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The complete conversion of cyclohexane into cyclohexanol and cyclohexanone by a simple silica-chromium heterogeneous catalyst
Applied Catalysis A: General 357 (2009) 93–99
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
The complete conversion of cyclohexane into cyclohexanol and cyclohexanone by a simple silica-chromium heterogeneous catalyst Farook Adam *, Premalatha Retnam, Anwar Iqbal School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2008 Received in revised form 12 January 2009 Accepted 13 January 2009 Available online 19 January 2009
Chromium and 4-(methylamino)benzoic acid (MBA) were incorporated into silica extracted from rice husk (RH). The chromium incorporated silica was labeled as RH-Cr and the chromium and MBA incorporated catalyst was labeled as RH-Cr-A. The specific surface area of RH-Cr and RH-Cr-A was determined to be 3.95 and 71.3 m2 g 1, respectively. The RH-Cr showed a wide pore distribution, while RH-Cr-A showed a very narrow pore structure. Both catalysts were shown to be amorphous by XRD. FTIR and EDX analysis showed that the amino benzoic acid and chromium had been incorporated into the silica matrix which resulted in nano-sized pores in RH-Cr-A. Complete conversion of cyclohexane was achieved in 6 h with H2O2 as the oxidant and acetonitrile as the solvent for both catalysts at 70 8C, yielding only cyclohexanone and cyclohexanol as products. The catalyst was reusable over many cycles. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Oxidation of cyclohexane Chromium-silica catalyst Rice husk silica Nano-porous catalyst Amino benzoic acid template
1. Introduction Cyclohexanol and cyclohexanone are obtained by the oxidation of cyclohexane. These two compounds are important intermediates in the manufacture of nylon-6 and nylon-66. The industrial scale preparation of cyclohexanol and cyclohexanone is carried out by the oxidation of cyclohexane or hydrogenation of phenol. The present commercial process for cyclohexane oxidation is carried out at 150 8C and 1–2 MPa pressure. This affords ca. 4% conversion and 70–85% selectivity to cyclohexanone and cyclohexanol over metal cobalt salt or metal–boric acid [1]. The conversion is limited to less than 10% to avoid over oxidation to unwanted products, since the primary products – i.e. cyclohexanone and cyclohexanol are more reactive than cyclohexane. In addition, the industrial process produces more cyclohexanol and additional steps are needed to improve the cyclohexanol/ cyclohexanone ratio [2]. To overcome these defects attempts have been made to synthesize more efficient catalyst for the oxidation of cyclohexane. The Co/ZSM-5 catalyst [3] was reported to give about 10 mol% conversion and 97% selectivity to cyclohexanone and cyclohexanol at 120 8C and 1.0 MPa pressure of O2. A zirconium complex bonded to modified carbamate silica gel gave a product distribution ratio of 6.6:1 of cyclohexanol/cyclohexene mixture with 21% conversion at
* Corresponding author. Tel.: +60 46533567; fax: +60 46574854. E-mail addresses: [email protected], [email protected] (F. Adam). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.01.017
200 8C [4]. Copper(II) complexes were used with H2O2 to give a total yield of 68.9% of cyclohexanol, cyclohexanone and other products in 24 h [5]. The use of Co3O4 nanocrystals gave a 7.6% conversion yielding cyclohexanol and cyclohexanone at 120 8C in 6 h with molecular oxygen [6]. A chromium containing complex, CrCoAPO-5(CH3COOH) [7] gave 50% conversion, yielding 55%, 8%, 15% and 22% selectivity towards cyclohexanol, cyclohexanone, adipic acid and others at 115 8C and 1 MPa of oxygen. Bellifa et al. [8] used a V2O5-TiO2 catalyst which resulted in 8% conversion and 76% selectivity to cyclohexanol using acetic acid as solvent and acetone as initiator. The Au/Al2O3 system using molecular oxygen in a solvent free system resulted in 12.6% conversion with a selectivity of 52.6% for cyclohexanol and 32.1% cyclohexanone [9]. A similar use of Au/MCM-41 with oxygen resulted in 19% conversion with 21.3% and 6% selectivity towards cyclohexanol, cyclohexanone and other products, respectively [10]. The use of titanium silicate [11] gave 27.8 mol% conversion with 44 mol% selectivity towards cyclohexanol, 45 mol% cyclohexanone and 11 mol% other products in a 5 h reaction at 100 8C with hydrogen peroxide as the oxidant. Only recently Yao et al. [12] reported a conversion of ca. 95% over Ce-MCM-41 at 100 8C over 12 h. This gave 82% selectivity to cyclohexanol. Sakthivel and Selvam [13] reported a conversion of 99% using calcined Cr-MCM-41 catalyst. Here again, the reaction was carried out at 100 8C and over 12 h duration. This was by far the most promising catalyst in the literature to date. However, the long reaction time and high temperature are grounds for further improvement.
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With the exception of Cr- and Ce-MCM-41 catalysts, all other catalyst reviewed gave less than 60% conversion. In general, serious pollution and low cyclohexane conversion are the main problems in the oxidation of cyclohexane. However, due to the importance of cyclohexanone and cyclohexanol in the manufacturing industry, it is thus important to develop a cheaper and more efficient and environmentally benign catalytic system. Herein we report a cheap chromium based silica catalyst from rice husk (RH) that seems to hold promise for the oxidation of cyclohexane under mild reaction conditions. The silica from RH can be obtained by simply burning [14] the rice husk to obtain the ash or by solvent extraction [15]. We have recently published several reports on the synthesis and catalytic activity of metal incorporated silica catalysts [15–18] from RH. We had also studied the incorporation of 4-(methylamino)benzoic acid (MBA) together with the metal [15,18] into the extracted silica. In this work we report an improved silica-chromium catalyst incorporating MBA into the RH silica framework which gave 100% conversion of cyclohexane in a much shorter time, which to our knowledge is unprecedented. 2. Experimental 2.1. Raw material Rice husk obtained from a local rice milling company, Leong Guan Rice Mill Sdn. Bhd, Penang, was first washed thoroughly with water to remove the adhering soil and dust. It was then dried in air for 48 h. The clean RH was washed with dilute nitric acid to remove inorganic metal absorbed from the soil. The washed and dried RH was labeled as RH-A1.
