Tungsten Catalyst Incorporating a Well-Defined Tetracoordinated Aluminum Surface Ligand for Selective Metathesis of Propane, [(≡Si?O?Si≡)(≡Si?O?)2Al?O?W(≡CtBu) (H)2]

Article abstract of DOI:10.1002/cctc.201801779

A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si?O?Si≡) (≡Si?O)₂ Al?OH], 3 was obtained by reacting di-isopropyl aluminium hydride with SBA-15 treated at 700 °C. The resulting surface [(≡Si?O?Si≡) (≡Si?O) ₂ Al (isobutyl) fragment undergoes β-H elimination at 400 °C leading to [(≡Si?O?Si≡)(≡Si?O?)₂Al?O) Al?H]. Further oxidation of this Al-hydride with N₂O leads to 3. This acidic support was used to create a well-defined surface organo-tungsten fragment [(≡Si?O?Si≡)(≡Si?O?)₂Al?O?W(≡CtBu)(CH₂tBu)₂] by reacting 3 with W(≡C-tBu)(CH₂-tBu)₃. A further reaction with hydrogen under mild conditions afforded the tungsten carbyne bis-hydride [(≡Si?O?Si≡)(≡Si?O?)₂Al?O?W(H)₂(≡C-tBu)]. The performance of each of the W-supported catalysts was assessed for propane metathesis in a flow reactor at 150 °C. [(≡Si?O?Si≡)(≡Si?O?)₂ Al?O?W(≡CtBu)(H)₂] was found to be a single-site catalyst, giving the highest turnover number (TON=800) and the highest reported selectivity for butane (45 %) vs. ethane (32 %) known for oxide-supported tungsten complex catalysts (with the supports being silica, silica-alumina, and alumina). The results demonstrate that modification of the oxide ligands on silica via the creation of Al Lewis acid center as an anchoring site for organometallic complexes opens up new catalytic properties, markedly enhancing the catalytic performance of supported organo-tungsten species.

Full text of DOI:10.1002/cctc.201801779

Accepted Article
Title: Tungsten Catalyst incorporating a Well-Defined Tera-coordinated
Aluminum Surface Ligand for Selective Metathesis of Propane,
[(≡Si−O−Si≡)(≡Si–O–)₂Al–O–W(≡CtBu) (H)₂
Authors: Jean-Marie Basset, Baraa Werghi, Anissa Bendjeriou-
Sedjerari, Abdesslem Jedidi, Natalia Morlanes, Edy Abou-
Hamad, Kushal Bhatte, Erjia Guan, Tao Ma, Antonio Aguilar-
Tapia, Samy Ould-Chikh, Luigi Cavallo, and Bruce C. Gates
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201801779
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Tungsten Catalyst incorporating a Well-Defined Tetra-coordinated
Aluminum Surface Ligand for Selective Metathesis of Propane,
[(≡Si−O−Si≡)(≡Si–O–)2Al–O–W(≡CtBu) (H)2]
Baraa Werghi,a Anissa Bendjeriou-Sedjerari,*a Abdesslem Jedidi,b Natalia Morlanes,a Edy Abou-
Hamad,c Kushal Bhatte,a Erjia Guan,d Tao Mae, Antonio Aguilar-Tapia,f Samy Ould-Chikh,a Luigi
Cavallo,*a Bruce C. Gates,*e and Jean-Marie Basset*a
Abstract: A well-defined aluminium-bound hydroxyl group on the surface of mesoporous SBA-15, [(≡Si–O–Si≡) (≡Si–O)2 Al–OH], 3 was
obtained by reacting di-isopropyl aluminium hydride with SBA-15 treated at 700 °C. The resulting surface [(≡Si–O–Si≡) (≡Si–O) 2 Al (isobutyl)
fragment undergoes β-H elimination at 400 ᴼC leading to [(≡Si–O–Si≡)(≡Si-O–)2Al–O) Al-H]. Further oxidation of this Al-hydride with N2O
leads to 3. This acidic support was used to create a well-defined surface organo-tungsten fragment [(≡Si–O–Si≡)(≡Si–O–)2Al–O–
W(≡CtBu)(CH2tBu)2] by reacting 3 with W(≡Ct-Bu)(CH2-tBu)3. A further reaction with hydrogen under mild conditions afforded the tungsten
carbyne bis-hydride [(≡Si–O–Si≡)(≡Si-O–)2Al–O–W(H)2(≡C-tBu)]. The performance of each of the W-supported catalysts was assessed for
propane metathesis in a flow reactor at 150 °C. [(≡Si–O–Si≡)(≡Si–O–)2 Al–O–W(≡CtBu)(H)2] was found to be a single-site catalyst, giving the
highest turnover number (TON = 800) and the highest reported selectivity for butane (45%) vs. ethane (32%) known for oxide-supported
tungsten complex catalysts (with the supports being silica, silica-alumina, and alumina). The results demonstrate that modification of the oxide
ligands on silica via the creation of Al Lewis acid center as an anchoring site for organometallic complexes opens up new catalytic properties,
markedly enhancing the catalytic performance of supported organo-tungsten species.
