Probing the Interface between Encapsulated Nanoparticles and Metal-Organic Frameworks for Catalytic Selectivity Control

Metal −Organic Frameworks for Catalytic Selectivity Control

Article

Probing the Interface between Encapsulated Nanoparticles and

Wei-Shang Lo, Lien-Yang Chou, Allison P. Young, Chenhao Ren, Tian Wei Goh,

Benjamin P. Williams, Yang Li, Sheng-Yu Chen, Mariam N. Ismail, Wenyu Huang,*

and Chia-Kuang Tsung *

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sı Supporting Information

ABSTRACT: Encapsulating metal nanoparticles (NPs) in metal−

organic frameworks (MOFs) to control catalytic selectivity has

recently attracted great attention; however, an understanding of the

NP−MOF interface is lacking. In this work, we used spectroscopy to

investigate the interfacial structure and then demonstrate its impact

on selectivity. Metal NPs were encapsulated in MOFs using three

approaches, impregnation, coating, and one-pot methods. Chemical

interactions at the interface were probed through IR and Raman

spectroscopy. The ﬁne diﬀerences between the interfacial structures

generated by the three methods aﬀected their selectivity for the

hydrogenation of crotonaldehyde. The direct interface generated by

the one-pot method provides the highest selectivity toward the

desired crotyl alcohol. This work provides a spectroscopic protocol to study hybrid interfaces and sheds light on their design for

eﬀective NP−MOF catalysts.

ontrolling the catalytic performance of metal nano-

particles (NPs) is a longstanding research topic in

mediate-directing agents control selectivity by modifying the

electronic structure of NPs or directing the conformation of

heterogeneous catalysis because they are at the center of many

intermediates. For example, in α, β-unsaturated aldehyde

hydrogenation, both adsorbed ligands and coated MOFs on

the catalytic surface can generate steric eﬀects to prevent

intermediates lying down in a ﬂat conformation, allowing only

the terminal CO bond to be hydrogenated and leading to

−3

key industrial processes.

Within this ﬁeld, expanding and

improving selectivity remains one of the most challenging aims.

It is well-known that selectivity can be inﬂuenced by reaction

conditions, NP sizes, and NP compositions, but a molecular-

,15

high chemoselectivity for unsaturated alcohol.

The

level design of selectivity control remains elusive. More

recently, introducing organic ligands to the NP surface has

proved to be an eﬀective way to rationally tune selectivity by

crystalline nature of MOFs could further provide a more

uniform steric control to direct intermediates at the interfaces

between NPs and MOFs. The tunable nature of MOFs

directing intermediates on a molecular level, and numerous

allows this steric control to be ﬁne-tuned.

organic ligands have been applied to eﬀectively control the

−9

Based on this mechanism of intermediate directing, the

detailed structure at the interface is very important. In order

for the steric eﬀect of the MOF pores to direct the

intermediate for selectivity control, the MOF must be directly

coated on the metal surface without any gap or chemicals in

between (Scheme 1 and notes in the Supporting Information,

SI). However, this important feature has been overlooked in

many previous works. Only a well-deﬁned and direct interface

selectivity.

Recently, intermediate control has been

extended from surface adsorbed ligands to crystalline solids,

metal−organic frameworks (MOFs). The crystalline nature

of the frameworks prevents the transient structure of the

organic ligands that could be a potential issue during certain

catalytic reactions. Selectivity for a great number of

important reactions has indeed been demonstrated using

MOFs as intermediate-directing agents, such as the selective

hydrogenation of α, β-unsaturated aldehydes, the selective

oxidation of diols, and the isomerization of methylcyclopen-

Received: July 20, 2020

Revised: January 14, 2021

Published: January 25, 2021

2−19

tane.

Even the challenging selective hydrogenation of

acrolein has been demonstrated, which has never been

reported using organic ligands alone.

The superior selectivity control provided by MOFs can be

understood through the underlying mechanisms. The inter-

2021 American Chemical Society

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sodium tetrachloropalladate) and MOF precursors (zirconium

Article

Scheme 1. Schematic Illustration of the Possible

Intermediate Conformation at the Interface

Scheme 2. Schematic Illustration of Three Diﬀerent

Approaches to Form the NP@MOF Catalyst

(

a) Ordinary intermediate conformation on a pure metal surface. (b)

Conformation directed by a direct NP−MOF interface. (c)

Conformation poorly controlled by an ill-deﬁned NP−MOF interface.

