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Hierarchical porous carbon synthesized from novel porous amorphous calcium or magnesium citrate with enhanced SF6 uptake and SF6/N2 selectivity Rui Sun, Cheuk-Wai Tai, Maria Strømme, and Ocean Cheung ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02005 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Nano Materials

Hierarchical porous carbon synthesized from novel porous amorphous calcium or magnesium citrate with enhanced SF6 uptake and SF6/N2 selectivity Rui Sun,† Cheuk-Wai Tai,‡ Maria Strømme*,† and Ocean Cheung*,† † Division of Nanotechnology and Functional Materials, Department of Engineering Sciences, Uppsala University, SE-751 21, Uppsala, Sweden ‡ Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91, Stockholm, Sweden KEYWORDS: porous carbon, SF6 adsorption, CO2 adsorption, amorphous calcium citrate, amorphous magnesium citrate

ABSTRACT: The emission of greenhouse gases such as CO2 and SF6 is believed to contribute significantly towards global warming. One way to reduce their release is by adsorption at point sources using a suitable adsorbent. In this work we present the synthesis of two hierarchical porous carbon materials (referred to as PC-CaCit and PC-MgCit) with high uptake of SF6 (5.23 mmol/g, 0 °C, 100 kPa) and a reasonable uptake of CO2 (> 3 mmol/g). PC-CaCit and PC-MgCit were obtained by pyrolysis of the most porous calcium citrate and magnesium citrate ever reported,

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which were synthesized by us. The Langmuir specific surface area of PC-CaCit and PC-MgCit was over 2000 m2/g (BET surface area also close to 2000 m2/g). We characterized PC-CaCit and PC-MgCit using a range of advanced characterization techniques including N2 adsorption, highresolution electron microscopy, powder X-ray diffraction and X-ray photoelectron spectroscopy. PC-CaCit and PC-MgCit also showed a SF6-over-N2 selectivity of ~33 at 0 °C (100 kPa), good cyclic performance and moderately low heat of adsorption. The porous carbons synthesized in this work are good candidate adsorbents for greenhouse gases.

1. Introduction Global warming and climate change observed in the recent years has become the subject of a widespread public concern. The emission of greenhouse gases, such as CO2 and SF6, is generally believed to contribute towards the greenhouse effect and global warming. CO2 is one of the most emitted greenhouse gases due to human activities, with the main source being the combustion of fossil-fuel and biogas.1-2 SF6 is widely used as a component gas in high voltage breakers in electrical equipment due to its good electrical insulating properties. SF6 is also highly stable, nontoxic and non-flammable.3 Despite it’s good electrical insulating properties, SF6 is a strong greenhouse gas with a global warming latency 23900 times that of CO2.4 It is important to reduce the emission of greenhouse gases in order to limit their potential impact towards climate change. Various techniques have been developed for greenhouse gas adsorption/separation that can be applied to existing industrial processes. These technologies include chemical and physical absorption,5-6 cryogenic distillation,7 membrane separation,8-9 and more recently, adsorption by porous solids adsorbents10-14. Adsorption processes have received significant interests in recent years due to their high energy efficiency, low running costs and the


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possibility to be retrofitted to emission sources (e.g. power plants or industries). For adsorptionbased gas separation to work effectively, a good adsorbent is needed. A good adsorbent should have high capacity for the gas to be captured (e.g. SF6 or CO2), high selectivity, good cyclic performance and moderate heat of adsorption. Porous materials including: porous carbons,15-16 zeolites,17-19, amine-grafted silica20-21 and a series of organic microporous materials (e.g., metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), organic cages, porous polymer networks),22-25 have been investigated as adsorbents in greenhouse gas adsorption previously with promising results. Table 1 lists and compares the SF6 and CO2 uptake of these different types of adsorbents. The pore structure, pore opening size and the microstructure of a porous adsorbent are critical factors for its performance. Liu et al.26 showed that CO2 gas (kinetic diameter 0.33 nm) can be selectively adsorbed over N2 by adjusting the pore size of zeolite A (to < 0.36 nm by K+ ion exchange) to limit the diffusion of N2 (0.36 nm). Monte Carlo simulations carried out by Builes et al.27 suggested that one of the most important properties for selective SF6 (0.55 nm) adsorption on porous materials is the pore diameter. The optimal pore diameter for SF6 adsorption was identified to be around 1.1 nm, although the adsorption mechanism is not fully understood. Porous carbon (or activated carbon) is considered to be a promising greenhouse gas adsorbent due to its high surface area, good chemical and thermal stability, easy-to-control microstructure and low cost of production. The intrinsic hierarchical structure of porous carbon provides a lowresistance mass transportation path. The micropores of porous carbon provide high surface area and pore volume for gas adsorption.


