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Open AccessCommunication

Is the Formation of Poly-CO2 Stabilized by Lewis Base Moieties in N- and S-Doped Porous Carbon?

by Saunab Ghosh 1 and Andrew R. Barron 1,2,3,*
1
Department of Chemistry, Rice University, Houston, TX 77005, USA
2
Department of Materials Science and Nanoengineering, Rice University, Houston, TX 77005, USA
3
Energy Safety Research Institute, Swansea University Bay Campus, Swansea SA2 8QQ, UK
*
Author to whom correspondence should be addressed.
Academic Editor: Enrico Andreoli
Received: 8 January 2016 / Revised: 19 January 2016 / Accepted: 5 February 2016 / Published: 15 February 2016
(This article belongs to the Special Issue Materials and Processes for Carbon Dioxide Capture and Utilisation)

Abstract

The polymerization of CO2 by Lewis basic moieties has been recently proposed to account for the high adsorption ability of N and S-doped porous carbon materials formed from the pyrolysis of sulfur or nitrogen containing polymers in the presence of KOH. Ab initio calculations performed on the ideal CO2 tetramer complex LB-(CO2)4 (LB = NH3, H2O, H2S) showed no propensity for stabilization. A weak association is observed using Lewis acid species bound to oxygen (LA = H+, AlF3, AlH3, B4O6); however, the combination of a Lewis acid and base does allow for the formation of polymerized CO2 (i.e., LB-C(O)O-[C(O)O]n-C(O)O-LA). While the presence of acid moieties in porous carbon is well known, and borate species are experimentally observed in KOH activated porous carbon materials, the low stability of the oligomers calculated herein, is insufficient to explain the reported poly-CO2.
Keywords: porous carbon; nitrogen; sulfur; poly-CO2; ab initio; Lewis acid; Lewis base porous carbon; nitrogen; sulfur; poly-CO2; ab initio; Lewis acid; Lewis base

1. Introduction

It has recently been reported by Hwang et al. [1] that pyrolysis of sulfur or nitrogen containing polymers in the presence of KOH results in a highly porous carbon materials, which were shown to exhibit a relatively high uptake of CO2 (0.82 g CO2/g sorbent @ 30 bar). Nitrogen-containing porous carbon (NPC) materials have previously been reported to be active for CO2 capture. In general, their uptake was in the range of 0.14–0.19 g CO2/g sorbent @ 1 bar [2,3,4,5]. Analysis of the reported data by Hwang et al., shows an uptake of between 0.11–0.19 g CO2/g sorbent @ 1 bar [1], i.e., within the range exhibited for prior materials. The high surface area/high porosity was proposed to be as a consequence of reactivity with KOH prior to pyrolysis. However, this approach is well known in the literature, for example, a similar method has been reported by Chandra, et al. [5] and investigated in detail [6], while the same N-containing polymers were reported to be porous carbon precursors that could be activated by KOH by Sevilla et al. [3]. Furthermore, sulfur containing porous carbon (SPC) has also been reported by Sevilla et al. [7] using the identical route to that subsequently reported by Hwang et al. [1]; the only difference was the measurement at high pressures in the latter study.
The suggested adsorption at high pressure was proposed [1] to be due to the polymerization of the CO2 to form poly-CO2 in which the N- or S-dopant acted as a Lewis base for the initiation of polymerization, see Figure 1. Spectroscopic data was presented as evidence for the formation of poly-CO2. In the case of Raman spectroscopy, the presence of a band at 798 cm−1 was similar to known spectroscopic characterization [8], while, the proposal of an 13C NMR peak at δ = 166.5 ppm and IR band at 1730 cm−1 due to poly-CO2 were based upon comparison with organometallic compounds of C2O4 [9,10]. The rationalization for the, heretofore, unknown formation of poly-CO2 in a porous carbon matrix was by suggesting that the nitrogen (or sulfur) Lewis base species were responsible for initiating polymerization. However, as noted above, N-doped porous carbon materials are both well-known and shown (under comparable conditions) to be efficient absorbers of CO2 [2,3,4,5]. Furthermore, except at high pressures and temperatures (40 GPa, 1800 K [8]) the only examples of oligomerized CO2 require both Lewis acid as well as Lewis base moieties [9,10]. It seems that the explanation provided is incomplete since this would have been observed previously, especially since studies at equally high pressures have been reported for a wide range of porous carbon materials [11,12,13,14].
Figure 1. Proposed polymerization mechanism for the Lewis base initiated formation of poly-CO2, where E = N or S [1].
Figure 1. Proposed polymerization mechanism for the Lewis base initiated formation of poly-CO2, where E = N or S [1].
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Given that both N- and S-doped porous carbon materials have been known for some time, and their chemistry well characterized, it is curious that the formation of poly-CO2 has not been previously reported. The report raises the question as to what is the nature of the activation of CO2 to polymerization? Can N- or S-donor atoms alone be responsible for CO2 polymerization? An understanding of this process would allow for the synthesis of other porous adsorbents for CO2, in particular those with a greater selectivity as compared CH4, N2 etc. We have therefore, investigated the stabilization of poly-CO2 with various Lewis base and Lewis acid species using ab initio methods.