2.5. Sample characterization The prepared samples were characterized by FTIR (PerkinElmer System 2000) spectroscopy, nitrogen adsorption porosimetry (Micromeritics Instrument Corporation model ASAP 2000, Norcross), powder X-ray diffractometry (Siemens Diffractometer D5000, Kristalloflex), scanning electron microscopy (SEM) (Leica Cambridge S360), energy dispersive spectrometry (EDX) (EDAX FALCON SYSTEM). The UV–vis spectrum was obtained from a PerkinElmer Lambda 35 UV–vis spectrophotometer. 2.6. Catalysis experiments Reactions were carried out in a 50 mL two-necked round bottom flask fitted with a reflux condenser. Acetonitrile (20 mL) was pipetted into the round bottom flask containing the weighed amount of catalyst and a magnetic stirring bar. This was followed by 2.6 mL (24 mmol) of cyclohexane. H2O2 (1.5 mL, 52 mmol) was then added drop by drop in approximately 30 s into the reaction vessel. Reaction at 80 8C is known to decompose the H2O2 [20]. Hence in this study the mixture was refluxed at 70 8C for the prescribed duration. After the reaction, the mixture was cooled and filtered. The products were analysed by GC (Hitachi GC-3000 with FID detector and a 30 m polar column) and the products were identified by GCMS (Hewlett Packard 5890 Series II and a Mass Selective Detector). A 0.2 mL of the product mixture was injected into the GC for each analysis. The temperature program used for the analysis were: initial oven temperature – 40 8C; ramp – 5 8C min 1; final oven temperature – 150 8C; injector port temperature – 250 8C; detector temperature – 250 8C. 3. Results and discussion
2.2. The sample preparation of RH-Cr
3.1. The characterization of RH-Cr and RH-Cr-A
30 g of RH-A1 was stirred in 500 mL 1.0 M NaOH for 24 h at room temperature. The solution was filtered and the resulting sodium silicate was titrated with 10% (w/w) (4.92 g, Cr(NO3)39H2O, Aldrich, 99.9%) chromium nitrate in 3 M nitric acid. The titration was done slowly until pH 5 was reached [19]. A green suspension formed when the solution reached pH 11. The sample was then aged in the mother liquor for 2 days. The gel formed was filtered through a suction filter and washed with distilled water. The gel was then dried in an oven at 110 8C for 24 h. The product was then ground into powder. The powder was then further washed with distilled water to wash off residual nitrate ions and dried at 110 8C for 24 h. It was then ground again and labeled as RH-Cr. The yield was ca. 4.7 g.
Nitrogen adsorption analysis was carried out for the three catalysts, i.e. RH-Cr, RH-Cr-A and RH-Si. Fig. 1 displays the BET isotherm plots for RH-Cr and RH-Cr-A. The BET results for RH-Si had been reported elsewhere [18]. Table 1 shows the results obtained for the BET analysis. Pore size distribution was obtained by applying the BJH pore analysis to the desorption branch of the isotherms. If the material had a regular pore structure, it will show a steep increase in the adsorption isotherm due to capillary condensation. Generally, the inflection position in P/Po depends on the diameter of the mesopores and its sharpness relates to its uniformity [21]. By observing the isotherms of RH-Cr, the adsorption curve can most closely be related to type III isotherm as in the IUPAC
2.3. The preparation of RH-Cr-A The same procedure for the preparation of RH-Cr was used to prepare RH-Cr-A, except 5% (w/w) of 4-(methylamino)benzoic acid (0.32 g) was added into the sodium silicate from RH. This solution was titrated with the chromium nitrate solution as before. The resulting catalyst was labeled as RH-Cr-A. The yield was ca. 4.8 g. 2.4. The preparation of RH-Si Rice husk silica without metal incorporation was prepared as control. A 100 mL sodium silicate solution from rice husk was titrated with 3.0 M HNO3 solution following the same preparation method for RH-Cr. In general, 100 mL of sodium silicate yields ca. 1.0 g RH-Si.
Fig. 1. The BET nitrogen adsorption program for RH-Cr and RH-Cr-A.
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Table 1 BET analysis for RH-Cr, RH-Cr-A and RH-Si.. Property
Catalyst
2
1
Specific surface area (m g ) Micropore volume (cm3 g 1) Micropore area (m2 g 1) Average pore diameter (A˚)
RH-Cr
RH-Cr-A
RH-Si
3.95 0.00029 0.599 63.4
71.3 0.00336 6.76 29.4
305 0.00510 12.2 69.1
classification. Adsorption isotherm of RH-Cr-A was closely similar to isotherm of type IV [22]. The hysterisis loop shown by RH-Cr and RH-Cr-A was closely related to type H3. In type H3 the closure of the loop is at P/Po > 0. However, for the case of RH-Cr and RH-Cr-A the closure of the loop was at the lowest P/Po value, i.e. very close to 0 which gives rise to sigmoid type isotherm. The sigmoid type isotherm shown by RH-Cr and RH-Cr-A is typical for adsorbent that swells [23]. With this it can be perceived that RH-Cr and RH-Cr-A undergo swelling as the relative pressure increased and returns to its normal condition at lower P/Po, as this is what leads to the closure of the loop at low P/Po. It can be concluded that RH-Cr and RH-Cr-A can be associated with slit-shaped pores or plate like particles. This conclusion was further illustrated by the SEM micrographs (not shown) which showed the presence of irregular cracks and slits in RH-Cr which can be related to slit shape and plate like particles as inferred by the BET analysis. The SEM of RH-Cr-A also showed irregular cracks and slits accompanied by pores and holes that can be related to the characteristics of the H3 hysterisis associated with slit-shaped pores or plate like particles. The isotherm of RH-Cr and RH-Cr-A are actually rather rare. With further observation, it can be seen that the desorption branch of both RH-Cr and RH-Cr-A are not smooth. This can be related to the fact that RH-Cr consist of pores in the mesoporous range. The bigger pore size within the mesoporous range dominating compared to mesopores with smaller pore size. This is reflected in the low specific surface area obtained for RH-Cr of 3.95 m2 g 1. However, the specific surface area found in this work is a great improvement compared to the chromium-silica catalyst [17] which was reported earlier, which had a surface area of 0.542 m2 g 1.
