Efficient One-Pot Synthesis of a Hexamethylenetetramine-Doped

nanomaterials

Article Eﬃcient One-Pot Synthesis of a Hexamethylenetetramine-Doped Cu-BDC Metal-Organic Framework with Enhanced CO2 Adsorption

Aisha Asghar 1 , Naseem Iqbal 1,* , Tayyaba Noor 2, Majid Ali 1 and Timothy L. Easun 3,*

1 U.S Pakistan Centre for Advanced Studies in Energy, National University of Sciences and Technology, H-12, Islamabad 44000, Pakistan 2 School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12, Islamabad 44000, Pakistan 3 School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK * Correspondence: [email protected] (N.I.); easuntl@cardiff.ac.uk (T.L.E.)

Received: 14 May 2019; Accepted: 30 June 2019; Published: 24 July 2019

Abstract: Herein we report a facile, efficient, low cost, and easily scalable route for an amine-functionalized MOF (metal organic framework) synthesis. Cu-BDC HMTA (HMTA = hexamethylenetetramine) has ⊃ high nitrogen content and improved thermal stability when compared with the previously reported and well-studied parent Cu-BDC MOF (BDC = 1,4-benzenedicarboxylate). Cu-BDC HMTA was ⊃ obtained via the same synthetic method, but with the addition of HMTA in a single step synthesis. Thermogravimetric studies reveal that Cu-BDC HMTA is more thermally stable than Cu-BDC MOF. ⊃ Cu-BDC HMTA exhibited a CO uptake of 21.2 wt % at 273 K and 1 bar, which compares favorably ⊃ 2 to other nitrogen-containing MOF materials.

Keywords: energy eﬃciency; functional metal organic frameworks; Cu-BDC; HMTA; CO2 adsorption

1. Introduction The world is currently facing the urgent and demanding challenges of saving and utilizing energy as efficiently as possible. Ever increasing carbon dioxide levels in the atmosphere are a serious threat to the environment [1]. Various carbon capture techniques have been explored to mitigate carbon dioxide levels in the atmosphere, including point source CO2 capture using advanced materials [2]. Metal organic frameworks (MOFs) are an advanced class of microporous and often − crystalline nanomaterials comprised of metal coordination sites bridged by organic linkers [3,4]. The resulting organic/inorganic hybrid 3-D networks that form often contain well-defined porosity, high surface area, and tunable chemical functionalities with potential for versatile applications in catalysis [5,6], separations [7], and gas storage [8]. With respect to the last of these, many studies have investigated the capture of carbon dioxide gas. Amine sites have an affinity towards carbon dioxide, are known to be highly effective at enhancing CO2 adsorption, and are amenable to use under dry or humid conditions. [9]. In this paper, we describe the synthesis, characterization and CO2 sorption of a hexamethylenetetramine-doped metal-organic framework. This study is an effort to incorporate hexamethylenetetramine within a Cu-BDC (BDC = 1,4-benzenedicarboxylate) framework, using an in-situ modification during synthesis, and to study the effect on carbon dioxide gas sorption capacity. Herein, we report a very straightforward method for modification of already reported Cu-BDC [10]. The strategy has several advantages. First, the entire synthetic procedure is quite simple. Second, this method is efficient, with the potential for high-yields, and hexamethylenetetramine is

Nanomaterials 2019, 9, 1063; doi:10.3390/nano9081063 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1063 2 of 10

Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 10 a low-cost chemical (£15.68/kg [11]). Third, a high nitrogen content can be achieved in the resulting yields, and hexamethylenetetramine is a low-cost chemical (£15.68/kg [11]). Third, a high nitrogen Cu-BDC HMTA material. content can ⊃be achieved in the resulting Cu-BDC⊃HMTA material. 2. Materials and Methods 2. Materials and Methods All the chemicals were purchased from Sigma Aldrich/Merck (St. Louis, MO, USA) and used asAll received. the chemicals were purchased from Sigma Aldrich/Merck (St. Louis, MO, USA) and used as received. 2.1. Synthesis 2.1. Synthesis To prepare Cu-BDC HMTA, equimolar quantities (1:1:1) of Cu(NO ) 6H O (296 mg, 1 mmol), ⊃ 3 2· 2 terephthalicTo prepare acid Cu-BDC (166⊃ mg,HMTA 1 mmol), equimolar and hexamethylenetetramine quantities (1:1:1) of Cu(NO (140 mg,3)2·6H 1 mmol)2O (296 were mg, 1 dissolved mmol), in terep10h mLthalic DMF acid in (166 a 50 mg, mL 1 beaker. mmol) Theand contentshexamethylenetetramine were ultrasonicated (140 at mg 25, ◦1C mmol) for 30 were min, anddissolved then the in 10solution mL DMF was in transferred a 50 mL beaker. to a 23 The mL c Teﬂonontents vial were in ultrasonicated a steel Parr vessel. at 25 The °C for Parr 30 vessel min, and was then sealed the and solutionheated was in antransferred oven at 110 to ◦aC 23 for m 24L Teflon h to yield vial greenish-blue in a steel Parr crystals. vessel. The reactionParr vessel mixture was sealed was decanted, and heatedthe productin an oven washed at 11 three0 °C timesfor 24 with hours DMF to yield (5 mL), greenish and then-blue three crystals. times withThe THFreaction (5 mL). mixture This yieldedwas decanted, the product washed three times with DMF (5 mL), and then three times with THF (5 mL). blue crystals. The sample was activated in a vacuum oven at 130 ◦C for 12 h before further analysis. ThisThe yielded same synthesisblue crystals. strategy The wassample used w foras activated obtaining in Cu-BDC a vacuum MOF, oven without at 130 the °C additionfor 12 hours of HMTA before [10 ]. furtherThis analysis.yielded blue The crystals. same synthesis A schematic strategy reaction was schemeused for for obtaining Cu-BDC CuHMTA-BDC MOF,synthesis without is shown the in ⊃ additionFigure 1 of. HMTA [10]. This yielded blue crystals. A schematic reaction scheme for Cu- BDC⊃HMTA synthesis is shown in Figure 1.