A crotonaldehyde molecule was used as a model reactant. These

possible conformations are adopted from the DFT literature

24,31

references.

24−26

between NPs and MOFs could provide selectivity control.

In this work, we aim to probe the interfacial structures and

their impact on selectivity control. We ﬁrst generate NP−MOF

interfaces by three commonly used approaches, impregnation,

27−30

coating, and one-pot.

Then, we use infrared and Raman

spectroscopy to gain chemical information at the interface

generated by each approach. After understanding the interface,

we tested the selectivity of the catalysts for a model reaction,

the hydrogenation of an α, β-unsaturated aldehyde.

(

a) Impregnation, (b) coating, and (c) one pot. The illustrated sizes

of the NPs in the scheme are used to emphasize the diﬀerences

between the methods.

As mentioned, the three methods used to form NP@MOF

catalysts are impregnation, coating, and one-pot. In the

impregnation method, MOFs are synthesized ﬁrst and serve

as a matrix to immobilize metal precursors that are reduced to

37−39

high aﬃnity to metal NPs

15,37

and also provides a probe for

We used the reported methods

form encapsulated NPs. In the coating method, colloidal

later spectroscopic study.

metal NPs are synthesized ﬁrst and then introduced into an

for the encapsulation of Pd NPs in UiO-66-NH to generate

MOF growth solution to coat MOF on the NPs. In the one-

pot method, metal NPs and MOF precursors are mixed in one

pot and then heated to form metal NPs and simultaneously

coat MOF. These three methods all generate an NP−MOF

interface; however, we hypothesize that their detailed

interfacial structure could vary due to diﬀerences in their

formation process (Scheme 2). The impregnation method is

the most conventional way to make heterogeneous catalysts. It

could generate a direct NP−MOF interface, but it is

challenging to completely avoid the growth of NPs on the

external surface of the preformed MOFs. In addition, the size

of formed NP is less tunable, as most of the previous methods

the impregnation and coating samples (details in the SI;

15,39−41

Figures 1a,b and S1 ).

Because there is no reported one-

pot method for NP@UiO-66-NH , we have developed a

method modiﬁed from previous reports for NP@UiO-66. In

a typical experimental condition, metal precursors (salts of

15,34

could only generate NPs with small size (Scheme 2a).

For

the coating method, because MOFs form around presynthe-

sized NPs, the colloidally synthesized NPs could be of well-

deﬁned morphologies and compositions. However, the capping

agent used to stabilize colloidal NPs and provide an anchor for

MOF growth could be trapped at the interface, preventing

direct interaction between the NP and the MOF (Scheme 2b).

For the one-pot method, it has a great potential to form direct

interfaces because no capping agent is added (Scheme 2c);

however, the synthetic condition of this relatively new method

is not ﬁne-tuned for the interface control. These hypothe-

sized diﬀerences motivated us to perform a detailed study to

provide direct comparisons between the methods.

Figure 1. Characterization of NP@UiO-66-NH . TEM images of

Pd@UiO-66-NH generated by (a) impregnation, (b) coating, and

^C^h^e^mⁱ^c^a^l^l^y^s^t^a^b^l^e^Uⁱ^O^-⁶⁶^-^N^H2 (University of Oslo-66-

NH ) is selected as the host MOF for this study. UiO-66-

(d) impregnation, (e) coating, and (f) one-pot methods. See the

Supporting Information for the loading and size distribution of the

encapsulated NPs.

NH is formed by the assembly of zirconium clusters and 2-

aminoterephthalic acid (BDC-NH ). The amino group has a

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Chemistry of Materials

Article

tetrachloride and BDC-NH ) are mixed in the solvent. The

solution is kept at 120 °C for 24 h without stirring. Pd NPs

form within the ﬁrst 15 min of heating, followed by the

formation of UiO-66-NH (Figure S2 ). Uniform Pd@UiO-66-

NH nanocrystals were produced (Figure 1c). Powder X-ray

diﬀraction (PXRD) patterns of Pd@UiO-66-NH composites

show no diﬀerence compared to simulated UiO-66 (Figure

S3). The presence of metal salts does not aﬀect MOF

formation (Figure S4). The loading of Pd in Pd@UiO-66-NH₂

was identiﬁed by inductively coupled plasma optical emission

spectrometry (ICP-OES) as 3.0 wt %.