addressed the design of a mesoporous silica support bearing well-
Introduction
defined tetrahedral aluminum hydroxide functionality or investigated
Heterogeneous catalysis is central to chemical manufacturing,
of the applications of such sites potential supports for molecular
petroleum refining, and natural gas conversion and is expected to be
catalysts.
the primary technology for meeting environmental and energy-
Recently, we reported the reaction of di-isopropylaluminum hydride
related challenges worldwide.[1]
(1 eq., room temperature) with isolated silanol surface groups (1.8
The main benefit of solid catalysts vs. soluble catalysts is their ease
mmol/g [≡Si–OH]) of mesoporous SBA-15 (highly dehydroxylated at
700 °C and 10-5 mbar).[5b] The mechanism involved was elucidated
of separation from fluid-phase products, but this advantage is offset
by the typical limited selectivity of solid catalysts, which is associated
by combining experimental data from a broad range of advanced
with their heterogeneous surfaces. To overcome this limitation,
spectroscopic characterizations including FT-IR (Figure S1, SI) solid-
state NMR spectroscopy [1H, 13C, multiple quanta (MQ) 2D 1H-1H
researchers have developed the idea of “catalysis by design” to
synthesize essentially single site catalysts on surfaces by using well-
defined molecular organometallic precursors. The syntheses are
(Figures S2-S4, SI),], and DFT calculations. The resulting bipodal
single-site supported tetrahedral isobutyl-aluminum complex, [(≡Si–
O–Si≡) (≡Si–O–)2 Al–iBu], 1a, was formed on the surface along with
silicon hydride, [≡Si–H], 1b, and silicon isobutyl, [≡Si–CH2CH(CH3)2],
1c. Surprisingly, the data showed that the hydride homologue of 1a,
[(≡Si–O–Si≡)(≡Si–O–)2Al–H], 2a, (represented simply as [Al–H]) was
guided by the principles and tools of surface organometallic
chemistry (SOMC),[2] often with the goal of providing single-site
catalysts that incorporate well-defined surface organometallic
fragments to participate in the catalytic cycle.
not generated from gaseous H2 but rather by a simple thermal
treatment of the isobutyl moiety via β-H elimination and isobutene
(Scheme S1, details in SI). [5a]
In previous decades, metal oxides such as silica and alumina have
been commonly used as supports for such single-site supported
catalysts,[3] and well-investigated examples include catalysts for
alkane metathesis and related reactions.[2b, 4]
To design better alkane metathesis catalysts, we were motivated by
the idea of creating more electron-deficient metal center, which,
might alter the coordination of the tungsten to the Al–O– ligand and
markedly affect the catalytic properties for alkane metathesis.[5]
Results and Discussion
1. Formation and characterization of Aluminum Hydroxide
Surface on SBA-15700
In the work reported here, we focused on the synthesis of an acidic
surface ligands by creating well-defined aluminum hydroxyl groups
within the mesopores of SBA-15, [(≡Si–O–Si≡)(≡Si–O)2Al–OH]
(represented here simply as [Al–OH] (3)) to serve as anchoring sites
for tungsten complexes.
