Highly porous organic polymers bearing tertiary amine group and their exceptionally high CO2 uptake capacities

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Highly porous organic polymers bearing tertiary amine group and their exceptionally high CO2 uptake capacities Ruth Gomes, Asim Bhaumik

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Cite this article as: Ruth Gomes, Asim Bhaumik, Highly porous organic polymers bearing tertiary amine group and their exceptionally high CO2 uptake capacities, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j. jssc.2014.10.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly porous organic polymers bearing tertiary amine group and their exceptionally high CO2 uptake capacities Ruth Gomes and Asim Bhaumik* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata - 700032, India *Address for correspondence. E-mail: [email protected]

ABSTRACT We report a very simple and unique strategy for synthesis of a tertiary amine functionalized high surface area porous organic polymer (POP) PDVTA-1 through the copolymerization of monomers divinylbenzene (DVB) and triallylamine (TAA) under solvothermal reaction conditions. Two different PDVTA-1 samples have been synthesized by varying the molar ratio of the monomers. The porous polymeric materials have been thoroughly characterized by solid state

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C CP MAS-NMR, FT-IR and UV-Vis spectroscopy, N2 sorption,

HR TEM and FE SEM to understand its chemical environment, nanostructure, bonding, morphology and

related surface properties.

PDVTA-1 with higher amine content

(DVB/TAA=4.0) showed exceptionally high CO2 uptake capacity of 85.8 wt% (19.5 mmolg-1) at 273 K and 43.69 wt% (9.93 mmolg-1) at 298 K under 3 bar pressure, whereas relatively low amine loaded material (DVB/TAA=7.0) shows uptake capacity of 59.2 wt% (13.45 mmolg-1) at 273 K and 34.36 wt% (7.81 mmolg-1) at 298 K. Highly porous nanostructure together with very high surface area and basicity at the surface due to the presence of abundant basic tertiary amine N-sites in the framework of PDVTA-1 could be responsible for very high CO2 adsorption. 1

KEYWORDS CO2 storage materials; porous organic polymers; amine functionalization; BET surface area; surface basicity. 1. INTRODUCTION Carbon dioxide considered as the primary anthropogenic greenhouse gas accumulated through the human activities, is responsible for global warming and climate change, which are major threats to the mankind in near future. The level of CO2 in the atmosphere is increasing everyday through the combustion of fossil fuels and wastes, and also via several chemical reactions used in the industrial processes [1-3]. Thus, in order to decrease the CO2 level in atmosphere, its removal from flue-gas streams via adsorption or sequestration is very important. Although, commercial carbon dioxide capture systems are known but their capture quantity/capacity is much smaller than that of the demand. Hence, research on the design and synthesis of high CO2 adsorbent material for carbon capture and sequestration (CCS) is an emerging and first growing area of research [4-6]. In this context, CO2 responsive porous polymers [7], various functionalized microporous and mesoporous materials [8-13], zeolitic imidazole frameworks (ZIF) [14-16], porous organic polymers (POP) [17-22], cation exchanged zeolites [23], porous carbons [24-27], metal organic frameworks (MOF) [28-31] having high CO2 storage capacity utilizing the physical adsorption pathway have been reported in the recent times. Among these materials POPs are gaining further interest as suitable support for loading active metal ions to form highly reactive heterogeneous catalysts, which can catalyze a wide range of liquid phase chemical transformations [32,33]. Among these, porous organic polymers bearing electron donor nitrogen atoms such as 1°/2°/3° amine, imidazole, pyridine, etc. are particularly interesting as the lone electron pair of N 2

