Amine-Functionalized Microporous Organic Nanotube Frameworks

Article pubs.acs.org/JPCC

Amine-Functionalized Microporous Organic Nanotube Frameworks Supported Pt and Pd Catalysts for Selective Oxidation of Alcohol and Heck Reactions Hui Zhang, Minghong Zhou, Linfeng Xiong, Zidong He, Tianqi Wang, Yang Xu, and Kun Huang* School of Chemistry and Molecular Engineering, East China Normal University, 500 N, Dongchuan Road, Shanghai 200241, P. R. China S Supporting Information *

ABSTRACT: We report a synthesis of amine-functionalized microporous organic nanotube frameworks supported Pt or Pd catalysts (Pt or Pd@NH2-MONFs) by a combination of hyper cross-linking tricomponent bottlebrush copolymers and subsequent gentle reduction. In this method, the intrabrush and interbrush cross-linking of polystyrene (PS) shell layer in the tricomponent bottlebrush copolymers led to the formation of micropores and large-sized nanopores (meso/macrospores) in NH2-MONFs, respectively, while selective removal of polylactide (PLA) core layer generated mesoporous tubular structure. Interestingly, the middle functional poly(Boc-aminoethyl acrylamide) (PBAEA) component could be deprotected to produce the free amine moieties in the channel of MONFs, which will play a key role to anchor Pt or Pd nanoparticles on the support by an in situ reduction with NaBH4. Owing to their high special surface area, robust organic framework, and hierarchically porous structure, the resultant Pt or Pd@NH2-MONFs show high heterogeneous catalytic activity and excellent reusability in the selective oxidation of alcohol and Heck reactions, respectively.

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INTRODUCTION Metal nanoparticles (NPs) have attracted considerable attention due to their excellent electronic, optical and catalytic properties.1−4 Especially, owing to the high surface area-tovolume ratio and small particle size, these nanoparticles have been intensively studied as catalysts for chemical reactions.5−11 Among all kinds of metal nanoparticles, Pt and Pd NPs have been extensively explored because of their high activity and selectivity in some valuable catalysis applications, such as the catalytic oxidation of alcohol, the hydrogenation of nitroarenes, and the Heck reaction.12−14 However, the aggregation of the metal NPs driven by the high surface energy will lead to the deactivation of the metal NPs catalysts system. Moreover, the deficiency in separation and recovery also limits their application in chemical catalysis. The heterogeneous metal-loaded catalyst system is an effective way to conquer the above defects by stabilizing the metal NPs onto solid supports. Porous materials as a series of excellent supports have been intensively explored for loading the metal nanoparticles due to the high surface area, special porous structure, and easy separation from reaction mixture.15−19 However, most of the research is focused on the © 2017 American Chemical Society

conventional carbon or inorganic porous materials including silica, zeolites, and aluminum oxides.20−27 Recently, porous organic polymers (POPs) have obtained much attention as an emerging class of porous materials owing to their high surface area, robust organic framework, and the excellent chemical modifiability.28 In particular, many efforts have recently been paid to use POPs as the catalytic supports for loading Pt and Pd NPs.29−34 For example, Spontak and co-workers synthesized the metal/polymer nanocomposites derived from hyper crosslinked polystyrene and investigated their catalytic properties in the direct oxidation of L-sorbose to 2-keto-L-gulonic acid.35 Wang and co-workers reported the preparation of the core/ shell POPs nanocomposite microspheres with the Fe3O4 NPs located in the core and the Pt NPs in the POPs shell, which can be applied in the enantio selective hydrogenation of ethyl pyruvate.36 Wang’s team also reported the POPs supported Pd NPs catalysts for hydrogenation of olefins.37 Recently, our group developed a novel method to fabricate the thiol or Received: March 14, 2017 Revised: May 26, 2017 Published: May 30, 2017 12771

DOI: 10.1021/acs.jpcc.7b02425 J. Phys. Chem. C 2017, 121, 12771−12779

Article

The Journal of Physical Chemistry C

Scheme 1. Schematic Presentation of the Synthesis of Pt or Pd@NH2-MONFs from Tricomponent Bottlebrush Copolymers Precursors

