The Preparation of a Residue-free, Alumina-supported Gold Catalyst by Decomposition of an Azido-Gold(III) Complex and an Evaluation of the Effectiveness of the Catalyst for the Hydrogenation of Propyne

Article abstract of DOI:10.1002/zaac.201400506

A 1% w/w alumina-supported gold catalyst is described that is prepared from the impregnation of alumina with a solution of ammonium tetraazidoaurate(III), NH₄[Au(N₃)₄]. Decomposition of the azide precursor leads to a residue-free catalyst comprised of well-dispersed gold particles. This novel material is tested for propyne hydrogenation using pulse-flow techniques and displays 100% selectivity to the partial hydrogenation product, propene, albeit in the presence of a large retention of hydrocarbon by the catalyst. The latter characteristic is associated with a support-mediated deactivation pathway. Comparing catalytic performance with a 1.45% w/w Au/TiO₂ reference catalyst indicates that chlorine-free precursor compounds are not required to enhance catalytic activity for this particular reaction.

Full text of DOI:10.1002/zaac.201400506

Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201400506
The Preparation of a Residue-free, Alumina-supported Gold Catalyst by
Decomposition of an Azido-Gold(III) Complex and an Evaluation of the
Effectiveness of the Catalyst for the Hydrogenation of Propyne
[
a,b]
[a]
[a]
Jose Antonio Lopez-Sanchez,
Clément Morisse, John M. Winfield,
[c]
[c]
[a]
Burkhard Krumm, Thomas M. Klapötke, and David Lennon*
Keywords: Supported catalysts; Gold; Alumina; Propyne hydrogenation; Propene hydrogenation; Gold azide precursor
compound
Abstract. A 1% w/w alumina-supported gold catalyst is described that tial hydrogenation product, propene, albeit in the presence of a large
is prepared from the impregnation of alumina with a solution of ammo-
nium tetraazidoaurate(III), NH [Au(N ]. Decomposition of the azide associated with a support-mediated deactivation pathway. Comparing
precursor leads to a residue-free catalyst comprised of well-dispersed catalytic performance with a 1.45% w/w Au/TiO reference catalyst
retention of hydrocarbon by the catalyst. The latter characteristic is
4
3 4
)
2
gold particles. This novel material is tested for propyne hydrogenation
using pulse-flow techniques and displays 100% selectivity to the par-
indicates that chlorine-free precursor compounds are not required to
enhance catalytic activity for this particular reaction.
Introduction
activity relationship for the hydrogenation of an α,β-unsatu-
rated aldehyde over supported Pd catalysts to be developed.
[
4]
Azido complexes of d block metals are characterized by
thermal instability and their decomposition, which is often ex-
plosive, has limited their use in synthesis. However, we have
exploited this aspect of their chemistry to prepare an alumina-
supported palladium catalyst by thermal decomposition of a
Klapötke and co-workers have previously synthesized and
characterized a number of highly sensitive ammonium tetraazi-
[5]
doaurates(III) compounds. This article describes a procedure
for combining ammonium tetraazidoaurate(III) with a high sur-
face area alumina to produce a new Au/Al O reference cata-
2
3
II
[1]
Pd azido complex in the presence of alumina. Salts such as
PdCl or Pd(NO ) are typically used as Pd precursor com-
lyst, subsequently referred to as Au(azide)/Al O .
2
3
2
3 2
The selective hydrogenation of propyne has been used as a
test reaction, enabling comparisons to be made with a recent
pounds but these can potentially leave chemical residues,
which could affect catalytic performance. Decomposition of
tetramminepalladium(II) tetraazidopalladate(II) leads to the
formation of Pd atoms, ammonia, and dinitrogen.[ Thus, a
controlled decomposition in the presence of alumina can be
used to produce a residue-free Pd/Al O catalyst, with the gas-
study using a World Gold Council (WGC) reference catalyst:
[6]
1
.45 wt% Au/TiO2. The Au precursor compound for the ref-
2]
erence catalyst is tetrachloroauric(III) acid; catalysts prepared
from this precursor can contain residues of chloride ion, and
even sodium when a sodium salt is used for precipitation in the
commonly used deposition-precipitation and co-precipitation
2
3
eous co-products readily separable from the solid Pd/alumina
matrix. The Pd is well dispersed and the catalyst exhibits a
distinct infrared spectrum for chemisorbed carbon mon-
oxide.[ Moreover, the Pd(azide)/Al O catalyst subsequently
techniques. Either may act as a catalyst poison, which has been
[7–9]
examined in some detail.
