Chinese Chemical Letters  2016, Vol.27 Issue (01): 77-80   PDF    
Fabrication of palladium nanoparticles as effective catalysts by using supramolecular gels
Wei Zhanga,b, Zhi-Gang Xiea     
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China;
b University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Two-component supramolecular gels were made through self-assembly of tetrazolyl derivatives and Pd(OAc)2. The robust gels indicated high storage modulus(>10,000 Pa) and loss modulus, which were studied by rheological measurements. The formed Pd nanoparticles(~9 nm) obtained during the formation of the gel showed effective catalytic hydrogenation of nitrobenzene and could be recovered and reused without loss of activity.
Key words: Pd nanoparticles     Self-assembly     Supramolecular gels     Catalysts    
 1. Introduction

Supramolecular chemistry and molecular self-assembly have been widely used to fabricate various functional new materials [1- 8]. For example,supramolecular gels (SGs) obtained from lowmolecular- weight gelators via non-covalent interactions have received considerable attention because of their potential applications in drug delivery and chemical sensor [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. The application of gels is dependent on their mechanical properties in a great extent. For example,the gel for the culture of isolated stem cells needs to have the elastic modulus of about 104 Pa to provide the enough mechanical resistance,which promotes cell adhesion and spreading [22]. Compared with the covalently bonded gels,SGs generally lack enough mechanical strength and viscoelasticity due to the relatively weak and reversible non-covalent interactions. So it still remains a challenge to control the mechanical strength for developing functional gels.

The synthesis of metal nanoparticles for heterogeneous catalysis has received great attention because of their unique catalytic properties and wide-ranging applicability. However,metal nanoparticles with ultra-small size are prone to aggregate because of their high surface energy. Much effort has been devoted to immobilization of metal nanoparticles inside the porous frameworks to avoid the aggregation [23, 24]. Synthesis of metal nanoparticles supported by robust frameworks usually was finished by the direct nanoparticle encapsulation or postmodification of frameworks [25, 26, 27]. However,these methods possess multiple steps and harsh reaction conditions,like high temperature and pressure,or even a large amount of catalysts. It is necessary to develop a straightforward method to prepare the framework-supported metal nanoparticles [28]. The unique stable gels are suitable matrices for the synthesis of metal nanoparticles. We envisioned the ultrafine metal nanoparticles could be obtained during the preparation of gels. In this work,we reported the in situ formation of Pd nanoparticles during the process of gel formation.

Tetrazoles and their derivatives are interesting materials and have been used in various areas,such as materials science,pharmaceutics and biological fields due to their easy functionalization. Recently,two-component SGs were prepared based on the tetrazolyl derivatives by hydrogen bonds and coordination chemistry and used for self-healing materials and oil spill recovery [29, 30]. In our recent work,we have reported synthesis of crosslinked polymers by the chemistry of 2,5-disubstuted tetrazoles [31, 32].

Herein,SGs were synthesized by simple two-component supramolecular chemistry,and showed quite high mechanical strength by changing the non-covalent interaction. More importantly,the ultrafine palladium nanoparticles were obtained during the self-assembly of gel,and could be used as effective heterogeneous catalysts for hydrogenation of nitrobenzene.

2. Experimental

All starting materials were purchased from Aldrich and Fisher and used without further purification,unless otherwise noted. Anhydrous solvents were dried by standard procedures. The thermogravimetric analysis (TGA) was performed by a Netzch Sta 449c thermal analyzer system at the heating rate of 10 ℃/min in N2 atmosphere. The FT-IR spectra were measured by a Nicolet Impact 410 Fourier transform infrared spectrometer. TEM micrographs were recorded using JEM 1011 with an acceleration voltage of 100 kV. The specimens were prepared by gently placing the sample on a surface of a carbon-coated copper grid,dried for 2 h at room temperature,and then subjected to observation. High-resolution transmission electron microscopy (HR-TEM) imagesweremeasured on a Tecnai G2 F20 S-TWIN electron microscope operated at 200 kV. Particle size data were obtained by analyzing TEM images using software Nano Measurer 1. 2. 5. SEM micrographs were performed on JEOL JXA-840 under an accelerating voltage of 15 kVX and the elemental mapping was also characterized on it under an accelerating voltage of 20 kVX. For SEM imaging,a drop of freshly prepared sample solution was cast onto a silicon slice,and then Au (1-2 nm) was sputtered onto the grids to prevent charging effects and to improve the image clarity. 1H NMR spectra were recorded at 400 MHz inDMSO-d6 and CDCl3 as internal standardwith TMS,and the solid-state 13C NMR spectra were recorded at 5 kHz. The Pd content of the sample was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis with an ICPX series II,Thermoscien-tific. Rheology test was also performed to investigate the sol-gel transition of the gelation process by a dynamic stress-controlledUS302Rheometer (AntonPaar) equipped with a parallel plate (diameter = 25 mm,gap = 0.5 mm) in oscillatory mode. After the samples were loaded,the edge of the plate was overlaid with a layer of low viscosity silicone oil to minimize the evaporation of solvent. The storage moduli G' and loss moduli G'' were thenmeasured as a functionof time at a frequency of1 Hzanda strainof1%,and the sampleswere incubated at37 ℃ for 30 minwith 60 data points were collected over the entire process.

