Transition metals are commonly used in the oxidation of cyclohexane, but their robust reaction conditions and lack of efficiency make further use challenging. This work aimed to improve the efficiency of the reaction further, a heterogeneous catalyst for the oxidation of cyclohexane was developed using chromium oxide nanoparticles immobilized on layered double hydroxide (Cr2O3/LDH). Various spectroscopy methods were used to identify the synthesized catalysts. The synthesized Cr2O3/LDH nanocatalyst was used for the selective oxidation of cyclohexane in the liquid phase without solvent. The structural characterization showed that the Cr2O3-NPs were well anchored on the LDH layers, resulting in a fine dispersion. The Cr2O3/LDH hybrid significantly increased conversion and selectivity, achieving a maximum conversion of 34.73% cyclohexane and 97.85% selectivity to K/A oil. The leaching experiment revealed that the Cr2O3/LDH exhibited the properties of a heterogeneous nanocatalyst, which could be reused for at least six cycles without affecting its catalytic efficiency.
Research Article
Coherent design of chromium oxide nanoparticles immobilized on layered double hydroxide as competent nanocatalyst for the oxidation of cyclohexane to KA oil
https://doi.org/10.21203/rs.3.rs-4619279/v1
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The development of highly efficient catalysts for specific oxidation of the C-H bond in alkanes at low temperatures is crucial in both scientific research and industrial conversion of coal, petroleum, natural gas, and fine chemicals [1, 2]. The oxidation of cyclohexane yields cyclohexanol and cyclohexanone, which serve as important intermediates in the production of nylon-6 and nylon-6,6 [3–5]. A commercial method for the production of KA oil involves the oxidation of cyclohexane with air or oxygen in the presence of homogeneous cobalt or manganese salt catalysts. The method achieved a low cyclohexane conversion rate of 3–5% and a KA oil selectivity of 82–84% [6, 7]. The above methods are unsatisfactory due to their limitations, including low conversion rates, lack of selectivity, generation of toxic waste, cost inefficiency, and the presence of toxic reagents. To overcome these disadvantages, the focus is on developing an innovative catalyst system that offers improved conversion, selectivity, and catalyst recovery. Additional benefits include cost-effectiveness and the use of safer reagents. The development of high-performance heterogeneous catalytic systems for cyclohexane oxidation is highly desirable. Therefore, the immobilization of metal nanoparticles and metal complexes on solid insoluble materials has attracted the attention of numerous researchers. Heterogeneous catalysts offer several advantages due to their high thermal stability, easy separation, and recyclability. They can be used without solvents, under milder conditions, and without generating harmful waste products [8]
Layered double hydroxides (LDHs) are a special type of inorganic materials that consist of positively charged two-dimensional layers with charge-compensating anions and water molecules between the layers [9]. LDHs have unique properties such as adjustable acidity, basicity, good layer expansion properties, chemical inertness, excellent surface area, and gallery anions between the layers. These properties make LDHs an excellent choice as supports for the preparation of heterogeneous catalysts for catalysis [10–12]. As a typical hydroxide support, active metal species can be supported on LDHs using traditional methods such as impregnation [13], precipitation-deposition [14], or by utilizing electrostatic interactions to introduce metal complexes into the interlayers of LDHs [15, 16]. The different ways of introducing metals may result in differences in metal/LDHs interface structures, which can affect the catalytic performances [17, 18]. In a recent study, we reported that Cu and CeO2 nanoparticles and Co Schiff base complexes were synthesized and immobilized on layered double hydroxide catalysts. Their catalytic activity has been investigated for the oxidation of cyclohexane and ethylbenzene [19–21].
For the first time, we report the preparation of Cr2O3 nanoparticles based on ZnAl-LDH as an effective and robust heterogeneous nanocatalyst for the liquid-phase oxidation of cyclohexane to KA oil under solvent-free conditions. Furthermore, the catalytic performance and stability of Cr2O3/LDH catalysts were tested and the reaction conditions were optimized. Furthermore, the hot filtration test revealed that Cr2O3/LDH was heterogeneous and could be recycled at least six times without significantly decreasing the catalytic activity.