Two main effects have been de-
1]
2
3
scribed: (i) chloride residues facilitate the sintering of Au par-
ticles during heat treatment, and (ii) they inhibit the catalytic
proved invaluable in discerning morphological trends in alu-
mina supported Pd catalysts,[ which then enabled a structure-
3]
[7]
activity by poisoning the active site. The performance of the
Au(azide)/Al O catalyst in the designated test reaction com-
2
3
*
Dr. D. Lennon
pared with the reference catalyst provides information on how
chloride residues may affect the application of supported gold
catalysts in hydrogenation reactions.
E-Mail: David.Lennon@glasgow.ac.uk
[
[
[
a] School of Chemistry
University of Glasgow
Joseph Black Building
Glasgow G12 8QQ, Scotland, UK
b] Department of Chemistry
University of Liverpool
Results and Discussion
Crown Street
McEwan and co-workers[
10]
and Cárdenas-Lizana and
Liverpool L69 7ZD, UK
c] Department of Chemistry
Ludwig-Maximilian University of Munich
Butenandstrs. 5–13 (Haus D)
[11]
Keane
have reviewed the application of supported gold
catalysts for selective hydrogenation reactions. Propyne
hydrogenation is selected as a test reaction in this work. Pérez-
81377 Munich, Germany
Z. Anorg. Allg. Chem. 0000, ᭿,(᭿), 0–0
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Ramírez and co-workers have shown that supported gold cata- cated the presence of gold particles of mean particle size
[12,13]
lysts can exhibit favorable propene yields.
Lopez-Sanchez 3.0Ϯ1.7 nm, with the error representing one standard devia-
and Lennon used pulse-flow methods to investigate the hydro- tion. Thus, the metal is well dispersed and exhibits a narrow
genation of propyne over two WGC reference catalysts: Au/ particle size distribution. A Perkin-Elmer Lambda 9 UV/Vis/
[6]
TiO and Au/Fe O . That work showed the Au/TiO catalyst NIR spectrophotometer fitted with a PE Integrating Sphere
2
2
3
2
to display complete selectivity to propene and progressive de- coated with barium sulfate was used to obtain the diffuse re-
activation, whereas Au/Fe O exhibited selectivity and deacti- flectance UV spectrum of the Au(azide)/Al O catalyst and
2
3
2 3
vation patterns dependent on ageing, catalyst pre-treatment and revealed the presence of a broad band at ca. 520 nm. This fea-
reaction temperature; with the variability in performance of ture corresponds to a gold plasmon transition and confirmed
[
6]
Au/Fe O attributed to support effects. The same pulse-flow the presence of metallic gold nanoparticles deposited on the
2
3
methodology is adopted here, permitting direct comparison be- alumina.[
14]
As with other supported gold catalysts,
[15]
the
tween Au(azide)/Al O and Au/TiO . The Au/Fe O catalyst presence of oxidized gold cannot be excluded.
2
3
2
2 3
is a less suitable comparator in this instance, due to the com-
plexity evident in the Au/Fe O -C H -H reaction system.
2
3
3
4
2
Au(azide)/Al O3
2
The Au(azide)/Al O catalyst activity in the gas phase hy-
2
3
Catalyst Characterization
The novel preparation of the Au(azide)/Al O catalyst is de- of 323, 423, 523 and 663 K using a pulse-flow methodology.
drogenation of propyne was studied at reaction temperatures
2
3
scribed in the Experimental Section. Atomic absorption spec- Fresh amounts of sample reduced at 523 K were used for each
trophotometry (Perkin-Elmer 1100B Atomic Absorption Spec- measurement. The C pulse composition after reaction at the
3
trophotometer) revealed the gold content of the Au(azide)/ four selected reaction temperatures is displayed in Figure 1.
