Accelerating Chemo- And Regioselective Hydrogenation of Alkynes over Bimetallic Nanoparticles in a Metal-Organic Framework

Article abstract of DOI:10.1021/acscatal.0c00177

Selective semihydrogenation of alkynes has been a long-term and significant target, yet it remains a great challenge. Herein, bimetallic nanoparticles in a metal-organic framework (MOF), i.e., CuPd&at;ZIF-8 composite, featuring a cubic CuPd core and a porous ZIF-8 shell, have been rationally fabricated for this end. Given the unique physicochemical properties, the Cu nanocubes can not only convert solar energy into heat to accelerate the reaction but also serve as the seed for in situ formation of Pd nanoparticles (NPs) on their external surface to regulate the chemoselectivity of Pd active sites. The additional growth of the MOF shell is helpful to stabilize the CuPd core and offer regioselectivity via the steric hindrance effect. Ammonia borane provides active hydrogen species to significantly boost the hydrogenation and ensure the high selectivity. As a result, the CuPd&at;MOF exhibits high efficiency, featuring a turnover frequency (TOF, 6799 min-1) of 5-105 times higher than that in previous reports, and high chemo- and regioselectivity toward the semihydrogenation of alkynes, in the presence of NH3BH3 as a hydrogen source, under visible-light irradiation at ambient temperature.

Full text of DOI:10.1021/acscatal.0c00177

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Article
Accelerating Chemo- and Regio-Selective Hydrogenation of Alkynes
over Bimetallic Nanoparticles in a Metal-Organic Framework
Luyan Li, Weijie Yang, Qihao Yang, Qiaoqiao Guan, Junling Lu, Shu-Hong Yu, and Hai-Long Jiang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c00177 • Publication Date (Web): 11 Jun 2020
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Accelerating Chemo- and Regio-Selective
Hydrogenation of Alkynes over Bimetallic
Nanoparticles in a Metal-Organic Framework
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Luyan Li,† Weijie Yang,‡ Qihao Yang,† Qiaoqiao Guan,† Junling Lu,† Shu-Hong Yu,† and Hai-
Long Jiang*†
†Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Soft
Matter Chemistry, Collaborative Innovation Center of Suzhou Nano Science and Technology,
School of Chemistry and Materials Science, University of Science and Technology of China
Hefei, Anhui 230026, P. R. China
‡School of Energy and Power Engineering, North China Electric Power University, Baoding,
Hebei 071003, P.R. China
KEYWORDS： metal-organic framework, photothermal effect, alloy nanocrystals, plasmonic,
selective catalysis
ABSTRACT：Selective semi-hydrogenation of alkynes has been a long-term and significant
target yet remains a great challenge. Herein, bimetallic nanoparticles in a metal-organic
framework (MOF), i.e. CuPd@ZIF-8 composite, featuring a cubic CuPd core and a porous ZIF-8
shell, have been rationally fabricated for this end. Given the unique physicochemical properties,
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the Cu nanocubes can not only convert solar energy into heat to accelerate the reaction but also
serve as the seed for in situ formation of Pd NPs on their external surface to regulate the
chemoselectivity of Pd active sites. The additional growth of MOF shell is helpful to stabilize
CuPd core and offer regioselectivity via steric hindrance effect. Ammonia borane provides active
hydrogen species to significantly boost the hydrogenation and ensure the high selectivity. As a
result, the CuPd@MOF exhibits high efficiency, featuring a turnover frequency (TOF, 6799 min-
1) of 5 ~ 105 times higher than that in previous reports, and high chemo- and regio-selectivity
toward the semi-hydrogenation of alkynes, in the presence of NH3BH3 as a hydrogen source,
under visible-light irradiation at ambient temperature.
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INTRODUCTION
Selective semi-hydrogenation of alkynes to alkenes is of great importance and has attracted
particular attention in both academia and chemical industry, especially relevant to commodity
chemical production (e.g. purification of phenylethylene feeds for polymerization). Typically,
the alkynes, especially terminal alkynes, are prone to over-hydrogenation to alkanes because of
their high reactivity; the selective semi-hydrogenation of alkynes remains a particular
challenge.1-10 To date, heterogeneous selective semi-hydrogenation reactions have been generally
investigated with metal-based catalysts in hydrogen (H2) atmosphere. Amongst them, Pd-based
bimetallic catalysts are suggested to be most promising for this reaction. The classical Lindlar
catalyst, partially poisoning Pd with a secondary metal such as Pb to improve selectivity, remains
the choice in industry. However, the toxic lead, unsatisfied activity and high price are the driving
forces for the search of alternative catalysts. With the research effort, CuPd has been found based
on theoretical simulations to be very promising owing to its non-toxic and high selectivity for
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hydrogenation reactions.11,
Unfortunately, experimental reports show that the semi-
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hydrogenation reaction over CuPd usually requires high H2 pressure and still causes over-
hydrogenation to alkanes at high conversion in the prolonged reaction time.1, 13-15 Therefore,
surpassing the limitation of the long-term “trade-off” effect to efficiently produce alkenes with
constant selectivity at high conversion level still poses a challenge by using the CuPd catalysts,
especially under ambient/moderate pressure, which is highly desired.