2.1 Materials and Methods
Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), aluminium nitrate nonahydrate (Al(NO3)3.9H2O), chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O), salicylic acid (C7H6O3), and sodium hydroxide (NaOH). These chemicals were all analytical grade and were purchased from E. Merck. The commercial 70% aqueous solution of TBHP and cyclohexane was also purchased from E. Merck. The cyclohexane (C6H12) used in this experiment was of high-performance liquid chromatography (HPLC) grade and was purchased from E. Merck. The purity was assessed using gas chromatography (G.C.) to confirm the absence of any oxidation products. To obtain a 1:3 molar ratio of cyclohexane to anhydrous TBHP system, a solution of TBHP in cyclohexane was prepared according to the previously reported procedure [22].
The samples were analyzed using powder X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer. The measurements were taken in the 2θ range of 2°-70°, using CuKα radiation (λ = 1.5418 Å) at a scanning speed of 2º per minute, with a step size of 0.02º. The scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDX) studies were performed using a JEOL 6390LA/O XMX N electron microscope operating at 20 kV. TEM investigations were performed using a JEOL/JEM 2100 microscope. The Fourier transform infrared (FTIR) spectra were obtained by taking samples in KBr using a Perkin Elmer model 1750 instrument. The concentrations of chromium, aluminum, and zinc were determined using the Thermo Electron IRIS INTREPID II XSP DUO, ICP-AES instrument. The X-ray photoelectron spectroscopy (XPS) experiments were conducted using a VSW, UK-manufactured X-ray photoelectron spectrometer. The BET surface area was determined by analyzing N2 adsorption data obtained at 77 K using a volumetric adsorption setup (Micromeritics ASAP-2010, USA). Analytical gas chromatography was performed using a Shimadzu Gas Chromatograph GC-14B equipped with a dual-flame ionization detector (FID) and an Agilent SE-30 capillary column; 30 m 0.32 mm. GC-MS (Perkin-Elmer Clarus 500 column; 30 m 60 mm) was used to identify the products.
2.2 Synthesis of LDH-[OH-C6H4COO]
The support, LDH-[OH-C6H4COO], was synthesized by the co-precipitation approach as a previously reported method [23]. A solution containing 13.2 g of zinc acetate dihydrate and 7.5 g of aluminium(III) nitrate nonahydrate was decarbonized water, with a zinc to aluminium molar ratio of 3:1. Another solution containing 8.6 g salicylic acid and 7.8 g NaOH in decarbonated water was added to this mixture with constant stirring. A viscous gel-like mixture immediately formed and was digested at 348 K for 50 h. After the cooling, the product was separated by filtration. It was then washed with water and then with methanol. Finally, it was dried overnight at 333 K.
2.3 Synthesis of Cr2O3 nanoparticles
To synthesize Cr2O3-NPs, heat 200 mL of 0.1 M chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O) at 80°C and stir with a magnetic stirrer. And then add ascorbic acid (20 mM) dropwise with constant stirring until the color changes from orange to green, indicating the formation of Cr2O3-NPs [24]. The resulting precipitates were filtered through a Buckner funnel and washed several times with distilled water. The precipitates were dried in an oven at 70°C for 24 h. Further investigations were carried out on the green powder form of Cr2O3-NPs.
2.4 Synthesis of Cr2O3/LDH nanocatalyst
The LDH-[OH-C6H4COO] was used to immobilize Cr2O3-NPs to develop the Cr2O3/LDH nanocatalyst. The LDH (1.0 g) was dispersed in 50 mL of distilled water, followed by the addition of an aqueous solution of chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O) (0.4 g) at 70°C with constant agitation for 1 h. After that, a 0.2 M solution of sodium citrate was gradually added to the mixture while stirring continuously until the colour changed from orange to green. This process took place at 70°C and continued for 12 h. The green powder was isolated from the solution using filtration, subjected to several washes with de-ionized water and ethanol, and then dried at 70°C overnight.
