Thiols make for better catalysts: Au nanoparticles supported on functional SBA-15 for catalysis of Ullmann-type homocouplings

Article abstract of DOI:10.1039/c7cc06146c

A strategy for arraying small gold nanoparticles on a mesoporous support modified with single-component or mixed self-assembled monolayers is described. The use of mixed surface modifiers allows easy access to a range of surface chemistries and modalities of interaction between nanoparticles and supports. A combination of thiol groups and linear semifluorinated chains effectively stabilized the nanoparticles against aggregation, while preserving their catalytic activity. The thiol-fluorous-supported catalyst was found active in Ullmann-type homocoupling of aryl halides and showed exceptional selectivity in this reaction.

Full text of DOI:10.1039/c7cc06146c

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Thiols Make for Better Catalysts: Au Nanoparticles Supported on
Functional SBA-15 for Catalysis of Ullmann-type Homocouplings
a
a
a†
*a
Tianyou Chen, Ba-Tian Chen, Konstantin V. Bukhryakov, and Valentin O. Rodionov
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A strategy for arraying small gold nanoparticles on a mesoporous controlled through the formidable arsenal of synthetic organic
support modified with single-component or mixed self-assembled chemistry. Synthesizing a variety of organic SAMs on a given surface
monolayers is described. The use of mixed surface modifiers allows is easier than synthesizing an equivalent number of chemically or
easy access to a range of surface chemistries and modalities of morphologically distinct nanomaterials. Thus, the strategy relying on
interaction between nanoparticles and supports. A combination of the variation of surface adsorbates could enable sampling large
thiol groups and linear semifluorinated chains effectively stabilized combinatorial spaces of NP-catalytic systems.
the nanoparticles against aggregation, while preserving their
Here, we extend the concept of reactivity-modulating functional
catalytic activity. The thiol-fluorous-supported catalyst was found SAMs to solid mesoporous supports. We reasoned that NPs could be
active in Ullmann-type homocoupling of aryl halides and showed covalently immobilized on a support through a SAM containing
exceptional selectivity in this reaction.
“sticky” functional groups with a high affinity for the NPs. Due to the
effectively two-dimensional nature of such a “Scotch tape” SAM, the
NPs could get tethered to the surface with only one side, leaving the
other accessible and catalytically active (Figure 1A). If the receiving
surface is highly curved, such as inside a mesopore channel, a variety
of non-covalent interactions between the SAM and the supported
particle become possible (Figure 1B). The use of mixed SAMs should
allow for easy access to a range of surface chemistries and modalities
of interaction between NPs and supports, and provide a better
understanding of the NP-support interactions through establishing
structure-activity relationships.
The activity and selectivity of heterogeneous catalysts is profoundly
influenced by their state of dispersion, as well as the accessibility and
the specific chemistry of their surfaces. Small nanoclusters and
1
nanoparticles (NPs) with clean/pristine surfaces are some of the
most catalytically-active materials due to their high fraction of
exposed atoms and active sites. As unprotected metal colloids are
prone to aggregation in solution, many practical NP-catalytic systems
rely on solid supports to improve the stability and recyclability of the
2
, 3
catalyst.
NPs protected by surface adsorbates, such as polymers,
surfactants, or self-assembled monolayers (SAMs) of small
4
molecules, can be as stable to aggregation as solid-supported NPs.
Many stabilizing adsorbates, most notably thiols, are quintessential
catalytic poisons, as they block access to the metal surface and
5
interact with the active sites. However, adsorbate-surface
interactions have also been exploited to improve or change the
6
-8
chemoselectivity of NP catalysts,
transformations.
and to effect chiral
Recently, we described a system where the
presence of a perfluorinated adsorbate improves both the activity
9
-11
1
2
and selectivity of a NP catalyst. The effects of adsorbates can be
Figure. 1 Au NPs immobilized on “Scotch tape” supports. (A) Flat receiving surface. The
top face of the NP is clean and exposed. (B) Highly-curved surface (mesopore channel).
The local solvent environment around the NP is defined by the nature of the SAM
a.