2. Results and Discussion

Initial studies used an ideal CO2 tetramer complex with a suitable Lewis base model (Figure 2a, LB = NH3, H2S, H2O). The choice of NH3 and H2S was intended as a simplified model for the N- and S-dopant within the porous carbon [1]. The addition of H2O to the study was because all the NPC and SPC materials reported also contain significant oxygen [1,2,3,4,5].
Figure 2. Ideal CO2 tetramer complexes used as the starting point for ab initio calculations with (a) Lewis base terminus (LB = NH3, H2S, H2O); (b) Lewis acid terminus (LA = AlF3, B4O6, H+); and (c) both Lewis acid and base termini. The numbers for the CO2 carbon atoms represent the position with respect to the Lewis base end. The syn conformation is shown for simplicity.
Figure 2. Ideal CO2 tetramer complexes used as the starting point for ab initio calculations with (a) Lewis base terminus (LB = NH3, H2S, H2O); (b) Lewis acid terminus (LA = AlF3, B4O6, H+); and (c) both Lewis acid and base termini. The numbers for the CO2 carbon atoms represent the position with respect to the Lewis base end. The syn conformation is shown for simplicity.
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Structural optimization, through energy minimization, resulted, in each case, in the complete dissociation of at least three of the CO2 molecules. In the case of NH3 and H2O the remaining CO2 is weakly associated as seen in Figure 3a,b, respectively. The NC (2.561 Å) and OC (2.373 Å) distances are outside of ordinary covalent interactions (1.47 and 1.43 Å, respectively), but within the sum of the van der Waals radii (3.25 and 3.22 Å, respectively) [15]. In order to determine local minima a series of starting models were used. Thus, if an all anti starting configuration is used as an initial model then a slightly different spatial arrangement of the uncoordinated CO2 molecules results in an alternative local minimum, but the Lewis baseCO2 interaction remains essentially constant. For the weaker Lewis base (H2S), complete dissociation results are irrespective of starting geometry.
Figure 3. Optimized local minima structures for (a) NH3 and (b) H2O interactions with CO2.
Figure 3. Optimized local minima structures for (a) NH3 and (b) H2O interactions with CO2.
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A convenient measure of the weak interactions is the O–C–O bond angle as a function of each CO2’s distance from the Lewis base. As may be seen from Table 1 the effects of NH3 and H2O are similar and limited to the first two CO2 molecules, while that of H2S is negligible. The dipole-dipole or van der Waal interaction is slightly exothermic; however, any attempt to move the CO2 molecules closer results in a destabilization.
Table 1. Ab initio calculated geometry of the CO2 molecules as measured by the O-C-O angle (°) for various Lewis base (LB) and Lewis acid (LA) systems a.
Table 1. Ab initio calculated geometry of the CO2 molecules as measured by the O-C-O angle (°) for various Lewis base (LB) and Lewis acid (LA) systems a.
LB1234LA
NH3170.9177.7177.2177.2-
H2S179.9179.8179.9179.0-
H2O171.7176.4178.4178.4-
-177.1176.8175.9173.9AlF3 b
-177.3 c178.9179.5174.7B4O6
-179.6179.4178.1 d144.3H+
NH3 e129.8131.2128.2131.7AlF3
NMe3128.5128.9125.6129.8B4O6
NH3134.4131.9130.9129.9H+
H2S135.4132.2131.0129.9H+
H2O141.1133.8131.7130.3H+
a The numbers represent the position of the CO2 molecule within the starting structure, see Figure 2; b Local minima involving a single CO2AlF3 interaction; c Also exhibits an OB interaction; d Involved in secondary interaction with H+; e Involves N–HF intra-oligomer H-bond.
It would thus appear that the view provided by Hwang et al. [1] is insufficient to explain the presence of the spectroscopically characterized poly-CO2. Given the isolation of oligomerized CO2 in the presence of both Lewis acid and base moieties [9,10], we have investigated their effect on nitrogen and sulfur stabilized poly-CO2 of a Lewis acid. In order to remove excess KOH from the reaction, the samples were reported to be washed with acid [1]. Thus, the simplest Lewis acid will be acid residue, which is simplified as H+.
It has been found that samples prepared according to the reported procedure [1] contain variable amounts of boron (1.8%–3.5%) as determined from X-ray photoelectron spectroscopy (Figure 4). Given that the activation of the carbon precursor was with KOH and uses glassware [1] this is not unexpected. The high resolution B 1s spectrum (Figure 4b) showed a peak (193.63 eV) consistent with the B 1s peak position for sodium borate glasses (192.45 eV) [16]. As such, we have modeled potential boron Lewis acid species using the B4O6 cluster. Finally, AlF3 was used as a strong Lewis acid for comparison with H+ and B4O6.
