Catalytic oxidation of cyclohexane to cyclohexanone.pdf

Bull. Mater. Sci., Vol. 34, No. 5, August 2011, pp. 11271135. c Indian Academy of Sciences.

Catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanolby tert-butyl hydroperoxide over Pt/oxide catalysts

I REKKAB-HAMMOUMRAOUI, A CHOUKCHOU-BRAHAM,L PIRAULT-ROY and C KAPPENSTEINLaboratoire de Catalyse et Synthse en Chimie Organique, Facult des Sciences, Universit A. Belkaid,B.P. 119 Tlemcen 13000, AlgeriaLACCO UMR CNRS 6503, Laboratoire de Catalyse en Chimie Organique, Facult des Sciences,86022 Poitiers Cedex, France

MS received 6 November 2010; revised 2 March 2011

Abstract. Heterogeneous oxidation of cyclohexane with tertiobutyl hydroperoxide was carried out on Pt/oxide(Al2O3, TiO2 and ZrO2) catalysts in the presence of different solvents (acetic acid and acetonitrile). The catalystswere prepared using Pt(NH3)2(NO2)2 as a precursor and characterized by chemical analysis using the ICPAESmethod, XRD, TEM, FTIR and BET surface area determination. The oxidation reaction was carried out at 70Cunder atmospheric pressure. The results showed the catalytic performance of Pt/Al2O3 as being very high in termsof turnover frequency.

Keywords. Oxidation; cyclohexane; platinum; TBHP; cyclohexanol; cyclohexanone.

1. Introduction

A mixture of cyclohexanone and cyclohexanol known as K/Aoil is obtained by cyclohexane oxidation. These two com-pounds are important intermediates in the manufacture ofnylon-6 and nylon-66 which have become important refe-rence materials in the industrial production of polymers,and the demand has expanded over the last few years. Theindustrial scale preparation of cyclohexanol and cyclohexa-none is carried out by oxidation of cyclohexane or byhydrogenation of phenol (Schuchardt et al 1993, 2001; Luet al 2005). The earlier process is carried out at around150C and 12 MPa pressure, employing metal cobalt saltor metal-boric acid as homogeneous catalyst worldwide. Thedrawback of this process is that the oxidation must be ope-rated in 36% conversion of cyclohexane to maintain a highselectivity (7580%) for the K/A oil (Ingold 1989; Sawatariet al 2001). Many attempts have been made to synthe-size more efficient catalysts for the oxidation of cyclo-hexane. Bellifa et al (2006) used a V2O5TiO2 catalystwhich resulted in an 8% conversion and a 76% selecti-vity to cyclohexanol using acetic acid as solvent and ace-tone as initiator. Copper (II) complexes were used withH2O2 to give a total yield of 689% of cyclohexanol, cyclo-hexanone and other products in 24 h (Silva et al 2007).The use of Co3O4 nanocrystals gave a 76% conversionyielding cyclohexanol and cyclohexanone at 120C in 6 hwith molecular oxygen (Zhou et al 2005). A chromium

Author for correspondence ([email protected])

containing complex, CrCoAPO5(CH3COOH) (Masterset al 2001) gave a 50% conversion, yielding 55%, 8%,15% and 22% selectivities towards cyclohexanol, cyclohexa-none, adipic acid and others, respectively at 115C and1 MPa of oxygen. The Co/ZSM-5 catalyst (Yuan et al2006) was reported to give about 10% mol conversionand 97% selectivity to cyclohexanone and cyclohexanol at120C and 10 MPa pressure of O2. A zirconium com-plex bonded to modify carbamate silica gel gave a pro-duct distribution ratio of 66:1 of cyclohexanol/cyclohexenemixture with 21% conversion at 200C (Anisia and Kumar2004). Titanium silicate (Reddy and Sivasankar 1991) gave278 mol % conversion with selectivities of 44 mol %,45 mol % and 11 mol % towards cyclohexanol, cyclohexa-none and other products, respectively in a 5 h reaction at100C with hydrogen peroxide as oxidant. The Au/Al2O3system using molecular oxygen in a solvent free systemresulted in 126% conversion with a selectivity of 526% forcyclohexanol and 321% for cyclohexanone (Xu et al 2007).A similar use of Au/MCM-41 with oxygen resulted in 19%conversion with 213% and 6% selectivity towards cyclo-hexanol, cyclohexanone and other products, respectively (Luet al 2004). Ebadi et al (2007) reported Fe, Mn and CoPcsupported on gammaalumina as catalysts for aerobic oxi-dation of cyclohexane in the gas phase under atmosphericpressure. They obtained 38% of selectivity for cyclohexanoland cyclohexanone with 29% conversion of cyclohexane.Recently, Yao et al (2006) reported a conversion of 95% overCeMCM-41 at 100C over 12 h; this gave 82% selecti-vity to cyclohexanol. Sakthivel and Selvam (2002) reported