Waters’ HPLC columns. DMF (HPLC grade) with 0.1 M LiBr was used as the solvent for polymers and eluent for GPC with a flow rate of 1 mL/min at 65 °C or THF (HPLC grade) with the same rate at 30 °C. The GPC instrument was calibrated with narrowly dispersed linear polystyrene standards. Transmission electron microscopy (TEM) images were obtained using a JEM-2100F TEM instrument. Samples were prepared by dip-coating a 400 mesh carbon-coated copper grid from a dilute sample solution allowing the solvent to evaporate. The infrared (IR) spectra were recorded using the FTIR (Thermo NICOLET is50). GC/MS analyses were obtained on an Agilent 6890 Series GC System with a Hewlett-Packard 5973 Mass Selective Detector (70 eV) using a HP-5MS fused silica capillary column (cross-linked 5% phenyl ethyl siloxane, 30 m × 0.25 mm ID × 0.25 μm film thickness) and argon as a carrier gas (1 mL/min). The split ratio was 1:50. The injector temperature was kept at 270 °C, and the detector was kept at 280 °C. The column temperature was held at 60 °C for 3 min, increased to 280 °C at a rate of 25 °C/min, and then kept at 280 °C for 5 min. A Quantachrome Autosorb IQ surface area and porosity analyzer was utilized to study the pore structure of the samples. Before measurements, the polymer samples were degassed for more than 10 h at 120 °C. The Brunauer− Emmett−Teller surface area and the micropore surface area were determined by the BET equation and the t-plot equation, respectively. The pore size distribution was analyzed by original density functional theory (DFT). Elemental analyses were determined by an elementar Vario EL III, and the sample was dried in vacuo at ambient temperature. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB 250Xi spectrometer. The powder X-ray diffraction (XRD) pattern of the sample was collected using a D8Advance X-ray diffractometer (Bruker AXS, Germany) with Cu K radiation. TG analyses were carried out with a NETZSCH STA449F3 simultaneous thermal analyzer at a heating rate of 10 K min−1 from 30 to 800 °C in a nitrogen atmosphere. Synthesis of the Metal-Loaded Amino-Functionalized Microporous Organic Nanotube Networks (Pt or Pd@NH2MONFs). The NH2-MONFs was prepared as the methods in the previous work.42 In a 50 mL flask, the NH2-MONFs (200 mg) and H2O (15 mL) was stirred for 12 h. The aqueous solutions of K2PtCl4 or PdCl2 (0.42 mL, 50 mM, 1 equiv to amine) were added, and the mixture was deoxygenated with nitrogen for 24 h. NaBH4 (16 mg, 2 equiv to amine) was added into the mixture at 0 °C and stirred for 9 h, centrifuged and

carboxyl-containing microporous organic nanotube networks supported Au and Pd NPs for the reduction reaction of 4nitrophenol and Suzuki−Miyaura cross-coupling reaction, respectively.6,38 Although great progress has been made in POPs heterogeneous catalyst system, it is still desirable for developing the high-loading Pt or Pd heterogeneous catalysts supported by POPs with high catalytic activity. Comparing with carboxyl and thiol groups, the amino groups also show stable and strong chelation ability with metal species, especially for Pt and Pd NPs. The strong interactions between amino groups and metal NPs will also promote the highloading capacity and structural stability of the catalyst system. Herein, we report a synthesis of the amine-functionalized microporous organic nanotube frameworks (NH2-MONFs) supported Pt and Pd NPs catalysts by a combination of hyper cross-linking tricomponent bottlebrush copolymers and subsequent gentle reduction (Scheme 1), in which amino groups are installed in the channel of the NH2-MONFs by rational molecular strategy. Promoted by strong interactions between amino groups and metal NPs, the high-loading Pt or Pd@NH2MONFs could be obtained via an in situ reduction of K2PtCl6 or PdCl2 with NaBH4. By introducing the PLA-templating mesoporous structure into the NH2-MONFs, it will facilitate the accessibility of the active sites and the diffusion of the molecules. Therefore, the obtained Pt or Pd@NH2-MONFs heterogeneous catalysts can be expected to show high catalytic activity and excellent stability in the oxidation of alcohol and the Heck reaction, respectively.

