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Journal of Molecular Catalysis A: Chemical 370 (2013) 104– 110

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Catalysis A: Chemical

j o ur nal ho me p age: www.elsev ier .com/ locate /molcata

olybdena–vanadia supported on alumina: Effective catalysts for thesterification reaction of acetic acid with n-butanol

heorghita Mitran, Octavian-Dumitru Pavel, Ioan-Cezar Marcu ∗

aboratory of Chemical Technology and Catalysis, Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry,niversity of Bucharest, 4-12, Boulevard Regina Elisabeta, 030018 Bucharest, Romania

r t i c l e i n f o

rticle history:eceived 4 October 2012eceived in revised form2 December 2012ccepted 8 January 2013vailable online 19 January 2013

a b s t r a c t

The paper describes the preparation of alumina-supported molybdena–vanadia catalysts, their structuraland textural characterization using XRD, N2 adsorption, UV–vis and NH3-TPD techniques as well as theircatalytic properties in the esterification reaction of acetic acid with n-butanol. The effects of esterificationconditions including reaction time, catalyst loading and acid-to-alcohol mole ratio and of reactant pre-adsorption on the conversion were investigated. The catalytic activity correlated well with the number ofstrong acid sites which increased by increasing the vanadia content. In optimized conditions, conversions

eywords:sterificationcetic acid-Butanol-Butyl acetateupported molybdena–vanadia catalysts

higher than 85% with 100% selectivity for n-butyl acetate can be obtained. Reactant pre-adsorption exper-iments suggested that the reaction follows the Langmuir–Hinshelwood mechanism. A good reusabilityof the catalysts after three reaction cycles was observed. A local interaction between molybdenum andvanadium on the catalyst surface has been evidenced.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Nowadays, there is a great interest in the esterification reactionecause organic esters are largely used in the production of plasticerivatives, perfumery, agro-chemistry and other branches of finehemistry [1].

The n-butyl acetate is an important chemical having extensivepplications in the industry being used in large quantities as a sol-ent in the lacquer industry and coating manufacture, extractantnd dehydrator [1–3]. Additionally, it is able to replace the toxic anderatogenic ethoxy ethyl acetate, which is often used as a solvent4]. The n-butyl acetate is commonly synthesized by esterificationf acetic acid with n-butanol in the presence of strong acid catalysts,hich is the most viable route for producing this value added prod-ct. Esterification being an equilibrium reaction, an excess of oneeactant and/or continuously removing of water by adsorption onrying agents or by co-distillation with entrainers such as benzener toluene can be used to enhance the performance of the reaction3,5].

In last decade, numerous solid acid systems were used insterification reactions, including classical solid acids such as ion-xchange resins [6–8], zeolites [9,10], superacids like sulfated

∗ Corresponding author.E-mail addresses: ioancezar.marcu@g.unibuc.ro, ioancezar marcu@yahoo.com

I.-C. Marcu).

381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.molcata.2013.01.001

zirconia [11,12], heteropoly acids (HPA) having the Keggin-typestructure [13–16] but also oxides of elements with valence fiveor higher [17–19] and supported chlorides [20]. The commerciallyavailable solid acid catalysts have recently been comparativelystudied in the esterification of acetic acid with butanol [21].

It is well known [22] that among the oxides of elements withvalence five or higher, molybdena and vanadia present strong tovery strong Brønsted acidity which make them good candidatesas acid catalysts. The high surface area and good thermal stabilityof alumina could put together with their surface acidity lead-ing to excellent solid acid catalysts. Indeed, it has recently beenshown that alumina-supported molybdena and vanadia were ableto catalyze the esterification reaction of acetic acid with n-butanolshowing conversions of 81 and 63%, respectively, and 100% selec-tivity for n-butyl acetate at 100 ◦C and 2 h reaction time [23,24].Nevertheless, while MoO3/Al2O3 was stable during the catalyticreaction, a loss of the catalytic activity of the V2O5/Al2O3 wasobserved due to the leaching of vanadia from the alumina support.It is expected that the addition of MoO3 to V2O5/Al2O3 will increaseits surface acidity [25] and, consequently, its catalytic activity in theesterification reaction. Moreover, Mo and V could interact on thesupport surface [26,27] resulting in an increased catalyst stability.Hence, it is interesting to investigate MoO3–V2O5/Al2O3 catalysts

for the esterification reaction. To the best of our knowledge, thereare no studies investigating alumina-supported MoO3–V2O5 mixedoxides as catalysts for the esterification reaction in general and forthe esterification of acetic acid with n-butanol in particular.

