Sorption of Carbon Dioxide, Methane, and Nitrogen

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The selectivity of CO2 over N2 in zeolite-F was 38 at 303 K and. 850 mmHg pressure ... CO2 capture technologies such as absorption, adsorp- tion, and membrane ..... Novel Polyethylenimine-Modified Mesopo- rous Molecular Sieve of ...

Sorption of Carbon Dioxide, Methane, and Nitrogen on Zeolite-F: Equilibrium Adsorption Study Mihir R. Belani, Rajesh S. Somani, and Hari C. Bajaj Inorganic Materials and Catalysis Division, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, (CSIR - CSMCRI), Bhavnagar, Gujarat 364002, India; [email protected] (for correspondence) Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12524 Carbon dioxide is a major contributor in greenhouse gases. Unswerving increase of CO2 in atmosphere, due to growing industries and automobile field, is a wakeup call to the world as excess of CO2 will affect humans in many ways among which greenhouse effect dominated the most. In the present work potassium based zeolite-F was synthesized by hydrothermal method as reported by International Zeolite Association. Synthesized zeolite-F was characterized with powder X-Ray diffraction analysis, FT-IR spectroscopy, scanning electron microscopy, thermo gravimetric analysis and BET surface area measurement. The equilibrium adsorption capacities of pure gases CO2, N2, and CH4 were measured at 273, 288, and 303 K up to 850 mmHg. Although zeolite-F possesses low CO2 adsorption capacity (43.6cc/g) at 303 K, the selectivity towards N2 and CH4 reflects its potential as an efficient CO2 adsorbent. The selectivity of CO2 over N2 in zeolite-F was 38 at 303 K and 850 mmHg pressure, whereas selectivity for CO2 over CH4 was 30. Thus, zeolite-F has shown potential for selective CO2 C 2017 Ameriadsorption from its gas mixture with N2 and CH4. V can Institute of Chemical Engineers Environ Prog, 00: 000–000, 2017

Keywords: zeolite-F, carbon dioxide, methane, nitrogen, adsorption INTRODUCTION

Increase in the atmospheric CO2 concentration (407.7 ppm, May, 2016 [1]) has gained interest among researchers worldwide to put their efforts on reduction of CO2 emissions. Fossil fuel based power plants, vehicles, hydrogen, and ammonia production and residential buildings are among those sources, which generate CO2 in larger proportions. CO2 capture and sequestration may be termed as first step in order to achieve optimum level of CO2 concentration back. CO2 capture technologies such as absorption, adsorption, and membrane separation are useful. Absorption is more of energy consuming while membrane separation requires use of hazardous reagents as initiators. Therefore, adsorption is considered to be more useful than other two discussed techniques. The well-known potential adsorbents reported for CO2 separation are activated carbon [2,3], graphite/graphene-based adsorbents [4,5], zeolites [6,7], silica based adsorbents [8], MOFs [9], carbon nanotubes [10], and nanoporous silica-based molecular basket [11]. C 2017 American Institute of Chemical Engineers V

Zeolites, a class of porous crystalline aluminosilicates, constructed of an episodic array of TO4 tetrahedral (T 5 Si or Al), which are widely used in separation applications mainly because of their unique molecular sieving property for liquid and gaseous molecules [12,13]. The presence of aluminum atoms in silica based molecular sieve materials introduce negative framework charges that are compensated by exchangeable cations in the pore space (often alkali cations) and these structural characteristics of zeolites enable them to adsorb a wide variety of gas molecules, including modern world’s antagonist CO2. Today there are over 250 unique zeolite topologies indexed by the International Zeolite Association (IZA). Zeolite is a conventional adsorbent that is useful in capturing CO2 due to its pore structure and high surface area. Brown et al. studied CO2 adsorption on SSZ-13 [14], wherein Cu-SSZ-13 showed significantly higher CO2 adsorption (89 cc/g), its modified Cu-form showed slight lower CO2 adsorption (84 cc/g). Dangi et al. [15] reported the CO2 adsorption (59 cc/g) at 303 K on zeolite-L. Chatti R. et al. [16] have reported breakthrough study on modified Na-X zeolite with 24 mg/g CO2 adsorption. Lei Wu et al. [17] reported amino-modified MIL-68(In) with enhanced hydrogen and CO2 sorption enthalpy. Among the modifiers used, 2-amino-terephthalic acid showed 53 cc/g CO2 adsorption at 273 K. Particularly studies reported by Brown and Dangi et al. motivated us to pursue CO2 adsorption on zeolite-F. Zeolite-F showed comparable volumetric equilibrium adsorption capacity of CO2 (64 cc/g) at 273 K (Table 1) with those adsorbents reported in the literature. In continuation of our work on potassium based zeolites [15], in this communication we focused on zeolite-F, having formula K10(Al10Si10O40)• wH2O (w 5 8). It was synthesized from the batch composition 5.26 K2O: Al2O3: 3 Si02: 94.5 H2O [18] that have been hardly studied for gas adsorption [19,20]. In this study, we used zeolite-F for adsorption of CO2, CH4, and N2. Zeolite-F was synthesized in our laboratory using known protocol [18]. Its structure resembles with the naturally occurring Edingtonite type zeolite. Unit cell data shows that zeolite-F contains tetragonal phase. It also contains edge length of a 5 6.9 and c 5 6.4 A˚ [21]. MATERIALS AND METHODS

