Single-step conversion of dimethyl terephthalate into cyclohexanedimethanol with Ru5PtSn, a trimetallic nanoparticle catalyst

Article abstract of DOI:10.1002/anie.200600359

Highly active and selective: A supported Ru₅PtSn nanoparticle cluster (the picture shows an axial projection of a tomogram), prepared from the carbonyl cluster [PtRu₅(CO)₁₅(μ-SnPh ₂)(μ₆-C)], is an excellent catalyst in the single-step hydrogenation of dimethyl terephthalate to cyclohexanedimethanol under mild conditions (100°C, 20 bar H₂). (Figure Presented).

Full text of DOI:10.1002/anie.200600359

Communications
ethylene glycol as a stepping stone in the production of
polyester fibers for applications involving polycarbonates and
[
1,3]
polyurethanes.
Industrially, CHDM is prepared by a two-
[
1,4]
step process using two reactors.
The first step is the highly
exothermic conversion of dimethyl terephthalate (DMT) into
dimethyl hexahydroterephthalate (DMHT) by using a sup-
ported Pd catalyst in the temperature range of 160–1808C and
an H pressure of 30–48 MPa (300–480 bar).The intermediate
2
DMHT is then converted into CHDM by using a copper–
chromite catalyst at temperatures about 2008C and an H
2
[
4]
pressure of about 40 bar. We have already shown that
[
5]
bimetallic nanoparticle catalysts, such as (silica-supported)
Ru Pt, Ru Pt , Ru Pd , and Ru Cu , can promote the single-
5
10
2
6
6
12
4
step conversion of DMT into CHDM and are more efficient
[
6]
than the Angelici-type catalyst, which consists of a rhodium
complex tethered on silica-supported palladium.Herein, we
report that of the wide range of possible hydrogenation
products of DMT (Scheme 1) mainly the desired products
DMHT and CHDM are formed with supported nanoparticles
of the trimetallic cluster Ru PtSn.This catalyst shows both the
5
highest activity and selectivity yet observed in any single-step
conversion of DMT into CHDM under mild conditions
Efficient Hydrogenation
(
1008C, 20 bar H ).
2
DOI: 10.1002/anie.200600359
The parent material, [PtRu (CO) (m-SnPh )(m -C)] (1;
5
15
2
6
[
7]
Figure 1), was obtained from the reaction of Ph SnH with
3
Single-Step Conversion of Dimethyl
[8]
the hexanuclear bimetallic complex [PtRu (CO) (m -C)].
5
16
6
Terephthalate into Cyclohexanedimethanol with
For comparison, we also carried out a few experiments (see
Ru PtSn, a Trimetallic Nanoparticle Catalyst**
5
below) using [PtRu (CO) (m-GePh )(m -C)] (2) prepared
5
15
2
6
from the same hexanuclear complex and Ph GeH.The
3
heterotrinuclear (Ru-Pt-Sn) complex was anchored onto a
mesoporous silica (Davison 38 Š) through silanol groups, as
Ana B. Hungria, Robert Raja, Richard D. Adams,*
Burjor Captain, John Meurig Thomas,*
Paul A. Midgley, Vladimir Golovko, and
Brian F. G. Johnson
[
5a,9]
described previously.
The reaction conditions and the
results for the hydrogenation of DMT are presented in
Table 1.From these data and from Figure 2, the superior
performance of the trimetallic nanoparticle catalyst is clear.
This is not surprising in view of the well-known role of tin as a
As a linker molecule in the polymer industry, 1,4-cyclo-
hexanedimethanol (CHDM) is a highly valued and exten-
[
10]
modifier in bimetallic petroleum-reforming catalysts. It has
also been shown that Pt enhances the activity of RuSn
catalysts for the hydrogenation of 1,4-cyclohexanedicarbox-
[
1]
[2]
sively used reagent. It is, for example, preferred over
[
11]
ylic acid to CHDM.
[
9a]
[
*] Prof. Dr. R. D. Adams, Dr. B. Captain
Department of Chemistry and Biochemistry
University of South Carolina
The significant improvement in performance that tin
effects to bimetallic catalysts is also relevant here.In previous
studies, EXAFS measurements showed that the tin atom
serves to anchor the metal clusters firmly to the siliceous
Columbia, SC 29208 (USA)
Fax: (+1)803-777-6781
E-mail: adams@mail.chem.sc.edu
[
12]
supports.
needed to shed light on the dramatic difference in behavior
between Ru PtSn and Ru PtGe, but we can say right away
Similar measurements (now underway) are
Dr. A. B. Hungria, Prof. J. M. Thomas, Dr. P. A. Midgley
Department of Materials Science
University of Cambridge
5
5
that it was extremely difficult to anchor the germanium
Cambridge, CB23QZ (UK)
Fax: (+44)1223-334-563
E-mail: jmt2@cam.ac.uk
parent 2 onto the silica under the same conditions for which 1
IV
IV
became firmly attached.Both Sn
and Ge are well-
[13,14]
known
to be able to replace tetrahedrally coordinated
Dr. R. Raja, Dr. V. Golovko, Prof. B. F. G. Johnson
University Chemical Laboratory
University of Cambridge
IV
Si ions in open-framework solids.However, in this work,
only infinitesimal quantities of Ge could be detected by
electron-induced X-ray emission.We believe that it is more
difficult to attach Ge to the siliceous surface because it is
more difficult to cleave GeꢀC bonds than SnꢀC bonds in the
Cambridge, CB21EW (UK)
[
**] This research was supported by the Office of Basic Energy Sciences
of the US Department of Energy under Grant No. DE-FG02-
00ER14980.
EPh ligands of the precursors 1 and 2, respectively.
High-angle annular-dark-field (HAADF) examination
of the monodispersed trimetallic catalysts showed high
2
[
15]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4
782
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4782 –4785

