Article pubs.acs.org/Organometallics
Vanadium Oxo Organometallic Species Supported on Silica for the Selective Non-oxidative Dehydrogenation of Propane Kai C. Szeto,† Björn Loges,† Nicolas Merle,† Nicolas Popoff,† Alessandra Quadrelli,† Hongpeng Jia,† Elise Berrier,‡ Aimery De Mallmann,*,† Laurent Delevoye,‡ Régis M. Gauvin,‡ and Mostafa Taoufik*,† †
Université Lyon 1, Institut de Chimie Lyon, CPE Lyon, CNRS, UMR 5265 C2P2, LCOMS, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France ‡ Université Lille Nord de France, CNRS UMR8181, Unité de Catalyse et de Chimie du Solide, UCCS USTL, F-59655 Villeneuve d’Ascq, France S Supporting Information *
ABSTRACT: The molecular complex [V(O)(Mes)3] (1) reacts with silica partially dehydroxylated at 700 °C (SiO2‑700) to afford [(SiO)V(O)(Mes)2] (2) as the major surface species, while a bis-grafted surface species [(SiO)2V( O)(Mes)] (3) is obtained on SiO2‑200. These surface species were characterized by DRIFT, Raman, 1H, 13C, and 51V solidstate NMR, and XAFS spectroscopy and show an activity in the non-oxidative dehydrogenation of propane at 500 °C. The bipodal surface organometallic complex displays a higher selectivity in propylene in comparison to the monopodal species.
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elimination is still a matter of debate.21 Previous research efforts have shown that the activity of this reaction depends on the size of the metal oxo units: when CrOx is deposited on high-surfacearea supports, the activity of the catalyst increases with the chromium dispersion.18 This suggests that CrOx units with different degrees of oligomerization and connectivities to the support show different catalytic behavior. Surface chromium sites may be present as either monomeric or polymeric species. Consequently, high loadings are often required. Conversely, vanadium(V) oxide supported on silica, alumina, or titania show promising activities in catalytic light alkane dehydrogenation.11,21−25 These vanadium catalysts are generally prepared via impregnation or sol−gel processes using oxovanadium(V) complexes.26,27 However, as observed for their chromium counterparts, the classical preparation methods yield a mixture of monomeric, dimeric, and polymeric VOx species on the surface, which prevents a reliable identification of the active sites. Nevertheless, the active specie has been proposed to be an isolated vanadium center on the surface bearing an oxo ligand in its coordination sphere.28−30 Many attempts to characterize the surface active site of supported VOx species using spectroscopic techniques for bulk systems such as XAFS and 51V MAS NMR led to inconsistent results.31−38 This active center is in low concentration in classical heterogeneous catalysts, thus further hampering their characterization along with the determination of the mechanism and of the initiation and deactivation steps. The
INTRODUCTION Propylene is one of the major intermediates in the chemical industry1 and is generally obtained as a coproduct from steam cracking of heavier hydrocarbons. Because of the increasing demand of propylene,2 several on-purpose methods for its production have been developed, such as olefin metathesis,3−6 methanol to olefin,7,8 and catalytic dehydrogenation of propane over a heterogeneous catalyst.9,10 Dehydrogenation reactions can be performed under oxidative or non-oxidative conditions. The most common path in industrial plants is a straight dehydrogenation (Oleflex process), without O2, but this reaction suffers from some problems. The main issues are limitation in equilibrium conversion, high heat requirements, deactivation of the catalyst (and so the necessity to regenerate it), and formation of byproducts due to cracking of propane.11 On the other hand, non-oxidative dehydrogenation catalysis often affords higher selectivity to the desired alkene, avoids CO2 formation, and produces valuable hydrogen. Most of the catalysts are based on supported transition-metal systems. For instance, supported noble-metal particles with or without promoter have been widely used.12−15 On the other hand, a rather important and less expensive class of heterogeneous dehydrogenation catalysts involving group V and VI isolated metal centers (Cr, V) supported on conventional materials (silica, alumina, titania) has a central role in an industrial context.16,17 For example, the chromium system has been extensively studied, and the active sites are believed to be chromium in its tri- or divalent state.18−20 Nevertheless, the mechanism of the C−H activation of propane and propylene release by β-H © 2013 American Chemical Society
Received: August 8, 2013 Published: October 29, 2013 6452
dx.doi.org/10.1021/om400795s | Organometallics 2013, 32, 6452−6460
Organometallics
Article
preparation of well-defined “single site” supported metal oxo catalysts therefore presents a significant scientific interest, especially for non-oxidative dehydrogenation,22 which has been poorly described. The structural uniformity inherent to molecular organometallic precursors will lead to well-defined supported vanadium oxo catalysts by surface organometallic chemistry (SOMC). Furthermore, Stair and Marks recently probed the influence of the structure of vanadium organometallic precursors onto the nuclearity and mode of interaction with θ-Al2O3 upon their grafting.39 In this paper, we investigate the reactivity of [V( O)(Mes)3] (Mes = 2,4,6-trimethylphenyl)40−42 with silica, as a continuation of our ongoing study on supported metal oxo complexes43,44 as models for industrial catalysts. These welldefined surface species have further been characterized by DRIFT, solid-state NMR, UV−vis, Raman, and XAFS spectroscopy and studied in the non-oxidative dehydrogenation of propane in a continuous-flow reactor.
