Biomolecule-derived supported cobalt nanoparticles for hydrogenation of industrial olefins, natural oils and more in water

Article abstract of DOI:10.1039/c9gc01276a

Catalytic hydrogenation of olefins using noble metal catalysts or pyrophoric RANEY nickel is of high importance in the chemical industry. From the point of view of green and sustainable chemistry, design and development of Earth-abundant, less toxic, and more environmentally friendly catalysts are highly desirable. Herein, we report the convenient preparation of active cobalt catalysts and their application in hydrogenations of a wide range of terminal and internal carbon-carbon double bonds in water under mild conditions. Catalysts are prepared on multi-gram scale by pyrolysis of cobalt acetate and uracil, guanine, adenine or l-tryptophan. The most active material Co-Ura/C-600 showed good productivity in industrially relevant hydrogenation of diisobutene to isooctane and in natural oil hardening.

Full text of DOI:10.1039/c9gc01276a

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Biomolecule-derived supported cobalt
nanoparticles for hydrogenation of industrial
Cite this: DOI: 10.1039/c9gc01276a
olefins, natural oils and more in water†
Anahit Pews-Davtyan, Florian Korbinian Scharnagl,
Carsten Kreyenschulte, Stephan Bartling, Henrik Lund, Ralf Jackstell and
Matthias Beller
Maximilian Franz Hertrich,
*
Catalytic hydrogenation of olefins using noble metal catalysts or pyrophoric RANEY® nickel is of high
importance in the chemical industry. From the point of view of green and sustainable chemistry, design
and development of Earth-abundant, less toxic, and more environmentally friendly catalysts are highly
desirable. Herein, we report the convenient preparation of active cobalt catalysts and their application in
hydrogenations of a wide range of terminal and internal carbon–carbon double bonds in water under
mild conditions. Catalysts are prepared on multi-gram scale by pyrolysis of cobalt acetate and uracil,
guanine, adenine or L-tryptophan. The most active material Co-Ura/C-600 showed good productivity in
industrially relevant hydrogenation of diisobutene to isooctane and in natural oil hardening.
Received 17th April 2019,
Accepted 13th August 2019
DOI: 10.1039/c9gc01276a
rsc.li/greenchem
defined systems, more stable and practical heterogeneous
cobalt catalysts have attracted considerable attention. For
Introduction
1
8
Catalytic hydrogenation of olefins is one of the central syn- example, the synthesis of olefin-stabilized Co nanoparticles
2
0
thetic methods in chemistry and intensively used in the petro- and solid Zr-MTBC-CoH catalysts has been reported for cata-
chemical, pharmaceutical, agrochemical, fine chemical and lytic hydrogenations of alkenes, imines, carbonyls, nitroarenes,
1
,2
9,16,22
food industries.
Typically, these reductions are accom- and heterocycles.
plished by using molecular hydrogen in the presence of sensi-
tive RANEY® nickel or heterogeneous precious metal catalysts, cobalt-based homogeneous and heterogeneous
During the last decade, our group also developed novel
2
3
24–30
catalysts.
predominantly based on palladium, rhodium, iridium, or In this regard, herein, we report the convenient preparation of
ruthenium. Their limited availability, price, and sometimes novel heterogeneous catalysts via pyrolysis of cobalt acetate
toxicity created enormous interest in alternative technologies with biological N-containing ligands on different supports
3
,4
and catalysts. In this respect, design and development of (carbon and aluminium oxides). N-Ligands were chosen from
Earth-abundant 3d-metal based hydrogenation catalysts are commercially available and relatively cheap amino acid trypto-
highly desirable. Among the different 3d metals, especially phan (Trp) and purine or pyrimidine nucleobases guanine
cobalt showed promising potential to be a good alternative to (Gua), adenine (Ade) and uracil (Ura). To the best of our knowl-
the noble metals regarding activity and generality. Therefore in edge to date these nitrogen-rich ligands (Fig. 1) have not been
5
,6
31
the past decade, the development of efficient homogeneous
used in the preparation of cobalt heterogeneous catalysts.
has gained Similarly, cobalt homogeneous complexes with nucleobases,
attention. Exemplarily, efficient homogeneous complexes for e.g. uracil or modified uracil, have rarely been prepared before
7
8–10
and heterogeneous cobalt based catalysts
1
1
3
2–34
directed or asymmetric olefin hydrogenation were synthesized and have not been used in catalysis.
