Single-Site VOx Moieties Generated on Silica by Surface

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Single-site VOx moieties generated on silica by surface organometal-lic chemistry: a way to enhance the catalytic activity in the oxidative dehydrogenation of propane Samir Barman, Niladri Maity, Kushal Bhatte, Samy Ould-Chikh, Oliver Dachwald, Carmen Haessner, Youssef Saih, Edy Abou-Hamad, Isabelle Llorens, Jeanlouis HAZEMANN, Klaus Köhler, Valerio D'Elia, and Jean-Marie Basset ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01263 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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Single-site VOx moieties generated on silica by surface organometallic chemistry: a way to enhance the catalytic activity in the oxidative dehydrogenation of propane Samir Barman,†,‡ Niladri Maity,†,‡ Kushal Bhatte,† Samy Ould-Chikh,† Oliver Dachwald,⊥ Carmen Haeßner,⊥ Youssef Saih,† Edy Abou-Hamad,† Isabelle Llorens,§ Jean-Louis Hazemann,Klaus Köhler,⊥ Valerio D’ Elia,*,†, # Jean-Marie Basset*† †

King Abdullah University of Science & Technology, KAUST Catalysis Center (KCC), 23955-6900 Thuwal, Saudi Arabia. ⊥Department

of Chemistry, Inorganic Chemistry, Lichtenbergstrasse 4 and Catalysis Research Center, Ernst-OttoFischer-Strasse 1, Technical University of Munich, 85747 Garching, Germany. #

Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), 21210, WangChan, Rayong, Thailand. §

Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256, CNRS – Université Lyon 1, 2 av A. Einstein, 69626 Villeurbanne Cedex, France 

Institut Neel, CNRS, 25, avenue des Martyrs, F-38042 Grenoble Cedex 9, France

ABSTRACT: We report here an accurate surface organometallic chemistry (SOMC) approach to propane oxidative dehydrogenation ODH using as a precursor a μ2-oxo-bridged, bimetallic [V2O4(acac)2] (1) (acac = pentane-2,4-dione) complex. The identity and the nuclearity of the product of grafting and of the subsequent oxidative treatment have been systematically studied by means of FT IR, Raman, solid state (SS) NMR, UV-Vis DRS, EPR and EXAFS spectroscopies. We show that the grafting of 1 on the silica surface under a rigorous SOMC protocol and the subsequent oxidative thermal treatment lead exclusively to well-defined and isolated monovanadate species. The resulting material has been tested for the oxidative dehydrogenation of propane in a moderate temperature range (400-525 °C) and compared with that of silica supported vanadium catalysts prepared by standard impregnation technique. The experimental results show that the catalytic activity in propane ODH is strongly upgraded by the degree of isolation of the VOx species that can be achieved by employing the SOMC protocol.

KEYWORDS: surface organometallic chemistry, silica-supported vanadium catalysts, oxidative dehydrogenation, single-site catalysis, propylene production, olefins INTRODUCTION The oxidative dehydrogenation (ODH) of light alkanes is regarded as a promising and advantageous strategy for the industrial scale production of olefins highly demanded by the industry. ODH represents a viable strategy to complement currently applied technologies such as cracking and anaerobic dehydrogenation; the formation of water as a by-product allows the process to operate under a thermodynamically favourable regime, while the presence of oxygen prevents coking at the catalyst surface.1 The process is affected by the formation of products of deep oxidation of alkanes (COx x=1,2), ultimately lowering selectivity and representing a significant drawback towards large scale application. Since decades there has been a steady upsurge in the application of supported vanadium in propane ODH due to its outstanding catalytic performance.2 In spite of recent advances in the applications of such catalysts, the search for a deeper understanding of the mechanistic and structural features leading

to higher catalytic activity and selectivity is still open.3 In this regard, a point of debate is represented by the structureactivity relationship of the various “vanadia species” that populate the support surface. These include tetrahedral (support-O)3V=O moieties, oligo- and polymeric VxOy species presenting simultaneously briding O-atoms, vanadyls (V=O) and V-O-support bonds and particles of bulk V2O5 formed at higher V-density. Opposing views exist on the catalytic relevance of the isolated/polymeric vanadia species. Based on the comparison of the observed catalytic turnover frequencies (TOF), a number of authors has inferred the lack of a specific influence of the degree of polymerization of the VOx species on catalyst activity.3, 4 In many of these cases, the choice of the support has been regarded as the key feature able to influence the catalytic activity by tuning the electronic properties of the vanadium center.3,4c,5 Using silica as a support, high vanadia dispersion has been often observed to play a crucial role in order to achieve higher cata-

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lytic activity and selectivity.6 According to these observations, vanadyl and V-O-support bonds should play the major role in this kind of catalysis. Nevertheless, in some cases higher turnover frequencies have been observed with an increase in the density of vanadium atoms and ascribed to an increase in the relative population of polymeric vanadia.7 Interestingly, a comparable observation in an ethane ODH study has been attributed to the formation of dimers on the support surface.8 Theoretical approaches have proposed a relevant catalytic role for the (O=)V-O-V(=O) motif in propane dehydrogenation,9 although there is a general consensus that the first proton abstraction takes place at a vanadyl site.3,10 In particular, Sauer et al. have reported that alkane dehydrogenation at two cooperating or dimeric sites in a (VV/VV) to (VIV/VIV) fashion is favorable upon a (VV) to (VIII∙H2O) two-electron pathway taking place on isolated vanadia species.11 Recent studies have shown that the presence of polymeric vanadia on the support surface cannot be excluded even at low vanadium density (less than 1 V/nm2) which is at variance with what had been previously assumed.4b,12 Therefore, the poor definition of the catalytically active centres on the support surface might cause the difficulties in discerning the catalytic contribution of mono- and polyvanadate species. In this complex frame, it is evident that the application of new preparative techniques able to produce single-site, isolated catalysts presenting exclusively one type of (monomeric or dimeric/polymeric) VOx structure might significantly contribute to shed light on the catalytic role of the targeted structures. The grafting of pure molecular precursors under surface organometallic chemistry (SOMC) methods represents a powerful tool for the preparation of well-defined, single-site catalysts.13 The systematic study of these surface complexes and of the subsequently formed, immobilized reaction intermediates via IR, SS NMR, EXAFS, UV-vis, EPR spectroscopies may reveal crucial mechanistic information.14 Differently from the traditional impregnation techniques, grafting by SOMC is conducted under moisture-free conditions from diluted solutions of pure precursors that are reacted in stoichiometric proportions (by controlling the precursor to surface hydroxyls ratio) with highly dehydroxylated oxide supports. The target is to generate identical molecular species at each accessible and isolated silanol grafting site, leading to singlesite, well-defined species. The so-synthesized catalysts are not calcined in air or exposed to moisture at any stage, to avoid irreversible structural modifications at the surface and/or at the grafted species. When compared to techniques such as incipient wetness impregnation or flame pyrolysis, this method should afford a non-statistical distribution of the surface species by attaining “molecular level control” on the identity and nuclearity of the species formed on the support surface. Such tailored materials may allow a deeper understanding of the structure-activity relationship (SAR) in catalysis in pursue of higher and more predictable rates and selectivity in oxidative dehydrogenation. We report here the first SOMC approach to propane ODH using as a precursor the μ2-oxo-bridged, bimetallic [V2O4(acac)2] (1) (where acac = pentane-2,4-dione) complex.15 The product of the grafting step (2, Scheme 1) and of the subsequent thermal treatment (3) are systematically studied by mean of Raman, FT IR, SS NMR, UV-Vis DRS, EPR and EXAFS. 3 has been tested for

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the oxidative dehydrogenation of propane in a moderate temperature range (400-525 °C) and its activity is compared with that of silica supported vanadium catalysts prepared by standard impregnation techniques. We demonstrate here that the high degree of isolation achieved by the SOMC protocol is a crucial factor in enhancing the catalytic activity of the vanadia species and that the presence of polymeric vanadate is not crucial for affording high ODH activity.

