Highly dispersed ultrafine palladium nanoparticles encapsulated in a triazinyl functionalized porous organic polymer as a highly efficient catalyst for transfer hydrogenation of aldehydes

Article abstract of DOI:10.1039/c8ta07502f

Fabrication of highly dispersed ultrafine noble metal nanoparticle (NMNP) based catalysts with high stability and excellent catalytic performance is a challenging issue for heterogeneous catalysis. As an alternative complement to existing solutions, herein, we designed and synthesized a stable triazinyl-pentaerythritol porous organic polymer (TP-POP) through a facile polycondensation between cyanuric chloride and pentaerythritol. The obtained TP-POP material has a three-dimensional folded structure, rich triazinyl groups, abundant hydrophobic pores and high thermal stability. Ultrafine Pd NPs with a narrow size distribution (1.4-2.8 nm) are then successfully confined in the organic pores of the TP-POP, through a reversed double solvent approach (RDSA). It is worth noting that the current strategy can effectively confine Pd NPs in the inner space of the TP-POP, and successfully avoids the agglomeration of Pd NPs as compared with the common impregnation-reduction method. The as-prepared Pd@TP-POP catalyst shows excellent catalytic activity in the reduction of 4-nitrophenol and transfer hydrogenation of aromatic aldehydes under very mild conditions. The excellent performance of the Pd@TP-POP catalyst is attributed to the abundant mesopores of the TP-POP which can enhance the accessibility of the highly dispersed ultrafine Pd NP active sites that are confined in the organic pores. More importantly, the Pd@TP-POP catalyst is easily recycled and highly stable without loss of its catalytic activity even after ten reaction cycles. Therefore, this study provides a new platform for designing and fabricating stable POP materials to confine size-controlled NMNPs with superior catalytic performance for various potential catalysis applications.

Full text of DOI:10.1039/c8ta07502f

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Highly dispersed ultrafine palladium nanoparticles encapsulated in
triazinyl functionalized porous organic polymer as a highly efficient
catalyst for transfer hydrogenation of aldehydes
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a
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Jin Yang, Man Yuan, Dan Xu, Hong Zhao, Yangyang Zhu, Menying Fan, Fengwei Zhang*
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and Zhengping Dong*
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State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Gansu Provincial Engineering Laboratory for Chemical Catalysis, Laboratory of
Special Function Materials and Structure Design of the Ministry of Education, Lanzhou University,
Lanzhou, 730000, PR China.
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Institute of Crystalline Materials, Shanxi University, Taiyuan, 030006, PR China.
E-mail: fwzhang@sxu.edu.cn (Fengwei Zhang), dongzhp@lzu.edu.cn (Zhengping Dong).
Fax: +86 0931 8912582; Tel: +86 0931 8912577.
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Abstract
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Fabrication of highly dispersed ultrafine noble metal nanoparticles (NMNPs) based
catalysts with high stability and excellent catalytic performance is a challenging issue for
heterogeneous catalysis. As an alternative complement to existing solutions, herein, we
designed and synthesized a stable triazinyl-pentaerythritol porous organic polymer (TP-POP)
through a facile polycondensation approach between the cyanuric chloride and pentaerythritol.
The obtained TP-POP material has three-dimensional fold structure, rich triazinyl groups,
abundant hydrophobic pores and high thermal stability. Ultrafine Pd NPs with a narrow size
distribution (1.4-2.8 nm) are then successfully confined in the organic pores of TP-POP,
through a reversed double solvent approach (RDSA). It is worth noting that the current
strategy can effectively confine Pd NPs in the inner space of the TP-POP, and successfully
avoids the agglomeration of Pd NPs as compared with the common impregnation-reduction
method. The as-prepared Pd@TP-POP catalyst shows excellent catalytic activity in reduction
of 4-nitrophenol and transfer hydrogenation of aromatic aldehydes under very mild conditions.
