Sapir Shekef
Aloni
a,
Milena
Perovic
b,
Michal
Weitman
a,
Reut
Cohen
a,
Martin
Oschatz
bc and
Yitzhak
Mastai
*a
aDepartment of Chemistry, The Institute of Nanotechnology, Bar-Ilan University, Ramat-Gan 5290002, Israel. E-mail: mastai@biu.ac.il
bMax Planck Institute of Colloids and Interfaces, Potsdam-Golm Science Park, Am Mühlenberg 1 OT Golm, Potsdam 14476, Germany
cInstitute of Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany
First published on 16th November 2019
The synthesis of chiral nanoporous carbons based on chiral ionic liquids (CILs) of amino acids as precursors is described. Such unique precursors for the carbonization of CILs yield chiral carbonaceous materials with high surface area (≈620 m2 g−1). The enantioselectivities of the porous carbons are examined by advanced techniques such as selective adsorption of enantiomers using cyclic voltammetry, isothermal titration calorimetry, and mass spectrometry. These techniques demonstrate the chiral nature and high enantioselectivity of the chiral carbon materials. Overall, we believe that the novel approach presented here can contribute significantly to the development of new chiral carbon materials that will find important applications in chiral chemistry, such as in chiral catalysis and separation and in chiral sensors. From a scientific point of view, the approach and results reported here can significantly deepen our understanding of chirality at the nanoscale and of the structure and nature of chiral nonporous materials and surfaces.
Chiral nanoparticles,9–14 chiral surfaces,15–20 and chiral mesoporous materials,20–28 are attracting great attention owing to their significant advantages such as high surface area and diverse chiral functionalities that confer many advantages for catalysis, bio-recognition, chiral separation processes, and other applications. Over the years, various synthetic methods were developed for preparing chiral mesoporous materials with a strong focus on chiral mesoporous silica.21,24,26,29–43 One approach for the preparation of chiral mesoporous materials is based on molecular imprinting which is very similar to chiral imprinting of polymers.44 In other studies, various types of chiral templates led to the formation of chiral silica. For example, Álvaro et al.45 proved that combining binaphthyl and cyclohexadiyl organic template groups with a tetraethyl orthosilicate precursor induces chiral porous silica formation. The group of Avnir contributed greatly to this study of chiral mesoporous silica. In several papers, it was shown that various chiral template molecules, such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol, or tyrosine, can be used for the synthesis of chiral mesoporous silica using sol–gel matrix production.32,33,35,36 Our group also reported on the use of various chiral block copolymers as templates for the formation of chiral silica.12,30,43,46 Furthermore, Oda et al.41,47,48 investigated the self-assembly organization of chiral amphiphilic molecules by controlling several parameters and studied the mechanism of formation of chiral mesoscopic molecular assemblies. Finally, chiral anionic surfactant-templated mesoporous silica materials yielded particularly chiral well-ordered structures, and in some cases even generated complex helical rod-like structures.26,37,49 Another important class of porous materials where chirality can be easily incorporated are metal–organic frameworks (MOFs).50 The mild conditions for the synthesis of these materials allow direct incorporation of various functionalities directly into the framework.
Although nanoporous substrates have proven to be very useful in the area of chiral nanomaterials, most of those nanoporous materials are still based on silica. However, carbon-based porous materials have advantages over silica such as higher thermal and chemical stability, electric conductivity, tuneable porosity, diverse ways of syntheses, as well as a wide range of possibilities for the choice of precursors. On the other side, the significant disadvantage of porous carbon materials over silica is their less defined surface chemistry and surface energy which makes the controlled functionalization of their surface with chiral functional groups rather difficult.
Recently we have introduced a new type of chiral mesoporous carbon materials based on chiral ionic liquids (CILs). CILs, mostly based on amino acids, were used as precursors and after a carbonization process, enantioselective nanoporous carbon has been formed. In recent years, the field of CILs51–55 experienced rapid growth and new synthetic methods were established with many new applications, such as asymmetric organic synthesis, chromatography, and chiral separation but their conversion to high-surface-area chiral porous carbons remained poorly investigated.
