Cost-Effective Synthesis of Amine-Tethered Porous Materials for Carbon Capture Weigang Lu,[A] Mathieu Bosch,[A] Daqiang Yuan,[B] and Hong-Cai Zhou*[A]

CHEMSUSCHEM COMMUNICATIONS

DOI: 10.1002/cssc.201402622 Cost-Effective Synthesis of Amine-Tethered Porous Materials for Carbon Capture Weigang Lu,[a] Mathieu Bosch,[a] Daqiang Yuan,[b] and Hong-Cai Zhou*[a]

A truly cost-effective strategy for the synthesis of amine-teth- amine-functionalized porous materials, their selectivity towards ered porous polymer networks (PPNs) has been developed. CO2 could be drastically affected by the presence of water A network containing diethylenetriamine (PPN-125-DETA) ex- vapor; thus, they are only applicable for carbon capture if the hibits a high working capacity comparable to current state-of- water vapor is removed prior to the adsorption.[6] On the other art technology (30% monoethanolamine solutions), yet it re- hand, amine-functionalized porous materials have been shown quires only one third as much energy for regeneration. It has to have increased stability in the presence of water vapor, as also been demonstrated to retain over 90 % capacity after 50 the water vapor inhibits urea formation.[7] Thus, incorporation adsorption–desorption cycles of CO2 in a temperature-swing of alkyl amine groups could produce porous materials with not adsorption process. The results suggest that PPN-125-DETA is only high CO2 capture capacity at very low pressures but also a very promising new material for carbon capture from flue large CO2 selectivity over other small gas molecules, including gas streams. water vapor, which is of practical significance because flue gas contains about 6% water.[8] It has indeed been demonstrated that amine-tethered porous materials can achieve several-fold

Over the past 50 years, atmospheric CO2 concentrations have the working capacity of aqueous amine solutions with energy been increasing at an accelerating rate primarily due to the efficiency increased by a factor of roughly three on materials burning of fossil fuels to satisfy ever-growing energy demand. basis.[9]

Mounting evidence suggests rising CO2 emissions are already Aside from having comparable working capacity, being less producing economic damage, and the risk of catastrophic cli- energy intensive, and possessing tolerance to the occurrence mate change is getting worse.[1] Carbon capture and storage/ of moisture in the feed, the production cost and long-term sta- sequestration (CCS), a process including the capture of CO2 bility are also important factors for the new materials to re- from large point sources, such as fossil-fuel-/coal-burning place amine solutions. Herein, we report a truly cost-effective power plants, has been proposed to mitigate anthropogenic synthesis of an amine-tethered porous material, namely PPN- [2] CO2 emissions. Of all the strategies that could be applied to 125-DETA (PPN stands for porous polymer network; DETA power plants, post-combustion carbon capture has been the stands for diethylenetriamine). All the solvents and chemicals most explored strategy to date as the capture system could be used in the synthesis are inexpensive stock commodities; in readily retrofitted to existing power plants.[3] For a tempera- addition, there are no air- or moisture-sensitive reactions in- ture-swing adsorption (TSA) process, CO2 is adsorbed at mod- volved in the synthetic process. Under simulated flue gas con- erate temperatures (~40 8C) and later released by heating ad- ditions (15% CO2 balanced with N2,408C), the working capaci- sorbents up to high temperatures (~1208C). In this context, ty of PPN-125-DETA can reach 0.7 (3.0 wt%) and 1.0 mmol gÀ1 aqueous amine solution systems are a benchmark and (4.0 wt%) at desorption temperatures of 1008C and 1208C, re- a mature technology for the separation of dilute CO2 from gas spectively; the values are comparable to those of monoetha- streams, but the regeneration uses up to approximately 25– nolamine (MEA) aqueous solutions, which can achieve about 40% of the power produced by the power plant.[2b,4] The spe- 2.1–5.5 wt% depending on the scrubbing process used[5g, 10] cific heat capacity of aqueous amine solutions is notoriously (typically heating under flue gas conditions to about 1208C). high, which is the leading cause of this huge energy penalty. An initial study to understand the dynamic cycling behavior of