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1
H and 13C NMR. The 51V MAS NMR spectrum was recorded using a single pulse excitation with a small pulse angle (π/8). Two spinning rates were used to determine the isotropic chemical shift. Further experimental details are given in the corresponding figure captions. The 51V chemical shift was referenced with respect to crystalline V2O5 used as an external standard (δ −609 ppm). Electron paramagnetic resonance (EPR) experiments were performed on a Bruker EXELSYS spectrometer at room temperature. A microwave frequency ν of 9.813 GHz and a power of 30 mW were used. The relative spin concentration and the g factor were calculated using g mark as an internal standard. The reactor cell for EPR measurements was made of quartz (4 mm i.d.) and was normally charged with 0.01 g samples. Xray absorption spectra were acquired at the A1 beamline of the DORIS ring, DESY, in Hasylab, Hamburg, Germany (project no. I20110747EC), at room temperature at the vanadium K edge, with a double-crystal Si(111) monochromator detuned 70% to reduce the higher harmonics of the beam. The spectra were recorded in the transmission mode between 5.3 and 6.5 keV. In the edge area, spectra were acquired from 5435 to 5540 eV with 0.5 eV monochromator steps. The molecular complexes homogeneously mixed with dry BN and the supported V samples were packaged as pellets within an argonfilled drybox in a double-airtight sample holder equipped with Kapton windows. This type of cell has already been used for air-sensitive compounds.45,46 Energy calibration was performed by using the spectrum of the V calibration foil (K edge 5465.1 eV), recorded simultaneously for each sample. The spectra analyzed were the results of four such acquisitions, and no evolution could be observed between the first and last acquisition. The data analyses were performed by standard procedures, using in particular the program Athena47 and the EXAFS fitting program RoundMidnight,48 from the “MAX” package, using spherical waves. The program FEFF8 was used to calculate theoretical files for phases and amplitudes based on model clusters of atoms.49 The value of the scale factor, S02 = 0.77, was determined from the k1, k2, and k3·χ(k) spectra of a reference compound, a sample of the molecular complex V(O)(mesityl)3 diluted in BN and carefully mixed and pressed as a pellet (one oxygen at 1.58(1) Å and three carbons at 2.05(2) Å in the first coordination sphere, with further carbon atoms, six at 3.06(3) Å and six at 3.35(4) Å). The refinements were performed by fitting the structural parameters Ni, Ri, and σi and the energy shift ΔE0 (the same for all shells). The fit residue, ρ (%), was calculated by the formula
EXPERIMENTAL SECTION
General Procedure. All experiments were carried out using standard air-free methodology in an argon-filled glovebox, on a Schlenk line, or in a Schlenk-type apparatus interfaced to a highvacuum line (10−5 Torr). Solvents were purified and dried according to standard procedures. [V(O)(Mes)3] was synthesized in two steps according to published methods.40−42 Elemental analyses were performed at Mikroanalytisches Labor Pascher. Released compounds during the grafting were performed on a Hewlett-Packard 5890 series II gas chromatograph equipped with a flame ionization detector and HP5 column (30 m × 0.32 mm). Transmission infrared spectra were recorded on a Nicolet 5700 FT-IR using an infrared cell equipped with CaF2 windows, allowing in situ studies. Typically 16 scans were accumulated for each spectrum (resolution 4 cm−1). Diffusereflectance infrared spectra were collected by a Nicolet 6700 FT-IR spectrophotometer in 4 cm−1 resolution. An airtight IR cell with CaF2 window was applied, and the final spectra comprise 64 scans. Confocal Raman spectra were acquired using the 488 nm line of an Ar ion laser (Melles Griot). The excitation beam was focused on the sample by a 50× working distance microscope, and the scattered light was analyzed by an air-cooled CCD (Labram HR, Horiba Jobin Yvon). The fluorescence was subtracted from the spectra for clarity. Diffusereflectance UV−vis spectra in the range 200−850 nm were taken on a PerkinElmer λ1050 UV−vis−near-IR spectrophotometer adapted with the Praying Mantis optical unit provided by Harrick. The