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Effect of Water Adsorption on Retention of Structure and Surface Area of Metal–Organic Frameworks

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Effect of Water Adsorption on Retention of Structure and Surface Area of Metal–Organic Frameworks
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  E ff  ect of Water Adsorption on Retention of Structure and SurfaceArea of Metal − Organic Frameworks Paul M. Schoenecker , Cantwell G. Carson , Himanshu Jasuja , Christine J. J. Flemming, and Krista S. Walton * School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332,United States * S  Supporting Information  ABSTRACT:  This work presents an experimental investigation of water adsorption in metal − organic frameworks (MOFs) atroom temperature and up to 90% relative humidity. Structural degradation of the materials after regeneration is analyzed viapowder X-ray di ff  raction (PXRD) and nitrogen adsorption measurements. MOFs with open metal sites are quite hydrophilic butappear to maintain their structure according to PXRD. However, signi 󿬁 cant surface area loss indicates that decomposition isoccurring and is likely an attribute of oxygen presence during the regeneration procedure. Materials with copper paddle-wheel(HKUST-1), 5-coordinated magnesium (Mg MOF-74), and 7-coordinated zirconium (UiO-66(-NH 2 )) maintain good structuralstability, while Zn-COOH containing MOFs (DMOF-1; DMOF-1-NH 2 ; UMCM-1) undergo complete loss of crystallinity. 1. INTRODUCTION The ability to synthesize metal − organic frameworks (MOFs) with prescribed structural features has led to intense interest inthe materials for selective adsorption processes. The hybridnature of MOFs provides an almost in 󿬁 nite set of building blocks that can be manipulated to target speci 󿬁 c adsorption behavior by introducing open metal sites and functional groups,or by further modulating the properties by postsyntheticmodi 󿬁 cation. 1 − 8 To date, much of the experimental andtheoretical research on MOF applications has centered onadsorption simulations and measurements. Investigations of gasstorage (hydrogen/methane) and car bon dioxide capture from 󿬂 ue gas have been a particular focus. 2 ,3 ,8  Aside from good adsorption loadings and high selectivities,the stability of an adsorbent in humid environments is a criticalproperty that must be considered when designing anadsorption process. The water sensitivity of certain MOFs has been well-documented, 9 − 13  but a variety of MOFsincluding pyrazolate 14 and imidazolate 15 frameworks andzirconium-based MOFs 16 have been reported in recent yearsthat do not lose structural integrity in the presence of water.Long and co-workers have reported pyrazolate-based MOFsthat show remarkable structural integrity after exposure to boiling water and other solvents due to the high p  K  a  value of the ligands. These materials also possess open metal sites. 14 Lillerud and co-workers have reported an interesting family of isoreticular zirconium MOFs built from various aromaticcarboxylates. 16 UiO-66 shows no change in PXRD patternafter exposure to liquid water and other solvents. The stability is attributed to the strength of the inorganic unit; eachzirconium atom is 8-coordinated via terephthalic acid ligands.Several MIL materials are known to maintain good structuralintegrity  af ter water exposure due to high coordination numbers, 17 − 19 and the zeolitic imidazolate frameworks (ZIFs) have also shown good stability under aqueous conditions. 