By shifting from conventional absorbing scrubbers to PPN-125-DETA shows that it can retain over 90 % of its CO2 porous solid materials the energy penalties can be mini- capture capacity after 50 adsorption–desorption cycles in [5] mized. Although high energy efficiency is expected for non- a temperature-dependent gravimetric adsorption under CO2 gas conditions. [a] Dr. W. Lu, M. Bosch, Prof. Dr. H.-C. Zhou PPN-125, a phenolic resin-type porous polymer, was synthe- Department of Chemistry sized from phloroglucinol (1,3,5-trihydroxybenzene) and ter- Texas A&M University College Station, TX, 77843 (USA) ephthalaldehyde in dioxane using HCl (aq) as a catalyst. The re- E-mail: [email protected] action was carried out at 1008C overnight in glassware instead [b] Prof. Dr. D. Yuan of over 2208C in an autoclave for four days as reported in the State Key Laboratory of Structure Chemistry literature.[11] The precipitate was collected and washed thor- Fujian Institute of Research on the Structure of Matter oughly with water and tetrahydrofuran (THF); the resulting Chinese Academy of Sciences Fuzhou, 350002 (P.R. China) reddish-brown material is rich in phenol groups, which can be Supporting Information for this article is available on the WWW under used as anchors to introduce amine chains. http://dx.doi.org/10.1002/cssc.201402622.

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To covalently attach amine chains inside this phenol-en- riched framework, a bifunctional linker with one end primed to react with amine and the other with phenol is desired. Our experiments showed that linkers with two identical or similar bonding sites, such as 1,2-dibromoethane, were poor perform- ers, probably because the bromine on either end could be substituted by phenols, resulting in a low density of bromines for subsequent amine substitutions. To avoid oversubstitution for the first step, a bifunctional linker with two ends having distinctive reaction behaviors should be judiciously selected. Epichlorohydrin, with chlorine on the one end and an epoxide functionality on the other end, appears to be the exact linker we are looking for. Our experiments demonstrated that phenol would only substitute the chlorine of epichlorohydrin if a weak organic base, such as triethylamine (TEA), was used (Scheme 1). Thus, the intact epoxide could be ring-opened

Figure 1. a) N2 sorption isotherms at À196 8C; b) CO2 sorption isotherms at 258C, adsorption (*)–desorption (*), PPN-125 (black), PPN-125-DETA (red).

friendly and easily scalable procedure with high potential for industrial application. Not surprisingly, the surface area decreased significantly along with epoxide introduction and subsequent amine addition; as a result, PPN-125-DETA shows a moderate BET surface area (229 m2 gÀ1), which is about one third of PPN-125 (Fig- Scheme 1. Synthetic procedure for PPN-125-DETA (PPN: porous polymer net- ure 1a). Nevertheless, its CO2 uptake at ambient temperature work; TEA: triethylamine; DETA: diethylenetriamine). and low pressures increased significantly. PPN-125-DETA can

take up 1.4 mmol (5.9 wt%) of CO2 at 258C and 0.15 bar, which using any alkyl amines at elevated temperatures. Moreover, ep- is about three times as high as PPN-125 (0.5 mmolgÀ1, ichlorohydrin is an inexpensive stock commodity and can be 2.1 wt%) under the same conditions (Figure 1b). PPN-125- used as a solvent for the reaction, which is extremely impor- DETA shows a typical chemisorption isotherm, exhibiting a pla- tant for the reaction to reach completion given that one of the teau at lower pressures associated with fast a adsorption rate reactants is solid. and a hysteresis loop associated with a slow desorption rate. PPN-125 was calculated to have a Brunauer–Emmett–Teller To better understand the adsorption properties, the isosteric 2 À1 (BET) surface area of 702 m g based on nitrogen gas adsorp- heat of adsorption–adsorption enthalpy (Qst) was calculated tion–desorption isotherms at À1968C (77 K, Figure 1a). The from CO2 adsorption isotherms at different temperatures and 2 À1 value is slightly lower than that of POF1B (917 m g ), which used to quantify the interaction between the material and CO2. was synthesized in an autoclave at a much higher tempera- A single-site Langmuir model was used to fit the CO2 adsorp- ture.[11c] We suspect that high temperature and high pressure tion isotherms of PPN-125. For PPN-125-DETA, on the other could tip the balance of reaction equilibrium in favor of the hand, the CO2 adsorption isotherms are much steeper in the polymeric product. In this work, however, the reaction condi- low-pressure regime (Figure 1b), which necessitates a multi- tions were deliberately chosen with a desire to provide a cost- site Langmuir model to properly fit the data.[12] In this case,

