Selective liquid phase oxidation of cyclohexane over Pt/CeO2-ZrO2-SnO2/SiO2 catalysts with molecular oxygen

Partial oxidation of cyclohexane into cyclohexanone and cyclohexanol (KA-oil) is an industrially significant reaction for producing precursors for the synthesis of ɛ-caprolactam and adipic acid, which are the building blocks of nylon. However, to date, the cyclohexane conversion ratio has usually been limited to less than 6% to prevent further oxidation of the cyclohexanol and cyclohexanone targets. In this study, we report that Pt/CeO2-ZrO2-SnO2/SiO2, in which CeO2-ZrO2-SnO2 provide reactive oxygen molecules from inside the bulk, can act as efficient catalysts. Optimization of the catalyst composition and reaction conditions provided a cyclohexane conversion ratio of 24.1% and a total selectivity for cyclohexanol and cyclohexanone of 83.4% at 130 °C in 0.5 MPa (4.9 atm) air for 7 h over a 5wt%Pt/16wt%Ce0.68Zr0.17Sn0.15O2.0/SiO2 catalyst. This catalyst has significant advantages over conventional catalysts because the reaction proceeds at a lower pressure, and there is no need for toxic radical initiators or free-radical scavengers.


Introduction
The selective oxidation of cyclohexane (CyH) to cyclohexanone (Cy=O, denoted as "K") and cyclohexanol (CyOH, denoted as "A"), which is a radical chain reaction, is a significant process in the chemical industry, because these are important intermediates for the manufacture of nylon-6 and nylon-6,6 [1]. The conventional industrial process for CyH oxidation to a mixture of K and A (known as KA-oil) involves the use of soluble cobalt or manganese salts as homogeneous catalysts that are radical generation agents, and the reaction is conducted at 140-180 ℃ and 0.8-2 MPa (7.9-19.8 atm) in air for 15-60 min [1]. KA-oil is produced by this method with 70%-85% selectivity; however, the CyH conversion ratio is usually limited to less than 6% to prevent further oxidation of the target KA-oil.
A number of alternative processes that employ heterogeneous catalysts have been proposed for selective CyH oxidation using molecular oxygen with a particular focus on improving the CyH conversion ratio without a significant decrease in the selectivity for KA-oil. Catalysts consisting of gold nanoparticles supported on zeolites such as ZSM-5 (Zeolite Socony Mobil No. 5) [2] and MCM-41 (Mobil Composition of Matter No. 41) [3] can convert CyH to KA-oil at 150 ℃ in 1 MPa (9.9 atm) O 2 atmosphere for 2-6 h with a 6%-19% conversion ratio and 92%-100% selectivity, even in the absence of free-radical  scavengers [4,5]. An Au/SiO 2 catalyst, in which gold nanoparticles were embedded in amorphous silica, also exhibited a high catalytic activity of 22.7% for CyH conversion and 80.6% selectivity for KA-oil at 150 ℃ and 1.5 MPa (14.8 atm) O 2 for 3 h, although toxic and explosive tert-butyl hydroperoxide (TBHP) was used as a radical initiator [6]. Size-controlled Au clusters on hydroxyapatite exhibited a CyH conversion ratio of 6.7%-14.9% with 94%-99% selectivity for KA-oil at 150 ℃ under 1 MPa (9.9 atm) O 2 for 4 h [7], although the addition of a small amount of TBHP was also essential in this case to initiate the reaction.
Mesoporous chromium and iron terephthalates (Crand Fe-MIL-101, respectively), which are categorized as metal-organic frameworks, have demonstrated high catalytic activity and selectivity for the oxidation of CyH [8]. The Cr-MIL-101/TBHP system produces KA-oil with 92% selectivity at a 25% CyH conversion ratio, while Fe-MIL-101/TBHP/O 2 gives a mixture of cyclohexyl hydroperoxide, K, and A with 99% total selectivity (49% selectivity for KA-oil) and a 38% conversion ratio at 70 ℃ in air (1 atm) for 8 h. However, the addition of TBHP is also essential in these reactions. Fe-filled carbon nanotubes are also active catalysts that have yielded a CyH conversion ratio of about 37% at 125 ℃ and 1.5 MPa (14.8 atm) O 2 for 8 h, although the selectivity to KA-oil was about 30% due to over-oxidation [9]. This situation has prompted the examination of novel and advanced CyH oxidation catalysts that can realize high CyH conversion ratios with high selectivity for KA-oil in a liquid phase reaction. We have previously prepared novel catalysts to realize the complete oxidation of volatile organic compounds under moderate conditions [10,11]. These catalysts involved a combination of platinum and a solid promoter to supply reactive oxygen molecules, which was important in order to allow oxidation of hydrocarbons in the gas phase [11,12]. Complete oxidation of ethylene, toluene, and acetaldehyde was realized at temperatures as low as 55, 110, and 140 ℃, respectively, over a 10wt%Pt/16wt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /γ-Al 2 O 3 catalyst [13].
In the present study, we have designed advanced catalysts that are optimized for CyH oxidation to KA-oil in the liquid phase without the need for radical initiators or free-radical scavengers. To realize such advanced catalysts, we have focused on a combination of Pt, Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 , and SiO 2 (silica gel). In this catalyst, platinum acts as the main oxidation agent and the Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 solid solution is the promoter from which reactive oxygen molecules are provided. Silica gel is employed as a catalyst support instead of γ-Al 2 O 3 because it has superior adsorption capabilities for C 6 and C 7 hydrocarbons [14]. In this study, xwt%Pt/16wt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /SiO 2 (hereafter denoted as Pt(x)/CZS/SiO 2 , where 1 10 x   ) catalysts were prepared and their oxidation activity for the conversion of CyH into K and A was investigated. Furthermore, the catalyst composition was optimized to simultaneously realize high activity and high selectivity.