RESULTS AND DISCUSSION Grafting of precursor complex 1 on SiO2-700 surface by SOMC protocol: The reaction of 1 with the surface of silica was not previously investigated. Upon grafting, this precursor can potentially generate surface species containing both vanadyl and organic moieties (acac). These can be excellent probes for the systematic and complementary characterization of the surface complex.14 Moreover, the dimeric nature of the precursor allows an investigation of the possibility to form dimeric surface complexes analogous to those recently studied by Rozanska et al. by a theoretical approach.11a 1 was reacted at 60 °C with silica (Aerosil) that had been previously dehydroxylated under dynamic vacuum ( 10-5 torr) at 700 °C (SiO2-700, See the SI for experimental details) in dichloromethane (DCM) in a closed rotor flask.

Scheme 1. a) Proposed and hypothetical surface species formed by the grafting of 1 at 60 °C on SiO2-700 to yield 2 and subsequent oxidative thermal treatment in dry O2 to afford 3. b) Possible surface species arising from the interaction of the acac moiety with the hydroxyl groups of the support. The possible grafting pathways of 1 and the relative surface species are displayed in Scheme 1. Although complex 1 is precipitated from acidic aqueous media as a dimer,15 it is known

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to dissociate to afford monomeric species VO2acac (A, Scheme 1) in alcohol medium15,16 and by reaction with diols.17 Therefore, the dimeric nature of the initial complex and the possible dissociation before grafting have been both taken into account. On the basis of the chemical structure and the reactivity reported for 1,15a,15b various grafting routes could be proposed for the reaction of dimeric complex 1 or monomeric complex A with the isolated silanols of SiO2-700. Protonolysis of the V-acac bond with formation of acetyl acetone (acacH) would lead to SOMFs (F: fragment) B and D’. 16b,17 Other possible pathways involve the opening of the dioxo bridge of 1 (C) or the formation of species D from A by sigma-bond metathesis via the surface silanol and liberation of acetylacetone.16a Moreover, the possibility that the vanadium complexes might be absorbed on the silica surface by H-bond between the acac moiety and the surface silanol should be taken into account (Models E, E’, Scheme 1b). This has been reported for the grafting of V(IV) complex V(=O)(acac)2 on the silica surface at room temperature.18 However, at higher grafting temperatures or by thermal treatment, an higher fraction of the vanadium complexes was observed to be directly grafted on silica via a Si-O-V bond (Scheme 1b).18a Following oxidative thermal treatment to produce 3, 2 may rearrange with the adjacent strained siloxane bridges present on the surface of SiO2-700.13,19 This might lead to dimeric V-complexes (F) or to isolated monovanadate species (G) according to the grafting pathway (dimeric vs monomeric) and to the stability of the surface dimers. Characterization of 2 by FT-IR, 1H and 13C CP/MAS NMR, ICP and elemental microanalysis: The isolated silanols on the surface of SiO2-700 display a characteristic and sharp Infrared (IR) band (O-H) at 3744 cm-1 (Figure 1). The density of the surface silanol groups has been reported in the range of 0.23 - 0.3 mmol SiOH/g of silica).20 The reaction of 1.0 g of SiO2-700 with about half equivalent of 1 (0.115 mmol) in DCM resulted in a purple solid (2).

a broad band in the 3300-3700 region (Figure 1). The observation of a set of IR bands in 2 at 1577, 1534 cm-1 as in 1 (that can be readily assigned to C=O and C=C of the acac moiety in its enolic form21), at 1430 cm-1 (for C-H) and of weak signals in the 3140-2780 cm-1 region (C-H) suggest the presence of the acac ligand also in 2 (Species B, C, D, E, E’ of scheme 1). The broad band centred at around 3600 cm-1 is typical for the (O-H) of H-bonded silanols.20d Whereas the stretching of isolated VO-H bonds has been reported at 3660 cm-1,22 the broad band in the 3300-3700 cm-1 region might indicate the interaction of the acac ligand with the unreacted surface silanols (species E, E’ of Scheme 1)18a,18b or the formation of VO-H functionalities during the grafting reaction (species C, D of Scheme 1) but in a H-bond to silanols.22 A vanadium loading of 1.04 wt.% (0.2 mmol V/g of silica; corresponding to 0.73 V/nm2 based on a BET surface area measurement of 165 m2/g) and a carbon content of 1.41 wt.% (1.1 mmol C/g of silica; corresponding to 0.22 mmol acac/g of silica) were obtained based on the elemental analysis of 2. Within the experimental error of the elemental analysis (±10 %), the V/acac ratio of 0.91 is in agreement with the presence of one acac group per each grafted V center (Species C, D, E, E’ of scheme 1). This suggests that the acac moiety does not act as a leaving group. The vanadium loading of 0.2 mmol/g is slightly lower than the maximum theoretical amount expected on SiO2-700 based on the number of surface silanols (0.23-0.3 mmol SiOH/g of SiO2-700) and it is consistent with the presence of a residual silanol stretching band at 3744 cm1. Based on the initial amount of complex 1 and SiO 2-700 and on the V loading of 2 it is possible to infer that the largest part of the V employed (ca. 87 %) was grafted on the silica surface. The 1H solid-state magic angle spinning (MAS) NMR spectrum of 2 displays two signals at 0.8 and 4.9 ppm, which correspond to the CH3 and CH protons of the enolic form of acac ligand and that are comparable to the analogous signals observed in the solid-state 1H NMR of 1 (Figure 2). 23 The 13C CP/MAS NMR spectrum presents three signals at 22, 103 and 193 ppm respectively. The first signal is assigned to the CH3 group and the other two peaks can be attributed to the CH and carbonyl C-atoms in the metal coordinated acac moiety respectively (Figure 2).23 Also in this case, the signals observed for 2 are comparable to those present in the solid state 13C NMR spectrum of 1.

Figure 1. Infrared spectra of SiO2-700 (olive), the silica grafted material 2 (red) and the spectrum of complex 1, [V2O4(acac)2], diluted with anhydrous KBr (black).

Figure 2. Comparison of the solid state 1H (left) and 13C CP/MAS (right) spectra of 1 and 2 (See the SI for experimental details).

This reaction is accompanied by a sharp decrease of the intensity of the IR band at 3744 cm-1 and with the formation of

Based on FT-IR and on the chemical shift values in both the 1H and 13C NMR spectra, the acac ligand is found in its enolic form as expected for the case in which it is coordinated to a

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metal centre. This clearly demonstrates that the signals observed in the FT IR and in the solid state NMR spectra do not arise from the absorption of acacH on the silica surface and further support the hypothesis that the acac group is not cleaved upon grafting. Oxidative thermal treatment of 2 to afford 3: The oxidative thermal treatment of 2 under a flow of dry O2 at 350 °C was monitored by in situ diffuse reflectance Fourier transform infrared (DRIFT-IR) spectroscopy (Figure S2 and Section S5 in the Supporting Information (SI)). A complete removal of the acac moiety was observed as indicated by the disappearance of the IR bands at 1577, 1534 cm-1 and in the 3140-2780 cm-1 region. This was confirmed by elemental microanalysis that detected the absence of carbon in 3. Interestingly, following the removal of the organic ligands under O2 a new week band became evident at 3660 cm-1. The appearance of this band has been attributed in the literature to the formation of isolated VO-H groups.22 As observed by other authors,18b this band appears only after the complete removal of the acac moiety and it does not seem to be clearly correlated to its decomposition product. Its appearance after treatment in dry O2 at 350 °C could be due to the progressive dehydration of the H-bonded hydroxyls responsible for the broad IR absorption at 3300-3700 cm-1. This could have the effect to leave on the surface some residual, isolated VOH functionalities that were already present in complex 2 (See for instance species D of scheme 1). Preparation of incipient wetness impregnation catalysts as reference materials: The standard incipient wetness impregnation (IWI) protocol using NH4VO3 as a precursor was applied for the synthesis of vanadia catalysts (herein generically referenced as IWIcatalysts) to apply as reference materials in order to compare the structural properties and the catalytic performance of 3 with catalysts prepared by traditional techniques. Accordingly, three silica supported compounds were prepared (See section S3 of the SI for details on the preparation of these species) with a V-loading of 0.98 wt.% (IWI-0.98; 0.65 V/nm2), 1.96 wt.% (IWI-1.96; 1.36 V/nm2) and 3.61 wt.% (IWI3.61; 2.6 V/nm2).