atom can interact with the Lewis acidic CO2 molecules [34-37]. Sayari et al have shown that amine loading and surface alkyl chains can significantly affect the CO2 adsorption amount among the amine functionalized porous materials [38-39]. On the other hand, for porous materials having high surface area and pore volume, the CO2 uptake is increased several times compared to their non-porous analogue as the adsorbate gas molecule can be efficiently trapped inside the micro- and mesopores of the large surface area material and suitable dipolar interaction could occur with the nitrogen-donor sites located at the surface as well as inside the pore. Herein, we report a simple strategy for the synthesis of very high surface area porous organic polymers (designated as PDVTA-1) bearing tertiary amine fraction via radical polymerization of different concentrations of two monomers divinylbenzene and triallylamine in the network under non-aqueous synthesis conditions. PDVTA-1 has been characterized by solid state MAS-NMR, Fourier transform infrared spectra (FT-IR), UV-Vis spectroscopy, high resolution transmission electron microscopy (HR TEM), field emission scanning electron microscopy (FE SEM), nitrogen sorption and CHN analysis. High surface area and basic nitrogen sites of the polymeric framework have been explored in CO2 adsorption. It is found that higher amine loading is responsible for larger CO2 uptake under identical conditions. 2. EXPERIMENTAL SECTION 2.1. Materials Divinylbenzene (DVB, monomer) and triallylamine (TAA, monomer) were obtained from Sigma Aldrich. Azobisisobutyronitrile (AIBN, radical initiator) was obtained from SRL and it was recrystallized from hot ethanol prior to use. All other chemicals used in the experiments were of analytical grade produced by E-Merk. 3

2.2. Instrumentation Quantachrome Autosorb 1-C was used to record the N2 adsorption/desorption isotherms of the porous organic polymers at 77 K. CO2 adsorption isotherm of the porous polymer was recorded by using a Bel Japan Inc. Belsorp-HP at 273 K. The sample was degassed at 373 K for 8 h under high vacuum condition prior to both of the above mentioned measurement.

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C CP

MAS NMR spectrum of the sample was obtained on a Bruker Advance 500 MHz NMR spectrometer using a 4 nm MAS probe under static condition (spinning rate 5000 Hz, with side band suppression). Nicolet MAGMA-FT IR 750 Spectrometer Series II was used to record the FT IR spectra of the sample. The UV-Visible diffuse reflectance spectra of the sample was recorded in a UV 2401PC with an integrating sphere attachment. BaSO4 was used as the background standard. High resolution transition electron microscopy (HR TEM) images of the mesoporous material were recorded using a JEOL 2010 TEM operated at 200 kV. Morphology and particle size of the samples was analyzed by using a Jeol JEM 6700 field emission scanning electron microscope (FE SEM). Elemental analyses of the materials were carried out using a Perkin Elmer 2400 Series II CHN analyzer. 2.3. Synthesis of porous polymer PDVTA-1 PDVTA-1 materials were synthesized via non-aqueous polymerization of a mixture of divinylbenzene and triallylamine under solvothermal condition using AIBN as a radical initiator [40]. The synthetic procedure for preparation of the porous polymer has been illustrated in Scheme. 1. In a typical synthesis for PDVTA-1, 781 mg divinylbenzene (6 mmol) and 206 mg triallylamine (1.5 mmol) were taken in 15 ml acetone in a round bottom flask and the mixture was continuously stirred for 15 min. The flask was purged with nitrogen gas and 25 mg AIBN (0.15 mmol) was added to the mixture and resulting slurry was stirred for 4 h under nitrogen 4

