**3. Solids containing group VI (Mo, W) metal ions used in heterogeneous catalysis**

Oxides of group VI (molybdenum and tungsten) and group VII (rhenium) are often used in industrial processes when they are supported on silica or alumina. The triolefin process, developed by Phillips (**Figure 9**), was the first commercial application using WO<sup>3</sup> supported on silica for olefin metathesis [36]. Initially, this process was developed in order to convert propene into ethylene and but-2-ene. Later, due to the increasing request of propene for the synthesis of numerous chemicals (polypropylene, acrylonitrile, propene oxide, cumene, and acetone), new processes were developed for the production of propene.

Actually, the propene production by metathesis is mainly made by use of the OCT (Olefins Conversion Technology) process, developed by ABB Lumus Technology at Houston. This reaction is the reverse of the triolefin process, with quite the same catalyst [37, 38]. It produces ca. 6% of the world production (6.5 Mtons/year in 2014). The SHOP (Shell Higher Olefins Process) is one of the main industrial processes using olefin metathesis for the production of α-olefins, which are precursors for plasticizers and detergents [37, 39]. The catalyst is based on MoO<sup>3</sup> /Al<sup>2</sup> O3 or WO<sup>3</sup> /SiO<sup>2</sup> , the production ability being ca. 1.2 Mtons/year [37]. Another industrial process using olefin metathesis is the synthesis of neohexene from di-isobutene and ethylene (**Figure 10**). Neohexene is mainly used for perfumes where it is a starting material for the obtention of synthetic musks [40].

The main application of these heterogeneous systems is in petrochemistry and their use in other domains such as organic synthesis, oleochemistry, or polymerization remains very limited, mainly due to the drastic conditions which are required and to their intolerance of functional groups. The most often used catalyst is WO<sup>3</sup> /SiO<sup>2</sup> , due to the following reasons: (i) it is resistant to poisoning by oxygenated and sulfided compounds due to the high reaction temperature (more than 350°C) [41, 42]; (ii) even if the coke formation is important at high temperature, the catalyst can be regenerated easily by calcination in air [42], without decomposition of the active sites, in

**Figure 9.** The Triolefin process developed by Phillips.

phenoxide complexes. They are less active and less selective. This was attributed to a higher electronic density around the metal, due to a stronger *σ*-donor effect of thiophenoxides compared to phenoxides [33]. The bis-siloxy oxo alkylidene tungsten complex **14** has been described recently. However, it displays a low initial activity (Turnover frequency (TOF) = 12 min−1) in metathesis of cis oct-4-ene at 80°C. This low activity has been explained by the low

Buchmeiser et al. have increased the catalytic activity of these oxo complexes by increasing the electrophilicity of the metal by transforming them into cationic species. They have reported recently the synthesis and X-ray structure of the first stable cationic complex of tungsten by

complex is highly active (the TONs can reach 10,000) in metathesis of olefins functionalized

B(ArF)<sup>4</sup>

(**Figure 8**). This

thermal stability of this complex in absence of phosphine ligands [34].

removing chlorine of the W─Cl bond by reaction with Ag(MeCN)<sup>2</sup>

by nitrile, sec-amine, or thioether groups [35].

**Figure 8.** Synthesis of the cationic tungsten(VI) complex.

**Figure 7.** Oxo carbenic complexes synthesized by Schrock et al.

72 Alkenes

**Figure 10.** Synthesis of neohexene by olefin metathesis.

contrast to other systems such as MoO<sup>3</sup> /Al<sup>2</sup> O3 or Re<sup>2</sup> O7 /Al<sup>2</sup> O3 ; and (iii) its preparation is easy, by impregnation of a high surface area silica by an aqueous solution of ammonium metatungstate [(NH<sup>4</sup> ) 6 H2 W12O40 • xH<sup>2</sup> O].

