Isolated, well-defined organovanadium(III) on silica: Single-site catalyst for hydrogenation of alkenes and alkynes

Article abstract of DOI:10.1039/c7cc01876b

A well-defined, isolated, single-site organovanadium(iii) catalyst on SiO₂ [(SiO₂)V(Mes)(THF)] was synthesized via surface organometallic chemistry, and fully characterized using a combination of analytical and spectroscopic techniques (EA, ICP, ¹H NMR, TGA-MS, EPR, XPS, DR-UV/Vis, UV-Raman, DRIFTS, XAS). The catalyst exhibits unprecedented reactivity in liquid-and gas-phase alkene/alkyne hydrogenation. Kinetic poisoning experiments revealed that 100% of the V sites are active for hydrogenation.

Full text of DOI:10.1039/c7cc01876b

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Camacho-Bunquin, R. R. Langeslay, P. A. A. Ignacio-de Leon, J. Niklas, O. G. Poluektov, C. Liu, J. G.
Connell, D. Yang, A. J. Kropf, H. Kim, P. C. Stair, M. Ferrandon and M. Delferro, Chem. Commun., 2017,
DOI: 10.1039/C7CC01876B.
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Isolated, Well-Defined Organovanadium(III) on Silica: Single-Site
Catalyst for Hydrogenation of Alkenes and Alkynes
H. Sohn,a J. Camacho-Bunquin,a,* R. R. Langeslay,a P. A. Ignacio-de Leon,b J. Niklas, a O. Poluektov,a
C. Liu,a J. G. Connell,c D. Yang,a J. Kropf,a H. Kim,a,d P.C. Stair,a,d M. Ferrandona and M. Delferroa,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Well-defined, isolated, single-site organovanadium(III) catalyst on systems reported, the [(CatPOP)V(Mes)(THF)] system showed
SiO2 [(SiO2)V(Mes)(THF)] was synthesized via surface significantly higher activity for diphenylacetylene reduction,
organometallic chemistry, and fully characterized using
a
with turnover frequency (TOF) values ~40 times faster than its
combination of analytical and spectroscopic techniques (EA, ICP, heavier analogues.11 The higher reactivity of the
1H NMR, TGA-MS, EPR, XPS, DR-UV/Vis, UV-Raman, DRIFTS, XAS). [(CatPOP)V(Mes)(THF)] catalyst is attributed to the ability of
The catalyst exhibits unprecedented reactivity in liquid- and gas- the CatPOP matrix to stabilize low-coordinate (10-electron),
phase alkene/alkyne hydrogenation. Catalyst poisoning monomeric organovanadium(III) fragments, with a reactive
experiments revealed that 100% of the V sites are active for V3+–C moiety that can activate hydrogen. However, it remains
hydrogenation.
unclear from this previous study whether the hydrogenation
activity of these catalysts arose mainly from the low
coordination of V(III) and/or from the redox-active ligand of
the CatPOP support. In order to verify which of the two
variables impart(s) this unprecedented reactivity, we omitted
redox contributions from the solid catalyst support by
replacing CatPOP with redox-inactive SiO2. Our findings
showed that the SiO2, as a support, is superior to CatPOP,
revealing that a redox-active ligand is not a prerequisite for the
reported vanadium’s hydrogenation system. This finding
establishes that site isolation can impart unprecedented
reactivity, and opens up new areas for catalysis with early- and
mid-first-row transition metals.
