ORIGINAL PAPER

Preparation of polymer networks for hydrogen storageusing the Ullmann synthetic protocol

K. A. Shaffei & Ayman M. Atta & Clara S. B. Gomes &

Pedro T. Gomes & R. A. El-Ghazawy &

Abdallah G. Mahmoud

Received: 6 January 2014 /Accepted: 9 April 2014# Springer Science+Business Media Dordrecht 2014

Abstract Based on the Ullmann synthetic protocol,melamine-based porous organic networks, to be used forhydrogen storage, were prepared by a facile one-pot approach.These polymers were produced by condensation of melaminewith different aryl halides under mild conditions, using asoluble copper(I) complex as catalyst. These materials exhibitBrunauer-Emmett-Teller (BET) surface areas up to 19.8 m2/gas measured by N2 adsorption at 77 K, and exhibit gravimetricstorage capacities up to 1.17 wt.%, at 20 bar and 77 K.

Keywords Hydrogen storage . Organic networks . Ullmanncoupling . Porous polymers

Introduction

Hydrogen is one of the alternative fuels envisioned to decreasethe carbon dioxide emissions and dependency on oil relatedproducts [1]. The development of a hydrogen economy re-quires the development of safe, lightweight, and high capacitystorage systems for hydrogen. The U.S. Department of Energy(DOE) targets for onboard automotive hydrogen storage have

slightly varied in the last 13 years [1–3], the gravimetric andvolumetric targets for 2017 being defined as 5.5 wt.% and40 g H2/L, respectively [4]. These values are very challengingbecause the H2 storage capacity of a material is related mainlyto the pressure and temperature at which the measurements areperformed. Using elevated pressures and low temperatureswill increase the system weights due to the presence of nec-essary containment technology and associated cooling sys-tems that are required for such conditions.

Metal hydrides, metal-organic frameworks (MOFs) and car-bon sorbents (carbon nanotubes, fibers, fullerenes, and activat-ed carbons) are state-of-the-art materials for hydrogen storage,and have been among the most studied substances of this area[3]. However, porous organic polymers have drawn great sci-entific interests in recent years due to the wide variety ofapplications, not only in gas storage [3, 5–9], but also in thesorption of organic vapors [10–12], gas separation [13], andheterogeneous catalysis [14, 15]. They have several advan-tages: a wide variety of organic reactions that can be used toprepare and modify them, high chemical and mechanical sta-bility, low skeleton density, and potential low cost [3, 9]. Porousorganic polymers have recently been found as promising ad-sorbents for hydrogen storage, showing a potential to meet theDOE targets [5, 16–20]. Because the porous polymers, asadsorbents, exhibit low adsorption enthalpies, almost all studiesare carried out at 77 K (boiling temperature of liquid nitrogen)in order to store appreciable quantities of hydrogen [9].

Polymers of intrinsic microporosity (PIMs) represent avery important class of porous polymers, in which the poresare formed due to the rigid subunits inefficient packing [9, 17,20]. Recently, PIMs have been demonstrated to adsorb up to2.7 wt.% H2 at 77 K and 10 bar [18].

Covalent organic frameworks (COFs) are crystalline po-rous polymers prepared by linking organic monomers throughstrong covalent bonds. COFs shows very high surface areas,up to 2930 m2/g, and H2 storage capacities up to 1.7 wt.% H2

at 77 K and 1 bar [21].

C. S. B. Gomes : P. T. Gomes (*) :A. G. MahmoudCentro de Química Estrutural, Departamento de EngenhariaQuímica, Instituto Superior Técnico, Universidade de Lisboa, Av.Rovisco Pais, 1049-001 Lisboa, Portugale-mail: pedro.t.gomes@tecnico.ulisboa.pt

A. M. Atta (*)Surfactants Research Chair, Chemistry Department, College ofScience, King Saud University, Riyadh( 11451, Saudi Arabiae-mail: khaled_00atta@yahoo.com

K. A. Shaffei :A. G. MahmoudDepartment of Chemistry, Faculty of Science, Helwan University,Ain Helwan, Cairo, Egypt

A. M. Atta :R. A. El-GhazawyPetroleum Application Department, Egyptian Petroleum ResearchInstitute, 1 Ahmad Elzomor St, Nasr city 11727, Cairo, Egypt

J Polym Res (2014) 21:445DOI 10.1007/s10965-014-0445-0

Hypercrosslinked polymers (HCPs) represent another class ofporous organic polymers, which have a permanent porosity dueto extensive chemical crosslinking that prevents the collapsing ofpolymer chains into a nonporous material [5, 9, 19]. Wood et al.produced a series of HCPs demonstrating hydrogen storagecapacities up to 3.7 wt.% at 77 K and 15 bar [5].

