Arginine-Assisted Hydrothermal Synthesis of Urchinlike Nb2O5

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Arginine-Assisted Hydrothermal Synthesis of Urchinlike NbO Nanostructures Composed of Nanowires and Their Application in Cyclohexanone Ammoximation 2

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Yong Wang, Feng Xin, Xiaohong Yin, Yuexiao Song, Tianyu Xiang, and Junzheng Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10312 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Arginine-Assisted

Hydrothermal

Synthesis

of

Urchinlike Nb2O5 Nanostructures Composed of Nanowires and Their Application in Cyclohexanone Ammoximation †

†

‡

†

†

Yong Wang, Feng Xin , Xiaohong Yin, Yuexiao Song , Tianyu Xiang , and Junzheng Wang*,

†

† School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China ‡ School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China

Abstract: Urchin-like Nb2O5 nanostructures have been successfully synthesized by a novel and simple L-arginine-assisted hydrothermal method. They are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), nitrogen adsorption–desorption isotherms, thermogravimetric and differential thermal analysis (TG-DTA), and Fourier transform infrared spectroscopy (FT-IR). The results show that the urchin-like nanostructures are composed of nanowires with diameter less than 15 nm and possess a high specific surface area of 249.9 m2•g-1. Urchin-like Nb2O5 nanostructures have been used for the first time as a novel catalyst instead of conventional titanosilicate in the liquid-phase ammoximation of cyclohexanone. The as-prepared urchin-like Nb2O5 nanostructures exhibit high

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catalytic activity in cyclohexanone ammoximation. Under the optimal reaction conditions, the conversion of cyclohexanone and selectivity of oxime are as high as 98.0% and 88.9%, respectively. Finally, a possible formation mechanism of urchin-like Nb2O5 nanostructures is proposed.


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1. Introduction Niobium pentoxide (Nb2O5), the most thermodynamically stable phase among various niobium oxides,1 has attracted much attention due to its application in the fields of catalysis,2-6 gas sensors,7 energy storage,8-10 and electrochromic devices.11 Since its particular electronic, optical, chemical, and physical properties greatly depend on their structures and morphologies, considerable progresses have been achieved to control the Nb2O5 nanostructures, such as nanoparticles,12,13

nanorods,14

nanowires,15,16

nanotubes,17,18

nanofibers,19

nanobelts,20

nanoplates,21 and nanosheets.2,22 Among them, the one-dimensional (1D) nanowires have been recently studied extensively, because they can provide more catalytic active sites and enhance the diffusion of reactants and products. However, it is still a significant challenge to develop a simple and efficient approach to prepare Nb2O5 nanowires with a large surface area. Amino acids with two functional groups (-NH2, -COOH) can interact with intermediate by electrostatic binding, covalent coupling, and physical adsorption during reaction process for preparation of nanomaterials.23 Recently, several groups reported that 1D nanostructures could be formed using basic amino acids including lysine and arginine. For example, Yuan’s group produced ordered arrays of mesoporous Co(OH)2 nanowires with the assistance of lysine.24 Sethi et al. prepared the linear gold nanochains by assembling of gold nanoparticles in the presence of arginine.25 In addition, arginine can be used for the preparation of highly dispersed 1D bean-like TiO2 nanoparticles.26 However, the formation mechanism of 1D nanostructures with basic amino acids is still not very clear. Cyclohexanone oxime is a key intermediate in the production of ε-caprolactam (a precursor of nylon-6) through the Beckmann rearrangement.27 Conventional titanosilicate catalysts (Ti-MWW and TS-1) exhibit excellent performances in the liquid-phase ammoximation of