The excellent performance of Pd@TP-POP catalyst is attributed to the abundant mesopores of
TP-POP which can enhance the accessibility of the highly dispersed ultrafine Pd NPs active
sites that confined in the organic pores. More importantly, Pd@TP-POP catalyst is easily
recycled and highly stable without loss of its catalytic activity even after ten reaction cycles.
Therefore, this study provides a new platform for designing and fabricating stable POPs
materials to confine size-controlled NMNPs with superior catalytic performance for various
potential catalysis applications.
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1. Introduction
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Highly dispersed ultrafine noble metal nanoparticles (NMNPs) based catalysts have drawn
much research attention for various catalysis applications in recent years. Due to the large
surface areas, edge and angular atoms of ultrafine NMNPs, they perform superior activity for
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catalysis. As the NMNPs size decreases, a large fraction of active noble metal atoms could
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be exposed to the surfaces, and act as active sites, thus dramatically increase the utilization
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efficiency of precious metals. Therefore, ultrafine NMNPs exhibit better catalytic activity
as compared with large NMNPs based catalysts. However, too small size of the NMNPs could
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lead to the serious aggregation of NMNPs due to their high surface energy. To solve these
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critical issues, a variety of supporting materials such as carbon nanofibers (CNFs),
pristine
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graphene (PG),
reduced graphene oxide (RGO),
C N4
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and porous SiO are
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employed to immobilize small NMNPs.
However, these inorganic materials supported
catalysts also suffer from some disadvantages, such as the leaching and aggregation of
NMNPs during reaction process. Therefore, it is of great importance to design supporting
materials that can not only limit NMNPs’ growth, but also make ultrafine NMNPs uniformly
and highly dispersed, as well as render the obtained catalysts with excellent stability and
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catalytic activity.
Recently, the emerging of novel porous organic polymers (POPs) including covalent
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organic frameworks (COFs), polymers of intrinsic microporosity (PIMs), conjugated
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hypercrosslinked polymers (HCPs) and covalent
microporous polymers (CMPs),
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organic polymers (COPs), have been reported to be potential candidates for catalysis
applications due to their unique skeletons and porous structural properties. These POPs
materials have high specific surface area, abundant heteroatoms content, adjustable pores,
high thermal stability and excellent physicochemical properties that make them suitable for
constructing NMNPs based catalysts. For example, Wang and co-workers reported
imidazolium-based POPs for supporting Pd NPs with excellent catalytic property and showed
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no obvious aggregation or leaching after multiple catalytic runs. Hongwei Gu’s group
reported the synthesis of a thioether-containing COFs for controlling growth of ultrafine
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NMNPs in their frameworks. Gold NPs were confined in imidazolium-based POPs for
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catalytic reduction of nitrobenzene derivatives. Banerjee’s group reported covalent POPs
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supported Au and Pd NPs catalysts for catalytic applications.
NMNPs on the POPs for the above reported catalysts is in aqueous media via common
impregnation-reduction method using NaBH as the reductant. Although such approach can
The method for loading
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immobilize the NMNPs in the pores of POPs, most of NMNPs with large particle size are still
deposited on the outer surface of the POPs, thus may block the micropore or leaching during
the catalytic process.
Catalytic hydrogenation with common hydrogen source H2 is an economical and
eco-friendly strategy for hydrogenation of various substrates including alkenes, alkynes,
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nitriles, aldehydes and nitro compounds.
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However, molecular H is explosive, difficult
and dangerous to store in the laboratory. The catalytic transfer hydrogenation (CTH) with
hydrogen donors such as alcohols, formic acid, formate and hydrazine is a simple and
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versatile approach, which is considered as an alternative for traditional hydrogenation with H .
The experimental process of CTH is easy to control and safe, and the reaction condition is
mild without high pressure. Among various hydrogen donors that can be used in CTH,
alcohols are the most attractive because they can not only serve as both hydrogen donors and
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solvents, but also the produced ketones have high added value.