A lot of studies were made on the WO<sup>3</sup> /SiO<sup>2</sup> system, before and after activation by propene, by using various spectroscopic methods such as Raman, UV-visible, EPR, XANES, and EXAFS. The first studies were made by Raman and led to the conclusion that the active site was an isolated surface complex of tungsten but of unknown structure [43]. The first postulated surface species was a pentacoordinated tungsten complex but no experimental justification was given [44].

In 1991, Basrur et al. have proposed that the active species of the WO<sup>3</sup> /SiO<sup>2</sup> catalyst was a bisoxo bis-siloxy tungsten complex [(≡SiO)<sup>2</sup> W(═O)<sup>2</sup> ] and that the activation by propene led to a reduction of tungsten and formation of acetone or to a transformation of the W═O double bond into a metal-carbene bond with liberation of acetaldehyde [45]. A characterization by EXAFS at room temperature of WO<sup>3</sup> /SiO<sup>2</sup> has shown that polytungstic species are present on the surface of silica [46]. By using a combination of Raman and UV-visible spectroscopies *in situ*, Wachs et al. have shown that, at room temperature, the tungsten oxide phase is composed of nanoparticles of WO<sup>3</sup> and of polyoxotungstate clusters (W12O39)6−, **17** (**Figure 11**). After dehydration at 450°C under air, these polyoxotungstate clusters evolve into bis-oxo bissiloxy tungsten species **15** and mono-oxo tetra-siloxy tungsten species **16** while the nanoparticles remain unchanged [47, 48].

At low coverage, the ethylene/butene ratio is equal to ca. 1 as expected while at high coverage

Recently operando methods (UV-Vis, Raman, XANES/EXAFS) were used in order to characterize the catalyst during its pretreatment and in presence of propene, the aim being to establish a structure-activity relationship. Wachs et al. studied by Raman the effect of the

coverages, the Raman spectrum shows new bands at 1016 and 958 cm−1, which were attributed to the symmetric vibrations of di-oxo and mono-oxo species, respectively. The di-oxo species displays also an asymmetric vibration band at 968 cm−1. The absence of the W─O─W band at

agreement with the UV-visible results. When the tungsten amount is higher than 0.6%, the Raman peak at 990 cm−1 increases with the amount of tungsten. At high loadings, three new bands appear at 270, 720, and 805 cm−1, characteristic of tungsten oxide nanoparticles. The main conclusion of this study is that tungsten is well dispersed on the silica surface for WO<sup>3</sup>

The catalyst containing 4 wt.% was also studied by operando Raman spectroscopy during the metathesis reaction of propene (1% in argon) at 300°C. The bands characteristics of the mono-oxo and di-oxo species (which are the sole species on the solid) decrease simultaneously with time and disappear after 100 minutes [51]. This proves that the two species were activated by propene and led to the formation of carbene species with elimination of oxygen

the initial bands of the tungsten oxide species are restored with their intensity and no formation of nanoparticle is detected by Raman. For catalysts with high loadings (8 wt.% WO<sup>3</sup>

the activation under propene leads to a strong decrease of the bands characteristic of the nanoparticles with formation of oxygenates such as formaldehyde or acetaldehyde but no acetone is formed. In addition, a study by ESR and UV-visible spectroscopy has shown that

from the coordination sphere of tungsten (**Figure 13**). After reoxidation by an O<sup>2</sup>

─C6

alkanes. This was interpreted as due to

loading during propene metathesis at 300°C.

aggregates. These results are in

/Ar mixture,

),

crystallites (or nanoparticles), which led to

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75

catalyst as a function of the oxide loading [51]. For low

it decreases strongly, due to the formation of C<sup>4</sup>

**Figure 12.** Conversion and ethylene/butene ratio as a function of the WO<sup>3</sup>

the presence of Brönsted acid sites on the WO<sup>3</sup>

tungsten is mainly in the +VI oxidation state.

/SiO<sup>2</sup>

200–300 cm−1 confirms the absence, at low coverage, of WO<sup>3</sup>

oligomerization and cracking [51].

pretreatment in air on a WO<sup>3</sup>

loadings below 8 wt.%.

