Characterization of La2O3-TiO2 and V2O5/La2O3-TiO2 catalysts

Journal of Molecular Catalysis A: Chemical 169 (2001) 207–223

Characterization of La2O3-TiO2 and V2O5/La2O3-TiO2 catalystsand their activity for synthesis of 2,6-dimethylphenol

Benjaram M. Reddy∗, Ibram GaneshInorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India

Received 4 August 2000; received in revised form 8 November 2000; accepted 5 December 2000

Abstract

The La2O3-TiO2 (1:5 molar ratio) mixed oxide was prepared by a co-precipitation method with in situ generated ammoniaand was impregnated with various amounts of vanadia (4–12 wt.%). The La2O3-TiO2 and the V2O5/La2O3-TiO2 catalystswere subjected to thermal treatments from 773 to 1073 K and were investigated by X-ray diffraction, FT-infrared, BETsurface area, and O2 chemisorption methods to establish the effects of vanadia loading and thermal treatments on the surfacestructure of the dispersed vanadium oxide species and the temperature stability of these catalysts. Conversion of cyclohexanolto cyclohexanone/cyclohexene was investigated as a model reaction to assess the acid–base properties of these materials.The catalytic property was evaluated for a single step synthesis of 2,6-dimethylphenol from cyclohexanone and methanol inthe vapor phase at normal atmospheric pressure. Characterization results suggest that the co-precipitated La2O3-TiO2 whencalcined at 773 K is in X-ray amorphous state and exhibits reasonably high specific surface area. The amorphous La2O3-TiO2

is converted into crystalline compounds La2Ti2O7 and La4Ti9O24 at 873 and 1073 K, respectively. The La2O3-TiO2 alsoaccommodates a monolayer equivalent of V2O5 (12 wt.%) in a highly dispersed state on its surface. The V2O5/La2O3-TiO2

catalyst is thermally quite stable up to 873 K calcination temperature. When subjected to thermal treatments beyond 873 K,the dispersed vanadium oxide selectively interacts with La2O3 portion of the mixed oxide and forms a LaVO4 compound. Theremaining TiO2 appears in the form of anatase phase. The La2O3-TiO2 and the V2O5/La2O3-TiO2 differ in terms of acid–baseproperties and the 12% V2O5/La2O3-TiO2 catalyst provided a maximum yield of 2,6-dimethylphenol among various catalystsinvestigated. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Vanadium oxide; La2O3-TiO2; Mixed oxide; Acid–base properties; Redox properties; Oxygen chemisorption; Dispersion; Thermalstability; 2,6-Dimethylphenol; Cyclohexanol; Methanol

1. Introduction

The activity and selectivity of supported vanadiacatalysts is known to be very sensitive to the composi-tion of the support and in case of TiO2-supported cat-alysts, the phase of support [1–3]. Previous researchhas clearly established that the preferred phase of TiO2is anatase and that the activity per gram of catalyst

∗ Corresponding author. Fax:+91-40-717-3387.E-mail address:[email protected] (B.M. Reddy).

increases with increase of vanadia loading up to thepoint where the surface of the support is covered bya theoretical monolayer of vanadium pentoxide [2–5].Several methods have been tried in the literature to in-crease the specific surface area of titania and thermalstability of titania anatase structure [6–9]. These meth-ods include making of nano-composites with a secondmaterial such as alumina [10,11]; adding palladium asa stabilizing agent [12]; and doping with cerium [13],silica [14,15], zirconia [16], and tungsten oxide [14].Recently, we have reported that the titania anatase

1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S1381-1169(00)00564-1

208 B.M. Reddy, I. Ganesh / Journal of Molecular Catalysis A: Chemical 169 (2001) 207–223

structure can be stabilized even up to 1273 K withincorporation of other stable oxides such as Al2O3,SiO2, and ZrO2 and having vanadium oxide present asa monolayer on these mixed oxide surfaces [10,15,16].

The La2O3-TiO2 composite material has been em-ployed for various purposes in the literature, suchas oxidation of chlorodifluromethane [17], oxidativecoupling of methane [18,19], automobile catalyticconverter [20], ceramic membrane top layer [21], andadsorbent [22]. Lanthana alone is also an interest-ing material and has been widely used as a catalyst[23,24] and catalyst support [25], particularly in theoxidative coupling of methane. The lanthanum titanatecatalyst activates methane more selectively than thecorresponding individual oxides [24]. Thus, La2O3and TiO2 have often been utilized as promoters,supports and catalysts; however, the combination ofLa2O3-TiO2 containing V2O5 has not been reportedpreviously.

The aim of the present investigation was to pro-vide basic insights into the structure of La2O3-TiO2and V2O5/La2O3−TiO2 catalysts, shedding light onthe influence of thermal treatments, oxide loading,and preparation method on both thermal stability andphysicochemical characteristics of these materials. Inthis study, a La2O3-TiO2 (1:5 molar ratio) binary ox-ide support was prepared by a co-precipitation methodand was impregnated with various amounts of vana-dium pentoxide. The prepared samples were then sub-jected to thermal treatments from 773 to 1073 K inorder to understand the temperature stability, disper-sion of vanadium oxide, and physicochemical proper-ties of these materials. These effects were monitoredby means of XRD, FTIR, BET surface area, and oxy-gen chemisorption methods. To understand acid–baseproperties, the conversion of cyclohexanol to cyclo-hexanone and cyclohexene was investigated. Finally,the catalytic property of these materials was evalu-ated for a single step synthesis of 2,6-dimethylphenolfrom methanol and cyclohexanone in the vapor phaseat normal atmospheric pressure.