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EXPERIMENTAL SECTION Materials. All reagents were used as received unless stated otherwise. Dichloromethane (DCM) and N,N-dimethylformamide (DMF) were dried using CaH2 and distilled. Glycidyl methacrylate (GM, Acros 97%) was purified by vacuum distillation and styrene was purified by passing over a basic alumina column. 2,2-Azoisobutyronitrile (AIBN) was purified by recrystallization from methanol. S-1-Dodecyl-S′-(α,α′dimethyl-α′-acetic acid)trithiocarbonate (TC),39 2-cyanoprop2-yl-4-cyanodithiobenzoate (CPD),40 and Boc-aminoethyl acrylamide (BAEAM)41 were synthesized according to literature procedures. Measurements. All 1H NMR spectra were recorded with a Bruker AVANCE III 500 spectrometer (500 MHz) by using CDCl3 or DMSO-d6 as a solvent. GPC data were obtained from Waters GPC system equipped with a Waters 1515 isocratic HPLC pump, a 2414 refractive index (RI) detector, and two 12772


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Figure 1. TEM images of the Pt@NH2-MONFs (A) and Pd@NH2-MONFs (B). The scale bars represent 50 nm, unless otherwise noted.

Figure 2. (A and B) N2 adsorption−desorption isotherms and pore size distributions calculated using DFT methods of Pt@NH2-MONFs. (C) TGA thermograms of NH2-MONFs (a) and Pt@NH2-MONFs (b). (D) XPS spectrum of Pt@NH2-MONFs for Pt 4f.

washed with H2O and methanol, dried in vacuum at 50 °C, and obtained the Pt or Pd@NH2-MONFs as a black powder. Synthesis of the Pt-Loaded Microporous Organic Nanowire Networks (Pt@NH2-MONWFs). Synthesis of Poly(GM-gBAEAM). Poly(GM-g-TC) was synthesized as the methods previously developed in our group.43 Poly(GM-g-TC)(10 mg) was mixed with AIBN (0.344 mg), BAEAM (223 mg), and 1,4dioxane (1.5 mL) in a reaction vessel and degassed by three freeze−pump−thaw cycles. The polymerization was then conducted at 60 °C for 3 h. The resulting polymer was precipitated from THF into diethyl ether three times and dried under vacuum at ambient temperature for 24 h. Yield = 100 mg (9%). GPC (PS standards, DMF as an eluent): Mn = 8.7 × 105g/mol, Mw/Mn = 1.02. Synthesis of Poly(GM-g-BAEAM-g-PS). Poly(GM-gBAEAM) (80 mg) was mixed with AIBN (0.25 mg), styrene (2.1 mL), and 1,4-dioxane (3.2 mL) in a reaction vessel and degassed by three freeze−pump−thaw cycles. The polymerization was then conducted at 50 °C for 20 h. The polymer was precipitated from DCM into methanol three times and dried under vacuum at ambient temperature for 24 h. Yield = 100 mg (13%). GPC (PS standards, DMF as an eluent): Mn = 4.5 × 106 g/mol, Mw/Mn = 1.16.

Synthesis of the Pt Nanoparticles Immobilized on the NH2-MONWFs (Pt@NH2-MONWFs). The NH2-MONWFs was synthesized from poly(GM-g-BAEAM-g-PS) by the same method as the NH2-MONFs. According to the preparation of the Pt@NH2-MONFs, the NH2-MONWFs was treated with K2PtCl4 and NaBH4 in the aqueous solution to obtain the Pt@ NH2-MONWFs as black powder. Synthesis of the Pt-Loaded Microporous Organic Nanotube Frameworks without the Amino Groups (Pt@MONFs). The MONFs without amino groups were synthesized as the previous work.42 The Pt@MONFs was synthesized by the same method as the Pt@NH2-MONWFs and obtained as black powder. Catalysis Experiment. Catalysis experiments were conducted in the abroach vials. The yield was determined using gas chromatography (GC). A mixture of benzyl alcohol (9.5 uL, 0.09 mM), K2CO3 (26 mg, 0.18 mM), the Pt@NH2-MONFs (15 mg, 0.009 mM), and H2O (0.75 mL) were added to an abroach vial. The reaction was stirred at room temperature for 1.5 h. After the reaction, the mixture was centrifuged to remove the solid catalyst, and the liquid was extracted with DCM and analyzed by GC. To investigate the recyclability of the Pt@NH2-MONFs catalysts, a ten cycle test was performed as follows. After the 12773