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On the other hand, the increase of reaction temperature is a general way to accelerate the
kinetics of catalytic reactions. From the viewpoint of energy saving, the conversion of solar
radiation into heat energy, via the localized surface plasmon resonance (LSPR) of metal
catalysts,16-19 to elevate the reaction temperature is desirable. Although Cu nanoparticles (NPs)
possess LSPR-induced photothermal effect, it is rarely reported because they are easily oxidized
and/or chemically etched during the reaction/illumination.20 To address this issue, encapsulating
Cu NPs into a porous material with appropriate pore sizes has shown to be an advisable
solution.21 The porous shell will not only prevent the Cu NPs from being further oxidized or
etched during the reaction, to some extent, but also facilitate the enrichment of substrates.
Moreover, uniform pore size might give rise to size-/regio-selective catalysis via the steric
hindrance effect. To meet these requirements, metal-organic frameworks (MOFs),22-30
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relatively new class of crystalline porous materials with high surface area and tunable pore sizes,
might be ideal candidates.31-48
Bearing the above considerations in mind, we have rationally fabricated a core-shell structured
MOF composite, namely CuPd@ZIF-8, which features cubic Cu supported Pd NPs together as a
core and a representative MOF, ZIF-8 (Zn(2-methylimidazole)2),49, 50 as a shell (Scheme 1).
Particularly, to avoid high-pressure H2 gas and tune the selective hydrogenation, ammonia
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borane (NH3BH3), as a particular hydrogen source, has been introduced into the alkyne
hydrogenation system. Thanks to the plasmon-driven photothermal effect of Cu nanocubes
(NCs), the high Pd activity boosted by active hydrogen species from NH3BH3, the
chemoselectivity influenced by Cu-Pd interaction while dominated by NH3BH3 hydrogen source,
and the protection and steric hindrance effect of ZIF-8 shell, the CuPd@ZIF-8 composite
exhibits excellent activity, both chemo- and regio-selectivity without “trade-off” effect, as well
as good recyclability, for alkyne semi-hydrogenation under visible light irradiation at ambient
temperature. Remarkably, the turnover frequency (TOF) of phenylacetylene semi-hydrogenation
reaches 6799 min-1, surpassing all previously reported metal-based catalysts, and the high
selectivity can be well maintained with extended reaction time. Based on our knowledge of the
literature, this is an unusual report on a heterogeneous catalyst with high activity, chemo- and
regio-selectivity toward hydrogenation of alkynes.
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Scheme 1. Schematic illustration showing the preparation process of CuPd@ZIF-8.
RESULTS AND DISCUSSION
The Cu NCs were prepared in H2O in presence of CuCl2·2H2O, glucose and hexadecylamine
(HAD).51 The Pd NPs were formed on the external surface of Cu NCs to give CuPd NCs via the
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2+
2+
galvanic reaction between Pd2+ and Cu0 (EPd
= +0.951 eV vs SHE; ECu
= +0.337 eV vs
/Cu
/Pd
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SHE). Following that, the Zn2+ and 2-methylimidazole were assembled onto the surface of CuPd
NCs with the help of interfacing polyvinyl pyrrolidone (PVP), leading to the formation of core-
shell structured CuPd@ZIF-8 composites (Scheme 1).
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Powder X-ray diffraction (XRD) pattern of CuPd NCs shows two peaks at 43.5 and 50.7
degree, which can be assigned to the (111) and (200) diffractions of face-centered cubic Cu,
respectively, indicating the successful synthesis of Cu NCs (Figure S1a). The absence of
identifiable diffraction peak of Pd indicates the possibly low Pd content and/or small Pd sizes.
Powder XRD pattern of CuPd@ZIF-8 clearly demonstrates the formation of ZIF-8 with good
crystallinity (Figure S1a). N2 sorption of CuPd@ZIF-8 indicates its high Brunauer-Emmett-
Teller (BET) surface area (1077 m2/g) and typical microporous character (Figure S1b). The
slight decrease in surface area compared to parent ZIF-8 (1296 m2/g) is ascribed to the mass
occupation of CuPd NCs.
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Figure 1. (a) SEM image of Cu NCs; TEM images of (b, c) CuPd NCs and (d) CuPd@ZIF-8.
Inset of c: HRTEM image of Pd NPs; (e) HAADF-STEM of CuPd and (f) the corresponding
elemental mapping of Cu (red), Pd (green).