Earth-abundant and inexpensive late first-row transition
metals such as Fe, Co, Ni and Cu have been extensively
researched as alternative catalysts to noble metals (Ru, Rh, Pd
and Pt) for the hydrogenation of unsaturated organic
functional groups.1-3 While early- and mid-first row transition
metals (e.g. Sc, Ti, V, Cr, Mn) have been largely studied for
their role as promoter ions for hydrogenation-active metals,
much less is known about how the manipulation of their
coordination environment and electronic structures can impart
hydrogenation reactivity.4-8 Some of the rare examples of early
transition metal hydrogenation catalysts include (1) in situ
reduced
titanocenes9
and
(2)
a
high-valent
Here, we support organovanadium(III) sites on a redox-
innocent bulk oxide surface, silica (SiO2), via surface
organometallic chemistry (SOMC).13, 14 This strategy afforded
conceivably the first example of well-defined, low-coordinate
bis(imido)vanadium(V) catalyst, which operates via heterolytic
hydrogenation mechanism.10
Our group recently developed a series of well-defined, site-
isolated first-row organo-transition metal catalysts (V3+, Cr3+,
Mn2+, Co2+ and Ni2+) supported on a catechol-containing
porous organic polymer (CatPOP), which were demonstrated
to be active for alkyne semi-hydrogenation.11, 12 Among the
d) Center for Catalysis and Surface Science and Department of Chemistry, Northwestern
University, Evanston, IL 60208, USA
Fig. 1. Synthesis of supported organovanadium catalyst on silica
[(SiO2) V(Mes)(THF)] (1).
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organovanadium(III) catalysts on SiO2 with remarkable activity of V4+ was detected). To confirm this assignment, the sample
for liquid- and gas-phase hydrogenation of alkenes and alkynes was exposed to air in order to oxidize the V3+ sites to higher
under mild conditions. Catalyst thermal stability and oxidation species, V4+ and V5+. The resulting EPR spectrum (Fig.
recyclability were studied under liquid- and gas-phase S4) shows an intense V4+ multiline signal in the g = 2 region,
hydrogenation conditions. Ultimately, quantitative catalyst centered at 330 mT, which is a characteristic EPR feature for
poisoning experiments confirm that 100% of all V sites are vanadyl(IV) ion with oxygen coordination,18 while the V3+ signal
catalytically active.
Metalation of partially dehydroxylated silica (SiO2(200 °C); 4.9 exposed
OH/nm2) with the molecular organometallic [V(Mes)3(THF)]15
The XAS pre-edge and edge energies of
affords an air-sensitive, paramagnetic, dark purple material consistent with V3+ standards (entries 2 and 3, Table 1),
[(SiO2)V(Mes)(THF)] ( ; Fig. 1).† [V(Mes)3(THF)] was chosen as confirming the +3 oxidation state of V observed in the
at g ≈ 4 completely disappeared. The presence of V5+ in the air-
was confirmed by XPS measurements (Fig. S5).
(Fig. 3) are
1
1
1
the organometallic precursor since the resulting supported
organovanadium species have at least one V–C(Mes) that can Table 1. Summary of V XANES fitting data.
likely activate H2 via heterolytic cleavage to give an active V–H
species.16 NMR-scale vanadation experiments in C6D6 with
1,3,5-tri-tert-butylbenzene as internal standard show the
release of ~2.1 equiv. of Mes-H and negligible amount of THF
(Fig. S1), indicating the presence of vanadium-bound mesityl
and THF groups. This metalation stoichiometry is consistent
Pre-edge energy Edge energy
Sample
(eV)
(eV)
[(CatPOP)V(Mes)(THF)
5468.6
5465.9
5466.4
5470.1
5470.6
5477.4
5471.1
5473.8
5478.0
5481.6
[V(Mes)3(THF)
1
V2O4
V2O5
with
[V(O)(Mes)3(THF)] sites on SiO2(200 °C)
molecule and
previously
reported
organovanadium(V)
17
.
The presence of a THF
a
mesityl ligand is further confirmed by
thermogravimetric analysis (TGA-MS, Fig. S2) and combustion
analysis. Upon heating under a flow of N2, peaks for both THF
and mesitylene were observed in the mass spectrum between
75 and 200 °C, corresponding to approximately 1 equiv. of
each ligand per vanadium. Carbon and hydrogen combustion
analysis also confirmed the presence of 1 equiv. of mesitylene
and THF per vanadium (Table S1). The diffuse-reflectance FT-IR
spectroscopic monitoring of the metalation reaction (Fig. 2)
shows: 1) the isolated Si-OH groups are consumed (3740 cm-1),
2) the internal silanols are unreacted (a broad band at 3520
cm-1), and 3) new bands are observed which are attributed to
the presence of V-bonded mesitylene and THF moieties (C–H =
3100-2870 cm-1 and C=C = 1610, 1590 cm-1).
