Catalytic activity of nanoscale borides: Co2B and Ni7B3 in the liquid-phase hydrogenation of citral

Article abstract of DOI:10.1016/j.jcat.2017.06.009

Metal borides are unconventional heterogeneous catalysts. Now, two compounds – the new phase Ni₇B₃ and well-known Co₂B – were synthesized as well-defined, nanoscale material. After characterization (X-ray diffraction, scanning electron microscopy, nitrogen physisorption) they were tested for the liquid phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) in n-hexane at different temperatures. Hydrogenation products such as geraniol, nerol, citronellal and citronellol were analyzed. The Ni-free catalyst Co₂B results in the formation of nerol and geraniol or citronellol selectively, depending on the reaction time. The new compound Ni₇B₃ yields citronellal or citronellol, depending on the temperature. Thus, the hydrogenation potential of borides – obtained as well-characterized, unsupported heterogeneous catalysts by a one-pot synthesis procedure and post-synthetic annealing – is demonstrated. The cobalt boride preferentially hydrogenates C[dbnd]O bonds, while the nickel boride is selective for C[dbnd]C double bonds.

Full text of DOI:10.1016/j.jcat.2017.06.009

Journal of Catalysis 352 (2017) 436–441
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Catalytic activity of nanoscale borides: Co2B and Ni7B3 in the
liquid-phase hydrogenation of citral
a,
N. Kalyon a, K. Hofmann a, J. Malter b, M. Lucas b, P. Claus b, , B. Albert
⇑
⇑
a Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
b Ernst-Berl-Institute of Technical and Macromolecular Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 8, 64287 Darmstadt, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal borides are unconventional heterogeneous catalysts. Now, two compounds – the new phase Ni7B3
and well-known Co2B – were synthesized as well-defined, nanoscale material. After characterization
(X-ray diffraction, scanning electron microscopy, nitrogen physisorption) they were tested for the liquid
phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) in n-hexane at different temperatures.
Hydrogenation products such as geraniol, nerol, citronellal and citronellol were analyzed. The Ni-free
catalyst Co2B results in the formation of nerol and geraniol or citronellol selectively, depending on the
reaction time. The new compound Ni7B3 yields citronellal or citronellol, depending on the temperature.
Thus, the hydrogenation potential of borides – obtained as well-characterized, unsupported heteroge-
neous catalysts by a one-pot synthesis procedure and post-synthetic annealing – is demonstrated. The
cobalt boride preferentially hydrogenates C@O bonds, while the nickel boride is selective for C@C double
bonds.
Received 23 March 2017
Revised 5 June 2017
Accepted 6 June 2017
Keywords:
Boride-based catalysts
Nanostructured catalysts
Hydrogenation
Citral
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
between citral and hydrogen in principle is very well investigated
[10]. A variety of supported catalysts and ionic liquid modified cata-
The reduction of metal salts with alkali metal tetrahydridobo-
rates in solvents was reported as early as 1953 to yield products
which contain boron and are catalytically active, but the exact
composition and active species of the catalysts has remained in
the dark [1,2]. Earlier, we investigated in detail the formation,
identity and crystallization of such precipitates, which were
obtained as amorphous, black solids when nickel, iron, and cobalt
salts were reacted with LiBH4 or NaBH4 [3–7]. They were found
to consist mainly of well-known metal borides such as Ni3B,
Ni2B, FeB, or Co2B as nanoscale powders. Surprisingly, an unknown,
metastable phase, Ni7B3, was discovered recently [8]. Since there
are very few studies on the catalytic behavior on well-defined,
single-phase and nanoscale borides, two of these phases were
now chosen to test their potential as heterogeneous catalysts in
the liquid phase hydrogenation of citral (3,7-dimethyl-2,6-
octadienal) at different temperatures. Products of the hydrogena-
tion of citral are industrially produced in huge quantities
(40.000 t/a of citral for example by the company BASF SE in
20151) and used in cosmetics and flavoring [9]. Thus, the reaction
lysts have been tested for this ‘‘wide-spread model reaction” [11]
before, mostly consisting of Pt, Pd, Ni or Ru [12–17]. Furthermore,
it is well known, that the (selective) hydrogenation of C@C, C@O,
and C„N bonds is possible with Ni or Co metals or borides
[7,10,18,19]. Depending on temperature, pressure, solvent, and cata-
lyst, the selectivity for products that are differently hydrogenated
was shown to vary, but earlier catalytic investigations on nanoscale
borides were performed with powders of unclear composition.
In this work we will focus on the hydrogenation of citral using
unsupported nanoscale borides of well-defined composition and
structure, Co2B and Ni7B3, to explore their potential as catalysts.
Only a few unsupported systems have been tested for these reac-
tions (e.g. in [20,21]), but borides certainly open up new possibili-
ties. Earlier, amorphous metal borides were shown to be active in
the hydrogenation of glucose [22] or cinnamaldehyde [23]. Also,
another application discussed for Co2B in the literature is the gen-
eration of hydrogen for proton exchange membrane fuel cells (e.g.
in [24]). Recently, a semi-amorphous sample of CoOOH/Co2B was
shown to be an exceptionally efficient electrocatalyst for the
oxygen evolution reaction in water splitting [25].
Citral is an isomer mixture of trans- and cis-3,7-dimethyl-2,6-
octadienal (geranial and neral). The unsaturated aldehyde exhibits
an isolated C@C bond as well as conjugated C@C and C@O bonds.
⇑
Corresponding authors.
E-mail address: albert@ac.chemie.tu-darmstadt.de (B. Albert).
BASF Report 2015: Economic, Environmental and Social Performance (http://
1
bericht.basf.com/2015/en/; accessed Feb. 3rd, 2017).
http://dx.doi.org/10.1016/j.jcat.2017.06.009
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

