High Surface Area Vanadium Phosphate Catalysts for n-Butane

Ind. Eng. Chem. Res. 2009, 48, 7517–7528

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High Surface Area Vanadium Phosphate Catalysts for n-Butane Oxidation Ali Asghar Rownaghi,*,† Yun Hin Taufiq-Yap,‡ and Fateme Rezaei† Department of Chemical Engineering, Monash UniVersity, Victoria 3800, Australia, and Department of Chemistry, UniVersiti Putra Malaysia, 43400, Selangor, Malaysia

Vanadium phosphorus oxide (VPO) was prepared using the precipitation procedure and tested for potential use in the partial oxidation reaction of n-butane to maleic anhydride. In particular, the effect of reducing agents such as the isobutanol, 1-butanol, and glycol, subsequent water treatment, and microwave heating were investigated in detail. The optimum synthesis conditions were identified with respect to catalyst activity for the oxidation of n-butane. The activity and selectivity of VPO prepared catalysts have been evaluated in a fixed bed microreactor and in situ gas chromatography (GC) was used to evaluate the system efficiency and analyze the product effluent stream. The different catalysts exhibited a range of activities and selectivities under the same reaction conditions. The range in catalyst performance may be attributed to the crystal size as well as particle size of catalyst. The results were interpreted in terms of surface area and catalyst nanostructure, and it has been generally concluded that the catalyst surface area is enhanced by the employment of glycol as the reducing agent, followed refluxing by distilled water and drying by microwave irradiation. The catalyst produced using this method is the most active and selective catalyst for partial oxidation of n-butane to maleic anhydride. The catalyst lifetime was tested under the optimum reaction conditions, and the catalyst was found to be highly stable for more than 70 h. The characterization of both precursors and calcined catalysts was carried out using X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Brunauer-Emmer-Teller (BET) surface area measurement, temperature programmed reduction (H2-TPR), and scanning electron microscopy (SEM). 1. Introduction The low alkanes, particularly propane and butane, can be easily obtained from liquefied petroleum gas. Propane and butane can also be used as feedstock in the production of chemical intermediates. Valuable oxygenated compounds such as alkenes are produced from the partial oxidation or dehydrogenation processes of low alkanes. Vanadium phosphate materials have been used as catalysts for selective conversion of butane since their discovery in 1966. This catalyst is extensively used for commercial production of maleic anhydride (MA) and typically butane, below the lower explosive limit, is reacted with air at temperatures in the range of 350-480 °C. Several technologies have been commercialized for MA productions which are based on the selective gas phase oxidation of butane over vanadium phosphorus oxide (VPO) catalysts including fixed beds, fluid beds, and circulating fluidized bed (CFB) reactor technology. In particular, the reason for the lack of understanding of reaction kinetics is related to the operating conditions of the reactors. Fixed beds typically operate under oxidizing conditions (∼1.8% vol butane in air). Fluid beds operate with higher butane concentrations of up to 4%, while circulating fluidized beds may operate with butane concentrations as high as 10% and even up to 20%.1,2 Under fuel-lean conditions (1.5% butane in air), the catalysts have to be operated at high conversion to achieve a significant yield of maleic anhydride. At any temperature, selectivity is a function of conversion which can be significantly increased if the catalysts can be operated at a lower temperature while maintaining high conversion. In this respect, preparation methods that produce * To whom correspondence should be addressed. Tel: +613 9905 1870. Fax: +613 9905 5689. E-mail address: ali.rownaghi@ eng.monash.edu.au; [email protected]. † Monash University. ‡ Universiti Putra Malaysia.

high-activity catalysts are favored, providing lower operational temperatures, thereby giving improved selectivity.3-6 As with VPO catalyst, it is well-established that vanadyl pyrophosphate behaves as a substrate for other VPO phases such as R-, -γ, and β-VOPO4 and plays an important role in hydrocarbon oxidation processes. In contrast, some researchers believe that the V5+/V4+ dimeric species present in the topmost oxidized layer of vanadyl pyrophosphate are responsible for catalyst activity.7-9 However, the fact is that both phase composition and catalytic activity of VPO catalysts are highly dependent on the synthesis route of precursors. To date, too many works have been devoted to study the synthesis of VPO catalysts using VOHPO4 · 0.5H2O as catalyst precursor which, under certain reaction conditions, is topotactically transformed to give (VO)2P2O7.10,11 Basically, catalyst precursor is synthesized by the reaction of V2O5 with H3PO4 using an appropriate reducing agent along with a solvent material. A large number of reducing agents and solvents have been utilized for the synthesis of VOHPO4 · 0.5H2O precursor. Successful preparation of precursor (VOHPO4 · 0.5H2O) is the key factor to obtain an effective catalyst.10-15 In the study performed by Bartley et al.,16 although water was employed as a solvent, however, the resulted catalyst showed a poor selectivity in maleic anhydride production reaction. Microwave irradiation has been shown to improve catalyst properties, and catalysts synthesized by the microwave route represent better performance than conventional catalysts.17 Solid oxide materials are more susceptible to absorb microwave energy and subsequently convert to heat, providing an increased heating rate and efficient drying.18-21 The use of ethylene glycol as reducing agent for replacement of C2-C4 and aromatic alcohols has not been widely investigated before.11 Therefore, it is the aim of this study to investigate the effects of water treatment and reducing agents such as isobutanol, 1-butanol,

10.1021/ie900238a CCC: $40.75  2009 American Chemical Society Published on Web 07/16/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009