Fig. 3. XANES and EXAFS spectra of [V(Mes)3(THF] (green),
1
(red) and [(CatPOP)V(Mes)(THF)] (blue).
EPR experiments. The extended X-ray absorption fine structure
(EXAFS) spectrum of is consistent with our structural
1
proposal of four-coordinate V3+ sites stabilized by bipodal
siloxides, a mesitylene group and a neutral THF molecule. The
spectrum is similar to that of the four-coordinate V3+ sites in
[(CatPOP)V(Mes)(THF)] and the molecular precursor
[V(Mes)3(THF)]. No higher-shell scattering that could
correspond to adjacent oligomeric V centers was observed,
ultimately confirming the monomeric nature of the majority of
the V sites. The UV-vis spectral features of
1 are also
consistent with the monomeric nature of the V centers (energy
edge Eg = 5.4 eV).19 The adsorption between 200 to 300 nm is
attributed to the V-O charge transfer and π−π∗ transition of
the mesitylene molecule. (Fig. S6 and S7). The combination of
these analytical and spectroscopic data, coupled with the
results of DFT modelling calculations (Fig. S8), is consistent
with the formation of isolated organovanadium(III) sites on
SiO2 with a general formula [(SiO2)V(Mes)(THF)].
Fig. 2. DRIFTS spectra of SiO2(200 °C), 1 and references (mesitylene
and THF).
The electronic structure of the V sites in
1 was elucidated
using a combination of spectroscopic techniques. Electron
paramagnetic resonance (EPR) spectroscopy and X-ray
photoemission spectroscopy (XPS) were employed to establish
Catalyst
1 was found to be active for hydrogenation of
alkenes and alkynes under the conditions employed in our
previous [(CatPOP)V(Mes)(THF)]-catalyzed study (75 °C, 200
psi-g H2, 20 h), confirming that redox activity of the CatPOP
support is not a prerequisite for the observed hydrogenation
activity of vanadium. At identical V loadings (0.5 mol%),
the oxidation state of vanadium. The EPR spectrum of
1 (Fig
S3) features a broad signal at g≈4, centered around 160 mT,
attributed to high-spin V3+ ions in the triplet state (S = 1; <1%
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1
effects 90% conversion and 78% selectivity for silica surface. The XPS spectrum of the spent catalyst not
stilbenes, while the [(CatPOP)V(Mes)(THF)] catalyst shows exposed to air indicates a significant oxidation of V3+ to a
lower activity (26% conversion, 91% selectivity) (Table S2). mixture of V+4 and V5+ (Fig. S5), indicating the latter
Despite the breadth of organovanadium species supported on deactivation pathway above could be also operative, likely due
SiO2,20 which are mostly active for polymerization,21-23 group- the presence of O2 in the feed or due to the activation of H2 via
28
transfer,24-27 and dehydrogenation catalysis,17,
catalyst
1
an oxidative addition mechanism.30
comprises the first example of hydrogenation-active supported
3+ sites.
To gain more insights into the hydrogenation reactivity of
V
1
, gas-phase hydrogenation of ethylene was performed under
Real-time monitoring of diphenylacetylene hydrogenation extremely air-free conditions using a fixed-bed flow reactor.
with
shows high stilbene selectivity (80%) at substrate Catalyst 1a hydrogenates ethylene at a rate of 0.58 ± 0.03 h-1
1
conversions as high as 80% (Fig. 4), along with 1 equiv. of (intrinsic TOF) at 50 °C. Note that under identical conditions,
mesitylene and THF per vanadium. Initial TOF value of 245 ± the reference V2O5/SiO2 (containing V5+) system and the air-
10 h-1 was achieved at 20% conversion. However, prolonging exposed
1
(containing V4+ and V5+) are both inactive,
the reaction resulted in the eventual exhaustive hydrogenation confirming that low-valent V3+ centers are the active catalytic
to 1,2-diphenylethane, indicating that the catalyst is not species. Furthermore, catalyst shows hydrogenation activity
selective to alkyne semi-hydrogenation. This is further even at room temperature (Fig. S12) making it, by far, the
confirmed by the catalytic activity of for 1-octene most active single-atom catalyst on SiO2 that our group has
1
1
1
hydrogenation. Quantitative hydrogenation to octane was developed; our in-house-developed single-site Ga- and Zn-SiO2
observed within an hour (TOF = 1740 ± 50 h-1 at 20% catalysts require higher reaction temperatures (T > 110 °C).31-33
conversion). Under the given conditions, a low tendency to Catalyst
1
exhibits partial stability at temperatures lower than
isomerize 1-octene to internal alkenes was observed.