For the first time, the
1
2–14
by Chirik and co-workers.
groups of von Wangelin,
In addition, in recent years the
1
5–18
9,10
19–21
Yang,
and Lin
reported
notable advancements. Complementary to such molecularly
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße
29a, D-18059 Rostock, Germany. E-mail: matthias.beller@catalysis.de
†
Electronic supplementary information (ESI) available: General experimental
procedures, characterisation data and NMR spectra. See DOI: 10.1039/
c9gc01276a
Fig. 1 N-Containing biological ligands used for catalyst preparation.
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catalytic activity of such materials is demonstrated in hydro- alysts. However, comparably harsh conditions (150 °C, 40 bar
3
0
genations of a wide range of substrates. Advantageously, reac- hydrogen) were necessary for the latter reaction. To identify
3
5,36
tions can be performed in water without using any additives.
more active and improved cobalt catalysts, our newly prepared
materials and commercial ones were screened in the model
reaction (Scheme 1) under mild conditions in water as a green
solvent (Table 1; ESI – Table S1†). More specifically, the experi-
ments were performed in high pressure equipment in parallel
vials, using 1.5 mmol diisobutene and 1 mol% of catalyst in
Results and discussion
We initiated our study on the development of new hydrogen-
ation catalysts with the preparation of more than 20 potentially
active cobalt catalysts.
All the materials were prepared in a straightforward
manner on multi-gram scale starting from commercially avail-
able cobalt(II) acetate, the corresponding N-ligands (Fig. 1),
and Vulcan XC72R or aluminium oxides as inorganic supports
see Experimental: catalyst preparation, ESI†). Then, the
obtained cobalt pre-catalysts with a 3 wt% metal content were
subjected to pyrolysis at 500, 600, 700, 800 or 1000 °C for 2 h
1.5 ml of water at 60 °C and under 30 bar of molecular hydro-
gen pressure for 18 h. As shown in Table 1, commercial cata-
lysts (Table 1, entries 1 and 2), non-pyrolyzed materials
(
Table 1, entries 3 and 10) and cobalt samples prepared
without biological ligands (Table 1, entries 4 and 11) were
completely inactive, indicating the crucial role of ligands and
importance of pyrolysis conditions in the formation of the cat-
alytically active cobalt species. Among the pyrolyzed materials,
adenine (Ade), guanine (Gua) and tryptophan (Trp) based
cobalt catalysts demonstrated low or moderate catalytic activity
(Table 1, entries 14–20). To our delight, uracil (Ura) based and
Vulcan supported catalysts were more active. Furthermore, at
(
under argon and are denoted as Co-ligand/support-Tpyrolysis
(
additionally (a) or (b) was used, if the Al O support was
2 3
acidic or basic, respectively; dry – if non pyrolyzed; air – if pyro-
lyzed under static air, ESI†). The catalytic performance of all
materials was evaluated in the industrially relevant diisobu-
tene hydrogenation as a model reaction (Scheme 1).
Diisobutene is formed by dimerization of isobutene and is
generally available as an equilibrium mixture (∼4 : 1) of its
isomers, namely terminal olefin 1 (2,4,4-trimethylpent-1-ene)
and its internal isomer 2 (2,4,4-trimethylpent-2-ene). In recent
years, there has been increasing interest in the hydrogenation
of diisobutene for the preparation of isooctane 3 (2,2,4-tri-
methylpentane) as an alternative anti-knock gasoline addi-
600 and 700 °C, pyrolyzed materials turned out to be excellent
catalysts for diisobutene (1 + 2) hydrogenation ensuring quan-
titative conversions and excellent yields of isooctane 3 even
with 0.5 mol% catalyst under very mild conditions (Table 1,
entries 6 and 7).
