Potassium Tethered Carbons with Unparalleled Adsorption Capacity

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Potassium tethered carbons with unparalleled adsorption capacity and selectivity for low cost carbon dioxide capture from flue gas Hongyu Zhao, Lei Shi, Zhongzheng Zhang, Xiaona Luo, Lina Zhang, Qun Shen, Shenggang Li, Haijiao Zhang, Nannan Sun, Wei Wei, and Yuhan Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14418 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Applied Materials & Interfaces

Potassium tethered carbons with unparalleled adsorption capacity and selectivity for low cost carbon dioxide capture from flue gas

Hongyu Zhao,ab Lei Shi,ac Zhongzheng Zhang,a Xiaona Luo,ab Lina Zhang,a Qun Shen,a Shenggang Li,ad Haijiao Zhang,b Nannan Sun, ＊a Wei Wei, ＊ade Yuhan Sun, ad

a. CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China. b. Institute of Nanochemistry and Nanobiology, School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China. c. Department of Chemistry, Shanghai University, Shanghai 200444, China. d. School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China. e. Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China.

KEY WORDS Carbon, Adsorbents, Adsorption, CO2 Capture, CCS

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ABSTRACT Carbons are considered less favorable for post-combustion CO2 capture due to their low affinity towards CO2, and nitrogen doping was widely studied to enhance CO2 adsorption, but the results are still unsatisfactory. Herein we report a simple, scalable, and controllable strategy of tethering potassium to a carbon matrix, which is showed to be able to enhance carbon-CO2 interaction effectively, and a remarkable working capacity of ca. 4.5 wt.% at flue gas conditions was achieved, this is among the highest for carbon-based materials. More interestingly, a high CO2/N2 selectivity of 404 was obtained. DFT calculations evidenced the introduced potassium carboxylate moieties are responsible to such excellent performance. We also show the effectiveness of this strategy is universal, and thus cheaper precursors can be used, leading to great promise for low cost carbon capture from flue gas.

1. INTRODUCTION As fossil fuels will dominate the global energy consumption for at least more than a decade, effective means for CO2 reduction are urgently needed. According to the IEA, the “no more than 2 oC” target can hardly be achieved without CO2 capture and storage (CCS),

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but deployment of related technologies is still hindered particularly by the

prohibitive high cost of CO2 capture.

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Since flue gas from power plant is the largest

stationary emission point, targeted CO2 capture solutions are obviously the current priority (post-combustion capture, PCC). Amine scrubbing is a well-established process for CO2 separation,

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However, the technology is highly energy-intensive, and suffers

from high environmental footprint (e.g. via amine evaporation). During the past few years, adsorption-based CO2 capture using solid materials has been widely studied as an 2 ACS Paragon Plus Environment

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alternative to amine scrubbing that may lead to cost effective CO2 capture from flue gas, and a wide spectrum of porous materials was investigated including zeolites, 4-5 carbons, 68

metal oxides,

9-11

immobilized amines,

covalent organic frameworks,

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12-13

metal-organic frameworks (MOFs),

organic porous materials,

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14-18

etc. Among these

materials, immobilized amines and MOFs with unsaturated metal centers (UMCs) are of particular interests due to their unparalleled CO2 adsorption capacity from simulated flue gas. 29-35 In general, operational conditions of practical PCC are very complicated, aspects such as low pressure, low CO2 concentration, presence of contaminants (SOx, NOx, H2O, etc.), and others induced a trade-off between different adsorption-desorption behaviours. For example, one of the general principles for immobilized amine and MOFs to have high adsorption capacity is the presence of strong adsorption sites to facilitate selective CO2 adsorption from a low concentration stream such as PCC, however these active sites are more difficult to regenerate and less stable in terms of cycling stability or even structural integrity of the materials.

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Thus, instead of considering adsorption capacities as an

exclusive factor, balanced adsorption behaviours are of paramount importance in order to decrease the cost of PCC. One excellent example was recently reported by Nandi and coworkers, who prepared a nickel isonicotinate MOF with ultralow parasitic energy costs for PCC, interestingly, this sample is not the highest performing material in any single aspect of adsorption behaviours. 36 Similarly, Jo et al. used different diamines to modify a MOF material in order to circumvent the mutually antagonistic adsorption behaviours during PCC, and it was also found that the sample with the highest equilibrium adsorption capacity did not lead to the highest working capacity. 37


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In essence, the adsorption behaviour of a material is thermodynamically determined by the interaction between adsorbent and adsorbate, which should be optimized to achieve the “balanced adsorption behaviour” mentioned above. To this end, one can either deactivate the excessively strong adsorption sites, or enhance CO2 affinity of weak adsorption sites. For example, Choi and co-workers selectively deactivated the most active amine groups (primary amine) in polyethyleneimine (PEI), by immobilizing this modified amine onto a silica support, heat of adsorption was reduced substantially as compared with un-modified PEI, leading to enhanced cycling and chemical stability.