1 Preparation of the Pt/CeO 2 ZrO 2 SnO 2 /SiO 2 catalysts
The 16wt%CZS/SiO 2 support was synthesized by co-precipitation and impregnation methods. SnC 2 O 4 was dissolved in a mixture of 1.0 mol·L −1 Ce(NO 3 ) 3 and 0.1 mol·L −1 ZrO(NO 3 ) 2 aqueous solutions in a stoichiometric ratio, and the mixture was then impregnated into commercially available silica gel (Fuji Silysia Chemical, CARiACT Q-3). The CZS content was adjusted to 16 wt% of the total support to optimize the amount of promoter available so as to provide the highest oxidation activity [13]. The pH of the aqueous mixture was adjusted to 11 by dropwise adding aqueous ammonia (5%). After stirring for 12 h at room temperature, the resulting precipitate was collected by filtration, washed several times with deionized water, and then dried at 80 ℃ for 12 h. The sample was ground in an agate mortar and then calcined at 600 ℃ for 1 h in the ambient atmosphere. Supported platinum catalysts (xwt%Pt/16wt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /SiO 2 ) (1 10 x   ) were prepared by impregnating the 16wt%CZS/SiO 2 support with a 4 wt% platinum colloid aqueous solution stabilized with polyvinylpyrrolidone (Tanaka Kikinzoku Kogyo Co., Ltd.). After impregnation, the sample was dried at 80 ℃ for 6 h and then calcined at 500 ℃ for 4 h. As references, 5wt%Pt/ ywt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /SiO 2 catalysts (hereafter denoted as Pt(5)/CZS(y)/SiO 2 , where 1 30 y   ) were also prepared to identify the feature of CZS.

2 Cyclohexane oxidation
The cyclohexane (CyH) oxidation reaction was conducted in batch mode using a mechanically stirred 50 mL autoclave made from SUS316 stainless steel. Prior to the reaction, CyH (5.0 g) and the catalyst (10 mg) were loaded into the autoclave, and dry air was supplied to the reactor. The reactor was sealed and heated to a stable operational temperature. After the reaction, the products were collected by decantation and analyzed using gas chromatograph mass spectrometry (GCMS; Shimadzu GCMS-QP2010 Plus). The turnover frequency (TOF) is defined as the number of cyclohexane molecules that a catalyst can convert to KA-oil per surface Pt site per unit of time. The amount of Pt atom exposed on the surface (Pt s ) was calculated from the amount of CO chemisorption, assuming that the metallic particles are spherical and one CO molecule adsorbs on one surface platinum atom (CO/Pt s = 1). Accordingly, TOF can be calculated from the following equation:

