Water-tolerant Ru-Starbon materials for the hydrogenation of organic acids in aqueous ethanol

Article abstract of DOI:10.1016/j.catcom.2010.03.015

Highly active and stable ruthenium-supported nanoparticles show excellent activities and selectivities to target products in the aqueous hydrogenation of organic acids including succinic, fumaric, itaconic, levulinic and pyruvic acids. The materials were also highly reusable and comparably more active than related supported Ru materials.

Full text of DOI:10.1016/j.catcom.2010.03.015

Catalysis Communications 11 (2010) 928–931
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Water-tolerant Ru-Starbon® materials for the hydrogenation of organic acids in
aqueous ethanol
Rafael Luque a,b,⁎, James H. Clark b
a
Departamento de Química Orgánica, Campus de Rabanales, Edificio Marie Curie, Ctra Nnal IV, Km 396, Universidad de Córdoba, E-14014, Córdoba, Spain
Green Chemistry Centre of Excellence, The University of York, Heslington, YO10 5DD, York, UK
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly active and stable ruthenium-supported nanoparticles show excellent activities and selectivities to
target products in the aqueous hydrogenation of organic acids including succinic, fumaric, itaconic, levulinic
and pyruvic acids. The materials were also highly reusable and comparably more active than related
supported Ru materials.
Received 24 February 2010
Received in revised form 20 March 2010
Accepted 26 March 2010
Available online 1 April 2010
©
2010 Elsevier B.V. All rights reserved.
Keywords:
Platform molecules
Supported nanoparticles
Starbon
Hydrogenations
Organic acids
Reactions in aqueous media
1
. Introduction
provide routes to higher value chemical products. Recent work with
succinic acid proves this platform chemical can be turned into a wide
variety of high added-value chemicals including esters, alcohols and
lactones using different chemical transformations such as esterifica-
tions, etherifications, and hydrogenations [8,10–12], with most of
them conducted under fermentation-like conditions. These reports
prove that the biochemistry can be integrated in one process that
potentially turns crude biomass into high valuable chemicals.
Hydrogenation of organic acids can provide access to important
commodity and specialty chemicals that find applications as solvents,
plastics, polymers, additives and important intermediates [1–3,10].
We have recently reported the hydrogenation of succinic acid using
various supported nanoparticles on Starbon® materials, with a
maximum activity found for 5%Pt-Starbon® [13]. Nevertheless, an
interesting selectivity switch to tetrahydrofuran (THF) was found
under certain reaction conditions for Ru-Starbon®, which makes this
catalyst very interesting from a commercial point of view.
Green chemistry offers protocols that have already proved
important in facilitating commodity chemical and materials produc-
tion from biomass when developing biorefinery processes [1–4]. The
range of high added-value chemicals and (bio)materials that future
biorefineries could produce is extensive and is expected to become
greater with further research [1–4]. Considerable research work in the
context of green chemistry has been carried out on the replacement of
organic solvents with water in synthetic reactions [5–7]. Recently, this
has been extended to studying systems under fermentation-like
conditions [8]. An increasing range of chemicals can be currently
produced in large volumes through fermentation processes operating
in dilute aqueous solutions [9]. These are the so-called platform
molecules that are generally defined as chemicals containing multiple
functional groups (e.g. acids, ketones, alcohols, amines, etc). An
important family of these platform molecules are C
4
organic (di)acids
(
e.g. succinic, fumaric, itaconic, and levulinic acids). These acids were
In this work, we report the efficient Ru-catalysed hydrogenation of
a variety of biomass platform molecules including succinic, fumaric,
itaconic, levulinic and pyruvic acids in aqueous solutions under mild
reaction conditions.
included in the top biomass platform molecules as forecast for near
future large scale applicability [9].
Equally important to the production of these new building block
molecules is cost-effective and clean downstream chemistry that can
2
. Experimental
2.1. Materials preparation
⁎
Corresponding author. Departamento de Química Orgánica, Campus de Rabanales,
Edificio Marie Curie, Ctra Nnal IV, Km 396, Universidad de Córdoba, E-14014, Córdoba,
Spain. Tel.: +34 057212065; fax: +34 957212066.
Starbon® supports were synthesized as previously reported in the
E-mail address: q62alsor@uco.es (R. Luque).
literature [14]. The material carbonised at 300 °C was chosen for its
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.03.015

