Aerobic oxidation of alcohols over novel crystalline Mo–V–O oxide

Feng Wang a,*, Wataru Ueda b,*a CREST, Japan Science and Technology Corporation (CREST-JST), Kita 21, Nishi 8, Kawaguchi, Saitama 332-0012, Japanb Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan

Applied Catalysis A: General 346 (2008) 155–163

A R T I C L E I N F O

Article history:

Received 12 February 2008

Received in revised form 8 May 2008

Accepted 14 May 2008

Available online 25 May 2008

Keywords:

Mo–V–O crystalline

Alcohol oxidation

Oxygen

Heterogeneous catalysis

Liquid phase

A B S T R A C T

Novel crystalline Mo–V–O oxide was employed as the catalyst in the aerobic oxidation of alcohols to the

corresponding carbonyl compounds. Reactions were mainly conducted at 353 K in pure oxygen or air

(1 atm). The selectivities for benzaldehydes were more than 95% in all cases. The conversions of benzyl

alcohols varied from 10% to 99% depending on the substituent. A Hammett plot gave a moderate r-value

of �0.249 (r2 = 0.98), suggesting that the reaction processes may involve hydride abstraction. The

oxidation of primary alkanols afforded aldehydes, and secondary alcohols were mainly dehydrated to

olefins. It was found that the conversion of linear alkanols decreased with the length of alkanols. Kinetic

analysis showed that catalytic reaction rate was first-order dependent on the concentrations of substrate

and of catalyst. The apparent activation energy was estimated to be 45.7 kJ mol�1. Catalytic reactions

took place on the 6- or 7-member rings on the a–b basal plane, where highly dense unsaturated metal

cation centers and oxygen anion might serve as catalytic active sites.

� 2008 Elsevier B.V. All rights reserved.

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Applied Catalysis A: General

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1. Introduction

Mo–V–O crystalline-based catalysts for selective oxidation ofalkanes have attracted much attention [1–3]. For example, the Mo–V–Nb–Te–O system has been reported as a promising candidate forreplacing of propene feeds with propane for selective oxidationand ammoxidation processes [4]. Our group has developed amethod of preparing crystalline Mo–V–O oxide with pureorthorhombic phase. The scheme of the crystalline material isshown in Fig. 1. The crystal [Pbam (55), lattice constants:a = 21.19 A, b = 26.57 A and c = 4.00 A] forms rods by anisotropicgrowth along the c-axis direction. The interval layer distance isaround 4 A. Our analysis has shown that the crystal consists of 6- or7-member rings (6MRs and 7MRs) on the a–b basal plane; fiveMoO6 octahedrons surround one MoO7 pentagonal bi-pyramid byedge-sharing; the 6MRs and 7MRs are interconnected with MoO6

and/or VO6 octahedrons by corner-sharing [5]. It has been reportedthat the Mo–V–O crystalline is an excellent catalyst for selectiveoxidation of ethane or propane to carboxylic acid [6,7].

The mechanism of catalytic oxidation of hydrocarbons over theMo–V–O crystalline material is not clear yet. According to themodel proposed by Grasselli et al. [4], the active and selectivesurface sites locate on the a–b plane of the M1 phase, and consist of

* Corresponding authors. Tel.: +81 11 706 9165; fax: +81 11 706 9164.

E-mail addresses: wangfeng@cat.hokudai.ac.jp (F. Wang),

ueda@cat.hokudai.ac.jp (W. Ueda).

0926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.05.021

an assembly of Mo6+/5+/V5+/4+ redox sites. We postulate that theunique ability of the orthorhombic Mo–V–O-based oxide catalystsoriginates from the uniform 6- or 7-member rings on the a–b basalplane [6]. The postulation is based on the results of selectiveoxidation of propane to acrylic acid. Due to the big molecule size ofthe substrate, the reaction may take place on the surface ofheptagonal channels.