In this article, we extend our research on enantioselective microporous carbon in view of our previous article. In this article, we describe the synthesis of a variety of CILs based on amino acids and the synthesis chiral nanoporous carbonaceous materials that were not reported in our previous article. Moreover, we apply several new techniques to characterize the chirality of chiral nanoporous carbonaceous materials such as isothermal titration calorimetry (ITC) and cyclic voltammetry (CV). Furthermore in our previous article the mechanism for the formation of chirality and the origin of enantioselective nature of the nanoporous carbonaceous materials was unclear therefrom in the article we focused on exploration of the mechanism of chirality in our nanoporous carbonaceous materials using technique of thermogravimetric analysis combined with gas chromatography and mass spectrometry (TGA-GC-MS) to investigate and confirm the chiral nature of the nanoporous carbons.
The XRD pattern of L-CIL(Tyr)-C shows an amorphous carbon structure that is typical for templated carbon materials with narrow pores, together with several distinct peaks that can be ascribed to the inorganic salts used for the synthesis of carbonaceous material (Fig. S1–S3†). This is in agreement with the results of thermogravimetric analysis (TGA), performed under the flow of synthetic air (Fig. S4†). The residual mass of approximately 24% at 1000 °C confirms the presence of remaining inorganic salts and due to organic residues that are originated from the chiral ionic liquids (see below at description of TGA-GC/MS results). The minor weight loss at low temperatures is likely due to adsorbed water molecules.
Low-pressure Ar physisorption experiments were carried out to analyze the pore structure of the chiral carbonaceous materials (Fig. 2 and Table S1†).
The use of argon as an adsorptive at 87 K offers advantages over nitrogen at 77 K in particular for micropore analysis. Namely, the absence of quadrupole moment in the case of Ar prevents the specific interactions with surface functional groups, allowing to establish a more straightforward correlation between the pressure at which adsorption occurs and the pore size. This is of particular importance for the highly polar IL-based carbons discussed here with a high density of functional groups. L-CIL(Tyr)-C exhibits a type I(b) isotherm, indicating its mainly microporous structure, with a small amount of narrow mesopores.56 The SSA of 625 m2 g−1 is significantly higher than for previously reported CIL-derived carbon materials.57 The PSD analysis (Fig. 2b) shows that the pores are centered at a diameter of 1.69 nm. The sample shows a micropore volume of 0.22 cm3 g−1 and a total pore volume of 0.26 cm3 g−1. This indicates that the salt melt acts as a template in this synthesis and is responsible for the development of significant porosity.
Raman spectroscopy is another useful tool to characterize sp2-based carbon structures by their so-called disordered (D) and graphite (G)-like bands with varying intensity, position, and width.58 In the Raman spectrum of L-CIL(Tyr)-C (Fig. S3†), the D band near 1350 cm−1 originates from the breathing modes of the six-fold sp2-hybridized carbon rings in the presence of defects and disorder. The G band near 1590 cm−1 is caused by bond stretching of sp2-hybridized carbon in either rings or chains. The peak height ratio of the D- and G-band (ID/IG) is proportional to the amount of six-membered sp2 carbon rings, which is commonly employed to evaluate the level of carbon ordering in porous carbons. After fitting the spectra with a 4-band model, the ID/IG ratio for L-CIL(Tyr)-C is determined to be 1.87, which is a typical value for porous carbon materials with high local disorder.58
In summary, it was shown that micro-sized particles of porous carbon with high surface area can be synthesized using chiral ionic liquids based on different amino acids, combined with a salt melt carbonization process. Apart from CILs based on tyrosine CIL(Tyr), phenylalanine CIL(Phe) and proline and CIL(Pro) other CILs were also successfully synthesized and carbonized.
