Solution-combustion Synthesized Nano-pellet α-Al2O3 and Catalytic Oxidation of Cyclohexane by Its Supported Cobalt Acetate

KONG Xiangyou, LIU Wenqiang, LIU Xuguang＊, ZHANG Pingping, LI Xia, WANG Zhiyi,2＊

(1. College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China; 2. Guangdong Sitong Group Co. Ltd, Chaozhou 521000, China)

Abstract: Nano-pellet α-Al2O3 was prepared using aluminum nitrate as precursor and urea as fuel by a fast method of solution combustion synthesis. The formation of the nano material was dependent on the molar ratio of fuel/oxidant, calcination temperature, and foreign metallic ions. The prerequisite conditions of the formation were a suitable fuel/oxidant molar ratio larger than two and calcination temperature higher than 673 K. Foreign ions, Ce4+ or Co2+, hindered this formation via promoting the generation of stable penta-coordinated Al3+ ions due to strong interaction with alumina, were revealed by 27Al NMR spectra. Such Al3+ ions were recognized as a critical intermediate state for the phase transformation of alumina and their presence deterred the transformation. The nano-pellet morphology of the product demonstrated a specific surface area of 69 m2/g, of which the external surface area occupied 59 m2/g. It was found that the supported cobalt acetate on such nanopellets existed as nanoparticles attached to the external surface, evidenced by the TEM characterization. The prepared catalyst could efficiently catalyze the selective oxidation of cyclohexane under the reaction condition of pressure under 0.8 MPa, temperature at 373 K, and time for 4 hours. The conversion of the reaction achieved up to 7.9%; while the cyclohexanone selectivity was 42.7% and the cyclohexanone and cyclohexanol selectivity was 91.6%. This catalytic performance recommends the supported cobalt acetate on the inert nano-pellet α-Al2O3 as a promising catalyst for the selective oxidation of cyclohexane.

Key words: solution combustion synthesis; alumina; cyclohexane; catalytic oxidation

1 Introduction

Alumina, an inorganic non-metallic material, plays a major role in diverse industries,eg, energy conversion, chemical production, and information technology, due to its stable structure, outstanding performance, and relatively low cost. This material, specifically, also finds its purpose as catalyst, catalyst carrier, or adsorbent[1-4]. The production of the material generally starts from dehydration and calcination of aluminum hydroxide as an essential precursor. Alumina can be divided into two main categories, namely, low-temperature and high-temperature alumina, according to the calcination temperature[5]. Low-temperature alumina refers to the alumina formed at calcination temperatures below 873 K, includingρ-Al2O3,χ-Al2O3,γ-Al2O3, andη-Al2O3[6]. High temperature alumina refers to the alumina formed at calcination temperatures above 873 K, includingκ-Al2O3,δ-Al2O3,θ-Al2O3, andα-Al2O3[7,8]. The above crystals of alumina in specific applications may vary in physical and chemical properties, resulting from different processing conditions. The most stable form among them isα-Al2O3which is calcinated at temperatures exceeding 1 473 K. This type of alumina, commonly known as sapphire, belongs to a hexagonal crystal structure with lattice parameters asa=b=0.476 nm andc=1.299 nm. It is the highest crystallization and most stable phase of all crystalline alumina.α-Al2O3possesses a variety of superior properties, like excellent mechanical properties, strong corrosion resistance, high temperature resistance, prominent insulation,etc., widely applied in wear resistance of polymer materials, ceramic materials, refractory materials, abrasives, ceramic substrates, and optical materials[9-15]. It has been recently found that α-Al2O3assumes unique advantages as the catalyst carrier due to its inertness at high temperatures, especially, catalytic properties of the support phase being not impeded[16-20]. The preparation method of α-Al2O3, however, is based on a phase transformation process in which high temperature is required to dehydrate the precursor of aluminum hydroxide. This high temperature will lead to an absence of certain morphological structure of the alumina crystal, resulting in a lower specific surface area and therefore inferior catalyst-supporting performance[21,22].

Solution combustion synthesis (SCS) is an emerging method of wet chemical synthesis and it provide a fast and effective route for the preparation of fine and nanooxide powders[23]. This synthesis is specifically based on a redox reaction between metal salts and reducing agents like glycine or urea[24], successfully applied to the preparation of aluminum-based compounds such as alumina and other oxides[25,26]. It possesses specific advantages over the gas phase method, the sol gel method, the mechanical ball mill method, or the molten salt method[19,27-30]. Its advantages lie in the occurrence of the reaction of the raw material in a solution so that the components can be uniformly mixed at molecular or ionic levels. Additionally, the synthesis temperature is low, the reaction time is short, and the synthetic powder size is small. The combustion reaction of the fuel and metal salts occurs in a very short period of time during the SCS process, resulting in flames and high temperatures. The combustion, therefore, will produce a large amount of gas by-products which can not only cause the solid product to expand significantly but also cause the temperature after the reaction to drop rapidly. Porous and well-dispersed solid products are obtained with relatively high specific surface area. This method, when applied to the preparation of porousα-Al2O3, obviously wins significant benefits than others. The preparation process of this method is simple and rapid, and synthesis conditions can be adjusted on combustion temperature, reduction capacity, coordination of metal ions, and amount of gas generation. The composition and properties of the material could be changed accordingly[31,32]. The main controlling factors of SCS are fuel type, fuel selection ratio, combination of fuel use, pH value, additive, and auxiliary energy supply.