Figure 4. X-ray photoelectron spectroscopy (XPS) survey spectrum (a) and high resolution B 1s spectra (b) showing the presence of B–O species.
Figure 4. X-ray photoelectron spectroscopy (XPS) survey spectrum (a) and high resolution B 1s spectra (b) showing the presence of B–O species.
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Initially, structural optimization was undertaken for the LA stabilized CO2 tetramer (Figure 2b). As may be seen from Table 1, the strong Lewis acid AlF3 shows two potential structures. The first higher energy form (Figure 5a) is of the same manner to the Lewis base derivatives (see Figure 3). The AlO distance (1.881 Å) is within the range for typical Lewis acid-base interactions observed from X-ray crystallography (1.8–2.0 Å) [17,18,19]; however, there appears little effect on subsequent CO2 molecules. The strong Lewis acid nature of AlF3 and the Lewis basic nature of the fluorine ligands results in a more stable (27 kJ/mol) structure involving two CO2 molecules occupying the axial positions of a trigonal bipyramidal aluminum; again the AlO distances (2.028, 2.049 Å) are within the range for such coordination geometries about aluminum (1.890(6)–2.283(2) Å) [20]. The additional CO2 molecules appear to show interactions with the fluorine ligands via Al–FC interactions (2.44, 2.49 Å); however, again there is no poly-CO2 stabilization. Like AlF3, borate (B4O6) shows a significant primary effect (Table 1, Figure 5b) with BO distances (1.594 and 1.646 Å) is slightly longer than typical Lewis acid-base interactions observed from X-ray crystallography (1.451(3)–1.502(4) Å) [21,22]. In contrast, H+ shows a significant structural effect on both the 1st and 2nd CO2, with diminishing results with distance (Table 1, Figure 5c). Again, as with AlF3 the more stable form involves the H+ hydrogen bonding between two CO2 molecules, with the OO distance (2.410 Å) being typical of an O–HO hydrogen bond [23,24].
Figure 5. Optimized local minima structures for (a) AlF3; (b) B4O6; and (c) H+ interactions with CO2.
Figure 5. Optimized local minima structures for (a) AlF3; (b) B4O6; and (c) H+ interactions with CO2.
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From the proceeding it is clear that Lewis acid species have a similar effect on the stabilization of oligomeric CO2 that is observed with Lewis bases. As prior structural characterization of oligomeric CO2 involved both Lewis acid and Lewis base moieties [9,10], we have investigated the stability of LB–(CO2)4–LA, where LB = NH3, NMe3, H2O, and H2S, while LA = AlF3, B4O6, and H+ (Figure 2c). Based upon the distortion of the CO2 molecules from linear (Table 1) and the CO distances (Table 2) the combination of both Lewis acid and Lewis base has a profound effect on the geometrical changes of each CO2, and hence the stability of the oligomer. Energy minima are calculated starting with both the all-syn (c.f., Figure 2c) and all-anti initial structures. As such the anti isomer is found to be more stable than the syn. The data shown in Table 1 and Table 2 is, therefore, for the anti isomer, except for H3N–(CO2)4–B4O6 which upon starting with the anti isomer undergoes a proton transfer from ammonia to oxygen resulting in opening of the borate cage. To obviate this effect, NMe3 was used as an “inert” analog. It is interesting to note that the “anti” conformer is actually a spiral structure, while the syn retains its planar confirmation. Representative structures are shown in Figure 6.
Table 2. Ab initio calculated geometry of the CO2 molecules as measured by the intermolecular distances (Å) for various Lewis base (LB) and Lewis acid (LA) systems. a
Table 2. Ab initio calculated geometry of the CO2 molecules as measured by the intermolecular distances (Å) for various Lewis base (LB) and Lewis acid (LA) systems. a
LBLBC1O1C2O2C3O3C4O4LALA
NH31.4671.4041.4091.4351.744AlF3
NMe31.4791.3851.4041.3851.466B4O6
NH31.5231.4171.4011.3940.969H+
H2S1.8761.4231.4021.3940.969H+
H2O1.5171.4391.4111.3980.969H+
a The numbers represent the position of the CO2 molecule within the starting structure, see Figure 2.
Figure 6. Optimized energy minima structures for (a) H3N-(CO2)4-H+; (b) H2O-(CO2)4-H+; (c) Me3N-(CO2)4-B4O6; and (d) H3N-(CO2)4-AlF3.
Figure 6. Optimized energy minima structures for (a) H3N-(CO2)4-H+; (b) H2O-(CO2)4-H+; (c) Me3N-(CO2)4-B4O6; and (d) H3N-(CO2)4-AlF3.
