Enhanced selectivity in acetylene hydrogenation by ligand modified Pd/TiO2 catalysts

Article abstract of DOI:10.1039/c0cc01742f

Modification of Pd/TiO₂ catalyst by adsorption of triphenylphosphine and phenyl sulfide leads to markedly enhanced selectivity for acetylene hydrogenation in the presence of ethylene and excess hydrogen. Similar selectivities were maintained in cases where ligand decomposition took place and sulfur was retained on the catalyst surface.

Full text of DOI:10.1039/c0cc01742f

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COMMUNICATION
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Enhanced selectivity in acetylene hydrogenation by ligand modified
Pd/TiO2 catalysts
Fiona-Mairead McKenna, Richard. P. K. Wells and James. A. Anderson*
Received 4th June 2010, Accepted 1st October 2010
DOI: 10.1039/c0cc01742f
Modification of Pd/TiO2 catalyst by adsorption of triphenyl-
phosphine and phenyl sulfide leads to markedly enhanced
selectivity for acetylene hydrogenation in the presence of ethylene
and excess hydrogen. Similar selectivities were maintained in
cases where ligand decomposition took place and sulfur was
retained on the catalyst surface.
hexane. The amount of ligand was selected to produce a
2.5 to 1 molar ligand to (bulk) palladium metal ratio. The
catalyst was stirred for 30 min and then dried on a rotary
evaporator. Results for 3 samples are presented here: 1%
Pd/TiO2, 2.5 : 1 PPh3 : Pd/TiO2 (triphenylphosphine) and
2.5 : 1 Ph2S : Pd/TiO2 (phenyl sulfide). Note that the Pd
loading is significantly greater than that used in the current
industrial process however, such low loadings lead to difficulties
when attempting to characterise the catalysts and thus a higher
loading has been employed.
Removal of trace acetylene from ethylene streams is a key step
in obtaining commercially valuable stocks of the world’s most
produced organic compound.1 Industrially, carbon monoxide
is employed as a competitive adsorbate and enhances selectivity
by lowering the surface availability of ethylene and hydrogen.2
The presence of CO together with a limited hydrogen supply
are however linked to increased oligomer or ‘‘green oil’’
formation and accelerated catalyst deactivation.3 Palladium
catalysts used in this reaction have been modified by alloying,
in particular with silver but other, non-precious metal catalysts
have also been developed.4 The origin of selectivity enhance-
ment is attributed to either reduced active ensemble size effects
or modified electronic characteristics of the active metal or
more favourable hydrogenation to a particular intermediate.5
Distinction between ensemble and electronic effects is often a
challenge using alloyed metals. Further theories attribute
selective behaviour to the availability of sub-surface hydrogen.6
The rationale behind experiments reported here was that, by
using ligands, one can exaggerate steric or electronic effects by
careful consideration of the ligand chosen and thereby assess
the mode in which selectivity is primarily achieved. The use of
ligands and surface modifiers of supported metal catalysts is
well documented7 and the particular modifiers exploited in
this study have been used successfully in previous studies.
For example, diphenyl sulfide has been shown to enhance
chemoselective hydrogenation over Pd/C8 and triphenyl
phosphine has been used to enhance activity and selectivity
of supported rhodium in olefin hydroformylation.9,10 How-
ever, to the best of our knowledge this is the first time this
method of modification has been studied in the reaction of
acetylene hydrogenation.
Confirmation of the presence, location (metal and support),
loading and thermal stability of the modifier were determined
by a combination of TGA-MS analysis and CO chemisorp-
tion. It was observed that the triphenyl phosphine modifier
was stable in hydrogen up to 393 K but the phenyl sulfide
modifier was decomposed to benzene and sulfur at temperatures
around 393 K. The extent to which the modifier ‘‘blocked’’ the
surface of the metal was estimated by the changes in CO
chemisorption however it should be recognised that any
changes in the electronic density of the metal may alter the
strength of CO chemisorption which does not necessarily
correspond to the general inaccessibility of metal sites.
To study the acetylene hydrogenation reaction under excess
hydrogen conditions, gas mixture composed of 1% C2H2 in
9% C2H4/N2 was used and this was diluted using 100% H2 to
give a H2 : C2H2 ratio of 25 : 1. This choice of conditions
permitted a better assessment of selectivity effects as excess
hydrogen conditions are more challenging in terms of obtaining
good selectivity to ethylene. Such conditions also lead to
reduced propensity to create carbonaceous deposits and
formation of ‘‘green oil’’ which might otherwise distort
findings related to modifications induced by the ligands.
Selection of titania as a support was intentional to diminish
potential contributions due to oligomerisation over a more
acidic oxide support such as alumina. Due to the exothermic
nature of the reaction under these conditions, a water cooled
reactor vessel was employed that maintained the temperature
at 323
K during reaction over all studied catalysts.
For comparative purposes, experiments were also performed
without ligand modifiers but in the presence of either 3 or
200 ppm CO.