2. Experimental methods

2.1. Catalyst preparation

The lanthana-titania (1:5 molar ratio) mixedoxide support was prepared by a homogeneous

co-precipitation method where ammonia was gener-ated in situ by decomposition of urea at 363 K. [16].In a typical experiment, the requisite quantities of tita-nium tetrachloride (Fluka, AR grade) and lanthanumnitrate (Loba, GR grade) were dissolved separately indeionized water, and were mixed together (pH= 2).An excess amount of solid urea with a metal to ureamolar ratio of 1:1.5 was also added to this mixturesolution. The resulting mixture was heated slowlyto 363–368 K on a hot plate with vigorous stirring(pH > 7). Heating was continued at this temperaturefor six more hours and the pH of the solution wasincreased to 8.5 by adding dilute ammonia in order tocomplete the precipitation and to facilitate aging. Theresulting precipitate was filtered off, washed severaltimes with deionized water until no chlorides couldbe detected with AgNO3 in the filtrate. The resultingcake was then oven dried at 383 K for 12 h and finallycalcined at 773 K for 6 h in an open-air furnace. Someportions of this calcined support were once againheated at 873, 973, and 1073 K for 6 h in a closedelectrical furnace in open-air atmosphere.

The V2O5/La2O3-TiO2 catalyst, containing differ-ent amounts of V2O5 (4–12 wt.%) was prepared by awet impregnation method. To impregnate vanadia therequisite quantity of ammonium metavanadate (Fluka,AR grade) was dissolved in aqueous oxalic acid solu-tion (2 M). To this clear solution, the finely powderedcalcined (773 K) mixed oxide support was added. Theexcess water was then evaporated on a water-bath andthe resulting material was oven dried at 393 K for 12 hand subsequently calcined at 773 K for 6 h under aflow of dry oxygen. The finished catalysts were onceagain treated at 873, 973 and 1073 K for 6 h in a closedelectrical furnace in open-air atmosphere. The rate ofheating as well as cooling was always maintained at10 K per minute.

2.2. Catalyst characterization

The X-ray powder diffraction patterns have beenrecorded on a Siemens D-5000 diffractometer byusing Cu Ka radiation source and ScintillationCounter detector. The XRD phases present in the sam-ples were identified with the help of ASTM PowderData Files. The crystallite size of TiO2 anatase wasestimated with the help of Debye–Scherrer equationusing the XRD data of the anatase (0 1 0) reflection

B.M. Reddy, I. Ganesh / Journal of Molecular Catalysis A: Chemical 169 (2001) 207–223 209

[26]. The FTIR spectra were recorded on a Nicolet740 FTIR spectrometer at ambient conditions, usingKBr disks, with a normal resolution of 4 cm−1 andaveraging 100 spectra. A conventional all glass vol-umetric high-vacuum (upto 1× 10−6 Torr) systemwas used for both BET surface area and O2 uptakemeasurements. The BET surface area was measuredby nitrogen physisorption at liquid nitrogen tempera-ture (77 K) by taking 0.162 nm2 as the area of crosssection of N2 molecule. Before measurements, thesamples were evacuated (1× 10−6 Torr) at 473 K for2 h. The O2 uptake measurements were carried out asper the procedure described elsewhere [27].

2.3. Catalyst evaluation

The conversion of cyclohexanol and one step syn-thesis of 2,6-dimethylphenol were investigated invapor phase under normal atmospheric pressure, ina down flow fixed bed differential micro-reactor, atdifferent temperatures. In a typical experiment ca.0.5–2.0 g of a catalyst sample was secured betweentwo plugs of Pyrex glass wool inside the glass reactor(Pyrex glass tube i.d. 0.8 cm) and above the catalystbed filled with glass chips in order to act as preheat-ing zone. The reactor was placed vertically insidea tubular furnace, which can be heated electrically.The reactor temperature was monitored by a thermo-couple with its tip located over the catalyst bed andconnected to a temperature indicator–controller. Thecatalyst was heated in a flow of air at 723 K for 5 h,prior to the reaction in order to facilitate activation.After the activation, the temperature was adjusted tothe desired temperature and the mixture of methanoland cyclohexanone or cyclohexanol was fed from amotorized syringe pump (Perfusor Secura FT, Ger-many) into the vaporizer where it was allowed tomix uniformly with air or nitrogen before enteringthe preheating zone of the reactor. The liquid prod-ucts collected through spiral condensers in ice cooledfreezing traps were analyzed by a gas chromato-graph. In the case of synthesis of 2,6-dimethylphenol,the liquid products were quantified by FID witha 10% Carbowax 20 M (length 2 m) column. Themain products observed were 2,6-dimethylphenol,2,6-dimethylcyclohexanone, 2-methylcyclohexanone,1-methoxycyclohexene and some unidentified prod-ucts. At higher temperatures, some small amounts of

CO and CO2 were also noticed. In case of cyclohex-anol conversion, the reaction products were analyzedby FID with OV-17 (length 2m) column. The mainproducts obtained were cyclohexene, cyclohexanone,phenol, CO and CO2 along with some unidentifiedproducts. In both cases, the activity data were col-lected under steady state conditions. The conversion,selectivity, and yield were calculated as per the pro-cedure described elsewhere [28].

3. Results and discussion

3.1. Thermal effects on XRD phase composition

The BET surface area of La2O3-TiO2 support cal-cined at 773 K was found to be 123 m2g−1. The quan-tity of vanadia required to cover the support surfaceas a unimolecular layer can be estimated from thearea occupied per VO2.5 (10.3 × 104 pm2) unit ofthe V2O5. This theoretical estimation yields a loadof 0.145 wt.% per m2 of the support to cover its sur-face with a compact single lamella of the vanadiumpentoxide structure [29]. However, in order to see theinfluence of vanadia on the thermal stability of theLa2O3-TiO2 support, a range of V2O5 loadings from4 to 12 wt.% have been selected in the present investi-gation.

The X-ray powder diffraction patterns of theLa2O3-TiO2 mixed oxide calcined at various temper-atures from 773 to 1073 K are shown in Fig. 1. As canbe noted from this figure, the lanthana–titania binaryoxide obtained via a homogeneous co-precipitationmethod and calcined at 773 K is in X-ray amorphousstate with broad background diffraction lines due to apoorly crystalline TiO2 anatase phase (JCPDS FilesNo. 21-1272). With increase in calcination tempera-ture from 773 to 973 K, in addition to an increase in theintensity of lines due to TiO2 anatase phase, presenceof a new phase with broad peaks atd = 2.99, 2.71and 3.22 Å can be seen, which are attributed due tothe formation of La2Ti2O7 (JCPDS Files No. 28–517)compound. On further increase of calcination temper-ature from 973 to 1073 K, the lines due to La2Ti2O7compound vanished and another group of new broadpeaks atd = 3.36, 3.28 and 2.54 Å are noted, whichare attributed to the presence of La4Ti9O24 compound(JCPDS Files No. 15–324). Recently, Gopalan and


Fig. 1. X-ray diffraction patterns of La2O3-TiO2 mixed oxide support calcined at different temperatures: (A) lines due to TiO2 anatase;(C1) lines due to La2Ti2O7; (C2) lines due to La4Ti9O24.

