Highly Efficient AuPd Catalyst for Synthesizing Polybenzoxazole with Controlled Polymerization

Abstract

•Establishes a controllable seed-mediated growth of AuPd nanoparticles•Builds a relationship between the sizes of nanocatalyst and macroscopic product•Develops a heterogeneous and stable nanomaterial for tandem polycondensation•Offers a greener synthesis to solve the practical hydrolysis problem of Zylon Dimension- and property-controlled nanoparticles (NPs) have become attractive new classes of heterogeneous catalysts for various chemical reactions, especially tandem reactions, by reducing the numbers of reaction steps and by increasing the reaction efficiencies toward targeted products. Here, we report an AuPd NP system as an active and stable catalyst to catalyze one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde, leading to the controlled polymerization and formation of polybenzoxazole (PBO). The highly pure PBO shows excellent thermal stability up to 600°C and improved chemical and mechanical stability compared with phosphoric acid-contaminated commercial PBO (Zylon, Mw = 40 kDa). The reported NP-catalyzed one-pot polymerization can be easily extended to prepare various rigid organic polymers that are important for ballistic fiber, anti-flame, smart-textile, and ionic separation membrane applications. Using nanoparticles (NPs) to catalyze chemical reactions for the formation of functional polymers with controlled polymerization is an important field of chemical research. In this article, we report an AuPd NP system as an active and stable catalyst to catalyze one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde, leading to the controlled polymerization and the formation of polybenzoxazole (PBO) (Mw = 3.6 kDa). The one-pot reaction is AuPd NP size and composition dependent, and 8-nm Au39Pd61 NPs are the best catalyst for the polymerization. The highly pure PBO shows excellent thermal stability up to 600°C and improved chemical and mechanical stability under challenging environmental conditions compared with commercial PBO (Zylon, Mw = 40 kDa). The reported NP-catalyzed one-pot reaction to polymerization is not limited to the formation of PBO but can be extended as a general approach to rigid polymers that are important for ballistic fiber, anti-flame, and smart-textile applications. Using nanoparticles (NPs) to catalyze chemical reactions for the formation of functional polymers with controlled polymerization is an important field of chemical research. In this article, we report an AuPd NP system as an active and stable catalyst to catalyze one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde, leading to the controlled polymerization and the formation of polybenzoxazole (PBO) (Mw = 3.6 kDa). The one-pot reaction is AuPd NP size and composition dependent, and 8-nm Au39Pd61 NPs are the best catalyst for the polymerization. The highly pure PBO shows excellent thermal stability up to 600°C and improved chemical and mechanical stability under challenging environmental conditions compared with commercial PBO (Zylon, Mw = 40 kDa). The reported NP-catalyzed one-pot reaction to polymerization is not limited to the formation of PBO but can be extended as a general approach to rigid polymers that are important for ballistic fiber, anti-flame, and smart-textile applications. 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Controlling the degree of polymerization is an essential step in obtaining polymer materials with desired macroscopic chemical and physical properties for various applications. Conventional syntheses rely on reactions among monomers under reaction conditions whereby chain transfer agents are typically needed to control polymerization degrees. These processes have been applied to produce many different kinds of polymers of commercial importance, such as Kevlar, Dacron, Kodel, Lexan, Lycra, and Zylon. Despite the common uses of these conventional methods for polymer productions, the reactions still have issues in rationally controlling the degrees and purities of polymerizations. Here, we report a new process of synthesizing highly pure polybenzoxazole (PBO) with controlled polymerization via AuPd NP-catalyzed one-pot reaction. PBO, or Zylon for the commercial product, is a subclass of polybenzoazoles. 