- Email: [email protected]
Sol–Gel synthesis and characterization of Nb-Mo and Nb-Mo-V mixed oxides as potential catalysts for the selective oxidation of propane
Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.
841 841
Sol-Gel synthesis and characterization of Nb-Mo and Nb-Mo-V mixed oxides as potential catalysts for the selective oxidation of propane Carlo Lucarellia, Pietro Moggi^*, Fabrizio Cavanib, Michel Devillersc "Dip. Chimica Organica e Industriale, Universita di Parma, Viale G.P. Usberti 17/A Campus, 43100 Parma, Italy; E-mail: [email protected] Dip. Chimica Industriale e dei Materiali, Universita di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy ' Unite de Chimie des Materiaux Inorganiques et Organiques, Universite Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium
Abstract In this work a modified hydrolytic sol-gel technique has been applied to synthesize binary and ternary systems based on Nb, Mo and V, as potential catalysts for the selective oxidation of propane to acrolein and/or acrylic acid. Crystalline mixed oxide phases were obtained and investigated by XRD, Raman and UV-VIS spectroscopy. 1. Introduction Mixed oxides of 4-6 group transition metals are important inorganic materials used in different fields such as catalysis, ceramics and microelectronics. Complex oxide formulations including Nb and Mo are extensively used in partial oxidation catalysis [1]. The current abundance and low cost of natural gas-based alkane feedstocks have recently generated a strong interest in the oxidative catalytic conversion of alkanes to olefins, oxygenates and nitriles. Many research results have shown that Mo- and V-containing mixed metal oxides and more complex systems containing Mo, Nb and V oxides are catalytically active for the partial oxidation of lower alkanes [2]. There is an increasing interest in the development of a process for direct oxidation of propane to acrylic acid as an alternative to the two-step conventional industrial process based on propylene as feedstock [3]. A high selectivity is necessary for reaching a high efficiency in the use of new raw materials and energy resources, as well as for reducing the cost of product separation and the downstream emissions according to the Kyoto Protocol [4].
842
C. Lucarelli et al.
2. Experimental 2.1 Preparation of catalysts The preparation of all catalytic systems has been performed by a sol-gel technique, following a standardized procedure to warrant a good reproducibility. The samples were prepared under similar conditions using Nb(OPr%, MoCl5 and VCKOPr1^, as starting materials. MoCl5 and VOCOPr^ are commercially available products, while Nb(OPr')5 has been synthesized starting from NbCl5 and 2-propanol by adding triethylamine, according to the following reaction: Cl Ck I Nfc-CIt ^CHJjCHOH 1-SNEla CK II Cl
OCH[CHd, HCJiHCO^ I *,M> — OCH(CH]]i +• 3 (ltCJHCO^ I
The reactants have been introduced with the following molar ratios: NbCl5/(CH3)2CHOH/NEt3=l/42/ll. 2-propanol has been added dropwise to niobium chloride under stirring at room temperature, then NEt3 has been added to the solution maintained in an ice bath under an anhydrous atmosphere to prevent the early hydrolysis of the product. After 7 hours of reaction at 110-120 °C, the solvent excess has been distilled out, then the solution has been cooled down to room temperature and the precipitated ammonium salt has been removed by filtration under nitrogen atmosphere. The reaction yield was about 75% based on NbCl5. The synthesized product was characterized by 'H-NMR. To prepare the Nb/V systems, a 2-propanol solution of Nb(OPr')5 has been added, under anhydrous conditions, to a stirred solution of VO(OPr')3 at room temperature, then a solution of mixed water and 2-propanol (hydrolysis molar ratio water-to-alkoxides R = 2) has been added dropwise to the stirred mixture of the metal oxide precursors getting the formation of a gel in few hours. Many systems with different compositions have been prepared, generally obtaining the formation of a homogeneous gel. In only one case (Nb/V=l :1) a gel was not obtained, probably because the hydrolysis of the vanadyl isopropoxide is much faster than the hydrolysis of the niobium isopropoxide used as precursors. Therefore, the large amount of partially hydrolyzed vanadyl isopropoxide, which quickly precipitates, has influence on the subsequent polycondensation reaction between the two metal hydroxide precursor species. In order to avoid the precipitation of vanadium hydroxide species, another sample with the same Nb/V ratio has been prepared by adding during the synthesis a stoichiometric amount of citric acid as complexing agent. The Mo/V, Mo/Nb and Mo/Nb/V samples have been synthesized under the same previously described conditions, but the hydrolysis ratio has been changed in some cases in order to obtain a homogeneous gel phase. Actually, in these
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
843