Lin [30] also reported the formation of La2Ti2O7compound at 973 K, and La4Ti9O24 at 1073 K andabove temperatures in a sample having the appropri-ate composition of titania and lanthana components.Very interestingly, no diffraction lines due to TiO2rutile phase (JCPDS Files No. 21-1276) are observedin the present study even up to the highest calcinationof 1073 K. The La2O3-TiO2 mixed oxide apparentlyexists in an amorphous phase when calcined at 773 K,and at higher temperatures some portions of thismixed oxide is converted into La2Ti2O7 compound

and subsequently to La4Ti9O24, and the latter phaseis thermally quite stable up to 1073 K. In a differentstudy, Limar et al. [31] also reported the formationof an amorphous La2O3·6TiO2, which after dehydra-tion passes into crystalline phases at 973–1073 K asobserved in the present study.

The XRD profiles of 4, 8, and 12% V2O5/La2O3-TiO2 catalysts calcined at different temperatures from773 to 1073 K are shown in Figs. 2–4, respectively.As can be noted from Fig. 2, there are no lines eitherdue to V2O5 or to a compound between vanadia and


Fig. 2. X-ray diffraction patterns of 4 wt.% V2O5/La2O3-TiO2 catalyst calcined at different temperatures: (C3) lines due to LaVO4; othersymbols as in Fig. 1.

lanthana–titania up to a calcination of 973 K, exceptsome broad diffraction lines due to titania anatasephase. The XRD results, thus, indicate that vanadiumoxide is present in a highly dispersed state on themixed oxide support up to 973 K calcination tem-perature. This observation is further supported fromoxygen uptake measurements as described in the laterparagraphs. However, at 1073 K calcination tempera-ture in addition to sharp TiO2 anatase lines, some new

lines with less intensity were observed atd = 3.22,2.98 and 3.41 Å. These lines can be attributed to theformation of LaVO4 (JCPDS Files No. 25–427) com-pound. It is well known in the literature that for vana-dia contents of less than monolayer coverage the activecomponent will be present as a two-dimensional vana-dium oxide over layer on the oxide support. Quantitiesin excess of monolayer coverage will have micro-crystalline V2O5 particles in addition to the surface


Fig. 3. X-ray diffraction patterns of 8 wt.% V2O5/La2O3-TiO2 catalyst calcined at different temperatures: symbols as in Fig. 2.

vanadium oxide overlayer [2,3]. However, it is inter-esting to note from Fig. 2 that the formation of LaVO4compound at 1073 K in the present V2O5/La2O3-TiO2catalyst system, which is expected to be formed atthe expense of dispersed vanadium oxide on theLa2O3-TiO2 surface.

In the case of 8 wt.% V2O5/ La2O3-TiO2 catalyst(Fig. 3), the presence of less intense lines due toLaVO4 can be seen at the calcination of 873 K, alongwith sharp diffraction lines due to titania anatase

phase. With increase of calcination temperature from873 to 1073 K an improvement in the intensity oflines due to both anatase and LaVO4 compound canbe noted. In the case of 12 wt.% V2O5/La2O3-TiO2catalyst (Fig. 4), the intense lines due to LaVO4compound and TiO2 anatase can be seen even at thecalcination of 873 K. On further increase of calcina-tion temperature from 873 to 1073 K, an appreciableimprovement in the intensity of lines due to bothtitania anatase and LaVO4 can be noted.


Fig. 4. X-ray diffraction patterns of 12 wt.% V2O5/La2O3-TiO2 catalyst calcined at different temperatures: symbols as in Fig. 2.

From XRD measurements, the crystallite size ofanatase phase in various V2O5/La2O3-TiO2 sampleswas estimated and the results are shown in Fig. 5. Ascan be noted from Fig. 5 that the impregnated vanadiaappears to increase the grain size of anatase in thesamples after calcination at higher temperatures. Thecrystallite size of TiO2 anatase in V2O5/La2O3-TiO2samples is larger than that of pure lanthana–titaniamixed oxide. Increase in vanadia loading also seems

to increase the size of TiO2 anatase. These results,thus, suggest that the doped vanadia also acceleratethe grain growth of titania. Another interesting pointto be mentioned from this figure is the retarding effectof added lanthana on the titania anatase to rutile phasetransformation.

A closer examination of Figs. 1–5 reveal that thecalcination temperature and presence of vanadia havethree major effects on the La2O3-TiO2 mixed oxide


Fig. 5. Effect of calcination temperature on TiO2 anatase crystal growth in various samples. TiO2 anatase (0 1 0) reflection was consideredfor XRD crystallite size measurements.

support. (a) In the case of pure support, transforma-tion of amorphous La2O3-TiO2 mixed oxide into def-inite crystalline compounds La2Ti2O7 and La4Ti9O24at 873 and 1073 K, respectively. Crystallinity of theremaining titania anatase increases with increase ofcalcination temperature. The La4Ti9O24 compoundformed at higher temperatures is quite stable in theabsence of vanadia. (b) In the case of vanadia con-taining samples formation of LaVO4 is noted evenat 4 wt.% loading (Fig. 2) on increase of calcinationtemperature from 773 to 1073 K. With increase in thequantity of vanadia, a gradual increase in the intensityof the lines due to LaVO4 and at the same time dis-appearance of lines due to La2Ti2O7 and La4Ti9O24compounds can be noted. It can be understood fromthese figures that the LaVO4 compound is formedat the expense of La2Ti2O7 and La4Ti9O24 phases.Another interesting point to be noted from this figureis that the quantity of vanadia is also an equally im-portant factor in the formation of LaVO4 compound,in addition to the calcination temperature. (c) Ab-sence of a crystalline V2O5 and appearance of LaVO4compound accompanied by TiO2 anatase phase forcalcination temperatures beyond 773 K was the an-other important observation. Further, the intensityof lines due to both LaVO4 and TiO2 anatase phasewere found to increase with increase of calcinationtemperature.

