Silica supported copper nanoparticles prepared via surface organometallic chemistry: active catalysts for the selective hydrogenation of 2,3-dimethylbutadiene

Article abstract of DOI:10.1039/c6nj03350d

2,3-Dimethylbutadiene can be highly selectively hydrogenated to 2,3-dimethyl-1-butene with a new catalyst based on silica supported copper nanoparticles (Cu-NPs) prepared via surface organometallic chemistry. Mesityl-copper was firmly grafted onto silica and the reduction of the resulting surface species under hydrogen at 350 °C led to well-dispersed Cu-NPs. Prior to catalytic tests, the final catalysts as well as the intermediates were characterised by DRIFT, SS NMR, EPR, TEM, XRD and elemental analyses.

Full text of DOI:10.1039/c6nj03350d

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MALLMANN, K. Boukebbous, N. Merle, C. Larabi, A. Garron, W. Darwich, L. El-Alaoui, K. C. Szeto and M.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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LETTER
Silica supported copper nanoparticles prepared via Surface
Organometallic Chemistry, active catalysts for the selective
hydrogenation of 2,3-Dimethylbutadiene
Received 00th January 20xx,
Accepted 00th January 20xx
K. Boukebbous, N. Merle, C. Larabi, A. Garron, W. Darwich, L. El-Adoui, K. Szeto*,
A. De Mallmann*, M. Taoufik
DOI: 10.1039/x0xx00000x
www.rsc.org/
2,3-Dimethylbutadiene can be highly selectively hydrogenated can be easily obtained from the dehydratation of pinacol
into 2,3-Dimethyl-1-butene with a new catalyst based on silica (Scheme 1).6
supported copper nanoparticles (Cu-Nps) prepared via surface
organometallic chemistry. Mesityl-copper was firmly grafted onto
silica and the reduction of the resulting surface species under
hydrogen at 350°C led to well-dispersed Cu-Nps. Prior to catalytic
tests, the final catalysts as well as the intermediates were charac-
terised by DRIFT, SS NMR, EPR, TEM, XRD and elemental analyses.
Scheme 1. Selective hydrogenation reaction of 2,3-dimethylbutadiene
(DMBD) that can be obtained from pinacol to 2,3-dimethyl-1-butene
(DMB-1)
Branched C6
α-olefins are valuable intermediates within fine
and petrochemical industry, in particular in the field of
perfume synthesis and drop in fuel. For instance, 2,3-dimethyl-
1-butene (DMB-1) is used in the preparation of fine chemicals
such as aromatic musks, which are widely used in perfumery,
household soaps and detergents.1,2 For example Tonalide®
(1,1,3,4,4,6-hexamethyl-1,2,3,4 tetra-hydronaphthalene) is
classically prepared by acid-catalysed cycloalkylation via
Friedel–Crafts cyclisation of p-cymene with DMB-1.1
Furthermore, branched hexanes are valuable additives for
gasoline. Especially 2,3-dimethylbutenes (DMBs) after a simple
hydrogenation to 2,3-dimethylbutane (DMBH) known to have
a high research octane number (RON) of 103.5 and low RVP
(Reid Vapour Pressure).3 DMBs are generally prepared by
dimerization of propylene on nickel catalysts. In particular, in
DifasolTM IFPEN process, a nickel salt is used in the presence of
a bulky basic phosphine and an Al-alkylating agent.4 However,
this process is dependent on propylene, where the demand for
such compound has incessantly risen in recent years, resulting
in an important increase in its price.5 Consequently, the
development of new and less costly processes to obtain 2,3-
DMBs and neohexene directly from other readily available
feeds, are highly desirable. An alternative route is by selective
hydrogenation of 2,3-dimethyl-1,3-butadiene (DMBD), which
The major challenge of this approach is to develop catalysts
that are selective for the mono-hydrogenation of this
substrate and that avoid the isomerization of DMB-1 into 2,3-
Dimethyl-2-butene (DMB-2). Only few studies have
exemplified the partial hydrogenation of DMBD in batch
reactor in the presence of homogenous catalyst. However, the
catalysts developed showed low activity toward DMB-1
formation.7-9 Moreover, no example employing hetreogenous
catalyst for this reaction has been reported in the literature.
On the other hand, selective hydrogenation of 1,3-butadiene
over supported monometallic and bimetallic nanoparticles
based on Ni, Au, Ag, Cu and Pd carried on various oxide
supports are largely studied.10-15 Interestingly, supported Cu-
Nps can partially hydrogenate both butadienes and alkynes
derivatives into alkenes.16-19 The metallic Nps are mainly
obtained by either deposition-precipitation or incipient
wetness impregnation and deposition of colloidal solution.