Fig. 2. The BJH pore size distribution for (a) RH-Cr and (b) RH-Cr-A. The diameter of the pores are given in A˚.
The MBA incorporated catalyst, RH-Cr-A consists of mesopores with an average pore diameter of ca. 2.94 nm (Fig. 2(b)) and had a specific surface area of 71.2 m2 g 1. This was a twenty-fold increase in the specific surface area compared to RH-Cr. A significant difference in RH-Cr-A was the presence of very small sized mesopores which was not found in RH-Cr (Fig. 2(a)). The increased number of mesopores with uniform size in RH-Cr-A can be attributed to the incorporation of the MBA acting as a template in the catalyst. It must be noted that these results were obtained without calcinating the catalysts. Fig. 3 shows the differential FTIR spectra between RH-Cr-A and RH-Cr. Bands at 2851 and 2924 cm 1 can be attributed to the C–H stretching vibration of the methyl group from the incorporated MBA. The C O stretching vibration of the carbonyl group was not
Fig. 3. The FTIR spectrum of RH-Cr and RH-CR-A and its differential spectrum.
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Fig. 4. Diffuse reflectance UV–vis spectrum for RH-Cr.
seen in the differential spectrum. This could indicate the possible bidentate coordination of the two oxygen atoms in the COO-group. The band at 875 cm 1 shows a slight shift in RH-Cr-A (873 cm 1) in the differential spectrum and this peak was assigned to the C–N stretching vibration. This band (although weak in RH-Cr-A) was not observed in RH-Cr. This is an indication that MBA had incorporated into the silica matrix. The strong N–H vibrations (at 3424, 3396 and 3379 cm 1) of the free MBA did not appear in the FTIR spectrum of RH-Cr-A and the differential spectrum. This was attributed to the formation of bonds between the nitrogen and chromium or the silicon atoms in RH-Cr-A. The UV–vis diffuse reflectance spectrum for RH-Cr and RH-Cr-A were obtained using potassium bromide as blank and is shown in Figs. 4 and 5. The two absorption bands at ca. 424 and 697 nm in RH-Cr and RH-Cr-A are typical of d–d transitions corresponding to the 4A2g(F) to 4T1g(P) and 4A2g(F) to 4T2g(F) transitions for the
trivalent chromium ions (Cr3+) in octahedral coordination [2]. The band at 270, 329 and 381 nm in both RH-Cr and RH-Cr-A could be assigned to the O to Cr(III) charge transfer transitions. Fig. 6 shows the XRD diffraction spectrum for RH-Si, RH-Cr and RH-Cr-A. The samples showed amorphous characteristics even after the incorporation of chromium and MBA. These XRD results were similar to other metal incorporated rice husk silica complexes reported earlier [14–19]. 3.2. Catalytic activity Three parameters namely the mass of catalyst, refluxing time and the type of oxidizing agent were examined to determine the catalytic activity of the prepared catalyst. Literature review showed that the main products formed in the oxidation of cyclohexane are cyclohexanone, cyclohexanol and adipic acid
Fig. 5. Diffuse reflectance UV–vis spectrum for RH-Cr-A.
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Fig. 6. The X-ray diffraction pattern of RH-Cr, RH-Cr-A and RH-Si.
along with several minor products (valeric acid, butyric acid, cyclohexene) [4]. Acetonitrile was used as the solvent in this study. No reaction was observed with the neat cyclohexane solution. The oxidation of cyclohexane is known to depend on the nature of the solvent used [12]. 3.2.1. Oxidation of cyclohexane with hydrogen peroxide with reaction time as variable The use of RH-Cr and RH-Cr-A as catalyst resulted in the gradual increase in the conversion with respect to time. The percentage conversion with RH-Si as catalyst was low, i.e. only 6.67%. Thus, the introduction of active transition metal (chromium) into the framework of the silica from rice husk creates active catalytic sites that are believed to result in an exceptional catalytic activity. Fig. 7 shows the graph of percentage conversion of cyclohexane with respect to time. The use of both catalysts resulted in 100% conversion with 0.1 g of the respective catalysts at a reaction temperature of 70 8C. Complete conversion of cyclohexane has never been achieved to our knowledge. RH-Cr seems to be very active in the oxidation of cyclohexane. The activity of RH-Cr-A was even better, possibly due to the presence of the phenyl group from MBA which can impart greater hydrophobic character to the surface. This permits the approach of the cyclohexane molecule to the polar catalyst surface for adsorption and subsequent transformation.
Fig. 7. The graph of percentage conversion of cyclohexane versus reaction time for RH-Cr and RH-Cr-A. The mass of catalyst used was 0.1 g and the temperature of reaction was 70 8C.
Fig. 8 shows the percent selectivity when RH-Cr was used as the catalyst. A product distribution of ca. 80:20 mixture was found in 6 h with 0.1 g of the catalyst at 70 8C. From Fig. 8, it can be inferred that the first formed species is the cyclohexanol. As the concentration of the cyclohexanol increased, it was further oxidized to cyclohexanone. Hence the selectivity towards cyclohexanol shows a maximum before decreasing. Under the same reaction conditions, RH-Cr-A yielded a mixture of ca. 50:50 of the same products as shown in Fig. 9. With this distinct difference in the selectivity ratio of the two main products, the MBA in RH-Cr-A certainly seems to play an important role in the reaction mechanism. Due to the narrow pore structure and the possible interaction of the lone pair electron in the N (contributed via MBA) atom plus the hydrophobic nature, cyclohexanol is retained on the active site of RH-Cr-A for a longer period of time. This leads to further oxidation to form cyclohexanone. The first formed cyclohexanol may have been fixed on the active site of RH-Cr-A due to the formation of hydrogen bonds between the hydrogen of the hydroxyl group and the nitrogen atom of the MBA within the catalyst matrix. Thus it can be said that the difference in selectivity between these two catalysts is due to the MBA in RH-Cr-A which influence the product selectivity.