The XRD observations described above reveal aninteresting information about the reactivity of vana-dia towards La2Ti2O7 and La4Ti9O24 compounds. Thedispersed vanadia reacts preferably with La2O3 por-tion of La2Ti2O7 and La4Ti9O24 compounds to formLaVO4, thus, liberating TiO2. The portion of TiO2 re-leased from La2Ti2O7 and La4Ti9O24 compounds ap-pear as the crystalline anatase phase as shown in thefollowing equations:

La2Ti2O7 + V2O5 → 2LaVO4 + 2TiO2 (anatase)

(1)

La4Ti9O24 + 2V2O5 → 4LaVO4 + 9TiO2 (anatase)

(2)

It is a well established fact in the literature [2,6,7,32]that a highly dispersed vanadia on TiO2 support ac-celerates the anatase-into-rutile phase transformationby lowering the activation temperature of this phe-nomenon, which is normally expected to be 823 Kand above in impurity-free TiO2 samples [8]. Duringthis transformation some of the vanadia is normallyreduced and gets incorporated into the rutile struc-ture as VxTi(1−x)O2 (rutile solid solution) [2,6,7,32].However, in the case of La2O3-TiO2 mixed oxide sup-port, the reactivity of vanadia towards La2Ti2O7 andLa4Ti9O24 compounds appears to be quite different. It


interacts preferably with the lanthana portion of thesecompounds to form LaVO4 as shown in Eqs. (1) and(2). The liberated titania appears in the form of crys-talline anatase phase. This is an interesting observa-tion from this study.

3.2. FT-infrared measurements

The FTIR spectra of La2O3-TiO2 mixed oxidesupport calcined at 773–1073 K revealed the pres-ence of strong absorption bands between 3400 and3600 cm−1 and at 1625 cm−1. In addition, a new peakat 650–830 cm−1 was also observed in the case of1073 K calcined samples. The absorption peaks be-tween 3400 and 3600 cm−1, known to be due to thepresence of surface hydroxyl groups, were graduallydecreased with increase in calcination temperature[33]. The peak at 1625 cm−1, due to the deforma-tion vibrations of adsorbed water, was also gradu-ally decreased after calcination at high temperatures[34]. Anatase and rutile phases of titania exhibitstrong absorption bands in the region of 850–650 and800–650 cm−1, respectively. A gradual improvementin the region between 850–650 cm−1 with increase incalcination temperature was observed suggesting thatthe titania is gradually transforming from an amor-phous to a crystalline anatase phase in line with XRDobservations.

The FTIR spectra of various V2O5/La2O3-TiO2 cat-alysts calcined at different temperatures are recordedin the range of 400–1800 cm−1, where those bandsdue to theνV=O are expected to be observed. Nor-mally, the IR spectrum of pure crystalline V2O5shows sharp absorption bands at 1020 and anotherat 820 cm−1 due to V=O stretching and V–O–Vdeformation modes, respectively [35]. The spec-trum of 4 wt.% V2O5/La2O3-TiO2 sample calcinedat 773 K was identical to that of pure support, inagreement with the XRD observations, where onlythe anatase phase of TiO2 is noted. With increase incalcination temperature from 773 to 1073 K a grad-ual change from amorphous to a crystalline anataseformation was noted from FTIR in line with XRDresults. A weaker band at around 980 cm−1 wasnoted in the spectrum of the sample calcined at873 K. On further increase of calcination tempera-ture from 873 to 1073 K the absorption band in theregion of 990–960 cm−1 was completely diminished,

which suggest that the dispersed vanadium oxideis transformed from amorphous VOx to a differentform. However, no absorption band at 1020 cm−1

due to V=O stretching mode of crystalline V2O5wasseen. The disappearance of absorption bands at990–960 cm−1 were also noted in the case of 8 and12% V2O5/TiO2-La2O3 samples at the calcinationof 873 K and above. The formation of crystallineLaVO4 may be the reason for the disappearance ofthe absorption band at 990–960 cm−1 of the dispersedVOx species [36]. An improvement in the intensity ofthe absorption band at 830–650 cm−1 was also notedwhen the samples were subjected to calcinations at>773 K. This improvement is more in the case of12% V2O5/La2O3-TiO2 catalyst. Crystallization oftitania anatase was found to be more in the case of12% catalyst from XRD study. All these observationmade from FTIR are in line with the findings of XRDstudy.

3.3. BET surface area and oxygen uptakemeasurements

The change in BET surface area of sample as a func-tion of calcination temperature is shown in Fig. 6. Aconsiderable decrease in the specific surface area ofthe catalyst, depending on the calcination temperatureand vanadia loading, can be noted from this figure. Thedecrease in specific surface area with increase in cal-cination temperature is a general phenomenon and isexpected to be due to sintering of the samples at highertemperatures [2,10,15]. Similarly, the decrease in spe-cific surface area with increase of vanadia loading maypresumably due to blocking of pores of the support bythe dispersed vanadium oxide and also due to the for-mation of LaVO4 compound. It is an established factin the literature that a highly dispersed vanadia nor-mally decreases the specific surface area of the supportby penetrating in to the pores of the support material[2,10,15,27]. The important point that can be noticedfrom Fig. 6 is that the decrease in surface area is morein the case of vanadia impregnated catalysts than thatof pure support. This observation clearly indicates thatthe pure lanthana–titania support is more stable whencompared to that of vanadia impregnated samples.

Oxygen uptakes at 643 K on pre-reduced V2O5/La2O3-TiO2 catalysts, calcined at different temperatures,as a function of vanadia loading are shown in Fig. 7.


Fig. 6. Effect of calcination temperature on the BET surface area of La2O3-TiO2 support and various V2O5/La2O3-TiO2 catalysts.