Angewandte
Chemie
dispersion on the supports
both before and after catalysis
Figure 3).There was clearly
(
no sintering of the nanoparti-
cles during the exothermic
hydrogenation reaction.More-
over,
nanoanalysis—by
induced X-ray
extremely
localized
electron-
emission
(Figure 4)—showed that the
composition of individual par-
3
ticles (volumes of ca.1 nm
analyzed) is very close to the
stoichiometry Ru PtSn of the
5
Scheme 1. Possible hydrogenation products of DMT.
precursor 1.
Figure 2. Bar chart comparing the activity and selectivity of the
Ru PtSn catalyst with those of other bi- and trimetallic catalysts for the
5
hydrogenation of DMT. H pressure: 20 bar; T=373 K; t=24 h. See
2
Table 1 and Experimental Section for further details.
Figure 1. Molecular structure of complex 1.
HAADF tomography has been successfully applied
before to explore the spatial distribution and morphology of
[
5,16]
supported nanoparticles.
With this purpose, a series of Z-
contrast (Z = atomic number)
images of the Ru PtSn catalyst
was acquired every 28 between
[
a]
5
Table 1: Comparison of catalysts for the hydrogenation of DMT.
[
b]
Catalyst
t
T
Conv.
TOF
Product distribution [mol%]
ꢀ708 and 708.Figure 5 shows the
ꢀ
1
[
h]
[K]
[mol%]
[h
]
DMHT
CHDM
A
B
axial projection of a specimen
tomogram together with a series
of single slices through the tomo-
gram, from which a tendency of the
particles to be evenly distributed
mainly close to the surface of the
material can be observed.In cer-
tain slices (Figures 5b and c) there
are some zones with a greater
density of particles inside the
volume of the material.(A movie
showing the tomographic retrieval
of the interior distribution of the
trimetallic nanoparticle catalysts is
provided in the Supporting Infor-
mation.)
Ru PtSn/SiO2
4
8
4
8
4
8
8
4
8
4
4
8
4
4
8
4
373
373
373
393
393
413
393
373
373
373
373
373
373
373
373
373
16.8
36.5
64.9
45.4
76.6
61.2
63.9
2.2
4.1
6.2
6.8
21.2
41.3
–
5.3
8.0
247
278
242
339
290
376
387
19
35
22
148
198
145
–
54.7
41.5
36.2
34.9
29.7
30.3
9.5
85.0
83.3
81.5
55.3
51.2
54.6
–
45.2
57.5
62.5
60.0
45.1
43.9
71.2
10.2
10.5
10.8
31.9
11.5
6.5
–
–
–
–
5
0.9
1.5
5.0
16.4
15.8
11.2
2.5
3.7
4.7
12.7
37.2
38.8
–
2
2
Ru PtSn/SiO2
5
–
8.9
9.9
8.3
2.2
2.5
2.9
–
–
–
–
22.6
18.6
Ru PtSn/SiO2
5
[
c]
Ru PtSn/SiO2
5
Ru PtGe/SiO2
5
2
2
2
Ru Pt/SiO2
5
[d]
Ru Sn/SiO2
6
–
–
–
54
27
77.2
81.0
–
–
[a] Conditions, unless otherwise given: SiO : mesoporous Davison silica with a pore diameter of 38 Š;
2
ꢀ1
ꢀ1
In summary, we have discov-
ered the synergistic value of the
H pressure: 20 bar. [b] TOF=[(molsubstr)(molcluster
) h ]. [c] H pressure: 40 bar. [d] Mesoporous SiO
2 2
2
of the MCM-41 type.
Angew. Chem. Int. Ed. 2006, 45, 4782 –4785ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4783

