Promoting the catalytic activities of polyanilines for L-lactic acid condensation by calcium-doping: A biocompatible strategy
Yiyang Zhang , Guangshu Yuan , Xiangkun Meng , Xu Zhang , Lei Yu
L-Lactide is the key basic raw material for the synthesis of poly(lactic acid) (PLA), which is employed to produce bio-degradable materials [1] and the biomedical polymers [2]. From the sustainable development viewpoint, it is the general trend for biodegradable materials to replace traditional plastics [3]. By contrast, the added values of biomedical polymers are much higher than that of bio-degradable materials. The quality of L-lactide significantly affects the value of the downstream polymers. The optical purity of L-lactide and the metal residue content are the key indexes to evaluate the L-lactide quality: the former determines the mechanical properties of the downstream polymers [4], while the latter determines whether the product can be applied to high-end occasions, e.g., to produce biomedical polymers such as the surgical sutures and cosmetic fillers [5]. However, the current methods for the synthesis of L-lactide require the use of transition metal catalysts, such as Sn, Pb, Ce, Zn [6–8]. Methods using the light metal catalysts such as Mg and Al are also reported, but the optical purities of the produced L-lactide are poor [9]. The reaction can be catalysed by organic molecules, but the cost of the catalysts was relatively high for large-scale application [10]. Therefore, developing transition metal-free catalyst system [11] with low cost for producing high optical purity L-lactide is still a great challenge that needs to be resolved at present stage.
On the other hand, polyanilines (PANIs) are practical engineering materials because they can be synthesized from the easily available aniline monomers via the oxidative polymerization reaction [12]. They are widely employed as the conductive polymers to fabricate the electronic devices [13], as electrode material for new battery development [14] and as coating materials to protect the metal surfaces from corrosion [15]. The nitrogen groups in PANIs can well coordinate with metals. Thus, PANIs are also employed as the support to anchor catalytic metals [16]. In comparison with traditional catalysts, PANIs-supported metal catalysts (M@PANIs) run with high turnover numbers [17], and can be recycled and reused without deactivation [18]. The catalytic activities of M@PANIs can be adjusted by introducing functional groups into the aniline monomers, making this catalytic material versatile for complex organic reactions [19]. The surface morphologies of PANIs are also tunable factors for catalyst design, and the materials with large specific areas and pore volumes are considered to be preferable catalyst support endowing the sufficient contact of the reactants with the catalytic sites [20]. Recently, we unexpectedly found that, the surface properties of poly-p-anisidine (PANI-OMe) could be improved by calcium-doping. The Ca-doped PANI-OMe (PANI-OMe/Ca) could catalyse the L-LA condensation to produce L-lactide without using any transition metals. Herein, we wish to report our findings.
PANIs are synthesized via the oxidative polymerization of anilines with (NH4)2S2O8, and the doped metals can be introduced during the polymerization process using the related salts as the metal ion sources [21]. The materials are washed by water and ethanol before usage. Using this strategy, Ca-doped polyaniline and poly-p-anisidine were synthesized as typical examples of PANI catalysts, and they were marked as PANI-H/Ca and PANI-OMe/Ca for short. The related PANIs without Ca-doping (PANI-H and PANI-OMe) were also prepared for comparison. The materials were then employed as catalyst for L-LA condensation to produce L-lactide. As shown in Fig. 1a, catalyzed by PANI-H, the reaction of 50 g of L-LA produced L-lactide in only 34.5% yield with 71.4% optical purity. By using PANI-OMe as catalyst, both product yield and purity of L-lactide were enhanced to 38.9% and 82.5% respectively. Improving the catalysts by Ca-doping could enhance their catalytic activity: The reaction with PANI-H/Ca produced L-lactide in 58.9% yield with 92.0% optical purity, while the reaction PANI-OMe/Ca led to L-lactide in 74.5% yield with 98.1% optical purity. During the process, the produced L-lactide was removed by distillation under vacuum. The nonvolatile residues left inside the reaction flask contained PANI-OMe/Ca could be reused as catalyst directly just by adding fresh L-LA and heated again to restart the next run of reaction. It was found that the catalyst was robust and could be used for at least five times without deactivation, producing L-lactide in more than 70% yields with its optical purity over 98% (Fig. 1b). Notably, the reaction can be magnified to the scale using 4 kg of L-LA to produce L-lactide in 75.0% yield with 98.2% optical purity. Chiral HPLC analysis verifies the high purity of the product (Fig. S4 in Supporting information).
