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Synthesis and characterization of highly porous borazine-linked polymersand their performance in hydrogen storage application†

Thomas E. Reich, Karl T. Jackson, Sa Li, Puru Jena and Hani M. El-Kaderi*

Received 7th April 2011, Accepted 2nd June 2011

DOI: 10.1039/c1jm11455g

A new class of highly porous borazine-linked polymers (BLPs) has

been synthesized utilizing arylamines and boron trihalides. BLPs

exhibit Brunauer–Emmett–Teller (BET) surface areas in the range

of 503 to 1364 m2 g�1 and store up to 1.3 wt% of hydrogen with 7.1

kJ mol�1 isosteric heat of adsorption.

The synthesis of highly porous organic polymers with predefined

porosity has attracted considerable attention due to their potential in

a wide range of applications that encompasses gas storage and

separation, conductivity, and catalysis.1 Among their attractive

features are linkage by strong covalent bonds and usual composition

of light elements (C,N, B, O) which impart several alluring properties

such as low density, high porosity, and physical stability as in the case

of microcrystalline covalent-organic frameworks (COFs).2–6 In the

absence of reversible bond formation processes, covalent polymers

lack long range ordering and tend to be amorphous as in the case of

polymers of intrinsic microporosity (PIMs),7 porous aromatic

frameworks (PAFs),8 conjugated microporous polymers (CMPs),9

porous organic frameworks (POFs),10 and porous polymer networks

(PPNs).11 Nevertheless, such polymers can be synthetically targeted

to allow for well-defined cavities through the use of rigid building

blocks that direct growth of the polymer without the aid of tem-

plating agents.1 In addition to customized porosity, some polymeri-

zation processes can lead to pore wall functionalization that

significantly enhances gas uptake and selectivity as we have demon-

strated recently for the benzimidazole-linked polymer, BILP-1.12

Alternative methods for enhanced gas uptake (i.e. hydrogen) by

porous architectures can also be attained by including polarizable

building units to improve hydrogen-framework interactions.13 Along

this line, we sought after the inclusion of borazine (B3N3) as a func-

tionalized polar building block into porous organic polymers. Bor-

azine is isostructural to the boroxine units found in COFs prepared

by boronic acid self-condensation reactions2 and has been mainly

used for the fabrication of BN-based ceramics or in organic opto-

electronics.14–16 However, up to date, the use of borazine for the

Department of Chemistry, Department of Physics, VirginiaCommonwealth University, Richmond, VA, 23284-2006, United States.E-mail: helkaderi@vcu.edu; Fax: +1 804 828 8599; Tel: +1 804 828 7505

† Electronic supplementary information (ESI) available: Experimentalprocedures, characterization methods, and gas sorption studies. SeeDOI: 10.1039/c1jm11455g

This journal is ª The Royal Society of Chemistry 2011

preparation of porous polymers for gas storage remains

unprecedented.

With these considerations in mind we report herein on the first use

of borazine in the construction of highly porous borazine-linked

polymers (BLPs) featuring halide decorated cavities for investigation

of their impact on porosity and hydrogen uptake under cryogenic

conditions. Monomeric halogen-functionalized borazine molecules

have been prepared previously by thermal decomposition of aryl-

amines–boron trihalides in aprotic solvents at elevated tempera-

tures.17 In a similar fashion, treatment of 1,4-phenylenediamine or

1,3,5-tris-(40-aminophenyl)benzene in dichloromethane at �78 �Cwith BCl3 or BBr3 followed by reflux in toluene afforded BLP-1(Cl),

BLP-1(Br), BLP-2(Cl), and BLP-2(Br), respectively, as white

powders in good yields as depicted in Scheme 1 (ESI†). The use of

dichloromethane in the first step was essential to dissolve the amine

building units which have very poor solubility in toluene at �78 �Cwhile the use of reduced temperature conditions was employed to

prevent premature side reactions of the highly reactive boron triha-

lides with amines. The chemical composition and structural aspects

Scheme 1 Synthesis of BLP-1 and BLP-2 from in situ thermal decom-

position of arylamines with boron trihalides.