cases the synthesis method was a combination between the hydrolytic and the non-hydrolytic techniques because M0CI5 and VO(OPr')3, M0CI5 and Nb(OPr1)5, and/or VO(OPr')3 have been added as precursors to the water/2propanol mixture. The condensation reaction always occurred under acidic conditions due to the fact that part of the HCl developed by the hydrolysis of M0CI5 remained in the solution. After a preliminary differential thermal analysis (DTA) test to evaluate the solid phase transformations at increasing temperatures, the xerogels obtained after ageing 7 days in air at room temperature and drying in vacuum, were thermally treated in static air as follows: 15 h at 623 K, 2 h at 723 K, and finally 4 h at 823 K or 5 h at 773 K or 4 h at 973 K or 4 h at 1173 K. 2.2 Characterization of catalysts The obtained mixed oxides were characterized by BET surface area determinations, X-ray powder diffraction (XRD) analyses, Raman spectroscopy, UV-VIS spectroscopy and scanning electron microscopy (SEM). The BET specific surface area measurements were carried out on a Micromeritics Pulse Chemisorb 2705 analyser using nitrogen at 77 K ("single point" method). The samples were previously outgassed under helium flow at 473 K. The powder X-ray diffraction spectra were recorded on a Philips PW 3710 diffractometer using the Cu Ka (k = 1.54178 A) radiation. The crystalline phases were identified with reference to the powder diffraction data files (JCPDS - ICDD). Raman spectroscopy was performed on a Renishaw 1000 spectrometer equipped with green laser (k = 514 nm, power 25 mW, confocal objectives 5X, 20X, 5 OX). SEM micrographs were taken with a JEOL 3120 instrument equipped with a Philips XL 30 ESEM. 3. Results and Discussion In Table 1 the measured BET surface area values of the synthesized systems are reported. All samples are characterized by low surface areas according to the values reported in literature for these materials [5], only the samples containing a large amount of Nb showing values higher then 10 m g"1. A homogeneous gel was obtained from all preparations with the only exception of the sample Nb/V=l/1. For the corresponding modified sample which was synthesized by adding citric acid, a homogeneous gel was obtained as in all other cases. From these preliminary results it was concluded that a high Nb(OPr')5 or M0CI5 amount, or the presence in the case of the sample Nb/V=l :1 of a vanadium ion complexing agent such as citric acid, are necessary for preparing homogeneous gels containing Nb, V and Mo mixed oxide precursors.
844
C. Lucarelli et al.
Table 1. Synthesized catalysts, hydrolysis ratio, calcination temperature/time and surface areas Surface area
R
Calcination temperature/time (K/h)
2
823/4
5.1
2
823/4
6.3
Nb/V=3/1
2
823/4
22.4
Nb/V=4.5/1
2
823/4
23.7
Nb/V=9/1
2
823/4
20.2
Mo/Nb=l/12
1
773/5
38.8
Mo/Nb=l/8
2
773/5
17.9
Mo/Nb=3/14
J
1173/4
4.1
Mo/Nb=3/14
3
973/4
4.3
Mo/Nb=l/4
3
973/4
2.5
Mo/Nb=3/2
5
1173/4
4.1
Mo/V=3/2
10
823/4
5.8
Mo/Nb/V=3/1.5/0.5
5
823/4
6.7
Mo/Nb/V=3/1/1
5
823/4
7.2
Mo/Nb/V=3/0.5/1.5
5
823/4
6.3
Hydrolysis Ratio
Sample Nb/V=l/1 Nb/V=l/lcit
a
(m2g')
Citric acid added as complexing agent
In Table 2 the crystalline phases detected by XRD and Raman analyses are shown for all the prepared samples. The sample Nb/V=l/1 (precipitate) reveals the stoichiometric crystalline phase NbVO5; instead, the sample Nb/V=l/1 cit, synthesized by adding citric acid as vanadium ion complexing agent, confirms a second crystalline phase (NbigVzjOss) in addition to NbVO5. It is reasonable to assume that the formation of a homogeneous gel has some influence on the structure of the material because it can favour a better atomic dispersion. The other samples containing Nb and V also show the presence of the crystalline mixed phase NbisV^ss, with the exception of sample Nb/V=9/1 which displays only Nb2O5. Only the sample Nb/V=l/1 shows evidence of a stoichiometric crystalline phase; the other Nb/V catalysts show signs of a loss of vanadium, probably because it forms an amorphous or microcrystalline phase with crystal size lower than 4 nm; in the case of Nb/V=9/1 vanadium containing phases were not detected. For the samples containing Nb and Mo, mixed oxide phases have not been detected, but only either Nb2O5, or a slightly reduced molybdenum oxide or a mixture of the two single oxides, depending on the
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
845
Mo/Nb ratio and the calcination temperature. Instead, the sample Mo/V=3/2 showed a mixed oxide phase containing Mo and V. Table 2. Crystalline phases detected by XRD and Raman analyses Sample
Crystalline Phases
Raman shift (cm"')
Nb/V=l/1
N b V O 5 [46-0046]
243; 318;643;690 990; 900; 742; 703; 472; 424; 359;232;147
Raman attributions Nb 2 O 5 V 2-\l* D2-x*J5
Nb/V=l/lcit a
Nb| 8 V 4 O5 5 [46-0087]/ NbVO 5 [46-0046]
148; 232; 315; 702;950
Nb 2 O 5
Nb/V=3/1
Nb 1 8 V 4 O 5 5 [46-0087J
703; 317; 233; 147 1011;952;732;239
Nb,O 5 Nb 18 V 4 O 55
Nb/V=4.5/1
Nb, 8 V 4 O 5 5 [46-0087]/Nb 2 O 5
Nb/V=9/1
128; 466; 627 158; 256; 725; 889; 981
Nb 2 O 5 Nb, s V 4 O 5 5
Nb 2 O 5
952; 703; 310; 233;147
Nb,O,
Mo/Nb=1/12
Nb 2 O 5 [05-0352]
952; 7 7 5 ; 7 0 4 ; 3 1 1 ; 233
Nb 2 O 5
Mo/Nb=l/8
Nb2O5 [07-00611
955; 718; 310;240
Nb 2 O 5
MO13O33 [82-1930], Nb 2 O 5
990; 865;660 910; 745; 360; 262;240
MoO.,
Mo/Nb=3/14
Mo/Nb=3/14
[37-1468] M013O33 [82-1930]
990; 874; 632 916; 751; 471; 367;264
Nb 2 O 5 MOO3 Nb 2 O 5
M013O33 [82-1930], Nb, 2 O 2 9 [16-0734]
987;674 934; 900; 730; 630; 470; 310; 258
Mo/Nb=3/2
Mo r3 O 3 3 [82-1930]
988;879;650 970; 914; 743; 629; 470; 419; 369; 263
Nb 2 O 5
lVTn/V=3/2 1VJ.L»/ V _?/ A*
MoV 2 O 8 [74-1510],MoO 3 [01-0706], V4O9 [23-0720]
989; 817; 659; 378; 330; 281; 233; 149.
