Anchor installation on porous polymer networks (PPNs) for high CO2 uptake

Article abstract of DOI:10.1016/j.polymer.2017.07.028

Porous polymer networks (PPNs) is an emerging category of advanced porous materials that are of interest for carbon capture due to their high physicochemical stability and convenient functionalization process. Herein, a series of alkylamine tethered PPN-200 (PPN-200-DETA, PPN-200-TETA and PPN-200-TAEA) were prepared through a novel post-synthetic strategy. Due to the presence of alkylamine groups, PPN-200-TAEA has CO₂ uptakes of 42 cm³/g (at 298 K and 0.15 bar) and 55 cm³/g (at 298 K and 1 bar), and a calculated CO₂/N₂ selectivity of 289, which demonstrate its potential in postcombustion carbon capture application.

Full text of DOI:10.1016/j.polymer.2017.07.028

Accepted Manuscript
Anchor installation on porous polymer networks (PPNs) for high CO uptake
2
Xinyu Yang, Lanfang Zou, Hong-Cai Zhou
PII:
S0032-3861(17)30687-0
10.1016/j.polymer.2017.07.028
JPOL 19840
DOI:
Reference:
To appear in: Polymer
Received Date: 10 January 2017
Revised Date: 29 June 2017
Accepted Date: 9 July 2017
Please cite this article as: Yang X, Zou L, Zhou H-C, Anchor installation on porous polymer networks
(PPNs) for high CO uptake, Polymer (2017), doi: 10.1016/j.polymer.2017.07.028.
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Anchor Installation on Porous Polymer Networks (PPNs) for High CO2 Uptake
Xinyu Yang,a Lanfang Zou,a Hong-Cai Zhoua, b, *
a Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
b Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77842, United
States
X.Y and L.Z contributed equally to this work
Corresponding author: zhou@chem.tamu.edu
Abstract
Porous polymer networks (PPNs) is an emerging category of advanced porous materials that are of interest for
carbon capture due to their high physicochemical stability and convenient functionalization process. Herein, a
series of alkylamine tethered PPN-200 (PPN-200-DETA, PPN-200-TETA and PPN-200-TAEA) were prepared
through a novel post-synthetic strategy. Due to the presence of alkylamine groups, PPN-200-TAEA has CO2
uptakes of 42 cm3/g (at 298 K and 150 mbar) and 55 cm3/g (at 298 K and 1 bar), and a calculated CO2/N2
selectivity of 289, which demonstrate its potential in postcombustion carbon capture application.
1. Introduction
With the rapid increase of the global population and the industrialization, the consumption of energy is growing
explosively. Currently over 85% of the global energy demand is being supported by fossil fuels, which will
continue to play an important role in the foreseeable future.(1-4) The burning of the fossil fuels releases large
amounts of CO2 into the atmosphere, which has made the CO2 concentration in the atmosphere increase from 280
ppm to 390 ppm since the beginning of the industrial age.(5-8) The increase of the CO2 concentration affects the
incoming and outgoing energy in the atmosphere, resulting in an increase of the average atmospheric temperature.
Though researchers are exploring cleaner forms of energy (such as solar energy and hydrogen) (9-11) to replace the
fossil fuels, these are far from practical application. Subsequently, strategies for carbon capture and sequestration
(CCS) are urgently required.(12, 13) In real applications, the combustion of fossil fuels in air generates flue gas
consisting of a majority of N2 with 15% CO2 and other minor components such as H2O, CO, NOx and SOx; thus,
CO2-uptake capacity at about 0.15 bar is more relevant to realistic postcombustion applications. Aqueous
alkanolamines, such as monoethanolamine (MEA) solutions, have been utilized as postcombustion “wet scrubbing”
methods in power plants due to their large CO2 capacity and selectivity. However, this method suffers from many
complications, such as high regeneration energy, toxicity and the corrosive nature of amines.
