Propane reacts with O2 and H2 on gold supported TS-1 to form oxygenates with high selectivity

Article abstract of DOI:10.1039/b800620b

Gold nanoparticles supported on a microporous titanosilicate (TS-1) were found to be highly selective (95%) towards the formation of acetone and isopropanol from propane, O₂, and H₂ at moderate temperatures (443 K). The Royal Society of Chemistry.

Full text of DOI:10.1039/b800620b

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Propane reacts with O and H on gold supported TS-1 to form
2
2
oxygenates with high selectivityw
a
a
b
a
c
ad
J. J. Bravo-Suarez, K. K. Bando, T. Akita, T. Fujitani, T. J. Fuhrer and S. T. Oyama*
´
Received (in Berkeley, CA, USA) 15th January 2008, Accepted 6th May 2008
First published as an Advance Article on the web 13th June 2008
DOI: 10.1039/b800620b
Gold nanoparticles supported on a microporous titanosilicate
TS-1) were found to be highly selective (95%) towards the
2 2
was carried out at 443 K and 0.3 MPa, with O and H . The
(
catalysts (Ti : Si = 2 : 100, 3 : 100, and 10 : 100) formed mostly
acetone with some isopropanol in combined selectivities of
over 85% and propane conversions of 2.1–6.5% (Table 1). In
particular, the Au–Ba/TS-1 catalyst with an intermediate Ti :
Si = 3 : 100 ratio presented the most remarkable catalytic
results with an oxygenate selectivity of 95% at a propane
formation of acetone and isopropanol from propane, O
at moderate temperatures (443 K).
2 2
, and H
Propane partial oxidation to high value products has been a
long sought goal. Examples of such products include acrylic
1
acid and acrylonitrile, as well as acetone and isopropanol.
conversion 6.5%, which corresponded to acetone and isopro-
À1 À1
Acetone, for example, is mainly produced as a co-product in
panol space-time yields (STY) of 33 and 2 g kgcat
h ,
À1
the synthesis of phenol by the cumene process, with a world
2
production of 5.2 million metric tons in 2004. Future planned
respectively. The resulting acetone TOF was 0.079 s based
on exposed Au. The Au loadings in the samples were similar,
with a slightly higher value for the catalyst with Ti : Si = 3 :
100. The difference in reactivities is likely due to an optimum
Ti : Si ratio that allows the formation of a maximum number
processes for directly obtaining phenol from benzene will
3
necessitate alternative routes for acetone. Isopropanol is
produced by hydration of propylene using acid catalysts, with
4
a total world capacity of 1.9 million metric tons in 1996. This
11
of effective Ti sites in the zeolite framework. Here, the best Ti
process requires high pressures and suffers from corrosion
problems. Alternative acetone and isopropanol production
methods from propane have been studied. In these methods,
: Si ratio of 3 : 100 corresponds well with previously reported
maximum contents of Ti in TS-1 zeolite framework
1
2
positions.
Catalytic oxidation with O
occurs by in situ formation of H
propane reacts with O
2
or hydroperoxides at mild tempera-
2
and H
2
on Au/TiO likely
2
2
–SiO
tures and high pressures using biomimetic complexes (e.g., Fe,
2
O
2
2 2
on Au. The H O then
Co, or Cu porphyrins, phthallocyanines, and oxometal-
transfers to Ti tetrahedral sites to form Ti–OOH species active
13,14
5
–9
6–9
lates)
process from low-cost abundant propane should be continu-
ous, produce oxygenates with high selectivity, use O as the
or Ti, V, or Fe containing catalysts.
An ideal
for selective oxidations, such as propylene epoxidation.
In
the case of propane, a similar mechanism may take place, and
the involvement of isopropoxy intermediate species could
explain the formation of acetone and isopropanol.
2
oxidant at moderate conditions, and use a recyclable catalyst.
None of the above methods fulfils all these requirements.
Here it is shown that propane can be converted continu-
ously to acetone and, in lesser amounts, isopropanol with high
selectivity (95%) under mild conditions (443 K and 0.3 MPa)
In situ UV-Vis spectra of Au–Ba/TS-1 (Ti : Si = 3 : 100)
under propane oxidation conditions show evidence for the
formation of the Ti–OOH species. Changes occur in several
regions (Fig. 1 and ESIw): (1) a band at about 540 nm due to a
1
5
using a mixture of O
2
and H
2
as the oxidizing agent and
plasmon resonance (PR) of Au nanoparticles (Au is mostly
metallic as evidenced by the absence in the Au L -edge
Au–Ba/TS-1 as the catalyst. Evidence is presented for the
presence of intermediates during the reaction, including ad-
sorbed oxygen on Au and hydroperoxide species on Ti.