2.5 General route for the catalytic oxidation of cyclohexane
Liquid-phase oxidation of cyclohexane with TBHP as the oxidant was carried out to evaluate the catalytic activity of the synthesized Cr2O3/LDH nanocatalyst. The oxidation reaction takes place in a three-necked round bottom flask equipped with a thermometer. The flask is charged with cyclohexane, TBHP, and 100 mg catalyst. The reaction is carried out at 80°C for 6 h. After 6 h of reaction, the catalyst was filtered and washed many times with methanol and then with acetone. The resulting mixture was immediately analyzed using GC 14-B and an Agilent SE-30 capillary column. It was then dried at 70°C for future use. Gas chromatography-mass spectrometry (GC-MS) was used to identify the products as shown in supplementary material Fig. S1 and S2. An internal standard, dodecane, was used to determine the selectivity of the products. The conversion (%) and selectivity (%), turnover number (TON) and carbon balance in the oxidation reaction were defined as follows [25, 6].
Conversion (%) = cyclohexane converted (mmol)/cyclohexane used (mmol) × 100
Selectivity (%) = product formed (mmol)/substrate used (mmol) × 100
Turnover number (TON) = mmol of products/ mmol of catalyst
Carbon balance (%) = 100 - [mmol(A + K + CHHP)]/[(mmol cyclohexane)in-(mol cyclohexane)out] × 100
3.1 Characterization of LDH-[OH-C6H4COO] and Cr2O3/LDH nanocatalyst
Figure 1 shows the FTIR spectra of LDH-[OH-C6H4COO], Cr2O3-NPs and Cr2O3/LDH. The broad peak at 3412 cm− 1 in the spectra of LDH-[OH-C6H4COO], Cr2O3-NPs and Cr2O3/LDH is attributed to the stretching vibration of the hydroxyl (OH) group in the brucite-like layer. This peak often appears in LDHs and indicates the presence of adsorbed water. The bands observed in the spectra of LDH-[OH-C6H4COO] and Cr2O3/LDH, in the range of 552–759 cm− 1 correspond to the stretching modes of O-M-O and M-O (M = Zn, Al) groups are present in the LDH sheets [26]. The M-OH vibration was observed at around 1629 cm− 1 [27]. The presence of two bands at 1458 and 1735 cm− 1 in the spectra of LDH-[OH-C6H4COO] may be attributed to the stretching of C = C bonds in the aromatic ring and the stretching of C = O bonds in the carboxyl groups of salicylic acid in LDH sheets [28, 29]. The bands in the ranges 3405–3425 and 550–565 cm− 1 were found to have the typical bands of hydroxyl groups and Cr–O distortion vibrations, respectively [30, 31]. The results suggest that salicylic acid is incorporated into the ZnAl-LDHs, and Cr2O3-NPs are effectively anchored onto LDH sheets.
The XRD patterns of LDH-[OH-C6H4COO], (b) Cr2O3-NPs, and (c) Cr2O3/LDH are shown in Fig. 2. The X-ray diffraction (XRD) patterns of LDH-[OH-C6H4COO] showed the presence of four different reflections at certain angles (2θ). These reflections were observed at 5.8° (003), 11.7° (006), 23.5° (009), and 60.4° (110), which are characteristic of LDH materials. The distinct reflection seen at about 60.4° is a result of the (110) plane, meaning that the density of the atomic distribution depends on the Zn-Al molar ratio [19]. The estimated interlayer distance of 15.22 Ǻ at the base peak (003) is consistent with the expected value for LDH in the presence of the salicylate anion. The XRD spectrum of Cr2O3-NPs shows two distinct and easily identifiable peaks. They can all be accurately correlated with crystalline Cr2O3, not only in terms of peak location but also in terms of their relative strength. The peaks with 2θ values of 32.10° and 45.17° correspond to the crystal planes (111) and (200) of crystalline Cr2O3, respectively [32]. Furthermore, the X-ray diffraction (XRD) patterns of Cr2O3/LDH show clear reflections at certain angles: 5.8° (003), 11.7° (006), 23.5° (009), and 60.4° (110). as well as 32.10° (111) and 45.17° (200), indicating effective immobilization of Cr2O3-NPs on the LDH-[OH-C6H4COO] host.