At this point, isopropanol
(i-PrOH) is one of the best hydrogen sources to be considered. As for various unsaturated
compounds that can be hydrogenated, hydrogenation of aldehydes to alcohols is significant
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for pharmaceutical synthesis and other fine organic chemicals synthesis. Great efforts have
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been devoted to develop efficient and highly selective catalytic systems for aldehydes
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Rh,
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hydrogenation. In general, some noble metals (e.g., Ru,
and Ir
) based catalysts
were the most common studied for hydrogenation of aldehydes using H
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, while high H
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pressure is inevitable, poor selectivity for alcohols and poor catalysts stability are always
involved. Consequently, it is necessary to develop stable NMNPs based catalysts for CTH of
aldehydes under mild reaction conditions.
Considering the above statements, herein we designed and synthesized a stable
triazinly-pentaerythritol porous organic polymer (TP-POP), which contains abundant
hydrophobic pores and rich triazinyl groups. Then, the TP-POP was devoted to encapsulate
highly dispersed ultrafine Pd NPs in its pores via a reverse double-solvents approach
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(RDSA). Briefly, TP-POP was firstly dispersed in H O solvent, which facilitated the
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molecule diffusion of the micro-droplets of Pd(AcO)
hydrophobic pores of TP-POP. Subsequently, a high concentration of NaBH
was added to reduce the Pd(AcO) , and highly dispersed ultrafine Pd NPs (1.4-2.8 nm) were
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-containing CH
2
Cl
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into the
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aqueous solution
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immobilized in the TP-POP, rather than dispersed on its surface with large Pd NPs. The
experimental results demonstrated that the obtained Pd@TP-POP catalyst exhibited superior
catalytic performance and recyclability in the reduction of 4-nitrophenol (4-NP) and CTH of
aldehydes compounds. Considering a series of advantages such as excellent size
controllability, remarkable ultrafine Pd NPs dispersibility and extremely high stability of
as-obtained Pd@TP-POP, this work is expected to provide a useful platform for fabricating
size-controlled, highly dispersed ultrafine NMNPs modified porous organic polymers based
heterogeneous catalysts for various catalysis applications.
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2. Experimental Section
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2.1. Materials
Cyanuric chloride and pentaerythritol were purchased from Tianjin Heowns Biochem Co.,
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Ltd.. Pd(AcO) was purchased from Sigma-Aldrich. 4-NP, i-PrOH, benzaldehyde and other
aldehydes used in the catalytic studies were supplied by Sinopharm Chemical Reagent Co.,
Ltd. (China). 1, 4-dioxane and other reagents were of analytical grade and used without
further purification. Deionized water was prepared in our lab and used throughout the
experiment.
2.2. Catalyst synthesis
Synthesis of TP-POP
Cyanuric chloride (0.08 mol) and pentaerythritol (0.06 mol) were added to a 50 mL
round-bottomed flask and dissolved by adding 30 mL of 1, 4-dioxane, followed by the
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addition of Na CO (0.24 mol). After stirring at 90 °C for 24 h, the reaction mixture was
cooled to room temperature and the solid was collected by filtration and washed with ethyl
acetate (50 mL) and deionized water (50 mL). Then, the product was dried at 30 °C for 24 h
in a vacuum oven and finally the white powder TP-POP was obtained.
Reverse double-solvents approach for synthesis of Pd@TP-POP catalyst
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To encapsulate Pd NPs within the TP-POP, 300 mg of TP-POP was firstly dissolved in 90
mL of deionized water and sonicated for 30 minutes. Next, a solution of Pd(AcO) (0.4 mmol)
dissolved in CH Cl (0.24 mL) was gradually added to the above solution and stirred for 3 h.
Then, the as-prepared mixture was reduced by adding a high concentration NaBH aqueous
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solution (2.7 M, 1.5 mL). Finally, the mixture was filtered and washed with methanol and
deionized water, and dried in a vacuum oven to obtain a 1.31%Pd@TP-POP catalyst.
Different metal loadings of 0.71%Pd@TP-POP and 0.21%Pd@TP-POP catalysts were also
prepared using the same method. Note that the Pd contents of all the prepared catalysts were
determined by inductively coupled plasma-optical emission spectrometers (ICP-OES).
