环己烷过钴液相高效氧化促进

环己烷过钴液相高效氧化促进

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Journal of the Taiwan Institute of Chemical Engineers000(2015)

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Journal of the Taiwan Institute of Chemical Engineers

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Liquid-phase e?cient oxidation of cyclohexane over

promoted

VPO catalyst using tert-butylhydroperoxide

Vahid Mahdavi?,Hamid Reza Hasheminasab

Department of Chemistry,Surface Chemistry and Catalysis Division,Faculty of Sciences,Arak University,Arak38156-8-8349,Iran

a r t i c l e i n f o

Article history:

Received19October2014

Revised13January2015

Accepted18January2015

Available online xxx

Keywords:

Vanadylpyrophosphate

Promoter

Co

Cyclohexane oxidation

Liquid phase oxidation

A series of cobalt-doped vanadium phosphorus oxides(VPO-Co)catalysts,as well as unpromoted sample

was prepared using classical organic method via VOHPO4·0.5H2O precursor followed by calcinations in

butane/air environment at400°C for24h.Techniques such as XRD,BET surface area,chemical titration,

4.43due to increase of V5+oxidation state from28to43%.

Oxidation of cyclohexane,for the?rst time,was studied in the liquid phase over VPO and VPO-Co catalysts,

using(TBHP)as an oxidant.The catalytic tests showed that cobalt doping signi?cantly

overall activity for the oxidation of cyclohexane.At90°C,the obtained activities were0.076

and/g VPO/h over the VPO and VPO-Co(molar ratio Co/V=0.1)catalysts,respectively.The effects

of Co loading,TBHP/cyclohexane molar ratio,amount of the catalyst,solvents and catalyst recycling were

investigated.The kinetic of cyclohexane oxidation was investigated at different temperatures using VPO-

Co(0.1)and excess TBHP.The order of reaction with respect to cyclohexane was determined to be pseudo-?rst

order.The value of the apparent activation energy was also determined.

©2015Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.

1.Introduction

Catalytic partial oxidations of hydrocarbons,particularly alkanes,

using oxygen or air as oxidant are signi?cant and economical to the

chemical industry[1,2].Among various alkanes oxidation,the selec-

tive oxidation of cyclohexane is much attractive because its products,

cyclohexanol and cyclohexanone,are the intermediates for the manu-

facture of Nylon-6and Nylon-6-6[2,3].In addition,cyclohexanol and

cyclohexanone are also used as solvents for lacquers and varnishes as

well as stabilizers and homogenizers for soaps and synthetic deter-

gent emulsions.The other uses of cyclohexanone are in the synthesis

of insecticides,herbicides and pharmaceuticals[3–5].

Homogeneous catalysis using soluble transition metal salts(such

as cobalt naphthenate)and O2as oxidant at a considerably high tem-

perature(150°C)is the only technology,which had actually been de-

veloped until now[6,7].Since the cyclohexanol and cyclohexanone

products are substantially more reactive than the cyclohexane

reactant,high selectivities(>80%)to the sum of cyclohexanol

and cyclohexanone only could be observed at low cyclohexane

?Corresponding author.Tel.:+988634173415;fax:+988634173406.

E-mail address:v-mahdavi@araku.ac.ir,vmahdavius@http://www.77cn.com.cn(V.Mahdavi).

conversion(<5%).Moreover,it is very di?cult to separate the cat-

alysts from reaction mixture in the homogeneous system.Therefore,

the development of effective heterogeneous solid catalysts could offer

advantages[6–10].

There were reports on the oxidation of cyclohexane over

Ti-MCM-41[11],V-MCM-41[12],V-MCM-48[13],Bi/MCM-41[14],

Au/MCM-41[15]and Au/ZSM-5[16].Fe(III)and Mn(III)complexes

were also employed as catalysts in the oxidation of cyclohexane un-

der mild conditions[17].Fe-MCM-41was tested as a catalyst for the

selective oxidation of cyclohexane using acetic acid as solvent and

methyl ethyl ketone as initiator[18].Mixed metal oxides were re-

ported to be e?cient for selective oxidation of cyclohexane[19,20].

In recent years,some transition metals such as Au,Co,V,Pt,Cr,Ce,

Fe,and Mo metal oxides on the various supports have been exten-

sively studied as the cyclohexane oxidation catalyst[21–30].Up to

now,the development of new heterogeneous catalysts for the oxida-

tion of cyclohexane is still of considerable commercial and academic

interest.

Vanadium phosphorus oxide(VPO)catalysts have long been rec-

ognized as the most fascinating catalyst in achieving high conver-

sion with good selectivity in n-butane partial oxidation to maleic

anhydride[31].It is generally accepted that VPO catalysts composed

mainly of vanadyl pyrophosphate((VO)2P2O7)are effective for the

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1876-1070/©2015Taiwan Institute of Chemical Engineers.Published by Elsevier B.V.All rights reserved.