Table 1 Hydrogenation of diisobutene (Scheme 1) in water with
a
different supported and biomolecule ligated cobalt catalysts
b
b
Entry
Catalyst
Conversion 1 + 2 [%]
Yield 3 [%]
3
7
tive. Due to environmental concerns of existing products, e.g.
methyl tert-butyl ether (MTBE), the petrochemical industry is
seeking more environmentally friendly octane booster com-
pounds. In this respect, isooctane is one of the leading candi-
dates because of its high octane number and economically
attractive production using existing infrastructure. In fact, this
process was successfully implemented in a world-scale plant
1
2
3
4
5
6
7
8
9
Co
CoO
3
O
4
0
2
0
5
0
4
W
Traces
Co-Ura/C-dry
Co/C-600
Co-Ura/C-500
Co-Ura/C-600
Co-Ura/C-700
Co-Ura/C-800
Co-Ura/C-1000
Co-Ura/Al
Co/Al (b)-800
Co-Ura/Al
Co-Ura/Al
Co-Trp/C-700
Co-Trp/Al
Co-Trp/Al
Co-Trp/Al
Co-Ade/C-700
Co-Ade/Al (a)-800
Co-Gua/C-700
0
0
65
65
c
c
100 (100 )
>99 (>99 )
>99
94
35
0
0
100
96
37
4
8
2
3
8
(Fortum’s NExOCTANE technology).
10
11
12
13
14
15
16
17
18
19
20
2
O
3
(b)-dry
2
O
3
2
O
O
3
(b)-700
(b)-800
Traces
5
2
3
6
3
2
2
2
2
O
3
O
3
O
3
(a)-700
(b)-700
(b)-800
33
40
64
18
34
10
31
38
62
17
25
0
Scheme 1 Diisobutene (mixture of isomers 1 + 2) hydrogenation to iso-
octane 3.
2 3
O
a
Diisobutene hydrogenation to isooctane
3
has been
Reaction conditions: 1.5 mmol substrate, 1.5 ml water, 1 mol% cata-
b
2
lyst, 30 bar H , 60 °C, 18 h. Yields were determined via GC, using
c
reported in the past mainly with noble-metal catalysts, such as
hexadecane as the internal standard. 0.5 mol% catalyst was used.
3
9,40
41
Pd
and Pt. Besides, studies on kinetics of diisobutene
hydrogenation were performed by Lylykangas et al. using com-
4
2
43
mercial nickel–alumina Ni/Al
2
O
3
and Co/SiO
2
catalysts.
Having the first efficient catalysts in hand, we investigated
Other cobalt catalysts have scarcely been studied in this hydro- the influence of critical reaction parameters for this transform-
genation process, although there is an increasing interest in ation (Scheme 1; Table 2; ESI – Table S2†). Interestingly,
2
3
cobalt-catalysed hydrogenation. As an example, in 2018, we testing various solvents showed that the active catalyst is
reported the hydrogenation of different olefins including diiso- effective in water even at 40 °C and 10 bars of molecular hydro-
butene in the presence of biowaste-derived cobalt chitosan cat- gen (Table 2, entries 8 and 9). Other tested solvents gave
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Table 2 Hydrogenation of diisobutene (1 + 2) in water with Co-Ura/C before being used in the next run. As shown in Fig. 2, in the
a
(
Scheme 1), and reaction optimization
second run Co-Ura/C-600 still showed >90% of the previous
productivity, while that of Co-Ura/C-700 dropped down to 54%.
However, during the third run a significant decrease of activity
was observed for both materials, while in the fourth run the
catalysts were almost inactive. To determine the reasons for
this loss of activity, leaching tests were performed. Thus, the
b
Conversion
1 + 2 [%]
Yield 3
[%]
b
Entry
Catalyst
p (H
2
) [bar]
1
2
3
4
5
6
7
8
9
Co-Ura/C-600
Co-Ura/C-700
Co-Ura/C-800
Co-Ura/C-600
Co-Ura/C-700
Co-Ura/C-600
Co-Ura/C-700
Co-Ura/C-600
Co-Ura/C-700
50
50
50
30
30
20
20
10
10
100
100
83
99 (90 )
98 (90 )
>99
>99
82
c
c
99 (90 ) reaction mixtures were filtered, all volatile components were
c
c
98 (90 )
removed and the residues were dissolved in aqua regia.