Figure 1. Reaction scheme for Cu-BDC HMTA synthesis. ⊃ 2.2. Characterization Figure 1. Reaction scheme for Cu-BDC⊃HMTA synthesis.

2.2. CharacterizationPowder X-ray diffraction (PXRD) patterns were collected on an X’PertPro Panalytical Chiller 59 diffractometer (Malvern, UK) using copper Kα (1.54 Å) radiation. Diffraction patterns were recorded inPowder the 2θ Xrange,-ray diffraction from 4.00 (P toXRD) 39.09 patterns degrees, were with collected a 2θ step on sizean X'Pert of 0.017,Pro P andanalytical a scan Chiller per step 59 of diffractometer34.9 s. Elemental (Malvern, analyses UK) (N,using C andcopper H) of K preparedα (1.54 Å Cu-BDC) radiationHMTA. Diffraction samples patterns were performed were ⊃ recordedto confirm in the presence 2θ range, of from amine 4.00 in to the39.09 prepared degrees, material, with a 2θ using step size a FlashSmart of 0.017, and NC a scan ORG per elemental step of 34.9analyzer s. Elemental (Oxford, analyses UK). Thermogravimetic (N, C and H) of analysesprepared (TGA) Cu-BDC were⊃HMTA performed samples using were a Perkin performed Elmer to Pyris confirm presence of amine in the prepared material, using a FlashSmart NC ORG elemental 1 thermo-gravimetric analyzer (Champaign, IL, USA). The temperature was increased from 25 ◦C to analyzer (Oxford, UK). Thermogravimetic1 analyses (TGA) were performed1 using a Perkin Elmer 700 ◦C at a heating rate of 5 ◦C min− under a flow of air (20 mL min− ). SEM images were collected Pyrisusing 1 thermo TESCAN-gravimetric/VEGA-3 equipmentanalyzer (C (Brno,hampaign, Czech IL, Republic). USA). The A SHIMADZUtemperature IRwas Affi increasednitt-1S spectrometer from 25 °C (Kyoto,to 700 ° Japan)C at a was heating used rate to obtainof 5 °C IR min spectra.−1 under a flow of air (20 mL min−1). SEM images were collected using TESCAN/VEGA-3 equipment (Brno, Czech Republic). A SHIMADZU IR Affinitt-1S Prior to CO2 sorption studies, the samples were degassed at 130 ◦C for 10 h and then back-filled with spectrometer (Kyoto, Japan) was used to obtain IR spectra. helium gas. CO2 adsorption experiments were performed on a Quantachrome Isorb-HP100 volumetric typePrior sorption to CO2 analyzer sorption (Boynton studies, Beach,the sample FL, USA).s were The degassed sample at was 130 tested °C for for 10 adsorption hours and at then two back different- filled with helium gas. CO2 adsorption experiments were performed on a Quantachrome Isorb- temperatures: 0 ◦C and 25 ◦C, at pressures from 1–14 bar. N2 adsorption studies of the MOF were HP100 volumetric type sorption analyzer (Boynton Beach, FL, USA). The sample was tested for conducted to analyze surface area and pore volume using a Quantachrome Nova 2200e at 196 ◦C at a adsorption at two different0 temperatures: 0 °C and 25 °C , at pressures from 1–14 bar. N2 adsorption− relative pressure of P/P = 0.1–1.0 and prior to the measurement, samples were degassed at 160 ◦C studiesunder of vacuum the MOF for 11 were h. conducted to analyze surface area and pore volume using a Quantachrome Nova 2200e at −196 °C at a relative pressure of P/P⁰ = 0.1–1.0 and prior to the measurement, samples were degassed at 160 °C under vacuum for 11 hours.