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Table 1. SF6 and CO2 adsorption by different adsorbents and the corresponding selectivity Material

SF6 uptake

SF6/N2 Selectivity

Reference

Mg-MOF-74

6.4 mmol/g, 25 °C, 100 kPa 3.8 mmol/g, 25 °C, 100 kPa 5.34 mmol/g, 25 °C, 100 kPa ~2.4 mmol/g, 0 °C, 100 kPa; ~1.75 mmo/g, 0 °C, 10 kPa 1.45 mmol/g, 20 °C, 100 kPa; ~0.74 mmol/g, 20 °C, 10 kPa 1.75, 20 °C, 100 kPa; ~0.92, , 20 °C, 10 kPa 2.59 mmol/g, 20 °C, 100 kPa; ~0.3 mmol/g , 20 °C, 10 kPa 1.61 mmol/g, 0 °C, 100 kPa ~3.6 mmol/g, 0 °C, 100 kPa

19, 25 °C, 100 kPa SF6:N2 = 10%:90% 46.1, 25 °C, 100 kPa SF6:N2 = 10%:90% 34.6, 25 °C, 100 kPa SF6:N2 = 10%:90% 178, 0 °C, 100 kPa SF6:N2 = 10%:90%

28

74, 20 °C, 100 kPa SF6:N2 = 10%:90%

30

43, 20 °C, 100 kPa; SF6:N2 = 10%:90% 18, 20 °C, 100 kPa SF6:N2 = 10%:90%

30

~31, 0 °C, 100 kPa SF6/N2 = 10:90 ~44, 0 °C, 100 kPa SF6/N2 = 10:90

31

3-10 mmolg/g, 25 °C, 100 kPa (simulated) 0.75 mmol/g, 25 °C, 100 kPa 2.8 mmol/g, 30 °C, 101 kPa

-

33

-

34

-

35

5.6 mmol/g, 0 °C, 70 kPa

-

36

CO2 uptake

CO2/N2 Selectivity

Reference

N-doped polypyrrole-based porous carbons nitrogen-rich porous carbons

6.2 mmol/g, 0 °C, 101 kPa; 3.9 mmol/g, 25 °C, 101 kPa 4.40 mmol/g, 0 °C, 100 kPa

-

37

38

porous carbons from fungi

5.5 mmol/g, 0 °C, 100 kPa

bifunctional porous carbon from algae

5.7 mmol/g, 0 °C, 100 kPa; 3.9 mmol/g, 25 °C, 100 kPa 6.24 mmol/g , 0 °C, 100 kPa; 1.25 mmol/g, 0 °C, 10 kPa 2-4 mmol/g, 0 °C, 100 kPa; 0.8-1.45 mmol/g. 0 C, 10 kPa

77.94, 25° C, 100 kPa CO2/N2=15:85 27.3, 0 °C, 100 kPa (Initial slopes from CO2 and N2 adsorption isotherms at 0 °C) 70.4, 0 °C, 100 kPa CO2/N2=15:85 -

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Apparent selectivity n(CO2)/n(N2) 14-26.7, 0 °C, 10 kPa; 15, 0 °C, 100 kPa CO2/N2 = 15:85 18.32, 0 °C, 100 kPa CO2/N2=15:85

42

30, 25 °C, 100 kPa CO2/N2=15:85

45

Zn-MOF-74 Co-MOF-74 porous organic cages CC3α MOFUiO-66-Zr zeolite-13X MOFMIL-100(Fe) CAU-17 partially oxidized single walled carbon nanohorns (CNHs) single walled armchair nanotubes Template-derived carbon coconut-shell based commercial activated carbon activated carbon MAXSORB

porous carbons from rice husk char Activated carbons from biomass magnetic activated carbons alkali-activated carbon N-doped active carbons

