Copper-gold nanoparticles encapsulated within surface-tethered dendrons as supported catalysts for the Click reaction

Article abstract of DOI:10.1016/j.apcata.2018.07.007

The synthesis, characterization, and catalytic testing of copper and copper:gold nanoparticles in melamine-based dendrons supported on SBA-15 is reported. It was found that by first making small gold nanoparticles using the dendrons as templates, stable c

Full text of DOI:10.1016/j.apcata.2018.07.007

Accepted Manuscript
Title: Copper-Gold Nanoparticles Encapsulated Within
Surface-Tethered Dendrons as Supported Catalysts for the
Click Reaction
Authors: Aibolat Koishybay, Daniel F. Shantz
PII:
S0926-860X(18)30330-2
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.07.007
APCATA 16733
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
2-4-2018
18-6-2018
3-7-2018
Please cite this article as: Koishybay A, Shantz DF, Copper-Gold
Nanoparticles Encapsulated Within Surface-Tethered Dendrons as Supported
Catalysts for the Click Reaction, Applied Catalysis A, General (2018),
https://doi.org/10.1016/j.apcata.2018.07.007
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Copper-Gold Nanoparticles Encapsulated Within Surface-Tethered Dendrons as Supported
Catalysts for the Click Reaction
Aibolat Koishybay and Daniel F. Shantz*
Department of Chemical and Biomolecular Engineering, Tulane University, 6823 St. Charles Avenue,
New Orleans, LA 70118, United States. Email: dshantz@tulane.edu
Graphical Abstract
Koishybay and Shantz Highlights




1. Stable copper overlayers on gold nanoparticles were synthesized
2. Better catalytic activity observed than unsupported CuNPs in the click reaction
3. Copper leaching was less than CuNPs encapsulated on bare SBA-15
4. Intermediates can be studied by in-situ FTIR
Keywords. Hybrid materials, dendrons, copper-gold nanoparticles, click chemistry
1

Abstract
The synthesis, characterization, and catalytic testing of copper and copper:gold nanoparticles in
melamine-based dendrons supported on SBA-15 is reported. It was found that by first making small gold
nanoparticles using the dendrons as templates, stable copper(0) overlayers could then be successfully
deposited. These samples were used as catalysts for the click reaction, with the most active samples
possessing rates of 8.9 mol triazole produced/(mol copper – hour), nearly 50 times high than literature
reports of copper nanoparticles in solution. In constrast, the gold nanoparticles themselves are inert for
this reaction, consistent with the copper being deposited as an overlayer on the gold nanoparticles.
Copper leaching was shown to be less pronounced for particles formed in the dendrons versus copper
simply deposited on SBA-15. Larger dendrons lead to suppressed rates due to diffusion resistance with
rates decreasing from 8.9 to 5.2 to 0.69 mole triazole produced/(mole Cu-hour) for G1, G2, and G3
respectively. Infrared spectroscopy was used to monitor the reaction in real time, and provided
information about the various surface intermediates formed.
2

1
. Introduction
Ordered mesoporous silica (OMS) has enjoyed a wide range of interest as a catalyst/catalyst support in
innumerable investigations.[1-3] This is a result of the OMS possessing a high surface area and highly
uniform and tunable pore sizes. OMS has been used in many applications, including both the synthesis of
supported metal nanoparticles as catalysts on OMS as well as being used as a support for tethering
organometallic complexes.[4-10] There has also been substantive research in the growth of polymer
brushes off the surface of mesoporous silicas. The contribution the Shantz lab has made to this area is the
synthesis of melamine based dendrons using amine-functionalized SBA-15 as a support.[11-14] Our
initial report coincided with a similar report from the Alper and Al-Sayari lab of PAMAM dendrons
grown off mesoporous silicas.[15] This has been a promising area of research both for our labs, as well
as other labs following up on our initial discovery.
There have been numerous studies that are well reviewed in the literature reporting metal nanoparticles
formed in OMS either from bare OMS or OMS functionalized with simple groups such as
aminosilanes.[16, 17] The synthesis of surface-tethered organometallics and how those materials
compare to their homogeneous counterparts has also been reported and well-reviewed.[7, 18, 19] An area
that has received less attention, is the synthesis of metal nanoparticles on OMS materials that have been
surface functionalized with more complex ligands such as polymers and dendrons. The ability to generate
hybrid materials with both the function of a metal and an organic moiety will be potentially of interest in
a range of problems including selective capture and conversion schemes as well as cascade or domino
reaction schemes. To that end our lab is investigating the feasibility of growing uniform metal
nanoparticles in the surface-tethered melamine dendron materials we have reported previously.[11-13, 20-
22]
There are numerous literature studies forming metal nanoparticles in dendrons in solution, or so called
dendrimer encapsulated nanoparticles (or DENs). This approach was pioneered by the Crooks lab in the
late 1990s using metals such as palladium, gold, copper, and platinum.[23-28] Those works showed the
3