Please cite this article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert-butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers(2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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oxidation of n -butane to maleic anhydride (MA)[32–35].One of the effective means of improving the catalytic properties is the introduc-tion of metal ions into the lattice [36–38].The effect of the dopant is to change the structure characteristics of catalyst phases [37,39].It also plays a role in having an effect on the adsorption of oxygen and its diffusion within the lattice,by which a nonselective route of butane oxidation is suppressed [40].

One of the most studied metal dopant introduced in VPO catalysts is cobalt.Kladekova et al.[41]found that the catalyst modi?ed by Co increased the speci?c rate of butane oxidation and maleic anhydride formation three times compared to the unmodi?ed catalyst.

VPO has been applied as a heterogeneous catalyst in gas phase oxidation reactions and its application is very limited in liquid phase reactions.Recently,VPO catalyst has been used for oxidation of alco-hols in liquid phase in the presence of hydrogen peroxide and TBHP as oxidant [42,43].We have recently reported the oxidation of ben-zyl alcohol on the cobalt promoted VPO catalyst in the presence of TBHP as oxidant [44].Unnikrishnan et al.have recently reported the oxidation of cyclohexane over VPO catalyst in an aqueous hydrogen peroxide [45].

In present study for the ?rst time,we developed the use of cobalt-doped VPO catalyst (VPO-Co)for the oxidation of cyclohexane with tert-butylhydroperoxide (TBHP)in the liquid phase.2.Experimental 2.1.Materials

All materials were of commercial reagent grade.V 2O 5and cyclohexane were obtained from Aldrich.H 3PO 4(85%)and tert -butylhydroperoxide (TBHP)70%solution in water were purchased from Merck chemical company.All of the solvents acetonitrile,toluene,p -dioxane,chloroform and ethanol used were of the highest commercial quality supplied from Merck Company and used without further puri?cation.

2.2.Preparation of unpromoted and Co-promoted VPO catalysts

The unpromoted VOHPO 4·0.5H 2O precursor was prepared by adding V 2O 5(15.0g)to a mixture of isobutanol (90mL)and benzyl al-cohol (60mL).The vanadium oxide-alcohol mixture was re?uxed for 4h at 105°C with continuous stirring.The mixture was then cooled to room temperature and aged at this temperature overnight.Ortho -phosphoric acid (11mL,85%)was added to obtain the P:V atomic

ratio of 1.The resulting solution was then heated again to 120°C and maintained under re?ux with constant stirring for 4h.The precipi-tate was removed by ?ltration,washed thoroughly and dried at 150°C overnight.

For the preparation of Co-doped catalyst precursor,V 2O 5(15.0g)and the cobalt promoter in the nitrate form was suspended by rapid stirring into a mixture of isobutyl alcohol (90mL)and benzyl alcohol (60mL).The amount of cobalt nitrate was adjusted to obtain 0.01–1.0molar ratio of Co/V.The vanadium oxide–cobalt nitrate–alcohol mixture was re?uxed for 7h at 110°C with continuous stirring.The mixture was then cooled to room temperature and aged at this tem-perature overnight.Ortho -phosphoric acid (11mL,85%)was added in such a quantity as to obtain P:V atomic ratio of 1.0.The resulting solution was then heated again to 120°C and maintained under re?ux with constant stirring for 4h.The precipitate was removed by ?ltra-tion,washed thoroughly with water and dried at 150°C overnight.Both unpromoted and promoted precursors were heated from room temperature to 400°C at a rate of 5°C/min in a ?ow of a 1.5%butane-in-air mixture (30mL/min)and kept at this temperature for at least 24h.The contents of cobalt and vanadium were deter-mined by atomic absorption spectroscopy (AAS)using a PerkinElmer Analyst instrument,after dissolving of samples in H 2SO 4(2M).

Cobalt-containing VPO catalysts prepared are designated as VPO-Co (x ),x being the Co/V molar ratio.These samples with grain size of 200–230mesh were separated and in the next step,were used in liquid phase to catalyze the oxidation of cyclohexane by TBHP.2.3.Catalysts characterization

The total surface areas of the catalysts were measured by the BET (Brunauer–Emmer–Teller)method using nitrogen adsorption at 77K.This was done by the Micromeritics ASAP 2000nitrogen adsorp-tion/desorption analyzer.

The average oxidation states of vanadium in the catalysts were determined by redox titration following the method of Niwa and Murakami [46].About 0.1g of each catalyst was dissolved in 100mL of 2M H 2SO 4at 80°C.The vanadium (IV or III)content was deter-mined by titration with a solution of KMnO 4(0.01N).The vanadium (V)content was determined by titration with a solution of a Mohr-salt (FeSO 4·(NH 4)2SO 4·6H 2O,0.01N)using diphenylamine as an indicator.

