Energy & Fuels 2009, 23, 2785–2789
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CO2 Adsorption-Based Separation by Metal Organic Framework (Cu-BTC) versus Zeolite (13X) Zhijian Liang,†,‡ Marc Marshall,‡ and Alan L. Chaffee*,†,‡ CooperatiVe Research Centre for Greenhouse Gas Technology (CO2CRC) and School of Chemistry, Monash UniVersity, Victoria 3800, Australia ReceiVed October 27, 2008. ReVised Manuscript ReceiVed February 23, 2009
The potential for the metal organic framework (MOF) Cu-BTC to selectively adsorb and separate CO2 is considered. Isotherms for CO2, CH4, and N2 were measured from 0 to 15 bar and at temperatures between 25 and 105 °C. The isotherms suggest a much higher working capacity (×4) for CO2 adsorption on Cu-BTC relative to the benchmark zeolite 13X over the same pressure range. Higher CO2/N2 and CO2/CH4 selectivities in the higher pressure range (1-15 bar) and with lower heats of adsorption were also demonstrated. Cu-BTC was observed to be stable in O2 at 25 °C, but its crystallinity was reduced in humid environments. The CO2 adsorption capacity was progressively reduced upon cyclic exposure to water vapor at low relative humidity ( 0.7, can be attributed to adsorption on the external crystallite surface. It was observed that Cu-BTC had very high H2O adsorption capacity (e.g., 50 wt % at saturation sorption at 25 °C). The high H2O capacities on CuBTC can be attributed to the high pore volume, together with the open metal sites present in the framework. The amount of H2O adsorbed at P/P0 < 0.4 was much greater than the amount of coordinated H2O (9 wt %), so that, even though the coordinated H2O does not alter the structure significantly,20 the large amount of H2O adsorbed even at low P/P0 raises the possibility that structure change could take place. As can be seen from Figure 7, the desorption of H2O does not follow the adsorption branch for the sample, showing large hysteresis. For the 32 °C isotherm the desorption curve lies below the adsorption curve, an unusual phenomenon, strongly suggesting that structural change has occurred. In other words, the structure of the sample during the 32 °C isotherm is probably not the same as during the 25 °C isotherm. The adsorbed H2O is not completely removed during desorption at 25 or 32 °C, suggesting that some kind of chemical interaction occurs. It is anticipated that some H2O molecules coordinate directly to the unsaturated Cu(II) centers, but it is also noted that after H2O sorption experiments the color changes noticeably from blue to green. The XRD pattern of the sample after water sorption experiments was compared with that of the as-synthesized samples (Figure 8). It was found that the sample after water sorption experiments exhibited a different XRD pattern (trace c) from that of the synthesized sample (trace a), indicating that the structure of the sample changed as a result of water sorption. (20) Prestipino, C.; Regli, L.; Vitillo, J. G.; Bonino, F.; Damin, A.; Lamberti, C.; Zecchina, A.; Solari, P. L.; Kongshaug, K. O.; Bordiga, S. Chem. Mater. 2006, 18, 1337.
Liang et al.
Figure 8. XRD patterns of samples after water sorption experiments under 97% humidity (c) and under 30% humidity (b), both at 25 °C, are compared with that of the as-synthesized samples (a). The intensities for b and c are emphasized by multiplying the raw data by 10, then shifting the baseline by 5000 and 10 000 au, respectively.
The CO2 sorption isotherm of the sample was measured again after water sorption. The CO2 adsorption capacity was now significantly reduced, to just 1.8 mol/kg (from 10.5 mol/kg) at 25 °C and 5 bar, again indicating structural change. This change was not due to the water retained in the sample after the H2O isotherms, since removal of this H2O by heating the sample at 100 °C in vacuum did not restore the original CO2 adsorption capacity. That is, it remained at 1.8 mol/kg. In a previous study it was reported that Cu-BTC was stable to water sorption,21 in contrast to our results. In this earlier study, Cu-BTC was exposed to ambient air (40% humidity), at 25 °C, for 7 days and the XRD pattern was unchanged. To gather the data reported in Figure 7, our Cu-BTC sample was exposed to higher relative humidities (up to 97%) for relatively long periods (2 to 8 h per point) so as to reach equilibrium at each point. As such, our experiments exposed the Cu-BTC to “harsher” (more moist) conditions than the prior study, so the results of the two studies are not necessarily inconsistent with each other. It can be noted that the stability of other MOFs, specifically MOF-5 and MOF-177, in the presence of water has recently been reported.22-25 These materials are not stable to water adsorption. To determine the stability of the Cu-BTC structure in the presence of unremoved solvents (e.g., water and ethanol), we measured the N2 sorption isotherm (not shown) and XRD (not shown) of the blue crystals of Cu-BTC after isolation from reaction solution by filtration, washing with water and ethanol 3 times, and storing in a sealed container for 3 months without initial drying. Its surface area is reduced to half of its initial surface area. However, the XRD pattern was almost unchanged. To assist our understanding of the Cu-BTC behavior in the presence of moisture, a series of experiments were carried out for which CO2 sorption isotherms were measured before and after each of three successive water sorption experiments for which the humidity was limited to 30%, at 25 °C (Figure 9). The CO2 adsorption capacity of Cu-BTC was observed to decline after each water sorption, but the decline became more gradual with each step such that it appeared to level out at 75% of its original level (at 5 bar) after three water adsorption/ desorption cycles. Thus, the results indicate that the material might be suitable for application in the presence of water under certain conditions (e.g., relative humidity maintained at less than 30% at 25 °C). The XRD pattern (Figure 8b) of the sample (21) Li, Y.; Yang, R. T. AIChE J. 2008, 54, 269. (22) Greathouse, J. A.; Allendorf, M. D. J. Am. Chem. Soc. 2006, 128, 10678. (23) Panella, B.; Hirscher, M. AdV. Mater. 2005, 17, 538. (24) Huang, L. M.; Wang, H. T; Chen, J. X.; Wang, Z. B; Sun, J. Y.; Zhao, D. Y.; Yan, Y. S. Micro. Meso. Mater. 2003, 58, 105. (25) Li, Y.; Yang, R. T. Langmuir 2007, 23, 12937.