Although the newly developed one-pot synthesis is not the

focus of this work, we want to emphasize its advantages,

primarily its potential to form controlled NP−MOF interfacial

structures. The procedure has improved upon previously

reported one-pot methods through the use of BDC-NH₂,

which plays two roles here. First, BDC-NH can promote the

formation of NP nuclei and accelerate the formation of NPs.

Second, its aﬃnity for metals allows it to stabilize the metal

surface and prevent NP aggregation without additional capping

Figure 2. Size-selective alkene hydrogenation over Pd NPs on UiO-

6-NH and Pd@UiO-66-NH obtained using the coating, one-pot,

and impregnation methods. The catalyst activity was normalized by

ethylene hydrogenation held at 40% conversion. Ethylene hydro-

genation is run at 0 °C, cyclohexene hydrogenation is run at 25 °C,

and cyclooctene hydrogenation is run at 40 °C.

agents. The BDC-NH stabilized NPs are thus formed ﬁrst and

then encapsulated into MOFs (Scheme 2c). When native BDC

is used as the MOF linker under the same reaction conditions,

the synthesized NPs formed a series of aggregates outside of

After investigating the encapsulation of NPs into MOFs, we

next studied the NP−MOF interfacial structure. Although the

structures look similar under TEM and identical under XRD,

these characterizations method provide little insight into the

interfacial structure (Figure 1). In light of this, the formation of

chemical interactions at the interface was investigated through

IR and Raman spectroscopy. Diﬀerent NP−MOF interfacial

structures lead to diﬀerent chemical bonding between NP and

MOF. These chemical interactions can be probed by

measuring changes in the chemical properties of either the

MOF linkers or the NP surface. To study change in the MOF

linkers after the formation of NP−MOF, we used diﬀuse

reﬂectance infrared Fourier transform spectroscopy (DRIFTS)

the UiO-66 crystals (Figure S5 ). Using BDC-NH , the size and

composition of encapsulated NPs can further be easily tuned

by changing the reaction temperature and metal precursor(s),

respectively. At higher temperatures, the nucleation rate is

accelerated, leading to smaller particles. Pd NP sizes can be

tuned from 3.6 ± 1.0 nm to 9.9 ± 1.8 nm by decreasing the

synthesis temperature from 150 °C to 90 °C (Figures S6 and

S7 ). PdPt alloy NPs were used to demonstrate composition

control. When a mixture of Pd and Pt precursors were

introduced (details in the SI), a series of alloyed NPs formed

(Figures S8 ). A volcano relationship between the activity and

the Pd:Pt ratio was observed (Figures S9 and Table S1),

indicating that the encapsulated metal NPs are indeed

to monitor the stretching of the amine group of BDC-NH .

alloyed. A linear relationship was observed while using a

mixture of pure Pt and Pd NPs (Figure S10).

Interaction between NP and MOF should lead to a weaker N−

H bond, resulting in a red-shift of the N−H stretch IR

Before studying the interfacial structures generated by the

three diﬀerent methods, we tested whether the metal NPs are

encapsulated by carrying out size-selective alkene hydro-

genation, using ethylene, cyclohexene, and cyclooctene, and

comparing the results with a control sample, in which NPs are

frequency. As shown in Figure 3, four samples were

compared: pure UiO-NH and Pd@UiO-NH generated by

each of the three methods, impregnation, coating, and one-pot.

−1

In the spectra, the bands at 3520 and 3401 cm are attributed

to the N−H asymmetric and symmetric stretching vibration

on the external surface of UiO-66-NH . The loading and size

−1

peak, respectively, and the band at 3670 cm corresponds to

distribution of encapsulated Pd NPs in each sample are

summarized in Table S2 and Figure S11. Due to the smaller

the μ −OH stretching of the zirconium cluster. The

impregnation sample showed the greatest red-shift, indicating

that the MOF linkers are chemically interacting with the

encapsulated metal NPs. While lesser, the red-shift of the one-

pot sample also indicates the MOF linkers are chemically

interacting with encapsulated NPs. In contrast, no noticeable

shift was observed for the coating sample, signifying that there

is little interfacial interaction between MOF and NP. We

attribute this limited contact to residues trapped at the

interface (Scheme 2b), which will be discussed in detail in a

later section. The greater shift observed in the impregnation

sample indicates that there are more chemical bonds at the

NP−MOF interface likely due to the smaller NP size generated

by the impregnation method (Figure S10).