15 ,19 Cychosz and Matzger 20 recently reported an investigation of the stability of several MOFs after exposure to aqueoussolutions with varying amounts of DMF. MOF structuresutilizing Zn-carboxylate connectivity (MOF-5, MOF-177) werefound to be unstable after exposure to liquid water, whilecopper paddle-wheel MOFs (HKUST-1, MOF-505) showedgood structure retention after similar testing. An investigation by Low et al. 21 using high throughput steam treatment foundthat metal − ligand bond strength and oxidation state of themetal cluster are important contributors to MOFs stability.Kaskel et al. 19 reported water adsorption isotherms for severalMOFs; HKUST-1, ZIF-8, DUT-4, MIL-100, and MIL-101. Water stability was analyzed following the water vaporadsorption as well as after immersing the MOFs in liquid water at 323 K. The Dietzel group has investigated the stability of the MOF-74/CPO-27 materials (Co, Mg, Ni) throughout dehydration/rehydration cycling. 22 − 24 The MOF-74 analogues were found to be stable during cyclic adsorption testing whileusing inert gases (Ar/N 2 ). However, when the same experiment was conducted in air, the Ni MOF-74 degraded. 23 Sensitivity to water vapor is widely considered to be a major weakness of MOFs that could negate potential advantages of the hybrid materials from an applications perspective. Under-standing the behavior of MOFs under humid conditions isquite important for applications such as CO 2  capture from  󿬂 uegas or air puri 󿬁 cation. The importance of MOF performance inhumid environments cannot be overstated, and understandingthe parameters that contribute to this sensitivity is critical forelevating MOFs to the applied level. Nevertheless, few systematic studies on the stability of MOFs after exposure tohumid streams have been reported. In this work, we present an Received:  October 12, 2011 Revised:  March 12, 2012  Accepted:  March 28, 2012 Published:  March 28, 2012 Articlepubs.acs.org/IECR © 2012 American Chemical Society  6513  dx.doi.org/10.1021/ie202325p  |  Ind. Eng. Chem. Res.  2012, 51, 6513 − 6519  experimental investigation of water adsorption in MOFs atroom temperature and up to 90% relative humidity (RH),followed by an analysis of structural degradation and surfacearea change. Speci 󿬁 cally, we examine structure retention after water exposure and regeneration via dynamic vacuum andelevated temperature, due to the direct link with many gasseparation applications.Seven MOFs were selected for this study to represent a rangeof features that are common in MOFs. These include open- metal sites (HK UST-1, 25 Mg-MOF-74 26 ), amine-functionalgroups (UiO-66, 27 DMOF-1 5,28 ), carboxylate coordination(UMCM-1, 29 HKUST-1, Mg-MOF-74), and nitrogen coordi-nation (DMOF-1). There are also di ff  erences in the metalcoordination as these MOFs are synthesized from zinc, copper,magnesium, or zirconium. The isoreticular family of UiO-66materials also contain open-metal sites upon activation or dehydroxylation. 16,30 ,31 However, Llewellyn et al. 32 shows thatthese open Zr sites do not interact with gases like CO and CO 2 in the same fashion as other open-metal site MOFs, e.g.,HKUST-1. So, UiO-66 and UiO-66-NH 2  are not considered inthe same category as Mg-MOF-74 and HKUST-1 for this study.HKUST-1, or Cu-BTC, is one of the most widely studied MOFs over the past decade. 19 ,33 − 36 This material is synthesizedfrom a mixture of Cu(NO 3 ) 2 · 3H 2 O and 1,3,5-benzenetricarboxylic acid (BTC) in deionized water and ethanol. Thesecondary building unit is formed by the copper paddlewheel,in which Cu − Cu dimers are connected by BTC ligands. Thelarge pore diameter is 9 Å, and the small pores are around 6 Å. An open coordination site is generated at each copper uponactivation of the material. Wang et al. 36 reported the  󿬁 rst waterisotherm for HKUST-1, but no structure or surface areaanalyses were performed. Low et al. 21 reported that HKUST-1is stable up to 200  ° C w hen exposed to 50 mol % steam, andCychosz and Matzger 20 