atmosphere at room temperature. Then the resultant mixture was autoclaved at 393 K for 24 h without stirring. The final product was washed thoroughly with acetone and dried in air. Five different PDVTA-1 samples were prepared by varying the molar ratio of divinylbenzene and triallylamine. With increasing the percentage of triallylamine from 0% to 20% gradually increase the surface area of the polymer, while the polymer remains hydrophobic in nature as that of pure divinylbenzene polymer. But further increase in the percentage of triallylamine (beyond 20%) leads to a decrease in the surface area and the polymer turns hydrophilic. The polymer PDVTA-1 (1), with highest surface area and another polymer PDVTA-1 (2) (molar ratio of divinylbenzene and triallylamine in the synthesis mixtures 80:20 and 87.5:12.5, respectively) were further characterized and their CO2 adsorption behaviors were studied. It is pertinent to mention that there is not much difference between the materials PDVTA-1(1) and (2), as far as molecular structure is concerned (Scheme 1). Only they differ in the molar composition of monomer units in the framework, former being rich in triallyl amine moieties. 2.3. Acid-base titration for surface basicity of the polymers The surface basicity of the polymers PDVTA-1(1) and PDVTA-1(2) were estimated by acid-base titration using (M/100) oxalic acid and standardized (M/100) NaOH solutions. The materials were dispersed in oxalic acid solutions and stirred for 2 h. After filtration, the concentrations of the solutions were measured by titrating with NaOH and the basicity of the materials were calculated from the change in acidity of the oxalic acid solutions over that in absence of PDVTA-1(1) and PDVTA-1(2). The estimated basicity for the polymers PDVTA1(1) and PDVTA-1(2) were 1.625 mmolg-1 and 0.989 mmolg-1, respectively.

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3. RESULTS AND DISCUSSION 3.1. Solid state NMR and chemical environments at different C atoms The chemical environment of different C atom in the polymeric network of PDVTA-1 and the presence of various organic functional groups have been confirmed by 13C MAS NMR experiment. The 13C CP MAS NMR spectrum for the porous polymer PDVTA-1(1) is shown in Fig. 1a, which exhibit strong signals at 28.5, 41, 48, 126.9, 137 and 144 ppm due to different aliphatic and aromatic C atoms present in the polymer network as shown in the structure of Fig. 1. The weak signal at 111 ppm indicates the presence of little amount of unreacted double bonds of the substrates. Further, the weak signal at 15 ppm resembles the the aliphatic sp3 C atom as shown in the framework structure of PDVTA-1 in Fig. 1. The

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C CP MAS NMR spectrum of

PDVTA-1(2) (Fig 1b), also exhibits similar chemical shifts of these carbon atoms as that of PDVTA-1(1). Thus, this

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C CP MAS NMR analysis confirms the almost complete co-

polymerization of divinylbenzene and triallylamine in PDVTA-1. The ratio of aromatic to aliphatic carbons in PDVTA-1(1), obtained through intigrating the area under the peaks in aromatic and aliphatic regions, is 1.25, whereas that for PDVTA-1(2) is 1.43. This clearly indicates that the polymer PDVTA-1(1) has greater percentage of aliphatic carbons corresponding to the higher amount of triallylamine used compared to PDVTA-1(2). 3.2. Spectroscopic and elemental analysis Fig. 2a illustrates the FT IR spectrum of the polymer PDVTA-1(1), which exhibit absorption peaks at 3100-3000, 2932, 2848, 1605, 1512, 1449, 1370, 794 and 710 cm-1 [41,42]. The peaks at 3100-3000 cm-1 are due to the aromatic stretching vibration whereas the peaks at 2932 and 2848 cm-1 correspond to the methylene C-H stretching vibration associated with a