15,34

size of the NPs formed in the impregnation sample,

the

metal NP surface areas of the four samples were normalized

through a standard method of ethylene hydrogenation (to 40%

conversion, Table S3 ). Because the MOF aperture size is

bigger than the cyclohexene and smaller than cyclooctene, we

observe low activity for cyclooctene hydrogenation over

impregnation, coating, and one-pot samples (Figure 2), while

the control sample (Pd-on-MOF) shows high activity for both

cyclohexene and cyclooctene hydrogenation. This result

indicates that most of the metal NPs are indeed encapsulated.

Because we used the impregnation method, the higher activity

of cyclooctene hydrogenation could be related to NPs on the

external surface, which has been reported. It is worth

mentioning that these NPs on the external surface of MOFs

To probe changes in the NP electronic structure, we

introduced carbon monoxide (CO) as a probe molecule,

because the stretching frequency of CO adsorbed on a metal

can be avoided when extra synthetic steps were introduced.

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Figure 5 a, the noncoated Au-PVP NPs showed a band at 1530

Article

Figure 4. DRIFTS of CO adsorbed on Pt-on-SiO and Pt@UiO-66-

NH obtained using the impregnation, one-pot and coating methods.

Signiﬁcant shifts in CO_a_t_o_pindicate that the MOF chemically interacts

with the Pt surface, changing the electronic structure of Pt.

Figure 3. DRIFTS, with the characteristic stretches of the amine

^g^r^o^u^p^hⁱ^g^h^lⁱ^g^h^t^e^d^,^f^o^r^Uⁱ^O^-^N^H2 ^aⁿ^d^P^d^@^Uⁱ^O^-⁶⁶^-^N^H2 obtained

using the impregnation, one-pot, and coating methods. Signiﬁcant

shifts in the N−H stretch indicate that the MOF chemically interacts

with the Pd surface.

surface of colloidal NPs are trapped at the interface after MOF

coating. These trapped organic ligands block the chemical

interaction between the MOF and the NP, generating an ill-

deﬁned interface. To test this hypothesis, we used Raman

spectroscopy to investigate the interfacial capping agents. The

same coating method and capping agent, polyvinylpyrrolidone

surface is sensitive to the electronic structure of the metal.

The small size of CO further allows it to easily pass through

the UiO-66 aperture. For these studies, Pd NPs were replaced

by Pt NPs, as the Pt surface yields well-deﬁned (and well-

(

PVP), were used, but the metal NPs were switched to Au for

48,49

its surface enhancement (details in the SI, Figures S14 −16).

Because the surface plasmon resonance of the Au NPs largely

promotes the Raman signal of molecules close to the Au

understood) CO signals.

Pt@UiO-66-NH samples were

synthesized via the same routes as Pd@UiO-66-NH (TEM

images and PXRD pattern are shown in Figure 1d−f and

15,40

surface, the information is very interface speciﬁc. As shown in

Figure S12 , respectively).

The loading and size distribution

of encapsulated Pt NPs in each sample are summarized in

Table S2 and Figure S13. The more bonds formed at the NP−

MOF interface, the more red-shifted the CO stretch frequency,

as the electron-donating amine group increases the charge

density on the metal surface, resulting in increased electron

back-donation to CO antibonding orbitals, weakening the CO

bond. As shown in Figure 4, four samples were again

compared: Pt NPs-on-SiO , as the control, and Pt@UiO-66-

NH synthesized using the impregnation, coating, and one-pot

methods. In the spectra, the band between 2050 and 2100

−

cm is attributed to CO linearly bound on Pt atoms

(

∼

CO_a_t_o_p). The frequency of CO in the Pt-on-SiO sample,

atop

−

2090 cm , was used as a reference for comparison.

Compared to this reference frequency, the CO frequency of

atop

the impregnation sample shows a red-shift from 2090 to 2066

−

cm , further indicating the chemical interaction between the

linkers and metals in the impregnation sample. As expected,

Figure 5. (a) Raman spectra of Au-PVP NPs, UiO-NH , and Au@

^Uⁱ^O^-⁶⁶^-^N^H2 formed using the coating method. (b) Ethylene

^h^y^d^r^o^g^eⁿ^a^tⁱ^oⁿ^o^v^e^r^P^t^@^Uⁱ^O^-⁶⁶^-^N^H2 obtained using the one-pot

and coating methods. Ethylene hydrogenation is run at 0 °C.

