Synthesis and characterisation of multi-element (Nb, V, Mo) MCM-41 molecular sieves

848

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

SYNTHESIS AND CHARACTERISATION OF MULTI-ELEMENT (Nb, V, Mo) MCM-41 MOLECULAR SIEVES Kilos, B.\ Volta, J.-C.^ Nowak, l.\ Decyk, P.^ and Ziolek, M.^ ^Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznah, Poland. E-mail: [email protected] ^Institut de Recherchers sur la Catalyse, CNRS, 2 Av. A. Einstein, 69626 Villeurbanne, Cedex, France. ABSTRACT This work describes the syntheses, the textural and structural characterisation of NbVSi, NbVMoSi MCM-41 type materials in comparison with NbSiMCM-41. It is evidenced that the second transition metal used (V or Mo) during the synthesis affects the location of the elements in the extra framework or framework position (on the walls or inside the walls). The role of each metal for texture and surface properties is discussed. Keywords: Nb, V, Mo containing MCM-41, texture, structure, and surface properties. INTRODUCTION The catalysts containing more than one metal component have become the area of interest in many research groups due to their unique catalytic activity [1-4]. The interaction between metals in such catalysts changes the properties of separate metals, which will be exemplified in a following way. The redox potential of niobia enhances the redox properties of some metal oxides (V, Cr, Mo, etc.) if they are supported on niobia [5]. If niobia is applied as a support for vanadia, the solid-state reaction between vanadia layer and the underlying support inhibits the activity of the catalyst [6]. In order to obtain a promoting effect a low quantity of niobium oxide species is usually added. Nb-V-Si [7] and Mo-V-Sb-Nb [8] oxide systems are the examples of materials in which niobium species acts as a promoter. The application of mesoporous molecular sieves of MCM-41 type as matrices for various metals has opened a new chapter in the catalytic studies. Multifunctional catalysts allow enhancement of both the catalytic activity and the selectivity. The aim of this paper is the preparation and characterisation of MCM-41 materials of various compositions (Nb, V, Mo, Si) addressed to the partial oxidation of hydrocarbons. EXPERIMENTAL Catalyst preparation Niobium, vanadium, and molybdenum were incorporated to the mesoporous molecular sieves of MCM41 type during the hydrothermal synthesis. Sodium silicate and niobium(V) oxalate were used as a silicon and niobium sources respectively, and cetyltrimethylammonium chloride was applied as a surfactant. For the preparation of catalysts containing vanadium and molybdenum, vanadyl(IV) sulphate and ammonium molybdate(VI) were sources of V and Mo respectively. The composition of the reactant mixture depended on the assumed Si/T atomic ratio (where T = Nb, V, Mo), which had been planned as 16, or/and 32, or/and 64, or/and 128, or/and 256. The gels that were formed from these components were loaded into a stoppered PP bottles and heated at 373 K for 24 h. The mixtures were then cooled down to the room temperature (RT). The pH level was adjusted to 11 by dropwise addition of diluted sulphuric (for Mo and V) or oxalic acids (for Nb or Nb + V and/or + Mo) with vigorous stirring. These reaction mixtures were heated again to 373 K for 24 h. The resulting precipitated products were washed with distilled water and dried in air at the ambient temperature. The template in the catalysts was removed by calcination at 823 K, 1 h in helium flow and 12 h in air under static conditions. The general formula of the obtained materials is denoted as TMCM-41.

