Effect of canopy structures and their steric interactions on CO₂ sorption behavior of liquid-like nanoparticle organic hybrid materials

Abstract

Liquid-like NOHMs with different grafting densities of polymeric canopy were synthesized to evaluate their solvating properties as CO₂ solvents. The in situ ATR FT-IR study of NOHMs with linear and branched canopies revealed distinct CO₂ capture and corresponding swelling behaviors. These observations suggested that the entropic contribution for CO₂ sorption in NOHMs can be tuned via the canopy design.

---

Liquid-like nanoparticle organic hybrid materials (NOHMs), which are synthesized by ionically or covalently grafting polymeric canopy species onto surface-modified inorganic nanoparticles, exhibit versatile physicochemical characteristics, ranging from glassy solids to solvent-free nanoparticle fluids.^1,2 A series of NOHMs has been synthesized with various types of inorganic cores,^3–5 and the hybrid's unique and striking diversity allows NOHMs to be potentially applicable in a variety of areas, such as electrolytes, reaction solvents, thermal management materials, magnetic fluids, and lubricants.^6,7

As a smart platform material, liquid-like NOHMs possess promising potential for energy and environmental applications, offering environmentally benign and highly tunable properties. Owing to their canopy, which consists of oligomeric or polymeric species, NOHMs exhibit a fluid character without the addition of solvent. Moreover, since NOHMs have a negligible vapor pressure and a high thermal stability, they can be utilized as a “green solvent”. The introduction of task-specific functionalities along the canopy chains further enhances the selectivity of chemical interactions of NOHMs for specific molecules. Furthermore, inorganic nanocores can also be designed with catalytic functionalities to facilitate different reactions.

In previous studies, it has been proposed that the novel liquid-like organic–inorganic hybrids can capture CO₂ effectively through both enthalpic and entropic contributions. Hypothetically, the free energy of the frustrated canopy species due to steric and/or entropic effect could be reduced by introducing small gaseous molecules into the organic canopy matrix, whereas grafting task-specific functional groups, such as amines, along the canopy chains provides enthalpic CO₂ capture via specific intermolecular interactions.⁸⁻¹¹ Thus, both the enthalpic and entropic contributions of NOHMs in CO₂ capture should be optimized in order to develop liquid-like NOHMs as an alternative CO₂ capture media.

Thus far, different designs of NOHMs have been synthesized to investigate their solvating properties, focusing on the conformational structures of canopy species induced by CO₂ sorption through spectroscopic observations such as nuclear magnetic resonance (NMR) and Fourier-transform infrared (FT-IR) spectroscopies.^8,10 The findings imply that the inter-canopy interactions associated with CO₂ sorption is highly tunable through the design of structures of organic canopy species; further, it may create a favorable pathway of polymeric canopy network and/or ‘unoccupied spaces’ for CO₂.

Here, the CO₂ sorption behavior of newly synthesized NOHMs with branch-structured canopy species was investigated by employing in situ attenuated total reflectance (ATR) FT-IR spectroscopy in order to investigate the effect of canopy structures on the entropic CO₂ sorption behavior of NOHMs. The results, including the solubility of CO₂ in the newly synthesized NOHMs with varying grafting canopy density, were then compared with NOHMs synthesized with linear-structured canopies to reveal the effect of the steric interaction on CO₂ solvating behaviors.

NOHMs used in this study we prepared as described in previous papers,^1,8,10 and their schematic syntheses are shown in Scheme 1. Briefly, surface-modified nanoparticles (1) were first prepared by grafting sulfonic group-terminated silane molecules onto the surface of colloidal silica suspension (7 nm diameter, LUDOX SM-30, Sigma-Aldrich). This step was conducted at 70 °C and took 24 hours. Next, either one of two different canopy materials – linear- or branched polyether amine (2, Jeffamine M-2070, Huntsman Co. (The Woodlands, TX)) and ethoxylated amine (3, Ethomeen 18/25, Akzo Nobel Surface Chemistry LLC (Chicago, IL)) – was added to the solution containing the functionalized silica nanoparticles. Various amounts of these polymers were used in order to synthesize NOHMs with varying grafting densities. The highest grafting density corresponded to the case where all sulfonic groups of the silane reacted with the polymers. The resulting NOHMs structures (4 & 5) are displayed in Scheme 1. Table 1 shows the grafting densities of polyether amine and ethoxylated amine tethered NOHMs. The grafting densities were determined using thermogravimetric tests. These analyses enable to determine the relative proportions of inorganic (SiO₂ nanoparticles) and organic components (silane + polymeric canopy). Then, by considering the molecular weight of the polymers and the surface area of the silica nanoparticles, the grafting densities were calculated. The structural conformation of the materials was also investigated using spectroscopic approaches including FT-IR, 2D NMR and Raman. The results can be found in prior studies.^8–11 Briefly, FT-IR was used to confirm the grafting of the polymer chains onto the functionalized nanoparticles. Raman and 2D NMR on the other hand, enabled the characterization of the degree of ordering of the polymer chains along with the extent of interactions between them. It was found that NOHMs exhibited greater ordering and less chain interactions than in pure polymer and this effect was further enhanced by decreasing the grafting density.