The structure of the catalysts was studied by X-ray diffraction (XRD)experiments.A diffractometer Philips model PW 1800instru-ment with Cu K αradiation and Ni ?lter was used to collect the X-ray data.The SEM image was obtained with a Philips XL30instrument.The infrared spectra of the catalysts were taken as KBr pellets on a Galaxy-5000Fourier transform infrared (FT-IR)spectrometer.Temperature-programmed reduction in H 2(H 2-TPR)was carried out in order to observe the reducibility of the VPO catalyst by using a Thermo Finnigan TPDRO 1110apparatus utilizing a thermal con-ductivity detector (TCD).H 2-TPR experiment was performed using a quartz reactor tube (4mm i.d.),in which a 40mg sample was mounted on loosely packed quartz wool.Prior to H 2–TPR measurement,a cat-alyst was pretreated in N 2at 473K (heating rate of 10K/min and hold time 30min),then cooled down under He.The reduction gas was composed of 5vol%H 2in Ar.The reaction temperature was pro-grammed to rise at a constant rate of 10K/min.A thermocouple in contact with the catalyst allowed the control of the temperature.The amount of H 2uptake during the reduction was measured by a thermal conductivity detector (TCD).The e?uent H 2O formed during H 2-TPR was adsorbed by a 5A molecular sieve adsorbent.2.4.Oxidation of cyclohexane

In a typical procedure,a mixture of 0.1g catalyst (VPO or VPO-Co),with a grain size of 200–230mesh 15mL acetonitrile and 10mmol of cyclohexane was stirred in a three-necked ?ask under nitrogen atmosphere at 50°C for 30min.The stirring rate of the solution was set at 750cycle/min.Then 10mmol of the oxidant (TBHP 70wt%solution in water)was added and the mixture was re?uxed at 90°C for 8h under nitrogen atmosphere.After ?ltration,the solid was washed with ethanol and the reaction mixture was analyzed by GC.A GC (PerkinElmer Model 8500)equipped with a ?ame ionization detector (FID)connected to a 3%OV-17column with a length of 2.5m and diameter of 1/8in.was used for product analysis.3.Results and discussion 3.1.Characterization of the catalysts

3.1.1.X-ray diffraction (XRD)

Fig.1shows the XRD patterns of unpromoted and Co-promoted VPO catalysts with different Co/V molar ratio.XRD patterns showed well crystalline materials.The appearance of lines at 2θ=22.8,28.3,29.9,33.6,37.7,46.2and 49.5°indicates the presence (VO)2P 2O 7(JCPDS:41-698),while the appearance of peaks at 2θ=12.1,19.3and 25.2°con?rms the presence of β-VOPO 4(JCPDS:27-948).The X-ray lines due to (VO)2P 2O 7broadened and diminished with the increase

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article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert -butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers (2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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Fig.1.XRD patterns of unpromoted and Co-promoted VPO catalysts:(a)VPO;(b)VPO-Co(0.01);(c)VPO-Co(0.06);(d)VPO-Co(0.1).Symbols:(VO)2P 2O 7(?);β-VOPO 4(?);CoPO 4( ).

of the Co/V molar ratio.As a result,Co promoted VPO gave poorer crystallinity compared to the unpromoted catalysts.All the patterns can be indexed to VPO catalyst although there are differences in the relative intensities of the main re?ections.On the other hand,CoPO 4phase (with the most representative peak at 2θ=24.6°)was ob-served in the VPO-Co series.In this case,the intensities of the X-ray lines of CoPO 4increased and those of the (VO)2P 2O 7and β-VOPO 4decreased with the increase of the Co/V molar ratio.The addition of Co from 0.01to 0.06Co/V molar ratios,does not signi?cantly affect on the XRD pattern.However,the addition of higher concentrations (for example,sample with Co/V =0.1)leads to a signi?cant decrease in the intensity of the (VO)2P 2O 7and β-VOPO 4re?ection.

The average crystallite size of VPO-Co(0.1)sample deter-mined from the diffraction peak broadening (for 2θ=29.9°and β=0.571°=0.00996rad)by using the Scherer’s formula was around 14nm (Fig.1d).

3.1.2.BET surface area measurement and redox titration

According to the XRD results (Fig.1),unpromoted and Co-promoted VPO catalysts contained V 5+and V 4+ions.In order to in-vestigate the effect of Co promoter on BET surface area,percentage of vanadium (V)and vanadium (IV)and average oxidation number of vanadium,BET and redox titration experiments on the VPO and VPO-Co(0.1)samples were performed and these results are shown in Table 1.

BET surface area of VPO gave a higher value,23.7m 2/g compared to 10.4m 2/g for VPO-Co(0.1).It suggests that the addition of Co cation into the VPO matrix may inhibit the formation of crystal phases with higher surface area.

Table 1

BET surface area,average vanadium valence and percentages of V 4+and V 5+oxidation states present in undoped and Co-promoted VPO catalysts.Catalyst

BET surface V 5+V 4+Average oxidation area (m 2/g)(%)(%)number of V in bulk a VPO

23.72872 4.28VPO-Co(0.1)

10.4

43

57

4.43

a

Determined by redox titration method.