As an alternative choice, the solid porous materials are very promising for CCS since they can overcome the
downsides of aqueous alkanolamine solutions. During the past decades, metal−organic frameworks (MOFs) have
been studied extensively due to their high surface area, tunable pore size, adjustable functionalities and the
potential in gas adsorption and separation.(14-16) Even though some MOFs with high densities of open metal sites
(Mg-MOF-74) or high amounts of amine loading (mmen-MOF-74) have shown high CO2 uptakes, the practical
application of most MOFs is limited by their poor physicochemical stability and, particularly, by sensitivity to
water.(17-19)
PPNs are another class of very promising candidate materials for carbon capture, which are constructed from
rigid monomers through covalent bonds and possess permanent porosity.(20-25) Due to their covalent connections,
PPNs normally display exceptional chemical and water stability, enabling them to be easily functionalized by
pre-synthetic or post-synthetic methods.(20-23) The introduction of alkylamines into PPNs can significantly
enhance the CO2 adsorption and CO2/N2 selectivity by utilizing the polarizability and quadrupole moment of CO2,
which have been demonstrated by Zhou and co-workers.(20) Nevertheless, till now, there are few works that
present novel methods of introducing alkylamines into PPNs.
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Herein, a novel post-synthetic strategy was designed to construct a series of alkylamine tethered PPNs, which
demonstrated high carbon capture capability and CO2/N2 selectivity. First, PPN-200 was synthesized by
Suzuki-Miyaura cross-coupling reaction using L1 and 1,4-phenylenediboronic acid. The default diamondoid
framework topology (dia topology) constructed by the combination of tetrahedral unit and linear unit provides
widely open and interconnected pores to efficiently eliminate the “dead space”. More importantly, the extremely
robust scaffold of PPN-200 makes it an ideal platform for introducing CO2-philic groups under harsh reaction
conditions. By reaction with N-bromosuccinimide (NBS), PPN-200 was modified to give PPN-200Br. The
methylbromide groups can be further substituted by alkylamines (diethylenetriamine (DETA), triethylenetetramine
(TETA) and tris(2-aminoethyl)amine (TAEA)) to produce PPN-200-amines (PPN-200-DETA, PPN-200-TETA and
PPN-200-TAEA) (Scheme 1). The resulting alkylamine tethered PPN-200s showed significant increases in CO2
uptake capacities (~55 cm3/g at 298 K and 1 bar) and high CO2/N2 selectivity (289).
Scheme 1. The synthetic route of PPN-200amines.
2. Experimental
2.1 Materials
All chemicals were purchased from Sigma-Aldrich or Acros Organics and were used as received without
further purification.
2.2 Instruments
Fourier transform infrared measurements (FT-IR) were performed on a SHIMADZU IR Affinity-1
spectrometer. Nuclear magnetic resonance (NMR) data were collected on a Mercury 300 spectrometer. N2 and
CO2 adsorption–desorption isotherms were measured using a Micromeritics ASAP 2020 system at different
temperatures. Powder X-ray diffraction (PXRD) was performed with a BRUKER D8-Focus Bragg–Brentano
X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at 40 kV and 40 mA.
Thermogravimetric analysis (TGA) was conducted on a TGA-50 (SHIMADZU) thermogravimetric analyzer
from room temperature to 600 °C at a ramp rate of 5 °C/min in a flowing nitrogen atmosphere. Scanning
electron microscope (SEM) was performed on QUANTA 450 FEG and energy dispersive X-ray spectroscopy
(EDS) was carried out by X-Max20 with Oxford EDS system equipped with X-ray mapping
2.3 Synthesis of 2,2’,4,4’,6,6’-hexamethylbiphenyl
To a 1000 mL three-necked flask containing 7.0 g of magnesium pieces, degassed anhydrous tetrahydrofuran
(THF, 600 mL) and a pinch of iodine were added under a nitrogen atmosphere. The resulting mixture was
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heated to 80 °C, and then 2-bromomesitylene (35 mL) was added dropwise. The reaction mixture was refluxed
for another 3h. After it was cooled down to room temperature, a mixture of anhydrous FeCl3 (1.1 g), 1,
2-dibromoethane (12 mL) and anhydrous THF (15 mL) was added under a nitrogen atmosphere. Stirring was
continued for another 1 h, and then the reaction was quenched by the addition of hydrochloric acid. Organic
solvents were evaporated under reduced pressure. The residue was extracted with dichloromethane. The
dichloromethane phase was dried over anhydrous MgSO4 and filtered. Most of dichloromethane was removed
under reduced pressure, and then methanol was added. 2,2’,4,4’,6,6’-hexamethylbiphenyl was collected as
white solid. (8.5 g, 31% yield). 1H NMR (300 MHz, CDCl3) δ: 6.93 (s, 4H), 2.33 (s, 6H), 1.86 (s, 12H).