Scheme 1 Schematic structures of NOHMs decorated with different canopy materials: surface modified nanoparticle (1), linear (2) and branched (3) canopy materials, and the synthesized NOHMs (4 & 5).

Table 1 Grafting densities of NOHMs

Canopy material	Grafting density,^a (canopy nm^−2)	NOHMs' notation
a Grafting densities of polymeric canopy species were measured by using thermogravimetric analyzer (ESI^1).
2	2.5	4a
2	3.8	4b
3	3.3	5a
3	4.2	5b

---

In order to investigate the effect of canopy structures on the entropic CO₂ capture, task-specific functional groups, such as amines, which are expected to offer significant CO₂ capture performance through enthalpic contributions, were deliberately excluded from the organic species in NOHMs. Although the materials used in this study include ether groups in polymeric canopies, which can interact with CO₂ via Lewis acid–base interaction,¹² the binding energy between CO₂ and Lewis basic sites of ethers is relatively weak (i.e. ΔH = ∼−16 KJ mol⁻¹ for CO₂–dimethylether complex)¹³ compared to the amine's (i.e. ΔH = −80 to −130 KJ mol⁻¹ for bicarbonate or carbonate).¹⁴ Thus, strong inclinations for chemical reactions with CO₂ could be minimized.

It has been known that only the ν₂ band in the IR-active fundamental vibrations for CO₂ exhibits distinguishable peak patterns coupled with weak Lewis acid–base interaction.¹⁵ Thus, an investigation of ν₂ modes in differently structured NOHMs may be useful in identifying the underlying CO₂ capture mechanisms of NOHMs and the relationship between structural variation and CO₂ capture capacity. The FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific Inc. (Madison, WI)) used in this study was equipped with a deuterated triglycine sulfate (DTGS) detector and an ATR accessory (Golden Gate™ Supercritical Fluids analyzer, Specac Ltd. (UK)).

Fig. 1 shows ATR FT-IR spectra of impregnated CO₂ in linear- and branch-structured NOHMs in the range of ν₂. In general, CO₂ in vapor phase exhibits a singlet at the frequency of ∼668 cm⁻¹ in mid-IR range. In the case of CO₂ absorbed in NOHMs, however, band splitting was observed due to the Lewis acid–base interaction between CO₂ and ether groups in NOHMs eliminating the double degeneracy. The broad bands were assigned to doublets of ∼660 and ∼650 cm⁻¹, which are associated with the out-plane and in-plane modes of ν₂, respectively.¹⁵ The band at 668 cm⁻¹ was confirmed to be vapor CO₂ entrapped in the interface between samples and the diamond crystal in ATR optics. As shown in Fig. 1, two types of NOHMs exhibit similar broad band in ν₂ region with opposite band maximum.

Fig. 1 ATR FT-IR spectra of ν₂ (CO₂) absorbed in (a) linear- and (b) branch-structured canopies in NOHMs. Each spectrum was measured at 4.1 MPa and 298 K. The dotted lines are the curve fit of measured spectra (×).

Although each pure unbound canopy species and their corresponding NOHMs exhibit very similar band shapes, the area ratios between deconvoluted peaks at ∼660 and ∼650 cm⁻¹ exhibit quite different patterns of CO₂ pressure and/or capture capacity as reported previously (for the deconvolution data of ν₂ mode see ESI†).^8,10 It was speculated that the differences in the area ratios between unbound polymers and those grafted onto the nanocores resulted from their structural/conformational differences. Since Lewis acid–base interaction was the only presumable intermolecular interaction of NOHMs with CO₂, the frustrated and more ordered structural arrangement of canopy species in NOHMs may have altered its CO₂ ‘packing’ behavior toward improved accessibility to the Lewis basic sites, i.e. the ether groups of the canopies.