Some authors have studied propene metathesis on WO<sup>3</sup> /SiO<sup>2</sup> and have shown that the reaction rate is linearly dependent on the propene partial pressure [49]. It has also been reported that the amount of surface tungsten and the treatment of the catalyst by an inert gas (nitrogen, argon, helium) [45] or by hydrogen [50] have a significant effect on the catalytic activity.

Recently, Wachs et al. have studied the effect of the WO<sup>3</sup> amount on silica on the catalytic activity in propene metathesis at 300°C. The results are depicted in **Figure 12**. The catalytic activity increases with the amount of WO<sup>3</sup> until a value of ca. 8 wt.% and then remains quite constant. These results were explained as follows: At low coverage (WO<sup>3</sup> < 8 wt.%) the catalytic activity is proportional to the amount of isolated mono-oxo and di-oxo species (which are all assumed to be active in olefin metathesis). At high coverage, the reaction rate is not dependent on the tungsten loading, due to the formation of WO<sup>3</sup> crystallites which are inactive [51]. There is also an effect of the WO<sup>3</sup> loading on the amount to ethylene and butenes.

**Figure 11.** Proposed molecular structures for WO<sup>3</sup> /SiO<sup>2</sup> .

contrast to other systems such as MoO<sup>3</sup>

A lot of studies were made on the WO<sup>3</sup>

oxo bis-siloxy tungsten complex [(≡SiO)<sup>2</sup>

EXAFS at room temperature of WO<sup>3</sup>

posed of nanoparticles of WO<sup>3</sup>

ticles remain unchanged [47, 48].

Some authors have studied propene metathesis on WO<sup>3</sup>

Recently, Wachs et al. have studied the effect of the WO<sup>3</sup>

constant. These results were explained as follows: At low coverage (WO<sup>3</sup>

/SiO<sup>2</sup> .

dependent on the tungsten loading, due to the formation of WO<sup>3</sup>

activity increases with the amount of WO<sup>3</sup>

tive [51]. There is also an effect of the WO<sup>3</sup>

**Figure 11.** Proposed molecular structures for WO<sup>3</sup>

O].

W12O40 • xH<sup>2</sup>

[(NH<sup>4</sup> ) 6 H2

74 Alkenes

given [44].

/Al<sup>2</sup> O3 or Re<sup>2</sup> O7 /Al<sup>2</sup> O3

/SiO<sup>2</sup>

/SiO<sup>2</sup>

In 1991, Basrur et al. have proposed that the active species of the WO<sup>3</sup>

impregnation of a high surface area silica by an aqueous solution of ammonium metatungstate

using various spectroscopic methods such as Raman, UV-visible, EPR, XANES, and EXAFS. The first studies were made by Raman and led to the conclusion that the active site was an isolated surface complex of tungsten but of unknown structure [43]. The first postulated surface species was a pentacoordinated tungsten complex but no experimental justification was

W(═O)<sup>2</sup>

a reduction of tungsten and formation of acetone or to a transformation of the W═O double bond into a metal-carbene bond with liberation of acetaldehyde [45]. A characterization by

on the surface of silica [46]. By using a combination of Raman and UV-visible spectroscopies *in situ*, Wachs et al. have shown that, at room temperature, the tungsten oxide phase is com-

After dehydration at 450°C under air, these polyoxotungstate clusters evolve into bis-oxo bissiloxy tungsten species **15** and mono-oxo tetra-siloxy tungsten species **16** while the nanopar-

tion rate is linearly dependent on the propene partial pressure [49]. It has also been reported that the amount of surface tungsten and the treatment of the catalyst by an inert gas (nitrogen, argon, helium) [45] or by hydrogen [50] have a significant effect on the catalytic activity.

activity in propene metathesis at 300°C. The results are depicted in **Figure 12**. The catalytic

lytic activity is proportional to the amount of isolated mono-oxo and di-oxo species (which are all assumed to be active in olefin metathesis). At high coverage, the reaction rate is not

; and (iii) its preparation is easy, by

system, before and after activation by propene, by

/SiO<sup>2</sup>

] and that the activation by propene led to

and have shown that the reac-

amount on silica on the catalytic

< 8 wt.%) the cata-

crystallites which are inac-

has shown that polytungstic species are present

until a value of ca. 8 wt.% and then remains quite

loading on the amount to ethylene and butenes.

and of polyoxotungstate clusters (W12O39)6−, **17** (**Figure 11**).

