Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Honeycomb-like Bicontinuous P‑Doped Porous Polymers from Hyper-Cross-Linking of Diblock Copolymers for Heterogeneous Catalysis Yang Xu, Tianqi Wang, Zidong He, Minghong Zhou, Wei Yu, Buyin Shi, and Kun Huang* School of Chemistry and Molecular Engineering, East China Normal University, 500N, Dongchuan Road, Shanghai 200241, P. R. China S Supporting Information *
ABSTRACT: This work reports a triphenylphosphine-guided hyper-cross-linking self-assembly strategy to construct honeycomb-like bicontinuous P-doped porous polymers (HBPs) based on polylactide-b-polystyrene/4-diphenylphosphinostyrene (PLA-b-P(S/DPPS)) diblock copolymers. The triphenylphosphine (PPh3) groups derived from DPPS not only play as the cross-linkable monomer with S and DPPS but also serve as the strong P ligands for binding the metal species. Subsequently, Pd nanoparticles (NPs) can be effectly encapsulated into the synthesized HBPs by a simple impregnationreduction method. The resultant Pd@HBPs show more excellent catalytic performance for selective hydrogenations than the corresponding homogeneous catalysts and synthesized heterogeneous analogues. The great performance could be attributed to the advantage of the three-dimensionally (3D) honeycomb-like interconnected mesoporous structure, which allows the accessible catalytically active sites to be efficiently exposed toward reactants. This strategy represents a new method for the preparation of porous organic polymers with special morphologies and various functionalizations for potential applications including energy storage, adsorption, separation, and catalysis.
1. INTRODUCTION As one of the most important industrial processes, selective catalytic hydrogenation is normally performed by the metalbased homogeneous catalysts.1−4 Thanks to the high mobility of active sites and the full accessibility toward reactants, the homogeneous catalysts always have superior catalytic activity and selectivity over the heterogeneous catalysts. Nevertheless, from a practical point of view, the heterogeneous catalysts are more desirable for industrial applications because of their convenience of recovering and recycling.5−11 With the rapid development of catalysis and organometallic chemistry in the past decades, great efforts have been devoted to combine the advantages of heterogeneous and homogeneous catalytic systems.8,11−14 As a rule, the metal-based heterogeneous catalytic systems usually consist of a solid support and active species, which are mainly prepared by immobilizing metal nanoparticles (NPs) onto the inner surface, outer surface, or interface of the support. Therefore, the morphologies of supported active sites (metal NPs) and properties of the supports play very important roles in catalytic performance.10,12,15 However, the immobilization process often results in low catalytic activity and selectivity owing to the poor accessibility of supports and the leaching of active species for some heterogeneous catalysts. Therefore, it is still an increasing © XXXX American Chemical Society
demand to prepare highly efficient solid heterogeneous catalytic systems with both abundant accessibly catalytically active sites toward reactants and avoidance active species leaching. Porous polymers with percolating accessible mesopores are very useful nanomaterials for biomedical applications, liquid separations, and catalysis.16−19 The existence of mesoporous spaces in porous materials can substantially increase accessibility of the active sites by enhanced diffusion, resulting in overall efficiency increase.20 Such porous materials usually possess a three-dimensionally (3D) interconnected pore structure to facilitate diffusion, and the morphologies of pore structure and properties of the supports can be controlled during fabrication. Recently, Seo and Hillmyer et al. reported the use of polymerization-induced microphase separation strategy to prepare nanostructured polymer monoliths with interconnected mesopores.19,21,22 The nanoporous interconnected bicontinuous structure was produced via in situ crosslinking of the incorporated styrene (S)/divinylbenzene (DVB) and selective hydrolysis of the poly(lactide) (PLA) domains. Subsequent work demonstrated the versatility of this strategy to Received: October 18, 2017 Revised: November 29, 2017
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DOI: 10.1021/acs.macromol.7b02222 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Synthetic Route to Pd@HBPs from PLA-b-P(S/DPPS) Diblock Copolymer Precursors (Small Balls Represent Pd NPs)
Figure 1. (A) 1H NMR spectrum of PLA-b-P(S/DPPS) diblock copolymer in [D8] THF. (B) 31P{1H} NMR spectra of (a) DPPS monomer and (b) diblock copolymer in [D8] THF. (C) FT-IR spectra of (a) PLA184-b-P(S368/DPPS75) diblock copolymer, (b) HBPs-1, (c) Pd@HBPs-1, and (d) Pd@HBPs-1 after eight runs. (D) 13C MAS NMR spectrum and (E) 31P MAS NMR spectrum of HBPs-1. (F) TG curve of HBPs-1.