According to the XRD results (Fig.1),Co-promoted VPO catalysts gave lower crystallinity compared to the unpromoted catalysts.Also BET surface area of promoted nanosized VPO catalysts were relatively proper.The BET surface area value is in agreement with the crystal-lite size distribution.Smaller crystallites size displays higher surface area.On the basis of the crystallite size results calculated by Scherrer equation (from XRD results),Co-promoted VPO catalysts had decre-ment in crystallite size and lead to the increment of the BET surface area.The surface area is one of the factors that control the activity of the catalyst.There is usually,a linear relationship between activities with catalyst’s speci?c surface area [47].This implies that the surface structure of the activated catalysts are very similar and the activity differences just due to higher surface area of vanadium phosphorus oxide catalysts which have higher numbers of active sites per unit mass of catalyst.

As results show in Table 1,the promotion of Co resulted in larger amount of V 5+species.Co was found to increase the average oxidation number from 4.28to 4.43for VPO-Co(0.1)due to an addition of V 5+oxidation state from 28to 43%.It is apparent that the valence state of surface vanadium plays an important role in the selective oxidation of n -butane to maleic anhydride [48,49].According to the literature by Abon and Volta [48]the active and selective vanadium phosphate catalysts usually display a mean oxidation state of vanadium slightly higher than 4.

3.1.3.Scanning electron microscopy (SEM)

The surface morphology of unpromoted and promoted VPO cata-lysts is shown in Fig.2.The unpromoted VPO catalyst (Fig.2a)shows thin platelets with uniform crystal size and rosette plate-like struc-ture.This data corroborates the X-ray diffractograms obtained in Fig.1,where a dominant (020)re?ection (line at 2θ=22.8)ob-tained is associated with a (VO)2P 2O 7phase having stacked platelet morphology.Promotion of VPO with Co-promoter caused changing in morphology of catalyst,thus causing the lost in rosette shape mor-phology and displaying nano-structured platelet and nano-rod struc-ture morphology (Fig.2b and c).SEM of VPO-Co with Co/V =0.5(Fig.2c)shows that the VPO structure was not retained after Co promotion and rosette shape morphology of VPO breaks down to irregular and aggregated particles.VPO and Co-promoted VPO cata-lyst(Co/V =0.06)are different slightly in the shape of crystallites,the addition of cobalt decrease the sizes of crystallites.The particles of VPO are composed of lamellar crystallites,the size of them is 1–3μm in diameter and 150–250nm in thickness,while the VPO-Co sample is composed of the fastener-like crystallites and nano-rod structure and size of them is smaller than the former (1–1.5μm in diameter and 50–100nm in thickness).

These results were in good agreement with the TEM results re-ported by Tau?q-Yap et al.[50].The TEM images demonstrated clearly that the addition of Co promoter into VPO catalyst affected the parti-cle size of the synthesized catalyst.However,the synthesized catalyst composed with nanosized particles which distributed in the nano-scale ranged.

3.1.

4.Fourier transforms infrared spectroscopy (FT-IR)

The FT-IR spectra of the precursors and activated catalysts in the 250–2250cm ?1region are shown in Fig.3A and B respectively.In Fig.3A,all of them show the characteristic vibrations of the vanadyl hydrogen phosphate hemihydrate (VOHPO 4·0.5H 2O).The band cen-tered at 1645cm ?1is characteristic of coordinated water as expected from the crystalline structure,and a marked shoulder at 1600cm ?1,which could be due to hydrogen-bonded uncoordinated water.In the VPO-Co precursors,Fig.3A(b,c,d)with different Co/V ratios,there appears a strong band at 975cm ?1[V 4+=O].Other characteristic bands corresponding to VOHPO 4·0.5H 2O phases are:790cm ?1(sym-metric stretching of P –O –P bond),1201,1105and 1047cm ?1(P –O stretching).The peaks at 929cm ?1,644cm ?1,and 530cm ?1were

Please cite this article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert -butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers (2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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Fig.2.Scanning electron micrographs of activated catalysts:(a)unpromoted VPO;(b)VPO-Co(0.06);(c)VPO-Co(0.5).

Table 2

IR absorption bands due to V 4+=O and P –O –P of different catalysts.Catalyst

V 4+=O wave P –O –P wave number (cm ?1)number (cm ?1)VPO

985780VPO-Co(0.01)982785VPO-Co(0.06)975797VPO-Co(0.1)

972

799

assigned to υP –(OH)[51],δO –P –O in β-VOPO 4[52]and δO –P –O,respectively.In the cases where Co has been added during the pro-duction step,it was observed that the P –O –P vibration was modi?ed [53].The υ(P –O –P)at 644cm ?1has a lower intensity in the promoted catalysts compared to the unpromoted sample and this decreasing is independent of the Co content.

