Mixed-valence manganese oxide catalysed ... - IngentaConnect

Progress in Reaction Kinetics and Mechanism, 2014, 39(4), 375 – 390 doi:10.3184/146867814X14139853537970

RESEARCH PAPER

Mixed-valence manganese oxide catalysed oxidation of benzyl alcohol and cyclohexanol in the liquid phase Mohammad Ilyasa,b, Muhammad Saeeda,c*, Mohammad Sadiqa,d and Mohsin Siddiquea,e a

National Centre of Excellence in Physical Chemistry, University of Peshawar, Pakistan b

c

Department of Chemistry, Qurtuba University of Science and Information Technology, Pakistan

Department of Chemistry, Government College University Faisalabad, Pakistan d

Department of Chemistry, University of Malakand, Pakistan

e

Department of Chemistry, Bacha Khan University, Pakistan *E-mail: [email protected]

ABSTRACT Two types of mixed-valence manganese oxides were synthesised by a mechanochemical process in the solid phase by chemical reaction of manganese(II) chloride and potassium permanganate at room temperature. The prepared catalysts were characterised by surface area and pore size, particle size, XRD analyses, SEM analyses and oxygen content measurements. These oxides were used as catalysts for oxidation of benzyl alcohol and cyclohexanol in the presence of solvent and also in solvent-free conditions using molecular oxygen as oxidant. The oxidation reactions were heterogeneous in nature where the catalysts were separated from the reaction mixture by simple filtration. Reaction took place in two steps according to the Langmuir – Hinshelwood mechanism. Benzyl alcohol/cyclohexanol and oxygen adsorb at the surface of the catalyst in the first step followed by reaction between adsorbed benzyl alcohol/cyclohexanol in second step. 69.8 kJ mol – 1 and 140.8 kJ mol – 1 were calculated as the activation energy for benzyl alcohol and cyclohexanol in n‑octane as the solvent respectively, with values of 65.1 and 76 kJ mol – 1 for benzyl alcohol and cyclohexanol in solvent-free conditions respectively.

KEYWORDS: Langmuir adsorption isotherm, Langmuir – Hinshelwood

mechanism, manganese oxide, cyclohexanol, benzyl alcohol 375

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M. Ilyas, M. Saeed, M. Sadiq and M. Siddique

1. INTRODUCTION As carbonyl compounds serve as important intermediates in many synthetic transformations, the oxidation of alcohols to the corresponding carbonyl compounds is one of the most important and widely used reactions in laboratory scale organic syntheses as well as in large scale chemical industries [1,2]. A variety of processes with different reagents and methods are available for these transformations. Various stoichiometric oxidants such as peroxides or high oxidation state metal compounds like permanganate and dichromate are frequently used reagents for these oxidation processes, but these reagents are expensive, toxic and produce large amounts of wastes and hence separation and disposal of such waste increases the number of steps [3]. The use of these stoichiometric oxidants is therefore not an attractive option for large scale oxidation processes. For these kinds of oxidation reaction, an alternative and environmentally friendly oxidant is desirable. An ideal oxidant for any largescale oxidation reaction is the one which has low price, high quality, is nontoxic and has secure availability. Molecular oxygen is the one which meets this criterion [4]. There are a few points, however, which make the use of molecular oxygen challenging. Although molecular oxygen has a high oxidation potential, it is not very reactive towards organic molecules due to the low reactivity of its triplet ground state, firstly and secondly, those reactions where molecular oxygen is present are often radical reactions, which are hard to control. To make an efficient use of molecular oxygen as oxidant, an appropriate catalyst is needed, which can activate the oxygen molecules for the oxidation of alcohols [5]. Both homogenous and heterogeneous catalysts can be used for such oxidation reactions; however, with homogeneous catalytic reactions both the reactants and catalysts are present in one phase and from an engineering viewpoint, a major disadvantage is the difficulty in separating the products from the catalyst which increases the number of steps, therefore the latter approach is superior due to the easier separation of products and catalysts and reuse of catalysts [6]. Metal oxides and supported metal oxides have been proposed as effective heterogeneous catalysts for the oxidation of alcohols. In many cases, aluminaand zirconia-supported metal oxides of precious metals like Pt and Pd have shown high activity for oxidation of alcohols. From the economical point of view, however, using oxides of non-precious transition metals like manganese as heterogeneous catalysts for oxidation of alcohols, using clean oxidants such as molecular oxygen is of great importance [7,8]. Manganese oxide, a non-precious metal oxide, has been used as an oxidation catalyst in several chemical processes, e.g. ozone decomposition [9], photocatalytic oxidation of organic pollutants [10], nitric oxide reduction [11], selective oxidations of www.prkm.co.uk