ability to use the dendrimer as a template to make uniform nanoparticles. Subsequent work from the
Crooks lab and other have expanded this work to a wide range of metals as well as the formation of
bimetallic nanoparticles.[15, 28-32] These works in part served as an inspiration for recent work forming
palladium nanoparticles in melamine dendrons[22] as well as the current work to determine if such an
approach would be as successful for forming supported copper nanoparticles.
Alkyne-azide cycloaddition or so called click reaction is an elegant approach to the synthesis of 1,4-
disubstituted 1,2,3-triazoles that have a wide range of applications, such as organic chemistry, material
science, polymer science and medicinal chemistry.[33-37] The click reaction continues to garner much
interest because it is a highly efficient and reliable reaction that runs in mild conditions with high
regioselective yields, allows connection of a wide range of functional groups and involves straightforward
product isolation procedures.[38-40] Click chemistry primarily evolved around homogeneous Cu(I)
catalysts which limit industrial chemical and pharmaceutical applications due to the challenges related to
the catalyst removal, recovery, reuse as well as toxic Cu contamination of products.[38, 41]
Heterogeneous catalysis is an alternative that can overcome these drawbacks.[41-44] Bulk copper was
proposed as an alternative for homogeneous Cu(I) catalysts but the use of CuNPs proved to be more
efficient. However, unsupported CuNPs have some limitations such as rigorous CuNP synthesis steps,
aggregation, high temperatures, use of commercially unavailable reagents and use of stabilizing agents
that add to the cost.[45] Therefore, the use of supported CuNPs promises to be advantageous and is the
focus of this study.
2
. Experimental
2.1 General. Tetraethoxysilane (TEOS, ≥99%, Alfa Aesar), hydrochloric acid solution (HCl, 2.0N,
BDH), and Pluronic P123 (PEG-PPG-PEG, MW = 5800, Sigma-Aldrich) were used in the SBA-15
synthesis. 3-aminopropyldimethylethoxysilane (APDMTES, Gelest) was used for aminosilane grafting.
ACS grade toluene was obtained from BDH. It was dried in an MBraun solvent purification system and
stored over activated molecular sieves (Alfa Aesar, 3A, 3-5mm beads) to remove residual water. Cyanuric
4

chloride (CC, ≥98, Alfa Aesar), tetrahydrofuran (THF, ≥99.9, no stabilizer, EMD Millipore), N,N-
diisopropylethylamine (DIPEA, ≥99%, Sigma-Aldrich), methanol (BDH, ACS grade), dichloromethane
(
DCM, BDH, ACS grade), 4-(aminomethyl)piperidine (AMP, ≥96%, Sigma-Aldrich), and anhydrous
piperazine (PIP, ≥99%, Alfa Aesar) were used in the dendrimer synthesis. Copper(II) chloride dihydrate
Baker Analyzed, ACS Reagent), sodium borohydride (≥99.99%, Sigma-Aldrich) and gold(III) chloride
(
trihydrate (≥99%, Sigma-Aldrich) were used for the metal loading experiments. Benzyl azide (≥94%,
Alfa Aesar), cyclohexylacetylene (≥98%, Alfa Aesar) and triethylamine (≥99%, Sigma-Aldrich) were
used in the click reaction. Deuterated chloroform with +0.03% v/v tetramethylsilane (TMS) was obtained
from Cambridge Isotope Laboratories and used for the NMR analysis of the click reaction. THF, DCM
and toluene were dried prior to use with an MBraun solvent system. All other chemicals were used as
received.
2.2 SBA-15 synthesis. SBA-15 samples were synthesized using the method reported by Zhao et al.[46]
As an example, 20 g of Pluronic P123 was dissolved in 125 mL of deionized water and 600 mL of 2 M
HCl by stirring for 5 h at room temperature. Then, 42.5 g (45.5 mL) of TEOS was added to the solution
and stirred for 24 h at 35 °C. The mixture was then aged at 80 °C for 24 h under static conditions. The
reaction product was collected by vacuum filtration, washed with deionized water and air-dried overnight.
The Pluronic was removed by calcination. The sample was heated from room temperature to 100 °C at a
rate of 1 °C/min, then held at 100 °C for 2 hours, then the temperature increased from 100 °C to 550 °C at
a rate of 1 °C/min, and finally held at 550 °C for 8 h.
2.3 Amine-functionalization of SBA-15. Amine-functionalized SBA-15 was synthesized via post-
synthetic amine grafting. Typically, 1.0 g of calcined SBA-15 was mixed with 0.5 mmol (94 μL)
APDMTES in 100 mL of anhydrous toluene. The mixture was stirred at room temperature for 24 h. The
solid product was collected by filtration, washed with copious amounts of toluene and methanol, and air
dried overnight.
5

2.4 Synthesis of dendron-SBA-15 hybrid materials. Dendron hybrid materials were synthesized per prior
work.[11-13, 22] As an example, 4-aminomethylpiperidine (AMP) dendrons were made by adding 1.0 g
of amine-functionalized SBA-15 to 25 mL of a 0.3M cyanuric chloride in THF solution and 2.5 mL of
DIPEA. The mixture was stirred for 24 h at room temperature in a closed container. The reaction products
were collected by filtration and washed with methanol, DCM and THF, respectively. The solid product
was then mixed with 25 mL of a 0.4 M AMP in THF solution and stirred for 24 h at room temperature.
The sample was again filtered and washed with methanol, DCM and THF, respectively. This sample was
denoted as G1-AMP-SBA-15. Higher generation samples were synthesized by repeating the procedure
stated above one or two times to obtain G2-AMP and G3-AMP dendrimers, respectively (Scheme 1). To
synthesize piperidine (PIP) containing dendrons, 25 mL of 0.4 M PIP in THF solutions were used instead
of 0.4 M AMP solutions. Throughout samples are denoted as Gx-AAA-SBA-15 where x is the dendron
generation and AAA is either AMP for dendrons containing 4-aminomethylpiperidine as the diamine or
PIP for dendrons containing piperidine as the diamine.
2
.5 Metal–dendron SBA-15 materials. Copper and gold loading on dendron-SBA-15 was performed by
dispersing 0.6 g of Gx-AAA-SBA-15 in 2.6 mL of 0.025 M HAuCl solution. The mixture was stirred for
0 min before gold was reduced with fivefold excess of a 50 mM aqueous solution of NaBH . The solid
product was separated by centrifugation and washed with deionized water. The obtained material was
dispersed in 6 mL of 0.032 M CuCl ·3H O solution
4
1
4
2
2
6

