Applied Catalysis A: General 376 (2010) 25–32

Catalytic performance of vanadium pyrophosphate oxides (VPO) in theoxidative dehydration of glycerol

Feng Wang a,*, Jean-Luc Dubois b, Wataru Ueda a,*a Catalysis Research Center, Hokkaido University, North 21, West 10, Sapporo 060-0811, Japanb Arkema France, 420 Rue d’Estienne d’Orves, 92705 Colombes, France

A R T I C L E I N F O

Article history:

Received 21 August 2009

Received in revised form 20 November 2009

Accepted 23 November 2009

Available online 27 November 2009

Keywords:

Dehydration

Glycerol

Acrolein

Vanadium pyrophosphate oxide

Molecular oxygen

A B S T R A C T

A series of vanadium pyrophosphate oxide catalysts were prepared by thermally treating catalyst

precursor VOHPO4�0.5H2O in the temperature range of 500–900 8C in nitrogen and their catalytic

activities were evaluated in glycerol dehydration with molecular oxygen. The catalysts were

characterized by X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric–differential

thermal analysis and temperature programmed desorption of ammonia. The dehydration activity

depends on treatment temperature on the catalyst precursor; the catalyst treated at 800 8C exhibits the

best result with 100% glycerol conversion and more than 95% selectivity for useful products, including

acrolein (64%), acetaldehyde, acetic acid and hydroxyacetone. We observed that the addition of

molecular oxygen into reaction could greatly reduce the selectivity for by-products from 20% to 4% and

can avoid coke formation. The dehydration activities of the catalysts were correlated with the

characteristics of the catalysts.

� 2009 Elsevier B.V. All rights reserved.

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Applied Catalysis A: General

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1. Introduction

The utilization of biomass as renewable resources for thesubstitution of fossil fuels is a key issue in moving our economytoward a more sustainable future. As a rule of green chemistry, therealization of 100% atom efficiency of these processes requires that100% of the starting materials actually end up in the usefulproducts [1]. At present, however, the production of renewablefuels from biomass usually generates substantial amounts of by-products. For example, biodiesel is currently produced bytransesterification of vegetable oils (triglyceride) and methanol,with 1 mol of glycerol produced per mole of triglyceride feed. Withthe rapid growth of biodiesel industry, glycerol production isremarkably increasing. Currently, the amount of glycerol that goesannually into technical applications is around 160 000 tons and isexpected to grow at an annual rate of 2.8% [2]. Thus, the upgrade ofglycerol as a useful feedstock is important to achieve a high atomefficiency of biodiesel production.

Catalysis method is considered as a promising way of utilizingglycerol. Glycerol could be processed through selective transes-terification or esterification to monoglycerides or polyglycerolesters [3,4]. Pyrolysis reactions taking place at 400–600 8C and

* Corresponding authors. Tel.: +81 11 706 9164; 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 � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.11.031

gasification processes occurring at more than 800 8C in airgenerated carbon oxide, hydrogen or carbon [5,6]. Hydrochlorina-tion of glycerol with gas HCl or concentrated hydrochloric acidproduced dichloropropanols and chloropropandiols, importantintermediates for the production epichlorhydrin [7]. Dehydrationof glycerol on the central or the terminal hydroxyl generatesacrolein or hydroxyacetone. These reactions are catalyzed by acidcatalysts in liquid phase or gas phase [8–15]. The glyceroloxidation was primarily reported in liquid-phase over supportedprecious-metal-catalysts [16–18]. Catalytic oxidation of theterminal hydroxyl gave glyceric acid and tartronic acid, oxidationof the central hydroxyl offered dihydroxyacetone, and oxidation ofboth hydroxyls produced hydroxypyruvic acid and mesoxalic acid.Products such as oxalic acid, hydroxyethanoic acid and formic acidwere obtained with C–C bond breaking. These processes have beenrecently summarized in several review papers [19–26].