Table 1. Preparation Condition of the VPO Catalyst precursor

catalysts

preparation condition

precursor heating

VHPA1 VHPA2 VHPB1 VHPB2 VHPC1 VHPD1

VPOA1 VPOA2 VPOB1 VPOB2 VPOC1 VPOD2

microwave conventional conventional conventional microwave microwave

VHPC2 VHPD2

VPOC2 VPOD2

VPD1 + isobutyl alcohol VPD2 + isobutyl alcohol VPD1 + 1-butanol VPD2 + 1-butanol VPD1 + ethylene glycol VHPC1 + 2 h reflux with distilled water (DW) VPD2 + ethylene glycol VHPC2 + 2 h reflux with distilled water (DW)

conventional conventional

and ethylene glycol on the catalytic performance of VPO for n-butane oxidation reaction. In particular, the aim of this work is to investigate the effects of alcohol solvents, water treatment, and microwave heating as drying devices on catalytic performance of precipitated vanadium phosphate catalysts for selective oxidation of n-butane. Structural and morphological investigation by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), Brunauer-Emmer-Teller (BET) surface area measurement, temperature programmed reduction (H2-TPR), and scanning electron microscopy (SEM) is discussed. 2. Experimental Section 2.1. Catalyst Preparation. All reagents were commercially obtained and employed without performing any purification. Preparation conditions under which the samples were synthesized are listed in Table 1. 2.1.1. VOPO4 · 2H2O. Vanadyl phosphate dihydrate (VOPO4 · 2H2O, VPD) was prepared according to the procedure described by Johnson et al.22 A mixture of vanadium(V) pentoxide, V2O5 (8.0 g from Fisher Chemicals), aqueous 85% ortho-phosphoric acid (38.3 mL, from Fisher Chemicals), and H2O (24 mL H2O/g solid) was heated to 393 K while stirring for 16 h and a homogeneous yellow precipitate was obtained. The resulting precipitate was separated by filtration, and washed with hot distilled water (100 mL) and acetone (100 mL). The yellow solid was then divided into two parts. One part was heated by microwave irradiation method (denoted VPD1) for 5 min. The heating conditions were set at 2450 MHz and an output power of 300 W. The second part was heated in the oven (denoted VPD2) for 16 h at 393 K under ambient atmosphere. The resulting yellow solids were identified to be VOPO4 · 2H2O by XRD. 2.1.2. VOHPO4 · 0.5H2O. Vanadyl hydrogen phosphate hemihydrate (VOHPO4 · 0.5H2O, VHP) was synthesized by the reduction of VOPO4 · 2H2O in isobutanol, 1-butanol, and ethylene glycol. The detailed description of precursor production was reported previously.22 A mixture of VOPO4 · 2H2O was reduced by isobutanol, 1-butanol, and ethylene glycol (80 mL, from BDH); the solution then was refluxed for 24 h until a light blue precipitate was formed. Once the reduction reaction was completed, the obtained blue precipitate was recovered by filtration and washed with hot distilled water (100 mL) and acetone (100 mL). The products were dried with both microwave heating and oven for 2 min and 12 h, respectively, to obtain precursor VOHPO4 · 0.5H2O. All precursors synthesized by both heating methods contained VOHPO4 · 0.5H2O which were identified by powder X-ray diffraction. In following steps, 2 g of precursor obtained by ethylene glycol was first refluxed in distilled water (20 mL H2O/g solid) for 2 h and then recovered by hot filtration followed by microwave and conventional heating drying as described above. All dried precursors were calcined in a flow of n-butane/air mixture for 6 h at 733 K.

2.2. Catalyst Characterization. 2.2.1. X-ray Diffraction (XRD). The X-ray diffraction (XRD) analyses were carried out using a Shimadzu diffractometer model XRD 6000 employing Cu KR radiation to generate diffraction patterns from powder crystalline samples at ambient temperature. 2.2.2. Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). The bulk chemical composition was determined by using a sequential scanning inductively coupled plasma-atomic emission spectrometer (ICP-AES) Perkin-Elmer Emission Spectrometer Model Plasma 1000. 2.2.3. BET Surface Area. The total surface area of catalysts was measured by the BET (Brunauer-Emmer-Teller) method using nitrogen adsorption at 77 K. The experiment was performed by Sorptomatic 1990 series nitrogen adsorption/ desorption analyzer. 2.2.4. Redox Titration. Redox titration was carried out using the method of Niwa and Murakami23 to estimate the average oxidation number of vanadium. 2.2.5. Scanning Electron Microscope (SEM). Surface morphology of the catalysts was observed under a scanning electron microscope (SEM), using a LEO operated at accelerating voltages of 15 kV. The samples were prepared by dispersing the catalyst powder on a metallic sample holder using a doublesided tape to keep them on the holder. The samples were coated with a thin layer of gold using a BIO-RAS Sputter Coater. Micrographs were recorded at various magnifications. 2.2.6. Temperature-Programmed Reduction in H2 (H2TPR). H2-TPR was carried out in order to observe the reducibility of the VPO catalyst by using a ThermoFinnigan TPDRO 1110 apparatus utilizing a thermal conductivity detector (TCD). The H2-TPR experiment was performed using a quartz reactor tube (4 mm i.d.), in which a ∼25 mg sample was mounted on loosely packed quartz wool. Prior to H2-TPR measurement, a catalyst was pretreated in N2 at 473 K (heating rate of 10 K min-1 and hold time 30 min) and then cooled down under He. The reduction gas was composed of 5 vol % H2 in Ar. The reaction temperature was programmed to rise at a constant rate of 10 K min-1. A thermocouple in contact with the catalyst allowed the control of the temperature. The amount of H2 uptake during the reduction was measured by a thermal conductivity detector (TCD). The effluent H2O formed during H2-TPR was adsorbed by a 5A molecular sieve adsorbent. The error on the peak temperature was shown to be (15 °C. 2.3. Catalytic Test. The oxidation of n-butane to maleic anhydride was carried out in a fixed-bed flow microreactor containing a standard mass of catalyst (0.25 g) at 673 K with a gas hourly space velocity (GHSV) of 2400 h-1. Prior to use, the catalysts were pelleted and sieved to give particles with 250-300 µm diameter. n-Butane and air were fed to the reactor via calibrated mass flow controllers to give a feedstock composition of 1.7% n-butane in air. The products were injected to an online gas chromatography for in situ analysis. The reactor is composed of stainless steel tube with the catalyst held in place by plugs of quartz wool. A thermocouple was located in the center of the catalyst bed, and the temperature difference was typically (1 K. Carbon mass balances of g95% were typically observed. 3. Results 3.1. Microstructures of Precursors and Catalysts. The XRD diffractograms of both vanadyl phosphate dihydrates (Figure 1) were identical to VOPO4 · 2H2O with peaks at 2θ ) 11.89, 23.93, and 28.71°.25 The microstructure of microwave heated vanadyl phosphate hydrate was significantly changed as