Scheme 1. Synthesis of dendrons illustrated with G2-AMP.
and stirred for another 10 min before it was reduced with a fivefold excess amount of 50 mM aqueous
4
solution of NaBH . The final product was collected by filtration, washed several times with deionized
water, dried in a Schlenk flask under vacuum at room temperature and labeled as Xwt% Cu
Y
Au-Gx-
AAA-SBA-15 where X, Y, Gx and AAA are weight percent loading of copper, copper to gold molar
ratio, dendrimer generation and dendron type, respectively. As an example, a second-generation AMP
dendron with two weight percent copper and a copper to gold molar ratio of 3 is denoted as 2wt% Cu
G2-AMP-SBA-15.
3
Au-
2.6 Analytical. Powder X-ray diffraction (PXRD) measurements were performed using a Rigaku
MiniFlex 600 diffractometer with CuKα radiation over a range of 20–80° 2θ. For SBA-15, the
measurement was performed from 0.5–4° (2θ). In the case of SBA-15, the instrument background at a
low angle (< 1° 2θ) was subtracted by measuring a zeolite ZSM-5 material (MFI, pure silica) over the
same range. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) measurements were
performed on a Nicolet iS50R spectrometer. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy
measurements were performed using an Evolution 300 instrument equipped with a diffuse reflectance
stage. Barium sulfate was used as the reference standard. Thermal gravimetric analyses (TGA) were
performed using a TA Instruments Q500 over a temperature range of 25–600 °C using air as a carrier gas
-1
and a temperature ramp rate of 1 °C min . Nitrogen adsorption experiments were performed using a
Micromeritics ASAP 2020 with approximately 100 mg of sample. The samples were degassed under
vacuum at 150 °C for 24 h before analysis. The mesopore volumes and surface areas were determined
using the α -method. The mesopore size distributions were calculated from the adsorption branch of the
s
isotherms using the Barrett–Joyner–Halenda (BJH) method with a modified equation for the statistical
film thickness. X-ray photoelectron spectroscopy (XPS) experiments were performed using a VG
Scientific MKII system with a Mg Kα X-ray source. The pressure in the chamber during analysis was <
-8
5
x10 mbar. Peak fitting was performed with a custom VBA program in Microsoft Excel using Voigt
7

profiles together with a Shirley background function. Transmission Electron Microscopy (TEM) and
Scanning Transmission Electron Microscopy (STEM) measurements were performed using a FEI Tecnai
G2 F30 Twin with Oxford EDS, Gatan Imaging Filter and EEL Spectrometer. The samples were
dispersed in 95% ethanol solution with sonication for 1 min and deposited on carbon-coated Cu grids
1
(
200 mesh, Electron Microscopy Science) and the solvent was evaporated. All H NMR spectra were
collected on a Bruker Avance 500 MHz using CDCl as a solvent and TMS as an internal standard.
3
2.7 Reaction monitoring with IR. Infrared spectroscopy measurements were performed using a flow cell
-1
setup inside a Bruker-Vertex 70 FT-IR instrument. The measurement parameters were 2 cm resolution
and 30 scans. The measurements were performed with a room temperature DLaTGS detector. The
reaction mixture was prepared by dispersing 0.033 g of catalyst, which corresponds to 5 mol% of Cu per
1
mole of reactant in 10 mL of THF. 0.2 mmol of triethylamine was added to the solution and subtracted
as a background. 0.2 mmol of benzyl azide and 0.2 mmol of cyclohexylacetylene were added to the
o
reaction mixture and heated to 50 C. The reaction progress was recorded every 20 min.
2
.8 Catalytic testing. The click reaction (scheme 2) was carried out as follows. Unless noted otherwise,
two mmol of benzyl azide, 2 mmol of cyclohexylacetylene and 2 mmol of triethylamine were added to
.16 g of catalyst, which
0
Scheme 2. Cycloaddition of benzyl azide to cyclohexyl acetylene.
corresponds to 2.5 millimoles of copper per 100 millimoles of reactant in 4 mL of THF. The reaction
1
mixture was then heated to 50 °C and the reaction conversion was monitored by H NMR. At different
intervals, 100 L of the reaction mixture was transferred into a small centrifuge tube and the solid catalyst
was removed from the solution by centrifugation at 13800 xg for 3 min. The liquid was diluted with 750
8

1

L of deuterated chloroform and analyzed by H NMR at 25 °C. Conversion was calculated by integrating
the peaks for the 1-benzyl-4-cyclohexyl-1H-1,2,3-triazole (s 7.16 ppm, 1H, CH) and (s, 2H, CH ) product
with respect to that of the benzyl azide (s 4.35 ppm, 2 H, CH ) and cyclohexylacetylene (d 2.04–2.05
ppm, 1 H) peaks. Note that in all experiments performed there were no side products observed.
2
2
3
. Results and Discussion
3.1 General Characterization. The dendron-SBA-15 hybrids, prior to metal uptake, were characterized
using a battery of methods to validate that the target dendrons were formed on the SBA-15 surface. X-ray
diffraction, nitrogen porosimetry, thermal gravimetric analysis, and infrared spectroscopy (Figures S1-S8,
Supporting Information) are all consistent with prior work from our lab. The supports are well-ordered
SBA-15 materials, the textural properties change as expected as upon increasing the dendron generation,
the target groups are present and that the ligand loading is consistent with prior work showing the vast
majority of the organic fragments in fact have the dendron structure shown in Scheme 1.
3.2 Copper nanoparticle formation. Recent work from our lab demonstrated the ability to form small (<
2
nm), uniform palladium nanoparticles on G2-PIP-SBA-15 materials.[22] Expanding on that work, the
synthesis of small copper nanoparticles on PIP-SBA-15 and AMP-SBA-15 materials was explored. As an
initial control copper was loaded onto SBA-15 and reduced. In that case, PXRD confirmed the presence
of copper(0) particles, however the copper species formed were large (> 10 nm) and generally found by
STEM to not be in the SBA-15 mesopores. In contrast, attempts to make copper(0) particles on PIP-
SBA-15 samples led to smaller particles. However, PXRD, XPS, and diffuse-reflectance UV-Vis
measurements showed that the copper was not exclusively copper(0), and that over the span of a few days
of storage the fraction of oxidized copper increased significantly. By contrast, using AMP dendrons
supported on SBA-15 it was possible to make Cu(0) nanoparticles based on PXRD; however these
samples suffered from stability and tended to oxidize. It was not possible to form pure, stable Cu(0)
nanoparticles in the dendrons.
9

