Communication
Isolated gold atoms and dimers were prepared on function-
alized multiwalled carbon nanotubes and detected by high-
angle annular dark field STEM (HAADF-STEM). The synthesis
and characterization procedures were described in detail earli-
ment factors were calculated: 437, 539, and 1090 for the CH3,
CH ,and vinyl CH groups of 1-butene, respectively; 185 and
2
2
244 for the CH and CH groups of butane, respectively; 220
3
2
for the CH group of 2-butene. The NMR signal of 2-butene in
3
[14]
er. This material was previously used in the aerobic oxidation
the spectrum detected using normal hydrogen is clearly ob-
servable, therefore the ratio of the PHIP-enhanced and equilib-
rium signals for this product is straightforward to calculate.
The percentage of the pairwise hydrogen addition for the for-
mation of 2-butene was thus estimated as 2%. For other prod-
ucts, the exact determination of the percentage of pairwise ad-
dition and/or signal enhancement from our data is not possi-
ble, because the NMR signals of 1-butene and butane are not
observed at all in the NMR spectrum recorded with the use of
normal hydrogen. However, an estimate of the minimum value
of signal enhancement that would be consistent with the re-
sults obtained is still possible. As the NMR signal intensities of
these products in the spectrum obtained using normal hydro-
gen do not exceed the noise level, the observed signal en-
hancement cannot be smaller than the ratio of the PHIP-en-
hanced signal to the level of noise. As a result, for 1-butene
formed in the hydrogenation of 1,3-butadiene with parahydro-
gen, the contribution of the pairwise route was estimated to
of thiophenol with O . In those studies, under the reaction
2
conditions the initially present single gold atoms were found
to aggregate to form clusters of 5–10 gold atoms that were
[
14]
highly active for the above-mentioned reaction. The original
material, containing isolated Au atoms and dimers on
MWCNTs, has been used here in the heterogeneous hydroge-
nation of 1,3-butadiene and 1-butyne.
Initially, the monoatomic Au/MWCNTs catalyst was used in
the heterogeneous hydrogenation of 1,3-butadiene. The
1
H NMR spectra for 1,3-butadiene hydrogenation with normal
hydrogen (ratio of ortho/para isomers=3:1) and parahydrogen
(ortho/para=1:1) over the Au/MWCNTs catalyst are presented
in Figure 1. It can be seen that all hydrogenation reaction
be not less than 6% from the CH group NMR signal and not
3
less than 3% from the CH group signal. Similarly, the percent-
2
age of the pairwise hydrogen addition was found to be not
less than 11 % and 7% from the NMR signals of the CH and
3
CH groups of butane, respectively. It should be noted that the
2
estimated percentage values are the lower limits for the pair-
wise addition contributions. Indeed, the intensities of the NMR
signals of 1-butene and butane in the NMR spectrum recorded
with the use of normal hydrogen are smaller than the noise
level, so the actual pairwise percentage values can be larger. In
addition, polarization losses caused by nuclear spin relaxation
can reduce the intensity of the enhanced NMR signals in the
spectra detected using parahydrogen, also potentially leading
to the underestimation of signal enhancement and the contri-
1
[10e]
Figure 1. Reaction mechanism (a) and H NMR spectra (b,c) recorded during
1
bution of the pairwise reaction route.
,3-butadiene hydrogenation with parahydrogen and normal hydrogen over
We note that when a fluid (in this case, 1,3-butadiene) rapid-
ly flows into an NMR magnet, its NMR signal is initially much
lower than its equilibrium value in the high magnetic field.
This incomplete equilibration of the nuclear spin magnetiza-
tion is carried over to all product molecules when 1,3-buta-
diene is hydrogenated. As a result, the NMR signal intensities
of the products may be significantly suppressed and can lead
to a significant over-estimation of the signal enhancements
obtained. These effects were not taken into account in
a recent report on the heterogeneous hydrogenation of pro-
the monoatomic Au/MWCNTs catalyst.
products (1-butene, 2-butene, and butane) exhibit PHIP effects,
indicating that they can be formed via the pairwise hydrogen
addition route. To estimate the contribution of the latter to the
overall reaction mechanism, 1,3-butadiene was also hydrogen-
ated with normal hydrogen. The results (Figure 1) showed that
in this case only the NMR signals of 2-butene were possible to
detect.
[16]
The percentage of pairwise hydrogen addition was evaluat-
ed by calculating the ratio of the signal enhancement ob-
served experimentally and the maximum signal enhancement
pene over ligand-capped platinum nanoparticles. In the ex-
periments reported herein, an insert filled with charcoal was
placed upstream of the reaction region to accelerate the nucle-
ar spin relaxation of 1,3-butadiene before it enters the reac-
tion, and thus to ensure that the NMR signals of 1-butene, 2-
butene, and butane are in thermal equilibrium in the strong
magnetic field, that is, that they are not artificially sup-
predicted theoretically if only the pairwise addition of H to
2
a substrate takes place. This procedure was described in detail
[
15]
previously. In addition, the influence of the NMR line width
on the observed signal enhancement was taken into account
here, because line broadening may lead to the loss of observa-
[17]
pressed.
[6a]
ble signal intensity for the antiphase PHIP (PASADENA ) pat-
tern. As a result, the following theoretical maximum enhance-
It is quite remarkable that strong PHIP signals are observed
for 1-butene and butane produced in the heterogeneous hy-
Chem. Eur. J. 2015, 21, 7012 – 7015
7013
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