Regeneration performance of metal–organic framewor

Authors: Izabela MajchrzakKucęba Dominika BukalakGaik

Publish Date: 2016/06/21

Volume: 125, Issue: 3, Pages: 1461-1466

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Abstract

The increasing attractiveness of the adsorption methods of CO2 capturing from coal power plant flue gas which can be observed in recent years is linked directly with the appearance and development of new efficient CO2 adsorbents The success of an adsorbent depends on the development of the material that under flue gas temperature conditions will have high sorption capacity and selectivity for CO2 At the same time the ease of regeneration and the usable lifetime of the adsorbent are of key importance The paper presents the potential of metal–organic frameworks MOFs for VPSA method for flue gas CO2 capture In this study the thermogravimetric test has been used to screen two kind of MOFs CuBTC MIL53Al and identification of promising materials for CO2 capture The examination of the sorption capacity stability and regeneration performance of metal–organic frameworks was carried out using a Mettler TGA/SDTA 851e thermobalance and TGVacuum system The studies of adsorption/desorption on the MOFs showed complete desorption of CO2 which confirmed the reversible nature of the process and the ability to use in multiple cycles in VPSA unitThe increasing attractiveness of the VPSA Vacuum Pressure Swing Adsorption method of CO2 capturing from coal power plant flue gas compared to the preferred currently absorption methods which can be observed in recent years is linked directly with the appearance of new efficient CO2 adsorbents such as metal–organic frameworks MOFs 1 The most important step in the design of a VPSA plant—cycles—is the selection of the adsorbent To be used in the VPSA adsorption method an ideal adsorbent should exhibit not only high selectivity and sorption capacity with respect to CO2 but also easy regeneration and stability during extensive adsorption–desorption cycling Currently only activated carbons and zeolites can be used for a largescale VPSA units 2 3 4 The metal–organic frameworks MOFs are a new class of nanoporous solids proposed to be used in adsorption VPSA units 5 Metal–organic frameworks MOFs are considered for CO2 capture owing to their large specific surface area of up to 6240 m2/g ordered pore structure and large pore volume 5 6 The majority of MOFs exhibit high sorption capacity with respect to CO2 in a higher pressure range of up to 42 bar thus excelling other physical adsorbents such as zeolites and activated carbons In turn the heat needed for the desorption of CO2 from these compounds is significantly lower than that of zeolitesUnfortunately in the range of lower pressures typical of flue gas only selected MOFs exhibit the adequate sorption properties with respect to CO2 Among the whole range of MOFs especially recommended for CO2 capture is CuBTC copper benzene135tricarboxylate CuBTC was used by Raganati et al 7 on a laboratory scale for CO2 capture on a fluidized bed The key parameter of MOFs which determined their application potential is porosity and particularly the diameter of the largest nanopore The larger the nanopore diameter the easier access by the adsorbed substance to the inner space Among MOFs the largest nanopore diameters are exhibited by eg MIL53 frameworks This metal–organic structure is also characterized by the highest pore homogeneity This is one of the reasons why these MOFs were selected for the evaluation of the potential of its use in the VPSA gas stream CO2 separation installation Due to very high sorption capacity of some MOFs compared to zeolites these compounds are mentioned as potentially the best adsorbents for VPSA systems A limitation is the lack of knowledge regarding the behavior of these compounds in actual VPSA systems and a considerable cost at the present stage of their development 5 6 7The regenerability and cyclic stability of adsorbents are among the most important characteristics taken into consideration when looking for CO2 efficient adsorbents It is essential that the adsorbent does not decrease in its sorption capacity during multiple adsorption–desorption cycles and does not change in its structure and propertiesThe ease of adsorbent regeneration is one of the key parameters in choosing the adsorbent for an adsorption CO2 capture plant To be useful for applications in multistage cycles the adsorbent should be regenerable while not losing its sorption properties Depending on the structural and chemical properties of the adsorbent the adsorption–desorption cycling can be carried out by changing the temperature and/or pressure or vacuum Therefore the most suitable adsorbent needs to be selected for respective adsorption processes TSA PSA VSA PTSA The stability of the adsorbent during extensive adsorption–desorption cycling is another parameter in its selection for adsorption CO2 capture installations The stability is the key parameter of an adsorbent as it determines its life and thus the frequency of its replacement in the adsorption plant The life of adsorbents is therefore equally important as their CO2 sorption capacity selectivity or kinetics because it directly influences the economy of the process of CO2 capture by the adsorption method and the possibility of using the adsorbents on a commercial scaleThe sorption capacity regenerability and sorbent stability in multiple cycles can be determined using thermogravimetric methods a TGA/SDTA 851e analyzer and TGVacuum system which are commonly used for assessing the sorption capacity of adsorbent samples with respect to CO2 The TGVacuum system is used as a straightforward fast laboratory method enabling the evaluation of the regeneration performance of materials and the determination of their suitability for CO2 separation in VPSA unitsThe aim of this study was comparison of regeneration performance of two metal–organic frameworks MOFs proposed for VPSA adsorption method for flue gas CO2 capture The evaluation of sorption capacity and sorbent regeneration in TGVacuum system was used to obtain preliminary information about the usefulness of MOFs for use in VPSA installationsTwo types of MOFs were used in test CuBTC Basolite C300 produced by BASF and MIL53Al Basolite A100 produced by BASF The metal–organic frameworks CuBTC and MIL53Al were characterized by nitrogen adsorption–desorption TG and SEM analysis The porous properties of the adsorbents were investigated by determining their N2 gas adsorption and desorption isotherms at −19615 °C using an ASAP 2010 Instrument Micromeritics The specific surface area was calculated by the BET method from the linear part of BET plot according to IUPAC recommendations using the adsorption isotherm relative pressure p/p o = 005–023 The pore size distribution was calculated by the BJH method and the pore volume was obtained from the maximum amount of adsorption at p/p o of 099 The microstructures of the adsorbents were observed using an Electron Microscopes EVO40 Series Carl Zeiss SMT

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