Afterward, we studied the chiral recognition capability of the chiral nanoporous carbon. In the first set of experiments, we examined the chiral recognition of CIL(Tyr)-C. We chose adsorption of L-phenylalanine from solution on the CIL(Tyr)-C as a representative case to demonstrate the chiral recognition ability of the nanoporous carbon using circular dichroism (CD) spectroscopy. We performed selective chiral adsorption measurements of 5 mM enantiomeric solutions of L-phenylalanine. Solutions were added to different amounts of nanoporous carbon (20, 30 and 40 mg L- or D-CIL(Tyr)-C), and CD signals were measured. The optical activity, namely the CD signals, of the chiral solutions was measured after adsorption equilibrium was achieved for approximately one day. The results of L-phenylalanine adsorption onto L- or D-CIL(Tyr)-C can be seen in Fig. S8.† A comparison of the adsorption measurements clearly shows the stereoselective uptake of the nanoporous carbon, displaying a significant difference in the adsorption of the two enantiomers. For example, approximately 2.90 mmol of L-Phe were adsorbed on 40 mg of D-CIL(Tyr)-C, while only 2.28 mmol of L-Phe was adsorbed on 40 mg of L-CIL(Tyr)-C. The CD results confirm the preference of L-phenylalanine for chiral adsorption on the D enantiomer of CIL(Tyr)-C. Based on the equilibrium concentrations of L-Phe, an equilibrium chiral discrimination ratio of 1.34 was calculated. This figure is relatively high for nanoporous materials with chiral functionalization and is sufficient for carrying out a successful enantiomeric separation on the chiral nanoporous carbon.
In order to support these findings and further demonstrate the enantioselective nature of CIL(Tyr)-C, isothermal titration calorimetry (ITC) was performed to measure the adsorption enthalpy of enantiomers onto CIL(Tyr)-C. In recent years it was shown that the ITC method can be used for detecting and measuring the chirality of nanomaterials;59–62 the enantioselective factor is obtained by calculating the difference between the adsorption enthalpies of the two enantiomers.
In a typical chiral ITC experiment, a solution of a chiral probe (e.g., enantiomers of amino acids) is titrated into the sample cell with the nanoporous carbon powders. The heat (ΔH) released due to molecular interactions is monitored as a function of time. Each peak represents the change in heat associated with the injections of the chiral probe solutions into the nanoporous carbon in the ITC reaction cell. The total heat of interaction is determined by the area under the peaks. The main contribution to the total change in enthalpy (ΔH) is due to the enthalpy of chiral binding between the D and L enantiomers and the chiral carbon. However, other enthalpy changes can result from the heat of dilution of the chiral solutions and from nonspecific heat effects, and these are calculated and corrected using a number of control experiments. In our ITC experiments, we used enantiomers of L- and D-tartaric acid (TA) as the chiral probe, and the ITC experiments were carried out in the following manner: a 1 mg mL−1 sample of L- or D-CIL(Tyr)-C was placed in the ITC cell, and 5 μL of a 10 mM D- or L-TA solution was injected into the suspension of the microporous carbon. The distinct difference between the D and L microporous carbon is shown in Fig. 4. It is evident that when L-TA is injected into L-CIL(Tyr)-C microporous carbon, the maximum heat flow for the first ITC peak is ca. 1.79 μcal s−1, whereas when D-TA is injected, the maximum heat flow in the first peak reduces significantly to ca. 1.56 μcal s−1. From the integration of the ITC peaks vs. injection number, the heat of adsorption for tartaric acid on L-CIL(Tyr)-C can be calculated, as shown in Fig. 3c. Moreover, the integration of the ITC curves in Fig. 3c represents the total heat of adsorption, of each enantiomer (ΔHads). We calculated ΔHabs values of 5.82 cal mol−1 for L-TA and 4.94 cal mol−1 for D-TA. These results prove the chiral nature of CIL(Tyr)-C and show an enantioselectivity value of about 1.17, similar to the results obtained in the CD chiral adsorption measurements. A similar ITC experiment was performed with L- and D-TA solutions injected into D-CIL(Tyr)-C nanoporous carbon, and the results of these experiments are presented in Fig. 3b and d. In this case, the