−

the one-pot method also shows a similar red-shift (∼20 cm )

due to the chemical interaction. In contrast, a relatively small

−

red-shift (∼8 cm ) of CO

is observed in the coating

atop

−

sample. Similar to the NH stretching study, it implies less

cm , corresponding to stretching vibrations of the CO

moiety on PVP, in agreement with the literature. After

chemical interaction at the interface in the coating sample

(

Scheme 2b). Importantly, this study also suggests that insights

coating with UiO-66-NH , which has no characteristic peaks in

provided by the Pd@UiO-66-NH system can be extended to

other metal NPs. Next, we studied the mechanisms causing the

diﬀerences in NP−MOF interactions.

this region, the peak can be clearly seen. The presence of the

PVP signal in the composite suggests that the capping agents

used to stabilize colloidal metal NPs are indeed trapped inside

the coated MOF. In contrast, no PVP signal was observed for

the samples generated by impregnation and one-pot methods

Capping agents are used to stabilize NPs for colloidal

1,52

synthesis.

We hypothesize that the capping agents on the

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Figure S17). To further examine the inﬂuence of this trapped

Article

crotonaldehyde hydrogenation (Figure 6 a). Numerous studies

pot methods. The sample of pure Pt NPs on the MOF surface,

which serves as a control (Pt-on-MOF), shows a 6.0%

selectivity to crotyl alcohol. For the impregnation sample,

selectivity increased (42.8%), indicating the promotion of the

hydrogenation of CO due to the presence of a direct NP−

MOF interface. However, the sample generated by the coating

method shows only a marginal increase in selectivity (9.8%),

which is attributed to the ill-deﬁned interface suggested by our

spectroscopic studies. The highest selectivity of crotyl alcohol

(70.4%) was observed in the one-pot sample, also due to the

direct interface revealed by our spectroscopy study. Although

both samples have a direct interface, NP size control allows the

one-pot sample to show higher catalytic activity than the

impregnation samples (Figures S11 and S13). It has been

reported that larger Pt NPs promote the formation of

interfacial capping agent, we compared the activity for ethylene

hydrogenation of Pt@UiO-66-NH synthesized by the coating

and one-pot methods with similar Pt loading and particle size.

(

The NP sizes generated by the impregnation method cannot

be controlled, so it was excluded from this study.) As shown in

Figure 5b, the coating sample shows a much lower activity than

the one-pot sample, which is attributed to active site blocking

6,57

by PVP residues.

After gaining an initial understanding of the NP−MOF

interfacial structure, we carried out a model reaction,

63,64

unsaturated alcohol.

The impregnation method can only

generate NPs with a small size that disfavors the formation of

15,34

unsaturated alcohol.

Besides providing a fundamental

understanding of the interface, our study also reveals the

potential of the one-pot synthesis method when the metal−

linker interaction is controlled and the formation sequence is

regulated. We hypothesize that, if the conditions are optimized,

NPs will form ﬁrst, stabilized by linkers with a high metal

aﬃnity. At this stage, the morphology and composition of the

NPs can be controlled by colloidal methods because the MOF

is not formed yet. Then, the MOF precursors will bond to the

linkers on the NP surface and form a MOF around the NPs,

forming a clean and direct interface. We believe that this

process could lead to highly controlled and active interfacial

structures.

In summary, we have developed a spectroscopic protocol to

probe the chemical interactions at the interface by combining

IR and Raman spectroscopies. We have used this toolbox to

reveal diﬀerences in the chemical interactions at the interfaces

generated by several common synthesis methods. We have

found that although the coating method allows for better

control over the encapsulated NPs than the impregnation

method, the interface generated by the coating approach

contains a trapped capping agent. This trapped capping agent

reduces crotyl alcohol selectivity for the hydrogenation of

crotonaldehyde. Our developed one-pot method, on the other

hand, shows the highest selectivity to the unsaturated alcohol,

due to the direct NP−MOF interface and size control of the

encapsulated NPs. While the encapsulation of metal NPs into

MOFs has been shown to be a route to promising composite

catalysts, our current study shows that it is essential to ﬁne-

tune the interfacial structures between NPs and MOFs. The

toolbox and understanding established in this work could oﬀer

a perspective for further optimization of MOF-based

heterogeneous catalyst design.