The SEM and TEM images seen in Fig. 3a, c and 4a, c demonstrate the plate-shaped crystalline morphologies and layered structures of the LDH-[OH-C6H4COO] and Cr2O3/LDH. The brucite-like layers retained their original shapes after immobilizing Cr2O3-NPs. The average particle size of LDH-[OH-C6H4COO] and Cr2O3/LDH was determined by analyzing TEM images and was determined to be 17 nm and 15 nm, respectively. The particle size distribution histogram is shown in Fig. 4b, d. EDX analysis was used to determine the chemical composition of LDH-[OH-C6H4COO] and Cr2O3/LDH. Figure 3c, d shows the EDX spectra of the support and LDH-[OH-C6H4COO] containing carbon (19.58%), oxygen (39.61%), zinc (31.32%), and aluminium (9.49%) included. On the other hand, Cr2O3/LDH contains carbon (17.41%), oxygen (40.43%), zinc (27.2%), aluminium (8.84%), and chromium (5.72%). Furthermore, the metal content of LDH-[OH-C6H4COO] and Cr2O3/LDH was identified by ICP-AES analysis. The results are listed in Table 1. The fresh Cr2O3/LDH nanocatalyst contains Zn (33.26%), Al (6.25%), and Cr (4.79%), whereas the metal content of LDH-[OH-C6H4COO] was determined to be Zn (35.54%), and Al (7.18%). Furthermore, the recycled Cr2O3/LDH nanocatalyst contains Zn (32.63%), Al (6.23%), and Co (5.75%) during six catalytic cycles. The results provide strong evidence that the Cr2O3 nanoparticles were evenly distributed throughout the LDH scaffolds and that the immobilization process did not change the structure of the LDH-[OH-C6H4COO].
The XPS survey spectra of Cr2O3/LDH in Fig. 5a exhibit different peak values at 73.5, 284.5, 529.8, 576.2, and 1021.3 eV, indicating the existence of Al 2p, C 1s, O 1s, Cr 2p, and Zn 2p. The core energy spectrum of the C 1s may be separated into three separate peaks, representing different oxygen-containing functional groups. These groups include sp2-hybridized aromatic carbons (C = C, 284.5 eV), carbonyl carbon (C = O, 288.7 eV), and carboxyl groups (O-C = O, 292.8 eV) [33]. The high-resolution O 1s core-level spectrum XPS spectra of Cr2O3/LDH (Fig. 5b) can be fitted and separated into two peaks. The peaks at 529.7 and 531.2 eV are often attributed to metal-oxygen bonds and oxygen in hydroxyl groups, respectively. The Al 2p core-level spectrum peak is seen at a binding energy of 73.4 eV, as shown in Fig. 5e. The Zn 2p core-level binding energies of Zn-Al LDH exhibit two distinct peaks at 1021.2 eV and 1044.3 eV, corresponding to Zn 2p3/2 and 2p1/2 respectively (Fig. 5c) [34]. The chromium state of Cr2O3-NPs (576.2 and 585.4 eV) was revealed by the Cr 2p core-level spectra of Cr2O3/LDH (Fig. 5f) [35, 36].
The catalytic efficiency of materials can be improved by increasing their surface area, achieving a fine distribution of active particles, and controlling their shape [37]. This makes the development of supported nanocatalysts extremely attractive. Table 1 shows the BET-specific surface areas, pore volume, and pore diameter of LDH-[OH-C6H4COO], and Cr2O3/LDH. The LDH-[OH-C6H4COO] has a BET-specific surface area of 18.26 m2/g, while the Cr2O3/LDH nanocatalyst has a BET-specific surface area of 29.42 m2/g. The LDH-[OH-C6H4COO] material has a pore volume of 0.06 cc/g and a pore diameter of 2.87 nm. In the Cr2O3/LDH nanocatalyst, these values increase to 0.19 cc/g and 4.88 nm, respectively. The immobilization of Cr2O3 nanoparticles on the LDH-[OH-C6H4COO] resulted in a significant increase in the surface area of the Cr2O3 nanoparticles supported on LDH. This suggests that the Cr2O3 nanoparticles are likely to be distributed on the LDH-[OH-C6H4COO].