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stretch of formed ether in PPN-125-EPOXIDE, and the appear- ance of an alkyl CÀH vibration (2840–2990 cmÀ1) is clear evidence of successful substitution. The epoxide-function itself in PPN-125-EPOXIDE exhibits two bands at 907 cmÀ1, associated with epoxide-ring deformation vibration, and at 1254 cmÀ1, which is caused by its CÀO stretching vibration.[14] These two bands vanished after the epoxide ring was opened by DETA, strong evidence for a high yield of the epoxide–DETA adduct. In the context of post-combustion carbon capture from fossil-fuel-/coal-burning power plants, the regeneration of capture materials can be accomplished through pressure-swing adsorption (PSA), vacuum-swing adsorption (VSA), TSA, or a combination of these processes. From the point of view of

Figure 2. Isosteric heats of adsorption Qst for the adsorption of CO2 ; single- energy conservation, TSA is particularly promising because the site and triple-site Langmuir models were used to fit the CO2 adsorption iso- energy requirement for carbon capture in the TSA process therms of PPN-125 and PPN-125-DETA, respectively. could be significantly less than for the PSA or VSA processes due to the potential availability of low-grade heat in a power a triple-site Langmuir model is appropriate because the chemi- plant as a source of energy for regeneration.[15] A TSA cycle in- cal structure of PPN-125-DETA is made up of primary and sec- volves heating the saturated adsorbent under ambient pres- ondary amines, and the backbone (phenyl/ether) moieties, all sure to desorb the captured CO2 and regenerate the capture of which might cause distinctive adsorption behaviors for CO2. material. To find out the appropriate regeneration temperature,

Figure 2 shows a plot of the adsorption enthalpies as a function thermogravimetric analysis (TGA) for PPN-125-DETA in N2 or of CO2 loading. Particularly noteworthy are the high enthalpy CO2 atmosphere was evaluated, as we can see from Figures 4 values for PPN-125-DETA for high CO2 loadings, which is con- sistent with its large CO2 capacity at low pressures (Figure 1b). Without taking heat exchange into account, heat of adsorption and desorption temperature are the two most important pa- rameters for an optimal solid sorbent. The initial heat of adsorption of PPN-125-DETA is calculated to be 61 kJmolÀ1 (Figure 2), which is close to the estimated optimal heat of adsorption to achieve minimum energy consumption in the TSA process.[13] FTIR spectroscopy (Figure 3) was performed to analyze the chemical processes of epoxide introduction and subsequent

Figure 4. Thermogravimetric analysis data for PPN-125-DETA in N2 atmosphere with a heating rate of 38CminÀ1. Inset shows no clear weight loss at 1208C for 999 min.