Catalysis A: Chemical 370 (2013) 104– 110 105

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G. Mitran et al. / Journal of Molecular

In the present contribution, we prepared �-alumina-supportedoO3–V2O5 catalysts with different loadings and Mo/V ratios

nd characterized them by several techniques, such as XRD,2 physisorption, NH3-TPD and UV–vis diffuse reflectance spec-

roscopy. Their catalytic activity was examined by carrying out thesterification reaction of acetic acid with n-butanol under suitableonditions. Further, various reaction parameters such as effect ofeaction time, temperature, molar ratio of the reactants were eval-ated to optimize the reaction conditions.

. Experimental

.1. Catalysts preparation

Al2O3 support was prepared from Al(NO3)3·9H2O (Fluka Ana-ytical) by precipitation with ammonium carbonate (Lachema)t controlled pH of 6.5. Four different MoO3–V2O5/Al2O3 cata-ysts with two MoO3–V2O5 loadings, i.e. 5 and 10 wt%, and two

oO3/V2O5 weight ratios, i.e. 9/1 and 1/9, were prepared using ane-step procedure. Thus, the two components were impregnatedimultaneously by the incipient wetness method with an aqueousNH4)6Mo7O24·4H2O (Fluka Analytical) and NH4VO3 (Fluka Ana-ytical) solution containing appropriate amounts of molybdenumnd vanadium. After impregnation, the samples were dried in airt 100 ◦C and then calcined at 600 ◦C for 4 h. They were denotedMo9V1Al, 5Mo1V9Al, 10Mo9V1Al and 10Mo1V9Al, respectively.

.2. Catalysts characterization

The crystalline phases were investigated by the X-ray diffrac-ion (XRD) method. XRD patterns were obtained with a Philips PW710 type diffractometer equipped with a Cu K� source (� = 1.54 A),perating at 50 kV and 40 mA. They were recorded over the 10–70◦

ngular range with 0.02◦ (2�) steps and an acquisition time of 1 ser point.

Surface areas of the catalysts were measured from the adsorp-ion isotherms of nitrogen at −196 ◦C using the BET method with

Micromeritics ASAP 2020 sorptometer. Samples were out-gassedt 300 ◦C for 4 h in the degas port of the adsorption apparatus. Theore size distribution curves were calculated using the desorptionranch of the isotherms with the Barrett–Joyner–Halenda (BJH)ethod.The acidity of the catalysts was estimated by temperature-

rogrammed desorption of ammonia (NH3-TPD). About 0.1 g of theatalyst sample was dehydrated at 500 ◦C in dry air for 1 h andurged with N2 for 0.5 h. The sample was then cooled down to00 ◦C under the flow of N2, and NH3 was supplied to the samplentil its saturation. For desorption of the physisorbed ammonia, aitrogen stream was passed over the sample, at the same temper-ture, until no more NH3 was observed in the exit flow. Finally, thehemisorbed NH3 was desorbed in a N2 flow by increasing the tem-erature successively up to 350 ◦C and 500 ◦C with a heating rate of0 ◦C/min. The ammonia desorbed was bubbled through a solutionf sulfuric acid. The acid in excess was titrated with a solution ofaOH, the amount of ammonia desorbed being then calculated. Themmonia desorbed at temperatures lower than 350 ◦C accountedor the weak and medium-strength acid sites while that desorbedn the temperature range from 350 to 500 ◦C, for the strong acidites.

Diffuse reflectance UV–vis spectra were recorded in the range

00–650 nm, using Spectralon as a standard, in a Jasco V 670 spec-rophotometer. The obtained reflectance spectra were convertednto the dependencies of Kubelka–Munk function on the absorptionnergy.

supported MoO3–V2O5 catalysts: (a) the alumina support, (b) 5Mo9V1Al, (c)5Mo1V9Al, (d) 10Mo9V1Al, (e) 10Mo1V9Al, (f) 5Mo9V1Al used in three reactioncycles, and (g) 10Mo1V9Al used in three reaction cycles.

