Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves

Applied Catalysis A: General 232 (2002) 189–202

Oxidative dehydrogenation of isobutane on MCM-41 mesoporous molecular sieves B. Sulikowski a,∗ , Z. Olejniczak b , E. Włoch a , J. Rakoczy c , R.X. Valenzuela d , V. Cortés Corberán d,1 a

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland b Institute of Nuclear Physics, Radzikowskiego 152, 31-342 Kraków, Poland c Kraków University of Technology, Warszawska 24, 31-155 Kraków, Poland d Instituto de Catálisis y Petroleoqu´ımica, CSIC, Campus UAM-Cantoblanco, 28049 Madrid, Spain Received 10 August 2001; received in revised form 11 February 2002; accepted 12 February 2002

Abstract MCM-41 mesoporous molecular sieves containing silicon and vanadium have been prepared by a direct hydrothermal route. Aluminium-containing [Si,Al]-MCM-41 and purely siliceous [Si]-MCM-41 solids were synthesised for comparison purposes. 51 V static and magic-angle-spinning NMR studies of [Si,V]-MCM-41 showed one type of VO4 in the as-prepared samples (site I) and two types of monodispersed and distorted to various extent VO4 tetrahedra, chemically bound to the walls of MCM-41 (sites IVa and IVb), in the calcined and rehydrated samples. NMR parameters of vanadium units are given. The catalytic performance of MCM-41 materials was probed in the oxidative dehydrogenation (ODH) of isobutane. The highest selectivities to isobutene (48–59%) were observed for the [Si,V]-MCM-41 materials with Si/V = 85 and 30, respectively. On the contrary, a sample containing V2 O5 bulk-like species exhibited much lower selectivity to isobutene, especially at the isobutane conversion higher than 5%. The presence of isolated vanadium species, as found by 51 V NMR, was responsible for the high olefin selectivity in the ODH of isobutane. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxidative dehydrogenation; Isobutane; MCM-41; 51 V NMR; Vanadium species

1. Introduction Two classes of materials that are used extensively as heterogeneous catalysts and for adsorption purposes are microporous and mesoporous inorganic solids. Crystalline aluminosilicates (i.e. zeolites) and aluminophosphates belong to the first class, while a number of inorganic solids, including mesoporous ∗ Corresponding author. Tel.: +48-12-6395127. E-mail addresses: [email protected] (B. Sulikowski), [email protected] (V. Cort´es Corber´an). 1 Co-corresponding author. Tel.: +34-915854783.

molecular sieves, to the second. The utility of zeolites have already had a great impact on the chemical industry. The wide application of zeolites stems from the fact that they are versatile materials and can be prepared with various structures and with a broad composition range. Incorporation of heteroatoms (different from aluminium and silicon) into zeolitic solids in order to change their catalytic and adsorption properties remains one of the main objectives of heterogeneous catalysis research. For example, it is known that substitution of titanium or vanadium in zeolites leads to the formation of redox centres and application of such materials in oxidation reactions (industrial processes

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 1 0 2 - 3

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have been already implemented) [1]. In this context, of considerable interest is vanadium, a component of numerous industrial catalysts for selective oxidation, ammoxidation and NOx reduction [2,3]. Dispersion of vanadia on different supports was widely studied [4–6]. It is, therefore, not unexpected that the idea of isolation of a particular metal species within either oxide or zeolite matrices attracted the attention of many researchers. Discovery at Mobil (in the early 1990s) of a novel class of silica-based materials (MCM type) characterised by a regular arrangement of uniform mesopores bridged the gap between zeolites and other amorphous forms of silica, due to pores size and their spatial distribution [7]. It was immediately recognised that this finding would have a major impact on catalysis. Incorporation of various elements into the MCM type materials [8], in analogy to isomorphous substitution of cations in zeolites [9], has aroused continuing interest world-wide and indeed led to the synthesis of novel materials. Attempts to prepare vanadium-containing MCM-41 type solids were successful [10–12] and the solids exhibiting interesting catalytic properties were obtained [13]. Despite a relatively abundant literature concerning the application of modified MCM-41 as catalysts in the liquid phase, a very few studies have been devoted to the gas phase oxidation of organic molecules [14,15]. The objective of our work was to study the oxidative dehydrogenation (ODH) of isobutane on the MCM-41 type materials containing vanadium. While several processes of lower alkanes dehydrogenation are known, they all suffer from the major drawbacks: high temperature required for the endothermic process and a very rapid deactivation of catalyst due to coking. An alternative to dehydrogenation is the ODH of alkanes, the process much more favourable from the thermodynamic standpoint [16]. The ODH of alkanes is also a demanding process, because the olefins formed are much more reactive than the starting substrates. Hence, the knowledge of the possible reaction centres and local environments of the vanadium ions are of great importance in designing an active and selective catalyst [2,16]. We studied catalytic properties of the [Si,V]-MCM-41 samples with different vanadium content. Vanadium was inserted by a direct hydrothermal route. The [Si]-MCM-41 sample was used as a reference material.