and S9 (Supporting Information); the sample was heated and kept at 1208C for 999 min and no clear weight loss was ob- served during this period of time. Thus, 1208C appears to be a safe regeneration temperature for this material. Figure 3. Comparison of the FTIR spectra of PPN-125, PPN-125-EPOXIDE, and After measuring the thermal stability, we then collected CO PPN-125-DETA. 2 adsorption isotherms for PPN-125-DETA at potential adsorption and desorption temperatures in a TSA process; Figure 5 illus- formation of the epoxide–DETA adduct. PPN-125 shows a trates the working capacity of PPN-125-DETA with a tempera- À1 CÀCÀO asymmetric stretch for the phenol group at 1215 cm , ture swing from 40 to 1208C for a flue gas simulant (15% CO2 of which the strength significantly decreases in PPN-125-EPOX- balanced with N2). The amount adsorbed at 408C, 0.15 bar IDE, suggesting most phenols have been utilized (though not (Q1) is 5.2 wt%, and upon heating the sample to 1208C, most À1 quantitatively). The band at 1103 cm corresponds to the CÀO of the CO2 will be released at 1 bar (Q2). Under these condi-

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Table 1. Estimated working capacities and regeneration energies for 30% [16,17] [9, 16] MEA, PPN-6-CH2-DETA, and PPN-125-DETA in a TSA process, data

for 30% MEA and PPN-6-CH2-DETA were extracted from literature.

Parameter 30% PPN-6-CH2- PPN-125- MEA DETA[a] DETA[b] working capacity [mmol gÀ1] 0.83 2.1 0.97 heat capacity at 40 8C[JgÀ1 KÀ1] 3.5 1.2 1.0 À1 DHcap [Jg ] 280 84 86 À1 DHads [Jg ] 100 194 48 regeneration energy [JgÀ1] 380 278 134 working capacity/regeneration ener- 2.2 7.5 7.2 gy [mmolkJÀ1] À1 energy efficiency [kJkgCO2 ] 10519 3019 3156 [a] Desorption temperature is 115 8C. [b] Desorption temperature is 1208C. Figure 5. TSA working capacity for PPN-125-DETA under flue gas conditions

(15% CO2 balanced with N2), calculated using single-gas (CO2) isotherms at different temperatures; the amount adsorbed at 408C, 0.15 bar (Q1) sub- Although the working capacity of PPN-125-DETA is only half tracted by the amount adsorbed at 100 or 120 8C, 1 bar (Q2). that of PPN-6-CH2-DETA, its regeneration energy is also only

half that of PPN-6-CH2-DETA (Table 1). As a result, their working tions, only 1.2 wt% is still adsorbed, thus generating a working capacities per energy values are about the same. Energy effi- capacity of 4.0 wt% (Q1À Q2) at 120 8C. The working capacity ciency, defined as energy penalty per unit of CO2 captured and drops to 2.9 wt% if regeneration temperature is 1008C. We formulated as the reciprocal of working capacity per energy, could achieve higher working capacity if vacuum purging was should also have similar values for these two materials (Table 1, applied after heating, but this would come at the expense of see the Supporting Information for further explanation). extra energy use. The working capacity value of PPN-125-DETA We assume that the dominant adsorption sites in both PPN- is comparable to the working capacity of MEA solutions, which 6-CH2DETA and PPN-125-DETA are amine groups. The more are typically heated under flue gas conditions to about 