Using modifiers to specify stereochemistry and enhance selectivity and activity in palladium-catalysed, liquid phase hydrogenation of alkynes

Article abstract of DOI:10.1016/j.cattod.2010.10.012

Enhancing selectivity is a key parameter in green chemistry. In this study, we have examined the liquid phase hydrogenation of alkynes over a palladium catalyst and used modifiers to enhance selectivity and activity. The reactions studied were the hydrogenation of 1-pentyne and 2-pentyne. Five modifiers were used, pentane nitrile and its respective amine, pentyl amine, 3-phenyl propionitrile and its respective amine, 3-phenyl propylamine and trans-cinnamonitrile. These modifiers were not hydrogenated under reaction conditions. It was possible to obtain high (>90%) selectivities to 1-pentene and cis-2-pentene at high conversion. The effect on rate was dependent upon the modifier and the alkyne. The effect of the modifier was the same whether added with or before the reactants. Competitive reactions confirmed that terminal alkynes and internal alkynes are hydrogenated on separate sites and do not interfere and that the modifier influences each separately.

Full text of DOI:10.1016/j.cattod.2010.10.012

Catalysis Today 164 (2011) 548–551
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Using modifiers to specify stereochemistry and enhance selectivity and activity
in palladium-catalysed, liquid phase hydrogenation of alkynes
∗
Paloma E. Garcia, Ailsa S. Lynch, Andy Monaghan, S. David Jackson
Centre for Catalysis Research, WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, G12 8QQ, Scotland, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2010
Received in revised form
Enhancing selectivity is a key parameter in green chemistry. In this study, we have examined the liquid
phase hydrogenation of alkynes over a palladium catalyst and used modifiers to enhance selectivity and
activity. The reactions studied were the hydrogenation of 1-pentyne and 2-pentyne. Five modifiers were
used, pentane nitrile and its respective amine, pentyl amine, 3-phenyl propionitrile and its respective
amine, 3-phenyl propylamine and trans-cinnamonitrile. These modifiers were not hydrogenated under
reaction conditions. It was possible to obtain high (>90%) selectivities to 1-pentene and cis-2-pentene at
high conversion. The effect on rate was dependent upon the modifier and the alkyne. The effect of the
modifier was the same whether added with or before the reactants. Competitive reactions confirmed
that terminal alkynes and internal alkynes are hydrogenated on separate sites and do not interfere and
that the modifier influences each separately.
1
5 September 2010
Accepted 2 October 2010
Available online 30 October 2010
Keywords:
Hydrogenation
Palladium
Pentyne
Selectivity modifiers
Amines
© 2010 Elsevier B.V. All rights reserved.
Nitriles
1
. Introduction
The selective hydrogenation of alkynes to alkenes is an area
eral they are thought to compete for the surface in the same way as
CO in ethyne hydrogenation, inhibiting the secondary reaction of
the alkene. However the study by Moulijn and co-workers [13] indi-
cates that on its own this effect did not explain the role of quinoline
on alkyne hydrogenation. It has been suggested [12] that donation
of the lone pair on the nitrogen to the metal can potentially change
the electronic nature of the metal and hence change activity and
selectivity, and indeed work by Yu et al. has shown evidence for
this [15]. In this study, we have examined the role of nitrile and
amine modifiers with both terminal and internal alkynes, looking
at their effect on activity and selectivity.
of catalysis that has been active for over 100 years and yet our
understanding is not complete. Recent results [1–5] have revealed
the importance of sub-surface hydrogen and carbon in determin-
ing the activity and selectivity of palladium catalysts. While work
by Boitiaux et al. [6] showed that a particle size effect was in evi-
dence when highly dispersed catalysts were used. Once having
produced the alkene there are issues of isomerisation and sub-
sequent hydrogenation. In some very elegant work Zaera [7] and
references therein investigated alkene isomerisation and showed
over Pt that the shape of the crystallite, and hence the crystal face,
had a significant effect on trans–cis and cis–trans isomerisation
such that the rate of each reaction was different depending on the
starting isomer.