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MONFs. Second, PtCl62− or Pd2+ could be anchored in the pore canal of the organic network by mixing the K2PtCl6 or PdCl2 aqueous solution with the NH2-MONFs through the interaction between the NH2 group and the metal ions. Finally, Pt@NH2-MONFs or Pd@NH2-MONFs can be obtained by an in situ reduction using NaBH4. Figure 1A shows transmission electron microscopy (TEM) image of the obtained Pt@NH2 -MONFs. The porous architecture of the NH2-MONFs is still maintained after the reduction of K2PtCl6 by NaBH4. Especially, hollow tubular channels could be clearly observed crossing through almost every unit in the polymer networks, which show the excellent stability of the NH2-MONFs as the support. As a result of the strong interaction between Pt and the amino sites, the Pt NPs are well dispersed in the tubular channel of the NH2-MONFs with the particles diameter of 3 ± 0.5 nm and the aggregation of Pt NPs could also be effectively prevented in the Pt@NH2MONFs catalyst system. Similarly, Pd@NH2-MONFs also possesses the well-organized three-dimension (3D) tubular structure and the controlled distribution of Pd nanoparticles with the diameter of 2.7 ± 0.5 nm (Figure 1B). The permanent porous properties of Pt or Pd@NH2MONFs are further analyzed by N2 adsorption−desorption isotherms (Figure 2A,B). A steep adsorption uptake at low relative pressure is observed, which prove the existence of abundant microporous structures. However, the apparent hysteresis loop in the desorption isotherm demonstrates the presence of accessible mesopores in the [email protected] As shown in Table 1, the Pt@NH2-MONFs shows a high BET surface area and large pore volume (794 m2/g and 1.41 cm3/g, respectively). The pore size distribution is calculated by the density functional theory (DFT) method, and the population ranges from 1.4 to 14.0 nm, as shown in Figure 2B, which suggests that the resulting Pt@NH2-MONFs has a hierarchically porous structure. However, the pore volume of the Pt@ NH2-MONFs is slightly inferior to that of the NH2-MONFs (1.41 cm3/g Vs 1.63 cm3/g), which indicates the successful immobilization of the Pt NPs in the organic networks. Interestingly, there is almost no change in the micropore volume, which means the Pt NPs were not anchored into the microporous shell but the mesoporous tubular channel of the networks. The above results also agree with the TEM images. Similarly, the Pd@NH2-MONFs also possesses a high surface area and abundant micropores and mesopores (Figure S2 and Table 1). These results further confirm that Pt or Pd@NH2MONFs is beneficial for catalysis process, as the uniform mesoporous channel will facilitate the mass transfer of reactants and products. Moreover, the suitable microporous wall of nanotube frameworks as well as strong interactions between amino sites and metal NPs are in favor of locking and protecting metal species. To further verify the existence and content of the metal nanoparticles in the NH2-MONFs, the obtained Pt or Pd@ NH2-MONFs are characterized by means of X-ray diffraction (XRD), thermo gravimetric analysis (TGA), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS). The characteristic diffraction peaks at 40°, 46°, 67°, and 84° could be observed from the XRD pattern for Pt@NH2MONFs (Figure S1A), which are designated to the Pt {111}, {200}, {220}, and {311} facets of the face-centered cubic (fcc) crystal structure. TGA analyses show Pt content in Pt@NH2MONFs is 18 wt % (Figure 2C), which is in good agreement with ICP analyses. The high content of the Pt NPs in the