Scanning electron microscope (SEM) observation shows that the Cu NCs have a regular shape
with average sizes of 61 ± 6 nm (Figure 1a). Transmission electron microscopy (TEM), high-
resolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM)
observation for CuPd NCs suggest that Pd NPs (~2.5 nm) are formed on the surface of Cu NCs
and almost do not change the original morphology of the latter (Figure 1b-c, e, S2a). Elemental
mapping results further support the uniform Pd dispersion on the Cu NCs (Figure 1f). The
well observable lattice fringe with spacing of 1.37 Å could be indexed to the (220) plane of Pd
NPs (Figure 1c inset). The TEM image of CuPd@ZIF-8 indicates that the CuPd NCs are well
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encapsulated by ZIF-8 shell and the size of an individual CuPd@ZIF-8 particle is 600-1000 nm
(Figure 1d, S2b). The actual contents of Cu and Pd are determined to be 5.5 mol% and 0.1
mol%, respectively, in CuPd@ZIF-8 composite by inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The trace Pd loading signifies the low cost of this catalyst. Notably, the
LSPR-induced absorption of Cu NCs in 575-700 nm can be well inherited to CuPd@ZIF-8
composite (Figure S3), suggesting that the ZIF-8 shell can be successfully penetrated by light
irradiation. To investigate the interaction between Pd and Cu, the temperature-programmed
reduction (TPR) for CuPd@ZIF-8 was conducted. Two reduction peaks at around 160 °C and
340 °C can be assigned to the oxidative species of PdO and Cu2O,14 respectively (Figure 2a).
Apparently, the reduction temperature of Cu2O at ~340 °C for CuPd@ZIF-8 is lower than that
(369 °C) for Cu@ZIF-8, revealing the interaction between Pd and Cu. The reduction of Cu2O in
CuPd@ZIF-8 might be promoted by the H2 spillover from Pd to the neighboring Cu2O.52 This
Cu-Pd interaction is further supported by X-ray photoelectron spectroscopy (XPS) data. The
3d5/2 peak for Pd0 shifts to the higher binding energy in CuPd due to the electron transfer from
Pd to Cu and the appearance of Cu2+ peak would be due to the partial oxidation of Cu in CuPd
NCs (Figure 2b, S4).19
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a
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ZIF-8
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Bonding Energy (eV)
Figure 2. (a) TPR profiles for Cu@ZIF-8, CuPd@ZIF-8 and ZIF-8; (b) XPS spectra of Pd 3d.
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The above rationally fabricated CuPd@ZIF-8 composite is expected to afford excellent
performance toward selective hydrogenation of alkynes under light irradiation at ambient
temperature. The phenylacetylene (1a) hydrogenation was firstly investigated to optimize the
reaction parameters (Figure 3). As expected, a high selectivity (97%) to phenylethylene (1b) can
be achieved over CuPd@ZIF-8 in H2 atmosphere at room temperature, which is distinctly
different from the poor selectivity (<1%) over Pd@ZIF-8 (Figure 3, Figure S5). The results
suggest that the introduction of Cu is crucial to the high selectivity, probably due to the lower
adsorption stability between alkene and the formed Pd NPs outside Cu NCs.1 Unfortunately, the
conversion of 1a over CuPd@ZIF-8 is only 19% under these conditions, which is even lower
than Pd@ZIF-8, as the Cu introduction reduces the adsorption energy of substrates and
suppresses the H2 dissociation based on the previous reports.1, 53 The photothermal effect of Cu
nanocubes in the catalyst is thought to be able to improve the conversion. To evaluate the
efficiency of photothermal conversion of CuPd@ZIF-8, the temperature change in the solution
along with time is detected. Results clearly show that light energy can be efficiently converted
into heat in the presence of CuPd@ZIF-8 (Figure S6). As expected, by simply introducing light
irradiation, the conversion of 1a over CuPd@ZIF-8 can be promoted to 40%, reflecting the great
contribution of photothermal effect (Figure 3). While the activity is improved, the selectivity
slightly decreases to 90% at this conversion (40%), and continues to fall off along with higher
conversion (Figure S7), in H2 atmosphere. The results indicate that, though the contribution of
Cu-Pd interaction in CuPd@ZIF-8, the over-hydrogenation trend might be hardly avoidable in
H2 atmosphere, similar to the results in previous reports.1, 15
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Figure 3. (a) Semi-hydrogenation of phenylacetylene in CH3OH with hydrogen donor of H2
balloon (or NH3BH3) over Pd@ZIF-8 (or CuPd@ZIF-8) in the absence (left side, with grey
background) or presence (right side, no background) of light irradiation (>400 nm, 160
mW/cm2).
The above results drive us to explore suitable hydrogen source to afford constantly excellent
selectivity at even high conversions toward the semi-hydrogenation of alkynes. Ammonia borane
(NH3BH3) with high hydrogen content of 19.6 wt% has been recognized as a highly potential
hydrogen storage medium.54-59 The NH3BH3 displays excellent solubility in methanol to in situ
generate active hydrogen species, facilitating subsequent hydrogenation.57-59 Therefore, it is
assumed that NH3BH3 might be a suitable hydrogen source to meet this challenge. The
alcoholysis of NH3BH3 over CuPd@ZIF-8 was found to steadily proceed, producing pure H2,
and the reaction can be promoted under the light irradiation (Figure S8). Encouraged by this, the
semi-hydrogenation of 1a was examined with NH3BH3 instead of H2. Significantly, in reference
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to the 70% conversion of 1a, light irradiation further leads to its almost complete conversion
(99%) with 96% selectivity to 1b within 3 min, based on the CuPd@ZIF-8 catalyst (Figure 3).
