A novel Cu-nanowire@Quasi-MOF via mild pyrolysis of a bimetal-MOF for the selective oxidation of benzyl alcohol in air

Yan Shenab, Li-Wei Baoab, Fang-Zhou Sunab and Tong-Liang Hu*abc
aSchool of Materials Science and Engineering, Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, National Institute for Advanced Materials, Nankai University, Tianjin 300350, China. E-mail: tlhu@nankai.edu.cn
bTianjin Key Lab for Rare Earth Materials and Applications, Nankai University, Tianjin 300350, China
cKey Laboratory of Advanced Energy Material Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China

Received 29th April 2019 , Accepted 17th July 2019

First published on 18th July 2019


The preparation of metal nanocomposite catalysts with MOFs as precursors has attracted extensive attention, as the design and preparation of catalysts by this method could maximize the advantages of the precursors and ensure the stability of the catalysts as much as possible. Here, a benzimidazole-modified Cu/Co bimetal-MOF as a precursor was used to synthesize a Cu-nanowire@Quasi-MOF material via a simple and low-cost thermal decomposition strategy for the first time. Experimental evaluation suggested that the Co-MOF part in Cu/Co bimetal-MOF was a morphology retainer, and the special channel composition of the Quasi-MOF played a space-confined role in the formation of the unique Cu-nanowires. Based on the detailed investigation, a possible formation mechanism was proposed. Furthermore, Cu-nanowire@Quasi-MOF exhibited excellent catalytic performance and good stability in the selective oxidation of benzyl alcohol in air. This study demonstrated that using MOFs as a precursor, by adjusting the calcination temperature and adding heteroatoms, can control the morphology of metal nanoparticles (MNPs) in MOF derivatives, reduce agglomeration and enhance catalytic activity, which will be a promising approach to in situ preparation of complex metal nanocomposites.


Introduction

To date, metal–organic frameworks (MOFs) and their composite materials have been widely used in catalysis, storage and separations, sensing, drug delivery, and so forth.1–11 The distinct crystalline and porous structures and tailored compositions of MOFs make them outstanding templates and precursors to create a variety of nano- or microstructures by thermal decomposition.12 In recent years, the integration of metal nanoparticles (MNPs) with MOFs for novel or enhanced properties has been frequently explored, especially in the fields of catalysis.13,14

Amongst the developed techniques, thermal decomposition is a relatively new approach to synthesize MNPs@MOF, MNCs@MOF, MNPs@C, MNPs@C–N and single-atom catalyst (SAC) materials.15–22 In theory, rational selection of an appropriate heating temperature can achieve the partial decomposition of MOFs to form MNPs and the remainder can retain the porosity of the framework to a certain extent, forming so-called ‘‘Quasi-MOFs’’. The ‘‘Quasi-MOFs’’, which possess a transition-state between MOFs and metal oxides, could expose the inorganic nodes to MNPs, resulting in strong metal–support interactions. It is noteworthy that the significantly enhanced stability of the MOF derivatives makes them more suitable for catalytic heterogeneous organic reactions, such as oxidation, hydrogenation, dehydrogenation etc. Only a few successful cases of fabricating MNPs@MOFs by the above-mentioned thermal decomposition method have been reported so far.23,24 Xu et al. synthesized “Au/Quasi-MIL-101’’ by controlled deligandation, which could catalyze carbon monoxide at low temperatures.23 Fan et al. fabricated a Quasi-MOF Au–Cu/CeBTC-600N-270 through selective elimination of the carbon under a dilute O2 atmosphere, and the resultant catalysts exhibited excellent performance for CO oxidation without any activity loss over 250 h of evaluation time.24 However, these examples require additional introduction of metal ions, and in situ synthesis of MNPs@Quasi-MOF has not been reported.

