Chemoselective Hydrogenation of Crotonaldehyde Catalyzed by an Au@ZIF-8 Composite

Article abstract of DOI:10.1002/cctc.201501171

Au nanoparticles of size 2.2 nm were encapsulated in a zeolitic imidazolate framework ZIF-8 framework (Au@ZIF-8). The composite was used as a catalyst for the selective hydrogenation of crotonaldehyde to crotyl alcohol with 90-95 % selectivity. Hydrogenation of 1-hexene yielded n-hexane in approximately the same conversion as crotonaldehyde, whereas cis-cyclohexene did not detectably react. To account for the confined environment of the ZIF, we supported Au nanoparticles on the outside of ZIF-8 (Au/ZIF-8). Using Au/ZIF-8 as a catalyst, 1-hexene and cis-cyclohexene were hydrogenated in low conversion whereas crotonaldehyde would only react at higher pressures in 70 % selectivity towards crotyl alcohol. Post-experiment TEM analysis of Au/ZIF-8 showed significant sintering whereas the median particle size of Au@ZIF-8 remained almost unchanged. Au@ZIF-8 was recycled as a catalyst for the hydrogenation of crotonaldehyde three times without significant loss of activity or selectivity.

Full text of DOI:10.1002/cctc.201501171

DOI: 10.1002/cctc.201501171
Full Papers
Chemoselective Hydrogenation of Crotonaldehyde
Catalyzed by an Au@ZIF-8 Composite
[a]
[b]
[a]
[a, c]
Casey J. Stephenson, Cassandra L. Whitford, Peter C. Stair, Omar K. Farha,*
Joseph T. Hupp*
and
[a]
Au nanoparticles of size 2.2 nm were encapsulated in a zeolitic
imidazolate framework ZIF-8 framework (Au@ZIF-8). The com-
posite was used as a catalyst for the selective hydrogenation
of crotonaldehyde to crotyl alcohol with 90–95% selectivity.
Hydrogenation of 1-hexene yielded n-hexane in approximately
the same conversion as crotonaldehyde, whereas cis-cyclohex-
ene did not detectably react. To account for the confined envi-
ronment of the ZIF, we supported Au nanoparticles on the out-
side of ZIF-8 (Au/ZIF-8). Using Au/ZIF-8 as a catalyst, 1-hexene
and cis-cyclohexene were hydrogenated in low conversion
whereas crotonaldehyde would only react at higher pressures
in 70% selectivity towards crotyl alcohol. Post-experiment TEM
analysis of Au/ZIF-8 showed significant sintering whereas the
median particle size of Au@ZIF-8 remained almost unchanged.
Au@ZIF-8 was recycled as a catalyst for the hydrogenation of
crotonaldehyde three times without significant loss of activity
or selectivity.
Introduction
[
5]
As many industrially relevant precursors are derived from allylic
alcohols, the chemospecific (C=O specific) hydrogenation of
a,b-unsaturated ketones and aldehydes is a highly desirable
chemical transformation. Traditional metals such as Ni, Pd, and
Pt are potent hydrogenation catalysts, but preferentially hydro-
genate the C=C bonds resulting mainly in saturated ketones
Au particle sizes. Although traditional Au nanoparticle cata-
lysts have never achieved 100% selectivity towards C=O hy-
[
6]
drogenation, selectivity towards allylic alcohols can be im-
proved by selective chemical poisoning of specific crystallo-
[
7]
graphic facets or by encapsulating the nanoparticles within
[
7b,8]
the pores of zeolites.
Herein, we report that encapsulation
[
1]
and aldehydes. Notably, although both are exoergic, hydro-
of Au nanoparticle catalysts by the zeolitic imidazolate frame-
[
9]
genation of a C=O bond is thermodynamically disfavored by
work compound ZIF-8 greatly boosts their regio/chemoselec-
tivity toward hydrogenation of a representative a,b-unsaturat-
ed aldehyde, crotonaldehyde, even relative to the above men-
tioned approaches. Indeed, the overall selectivity for allylic al-
cohol formation is approximately 95% and the preference for
the allylic alcohol over the alkyl aldehyde product is, within ex-
perimental uncertainty, quantitative.