The [Al−H] (2) sample was exposed to N2O (0.7 bar, 5.5 eq. per Al,
100 ºC, 2 days),[4, 6] and the IR spectra (Figure 1) show that only the
surface species 2a was converted to [Al–OH], 3a (Scheme S2). The
spectra show that [≡Si–H]x and [ ≡ Si–CH2CH(CH3)2] were not
affected by this process.[6b] Elemental analyses of materials 2 and 3
indicate that the aluminum content (3.8 wt.%, 1.4 mmol/g) and
carbon content (3.5 wt%, 3 mmol/g) remained the same after the
reaction of 2 with N2O (Tables S1 and S2, SI). The amount of [Al-
OH], 3a was further quantified by MeLi titration to be 0.9 mmol/g
(details in SI). In other words, 65% of Al sites are identified as [(≡Si–
O–Si≡) (≡Si–O–)2 Al–OH], 3a.
To the best of our knowledge, no previous investigations have
[a] B. Werghi, Dr. A. Bendjeriou-Sedjerari, Dr. N. Morlanes, Dr. S. Ould-Chikh
Prof. L. Cavallo, Prof. J.M. Basset, 4700 King Abdullah University of Science and
Technology, 23955-6900 Thuwal, KSA, E-mail: jeanmarie.basset@kaust.edu.sa
[b] Dr. A. Jedidi, King Abdulaziz University (KAU), Department of Chemistry, Faculty
of Science, Jeddah 21589, Saudi Arabia
The synthesis process was followed by FT-IR where the spectrum of
Figure 1a is assigned to [Al–H], 2. The bands at 2950 [as(CH3)],
2865 [s(CH2)], 1460 [δas(CH3)], and 1365 cm−1 [δs(CH3)] are
attributed to silicon isobutyl groups. The bands that appeared at
2251 and 2163 cm−1 are characteristic of [≡Si–H] and [=SiH2],
respectively.[7] These results lead to the summary of the surface
chemistry presented in Scheme S1.
[c] Dr. E. Abou-Hamad, King Abdullah University of Science and Technology
(KAUST), Core Labs, Thuwal, 23955-6900, Saudi Arabia
[d] Dr. E. Guan, Department of Materials Science and Engineering, University of
California, Davis, CA 95616, USA
[e] Dr. T. Ma, Prof. B. Gates, Department of Chemical Engineering, University of
California, Davis, CA 95616, USA
[f]
Dr, A. Aguilar-Tapia, Institut Néel, UPR 2940 CNRS–Université Grenoble Alpes,
The band observed at 1945 cm-1 was assigned to the terminal [Al–
F-38000 Grenoble, France
H], [(Al−H)].[5] After the reaction with N2O, the IR spectrum of [Al–OH],
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3, shows a sharp decrease in intensity of this [ (Al−H)] band,
accompanied by the appearance of a band at 3783 cm-1 (Figure 1b).
The latter is assigned to the [(OH-μ1-AlIV)] of the [Al–OH] species
3.[8] It is noteworthy that the IR band calculated for [ν(OH-μ1-AlIV)]
(3790 cm-1, SI) is consistent with the experimental value (3783 cm-1)
and with the literature.[8a]
[6b]
As expected and previously reported by Rataboul et al.
the [≡Si–
H]x surface groups are unaffected by this oxidation process.
Scheme 1: Schematic representation showing the proximities of
[(≡Si−O−Si≡)(≡Si−O)2Al−OH], [≡Si–CH2CH(CH3)2], and [≡Si−H] by (DQ) 2D
1H−1H demonstrated by the solid-state NMR spectra.
To complement the experimental results, we investigated the
mechanism involved in the [Al–H],
2 oxidation with N2O by
performing density functional theory (DFT) calculations, using the
same model described in previous work.[5] Various plausible reaction
pathways were proposed (Figures S24 and S25, SI). The lowest
energy was obtained via five center transition states (Figure S26,
SI).