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carbon monoxide [12], ethylbenzene [13], ethanol [14], propanol [15], and decomposition of hydrogen peroxide [16]. Su et al. [3] have reported four types of oxides of manganese i.e. MnO, MnO2, Mn2O3 and Mn3O4 as catalyst for oxidation of benzyl alcohol in solvent-free conditions. About 37.7% conversion of benzyl alcohol was achieved in 3 hours at 353 K under microwave irradiation while using molecular oxygen as an oxidant. Suib et al. [17] have published a patent report about the catalytic oxidation of benzyl alcohol using manganese oxide (octahedral molecular sieves) as catalyst and toluene as solvent. Benzaldehyde was the sole reaction product with 97% conversion of benzyl alcohol in 4 hours under reflux conditions, using excess of air or oxygen as stoichiometric oxidant. Similarly Yang et al. [18] have studied the oxidation of benzyl alcohol to benzaldehyde catalysed by C – Mn oxide catalyst supported on carbon at 353 K. Yang et al. [19] studied the oxidation of benzyl alcohol with 100% selectivity to benzaldehyde using MnO2 as catalyst in the presence of 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO). About 23% and 36% benzyl alcohol was converted to benzaldehyde in 6 hours at 393 K with and without TEMPO. Cecchetto et al. [20] have reported Mn – Cu and Mn – Co – TEMPO-catalysed oxidation of cyclohexanol to cyclohexanone in different solvents. About 19 – 100% conversion was reported in 4 – 9 hours with different manganese catalysts. Brinksma et al. [21] have reported manganese oxide-catalysed oxidation of various alcohols with a 90 turnover frequency for cyclohexanol. Similarly Chavan et al. [22] and Shulpin et al. [23] have reported manganesebased complexes for oxidation of cyclohexanol and other alcohols. In the present investigation manganese oxide is reported as a catalyst for the oxidation of benzyl alcohol and cyclohexanol in the presence and absence of solvent. An attempt has been made to compare the kinetics of manganese oxide catalysed oxidation of benzyl alcohol and cyclohexanol in the liquid phase.

2. RESULTS AND DISCUSSION 2.1 Characterisation of catalysts Surface area and pore size analyses were carried out by investigation of nitrogen adsorption at 77 K. Before the investigation of nitrogen adsorption, the catalyst samples were degassed at 383 K for 2 hours to remove the already adsorbed gases. The BET surface area of the catalysts was determined by applying the BET equation to the volume of nitrogen adsorbed at the surface of the catalysts in the range of P/P° ~ 0.05 – 0.3 after degassing the samples at 383 K for 2 hours. The cross-sectional area of the nitrogen was taken as 16.2 Å2/molecule. The average pore radius and pore volume were determined www.prkm.co.uk


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from the desorption branch of the isotherm by the BJH method. The results of surface area and pore size analyses are given in Table 1. Particle sizes for reduced and unreduced manganese oxide were found as 50 µm and 11 µm, respectively by the wet method of particle size analysis. In the XRD pattern of the prepared catalyst, peaks at 2θ = 12.7°, 25.2°, 35.1°, 40.7° and 58.85°are observed which show the crystalline nature of the catalyst. Among the peaks, those at 2θ = 12.7° and 25.2° are assigned to β‑MnO2, and at 35.1°, 40.7° and 58.9°to MnO. Lia et al. [24] assigned the peak at 2θ = 25.2° to Mn2O3. Wang et al. [25] assigned the peak at 2θ = 36.2° and 44.4° to Mn3O4 and at 2θ = 38.2° to α‑Mn2O3. On comparison of the XRD of our prepared catalyst with reported XRD patterns, it was concluded that our catalyst is a mixture of β‑MnO2, Mn2O3, and MnO in nature. The crystallite size of the catalyst used for oxidation of benzyl alcohol was calculated to be 11.56 nm and 3.84 nm for reduced and unreduced catalysts, respectively with the use of the Debye – Scherrer equation [1,26]. SEM analyses of the catalyst samples were carried out to explore their morphology. The micrographs revealed that the morphology of the catalysts do not change in the oxidation of alcohols, as described earlier [26]. The bulk and surface oxygen content of the reduced manganese oxide sample were determined as 2.2 × 10 – 3 and 1.1 × 10 – 3 g atom of oxygen per g of sample respectively. Similarly for unreduced manganese oxide, the bulk and surface oxygen content were determined as 4.1 × 10 – 3 and 2.3 × 10 – 3  g atom of oxygen per gram of manganese oxide, respectively. Table 1 Surface area and porosity of the catalysts Parameters BET surface area (m 2 g – 1) Pore volume (cm3 g – 1) Pore radius (Å) Pore surface area (m 2 g – 1)

