Selective reduction of phenylacetylene with AlMgO particles used as an alternative water-reactive generator of hydrogen

Article abstract of DOI:10.1007/s10562-013-1019-1

A new inorganic material was used to produce hydrogen in situ by contact with water for reduction of unsaturated hydrocarbons. Phenylacetylene reduction into styrene and ethylbenzene was chosen as a model reduction reaction. We show that a fine control over the reaction yield and selectivity can be obtained by controlling the hydrogen release and such parameters of the synthesis as the temperature, water content and the speed of stirring. We found that the largest amount of water at 80 C and vigorous stirring result in the highest yield and selectivity of the reaction. An alternative way of reaction control is suggested as compared to a conventional way of a catalyst inhibition. The new possibility of the reaction control may be of a wide interest for the laboratory synthesis for a good number of hydrogenation reactions.

Full text of DOI:10.1007/s10562-013-1019-1

Catal Lett (2013) 143:739–747
DOI 10.1007/s10562-013-1019-1
Selective Reduction of Phenylacetylene with AlMgO Particles used
as an Alternative Water-Reactive Generator of Hydrogen
Anna Kozina
•
Jose L. Iturbe Ignacio A. Rivero
•
Received: 14 December 2012 / Accepted: 29 April 2013 / Published online: 14 May 2013
Ó Springer Science+Business Media New York 2013
Abstract A new inorganic material was used to produce
hydrogen in situ by contact with water for reduction of
unsaturated hydrocarbons. Phenylacetylene reduction into
styrene and ethylbenzene was chosen as a model reduction
reaction. We show that a fine control over the reaction
yield and selectivity can be obtained by controlling the
hydrogen release and such parameters of the synthesis as
the temperature, water content and the speed of stirring.
We found that the largest amount of water at 80 °C and
vigorous stirring result in the highest yield and selectivity
of the reaction. An alternative way of reaction control is
suggested as compared to a conventional way of a catalyst
inhibition. The new possibility of the reaction control may
be of a wide interest for the laboratory synthesis for a good
number of hydrogenation reactions.
1 Introduction
The hydrogenation (reduction) of unsaturated organic
compounds is a widely used chemical reaction for many
industrial applications in such as petro-, food and health
industry. The hydrogenation of alkynes is an important
industrial process to obtain alkenes and alkanes. The
interest in reduction of phenylacetylene (PA) has its origin
in the reaction products styrene (ST) and ethylbenzene
(EB). Styrene is an extremely important monomer in
numerous polymerization reactions, therefore, its produc-
tion from phenylacetylene is important both from scientific
and industrial standpoints. However, the styrene production
is not a trivial task, since the hydrogenation reaction
(Eq. 1) usually does not stop on the first stage and results in
a mixture of styrene with ethylbenzene or even proceeds
completely to ethylbenzene alone.
Keywords Hydrogenation ꢀ AlMgO particles ꢀ
Hydrogen source ꢀ Phenylacetylene
Ph--CꢁCH!Ph--C=CH2!Ph--CH2CH3
ð1Þ
To obtain styrene as a main product the reaction must be
thoroughly controlled by tuning the reaction conditions.
Therefore, PA partial reduction is a convenient model to
evaluate the process design, performance of catalyst or to
optimize reaction conditions. There are two main
conditions to be fulfilled in any typical hydrogenation
reaction: (i) the presence of a catalyst (usually platinum
group metals) and (ii) the hydrogen source. Also, there are
at least two ways to carry out the hydrogenation: first,
hydrogen may be added to the reaction in a gaseous form as
a batch (from a balloon under atmospheric pressure or a
built hydrogen source under high pressure and/or
temperature) or a dilute steam of reactant mixed with
hydrogen is made to flow above or through the catalyst
A. Kozina ꢀ J. L. Iturbe ꢀ I. A. Rivero
Departamento de Qu ´ı mica, Instituto Nacional de Investigaciones
Nucleares, 52750 Ocoyoacac, Estado de M e´ xico, Mexico
e-mail: anna.kozina@fisica.unam.mx
Present Address:
A. Kozina
Instituto de F ´ı sica, Universidad Aut o´ noma Nacional de M e´ xico,
CU, 04510 Mexico, DF, Mexico
Present Address:
I. A. Rivero (&)
Centro de Graduados e Investigaci o´ n en Qu ´ı mica, Instituto
Tecnol o´ gico de Tijuana, 22000 Tijuana, Mexico
e-mail: irivero@tectijuana.mx
(flow hydrogenation) [1]. Thus, there are mainly two ways
to gain control over the process rate—by decreasing the
123

7
40
A. Kozina et al.
activity of a catalyst or slowing down the hydrogen
income.