Fig. 8. The percentage selectivity towards cyclohexanol and cyclohexanone with 0.1 g of RH-Cr at 70 8C.
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3 h was not studied for this paper. The details of the reaction rate which will include reaction below 3 h will be communicated later.
Fig. 9. The percentage selectivity towards cyclohexanol and cyclohexanone with 0.1 g of RH-Cr-A at 70 8C.
From Fig. 7, it can be seen that RH-Cr-A produced a higher conversion after 3 h compared to RH-Cr. In the case of RH-Cr, the cyclohexanol concentration (Fig. 8) reached a maximum at ca. 6 h and thence decreased over time. However, with RH-Cr-A the cyclohexanol concentration (Fig. 9) seems to decrease immediately and reaches an equilibrium value after ca. 5 h. This can only be possible if the first formed cyclohexanol is retained on the catalyst surface to further undergo oxidation to yield the ultimate oxidation product, i.e. cyclohexanone. The reaction profile below
3.2.2. Oxidation with hydrogen peroxide with variable catalyst mass Figs. 10 and 11 show the product distribution with RH-Cr and RH-Cr-A, respectively. With respect to RH-Cr, there was an initial change in the product distribution when the mass of catalyst was increased from 0.1 to 0.3 g. The product distribution remained almost constant at 50:50 towards the two products. With respect to RH-Cr-A (Fig. 11), a constant distribution of product was observed up to 0.3 g catalyst mass. Further increase in catalyst mass resulted in the decrease in selectivity to cyclohexanone. It can thus be concluded that the mass of catalyst required for the maximum yield of cyclohexanone (53%) was 0.3 g which resulted in 100% conversion with respect to cyclohexane. With regards to the Ce- and Cr-MCM-41 catalysts reported [12,13] which resulted in ca. 95% and 99% conversion, it must be stated that these catalysts were prepared by the conventional hydrothermal process using TEOS as the starting material. The whole process was reported to take 7 days [12] and the catalyst needs to be calcined at 550 8C. The optimum yields were achieved at 100 8C after 12 h of reaction in both cases. In contrast, RH-Cr and RH-Cr-A was prepared by a simple titration method and the catalyst can be prepared within 6 h starting from RH. These catalysts need not be calcined and it can be used after drying at 100 8C with 100% conversion achieved within 6 h of reaction time. In conclusion, the optimum reaction time and mass for RH-Cr and RH-Cr-A was found to be 6 h and 0.3 g, respectively. These catalysts are probably the cheapest catalyst prepared to effect maximum conversion of cyclohexane at a low temperature of 70 8C. 3.2.3. The reusability and leaching effect of the catalyst Reusability of RH-Cr for the oxidation of cyclohexane was tested over five cycles as shown in Fig. 12. During the first cycle, 100% cyclohexane was converted. The conversion dropped about 10– 20% during first until third recycle. During the fourth recycle, the conversion increased again to 95.70%. These observations could not be accounted for at this time but could be due to experimental error. Leaching test was done on the fresh and final cycle to identify the heterogeneity of RH-Cr. The results are shown in Fig. 13. The heterogeneity test was done according to the method described by Shylesh et al. [24]. The first used catalyst, RH-Cr was removed after an hour using the hot filtration technique and the reaction was continued using the
Fig. 10. The product distribution with increasing mass of RH-Cr. Reaction temperature of 70 8C and 6 h reaction time.
Fig. 11. The product distribution when increasing mass of RH-Cr-A was used as the catalyst with a reaction temperature of 70 8C and 6 h reaction time.
Fig. 12. The recyclability of RH-Cr over five reaction cycles.
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4. Conclusion The heterogeneous catalyst RH-Cr and RH-Cr-A from rice husk silica had been synthesized and characterized. The catalysts showed unique physical characteristics as shown by the BET analysis. The catalysts showed superior catalytic activity for the oxidation of cyclohexane, achieving 100% conversion within 6 h. The catalyst could be prepared easily from rice husk and there was no need for calcination prior to their use in the catalysis. These two catalyst hold great promise in the industrial preparation of cylcohexanol and cyclohexanone. Acknowledgements The authors wish to thank Leong Guan Rice Mill Sdn. Bhd, Penang for providing the rice husks for this study. We would also like to thank the Ministry of Education, Malaysia for the FRGS grant (No. 203/PKIMIA/671021) which partly supported this work. Fig. 13. The graph of percentage conversion versus time during the leaching test involving the fresh catalyst, RH-Cr and the recycled (fifth use) catalyst. The catalyst was removed after the first hour and the reaction allowed to continue to completion in 6 h.
filtrate under the same condition. Conversion of cyclohexane continued to completion without the presence of the catalyst. Similar result was observed when the fourth recycled catalyst (fifth use) was subjected to leaching test. Almost complete conversion, i.e. 98.80% of cyclohexane was observed in this case. This suggests that the leaching of chromium (homogeneous catalysis) species may not be the only cause of the continued activity in the absence of the heterogeneous catalysts. It could indicate a free radical mechanism is operative in which the RH-Cr plays an important role in the initial step. However, this could not be verified in this work. Similar results were obtained for RH-CrA. 3.2.4. Oxidation with molecular oxygen The oxidation of cyclohexane was also carried out using oxygen gas as the oxidizing agent. No oxidation products were detected under these conditions. Thus it can be concluded that H2O2 is the best oxidizing agent in the oxidation of cyclohexane when RH-Cr or RH-Cr-A was used as the catalyst. These results are reasonable since it is known that in order to accelerate the initiation step in the auto oxidation process using oxygen as the oxidant, addition of H2O2 or tert-butyl hydroperoxide (TBHP) as co-catalyst was essential [25]. There is however, conflicting reports regarding the heterogeneity of chromium loaded catalyst. The high conversion rates are supposedly due to the few ppm of chromium ions leached out into the solution. Thus, resulting in a homogeneous catalytic effect [26,27].