Reduced Mn‑oxide 10.3 1 × 10 – 2 18.2 5.5

Unreduced Mn-oxide 35.9 0.07 17.8 31.2

2.2 Catalytic activity The catalytic efficiencies of the prepared manganese oxides were explored by investigating the oxidation of benzyl alcohol and cyclohexanol in the liquid phase. The oxidation reactions were carried out in both the presence and absence of solvent. n‑octane was used as solvent. In solvent-free conditions, 10 mL of alcohol was taken, while in the case of solvent, 2 mmol of alcohol was taken in 10 mL of solvent. In the first case 0.3 g of catalyst was used while in the second case 0.2 g of manganese oxide was used as catalyst. Molecular oxygen was passed through the benzyl alcohol/cyclohexanol at the rate of 60 mL www.prkm.co.uk


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min . Before passing the oxygen through benzyl alcohol/cyclohexanol, it was saturated with benzyl alcohol/cyclohexanol by passing it through a saturator containing benzyl alcohol/cyclohexanol at the temperature of the condenser (277 – 281 K). This saturation of oxygen with alcohol minimised the loss of alcohol from the reactor with the flow of oxygen. The time course study for oxidation of benzyl alcohol and cyclohexanol was monitored periodically in which both reduced and unreduced manganese oxides were used as catalyst at 101 kPa partial pressure of oxygen. Reduced manganese oxide was a more reactive catalyst than unreduced manganese oxide, indicating that an increase in transition metal content (Mn) in the catalyst increases the catalytic activity. Table 2 shows the comparison of the catalytic efficiency of the two catalysts for oxidation of benzyl alcohol and cyclohexanol at 363 K. Reduced manganese oxide was used as the catalyst in further experiments. To confirm whether the mixed-valence manganese oxide in the present investigation acts as an oxidant or a catalyst, separate experiments under the flow of molecular oxygen and nitrogen with 2 mmol benzyl alcohol were carried out. The results revealed that conversion of the benzyl alcohol under the flow of molecular oxygen was more (82.3%) than conversion of benzyl alcohol under the flow of molecular nitrogen (31.5%) in one hour at 363 K. In another experiment, the amount of benzyl alcohol was doubled from 2 mmol to 4 mmol, and almost the same conversion of benzyl alcohol was achieved under the flow of oxygen in 2 hours as it was in 1 hour. These results show that the present mixed-valence manganese oxide acts as a catalyst for the oxidation reaction between benzyl alcohol and molecular oxygen. Furthermore, evidence for the catalytic role of the mixed-valence manganese oxide was obtained by exploring the dependence of benzyl alcohol conversion on the partial pressure of oxygen. The conversion of benzyl alcohol was reduced from 82.3 to 53% when the partial pressure of oxygen was reduced from 101 to 34 kPa. Finally, the used catalyst was washed, dried and re‑used for oxidation of benzyl alcohol. It was observed that used catalyst has same catalytic performance as fresh catalyst. In another experiment, the used catalyst was re‑used without washing. Almost the same catalytic performance was observed as given in – 1

Table 2 Comparison of catalytic efficiency of reduced and unreduced manganese oxide for oxidation of benzyl alcohol and cyclohexanol in terms of % conversion at 363 K and 101 kPa partial pressure of oxygen Alcohols BzOH CyOH

Conditions No solvent Solvent No solvent Solvent

Time (min) 300 120 300 120

Reduced Mn‑oxide 21.1 95.6 16.1 72.2

Unreduced Mn‑oxide 3.5 55 2.6 37.5

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Figure 1. From these observations it could be concluded that mixed-valence manganese oxide in the present case acts as the catalyst rather than a stoichiometric oxidant. Time-profile investigation of the oxidations of benzyl alcohol and cyclohexanol was carried out at 101 kPa partial pressure of oxygen in temperature range of 353 – 373 K in solvent-free conditions and at 343 – 363 K in solvent (Figures 2 and 3). A linear increase in conversion was observed with the passage of time. Benzaldehyde and cyclohexanone were detected as the reaction products of the oxidation of benzyl alcohol and cyclohexanol respectively. Results of timeprofile investigations are summarised in Table 3. Table 3 Oxidation of benzyl alcohol and cyclohexanol catalysed by reduced manganese oxide at 101 kPa partial pressure of oxygen Alcohols Conditions