reactions may be used to produce hydrogen for further
usage as a fuel or as a reduction agent in the reaction
mixture. In fact, aluminium nanoparticles currently raised
some interest in the scientific community because of their
application for production of hydrogen [15–18]. However,
there are two problems that one may face with such
systems: first, the stable rate of reaction must be achieved
and, second, passivating oxide coating formed on the
particle surface on contact with air must be possibly
avoided to obtain the highest metal conversion. There are
different approaches that are commonly used to overcome
the mentioned drawbacks. One of the approaches is the
application of more extreme conditions of water/
aluminium reaction such as high temperature or high pH
[19, 20]. Another way is to modify the surface of
aluminium with metal oxides, such as gamma-Al O [21,
The first approach is more widely used and there is a
good number of works dedicated to selective hydrogena-
tion of unsaturated hydrocarbons. While the field is still
quite open to research, there are a few directions that are
worth mentioning. Usually, the catalyst is partially deac-
tivated by selective metal site poisoning with certain
additives such as lead or quinoline [2]. Also, different
supports such as MCM-41, alumina, zeolites, pumice, hy-
drotalcite, among others, are used instead of conventional
activated carbon to decrease the catalyst activity [3–7].
Compounds containing Os, Ru and Rh is another approach
[
[
8–10]. Finally, bimetallic particles Ni/Pd, Fe/Pd, Mg/Pd
11] or gold nanoparticles [12, 13] proved to work selec-
tively for hydrogenation of alkynes, although the search for
more effective catalyst is still ongoing.
2
3
22]. In this case it is possible to obtain a continuous
reaction with water and to generate hydrogen at room
temperature under atmospheric pressure. In the third
approach the high hydrogen release rates are achieved by
mixing aluminium with other metals such as Ga, In, Sn or
Zn [23–25], which prevents the formation of aluminium
oxide. The mentioned approaches allow creation of such
materials that would allow the total metal conversion with
a constant reaction rate. The control of the hydrogen
synthesis may be then gained mainly by controlling
reaction temperature and water content.
The second approach in the reduction control is to allow a
very slow hydrogen flow because its quantity needed for
reaction is extremely small. Thus, a very slow controlled
hydrogen release may allow one to obtain the whole variety
of the products with a desirable ratio. However, if a con-
ventional hydrogen source is used (tank or a balloon), the
control of hydrogen supply is incredibly difficult to achieve.
Even the dilution of hydrogen with an inert gas (argon or
nitrogen) would bring a lot of technical problems in the flow
control and, in practice, cannot be considered as a good
alternative. Moreover, the operation of such processes often
requires special high-pressure resistant reactors used with
extra safety precautions and effective heat removal. There-
fore, an ‘in-situ’ slow hydrogen production performed under
mild conditions (a standard vial under atmospheric pressure,
slightly high temperature and a moderate stirring) might be
the right solution of the problem.
In the present study we use the material that consists of
Al mixed with MgO, following the second (Al/metal oxide)
approach. Al mixed with MgO in the presence of water can
serve as a controllable portable source of hydrogen for fine
laboratory syntheses. We present an elaborate study of
controlled hydrogenation of phenylacetylene using this
new material as a water-reactive source of hydrogen. We
vary reaction parameters to carry out a systematic inves-
tigation of their influence on the relative amount of each
hydrogenation product. We optimized the reaction param-
eters to obtain the largest reaction conversion or the largest
yield of styrene.
It is well-known that a metal reaction with water results
in a formation of a metal product and hydrogen. For
example, aluminium can react with water in the following
ways [14]:
2
2
2
Al + 6H2O = 2Al(OH Þ + 3H2
Al + 4H2O = 2AlO(OH) + 3H2
ð2Þ
ð3Þ
ð4Þ
3
2
Experimental
Al + 3H2O = Al2O3 + 3H2
The first possible reaction product is Al(OH) (bayerite).
3
2.1 Synthesis and Characterization of AlMgO material
The second one is AlO(OH) (boehmite). The third product
is Al O (alumina). These reaction products differ in their
2
3
The water-reactive material was synthesized as follows:
first, Mg/Al alloy (70:30 by weight) was obtained in a
tubular furnace. The alloy was cooled down and milled.
Then, the powder was placed in a furnace and heated in the
temperature range of 250–600 °C with a rate of 10 °C/min.
The system was maintained under atmospheric pressure
with the access of air during 1.5 h. As a result, the material
with mixed Al and MgO was obtained.
level of hydration, however, all three of them produce the
same amount of hydrogen with respect to the amount of
aluminium reacted. However, they differ in the amount of
water that is required for the reaction. These reactions are
all thermodynamically favorable over the wide temperature
range from room temperature to temperatures far in excess
of the melting point of aluminium (660 °C). Thus, these
1
23

Reduction of Phenylacetylene with AlMgO Particles
741
Energy dispersive X-ray (EDS) analysis was used for the
elemental analysis of the selected area observed under
scanning electron microscope (SEM) (Jeol JSM 5600, Jeol
Ltd., Japan). The structural characterization was carried out
with the powder X-ray diffraction (XRD) in a Siemens
D5000 diffractometer with Cu Ka radiation (k = 1.5,416
3 Results
3.1 Material Characterization
The elemental composition of the obtained material is the
following by weight: O, 26.97; Mg, 43.72 and Al, 29.31.