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Table 2. XRD Data of Precursors fwhmb/Å

relative intensity sample

001

130

I001/I130a

001

130

VHPA1 VHPA2 VHPB1 VHPB2 VHPC1 VHPD1 VHPC2 VHPD2

255 223 119 678 2262 2766 267 272

292 294 289 356 376 436 246 222

0.87 0.76 4.10 1.90 7.08 6.34 1.08 1.22

0.58 0.61 0.29 0.34 0.16 0.16 0.67 0.66

0.44 0.44 0.22 0.22 0.15 0.15 0.50 0.46

a Intensity ratios of (001) and (130) reflection planes. half-maximum (fwhm) of (001) and (130) reflection.

b

Full width at

Figure 1. X-ray powder diffraction patterns of vanadyl phosphate dihydrate (VOPO4 · 2H2O, VPD) heated by microwave and conventional methods. (001) is the main peak for VOPO4 · 2H2O.

Figure 3. X-ray powder diffraction patterns of vanadium phosphorus oxide ((VO)2P2O7, VPO) catalysts. (020) and (204) are the main peaks for (VO)2P2O7.

Figure 2. X-ray powder diffraction patterns of isobutanol, 1-butanol, and ethylene glycol of vanadyl hydrogen phosphate hemihydrate (VOHPO4 · 0.5H2O, VHP) precursors heated by microwave and conventional methods. (001) and (130) are the main peaks for VOHPO4 · 0.5H2O.

evident from the enhanced intensity of peak at 2θ ) 11.89° corresponding to plane (001). The XRD patterns of the precursors obtained by the reduction of VOPO4 · 2H2O using primary alcohols (i.e., isobutyl alcohol, 1-butanol, and ethylene glycol) are shown in Figure 2. All precursors exhibited patterns ralated to the VOHPO4 · 0.5H2O phase, but with different intensities and line widths of diffraction lines, depending on the reducing agents used. These results reveal that the solvent and microwave irradiation do not change the precursor phase structure. The main peaks appeared at 2θ ) 15.5, 19.7, 24.2, 27.1, 28.7, 30.4, 37.5, and 49.2° which correspond to the (001), (101), (021), (121), (201), (130), (040), and (331) planes, respectively.26 The intensity of (001) plane was significantly increased by using 1-butanol and ethylene glycol; however, changes in intensity and line widths of the (130) plane were almost negligible. Moreover, as it is apparent from Figure 2, VHPA2, VHPC2, and VHPD2 catalysts showed broad planes (001) with weak intensity. Table 2 summarizes the relative intensity, intensity ratio, and full width at half-maximum of (001) and (130) for all precursors. The microstructure of the

catalyst prepared by 1-butanol and microwave irradiation was changed as can be observed from peak intensities and the values of the full width at half-maximum (fwhm). For instance, the relative peak intensity ratios of I(100)/I(130) were significantly increased from 0.87 (VHPA1) to 4.10 and 7.08 for VHPB1 and VHPC1 precursors, respectively. The fwhm of the microwave heated precursors synthesized by 1-butanol (VHPB1) and ethylene glycol (VHPC1) were decreased from 0.58 (VHPA1) to 0.29 and 0.16 along the (001) plane. As evident from Figure 3, XRD patterns of catalysts correspond to the (VO)2P2O7 phase (JCPDS File No. 34-1381) with the main peaks at 2θ ) 22.6, 28.2, and 29.7°. These peaks are related to (020), (204), and (221) reflections, respectively. This result indicates the transformation of VOHPO4 · 0.5H2O to (VO)2P2O7, with variations in relative intensity and peak broadness depending on the reducing agent used. Furthermore, for glycol catalyst prepared conventionally, some additional weak peaks emerged at 2θ ) 19.4° and 24.3° (JCPDS No. 270948) and 2θ ) 22.3°, 27.6°, and 36.3° (JCPDS No. 34-1433), which correspond to the VOPO4 and VO(PO3)2 phases, respectively. Table 3 summarizes the relative intensity, fwhm, and crystallite size results calculated from the Debye-Scherrer equation.24 Also, the use of 1-butanol and ethylene glycol led to a significant increase in the values of I020/I204, indicating the increase in exposure of the (020) plane which contains the vanadyl group. Table 4 summarizes the surface area, P/V atomic ratio, and oxidation number of V in the bulk of the catalysts. By replacing isobutanol with 1-butanol, the surface area of the catalysts was reduced to 26 and 24 m2 g-1 for VPOB1 and VPOB2,