2.1. Synthesis of 5,5-(1,4-phenylene)bis(1H-tetrazole) (PBTZ)

A mixture of terephthalonitrile (10mmol),sodium azide (60 mmol) and triethylamine hydrochloride (60 mmol) in dry toluene (70 mL) was heated under reflux for three days. The reaction mixture wasmixed with NaOH solution (1 mol/L,100 mL),stirred for 30min and filtered,then the aqueous part was acidified (pH 1) with concentrated HCl and filtered to give the crude powder of 1. The crude powder was suspended in NaOH solution (50 mL,1 mol/L),stirred atambient temperature for 30min and filtered. The filtrate was adjusted to pH 5 with addition of HCl (1 mol/L) solution and filtered to give pale grayish powder,which was washed with distilled water until no Cl- remained. 1H NMR spectrum was deposited in Fig. S1 in Supporting information.

2.2. Synthesis of SG-1

PBTZ (13.9mg,0.065mmol) and Pd(OAc)2 (14.7mg,0.065 mmol) were dissolved inDMF(100 mL) in a Scintillation vial by heating up to 100 ℃. Then the two solutions were mixed together quickly. The brown opaque gel could be achieved within about 30 s when the mixturewas cooled down to the room temperature. The sample was simply confirmed by the "stable to inversion of a test tube"method.

2.3. Catalytic test

In a typical run of catalytic activity test of SG-1,nitrobenzene (0.5 mmol) and catalysts were added to ethanol (2 mL) under the atmosphere of H2 (1 atm),then the solution was stirred at 25 ℃ for 1 h. SG-1 (4 mg),Pd/C (10%,10 mg),Pd(OAc)2 or PBTZ was used as catalysts,respectively. After the reaction was completed (monitored by TLC),the mixture was centrifuged at 5000 rpm and the solid was washed with dichloromethane (3 × 5 mL). Then the product was collected and the conversions were determined by 1H NMR spectroscopy. For comparatively and recycle test,the catalyst was isolated by centrifugation after the same cycle and washed with dichloromethane (3 × 5 mL). Then the isolated catalyst was used for the next cycle reaction with further treatment and the process was repeated for 3 times. For the Suzuki-Miyaura coupling reactions,SG-1,1,3,5-tribromobenzene or 2-bromopyridine (1.0 mmol),phenylboronic acid (1.5 mmol),potassium carbonate (276 mg,2.0 mmol),and SG-1 (5 mg,1.2 mol%) were added to a pressure flask with dry toluene (4 mL). Then the reaction mixture was stirred at 120 ℃ at ambient atmosphere. After 2 h,the mixture was centrifuged,and the solid was washed with dichloromethane (3 ± 5 mL). The combined organic phase was washed with water (3 ± 15 mL) to remove potassium carbonate. The organic phase was then evaporated under reduce pressure to leave the crude products which were further purified by column chromatography over silica gel to obtain the desired products.

3. Results and discussion

The ligand 5,50-(1,4-phenylene)bis(1H-tetrazole) (PBTZ) was prepared according to literature method [33]. SG-1 was formed immediately after mixing PBTZ and Pd(OAc)2 in DMF at 100 ℃. As shown in Fig. 1,inversion tests were used to confirm formation of SG-1 with color of brown. The gel formation was ascribed to the cooperative hydrogen bonding interaction between tetrazoles and Pd. In order to obtain an aerogel of the SG,the wet gel was immersed sequentially in methanol and water to exchange the DMF,and then was freeze-dried. The lyophilized powder of SG-1 was insoluble in all of organic solvents tested and water,even in concentrated hydrochloric acid. SG-1 was characterized by transmission and scanning electron microscopy (TEM,HR-TEM and SEM),infrared spectroscopy (IR),thermogravimetric analysis (TGA),solid-state 13C NMR spectrum,(ICP-AES) analysis and rheological measurements.

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Fig. 1.Picture of SG-1 and a schematic representation of the DMF-mediated possible coordination mode between the PBTZ and the Pd2+. Color code: C(gray),N(blue),O(red),H(white) of the ligand PBTZ,Pd(yellow),and the OAc-1 anion was omitted for clarity (drawn in ChemBio3D Ultra 12.0 with MM2 stimulation with the lowest energy).