M0O3
Nb 1 8 V 4 O 5 5 [46-0087], NbO 2
753; 246. 950; 853; 658.
Nb 2 O 5 M0O3
(V 0 , 07 Mo 0 , 9 3) 5 O 14 [31-1437], MOO3 [01-0706]
756; 246. 956; 866; 656.
Nb 2 O 5 M0O3
(Nbo,o9Moo,,i)02,8o [27-1310], V 2 MoO 8 [74-1510],MoO 3 [01-0706]
753; 246.
Nb,O 5
950; 853; 658.
1V10U3
Mo/Nb=l/4
Mo/Nb/V=3/1.5/0.5
[19-0959]
MoOs Nb 2 O 5 MOO3
(Nb 0 , 09 Mo 0 , 91 )O 2 , 80 [27-1310], Mo/Nb/V=3/1/1
Mo/Nb/V=3/0.5/1.5
Citric acid added as complexing agent
846
C. Lucarelli et al.
In the ternary systems Mo-Nb-V mixed oxide phases containing Mo-Nb and Mo-V have been detected in addition to molybdenum oxide, as revealed in Table 2 for the samples Mo/Nb/V=3/1/1 and 3/0.5/1.5, respectively. The Raman analyses also demonstrate the presence of different metal oxides. The sample Nb/V=l/1 shows the typical Raman bands of Nb2O5 (243; 318; 643; 690 cm" ), assignable to the angular distortion of O-Nb-0 bonds, to the bending of Nb-O-Nb bonds and to the distortion of Nb-0 in the Nb-O-Nb bonds, respectively, according to Zhen Zhao et al. [6-7]. In addition, according to the distortion vibrations of V-O-Nb and the vibration of terminal V=O in V-O-Nb groups [6,8-9], the bands confirm the formation of a solid solution V2.xNb2.xO5. The Raman analysis for the sample Nb/V=l/1 cit confirms only the bands due to the Nb(V) oxide, but they are shifted towards higher wave number values, that implies the formation of a solid solution containing Nb and V [6]. The Nb/V=3/1 sample shows the Raman bands at 961 and 1011 cm"' due to the vibration of terminal V=O in the Nb|SV4O55 phase [6,8-9], in addition to the Nb(V) oxide Raman bands shifted like in the previous sample. In the Raman spectrum of the sample Nb/V=4.5/1 it is possible to recognize the presence of Nb(V) oxide (128; 466; 627 cm"1) and that one of a solid solution containing both metals (158; 256; 725; 889; 981 cm"1) [6-8]. Finally, the catalyst Nb/V=9/1 shows only the Raman bands due to Nb2O5. The Raman spectroscopy, for these samples, gives some useful informations about the oxide structure but does not explain the apparent loss of vanadium. In the other samples containing Mo and Nb the Raman bands due to Mo=O bonds vibration (990-1000 cm"') and MoO3 (660 cm"'), in addition to Nb oxides have been detected. The samples containing Mo, Nb and V showed the bands due to the terminal V=O bonds (970, 998 cm"'), the bands due to MoO3 (817, 663 cm"') and the bands due to the Nb oxides. In the case of the sample Mo/Nb/V=3/1/1 some bands probably due to a VxNb2-x05 solid solution (700 cm"' [V-O-Nb; Nb-O-Nb], 290 cm"' [VxNb2-xO5 distortions]) have been detected [6], while in the sample Mo/Nb/V=3/1.5/0.5 the bands due to V4Nbi8055 have also been detected [10]. For the sample Mo/V=3/2 only the bands due to the MoO3 oxide have been detected. Also in this case, by Raman spectroscopy it was possible to collect useful information about the structure of the samples, but it was also evident the apparent loss of some metal phases. Instead, the presence of vanadium in all samples has been detected by UV-VIS spectroscopy. The sample Nb/V=9/1 even shows the presence of a microcrystalline V4+ phase (UV band at about 420 nm) as reported in figure 1; the presence of V4+ was determined by recording the spectrum of the sample and subtracting to this one the spectrum of pure Nb2O5 calcined at the same temperature.