Al O sample to be 0.83Ϯ0.01 wt-%, with the error represent-
At 323 K no reaction products are detected. The lack of hy-
2
3
ing one standard deviation of three replicate measurements. No drogenation activity is attributed to a minimal supply of atomic
nitrogen was detectable via elemental analysis (Exeter Analyti- hydrogen at this temperature. Dissociative adsorption of
[
16]
cal Elemental Analyser CE440) and no chloride was detectable hydrogen over gold foil does not occur below 473 K,
with a Sherwood chloride analyzer 926 (Mk II) at a detection though Bond and co-workers have shown that this temperature
al-
–
1
limit of 10 mg·L chlorine. The surface area of the catalyst can be lowered to ca. 400 K to affect hydrogenation processes
determined by a Micrometrics Gemini III 2375 Surface Area when small gold particles are used.[ Indeed, consistent with
17]
2
–1
Analyzer was ca. 120 m ·g , indicating the preparative tech- that work, Figure 1b indicates a small degree of activity at
nique to have a negligible effect on total surface area. After 423 K, with propene detected as the only reaction product
2
–1
–1
reaction testing at 523 K a surface area of 106 m ·g was de- (propene: 0.86 μmol·gcat for pulse 1). Over the pulse se-
termined. Transmission electron microscopy (TEM) was per- quence studied, no significant deactivation is apparent al-
formed with a JEOL-1200EX microscope, operating at a pri- though some propyne retention is observed, which is approxi-
mary beam energy of 80 kV. The Au(azide)/Al O sample indi- mately constant after pulse 4 (Figure 1b). This corresponds to
2
3
Figure 1. Carbon mass balance profiles corresponding to propyne hydrogenation over the Au(azide)/Al
2 3
O catalyst at the following reaction
temperatures: (a) 323 K, (b) 423 K, (c) 523 K, and (d) 623 K. The catalyst was reduced at 523 K prior to reaction testing in all cases.
Z. Anorg. Allg. Chem. 0000, 0–0
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cumulative propyne retention of 13.4 μmol·g–1 over the 9
pulses studied.
Figure 1d shows that increasing the temperature further to
623 K accentuates this deactivation pathway, as evidenced by
Pulse-flow propyne hydrogenation studies on a reference lower activity and almost complete retention of the incident
Au/TiO catalyst at 423 K are characterized by selective pro- propyne molecules by the catalyst.
2
pene production similarly accompanied by propyne reten-
Benchmarked against the performance of the Au/TiO refer-
2
tion.[ At approximately steady-state conditions, propene pro- ence catalyst, Figure 1 shows the Au(azide)/Al O catalyst not
6]
2
3
duction for the Au/TiO catalyst corresponds to approximately to exhibit remarkable performance in selectively hydrogenat-
2
2
μmol propene per gcat,[6] compared with 0.9 μmol propene ing propyne to propene. However, with respect to the selection
per gcat seen in Figure 1b. Specific activity, as defined in the of Au precursor compounds for selective hydrogenation reac-
Experimental Section, is a means of assessing the reactivity tions, this outcome does indicate that chlorine-free precursor
of supported gold catalysts.[18] Table 1 compares the specific compounds do not convey any specific benefits regarding en-
activity of the Au(azide)/Al O and Au/TiO2 catalysts at hanced catalytic activity for this particular reaction. Indirectly,
2
3
pseudo-steady state for a reaction temperature of 423 K. This this work then endorses the continued use of tetrachloroauric
normalization of alkene production with respect to mass of acid as the precursor of choice for supported gold catalysts.
gold shows the Au(azide)/Al O catalyst to exhibit 79% of the
2
3
activity achievable with the reference catalyst. Although the
propene yields are lower for the Au(azide)/Al O catalyst, the
The Alumina Support
2
3
general characteristics, as evidenced by low propene yields ac-
companied by propyne retention, are broadly comparable to
the trends reported for the reference material.