maximum heat flows for the first ITC peak are about 1.8 and 1.66 μcal for D- and L-TA, respectively, with corresponding enthalpies of chiral binding of 5.82 and 4.94 cal mol−1, the exact opposite values with respect to L-CIL(Tyr)-C, confirming the enantioselectivity value of 1.18. Hence, ITC nicely shows the chiral recognition of CIL(Tyr)-C. The chirality of the nanoporous carbon is shown to be dependent on the chirality of the tyrosine; nanoporous carbon prepared from L-tyrosine shows a preference for adsorption of L enantiomers, while the nanoporous carbon of D-tyrosine shows a preference for adsorption of D enantiomers. It should be noted that our results from the CD selective chiral adsorption and ITC measurements are in line with results reported for chiral nanoporous materials, for example, silica, that were prepared by molecular imprinting methods.36,43 In other words, the chirality of the nanoporous materials is determined by the chirality of the precursor molecules. In our case, nanoporous tyrosine-derived carbon, which originated from the chiral ionic liquid of L-tyrosine, had L chiral recognition, while carbon synthesized from the CIL of D-tyrosine had D chiral recognition.
Next electrochemical techniques such as cyclic voltammetry (CV) can be used to investigate the chirality of surfaces.63,64 In a set of CV experiments, we examined the chiral recognition ability of CIL(Tyr)-C by using it as an electrode. We prepared carbon electrodes for the CV experiments from CIL(Tyr)-C as follows: 6 mg of the mesoporous carbon was mixed with 4 mg of carbon black and 0.2% NAFION wt% to give the optimal combination for preparing the carbon electrode. The chiral solutions for the CV measurements were L- and D-TA (5 mM), with Na2SO4 (0.1 mM) as supporting electrolyte. Cyclic voltammetry was performed with D and L carbon electrodes, and their electrochemical activities were measured. From the electrochemistry literature, it is known that oxidation of TA occurs at about 0.6–0.7 V vs. RHE on the carbon electrodes. The results of the CV measurements are shown in Fig. 4; the oxidation peaks for solutions of the L and D enantiomers of TA show a different shape, depending on the chiral nature of the mesoporous carbon electrodes. In order to quantify the different electrochemical behavior, the total electrochemical current for the oxidation process, Q, can be calculated from the areas below the oxidation of the CV curves. The ratio between the total current for oxidation of D and L-CIL(Tyr)-C carbons (QD/QL) can give an indication of the chiral selectively of the carbon electrodes. The results show that the D-CIL(Tyr)-C electrode selectively oxidizes more of the L enantiomer of TA, while the L-CIL(Tyr)-C electrode selectively oxidizes more of the D enantiomer. The chiral recognition of tartaric acid enantiomers can be calculated by the total current ratios: for the D-CIL(Tyr)-C electrode the enantioselectivity factor is QD/QL = 1.068, while for the L-CIL(Tyr)-C electrode the enantioselectivity factor is much greater: QL/QD = 1.393.
It should be emphasized that the CV measurements display a chiral behavior that is inverse compared to the CD and ITC measurements. The electrochemical measurements demonstrate chiral recognition that is inverse with respect to the chirality of the mesoporous carbon, i.e. the electrode based on L-CIL(Tyr)-C shows high chiral recognition for the D enantiomer of TA, while the other electrode, based on D-CIL(Tyr)-C, shows high chiral recognition for the L enantiomer. It is still unclear to us what might be the reason for the observed difference in chiral recognition. One possible explanation is based on the nature of the CV measurements which differs from that of the chiral adsorption from CD and ITC measurements. In the latter, we measured the chiral recognition at chemical equilibrium, while in the electrochemical measurements we probed the chiral recognition under dynamic and diffusion conditions of electrochemical reactions. This explanation is supported by other studies that reported the preferred formation of homochiral domains of TA enantiomers on metallic surfaces due to weak interactions between TA and the metallic surface. Nevertheless, the electrochemical studies have clearly demonstrated the chiral nature and enantioselective properties of the carbon electrodes.