This much higher activity than that using H2 gas discussed above implies that NH3BH3 might
generate special active hydrogen species to boost the reaction. In contrast, Pd@ZIF-8 gives only
68% selectivity and no reaction occurs with ZIF-8, implying the importance of Pd for the activity
while Cu for promoting the selectivity (Table S1). To our delight, the ZIF-8 shell almost does not
bring effects on the catalytic rate, based on the comparison of phenylacetylene hydrogenation
kinetics over CuPd NCs and CuPd@ZIF-8 (Figure S9). The transport limitation of ZIF-8 has
been further excluded by examining the yield dependence on the catalyst amount (Figure S10).
The TOF of CuPd@ZIF-8 is calculated to be 6799 min-1, which represents the highest value
among ever reported metal catalysts for this reaction (Table S2). The high efficiency implies that
the introduction of NH3BH3 might boost the conversion by the sufficient contact between the
substrate and the hydrogen donor, which in situ generates hydrogen active species.57
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Phenylethylene
Phenylethylene
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1a (0.1 mmol) CuPd@ZIF-8, NH BH
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1b (100%)
+
+
CH3OH, light on
Phenylacetylene
Phenylethane
1b (10 mmol)
1c undetectable
Phenylethane
0
1
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20 40 60 80 100 120
Time (min)
Time (min)
Figure 4. Time-dependent conversion of the hydrogenation of (a) phenylacetylene or (b)
phenylethylene using CuPd@ZIF-8 with NH3BH3 as the hydrogen source under light irradiation
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at ambient temperature. Inset of b: selective reduction of a 1:100 mixture of phenylacetylene and
phenylethylene.
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More importantly, the selectivity shows an apparent decrease in the prolonged reaction time in
H2 atmosphere, while almost no over-hydrogenated product can be detected even the reaction
time length is extended to 2 h in presence of NH3BH3 (Figure 4a, S11). This point has been
further clarified by employing 1b as the reactant for hydrogenation over CuPd@ZIF-8, where
almost no phenylethane (1c) can be produced (Figure 4b). The results unambiguously
demonstrate that, in addition to the Cu in the catalyst, NH3BH3 also plays a critical role in the
high chemoselectivity for the semi-hydrogenation of alkyne. Encouraged by these results, the
experiment, the removal of a small amount of 1a (0.1 mmol) from 1b (10 mmol), which
represents the important purification process in steam cracking of alkenes (usually containing a
small fraction of alkyne impurities) in industry,60, 61 was investigated. Strikingly, 1a can be
completely converted to 1b rapidly without over-hydrogenation to 1c (Figure 4b inset). These
results are very exciting and can be achieved in very few reports,60, 61 as the conventional
catalysts (such as the commercial Lindlar catalyst) and/or hydrogen gas usually result in rapid
over-hydrogenation to alkanes with prolonged reaction time.
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To get insight into the hydrogenation process over the CuPd@ZIF-8 in the presence of
NH3BH3, controlled experiments and density functional theory (DFT) calculations were carried
out. The dependency of the reaction rate on the NH3BH3 amount toward the hydrogenation of 1a
suggests that it follows the first-order reaction, manifesting that NH3BH3 is truly participated in
the reaction (Figure 5a).62 To understand the adsorption behavior of phenylacetylene (C8H6) and
NH3BH3 substrates over the active sites, their adsorption on the catalyst surface has been
investigated through calculating the adsorption energy and adsorption configuration (Figure 5b,
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S12). The adsorption energy of C8H6 on Pd site is -2.43 eV, apparently larger than that of
NH3BH3 (-1.49 eV), which suggests the stronger interaction between C8H6 and Pd sites.
Accordingly, C8H6 is preferentially adsorbed on Pd sites. In reverse, NH3BH3 is mainly adsorbed
on Cu sites, due to its larger adsorption energy (-1.38 eV) than that of C8H6 (-0.61 eV) on Cu
sites.
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Figure 5. (a) Dependency of the reaction rate on the amount of NH3BH3. The correlation
coefficient (R2) is 0.98. (b) Adsorption energy of C8H6 and NH3BH3 on Pd and Cu sites,
respectively. (c) Semi-hydrogenation of phenylacetylene using CuPd@ZIF-8 with NH3BH3 in
CH3OH (red), NH3BH3 in CH3OD (black) and NH3BD3 in CH3OH (blue). (d) Energies of
intermediates and TSs in the mechanism of phenylacetylene hydrogenation to afford the 1st
molecule of alkene (step I) over CuPd@ZIF-8 in presence of NH3BH3 based on DFT
calculations. Pd atom (black), Cu atom (orange).
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Subsequently, the deuterium-labeling experiments were conducted to figure out the real role of
NH3BH3 in the reaction (Figure S13, S14). Whatever NH3BD3 or ND3BH3 is introduced into
CH3OH, the deuterated phenylethylene can be obtained (Figure S14), clearly approving the
cleavage of both B-H and N-H bonds is involved in the reaction and probably the generated
hydrogen species (Hδ- and/or Hδ+)54 participate in the formation of phenylethylene product.