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
847
It M
w V
t» V
W M
u
u
n / /
V
J2*
\ '
it it
Dlff. (NbA/=9/1-Nb2O5)
u "
wn
M
T
w
I
\
\
1 V V •«
\
M
M
V.
'•
M
IW
ID*
IMM
Figure 1: UV-VIS analysis of the sample Nb/V=9/1.
To evaluate the effective relative amount of different metals and their interdispersion SEM microanalyses have been performed. The samples calcined at temperatures higher than 973 K showed a molybdenum amount lower than the theoretical one, probably because at the highest temperature Mo oxide volatilizes from the lattice. The theoretical amount of different metals was respected for all the other samples. The effective presence of metal oxides, yet not detectable by XRD and Raman spectroscopy, was detected by SEM microanalysis in all samples. 4. Conclusions The synthesis method applied in this work has produced good results, in terms of crystalline mixed phases obtained. In fact, many systems showed, after calcination, crystalline mixed oxide phases. The sample withNb/V=l:l showed a stoichiometric crystalline phase NbVO5, such as the Nb/V=4.5:l system, which presented the stoichiometric crystalline phase Nb|SV4O55 in addition to Nb2O5. The XRD spectrum of the sample Nb/V=9:l only showed the Nb2O5 phase, but the presence also of a microcrystalline V4+ phase was observed by UV-VIS analysis. It is important to remark that only in the case of the Nb/V=l/1 system the stoichiometry of the obtained compounds was according to that of the preparation atomic ratio. For example, the sample Nb/V=4.5:l would give only the Nbi8V4O55 phase; on the contrary, it also showed the presence of Nb2O5. This fact implies that (i) the vanadium is present as a dispersed amorphous or microcrystalline phase (this is the case of Nb/V=9/1) or (ii) the mixed phases are non-stoichiometric solid solutions (V3+xNb2-xO5), that could "give hospitality" to a higher amount of vanadium than the stoichiometric
848
C. C. Lucarelli et al.
one; the last hypothesis is probably true because the formation of solid solutions was actually detected by Raman spectroscopy. The ternary samples also showed in all cases crystalline mixed oxide phases, such as (Nbo.o9Moo.9i)02.8o and ( )
The Raman analyses have supplemented the XRD results. In some cases the Raman analyses have shown vibration bands assignable to different phases than XRD; these results suggest as hypothesis that some crystalline phases are not detectable by XRD analysis because they are microcrystalline with a size lower then 4 nm. For the ternary Mo-Nb-V samples it was observed that the formation of the Mo/V and Mo/Nb mixed phases occurs for a restricted range of Mo/V and Mo/Nb ratios. Finally, the preparation method has a pronounced influence on the obtained products. In fact, the preparation of mixed oxides by traditional techniques (such as the pH controlled co-precipitation) does not result in single phases but several multi-component phases with various atomic ratios are obtained. In the system under investigation it seems to be impossible to obtain a contemporary precipitation of two components, like hydroxides or non soluble salts, instead non uniform precipitates are always obtained [11]. The thermal transformation of these precipitates gives compounds with a variable atomic ratio as a function of their local concentration in the precipitate. On the contrary, the sol-gel method can get to the formation of a truly homogeneous gel (atomic interdispersion), that is the precursor of a very well interdispersed final material. In all cases the calculated metal amounts and a good metal interdispersion have been verified by SEM-EDS microanalyses. References 1. P. Afanasiev, I Phys. Chem. B, 109 (2005) 18293. 2. J. L. Al-Saeedi, V. V. Guliants, Appl. Catal. A: General, 237 (2002) 111. 3. T. Blasco, P. Botella, P. Conception, J. M. Lopez-Nieto, A. Martinez-Arias, C. Prieto, J. Catal., 228 (2004) 362. 4. V. Cortes Corberan, Catal. Today, 99 (2005) 33. 5. M. Cimini, J. M. M. Millet, N. Ballarini, F. Cavani, C. Ciardelli, C. Ferrari, Catal. Today, 91-92(2004)259. 6. Zhen Zhao, Xingtao Gao and I. E. Wachs, 1 Phys. Chem. B, 107 (2003) 6333. 7. B. X. Huang, Kang Wang, J. S. Church, Ying-Sing Li, Electrochim. Acta, 44 (1999) 2571. 8. M. Sarzi-Amade, S. Morselli, P. Moggi, A. Maione, P. Ruiz, M. Devillers, Appl. Catal. A: General, 284(2005)11. 9. I.E. Wachs, J.M. Jehng, G. Deo, H. Hu, N. Arora, Catal. Today, 28 (1996) 199. 10. P. Moggi, S. Morselli, C. Lucarelli, M. Sarzi-Amade, M. Devillers, Stud. Surf. Sci. Catal., 155(2005)427. 11. N. Ballarini, G. Calestani, R. Catani, F. Cavani, U. Cornaro, C. Cortelli, M. Ferrari, Stud. Surf. Sci. Catal., 155 (2005) 81.