Synthesis of the 1.81%Pd/TP-POP catalyst via common impregnation-reduction method
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Typically, TP-POP (300 mg) was ultrasonically dispersed in 30 mL of Pd(AcO) (0.07
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mmol) aqueous solution with magnetic stirring for 3 h at room temperature. Then, 0.50 mL of
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NaBH (1.4 mmol) aqueous solution was added into the above solution with vigorous stirring.
The sample was obtained by centrifugation, washing and vacuum drying at 40 °C overnight.
2.3. Characterization
The thermal stability of the TP-POP was measured by a TA-Q50 thermogravimetric
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atmosphere from room temperature to 800 C. The changing of the
analyzer under N
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chemical bonds during the preparation of the materials was characterized by Fourier
transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker) using KBr pressed pellet
method. The morphology and structure of TP-POP and Pd@TP-POP catalysts were observed
by scanning electron microscopy (SEM, JSM-6701F) and transmission electron microscopy
(TEM, Tecnai G2F30) coupled with energy dispersive spectroscopy (EDS). X-ray
photoelectron spectroscopy (XPS, Perkin-Elmer PHI-5702) was employed to analyze the
surface electronic states of the prepared catalysts. Powder X-ray diffraction (XRD, Rigaku
D/max-2400) measurements were performed in a diffractometer using the Cu-Kα radiation as
the X-ray source within the 2θ range of 10-90°. Brunauer-Emmett-Teller (BET, Micromeritics
ASAP 2010) analysis was used to study the pore size distribution and surface area of the
catalysts. Pd content was measured using ICP-OES (PQ9000). The contents of C, H, and N
elements in catalysts were measured by elemental analysis (GmbHVario, EI Elementar). An
Agilent Cary 5000 UV-Vis spectrophotometer was utilized for monitoring the progress of the
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catalytic reduction of 4-NP in the scanning range of 250-500 nm. The CTH of aldehydes were
monitored by gas chromatography-mass spectrometry (GC-MS, Agilent 5977E).
2.4. Catalytic reduction of 4-NP
Typically, 2.7 mL of light yellow 4-NP aqueous solution (0.01 M) was added into a cuvette.
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Then, 20 ꢀL of dispersive catalyst aqueous mixture (15 mg mL ) was added to the above
yellow solution. Subsequently, 0.3 mL of freshly prepared NaBH (0.1 M) aqueous solution
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was injected into the above mixture. The conversion of 4-NP to 4-aminophenol (4-AP) was
monitored by UV-Vis measurement.
2.5. Catalytic transfer hydrogenation of aldehydes
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Briefly, benzaldehyde (1 mmol), 1.31%Pd@TP-POP (10 mg), and KOH (0.2 mmol) were
refluxed in 5 mL of i-PrOH (serve as both solvent and hydrogen resource). The reaction
mixture was stirred at 80 °C. The reaction process was monitored by GC-MS. After the
completion of the reaction, the catalyst was washed several times with deionized water and
ethanol, dried under vacuum overnight and used for the next cycle.
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3. Results and discussion
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In the current work, the TP-POP supporting material was synthesized by an ingenious
polycondensation between cyanuric chloride and pentaerythritol as depicted in Scheme 1.
Interestingly, the TP-POP itself contains abundant hydrophobic pores and nitrogenous ligand
sites, thus the tiny Pd(AcO)
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-containing CH Cl micro-droplets can enrich in the pores. Under
the quick reduction by NaBH
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, ultrafine Pd NPs are precisely confined in the organic pores
with controlled size and high dispersity, rather than deposited on the outer surface of TP-POP.
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Scheme 1. Schematic illustration for fabrication of Pd@TP-POP catalyst.