1127

1128 I Rekkab-Hammoumraoui et al

a conversion of 99% yielding 82% and 66% selectivi-ties towards cyclohexanol, and cyclohexanone, respectivelyusing calcined CrMCM-41 catalyst. Here again, the reac-tion was carried out at 100C and over a 12 h duration. Thisis by far the most promising catalyst reported in the litera-ture to date. However, the long reaction time and high tem-perature are grounds for further improvement, and it is stilldifficult to apply these technologies to industrial processes.

Platinum catalyst, Pt/C, in the presence ofH3PMo12O4014H2O, an heteropoly compound, represent-ing active catalytic systems for cyclohexane oxidation witha mixture of O2 and H2 gases and a selectivity of 648 molfor cyclohexanol and 169 mol for cyclohexanone wasreported (Kuznetsova et al 2003). Radic et al (2004) con-ducted studies on the complete oxidation of hexane andtoluene in the presence of Pt/Al2O3. They demonstratedthat the specific activity of hydrocarbons oxidation in thepresence of catalysts based on supported platinum and pa-lladium is known to depend on the size of metal crystallites(Carballo et al 1978; Kobayashi et al 1988; Marecotet al 1994; Labalme et al 1996; Pliangos et al 1997;Papaefthimiou et al 1998; Garetto and Apesteguia 2000).The development of specific activities based on the particlesize of noble metals has been attributed to morphologicaleffects of the metals and not to their chemical effects. TheTOF of supported noble metals improved while increasingtheir size.

Here, we report Pt/oxides (Al2O3, TiO2 and ZrO2) cata-lysts which show satisfactory conversion to the cyclohexaneoxidation and selectivity of cyclohexanol and cyclohexanoneusing tertiobutyl hydroperoxide in CH3CN and CH3COOHas solvents.

2. Experimental

2.1 Catalyst preparation

The following supports, Al2O3 (Oxid C Degussa), TiO2(Titandioxid Degussa), ZrO2 (Aldrich), of commercial gradewere purchased in the form of nanopowder (in the range,1050 nm). They required further treatment before theimpregnation of the metal precursors. The supports weremixed with water (200 mL H2O for 100 g of support) in orderto form a paste. The latter was dried overnight at 120C,and then sieved to retain only the particles having a diameterranging between 01 and 025 nm. Then the support under-went calcination at 400C under oxidative flow (20% O2,80% Ar, 60 mL/min) during 4 h.

An aqueous solution of Pt (NH3)2(NO2)2 (Alfa Aesar)was then impregnated on these oxides in order to obtain 1and 5% wt. of platinum catalyst. After solvent evaporation,the solids were dried at 120C overnight, then calcined at400C for 4 h under oxidative atmosphere (Ar: 48 mL/min O2: 12 mL/min). Finally, the solids were reduced in H2(60 mL/min) at 400C for 4 h.

2.2 Characterization

Chemical composition of the samples were analysed byinductively coupled plasma-atomic emission spectroscopy(ICP-AES) using an OPTIMA 2000 DV spectrometer.

Diffractograms of the catalysts were obtained by X-raydiffraction (XRD) experiments performed on a SiemensD5005 powder diffractometer using Cu K radiation ( =015186 nm) and a backmonochromator. XRD patterns wererecorded using 2 s dwell time, 004 step size and a constantdivergence slit of 18. Crystalline planes were identified bycomparison with PDF standards from ICDD.

The N2 adsorptiondesorption measurements of supportswere carried out at 196C using Micromeritics Tristar.Surface areas of all the supports were calculated usingBrunauerEmmettTeller method, whereas BarrettJoynerHalenda method was employed to get pore size distribution.Pore size distribution was obtained from desorption branchof isotherms.