Figure 1. IR spectra of (a) [Al–H], 2, and (b) [Al–OH], 3; (c) difference
spectrum determined by subtraction of spectrum of [Al−OH], 3, from spectrum
of [Al–H], 2.
To better understand the nature of the surface aluminum hydroxyl
group, we performed solid-state NMR spectroscopy experiments.
1
The H SSNMR (Figure S5, ESI) displays a new signal at 4.7 ppm
tentatively attributed to the proton of [(≡Si–O–Si≡)(≡Si–O-)2Al–OH].[9]
This attribution is confirmed by 1H solid-state NMR after
adsorption/desorption of 15N isotopically labeled pyridine onto
materials 3 (vide supra). Additionally, DFT simulation predicts a
resonance of the Al-OH model at 3.5 ppm. The other resonances at
0.9 ppm corresponding to [≡Si–CH2CH(CH3)2], at 1.9 ppm for [≡Si–
CH2CH(CH3)2] remained unchanged as a result of the grafting of di-
isopropylaluminum hydride to the SBA-15700 surface. The
characteristic signal of silicon hydride surface groups was also
unchanged (the resonances characteristic of [≡Si–H] and [= SiH2]
appear at 4.2 ppm).
Figure 2. DQ rotor-synchronized 2D 1H-1H MAS NMR spectrum of [Al−OH], 3.
We further investigated the nature of the acidity of the Al-modified
SBA-15700 surface groups [Al–H], 2, and [Al–OH], 3, by measuring
the adsorption/desorption of the probe molecule pyridine (pKb
=
1
The double quanta (DQ) 2D H–1H spectrum of [Al–OH], 3, (Figure
5.21) characterized by IR spectroscopy. IR spectra of adsorbed
pyridine are commonly used to determine the qualitative nature of
acidic sites on solids, such as zeolites, silica, and alumina.[10] The
adsorption of pyridine at Brønsted acid sites results in the formation
of pyridinium ions, whereas adsorption on Lewis acid sites forms
coordinately bonded pyridine, and their IR spectra readily
distinguishes these two species.19
2) exhibits a strong autocorrelation on the 2:1 diagonal centered at
approximately 0.9 ppm in F2 and 1.8 ppm in F1. It corresponds to
the proton signal of [≡Si–CH2–CH(CH3)2].[5a] The autocorrelation
observed at 4.2 ppm is assigned to the [=SiH2] moiety (5’ in the
spectrum).
The close proximity (< 5 Å) of the aluminum hydroxyl, silicon
hydride, and silicon isobutyl surface groups is retained in sample 3,
which is consistent with our reported results (Scheme 1).[5b] Thus,
outside the diagonal, the two signals centered at approximately 5.6
and 6.6 ppm are attributed to the correlation between the proton of
aluminum hydroxyl and the following: (a) one proton of the methyl
group of silicon isobutyl groups, [δH(Al–OH) + δH(Si–CH2–CH(CH3)2)
= 4.7+ 0.9] and (b) one proton of silicon isobutyl groups, [δH(Al–OH)
+ δH(Si–CH2–CH(CH3)2) = 4.7+ 1.9], respectively. As expected from
the mechanism of formation of Si−H and Si−isobutyl, a through-
space correlation is observed at 9 ppm between the proton of
[(≡Si−O−Si≡)(≡Si−O-)2Al−OH] and one proton of silicon hydride
moieties, [δH(Al-OH) + δH(Si-H) = 4.7 + 4.2].