Surface Organometallic Chemistry (SOMC) is an established
methodology that affords well-defined heterogeneous
catalysts through the grafting of organometallic complexes on
surface hydroxyls of classical supports such as alumina, silica,
mesoporous silica, silica-alumina…20,21 When the grafted
organometallic complexes are treated under hydrogen,
supported Nps can be obtained for late transition metals.
However, for oxophilic elements such as group IV, V and VI,
the stable M-O bond prevents agglomeration and hard
conditions are usually required, and as a consequence, low
control on the formed systems can be obtained with this
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methodology. For late transition metals, Gates and co-workers
reported the formation of rhodium Nps by reaction of silica
supported
diallylrhodium
species
[≡
SiO-Rh(ɳ3-C3H5)2]
(obtained by the reaction of silica surface with [Rh(ɳ3-C3H5)3])
with hydrogen at room temperature, affording nanoparticles
from 1 to 3 nm sizes.22
In this paper, copper nanoparticles supported on silica were
prepared via Surface Organometallic Chemistry (SOMC) in two
steps. This approach involves the grafting of mesitylcopper
onto partly dehydroxylated silica. In fact, during the
immobilization of the organometallic precursor, the silanol
groups react with copper complexes resulting in a chemical
bond between silica and copper with concomitant mesitylene
release. The amount of copper loaded on the surface can be
controlled by the surface density of the silanol groups, which
depends on the dehydroxylation temperature. Thus, this
strategy should provide a homogeneous distribution of the
copper precursor, which in principle after the reduction of the
supported copper moieties yields a narrow size distribution of
the Nps. The obtained intermediates as well as the final
catalysts were characterized, and their catalytic performances
were evaluated in the selective hydrogenation of DMBD using
a fixed bed flow reactor.
Fig. 1 Characterizations of the surface complex resulting from the grafting of
Mesitylcopper onto SiO2-700: (A) DRIFT spectra of a) silica dehydroxylated at
700 °C, b) After mesitylcopper grafting; (B) 1H MAS NMR; (C) 13C CPMAS NMR
(peaks with an asterisk are spinning side bands).
The grafting of mesitylcopper (1), mainly Cu4 tetrameric
clusters in toluene,23 onto a non-porous silica dehydroxylated
at 700°C, SiO2-700, was carried out in a glove-box at room
temperature. The reaction is followed by IR (Fig. 1 (A)). After
the grafting reaction, the isolated ν(SiO-H) at 3747 cm-1
completely disappeared and new bands appeared in the 3100-
2850 cm-1 as well as in the 1620-1400 cm-1 ranges. These peaks
Cu
Cu
Cu
Cu
Cu Cu
O
Si
O
O
Cu
Cu
O
O
Si
O
O
O
O
Si
O
O
Si
O
O
O
50%
Cu
are characteristic of aromatic and aliphatic ν(C−H),
ν(C=C) and
+
SiO2-200
SiO2-700
80%
Cu
Cu
2a
2b
+
δ(C−H) vibraꢀons of the ligands of the surface chemisorbed Cu.
1H MAS NMR spectrum of the resulting material shows signals
at 2.2 ppm and 7.0 ppm respectively attributed to aliphatic CH3
and aromatic C-H protons of the mesityl ligand of copper (Fig1
(B)). Moreover, 13C CP MAS NMR data highlighted in Fig.1 (C)
shows signals at 18 ppm and 27 ppm, assigned to o-CH3 and p-
CH3 correspondingly, likewise aromatic signals of o-Csp2, m-
Csp2 and p-Csp2 were observed at 155, 125, 141 ppm,
respectively. Furthermore, no EPR signal is observed,
indicating that no Cu(II) is formed upon the grafting reaction.
Elemental analysis of this material showed the presence of
4.61 %wt and 5.51 %wt of Cu and C respectively (C/Cu = 6.3). On
SiO2-700, monoatomic organometallics are usually grafted as
monosiloxy (monopodal) complexes.24-26 However the case of
such a large cluster seems more complex. Indeed, in this case,
almost all the surface silanols, ca. 220 µmol/g, have reacted
according to IR, while only ca. 180 µmol/g of Cu4 clusters are
grafted. This strongly suggests that a mixture of mainly
monopodal (about 80%) and bipodal (about 20%) tetranuclear
species are formed (left part of Scheme 2). The reaction of the
copper cluster with silanols was also characterized by a release
Cu
Cu Cu
O
Si
O
Cu
Cu
O
Cu Cu
O
1
O
O
Si
O
Si
O
O
Cu Cu
O
O
O
O
Si
Si
O
O
O
O
O
O
O
20%
50%
Scheme 2 Grafting of tetrameric mesitylcopper (1) onto SiO2-700 or SiO2-200
.