R. Luque, J.H. Clark / Catalysis Communications 11 (2010) 928–931
929
Table 1
ideal hydrophilicity/hydrophobicity ratio and its proven water-
tolerant properties [8,11,12].
Textural properties of Ru-Starbon® materials.
Ru nanoparticles were deposited on Starbon® following a simple
previously reported impregnation/reduction protocol [13] using
Materials
Metal loading
wt.%)
Surface area
Pore volume
Pore size
(nm)
2
−1
−1
(
(m
g
)
(mL g
)
RuCl
3
.xH
2
O as Ru precursor and ethanol/acetone as solvent under
1%Ru-Starbon®
0.89
3.01
4.61
6.26
252
279
228
225
0.56
0.54
0.52
0.55
4.4
4.2
3.9
4.0
3
5
7
%Ru-Starbon®
%Ru-Starbon®
%Ru-Starbon®
mild heating (45–55 °C). The final catalysts were then filtered off,
washed with acetone and activated at 100 °C overnight prior to their
use in the reaction. Different loadings were investigated (1–7 wt.%) to
find out the optimum catalyst loading in the reaction. Materials were
previously characterised by Aberration Corrected (Scanning) Trans-
mission Electron Microscopy (AC-TEM/STEM), N
X-Ray Photoelectron Spectroscopy (XPS) [13].
2
physisorption and
metal loadings of 3% or above provided 80–95% conversion of starting
material with varying selectivities to γ-butyrolactone (GBL) and THF
(Fig. 2), the main products obtained in the reaction [13]. Nevertheless, an
2
.2. Materials characterisation
increase of metal loading above 5% did not seem to have any effecton the
activity of the systems, although slightly higher selectivities to THF were
observed (Fig. 2). A maximum selectivity of 80–90% was obtained for the
higher loading Ru materials (5 and 7%Ru-Starbon®, Fig. 2). In view of
these results, 5%Ru-Starbon® was selected as optimum metal loaded
catalyst for further studies. A comparison between Ru supported
Starbon® and similarly loaded Ru on other supports was then performed
to further prove the advantages of our catalyst.
Nitrogen adsorption measurements were carried out at 77 K using
an ASAP 2010 volumetric adsorption analyzer from Micromeritics. The
samples were outgassed for 2 h at 100 °C under vacuum (pb10 Pa)
and subsequently analysed. The linear part of the BET equation (relative
pressure between 0.05 and 0.22) was used for the determination of the
specific surface area.
−
2
The metal content in the materials was determined using Inductively
Coupled Plasma (ICP) in a Philips PU 70000 sequential spectrometer
equipped with an Echelle monochromator (0.0075 nm resolution).
Results shown in Table 2 demonstrate the superior performance of
our systems (5-10 times more active) compared to Ru supported on
conventional commercial carbons in terms of conversion, activity per
metal atom and, most critically, selectivities to THF. Very low selectivities
to THF were found for Ru supported on commercial supports, with GBL
and1,4-butanediol (BDO) obtained asmajor reaction products under the
investigated reaction conditions. Despite the similar metal loadings
achieved in all materials, the remarkably observed differences in activity
of our systems is believed to be mostly due to the exceptional stability of
the Starbon® materials in aqueous media compared to related
microporous carbonaceous materials [11,12]. The homogeneous, narrow
and small nanoparticle size distribution in the support is also an
important advantage, in contrast to the appreciable formation of
domains with larger aggregates observed for DARCO® and similar
microporous carbons supported nanoparticles [13,15].
To further extend the scope of the protocol, various organic acids
were subsequently tested in the hydrogenation reaction. Results
included in Table 3 for the particular case of 5%Ru-Starbon®
demonstrate the protocol was also amenable to a range of substrates
including fumaric, itaconic, levulinic and pyruvic acids via reduction of
different functionalities (e.g. endo/exo double bonds, carbonyl groups).
The selectivities to the target product were excellent in almost all cases
with only traces of esterification products being observed.
3
Samples were digested in HNO and subsequently analysed by ICP at the
University of Newcastle.
2
.3. Catalytic experiments
A typical hydrogenation experiment was performed as follows:
0 mmol succinic acid, 30 mmol EtOH (2.7 mL), 50 mmol water
1
(
0.9 mL) and 0.1 g of Ru-Starbon® were added to the reaction vessel
of a Parr hydrogenator (model 5RH35HN60T). The reactor was then
filled and purged with H several times prior to its filling with H at
0 bar pressure. The reaction temperature was then set to 100 °C and
2
2
1
the reaction started after the vessel reached the target temperature. The
evolution of the reaction was monitored by periodically sampling
aliquots of reaction mixture that were subsequently analysed by GC/
GC–MS using an Agilent 6890N GC model equipped with a 7683B series
autosampler and fitted with a DB-5 capillary column and a FID detector.
For the reuse experiments, the catalyst was filtered off upon reaction
completion (more than 24 h), washed with acetone and methanol and
activated at 150 °C in an oven overnight prior to its reuse in the
hydrogenation reaction.
3
. Results and discussion
Ru nanoparticles were previously reported to be homogeneously
distributed on the support, being primarily single-crystalline with
.7 nmaverage particle size for the particularcase of the 5%Ru-Starbon®
2
material [13]. Interestingly, the nanoparticle size was found to be pretty
similar between materials regardless of the metal content (typically
between 2 and 3 nm for the 1 to 7 wt.% Ru loaded materials). Ru
0
particles were composed of a major Ru core (277.3 eV) with minor
contributions from metal oxide components, probably as passivating
2
metal oxide layers on some Ru particles [13]. N physisorption of the Ru-
2
−1
Starbon® showed high surface area materials (N200 m g ) with high
pore volumes were obtained. The average pore size was about 4 nm,
with some microporosity contribution [11,12,14] (Table 1).
The hydrogenation of succinic acid (SA) in aqueous ethanol was
chosen as a model reaction to investigate the activity of the Ru-Starbon®.
Blank runs in the absence of catalyst gave virtually no conversion of
starting material. Four different loadings were prepared in order to see
the effect of the metal loading in the conversion and selectivity of the
systems. Results included in Fig. 1 prove low conversions and
selectivities were obtained at low metal loadings (b3 wt.%) and only
Fig. 1. Activity of Ru-Starbons with various metal loadings in the hydrogenation of succinic
acid. Reaction conditions: 10 mmol succinic acid, 30 mmol EtOH, 50 mmol H
2 2
O, 10 bar H ,
100 °C, 0.1 g catalyst, 24 h reaction.