Figure 1. STEM images of 2wt% Cu-SBA-15 (top left) and 2wt% Cu-G2-AMP-SBA-15 (top right.
Bottom) PXRD patterns of the samples.
.3 Copper-gold nanoparticles. The above results highlight the challenges of forming stable, small
(
3
copper(0) nanoparticles in dendrons. This was in contrast to our previous work using palladium where it
was found one could obtain stable Pd(0) 2 nm particles in dendrons. Given prior literature that indicated
one route to stabilizing copper(0) was to overlayer it onto gold this was attempted.[47-49] Figure 2
shows PXRD results for samples containing 1 wt% Cu and a 3:1 molar ratio of copper:gold on G2-AMP-
SBA-15. The sample where the two elements were reduced simultaneously resulted in large domains of
gold based on the results in Figure 2 (red trace). The sample where gold was added and reduced then,
followed by copper addition and reduction (blue trace), by contrast resulted in small nanoparticles based
on PXRD. The same phenomena was seen for samples where the addition order was reversed, i.e. copper
addition then reduction, followed by gold addition and reduction (black trace). In this last sample the
1
0

Figure 2. Effect of reduction sequence on copper / gold bimetallic systems at one weight percent copper
and 3:1 Cu:Au molar ratio on a G2-AMP-SBA-15 sample. Red trace simultaneous reduction of copper
and gold. Black trace, sequential addition and reduction of copper then gold. Blue trace, sequential
addition and reduction of gold then copper.
diffraction peaks for gold could be observed but were dramatically broader. None of these samples
exhibited features consistent with the presence of copper oxides or large copper(0) domains. The effect
of dendron generation and Cu:Au ratio at a fixed copper loading were also explored. In all cases, no
copper oxide species are observed by PXRD. As the generation of dendron increases the gold domain
size decreased based on PXRD and as the amount of gold in the copper:gold system increased an increase
in the intensity of the gold peaks, but not a decrease in the peak widths, was observed in PXRD (Figures
S9-S10). These results are consistent with the picture that gold(0) nanoparticles can be used to stabilize
the presence of copper(0) species. Further evidence for this was found by XPS (Figure 3) and UV-Vis
(
Supporting Information Figure S11) which did not give any indication for the presence of oxidized
copper.
1
1

Figure 3. (Top left) TEM and (top right) STEM images of CuNP inside SBA-15 mesopores. (Bottom)
XPS spectrum of 2wt%-Cu Au-G2-AMP-SBA-15.
3
3.4 Click reaction. The copper:gold samples described above were investigated for the coupling of an
alkyne with an azide, i.e. click chemistry. Control measurements indicated that bare SBA-15, G2-AMP-
SBA-15, and 2 wt% Au-G2-SBA-15 gave no conversion of the reactants after 24 hours at 323 K. 2wt%
Cu-SBA-15 was active for this reaction with a conversion of 94% after 18 hours at 323 K, which
corresponds to an approximate rate at short reaction times (< 5hrs) of 4.3 mol triazole produced/(mol
copper-hour). 2 wt%-Cu-G2-AMP-SBA-15 was also screened for this reaction, and gave an 81%
conversion after 24 hours of reaction at 323 K. These results are consistent with prior literature that
shows copper is active for the click reaction, and that gold is not.[35] With this information in hand the
Cu-Au materials were tested.
1
2

Figure 4 shows catalytic data for the Cu-SBA-15, 2 wt%-Cu-G2-AMP-SBA-15, and 2 wt% Cu
AMP-SBA-15. The Cu Au sample was more active than the Cu-G2-AMP sample. This was consistent
with our characterization work that suggested the copper formed an effective overlayer on the gold, i.e.
3
Au-G2-
3
we effectively realized higher copper utilization as the same amount of copper in the Cu
being overlaid onto the gold versus forming pure
3
Au sample was
Figure 4. Catalytic results for 2wt%-Cu-SBA-15, 2wt%-Cu-G2-AMP-SBA-15, and 2wt%-Cu
AMP-SBA-15.
3
Au-G2-
copper particles. In the case of the Cu Au sample the early time data gives an estimated rate of 5.2 mol
3
triazole produced/(mol Cu – hour). For comparison, comparable conditions were used by Alonso and
coworkers with copper nanoparticles[50] and gave rates in the range of 0.001 to 0.04 mol triazole
produced/(mol Cu-hour). Thus our samples are notably more active on a copper basis. That the Cu-SBA-
15 sample was more active than the Cu-G2-AMP-SBA-15 sample could have two explanations. The first
is that for Cu-SBA-15 more copper was leached into solution, and thus both homogeneous and
heterogeneous catalysis were being observed. The second is that there was more diffusion resistance in
the case of the G2-AMP sample. It is likely that both factors are at work, but we did not attempt to
uncouple their relative contributions. Leaching studies of the samples are described in more detail below.
1
3