catalytic reaction, the catalysts were separated by centrifugation and washed by methanol and then dried in vacuum for the following catalysis. The liquid was extracted with DCM and analyzed by GC. Controlled experiments were designed to demonstrate the superiority of the Pt@NH2-MONFs catalysts. In two sealed vials, a mixture of benzyl alcohol (9.5 uL, 0.09 mM), K2CO3 (26 mg, 0.18 mM), and H2O (0.75 mL) was added. The Pt@NH2-MONWFs (35 mg, 0.009 mM) and Pt@ MONFs (60 mg, 0.009 mM) catalysts were used for the controlled experiments. The mixture was stirred at room temperature with constant stirring for 1.5 h. The reaction yields were monitored by GC. To explore the catalytic activity of the Pd@NH2-MONFs catalysts, the Heck−Mizoroki reactions with aryl iodide and methyl acrylate were chosen as the model reaction. A mixture of aryl iodide (0.23 mmol), methyl acrylate (0.46 mmol), triethylamine (0.58 mmol), and Pd@NH2-MONFs catalyst (1.3 × 10−3 mmol) were conducted in the mixture of DMF (0.8 mL) and H2O (0.2 mL) at 90 °C. After the reaction, the mixture was centrifuged to remove the solid catalyst, and the liquid was analyzed by GC. A six cycle test was performed to investigate the recyclability of the Pd@NH2-MONFs catalysts. After the catalytic reaction of p-iodotoluene and methyl acrylate, the catalysts were separated by centrifugation, washed by methanol, and then dried in vacuum for the following catalysis. The liquid was extracted with DCM and analyzed by GC.

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RESULTS AND DISCUSSION Scheme 1 describes the synthetic strategy of the aminefunctionalized microporous organic nanotube frameworks Table 1. Textural Parameters of Different Nanomaterials samples NH2-MONFs Pt@NH2-MONFs Pd@NH2-MONFs Pt@NH2MONWFs Pt@NH2-MONFs after recycled

SBETa (m2/g)

Vtotalb (cm3/g)

Vmicroc (cm3/g)

Vmeso/macro (cm3/g)

content of Pt NPs

827 794 792 573

1.63 1.41 1.25 1.33

0.12 0.12 0.10 0.02

1.51 1.29 1.15 1.31

18.0% 17.7% 7.7%

795

1.42

0.11

1.31

a

BET special surface area calculated from N2 adsorption isotherm. Total pore volume at P/P0 = 0.995. cMicroporous volume obtained from t-plots method.

b

Scheme 2. Illustration of Pt-Loaded Catalysts for the Oxidation of Benzyl Alcohol

supported Pt and Pd catalysts. First, according to our previous reported method,42 the amine-functionalized microporous organic nanotube frameworks (NH2-MONFs) composed of a hyper cross-linking microporous shell and a PLA-templating mesoporous tubular core are prepared by the combination of the hyper cross-linking and the molecular-templating tricomponent bottlebrush copolymers via the Friedel−Crafts (F−C) alkylation reaction. Simultaneously, the middle poly(Bocaminoethyl acrylamide) (PBAEA) component will be deprotected to produce the free amino moieties in the channel of 12774


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Figure 3. (A) Conversion (%) vs time (min) for the oxidation reaction of benzylalcohol in H2O catalyzed by Pt@NH2-MONFs (a), Pt@NH2MONWFs (b), and Pt@MONFs (c). Each conversion was detected by GC based on starting material. (B) Catalytic performance of the recycled Pt@NH2-MONFs.