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only one resonance at -561 ppm as determined by variation of the spinning frequencies (Figure 3). The signal presents an up-field shift of 105 ppm in comparison to the molecular complex 1, resonating at -456 ppm (Figure 3). This relatively large shift suggests a change in the chemical environment and symmetry of the vanadium centre upon grafting that would be unlikely in the case complex 1 were simply absorbed on the surface by H-bonding interaction (Species E, E’ of Scheme 1). This is also consistent with the results obtained from the EXAFS investigation that suggest a change in symmetry from square pyramidal (1) to (pseudo)tetrahedral (2) upon grafting (vide infra). Whereas the isotropic shift of 2 falls in the spectral region assigned to isolated tetrahedral VO43- complexes,24a,25 also oxygen bridged (V-O-V) distorted tetrahedral surface complexes4e,24a,28and reference vanadate salts29 have been proposed to generate 51V NMR shifts between -550 and -600 ppm. Therefore, it is challenging to assess the presence of monomeric or dimeric complexes in 2 by 51V NMR spectroscopy. The 51V MAS NMR of 3 displays a signal with an isotropic chemical shift of -615 ppm. This signal is shifted up-field by 54 ppm relative to the grafted complex 2 (Figure 3). The impregnation catalysts with a comparable vanadium loading as 3 (IWI-0.98), presents a very similar shift at -612 ppm (Figure S1). Therefore, differences in the chemical environment around the V centres that might exist between these catalysts prepared by different techniques are not obviously clarified by solid state 51V NMR. The vanadium resonances at about -615 ppm for both 3 and IWI-0.98 are consistent with those reported in other 51V NMR studies on silica-supported tetrahedral vanadia species.28 However, they are shifted 60-100 ppm lower field than observed for other silica supported vanadia species prepared by incipient wetness impregnation.4e,26 To notice, the 51V NMR shifts for 3 and IWI-o.98 are very close to the value assigned to crystalline V2O5 at around -610 ppm.4e,24a,24b,25a Nevertheless, the absence of bulk-like vanadia in 3 and in IWI-0.98 could be readily confirmed based on Raman spectroscopy as discussed in the following section.

Solid-state 51V NMR of 1-3 and IWI-0.98: 51V

MAS NMR is a powerful technique for the investigation of the coordination environment and geometry of the vanadium centres in supported vanadia catalysts.24 Peak assignment based on prior literature studies is made challenging by the different experimental conditions under which the spectra were collected and by the lack of suitable unsupported standards.24a The isotropic 51V NMR shift of isolated tetrahedral VO4 complexes on various supports (SiO2, TiO2, Al2O3) has been often identified in the -500 to -600 ppm range.24a,25 However, signals with a 51V chemical shift as low as -460 ppm6b or as high as -710 ppm have been also assigned to silica-supported isolated tetrahedral vanadia, the latter, based on a comparison with bulk crystalline compounds.26 Grant et al.4e have recently assigned to monomeric VO4 species an isotropic shift at -675 ppm in the 51V MAS NMR of silica-supported vanadia catalysts prepared by impregnation. This assignment is based on that of Feher et al. in the 51V NMR investigation of homogeneous vanadium-silsesquioxanes in toluene.27 The 51V MAS NMR spectrum of 2 reveals

Figure 3. 51V MAS NMR spectra of 1 (bottom), 2 (middle) and 3 (top) acquired at 900 MHz 1H NMR spectrometer with a 20 KHz MAS frequency, a repetition delay of 0.3 s and 4096 scans.

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Raman Spectroscopic Studies of 1-3 and IWI-catalysts: The Raman spectrum of 2 (Figure 4, See also Section S1 of the SI for experimental methods) displays an absorption signal at 1032 cm-1 typical for the vanadyl (V=O) moiety in noncrystalline vanadia species.3 This signal is red-shifted by 30 cm-1 when compared to the corresponding vibrational band in precursor complex 1 that contains V-O-V bonds. The Raman spectrum of 3, acquired under an Ar atmosphere, displays a strong vanadyl signal at 1037 cm-1 which is red-shifted by 5 cm-1 relative to the band observed for grafted material 2. Raman bands in the 1020-1035 cm-1, as those observed for materials 2 and 3 have been often attributed to tetrahedral, non-polymeric mono-oxo VO4 species.4a,4c,4e Broad spectral features in the 350-550 cm-1 and 600-625 cm-1 regions originate from the SiO2-700 support (See the Raman spectrum of SiO2-700 in Figure S4 of the SI) and can be attributed to cyclic siloxane structures.30

Figure 4. Raman spectra of the precursor complex [V2O4(acac)2] (1) (black), the grafted material 2 (red) and material 3 (blue). The Raman spectra of the IWI-catalysts are reported in Figure S3 and compared with those of complexes 2 and 3 (See Figure S4 in the SI). Species IWI-0.98, having a very similar V-loading as 3, displays a nearly identical Raman spectrum as those of species 2, 3 with a vanadyl vibration band at 1036 cm-1 and comparable broad spectral features arising from the support. In the case of IWI-3.61 a signal at 995 cm-1 is clearly visible along with the main band at 1040 cm-1. The presence of additional bands at 690 cm-1 and in the 250-300 cm-1 region is indicative of the existence of tridimensional bulk vanadia on the silica surface.3,4e This is in a good agreement with literature reports observing the formation of tridimensional vanadia above a vanadium density of just 1.7 V/nm2.4e In our case, the incipient formation of trace amounts of crystalline vanadia is observed also for species IWI-1.96 (1.36 V/nm2) based on the presence of a weak band at 995 cm-1 and of the other spectral features observed also for IWI-3.61 (2.59 V/nm2). UV-vis DRS characterization of 1-3 and IWI-catalysts: The UV-vis DRS spectra of materials 1-3 are shown in Figure 5. The spectrum of 2 shows a broad absorption centred at 530 nm attributable to the LMCT transition of the acac group that is also observed in complex 1 but slightly red-

shifted (488 nm). This further supports the IR and NMR observation that the acac ligand remains coordinated to the vanadium centre after grafting.

Figure 5. UV-vis DRS spectra of complex 1 (black), the grafted material 2, (red) and material 3 (blue). 2 displays a main absorption band with a maximum at 297 nm. This band is expected to originate from the O → V charge transfer.31 As expected, following the oxidative thermal treatment of 2, the acac based LMCT transition is not observed in the UV-vis spectrum of 3 leaving only a band centred at 292 nm corresponding to the O → V charge transfer. The increase in the number of V-O-V coordinations at the vanadium centre has been correlated to a shift of the absorption relative to the O → V charge transfer towards higher wavelengths (lower energies).30,32 Accordingly, UVVis absorption bands with maxima close to 300 nm as observed for 2 and 3 would indicate the presence of polymeric vanadia species on the surface rather than isolated vanadyl groups. Nevertheless, the edge energy values derived from the O → V charge transfer peaks of 3 (3.6 eV, Figure S6, SI) and of 2 and IWI-0.98 (3.7 eV and 3.5 eV respectively, Figures S5 and S7, SI) would rather be attributed to isolated vanadyl groups.33 IWI-1.96 and IWI-3.61, showing clear evidence of crystalline V2O5 in the Raman spectra (Figure S3), display comparable UV-Vis DRS spectral features (Figures S8 and S9) as the other materials with regards to the position of the O → V charge transfer peaks and of the edge energy values. This implies that it is very challenging to estimate the degree of vanadia polymerization based exclusively on UV-Vis spectroscopy. Structural elucidation of compounds (1-3) by X-ray absorption spectroscopy To provide detailed information on the vanadium coordination geometry, on its structural environment and formal oxidation state, materials 1-3 were studied by X-ray absorption spectroscopy (XAS). The normalized XAS spectra depicted in Figure 6 show a distinct pre-edge feature, a broad XANES region and a featureless EXAFS region. The XANES regions is of a special interest and it is considered to be rich in chemical and structural information on the absorber atom.34The interpretation of the shape and energy position of XANES V K-edge features for vanadium compounds presenting different geometry (tetrahedral, octahedral, and square pyramidal) and oxidation state (+III, +IV, +V) has been reviewed.35 Further research in the last decade has led to the refinement