Division of Physical Sciences and Engineering and KAUST Catalysis Center, King
Our initial design target was a selective catalyst for Ullmann-type
Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900,
Kingdom of Saudi Arabia. Email: valentin.rodionov@kaust.edu.sa
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Present addresses: Department of Chemistry, Massachusetts Institute of
1
3, 14
homocoupling (UHC) of aryl halides.
reactions are conducted in the presence of Cu, Ni and Pd catalysts.
Conventionally, UHC
15-
1
7
A major competing reaction that occurs with all these catalytic
†
Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307
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Table 1 Synthesis and fundamental characterization data of functional mesoporous supports.a
View Article Online
DOI: 10.1039/C7CC06146C
Molar feed ratio, %a
Pore diameter,
BET surface
area, m /g
Pore volume,
S loading,
mmol/gd
-
Entry
Material
nmc
6.1
6.1
6.1
6.1
6.1
6.1
6.1
5.6
2
c
cm /g
3
c
H10
-
H2F8
H11SH
-
1
2
3
4
5
6
7
8
SBA-15 SiO
SBA-FC
2
-
100
99
97.6
95.2
-
685
642
593
594
641
604
639
575
0.69
0.66
0.60
0.61
0.65
0.60
0.64
0.61
-
-
-
SBA-FC-S1
-
1
0.017
0.027
0.061
0.030
0.032
0.182
SBA-FC-S2.5
SBA-FC-S5
SBA-HC-S5
SBA-S5
-
-
2.4
4.8
4.8
100b
100
95.2
-
-
-
-
SBA-S100
aConditions for functionalization: 60 mg of activated SBA-15 SiO
for a final concentration of 2 mM (except Entry 7), and the suspensions were stirred for 24h under Ar. 0.1 mM of H11SH used for functionalization. N
physisorption data. Results reported based on the adsorption branch. Determined by ICP-OES.
was dispersed in 300 mL of dry toluene at 80°C. Mixtures of silanes were then introduced
2
b
c
2
d
systems is hydrodehalogenation. While the conditions have been mechanistic steps in the formation of silane monolayers happen in
2
7, 28
optimized in many cases to improve the conversion of substrate, solution rather than directly on the receiving surface.
As
perfect selectivity for the homocoupling product and mechanistic perfluorinated molecules tend to aggregate readily, both the kinetics
understanding of the process remain elusive. Recent studies of the deposition and the structural features formed on the surfaces
18-20
21, 22
29
highlighted the utility of Au
and AuPd
NPs for UHC. These by fluorous silanes are determined by their state of agglomeration.
catalysts can operate at lower temperatures, thus providing an The mechanism of deposition and the fundamental structure of the
opportunity for achieving improved selectivity. The catalytic resulting monolayer can be different in the presence and in the
3
0
competence of Au NPs in UHCs is strongly dependent on the nature absence of a nucleophilic catalyst. Furthermore, deposition of
1
9, 23, 24
as well as Au particle size and silanes bearing nucleophilic functional groups can be autocatalytic.31
of the supporting material,
2
0, 24, 25
morphology.
We synthesized SBA-15-type mesoporous SiO
activated by calcining at 500°C, and subsequently modified by aggregates in solution, where nucleophilic thiol groups catalyze
functional silanes. We chose hydrocarbon silane (n- alkoxysilane condensation more efficiently than they could do in a
decyltriethoxysilane, H10), fluorous silane (1H,1H,2H,2H- truly homogeneous system.