In order to study the tubular mesopore structure for the influence on the catalysis reaction, microporous organic nanowire frameworks (NH2-MONWFs) with dominated micropores are prepared by hyper-cross-linking of the bottlebrush copolymers precursors P(GM-g-PBAEAM-g-PS) according to previous reported method,43 in which no PLAtemplating core exists (Scheme S1). Subsequently, the Ptloaded microporous organic nanowire frameworks (Pt@NH2MONWFs) are synthesized with the similar method as Pt@ NH 2 -MONFs. TEM images also show that Pt@NH 2 MONWFs has disordered structure without the existence of tubular channels and the Pt species tend to agglomeration on the surface of NH2-MONWFs with a nonuniform distribution (Figure S3).The BET surface area and total pore volume of the Pt@NH2-MONWFs (573 m2/g and 1.33 cm3/g, respectively, as show in Table 1) are apparently lower than that of Pt@NH2MONFs. Moreover, no obvious pronounced hysteresis loop observed in the desorption isotherm indicates the comparative Pt@NH2-MONWFs catalyst has a dominated micropore structure (Figure S4). For the lacking of sufficient space provided by the inner channels for immobilizing the Pt NPs, the content of Pt NPs is only 7.7%, which is distinctly inferior to the Pt@NH2-MONFs. The above results indicate the tubular channels in the network units play a great role for the high loading-capacity of the NH2-MONFs and the well distribution of Pt NPs. As compared, we also prepare a porous organic framework without the amino groups in the channel of the network units (MONFs), which is synthesized according to our previous work.42 Then, the Pt-loaded organic nanotube networks (Pt@MONFs) are synthesized as Pt@NH 2 MONFs. A lower Pt loading-capacity of the Pt@MONFs (4.2%, obtained from ICP results) is observed due to lacking the strong interactions between the Pt NPs and the amino sites. Without the interaction between the Pt NPs and the amino sites, the Pt NPs are not dispersed in the inner channels of the network units but anchored disorderly in the MONFs (Figure. S5). Based on the above results, the amino groups in the channel of the tubular units have a significant impact on improving the Pt loading-capacity of the NH2-MONFs and the regular distribution of the Pt NPs. To explore the catalytic activity of the Pt@NH2-MONFs as a heterogeneous catalyst, the oxidation of benzylalcohol is chosen as the model reaction (Scheme 2). In this work, the selective oxidation of alcohol to aldehyde is catalyzed by the Pt@NH2MONFs in a mild aqueous media at room temperature, with

Table 2. Comparison of Catalytic Activity for the Oxidation Reaction with Various Catalysts no.

catalyst

1

MgO−Ti− N-Pt Pt/CNTb Pt−Bi/CNT Pt/ZrO

2 3 4 5 6 7 a

Pt−Fe3O4/ rGOc Pt/BWTd Pt@NH2MONFs

Pt [mol %]

solvent/ oxidant

temp./time [h]

yielda [%]

ref

5

H2O/O2

90 °C/3

100

48

0.1 0.1 0.5

75 °C/1 75 °C/3 90 °C/3

13.2 55 100

49 50 51

0.3

H2O/O2 H2O/O2 nheptane/ O2 H2O/O2

80 °C/2

33.6

52

2 10

H2O/air H2O/air

50 °C/24 RT/1.5

89 100

53 this work

GC yield. bCarbon nanotube. cReduced graphene oxide. wattletannin.

d

Black

organic frameworks benefits from the strong chelation interaction between the amino sites and the Pt NPs. The element states of Pt in the Pt@NH2-MONFs and the coordination ability of amino groups with Pt NPs are also further investigated by subsequent XPS characterizations (Figure 2D). In the high-resolution XPS spectrum, the peaks of Pt 4f7/2 and Pt 4f5/2 binding energies at 72.1 and 75.2 eV for Pt(0) species are observed slightly higher than the bulk Pt 4f levels (4f7/2 = 71.4 eV and 4f5/2 = 74.5 eV), which is attributed to the interaction between the amino sites and Pt NPs. These results reveal that the Pt NPs is successfully anchored on the Pt@NH2-MONFs by coordination to amino ligands rather than by physical adsorption of PtCl62− on the support surface. Similarly, the XRD pattern of Pd@NH2-MONFs demonstrates the formation of Pd NPs and ICP result shows a 17.7% content of Pd in Pd@NH2-MONFs (Figure S1B and Table 1). The state of Pd in Pd@NH2-MONFs is also investigated by XPS characterization, as shown in Figure S2D. It can be seen that, in the Pd 3d region, two peaks at 341.4 and 336.2 eV for Pd@ NH2-MONFs correspond to 3d5/2 and 3d3/2 for Pd(0) species. The peaks shift negatively by 0.4 eV due to the interaction of the amino sites and Pd NPs. Moreover, the peaks of 337.7 (Pd2+ 3d5/2) and 342.9 eV (Pd2+ 3d3/2) also appear in XPS spectrum, which suggests a part of unreduced Pd2+ ions still exist in catalyst, and the ratio of reduced Pd is 47%, which is consistent with the reported results.45−47 12775