Reasonable and purposeful design of the architecture and composition of the MOF precursors is required to generate the desired nanostructure. Jiang et al. rationally grew a representative MOF, Zn(2-methylimidazole)2 (ZIF-8), on Pd NCs to obtain Pd NCs@ZIF-8 with a core–shell structure.25 More specifically, the incorporation of doping heteroatoms and other components into MOFs, could control the nucleation and growth process, which affect the final structures and morphologies.18,26–28 Yang et al. demonstrated the introduction of dicyandiamide (DCDA) as both an inducer and an extra N source into Zn–Fe-ZIFs to synthesize a N-doped carbon nanotube material through pyrolysis.19,29 In general, bimetallic MOFs should have a clear benefit over a mixture of the respective monometallic MOFs. Yin et al. obtained Co single atoms on C–N-doped materials via a simple pyrolysis process of bimetallic ZIF precursor, which exhibited outstanding ORR performance.30

Herein, we designed a facile and controlled strategy for the fabrication of a Cu-nanowire@Quasi-MOF composite for the first time by employing a benzimidazole-modified Cu/Co bimetal-MOF as a self-template. Interestingly, numerous Cu-nanowires were prepared in situ, and they were curly and embedded in the Quasi-MOF octahedrons which retained the morphology of the bimetal-MOF template. The formation mechanism, structural characteristics and formation process of the composite were explored in detail.31 Experimental evaluation suggested that the Co-MOF part in the Cu/Co bimetal-MOF template served as a morphology retainer, and the special channels formed in the Quasi-MOF played a space-confined role in the formation of Cu-nanowires. The synthesized Cu-nanowire@Quasi-MOF composite exhibited excellent catalytic activity in selective oxidation of benzyl alcohol in air.

Experimental

Materials and methods

All reagents used in the experiments were of analytical grade without further purification. The PXRD patterns of the as-prepared samples were obtained from an X-ray diffractometer (Rigaku MiniFlex600) in transition mode with Cu Kα radiation. The morphology and structure of the samples were analyzed by a field emission SEM (FEI Nanosem 430; Hitachi S-4800), energy dispersive spectroscopy mapping and a high-resolution TEM (FEI Tecnai, model G2F-20 field emission TEM). The Raman spectrum was recorded by a Renishaw in Via Raman spectrometer with laser excitation at 532 nm wavelength. The nitrogen adsorption–desorption isotherms were measured at 77 K by a Micrometrics ASAP 2020 M volumetric gas adsorption analyzer (Micrometrics Instrument Corp.). The pore-size distribution was determined from BJH calculations based on the desorption branch of the corresponding isotherms. The specific surface areas of the materials were calculated using the Brunauer–Emmett–Teller method. Fourier transform infrared spectra of the samples were recorded at room temperature by a Bruker TENSOR 27 FT-IR spectrometer.

Synthetic procedures

Preparation of bimetal-MOFs. At first, three solutions were prepared. Solution A: 4.020 g BZI (benzimidazole, 20 mmol) was dissolved in 90 mL of ethanol and DMF (V[thin space (1/6-em)]:[thin space (1/6-em)]V = 1[thin space (1/6-em)]:[thin space (1/6-em)]1, same as below). Solution B: 0.480 g Cu(NO3)2·3H2O (2 mmol) and 0.578 g Co(NO3)2·6H2O (2 mmol) were dissolved in 90 mL of ethanol and DMF. Solution C: 4.200 g of BTC (1,3,5-benzenetricarboxylic acid, 20 mmol) in 180 mL ethanol and DMF. Then solution A was added to solution B in a three-necked round-bottom flask under stirring for 30 min, and then solution C was added to the above mixed solution under stirring for 12 h at 90 °C. After cooling down to room temperature, the product/solid was filtered and washed with ethanol several times, then dried at 60 °C for 6 h under vacuum.
Preparation of Cu-nanowire@Quasi-MOF nanocomposites. The as-synthesized Cu/Co bimetal-MOF was pyrolyzed in a tube furnace at 300 °C for 30 min with a heating rate of 2 °C min−1 under N2 atmosphere. After cooling down to room temperature naturally under N2 atmosphere, the Cu-nanowire@Quasi-MOF was obtained. For comparison, Cu-MOF crystals were synthesized without addition of Co2+ by using the same carbonization procedure.