found that HKUST-1 exhibits goodstructure retention in a 7:1 mixture of H 2 O:DMF even after 21months of exposure. In contrast, Kaskel et al. 19 determinedfrom powder X-ray di ff  raction that HKUST-1 breaks downafter immersion in pure water at 323 K for 24 h.Mg-MOF-74 is synthesized from magnesium and 2,5-dihydroxyterephthalic acid. It possesses one-dimensionalchannels of approximately 11 Å diameter in a honeycombtopology, with removable solvent molecules coordinating at themetal sites. Of the open-metal site MOFs, Mg MOF-74 exhibitsamong the highest loadings of CO 2  at low  pressure (e.g., 35.2 wt % uptake of CO 2  at 298 K and 1 atm). 3 It also has beenreported to exhibit a strong a ffi nity for water, 34,36 ,38  but thee ff  ect of water adsorption and regeneration on available surfacearea has not been investigated.UiO-66 contains [Zr 6 O 4 (OH) 4 ] clusters linked with twelveterephthalate moieties. The pore diameters are approximately 6 Å. Upon dehydroxylation [Zr 6 O 4 (OH) 4 ] clusters change to[Zr 6 O 6 ], and the Zr metal-centers undergo a transition fromthe as-synthesized 8-coordinated state to 7-coordinated state. 31 Previous studies have shown that UiO-66 maintains itsstructure after immersion in w ater and other solvents, but no water isotherms were reported. 16 Nitrogen-coordinated DMOF-1 (Zn 2 (BDC) 2 (DABCO)) hassquare-shaped channels of 7.5 Å diameter that are inter-connected by smaller pores with diameters of 4.8  ×  3.2 Å. It has been shown to be hydrophobic up to 42% RH, but no structureanalysis w as performed postexposure. 28 On the other hand,Liang et al. 38 investigated the water tolerance of zinc and nickel versions of this material and found that both appear to losestructural integrity after exposure to relative humidity above60%. The corresponding surface area analysis was notperformed.MOFs with amine-functionalized ligands often provide thefunctional sites capable of facilitating postsynthetic modi 󿬁 cation(PSM), 7 ,39 and therefore, are of great importance whenconsidering PSM materials for humid gas separation processes.There have been no water adsorption or stability studiesreported for the amine containing analogues, UiO-66-NH 2  andDMOF-1-NH 2 .UMCM-1 is a mesoporous MOF synthesized from zinc andtwo organic ligands, terephthalic acid and 1,3,5-tris(4-carboxyphenyl)benzene (BTB). This MOF has shown somepromise for dry gas separation applications. 40 However, with acoordination environment similar to MOF-5, it is unlikely thatUMCM-1 will exhibit great water stability. 2. EXPERIMENTAL SECTION 2.1. Synthesis Methods.  All chemicals were procured fromcommercial sources (Fisher and Sigma Aldrich) and used without further puri 󿬁 cation. Samples were stored in sealed vialsprior to use. UMCM-1.  A  modi 󿬁 ed version of the previously reportedsynthesis method 29  was used. Zn(NO 3 ) 2 · 6H 2 O (3.87 g, 13.0mmol), terephthalic acid (TPA) (0.540 g, 3.25 mmol), and1,3,5-tris (carboxyphenyl) benzene (1.28 g, 2.92 mmol) weredissolved in 120 mL of diethylformamide (DEF). The solution was then  󿬁 ltered twice to remove undissolved solids anddivided into twelve 20 mL scintillation vials in a sand bath. Thesand bath was heated to 85  ° C for 48 h. The product was rinsed with dimethylformamide (DMF) three times before solventexchanging with dichloromethane (CH 2 Cl 2 ) for 96 h viaSohxlet extraction prior to activation at 150  ° C. Mg MOF-74.  The reported synthesis method 3  wasimplemented as follows: Mg(NO 3 ) 2 · 6H 2 O (1.90 g, 7.4mmol) and 2,5-dioxido-1,4-benzenedicarboxylic acid(H 4 DOBDC) (0.444 g, 7.40 mmol) were placed in a 200 mLof DMF:ethanol:water (15:1:1, by volume). The resultantmixture was sonicated until homogeneous. Then, 10 mLportions of the solution were placed in 20 mL scintillation vialsand placed in a sand bath. The sand bath was heated to 125  ° C,and the solution was allowed to react for 20 h. The resultantproduct was collected and placed in a Sohxlet extractor for 96 hto exchange with methanol before activation at 250  ° C. HKUST-1.  