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The Journal of Physical Chemistry C Table 3. Oxidation Reaction of Alcohol with Different Substratesa

Reaction conditions: alcohol (0.09 mmol), K2CO3 (0.18 mmol), Pt@NH2-MONFs catalyst (9 × 10−3 mmol), H2O (0.75 mL), and room temperature. bGC yield.

a

Scheme 3. Illustration of Pd@NH2-MONFs Catalysts for the Heck−Mizoroki Reaction

Table 4. Heck−Mizoroki Reaction with Different Substratesa

the existence of K2CO3 as an alkali addition and oxygen in the air as the oxidizing agent via an environment-friendly synthetic route. As shown in Figure 3A, the conversion is nearly up to 100% when the reaction is conducted at ambient temperature for 1.5 h. To our best knowledge, the catalytic activity of the Pt@NH2-MONFs is one of the highest values for the green oxidation of the alcohol in various Pt-loaded heterogeneous catalysts (Table 2). The high catalytic activity of the Pt@NH2MONFs might be related to their hierarchical porosity, welldispersed Pt NPs distribution and high Pt loading-capacity. Meanwhile, the introduction of PLA-templating mesopores will

entry

R

time (h)

yield (%)b

1 2 3 4 5

H CH3 OCH3 COCH3 3-Br

3 3 3.5 3 3

94 98 96 95 99

a

Reaction conditions:aryl iodide (0.23 mmol), methyl acrylate (0.46 mmol), triethylamine (0.58 mmol), Pd@NH2-MONFs catalyst (1.3 × 10−3 mmol), DMF (0.8 mL), H2O (0.2 mL), and 90 °C. bIsolated yield.

facilitate the diffusion of the substances and the immobilization of Pt NPs in the tubular channel will also promote the accessibility of the Pt NPs. To prove the above hypothesis, the oxidation reactions are also performed using the Pt@NH2MONWFs and the Pt@MONFs as the catalysts with a same Pt 12776


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performance of Pd@NH2-MONFs, the Heck−Mizoroki reactions with aryl iodide and methyl acrylate are conducted in the mixture of DMF and H2O at 90 °C (Scheme 3). As shown in Table 4, high yields and catalytic efficiency in the reactions show the excellent catalytic activity of the Pd@NH2MONFs. It demonstrates that the NH2-MONFs can be widely applied in various metal-loaded heterogeneous catalysts. Identically, the recycling experiments are also conducted in the same condition to explore the recoverability of the Pd@ NH2-MONFs (Figure S9). The Pd@NH2-MONFs can be recycled for more than six times with no apparent decrease in the catalytic activity. In addition, the result of the deactivation experiment shows there is no leaching of Pd from catalyst, which further demonstrates the good recoverability and stability of the obtained Pd@NH2-MONFs.