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As a result, they are inevitably contaminated with phosphoric acid (PA) units that can catalyze the hydrolysis of the benzoxazole ring upon its exposure to humid and lighted environments (Figure S1), causing unexpected and fast degradation of the mechanical integrity of the polymer fibers.34Kanbargi N. Hu W.G. Lesser A.J. Degradation mechanism of poly(p-phenylene-2,6-benzobisoxazole) fibers by P-31 solid-state NMR.Polym. Degrad. Stabil. 2017; 136: 131-138Crossref Scopus (5) Google Scholar, 35Froimowicz P. Zhang K. Ishida H. Intramolecular hydrogen bonding in benzoxazines: when structural design becomes functional.Chemistry. 2016; 22: 2691-2707Crossref PubMed Scopus (68) Google Scholar, 36Park E.S. Sieber J. Guttman C. Rice K. Flynn K. Watson S. Holmes G. Methodology for detecting residual phosphoric acid in polybenzoxazole fibers.Anal. Chem. 2009; 81: 9607-9617Crossref PubMed Scopus (21) Google Scholar Our new AuPd NP-catalyzed one-pot reaction of formic acid, 1,5-diisopropoxy-2,4-dinitrobenzene, and terephthaldehyde to PBO is NP size- and composition-dependent, and among the 4-, 6-, 8-, and 10-nm AuPd NPs studied, 8-nm Au39Pd61 NPs are the most efficient in catalyzing the reaction to the highest degree of polymerization (molecular weight [Mw] = 3.6 kDa). Compared with the Zylon (Mw = 40 kDa), our “lighter” PBO has not only comparable thermal stability (over 600°C) but also much improved chemical and mechanical stability against water- and organic-solvent-induced polymer degradation. Molecular mechanics (MM) and density functional theory (DFT) simulations reveal that both surface strain and surface geometry of the AuPd NPs contribute to the size-dependent polymerization. Our studies show a general approach to NP-controlled catalysis applied to polymerization. Recently, AuPd alloy NPs were studied as stable catalysts to dehydrogenate formic acid into H2 for hydrogenation reactions.37Yang Y. Xu H. Cao D. Zeng X.C. Cheng D. Hydrogen production via efficient formic acid decomposition: engineering the surface structure of Pd-based alloy catalysts by design.ACS Catal. 2019; 9: 781-790Crossref Scopus (46) Google Scholar, 38Yu W.Y. Mullen G.M. Flaherty D.W. Mullins C.B. Selective hydrogen production from formic acid decomposition on Pd-Au bimetallic surfaces.J. Am. Chem. Soc. 2014; 136: 11070-11078Crossref PubMed Scopus (189) Google Scholar, 39Muzzio M. Yu C. Lin H.H. Yom T. Boga D.A. Xi Z. Li N. Yin Z.Y. Li J.R. Dunn J.A. et al.Reductive amination of ethyl levulinate to pyrrolidones over AuPd nanoparticles at ambient hydrogen pressure.Green. Chem. 2019; 21: 1895-1899Crossref Google Scholar, 40Gu X. Lu Z.H. Jiang H.L. Akita T. Xu Q. 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Soc. 2013; 135: 16833-16836Crossref PubMed Scopus (1006) Google Scholar Then, in the presence of Au seeding NPs, a controlled amount of Pd(acac)2 was reduced in oleylamine (OAm) and oleic acid (OAc) (v/v = 50:1) at 260°C to yield AuPd NPs (see Supplemental Information). We further deposited these AuPd NPs onto a Ketjen carbon support (C) and annealed the supported NPs, NPs/C, under forming gas at 500°C for 10 min to obtain catalytically active AuPd alloy NPs. Figure 1 shows representative transmission electron microscopy (TEM) images of 8-nm Au seeding NPs (Figure 1A) and 8.2 ± 0.4 nm AuPd/C (Figure 1B) (TEM images of other Au NPs and AuPd NPs are given in Figures S2A–S2C and S3A–S3C, respectively). Starting from 4-, 6-, 8-, and 10-nm Au NPs, we obtained 4.3 ± 0.2, 6.4 ± 0.3, 8.2 ± 0.4, and 10.5 ± 0.2 nm Au39Pd61 alloy NPs, showing a very small NP size increase after the alloy formation. For simplicity of presentation, these AuPd NPs are denoted as 4-, 6-, 8-, and 10-nm NPs in this paper. The alloy structure of the NPs was characterized by high-resolution TEM (HRTEM) (Figure S4), high-angle annular dark field (HAADF) scanning TEM (STEM) and elemental mapping (Figure 1C), X-ray diffraction analysis, and X-ray photoelectron spectroscopy (XPS) (Figures S5–S7 and Table S1). From these analyses, we conclude that homogeneous alloy AuPd NPs with a face-centered cubic structure are obtained. The controls achieved in preparing AuPd NPs allow us to study size- and composition-dependent catalysis for reactions leading to the formation of small subunits of PBO. In this test, we studied formic acid (FA)-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and the subsequent condensation of the 1,5-diisopropoxy-2,4-aminobenzene product with benzaldehyde to form (1E,1′E)-N,N′-(4,6-diisopropoxy-1,3-phenylene)-bis(1-phenylmethanimine) (denoted as bis-imine) in N-methylpyrrolidone (NMP) solvent (Figure 2A ). We attached isopropyl groups to the oxy-nitrobenzene structure to ensure that both reactants and products were soluble.42Fukumaru T. Saegusa Y. Fujigaya T. Nakashima N. Fabrication of poly(p-phenylenebenzobisoxazole) film using a soluble poly(o-alkoxyphenylamide) as the precursor.Macromolecules. 