The pure La2O3-TiO2 support was also found tochemisorb some small amount of O2 (5.2mmol g−1)under the experimental conditions used in this study.Therefore, the contribution of pure support alone wassubtracted from the results. As can be noted fromFig. 7, the oxygen uptake increases with increase ofvanadia loading. However, the numerical values of O2uptake varied appreciably and are maximum in thecase of catalysts calcined at 773 K and minimum at1073 K. A large decrease in the O2 uptakes is noted

Fig. 7. Effect of calcination temperature on the oxygen uptake capacity of various V2O5/La2O3-TiO2 catalysts. O2 uptake was obtainedby a volumetric method at 643 K.

between 873 and 973 K treated samples and such adifference is not found neither between 773 and 873 Knor between 973 and 1073 K treated samples. It canbe inferred from these results that the calcination ofV2O5/La2O3-TiO2 catalysts at 973 K (the meltingpoint of V2O5 is 963 K) and above apparently resultsin the formation of more crystalline LaVO4 com-pound. The LaVO4 may be formed due to a reactionbetween La2O3-TiO2 and the vanadia being leachedout from the micro-pores of the support material where


Fig. 8. Effect of calcination temperature on the dispersion ofvanadia on La2O3-TiO2 support. Dispersion was calculated fromO2 uptake measurements by assuming O2/V2O5 = 1.

it was in a highly dispersed state. A substantial lossin the specific surface area of the catalyst depend-ing on the calcination temperature and vanadia con-tent has been noted (Fig. 6). Thus, calcination of thevanadia containing catalysts brings about a decreaseof the specific surface area in all samples studied, be-ing proportional to the temperature of calcination. Re-cently, Reddy et al. [27] demonstrated that the slopeof the plot between oxygen uptake and V2O5 load-ing approaches unity at very low loadings. This unityindicates a limiting stoichiometry of O2/V2O5 = 1.Using this stoichiometry, Oyama et al. [37] defineddispersion as the ratio of molecular oxygen uptake toV2O5 content. Fig. 8 represents the dispersions derivedfrom O2 uptake measurements. The apparent disper-sion decreases with increase in vanadia loading at agiven calcination temperature. This is a general phe-nomenon observed on any supported catalyst system[10,15,27,38]. Increasing calcination temperature alsoresults in a tremendous decrease in the dispersion ofvanadia, especially in the case of catalysts calcined at1073 K. However, in the case of samples calcined at773 K the dispersion remains reasonably high at allloadings. Another interesting point to note Fig. 8 isthat the vanadia dispersion is reasonably high even at873 K. The decrease in the dispersion with increase incalcination temperature as well as V2O5 loading areprimarily due to the formation of LaVO4 compound.Thus, O2 uptake results are in line with XRD and FTIRobservations.

Fig. 9. Conversion of cyclohexanol to different products onLa2O3-TiO2 and V2O5/La2O3-TiO2 samples at 673 K and undernormal atmospheric pressure.

3.4. Conversion of cyclohexanol

For the purpose of better comparison, the steadystate activity and selectivity results of cyclohex-anol conversion over lanthana–titania and vanadia/lanthana–titania catalysts, at a constant temperatureof 673 K and under normal atmospheric pressure, arepresented in Fig. 9. As can be noted from this fig-ure, the La2O3-TiO2 support exhibits more selectivitytowards phenol, whereas the V2O5/La2O3-TiO2 cat-alysts show more selectivity towards cyclohexene. Ina recent study, Bezouhanova and Al-Zihari [39] usedcyclohexanol conversion as a test reaction to investi-gate the acid–base properties of different metal oxidecatalysts. They reported a direct correlation betweenBrønsted acid sites and the amount of cyclohexene,the dehydration product, formed. The dehydrogena-tion product, cyclohexanone, was directly correlatedwith the basic sites. There is a possibility of forma-tion of coordinatively unsaturated metal cations andoxygen anions on the surface of oxide/binary oxidewhen obtained from co-precipitation method from thecorresponding metal aqueous salt precursors [33]. Itis a well-established fact in the literature that exposedcoordinatively unsaturated metal cations and oxygenanions on the surface of the catalyst act as Lewis acidand Brønsted base sites [33]. The formation of moreamount of phenol over La2O3-TiO2 than on vana-dia impregnated samples may presumably be due tothe presence of more surface oxygen anions, which


are expected to be formed in the preparation stage.As the vanadia content increases on the surface oflanthana–titania binary oxide there is a drastic de-crease in the selectivity of phenol and a significantimprovement in the selectivity of cyclohexene. As perthe literature reports [39,40], the V2O5 is a typicalacid, possesses only Brønsted acid sites and producescyclohexene as the sole product in the reaction ofcyclohexanol. The increase in selectivity towards cy-clohexene on vanadia impregnated lanthana–titaniasuggests that vanadia has covered the surface of thecarrier. Increasing the content of vanadia on the sur-face of the support may lead to the formation of octa-hedral vanadia species having more surface exposedhydroxyl groups [2,41]. It can also be suggested thatas the vanadia content increases the exposed coordi-nately unsaturated sites will be decreased drasticallyon the surface. It was also noticed that as the reactiontemperature increases the selectivity to phenol andcyclohexanone were found to decrease, however, theselectivity towards cyclohexene was increased overall the catalysts investigated. This is primarily due tothe fact that as the temperature increases there will bean enhancement in the dehydroxylation of the surfaceleading to more coordinately unsaturated cations (i.e.Lewis acid sites). More of the surface Lewis acid sitepopulation facilitate more interaction of alcohol withthe surface leading to more dehydration product, cy-clohexene. From this study, it can be inferred that thevanadia impregnated lanthana–titania contains moreacidic sites on the surface than that of pure support.