/SiO<sup>2</sup>

catalyst was a bis-

**Figure 12.** Conversion and ethylene/butene ratio as a function of the WO<sup>3</sup> loading during propene metathesis at 300°C.

At low coverage, the ethylene/butene ratio is equal to ca. 1 as expected while at high coverage it decreases strongly, due to the formation of C<sup>4</sup> ─C6 alkanes. This was interpreted as due to the presence of Brönsted acid sites on the WO<sup>3</sup> crystallites (or nanoparticles), which led to oligomerization and cracking [51].

Recently operando methods (UV-Vis, Raman, XANES/EXAFS) were used in order to characterize the catalyst during its pretreatment and in presence of propene, the aim being to establish a structure-activity relationship. Wachs et al. studied by Raman the effect of the pretreatment in air on a WO<sup>3</sup> /SiO<sup>2</sup> catalyst as a function of the oxide loading [51]. For low coverages, the Raman spectrum shows new bands at 1016 and 958 cm−1, which were attributed to the symmetric vibrations of di-oxo and mono-oxo species, respectively. The di-oxo species displays also an asymmetric vibration band at 968 cm−1. The absence of the W─O─W band at 200–300 cm−1 confirms the absence, at low coverage, of WO<sup>3</sup> aggregates. These results are in agreement with the UV-visible results. When the tungsten amount is higher than 0.6%, the Raman peak at 990 cm−1 increases with the amount of tungsten. At high loadings, three new bands appear at 270, 720, and 805 cm−1, characteristic of tungsten oxide nanoparticles. The main conclusion of this study is that tungsten is well dispersed on the silica surface for WO<sup>3</sup> loadings below 8 wt.%.

The catalyst containing 4 wt.% was also studied by operando Raman spectroscopy during the metathesis reaction of propene (1% in argon) at 300°C. The bands characteristics of the mono-oxo and di-oxo species (which are the sole species on the solid) decrease simultaneously with time and disappear after 100 minutes [51]. This proves that the two species were activated by propene and led to the formation of carbene species with elimination of oxygen from the coordination sphere of tungsten (**Figure 13**). After reoxidation by an O<sup>2</sup> /Ar mixture, the initial bands of the tungsten oxide species are restored with their intensity and no formation of nanoparticle is detected by Raman. For catalysts with high loadings (8 wt.% WO<sup>3</sup> ), the activation under propene leads to a strong decrease of the bands characteristic of the nanoparticles with formation of oxygenates such as formaldehyde or acetaldehyde but no acetone is formed. In addition, a study by ESR and UV-visible spectroscopy has shown that tungsten is mainly in the +VI oxidation state.

UV-Vis, XANES, or EXAFS show all surface species, not only those which are active in olefin metathesis. As a consequence, it is very difficult to understand the activation mechanism of the

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77

The preparation of systems containing a higher amount of active sites could lead to more active (and easily regenerated) systems and could allow a better characterization and mechanistic study of the initiation and deactivation steps and their rationalization in terms of classical organometallic chemistry. Surface organometallic chemistry (SOMC) is a choice method for the preparation of silica supported complexes. Numerous tungsten complexes with a variety of ligands (alkyl, carbene, carbyne, oxo, imido, aryloxy, etc.) were immobilized on oxide supports in order to obtain single site species which can be applied for the valorization of hydrocarbons via various reactions (alkane or alkene metathesis, methane coupling, etc.) [55, 56]. These materials can be characterized by the same spectroscopic methods than the conven-