N. Kalyon et al. / Journal of Catalysis 352 (2017) 436–441
437
Thus, it is a very suitable test molecule for analyzing the selectivity
of a hydrogenation reaction and investigating the discrimination
between C@O and C@C bonds. The thermal and transport proper-
ties of citral have been investigated before [26]. Hydrogenation
of the C@C bond of the conjugated part leads to citronellal,
whereas hydrogenation of the C@O bond yields nerol and geraniol.
Citronellol is obtained by hydrogenation of the C@C bond of nerol
and geraniol or the C@O bond of citronellal. The reaction of hydro-
gen with the isolated C@C bond of citronellol will lead to the unde-
sired, fully-hydrogenated product, dimethyloctanol [14,27,28].
Obtaining menthol from citral or citronellal with high selectivity
is feasible by suitable catalytic reactions [29–32]. The reaction
pathways of the hydrogenation of citral are depicted in Fig. 1.
with degassed water and acetone. The product was dried in vacuo
(r.t., 16 h), and then annealed at 648 K for two hours. The catalyst
was handled under purified argon.
2.2. Catalyst characterization
Both products were characterized before and after annealing
using X-ray diffraction (XRD) on a powder diffractometer (Stadi
P, Stoe&Cie GmbH, Darmstadt) with transmission geometry.
Molybdenum radiation was used (MoKa1, k = 0.70930 Å) and
monochromatized (Ge [111]), and reflections in the 2h range of
5–50° were recorded with a position sensitive detector. The
samples were prepared on a flat sample holder.
High-resolution scanning electron microscopy (HRSEM)
measurements were carried out on a Philips XL30 FEG instrument.
The specific surface areas were measured by N2 physisorption
at 77.3 K (Quadrasorb SI, Quantachrom). Prior to the analyses, sam-
ples were heated in vacuo to 523 K for 10 h. Adsorption isotherms
were measured, specific surfaces were calculated using the Bru-
nauer, Emmet, and Teller (BET) equation.
2. Materials and methods
2.1. Synthesis of Co2B and Ni7B3
The nanoscale borides Co2B and Ni7B3 were prepared in aque-
ous solutions by reduction of metal halides with NaBH4 under
argon atmosphere at room temperature [8,33,34].
For the synthesis of Co2B, CoCl2 6H2O (Fluka, 1 eq., 1.903 g) was
dissolved in 200 ml of degassed water. Sodium tetrahydridoborate
(Alfa Aesar, 2 eq.) was added to the solution. Immediately, a black
product precipitated and the formation of hydrogen was observed.
After two hours the hydrogen evolution ended; the solvent was
decanted, the product was washed three times with degassed
water, and dried in vacuo (r.t., 16 h). Annealing was done under
purified argon (traces of oxygen and nitrogen removed by reaction
with titanium sponge at 925 K) for two hours at 673 K.
Ni7B3 was prepared from NiCl2 6H2O (Grüssing GmbH, 1 eq,
1.902 g) which was dissolved in 400 ml of degassed water and
dropped quickly onto NaBH4 (Alfa Aesar, 1 eq). Hydrogen evolution
was observed and the amorphous boride precipitated immediately.
The reaction system was stirred for two hours, and then the sol-
vent was decanted and the precipitate was washed three times
2.3. Hydrogenation of citral
Catalyzed test reactions were performed in high-pressure batch
autoclaves (Parr, 300 mL, stainless steel). To enhance comparabil-
ity with experiments on other catalysts described in the literature,
the catalysts were pre-treated at 423 K and a H2 pressure of
1.5 MPa in situ by stirring 250 mg cobalt or nickel boride in
110 ml n-hexane. After this procedure, the desired reaction tem-
peratures (353, 393, 423 K) and pressures (5 MPa) were set, and
citral (Merck, 10 mL) in hexane (25 mL, with tetradecan as internal
standard) was added from a separate tank. Samples (50 ll) were
taken periodically after 0, 20, 30 or 40, 60, 90, 120, and 240 min
and analyzed offline by gas-chromatography for nerol, geraniol,
citronellol, citronellal, and dimethyloctanol (Shimadzu GC 2010
Plus).
Fig. 1. Hydrogenation reaction network of citral.