content under the similar reaction condition. When the Pt@ NH2-MONWFs work as the catalyst in the reaction, the conversion of the aldehyde ascends slowly and is only up to 32.6% after 1.5 h. The inferior catalytic ability of Pt@NH2MONWFs might result from the absence of tubular mesopore, which might confine the mass transfer of reactants and products and accessibility of the Pt NPs. Moreover, the low loadingcapacity of the Pt@NH2-MONWFs also restrains the catalytic activity of the Pt-loaded catalyst. However, the conversion of the aldehyde is just 15% after 1.5 h when the Pt@MONFs is used as catalyst. Due to lacking the amino groups in MONFs, it is difficult to immobilize the Pt NPs on the supports. This will lead to a lower Pt loading-capacity for Pt@MONFs, which will result in the poor catalytic performance in the oxidation reaction. To study the effect of Pt loadings in Pt@MONFs on catalytic activity, various catalysts with different Pt loading are prepared, and the corresponding catalytic activities are investigated for oxidation of alcohols (Figure S6). The results show that the catalytic activity improved with the increase of Pt loading, which demonstrates the high Pt loading plays a positive role in the catalytic activity. To explore the recoverability of the Pt@NH2-MONFs, the recycling experiments are also conducted in the same condition. As shown in Figure 3B, the Pt@NH2-MONFs can be recycled for more than ten times with no apparent decrease in the catalytic activity. TEM shows that there is no destruction in the porous structure of the Pt@NH2-MONFs after recycled for ten times (Figure S7). Moreover, the Pt NPs are still well-dispersed in the tubular channels of the network units after recycled. N2 adsorption−desorption isotherms also demonstrate that the high surface area and large pore volume of the recycled Pt@ NH2-MONFs could still be maintained (Figure S8). The above results indicate the excellent stability of the Pt@NH2-MONFs. In addition, we design a deactivation experiment to further investigate the stability of the Pt@NH2-MONFs. When the oxidation reaction proceeds for 45 min, the solid catalyst is separated by centrifugation. Further oxidation of the alcohol is conducted in the residual liquid under the same condition. The result shows there is no change for the conversion of aldehyde in the residual after the removal of the catalyst, which means the Pt NPs are not leaded from the NH2-MONFs support in the catalytic process due to the strong interaction between Pt and amino sites. ICP analysis of the residual liquid reveal that there is nearly no Pt NPs (0.05 ppm) leaching from the Pt@ NH2-MONFs, which is in accord with the result of the deactivation test. In contrast, there is 3.4% Pt NPs leached from the Pt@MONFs when the oxidation reaction is catalyzed by the Pt@MONFs, which resulted from the absence of amino groups in the channel of Pt@MONFs. The above results indicate the Pt@NH2-MONFs possess excellent stability and recyclability in the heterogeneous catalysis. In addition, more alcohols are selected as substrates to explore the wide range of application for the Pt@NH2-MONFs (Table. 3). All catalysis reactions can be achieved in the air at ambient temperature under an environmental-friendly condition. High catalytic activity in different oxidation reactions indicates that the Pt@NH2-MONFs catalyst exhibits the excellent catalytic performance in various substrates and provides great potential in the application of different organic synthesis. Palladium-catalyzed carbon−carbon coupling reactions are important for the generation of C−C bonds and play a great role in modern organic synthesis. To study the catalytic

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CONCLUSION In summary, we report metal-loaded heterogeneous catalyst supported by amine-functionalized microporous organic nanotube frameworks. Based on the creative synthetic strategy, the Pt or Pd@NH2-MONFs possesses high surface areas, high loading-capacity, hierarchically porous structure, and welldistribution of metal nanoparticles. The hierarchically porous structure facilitates the accessibility of the active sites and the diffusion of the molecules to promote high catalytic activity. The structural stability can be improved efficiently by the strong interaction between Pt or Pd and the amino sites. The Pt-loaded heterogeneous catalysts show high catalytic activity and excellent recoverability in the selective oxidation of alcohol with a green reaction condition. The new metal−organic hybrid nanomaterials provide a kind of way for the metal-loaded heterogeneous catalyst and have great potential application in modern organic synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02425. Synthetic route of the amino-protected bottlebrush copolymer precursors and Pt@NH2-MONWFs, XRD patterns, TGA data, XPS spectra, TEM images, catalytic data, N2 adsorption−desorption isotherms, and pore size distributions. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail:[email protected]. ORCID

Kun Huang: 0000-0003-2737-1189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China Grants 51273066 and 21574042, Shanghai Pujiang Program Grant 13PJ1402300, and large instruments Open Foundation of East China Normal Universtiy. 12777


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Amine-Functionalized Microporous Organic Nanotube Frameworks

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