2.4 Synthesis of 3,3’,5,5’-tetraiodo-2,2’,4,4’,6,6’-hexamethylbiphenyl (L1)
To a mixture of 2,2’,4,4’,6,6’-hexamethylbiphenyl (4.0 g), solid iodine (7.0 g) and H5IO6 (3.1 g) in a 500 mL
flask, CH3COOH/H2O/H2SO4 was added (240/48/7.2 mL). The resulting mixture was stirred at 90°C for 3 days.
The reaction mixture was diluted with large amount of water. The precipitate was filtered and washed with
water. The pink solid was collected and dissolved in CHCl3, then washed with a saturated Na2S2O3 solution to
remove iodine residue. The organic phase was dried over anhydrous MgSO4, filtered and evaporated under
reduced pressure to produce 3,3’,5,5’-tetraiodo-2,2’,4,4’,6,6’-hexamethylbiphenyl as a white solid (10.6 g, 85%
yield). 1H NMR (300 MHz, CDCl3) δ: 2.05 (s, 12 H), 3.02 (s, 6 H).
2.5 Synthesis of PPN-200
3,3’,5,5’-tetraiodo-2,2’,4,4’,6,6’-hexamethylbiphenyl (148.2 mg), 1,4-phenylenediboronic acid (66.4 mg), CsF
(1.216g) and Pd(PPh3)4 (20mg) were charged in a three-necked round bottom flask, followed by three
evacuation and refill cycles with nitrogen on a Schleck line. A 20 ml degassed mixture of dioxane/water (9:1 in
volume) was added to the reaction flask through a cannula. The mixture was stirred at 100 °C under a N2
atmosphere for 48 h. After cooling to room temperature, the mixture was filtered and the solid was washed
with DMF (30 ml × 3), methanol (30 ml × 3), H2O (30 ml × 3), acetone (30 ml × 3) and dried in vacuo to
obtain PPN-200 (71.4 mg, yield 92.4%) as pale yellow solid.
2.6 Synthesis of PPN-200Br
PPN-200 (280 mg), N-bromosuccinimide (NBS) (0.852 g) and benzoyl peroxide (BPO) (100 mg) were
combined in anhydrous CCl4 (150 ml), and the mixture was refluxed for 48 h. After cooling to room
temperature, the mixture was filtered, and the solid was washed with DMF, methanol, H2O and acetone and
dried in vacuo to obtain PPN-200Br (380 mg, yield 28%).
2.7 Synthesis of PPN-200-amines
PPN-200Br (80 mg) in 20 ml amine (DETA/TETA/TAEA) was heated to 90℃ for 72 h. The resulting solid was
centrifuged and washed with DMF (30 ml × 3), methanol (30 ml × 3), H2O (30 ml × 3), acetone (30 ml × 3)
and then dried in vacuo to afford the final product. (PPN-200-DETA, yield 85%; PPN-200-TETA, yield 81%;
PPN-200-TAEA, yield 96%)
3. Results and discussion
A high surface area is critical for PPN-200 to investigate its properties and is also essential for postsynthetic
modification. So we first optimized the synthetic conditions of PPN-200 by a series of control experiments (Figures
1a-1c, Table S1). The first series of experiments were designed to determine an appropriate solvent system. We
chose different ratios of dioxane and water as solvents, ranging from 20:0, 18:2, 16:4, 14:6, 12:8 to 10:10. As
shown in Figure 1a, to obtain a porous polymer, it was essential to utilize a mixture of dioxane and water. If pure
dioxane was used, we only observed nonporous polymers, whose N2 adsorption was very close to 0, due to the poor
solubility of base in dioxane. But, if too much water was used, the decreasing solubility of L1 in the mixed solvent
led to the decreased N2 adsorption of PPN-200, which peaked at the ratio of 18:2. After determining a suitable
solvent, the next step was to optimize the type and amount of base. From the Figure 1b and Figure 1c, we can see
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that the porosities of PPN-200 synthesized by using CsF were much higher than those with K2CO3. With the
increase of the amount of CsF, the N2 adsorption of PPN-200 increased and peaked at 10 equivalents; after that, the
N2 adsorption decreased as more CsF was used (Figure 1c).
By combining best synthetic conditions, we determined the PPN-200 with highest N2 adsorption (486 m3/g)
under 1 bar with an adsorption/desorption hysteresis loop (Figure 1d). Its Langmuir surface area,
Brunauer-Emmett-Teller (BET) surface area, and pore volume are 1483 m2/g, 944 m2/g, and 0.48 cm3/g,
respectively. Its pore size distribution was calculated by the Density Functional Theory (DFT) method (Figure 2b).