Synthesis of Zeolite-F Zeolite-F was synthesized using method recommended by IZA [18]. Accordingly, 9.26 g potassium hydroxide (AR, 85%,

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Table 1. Comparison of CO2 adsorption capacity of various adsorbents reported in the literature.

Adsorbent

Modification

SSZ-13 SSZ-13 K-L Na-X MIL-68

H-form Cu-form Cation exchange Monoethanolamine (MEA) 2-Aminoterephthalic acid Terephthalic acid/amino terephthalic acid

Zeolite-13X Silica b-zeolite b-zeolite

As such APTES-grafting As such MEA

CO2 adsorption capacity (cm3/g) 89 84 59 25 53 50 35 04 47 39 17

Conditions 298 298 303 348 273 273

K, K, K, K, K, K,

760 760 760 760 760 760

mmHg mmHg mmHg mmHg mmHg mmHg

298 K, 760 mmHg Relative humidity study 298 K, 760 mmHg 303 K, 760mmHg 303 K, 760 mmHg

References [14] [14] [15] [16] [17]

[6] [29] [30] [30]

Rankem, India) is added to 20 mL distilled water under stirring. After KOH gets dissolved in water it is divided in to two parts. In part-1, 0.719 g Aluminium wire (99.9%, Sigma Aldrich) was added and refluxed for 5 h. In part-2, fumed silica (Cab-O-Sil) was added and then stirred for 5 h. Both the solutions were filtered and stored in plastic bottles. Afterwards both the solutions were mixed and gel formation takes place. Mechanical stirring at 1000 rpm was applied for 5 min. Crystallization of gel was accomplished at 958C 96 h in a closed system. The required product (zeolite –F) was recovered by centrifugation and washed with distilled water till the filtrate pH was 8 to 9. Characterization of Zeolite-F The powder X-ray diffraction (PXRD) patterns of synthesized samples were recorded with a PANalytical (model: Empyrean X’PERT-PRO XRD) with CuKa radiation (k 5 1.5406 A˚) on an advance X-ray powder diffractometer. PXRD patterns were collected using scan step time 20, 40, 60, and 80 s for each step size (2 h) considering as 0.0130, and 2h ranging from 5 to 508. Thermogravimetric analyzer (NETZSCH TG 209F1 Libra TGA209F1D-0105-L) was used for the investigation of thermal stability of the synthesized zeolite-F under the argon atmosphere at the heating rate of 108C min21 from 30 to 8008C. Microscopic images of zeoliteF samples were collected using FE-SEM JEOL JSM 7100F. The surface area measurement of the zeolite-F samples was carried out using static volumetric gas adsorption analyzer system (Model-ASAP 2020, Micromeritics) by obtaining N2 adsorption/desorption isotherms at 77 K. Prior to the adsorption measurement the sample was degassed overnight under vacuum (5 3 1026 mmHg) at 623 K. Equilibrium Adsorption Measurements Zeolite samples being hydrophilic in nature were dried at 383 K for 10 h in an oven. Prior to adsorption measurements, the samples were out gassed by heating up to 623 K, at a heating rate of 1 K min21 under vacuum (5 3 1023 mmHg) for 5 h using a degassing system. CO2, N2, and CH4 adsorption isotherms were measured at 273, 288, and 303 K using a static volumetric system (Model-ASAP-2020, Micromeritics, Figure 1). Adsorption temperature was maintained by circulating water from a constant temperature water bath (Julabo F25, Seelbach, Germany). Adsorption capacity, as the volume of gas adsorbed per gram of adsorbent, was measured at 278, 288, and 303 K up to 850 mmHg. RESULTS AND DISCUSSION

Powder X-Ray Diffraction The PXRD pattern for zeolite-F is shown in Figure 2a. The diffraction pattern indicates well crystalline nature of 2 Month 2017