841 841
Sol-Gel synthesis and characterization of Nb-Mo and Nb-Mo-V mixed oxides as potential catalysts for the selective oxidation of propane Carlo Lucarellia, Pietro Moggi^*, Fabrizio Cavanib, Michel Devillersc "Dip. Chimica Organica e Industriale, Universita di Parma, Viale G.P. Usberti 17/A Campus, 43100 Parma, Italy; E-mail: [email protected] Dip. Chimica Industriale e dei Materiali, Universita di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy ' Unite de Chimie des Materiaux Inorganiques et Organiques, Universite Catholique de Louvain, Place Louis Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium
Abstract In this work a modified hydrolytic sol-gel technique has been applied to synthesize binary and ternary systems based on Nb, Mo and V, as potential catalysts for the selective oxidation of propane to acrolein and/or acrylic acid. Crystalline mixed oxide phases were obtained and investigated by XRD, Raman and UV-VIS spectroscopy. 1. Introduction Mixed oxides of 4-6 group transition metals are important inorganic materials used in different fields such as catalysis, ceramics and microelectronics. Complex oxide formulations including Nb and Mo are extensively used in partial oxidation catalysis [1]. The current abundance and low cost of natural gas-based alkane feedstocks have recently generated a strong interest in the oxidative catalytic conversion of alkanes to olefins, oxygenates and nitriles. Many research results have shown that Mo- and V-containing mixed metal oxides and more complex systems containing Mo, Nb and V oxides are catalytically active for the partial oxidation of lower alkanes [2]. There is an increasing interest in the development of a process for direct oxidation of propane to acrylic acid as an alternative to the two-step conventional industrial process based on propylene as feedstock [3]. A high selectivity is necessary for reaching a high efficiency in the use of new raw materials and energy resources, as well as for reducing the cost of product separation and the downstream emissions according to the Kyoto Protocol [4].
842
C. Lucarelli et al.
2. Experimental 2.1 Preparation of catalysts The preparation of all catalytic systems has been performed by a sol-gel technique, following a standardized procedure to warrant a good reproducibility. The samples were prepared under similar conditions using Nb(OPr%, MoCl5 and VCKOPr1^, as starting materials. MoCl5 and VOCOPr^ are commercially available products, while Nb(OPr')5 has been synthesized starting from NbCl5 and 2-propanol by adding triethylamine, according to the following reaction: Cl Ck I Nfc-CIt ^CHJjCHOH 1-SNEla CK II Cl
OCH[CHd, HCJiHCO^ I *,M> — OCH(CH]]i +• 3 (ltCJHCO^ I
The reactants have been introduced with the following molar ratios: NbCl5/(CH3)2CHOH/NEt3=l/42/ll. 2-propanol has been added dropwise to niobium chloride under stirring at room temperature, then NEt3 has been added to the solution maintained in an ice bath under an anhydrous atmosphere to prevent the early hydrolysis of the product. After 7 hours of reaction at 110-120 °C, the solvent excess has been distilled out, then the solution has been cooled down to room temperature and the precipitated ammonium salt has been removed by filtration under nitrogen atmosphere. The reaction yield was about 75% based on NbCl5. The synthesized product was characterized by 'H-NMR. To prepare the Nb/V systems, a 2-propanol solution of Nb(OPr')5 has been added, under anhydrous conditions, to a stirred solution of VO(OPr')3 at room temperature, then a solution of mixed water and 2-propanol (hydrolysis molar ratio water-to-alkoxides R = 2) has been added dropwise to the stirred mixture of the metal oxide precursors getting the formation of a gel in few hours. Many systems with different compositions have been prepared, generally obtaining the formation of a homogeneous gel. In only one case (Nb/V=l :1) a gel was not obtained, probably because the hydrolysis of the vanadyl isopropoxide is much faster than the hydrolysis of the niobium isopropoxide used as precursors. Therefore, the large amount of partially hydrolyzed vanadyl isopropoxide, which quickly precipitates, has influence on the subsequent polycondensation reaction between the two metal hydroxide precursor species. In order to avoid the precipitation of vanadium hydroxide species, another sample with the same Nb/V ratio has been prepared by adding during the synthesis a stoichiometric amount of citric acid as complexing agent. The Mo/V, Mo/Nb and Mo/Nb/V samples have been synthesized under the same previously described conditions, but the hydrolysis ratio has been changed in some cases in order to obtain a homogeneous gel phase. Actually, in these
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
843