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ACS Catalysis of the experimental methodologies and to theoretical developments.36,37The exclusive use of the absorption edge position is of limited applicability because several oxidation states display overlapping absorption edge position. On the other hand, the use of the pre-edge features along with the absorption edge position was found to be by far more accurate for XANES interpretation.36a In the following work, we use a method based on the interpretation of both the preedge peak area and its centroid position which was originally applied for the determination of the Fe (III)/Fe (II) ratio in minerals38 and later for the determination of the vanadium speciation in steel slag.36a All XANES spectra depict a very intense pre-edge located around 5470 eV (Figure 6). The intensity of the pre-edge is regarded as a probe of 3d-4p orbitals mixing and thus as an indirect probe of the symmetry around the metal centre.36b,39,40 For a better qualitative description, the decomposition of the pre-edge peaks into Voigt components is shown in the inset of Figure 6. The total area (sum of each component area) and the centroid positions (area-weighted average of the position energy in energy of each component) are reported in Table 1. The preedge features for the initial precursor complex, 1, the grafted material 2, and the thermally oxidized material 3 appear different upon close examination. The total area of the preedge is decreasing from 1.67 down to 1.49 after grafting of 1, and further increase up to 2.07 after the oxidative thermal treatment of 2 in O2. The centroid position is also following a similar trend with a decrease from 5470.5 eV down to 5469.8 eV after the grafting of 1, and an increase up to 5470.0 eV after the oxidative thermal treatment of 2 in O2. This suggests that a small fraction of the V(V) initially present in V2O4(acac)2 is reduced to V(IV) after grafting. The V(V) oxidation state is fully re-establish after oxidative thermal treatment. This could be unambiguously verified by the EPR spectroscopic measurement which will be discussed in the following section. Initially, the symmetry of vanadium center in 1 is approximated with a square pyramidal symmetry.15a,15b For complex 2, the components used to decompose the pre-edges are different in terms of number, energy positions and areas compared to 1 (Table 1). This is indicative of a complete change of symmetry after grafting -the latter being undetermined. After the oxidative thermal treatment, the pre-edge becomes very similar to what is observed with a V(=O)(OiPr)3 reference (see supplementary information, Section S8 and Figure S11) which demonstrate a pseudo tetrahedral symmetry (C3v) for 3. Vanadium K-edge k2-weighted Fourier transform of EXAFS signals for V2O4(acac)2 (1), grafted material (2), thermally oxidized (3) and the corresponding back Fourier transforms are shown in Figure 7. The parameters extracted from the fit results are regrouped in Table 2. To the best of our knowledge, no crystal structure has been reported for the V2O4(acac)2 precursor complex. Hence, to provide a set of initial atomic coordinates for the EXAFS study, molecular models of V2O2(acac)2 were built and optimized at the DFT level (See in the SI, Section S8). For the sake of clarity, we also investigated the nuclearity of the precursor by modelling the monomeric (VO2(acac)) and the dimeric form V2O4(acac)2 (Scheme 1 and Figure S12). Comparison of the statistical agreement between the two models points at a dimeric form for 1 (R-factor: 0.08 vs 0.01, 2 = 593 vs 93, See Figure S13 for additional details).

1 2 3

1.6

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5470

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5474

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5400 5420 5440 5460 5480 5500 5520 5540 5560 5580 5600

Energy [eV]

Figure 6. Experimental vanadium K-edge XANES spectra of 1, 2 and 3. The inset shows the extracted pre-edges with a subtracted background and their decomposition with Voigt functions using a fixed Lorentzian width of 0.8 eV. Table 1. Pre-edge characteristics for 1, 2 and 3. Material

1

2

3

Component area position [eV] 5468.2

0.09

5470.2

1.39

5472.5

0.19

5468.2

0.11

5469.4

0.73

5470.5

0.64

5468.2

0.34

5469.3

0.32

5470.4

1.23

5472.0

0.18

total area

main Centroid edge [eV] [eV]

1.67

5470.4

5480.3

1.49

5469.8

5479.3

2.07

5470.0

5480.7

Table 2: Parameters extracted from the EXAFS fit for 1, 2, and 3. Underlined characters denote a fixed parameter. d [Å]

σ2[Å2] ΔE0 x103 [eV]

V-O1 3

1.690 ±0.008

11.3 ±0.9

V-O2 2

1.947 ±0.008

1.3 ±0.5

V-V

3.22 ± 0.02 3 ± 1

Materials Shell N(a)

1

2

3 a

1

V-O1 1.0 ± 0.2 1.68 ± 0.02

0.9

V-O2 3.2± 0.9 1.98 ± 0.02

6±3

V-O1 1.1 ± 0.1

0.9

1.57 ± 0.02

V-O2 3.1 ± 0.2 1.76 ± 0.02

3±1

Refers to the coordination number of atoms.

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R

9 ±2

0.01

7 ±5

0.01

-4 ±4

0.01

The DFT molecular model of V2O4(acac)2 involves three different oxygen atoms located at distances of 1.60 Å (1 x V=O), 1.82 Å (2 x 2-O) and 2.01 Å (2 x O from acac) Å. The DFTcalculated V-V distance for the µ2-oxo bridged dimer is 2.7 Å. During EXAFS fitting, the scattering paths of the vanadyl oxygen atom and of the two bridging oxygen atoms were merged into a triply degenerated V-O scattering path. Thus, the first shell of V2O4(acac)2 is described here by 3 V-O1 bonds at 1.68 ± 0.01 Å and 2 V-O2 bonds at 1.94 ± 0.01 Å for the acac ligand. Note the large increase of the mean square displacement parameter for V-O1 due to the modelling of two types of oxygen atoms (Table 2). The long distance scatterer was best fitted with a single V-V scattering path at 3.22 ± 0.02 Å. However, the latter distance is rather unexpected for a structure displaying two bridging 2-O oxygen atoms and seems more compatible with a single bridging oxygen. The presence of a µ2-oxo bridged V2O2 core in this compound has been inferred in the early work of Doadrio et al. based on the observation of a band at 775 cm-1 in the IR spectrum of the complex.15a Nevertheless, the position of this band does not allow a clear discrimination between the existence of a single V-O-V bond or of a V2O2 core, although the µ-oxo bond is likely found in a bent geometry (νV-O-V > 700 cm-1).41 Indeed, the FT IR spectrum of 1 displays a clear indication of the presence of H-bonded VO-H groups (broad band at 3411 cm-1) that could arise from the opening of one of the V-O-V bridges by a molecule of water.42 Based on these observations, a revised structure (1’) for 1 is proposed in Scheme 2. An unequivocal answer to the doubts concerning the structure of 1 reported in the literature15a,15b remains tied to a structure resolution of its X-Ray diffraction pattern (See Figure S16 for a PXRD pattern of 1).