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Table 3. XRD Data of Nanostructure Catalysts fwhmb/Å

crystallite sizec/nm

catalysts

I020/I204a

020

204

020

204

VPOA1 VPOA2 VPOB1 VPOB2 VPOC1 VPOD1 VPOC2 VPOD2

0.98 0.77 1.61 1.60 1.36 1.40 3.20 4.66

0.5740 0.5672 0.5400 0.4800 0.8031 0.8467 0.5698 0.5902

0.4550 0.4253 0.4100 0.4100 0.4785 0.5233 0.5322 0.5362

13.9 14.1 14.7 16.6 10.0 9.5 14.2 13.6

17.8 19.0 19.6 19.8 16.9 15.5 15.2 15.1

a Intensity ratios of (020) and (204) reflection planes. b Full width at half-maximum (fwhm) of (020) and (204) reflection. c Crystallite size were calculated accordingly to Debye-Schrrer equation.24

Table 4. Physical and Chemical Properties of Catalysts

catalyst VPOA1 VPOA2 VPOB1 VPOB2 VPOC1 VPOD1 VPOC2 VPOD2

surface P/V areaa (m2 g-1) ratiob V5+ (%) V4+ (%) 32 27 26 24 44 46 41 42

1.08 1.04 1.09 1.07 1.07 1.05 1.09 1.07

27 25 27 20 15 9 31 13

73 75 73 80 85 91 69 87

average oxidation number of V in bulkc 4.27 4.25 4.27 4.20 4.15 4.09 4.31 4.13

a After pretreatment at 423 K in a vacuum. b Estimated from ICP-AES. c Average oxidation number of vanadium estimated by redox titration.

respectively. On the other hand, BET results indicated that surface area of catalyst prepared by ethylene glycol was larger than catalysts prepared under the standard dihydrate method. The obtained surface area values were consistent with crystalline size data and SEM morphology. On the basis of the crystalline size results calculated from the Debye-Scherrer equation, catalysts synthesized by ethylene glycol exhibited smaller particle size with higher line width as compared to catalysts prepared via the standard VPD method. For all catalysts, the crystals present in 020 phases display smaller size and hence higher surface area. The P/V ratio of bulk and average oxidation number of vanadium were found to be near unity and 4.2, respectively for all catalysts. However, for catalyst prepared by glycol, the average oxidation number was decreased with the higher amount of V4+ phase. On the other hand, refluxing by distilled water reduced the amount of V4+ phase and, therefore, enhanced the reactivity of the catalyst. Figure 4 shows SEM images of the catalyst particles. The SEM micrographs of catalysts revealed that refluxing by isobutyl alcohol resulted in rosette structures with thin platelets as characterized by an XRD pattern with the (020) reflection being dominant.27,28 All catalysts exhibited rosette-shape species, adhered to the aggregated particles which were recognized as (VO)2P2O7 phase, one of the most typical morphologies observed. For ethylene glycol catalyst, the platelets are smaller which is attributed to higher amounts of rosette-shape clusters in the catalyst. These smaller pieces are responsible for the reduced exposure of the surface plane. 3.2. Temperature Programmed Reduction (TPR) in H2/ Ar. In order to investigate the effect of organic media (i.e., isobutyl alcohol, 1-butanol, and ethylene glycol), distilled water and microwave irradiation on the redox properties, and the amount and nature of the oxygen species of the catalysts, H2TPR experiments were performed. Figure 5 shows H2-TPR profiles of all active catalysts. Total amounts of oxygen removal as well as the values of reduction activation energies are shown

in Table 5. The reduction temperatures for all microwave assisted catalysts were shifted to slightly higher temperatures, and amount of released oxygen species associated with V5+ was significantly increased for VPOB1, VPOB2, VPOC2, and VPOD2 catalysts. 3.3. Selective Oxidation of n-Butane. Table 6 summarizes the catalytic performance of catalysts at reaction temperature of 673 K and GSHV of 2400 h-1. A significant improvement of catalytic performance was observed for n-butane conversion and MA selectivity when V2O5 was synthesized using ethylene glycol, water, and microwave as the reducing agent, solvent, and heating media, respectively. Figure 6 shows n-butane conversion and selectivity to MA. The VPOD1 catalyst was found to be the most active, and the VPOD2 catalyst was shown to give the highest MA selectivity. The selectivity to MA of VPOD1 was reached to about 81% at 63% conversion. In addition, by using 1-butanol as the reducing agent, the n-butane conversion was reduced to 45% while the MA selectivity was enhanced to 68%. 4. Discussion 4.1. Influence of 1-Butanol, Ethylene Glycol, Water Treatment, and Microwave Irradiation on Microstructure of Precursors. In this study, it was found that 1-butanol and ethylene glycol influence catalytic performance by increasing the relative peak intensity (001) and reducing fwhm values (Table 2). For instance, the relative peak intensity ratio of I001/ I130 was significantly increased from 0.87 (VHPA1) to 4.10 and 7.08 for the microwave heated precursor prepared by 1-butanol and ethylene glycol, respectively. This ratio was remarkably increased to 6.34 when distilled water was employed. The fwhm value of microwave heated precursor synthesized by ethylene glycol was decreased for both planes, i.e. from 0.58° (VHPA1) to 0.16° along the (001) plane and from 0.44° (VHPA1) to 0.15° along the (130) plane. On the other hand, the relative peak intensity ratio of I001/I130 of VHPC1 precursor prepared by ethylene glycol was slightly increased from 0.76 (VHPA2) to 1.08 and then exhibited a sharp increase to 1.22 due to the effect of water reflux. The fwhm value of the VHPC1 precursor was increased for both planes, i.e. from 0.61° (VHPA2) to 0.67° along the (001) plane and from 0.44° (VHPA2) to 0.50° along the (130) plane. However, by further refluxing of VHPC1 precursor by water, the values of fwhm was slightly decreased to 0.66 and 0.46° along the (001) and (204) planes, respectively. These planes were chosen because the reflections of (020) and (204) planes in the (VO)2P2O7 are related to (001) and (130) planes in the VOHPO4 · 0.5H2O phase.7 Microwave irradiation is believed to offer substantial benefits in the synthesis of heterogeneous catalysts by increasing the catalyst activity. The microwave coupling dielectric heating effect lies in the capacity of an electric field to polarize charges in the material. This effect is enhanced when irradiated material has a strong dipolar character. This is particularly the case for catalysts with oxide supports containing OH groups.29-32 Moreover, when polar molecules exist on the surface of the dehydrate (VOPO4 · 2H2O) and precursor (VOHPO4 · 0.5H2O), stronger interaction with microwave energy will be formed. The other substantial advantages offered by microwave heating are (i) significant reduction in catalyst synthesis costs due to shorter processing time and (ii) improved catalytic properties of synthesized catalysts.32 X-ray diffraction patterns of precursors, refluxed by distilled water (VHPD1 and VHPD2) are presented in Figure 2. Previous studies10,13 showed that the water extraction of vanadium phosphate