According to the analysis by SEM (at 60000X), small
clusters were observed at the range of 300 nm that are
formed by much smaller particles with a very broad par-
ticle size distribution(polydispersity index is about
30–40 %) .
˚
A) operated at 35 kV and 25 mA. The samples were reg-
istered between 5 and 90° (2h) with a step time of 1 s and a
step size of 0.03°.
2
.2 Reaction Scheme
To verify the presence of aluminium and magnesium
oxide, the material was characterized by X-ray diffraction.
Figure 1 shows representative XRD patterns. The 2h angles
and intensities of the peaks in the pattern are in a good
agreement with the standard patterns for pure aluminium
(JCPDS 4-787) and MgO crystalline phases (JCPDS 7-239).
It is worth to remark on the stability of the material,
which consists of two compounds: Al and MgO. The oxide
is stable in the air, the metal Al is oxidized by the oxygen
forming the oxide layer Al O but the oxidation process is
According to the standard measurements performed with the
home-made mass flow meter, 10 mg of the material in excess
of water (20 mL or 2,200 times more than it is stechiometri-
cally necessary and in the presence of NaOH to speed up and
-4
complete the reaction) release 3.6 mL (1.5 9 10 mol) of
hydrogen. Taking into account the reaction of phenylacety-
lene (98 %, Sigma–Aldrich) and 10 % losses (that may not
react because the gas may escape from the reactant solution or
even the vial), we estimated the concentration of the PA
solution that should be, in principle, reduced completely by
the amount of hydrogen released by 10 mg of the material.
Thus, the PA concentration was fixed along all the experi-
ments and the solution of CPA = 0.435 mg/mL in methanol
2
3
very slow (days to months depending on the way of stor-
age). Usually it is stored under nitrogen atmosphere to avoid
oxidation and is used within the days after preparation.
(
99.8 %, Sigma–Aldrich) was prepared. To carry out the
3.2 Influence of Reaction Conditions on Product
Outcome
reaction, a certain amount of the metal particles and 5 mg of
Pd/C (10 wt % loading, Sigma–Aldrich, cat. number 205699)
catalyst were weighed out into a vial with a magnetic stirrer.
Then, 10 mL of the PA solution was added. Then, a certain
amount of water was added and the vial was sealed immedi-
ately. After that, the vialwas placed into a thermostatted water
cell and the stirrer was switched on. The maximum amount of
water that can be added to the reaction mixture without pH
change to basic (8.2–12.0) was defined by simple titration of
First of all, we conducted the reaction the same as Exp 1 in
Table 1 at 18 °C but without the catalyst Pd/C to check a
possible catalytic activity of AlMgO material. However, no
catalytic activity was observed because after 24 h of stirring
no other products than PA were found. Thus, and Pd/C cat-
alyst was then constantly used. Then, we used a conventional
way of reaction with the hydrogen source from a pressure
tank without AlMgO material. The conditions were chosen
the same as in Exp 1 at 18 °C. The reaction was conducted in
a pressure resistant reactor, where all the reactants (except
AlMgO) were mixed. Then, the hydrogen from a hydrogen
tank was added until the gas pressure reaches of about 3 atm.
The mixture was left without stirring for 1 h and then was
analyzed with the mass-spectrometer. In this case the reac-
tion proceeded very rapidly and in 1 h 100 % of phenyl-
acetylene was reduced to ethylbenzene. This experiment
shows that when the rate of reaction is high, it is extremely
difficult to control the yield of intermediate products.
Therefore, it is necessary to slow down the reaction to obtain
finer control over the process, which we suggest may be done
by the slow hydrogen input. Therefore, we fixed all the other
variables to the following values: mAlMgO = 10 mg,
VPAsol = 10 mL, Vwater = 10 mL, low stirring, and varied
the reaction temperature between 18 and 80 °C, which cor-
responds to Exp 1 in Table 1. Figure 2 shows the relative
1
0 mg of the material in 10 mL of methanol in the presence of
phenolphthalein and turned out to be 14 mL. Thus, the largest
amountofwaterusedforthereactionwastakenas10 mL. The
following parameters of reaction conditions were varied:
temperature, water content, stirring speed and the amount of
the inorganic material. The catalyst (Pd/C) amount of 5 mg
was always fixed. All the experiments with the synthesis
variables are summarized in Table 1.
The reaction was left during 24 h and every certain time
a sample of 1 mL was taken from a vial. The samples were
then analyzed with a direct injection gas chromatograph
coupled to a mass-spectrometer (Agilent 6,890 N Network
Gas Chromatograph and Agilent 5973 Network Mass
Selective Detector, Agilent Technologies, USA). A relative
percentage of each substance in the mixture was defined by
integration of the area below each peak in the chromato-
grams. Each experiment was repeated twice and the aver-
age was taken.
123

7
42
A. Kozina et al.
Table 1 Summary of experiments performed with the fixed and varied parameters of the synthesis
Experiment
C
PA
V
(mL)
PA/Methanol
m
(mg)
pd/c
V
water
(mL)
T (°C)
Stiring
(rpm)
m
AlMgO
(mg)
(mg/mL)
Exp 1
Exp 2
Exp 3
Exp 4
0.435
0.435
0.435
0.435
10
10
10
10
5
5
5
5
10
Variation 18, 40, 80
540
10
Variation 3, 6, 10
80
80
80
540
10
10
10
Variation 440, 990
990
10
Variation 10, 15, 20
Fig. 1 XRD pattern of AlMgO with the peaks corresponding to pure
aluminium (blue) and MgO (red) crystalline phases
amounts of all the reactants for each temperature. The error
bars for this and the following results are between 2 and 6 %.
As one can see from Fig. 2, the most significant feature
of all the plots is that already at 18 °C both possible
products can be obtained. This implies that Al actually
reacts with water and hydrogen is produced. Moreover, the
fact that the reduction does not proceed rapidly and
quantitatively to ethylbenzene suggests that the reaction is
actually controlled by the release of hydrogen. This is one
of the most important characteristics of the used material:
no catalyst inhibition is necessary to control the reaction
outcome. The second important result is that with the
increase of temperature the relative amount of each product
changes, as well as the total percentage of reduction. Thus,
the PA reduction increases from 40 to about 60 % on
increase of temperature up to 80 °C. At the same time, the
production of styrene increases from 15 to 50 % with
heating, so that at 80 °C styrene becomes the main reaction
product. This opens up a unique possibility of the reaction
control. Reaction conversion and selectivity for styrene for
each temperature are summarized in Table 2.
Fig. 2 Relative amounts of each reactant as a function of time for a
given temperature: black circles are phenylacetylene, light gray
squares are styrene and gray diamonds are ethylbenzene. The reaction
temperature is given in the legend. Solid lines of corresponding colors
are the fits of the first-order reaction kinetic equations for PA
content. Reaction conversion and selectivity for styrene for
each water volume are summarized in Table 3.
The total reduction of PA increases with the increase of
water content from 41.1 up to 72.7 % for 10 mL of water.
This results from the larger amount of released hydrogen
with the larger water content. Interestingly, if with 3 mL of
water both reaction products (ST and EB) form simulta-
neously almost at the same ratio, with 6 mL of water the
production of styrene is favored, while with 10 mL the
outcome of the both products increases in the ratio similar to
Vwater = 6 mL. Thus, the increase in hydrogen production
results in the increase of the total PA reduction Fig. 3.
The next parameter we varied was the intensity of stir-
To evaluate the effect of water, we fixed all the
parameters to mAlMgO = 10 mg, VPAsol = 10 mL, low
stirring, T = 80 °C, and varied the volume of added water
between 3 and 10 mL (Exp 2 in Table 1). Figure 3 shows
the relative amounts of all the reactants for each water
ring. The fixed variables are mAlMgO = 10 mg, VPAsol
=
10 mL, Vwater = 10 mL, T = 80 °C, and the intensity of
stirring was varied as follows: low stirring (LS) corre-
sponds to 540 rpm and fast stirring (FS) to 990 rpm of a
1
23

Reduction of Phenylacetylene with AlMgO Particles
743
Table 2 Conversion of PA XPA and reaction selectivity for styrene
S
ST at different temperatures as indicated
PA (%) ST (%) PA (%) ST (%)
.5
X
S
X
S
XPA (%)
SST (%)
6
100.0
67.2
48.6
51.2
33.9
27.6
37.6
45.5
48.3
54.5
37.6
30.8
27.1
25.3
31.3
58.8
64.2
65.0
67.6
72.7
75.0
73.8
72.2
72.0
70.6
1
2
2
3
6.1
3.6
8.4
9.4
Fig. 4 Relative amounts of each reactant as a function of time for a
given intensity of stirring: (a) low stirring (540 rpm) and (b) fast
stirring (990 rpm); black circles are phenylacetylene, light gray
squares are styrene and gray diamonds are ethylbenzene. Solid lines
of corresponding colors are the guides for the eye
Table 4 Conversion of PA XPA and reaction selectivity for styrene
SST at different intensities of stirring as indicated
X
PA (%) ST (%) PA (%)
S
X
SST (%)
990 rpm
5
40 rpm
540 rpm
990 rpm
Fig. 3 Relative amounts of each reactant as a function of time for a
given volume of added water: black circles are phenylacetylene, light
gray squares are styrene and gray diamonds are ethylbenzene. The
ratio water/PA solution is given in the legend. Solid lines of
corresponding colors are the guides for the eye
5
6
8.8
4.2
75.0
73.8
72.2
72.0
70.6
43.3
51.0
55.1
59.6
67.5
100.0
93.1
94.9
91.9
96.6
65.0
6
7
7.6
2.7
Table 3 Conversion of PA XPA and reaction selectivity for styrene
SST at different water/PA solution ratios as indicated
X
PA (%) ST (%) PA (%) ST (%) PA (%)
S
X
S
X
SST (%)
different: on more vigorous stirring the amount of styrene is
much larger than that of ethylbenzene (Fig. 4b). Also, the
selectivity for styrene increases significantly at 990 rpm.
This suggests that a liquid–solid mass transfer may also be
responsible for the reaction selectivity, together with the
other parameters mentioned above. Possible reasons for this
selectivity will be discussed below.
2
3
3
3
4
7.4
1.9
3.0
4.6
1.1
33.1
33.8
40.6
43.7
39.7
32.3
35.5
35.0
36.7
46.9
75.5
78.6
81.8
79.3
77.5
58.8
64.2
65.0
67.6
72.7
75.0
73.8
72.2
72.0
70.6
Finally, taking into account the influence of all the
variables, we conducted the synthesis with optimal
parameters with the intention to obtain the largest yield
and, possibly, selectivity of the controlled reduction. Thus,
tiny magnetic stirrer (Exp 3 in Table 1). Figure 4 and
Table 4 summarize the results.
Figure 4 shows that the total reduction of PA is practically
the same for the both cases: it stays at about 70 %. This
implies that the reaction reduction yield is not sensitive to
stirring. However, the yields of each product are quite
we used the following parameters: VPAsol = 10 mL, Vwater
=
10 mL, T = 80 °C, fast stirring of 990 rpm, and we varied
the amount of AlMgO material between 10 and 20 mg to
123

7
44
A. Kozina et al.
on the catalyst surface, (iii) hydrogenation step,
(iv) product desorption and (v) product diffusion from the
surface. The first and the last steps may be affected by the
concentration of the reactants in bulk (concentration gra-
dient between the bulk and the catalyst), the molecule size
and structure (especially for small pores) and the temper-
ature. The second and the fourth steps are mainly governed
by the molecular structure and the temperature. Thus, as
one can see, the heterogeneous catalytic hydrogenation is a
rather complex process, where the contribution of all the
factors matters. In our case, the additional complicity is
added by the hydrogen production in situ, which also may
be affected by the varied reaction parameters (Table 1).
The general mechanism of phenylacetylene reduction
may be described by the following scheme (Fig. 6)
[
26–28]. It is proposed that phenylacetylene reduction
proceeds by two routes: either first PA is reduced to ST (1)
and then ST is reduced to EB (2), or the reduction of PA
proceeds directly to EB (3) [5]. The latter case occurs with
the formation of alkylidene that occupies three active cat-
alyst sites and then three hydrogen atoms are attached
simultaneously. Since in our case the concentrations of
phenylacetylene and hydrogen are very low (1:4,166 by
volume) and the catalyst particles are quite small
Fig. 5 Relative amounts of each reactant as a function of time for a
quantity of the metal oxide: (a) 10 mg and (b) 15 mg; black circles
are phenylacetylene, light gray squares are styrene and gray
diamonds are ethylbenzene. Solid lines of corresponding colors are
the guides for the eye. Addition of 20 mg of the material resulted in
1
00 % EB after 1 h
(2.5–5 nm), the probability of such a direct hydrogenation
produce more hydrogen in the reaction mixture (Exp 4 in
Table 1). The results are shown in Fig. 5.
is negligible as compared to the first route, therefore, we
will neglect this route in the further discussion. Low initial
concentrations of the reactants and a limited hydrogen
transfer also explain the first order of the reaction (Fig. 2),
which is different from zero-order PA hydrogenation in
excess of hydrogen reported previously [5, 11, 29, 34].
Since there is no equilibrium for hydrogenation of
phenylacetylene, the reaction proceeds quantitatively.
However, we do not obtain the total 100 % conversion of
PA even at high temperature and after 24 h of reaction
time, as it is shown in Table 2. This implies that it is not
enough of released hydrogen to reduce all of PA. There are
at least three possible reasons for that: (i) the total release
of hydrogen is not complete after 24 h; (ii) the losses of
hydrogen for the reduction are more than 10 % as we
assumed in the beginning; (iii) there is a mass transfer
limitation for hydrogen that depends on temperature. The
fact that PA conversion grows with the increase of tem-
perature indicates that the hydrogen availability for the
reaction also increases on heating. Deng et al. [22] con-
ducted an experiment showing that the hydrogen release
As it is clear from Fig. 5, on addition of 15 mg of the
inorganic material, the reduction on phenylacetylene is
1
00 %, while with 10 mg it is 70 %. Also, after 1 h of
reaction with 15 mg of AlMgO (Fig. 5b), the yield of
styrene is about 70 %, which is the maximum we could
obtain as compared with all the other reaction conditions.
However, the yield of ethylbenzene also increases signifi-
cantly with more hydrogen present. It is curious that when
all phenylacetylene is used, the production of ethylbenzene
occurs at the expense of styrene. Thus, if styrene is the
preferable product, one can stop or slow down the reaction
after 1 h by separation of the catalyst and AlMgO material
from the reactant mixture. Interestingly, on addition of
2
0 mg of AlMgO, all phenyalcetylene was completely
reduced to ethylbenzene already after 1 h. This implies that
a slightly larger amount of hydrogen significantly accel-
erates the reaction, which makes it quite difficult to detect
an intermediate product. This is often observed when a
conventional hydrogen source is used.
4
Discussion
As it is well known, there are five main steps in a hetero-
geneously catalyzed hydrogenation reaction: (i) diffusion
of reactants to the catalyst surface, (ii) reactants adsorption
Fig. 6 Hydrogenation scheme
1
23

Reduction of Phenylacetylene with AlMgO Particles
745
rate from the reaction of Al with water grows with the
temperature increase, which is in an agreement with our
observation. Another factor is the mass transfer limitation.
Since the stirring is fixed to 540 rpm for all the experi-
ments of series Exp 1, the effect of stirring on the mass
transfer may be considered the same for all the tempera-
tures. However, the solubility of hydrogen in methanol
grows with temperature [30], which should increase its
availability for the reaction. The contribution of these two
factors results in the larger PA conversion at high
temperatures.
heterogeneously Pd catalyzed hydrogenation, in order to
react, both hydrogen and the reactants (in this case PA and
styrene) should first adsorb to the metal active sites.
Hydrogen molecules are small and it is the easiest for them
to be adsorbed. Because of the benzene ring and the car-
bon–carbon triple bond PA has a flat structure, allowing it
to adsorb parallel to the metal surface and expose its triple
bond for the reaction. Styrene has the carbon–carbon
double bond, which makes its structure more bulky (the
benzene ring protruding above the double bond), which
hinders its adsorption. Thus, there is always a competition
between all the reactants for the catalyst active sites. The
production of larger amount of hydrogen results in the
stronger competition of its molecules with PA and ST.
The increase of selectivity with the increase of tem-
perature observed in Fig. 2 may be explained in terms of
competition in adsorption of PA and ST on Pd/C catalyst. It
is known that alkynes have a higher affinity to the carbon
surface [31–33, 35] and are less thermodynamically stable
than alkenes, since the heats of hydrogenation of alkynes
are greater than twice the heats of hydrogenation of anal-
ogous alkenes [36, 37]. Therefore, PA has a higher prob-
ability to adsorb on the catalyst and be hydrogenized. At
low temperature, the desorption of styrene is not that
pronounced as at higher temperature, thus, when it is
formed from PA, it stays longer on the catalyst surface and
its probability of being reduced to EB is higher. This
results in a very low selectivity towards styrene at 18 °C.
With increase of temperature, the selectivity towards sty-
rene increases because the desorption of styrene is pro-
moted [38] and after formation it diffuses away from the
catalyst rather than stays for the further reduction. At
intermediate temperatures (40 °C), apparently, there is a
competition of the rate of hydrogen production and transfer
with the desorption of styrene: more hydrogen is available
at 40 °C than at 18 °C but less styrene is desorbed than at
Among all the three components (H , PA and ST), styrene
2
looses the competition for structural reasons mentioned
above, allowing more adsorption of PA and hydrogen. This
produces more styrene in the reaction as compared to
ethylbenzene. At the largest water content of 10 mL, the
amount of released hydrogen is the largest and we specu-
late that the probability of styrene to react also grows just
because more PA molecules convert to styrene and the
competition between PA and ST might decrease. That is
reflected in a slightly lower selectivity for styrene at 10 mL
of water (Table 3).
It is worth to notice that the amounts of water used in the
experiments are far above its stoichiometrical equivalent,
which was calculated equal to about 9 mg (0.009 mL).
Theoretically, this amount of water should be sufficient to
react with all Al in the material. However, in reality this
amount is too small to even start the reaction, thus, an
excess of water is required. Also, as we see, even the
variation in the excess of water changes the reaction pro-
cess, which is the consequence of the hydrogen release. We
speculate that the hydrogen release from the AlMgO
material is a rather slow process and it may be governed by
the flow of water into the material pores and channels and
its diffusion inside them. One of the parameters that help to
reach the equilibrium of water diffusion inside the material
is the water concentration in the reactant mixture: more
water outside the material helps to increase its amount
inside and, therefore, release hydrogen faster. However, as
it was mentioned above, the hydrogen release may be not
complete even after 24 h even with the maximum amount
(10 mL) of water.
8
0 °C. This results in the larger overall PA conversion but
low reaction selectivity for styrene. Less competition
between ST and PA and faster hydrogen production at
80 °C results in larger total PA reduction and better
selectivity for styrene.
The activation energies obtained for hydrogenation
of phenylacetylene and styrene are 30.6 ± 0.8 and
1
7.9 ± 0.2 kJ/mol, respectively. This is in a very good
agreement with the previous studies where the activation
energy of PA is in the range of 22.3–33.5 kJ/mol and that
of ST is between 21.2 and 45.0 kJ/mol [29, 34, 39–42].
The increase of the amount of water or AlMgO material
in the reaction mixture besides the increase of the total PA
reduction also shifts the selectivity towards styrene going
from 3 to 6 mL of water (Fig. 3, Table 3). Further increase
of water (10 mL), however, slightly lowers the selectivity,
though the conversion grows. The governing parameter of
these experiments is the hydrogen concentration, which
grows with larger amount of water. Higher hydrogen
availability results in the larger PA conversion. In
The increase of the amount of AlMgO material in the
reaction mixture significantly increases the reaction rate
and PA conversion: if with 10 mg only 67.5 % of PA is
reduced by 24 h of reaction time, with 15 mg 100 % of PA
is reduced by 1 h giving both ST and EB and with 20 mg
100 % conversion yields only EB by 1 h (Fig. 5). Because
of such a fast reduction, the selectivity cannot be calculated
in the last two cases. Since the stirring is fixed to 990 rpm
123

7
46
A. Kozina et al.
for all the three experiments, we can conclude that the
hydrogen mass transfer connected with stirring does not
limit the reaction, otherwise the conversion would not
change with increase of hydrogen input to the system.
Moreover, the experiment at 540 rpm (Table 4) resulted in
a similar total PA conversion, which allows us to conclude
that in the used velocities of stirring the mass transfer is not
limited by agitation. Thus, the only limiting factor is the
concentration of hydrogen in the mixture. Obviously, the
increase of the amount of AlMgO in the stoichiometrical
excess of water as compared to Al provides more hydrogen
and in the higher rate, which results in the observed growth
of reaction rate and conversion.
initial conditions these effects can be neglected. The last
two factors are connected with the PA and ST molecules
behavior. Since the molecule effective size is comparable
for these two components, we can conclude that the dif-
fusion in the large catalyst pores should not be affected by
the type of the molecule (since the temperature is the same
for both of them). However, the adsorption—desorption of
these molecules may differ significantly. Moderate stirring
may promote adsorption, whereas on very vigorous stirring
adsorption is hindered and desorption promoted. Since
styrene has less affinity to the Pd/C surface, a vigorous
stirring decreases its adsorption even stronger. Also, sty-
rene just formed from PA desorbs more easily from the
surface on fast stirring moving away in the flow. As a
result, significantly more styrene stays in the bulk reaction
mixture and not on the catalyst surface when it is vigor-
ously stirred resulting in the higher selectivity for styrene at
990 rpm.
There are several processes occurring during the reaction
that may be affected by stirring. We will discuss them one by
one. First, the production of hydrogen from AlMgO material
and water may be affected. Therefore, first, we measured the
profile of hydrogen production as a function of stirring.
Figure 7 shows the profiles of released hydrogen from
1
00 mg of AlMgO in the presence of 3 mL of water.
We found that within the experimental error the profiles
5 Conclusions
overlap for 540 and 990 rpm. Thus, we conclude that if all
the other reaction variables are fixed, the same amount of
hydrogen is released in the case of both stirring velocities
and this factor may be excluded.
The reduction reactions of phenylacetylene, which is a
good model to evaluate new catalysts in the reduction of
organic materials, were conducted using an inorganic
AlMgO material as a water reactive generator of hydrogen.
We demonstrated that a slow and controlled input of
hydrogen in situ is an alternative way of selective hydro-
genation as compared to a conventional catalyst modifi-
cation. We varied reaction conditions, such as the
temperature, the amount of water and material, and the
speed of stirring, to investigate their influence on the
reaction conversion and selectivity. We suggest that this
water reactive hydrogen generating material has a wide
potential laboratory application.
When the hydrogen is released from the material its
concentration in the bulk may be affected because the
vigorous stirring may favor hydrogen desorption from the
solution and its escape from the vial. This effect was not
quantified but it should be kept in mind. Also, the diffusion
of hydrogen into the catalyst pores towards the Pd active
sites may be affected by stirring. However, the same total
PA conversion (Fig. 4) indicates that if these effects take
place, their influence is the same independently from the
velocity of stirring (at least in the mentioned rpm interval).
Thus, we have an indirect evidence that under the same
Acknowledgments The financial support of the Mexican National
Council of Science and Technology (CONACyT) is highly
appreciated.