Catalytic dehydration of glycerol to acrolein has attracted greatinterests recently. Acrolein is produced by the oxidation of propeneover mixed oxide catalysts, such as Bi–Mo catalysts [27]. Theoxidation of acrolein gives acrylic acid, which is the maincomponent of hygienic pads and diapers owing to its superabsorbent properties. Mixed oxides, such as Mo–V–W–Cu–O, Mo–V–Te–O and Mo–V–Sb–O oxides, were reported as effectivecatalysts in acrolein oxidation [28,29]. Currently 85% of acrylicacid is produced by acrolein oxidation. The production of acrylicacid greatly depends on the production of acrolein, which highlyrelies on proplylene obtained by cracking petroleum fractions.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–3226

Therefore, it is very important to develop biosource-based strategyto produce acrolein.

Dubois et al. reported in patents that WO3–ZrO2 systems couldcatalyze glycerol dehydration to acrolein in high selectivity [30–32]. The 65% yield of acrolein was achieved at 100% conversion ofglycerol by using a 15 wt% WO3/ZrO2 catalyst. The addition ofmolecular oxygen with glycerol made it possible to maintain theglycerol conversion and the acrolein yield, while reducing theformation of by-products. Recently Ulgen and Holderich investi-gated the system in details [11]. A maximum selectivity of 74%toward acrolein was achieved at 73% conversion of glycerol. Theyalso found that the addition of oxygen could reduce the formationof side-products.

Liu et al. recently reported glycerol dehydration over rare earthpyrophosphates catalysts in gas phase [14]. They found that thepyrophosphate catalysts were active and selective toward acroleinproduction. They reported that catalytic results were greatlyinfluenced by the doped rare earth elements and by catalystsurface acidity. Among the tested catalysts, Nd4(P2O7)3 exhibitedthe best result with 80% selectivity to acrolein at 87% conversion ofglycerol.

The well-known pyrophosphate phase catalysts are vanadiumpyrophosphate oxides (VPO). These catalysts have shown excellentactivities in selective oxidation of butane to maleic anhydride andthus are used as the main components of industrial catalysts forthe reaction. The oxidation mechanism of the VPO catalysts is toapply the redox circle of V5+–V4+–V3+ located in a certaincrystalline phase or crystalline plane. Kamiya et al. reported thatthe acidity of the VPO catalysts was affected by surface P/V ratio,which was influenced by thermal treatment temperature [33].They reported that the total amount of Bronsted and Lewis acidsites of several VPO catalysts prepared in different methods variedin the range of 0.26–0.31 mmol m�2.

We were interested in glycerol dehydration over VPO oxidescatalysts because of their mild acid-basic properties. We addedmolecular oxygen into reaction in order to avoid coke formationand to reduce side reactions. The present research was alsomotivated by direct conversion of glycerol to acrylic acid. As analternative to current proplylene route, acrylic acid can beproduced in tandem-type reactors with glycerol dehydrationand acrolein oxidation reactions in two reactors since bothreactions have been well progressed. However, it will be simplerand more economic to integrate two reactions on one catalyst.Because the dehydration reaction is endothermic and the oxidationreaction is exothermic, by the integration of two reactions on onecatalyst bed we can realize a better heat balance.

2. Experimental

2.1. Materials

All reagents were reagents of analytical-reagent grade pur-chased from Wako Pure Chemical Industries Ltd. and were usedwithout further purification. Distilled water prepared by using aYamato Autostill WG25 (Tokyo, Japan) was used throughout thiswork.

2.2. Preparation of catalyst

Dihydrate VOPO4�2H2O was prepared by a method described inthe literature [34]. Briefly, a mixture of V2O5 (24 g), H3PO4 (85% (w/w), 133 mL), and H2O (577 mL) was refluxed at 115 8C for 16 h. Theresulting precipitate was collected by filtration, washed with100 mL acetone, and dried for 16 h under ambient conditions. BothX-ray diffraction patterns (XRD) and infrared (IR) characterizationsindicated that the solid was VOPO4�2H2O. A suspension of

VOPO4�2H2O (5 g) powder in 2-butanol (50 mL) was stirred underreflux for 23 h. The resulting light-blue solid was collected byfiltration, washed with 100 mL acetone and dried for 16 h underambient conditions. The solid was confirmed to be hemihydrateVOHPO4�0.5H2O by XRD and IR. Different batches of VOH-PO4�0.5H2O were identical as checked by XRD and IR. Severalbatches were collected together to obtain enough weight for nextstep preparation. Thermal treatment of VOHPO4�0.5H2O (2.0 g) togive pyrophosphate phase (VO)2P2O7 was conducted in a 40-mLnitrogen flow at a temperature of 500, 600, 700, 800 and 900 8C for2 h, respectively. These catalysts are designated as VPO-n. Forexample, VPO-5 represents a sample with the treatment tempera-ture of 500 8C.