the best. To notice, the presence of the acac ligand in 2 generating the V-C scattering path is supported by FT-IR, SS NMR and UV-Vis DRS spectroscopies. All the better-fitting models are characterized either by the absence of a V-V contribution (V-Si + V-C) or by a decreased coordination number for the V-V scattering path (N(V-V) = 0.3-0.4). These observations suggest that before, or as an effect of grafting, monomeric or a mixture of monomeric and dimeric surface complexes are formed from dimeric precursor 1. To notice, a minor fraction of the V atoms in complex 2 are found in a V(IV) oxidation state as supported by XANES and EPR (See below) investigation. 1 2 3

1.2

|(R)| [Å-3]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.4

0.0 0

1

2

3

4

5

R [Å] 1.6

1.2

0.8

Im[(q)] [Å-2]

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0.4

0.0

1 2 3

-0.4

-0.8 0

Scheme 2. Revised structure (1’) of precursor 1 based on the EXAFS analysis. The first coordination shell of species 2 was best fitted with two different V-O scattering paths. The parameters extracted from the EXAFS fit for the [0.7; 2.4] Å range highlight a short V=O double bond at 1.68 ± 0.02 Å (N = 1.0 ± 0.2) and a longer V-O single bond at 1.98 ± 0.02 Å (N= 3.2 ± 0.9). A model with two V=O double bonds (≡SiO-V(=O)2, D’, Scheme 1) was also considered to investigate the occurrence of this species but led to a lesser statistical agreement (Rfactor: 0.02 vs 0.01, 2 = 168 vs 141). The longer R-range ([0.7; 3.8] Å) was investigated using six different models in which the first shell, modelled by the two oxygen scattering paths, was augmented by V-V, V-C, and V-Si long distance scatterers or by a combination thereof. (See Table S2, and Figures S14 and S15 for EXAFS best fits, related Fourier and Cauchywavelet transforms of data and fits). Close examination of the results indicate that models including just a single V-Si or a V-C scattering path can be discarded. The three other models provide a reasonable agreement with the data; the model including both a V-V and a V-C scattering paths being

2

4

6

8

10

12

14

k [Å]

Figure 7. Fourier transforms of EXAFS k2.(k) functions (top) without phase correction and imaginary part of the back Fourier transform (bottom) in [0.7; 4] Å range for V2O4(acac)2 (1), grafted material (2) ([0.7; 2.4] Å range), thermally oxidized material (3) ([0.7; 2.4] Å range). The empty symbols are the experimental data while the full lines are the fit result Further treatment under O2 flow at 350 °C results in material 3 with a very different EXAFS spectrum from the previous two samples. A distance contraction of the oxygen shells is noticed with a shorter V=O bond at 1.57 ± 0.01 Å and three V-O bond at 1.76 ± 0.01 Å. There are no obvious contributions from higher order shell observable on the Fourier transform. This demonstrates that after heat treatment under O2, the vanadium centres are exclusively monomeric and isolated in spite of the dimeric nature of the initial precursor employed. EPR spectroscopic characterization of surface complexes 2,3. In order to get additional information on the oxidation state and structure of the vanadium species formed upon grafting,

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electron paramagnetic resonance (EPR) investigations were performed. The EPR spectra depicted in Figure 8 (black) evidence that the grafting of complex 1 leads to the formation of non-negligible amounts of vanadium(IV). Due to the remarkably good resolution of the spectra, the vanadium(IV) centres (3d1, S = ½) in material 2 must be well-separated from each other (diamagnetic dilution). The characteristic 51V hyperfine structure (octet of lines, I = 7/2) shows that isolated mononuclear oxo-vanadium(IV) centers were grafted on silica during synthesis. The spectra are described best by a spin Hamiltonian with axially symmetric g and hyperfine tensors. Their simulation (Figure 8, red spectra) yielded spectral parameters which are typical of oxo-vanadium(IV) sites in tetragonal coordination geometry (see Table 3).43 The extraordinarily narrow line widths (Bpp < 2.5 mT) demonstrate the well-defined molecular structure of the grafted vanadium(IV) surface complexes. The high resolution allowed the identification of a second tetragonal oxovanadium(IV) surface complex (ratio about 1:2) with very similar spectral parameters (see Table 3). This finding underlines the uniqueness of SOMC grafting technique for preparation of supported isolated and well-defined oxovanadium catalysts. To further understand the degree of V(V) reduction occurring during the grafting step, we carried out a quantitative determination of isolated V(IV) sites in 2 as a fraction of the total vanadium content. The double integration of the EPR spectrum of 2 in Figure 8 and the comparison to peak areas of calibration solutions (see the SI) suggests that about 12 % of the total vanadium atoms are V(IV). It is known for EPR that the experimental errors of such quantitative determinations can be rather high (Δrel = ± 25 %). However, even considering the error margin, 15-20 % of the V atoms is in oxidation state IV at maximum. This percentage is in good agreement with the XAS results presented in the previous section and it clearly shows that a minor fraction of the V(V) precursor has been reduced to V(IV) during the grafting procedure. Concerning the structure of the V(IV) complex the EPR spectrum suggests that a V=O bond is clearly present, forming the dominating symmetry axis of the molecule. Comparison with the EPR parameter of similar complexes indicates that the first coordination sphere is completed by singly bounded oxygen atoms.43 Therefore, considering the formation of at least one bond with a surface oxygen atom upon grafting, a general structure can be proposed for the V(IV) species (Scheme 3). The coordination sphere of the V(IV) complex might be completed by the acac ligand or by other oxygen containing ligands as the reduction of V(V) to V(IV) might be related to the oxidative degradation of the acac ligand as observed for other transition metal complexes.44

Scheme 3. Plausible structure of the surface V(IV) complexes observed by EPR spectroscopy. Treating 2 in a flow of pure oxygen at 350 °C results in the complete disappearance of the EPR signal (see Figure S17b in

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the SI, strongly amplified) being in good agreement with total oxidation to EPR silent vanadium(V) (3d0, S = 0) and with the other spectroscopic results. Treatment with propane at 400 °C re-established EPR-active isolated V(IV) species qualitatively. The spectral resolution is not as good as in the fresh sample, yet it does not deteriorate excessively either. This robustness against overall surface shape transformation like thermally induced atom mobility under reactive atmosphere is vital evidence for grafting being capable of providing sintering-resistant catalysts.

Figure 8. X-band EPR spectra of the isolated oxo-vanadium(IV) surface complexes (2) formed by grafting of 1 on SiO2-700. Experimental (black) and simulated spectra of 2 (red, see the SI for recording parameters and simulation). Table 3: Experimental and simulated45 EPR spectral parameters of the isolated oxo-vanadium(IV) surface complexes formed after grafting of 1 on SiO2-700 (Figure 8). Exp. 285 K

Exp. 153 K

Sim. 285 Ka

Sim. 153 Ka

S1

S2

S1

S2

g||

1.942

1.942

1.942

1.942

1.942

1.942

g┴

1.989

1.988

1.982

1.981

1.982

1.981

A|| [10-3 cm-1]

17.4

17.4

17.2

17.4

17.4

17.6

A┴ [10-3 cm-1]

6.8

6.8

6.6

6.3

6.6

6.2

a Error:

g = ± 0.002, A = ± 0.3 x 10-3 cm-1.

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Final proposal on the precursor grafting pathway and on the structures of species 2,3: Based on the spectroscopic and analytical evidences collected above and on the revised structure (1’, Scheme 2) for precursor 1, we propose a revised grafting pathway and the structures in Scheme 4 for surface complexes 2 and 3 (Scheme 4).