The controlling factors of SCS were investigated for the preparation of porous α-Al2O3in this paper. It was found that the α-Al2O3was obtained in a pellet form, applied as a catalyst carrier. It was loaded with cobalt acetate to catalyze the oxidation of cyclohexane, demonstrating excellent catalytic properties. The selective oxidation process of cyclohexane is dependent on the catalytic oxidation reaction to convert into cyclohexanone and cyclohexanol, generally termed as KA oil, which are valuable raw organic chemical material[33,34]. This study may shed light on the developing technique of utilizing α-Al2O3as a catalyst carrier.

2 Experimental

2.1 Preparation of α-Al2O3

The α-alumina was prepared with urea as the fuel and aluminum nitrate as the oxide precursor by a solution combustion synthesis method [2Al(NO3)3+5(NH2)2CO→Al2O3+8N2+5CO2+10H2O][25]. The preparation procedure was as follows. 60 g of Al(NO3)3∙9H2O and 19.2 g of urea (2:1 molar ratio of fuel to oxidant) were added into 36 mL of deionized water, stirring until completely dissolved. The resulting mixture was transferred to a muラe furnace and heated at a heating rate of 6 K/min to 873 K for 5 hours in an air atmosphere. The obtained samples were designated as Al2O3(φ-T) whereφstands for the molar ratio of the fuel (urea) to the aluminum nitrate, andTrefers to the calcination temperature. Various Al2O3(φ-T) samples were prepared by changing the amount of urea, 1≤φ≤2.5, and calcining reaction temperatures, 573 K≤T≤1 073 K.

The preparation of Al2O3containing different amounts of CeO2is as follows. 59.4 g of Al(NO3)3∙9H2O, 0.07 g of Ce(NO3)3∙6H2O, and 19.2 g of urea were added into 36 mL of deionized water, stirring until all dissolved. The mixed solution was transferred to the furnace and heated at a heating rate of 6 K/min to 1 073 K for 5 hours in an air atmosphere to obtain a 1% CeO2/Al2O3(2-1 073) sample. The 1% in the sample name refers to the molar ratio of the Ce/Al in the solution. Ce and Al ions in the solution both existed in the forms of CeO2and Al2O3, respectively, after the combustion. The above steps were repeated by changing the amount of added aluminum nitrate and cerium nitrate, with resultant samples of 2%-8% CeO2/Al2O3(2-1 073).

The preparation of Al2O3containing different amounts of Co3O4is similar to the above procedures, except for 0.47 g of Co(NO3)2∙6H2O as the replacement of 0.07 g of Ce(NO3)3∙6H2O in the solution. 5% Co3O4/Al2O3(2-1 073) samples were obtained.

2.2 Preparation and evaluation of the catalytic cyclohexane oxidation reaction over CoAc2/Al2O3 (φ-T)

A series of Al2O3(φ-T) samples were obtained to support cobalt acetate (CoAc2), for the purpose of optimizing the preparation conditions of SCS. The loaded cobalt acetate catalysts, labeled as CoAc2/Al2O3(φ-T), were prepared by an incipient wetness impregnation method. The steps are as follows: 0.1 g of cobalt acetate (CoAc2∙4H2O) was dissolved in 1.0 mL of deionized water. The solution of cobalt acetate was slowly added into 0.4 g of Al2O3(φ-T) with simultaneously stirring. The mixture was dried at 333 K for 6 hours in an oven for 2 hours to acquire the final catalyst product, CoAc2/Al2O3(φ-T).

The experimental setting of selective catalytic oxidation of cyclohexane is illustrated in Fig.1. The detailed operating procedure for the oxidation reaction is as follows, 1.0 g of the catalyst and 102.5 mL of cyclohexane were placed in a stainless-steel reactor. The reactor was filled with oxygen (O2, 99%) repeatedly for three times, on a purpose of removing impurity gases and detecting the airtightness of the reactor. The reaction conditions were O2pressure of 0.8 MPa, stirring rotation speed of 200 r/min, reaction temperature of 373 K, and reaction time of 4 hours. After the reaction, the supernatant of the reaction fluid was extracted for gas chromatography coupled with mass spectrometry (GC-MS) analysis on an Agilent 7890A-5975C gas chromatograph coupled mass spectroscopy equipped with a HP-5 chromatographic column (30 m × 0.25 mm × 0.25 μm).