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
The complete conversion of cyclohexane into cyclohexanol and cyclohexanone by a simple silica-chromium heterogeneous catalyst Farook Adam *, Premalatha Retnam, Anwar Iqbal School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2008 Received in revised form 12 January 2009 Accepted 13 January 2009 Available online 19 January 2009
Chromium and 4-(methylamino)benzoic acid (MBA) were incorporated into silica extracted from rice husk (RH). The chromium incorporated silica was labeled as RH-Cr and the chromium and MBA incorporated catalyst was labeled as RH-Cr-A. The specific surface area of RH-Cr and RH-Cr-A was determined to be 3.95 and 71.3 m2 g 1, respectively. The RH-Cr showed a wide pore distribution, while RH-Cr-A showed a very narrow pore structure. Both catalysts were shown to be amorphous by XRD. FTIR and EDX analysis showed that the amino benzoic acid and chromium had been incorporated into the silica matrix which resulted in nano-sized pores in RH-Cr-A. Complete conversion of cyclohexane was achieved in 6 h with H2O2 as the oxidant and acetonitrile as the solvent for both catalysts at 70 8C, yielding only cyclohexanone and cyclohexanol as products. The catalyst was reusable over many cycles. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Oxidation of cyclohexane Chromium-silica catalyst Rice husk silica Nano-porous catalyst Amino benzoic acid template
1. Introduction Cyclohexanol and cyclohexanone are obtained by the oxidation of cyclohexane. These two compounds are important intermediates in the manufacture of nylon-6 and nylon-66. The industrial scale preparation of cyclohexanol and cyclohexanone is carried out by the oxidation of cyclohexane or hydrogenation of phenol. The present commercial process for cyclohexane oxidation is carried out at 150 8C and 1–2 MPa pressure. This affords ca. 4% conversion and 70–85% selectivity to cyclohexanone and cyclohexanol over metal cobalt salt or metal–boric acid [1]. The conversion is limited to less than 10% to avoid over oxidation to unwanted products, since the primary products – i.e. cyclohexanone and cyclohexanol are more reactive than cyclohexane. In addition, the industrial process produces more cyclohexanol and additional steps are needed to improve the cyclohexanol/ cyclohexanone ratio [2]. To overcome these defects attempts have been made to synthesize more efficient catalyst for the oxidation of cyclohexane. The Co/ZSM-5 catalyst [3] was reported to give about 10 mol% conversion and 97% selectivity to cyclohexanone and cyclohexanol at 120 8C and 1.0 MPa pressure of O2. A zirconium complex bonded to modified carbamate silica gel gave a product distribution ratio of 6.6:1 of cyclohexanol/cyclohexene mixture with 21% conversion at
* Corresponding author. Tel.: +60 46533567; fax: +60 46574854. E-mail addresses: [email protected], [email protected] (F. Adam). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.01.017
200 8C [4]. Copper(II) complexes were used with H2O2 to give a total yield of 68.9% of cyclohexanol, cyclohexanone and other products in 24 h [5]. The use of Co3O4 nanocrystals gave a 7.6% conversion yielding cyclohexanol and cyclohexanone at 120 8C in 6 h with molecular oxygen [6]. A chromium containing complex, CrCoAPO-5(CH3COOH) [7] gave 50% conversion, yielding 55%, 8%, 15% and 22% selectivity towards cyclohexanol, cyclohexanone, adipic acid and others at 115 8C and 1 MPa of oxygen. Bellifa et al. [8] used a V2O5-TiO2 catalyst which resulted in 8% conversion and 76% selectivity to cyclohexanol using acetic acid as solvent and acetone as initiator. The Au/Al2O3 system using molecular oxygen in a solvent free system resulted in 12.6% conversion with a selectivity of 52.6% for cyclohexanol and 32.1% cyclohexanone [9]. A similar use of Au/MCM-41 with oxygen resulted in 19% conversion with 21.3% and 6% selectivity towards cyclohexanol, cyclohexanone and other products, respectively [10]. The use of titanium silicate [11] gave 27.8 mol% conversion with 44 mol% selectivity towards cyclohexanol, 45 mol% cyclohexanone and 11 mol% other products in a 5 h reaction at 100 8C with hydrogen peroxide as the oxidant. Only recently Yao et al. [12] reported a conversion of ca. 95% over Ce-MCM-41 at 100 8C over 12 h. This gave 82% selectivity to cyclohexanol. Sakthivel and Selvam [13] reported a conversion of 99% using calcined Cr-MCM-41 catalyst. Here again, the reaction was carried out at 100 8C and over 12 h duration. This was by far the most promising catalyst in the literature to date. However, the long reaction time and high temperature are grounds for further improvement.
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With the exception of Cr- and Ce-MCM-41 catalysts, all other catalyst reviewed gave less than 60% conversion. In general, serious pollution and low cyclohexane conversion are the main problems in the oxidation of cyclohexane. However, due to the importance of cyclohexanone and cyclohexanol in the manufacturing industry, it is thus important to develop a cheaper and more efficient and environmentally benign catalytic system. Herein we report a cheap chromium based silica catalyst from rice husk (RH) that seems to hold promise for the oxidation of cyclohexane under mild reaction conditions. The silica from RH can be obtained by simply burning [14] the rice husk to obtain the ash or by solvent extraction [15]. We have recently published several reports on the synthesis and catalytic activity of metal incorporated silica catalysts [15–18] from RH. We had also studied the incorporation of 4-(methylamino)benzoic acid (MBA) together with the metal [15,18] into the extracted silica. In this work we report an improved silica-chromium catalyst incorporating MBA into the RH silica framework which gave 100% conversion of cyclohexane in a much shorter time, which to our knowledge is unprecedented. 2. Experimental 2.1. Raw material Rice husk obtained from a local rice milling company, Leong Guan Rice Mill Sdn. Bhd, Penang, was first washed thoroughly with water to remove the adhering soil and dust. It was then dried in air for 48 h. The clean RH was washed with dilute nitric acid to remove inorganic metal absorbed from the soil. The washed and dried RH was labeled as RH-A1.
