The selective partial oxidation of alkanes is an important transformation in the production of valuable chemicals and intermediates such as ketones and alcohols [1]. For example, cyclohexanone and cyclohexanol (also known as KA-oil), the two main products of the partial oxidation of cyclohexane, are the key intermediates in the fabrication of polyamide and plastics such as Nylon 6 and Nylon 66 [2]. However, control over the selective oxidation of saturated hydrocarbons is challenging because of the inertness of the saturated C-H bonds and subsequent oxidation issues resulting from the higher reactivity of the products when compared with that of the reactants. Accordingly, the industrial manufacture of KA-oil from cyclohexane oxidation needs careful control (at conversions of <5% with 75%-80% selectivity to KA-oil) to prevent the production of excessive amounts of byproducts owing to over-oxidation [3-5].
Over the past few decades, considerable research efforts have been devoted to exploring efficient and environmentally sound methodologies for the selective oxidation of alkanes. In this regard, heterogeneous catalysis has received the most attention, and various efficient metal catalysts have been developed [6-23]. These catalysts include metal-incorporated molecular sieves [12], metal-substituted molecular sieves [13, 15], supported Au nanoparticles [6-10, 23], metal-organic frameworks (MOFs) [22], and carbon-based materials [24, 25]. However, the development of cost-effective, and highly active and selective heterogeneous catalysts for the oxidation of alkanes remains a great challenge to date.
Of particular interest, MOFs are a well-known, new class of porous functional materials constituting metal ions and organic ligands. Owing to their ordered structures and relatively low thermal stability, MOFs have been used for the preparation of new metal oxides or carbon nanomaterials by thermal decomposition. As reported, MOF-derived materials exhibit excellent performance in various applications including heterogeneous catalysis [26-36], electrochemistry [37, 38], and gas adsorption [39-41]. To our knowledge, reports on the use of MOF-derived materials as catalysts for liquid-phase organic synthesis are rare.
Herein, we report a novel, non-noble Ni-based heterogeneous catalytic system for the selective oxidation alkanes. Various alkanes, such as cyclohexane and ethylbenzene, could be selectively transformed into the desired aldehydes or ketones in high yields under mild conditions. The catalysts were prepared by simple thermolysis of a Ni-containing MOF under inert atmosphere.
All chemicals were purchased from commercial corporations and used without further treatment.
Typically, a mixture of Ni(NO)3∙6H2O (0.9 mmol), 1, 4-benzenedicarboxylic acid (H2bdc; 0.72 mmol), triethylenediamine (dabco; 0.58 mmol), and dimethylformamide (DMF; 15 mL) were added to a Teflon-lined autoclave. The reactor was heated in an oven at 120 ℃ for 2 d. The obtained green powder was then washed with DMF and methanol, and finally dried under vacuum at 150 ℃ for 2 h.
The Ni@C-N catalysts were prepared by pyrolysis of Ni-MOF in an inert atmosphere. Ni-MOF was heated at a heating rate of 1 ℃/min from room temperature to 200 ℃ and maintained at this temperature for 2 h in Ar. Then, the temperature was increased to the target temperature (i.e., 500, 600, 700, 800, or 900 ℃) at the same heating rate and subsequently maintained for 8 or 15 h. The resulting samples were denoted as Ni@C-N-x-8h or Ni@C-N-x-15h, where x indicates the target thermolysis temperature (℃).
Powder X-ray diffraction (PXRD) patterns of the prepared materials were recorded on a Rigaku diffractometer (D/MAX-ⅢA; 3 kW) using Cu Kα radiation (40 kV, 30 mA, λ = 0.1543 nm). N2 adsorption-desorption isotherms were measured on a Micromeritics ASAP 2020M instrument at -196 ℃ . The Ni content of the samples was determined quantitatively by atomic absorption spectroscopy (AAS) on a HITACHI Z-2300. The surface topography of Ni-MOF and the Ni@C-N materials was investigated by scanning electron microscopy (SEM; MERLIN of ZEISS). Transmission electron microscopy (TEM) images of the samples were recorded on a JEM-2010HR (JEOL) microscope. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD system with a base pressure of 1.33 x 10-7 Pa.