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The FT-IR spectra of the precursors and TP-POP are shown in Fig. 1a. The absorption peak
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at 851 cm in cyanuric chloride indicates the stretching vibration of the C-Cl bond, while in
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TP-POP the absorption peak of C-Cl bond disappeared. The bands at 1570 cm and 1249 cm
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of the TP-POP indicate the presence of triazine units and C-O-C bonds. The peak at 806
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cm in TP-POP represents the frame vibration of the triazine ring. The OH stretching at 3325
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cm in TP-POP has significantly been weakened compared with the pentaerythritol,
indicating that the starting material pentaerythritol has been converted. The weak peaks at
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2955 cm and 2885 cm appear in TP-POP are asymmetric stretching and symmetric
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stretching vibration of the C-H bond. In addition, the strong bands in the 1200-1600 cm
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region correspond to the stretching modes of triazine units on the TP-POP. These results
prove the successful polycondensation between cyanuric chloride and pentaerythritol, and the
TP-POP has been successfully prepared.
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Fig. 1. (a) FT-IR spectra of TP-POP, cyanuric chloride and pentaerythritol; (b) XRD patterns of
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TP-POP and 1.31%Pd@TP-POP samples; (c) N adsorption/desorption isotherms of TP-POP and
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1.31%Pd@TP-POP; (d) pore size distribution curve of TP-POP and 1.31%Pd@TP-POP.
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The crystallinity and regularity of the materials’ structure were investigated by PXRD (Fig.
1b). A wide peak in the range of 15-30° refers to the triazine carbon in the partial crystallinity
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TP-POP framework. Meanwhile, the wide peak existing in 1.31%Pd@TP-POP suggests the
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encapsulation of Pd NPs did not damage the structure of TP-POP. Comparing with the XRD
standard card of Pd (JCPDS No.87-637), the 1.31%Pd@TP-POP catalyst exhibits the
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characteristic peaks of Pd(111) at 2θ = 40°. Owing to the low loading amount of Pd NPs are
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encapsulated inside the TP-POP, no other obvious diffraction peaks for Pd NPs are detected.
According to the above analysis, the prepared catalyst has a certain order structure and Pd
NPs are successfully encapsulated in the framework of TP-POP.
The porous properties of TP-POP and 1.31%Pd@TP-POP were investigated by nitrogen
adsorption-desorption measurement at 77 K. The isotherms of TP-POP and
1.31%Pd@TP-POP exhibit characteristics of type Ⅳ according to the classification of the
International Union of Pure and Applied Chemistry (Fig. 1c), indicating the mesoporous
structure of the prepared materials. The average pore size of TP-POP was determined to be 3
nm (Fig. 1d). This is a bit larger than the average particle size of Pd NPs and illustrates the
successful encapsulation of Pd NPs in the pores of the TP-POP. There are also a few larger
pores with the size of 5 to 7 nm, which embody the richness of the mesopores and promote
the diffusion and mass transfer of the reactants in Pd@TP-POP catalyst.
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The as-synthesized TP-POP and 1.31%Pd@TP-POP were further analyzed by SEM and
TEM. It can be observed from Figs. 2a-b that, the TP-POP presents three-dimensional fold
structure and organic pores. Notably, no morphological changes in 1.31%Pd@TP-POP are
observed after Pd NPs encapsulation using RDSA strategy (Fig. 2c). TEM image of
1.31%Pd@TP-POP catalyst shows more organic pores structure with the encapsulation of Pd
NPs (Fig. 2d). The TP-POP has an average pore size of 3 nm. Its advantage is that Pd NPs are
uniformly encapsulated in TP-POP with a controlled size. Specifically, Pd NPs are not only
hard to agglomerate, but also not easily leaching, thus the catalyst has a good stability. For
comparison, TEM image of the 1.81%Pd/TP-POP catalyst prepared by the common
impregnation-reduction method is shown in Fig. S1. It can be clearly seen that, Pd NPs are
seriously agglomerated at the outer surface of the TP-POP. Therefore, the RDSA strategy has
a great advantage in size-controlled preparation of uniform Pd NPs and confines them inside
the TP-POP. The particle size distribution of the encapsulated Pd NPs by RDSA is
approximately 1.4-2.8 nm (Fig. 2e). Due to Pd NPs are embedded in the internal surface of
TP-POP, the lattice of Pd NPs cannot be seen from HRTEM (Fig. S2). Moreover, EDS pattern
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confirms that the 1.31%Pd@TP-POP catalyst contains C, N, O and Pd elements (Fig. 2f).