SOMC can be considered as a bridge between homogeneous and heterogeneous catalysis [55–57]. Its aim is to graft organometallic complexes on oxide surfaces (silica, alumina, titania, zirconia, etc.) or on metal surfaces. In the case of oxides, the complex can be linked to the support by one or more bonds with surface oxygen atoms. When the support has been previously functionalized, the bonding can be made via other atoms such as P, N, Si, etc. As it is the case in homogeneous catalysis, these surface organometallic species can be defined by their ligands around the metal. Two types of ligands can be considered, those which will be involved in the catalytic cycle and those which are only spectators (such as oxo, alkoxo, amido, or imido groups). The modification of both types of ligands can have a drastic effect on the activity and selectivity of a given catalytic reaction, allowing to establish structure-activity relationships. For example, pretreatment of the support at different temperatures will lead to the synthesis of surface complexes with one, two, or three bonds with the surface. This new

• The catalyst can be easily separated from the reaction products and recycled.

• The catalysts are single sites, as in homogeneous catalysis.

tional catalysts (solid-state NMR, EXAFS, DRIFT, ESR, UV-Vis, etc.).

**Figure 14.** Conversion of the mono-oxo tungsten(VI) species into the di-oxo one.

**4. Supported tungsten catalysts prepared by SOMC**

catalyst (and also its deactivation).

approach has many advantages:

**Figure 13.** Possible mechanisms for the activation of the di-oxo tungsten species by propene.

However, Bell et al. [52] have recently shown by an *in situ* XANES study that there is a reduction of W(VI) to W(IV) during the activation of WO<sup>3</sup> /SiO<sup>2</sup> at 500°C. This result is in agreement EXAFS data and with the formation of acetone during the activation (**Figure 13**). The amount of evolved acetone shows that for a catalyst containing 5.4 wt. only 5% of tungsten atoms are active in olefin metathesis. This value is similar to what had been reported for MoO<sup>3</sup> supported on silica [53] and that proposed by Wachs et al. for WO<sup>3</sup> on silica [51]. Unfortunately, no mechanism was proposed to explain the transformation of the mono-oxo tungsten (IV) species into the bisiloxy-oxo-carbene (≡SiO)<sup>2</sup> (O)W═CHR (R = H, CH<sup>3</sup> ). Very recently Stair et al. have observed, during a Temperature Programmed Reduction (TPR) study of WO<sup>3</sup> /SiO<sup>2</sup> in presence of propene, the formation of a mixture of methane, carbon monoxide, and hydrogen [54]. They proposed that the activation at 700°C is made via a pseudo-Witting reaction with evolution of aldehydes, not stable at high temperature and which decompose into small molecules (**Figure 13**).

EXAFS spectra of a pretreated 5.4 wt.% WO<sup>3</sup> /SiO<sup>2</sup> catalyst show the presence of mono-oxo and di-oxo tungsten species with contributions in the Fourier transform at 0.12 nm (W═O) and 0.16 nm (W─O) (the true distances take into account a phase correction and are slightly larger by 0.04 nm than those deduced from the Fourier transform). After treatment at 600°C under inert gas (helium), Bell et al. observed a decrease of the peak at 0.16 nm [52]. This decrease was attributed to the transformation of the mono-oxo species into the di-oxo one (**Figure 14**). This increase of the di-oxo concentration could explain the higher activity of this system compared to that obtained after activation under air.

Stair et al. reported recently that a pretreatment at high temperature of the WO<sup>3</sup> /SiO<sup>2</sup> catalyst by a gas containing propene increased by two to three orders of magnitude its activity at low temperature [54]. Surprisingly, these catalysts can be regenerated by a treatment under nitrogen at high temperature.

