A Porous Aromatic Framework Functionalized with Nitro Groups and Its Carbon Dioxide Adsorption
Bulletin of the Korean Chemical Society (Print)
Linking of rigid organic molecules by coupling and/or condensation reactions can yield extended organic polymer networks usually as amorphous solids except for the COFs (covalent organic frameworks) developed by Yaghi and coworkers. 1,2 As organic polymer networks are formed through covalent bonds, they are usually stable in water and even in acidic or basic conditions. In addition these are comprised of light elements (i.e. C, H, N, O, and B), 2 some organic polymer networks demonstrate very
... demonstrate very high Brunauer-Emmett-Teller (BET) surface areas. 3 It is believed that high chemical stability and porosity of organic polymer networks allow more opportunity of their industrial applications such as CO 2 capture in humid flue gases emitted by coal-fired power plants. Connection of the tetrahedral building units by C-C coupling reactions is expected to produce a diamond-like network, although a long-range order is not always guaranteed. 1,3 For instances, Yamamoto-type Ullmann coupling was applied for linking tetrakis-4-bromophenyl-methane (BPM) monomers to give amorphous PAF-1 (porous aromatic framework-1). 4 PAF-1 showed a high surface area of 5600 m 2 /g. Even higher surface area (6461 m 2 /g for PPN-4 (porous polymer network-4)) was obtained by linking of tetrakis-4-bromophenyl-silanes. 5 The emergent of organic polymer networks with high-surface areas can provide the opportunity to tune the property of these materials by post-synthetic modification. Indeed, PAF-1 (and PPN-6 having a same chemical composition as PAF-1) was directly able to be sulfonated, 6 lithiated, 7 and decorated with various organic amines. 8 However, there are at least two challenges to be addressed. One is development of simple and inexpensive synthetic route because the crystallinity and porosity of organic porous network are generally poor. Another challenge is improvement of thermal stability. Here we report a new synthetic procedure of PAF-1, and its easy nitration (Scheme 1). The nitro-functionalized PAF-1 shows enhanced CO 2 adsorption properties. Interestingly, in due course we found that a thermolysis product of PAF-1 has very high thermal stability in air. PAF-1 was prepared by a Yamamoto-type Ullmann coupling reaction conducted at 80°C according to the published procedure. 4 However, we were only able to obtain PAF-1 with the surface area of 2100 m 2 /g even after repeated trials. One of the reasons for the low surface area may be ascribed to the incomplete removal of occluded Ni metal particles. Thermogravimetric analyses imply that crude products contain a significant amount of Ni metal (15 wt %, Fig. 3S ). The embedded Ni was not fully removed even after the treatment of crude products with concentrated HCl. Instead of the homo-coupling reaction, we next prepared PAF-1 by Suzuki-Miyaura reactions 9,10 between tetrakis(4phenylboronic acid pinacol ester)methane (PBPM) and BPM in the presence of a Pd(0) catalyst. The product (termed 1S) is insoluble in most of organic solvents, which is indicative of the formation of extended structure. Progress of the coupling reaction was also confirmed by IR spectra; absorbance for C-Br stretching bands at 512 and 532 cm 1 was almost disappeared (Fig. 6S ). To our disappointment, the BET surface area of 1S was 630 m 2 /g only. Besides the occluded Pd (ca. 10 wt % based on TGA analyses, Fig. 8S ), the structural regularity of 1S may be lower than the PAF-1 obtained by a Yamamoto-type Ullmann reaction. As the Suzuki reaction requires alternative attachments of two units (i.e. BPM and more bulky PBPM), uniform growth of ordered polymer network seems to be disturbed during the coupling reaction. There is a trend that less-ordered polymer network structures are obtained by a rapid coupling reaction of organic units. In order to improve the crystallinity of polymeric networks, we implemented a Yamamoto-type Ullmann reaction again at a lower temperature, 25°C. To evaluate the porosity, N 2 isotherm of the final product (termed 1U) was recorded. As shown in Figure 1(b) , 1U demonstrated a steep rise of N 2 uptake below P/P 0 < 0.1, indicating the presence of microporosity. The BET surface area of 1U is estimated to be 3270 m 2 /g. The small hysteresis of the isotherm may reflect the flexible nature of the polymer networks. Indeed, the powder X-ray diffraction pattern Scheme 1. Preparation of PAF-1, and its functionalization with nitric acid: i) 1,5-cyclooctadiene, bis(1,5-cyclooctadiene)nickel(0), 2,2'-bipyridyl, DMF, 25 °C; ii) 3 M HNO3, DMF, 50 °C.