À1
3
5 kJmol versus that of a C=C bond. Nevertheless, nanoparti-
culate Au catalysts have been shown to be selective for the hy-
drogenation of the C=O bonds of a, b-unsaturated ketones
[
2]
and aldehydes.
The selectivity of Au nanoparticles as catalysts for organic re-
actions is influenced by many factors, including reaction condi-
[
2a]
[2a,b,3]
tions, nanoparticle size and morphology,
and the type
ZIFs belong to a class of materials known as metal-organic
[
2a,4]
[10,11]
of support.
Nanoparticle size is thought to have perhaps
frameworks (MOFs).
These versatile materials have been in-
the greatest influence on selectivity, with the best selectivity
vestigated for many potential applications including chemical
[
12]
[13]
[10b]
towards a,b-unsaturated alcohols obtained from the smallest
separations, sensing, and catalysis.
The overall stoichi-
ometry of homoleptic ZIFs is generally M(Im) where the node,
2
M, is a tetrahedrally coordinated metal in oxidation state II,
and the bond angle is similar to the SiÀOÀSi bond angle found
in zeolites and, by definition, the topology of a ZIF is one
known for inorganic zeolites.
[
a] C. J. Stephenson, Prof. P. C. Stair, Prof. O. K. Farha, Prof. J. T. Hupp
Department of Chemistry
Northwestern University
2
145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: j-hupp@northwestern.edu
o-farha@northwestern.edu
The encapsulation of nanoparticles within MOFs—typically
for catalysis, but also for applications such as chemical sens-
[
b] C. L. Whitford
[
14]
[15]
ing —has been the subject of considerable study. The two
general approaches to obtaining nanoparticle@MOF compo-
sites are: a) to form the particles within the preexisting MOF—
typically by reducing metal complexes that have infiltrated the
MOF pores (e.g., by chemical vapor deposition or incipient
Department of Chemical and Biological Engineering
Northwestern University
2
145 Sheridan Road, Evanston, IL 60208 (USA)
[
c] Prof. O. K. Farha
Department of Chemistry
King Abdulaziz University
Jeddah (Saudi Arabia)
[
7b,15a,16]
wetness impregnation),
or b) to grow the MOF around
the preexisting nanoparticle(s). The second strategy has
proven increasingly popular because it allows for independent
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501171.
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control (precontrol) over particle size and because it typically
yields pinhole-free MOF coatings—thereby ensuring that gas-
or solution-phase molecules must traverse the MOF to reach
[
16f,i,17]
a
potentially catalytic or chemoresponsive particle.
Indeed, since its first description for ZIF-8 and various metal,
[18]
metal–oxide, and metal–sulfide nanoparticles, the approach
[
17d,19]
has been extended to a variety of MOFs, including UiO-66
[
17d]
[19]
[17d]
[16f,i,17a,d,f,19,20]
and UiO-67,
MIL-53, MOF-801,
and others.
[21]
ZIF-8 presents small, but flexible, apertures of crystallo-
graphic width 3.4 Š that open into cavities of diameter 11.6 Š.
Recognizing that small linear alkanes, alkenes, and alcohols
have kinetic diameters in the range of approximately 4.4–6 Š,
crotonaldehyde is expected to pass through the apertures. In
turn, at the ZIF-8/gold-nanoparticle interface, only the perme-
ating molecule’s terminus would be able to contact the en-
shrouded particle’s surface. This could provide a basis for che-
moselective (aldehyde selective) hydrogenation of crotonalde-
hyde to yield crotyl alcohol. We previously observed that ZIF-
8
-encapsulated platinum nanoparticles display superb regiose-
lectivity towards terminal olefins in the hydrogenation of linear
[
18,22]
Figure 1. TEM images of a) Au@ZIF-8 showing Au nanoparticles encapsulat-
ed in ZIF crystallites and b) higher magnification image of Au@ZIF-8 show-
ing small Au nanoparticles in a single ZIF crystallite; STEM images of Au/ZIF-
alkenes and alkynes.