Even if some tentative structure-activity relationships were made for the MO<sup>3</sup> /SiO<sup>2</sup> (M = Mo, W) catalysts, the structure of the true active species is not really known up to now. The main problem is due to the low amount of active sites. Spectroscopic methods such as in-operando Raman,

**Figure 14.** Conversion of the mono-oxo tungsten(VI) species into the di-oxo one.

However, Bell et al. [52] have recently shown by an *in situ* XANES study that there is a reduc-

EXAFS data and with the formation of acetone during the activation (**Figure 13**). The amount of evolved acetone shows that for a catalyst containing 5.4 wt. only 5% of tungsten atoms are active in olefin metathesis. This value is similar to what had been reported for MoO<sup>3</sup>

no mechanism was proposed to explain the transformation of the mono-oxo tungsten (IV)

presence of propene, the formation of a mixture of methane, carbon monoxide, and hydrogen [54]. They proposed that the activation at 700°C is made via a pseudo-Witting reaction with evolution of aldehydes, not stable at high temperature and which decompose into small mol-

/SiO<sup>2</sup>

and di-oxo tungsten species with contributions in the Fourier transform at 0.12 nm (W═O) and 0.16 nm (W─O) (the true distances take into account a phase correction and are slightly larger by 0.04 nm than those deduced from the Fourier transform). After treatment at 600°C under inert gas (helium), Bell et al. observed a decrease of the peak at 0.16 nm [52]. This decrease was attributed to the transformation of the mono-oxo species into the di-oxo one (**Figure 14**). This increase of the di-oxo concentration could explain the higher activity of this

by a gas containing propene increased by two to three orders of magnitude its activity at low temperature [54]. Surprisingly, these catalysts can be regenerated by a treatment under nitro-

catalysts, the structure of the true active species is not really known up to now. The main problem is due to the low amount of active sites. Spectroscopic methods such as in-operando Raman,

have observed, during a Temperature Programmed Reduction (TPR) study of WO<sup>3</sup>

/SiO<sup>2</sup>

(O)W═CHR (R = H, CH<sup>3</sup>

at 500°C. This result is in agreement

catalyst show the presence of mono-oxo

on silica [51]. Unfortunately,

). Very recently Stair et al.

/SiO<sup>2</sup>

/SiO<sup>2</sup>

catalyst

(M = Mo, W)

sup-

/SiO<sup>2</sup> in

tion of W(VI) to W(IV) during the activation of WO<sup>3</sup>

species into the bisiloxy-oxo-carbene (≡SiO)<sup>2</sup>

EXAFS spectra of a pretreated 5.4 wt.% WO<sup>3</sup>

ecules (**Figure 13**).

76 Alkenes

gen at high temperature.

ported on silica [53] and that proposed by Wachs et al. for WO<sup>3</sup>

**Figure 13.** Possible mechanisms for the activation of the di-oxo tungsten species by propene.

system compared to that obtained after activation under air.

Stair et al. reported recently that a pretreatment at high temperature of the WO<sup>3</sup>

Even if some tentative structure-activity relationships were made for the MO<sup>3</sup>

UV-Vis, XANES, or EXAFS show all surface species, not only those which are active in olefin metathesis. As a consequence, it is very difficult to understand the activation mechanism of the catalyst (and also its deactivation).

The preparation of systems containing a higher amount of active sites could lead to more active (and easily regenerated) systems and could allow a better characterization and mechanistic study of the initiation and deactivation steps and their rationalization in terms of classical organometallic chemistry. Surface organometallic chemistry (SOMC) is a choice method for the preparation of silica supported complexes. Numerous tungsten complexes with a variety of ligands (alkyl, carbene, carbyne, oxo, imido, aryloxy, etc.) were immobilized on oxide supports in order to obtain single site species which can be applied for the valorization of hydrocarbons via various reactions (alkane or alkene metathesis, methane coupling, etc.) [55, 56]. These materials can be characterized by the same spectroscopic methods than the conventional catalysts (solid-state NMR, EXAFS, DRIFT, ESR, UV-Vis, etc.).