In order to estimate the strength of inter-molecular interaction between CO₂ and Lewis basic sites of NOHMs' canopies, the band width of the ν₂ doublets was measured and the result is shown in Table 2. It has been reported that the ν₂ band width increases with the increase of the strength of the interaction of CO₂ with the Lewis bases in polymers or ionic liquids.^15,16 In both cases of linear and branched NOHMs, the band width increased as the polymeric canopy species were tethered onto the functionalized nanocores to create liquid-like NOHMs. Interestingly, the difference in the band width of the NOHMs with branch-structured canopy (5a & 5b vs. 3) was notably larger than that of linear chain NOHMs (4a & 4b vs. 2). The result suggests that varying the grafting density and/or the canopy conformational structure can alter the strength of inter-molecular interactions in liquid-like NOHMs, in this case interactions between CO₂ and ether groups in NOHMs.

Table 2 Band maximum and half-width of absorption band of ν₂ mode of CO₂ in NOHMs

Measurement	Pressure,^a (MPa)	Wavenumber,^b (cm^−1)	Δν1/2,^c (cm^−1)
a At 298 K.b Band maximum of ν2 absorption band.c Δν1/2 = averaged value of full width at half-maximum (FWHM) of the ν2 absorption band in the pCO2 range of 0.6–5.4 MPa.
2	5.5	650.8	20.0
4a	5.5	651.8	20.1
4b	5.5	651.8	20.4
3	5.5	657.6	18.1
5a	5.5	658.6	18.5
5b	5.5	658.6	18.9

---

In order to further investigate the steric hindrance effect on CO₂ sorption in NOHMs, the CO₂-induced swelling behaviors of NOHMs were observed at various CO₂ loadings. The in situ ATR FT-IR was employed to simultaneously measure the swelling and CO₂ solubilities, and the detailed experimental method can be found in previous studies.^8–11 It is believed that polymers with highly branched chains may create more ‘free volume’, since polymeric species cannot pack tightly together due to steric interactions.¹⁷ As shown in Fig. 2(a), the canopy species bound to NOHMs (4a & 4b and 5a & 5b) exhibited a lower volume increase than their corresponding unbound canopy chains (2 and 3) at the same CO₂ pressure conditions. For both the branch-structured and linear canopy cases, NOHMs with lower grafting density (4a and 5a) resulted in further reduced CO₂-induced swelling, which was consistent with the previous study.^8,10 A recent study revealed that the increasing grafting density of polymeric canopy can increase inter-canopy interaction while decreasing degree of order of bound canopy, and vice versa.¹⁰ Thus, it may be possible to tune the frustrated canopy chains bound onto the nanoparticle by varying the canopy’s structure and grafting density, and as a result, introducing CO₂ into the NOHMs can exhibit distinct sorption behavior, particularly in terms of swelling and/or CO₂ capture capacity.

Fig. 2 CO₂-induced swelling behavior of linear- and branch-structured canopy tethered NOHMs with varying grafting densities as a function of (a) CO₂ pressure and (b) CO₂ capture capacity measured at 298 K.

Comparing NOHMs with different steric hindrance, the differences of the CO₂-induced swelling between unbound and bound canopy species were larger in NOHMs with branched canopy (3 vs. 5a & 5b) than NOHMs with a linear polymer chain (2 vs. 4a & 4b). As shown in Fig. 2(b), when the swelling data were correlated to the CO₂ capture capacity, the structural effect on NOHMs' CO₂ capture and swelling behaviors became even more evident. Interestingly, the NOHMs with branched canopy (5a & 5b) exhibit relatively higher CO₂ capture capacity than the corresponding unbound canopy (3) and, moreover, NOHMs with higher grafting density (5b) exhibit higher CO₂ capture capacity than those with lower grafting density (5a). On the other hand, significant differences between them were not observed in the NOHMs having linear canopies. The result indicate that the canopy's conformational structures induced by parameters such as canopy structure and grafting density can create a favorable pathway for CO₂ interaction through the specific functional sites in the NOHMs' canopy matrix with improved accessibility.

In summary, the effect of the canopy structures and steric hindrance in NOHMs on CO₂ sorption behavior was investigated via in situ ATR FT-IR spectroscopy. The results indicated that the CO₂ sorption and CO₂-induced swelling behaviors of NOHMs were tunable by altering the canopy materials' conformational structures. Both the branched canopy structure and lower grafting density led to more ordered arrangement of canopy chains within NOHMs, and this resulted in higher CO₂ loading with smaller volume increase for NOHMs compared to the unbound canopy. The findings from this study will be valuable for further optimization of NOHMs' design for CO₂ capture.

This publication was based on work supported by Award no. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST).