438
N. Kalyon et al. / Journal of Catalysis 352 (2017) 436–441
Fig. 2. X-ray powder diffraction patterns of Co2B and Ni7B3 (blue: before hydrogenation reaction, red: after hydrogenation reaction), compared to the literature data (black
[35,8]).
Fig. 3. Unit cells of Co2B (left) and Ni7B3 (right) with cobalt atoms (black), boron atoms (red) nickel atoms (blue, pink).
Fig. 4. HRSEM images of Co2B (a) and Ni7B3 (b).
2.4. Recycling test
3. Results and discussion
To check for recyclability, the reaction mixture was removed
using a filter cartridge after one of the reactions, and the catalyst
was washed with hexane without opening the reactor. Then, the
reaction was again set up, run and analyzed as described above.
After the reaction, the liquid phase was boiled down to check for
a possible metal residue by energy-dispersive spectroscopy. This
test did not indicate any leaching of the metal catalyst.
3.1. Synthesis and characterization of the catalysts
Precipitates obtained by reduction of metal salts proved to be
X-ray amorphous. Then, after annealing at 648 K, the powder
diffraction patterns of the samples exhibited the crystallinity of
the borides and showed the typical reflections of single-phase
Co2B or Ni7B3 (Fig. 2). The catalysts were re-investigated after the

N. Kalyon et al. / Journal of Catalysis 352 (2017) 436–441
439
atoms are surrounded antiprismatically by eight cobalt atoms as
shown in Fig. 3. Ni7B3 crystallizes with a disordered Th7Fe3-type
structure (hexagonal crystal system, space group P63mc), as
described in Ref. [8]. Here, the boron atoms are located within trig-
onal prisms of nickel atoms. The unit cell is shown in Fig. 3.
To prove the nanoscale character of the powders, HRSEM
images of Co2B and Ni7B3 were recorded (Fig. 4). The spongy struc-
ture is typical for nanoscale borides. For Ni7B3 it was observed by
transmission electron microscopy that the intergrowth of several
nanoparticles (diameter < 20 nm) results in coherent scattering
and larger crystallites according to XRD (diameter according to
the Scherrer formula: ꢀ95 nm). The nanoparticles are highly
agglomerated, especially in the case of Co2B which is a ferromag-
netic compound.
The specific surface area according to BET led to values of
6.0 m2/g for Co2B and 13.6 m2/g for Ni7B3 at. Wu et al. observed a
type III isotherm and a specific surface area of 21 m2/g for a differ-
ent set of samples of Co2B after thermal treatment at 773 K, but
these samples contained metallic Co [36]. In Ref. [19], the surface
area of an amorphous powder containing Co and B was described
to amount to 14.9 m2/g. In the work of Mahdavi et al., Ni2B with
a specific surface area of 10.7 m2/g was discussed [37].
Fig. 5. Conversion of citral (XCitral
(citronellal, CAL; citronellol, COL; dimethyloctanol, DMO; Nerol/Geraniol) during
the hydrogenation of citral with Co2B at 423 K.
) and yields of different reaction products
hydrogenation reaction, to check for changes upon reaction, and no
structural changes were observed. So, the identity of the phase-
pure borides and their stability during hydrogenation reactions
were proven.
3.2. Catalytic activity
Co2B crystallizes in the well-known CuAl2-structure type
(tetragonal crystal system, space group I4/mcm [35]). The boron
The catalytic activity of Co2B for the hydrogenation of citral was
measured as a reference for the investigation of the new compound
Table 1
Conversion (X) and product yields (Y) of the hydrogenation of citral with Co2B and Ni7B3.
T/K
t/min
XCitral/%
YCOL/%
YCAL/%
YNerol
/
Geraniol/%
YDMO/%
Co2B
423
60
90
120
240
60
78
97
100
100
36
52
66
94
85
24
47
76
60
2
10
2
0
41
41
9
0
1
1
1
1
2
2
1
0
3
2
2
1
0
1
4
29
0
0
0
1
1
3
5
21
4
7
0
Ni7B3
Ni7B3
Ni7B3
353
393
423
31
46
56
74
47
39
26
3
90
3
5
120
240
60
11
14
25
35
41
58
68
71
63
90
94
98
120
240
60
90
120
240
100
100
100
100
100
25
11
5
11
23
0
Fig. 6. Selectivities for different reaction products and conversion (XCitral) of citral in recycling tests, each run analyzed after 90 min of hydrogenation with Co2B at 423 K.