Figure 6. (a) Two pathways for the hydrogenation of crotonaldehyde.

(

b) Selectivity for crotonaldehyde hydrogenation over Pt NPs on

UiO-66-NH₂and Pt@UiO-66-NH₂obtained using the coating,

impregnation, and one-pot methods. To compare the selectivities of

our samples, the conversion of each reaction was kept at 30%. Crotyl

alcohol is the preferred product, and selectivity was determined by its

ratio to the sum of all three products. Reaction conditions: 2 mL of

isopropanol, 100 μL of crotonaldehyde, 30 bar H , 70 °C, and

reaction time of 18 h.

suggest that this type of α, β-unsaturated aldehyde hydro-

genation over a NP@MOF catalyst favors the hydrogenation

of the CO bond, which is originally thermodynamically less

favorable than the hydrogenation of the CC

5,16,20,58−61

bond.

Although many diﬀerent mechanisms have

15 20

been proposed, such as steric or activation eﬀects, a direct

interface is required for high selectivity in all hypothesized

mechanisms (Scheme 1). For example, it has been proposed

that the improved selectivity is due to the MOFs ability to

EXPERIMENTAL SECTION

■

Chemicals and Characterization. See the section of chemicals

and characterization details in the Supporting Information for

information on instrumentation and the sources of chemicals.

regulate the orientation of intermediates. Such regulation is

Synthesis of Pd@UiO-66-NH Using the Impregnation

Method. See the Supporting Information for the synthesis of

dependent on a clean and direct NP−MOF interface. For our

encapsulated Pt NPs and Au NPs in UiO-66-NH₂using the

study, Pt@UiO-66-NH was chosen because it produces fewer

impregnation method. To form Pd@UiO-66-NH

composites using

byproducts than Pd. To compare the selectivity of our

samples, the conversion of each reaction was kept at 30%

the impregnation method, 100.0 mg of activated UiO-66-NH powder

(see the Supporting Information for the synthesis of UiO-66-NH₂)

(

Table S4 ). As shown in Figure 6, four samples were

was dispersed in 5 mL of dichloromethane followed by sonication for

40 min at room temperature. Then, 5 mL of dichloromethane

containing 7.0 mg of palladium acetate was added dropwise to the

compared: pure Pt NPs on UiO-66-NH and Pt@UiO-66-

NH synthesized through the coating, impregnation, and one-

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Chemistry of Materials

The Supporting Information is available free of charge at

Article

above solution with magnetic stirring at 900 rpm. The mixture was

stirred at 500 rpm for 24 h at ambient temperature. The as-

Crotonaldehyde Hydrogenation. Catalysts were dispersed in 2

mL of isopropanol solution in the 10 mL ampule, and then 40 μL of

crotonaldehyde (0.4 mmol) was added into the above solution. Before

catalysis, the high-pressure reactor vessel (Wattacas Inc., 500 mL) was

preheated to 70 °C. The temperature was controlled using a hot plate

(RCT basic, IKA) with a type-K thermocouple. Subsequently, the

ampule was transferred into a high-pressure reactor vessel. The

synthesized Pd -impregnated UiO-66-NH was centrifuged and

washed with dichloromethane three times. The sample was further

washed by dichloromethane three times every 12 h to remove the

remaining palladium precursor adsorbed on the surface of UiO-66-

NH . The resulting sample was then air-dried and reduced under a

0% H ﬂow (50 mL/min, balanced with helium) at 200 °C for 2 h.