3.4.1 The effect of the copper:gold ratio. Figure 5 shows the catalytic results for samples with Cu:Au
ratios of 1:1, 3:1, and 5:1 (XRD and STEM of 5:1 and 1:1 samples in Support Information, S12 and S13).
As can be seen the 1:1 and 3:1 samples were
Figure 5. Effect of the Cu:Au ratio at a given copper weight loading in G2 dendrons on the catalytic
properties of the samples. All three samples contain two weight percent copper.
comparable. The 5:1 sample showed decreased reactivity on a copper normalized basis. A simple
explanation for this finding is that upon increasing the copper to gold ratio that thicker layers of copper
were being formed on the gold particles, leading to a lower utilization, i.e. a smaller fraction of the copper
atoms were on the surface of the nanoparticles. Qualitative calculations indicate that to coat a 2 nm gold
particle that an approximately 1:1 Cu:Au ratio is needed, consistent with our findings in figure 5 and our
conclusion of an overlayer. Two additional controls were performed to test this idea. The first was to
make copper nanoparticles followed by addition and reduction of gold. In this case the conversion was
only 17% after 24 hours. The second was to take a 2wt%-Cu Au-G2-AMP-SBA-15 sample and anneal it
3
at 150 °C under nitrogen to form the intermetallic[51], with no change in the particle size observed by
STEM or PXRD. In that case the conversion declined from 98% at 15 hours to 80% at 24 hours (71% at
12 hours) after annealing. Both of these control experiments support our hypothesis that the copper
1
4

formed an overlayer on the gold, and that the enhanced reactivity observed was a result of higher copper
utilization, and not due to changing the electronic properties of the copper. This was explored more
thoroughly using infrared spectroscopy (vide infra).
3
.4.2 The effect of the dendron generation. A series of 2wt%-Cu Au-Gx-AMP-SBA-15 samples of
3
different dendron generation were synthesized (STEM in Supporting Information, S13). As can be seen
in Figure 6 there is a systematic decrease in the rate on a normalized copper basis when going from G1 to
G2 to G3. Estimates of the rates for these samples based on the early time data are 8.9 to 5.2 to 0.69 mole
triazole produced/(mole Cu-hour) for G1, G2 and G3 respectively. This same trend was observed in prior
work in our lab using these dendrons without
Figure 6. Effect of dendron generation on the observed rate for a series of 2wt%-Cu3Au-AMP-SBA-15
samples.
metals as catalysts for the Nitroaldol reaction. This finding was ascribed to the increase in diffusion
resistance as one goes to larger dendrons, and again consistent with prior work a more substantive change
was seen in going from G2 to G3 than from G1 to G2.
1
5

3.4.3 Catalyst Stability and Metal leaching. The stability of our CuAu-catalysts was also studied (Table
S3 and Figures S14-S17 in Supporting Information). Two sets of recycle studies for the 2wt% Cu Au-
3
G2-AMP-SBA-15 were performed. The first studies used a 17.5 h on stream interval. In this case the
conversions went from 96% to 71% (first recycle) to 7% (second recycle). In the second set of
experiments a reaction time of 24 hours was used. In that case the conversion went from 100% to 48% to
approximately 1% for the fresh catalyst, first recycle, and second recycle. Characterization of the spent
catalyst points to two causes for this deactivation. The first was fouling of the catalyst. TGA analysis of
the sample after two 24 hour reaction cycles showed a weight loss of 33% for the G2 sample, whereas the
weight loss due to dendrons was 22% for the sample before catalytic testing. This finding shows that
organic matter deposits on the catalyst during reaction. The second cause for deactivation was metal
leaching and subsequent homogeneous catalysis by copper. To probe this the hot filtrate method was
employed. Table one summarizes the results obtained for three samples
Conversion
Click reaction @ 50 °C for 6 Hot filtrate after 12 hours
Catalyst
hours
post separation @ 50 °C
2
2
2
wt% Cu-SBA-15
wt% Cu-G2-AMP-SBA-15
64 %
39 %
62 %
93 %
49 %
76 %
wt% Cu Au-G2-AMP-SBA-15
3
Table 1. Summary of conversion before and after catalyst separation.
wherein the conversion was measured at six hours, the catalyst separated from the reaction, and the
filtrate heated for an additional twelve hours. As can be seen the Cu-SBA-15 had the most metal leaching
based on the increase in conversion, and has effectively the same conversion as determined after 18 hours
when the SBA-15 sample was not separated. The dendron samples, both 2wt% Cu-G2-AMP and 2wt%
Cu Au-G2-AMP, had some leaching though the increase in conversion after separation of the catalyst was
3
smaller than for the copper SBA-15 material. Additional, EDS of the spent catalyst (Supporting
Information) shows that while the parent material has the proper copper to gold ratio, after reaction the
catalyst no longer contains gold, and has a slightly reduced copper content. Thus under reaction
conditions the gold leaches, resulting in a material similar to the copper supported dendron sample which
1
6

according to table 1 shows some copper leaching via the hot filtrate test. In regards to the state of the
copper, XPS and XRD before and after testing only show the presence of Cu(0). UV-Vis is somewhat
more ambiguous as the changes are subtle but could be interpreted as the presence of mixed oxidation
state copper.
3.5 Infrared spectroscopy. The results above demonstrated that the materials made were catalytically
active for the click reaction. IR was chosen to follow the reaction and develop deeper insights into the
reaction, with two specific aims. The first aim was to observe the kinetics and any intermediates in the
catalytic cycle to see if our system was similar/different to homogeneous copper. The second aim was to
try and further validate our hypothesis that we were forming copper ovelayers and simply observing
improved rates due to higher efficiency of the copper, i.e. the reactivity variations did not have an
electronic effect origin.
-1
Figure 7 shows the time resolved IR data for the Cu
3
Au sample. The feature at 3257 cm is due to the
-1
-1
alkyne (1), the feature at 2094 cm is due to the azide (2), and the feature at 1780 cm is due to the
-1
triazole (5). There are two additional features, one at 2337 cm due to an intermediate bound to copper
-1
(
3), and a feature at 1727 cm that is due to the bound product (4). The results in Figure 7 indicate that
IR can be used to follow the reaction in real time. Variations
1
7