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cyclohexanone with ammonia and hydrogen peroxide to prepare cyclohexanone oxime.28 Nevertheless, synthesis of titanosilicate is a complex and tedious procedure, and requires the use of a large amount of expensive organic structure-directing agent (e.g. tetraalkylammonium hydroxides and piperidine). Generally, the use of solvent tert-butanol in the process is not environmentally friendly and will increase energy consumption for separation and recycling. Furthermore, it is difficult to separate and recover titanosilicate catalysts due to their small particle size. Hence, it is still highly demanded to prepare an alternative efficient catalyst for the cyclohexanone ammoximation. Here we firstly report a novel and simple L-arginine-assisted hydrothermal method for the high-yield preparation of urchin-like Nb2O5 nanostructures composed of nanowires with a large surface area (249.9 m2 ·g-1), using low cost niobium oxalate as the starting material. The influences of concentration of L-arginine, concentration of niobium oxalate, reaction time, and synthesis temperature on the morphology and structure of Nb2O5 are investigated systematically. When urchin-like Nb2O5 nanostructures are used as catalysts in the liquid-phase ammoximation of cyclohexanone, the high conversion of cyclohexanone (98.0%) and selectivity of oxime (88.9%) are achieved.

2. Experimental methods 2.1 Reagents and chemicals. Niobium oxalate was purchased from Shanghai D&B Biological Science and Technology Co., Ltd. (Shanghai, China). L-Arginine was purchased from J&K China Chemical Co., Ltd. (Beijing, China). Ammonia solution (25 wt%), hydrogen peroxide (30 wt%), and cyclohexanone were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Dichloromethane and toluene were purchased from Tianjin Yuanli


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Chemical Reagent Company (Tianjin, China). All chemicals were of analytical grade and used without any further purification. 2.2 Preparation of Nb2O5 nanostructures. Urchin-like Nb2O5 nanostructures were synthesized through a simple hydrothermal method. In a typical synthesis, niobium oxalate (2 mmol) was dissolved in 25 mL deionized water with stirring at 60 °C for 10 min to form solution A. L-Arginine (1 mmol) was dissolved in 5 mL deionized water with stirring for 5 min to form solution B, and then it was added dropwise into solution A with stirring for 10 min at room temperature. The mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave, heated to 180 °C and maintained for 12 h. After the autoclave was cooled down to room temperature naturally, the white precipitate was collected by centrifugation, washed repeatedly with deionized water and dried in vacuum at 60 °C overnight. The as-prepared product was denoted as NBO-Arg. The concentrations of L-arginine were changed from 0 to 100 mmol·L-1. The experiments were carried out at different reaction temperatures (120 °C, 140 °C, 160 °C, 180 °C, and 200 ℃) and reaction time (0.5 h, 1 h, 2 h, and 4 h). The obtained products after calcination at different temperatures for 4 h were denoted as NBO-Arg-x, where x stands for calcination temperature. The amorphous Nb2O5 was synthesized by a co-precipitation method. In brief, niobium oxalate precursor (2 mmol) was dissolved in 25 mL deionized water. Then, the proper amounts of ammonia solution were added into the above solution to adjust the pH value to 9.0. The resulting precipitate was collected by filtration, washed thoroughly with deionized water and dried at 60℃ under vacuum for 10 h. The obtained amorphous Nb2O5 was denoted as NBO-NH3. 2.3 Catalyst characterization. The X-ray diffraction measurements were carried out on a Rigaku Ultima IV X-ray diffractometer operating at 200 mA and 40 kV, using Cu-Kα radiation


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(λ = 1.5418 Å). The 2θ data was collected in the range from 10 to 70° with a step size of 0.2° per second. The scanning electron microscopy images of the samples were taken on a Hitachi S-4800 electron microscope. The high-resolution transmission electron microscopy images were obtained by using a JEOL JSM-3010 microscope. The samples were prepared by dropping a droplet of ethanol suspension onto a copper grid coated with a 5 nm thickness of amorphous carbon film. Nitrogen adsorption–desorption isotherms were measured on a Quantachrome Quadrasorb SI apparatus at liquid nitrogen temperature (77 K). Prior to adsorption experiments, the samples were degassed at 300 °C for 4 h. The specific surface area was calculated using the multipoint Brunauer-Emmett-Teller (BET) method. The thermogravimetric and differential thermal analysis was performed on a Mettler Toledo TGA/SDTA851e apparatus from room temperature to 800 °C at a heating rate of 10 °C·min−1 in air. Fourier transform infrared spectroscopies were recorded from 400 to 4000 cm-1 on a Nicolet-380 FT-IR using KBr pellet method. Fourier-transform infrared spectroscopies of adsorbed pyridine were measured using a Nicolet Nexus instrument with a MCT detector. NH3 temperature-programmed desorption (TPD) was carried out on a Micromeritics AutoChem 2920 II equipment.. 2.4 Cyclohexanone ammoximation. The liquid-phase cyclohexanone ammoximation was carried out in a three-necked glass flask (100 mL) equipped with a water-circulated condenser and a magnetic stirrer according to the reaction equation as shown in Scheme 1. Scheme 1. Chemical reaction equation of the cyclohexanone ammoximation. O