849 Characterisation techniques The metal contents (Nb, V, Mo) in the calcined TMCM-41 samples were determined by inductively coupled plasma emission spectroscopy (ICP) (AES-Flammes Perkin Elmer M 1100) after solubilisation of the samples in H2S04:HN03:HC1 solutions. X-ray diffraction (XRX)) patterns were recorded between 1 and 40° (29) on a Philips PW 1710 diffractometer (Cu Ka radiation) with a step size 0.02°. The specific surface area (BET) and pore volume of the TMCM-41 materials were measured by nitrogen adsorption at 77 K on a Micromeritics ASAP 2010 apparatus. Scanning electron micrographs with EDX analyses were recorded using a JSM-840A JOEL scanning electron microscopy operated at 15 kV. The samples were deposited on a sampler holder with an adhesive carbon foil and sputtered with gold. UV-VIS spectra were obtained in a Perkin Elmer Lambda 9 spectrometer equipped with a reflectance accessory and a homemade sample holder containing 0.2 g solid powder. BaS04 was used as a reference in the measurements. X-ray Photoelectron Spectroscopy (XPS) was used to analyse the binding energy values and the atomic surface concentration transition metals incorporated in MCM-41. The XPS measurements were conducted on a Escalab 200R X-ray photoelectron spectrometer with MgKa. The effects of the sample charging were eliminated by correcting the observed spectra for a Cls binding energy value of 284.5 eV. The temperature-programmed reduction (H2-TPR) of the samples was carried out using H2/Ar (10 vol%) as reductant (flow rate = 32 cm^ min'^). 0.03 g of the sample was filled in a quartz tube, treated in a flow of helium at 723 K for 1 h and cooled to RT. It was then heated at the rate of 10 K min'' to 1300 K in the presence of the reducting mixture. Hydrogen consumption was measured by a thermal conductivity detector in PulseChemiSorb 2705 (Micromeritics) apparatus. The electron spin resonance (ESR) investigations were carried out using RADIOPAN SE/X 2547 spectrometer. A cavity operating at a frequency of 8.9 GHz (X - band) was used. The ESR measurements were performed at 77 K. Before the registration of spectra, the catalyst was activated under vacuum in the temperature range of RT - 823 K for 4 h at the desired temperature. RESULTS AND DISCUSSION The possibility of isomorphous substitution of silicon in MCM-41 materials with transition metals depends strongly on the metal's nature. If two or three various metals are added during the MCM-41 synthesis, a competition will occur in their location in different positions. One may consider their location in the extra framework positions, on the surface, and inside the walls (in the skeleton) of the amorphous material. Table 1, which presents the assumed and real silicon to transition metal atomic ratios, shows clearly that the amount of metals in the final material differs from that in the gel and this behaviour depends strongly on the metal's nature. Using the same atomic amount of Nb, V, and Mo sources (together with sodium silicate) for preparing one transition metal containing MCM-41, the following order of incorporation of metal content was observed: N b > V » M o [9]. If two (Nb, V) or three (Nb, V, Mo) metals are introduced together during the synthesis, the V content is enhanced in comparison with one metal containing samples. Niobium seems to be responsible for this feature. More vanadium is incorporated into the final mesoporous material when V is introduced together with Nb. The introduction of Mo into the framework together with Si is extremely difficuh (Si/Mo ratios >3000). The use of higher amount of Mo source leads to a higher content of Mo, but no more than -0.01 part of Mo used in the reagent mixture is located in the final material. It is worthy of notice that in the case of high amount of Nb source used during the synthesis of NbMCM-41 (Si/Nb = 32 or 64) the real Si/Nb ratio in the final material is lower. It suggests that not all of Nb is in the Si-O-Nb framework but part of Nb is localised in the extra framework position, which will be considered in the discussion of XRD patterns below. As a consequence, chemical analysis gives rise to a lower Si/Nb ratio than assumed one, which indicates that part of Si from the reactant mixture was not placed in the final material. A similar feature was indicated in [10] for Co-MCM-41. If Si/Nb = 128 is used in the gel, the obtained material exhibits a similar Si/Nb ratio from chemical analysis.

850 Table 1. The chemical composition of the samples (obtainedfromICP analysis). Assumed atomic ratios

Atomic ratios Si/Nb

Atomic ratios Si/V

Atomic ratios Si/Mo

Atomic ratios V/Nb

Atomic ratios Mo/Nb

Atomic ratios MoA^

NbMCM-41-32 NbMCM-41-64 NbMCM-41-128 VMCM-41-32 VMCM-41-128 MoMCM-41-32 MoMCM-41-128 NbVMCM-41-51

Si/Nb=32 Si/Nb=64 Si/Nb=128 SiA^=32 SiA^=128 Si/Mo=32 Si/Mo=128 Si/Nb=256 SiA^=64 V/Nb=4/1

26 45 129

-

-

259 1950

-

-

3274 9440

-

-

-

216

272

NbVMCM-41-64

Si/Nb=128 SiAV=128 Si/Nb=128 SiA^=32 Si/Mo=16 V/Nb=4/1 Mo/V=2/l

129

879

-

0.08

-

-

134

133

1712

2

0.08

0.08

Si/Nb=256 SiAA=64 Si/Mo=32 V/Nb=4/1 Mo/V=2/l

230

255

<4043

0.9

<0.06

<0.06

Catalyst

NbVMoMCM-419.8

NbVMoMCM-4119.7

0.8

The last number in the catalyst symbol denotes Si/T assumed ratio; T=X of transition metal atoms All the materials synthesised within this work exhibited well hexagonal ordered mesoporous structure of MCM-41 type with one system of pores (ca 4 nm of diameter) and a very high specific surface area (ca 1000 Table 2. Texture parameters from low temperature N2 dsorption/desorption. a Catalyst MCM-41 NbMCM-41-32 NbMCM-41-64 NbMCM-41-128 VMCM-41-32 VMCM-41-128 MoMCM-41-32 MoMCM-41-128 NbVMCM-41-51 NbVMCM-41-64 NbVMoMCM-41-9.8 NbVMoMCM-41-19.7 '' ao=2di,

Surface area (BET) (m^g-^) 1090 960 1040 930 1070 1090 1040 1100 970 970 1020 1020