Figure 4. Continued.

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Figure 4. Scanning electron micrographs (SEM) of VPO catalysts.

precursors acts as an essential step in the removal of the VO(H2PO4)2 phase since, in presence of this impurity, catalysts with low activity and surface area are produced. However, unlike VOHPO4 · 0.5H2O, VO(H2PO4)2 is soluble in water and, therefore,

could be readily removed from catalyst precursors by hot water extraction.10 No evidence of the presence of VO(H2PO4)2 as an impurity in VOHPO4 · 0.5H2O precursor was observed from the powder XRD diffraction (Figure 2).


Figure 5. Temperature programmed reduction (H2-TPR) profiles of VPO catalysts.

4.2. Microstructures of Catalysts Derived from Precursors with Different Crystallite Size. The transformation from VOHPO4 · 0.5H2O to (VO)2P2O7 proceeds via a complex process involving dehydration, oxidation, and reduction. Previous literature5,7,10,15 reported that topotactic dehydration to (VO)2P2O7 occurred preferentially at the periphery of the VOHPO4 · 0.5H2O crystallites, whereas in the interior of the crystallites, VOHPO4 0.5H2O was initially oxidized to VOPO4 and then reduced to (VO)2P2O7, while the size and shape of the precursor was retained during transformation. As can be seen from Table 3, by replacing isobutyl alcohol with 1-butanol, the values of I020/I204 were increased for microwave assisted catalysts from 0.98 (VPOA1) to 1.61 (VPOB1). Previous literature28,35 reported that the best way to improve the catalytic performance of VPO catalysts is to increase the relative exposure of the (020) plane of (VO)2P2O7 since this plane effectively participates in the reaction of partial oxidation of n-butane to maleic anhydride. Moreover, as shown in Table 3, the crystalline size of catalysts prepared by 1-butanol were slightly increased namely from 14.1 to 16.6 nm along the (020) plane and 19.0 to 19.8 nm along the (204) plane for conventional VPOA2 and VPOB2 catalysts, respectively. In the case of 1-butanol, the BET surface area measurement values are in contrast with previous study.36 However, as evident from Figure 3 and Table 3, microwave catalyst (VPOB1) represented a lower reflection intensity (020) than conventional catalyst (VPOB2), but the total surface area of the VPOB1 catalyst (26 m2 g-1) was higher than that of VPOB2 (24 m2 g-1) which demonstrates that the exposure of the (020) plane is not the only factor that enhances BET surface area. Therefore, the polarity and type of the alcohol used as the reducing agent have a stronger impact on catalysts properties. As discerned in Table 4, the P/V atomic ratios obtained via chemical analysis using ICP, are approximately unity. Some of the values slightly deviated from the nominal P/V atomic ratio value of 1:1; however, these were still in the optimal P/V atomic ratio range to produce an active and selective (VO)2P2O7 phase. This is also well-agreed upon in numerous literature works, indicating that the V4+ species responsible for the activity of the respective catalyst could be stabilized by higher P/V atomic ratios.12 The average oxidation number of vanadium and percentage of V+4 and V+5 oxidation states are also summarized in Table 4. It is widely accepted that the valence state of vanadium plays an important role in the selective oxidation of n-butane to MA.37 The process for preparation and drying