Communications
Figure 3. HAADF images of Ru PtSn nanoclusters on Davison 38 Š
5
silica before (a) and after (b) catalysis.
trimetallic nanoparticle catalyst Ru PtSn as a highly efficient
5
and selective means of effecting the catalytic single-step
hydrogenation of DMT to CHDM under mild conditions.The
presence of tin in the nanoparticle catalyst appears to play a
key role in anchoring the particle owing to its oxophilicity for
the support, which in turn diminishes the tendency for the
nanoparticles to sinter.
Experimental Section
The organometallic complexes 1 and 2 were prepared according to a
[
7]
published procedure. The catalysts were prepared following stan-
[
5a,9]
dard procedures reported earlier.
porous support the popular (organic-template-derived) MCM-type
which have non-intersecting pores) and related silicas such as SBA-
5, we used a commercially available (from Grace Davison) desiccant
Rather than employing as
(
1
Figure 5. Axial projection of a specimen tomogram (a), with successive
slices (b–d) through a scanning electron tomogram of Ru PtSn
5
silica with a narrow pore-size distribution (designated Davison 923;
pore diameter 38 Š).This support is made by reacting sodium silicate
with a strong mineral acid (usually sulfuric acid); the pore size is
controlled by gel time, final pH value, temperature, concentration of
reactants, etc.Compared to MCM-41 type silica, the Davison silica is
much lower in cost, thermally and mechanically more stable, and less
susceptible to structural collapse.It also has some intersecting pores
that facilitate the diffusion of the reactant species to the immobilized
supported on Davison 38 Š silica. See Supporting Information for a
video of the full dynamic tomogram.
support (with the anchored cluster) was washed twice with diethyl
ether (20 mL) and dried in vacuum.The catalyst was activated
(decarbonylated) by calcination in vacuum at ca.200 8C for 2 h.The
loading of the clusters on the mesoporous support varied from sample
to sample; it was accurately determined by inductively coupled
2
ꢀ1
catalyst, and its surface area is close to 700 m g
.
A slurry of cluster 1 or 2 (100 mg) in dichloromethane (20 mL)
and diethyl ether (20 mL) was added under inert atmosphere (Ar) to
plasma (ICP) analysis for each catalytic experiment.For the Ru
cluster, the loadings on the support varied from 0.0015 to
0.0026 mmol, whereas for the Ru PtGe cluster the loadings were in
the range of 0.00165 to 0.00372 mmol.
5
PtSn
4
00 mg of thoroughly degassed mesoporous silica Davison 923.The
mixture was stirred for 12 h.The support with the adsorbed cluster
was allowed to settle and the liquid (on top) was removed.The
5
Figure 4. a) Electron-induced X-ray emission spectrum of nanoparticles of Ru PtSn nanoclusters on Davison 38 Š silica. The arrow in the inset
5
points to the particle for which this emission spectrum was recorded. The peak for Cu in this spectrum originates from the sample holder. b) Plot
showing the uniformity of the composition of several nanoparticles of the Ru PtSn catalyst: triangles Ru; circles Sn; squares Pt.
5
4
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 4782 –4785