Notes and references

1. (a) A. B. Bourlinos, S. R. Chowdhury, R. Herrera, D. D. Jiang, Q. Zhang, L. A. Archer and E. P. Giannelis, Adv. Funct. Mater., 2005, 15, 1285; (b) R. Rodriguez, R. Herrera, L. A. Archer and E. P. Giannelis, Adv. Mater., 2008, 20, 4353.
2. (a) P. Agarwal, H. Qi and L. A. Archer, Nano Lett., 2010, 10, 111; (b) M. L. Jespersen, P. A. Mirau, E. Meerwall, R. A. Vaia, R. Rodriguez and E. P. Giannelis, ACS Nano, 2010, 7, 3735.
3. (a) A. B. Bourlinos, S. R. Chowdhury, D. D. Jiang, Y.-U. An, Q. Zhang, L. A. Archer and E. P. Giannelis, Small, 2005, 1, 80; (b) A. B. Bourlinos, R. Herrera, N. Chalkias, D. D. Jiang, Q. Zhang, L. A. Archer and E. P. Giannelis, Adv. Mater., 2005, 17, 234; (c) A. B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Herrera, S. H. Anastasiadis, D. Petridis and E. P. Giannelis, Small, 2006, 2, 513; (d) A. B. Bourlinos, E. P. Giannelis, Q. Zhang, L. A. Archer and G. Floudas, Eur. Phys. J. E, 2006, 20, 109; (e) Y. Zheng, J. Zhang, L. Lan, P. Yu, R. Rodriquez, R. Herrera, D. Wang and E. P. Giannelis, ChemPhysChem, 2010, 11, 61.
4. (a) S. C. Warren, M. J. Banholzer, L. S. Slaughter, E. P. Giannelis, F. J. DiSalvo and U. Wiesner, J. Am. Chem. Soc., 2006, 128, 12074; (b) A. A. Voevodin, R. A. Vaia, S. T. Patton, S. Diamanti, M. Pender, M. Yoonessi, J. Brubaker, J.-J. Hu, J. H. Sanders, B. S. Phillips and R. I. MacCuspie, Small, 2007, 3, 1957.
5. (a) A. B. Bourlinos, V. Georgakilas, N. Boukos, P. Dallas, C. Trapalis and E. P. Giannelis, Carbon, 2007, 45, 1583; (b) A. B. Bourlinos, V. Georgakilas, V. Tzitzios, N. Boukos, R. Herrera and E. P. Giannelis, Small, 2006, 2, 1188; (c) A. W. Perriman, H. Colfen, R. W. Hughes, C. L. Barrie and S. Mann, Angew. Chem., Int. Ed., 2009, 48, 6242; (d) N. Fernandes, P. Dallas, R. Rodriguez, A. B. Bourlinos, V. Georgakilas and E. P. Giannelis, Nanoscale, 2010, 2, 1653.
6. D. Kim and L. A. Archer, Langmuir, 2011, 27, 3083.
7. (a) S. S. Moganty, N. Jayaprakash, J. L. Nugent, J. Shen and L. A. Archer, Angew. Chem., Int. Ed., 2010, 49, 9158; (b) J. L. Nugent, S. S. Moganty and L. A. Archer, Adv. Mater., 2010, 22, 3677.
8. Y. Park, J. Decatur, K.-Y. A. Lin and A.-H. A. Park, Phys. Chem. Chem. Phys., 2011, 13, 18115.
9. Y. Park, D. Shin, Y. N. Jang and A.-H. A. Park, J. Chem. Eng. Data, 2011, 57, 40.
10. C. Petit, Y. Park, K.-Y. A. Lin and A.-H. A. Park, J. Phys. Chem. C, 2011, 116, 516.
11. K.-Y. A. Lin and A.-H. A. Park, Environ. Sci. Technol., 2011, 45, 6633.
12. S. P. Nalawade, F. Picchioni, J. H. Marsman and L. P. B. M. Janssen, J. Supercrit. Fluids, 2006, 36, 236.
13. K. H. Kim and Y. Kim, J. Phys. Chem. A, 2008, 112, 1596.
14. I. Kim and H. F. Svendsen, Int. J. Greenhouse Gas Control, 2011, 5, 390.
15. S. G. Kazarian, M. F. Vincent, F. V. Bright, C. L. Liotta and C. A. Eckert, J. Am. Chem. Soc., 1996, 118, 1729.
16. S. G. Kazarian, B. J. Brisco and T. Welton, Chem. Commun., 2000, 2047.
17. A. Peacock and A. Calhoun, Polymer Chemistry: Properties and Applications, Carl Hanser Verlag, Munich, 2006.

---

Footnote

† Electronic supplementary information (ESI) available: TGA data and deconvolution data of ATR FT-IR. See DOI: 10.1039/c3ra46801a

---