The Au–Ba/TS-1 catalysts were prepared by the deposi-
3
XANES of a near-edge resonance around 11 920 eV due to
oxidized Au, see ESIw); (2) band growth at 320–410 nm and
1
2,16
210–240 nm due to Ti–hydroperoxo species;
and (3) a
1
0
2 3
tion–precipitation (DP) method using Na CO as the neu-
band decrease at 250–300 nm due to tripodal Ti sites,
1
2
tralizing agent, a pH of 9, and TS-1 as the support (see ESI for
Ti(OSi)
3
–OHÁ(H
2
O)
2
.
These assignments are supported by
detailsw). Ba was added to improve the Au capture by the
support. The Au loadings were about 0.1 wt% and Au
density functional theory (DFT) calculations (see ESIw). The
Au PR position as a function of reaction time (Fig. 1a) shows
an initial increase (red shift) during the first 0.1 h, followed by
a decrease (blue shift), reaching a constant value (539.4 nm)
1
0
particle sizes about 4.0 nm (see ESIw). Propane oxidation
a
after 3 h. The initial red shift is related to O
2
adsorption on
Au, while the subsequent blue shift is explained by reaction
of O , most likely with H to form H . The steady-state Au
PR position (539.4 nm) was higher than that observed under
flow (534.1 nm), confirming the presence of adsorbed O on
Au during reaction. Although an adsorbed superoxide O
species has been observed by electron paramagnetic resonance
National Institute of Advanced Industrial Science and Technology
(AIST), 16-1 Onogawa, Tsukuba, 305-8569, Japan
AIST, Midorigaoka 1-8-31, Ikeda 563-8577 Osaka, Japan
Chemistry and Physics, Radford University, Radford, VA 24142,
USA
Chemical Engineering, Virginia Tech, Blacksburg, VA 24061, USA.
E-mail: oyama@vt.edu; Fax: +1 540 231 5022;
Tel: +1 540 231 5309
1
7
b
c
2
2
2 2
O
d
H
2
2
À
2
w Electronic supplementary information (ESI) available: Experimental
section, TEM, Au L edge XANES and UV-Vis data. See DOI:
1
6
spectroscopy, to the best of our knowledge, this is the first
3
10.1039/b800620b
in situ experimental evidence of adsorbed oxygen on Au
3
272 | Chem. Commun., 2008, 3272–3274
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Table 1 Catalytic reactivity of Au–Ba/TS-1 catalysts for propane partial oxidation
b
/nm Conversion (%) Selectivity (%)
c
efficiency (%) Main product
d
Nominal Ti : Si [Au]
ratio
D
p
H
2
a
(wt%)
catÀ1 À1
À1
C
3
H
8
H
2
Acetone 2-Propanol
C
3
H
6
CO
2
STY/g kg
h
TOF/s
2
3
1
1
N
a
: 100
: 100
0.09
0.11e
0.07
0.02
1.2
4.4
4.0
4.2
—
2.7
b
2.1
6.5
2.5
0.4
1.4
7
10
5
5
15
92
90
83
89
4
2
5
2
0
0
0
0
0
0
69
6
5
15
11
27
10
17
42
16
16
10
33
12
2
120
0.032
0.079
0.047
0.031
0.029
0 : 100
0 : 100
f
g
c
d
Au content by ICP-AES. Au average size by TEM.
H
2
3
selectivity to C -products. (STY: space-time yield; TOF: turnover frequency) TOF
e
3
based on exposed Au. Au dispersion is calculated from particle size, assuming Au fcc packing and spherical particles. Estimated from Au L -
f
XAFS edge-jump intensity. Without Ba. Au/TiO
g
2
(P25) with C
3 8 2 2
H : H : O : Ar = 2 : 1 : 1 : 6, 443 K, 0.1 MPa, SV (space velocity) = 36 000
3
cm h
À1
À1
3
À1 À1
g
cat . Reaction conditions: C
3
H
8
: H
2
: O
2
: Ar = 0.5 : 3 : 1 : 5.5, 443 K, 0.3 MPa, SV B 4500 cm h
gcat . Reactivity after 0.5 h.
supported on a titanosilicate. Propane conversion and selec-
tivities on Au/TS-1 appeared to track with spectroscopic
changes (see ESIw); however, to prove that the observed
species are true intermediates would require the use of tran-
1
4,18
sient spectroscopic techniques.