2. Experimental 2.1. Samples preparation Hydrothermal synthesis of vanadium-containing mesoporous molecular sieves was carried out using tetraethyl orthosilicate (TEOS) as a silicon source, vanadyl sulphate as a vanadium source, cetyltrimethylammonium chloride (CTACl), and sodium hydroxide. The molar compositions of the gels prepared for syntheses were: 1 TEOS: 0.25 Na2 O: 0.65 CTACl: (0.016–0.066) VOSO4 : 62 H2 O. The syntheses were performed in a Teflon-lined stainless steel autoclaves at 100 ◦ C for 96 h. The resulting solids were carefully washed with doubly-distilled water, dried at 80 ◦ C, and finally calcined in air at 550 ◦ C overnight to remove the organic template. The as-prepared and calcined samples were analysed by atomic absorption spectroscopy. Purely siliceous and [Si,Al]-MCM-41 samples were prepared similarly by a direct hydrothermal route adjusting appropriately the gel composition. Aluminium sulphate was used for the latter sample. 2.2. X-ray diffraction (XRD) Powder XRD patterns were acquired on a Siemens D5005 automatic diffractometer using Cu K␣ radiation (55 kV, 30 mA) selected by a graphite monochromator in the diffracted beam. Silicon powder was used as an internal standard if necessary. 2.3. Argon adsorption studies Sorption of argon was measured in a volumetric sorption unit of a standard design. The samples were outgassed at 350 ◦ C before the measurement. 2.4. UV–VIS spectra The diffuse reflectance UV–VIS spectra were collected on a Shimadzu UV-2100 spectrometer. The calcined and rehydrated samples were ground into powders and deposited onto a holder, and the spectra were acquired in the range of 200–800 nm at room temperature. BaSO4 was used as a reference material. The percentage of reflectance was converted into f(R)-values using the Kubelka Munk transform.

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2.5. Catalytic tests The ODH of isobutane was performed in a tubular down-flow quartz reactor (length 39 cm, diameter 0.9 cm). The catalyst particles (0.10 g, 0.315– 0.400 mm) were mixed with 2 cm3 of quartz chips (0.40–0.63 mm) and loaded into the reactor. Additional amounts of SiC particles (0.50–0.80 mm) were placed above the catalyst bed to decrease the reactor void, thus suppressing the undesirable gas phase reactions. The ODH of isobutane was studied using helium (99.999%): isobutane (>99%): oxygen (99.7%) = 82:12:6 (total gas flow 100 cm3 /min; W/F = 3.54 g h/mol i-C4 ). The SiC and quartz chips were cleaned with concentrated HNO3 and intensively washed with distilled water prior to use. Before the test a catalyst was preheated in the dry helium flow (40 cm3 /min) at 550 ◦ C for 1 h. The conversion of isobutane was investigated at 400–600 ◦ C under the atmospheric pressure and steady-state conditions, which were established after 10–30 min on stream at given temperature. The process was performed until the temperature at which the oxygen was completely consumed. The substrate and products were analysed by a Hewlett–Packard gas chromatograph using AT-Alumina capillary column (30 m, 0.53 mm i.d.) and FID detector for hydrocarbons and Unibeads 1S packed column (3 m, 2.0 mm i.d.) and TCD detector for O2 , CO and CO2 . Conversion of isobutane is expressed in mol%. Selectivities were calculated as amount of each compound in the product (mol%) related to the isobutane conversion. Note the selectivity to linear butenes is not involved in the selectivity to isobutene. The dehydrogenation selectivity of the samples is lowered further by the presence