Modifiers are common in alkyne hydrogenation [8–10]. In
ethyne hydrogenation carbon monoxide is typically added to
inhibit ethane hydrogenation by what is normally considered a site
blocking mechanism with the CO being more strongly bound to the
surface than the ethane. Modifiers have also been used in the liq-
uid phase the liquid phase and nitrogen containing modifiers have
been reported in the literature [11–13] and in patents [14]. In gen-
2. Experimental
The catalyst used throughout this study was a 1% w/w
Pd/␪-alumina (Johnson Matthey, characterised by a BET area of
2
−1 −1
9
7.6 m g , a pore volume of 0.49 ml g , a metal loading of 1 wt.%
and a metal dispersion of 32.5%). All reactants were used with-
out further purification. The reaction was carried out in a 0.5l
Buchi stirred autoclave. 0.05 g of catalyst was added to 330 ml of
degassed solvent, hexane. Reduction of the catalyst was performed
3
−1
in situ by sparging the system with H2 (300 cm min ) for 30 min
at 313 K while stirring the contents of the autoclave at 800 rpm.
After reduction, the autoclave was adjusted to the appropriate
reaction temperature of between 298 and 333 K under a nitrogen
atmosphere. For both 1-pentyne and 2-pentyne, 1 ml was injected
into an unstirred solution, followed by 20 ml of degassed hexane
∗ Corresponding author. Tel.: +44 141 330 6867.
E-mail address: sdj@chem.gla.ac.uk (S.D. Jackson).
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.012

P.E. Garcia et al. / Catalysis Today 164 (2011) 548–551
549
Table 1
First order rate constants (×10− min ) for alkyne hydrogenation.
3
−1
Alkyne
No mod.
PA
3-PPA
PN
3-PPN
TCN
1
2
-Pentyne
-Pentyne
32.9 ± 0.6
18.8 ± 0.3
42.2 ± 1.1
9.8 ± 0.6
36.6 ± 0.6
7.5 ± 0.4
16.9 ± 0.4
16.6 ± 0.2
14.1 ± 0.2
18.0 ± 0.3
27.7 ± 0.7
13.5 ± 0.1
Table 2
Selectivity (%) at 20% conversion for 1-pentyne hydrogenation.
1
00
9
8
7
6
5
4
0
0
0
0
0
0
Pentane
Trans-2-pentene
1-pentene
Cis-2-pentene
No modifier
PPA
PA
17.1
8.4
6.7
21.9
27.3
29.9
0.6
16.1
3.0
5.3
18.5
20.3
19.9
1.6
60.4
87.1
85.8
52.0
45.2
43.3
96.9
6.4
1.4
2.2
7.6
7.2
7.0
0.8
3
3
PPN
PN
PN
a
TCN
a
PN added 30 min before 1-pentyne.
0
20
40
60
80
100
to ensure that all the reactant was washed into the reactor. For
the competitive reactions, 1 ml of modifier was added with the
reactant. The autoclave was then mixed briefly at a stirrer speed
of 800 rpm and pressurised to 1 barg with N2 and a sample was
taken. The vessel was depressurised and then pressurised with H2
to 2 barg. Following this the stirrer was set to a speed of 1000 rpm
and samples taken. Liquid samples were analysed by GC using a
Conversion (%)
No mod.
3PPA
PA
3-PPN
PN
TCN
Fig. 1. Selectivity/conversion plot for 1-pentyne hydrogenation.
5
0 m CP-Al O /Na SO column. Standard checks were undertaken
2 3 2 4
tivity. Trans-cinnamonitrile however is the most effective modifier
with selectivity to the alkene >90% at 95% conversion. For 2-pentyne
hydrogenation all the modifiers enhance selectivity and enable
>90% alkene selectivity to be achieved at high conversion.
The competitive reaction between 1-pentyne and 2-pentyne
was also examined. The rate constant for 1-pentyne hydrogena-
tion, in the presence of equimolar 2-pentyne, was 32.7 ± 4.8 (c.f.
32.9 ± 0.6 in the absence of 2-pentyne), while the rate constant for
2-pentyne hydrogenation, in the presence of equimolar 1-pentyne,
was 17.2 ± 3.2 (c.f. 18.8 ± 0.3 in the absence of 1-pentyne). These
results indicate that the rate of hydrogenation of each alkyne is not
influenced by the presence of the other.
to confirm that the system was not under mass transport control.