In situ UV-Vis spectra (Fig. 1b) initially show hydrated
hydroxyl species Ti(OSi) O) that react (decrease in
–OHÁ(H
70 nm band) with H to form hydroperoxide species. The
Scheme 1 Sequence of steps in oxygen activation: (a) H O formation
2
on gold; (b) hydroperoxide formation on Ti centers.
2
3
2
2
2
2 2
O
latter are dehydrated Ti(OOH) (225 nm band) and hydrated
Ti–OOH (360 nm band) species. DFT calculations provide the
assignments (see ESIw). Overall, the in situ PR and UV-Vis
spectroscopy results are consistent with the adsorption of
oxygen on the gold nanoparticles followed by activation with
H to produce H O , which transfers to a Ti center to form
The main products of reaction on the gold catalysts were
acetone (TS-1 support) and propylene (TiO support). It has
been proposed that propane activation on TS-1 yields an
2
1
9
isopropoxy intermediate, and this is likely the case for these
catalysts (Scheme 2a). A 21 proton on propane (most acidic)
undergoes attack by the terminal OH on the hydroperoxide,
and the electrons from the C–H bond transfer to the proximal
peroxide oxygen (most electropositive). The propoxy species
could produce acetone (Scheme 2b) or propylene (Scheme 2c),
as is observed for isopropanol on basic oxides.
2
2
2
hydroperoxide species (Scheme 1). In the scheme Au repre-
sents a gold cluster, not an isolated gold atom.
The different selectivities of the TS-1 and TiO
2
supports
2
could arise from differences in their acid–base properties.
0
Temperature-programmed desorption (TPD) traces of CO
2
2
and NH from Au–Ba/TS-1 (Ti : Si = 3 : 100) and Au/TiO
3
catalysts, and their corresponding supports clearly show
marked differences (Fig. 2). The Au–Ba/TS-1 sample mainly
has small amounts of weak acid and base sites, while the Au/
2
TiO catalyst also has stronger basic and acidic sites and in
larger concentrations.
Under the oxidizing conditions of the present study acetone
formation is expected to proceed by reaction of the
Scheme 2 Possible sequence of steps during propane selective oxida-
tion on gold supported Ti-containing catalysts: (a) formation of the
isopropoxide intermediate; (b) oxidation of the isopropoxide species to
acetone; (c) dehydration of the isopropoxide species to propylene.
Fig. 1 In situ UV-Vis spectra for Au–Ba/TS-1 (Ti : Si = 3 : 100)
under reaction conditions: (a) Au plasmon resonance position, and (b)
difference spectra with catalyst before reaction under Ar.
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Fig. 2 (A) CO
2
and (B) NH
3
temperature-programmed desorption traces for different gold supported catalysts and supports. (a) TS-1 (Ti : Si =
3
: 100); (b) Au–Ba/TS-1 (Ti : Si = 3 : 100); (c) TiO
2 2
(P25); and (d) Au/TiO .
isopropoxide intermediate with H O (Scheme 2b). On the
2
to a better understanding of gas-phase catalyzed oxidations on
gold supported catalysts.
2
other hand, propylene formation likely occurs by dehydration
of isopropoxy species. In analogy with isopropanol reactions
on metal oxides, this would be expected to occur on
strong–weak or weak–strong acid–base pair sites present on
Support was provided by the Ministry of Economy, Trade
and Industry, the National Science Foundation, the Japan
Society for the Promotion of Science, and KEK-PF.
the TiO
2
support, possibly via concerted E
2
or E1cB mechan-
2
isms, respectively. The E
1
Notes and references
2
dinuclear elimination mechanism
involves transfer of a proton on the adsorbed propoxy species
to a surface OH group, with simultaneous loss of the surface
1. J. F. Brazdil, Top. Catal., 2006, 38, 289.
2. Z.-C. He, Xiandai Huagong, 2005, 25(11), 61.
2
2,23
3. P. P. Notte
´
. A. J. Papa, Propanols, in Ullmann’s Encyclopedia of Industrial
, Top. Catal., 2000, 13, 387.
oxygen link (Scheme 2c).
The E1cB mononuclear elimina-
4
tion mechanism is similar, but occurs in two steps, with the
2
Chemistry [online], Wiley Interscience, Weinheim, Germany,
posted: June 15, 2000.
. H. Sun, F. Blatter and H. Frei, Catal. Lett., 1997, 44, 247.
. N. Mizuno, Catal. Surv. Jpn., 2000, 41, 49.
. R. Raja, C. R. Jacob and P. Ratnasamy, Catal. Today, 1999, 49,
2,23
proton abstracted first.