6.0 mmol/g, 0 °C, 101 kPa; 1.8 mmol/g, 0 °C, 15 kPa 6.47 mmol/g, 0 °C, 100 kPa; 2.46 mmol/g, 0 °C, 15 kPa 4.26 mmol/g, 25 °C, 100 kPa;

28

28

29

30

32

39

40

43

44


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1.77 mmol/g, 25 °C, 15 kPa; 1.42 mmol/g, 25 °C, 10 kPa 1.5 mmol/g, 25 °C, 100 kPa ~1.7 mmol/g, 25 °C, 100 kPa 8.6 mmol/g, 0 °C, 100 kPa

Order mesoporous carbon mesoporous carbon CMK-3 carbon molecular sieves Zeolite 4A

4.1 mmol/g, 0 °C, 100 kPa

Zeolite ZK-4

4.1 mmol/g, 0 °C, 100 kPa;

ZIF-95/ZIF-100

~0.9-1.0 mmol/g, 25 °C, 107 kPa

-

46

-

47

2-14, 25 °C, 100 kPa 5.9, 0 °C, 100 kPa CO2/N2=15:85 44, 0 °C, 100 kPa CO2/N2=15:85 11.1, 25 °C, 101 kPa CO2/N2=50:50

48

17, 49

19

50

Highly porous amorphous calcium carbonate (HPACC) and mesoporous magnesium carbonate (MMC) with high surface area have been reported recently.51-53 HPACC and MMC are constructed with nanoparticles with an estimated average diameter less than 10 nm that have aggregated together to form a porous solid. The porosity of HPACC and MMC comes from the space between the aggregated nanoparticles. Porous materials similar to HPACC and MMC can be used as templates to synthesize other porous/hybrid materials.54-56 The weak acidity of carbonic acid means that HPACC and MMC (which are amorphous CaCO3 and MgCO3, respectively) can also be used to form other porous materials.51, 57 In this study, we report the synthesis of a novel porous amorphous calcium citrate (referred to as CaCit) and magnesium citrate (referred to as MgCit) by modifying the synthesis procedures of HPACC and MMC. Neither calcium citrate nor magnesium citrate have been reported in an amorphous form or in a porous form previously, to our best knowledge. We characterized CaCit and MgCit extensively and subsequently used them as precursors for the synthesis of porous carbon. The potential application of the porous carbon obtained from CaCit (referred to as PCCaCit) and MgCit (referred to as PC-MgCit) in greenhouse gas separation was explored. The uptake of SF6, CO2, CH4 and N2 at 0 °C, 10 °C and 25 °C was measured. The cyclic performance,


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the adsorption kinetics and the heat of adsorption of SF6 and CO2 on PC-CaCit and PC-MgCit were also investigated. 2. Experiment Section 2.1. Materials Magnesium oxide (MgO, >99%), citric acid (CA, >99.5%) and calcium citrate tetrahydrate (Ca3(C6H5O7)2·4H2O, 99%) were purchase from Sigma-Aldrich. Calcium oxide (CaO, Reagent Grade) was purchased from Alfa-Aesar. Methanol (>99.8%) was purchased from VWR Sweden. CO2 (>99.998%) and SF6 (>99.97%) were purchased from Air Liquide AB. All the chemicals were used without further purification. 2.2. Synthesis of porous amorphous calcium citrate (CaCit) and magnesium citrate (MgCit) CaCit was synthesized using a novel method developed from the synthesis of HPACC.51 3.33 g of CaO was mixed in 200 cm3 of methanol at 50 °C under constant stirring in a glass reaction vessel purchased from Andrew Glass Co. Ltd (Vineland, USA). When the mixture appeared to be homogeneous the reaction vessel was sealed and 4 bar of CO2 was applied. The pressurized mixture was left stirring for 4 hours. After 4 hours the CO2 pressure was released and the reaction mixture was centrifuged at 3800 rpm for 15 minutes to remove the unreacted CaO. After centrifugation, a sol containing highly dispersed amorphous calcium carbonate (ACC) nanoparticles was obtained. We have previously shown that these ACC nanoparticles have an average particle size of less than 10 nm – calculated using the BET specific surface area (~360 m2/g) and the density of the material (~2.4 g/cm3) obtained from He pycnometery (assuming spherical, non-fused particles).51 This ACC sol was used to prepare CaCit.