930
R. Luque, J.H. Clark / Catalysis Communications 11 (2010) 928–931
Table 3
Optimised catalytic activity [Conversion, selectivity (mol%) and Turnover frequency
−
1
(
TOF, h )] of 5%Ru-Starbon® in the aqueous phase hydrogenation of a range of organic
a
acids .
Substrate
Time of reaction Conversion Sproduct
TOF
(min)
(mol%)
(mol%)
(h−1)
9
4
0
5
N99
N99 (SA)
138
269
N99
N95 (methyl SA)
Fig. 2. Selectivity to THF of differently loaded Ru-Starbons in the hydrogenation of
succinic acid. Reaction conditions: 10 mmol succinic acid, 30 mmol EtOH, 50 mmol
H O, 10 bar H , 100 °C, 0.1 g catalyst, 24 h reaction. The difference to 100 in selectivity
2 2
corresponds mostly to GBL, with minor quantities (b15%) of 1,4-butanediol.
130
N99
N95
N95
92
(4-hydroxypentanoic
acid)
9
4
0
5
90 (lactic acid)
121
Most importantly, the catalysts were also stable and highly reusable
under the investigated reaction conditions, preserving over 95% of their
initial activity and selectivities after 5 reaction cycles (Fig. 3), irrespec-
tive of the hydrogenated substrate. The selectivity remains almost
unchanged after 5 uses in all cases (Table 3, last entry). The leaching of
the materials into solution was also checked by ICP/AES of the final
reaction mixture. No detectable metal traces in solution (b0.5 ppm)
were determined by this method, confirming the strong binding
between the Ru nanoparticles and the Starbon® surface that prevented
metal leaching during/after the reaction.
94b
N95b (methyl SA)
1292c
a
2 2
Reaction conditions: 10 mmol acid, 30 mmol EtOH, 50 mmol H O, 10 bar H ,
1
00 °C, 0.1 g catalyst.
b
These important findings constitute the first report of aqueous
hydrogenation of organic acids under mild and benign reaction
Reused 5%Ru-Starbon® after 5 reaction cycles (Fig. 3, 5th use).
Cumulative TOF after 5 uses (sum of the TOF values of every reuse cycle).
c
2
conditions (10 bar H , 100 °C, aqueous ethanol as solvent) using
biomass derived catalysts.
4
. Conclusions
We have demonstrated an efficient protocol for the mild hydroge-
nation of platform molecules in aqueous environments using Ru
nanoparticles supported on biomass derived materials (Starbons®).
Our nanomaterials proved to be very active and selective in the process
due to a combination of very good dispersions and homogeneity of the
metal on the catalyst, very small and narrow particle sizes and particle
distribution as well as outstanding water-tolerant properties of the
Starbon®-300 support in water. This novel methodology may pave the
way to the utilisation of biomass derived materials in biorefineries both
to improve the green credentials of chemical products using low impact
technologies and the utilisation of cheap, readily available and
renewable catalyst supports in chemical processes.
Fig. 3. Reuses of the 5%Ru-Starbon® catalyst in the aqueous hydrogenation of itaconic
acid to methyl succinic acid. Reaction conditions: 10 mmol itaconic acid, 30 mmol
EtOH, 50 mmol water, 0.1 g of the catalyst, 100 °C, 10 bar H pressure, 45 min reaction.
2
Acknowledgments
Table 2
Activity comparison of Ru-Starbon® [Turnover number (TON) and turnover frequency
−
1
RL gratefully acknowledges Ministerio de Ciencia e Innovación for
the concession of a Ramon y Cajal contract (RYC-2009-04199).
(
TOF, h )] with a variety of Ru supported materials in the aqueous hydrogenation of
a
succinic acid .
Catalyst
Metal loading
wt.%)
Conv.
(mol%)
S
THF
TON
TOF
(h−1)
References
(
(mol%)
5
5
5
5
%Ru-Starbon®
%Ru-Norit®
%Ru-Vulcan®
%Ru-DARCO®
4.61
4.76
4.87
5.12
90
42
65
72
60
223
23
38
10
1
2
[1] J.H. Clark, F.E.I.D. Deswarte, T.J. Farmer, Biofuels Bioprod. Bioref. 3 (2009) 72–90.
[2] Y.C. Lin, G.W. Huber, Energy Environ. Sci. 2 (2009) 68–80.
[3] J.H. Clark, F.E.I.D. Deswarte, Introduction to Chemicals from Biomass, John Wiley &
Sons, Chichester, 2008.
b
b30
b30b
b35b
41
2
[
4] B. Kamm, P.R. Gruber, M. Kamm (Eds.), Biorefineries—Industrial Processes and
Products, Status Quo and Future Direction, Wiley-VCH, Weinheim, Germany,
2006, pp. 16–20.
a
Reaction conditions: 10 mmol SA, 30 mmol EtOH, 50 mmol H
2
O, 10 bar H
2
, 100 °C,
0
.1 g catalyst, 24 h reaction.
b
GBL (N40%) and BDO (b20%) were detected as major hydrogenation products.
[5] C.X.A. Da Silva, V.L.C. Gonçalves, C.J.A. Mota, Green Chem. 11 (2009) 38–41.