Catalyst evaluation

The selective aerobic oxidation of benzyl alcohol was chosen as a model reaction to evaluate the activity of the nanocomposites. Reactions were carried out under an air atmosphere and base-free conditions in a two-necked glass flask. Typically, 1 mmol benzyl alcohol and 20 mg normalized catalyst were amalgamated into a 25 mL two-necked glass flask equipped with a reflux condenser. The system was under bubbling dry air at a flow rate of 15 mL min−1, and the glass flask was immersed into a water bath at 90 °C to initiate the reaction. During each reaction, the mixture was vigorously stirred to exclude any mass transfer limitation. After the reaction completed, the products were analyzed using a Shimadzu GC-2014C gas chromatograph, equipped with a CBP1-S25-050 capillary column and an auto injector.

Results and discussion

Synthesis and characterization of Cu/Co bimetal-MOF

In previous studies, Han et al. confirmed that the amount of BZI could change the morphology of Cu-MOF nanoparticles.32 BZI might compete with the BTC during coordination with copper ions, and this could control the crystal growth. Referring to the literature, in order to obtain a Cu-MOF with a regular octahedral structure, the molar ratio of BZI and BTC was fixed to 1.0 in this work. With the fixed feeding molar ratio of BZI and BTC, the uniform Cu/Co bimetal-MOF octahedrons with an average size of approximately 2.0 μm (Fig. S1b, ESI) were synthesized by adjusting the input amount of Co(NO3)2·6H2O to make the feeding molar ratio of Co/Cu as 1.0. When the amount of Co(NO3)2·6H2O was excessive, the morphology of the Cu/Co bimetal-MOF was no longer uniform, as shown in Fig. S1a (ESI). The uniform morphology of the Cu/Co bimetal-MOF octahedrons was reliably controlled by adopting different Co(NO3)2·6H2O concentrations. The PXRD patterns of the synthesized Cu-MOF and Cu/Co bimetal-MOF matched well with the simulated one of HKUST-1 (Fig. 1a). No peaks for other impurities were detected, indicating the high purity of the synthesized materials. The uniform distribution of Cu, Co and N in the as-prepared Cu/Co bimetal-MOF has been further proven by elemental mapping for the representative Cu/Co bimetal-MOF octahedrons (Fig. 1c and d). These results indicated that Cu/Co bimetal-MOF had been successfully synthesized by a one-pot method.
image file: c9qm00277d-f1.tif
Fig. 1 (a) PXRD patterns of HKUST-1, Cu-MOF, and Cu/Co bimetal-MOF, (b) TG curves of the as-prepared Cu-MOF and Cu/Co bimetal-MOF, (c) FE-SEM of Cu/Co bimetal-MOF, (d) TEM image of Cu/Co bimetal-MOF and EDX element mapping of Cu, N and Co.

As displayed in the thermogravimetric analysis (TG) curves, both Cu-MOF and Cu/Co bimetal-MOF were stable up to 250 °C, which coincided with the results in the literature (Fig. 1b). It is worth mentioning that the weight loss rate of Cu/Co bimetal-MOF was slower than that of Cu-MOF, which illustrated that the addition of Co could enhance the thermal stability of the MOF within a certain temperature range.

Synthesis and characterization of Cu-nanowire@Quasi-MOF

It was reported that direct calcination of HKUST-1 under N2 could obtain Cu nanoparticles (NPs) embedded in porous carbon, during which coordinated copper ions were reduced to Cu NPs and BTC was carbonized to graphite.33–35 Particularly worth mentioning here is that the pyrolysis temperatures were intentionally set around those of the maximum mass loss based on the TG profiles. The variable temperature PXRD of Cu/Co bimetal-MOF shown in Fig. 2 revealed that metal Cu was substantially formed at about 280 °C. The crystal structure of HKUST-1 was still retained at 300 °C, and as the temperature rose, the characteristic peaks gradually decreased, while they completely disappeared at about 400 °C and only strong Cu related diffraction peaks were found, indicating that the crystal structure of Cu/Co bimetal-MOF was completely destroyed. Therefore, in order to transform the Cu/Co bimetal-MOF phase into the MNPs/Quasi-MOF nanocomposites, annealing conditions for the conversion of Cu/Co bimetal-MOF were selected at 300 °C for 1/2 h under N2 gas atmosphere, affording Cu-nanowire@Quasi-MOF.
image file: c9qm00277d-f2.tif
Fig. 2 The variable temperature PXRD patterns of Cu/Co bimetal-MOF.