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The preparation of CaCit from the ACC sol was carried out as follows: 0.55 g CA was dissolved into 5 cm3 methanol then added to 15 cm3 ACC suspension dropwise under stirring. The mixture was then left stirring for 1 hour. After 1 hour, the methanol solvent was evaporated by heating the mixture in a ventilated oven at 150 °C for 5 hours. CaCit in the form of white powder was acquired after drying. MgCit was synthesized with the method described above with several alterations: 1) MgO was used as the starting material. 2) The MgO/methanol/CO2 reaction mixture was left at room temperature for 24 hours to form the MMC sol as described in our previous work.53 3) 3.0 g CA was dissolved in 15 cm3 methanol and then added to 15 cm3 of MCC suspension. The reaction mixture was kept stirring for 1 hour. After drying the mixture at 150 °C for 5 hours, MgCit powder was obtained. The larger amount of CA used in the synthesis of MgCit as compared to the corresponding synthesis of CaCit was due to the different concentrations between the ACC and MMC sol solutions. 2.3. Synthesis of porous carbon Porous carbon was obtained by calcination and pyrolysis of CaCit and MgCit in a tube furnace. Prior to the heating step, the sample (CaCit or MgCit) was placed in the furnace and kept at 30 °C for 1 hour under a flow of N2. After 1 hour, the sample was heated under a flow of N2 up to 800 °C at 3 °C /min, then kept at 800 °C for 1 hour before cooling down to room temperature. Porous carbon containing residue CaO or MgO was then obtained. The oxide residue was removed by mixing the porous carbon with an excess of a 10 w/w % HCl solution. Porous carbon was then separated from the HCl solution by centrifugation and washed with deionized water by repeated centrifugation until the pH of the water was ~7. Porous carbon was then separated and dried at 120


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°C for 20 hours. The yield of PC-CaCit and PC-MgCit (with respect to the mass of CaCit and MgCit) was 9.14 wt.% and 9.67 wt.%, respectively (discussed further in the Supporting Information). 2.4. Characterization Infrared (IR) spectra of samples were recorded using a Bruker Tensor 27 spectrometer (Bruker, Bremen, Germany) coupled with a Platinum attenuated total reflection (ATR) diamond sample stage. Raman spectroscopy experiments were carried out using a Renishaw inVia Qontor (Wotton-underEdge, United Kingdom) Raman microscope. The sample was placed on a glass slide and exposed to a frequency-doubled Nd:YAG lasers (wavelength 532 nm) during data collection. Powder X-ray diffraction (XRD) measurements were carried out in a Bruker D8 advance XRD Twin-Twin instrument (Bruker, Bremen, Germany) with Cu-Kα radiation (λ=0.15418 nm) with a step size of 0.04° and a measuring time of 2 s per step in the 2Ɵ range between 10 and 70 °. The instrument was operated at 45 kV and 40 mA. Thermogravimetric analysis (TGA) of the samples was performed using a Mettler Toledo TGA2 (Schwerzenbach, Switzerland) in air or N2 with flow rate 40 mL/min from room temperature to 900 °C at a heating rate of 10 °C /min. Scanning electron microscopy (SEM) secondary electron images were recorded using a Zeiss LEO 1530 scanning electron microscope (Oberkochen, Germany) operated at 2 kV. The samples were mounted on aluminum stubs with double-sided carbon tape and coated with a layer of gold-


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palladium prior to analysis to avoid the charging effect. An in-lens secondary electron detector was used for imaging. Transmission electron microscopy (TEM) measurements were carried out at room temperature using a JEOL JEM-2100F microscope (Tokyo, Japan) (Cs = 0.5 mm and point resolution of 1.9 Å) operated at 200 kV equipped with a Schottky field-emission gun and Gatan Ultrascan 1000 camera. The crushed powders were dispersed on TEM grids with carbon supporting films. X-ray photoelectron spectroscopy (XPS) experiments were conducted on a PHI Quantera II scanning XPS microprobe (Chanhassen, NM, USA). The samples were sputter-cleaned using argon ions for 5 min at 200 V to remove surface contamination. Full spectra along with highenergy resolution spectra for C 1s, O 1s and Ca 2p or Mg 2s were recorded. The spectra were calibrated against the C 1s peak at 284.8 eV for adventitious carbon. 2.5. Gas adsorption The porosity of CaCit, MgCit, PC-CaCit and PC-MgCit were tested by N2 gas adsorption in a Micromeritics ASAP 2020 (Norcross, GA, USA) volumetric gas adsorption analyzer. Prior to the adsorption measurements, CaCit and MgCit were degassed at 100 °C for 6 hours, and PC-CaCit and PC-MgCit were degassed at 120 °C for 10 hours under dynamic vacuum (1 x 10-4 Pa) using a Micromeritic Smart VacPrep060 (Norcross, GA, USA) sample preparation unit. N2 adsorption/desorption isotherms were obtained at liquid nitrogen temperature (-196 °C). Additionally, SF6, CO2, CH4 and N2 adsorption/desorption isotherms of PC-CaCit and PC-MgCit were recorded at 0 °C, 10 °C and 25 °C. Temperature control was carried out in a Dewar flask using a temperature-controlled water bath or an ice slurry. The isosteric heat of SF6 and CO2 adsorption at moderate loading was calculated using the Clausius–Clapeyron Equation;58