The IR spectra collected after the exposure of each of the samples
Al-modified SBA-15 [Al–H], 2, and [Al–OH], 3, to pyridine and the
subsequent evacuation at 100, 200, 300, and 400 °C are reported in
11]
Figures S6 and S7 of the SI, respectively.[10b,
The pyridine
adsorption/desorption data show that whatever temperature of
desorption, the Lewis acid sites are mainly present and the most
intense signal centered at 1622 cm-1 is assigned with a tetra-
coordinated aluminum, AlIV. [12] This observation fits with our surface
characterization data and with the predominance of AlIV sites.[13]
The nature of these [(≡Si−O−Si≡)(≡Si−O)2 Al−OH] sites has been
further investigated by the 15N solid-state NMR spectrum of the solid
contacted with 15N labeled pyridine and evacuated at 400 ⁰C (10-5
mbar) displays one signal at 243 ppm (Figure S9 and S10, SI),
corresponding to a typical Lewis acid-Lewis base interaction.[14]
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This conclusion is consistent with the nucleophilic character of the
[Al–OH] groups and suggests that the Lewis acidity arises from the
breaking of the coordination bond between the siloxane bridge
[(≡Si−O−Si≡)] and that the Al−OH group from [(≡Si−O−Si≡)(≡Si−O)2
Al−OH] makes the aluminum orbital accessible to the lone pair of the
nitrogen atom in pyridine.[15] This hypothesis was supported by DFT
calculations where two different possibilities were recorded
coordination of pyridine and formation of the ion pyridinium (SI). The
results clearly show a more stable structure Al coordination with E =
(-23.8) kcal/mol (Figure 3) which also indicates that the pyridine
coordination leads to an elongation of the Al−O bond from 2.01 to
2.85 Å, with the Al−N bond length being 2.02 Å (SI). These results
clearly demonstrate that the Al coordination number remains the
same after pyridine adsorption and, further, that surface Si−O−Si is a
ligand in the sense of the Green formalism.[16]
values, the good overall agreement between the data and the fit, and
the goodness of the fits of all the individual absorber–back scatterer
shells. The data characterizing the sample and the details of the
candidate models and fits are presented in the SI.
Scheme 2. Reaction of 4 with H2 (0.5 bar, 72 equiv. / W) to yield 5.
The supported tungsten species 4 was treated with H2 under mild
conditions (0.6 mbar, 3 h, room temperature) to form a tungsten
hydride species.
Indeed, EXAFS data were collected to characterize the sample 5 for
the incorporation of hydride ligands. The data give good evidence of
one W–C contribution (Table S11, ESI), with a coordination number
of approximately 1, at a distance of 1.85 Å, which is consistent within
error with the presence of one (≡CtBu) group bonded to tungsten
and the removal of two (CH2tBu) ligands from sample 4. Two W–O
contributions were observed, each with a coordination number of
approximately 1, at distances of 1.99 and 2.53 Å. These findings
indicate one support oxygen atom strongly bonded to tungsten and
one support oxygen atom not strongly bonded to tungsten (at 2.53
Å). One W–Al shell, with a coordination number of approximately 1
at a distance of 3.37 Å, was also observed. Recall, too, that W–H
contributions are not detectable by EXAFS spectroscopy.
(a) E= -23.8 kcal/mol
(b) E= -10.5 kcal/mol
Figure 3. DFT-optimized geometry of pyridine adsorption on the
[(≡Si−O−Si≡)(≡Si−O-)2Al−OH, 3a. (a) Pyridine bound to Al (b) Pyridine bound
to H of Al-OH.
Thus, the results are consistent with the inferences that the tungsten
atoms in the sample were chemically bonded to single Al–O sites on
the support and that the H2 treatment removed two (CH2tBu) ligands
and left one (≡CtBu) ligand on the tungsten.
2. Demonstration of Concept: Design of W-Containing Catalysts
Supported Al-OH (3) group and Formation its Hydride
Homologous
The IR spectra of the sample after H2 treatment (Figure S14, SI)
indicate a decrease in intensity of the bands in the 3000−2700 and
1470−1330 cm-1 regions characterizing the CH) and δ(CH)
We sought to take advantage of the new structure and the unique
chemical properties of [Al–OH], 3, as an anchoring site for novel
SOMC applications. The W(≡CtBu)(CH2tBu)3 reacts with (3),
resulting in a covalently bonded tungsten complex. The analysis of
the gas-phase products that formed during the synthesis (1.16
molecules of neopentane per W atom) is consistent with the
formation of the supported W species shown in Scheme 2.
vibrations, respectively, and the formation of a band at 1960 cm-1,
[19]
the characteristic frequency of (W-H) vibrations.