2014; 47: 2088-2095Crossref Scopus (23) Google Scholar The results of the AuPd-catalyzed reactions are summarized in Figures 2B and 2C, from which we conclude that 8-nm Au39Pd61 NPs are the most active catalyst for the reduction/condensation reaction (Figure 2A). The 8-nm Au39Pd61/C (2.5 mol %) was used to catalyze FA-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and subsequent condensation with terephthalaldehyde in NMP to form poly(p-phenylene-(4,6-diisopropoxy-1,3-phenylene)diethanimine), denoted as pre-PBO, which was further subject to heating treatment at 300°C under a N2 atmosphere for 6 h to remove isopropyl groups and promote ring closure for the formation of PBO (Figure 3). From TEM (Figure S8) and ICP-AES (inductively coupled plasma atomic emission spectroscopy) elemental analyses, we concluded that the NPs have no composition change before and after polymerization reaction. We further performed cyclic voltammetry (CV) studies of the AuPd catalyst to study NP redox property change, which is sensitive to the NP surface structure. We studied the catalyst CV in 0.5 M H2SO4 at a scan rate of 50 mV s−1 in the potential range from 0 V to 1.7 V (versus reversible hydrogen electrode [RHE]) (Figure S9). We can see that the reduction peaks of the oxidized Au (at 1.12 V) and Pd (at 0.55 V) are nearly identical, indicating that there is no surface structure change of the catalyst during the polymerization process. The reduction peak area can be integrated to obtain the Au/Pd composition information, and we see no obvious surface composition change after the polymerization reaction (Table S2). We also studied XPS of the alloy catalyst (Figure S10). We see no obvious Au/Pd binding energy (Figure S10) and atomic percentage change of the catalyst before and after the reaction (Table S2). Summarizing the Au/Pd composition information we obtained from ICP-AES, CV, and XPS (Table S2), we conclude that our AuPd alloy catalyst is not only active but also stable in the polymerization process. Thermal gravimetric analysis (TGA) under a N2 atmosphere showed that the pre-PBO has a weight loss of 25.7%, which agrees well with the calculated weight loss of 27.3% for the pre-PBO/PBO conversion (Figure S11). Infrared spectra of the newly prepared PBO show characteristic benzoxazole C=N, C–N, and C–O vibration peaks at approximately 1,620, 1,360, and 1,054 cm−1, respectively, which are similar to that of the commercial PBO, Zylon (Figure S12). UV-visible (UV-vis) absorption spectra taken in methanesulfonic acid solutions of PBO and Zylon show the nearly identical absorption and photoluminescence (PL) peaks (Figure S13), indicating the highly aromatic nature and conjugated structure of alternating benzoxazole and phenyl rings within PBO and Zylon.43Feng D.D. Wang S.F. Zhuang Q.X. Guo P.Y. Wu P.P. Han Z.W. Joint theoretical and experimental study of the UV absorption spectra of polybenzoxazoles.J. Mol. Struct. 2004; 707: 169-177Crossref Scopus (27) Google Scholar The two split absorption peaks of 404 nm and 428 nm for PBO are induced by intermolecular interactions, consistent with that of the Zylon sample.44DiCesare N. Belletete M. Leclerc M. Durocher G. Intermolecular interactions in conjugated oligothiophenes. 2. Quantum chemical calculations performed on crystalline structures of terthiophene and substituted terthiophenes.J. Phys. Chem. A. 1999; 103: 803-811Crossref Scopus (53) Google Scholar A more interesting aspect of this reaction is that the degree of polymerization is dependent on the size of the AuPd NPs. Among 4-, 6-, 8-, and 10-nm AuPd NPs tested, pre-PBO was formed with an Mw of 2.1, 2.4, 3.6, and 3.0 kDa, respectively, as measured by gel-permeation chromatography (GPC) (Figure S14A). The 8-nm NPs induced the highest degree of polymerization in the one-pot reaction process. The catalyst was also reusable: our preliminary tests show that after three reaction runs, the polymer prepared from each run had similar Mw and the NP catalyst showed no obvious composition and morphology change (Figures S8 and S14B). ICP-AES measurements show that the PBO synthesized using our method is metal- and PA-free. As a comparison, Zylon contains 0.5% (by weight) of P, which means that there is one PA group for every ∼25 repeating PBO units. A large concern of PBO-based materials has been its long-term thermochemical and mechanical stability, which has been attributed to accelerated hydrolysis due to residual phosphorus contaminants from the PA used in their synthesis.34Kanbargi N. Hu W.G. Lesser A.J. Degradation mechanism of poly(p-phenylene-2,6-benzobisoxazole) fibers by P-31 solid-state NMR.Polym. Degrad. Stabil. 