NOH H2O2

NH3

NBO-Arg 2H2O

The cyclohexanone ammoximation was conducted in pure water in the absence of any organic solvent (e.g. tert-butanol). For a typical reaction, cyclohexanone (2.59 g), deionized water (18 g),


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and catalyst (NBO-Arg, 0.75 g) were added into the flask. The reaction mixture was slowly heated to 75 °C under stirring in a water bath. Subsequently, the reaction was initiated by the addition of the proper amounts of ammonia solution (25 wt%) and hydrogen peroxide (30 wt%) at a constant rate for 2 h with peristaltic pumps. The molar ratio of reactants was nC6H10O: nNH3⋅H2O: nH2O2 = 1: 2.5: 1.6. After the required amounts of ammonia and hydrogen peroxide solutions were added, the reaction was allowed to proceed for another 0.5 h. When the reaction was completed, cyclohexanone oxime and the remaining unreacted cyclohexanone in the upper liquid were extracted by dichloromethane (41 g) before analysis. The concentrations of cyclohexanone and cyclohexanone oxime in the extracted solution were analyzed by gas chromatography (Agilent 7890A equipped with a FID detector and HP-5 capillary column). Toluene was used as internal standard for quantification. The conversion of cyclohexanone (XCyc) and the selectivity of oxime (SOxime) were calculated from equation (1) and (2), respectively. X =

× 100%

(1)

S = × 100%

(2)

where is the initial mass fraction of cyclohexanone.

is the mass fraction of cyclohexanone after reaction. is the mass fraction of cyclohexanone oxime after reaction.

3. Results and Discussion ACS Paragon Plus Environment

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3.1 Morphology and structure characterization. Figure 1 exhibits the XRD patterns of the asprepared products NBO-Arg and NBO-NH3. The NBO-NH3 shows only broad diffraction features and no sharp peaks indicative of crystalline phases (Figure 1a). The NBO-Arg has four peaks at 22.9°, 26.6°, 46.6°, and 55.4°, corresponding to (001), (100), (002), and (102) of pseudo-hexagonal Nb2O5 structure (JCPDS, Card No. 18-0911), respectively, which is consistent with previous report.15 No other impurity peaks are detected, indicating the high purity of the sample. Moreover, elemental analysis (see Figure S1 in the Supporting Information) by the energy dispersive X-ray (EDX) confirms that the NBO-Arg is only composed of niobium and oxygen elements (the C signal comes from the conducting resin).

Figure 1. XRD patterns of as-synthesized samples: (a) NBO-NH3 and (b) NBO-Arg. Figure 2a shows a typical NBO-Arg sample with a novel urchin-like hierarchical nanostructure. With careful observation of a high magnification SEM image (Figure 2b), one can find urchin-like Nb2O5 nanostructures consisting of large quantities of nanowires. The TEM image in Figure 2c further confirm that the sample NBO-Arg is constructed by lots of disordered Nb2O5 nanowires with sharp edges and small diameters. The HRTEM image of an individual Nb2O5 nanowire shows that the diameters of NBO-Arg nanowires are less than 15 nm


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(Figure 2d). Apparent lattice fringes are perpendicular to the nanowire axis with the lattice spacing of about 0.392 nm, which matches well with the d value of 0.388 nm from the XRD result in Figure 1. It is indicated that the crystal growth of Nb2O5 nanowires follows the (001) direction in the presence of L-arginine during hydrothermal treatment.