1208C CO2 is adsorbed, the more energy is needed to break the and can achieve working capacities of about 2.1–5.5 wt% de- amine–CO2 interaction in the desorption process. Accordingly, [5g,10] pending on the scrubbing process used in the study. the working capacity (Q1ÀQ2) is roughly proportional to DHads As mentioned earlier, one of the primary reasons that we are for amine-tethered solid sorbents. On the other hand, amine– looking for alternatives for MEA solutions is because the regen- CO2 interaction in the case of aqueous amine solutions mani- eration of MEA solutions is exceedingly energy intensive. To fests itself as a higher DHads per working capacity possibly due evaluate the viability of capture material, regeneration energy to the formation and stabilization of zwitter ionic and charged is equally as important as working capacity in CCS technolo- species in the polar medium.[18] The value of energy efficiency gies. In a TSA process, total energy input to reach the desorp- in kJ per kg CO2, calculated by dividing regeneration energy tion conditions depend on two main energy uses. One is the with working capacity, is clearly in favor of PPN-125-DETA over energy needed to raise the temperature from the capture tem- 30% MEA solutions. Interestingly, all amine-tethered PPNs have perature to the desorption temperature, which can be inte- energy efficiencies roughly in the same range despite of the [16] grated from the measured heat capacity (DHcap); the other is differences in their working capacities, which can be easily the energy required to overcome the endothermic desorption traced back to their low heat capacities. Similar to other process, which can be integrated from the calculated heat of amine-tethered solid sorbents, the heat capacity of PPN-125- [16] À1 À1 adsorption (DHads). DETA was measured to be around 1.0 Jg K , which is sig- The energy required to heat MEA solutions to reach a capaci- nificantly lower than that of aqueous MEA solutions À1 À1 À1 À1 [19] ty of 0.83 mmolg is around 380 Jgsol opposed to (~3.5 Jg K ). À1 134.1 Jgmat for PPN-125-DETA to reach a capacity of In the context of the TSA carbon capture process, amine- 0.97 mmolgÀ1. As a result, working capacity per energy of tethered materials, especially primary amines, could potentially À1 [20] PPN-125-DETA (7.2 mmol kJ ) is more than three times higher suffer from thermal, oxidative, and CO2-induced degradation. than that of MEA solutions (2.2 mmolkJÀ1). In other words, The topic of material stability is especially important, as it can PPN-125-DETA can achieve an energy efficiency that is by affect the adsorbent lifetime and, therefore, the operating a factor of three higher compared to MEA solutions on a mate- costs of the process. To understand the dynamic cycling be- rial basis. To be specific, 10519 kJ of energy is needed to cap- havior of PPN-125-DETA, temperature-dependent gravimetric ture 1 kg CO2 using MEA solutions, and only 3156 kJ is re- adsorption was studied by using a TGA under CO2 gas condi- quired for PPN-125-DETA to capture the same amount of CO2. tions. PPN-125-DETA shows 50 adsorption–desorption cycles of