J. Mater. Chem., 2011, 21, 10629–10632 | 10629

Fig. 1 Nitrogen uptake isotherms (A); hydrogen isosteric heat of

adsorption (B). Hydrogen uptake isotherms at 77 K (C) and 87 K (D);

adsorption (filled) and desorption (empty).

Table 1 Porous properties and hydrogen uptake of BLPs

PolymerSABET/SALang

a/m2 g�1

Pvolb/

cm3 g�1

PSDc/nm

H2d

(wt%)Qst

e/kJ mol�1

BLP-1(Cl) 1364/1828 0.746 1.3 1.10 7.1BLP-1(Br) 503/730 0.303 1.3 0.68 7.1BLP-2(Cl) 1174/1699 0.649 1.3 1.30 7.2BLP-2(Br) 849/1221 0.571 1.3 0.98 7.5

a Calculated by BET/Langmuir methods. b Calculated from nitrogenadsorption at P/Po ¼ 0.9. c Calculated from NLDFT. d Calculated at77 K/1 bar. e Calculated from the virial method.

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were investigated by spectral and analytical methods while porosity

was examined by nitrogen sorption measurements. The BLPs were

isolated by filtration under an inert atmosphere and were washed

with dichloromethane and dried at 1� 10�5 torr/80 �C for 18 h prior

to analytical and spectral characterizations.

All BLPs are amorphous as evidenced by their powder X-ray

diffraction (PXRD) studies that were performed on both as-prepared

and activated samples. The phase purity and morphology of each

polymer were investigated by scanning electron microscopy (SEM)

which revealed irregular particles of�0.2 to 0.3 mm in size (ESI, S8†).

Furthermore, the micro-elemental analysis on activated BLPs further

verified their chemical composition and purity. To establish the

chemical connectivity and geometry of the boron sites, we collected

FT-IR spectra and carried out solid-state 11B multiple quantum

magic angle spinning (MQMAS) NMR experiments. FT-IR spectra

show significant depletion of the stretching and bending bands of

amine protons around 3420 cm�1 and 1610 cm�1, respectively, and

the formation of new bands �1487, 1406, and 819 cm�1 which are

characteristic of the B3N3 ring (ESI, S1–S4†).18 We have selected one

representative material for 11B and 13C NMR experiments. The data,

which are very sensitive to the boron magnetic and chemical envi-

ronments, revealed one signal at 30 ppm with a quadrupole coupling

constant Qcc of 2.39 MHz (ESI, S5–S7†).19 This signal falls in the

reported range for tri-coordinated boron atoms in borazines, which

usually appear at �25 to 40 ppm, and are in sharp contrast to tetra-

coordinated boron which appears upfield (0 to �45 ppm). In addi-

tion, the Qcc value, which depends on the boron site symmetry, is in

agreement with trigonal sites (Qcc ¼ 2.8 MHz).19 The data collected

from these spectral experiments in conjunction with elemental anal-

ysis results (ESI†) clearly authenticate the formation of the borazine

building block for each of these polymers. To assess the thermal

stability of the BLPs, thermogravimetric analysis was conducted

under a flow of nitrogen. TGA traces show that BLPs remain stable

up to about 200 �Cwhere a major weight loss takes place which may

result from halogen evolution due to boron-halide bond decompo-

sition (ESI, S9–S12†). Therefore, BLPs are thermally less stable than

covalent organic frameworks or organic polymers which generally

remain stable up to �400 �C.2–6

To investigate the permanent porosity of BLPs, we carried out N2

sorption experiments at 77 K on activated samples. The activation

process involved guest molecule exchange with anhydrous dichloro-

methane followed by filtration then degassing at 80 �C/1� 10�5 torr

for 18 h prior to data collection. The N2 uptakes (Fig. 1A) display

type I isotherms with a sharp increase in the P/Po ¼ 0 to 0.1 range

which is indicative of their permanent andmicroporous nature, while

the final rise at P/Po ¼ 0.9–1.0 is due to nitrogen condensation in

macro- and meso-porous interparticulate cavities. A very minor

hysteresis for all samples is consistent with their fine powder nature.