The cobalt contents of the obtained aqueous solutions were
determined via Atomic Absorption Spectroscopy (AAS), which
98
98
64
63
97
97
63
62
confirmed that they were below the detection limit of 0.04 mg
a
−1
Reaction conditions: 1.5 mmol substrate, 1.5 ml water, 1 mol% cata-
lyst, 10–50 bar H , 40 °C, 18 h. Yields were determined via GC, using
L . Consequently, the observed productivity loss cannot be an
b
2
effect of cobalt leaching.
c
hexadecane as the internal standard. 0.5 mol% catalyst was used.
Subsequently, for characterization of the active catalysts
and their deactivation mechanism, detailed analyses of both
fresh and recycled catalysts were performed by means of
powder X-ray diffraction (XRD), X-ray photoelectron spec-
troscopy (XPS) and transmission electron microscopy (TEM)
inferior results under identical conditions (ESI – Table S2†).
Some of these solvents (propylene carbonate, acetonitrile or
methanol) are even inactive at higher reaction temperature,
which might be due to their adsorption/coordination behav-
iour on the catalyst surface (ESI – Table S2†). Consequently, all
further experiments were performed in water. From the three
most active catalysts (Table 1, entries 6–8) tested at 40 °C and
(
ESI – S10†).
The samples obtained after pyrolysis at different tempera-
tures were investigated by powder X-ray diffraction in the first
step. As seen from the diffraction pattern, Co O is obtained as
3
4
the main crystalline phase if a 500 °C temperature is applied
in the synthesis procedure. If the pyrolysis temperature is
further increased to 700 °C the Co-oxide is further reduced to
metallic cobalt (cubic, ESI – Fig. S10B†). When the synthesis is
performed at 1000 °C, the oxide seems to be fully reduced to
cobalt. However, due to the low catalyst loading and broad
overlapping diffraction peaks the presence of CoO or Co-carbide
species cannot be ruled out from the diffraction data.
5
0 bar hydrogen, Co-Ura/C-800 still showed very good activity
(Table 2, entry 3).
Furthermore, variations of hydrogen pressure and the cata-
lyst amount (Table 2, entries 4 and 5) revealed Co-Ura/C-600
and Co-Ura/C-700 as the best catalytic systems with similar per-
formances. As is evident from Table 2, both catalysts ensure
excellent yields of the desired hydrogenation product 3 even at
4
0 °C/20 bar hydrogen pressure (Table 2, entries 6 and 7) and
The XPS analysis of catalysts pyrolyzed at 500, 600, and
function equally throughout all experiments. Next, both Co-Ura/
C-600 and Co-Ura/C-700 were used in diisobutene hydrogen-
ation under standard reaction conditions (1.5 mmol substrate,
700 °C shows very similar results in their C 1s, N 1s and Co 2p
spectra as well as in the quantitative analysis of the surface
composition (ESI – S10C† for a quantification table). The C 1s
spectra display strong peaks of carbon–carbon bonds (ESI –
Fig. S10C-2†). In the N 1s region binding energies character-
istic of pyridinic and pyrrolic nitrogen can be found in all
spectra (ESI – Fig. S10C-4†). With increasing pyrolysis tempera-
ture an increasing oxygen concentration can be observed.
When 1000 °C was used no cobalt and nitrogen could be
detected at the surface of the catalyst while for carbon distinct
C–O bonds could be found (ESI – Fig. S10C-2†). The Co 2p
spectra for the fresh catalysts and pyrolysis temperatures up to
1
mol% catalyst, 60 °C, 30 bar hydrogen, 1.5 mL water and 18 h)
for four consecutive runs (Fig. 2, ESI –Table S5†).
After each run the catalyst was separated by filtration and
thoroughly washed with acetone to remove traces of hexade-
cane (internal standard) and the product or substrate. The sep-
arated catalysts were dried at 60 °C under high vacuum for 4 h,
7
7
00 °C are quite similar and show two main features at
79.8 eV and 797 eV corresponding to Co 2p3/2 and Co 2p1/2
,
which are characteristic of Co O (Fig. 4 for Co-Ura/C-600 and
3
4
4
4
ESI – Fig. S10C-3†).