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Not all of the structures provided a simple oligomerization of the CO2. It was found that local minimum for AlF3 and B4O6 complexes sometimes involved secondary interactions. For example, in the cases of H3N–(CO2)4–B4O6 proton extraction (from the Lewis base) by the strongly Lewis basic oxo-sites resulted irrespective of the initial conformation employed. Thus, Me3N–(CO2)4–B4O6 was used as a model (Figure 6b). Not as marked was the secondary N–HF intra-oligomer interaction observed for H3N–(CO2)4–AlF3 (Figure 6c).
It is worth noting that the CO distances (Table 2) in the chain range between 1.385–1.439 Å, which is to the shorter side of that expected for a typical C–O single bond (1.43 Å). As may be seen, the relative effect of the Lewis acid terminus on the CO distances (Table 2) is AlF3 > H+ ≈ B4O6, while that of the Lewis base is NH3 ≈ H2O > H2S. In contrast, the O–C–O angle changes (Table 1) follow the orders H+ ≈ B4O6 > AlF3 for the Lewis acid and NH3 ≈ H2S > H2O for the Lewis base.
Based upon these results, it is clear that if poly-CO2 were formed within a N- or S-doped porous carbon material then the presence of a Lewis acid would be necessary. It should be noted that in the original report [1], HCl was used to remove excess KOH from the porous carbon after pyrolysis. Although extensive washing with H2O is described there is a possibility that residual acid was retained prior to CO2 adsorption measurements that provided the Lewis acid species. However, experimentation with different washing protocols does not alter the CO2 uptake of the NPC or SPC. As noted above, the use of KOH as an activator in glassware results in borate species being present. We have, therefore, investigated the activation of NPC samples using sodium metaborate tetrahydrate (NaBO2·4H2O) and borax (Na2B4O7), see Experimental. As may be seen from Figure 7, the CO2 uptake for these borate activated NPC materials is significantly lower than that for KOH activated analog. Thus, the presence of borate species is not associated with high CO2 adsorption in NPC; however, clearly KOH activation is important [3,5,6,7].
Figure 7. Plot of CO2 uptake (mmol CO2/g NPC) as a function of CO2 pressure (Bar) for NPC activated with (a) sodium metaborate tetrahydrate and (b) borax as compared to NPC activated with KOH.
Figure 7. Plot of CO2 uptake (mmol CO2/g NPC) as a function of CO2 pressure (Bar) for NPC activated with (a) sodium metaborate tetrahydrate and (b) borax as compared to NPC activated with KOH.
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The presence of Lewis acid sites in porous carbons has been well characterized [25,26,27]. While we acknowledge that Lewis acid sites could be associated with the N content, they are ordinarily associated with oxygen moieties, such as OH groups, in porous carbon materials. Thus, it would be reasonable to propose that if poly-CO2 species are formed at high pressure within NPC or SPC it would be stabilized by the presence of oxygen based Lewis acid sites. Attempts to model such an interaction using phenol (C6H5OH) as the protic source was unsuccessful; however, interaction of a Lewis acid/base model compound derived from dibenzo[a,j]anthracene (Figure 8) with excess CO2 resulted in the formation of the trimeric CO2 species shown in Figure 9 as a energy minimum. While only a simplified approximation of structures that could be present within NPC, this suggests that the presence of O in porous carbon materials (see Figure 4) could provide Lewis acid sites that in concert with O, N or S Lewis base species could explain the formation of poly-CO2. However, the weakness of the CO2CO2 interaction still posits against significant poly-CO2 under the conditions of 25 °C and 30 Bar.
Figure 8. Structure of the bi-functional model Lewis acid/base functionalized dibenzo[a,j]anthracene.
Figure 8. Structure of the bi-functional model Lewis acid/base functionalized dibenzo[a,j]anthracene.
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Figure 9. Optimized energy minimum structure for the interaction of the model Lewis acid/base functionalized dibenzo[a,j]anthracene (Figure 8) with CO2. Selected bond lengths and angles: NC = 1.633 Å, HO = 2.994 Å, CO = 2.545 and 2.729 Å, O–C–O = 140.1°, 175.3° and 173.8°.
Figure 9. Optimized energy minimum structure for the interaction of the model Lewis acid/base functionalized dibenzo[a,j]anthracene (Figure 8) with CO2. Selected bond lengths and angles: NC = 1.633 Å, HO = 2.994 Å, CO = 2.545 and 2.729 Å, O–C–O = 140.1°, 175.3° and 173.8°.
Carbon 02 00005 g009