3. Results and Discussion

3.1. PXRD Patterns of Cu-BDC and Cu-BDC⊃HMTA Nanomaterials 2019, 9, 1063 3 of 10

Nanomaterials3. Results and2019,Discussion 9, x FOR PEER REVIEW 3 of 10

3.1. PXRD Patterns of Cu-BDC and Cu-BDC HMTA Powder X-ray diffraction patterns⊃, for products of the synthesis of Cu-BDC and Cu-BDCPowder⊃HMTA X-ray, were diffraction collected patterns, and are for shown products in Figure of the synthesis2. Both synthesized of Cu-BDC materials and Cu-BDC showHMTA, sharp ⊃ werediffraction collected peaks and indicating are shown predominantly in Figure2. Both crystalline synthesized material materials. The show peak sharp positions di ffraction are in peaks good agreementindicating predominantlywith the PXRD crystalline of Cu-BDC material. previously The peak reported positions by Carson are in goodet al. agreementin 2009, indicating with the successfulPXRD of Cu-BDC synthesis previously of the Cu- reportedBDC [10] by. There Carson are etseveral al. in additional 2009, indicating peaks in successful the PXRD synthesis pattern of Cuthe-BDC Cu-BDC⊃HMTA [10]., Theremost are notably several at additional2θ = 8.31° peaks. These in cannot the PXRD be patternascribed of to Cu-BDC a simpleHMTA, mechanical most mixture of Cu-BDC and HMTA as, firstly, HMTA is soluble in the organic solvents⊃ used to wash notably at 2θ = 8.31◦. These cannot be ascribed to a simple mechanical mixture of Cu-BDC and HMTA theas, firstly, reaction HMTA product, is soluble which in themakes organic this solventsoption unlikely. used to wash Secondly, the reaction the increased product, CO which2 sorption makes observedthis option in unlikely. the Cu-BDC Secondly,⊃HMTA the product increased (described CO sorption below) observed is not consistent in the Cu-BDC with a simpleHMTA physical product 2 ⊃ mixture(described of below) Cu-BDC is and not consistentHMTA, which with we a simple would physical anticipate mixture having of a Cu-BDC lower gas and uptake HMTA, than which the wepure would Cu-BDC anticipate alone. Thirdly,having a and lower most gas conclusively, uptake than the powderpure Cu-BDC XRD pattern alone. Thirdly,of pure HMTAand most is shownconclusively, in Figure the powder 2, and XRD no patternHMTA ofpeaks pure correspondHMTA is shown to the in Figure new peaks2, and observed no HMTA in peaks Cu- BDCcorrespond⊃HMTA. to the We new tentatively peaks observed ascribe in theCu-BDC additionalHMTA. peaks We tentatively to well-ordered ascribe the HMTA additional molecules peaks ⊃ bindingto well-ordered to the axial HMTA copper molecules sites in binding the framework. to the axial Inspecting copper sites the in ( thepreviously framework. reported Inspecting) Cu-BDC the crystal(previously structure reported) [10] shows Cu-BDC that crystal the 2D structure layers pack [10] with shows significantly that the 2D offset layers paddlewheels pack with significantly from one layeroffset paddlewheels to the next, fromwith one copper layer cente to thersnext, separated with copper by 6.331 centers Å separated (Cu–Cu distance).by 6.331 Å Looking (Cu–Cu approximatelydistance). Looking along approximately the Cu–Cu axis, along although the Cu–Cu still withaxis, althoughsome notable still withoffset, some the nextnotable available offset, paddlewheelthe next available is 9.068 paddlewheel Å (Cu–Cu distance) is 9.068 Åaway (Cu–Cu. HMTA distance) has four away. potentially HMTA coordinating has four potentially nitrogen atomscoordinating separated nitrogen by ~2.47 atoms Å (N separated–N) across by ~2.47a tricyclic Å (N–N), adamantane across a tricyclic,-like cage adamantane-like [12]. There are examples cage [12]. ofThere HMTA are examples bridging ofcopper HMTA nodes bridging in a coppermetal- nodesorganic in framework a metal-organic, but framework,these typically but theseinvolve typically much shorterinvolve Cu much–Cu shorter distances Cu–Cu (5.761 distances Å in the (5.761 cited Å example) in the cited with example) an angle with between an angle the between Cu-N thebonds Cu-N of adjacentbonds of adjacentcentres across centres the across HMTA the HMTA of ~108°, of ~108 which, which is not is not possible possible in in our our Cu Cu-BDC-BDC⊃HMTA ◦ ⊃ framework [ [12]].. Fur Furtherther indirect indirect evidence evidence for for bonding bonding of of HMTA HMTA to to the the framework framework is is that that it it is not readily removed by by simple simple washing washing with with organic organic solvents. solvents. It Ithas has not not been been possible possible at atthis this stage stage to synthesito synthesizeze crystals crystals of Cu of Cu-BDC-BDC⊃HMTAHMTA of sufficient of sufficient size size and and quality quality to obtain to obtain the the single single-crystal-crystal X- ⊃ rayX-ray structure. structure.

BDC Cu(NO ) .6H O 3 2 2 HMTA Cu-BDC HMTA Cu-BDC (Present study)

Cu-BDC (Cantwell et al., 2009) Counts

5 10 15 20 25 30 35

2

Figure 2. 2. PXRDPXRD patterns pattern fors for Cu-BDC Cu-BDC (black, (black reported, reported by Carson by etCarson al., 2009), et al., Cu-BDC 2009) (red,, Cu synthesized-BDC (red, herein), Cu-BDC HMTA (blue), HMTA (green), Cu(NO ) 6H O (lilac) and the H BDC linker synthesized herein)⊃ , Cu-BDC⊃HMTA (blue), HMTA (green3),2 Cu(NO· 2 3)2·6H2O (lilac) and2 the H2BDC linker(dark yellow).(dark yellow).