Although Pt@ZIF-8 composites
should be capable of regioselectively catalyzing the hydroge-
nation of crotonaldehyde, decarbonylation forms CO, which
readily adsorbs and poisons Pt surfaces. Gold nanoparticles, in
8
showing Au nanoparticles on the exterior of ZIF-8 crystallite in c) scanning
mode and d) Z-contrast mode.
[23,24]
contrast, resist CO poisoning,
catalytic encapsulants.
making them attractive as
where that PVP-coated Au nanoparticles are active for the hy-
Results and Discussion
[
3c]
drogenation of a,b-unsaturated aldehydes.
Poly(vinylpyrrolidone) (PVP) coated Au nanoparticles were syn-
thesized by using a procedure adapted from the literature (see
Catalytic hydrogenation runs were conducted in a 25 mL
Parr bomb with 3.6 mL solvent (THF or ethanol) with
2000 equivalents of substrate, under 5 bar of H2 for 24 h at
808C, using 0.200 mL undecane as an internal standard
(Table 1). To gauge whether nanoparticles were fully encapsu-
lated and that pinhole defects in the MOF were absent, we
first evaluated the substrate-size selectivity of the composite
catalyst. As anticipated for well-encapsulated active sites, cis-
cyclohexene proved unreactive if exposed under H2 to
Au@ZIF-8. In contrast, under the same conditions exposure to
Au/ZIF-8 converted cis-cyclohexene to cyclohexane with 4% in-
itial yield (Table 1, entries 1 and 2), demonstrating that ex-
posed nanoparticles are indeed catalytically competent for
alkene hydrogenation. Thus, the absence of reactivity in the
presence of Au@ZIF-8 can be ascribed to substrate exclusion.
Extending the comparison to 1-hexene, a candidate reactant
that is known to be capable of permeating ZIF-8, we observed
30% conversion to n-hexane with Au/ZIF-8 as the catalyst, but
only 10% conversion with Au@ZIF-8 (entries 2 and 3). The dif-
ference can be viewed as an indication of the extent to which
the rate of reactant transport through the MOF limits the ki-
netics of composite-catalyzed hydrogenation. To access the
composite material’s ability to hydrogenate internal C=C unsa-
turated bonds, we performed hydrogenations with Au@ZIF-8
and Au/ZIF-8 with trans-3-hexene as a substrate. The substrate
went unreacted with Au@ZIF-8 as a catalyst (entry 5) whereas
Au/ZIF-8 hydrogenated trans-3-hexene to n-hexane in 10%
conversion (entry 6).
[25]
Experimental Section). From transmission electron microsco-
py (TEM) (see Supporting Information Figure S1) measure-
ments, the particles averaged 1.6 nm in diameter, with the vast
majority below 2 nm (and a smaller number of ꢀ4 nm). The
Au@ZIF-8 composite was synthesized by the addition of meth-
anolic solutions of zinc nitrate and linker followed by addition
À1
of a dilute (0.22 mgAu mL ) aqueous solution of Au nanoparti-
cles. The brown Au@ZIF-8 composite was characterized by
scanning transmission electron microscopy (STEM), inductively
coupled plasma atomic emission spectroscopy (ICP–AES), and
powder X-ray diffraction (PXRD) measurements. The Au con-
tent was found to be approximately 0.6 wt.% by ICP–AES.
STEM revealed crystallites of approximately 400–600 nm size
(
Figure 1a and Figure S3). From STEM of Au@ZIF-8, the Au
nanoparticles were of average particle diameter of 2.5 nm with
a median particle diameter of 2.2 nm (Figure 1aand 1b). The
smallest observed particle was approximately 1.1 nm and the
largest was approximately 6 nm.
