Langmuir 2006, 22, 4225-4229
4225
Adsorption Studies of a Microporous Phthalocyanine Network Polymer A. Verena Maffei,† Peter M. Budd,*,† and Neil B. McKeown‡ Organic Materials InnoVation Centre, School of Chemistry, The UniVersity of Manchester, Manchester M13 9PL, U.K., and School of Chemistry, Cardiff UniVersity, Cardiff CF10 3AT, U.K. ReceiVed January 10, 2006. In Final Form: February 9, 2006 The adsorption/desorption of N2 at 77 K and the adsorption from aqueous solution at 298 K of four organic probe molecules of different sizes (phenol, 4-nitrophenol, orange II, naphthol green B) were studied for a phthalocyanine network polymer of intrinsic microporosity (PIM) and for an activated carbon (Darco 20-40 mesh). N2 sorption analysis gave similar surface areas for the PIM and the carbon (610 and 545 m2 g-1, respectively) but showed differences in pore size distribution, the PIM being essentially microporous (pore size < 2 nm), with a high proportion of ultramicropores (50 nm).1 Microporous materials, such as zeolites and activated carbons, find wide application in heterogeneous catalysis and molecular separations. We have demonstrated that organic network polymers comprising large planar units (e.g., phthalocyanine,2 porphyrin,3 and hexaazatrinaphthylene4), connected by rigid, nonlinear linkers, behave in a similar way to microporous materials and exhibit high apparent surface areas (5001000 m2 g-1) by low-temperature nitrogen adsorption. We have further shown that nonnetwork polymers with sufficiently rigid and contorted backbones behave like microporous materials in the solid state.5 We are thus developing a range of polymers of intrinsic microporosity (PIMs),6,7 including both insoluble network-PIMs with potential for use as heterogeneous catalysts and soluble PIMs that can be cast into membranes for liquid8 and gas9 separations. These are glassy polymers that degrade at temperatures above 300 °C. This paper provides the first detailed study of the sorption properties of a phthalocyanine network-PIM and compares it with an activated carbon of comparable surface area (DARCO 20-40 mesh). The network-PIM investigated here (denoted CoPc20) comprises cobalt phthalocyanine units interconnected * To whom correspondence should be addressed. Telephone: (++)161-275-4711. E-mail:
[email protected]. † The University of Manchester. ‡ Cardiff University. (1) Everett, D. H. Pure Appl. Chem. 1972, 31, 577-638. (2) McKeown, N. B.; Makhseed, S.; Budd, P. M. Chem. Commun. 2002, 27802781. (3) McKeown, N. B.; Hanif, S.; Msayib, K.; Tattershall, C. E.; Budd, P. M. Chem. Commun. 2002, 2782-2783. (4) Budd, P. M.; Ghanem, B.; Msayib, K.; McKeown, N. B.; Tattershall, C. J. Mater. Chem. 2003, 13, 2721-2726. (5) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E. Chem. Commun. 2004, 230-231. (6) McKeown, N. B.; Budd, P. M.; Msayib, K. J.; Ghanem, B. S.; Kingston, H. J.; Tattershall, C. E.; Makhseed, S.; Reynolds, K. J.; Fritsch, D. Chem. Eur. J. 2005, 11, 2610-2620. (7) Budd, P. M.; McKeown, N. B.; Fritsch, D. J. Mater. Chem. 2005, 15, 1977-1986. (8) Budd, P. M.; Elabas, E. S.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E.; Wang, D. AdV. Mater. 2004, 16, 456-459. (9) Budd, P. M.; Msayib, K. J.; Tattershall, C. E.; Ghanem, B. S.; Reynolds, K. J.; McKeown, N. B.; Fritsch, D. J. Membr. Sci. 2005, 251, 263-269.
by spirocyclic linkers. It was prepared by the cyclotetramerization of a bis(phthalonitrile) monomer formed from 4,5-dichlorophthalonitrile and 3,3,3′,3′-tetramethyl-1,1′-spirobis(indan5,5′,6,6′-tetrol) (Figure 1).2 The spiro center is a site of contortion that forces neighboring phthalocyanine units to point in different directions, while the fused-ring structure prohibits rearrangement to a dense solid. Since the directions imposed by the spiro centers are statistically distributed, the material is amorphous. In this work, nitrogen adsorption is used to investigate the apparent pore size distribution. The molecular sieve nature of the material is further demonstrated by studies of the adsorption from aqueous solution of four compounds of different molecular sizes and characteristics: phenol, 4-nitrophenol, orange II, and naphthol green B. The structures of these probe molecules are shown in Figure 2 and their key characteristics listed in Table 1. Experimental Section Materials. Preparation of Bis(phthalonitrile) Monomer. 4,5Dichlorophthalonitrile (15 g) and 3,3,3′,3′-tetramethyl-1,1′-spirobis(indan-5,5′,6,6′-tetrol) (12 g) were stirred under N2. Dry dimethylformamide (150 cm3) and excess K2CO3 (30 g) were added slowly, and then the mixture was heated for 3 h at 80 °C. The product was isolated, washed with distilled water, dried, and recrystallized from methanol and then dichloromethane (yield, 80%). Anal. Calcd for C37H24N4O4: C, 75.5; H, 4.1; N, 9.5%. Found: C, 75.3; H, 4.1; N, 9.6%. IR (KBr): 2235 cm-1 nitrile, 1333 cm-1. MS (EI): m/z 623 (100%) (M+ + Cl-). 1H NMR (300 MHz, pyridine): δ 1.38 (3H, s, Me), 1.35 (3H, s, Me), 2.62 (1H, d, CH2), 2.67 (1H, d, CH2), 6.8 (1H, s, Ar-H), 7.25 (1H, s, Ar-H), 8.0 (1H, s, Ar-H), 8.2 (1H, s, Ar-H). Formation of Network-PIM. Bis(phthalonitrile) monomer (5.4 g) and anhydrous cobalt(II) acetate (0.81 g) were stirred under N2 and heated to 100 °C (15 min). Dry quinoline (18.9 cm3) was added and the temperature increased first to 120 °C (30 min) and then to 160 °C. After 105 min a dark green precipitate began to form. The reaction mixture was maintained at 160 °C for 18 h and then cooled and the precipitate filtered off. The product was washed with ethanol (2 h), acetone (1 h), methanol (2 h), and acetone (1 h), refluxed with tetrahydrofuran (24 h), refluxed with dichloromethane (24 h), washed with acetone, refluxed with dimethylacetamide (24 h), washed with acetone, refluxed with dimethylformamide (24 h), washed with acetone, and then Soxhlet-extracted with tetrahydrofuran (6 days). The purified product was dried at 127 °C under vacuum (20 h) (yield, 85%). Anal. Calcd for CoC74H48,N8O8: C, 71.9; H, 3.9; N, 9.1; Co, 4.8%. Found: C, 68.7; H, 3.5; N, 7.9; Co, 3.8%.
10.1021/la060091z CCC: $33.50 © 2006 American Chemical Society Published on Web 03/14/2006
4226 Langmuir, Vol. 22, No. 9, 2006
Maffei et al.
Figure 1. Formation of cobalt phthalocyanine network-PIM. Reagents and conditions: (i) K2CO3, DMF, 80 °C; (ii) (CH3CO2)2Co, quinoline, 160 °C.
Figure 2. Chemical structures of probe molecules used for the study of adsorption from aqueous solution. Table 1. Characteristics of Probe Molecules Used in Studies of Adsorption from Aqueous Solution probe
dimensions (nm)
phenol 4-nitrophenol orange II naphthol green B
0.53 × 0.43 × 0.33 0.64 × 0.43 × 0.33 1.27 × 0.70 × 0.33 length 1.6
Carbon. Granular carbon, DARCO 20-40 mesh, was obtained from Aldrich. Adsorbates. Phenol, 4-nitrophenol, orange II, and naphthol green B were obtained from Aldrich. Methods. Nitrogen Adsorption. Nitrogen adsorption/desorption at 77 K was measured using a Micromeritics ASAP 2020 system. The adsorbate sample was first dried under vacuum (10-4 bar) at 127 °C for 20 h and then degassed under ultrahigh vacuum (10-9 bar) at 120 °C for 16 h. The sample was back-filled with nitrogen, transferred to the analysis system, and then again degassed under ultrahigh vacuum at 100 °C overnight, prior to sorption analysis. The free space was measured at the end of the sorption experiment, after a further 2 h degassing at 100 °C. Helium gas was used for the free-space determination at ambient temperature and at 77 K. Apparent surface areas were calculated from N2 adsorption data at relative pressures below 0.23, by the multipoint Brunauer-EmmettTeller (BET) method.10 Data were also analyzed by the t-plot method11 and by the Barrett-Joyner-Halenda (BJH) method,12 using the (10) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (11) Lippens, B. C.; Linsen, B. G.; de Boer, J. H. J. Catal. 1964, 3, 32-37. (12) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.
solubility at 25 °C (g dm-3) 84 25 16 25 14 26
λmax (nm)
max (dm3 mol-1 cm-1)
270 318 486 284
1 449.6 14 9 233.5 19 952 26 451
manufacturer’s software. Apparent micropore distributions were calculated from N2 adsorption data by the Horvath-Kawazoe (HK) method.13 Adsorption from Solution. Prior to analysis, powdered samples of adsorbent were dried at 127 °C under vacuum for 20 h and degassed at 120 °C for 16 h. Adsorbate solution (10 cm3) of known concentration was added to a sample of adsorbent (50 mg) and the suspension stirred at 25 °C for a specified time period. The liquid was decanted off and centrifuged at 5000 rpm for 15 min and the supernatant removed. The concentration of the supernatant was determined by UV absorbance at λmax, using a Hewlett-Packard 8425A diode array spectrometer with a path length of 1 cm. Values of λmax and molar absorption coefficient, max, were determined in the laboratory or, in the case of phenol, obtained from the literature14 (Table 1). The amount of adsorbate adsorbed per gram of adsorbent was calculated from the difference between the initial and final concentrations of the adsorbate solution. The experiment was repeated for different time periods to establish the equilibration time and repeated for various adsorbate concentrations. (13) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470-475. (14) Shu, H.-T.; Li, D.; Scala, A. A.; Ma, Y. H. Sep. Purif. Technol. 1997, 11, 27-36.