3.2. FTIR of Cu-BDC and Cu-BDC⊃HMTA Fourier transform infrared spectra (FTIR) collected for prepared materials confirm the presence of representative functional groups indicative of Cu-BDC MOF formation (Figure 3). Sharp peaks representative of symmetric and asymmetric stretching of carboxylates bonded to Cu are observed Nanomaterials 2019, 9, 1063 4 of 10

3.2. FTIR of Cu-BDC and Cu-BDC HMTA ⊃ Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 10 Fourier transform infrared spectra (FTIR) collected for prepared materials conﬁrm the presence ofat representative 1521 cm−1 and functional 1362 cm−1 groups in the Cu indicative-BDC sample of Cu-BDC [10]. Both MOF materials formation show (Figure the presence3). Sharp of peaks what is representativelikely to be of water symmetric (even and after asymmetric vacuum-oven stretching drying of the carboxylates samples) in bonded the form to Cu of are a observedbroad peak 1 1 atcentered 1521 cm −aroundand 1362 3400 cm cm−−1, in which the Cu-BDC is much samplemore evident [10]. Bothin the materials Cu-BDC show sample the than presence in the ofCu- whatBDC is⊃ likelyHMTA to material be water and (even is likely after due vacuum-oven to the rapid dryinguptake theof atmospheric samples) in water the form when of performing a broad 1 peakthe centeredmeasurement around in 3400air. The cm −relatively, which isreduced much morewater evident content inin thethe Cu-BDCCu-BDC⊃ sampleHMTA than sample in the may Cu-BDC HMTA material and is likely due to the rapid uptake of atmospheric water when performing indicate⊃ slower water adsorption as a result of pore-blocking by adsorbed HMTA and, in agreement the measurement in air. The relatively reduced water content in the Cu-BDC HMTA sample may with the binding of HMTA to copper nodes proposed above, the occupation⊃ of axial sites on the indicatecopper slower paddlewheels water adsorption by HMTA as which a result otherwise of pore-blocking could rapidly by adsorbed adsorb HMTAwater. and,In addition in agreement to peaks withcoincident the binding with ofthose HMTA of Cu to-BDC copper MOF nodes, the proposedCu-BDC⊃ above,HMTAthe sample occupation illustrates of axialsome sitesnew onfeatures the . copperA characteristic paddlewheels peak by for HMTA amine which-containing otherwise functional could rapidly groupsadsorb is observed water. at In 1089 addition cm−1,to consistent peaks coincidentwith C–N with bond those stretching of Cu-BDC [13 MOF,,14]. P theeaks Cu-BDC at 2915HMTA and 2845 sample cm− illustrates1 can be ascribed some new to features. stretching ⊃ 1 A characteristicvibrations of peakC–H bonds for amine-containing introduced by functionalthe incorporation groups isof observedHMTA [14 at, 108915]. cm− , consistent with 1 C–N bond stretching [13,14]. Peaks at 2915 and 2845 cm− can be ascribed to stretching vibrations of C–H bonds introduced by the incorporation of HMTA [14,15].

0.020 Cu-BDC

Cu-BDC ⊃ HMTA

0.016 Abs

0.012

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber cm

Figure 3. FTIR spectra for Cu-BDC and Cu-BDC HMTA. ⊃ Figure 3. FTIR spectra for Cu-BDC and Cu-BDC⊃HMTA. 3.3. Thermal Stability of Cu-BDC and Cu-BDC HMTA ⊃ 3.3.The Thermal two materials Stability of were Cu-BDC studied and byCu TGA-BDC and⊃HMTA the results are shown in Figure4. For both MOFs there isThe less two than materials 2% weight were loss studied observed by TGA below and 150 the◦C, results indicating are shown the pre-treatment in Figure 4. hasFor removedboth MOFs thethere majority is less of than the residual 2% weight solvent, loss observed and there below is only 1 minimal50 °C, indicating adsorbed the moisture. pre-treatment The small has weightremoved lossthe in majority Cu-BDC of between the residual 170 ◦ Csolvent and 320, and◦C there (approx. is only 8%) minimal is consistent adsorbed with moisture. loss of surface The small adsorbed weight DMFloss [13 in]. Cu For-BDC Cu-BDC, between decomposition 170 °C and of 320 the °C benzene (approx. dicarboxylate 8%) is consistent starts at with about loss 375 of◦ C,surface above adsorbed which temperatureDMF [13]. thereFor Cu is rapid-BDC, degradation decomposition to the of metalthe benzene oxide. Notably,dicarboxylate there isstart multi-steps at about degradation 375 °C, above of Cu-BDCwhich temperatureHMTA, with massthere lossesis rapid from degradation ~250 ◦C and to 425 the◦C metal consistent oxide with. Notably, HMTA t sublimationhere is multi and-step ⊃ thermaldegradation degradation, of Cu- respectively.BDC⊃HMTA In, with Cu-BDC mass HMTA,losses from linker ~250 degradation °C and 425 appears °C consistent to be overlapped with HMTA ⊃ withsublimation HMTA degradation. and thermal Nodegradation further weight, respectively loss was. In observedCu-BDC⊃ aboveHMTA 450, linker◦C fordegradation Cu-BDC appears MOF, andto abovebe overlapped 550 ◦C for with Cu-BDC HMTAHMTA. degradation The Cu-BDC. No further thermal weight degradation loss was results observed in 25% above metal 450 oxide, °C for ⊃ andCu 74%-BDC linker MOF+, DMFand above in the initial550 °C mass, for Cu consistent-BDC⊃HMTA with the. The expected Cu-BDC metal:linker:DMF thermal degradation ratio ofresults 1:1:1. in The25% relative metal proportions oxide, and of Cu-BDC 74% linkerHMTA + DMF components in the cannot initial reliablymass, consistent be extracted with from the these expected data ⊃ duemetal: to thelinker: diﬃcultyDMF disentangling ratio of 1:1:1. the The overlapping relative proportions mass losses of HMTACu-BDC and⊃HMTA linker incomponents this experiment. cannot reliably be extracted from these data due to the difficulty disentangling the overlapping mass losses of HMTA and linker in this experiment. Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 10