In PANIs-catalyzed condensation reaction of L-LA, the nucleophilic attack of nitrogen groups in PANIs to PLA oligomer was the key of the process [22]. Thus, introducing electron-donating MeO- into the aniline monomers could improve the nucleophilicity of nitrogen in PANIs to promote the activity of the catalyst, as being attested by the results in Fig. 1a. It is interesting that Ca-doping exerts even greater influences on the catalytic activity of the materials. In order to clarify the principles causing this phenomenon and get sufficient information for catalyst design in further investigations, a series of experiments characterizing the materials were then performed.
The Fourier transform infrared (FT-IR) spectra of the materials can well reflect the information of the functional groups in the materials (Fig. 2a). The quinone structure N=Q=N is reflected by the peak at 1592 cm−1, while the existence of the benzene structure N-B-N can be attested by the signals at ca. 1500 cm−1 [23], indicating that the aniline monomers have successfully polymerized. Besides, the N-H absorption peak emerges at 3227 cm−1, and the C-N stretching vibration can be reflected by the peak at 1360 cm−1 [24]. The C-H in-plane bending vibration and C-H out-of-plane bending vibration peaks are at 1152 cm−1 and 780 cm−1 respectively [25]. For MeO-containing PANI-OMe and PANI-OMe/Ca, the absorption peak of C-O can be observed at 1033 cm−1 [26]. The FT-IR spectra show that Ca-doping does not exert obvious influences on the functional group structures of the materials.
The powder X-ray diffraction (XRD) patterns of the materials show that without metal-doping, PANI-H and PANI-OMe are amorphous materials, and only typical polyaniline skeleton peaks can be observed (Fig. 2b) [27]. For PANI-H/Ca and PANI-OMe/Ca, the Ca residues exist in the form of CaSO4·2H2O, as being reflected by the typical peaks (JCPDS No. 33-0311) in XRD pattern (Fig. 2b) [28]. The narrower FWHM of Ca residue peaks in PANI-OMe/Ca indicates that they exist with better crystallinity [29]. In the XRD patterns of PANI-H/Ca and PANI-OMe/Ca, the broad peak at 2θ = 25.3° is in accordance with the signal of the facet {110} of PANI [30], showing that even the Ca-doped PANI materials are of certain crystallinity, and this is quite different to the amorphous PANI-H and PANI-OMe. The crystallinities of Ca-doped PANIs attribute to the overlaps of benzene and quinone rings in PANI chain [31].
Scanning electron microscope (SEM) image of PANI-OMe/Ca indicates that the material is formed by granular structures (Fig. 3a). Comparatively, the surface structures of PANI-OMe are even larger and this may result in less specific surface area (Fig. 3b). Obviously, Ca-doping facilitates the generation of the tiny surface structures of the materials. In the transmission electron microscope (TEM) images (Figs. 3c–e), Ca residues are reflected by the nanoscale black dots, which are firmly anchored onto the PANI-OMe fibers in gray zones. In HR-TEM image (Fig. 3f), CaSO4·2H2O d{131} spacing of 0.27 nm (JCPDS No. 33-0311) can be observed, in accordance with the XRD result (Fig. 2b). The existence of CaSO4·2H2O crystals in the material is also reflected by its electron diffraction pattern (Fig. 3g), while the energy dispersive X-ray (EDX) spectrum (Fig. 3h) shows that the material is formed by Ca, S, O, C and N elements.