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Similarly, Jung et al. used the primary amine groups in the PEI molecule as knots to crosslink the polymer chain, in addition to improved thermal stability, adsorption stability was also increased due to the avoidance of generating unfavourable urea structures. 38 For the other approach, nitrogen doped carbons (NDCs) have been widely proved to have better CO2 adsorption than pure carbons, and higher adsorption enthalpies were obtained as well.

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Alternatively, Nugent used a reticular chemistry strategy to insert

periodically arrayed hexafluorosilicate into a MOF without UMCs, which resulted in contracted pores, and thus higher CO2 affinity due to better overlap of attractive potential fields from the opposite walls, leading to higher CO2 uptake. 44 Nevertheless, complicated and tedious preparation steps are involved in most of the above approaches, among which NDCs are relatively cheap and easy to synthesize, therefore these materials have been investigated extensively. Unfortunately, the best carbonization temperature to achieve an appropriate porosity differs from the one to obtain optimized type/content of N-functionalities, this is because porosity of NDCs increases with the increasing of carbonization temperature until an peak value is achieved,


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while N-functionalities, particularly the chemically more active ones (stronger adsorption sites), are prone to decompose with the increase of temperature, this issue manifests NDCs with limited enhancement on CO2 affinity, and thus less favorable for PCC. 45-47 As a matter of fact, the role of nitrogen-bearing functionalities in enhancing CO2 affinity of NDC lies on the disturbance of the evenly distributed surface electric field of carbons, which led to stronger interaction with the quadrupole of CO2, this is to say involving any element with different electron negativity from pure carbon should work similarly. As proof-of-concept, both Zhao

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and some of us

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reported the involvement

of potassium as extra-framework cations to enhance CO2 adsorption on carbons. In these works, carbons were firstly activated with KOH at elevated temperatures, washing of the obtained mixture with ethanol or H2O instead of HCl solution allowed preservation of some potassium species in the carbon matrix. Although the synthesized carbons were effective as CO2 adsorbents, their preparation suffered from use of large amounts of corrosive KOH and less flexibility in controlling the potassium content in the final materials. Based on these pioneering works, we report herein simple, scalable, and controllable synthesis of potassium tethered carbons from low cost precursors, namely the controlled oxidation of carbons followed by potassium exchange. Based on comprehensive investigations on their CO2 adsorption performance, we demonstrate that the current strategy is more effective in enhancing CO2 affinity compared to NDCs, which resulted in a higher and more appropriate adsorption enthalpy of 30 kJ/mol at a medium to high CO2 loading range of 0.6-2.0 mmol/g. Consequently, excellent CO2 adsorption capacities of 4.50-5.76 wt.% in flue gas conditions (40 oC, 1 bar, 15 vol.% CO2) were achieved, which


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are among the highest for carbon-based materials. Meanwhile, the samples were also characterized by fast adsorption kinetics, high cycling stability, and easy regeneration (115 oC). Besides, DFT calculations demonstrated for the first time that compared with unmodified carbon or NDCs, potassium tethering introduced new adsorption sites that attract CO2 molecule with appropriate strength, and owing to the electrostatic nature of such interaction, several CO2 molecules could be accommodated by only one potassium atom without obvious decrease in their binding energies. On the other hand, the moderate surface areas (~500 m2/g) of PTCs limited N2 adsorption, therefore a high CO2/N2 adsorption selectivity was obtained for PTCs, which demonstrated their great potential to be used in PCC. Finally, we also established different preparation protocols according to the nature of different carbon precursors to achieve highly dispersed tethering of potassium, so that cheaper carbons can be used, making these materials highly competitive for low cost PCC.

2. EXPERIMENTAL 2.1. Synthesis procedure Three parent carbons (AC, CB, and MC) were used for modification. AC and CB were activated carbon and carbon black commercially available from Sinopharm and Black Diamond Co., Ltd., respectively. MC (mesoporous carbon) was prepared following a solvent-free method, 50 typically, 1.12 g of p-phthalaldehyde, 0.88 g of resorcinol and 3 g of Pluronic F127 were thoroughly mixed and grounded, and then subjected to thermal treatment in an autoclave for 8 h at 250 oC. The obtained material was further carbonized in nitrogen at 800 oC for 5 h (2 oC/min heating rate) to obtain the final MC.