2.3. Catalytic test

The esterification reactions of acetic acid (Chimactiv, 99.5%)with n-butanol (Riedel-de Haën, 99.5%) were performed in a 150 mLtwo-neck flask equipped with a condenser and an additional portfor sample withdrawal. The above assembly was heated using athermostated hotplate. The reaction was carried out at 100 ◦C with amolar quantity of acetic acid of 0.09 and an n-butanol-to-acetic acidmolar ratio varied from 1 to 3. Cyclohexane (Riedel-de Haën, 99.5%)was always added to the reaction mixture for water removal, thecyclohexane-to-acetic acid molar ratio being kept at 1. The amountof catalyst was varied between 0.5 and 1.3% of the mass of mix-ture charge in the reaction. All the experiments were conductedat a speed of agitation of 600 rpm to avoid diffusional limitationsas reported elsewhere [23,24]. All the catalysts used in the reac-tion were in the powder form. Pre-adsorption experiments wereperformed by premixing the catalyst with one of the reactants orboth at room temperature for 24 h followed by heating to 100 ◦Cand charging the preheated remaining reactant. Samples from theorganic layer were withdrawn at regular intervals and analyzedwith a Thermo Finnigan chromatograph using a DB-5 column anda flame ionization detector. Under the employed conditions ofreaction butyl acetate was the only product detected. The massbalances, calculated after a reaction time of 120 min, were alwayshigher than 95%.

3. Results and discussion

3.1. Characterization of the catalysts

The X-ray diffraction measurements (Fig. 1) showed that theMoOx–VOx species are in all cases well dispersed on the support

106 G. Mitran et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 104– 110

Table 1Physico-chemical properties of the catalysts.

Catalyst BET surface area (m2/g) Pore volume (cm3/g) Number of acid sites (mmol/g)

Weak and medium Strong Total number

Al2O3 227 0.38 0.097 0.027 0.125Mo9V1Al 220 0.39 0.066 0.184 0.2510Mo9V1Al 204 0.37 0.272 0.218 0.49

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one hand, the isolated and polymerized octahedral VO6 speciesdecrease then disappear by decreasing the vanadia content from10 wt% in V2O5/Al2O3 sample to 0.5 wt% in 5Mo9V1Al sample [35]and, on the other hand, an interaction between molybdenum and

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s only broad lines corresponding to �-alumina (PDF 10-425) werebserved. Neither V2O5 nor MoO3 crystallites were detected at anyoOx–VOx loading on alumina. No major changes could be noted

n the XRD patterns of the 5Mo9V1Al and 10Mo1V9Al catalysts afterhree reaction cycles (Fig. 1).

In general, the specific surface area and pore size of the cata-yst have significant influence on its catalytic activity. The surfacereas and pore volumes of the supported catalysts and of thelumina support were given in Table 1. After impregnation oflumina with MoO3–V2O5 and by increasing the catalyst loadingnd V2O5 content, the surface area of the samples decreased. Theeduction in surface area has already been observed for alumina-upported molybdena–vanadia [28] and may be due to the blockagef pores by MoOx–VOx species. Nevertheless, the supported sam-les possess high surface areas due to the dispersion effect oforous carrier. All the catalysts displayed typical type IV nitrogendsorption/desorption isotherms (according to IUPAC classifica-ion) with a clear hysteresis loop at the relative pressure of 0.5–0.85,haracteristic of mesoporous materials with uniform cylindricalores [29], as exemplified in Fig. 2a for 5Mo9V1Al and 10Mo1V9Alatalysts. The catalysts show uniform and narrow pore size dis-ributions with average peak pore diameters in the range 5–9 nmFig. 2b) and, therefore, they should be readily accessible to theeactants. No correlation can be made between the average poreiameter and the catalyst loading.

Surface acidity is the most important function of the catalystssed in the esterification reaction. Therefore, the total acidities ofhe catalysts, expressed as the total number of acid sites per gramf catalyst, have been determined by NH3-TPD and are presentedn Table 1. It can be observed that the total acidity increased bydding MoO3–V2O5 to alumina and by increasing the MoO3–V2O5oading and that at similar MoO3–V2O5 loadings, it was higheror the samples with higher vanadia content, following therder: Al2O3 < 5Mo9V1Al < 5Mo1V9Al < 10Mo9V1Al < 10Mo1V9Al.evertheless, the number of strong acid sites increased con-

inuously with the vanadia content following the order:l2O3 < 5Mo9V1Al < 10Mo9V1Al < 5Mo1V9Al < 10Mo1V9Al. Thisuggests that the strong acid sites are mainly associated to theresence of vanadia.

