Ind. Eng. Chem. Res. 2008, 47, 6333–6335
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RESEARCH NOTES Metal Organic Framework Adsorbent for Biogas Upgrading Simone Cavenati, Carlos A. Grande,* and Alı´rio E. Rodrigues Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto, Rua Dr. Roberto Frias, s/n, 4200-465, Porto, Portugal
Christoph Kiener and Ulrich Mu¨ller BASF SE, Ludwigshafen, Germany
The discovery of new materials with enhanced selectivity or capacity will boost adsorption applications, especially for environmental control. In particular, it was mentioned that metal-organic frameworks (MOF) with different tailored properties may be prepared for desired separations. To promote industrial application of MOF materials, up-scaling and process design still must be completely developed. In this work, we report adsorption equilibrium data of methane and carbon dioxide on Cu-MOF extrudates. The most important properties observed are the high-capacity for CO2 and the small nonlinearity of the isotherms. In this context, this adsorbent can be used for biogas upgrading to produce biomethane and reduce fossil-fuel CO2. Introduction Biogas is the gaseous product of anaerobic decomposition of organic matter and a renewable source to produce methane. Unfortunately, not only methane is produced, and the biogas composition at the exit of digesters and from landfills is generically comprised of four gases: contaminants (sulfur compounds), water, carbon dioxide (CO2), and methane (CH4). These gases may be removed sequentially,1 starting with sulfur compounds and then water.2 Finally, the mixture of CO2 and CH4 must be separated. CO2 is the major contaminant (25%-45%), and the economics of its removal is the most critical step, in regard to upgrading biogas for transport applications or satisfying pipeline specifications. Water washing, membranes, and amine scrubbing, as well as vacuum pressure swing adsorption (VPSA) processes are being used.2 We have studied the VPSA process by applying equilibrium and kineticbased adsorbents3 and also using different layers of these materials.4 The main drawbacks of equilibrium-based adsorbents were either low selectivity (activated carbon materials) or low regenerability (zeolite 13X). Thus, kinetic-based materials were preferred.3,5 Metal organic framework (MOF) solids are highly crystalline materials, with robustness provided by the strong bonding of inorganic molecules and linking organic units that may allow specific tailoring according to the desired application.6 The high surface area of MOF compounds may boost new development in adsorption technologies for gas molecules.7 In fact, these new adsorbents were studied in several applications: gas storage,8 CO2 storage,9 H2 adsorption,10,11 and other gas cleaning and separation processes.12,13 However, despite their potential aspects, no industrial application is yet available using MOF compounds. For the purpose of finding a suitable large-scale application of MOF materials, we have determined adsorption equilibrium of CO2 and CH4 in metal organic framework (Cu-MOF) * To whom correspondence should be addressed: Tel.: +351 22 508 1618. Fax: +351 22 508 1674. E-mail address:
[email protected].
extrudates. The novel aspect of this work is that measurements were performed in extrudates that were comprised of the MOF adsorbent linked by a binder to provide mechanical resistance. The MOF tablets used in this publication, with dimensions of 3 mm × 3 mm, are based on Cu-BTC powder synthesized by a well-described synthesis recipe.14 The shaping of the powder has been performed on a S300 Excenter press (Fa. Kilian) using 2% Alox C (Degussa) and 3% graphite as additives. The final tablets have been dried at 373 K under vacuum. Bulk density of the material is 582 kg/m3. The lateral stiffness of the shaped body was determined to be 24 N. Adsorption data of CO2 and CH4 at 303 K are shown in Figure 1. The data were measured using a gravimetric microbalance (Rubotherm, Germany). For comparison purposes, we have made comparisons to CO2 adsorption equilibrium on Cu-BTC powder at 298 K.9 The higher loading of CO2 in the Cu-BTC pelletized sample than in the powder form is due to a higher
Figure 1. High-pressure adsorption equilibrium of carbon dioxide ([) and methane (2) at 303 K on MOF extrudates. For comparison, we present data for CO2 on Cu-BTC powder (denoted by solid squares, 9).9 Solid lines are fittings of the data using the Langmuir model.
10.1021/ie8005269 CCC: $40.75 2008 American Chemical Society Published on Web 07/25/2008
6334 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 1. Parameters of the Langmuir Model for the Adsorption Equilibrium of CO2 and CH4 on Cu-MOF Extrudatesa gas CO2 CH4
qm,i [mol/kg] 12.95 9.98
Ki° [bar-1] -6
5.56 ×10 9.76 ×10-5
-∆Hi [kJ/mol] 28.1 16.6
a The parameter qi is defined as qi ) qm,i KiP/(1 + KiP), where Ki ) Ki° exp(-∆Hi/RT).