Blank experiments performed on the alumina support mate-
rial (Figure 2) showed no propene or propane formation for a
range of temperatures up to 623 K, establishing the propene
formation observed in Figure 1 to be gold-mediated. Figure 2
Table 1. Specific activity values for the formation of propene over also indicates that the support material is primarily responsible
Au(azide)/Al and Au/TiO catalysts at 423 K. The propene pro- for the substantial degree of propyne retention. This is particu-
duction values represent a pseudo-steady state regime and correspond
to the 9th and 8th pulse values observed for the Au(azide)/Al and
Au/TiO catalysts, respectively. The Au/TiO data is obtained from the
2
O
3
2
larly evident at elevated temperatures (Ն 523 K). The con-
sumption of incident hydrocarbon is attributed to a propyne
oligomerization process that is accelerated at higher tempera-
tures. This process will additionally block active sites on the
metal and is thought to contribute to the deactivation trends
observed in Figure 1. The origin of this phenomenon is thought
to be Lewis acid sites on the alumina that form via dehydration
2 3
O
2
2
[6]
literature.
Catalyst
Au
wt-%
Propene production Specific activity
–
1
–1
/
/propene μmol·gcat
/propene μmol·gAu
Au(azide)/Al
Au/TiO
2
O
3
0.83
1.45
0.90
2.00
108
138
2
[
20]
and de-hydroxylation stages at elevated temperatures.
ther work using support materials exhibiting different degrees
Fur-
[21]
Figure 1c shows the results for experiments performed at of Lewis acidity (e.g. β-AlF
23 K. Compared with the 423 K data (Figure 1b), the quantity establishing the validity of this assertion. Given the extent of
and α-AlF
)
would assist in
3
3
5
of propene increased significantly in the first pulses (pulse 1 retention evident in Figure 1d and Figure 2, the majority of the
produced 3.42 μmol propene per g ) but this progressively oligomeric material is thought to be associated with the alu-
cat
decreased on increasing pulse number. The amount of propene mina surface. Ossipoff and Cant have shown that oligomeriza-
–
1
stabilized at ca. 0.9 μmol·gcat by pulse 9 with a propyne con- tion processes play a significant role in the hydrogenation of
version of 34.3%. Figure 1c also shows propyne retention to propyne over supported copper catalysts.[22]
become more substantial at this temperature and corresponds
–
1
to 4–6 μmol·gcat per pulse, signifying 20–30% propyne re-
tention for each pulse. Previous elevated temperature propyne
hydrogenation pulse-flow studies on two WGC reference cata-
lysts – Au/TiO and Au/Fe O – similarly showed decreasing
2
2 3
propene yields on successive pulsing accompanied by large
degrees of propyne retention.[
6]
Increasing reaction temperature is expected to favor the dis-
sociative adsorption of molecular hydrogen and enhance
hydrogen supply at the catalyst surface. However, Figure 1c
indicates the increasing temperature also permits access to a
deactivation channel, as evidenced by decreasing propene
yields on continued pulsing. Deactivation has previously been
reported for pent-1-ene hydrogenation over gold catalysts, and
was associated with the formation of carbonaceous resi-
dues.[ Further, Azizi et al. reported significant deactivation
above 573 K for the hydrogenation of ethyne over a ceria-sup-
Figure 2. Carbon mass balance profile for propyne hydrogenation over
alumina at a range of temperatures: pulses 1–7, 523 K; pulses 8–10,
17]
423 K; pulses 11–12, 323 K and pulses 13–14, 623 K. The alumina
[19]
ported gold catalyst due to polymerization processes.
was exposed to a reduction treatment at 523 K prior to reaction testing.
Z. Anorg. Allg. Chem. 0000, 0–0
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Catalyst Reduction Temperature
Conclusions
Earlier studies on a Au/Fe O catalyst showed that propene
2
3
A 1% alumina-supported gold catalyst was synthesized by
yields could be improved and maintained by adoption of a high
temperature reduction process, where the reduction in
hydrogen was performed at 663 K.[ This approach was ap-
the photo-decomposition of a gold-azide precursor complex
and assessed using propyne hydrogenation as a test reaction.
