Adsorption of carbon dioxide on elastic layered metal-organic frameworks (ELMs) was investigated during and after exposure to water. Two ELM variants, ELM-11 and ELM-12, were contacted with water vapor and the impact of cyclical exposure on the CO2 capacity of the adsorbents was observed.
Microporous and Mesoporous Materials 304 (2020) 110377 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso Carbon dioxide capacity retention on elastic layered metal organic frameworks subjected to hydrothermal cycling Francisco J Sotomayor b, Christian M Lastoskie a, * a b Department of Civil and Environmental Engineering, University of Michigan, 1351 Beal Avenue, Ann Arbor, MI, 48109-2125, USA Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL, 33426, USA A R T I C L E I N F O A B S T R A C T Keywords: Flexible adsorbent Water vapor Density functional theory Isotherm Regeneration Adsorption of carbon dioxide on elastic layered metal-organic frameworks (ELMs) was investigated during and after exposure to water Two ELM variants, ELM-11 and ELM-12, were contacted with water vapor and the impact of cyclical exposure on the CO2 capacity of the adsorbents was observed ELM-11 was found to lose CO2 capacity with each successive exposure to water, whereas ELM-12 retained CO2 capacity through four exposure cycles Density functional theory calculations were performed to interpret these observations Changing the counter-ion from the simple tetrafluoroborate (BFÀ4 ) to the larger and more complex trifluoromethanesulfonate (CF3SOÀ3 ) anion expands the number of potential binding sites for adsorbate molecules While CO2 directly competes with other adsorbates for binding sites in ELM-11, CO2 does not directly compete with other adsorbates in ELM-12 due to its preference for direct interaction with both fluorine and oxygen atoms in CF3SOÀ3 Introduction Metal-organic frameworks (MOFs), also known as porous coordina tion polymers (PCPs), are a novel class of hybrid materials assembled from metal ions with well-defined coordination geometry and organic bridging ligands [1] Through careful selection of the metal and organic building blocks, MOFs can be conceptually designed into networks possessing finely tuned pore size and crystal structure Over 20,000 different MOFs have been reported within the past decade [2] The structural and chemical diversity of MOFs has fostered extensive research into their potential applications for gas storage, ion exchange, molecular separation, and heterogeneous catalysis [3] Notably, the exceptional tunability of MOFs has made possible the synthesis of structures with record-breaking porous material properties such as surface area and capacity for the storage of hydrogen, methane, or carbon dioxide by physical adsorption [4] The large surface areas, adjustable pore sizes, and controllable surface properties of certain MOFs make them attractive prospective adsorbents for CO2 capture from mixed gas streams Flexible MOFs, also known as soft porous crystals (SPCs) [5], are a subset of MOFs that possess both a highly ordered network and struc tural transformability In contrast with rigid MOFs, which retain their structure and their porosity irrespective of environmental factors, SPCs undergo structural transformations that are dependent on external stimuli such as temperature, mechanical pressure, or guest adsorption, on account of their bi-stable or multi-stable attributes [6] This facet of SPCs has led to the observation of previously unanticipated gas adsorption phenomena A subset of SPCs are the so-called elastic layered metal-organic frameworks (ELMs) [7,8] ELMs are composed of metal vertex ions, connecting ligands, and charge-balancing counter-ions that are arranged into two-dimensional sheets that in turn assemble into three-dimensional stacked structures These materials show a latent porosity [9] for the adsorption of gas molecules above a specific pres sure, termed the “gate pressure”, that results in an expansion of the interlayer spacing between the two-dimensional sheets and a corre sponding jump in the adsorption isotherm that cannot be classified in accordance with the conventional IUPAC isotherm designations The exotic adsorption characteristics of ELMs are not observed in more familiar commercial adsorbents such as activated carbons or zeolites Nor are they found in the adsorption isotherms of MOFs with rigid pore structures These unusual features confer upon ELMs potential advan tages for CO2 capture, inasmuch as they combine a high selectivity for separation of CO2 from gas mixtures with a low energy requirement for adsorbent regeneration and CO2 recovery [8] DOI of original article: https://doi.org/10.1016/j.micromeso.2019.03.019 * Corresponding author E-mail address: cmlasto@umich.edu (C.M Lastoskie) https://doi.org/10.1016/j.micromeso.2020.110377 Received 19 June 2018; Received in revised form 14 February 2019; Accepted 11 March 2019 Available online 17 June 2020 1387-1811/© 2019 Elsevier Inc All rights reserved F.J Sotomayor and C.M Lastoskie Microporous and Mesoporous Materials 304 (2020) 110377 However, to be suitable for post-combustion carbon capture (PCC), prospective adsorbents must selectively adsorb CO2 at low concentra tion (4–15 vol%) in the presence of other flue gas constituents Coal combustion flue gas, for example, typically contains to vol% water vapor and tens to hundreds of parts per million of SOx, NOx, and CO [10], any of which may significantly impact the CO2 capture perfor