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strong peak at 1449 cm-1. The peak at 1512 and 1605 cm-1 originates from the aromatic C=C stretching vibration of the benzene ring. The peak at 1370 cm-1 could be attributed to the C-N stretching. The other peaks at 794 and 710 cm-1 comes from the aromatic C-H bending vibration. The FT-IR spectra of PDVTA-1(2) (Fig 2b) exhibits similar adsorption peaks as that of PDVTA1(1). Further, the UV-Vis spectra (Fig. 3) of porous PDVTA-1(1) shows strong peaks at 218, 261, 291, 329 nm corresponding to different aliphatic and aromatic chromophoric moieties. Elemental analysis reveals the C, H and N content of PDVTA-1(1) as: C = 88.95%, H = 6.56% and N = 2.15%, this result agrees well with theoretical concentration of 80:20 for divinyl benzene and triallylamine (theoretical value, C= 89.56%, H= 8.39%, N=2.04%), that have been used in the during synthesis. On the other hand, the elemental analysis of PDVTA-1 (2) gives C = 89.53%, H = 6.77% and N = 1.34% (theoretical value, C= 90.57%, H= 8.15%, N=1.28%). 3.3. Porosity and surface area measurement Radical polymerization of two olefin moieties of divinylbenzene with three alkyl branches of triallylamine in the presence of AIBN radical initiator could result the intrinsic porosity and void spaces due to rapid cross-linking [43]. Nitrogen adsorption/desorption studies of the porous polymers gives a typical type IV isotherm (Fig. 4) with a significant hysteresis loop in the high pressure region [44]. BET surface area and pore volume of PDVTA-1 samples 1 and 2 were 903 and 763 m2g-1 and 0.472 and 0.368 ccg-1, respectively. Nonlocal density functional theory (NLDFT) was employed to estimate the pore size distribution of PDVTA-1 (1) and PDVTA-1(2), and these are shown in Fig. 5, which indicates two types of pores having dimensions 1.5 nm and 4.1 nm, respectively. De Boer statistical thickness (t-plot) analysis gives the micropore and mesopore contributions to the surface area to be 779 m2g-1 and 123 m2g-1 respectively for PDVTA-1(1), suggesting highly microporous nature of the material. Further, we 7

have analyzed the nanostructure of this co-polymer through high resolution TEM and corresponding image is shown in Fig. 6. From this TEM image of PDVTA-1 micropores having dimension 1.3 nm is clearly seen throughout the polymer matrix. On the other hand the mesopore contribution to the surface area could be attributed to the interparticle porosity. Further, the FESEM image (Fig. 7) of the sample PDVTA-1(1) shows self-assembled spherical structures (dimension 1.8-2.6 µm) consisting of tiny spherical nanoparticles of the copolymer having of dimensions ca. 25 nm. From this FE SEM image the interparticle void between these tiny polymer nanoparciles is quite clearly visible and this corresponds to the average mesopores of ca 4.0 nm. This result agrees well with the BET analysis data (Fig. 5). 3.5. Carbon Dioxide Uptake CO2 adsorption isotherms of the polymers PDVTA-1 (1) and PDVTA-1 (2) at 273 K and 298 K are shown in Fig. 8. As seen from these isotherms, with the increase in the relative pressure of CO2, adsorption capacity goes on increasing monotonically. Although, at high pressure the rate of adsorption decreases considerably. The total CO2 uptake amount for PDVTA-1 (1) is 19.5 mmolg-1 (i.e. 85.8 wt %) at 273 K and 9.93 mmolg-1 (i.e. 43.69 wt %) at 298 K under 3 bar pressure, which is much higher compared to previously reported porous organic polymeric materials, whereas the CO2 uptake for PDVTA-1 (2) is 13.45 mmolg-1 (i.e. 59.2 wt%) at 273 K and 7.81 mmolg-1 (i.e. 34.36 wt%) at 298 K under the same pressure condition. Among the high CO2 adsorbent porous organic polymers, covalent triazine framework CTF-0 prepared through the trimerization of 1,3,5-tricyanobenzene showed an uptake of 4.22 mmolg-1 (15.7 wt %) [45]. To the best of our knowledge, the highest CO2 storage obtained over porous organic polymer till date is for BILP-4, which shows 5.45 mmolg-1 (i.e. 24 wt %) at 273 K at 1 bar pressure [46], whereas the CO2 adsorption amount for PDVTA-1 (1) under same 8