Pd (1 wt%) on titania (Degusasa, P25) was prepared by
dropwise impregnation and stirred at room temperature for
6 h. The resulting solution was left in a furnace at 373 K
overnight to remove some of the excess water after which it
was stirred for another 6 h and subsequently evaporated to
dryness. The resulting powder was calcined for 3 h at 673 K
and stored. Deposition of the two ligands was carried out from
Table 1 displays conversion data obtained over the catalysts
under these conditions. Residual acetylene concentrations are
presented based on ppm of the C2 components of the feed-
stream. At steady state, the stream passing over unmodified
Pd/TiO2 was reduced to 1415 ppm acetylene (98.6% conversion)
but led to some loss of ethylene as a result of non-selective
hydrogenation and also yielded a significant proportion of
by-products. Addition of 3 ppm CO to the feedstream led to
enhanced conversion of the acetylene over Pt/TiO2 such that
Surface Chemistry and Catalysis Group, Dept of Chemistry,
University of Aberdeen, UK AB24 3UE.
E-mail: j.anderson@abdn.ac.uk; Fax: +44 (0)1224 272921;
Tel: +44 (0)1224 272905
c
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Table 1 Conversion of acetylene, formation/removal of ethylene and
by product formation over modified and unmodified Pd/TiO2 catalysts
in a mixed ethylene/acetylene feedstream at 323 K after 120 min on
stream
Acetylene
conc/ppm
Ethylene gain Total by-product
(loss)/%
Catalyst
signal/mV
Pd/TiO2
1415
0
57
(À81)
(À99)
(À4.6)
3
67 178
174 911
14 363
8935
3ppm CO–Pd/TiO2
PPh3–Pd/TiO2
Ph2S–Pd/TiO2
reduced at 323 K
Ph2S–Pd/TiO2
reduced at 393 K
571
295
(À23.5)
26 869
no residual acetylene could be detected. However, this was
balanced by the almost complete loss of ethylene via hydrogena-
tion to produce ethane and other by-products. Under the same
reaction conditions, PPh3 modified catalyst reduced acetylene
concentration in the stream to 57 ppm (99.94% conversion)
while hydrogenating a significantly smaller proportion of the
ethylene and producing only ca. 20% as much by-product as
the unmodified Pt/TiO2. Diphenyl sulfide modified catalyst
produced even less by-product and this catalyst was also
able to hydrogenate acetylene without leading to reduction
in ethylene levels in the feed-stream. Acetylene conversion,
however, was 99.43%, slightly lower than that obtained by
PPh3 modification. Ligand modification under these con-
ditions gave favorable performance compared to the use of
CO in the feed-stream which led to either complete acetylene
removal but at the cost of almost complete (99%) removal of
ethylene (3 ppm CO) or with 48% loss of ethylene but
with much poorer levels of conversion (63.99%) of acetylene
(200 ppm CO).
Fig. 1 Hydrogenation rates of acetylene (filled columns) and ethylene
(empty columns) during non-competitive reactions over: (a) Pd/TiO2,
(b) Pd/TiO2 with 3 ppm CO, (c) PPh3–Pd/TiO2, (d) Ph2S–Pd/TiO2
reduced at 323 K and (e) Ph2S–Pd/TiO2 reduced at 393 K.
acetylene hydrogenation over all samples fell within a narrow
range between 9.3 Â 10À5 to 11 Â 10À5 mol gÀ1
s
À1. The
standard deviation based on repeat experiments for any one
catalyst was marginally less than the range exhibited across the
series and so differences should not be taken as being of
considerable significance. However, when these activities are
calculated based on the different CO chemisorption capacities,
then the range in TOFs is significant for the series. For the
unmodified Pt/TiO2, the relative rates of both reactants were
similar suggesting that the higher conversion of acetylene
compared with ethylene when they were both present
(Table 1) arises from the more favourable adsorption of the
alkyne.
Under conditions of excess hydrogen, catalysts modified by
the use of ligands performed substantially better than catalyst
modified by continuous feeding of CO into the reagent stream.
It should be noted that when the phenyl sulfide catalyst was
reduced at 393 K rather than 323 K the selectivity pattern was
altered (Table 1) with increased conversion of acetylene
(99.7%) but at the expense of greater ethylene conversion
and enhanced by-product formation. TGA, TPR and FTIR
analyses indicate that increasing the reduction temperature
from 323 to 393 K leads to loss of the phenyl ligands but with
retention of the sulfur by the catalyst surface.11 This is
consistent with the relative magnitude of the CO chemisorp-
tion capacity of the Pd (13.4, 1.7 and 0.2 mmol gÀ1 for
unmodified, 323 K reduced and 393 K reduced, respectively:
no CO uptake was detected for the support alone). From the
decrease in CO uptake it can be deduced that the presence of
sulfur continues to modify the adsorption characteristics of the
metal but that the rate per exposed metal atom increases for
both acetylene and ethylene hydrogenation. However, the
relative rates of these two reactions in the absence of phenyl
ligand are less favorable than in its presence.