Figure 2. (a) Low and (b) high magnification SEM images of NBO-Arg nanostructures; (c) TEM image of as-synthesized sample NBO-Arg; (d) HRTEM image of an individual Nb2O5 nanowire. Compared with NBO-Arg, the NBO-NH3 has irregular compact structure (see Figure S2a in the Supporting Information). The TEM image of NBO-NH3 displays that the compact structure is composed of a large number of aggregated Nb2O5 nanoparticles (see Figure S2b in the Supporting Information). The directional growth of Nb2O5 nanoparticles is not allowed under the


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rapid non-equilibrium precipitation conditions. Thus, Nb2O5 nanoparticles aggregate together to form compact structure. The nitrogen adsorption-desorption isotherms were measured to further examine the inner feature of the Nb2O5 nanostructure (see Figure S3 in the Supporting Information). The specific surface area of NBO-Arg is as much as 249.9 m2·g-1, which is substantially larger than that of NBO-NH3 (only about 80 m2·g-1). Thermogravimetry and differential thermal analysis was performed to determine the thermal decomposition behavior of the sample NBO-Arg as shown in Figure 3. The weight loss (~4.7%) from 20 °C to 200 °C is attributed to the removal of moisture and physically adsorbed water. A further weight loss in the range of 200–500 °C is caused by the thermal decomposition of L-arginine

molecules and the oxalate precursor adhering to the Nb2O5 surface, which is supported

by two exothermic peaks centered at 280 °C and 445 °C in DTA curve. The strong peak at 280 °C indicates the oxidation of most of the organic components while small peak at 445 °C is probably caused by the decomposition of a small portion of the residual organic component. A weak exothermic peak around 630 °C is due to the collapse and agglomeration of Nb2O5 nanowires, which can be confirmed in the Figure S4 (see Supporting Information). Finally, there is no weight loss beyond 500°C and the total weight loss is about 20.5%.

Figure 3. (a) TG and (b) DTA curves of the as-synthesized sample NBO-Arg.


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The FT-IR spectra of the sample NBO-Arg and NBO-Arg-500 are showed in Figure 4. The typical absorption bands of L-arginine can be found in the range of 1700～1660 cm−1 and 1300～ 1000 cm-1 (Figure 4a), indicating that L-arginine molecules are adsorbed on the surfaces of NBOArg. Furthermore, the peak at 1406 cm−1 can be assigned to residual oxalate precursor in the sample. Another strong band, found at 1656 cm-1, is due to the bending vibrations of H2O molecules. After calcination of sample NBO-Arg at 500 °C, the typical absorption bands of L-arginine

disappear and only a small intensity band at 1406 cm−1 exists (Figure 4b). It means

that the organic components are almost completely removed from the sample after calcination at 500 °C, which is consistent with TG-DTA data in Figure 3a.

Figure 4. FT-IR spectra of (a) NBO-Arg and (b) NBO-Arg-500. 3.2 Effect of the concentration of L-arginine. The effect of L-arginine concentration on the morphology and structure of Nb2O5 is investigated. Figure 5 displays the SEM images of the NBO-Arg nanostructures synthesized by using different concentrations of L-arginine ranging from 0 to 100 mmol·L-1. Irregular nanoparticles with rough surface are observed in the absence of L-arginine (Figure 5a). The Nb2O5 particles with burr-like protuberances are obtained in the presence of L-arginine with 6.67 mmol·L-1 (Figure 5b). Hierarchical interconnected urchin-like


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Nb2O5 nanostructures are found until the concentration of L-arginine increases to 16.67 mmol·L-1 (Figure 5c). With increasing the concentration of L-arginine from 16.67 to 33.33 mmol·L-1 (Figure 5d), although urchin-like nanostructure composed of nanowires is maintained, the lengths of nanowires and sizes of urchin-like nanostructures increase gradually. When the concentration of L-arginine is further increased to 66.67 mmol·L-1 (Figure 5e), the urchin-like nanostructures are almost destroyed. The urchin-like nanostructures composed nanowires almost disappear when the concentration of L-arginine increases to as high as 100 mmol·L-1 (Figure 5f).