As a control for ZIF environmental effects other than con-
finement, we also immobilized Au nanoparticles on the exteri-
or surface of the MOF (Au/ZIF-8). Electron microscopy revealed
a composite consisting of ZIF-8 crystallites of approximately
2
00 nm, with Au nanoparticles of 2–5 nm diameter (plus a few
nanoparticles greater than 5 nm) sited on the crystallite surface
Figure 1c and 1d). Although one might anticipate interference
by PVP, for example, substrate binding, it has been shown else-
(
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From the literature, the selectivity of non-encapsu-
lated gold nanoparticles for the unsaturated alcohol
product in the catalysis of hydrogenation of a,b-un-
saturated alcohols is strongly dependent on nanopar-
ticle size. Bus et al. examined alumina-supported Au
nanoparticles of various sizes for the liquid phase hy-
Table 1. Catalytic results for the hydrogenation of unsaturated substrates using
Au@ZIF-8, Au/ZIF-8, Au @ZIF-8, or Au /ZIF-8 as catalysts.
6 6
[
a]
drogenation of cinnamaldehyde (80 bar H ; 1008C).
[
b,c]
[b,c]
2
Entry Catalyst Substrate
Product
Conversion
Selectivity
[%]
For 1–2 nm nanoparticles, they reported 90% selec-
tivity for the allylic alcohol, with selectivity decreasing
[
%]
1
2
Au@ZIF-8
Au/ZIF-8
0
4
–
–
[
27]
sharply with increasing particle size. Zanella et al.
looked at titania-supported Au nanoparticles (1–
2 nm) on titania for the gas-phase hydrogenation of
3
4
5
6
7
8
9
1
Au@ZIF-8
Au/ZIF-8
Au@ZIF-8
Au/ZIF-8
Au@ZIF-8
Au/ZIF-8
Au/ZIF-8
10
30
0
10
13
–
–
–
–
–
93
–
[
28]
crotonaldehyde at 1208C.
The highest selectivity
observed for crotyl alcohol was approximately
[
28]
7
0%. Volpe explored the hydrogenation of croto-
naldehyde over Au/CeO catalysts in the gas phase
2
[
d]
50
20
70
85
and found that high selectivity (73%) was obtained
[
d]
d]
0
Au
8
6
@ZIF-
[4b,29]
with the smallest particles (sub 4 nm).
Intriguing-
[
ly, if the Au/CeO catalysts were used in the liquid
2
1
1
Au
6
/ZIF-8
80
11
14
9
50
90
95
89
[
29b]
[
e]
e]
e]
12
13
14
Au@ZIF-8
Au@ZIF-8
Au@ZIF-8
phase, the highest selectivity obtained was 29%.
[
[
The drop in selectivity was attributed to competitive
adsorption of solvent and other products. This high-
lights the importance of encapsulation as a means of
protecting the nanoparticle surface from reactants
that might be detrimental to selectivity. Given the
distribution of particle sizes in our composites, it is
conceivable that the approximately 5% diversion
away from crotyl alcohol and toward butyraldehyde-
[
0
a] General catalytic conditions: 2000 equivalents of substrate in 3.6 mL solvent with
.200 mL undecane as an internal standard at 808C under 5 bar H performed in du-
plicate. [b] Determined by GC–TOF or GC–FID techniques. [c] Selectivity relative to all
2
reported products. [d] Performed under 15 bar H
catalyst sample.
2
. [e] Performed by reusing the same
For crotonaldehyde, Au@ZIF-8 catalyzed its hydrogenation
with 93% selectivity for the desired allylic alcohol (Table 1,
entry 7). Zero amounts of butyraldehyde were detected, sug-
gesting quantitative selectivity for C=O hydrogenation relative
to C=C hydrogenation. Also detected, however, were small
amounts of side products having masses matching those of
dimers of butyraldehyde. Butyraldehyde can form from C=C
hydrogenation of crotonaldehdye or from keto–enol tautome-
rization of crotyl alcohol (Scheme 2). The side products pre-
sumably arise from aldol condensation of butyraldehyde. We
conclude, therefore, that the observed overall selectivity of
derived products, is the result of comparatively unselective hy-
drogenation catalysis by larger particles. If so, then even
higher selectivities for crotyl alcohol (i.e., >95%) would be ob-
tained if composites containing only the smallest catalytic par-
ticles could be used. Regardless, the selectivity that we ob-
serve for the allylic alcohol is among the highest reported for
[
5,6,30]
the hydrogenation of crotonaldehyde.