produce interconnected porous materials through the introduction of a nonreactive functional additive, such as polymer or a small molecule, and then to selectively remove them.23−25 However, it is worth noting that the most of the researches for the above porous polymers are focused on disordered bicontinuous structure, and reports on the synthesis of
functional bicontinuous porous polymers with special morphology and efficient metal-based catalysts remain rare. In this paper, we develop a triphenylphosphine-guided hypercross-linking self-assembly strategy to construct honeycomblike bicontinuous P-doped porous polymers (HBPs) based on polylactide-b-polystyrene/4-diphenylphosphinostyrene (PLA-bB
DOI: 10.1021/acs.macromol.7b02222 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (A, B) TEM images and (C, D) SEM images of HBPs-1.
= −6.2 ppm) in the PLA-b-P(S/DPPS) diblock copolymer precursors (Figures 1B).27 These results indicated the successful introduction of DPPS and S monomers. The structure parameters of the repeat units of LA, S, and DPPS could be tuned from the molar ratio of monomers in RAFT polymerizations, which can be calculated by NMR end-group analysis. Gel permeation chromatography (GPC) traces in Figure S1 showed that the synthesized polymers exhibited low polydispersities (Mw/Mn < 1.30), revealing the formation of well-defined diblock copolymer precursors. As a representative sample among our synthesized PLA-bP(S/DPPS) diblock copolymer precursors, PLA184-b-P(S368/ DPPS75) was selected for subsequent studies in detail. According to the previous reported strategy,28,29 a one-step Friedel−Crafts hyper-cross-linking reaction of PLA184-b-P(S368/ DPPS75) to prepare the HBPs-1 was carried out in CCl4 solution with anhydrous FeCl3 as catalyst. Fourier transform infrared (FT-IR) spectra analysis showed a complete disappearance of the characteristic PLA carbonyl stretch peak (1758 cm−1) after Friedel−Crafts alkylation reaction, indicating efficient removal of PLA domains during the preparation (Figure 1C).28 This can be owed to the FeCl3 which supplied a Lewis acidic hydrolysis environment as well as HCl generated from the byproduct of Friedel−Crafts alkylation reaction. A new peak at 1660 cm−1 can be observed in the spectra of crosslinked samples, which could be assigned to the −CO− carbonyl groups converted from −CCl2− cross-linking bridges between phenyl rings.30 However, the peak intensity of the −CO− carbonyl groups is really weak, indicating a low cross-linking degree structure. An additional peak which appeared at 1437 cm−1 in all samples arised from the peak of the vibration of the P−C bond in DPPS moieties (Figure 1C).28 13C and 31P solidstate NMR spectra were further performed to characterize the structure of HBPs-1. As shown in the Figure 1D, the strong signal at 41.3 ppm is ascribed to the polymerized vinyl groups, while the signal at 128.8 ppm is assignable to the aromatic carbons derived from the phenyl of S and DPPS. The signal at 217.5 ppm could be attributed to the −CO− carbonyl groups from cross-linking bridges, which is consistent with the FT-IR
P(S/DPPS)) diblock copolymers (Scheme 1). Distinct from previous reports, the triphenylphosphine (PPh3) functional groups from DPPS not only play as the cross-linkable monomer with S and DPPS but also serve as a strong P ligand to bind the metal species. Importantly, the space effect from the three aromatic rings on the P atom could influence the hyper-crosslinking self-assembly process to generate final honeycomb-like bicontinuous mesoporous structure. The honeycomb-like mesoporous size also could be tuned by changing the length of degraded PLA domains. Because of the 3D interconnected pore structure and extensive distribution of P ligand inside, the highly active well-defined Pd NPs could be decorated on HBPs (Pd@HBPs) through a simple impregnation-reduction method. The obtained Pd@HBPs exhibited excellent catalytic performance for nitroarenes selective hydrogenation.