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Figure 5. SEM images of Nb2O5 nanostructures synthesized by using different concentrations of L-arginine:

(a) 0 mmol·L-1, (b) 6.67 mmol·L-1, (c) 16.67 mmol·L-1, (d) 33.33 mmol·L-1, (e) 66.67

mmol·L-1, and (f) 100 mmol·L-1.


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During the hydrothermal process, hydroxyl ions can be generated from the guanidine group of basic amino acid L-arginine under aqueous conditions, and the pH value of the solution increases greatly. 29, 30 The high concentration of L-arginine leads to the alkali decomposition of niobium oxalate, which facilitates the formation of Nb2O5 nanostructures. In addition, a part of L-arginine molecules adsorbed on the surfaces of Nb2O5 nanostructures is important for the formation of 1D nanostructure, which is similar to that observed in the formation of TiO2.26 Moreover, it is known that Nb2O5 can be dissolved into various niobium oxide ionic species with pH>6.5.31 Therefore, the Nb2O5 nanowire is destroyed at relatively high pH condition. It is demonstrated that the concentration of L-arginine plays a vital role in the formation of Nb2O5 nanowires. 3.3 Effect of the hydrothermal temperature. To gain more information during the hydrothermal process, the effect of hydrothermal temperature is investigated. Under the relatively low hydrothermal temperature (120 ℃ ), only irregular Nb2O5 nanoparticles are obtained (Figure 6a). Upon increasing the temperature to 140 ℃, aggregated Nb2O5 nanowires are observed (Figure 6b). With the temperature rising to 160 ℃ , needle-like Nb2O5 nanostructures are found (Figure 6c). The urchin-like Nb2O5 nanostructures cannot be observed clearly until the temperature increases to 180 ℃ (Figure 6d ). When the hydrothermal temperature is further increased to 200 ℃, most of the urchin-like Nb2O5 nanostructures are collapsed into compact nanostructures (Figure 6e), which is likely due to the

L-arginine

decomposition at elevated temperatures.26 It is indicated that the hydrothermal temperature has an important effect on the formation of fine urchin-like Nb2O5 nanostructures.


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Figure 6. SEM images of Nb2O5 nanostructures synthesized at different hydrothermal temperatures: (a) 120 ℃, (b) 140 ℃, (c) 160 ℃, (d) 180 ℃, and (e) 200 ℃. 3.4 Effect of the concentration of niobium oxalate. The effect of the niobium oxalate concentration on the morphology of the Nb2O5 nanostructures is examined under otherwise


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identical conditions. The concentration of niobium oxalate has a relatively minor influence on the morphology of Nb2O5 nanostructures, except that the size of the urchin-like nanostructure becomes larger with the increase of the concentration of niobium oxalate (Figure 7). It is revealed that the concentration of L-arginine rather than the ratio of L-arginine concentration to niobium oxalate concentration plays a great role in the formation of urchin-like Nb2O5 nanostructures.

Figure 7. SEM images of Nb2O5 nanostructures synthesized by using different concentrations of niobium oxalate: (a) 16.67 mmol·L-1, (b) 33.33 mmol·L-1, (c) 66.66 mmol·L-1, and (d) 100 mmol·L-1.


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3.5 Possible formation mechanism of urchin-like Nb2O5 nanostructures. To help trace the growth process of urchin-like Nb2O5 nanostructures and further understand the related formation mechanism, a series of experiments are carried out with different reaction periods to monitor the evolution of the Nb2O5 shapes. The morphology of the Nb2O5 nanostructure can be controlled by adjusting the reaction period, as shown in SEM images (Figure 8). At the early stage of the reaction (30 min), irregular Nb2O5 nanoparticles are observed (Figure 8a). After hydrothermal reaction for 1 h, the surfaces of the Nb2O5 nanoparticles become rough and some protuberances begin to appear (Figure 8b). When the reaction time is prolonged to 2 h, urchin-like nanostructures composed of Nb2O5 nanowires are obtained (Figure 8c). Further increasing the reaction time to 4 h, the urchin-like Nb2O5 nanostructures with larger size are formed (Figure 8d). It is noted that similar urchin-like Nb2O5 nanostructures are found even the reaction time is prolonged to 12 h (Figure 1a).