The FT-IR spectra of the activated VPO catalysts are shown in Fig.3B.Examination of the FT-IR spectrum suggests that the peaks of characteristic bands of VPO are weaker than cobalt-doped ones,while,the signi?cant differences observed in the spectra are for vi-brations of the linkages between the layers of the vanadyl pyrophos-phates and this region is shown in Table 2.The V =O absorption bands of the catalysts containing cobalt are signi?cantly shifted to lower wave number,while the P –O –P absorption bands of them shifted to higher wavenumber.The introduction of promoter into the crystal lattice,in other words,the substitution of vanadium by metal bring

about a shift of V =O wave number to lower frequencies [54],and the higher shift in P –O –P wavenumber indicates the promoter atoms affect the layer linkages [55].So these above results strongly sug-gest that promoter element cobalt is located in the crystal lattice of vanadyl pyrophosphate.

3.1.5.H 2temperature-programmed reduction (H 2-TPR)

In order to investigate the effect of Co promoter on redox proper-ties,H 2-TPR experiments were performed and total amount of oxygen removed are presented in Table 3.VPO and VPO-Co(0.1)samples have two reduction peaks that the maximum temperatures of peaks 1and 2for VPO were observed at 577and 844°C,respectively.The maximum temperatures of peaks 1and 2for VPO-Co(0.1)were also observed at 535and 805°C,respectively.The observation of two kinetically dif-ferent reduction peaks for these catalysts implies the presence of two types of oxygen species.The ?rst peak at low temperature be-tween 527°C and 587°C was assigned to the removal of oxygen species associated with V 5+(V 5+→V 4+and V 5+→V 3+)whereas,the second peak at high temperatures range from 777to 877°C was due to the removal of the oxygen species associated with V 4+phase (V 4+→V 3+).Electrical conductivity investigation suggested that O 2?is related to V 5+phase while O ?is associated to V 4+phase [56–58].As evident from Table 3,the total amount of oxygen removed from both peaks (V 5+–V 4+)of VPO were found to be 25.85×1020atom/g,at oxygen ratio of 0.54.The total amount of oxygen also removed

Please cite this article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert -butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers (2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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2015;17:49]Fig.3.FT-IR of VPO and Co-VPO:(A)before calcinations:(a)VPO,(b)VPO-Co(0.01),(c)VPO-Co(0.06),(d)VPO-Co(0.1)and (B)after calcinations:(a)VPO,(b)VPO-Co(0.01),(c)VPO-Co(0.06),(d)VPO-Co(0.1).

from the Co doped sample (VPO-Co(0.1))was slightly higher than

VPO catalyst (32.33×1020atom/g,at oxygen ratio of 0.67).H 2-TPR

results presented in Table 3illuminate this fact that the promotion of

VPO catalyst by Co element slightly increased the amount of oxygen

species removed from V 5+phase.TPR results show that the addition

of Co to VPO catalyst leads to a decrease in the reduction temperature

of lattice oxygen and an increase in the quantity of reducible lattice

oxygen species at lower temperature. 3.2.Oxidation of cyclohexane with TBHP/effect of Co loading Firstly,the model compound cyclohexane was tested for reactivity under a variety of experimental conditions.Results for oxidation of cy-clohexane with TBHP in the presence of VPO and VPO-Co with 0.01–1molar ratio of Co/V,are shown in Table 4.All reactions were conducted at re?ux temperature (90°C)for 8h with 0.1g of the catalyst,15mL acetonitrile,10mmol of the cyclohexane and 10mmol TBHP 70wt%Please cite this article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert -butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers (2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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Table 3

Total amount of oxygen removed and ratio for oxygen removed of V 5+/V 4+estimated by reduction in H 2/Ar for VPO and VPO-Co(0.1)catalysts.Catalyst

Peak

T max (°C)

Total amount of oxygen Total amount of oxygen Ratio of oxygen removed ×10?3(mol/g)removed ×1020(atom/g)removal of V 5+/V 4+VPO

1577 1.529.1500.54

2844 2.7816.70Total oxygen atoms removed 4.3025.85VPO-Co(0.1)

1535 2.1613.000.67

2

805

3.2119.32Total oxygen atoms removed

5.37

32.33

Table 4

Oxidation of cyclohexane with TBHP in presence of VPO-Co catalyst.Catalyst

Co/V a (molar ratio)

Conversion (%)

Activity (g pro/g VPO /h)

Selectivity (%)Cyclohexene

Cyclohexanol Cyclohexanone No catalyst –20.01410000VPO

–100.076453322VPO-Co(0.01)0.01260.198433225VPO-Co(0.03)0.03290.223393625VPO-Co(0.06)0.06380.289443323VPO-Co(0.10)0.1650.491472033VPO-Co(0.20)0.20360.269413821VPO-Co(0.50)0.50330.242394219VPO-Co(1.0)

1.0

29

0.215

45

38

17

Reaction condition:0.1g catalyst with a grain size of 200–230mesh;cyclohexane 10mmol;TBHP 10mmol;15mL acetonitrile;stirring rate 750rpm;re?ux temperature(90?C);reaction time 8h.