Figure 7. Time resolved IR data for 2wt% Cu Au-G2-AMP-SBA-15 catalyzed click reaction.
3
of the ratio of the alkyne and azide, i.e. 2:1 or 1:2 versus 1:1, appears to make little or no difference in the
intensity evolution of the IR spectrum (Figure S18). The results indicate that the alkyne adsorbs rapidly,
there is an induction period before azide consumption is observed, and we believe that the formation of
the intermediate prior to ring closure and triazole formation is the rate limiting step.
Figure 8 shows our putative mechanism for this reaction, and Figure 9 shows normalized absorbance as a
function of the reaction time extracted from the results in Figure 7. There are a few important
observations from the results in Figure 9. First, the triazole product absorbance saturates at very low
conversion (the conversion determined for these experiments is less than 2% in all cases). Second, as
mentioned above, it is clear that an appreciable amount of the alkyne rapidly adsorbs on the copper
between when the
Figure 8. Putative mechanism for the click reaction.
1
8

1
9

Figure 9. Normalized IR absorbance profiles as a function of reaction time for the (top) CuAu, (middle)
Cu Au, and (bottom) Cu Au G2-AMP-SBA-15 samples.
3
5
reaction mixture is prepared and the experiment started (5-10 minutes). Third, as the copper to gold ratio
increases the azide and alkyne absorbances decrease more quickly. Finally, at a copper to gold ratio of
three the maximum in the surface intermediate (3) takes much longer to occur than the other ratios. The
reason for this last point is unclear. The IR spectra for the CuAu and Cu Au samples are included in the
5
Supporting Information (Figure S19). Efforts were made to fit the absorbance versus time data (both
normalized and un-normalized) based on the mechanism shown in Figure 8, however it was not possible
to properly capture the measured profiles. Some possible complications include large differences in the
absorbance of the products and intermediates versus the reactants. That the triazole product plateaus (i.e.
saturates) in the IR data at very low conversion is consistent with this. Consistent with this we found in
separate experiments that under the conditions employed low concentrations of triazole (5 mM) led to
non-linearities in the absorbance, i.e. we were outside the beer’s law limit. Despite this, the IR clearly
allowed us to track the reaction and observe intermediates that have been observed previously in the
homogeneous catalysis click chemistry literature. That all samples including a G2-AMP-SBA-15 (not
shown) displayed very similar trends in the IR is consistent with our hypothesis that overlayering the
copper on gold does not lead to dramatic changes in the electronic properties of the copper.
2
0

4
. Conclusions
The synthesis, characterization, and catalytic testing of copper and copper:gold nanoparticles in
melamine-based dendrons supported on SBA-15 was reported. The key to controlling the oxidation state
of the copper was to form overlayers of copper on gold nanoparticles as evidenced by ultraviolet visible
spectroscopy and X-ray photoelectron spectroscopy. Regarding the catalytic properties of these materials,
copper:gold ratios of three to one and lower were found to be preferred for catalysis. Copper leaching
was shown to be less pronounced for particles formed in the dendrons versus copper simply deposited on
SBA-15. The various controls performed were consistent with the picture of the synthetic approach
depositing copper overlayers on gold. Infrared spectroscopy provided information about the various
surface intermediates formed consistent with prior literature mechanisms from homogeneously copper
catalyzed click reaction studies. The current works shows the metal-dendron hybrids to be promising
catalysts for this reaction and offers design strategies for building complex materials for use in
heterogeneous catalysis.
5
. Acknowledgments
We gratefully acknowledge Professor Russ Schmehl and S. L. Guertin at Tulane for access to the IR
spectrometer useful discussions. The IR instrument was acquired through support from the Louisiana
Board of Regents award TR_24_2014-ENH-TR-25. D.F.S. gratefully acknowledges partial support from
Entergy and Tulane University.
6
. Supporting Information.
Powder X-ray diffraction (PXRD) patterns of parent SBA-15, nitrogen adsorption isotherms, TGA
results, and IR spectra of the PIP and AMP SBA-15 samples investigated. PXRD patterns of Cu:Au 3:1
AMP samples, PXRD patterns of G2-AMP samples with varying Cu:Au ratios, UV-Vis of 2wt% Cu
G2-AMP-SBA-15 sample, STEM of 2wt% Cu Au and CuAu – G2-AMP-SBA-15 samples, STEM of the
G1 and G3 2wt%-Cu Au-AMP-SBA-15 samples. EDS, PXRD, XPS, UV-Vis and IR spectra of 2wt%-
Cu Au-AMP-SBA-15 sample before and after the click reaction. Time resolved IR data for the 2wt%
3
Au
5
3
3
2
1