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Figure 8. SEM images of Nb2O5 nanostructures synthesized with different reaction periods: (a) 0.5 h, (b) 1 h, (c) 2 h, and (d) 4 h. The XRD patterns of Nb2O5 synthesized after different reaction time show that all samples are pseudo-hexagonal phase Nb2O5 (Figure 9, JCPDS, Card No. 18-0911). The intensity ratios of (001) peak to (100) peak, denoted as I (001)/I

(100),

are described in Table S1 (see Supporting

Information). The value of I (001)/I (100) is a measure of orientation degree of Nb2O5 nanostructures. From estimates of their relative intensities one can infer the preferential growth of the Nb2O5 nanowires. The value of I (001)/I

(100)

increases as the increasing reaction time, implying that a

preferential growth of the nanowires along the c-axis. These results are consistent with those of SEM observation (Figure 8).


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Figure 9. XRD patterns of Nb2O5 nanostructures synthesized with different reaction periods: (a) 0.5 h, (b) 1 h, (c) 2 h, and (d) 4 h. Based on the results and discussion above, the possible formation mechanism of the hierarchical urchin-like Nb2O5 nanostructures can be proposed. Firstly, Nb(OH)5 is formed by hydrolysis of niobium oxalate in the presence of L-arginine at relatively high temperature (180 ℃). Subsequently, a large number of Nb2O5 nuclei are generated by the condensation reaction, followed by growth into Nb2O5 nanoparticle seeds. Finally, Nb2O5 nanowires are then grown on the surface of the seeds and they aggregate together to achieve the final urchin-like structure to decrease their high surface energy.24 According to the results of FT-IR (Figure 4), L-arginine

can be adsorbed on the surfaces of Nb2O5 seeds, which leads to a faster growth rate

along the (001) direction than the (100) direction. With increasing hydrothermal reaction time, Nb2O5 nanowires continuously grow along the (001) direction, leading to the formation of the urchin-like Nb2O5 nanostructure with increased size. 3.6 Catalytic activity. The catalytic performances of urchin-like Nb2O5 nanostructures are tested in the liquid-phase ammoximation of cyclohexanone for the first time. In order to obtain the optimal conditions, the effects of reaction temperature, molar ratio of H2O2 to cyclohexanone,


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H2O2 concentration, molar ratio of NH3 to cyclohexanone, and catalyst amount on cyclohexanone ammoximation are investigated systematically.

Figure 10. Effect of (a) reaction temperature, (b) molar ratio of H2O2 to cyclohexanone, (c) H2O2 concentration, (d) molar ratio of NH3 to cyclohexanone, and (e) catalyst amount on cyclohexanone ammoximation. Reaction conditions: (a) catalyst, 0.75 g; cyclohexanone, 2.59 g; nC6H10O: nNH3⋅H2O: H2O2 (30 wt%)= 1: 2.5: 1.6; (b) catalyst, 0.75 g; cyclohexanone, 2.59 g; nC6H10O: nNH3⋅H2O = 1: 2.5; H2O2, 30 wt%; temperature, 75 °C; (c) catalyst, 0.75 g;