(est. 10-15 % of the total V content). The reduced V-species might have formed as an effect of the oxidative degradation of the acac ligand as observed for other transition metal complexes.44 Finally, absorption of 1 via coordination of the acac ligand to the silica surface (models E or E’ of Scheme 1) can be discarded given the large variation in the V chemical shift (over 100 ppm) observed upon formation of 2 (Figure 3) and on the complete change of symmetry observed by XANES when compared to the pristine precursor 1. Material 3 is represented as an isolated tetrahedral VO4 surface complex based on 51V-NMR, Raman spectroscopy and on the complete absence of V-V contributions in the XAS study. Therefore, the structural study shows that the dimeric structure of complex 1 is not, or it is only in minor part preserved upon grafting on the silica surface to afford 2. No traces of dimeric species are found when 2 undergoes an oxidative thermal treatment to yield 3 as isolated monovanadate species. Propane oxidative dehydrogenation (ODH) by compound 3 and IWI-catalysts:

Scheme 4. Proposed grafting pathway for complex 1’ and structures of 2, 3. The grafting of 1’ might proceed via cleavage of the V-O-V bond yielding a monomeric surface V complex and a monomeric V fragment (V(O)(OH)2acac) acting as a leaving group (Scheme 4). The proposed monomeric surface complex in 2 is consistent with the presence of a metal-coordinated acac moiety (FT IR, SS 1H and 13C NMR, UV-Vis) in a 1:1 ratio to V (elemental analysis) and it is also in a good agreement with the EXAFS model VI of Table S2. The presence of the vanadyl moiety is supported by Raman spectroscopy (Figure 4), EPR and EXAFS investigation. The presence of a VOH functionality completing the coordination shell of the V centre is challenging to demonstrate by FT IR because of the broad absorption in the 3300-3700 region of the FT-IR spectrum of 2. This could arise from the interaction of the acac group or of the VO-H moieties with the residual surface silanols. This broad band might overlap the stretching of the isolated VO-H species expected to appear at 3660 cm-1 ν(VO22 H). Interestingly, the in situ DRIFT IR study of the thermal treatment of 2 under O2 to afford 3 shows the appearance of a peak at 3660 cm-1 after the complete removal of the acac ligand. This signal could be tentatively attributed to residual VOH functionalities becoming evident by IR only after the complete removal of the H-bonded hydroxyls generating the broad absorption in the 3300-3700 cm-1 region. Given the high yield of the vanadium grafting (ca. 90 % according to the V loading), the (V(O)(OH) 2acac) fragment produced by the reaction of 1’ with the surface is expected to undergo a subsequent grafting reaction (See Scheme S1 in the SI for an extended picture of the grafting pathways) also leading to 2. The results of the EXAFS investigation on complex 2 do not allow to unequivocally discard the presence of dimers in this compound (Table S2, Models II and IV). Nevertheless, the dimeric species seems to be present, at most, as a minor component of 2. Dimeric species might arise from the condensation of a (V(O)(OH)2acac) fragment with a previously grafted V monomer. Part of the monomeric and/or of the minor component dimeric complexes in 2 are found in a VIV oxidation state according to XAS and EPR spectroscopies

Catalyst 3 has been applied in the catalytic ODH of propane in the 400 - 525 °C temperature range under dynamic conditions (Figure 9; see Section 11 of the SI for experimental details and Table S3). As expected, the conversion of propane catalysed by 3 increases linearly with the contact time (W/F; W: mass of catalyst in g, F: gas flow rate in mL/s) at all temperatures. A high degree of propane conversion (about 20%), with a propylene selectivity of 55 %, was observed at 525 °C for a residence time of 0.3 g·s·mL-1. The original activity was preserved even after a prolonged experimental time of 22 h (See Figure S18 for a time-on-stream experiment) thus showing that the catalyst is stable under the reaction conditions. The catalytic performance of 3 has been compared with that of the IWI-catalysts prepared by traditional incipient wetness impregnation technique (See Figures S19-S21 for the propane conversion versus contact time curves in the 400525 °C range for the IWI-catalysts). The catalytic results for the IWI-catalysts are summarized in Tables S4-S6 of the SI. In all cases, up to a reaction temperature of 475 °C, propylene, CO2 and CO were observed as the only reaction products. Additional minor products (such as methane, ethylene) could be detected at T ≥ 500 °C along with traces (0.01 vol. %) of other oxidation products. For the IWI-catalysts, the conversion of propane was found to increase linearly with the V-density (and loading) on the silica surface (Figure 10 for T = 525 °C and W/F = 0.3 g∙s∙mL-1) as reported by other authors.4e However, the propane conversion observed for 3 does not appear to match the trend defined by the IWIcatalysts. Catalyst 3 was found to clearly outperform the comparable (with regards to spectroscopic features, catalyst loading and vanadium density) IWI-catalyst (IWI-0.98) at each contact time (Figure 10 and Figure S22, Tables S3 and S4). Indeed, in absolute terms, the performance of 3 in the conversion of propane was found to be even higher than that of IWI-1.96 bearing nearly twice the V loading as 3 and just slightly below that of IWI-3.61. It is worth noticing that all catalysts were prepared using the same batch of silica (Aeorosil-200) therefore the differences in activity should not depend on the different quality, degree of purity or preparation technique of the support.

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what has been observed in the literature,6,46 although at variance with the results of other authors.3,4 For instance, Kondratenko et al. reported that samples with different loadings of vanadium (in the 0.2 to 5 V/nm2 range) supported on MCM-41 did not display obvious differences in turnover frequency.4f Similarly, Trunschke et al. could not observe a strong effect of V loading on the propane ODH activation energies in a study on SBA-15.4b

Figure 9. Propane conversion as a function of contact time for 3 (Reaction conditions: T = 400-525 ◦C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10 - 50 mL / min). W/F (residence time; g·s·mL-1): reciprocal value of the ratio between propane flow (F, mL·s-1) per mass of catalyst employed (W, g). Figure 11. Productivity of propylene as a function of contact time for 3 and for the IWI-catalysts (Reaction conditions: T = 525 °C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10 - 50 mL / min).

Figure 10. Rate of propane consumption plotted as a function of the vanadium surface coverage. (Reaction conditions: T = 525 ◦C; C3H8:O2:N2:He = 15:7.5:15:62.5; W/F= 0.3 g∙s∙mL-1) More importantly, in terms of propylene productivity (µmol of propylene per V atom per hour) catalyst 3 outperforms all the IWI-catalysts (see Figures 11 and S22 and Tables S3-S6 in the SI, see also Figure S23 for the trend obtained at 500 °C) displaying a rate of propylene formation that is 55 % (at 500 °C) and nearly 40 % (at 525 °C) higher than that of IWI-0.98 and 2 to 3 times higher than that of IWI-1.96 under the same conditions (contact time: 0.30 g∙s∙mL-1). For the IWIcatalysts, the propylene TOF values were found to drop significantly with the increase in V loading. For IWI-3.61, this trend is attributable to the formation of crystalline vanadia on the surface as observed by Raman spectroscopy (Figure S3), leading to the unselective oxidation of propane and depressing the selectivity and the yield of propylene.3 The observed decrease in TOF by the V loading is in agreement with

At 525 °C, the trends relative to the different kind of catalysts (SOMC and IWI) appear strongly different also with regards to propylene selectivity (Figure 12). A recent literature survey by Carrero et al.3 has highlighted that, in the absence of crystalline vanadia, the profiles of propylene selectivity versus propane conversion are similar for all catalysts; the extrapolated propylene selectivity at propane conversion values close to zero tends to nearly 100 %. The decrease of selectivity at higher conversion values is generally attributed to the consecutive oxidation of propylene.47 These observations are in a good agreement with the behaviour of the IWI-catalysts at 525 °C. The selectivity for IWI-0.98 is high at low propane conversion (a value of ca. 85 % can be extrapolated at zero propane conversion, in a good agreement with recently reported, well-dispersed silica supported vanadia catalysts4e) and drops rapidly to a value of roughly 62 % by 13 % propane conversion. The extrapolated selectivity for the catalysts containing bulk V2O5 (IWI-3.61) is much lower than 100% even at zero propane conversion as expected based on the presence of crystalline vanadia.3,48,49Also for this catalysts the selectivity drops rapidly with the conversion of propane. Within the IWI-catalysts series, selectivity is found to decrease with the increase of V density as observed by other authors.2b,3,4b,6a,48 The propylene selectivity of 3 at 525 °C is observed to remain constant (66 %) up to ca. 12 % propane conversion and it drops slightly (ca. 55 %) for a propane conversion of about 20 %. As an effect, the propylene selectivity of 3 extrapolated at low propane conversion (66 %) is much lower than that of IWI-0.98 and IWI-1.96. However, at higher propane conversion the selectivity curves for IWI-0.98 and IWI-1.96 eventu-

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ally cross that of 3; the selectivity for catalyst 3 becomes comparable or higher than that extrapolated for the IWIcatalysts at higher propane conversion rates.

