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 1of. 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) ofK 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 washeating 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 peaksgood agreementindicating predominantlywith the PXRD crystalline of Cu-BDC material.

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