2.5. Sample characterization The prepared samples were characterized by FTIR (PerkinElmer System 2000) spectroscopy, nitrogen adsorption porosimetry (Micromeritics Instrument Corporation model ASAP 2000, Norcross), powder X-ray diffractometry (Siemens Diffractometer D5000, Kristalloflex), scanning electron microscopy (SEM) (Leica Cambridge S360), energy dispersive spectrometry (EDX) (EDAX FALCON SYSTEM). The UV–vis spectrum was obtained from a PerkinElmer Lambda 35 UV–vis spectrophotometer. 2.6. Catalysis experiments Reactions were carried out in a 50 mL two-necked round bottom flask fitted with a reflux condenser. Acetonitrile (20 mL) was pipetted into the round bottom flask containing the weighed amount of catalyst and a magnetic stirring bar. This was followed by 2.6 mL (24 mmol) of cyclohexane. H2O2 (1.5 mL, 52 mmol) was then added drop by drop in approximately 30 s into the reaction vessel. Reaction at 80 8C is known to decompose the H2O2 [20]. Hence in this study the mixture was refluxed at 70 8C for the prescribed duration. After the reaction, the mixture was cooled and filtered. The products were analysed by GC (Hitachi GC-3000 with FID detector and a 30 m polar column) and the products were identified by GCMS (Hewlett Packard 5890 Series II and a Mass Selective Detector). A 0.2 mL of the product mixture was injected into the GC for each analysis. The temperature program used for the analysis were: initial oven temperature – 40 8C; ramp – 5 8C min 1; final oven temperature – 150 8C; injector port temperature – 250 8C; detector temperature – 250 8C. 3. Results and discussion
2.2. The sample preparation of RH-Cr
3.1. The characterization of RH-Cr and RH-Cr-A
30 g of RH-A1 was stirred in 500 mL 1.0 M NaOH for 24 h at room temperature. The solution was filtered and the resulting sodium silicate was titrated with 10% (w/w) (4.92 g, Cr(NO3)39H2O, Aldrich, 99.9%) chromium nitrate in 3 M nitric acid. The titration was done slowly until pH 5 was reached [19]. A green suspension formed when the solution reached pH 11. The sample was then aged in the mother liquor for 2 days. The gel formed was filtered through a suction filter and washed with distilled water. The gel was then dried in an oven at 110 8C for 24 h. The product was then ground into powder. The powder was then further washed with distilled water to wash off residual nitrate ions and dried at 110 8C for 24 h. It was then ground again and labeled as RH-Cr. The yield was ca. 4.7 g.
Nitrogen adsorption analysis was carried out for the three catalysts, i.e. RH-Cr, RH-Cr-A and RH-Si. Fig. 1 displays the BET isotherm plots for RH-Cr and RH-Cr-A. The BET results for RH-Si had been reported elsewhere [18]. Table 1 shows the results obtained for the BET analysis. Pore size distribution was obtained by applying the BJH pore analysis to the desorption branch of the isotherms. If the material had a regular pore structure, it will show a steep increase in the adsorption isotherm due to capillary condensation. Generally, the inflection position in P/Po depends on the diameter of the mesopores and its sharpness relates to its uniformity [21]. By observing the isotherms of RH-Cr, the adsorption curve can most closely be related to type III isotherm as in the IUPAC
2.3. The preparation of RH-Cr-A The same procedure for the preparation of RH-Cr was used to prepare RH-Cr-A, except 5% (w/w) of 4-(methylamino)benzoic acid (0.32 g) was added into the sodium silicate from RH. This solution was titrated with the chromium nitrate solution as before. The resulting catalyst was labeled as RH-Cr-A. The yield was ca. 4.8 g. 2.4. The preparation of RH-Si Rice husk silica without metal incorporation was prepared as control. A 100 mL sodium silicate solution from rice husk was titrated with 3.0 M HNO3 solution following the same preparation method for RH-Cr. In general, 100 mL of sodium silicate yields ca. 1.0 g RH-Si.
Fig. 1. The BET nitrogen adsorption program for RH-Cr and RH-Cr-A.
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Table 1 BET analysis for RH-Cr, RH-Cr-A and RH-Si.. Property
Catalyst
2
1
Specific surface area (m g ) Micropore volume (cm3 g 1) Micropore area (m2 g 1) Average pore diameter (A˚)
RH-Cr
RH-Cr-A
RH-Si
3.95 0.00029 0.599 63.4
71.3 0.00336 6.76 29.4
305 0.00510 12.2 69.1
classification. Adsorption isotherm of RH-Cr-A was closely similar to isotherm of type IV [22]. The hysterisis loop shown by RH-Cr and RH-Cr-A was closely related to type H3. In type H3 the closure of the loop is at P/Po > 0. However, for the case of RH-Cr and RH-Cr-A the closure of the loop was at the lowest P/Po value, i.e. very close to 0 which gives rise to sigmoid type isotherm. The sigmoid type isotherm shown by RH-Cr and RH-Cr-A is typical for adsorbent that swells [23]. With this it can be perceived that RH-Cr and RH-Cr-A undergo swelling as the relative pressure increased and returns to its normal condition at lower P/Po, as this is what leads to the closure of the loop at low P/Po. It can be concluded that RH-Cr and RH-Cr-A can be associated with slit-shaped pores or plate like particles. This conclusion was further illustrated by the SEM micrographs (not shown) which showed the presence of irregular cracks and slits in RH-Cr which can be related to slit shape and plate like particles as inferred by the BET analysis. The SEM of RH-Cr-A also showed irregular cracks and slits accompanied by pores and holes that can be related to the characteristics of the H3 hysterisis associated with slit-shaped pores or plate like particles. The isotherm of RH-Cr and RH-Cr-A are actually rather rare. With further observation, it can be seen that the desorption branch of both RH-Cr and RH-Cr-A are not smooth. This can be related to the fact that RH-Cr consist of pores in the mesoporous range. The bigger pore size within the mesoporous range dominating compared to mesopores with smaller pore size. This is reflected in the low specific surface area obtained for RH-Cr of 3.95 m2 g 1. However, the specific surface area found in this work is a great improvement compared to the chromium-silica catalyst [17] which was reported earlier, which had a surface area of 0.542 m2 g 1.