HKUST-1 was synthesized as follows: Cu-(NO 3 ) 2 · 3H 2 O (4.55 g, 18.8 mmol) was dissolved in 60 mLof deionized water and trimesic acid (2.10 g, 9.99 mmol) wasdissolved in 60 mL of ethanol via sonication. The solutions were added together and placed in 23 mL PTFE lined aciddigestion vessels. The reaction was conducted at 100  ° C for 18h. The product was rinsed with methanol and water beforeactivation at 150  ° C. UIO-66.  The previously reported synthesis method 16  wasimplemented as follows: ZrCl 4  (0.636 g, 2.73 mmol) andterephthalic acid (TPA) (0.453 g, 2.73 mmol) were dissolved via stirring in 106 mL of DMF. The solution was divided upequally and placed in ten 20 mL scintillation vials. The vials were placed in a sand bath at 120  ° C and reacted for 20 h. Theresultant product was rinsed with DMF three times beforeactivation at 200  ° C. The amino version of UiO-66 wassynthesized following the same procedure, substitutingterephthalic acid with amino-terephthalic acid (ATPA). Industrial & Engineering Chemistry Research  Article dx.doi.org/10.1021/ie202325p  |  Ind. Eng. Chem. Res.  2012, 51, 6513 − 6519 6514  DMOF-1.  The procedure reported by Wang et al. 5  was usedto solvothermally synthesize DMOF-1. Zn(NO 3 ) 2 · 6H 2 O (1.74g, 6.00 mmol), TPA (1.02 g, 6.00 mmol), and 1,4-diazabicyclo[2.2.2]octane or DABCO (1.08 g, 9.63 mmol) were dissolved in 150 mL of DMF. The solution was then 󿬁 ltered three times to remove the white precipitate and placedin ten 20 mL scintillation vials in a sand bath. The sand bath was heated from 35 to 120  ° C at a rate of 2.5  ° C/min andreacted for 12 h. The product was rinsed with DMF three timesand activated at 110  ° C. DMOF-1-NH  2 .  Again, the procedure reported by Wang et al. 5  was used to solvothermally synthesize DMOF-1-NH 2 . Zn-(NO 3 ) 2 · 6H 2 O (1.79 g, 6.02 mmol), ATPA (1.10 g, 6.09 mmol),and DABCO (1.08 g, 9.63 mmol) were dissolved in 150 mL of DMF. The solution was then  󿬁 ltered three times to remove the white precipitate and placed in  󿬁 fteen 20 mL scintillation vialsin a sand bath. The sand bath was heated from 35 to 120  ° C ata rate of 2.5  ° C/min, and the vials were allowed to react for 12h. The product was rinsed with DMF three times and thenplaced in a Sohxlet extractor for solvent exchange withchloroform for 72 h at 90  ° C. 2.2. Adsorption Isotherm Measurement.  Water adsorp-tion isotherms were measured at 298 K and 1 bar on an IGA-003 microbalance from Hiden Isochema. All MOF samples were activated in situ to remove residual solvent and wateradsorbed during sample loading, which requires brief exposure(c.a. 3 min) to ambient air. Dry air was used as the carrier gas, with a portion of the carrier gas being bubbled through a vesselof deionized water. The relative humidity (RH) was controlled by varying the ratio of saturated air and dry air via two mass 󿬂 ow controllers. Experiments were conducted up to 90% RHdue to water condensation in the apparatus at higherhumidities. The total gas  󿬂 ow rate was 200 cm 3 /min for theentire experiment. Variable timeouts were used with amaximum limit of 24 h per isotherm point. Due to  󿬂 uctuatingclimate control of the laboratory itself, condensation inside thegravimetric adsorption apparatus can occur at 90% RH. In thiscase, equilibrium was not reached due to continued mass gainfrom condensation, and desorption data were not reported. After the isotherms were collected, the samples wereregenerated under dynamic vacuum and elevated temperature.Reactivation temperatures for each MOF are given in theSupporting Information (Table S1). 