Microporous Polystyrene Particles for Selective Carbon Dioxide

Article pubs.acs.org/Langmuir

Microporous Polystyrene Particles for Selective Carbon Dioxide Capture Maria Kaliva,†,‡ Gerasimos S. Armatas,‡ and Maria Vamvakaki*,†,‡ †

Institute of Electronic Structure and Laser, Foundation for Research and Technology − Hellas, P.O. Box 1527, 711 10 Heraklion Crete, Greece ‡ Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 710 03 Heraklion Crete, Greece S Supporting Information *

ABSTRACT: This study presents the synthesis of microporous polystyrene particles and the potential use of these materials in CO2 capture for biogas purification. Highly crosslinked polystyrene particles are synthesized by the emulsion copolymerization of styrene (St) and divinylbenzene (DVB) in water. The cross-link density of the polymer is varied by altering the St/DVB molar ratio. The size and the morphology of the particles are characterized by scanning and transmission electron microscopy. Following supercritical point drying with carbon dioxide or lyophilization from benzene, the polystyrene nanoparticles exhibit a significant surface area and permanent microporosity. The dried particles comprising 35 mol % St and 65 mol % DVB possess the largest surface area, ∼205 m2/g measured by Brunauer−Emmett−Teller and ∼185 m2/g measured by the Dubinin−Radushkevich method, and a total pore volume of 1.10 cm3/g. Low pressure measurements suggest that the microporous polystyrene particles exhibit a good separation performance of CO2 over CH4, with separation factors in the range of ∼7−13 (268 K, CO2/CH4 = 5/95 gas mixture), which renders them attractive candidates for use in gas separation processes.

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polymers (ICPs).7,8 Recently, “all-organic” porous materials have attracted considerable scientific and technological interest for use in separation, sensing, gas storage, and catalysis.9,10 Organic porous polymers possess certain advantages compared to their inorganic counterparts, such as better performance, facile processing, lighter weight, and higher tolerance to moisture.10,11 There are four main classes of microporous organic polymers (MOPs), hyper-cross-linked polymers (HCPs), polymers with intrinsic microporosity (PIMs), conjugated microporous polymers (CMPs), and covalent organic frameworks (COFs). HCPs possess a surface area as high as 2000 m2/g.12,13 The porosity of these materials arises from the extensive crosslinked network which prevents the polymer chains from collapsing into a dense, nonporous material. PIMs possess a surface area in the range of 500−1064 m2/g.14,15 The porosity is the result of their rigid and contorted molecular structure which hinders the efficient packing of the polymer chains in space. CMPs possess surface areas up to 1200 m2/g when a tetrahedral monomer is utilized.16,17 Finally, COFs are crystalline porous materials, which comprise nonmetallic elements (C, H, O, B) that are linked by strong covalent bonds.18,19 The performance of MOPs in gas separation and storage is primarily determined by their microporous structure and internal surface area, whereas differences in gas solubility,

INTRODUCTION Alternative, environmentally friendly energy sources have become very attractive nowadays, due to the limitations in energy from fossil oil resources and the constant increase of world energy needs. One of the most promising candidates for the energy power solution is biogas which is both environmentally and economically attractive. However, despite these advantages the use of biogas as an alternative energy source is limited mainly because of its low quality. Apart from its main component CH4 (55−80 vol %) biogas often also contains substantial amounts of CO2 (20−45 vol %) and H2S (0−1 vol %). In particular, the presence of CO2 reduces its energy content and leads to the corrosion of natural gas pipelines. Thus the purification of biogas and the selective removal of CO2 is critical for its utilization as an efficient energy source. Numerous methods have been developed lately for the elimination of CO2 from biogas, among which are chemical absorption, cryogenic distillation, membrane separation and pressure swing adsorption (PSA).1,2 The latter method employs an adsorbent material to separate gas species from a mixture of gases, under pressure, based on the molecular characteristics of the gases and their relative affinity for the adsorbent material. PSA has certain advantages related to its high efficiency and low cost and therefore has been extensively employed in gas separation processes. So far, studies on CO2/CH4 separation utilize inorganic and hybrid microporous materials such as zeolites,1,3,4 metal− organic frameworks (MOFs)5,6 and infinite coordination © 2012 American Chemical Society