2. RESULTS AND DISCUSSION Synthesis and Characterization. Our strategy for preparing honeycomb-like bicontinuous P-doped porous polymers (HBPs) was based on the hyper-cross-linking selfassembly behavior of the well-defined diblock copolymer precursors PLA-b-P(S/DPPS), which were synthesized by reversible addition−fragmentation chain transfer (RAFT) copolymerization of styrene (S) and 4-diphenylphosphinostyrene (DPPS) with PLA-CTA macromolecular initiator (Scheme 1). Herein, polylactide with a trithiocarbonate chain transfer agent at one terminus (PLA-CTA) was chosen as the chemically degradable block, and poly(styrene/4-diphenylphosphinostyrene) (P(S/DPPS)) was selected as the crosslinkable and functional block. The incorporation of S and DPPS was corroborated by 1H and 31P{1H} NMR spectroscopic analysis (Figure 1). The resonance of aromatic protons of triphenylphosphine moiety at δ = 7.6−7.2 ppm and styrene moiety at δ = 7.0−6.2 ppm were observed in the 1H NMR spectrum of PLA-b-P(S/DPPS) (Figure 1A).26 In addition, the 31 1 P{ H} NMR spectra also showed the narrow phosphorus resonance for pure DPPS monomer (δ = −5.9 ppm) was replaced by a broadened and shifted phosphorus resonance (δ C
DOI: 10.1021/acs.macromol.7b02222 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Scheme Illustration of the Triphenylphosphine-Guided Hyper-Cross-Linking Self-Assembly Process
spectra analysis.31 Additionally, 31P solid-state NMR spectrum of HBPs-1 showed two signals at 28.5 and −6.5 ppm, which corresponded to the −PO moiety and tertiary phosphine atoms, respectively (Figure 1E).14,32 It is well-known that the P atom of a triphenylphosphine group is very easy to be oxidized to generate the −PO moiety under the action of the oxidant, such as H2SO4.33 In this work, the FeCl3 as the catalyst of Friedel−Crafts hyper-cross-linking reaction also has a good oxidizability, which could oxidize the triphenylphosphine group to form the −PO moiety under the reported conditions. Moreover, thermogravimetric analysis (TGA) demonstrated that the decomposition of the HBPs-1 started from 400 °C, indicating its high thermal stability (Figure 1F). The morphology of HBPs-1 was then investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analysis. Interestingly, spherical morphology and distinguishable honeycomb-like interconnected pore structure could be clearly observed for the obtained HBPs-1 (Figure 2 and Figure S2). The average pore sizes in the HBPs-1 frameworks were measured to be 18 ± 2 nm, which might be produced from PLA degradation. A possible mechanism of triphenylphosphine-guided hyper-crosslinking self-assembly process was proposed to interpret the formation of the honeycomb-like bicontinuous structure (Scheme 2). In order to prove our hypothesis, a series of kinetic experiments for PLA184-b-P(S368/DPPS75) as a typical example were carried out to study the formation of bicontinuous structure. From the TEM images of different cross-linking reaction times (Figure S3), a bicontinuous structure with irregular hollow cavities could be observed clearly even at the beginning of reaction about 30 min, which indicated that the bicontinuous structure would be formed at the preliminary stage of hyper-cross-linking process. Upon prolonging the cross-linking reaction time, the regular honeycomb-like bicontinuous structure could be produced finally. In addition, according to our previous report,29 at the beginning of the Friedel−Crafts alkylation reaction, a large amount of HCl gas byproducts is produced. Then, the dissolved HCl gas will reduce the solubility of PLA in CCl4 for the PLA-b-PS system, which will mediate the PLA-b-PS copolymers to self-assemble in CCl4 solution to form spherical micelle-like intermediate. Therefore, it is reasonable to suppose that the decreasing solubility of PLA will also promote a microphase separation of PLA184-b-P(S368/DPPS75) diblock copolymers in CCl4 to form a honeycomb-like bicontinuous structure due to the space effect
