to observe the morphology and porosity of the MOPAs. The MOPAs we obtained all have relatively homogeneous, porous self-assembled structures, ranging from spherical vesicles, to sheet-like, to sponge-like morphologies as the transition-metal ions varies. Spherical vesicle structures were observed for the MOPAs (M(III), M = Cr, Fe). MOPA2 and MOPA5 were products of H2TeCPp, H2TrCPp, and Al(NO3)3·9H2O, respectively, belonging to selfassembled sheet structures. Designing and synthesizing different kinds of MOPAs would be relatively easy by changing the

shifts of the Soret band could be found in the MOPAs spectra compared with H2TeCPp. MOPAs formed by H2TrCPp and H2DiCPp followed the same rules, which could indirectly suggest that porphyrin molecules in MOPAs belong to H-type packing.55,56 Spectroscopic characterizations of MOPAs have verified the porous metal organic polymer materials we obtained were constituted by free porphyrin building blocks. 3.3. Morphology Analysis. The foregoing view is further confirmed by SEM (Figure 4a) and TEM (Figure 4b) E


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Inorganic Chemistry

Figure 5. N2 isotherms of MOPAs at 77.3 K (solid symbols: adsorption; open symbols: desorption) of MOPAs.

morphology-related factors of the MOPAs. TEM images of the MOPAs confirmed a continuous amorphous network with the absence of long-range order but connected particles. Both SEM and TEM analyses are consistent with the sorption results of MOPAs and revealed a porous structure of the MOPAs. 3.4. Gas Sorption. To evaluate the porous properties of the MOPAs, nitrogen adsorption analyses were performed at 77.3 K following a degasification at 170 °C for 6 h to remove adsorbed molecules. MOPAs exhibit typical type IV isotherms, which have H3-type hysteresis loops according to the IUPAC classifications (Figure 5), revealing an uptake capacity of 855− 1850 cm−3 g−1 for both micro- and mesopore materials.57 The initial saturation of MOPAs at low pressure indicates the existence of micropores. The uptake volumes near 1 atm increase and there is an absence of saturation due to the condensation of nitrogen molecules in interparticle voids.50,58 The relatively high BET surface areas of MOPA1, however, may be explained by the inertness or lability of the metal ions and the coordination efforts. Cr(III) is a well-known inert ion serving as an inorganic node to support porous materials as compared with other trivalent metal species.59,60 The fourcarboxyl MOPA1 coordinates with Cr(III) will also have a noticeable effect on its stability due to the kinetic inertness of Cr(III) and strong coordination. Therefore, the effect of metal ions has a dominant role in the BET surface areas study. A comparison is made regarding BET surface area, total pore volume, and N2 uptake among porous materials61−67 in Table 5. The MOPAs exhibited a variable BET surface area (249−779 m2 g−1) and prominent N2 uptake capacity (855−1850 cm−3 g−1), which are significantly superior than many porous materials reported to date under similar conditions. Gelators H2TeCPp, H2TrCPp, and H2DiCPp with various numbers of carboxyl groups varied in their abilities to immobilize solvents, resulting from the possible interaction between carboxyl and solvents molecules. MOPA1 and MOPA2 gelated by H2TeCPp, with a specific surface area of 779 and 420 m2 g−1, respectively, performed better than MOPA4−6 (formed by H2TrCPp and H2DiCPp with the same salts). Therefore, H2TeCPp as a multidentate organic ligand coordinates more easily with Cr, Al, or Fe atoms to form a 3D network structure. Compared to MOPA2 and MOPA3, MOPA1 has a stronger capability of absorbing N2 gas primarily due to Cr(III), Al(III), and Fe(III) possibly forming different micro−mesoporous structures. Based on Barrett−Joyner− Halenda analysis, MOPAs possess a wider pore size distribution

Table 5. Comparison of BET Surface Area, Total Pore Volume, and N2 Uptake in Reported Porous Materials

MPFs MOFs MOGs

HOFs zeolite MOPAs (this work)

a

adsorbent

BET surface area (m2 g−1)a

[Pd2(MDDCPP)]61 JUC-13262 G1-Pd63 MOG-164 GP1-Pd65 1a66 zeolite beta/PSSF67 MOPA1 MOPA2 MOPA3 MOPA4 MOPA5 MOPA6

379 253 38.8 80.4 22.1 97.7 118 779 420 249 386 355 376

total pore volume (cm3 g−1)

N2 uptake capacity (cm3 g−1)

0.157

182 29 87 292.1 49 47 42 1100 855 890 1050 1850 1248

0.13 4.52 0.074 0.067 1.72 1.32 1.39 1.62 2.86 1.93

The range of P/P0 was 0.05−0.3 for BET calculation.