Fig.1 Experimental setting of selective catalytic oxidation of cyclohexane

2.3 Material characterization and analysis

The composition and crystal structure of the samples were characterized by X-ray diffraction (XRD) analysis using a Rigaku D-max2500v/pc X-ray diffractor, testing conditions being Cu target, 40 kV, scanning range of 5° to 75°, scanning rate of 10 °/min. The Fourier transform infrared spectroscopy (FT-IR) was recorded within an infrared range of 4 000-400 cm-1on a Nicolet Protege 460 spectrometer. Thermogravimetry/differential scan analysis (TG/DSC) was conducted at a heating rate of 20 K/min under air atmosphere of 10 mL/min on a NETZSCH TG 209 thermal analyzer. The27Al solid-state nuclear magnetic resonance (27Al MAS NMR) was obtained with a pulse length of 3.2 s, a cycle delay of 5 s, a rotation frequency of 14 kHz, and a spectral record at 600 MHz on a JNM-ECA 500 (JEOL) MRI spectrometer. The scanning electron microscope (SEM) images were captured using a JSM-6700 (JEOL) field emission scanning electron microscope. The transmission electron microscope (TEM) observation was performed on a TEM-2100Plus (JEOL) transmission electron microscope. Nitrogen physical adsorption was measured by an ASAP2020 ratio surface area and porosity analyzer manufactured by an American micromeritics company. The samples were pre-treated under N2(99.99%) atmosphere and dried at 473 K for 3 hours before analysis. The adsorption was carried out at 77 K and the non-local density functional theory (NLDFT) model was used to determine the porosities of the samples. The Brunauer-Emmett-Teller (BET) andt-plot methods were adopted to estimate the samples surface areas (experimental error ＜ 1 m2/g) based on the adsorption isotherms.

3 Results and discussion

3.1 The controlling factors of SCS preparation of alumina and corresponding phases

3.1.1 Ratio of fuel to metal salt

The flames and high temperatures generated by the combustion reaction of the fuel led to various phases of the formation products during the preparation process of the SCS method[35]. Fig.2 presents XRD patterns of the alumina products prepared at the same calcination reaction temperature of 873 K and varied fuel-to-oxidant ratio,φ. There were no evident diffraction peaks on the XRD patterns of the Al2O3(1-873) and (1.5-873) samples,i e, amorphous phase. This result indicates that the increment of the fuel ratio to 1.5 did not modify the crystallization state of the Al2O3product, compared to the 1:1 fuel ratio. Higher increment in the fuel ratio, whereas, significantly changed the form of alumina. XRD diffraction peaks of the Al2O3(2-873) and Al2O3(2.5-873) samples simultaneously at 25.6°, 35.1°, 37.8°, 43.4°, 52.6°, 57.5°, 59.8°, 61.3°, 66.5°, and 68.2°, corresponded to the structure of alpha-alumina (α-Al2O3, PDF#10-0173)[17,24]and there are no other diffraction peaks. This demonstrates that both of the two samples are single alpha-phase with no other phases. The Al2O3(2-873) sample underwent a more distinctive transition from amorphous to crystalline structure as the fuel ratio increased, compared to the Al2O3(1.5-873) sample. The crystallization state of the Al2O3(2.5-873) did not undergo further change with the proportion of the fuel. This trend could be attributed to the disparate maximum combustion temperatures resulted from differentφvalues. The maximum temperature which reached atφ= 2 was sufficient to produce a stable pure phase of α-Al2O3. Increasing the fuel ratio could not transform the crystallization state of the product under the relatively low fuel ratio ofφ= 1-1.5. Increasing the ratio, conversely, was conducive to transition to the crystallization of the alumina product under an intermediate fuel ratio (φ= 1.5-2). Higher fuel ratio (φ= 2-2.5) did not modify the crystallization state of the alumina. The temperature of the burning flames in the solution generated by the fuel would lead to various formation environment of the alumina phase[24,32,36].

Fig.2 XRD patterns of alumina products prepared by varied fuelto-oxidant ratio

3.1.2 Calcination temperature

Another vital factor of the SCS method is calcination temperature of the reaction solution. XRD patterns in Fig.3 represent the alumina products prepared at different calcination temperatures under the condition ofφ= 1.5. The reactants could gain ample energy to trigger the combustion reaction at the calcination temperature which directly dominates the reaction process and the products. The Al2O3(1.5-873) sample, as seen from the figure, only demonstrated one weak diffraction peak at 67.0°, the rest diffuse peaks being related to an amorphous state. The Al2O3(1.5-1 073) sample, whereas, possessed strong diffraction peaks at 37.6°, 39.5°, 45.9°, 60.1°, and 67.0°, which corresponds toγ-Al2O3(PDF#10-0425), verifying that the Al2O3(1.5-1 073) is completely composed ofγ-Al2O3phase. Comparing the XRD diffraction peaks of the two samples, it was found that higher calcination temperature is favorable of promoting phase transitions and improving the crystallization of the product under the same fuel ratio (φ= 1.5).