autoclave was purged 5 times with H to remove air. Then, the

Synthesis of the Pd@UiO-66-NH ₂ Composite Using the

hydrogenation of crotonaldehyde was carried out at 30 bar H at 70

Coating Method. See the Supporting Information for the synthesis

of encapsulated Pt NPs and Au NPs in UiO-66-NH using the coating

method. In general, 25.8 mg of ZrOCl ·8H O (0.08 mmol) and 14.5

mg of BDC-NH (0.08 mmol) were dissolved in 7.622 mL of DMF in

a 20 mL scintillation vial, and then 1.378 mL of acetic acid was added

into the solution to take the ﬁnal volume to 9 mL. Next, 1 mL of Pd

NPs (0.5 mg/mL in DMF, synthesized from the colloidal method

using PVP as capping agents) was introduced into the above solution,

and the solution was heated at 120 °C in an oil bath for 2 h (see the

Supporting Information for the synthesis of Pd NPs). After cooling,

°C for 18 h with magnetic stirring at 500 rpm. After that, the catalysts

were separated by centrifugation, and the reaction solution was

ﬁltered through a ﬁlter membrane (0.22 μm). The products were

analyzed using gas chromatography−mass spectrometry (Shimadzu

QP2010 Ultra, column: Rtx-5, 30 m × 0.25 mm × 0.25 μm).

ASSOCIATED CONTENT

■

sı

Supporting Information

https://pubs.acs.org/doi/10.1021/acs.chemmater.0c03007.

the formed Pd@UiO-66-NH powder was collected by centrifugation

(

8000 rpm, 10 min). The sample was washed with DMF three times.

Then, the as-synthesized Pd@UiO-66-NH powder was activated by

Detailed experimental procedures, characterizations of

the synthesized samples, ICP-OES measurement, Raman

study, DRIFTS measurement, alkene hydrogenation,

and crotonaldehyde hydrogenation (PDF )

solvent exchange to remove the residues trapped in the framework.

The activation process was carried out by washing three times with

DMF and methanol every 12 h.

Synthesis of Pd@UiO-66-NH Using the One-Pot Method.

See the Supporting Information for the one-pot synthesis of

encapsulated Pt NPs, the PdPt alloy, and Au NPs in UiO-66-NH₂

AUTHOR INFORMATION

using the one-pot method. In general, 14.5 mg of BDC-NH (0.08

■

mmol) and 2.78 mg of Na PdCl ·3H O (0.008 mmol) were dissolved

Corresponding Author

in 7.622 mL of DMF in a 20 mL scintillation vial, and then 1.378 mL

of acetic acid was added into the solution to make the ﬁnal volume 9

Wenyu Huang − Department of Chemistry, Iowa State

University, Ames, Iowa 50011, United States; orcid.org/

mL. Next, 1 mL of DMF containing 18.6 mg of ZrCl (0.08 mmol)

0000-0003-2327-7259 ; Email: whuang@iastate.edu

was introduced into the above solution. The solution was heated to

20 °C in an oil bath for 24 h. After cooling, the formed Pd@UiO-66-

Authors

NH powder was collected by centrifugation (8000 rpm, 10 min).

The sample was washed with DMF three times. Then, the as-

synthesized Pd@UiO-66-NH₂powder was activated by solvent

exchange to remove residues trapped in the framework. Activation

was performed by washing with DMF and methanol three times every

Wei-Shang Lo − Department of Chemistry, Merkert Chemistry

Center, Boston College, Chestnut Hill, Massachusetts 02467,

United States; orcid.org/0000-0002-4401-2922

Lien-Yang Chou − Department of Chemistry, Merkert

Chemistry Center, Boston College, Chestnut Hill,

2 h. The activated Pd@UiO-66-NH sample was dried at 80 °C for

4 h.

Massachusetts 02467, United States; orcid.org/0000-

DRIFTS Measurement. DRIFTS spectra were collected using a

0001-8171-1346

Bruker Tensor 27 with a mercury−cadmium−telluride detector

Allison P. Young − Department of Chemistry, Merkert

Chemistry Center, Boston College, Chestnut Hill,

Massachusetts 02467, United States

Chenhao Ren − Department of Chemistry, Merkert Chemistry

Center, Boston College, Chestnut Hill, Massachusetts 02467,

United States

(

MCT) and a Harrick diﬀuse reﬂectance accessory. The beam

alignment and calibration were performed before each measurement.

Both CO-DRIFTS spectra and DRIFTS spectra and were carried out

using this instrumental setup.

Raman Measurment. Raman spectra were collected on a Micro-

Raman system (XploRA, Horiba) with a 532 nm laser excitation at

room temperature. Before each measurement, samples were dropcast

on a Si wafer (1.0 cm × 1.0 cm) and dried at 80 °C for 24 h.

Considering the sensitivity of the MOFs to laser excitation, a low

output power (10 mW) was used to carry out the Raman

measurement. The laser alignment and calibration at 532 nm were

performed before each measurement using a Si wafer. See the

Supporting Information for the detailed procedure.