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of saturated alkanes and COx in the products. Carbon and mass balances were kept within 5% or below. 2.6. Solid state NMR Solid state NMR spectra of 51 V were measured on a home-made pulse NMR spectrometer at 78.75 MHz (magnetic field 7.05 T). A Bruker HP-WB high-speed MAS probe equipped with the 4 mm zirconia rotor and KEL-F cap was used to record the MAS spectra at the spinning speed ranging from 7 to 8 kHz. The free induction decay was recorded after a single 2 ␮s rf pulse, which corresponded to a π /8 tipping angle for the liquid sample. The repetition rate was 0.2 s, the spectrum width 200 kHz, and 1 k of complex points were acquired. The number of acquisitions ranged from 16,000 up to 200,000 for low vanadium content samples. The recorded spectra were interpreted on the basis of large experimental data in the literature, as well as by comparison with some model compounds containing well characterised vanadium environment (V2 O5 ). All the samples were hydrated before the measurements.

3. Results and discussion The calcined solids were characterised by XRD and argon adsorption studies. XRD revealed a typical pattern of the MCM-41 type structure (except the sample with Si/V = 15). It is also apparent that the repeat distance (a0 ) between the pores centres is smaller upon insertion of vanadium. Argon adsorption studies showed good sorption properties (Table 1).

Table 1 Characteristics of the as-prepared and calcined/rehydrated MCM-41 materials, containing vanadium, silicon and aluminium Sample

Si/V in gel

Si/V in calcined sample

BETAr (m2 /g)

d100 (nm)

a0 (nm)

[Si,V]-amorphous (15) [Si,V]-MCM-41 (30) [Si,V]-MCM-41 (85) [Si]-MCM-41 [Si,Al]-MCM-41 (30)a

15 30 60

15 30 85

21 714 841 840 959

3.35 3.29–3.44 3.80 3.89

3.87 3.80–3.97 4.39 4.49

The amorphous sample is included for comparison. The repeat distance (a0 ) between the pore centres was calculated with the formula √ a0 = 2 d100 / 3. a Si/Al = 30.

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Note that BET area for samples [Si]-MCM-41 and [Si,V]-MCM-41 with Si/V = 85 is the same and decreases further upon increasing the content of vanadium for sample with Si/V = 30. The environment of vanadium in the as-prepared and calcined MCM-41 type solids was investigated by static and dynamic 51 V NMR. Vanadium-51 NMR spectroscopy has become a very useful experimental method for studying the structure of vanadium complexes in solid state. In contrast to other nuclei that are extensively used in the structure determination of zeolites, molecular sieves and glasses, such as 29 Si, 27 Al, 71 Ga and 11 B, the NMR lineshape of 51 V in high magnetic field is dominated by chemical shift anisotropy [17,18]. It should be stressed that the isotropic chemical shift alone, as determined from the position of the centre band in the MAS NMR spectrum, is insufficient to discriminate between different coordinations of vanadium complexes. It is therefore necessary to measure both the static and MAS NMR spectra. A

crucial information provided by the MAS NMR spectrum is the number of nonequivalent positions of vanadium ions in the studied sample, as determined from the number of centre bands. Based on this, a detailed analysis of the static spectrum can be carried out. In favourable cases of only few nonequivalent positions, it is possible to determine the coordination, local symmetry, and the type of association of vanadium polyhedra by comparison with the spectra of model compounds, accompanied by numerical simulation. Since the 51 V NMR spectra are extremely sensitive to any distortions of local symmetry, the hydration and calcination processes and their influence on the coordination of the vanadium complexes can also be studied. 3.1. As-prepared samples The absolute scale MAS NMR spectra of as prepared V/MCM-41 samples with Si/V ranging from 15 to 85 are shown in Fig. 1. The absolute intensities of

Fig. 1. 51 V MAS NMR spectra of the as-prepared samples. The spectra were acquired in the absolute mode with spinning rate 7.7 kHz. The central band is marked with the asterisk. (a) Amorphous sample, Si/V = 15; (b) [Si,V]-MCM-41, Si/V = 30; and (c) [Si,V]-MCM-41, Si/V = 85.