Fig.3 XRD patterns of the Al2O3 (1.5-873) and (1.5-1 073) samples

XRD patterns of the alumina products prepared underφ= 2 are plotted as a function of the calcination temperature in Fig.4. As seen from the figure, the calcination temperature directly dominated the reaction process and the reaction product. The strong diffraction peaks of the Al2O3(2-573) sample at 39.5°, 45.9°, 60.1°, and 67.0° correspond toγ-Al2O3, while the weak peaks at 25.6°, 35.1°, 37.8°, 43.4°, 52.6°, 57.5°, 68.5°, and 68.2° match withα-Al2O3. This phenomenon indicates that there are two phases in the Al2O3(2-573) sample, namely,γ-Al2O3as the main component overα-Al2O3. The Al2O3(2-673), (2-873), and (2-1 073) samples all demonstrated the same diffraction pattern, that is, strong diffraction peaks at 25.6°, 35.1°, 37.8°, 43.4°, 52.6°, 57.5°, 59.8°, 61.3°, 66.5°, and 68.2° and no other peaks, indicating a singleα-Al2O3phase. The phase of the product changed from γ- to α-Al2O3phase with the elevation of the calcination temperature under the same fuel ratio. The enhancement of the calcination temperature could not modify the crystal structure of the product when the temperature was greater than 673 K. The above results illustrate that a critical temperature of 673 K could trigger solution combustion and render a formation condition ofα-Al2O3phase. The temperature of 573 K was not high enough to trigger the combustion and, therefore, the formation. Similarly, the flame temperature originated by the combustion of the solution was not sufficient to produce theα-Al2O3phase in the case ofφ= 1.5.

Fig.4 XRD patterns of the Al2O3 (2-573), (2-673), (2-873), and (2-1 073) samples

3.1.3 Metal ions in the precursor solution

Metal ions in the precursor solution may also influence the combustion process. Fig.5 presents XRD patterns of the alumina products prepared by the SCS method after adding various amounts of Ce to the solution at calcination temperature of 1 073 K andφ= 2. It is evident that only theα-Al2O3phase with high crystallinity was produced without addition of cerium. The strength of the diffraction peaks corresponding to theα-Al2O3phase gradually dropped with the increment of cerium content. The diffraction peaks of the phase almost disappeared when the cerium proportion reached 5% (molar ratio of Ce/Al). The intensities of the diffraction peaks related to theγ-Al2O3phase (37.6°, 45.9°, and 67.0°), conversely, increasingly climbed with the cerium content. The sample was composed of an almost completeγ-Al2O3phase at the cerium content of 5%. There were also distinctive diffraction peaks at 28.6°, 33.1°, 47.5°, 56.3°, 59.1°, and 69.4°, matching with CeO2(PDF#34-0394). The intensities of those peaks were enhanced when the cerium addition was further increased to 8%. The diffraction peaks of theγ-Al2O3phase vanished, and the phase appeared amorphous. The results indicate that adding a small amount of Ce4+ions to the precursor solution could inhibit the production of theα-Al2O3phase and stabilized theγ-Al2O3phase; it would be attributed to the suppression of the Ce3+ions on the phase transition of alumina. The more the amount of Ce3+ions are, the less favorable phase transformation toα-Al2O3phase is. The influence of Ce3+ions on the alumina phase will be further analyzed using27Al NMR in this paper.

Fig.5 XRD patterns of 1%-8% CeO2/Al2O3 (2-1 073) samples

Fig.6 presents XRD patterns of the alumina products prepared by the SCS method after adding various amounts of cobalt at calcination temperature of 1 073 K andφ= 2. As can be seen, the diffraction peaks of the singleα-Al2O3phase product without adding cobalt decreased with the content of cobalt. Those peaks, also similarly to the samples with cerium addition, vanished when the content reached 5%. The diffraction peaks of theγ-Al2O3phase whereas became more evident, and also appeared to completely dominate at the content of 5%. The effect of adding Co2+ions to the precursor solution was akin to that of adding Ce4+. That is, a small amount of Co2+ions could inhibit the formation of theα-Al2O3phase and promote theγ-Al2O3phase. More addition of the ions would facilitate the transition of theγ-Al2O3phase to the amorphous state because adding Co2+also inhibits the phase transformation of alumina.

Fig.6 XRD patterns of 1%-5% Co3O4/Al2O3 (2-1 073) samples

3.2 Characterization of the SCS prepared alumina

3.2.1 The morphology of the alumina products

The alumina obtained by the SCS method assumed a specific nano-pellet morphological appearance[24]. The SEM images of the sample with an alpha-phase are presented in Fig.7 for different calcination temperatures at the ratio ofφ= 2. The lower magnification images display that the appearance of the product did not visibly change with the increase of the calcination temperature, exhibiting a two-dimensional thin sheet structure. The higher magnification images reveal the thickness of the sheet was about 100 nm and the entire whole-piece of a sheet was assembled by small pieces of relatively regular sheets. The joints between the adjacent small sheets had a tendency to become larger with lower calcination temperature. This can be supposed that higher calcination temperatures may lead to sintering of the alumina sheets and result in fewer cracks. This type of nano-pellet alumina, as a catalytic carrier, possess a specific loading dispersion function on the loaded active phases, beneficial to the elimination of external diffusion. It will be further discussed in conjunction with the specific surface area analysis.