2017; 136: 131-138Crossref Scopus (5) Google Scholar, 35Froimowicz P. Zhang K. Ishida H. Intramolecular hydrogen bonding in benzoxazines: when structural design becomes functional.Chemistry. 2016; 22: 2691-2707Crossref PubMed Scopus (68) Google Scholar, 36Park E.S. Sieber J. Guttman C. Rice K. Flynn K. Watson S. Holmes G. Methodology for detecting residual phosphoric acid in polybenzoxazole fibers.Anal. Chem. 2009; 81: 9607-9617Crossref PubMed Scopus (21) Google Scholar As the PBO generated through our method is PA-free, it allows us to study for the first time the intrinsic stability properties of this PBO in different environmental conditions. We first performed the TGA of our PBO and the commercial PBO Zylon under a N2 atmosphere (Figure 4A ). Our pristine PBO (Mw = 3.6 kDa) displays an onset decomposition temperature at 600°C, whereas Zylon (Mw = 40 kDa) has it at 650°C. After immersion in water or DMSO under ambient conditions for 1 month, the onset decomposition temperature for Zylon and PBO were comparable (610°C and 600°C, respectively; Figure 4A). After the samples were immersed in boiling water for 5 days, the onset decomposition temperature of Zylon was reduced to 500°C, while the PBO was still at ∼600°C (Figure 4B). The difference in PBO and Zylon thermal stability was also observed in 5% (T5) and 20% (T20) mass loss temperatures. Zylon displayed a significant depression of T5 (587°C) and T20 (689°C) than the PBO (T5/T20 at 635°C/693°C). To confirm that use of PA in the synthesis can lead to the fast hydrolysis of PBO, we immersed both Zylon and our PBO samples in boiling 0.5% PA aqueous solution for 5 days and then measured their thermal and mechanical properties (Figures 4B and 4C). The onset decomposition temperature of Zylon drops even further to ∼450°C, while that of the PBO is at ∼550°C. This PA-induced hydrolysis study supports the notion that the presence of PA can accelerate PBO hydrolysis, and our PA-free PBO made from the one-pot catalytic reaction is more stable against this hydrolysis degradation than the PA-contaminated Zylon. Tensile stress measurements on 7.6-μm-thick PBO and 10.5-μm-thick Zylon films (Figure S15) revealed that the higher Mw Zylon film was stronger prior to environmental challenges (Figure 4C). After immersing the samples in boiling water or boiling 0.5% PA aqueous solution for 5 days, Zylon was subject to a more significant drop in mechanical strength to 15.1 MPa than the PBO film to 21.4 MPa (Figure 4C). Our studies demonstrate that the highly pure PBO, even at a significantly lower degree of polymerization than Zylon, can display improved thermal and mechanical stability after accelerated hydrolysis conditions. To understand why the catalytic formation of PBO in the one-pot reaction depends on the size of AuPd NPs, we analyzed the model reaction of FA-induced reduction of 1,5-diisopropoxy-2,4-dinitrobenzene and the amine condensation with benzaldehyde (Figure 2A) in three reaction steps: FA dehydrogenation, reduction of the nitro groups, and condensation of the diamine with aldehyde. From the NP size-dependent dehydrogenation of FA (Figure S16A), we can see that the 4-nm NP catalyst provides the highest initial turnover frequency (TOF) value. As the size increases, the activity drops and the TOF decreases from 223 to 170 h−1. A similar trend is observed for the hydrogenation of 1,5-diisopropoxy-2,4-dinitrobenzene (Figure S16B). However, for the condensation of two equivalents of benzaldehyde with 1,5-diisopropoxy-2,4-diaminobenzene, larger NPs (8 and 10 nm) are more efficient, and 8-nm NPs are the best catalyst for the reaction (Figure S16C), which is consistent with what we observed in Figure 2C and in the PBO synthesis. We should clarify here that the presence of AuPd NPs in the reaction solution is essential for the condensation reaction step (Table S3). Without AuPd NPs as the catalyst, the condensation reaction could not proceed smoothly and could only produce the imine product in <35% yield. FA dehydrogenation on metal surfaces has been studied by DFT,45Yoo J.S. Zhao Z.J. Norskov J.K. Studt F. Effect of boron modifications of palladium catalysts for the production of hydrogen from formic acid.ACS Catal. 