During the Ca-doped PANI synthesis processes, the reaction of CaO with aqueous HCl initially furnished CaCl2. Subsequently, the oxidation reaction of anilines with (NH4)2S2O8 led to SO42−, which then reacted with Ca2+ (from CaCl2) to generate CaSO4·2H2O residue precipitated in the materials. Washing the materials with water and ethanol could remove most of the soluble inorganic species, but partial of CaSO4·2H2O and chlorides were still left in the material. This can be reflected by elemental mapping studies (Fig. 4). First, high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) images are highly sensitive to variations in the atomic number of atoms in the sample (Z-contrast images) [32]. Thus, the almost white HAADF-STEM image of PANI-OMe/Ca (Fig. 4b vs. Fig. 4a) reveals that the material exhibits a uniform composition, as evidenced by the presence of atoms with comparable weights, i.e., most of the material is formed by light elements such as C, N and O coming from the PANI-OMe skeleton, while the relatively heavier atoms such as Cl, S and Ca are rare. Moreover, as shown in Figs. 4f–h, Cl, S and Ca elements disperse within the skeleton of PANIs. The distribution of Ca is not as clear as that of Cl and S (Fig. 4h vs. Figs. 4e–g), showing that the later elements may also exist in other non-Ca salt forms, such as the Cl− and SO42− anions combined with the ammonium salt species on PANI skeleton.
Fig. 5 shows the high-resolution X-ray photoelectron spectroscopy (XPS) of Ca 2p and N 1s in PANI-H/Ca and PANI-OMe/Ca (detail data given in Tables S1 and S2 in Supporting information). The spectrum of N 1s of PANI-H/Ca can be deconvoluted into four regions. The peak at 399.4 eV attributes to =N− [33,34]. The peak near 399.9 eV reflects −NH− [33]. The signal at 401.2 eV reflects the Nα+, which is related to the polaron and bipolar nitrogen cations [35]. The peak at 402.6 eV attributes to Nβ+, related to the protonated amine unit. Due to the strong electron localization caused by the poor conjugation on the sp3 bond, the binding energy of the protonated amine unit is high [36]. For PANI-OMe/Ca, we found that the content of Nβ+ decreased (4.2% vs. 10.0%), while the content of Nα+ elevated (24.8% vs. 15.1%), indicating the increased doping of the material [37]. There is an obvious shift for the binding energy of Nβ+, verifying that introducing the electron-donating MeO- can affect the electron density around the nitrogen groups coordinating with calcium [37]. In the spectrum of PANI-OMe/Ca, the contents of =N− and −NH− are similar, resulting in better overlaps of the quinone structure and benzene structure to enhance the material crystallinity, which is consistent with the analysis result of XRD (Fig. 2b).
The XPS spectra of Ca 2p indicate that the element exists in two forms. The forms Ⅰ and Ⅱ marked in the spectra attribute to the Ca-N and Ca(Ⅱ) species respectively. Shifts of the binding energies of the Ca species verify that introducing MeO- can change the interaction of Ca with the PANI supports. In PANI-OMe/Ca, the electron density around Ca elevates to enhance the shielding effect and reduce the binding energy of the element. Furthermore, it is evident that the concentration of Ca-N species in PANI-OMe/Ca is significantly greater compared to PANI-H/Ca (59.96% vs. 53.01%). This observation confirms the enhanced interaction between calcium and the nitrogen species present in the material. Thus, in terms of the XPS spectra of both N and Ca, it can be concluded that introducing the electron-donating MeO- into aniline monomers can enhance the electron density of the nitrogen groups in prepared PANIs. This may be related to the improved catalytic activity of PANI-OMe/Ca vs. PANI-H/Ca.
Nitrogen adsorption-desorption experiments were performed to clarify the influences of Ca-doping on the material specific surface area, pore volume, as well as the pore size of the material (Fig. 6). By comparing the isotherms, it is found that the specific surface area of PANI-OMe elevates from 17.062 m2/g to 33.578 m2/g after Ca-doping (increased by 96.8%), while the pore volume of the material rises from 0.103 cm3/g to 0.380 cm3/g (increased by 268.9%). The effect of Ca-doping on the pore size of the material is even more obvious. The average pore size in PANI-OMe is 3.059 nm, indicating that it is a mesoporous material. Interestingly, by Ca-doping, the average pore size increases to 291.5 nm, i.e., PANI-OMe/Ca is a macroporous material (Fig. 6a).