Cu
5 sample and 2wt%-Cu
. References
1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu,
3
Au-G2-AMP samples with varying and azide:alkyne ratios and for the 2wt%-CuAu-G2-AMP-SBA-
1
5
Au-G2-SBA-15 sample.
7
[
D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, A New Family of
Mesoporous Molecular-Sieves Prepared with Liquid-Crystal Templates, J. Am. Chem. Soc., 114
(1992) 10834-10843.
[
2] U. Ciesla, F. Schüth, Ordered Mesoporous Materials, Micropor. Mesopor. Mat., 27 (1999) 131-
1
[
49.
3] W. Li, D.Y. Zhao, An overview of the synthesis of ordered mesoporous materials, Chemical
Communications, 49 (2013) 943-946.
4] R.K. Sharma, S. Sharma, S. Dutta, R. Zboril, M.B. Gawande, Silica-nanosphere-based organic-
[
inorganic hybrid nanomaterials: synthesis, functionalization and applications in catalysis, Green
Chemistry, 17 (2015) 3207-3230.
[
(
[
(
[
5] K. Moller, T. Bein, Inclusion Chemistry in Periodic Mesoporous Hosts, Chem. Mater., 10
1998) 2950-2963.
6] K. Moller, T. Bein, Talented Mesoporous Silica Nanoparticles, Chemistry of Materials, 29
2017) 371-388.
7] C. Coperet, A. Comas-Vives, M.P. Conley, D.P. Estes, A. Fedorov, V. Mougel, H. Nagae, F.
Nunez-Zarur, P.A. Zhizhko, Surface Organometallic and Coordination Chemistry toward Single-
Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities, Chemical Reviews,
1
[
16 (2016) 323-421.
8] Y. Chen, J.L. Shi, Chemistry of Mesoporous Organosilica in Nanotechnology: Molecularly
Organic-Inorganic Hybridization into Frameworks, Advanced Materials, 28 (2016) 3235-3272.
9] U. Diaz, D. Brunel, A. Corma, Catalysis using multifunctional organosiliceous hybrid
materials, Chemical Society Reviews, 42 (2013) 4083-4097.
10] E.L. Margelefsky, R.K. Zeidan, M.E. Davis, Cooperative catalysis by silica-supported organic
functional groups, Chemical Society Reviews, 37 (2008) 1118-1126.
11] E.J. Acosta, C.S. Carr, E.E. Simanek, D.F. Shantz, Engineering nanospaces: Iterative
[
[
[
synthesis of melamine-based dendrimers on amine-functionalized SBA-15 leading to complex
hybrids with controllable chemistry and porosity, Advanced Materials, 16 (2004) 985-+.
[
12] Q.Q. Wang, V.V. Guerrero, A. Ghosh, S. Yeu, J.D. Lunn, D.F. Shantz, Catalytic properties
of dendron-OMS hybrids, Journal of Catalysis, 269 (2010) 15-25.
13] S. Yoo, J.D. Lunn, S. Gonzalez, J.A. Ristich, E.E. Simanek, D.F. Shantz, Engineering
[
nanospaces: OMS/dendrimer hybrids possessing controllable chemistry and porosity, Chemistry
of Materials, 18 (2006) 2935-2942.
[
14] J. Han, Y. Lou, X. Cai, B.C. Gibb, D.F. Shantz, Pore Modified FDU-12 as a Novel Container
for Dendron Growth, The Journal of Physical Chemistry C, (2017).
15] J.P.K. Reynhardt, Y. Yang, A. Sayari, H. Alper, Polyamidoamine dendrimers prepared inside
the channels of pore-expanded periodic mesoporous silica, Advanced Functional Materials, 15
[
(2005) 1641-1646.
[
16] Q.Q. Wang, D.F. Shantz, Ordered mesoporous silica-based inorganic nanocomposites,
Journal of Solid State Chemistry, 181 (2008) 1659-1669.
2
2

[
17] P.C. Angelome, L.M. Liz-Marzan, Synthesis and applications of mesoporous nanocomposites
containing metal nanoparticles, Journal of Sol-Gel Science and Technology, 70 (2014) 180-190.
18] A. Corma, H. Garcia, Silica-bound homogenous catalysts as recoverable and reusable
catalysts in organic synthesis, Advanced Synthesis & Catalysis, 348 (2006) 1391-1412.
19] M. Perez-Cabero, J. El Haskouri, B. Solsona, I. Vazquez, A. Dejoz, T. Garcia, J. Alvarez-
[
[
Rodriguez, A. Beltran, D. Beltran, P. Amoros, Stable anchoring of dispersed gold nanoparticles
on hierarchic porous silica-based materials, Journal of Materials Chemistry, 20 (2010) 6780-6788.
[
20] S. Yoo, S. Yeu, R.L. Sherman, E.E. Simanek, D.F. Shantz, D.M. Ford, Reverse-Selective
Membranes formed by Dendrimers on Mesoporous Ceramic Supports, J. Membrane Sci., 334
(2009) 16-24.
[
21] J.X. Han, Y.Y. Lou, X.Y. Cai, B.C. Gibb, D.F. Shantz, Pore Modified FDU-12 as a Novel
Container for Dendron Growth, Journal of Physical Chemistry C, 121 (2017) 22031-22039.
22] Y. Lou, D.F. Shantz, Palladium dendron encapsulated nanoparticles grown from MCM-41
and SBA-15, Journal of Materials Chemistry A, 5 (2017) 14070-14078.
23] R.M. Crooks, M.Q. Zhao, L. Sun, V. Chechik, L.K. Yeung, Dendrimer-encapsulated metal
[
[
nanoparticles: Synthesis, characterization, and applications to catalysis, Accounts of Chemical
Research, 34 (2001) 181-190.
[
24] R.W.J. Scott, O.M. Wilson, R.M. Crooks, Synthesis, characterization, and applications of
dendrimer-encapsulated nanoparticles, Journal of Physical Chemistry B, 109 (2005) 692-704.
25] R.W.J. Scott, H. Ye, R.R. Henriquez, R.M. Crooks, Synthesis, Characterization, and Stability
of Dendrimer-Encapsulated Palladium Nanoparticles, Chem. Mat., 15 (2003) 3873-3878.
26] O.M. Wilson, M.R. Knecht, J.C. Garcia-Martinez, R.M. Crooks, Effect of Pd Nanoparticle
Size on the Catalytic Hydrogenation of Allyl Alcohol, Journal of the American Chemical Society,
28 (2006) 4510-4511.
27] L.K. Yeung, R.M. Crooks, Heck Heterocoupling within a Dendritic Nanoreactor, Nano
Letters, 1 (2001) 14-17.
28] M. Zhao, L. Sun, R.M. Crooks, Preparation of Cu Nanoclusters within Dendrimer Templates,
Journal of the American Chemical Society, 120 (1998) 4877-4878.
29] W. Huang, J.N. Kuhn, C.-K. Tsung, Y. Zhang, S.E. Habas, P. Yang, G.A. Somorjai,
[
[
1
[
[
[
Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous
Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation, Nano Letters, 8 (2008) 2027-
2
[
034.
30] Y.-G. Kim, S.-K. Oh, R.M. Crooks, Preparation and Characterization of 1−2 nm Dendrimer-
Encapsulated Gold Nanoparticles Having Very Narrow Size Distributions, Chem. Mat., 16 (2004)
67-172.
31] P. Maity, S. Yamazoe, T. Tsukuda, Dendrimer-Encapsulated Copper Cluster as a
Chemoselective and Regenerable Hydrogenation Catalyst, ACS Catalysis, 3 (2013) 182-185.
32] X. Peng, Q. Pan, G.L. Rempel, Bimetallic dendrimer-encapsulated nanoparticles as catalysts:
a review of the research advances, Chemical Society Reviews, 37 (2008) 1619-1628.
33] V.V. Rostovtsev, L.G. Green, V.V. Fokin, K.B. Sharpless, A stepwise Huisgen cycloaddition
1
[
[
[
process: Copper(I)-catalyzed regioselective "ligation" of azides and terminal alkynes, Angewandte
Chemie-International Edition, 41 (2002) 2596-+.
[
34] C.W. Tornoe, C. Christensen, M. Meldal, Peptidotriazoles on solid phase: [1,2,3]-triazoles by
regiospecific copper(I)-catalyzed 1,3-dipolar cycloadditions of terminal alkynes to azides, Journal
of Organic Chemistry, 67 (2002) 3057-3064.
2
3