The oxidation of alkanes (e.g., ethylbenzene) was conducted in a 25-mL Schlenk tube. Typically, ethylbenzene (0.5 mmol), catalyst (10 mol% based on Ni), tert-butyl hydroperoxide (TBHP; 1.5 mmol), and DMF (2 mL) were added to the tube and heated at 80 ℃. After reaction, the catalyst was isolated from the mixture solution by centrifugation, then washed with DMF and methanol. The liquid mixtures were then analyzed by gas chromatography mass spectrometry (Agilent Technologies 7890B-5977A) on a chromatograph equipped with a 0.25 mm × 30 m HP-5MS capillary column.
For the recyclability tests, the catalyst was separated from the reaction mixture and washed with DMF and methanol for several times. The solid was then dried at 80 ℃ in an oven and reduced by H2 at 200 ℃ for 2 h before use for the next run.
Ni-MOF was prepared according to the procedures described in previous reports [42, 43]. The PXRD pattern of the as-synthesized Ni-MOF (Fig. 1(a)) matched well with the published XRD patterns [42, 43]. The N2 adsorption-desorption isotherm measured at -196 ℃ indicated that Ni-MOF was mostly microporous (Fig. 1(b)). The Brunauer-Emmett-Teller (BET) surface area was calculated to be ~1700 m2/g.
The Ni@C-N materials were prepared by pyrolysis of Ni-MOF under a continuous flow of Ar for 8 or 15 h at varying temperatures. The Ni contents in the samples were ~35-40 wt%. Fig. 2 shows the PXRD patterns of the Ni@C-N materials. All the prepared materials displayed five diffraction peaks at ~44.4°, 51.8°, 76.4°, 92.9°, and 98.5°, which are characteristics of metallic Ni (JCPDS No. 04-0850) [44]. The intensity of the Ni diffraction peaks increased with increasing pyrolysis temperatures employed, indicating the formation of Ni phase with a higher crystallization degree. The crystallite sizes of Ni Particles in the materials were in the range of 7-10 nm, as calculated by the Scherrer formula.
Fig. 3 shows the N2 adsorption-desorption isotherms of the Ni@C-N materials measured at -196 ℃ . All isotherms featured typical adsorption curves of types Ⅰ and Ⅳ, with an apparent hysteresis loop in the relative pressure (P/P0) range of 0.5-1.0, suggesting the presence of a micro-/meso-/macroporous structure in the Ni@C-N materials. The BET surface areas of the Ni@C-N materials were calculated to be ~100-200 m2/g.
The surface morphology of the Ni@C-N materials was examined by SEM. The SEM images in Fig. 4(a)-(d) depict the initial shape of the Ni-MOF crystals when MOF was calcined below 700 ℃ for 8 h. The surface of the materials became distorted and considerably rougher upon decomposition and carbonization of the MOF. When the pyrolysis temperature increased to 800 ℃, numerous nanotubes were obtained (Fig. 4(e)), which might be catalyzed by the generated Ni nanoparticles.
The TEM images in Fig. 5 revealed that the metallic Ni nanoparticles were highly dispersed in the carbon matrix derived from carbonization of the organic linker in Ni-MOF. At the higher MOF pyrolysis temperatures, aggregation of the Ni nanoparticles into larger particles was observed. When the thermolysis temperature increased to 900 ℃, most of the particles agglomerated into large particles with an average size of ~18 nm (Fig. 6). High-resolution TEM analysis of an individual large particle indicated that each Ni nanoparticle was tightly embedded in the graphitic carbon (Fig. 6(c)).