Figure 2. Adsorption of (a) carbon dioxide (CO2) and (b) methane (CH4) on metal organic framework (MOF) extrudates at 303, 323 and 373 K. Solid lines are fittings with Langmuir models.
surface area. The surface area of the powder sample is 1781 m2/g,9 while the surface area of this sample is >2000 m2/g.14 The material has higher selectivity toward CO2, decreasing at high pressures when the saturation plateau is reached. This material does not present the “knee” of other high-surface isoreticular metal-organic frameworks (IRMOFs)9 with strong attractive electrostatic interactions between CO2 molecules.17 The data shown in Figure 1 were recorded over a wide pressure range and at a single temperature. Almost all industrial applications of new materials for gas separation processes involve CO2 partial pressures of 40 kJ/mol).16 Another material that is used for comparison purposes is MOF-5. This material has a capacity of ∼20 mol/kg at high pressures, but at low pressures, its selectivity toward CO2 is not high, because of the shape of the isotherm (see Figure 3). The adsorption data reported in this study indicate that CuMOFs can be applied “as it is” in CO2/CH4 separation for biogas upgrading. Because of the slight nonlinearity of the CO2 isotherms, when compared with other “equilibrium-based” materials, the regeneration can be performed using only a slight vacuum (instead of 0.1 bar)3 for smaller periods of time. Also, taking into account the high loading that can be achieved per mass unit of adsorbent, the VPSA process can be significantly
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smaller, eventually allowing “home-based” or portable units for biogas upgrading. Conclusions Metal organic framework extrudates (Cu-MOFs) were evaluated for the adsorption of CO2 and CH4 at 303, 323 and 373 K. The materials show selectivity ranging from 4-6 at a pressure of 0.1-3 bar and a very high capacity for CO2 (6.6 mol/kg at 2.5 bar and 303 K). This new adsorbent can be successfully applied in vacuum pressure swing adsorption (VPSA) units for carbon dioxide/methane (CO2/CH4) separation for biogas upgrading. Using this adsorbent, important savings, with regard to power consumption, and also drastic size reduction can be achieved. Literature Cited (1) Knaebel, K. S.; Reinhold, H. E. Landfill Gas: From Rubbish to Resource. Adsorption 2003, 9, 87. (2) Hagen, M.; Polman, E.; Jensen, J. K.; Myken, A.; Jo¨nsson, O.; Dahl, A. Adding gas from biomass to the gas grid. Report SGC 118, Swedish Gas Centre, 2001. (3) Grande, C. A.; Rodrigues, A. E. Biogas to Fuel by Vacuum Pressure Swing Adsorption I. Behaviour of Equilibrium and Kinetic Adsorbents. Ind. Eng. Chem. Res. 2007, 46, 4595. (4) Grande, C. A.; Rodrigues, A. E. Layered Vacuum Pressure-Swing Adsorption for Biogas Upgrading. Ind. Eng. Chem. Res. 2007, 46, 7844. (5) Kim, M.-B.; Bae, Y.-S.; Choi, D. K.; Lee, C.-H. Kinetic Separation of Landfill Gas by a Two Bed Pressure Swing Adsorption Process Packed with Carbon Molecular Sieve: Nonisothermal Operation. Ind. Eng. Chem. Res. 2006, 45, 5050. (6) Rowsell, J. L. C.; Yaghi, O. M. Metal-organic Frameworks: A New Class of Porous Materials. Microporous Mesoporous Mater. 2004, 73, 3. (7) Ma, S.; Sun, D.; Wang, X.-S; Zhou, H.-C. A Mesh-Adjustable Molecular Sieve for General Use in Gas Separation. Angew. Chem., Int. Ed. 2007, 46, 2458.
(8) Du¨ren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Design of New Materials for Methane Storage. Langmuir 2004, 20, 2683. (9) Millward, A. R.; Yaghi, O. M. Metal-organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998. (10) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. Impact of Preparation and Handling on the Hydrogen Storage Properties of Zn4O(1,4benzenedicarboxylate)3 (MOF-5). J. Am. Chem. Soc. 2007, 129, 14176. (11) Furukawa, H.; Miller, M. A.; Yaghi, O. M. Independent verification of the saturation hydrogen uptake in MOF-177 and establishment of a benchmark for hydrogen adsorption in metal-organic frameworks. J. Mater. Chem. 2007, 17, 3197. (12) Wang, Q. M.; Shen, D.; Bu¨low, M.; Lau, M. L.; Deng, S.; Fitch, F. R.; Lemcoff, N. O.; Semanscin, J. Metallo-organic Molecular Sieve for Gas Separation and Purification. Microporous Mesoporous Mater. 2002, 55, 217. (13) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre´, J. Metal-organic Frameworks;Prospective Industrial Applications. J. Chem. Mater. 2006, 16, 626. (14) Schubert M.; Mu¨ller, U.; Hesse, M.; Diehlmann, U. Process for Preparing Porous Metal-Organic Framework Materials, WO/2007/090809, 2007. (15) Himeno, S.; Komatsu, T.; Fujita, S. High-Pressure Adsorption Equilibria of Methane and Carbon Dioxide on Several Activated Carbons. J. Chem. Eng. Data 2005, 50, 369. (16) Cavenati, S.; Grande, C. A.; Rodrigues, A. E. Adsorption Equilibrium of Methane, Carbon Dioxide and Nitrogen on Zeolite 13X at High Pressures. J. Chem. Eng. Data 2004, 49, 1095. (17) Walton, K. S.; Millward, A. R.; Dubbeldam, D.; Frost, H.; Low, J. J.; Yaghi, O. M.; Snurr, R. Q. Understanding Inflections and Steps in Carbon Dioxide Adsorption Isotherms in Metal-organic Frameworks. J. Am. Chem. Soc. 2008, 130, 406.
ReceiVed for reView April 02, 2008 ReVised manuscript receiVed June 13, 2008 Accepted July 12, 2008 IE8005269