of the three aromatic rings on the P atom and the relatively low cross-linking degree for PLA184-b-P(S368/DPPS75). Although the formation mechanism of the honeycomb-like bicontinuous structure is not fully clear at this stage, the above-mentioned results allow us to propose a bicontinuous structure model produced by a triphenylphosphine-guided hyper-cross-linking self-assembly process to explain the formation of such unusual HBPs. In order to further explore the formation mechanism in this work, we synthesized a series of diblock copolymers (PLAb-P(S/DPPS)) with different ratios of S/DPPS to observe the formation of the structures. The TEM images of samples with different ratios of S/DPPS show an obvious transformation after cross-linking reaction 24 h (Figure S4). When the ratio of S/DPPS is 400:0, the relatively dispersed hollow spherical morphology can be found in Figure S4A, which is similar to our previous report.29 As the ratio of S/DPPS changes to 273/13, the closely packed hollow spherical morphology can be observed clearly (Figure S4B). These results confirmed that spherical micelle-like intermediate was formed at first, and the space effect of the three aromatic rings on the P atom could promote the spherical micelle-like intermediates to intertwine and fuse together during the fast hyper-cross-linking process. As the ratio of S/DPPS increases to 360/30 and 368/75, the fusion of spherical micelle-like intermediate is getting closer and an obvious honeycomb-like bicontinuous structure can be produced (Figure S4C,D). However, when the content of DPPS further increases and the ratio of S/DPPS reaches up to 355/120, the honeycomb-like bicontinuous structure tends to collapse (Figure S4E). Therefore, we believe, with the increase of content of DPPS in S/DPPS, a transformation from the spherical micelle-like model to honeycomb-like bicontinuous model for PLA-b-P(S/DPPS) system will occur and the content of DPPS in S/DPPS plays a key role for the formation of honeycomb-like bicontinuous structure. Furthermore, the Brunauer−Emmett−Teller (BET) surface area and total pore volume of HBPs-1 are measured to be 78 m2/g and 0.19 cm3/g, respectively (Figure S5). Compared with the reported PS-based cross-linking polymers,19 the relative lower surface area might be attributed to the spacial and electronic effect of the PPh3 groups, which will seriously hinder the hyper-cross-linking reaction among phenyl groups and lead to the low cross-linking degree. The honeycomb-like bicontinuous structure for the obtained HBPs may be very favorable for the accessibility of catalytically active sites and the diffusion of the substrates in the catalytic processes. D
DOI: 10.1021/acs.macromol.7b02222 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. TEM images for (A, B) Pd@HBPs-1 and (C, D) Pd@HBPs-1-B. Particle size distributions and HRTEM images of Pd NPs are provided in the insets.
Figure 4. (A) XPS spectra of the Pd 3p region for (a) Pd@HBPs-1 and (b) Pd@HBPs-1 after eight runs. (B) XPS spectra of the P 2p region for Pd@HBPs-1.
Figure 5. (A) Catalytic performance in nitrobenzene hydrogenation of (a) PdCl2, (b) Pd(PPh3)4, (c) Pd@HBPs-2, (d) Pd@HBPs-1-B, and (e) Pd@HBPs-1. Reaction conditions: nitrobenzene (0.5 mmol), Pd (0.6 mol %), ethanol (2 mL), 1 atm of H2, room temperature, and 1 h. (B) Kinetic profiles of nitrobenzene hydrogenation for Pd@HBPs-2, Pd@HBPs-1-B, and Pd@HBPs-1.
Next, Pd-loaded honeycomb-like bicontinuous P-doped porous polymers (Pd@HBPs) are prepared by treating HBPs with PdCl2 in the acetonitrile solvent and subsequently reduced by H2. The TEM image shows ultrafine Pd nanoparticles (NPs) are uniformly distributed in Pd@HBPs-1, and the average sizes are 1.5 ± 0.7 nm (Figure 3A,B). No obvious characteristic peaks of Pd NPs were observed in the powder X-ray diffraction (XRD) pattern of Pd@HBPs-1, revealing that the Pd NPs are
too small and/or well-dispersed in the support (Figures S6). Xray photoelectron spectroscopy (XPS) was further employed to confirm successful incorporation of Pd species. As shown in Figure 4A, Pd 3d5/2 at 335 eV and Pd 3d3/2 at 337 eV are attributed to Pd0 species and Pd2+ species, respectively.34 The ratio of Pd0/Pd2+ in Pd@HBPs-1 is 0.55. In comparison with P 2d binding energy of Pd@HBPs-1 and the reported PPh3-based polymer (130.9 eV),11 an obvious higher value of P 2d binding E