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Mixed-acid oxidation and potassium tethering: Typically, 3 g of the parent carbons and 120 mL mixed-acid (concentrated HNO3 and H2SO4 with a volume ratio of 3) were added to a three-necked flask and heated to different temperatures (60, 80 and 100 oC) for 8 h. Subsequently, the mixture was cooled to room temperature and washed adequately with deionized water and dried at 70 oC for 24 h. The oxidized samples were further stirred in 2 mol/L KOH aqueous solution (mass:volume = 1.5 g:300 mL) for 24 h at room temperature, followed by filtration, washing with deionized water to neutral to remove physically deposited potassium ions, and then dried at 70 oC to recover the potassium tethered carbons (PTCs). 2.2. Characterization and evaluation N2 adsorption at -196 oC was measured on a Micromeritics ASAP2420 apparatus (pressure and temperature accuracy are ± 0.15% of reading and ± 0.1 oC, respectively), samples were degassed at 300 oC for 10 h under vacuum before any tests. A TA Q50 thermal gravimetric analyzer (TGA, weight and temperature accuracy are ± 0.1% and ± 0.1 oC, respectively) was used to measure CO2 adsorption under ambient pressure, the samples were firstly treated at 115 oC in Ar (100 mL/min) for 30 min. CO2 and N2 adsorption isotherms were measured and crosschecked by a Quantachrome iSorb HP1 analyzer, a Belsorp max instrument, and a Micromeritics ASAP2020 instrument, respectively, a degas temperature of 120 oC and a vacuuming level of ca. 1×10-2 mbar were used. Purities of the all gases (CO2, N2, and He) used in adsorption measurement are higher than 99.99%. Raman spectra were collected on an RM2000 microscopic confocal Raman spectrometer with an excitation laser of 532 nm. FT-IR spectrum was collected using the KBr/sample pellet method on a Thermo Fisher Nicolet 6700 FT-IR spectrometer at ambient temperature. X- Ray photoelectron spectroscopy (XPS) of the samples was analyzed on a PHI-


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5000C ESCA System. The morphology of samples was investigated by scanning electron microscopy (SEM) on a SUPRRATM55 apparatus with an acceleration voltage of 2.0 kV, and transmission electron microscopy (TEM) images were taken with JEOL 2100F operating at 200 kV. Elementary analysis (EA) was performed on a Thermo Flash 2000 CHNS analyzer. A C22H12 molecule with six fused benzene rings and its derivatives with –COOH and –COOK groups were used as the models of pure, oxidized graphite, and potassium tethered graphite during DFT calculation. The AUG-cc-pVDZ basis set

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was used for

the non-metal atoms, whereas the Stuttgart relativistic small core (RSC) effective core potential (ECP) based basis set functional, i.e. B97D

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was used for potassium. The B97 exchange-correlation

was used to improve the description of the dispersion interaction

between CO2 and graphite at the DFT level due to the consideration of Grimme's D2 dispersion correction and reduced computational cost by ignoring the Hartree-Fock exchange. Geometry optimizations were carried out in redundant internal coordinates with the Berny algorithm

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for local minima, and analytic harmonic frequencies were

calculated to verify the nature of the stationary states and to obtain the zero-point energy corrections. All calculations were performed with the Gaussian 09 program package,

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molecular visualization was accomplished using the AGUI graphical interface from the AMPAC program package, 56 and atoms-in-molecule (AIM) analysis was carried out with the Avogadro program. 57

3. RESULTS AND DISCUSSION 3.1. Parent carbons In this study, three carbons, namely AC, CB, and MC, were selected for surface modification. Among these, AC is an activated carbon commercially available from Sinopharm, CB is a carbon


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black sample supplied from Black Diamond Co., Ltd. MC is prepared via a solvent-free method reported previously with minor modifications. 50, 58 Figures 1a, b, and c show SEM images of the three parent carbons. At lower magnification, AC and MC showed granular morphologies, while CB was strawberry-like with rough surface (Inserts of Figure 1). In high-magnification SEM images, highly-developed porous structures can be seen on the surface of AC and CB, but it seems that for AC, the pores were mainly located within the carbon matrix indicating their formation by steam activation, while the pores in CB were mainly generated by the voids between large amounts of microspheres. In comparison, MC is more condense which might be related to severe contraction of its precursor during high temperature carbonization.

Figure 1. SEM images of a) AC, b) CB, and c) MC

Detailed textural properties of the samples were further characterized by N2 adsorption isotherms. As can be seen from Table 1, BET surface areas higher than 1000 m2/g were obtained for AC and CB, while the value for MC is lower (599 m2/g). As can be seen from Figure 2a, all the N2 isotherms showed rapid increase of N2 adsorption at low pressures, indicating the presence of micropores. According to a recent update from IUPAC, 59 the hysteresis loop of AC and MC can be classified as H2 (b) type, and the sharper step-down of the desorption branch of MC suggests higher ordering of its mesoporous as reported previously.

50, 58

For sample CB, a H4 hysteresis loop was

obtained which agrees with the accumulation of voids between the microspheres (Figure


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1b). Accordingly, pore size distribution calculated by Non-Local Density Functional Theory (NLDFT) confirmed the presence of both micro- and meso- pores in AC and MC, while larger pores were favourably found in CB (Figure 2b).

Figure 2. a) N2 isotherms at -196 oC and b) NLDFT pore size distribution of parent carbons

Surface composition of the three parent carbons were quantified by XPS. From the full-scale survey (Figure S1), oxygen element was identified apart from carbon for all the three samples, which is indicative for the presence of oxygen-containing functionalities. Furthermore, the oxygen concentrations of the samples increased in the order of CB

Potassium Tethered Carbons with Unparalleled Adsorption Capacity

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