Besides, it
includes strong C–H vibrations indicating the continued presence of
hydrocarbon ligands on the tungsten, in agreement with the EXAFS
data and SS 13C NMR spectra. Indeed, the 13C NMR data show the
persistence of the signal at 32.3 ppm[17] (Figure S16, SI), and they
are supported by the elemental analysis which shows a C/W ratio of
5.9 (theoretical value of 5) corresponding to the remaining (≡CtBu)
ligand (Table S4, SI). Furthermore, we modeled by DFT the
[(≡Si−O−Si≡)(≡Si−O-)2Al−O-W(≡CtBu)(H)2] (Figure S29, SI). The
optimization of our model leads to a stable structure, which has been
checked by vibrational frequency calculations.
The newly formed surface species is formulated as
[(≡Si−O−Si≡)(≡Si−O-)2Al−O-W(≡CtBu)(CH2tBu)2],
4,
and
the
spectroscopic evidence for its identification is as follows: The IR
spectrum of Figure S11 (SI) shows that two groups of new bands
appeared, at 3000-2700 and 1470-1330 cm-1, attributed respectively
to the [CH)] and [δ(CH)] vibrations of the perhydrocarbyl ligands
on the tungsten. Accompanying the formation of these new bands,
the [(OH-μ-AlIV)] band at 3783 cm-1 completely disappeared,
confirming the successful immobilization of W(≡CtBu)(CH2tBu)3 via
reaction with Al−OH sites. The 13C SS NMR signals characteristic of
the neopentyl ligands were observed at 34 and 102 ppm and
assigned to CH2 and CH3, respectively. Nonetheless, we observed a
weak signal at 32 ppm that we assigned to the CH3 group of the
neopentylidyne fragment (Figure S13, SI).[17]
The W low loading in 4 (1.43 wt.%, Table S3, SI) does not allow
clear observation of the quaternary carbon of W-carbyne. This result
was expected, since earlier work demonstrated that to measure the
resonance of the carbyne in a species immobilized on oxide support
it is necessary to have a high W loading (8 wt.%) and/or isotopically
labeled W-containing species.[17-18]
3. Catalytic test: Propane metathesis and Kinetic determination
of Primary Products
Propane metathesis experiments were performed in a continuous
flow reactor (5 mL/min of propane, weight hourly space velocity,
WHSV = 5.4 h-1, volume hourly space velocity, VHSV = 2700 h-1) at
150 °C until the catalyst was deactivated (50 h). The products of the
reaction were identified online and quantified by gas
chromatography (GC). The catalytic properties of catalyst 4 were
compared with W-containing catalysts (Figure 4), namely,
[≡Si−O−W(CH3)5] and its homologous hydride [≡Si−O−W(H)x]
(Scheme S4, SI).[19-20]
The cumulated TON (turnover number) of catalyst 4 and 5 was
found to be 285/W and 800/W, respectively after 200 h (Table S5,
ESI). These values are higher than those previously obtained with
To solve this characterization issue, we used extended X-ray
absorption fine structure (EXAFS) spectroscopy. This technique
provides additional structural information characterizing the surface
species 4 and its hydride homologous 5 (scheme 2), recording data
at the W LIII edge from a sample of 4 in an argon-filled fluorescence
cell and a sample of 5 in a transmission cell, each at room
temperature. The parameters of the best-fitting model are
summarized in Table S11,SI.
W-alkyl-containing catalysts grafted onto silica, alumina, or silica-
[21]
alumina
and any single-site monometallic catalyst for propane
metathesis.[19-20] In particular, the data demonstrate a higher activity
than that of W neopentylidyne neopentyl grafted on alumina (TON:
121/W) and silica-alumina (TON: 123/W).[21]
In complementary experiments, the reaction with catalyst 5 was
investigated with a flow reactor to identify the primary products by
These EXAFS data show good agreement with the structure of 4,
[(≡Si−O−Si≡)(≡Si−O-)2Al−O-W(≡CtBu)(CH2tBu)2]. The good fit of the
data to this model is verified regarding physically realistic parameter
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examining the effect of the contact time (proportional to the inverse
of the space velocity) on propane conversion and product selectivity.