Alkene Hydrogenation. Samples were diluted with low surface

area quartz and loaded into U-shaped glass reactors. The glass reactor

was then connected to a home-built gas-phase ﬂow system for alkene

hydrogenation. Gas ﬂows, including helium, hydrogen, and ethylene,

were regulated using calibrated mass ﬂow controllers. The desired

partial pressure of cyclohexene or cis-cyclooctene was achieved by

bubbling He through the liquid and assuming saturation. For all the

reactions, the gas composition was analyzed with a mass spectroscope

Tian Wei Goh − Department of Chemistry, Iowa State

University, Ames, Iowa 50011, United States; orcid.org/

0000-0002-4141-3392

Benjamin P. Williams − Department of Chemistry, Merkert

Chemistry Center, Boston College, Chestnut Hill,

Massachusetts 02467, United States

Yang Li − Department of Chemistry, Merkert Chemistry

Center, Boston College, Chestnut Hill, Massachusetts 02467,

United States

Sheng-Yu Chen − Department of Chemistry, Merkert

Chemistry Center, Boston College, Chestnut Hill,

Massachusetts 02467, United States

Mariam N. Ismail − Chemistry and Physics Department,

Simmons University, Boston, Massachusetts 02115, United

States; orcid.org/0000-0002-8264-6715

(

MKS special V2000P). The temperature was controlled by a furnace

(

CARBOLITE) and PID controller (Diqi-Sense) with a type-K

thermocouple. See the Supporting Information for the detailed

procedure.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.chemmater.0c03007

951

Chem. Mater. 2021, 33, 1946−1953

Chemistry of Materials

(14) Hou, C.; Zhao, G.; Ji, Y.; Niu, Z.; Wang, D.; Li, Y.

Article

Author Contributions

Hydroformylation of alkenes over rhodium supported on the metal-

organic framework ZIF-8. Nano Res. 2014, 7 (9), 1364−1369.

(

W.-S.L., L.-Y.C., and A.P.Y.) These authors contributed

equally.

(

15) Guo, Z.; Xiao, C.; Maligal-Ganesh, R. V.; Zhou, L.; Goh, T. W.;

Notes

Li, X.; Tesfagaber, D.; Thiel, A.; Huang, W. Pt Nanoclusters Confined

within Metal −Organic Framework Cavities for Chemoselective

Cinnamaldehyde Hydrogenation. ACS Catal. 2014, 4 (5), 1340−

1348.

The authors declare no competing ﬁnancial interest.

∥

Chia-Kuang Tsung died on January 5, 2021, from

complications due to COVID-19.

(16) Stephenson, C. J.; Whitford, C. L.; Stair, P. C.; Farha, O. K.;

Hupp, J. T. Chemoselective Hydrogenation of Crotonaldehyde

Catalyzed by an Au@ZIF-8 Composite. ChemCatChem 2016, 8 (4),

ACKNOWLEDGMENTS

■

(

This work is supported by National Science Foundation

CHE-1566445) and Boston College. Authors thank Prof.

55−860.

(17) Choe, K.; Zheng, F.; Wang, H.; Yuan, Y.; Zhao, W.; Xue, G.;

Qiu, X.; Ri, M.; Shi, X.; Wang, Y.; Li, G.; Tang, Z. Fast and Selective

Semihydrogenation of Alkynes by Palladium Nanoparticles Sand-

wiched in Metal −Organic Frameworks. Angew. Chem., Int. Ed. 2020,

59 (9), 3650−3657.

Matthias Waegele at Boston College for helpful comments on

the spectroscopic study. Lastly, the authors would like to thank

Chia-Kuang “Frank” Tsung for the enormous contribution to

this work. Frank was an excellent teacher, a compassionate

adviser, and a kind mentor. And in this time of grief, we will

have our memories of a great scientist and ﬁne man, Chia-

Kuang Frank Tsung.

(

18) Tong, Y.; Xue, G.; Wang, H.; Liu, M.; Wang, J.; Hao, C.;

Zhang, X.; Wang, D.; Shi, X.; Liu, W.; Li, G.; Tang, Z. Interfacial

coupling between noble metal nanoparticles and metal −organic

frameworks for enhanced catalytic activity. Nanoscale 2018, 10 (35),

6425−16430.

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