Along the same prospect, mesitylcopper (1) was also grafted on
silica dehydroxylated at 200 °C, SiO2-200, as previously described up
above for (SiO2-700). The DRIFT spectrum of SiO2-200 showed in the
ν(O-H) stretching vibration region a band at 3741 cm-1 for isolated
silanols with a large shoulder centered at 3600 cm-1 characteristic
of bridged silanols28 (Fig. S3 a)). After the grafting reaction, the
3741 cm-1 band of isolated silanols almost completely disappeared
and the band of the bridged silanols at 3624 cm-1 increased in
intensity (spectrum b Fig. S3), most probably due to an interaction
of the residual surface silanols with the methyl and aromatic groups
of the grafted complex in 2b. Simultaneously ν(C−H) and δ(C−H)
vibration bands characteristic of the ligands of the chemisorbed
species appear. Moreover, no EPR signal was either observed after
the grafting reaction of Cu(I) mesityl on SiO2-200, meaning that most
likely the copper centers keep their Cu(I) oxidation state. Besides,
1
the H MAS NMR spectrum of the resulting material shows signals
of mesitylene, detected by GC/MS.
A similar reactivity
at 2.2 ppm and at 7.1 ppm attributed respectively to aliphatic CH3
and aromatic C-H protons of the mesityl ligand of copper (spectrum
(A) in Fig. S4). Moreover, 13C CP MAS NMR data show signals of the
two o-CH3 and p-CH3 at 19 ppm and at 28 ppm respectively,
between a copper mesityl Cu4 cluster and molecular
β
-
diketiminato aluminum-monohydroxide, has been observed by
Roesky et al..27
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likewise aromatic signals of o-Csp2, m-Csp2 and p-Csp2 at 154, presented a much more important activity compared to the catalyst
123 and 139 ppm, respectively (spectrum (B) in Fig. S4). Mass prepared by a more classical method (Fig. 3).
balance analysis of the material indicates a Cu and C content of The conversion of DMBD in the presence of Cu@SiO2-200, 3b, slowly
4.09 %wt and 2.61 %wt respectively (C/Cu = 3.4). These results decreased with time from 75% to 50% after 20 h (curve a in Fig. 3).
may be interpreted by the formation of a 50/50 mixture of For Cu@SiO2-700, the conversion drops drastically from 45% to 18%
tetrameric Cu4 cluster grafted respectively on two or three silanols during the first hour of reaction and then decreases smoothly to
with two or one remaining mesityl groups, as shown on the right 13% after 10 h (curve b) in Fig. 3). In the presence of Cu@SiO2-wi
,
part of Scheme 2.
the conversion is solely around 15% at the beginning of the catalytic
The reduction of the grafted complexes on SiO2-700 and SiO2-200
under H2, at 350°C for 18h, gives dark-brown powders Cu@SiO2-700
(3a) and Cu@SiO2-200 (3b). The mechanism for Cu-Nps formation
during the reduction is still not well understood in spite of many
general proposed suggestions 29, 30. The infrared spectrum on Fig.S5
reaction and then falls very near zero after 1 h. Remarkably, the
analysis of the hydrogenated products shows a high selectivity
towards DMB-1, around 89% and about 10 % towards DMB-2 for
both SOMC-prepared catalysts, Cu@SiO2-200 and Cu@SiO2-700 (Fig.
S8 (A) and (B)). Concomitantly, only traces of fully hydrogenated
DMBH are observed. The DMB-1/DMB-2 ratio is much higher than
predicted by thermodynamic at 75°C (9 instead of 0.35).31
−H) bands,
and Fig. S6 show a depletion of the ν(C−H) and δ(C
and
the reappearance of the signal at 3747 cm-1 characteristic of
isolated silanols, due to the hydrogenolysis of Cu-O bonds and the
formation of Cu-Nps.