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the spectra scale well with the vanadium content, and the overall shape of the spinning sidebands pattern is essentially the same in all the samples. A single set of spinning sidebands is observed, indicating that only one kind of diamagnetic V5+ is present. The central band is indicated by an asterisk. However, only a residual signal is detected, which is approximately 10% of that observed in the calcined samples. It suggests that most of vanadium in the as-synthesised samples exists as a paramagnetic V4+ species that is unobservable by NMR. Due to low signal intensity it was not possible to obtain any static NMR spectra. The isotropic chemical shift δ iso = 620 ppm and the chemical shift anisotropy δ = |δ33 − δ11 | = 500 ppm as estimated from the envelope of the spinning sidebands pattern can be interpreted as arising from the distorted tetrahedral complex of Q(2) type, i.e. possessing the two bridging oxygens. An analogous observation was made by Chatterjee et al. [19], who could not see any NMR signal at all in the as-synthesised samples of V/MCM-41. Chao et al. [20] suggested on the basis of EPR, UV–VIS and NMR results that two types of vanadium coexist in the as-synthesised samples of V/MCM-41, namely paramagnetic VO2+ ions and diamagnetic (SiO)2 OHV=O complexes, both located in the distorted tetrahedral units. A much stronger MAS NMR signal in the as-synthesised samples of V/MCM-41 was observed by Luan et al. [11]. The isotropic chemical shift δ iso = −622 ppm was assigned to tetrahedral V5+ units located either on the tubular walls or inside the walls. The EPR results also indicated the existence of vanadyl VO2+ complexes in a square pyramidal configuration. In the V/SiO2 system, an isotropic peak in that range Mastikhin and Lapina [18] assigned to the tetrahedral species bound to the surface via two V–O bonds, with probability of V–O–V bonds as well. Since the shape of the MAS NMR spectra remains roughly the same for all the samples despite the vanadium content, it is rather unlikely that such V–O–V linkages exist in our samples. In the V/MEL system, Sen et al. [21] observed a similar MAS NMR spectrum in the as-prepared sample, with δ iso = −621 ppm and the ratio V5+ /V4+ increasing with decreasing vanadium content. The authors suggested that the HVO4 2− ions present in the gel transformed upon crystallisation into a pseudotetrahedral (SiO)2 OHV=O complex. It is also interesting

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to note that similar isotropic chemical shift (δ iso = −633 ppm), but somewhat smaller δ was observed in the as-synthesised samples of V/␤ zeolite, probably due to high degree of crystallinity [22,23]. On the basis of UV–VIS, FT-IR and NMR it was assigned to a distorted tetrahedral (SiO)3 V=O complex, which was resistive to washing with H2 O and NH4 OAc. It can be concluded that in our as-synthesised samples of V/MCM-41 only about 10% of vanadium exists in the V5+ oxidation state, bound to the tubular walls by two oxygen bridges in the form of (SiO)2 OHV=O complexes. 3.2. Calcined and rehydrated samples The absolute scale 51 V MAS and static NMR spectra of three calcined and rehydrated MCM-41 samples with different vanadium coverage are shown in Figs. 2 and 3, respectively. As it was already mentioned above, the signal intensity is an order of magnitude larger than in the as-prepared samples, which means that most of vanadium in the V4+ state has been oxidised to the V5+ state during calcination. Upon calcination, vanishing of the EPR signal of the V/MCM-41 samples was noticed by other groups [11,20]. The important point is that the residual signal observed in the samples before calcination arising from tetrahedral (SiO)2 OHV=O complexes cannot be distinguished from the other signals that are dominant in the calcined samples. Nevertheless, such complexes may still exist as the most stable units which either do not change upon calcination and hydration [23], or transform into (SiO)3 V=O complexes [20,21], in both cases being resistive to washing. The top spectrum in Fig. 2, corresponding to the sample with the highest vanadium content (sample with Si/V = 15), consists of two families of spinning sidebands, clearly representing two different species in approximately ratio 1:1. The corresponding static spectrum presented at the top of Fig. 3 is difficult to interpret being a superposition of two different anisotropic powder patterns. Still, the presence of a strong shoulder at −300 ppm in the static spectrum suggests that at least one of the species may have an octahedral coordination. In order to identify the origin of the two different coordinations, these two families of spinning sidebands are shown separately in Fig. 4, accompanied by the original spectrum at the top. The