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―𝐸𝑎𝑑𝑠 𝑑𝑙𝑛(𝑃) = 1 𝑅 𝑑( ) 𝑇 Here P is the pressure at a particular gas loading, T is the temperature, R is the ideal gas constant and Eads is the heat of adsorption. 3. Results and Discussion 3.1. Characterization of CaCit and MgCit As detailed in the experimental section, CaCit and MgCit with high porosity were obtained by adding CA into ACC and MMC sol, respectively. The ACC and MMC nanoparticles that were dispersed in methanol solution reacted with CA to form nanoparticles of CaCit and MgCit due to the difference in pKa (carbonic acid pKa1=3.6, citric acid pKa1=3.13). CO2 was given off as a product of the reaction. Unlike the ACC and MMC nanoparticles dispersed in methanol, the CaCit and MgCit nanoparticles formed aggregates immediately and precipitated out of the methanol solution. SEM images shown in Figure S1 confirmed that CaCit and MgCit were in the form of small nanoparticles and that they had similar morphology as that of HPACC and MMC.51, 53 Note that although we did not perform any specific analyses on the particle size of CaCit and MgCit, we observed from the SEM images that the CaCit particles appeared to be smaller than the MgCit particles, as well as being more closely packed together. The difference may be related to the concentration of the sol during synthesis (the HPACC sol had a lower concentration than the MMC sol). Further studies are required in order to fully understand this observation. The IR spectra of CaCit and MgCit (Figure S2) showed the characteristic IR bands of calcium citrate and magnesium citrate. The C=O antisymmetric stretching band could be detected for both


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CaCit and MgCit at around 1575 and 1600 cm-1, respectively. The C=O symmetric stretching band was located at around 1410 cm-1 for both CaCit and MgCit. The small shoulder band at ~1700 cm-1 was attributed to the antisymmetric stretching C=O band of the small amount of excess CA on the materials. Powder X-ray diffractogram of CaCit and MgCit (Figure S3) exhibited no diffraction peaks, which demonstrated that the materials were X-ray amorphous. The N2 adsorptiondesorption recordings illustrated that both CaCit and MgCit were porous with BET specific surface areas of ~217 m2/g and ~136 m2/g, respectively (Figure S4). We were not aware of any previous reports on high surface area calcium citrate or magnesium citrate materials. The CaCit and MgCit materials presented here are therefore, the most porous forms of calcium citrate and magnesium citrate ever reported in literature. The porosity characteristics of CaCit and MgCit are listed in Table 2. Table 2. Porosity characteristics of CaCit, MgCit, PC-CaCit and PC-MgCit, obtained from N2 adsorption

Sample

S

BET 2

S

Langmuir 2

S

1 micro 2

S

V

1 ext 2

total 3

V

1 micro 3

(m /g)

(m /g)

(m /g)

(m /g)

(cm /g)

(cm /g)

CaCit

217

278

15

202

1.29

0.002

MgCit

136

174

10

126

0.51

0.002

PC-CaCit

1971

2480

313

1658

1.51

0.13

PC-MgCit

2103

2641

268

1835

3.49

0.11

1Micropore

area (Smicro), external surface area (Sext) and micropore volume (Vmicro) were calculated

using the t-plot method.59 Total pore volume (Vtotal) at P/P0=0.9738. TGA curves of CaCit and MgCit recorded in air are shown in Figure S5 and Figure S6, respectively. The TGA curve of CaCit is compared with that of crystalline calcium citrate [Ca3(C6H5O7)2·4H2O] in Figure S5. The first mass drop in CaCit occurred earlier than in crystalline calcium citrate due to the amorphous nature of the former. The overall weigh loss of CaCit and