440
N. Kalyon et al. / Journal of Catalysis 352 (2017) 436–441
Fig. 7. Conversion of citral (XCitral
)
and yields of different reaction products (citronellal, CAL; citronellol, COL; dimethyloctanol, DMO; nerol/geraniol) during the
hydrogenation of citral with Ni7B3 at 353 K (left) and 423 K (right).
Fig. 8. Results of the hydrogenation of citral emphasizing the selectivities for certain products after 60 min with nanoscale Ni7B3 or Co2B used as catalysts.
Ni7B3. To allow for comparison with the literature, Co2B was inves-
tigated at 423 K and a pressure of H2 of 5 MPa. The results are
shown in Fig. 5 and Table 1. It becomes evident, that nerol and
geraniol as intermediates are formed initially with a maximum
yield of 41%. The yield of citronellol increases up to 76% after
120 min. It becomes the main product when 100% of citral are con-
verted. Citronellal is obtained in low amounts, similar to the unde-
sired product that is hydrogenated in full, dimethyloctanol. These
results are very well in accordance with the observations of Chen
et al., which, however, were obtained for Co-B catalysts with
less-defined composition and structure [19].
One of the reactions was repeated twice without changing the
catalyst, to check for its recyclability. The results of three consecu-
tive runs proved to be very similar to those shown in Fig. 5, with
selectivities for gerol and neraniol even improving upon repetition.
Fig. 6 gives the composition of the product mixtures for each of the
three runs after 90 min at 423 K. The increase in activity is attrib-
uted to a possible deagglomeration of the powder of unsupported
catalyst upon prolonged stirring of the reaction mixture.
geraniol, citronellol and dimethyloctanol were detected with
minor selectivity. In terms of selectivity, the situation proved to
be similar at higher reaction temperatures, citronellal being the
main product also at 393 K during the first 90 min (Table 1). But,
increasing the reaction temperature up to 423 K resulted in consid-
erable changes. As expected, the conversion of citral became
higher, e.g. after 60 min it was only 36% at 353 K but 100% at
423 K. At 423 K, at the beginning of the reaction, citronellal was
observed to be the main product, but after 40 min citronellol
became the main product. The reaction to the undesired, fully
hydrogenated products was slow, but after 90 min significant
amounts of dimethyloctanol started to form. Nerol/geraniol was
obtained with minor yields. The distinct selectivity for citronellol
is attributed to the well-defined character of the single-phase cat-
alyst. According to our magnetic measurements [8], Ni7B3 is very
pure and does not contain elemental nickel.
4. Conclusions
Using Ni7B3 a conversion of citral of 94% was observed after
240 min at 353 K, and the yield of citronellal then amounted to
74 %. As shown in Fig. 7, at this temperature citronellal remained
the main product over the time span investigated, and the conver-
sion of citral continued to raise. Other products such as nerol and
Co2B and Ni7B3 can be obtained as nanoscale and single-phase
powders. These materials were characterized in detail by means
of XRD, N2 physisorption and HRSEM. Using the hydrogenation of
citral in n-hexane as test reaction, the catalytic potential of

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441
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were used as catalysts that were very well-defined concerning
their composition, phase-purity, and particle size. During the
preparation of the catalysts and the test reactions, an emphasis
was put on the absence of oxygen, since nickel and cobalt oxides
and hydroxides at the surface of nanoparticles are known to influ-
ence the catalytic behavior significantly, as it was shown very
recently in this journal [38]. A selection of the results in terms of
selectivities is comprised in Fig. 8. With Co2B, the C@O bond was
hydrogenated preferentially and nerol and geraniol were produced
with high selectivity at the beginning of the reaction, later citronel-
lol became the main product. With Ni7B3, the C@C bond was
hydrogenated preferentially and either citronellal or citronellol
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The funding of the Deutsche Forschungsgemeinschaft of the
Darmstadt Graduate School of Excellence of Energy Sciences and
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