3 Characterization
The sample compositions were analyzed using X-ray fluorescence spectrometry (XRF; Rigaku ZSX-100e). The crystal structures of the catalysts were identified by X-ray powder diffraction (XRD; Rigaku SmartLab) analysis using Cu K radiation (40 kV, 30 mA). The Brunauer-Emmett-Teller (BET) specific surface areas were measured by nitrogen adsorption at −196 ℃ (Micromeritics Tristar 3000). Transmission electron microscopy (TEM) was performed at an accelerating voltage of 300 kV (Hitachi H-9000NAR). The metal dispersion analysis was carried out using a pulse method at -50 ℃ with 10%CO-He (0.03 mL). The Pt metal dispersion was defined as the following equation, where the amount of surface Pt atoms was calculated from the CO chemisorption values as mentioned above. Dispersion (%)  s Amount of Pt atoms exposed on the surface (Pt ) 100 Total amount of Pt atoms (Pt) 

1 Characterization of the catalysts
The chemical composition of the samples was confirmed using XRF. The BET specific surface area and platinum dispersion data for the catalysts are summarized in Table 1. The BET specific surface area decreases with increasing platinum content, which may be due to the much larger density of Pt than that of the oxide promoter and support. The platinum dispersion in the catalysts decreases sharply when the amount of platinum becomes higher than 5 wt%, which suggests significant aggregation of the platinum particles. To confirm this, the degree of aggregation of platinum on the Pt(x)/CZS/SiO 2 (x = 1, 5, and 7) catalysts was observed using TEM. Figure 1 shows that there are no notable differences between the Pt(1)/CZS/SiO 2 and Pt(5)/CZS/SiO 2 samples, which both have a particle

2 Catalytic performance
The catalytic performance for the selective oxidation of CyH over the Pt(x)/CZS/SiO 2 catalysts was investigated in an air atmosphere (Table 1). Highly efficient and quantitative CyH oxidation to KA-oil could be accomplished at 130 ℃ for 7 h under a 0.5 MPa (4.9 atm) air atmosphere for all samples. Even without the use of radical initiators and free-radical scavengers, the results for both conversion and selectivity are excellent. In particular, the reaction pressure (0.5 MPa or 4.9 atm) employed in the catalytic system is significantly lower than that used with conventional methods (1-1.5 MPa or 9.9-14.8 atm) [4][5][6][7][8][9]. The highest activity is obtained for Pt(5)/CZS/SiO 2 , for which a conversion ratio of 24.1% and a selectivity of 83.4% for KA-oil are achieved. The slight decrease in the conversion ratio for high Pt concentrations may be due to the large amount of aggregation of the Pt particles. Figure 3(a) shows the dependence of the CyH conversion ratio and the KA-oil selectivity on the reaction pressure for the Pt(5)/CZS/SiO 2 catalyst, where the reaction temperature and time are fixed at 130 ℃ and 7 h, respectively. The CyH conversion ratio is improved by the application of pressure and the highest selectivity for KA-oil is obtained at 0.5 MPa (4.9 atm). However, the selectivity decreases as the pressure is increased further due to the over-oxidation of CyH into by-products such as pentanoic acid, glutaric acid, and carbon dioxide. Figure 3(b) shows the influence of the reaction temperature; the conversion ratio becomes saturated at 130 ℃ and is virtually constant at higher temperatures. However, the total selectivity for KA-oil decreases with increasing temperature. Figure 3(c) depicts the effect of the reaction time on the CyH conversion ratio and KA-oil selectivity for the Pt(5)/CZS/SiO 2 catalyst. The CyH oxidation reaction progresses from 3 to 12 h, and a conversion ratio of 24.1% and a selectivity of 83.4% for the two desired oxygenates are achieved at 7 h. Similar to previously reported results [6], the oxidation reaction does not appear to slow down with time, which indicates that there is no loss of catalytic activity. However, the selectivity for KA-oil decreases with increasing reaction time due to over-oxidation of CyH. The detailed results are summarized in Table 2.   To identify the feature of CZS, Pt(5)/CZS(y)/SiO 2 1 30 y   catalysts were also prepared and their catalytic activities were evaluated. Dependence of the catalytic performance of the Pt(5)/CZS(y)/SiO 2 catalysts on the amount of CZS is shown in Fig. 4, where the reaction conditions are 10 mg catalyst, 5.0 g cyclohexane, 130 ℃, 0.5 MPa (4.9 atm), and 7 h. The CyH conversion increases with increasing the amount of CZS. The KA-oil selectivity is almost maintained until y = 16, but it gradually decreases when an excess amount of CZS is loaded on the catalyst due to over-oxidation. From these results, the feature of CZS is evidenced that appropriate amount of CZS addition is effective to increase the CyH conversion without decreasing the KA-oil selectivity.