Analytical scanning transmission electron microscopy
STEM) was used to characterize the morphology of Co-Ura/
(
C-600. The general overview confirms different types of struc-
tures, comprising Co oxide, Co metal and core–shell type par-
ticles with the Co metal at the core surrounded by a Co oxide
shell (Fig. 3a, ESI – S10D-1†). Co metal surfaces not sur-
rounded by its oxide are usually covered by several layers of
carbon (Fig. 3b, ESI – S10D†).
Fig. 2 Recycling tests of Co-Ura/C-600 and Co-Ura/C-700 catalysts by
hydrogenation of diisobutene in water.
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Fig. 3 High angle annular dark field (HAADF) STEM image (a) showing
the general morphology of fresh Co-Ura/C-600 and an annular bright
field (ABF) STEM image (b) highlighting a Co metal core partially covered
by carbon and Co oxide of the same sample. HAADF-STEM image of a
one time used Co-Ura/C-600rec1 sample (c) shows formation of a Co
oxide type structure growing around C-support particles. HAADF-STEM
image of catalyst Co-Ura/C-600rec4 recovered four times (d) empha-
sizes further evolution of the new Co oxide phase.
Fig. 4 Co 2p XPS spectra of fresh Co-Ura/C-600, and recycled Co-
Ura/C-600rec1 and Co-Ura/C-600rec4 from top to bottom, respect-
ively. The dashed lines at 779.8 eV and 803 eV mark pronounced
3 4
changes in the spectra from predominantly Co O in the fresh catalyst
to CoO or Co(OH) in the catalyst recycled four times.
2
To gain insights into the deactivation of the Co-Ura/C-600
catalyst the recycled material was studied by XRD, XPS and
TEM after its fourth usage. X-ray diffraction reveals that no
crystalline Co species can be found. This either indicates that
the Co/Co-oxides are transformed into an amorphous state or
their crystallite size drops significantly. After recycling of the
catalyst Co-Ura/600 pronounced changes are observed in the
XPS spectra of Co 2p indicating a change in the oxidation state
of cobalt (see Fig. 4). The Co 2p signal is shifted to slightly
higher binding energies and a new satellite feature around
8
03 eV becomes visible, indicating the formation of CoO or
4
4
Co(OH)
2
phases
with each recycling step. Only minor
changes can be observed in the C 1s (ESI – Fig. S10C-2†) and
N 1s (ESI – Fig. S10C-4†) spectra after each cycle.
STEM measurements of recovered catalysts Co-Ura/
C-600rec1 and rec 4 further support the XRD and XPS results
Fig. 5 Reaction profile for the hydrogenation of diisobutene over Co-
as the morphology of Co phases changes during the reaction. Ura/C-600 at 30 bar H
After recovering once used Co-Ura/C-600rec1 a veil like struc-
2
, 60 °C, 1–18 h.
ture attached to the surface of Vulcan support particles could
be identified (Fig. 3c). Electron energy loss spectroscopy of both substrates reached 96% and 42% conversion, respect-
these structures indicates a Co–oxygen bond (ESI – D-2†) ively. After 7 h, the reaction mixture consisted of 96% of
although a content of hydrogen is also possible. However, it is desired product isooctane 3 along with 4% of remaining
not possible to verify the hydrogen content by this method. internal olefin 2.
The four times recovered catalyst Co-Ura/C-600rec4 shows
further evolution of the veil type structure at the cost of other C-600, it was applied in the hydrogenation of 15 diverse olefins
Co metal or Co oxide particles (Fig. 3d; ESI – S10D-3†). including various functional groups. The results shown in
To explore the scope and limitations of our catalyst Co-Ura/
Next, kinetic investigations on diisobutene hydrogenation Table 3 demonstrate excellent catalytic activity of this material
were performed under our standard conditions. The hydrogen- often leading to full conversion and quantitative yield under
ation of diisobutene (1 + 2) to isooctane 3 was stopped after mild conditions in water (Table 3, entries 1–4, 6, and 11). A
1
h, 2 h, 4 h, 7 h and 18 h reaction times and the reaction number of functional groups such as alcohol, amine, nitrile,
mixture was analysed by GC (Fig. 5). As expected, terminal ether, ester, aldehyde, and sulfonamide groups are well
olefin 1 is more reactive than the internal one 2 and after 4 h tolerated.