The Brunauer–Emmett–Teller (BET) surface areas (Table 1) were

higher for the Cl-decorated materials BLP-1(Cl): 1364 and BLP-2

(Cl): 1174 m2 g�1 than the corresponding Br-decorated BLP-2(Br):

849 and BLP-1(Br): 503 m2 g�1. Pore size distribution (PSD) was

calculated using Non-Local Density Functional Theory (NLDFT)

and found to be centered about 1.3 nm (ESI, S14, S18, S22, and

S26†). These values are inline with 2D COFs such as CTF-15a

(SABET¼ 791m2 g�1; PSD¼ 1.2 nm) andCOF-12c (SABET¼ 711m2

g�1; PSD¼ 0.9 nm). The irregular trend in surface area values (Table

S1†) may arise from solid state packing that can lead to a more

accessible surface as indicted by a recent theoretical investigation of

10630 | J. Mater. Chem., 2011, 21, 10629–10632

2D COFs.2d The inclusion of Br into the frameworks of BLPs

significantly reduces pore volume (Table 1). Despite the structural

similarity between borazine building units of BLPs and their boroxine

counterparts in COFs, the reactive nature of the boron-halide bonds

might promote the formation of cross-linked and hyper-branched

polymers. This kind of reactivity in addition to the more robust B–N

linkage might be the reasons behind the amorphous nature of all

BLPs reported in this study. Additionally, the large halide atoms

might sterically preclude the close stacking of 2D sheets typically

found in crystalline COFs. This stacking aversion would result in

more accessible sheets and explain why BLPs exhibit higher surface

areas than their analogous COFs. Nevertheless, estimating porosity

and investigating potential solid-state packing for amorphous organic

polymers have been established using materials modeling that takes

advantage of targeted synthesis where the topology of the system is

rationalized based on the geometric shape of employed building

units.8 Accordingly, we considered two solid-state arrangements for

each polymer where the borazine ring participates in the formation of

eclipsed and staggered 2D sheets to form hexagonal channels in

a fashion that has been thoroughly investigated for 2D COFs. The

BLPs’ models were constructed using Materials Studio and their

geometry was optimized using Forcite (ESI†).20 Interestingly, all

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eclipsed conformations lead to channels of �1.2 nm which are very

similar to the experimental PSD values reported in Table 1 (�1.3 nm)

using the NLDFT method. These results point out BLPs may adopt

a short-range eclipsed conformation similar to CTF-1 and in contrast

to COF-1 which exhibits ‘‘graphite’’-type packing with noticeably

narrower pores (�0.9 nm). Worth noting is the estimated pore width

for an eclipsed COF-1 model (�1.5 nm) is fairly similar to the PSDs

of BLPs given the large size of the Cl and Br that decorate the pores

of BLPs.