Recently, Germain et al. [22] developed a new class of poroussize-selective hypercrosslinked polymer networks containing 37-92%of pores small enough for hydrogen adsorption but too smallto allow penetration of nitrogen. They used the Ullmann syntheticroute, which was reported by Gujadhur et al. [23], to couplephenylenediamine with diiodobenzene and tribromobenzene.The resulting porous polymer networks consisted of aromaticrings linked through a trivalent nitrogen atom.

The aim of the present work is to produce a new type of N-enriched porous organic networks by a facile one-pot approachto be used for hydrogen storage. These polymers are producedthrough the Ullmann synthetic protocol by condensation ofmelamine with different aryl halides, under mild conditions,using a soluble air-stable copper(I) complex, Cu(PPh3)3Br, ascatalyst and cesium carbonate as base. The reaction procedurewas represented in Scheme 1. The structures and properties ofthe resulting materials were systematically characterized byvarious important techniques.

Experimental

Materials

Solvents and starting materials were obtained from commer-cial sources (e.g. Alfa Aesar) and used as received withoutfurther purifications. The complex Cu(PPh3)3Br was preparedaccording to a literature procedure [23].

Synthesis of polymer networks

Condensation of Melamine with 1,4-Diiodobenzene A two-necked round bottom Schlenk flask fitted with a con-denser, dropping funnel and a magnetic stir bar wascharged with melamine (0.94 g, 7.45 mmol) ,bromo(triphenylphosphine)copper(I) (1 g, 1.1 mmol), ce-sium carbonate (2.5 g, 7.7 mmol) and dimethylformamide(80 ml). After degassing by nitrogen bubbling, the mixturewas stirred under reflux for a few minutes (typically 5 min),then a solution of 1,4-diiodobenzene (0.85 g, 2.6 mmol) in5 ml of DMF was added dropwise to the flask. The reactionmixture was heated to 200 °C for 72 h, under an inert atmo-sphere, with continuous stirring.

Condensation of Melamine with 1,4-Dibromobenzene Thereaction was performed using a procedure similar to the

previous one, except that 1,4-diiodobenzene was replaced by1,4-dibromobenzene (0.6 g, 2.6 mmol).

Condensation of Melamine with 1,3,5-tribromobenzene Usinga similar reactor, melamine (1.02 g, 8.1 mmol),bromo(triphenylphosphine)copper(I) (0.75 g, 0.8 mmol), ce-sium carbonate (1.95 g, 6 mmol) and dimethylformamide(80 ml) were mixed together and degassed by nitrogen bub-bling. The mixture was stirred under reflux for a few minutes(typically 5 min), then a solution of 1,3,5-tribromobenzene(0.425 g, 1.345 mmol) in 5 ml of DMFwas added dropwise tothe flask. The reaction mixture was heated to 200 °C for 72 h,under an inert atmosphere, with continuous stirring.

Work-up of Reactions and Purification of Polymers Followingeach condensation reaction, the polymer was collected andextracted with DMF in a Soxhlet apparatus until the solventaround the thimble was visually colorless for at least one day.The polymers were collected by filtration, and rinsed withwater. The polymers were then rinsed with diethyl ether beforebeing dried in vacuum at 60 °C.

Characterization

Fourier transform infrared (FTIR) Infrared spectra (4000 –400 cm−1) were recorded on a Jasco FT/IR-4100 instrument asKBr pellets.

Solid State NMR Solid-state NMR spectra were measured ona TecMag/Bruker 300 “wide bore” spectrometer. For cross-polarization magic angle spinning (CP-MAS) 13C NMR spec-trum: the employed frequency was 75.49 MHz, the magicangle spinning rate was 3.8 kHz, the contact time was 3 ms,the relaxation delay was 3 s, and the external reference usedwas Glycine. For CP-MAS 15N NMR spectrum: theemployed frequency was 30.42 MHz, the magic angle spin-ning rate was 3.8 kHz, the contact time was 3 ms, the relax-ation delay was 3 s, and the external reference used wasGlycine. All spectra were acquired at room temperature. Theanalysis of the spectra was carried out using Tecmag software.