Figure 1. Volumetric CO2 adsorption unit (ASAP-2020, Micromeritics). [Color figure can be viewed at wileyonlinelibrary.com]

synthesized material having a good agreement in the peak positions with those of the zeolite-F [18] (Figure 2b) JCPDS data file no. 00-038-0216. Effect of variation of scan step time on the XRD pattern is shown in Figure 3, which confirms that higher scan step time gives more precise data. If the scan step time is reduced, it may give erroneous results while analyzing the data [22]. By increasing scan step time, we observed high increment in intensity at 2h 5 138, which corresponds to (001) plane, from this one can say that it is the case of preferential orientation [23]. Thermogravimetric Analysis The thermogravimetric and differential thermal profile for zeolite-F is shown in Figure 4. The weight loss, in the temperature range of 50–2608C can be attributed to adsorbed water loss. The observed weight loss was 10 wt % only. After removal of water zeolite-F structure remained almost stable upto 8008C that confirms that zeolite-F is thermally stable adsorbent. FTIR Spectroscopy The FTIR spectrum of zeolite-F is shown in Figure 5. The deformation and mixed vibration of silicon aluminum lattice absorption peaks in the range 760–440 cm21 are observed. The strong infrared absorption band at 1010 cm21 is

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Figure 4. TGA and DTA profiles of zeolite-F. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2. (a) PXRD pattern of zeolite-F and (b) Comparison of XRD patterns of Zeolite-F (1) reported [15] and (2) Laboratory synthesized sample. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5. IR spectrum of zeolite-F.

assigned to vibration of (Al, Si) – O bonds. The absorption bands in the range 3607–3460 cm21 are vibration of AOH bond. The vibrations in the region 1657 cm21 can be assigned to the presence of zeolitic water. Most intense band, assigned to antisymmetric O-T-O stretching vibration, lies in the 1200–950 cm21 range.

N2 Adsorption-Desorption Isotherm The N2 adsorption-desorption isotherm of zeolite-F measured at 77 K for powder sample is shown in Figure 7. The isotherm reveals slow uptake up to p/p0 5 0.6 and then gives continuous increase in uptake till relative pressure reached p/p0 5 0.9. After that, it increased drastically till the saturation. The BET and Langmuir surface area of powder zeoliteF, was calculated from N2 adsorption-desorption data at 77 K using BET and Langmuir equations, and were found to be 54 and 78 m2/g, respectively, with total pore volume of 0.14 cm3/g at p/p0 5 0.97. The N2-isotherm can be classified as Type-III, which indicates adsorption on non-porous/macroporous solid with weak adsorption of nitrogen. The t-plot method was adopted for micropore analysis to obtain micropore volume, micropore surface area, and external surface area. These values were found to be 0.002 cm3/g, 4.47 m2/g, and 48.05 m2/g, respectively. Higher value of external surface area as compared with micropore surface area is a proof of less entrapment of N2 in the pores of zeolite-F. The t-plot curve was well fitted to linear, as shown in Figure 8.

Scanning Electron Microscopy SEM images of zeolite-F showed assembly of cottoncandy-like [24] morphology (Figures 6a–6d) in contrast to the reported prism like morphology [25]. This may be due to the difference in synthesis protocol and conditions.

Differential Pore Volume and Surface Area Distribution Differential pore volume was calculated using BarretJoyner-Halenda (BJH) model and the Horvath-Kawazaoe (HK) method. The BJH model is used for calculation of the pore size distributions (PSD) in the mesoporous range and a

Figure 3. Effect of scan time on PXRD pattern of zeolite-F. [Color figure can be viewed at wileyonlinelibrary.com]

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Figure 6. SEM images of zeolite-F at different magnifications. (a) 33300, (b) 38500, (c) 313,000, and (d) 323,000.

Figure 7. N2 adsorption-desorption isotherms of zeolite-F at 77 K. [Color figure can be viewed at wileyonlinelibrary. com]

small part of a microporous range [26]. The HK model is developed for Slit-like micropores [27]. It is priggishly used to determine the microporous materials. The comparison of PSD obtained using BJH and HK models is shown in Figure 9. PSD observed by BJH method shows comparatively high pore volume at 2.5 nm. Same phenomenon was observed by HK model, which depict 5–10 nm pores are present in zeolite F. CO2 Adsorption The adsorption isotherms of CO2, N2, and CH4 measured at three different temperatures, that is, 273, 288, and 303 K of zeolite-F up to 850 mmHg is shown in Figures 10–12, 4 Month 2017

Figure 8. The t-plot for zeolite-F.