NanomaterialsNanomaterials2019 2019, 9, ,9 1063, x FOR PEER REVIEW 55 of of 10 10

Figure 4. TGA of Cu-BDC MOF (dashed line) and Cu-BDC⊃HMTA (solid line).

Figure 4. TGA of Cu-BDC MOF (dashed⊃ line) and Cu-BDC ⊃HMTA (solid line). 3.4. Elemental CompositionFigure 4. TGA of Cu of -CBDCu-BDC and MOF Cu- BDC(dashedHMTA line) and Cu-BDC⊃ HMTA (solid line). 3.4. ElementalTo confirm Composition the chemical of Cu-BDC composition and Cu-BDC of bothHMTA samples, elemental analysis and EDS were 3.4. Elemental Composition of Cu-BDC and Cu-BDC⊃⊃HMTA performed (Table 1). The empirical formulae calculated on the basis of EDS, and elemental analysis To conﬁrm the chemical composition of both samples, elemental analysis and EDS were performed for CuTo-BDC confirm, and the Cu - chemicalBDC⊃HMTA composition are: C11 H of11 CuNO both samples,5 and C 14 elementalH16CuN4O analysis4, respectively and EDS. This were is (Table1). The empirical formulae calculated on the basis of EDS, and elemental analysis for Cu-BDC, consistentperformed with (Table metal: 1). The linker: empirical {DMF formulaeor HMTA calculated, respectively} on the molar basis ratios of EDS of, 1:1:1and ,e lementaland in line analysis with and Cu-BDC HMTA are: C11H11CuNO5 and C14H16CuN4O4, respectively. This is consistent with eachfor of Cu the-BDC copper⊃, and axial Cu- sitesBDC ⊃beingHMTA occupied are: C by11H HMTA11CuNO in5 Cuand-BDC C14⊃HHMTA.16CuN4O 4, respectively. This is metal: linker: {DMF or HMTA, respectively} molar ratios of 1:1:1, and in line with each of the copper consistent with metal: linker: {DMF or HMTA, respectively} molar ratios of 1:1:1, and in line with axial sites being occupied by HMTA in Cu-BDC HMTA. each of the copper axialTable sites 1. Elemental being occupied composition by⊃ HMTA of Cu-BDC in Cu and-BDC Cu-BDC⊃HMTA.⊃HMTA . Elemental TableCalculated 1. Elemental by composition Elemental of Cu-BDC and Cu-BDC HMTA. Table 1. Elemental composition of Cu-BDC and Cu-CalculatedBDC⊃⊃HMTA by. EDS Composition Analyzer Elemental Composition Calculated by Elemental Analyzer Calculated by EDS MOFElemental Sample CCalculated H by ElementalN C O N Cu MOF Sample C H N CCalculated O by N EDS Cu Composition 44.03 Analyzer3.64 4.63 45.94 27.27 5.70 21.09 44.03 3.64 4.63 45.94 27.27 5.70 21.09 Cu-BDCCu-BDC MOF Sample (44.07)C (44.07)(3.69)H (3.69)(4.67)N (4.67)(44.0)C (44.0) (26.62)(26.62)O (4.67)(4.67)N (21.13)(21.13)Cu 45.8044.03 45.80 4.43.64 4.4 15.304.63 15.3047.8145.94 47.81 18.5927.2718.59 16.3016.305.70 17.3017.3021.09 Cu-BDCCuCu-BDC-⊃BDCHMTA HMTA ⊃ (45.77)(44.07) (45.77)(4.39)(3.69) (4.39)(15.26)(4.67) (15.26)(45.77)(44.0)(45.77) (17.41)(26.62)(17.41) (15.26)(15.26)(4.67)(17.27) (17.27)(21.13) Note:45.80 Theoretical values4.4 in brackets15.30 and calculated47.81 values outside18.59 brackets. 16.30 17.30 Cu-BDC⊃HMTANote: Theoretical values in brackets and calculated values outside brackets. (45.77) (4.39) (15.26) (45.77) (17.41) (15.26) (17.27) 3.5. Morphology of Note: Cu Cu-BDC-BDC Theoretical and Cu Cu-BDC values-BDC⊃ inHMTA brackets and calculated values outside brackets. ⊃ Scanning electron electron microscopy microscopy images images of the of prepared the prepared samples samples are shown are shownin Figure in 5. Figure The Cu5. - 3.5. Morphology of Cu-BDC and Cu-BDC⊃HMTA TheBDC Cu-BDC crystallites crystallites have a plate have alike plate structure, like structure, while Cu while-BDC Cu-BDC⊃HMTAHMTA shows shows a more a moreregular regular flat rod ﬂat- ⊃ rod-likelike structure.Scanning structure. electron microscopy images of the prepared samples are shown in Figure 5. The Cu- BDC crystallites have a plate like structure, while Cu-BDC⊃HMTA shows a more regular flat rod- like structure. a b

