A transition-metal-free hydrogenation catalyst: Pore-confined sodium alanate for the hydrogenation of alkynes and alkenes

Article abstract of DOI:10.1016/j.jcat.2016.09.024

Hydrogenation catalysis is dominated by transition metals, but interest in alternative catalysts has been growing over recent years. Herein, a transition-metal-free catalyst is discussed consisting of carbon supported NaAlH₄ as a selective catalyst for hydrogenation. This is illustrated using a range of substrates, and in more detail for the case of diphenylacetylene. Catalytic activity depends on the solvent utilized; in cyclohexane the activity is 2.3?mol?(DPA)?mol^?1 (NaAlH₄) h^?1 at 100?bar H₂, 150?°C with a slight preference for the formation of trans-stilbene. The catalyst selectivity is influenced by the loading, yielding a high selectivity toward the thermodynamically less stable cis-stilbene at low catalyst loadings. This proof of principle shows promise for using metal hydrides based on earth-abundant elements as effective hydrogenation catalysts.

Full text of DOI:10.1016/j.jcat.2016.09.024

Journal of Catalysis 344 (2016) 129–135
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
A transition-metal-free hydrogenation catalyst: Pore-confined sodium
alanate for the hydrogenation of alkynes and alkenes
Peter L. Bramwell a, Jinbao Gao a, Bernd de Waal a, Krijn P. de Jong a, Robertus J.M. Klein Gebbink b,
Petra E. de Jongh a,
⇑
a Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3583CG, The Netherlands
b Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3583CG, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Hydrogenation catalysis is dominated by transition metals, but interest in alternative catalysts has been
growing over recent years. Herein, a transition-metal-free catalyst is discussed consisting of carbon sup-
ported NaAlH4 as a selective catalyst for hydrogenation. This is illustrated using a range of substrates, and
in more detail for the case of diphenylacetylene. Catalytic activity depends on the solvent utilized; in
cyclohexane the activity is 2.3 mol (DPA) molÀ1 (NaAlH4) hÀ1 at 100 bar H2, 150 °C with a slight prefer-
ence for the formation of trans-stilbene. The catalyst selectivity is influenced by the loading, yielding a
high selectivity toward the thermodynamically less stable cis-stilbene at low catalyst loadings. This proof
of principle shows promise for using metal hydrides based on earth-abundant elements as effective
hydrogenation catalysts.
Received 20 June 2016
Revised 25 August 2016
Accepted 19 September 2016
Keywords:
Hydrogenation
Heterogeneous catalysis
Alkene
Ó 2016 Elsevier Inc. All rights reserved.
Alkyne
Sodium alanate
1. Introduction
than as catalysts. There is one isolated example of NaAlH4 being
utilized as a hydrogenation catalyst where Ti and NaAlH4 were
Hydrogenation of C-C multiple bonds is a key step in the syn-
thesis of many important compounds [1–3]. As with many other
fields of catalysis, transition metals dominate. For example, Pd is
widely employed in hydrogenation of alkynes to alkenes [4–8],
particularly in the case of the famous Lindlar catalyst [9]. Pd-
based catalysts generally show very high activity (turnover fre-
quencies, TOF, of 2500 hÀ1 at 22 °C) [10] under mild conditions
but often relatively low selectivities [11,12].
In recent years a new family of transition-metal-free catalysts,
frustrated Lewis pairs, has emerged which boast reasonably high
activities (TOFs of 5–40 hÀ1 at 80 °C) [13] and selectivities toward
cis-alkenes without utilizing transition metals [14,15]. Light metal
hydrides such as LiAlH4 and NaBH4 are based on earth-abundant
elements and are typically used in the stoichiometric reduction
of polar groups such as carbonyls [16,17]. However, these materi-
als so far have seldom been applied to the reduction of alkenes or
alkynes and in such cases only as stoichiometric reagents, rather
milled together and tested in the hydrogenation of dipheny-
lacetylene [18]. At 130 °C and 100 bar of hydrogen pressure the
alkyne starting material was fully hydrogenated to the saturated
product within three hours. Ti-doped sodium alanate is known to
reversibly store and release hydrogen, which might explain its cat-
alytic activity.