3.2 Catalytic performance
3.2.1. Reaction condition optimization
The catalytic efficiency of Cr2O3/LDH in the oxidation of cyclohexane was studied using cyclohexane as a model substrate to obtain optimized conditions. The catalytic activities of Cr2O3/LDH were thoroughly investigated under different conditions and the results are shown in Table S1. The Cr2O3-NPs exhibit low conversion and selectivity, whereas LDH shows catalytic inactivity. Integration of Cr2O3-NPs with LDH resulted in remarkable conversion (34.73%) and high selectivity of KA oil (97.85%). To ensure the importance of the active sites in the liquid-phase oxidation of cyclohexane, some blank experiments were carried out. The results show that in the absence of a catalyst and the presence of pure LDH, no oxidation products were formed. Cr2O3 nanoparticles and Cr2O3/LDH nanocatalyst were oxidized with cyclohexane to KA oil and other products. The Cr2O3/LDH nanocatalyst shows maximum conversion of cyclohexane (34.73%), which is higher than Cr2O3 nanoparticles (22.48%) under optimized conditions. This study demonstrated the excellent efficiency of Cr2O3/LDH nanocatalyst in the oxidation of cyclohexane. Therefore, the large surface area of the Cr2O3/LDH nanocatalyst may not be the only factor for its good catalytic activity, and the basic sites provided by ZnAl-LDH may also play an important role.
To figure out the most efficient oxidizing agent for the oxidation of cyclohexane, at first, the oxidants used were hydrogen peroxide (H2O2), commercial 70% tert-butyl hydroperoxide (70%TBHP), and TBHP in cyclohexane (anhydrous). The reaction involving H2O2 and 70%TBHP, with an oxidant to cyclohexane ratio of 1:1, resulted in modest conversions of cyclohexane, around 06.29% and 04.70% respectively. The selectivity of KA oil was 74.58% and 69.51% at 70°C, using a catalyst dosage of 50 mg after 6 hours. Alternatively, when subjected to identical reaction conditions, TBHP demonstrates a conversion of 7.59% with selectivity towards KA oil of 91.95%. The findings indicate that TBHP provides better conversion and selectivity. Thus, TBHP without water was selected as the most suitable oxidant for conducting the desired reaction in the absence of a solvent.
The stoichiometric ratio of cyclohexane to TBHP is crucial for the oxidation process. To optimize the reaction, four different molar ratios of cyclohexane to TBHP were examined: 1:1, 1:2, 1:3, and 1:4. The catalyst concentration used was 50 mg and the reaction was carried out at a temperature of 70°C. The results are presented in Table S1. A molar ratio of 1:1 results in a cyclohexane conversion of 7.59% and a selectivity of 91.95% for KA oil. Increasing the molar ratio to 1:2 and 1:3 results in cyclohexane conversion of 11.64% and 18.23%, with selectivity of 92.04% and 94.54% for KA oil, respectively. However, when the molar ratio was increased to 1:4, the conversion of cyclohexane increased only slightly to 22.50%. At this point, the selectivity of the KA oil drops to 92.05%. The maximum conversion and the maximum selectivity of KA oil occur at a molar ratio of 1:3. The turnover number and carbon balance increased when the molar ratio of cyclohexane to TBHP increased from 1:1 to 1:4. Therefore, the 1:3 ratio is considered optimal molar ratio for further observations of the oxidation reaction.
The presence of a catalyst is a crucial factor for any catalytic reaction because without it the reaction cannot proceed effectively. The catalyst allows the substrate and the oxidant molecules on its surface to come into close contact, thereby fulfilling its intended purpose. The results are listed in Table S1. The first observation was recorded at a catalyst concentration of 50 mg, achieving a conversion of cyclohexane of 21.33% with a selectivity of KA oil of 94.54% at a molar ratio of cyclohexane to TBHP of 1:3 and 70°C. Under similar reaction conditions, when the catalyst concentration is kept at 75 and 100 mg, the conversion of cyclohexane reaches 25.64% and 34.73% with selectivity of 96.76% and 97.85% for KA oil, respectively. In the last round of influence of catalyst concentration, when the catalyst concentration increases to 125 mg, the conversion of cyclohexane unexpectedly drops to 33.38% with a selectivity of 97.72% for KA oil. The turnover number increased from 396 to 434 while the carbon balance decreased from 1.15 to 0.81% when the concentration of catalyst increased from 50 to 100 mg. Based on the data obtained above, a catalyst concentration of 100 mg is assumed to be the optimal catalyst concentration. The results of the comparison of the catalytic activity of Cr2O3/LDH nanocatalyst with the previous studies are shown in Table S2 [38–42].