3.5. One step synthesis of 2,6-dimethylphenol frommethanol and cyclohexanone

2,6-Dimethylphenol is an important chemical inter-mediate in the polymer industry for engineering plas-tics [42]. The commercial synthetic method is basedon a liquid phase process, where phenol is methy-lated with methanol using an alumina catalyst. How-ever, this process not only requires high pressure andtemperature, but also produces wide range of byprod-ucts, including various isomers of xylenol [43]. In fact,there are several advantages like continuous produc-tion, simplified product recovery, catalyst regenera-bility, etc. for carrying out this reaction in the vaporphase from cyclohexanone and methanol. In a recentpublication, Wang et al. [44] reported the synthesis

of 2,6-dimethylphenol from methanol and cyclohex-anone over vanadia/titania catalyst. However, the con-version and product selectivities reported are limitedon this catalyst. The mixed oxide supports have at-tracted much attention recently because of their bet-ter performance than their constituent single oxidesfor various reactions [45–47]. Therefore, this particu-lar reaction was selected in the present investigationto study the catalytic properties of La2O3-TiO2 andV2O5/La2O3-TiO2 catalysts.

The activity and selectivity for the single stepsynthesis of 2,6-dimethylphenol was investigated be-tween 573 and 698 K. The activity and selectivitytrends on various catalysts followed the same patternwith temperature. In general, an increase in the con-version with an increase in temperature was observed.The formation of some additional side products withtraces of CO and CO2 were also occasionally notedat higher temperatures. The change in conversion asa function of contact time at a fixed temperature of673 K was also studied on various catalysts. A de-crease in the conversion of cyclohexanone during theinitial reaction period was observed for all the cat-alysts used, but stable activity was obtained withina few hours. The conversion and selectivity are cal-culated on cyclohexanone basis and almost all theexcess methanol was recovered after the reaction.The conversion and selectivity results as a function oftemperature on the La2O3-TiO2 support and the 12%V2O5/TiO2-La2O3 catalyst are presented in Figs. 10and 11, respectively. As can be noted from these fig-ures, the conversion of cyclohexanone was found toincrease as the reaction temperature increases. On theLa2O3-TiO2 support, at low reaction temperatures theformation of 1-methoxycyclohexene, methyl formate,and dimethyl ether in large amounts was noted inaddition to small amounts of 2-methylcyclohexanone,2,6-dimethylcyclohexanone, and 2,6-dimethylphenol.At 648 K and above temperatures more selectivitytowards 2-methylcyclohexanone and 2,6-dimethylcy-clohexanone and a less selectivity towards 2,6-dimethylphenol can be seen. However, at all thesetemperatures the formation of methyl formate,dimethyl ether, and COx in small quantities wasnoted. Upon impregnating the support with vana-dia a major change in the product distribution wasnoticed. In the case of vanadia impregnated catalysts,the main products observed were 2,6-dimethylphenol,


Fig. 10. Activity and selectivity results of one step synthesis of 2,6-dimethylphenol from cyclohexanone and methanol overLa2O3-TiO2 support at different temperatures: (a) conversion; (b) selectivity to 2,6-dimethylphenol; (c) 2,6-dimethylcyclohexanone; (d)2-methylcyclohexanone; (e) 1-methoxycyclohexene.

Fig. 11. Activity and selectivity results of one step synthesis of 2,6-dimethylphenol from cyclohexanone and methanol over 12 wt.%V2O5/La2O3-TiO2 catalyst at different temperatures: symbols as in Fig. 10.


Table 1Catalytic and physicochemical properties of various mixed oxide based vanadium oxide catalysts

Catalysta Conversion(%)

2,6-DMPselectivity (%)

2,6-DMPyield

BET SA(m2 g−1)

O2 uptake(mmol g−1)

CyclohexanoneTOF (× 10–3)

Compositiond

(molar ratio)

14%V2O5/TiO2-Al2O3 87 54 47 108 319 0.389b 0.940c 1:114%V2O5/TiO2-ZrO2 76 56 43 126 233 0.396b 1.123c 1:120%V2O5/TiO2-SiO2 92 64 59 151 903 0.286b 0.359c 1:112%V2O5/In2O3-TiO2 64 19 12 61 911 0.332b 0.240c 1:1312%V2O5/Ga2O3-TiO2 90 65 58 51 532 0.931b 0.671c 1:54%V2O5/La2O3-TiO2 92 48 44 93 161 2.830b 5.450c 1:58%V2O5/La2O3-TiO2 93 55 51 78 216 1.410b 2.870c 1:512%V2O5/La2O3-TiO2 96 84 81 63 256 0.990b 2.550c 1:5

a V2O5 content in wt.%.b Number of molecules of cyclohexanone converted per vanadium atom per second.c Number of molecules of cyclohexanone converted per active oxygen atom per second.d Support composition.

2,6-dimethylcyclohexanone, 2-methylcyclohexanoneand 1-methoxycyclohexene, respectively. In the caseof vanadia/lanthana–titania catalysts, maximum se-lectivity towards 2,6-dimethylphenol was noted at648 K and above temperatures (Fig. 11). Similar re-sults were also noted on 4 and 8% catalysts. In gen-eral, the selectivity of 2,6-dimethylphenol increasedwith increase in temperature and at the same timethe selectivity towards 2,6-dimethylcyclohexanone,2-methylcyclohexanone and 1-methoxycyclohexenedecreased drastically. An improvement in the con-version and selectivity was observed upon increasingthe loading of vanadium oxide on the support. The12 wt.% V2O5/La2O3-TiO2 catalyst exhibited moreconversion and selectivity.

For the purpose of comparison, the activity andselectivity trends over various vanadia-containingcatalysts were obtained and shown in Table 1. Theinvestigated TiO2-SiO2, TiO2-Al2O3, TiO2-ZrO2,La2O3-TiO2, Ga2O3-TiO2, and In2O3-TiO2 mixedoxides are known to possess different solid acid–baseproperties. As can be noted from Table 1, the cyclo-hexanone TOF is highly dependent on the vanadialoading. Among various metal oxides tested the 12%V2O5/La2O3-TiO2 exhibited favorable characteristicsfor the single step synthesis of 2,6-dimethylphenoland provided maximum yield reported so far.