Assuming the regeneration of MEA solutions requires roughly CO2 with retention of over 90% capacity (Figure 6); a cycle in- 30% of the energy produced by the power plants,[4c] it could cludes heating and keeping PPN-125-DETA at 1008C for be reduced to approximately 10% if amine-tethered PPNs 30 min, then cooling and keeping at 408C for 30 min. The loss were used instead. of capacity could be due to irreversible urea formation. We

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analysis. Elemental analysis calcd. (found): C, 65.78 (64.20); H, 6.57 (7.07); N, 10.96 (10.05); Cl, 0.00 (0.35).

Acknowledgements

This work has been financially supported by Office of Naval Re- search. M.B. acknowledges the Texas A&M Graduate Merit Fellow- ship.

Keywords: carbon capture · energy efficiency · microporous materials · networks · polymers

Figure 6. 50 adsorption–desorption cycles of CO2 with retention of over 90% capacity; temperature range 40–100 8C. [1] a) Atmospheric CO2 concentration, US Earth System Research Laborato- ry, Mauna Loa, Hawaii, 2014. Web-accessed at: http://www.esrl.noaa.- gov/gmd/dv/iadv/graph.php?code =MLO&program=ccgg&type=ts; expect PPN-125-DETA to have a better performance under hu- b) EPA (US Environmental Protection Agency), Climate Change, 2014. midified conditions, as the presence of moisture has been Web accessed at: http://www.epa.gov/climatechange/. demonstrated to stabilize the material toward oxidative degra- [2] a) M. E. Boot-Handford, J. C. Abanades, E. J. Anthony, M. J. Blunt, S. Bran- dation and prevent the formation of urea from occurring.[7a] dani, N. Mac Dowell, J. R. Fernandez, M.-C. Ferrari, R. Gross, J. P. Hallett, R. S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, In conclusion, a truly cost-effective procedure for amine- R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N. Shah, J. G. Yao, P. S. tethered porous polymer networks (PPNs) has been developed. Fennell, Energy Environ. Sci. 2014, 7, 130– 189; b) N. MacDowell, N. A PPN containing diethylenetriamine (PPN-125-DETA) exhibits Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C. S. Adjiman, C. K. a working capacity comparable to monoethanolamine (MEA) Williams, N. Shah, P. Fennell, Energy Environ. Sci. 2010, 3, 1645– 1669. [3] a) K. Z. House, C. F. Harvey, M. J. Aziz, D. P. Schrag, Energy Environ. Sci. solutions in a temperature-swing adsorption process over 2009, 2, 193– 205; b) M. Ishibashi, H. Ota, N. Akutsu, S. Umeda, M. a temperature range of 40–120 8C. More importantly, the re- Tajika, J. Izumi, A. Yasutake, T. Kabata, Y. Kageyama, Energy Convers. generation cost of PPN-125-DETA is only one third of MEA sol- Manage. 1996, 37, 929 –933; c) M. Zhao, A. I. Minett, A. T. Harris, Energy utions, primarily due to the large difference in their heat ca- Environ. Sci. 2013, 6, 25–40. [4] a) J. D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R. D. Srivastava, Int. J. pacities. On a material basis, the energy penalty can be re- Greenhouse Gas Control 2008, 2, 9– 20; b) T. Lewis, M. Faubel, B. Winter, duced from 30% to 10% of the energy produced by power J. C. Hemminger, Angew. Chem. Int. Ed. 2011, 50, 10178–10181; Angew. plants if MEA solutions are replaced by amine-tethered porous Chem. 2011, 123, 10360 –10363; c) G. T. Rochelle, Science 2009, 325, materials. It is demonstrated that PPN-125-DETA retains over 1652 –1654. [5] a) T. C. Drage, C. E. Snape, L. A. Stevens, J. Wood, J. Wang, A. I. Cooper, 90% working capacity after 50 adsorption–desorption cycles of R. Dawson, X. Guo, C. Satterley, R. Irons, J. Mater. Chem. 2012, 22, 2815 – CO2. Overall, PPN-125-DETA is a very promising new material 2823; b) M. Sevilla, A. B. Fuertes, Energy Environ. Sci. 2011, 4, 1765 – for carbon capture from flue gas streams. Efforts are currently 1771; c) G.-P. 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Guo, J. Jiang, D. Zhao, (10 mL), concentrated HCl(aq) (1.0 mL; 37%) was added. The result- ChemSusChem 2014, 7, 2791 –2795; k) W. Lu, W. M. Verdegaal, J. Yu, P. B. ing mixture was stirred and heated to 1008C overnight. After cool- Balbuena, H.-K. Jeong, H.-C. Zhou, Energy Environ. Sci. 2013, 6, 3559 – ing to room temperature, the precipitate was collected and 3564. [6] a) B. Danielle, K. Mourad, N. Patrick, M. Murielle, H. Robert, Sci. Technol. washed several times with THF. The product was dried in air and Adv. Mater. 2008, 9, 013007; b) A. C. Kizzie, A. G. Wong-Foy, A. J. Matzg- used for the next step or dried in vacuum for analysis. Elemental er, Langmuir 2011, 27, 6368 –6373. analysis calcd. (found): C, 75.00 (74.35); H, 3.60 (4.04); N, 0.00 [7] a) A. Sayari, Y. Belmabkhout, J. Am. Chem. Soc. 2010, 132, 6312– 6314; (0.00); Cl, 0.00 (0.42). b) P. J. E. Harlick, A. Sayari, Ind. Eng. Chem. Res. 2007, 46, 446 –458; c) N. 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Energy-efficient carbon capture: A W. Lu, M. Bosch, D. Yuan, H.-C. Zhou* truly cost-efficient strategy for the syn- && – && thesis of amine-tethered porous polymer networks has been developed. A Cost-Effective Synthesis of Amine- network containing diethylenetriamine Tethered Porous Materials for Carbon exhibits a high working capacity compa- Capture rable to current state-of-art technology (30% monoethanolamine solution), yet it requires only one third as much energy for regeneration. It has also been demonstrated that no capacity loss occurs after 50 adsorption–desorption cycles of CO2 in a temperature- swing adsorption process.

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