The morphologies and microstructures of the Cu-nanowire@Quasi-MOF were firstly investigated by using SEM, TEM, PXRD and FTIR spectra. The SEM images in Fig. 3a demonstrated that the Cu-nanowire@Quasi-MOF still retained the octahedral morphology. Interestingly, the octahedrons were embedded with numerous curly nanowires, which was further confirmed by the TEM image in Fig. 3b. As seen in Fig. 3c, the elemental mapping demonstrated that the elements Cu, Co, N, C and O were evenly distributed throughout the Cu-nanowire@Quasi-MOF, which suggested that the original frame structure might still remain in the Cu-nanowire@Quasi-MOF. In order to further investigate the textural properties of the Cu-nanowire@Quasi-MOF nanocomposite phase, PXRD was performed. The diffraction peaks below 20° in the PXRD pattern shown in Fig. 4a were similar to those of Cu/Co bimetal-MOF (HKUST-1). And the remaining peaks located at 2θ = 43.30°, 50.43°, and 74.13° could be attributed to the (111), (200), and (220) crystal planes of Cu0 (PDF#04-0836).36 Peaks for the carbon layer were not detected in the PXRD pattern because of the amorphous structure of the carbon. The above results implied that the framework of HKUST-1 was partially retained and metallic Cu was formed, which agreed well with the elemental mappings obtained by TEM. The FTIR spectrum in Fig. 4b further confirmed the chemical composition and phase change of the pyrolytic product. The asymmetric stretching of the carboxylate groups in BTC was detected at 1508–1623 cm−1, and the symmetric stretching of the carboxylate groups in BTC was observed at 1384 and 1405 cm−1. Several bands in the region of 1300–600 cm−1 were assigned to the out-of-plane vibrations of BTC,37 indicating that the Cu/Co bimetal-MOF was preserved partially. In addition, magnified TEM images provided clear architectures of the nanowires (Fig. 3b). The elemental composition of the nanowires inside the octahedron was examined by EDX spectroscopy and the results are shown in Fig. 3d. These results suggested that Cu0 was the main copper species in the nanowires, which was also proved by EDX of a single nanowire (Fig. S2, ESI).


image file: c9qm00277d-f3.tif
Fig. 3 FESEM (a), TEM (b) and EDX spectra (c) images of the as-prepared Cu-nanowire@Quasi-MOF; (d) the EDX spectra of selected area enlargement in (b).

image file: c9qm00277d-f4.tif
Fig. 4 (a) PXRD and (b) FTIR patterns of Cu-nanowire@Quasi-MOF. (c–f) XPS of Cu-nanowire@Quasi-MOF, (c) survey spectrum, data in the inserted table is the ratio of various atoms, (d) Cu 2p spectrum, (e) Co 2p spectrum, and (f) N 2p spectrum.

To further investigate the ingredients of Cu-nanowire@Quasi-MOF, XPS measurements were conducted. The survey spectrum (Fig. 4c) revealed the ratio of Cu, Co, O, N and C atoms. Fig. 4d showed the curve fitted spectra of the Cu 2p and the main peaks at 932.7 and 952.5 eV for Cu 2p3/2 and Cu 2p1/2, respectively, which could be ascribed to Cu0 species, indicating that part of the Cu2+ had converted into Cu0 during thermolysis. Other satellite peaks indicated the presence of a small amount of Cu2+.38 The Co 2p peaks at 781.98 eV and 797.98 eV could be assigned to the Co2+ in Cu/Co bimatel-MOF (Fig. 4e). In the N 2p spectrum (Fig. 4f), the two peaks located at 398.5 and 399.9 eV could be attributed to the C[double bond, length as m-dash]N–C and amino functional groups, respectively.39 The above results showed that the Cu2+, Co2+ and organic ligands still existed in Cu-nanowire@Quasi-MOF, which was consistent with the PXRD, FTIR and EDX results as well.