(3)

The formation of 2,6-dimethylphenol frommethanol and cyclohexanone is shown schematicallyin Eq. (3). The product distribution observed gives animpression that this reaction proceeds in a differentmanner from that of normal methylation of aromaticcompounds. In general, in the alkylation of substitutedaromatic compounds, one can occasionally observe amixture ofortho-, para-, or meta-substituted productsdepending on the type of precursors, reaction condi-tions and the catalyst used. However, in the presentcaseortho-substituted phenol and/or cyclohexanoneare exclusively formed from methanol and cyclo-hexanone mixtures. The non-formation ofmeta- orpara-substituted products and the selective formationof 1-methoxycyclohexene, 2-methylcyclohexanone,2,6-dimethylcyclohexanone, and 2,6-dimethylphenolsuggest that the reaction may proceed as shown inScheme 1.

As shown in Scheme 1, in the first step of reac-tion the cyclohexanone condenses with a methanolmolecule, in its another isomeric form enol, andproduces 1-methoxycyclohexene, which was ob-tained as one of the side products. Thus, produced1-methoxycyclohexene over isomerization produces2-methylcyclohexanone. Thus, formed 2-methylcyclo-hexanone by condensing with another molecule ofmethanol produces 2,6-dimethylcyclohexanone as


Scheme 1.

shown in the Scheme 1. It is a well-establishedfact in the literature that the transition metal ox-ides, such as V2O5, MoO3, etc. are very active forthe oxidative dehydrogenation of organic molecules[48,49]. Thus, formed 2,6-dimethylcyclohexanoneby oxidative dehydrogenation probably produces the2,6-dimethylphenol. However, further studies arehighly essential to establish the mechanism of thiscomplex reaction.

Under dehydrated conditions, surface vanadiaspecies possesses terminal V=O groups and polymericfunctionalities associated with isolated and polymer-ized species on the support surface [50]. The molec-ular structures of the surface VOx species are knownto consist of a terminal V=O bond and three bridgingV–O–Support bonds in the case of isolated species,and a terminal V=O bond with one bridging V–O–Sand two bridging V–O–V bonds for the polymerizedspecies [51]. These structures are highly dependent onthe nature of the support material and on the amountof vanadium oxide deposited. It is known that theconcentration of polymerized surface functionalityincreases with vanadium oxide coverage on the sup-port surface. Hence, as the vanadium oxide loading

increases up to monolayer coverage on the surfaceof the support material, along with V=O bonds, theV–O–S bonds also increases. However, at abovemonolayer coverages the increase of vanadia loadingincreases only the V=O and V–O–V bonds and notthe V–O–S bonds. Reddy et al. [27,28] have recentlyobserved that the yield of isobutyraldehyde increasesup to the monolayer coverage and declines slightly be-yond the monolayer loading. However, in the presentcase the TOF of cyclohexanone conversion decreaseswith increase of vanadia loading. A comparison ofactivity trends with vanadia loading clearly indicatesthat the V–O–S bonds may be the probable activeparticipants in the synthesis of 2,6-dimethylphenolfrom methanol and cyclohexanone. Change of specificmixed oxide support does have a significant effecton the cyclohexanone or 2,6-dimethylphenol TOFs(Table 1). This observation further supports that thebridging V–O–Support bonds may be involved in thecritical kinetic steps of the cyclohexanone conversionor 2,6-dimethylphenol formation.

Comparison of cyclohexanone conversion or2,6-dimethylphenol formation with other reactionsover the same supported vanadium oxide catalysts


can provide additional understanding on the fun-damental aspects of this reaction. The selectiveoxidation of 4-methylanisole to anisaldehyde oversupported vanadia catalysts was found to depend onthe acid–base properties of the support oxide apartfrom vanadia surface coverages [52]. Recently, Deoand Wachs [53] have studied the methanol oxidationon a number of supported vanadium oxide catalysts.They found that the activity of the alcohol oxidationcorrelates with the extent of reduction and strengthof V=O and V–O–S bonds. However, it is difficultto establish such a direct correlation in the presentstudy because of the complexity of the reactioninvolved.

4. Conclusions

The following conclusions can be drawn from thisstudy: (1) The lanthana–titania binary oxide is aninteresting and promising support material for thedispersion of vanadium oxide. (2) The co-precipitatedLa2O3-TiO2 mixed oxide when calcined at 773 Kis in X-ray amorphous state and exhibits reason-ably high specific surface area. The amorphousLa2O3-TiO2 gets converted into crystalline La2Ti2O7and La4Ti9O24 compounds at 873 and 1073 K, respec-tively. (3) The La2O3-TiO2 mixed oxide also accom-modates a monolayer equivalent of V2O5 (12 wt.%)in a highly dispersed state on its surface. Further,the V2O5/La2O3-TiO2 catalyst is thermally stable upto 873 K calcination temperature. When subjectedto thermal treatments beyond 873 K, the dispersedvanadium oxide selectively interacts with La2O3portion of the La2O3-TiO2 mixed oxide and readilyforms LaVO4 compound with the release of TiO2.The liberated TiO2 appears in the form of anatasephase. (4) Among various catalysts investigated, theV2O5/La2O3-TiO2 combination is a promising systemfor the one step synthesis of 2,6-dimethylphenol frommethanol and cyclohexanone mixtures. In particular,the 12 wt.% V2O5/La2O3-TiO2 catalyst provided amaximum yield of 2,6-dimethylphenol.

Finally, further studies are highly essential in orderto understand the microscopic mechanism of this reac-tion and the inter relationships between the acid–baseand redox characteristics of the catalyst and their cat-alytic properties.

Acknowledgements

Ibram Ganesh thanks the University Grants Com-mission, New Delhi for the award of a senior researchfellowship.

References

[1] D.J. Hucknall, Selective Oxidation of Hydrocarbons,Academic Press, London, 1974.

[2] G.C. Bond, S.F. Tahir, Appl. Catal. 71 (1991) 1 and referencestherein.

[3] G. Deo, I.E. Wachs, J. Haber, Crit. Rev. Sci. Eng. 19 (1994)141.

[4] I.M. Pearson, H. Ryu, W.C. Wong, K. Nobe, Ind. Eng. Chem.Prod. Res. Dev. 22 (1983) 381.

[5] A. Baiker, P. Dellenmeir, M. Glinski, A. Reller, Appl. Catal.35 (1989) 351.

[6] R. Kozlowski, R.F. Pettifer, J.M. Thomas, J. Phys. Chem. 87(1983) 5176.