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Table 3 (Contd.)
Table 3 Co-Ura/C-600 catalyzed hydrogenations in water: substrate
scope
a
b
b
Entry Products
T (°C) Conversion (%) Yield (%)
b
b
Entry Products
1
T (°C) Conversion (%) Yield (%)
14
60
50
48
60
100
>99
15
60
98
98
2
3
4
60
100
>99
a
2
Reaction conditions: 1.5 mmol substrate, 1 mol% catalyst, 30 bar H ,
.5 ml water, 18 h. Yields were determined via GC or NMR, using hex-
b
1
adecane or mesitylene, respectively, as the internal standard. Bonds in
blue indicate sites of double bond hydrogenation.
60
60
100
99
>99
99
In the case of vinylphthalimide (2-vinylisoindoline-1,3-
dione), hydrogenation led to low conversion under standard
conditions (Table 3, entry 7). However, increasing the tempera-
ture to 100 °C ensured 56% yield of product 10 due to partial
polymerization of the substrate. Even the highly hindered
double bond in triphenylethylene could be hydrogenated to
give product 11, albeit elevated temperature was required for
this hydrogenation (Table 3, entry 8).
The hydrogenation of substrates with multiple double
bonds is a demanding task, which is of interest in the valorisa-
tion of natural terpenes. Thus, we were pleased to find
that our best catalyst was able to ensure high regio- and
chemoselectivity in the hydrogenation of monocyclic terpene
R-(+)-limonene ((R)-1-methyl-4-(prop-1-en-2-yl)cyclohex-1-ene)
5
60
60
96
86
98
6
7
100
(
Table 3, entry 9), which is the main component of essential
60
00
16
100
10
56
1
oils from the rinds of citrus fruits. Both limonene and its
mono-hydrogenation product p-menthene 12 ((R)-4-isopropyl-
1
-methylcyclohex-1-ene) are valuable products and widely used
4
5
in cosmetics and in the fragrance industry as well as in the
synthesis of fine and bulk chemicals, e.g. aviation biofuel.
Notably, selective hydrogenation of R-(+)-limonene to (+)-p-1-
menthene has recently been reported by Leitner using com-
mercially available noble metal catalysts Pt/C and Pt/Al O , as
well as by Feldmann, von Wangelin and Wolf using unsup-
ported cobalt nanoparticles.
4
6
8
9
60
20
0
21
0
21
1
1
4
7
60
40
0
99
0
81
2 3
1
8
16
The Co-Ura/C-600 catalyst performed preferred reduction of
the external double bond against the internal one to yield par-
tially hydrogenated versatile product (+)-p-1-menthene 12 in
1
0
60
52
52
80
43
36
8
80
81% yield (Table 3, entry 9). In addition, a tiny amount (∼6%)
1
00
60
60
60
of fully hydrogenated product p-menthane (mixture of
isomers) was also formed. Similarly, selective hydrogenation of
allyl methacrylate was achieved, which occurred preferably at
the allylic double bond to form product 13 as a single product.
At higher temperature mainly polymerization of the substrate/
product was observed, which demonstrates the importance of
running such hydrogenations at lower temperature (Table 3,
entry 10).
1
1
1
1
2
3
100
100
83
>99
92
72
An example of another relevant hydrogenation is the selec-
tive reduction of myrcene (7-methyl-3-methyleneocta-1,6-
diene) (Table 4). Myrcene is an important natural monoter-
pene and represents a substantial component of the essential
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Table 4 Co-Ura/C-600 catalyzed hydrogenations in water: substrate oils of several plants including wild thyme, lemon grass,
a
scope – natural monoterpenes
48
mango, cannabis, parsley, cardamom, hops and more.
Structurally, this triene consists of two isoprene units like
other monoterpenes. Myrcene and its hydrogenation
products are highly valued as raw materials for the preparation
of pharmaceuticals, and flavour and fragrance chemicals
Products
Substrate
4
9–51
such as menthol, citronellal and nerol.