Furthermore, the ratio (4.08) of rate constants, KIE (KL/KH), which falls in the value range (2-7)
of the primary kinetic isotope effect,63 for the reactions involving light (KL) and heavy (KH)
isotopically substituted ammonia borane (NH3BD3), demonstrating that the activation of B-H
bond is the rate-determining step (RDS) in the reaction and revealing that the generated Hδ-
might be involved in the reaction (Figure 5c). In contrast, the KIE of 1.48 for the reactions
involving isotopically substituted methanol (CH3OD) suggests that the O-H activation in
methanol would not be the RDS in the reaction.
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In sharp contrast to the deuterated NH3BH3 experiments above, no H-D exchange takes place
and all the H atoms in phenylethylene can be detected in the 1H NMR spectrum with NH3BH3 in
CD3OD solvent (Figure S15), excluding the possibility of H atom in alkene product originated
from CH3OH. This is completely different from the reported in situ H2 generation from NH3BH3
for subsequent hydrogenation process in previous examples,60, 61 in which the H2 is originated
from both B-H cleavage and the other proton in CH3OH.54 According to the reported work,64 it is
assumed that the active hydrogen species from B-H and N-H cleavage are able to react with the
alkyne group directly to give product, which is supported by the fact that negligible hydrogen gas
production can be detected during our reaction (Figure S16).
To further get clear how the hydrogenation process takes place, we propose a plausible
mechanism by resorting DFT calculation and above experimental results (Figure S17, S18). It is
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proposed that the ideal alkyne hydrogenation in the presence of NH3BH3 involves three steps (I,
II, III) and generates three molar alkenes by consuming one molar NH3BH3. The process (step I)
of generating the 1st molecule of alkene has been confirmed by the DFT calculations. First of all,
NH3BH3 and C8H6 are adsorbed onto the Cu and Pd surfaces, respectively. Then, the breaking of
B-H bond in NH3BH3 gives the first hydrogen species (Hδ-) on the Cu surface, which
nucleophilically attacks the C≡C bond (TS1) (Figure S17c). Meanwhile, the other H atom (Hδ-)
is also released by the second B-H breaking, co-participating in the formation of the 1st molecule
of alkene, which is identified as the RDS process with an energy barrier of 1.46 eV (TS2)
(Figure 5d, S17e). The new ammonia alkoxyborane complex (BH2=NH2) is subsequently created
by transferring one H atom from -NH3 to -BH (TS3), which is validated by the infrared
spectroscopy (Figure S17g, S19). The 2nd molecule of alkene is produced following the similar
process as the first step (I) by consuming two Hδ- species from two B-H bond cleavage (Figure
S17i-j, S18). The 3rd molecule of alkene is generated via obtaining one H atom from the B-H
breaking. It is supposed that the other proton is from the N-H breaking via an inner-sphere
addition mechanism (Figure S18).64 This assumption is supported by the deuterated experiments
(ND3BH3 and CD3OD), which indicate the hydrogen species from N-H cleavage, but not proton
from CH3OH, participates in the production of alkene (Figure S14, S15). After producing the
three molecules of alkene, NH4B(OCH3)4 is accordingly generated, as evidenced by the 11B
NMR spectrum for the mixture (Figure S20).
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From the above experimental and calculated results, it is clear that the active hydrogen species
(Hδ- and Hδ+) from B-H and N-H cleavage directly react with alkynes instead of producing H2.
Given the weak interaction between the polar hydrogen species and alkenes,62 the hydrogenation
of alkenes is unfavorable, suppressing their over-hydrogenation and thus resulting in high
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selectivity to alkenes. Therefore, such hydrogenation process over CuPd@ZIF-8 is based on the
reaction between 1a and polar hydrogen species (Hδ- and Hδ+) from NH3BH3, distinctly different
from the reaction in H2 atmosphere.
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To further investigate the stability and reusability of the catalyst, recycling experiments have
been also carried out. To our delight, the activity of CuPd@ZIF-8 is well maintained in the five
consecutive runs, indicating its great recyclability and stability (Figure S21a, b). In comparison,
the conversion over CuPd NCs, in the absence of ZIF-8 shell protection, sharply decreases
during the three cycles due to the surface oxidation and possible chemical etching of Cu NCs
(Figure S21c, d).20, 31
Table 1. Selective hydrogenation of different alkynes over CuPd@ZIF-8.[a]
R2
R1
R2
R1
Con. Sel.
(%) (%)[c]
Entry
Substrate
Product
1
2
3
4
5
97
98
98
97
93
96
97
94
96
97
OCH3
CH3
F
OCH3
CH3
F
F
F
6
7
8
92
93
94
94
95
95
Cl
Cl
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OH
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9[b]
10
95
7
97
99
8
9
a Reaction conditions: substrate (0.1 mmol), time (5 min), CuPd@ZIF-8 (1 mg, Pd: 0.1 mol%),
CH3OH (6 mL), NH3BH3 (5 mg), light: >400 nm, 160 mW/cm2. b Time (10 min). [c] Catalytic
reaction products were analyzed and identified by gas chromatography.