The preparation and characterization of confined light metal
hydrides for the purpose of reversible uptake and release of hydro-
gen (hydrogen storage) at reasonably low temperatures with fast
kinetics have been demonstrated in recent years [19–27]. Melt
infiltration under hydrogen pressure (to prevent decomposition)
can be utilized to prepare these materials. The metal hydride and
support are mixed together and heated to the melting point of
the hydride. The molten hydride infiltrates the pores and, upon
cooling, produces solid metal hydride confined within the pores,
which is referred to as nanoconfinement. The resulting material
consists of a carbon matrix of which the pores are filled with metal
hydride. This pore-confined metal hydride lacks long range crys-
tallinity and demonstrates orders of magnitude faster kinetics of
absorption and desorption compared to the corresponding macro-
crystalline metal hydride material.
Abbreviations: XRD, X-ray diffraction; SEM, scanning electron microscopy; EDX,
energy dispersive X-ray spectroscopy; TPD, temperature programmed desorption;
TCD, thermal conductivity detector; DPA, diphenylacetylene; PEG, polyethylene
glycol; GC, gas chromatography.
In the case of NaAlH4 almost full reversibility of hydrogen
release and uptake was achieved at 150 °C [21]. Reduction of the
⇑
Corresponding author.
E-mail address: P.E.dejongh@uu.nl (P.E. de Jongh).
http://dx.doi.org/10.1016/j.jcat.2016.09.024
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

130
P.L. Bramwell et al. / Journal of Catalysis 344 (2016) 129–135
particle size improves the kinetics of hydrogen sorption as bulk
crystalline NaAlH4 displays much slower uptake kinetics and
requires temperatures above 180 °C to desorb hydrogen. Addition-
ally, the presence of carbon alters the hydrogen release profile
from a three-step pathway to a two-step pathway, bypassing the
intermediate Na3AlH6 phase altogether [21]. Nanoconfinement
promotes reversibility by preventing the macroscopic phase sepa-
ration of NaH and Al. All of these factors allow the NaAlH4/C
nanocomposite to release and reabsorb hydrogen in a reversible
manner at relatively low temperatures. This raises the interesting
question of whether this material could also be used as a hydro-
genation catalyst in the absence of any transition metals [18,28].
Hence we demonstrate carbon-confined NaAlH4, previously devel-
oped for hydrogen storage purposes, as a transition-metal-free
hydrogenation catalyst. We explore the hydrogenation of alkynes
and alkenes, while studying in detail the recyclability, selectivity,
and activity depending on solvent and catalyst loading in the case
of DPA hydrogenation (Scheme 1).
and nitrogen physisorption before use in catalytic tests. Multiple
batches of the catalyst were prepared using the above procedure
and their full characterization can be found in Section S1 of the
supporting information.
All hydrogenation reactions were carried out in a Parr 300 mL
autoclave at 150 °C, 100 bar hydrogen pressure and constant stir-
ring. The autoclave was loaded in the nitrogen glove box. 270 mg
of catalyst (1 mmol NaAlH4) and 890 mg of DPA (5 mmol) were
added to 180 mL of solvent. Dodecane was added as an internal
standard. Aliquots of reaction mixture were taken for analysis at
regular intervals using a sampling arm which was flushed with a
small amount of reaction mixture before the taking of each sample
(roughly 2 mL of reaction mixture per sample). Samples were then
analyzed by Gas Chromatography (GC-2010 Shimadzu gas chro-
matograph equipped with a Shimadzu AOC-20i Auto injector)
using a CP-Wax column to separate the reaction mixture by boiling
points; Boiling points: DPA (300 °C), cis-stilbene (307 °C), trans-
stilbene (305 °C), bibenzyl (284 °C).