Fig.7 SEM images of the Al2O3(2-1 073), (2-773), and (2-673) samples: (a) 2 000×; (b) 5 000×; and (c) 10 000×

3.2.2 Coordination of aluminum ions in the alumina products

The coordination of the species of aluminum ions could be determined by the27Al MAS NMR results.27Al MAS NMR patterns for three Al2O3(2-1 073) samples, with relevant fitted peaks are plotted in Fig.8. Conventional alumina, as seen in the figure, contains aluminum ions with coordination numbers of only 4 and 6 with associated chemical shifts around 65 and 10 ppm, marked as Al(T) and Al(O), respectively. The proportions of the two ions accounted for 31% and 69% in the product. There were possibly other aluminum ions with a coordination number of 5, with a chemical shift near 36 ppm and marked as Al(P)[37-40]. The 1% CeO2/Al2O3(2-1 073) sample, carrying 22% aluminum ions as Al(P), was obtained by addition of Ce4+ions to the precursor solution.This evidently higher proportion of Al(P) than that of the Al2O3(2-1 073) sample indicates adding Ce4+to the precursor solution could stabilize Al(P)[41]which plays a key role in the transition of the alumina phase intoα-Al2O3during the phase transformation[8,42,43]. Cerium ions could form a specific phase[44]with alumina and stabilize Al(P), inhibiting the transformation of this phase into theα-Al2O3. This phenomenon is consistent with the amorphousγ-Al2O3of the alumina samples added with cerium ions, demonstrated by the XRD characterization analysis. It was also reported that rare earth elements could stabilize the amorphous form of alumina by inhibiting the phase transition of alumina[45-48]. The action of the cobalt ions on the formation of the alumina phase was similar to that of cerium ions after the addition to the precursor solution. They also stabilized the Al(P) species and inhibits the transition to theα-Al2O3.

Fig.8 27Al MAS NMR spectra and corresponding fitting profiles of the 5% CeO2/Al2O3(2-1 073), 5% Co3O4/Al2O3(2-1 073), and Al2O3(2-1 073) samples

FT-IR spectra of the Al2O3(2-673/773/1 073) samples prepared at the same fuel ratio (φ= 2) and various calcination temperatures are presented in Fig.9. The water absorption or surface hydroxyl absorption peaks of the sample were at 3 600 cm-1and 1 640 cm-1[17,49].There was little difference among the infrared spectroscopy of the three samples. The weak absorption peaks of water within the three samples indicate that the samples contained only a small amount of adsorption water. It was verified by the above analysis that these samples were all composed of theα-Al2O3phase whose surface could only attach relatively low amount of hydroxyl and exhibit chemical inertness of adsorption to water. All the three samples displayed relatively strong vibration absorption peaks of the Al-O bond, corresponding to the presence of a large number of Al-O bond structures in the samples. In addition, none of the three samples manifested any vibration absorption peaks in carbon-related structures and functional groups, indicating no residual organic carbon being left. That is, a pure alpha-alumina phase could be obtained even at a calcination temperature of 673 K. In this regard, residual organic carbon species and the phase change of alumina could be analyzed by the TG/DSC technique. The Al2O3(2-1 073) sample revealed a similar TG/DSC curve to the standard alpha-alumina, as illustrated in Fig.10. A slight weight loss (1.5%) within 473 K was accompanied by the heat absorption, due to the detachment of the adsorption water on the sample surface. Decomposition or reaction could generally occur during the temperature range of 473-1 073 K. The Al2O3(2-1 073) and standardα-Al2O3samples underwent virtually no weight loss, indicating no organic carbon species in those samples. This was consistent with the previous infrared analysis results. The sample mass decreased slightly, accompanied by cert ain heat absorption, as the testing temperature further increased. This may be attributed to the sintering of the sample.

Fig.9 FT-IR spectra of the Al2O3(2-T) samples

Fig.10 TG/DSC curves of the Al2O3 (2-1 073) sample (solid line) and the standard α-Al2O3 sample (dash line)

3.2.3 The porous structure of alumina

The nitrogen adsorption/desorption isotherms of the prepared Al2O3(2-673/773/1 073) samples are plotted in Fig.11. The BET surface areas of the three samples are listed as 69, 44, and 16 m2/g in Table 1. The former two ratios are superior to that (24 m2/g) of the samples prepared by the sol-gel methods[28,29,50]. The results also demonstrate that the specific surface areas of the samples dropped with the calcination temperature since the high temperature would destroy the porous structure in the samples. The three isotherms in the figure assumed the typical characteristics of a type IV adsorption isotherm. That is, adsorption started by formation of monolayer followed by multilayer, and condensation occurred in the passages of the pores of the sample. A back-hysteresis ring came after the condensation, resulting in an adsorption saturation platform. All the three samples were thus proved as mesoporous materials. Small amount of microporous filling at pretty low pressures also indicated that there were certain micropores in those three samples. There-fore, the obtained products were named as micro-mesoporous materials.