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Encouraged by the outstanding catalytic performance of CuPd@ZIF-8 towards the semi-
hydrogenation of phenylacetylene, a variety of alkynes with different functional groups have
been investigated (Table 1). Delightedly, various terminal aryl alkynes with electron-donating (-
OCH3 and -CH3) or electron-withdrawing (-F) substituents can be completely transformed to the
corresponding alkenes with high chemoselectivity (>94%) (entries 1-5). Moreover, the selective
hydrogenation of aliphatic alkynes can be also achieved with excellent selectivity (>94%),
indicating the great substrate tolerance of the catalyst (entries 6-9). On the contrary, the
hydrogenation of diphenylacetylene was greatly restrained and only 7% conversion was achieved
over CuPd@ZIF-8 (entry 10). The result indicates that the transformation of internal alkynes to
the corresponding alkenes is impeded possibly due to the steric hindrance by the small pore size
of ZIF-8, which makes the internal carbon-carbon triple bond hardly accessible to the Pd active
sites. Inspired by this, the hydrogenation of the substrate with multiple functional groups,
phenylpropiolaldehyde, is attempted to get the alkynol, 2-propyn-1-ol, which is an important
corrosion inhibitor in petrochemical industry (Table 2).65 Incredibly, the desired product is
obtained with 98% selectivity over CuPd@ZIF-8 (entry 1). In contrast, both carbon- carbon
triple bond and terminal aldehyde group are reduced over the CuPd catalyst (entry 2). This
suggests that the small pore size of ZIF-8 allows the terminal aldehyde group only but restrains
the internal triple bond to interact with the Pd sites. Therefore, ZIF-8 shell not only contributes to
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the stabilization of CuPd NCs but also offers the high regioselectivity via the space steric
hindrance.
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Table 2. Selective hydrogenation of phenylpropiolaldehyde over CuPd@ZIF-8 and CuPda.
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OH
O
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OH
+
+
c
e
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Yield (%)b
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Entry
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Conv. (%)
c
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2
CuPd@ZIF-8
CuPd
>99
>99
98
2
68
32
-
aReaction conditions: substrate (0.1 mmol), time (12 min), CuPd@ZIF-8 (Pd: 0.1 mol%); CuPd
(Pd: 0.1 mol%), CH3OH (4.5 mL), H2O (1.5 mL), NH3BH3 (7.8 mg), light: >400 nm, 160
mW/cm2. bCatalytic reaction products were analyzed and identified by gas chromatography.
CONCLUSION
In summary, we have rationally fabricated a core-shell CuPd@ZIF-8 composite, featuring a
cubic CuPd core and a MOF shell, for selective hydrogenation of alkynes. In the catalyst, the Cu
not only achieves plasmonic photothermal conversion but also regulates the Pd electronic state to
improve the catalytic chemoselectivity; the MOF stabilizes the CuPd NCs and enables
regioselective hydrogenation of alkynes via steric hindrance effect. Thanks to the well dispersed
Pd site, the Cu photothermal effect, and the in situ generated active hydrogen species from
NH3BH3 that is well soluble in CH3OH solvent, the hydrogenation of alkynes can be greatly
boosted. Particularly, instead of traditional H2 gas, the active hydrogen species generated from
the alcoholysis of NH3BH3 ensure the high chemoselectivity in the hydrogenation of alkynes or
alkene/alkyne mixture even over extended reaction time. As a result, the CuPd@ZIF-8 is capable
of catalyzing the hydrogenation of alkynes to alkenes with high chemo- and regio-selectivity,
good recyclability and extremely high activity, featuring the highest TOF value reported thus far,
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toward phenylacetylene semi-hydrogenation. This work opens a new avenue to the rational
design of highly efficient, chemo- and regio-selective composite catalysts and suitable catalytic
systems toward high-performance catalysis.
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EXPERIMENTAL SECTOIN
Preparation of Cu nanocubes: The Cu NCs were prepared according to the previous work
with minor modifications.51 Typically, 52.5 mg of CuCl2·2H2O, 50 mg of glucose, 225 mg of
HAD and 25 mL water were mixed in a 50 mL round-bottom flask and capped. The mixture
solution was magnetically stirred at room temperature for 12 h. Then the flask was transferred
o
into an oil bath and heated at 100 C for at least 6 h under magnetic stirring. As the reaction
proceeded, the color of the solution was changing from blue to red-brown. After reaction, the
o
product was harvested by centrifugation and washed six times with hot water (65 C). Finally,
the sample was dispersed into 5 mL hot water (2 mg/mL).
Preparation of CuPd nanocubes: The obtained Cu nanocubes solution (1 mL) was diluted
into 10 mL deionized water in a 25 mL round-bottom flask, followed by the addition of 50 mg
PVP under vigorous stirring for 12 h. Then, 1 mL (0.91 M) of K2PdCl4 aqueous solution was
injected into the Cu nanocubes solution using a syringe pump at a rate of 0.25 mL/min under
stirring for 12 h. After the galvanic reaction, the obtained product was collected via
centrifugation and dispersed into 1 mL methanol (2 mg/mL).