Conver.=(moles of cyclohexane reacted/moles of cyclohexane in the feed)×100.Selec.i =(moles of cyclohexane converted to i /moles of cyclohexane reacted)×100.a

The contents of Co and V were determined by AAS.

solution in water.The conversion percentage was calculated for all reactions according to amounts of substrate (cyclohexane).The re-sults show that reactions with Co-promoted VPO catalysts had rela-tively high activity (0.491g pro /g vpo /h )compared to the unpromoted VPO sample (0.076g pro /g vpo /h ).The catalytic activity is enhanced when VPO is promoted (more than 6times order of magnitude),sug-gesting that the interaction between VPO and Co promoter results in a deep modi?cation of the catalytic properties.

From Table 4it can be found that cyclohexane conversion at 0.1molar ratios of Co/V,is highest (65%).The conversion decrease with the addition of Co promoter,for example in the presence of VPO-Co(0.5)sample,conversion is 33%.Nevertheless,the lower perfor-mances of VPO-Co(0.5)compared to those of VPO-Co(0.1)catalyst,despite the higher Co doping,con?rms that sample with un-like VPO structure or morphology has a poorer activity.When Co/V mo-lar ratio exceed 0.1,the crystalline (VO)2P 2O 7decreased,and which would decrease the conversion percentage.In fact,an increase in Co/V molar ratio from 0.01to 1produces signi?cant changes in phase composition,textural properties,morphology and relative content of (VO)2P 2O 7/VOPO 4species,of the VPO-Co catalysts.Such alterations in catalyst characteristics should account for the observed difference in performance.

The results of Table 4show that reactions in presence Co-promoted VPO catalysts had relatively high selectivity respect to cyclohexene compared to the cyclohexanol or cyclohexanone.Therefore,these cat-alytic systems promote an oxidation dehydrogenation (ODH)path-way.VPO-Co(0.1)catalyst,has higher %conversion with respect to other samples.On the VPO-Co(0.1)catalyst,the conversion of cyclo-hexane was 65%and the selectivity of cyclohexene,cyclohexanol and cyclohexanone were 47,20and 33%,respectively.Therefore VPO-Co(0.1)may be a better catalyst among all other catalysts listed in Table 4.

3.3.Effect of the amount of the catalyst

In these experiments,the amounts of VPO-Co(0.1)catalyst were varied from 0.02to 0.3g for reactions carried out at 90°C for 8h,and other reaction conditions remaining constant.The results (Table 5)demonstrate clearly that the oxidation reaction is strongly dependent upon the catalyst amount.Without addition of catalyst,the conver-sion%is 2.5%and in the presence of catalyst,there is a general trend of increasing conversion of cyclohexane by rising catalyst amounts be-cause of the increase in the total number of available active catalytic sites for the reaction.

Table 5

Effect of amount of VPO-Co(0.1)catalyst on cyclohexane oxidation with TBHP as oxidant.Entry

Amount of catalyst (g)

Conversion (%)

Selectivity (%)Cyclohexene

Cyclohexanol Cyclohexanone 10 2.51000020.023*********.054140144640.106445183750.156347203360.20675020307

0.30

64

28

35

37

Reaction condition:cyclohexane 10mmol;TBHP 10mmol;15mL acetonitrile;re?ux temperature (90?C);reaction time 8h.

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Table 6

Effect of solvents on oxidation of cyclohexane with TBHP in the presence of VPO-Co(0.1).Solvent

Protic/aprotic

Dielectric constant

Dipole moment (D )

Conversion (%)

Selectivity (%)Cyclohexene

Cyclohexanol Cyclohexanone Acetic acid Protic 6.1 1.6848563311Chloroform Aprotic 4.81 1.1544641125Toluene Aprotic 2.380.4339719201,4-Dioxane Aprotic 2.250.453656233Acetonitrile Aprotic 37.5 3.925438458n -Hexane

Aprotic

1.89

0.08

37

57

29

14

Reaction condition:0.1g catalyst;cyclohexane 10mmol;TBHP 10mmol;15mL solvent;reaction temperature (70°C);reaction time 8h.

When increasing the amount of loading catalyst from 0.02g to 0.1g the conversion of cyclohexane was increased from 33%to 64%.However,the conversion did not distinctly bene?t from increasing the catalyst amount beyond 0.1g.In this reaction,the reaction rate is determined by surface reaction and mass transfer.In VPO catalysts,the pore diameters are very low.Therefore,the cyclohexane oxidation reaction mostly takes place on the external surface of the catalysts and the internal diffusion has little impact on the reaction rate.By increasing the stirring speed,the reaction rate can be accelerated because the mass transfer can be enhanced.In this reaction condition,the surface reaction is the limiting step when the amount of catalyst is below 0.1g.However,the external diffusion becomes the limiting step when the amount of catalyst is exceeded 0.1g.Therefore,in this reaction condition,the optimum mass of catalyst is 0.1g.