The acidity of supports was characterized by pyridineadsorption followed by IR spectroscopy. IR spectra wererecorded on a Nicolet Magna IR spectrometer using a thinwafer (16 mm in diameter, 1015 mgcm2) activated in situin the IR cell under secondary vacuum (103 Pa) at 200Cfor 2 h. Pyridine was adsorbed on the sample at 150C.The IR spectra were recorded at room temperature afteractivation and pyridine thermodesorption under vacuum(103 Pa) for 1 h at 150C. The total amounts of Lewis acidsites accessible to pyridine were quantified by the subtrac-tion between P150 spectrum (spectrum of the catalyst afteradsorptiondesorption at 150C of pyridine) and Pref (spec-trum of the catalyst before adsorption of pyridine). This tech-nique allowed on one hand elimination of the solid intrinsicabsorbance in the wavelength range studied, and on anotherhand the ignorance of the pyridine physisorption influence(below 150C). Concentrations of Lewis acid sites able toretain pyridine adsorbed at 150C were determined fromthe normalized absorbance areas of the band at 1455 cm1(PyL), using extinction coefficients previously deter-mined. Spectra were also recorded after pyridine thermo-desorption under vacuum (103 Pa) for 1 h at 250C, 350Cand 450C.

Electron microscopic studies of the catalysts were carriedout on a JEOL 2000 FX instrument operating at 200 kV.Catalyst specimens for electron microscopy were preparedby grinding the powder samples in an agate mortar, suspend-ing and sonicating them in ethanol, and placing a drop ofthe suspension on a holey carbon copper grid. After evap-oration of the solvent, the specimens were introduced intothe microscope column. The area distribution of particleswas determined by counting a large number of particles(>500) on the TEM micrographs and by plotting ni as afunction of di (ni is the number of particles within differentintervals with given average diameters di ). The mean sur-face diameter (Perkas et al 2005) of particles is then givenby d = ni d3i /

ni d2i . The measurements were done

Catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol 1129

manually using the ImageJ program (Rasband and Image19972005).

The chemisorption measurements were carried out in aglass volumetric catalyst system. Catalyst samples of 02 gwere placed into a reactor and reduced with pure H2 for 2 hat 400C. After evacuating the catalysts at the same tem-perature for 2 h, two adsorption isotherms (figure 1) wereobtained at ambient temperature. The linear region of thefirst isotherm was extrapolated to zero pressure in orderto calculate the amount of physisorbed and chemisorbedH2 (HCT ). After an evacuation period of 1 h, the secondisotherm of H2 was obtained (HCR). The same procedure(as the first isotherm) was used to calculate the amount ofphysisorbed species. The irreversibly held H2 was calculatedfrom the difference between the two values. A 1/1 H/Pt stoi-chiometry was assumed in order to calculate the meta-llic dispersion. From the platinum dispersion, the mean

diameter of the platinum particles (d) was calculated usingthe formula:

d (nm) = 5 103

Pt SPt D ,

where Pt is the Pt density (214 gcm3), SPt the specific areaof Pt (275 m2 g1) and D the dispersion of platinum.

2.3 Catalytic reactions

The catalytic oxidation of cyclohexane with tertiobutylhydroperoxide (TBHP) as the oxidant was carried out in aglass round-bottom flask with a magnetic stirrer and refluxcondenser. First, commercial TBHP 70% in H2O (Aldrich)was stirred with cyclohexane in order to perform a phasetransfer from water to cyclohexane. In a typical reaction,

Figure 1. Example of hydrogen adsorption isotherms (HCT ) and (HCR).

Table 1. Textural properties of catalysts and supports.

Supports

ICP Lewis acidity Surface area Pore size Pore volumeCatalysts (wt. %) (mol g1) (m2g1) (nm) (cm3g1)

506Pt/Al2O3 150 95 31 072

102494

Pt/ZrO2 52 35 30 026098500

Pt/TiO2 6 43 26 027103


Table 2. Metallic accessibility and crystallite size for various catalysts.

Chemisorption TEM

Metal Dispersion Particle sizea Dispersion Particle sizebCatalyst content (%) (%) (nm) (%) (nm)

Pt/Al2O3 102 20 47 / /506 28 33 29 32

Pt/ZrO2 098 31 302 / /494 42 22 54 17

Pt/TiO2 103 60 15 / /500 52 18 54 17

aCalculated upon the number of accessible Pt using cubic particle model with 5 accessible facets;bestimated from particle sizes using cubic particle model with 5 accessible facets.The surface areaper Pt site (am) is taken as 808 2.