Fig.11 Nitrogen absorption (solid)/desorption (hollow) isotherms of the Al2O3 (2-T) samples

Table 1 Specific surface area and pore volume of the Al2O3 (2-T) samples

The pore-size distributions of the three above alumina products mentioned above are plotted in Fig.12 utilizing the NL-DFT method. There existed certai n amount of microporous structure in all three samples, with the volumes of the micropores ranking as Al2O3(2-673) ＞ Al2O3(2-773) ＞ Al2O3(2-1 073). The results, as explained before, would be the evidence showing that high calcination temperature could destroy the microporous structure of the sample. There were large amount of mesopores in all the samples except for the minor microporous structures. The sizes of most mesopores were 5 nm, 7 nm, and 9 nm for the

Fig.12 Pore-size distribution curves of the Al2O3 (2-T) samples

Al2O3(2-673), Al2O3(2-773), and Al2O3(2-1 073)sample, respectively. This elevation trend of the meso-pore size corresponded to the increment of the preparation temperature. The key feature of the nano-pellet Al2O3(2-673) sample is that the BET ratio was mainly contributed from the external area, up to 59 m2/g. This feature could facilitate more homogeneous dispersion of the active phase on the outer surface of the product, so that the catalytic activities could be enhanced by the possibility of exposing more active sites of the catalyst.

3.3 Structural simulation of alumina

The crystal structure ofα-Al2O3is discussed in detail elsewhere and will not be repeated here[51]. As can be seen from Fig.13, an Al3+ion is surrounded by six oxygen ions,i e, this hexa-coordinated ion forms a skewed octahedron with its ligands. The existence of the hexa-coordinated aluminum ions in the product was confirmed by the27Al NMR results. The simulation calculation indicates that the lattice constants area= 0.4809 nm,b= 0.4809 nm, andc= 1.3137 nm, comparable to the lattice constants obtained from the XRD analysis, namely,a= 0.4759 nm,b= 0.4759 nm, andc= 1.2993 nm. There was a simulation for the electronic structure of theα-Al2O3[52]. The energy belt of this material is illustrated in Fig.13. Its energy belt structure is relatively flat, the characteristics of ionic compounds. The Fermi energy at the top of the valence band matches up with typical insulator characteristics. The broad band gap ofEg= 6.0 eV restrains the electrons of the filled band from jumping into the empty band due to insufficient energy obtained from thermal excitation or external electric field. The theoretical calculation value of the band gap theory is 6.0 eV, by general gradient approximation (GGA) of the calculation method of density functional theory (DFT), which is less than the experimental value of 8.7 eV. This relatively accurate value does not impair the theoretical analysis of the electronic structure. The calculation results confirmed that the alpha-phase of the alumina exhibits typical insulated ceramic properties and does not meddle with the chemical properties of the loaded catalyst when used as a catalyst carrier[17]. The total density of states (TDOS) and partial density of states (PDOS) of theα-Al2O3are also presented in Fig.13. The PDOS data revealed the contributions of Al and O to the TDOS and they were plotted as the effective orbits of Al and O atoms in the unit cell. The state density of this material consists of three regions (A, B, C). The A region of low energy lies at the bottom of the valence band,i e, -20 to -15 eV, whose state density is majorly derived from the 2sorbitals of the Al and O atoms and minorly from the 3porbital of the Al atom. The B region sits at the upper part of the valence band,i e, -7.5 to 0 eV, as bonding orbital, whose state density comes mainly from the 3porbital of the Al atom and the 2porbital of the O atom. The C region is the conduction band with a range of 5-15 eV, primarily composed of antibonding orbitals of the Al and O atoms.

Fig.13 Crystal structure and electronic structure of α-alumina

In addition, there exists a certain degree of hybridization because of the similar energy between the 2porbital of the O atom and the 3sand 3porbitals of the Al atom, resulting in the formation of partial covalent bonds. The degree of hybridization is determined to be low from analyzing the peak shape in the figure, and the proportion of the covalent bond is therefore small.α-Al2O3is concluded as a mixed bond material with strong ion bonds and weak covalent bonds.

3.4 Selective catalytic oxidation reaction of cyclohexane to cyclohexanone and cyclohexanol

3.4.1 Morphology of CoAc2/Al2O3

The alumina optimally obtained by the SCS method assumed a nano-sized pellet shape according to the above characterization analysis. The specific surface area chiefly came from the external surface area which could be homogeneously dispersed with the supported active phase. Fig.14 presents SEM images of the supported CoAc2on the prepared alumina atφ= 2 under two calcination temperatures. There is no apparent distinction between the SEM images before and after the loading from the comparison between Fig.7 and 14. This result indicates that the microstructure of the alumina was not damaged and it maintained a two-dimensional thin sheet structure with a desirable stability after the incipient wetness impregnation treatment. The presence of CoAc2particles, however, was not visible in the SEM images because the particles were too small for SEM to observe. The TEM images of the Al2O3(2-673) sample and the CoAc2-loaded sample were obtained to display more details of the structure as shown in Fig.15. The two-dimensional thin-sheet structure in Fig.15(a) is consistent with the SEM results. There are a large number of pores, averaging 10 nm in diameter, distributed on the sheet with a uniform separation distance for the material without loading any chemicals in Fig.15(b). The presence of dispersed particles with smaller size of 3 nm is evident in the CoAc2-loaded sample as in Figs.15(c) and 15(d). The dispersion feature on the external surface, will be conducive to a catalytic reaction process by eliminating external diffusion resistance and elevating the accessibility of the active phase.