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Fig. 2. 51 V MAS NMR spectra (7.7 kHz, absolute mode) of the calcined and rehydrated samples prepared by a direct hydrothermal route. (a) Amorphous sample, Si/V = 15; (b) [Si,V]-MCM-41, Si/V = 30; and (c) [Si,V]-MCM-41, Si/V = 85. The central bands are indicated by asterisks.

first family of spinning sidebands which is characterised by δ iso = −523 ppm and δ = 700 ppm does not have any direct correspondence in the literature. Therefore it can be only tentatively assigned to immobile, distorted octahedral vanadium complexes having water in their first coordination sphere. The tetrahedral species characterised by δ iso in the same range would have much smaller δ [11,12,18]. On the other hand, the second family of spinning sidebands, with δ iso = −586 ppm, δ = 400 ppm, and nearly axial anisotropy of chemical shift, can be identified with the distorted tetrahedral vanadium complexes chemically bound to the surface, and possessing one short V=O bond [24]. A similar tetrahedral species was found by Wei et al. [12] in the calcined and rehydrated sample of V/MCM-41. On the basis of UV-Raman, UV–VIS, and NMR results it was interpreted as the (SiO)3 V–OH complex incorporated into the framework and stabilized by a nearby OH− ion produced by dissociation of H2 O. Another possibility

suggested in the literature for V/SiO2 system, namely a 5-6-coordinated V2 O5 -like hydrated surface phase characterised by similar δ iso , would have much bigger δ [20,25]. It should be noticed that the sample with the highest V content collapsed after calcination, as evidenced by X-ray results. The destruction of the V/MCM-41 (Si/V = 20) mesoporous structure after calcination was also observed by Park et al. [26]. The MAS and static 51 V NMR spectra of calcined and rehydrated V/MCM-41 samples containing lower concentration of vanadium and conserving their mesoporous structure upon calcination are shown in Figs. 2b and c and 3b and c, respectively. The MAS centre band consists of two closely spaced intense peaks in ratio 1:2, located around −510 ppm. The envelope of the whole spinning sideband pattern reproduces the shape of the static spectrum very well. The spinning sidebands linewidth increases with their order, providing additional evidence for the presence of two nonequivalent sites. Relatively small and

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Fig. 3. 51 V NMR static spectra acquired in the absolute mode of calcined and rehydrated samples: (a) amorphous sample, Si/V = 15; (b) [Si,V]-MCM-41, Si/V = 30; and (c) [Si,V]-MCM-41, Si/V = 85.

nearly axial chemical shift anisotropy is of the order of 400 ppm. Both the δ iso range and the magnitude of δ point to the vanadium species with slightly distorted tetrahedral symmetry. An analogous MAS NMR spectrum was obtained by Reddy et al. [10,27], who reported the first successful incorporation of vanadium into the framework of mesoporous MCM-41. The signals were assigned, basing on the small linewidths observed in the static spectrum, to two isolated tetrahedrally coordinated orthovanadate-type complexes. Similar, almost isotropic one or two peaks in the same frequency range were observed by other groups in the calcined and rehydrated samples of V/MCM-41 [12,19,26], as well as in V/MFI [28] and V/MEL [21]. Some controversy exists in the literature as to the physical origin of such narrow lines in the 51 V NMR spectra. Although in all cases the lines were assigned to symmetric tetrahedral sites, the proposed location of vanadium complexes and the reason for symmetrization of the spectrum were different. In the