Received: November 4, 2011 Published: January 3, 2012 2690

dx.doi.org/10.1021/la204991n | Langmuir 2012, 28, 2690−2695

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Lyophilization (Method 2). A second method used to dry the polystyrene particles and obtain the porous material is the lyophilization technique. In this method, the polymer particles were first swollen in benzene using a sonicator bath. Next, the benzene dispersion was frozen in liquid nitrogen and was placed under vacuum to remove the solvent by sublimation. Material Characterization. Transmission Electron Microscopy (TEM). A JEOL JEM-2100 instrument at an electron accelerating voltage of 80 kV was employed for the measurements. TEM samples were prepared by dispersing the polystyrene particles in ethanol (∼0.06 wt %) followed by filtration with a 0.2 μm pore size syringe filter. A drop of the diluted sample was then placed on a holey carbon grid and was allowed to dry in air overnight. Scanning Electron Microscopy (SEM). Scanning electron micrographs were recorded using a field-emission JEOL 7000 electron microscope operating at 10 kV. The samples were prepared by gently placing the polystyrene particles on carbon tape and were sputtercoated with a 10 nm Au film to reduce charging. Gas Adsorption Isotherms. Gas sorption isotherms were measured on a Micromeritics ASAP 2020 sorption analyzer. Before the measurement, the samples were outgassed at 100 °C under vacuum ( 0.85 is attributed to the capillary condensation and evaporation of N2 in large interparticle voids. The materials have moderately high Brunauer−Emmett−Teller (BET) surface area and total pore volume which is strongly depended on the choice of drying method and the cross-link density of the polymer network (3a: ∼83, ∼0.33 cm3 g−1, 3b: ∼71, ∼0.28 cm3 g−1, and 4: ∼205, ∼1.10 cm3 g−1) (see Table 1). The apparent Dubinin−Radushkevich (DR)30 surface area of (3a) and (4), which was determined from the CO2 adsorption isotherms at 268 K, was found to be ∼102 and ∼185 m2g−1, respectively (Figure 2b and Table 1). The good agreement between the DR surface area and that obtained from BET analysis suggests the intrinsic microporosity of these polymeric materials. Analysis of the CO2 adsorption isotherms with the density function theory (DFT) gives a bimodal size distribution of pores with the maximum of the peaks centered at ∼5.9 and ∼7 Å for (3a) and ∼5.9 and ∼6.8 Å for (4), respectively (Figure 2c and Table 1). It is worth noting that the volume of ultramicropores (4−6 Å) within the polymer structure increased slightly with increasing the degree of cross-linking from 25% in (3a) to 65% in (4), which is clearly reflected in the difference in the relative intensity of the peaks at ∼6 and ∼7 Å, respectively. CO2/CH4 Separation. The permanent microporosity and the unique surface functionality assigned to the pendant and main chain phenyl groups of the polymer led us to examine the ability of (4) to separate small molecules, including CO2 and CH4. Sample (4) showed a significantly higher adsorption capacity for carbon dioxide compared to methane even at 268 K (Figure 3a). Analysis of the single-component isotherms using the ideal adsorption solution theory (IAST)31,32 gives the multicomponent adsorptions and selectivities for CO2/CH4 mixtures (Figure 3b). The IAST is a simple and very powerful

pj0 pi0

(6)

j

p=

∑ pi0 xi i

(7)

Isosteric Heat of Adsorption. The gas adsorption isotherms acquired at different temperatures (−10 and 0 °C) were described and analyzed using the following virial-type equation:28 m n 1 ln p = ln v + ∑ aivi+ ∑ bivi T (8) i=0 i=0 where p is the pressure in Torr, v is the amount adsorbed in mmol g−1, T is the temperature in K, and ai, bi are adjustable parameters. m, n represent the order of polynomials that are required to adequately describe the isotherms. The coverage-dependent isosteric heat of adsorption, qst, was calculated using the following expression: m

qst = − R

∑ aivi i=1

(9)

where R is the universal gas constant.

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RESULTS AND DISCUSSION Synthesis of the Porous Polystyrene Particles. Highly cross-linked polymer particles were synthesized by free-radical emulsion copolymerization of St (1) with the bifunctional monomer DVB (2) in water (Scheme 1). The cross-link Scheme 1. Schematic Representation of the Procedure Followed for the Synthesis of the Highly Cross-Linked Polystyrene Particles

density of the polymer network was varied from 25 mol % (3) to 65 mol % (4) by altering the St/DVB molar ratio between 3.0 and 0.54 in the reaction feed. The decrease in the St/DVB mole ratio increases the cross-link density and decreases the pore size of the polymer network (see Table 1). After synthesis, the particles were purified by extensive ultrafiltration in water and the samples were dried under vacuum. Next, the dried polymer samples were dispersed in 2692

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Table 1. Synthesis Characteristics, Surface Area, Total Pore Volume, and Pore Size Distribution of the Microporous Polystyrene Particle Samples surface area (m2 g−1)a

a

sample

cross-link density (mol %)

dispersion medium

drying technique

BET

DR

total pore volume (cm3 g−1)

pore size (Å)

3a 3b 4

25 25 65

ethanol benzene ethanol

supercritical point lyophylization supercritical point

83 71 205

102

0.33 0.28 1.10

5.9, 7.0

185

5.9, 6.8

BET and DR surface area determined from N2 (77 K) and CO2 (268 K) adsorption isotherms, respectively.

Figure 1. Scanning electron micrographs (a) and transmission electron microscopy images (b and c) of sample 4. Figure 1c shows a magnification of the area in the red circle of Figure 1b.