The UV–vis diffuse reflectance spectra of the catalysts are com-aratively shown in Fig. 3. V2O5/Al2O3 and MoO3/Al2O3 with0 wt% metal oxide loading were used as reference samples. Thepectrum of V2O5/Al2O3 sample shows five main absorption bandst about 220, 260, 340, 380 and 480 nm. The 220 and 260 nmands can be assigned to isolated tetrahedrally coordinated V5+

pecies, while the 340 and 380 nm bands can be attributed to iso-ated octahedrally coordinated V5+ species. The 480 nm band cane associated to octahedral VO6 chains from well dispersed or crys-alline V2O5 clusters [30,31]. In the V2O5/Al2O3 sample crystalline2O5 has been evidenced [24]. The spectrum of MoO3/Al2O3 samplehows three main absorption bands at about 220, 260 and 300 nm.

he 220 nm and 260 nm bands can be attributed to the tetrahe-ral molybdate species, whereas the 300 nm band can be assignedo the octahedral molybdate species [32–34]. By adding molyb-ena and decreasing the vanadia content in the V2O5–MoO3/Al2O3

0.068 0.302 0.370.079 0.461 0.54

samples, the 480 and 380 nm bands disappear while the 340 nmband fall in the range 330–310 nm. These results suggest that, on

Pore d iame ter (nm)

Fig. 2. N2 adsorption–desorption isotherms (a) and pore size distribution curves (b)of 5Mo9V1Al (�) and 5Mo1V9Al (©) catalysts.

G. Mitran et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 104– 110 107

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Fig. 4. Conversion of acetic acid as a function of the reaction time: (a) on the differentcatalysts with a n-butanol-to-acetic acid mole ratio 2:1, 0.7 wt% catalyst and reaction

3.2.2. Effect of reactant pre-adsorption

f the V2O5/Al2O3 and MoO3/Al2O3 reference materials: (a) MoO3/Al2O3, (b)Mo9V1Al, (c) 10Mo9V1Al, (d) 5Mo1V9Al, (e) 10Mo1V9Al, and (f) V2O5/Al2O3.

anadium takes place with characteristic bands at 310–330 nm aslready shown by Volta and coworkers [27].

.2. Catalytic activity

The effects of esterification conditions including reaction time,atalyst loading and acid-to-alcohol mole ratio and of reactantre-adsorption on the reaction conversion were investigated, theesults obtained being presented below.

.2.1. Effect of reaction timeThe influence of the reaction time was studied in the following

eaction conditions: n-butanol-to-acetic acid mole ratio 2:1, theatalyst representing 0.7 wt% of the mass of mixture charge inhe reaction and the reaction temperature being kept at 100 ◦C.n these conditions, the selectivity of n-butyl acetate being, in allases, 100%, the conversion of acetic acid can represent the yieldf n-butyl acetate. The results obtained are shown in Fig. 4a. Asxpected, for all the catalysts and support a gradual rise in theonversion with increasing in duration of the reaction period cane observed. The activity of the different catalysts and the supportt 120 min reaction time ranged as follows (the conversion of aceticcid in parenthesis): Al2O3 (41%) < 5Mo9V1Al (46%) < 10Mo9V1Al62.5%) < 5Mo1V9Al (68.3%) < 10Mo1V9Al (71%). This order isimilar to that corresponding to the number of strong acid sitesuggesting that they play a key role in the esterification reactionf acetic acid with n-butanol. The conversion values observed forhe best catalysts in the series studied are comparable with thoseeported for the same reaction performed with commerciallyvailable catalysts [21]. It is noteworthy that, by adding low quan-ities of molybdena to �-alumina-supported vanadia, the catalytic

ctivity was improved as the acetic acid conversions for 5Mo1V9Alnd 10Mo1V9Al catalysts were higher compared with V2O5/Al2O3ith similar vanadia contents, i.e. 5 and 10 wt%, tested in similar

temperature 100 ◦C (× – alumina support; © – 5Mo9V1Al; � – 10Mo9V1Al; � –5Mo1V9Al; � – 10Mo1V9Al) and (b) on the 10Mo9V1Al catalyst with a n-butanol-to-acetic acid mole ratio 3:1, 1 wt% catalyst and reaction temperature 100 ◦C.

reaction conditions, for which the acetic acid conversions were50.3 and 66.4%, respectively [24].