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of precursors was observed to be directly proportional to the average oxidation state of vanadium. This could be attributed to the formation of a VV phase (VOPO4) after microwave heating, as shown in the XRD diffraction patterns (Figure 3). As shown in Figure 4, VPOB1 contains platelike crystals, arranged into characteristic rosette-shaped clusters. The size of platelets are relatively small resulting in larger numbers of rosette-shaped clusters and, hence, a higher surface area. The VPOB2 catalyst showed platelike species in the form of aggregated particles with secondary structure which are recognized as the (VO)2P2O7 phase, representing one of the most typical morphologies observed. Abon et al.38 reported that the increase of the crystalline (VO)2P2O7 phase would change the morphology of catalyst particles and lead to formation of more split agglomerates. As evident from SEM micrographs, the microwave irradiated catalysts represented a thin rosettetype structure with uniform crystal size. These results are in agreement with the XRD data and BET surface area measurements (Tables 3 and 4). On the basis of XRD results, it was apparent that the catalysts refluxed in glycol treatment contain smaller crystals than those refluxed in isobutyl alcohol. By using the Debye-Scherrer equation,24 it was found that the crystal size of the microwave catalyst synthesized in glycol and water reflex was decreased from 13.9 to 10.0 to 9.5 nm along the (020) plane for VPOA1 and from 17.8 to 16.9 to 15.5 nm along the (204) plane for VPOA1. Nevertheless, in the case of conventional catalysts, using glycol and water gave rise to the reduction of crystal size from 14.1 (VPOA2) to 14.0 to 13.6 nm along the (020) plane and from 19.0 to 15.2 to 15.1 nm along the (204) plane, respectively. For sample VPOA1, I020/ I204 was slightly increased from 0.98 to 1.36 followed by a steep increase to 1.40 when distilled water was employed. In the case of VPOA2, XRD patterns indicated that I020/I204 of the peaks was significantly increased from 0.77 to 3.20 and, as with VPOA1, refluxing with water, enhanced this ratio dramatically to 4.66. As can been seen from Table 3, employing ethylene glycol followed by water reflux increases the I020/I204 ratio, indicating the presence of more vanadyl group on the surface of catalyst due to the exposure of (020). Haber et al.35 reported that catalytic performance could be improved by increasing the relative exposure of the (020) plane of (VO)2P2O7, since this plane is directly involved in the reaction of partial oxidation of n-butane to maleic anhydride. Moreover, as noted above, performing the water reflux as an extra step during catalyst preparation gives rise to smaller crystals and hence enhances the catalytic performance of (VO)2P2O7 for selective oxidation of n-butane to maleic anhydride. In general, the catalyst precursors refluxed with distilled water and dried by microwave irradiation displayed higher specific surface area as compared with other catalysts. The mechanism of heat diffusion in the traditional heating process is based on the fact that heat gradually diffuses from the outer surface to the inner surface.39 However, the mechanism of heat generation in microwave irradiation is that the energy is transferred inside the complex and, hence, heat is produced from the interaction of irradiation with polar bonds in the complex. When a wet body is exposed to microwave radiation, the portion that contains the highest moisture content absorbs the microwave most strongly and hence becomes the hottest part of the body.40 Therefore, the rate of evaporation is the greatest from the wettest region which impacts morphology, pore value, pore size, and pore distribution of the catalyst. As reported earlier, there is a linear relationship between n-butane conversion (mole converted per gram per hour) and catalyst surface area.11 This implies

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Table 5. Total Amount of Oxygen Removed, Values of Reduction Activation Energies, and Ratio for Oxygen Removed of V5+/V4+ Estimated by Reduction in H2/Ar for Catalysts catalyst VPOA1 total oxygen atoms VPOA2 total oxygen atoms VPOB1 total oxygen atoms VPOB2 total oxygen atoms VPOC1 total oxygen atoms VPOD1 total oxygen atoms VPOC2 total oxygen atoms VPOD2 total oxygen atoms

peaka

Tmax/K

reduction activation energyb, Er (kJ mol-1)

total amount of oxygen removed ×10-3 (mol g-1)

total amount of oxygen removed ×1021 (atom g-1)

ratio of oxygen removal of V5+/V4+ c

1 2 removed 1 2 removed 1 2 3 removed 1 2 3 removed 1 2 3 removed 1 2 3 removed 1 2 3 removed 1 2 3 removed

800 1025

138 177 136 175

815 840 1071

140 145 185

807 818 1062

139 141 182

780 897 1045

130 150 175

782 895 1052

131 150 176

803 818 1065

134 137 178

825 851 1075

138 142 180

0.15 0.75 0.90 0.09 0.79 0.88 0.32 0.56 0.99 1.77 0.19 0.11 0.79 1.09 0.06 0.05 0.64 0.75 0.04 0.05 1.35 1.44 0.14 0.09 0.72 0.95 0.12 0.19 0.90 1.21

0.20

789 1012

0.26 1.25 1.51 0.14 1.31 1.45 0.53 0.76 1.64 2.93 0.31 0.18 1.32 1.81 0.10 0.08 1.07 1.25 0.07 0.08 2.25 2.40 0.24 0.14 1.19 1.57 0.20 0.32 1.49 2.01

a After pretreatment at 473 K in a N2 and cooling down under He. equation. c Estimated from H2-TPR.

b

Table 6. Catalytic Performance of Vanadium Phosphate for the Oxidation of n-Butane to Maleic Anhydridea selectivity (%)

b

catalyst

n-butane conversion (%)

MA

CO

CO2

yieldb (%)

VPOA2 VPOB1 VPOD1 VPOD2

52 45 81 65

56 68 63 74

32 15 24 16

12 17 13 10

29 30 51 48

a Reaction conditions 673 K, 1.7% n-butane in air, GHSV 2400 h-1. Yield (%) ) n-butane conversion (%) × MA selectivity (%).

that the surface structure of activated catalysts are very similar and the activity differences are just due to the higher surface area of VPO catalyst having a higher number of active sites per unit mass of catalyst. It is, therefore, very surprising that the structure of the activated catalysts are completely different. The SEM micrographs of the catalyst precursors prepared by glycol and microwave heating exhibited a more homogeneous distribution of the rosette-shaped surface species and thin structures as compared to the catalyst precursors dried by conventional heating. Catalyst dried by the conventional heating method showed particles with different shapes and sizes agglomerated together with secondary platelet morphology. 4.3. Influence of Reducing Agent (Catalyst Microstructure) on Amount and Nature of the Oxygen Species. In order to investigate the effect of organic media on redox properties and the amount and nature of oxygen species of catalysts, H2TPR experiments were performed on microwave and conventional catalysts. The conventional catalyst displayed two reduction peaks in the range of 600-1100 K. These peaks appeared at 789 and 1012 K, where the first peak corresponded to the reduction of V5+ phase whereas the second peak was related to the removal of lattice oxygen from the active V4+ phase.37 The