Time (min) Conversion (%) 300 *TOF

343 K

353 K 11.9 7.9

363 K 21.1 14.1

Solvent

Conversion (%) *TOF

60

58.9 294.5

72.4 362

82.3 411.5

No solvent

Conversion (%) *TOF

300

8.5 5.7

16.1 10.7

No solvent BzOH

CyOH

Conversion (%) 60 31.1 42.7 *TOF 155.5 213.5 *TOF (Turnover frequency): conversion per gram of catalyst per hour Solvent

373 K 38.1 25.4

29.4 19.6

52.7 263.5

Figure 1 Re‑use of the catalyst in oxidation of benzyl alcohol. Reaction conditions: benzyl alcohol 2 mmol/10 mL solvent, catalyst 100 mg, temperature 363 K, pressure of oxygen 101 kPa, agitation. 800 rpm, time 1 hour.

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min

Figure 2 Time profile investigation of reduced manganese oxide catalysed oxidation of alcohols in solvent-free conditions: (a) benzyl alcohol; (b) cyclohexanol. Reaction conditions: alcohol 10 mL (96 mmol), catalyst 0.3 g, partial pressure of oxygen 101 kPa, agitation 800 rpm.

2.3 Kinetic analysis Langmuir – Hinshelwood kinetic model can be used to describe the manganese oxide catalysed oxidation of alcohols, in which it is assumed that the reaction between alcohols (R) and molecular oxygen takes place at the surface of the catalyst particles. Recently this model has been described for reaction involving liquid and gaseous reactants at the surface of the catalyst, although it was initially developed to describe the reaction involving the gaseous reactants www.prkm.co.uk


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Figure 3 Time profile investigation of reduced manganese oxide catalysed oxidation of alcohols in solvent: (a) benzyl alcohol; (b) cyclohexanol. Reaction conditions: alcohol 0.2 M (10 mL), catalyst 0.2 g, partial pressure of oxygen 101 kPa, agitation 800 rpm.

[1,2,6,27 – 30]. According to Langmuir – Hinshelwood theory, first the gaseous oxygen and liquid alcohol (R) diffuse to the catalyst surface and become adsorbed. On the surface of catalyst, the adsorbed oxygen and alcohol undergo the catalytic reaction and generate a number of products. Finally the products desorb from the surface. According to Langmuir – Hinshelwood kinetic theory, the rate of reaction is proportional to the fraction of the surface covered by substrate, θ, www.prkm.co.uk


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Rate = k rθRθO

(1)

2

where k r, θR and θO represent the rate constant and the fraction of surface 2 covered by alcohol and molecular oxygen respectively. At constant partial pressure of oxygen, the rate expression becomes Rate = kθR

(2)

Adsorption of alcohol on the surface of catalyst may take place either according to the Langmuir, Temkin or Freundlich adsorption isotherm. Considering Langmuir adsorption isotherm, the rate expression becomes (3) where K represents the adsorption coefficient for alcohol. Considering the Temkin adsorption isotherm, the rate expression is given by Rate = k r(K1lnK2[R])

(4)

where K1 and K2 are constants related to the heat of adsorption, which decreases linearly with surface coverage. A plot of ln[R] against rate gives a straight line. Similarly considering the Freundlich adsorption isotherm, the rate of reaction is given by Rate = k rK[R]1/n

(5)

Where K is adsorption coefficient for alcohol and n (> 1) is a constant. A plot of ln[R] against ln rate gives a straight line. In the solvent-free conditions, due to the negligible change in the concentration of alcohol, θR (at all stages of conversion in the present case) can be taken equal to 1 and therefore, Eqn (1) changes to Rate = k′θO

2

(6)

Under these conditions at a constant oxygen pressure, Eqn (6) will become Rate = k′′

(7)

This is a pseudo zero order reaction and integration will change it to (R)t = –k′′t

(8)

where (R)t is the amount of alcohol left at time “t”. Equations (3), (4) and (5) were applied to the time-profile data in Figure 3 for www.prkm.co.uk