Thermal gravimetric analysis

Thermal gravimetric analysis (TGA) measurements were exam-ined using a SDT 2960 Simultaneous DSC-TGA from roomtemperature to 800 °C and a heating rate of 10 °Cmin−1 underN2.

Sorption isotherms: surface areas, total pore volumes,and H2-uptakes

The specific surface area (BET) and total pore volume (PV) ofthe polymers were determined from nitrogen adsorption iso-therms measured at 77 K using Quantachrome NOVA

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Automated Gas Sorbometer (NOVA 2200e). All sampleswere degassed at 150 °C overnight, under dynamic vacuum,before performing the tests.

The hydrogen storage capacities measurements of sampleswere determined at 77 K over a pressure range up to 20 barwith an Intelligent Gravimetric Analyzer (IGA-003, Hiden)that incorporates a microbalance. Degassing of all polymerswas performed at 150 °C overnight, under dynamic vacuum,before performing the tests.

Results and discussion

Preparation of porous polymers

In this study, we aimed to prepare networks consisting of aro-matic rings held in a porous framework by the smallest possiblelinking groups. The reasons for this approach are two-fold. First,to increase the structure rigidity, since it has been proved thatporous polymers prepared using smaller, more rigid cross-linksexhibit higher surface areas than those synthesized with longer,less-rigid cross-links [7]. Second, smaller cross-links add lessnon-adsorbingmass to the final porousmaterial.We also wantedto avoid the presence of electronwithdrawing substituents on thearomatic rings because they decrease the ability of materials tophysisorb hydrogen [7]. The simplest approach to prepare anetwork consisting of aromatic rings starts with precursors con-taining reactive groups directly attached to the aromatic ring.Based on the recent successful use of Cu(PPh3)3Br as a catalystfor the formation of aryl-nitrogen bonds [23], we relied on thistechnique to held the aromatic rings in porous networks. Asshown in Scheme 1, using the Ullmann reaction catalyzed withsoluble copper salt [23], polymeric networks were prepared bycondensation of melamine (1) with 1,4-diiodobenzene (2), 1,4-dibromobenzene (3) and 1,3,5-tribromobenzene (4). The final

products are in fact networks of aromatic and triazine ringsconnected through nitrogen atoms (Scheme 1).

Structure analysis of polymer networks

Successful build-up of a three-dimensional polymeric networkand high conversion of monomers functional groups wasconfirmed by Fourier transform infrared (FTIR) spectroscopy.As shown in Fig. 1, the absence or sharp decrease in theintensity of bands attributed to the primary amine groups ofmelamine at 3469 and 3419 cm−1 (NH2 stretching) gives anindication of successful condensation reaction. The distinctbands corresponding to the quadrant ring stretching vibrations(1529 cm−1) and semi-circle ring stretching vibrations(1440 cm−1) of triazine [24] are present in the networksspectra, indicating the successful incorporation of melamineinto the network.

Fig. 1 FTIR spectra of polymer networks

Scheme 1 Components andreaction used to prepare nitrogen-linked porous networks ofaromatic rings, and representativestructures

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Due to the poor solubility of the synthesized polymers incommon solvents, the detailed analysis of their chemicalstructures were performed by 13C solid-state NMR spectros-copy. The 13C cross-polarization magic angle spinning (CP-MAS) NMR spectra of polymeric networks are shown inFigs. 2, 3 and 4.

Figure 2 shows the 13C CP-MAS NMR spectrum ofthe network produced by the condensation of monomers 1

and 2. The resonance at 160.50 ppm corresponds to thearomatic carbons of the triazine ring. The resonanceappearing at 134.64 ppm corresponds to the substituted car-bons of benzene aromatic rings, whereas that at 129.13 ppmcan be correlated to the unsubstituted carbons of benzenearomatic rings.

Figure 3 shows the 13C CP-MAS NMR spectrum of thenetwork produced by the condensation of monomers 1 and 3.

Fig. 3 CP-MAS naturalabundance 13C NMR spectrum ofmaterial (1+3)

Fig. 2 CP-MAS naturalabundance 13C NMR spectrum ofmaterial (1+2)

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The resonance at 128.58 ppm can be correlated to theunsubstituted carbons of benzene aromatic rings. Theresonance at 136.92 ppm corresponds to the substitutedcarbons in benzene aromatic rings, whereas the reso-nance appearing at 166.82 ppm corresponds to the aro-matic carbons of the triazine ring.