DOI: 10.1021/acs.macromol.7b02222 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules energy of Pd@HBPs-1 was observed (132.5 eV) in Figure 4B, owing to the strong electron donation of the triphenylphosphine ligands to Pd.11 These results reveal that the Pd is successfully decorated on the HBPs-1 by coordination to triphenylphosphine ligands rather than by physical adsorption on the support surface. It is well-known that the size and location of supported Pd NPs have an important effect on their catalytic performance. As a comparison, Pd-loaded HBPs-1 were also prepared by reduction with NaBH4 (designated as Pd@HBPs-1-B). TEM images of Pd@HBPs-1-B also exhibit a good distribution of Pd NPs with an average diameter of 4.2 ± 1.0 nm (Figure 3C,D). The larger size of Pd NPs of Pd@HBPs1-B could be ascribed to the assumption that some palladium precursors would be redissolved and diffuse in the aqueous solution of NaBH4, resulting in the formation of relative large Pd NPs on the support. Inductively coupled plasma spectroscopy (ICP) analyses show that Pd contents in Pd@HBPs-1 and Pd@HBPs-1-B are 0.676 and 0.655 mmol/g, respectively. Evaluation of Catalytic Performance. The catalytic capabilities of the obtained Pd@HBPs catalysts in the hydrogenation were evaluated by employing nitrobenzene as a model substrate. In this study, the hydrogenation reactions were typically performed at room temperature under 1.0 atm of H2 over a period of 1 h. As shown in Figure 5B, Pd@HBPs-1 afforded full conversion of nitrobenzene within 1 h, while the Pd@HBPs-1-B under the same conditions gave rise to a complete conversion need 3 h. The higher activity of Pd@ HBPs-1 than Pd@HBPs-1-B probably results from the different size and location of supported Pd NPs, while the smaller Pd NPs exhibited a larger number of available active sites toward substrates. Besides the size of NPs, the mesoporous size of support also could influence the catalytic performance. PLA60b-P(S336/DPPS65) as a contrast diblock copolymer precursor was used to construct Pd@HBPs-2, in which the repeat units of LA were reduced to 60 compared with the 184 of Pd@HBPs-1. The TEM image of Pd@HBPs-2 shows that average mesoporous size of the support was 14 ± 2 nm and the average size of Pd NPs was 1.2 ± 0.2 nm (Figure S7). The Pd content in Pd@HBPs-2 was 0.693 mmol/g as determined by ICP analysis. Interestingly, when Pd@HBPs-2 was employed for the model reaction, a quantitative 54% conversion was achieved in 1 h, which is slower than Pd@HBPs-1 under the same conditions. A possible reason for this phenomenon is that enlarged mesoporous can enhance the diffusion of reactants to access Pd NPs deposited on the interior cavity of Pd@HBPs-1. All of the above experimental results indicate that the size of active sites (Pd NPs) and the mesoporous structure of the support are crucial for an outstanding heterogeneous catalysis. For comparison, homogeneous PdCl2 and Pd(PPh3)4 catalysts were also tested under the same conditions and showed very low efficiency as shown in Figure 5A (3% and 5% yields, respectively). Obviously, such honeycomb-like bicontinuous Pd@HBPs catalysts exhibited better catalytic hydrogenation than the corresponding homogeneous catalysts. Importance of Honeycomb-like Bicontinuous Structure and PPh3 Groups. To further investigate the importance of the special honeycomb-like bicontinuous structure of Pd@ HBPs for catalytic performances, a series of porous polymers supported Pd catalysts were prepared from different precursors. As shown in Scheme 3, PLA184-b-P(S368/DPPS75) was utilized to remove the PLA block first and produce a S368/DPPS75 block fragment. After hyper-cross-linking and metalation, catalyst A was prepared (for route A). The TEM image of the obtained
Scheme 3. Synthetic Routes to Catalysts A, B, and C from Different Precursorsa
a
Pd contents in catalysts A, B, and C are 0.835, 0.315, and 0.007 mmol/g, respectively.