60 mmol (65 mL) of cyclohexane and 60 mmol (85 mL)of oxidant (TBHP) were mixed in a closed Erlenmeyer flaskand magnetically stirred for 24 h. The organic phase wasthen separated from the aqueous phase. In order to con-trol the phase transfer, concentration of the remaining TBHPin the aqueous phase was determined by iodometric titra-tion, and was found to be ZrO2>TiO2. Thesmallest particle size was obtained for Pt/TiO2 catalyst(18 nm) while the biggest corresponded to Pt/Al2O3


(33 nm). The deposited catalysts on ZrO2 showed the for-mation of particles of 2 nm in size. This proved to beconsiderably smaller than the mesopore diameter, indicatingthat incorporation of the particles in the mesopores could

readily occur. Additionally, the 1% Pt catalysts gave lowerdispersion than the 5% Pt ones. By correlating the metal par-ticle sizes to pore volumes, we can deduce that large parti-cles are formed in the presence of Pt/Al2O3 (072 cm3 g1)

(a)

(b)

(c)

Figure 3. TEM photos of catalysts and particle size distribution: Pt/Al2O3 (a), Pt/TiO2 (b) and Pt/ZrO2 (c).


Others

OH OMe

Me OOH

Me

Pt/MxOy

CH3CN or CH3COOH

Scheme 1. Catalytic oxydation of cyclohexane to cyclohexanone and cyclohexanol.

Table 3. Oxidation of cyclohexane with different catalysts.

Acetic acid Acetonitrile

Selectivity (mmol) Selectivity (mmol)

Support wt. (%) d (nm) TOF (h1) Ol One Others TOF (h1) Ol One Others

Al2O3 506 334 782 28 07 1354 481 05 0 10102 468 2830 2 056 632 1377 15 0 28

TiO2 500 218 169 4 14 66 95 1 05 21103 156 793 092 045 763 520 04 0 44

ZrO2 494 180 440 18 2 136 334 05 0 127098 302 1412 1 03 53 1155 04 0 5

C6H12 = 65 mL; TBHP = 85 mL; solvent = 50 mL; catalyser = 005 g; t = 6 h; T = 70C;TOF = mole of converted cyclohexane per unit time per mole of dispersed platinum.

while smaller ones are formed in the presence of Pt/TiO2 andPt/ZrO2 (027 cm3 g1).

3.3 XRD characterization

All diffractograms (figure 2) confirm that metallic platinumis present in catalysts as a result of the reduction step andno platinum oxide is observed. Metallic Pt is indicated bypeaks at 3976, 4624 and 6745 2 . Alumina-based cata-lyst shows two crystalline phases with sharper peaks, indi-cating that alumina is in and forms. The diffractogramof the supported catalyst on TiO2 shows the two well knowntitanium oxide crystalline phases, viz. anatase and rutile, andonly weak diffraction peaks of platinum are shown. Zirconiasupport is highly crystalline resulting in very intense peaksand the Pt metallics species can be seen as well.

3.4 TEM characterization

The TEM pictures of the prepared catalysts are shown infigure 3. The 5% Pt/Al2O3 catalyst sample was well dis-persed with no indication of agglomeration of the Pt particles(figure 3a). The particle size was found to be in the rangeof 156 nm, the diameter being mostly 24 nm. The 5%Pt/TiO2 had a narrow size distribution of particles in therange of 125 nm with a mean surface diameter of 173 nm(figure 3b). From the direct TEM micrographs (figure 3c),

it may be noted that the ZrO2 support had a more irre-gular morphology. It was coated by small particles of plati-num. On the other hand, the TEM pictures of 5% Pt/ZrO2indicated showed particles in the range 125 nm but alsolarger particles of platinum up to 35 nm (figure 3c). Simi-lar surface diameters of Pt particles (173 nm) are shown in5% Pt/TiO2 and 5%Pt/ZrO2. The same particle sizes wereobtained for catalysts based on TiO2 and ZrO2 by Perkas et al(2005). On the other hand, the crystallite sizes estimated bychemisorption of H2 are the same which confirm the results(table 2).