Recently we have communicated a result of alcohol oxidationover Mo–V–O crystalline material [8]. The significance of theresearch is that the crystalline Mo–V–O oxide is reported for the firsttime as catalyst for selective oxidation of primary and secondaryalcohols (carbon number is �4) to corresponding carbonylcompounds in liquid phase. Alcohol oxidation was a suitable modelreaction due to versatile alcohol substrates in the cases of geometrichindrance of hydroxyl group, and has been extensively studied overprecious-metal-based catalysts [9–11]. Through the research, wecould clarify the active sites for the oxidation of alcohols. Thecatalytic results show that, in most reactions, more than 99%selectivities for benzaldehydes are achieved over the oxidation ofbenzyl alcohols. Furthermore, we systematically studied thestability of the catalyst in the oxidation in liquid reaction byrecycling, reuse, catalyst separation and ICP-MS and XRD techni-ques. It was confirmed that the structure and composition of thecrystalline material was stable during reaction, and the reaction wasan example of heterogeneous catalysis [12]. Thus the presentresearch combining high selectivity and high stability of nanocrys-talline catalyst is an important green process, without using additiveor precious metal, and with molecular oxygen as oxidant.

Fig. 1. The crystal structure and SEM image of the orthorhombic molybdenum vanadium oxide. (left upper) View of crystal from the c-axis direction; 6- and 7-member rings

(6MR and 7MR) are labeled; (left lower) view of crystal from the a-axis direction.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163156

Herein, we report the optimization of reaction conditions,kinetic analysis and mechanism of selective oxidation of alcohol.And we will provide a full view of the catalytic performance ofalcohol oxidation over Mo–V–O crystalline material.

2. Experimental

2.1. Catalyst preparation

All reagents were analytical grade. They were purchased fromWako Pure Chemical Industries, Ltd. and used without furtherpurification. Distilled water was prepared using Yamato AutostillWG25 (Tokyo, Japan).

Catalyst preparation procedure: VOSO4�nH2O (64.83% wt.)solution (V concentration 0.10 mol L�1, 120 mL) was poured into(NH4)6Mo7O24�4H2O ammonium heptamolybdate tetrahydrate(AHM) solution (Mo concentration 0.42 mol L�1, 120 mL). Theabove mixture was stirred for 10 min under ambient conditionbefore being transferred to a 300-mL Teflon-lined autoclave. Themixture was bubbled with a flow of nitrogen (50 mL min�1) for10 min to deaerate oxygen. The autoclave was then sealed, andplaced in a 175-8C oven for 48 h. The black solid forming on theTeflon liner was filtered out, and the material was dried at 80 8C for24 h. The purification of the dried sample was carried out byadding 1.0 g sample into an oxalic acid solution (0.4 mol L�1,50 mL) to remove the debris occluded in the pores or on externalsurfaces [5]. The solution was magnetically stirred at 60 8C for30 min and filtered; the filtration was washed with 500 mL waterand was dried at 80 8C for 24 h, and then degassed in nitrogen(50 mL min�1) at 400 8C (heating ramp rate 10 8C min�1) for 2 h.The calcined sample was taken out after the oven temperaturedecreased below 150 8C.

Reactant and product concentrations were measured by gaschromatography using a flame ionization detector (ShimazuClassic-5000, 60 m TC WAX column), operated with a heatingprogram: 100 8C for 10 min, ramp 10 8C min�1 to 230 8C (kept for25 min).

2.2. Catalytic test

We used a stirred batch reactor for catalytic tests. The volume ofthe reactor was ca. 15 mL. The gas balloon containing oxygen/air ofca. 1 L was connected with the reactor. It was calculated that lessthan 10% of the total oxygen could be consumed based on 100%

conversion of added alcohols. In a typical reaction, the catalyst andmagnetic stir bar were initially loaded into the reactor. Theoxygen/air provided by the gas tank was inflated into the reactor atroom temperature through a needle connected with a side-mouthsealed with Teflon septum, through which a mixture of alcohol,toluene, and internal standard was injected by a syringe. The totalreaction volume was 1.75 mL adjusted by toluene. The reactor wasplaced into an oil bath that was thermally controlled with constanttemperature during each reaction. Aliquots were collected atintervals. After reaction, the catalyst was filtered out using a 0.2-mm membrane syringe; the filtrate was analyzed using gaschromatography mass spectrometry (GC–MS). 1H NMR wasemployed to analyze acid products. 1H NMR analysis of the crudemixture after reaction involved the following procedures: ca.0.3 mL aliquot of reaction mixture was put into a small vial, andevaporated under 65 8C in a rotation evaporator in vacuum. About1 mL d6-dimethyl sulphoxide (DMSO) solvent was added todissolve the remaining material before spinning (21–24 8C).Trimethylsilane (TMS, 0.1%) was used as reference signal toestablish the zero.

3. Results and discussion

3.1. Influence of oxygen, air and Ar

For the aerobic oxidation of benzyl alcohol to benzaldehyde, theresults were comparable when using oxygen and air as oxygensource at 1 atm (Fig. 2). Conversions of benzyl alcohol were over20% with >99% selectivity to benzaldehyde. The reaction in argonafforded 3% conversion and 16% selectivity to benzaldehyde. Thesmall conversion in argon implied that the catalyst might havesome reducible sites, which acted as oxidants during reaction [13].In the absence of oxygen, the reduced sites cannot be re-oxidizedto the original state.

3.2. Influence of solvent on aerobic oxidation

We optimized a range of polar and apolar solvents in theaerobic oxidation of benzyl alcohol (Table 1). Water as solventwas weakly active and afforded ca. 1% conversion. We observedthat the reaction mixture was separated into two phases: theupper phase contained benzyl alcohol and internal standard andthe lower phase contained water and catalyst, suggesting thatmass transfer became limited [14]. The mass transfer is probably

Fig. 2. Catalytic test in different atmospheres.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163 157

retarded because water may form a film layer on the catalystsurface, and thus the possibility of benzyl alcohol and oxygendiffusing to active sites is reduced [15,16]. To solve the problem,we conducted a reaction of water solvent at 403 K with airpressure of 1.0 MPa. The catalytic conversion of benzyl alcoholwas 38% with 92% selectivity for benzaldehyde, indicating thatthe mass transfer was increased. Water may be a favorablesolvent for water-miscible substrates under elevated tempera-ture and pressure. On the other hand, water may be an unsuitablesolvent for alcohols that contain a high carbon/oxygen atomicratio [11]. For this reason, we did not employ water as solvent inthis research.

All polar solvents, such as acetonitrile and nitromethane,greatly influenced the catalytic performance, typically givinghigher activity and less selectivity to benzaldehyde. The majorbyproducts were acids and esters. It is understandable that theoxide has a polar surface, where the polar substance may form ahydrophilic environment around active centers, and thusprobably retain a polar product, such as benzaldehyde. Undersuch circumstance, the over-oxidation of benzaldehyde maytake place. Apolar solvents, such as toluene, benzene, triflur-omethylbenzene and cyclohexane, afforded similar conversion(17–22%), among which toluene was the best one with >99%selectivity at 22% conversion. It is likely that the catalystremains coated by apolar solvent and that benzyl alcohol andbenzaldehyde diffuse better through apolar solvent thanthrough water. Trifluromethylbenzene was more toxic andexpensive than toluene considering practical use. Interestingly,the solvent-free reaction gave 11% conversion with 95%selectivity to benzaldehyde.

Table 1The aerobic oxidation of benzyl alcohol in different solventsa

Solvent Conv. (%) Sel. (%)

Water 1 >99

Waterb 38 92

Acetonitrile 46 11

Nitromethane 25 80

1,2,3-Trifluromethylbenzene 19 98

Toluene 22 >99

Benzene 18 96

Cyclohexane 17 98

Neat reaction (benzyl alcohol) 11 95

Water (pH 4) 8 75

Water (pH 10) 10 40

a Reaction conditions: benzyl alcohol 0.7 m mol, solvent 1.6 mL, catalyst 0.03 g,

353 K, 24 h, oxygen balloon (1 atm).b In autoclave: 403 K, 24 h, air (1.0 MPa).

All reactions were carried out under the approximately neutralconditions (pH 7). The test of adjusting reaction mixture withacid/base to pH 4 (by H2SO4) and 10 (by NaOH), respectively, didnot enhance catalytic performance. The byproducts of ester andether were formed, indicating the character of acid or basehomogeneous catalysis. In contrast, for reactions over gold (e.g.the works by Hutchings and coworkers [17–19]) or palladiumcatalysts [11], strong bases, generally sodium hydroxide, areindispensible.

3.3. The time-on-stream profile of aerobic oxidation benzyl alcohol

The catalytic oxidation of benzyl alcohol was tracked byinterval sampling and the results are depicted in Fig. 3. Thecatalytic conversion in the absence of catalyst realized ca. 3%conversion after 35 h, due to autocatalysis. It can be seen that,previous to the 30 h reaction, benzaldehyde was the soloproduct. The conversion of benzyl alcohol sharply increased toca. 10% in 10 h without an induction period, and then slowlyrose to 25%. The profile of conversion versus time showed aninhibition due to products, such as water, which may stronglyadsorb on catalytic sites and thus limit mass transfer. A smallamount of byproducts such as benzyl benzoate and benzylatedtoluene were formed after 30 h reaction. An importantobservation was that no benzoic acid product was formed,suggesting the absence or suppression of free radical-typeautooxidation, since autooxidations are faster for aldehydeversus alcohol [20].

3.4. Catalytic oxidation of substituted benzyl alcohols

The oxidation of substituted benzyl alcohols was examined. Thecatalytic results are shown in Table 2. Among all benzyl alcohols,selectivities for benzaldehydes were over 95%, and did not greatlychange with substituents. On the other hand, the conversion ofbenzyl alcohol was remarkably dependent on substituent. As thesubstituent group varied from electron-withdrawing to electron-donating, the conversion was increased from 10% to 99%. A

Fig. 3. Time-on-stream of selective oxidation of benzyl alcohol over Mo–V–O

crystalline with O2 at 353 K. The symbol ( ) indicates the conversion of benzyl

alcohol in a reaction without catalyst. The symbols (4), (*) and (^) indicate the

selectivity of benzaldehyde, the conversion of benzyl alcohol and the selectivity of

ester and alkylated products in a reaction in the presence of catalyst.

Table 2Catalytic performances of crystalline Mo–V–O oxide in the aerobic oxidation of alcohols by molecular oxygen

Entry Substrate Main product Conv. (%) Sel. (%)

1 22 >99

2 10 98

3 14 97

4 15 97

5 16 >99

6 18 >99

7 18 96

8 23 >99

9 23 98

10 31 >99

11 49 >99

12 80 95

13 83 >99

14 >99 >99

Reaction conditions: substrate 0.7 m mol, toluene 1.6 mL, catalyst 0.03 g, 353 K, 24 h, 1-L oxygen balloon (1 atm). The conversion of alcohol and selectivity of carbonyl

compound were determined by GC with p-xylene (40 mL) as internal standard. The byproducts were analyzed by MS.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163158

complete conversion of 4-methyl benzyl alcohol was obtainedwith >99% selectivity for 4-methylbenzaldehyde, which was thebest result achieved over the catalyst. It should be emphasized thatthe oxidation of alcohols with bulky substituents, like phenethyl(Table 2, entry 2), benzyl (entry 3) and phenyl (entry 4), were alsoachieved in excellent selectivity with 10–15% conversion. Inciden-

tally, there was no report on entry 4 via catalysis reaction, exceptone example using CrO3/H2SO4 as oxidant [21]; thus entry 2represents the first example of a one-step synthesis of o-phenethylbenzaldehyde by oxidizing o-phenethyl benzyl alcohol. It wasnoted that 4-hydroxy benzyl alcohol was not oxidized overpolyoxometalate (POM) catalysts [22,23]. However, the same

Fig. 4. Hammett plot of catalytic oxidation of para-substituted benzyl alcohols.

Fig. 5. The reaction rates of alkanols (mol alcohol g�1 cat h�1) as a function of the

carbon numbers of alkanols.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163 159

oxidation using the crystalline Mo–V–O as catalyst afforded theconversion of 83% (entry 13) with selectivity of >99%. Althoughsome POM catalysts contain Mo and V elements [24], the Mo–V–Ocatalyst is completely different from POM catalysts in structuralassembly as it is a rigid oxide crystal with 3D frameworks, and hasmicro-pores. The crystalline Mo–V–O oxide can survive purifica-tion with a 0.4-mol L�1 oxalic acid solution (pH ca. 0.9) at 333 K. Incontrast, POMs have discrete structures, most of which are stablein the 3 � pH � 6.5 range, and thus tend to degrade by hydrolysisreaction [25].

We conducted competitive oxidation of p-substituted benzylalcohols in two pots, each of which included Hammett substituentconstant sþp � 0 and�0, respectively, which were referenced fromreports that considered resonance effect [26]. The shorter time of4 h at higher temperature of 383 K was chosen to avoid sidereactions. As depicted in Fig. 4, a plot of Hammett sþp against log kX/kH gave a moderate r-value of �0.249 (r2 = 0.98), suggesting thatthe reaction processes may involve hydride abstraction [27]. Thenoteworthy steps in the formation of benzaldehyde are postulatedas follows: (i) formation of surface benzyl species by activating

Table 3Catalytic performances of crystalline Mo–V–O oxide in the aerobic oxidation of alcoho

Entry Substrate Product

1

2

3

4

5

6

7

8

Reaction conditions: substrate 0.7 m mol, toluene 1.6 mL, catalyst 0.03 g, 353 K, 24 h,

compound were determined by GC with p-xylene (40 mL) as internal standard. The by

O–H bond; (ii) hydride abstraction and formation of benzaldehyde;(iii) elimination of hydride by oxygen.

3.5. Catalytic oxidation of alkanols

Distinctive product distribution was observed in the oxidationof various alkanols in terms of alcohol chain length and theposition of hydroxy groups (Table 3). The major products ofprimary alcohols were aldehydes, with selectivities more than 90%,and the remaining were olefins. The conversion of primary alcoholsgreatly depended on the chain length. For instance, from alcoholsof C4 to C8, the conversion decreased from 33% to 1%. The plot ofcatalytic reaction rate against carbon number resulted in adecreasing line (Fig. 5), suggesting that a steric hindrance by thetail CH3(CH2)n (2 < n < 6) chain played a key role. On the otherhand, secondary alcohols were mainly dehydrated to olefins withhigher conversion than their primary alcohol counterparts. Theselectivities of aldehydes were decreased to 4–29%. We attributethe great difference of product distribution to the adsorption-activation model. It has been reported that the a-methyl group ofC–OH exerts a steric effect on the adsorption of methylcyclohex-anols [28]. The same effect may occur with secondary alcohols,

ls by molecular oxygen

Conv. (%) Sel. (%)

33 96

19 94

11 95

1 91

92 4

22 22

40 20

30 29

1-L oxygen balloon (1 atm). The conversion of alcohol and selectivity of carbonyl

products were analyzed by MS.

Fig. 6. The adsorption model of primary alcohol (1-hexanol) and secondary alcohol (2-hexanol) on the pore area of (0 0 1) basal plane. The size of pore is ca. 0.4 nm. (A) Vertical

adsorption on the pore. (B) Parallel to pore due to steric hindrance.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163160

which may adsorb on the tops of pores in a parallel way. Due to thelack of steric hindrance, the primary alcohol may vertically adsorbon the tops of pores. Fig. 6 depicts the adsorption model takinghexanols as examples. Such an adsorption model would determinereachable C–H and O–H bonds activated on active sites. Although itwas unclear why the pore area could capture and activate thesubstrate, the unsaturated metal ion sites (Mo5+/6+ and V5+/4+) andthe electron-rich oxygen anions (O�) around the vicinity of poresmay provide geometrically suitable sites for activating hydro-carbons.

3.6. Kinetic analysis

The catalytic oxidation of benzyl alcohol mainly gave benzal-dehyde. The kinetic investigations of this reaction were carried outto establish the dependence of the rate of oxidation on thevariations in the concentrations of benzyl alcohol, catalyst andreaction temperature. The total volume of the reaction mixture inall kinetic experiments was 1.75 mL. The concentrations of benzylalcohol and catalyst and the oxygen pressure were constantthrough the reaction. Rates of oxidation of benzyl alcohol werecalculated from the plots of moles of benzyl alcohol consumedversus time (ks) using the initial rate approach concept and thekinetics were interpreted.

Fig. 7. Effect of benzyl alcohol concentration: 0.40, 0.46, 0.63 and 0.81 mol L�1, respec

reaction volume 1.75 mL. (A) The concentration of benzyl alcohol against time in kiloseco

alcohol (mol L�1).

3.6.1. Effect of benzyl alcohol concentration

Experiments on oxidation of benzyl alcohol were carried out at353 K by varying the concentrations of benzyl alcohol (0.40–0.81 mol L�1) while keeping the concentration of catalyst(17.4 g L�1) constant. The reaction rate for each concentrationwas determined by the slope of the lines in Fig. 7(A), showing theconcentration of benzyl alcohol versus time. The slope had the unitof mol L�1 ks�1. The reaction rate was thus obtained by multi-plying the slope value with 0.00175 L, which was the volume of thereaction. The effect of substrate concentration on the rate ofoxidation is shown in Fig. 7(B), as a plot of rate versusconcentration of benzyl alcohol. It was found that the rate ofoxidation showed first-order dependence with respect to substrateconcentration.

3.6.2. Effect of catalyst concentration

The catalyst concentration was varied between 2.9 and69.6 g L�1 at benzyl alcohol concentration of 0.40 mol L�1, oxygen1 atm, and temperature 353 K. The influence of the catalystconcentration upon the initial reaction rate is shown in Fig. 8. Thecatalytic rates increased with catalyst concentration. The reactionwas first-order dependent on catalyst concentration. As thepartial pressure of oxygen did not change during the reaction, theconcentration of oxygen in toluene (at 313 K, the mole fraction

tively. Concentration of catalyst 17.1 g L�1, O2 pressure 1 atm, temperature 353 K,

nd. (B) The plot of reaction rate (mol ks�1) against the initial concentration of benzyl

Fig. 8. Effect of catalyst concentration: 2.9, 5.8, 17.4, 34.8 and 69.6 g L�1, respectively, benzyl alcohol concentration 0.40 mol L�1, oxygen 1 atm, and temperature 353 K. (A)

Time-on-stream plot. (B) ln(1 � X) versus time (unit: kilo second). (C) The plot of k1cat versus catalyst concentration (g L�1). The slope value of k2cat is 0.0061 L ks�1 g�1.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163 161

solubility of oxygen in toluene at 1 atm partial pressure of gas is9.6 � 104. No data was available at 353–383 K is considered to beconstant [29]. The reaction showed zero-order dependence onoxygen, which was also supported by the negligible difference inresults between employing air and employing pure oxygen asoxygen source. Thus, the oxygen effect could be simplified asconstant. The rate was obtained from the integrated form of therate expression ln(1 � X) = k[cat][O][A]t = k2cat[cat]t = k1catt,where [X] is the conversion of benzyl alcohol, [cat] is theconcentration of catalyst (g L�1), [O] is the concentration ofoxidant, and [A] is the concentration of benzyl alcohol. A plotof ln(1 � X) versus t resulted in several straight lines, the slopes ofwhich, k1cat, are 1.2, 2.2, 6.7, 15.8, and 25.8 ks�1, respectively. Thek1cat versus [cat] gave a straight line, the slope of which, k2cat, was0.0061 L ks�1 g�1.

3.6.3. Effect of reaction temperature

The effect of reaction temperature on the rate of oxidationof benzyl alcohol was studied by varying the temperaturebetween 353 and 383 K. The other parameters were keptconstant: benzyl alcohol 0.40 mol L�1, oxygen 1 atm, and thecatalyst 17.1 g L�1.

In view of the linearity of the conversion data when plotted asln(1� X) (Fig. 9B), we use a rate law given by r = k[cat][O][A]. Therate was obtained from the integrated form of the rate expressionln(1� X) = k[cat][O][A]t = k2Temp.[A]t = k1Temp.t. A graph of ln(1� X)versus time has a slope of k1Temp., which is 6.71� 10�3, 1.19� 10�2,

and 2.48� 10�2 ks�1 for 353, 365 and 383 K, respectively. Therate equation was simplified as �d[A] = k[cat][O][A]dt = kobs[A]dt.The kobs values at 353, 365 and 383 K based on data of Fig. 9(A)were calculated to be 2.50� 10�6, 4.23� 10�6 and 8.45� 10�6

mol�1 L�1 ks�1, respectively. From the Arrhenius plot of ln k versus1/T shown in Fig. 9(C), the activation energy Ea was evaluated to be45.7 kJ mol�1, which was similar to the reported values of 45.8 kJmol�1 [30], 45.4 kJ mol�1 [31] and 43.5 kJ mol�1[32].

3.7. Mechanism consideration

We attributed product distributions to the different ways ofadsorption of alcohols on catalytic active sites, typically 6- or 7-member rings, which occupy the main part of the a–b basalplane. The window size of 6- or 7-member rings is around 4 A.The alcohols used generally have large molecule size, up to 5–10 A. Therefore, it is impossible for substrate molecules, such asbenzyl alcohol, to reach deep pores. However, hydroxyl group ofprimary alcohol RCH2OH is in the approximate dimensions ofthe pore window, and could adsorb onto 6- or 7-member ringsin a perpendicular direction. Thus, the abstraction of H from aC–H bond by (Mo–O–V) sites would lead to aldehyde RCHO. Onthe other hand, the adsorption of secondary alcohol isgeometrically hindered due to the bending of C–C bonds atthe neighborhood of the hydroxy group. As part of the externalsurface has acid sites, the dehydration of secondary alcohol toolefin is believed to take place. The detailed reaction mechanism

Fig. 9. Effect of reaction temperature: 353, 365 and 383 K, respectively. Benzyl alcohol 0.40 mol L�1, oxygen 1 atm, and the catalyst 17.1 g L�1. (A) The concentration of benzyl

alcohol as a function of reaction time using Mo–V–O as catalyst at 353, 365 and 383 K. Lines are fit using the first–order law. (B) A linear dependence of ln(1 � X) on time, and

the pseudo-first-order rate constants at three temperatures. (C) Arrhenius plot. The activation energy Ea was 45.7 kJ mol�1.

F. Wang, W. Ueda / Applied Catalysis A: General 346 (2008) 155–163162

and the function of each metal in the reaction need furtherinvestigation.

4. Conclusions

We have demonstrated that novel crystalline Mo–V–O oxide isan efficient catalyst for the aerobic oxidation of alcohols. Theoxidant source can be pure oxygen or air. All polar solvents gavehigher activity and less selectivity to desired aldehydes. Thebyproducts were acids and esters. Toluene was the best solvent,with >99% selectivity for benzaldehyde at the 22% conversion ofbenzyl alcohol. An important observation was that no benzoic acidproduct was observed, suggesting the absence or suppression offree radical type auto-oxidation. The selectivities for benzalde-hydes were more than 95% and did not change greatly withsubstituents. The conversions of benzyl alcohols were remarkablydependent on substrates. As the substituent varied from electron-withdrawing to electron-donating, the conversion was increasedfrom 10% to 99%. Distinctive differences of product distributionwere observed in the oxidation of various alkanols in terms of thechain length and of the position of the hydroxy group. The majorproducts of primary alcohols were aldehydes, with the selectivitiesmore than 90%, and the remaining were olefins. It was found thatthe rate of oxidation of benzyl alcohol was first-order dependentwith respect to substrate concentration and catalyst concentration.We postulate that the oxidation reaction takes place on the tops of6- or 7-member rings, where highly dense unsaturated metalcation centers and oxygen anions may function as catalytic activesites.

Acknowledgement

This work was supported by CREST-JST (Japan Science andTechnology Corporation).

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