2.3. Characterization.  Powder X-ray di ff  ractograms arecollected using a PANalytical X-ray di ff  ractometer. Initialsample sizes for each material are on the order of 200 mgand are able to  󿬁 ll a bulk XRD sample holder. However, due tosize limitations of the gravimetric adsorption sample pan,reactivated samples are on the order of 50 mg. Di ff  ractogramsare collected under ambient conditions, and humidity exposureis minimized via storage in sealed vials. Nitrogen adsorptionisotherms at 77 K were measured for each MOF before andafter water exposure using a Quadrasorb system fromQuantachrome Instruments. 3. RESULTS AND DISCUSSION Figure 1 shows water adsorption and desorption isotherms forHKUST-1 and Mg MOF-74 at 298 K. As expected, both openmetal site materials exhibit a strong a ffi nity for water, withloadings of 33 and 37 mol/kg for HKUST-1 and Mg MOF-74,respectively, at  ∼ 90% RH. These results are consistent withprevious reports that open-metal site MOFs have high a ffi nitiesfor small molecules containing accessible lone pairs of electronssuch as CO 2  and H 2 O. 3 ,19,33 ,34,38 In agreement with the  󿬁 ndingsof Kaskel et al., 19 the hysteresis curves show that a portion of the water cannot be desorbed under  󿬂 owing dry air. At 0%relative humidity, Mg MOF-74 retains more water thanHKUST-1 upon desorption; Mg MOF-74 retains 17% of maximum uptake compared to 9% retained by HKUST-1. Water adsorption plots for UiO-66, DMOF-1, and theamine-functionalized analogues are shown in Figure 2. Of thesefour MOFs, the zirconium based UiO-66 and amine-functionalized analogue exhibit the highest water uptake athumidity levels greater than or equal to 30% RH. UiO-66adsorbs relatively little water below 20% RH but then exhibits asharp step in the isotherm in which adsorption loadingsincrease from 3 to 16 mmol/g. This MOF has been reported toundergo a transition from the as-synthesized 8-coordinatedstate to 7-coordinated upon dehydroxylation. 16 The observedstep in the isotherm may correspond to a transition back to the8-coordinated state, but the step is more likely attributed topore  󿬁 lling also reported by Llewellyn et al. 32 This point will berevisited in later discussion. The desorption isotherms of UiO-66 and UiO-6-NH 2  (Supporting Information Figure S9 andS10) exhibit hysteresis indicative of the inability to remove aportion of the water under  󿬂 owing dry air. At 0% RH, UiO-66and UiO-66-NH 2  retain 1.7 and 2.8 mmol H 2 O/g, respectively, Figure 1.  Water adsorption and desorption isotherms for open metalsite MOFs, HKUST-1 and Mg MOF-74, at 298 K and 1 bar. Figure 2.  Water adsorption isotherms for parent materials, UiO-66and DMOF-1, along with their amine-functionalized analogues, UiO-66-NH 2  and DMOF-1-NH 2  , at 298 K and 1 bar. Industrial & Engineering Chemistry Research  Article dx.doi.org/10.1021/ie202325p  |  Ind. Eng. Chem. Res.  2012, 51, 6513 − 6519 6515   which suggests rehydroxylation of the samples during wateradsorption and is in agreement with the reported results fromLlewellyn and co-workers. 32 For UiO-66-NH 2  , the adsorptionisotherm exhibits more rectangular or Type I behavior below 20% RH compared to the parent material, which along with theincreased water retention under dry air  󿬂 ow are indicative of the preferred amine −  water interactions. In agreement with what others have reported, 28 ,38,41  we  󿬁 nd that DMOF-1displays the most hydrophobic character of all the materialsin this study up to 20% RH. Above this concentration, we seeatypical adsorption behavior in agreement with the  󿬁 ndings of Liang et al. 38 More speci 󿬁 cally, a sharp increase in uptake at30% RH followed by a decrease from 40 to 60% RH. In thiscase, the loss of