In this respect,
selective hydrogenation of myrcene is of commercial impor-
tance, since the obtained products are potential precursors for
further functionalized oxygenated derivatives. However, since
myrcene contains three different carbon–carbon double
bonds, the selective hydrogenation is highly challenging.
Previous studies on myrcene hydrogenation made use of pre-
b
Entry
T (°C)
Yields of 18a–d and total (%)
1
2
3
40
60
120
20
0
18
10
0
49
61
0
3
17
98
90
88
98
52
53
cious metal catalysts, such as Pd/SiO2, palladium nano-
0
5
4
particles, or chiral phosphine complexes of rhodium and
a
Reaction conditions: 1.5 mmol substrate, 1 mol% catalyst, 1.5 ml
water, 30 bar H , 18 h. Yields were determined via NMR, using mesi-
2
tylene as the internal standard. Bonds in blue indicate sites of double
bond hydrogenation.
55
ruthenium. Furthermore, also unsupported cobalt(0) nano-
b
particles were applied for the non-selective, complete
1
6
hydrogenation.
a,b
Scheme 2 Co-Ura/C-600 catalyzed hydrogenations in water: substrate scope
–
natural oils and fatty acid derivatives.
a
Reaction conditions: a) 1.5 mmol substrate, 1 mol% catalyst, 1.5 ml water, 30 bar H , 100 °C, 18 h; b) 300 mg substrate, 30 mg catalyst, 1.5 ml water,
0 bar H , 100 °C, 18 h; c) similar to b) at 120 °C; d) similar to b) at 140 °C; and e) similar to b) at 150 °C, 50 bar H . Conversions and yields were
2
2
b
3
2
determined via NMR, using mesitylene as the internal standard.
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In contrast, the Co-Ura/C-600 catalyst showed excellent oil was easily hydrogenated and converted to the white solid
activity and good selectivity towards mono-, di- or full hydro- product 21 almost quantitatively (Scheme 2B). Finally, hydro-
genation of myrcene leading to isolated dienes, internal genation of apricot kernel oil was performed both at 120 °C and
alkenes or branched alkanes depending on the reaction temp- 150 °C with 77% and 87% conversion, respectively, as deter-
1
erature (Table 4). Thus, at 40 °C high regioselectivity toward mined by H NMR (ESI – S9†). Here, the main product glycerine
mono- and dihydrogenated products 18a–c was observed with tristearate 22 was obtained as a waxy, white solid. Moreover, the
a very good combined selectivity of 90%. Product distribution hydrogenation degree of linseed oil was very high at 150 °C and
showed that the selectivity to 18c was 49% which is the highest the conversion to product 22 was almost complete.
value and dienes 18a and 18b were formed roughly in the
same proportion (18–20% each). The alkyl-substituted double
bond remained almost intact. Then, at 60 °C predominantly
the dihydrogenated product 18c was formed in 61% yield
Conclusions
along with diene 18b (10%) and internal alkane 18d (17%). Novel heterogeneous cobalt catalysts were prepared by impreg-
Increasing the reaction temperature to 120 °C led exclusively nation of inorganic supports with cobalt salts in the presence
to the complete hydrogenation product 18d in an excellent of 4 different bioorganic ligands (tryptophan (Trp), guanine
9
8% yield. This full hydrogenation of myrcene to 2,6-dimethyl- (Gua), adenine (Ade), and uracil (Ura)) and subsequent pyrol-
octane 18d can be considered as a renewable route to syn- ysis. Among the resulting materials, specifically Co-Ura/C-600
5
6
thesize fuel additives.
showed excellent catalytic activity for hydrogenation of diverse
Finally, we were attracted by the highly important hydrogen- substrates, such as industrially relevant olefins (diisobutene),
ation of vegetable oils, fatty acid esters and their triglycerides natural oils, fatty acid derivatives and monoterpenes in water,
under our mild conditions. Oils still constitute one of the without any additives. Broad functional group tolerance and
most important renewable raw materials of the chemical often high selectivity towards hydrogenation of terminal
industry. Their hardening over catalytic hydrogenation is a fun- olefins have been shown. The easy preparation of the Co-Ura/
damental industrial bulk scale process for the production of C-600 catalyst in multi-gram scale, no need for special hand-
important chemicals, such as stabilizers and surfactants, ling, long shelf life, and high air- and water-stability, makes it
environmentally friendly industrial fluids and lubricants or an attractive alternative to presently used homogeneous and
57–59
eatable fats, such as margarine.