3.5 Cyclohexane oxidation

Kinetic studies of the catalytic oxidation of cyclohexanewith TBHP as oxidant, in acetonitrile or acetic acid as sol-vent at 70C were performed on different samples. Thedesired products are cyclohexanol (C6H11OH), cyclohex-anone (C6H10O) (scheme 1), but other products like cyclo-hexyl hydroperoxide, adipic acid, ester dicyclohexyl adipate,hexanolactone, and other esters (Zhou et al 2005), cyclo-hexyl acetate (Kumar et al 2009) did also form. In general,cyclohexyl hydroperoxide can be easily converted into cyclo-hexanol by reduction with a variety of reducing agents, thusincreasing the yield of the target compounds. For this study,we focused on the selectivity towards olone only (cyclohexa-nol and cyclohexanone). The amount of the used catalyst was


005 g and the cyclohexane to TBHP mole ratio was (1:1).The results are shown in table 3.

In the preliminary experiments, a non-catalyzed oxidationreaction was carried out under typical reaction conditions andno oxidative products were formed.

Moreover, to check the impact of the supports on thecyclohexane oxidation reaction, we tested them as catalysts(figures 46). All supports exhibited conversions


cyclohexanone are produced and the selectivity in productsdoes not exceed 1 mmol. In acetonitrile medium,lower selectivities of olone mixtures (cyclohexanol +cyclohexanone) are obtained and the amount of byproductsbecomes important.

Regarding the catalysts of platinum supported on zirco-nium oxide, the evolution of cyclohexanol selectivity withregard to the metal content is less pronounced in the pre-sence of acetonitrile. Indeed, we observe no increase in selec-tivity when the platinum content increases and no productionof cyclohexanone is visible. The formation of byproductsincreases significantly with the platinum content to about126 mmol in the presence of acetonitrile and 136 mmol inthe presence of acetic acid. We can see that these catalystslead mainly to products other than olone mixing.

Apparently, Pt/TiO2 catalyst has the best performance inacetic acid. The higher selectivity of Pt/TiO2 in acetic acidcompared to acetonitrile may be explained in the same wayas in Du et al (2009). As is known, enhancement of the acti-vity by acetic acid in the oxidation of cyclohexane has beenreported in the literature (Shulpin et al 1999; Sooknoi andLimtrakul 2002). In acetic acid, TBHP is stabler and dimi-nishes the self-decomposition by forming peroxyacetic acid.Cyclohexane and TBHP are dissolved in acetic acid and thereaction products (cyclohexanol and cyclohexanone) are notonly soluble in the reaction mixture but can also be displacedfrom the catalyst surface as they formed.

As listed in table 3, acetonitrile is not a perfect solventfor cyclohexane oxidation over Pt catalysts. The best cata-lytic performance was obtained in the presence of acetic acidwhich presents a turnover frequency (TOF) of 2830 h1 over1% Pt/Al2O3. Furthermore and surprisingly, the high TOFwas attained over 1% Pt/oxides.

However, as the platinum loadings of our samplesincreased from 1 to 5%, both the conversion of cyclohexaneand the total selectivity to the products increased, and a sharpdecrease of the TOF value became evident at the same time.These effects are more likely caused by a decrease of highlyactive Pt particles since the particles grow bigger accordingto the TEM analysis.

4. Conclusions

Various oxide supported Pt catalysts were investigated foroxidation of cyclohexane with TBHP. The main conclu-sions are:(I) XRD analysis showed that metallic platinum is present incatalysts and no oxide was observed.(II) All the supports are active during oxidation of cyclohex-ane but the presence of metal clearly improves activity of thereaction.(III) Activity depends on the acidity of the support or Ptparticles size and can be ordered as follows:

Pt/Al2O3>Pt/ZrO2>Pt/TiO2.

(IV) Acetic acid as a solvent is better than acetonitrile,since acetic acid gave high conversion and high selectivity tocyclohexanol.

Acknowledgement

Authors thank the Algerian Ministry of Higher Educationand Scientific Research for the fellowship funding in Univer-sity of Poitiers, France.

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Catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol by tert-butyl hydroperoxide over Pt/oxide catalystsAbstractIntroductionExperimentalCatalyst preparationCharacterizationCatalytic reactions

Results and discussionTextural propertiesMetal dispersionXRD characterizationTEM characterizationCyclohexane oxidation

ConclusionsReferences

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Catalytic oxidation of cyclohexane to cyclohexanone.pdf