In summary, all the techniques used in this work clearly indicate the enantioselective nature of the carbons, however, still the main question regarding the origin of the chirality in our carbon materials is unclear. Overall, in chiral nanoporous materials such as silica, the molecular mechanism leading to the formation of chirality is based on the chiral templating mechanism. Nevertheless, in our case, we assume that the chiral nature of our carbon is due to the presence of chiral residues of the ionic liquids used as the source of carbon in our synthesis method. It is known from previous studies on nanoporous carbons prepared with zinc ions that zinc ions are able to coordinate and stabilize polar organic functional groups even at elevated temperatures65,66 In our previous work57 we speculated that chiral organic motifs remain in the carbonized scaffold after the carbonization process. This hypothesis was supported by thermogravimetric analysis combined with gas chromatography and mass spectrometry (TGA-GC-MS).
In order to verify our assumption regarding the mechanism for the chirality in our carbon, we decided to perform a series of TGA-GC-MS measurements of the pure chiral ionic liquids and of the carbons. TGA-GC-MS is a powerful analytical methodology for composite materials. In these experiments, we can analyze the volatile compounds that are released from the carbons so that a combination of GC/MS and TGA can give another dimension of understanding of the carbon structure and the origin of chirality. All the TGA-GC/MS measurements were carried out under nitrogen atmosphere. First, we measured the TGA-GC/MS profile for the pure L-CIL(Tyr) as shown in Fig. 5a(i) and b(i). From the TGA curve is can be seen that the pure L-CIL(Tyr) decomposition from ca.120 °C up to 550 °C and the maximum decomposition rate is observed at 310 °C. The pyrolysis at 310 °C of L-CIL(Tyr) gave three main products: I, II and III with designate mass (and their corresponding base fragment peak) characterized each compound: 178 (119), 192 (161), 178 (147) for I, II, III respectively. The analysis of L-CIL(Tyr)-C in TGA-GC/MS is not a trivial one since it consists of a mixture of organic and inorganic compounds and where the organic compounds are found only in trace amounts (12% weight loss). Consequently, the TGA thermogram feature is of a moderate slope, indicating a slow release of the organic components. The emitted gases from the L-CIL(Tyr)-C at 400 °C were collected and accumulated in a 100 μL loop and the results are shown in Fig. 5a(ii) and b(ii). Then the mixture was injected and separated using a capillary GC column. Since the extent of the analysts is extremely low, in the total ion chromatogram (TIC) we could not detect the presence of organic compounds that are related to chiral liquid L-CIL(Tyr). However, when we examined the extracted ion chromatograms (EICs) of the designated masses, the chromatogram contained the same three major components as in the L-CIL(Tyr). All three major thermal products are related to L-CIL(Tyr) that releases its quaternary ammonium cation. The mass spectrum of the peak at retention time (RT) 7.88 minutes closely matches to methyl 2,3-dihydro-1-benzofuran-3-carboxylate, I, that seems to be a thermal rearrangement product. The peaks at RTs 8.40 and 8.86 are the degradation products methyl p-methoxycinnamate II and methyl p-hydroxycinnamate III that may form by elimination reactions.
Fig. 5 Gas chromatography and mass spectrometry, (RT and MS chromatograms) and the TGAs profiles of L-L-CIL(Tyr) and L-CIL(Tyr)-C. |
In conclusion, the above have showed two important findings first that the carbon prepared at 450 °C still contain organic residues and that those organic residues are originated from the chiral ionic liquids. In general, these results prove our claims that chiral organic motifs remain attached to the carbonized scaffold in the final carbon, and those chiral functional groups provide the chiral recognition observed in our carbon.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9na00520j |
This journal is © The Royal Society of Chemistry 2019 |