Mesopore volume (BJH-KJS)** 0.99 0.87 0.88 0.80 0.99 1.01 1.02 1.02 0.84 0.89 0.88 0.93

Pore diameter (KJS)** (nm) 4.02 3.66 3.64 3.65 3.94 3.98 3.94 4.10 3.62 3.96 3.95 3.97

pF ;t=ao-w/1.05; ao=unit cell, w-pore size, t-wall thickness

V3•^r^/3 '"Vl+pF

' calculated according to Kruk, Jaroniec, Sayari (KJS) method [12]

Wall thickness*, (KJS)** (nm) 0.87 0.99 1.02 1.04 0.85 0.84 0.94 0.82 1.00 0.98 0.94 0.90

851 Figure 1 shows XRD patterns of calcined TMCM-41 with different Si/T ratios. All four XRD diffraction lines indexed to hexagonal regularity of MCM-41, i.e. (100), (110), (200), and (210) were observed, and the peak intensity is hardly changed. These patterns verify the presence of hexagonal mesoporous arrangements. This indicates that all the prepared catalysts have a structure comparable to that of SiMCM-41. The higher wall thickness of NbMCM-41 in comparison to that of MCM-41 (Table 2) is due to a longer Nb-O bonds [11] than Si-O ones, and may suggest the inclusion of metals into the walls.

B

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LA^

L/Lv 8

2

10

4

6

8

10

20," Figure 1. XRD patterns of calcined TMCM-41 with various Si/T ratios A - a) NbVMCM-41-64, b) NbVMCM-41-51, c) NbVMoMCM-41-19.7 and B - a) VMCM-41-32, b) NbMCM-41-32, c) MoMCM-41-32. The introduction of metals slightly decreases the unit cell parameter, and this decrease depends on the amount of metal introduced. Materials with a higher niobium content (NbMCM-41-32 and 64) exhibit peaks in XRD patterns in a high-angle range, assigned to Nb(V)-oxide species located in the extra framework position. The XRD patterns in this range are shown in Fig. 2 and the compounds concluded from them are listed in Table 3. Generally, if Nb content in the gel is higher, Nb205 is formed, whereas in the case of lower Nb content Na2Nb40ii is observed. It is important to note that V and Mo species were not found in the XRD patterns, but their concentration was very low (most probably not detectable for XRD). Table 3. XRD results Catalyst MCM-41 NbMCM-41-32 NbMCM-41-64 NbMCM-41-128 VMCM-41-32 VMCM-41-128 MoMCM-41-32 MoMCM-41-128 NbVMCM-41-51 NbVMCM-41-64 NbVMoMCM-41-9.8 NbVMoMCM-41-19.7

dioo values (nm)

Unit cell (ao) (nm)

Extra framework phase

3.96 3.71 3.76 3.75 3.79 3.87 3.94 3.80 3.90 3.73 3.78 3.86

4.57 4.29 4.34 4.33 4.38 4.47 4.55 4.39 4.51 4.31 4.36 4.46

NbsOs NbsOs Na2Nb40n Na2Nb40ii Na2Nb40ii Nb205 traces Na2Nb4011

852

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1 CO

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U..J.L>..L.-i,iAl,jHll

T^'^^TtWFjpW TVwip»*w

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d

1l|yi|l|Lu^y»iA^^

ii#riil litiiiililljii ^Mimllllllillf _^1 r'^T^rflfW'frTirFT'Trw' rV^VVWwW NbO^ —' 1

10

1

15

1

1

20

H

1

25

ll, . , ll.40

1—'

30

35

2©.° Figure 2. Extraframeworkphases formed during the synthesis - XRD pattems A - a) NbMCM-41-32, b) NbMCM-41 -64, c) NbVMoMCM-41-9.8 and B - a) NbVMCM-41-64, b) NbMCM-41-128 c) NbVMCM-41-51, d) NbVMoMCM-41-19.8. Table 4. Content of T elements in the catalysts from ICP (bulk) and XPS (surface). Catalyst NbMCM-41-32 NbMCM-41-64 NbMCM-41-128 MoMCM-41-32 MoMCM-41-128 NbVMCM-41-51 NbVMCM-41-64 NbVMoMCM-41-9.8 NbVMoMCM-41-19.7