a b

FigureFigure 5. 5. SEMSEM images images at at 10 10 µmµm for for ( (aa)) Cu Cu-BDC-BDC MOF MOF and and at at 20 20 µmµm for ( b)) Cu Cu-BDC-BDC⊃HMTA MOF.MOF. ⊃ Figure 5. SEM images at 10 µm for (a) Cu-BDC MOF and at 20 µm for (b) Cu-BDC⊃HMTA MOF. Nanomaterials 2019, 9, 1063 6 of 10

3.6. CO Adsorption Studies of Cu-BDC MOF and Cu-BDC HMTA 2 ⊃ The CO2 adsorption capacity for both MOF materials was evaluated by monitoring pseudo equilibrium adsorption uptake. Samples were initially degassed at 130 ◦C for 12 h. 200 mg of each sample was used for three consecutive adsorption-desorption cycles at 273 K or 298 K with adsorbate pressure ranging between 1 to 14 bar. For Cu-BDC HMTA, the CO2 uptake recorded at 1 bar was 1 1 ⊃ 4.8 mmol g− (21.1 wt %), and 2.1 mmol g− (9.24 wt %) at 273 K and 298 K, respectively (Figure6). Notably, for Cu-BDC, without the amine modiﬁcation, CO2 uptake was measured at 1 bar as only Figure 16. 1 1.2 mmol g− (5.28 wt %), and 0.8 mmol g− (3.53 wt %) at 273 K and 298 K, respectively. At 14 bar, the CO uptake at 273 C, and 298 C for Cu-BDC HMTA is 12 mmol g 1 (52.8 wt %), and 9 mmol g 1 2 ◦ ◦ ⊃ − − (39.6 wt %), respectively (Figure6), again markedly higher than for Cu-BDC (17.4 wt %, and 13.2 wt % at 273 K and 298 K, respectively).

Figure 6. CO adsorption isotherms for Cu-BDC and Cu-BDC HMTA at 273 K and 298 K, as shown. 2 ⊃ 3.7. Surface Area and Porosity of Cu-BDC HMTA MOF ⊃ The N adsorption isotherm for Cu-BDC HMTA was recorded at 77 K (Figure7B). The Langmuir 2 ⊃ and BET surface areas for Cu-BDC were found to be 868 m2/g and 708 m2/g, respectively, while Cu-BDC HMTA revealed lower values of 683 m2/g (Langmuir), and 590 m2/g (BET) (Table2). ⊃ Although the introduction of the amine into Cu-BDC HMTA reduces its surface area, the CO ⊃ 2 adsorption is increased. The presence of additional binding sites in MOFs by amine/amide incorporation has been shown to induce dispersion, and electrostatic forces that enhance CO2 gas adsorption (Table2)[13]. The isosteric heat of CO 2 adsorption (Qst) in Cu-BDC HMTA was calculated from the 1 ⊃ adsorption isotherms at 273 and 298 K (Figure7A) as 29.8 kJ mol − . Such a moderate value is lower than many other MOFs (see Table2), and is highly desirable because of the anticipated lower material regeneration energy demand. Qst is the heat Q released in a constant temperature calorimeter when a differential amount of gas is adsorbed at constant pressure. The Van’t Hoff isobar equation relates Qst to adsorption isotherms at different temperatures. It is derived from equating the chemical potential of the adsorbed phase, and the gas phase, applying the Gibbs Helmholtz relation, and assuming that the vapor phase behaves like an ideal gas. From experimentally obtained isotherms at a constant amount adsorbed and two different temperatures (T1 and T2), Qst is obtained by following equation:      (lnP1 lnP2)  Qst = R  −  (1)  1 1  T1 − T2 Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 10

[Cu2PDAI(H2O)] 1372 273 1 28.6 26.3 [20] [Cu2(TCMBT)(bpp)(μ3 − OH)] · 6H2O 808 298 20 25.5 26.7 [21] 4− [Cu2(BDPT )(H2O)2] 1400 273 1 30.7 22.5 [22] en-CuBTTri 345 298 1 58.2 90 [23] en@CuBTC - 298 1 19.5 30 [24] mmen-CuBTTri 870 298 1 15.4 - [25] Present Cu-BDC 708 273 1(14) 5.28 (17.4) - study Present Cu-BDC⊃HMTA 590 273 1(14) 21.2(52.8) 29.8 study