2.3. Characterization of samples

XRD characterizations were performed with a Rigaku, RINTUltima+ diffractometer with Cu Ka radiation (Ka 1.54056 A) and X-ray power of 40 kV/20 mA. Field emission scanning electronmicroscopy (FE-SEM) was performed on a JSM-7400F (JEOL).Samples for SEM were dusted on an adhesive conductive carbonpaper attached to a brass mount. Specific surface areas (SBET) weremeasured by nitrogen adsorption at �196.15 8C using Brunauer–Emmett–Teller method (BET) over an Autosorb 6AG (Quanta-chrome Instruments). Before the measurements, the samples werepretreated at 200 8C for 48 h under vacuum in order to desorbimpurities from surface. The surface area was calculated from 5adsorption isotherm points at relative nitrogen pressures (p/p0)between 0.05 and 0.3. The XP spectra (XPS) were obtained by usinga JPC-9010mC. A monochromated MgKa (10 kV; 10 mA) line wasused as X-ray source. The pressure in the analyzing chamber was1 � 10�6 Torr during measurement. Binding energies of peakswere corrected by referencing the C 1s signal of adventitiouscontamination hydrocarbon to 284.2 eV. The measurements ofthermogravimetric–differential thermal analysis (TG–DTA) wereperformed on a TG-8120 (Rigaku) thermogravimetric analyzer. Dryair provided by a pressure tank with a flow rate of 30 mL min�1

was used as the carrier gas. The catalyst sample and referencesubstance (a-alumina) were loaded into two separate pans. Theseexperiments were conducted from room temperature to 900 8C at aheating speed of 10 8C min�1. The baseline was subtracted from ablank run without loading sample. Temperature-programmeddecomposition mass spectrometry (TPD-MS) measurements wereperformed from 120 to 600 8C at a heating rate of 10 8C min�1 inhelium flow (50 mL min�1). The decomposed gas molecules weremonitored by a mass spectrometer (ANELVA, Quadrupole MassSpectrometer, M-100QA, BEL Japan). Several mass fragments werecollected: acetone (m/z 58), carbon dioxide (44), oxygen (32),carbon oxide (28), water (18, 17, 16). Temperature programmeddesorption (TPD) of ammonia (NH3-TPD) was employed tomeasure catalyst surface acidity. The experiment was carriedout on a BELSORP apparatus. The experimental procedure was asfollows: ca. 80 mg of catalyst was set up between two layers ofquartz wool and was pre-heated under helium (50 mL min�1) at200 8C for 1 h. Then, ammonia was introduced at 100 8C for 30 min.The desorption profile was recorded with a mass spectrometer(ANELVA, Quadrupole Mass Spectrometer, M-100QA, BEL Japan)from 100 to 600 8C under helium flow (50 mL min�1).

2.4. Catalytic reaction

Glycerol dehydration reactions were conducted in a verticalfix-bed reactor under atmospheric pressure. The home-madePyrex reactor with an internal diameter of 5 mm was used. Amixture of catalyst and corundum (50–70 mesh) was loaded in themiddle section of the reactor, with quartz wool packed in both

Fig. 1. XRD patterns of the fresh VOHPO4�0.5H2O and the thermally treated

VOHPO4�0.5H2O from 500 to 900 8C for 2 h at a heating rate of 10 8C min�1 in

nitrogen, (a) fresh VOHPO4�0.5H2O, (b) VPO-5, (c) VPO-6, (d) VPO-7, (e) VPO-8, and