samples were diluted in dry BaSO4, and the spectra were recorded against a BaSO4 baseline. An airtight cell with quartz windows was used. Solution NMR spectra were recorded on an Avance-300 Bruker spectrometer. All chemical shifts were measured relative to residual 1H or 13C resonances in the deuterated solvent: C6D6, δ 7.15 ppm for 1H, 128 ppm for 13C. Solid-state NMR spectra were acquired with an Avance II 800 spectrometer (1H, 800.13 MHz; 13C, 201.21 MHz; 51V, 210.30 MHz). For 1H experiments, the spinning frequency was 20 kHz, the recycle delay was 5 s, and 16 scans were collected using a 90° pulse excitation of 3 μs. The 13C CP MAS spectrum was obtained at a spinning frequency of 20 kHz, with a recycle delay of 5 s, and 10312 scans were collected. Optimal resolution was achieved using the PISARRO decoupling at an rf field strength of 70 kHz and with a decoupling pulse unit of 45 μs, corresponding to 0.9τR (where τR is the rotor period). The Hartmann−Hahn conditions were optimized with a ramped radio frequency (rf) field centered at 50 kHz applied on protons, while the carbon rf field was matched to obtain the optimal signal. The contact time was set to 10 ms. The same CP and decoupling conditions were used for the acquisition of the twodimensional HETCOR spectrum. The number of collected scans was limited to 688, with a recycling delay of 5 s and 50 t1 increments in the 1 H dimension, leading to a total experimental time of 48 h. Chemical shifts are given in ppm with respect to TMS as external reference for
ρ=
∑k [k3χexp (k) − k3χcal (k)]2 ∑k [k3χexp (k)]2
× 100
As recommended by the Standards and Criteria Committee of the International XAFS Society, the quality factor, (Δχ)2/ν, where ν is the number of degrees of freedom in the signal, was calculated and its minimization considered in order to control the number of variable parameters in the fits. The fits were conducted by first considering the coordinated ligands of the complexes and then by adding further backscatterers in order to improve the fits. For both silica-supported complexes 2 and 3, considering only the first sphere, one oxo ligand was considered and the coordination numbers of O (siloxy) and C (mesityl) ligands were set free to vary in noninteger values with one constraint (NO + NC = 4.0 for VV), in order to reduce the number of variable parameters in the fit but also to consider, for instance, the possibility of mixtures of mono- and bis-siloxy species in both preparations. Satisfactory fits were obtained with values in agreement with the foreseen coordination numbers, within the error margins, for the monosiloxy-supported vanadium complex 2 on SiO2‑700 (one oxo (1.58(2) Å), 1.1 ± 0.4 for NO (theory, 1; 1.78(3) Å) and 1.9 ± 0.4 for NC (theory, 2; 2.02(3) Å)) and the bis-siloxy-supported vanadium complex 3 on SiO2‑200 (one oxo (1.59(2) Å, 1.9 ± 0.3 for NO (theory, 2; 1.79(3) Å) and 1.1 ± 0.3 for NC (theory, 1; 2.08(3) Å)). In both cases the important decays of (Δχ)2/ν justified taking into account a third type of backscatterer in the fit of the first coordination sphere of these complexes (oxo, O and C) in comparison to fits with only two shells. Further improvements of the fit were carried out with integer 6453
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Organometallics
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Figure 1. (left) IR spectra of silica2‑700 (a) and sample after grafting of OVMes3 on SiO2‑(700) (b). (right) Raman spectra of molecular OVMes3 and sample after grafting on SiO2‑(700). values for neighboring numbers for silica-supported complexes 2 and 3. The inclusion of a supplementary shell with silicon did not lead to statistical improvements of the fits. Preparation and Characterization of 2 and 3. Prior to the grafting reactions, the molecular precursor [V(O)(Mes)3] (1) was synthesized in two steps.40,50 A mixture of 1 (300 mg, 0.70 mmol) and SiO2‑(700) (2 g) in pentane (10 mL) was stirred at 25 °C for 2 h. After filtration, the solid, 2, was washed five times with pentane and the liquid was transferred into another reactor in order to quantify mesitylene evolved during grafting by using p-xylene as internal standard. The resulting red powder was dried under vacuum (10−5 Torr). Analysis by gas chromatography indicated the formation of 503 μmol of mesitylene during the grafting (0.9 ± 0.1 mesitylene/V). Anal. Found: V, 1.40; C, 5.63. 