Fig.14 SEM images of the (a, b, c) CoAc2/Al2O3 (2-773) and the (d, e, f) CoAc2/Al2O3 (2-673) samples

Fig.15 TEM images of the (a, b) Al2O3 (2-673) and the (c, d) CoAc2/Al2O3 (2-673) samples

3.5 Catalytic oxidation of cyclohexane

The key point of cyclohexane oxidation is activation of oxygen molecules which is dependent on the catalyst. Catalytic performance for selective oxidation of cyclohexane are displayed for different cobalt catalysts loaded on alumina samples in Fig.16. This selective oxidation reaction being carried out at 373 K under 0.8 MPa, the conversion of cyclohexane without any catalyst was only 0.3%, and the selectivity of cyclohexanone was 36%. The conversion could moderately be ameliorated to 1.3%, 1.4%, or 1.5%, with the introduction of the Al2O3(2-1 073), Al2O3(2-773), or Al2O3(2-673) sample as the catalyst, respectively. The highest conversion of the Al2O3(2-673) sample could be attributed to its largest specific surface area which is conducive to the catalytic reaction. The results indicated that alumina has a certain adsorption and activation effect on molecular oxygen. The limited cyclohexane conversion whereas implied that it was relatively weak.

Fig.16 Catalytic performance of six catalysts in the oxidation of cyclohexane

The conversion rates of 1% Co3O4/Al2O3(2-1 073) and 5% Co3O4/Al2O3(2-1 073) catalysts were 0.8% and 1.2%, respectively, being remained unchanged. This proved that Co3O4has restricted promoting effect on the oxidation of cyclohexane in the composite metal oxide catalysts. The conversion of cyclohexane reached 1.9% when only CoAc2was used as a catalyst, greatly improved compared with the controlling results. The selectivity of the six catalysts involved in Fig.16 were all about 85% to KA oil, slightly different from that of cyclohexanone. The selectivity of the Al2O3(T) samples and the CoAc2catalyst to cyclohexanone were both about 40%, and the two Co3O4/Al2O3catalysts were about 35%, without obvious strengths over the catalytic selectivity. The above facts demonstrated that CoAc2possesses more advantages as the supported active phase in which the valence of Co2+ion is changeable, and the ion is easy to activate molecular oxygen. In practice, cobalt salt with variable valence utilized in the catalytic oxidation method is one of the common processes used in the preparation of KA oil from cyclohexane. Cobalt salts like cobalt naphthenate, cobalt stearate, and cobalt oleate are generally employed as catalysts in those processes. Those salts play two major roles in an oxidation process. They, firstly, promote the C-H bonds of cyclohexane to break and produce C6H11• free radicals under the action of oxygen molecules. They also promote the intermediate C6H11OOH to decompose into C6H11O• and •OH free radicals. The catalysis aided by those salts can reduce the reaction requirements and facilitate the chain reaction rate of free radicals. In a traditional process, cyclohexane conversion in the cobalt salt catalytic oxidation is about 5% at the reaction temperatures within 423-433 K and under the pressure of 1.1 MPa. The selectivity of cyclohexanol and cyclohexanone is about 80%. The advantage of the cobalt salt catalytic oxidation is that the reaction condition is mild and the equipment is less demanding, while the disadvantage is that the carboxylic acid produced during the reaction would readily react with the catalyst to form cobalt carboxylate which could contaminate the equipment and block pipes, as well as the low selectivity of KA oil. The application of the prepared alumina-supported cobalt acetate in this paper will not only inherit the catalytic advantage of cobalt salts, but also avoid the defects of using single cobalt salt catalyst.

The viability of applying the catalyst of alumina supported CoAc2in the selective oxidation of cyclohexane was tentatively explored in this paper, due to the above impediments. The catalytic performance of the supported CoAc2catalysts in the selective oxidation of cyclohexane is illustrated in Fig.17. The oxidation reactions were conducted at 373 K and under 0.8 MPa. The conversion of the Al2O3(1.5-1 073) sample (φ= 1.5) was 3.7%, lower than that of the Al2O3(2-1 073) sample (φ= 2) as 6.6%, owing to the composition difference of the carriers. The carrier in the former sample was a singleγ-Al2O3phase, while the latter carrier was a singleα-Al2O3phase which is more active in promoting the catalytic oxidation. The mechanism may be accounted by the insulation characteristics of the inert alpha-alumina (Fig.13), and the stable nature of the material. It avoided strong interaction between the carrier and the active phase, thus being more beneficial to the catalytic role of the active phase[16,36]. The catalytic activity was significantly ameliorated due to the fine dispersion on the carrier compared with the use of single CoAc2phase as the catalyst whose conversion was only 1.9%.