| Catalysts | Conversion (%) | TON | Selectivity (%) | K/A (molar ratio) | Carbon balance (%) | |||
|---|---|---|---|---|---|---|---|---|
| K | A | #others | KA oil | |||||
| Blank | - | - | - | - | - | - | - | - |
| LDH-[OH-C6H4COO] | - | - | - | - | - | - | - | - |
| Cr2O3-NPs | 22.48 | 281 | 49.83 | 30.17 | 13.36 | 80.0 | 1.65 | 6.64 |
| Cr2O3/LDH | 34.73 | 434 | 76.52 | 21.33 | 1.34 | 97.85 | 3.59 | 0.81 |
Reaction conditions: Cyclohexane (10 mmol), TBHP (30 mmol), Catalyst (75 mg), 80°C, Time (6 h), TON: turnover number; #cyclohexyl hydroperoxide, adipic acid, glutaric acid, succinic acid, hydroxyl hexylic acid, and ε-caprolactone.
The catalytic reaction is strongly influenced by the temperature used. It provides the required kinetic energy for the molecules to collide with each other, which is the first step of the reaction. Then the collision with the correct orientation yields products. To investigate the influence of temperature on the catalytic reaction, four different temperatures were considered, namely 70, 80, 90, and 100°C. The results are incorporated in Table S1. Experiments show that when the catalytic reaction was carried out at 70 ° C, a conversion of cyclohexane of 28.64% and selectivity of KA oil of 96.94% was achieved. When the temperature was increased to 70 to 80°C, the conversion increased to 34.73% with 97.85% selectivity of KA oil. Here too, the increase to 80 to 90°C leads to a decrease in conversion (34.15%) and selectivity (97.10%). Due to the formation of a significant number of acids and esters at higher temperatures, the selectivity of KA oil was reduced. The turnover number increased from 396 to 434 while the carbon balance decreased from 2.38 to 0.95% when the reaction temperature increased from 70 to 90°C. The analysis showed that the highest conversion was achieved at 80°C and is therefore considered the optimal temperature. The influence of time on the oxidation of cyclohexane is monitored over a time interval of 60 minutes to 360 minutes at a substrate-to-TBHP molar ratio of 1:3, a catalyst concentration of 100 mg and 80°C. The result shows that the conversion of cyclohexane increases linearly up to 6 h and a conversion of cyclohexane 34.73% is achieved with 97.85% selectivity of KA oil.
Therefore, the optimum conditions, at which conversion of cyclohexane is obtained maximum into its corresponding oxidation products, are 100 mg concentration of catalyst, 1:3 molar ratio of cyclohexane to TBHP, and 80°C temperature for 6 h of time. The conditions mentioned above provide 34.73% conversion of cyclohexane with 97.85% selectivity of KA oil.
3.2.2. Substrate scope
To investigate the substrate scope of Cr2O3/LDH under optimized conditions, different substrates were tested. The results are summarized in Table 3. Selectivity (> 80%) was achieved for all substrates and excellent applicability was observed for different substrate types. The results suggest that the Cr2O3/LDH nanocatalyst showed catalytic activity toward different types of substrates. Consequently, a wide range of substrates can be catalytically activated by the Cr2O3/LDH nanocatalyst.
| Entry | Substrate | Conversion (%) | Selectivity of products (%) |
|---|---|---|---|
| 1 | Cyclohexane | 34.73 | (KA oil) 97.85 |
| 2 | Tetralin | 30.27 | (α-Tetralol) 81.43 |
| 3 | Decalin | 21.94 | (Decalol) 90.59 |
| 4 | Methylcyclohexane | 17.58 | (Methylcyclohexanone) 88.37 |
Reaction conditions: Substrate (10 mmol), TBHP (30 mmol), Catalyst (100 mg), 80°C, Time (6 h).