[
35] H.C. Kolb, M.G. Finn, K.B. Sharpless, Click chemistry: Diverse chemical function from a
few good reactions, Angewandte Chemie-International Edition, 40 (2001) 2004-+.
36] Y. He, Y. Liang, D.J. Wang, The highly sensitive and facile colorimetric detection of the
[
glycidyl azide polymer based on propargylamine functionalized gold nanoparticles using click
chemistry, Chem Commun, 51 (2015) 12092-12094.
[
37] S. Lee, H. Koo, J.H. Na, S.J. Han, H.S. Min, S.J. Lee, S.H. Kim, S.H. Yun, S.Y. Jeong, I.C.
Kwon, K. Choi, K. Kim, Chemical Tumor-Targeting of Nanoparticles Based on Metabolic
Glycoengineering and Click Chemistry, Acs Nano, 8 (2014) 2048-2063.
[
38] T.N. Jin, M. Yan, Y. Yamamoto, Click Chemistry of Alkyne-Azide Cycloaddition using
Nanostructured Copper Catalysts, Chemcatchem, 4 (2012) 1217-1229.
39] F. Alonso, Y. Moglie, G. Radivoy, Copper Nanoparticles in Click Chemistry, Accounts of
Chemical Research, 48 (2015) 2516-2528.
40] R. Hudson, C.J. Li, A. Moores, Magnetic copper-iron nanoparticles as simple heterogeneous
catalysts for the azide-alkyne click reaction in water, Green Chemistry, 14 (2012) 622-624.
41] S. Jang, Y.J. Sa, S.H. Joo, K.H. Park, Ordered mesoporous copper oxide nanostructures as
highly active and stable catalysts for aqueous click reactions, Catalysis Communications, 81
[
[
[
(2016) 24-28.
[
42] S. Chassaing, V. Beneteau, P. Pale, When CuAAC 'Click Chemistry' goes heterogeneous,
Catalysis Science & Technology, 6 (2016) 923-957.
43] S.G. Pan, S. Yan, T. Osako, Y. Uozumi, Batch and Continuous-Flow Huisgen 1,3-Dipolar
[
Cycloadditions with an Amphiphilic Resin-Supported Triazine-Based Polyethyleneamine
Dendrimer Copper Catalyst, Acs Sustainable Chemistry & Engineering, 5 (2017) 10722-10734.
[
44] D. Astruc, D. Wang, C. Deraedt, L.Y. Liang, R. Ciganda, J. Ruiz, Catalysis Inside
Dendrimers, Synthesis-Stuttgart, 47 (2015) 2017-2031.
45] F. Alonso, Y. Moglie, G. Radivoy, M. Yus, Unsupported Copper Nanoparticles in the 1,3-
Dipolar Cycloaddition of Terminal Alkynes and Azides, Eur J Org Chem, (2010) 1875-1884.
46] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, Nonionic triblock and star
[
[
diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable,
mesoporous silica structures, Journal of the American Chemical Society, 120 (1998) 6024-6036.
[
47] R. He, Y.C. Wang, X.Y. Wang, Z.T. Wang, G. Liu, W. Zhou, L.P. Wen, Q.X. Li, X.P. Wang,
X.Y. Chen, J. Zeng, J.G. Hou, Facile synthesis of pentacle gold-copper alloy nanocrystals and their
plasmonic and catalytic properties, Nature Communications, 5 (2014).
[
48] J. Yin, S.Y. Shan, L.F. Yang, D. Mott, O. Malis, V. Petkov, F. Cai, M.S. Ng, J. Luo, B.H.
Chen, M. Engelhard, C.J. Zhong, Gold-Copper Nanoparticles: Nanostructural Evolution and
Bifunctional Catalytic Sites, Chemistry of Materials, 24 (2012) 4662-4674.
[
49] X. Liu, J. Ruiz, D. Astruc, Prevention of aerobic oxidation of copper nanoparticles by anti-
galvanic alloying: gold versus silver, Chem Commun (Camb), (2017).
50] F. Alonso, Y. Moglie, G. Radivoy, M. Yus, Unsupported Copper Nanoparticles in the 1,3-
[
Dipolar Cycloaddition of Terminal Alkynes and Azides (April, pg 1875, 2010), European Journal
of Organic Chemistry, (2010) 5913-5913.
[
51] R.E. Schaak, A.K. Sra, Synthesis of atomically ordered nanocrystals from bimetallic
nanoparticle precursors., Abstr Pap Am Chem S, 227 (2004) U1537-U1537.
2
4