3. Experimental Section

3.1. Computational Methods

Ab initio all electron molecular orbital (MO) calculations utilized non-local, gradient corrected density functional theory were performed using the Gaussian09 suite of programs [28]. For these molecules, relativistic effects were trivial and hence not taken into account. Optimization of all structures was carried out at the Hartree-Fock level with the 3-21G(*) basis set [29,30]. We have previously shown this basis set to give acceptable predictions of structural parameters for main group compounds [20,31,32,33]. To determine the relative energy of each species with electron correlation included, second order Møller-Plesset (MP2) calculations were performed [29].

3.2. Synthesis and Characterization of N- and S-Doped Porous Carbons

Sulfur and nitrogen containing porous carbon samples (SPC and NPC, respectively) were synthesized according to the protocol described previously [1,7]. Additional NPC samples were prepared using sodium metaborate tetrahydrate (NaBO2.4H2O, Sigma Aldrich, St. Louis, MO, USA) or borax (Na2B4O7, Sigma Aldrich) as the activating agent. In a typical preparation, 300 mg of polyacrylonitrile (PAN, average molecular weight 150,000, Sigma Aldrich) and 300 mg of sodium metaborate tetrahydrate were mixed in a mortar and the mixture was activated by heating at 600 °C under Ar flow (600 sccm) for 1 h. For activation by borax, 400 mg of PAN was mixed with 800 mg of borax and activated under similar condition. Finally, activated material was washed with 1 L distilled water and dried in an oven at 80 °C for 12 h.
Samples were characterized by X-ray photoelectron spectroscopy. Measurements carried out in a PHI Quantera scanning XPS microprobe (Physical Electronics, Chanhassen, USA). The wt % of chemical elements was determined by XPS survey scans with pass energy of 140 eV. For detailed elemental analysis high resolution multi-cycle elemental scans with pass energy 26 eV was performed. Each spectrum was then deconvoluted by appropriate basis functions. Before spectral fitting, each spectrum was corrected for reference binding energy for C1s to 284.8 eV. The volumetric uptake measurements (sorption and desorption) of CO2 and CH4 were performed in an automated Sievert instrument (Setaram PCTPRO, Caluire, France) [34]. Various PC samples were first crushed into powders and packed in a stainless steel autoclave sample cell. Initial sample pretreatment was carried out at 130 °C for 1.5 h under high vacuum. The free volume inside the sample cell was determined by a series of calibration procedures done under helium. Gas uptake experiments were carried out with high purity research grade CO2 (99.99% purity, Matheson TRIGAS, Basking Ridge, NJ, USA).

4. Conclusions

In summary, we have shown that the reported formation of poly-CO2 is unlikely to involve just the N or S substituents in NPC and SPC, respectively. Instead, if poly-CO2 is formed it most likely is due to the presence of a Lewis acid species in concert with a Lewis base species. The spectroscopic observation of poly-CO2 [1] may have been due to other factors in the reported materials rather than simply the presence of S or N. This fits with the lack of such spectroscopic reports in prior samples prepared by identical synthetic routes [3,5,6,7]. Based upon our simplified model structures, known molecular derivatives [9,10], and the known association of Lewis acid sites with oxygen [25,26,27], we propose that the presence of oxygen in high activity porous carbon materials is equally (or more) important than heteroatoms such as N or S. This is the subject of our future experimental studies.

Acknowledgments

This work was supported by Apache Corporation, Inc., the Robert A. Welch Foundation (C-0002), and the Welsh Government Ser Cymru Programme.

Author Contributions

Andrew R. Barron performed the calculations and analyzed the data. Saunab Ghosh prepared the porous carbon samples and performed XPS analysis. Both authors created the Figures and contributed to the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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