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cyclohexanone, 2.59 g; nC6H10O: nNH3⋅H2O: nH2O2 = 1: 2.5: 1.6; temperature, 75 °C; (d) catalyst, 0.75 g; cyclohexanone, 2.59 g; nC6H10O: nH2O2 (30 wt%)= 1: 1.6; temperature, 75 °C; (e) cyclohexanone, 2.59 g; nC6H10O: nNH3⋅H2O: H2O2 (30 wt%)= 1: 2.5: 1.6; temperature, 75 °C ; water 18 g and time 2 h were the other same reaction conditions. The effect of reaction temperature on cyclohexanone ammoximation is shown in Figure 10a. The reaction temperature plays the most significant role among various reaction parameters. The conversion and selectivity increase at first with the increasing temperature and reach maximum at 75 °C, because the increase of temperature could enhance the catalytic activity within a certain range. However, the further increase of the temperature gives rise to the rapid decrease of the conversion and selectivity. Since the higher temperature may not only accelerate the decomposition and volatilization of the H2O2 and dissolved ammonia, but also result in side reactions.32 Therefore, the optimum reaction temperature should be 75 °C for this reaction Figure 10b illustrates the influence of the molar ratio of H2O2 to cyclohexanone on cyclohexanone ammoximation. The concentration of H2O2 also plays a key role in the cyclohexanone ammoximation. The conversion and selectivity increase with the molar ratio of H2O2 to cyclohexanone ranging from 1.05 to 1.6, and then decrease slightly. As an active oxidant, decomposition of H2O2 easily occurs at high temperature. Therefore, excess H2O2 is needed to achieve high conversion and selectivity. However, the excessive dosage of H2O2 would speed up its decomposition, which makes the conversion and selectivity decrease. Taking into consideration all these factors, the optimal molar ratio of H2O2 to cyclohexanone is 1.6. To account for the effect of H2O2 concentration on the reaction, the cyclohexanone ammoximation was carried out in various H2O2 concentrations (10 wt%, 15 wt%, 20 wt%, 25 wt%, and 30 wt%). With the increase of the H2O2 concentration, the conversion almost


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remains stable and the selectivity increases (Figure 10c). It is likely due to the high H2O2 concentration has more positive influence on the cyclohexanone ammoximation than side reactions, thus the selectivity of cyclohexanone oxime increases. As a result, the suitable H2O2 concentration is 30 wt%. The influence of the molar ratio of NH3 to cyclohexanone is studied. The conversion increases and then keeps around at 98.0% with increasing molar ratio of NH3 to cyclohexanone from 1.5 to 3.0 (Figure 10d). When the molar ratio is below 2.5, the amount of NH3 is not enough to satisfy the requirement of reaction, because of rapid volatilization of ammonia at relatively high temperature. Nevertheless, the selectivity increases at first and then decreases slightly, since the excess ammonia leads to the decomposition of hydrogen peroxide, 33 which is a key factor for cyclohexanone ammoximation as mentioned before. Consequently, the suitable molar ratio of NH3 to cyclohexanone is 2.5. Figure 10e shows the influence of catalyst amount on cyclohexanone ammoximation. It can be seen that the conversion increases obviously until the catalyst amount reaches 0.75 g, and then remains almost unchanged. However, the selectivity increases at first, reaches a maximum at 0.75 g, and then decreases slightly. It is implied that the catalyst has active sites not only for the main reaction but also for side reactions, as the latter is dominant, the selectivity would decrease. Thus, the optimal amount of the catalyst is 0.75 g. The cyclohexanone ammoximation reactions over NBO-NH3 and NBO-Arg are carried out under the optimal conditions as shown above. The results are listed in Table 1.


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Table 1. Catalytic activity of NBO-NH3 and NBO-Arg a Lewis acid densityb

TOFc

No.

Catalyst

Conversion (%)

Selectivity (%)

Yield (%)

(mmol•g−1)

(h-1)