Preparation of CuPd@ZIF-8: Typically, CuPd nanocubes solution (1 mL) and the methanol
solution of zinc nitrate (25 mM, 15 mL), were mixed at room temperature under N2 bubbling for
5 min, then the equivalent volume of methanol solution of 2-methylimidazole (25 mM, 15 mL)
o
was added to react at 40 C for 30 min in a water bath with a N2 balloon without stirring.66 The
resultant composite was collected by centrifugation and washed twice with methanol. The
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synthesized red-brown powder was further dried overnight at 333 K under dynamic vacuum and
dispersed into 4 mL methanol (1 mg/mL).
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Preparation of Pd@ZIF-8: The Pd nanoparticles (~3 nm) were prepared firstly according to
the reported work with minor modification.67 Typically, 105 mg of Polyvinylpyrrolidone (PVP,
~55000 Mw), 60 mg of ascorbic acid and 5 mg of KBr were dispersed into 8 mL of deionized
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o
water and preheated at 80 C for 10 min. Then, 3 mL of K2PdCl4 (57 mg) aqueous solution was
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added and reacted for 3 hours at 80 C. The Pd nanoparticles were collected via centrifugation
and washed with acetone for five times. Finally, the obtained Pd nanoparticles are dispersed into
6 mL of deionized water (1 mg/mL). To synthesize Pd@ZIF-8, 750 μL of Pd nanoparticles
solution (1 mg/mL) was mixed with the methanol solution of zinc nitrate (Zn(NO3)2·6H2O, 1.069
g, 15 mL), then the equivalent volume of methanol solution of 2-methylimidazole was added
(C4H6N2, 1.61 g, 15 mL). The obtained mixed solvent reacted for 10 hours at room temperature.
After the reaction, the gray powder was collected via centrifugation and washed three times with
methanol. Finally, the obtained product was dried in vacuum oven at 60°C for 12 h.
Preparation of ND3BH3 and NH3BD3: ND3BH3 was prepared according to the reported
method with minor modifications.68 Firstly, 30 mg NH3BH3 and 2 mL CD3OD were added in a 5
mL round bottom flask and vigorously stirred for 4 h at room temperature, then ND3BH3 was
obtained after removing the solvent in vacuum. NH3BD3 was also prepared according to the
reported method. Briefly, NaBD4 (100 mg) and (NH4)2SO4 (315.7 mg) were added to a 100 mL
three-neck round bottom flask. Then, 14 mL THF was added and the mixture was vigorously
stirred at 40 °C for 2 h. Subsequently, it was cooled down to room temperature, filtered and dried
under vacuum to give NH3BD3 powder.
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Photothermal effect evaluation: In general, a mixture of 1 mg catalyst (1 mL, 1 mg/mL) and
6 mL methanol was placed in a two-necked round-bottomed flask (25 mL) and capped. The
mixture was irradiated under 160 mW/cm2 light with a 300 W Xe lamp and magnetic stirring.
Then, the temperature of solution was detected and recorded every 10 min to give the time-
temperature curve.
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Catalytic performance evaluation for the dehydrogenation of ammonia borane
(NH3BH3): In general, a mixture of 1 mg catalyst (1 mL, 1 mg/mL) and 5 mL methanol was
placed in a two-necked round-bottomed flask (25 mL) and capped. The mixture was irradiated
under 160 mW/cm2 light with a 300 W Xe lamp. A gas burette filled with water was connected
to the reaction flask to measure the volume of hydrogen evolved. The reaction started when
NH3BH3 (5 mg) was added into the flask. The volume of the evolved hydrogen gas was
monitored by recording the displacement of water in the gas burette. This reaction was
completed when there was no more gas generated.
Catalytic performance evaluation of ZIF-8 and Pd@ZIF-8 for hydrogenation of
phenylacetylene in the presence of NH3BH3: Typically, a mixture of Pd@ZIF-8 or ZIF-8 (1
mg), 0.1 mmol phenylacetylene, 5 mL methanol was placed in a two-necked round-bottomed
flask (25 mL) under 160 mW/cm2 light irradiation with a 300 W Xe lamp and mechanical
stirring. Then the reaction was initiated by adding 7.8 mg NH3BH3.
Catalytic performance evaluation of CuPd@ZIF-8 or CuPd for hydrogenation of
phenylacetylene and phenylethylene in the presence of NH3BH3: Typically, a mixture of 1 mg
CuPd@ZIF-8 (1 mL, 1 mg/mL), 0.1 mmol phenylacetylene, 5 mL methanol was placed in a two-
necked round-bottomed flask (25 mL) under 160 mW/cm2 light irradiation with a 300 W Xe
lamp and mechanical stirring. The reaction was initiated after adding 7.8 mg NH3BH3 and
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proceeded for 10 min. After the reaction completion, 200 µL of the resultant mixture was taken
into a mixed solvent with 1 mL H2O and 1 mL ethyl acetate. Upon the adequate shaking, the
mixture was allowed to be centrifuged and the product was extracted to the upper layer of ethyl
acetate. Then, 600 µL of sample in the upper solution was detected by GC. For the catalytic
recycling experiments, CuPd@ZIF-8 was separated by centrifugation after reaction, being
thoroughly washed by 5 mL methanol for 3 times. Then, it was dried in vacuum for 6 h for the
next run. The hydrogenation of phenylethylene was attempted under similar conditions.