Figure 12. Comparison of propylene selectivity as a function of propane conversion at different contact time for 3 and the IWI-catalysts (Reaction conditions: T = 525 °C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10-50 mL / min; contact time = 0.3 – 0.06 g·s·mL-1). At 500 °C (See Figure S24) lower propane conversion and higher selectivity than at 525 °C is achieved in general for all catalysts at the same contact time. In the 3-6 % propane conversion range, catalysts 3, IWI-0.98 and IWI-1.96 displays very similar selectivity (75-80 %), whereas the selectivity observed for IWI-3.61 is, once again, well below that of the other species. Finally, we determined the apparent activation energy (Eg) for catalyst 3 and IWI-0.98 from the Arrhenius plot (Figure S25) using the initial rates of propylene formation with a contact time of 0.3 g·s·mL-1. This gave a value of 132 kJ/mol for both catalysts. However, considering the initial rates of propane ODH in the Arrhenius plot a different activation energy of 145 kJ/mol could be determined once again for both materials. These values fall within the range of 117±28 kJ/mol reported in a recent survey of published activation energies for propane ODH.3 In spite of the different catalytic activity, no apparent difference in the activation energy between 3 and IWI-0.98 was observed, although a difference of a few kJ/mol could be hidden by the error margins relative to this measurement. Discussion of the Catalysis Results: A tentative explanation for the difference in activity and selectivity between species 3 and IWI-0.98 could be provided by considering the structure of the vanadia species on the surface support. To note, both catalysts present similar, and relatively low V loading and density. As commented in a recent review, a low density of V atoms allows a larger spatial freedom on the identity of the surface species depending on the preparation technique, choice of the precursor, etc. This, might play a relevant role in determining the differences in catalytic behaviour observed in the literature among differently prepared, low V loading catalysts.3 Therefore, a deeper insight on the structure of catalysts 3 and IWI-0.98 is required. Although some authors have proposed the exclusive

existence of isolated terminal vanadyl complexes on the surface of dehydrated silica up to the monolayer coverage,4c,6c,30 it is generally accepted that vanadium catalysts prepared by traditional preparation techniques present a mixture of isolated vanadyls and bidimensional polymeric vanadia species.3,4f,5,12a-12c,9a,50 This has been proposed even for V densities in the 0.6-0.7 V/nm2 range.4b,51 Systematic attempts have been directed at the individuation of such polymeric structures by Raman and UV-Vis spectroscopy,4c,6c but the general applicability of these methods is still a matter of debate.4b,12a-c,52 Therefore, in spite of the very similar spectroscopic features revealed in the 51V MAS NMR, Raman and UV-Vis spectra of catalyst 3 and of IWI-0.98, the application of these techniques might not be adequate to estimate the isolation or polymerization degree of vanadia in these catalysts and their pristine structures could differ. Remarkably, in a recent study on the grafting of VOCl3 on dehydroxylated silica surfaces,53 a comparable XAS analysis as the one presented here, was able to clearly identify polymeric structures for a V density of ca. 0.7 V/nm2 (for the grafting on SiO2-1000) In the same study, V-V interactions pertaining to polymeric vanadia could also be clearly identified for the V atoms supported on SiO2-700 but these were not observed for the case of 3. Moreover, all these polymeric vanadia containing catalysts displayed a Raman band at 1040 cm-1 comparable to those observed for 2, 3 and IWI-098. These observations based on EXAFS investigation support the high degree of vanadium atoms isolation afforded by the SOMC protocol in 3 and confirm that it is challenging to evaluate the degree of polymerization of vanadia exclusively by Raman spectroscopy. The likely structural dissimilarities between single-site catalyst 3 and the statistical distribution of monomeric and polymeric surface species obtained by the classical impregnation technique3,4f,4b,5,12a-12c,9a,50,51 could play an important role in determining the differences in catalytic activity between 3 and IWI-0.98. These might arise from the different electronic properties of the V centres and/or from different mechanistic pathways occurring on the isolated or polymeric (and potentially cooperating) sites.11,54 Indeed, understanding the effect on activity and selectivity of the isolated tetrahedral vanadia moieties present in 3 versus the polymeric species on the surface of the IWI-catalysts requires a clear picture of the role of the vanadyl and of the V-O-V bridging oxygens in the activation of propane and in the successive, undesired oxidative steps. Although the first H abstraction is believed to take place at the vanadyl oxygen,3,10,11 Alexopoulos et al. have proposed by a DFT approach that the initial propane physisorption is likely to take place at the bridging oxygen atoms.9a More DFT studies have proposed a relevant role in catalysis by vanadyl moieties where the respective V atoms are connected by bridging oxygen atoms ((O)V-O-V(O) motif).9b,11a Interestingly, the V-O-V bridging oxygen atoms have been proposed to be more selective towards propane dehydrogenation than the vanadyl oxygen.9a Therefore, the V-O-V bridges (when available) cannot be excluded to play an important role in the title reaction by participating to the binding of the substrate and by modulating the electronic properties of the vanadyl moiety. On the basis of these considerations, it is clear that the degree of polymerization of vanadia can play a key role in the different activity and selectivity observed between IWI-

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0.98 and 3. It is challenging to explain the constant selectivity trend observed for catalyst 3 in the 4-12 % propane conversion segment. This behaviour implies that for the isolated VO4 moieties the ratio between the rates of propylene formation from the C3H7* intermediate (Path 2, Scheme 5) and consecutive propylene oxidation (Path 3, Scheme 5) does not apparently increase with the concentration of propylene in the given propane conversion range. The observed trend could be attributed to a high rate of the direct deep oxidation of propane on 3 via a C3H7* intermediate (Path 4, Scheme 5), as discussed by Carrero et al.,3 with the oxidation of propylene playing a role in depressing the selectivity only at higher propane conversion. This hypothesis would be in agreement with the relatively low selectivity values extrapolated for 3 at zero propane conversion. In spite of the oxidation of propylene being favoured above that of propane, Dai et al. have shown that the oxidation of propane to acetone over V2O5, as an initial direct oxidation step following the formation of the C3H7* intermediate, can proceed with a barrier of 30.5 kcal/mol. This barrier is comparable to that determined for the dehydrogenation of C3H7* to propylene (26.7 kcal/mol), and therefore, the processes could compete.55 However, in the study of Dai et al., two vanadyl oxygens of V2O5, rather than isolated monovanadates, were able to cooperate in the consecutive steps of H-abstraction. Moreover, it should be noted that, whereas the selectivity trend of 3 at 525 °C is very different from that displayed by the IWI-catalysts, the trend at 500 °C is similar for all catalysts with the exclusion of IWI3.61 that presents bulk V2O5. Although the spent catalyst at 525 °C did not present traces of V2O5, future studies, for instance by EXAFS spectroscopy, should contribute to clarify if the specific selectivity trend displayed by 3 at 525 °C does also depend on a structural modification occurring above 500 °C. Structural changes such as VOx migration/aggregation might occur as an effect of temperature50,56 or due to the successive steps of hydration and dehydration during the catalytic process.50,57