Conversion (%)
Conversion (%) Conversion (%) Selectivity (%) Selectivity (%) Selectivity (%) Conversion (%) Selectivity (%) Figure 5 presents the relationship between the CyH conversion ratio and the selectivity for KA-oil under various reaction conditions for all of the catalysts in the present study. The selectivity for KA-oil decreases very gradually with increasing CyH conversion ratio until it reaches 24.1%. However, when the conversion ratio becomes greater than 24.1%, the selectivity decreases sharply due to over-oxidation. Therefore, it is crucial to limit the conversion ratio to just below this threshold in order to satisfy the requirements of both a high conversion ratio and high selectivity. Based on the results shown in Fig. 4, the optimum reaction conditions for obtaining the highest yield of KA-oil are 0.5 MPa (4.9 atm), 130 ℃, and 7 h with the Pt(5)/CZS/SiO 2 catalyst, for which 24.1% CyH conversion ratio and 83.4% selectivity are simultaneously realized. Furthermore, even though the catalysis test is repeated five times under the same conditions, the catalyst remains stable and no change is found in either the activity or selectivity.

3 Reaction mechanism
The CyH oxidation reaction proceeds via the free radical chain mechanism indicated in reactions (1) to (5) [15,16]: The chain reaction is initiated by homolytic bond cleavage of cyclohexyl hydroperoxide (CyOOH) according to reaction (1). Cerium ions can exhibit redox activity between Ce 3+ and Ce 4+ , and thereby this initiation is catalyzed via the Haber-Weiss cycle [17]. The CyO • radical produced by reaction (1) reacts with CyH via reaction (2) to form the cyclohexyl radical (Cy • ). This then rapidly reacts with molecular O 2 to form the cyclohexyl peroxy radical (CyOO • ) according to reaction (3), and the Cy • radical is then continuously regenerated in reaction (4). The formation of CyOH and Cy=O has been attributed to chain termination by mutual destruction of two CyOO • radicals according to reaction (5) [16,17].
The selectivity of CyOOH typically decreases with increasing conversion ratio [4]. Under the present reaction conditions, CyOOH is not detected in the products, because it decomposes into CyOH and Cy=O by the radical chain reaction. However, in the reaction mechanism shown in (1) to (5), CyOOH must be produced as the starting material. Since the present process does not involve a radical initiator, CyOOH is expected to be produced by catalytic oxidation of CyH [18,19], so that CyOOH formation is the rate-determining step [20]. In fact, there is an induction period for about 3 h, then the activity increases almost linearly until 7 h, and finally, it saturates after longer than 7 h due to over-oxidation, as seen in Fig. 3(c). This oxidation reaction is facilitated by the readily reducible Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 , which can provide active oxygen species from the catalyst bulk. This oxygen then reacts with CyH at the interface of the Pt and Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 phases [11,13]. The Pt(5)/CZS/SiO 2 catalyst has a significantly high oxidation activity, so that there is no need for a radical initiator. However, for high CyH conversion ratios, both Cy=O and CyOH are further oxidized, and the selectivities for Cy=O and CyOH are consequently reduced. As a result, there are optimum reaction conditions for balancing both the selectivity and conversion ratio.

Conclusions
Novel Pt/CeO 2 -ZrO 2 -SnO 2 /SiO 2 catalysts were successfully prepared for the partial oxidation of CyH into CyOH and Cy=O. The composition was optimized to simultaneously achieve both a high conversion ratio and a high selectivity; a cyclohexane conversion ratio of 24.1% and a selectivity of 83.4% for KA-oil were realized at 0.5 MPa (4.9 atm), 130 ℃, and 7 h using the 5wt%Pt/16wt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /SiO 2 catalyst. This catalytic process has significant advantages in that www.springer.com/journal/40145 117 not only are radical initiators and free-radical scavengers not required, but also the reaction pressure is significantly lower than that typically applied in conventional reactions, due to the combination of catalytic oxidation by platinum, oxygen provision from the bulk of the CZS promoter, and the adsorption of C6 hydrocarbons onto SiO 2 . These characteristics make 5wt%Pt/16wt%Ce 0.68 Zr 0.17 Sn 0.15 O 2.0 /SiO 2 an efficient partial oxidation catalyst for conversion of CyH.