For the reaction with CuPd NCs, 5 mg of CuPd NCs were measured accurately, then dispersed
into 5 mL methanol, giving the CuPd solution with the concentration of ~1 mg/mL. Then, 16.5
μL of CuPd catalyst (1 mg/mL) was took out and mixed with the 0.1 mmol phenylacetylene and
5 mL methanol in a two-necked round-bottomed flask (25 mL) under 160 mW/cm2 light
irradiation with a 300 W Xe lamp and mechanical stirring. The reaction was initiated after
adding 7.8 mg NH3BH3 and proceeded for 10 min. After the reaction completion, 200 µL of the
resultant mixture was taken into a mixed solvent with 1 mL H2O and 1 mL ethyl acetate. Upon
the adequate shaking, the mixture was allowed to be centrifuged and the product was extracted to
the upper layer of ethyl acetate. Then, 600 µL of sample in the upper solution was detected by
GC.
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Catalytic performance evaluation of CuPd@ZIF-8 for hydrogenation of phenylacetylene
by H2 gas: The hydrogenation of phenylacetylene in H2 atmosphere was proceeded as follows.
Typically, a mixture of 1 mg CuPd@ZIF-8 (1 mL, 1 mg/mL), 0.1 mmol phenylacetylene and 5
mL methanol was placed in a necked round-bottomed flask (25 mL) with a H2 balloon. The
reaction was accelerated under 160 mW/cm2 light irradiation with a 300 W Xe lamp.
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Catalytic performance evaluation of CuPd@ZIF-8 and CuPd for hydrogenation of
phenylpropiolaldehyde in the presence of NH3BH3: Typically, a mixture of CuPd@ZIF-8 (1
mL, 1 mg/mL) or equimolar amount of CuPd (18.8 μL, 2 mg/mL), 0.1 mmol
phenylpropiolaldehyde, 4.5 mL methanol and 1.5 mL H2O were placed in a two-necked round-
bottomed flask (25 mL) under 160 mW/cm2 light irradiation with a 300 W Xe lamp and
mechanical stirring. Then the reaction was initiated by adding 7.8 mg NH3BH3.
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Time-dependent conversion of phenylacetylene or phenylethylene hydrogenation:
Typically, a mixture of 1 mg CuPd@ZIF-8 (1 mL, 1 mg/mL), 0.1 mmol phenylacetylene or
phenylethylene, 5 mL methanol was placed in a two-necked round-bottomed flask (25 mL) under
160 mW/cm2 light irradiation with a 300 W Xe lamp under mechanical stirring. Then the
reaction was initiated by adding 7.8 mg NH3BH3. To obtain the kinetic curve of the catalytic
reaction, during the reaction process, 80 µL of the reaction mixture was sampled per 30 seconds.
Dependency of the reaction rate on the amount of NH3BH3: A mixture of 1 mg
CuPd@ZIF-8 (1 mL, 1 mg/mL), 0.1 mmol phenylacetylene or phenylethylene, 5 mL methanol
was placed in a two-necked round-bottomed flask (25 mL) under 160 mW/cm2 light irradiation
with a 300 W Xe lamp under mechanical stirring. Then, different amount of NH3BH3 was added
to initiate the reaction and the time-dependent conversion was completed following as the above
process, respectively.
Deuterium-labeling experiments: All controlled experiments were completed under the
same conditions as the experiment above except for that the substrate or reactant was replaced by
the corresponding deuterated reagent.
DFT Calculation Method: The Perdew-Burke-Ernzerhof (PBE) functional and projector
augmented wave (PAW) potentials were adopted to complete the DFT calculations by using
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Vienna ab initio simulation package.69, 70 In detail, the spin polarization correction was included
to consider the magnetism effect. Consistent with previous studies,71, 72 a 1×1×1 Γ-centered k-
point and 500 eV cutoff energy were adopted to obtain accurate electronic energy in ground-state.
The convergence standards of energy and force were selected as 10-5 eV and 0.02 eV/Å. The
vibrational frequency with finite displacements of ± 0.02 Å was calculated to obtain the zero
point energy correction and vibrational entropy correction.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/*****.
Materials and instrumentation; material characterizations and catalytic results.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jianglab@ustc.edu.cn
Author Contributions
The manuscript was written through contributions of both authors. Both authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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We gratefully thank all reviewers for their insightful comments and valuable suggestions. This
work was supported by the National Natural Science Foundation of China (21725101, 21871244,
21673213, 21521001), International Partnership Program of CAS (211134KYSB20190109),
Fundamental Research Funds for the Central Universities (WK2060030029), and Fujian Institute
of Innovation (CAS).
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REFERENCES
(1) Studt, F.; Abild-Pedersen, F.; Bligaard, T.; Sørensen, R. Z.; Christensen, C. H.; Nørskov, J.
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