We prepared a ZIF-8 composite with approximately 6–10 nm
Au nanoparticles encapsulated within to investigate the effect
of particle size on selectivity in crotonaldehyde hydrogenation
(Au @ZIF-8, Figure 2a and Figure 2b). We also prepared a con-
6
9
5% for hydrogenation of the aldehyde portion of crotonalde-
trol catalyst with 6–10 nm Au nanoparticles on the exterior of
hyde is likely also the initial selectivity for C=O versus
C=C hydrogenation. Consistent with this notion, no
evidence was found for formation of a third primary
product (such as propene; Scheme 1). Finally, notably
the surface sites of ZIF-8 (presumably including non-
coordinated nitrogen atoms of 2-methyl-imidazolate)
are known to be catalytically active for aldol conden-
[
26]
sations.
Scheme 2. Aldol condensation reaction between crotyl alcohol and butyrylaldehyde.
Scheme 1. Possible hydrogenation products and decarbonylation pathway for crotonaldehyde.
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Figure 3. TEM images of a) Au@ZIF-8 post-catalysis; b) Au/ZIF-8 post-cataly-
sis.
Given the negligible or nearly negligible changes in Au@ZIF-
8
nanoparticle size during catalysis, we examined the potential
of the composite for recovery and reuse. As shown in Table 1,
entries 12–14, the activity and selectivity of the catalyst
changed very little, if at all, over the course of four catalytic
runs. (Between trials, the sample was dried overnight at 1008C
under vacuum.) In contrast, conventionally supported Au
nanoparticle catalysts suffer from high rates of particle sinter-
Figure 2. TEM images of a) multiple Au
particles 6–10 nm on the exterior of ZIF crystallites; b) Au
nanoparticles; c) Au @ZIF-8 showing 6–10 nm Au nanoparticles encapsualt-
6
/ZIF-8 crystallites showing Au nano-
6
/ZIF-8 showing Au
6
[
23,32]
ing and aggregation.
Finally, while our study was in progress we learned of similar
ed in ZIF-8 crystallite; d) Au
tallite.
6
@ZIF-8 showing Au nanoparticles in ZIF-8 crys-
[
7b]
work by Huang and coworkers. Briefly, they examined the
hydrogenation of cinnamaldehyde as catalyzed by Pt@UiO-66-
the ZIF-8 framework (Au /ZIF-8 Figure 2c and 2d). The hydro-
NH . Cinnamaldehyde presents a larger steric cross-section
6
2
genation of crotonaldehyde was carried out at elevated pres-
sure (15 bar H ). For Au /ZIF-8, selectivity towards crotyl alco-
than crotonaldehyde, but UiO-66 presents larger apertures
than ZIF-8. The net result is similarly high MOF-engendered se-
lectivity of the nanoparticle catalyst for hydrogenation of the
C=O bond relative to the allyl C=C bond. However, notably the
authors used 1 mL of trimethylamine in their catalytic trials,
presumably to selectively poison more reactive facets and in-
crease selectivity towards C=O hydrogenation. In our work, we
only rely on the nanoparticles and the confining environment
of the framework to provide selectivity.
2
6
hol is approximately 50% with a large amount of aldol con-
densation products and n-butanol as the main side-products
(
Table 1, entry 11). Au @ZIF-8 converted 20% of crotonalde-
6
hyde to crotyl alcohol with 85% selectivity (entry 12). This
result indicates that the selectivity that we obtain with
Au@ZIF-8 might be attributed to the confined environment of
the ZIF and not strictly to particle size alone. Similarly, Rojas
et al. supported 6 nm Au nanoparticles on silica for the liquid-
phase hydrogenation of cinnamaldehyde at 808C and obtained
[
31]
a selectivity of 80%.