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Fig. 2. (a), (c) SEM images and (b), (d) TEM images of TP-POP and 1.31%Pd@TP-POP catalyst;
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(e) Pd NPs size distribution; (f) EDS pattern of 1.31%Pd@TP-POP catalyst.
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The XPS spectra of 1.31%Pd@TP-POP displayed four binding energy peaks at 284.6,
335.2, 398.6 and 533 eV, which were assigned to C 1s, Pd 3d, N 1s and O 1s, respectively
(Fig. 3a). The C 1s spectrum of 1.31%Pd@TP-POP included three components: C-C, C=N,
C-O, which confirms that the compositions of TP-POP are the triazinyl groups and C-O-C
bonds (Fig. 3b). It can be seen from Fig. 3c that, the N 1s binding energy peak at 398.6 eV
was ascribed to pyridinic N of the triazine unit. As shown in Fig. 3d for 1.31%Pd@TP-POP,
the peaks of Pd(0) were observed at 335.2 eV and 340.4 eV, corresponding to the Pd 3d5/2 and
Pd 3d3/2, respectively. Meanwhile, the other two peaks of Pd(II) oxidation state were observed
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at 337.7 eV and 342.9 eV. The XPS peak fitted data shows that the proportion of surface
metal Pd(0) accounts for 79%. The presence of oxidized Pd(II) in the 1.31%Pd@TP-POP
catalyst could be attributed to the interaction of highly dispersed ultrafine Pd NPs with the
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triazinyl groups through N atoms. Thus, the XPS results further indicate the successful
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preparation of TP-POP and immobilization of Pd NPs in TP-POP.
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Fig. 3. (a) the wide-range XPS spectrum of 1.31%Pd@TP-POP catalyst; (b), (c) and (d) the
high-resolution spectrum of C 1s, N 1s and Pd 3d of 1.31%Pd@TP-POP, respectively.
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7
8
We also investigated the thermo stability of TP-POP (Fig. S3). The initial weight loss of
35.6% up to 292.3 °C is due to loss of intercalated water and gas in lattice of TP-POP. The
high weight loss above 352.8 °C may be attributed to the thermolysis of the TP-POP. When
the pyrolysis temperature rise up to 400 °C, the weight decrease of the sample is very slow,
indicating the TP-POP is pyrolyzed to a stable material. These results demonstrate that the
TP-POP material possesses high thermal stability under 292 °C, which may be attributed to
the rigidity of the skeleton and the conjugated diamond topology. Therefore, TP-POP can be
used as supporting material for fabricating supported catalysts and used in relatively mild
reaction conditions.
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0
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2
3
4
5
6
The catalytic performance of Pd@TP-POP catalysts was evaluated via the catalytic
reduction of 4-NP by NaBH as a model reaction. The progress of such reaction can be easily
4
monitored by measuring the changes in UV-Vis absorbance because of the only product of
4-AP obtained from 4-NP reduction. As shown in Fig. S4a, the 4-NP aqueous solution
exhibits a strong absorption peak at 317 nm. After the addition of NaBH to the aqueous
4
solution, the peak of 4-NP is shifted to 400 nm and the light yellow color of solution changed
into a bright yellow due to the formation of 4-nitrophenolate ions. After adding a certain
amount of catalyst (Fig. 4a), the color of the solution changed rapidly from bright yellow to
colorless. Meanwhile, the declining absorption peak at 400 nm is accompanied by a new
growing absorption peak at 300 nm, which is ascribed to the formation of 4-AP from 4-NP. In
the absence of the Pd@TP-POP catalyst, the reduction process does not take place (Fig. S4b).
The reaction can be completed only in 345 s to achieve quantitative conversion of 4-NP,
confirming the 1.31%Pd@TP-POP catalyst exhibits excellent catalytic performance (Fig. 4b).