The XPS spectra of Ni-MOF and Ni@C-N-900-8h are shown in Fig. 7. The Ni 2p peaks at 856.0 and 873.6 eV could be assigned to Ni2+ in Ni-MOF (Fig. 7(a)). After thermolysis, the two Ni 2p peaks of Ni@C-N-900-8h respectively shifted to lower binding energies by ~3.2 and ~3.3 eV relative to those of Ni-MOF, indicating that most of the Ni(Ⅱ) ions in MOF were reduced to Ni(0) [45]. This result agreed with common experimental observations, whereby central metal ions of MOF with reduction potentials higher than 0.27 Ⅴ are typically reduced to their metallic states upon calcination in an inert atmosphere [46-48]. The N 1s peak of Ni-MOF appeared at 399.8 eV and could be assigned to the N of triethylenediamine coordinated with a metal ion [49]. Interestingly, this bonding energy was slightly higher than that of amino nitrogen (399.6 eV) owing to the influence of the magnetic metal center (i.e., Ni) [50, 51]. After thermolysis, the N 1s spectra of Ni@C-N-900-8h showed two new peaks at 398.9 and 401.0 eV, which could be assigned to pyridinic N and graphitic N, respectively [52]. This phenomenon was consistent with the TEM result (Fig. 6(c)) with respect to the formation of a graphitic structure.
First, ethylbenzene oxidation was employed as a model reaction to evaluate the catalytic activities of the as-synthesized Ni@C-N materials. The catalytic reaction was conducted at 80 ℃ using TBHP as oxidant, and the results are summarized in Table 1. The blank reaction, without any catalyst, achieved minimal conversion performance only (Table 1, entry 1). In contrast, in the presence of the Ni@C-N materials, the oxidation transformation was efficiently promoted. The activity of the catalysts was strongly dependent on the pyrolysis temperature and time. Generally, the Ni@C-N materials prepared under higher MOF thermolysis temperatures showed higher activities (Table 1, entries 2-11), possibly because of the higher carbonization degree of the organic linkers in the MOF. In contrast, a longer pyrolysis time reduced the catalytic activity of the obtained Ni@C-N materials. Among the examined catalysts, Ni@C-N-900-8h displayed the best ethylbenzene oxidation performance (Table 1, entry 6), achieving an ethylbenzene-to-acetophenone conversion of 77% within 48 h of reaction at 80 ℃. Moreover, a higher yield of acetophenone could be achieved by simply increasing the amount of TBHP, while maintaining other conditions constant (Table 1, entry 12).
Subsequently, the recyclability of Ni@C-N-900-8h catalyst toward ethylbenzene oxidation was examined. The results are presented in Table 1 (entries 6, 13-15); no significant decline in the conversion and selectivity performance was observed during repeated catalytic runs (up to four runs). These results demonstrate the high stability of the Ni@C-N catalyst under the investigated conditions.
To demonstrate the general applicability of the Ni@C-N catalysts, we extended the use of the present catalytic system (specifically, Ni@C-N-900-8h) to the oxidation transformations of various saturated alkanes under similar reaction conditions. The results are summarized in Table 2. As observed, the various secondary C-H bonds were oxidized smoothly with excellent selectivities to their corresponding partial oxidation products (Table 2, entries 1-5). For the oxidation of cyclohexane, a moderate conversion with 100% selectivity to cyclohexanone was achieved at 100 ℃ (Table 2, entry 5). Substituted ethylbenzene containing either electron-withdrawing or electron-donating functional groups also displayed excellent conversions and selectivities to the corresponding ketones (Table 2, entries 6-8). It was interesting to note that Ni@C-N was also highly active and selective toward the oxidation of 1, 4-diethylbenzene, achieving >95% selectivity to the diketone product (Table 3, entry 9). These results demonstrate the high versatility of the MOF-derived Ni@C-N catalysts prepared herein for the selective oxidation of alkanes.
We have developed novel, efficient, and cost-effective Ni-based catalysts for application in the selective oxidation of alkanes. The Ni@C-N materials were prepared by direct thermolysis of a Ni-containing MOF under inert atmosphere. The as-prepared Ni nanoparticles embedded in the C-N composites displayed high activity and selectivity in the oxidation of alkanes to their corresponding partial oxidation products under mild reaction conditions, thus showing a broad substrate scope for diverse saturated C-H bonds. Moreover, the catalysts are highly stable and maintain high activity and selectivity during numerous repeated catalytic cycles under the investigated conditions. The present strategy may be extended to the development of other novel metal-carbon composite catalysts for selective catalytic oxidation applications.
2016,
Vol. 37 Issue (6): 955-962 
PDF (1018 KB)
2016,
Vol. 37 Issue (6): 955-962 PDF (1018 KB)