Fig. 2. The BJH pore size distribution for (a) RH-Cr and (b) RH-Cr-A. The diameter of the pores are given in A˚.
The MBA incorporated catalyst, RH-Cr-A consists of mesopores with an average pore diameter of ca. 2.94 nm (Fig. 2(b)) and had a specific surface area of 71.2 m2 g 1. This was a twenty-fold increase in the specific surface area compared to RH-Cr. A significant difference in RH-Cr-A was the presence of very small sized mesopores which was not found in RH-Cr (Fig. 2(a)). The increased number of mesopores with uniform size in RH-Cr-A can be attributed to the incorporation of the MBA acting as a template in the catalyst. It must be noted that these results were obtained without calcinating the catalysts. Fig. 3 shows the differential FTIR spectra between RH-Cr-A and RH-Cr. Bands at 2851 and 2924 cm 1 can be attributed to the C–H stretching vibration of the methyl group from the incorporated MBA. The C O stretching vibration of the carbonyl group was not
Fig. 3. The FTIR spectrum of RH-Cr and RH-CR-A and its differential spectrum.
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Fig. 4. Diffuse reflectance UV–vis spectrum for RH-Cr.
seen in the differential spectrum. This could indicate the possible bidentate coordination of the two oxygen atoms in the COO-group. The band at 875 cm 1 shows a slight shift in RH-Cr-A (873 cm 1) in the differential spectrum and this peak was assigned to the C–N stretching vibration. This band (although weak in RH-Cr-A) was not observed in RH-Cr. This is an indication that MBA had incorporated into the silica matrix. The strong N–H vibrations (at 3424, 3396 and 3379 cm 1) of the free MBA did not appear in the FTIR spectrum of RH-Cr-A and the differential spectrum. This was attributed to the formation of bonds between the nitrogen and chromium or the silicon atoms in RH-Cr-A. The UV–vis diffuse reflectance spectrum for RH-Cr and RH-Cr-A were obtained using potassium bromide as blank and is shown in Figs. 4 and 5. The two absorption bands at ca. 424 and 697 nm in RH-Cr and RH-Cr-A are typical of d–d transitions corresponding to the 4A2g(F) to 4T1g(P) and 4A2g(F) to 4T2g(F) transitions for the
trivalent chromium ions (Cr3+) in octahedral coordination [2]. The band at 270, 329 and 381 nm in both RH-Cr and RH-Cr-A could be assigned to the O to Cr(III) charge transfer transitions. Fig. 6 shows the XRD diffraction spectrum for RH-Si, RH-Cr and RH-Cr-A. The samples showed amorphous characteristics even after the incorporation of chromium and MBA. These XRD results were similar to other metal incorporated rice husk silica complexes reported earlier [14–19]. 3.2. Catalytic activity Three parameters namely the mass of catalyst, refluxing time and the type of oxidizing agent were examined to determine the catalytic activity of the prepared catalyst. Literature review showed that the main products formed in the oxidation of cyclohexane are cyclohexanone, cyclohexanol and adipic acid
Fig. 5. Diffuse reflectance UV–vis spectrum for RH-Cr-A.
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Fig. 6. The X-ray diffraction pattern of RH-Cr, RH-Cr-A and RH-Si.
along with several minor products (valeric acid, butyric acid, cyclohexene) [4]. Acetonitrile was used as the solvent in this study. No reaction was observed with the neat cyclohexane solution. The oxidation of cyclohexane is known to depend on the nature of the solvent used [12]. 3.2.1. Oxidation of cyclohexane with hydrogen peroxide with reaction time as variable The use of RH-Cr and RH-Cr-A as catalyst resulted in the gradual increase in the conversion with respect to time. The percentage conversion with RH-Si as catalyst was low, i.e. only 6.67%. Thus, the introduction of active transition metal (chromium) into the framework of the silica from rice husk creates active catalytic sites that are believed to result in an exceptional catalytic activity. Fig. 7 shows the graph of percentage conversion of cyclohexane with respect to time. The use of both catalysts resulted in 100% conversion with 0.1 g of the respective catalysts at a reaction temperature of 70 8C. Complete conversion of cyclohexane has never been achieved to our knowledge. RH-Cr seems to be very active in the oxidation of cyclohexane. The activity of RH-Cr-A was even better, possibly due to the presence of the phenyl group from MBA which can impart greater hydrophobic character to the surface. This permits the approach of the cyclohexane molecule to the polar catalyst surface for adsorption and subsequent transformation.
Fig. 7. The graph of percentage conversion of cyclohexane versus reaction time for RH-Cr and RH-Cr-A. The mass of catalyst used was 0.1 g and the temperature of reaction was 70 8C.
Fig. 8 shows the percent selectivity when RH-Cr was used as the catalyst. A product distribution of ca. 80:20 mixture was found in 6 h with 0.1 g of the catalyst at 70 8C. From Fig. 8, it can be inferred that the first formed species is the cyclohexanol. As the concentration of the cyclohexanol increased, it was further oxidized to cyclohexanone. Hence the selectivity towards cyclohexanol shows a maximum before decreasing. Under the same reaction conditions, RH-Cr-A yielded a mixture of ca. 50:50 of the same products as shown in Fig. 9. With this distinct difference in the selectivity ratio of the two main products, the MBA in RH-Cr-A certainly seems to play an important role in the reaction mechanism. Due to the narrow pore structure and the possible interaction of the lone pair electron in the N (contributed via MBA) atom plus the hydrophobic nature, cyclohexanol is retained on the active site of RH-Cr-A for a longer period of time. This leads to further oxidation to form cyclohexanone. The first formed cyclohexanol may have been fixed on the active site of RH-Cr-A due to the formation of hydrogen bonds between the hydrogen of the hydroxyl group and the nitrogen atom of the MBA within the catalyst matrix. Thus it can be said that the difference in selectivity between these two catalysts is due to the MBA in RH-Cr-A which influence the product selectivity.