(Figure S3) up to ca. 6.2−37.2 nm in a mesopore range. CO2 uptake performances of MOPAs also have been investigated at 273 and 298 K. All sorption isotherms reveal moderate CO2 adsorption (Figure 6) and are comparable with the reported metal porphyrin-based frameworks69 (26.7−31.0 cm3 g−1 at 273 K, 13.7−14.2 cm3 g−1 at 298 K) and hydrogen-bonded organic frameworks (HOFs) HOF-3a70 (31 cm3 g−1 at 273 K, 21 cm3 g−1 at 296 K) and HOF-7a71 (18.4 cm3 g−1 at 273 K, 10.3 cm3 g−1 at 296 K). The phase purity of MOPAs was verified by powder X-ray diffraction patterns, which exhibited only weak and broad signals (broad “humps”) (Figure S4) with the absence of longrange order, suggesting that MOPAs are amorphous micro− mesoporous structures with random growth when forming gel spheres during self-assembly, which could be attributed to the aggregation of the nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) tests were performed to analyze the composition of the MOPAs (Figure S5). EDS analysis of MOPA1 shows that the atomic ratio of C:Cr was 11.88:1.00, which approximates the ideal maximally coordinated ratio 12:1. The rest of the MOPAs follow the same rules, where carboxyl porphyrins and M(III) (M = Cr, Al, Fe) in MOPAs are expected for a model of coordination in which a porphyrin unit is coordinated with the same number of carboxyl groups for F


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Inorganic Chemistry

Figure 6. CO2 sorption isotherms of the MOPAs at 273 K (red) and 298 K (blue) (solid symbols: adsorption; open symbols: desorption).

metal ions.68 On the basis of the results above, also proposed here is a possible mechanism for MOPA formation (Figure 7). Porphyrins and metal ions assembled porphyrin gels under solvothermal conditions and cross-linked porous MOPAs by the coordination between carboxyl groups and metal ions after supercritical CO2 drying. However, interactions such as hydrogen bonds and π−π stacking may also influence MOPA fabrication. 3.5. Dye Adsorption Tests. To examine the adsorption capacity of MOPAs for bulky molecule transportation, MOPA3 was selected as the representative adsorbent, and two typical types of dye molecules, methylene blue (MB) and rhodamine B (RhB), were used to test the adsorption property of MOPAs in further study. The general adsorption ability test could be described as follows: MOPA3 (7.5 mg) in 50 mL of dye solution (50 mg L−1) was continuously stirred for 10 h at room

temperature. Then the concentration of dyes remaining in solution was diluted 10 times for a UV−vis spectrophotometer determination after centrifuging. It can be seen in Figure 8 that MOPA3 exhibits an extraordinary adsorption both in MB and in RhB, with a removal efficiency of 94.54% and 98.91%, respectively. This demonstrates that the porous MOPAs materials have a fairly high capability in dye adsorption. 3.6. Thermal Stability Study. The thermal stability of the MOPAs was investigated with TGA in N2 at a heating rate of 10 °C min−1. In all cases, two main weight losses were observed (Figure S6). An apparent weight loss was observed from room temperature to 130 °C for the first stage. This may be due to physisorbed or chemisorbed solvent molecules. Then no obvious weight loss observed at the second stage (130 to 250 °C) suggests that the whole structure is relatively stable. The third stage of the weight loss above 250 °C corresponds to the G


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Inorganic Chemistry

Figure 7. Possible mechanism of MOPA formation.

Figure 8. UV−vis spectra of methylene blue (MB) (left) and rhodamine B (RhB) (right) before (solid lines) and after (dash-dotted lines) the addition of MOPA3. Photos (insets) show the dyes in aqueous solutions before and after adsorption by MOPA3.

■

decomposition of the host frameworks. The analysis of the MOPA thermogravimetric curve implies that MOPAs display an eminent thermal stability.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00274. 1 H NMR data for H2TeCPp-OMe, H2TrCPp-OMe, H2DiCPp-OMe, and H2CPp-OMe (Figure S1); 1H NMR data of ester porphyrins (Table S1); IR spectra of carboxyl porphyrins and MOPAs (Figure S2); UV− vis−DRS spectral data for MOPAs (Table S2); pore size distributions for MOPAs (Figure S3); PXRD patterns for MOPAs (Figure S4); EDS for MOPAs (Figure S5); TGA curves for MOPAs (Figure S6) (PDF)

4. CONCLUSIONS We have demonstrated a viable route for the generation of MOPAs based on rigid carboxyl porphyrins, and a series of MOPAs have been synthesized. UV−vis−DRS, FT-IR, TEM, SEM, EDS, powder XRD, and gas sorption experiments have been used to characterize their structural and porous properties. A gelation study has been carried out in various conditions, indicating the relation between gel formation and ambient environment. These materials feature the desirable physical properties of aerogels, exhibiting tunable permanent porosity structures and a relative high surface area, as well as the accessibility of the MOPAs for dye molecules. Their micro− mesoporous structures, proved as a continuous amorphous network with the absence of long-range order, are available for various guest molecules, as revealed by gas absorption/ desorption experiments. Thereby, due to the participation of porphyrins, these porous materials may have a variety of functional applications in catalysis, photoconduction, luminescence, and compound separation.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +0086 29 88302604. Fax: +0086 29 88303798. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors acknowledge the research grant provided by the National Natural Science Foundation of China (No. 21271148). H


Article

Inorganic Chemistry

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Synthetic Methodology for the Fabrication of Porous Porphyrin

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