(f) VPO-9. Spectra are offset for clarity.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–32 27

ends. A thermocouple to monitor the temperature of evaporatingzone and a syringe to add aqueous glycerol were placed in aposition of 10-cm higher than the top of catalyst bed. The reactiontemperature was monitored by a thermocouple inserted into themiddle of catalyst bed. Before reaction, the catalyst waspretreated at the reaction temperature of 300 8C for 1 h innitrogen at a flow rate of 18 mL min�1. Oxygen was mixed withnitrogen 10 min before starting reaction by turning on glycerolflowing. Typically, 0.2 g catalyst was used and was diluted with3.0 g corundum. Aqueous glycerol (20%, w/w) was fed at a speed of0.50 g h�1 by a syringe pump. The composition of fed gas wasnitrogen: oxygen: water: glycerol, equal to 66.6:1.7:30.3:1.5 inmolar ratio. The sampling interval was 10 h. The products werecollected in an ice-water cold trap directly connected to the outletof the reactor. About 0.02 g of 2-butanol as internal standard in5 mL water was first loaded into the cold trap. The collectedsamples were quickly analyzed by means of GC–MS (Shimadzu15A, Japan) with an auto-sampler equipped with a capillarycolumn (GL Sciences, TC-FFAP 60 m � 0.25 mm � 0.5 mm) and aflame ionization detector (FID). The chromatograph column wasrun at a program from 110 to 250 8C at a ramping rate of5 8C min�1 and kept at 250 8C for 10 min. The gas phase wasanalyzed by three online GCs with two thermal conductivitydetectors (TCD) and one FID detector. The analysis of gas phaseallowed the quantifications of carbon oxide, carbon dioxide,acetaldehyde, acrolein and acetic acid.

2.5. Data processing

The contributions of all GC analyses for liquid phase and gasphase were joined to evaluate conversion, selectivity and carbonbalance. Calculations of glycerol conversion (Xgly) were based onthe concentration of glycerol at the inlet and the outlet of thereactor:

Xgly ¼Mgly; in �Mgly; out

Mgly; in; (1)

where Mgly, in and Mgly, out are the quantities of fed and unreactedglycerol, respectively.

Nitrogen was used as internal standard for analyzing gas phaseproducts. The conversion of oxygen was determined by measuringintegration GC area ratios of oxygen to nitrogen for both the inletand the outlet gas (Eq. (2)). However, the values of summing uptotal detected oxygen from oxidized products, such as acetic acid,acrylic acid, CO and CO2, were always smaller than Xoxy. There arethree factors affecting the difference. First, water was not includedinto the calculation except through mass balance of eachindividual reaction. Second, some unknown products probablycontributing to oxygen consumption was not included into thecalculation. Third, some oxidized products were unable to bedetected by GC–MS due to small quantity or decomposition duringanalysis.

Xoxy ¼RO2=N2 ; in � RO2=N2 ; out

RO2=N2 ; in; (2)

where, RO2=N2 ; inand RO2=N2 ; out are the ratios of integrated GC area at

the inlet and the outlet of the reactor, respectively.Carbon balance was calculated by summing up the unreacted

glycerol and the detected and calibrated products. Carbon balanceswere always less than 100%. There are several reasons for thedifference. First, some heavy products, like coke and unknownproducts, were not included into calculations because of uncertainquantity. Second, some minor peaks in chromatograms were notquantified. Third, some products might be decomposed duringanalysis, such as in GC injector. The selectivity of product I (Si) was

calculated from the molar amount of the product I and theconverted glycerol according to Eq. (3):

Si ¼Mi

Mgly; in �Mgly; out� zi

3� 100%; (3)

where, z represents the number of carbon atom, for example, z = 3for glycerol; Mi represents the detected molar amount of theproduct I.

3. Results and discussions

3.1. Catalyst characterization by XRD

XRD patterns of the thermally treated samples and the catalystprecursor are shown in Fig. 1. The XRD pattern for the catalystprecursor matches that reported for the pure hemihydrateVOHPO4�0.5H2O phase with good crystallinity. As shown byXRD, the only crystalline phase detected in the thermally treatedsamples is the pyrophosphate (VO)2P2O7 [35], which is the phasethat is considered to be the most active one for n-butane partialoxidation reactions [36]. The crystallinity of our samples isstrongly dependent on treatment temperature. For example, it isobserved in Fig. 1 that diffraction peaks for the (2 0 0) at 23.048,(0 2 4) at 28.488 and (0 3 2) at 30.028 planes narrow and intensifywith increasing treatment temperature [37]. The average crystal-lite sizes (nm) estimated by the Sherrer’s equation from the mostintensive (2 0 0) peak are shown in Fig. 2. The crystallite sizesincreased slowly from 10.1 nm for VPO-5 to 23.7 nm for VPO-8 andrapidly increased to 110.0 nm at VPO-9. These variations can beassigned to the change in the crystal structure of the VPO catalystsduring thermal treatment. The ratios of XRD intensity of I200 to I024