This is unlikely in the present case
5
6
7
because the Ti group is not a sufficiently strong base. The Ba
component is probably not involved in the reaction, as un-
promoted Au/TS-1 catalysts show similar conversion and
selectivity. As mentioned earlier, its main role is to increase
171.
8. C. Espro, F. Arena, F. Tasselli, A. Regina, E. Drioli and
A. Parmeliana, Catal. Today, 2006, 118, 253.
1
0
the Au loading (Table 1).
Propane oxidation has been previously reported in a H
cell system at 298 K. Acetone (39%) and acetaldehyde (13%)
9
. M. G. Clerici, Appl. Catal., 1991, 68, 249.
0. J. Q. Lu, X. Zhang, J. J. Bravo-Suarez, K. K. Bando, T. Fujitani
and S. T. Oyama, J. Catal., 2007, 250, 350.
–O
2 2
1
´
À1 À1 24
were produced at a STY of only 6 g kgcathode
h
.
More
11. S. Bordiga, F. Bonino, A. Damin and C. Lamberti, Phys. Chem.
Chem. Phys., 2007, 9, 4854.
2+
recently, propane oxidation mediated by a Fe –H
2
O
2
Fenton
1
2. P. Ratnasamy, D. Srinivas and H. Kno
8, 1.
13. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 1998, 178, 566.
¨
zinger, Adv. Catal., 2004,
system was carried out in a three-phase catalytic hollow fiber
membrane reactor. Total oxygenate selectivity (acetone, iso-
4
1
4. J. J. Bravo-Suarez, K. K. Bando, J. Q. Lu, M. Haruta, T. Fujitani
´
propanol, n-propanol, and propanal) was 96%, but the STY
À1 À1 8
and S. T. Oyama, J. Phys. Chem. C, 2008, 112, 1115.
5. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters,
Springer, Berlin, 1995.
reached only 5 g kgmembrane
propane is oxidized with O
h . In the present study
1
2
and H
2
to acetone and isopropa-
À1
nol with a high selectivity of 95% and a STY of 35 g kgcat
À1
16. B. Chowdhury, J. J. Bravo-Sua
K. K. Bando, S. Tsubota and M. Haruta, J. Phys. Chem. B,
006, 110, 22995.
7. J. J. Bravo-Suarez, K. K. Bando, J. Q. Lu, T. Fujitani and
´
rez, N. Mimura, J. Q. Lu,
h
, which is much higher than previously reported yields and
2
2 2
is similar to that obtained in a batch system using costly H O
1
´
and TS-1 with a selectivity to oxygenates (acetone and iso-
À1 À1 9
S. T. Oyama, J. Catal., 2008, 255, 114.
1
1
8. C. Reed, Y. Xi and S. T. Oyama, J. Catal., 2005, 235, 378.
9. C. B. Khouw, C. B. Dartt, J. A. Labinger and M. E. Davis,
J. Catal., 1994, 149, 195.
20. J. Cunningham, B. K. Hodnett, M. Ilyas, J. Tobin, E. L. Leahy
and J. L. G. Fierro, Faraday Discuss. Chem. Soc., 1981, 72, 283.
propanol) close to 100% and a STY of 79 g kgcat
h .
In conclusion, it has been demonstrated that Au supported
on TS-1 are active catalysts for propane oxidation with O and
2
H , and highly selective towards the formation of acetone and
2
2
1. V. K. Dı
2000, 63, 53; J. I. Di Cosimo, V. K. Dı
C. R. Apesteguıa, J. Catal., 1998, 178, 499.
22. M. A. Aramendıa, V. Borau, C. Jimenez, J. M. Marinas, A. Porras
´
ez, C. R. Apesteguı
´
a and J. I. Di Cosimo, Catal. Today,
isopropanol, with space-time yields (STY) comparable to
9
those obtained with liquid H O in a batch reactor. These
´
ez, M. Xu, E. Iglesia and
2
2
´
results are encouraging as further improvements in the cata-
lytic and reactor system will likely yield much higher STYs. In
situ spectroscopic data and catalyst characterization reveal the
´
´
and F. J. Urbano, J. Catal., 1996, 161, 829.
3. J. March, in Advanced Organic Chemistry. Reactions, Mechanisms,
and Structure, Wiley, New York, 4th edn, 1992, p. 290.
4. I. Yamanaka, S. Hasegawa and K. Otsuka, Appl. Catal., A, 2002,
226, 305.
2
2
presence of possible reaction intermediates (O
2
adsorbed on
metallic Au, Ti–OOH, and isopropoxy species) and contribute
3
274 | Chem. Commun., 2008, 3272–3274
This journal is ꢀc The Royal Society of Chemistry 2008