The main results can be summarized as follows:
(i) The catalyst, exhibiting metal particles of ca. 3 nm dia-
meter is 100% selective in the partial hydrogenation of pro-
pyne. The complete selectivity to propene is an inherent prop-
erty of the catalyst, as hydrogenation to propane is not access-
ible within the temperature range studied.
6]
plied to the Au(azide)/Al O catalyst, where the catalyst was
2
3
reduced at 663 K then propyne hydrogenation examined at
23 K; the results are presented in Figure 3. Compared with
5
the catalyst reduced at 523 K and reaction tested at the same
temperature (Figure 1c), the quantities of propene are low, and
decrease with increasing pulse number: 2.04 to 0.30 μmol pro-
pene per gcat in pulse 1 and pulse 8, respectively. Additionally,
the propyne retention is increased.
(ii) Reaction temperatures Ն 423 K are required to induce
propene production but simultaneously this induces large re-
tention of hydrocarbon feedstock by the catalyst. Blank experi-
ments show the hydrocarbon retention pathway to be support-
mediated.
(iii) Under comparable reaction conditions, the Au(azide)/
Al O catalyst exhibits a reduced propene yield compared to
2
3
a reference Au/TiO catalyst. This implies that chlorine-free
2
precursor compounds are not required to enhance catalytic ac-
tivity for this particular reaction.
Experimental Section
Catalyst Preparation: Ammonium tetraazidoaurate(III) was synthe-
sized according to Equation (1) from ammonium tetrachloroaurate(III)
with an excess of silver azide in methanol.
2 3
Figure 3. Carbon mass balance profile for the Au(azide)/Al O cata-
lyst after reduction/pre-treatment at 663 K then propyne hydrogenation
reaction testing at 523 K.
4AgN /CH OH
3
3
NH
4
[AuCl
4
] ––––––––Ǟ NH
4
[Au(N
3
)
4
]
(1)
–
4AgCl
Clearly, a high temperature reduction stage is non-beneficial
for alumina-supported gold catalysts. Moreover, the negative
Caution! The red-orange salt is an extremely shock-sensitive material,
response of the Au(azide)/Al O catalyst compared with the which exploded spontaneously during several attempts of solvent
2
3
[
24–26]
Au/Fe O catalyst is a further indication of the importance of evaporation, similar as reported for Group 1 cation salts.
How-
2
3
the support material in affecting favorable transformations in ever, this ammonium salt can be handled with absolute caution in
[
5]
methanol solutions, whereas tetraazidoaurates(III) containing large
a relatively straight-forward hydrogenation reaction.
[
24–28]
cations are relatively safe to handle.
Propene Hydrogenation
A 1% w/w Au/Al
procedure. A clear red (filtered) solution of NH
methanol (approx. 20 mL), generated according to the above equation
under light exclusion, was poured onto a slurry of alumina (5.0 g, De-
gussa Oxid C, BET surface area = 109 m ·g ) in methanol (75 ml).
The resulting slurry was carefully shaken frequently and the container
kept open for drying over a period of several days at ambient tempera-
ture in the presence of sunlight. The light-initiated decomposition pro-
cess is described by Equation (2).
2
O
3
catalyst was prepared according to the following
Au(N (100 mg) in
4
3 4
)
In order to elucidate the origin of the 100% propene selec-
tivity observed (e.g. Figure 1c), several experiments were per-
formed using a hydrogen:propene (3:1) mixture over the
Au(azide)/Al O catalyst to evaluate if this catalyst could hy-
2
–1
2
3
drogenate propene. After a reduction stage at 573 K, a first
series of experiments studied the passing of three pulses of the
reactant mixture at increasing temperatures of 423, 523, 623,
7
23 and 773 K. A second series of experiments studied a
NH
4
[Au(N
3
)
4
]/Al
2
O
3
(s) Ǟ Au/Al
2
O (s) + 6N
3
2
(g) + NH
3
(g) + 1/2H
2
(g)
2)
Au(azide)/Al O sample reduced similarly but the testing was
performed isothermally at 523 K for a sequence of 10 pulses.