The following blank and reference measurements were per-
formed, the results of which can be found in Sections S2–S3 in
the supporting information. The influence of the carbon support
on the activity was tested by performing the above catalysis exper-
iments using only 215 mg of the carbon support in the absence of
NaAlH4. To rule out the effect of the metals in the autoclave walls
catalytic tests were performed using a Teflon lining in the auto-
clave. As the Teflon liner reduced the volume of the reaction vessel
the reaction was scaled down by 15%. The catalytic tests were
repeated to demonstrate reproducibility. The recyclability of the
catalyst was tested by recovering the catalyst at the end of the
hydrogenation reaction and re-using in another test. Since
150 mg of the NaAlH4/C catalyst was recovered (due to losses
through filtering and removing the catalyst from the reactor) the
catalytic loading in the second reaction was 150 mg instead of
270 mg. Isomerization studies were carried out by performing
the reaction in the same manner as above but instead of adding
DPA either trans-stilbene or cis-stilbene was added.
For the experiments on the adsorption of each substrate on the
catalyst in different solvents the following procedure was applied.
All experiments were performed at room temperature to ensure no
hydrogenation reaction occurred. In each experiment a substrate,
either DPA, cis-stilbene, trans-stilbene or bibenzyl, was dissolved
in 9 mL of the solvent and stirred for 5 min. This was then analyzed
by gas chromatography to determine the substrate concentration
at 100%. Then 100 mg of catalyst was added (with a volumetric liq-
uid:solid ratio of 203) to the solution and a sample was taken at
60 min for gas chromatography (GC) analysis. The decrease in
2. Experimental section
All materials were stored in a nitrogen-filled glove box (Mbraun
Labmaster I30, 1 ppm H2O, <1 ppm O2) prior to use, except for the
catalyst which was stored in an argon-filled glove box (Mbraun
Labmaster dp, 1 ppm H2O, <1 ppm O2) and transferred to the other
glove box immediately before use. NaAlH4 powder (hydrogen stor-
age grade), DPA (98%), cyclohexane (anhydrous, 99.5%), dodecane
(anhydrous, P99%), 1-octyne (97%), 4-octyne (99%) and styrene
(99%) were all obtained from Sigma-Aldrich. Toluene was dried
in a distillation apparatus before storage. All liquid reagents and
solvents were stored in the glove box over molecular sieves follow-
ing degassing by bubbling nitrogen gas through the liquid for sev-
eral hours. DPA was dried in vacuo overnight before storage in the
glove box. The carbon aerogel was prepared by the sol-gel resorci-
nol procedure [26] and was analyzed by nitrogen physisorption
(performed at À196 °C, Micromeritics TriStar) to determine the
pore characteristics (BET surface area 564 m2 gÀ1, pore volume
0.57 cm3 gÀ1, broad pore size distribution with a maximum around
18–20 nm). The aerogel was dried at 600 °C under argon flow for
12 h before storing in the argon-filled glove box.
The catalyst was prepared by melt infiltration according to pre-
viously reported literature [21] with a loading of 20 wt% NaAlH4.
The result was analyzed by X-ray Diffraction (Bruker AXS D8
advance 120 machine, Co-K radiation, air-tight sample holder
a
used), Temperature Programmed Desorption (Micromeritics Auto-
Chem II, equipped with a TCD detector, Ar flow of 25 mL minÀ1
)
Scheme 1. Reaction scheme showing the hydrogenation of diphenylacetylene to bibenzyl.

P.L. Bramwell et al. / Journal of Catalysis 344 (2016) 129–135
131
concentration was assumed to be due to substrate adsorption on
the catalyst surface, allowing the calculation of the quantity of
adsorbed substrate. Each sample was run in the GC three times
and an average was taken, yielding the points shown in Fig. S6.1,
and the variation in these three measurements was then used to
calculate the error given in Table 2.