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the oxidation of benzyl alcohol and cyclohexanol using n‑octane as the solvent at various temperatures, using Curve Expert software. Equation (3) gave the best fit to the experimental data which indicates that the adsorption of benzyl alcohol and cyclohexanol at the surface of the catalyst follow the Langmuir adsorption isotherm. There was a high correlation between experimental and calculated values as indicated by the regression coefficient (Figure 4). Rate constants and adsorption coefficient calculated at various temperatures are listed in Table 4. Similarly Eqn (8) was applied to the time-profile data (Figure 2) for the oxidation of benzyl alcohol and cyclohexanol at various temperatures under solvent-free conditions which resulted in straight lines as given in Figure 5. The slopes of these lines give rate constants, k”, which are listed in Table 5. Table 4 Rate constants and adsorption coefficient calculated by fitting Eqn (3) to time profile data from Figure 3 using Curve Expert software Alcohols BzOH

k (min – 1) K (L mol – 1) R2

343 K 0.0385 0.2979 0.99

353 K 0.0760 0.2615 0.99

363 K 0.1483 0.2237 0.99

CyOH

k (min – 1) K (L mol – 1) R2

0.0063 0.942 0.95

0.0444 0.179 0.99

0.0948 0.118 0.99

Table 5 Rate constant calculated by fitting Eqn (8) to time profile data from Figure 2 Alcohols BzOH

k′′ (min – 1) R2

353 K 0.0414 0.92

363 K 0.0781 0.97

373 K 0.1359 0.98

CyOH

k′′ (min – 1) R2

0.0287 0.92

0.0620 0.97

0.0974 0.98

2.4 Activation energy The activation energy was calculated by applying the Arrhenius equation to rate constants at various temperatures. The rate constants determined by the Langmuir – Hinshelwood model at various temperatures listed in Table 4 are true rate constants (true in the sense that the contributions of the adsorption equilibrium constants for benzyl alcohol and cyclohexanol have been excluded); hence the activation energies calculated using these rate constants are true activation energies, ET, which were found as 69.8  kJ  mol – 1 and 140.8  kJ  mol – 1 for benzyl alcohol and cyclohexanol respectively. The enthalpy (ΔH) and entropy www.prkm.co.uk


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Figure 4 Correlation between experimental and calculated values, calculated by fitting Eqn (2) to the time profile data of oxidation of benzyl alcohol and cyclohexanol in solvent from Figure 3 using Curve Expert software: (a) benzyl alcohol; (b) cyclohexanol.

(ΔS) of adsorption of benzyl alcohol and cyclohexanol were determined by applying the Van’t Hoff isochore [Eqn (9)] to the adsorption coefficients, K, at various temperatures from Table 4. The enthalpy (ΔH) of adsorption of benzyl alcohol and cyclohexanol were calculated as – 14.8  kJ  mol – 1 and – 108.1  kJ  mol – 1 respectively. Similarly entropy (ΔS) of adsorption of benzyl alcohol and www.prkm.co.uk


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cyclohexanol were calculated to be 53.2 Jmol – 1 and 317 Jmol – 1 respectively. (9) As adsorption is always exothermic, the enthalpy of adsorption, ΔH is negative from the system’s point of view and –Ha has a positive value [22]. The true activation energy is thus the sum of the enthalpy of adsorption as a positive quantity and the apparent activation energy (equation 10). Thus the apparent activation energies for benzyl alcohol and cyclohexanol in the presence of solvent are 55 kJ mol – 1 and 32.7 kJ mol – 1 respectively.

Figure 5 Fitting of Eqn (7) to the time profile data of oxidation of benzyl alcohol and cyclohexanol in solvent-free conditions from Figure 2: (a) benzyl alcohol; (b) cyclohexanol.

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ET = EA+ ΔH

(10)

Similarly, the activation energies for benzyl alcohol and cyclohexanol in solvent-free conditions were calculated by applying the Arrhenius equation to the rate constants listed in Table 5 and were found to be 65.1 kJ mol – 1 and 76 kJ mol – 1 respectively.