2
3
(
In neither of the experiments were any quantities of propane With this procedure, the precursor compound readily partitioned into
observed. Furthermore, none of the hydrocarbon retention pro- gaseous and solid phase components. The gaseous components com-
cesses, clearly apparent in the propyne experiments (Figure 1), prised distinct molecules of high vapor pressure, which were vented
readily from the remaining solid to leave an intimate mixing of met-
were observed. These results indicate that the gold catalyst
allic gold and the alumina support material, with no residue from the
cannot hydrogenate propene for temperatures up to 773 K and,
original precursor compound remaining. The solid, of light purple col-
oration, was air stable and was ground to produce a powder.
furthermore, indicate the catalyst to be inherently 100% selec-
tive to propene when exposed to mixtures of hydrogen and
propyne. This outcome is consistent with recent reports on sup-
Reaction Testing: A pulse-flow micro-reactor system was used, that
The catalyst sample was ground
and sieved to give a grain fraction of 250–500 μm. Approximately
[
6,29]
ported gold catalysts applied in alkyne hydrogenation reactions has been described elsewhere.
exhibiting high selectivity to the alkene.[
12,23]
Z. Anorg. Allg. Chem. 0000, 0–0
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Zeitschrift für anorganische und allgemeine Chemie
0
.220 g of the catalyst sample was reduced in a 25% H
2
in helium
[2] W. Beck, T. M. Klapötke, J. Knizek, H. Nöth, T. Schütt, Eur. J.
–1
Inorg. Chem. 1999, 523.
flow (67 mL·min ) while the temperature was raised from ambient
temperature to the desired reduction temperature (typically 523 K) at
5
perature in a flow of helium. Pulses of reactant gas (78 μmol·gcat ) of
a 3:1 hydrogen/propyne mixture were injected into the helium carrier
gas (50 mL·min ) by passing the carrier flow through a sample loop.
Each pulse corresponds to an incident propyne pulse of 19.5 μmol pro-
pyne per gcat. On elution from the catalyst bed, the full pulse was
analyzed by on-line gas chromatography, using a Philips PU 4500 gas
[
3] T. Lear, R. Marshall, J. A. Lopez-Sanchez, S. D. Jackson, T. M.
Klapötke, G. Rupprechter, M. Bäumer, H.-J. Freund, D. Lennon,
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–1
K·min , then held for 2 h. The sample was set to the reaction tem-
–
1
[
4] A. R. McInroy, A. Uhl, T. Lear, T. M. Klapötke, S. Shaikhutdinov,
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propyne (BDH, 96% purity) was purified through vacuum distillation
prior to use. Blank experiments performed on the empty reactor at
elevated temperatures displayed no activity. Blank experiments on alu-
mina Degussa Oxid C were performed following the same catalyst
pre-treatment and procedures described above. All experiments were
performed at least twice, with the figure presented here being represen-
tative of those observed for the catalyst under the specified reaction
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0 t
propyne – propyne
X
propyne (%) =
ϫ100,
ϫ100,
propyne
propene
t
propene + propane )
0
[19] Y. Azizi, C. Petit, V. Pitchon, J. Catal. 2008, 256, 338.
t
Spropene (%) =
[
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(
t
moles propene pulse–1
mass of Au
SA (mol propene pulse gAu–1) =
–
1
,
[
where Xpropyne and Spropene represent the propyne conversion and pro-
[
18]
pene selectivity, respectively. SA represents the specific activity
.
[
[
Acknowledgements
[
[
[
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technical assistance with electron microscopy and elemental analysis,
respectively.
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Received: October 31, 2014
Published Online: ᭿
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© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
www.zaac.wiley-vch.de
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
J. A. Lopez-Sanchez, C. Morisse, J. M. Winfield, B. Krumm,
T. M. Klapötke, D. Lennon* ............................................. 1–6
The Preparation of a Residue-free, Alumina-supported Gold
Catalyst by Decomposition of an Azido-Gold(III) Complex
and an Evaluation of the Effectiveness of the Catalyst for the
Hydrogenation of Propyne
Z. Anorg. Allg. Chem. 0000, 0–0
6
© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim