Hierarchically Micro- and Mesoporous Coordination Polymer

Apr 20, 2010 - Zhifeng Xin, Junfeng Bai*, Yongming Shen and Yi Pan .... Yan Ma , Ling Chen , Dan Ma , Bao-Xin Chen , Feng Gao , Yun-Qi Tian , Yu-Kun S...
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DOI: 10.1021/cg901520r

Hierarchically Micro- and Mesoporous Coordination Polymer Nanostructures with High Adsorption Performance

2010, Vol. 10 2451–2454

Zhifeng Xin, Junfeng Bai,* Yongming Shen, and Yi Pan State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China Received December 5, 2009; Revised Manuscript Received March 24, 2010

ABSTRACT: Hierarchically micro- and mesoporous supra-nanostructure, Cu-BDC and Cr-BDC (BDC=1,4-benzenedicarboxylate), were prepared in which the mesopores are arranged in a disordered fashion. N2, H2, CO2, and CH4 adsorption properties of them have been investigated at low and higher pressures, indicating that they are new porous materials with relatively high performance. Most interestingly, Cr-BDC exhibits high H2 adsorption isosteric heat and higher uptakes of CO2 and H2 among these coordination polymer particle materials.

*Corresponding author: Tel.: þ86 25 83593384. Fax: þ86 25 83314502. E-mail: [email protected].

after evacuation at 140 °C (Figure S3a, Supporting Information) does not exhibit significant weight change up to 400 °C. Although the diffractogram in the X-ray diffraction (XRD) pattern Cu-BDC after evacuation at 140 °C shows some noise and peaks shift compared to that of Cu-BDC as synthesized (Figure S2b, Supporting Information), the desolvated Cu-BDC diffractogram still has the characteristics of an ordered, crystalline material.14 The XRD pattern (Figure S2, Supporting Information), composition, and TGA of Cu-BDC is similar to those of Cu(tpa) 3 (dmf) (tpa=1,4-benzenedicarboxylate, dmf=N,N-dimethylformide).14a In addition, the calculated micropore diameter of Cu-BDC is about 0.53 nm which is in reasonably agreement with the structure of Cu(tpa) 3 (dmf).14 However, the XRD pattern of Cr-BDC (Figure S3b, Supporting Information) indicates that Cr-BDC is almost amorphous as synthesized and after evacuation at 140 °C. To characterize the morphologies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed. The SEM image reveals that Cu-BDC exhibits square morphology (Figure 1a), its diameter is about 300 nm, and thickness iss about 20 nm. Meanwhile, TEM image (Figure 1a) and the HRTEM image (Figure 1c) of Cu-BDC further reveal the mesoporous structure of the nanosquares. The mesoporous channels are not parallel to other channels, resulting in most of them being connected together. However, the fixed diameter of mesopores is hard to judge. The calculated average mesopore diameter of Cu-BDC is about 14 nm and mesopore distribution (Figure 1d) is in a wide range. Cr-BDC exhibits suprastructure based on irregular nanosphere in which the average diameter of the spheres is about 50 nm (Figure 2a). Notably, the mesopores are formed by the nanoparticles in a random manner, which can be seen in the SEM and TEM images (Figure 2a,b). Likewise, the calculated average mesopore diameter with BJH method based on N2 adsorption data is about 17 nm and the mesopore distribution is also very wide (Figure 2d). Meanwhile, HRTEM image (Figure 2c) gives the disordered microporous structure of each nanoparticle. To clearly present the relationship between micropores and mesopores of Cu-BDC and Cr-BDC, a sketch map is given (Scheme 1). The calculated BET surface area from N2 sorption of Cu-BDC and Cr-BDC is about 241 m2/g and 404 m2/g, respectively. The initially saturation at low pressure of N2 adsorption (Figure 3a) suggests the existence of micropores15 in the two samples. The micropore distribution is about 0.53 nm for Cu-BDC and 0.50 nm for Cr-BDC, respectively (Figure S4, Supporting Information). When the relative pressure goes up to 0.95, the curve increases sharply and the desorption branch has hysteresis between 0.8 and 1 relative pressure, which is contributed to the existence of mesopores in these nanostructures.16

r 2010 American Chemical Society

Published on Web 04/20/2010

Recent years have witnessed the rapid development of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs)1 due to their potential applications in catalysis,2 gas storage,3 separation,4 nonlinear optics,5 and drug delivery.6 However, compared with these counterparts and unlike inorganic micro/nanomaterials,7 coordination polymer particles (CPPs) have been very less investigated, which may exhibit a higher level of structural tailorability and other interesting properties as a function of their micro/nanoscale architectures.8 In addition, hierarchically porous materials have received considerable attention because of their high pore volumes and large surface areas, together with potentially larger pore sizes.9 However, to the best of our knowledge, few report8f concerns about the hierarchically micro- and mesoporous coordination polymer micro/nanoparticles and supra-nanostructures in which the interesting micropores and mesopores coexist. 1,4-Benzenedicarboxylate (BDC = 1,4-benzenedicarboxylate) is an excellent building block for constructing PCPs or MOFs.1d,10 However, nanoscale PCPs or MOFs based upon it has been initially investigated in the last several years,11,13c and there is a great need to expand this interesting field from it. Our group is mainly focused on construction and morphology control of coordination polymers with interesting properties.12 Herein, hierarchically micro- and mesoporous nanostructures, Cu-BDC and Cr-BDC were reported in which the mesopores were formed in a disordered fashion. Most interestingly, Cr-BDC exhibits high H2 adsorption isosteric heat and higher adsorption of H2 and CO2 among these coordination polymer particle materials. Cu-BDC and Cr-BDC were quickly prepared at the early stage of the particle formation process according to the early reported formation mechanism of ICP particles.8c,g The chemical compositions of the two samples were determined by thermal gravimetric analysis (TGA) and elemental analysis (EA), indicating the 1:1 composition of the deprotonated BDC and metal ion in both complexes. IR spectra of these samples show the expected strong characteristic adsorptions for the symmetric and asymmetric vibrations of BDC (1610-1550 and 1420-1335 cm-1) and adsorbed water (3500-3200 cm-1). Also they exhibit no absorptions of any protonated BDC (1715-1680 cm-1), thus confirming the complete deprotonation of H2BDC by triethylamine.13 The TGA curves of Cu-BDC indicate that Cu-BDC is stable before 300 °C. A TGA curve of Cu-BDC after evacuation at 140 °C (Figure S2a, Supporting Information) shows no weight loss before 300 °C. These results suggest that vacuum drying at 140 °C is sufficient to desolvate DMF in Cu-BDC. Likely, the TGA curve of Cr-BDC

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Figure 1. (a) SEM, (b) TEM, (c) HRTEM images, and (d) distributions of pore diameters obtained using the Barrett-Joyner-Halenda (BJH) method for Cu-BDC.

Figure 2. (a) SEM image, (b) TEM image, (c) HRTEM image, and (d) distributions of pore diameters obtained using the BarrettJoyner-Halenda (BJH) method of Cr-BDC.

To further investigate the adsorption properties of these materials, CO2, H2, and CH4 adsorption isotherms were measured. The low pressure adsorption isotherms of CO2 and H2 for the two samples are depicted in Figures 3 and 4. The CO2 uptake is 17.5 cm3 (STP)/g for Cu-BDC and 48.5 cm3 (STP)/g for Cr-BDC at 1 bar and 258K (Figure 3b, Table 1). And the H2

adsorption amount of Cu-BDC and Cr-BDC is 17.5 cm3 (STP)/g and 89 cm3 (STP)/g at 1 bar and 77 K, respectively. Quite interestingly, Cr-BDC exhibits higher CO2 and H2 uptakes (Figure 3b, Table 1) which may be attributed to its relative larger surface area.18 The H2 isotherms of Cu-BDC and Cr-BDC (Figure 4a) show hysteresis, which may due to the higher H2

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Scheme 1. Formation Process of Nanostructured Cu-BDC and Cr-BDC

