degradation to the hydrogenolysis and disproportionation
products. Catalyst choice makes a significant difference in
the dehydrogenation rate of indoline, and similar trends would
be expected for other N-heterocycles, where Pd/SiO2 and
Pd/Al2O3 show enhanced activity towards the dehydrogenation.
Steric hindrance about the nitrogen atom causes a marked
increase in the dehydrogenation of piperidine derivatives. The
extent of dehydrogenation of 2,6-di-tert-butylpiperidine
after 1 h at 100 1C was significantly greater than the ortho-
unsubstituted piperidine derivatives or 2,6-dimethylpiperidine.
Coupling this hindrance with an electron withdrawing and/or
conjugating group, as shown by Cui et al., could increase the
rate further.
Sol–gel catalyst procedure
Tetraethoxy orthosilicate (21.5 mmol, 4.8 ml), anhydrous
ethanol (2.0 ml), palladium(II) chloride (0.6 mmol, 108.0 mg)
and 30% aqueous ammonium hydroxide (0.3 ml) were
combined in a 15 ml round bottom flask equipped with a
magnetic stir bar and a water condenser under air. The
mixture was stirred magnetically and heated at 80 1C for
10 min under argon flow via needles at the top of the
condenser. Water (2.0 ml) was then injected directly into the
reaction mixture through the septum at the top of the condenser
using a 30 cm needle and then stirred for an additional 4 h,
during which time gel formation took place. The reaction
mixture was filtered, dried under dynamic vacuum for 18 h,
then further dried in an oven at 400 1C for 4 h. Other catalysts
were prepared by replacing tetraethoxy orthosilicate and/or
Pd(II)Cl2 with similar amounts of different support precursors
(aluminium triethoxide, titanium(IV) i-propoxide, zirconium(IV)
ethoxide) and metal precursors (platinum(II) chloride, rhodium(III)
chloride hydrate, iridium(III) chloride hydrate, ruthenium(III)
chloride trihydrate, iron(III) chloride, copper(II) chloride,
nickel(II) chloride, and cobalt(II) chloride hexahydrate).
Despite these findings, greater rates of endothermic dehydro-
genation are still required before the exothermic/endothermic
mixed fuel can be practical.
Experimental
All reagents except for 6 were purchased from chemical
suppliers and used as received. 1H NMR spectra were collected
at 300 K on a Bruker AV-400 spectrometer operating at
400.3 MHz and referenced to SiMe4. GC/MS analyses were
performed on an Agilent Technologies 6850 GC coupled with
an Agilent Technologies 5975C VL MSD with a Triple-Axis
Detector. MS spectra were analyzed by library comparison
using NIST MS Search 2.0.
Catalyst screening procedure
Indoline (3.0 mmol, 0.33 ml) was loaded into a 10 ml round
bottom flask containing the catalyst to be screened (1 mol%
loading, 0.03 mmol) under air. The flask, which was equipped
with a magnetic stir bar and a condenser, was immersed into
an oil bath pre-heated to 100 1C, under a constant argon flow
and stirred magnetically. A needle leading to an oil bubbler
allowed H2 gas, generated in situ to escape from the system.
After 60 min, the reaction was allowed to cool, and an aliquot
was obtained, dissolved in CDCl3, and filtered through Celite
545s. Yield was determined by 1H NMR spectroscopy. A
similar procedure was used for 2,6-di-tert-butylpiperidine
(1.92 mmol, 378.4 mg) with Pd/SiO2 (10 wt% Pd, 1 mol%
loading).
Synthesis of 2,6-di-tert-butylpiperidine (6)
To a 31 ml steel pressure vessel, equipped with a magnetic stir
bar, 2,6-di-tert-butylpyridine was added with 1–2 mol% Pd/C
under air. The vessel was then sealed, flushed three times with
H2, heated to 100 1C, pressurized to 80–90 bar with H2, and
stirred for 18 h. After cooling, the pressure was released from
the vessel. The desired product was isolated by filtration.
Purity was determined by 1H NMR spectroscopy. In all cases,
499% hydrogenation with 499% selectivity was observed.
The identity of the product was confirmed by comparing
1H NMR spectrum to published literature data.23
Acknowledgements
The authors gratefully acknowledge financial support from the
Natural Sciences and Engineering Research Council of
Canada, the Ontario Centres of Excellence (Energy), Defense
Research and Development Canada, AUTO21, Chrysler
Canada, and the Canada Research Chairs program.
Rate constant determination
Indoline (8.9 mmol, 1.0 ml) was injected via a syringe into a
10 ml round bottom flask containing 10 wt% Pd/C (1 mol%
loading, 0.089 mmol, 94.6 mg) and a magnetic stir bar under
air. The flask was equipped with a condenser, immersed in an
oil bath pre-heated to 100 1C and stirred magnetically for
60 min with constant argon flow provided via a needle through
a rubber septum at the top of the condenser to remove any H2
formed in situ. A second needle leading to an oil-bubbler
allowed gas to escape from the system. Small aliquots of the
reaction mixture were obtained after 5, 10, 15, 30, 45 and
60 min, dissolved in CDCl3, then filtered through Celite 545s.
Yields at each time were determined by comparing relative
Notes and references
1 L. Schlapbach and A. Zuttle, Nature, 2001, 414, 353–358.
¨
2 M. Felderhoff, C. Weidenthaler, R. von Helmholt and U. Eberle,
Phys. Chem. Chem. Phys., 2007, 9, 2643–2653.
3 L. J. J. Janssen, J. Appl. Electrochem., 2007, 37, 1383–1387.
4 U.S. Department of Energy, DOE Targets for Onboard Hydrogen
energy.gov/hydrogenandfuelcells/storage/pdfs/targets_onboard_
hydro_storage.pdf (accessed April 15, 2010).
5 F. H. Stevens, V. Pons and R. T. Baker, Dalton Trans., 2007, 2613.
6 E. Fakiogula, Y. Yurum and T. N. Veziroglu, Int. J. Hydrogen
¨
Energy, 2004, 29, 1371.
1
integrations in the H NMR spectra. A plot of ln(n/n0) of the
7 A. Moores, M. Poyatos, Y. Luo and R. H. Crabtree, New J.
Chem., 2006, 30, 1675.
8 D. Wechsler, Y. Cui, D. Dean, B. Davis and P. G. Jessop, J. Am.
Chem. Soc., 2008, 130, 17195–17203.
starting material (indoline) versus time was linear for the
duration of the experiment (one half life). The slope is equal
to the negative of the rate constant.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 417–422 421