3
. Results
The reactions studied were the hydrogenation of 1-pentyne and
-pentyne. Four modifiers were used, pentane nitrile (PN, valeroni-
2
trile) and its respective amine, pentyl amine (PA, amyl amine), and
-phenyl propionitrile (3-PPN) and its respective amine, 3-phenyl
3
propylamine (3-PPA). These modifiers were not hydrogenated
under reaction conditions. The first order rate constants for the
reactions are reported in Table 1.
Surprisingly, in 1-pentyne hydrogenation, the amine modifiers
enhance the rate of hydrogenation, whereas the nitrile modifiers
decrease the rate of hydrogenation. With 2-pentyne the effects
are almost reversed with the amine modifiers causing a reduc-
tion in hydrogenation rate while the nitriles have much reduced
effect. Clearly the dominant functional group in the adsorption is
the amine or nitrile whereas the aromatic ring has little effect. To
examine the effect of when the modifier was added, PN was added
to a reduced catalyst 30 min before 1-pentyne. The rate constant
for 1-pentyne hydrogenation was 23.3 ± 0.8.
The competitive reaction between the alkynes was repeated
with PN present. PN was chosen because with 1-pentyne it caused a
reduction in alkene selectivity whereas with 2-pentyne it enhanced
alkene selectivity. To determine whether the PN was affecting the
alkene selectivity in the same manner as when each alkyne was
present in the absence of the other, the selectivity was measured
and compared with a modelled selectivity taken from the individ-
ual reactions in the presence of PN at equivalent conversion. The
results are shown in Table 4.
The effect of the modifiers on selectivity was also examined.
A comparison of selectivity at 20% conversion for 1-pentyne is
reported in Table 2 and for 2-pentyne in Table 3.
100
It is also of interest to see the effect of the modifiers on selec-
tivity over the full range of conversion. This is shown in Fig. 1 for
9
0
0
0
0
0
1
-pentyne hydrogenation and Fig. 2 for 2-pentyne hydrogenation.
With 1-pentyne the modifiers split into two groups, the amines,
8
7
6
5
which increase selectivity and the nitriles, which decrease selec-
Table 3
Selectivity (%) at 20% conversion for 2-pentyne hydrogenation.
Pentane
Trans-2-pentene
1-pentene
Cis-2-pentene
No modifier
PPA
PA
15.5
0
0
1.7
0
0.8
17.7
3.2
6.1
13.5
7.5
3.6
1.1
0.3
0.5
1.1
0.7
2.1
65.7
96.5
93.4
83.8
91.8
93.6
0
20
40
60
80
100
3
Conversion (%)
3
PPN
No mod
3-PPA
PA
3-PPN
PN
TCN
PN
TCN
Fig. 2. Selectivity/conversion plot for 2-pentyne hydrogenation.

5
50
P.E. Garcia et al. / Catalysis Today 164 (2011) 548–551
Table 4
7.0E-02
Alkene selectivity (%) during competitive alkyne hydrogenation with and without
PN at equivalent conversions.
6
5
4
.0E-02
.0E-02
.0E-02
Trans-2-pentene
1-pentene
Cis-2-pentene
1
1
-PY/2-PY, no modifier
-PY/2-PY, modelled from
individual runs
16.6
19.5
30.4
29.5
53.0
51
3.0E-02
.0E-02
1.0E-02
.0E+00
2
1
1
-PY/2-PY, with PN
modifier
-PY/2-PY, modelled from
individual runs with PN
22.7
23
35.8
33.5
41.6
43.5
0
-8
-6
-4
-2
0
ln(modifier/reactant)
4
. Discussion
The effect of nitrile and amine modifiers in hydrogenation of C5-
Fig. 3. The relationship between 3-PPN modifier concentration and rate constant
for 1-phenyl-1-propyne [21].
alkynes has been investigated. It is clear for the results presented
that there is a difference in response to the modifier depending on
whether the reactant is a terminal or internal alkyne and whether
the modifier is an amine or nitrile.