Microporous Phthalocyanine Network Polymer
Langmuir, Vol. 22, No. 9, 2006 4227
Figure 4. t-plots for Darco 20-40 mesh carbon (circles) and CoPc20 network-PIM (squares). The straight lines show the linear extrapolations used for the estimation of micropore volume.
Figure 3. Nitrogen adsorption (filled symbols) and desorption (open symbols) isotherms at 77 K for (a) Darco 20-40 mesh carbon and (b) CoPc20 network-PIM. To investigate the effect of regenerating the adsorbent, the samples of CoPc20 after phenol adsorption were washed with ethanol. Removal of phenol from the samples was monitored by UV spectroscopy of the washings. After removal of phenol, the samples were washed with acetone, purified as above, and dried at 127 °C under vacuum (20 h). The regenerated CoPc20 was used again for phenol adsorption.
Results and Discussion Nitrogen adsorption/desorption isotherms for the carbon and the CoPc20 network-PIM are compared in Figure 3. BET analysis gives apparent surface areas of 545 m2 g-1 for the carbon (literature value, 650 m2 g-1)15 and 610 m2 g-1 for CoPc20. Both samples exhibit high uptake at very low relative pressures, typical of microporous materials. However, significant differences are observed at higher relative pressures. In the case of the carbon, there is a hysteresis loop (the desorption curve lies above the adsorption curve) which closes at a relative pressure above 0.4 (a type IV isotherm), indicating mesoporosity.16 In the case of CoPc20, the isotherm is much flatter and the hysteresis extends to low relative pressures. This latter type of hysteresis, which is characteristic of PIMs, is generally associated with microporous systems.17 It may arise where there is a network of micropores, with larger cavities connected by constrictions, or where an adsorbate causes a microporous material to swell. The isotherms therefore suggest that CoPc20 is essentially microporous, while the carbon possesses some mesoporosity in addition to microporosity. This is demonstrated further by more detailed analysis of the isotherms, as discussed below. (15) Wang, R.-C.; Kuo, C.-C.; Shyu, C.-C. J. Chem. Technol. Biotechnol. 1997, 68, 187-194. (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic: London, 1982. (17) Ilinitch, O. M.; Fenelonov, V. B.; Lapkin, A. A.; Okkel, L. G.; Terskikh, V. V.; Zamaraev, K. I. Microporous Mesoporous Mater. 1999, 31, 97-110.
Figure 5. BJH differential mesopore volume distributions for Darco 20-40 mesh carbon (circles) and CoPc20 network-PIM (squares).
In Figure 4 the N2 adsorption isotherms are presented as t-plots (amount adsorbed against the statistical thickness of the N2 film). This is a way of comparing the results with the standard isotherm expected for N2 on a nonporous solid.16 For an isotherm of identical shape to the standard, the t-plot would be a straight line passing through the origin. Mesoporosity gives rise to an upward deviation of the t-plot in the high-pressure region, as is clearly seen for the carbon in Figure 4, while microporosity distorts the t-plot in the low-pressure region. Analysis of the t-plot (using the Harkins-Jura thickness equation)18 for the carbon gives a micropore volume of 0.15 cm3 g-1 and a micropore surface area of 338 m2 g-1, suggesting that not much more than 60% of the total apparent surface area is associated with micropores. In contrast, for CoPc20, the micropore volume is 0.29 cm3 g-1 and the micropore surface area is 570 m2 g-1, indicating that most of the apparent surface area is associated with micropores. The mesoporosity of the carbon was further characterized by BJH analysis of the desorption curve, which indicated a distribution of pore sizes extending to above 40 nm pore width, while BJH analysis of CoPc20 indicated minimal porosity in the mesopore range (Figure 5). Micropore distributions determined by the HK method, assuming a slit-pore geometry and the original HK carbongraphite interaction potential, are shown in Figure 6. Similar results were obtained using a cylinder-pore geometry with a Ross-Olivier19 carbon-graphite interaction potential. The pore models used are obviously simplistic, and the absolute numbers should be treated with caution. Nevertheless, HK analysis indicates that CoPc20 has a high concentration of pores of size in the region of 0.6-0.7 nm. Pores of dimensions