increased by threefold from 1.5 to 4.5, indicating that the crystalstacking order changed, and the pellet structure was welldeveloped during thermal treatment. The most important mod-ifications occurred between 800 and 900 8C.

3.2. Catalyst characterization by SEM

Increase in treatment temperature normally brings aboutchanges in structural as well as textural properties. The lattercan be visible by microscopy images. Characterizations by SEM

Fig. 2. The dependence of calculated crystallite sizes on thermal treatment

temperature.

Fig. 4. UV–vis spectra of the fresh VOHPO4�0.5H2O and the thermally treated

VOHPO4�0.5H2O, (—) fresh VOHPO4�0.5H2O, (~) VPO-5, (&) VPO-6, (^) VPO-7, (5)

VPO-8, (*) VPO-9.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–3228

show that there was no big change of pellet morphology for thesamples from VPO-5 to VPO-7 (Fig. 3). Small pellets and pores werevisible in the sample VPO-8 and surface roughness was increased.Small pellets with smooth surface were formed in the sample VPO-9, which is indicative of minor sintering behavior.

3.3. Catalyst characterizations by UV–vis

UV–vis spectra are shown in Fig. 4. The spectra for the thermallytreated samples match that reported for (VO)2P2O7 [38]. Anintensive band observed at 300 nm is attributed to the O2�–V4+

charge transfer, which did not appear on the precursor VOH-PO4�0.5H2O. The band at 500 nm is due to V5+ in an octahedralenvironment. The two bands at 650 nm and at 850–900 nm arecorresponding to the same V4+ in the octahedral symmetry [39].The dependences of V4+ at 650 nm and V5+ at 500 nm on treatmenttemperature are depicted in Fig. 5. Our observation is consistentwith several suggestions that the V4/V5+ couple was formed on the(VO)2P2O7 surface of highly crystalline catalysts [40,41]. The ratio

Fig. 3. Representative microcopy images of the fresh VOHPO4�0.5H2O and the thermally t

VPO-6, (d) VPO-7, (e) VPO-8, and (f) VPO-9.

of V4+ to V5+ increases with increasing temperature. The UV–vischaracterization confirms that the intensification of the (2 0 0)diffraction peak in XRD pattern and the surface morphologychange in SEM images from 700 to 900 8C are caused by phasereduction.

3.4. Catalyst characterization by XPS

The results of XPS characterization were analyzed by usingliterature method [42]. The stoichiometric P/V ratio of vanadiumpyrophosphate oxide is 1. The XPS analyses reveal that the actual P/V ratios of our samples were in the rage of 2–1.8, indicating that thesample surface was enriched with phosphorous element (Fig. 6). Itwas considered by other authors that surface phosphorusenrichment is probably caused by phase reduction [42], whichwas also consistent with our UV–vis analyses. The P/V ratiosdecreased with increase in treatment temperature, indicating theloss of phosphorus. On the other hand, we observed the shift of theP(2p3/2) peak toward higher binding energy with increasing

reated VOHPO4�0.5H2O from 500 to 900 8C, (a) fresh VOHPO4�0.5H2O, (b) VPO-5, (c)

Fig. 5. The dependence of peak intensity on treatment temperature, (a) 500 nm

(V5+), (b) 650 nm (V4+), and (c) the ratio of 650–500 nm.

Fig. 7. Shift of P 2p3/2 bonding energy of the thermally treated VOHPO4�0.5H2O, (a)

VPO-5, (b) VPO-6, (c) VPO-7, (d) VPO-8, and (e) VPO-9. Spectra are offset for clarity.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–32 29

temperature (Fig. 7) [43]. The shift is probably due to the formationof P–O–P linkages. We used C(1s) charge referencing to determinethe shifts of the P(2p3/2) transitions, and it is reported that suchcarbon reference has potential inaccuracies inherent in such atechnique [42]. However, the C(1s) line shapes were similar in allsamples, suggesting that the nature of the carbon did not differbetween samples and that such small shift of P(2p3/2) bondingenergy is due to phase reduction. The P/V ratios decreased withincreasing treatment temperature, which may be attributed toprogressive loss of phosphorus or changes of crystalline stackingorder.