V/MCM-41 system the lines were interpreted as arising from the tetrahedral sites within the framework, possibly in the form of (SiO)3 V–OH ions stabilized by nearby OH− ions produced by dissociation of H2 O [12]. This species was reported not to be extractable by ammonium acetate [26]. In the V/MFI system, the authors suggested that the (SiO)2 OHV=O complexes were connected to the framework at the defect sites [28]. Another model was proposed in the V/MEL system, in which part of vanadium V4+ present in the gel in the form of H3 VO4 − ions appear after crystallization as the paramagnetic extralattice (OH)2 V=O ions, which transform upon calcination and then hydration into diamagnetic (SiO)(OH)2 V=O and (OH)3 V=O complexes [21]. Since they are loosely bound, or not bound to the surface at all, the motional averaging may lead to quasi-isotropic NMR spectra. This model was supported by the fact that these hydrated species could be easily extracted by ammonium acetate and were catalytically inactive.

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Fig. 4. 51 V MAS NMR spectrum (7.7 kHz) of the calcined and rehydrated amorphous sample (a) with Si/V = 15; (b) and (c) two families of spinning sidebands are shown separately.

It should be noticed that quite different effects of hydration were observed in many other systems of vanadium supported on siliceous substrates, like V/SiO2 [17,24,25], V/HMS [27], V/MCM-48 [29], and V/␤ zeolite [23]. The samples were prepared by impregnation, except for the V/HMS sample, which was made by hydrothermal synthesis. In all these systems the tetrahedral species present in calcined samples that were bound to the surface by two or three V–O–Si bonds transformed upon hydration into the square-pyramidal and then to octahedral species by incorporation of one or two water molecules into the first coordination sphere. The change of coordination number from 4 to 6 upon hydration/dehydration was reversible. The only exception was the VO4 encapsulated in the pores of SiO2 observed in the samples calcined at high temperature, which was found insensitive to hydration [24]. Our interpretation is supported further by the diffuse reflectance UV–VIS spectra (not shown). Thus, in the calcined and rehydrated samples with Si/V =

30 and 85 absorption band at 275 and 340 nm were found. These are characteristic for vanadium with the tetrahedral coordination [20,30]. With increasing vanadium content in the solid and after prolonged hydration under ambient conditions a weak, broad signal with the maximum at ca. 475 nm becomes visible. The absorption in this region may be due to overlapping signals of square pyramidal and octahedral vanadium species [20,29,30]. In summary, five different V5+ species were identified in the samples studied here. Three of them (labelled I, IVa and IVb), belong to the monodispersed vanadium VO4 tetrahedra chemically bound to the tubular walls of the [Si,V]-MCM-41. The NMR parameters and suggested coordination of the vanadium units are listed in Table 2. 3.3. Catalytic activity Catalytic properties of the purely siliceous [Si]-MCM-41 sample were studied, in addition to the

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Fig. 5. The conversion of isobutane as a function of temperature. (a) [Si]-MCM-41; (b) amorphous sample, Si/V = 15; (c) [Si,V]-MCM-41 (30); and (d) [Si,V]-MCM-41 (85). Conditions, He: C4 :O2 = 82:12:6; W/F = 3.54 (g h/mol i-C4 ).

other differing in vanadium content (Si/V = 15, 30 and 85). A sieve containing silicon and aluminium was also used in some experiments. In Fig. 5 the activity of samples in the ODH of isobutane is plotted as a function of temperature. The amorphous sample with the highest vanadium is the least active, probably due to its low BET area (Table 1). The purely siliceous sample is more active, especially at higher temperatures (curve a). Upon incorporation of vanadium into the [Si]-MCM-41, still more active solids are obtained (Fig. 5, curves c and d). However, the activity of both [SiV]-MCM-41 solids is comparable. NMR studies showed that in these samples vanadium is present in the form of two distinct monodispersed VO4 polyhedra exhibiting similar NMR parameters (Table 2). As seen, vanadium content is not a major factor affecting catalytic activity of the samples. Low effect of V content upon activity was also observed by other authors in the ODH of propane [15]. It is also of interest to compare the selectivity to olefin vs. the conversion level of isobutane (Fig. 6). It should be pointed out that the values of the apparent activation energies for olefin and CO2 formation, respectively, calculated from the reaction rate obtained at differential conversions less or equal to 6%, were equivalent within their confidence interval (around 20 ± 2 kcal/mol) for each catalyst. This precludes any artificial temperature dependence in the observed