π-electron density of the phenyl groups facilitate strong dispersion or induced-dipole interactions with quadrupolar CO2, but not with the essentially nonpolar CH4 molecules (small octupole moment). These adsorptive processes promote the discrimination of CO2 over CH4 and, thus, the effective separation of the CO2/CH4 mixture. Unlike solubility separation, size-sieving discrimination of CO2 and CH4, on the basis of their molecular kinetic diameter (dk(CO2) ∼3.3 Å and dk(CH4) ∼3.8 Å), in this copolymer cannot be excluded. To elucidate the role of dispersion and the induced-dipole interactions on the adsorption selectivity processes, we also determined the heat of adsorption (qst) for the probe molecules, that is, CO2 and CH4. The isosteric heat of adsorption of CO2 and CH4 for (4), which was obtained by fitting the adsorption isotherms at 268 and 278 K to appropriate virial-type equations,12 was calculated to be ∼22 and ∼16 kJ mol−1, respectively at the limit of zero coverage (Figure 3d). The high value of qst found for CO2 supports further the presence of favorable adsorption interactions between the quatrapolar CO2 and the electron-rich polymer surface. It is also noted that the internal surface area of the microporous polystyrene particles remains unaffected in the presence of moisture or water, due to the hydrophobic nature of the polymeric material. This was verified by redispersing the supercritical point dried sample (4) in water. After drying under

method that utilizes pure gas isotherms to effectively predict mixed-gas equilibria in many porous solids (especially at low loadings close to the Henry’s law regime), including zeolites, silicates, and coordination polymers (MOFs, zeolitic imidazolate frameworks (ZIFs) and COFs).5,33−35 Sample (4) exhibited an excellent separation performance of CO2 over CH4 with separation factors in the range of ∼7−13 (Figure 3c) at 268 K for a CO2/CH4 = 5/95 mixture. The selected concentration of the gas mixture is a typical feed composition used in natural gas purification. It is worth noting that the CO2/CH4 selectivities obtained in this study are comparable or exceed those of other high-performance polymeric membranes and inorganic materials including polyimides (12−28),23 ZIF (5−10),36 MOF-5 (2−3),33,35 Cu-BTC (6−10),35,37 MCM-41 (4−5),38 and zeolite 13X (2−14).39 However, in contrast to (4), other glassy polymers, such as polyimides, suffer from carbon dioxide plasticization and swelling of the polymer matrix, especially at high CO2 concentrations. This effect can increase significantly the permeation of CH4 compared to that of CO 2 and, consequently, will reduce the CO 2 /CH 4 selectivity.40 In general, highly cross-linked networks such as 4 exhibit excellent resistance to plasticization because of the rigid polymer network structure. We ascribe the CO2/CH4 mixed-gas selectivity of 4 to the different adsorptive interactions between the probe molecules and the porous surface of the polystyrene framework. The high 2693

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Figure 2. (a) Nitrogen adsorption−desorption isotherms for sample 3a (particles of 25% cross-link density dried by supercritical drying with liquid CO2) (□), 3b (particles of 25% cross-link density dried by lyophilization under vacuum) (Δ), and 4 (particles of 65% cross-link density dried by supercritical drying with liquid CO2) (○) at 77 K. Filled and open symbols are the adsorption and desorption data, respectively; P is the gas pressure and P0 is the saturation pressure (749 mmHg). STP: standard temperature and pressure. (b) Carbon dioxide adsorption isotherms for 3a (■) and 4 (○). (c) Density function theory micropore size distribution for 3a (■) and 4 (○) calculated from the CO2 adsorption isotherms at 268 K.

Figure 3. (a) Adsorption isotherms of CO2 (filled symbols) and CH4 (open symbols) for 4, measured at 268 K. The corresponding red lines are fits to the data. (b) Mixture isotherms and (c) selectivity of CO2 over CH4 for 4 predicted by IAST for CO2/CH4 = 5/95 mixtures at 268 K. (d) Isosteric heat of adsorption of CO2 and CH4 for 4 as a function of the loading amount.

vacuum, a surface area of 280 m2 g−1 was measured, which is similar to the value found for sample (4) before its dispersion in water (see Supporting Information Figure S1).

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dioxide separation from methane. The polystyrene particles obtained after supercritical point drying from ethanol exhibited a relatively large surface area with pore sizes in the range of ultramicropores (4−6 Å). The much larger internal surface area of the polystyrene particles compared to analogous systems reported in the literature was attributed to the controlled drying procedure employed for the removal of the solvent, which prevented the collapse of the particles by eliminating surface

CONCLUSIONS

A series of highly cross-linked polystyrene particles have been synthesized by the emulsion copolymerization of styrene with divinylbenzene in water and were tested in the selective carbon 2694

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tension. The microporous polysterene particles maintained their inner surface area after redespersion in water, which indicates that the material is not moisture sensitive in contrast to other microporous materials such as the MOFs. The sample with 65% cross-link density exhibited very good separation performance in CH4 purification by capturing polar gases such as CO2 with selectivities comparable to those found in the literature for high surface area materials. In addition to CO2/ CH4 discrimination, these polymeric particles may also be used for the removal of N2O, H2S, NH3 and other polar gas species from their mixtures with nonpolar gases.

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ASSOCIATED CONTENT

S Supporting Information *

BET plot for 4 after dispersion in water and drying under vacuum at ambient temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We thank Mr. I. Spanopoulos and Mr. P. Xydias for their help with the N2 adsorption−desorption isotherms.

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dx.doi.org/10.1021/la204991n | Langmuir 2012, 28, 2690−2695