Extensive studies on hydrogen storage have been reported recently

because of its renewable and clean aspects that make it very attractive

for future use in automotive applications.21 Physisorbed hydrogen

storage which takes advantage of high surface area materials, MOFs,

COFs, and organic polymers among others is very promising because

of the rapid uptake and release of hydrogen.6,22 However, the weak

interactions between hydrogen molecules and pore walls necessitate

the use of low temperatures and elevated pressure conditions to offset

hydrogen’s low heat of adsorption. Therefore, the ongoing quest for

new materials with enhanced storage properties remains a great

challenge and is tangibly beyond the targets set by the US Depart-

ment of Energy for 2015 (5.5 wt% hydrogen and 0.040 kg hydrogen

per L).23 We decided to investigate the potential use of BLPs in low

pressure hydrogen storage at 77 K and 87 K and calculated their

isosteric heat of adsorptions (Fig. 1). The H2 uptake listed in Table 1

was higher for BLPs of higher surface area, and the extent of halogen

decoration seems to have a modest impact on the final uptake at the

above mentioned conditions. All hydrogen isotherms are fully

reversible without a noticeable hysteresis, and the uptake ranges

between 0.68–1.30 wt% and 0.47–0.83 wt% at 77 K and 87 K,

respectively. The isosteric enthalpies of adsorption (Qst) for H2 were

calculated from these adsorption data using the virial method24 to

determine the interaction between H2 molecules and BLP networks.

At zero-coverage, theQst values ranged between 7.1 and 7.5 kJ mol�1

and drop gradually to reach values that fall in the range of 5.2 to 5.7

kJ mol�1 at higher loading (7 to 13 mg g�1) as illustrated in Fig. 1B.

These Qst values are similar to values reported for organic polymers

such as POFs (5.8–8.3),10 2D COFs (6.0–7.0 kJ mol�1),6 polyimide

networks (5.3–7.0 kJ mol�1),17a PPNs (5.5–7.6 kJ mol�1),11 and BILP-

1 (7.9 kJ mol�1)12 and are much higher than values reported for 3D

COFs: COF-102 (3.9 kJ mol�1), COF-103 (4.4 kJ mol�1), and the

carbon-based porous aromatic framework PAF-1 (4.6 kJ mol�1).8

Despite their amorphous nature, the performance of BLPs in

hydrogen storage is very comparable with other organic polymers of

similar surface areas.22 For example under similar conditions the

analogous crystalline COF-1 and CTF-1 store 1.28 and 1.55 wt% of

H2, respectively.5,6 It was also relevant to compare the hydrogen

uptake by BLPs with those of purely organic polymers such as

hypercrosslinked polymer networks synthesized by the self-conden-

sation of bischloromethyl monomers (0.89–1.69 wt%),25 nitrogen-

linked nanoporous networks of aromatic rings (0.01–0.85 wt%),26

and polymers of intrinsic microporosity (PIMs) which recently have

been reported to be among the best organic polymers for hydrogen

uptake (0.74–1.83 wt%).27 Moreover, BLPs show similar H2 uptake

as those reported for crystalline and high surface area COFs (0.9–1.2

wt%) or the amorphous porous aromatic framework (PAF-1, 1.5 wt

%).5,6 In contrast, activated carbon such as PICACTIF-SC, AX-21,

and zeolite-templated show a noticeably higher uptake (1.90, 2.40,

and 2.60 wt%, respectively) due to their ultrafine pores which are

typically less than 1 nm in size.28

This journal is ª The Royal Society of Chemistry 2011

In conclusion, we have prepared four new porous borazine-linked

polymers by thermolysis of arylamines–boron trihalides and investi-

gated their porosity and hydrogen uptake under cryogenic condi-

tions. The resultant polymers exhibit high surface areas and feature

halide-decorated cavities which should open the door for selective gas

uptake and post-synthesis modification studies. The hydrogen sorp-

tion experiments indicate that BLPs have moderate low-pressure

hydrogen storage capacities and display somewhat higher hydrogen

isosteric heat of adsorption; however, higher pressure conditions are

required to fully explore the potential of BLPs in hydrogen storage

applications. The flexibility in the boron trihalide type and arylamine

building units allows for a myriad of possible new BLPs which may

lead to enhanced porosity and increased gas storage capacity.

Acknowledgements

We are grateful to the US Department of Energy, Office of Basic

Energy Sciences (DE-SC0002576) for generous support of this

project. H.M.E. acknowledges support of the Donors of the Amer-

ican Chemical Society Petroleum Research Fund ACS-PRF (48672-

G5).

Notes and references

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