Pore characteristics were also used to investigate the material structure from microscopic analysis indirectly. Thus, we analyzed the pore composition through N2 sorption isotherms of the Cu/Co bimetal-MOF (Fig. 7a) and Cu-nanowire@Quasi-MOF (Fig. 7g). The pore distributions showed a sharp micropore peak at around 0.86 nm (Fig. 7b), a typical characteristic peak of HKUST-1. A pronounced hysteresis in the P/P0 was observed for the two samples, implying the presence of a large number of mesopores. Notably, compared with the N2 adsorption–desorption isotherms of Cu/Co bimetal-MOF, the isotherms of Cu-nanowire@Quasi-MOF displayed a significant increase in the adsorption at high relative pressures (P/P0 > 0.9), indicating the existence of large cavities. The pore-size distributions (Fig. 7b and h) calculated from the adsorption branch of the nitrogen isotherms according to the BJH method showed that there are many pores with apertures of less than 2 nm for both Cu/Co bimetal-MOF and Cu-nanowire@Quasi-MOF. The coexistence of micropore and mesopore characteristics in both Cu/Co bimetal-MOF and Cu-nanowire@Quasi-MOF could be regarded as indirect evidence that Cu-nanowire@Quasi-MOF possessed a Quasi-HKUST-1 structure. With the above investigations, it has been confirmed that Cu-nanowire@Quasi-MOF was composed of a Quasi-MOF matrix embedded with curly nanowires consisting of Cu.

Cu-Nanowire@Quasi-MOF formation process

Han et al. successfully synthesized in situ Cu2O nanoparticles anchored on a nitrogen-doped porous carbon yolk–shell cuboctahedral (CNPC) framework by direct derivation from a benzimidazole-modified Cu–BTC MOF as a precursor at 300 °C.32 Compared with Han's method, the most distinguishing characteristic found here was the addition of cobalt ions in the precursor. To investigate the influence and necessity of cobalt ions on the morphology and structure of the final pyrolysis product, Cu-MOF and Cu/Ni bimetal-MOF were synthesized for comparison under the same conditions in the absence of Co or by addition of Ni instead of Co, respectively. Subsequently, Cu-MOF(300) and Cu/Ni bimetal-MOF(300) were obtained after the same calcination process.

The SEM and TEM images of Cu-MOF(300), Cu/Ni bimetal-MOF(300) and Cu-nanowire@Quasi-MOF are shown in Fig. 5. The EDX spectroscopy images of Cu-nanowire@Quasi-MOF and Cu/Ni bimetal-MOF(300) are shown in Fig. S3 and S4 (ESI), and the presence of Ni and Co in the materials means that bimetal MOF Cu/Ni bimetal-MOF(300) and Cu-nanowire@Quasi-MOF were successfully prepared respectively. In Fig. 5, the Cu-MOF(300) composite showed an octahedral morphology with a smooth surface and core–shell structure, which is in agreement with Han's report basically. However, although Cu/Ni bimetal-MOF(300) still retained the octahedral morphology, its interior was almost filled with metal particles at an average diameter of approximately 100 nm rather than nanowires compared with the Cu-nanowire@Quasi-MOF. From these results, it was concluded that introducing Co ions in the precursor was necessary as they could adjust the structure and morphology of the final pyrolysis product. Therefore, we demonstrated that ion modulation (Co2+ as the indicator) was able to create Cu-nanowire@Quasi-MOF by a pyrolysis method.


image file: c9qm00277d-f5.tif
Fig. 5 Scheme and TEM images of Cu MOF(300), Cu/Ni bimetal-MOF(300), and Cu-nanowire@Quasi-MOF.