adsorbed water despite increasing water vaporconcentration is likely an attribute of both the structure loss of the adsorbent itself and the hydrophobic properties of thedegradation product, DABCO. DMOF-1-NH 2  exhibits anincrease in water adsorption compared to the hydrophobicparent material and does not demonstrate the same loss of adsorbed water. This is likely due to the hydrophilic characterof the proposed degradation product, ATPA.The compilation of adsorption isotherms is shown inSupporting Information Figure S6. For the zinc-carboxylateMOF, UMCM-1, the adsorption isotherm nearly mirrorsDMOF-1-NH 2  adsorption until 40% RH, but at higherhumidity levels there is a more rapid increase in uptake.From a pore volume perspective (2.41 cm 2 /g), the low uptakeresults for UMCM-1 are surprising.To examine the possible degradation of the tested materials,powder XRD data were collected for the as-synthesized samplesand for the samples exposed to humid conditions andreactivated. Figure 3 illustrates the apparent structure retentionof UiO-66-NH 2  and Mg-MOF-74 and the signi 󿬁 cant loss of crystallinity for DMOF-1-NH 2  and UMCM-1. UiO-66 andHKUST-1 also retained their crystallinity based on PXRD(Supporting Information), but it is important to note that slightdegradation of the structure may not show up in the powder X-ray patterns due to high intensities of the peaks at low angles.The hydrolysis degradation reaction appears to describe theUMCM-1 structure loss; at higher values of 2 Θ  , there isevidence of Zn(OH) 2  , which is a direct product of thisreaction. 21 The degradation of UMCM-1 is not surprisingconsidering that the coordination environment is identical toMOF-5, which is well-known to decompose under humidconditions. The signi 󿬁 cant loss of crystallinity in the DMOF-1materials is somewhat surprising considering the hydrophobicnature of the parent material under low relative humidity andthe relatively high p  K  a  of the DABCO ligand. However, thezinc-carboxylate coordination is notoriously unstable and likely the weakest link which initiates the framework collapse, in spiteof the Zn − N coordination from the DABCO ligand.Further support of the stability analysis is captured by BETsurface area analysis of the initial and reactivated samples.These results are shown in Table 1. The water uptake at 80%RH for each MOF does not directly correlate with pore volumeor diameter. Instead, site preferences and degradation dictatethe water adsorption. For example, the open-metal sitematerials with relatively small pore volumes, HKUST-1 andMg MOF-74, show signi 󿬁 cantly higher uptake than UMCM-1.This is due to the open-metal site MOFs ’  a ffi nity for water andalso due to the degradation of UMCM-1. Similarly, DMOF-1and DMOF-1-NH 2  have signi 󿬁 cantly lower water loadings of  Figure 3.  PXRD results for (a) UiO-66-NH 2  , (b) Mg-MOF-74, (c) DMOF-1-NH 2  , and (d) UMCM-1. Industrial & Engineering Chemistry Research  Article dx.doi.org/10.1021/ie202325p  |  Ind. Eng. Chem. Res.  2012, 51, 6513 − 6519 6516  0.04 and 0.11 cm 3 /g at 80% RH, respectively compared to theiraccessible pore volumes of 0.58 cm 3 /g from N 2  adsorption at77 K. This is attributed to the degradation of DMOF-1 andDMOF-1-NH 2  throughout the water isotherm collection, whichis in agreement with the PXRD data (Figure 4 and Supporting Information Figure S1). The increased water uptake exhibited by the amine-functionalized version is likely due to thehydrophilic character of the ATPA ligand itself. In agreement with PXRD results, UiO-66 and the amine-functionalizedanalogue display negligible loss of surface area. Water loadingsof 0.37 cm 3 /g at 80% RH for both UiO-66 and UiO-66-NH 2 match well with the value previously reported. 