perfluorodecyltriethoxysilane, H2F8), and a thiol silane (11- Small AuNPs protected with PPh
triethoxysilyl)undecane-1-thiol, H11SH) with similar main-chain following the method of Hutchinson.32, 33 Triphenylphosphine is a
lengths for this modification, as we expected this will provide SAMs labile ligand in this context, and can be easily replaced by a variety of
with -SH groups exposed on their outer boundaries. The silanes were thiols. The Au101∙PPh NPs were suspended together with different
Thus, the anomalous kinetics of the incorporation of H11SH into
,26 which was fluorous monolayers could be explained by the formation of mixed
2
a
a
3
3
(Au101∙PPh ) were synthesized
(
3
used either in pure form, or combined to yield mixed monolayers. SBA supports in dichloromethane. After 24 h of stirring, the resulting
The modification reactions were performed in dry toluene at 80°C SBA-Au materials were collected and thermally activated in vacuum
over 24 h. Successful modification was confirmed by the appearance at 200°C over 2 h. The Au loading achieved in this way was similar for
of characteristic absorption bands in the Raman spectra of materials all the supports, although thiolated materials retained slightly more
modified with fluorocarbons (Figure S2, ESI), and by Au than the parent SBA-15 SiO
thermogravimetric analysis (TGA, Figure S3, ESI). For convenience,
Transmission electron microscopy (TEM) imaging of the SBA-Au
the modified SBA-15 SiO
supports will be referred to as SBA-FC(HC)- catalysts indicated that the NPs readily leached and aggregated on
2
or SBA-FC (Table S2, ESI).
2
SX, where FC and HC designate fluorous and hydrocarbon chains supports lacking thiol groups due to weak particle-support
respectively; S designates thiol-terminated chains; and X refers interactions (Figures 2A-B, and Figures S6-S7, ESI). Free NPs were
either to the molar feed ratio of H11SH silane in the mixture, or the clearly visible on the carbon film supporting the samples. The
concentration of pure H11SH used in the modification (Table 1).
average particle size and polydispersity underwent a marked change
TGA of the modified SBA materials revealed that all of them after thermal activation (Figures S6-S7, ESI). In contrast, all the SBA-
exhibited two weight-loss regions: 50 to 150°C, attributable to the SX materials proved to be “sticky” for AuNPs. Few if any stray
desorption of H
decomposition of organic SAM. Supports modified with Au materials, even the low-sulfur SBA-FC-S1-Au (Figures 2C-D, and
semifluorinated silanes exhibited higher
decomposition Figures S8-S13, ESI). The changes in particle size and distribution
temperatures (>500°C) than the supports bearing only hydrocarbon upon thermal activation were smaller for all the SBA-SX-Au materials
chains (<250°C).
than they were for SBA-Au and SBA-FC-Au. Support materials with
2
O; and 200 to 650°C, attributable to the unsupported particles could be seen in the TEM images of SBA-SX-
The sulfur content in the modified SBA-SX materials was higher sulfur content fared better: while the average particle size and
determined by inductively-coupled plasma optical emission polydispersity remained virtually unchanged for SBA-FC-S5-Au and
spectrometry (ICP-OES, Table 1). SBA-S100 was found to have the SBA-S5-Au (Figures S8 and S12, ESI), some aggregation was
highest S content, and SBA-FC-S1 the lowest. Interestingly, the S immediately apparent in other samples. The degree of this thermally
content in SBA-FC-S5 was approximately twice as high as it was in induced aggregation could be loosely correlated with the S content
SBA-HC-S5 or SBA-S5, even though all three materials were of the corresponding supports. It is important to note that
synthesized using the same concentration of H11SH. The key unsupported thiol-protected AuNPs undergo rapid increase in size
2
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low degree of conversion, at the cost of a considerable reduction in
View Article Online
DOI: 10.1039/C7CC06146C
reaction rate (Table 2, Entry 4). This selectivity advantage was lost
after a longer reaction time/at a higher degree of conversion.
The activity of SBA-FC-Au was similar to that of SBA-Au, although
the fluorous catalyst was more selective for the UHC product at low
conversion (Table 2, Entry 5). Interestingly, combining fluorous and
thiol-terminated chains on the catalyst support resulted in a
significant synergistic effect. The SBA-FC-SX-Au catalysts were
remarkably selective for the UHC product (Table 2, Entries 6-8), even
after prolonged reaction times. SBA-FC-S5-Au in particular retained
almost quantitative preference for the homocoupling product after
9
6 hours at 120°C in toluene (Table 2, Entry 8).
The observed differences in selectivity cannot be explained solely
by the quantitative variation in sulfur content between the supports.