Fig.17 Catalytic performance of alumina-supported catalysts in the oxidation of cyclohexane

The CoAc2/Al2O3(2-T) catalyst consisted of alumina supported CoAc2prepared atφ= 2 and different calcination temperatures. The conversion of the CoAc2/Al2O3(2-1 073), (2-773), and (2-673) samples were 6.6%, 7.4%, and 7.9%, respectively. The dissimilar catalytic activity indicates that the specific surface area of a catalyst carrier exerts considerable impact on its catalytic performance. As mentioned earlier, raising the calcination temperature could destroy the hollow structure in the alpha-alumina carrier and reduce its surface area. The Al2O3(2-673) sample possessed the largest specific surface area mainly from the external area and optimal hollow structure which helps the dispersion of the active phase and the exposure of the active sites. The CoAc2/Al2O3(2-673) catalyst thus exhibited the most outstanding catalytic performance. In addition, the four catalysts in the figure were more than 90% selective in KA oil, with slight variances in the selectivity of cyclohexanone which increased with decreasing calcination temperature. The catalytic performance of the CoAc2/Al2O3(2-673) catalyst was significantly elevated at 373 K with a conversion rate of 7.9% and the KA oil selectivity reached 91.6%, compared to the single alumina or CoAc2catalysts in Fig.16. It could be attributed to the formation of CoAc2nanoparticles via dispersion the alumina carrier (Fig.15). The larger the specific surface area of the nano-pellet alumina is, the better dispersion of the catalyst and therefore the higher the catalytic activity. It could be recommended that the CoAc2/Al2O3(2-673) catalyst be selected as a promising catalyst for the oxidation of cyclohexane due to its superior performance over others reported in literatures. The relevant catalytic properties are listed in Table 2. At present, the reaction of molecular oxygen with liquid cyclohexane in solvent-free conditions is generally performed at 413-423 K and under 1.0-1.5 MPa. The conversion rate of cyclohexane and selectivity of KA oil are different dependent on specific catalysts. A high conversion rate is often accompanied by a low selectivity since the conversion rate and selectivity are restricted by each other. The conversion is normally controlled at less than 9% based on the safety consideration of the oxidation reaction, and the selectivity will exceed 90%. It is worth noting that the catalytic oxidation of cyclohexane in this paper was implemented at 373 K, an evidently lower temperature which is more favorable for the controlling of the oxidation process and consequently safer. This advantage may be accounted for the dispersion of CoAc2on the inertα-Al2O3. The dispersion form of nanoparticles could give full play to the catalytic activation on oxygen molecules. The position of the active phase, on the external surface of the nano-pellet alumina, would eliminate possible inhibition of pore diffusion on the catalytic properties. Last but not the least, the inert alpha phase of alumina, different from the active gamma phase, could avoid any meddling of the carrier on the active nanoparticles.

Table 2 Catalytic performance of different catalysts for the oxidation of cyclohexane

The effect of reaction time on the cyclohexane oxidation was investigated with respect to the CoAc2/Al2O3(2-673) catalyst, as illustrated in Fig.18, being conducted at 373 K and under 1.0 MPa. The conversions of cyclohexane after 1 h, 2, 3, 4, and 5 hours, were 2.8%, 4.4%, 5.7%, 7.9%, and 8.0%, respectively. That is, the conversion firstly climbed with prolonged reaction time and it leveled off after 4 hours. The result indicates that the catalytic reaction reached temporary equilibrium at that moment. Further oxidation with longer reaction time would generate undesired by-products. It can also be seen from the figure that the total selectivity of KA oil remained unchanged, and the selectivity of cyclohexanone slightly increased with longer reaction time. The latter selectivity reached the highest value as 42.7% at 4 hours and dropped with longer reaction time. It could be concluded that the time of 4 hours is the optimal time for this catalytic oxidation.

Fig.18 The conversion and selectivity of the catalytic oxidation of the CoAc2/Al2O3 (2-673) catalyst as a function of the reaction time

4 Conclusions

Alpha-alumina (α-Al2O3) products were prepared by a method of solution combustion synthesis, with urea as the fuel and aluminum nitrate as the oxidant. It was found that the ratio of fuel to metal salt, calcination temperature, and species of metal ions chiefly controlled the phase of the products. The relatively high ratio of fuel to metal salt and calcination temperature facilitated the formation of alpha-alumina phase. The Ce4+and Co2+ions inhibited the phase transformation of alumina, not favorable on the formation of the desired phase. The prepared two-dimensional nano-pellet alpha-alumina, assuming a relatively large external surface area, could be used as a catalyst carrier to support an active phase of CoAc2. This catalyst effectively promoted the selective catalytic oxidation reaction of cyclohexane. The conversion of cyclohexane was 7.9% and the selectivity of cyclohexanone was 42.7%, due to specific structural advantages of the catalyst. That is, the active phase could be well dispersed as nanoparticles on the external surface of the carrier, eliminating the inhibition of catalytic reactions imposed by the pore diffusion. And the inert characteristics of alpha-alumina would not evidently affect the catalytic properties of the nanoparticles.