Scheme 5. Schematic representation of the reaction framework of propane ODH to afford propylene and the undesired oxidation products. In the light of these considerations it is clear that the difference in activity and selectivity between apparently similar catalysts such as 3 and IWI-0.98 may arise from a different chemical environment around the V centres. System-specific theoretical analysis of all the viable pathways of consecutive oxidation and of the activation barriers for the whole reaction framework will need to be taken into account to explain the directionality of the effect of site isolation on this process. It is clear that along with the good catalytic activity displayed by single-site catalyst 3, a higher degree of selectivity towards propylene would be desirable. As for a comparison,

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non-oxidative dehydrogenation of propane on vanadia catalysts can proceed with a selectivity for propylene over 80 % at a conversion of ca. 45 %.58. Experimental and fundamental studies by Kondratenko et al. have shown that propylene selectivity in the oxidative dehydrogenation of olefins is significantly improved when the reaction is performed using N2O in place of O2 as an oxidant.4f,59 The direct combustion of propane appears to be inhibited and the selectivity decreases less markedly with the conversion of propane.4f,60 On a kinetic and mechanistic standpoint, this has been correlated to the fact that catalyst reoxidation by N2O is less favourable than by O2. This causes a decrease in the steady-state density of oxidising sites on the surface thus favouring the formation of propylene over COx, as the latter species requires multiple active oxygen sites.4f,61 Moreover, the reoxidation of the surface vanadium atoms by O2 has been proposed to produce peroxovanadates that are able to promote the consecutive oxidation of propylene; a pathway that is inhibited when N2O is used as an oxidant.61b,62 In the light of these considerations, future investigation should be focused to increase the selectivity of highly active single site catalysts such as 3 by application of oxidants different from O2. Given the limited availability of large volumes of N2O, CO2 could be also studied as an oxidant able to afford higher selectivity.63 This could also represent an interesting case for the integrated re-utilization of CO2.64

Conclusions Vanadium based catalysts for the oxidative dehydrogenation of propane were prepared for the first time according to a stringent SOMC protocol. The μ2-oxo-bridged, bimetallic [V2O4(acac)2] complex (1) was chosen as a precursor and its grafting on the silica surface was studied by elemental microanalysis, FT IR, solid state NMR, UV Vis, Raman, EPR and XAS spectroscopies. Further structural analysis of the precursor, led to a refinement of its reported geometry hinting at the presence of a single oxo bridge. This structural arrangement might be seen as a crucial factor in the grafting pathway of 1 that was found to proceed mostly by the formation of isolated [≡Si-O-V(=O)(acac)OH] units (2), although the presence of a fraction of dimeric complexes could not be excluded by XAS analysis. Upon oxidative thermal treatment, the exclusive formation of well-defined isolated monovanadate species was observed (3). The catalytic activity of the thus prepared catalyst was tested in the oxidative dehydrogenation of propane and compared to that of various silica supported vanadia catalysts prepared by a conventional impregnation technique. A remarkable enhancement of the catalytic activity in the conversion of propane and in the propylene productivity was observed for the SOMC complex 3 when compared to that of standard impregnation catalysts. This stark difference is attributable to the different chemical environment of the catalysts. In particular, the isolated and well-defined monomeric complexes obtained through the SOMC protocol are by far more active catalysts for propane ODH than the mixture of monomeric and polymeric species obtained via incipient wetness impregnation in terms of propane conversion and propylene productivity. At low propane conversion, catalyst 3 is less selective than the best IWI-catalyst at T > 500 °C (525 °C). However, it displays comparable or even higher selectivity than the impregnation catalysts at higher propane conversion or at 500 °C.

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The relatively low, and propane conversion-independent selectivity of the SOMC catalyst observed at 525 °C calls for future studies on the mobility and catalytic mechanism of the single-site VOx complexes. Overall this work represents an unequivocal contribution to the long standing question on the role of isolated and polymeric vanadia in propane ODH: The presence of V-O-V bonds does not lead to an improvement of the catalytic activity per V atom, with the isolated complexes being intrinsically more active, however, it seems to have a slightly beneficial effect on the selectivity at T > 500 °C. We believe that the current study opens new perspectives for the application of SOMC for the preparation of novel single-site catalysts for propane ODH on other supports (TiO2, CeO2 and ZrO2) able to provide a dramatic enhancement of the TOF values and stronger anchoring of the VOx complexes to the surface when compared to SiO2.56,65

AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected]; [email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The research was supported by the King Abdullah University of Science and Technology (KAUST). OD would like to thank KAUST ORS for research support.

REFERENCES

ASSOCIATED CONTENT General procedures, analytical data, spectroscopic characterization and catalysis details. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Infrared spectra of SiO2-700 (olive), the silica grafted material 2 (red) and the spectrum of complex 1, [V2O4(acac)2], diluted with anhydrous KBr (black). 270x152mm (120 x 120 DPI)

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Figure 2. Comparison of the solid state 1H (left) and 13C CP/MAS (right) spectra of 1 and 2 (See the SI for experimental details). 270x152mm (120 x 120 DPI)

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Figure 3. 51V MAS NMR spectra of 1 (bottom), 2 (middle) and 3 (top) acquired at 900 MHz 1H NMR spectrometer with a 20 KHz MAS frequency, a repetition delay of 0.3 s and 4096 scans. 270x152mm (120 x 120 DPI)

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Figure 4. Raman spectra of the precursor complex [V2O4(acac)2] (1) (black), the grafted material 2 (red) and material 3 (blue). 270x152mm (120 x 120 DPI)

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Figure 5. UV-vis DRS spectra of complex 1 (black), the graft-ed material 2, (red) and material 3 (blue). 270x152mm (120 x 120 DPI)

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ACS Catalysis

Figure 6. Experimental vanadium K-edge XANES spectra of 1, 2 and 3. The inset shows the extracted preedges with a subtracted background and their decomposition with Voigt functions using a fixed Lorentzian width of 0.8 eV. 270x152mm (120 x 120 DPI)

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Figure 7. Fourier transforms of EXAFS k2·χ(k) functions (top) without phase correction and imaginary part of the back Fourier transform (bottom) in [0.7; 4] Å range for V2O4(acac)2 (1), grafted material (2) ([0.7; 2.4] Å range), thermally oxidized material (3) ([0.7; 2.4] Å range). The empty symbols are the experimental data while the full lines are the fit result. 270x152mm (120 x 120 DPI)

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ACS Catalysis

Figure 8. X-band EPR spectra of the isolated oxo-vanadium(IV) surface complexes (2) formed by grafting of 1 on SiO2-700. Experimental (black) and simulated spectra of 2 (red, see the SI for recording parameters and simulation). 270x152mm (120 x 120 DPI)

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Figure 9. Propane conversion as a function of contact time for 3 (Reaction conditions: T = 400-525 ◦C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10 - 50 mL / min). W/F (residence time; g·s·mL-1): reciprocal value of the ratio between propane flow (F, mL·s-1) per mass of catalyst employed (W, g). 338x190mm (96 x 96 DPI)

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ACS Catalysis

Figure 10. Rate of propane consumption plotted as a func-tion of the vanadium surface coverage. (Reaction conditions: T = 525 ◦C; C3H8:O2:N2:He = 15:7.5:15:62.5; W/F= 0.3 g∙s∙mL-1). 270x152mm (120 x 120 DPI)

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Figure 11. Productivity of propylene as a function of contact time for 3 and for the IWI-catalysts (Reaction conditions: T = 525 °C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10 - 50 mL / min). 338x190mm (96 x 96 DPI)

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ACS Catalysis

Figure 12. Comparison of propylene selectivity as a function of propane conversion at different contact time for 3 and the IWI-catalysts (Reaction conditions: T = 525 °C; C3H8:O2:N2:He = 15:7.5:15:62.5; total flow = 10-50 mL / min; contact time = 0.3 – 0.06 g·s·mL-1). 338x190mm (96 x 96 DPI)

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Entry for Table of Contents 257x137mm (120 x 120 DPI)

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