D. Ventura-Espinosa, S. Martín, H. García et al.
Journal of Catalysis 394 (2021) 113–120
sulphur and gold confirms the presence of NHC ligands and triflate
-
anions (OTf ) on the surface of Au NPs (see SI for the high-
resolution spectra and assignment of these elements). The high-
resolution spectrum of Au(4f) region for
doublet peaks, due to the spin–orbit splitting effect (4f7/2 and
5/2) at binding energies of 88.7 and 85.0 eV attributed to Au(I)
a-NPs-rGO shows two
4
f
[
(
50,51] and at 88.0 and 84.3 eV attributed to Au(0) [52,53]
Fig. 3d). Analysis of the corresponding areas associated with each
signal in the convoluted spectrum shows an area ratio of 2:1 for
the peaks attributed to Au(0) respect to the peaks attributed to
Au(I). In the case of the hybrid material containing a naphthalene
group (b-NPs-rGO), the two doublet peaks appear at the same
binding energies than that for a-NPs-rGO, confirming the presence
of Au(I) and Au(0), albeit in this case the relative area ratio is
inverted with Au(I) being the prevalent (Fig. 3h). Surprisingly, in
the case of c-NPs-rGO only a doublet peak at the binding energies
corresponding to Au(I) is observed without detecting the presence
of Au(0) (Fig. 3i). This last result is in agreement with the findings
of Toste and Somorjai who reported that XPS of Au NPs functional-
ized with chiral-NHC ligands showed only the presence of Au(I). A
further study using EXAFS revealed a major contribution Au(0)
[
48]. In our case, the differences in the XPS analysis are ascribed
to a ligand effect. The three hybrid materials have the same Au
environment but different remote ligand functionalization and in
the case of the bulkier polyaromatic group (pyrenyl) only Au(I) is
observed.
3.2. Ligand effects in the catalytic hydration of alkynes.
The evaluation of ligand effect in the stabilization of Au NPs was
assessed in hydration and intramolecular hydroamination of
alkynes. Gold is an efficient catalysts in the hydration of alkynes
for a variety of different substrates at low catalysts loadings and
under mild reaction conditions [54–56]. We chose 4-octyne as a
model substrate. In all cases the catalytic reactions were carried
out under the same conditions and using a low catalyst loading
(
[Au] 0.05 mol%). The activity and stability of the three hybrid
materials was evaluated by monitoring the reaction progress by
gas chromatography and by reusing the hybrid materials (Fig. 4).
After each run, the catalysts is removed from the solution by
decantation, washed with MeOH and dried with pentane. The
Fig. 4. Performance of Au-NPs-rGO as catalysts in hydration of alkynes. Reaction
results in the first run using
a
-NPs-rGO show full conversion in
conditions: 4-octyne (0.2 mmol), catalyst loading (0.05 mol% based on Au), MeOH
(1.4 mL) as solvent, H O (2 Eq, 7.2 lL) at 50 °C. Conversion determined by GC/FID
2
and using 1,3,5-trimethoxybenzene as an internal standard.
the hydration of 4-octyne in less than 100 min at 50 °C. In the sec-
ond run an important decrease in activity was observed, but still
full conversion was achieved in 200 min. The activity decrease is
more pronounced in run 3, where less than 50% conversion was
achieved in 500 min (Fig. 4a). These results indicate that in the case
conversions were obtained in less than 150 min. In runs 9 and 10,
there is a considerable catalyst deactivation but still full conversion
is achieved in 300 min. The differences in the activity and stability
using the three catalytic systems indicates a pronounced effect of
the ligand in the stabilization of the Au NPs on the surface of
graphene.
of the hybrid material
a-NPs-rGO deactivation in the first run
occurs in a significant extent. Similar results were obtained using
b-NPs-rGO as catalyst. Thus, there was a considerable catalyst
deactivation from run 1 to run 2, but the decrease in activity was
even more important in run 3, where only 50% conversion could
be achieved in 500 min (Fig. 4b). Therefore, the catalytic behaviour
of a-NPs-rGO and b-NPs-rGO is similar and characterized by a high
activity in the first run at low catalyst loadings and a notable deac-
3.3. Characterization of Au NPs after the hydration of alkynes.
tivation in the second and third runs.
Completely different behavior was observed for
terms of activity, -NPs-rGO is even a better catalyst than
NPs-rGO or b-NPs-rGO, a fact that can be attributed to the major
amount of Au(I) as observed by XPS. Using -NPs-rGO full conver-
sion of 4-octyne was achieved in 60 min. The -NPs-rGO material
could be recycled up to ten times (Fig. 4c) indicating its high stabil-
ity. The results show that from run 1 to run 3 the activity is main-
tained according to the similar apparent rate constants and the
coincidence in the reaction-time profiles (Fig. 4c inset). Then, there
is a gradual catalyst deactivation from run 4 to run 8, but still full
c
-NPs-rGO. In
In order to clear up the differences in activity/stability and
understand deactivation pathways of Au NPs, the hybrid materials
after the recycling experiments were characterized by HRTEM
microscopy, XPS and ICP/MS (Fig. 5). First, we focused on the mor-
phological assessment of the support in the three hybrid materials.
The HRTEM images before and after the recycling experiments
show similar properties for the graphene, indicating that the sup-
port is not altered under the conditions of hydration of alkynes,
c
a-
c
c
even in the case of the material
c-NPs-rGO after ten consecutive
runs. Regarding the presence of Au NPs, it is important to note that
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