SBA-S5, SBA-HC-S5, and SBA-FC-S2.5 have similar sulfur loadings
(Table 1, Entries 4, 6 and 7), yet only SBA-FC-S2.5-Au exhibits a near-
exclusive selectivity for the desired UHC product. Thus, it is likely the
selectivity is defined by the differences in spatial distribution and
accessibility of the -SH groups on the support. Depending on the
specific structure of the SAM, the proportion of the thiols bound to
Figure. 2 Transmission electron microscopy (TEM) images of SBA-Au materials before
and after thermal activation at 200°C in vacuum. (A) SBA-Au. (B) SBA-Au after activation. Au, the specific attachment points, and accessibility of the supported
(
C) SBA-FC-S5-Au. (D) SBA-FC-S5-Au after activation.
NPs can vary. Due to the immiscibility of CF and CH chains, mixed
fluorous-hydrocarbon SAMs can self-sort into patches even on
surfaces with high curvature. Thus, compact, thiol-rich patches
under similar conditions.34
36
The Au NP catalysts thus prepared were evaluated for their
activity and selectivity in the Ullmann-type homocoupling of
iodobenzene (Table 2). A modest yield of biphenyl was obtained
using unsupported Au101∙PPh NPs, along with a significant amount
3
of hydrodehalogenation product, benzene (Table 2, Entry 1). This
result was broadly in line with the previously reported AuNP catalytic
systems for UHC,
selectivity explicitly. The Au NP catalyst was readily poisoned by
metallic mercury (ESI, Section 6), supporting the assumption of the
could provide perfectly-sized multivalent tethering points for AuNPs.
The particles in SBA-FC-SX-Au catalysts could be tethered upon a sea
of fluorous chains, which is very likely to strongly affect both the local
solvent environment and the orientation of the particles relative to
the support surface. In contrast, thiol groups in SBA-S5 and SBA-HC-
S5 could be too broadly distributed, providing fewer effective
tethering interactions per AuNP. While SBA-S100 has the highest
content of sulfur among the materials surveyed, it is apparent that
this higher amount of thiolates is sufficient to poison the catalyst. It
is also possible that the H11SH chains have a lower degree of
organization and looser packing in the SBA-S100 material than they
have in the SBA-FC-SX series, which allows for additional non-
productive thiol-gold interactions, and a weaker AuNP-support
association. Efforts towards improved characterization of the NP-
surface thiol interactions using X-Ray photoelectron spectroscopy
1
8, 20
though not all the works address the issue of
3
5
2
0, 25
heterogeneous nature of the operational species.
The conversion improved markedly for the supported SBA-Au
catalyst (Table 2, Entry 2), although selectivity for biphenyl remained
poor. The low-sulfur SBA-S5-Au catalyst was somewhat less active
than SBA-Au, without any improvement in selectivity (Table 2, Entry
3
). The catalyst with the highest sulfur content, SBA-S100-Au, was
substantially more selective than either SBA-Au or SBA-S5-Au at a
(XPS) are underway in our laboratories.
Table 2 Performance of SBA-Au catalysts in Ullmann-type homocoupling of iodobenzene.a
4
8h
96h
Entry
Catalyst
Conversion, %
Selectivity, %
Conversion, %
Selectivity, %
1
2
3
4
5
6
7
8
9
Au101*PPh
SBA-Au
3
41
68
63
49
60
52
56
56
53
76
72
76
90
82
94
98
98
87
56
93
76
65
79
78
84
74
75
79
78
72
71
78
91
87
99
80
SBA-S5-Au
SBA-S100-Au
SBA-FC-Au
SBA-FC-S1-Au
SBA-FC-S2.5-Au
SBA-FC-S5-Au
SBA-HC-S5-Au
a
b
Reaction conditions: iodobenzene (50 μmol), dimethylformamide (DMF, 0.5 mL), catalyst (2 mol% Au), K
Determined by GC-MS. Selectivity for biphenyl.
3
PO
4
(2 equiv), and n-decane as internal standard, 120°C in air.
c
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In conclusion, we synthesized a set of AuNP catalysts supported
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thiolated supports exhibited thermal stability well beyond that of
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