Metal Organic Frameworks (MOFs) are nanoporous materials suitable for gas purification and CO2 capture by means of adsorption processes. MOFs are endowed with a large surface area, adjustable pore sizes and tunable chemical and physical properties, as well as acceptable thermal stability. The chemical structure of MOFs is characterized by metal centers (nodes) coordinated by organic linkers, to form a regular crystalline structure characterized by cages where the gas molecules are entrapped. By varying the chemical composition, one can thus tune the size of the pores and the separation performance of such structures. The reference material in the class of MOFs is the so-called HKUST-1, formed by Cu (copper) centers coordinated by organic benzene tricarboxylate (BTC) linkers. The chemical formula of HKUST-1 is Cu3 (BTC)2 and is sold under the name of Basolite® C300 by BASF: such material has outstanding CO2 capture properties. In this work, we synthesized HKUST-1, as well as new materials obtained from modification of HKUST-1, and studied the effect of such modifications on the microstructure and CO2 capture performance. Firstly, two materials were obtained by including in to the HKUST-1 crystal different amounts of graphene-like (GL) layers (5 wt % and 15 wt %, named MGL-1 and MGL-2, respectively), a new graphene related material (GRM) obtained from the carbon black demolition [1]. The incorporation of those layers in the HKUST-1 crystal moiety is able to modulate the chemicophysical characteristics of the pristine MOF structure, thus enhancing the non-specific adsorption capacity of HKUST-1, the adsorption kinetics and the thermomechanical resistance additionally providing electrical conductivity.[2] Then, another modification of HKUST-1 was explored by replacing part of the Cu nodes with zinc atoms: two samples of this kind were prepared and named ZMOF-15 and ZMOF-50, whereas 15 and 50 stands for the nominal Zn salt content (the actual measured substitution of the metal centers is 0.83 and 9.63 wt. %, respectively). Finally, both modification strategies were applied simultaneously to obtain samples intercalated with GL layers in which the Cu centers are partially substituted with Zn atoms. Two materials of this kind were fabricated and named ZMOF-15-GL5 and ZMOF-50-GL5, both containing 5 wt. % of GL layers, and 15 and 50 wt.% nominal Zn salt content (the actual measured substitution of the metal centers 1.13 and 7.84 wt. %, respectively). The list of samples studied, together with their composition, is shown in Table 1. We measured the BET surface area, as well as the micropores and mesopores. Then, we measured CO2, N2 and CH4 adsorption at 35°C and pressures up to 25 bar using a manometric technique.[3] The properties were compared to the corresponding values measured in HKUST-1. All samples show preferential adsorption for CO2 with respect to the other gases: the best samples compete with HKUST-1 in terms of CO2 adsorption capacity, although sometimes showing lower BET area. Such results indicate that the separation performance is affected by the chemical structure of the material and not only by the internal porosity. The best samples, as far as CO2 sorption capacity is concerned, are those containing either a small amount of Zinc (ZMOF-15) or those that contain only GL layers (MGL-1, MGL-2), whose CO2 sorption properties are shown in Figure 1. Samples containing larger amounts of Zn (ZMOF-50), and samples obtained by simultaneous application of the two strategies (ZMOFGL5 and ZMOF-50-GL5) show unsatisfactory performances toward CO2 absorption. The pure gas sorption isotherms of the three gases inspected were modeled with the Langmuir model, and mixed gas selectivities were estimated using the multicomponent Langmuir model, indicating that the CO2/CH4 selectivity of the modified samples is equal or lower than the HKUST-1 sample. On the other hand, some of the modified samples have a higher CO2/N2 selectivity than HKUST-1. Thus, such materials are promising candidates for CO2 capture through adsorption in post-combustion applications.
Novel metal organic frameworks (MOFs)-based composites for CO2 capture from gaseous streams
Michela Alfè;Valentina Gargiulo;
2017
Abstract
Metal Organic Frameworks (MOFs) are nanoporous materials suitable for gas purification and CO2 capture by means of adsorption processes. MOFs are endowed with a large surface area, adjustable pore sizes and tunable chemical and physical properties, as well as acceptable thermal stability. The chemical structure of MOFs is characterized by metal centers (nodes) coordinated by organic linkers, to form a regular crystalline structure characterized by cages where the gas molecules are entrapped. By varying the chemical composition, one can thus tune the size of the pores and the separation performance of such structures. The reference material in the class of MOFs is the so-called HKUST-1, formed by Cu (copper) centers coordinated by organic benzene tricarboxylate (BTC) linkers. The chemical formula of HKUST-1 is Cu3 (BTC)2 and is sold under the name of Basolite® C300 by BASF: such material has outstanding CO2 capture properties. In this work, we synthesized HKUST-1, as well as new materials obtained from modification of HKUST-1, and studied the effect of such modifications on the microstructure and CO2 capture performance. Firstly, two materials were obtained by including in to the HKUST-1 crystal different amounts of graphene-like (GL) layers (5 wt % and 15 wt %, named MGL-1 and MGL-2, respectively), a new graphene related material (GRM) obtained from the carbon black demolition [1]. The incorporation of those layers in the HKUST-1 crystal moiety is able to modulate the chemicophysical characteristics of the pristine MOF structure, thus enhancing the non-specific adsorption capacity of HKUST-1, the adsorption kinetics and the thermomechanical resistance additionally providing electrical conductivity.[2] Then, another modification of HKUST-1 was explored by replacing part of the Cu nodes with zinc atoms: two samples of this kind were prepared and named ZMOF-15 and ZMOF-50, whereas 15 and 50 stands for the nominal Zn salt content (the actual measured substitution of the metal centers is 0.83 and 9.63 wt. %, respectively). Finally, both modification strategies were applied simultaneously to obtain samples intercalated with GL layers in which the Cu centers are partially substituted with Zn atoms. Two materials of this kind were fabricated and named ZMOF-15-GL5 and ZMOF-50-GL5, both containing 5 wt. % of GL layers, and 15 and 50 wt.% nominal Zn salt content (the actual measured substitution of the metal centers 1.13 and 7.84 wt. %, respectively). The list of samples studied, together with their composition, is shown in Table 1. We measured the BET surface area, as well as the micropores and mesopores. Then, we measured CO2, N2 and CH4 adsorption at 35°C and pressures up to 25 bar using a manometric technique.[3] The properties were compared to the corresponding values measured in HKUST-1. All samples show preferential adsorption for CO2 with respect to the other gases: the best samples compete with HKUST-1 in terms of CO2 adsorption capacity, although sometimes showing lower BET area. Such results indicate that the separation performance is affected by the chemical structure of the material and not only by the internal porosity. The best samples, as far as CO2 sorption capacity is concerned, are those containing either a small amount of Zinc (ZMOF-15) or those that contain only GL layers (MGL-1, MGL-2), whose CO2 sorption properties are shown in Figure 1. Samples containing larger amounts of Zn (ZMOF-50), and samples obtained by simultaneous application of the two strategies (ZMOFGL5 and ZMOF-50-GL5) show unsatisfactory performances toward CO2 absorption. The pure gas sorption isotherms of the three gases inspected were modeled with the Langmuir model, and mixed gas selectivities were estimated using the multicomponent Langmuir model, indicating that the CO2/CH4 selectivity of the modified samples is equal or lower than the HKUST-1 sample. On the other hand, some of the modified samples have a higher CO2/N2 selectivity than HKUST-1. Thus, such materials are promising candidates for CO2 capture through adsorption in post-combustion applications.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