condition is 8.4 mmolg-1. Also the adsorption showed gradually increasing trend, suggesting that these materials will further adsorb CO2 upon increasing the pressure. Such high CO2 adsorption capacity of the materials can be explained on the basis of high surface basicity and surface area of the polymers. The isosteric heat of adsorption (Qst) for PDVTA-1(1) and PDVTA-1(2) were calculated from the adsorption isotherms at 273 K and 298 K using Clausius-Clapeyron equation the corresponding plots are shown in Fig. 9. The initial Qst values for PDVTA-1(1) and PDVTA1(2) were found to be 22.2 and 19.0 kJmol-1, respectively. These high initial Qst values suggest strong interaction of the CO2 molecules with the adsorbent materials. The slightly lower Qst value of PDVTA-1(2) compared to that of PDVTA-1(1) agrees well with the lower N-content, lower basicity and relatively lower surface area and pore volume of PDVTA-1(2) compared to PDVTA-1(1). 4. CONCLUSION In conclusion, we have reported an efficient synthesis method for high surface area porous organic polymers via copolymerization of divinylbenzene and triallylamine under nonaqueous polymerization pathway. The copolymer material (PDVTA-1) has been characterized thoroughly using

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C CP MAS-NMR spectroscopy, FT-IR and UV-Vis spectroscopy, N2

sorption and HR TEM. PDVTA-1 bearing higher amine fraction showed CO2 uptake capacity of 85.8 wt %, at 273 K and 43.7 wt % at 298 K, whereas lower amine loaded sample showed 59.2 wt% CO2 uptake at 273 K and 34.4 wt % CO2 uptake at 298 K under 3 bar pressure. High CO2 uptake over these porous organic polymers may contribute significantly in developing the efficient carbon capture and storage medium for the environmental remediation of greenhouse gas CO2.

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ACKNOWLEDGMENT RG thanks CSIR, New Delhi for a junior research fellowship. AB wishes to thank DST, New Delhi for instrumental facilities through DST Unit on Nanoscience and DST-SERB project grants. REFERENCES [1]

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Figure 1 [Gomes and Bhaumik]

Figure 1

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C CP MAS NMR spectrum of the porous polymer a) PDVTA-1(1) and b)

PDVTA-1(2).

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Figure 2 [Gomes and Bhaumik]

Figure 2

FT IR spectra of a) PDVTA-1(1) and b) PDVTA-1 (2).

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Figure 3 [Gomes and Bhaumik]

Figure 3

UV-Vis absorbance spectrum of PDVTA-1 (1).

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Figure 4 [Gomes and Bhaumik]

Figure 4

N2 adsorption/desorption isotherms of PDVTA-1(1) and PDVTA-1(2) at 77 K. Adsorption points are marked by filled circles and desorption points by empty cycles.

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Figure 5 [Gomes and Bhaumik]

Figure 5

Pore size distributions of PDVTA-1-(1) and PDVTA-1-(2) calculated from their respective N2 sorption isotherm employing NLDFT method.

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Figure 6 [Gomes and Bhaumik]

Figure 6

HR TEM images of the porous polymer PDVTA PDVTA-1.

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Figure 7 [Gomes and Bhaumik]

Figure 7

FE SEM images of the porous polymer PDVTA PDVTA-1.

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Figure 8 [Gomes and Bhaumik]

Figure 8

CO2 adsorption isotherm of PDVTA-1 (1) and PDVTA-1 (2) at 273 K and 298 K.

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Figure 9 [Gomes and Bhaumik]

Figure 9

Isosteric heat of adsorption (Qst) of PDVTA-1(1) and PDVTA-1(2) as a function of CO2 adsorbed.

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SCHEMES Scheme 1. Copolymerisation of divinylbenzene and triallylamine to obtain porous polymer PDVTA-1.

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Research Highlights •

Designing the synthesis of a new N-rich cross-linked porous organic polymer PDVTA-1.

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PDVTA-1 showed mesoporosity with very high surface area of 905 m2g-1

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High surface area and presence of basic sites facilitates the CO2 uptake.

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PDVTA-1 showed exceptionally high CO2 adsorption capacity of 85.8 wt% at 273 K, 3 bar pressure.

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