Qst is the heat Q released in a constant temperature calorimeter when a differential amount of gas is adsorbed at constant pressure. The Van't Hoff isobar equation relates Qst to adsorption isotherms at different temperatures. It is derived from equating the chemical potential of the adsorbed phase, and the gas phase, applying the Gibbs Helmholtz relation, and assuming that the vapor phase behaves like an ideal gas. From experimentally obtained isotherms at a constant amount adsorbed and two different temperatures (T1 and T2), Qst is obtained by following equation: (푙푛푃1 − 푙푛푃2) (1) 푄푠푡 = 푅 ( 1 1 ) ( − ) 푇1 푇2

where R is ideal gas constant. Isosteric heats of adsorption for Cu-BDC⊃HMTA were calculated using 273 K and 298 K isotherms using the slope of a Van’t Hoff plot against the amount adsorbed. Nanomaterials 2019, 9, 1063 7 of 10 Here, the Qst value decreases with loading, indicating strong interaction between the quadrupole moment of carbon dioxide and the adsorbent surface. whereThe R pore is ideal size gas distribution constant. obtained Isosteric from heats BET of adsorption–BJH N2 adsorption for Cu-BDC showsHMTA micro werepores calculatedat around ⊃ using8.3 Å in 273 Cu K-BDC, and 298 and K larger isotherms pores using at 14the Å that slope may of aoriginate Van’t Ho fromﬀ plot defects against or theinter amount-crystalline adsorbed. gaps Here,[14]. Unsurprisingly, the Qst value decreases a narrow with pore loading, distribution indicating of smaller strong pores interaction around between7.1 Å is observed the quadrupole in the momentCu-BDC⊃ ofHMTA carbon sample. dioxide and the adsorbent surface.

A B

Figure 7. (A) IsostericIsosteric heatsheats ofof COCO2 adsorption onto Cu-BDCCu-BDC⊃HMTA. ((BB)N) N2 adsorption-desorptionadsorption-desorption 2 ⊃ 2 isotherms at 77 K. Adsorption is representedrepresented byby hollowhollow circlescircles (Red(Red == Cu-BDCCu-BDC⊃HMTA, GreenGreen == Cu ⊃ BDC),BDC), and desorption is marked by closed circles.

Table 2. Surface area, CO2 uptake and Qst values for selected Cu-based MOFs. The uptake of CO2 is only part of the utility of these materials. They also need to have reproducible uptake on more thanBET oneTemperature sorption-desorptionPressure CO cycle.Adsorption Cu-BDC revealedQst (KJ a significant Material 2 Reference (m2/g) (K) (bar) (wt %) 1 loss in adsorption capacity over successive operations compared to Cu-BDCmol⊃HMTA− ) (Figure 8). Here, adsorptionCu (TATB) capacity calculated 3360 at 298 293 K and 14 bar - for Cu-BDC - lowered 61by about 14% [13] after [Cu3(TDPAT)] 1938 273 1 25.8 42.2 [14]

Cu2(H2O)2BDPO 2447 273 1 40.1 25.4 [15]

[Cu4(µ4-O)Cl2(COO)4N4] 2690 273 10 27.3 36.5 [16]

Cu(pia)2(SiF6)(EtOH)2(H2O)12 285 296 1 5.5 30 [17] i [Cu BTB) 6 3288 273 20 157 - [18] 3 − n

[Cu3L2(H2O)5] 2690 273 1 27.3 - [19]

[Cu2PDAI(H2O)] 1372 273 1 28.6 26.3 [20] [Cu (TCMBT)(bpp)(µ OH)] 6H O 808 298 20 25.5 26.7 [21] 2 3 − · 2 h 4 i Cu2 BDPT − (H2O)2 1400 273 1 30.7 22.5 [22] en-CuBTTri 345 298 1 58.2 90 [23] en@CuBTC - 298 1 19.5 30 [24] mmen-CuBTTri 870 298 1 15.4 - [25] Cu-BDC 708 273 1 (14) 5.28 (17.4) - Present study Cu-BDC HMTA 590 273 1 (14) 21.2 (52.8) 29.8 Present study ⊃

The pore size distribution obtained from BET–BJH N2 adsorption shows micropores at around 8.3 Å in Cu-BDC, and larger pores at 14 Å that may originate from defects or inter-crystalline gaps [14]. Unsurprisingly, a narrow pore distribution of smaller pores around 7.1 Å is observed in the Cu-BDC HMTA sample. ⊃ The uptake of CO2 is only part of the utility of these materials. They also need to have reproducible uptake on more than one sorption-desorption cycle. Cu-BDC revealed a signiﬁcant loss in adsorption capacity over successive operations compared to Cu-BDC HMTA (Figure8). Here, adsorption capacity ⊃ calculated at 298 K and 14 bar for Cu-BDC lowered by about 14% after three cycles from 3 to 2.6 mmol/g. Nanomaterials 2019, 9, 1063 8 of 10 Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 10 three cycles from 3 to 2.6 mmol/g. This decrease in adsorption capacity over successive cycles was This decrease in adsorption capacity over successive cycles was more prominent at higher temperature more prominent at higher temperature compared to lower temperature (273 K) adsorption. In compared to lower temperature (273 K) adsorption. In contrast, Cu-BDC HMTA demonstrated much contrast, Cu-BDC⊃HMTA demonstrated much lower percentage decline⊃ in CO2 uptake over three lower percentage decline in CO2 uptake over three successive adsorption cycles (1.1% at 298 K and successive adsorption cycles (1.1% at 298 K and 0.83% at 273 K). 0.83% at 273 K).