Figure 3. (a) N2 adsorption isotherms for Cr-BDC (blue solid triangles: adsorption, blue open triangles: desorption) and Cu-BDC (black solid squares: adsorption, black open squares: desorption) measured at 77 K. (b) Low pressure CO2 uptakes for Cr-BDC (blue solid triangles: adsorption, blue open triangles: desorption) and Cu-BDC (black solid squares: adsorption, black open squares: desorption) measured at 258 K.

adsorption isosteric heat of (Qst, 4 and 8 kJ/mol, respectively) which was obtained by fitting the isotherms recorded at 77 and 87 K according to the virial equations (Figure 4b).19a In addition, the high pressure CO2, H2, and CH4 uptakes (Table 1, Figures S6 and S7) for these two materials have further been measured at 298 K (CO2, CH4) and 77 K (H2). At higher pressure, the CO2 and

Figure 4. (a) Low pressure H2 adsorption isotherms for Cu-BDC (blue solid triangles: adsorption, blue open triangles: desorption) and Cr-BDC (black solid squares: adsorption, black open squares: desorption) measured at 77 K; (b) Isosteric heat of adsorption Qst for H2 in Cu-BDC (black solid squares: Obtained from the data at 77K, black open squares: 87K) and Cr-BDC (red solid circles: Obtained from the data at 77K, red open circles: 87K).

H2 uptakes significantly increased compared to the low pressure adsorption amount (Table 1), and high pressure CH4 uptakes is 25.7 cm3 (STP)/g for Cr-BDC and 15.4 cm3 (STP)/g for Cu-BDC, respectively. Notably, the high pressure gases uptakes of coordination polymer nanoparticles have been investigated for the first time.

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Table 1. The Gases Sorption Parameter of the Amorphous ICP Particles 2

materials

SBET (m /g) 8c

Zn-ASFL (3) 5a8g 5b8g ZSM-517 Cu-BDC Cr-BDC

225 244 264 418 241 404

SMIC (m2/g)

SEXT/SMICa

SMESb/SMIC

CO2 uptakec (cm3 (STP)/g) 31 35 37.6

164 217

0.47 0.86

0.37 0.75

17.5 (1 bar), 25.7 (20 bar) 48.5 (1 bar), 68.4 (20 bar)

H2 uptaked (cm3 (STP)/g) 64 77 63.4 79 17.5 (1 bar), 59.7 (20 bar) 89 (1 bar), 253 (20 bar)

a

SEXT and SMIC are external surface area and micropore surface area, respectively, determined by t-plot analysis. b SMES is the mesopore surface area estimated by BJH adsorption cumulative volume of pores between 1.7 and 300 nm width. c The low pressure CO2 uptake was measured at 258 K and high pressure CO2 uptake was measured at 298 K. d H2 uptake was measured at 77 K.

In summary, hierarchically micro- and mesoporous Cu-BDC nanoparticles and Cr-BDC suprastructures based upon BDC were reported in which the mesopores are in a disordered fashion. H2, CO2, and CH4 adsorption properties of them have been investigated, indicating that they are new porous materials with relatively high adsorption performance. Most interestingly, Cr-BDC exhibits higher H2 adsorption isosteric heat and higher uptakes of H2 and CO2 among these CPP materials.

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Acknowledgment. The authors gratefully acknowledge support from the Major State Basic Research Development Programs (Nos. 2006CB806104 and 2007CB936302), the NSFC (Nos. 20771058 and 20931004), and the Science Foundation of Innovative Research Team of NSFC (No. 20721002). Supporting Information Available: Thermal gravimetric curves and XRD patterns of Cu-BDC and Cr-BDC as-synthesized and after evacuation at 140 °C for 8 h; N2 adsorption isotherms, pore distribution, low pressure H2 adsorption, and desorption isotherms measured at 77 K and 87 K of Cu-BDC and Cr-BDC; high pressure H2, CH4, CO2 adsorption and desorption isotherms measured at 298 K and low pressure CO2 adsorption and desorption isotherms measured at 258 K. This information is available free of charge via the Internet at http://pubs.acs.org/.

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