0.8), whereas the attachment of [W(≡CtBu)(H)2] to the classical
siloxy results in Mayer bond orders of Si–O = 1 and W–O = 0.95. In
the case of ≡Si–O–W(≡CtBu)(H)2, there is no contribution of the O
lone pair(s) to the W in the bond ≡Si–O–W. This result is consistent
with the energy value (-22 kcal/mol, SI) obtained for a release of a
hydride in ionic form (H-). In parallel, we investigated the possible
activation of an alkane by the -O-W(≡CtBu)(H)2 moiety in classic W-
carbyne grafted to a Si-OH or to Al-OH bond (Figure S30, SI).
The data (Figure S19), indicates that alkenes and H2 are the primary
products, consistent with earlier reports.[22] Moreover, the catalyst
performance data demonstrate new catalytic properties attributed to
the Al centers anchoring the catalyst. Propane metathesis typically
involves conversion to methane (6%), ethane (56%), butane (34%),
and pentane (4%) (Figure S17, SI).
[19-21]
A markedly different
3 was used as an
product distribution was obtained when
electrophilic surface ligand for the W-containing catalysts 4 and 5.
Thus, the data demonstrate that the new catalyst favors the
formation of higher instead of lower alkanes (methane 3.8%, ethane
32% vs. butane 45%, isobutane 1%, pentane 8%, isopentane 1%
and approximately 3% (C6 + C7 + C8)) (Figure S18, SI) with higher
rates of secondary olefin insertion (Figures S20 and S21, details SI).
Conclusions
The data reported here illustrate how the insertion of Lewis acid
center (Al ) between the silica surface and the active catalytic site
markedly alters the performance of a tungsten catalyst, as illustrated
for propane metathesis. The single well-defined W carbyne/hydride
species plays the role of a multifunctional catalyst for the elementary
steps of dehydrogenation/alkene metathesis/hydrogenation. The Al–
OH Lewis acid center acts both as an anchoring site for the W and
also modifies the steric and to some degree the electronic properties
of the W site, giving a catalyst with a higher TON than reported
propane metathesis catalysts and a different product distribution.
The relatively high TON achieved with catalysts 4 and 5 clearly
highlights the beneficial influence of the Al center on the catalytic
properties of the tungsten—increasing the catalyst lifetime from 12 to
200 h (Figure 4). We posit that this effect could be evidence of the
electronic combining steric properties of the catalyst.
Experimental section
General Procedure
All experiments were carried out under strict controlled atmospheres
(vacuum lines and or schlenck tubes). Treatments of the surface
species were carried out using high vacuum lines (1.34 Pa) and
glove box techniques. Pentane was distilled on Na-benzophenone
and degassed through freeze pump thaw cycles. Hydrogen was
dried over freshly regenerated molecular sieves (3Å) and deoxo
traps.
Infrared spectra were recorded on
a
Nicolet 6700 FT-IR
spectrometer equipped with a cell under controlled atmosphere.
Elemental analyses were performed at the KAUST Analytical Core
lab. The varian 720-ES ICP-Optical Emission Spectrometer (Al and
W) after the samples preparation by microwave digestions on
Milestone ETHOS 1. Analysis of C and H were done on Flash 2000
Elemental Analyzer from Thermo Scientific.
Nitrogen adsorption–desorption isotherms at 77 K were measured
using a Micromeritics ASAP 2024 physisorption analyzer. Specific
surface areas were calculated following typical BET procedures.
Pore size distribution was obtained using BJH pore analysis applied
to the desorption branch of the nitrogen adsorption/desorption
isotherm.