3.5. Catalyst characterizations by TG–DTA and TPD-MS

We investigated the thermal treatment process of the precursorVOHPO4�0.5H2O in nitrogen by TG–DTA and TPD-MS techniques(Fig. 8). A distinctive weight loss peak appearing from 376 to 478 8Cin TG curve and an endothermic peak in DTA curve is due to theformation of pyrophosphate phase by dehydration, which iscorresponding to a water peak in the MS curve. Further increase intemperature to 900 8C resulted in continuous and tiny weight loss,which may be due to the progressive dehydration and to the

Fig. 6. Surface P/V ratios as a function of treatment temperature. The results are

calculated based on XPS data.

formation of crystal defects owing to phase reduction or loss ofphosphorus. The observation of a peak of amu 32 (oxygen) at about780 8C implies phase reduction.

3.6. Catalytic results and discussions

The catalyst precursor VOHPO4�0.5H2O was treated at atemperature of 500–900 8C in nitrogen atmosphere to obtain fiveVPO catalysts. The SBET was 3.4 m2 g�1 for VPO-500 and increasedto 13.9 m2 g�1 for VPO-600 since the thermal treatment result insmaller particles. Further increase in temperature led to thedecrease of SBET to 7.0 m2 g�1 for VPO-9. The SBET reported here are

Fig. 8. TG–DTA analysis of the catalyst precursor VOHPO4�0.5H2O in nitrogen

atmosphere at a heating rate of 10 8C min�1 and TPD-MS results of VOHPO4�0.5H2O

in helium at a heating rate of 10 8C min�1.

Table 1Catalytic result over the VPO catalysts.

Catalysta: VPO-5 VPO-6 VPO-7 VPO-8 VPO-9

SBET [m2 g�1]: 3.4 13.9 12.2 10.0 7.0

Acidity [mmol g�1]b: 1.5 1.8 2.0 2.8 2.4

Xgly [mol/mol%]c 47 61 81 100 97

Xoxy [mol/mol%]d 100 100 95 80 58

CB [mol/mol%]e 85 78 78 77 60

S (individual products) [%]

Acetaldehyde 8 7 9 8 10

Acetic acid 1 1 2 2 2

Acrolein 39 33 55 64 35

1-Propanal 1 1 0 0 0

Hydroxyacetone 21 23 9 6 15

Acrylic acid 1 1 1 0 1

1,3-Dioxane-5-ol 3 3 4 2 6

Unknown1 (m/z 137)f 15 17 10 7 14

Unknown2 (m/z 129)f 9 7 9 7 6

CO 1 0 0 0 0

CO2 3 2 1 1 1

Othersg 0 4 1 4 11

a Reaction conditions: 0.2 g catalyst was used and was diluted with 3.0 g

corundum. Reaction temperature 300 8C. The aqueous glycerol (20%, w/w) was fed

at a speed of 0.50 g h�1. The composition of fed gas was N2:O2:H2O:glycerol, equal to

66.6:1.7:30.3:1.5 in molar ratio. The sampling interval was 10 h.b Catalyst acidity was measured by NH3-TPD.c Conversion of glycerol.d Conversion of oxygen.e Carbon balance.f The detailed chemical information of the two products remain unknown. Their

selectivities are determined by using the coefficient factor of glycerol. Unknown1

(m/z, intensity): 137 (2%), 117 (4%), 85 (2%), 73 (3%), 100 (5%), 57 (10%), 43 (100%).

unknown2 (m/z, intensity): 129 (2%), 117 (3%), 100 (8%), 73 (3%), 58 (8%), 43 (100%).g The selectivity to others is determined by 100-sum of all detected products.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–3230

all smaller than 30 m2 g�1, typical values of VPO catalysts preparedin organic media. The catalyst acidities measured by NH3-TPDincreased with treatment temperature and had a maximum valueof 2.7 mmol g�1 for VPO-8. The catalyst acidities were in the rangeof VPO catalysts acidities as reported by Kamiya et al. [33].