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MgCit was comparable to the theoretical weight loss of decomposition of pure calcium and magnesium citrate. Hence, the amount of impurities such as CaCO3, MgCO3 or citric acid in the materials was low (cf. Supporting Information for more details). 3.2. Characterization of porous carbon Figure 1 together with Figure S8 presented the formation mechanism of porous carbon (PC-CaCit and PC-MgCit) from CaCit and MgCit. PC-CaCit and PC-MgCit were obtained by heat treatment of the CaCit and MgCit in N2 as detailed in the Experimental Sections. During heat treatment, both calcination and pyrolysis occurred. CaCit decomposed into CaCO3 and carbon (Step i, left panel of Figure 1).60 This step was demonstrated by the mass drop between 372 and 542 °C (centered at ~ 460 °C) observed in Figure S8a. At this stage, carbon nanoparticles containing CaCO3 within their structures (CaCO3@C) was formed. When heated to a higher temperature (~730 °C), calcination or decomposition of CaCO3 to form CaO occurred with CO/CO2 gas being eliminated (Step ii, left panel of Figure 1). Physical activation of the carbon also occurred concurrently with CO/CO2 elimination and the micropores were created within the carbon nanoparticles. MgCit underwent a similar calcination/pyrolysis process (Right panel in Figure 1 and Figure S8b). However, there was no clear temperature gap between step i and ii for MgCit, which meant that carbon formed from MgCit concurrently with the decomposition of MgCO3 to MgO (with CO2/CO being eliminated). This may have contributed to the difference in the morphology of PC-CaCit and PC-MgCit as discussed later as well as in more detail in the Supporting Information. The subsequent acid washing steps (Step iii) removed the CaO and MgO nanoparticles from inside the carbon nanoparticles and further opened up the pores. It is worth noting that Zhou et al.61 has prepared porous carbon with crystalline calcium citrate [Ca3(C6H5O7)2·4H2O] previously, but the


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calcium citrate they used was crystalline and not constructed from nanoparticles (cf. Supporting Information for more details).

Figure 1. Schematic representation of the formation mechanism of porous carbon. i, the pyrolysis of CaCit or MgCit to form CaCO3@C or MgCO3@C; ii, CaCO3 or MgCO3 decompose to CaO or MgO at elevated temperature; iii, acid washing to remove the oxide residue to obtain PC-CaCit and PC-MgCit. The powder X-ray diffractogram of PC-CaCit and PC-MgCit showed that both materials were Xray amorphous (Figure S9). It should also be noted that no CaO or MgO was detected from powder XRD or XPS (Figure S10). We analyzed the purity of the PC-CaCit and PC-MgCit by TGA (Figure S11) in air and found that both materials were essentially pure carbon ( 3 mmol/g, 0 °C at 100 kPa) and comparable to many carbon or zeolite adsorbents. The CO2/N2 selectivity was ~11-13 under the test conditions. SF6 and CO2 were found to only physisorb with moderately low heats of adsorption on PC-CaCit and PC-MgCit and no uptake capacity was lost during cyclic adsorption measurements for both gases. PC-CaCit and PC-MgCit are good candidate adsorbents for greenhouse gas adsorption, particularly for SF6/N2 separation. PC-CaCit and PC-MgCit may have an advantage over zeolite adsorbents for gas separation applications as carbon materials are generally not water sensitive. This means that PC-CaCit and PC-MgCit can be used to treat flue gas from power generation or industries without a water removal step. Furthermore, the synthesis of PC-CaCit and PC-MgCit does not involve the use of expensive organic linkers or templates, which may also be another advantage of PC-CaCit and PC-MgCit over other adsorbents such as mesoporous silica and metal organic frameworks. Further studies involving the use of PC-CaCit and PC-MgCit in other applications, such as water purification and energy storage could expand the range of potential applications of these materials.

ASSOCIATED CONTENT Supporting Information. The additional characterization of CaCit, MgCit, PC-CaCit and PCMgCit, and the additional gas adsorption data of PC-CaCit and PC-MgCit (PDF).