1H MAS NMR (800 MHz): δ 6.3, 2.5, 1.9 ppm. 13C CP MAS NMR (200 MHz): δ 143, 137, 125, 23, 19 ppm. The same procedure was employed for SiO2‑200 to yield 3, with formation of 607 μmol of mesitylene during the grafting (1.9 ± 0.1 mesitylene/V). Anal. Found: V, 1.64; C, 3.06. 1H MAS NMR (800 MHz): δ 6.4, 2.5, 2.0 ppm.13C CP MAS NMR (200 MHz): δ 143, 136, 124, 24, 19 ppm. In Situ Characterization of the Formation of 2 and 3 by IR Spectroscopy. SiO2 (around 20 mg) was pressed into an 18 mm selfsupporting disk, adjusted in a sample holder, and introduced into a glass or quartz reactor equipped with CaF2 windows. The supports were dehydroxylated at 200 or 700 °C. Complex 1 was impregnated at 25 °C in pentane onto the silica disk. After 2 h of reaction, subsequent washings, and drying, an infrared spectrum was recorded. Catalyst Evaluation. The catalytic performance of propane conversion was carried out in a stainless steel continuous-flow reactor (Ptotal = 1 bar, T = 500 °C, total flow rate 5 mL min−1 (20% of propane in argon), catalyst 1 g). The gases were purified with a column of molecular sieves and activated Cu2O/Al2O3 and controlled by Brooks mass flow controllers. The catalyst was charged in the glovebox. A four-way valve allowed isolation of the charged catalyst in the reactor from the environment and extensive purging of the tubes before the reaction. The products were determined by an online HP 6890 GC instrument equipped with a 50 m KCl/Al2O3 column and FID.
perhydrocarbyl ligands. The IR vibration band of the VO being hidden by the silica network’s own vibrations, we relied on Raman spectroscopy. In Figure 1b, the Raman spectra of 1 and 2 are compared. Both spectra are very similar. A characteristic broad line of medium intensity is observed around 1035 cm−1 for 2, assigned to the stretching mode of the VO moiety.39,52−55 In comparison, 1 features a band at 1029 cm−1.50 The lines at about 1700−1100, 900, and 500−620 cm−1 result from mesityl-related modes. However, a notable difference between both spectra is the broadness of the 1035 cm−1 band in 2, related to the presence of Si−O−V in the grafted sample.56−63 A similar observation is obtained when 1 is grafted on a SiO2‑(200) disk (Figure S1, Supporting Information). The main difference is the presence of a larger amount of remaining silanols which are probably in interactions with perhydrocarbyl ligands, resulting in a broad band extending from 3740 to 3250 cm−1. The irreversible disappearance of the free νSiO‑H band and the appearance of νCH and δCH bands and the presence of mesitylene in the liquid phase are in full agreement with a chemical grafting of OVMes3 on silica. Mass Balance Analyses. In order to establish the mass balance of the grafting of 1 with silica, the reaction was carried out by impregnation. A pentane solution of 1 in excess was impregnated at 25 °C on amorphous silica (Evonik, Aerosil 200) previously partially dehydroxylated under vacuum (10−5 mbar) at 700 °C (SiO2‑(700)) or 200 °C (SiO2‑(200)). Upon reaction the initially white material yields the red solids 2 and 3, respectively. First, in the case of SiO2‑(700), the V loading obtained is 1.4 wt %, which corresponds to 0.27 mmol of V/g of silica (Table 1).
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Table 1. Elemental Analysis for the Grafting of 1 onto Silica
RESULTS AND DISCUSSION Vibrational Spectroscopic Studies of the Grafting of VO(Mes)3 on Silica. Silica pellets were used to follow the grafting reaction using transmission IR spectroscopy. The band attributed to isolated silanols νOH at 3747 cm−1 on SiO2‑(700) almost fully disappeared, in agreement with the expected silanolysis reaction (Figure 1a). The weak band centered at 3600 cm−1 is the result of OH vibration modes from residual silanols in interactions with aromatic moieties (Figure 1a), causing red shifts of 140−150 cm−1 with an almost doubled half-width.46,51 Concomitantly, two groups of bands appeared in the 3200−2700 and 1600−1300 cm−1 regions, assigned respectively to νCH, δCH, and aromatic C−C vibrations from the
support
V, wt %a
C, wt %b
C/V ratio
mesitylx/V
SiO2‑(200) SiO2‑(700)
1.64 1.4
3.06 5.63
7.9 17.1
0.9 1.9
The error on V is