For the attempted hydrogenation of crotonaldehyde using
Au/ZIF-8 as a catalyst, we observed no conversion (Table 1,
entry 8). The failure was accompanied by a change in catalyst
color from brown to pink (Figure S5; in contrast, the color of
Au@ZIF-8 was unchanged). The pink color is suggestive of par-
ticle aggregation and indeed TEM indicated a substantial in-
crease in particle size (to 6 nm, Figure 3b) with some exceed-
ing 15 nm. In contrast, for Au@ZIF-8 we saw no change in
median particle diameter (2.2 nm, Figure 3a) and only a small
increase in average particle size (2.5 nm to 3.3 nm). This differ-
ence might or might not be statistically meaningful and, there-
fore, indicative of slight sintering. We then attempted the hy-
drogenation of crotonaldehyde at higher pressure (15 bar H2).
Under these conditions, with Au/ZIF-8 as the catalyst, we ob-
served 30% conversion of crotonaldehyde with approximately
Conclusions
Encapsulation of small Au particles (ꢀ2 nm) in zeolitic imida-
zolate framework compound ZIF-8 renders them highly selec-
tive for aldehyde hydrogenation over internal unsaturated
bond hydrogenation. Encapsulation also prevents particle ag-
glomeration and concomitant loss of catalytic activity, enabling
the catalyst to be isolated, re-suspended and reused. The ob-
served enhanced selectivity for the formation of crotyl alcohol
from crotonaldehyde is attributed to a tendency for the small-
diameter apertures to permit only the termini of the reactant
to reach the Au nanoparticle surface. The terminal methyl
group is unreactive and the aperture-confined allyl group at
the center of crotonaldehyde is unable to access the catalyst
surface, leaving only the C=O group available and able to
react with nanoparticle-activated hydrogen.
7
0% selectivity towards the allylic alcohol (Table 1, entry 9). Bu-
tyraldehyde, butanol, and aldol condensation products were
the primary side products observed.
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Synthesis of Au@ZIF-8 composites
Experimental Section
Au@ZIF-8 composites were prepared by modifying the literature
procedure. Methanolic solutions of zinc nitrate hexahydrate
Materials
Hydrogen tetrachloroaurate trihydrate (49% Au, 99.9% purity
based on Au) and zinc nitrate hexahydrate (98%) was purchased
from Strem Chemicals; Au ICP standard, poly(vinylpyrrolidone)
powder, (K30, MW 40000; MW 10000), 2-methylimidazole (99%), 1-
hexene (99.9%), trans-3-hexene (99.9%), cis-cyclohexene (>99%),
crotonaldehyde (98% predominantly trans), undecane ( ꢁ99%),
ethanol (anhydrous, ꢁ99.5%), diethyl ether (ACS reagent, ꢁ99%),
nitric acid (traceSELECT for trace analysis), hydrochloric acid (ACS
regent grade, 37%), tetrahydrofuran (>99.9%, anhydrous), and
methanol (HPLC grade, 99.99%) were purchased from Sigma Al-
drich; Microsep Advanced Filter Devices (30k cutoff) were pur-
chased from Pall Life Sciences; hydrogen for catalytic experiments
(
500 mL, 15 mm) and 2-methylimidazole (500 mL, 15 mm) were
mixed in a 250 mL Erlenmeyer flask, followed by the immediate ad-
dition of PVP-coated Au nanoparticles. The solution slowly became
opaque and a light brown precipitate slowly formed. After stand-
ing for approximately 24 h, the precipitate was isolated by centrifu-
gation and washed several times with methanol. In between wash-
ings, the solid was allowed to soak in solvent for a few hours. The
solid was dried under vacuum, overnight.
Synthesis of Au/ZIF-8
ZIF-8 (200 mg) was stirred with Au–PVP (2 mg Au) in 20 mL H O in
5
washed twice with DMF and then ethanol, and dried under
(
UHP, 99.999%) was purchased from Airgas; TEM grids (300 mesh
2
0 mL DMF for 2 h. The brown solid was isolated by centrifugation,
Cu/lacey carbon; 400 mesh, Cu/C) were purchased from Ted Pella.
vacuum at 808C overnight.