3.4.Effect of solvents

Solvent effects are well documented in the organic synthesis literature.Similar effects have also been reported in the heteroge-neous catalysis literature;however,the mechanistic basis of the ob-served effects is not clear.Solvent effects in heterogeneous catalysis have been rationalized by correlating reaction rates and product dis-tributions with solvent polarity or dielectric constant.Solvent effects can also be manifested via competitive adsorption between the sol-vent and the reactant.Also in the absence of transport limitations,apparent solvent effects can be attributed to variations in reactant solubility in the liquid phase in some cases.In these experiments the solvent was changed for each run while the other conditions,(0.1g of the VPO-Co(0.1)catalyst with a grain size of 200–230mesh,10mmol cyclohexane,10mmol TBHP,the stirring rate of the reaction mix-ture 750cycle/min and reaction temperature,70°C for 8h)remain the same.The solvents have been varied from polar to nonpolar one.The results of cyclohexane conversion with the various solvents are shown in Table 6.

As demonstrated by

Table 6,the behavior of cyclohexane oxidation in various solvents is strikingly different.The conversion (%)of cyclohexane decreased in the order:acetonitrile >acetic acid >chloroform >toluene >n -hexane >dioxane.The variation of cyclohexane conversion in the presence of different solvents is mostly due to the competitive adsorption between the solvents and cyclohexane on the catalyst and thereby occupying a part of the active sites of the catalyst by the adsorbed solvent molecules.For example,the lower cyclohexane conversion percentage in dioxane maybe is due to the strong adsorption of dioxane on the catalytic active sites via lone pair of electrons on oxygen.

The selectivity of reaction products also have been varied in the presence of different solvents.In the presence of toluene as a solvent the selectivity of cyclohexene is 71%and in 1,4-dioxane as a solvent the selectivity of cyclohexene and cyclohexanol is 5%and 62%respec-tively.Therefore,in this solvent the major product is the cyclohexanol and oxidation dehydrogenation (ODH)pathway to be suppressed in presence of 1,4-dioxane.

01020304050607080900

2

4

6

8

10

12

14

16

18

20

22

C o n v e r s i o n (%)

Time (h)

T= 90 C T= 27 C T= 40 C T= 55 C T= 75 C

Fig.4.Conversion of cyclohexane as a function of time at 27,40,55,75and 90°C with VPO-Co(0.1)catalyst in the presence of excess TBHP.Reaction condition:0.1g catalyst with the grain size of 200–230mesh;the stirring rate of the reaction mixture 750cycle/min;cyclohexane 10mmol;TBHP 150mmol;15mL acetonitrile.

Acetonitrile is a polar solvent with a very high dielectric constant it may readily dissolve TBHP along with the cyclohexane and increas-ing the e?ciency of the catalytic system.Also,highly polar solvents like acetonitrile may facilitate formation of active oxygen species and thereby enhance the catalytic activity.The best conversion for cyclo-hexane in the acetonitrile as a solvent is 54%at reaction temperature 70°C.

3.5.Kinetics of reaction

Oxidation of cyclohexane was studied exclusively for kinetic eval-uation and results of the study are as follows.

In this kinetic study the depletion of cyclohexane concentration in the presence of excess TBHP was monitored and plotted with respect to time (Fig.4).The reaction was carried out in a mixture of 15mL acetonitrile,5mmol cyclohexane,75mmol TBHP and 0.1g of VPO-Co(0.1)catalyst with the grain size of 200–230mesh and stirring rate of the reaction mixture 750cycle/min at 90°C in a two-necked round bottom ?ask.Samples of 0.3μL were withdrawn at regular intervals and analyzed by GC.The rate expression [59]may be written as:

Rate =k [Cyclo ]n

[TBHP ]m

(1)

where Cyclo stands for cyclohexane,n is the order of reaction with respect to cyclohexane,m is the order of reaction with respect to TBHP,and k is the rate constant.If n =1and using excess concentration of TBHP,the integrated expression can be written as

?ln (1?X )=k t ,

(2)

X is the conversion of cyclohexane after time t .

According to expression (2),the plot of ?ln(1?X )with re-spect to time gives a linear relationship and as such represents a pseudo-?rst-order dependence on cyclohexane.The kinetic of cyclo-hexane oxidation at re?ux temperature (90°C)was investigated and

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R² = 0.9974

R² = 0.9969

R² = 0.9970

R² = 0.9972

R² = 0.9973

0.20.40.60.811.21.41.61.822.22.40

1

2

3

4

5

6

789101112131415

-L n (1-X )

Time (h)

T=90 C

T= 27 C T= 40 C T= 55 C T= 75 C

Fig.5.Pseudo-?rst order kinetics of cyclohexane oxidation at 27,40,55,75and 90°C with VPO-Co(0.1)catalyst in the presence of excess TBHP.

y = -3003.2x + 6.4965

R² = 0.999

-4-3.5-3-2.5-2

-1.5-10.0026

0.0029

0.0032

0.0035

L n K

1/T (K)

Fig.6.Effect of temperature on the rate constant of the oxidation of cyclohexane (Arrhenius plot).

according to Fig.5,it followed a pseudo-?rst order with respect to cyclohexane.

3.5.1.Effect of temperature on the rate of the oxidation of cyclohexane The temperature of reaction can in?uence reaction rate,because rate constants are strong functions of temperature.Therefore,the study of the effect of temperature is very important for a heteroge-neously catalyzed reaction.Oxidation of cyclohexane was carried out at 27,40,55,75and 90°C in the same reaction conditions and rate constants of reactions were determined,these are shown in Figs.4and 5.Evaluations of regression analysis,by ?tting function LINEST (Excel Software)for data of Fig.5are shown in Table 7.These results suggest that oxidation of cyclohexane over the VPO-Co(0.1)catalyst well follows the pseudo-?rst order kinetic model.

From the pseudo-?rst-order rate constants,the plot of In k vs.1/T (Arrhenius plot)was drawn (Fig.6)and the value of the apparent

Table 7

The regression analysis by ?tting Function LINEST (Excel Software).T (°C)

Equation of k (min ?1)

Standard R 2

SS resid

line Error of k 27Y =0.0362X 0.03620.000550.99690.00265840Y =0.0590X 0.05900.000930.99700.00531355Y =0.0922X 0.09220.001400.99720.01129075Y =0.1443X 0.14430.002190.99730.02711290Y =0.2246X

0.22460.00355

0.99740.045308

SS resid =The residual sum of

squares.

C o n v e r s i o n (%)

12345

No. of cycle

Fig.8.The effect of catalyst recycling.Reaction condition:0.1g VPO-Co(0.1)catalyst;re?ux temperature (90°C),cyclohexane 10mmol;TBHP 10mmol;15mL acetonitrile;reaction time of a run 8h.

activation energy (E a )was evaluated from the slope of the plot,it was determined as 24.97kJ/mol.3.6.Reaction mechanism

According to the results of VPO characterization,we can conclude that an oxidized phase such as (VO)2P 2O 7is not the only active phase for selective oxidation reaction and a suitable V 5+/V 4+balance is required for the best performance of this catalysts.Therefore,we suggest that the partial oxidation of cyclohexane pathway may be as a reversible V 4+/V 5+redox mechanism as illustrated in Fig.7.

The promotion of Co increase the average oxidation number of vanadium and resulted a larger amount of V 5+species,which are essential to the V 4+/V 5+redox mechanism,will be reversible.3.7.Catalyst recycling

In a recycling study conducted after each experiment,the catalyst was separated from the reaction mixture by ?ltration,washed with solvent,and dried carefully in order to reuse the catalyst and study its lifetime and stability.

The catalyst VPO-Co(0.1)was recycled ?ve times.The results are shown in Fig.8.The VPO-Co(0.1)catalyst maintained sustained

V 5+

V 4+H 3C

C OH CH 3

CH 3

(TBA)

O

Fig.7.Reaction pathway of partial oxidation of cyclohexane with TBHP over the VPO-Co catalyst.

Please cite this article as:V.Mahdavi,H.R.Hasheminasab,Liquid-phase e?cient oxidation of cyclohexane over cobalt promoted VPO catalyst using tert -butylhydroperoxide,Journal of the Taiwan Institute of Chemical Engineers (2015),http://www.77cn.com.cn/10.1016/j.jtice.2015.01.020

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activity even after being used for ?ve cycles,and the conversion (%)of cyclohexane was only slightly decreased.Therefore,the VPO-Co(0.1)catalyst shows great promise as a heterogeneous catalyst.

4.Conclusions

The VPO and VPO promoted catalysts with different cobalt load-ings were synthesized,characterized,and for the ?rst time,used for the oxidation of cyclohexane with TBHP in the liquid phase.The VPO catalyst was contained crystalline form of vanadyl pyrophosphate ((VO)2P 2O 7)and also a small amount of β-VOPO 4phase.However,Co promoted VPO gave poorer crystallinity compared to the unpro-moted catalysts.CoPO 4phase also observed in the VPO-Co series.

The promotion of VPO catalyst by Co element slightly increased the amount of oxygen species removed from V 5+phase,decreased the reduction temperature of lattice oxygen and may inhibit the forma-tion of crystal phases with higher surface area.The vanadium in VPO catalyst is predominantly in V 4+and V 5+states.Co was found to in-crease the average oxidation number of vanadium due to an addition of V 5+oxidation state.

In addition,the catalyst can be recycled several times and a nearly identical conversion percentage of the recovered catalyst,suggests its reusability and stability.Also,alkyl peroxides prove to be very e?cient and environmentally friendly oxidants since the by-products are only alkyl alcohols.Therefore this catalysis system is very active and suitable for the oxidation of cyclohexane.

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