cases the synthesis method was a combination between the hydrolytic and the non-hydrolytic techniques because M0CI5 and VO(OPr')3, M0CI5 and Nb(OPr1)5, and/or VO(OPr')3 have been added as precursors to the water/2propanol mixture. The condensation reaction always occurred under acidic conditions due to the fact that part of the HCl developed by the hydrolysis of M0CI5 remained in the solution. After a preliminary differential thermal analysis (DTA) test to evaluate the solid phase transformations at increasing temperatures, the xerogels obtained after ageing 7 days in air at room temperature and drying in vacuum, were thermally treated in static air as follows: 15 h at 623 K, 2 h at 723 K, and finally 4 h at 823 K or 5 h at 773 K or 4 h at 973 K or 4 h at 1173 K. 2.2 Characterization of catalysts The obtained mixed oxides were characterized by BET surface area determinations, X-ray powder diffraction (XRD) analyses, Raman spectroscopy, UV-VIS spectroscopy and scanning electron microscopy (SEM). The BET specific surface area measurements were carried out on a Micromeritics Pulse Chemisorb 2705 analyser using nitrogen at 77 K ("single point" method). The samples were previously outgassed under helium flow at 473 K. The powder X-ray diffraction spectra were recorded on a Philips PW 3710 diffractometer using the Cu Ka (k = 1.54178 A) radiation. The crystalline phases were identified with reference to the powder diffraction data files (JCPDS - ICDD). Raman spectroscopy was performed on a Renishaw 1000 spectrometer equipped with green laser (k = 514 nm, power 25 mW, confocal objectives 5X, 20X, 5 OX). SEM micrographs were taken with a JEOL 3120 instrument equipped with a Philips XL 30 ESEM. 3. Results and Discussion In Table 1 the measured BET surface area values of the synthesized systems are reported. All samples are characterized by low surface areas according to the values reported in literature for these materials [5], only the samples containing a large amount of Nb showing values higher then 10 m g"1. A homogeneous gel was obtained from all preparations with the only exception of the sample Nb/V=l/1. For the corresponding modified sample which was synthesized by adding citric acid, a homogeneous gel was obtained as in all other cases. From these preliminary results it was concluded that a high Nb(OPr')5 or M0CI5 amount, or the presence in the case of the sample Nb/V=l :1 of a vanadium ion complexing agent such as citric acid, are necessary for preparing homogeneous gels containing Nb, V and Mo mixed oxide precursors.
844
C. Lucarelli et al.
Table 1. Synthesized catalysts, hydrolysis ratio, calcination temperature/time and surface areas Surface area
R
Calcination temperature/time (K/h)
2
823/4
5.1
2
823/4
6.3
Nb/V=3/1
2
823/4
22.4
Nb/V=4.5/1
2
823/4
23.7
Nb/V=9/1
2
823/4
20.2
Mo/Nb=l/12
1
773/5
38.8
Mo/Nb=l/8
2
773/5
17.9
Mo/Nb=3/14
J
1173/4
4.1
Mo/Nb=3/14
3
973/4
4.3
Mo/Nb=l/4
3
973/4
2.5
Mo/Nb=3/2
5
1173/4
4.1
Mo/V=3/2
10
823/4
5.8
Mo/Nb/V=3/1.5/0.5
5
823/4
6.7
Mo/Nb/V=3/1/1
5
823/4
7.2
Mo/Nb/V=3/0.5/1.5
5
823/4
6.3
Hydrolysis Ratio
Sample Nb/V=l/1 Nb/V=l/lcit
a
(m2g')
Citric acid added as complexing agent
In Table 2 the crystalline phases detected by XRD and Raman analyses are shown for all the prepared samples. The sample Nb/V=l/1 (precipitate) reveals the stoichiometric crystalline phase NbVO5; instead, the sample Nb/V=l/1 cit, synthesized by adding citric acid as vanadium ion complexing agent, confirms a second crystalline phase (NbigVzjOss) in addition to NbVO5. It is reasonable to assume that the formation of a homogeneous gel has some influence on the structure of the material because it can favour a better atomic dispersion. The other samples containing Nb and V also show the presence of the crystalline mixed phase NbisV^ss, with the exception of sample Nb/V=9/1 which displays only Nb2O5. Only the sample Nb/V=l/1 shows evidence of a stoichiometric crystalline phase; the other Nb/V catalysts show signs of a loss of vanadium, probably because it forms an amorphous or microcrystalline phase with crystal size lower than 4 nm; in the case of Nb/V=9/1 vanadium containing phases were not detected. For the samples containing Nb and Mo, mixed oxide phases have not been detected, but only either Nb2O5, or a slightly reduced molybdenum oxide or a mixture of the two single oxides, depending on the
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
845
Mo/Nb ratio and the calcination temperature. Instead, the sample Mo/V=3/2 showed a mixed oxide phase containing Mo and V. Table 2. Crystalline phases detected by XRD and Raman analyses Sample
Crystalline Phases
Raman shift (cm"')
Nb/V=l/1
N b V O 5 [46-0046]
243; 318;643;690 990; 900; 742; 703; 472; 424; 359;232;147
Raman attributions Nb 2 O 5 V 2-\l* D2-x*J5
Nb/V=l/lcit a
Nb| 8 V 4 O5 5 [46-0087]/ NbVO 5 [46-0046]
148; 232; 315; 702;950
Nb 2 O 5
Nb/V=3/1
Nb 1 8 V 4 O 5 5 [46-0087J
703; 317; 233; 147 1011;952;732;239
Nb,O 5 Nb 18 V 4 O 55
Nb/V=4.5/1
Nb, 8 V 4 O 5 5 [46-0087]/Nb 2 O 5
Nb/V=9/1
128; 466; 627 158; 256; 725; 889; 981
Nb 2 O 5 Nb, s V 4 O 5 5
Nb 2 O 5
952; 703; 310; 233;147
Nb,O,
Mo/Nb=1/12
Nb 2 O 5 [05-0352]
952; 7 7 5 ; 7 0 4 ; 3 1 1 ; 233
Nb 2 O 5
Mo/Nb=l/8
Nb2O5 [07-00611
955; 718; 310;240
Nb 2 O 5
MO13O33 [82-1930], Nb 2 O 5
990; 865;660 910; 745; 360; 262;240
MoO.,
Mo/Nb=3/14
Mo/Nb=3/14
[37-1468] M013O33 [82-1930]
990; 874; 632 916; 751; 471; 367;264
Nb 2 O 5 MOO3 Nb 2 O 5
M013O33 [82-1930], Nb, 2 O 2 9 [16-0734]
987;674 934; 900; 730; 630; 470; 310; 258
Mo/Nb=3/2
Mo r3 O 3 3 [82-1930]
988;879;650 970; 914; 743; 629; 470; 419; 369; 263
Nb 2 O 5
lVTn/V=3/2 1VJ.L»/ V _?/ A*
MoV 2 O 8 [74-1510],MoO 3 [01-0706], V4O9 [23-0720]
989; 817; 659; 378; 330; 281; 233; 149.