races is significantly less than on the edge and corner sites and so
the 2-alkyne competes more effectively. When pentanenitrile was
added to a mix of 1-pentyne and 2-pentyne it can be seen from the
alkene selectivities (Table 4) that the modifier affects each alkyne
independently. Modelling the selectivity values at equal conver-
sion from the single alkyne/modifier reaction gives good agreement
with the values found for the co-hydrogenation. This reinforces the
separate site nature of the 1-alkyne and 2-alkyne hydrogenation
with 1-pentene formed at edge sites and cis-2-pentene formed on
the terraces. A recent study of 1-phenyl-1-propyne hydrogenation,
another internal alkyne, using variable amounts of 3-PPN showed
that, as the concentration of the modifier is reduced the rate con-
stant for the hydrogenation of the internal alkyne increases (Fig. 3)
Amines are known to be catalyst poisons and this action relates
to the lone pair on nitrogen and its ability to donate to the metal.
Hence aromatic amines where the lone pair is conjugated to the
aromatic ring are much less deleterious to a reaction than aliphatic
amines [16,17]. However, in contrast to this inhibition effect, the
rate of reaction of 1-pentyne increases (Table 1) when pentyl
amine and 3-phenyl-propyl amine are present. Similar behaviour
has been observed with 1-butyne hydrogenation [11] where addi-
tion of piperidine resulted in a rate enhancement. The reason for
this enhancement can be related to the strength of adsorption
of the alkyne. Typically alkynes are strongly adsorbed and show
either zero or slightly negative order kinetics. Hence the rational
for the enhancement effect of the amine molecules argues that the
amine donates electron density to the palladium and so reduces
the strength of alkyne adsorption allowing a faster rate of hydro-
genation to be achieved. However this argument only appears to
hold for primary alkynes (1-butyne and 1-pentyne) and not for
internal alkynes, where the effect of the amine modifiers on 2-
pentyne hydrogenation is to significantly reduce the rate rather
than enhance it. This difference in behaviour can be understood in
terms of adsorption characteristics of terminal and internal alkynes.
Terminal alkynes have been shown to hydrogenate at low coordina-
tion sites such as edge and corner atoms whereas internal alkynes
favour terraces [18–20]. The results of the competitive reaction
between 1-pentyne and 2-pentyne confirm that they react on dif-
ferent parts of the surface and do not influence each other. As the
adsorption of amines is strong one may expect that they would
preferentially adsorb at low coordination sites on the catalyst sur-
face; in doing so they affect the reactivity of the terminal alkyne
by reducing its strength of adsorption due to electron donation to
the Pd hence allowing faster hydrogenation. Because the adsorp-
tion takes place on the edges and corners and the flexible modes of
adsorption available to the terminal alkyne, we envisage that there
is the potential for co-adsorption rather than competitive adsorp-
tion. With the internal alkyne, adsorption takes place on terrace
and faces. Here we suggest the amine will compete directly with
the alkyne and cause the reduction in activity observed.
[
21] supporting the view that the effect of the nitrile modifier is
related to strength of adsorption.
Subsequent hydrogenation and isomerisation of the alkenes
formed is in general significantly inhibited by the presence of either
amine or nitrile and this is expected as re-adsorption of the alkene
will be inhibited by the presence of the more strongly bound amine
and nitrile. However the nitrile modifiers appear to enhance the
conversion of the alkene when used in the presence of 1-pentyne.
This is true whether the modifier is added before the alkyne or coin-
cidentally. Separate tests using a 3PPN/1-pentene mix showed that
the nitriles inhibited the hydrogenation of 1-pentene. Therefore the
enhancementofthe rate ofalkenehydrogenationmustoccurbefore
the 1-pentene desorbs from the surface. Note however that the
same enhancement is not seen when the primary alkene is the cis-
2
-alkene suggesting that, like the alkynes there are differences in
adsorption/hydrogenation for 1-pentene and cis-2-pentene. This is
supported by work by Zaera [7] who found that cis/trans isomerisa-
tion is sensitive to surface structure and that certain surfaces favour
cis/trans isomerisation whereas other favour trans/cis isomerisa-
tion. The effect of the modifiers on cis/trans isomerisation can be
seen in Table 3. Even though all the modifiers inhibit alkene hydro-
genation, isomerisation is sensitive to the modifier with the amines
giving high cis:trans ratios (>15) while the nitriles give low cis:trans
ratios (≤ 12). TCN, the conjugated nitrile has cis:trans values closer
to the amines.
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