3. Results and discussion
3.1. Catalyst characterization
The catalyst is prepared by melt infiltration of NaAlH4 into the
pore structure of a carbon aerogel support (volume average pore
diameter of 13 nm, prepared by the sol-gel resorcinol method)
[29]. A schematic of the melt infiltration procedure is shown in
Fig. 1. Following melt infiltration X-ray diffraction (XRD) shows
the loss of crystalline NaAlH4 and a small amount of crystalline
Al is present. Al is present due to decomposition of a minor fraction
of the NaAlH4 to NaH and Al during melt infiltration [21]. The low
intensity of the NaAlH4 signal shows that pore-confined NaAlH4
lacks long range crystallinity, where the proportion of crystalline
NaAlH4 is 29% compared to the physical mixture, indicating a con-
finement of 71% of the NaAlH4. This is supported by the pore vol-
ume loss upon melt infiltration of 26 vol% ( 2 vol%), which
accounts for 91% of the NaAlH4 present. Mapping of the distribu-
tion of NaAlH4 and Al through the catalyst surface has been well
studied previously through use of scanning electron microscopy
(SEM) and energy dispersive X-ray spectroscopy (EDX) [21]. As a
result it is known that the NaAlH4 is distributed in a homogeneous
manner. Hence the catalyst does not contain discrete NaAlH4 par-
ticles but rather the pores are either completely filled or empty,
with NaAlH4 exposed at the pore mouths at the external surface
of the carbon support particle, and at the boundaries between filled
and unfilled pore regions.
Fig. 2. Temperature programmed desorption profile showing the hydrogen release
of the NaAlH4/C catalyst performed at
a
heating rate of 5 °C minÀ1 under
25 mL minÀ1 Ar flow.
in order for NaAlH4 to perform hydrogenation reactions in a cat-
alytic manner and full reversibility at 150 °C has indeed been
demonstrated in the literature [20], where the optimal reversible
hydrogen capacity is achieved with rehydrogenation at 100 bar of
H2. Therefore a temperature of 150 °C was used for all catalytic
testing.
3.2. Catalyst testing
Fig. 3 shows the hydrogenation of DPA performed using the
NaAlH4/C catalyst under 100 bar H2 of pressure. It is clear that
the nanocomposite exhibits catalytic behavior by fully hydrogenat-
ing DPA when used in a 5:1 DPA:NaAlH4 molar ratio (turnover
number 10) with a molar activity, taken between 90% and 50%
DPA conversion, of 2.29 mol (DPA) molÀ1 (NaAlH4) hÀ1. Due to
the nature of the catalyst it is difficult to estimate a turnover fre-
quency (TOF) as it is not possible to measure the amount of
exposed active surface area. Only a small amount of the NaAlH4
in the catalyst is active, as it is confined within the pores; therefore,
for this catalyst a large fraction of the NaAlH4 is not exposed to the
reactants. For this reason a TOF range has been calculated with a
Characterization of the hydrogen content of nanoconfined
hydride materials is commonly performed using temperature pro-
grammed desorption (TPD) and the temperature at which the
hydrogen can be released is also relevant to hydrogenation cataly-
sis. In the NaAlH4/C nanocomposite hydrogen release (Fig. 2)
begins at 100 °C with a maximum release rate at 175 °C and a total
emission of 0.37 wt% of hydrogen (compared to a theoretical
capacity of 1.48 wt%). This hydrogen release should be reversible
Fig. 1. Schematic representation of the melt infiltration process used in preparing the NaAlH4 nanocomposite. Orange shading on the right picture represents how the pores
are filled by NaAlH4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

132
P.L. Bramwell et al. / Journal of Catalysis 344 (2016) 129–135
are at least partially responsible for the observed activity of the
NaAlH4/C catalyst. To rule this out the reaction was also performed
with a Teflon insert in the autoclave (Fig. S2.1). When the Teflon
liner is used the activity is 2.32 mol (DPA) molÀ1 (NaAlH4) hÀ1
,
which is only a 1% deviation from the activity seen when the Teflon
insert is not used. Further, the carbon support itself showed no
effect on the rate of DPA hydrogenation (see supporting informa-
tion Section S2). Therefore it can be concluded that the metals in
the autoclave walls and the carbon support are not the source of
the apparent catalytic activity of the NaAlH4/C catalyst.
To determine recyclability of the catalyst a reaction was per-
formed at 150 °C, 100 bar hydrogen pressure in a cyclohexane sol-
vent with a catalyst loading of 270 mg and DPA loading of 5 mmol.
After the reaction was finished the catalyst was recovered by filtra-
tion, dried and used again in a hydrogenation reaction under the
same conditions (Fig. 5). In the second run 150 mg of the catalyst
was recovered so the amount of catalyst was 44% lower. This is
reflected in the data as the rate of hydrogenation is 53% slower
in the second run.