3. EXPERIMENTAL 3.1 Preparation of catalyst Manganese oxide was prepared as reported earlier [26]. Solid manganese(II) chloride and potassium permanganate were first ground at a 2 : 3 mole ratio in an agate mortar at room temperature for 30 minutes and then heated at 323 K for 3 hours and ground again. This process was repeated three times. After grinding, it was kept at 373 K for 24 hours to complete the reaction. The resultant solid was washed with distilled water to remove the untreated precursor materials. After washing, it was dried at 383 K for 24 hours. The resultant black powder was designated as unreduced manganese oxide catalyst. A portion of this sample was reduced at 573 K under a flow of molecular hydrogen at a flow rate of 100 mL min – 1, for two hours. The resulting powder was designated as reduced manganese oxide catalyst. 3.2 Characterisation of catalysts The prepared catalysts were characterised by surface area and pore size analyses, particle size analyses, XRD, SEM, and determination of oxygen content as reported earlier [26]. A Quanta Chrome NOVA 1200e, USA, Surface Area and Pore Size Analyser instrument was used for surface area and pore size analyses of the catalysts. An Analysette 22 Compact, Fritsch, Germany, particle size analyser was used for particle size analyses of the prepared catalysts. A JEOL (JDX-3532) Japan, X‑ray diffractometer was used for XRD analyses using Cu – Kα radiation with a tube voltage of 40 kV and 20 mA, with 2θ ranges from 2 to 70°. Scanning electron microscope, JEOL, JSM-6490 Japan was used for SEM analyses. The bulk oxygen content was determined by dissolving 2.5 g of potassium iodide in 20 mL of 36% acetic acid solution followed by addition of 0.2 g of the catalyst. The solution was allowed to stand for 15 minutes under an atmosphere of molecular nitrogen to liberate iodine. After filtration, the liberated iodine was titrated against 0.1 N sodium thiosulfate solution using starch as indicator The surface oxygen of the catalyst was determined by adding about 0.2 g of the catalyst and 2 g potassium iodide to 15 mL buffer solution at pH 7.1 which was vigorously shaken in a nitrogen atmosphere for www.prkm.co.uk


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30 minutes. The reaction mixture was then filtered, acidified with 1 N HCl and the liberated iodine was titrated against 0.1 N sodium thiosulfate solution using starch as indicator. The following expression was used for determination of the oxygen content.

Oxygen( gatmOxygen / gCatalyst =

Na 2 S 2 O3 (mL) × N ( Na 2 S 2 O3 ) 1000 × Catalyst ( g ) × 2

(11)

3.3 Oxidation protocol Oxidation reactions of benzyl alcohol and cyclohexanol were carried out in a magnetically stirred round-bottom Pyrex glass three-necked batch reactor, provided with a reflux condenser and a mercury thermometer. The reaction temperature was maintained by using a hot plate, (ARE heating and magnetic stirrer, VELP Scientifica). A predetermined quantity of the benzyl alcohol/ cyclohexanol was placed in the reactor and was saturated with oxygen by passing molecular oxygen through the reaction mixture at a flow rate of 60 mL min – 1 for 30 minutes. After reaching the required temperature, a known amount of catalyst was added to the reactor and the flow of oxygen was kept constant throughout the run while stirring the reaction mixture at a constant speed of agitation (800 rpm). After the desired interval of time the reaction was stopped and the catalyst was separated from the reaction mixture using a Whatman Glass micro fibre filter No. 1825 055 with the help of a glass syringe. Analysis of the reaction mixture was carried out by UV‑Vis spectrophotometer (Shimadzu UV‑160A, Japan) and GC (Clarus 500, Perkin Elmer) equipped with FID and capillary column (Elite 5). Excellent separation of the components of the reaction mixture was obtained under the conditions of GC presented in Table 6. Table 6 Working conditions of GC for separation of reaction mixture Chamber Injector

Parameter Sample volume (µL) Temperature (K)

Description 3 7.9

Oven

Temperature (K) Ramping

473 373 K for zero min, 10 K min – 1 to 473

Detector

Detector Temperature (K) Air (mL min – 1) Hydrogen (mL min – 1)

FID 483 450 45

Channel parameters

Column Carrier gas Split flow (mL min – 1) Sampling rate (pts s – 1)

Capillary, Elite‑5, Cat. No. N9316076 Nitrogen 20 12.5

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4. CONCLUSIONS Two types of mixed-valence manganese oxides were prepared and were tested for the oxidation of benzyl alcohol and cyclohexanol in the liquid phase. Reduced mixed-valence manganese oxide was more reactive for the oxidation of benzyl alcohol and cyclohexanol. The oxidation of benzyl alcohol and cyclohexanol in the present case is taking place purely under the kinetically controlled region, where the Langmuir – Hinshelwood type of mechanism is operative. According to this mechanism, the reaction proceeds in two steps. In the first step, both the reactants, i.e. alcohol and oxygen, are adsorbed at the surface of the catalyst, while in the second step the adsorbed reactants react and give the final products. The adsorption of benzyl alcohol and cyclohexanol takes place according to the Langmuir adsorption isotherm which indicates that the adsorption sites for benzyl alcohol and cyclohexanol are homogeneous in nature. Under solventfree conditions, pseudo zero order kinetics were operative.

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