Figure 4 shows the 13C CP-MAS NMR spectrum ofthe network produced by the condensation of monomers1 and 4. The resonance at 173.33 ppm corresponds tothe carbons of the triazine ring. The resonanceappearing at 92.20 ppm corresponds to the unsubstitutedcarbons of benzene aromatic rings, whereas that at144.92 ppm can be correlated to the substituted carbonsof benzene aromatic rings.

Thermal stability

TGA curves of the polymer networks are displayed in Fig. 5.The polymers exhibit a small weight-loss process before250 °C, then a single major decomposition starting at a tem-perature above 250 °C for polymers (1+2) and (1+3), andstarting at 320 °C for polymer (1+4). The first gradual massloss can be related to out-gassing of moisture and solvent(DMF) trapped inside the polymers.

Porous properties and H2 storage capacities of networks

As the primary aim of this study was to identify polymernetworks with gravimetric hydrogen storage properties,

Fig. 5 TGA curves of polymernetworks

Fig. 4 CP-MAS naturalabundance 13C NMR spectrum ofmaterial (1+4)

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different monomers were used to produce polymer networkswith different porous properties and H2 storage capacities(Table 1).

The porous properties of the polymers were analyzed bynitrogen adsorption-desorption isotherms (Fig. 6). The

hysteresis, which is usually found for microporous networksduring desorption [25], is not observed. This indicates a veryhigh degree of crosslinking of small structural motifs.

The hydrogen storage capacity of each network samplewas determined from hydrogen adsorption isotherms at 77 Kand pressure up to 20 bar (Fig. 7). It is clear that H2 uptakes donot approach saturation even at a pressure of 20 bar, implyingthat higher uptake capacities are possibly attained at elevatedpressures.

Conclusions

In summary, we prepared porous polymer networks of aromaticrings through the Ullmann synthetic protocol by condensationof melamine with different aryl halides, which possess several

Fig. 7 Gravimetric H2 adsorptionisotherms of polymer networks upto 20 bar, at 77 K

Fig. 6 Nitrogen adsorption-desorption isotherms of polymernetworks. Solid lines foradsorption and dashed lines fordesorption

Table 1 Porous properties of polymeric networks, and their gravimetricH2 uptakes

Entry Composition BET surfaceareaa (m2/g)

PVb

(cm3/g)H2 uptake(1/20 bar) (wt.%)

1 (1+2) 19.8 0.033 0.16/1.17

2 (1+3) 11.6 0.019 0.12/1.04

3 (1+4) 6.9 0.015 0.10/0.94

a Calculated from nitrogen adsorption isotherms using the BETequation.b Total pore volume calculated from nitrogen adsorption at P/Po=0.95.

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unique characteristics: a facile one-pot synthesis, involvingcheap and simple monomers and catalyst, these materialsexhibiting gas sorption properties. Characterization techniques(IR spectroscopy, and 13C solid state NMR spectroscopy) wereused to identify the structure of the synthesized networks. Thenewly synthesized polymers exhibit hydrogen uptake capacitiesat 77 K up to 0.16 wt.% and 1.17 wt.%, at 1 and 20 bar,respectively.

The only attempt to produce a porous polymeric networkthrough Ullmann condensation using a soluble copper com-plex was reported by Germain et al. [22]. The H2 uptakevalues obtained in the current work are very similar to thoseclaimed by these authors (0.01-0.19 wt.% at 77 K, and1.2 bar), taking into consideration the difference in pressuresat which the H2 capacity values were measured.

Although the one-pot Ullmann condensation techniqueusing soluble copper catalyst has been proved to be asimple and efficient technique for the production of porouspolymeric networks, major challenges remain to be ad-dressed in order to increase significantly the surface areaand storage capacity. Potential improvements of these pa-rameters may lie on the use of different types of startingmaterials (e.g. polyamines and haloaryls). Also, attentionshould be paid to noncatalytic approaches because thecatalyst should be removed from the very small poresafter reaction completion.

Acknowledgments We thank the Fundação para a Ciência eTecnologia, Portugal, for financial support (Projects PTDC/EQU-EQU/110313/2009 and PEst-OE/QUI/UI0100/2013) and for a fellowship toC.S.B.G. (SFRH/BPD/64423/2009). We thank the Erasmus Mundusprogramme (WELCOME) for granting a scholarship to A.G.M.

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