Fig. 3. Concentrations during hydrogenation of DPA performed with cyclohexane as
a solvent. Reaction performed under 100 bar H2 pressure, at 150 °C, with a catalyst
loading of 270 mg (1 mmol of NaAlH4) and 5 mmol of DPA.
In order to further determine the scope of the catalyst a number
of other substrates were also tested under the same conditions
(Table 1, see supporting information Section S5 for reaction pro-
files). 1-octyne and 4-octyne were chosen as they are unfunction-
alized alkynes and can demonstrate whether the catalyst is able to
reduce terminal and internal alkynes respectively. Here it is clear
that the catalyst can effectively reduce the unfunctionalized mole-
cules and is more effective in reducing internal alkynes than termi-
nal alkynes. This is also true for the resulting alkenes, which are
further reduced to alkanes, demonstrating the ability of the cata-
lyst to also reduce unfunctionalized alkenes. Hydrogenation of a
conjugated alkene, styrene, further demonstrates the wide scope
of the applicability of the catalyst.
lower limit and a more realistic estimate. The lower limit is based
on the assumption that all of the BET surface areas measured by
nitrogen physisorption are composed of available catalytically
active sites. A more realistic estimation was made by using the vol-
ume fraction of NaAlH4 in the catalyst, and assuming that the frac-
tion of the total surface area consisting of NaAlH4 was the same as
the volume fraction of NaAlH4 (see supporting information Sec-
tion S4 for further details). The turnover frequency has been esti-
mated to be 2 or 179 hÀ1 for the lower and more realistic
estimate respectively. Although significantly lower than a typical
Pd-based catalyst, these values are comparable to those for Frus-
trated Lewis Pair catalysts [13]. The alkyne functionality in the
DPA is fully reduced to the alkane (bibenzyl) by the catalyst. This
demonstrates that the catalyst can in fact hydrogenate both alky-
nes and alkenes.
Fig. 4 shows the dependence of the rate of the DPA consumption
on catalyst loading. Each point was generated by performing the
reaction for 2 h with samples being taken every 10 min for GC
analysis. In this way the rate of DPA consumption up to 50% was
determined accurately for each catalyst loading. The rate increases
linearly with catalyst loading. Since there is a small amount of DPA
conversion in the absence of catalyst it is possible that the metals
in the walls of the autoclave, in which the reaction is carried out,
3.3. Solvent effects
In previous literature [27] the hydrogenation of DPA was per-
formed in toluene, and for this reason toluene was also used as a
solvent for comparison to cyclohexane (Fig. 6). In toluene 50% con-
version of DPA is achieved in roughly 350 min (activity of 0.48 mol
(DPA) molÀ1 (NaAlH4) hÀ1) whereas the same conversion when
cyclohexane is employed occurs around 90 min (2.29 mol (DPA)
molÀ1 (NaAlH4) hÀ1), which is comparable to that for a Ti-doped
ball-milled NaAlH4 catalyst under similar conditions.
The solvent-dependence of the activity is striking and may be a
result of either differences in hydrogen solubility or interaction
with the catalyst. At room temperature the solubility of hydrogen
in cyclohexane is 35% higher than for toluene at a pressure of
44 bar [30], but this can only account for a small portion of the dif-
ference in activity. Therefore it is very likely that something other
than hydrogen solubility is affecting activity, such as competitive
adsorption from the solvent. To verify this hypothesis we per-
formed experiments on the adsorption of each substrate onto the
catalyst surface in both solvents (Table 2. Details of the experi-
ments and calculations can be found in S6 of the supporting infor-
mation). They showed that there is a higher level of substrate
adsorption in cyclohexane compared with toluene. Therefore, it
can be inferred that toluene strongly adsorbs onto the catalyst sur-
face, hindering the adsorption and subsequent reaction of DPA.
3.4. Selectivity control
The catalyst favors the production of trans-stilbene, which is not
surprising as trans-stilbene is the thermodynamically more stable
isomer [31]. There are two possible explanations for this, either
this is the intrinsic selectivity of the catalyst or isomerization of
cis-stilbene to trans-stilbene during reaction is affecting the
Fig. 4. Average rate of DPA hydrogenation versus catalyst loading in cyclohexane.
Each data point was determined from the time taken to reach 50% conversion of
DPA. All experiments were performed at 150 °C, 100 bar H2 with GC samples taken
every 10 min for 2 h.

P.L. Bramwell et al. / Journal of Catalysis 344 (2016) 129–135
133
Fig. 5. Recyclability test of the NaAlH4/C catalyst. The first run is shown in black and the run using the spent catalyst is shown in red. Conditions in both experiments: 150 °C,
100 bar H2, cyclohexane solvent, 5 mmol DPA. Samples taken every 30 min for 6 h, with one sample taken after roughly 20 h. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Table 1
Conversion, yields and activities obtained when using the NaAlH4/C nanocomposite as a hydrogenation catalyst for a number of substrates. Activities are given as the number of
moles of hydrogenated substrate per hour, normalized to the total number of moles of NaAlH4 in the NaAlH4/C catalyst (roughly 1 mmol).
Substratea
DPA
Conversion/%
100
Activity
1.00
Product
Yield/%
Cis-stilbene
Trans-stilbene
Bibenzyl
1-Octene
Octane
4-Octene
Octane
Ethylbenzene
0
0
100
47
43
2
93
97
1-Octyne
4-Octyne
Styrene
100
95
0.19
0.23
0.21
97
a
Reactions performed at 150 °C, 100 bar H2 pressure for 48 h in a cyclohexane solvent, NaAlH4:substrate molar ratio of 1:5.
Table 2
Summary of adsorption data for each substrate in cyclohexane and toluene.
Solvent
Substrate
Concentration drop
after 60 min/%
% of catalyst
surface coveragea
Cyclohexane
DPA
17.8
16.0
15.9
10.8
3.82 0.03
4.04 0.04
3.38 0.02
2.79 0.03
Cis-stilbene
Trans-stilbene
Bibenzyl
Toluene
DPA
1.5
1.9
3.4
0.5
0.35 0.00(2)
0.82 0.01
0.61 0.00(4)
0.10 0.00(03)
Cis-stilbene
Trans-stilbene
Bibenzyl
a
Calculated from the BET surface area of the catalyst.
An interesting question given that isomerization of cis-stilbene
can take place at the free surface of the catalyst is whether the
number of active sites can affect the selectivity. Therefore a high
molar DPA:NaAlH4 ratio of 25:1 was applied, by reducing the cat-
alyst loading (Fig. 8). The lower loading drastically altered the
selectivity, yielding a cis:trans ratio of 12 at 50% conversion com-
pared to a ratio of 1 at the same conversion when the lower
DPA:NaAlH4 ratio of 5 is used. The reason for this is possibly block-
ing of the active site required for isomerization by DPA at lower
catalyst loadings. In any case the fact that the catalyst loading
has such a marked impact on the selectivity demonstrates that this
is not thermal isomerization. Compared to the more active Pd cat-
alysts this provides the unique opportunity to tune selectivity by
changing catalyst loading.
Fig. 6. Hydrogenation of DPA performed with toluene as a solvent. Each data point
corresponds to
a sample taken at the respective time and analyzed by gas
chromatography. Reactions performed under 100 bar H2 pressure, at 150 °C, with a
catalyst loading of 270 mg (1 mmol of NaAlH4) and 5 mmol of DPA.
observed selectivity. To verify this we monitored two separate
reactions: hydrogenation of trans-stilbene and hydrogenation of
cis-stilbene rather than DPA (Fig. 7). It was found that the rate of
trans-stilbene formation from cis-stilbene is roughly 1.2 mmol hÀ1
whereas isomerization from trans- to cis-stilbene does not occur at
all. This rate of isomerization can account for 100% of the trans-
stilbene that is formed during reaction. Therefore, the catalyst pri-
marily produces cis-stilbene through hydrogenation of DPA and
trans-stilbene is only formed through isomerization of the cis-
stilbene product.
In order to gain more insight into the mode of action of the cat-
alyst, particularly whether hydrogen can be directly extracted from
the NaAlH4 lattice or whether it is provided by the gas phase only,

134
P.L. Bramwell et al. / Journal of Catalysis 344 (2016) 129–135
Fig. 7. Hydrogenation of cis-stilbene (left) and trans-stilbene (right) to determine the effect the catalyst has on isomerization of cis-stilbene and its reversibility. Conditions:
150 °C, 100 bar H2, cyclohexane solvent, 270 mg catalyst (1 mmol NaAlH4), 5 mmol cis-/trans-stilbene.
the catalyst. Once hydrogen pressure is introduced, the reaction
proceeds much more rapidly to 100% conversion to bibenzyl (BB)
within 6 h. Possibly the extraction of hydrogen leads to a change
in the nanostructure of the material (increasing the accessibility
of the active sites) or an intrinsic change in the nature of the active
sites.
Interestingly, if the hydrogen gas is introduced after
a
hydrogen-free period the catalyst is more selective toward cis-
stilbene than when hydrogen is introduced at the start of the reac-
tion. At 50% conversion the cis:trans ratio is 5, which is a fivefold
increase compared to the reaction shown in Fig. 3. There are two
possible explanations for this, the first being the aforementioned
altered active surface area resulting from dehydrogenation of the
catalyst. The other is that the intrinsic properties of the catalyst
are changed during the Ar stage. However, in order to better under-
stand this further investigation is required.
Fig. 8. Hydrogenation of DPA performed with a catalyst loading of 54 mg (0.2 mmol
NaAlH4) and 5 mmol DPA. 150 °C, 100 bar hydrogen, cyclohexane solvent.
4. Conclusions
In conclusion, a transition-metal-free catalyst for hydrogena-
tion of terminal and internal alkynes and alkenes has been demon-
strated which consists of carbon supported NaAlH4 using a carbon
aerogel type support. The catalyst has the ability to hydrogenate a
number of substrates, including conjugated systems and unfunc-
tionalized molecules. Activity depends on solvent due to competi-
tive adsorption effects on the catalyst surface, as confirmed
experimentally. Using cyclohexane as a solvent the activity for
DPA hydrogenation is 1 mol (DPA) molÀ1 (NaAlH4) hÀ1 at 150 °C.
In contrast to typically used Pd catalysts the selectivity is 12 times
higher in favor of the cis-stilbene intermediate at low catalyst
loadings.
Additionally, we have shown that hydrogenation can occur via
direct hydrogen extraction from the NaAlH4 lattice. When the cat-
alyst is partially dehydrogenated before addition of hydrogen gas
pressure, selectivity is 5 times higher toward cis-stilbene than
when hydrogen pressure is applied at the beginning of the reac-
tion. As this new type of catalyst is based solely on light and abun-
dant elements and does not contain any transition metals, yet can
still display high selectivity toward the cis-alkene, it promises a
new avenue in the development of catalysts for selective
hydrogenation.
Fig. 9. Hydrogenation of DPA in the absence of gaseous hydrogen until 1400 min
where hydrogen pressure was applied. The reaction was performed in cyclohexane
at 150 °C with 80 bar Ar pressure initially before switching to 100 bar H2 pressure. A
DPA:NaAlH4 molar ratio of 5:1 was used.
the catalyst was tested in the absence of gaseous hydrogen (Fig. 9).
Hydrogenation occurs in the absence of gaseous hydrogen. This
demonstrates that hydrogenation can proceed via extraction of
hydrogen from the NaAlH4 lattice. However, the rate of hydrogena-
tion is low; reaching less than 10% conversion of DPA in 24 h,
which corresponds to the consumption of 18% of the hydrogen in
Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.

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135
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The Project has been conceived by Petra de Jongh. Experimental
work was carried out by Peter Bramwell, Jinbao Gao and Bernd de
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The authors declare no competing financial interests.
Acknowledgment
The authors thank the financial support of the Netherlands
Organisation for Scientific Research (NWO-ECHO grant). The
authors would also like to thank Rien van Zwienen and Ad Mens
for technical support and Dr. Michael Felderhoff of the Max-
Planck-Institut für Kohlenforschung for the stimulating
discussions.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.09.024.
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