M0O3
Nb 1 8 V 4 O 5 5 [46-0087], NbO 2
753; 246. 950; 853; 658.
Nb 2 O 5 M0O3
(V 0 , 07 Mo 0 , 9 3) 5 O 14 [31-1437], MOO3 [01-0706]
756; 246. 956; 866; 656.
Nb 2 O 5 M0O3
(Nbo,o9Moo,,i)02,8o [27-1310], V 2 MoO 8 [74-1510],MoO 3 [01-0706]
753; 246.
Nb,O 5
950; 853; 658.
1V10U3
Mo/Nb=l/4
Mo/Nb/V=3/1.5/0.5
[19-0959]
MoOs Nb 2 O 5 MOO3
(Nb 0 , 09 Mo 0 , 91 )O 2 , 80 [27-1310], Mo/Nb/V=3/1/1
Mo/Nb/V=3/0.5/1.5
Citric acid added as complexing agent
846
C. Lucarelli et al.
In the ternary systems Mo-Nb-V mixed oxide phases containing Mo-Nb and Mo-V have been detected in addition to molybdenum oxide, as revealed in Table 2 for the samples Mo/Nb/V=3/1/1 and 3/0.5/1.5, respectively. The Raman analyses also demonstrate the presence of different metal oxides. The sample Nb/V=l/1 shows the typical Raman bands of Nb2O5 (243; 318; 643; 690 cm" ), assignable to the angular distortion of O-Nb-0 bonds, to the bending of Nb-O-Nb bonds and to the distortion of Nb-0 in the Nb-O-Nb bonds, respectively, according to Zhen Zhao et al. [6-7]. In addition, according to the distortion vibrations of V-O-Nb and the vibration of terminal V=O in V-O-Nb groups [6,8-9], the bands confirm the formation of a solid solution V2.xNb2.xO5. The Raman analysis for the sample Nb/V=l/1 cit confirms only the bands due to the Nb(V) oxide, but they are shifted towards higher wave number values, that implies the formation of a solid solution containing Nb and V [6]. The Nb/V=3/1 sample shows the Raman bands at 961 and 1011 cm"' due to the vibration of terminal V=O in the Nb|SV4O55 phase [6,8-9], in addition to the Nb(V) oxide Raman bands shifted like in the previous sample. In the Raman spectrum of the sample Nb/V=4.5/1 it is possible to recognize the presence of Nb(V) oxide (128; 466; 627 cm"1) and that one of a solid solution containing both metals (158; 256; 725; 889; 981 cm"1) [6-8]. Finally, the catalyst Nb/V=9/1 shows only the Raman bands due to Nb2O5. The Raman spectroscopy, for these samples, gives some useful informations about the oxide structure but does not explain the apparent loss of vanadium. In the other samples containing Mo and Nb the Raman bands due to Mo=O bonds vibration (990-1000 cm"') and MoO3 (660 cm"'), in addition to Nb oxides have been detected. The samples containing Mo, Nb and V showed the bands due to the terminal V=O bonds (970, 998 cm"'), the bands due to MoO3 (817, 663 cm"') and the bands due to the Nb oxides. In the case of the sample Mo/Nb/V=3/1/1 some bands probably due to a VxNb2-x05 solid solution (700 cm"' [V-O-Nb; Nb-O-Nb], 290 cm"' [VxNb2-xO5 distortions]) have been detected [6], while in the sample Mo/Nb/V=3/1.5/0.5 the bands due to V4Nbi8055 have also been detected [10]. For the sample Mo/V=3/2 only the bands due to the MoO3 oxide have been detected. Also in this case, by Raman spectroscopy it was possible to collect useful information about the structure of the samples, but it was also evident the apparent loss of some metal phases. Instead, the presence of vanadium in all samples has been detected by UV-VIS spectroscopy. The sample Nb/V=9/1 even shows the presence of a microcrystalline V4+ phase (UV band at about 420 nm) as reported in figure 1; the presence of V4+ was determined by recording the spectrum of the sample and subtracting to this one the spectrum of pure Nb2O5 calcined at the same temperature.