Bulk Si/Nb 26 44 129 216 207 134 231

Si/Mo 3274 9440 1712 4044

Surface Si/Nb 60 104 263 349 285 272 293

Si/Mo 2400 1080 712 750

SEM EDX analyses indicated the non uniform dispersion of metals. The oxidation states of metals were observed as follows: Nb^^ V^"^, V^^, and Mo^^. Table 4 shows the Si/Nb and Si/Mo ratios measured in the bulk material (chemical analyses) and on the surface (XPS). It may be concluded that molybdenum prefers the location on the surface of the particles whereas niobium concentration in the bulk material is higher than that on the surface. It is true for both monometal and multimetal containing MCM-41. UV-VIS spectroscopy is one of the most commonly used techniques to access the local V environment in vanadium containing silicates. UV-VIS spectra of vanadium containing materials are exhibited in Fig 3. According to the literature [13-15], the region examined is associated with oxygen to vanadium electron transfer, mainly characteristic of V(V) ions. The lower energy charge transfer (LCT) band for octahedral coordination is falling in the 333 500 nm region. In tetrahedral vanadium(V) compounds, in contrast the LCT band is found at -333 nm and the second charge-transfer (CT) transition band appears at -278 nm. The LCT transition for V(IV) falls at higher frequencies in the 286 - 250 nm region. In the visible region the d-d transitions of VO^^ ions could be observed (near 600 and 770 nm). Since the d-d transitions of vanadyl VO^^ ions are generally 10-30 times weaker than those of charge transfer transitions [13] such an absorption is apparently undetected by diffuse reflectance UV-VIS. It is worthy of notice that the second coordination sphere (like in the siliceous mesoporous matrix) for vanadium - oxygen complex can slightly change the positions of UV-VIS bands above described. Taking into account the above description one can assign the bands shown in Fig. 3 as follows. In part A (Fig. 3), for the samples exhibiting higher vanadium loading (Si/V=133 to 259), the presence of octahedrally coordinated V^^ ions (a band around - 3 8 0 nm) was detected. One cannot exclude that the band at -380 nm covers the other one at -340 nm assigned to tetrahedrally coordinated V^^ ions, well visible in spectra shown in part B for low V-loaded materials (Si/V=272, 879 and 1950). The presence of UV band at 344 nm indicates the location of vanadium in the skeleton of MCM-41 materials. This

853 vanadium species is reduced upon evacuation at 573 K as evidenced from ESR spectra showing VO ^ species characterised by the hyperfme structure typical of isolated VO^^ ions [16]. Fig. 5 displays ESR spectra of NbVMCM-41-51 material, in which hyperfme structure from isolated VO^^ is not present in the fresh material, but is very well visible after evacuation at 573 K. In a fresh sample, a broad ESR signal indicates a high local spin concentration of vanadyl VO^^ ions, suggesting the presence of such species on the surface of MCM-41.

J251

A

0.2

B

224

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b

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600

•

r^^

/

800

^-i^ ~

1

200

1

400

'

—1

'

600

1

800

Wavelength, nm

Wavelength, nm

Figure 3. Diffuse reflectance UV-VIS spectra of V containing MCM-41 materials A - a) MCM-41, b) NbVMoMCM-41-19.7, c) VMCM-41-32, d) NbVMoMCM-41-9.8 and B - e) VMCM-41-128 f) NbVMCM-41 -51 g) NbVMCM-41 -64.

Field, [mT] Figure 4. ESR spectra of VNbMCM-41-51 a) fresh sample and b) after evacuation at 573 K for 4 h. The location of niobium in the skeleton of MCM-41 materials can be deducted from H2-TPR profiles (Fig. 5). It is known [17] that H2-TPR profiles of the extra framework niobium species result in a peak (or peaks) at temperatures below 1000 K, and Nb^^ located in the framework is reduced above 1000 K. In the profiles shown in Fig. 6 the broad peaks < 1000 K are difficult to assign to some species because it also covers reduction of vanadium - oxide forms. However, the presence of peaks > 1000 K indicates the reduction of niobium located in the MCM-41 skeleton.

854

400

600

800 1000 1200

Temperature, K Figure 5. H2 -TPR profiles of Nb containing materials a) NbVMoMCM-41-9.7, b) NbVMCM-41-64, c)NbMCM-41-64.

CONCLUSIONS Vanadium and molybdenum are difficult to locate in the MCM-41 materials in both extra framework and framework positions. However, when V and Mo are incorporated together with niobium (in the gel) the Si/W in the final material increases significantly. The same SiA^=128 ratio in the gel results in SiA^=879 in NbVMCM-41-64 and SiA^=1950 in VMCM-41-128. In all the materials studied, transition metal elements are partially located in the skeleton and partially in the extra framework positions. The skeletal location of vanadium was estimated on the basis of UV-VIS spectra (tetrahedrally coordinated V^^) and ESR study (VO^^ isolated - after evacuation). H2-TPR results indicate the presence of niobium in MCM-41 skeleton. Molybdenum is preferentially located on the surface of mesoporous as shown by XPS measurements. The oxidation states of transition metal elements were estimated as follows: Nb^^, V^^ and V^^, Mo^^. The valence state of vanadium depends strongly on the conditions of the material pre-treatment. ACKNOWLEDGMENTS Polish State Committee for Scientific Research (KBN - grant 2004-2006) is acknowledged for a financial support.

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