Its pore diameters are widely distributed from 10 Å to more than 60 Å, and no pattern could be identified from the
pore size distribution plot, which implies the amorphous nature of PPN-200. This can also be confirmed by powder
X-ray diffraction (PXRD) pattern (Figure S1) and scanning electron microscope (SEM) (Figures S8-S12). The most
prominent pores have diameters of 11 Å and 14 Å, which is consistent with the microporous nature of PPN-200.
Figure 1. N2 sorption at 77 K with changing solvent (a), different amount of K2CO3 (b) and CsF (c), (d) N2
isotherm of optimized PPN-200.
After acquiring the highly porous PPN-200, we brominated it by using NBS to form the active intermediate
PPN-200Br, which followed by substitution with excess alkylamines to yield PPN-200-amines (PPN-200-DETA,
PPN-200-TETA and PPN-200-TAEA). The PPN-200Br and PPN-200-amines showed dramatic decreases in N2
uptake (Figure 2a), which confirm that the functionalization occurs within the cavities. The occurrence of amine
substitution was confirmed by TGA (Figures S3-S7), EDS (Table S2) and infrared spectroscopy (IR) (Figure S2),
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from which we can clearly find there was a broad peak (from 3100 to 3500 cm-1) belonging to alkylamines. The
introduction of amine groups into PPN-200 resulted in materials with excellent CO2 adsorption at 298 K and low
pressures (Figure 2c). The trend of improvement in CO2 adsorption under low pressure (0.15 bar) in terms of
tethered amine groups was TAEA > TETA > DETA. Though the PPN-200-TAEA has the lowest N2 adsorption, it
presents the highest CO2 adsorption of all amine-tethered PPN-200s, which indicates the CO2 uptake capacity is
closely related to the amount of amine instead of the surface area. PPN-200-TAEA exhibits high CO2 adsorption:
42 cm3/g (8.3 wt%) at 298 K and 0.15 bar and 55 cm3/g (10.9 wt%) at 298 K and 1 bar. The CO2/N2 selectivity of
PPN-200-TAEA in flue gas conditions was evaluated by the ratio of the adsorbed gas quantity where the partial
pressure for CO2 is 0.15 bar and N2 is 0.85 bar. The calculated single-component CO2/N2 selectivity of
PPN-200-TAEA in flue gas at 298 K is 289 that is significantly larger than other N-riched PPNs.
Figure 2. (a) N2 isotherms of PPN-200, PPN-200Br, and PPN-200-amines, (b) pore size distribution of PPN-200,
and (c) CO2 adsorption of PPN-200Br and PPN-200-amines, (d) CO2 and N2 adsorption for PPN-200-TAEA at
298K.
4. Conclusion
In conclusion, we have successfully designed a novel strategy to synthesize a series of alkylamine functionalized
PPN materials (PPN-200-DETA, PPN-200-TETA, and PPN-200-TAEA), which demonstrated excellent carbon
capture properties. Among them, the CO2 uptake of PPN-200-TAEA reaches 42 cm3/g (8.3 wt%) at 298 K and 0.15
bar and its calculated CO2/N2 selectivity reaches 289. Given the outstanding physicochemical stability and high
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CO2 adsorption amount under 0.15 bar, the PPN-200-TAEA has great potential for practical application in
postcombustion carbon capture.
5. Acknowledgements
This work was supported by U.S. Department of Energy Office of Fossil Energy, National Energy Technology
Laboratory (DE-FE0026472) and as part of the Center for Gas Separations Relevant to Clean Energy Technologies,
an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy
Sciences under Award Number DE-SC0001015. The authors gratefully acknowledge Mr. Jeremy Willman for
proofreading.
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In this paper, a novel post-synthetic strategy was designed to construct a series of alkylamine
tethered PPNs (PPN-200-DETA, PPN-200-TETA, and PPN-200-TAEA), which demonstrated
excellent carbon capture properties. Among them, the CO2 uptake of PPN-200-TAEA reaches 42
cm3/g (8.3 wt%) and its calculated CO2/N2 selectivity reaches 289. Given the outstanding
physicochemical stability and high CO2 adsorption under 150 mbar, the PPN-200-TAEA has great
potential for practical application in a post-combustion CO2 capture.