more reluctant to undergo hydrogenation on CeO2, than when
the carbonyl group was activated on Ru/TiO2. These spectro-
scopic data agree very well with the catalytic results. There-
fore, according to the data obtained on the influence of Ru
particle size and the CQO shift in IR, the high activity
observed with the Ru/TiO2 catalyst may arise from the synergism
of small ruthenium crystallite size to activate the H2 and the
role of the TiO2 support to adsorb and activate the acid,
probably in the periphery of the metal crystallites.
differences in activity cannot be due to homogeneous reactions.
Additionally, a study of catalyst productivity was carried out
by performing the reaction with a ratio lactic acid/catalyst of
3482, and a turnover number of 893 was achieved with the
Ru(0.64%)/TiO2 catalyst. Also, the recyclability of the
catalyst was proved by performing three consecutive runs
without any loss of activity.
The scope of the catalyst has been demonstrated by carrying
out the hydrogenation of various acids. The results are
summarized in Table 1. It can be seen there that high yields
to the corresponding lactones are generally obtained with
TONs at least twice as high for Ru(0.64%)/TiO2 than for
Ru(5%)/C.
To evidence the heterogeneous nature of the hydrogenation
taking place in our system and to check if the homogeneous
catalysis can take place, two different leaching tests were
performed. The hot filtration test consists in stopping the
reaction at half conversion, and filtering off the catalyst
without cooling the solution, so to avoid re-deposition of the
ruthenium particles on the support. As shown in ESI,w after
hot filtration of the solid catalyst, no further conversion was
observed, indicating the absence of active species in the
solution formed by metal leaching.
From the results presented above, it can be concluded that
Ru(0.64%)/TiO2 is the most active catalyst for the reduction
of the hydroxycarboxylic acids reported until now. The
catalytic activity of Ru(0.64%)/TiO2 can be explained from
the synergism between the small particle size of Ru crystallites
and the activation of the carbonyl group on the TiO2. This
synergetic effect between the metal and the support enhances
more than three times the catalytic activity of Ru(0.64%)/TiO2
(TOF per surface metal atom) with respect to the conventional
Ru supported on active carbon.
When ICP analysis is performed to the reaction mixture
with Ru/TiO2 catalyst after 5 hours of reaction, only 0.1%
of the total ruthenium contained in the fresh catalyst was
determined in the solution. This amount is much lower than
for the commercial Ru(5%)/C, where 0.6% of leaching was
determined for the same reaction. The results indicate that
Financial support by the Spanish MICNN (Consolider-
Ingenio 2010 (Proyecto MULTICAT)) and Generalidad
Valenciana (Prometeo/2008/130) is gratefully acknowledged.
A.P. thanks the Generalitat Valenciana for a Prometeo
research associate contract.
Table 1 Comparison of the catalytic activity of Ru(5%)/C and
Ru(0.6%)/TiO2 for the catalytic hydrogenation of three functionalized
carboxylic acids. Reaction conditions: 0.4 mol%Ru, 150 1C and 35 bar
H2 pressure
Notes and references
Conversion
(%)/TON
z All the catalysts have been submitted to exhaustive washings until
they are free of chloride based on the silver nitrate test.
y Due to the lack of carbon transparency and experimental limitations,
IR spectroscopy was found to be unsuitable to study propanoic acid
adsorption on Ru/C samples.
Substrate
Selectivity (%)
Catalyst Ru(5%)/C
90
Levulinic acid
100/106
10
1 J. Lauridsen, J. Am. Oil Chem. Soc., 1976, 53, 400.
2 (a) A. E. Martin and F. H. Murphy, Krik-Othmer Encyclopedia of
Chemical Technology, New York, 4th edn, 1994; (b) D. T. Trent,
Krik-Othmer Encyclopedia of Chemical Technology, New York,
4th edn, 1996.
3 (a) A. J. McAlees and R. McCrindle, J. Chem. Soc. C, 1969, 2425;
(b) H. Adkins and H. R. Billica, J. Am. Chem. Soc., 1948, 70, 3118;
(c) E. Bowden and H. Adkins, J. Am. Chem. Soc., 1934, 56, 689.
4 H. S. Broadbent, G. C. Campbell, W. J. Bartley and J. H. Johnson,
J. Org. Chem., 1959, 24, 1847.
Succinic acid
Itaconic acid
75/12.5
90
30
100/28.6
44
25
5 (a) R. L. Augustine, Catal. Today, 1997, 37, 419; (b) M. J. Mendes,
O. A. A. Santos, E. Jordao and A. M. Silva, Appl. Catal., A, 2001,
217, 253; (c) K. Nomura, H. Ogura and Y. Imanishi, J. Mol. Catal.
A: Chem., 2002, 178, 105; (d) Y. Pouilloux, A. Piccirilli and
J. Barrault, J. Mol. Catal. A: Chem., 1996, 108, 161;
(e) K. Tahara, E. Nagahara, Y. Itoi, S. Nishiyama, S. Tsuruya
and M. Masai, Appl. Catal., A, 1997, 154, 75; (f) K. Tahara,
H. Tsuji, H. Kimura, T. Okazaki, Y. Itoi, S. Nishiyama,
S. Tsuruya and M. Masai, Catal. Today, 1996, 28, 267;
Catalyst Ru(0.6%)/TiO2
93
Levulinic acid
Succinic acid
Itaconic acid
100/247
7
(g) M. Toba, S. Tanaka, S. Niwa, F. Mizukami, Z. Koppa
L. Guczi, J. Sol-Gel Sci. Technol., 1998, 13, 1037; (h) M. Toba,
S. Tanaka, S. Niwa, F. Mizukami, Z. Koppany, L. Guczi,
K. Y. Cheah and T. S. Tang, Appl. Catal., A, 1999, 189, 243;
(i) M. Jahjah, Y. Kihn, E. Teuma and M. Gomez, J. Mol. Catal. A:
Chem., 2010, 332, 106.
´
ny and
100/38
100/68
98
´
´
32.4
58
9.6
6 (a) Z. Zhang, J. E. Jackson and D. J. Miller, Appl. Catal., A, 2001,
219, 89; (b) T. A. Werpy, J. G. Frye, A. H. Zacher and D. J. Miller,
2002, WO Patent 200335582.
7 M. Takasaki, Y. Motoyama, K. Higashi, S. H. Yoon, I. Mochida
and H. Nagashima, Chem.–Asian J., 2007, 2, 1524.
8 R. E. Benfield, J. Chem. Soc., Faraday Trans., 1992, 88, 1107.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3613–3615 3615