125
°C (Fig. 5a). However, gas-phase hydrogenation
experiments at temperatures above 150 °C resulted in
aggressive deactivation due to V sintering. Catalyst is
1
extremely sensitive to low concentrations of O2 and moisture
in the feed; stable conversions were achieved upon using
moisture-free SiC as solid catalyst bed diluent and by installing
a hydrogenation-inactive Fe/SiO2 oxygen scrubber upstream of
the reactor. Quantitative poisoning of the V3+ sites in
1 with
2,2’-bipyridine (bpy) revealed that almost 100% of all the V
sites are active,34, 35 further confirming the single-site nature of
the catalyst 1 (Fig. 5b).
Fig. 4. Results of solution-phase diphenylacetylene and 1-
octene hydrogenation activity of
1
at 75 °C.
Recycling experiments of catalyst
1
for diphenylacetylene
hydrogenation showed a marked decrease in activity after
each cycle (Fig. S9), rendering the catalyst completely inactive
after only three cycles. Catalyst filtration experiments showed
no detectable hydrogenation reactivity from the dodecane
filtrate, confirming absence of catalytic V ions in the solution.
Hence, the observed deactivation could potentially arise from
(1) agglomeration of V sites and/or (2) oxidation of V3+. TEM
analysis of the spent catalyst confirms the formation of large
vanadium agglomerates,29 confirming the former deactivation
pathway (Fig. S10a). Even though contributions from trace
amounts of water in solution cannot be completely ruled out,
the agglomeration of vanadium is mainly attributed to thermal
effects, as similarly observed in the gas phase (vide infra).
Similarly, the UV-Raman spectrum of the post-catalysis sample
showed polymerized V-O-V species present on the surface (Fig.
S11), which is in agreement with vanadium sintering on the
Fig. 5. Results of (a) gas-phase ethylene hydrogenation
experiments with at 100 °C, (b) Plot (linear best-fit) of
1
turnover frequency versus equivalents of added 2,2’-bipyridine
per vanadium center during titration poisoning of ethylene
hydrogenation mediated by
1 (feed: 0.8% ethylene; 1 : 2.3
ethylene-to-hydrogen ratio) at 100 °C.
In summary, the structure-property correlation study
presents design rules for imparting hydrogenation reactivity to
supported vanadium sites: site isolation and low valence states
(V3+) are prerequisites for catalysis. We also proved that the
redox-active ligand is not a necessary criterion for catalysis,
which has not been recognized prior to this study. In
particular, we purposefully synthesized a well-defined, single-
site organovanadium(III) catalyst on SiO2 via surface
organometallic chemistry. Spectroscopic and analytical
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This research was supported by the U.S. Department of
Energy (DOE), Office of Basic Energy Sciences, Division of
Chemical Sciences, Biosciences and Geosciences under
Contract DE-AC02-06CH11357. Use of the Advanced Photon
Source is supported by the U.S. Department of Energy, Office
of Science, and Office of Basic Energy Sciences, under Contract
DE-AC02-06CH11357. MRCAT operations are supported by the
Department of Energy and the MRCAT member institutions.
Use of the Center for Nanoscale Materials at Argonne National
Laboratory was supported by the U. S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Contract DE-AC02-06CH11357. R.R.L., P.I.-D. and J.C.-B. are
partly supported by the LDRD program at Argonne National
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‡ The UV-Vis spectra acquired in both air-exposed of 1 and
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