The conversions and product distributions in glycerol dehydra-tion in the presence of molecular oxygen are shown in Table 1. Theglycerol conversion increased with treatment temperature from47% for VPO-5 to 100% for VPO-8. Glycerol conversion increasedwith catalyst acidities as depicted in Fig. 9. The VPO-8 catalystexhibited the best result in this study with 64% selectivity foracrolein at 100% conversion of glycerol. The total selectivities ofdetected products were more than 95% except one reaction usingVPO-9 catalyst, which has the selectivity of 90%. The major

Fig. 9. The relationship between conversion of glycerol and catalyst acidity

determined by NH3-TPD.

products were acrolein, acetaldehyde and hydroxyacetone, ac-companied by acetic acid and acrylic acid. Some other by-products,such as acetone and propanoic acid with the selectivity of less than1% are not given in Table 1. These results showed that our catalystwas superior to WO3–ZrO2 catalyst (73% glycerol conversion and74% selectivity for acrolein) [11], and was comparable toNd4(P2O7)3 catalyst (99% glycerol conversion and 73% selectivityfor acrolein) [14]. The products over VPO catalysts were mainlyfrom dehydration and oxidation reactions, and in comparison,products of literature works were from dehydration, hydrogena-tion and C–C bond breaking reactions. This difference wasattributed to oxidative atmosphere in the present research. Theglycerol dehydration reaction without oxygen is generallyaccompanied by side reactions giving by-products such ashydroxypropanone, propanaldehyde, glycerol polycondensationproducts and cyclic glycerol ethers, which can cause the formationof coke on catalysts and will result in catalyst deactivation,reduction in yield and in the selectivity toward acrolein. Duboiset al. found that over W–Zr oxide catalysts, by carrying out thereaction in the presence of oxygen it was possible to obtain higherglycerol conversion by inhibiting, first, the formation of by-products, particularly those originating from hydroxypropanoneand the hydrogenation of dehydrated products such as propa-naldehyde, second, the deactivation of the catalyst by reducing theformation of coke [30,32]. However, we did not observe theformation of propionaldehyde and hydroxypropanone. These twoproducts are not desired because they are difficult to be isolatedand necessitate separation and purification steps which lead to ahigh recovery cost for the purified acrolein [2].

Previous studies have suggested that the dehydration ofglycerol proceeds in two different ways. When protonation occursat the central hydroxyl group of glycerol, a water molecule and aproton are eliminated from the protonated glycerol, and then 3-hydroxypropanal is produced by tautomerism, which is unstableand readily dehydrated into acrolein. On the other hand, whenprotonation proceeds at the terminal hydroxyl group of glycerol,hydroxyacetone is produced through dehydration and deprotona-tion accompanied by tautomerism. The dependence of theselectivity ratio of acrolein to hydroxyacetone is plotted againcatalyst treatment temperature in Fig. 10. The nature of acid sitesmay play a key role in determining the ratio. The change of the ratioreflects the treatment temperature influence on two types of activesites, dehydration at the central carbon site and the terminal one.Higher treatment temperature was favorable for the dehydrationat the central carbon site. The maximum ratio of acrolein tohydroxyacetone equal to 11.7 was achieved in the sample VPO-8.

We showed that in order to achieve high glycerol conversionand acrolein selectivity, it was necessary to pretreat catalyst

Fig. 10. The dependence of selectivity ratio of acrolein to hydroxyacetone (~) and

conversion of oxygen (*) on thermal treatment temperature.

Fig. 11. The comparison of XRD patterns of the fresh VPO-8 and the used VPO-8.

Fig. 13. The TG–DTA analyses of the used catalysts in the reactions with and without

oxygen.