AUTHOR INFORMATION Corresponding Author


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*[email protected], *[email protected] Funding Sources The Swedish Research Council is acknowledged for funding this work. The KAW Foundation is acknowledged for an equipment grant for the electron microscopy facilities at Stockholm University and financial support for C.-W.T under the project 3DEM-NATUR. Rui Sun thanks the China Scholarship Council (CSC) for financial support. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Daniel Koivisto and Marial Vall of Uppsala University are acknowledged for their help with material synthesis. ABBREVIATIONS HPACC, highly porous amorphous calcium carbonate; MMC, mesoporous magnesium carbonate; CaCit, porous amorphous calcium citrate; MgCit, porous amorphous magnesium citrate; ACC, amorphous calcium carbonate; IR, Infrared; ATR, attenuated total reflection; XRD, X-ray diffraction, TGA, thermogravimetric analysis; SEM, scanning electron microscopy; TEM, transmission

electron

microscopy;

XPS,

X-ray

photoelectron

spectroscopy;

BET,

Brunauer−Emmett−Teller (BET); DFT, density functional theory; IAST, the ideally adsorbed solution theory. Supporting Information


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More characterization data of CaCit, MgCit, PC-CaCit and PC-MgCit, additional discussion on the formation mechanism of porous carbon and additional adsorption data of PC-CaCit and PCMgCit

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37

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

CaCit

MgCit

i

i ~460 °C, N2

CaCO3@C

ii

800 °C, N2

CaO@C

iii

Page 38 of 44

Acid wash

PC-CaCit

~460 °C, N2

MgCO3@C

ii

800 °C, N2

MgO@C Acid wash

iii

PC-MgCit

Carbon matrix

Pore in carbon

CaCO3 nanoparticle

MgCO3 nanoparticle

CaO nanoparticle MgO nanoparticle ACS Paragon Plus Environment

PC-CaCit

40

20

0 0.00

0.25 0.50 0.75 Relative Pressure (P/P0)

1.00

(c) PC-MgCit

40

0 0.00

5

(b) 4

0.25 0.50 0.75 Relative Pressure (P/P0)

1.00

PC-CaCit Micropores

3 2 1 0 1

3

120

80

5

3

(a)

dV/dlog(W) Pore Volume (cm /g)

60

dV/dlog(W) Pore Volume (cm /g)

Quantity Adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Quantity Adsorbed (mmol/g)

Page 39 of 44

10 Pore Size (nm)

(d) PC-MgCit

4 3

Micropores

2 1 0 1


10 Pore Size (nm)

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

(a)

1 μm

(c)

1 μm

(b)

200 nm

(d)

200 nm


Page 40 of 44

Page 41 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


(a)

50 nm

(c)

50 nm

(b)

10 nm

(d)

10 nm


0 C

PC-CaCit SF6

4

10 C 25 C

2

0 0

40 80 Pressure (kPa)

6

120

(c) PC-CaCit CO2

4

0 C 10 C 25 C

2

0 0

1.0


120

(e) PC-CaCit N2

0 C 10 C 25 C

0.5

0.0 0


Amount adsorbed (mmol/g)

(a)


6


Amount adsorbed (mmol/g) Amount adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50



Page 42 of 44

6

(b)

0 C

PC-MgCit SF6

4

10 C 25 C

2

0 0


6

120

(d) PC-MgCit CO2

4

0 C 10 C 25 C

2

0 0


1.0

120

(f) PC-MgCit N2

0.5

0 C 10 C 25 C

0.0

ACS Paragon 120 Plus Environment 0


120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Relative SF6 uptake (%)

Page 43 of 44

100

PC-CaCit PC-MgCit

95

90 2

4 Cycle


6


6

40

2.5 mmol/g

6

ln(PSF )

4

(b) Eads (kJ/mol)

(a)

2 0.5 mmol/g

20

0 0 0.0034

0.0035 0.0036 -1 1/T (K )

0.0037

6

1 2 SF6 loading (mmol/g)

3

1 2 SF6 loading (mmol/g)

3

40

(c)

(d) Eads (kJ/mol)

4

2.5 mmol/g

6

ln(PSF )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 44 of 44

2 0.5 mmol/g

20

0 0 0.0034

0.0035 0.0036 -1 1/T (K )

0.0037


Hierarchical Porous Carbon Synthesized from ... - ACS Publications

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