3.3 Leaching experiment
We performed a leaching test to determine the stability of the Cr2O3/LDH nanocatalyst during the catalytic reaction (Fig. S3). In this experiment, the particles responsible for catalyzing the reaction were separated from the mixture after 1 h and the reaction continued with the filtrate for the stipulated time of 6 h. However, the gas chromatographic examination showed that we could not detect any increase in product formation after the specified time of 6 h. This suggests that no leaching of active species occurred throughout the catalytic process, as detected by ICP-AES. The experimental results of the hot filter test show that no chromium leaching occurred during the catalytic process.
3.4 Reusability of Cr2O3/LDH nanocatalyst
The recyclability of Cr2O3/LDH nanocatalyst was evaluated under optimized reaction conditions (Fig. S4). No noticeable decrease in the catalytic activity of the catalyst was observed even after six cycles were completed. From the XRD pattern, FTIR, and BET surface of the recycled catalyst, it is clear that the layered structure of the reused catalyst was maintained after six cycles of use. As additional data, ICP analysis of the recycled catalyst shows that the chromium content in the reused catalyst almost did not change during the recycling process. These results are presented in Table 1 and Fig. S5 and S6. The recycling experiments in this study demonstrate that the Cr2O3/LDH nanocatalyst possesses excellent structural and catalytic stability during the oxidation of cyclohexane under conditions tailored to optimize the reaction.
3.5 Possible reaction mechanism of cyclohexane with TBHP over Cr2O3/LDH nanocatalyst
The free radical mechanism was accepted in the liquid-phase oxidation of cyclohexane in free solvent with TBHP. To gain further insight into the reaction mechanism, a controlled experiment was conducted. The addition of 2,6-di-tert-butyl-4-methylphenol (BHT) as a radical scavenger indicates the presence of free radicals in the reaction system. Based on the findings of this investigation as well as other research, Scheme 1 depicts the possible mechanism for the cyclohexane oxidation reaction over a Cr2O3/LDH nanocatalyst in the presence of TBHP oxidants [43]. During the reaction, the initial TBHP molecules will be attracted towards the surface of Cr2O3/LDH due to the presence of their active species (Cr3+), leading to the formation of active radicals (t-BuO• and t-BuOO•). The t-BuO• radical would react with the C − H bond of cyclohexane, leading to the formation of cyclohexyl radicals. The cyclohexyl radical would then react with t-BuOO•, possibly formed by TBHP on the metal surface, resulting in the formation of cyclohexyl-t-butyl hydroperoxide. Cyclohexyl-t-butyl hydroperoxide decomposes to form cyclohexyl peroxy radicals and t-BuO• radicals. The cyclohexyl peroxy radical then decomposes further into cyclohexanol and cyclohexanone.
We report a low-cost and highly effective method for anchoring Cr2O3-NPs on LDH. The structure of Cr2O3/LDH is determined using several characterization methods. The efficiency of Cr2O3/LDH in catalyzing the oxidation of cyclohexane in a liquid phase was evaluated using TBHP as an oxidant. The as-synthesized Cr2O3/LDH showed superior catalytic activity compared to pure Cr2O3-NPs, with a cyclohexane conversion rate of 34.73% and a KA oil selectivity of 97.85%. The hot filtration experiment showed that the Cr2O3/LDH had heterogeneous catalytic properties and could be recycled for at least six cycles with almost identical activity. Furthermore, the XRD pattern, FTIR analysis, and BET surface analysis of the recycled nanocatalyst show that the Cr2O3/LDH nanocatalyst maintains its stability. The conversion and selectivity are maintained even after six consecutive cycles. The nanocatalyst is suitable for the oxidation of a wide variety of substrates. This work presents a novel approach to develop a very stable Cr2O3-based nanocatalyst, which can potentially be used in the oxidation of various substrates.
Declaration of Competing Interest
The author declares that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The author is thankful to the Principal, Govt. P. G. College, Guna and Head of the Department for providing necessary facilities. We are also thankful to STIC Cochin for providing SEM, EDX, ICP, XRD, and HRTEM and material analysis research centre, Bengaluru for providing the BET surface area facility.
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Scheme 1 is available in the Supplementary Files section
The authors declare no competing interests.
- Supplementarymaterial2.docx
- Scheme1.png
Scheme 1. Proposed mechanism for the oxidation of cyclohexane with TBHP over Cr2O3/LDH nanocatalyst.
- GA.png
Graphical abstract