trends we discuss further. As can be inferred from Fig. 6, large differences between performance of the samples exist. The sample [Si]-MCM-41 without vanadium is the least selective one (curve a). The amorphous sample, containing the largest amount of vanadium, although gives selectivity of 62 at 2% isobutane conversion, dramatically loses it with increasing the conversion level (Fig. 6b). When already a small amount of vanadium is incorporated into the siliceous MCM-41 matrix, the selectivity increases sharply to 47% for 10% conversion (Fig. 6d). The highest selectivity was found for the sample with Si/V = 30 (Fig. 6c). The sample [Si,Al]-MCM-41 (30) (not shown) gives selectivity to olefin below 40% at this conversion level, closer to that found for the purely siliceous [Si]-MCM-41 material. Another feature in Fig. 6 is a tendency to exhibit maximum of selectivity in the 0–3 mol% isobutane conversion. Two points deserve attention to understand this: (i) the only measurements that are breaking the expected trend of continuous decreasing of the ODH selectivity with increasing conversion are present at the lowest conversion. These points are the less precise ones (with the highest relative error) and on the other hand are the initial points in the catalytic protocol used; (ii) it is well recognised that in the all redox oxide catalysts the active phase is initially reduced until the dynamic steady state is reached. It is observed

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Fig. 6. The selectivity of isobutene formation as a function of isobutane conversion. (a) [Si]-MCM-41; (b) amorphous sample, Si/V = 15; (c) [Si,V]-MCM-41 (30); and (d) [Si,V]-MCM-41 (85). Conditions as in Fig. 5.

frequently during the starting period of time-on-stream and is accompanied by a (stoichiometric) reduction of the active phase by the hydrocarbon feed and converting it into COx . If reduction is slow enough we can observe an increased COx selectivity during the initial tests, as it was generally found during the tests. The observed maxima can be therefore explained by reduction phenomena and catalytic test precision at very low conversion. Apart from isobutene, the main product of the isobutane transformation, the other three butenes were found in the reactor effluents. The selectivity to trans-butene-2, cis-butene-2 and butene-1 was from 5 to 16%, thus the overall selectivity to the C4 dehydrogenation products is higher by this value. The distribution of the trans and cis isomers in the products is summarised in Table 3. Interestingly, from Table 3 it can be inferred that the equimolar amounts of trans-butene-2 and cis-butene-2 are typical for vanadium-containing samples, while the enhanced amount of the isomer trans was found for the aluminium-substituted sieve. The isomerization of the double bond in butenes is a reaction proceeding on the acidic sites of the solids and the distribution of the isomers, where the ratio trans:cis exceeds 1, may be observed even below 200 ◦ C. The results can be rationalised assuming that the initial ratio between trans- and cis-butene-2 can be changed further

Table 3 The ratio between trans-butene-2 (trans) and cis-butene-2 (cis) at thermodynamic equilibrium [32] and as found experimentally in the products over [Si,Al]-MCM-41 (30), [Si,V]-MCM-41 (30) and [Si,V]-MCM-41 (85) at 450–550 ◦ C Temperature (◦ C)

Equilibrium ratio trans:cis

450

Experimental ratio Catalyst

Trans:cis

1.62

Si,Al Si,V (30) Si,V (85)

1.4 1.0 1.1

500

1.56

Si,Al Si,V (30) Si,V (85)

1.3 1.0 1.1

550

1.50

Si,Al Si,V (30) Si,V (85)

1.3 1.1 1.1

by the secondary isomerization of butenes on the [Si,Al]-MCM-41. Further work is carried out along this line to understand behaviour of the samples. The combustion of isobutane to carbon oxides is visualised in Fig. 7. First, we note that combustion to COx is limited on all mesoporous samples. If the selectivity is calculated taking into account the number of carbon atoms in the product, these data would be still lower. Second, carbon dioxide is the main reaction product for all the catalysts studied. This shows