As shown in Figs. 4c and d, the 0.71%Pd@TP-POP and 0.21%Pd@TP-POP catalysts need
521 s and 992 s in the 4-NP reduction, respectively. Thus, it is reasonable that such an
enhanced catalytic feature of 1.31%Pd@TP-POP catalyst should be ascribed to the ultrafine
Pd NPs encapsulated in mesoporous TP-POP; which not only promotes the mass transfer and
diffusion of reactants into the organic pores to attach the Pd active sites, but also enhances the
utilization ratio of Pd atoms. To further prove this point, 1.81%Pd/TP-POP catalyst prepared
through the common impregnation-reduction method was used for the reduction of 4-NP for
comparison. The result showed that the 1.81%Pd/TP-POP gives a lower catalytic performance
with complete conversion of 4-NP in 470 s (Fig. 4e). The reason may be due to the
agglomeration of Pd NPs to larger NPs on the outer surface of TP-POP (Fig. S1). This fully
reflects the excellent advantage that RDSA method can encapsulate Pd NPs to prevent them
from agglomeration on the TP-POP surface, and also increase the utilization ratio of Pd atoms
in catalysis applications.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
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4
Fig. 4. (a) the image of Pd@TP-POP catalyzed 4-NP reaction process; UV-Vis spectra of the 4-NP
5
6
7
reduction catalyzed by the (b) 1.31%Pd@TP-POP, (c) 0.71%Pd@TP-POP, (d) 0.21%Pd@TP-POP
and (e)1.81%Pd/TP-POP catalysts; (f) plots of ln(C
reduction of 4-NP with different catalysts.
t 0
/C ) against the reaction time (t) for the
8
9
0
1
2
3
The kinetic of the catalytic reduction of 4-NP using NaBH
4
in the presence of the catalysts
was also studied according to the following equation:
1
1
1
1
ln(C /C ) = - kt
t 0
where C
time t = 0 and the rate constant, respectively. The rate constant k for the catalytic reduction of
4-NP using 1.31%Pd@TP-POP, 0.71%Pd@TP-POP, 0.21%Pd@TP-POP and
t 0
, C , k is the concentration of 4-NP in the reaction time t, the initial concentration at
13

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-
1
-1
1.81%Pd/TP-POP catalysts were 0.61 min , 0.37 min , 0.20 min and 0.41 min ,
-1
-1
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
respectively (Fig. 4f). Besides, the TOF value for the 4-NP reduction on 1.31%Pd@TP-POP
-1
-1
catalyst was 227.07 h which is higher than that of 0.71%Pd@TP-POP (TOF = 192.48 h ),
-1
-1
0.21%Pd@TP-POP (TOF = 153.24 h ) and 1.81%Pd/TP-POP (TOF = 114.43 h ). It is
important to state that the activity of 1.31%Pd@TP-POP catalyst is also superior to many
5
1-58
other Pd-based catalysts that have been reported as shown in Table 1.
The superior
performance of the 1.31%Pd@TP-POP catalyst can be attributed to the following reasons:
First, due to the three-dimensional mesoporous fold structure and organic pores in TP-POP,
the accessibility of Pd active sites can be obviously enhanced and the reaction can be
promoted to proceed faster. Second, uniformly encapsulated Pd NPs inside the TP-POP by
RDSA can effectively prevent the agglomeration of Pd NPs and make the catalytic effect
extremely good. Finally, compared with other catalysts, the 1.31%Pd@TP-POP catalyst has a
lower loading of ultrafine Pd NPs, thus large fraction of active Pd atoms could be exposed to
the NPs surfaces, further increasing the utilization ratio of Pd metal. Therefore, the
1.31%Pd@TP-POP obviously exhibits excellent catalytic activity.
1
1
1
1
1
1
1
1
6
7
Table 1. Comparison of catalytic activity of Pd@TP-POP catalyst with the reported Pd-based
catalysts used in the reduction of 4-NP.
Entry
Catalysts
T
o
Time Pd loading
C) (min) (%)
1.31
0.71
k
TOF
-1
(h )
Ref.