Fig. 8. The percentage selectivity towards cyclohexanol and cyclohexanone with 0.1 g of RH-Cr at 70 8C.
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3 h was not studied for this paper. The details of the reaction rate which will include reaction below 3 h will be communicated later.
Fig. 9. The percentage selectivity towards cyclohexanol and cyclohexanone with 0.1 g of RH-Cr-A at 70 8C.
From Fig. 7, it can be seen that RH-Cr-A produced a higher conversion after 3 h compared to RH-Cr. In the case of RH-Cr, the cyclohexanol concentration (Fig. 8) reached a maximum at ca. 6 h and thence decreased over time. However, with RH-Cr-A the cyclohexanol concentration (Fig. 9) seems to decrease immediately and reaches an equilibrium value after ca. 5 h. This can only be possible if the first formed cyclohexanol is retained on the catalyst surface to further undergo oxidation to yield the ultimate oxidation product, i.e. cyclohexanone. The reaction profile below
3.2.2. Oxidation with hydrogen peroxide with variable catalyst mass Figs. 10 and 11 show the product distribution with RH-Cr and RH-Cr-A, respectively. With respect to RH-Cr, there was an initial change in the product distribution when the mass of catalyst was increased from 0.1 to 0.3 g. The product distribution remained almost constant at 50:50 towards the two products. With respect to RH-Cr-A (Fig. 11), a constant distribution of product was observed up to 0.3 g catalyst mass. Further increase in catalyst mass resulted in the decrease in selectivity to cyclohexanone. It can thus be concluded that the mass of catalyst required for the maximum yield of cyclohexanone (53%) was 0.3 g which resulted in 100% conversion with respect to cyclohexane. With regards to the Ce- and Cr-MCM-41 catalysts reported [12,13] which resulted in ca. 95% and 99% conversion, it must be stated that these catalysts were prepared by the conventional hydrothermal process using TEOS as the starting material. The whole process was reported to take 7 days [12] and the catalyst needs to be calcined at 550 8C. The optimum yields were achieved at 100 8C after 12 h of reaction in both cases. In contrast, RH-Cr and RH-Cr-A was prepared by a simple titration method and the catalyst can be prepared within 6 h starting from RH. These catalysts need not be calcined and it can be used after drying at 100 8C with 100% conversion achieved within 6 h of reaction time. In conclusion, the optimum reaction time and mass for RH-Cr and RH-Cr-A was found to be 6 h and 0.3 g, respectively. These catalysts are probably the cheapest catalyst prepared to effect maximum conversion of cyclohexane at a low temperature of 70 8C. 3.2.3. The reusability and leaching effect of the catalyst Reusability of RH-Cr for the oxidation of cyclohexane was tested over five cycles as shown in Fig. 12. During the first cycle, 100% cyclohexane was converted. The conversion dropped about 10– 20% during first until third recycle. During the fourth recycle, the conversion increased again to 95.70%. These observations could not be accounted for at this time but could be due to experimental error. Leaching test was done on the fresh and final cycle to identify the heterogeneity of RH-Cr. The results are shown in Fig. 13. The heterogeneity test was done according to the method described by Shylesh et al. [24]. The first used catalyst, RH-Cr was removed after an hour using the hot filtration technique and the reaction was continued using the
Fig. 10. The product distribution with increasing mass of RH-Cr. Reaction temperature of 70 8C and 6 h reaction time.
Fig. 11. The product distribution when increasing mass of RH-Cr-A was used as the catalyst with a reaction temperature of 70 8C and 6 h reaction time.
Fig. 12. The recyclability of RH-Cr over five reaction cycles.
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4. Conclusion The heterogeneous catalyst RH-Cr and RH-Cr-A from rice husk silica had been synthesized and characterized. The catalysts showed unique physical characteristics as shown by the BET analysis. The catalysts showed superior catalytic activity for the oxidation of cyclohexane, achieving 100% conversion within 6 h. The catalyst could be prepared easily from rice husk and there was no need for calcination prior to their use in the catalysis. These two catalyst hold great promise in the industrial preparation of cylcohexanol and cyclohexanone. Acknowledgements The authors wish to thank Leong Guan Rice Mill Sdn. Bhd, Penang for providing the rice husks for this study. We would also like to thank the Ministry of Education, Malaysia for the FRGS grant (No. 203/PKIMIA/671021) which partly supported this work. Fig. 13. The graph of percentage conversion versus time during the leaching test involving the fresh catalyst, RH-Cr and the recycled (fifth use) catalyst. The catalyst was removed after the first hour and the reaction allowed to continue to completion in 6 h.
filtrate under the same condition. Conversion of cyclohexane continued to completion without the presence of the catalyst. Similar result was observed when the fourth recycled catalyst (fifth use) was subjected to leaching test. Almost complete conversion, i.e. 98.80% of cyclohexane was observed in this case. This suggests that the leaching of chromium (homogeneous catalysis) species may not be the only cause of the continued activity in the absence of the heterogeneous catalysts. It could indicate a free radical mechanism is operative in which the RH-Cr plays an important role in the initial step. However, this could not be verified in this work. Similar results were obtained for RH-CrA. 3.2.4. Oxidation with molecular oxygen The oxidation of cyclohexane was also carried out using oxygen gas as the oxidizing agent. No oxidation products were detected under these conditions. Thus it can be concluded that H2O2 is the best oxidizing agent in the oxidation of cyclohexane when RH-Cr or RH-Cr-A was used as the catalyst. These results are reasonable since it is known that in order to accelerate the initiation step in the auto oxidation process using oxygen as the oxidant, addition of H2O2 or tert-butyl hydroperoxide (TBHP) as co-catalyst was essential [25]. There is however, conflicting reports regarding the heterogeneity of chromium loaded catalyst. The high conversion rates are supposedly due to the few ppm of chromium ions leached out into the solution. Thus, resulting in a homogeneous catalytic effect [26,27].
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