Gas phase analysis was performed with an Agilent 6850 gas
chromatograph with split/splitless injector and FID. 10ꢀµl was
injected by the hot needle technique (thermospray) at an injector
temperature of 250°C using the split mode (split ratio 50:1; 100
ml/min split flow). A HP-AL/KCl 50 mꢀ×ꢀ0.32 mm; 8.00 μm capillary
column coated with a stationary phase divinylbenzene/ethylene
glycol dimethacrylate was used with nitrogen as carrier gas at 18.3
psi pressure. Each analysis was carried out with the same
conditions: a flow rate of 2 ml/min, an isotherm at 80°C for 12 min
then at 170°C for 3 min , and a detector sets with a data rate of 5 Hz
and a minimum peak width of 0.04 min.
Figure 4. Turnover number and product selectivity (after 48 h) obtained during
propane metathesis in a dynamic flow reactor with catalysts 4, 5, and 6
[≡Si−O−W(CH3)5] the homologous hydride [≡Si−O−W(H)x], 7, synthesized as
described in refs. 31 and 30, respectively.
Dehydroxylation of SBA-15 at 700 ºC
The SBA-15 was dehydroxylated at 700 ºC under dynamic vacuum
(10-5 mbar) in quartz reaction. The process takes around 30 h (3 h at
150 ºC and 16 h at 700 ºC with a rate of 60 ºC/h). The resulting
SBA15700 (1.8 mmol/g of ≡Si−OH, obtained by titration with MeLi
and GC measurments of evolved methane)[24] was stocked in the
glove box for further treatment.
By electronic, we mean that the Al Lewis acid center affects the
electron density of the tungsten. By steric, we suggest that some
elementary steps may be more favored when Al is present between
the silica surface and the active site. This agrees with previous
work,[23] inserting Zr atom between silica and Ta hydride increases
the TON from 58 to 100 and also increases the catalyst lifetime.
Generation of the Al-OH sites
m = 100 mg of (2) was exposed to 0.7 bar of N2O (grade 6) at RT
for 1 h then heated to 100°C for 48 h.
To elaborate on these points and help resolve the posited electronic
and steric effects, we performed DFT calculations by determining the
Mayer bond order. The results show that oxygen doublet transfer
occurs to the tungsten center (with a Mayer bond order of W–O =
1.2) rather than to the aluminum (with a Mayer bond order of Al–O =
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10.1002/cctc.201801779
ChemCatChem
FULL PAPER
The gas phase was evacuated under vacuum (10-5 mbar) at RT for 1
h. The resulting material (3) was kept in a glovebox for further
characterizations and reactions.
Huang, J.-M. Basset, Angew. Chem., Int. Ed. 2016, 55, 11162-
11166.
[18]
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Cazat, J. M. Basset, B. M. Maunders, G. J. Sunley, Adv. Synth.
Catal. 2007, 349, 231-237; bE. Le Roux, M. Taoufik, M. Chabanas,
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Acknowledgements
The research was supported by the King Abdullah University of
Science and Technology and by the U.S. Department of Energy,
Office of Science, Basic Energy Sciences, Grant DE-FG02-
04ER15513. We acknowledge the European Synchrotron Radiation
Facility for providing synchrotron radiation facilities.
L.C. and A. J. are grateful to the KAUST Supercomputing Laboratory
for the resources. A. J. thank High-Performance Computing Center
(AZIZ supercomputer) for the support.
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Keywords. Surface organometallic chemistry (SOMC), Alumium
tetracoordinated, Lewis acidity, Propane metathesis, SBA-15.
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10.1002/cctc.201801779
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FULL PAPER
The formation of tetra-coordinated Aluminum hydroxide [(≡Si–O–Si≡)(≡Si–O)2Al–OH] surface ligand, through [Al-H] oxidation using N2O.
These new sites were used as anchoring centers for the immobilization of a tungsten based complex W(≡CtBu)(CH2tBu)3 leading to the
generation of a monopodal well-defined surface organo-tungsten fragment [(≡Si-O-Si≡)(≡Si-O-)2Al–O–W(≡CtBu)(CH2tBu)2]. This new catalyst
activity was evaluated in the propane metathesis reaction, where a TON of 800 was obtained, the highest value obtained for a SOMC
monometallic catalyst.
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