0.11 0.88

0.38

0.17

0.07

0.31

0.34

Activation energies were calculated from the modified version of the Redhead

amount of oxygen removed from both peaks was 0.90 × 1020 and 7.90 × 1020 atom g-1, respectively, with an oxygen ratio of 0.11 for V5+ to V4+. The microwave irradiation catalysts showed similar reduction profiles as conventional materials. It was found that the microwave irradiation catalyst exhibited the reduction peaks at higher temperatures and the amount of oxygen significantly increased to 0.90 × 1021 atom g-1, with an oxygen ratio of 0.20 for V5+ to V4+. As 1-butanol is concerned, the catalyst prepared conventionally, represented three characteristic reduction peaks at 807, 818, and 1062 K. The amount of oxygen removed from both peaks was 1.90 × 1020, 1.10 × 1020, and 7.9 × 1020 atom g-1 (Table 5), respectively. On the other hand, for the microwave catalyst (VPOB1), the third reduction peak was shifted to higher temperature, i.e. 789 K, while the second one was appeared at lower temperature. In addition, the amount of oxygen species released associated with V5+ was significantly increased to 8.80 × 1020 atom g-1 while, for conventional catalyst (VPOB2), this amount was reduced to 3.00 × 1020 atom g-1 (see Table 5). As with ethylene glycol, for catalyst irradiated by microwave (VPOC1), employment of ethylene glycol as a reducing agent, gave rise to three reduction peaks at 782, 895, and 1052 K. The amounts of oxygen removed from these peaks were 0.60 × 1020, 0.50 × 1020, and 6.40 × 1020 atom g-1, respectively. On the other hand, the TPR profiles of VPOD1 showed that the microwave heating and water reflux shifted the peaks to lower temperatures, i.e. 780, 897, and 1045 K as compared to the former catalyst (VPOC1). The amount of oxygen species released associated with V4+ was significantly increased to 1.35 × 1021 atom g-1 while for VPOD1, the amount reached 1.44 × 1021 atom g-1, with an oxygen ratio of 0.07 for V5+ to V4+.


7525

Figure 6. n-Butane conversion and MA selectivity for different reducing agents.

In the case of conventional catalyst (VPOC2), the first and third reduction peaks were shifted to higher temperatures, namely, 803 and 1065 K, while the second reduction peak appeared at a lower temperature, 818 K, as compared to microwave catalyst (VPOC1). The amount of oxygen species removed from the catalyst, due to the reduction of V5+ and V4+ were shown to be (2.30 and 7.20) × 1020 atom g-1, respectively. The results also revealed that refluxing the VPOC2 precursor with distilled water to give VPOD2, slightly shifted these three reduction peaks to higher temperatures, i.e. 825, 851, and 1075 K compared to former catalyst, VPOC2. In addition, the amount of oxygen species released associated with V5+ was enhanced to 3.10 × 1020 atom g-1. By refluxing the precursor with distilled water, the total amount of oxygen species was dramatically increased to 1.21 × 1021 atom g-1. For conventional VPOC2 and VPOD2 catalysts, the oxygen atom released ratio of V5+ to V4+ were shown to be 0.31 and 0.34, respectively. These results indicated that using microwave heating and water reflux resulted in a highly active catalyst with the highest amount of active V4+-O- pair for n-butane activation. The total amount of oxygen species removed from the catalyst demonstrated the positive impact of microwave heating and water refluxing on catalyst performance. Electrical conductivity investigation on this catalyst suggested that O2- is due to the V5+ phase whereas O- was attributed to the V4+ phase. A good direct correlation was reported41 between the amount of oxygen species removed from both phases (V4+ and V5+) with the n-butane conversion and MA selectivity, respectively. Therefore, a higher amount of oxygen removed from microwave heated catalyst suggested that microwave process is capable of producing a more active and selective catalyst for n-butane oxidation reaction. 4.4. Influence of Catalyst Microstructure on Selectivity in the Selective Oxidation of n-Butane. As shown in Figure 6, VPOB1, VPOD1, and VPOD2 displayed higher selectivity to MA than VPOA2 catalyst. In particular, VPOD1 exhibited remarkably high conversion, reaching 81% at low selectivity levels (63%) and remaining at 65% even at 74% selectivity to MA. The catalytic evaluation of n-butane selective oxidation to maleic anhydride revealed that the water treated and microwave heated catalyst (VPOD1) significantly enhanced the reaction conversion (81%) compared to conventional catalyst (VPOA2) with 52% conversion. For VPOD1 catalyst, the MA