SelectiVe Adsorption and Separation of CO2
Figure 9. Succession of CO2 isotherms at 25 °C and 0 to 5 bar before and after each H2O isotherm at 25 °C in the relative vapor pressure range 0 to 30%; (a) before first H2O isotherm, (b) between first and second H2O isotherms, (c) between second and third H2O isotherms, (d) after third H2O isotherm.
Figure 10. Comparison of CO2, N2, and CH4 adsorption isotherms of the Cu-BTC with that of benchmark zeolite 13X at 25 °C.
following these experiments still showed some of the initial XRD peaks, but other peaks disappeared, indicating a loss of crystallinity. A comparison of the adsorption properties of the Cu-BTC and the benchmark zeolite 13X is shown in Figure 10. The data for zeolite 13X are taken from the literature.3 The dramatically different CO2 adsorption isotherm shapes, because of their different structural properties, are readily apparent. The comparison indicates that the pressure range for which Cu-BTC is suitable in a PSA process is not the same as for zeolite 13X. For zeolite 13X the CO2 reaches saturation at relatively low pressure range. The CO2 adsorption capacity increases almost linearly in the pressure range of 0 to 5 bar for Cu-BTC, but only in the relatively low pressure range of 0 to 1 bar for zeolite 13X. Clearly, the CO2 adsorption capacity for Cu-BTC (12.7 mol/kg) is much higher than for zeolite 13X (6.9 mol/kg) at 25 °C and 15 bar. The working capacity in a PSA case was calculated as the difference between the CO2 adsorption capacity at 1 and 15 bar (at 25 °C). Thus, the working capacity of Cu-BTC in a PSA system is 8.1 mol/kg. This can be compared with just 2.2 mol/ kg for the benchmark material zeolite 13X over the same pressure range. The heats of CO2 adsorption can also be compared. The heat of CO2 adsorption on Cu-BTC determined by differential thermal analysis (DTA) was 30 kJ mol-1. Thus, it can be
Energy & Fuels, Vol. 23, 2009 2789
Figure 11. CO2/N2 and CO2/CH4 selectivities of the Cu-BTC.
characterized as a physisorption phenomenon. The value is comparable to that reported for Cu-BTC,6 but somewhat lower than the heat of adsorption reported for CO2 with zeolite 13X (49 kJ mol-1).26 The lower values are probably beneficial from the perspective of a reduced energy requirement for adsorbent regeneration. The CO2/N2 and CO2/CH4 selectivities of Cu-BTC are compared with that of benchmark zeolite 13X in Figure 11. The selectivities for Cu-BTC were calculated from the single gas isotherms by dividing the CO2 adsorption capacity by that of N2 or CH4 at each pressure point.27 The Cu-BTC selectivity toward CO2 at 25 °C decreases slowly as pressure is increased. At low pressure the Cu-BTC shows lower selectivity toward CO2 (at 25 °C) than zeolite 13X, because of the relatively low heat of CO2 adsorption. However, its selectivity toward CO2 decreases more slowly than that of zeolite 13X with increasing pressure. Therefore Cu-BTC shows promise for separating CO2 from CO2/N2 or CO2/CH4 gas mixtures by pressure swing adsorption at higher pressure. Conclusion Cu-BTC was prepared and characterized. Its potential application in a PSA system to selectively capture CO2 was evaluated by measuring a series of sorption isotherms, for CO2, CH4, and N2 using an IGA between 0 and 15 bar and at temperatures ranging from 25 to 105 °C. The results show that the working capacity of Cu-BTC in a PSA system, at 25 °C, is almost four times that of the benchmark material zeolite 13X. It also showed higher CO2/N2 and CO2/CH4 selectivities in the higher pressure range (>1.0 bar) and a lower energy requirement for regeneration than zeolite 13X. Cu-BTC was also stable in O2 at 25 °C, but adsorbed 1 mol/kg at 2.5 bar. The CO2 adsorption capacity of Cu-BTC declined after water sorption, particularly at high relative humidity, but this capacity may level out at a reasonable level value after several water adsorption/ desorption cycles so long as it is protected from prolonged exposure to high relative humidity. Thus, the material may be suitable for PSA application in the presence of water vapor under certain conditions, e.g., less than 30% relative humidity at 25 °C. EF800938E (26) Dunne, J. A.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5896. (27) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. J. Ind. Eng. Chem. Res. 2008, 47, 6333.