The use of CO as modifier, even at the very low concentration
of 3 ppm, slightly reduced the rate of acetylene hydrogenation
but not to the extent to which ethylene hydrogenation became
disfavoured. This is consistent with the theory behind the use
of this surface modifier.12 The extent to which ethylene
hydrogenation was disfavoured by CO was, however, insignifi-
cant compared to the impact of the use of either triphenyl-
phosphine (Fig. 1, column c) or diphenylsulfide (Fig. 1, column d)
with, in particular the latter reducing the activity of ethylene
hydrogenation to zero. Acetylene rates were either similar
(diphenyl sulfide) or slightly enhanced (triphenylphosphine)
by the modifier when compared on a per gram basis. However,
given that both modifiers resulted in partial loss of exposed
metal surface after exposure to modifier (by 87.3 and 82.1%,
respectively) the rate of acetylene hydrogenation per site
(TOF) was considerably enhanced by the modifiers indicating
a promotional effect as a consequence of the presence of the
ligand on this reaction. However, note that even after activation
of the catalyst at 393 K, which led to loss of phenyl ligands for the
diphenyl sulfide modified catalyst,11 the relative rate of acetylene
hydrogenation was maintained (Fig. 1, columns d and e). The
only apparent consequence of the loss of phenyl ligands was the
recovery of some of the activity for ethylene hydrogenation.
Further insight into the selectivity enhancements induced
by the modifiers was obtained by separately studying the
hydrogenation rates of acetylene and ethylene. These were
performed using 5% of each gas in nitrogen at 150 ml minÀ1
with addition of hydrogen to give H2 : C2 ratio of approxi-
mately 3 : 1. Data compiled in Fig. 1 show that the rates for
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Experiments conducted with single reagents (Fig. 1) also
allowed assessment of the relative contributions of each
hydrocarbon towards by-product formation. In the ethylene
runs, by-products detected by GC were insignificant with the
exception of runs containing CO which led to detectable
quantities of methane and other hydrocarbons. Methane was
also abundant in the acetylene reaction which involved CO
whereas this product was absent in the cases of all other
modified catalysts. Other longer chain hydrocarbons were
formed with equivalent propensity on all samples.
frequencies observed for the reaction of acetylene alone over
the catalysts treated with ligand modifiers (Fig. 1).
One possible scenario is that adsorption of the modifier on
the surface of Pd results in a templating of the surface where
the areas of exposed retained Pd surface corresponds to the
physical size of the ligands (i.e. controlled by the packing of
the bulky phenyl ligands). If the ligands can then be removed
via low temperature hydrogenation/decomposition, such as in
the case of diphenyl sulfide, then the heteroatom may be
retained in its original location, thus retaining the same
templated area on the surface and consequently, retaining
similar catalytic properties (Table 1 and Fig. 1).
Spectroscopic and theoretical calculations provide evidence
for charge transfer from PPh3 to the metal centre13,14 and it
has been postulated that the impact of this ligand (and others)
on a metal surface is to polarise the metal–hydrogen bond
and subsequently enhance selectivity in semi-reduction of
alkynes15 (based on the relative vulnerability of the triple bond
relative to the double bond to nucleophilic attack).16 It
might be assumed that Ph2S similarly modifies the electronic
properties of the Pd metal and this may have led to the
enhanced semi-hydrogenation selectivity and enhanced reac-
tion rates observed here, and the enhanced chemoselectivity
observed in previous studies.8 It should be noted however, that
while previous studies illustrated that this ligand enhanced
chemoselectivity of Pd catalyst, substrates which contained
CRC triple bonds were fully hydrogenated under the con-
ditions employed8 whereas data here (Fig. 1 and Table 1) show
that catalyst modification enhanced selectivity to the partially
hydrogenated state.
Experiments conducted under high pressure (10 bar) condi-
tions, more akin to those employed by industry, confirm that
the enhanced selectivity effects observed here are still operative.
We thank SABIC for financial support of this project, Dr A.
M. Ward (SABIC) for helpful discussion and Prof. C. Hardacre
and Ms Loredana Mantarosie, Queen’s University Belfast, for
performing high pressure testing on these catalysts.
Notes and references
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While it might be tempting to ascribe the enhanced selectivity
of Pd/TiO2 catalysts solely to electronic modification resulting
from charge transfer from the phenyl ligands to the metal via
the P or S atom interacting with Pd, comparison of data for
unmodified catalyst with Pd/TiO2 modified by Ph2S and
prereduced at 393 K and which resulted in loss of the phenyl
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promoting acetylene hydrogenation and suppressing the
propensity to hydrogenate ethylene. Such observation would
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12
CO on Ni/SiO2 and/or suppressing the unselective path
involving hydrogenation of vinyl to ethylidene.5 Geometric
effects alone are insufficient to describe the higher turnover
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2351–2353 2353