The TEM images of SG-1 (Fig. S2 in Supporting information) indicated the SG-1 constituted with nanoparticles with average diameter of several nanometers. Small Pd nanoparticles with low dispersity could be seen in whole horizon. In addition,HR-TEM was used to see the details of Pd nanoparticles as shown in Fig. 2a. Uniform spherical Pd particles can be seen obviously with average diameter of 9 nm. SEM shows rough surface of SG-1 (Fig. S3 in Supporting information),and the EDS mapping gives clear homogeneous distribution of Pd element in SG-1 (Fig. 2b). The Pd content in SG-1 determined by ICP-AES is 26 wt%,which is similar to the calculated value of 24 wt%. The IR spectra of SG-1 exhibited obvious different patterns with PBTZ,indicating the interaction between PBTZ and Pd as shown in Fig. 3a. Solid-state 13C cross polarization/magic angel spinning nuclear magnetic resonance (CP/MAS/NMR) spectroscopy was used to confirm the composition of SGs. As shown in Fig. 3b,two characteristic signals for benzene and tetrazole carbon in SG-1 appeared at 128 and 163 ppm,respectively. TGA analysis showed that SG-1 were stable up to 250 ℃ at the heating rate of 10 ℃/min in N2 atmosphere,as shown in Fig. 4a.

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Fig. 2.HR-TEM (scale bar: 10 nm) (a) and SEM (scale bar: 40 mm) (b) of SG-1. The inside in (b) shows the corresponding EDS mapping images of the selected region of the SG-1.

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Fig. 3.FT-IR (a) and solid-state (b) 13C NMR spectra of SG-1.

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Fig. 4.TGA(a) and storagemodulus and lossmodulus as a function of time (b) of SG-1.

The mechanical intensity of the gel is an important parameter for their potential application. Rheological measurements are used to study the behavior of the gels when they are exposed to mechanical stress. The "storage" modulus G' represents the ability of the deformed materials to "snap back" to its original geometry,while the "loss" modulus G'' means the tendency of a materials to flow under stress [34]. The storage modulus G' and loss modulus G'' was measured as a function of time. As shown in Fig. 4b,the G' values exceeded the G'' value by about 1 order of magnitude for SG- 1,which was indicative of an elastic gel rather than viscous material [35, 36, 37]. It is worthy to mention that G' for SG-1 is more than 10,000 Pa,which is seldom seen for supramolecular gels [38, 39]. This high mechanical intensity of SG-1 benefits to the potential application in cell adhesion,but the biocompatibility needs to be investigated in details.

As revealed by HR-TEM,the ultrafine Pd nanoparticles were obtained after freeze-drying the wet gel of SG-1. The catalytic performances of SG-1 were investigated by hydrogenation of nitrobenzene at room temperature under 1 atm of H2 with 0.1 mol% of catalysts. The conversion of the nitrobenzene was determined by integrating the signals of the reactant (d 8.19) and the product (d 7.12) in the crude reaction mixtures (Fig. S4 in Supporting information). As shown in Table 1,SG-1 afforded almost complete conversion of nitrobenzene to aniline within 1 h,which is comparable to the commercial Pd/C catalysts with same content of Pd. We studied the recyclability and reusability of the SG-1 catalyst. SG-1 could be readily recovered from the reaction via filtration or centrifugation. The same amount of recovered SG-1 showed no decrease of conversion for hydrogenation of nitrobenzene after recycling three times. Morphologies of SG-1 revealed by TEM do not show obvious changes in Fig. S5 in Supporting information. The control experiments using only Pd(OAc)2 or PBTZ as catalyst did not form any product. These results revealed the SG-1 possessed high catalytic activity and could be reused easily. The applicability of the SG-1 catalysts in Suzuki coupling reaction was demonstrated in Fig. S6 in Supporting information. 2-Phenylpyridine and 1,3,5-triphenylbenzene was obtained after running silica column with isolated yield of 9% and 8% in 2 h,respectively. These results illustrate the generality of Pd nanoparticles in catalyzing organic transformations.

Table 1
Hydrogenation of nitrobenzene.a
4. Conclusion

In summary,two-component supramolecular gels were prepared by using tetrazolyl derivatives and Pd(OAc)2 by the metal coordination and hydrogen bonding interactions. The Pd nanoparticles obtained during the formation of SG-1 were shown to be highly active and recyclable heterogeneous catalysts toward hydrogenation of nitrobenzene. This work highlights the potential of using gelation as an in situ method for developing metal nanoparticles with catalytic activity.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 91227118).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2015.09.009.

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