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–32 31

precursor at higher temperature such as at 800 8C. The tempera-ture was much higher than usual treatment temperature in therange of 400–600 8C for applications in hydrocarbon oxidation. Itwas reported that strong Lewis acid catalysts such as Al2O3 andNb2O5 showed a poor selectivity for acrolein, and typical Bronstedacids like 5% H3PO4/a-Al2O3 and heteropoly acids offered highselectivity for acrolein [15]. Therefore, the possible reason forhigher treatment temperature was that under these conditionsacid sites, for example Lewis acid sites were generated on thecatalyst surface. Because the reactions were run at high humidity(gas phase composition: water 30.3 mol%), there is a greatpossibility that some Lewis acid sites were converted to Bronstedacid sites, which might catalyze glycerol dehydration. Character-izations of the used VPO-8 catalyst by XRD revealed that itscrystalline structure did not change after reaction (Fig. 11).

The conversion of oxygen is plotted against thermal treatmenttemperature in Fig. 10. The conversion starts to decrease from 100%for VPO-600 to 58% for VPO-900. Although all reactions werecarried out in the presence of excess oxygen to glycerol, we did notobserve remarkable oxidation of acrolein to acrylic acid. There aretwo products (unknown1 and unknown2) whose structures werenot exactly known yet. However, GC–MS analysis data suggested

Fig. 12. The comparison of two reaction results with and without adding oxygen.

that the two products were acetal of acetaldehyde and glycerol orisomers.

Catalytic results of two reactions with (reaction 1) and without(reaction 2) adding oxygen over the VPO-8 catalyst are given inFig. 12. The conversion of glycerol decreased to 95% in reaction 2,compared with 100% in reaction 1. Notably, the carbon balancegreatly decreased to 66% in reaction 2. The selectivity towardacrolein also decreased to 60% in reaction 2. On the contrary, theselectivities toward acetaldehyde and hydroxyacetone increased.Selectivity to other by-products increased from 4% in reaction 1 byalmost 6 times to 20% in reaction 2, indicating that side reactionsoccurred when molecular oxygen was not added. Similar positiveeffect of oxygen were reported in the works by Dubois et al. [30,32]and Ulgen and Holderich [11].

In the study, the addition of oxygen was proved to be efficientin avoiding the formation of carbon and reducing adsorption ofheave carbon species. After reaction, the catalyst color in reaction1 was still gray, but the catalyst color in reaction 2 became black,indicating that carbon deposit was formed on the latter catalyst.The used catalysts in the two reactions were characterized by TG–DTA technique (Fig. 13). The experiments were conducted in dryair at a heating rate of 10 8C min�1. Each weight loss step wascorresponding to an exothermal peak in the DTA curve. Theanalyses revealed that the used catalyst in the reaction 1 had atotal weight loss of 2.1% at 600 8C, which was much smaller than8.4% of the used catalyst in the reaction 2. The catalyst in thereaction 1 had only one weight loss step, with the centraltemperature of 340 8C, which was similar to the temperature ofthe first weight loss step of the catalyst in the reaction 2. The peakwas attributed to the combustion of adsorbed heavy hydrocarbonspecies, such as glycerol acetals. The second peak of the usedcatalyst in the reaction 2 appeared at 501 8C. The used catalyst inthe reaction 1 did not have the similar peak in its TG curve. Suchhigh combustion temperature suggested that intensive carbonspecies, such as carbon coke, were formed on the catalyst in thereaction 2.

4. Conclusion

The glycerol dehydration activity depended on treatmenttemperature on the catalyst precursor VOHPO4�0.5H2O. It wascharacterized that treatment temperature changed crystallinestacking order, textural morphology, surface P/V ratio and acidity.High temperature treatment led to phase reduction and progres-sive structural dehydration, brought out loss of phosphorus andgenerated acidic sites. The catalyst treated at 800 8C (VPO-8)exhibited the best result with 100% glycerol conversion and more

F. Wang et al. / Applied Catalysis A: General 376 (2010) 25–3232

than 95% selectivity for useful products, including acrolein (64%),acetaldehyde, acetic acid and hydroxyacetone. A comparisonbetween two reactions with adding oxygen and without addingoxygen showed that the addition of molecular oxygen into reactioncould greatly reduce the selectivity for by-products from 20% to 4%and could avoid coke formation. This study showed that vanadiumpyrophosphate oxides were less active toward the one-stepconversion of glycerol to acrylic acid. Future work will be focusedon improving oxidation ability of the catalysts by doping withother metals.

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