2015; 5: 6579-6586Crossref Scopus (78) Google Scholar which indicates that H* binds too strongly on pure Pd (111) but too weakly on pure Au (111). Using combined classical MM46Zhou X.W. Johnson R.A. Wadley H.N.G. Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers.Phys. Rev. B. 2004; 69: 144113Crossref Scopus (851) Google Scholar and DFT simulations, we can elucidate the observed size-dependent polymerization on the AuPd NPs. From the strain distribution on the Au39Pd61 NP (111) surface (Figure 5A ), we find that the smaller NPs exhibit a higher degree of compression (Figure 5B), and the average surface compression on 4-, 6-, 8-, and 10-nm Au39Pd61 NPs is 1.37%, 1.08%, 0.86%, and 0.66%, respectively (Figure 5C). The free energy diagrams are calculated to estimate the overpotential for each reaction step (Figures 5D–5F). We find that on an AuPd slab without the compressive strain, the metal surface binds H* too strongly, resulting in a high overpotential of 0.62 V; on an AuPd surface with 2% compression, the overpotential is lowered to 0.5 V. Since 4-nm NPs have the largest surface compression of 1.37%, they should be the most active catalyst for FA dehydrogenation. In the -NO2 hydrogenation reaction step, migration of surface H* toward O–NO is the only endothermic reaction step (Figure 5E); the surface compression decreases the migration energy barrier and promotes the hydrogenation reaction. Thus, the smaller the NPs (4 nm in the study), the higher the activity. In the condensation reaction step, the formation of the C–N bond is the rate-determining step, which is also enhanced by the surface compression thanks to weakened adsorption of the amino and carbonyl groups (Figure 5F). On the other hand, the condensation reaction involves two large molecules (amine and aldehyde), requiring larger NPs for the reaction (Figure S17). Otherwise, the reactants are too close to the undercoordinated edge sites, lowering the reaction activity (Figure S18).47Ruban A. Hammer B. Stoltze P. Skriver H.L. Norskov J.K. Surface electronic structure and reactivity of transition and noble metals.J. Mol. Catal. A Chem. 1997; 115: 421-429Crossref Scopus (1077) Google Scholar By taking both strain and geometric factors into consideration, we conclude that 8-nm NPs are the most active for catalyzing the condensation and further polymerization reactions. We report a new NP-based catalytic approach to synthesize functional polymers with controlled polymerization, purity, and properties. Using the rigid organic polymer PBO as the model system, we have demonstrated that AuPd alloy NPs are especially efficient at catalyzing multiple chemical reactions in one pot, including FA dehydrogenation, nitro-hydrogenation, and amine/aldehyde condensation, to form PBO. The AuPd NPs show both size- and composition-dependent catalytic polymerization, and 8-nm Au39Pd61 NPs are the most efficient catalysts for the formation of PBO (Mw = 3.6 kDa). The PBO shows excellent thermal stability up to 600°C, which is comparable with the commercial PBO (Zylon, Mw = 40 kDa). More importantly, this “lighter” PBO exhibits much improved chemical and mechanical stability compared with Zylon after exposure to water, DMSO, or even 0.5% PA aqueous solution under either ambient or boiling conditions. Our new synthesis solves the long-standing PBO purity and degradation problems by demonstrating that the AuPd NP-catalyzed one-pot reaction is key to obtaining highly pure PBO, and chemical purity is essential for the PBO to maintain its thermomechanical stability. The NP catalyst is stable in the polymerization conditions and can be separated easily from the reaction system and recycled for the next round of reaction. This AuPd alloy NP-catalyzed reaction is not limited to the formation of PBO, and we are actively working to extend this method as a general approach to prepare highly pure rigid organic polymers with more rational control of polymerization for broad ballistic fiber, anti-flame, smart-textile, and ionic separation membrane applications.