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Fig. 7. Combustion of isobutane and intermediate products to COx . (a) [Si]-MCM-41; (b) amorphous sample, Si/V = 15; (c) [Si,V]-MCM-41 (30); and (d) [Si,V]-MCM-41 (85). Conditions as in Fig. 5.

that the homogeneous reaction is very limited in our catalytic system [33]. The lowest combustion was detected on the amorphous sample (Si/V = 15); interestingly, the combustion level remains constant from 440 to 600 ◦ C. Total oxidation to COx is more pronounced for samples with Si/V = 30 and 85. Surprisingly, the highest combustion was observed for the purely siliceous [Si]-MCM-41 material. It is known that Brønsted acid sites are needed both for dehydrogenation and total oxidation [16]. The presence of acid centres changes simultaneously sorption properties of the samples in respect to the olefins formed. The more acidic the surface, the more difficult it is to desorb an olefin from it. Hence, the adsorbed molecules are more amenable to the consecutive reactions with oxygen leading finally to CO and CO2 . Thus the highest combustion level may be linked with the highest acidity, in agreement with the selectivity to primary cracking products (Fig. 8a). The third path of isobutane transformation, in addition to ODH to butenes and combustion to COx , is cracking of the substrate molecule to methane and propene (Fig. 8). The common feature exhibited by all the solids is a very similar level of cracking at all temperatures studied. The highest cracking level was found for the amorphous sample, in accord with the presence of V2 O5 crystallites and selectivity to carbon oxides.

The activity and selectivity of vanadium-containing catalysts is of paramount importance due to numerous oxidation reactions catalysed by them. In many cases a synergetic effect between the active vanadium phase and the oxide support led to improved catalytic performance. For example, the interaction between vanadium pentoxide and anatase brought about the stabilisation of a particular vanadium phase (V6 O13 ) on the surface [31]. Such an interaction was a prerequisite for enhanced selectivity in hydrocarbons oxidation. Many other oxide supports for vanadium pentoxide were studied [5]. In view of this, the dispersion of vanadium species on the surface of other solids, like microporous and mesoporous molecular sieves, is of considerable interest. In such a situation, the dispersion of vanadium oxide on the internal and the external surfaces of a molecular sieve may be considered, and the sieve acts merely as a support. Another possibility emerges when the isomorphous substitution of vanadium in the framework of SiO2 takes place. A novel class of solids, where V is atomically dispersed in the zeolite or mesoporous molecular sieve, would then be formed, thus giving an opportunity to study the activity of vanadium centres embedded in a regular crystalline (zeolite) or quasi-crystalline (MCM) environment. The hydrothermal synthesis applied in this work yields solids belonging to the last class.

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Fig. 8. The selectivity of isobutane transformation to the primary cracking products (methane + propene). (a) [Si]-MCM-41; (b) amorphous sample, Si/V = 15; (c) [Si,V]-MCM-41 (30); and (d) [Si,V]-MCM-41 (85). Conditions as in Fig. 5.

4. Conclusions Vanadium was incorporated into the structure of MCM-41 mesoporous molecular sieve by direct hydrothermal synthesis. 51 V NMR static and dynamic studies enabled the detailed description of the vanadium sites in the as-prepared and calcined samples. In summary, five different V5+ species were identified in the samples. Three of them (labelled I, IVa and IVb) belong to the monodispersed VO4 tetrahydra chemically bound to the tubular walls of the [Si,V]-MCM-41. The NMR parameters and suggested coordination of vanadium are given. Insertion of vanadium into the [Si]-MCM-41 mesoporous molecular sieve affects its activity and selectivity in the ODH of isobutane. Already a small amount of vanadium incorporated into the siliceous MCM-41 matrix increases the selectivity sharply from 30 to 47% at the 10% conversion of substrate. High selectivity to isobutene was found for two MCM-41 samples containing either small or medium amount of VO4 species. The highest selectivity was observed for the sample with Si/V = 30 (59 and 52% for 5 and 10% isobutane conversion, respectively). As revealed by NMR studies, both samples contained two tetrahedral, slightly distorted vanadium sites. It follows

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