catalyst A indicated the formation of a bulk structure without a honeycomb-like bicontinuous structure existing (Figure S8A). This phenomenon proves that the PLA block plays a very important role in the formation of honeycomb-like bicontinuous structure. According to the catalytic test (Figure 6), catalyst A showed an obvious lower catalytic activity in nitrobenzene hydrogenation than the Pd@HBPs-1 under the same conditions. The only difference in two catalysts is the morphology; therefore, it is reasonable to propose that honeycomb-like bicontinuous structure of Pd@HBPs-1 is an important factor for the excellent catalytic performance. The lack of the accessible mesopores might slow the reactants to approch the Pd active sites of catalyst A. Route B exhibited a simpler route to prepare catalyst B based on the direct crosslinking of benzene and triphenylphosphine molecules. The TEM image of the obtained catalyst B indicated the formation of a disordered structure (Figure S8B). In comparison with Pd@HBPs-1, catalyst B also showed a slightly lower catalytic activity in the hydrogenation of nitrobenzene under the same reaction conditions. However, the catalytic activity of catalyst B even better than catalyst A, which is probably attributed to its relatively higher surface area (171 m2/g of catalyst B toward 28 m2/g of catalyst A) (Figure S9). The higher surface area of microporous catalyst B will improve the interactions between the Pd nanoparticles and the reactants/products and also can be more benefit for the diffusion of reactants and products, which will lead to a better catalytic activity than catalyst A under the same reaction conditions.35 All above experimental results indicate that the morphologies and properties of the supports play the significant roles in heterogeneous catalysts. For route C as shown in Scheme 3, a PLA184-b-PS400 diblock copolymer without DPPS component was used as precursor to prepare catalyst C via the similar method as the perpartion of Pd@HBPs-1. The TEM result showed that the final morphology is really sensitive to the DPPS component. Without DPPS, the morphology of catalyst C tends to be the hollow nanospheres after hyper-cross-linking (Figure S8C), which is consistent with our latest results that hyper-crosslinking mediated self-assembly for PLA-b-PS diblock copolymer will produce hollow porous polymer.29 On the contrary, this also further confirmed that the triphenylphosphine (PPh3) groups play a crucial role for the formation of the honeycomblike bicontinuous structure. Owing to the absence of organic F
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Figure 6. (A) Kinetic profiles of nitrobenzene hydrogenation for Pd@HBPs-1 as well as catalysts A, B, and C. Reaction conditions: nitrobenzene (0.5 mmol), Pd (0.6 mol %), ethanol (2 mL), 1 atm of H2, and room temperature. (B) Recycling tests of the Pd@HBPs-1. Reaction conditions: nitrobenzene (0.5 mmol), Pd (0.6 mol %), ethanol (2 mL), 1 atm of H2, room temperature, and 1 h.
Table 1. Selective Hydrogenation of Various Nitroarenesa
a Reaction conditions: Pd@HBPs-1 (0.6 mol % Pd to substrate), 0.5 mmol of substrate, 2 mL of ethanol, 1 atm of H2, room temperature. bThe conversion and selective yields were determined by NMR. cHBPs-1 without Pd as the catalyst. dAfter 20 min, the reaction of filtration was performed for an additional 12 h. eCatalyst B as the catalyst.
Generality, Recyclability, and Stability of Pd@HBPs. The generality, recyclability, and stability of catalysts are essential features for potential use in industrial applications. To explore the generality of the catalytic system, nitroarenes bearing different electronic and steric characters were tested. As shown in Table 1, employing nitrobenzene as substrate, 99% of isolated yield of aniline was obtained with Pd@HBPs-1 as
ligands PPh3, catalyst C showed a very low Pd content (0.007 mmol/g), which will result in the low catalytic activity in the model reaction (Figure 6A). Therefore, the PPh3 groups here not only play as the cross-linkable monomer for guiding the formation of honeycomb-like 3D bicontinuous frameworks but also can serve as the uniformly distributed strong P ligand for binding the metal species. G
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to introduce a variety of functional monomers to construct various porous polymers for advanced applications are underway in our laboratory now.
catalyst (entry 1). It should be mentioned that no aniline was formed in the control experiment in the presence of HBPs-1 only, suggesting that Pd NPs are crucial for hydrogenation (entry 2). For various types of nitroarenes bearing different electronic groups, such as −OCH3, −CN, −NH2, and −Cl, Pd@HBPs-1 gave good conversions within 1−3 h (entries 4− 7). It is noteworthy that either electron-withdrawing or electron-donating of nitroarenes has little influence on the catalysis, desired hydrogenated products obtained with great selective yields. In addition, when 1-chloro-2-nitrobenzene was employed as a substrate, 99% conversion of 1-chloro-2nitrobenzene was obtained in 3 h via using Pd@HBPs-1 as the catalyst (entry 8). However, the same reaction over catalyst B reached only 48% conversion in 3 h and 95% conversion in prolonging 3 h under identical conditions (entry 9). The inferior catalytic performance of catalyst B further confirmed the important of the honeycomb-like bicontinuous mesoporous structure. To further investigate the stability of Pd@HBPs-1, a brief hot filtration test was adopted. After hydrogenation of nitrobenzene was run for 20 min (30%), Pd@HBP-1 was removed from the reaction by centrifugation, and the reaction was allowed to react for another 12 h with the mother liquor only. However, no obvious change in conversion was observed, indicating that the catalyst active phase was not the dissolved homogeneous Pd atoms leached from the support (entry 3). ICP analysis of the filtrate showed that nearly no Pd leaching (