32 Nevertheless,these water uptakes are less than pore volumes obtained via N 2 adsorption. This is likely an attribute of the rehydroxylation of the materials during water exposure. BET modeling of the N 2 adsorption at 77 K for UMCM-1 con 󿬁 rm the degradationapparent in the XRD analysis, showing almost total loss insurface area. Despite structure con 󿬁 rmation via XRD, both of the open metal site MOFs undergo signi 󿬁 cant reduction insurface area. HKUST-1 and Mg MOF-74 show 26 and 83%loss, respectively. Kaskel et al. 19 also report a signi 󿬁 cant loss inBET surface area, 48%, for HKUST-1 following wateradsorption and reactivation. The Dietzel group show thecomplete stability of MOF-74 materials during cyclicdehydration/rehydration experiments under inert atmos-phere, 22 ,24  but Ni MOF-74 was shown to degrade duringidentical testing in the presence of oxygen. 23 The dehydrationor reactivation procedure of our study utilized dynamic vacuum, which may have prevented oxygen exposure fromcontributing to structure degradation. However, since air wasused as a carrier gas, the entrained oxygen in the adsorbed water appears to su ffi ciently supply the degradation reactionduring reactivation. However, this could not be determinedconclusively from the available data.Greater insight into water adsorption behavior of MOFs can be gained by comparing our results with adsorption intraditional porous materials. Adsorption isotherms for UiO-66-NH 2  , Mg-MOF-74, and HKUST-1 are compared in Figure 4  with water adsorption in zeolites 42 5A and 13X. The calciumand sodium cations in 5A and 13X, respectively, provide strongadsorption sites for water at low relative pressure. The morerectangular Type I isotherms are indicative of this behavior.The MOFs with open-metal sites show analogous hydro-philicity but have much higher saturation loadings compared tothe zeolites due to larger pore volumes. The rectangular Type Iisotherm for UiO-66-NH 2  compared to the parent material(Figure 2) illustrates the favorable impact of amino functionalgroups on water adsorption. A comparison of  water isotherms for UiO-66 and DMOF-1  with BPL carbon 43 and mesoporous silicas 44 MCM-41 andSBA-1 is shown in Figure 5. These materials exhibit Type V isotherms, which are characteristic of strong  󿬂 uid − 󿬂 uidinteractions. The pore  󿬁 lling occurs  󿬁 rst for UiO-66, whichhas pore sizes of ca. 6 Å and is immediately followed by DMOF-1, with 7.5 Å and 4.5  ×  3.8 Å pores. SBA-1 (21 Å pores) undergoes condensation at the next lowest pressure,followed by BPL carbon, which possesses a distribution of pore Table 1. Adsorption Loadings at 80% Relative Humidity andBET Surface Area Comparison of Samples before Water Exposure and after Isotherm Measurement and Reactivation surface area (m 2 /g)materialpore volume b (cm 2 /g)porediameter(Å)loading ,80% RH c  (cm 3 /g) before after%lossMg-MOF-74 a 0.65 11 0.62 1400 238 83UiO-66-NH 2 0.57 <6 0.37 1040 1050 0UiO-66 0.52  ∼ 6 0.37 1160 1130 2DMOF-1 0.58 7.5  ×  7.5;4.8  × 3.20.04 1960 7 100DMOF-1-NH 2 0.58 7.5  ×  7.5;<4.8  × 3.20.11 2010 0 100HKUST-1 a 0.62 9; 6 0.49 1270 945 26UMCM-1 2.41 27  ×  32;14  ×  170.11 6010 205 97 a Contains open metal sites.  b Obtained from the Dubinin −  Astakov model of N 2  adsorption at 77 K.  c  Condensation e ff  ects observed athigher humidity levels. Figure 4.  Water adsorption isotherms for UiO-66-NH 2  , HKUST-1,and Mg MOF-74 compared with zeolites 5A and 13X from the work of Wang et al., 37 all at 298 K. Figure 5.  Water adsorption isotherms for UiO-66 (298 K) compared with MCM-41 (293 K), SBA-1 (293 K), and BPL carbon (298 K). Industrial & Engineering Chemistry Research  Article dx.doi.org/10.1021/ie202325p  |  Ind. Eng. Chem. Res.  2012, 51, 6513 − 6519 6517
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