The Raman spectra of Cu/Co bimatel-MOF, Cu-nanowire@Quasi-MOF, Cu/Ni bimatel-MOF(300) and Cu-MOF(300) are shown in Fig. S5 (ESI); the peaks located at 1314 to 1544 cm−1 were characteristic of carbon and indicated the presence of the carbon in Cu-nanowire@Quasi-MOF, Cu/Ni bimatel-MOF(300) and Cu-MOF(300).40 Besides, Cu-nanowire@Quasi-MOF showed two small peaks at 821 and 1003 cm−1 which were attributed to the C–H and C[double bond, length as m-dash]C, respectively.23 This indicated that some of the ligands were retained, which suggested that the Co-MOF in the Cu/Co bimetal-MOF template served as a morphology retainer.

It was found that calcination conditions have remarkable effects on the structure and activity of the catalysts and different supports have different optimum calcination temperatures. To better understand the growth process of Cu-nanowire@Quasi-MOF, the temperature-dependent evolution of shape and structure was elucidated by FTIR, SEM and TEM. The annealing conditions for the transformation of Cu/Co bimetal-MOF were selected from 150–400 °C for 1/2 h under a N2 gas atmosphere, affording a series of calcined Cu/Co bimetal-MOF(x) samples, where x indicates the calcination temperature. Then, FTIR measurement of the Cu/Co bimetal-MOF(x) samples was performed (Fig. S6, ESI), in order to analyse the decomposition degree of Cu/Co bimetal-MOF by the change of characteristic groups. The result showed that the peaks of the ligand-related group were retained up to 350 °C, meaning that the materials still partly retained the primary structures of Cu/Co bimetal-MOF. Fig. 6 presents FESEM and TEM images of the pristine Cu/Co bimetal-MOF and four representative Cu/Co bimetal-MOF(x) including Cu/Co bimetal-MOF(260), Cu/Co bimetal-MOF(280), Cu-nanowire@Quasi-MOF and bimetal-MOF(340). As shown in Fig. 6b and g, Cu/Co bimetal-MOF(260) maintained the regular octahedral shape, but when compared with Cu/Co bimetal-MOF, the surface began to become rough. TEM, in situ PXRD and FTIR investigations have indicated that around this temperature, a portion of the ligands was pyrolyzed to form carbon. When the calcination temperature increased to 280 °C (Fig. 6c and h), flocculent shadows appeared inside the octahedron. With an increase of temperature to 300 °C (Fig. 6d and i), it was obvious that the octahedral structure of the precursor was preserved well and the octahedron was inlaid with curly nanowires which exhibited good dispersity. The diameter of the nanowires is about 30–40 nm, as shown in Fig. S2 (ESI). At a higher calcination temperature of 340 °C (Fig. 6e and j), the octahedron slightly shrank, and spherical particles with an average diameter of 100 nm were observed, which might be formed by the agglomeration of nanowires along with an increase in temperature. The gradual change of contrast in the TEM images suggested that nanowires were wrapped inside the Quasi-MOF. With increasing pyrolysis temperature, the nanoparticles tended to aggregate owing to the higher thermal energy.


image file: c9qm00277d-f6.tif
Fig. 6 (a–e) FE-SEM and (f–j) TEM images of the products obtained at different calcination temperatures. (a and f) Cu/Co bimetal-MOF. (b and g) Cu/Co bimetal-MOF(260). (c and h) Cu/Co bimetal-MOF(280). (d and i) Cu-Nanowire@Quasi-MOF. (e and j) Cu/Co bimetal-MOF(340).

Besides, a time-dependent morphology evolution study was performed at 300 °C and under N2 atmosphere (Fig. S7, ESI). Metal nanopaticals began to accumulate 10 minutes after calcinated at 300 °C, and Cu-nanowires@Quasi-MOF was prepared by calcination for 30 minutes. Upon an extended calcination time of 60 minutes, nanowires were formed both inside and outside the octahedron, and the octahedron was then disintegrated at 120 minutes.