R. Luque, J.H. Clark / Catalysis Communications 11 (2010) 928–931
931
[
6] P. Makowski, R. Demir-Cakan, M. Antonietti, F. Goetmann, M.M. Titirici, Chem.
Commun. (2008) 999–1001.
[11] V. Budarin, R. Luque, D.J. Macquarrie, J.H. Clark, Chem. Eur. J. 13 (2007) 6914–6919.
[12] V. Budarin, J.H. Clark, R. Luque, D.J. Macquarrie, A. Koutinas, C. Webb, Green Chem.
9 (2007) 992–995.
[13] R. Luque, J.H. Clark, K. Yoshida, P.L. Gai, Chem. Commun. (2009) 5305–5307.
[14] V. Budarin, J.H. Clark, J.J.E. Hardy, R. Luque, K. Milkowski, S.J. Tavener, A.J. Wilson,
Angew. Chem. Int. Ed. 45 (2006) 3782–3785.
[
[
7] T. Okuhara, Chem. Rev. 102 (2002) 3641–3665.
8] R. Luque, C.S.K. Lin, C. Du, D.J. Macquarrie, A. Koutinas, R. Wang, C. Webb, J.H.
Clark, Green Chem. 11 (2009) 193–200.
[
9] T. Werpy, G. Petersen, Top Value added chemicals from biomass, U.S. Department
of Energy, NREL/TP-510-35523, 2004. http://www1.eere.energy.gov/biomass/
pdfs/35523.pdf.
[15] C. Gonzalez-Arellano, J.M. Campelo, D.J. Macquarrie, J.M. Marinas, A.A. Romero, R.
Luque, Chem. Sus. Chem. 1 (2008) 746–750.
[10] C. Delhomme, D. Weuster-Botz, F.E. Kühn, Green Chem. 11 (2009) 13–26.