References
1
2
3
. Irfan M, Glasnov TN, Kappe CO (2011) Chem Sus Chem 4:300
. Lindlar H, Dubuis R (1973) Org Synth 5:880
. Na-Chiangmai C, Tiengchad N, Kittisakmontree P, Mekasu-
wandumrong O, Powell J, Panpranot J (2011) Catal Lett 141:
1
149
4
. Dom ´ı nguez-Dom ´ı nguez S, Berenguer-Murcia A, Linares-Solano
A, Cazorla-Amor o´ s D (2008) J Catal 257:87
5
6
7
. Duca D, Liotta LF, Deganello G (1995) Catal Today 24:15
. Papp A, Moln a´ r A, Mastalir A (2005) Appl Cat A 289:256
. Mastalir A, Kiraly Z (2003) J Cat 220:372
Fig. 7 The volume of the released hydrogen from the reaction of
1
8. Andriollo A, Esteruelas MA, Meyer U, Oro LA, Sanchez-
Delgado RA, Sola E, Valero C, Werner H (1989) J Am Chem Soc
111:7431
00 mg of AlMgO with 3 mL of water as a function of time for
different stirring velocities: black circles for 540 rpm and light gray
diamonds for 990 rpm. The error bars are also given
9. Derien S, Dixneuf PH (2004) J Organomet Chem 689:1382
1
23

Reduction of Phenylacetylene with AlMgO Particles
747
1
0. Leitmannova E, Svoboda J, Sedl a´ ek J, Vohl ´ı dal J, Kaer P, erven y´
L (2010) Appl Cat A 373:34
25. Parmuzina AV, Kravchenko OV, Bulychev BM, Shkol’nikov EI,
Burlakova AG (2009) Russ Chem Bull 58:493
26. S a´ rk a´ ny A, Weiss AH, Guczi L (1986) J Catal 98:550
27. Anderson JR (1975) Structure of Metallic Catalysts. Academic
Press, London, p 296
1
1. Dominguez–Dominguez S, Berenguer-Murcia A, Cazorla-
Amoros D, Linares-Solano A (2006) J Catal 243:74
2. Nikolaev SA, Smirnov VV (2009) Gold Bull 42:182
3. Nikolaev SA, Permyakov NA, Smirnov VV, Vasil’kov AYu,
Lanin SN (2010) Kinet Catal 51:288
1
1
28. Al-Ammar AS, Webb G (1979) J Chem Soc Faraday Trans
l(75):1900
1
1
4. J. Petrovic, G. Thomas (2008) U.S. Department of Energy, ver-
sion 1.0
29. Jackson SD, Shaw LA (1996) Appl Catal A 134:91
30. Descamps C, Coquelet C, Bouallou C, Richon D (2005) Ther-
mochim Acta 430:1
31. Webb G (1978) In: Bamford CH, Tipper CFH (eds) Compre-
hensive Chemical Kinetic, 21st edn. Elsevier, Amsterdam, p 1
32. Webb G (1990) Catal Today 7:139
33. Jackson SD, Hardy H, Kelly GJ, Shaw LA (1997) In: Blaser HU,
Baiker A, Prins R (eds) Heterogeneous catalysis and fine chem-
icals iv. Elsevier, Amsterdam
34. Hardacre C, Mullan EA, Rooney DW, Thompson JM, Yablonsky
GS (2006) Chem Eng Sci 61:6995
5. M. N. Larichev, A. N. Jigatch, I. O. Leipunsky, M. L. Kuskov, A.
A. Pshechenkov (2003). In: Pros. of 9 International Workshop on
Combustion and Propulsion ‘Novel energetic materials and
applications’,14–18 Sept 2003. La Spezia, Italy, pp 14–1
6. Li Y, Song W, Xie C, Zeng D, Wang A, Hu M (2006) Mat Chem
Phys 97:127
1
1
1
7. Bunker CE, Smith MJ (2011) J Mater Chem 21:12173
8. Soler L, Macan a´ s J, Munoz M, Casado J (2007) J Pow Sour
1
69:144
1
9. Bunker BC, Nelson GC, Zavadil KR, Barbour JC, Wall FD,
Sullivan JP, Windisch CF Jr, Engelhardt MH, Baer DR (2002)
J Phys Chem B 106:4705
35. Tysoe WT, Nyberg GL, Lambert RM (1983) J Chem Soc Chem
Commun, 623
36. Rogers DW, McLafferty FJ (1971) Tetrahedron 27:3765
37. Abboud J-LM, Jimenez P, Roux MV, Turrion C, Lopez-Mardo-
mingo C, Podosenin A, Rogers DW, Liebman JF (1995) J Phys
Org Chem 8:15
38. Bartell FE, Thomas TL, Fu Y (1951) J Phys Chem 55:1456
39. Shutt E, Winterbottom JM (1971) Platinum Metals Rev 15:94
40. Terasawa M, Yamamoto H, Kaneda K, Imanaka T, Teranishi S
(1979) J Catal 57:315
41. Mironova LV, Belykh BL, Usova V, Shmidt FK (1985) Kinet
Katal 26:469
42. Chen SY, Smith JM, McCoy BJ (1987) Chem Eng Sci 42:293
2
2
2
2
2
0. Schoenitz M, Chen C-M, Dreizin EL (2009) J Phys Chem B
1
13:5136
1. Deng Z-Y, Liu Y-F, Tanaka Y, Ye J, Sakka Y (2005) J Am
Ceram Soc 88:977
2. Deng Z-Y, Ferreira JMF, Tanaka Y, Ye J (2007) J Am Ceram Soc
9
0:1521
3. Parmuzina AV, Kravchenko OV (2008) Int J Hydrogen Energy
3:3037
4. Kravchenko OV, Semenenko KN, Bulychev BM, Kalmykov KB
2005) J Alloys Compd 397:58
3
(
123