mance and stability of MOFs [11] In addition, because PCC systems often assume regeneration of the adsorbent and recovery of the captured CO2, prospective CO2 capture materials need to perform without ca pacity fade through many adsorption and regeneration cycles Regeneration of a solid adsorbent is typically accomplished by temperature swing adsorption (TSA), pressure swing adsorption (PSA), vacuum swing adsorption (VSA), or some combination of these pro cesses [10] Because of the availability of low-grade waste heat from a power plant as an energy source for regeneration, TSA is considered particularly promising for many carbon capture operations For their use in TSA recovery, ELMs must demonstrate consistent and reproducible CO2 capture performance after repeated exposure to unwanted gas components, thermal stresses, moisture, and trace components of flue gas Many rigid MOFs have relatively high thermal stability For example, room temperature CO2 adsorption on Zn4O(bdc)3 (bdc ¼ 1,4-benzene dicarboxylate), otherwise known as MOF-5, remains near 3.6 wt% when the adsorbent is cycled between 30 and 300 � C at atmospheric pressure [4] Only above 400 � C does MOF-5 undergo thermal decomposition and lose its capability to retain CO2 Conversely, flexible MOFs, by defini tion, have crystalline structures that are more susceptible to flexing and distortion upon exposure to external stimuli, leading to the question of whether the weaker inter-framework interactions characteristic of flexible MOFs such as ELMs reduces their thermal stability Adsorption-desorption cycling of CH4 on ELM-11 at 303 K showed no degradation of the gated adsorption capacity even after 50 cycles [8] Thermogravimetric analysis revealed that ELM-11 structure begins to lose bipyridine and BF4 molecules at around 500 K [12] The findings from these two experiments suggest that ELM-11 can be degassed at temperatures up to approximately 200 � C with no structural degrada tion ELM flexible framework adsorbents are thus expected to have thermal cycling stability comparable to that of rigid MOFs Certain MOFs are well known to be structurally unstable in contact with water [13] For example, Cu3(btc)2 (btc ¼ 1,3,5-benzene tri carboxylate), also known as HKUST-1, is stable in dry air at room tem perature, but its crystallinity progressively declines upon cyclic exposure to water vapor from air at 30% relative humidity, plateauing at 75% of its original crystallinity after repeated water cycling The MOF Ni/dobdc loses CO2 capacity after repeated H2O/CO2 mixture isotherm measurements Zn2(bdc)2(dabco) and Ni2(bdc)2(dabco) (dabco ¼ 1, 4-diazabicyclo[2.2.2]octane) are stable after O2 adsorption at 25 � C from air at 30% relative humidity, but collapse upon exposure to air at 60% relative humidity at the same temperature Kizzie and coworkers [14] investigated the effect of humidity on CO2 capture in the M/dobdc series (where M ¼ Zn, Ni, Co, or Mg; dobdc ¼ 2,5-dioxidobenzene-1, 4-dicarboxylate) They found that although Mg/dobdc had the highest initial CO2 capacity at the conditions used in their study, exposure to air at a relative humidity of 70% followed by thermal regeneration resulted in the retention of only 16% of the initial CO2 capacity In contrast, 85% of the CO2 capacity in Co/dobdc was retained under the same condi tions It is evident then that water vapor can both irreversibly after MOF structures and hinder their adsorption of CO2 For ELM class materials, Cheng et al [12] studied the evolution of the structure of ELM-11 upon a single cycle of dehydration and rehy dration, and reported that the CO2 adsorption capacity was largely preserved after exposure to water vapor The slight differences observed in the CO2 adsorption isotherms before and after water vapor exposure were attributed to stacking faults within the ELM-11 framework This study however considered only a single cycle of exposure to moisture, and did not investigate the stability of other ELM analogues to water Considering that other MOFs show significant performance loss under cyclic exposure to water, and that the substitution of different framework components can have a significant effect on hydrothermal stability, the work presented herein seeks to better understand the CO2 capacity retention of ELM variants after water vapor exposure through a combination of isotherm measurements after water vapor cycling and density functional theory calculations Experimental Material preparation Two isostructural ELM variants were experi mentally tested: Cu(bpy)2(BF4)2, termed ELM-11, and Cu(bpy)2(OTf)2, termed ELM-12, (where bpy ¼ 4,40 -bipyridine and OTf ¼ CF3SỒ3 ) Two methods were used to obtain samples of ELM-11 The first method was the purchase of the un-activated precursor of ELM-11, [Cu(bpy) (BF4)2(H2O)2]bpy, termed pre-ELM-11, sold commercially by Tokyo Chemical Industry Co., Ltd (CAS Number: 854623-98-6, Product Number: C2409) at >98% purity The second method was the synthesis of pre-ELM-11 following the procedure reported by Tran [15] 4, 40 -bipyridine (0.312 g; mmol) in mL of ethanol was slowly added to an 8-mL aqueous solution of Cu(BF4)2⋅H2O (0.309 g; mmol) at room temperature A blue precipitate gradually formed The mixture was stirred for h at room temperature, after which the solid was allowed to settle for two days and then filtered off, washed with water and ethanol, and dried in air at room temperature Once pre-ELM-11 is obtained, it is easily converted to ELM-11 by degassing under vacuum (