In order to investigate the relationship between the formation of Cu-nanowires and the pore structure characteristics, the N2 adsorption and desorption isotherms were measured for the samples calcined at 260 °C, 280 °C, 300 °C and 340 °C (Fig. 7c, e, g and i), and all of them displayed similar isotherms. The pore size distributions revealed that the mesoporous size of Cu/Co bimetal-MOF(280) and Cu-nanowires@Quasi-MOF (Fig. 7f and h) increased from 20 nm to 45 nm which matched with the diameter of the nanowires. The multi-stage mesoporous environment provided a space template for the growth of nanowires. When the calcination temperature increased to 340 °C (Fig. 7j), the micropores disappeared, resulting in the aggregation of Cu0 into spheres with a diameter of about 100 nm. Several reports have shown that nanomaterials with specific and fine morphologies can be synthesized by a space-confined effect.41–44 Li et al. synthesized ZIF-67 nanocrystals due to the nanoconfined environment inside hollow carbon nanospheres (HCSs), which was limited through the pores and channels of HCSs.45 Under carbonization conditions, the structures of the carbon matrix can also play a practical and effective role in affecting the morphologies of the metal or metal oxides generated from the pyrolytic MOFs.46 So, we assume that the “shaping effect” of Quasi-MOF plays an important role through the space-confinement effect of special channels formed via the Quasi-MOF.


image file: c9qm00277d-f7.tif
Fig. 7 N2 adsorption–desorption isotherms at 77 K (a, c, e, g and i) and pore size distribution (b, d, f, h and j) of (a and b) Cu/Co bimetal-MOF, (c and d) Cu/Co bimetal-MOF(260), (e and f) Cu/Co bimetal-MOF(280), (g and h) Cu-nanowire@Quasi-MOF and (i and j) Cu/Co bimetal-MOF(340).

On the basis of these characterizations, the growth process of Cu-nanowire@Quasi-MOF is illustrated in Scheme 1.


image file: c9qm00277d-s1.tif
Scheme 1 Illustration of the formation of the Cu-nanowire@Quasi-MOF.

The morphology of the MNPs may be mainly determined by the aggregation tendencies of the primary particles, which was influenced by the pyrolysis conditions.47 The above results highlight a facile pyrolysis route to the formation of Cu-nanowire@Quasi-MOF, and it is suggested that the formation of the composites comprises at least three steps: (1) solvent molecules evaporate and the ligands partially pyrolyze to form carbon skeletons; (2) metastable copper nanoparticles begin to aggregate; (3) redistribution and reassembly of Cu nanoparticles to produce Cu-nanowires within the inner space, resulting in the formation of Cu-nanowire@Quasi-MOF composite structures.

Specifically, on the one hand, with an increase intemperature, solvent molecules evaporate and a small portion of organic ligands begins to decompose from Cu/Co bimetal-MOF. Due to the difference in stability, the coordination bonds formed between the Cu2+ and the ligands were broken first. Meanwhile, the existence of the Co-MOF part could improve the thermal stability of Cu/Co bimetal-MOF, which contributes to retaining the octahedron morphology of the precursor well in the designated temperature range. By controlling the calcination temperature and the heating rate, the shrinkage stresses on different directions of the nanoparticles could be exactly balanced, which then results in the uniform distribution of Quasi-MOF in the octahedron. On the other hand, the carbon formed by ligands during pyrolysis could reduce free Cu2+ ions to Cu0, and under the influence of Ostwald ripening, the density of the initial octahedron changes during thermal decomposition and metal nanoparticles begin to form.48 It is well known that a template is usually needed for the preparation of MNPs with specific morphologies. MNP growth is limited by the nano-channel template as the “mould”, which is prepared by depositing target materials in the nano-channel template. The mesoporous skeleton formed during pyrolysis and the unique channel formed in the retained MOF might limit the growth of MNPs and provide a template for the formation of copper nanowires.