10 nm
(A)
10 nm
Fig. 3 Catalytic activities for hydrogenation of DMBD over a) Cu@SiO2-200, 3b;
b) Cu@SiO2-700, 3a and c) Cu@SiO2-wi
(B)
In conclusion, Mesitylcopper Cu(I) clusters could be firmly grafted
via SOMC onto non-porous silica supports dehydroxylated either at
700 °C (SiO2-700) or 200 °C (SiO2-200). The resulting materials were
characterized by DRIFT, elemental chemical analysis, ESR and solid-
state MAS NMR (1H, 13C). These surface species were further
reduced under hydrogen at 350°C and led to supported Cu-Nps,
with a size of ca. 2.3 nm onto SiO2-700 and 1.9 nm onto SiO2-200,
according to TEM. These new materials were then successfully used
to selectively hydrogenate DMBD into DMB-1. With a DMBD
conversion of up to 75%, a selectivity into DMB-1 of up to 90 % was
reached while that into 2,3-Dimethyl-2-butene (DMB-2) was less
than 10% and just traces of fully hydrogenated 2,3-dimethylbutane
(DMBH) were observed. Besides, these catalysts presented a good
stability with time on stream. Further investigations will be
conducted in order to explain the better activity of SOMC-prepared
Cu catalysts compared to more classical catalysts.
Fig. 2 TEM micrographs and corresponding copper nanoparticles size distri-
bution histograms for Cu@SiO2-700, 3a (A) and Cu@SiO2-200, 3b (B) catalysts.
TEM analyses performed on catalysts 3a and 3b showed highly
dispersed copper particles (Fig. 2). For 3b, smaller nano-particles
sizes were obtained with a narrower distribution (maximum of the
distribution at 1.9 nm, FWHM = 1.2 nm) than for 3a (2.3 nm, FWHM
= 1.4 nm). Despite the fact that the copper amounts grafted on
both supports are very close, according to TEM, supported Cu-Nps
are smaller on SiO2-200 than on SiO2-700, probably due to a better
anchoring of the Cu4 precursor on the SiO2-200 surface. Moreover,
XRD showed no peak that could be attributed to metallic copper on
the diffractogram (Fig. S7). It can thus be stated that no or very few
large copper particles (> 10 nm) are formed.
The NEXT-GTL European large-scale integrating collaborative
project is gratefully acknowledged for financial support.
Experimental
I General
The catalytic performances of both materials were evaluated for
All experiments (mesitylcopper synthesis, grafting reactions)
were performed under moisture and oxygen free argon using
either standard Schlenk or glove-box techniques, with carefully
dried and degassed solvents. Aerosil® silica was wet with
the hydrogenation of 2,3-dimethylbutadiene (DMBD) in
a
continuous flow reactor at 75°C under atmospheric pressure and
compared to Cu-Nps supported on silica prepared through incipient
wetness impregnation (Cu@SiO2-wi). Surprisingly, 3a and 3b
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water and compacted, calcined and dehydroxylated for 16 h at
two different temperatures, 200°C and 700°C that led to two
different supports, denoted as SiO2-200 and SiO2-700 respectively.
Elemental analyses were performed by the Mikroanalytisches
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Mesitylcopper was synthesized according to previously described
methods.23,32-33 A solution of 219 mg of mesitylcopper (1.2 mmol, 6
equiv) in 15 ml of toluene was added to 800 mg of SiO2-700 (0.250
mmol/g - 0.75 OH/nm2). After stirring the suspension overnight at
room temperature, the solid was filtrated and washed 5 times with
2 ml toluene and then 4 times with 2 ml pentane to remove the
excess. The filtrate solutions were combined and analyzed by GC in
order to quantify the amount of mesitylene released. The yellow
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In order to obtain a similar Cu loading on SiO2-200 (0.830 mmol/g -
2.5 OH/nm2), while the concentration of the surface OH anchoring
sites is much higher than on SiO2-700, a solution of 105 mg of
mesitylcopper (ca. 4.6 wt% Cu /SiO2-200, limited amount of Cu,
unsaturated solid, colorless resulting solution) dissolved in 15 ml
toluene was added to 800 mg of SiO2-200. The preparation was then
conducted as above. The powders were stored in a glovebox.
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This catalyst was prepared as reported in the literature from copper
(II) nitrate trihydrate (Cu(NO3)2. 3H2O) and 1g of Aerosil® silica.34
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temperature was maintained at 75°C. Every 17 min, an
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Table of Content
Mesitylcopper reacts with silica surface to give Cu(I) supported clusters. They were reduced
as nanoparticles, Cu-NPs, under H2 at 350°C. These materials allow the selective
hydrogenation of 2,3 dimethylbutadiene into 2,3 dimethyl-1-butene. Catalysts and precursors
were characterised by DRIFT, SS NMR, EPR, TEM, XRD and elemental analyses.