Angewandte
Chemie
[
5a]
Electron microscopy: For the tomograms shown in Figure 5, 71
images were recorded with an acquisition time of 20 s every 28 from
analogous to those reported earlier were carried out to rule out the
possibility of leaching, and analysis of the resulting filtrate at the end
of reaction (24 h) by ICP and AAS revealed only trace amounts
(< 5 ppb) of dissolved metal ions (Pt, Ru, Sn).
+
708 to ꢀ708 using a Fischione ultrahigh-tilt tomography holder
model 2020 and a FEI Tecnai F20 field emission gun transmission
electron microscope operated at 200 kV in scanning transmission
electron microscope (STEM) mode.The probe size was approx-
imately 0.5 nm in diameter, and each HAADF image was recorded
with a pixel size of 0.27 nm using a Fischione HAADF detector. The
Received: January 27, 2006
Revised: April 24, 2006
Published online: June 23, 2006
“
missing wedge” of data (at high tilts) leads to anisotropic spatial
resolution, with a degradation of resolution in the direction parallel to
the optical axis of about 30%.Image acquisition was undertaken
using the FEI software package Xplore3D.Images were then aligned
sequentially using Inspect 3D.Reconstructions, again with
Inspect 3D, were performed using either weighted back-projection
Keywords: electron microscopy · heterogeneous catalysis ·
hydrogenation · nanoparticles · tomography
.
[
1] S.R.Turner, Polym. Sci. 2004, 42, 5847.
(WBP) routines or an iterative routine (SIRT) that constrains the
[
2] B.J.Sublett, G.W.Connell (Eastman Chemical), US-A 5559159,
reconstructed volume to match the original images when re-projected
back along the original tilt directions.This constraint has the effect of
minimizing some of the unwanted effects of the limited data sampling
and greatly reduces the “fan” artifact that can be evident in many
WBP reconstructions.Voxel projections were constructed in
Inspect 3D, and surface rendering (after a segmentation process)
was undertaken using Amira software.
1995 [Chem. Abstr. 1996, 125, 277914v].
[
[
[
3] R.R.Amborse, J.B.OꢀDwyer, B.K.Johnston, D.P.Zielinski, S.
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Catalysis: The liquid-phase hydrogenation of DMT was carried
out in a high-pressure, teflon-lined, stainless steel catalytic reactor
(150 mL).The catalyst (50 mg), which was stored under inert
2
001, 40, 4638; b) J.M. Thomas, B.F.G. Johnson, R. Raja, G.
Sankar, P.A.Midgley, Acc. Chem. Res. 2003, 36, 20.
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58.
conditions (Ar), was transferred to the reactor (using a robotically
controlled catalyst-delivery unit) containing about 2.5 g of DMT
[
[
(
Aldrich, ꢁ 99% pure) dissolved in ethanol (75 mL) and 0.5 g of the
6
internal standard (hexadecane).The reactor was sealed, and its
contents were inertized (thrice) with dry N prior to reaction.The
2
1
contents of the reactor were stirred (1700 rpm) and heated to the
desired temperature (from a low of 373 K to a high of 413 K).Dry
hydrogen (dynamic pressure of 20 or 40 bar, see Table 1) was
pressurized into the reaction vessel and, using mini-robot liquid-
and gas-sampling valves, small aliquots (0.1 mL) of liquid and gas
samples were removed to study the kinetics of the reaction without
perturbing the pressure in the reactor.
The composition of the liquid and gaseous products was
continuously monitored by using an online computer-controlled
system linked to a GC and LC-MS system (Shimadzu QP 8000).The
products were analyzed (using hexadecane as the internal standard)
by gas chromatography (Varian, Model 3400 CX) employing an HP-1
capillary column (25 m ” 0.32 mm) and a flame ionization detector.
The identities of the products were first confirmed using authenti-
cated standards, and their individual response factors were deter-
mined by using a suitable internal standard (calibration method).The
conversions (Conv.) and selectivities (Sel.) were determined as
defined by Equations (1) and (2), and the yields were normalized
[
8] R.D.Adams, W.Wu, J. Cluster Sci. 1991, 2, 271.
[
9] a) S. Hermans, R. Raja, J.M. Thomas, B.F.G. Johnson, G.
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mol
ꢀmolsubstrate ðresidualÞ
[14] a) R.D. Oldroyd, J.M. Thomas, G. Sankar,
Chem. Commun.
substrate ðinitialÞ
Conv: ½%Š ¼
ꢂ 100
ð1Þ
ð2Þ
1997, 2025; b) R D. .Oldroyd, G.Sankar, J M. .Thomas, D.
mol
substrate ðinitialÞ
Ozkaya, J. Phys. Chem. B 1998, 102, 1849; c) J.M.Thomas, G.
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molindividual product
moltotal products
Sel: ½%Š ¼
ꢂ 100
[
[
with respect to the response factors obtained as described above.For
the internal-standard GC method, the response factor (RF) and
mol% of individual products were calculated using Equations (3) and
16] a) P.A. Midgley, M. Weyland, J.M. Thomas, P.L. Gai, E.D.
Boyes, Angew. Chem. 2002, 114, 3958; Angew. Chem. Int. Ed.
(
4).The identity of the products was further confirmed by GC-MS.
2002, 41, 3804; b) J.M.Thomas, P.A.Midgley, T.J.Yates, J.S.
molproduct
molstandard areaproduct
areastandard
Barnard, R.Raja, I.Arslan, M.Weyland, Angew. Chem. 2004,
116, 6913; Angew. Chem. Int. Ed. 2004, 43, 6745.
RF ¼
ꢂ
ð3Þ
ð4Þ
areaproduct
100
ꢂ mol
mol % product ¼ RF ꢂ molstandard
ꢂ
areastandard
sample
The Ru PtSn catalysts were reused three times without appreci-
5
able loss in catalytic activity or selectivity.Further experiments
Angew. Chem. Int. Ed. 2006, 45, 4782 –4785ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4785