For the 10Mo9V1Al catalyst in optimized conditions, i.e. n-butanol-to-acetic acid mole ratio 3:1, the catalyst representing1 wt% of the mass of mixture charge in the reaction and the reac-tion temperature being kept at 100 ◦C, the reaction time wasincreased until reaching constant conversion (Fig. 4b). The con-version reached a maximum of ca. 85% with 100% selectivity ofn-butyl acetate after 150 min of reaction, and then it remainedalmost unchanged. This suggests that 150 min is the optimum reac-tion time. Note that it was lower than that reported for othersupported catalysts, such as supported silicotungstic acid [15], sup-ported dodecatungstophosphoric acid [16] and supported niobia[17], but comparable with that reported for the commercially avail-able ion-exchange resins [21] tested in the same reaction.

Two types of mechanisms can be considered for the esterifica-tion reaction over solid acid catalysts: one involving the surfacereaction between a chemisorbed species and a species from the

1 Catalysis A: Chemical 370 (2013) 104– 110

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08 G. Mitran et al. / Journal of Molecular

iquid phase, known as Eley–Rideal type mechanism, and otherequiring the chemisorption of both acid and alcohol, known asangmuir–Hinshelwood type mechanism. Pre-adsorption of alco-ol, acid or both alcohol and acid on the solid catalyst beforehe esterification reaction can give information about the mecha-ism involved [36]. Thus, the effect of reactant pre-adsorption haseen studied with the 5Mo9V1Al catalyst for a n-butanol-to-aceticcid mole ratio of 1:1 and with 0.7 wt% catalyst. It has been pre-ixed with acetic acid, n-butanol or both acetic acid and n-butanol

or 24 h at room temperature and then heated until reaching theeaction temperature, i.e. 100 ◦C. At this moment the preheatedemaining reactant was added. The obtained conversion versusime curves are shown in Fig. 5 where the curve correspondingo the reaction without premixing of reactants is also presentedor comparison. It can be observed that the highest conversionalues where obtained when premixing the catalyst with the mix-ure of acetic acid and n-butanol. The conversion of acetic acidecreased following the order: premixing with both acetic acid and-butanol > no premixing > premixing with acetic acid > premixingith n-butanol. These results suggest that the esterification reac-

ion needs the chemisorption of both acetic acid and n-butanol andnvolves a Langmuir–Hinshelwood type mechanism. This result

as in disagreement with those obtained by Parida et al. for the

Fig. 5. The effect of reactant pre-adsorption on the 5Mo9V1Al catalyst with a n-butanol-to-acetic acid mole ratio of 1:1, 0.7 wt% catalyst and reaction temperature100 ◦C: (©) no pre-adsorption, (�) pre-adsorption of acetic acid, (�) pre-adsorptionof n-butanol, and (*) pre-adsorption of both acetic acid and n-butanol.

ig. 6. Effect of acid-to-alcohol mole ratio on the esterification of acetic acid with n-butanol using (a) 5Mo9V1Al, (b) 10Mo9V1Al, (c) 5Mo1V9Al and (d) 10Mo1V9Al as catalysts.eaction conditions: temperature 100 ◦C, 0.7 wt% catalyst.

G. Mitran et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 104– 110 109

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utanol using 5Mo9V1Al (©), 10Mo9V1Al (•), 5Mo1V9Al (�) and 10Mo1V9Al (�) asatalysts. Reaction conditions: temperature 100 ◦C, acetic acid-to-n-butanol molaratio 1:3 and reaction time 120 min.

sterification of acetic acid with n-butanol catalyzed by silico-ungstic acid supported zirconia [15] or vanadium phosphate [37]nd by Liu et al. [38] for the silica-supported Nafion-catalyzed ester-fication of acetic acid with methanol. However, our result agrees

ith that obtained by Lee et al. [39], Teo and Saha [40] and Akbaynd Altıokka [41] for the esterification of acetic acid with amyllcohol catalyzed by acid resin catalysts and by Miao and Shanks36] for the esterification of acetic acid with methanol over sulfoniccid-functionalized mesoporous silica.

It is noteworthy that the results above also showed that thehemisorption of n-butanol was stronger than that of acetic acids the conversion values obtained by premixing the catalyst with-butanol were lower than those obtained by premixing the cat-lyst with acetic acid. This is in line with other literature resultsroposing a stronger chemisorption of the alcohol than the acid40,42]. Moreover, it can be observed that the pre-adsorption ofither acetic acid or n-butanol inhibited the reaction suggesting aompetitive adsorption of the reactants on the catalyst surface.