[7] F. Roozeboom, M.C. Mittlemeijer-Hazeleger, J.A. Moulin, J.Medema, V.H.J. de Beer, P.J. Gellings, J. Phys. Chem. 84(1980) 2783.

[8] S.R. Yoganarasimhan, C.N.R. Rao, Trans. Faraday Soc. 58(1962) 1579.

[9] K.-N.P. Kumar, Appl. Catal. A: Gen. 119 (1994) 163.[10] B.M. Reddy, M.V. Kumar, E.P. Reddy, S. Mehdi, Catal. Lett.

36 (1996) 187.[11] H.K. Matralis, M. Ciardelli, M. Ruwet, P. Grange, J. Catal.

157 (1995) 368.[12] K. Jiratova, O. Solcova, H. Snajdaufova, L. Moravkova, H.

Zahradnikova, V. Ponec, M. Shamanska, H.U. Blaser, Stud.Surf. Sci. Catal. 75 (1993) 1235.

[13] K.-N.P. Kumar, K. Keizer, A.J. Burggraaf, J. Mater. Sci. Lett.13 (1994) 59.

[14] G. Olivery, G. Ramis, G. Busca, V.S. Escribano, J. Mater.Chem. 3 (1993) 1239.

[15] B.M. Reddy, S. Mehdi, E.P. Reddy, Catal. Lett. 20 (1993)317.

[16] B.M. Reddy, B. Manohar, S. Mehdi, J. Solid State Chem. 96(1992) 233.

[17] C.A. Le Duc, J.M. Campbell, J.A. Rossin, Ind. Eng. Chem.Res. 35 (1996) 2473.

[18] Z. Kalenik, E.E. Wolf, Stud. Surf. Sci. Catal. 61 (1991) 97.[19] G.S. Lane, Z. Kalenik, E.E. Wolf, Appl. Catal. 53 (1989) 183.[20] Z. Bocun, W. Ren, Zhongguo Xitu Xuebao (China), 9(1)

(1991) 40 CA Vol. 115: 264300k.[21] K.-N.P. Kumar, K. Keizer, A.J. Burggraaf, J. Mater. Chem.

3 (1993) 1412.[22] U. Trudinger, G. Muller, K.K. Unger, J. Cromatogr. 35 (1990)

111.[23] S.-J. Huang, A.B. Walters, M.A. Vannice, J. Catal. 173 (1998)

229.[24] H. Borchert, M. Baerns, J. Catal. 168 (1997) 315.


[25] A. Slagtern, Y. Schuurman, C. Leclercq, X. Verykios, C.Mirodatos, J. Catal. 172 (1997) 118.

[26] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures forPolycrystalline and Amorphous Materials, 2nd Edition, Wiley,New York, 1974.

[27] B.M. Reddy, B. Manohar, E.P. Reddy, Langmuir 9 (1993)1781.

[28] B.M. Reddy, E.P. Reddy, I. Ganesh, Res. Chem. Intermed.23 (1997) 703.

[29] F. Roozeboom, T. Franson, P. Mars, P.J. Gellings, Z. Anorg.Allg. Chem. 449 (1979) 25.

[30] R. Gopalan, Y.S. Lin, Ind. Eng. Chem. Res. 34 (1995) 1189.[31] T.F. Limar, N.G. Kisela, Neorg. Mater. 10 (10) (1974) 1826

(Russia), CA No. 82: 77618h.[32] A. Vejux, P. Courtine, J. Solid State Chem. 23 (1978) 93.[33] H.H. Kung, Stud. Surf. Sci. Catal (Transition Metal Oxides,

Surface Chemistry and Catalysis) 45 (1988) 57.[34] A. Jentys, G. Warecka, M. Derewinski, J.A. Lercher, J. Phys.

Chem. 93 (1989) 4837.[35] G.C. Bond, A.J. Sarkany, G.D. Parfitt, J. Catal. 57 (1979)

476.[36] Y. Nakagawa, O. Ono, H. Miyata, Y. Kubokawa, J. Chem.

Soc., Faraday Trans. 79 (1983) 2929.[37] S.T. Oyama, G.T. Went, A.T. Bell, G.A. Somarjai, J. Phys.

Chem. 93 (1989) 6786.

[38] B.M. Reddy, K. Narsimha, P.K. Rao, V.M. Mastikhin, J. Catal.118 (1989) 22.

[39] C.P. Bezouhanova, M.A. Al-Zihari, Catal. Lett. 11 (1991)245.

[40] A. Auroux, A. Gervasini, J. Phys. Chem. 94 (1990) 6371.[41] G. Deo, I.E. Wachs, J. Catal. 129 (1991) 307 .[42] J.M. Margolis, Engineering Thermoplastics: Properties and

Applications, Marcel Dekker, New York, 1985.[43] K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie,

Verlag Chemie, GmbH, Weium, Germany, 1976.[44] F.L. Wang, L. Yu, W.S. Lee, W.F. Wang, J. Chem. Soc.,

Chem. Commun. (1994) 811.[45] I. Wang, R.C. Chang, J. Catal. 107 (1987) 195.[46] B.E. Hand, M. Maciejewski, A. Baiker, J. Catal. 134 (1992)

75.[47] B.M. Reddy, B. Chowdhury, J. Catal. 179 (1998) 413.[48] B. Manohar, I. Ganesh, B.M. Reddy, J. Mol. Catal. A: Gen.

129 (1998) L5.[49] A. Lisovski, C. Ahroni, Catal. Rev. Sci. Eng. 36 (1994) 25.[50] N. Das, H. Eckert, H. Hu, I.E. Wachs, F. Feher, J. Phys.

Chem. 97 (1993) 8240.[51] G. Busca, G. Centi, F. Trifiro, Appl. Catal. 25 (1986) 265.[52] B.M. Reddy, I. Ganesh, B. Chowdhury, Chem. Lett. (1997)

1145.[53] G. Deo, I.E. Wachs, J. Catal. 156 (1994) 323.

Characterization of La2O3-TiO2 and V2O5/La2O3-TiO2 catalysts

Documents

Transcript of Characterization of La2O3-TiO2 and V2O5/La2O3-TiO2 catalysts