It is notable that using calcium lactate same weight of calcium lactate instead of PANI-OMe/Ca led to lactide in only 22% yield with the optical purity at 76%. Thus, it can be concluded that PANIs play key roles in the reaction. On the basis of the above experimental results, material characterizations as well as the literature reports, a plausible mechanism of the PANI-OMe/Ca-catalysed condensation of L-LA to L-lactide was proposed. As shown in Fig. 7, the dehydration of L-LA initially led to the PLA oligomer [6]. The formation of the hydrogen bond between the terminal hydroxyl of PLA oligomer with the nitrogen in PANI-OMe could draw close the catalytic site with the reactants and led to the intermediate A [22]. Nucleophilic attack of the imine site in PANI-OMe to the carboxyl carbon of PLA oligomer afforded the intermediate B [38]. The XPS studies demonstrates that introducing MeO- can enhance the electron density of nitrogen in the material (Fig. 5), which is beneficial for improving the nucleophilicity of the nitrogen species [39]. The intramolecular nucleophilic reaction of the terminal hydroxyl in PLA oligomer to the carboxyl carbon of the adjacent L-LA unit produced the intermediate C. A degraded molecule of PLA oligomer was also released and it could participate the next run of the cracking reaction to generate more L-lactide molecules. Decomposition of C released the L-lactide [40] and regenerated the PANI-OMe catalyst [22]. The dimensions of the PLA oligomers are substantial. Fortunately, Ca-doping can significantly enlarge the pore size of the PANI-OMe/Ca material to 291.5 nm (Fig. 6b), allowing the entrance of the PLA oligomer molecules into the pores of the material so that the catalytic sites inside the pores could be sufficiently utilized. Thus, the catalytic activity of PANI-OMe/Ca was obviously higher than the undoped materials.
In conclusion, Ca-doped PANI-OMe is a highly effective catalyst for the condensation process of L-lactic acid to produce L-lactide in 71.4%−74.5% yields with optical purity over 98%. The nucleophilicity of nitrogen in PANIs can be enhanced through the introduction of electron-donating MeO- groups to the aniline monomers, thereby amplifying the catalytic efficacy. Ca-doping results in a notable increase in the pore size of the material, reaching 291.5 nm. This modification transforms the material into a macroporous structure, facilitating enhanced contact between the catalytically active nitrogen sites and large molecules such as the PLA oligomer. The PANI-OMe/Ca catalyst has been evaluated on a kilogram scale, yielding 75.0% with an optical purity of 98.2%.
This discovery presents a novel approach for the production of environmentally friendly biodegradable materials and biomedical polymers. Using bio-compatible Ca as the dopant of catalyst makes the related product fit for the applications in biomedical field. The produced high-purity L-lactide obtained can be used to manufacture implants and drug-delivery carriers. The findings in this work may also inspire new strategies for designing the heterogeneous catalyst systems, which has broad application prospects for synthesis [41–43]. The new technology's cost-effectiveness, transition metal-free catalyst, and environmental friendliness make it a promising solution for both industrial production and scientific research, opening up new possibilities for sustainable development.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Yiyang Zhang: Writing – review & editing, Writing – original draft, Investigation. Guangshu Yuan: Investigation. Xiangkun Meng: Investigation. Xu Zhang: Investigation. Lei Yu: Writing – review & editing, Writing – original draft, Supervision, Conceptualization.
We thank the Yangzhou Key Research and Development Program: Industry Foresight and Key Core Technology (No. YZ2023019), Cooperation Project of Yangzhou City with Yangzhou University (No. YZ2023209), Sichuan Tianfu Talent Programme (No. A.2200732), Chengdu Rongpiao Talent Pro-gramme (No. 1043), SeleValley Scholars Basic Research Project (No. 2301) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support.
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 PANI derivatives-catalyzed L-LA condensation to produce L-lactide: (a) The reactions catalyzed by PANI-H, PANI-OMe, PANI-H/Ca and PANI-OMe/Ca; (b) Catalyst recycling and reusing for the reactions with PANI-OMe/Ca catalyst.
Figure 3 Characterization of the materials: (a, b) SEM images of PANI-OMe/Ca (a) and PANI- OMe (b); (c-e) TEM images of PANI-OMe/Ca; (f) HR-TEM image of PANI-OMe/Ca; (g) electron diffraction pattern of PANI-OMe/Ca; (h) EDX spectrum of PANI-OMe/Ca.
Figure 4 Composition analysis of PANI-OMe/Ca: (a) TEM image; (b) HAADF-STEM image; and (c–h) elemental mapping images.
Figure 6 Nitrogen adsorption-desorption isotherms of PANI-OMe (a) and PANI-OMe/Ca (b).
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