Nb-Mo and Nb-MoNb-Mo-V Sol-gel synthesis and characterization of ofNb-Mo V mixed oxides... oxides...
847
It M
w V
t» V
W M
u
u
n / /
V
J2*
\ '
it it
Dlff. (NbA/=9/1-Nb2O5)
u "
wn
M
T
w
I
\
\
1 V V •«
\
M
M
V.
'•
M
IW
ID*
IMM
Figure 1: UV-VIS analysis of the sample Nb/V=9/1.
To evaluate the effective relative amount of different metals and their interdispersion SEM microanalyses have been performed. The samples calcined at temperatures higher than 973 K showed a molybdenum amount lower than the theoretical one, probably because at the highest temperature Mo oxide volatilizes from the lattice. The theoretical amount of different metals was respected for all the other samples. The effective presence of metal oxides, yet not detectable by XRD and Raman spectroscopy, was detected by SEM microanalysis in all samples. 4. Conclusions The synthesis method applied in this work has produced good results, in terms of crystalline mixed phases obtained. In fact, many systems showed, after calcination, crystalline mixed oxide phases. The sample withNb/V=l:l showed a stoichiometric crystalline phase NbVO5, such as the Nb/V=4.5:l system, which presented the stoichiometric crystalline phase Nb|SV4O55 in addition to Nb2O5. The XRD spectrum of the sample Nb/V=9:l only showed the Nb2O5 phase, but the presence also of a microcrystalline V4+ phase was observed by UV-VIS analysis. It is important to remark that only in the case of the Nb/V=l/1 system the stoichiometry of the obtained compounds was according to that of the preparation atomic ratio. For example, the sample Nb/V=4.5:l would give only the Nbi8V4O55 phase; on the contrary, it also showed the presence of Nb2O5. This fact implies that (i) the vanadium is present as a dispersed amorphous or microcrystalline phase (this is the case of Nb/V=9/1) or (ii) the mixed phases are non-stoichiometric solid solutions (V3+xNb2-xO5), that could "give hospitality" to a higher amount of vanadium than the stoichiometric
848
C. C. Lucarelli et al.
one; the last hypothesis is probably true because the formation of solid solutions was actually detected by Raman spectroscopy. The ternary samples also showed in all cases crystalline mixed oxide phases, such as (Nbo.o9Moo.9i)02.8o and ( )
The Raman analyses have supplemented the XRD results. In some cases the Raman analyses have shown vibration bands assignable to different phases than XRD; these results suggest as hypothesis that some crystalline phases are not detectable by XRD analysis because they are microcrystalline with a size lower then 4 nm. For the ternary Mo-Nb-V samples it was observed that the formation of the Mo/V and Mo/Nb mixed phases occurs for a restricted range of Mo/V and Mo/Nb ratios. Finally, the preparation method has a pronounced influence on the obtained products. In fact, the preparation of mixed oxides by traditional techniques (such as the pH controlled co-precipitation) does not result in single phases but several multi-component phases with various atomic ratios are obtained. In the system under investigation it seems to be impossible to obtain a contemporary precipitation of two components, like hydroxides or non soluble salts, instead non uniform precipitates are always obtained [11]. The thermal transformation of these precipitates gives compounds with a variable atomic ratio as a function of their local concentration in the precipitate. On the contrary, the sol-gel method can get to the formation of a truly homogeneous gel (atomic interdispersion), that is the precursor of a very well interdispersed final material. In all cases the calculated metal amounts and a good metal interdispersion have been verified by SEM-EDS microanalyses. References 1. P. Afanasiev, I Phys. Chem. B, 109 (2005) 18293. 2. J. L. Al-Saeedi, V. V. Guliants, Appl. Catal. A: General, 237 (2002) 111. 3. T. Blasco, P. Botella, P. Conception, J. M. Lopez-Nieto, A. Martinez-Arias, C. Prieto, J. Catal., 228 (2004) 362. 4. V. Cortes Corberan, Catal. Today, 99 (2005) 33. 5. M. Cimini, J. M. M. Millet, N. Ballarini, F. Cavani, C. Ciardelli, C. Ferrari, Catal. Today, 91-92(2004)259. 6. Zhen Zhao, Xingtao Gao and I. E. Wachs, 1 Phys. Chem. B, 107 (2003) 6333. 7. B. X. Huang, Kang Wang, J. S. Church, Ying-Sing Li, Electrochim. Acta, 44 (1999) 2571. 8. M. Sarzi-Amade, S. Morselli, P. Moggi, A. Maione, P. Ruiz, M. Devillers, Appl. Catal. A: General, 284(2005)11. 9. I.E. Wachs, J.M. Jehng, G. Deo, H. Hu, N. Arora, Catal. Today, 28 (1996) 199. 10. P. Moggi, S. Morselli, C. Lucarelli, M. Sarzi-Amade, M. Devillers, Stud. Surf. Sci. Catal., 155(2005)427. 11. N. Ballarini, G. Calestani, R. Catani, F. Cavani, U. Cornaro, C. Cortelli, M. Ferrari, Stud. Surf. Sci. Catal., 155 (2005) 81.