respectively. It is observed that at a given pressure, adsorption capacity increases with the decrease in temperature. This can be understood simply on the basis of the kinetic energy of adsorbate molecules, which is proportional to the temperature. Increasing temperature makes adsorbate molecules energized enough to desorb from the surface. The adsorption isotherms (Figures 9–11) of different gases clearly indicates the adsorption capacity of CO2 is much higher than those of all other gases studied; and the order of adsorption capacity is CO2  CH4 > N2. Main highlight of the present study (Table 2) is the higher adsorption of CO2 at low pressure (51 cc/g at 15 mmHg and 65 cc/g at 850 mmHg pressure). It revealed that very high CO2 adsorption take place at very low absolute pressure (15 mmHg) and the adsorbent

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep

Figure 9. PSD of synthsized zeolite-F obtained by BJH and HK models. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 10. CO2 adsorption on zeolite-F at different temperatures (Inset image shows 80–85% saturation by CO2 at very low absolute pressure). [Color figure can be viewed at wileyonlinelibrary.com]

get saturated up to 80–85% (inset image in Figure 10). Favorably, lower uptake for CH4 and N2 were observed at all the three temperatures at 850 mmHg. These results revealed that zeolite-F can auspiciously adsorb CO2 over CH4 and N2. The higher uptake of CO2 was due to its higher quadrupole moment (13.4 3 10240 C m2) and polarizability (26.3 3 10225 cm3) compared with CH4 (0 C m2 and 26.0 3 10225 cm3) and N2 (4.7 3 10240 C m2 and 17.6 3 10225 cm3) [28]. The higher uptake of methane over nitrogen may be due to the higher polarizability of methane than nitrogen. Notable higher selectivity for CO2 in comparison to methane and nitrogen gas {CH4 (CO2/CH4 530).and N2 (CO2/N2 5 38)} is one of the key advancement from this study. The selectivity of the above gases at three different temperatures for zeoliteF is calculated from equilibrium adsorption capacities (Table 2). It is observed that with an increase in adsorption temperature the selectivity of CO2 and CH4 over nitrogen increases due to comparative poor electrostatic interactions between nitrogen and zeolite-F at higher temperature. The selectivity was found to be CO2/N2 > CO2/CH4.

Figure 11. N2 adsorption on zeolite-F at different temperatures. [Color figure can be viewed at wileyonlinelibrary. com]

Figure 12. CH4 adsorption on zeolite-F at different temperatures. [Color figure can be viewed at wileyonlinelibrary. com]

Table 2. Adsorption capacity of CO2 on zeolite-F at 15 and 850 mmHg at 273, 288, and 303 K. CO2 adsorption capacity (cm3/g) Temperature 273 K 288 K 303 K

15 mmHg

850 mmHg

51 39 31

65 55 44

The equilibrium selectivity (Table 3) of a gas “M” over gas “N” was calculated using Eq. 1. a5

qM qN

(1)

where, qM and qN are the quantity (cc/g) of M and N adsorbed, respectively.

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Table 3. Equilibrium adsorption selectivity of CO2/N2, CO2/ CH4, and CH4/N2 on zeolite-F. Equilibrium adsorption selectivity at 15 mmHg at 850 mmHg at temperature Adsorbate temperature pair 273 K 288 K 303 K 273 K 288 K 303 K CO2/N2 CO2/CH4 CH4/N2

673 426 2

584 1657 –

741 742 –

33 15 2

31 19 2

38 30 –

Figure 13. Heat of adsorption plot of zeolite-F for CO2.

Heat of Adsorption Isosteric heat of adsorption was calculated from carbon dioxide adsorption data collected at three different temperatures (273, 288, and 303 K) using Clausius-Clapeyron equation (Eq. (2)). 2DH 5R

" # olnP 1 o T

(2)

where, DH stands for heat of adsorption. Zeolite-F possesses heat of adsorption fairly within the range of physisorption (Figure 13). CONCLUSIONS

Zeolite-F was synthesized with reported protocol and was characterized using PXRD, FR-IR, TGA, and surface area. The adsorption isotherm CO2 on zeolite-F at 273 K gave CO2 adsorption of 65 cc/g at 850 mmHg. CO2 adsorption decreases study with an increase in the adsorption temperature. Key outcome of the present study is that zeolite–F’s ability to get saturated with CO2 at very low absolute pressure as it adsorbed almost 80–85% of CO2 at only 15 mmHg. Zeolite–F also depicted good CO2 selectivity over N2 (CO2/ N2 5 38) and CH4 (CO2/CH4 5 30) and can be exploited as CO2 selective adsorbent. ACKNOWLEDGMENTS

Authors are thankful to Analytical Division and Centralized Instrumentation Facility of CSIR-CSMCRI for providing analytical facilities. MB is grateful to CSIR for the funding support under the Network project TAPCOAL (CSC-0102). MB is also thankful to Maharaja Krishnakumarsinhji Bhavnagar University for PhD registration. 6 Month 2017

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