14 12 12 11.9 12

10 9 8.9 8.9

6 4.1 4 3.7

Adsorption (mmol/g) Adsorption 4 3 2.8 2.6 2

0 Cu-BDC 298K Cu-BDC HMTA 298K Cu-BDC 273 K Cu-BDC HMTA 273K

CO2 adsorption (mmol/g) during three consecutive cycles

Figure 8. CO adsorption in mmol/g calculated at 14 bar for Cu-BDC and Cu-BDC HMTA at 273 K 2 ⊃ Figureand 298 8. KCO as2 shown.adsorption in mmol/g calculated at 14 bar for Cu-BDC and Cu-BDC⊃HMTA at 273 K and 298 K as shown. 4. Conclusions 4. ConclusionIn summary,s we report the simple modification of a Cu-BDC MOF during synthesis by the incorporation of a hexamethylenetetramine additive. The Cu-BDC HMTA MOF material forms as a In summary, we report the simple modification of a Cu-BDC⊃ MOF during synthesis by the crystalline solid with rod-like crystallites. Thermogravimetric studies⊃ reveal that Cu-BDC HMTA incorporation of a hexamethylenetetramine additive. The Cu-BDC HMTA MOF material forms⊃ as ais crystalline more thermally solid with stable rod than-like Cu-BDC crystallites MOF.. Thermogravimetric Moreover, carbon studies dioxide reveal adsorption that Cu studies-BDC⊃ forHMTA these issamples more thermally reveal markedly stable than better Cu carbon-BDC dioxideMOF. M uptakeoreover, by c thearbon amine-modified dioxide adsorptio frameworkn studies (5.25 for wt these % for Cu-BDC, and 21.2 wt % for Cu-BDC HMTA, respectively, at 273 K and 1 bar). The addition of nitrogen samples reveal markedly better carbon⊃ dioxide uptake by the amine-modified framework (5.25 wt %atoms for Cu by-BDC the incorporation, and 21.2 wt of % HMTA for Cu leads-BDC to⊃HMTA the enhanced, respectively adsorption, at 273 of COK and2 gas, 1 whichbar). The we ascribeaddition to offavorable nitrogen interactions atoms by [the26] betweenincorporation CO2 molecules of HMTA and lead thes nitrogen-modifiedto the enhanced adsorption pores [27]. The of CO modified2 gas, MOF, Cu-BDC HMTA, also displays enhanced cyclic stability and can be reused over three cycles. which we ascribe⊃ to favorable interactions [26] between CO2 molecules and the nitrogen-modified poreThissstudy [27]. The describes modified a cost-e MOF,ffective Cu-BDC strategy⊃HMTA for the, also incorporation displays enhanced of amine cyclic groups stability in MOF and structures, can be reusedfor enhanced over three CO2 capture cycles. This applications, study describes using HMTA a cost as-effective a cheap additive.strategy for This the serves incorporation as a low-cost of aminealternative groups to in expensive MOF structures amine based, for enhanced ligands that CO are2 capture often custom-builtapplications, tousing make HMTA MOFs as for a carboncheap dioxide capture. Future studies are needed to address the longer-term stability of Cu-BDC HMTA additive. This serves as a low-cost alternative to expensive amine based ligands that are⊃ often customand stability-built to contaminants.make MOFs for carbon dioxide capture. Future studies are needed to address the longer-term stability of Cu-BDC⊃HMTA and stability to contaminants. Author Contributions: Conceptualization, T.L.E.; Data curation, M.A.; Investigation, A.A.; Methodology, N.I.; Supervision, T.N., T.L.E.; writing, all authors. Author Contributions: Conceptualization, T.L.E.; Data curation, M.A.; Investigation, A.A.; Methodology, N.I.; Funding: Funding for this project was provided by USPCAS-E, Cardiff University, NUST Pakistan and Higher Supervision,Education, Pakistan T.N., T.L.E.; and Thewriting, APC all was authors. funded by NUST, Pakistan. Funding:Acknowledgments: Funding forThe this authorsproject was are veryprovided grateful by USPCAS to Higher-E, EducationCardiff University, Commission, NUST Pakistan Pakistan for and providing Higher Education,financial support Pakistan to carryand The out someAPC partwas offunded this research by NUST, work Pakistan. at Cardi ff University, UK. TLE gratefully acknowledges the Royal Society for the award of a University Research Fellowship (6866), and Cardiff University for funding. Acknowledgments: The authors are very grateful to Higher Education Commission, Pakistan for providing Conflicts of Interest: The authors declare no conflict of interest. financial support to carry out some part of this research work at Cardiff University, UK. TLE gratefully acknowledges the Royal Society for the award of a University Research Fellowship (6866), and Cardiff University for funding.

Conflict of interests: The authors declare no conflict of interest.

References Nanomaterials 2019, 9, 1063 9 of 10

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