.2.3. Effect of initial reactants molar ratioThe initial acid-to-alcohol molar ratio was varied from 1:1 to

:3 for the reaction at 100 ◦C and with 0.7 wt% catalyst, the resultsbtained being showed in Fig. 6. It can be observed that, as expected,or all the catalysts studied and for all the reaction times con-idered, the conversion substantially increased by increasing thenitial acid-to-alcohol mole ratio from 1:1 to 1:2. By further increas-ng the acid-to-alcohol mole ratio to 1:3, the conversion increasedut to a lower extent. This could be explained by a competitivedsorption of acetic acid and n-butanol, with a stronger adsorptionf the alcohol in agreement with the adsorption behavior describedarlier. Note that the selectivity for n-butyl acetate was, in all cases,00% and the conversion of acetic acid reached 77% in the presencef 10Mo1V9Al catalyst for an acetic acid-to-n-butanol mole ratioqual to 1:3 and a reaction time of 120 min.

.2.4. Influence of catalyst amountThe amount of the catalyst was varied from 0.5 to 1.3% by mass of

he total reaction mixture using 5Mo9V1Al, 10Mo9V1Al, 5Mo1V9Alnd 10Mo1V9Al as catalysts, while keeping the n-butanol-to-aceticcid mole ratio at 3:1, the reaction temperature at 100 ◦C and theeaction time at 120 min (Fig. 7). In all cases, the conversion of acetic

Fig. 8. Reusability of the (a) 5Mo9V1Al and (b) 10Mo1V9Al catalysts in the esterifi-cation of acetic acid with n-butanol. Reaction conditions: temperature 100 ◦C, aceticacid-to-n-butanol molar ratio 1:3 and 1 wt% catalyst.

acid increased with increasing the catalyst amount from 0.5 to ca.1% and then it tends to a plateau. This suggests that 1 wt% is theoptimal mass fraction of the catalyst in the reaction medium.

3.2.5. Reusability of the catalystCompared with homogeneous acids, the advantages of solid acid

catalysts are their easy recovery and good reusability. Therefore,it is quite necessary to evaluate the reusability of the alumina-supported molybdena–vanadia catalysts, especially in viewpointof practical application. The 5Mo9V1Al and 10Mo1V9Al catalystswere used for recycling experiments carried out under the follow-ing reaction conditions: acetic acid-to-n-butanol mole ratio was1:3, 1 wt% catalyst, reaction temperature 100 ◦C. In order to regen-erate the catalyst after 2 h of reaction, it was separated by filtration,washed with distilled water several times, dried at 120 ◦C in air andthen used in the esterification reaction with a fresh reaction mix-

ture. For the regenerated 5Mo9V1Al and 10Mo1V9Al catalysts afterthree cycles, the conversion decreased by ca. 1% and 4%, respec-tively (Fig. 8), suggesting a good reusability of these catalysts in theesterification of acetic acid with n-butanol. It is noteworthy that,

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10 G. Mitran et al. / Journal of Molecular

ompared with the alumina-supported vanadia catalyst with a sim-lar vanadia content, i.e. 10 wt%, for which the conversion decreasedy 15% after three reaction cycles [24], the 10Mo1V9Al catalystad a much better stability. This suggests indeed that molybdenand vanadia interact on the catalyst surface increasing the catalysttability [26]. Additionally, based on the DR–UV–Vis results, theolymerized VO6 species present on the catalyst surface seem, inarticular, to be responsible for the catalyst instability during theatalytic test.

. Conclusion

MoO3–V2O5 catalysts supported on �-alumina act as efficientnd stable solid acid catalysts for the esterification of acetic acidith n-butanol. The catalytic activity correlated well with theumber of strong acid sites which increased by increasing the vana-ia content. It ranged as follows (the conversion of acetic acidt 120 min reaction time in parenthesis): Al2O3 (41%) < 5Mo9V1Al46%) < 10Mo9V1Al (62.5%) < 5Mo1V9Al (68.3%) < 10Mo1V9Al (71%).n all the esterification reactions the selectivity for butyl acetate

as 100%. Reactant pre-adsorption experiments suggested that theeaction follows the Langmuir–Hinshelwood mechanism with atronger adsorption of n-butanol than acetic acid.

The optimum reaction time was about 150 min and the optimalass fraction of the catalyst in the reaction medium was found

o be around 1 wt%. A good reusability of the catalysts after threeeaction cycles was observed. A local interaction between molyb-enum and vanadium on the catalyst surface has been evidenced.ompared with V2O5/Al2O3 with similar vanadia loadings, theoO3–V2O5/Al2O3 catalysts showed not only an increased stabil-

ty during the esterification reaction, but also an improved catalyticctivity.

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