Catalytic performance

The catalytic performance of the samples was evaluated in the selective oxidation of benzyl alcohol in air. The activity of different catalysts is compared in Fig. 8a. Over a pristine Cu/Co bimetal-MOF, the oxidation exhibited only 12.0% conversion of benzyl alcohol with 92.0% selectivity of benzaldehyde. Obviously, the catalytic activity of each Cu/Co bimetal-MOF(x) was superior to that of the Cu/Co bimetal-MOF. Previous investigations have indicated that with increasing calcination temperature, a large number of solvent molecules such as ethanol and DMF would escape, allowing active sites to be exposed.49 As a result, the activity of the catalysts would be significantly promoted. Cu/Co bimetal-MOF(260) gained 90.7% selectivity of benzaldehyde with 32.5% conversion of benzyl alcohol. Compared with Cu/Co bimetal-MOF(260), Cu-nanowire@Quasi-MOF dramatically increased to 98.8% selectivity of benzaldehyde with 62.8% conversion of benzyl alcohol. The Cu/Co bimetal-MOF(280) and Cu/Co bimetal-MOF(340) showed a lower activity than Cu-nanowire@Quasi-MOF under the same reaction conditions. As indicated above, Cu-nanowire@Quasi-MOF possesses both abundant mesopores and micropores which may enhance its activity by improving mass transfer in the catalytic process.50 At the same time, compared with MNP spheres, Cu-nanowires have larger surface area, which improves the contact probability between the reactants and catalyst.
image file: c9qm00277d-f8.tif
Fig. 8 (a) Catalytic performance of Cu/Co bimetal-MOF(x); 1, 2, 3, 4 and 5 represent Cu/Co bimetal-MOF, Cu/Co bimetal-MOF(260), Cu/Co bimetal-MOF(280), Cu-nanowire@Quasi-MOF and Cu/Co bimetal-MOF(340), respectively. (b) Effect of reaction time. Reaction conditions: 20 mg catalyst: Cu/Co bimetal-MOF(300), 1 mmol benzyl alcohol, 10 mL CH3CN, 90 °C.

The influence of reaction time on the catalytic performance of the present reaction system was also investigated using a Cu-nanowire@Quasi-MOF catalyst. The results indicated that under the studied conditions, 5 hours was the optimum reaction time (Fig. 8b).

Stability of the catalyst

The stability of the catalyst was evaluated with cyclic tests. In this study, the catalyst was centrifuged after each run and washed 5 times with ethanol to remove residuals from the catalyst.51 As seen in Fig. 9 the conversion of benzyl alcohol and the selectivity of benzaldehyde after 4 cycles is summarized. The activity of the catalyst decreases slightly after 3 cycles. Such a high stability may be attributed to the special structure of the Cu-nanowire@Quasi-MOF. The Cu-nanowires were stabilized by Quasi-MOF, which could hamper the aggregation of the Cu-nanowires effectively. The catalyst showed 76.5% benzaldehyde selectivity after four cycles. The decrease in the reaction activity might be attributed to the detachment of the ligand that remained in the Cu-nanowire@Quasi-MOF during the oxidation, which would weaken the confinement of the Quasi-MOF, leading to the aggregation of nano-metal into bulk metals, the morphology of the catalyst after four cycle tests is shown in Fig. S8 (ESI).
image file: c9qm00277d-f9.tif
Fig. 9 Cyclic tests. Reaction conditions: catalyst (Cu-nanowire@Quasi-MOF, 20 mg), benzyl alcohol (1 mmol), 10 mL CH3CN, 90 °C, 5 hours.

Conclusion

In conclusion, Cu-nanowire@Quasi-MOF was prepared by a mild thermal treatment using a BZI modified Cu/Co bimetal-MOF as a precursor for the first time. An in situ space-confined topography control strategy was proposed. The complex pore environment including micropores and mesopores in the Quasi-MOF physically confined the growth of Cu NPs, resulting in the in situ formation of nanowires in the octahedral structure. Moreover, Cu-nanowire@Quasi-MOF exhibited excellent catalytic activity for the oxidation of benzyl alcohol in air. Considering the important role of the morphology of MNPs in catalysis, this study provides a feasible way to prepare MNP composite catalysts with specific morphology using MOFs as the exclusive precursors. The space-confined in situ strategy of MNPs opens up a new avenue for the rational design of MNP composite catalysts, which will be helpful for the preparation of similar catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the NSFC (21673120), and the Fundamental Research Funds for the Central Universities, Nankai University (63196005).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qm00277d

This journal is © the Partner Organisations 2019