Physical and analytical measurements
Gas chromatography time-of-flight (GC–TOF) data were recorded
on a Waters Micromass GCT Premiere with an Agilent 7890 GC
inlet and a DB5 30 meter column in EI mode. Samples were diluted
in 1 mL diethyl ether. GC-FID data were recorded on an Agilent
Catalytic trials
Catalytic hydrogenations were conducted in a 25 mL Parr bomb
with 3.6 mL solvent (THF or ethanol) with 2000 equivalents of sub-
strate, under 5 bar of H . Au@ZIF-8 or Au/ZIF-8 were first heated at
7
820A with a 19091 J-413 column. TEM was performed on a Hitachi
2
1
208C under vacuum for 2 h. Samples were then reduced under
H-8100 Microscope. STEM was performed on a Hitachi HD-2300
Dual EDS Cryo STEM at 200 kV. Samples for STEM analysis were
prepared by dropping a methanolic solution of the compound
onto Cu/C TEM grids or lacey carbon TEM grids. ICP–AES was per-
formed on a Varian Vista MPX ICP spectrometer. Samples were di-
gested in aqua regia and diluted in Milli-Q water. PXRD analysis
was performed on a Rigaku Smartlab Thin-film Diffraction WorkSta-
tion with 9 kW copper rotation anode X-ray source coupled with
a multilayer optic. Spectra were recorded from 28<q<708.
a H atmosphere for 2 h at 1208C. The catalyst and reaction solvent
2
were loaded into the reactor under an inert atmosphere. The Parr
reactor was then interfaced to a Schlenk line and cooled in a dry
ice/acetone bath. The reaction solution was subjected to two or
three freeze–pump–thaw cycles. The reactor was then placed in an
8
08C oil bath. The system was equilibrated before introducing H2.
Samples were analyzed by taking an aliquot, diluting it in 1 mL di-
ethyl ether before analyzing it by GC.
Synthesis of sub 2 nm Au-PVP nanoparticles
Acknowledgements
The Au nanoparticles were synthesized by a procedure adapted
[25]
We gratefully acknowledge financial support from the National
Science Foundation (DMR-1334928). Acquisition of data on trace
analysis, GC instruments, and X-ray used in the IMSERC facility of
Northwestern University was made possible by support from
Northwestern University and grant CHE-0923236 from the Na-
tional Science Foundation, respectively. This work made use of
the TEM and STEM instruments located in the EPIC facility
from the literature. HAuCl ·3H O (20.0 mg, 0.051 mmol) and PVP
4
2
(
MW 40000; 555 mg) in 50 mL water were stirred in an ice–water
bath for 20 min before NaBH (20 mg, 0.95 mmol) in 5 mL H O was
4
2
rapidly added to the stirring solution. The solution changed color
from yellow to brown indicating the formation of small Au nano-
particles. After stirring for 20 min, chilled acetone was added to
precipitate nanoparticles. The nanoparticles were isolated by cen-
trifugation and then dissolved in ice-cold water and subjected to
filtration using a Pall Life Sciences Microsep Advanced Filter Devi-
ces (30k cutoff). The solution/suspension was kept in an ice–water
bath throughout the purification process.
(
NUANCE Center-Northwestern University), which has received
support from the MRSEC program (NSF DMR-1121262) at the Ma-
terials Research Center; the Nanoscale Science and Engineering
Center (NSF EEC-0647560) at the International Institute for Nano-
technology; and the State of Illinois, through the International In-
stitute for Nanotechnology.
Synthesis of 6 nm Au–PVP nanoparticles
Larger Au nanoparticles were synthesized by adapted literature
[
33]
methods. HAuCl ·3H O (10.0 mg, 0.025 mmol) was dissolved in
5
9
4
2
Keywords: aldehydes
hydrogenation · metal–organic frameworks
·
chemoselectivity
·
gold
·
mL Millipore water. PVP (MW 10000; 62 mg) was dissolved in
0 mL ethylene glycol and heated at 808C for 2 h. The solution of
gold chloride was added to the PVP solution at 08C. The pH was
adjusted to 9 by the addition of 1m NaOH (1m in Millipore water).
The reaction was then stirred at 1008C for 90 min. The pink solu-
tion was cooled to RT and subsequently subjected to filtration
using a Pall Life Sciences Microsep Advanced Filter Deice (30k
[
[
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Received: October 27, 2015
Published online on January 20, 2016
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