1

NBO-NH3

57.1

38.2

21.8

0.213

18.0

2

NBO-Arg

98.0

88.9

87.1

0.436

35.1

a

Reaction conditions: catalyst, 0.75 g; cyclohexanone, 2.59 g; nC6H10O: nNH3⋅H2O: nH2O2 (30 wt%)= 1: 2.5: 1.6; water, 18 g; temperature, 75 °C; time, 2 h. bThe Lewis acid density was evaluated by pyridine-adsorption experiments with Fourier transform infrared spectroscopy (FTIR) . cTOF was evaluated by the rate of oxime formation per Lewis acid site. Table 1 presents that the catalytic activity of urchin-like Nb2O5 nanostructures is significantly more efficient than that of the Nb2O5 compact structure obtained by precipitation method for the cyclohexanone ammoximation. The conversion of cyclohexanone and selectivity of oxime over the urchin-like Nb2O5 nanostructures are as high as 98.0% and 88.9%, respectively. The much higher catalytic activity of NBO-Arg catalysts might be attributed to their large specific surface area, which usually provides more active sites. In addition to the large surface area, the acid density and acid acidity of Nb2O5 also play a key role in determining the catalytic activity. As shown in table S2 (see Supporting Information), the Nb2O5 nanosheets without Lewis acid sites are inactive while the Na+/NBO-Arg without Brønsted acid sites exhibits almost the same activity as NBO-Arg for the cyclohexanone ammoximation reaction. Therefore, it suggests that the Lewis acid sites of Nb2O5 are the active sites for the cyclohexanone ammoximation reaction. The Lewis acid density for NBO-Arg is 0.436 mmol•g−1, which is much higher than that of NBO-NH3 (0.213 mmol•g−1). We also investigated the acidity of samples by the NH3-TPD technique (see Figure S5 in the Supporting Information). The sample NBO-NH3 shows two main desorption peaks of NH3 at about 230°C and 360 °C. Whereas the main desorption peaks of NH3


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for the NBO-Arg appear at around 260 °C and 420 °C. This result indicates that the acidity of the NBO-Arg is stronger than that of the NBO-NH3. Both the higher acid density and stronger acidity for NBO-Arg are beneficial to the much higher catalytic activity of NBO-Arg catalyst. To make a better comparison of the catalytic activity between the NBO-Arg and NBO-NH3, we have further evaluated the turnover frequency (TOF) of the catalysts. The TOF obtained from the NBO-Arg (35.1 h-1) is also significantly higher than that over NBO-NH3 (18 h-1). We propose that, besides the larger specific surface area, higher acid density and stronger acidity, the accessibility of the acid sites also plays an important role in determining the catalytic behaviors. The 1D open structure of the urchin-like Nb2O5 nanostructures composed of Nb2O5 nanowires can facilitate the diffusion of reactant molecules and allow them to access the active sites more easily.

4. Conclusions In summary, urchin-like Nb2O5 nanostructures composed of nanowires are synthesized through a simple hydrothermal method with the assistance of

L-arginine.

The proper

concentration of L-arginine and hydrothermal temperature are crucial for the formation of the well-defined hierarchical urchin-like Nb2O5 nanostructures. Furthermore, the as-prepared urchinlike Nb2O5 nanostructures exhibit excellent catalytic performance for the cyclohexanone ammoximation, and high conversion of cyclohexanone (98.0%) and selectivity of oxime (88.9%) are achieved. The excellent performance benefits from the large surface area, high Lewis acid density, strong acidity and the unique nanostructure of the Nb2O5 nanowires. Additionally, it is expected that the unique urchin-like Nb2O5 nanostructures may have potential applications in other fields, such as photo-catalysis and energy storage. ASSOCIATED CONTENT


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Supporting Information. The supporting information includes the EDX pattern of the assynthesized sample NBO-Arg, the SEM and TEM images of as-synthesized sample NBO-NH3, the N2 adsorption-desorption isotherms of as-synthesized sample NBO-Arg, the SEM image of the sample NBO-Arg-650, and the intensity ratio of XRD peaks for Nb2O5 samples synthesized after various reaction time. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +86-22-2740-9533. Fax: +86-22-2740-9533. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge Kui Ma for the assistance of catalyst characterization (nitrogen adsorption–desorption data and thermogravimetric and differential thermal analysis). REFERENCES (1) Varghese, B.; Haur, S. C.; Lim, C. T. Nb2O5 Nanowires as Efficient Electron Field Emitters. J. Phys. Chem. C 2008, 112, 10008-10012. (2) Fan, W.; Zhang, Q.; Deng, W.; Wang, Y. Niobic Acid Nanosheets Synthesized by a Simple Hydrothermal Method as Efficient Brønsted Acid Catalysts. Chem. Mater. 2013, 25, 3277-3287.


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Table of Contents


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Arginine-Assisted Hydrothermal Synthesis of Urchinlike Nb2O5

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