Since biodiesel (commonly noble metal based catalysts.
fatty acid methyl esters) and oil based bio-paraffins are non-
toxic, biodegradable renewable alternatives to fossil based fuel,
their feedstock diversification has become an important topic Experimental
60
for scientists in industry and academia. In this context, also
General preparation of Co-Ura\C catalysts (3 wt% cobalt-based,
uracil ligated and Vulcan supported)
6
1
62
the potential of apricot kernel, linseed or castor oils was
explored, besides their use as important components in cos-
metic preparations or pharmaceutical formulations.
In a 500 mL two-neck round bottomed flask, equipped with a
reflux condenser and a magnetic stir bar, Co(OAc) ·4H O
The following industrially relevant substrates were explored:
fatty acid methyl esters (Scheme 2A); triglyceride glycerine
trioleate and trilinoleate (Scheme 2C) as well as natural apricot
kernel, linseed and castor oils (Scheme 2B and C; ESI – Fig. S4,
Table S6†). As is evident from Scheme 2, Co-Ura/C-600 turned
out to be a very good catalyst for hydrogenation of fatty acid
derivatives applied in our study. Hence, methyl oleate and its
isomer methyl elaidate were hydrogenated at 100 °C to the
corresponding methyl stearate 19 and methyl ricinoleate to
produce 20 quantitatively (Scheme 2A). Moreover, methyl
oleate and methyl ricinoleate were hydrogenated already at
2
2
(
6
762.2 mg, 3.06 mmol, 1.0 equiv.) and ligand uracil (692.9 mg,
.12 mmol, 2.0 equiv.) were dissolved in ethanol (360 mL). The
flask was immersed in an oil bath and heated at 70 °C. After
0 min, 4.77 g of Vulcan powder was added to the reaction
3
mixture and the resulting heterogeneous mixture was heated
for 4 h at 80 °C. The solvent was removed using a rotary evap-
orator and the residue was dried overnight at 65 °C under
high-vacuum. The dried sample was ground in an agate
mortar to a fine powder (5.9 g), from which a 0.5 g portion was
transferred to a ceramic crucible and pyrolyzed at tempera-
tures between 500 and 1000 °C (the oven was evacuated to ca.
6
0 °C with almost 90% yield, whereas methyl elaidate conver-
5
mbar and then flushed with argon three times, and the
sion was 55% (Scheme 2A; ESI – Table S6†). Additionally,
heating rate was 25 °C per minute and held at pyrolysis temp-
erature for 2 h under an argon atmosphere). The pyrolyzed cat-
alysts were ground again in an agate mortar, and stored in
glass vials in air, without special protection. The catalysts were
labelled as Co-ligand\support-temperature (e.g. Co-Ura\C-600).
hydrogenation of glycerine trioleate led to glycerine tristearate
2
2 in 88% yield at 60 °C (ESI – Table S6†) and to excellent
quantitative yield at 100 °C. In the case of multiply unsaturated
glycerine trilinoleate, the hydrogenation degree increased with
temperature and conversion (Scheme 2C).
To our delight, natural vegetable oils also were hydrogen-
ated smoothly. Their main component triglycerides are shown
General procedures for hydrogenation reactions
in Scheme 2 and their detailed composition and NMR spectra A 4 mL screw-cap vial was charged with a catalyst (30 mg,
are available in the ESI in the S9 section.† Hence, refined castor ∼1 mol%), a substrate (1.5 mmol), 1.5 mL of deionized water
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(
or screened solvent) and a Teflon-coated stirring bar. The vial
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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This work has been supported by the State of Mecklenburg –
Western Pomerania, the BMBF (Grant No. 03XP0060,
KataPlasma). We thank Dr C. Fischer, S. Buchholz, S. Schareina,
A. Lehmann, and K. Schubert for their excellent technical and
analytical support (all from LIKAT). We also thank Reinhard
Eckelt for performing BET measurements and Dr Angela
Koeckritz for fruitful discussions on heterogeneous catalysts.
2
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