selectivity was slightly increased from 56% to 63%. The enhancement of catalytic activity of VPOD1 was attributed to a higher BET surface area resulting from the development of the (020) plane of the (VO)2P2O7 phase. VPOD1 catalyst with higher surface area (46 m2 g-1) exhibited higher activity than VPOA2 catalyst with lower surface area (27 m2 g-1). As evident from Figure 6, the conversion of n-butane was decreased when VPOB1 catalyst was used. VPOD1 catalyst with higher surface area (46 m2 g-1) represented significantly higher activity than VPOD2 catalyst with lower surface area (42 m2 g-1). On the other hand, the catalytic evaluation results showed that, the microwave irradiation significantly affects the catalytic performance of the catalysts by giving beneficial effect to the n-butane conversion. The value of n-butane conversion remarkably rises from 65% for conventional catalyst (VPOD2) to 81% for microwave VPOD1 catalyst. This effect can be attributed to the increase of BET area which is connected to the development of the (020) plane of (VO)2P2O7. A higher amount of active site (V4+) and oxygen species are responsible for activation of n-butane as well as enhancing catalyst activity. In general, the results revealed that using ethylene glycol and microwave irradiation in the preparation of catalysts can improve catalytic performance, selectivity, and conversion of n-butane. Higher yields of VPOD1 and VPOD2 catalysts are due to higher surface area and imply that they have highly reactive and labile oxygen species originated from V4+ which are capable of increasing the breaking rate of C-H bonding. The enhancement of catalytic performance of these new catalysts was attributed to the higher number of oxidants associated with active V4+ phase as observed in TPR experiment (see Figure 6). A good correlation was found between the oxygen species associated with V4+ and the n-butane conversion by plotting conversion versus oxygen species associated with V4+ as shown in Figure 7. Taufiq-Yap et al.42 have reported a good correlation between the oxygen species associated with V4+ and the n-butane conversion and concluded that V4+sO- acts as a center for n-butane activation. Furthermore, a higher amount of surface lattice oxygen, which act as active species, are also contributed to improved catalyst activity.43 The result indicates that these specific oxygen species were highly active for partial oxidation of n-butane and confirmed the electrical conductivity data obtained by Hermann and co-workers.44 These authors suggested

7526


Figure 7. n-Butane conversion as a function of amount of oxygen removed associated with V4+.

Figure 8. MA selectivity as a function of amount of oxygen removed associated with V5+.

that the O- species associated with V4+ act as the center for the n-butane activation. As revealed by TPR results (Figure 8, Table 5), the amount of V5+ species were significantly increased to 3.10 × 1020 and 8.80 × 1020 atom g-1, respectively, for VPOD2 and VPOB1catalysts as compared with conventional

VPOA2 catalyst (9.00 × 1019). The increase of n-butane conversion rate suggests a correlation between the amount of V5+ species and catalytic performance of the catalyst. It has been reported that V5+ species play an important role in hydrogen abstraction from n-butane.45 A synergetic effect of a

Ind. Eng. Chem. Res., Vol. 48, No. 16, 2009 5+

well-dispersed nanocrystalline V phase in the major very crystalline pyrophosphate phase led to enhancement of the n-butane activation and production of maleic anhydride. However, the new solvents induced the formation of VOPO4 phases which changed the ratio of V5+/V4+ and influenced the catalytic performance. The highly crystalline structure of the (VO)2P2O7 phase along with VOPO4 phases significantly improved the catalytic performance. Ebner and Thompson46 suggested that V5+ species were inevitable species for activation of the CsH bond of butane molecule. Therefore, increments of the V5+dO center from 11% (for conventional VPOA) to 34% and 88% for VPOD2 and VPOB1, respectively, have dramatically improved the butane conversion. The utilization of ethylene glycol gave rise to a well-dispersed nanocrystal of VOPO4 and (VO)2P2O7 phases and hence altered the ratio of V5+/V4+ resulting in the enhancement of n-butane activation. Furthermore, the smaller particle size at the (020) plane obtained for VPOD1, strongly improved the MA selectivity. In general, the catalytic test results showed that the employment of ethylene glycol, water treatment, and microwave irradiation in the preparation of the catalysts, improve the catalytic performance, selectivity, and n-butane conversion. 5. Conclusions Experimentally, the successful synthesis of vanadyl pyrophosphate by using isobutanol, 1-butanol, and ethylene glycol as reducing agents and distilled water as a solvent has been demonstrated. It was shown that the improvement of the catalyst synthesis method by using reducing agents such as 1-butanol and ethylene glycol and refluxing by distilled water and microwave heating potentially alters the phase composition, morphology, redox properties, and catalytic behavior of the vandyl pyrophosphate catalysts, although the principal phase structures of VOHPO4 · 0.5H2O and (VO)2P2O7 are still retained. Total surface area of catalysts prepared by isobutyl alcohol as organic media are slightly higher than those synthesized by 1-butanol, while by utilizing ethylene glycol the surface area was found to be larger than those prepared under the standard VPD method. Moreover, ethylene glycol creates smaller particles with higher surface area (>40 m2 g-1) compared to previous C4-alcohol based methods which give rise to surface area of about ∼20-27 m2 g-1. The results of this study revealed that more efficient materials can be produced using ethylene glycol as a reducing agent. Furthermore, no impurities would be formed if the materials are recovered by filtration and washed with hot water. Refluxing of the precursor by distilled water decreases the catalyst crystallite size and consequently enhances the surface area as confirmed from the thin platelike structure. The adoption of microwave assisted synthesis was observed to be an effective way to improve the surface area and morphology of catalysts. As the TPR results showed, ethylene glycol and microwave enhanced both the activity and selectivity of VPO catalyst by inducing the mobility and availability of the O- and O2- surface ions. The lower onset temperature and higher peak area indicate the effect of ethylene glycol and microwave heating on the improved activity and selectivity of (VO)2P2O7 catalyst for selective oxidation of n-butane. On the whole, the results of this study emphasize the role of ethylene glycol, water treatment, and microwave irradiation in the improvement of the selectivity and activity of VPO catalyst being used in partial oxidation of n-butane to MA.

7527

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ReceiVed for reView February 12, 2009 ReVised manuscript receiVed June 8, 2009 Accepted June 22, 2009 IE900238A

High Surface Area Vanadium Phosphate Catalysts for n-Butane

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