-1
(min )
(
1
2
3
4
5
6
7
8
9
1.31%Pd@TP-POP
0.71%Pd@TP-POP
0.21%Pd@TP-POP
1.81%Pd/TP-POP
25
25
25
25
25
25
25
25
25
25
5.75
8.68
0.61
0.37
0.20
0.41
0.22
2.22
0.37
1.29
1.22
0.36
0.28
-
227.07 This work
192.48 This work
153.24 This work
114.43 This work
228.69 51
16.53 0.21
7.83
14
1.81
3.5
Pd@DANI-CS-Fe
Pd/Fe @γ-AlOOH-YSMs
Pd/NHPC
Fe @CeO
Ni@Pd/KCC-1
3 4
O
3
O
4
2.83
8
10.0
3.22
3.0
136.80 52
122.40 53
3
O
4
2
/Pd
3.0
4.8
106.57 54
10.08
70.10
93.60
24.16
3.30
55
56
57
58
10
Pd@h-mSiO
Fe @CeO
Pd-Nylon
2
2
-4.9%
-SO
12.33 4.9
11
3
O
4
3
H@PPy-Pd 25
22
13
35
6.1
12
2.70
1
1
2
8
9
0
To further prove the superior catalytic performance of Pd@TP-POP catalyst, the CTH of
aldehyde compounds was chosen as the research object. Herein, benzaldehyde was used as a
model substrate for screening the CTH reaction conditions. In general, many experimental
14

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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
factors such as reaction temperature, time, and hydrogen donors can affect the catalytic
activity. As shown in Table 2, entries 1-3, the conversion of benzaldehyde reached 98% at
80 °C at the same reaction time. It reveals that the higher the reaction temperature, the better
the CTH efficiency of benzaldehyde, which proves that the reaction progress is
thermodynamically controlled. Subsequently, the influence of Pd loading on the benzaldehyde
conversion was discussed. The conversion of benzaldehyde to benzyl alcohol was only 43%
when 0.21%Pd@TP-POP catalyst was used (Table 2, entry 4). Using the 0.71%Pd@TP-POP
catalyst, the conversion was only 71% (Table 2, entry 5). The above results show that 1.31%
Pd@TP-POP catalyst has the best catalytic performance. In addition, the amount of catalyst
also plays a key role in catalytic reactions, such as the CTH conversions of benzaldehyde
were 86% and 99% when using 5 mg and 15 mg of 1.31%Pd@TP-POP, respectively (Table 2,
entries 6-7). Because the CTH effect has no significant improvement using 15 mg of catalyst,
thus the optimum amount of catalyst is 10 mg.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
Alcohols are regarded as hydrogen donors may affect the CTH efficiency to a certain extent,
and then various alcohols were evaluated. The CTH reaction showed poor conversion using
the EtOH, glycol and cyclohexanol solvent, indicating that these alcohols were the negative
hydrogen donors used for the CTH of benzaldehyde (Table 2, entries 8-10). In contrast,
i-PrOH exhibited optimal performance that the conversions and selectivity were 98% and
100%. As shown in Table 2, entry 11, the presence of base is essential to initiate the reaction.
2 3
When K CO and triethylamine were used as base, the CTH of the benzaldehyde didn’t occur
(Table 2, entries 12-13). So the best base here is KOH, which facilitates the adsorption of
hydroxyl groups and the removal of hydrogen atoms in the hydrogen transfer reaction.
In addition, the conversions of CTH of benzaldehyde were only 15% and 17% for without
catalyst and the TP-POP as a catalyst, respectively (Table 2, entries 14-15), indicating that
metal Pd is the reactive site for the CTH of benzaldehyde and the 1.31%Pd@TP-POP catalyst
exhibits superior catalytic activity. The commercial 5%Pd/C was also used as catalyst under
the same reaction conditions; and the conversion of benzaldehyde was only 35% (Table 2,
entry 16). Compared with the 1.31%Pd@TP-POP catalyst, the conversion was only 61%
when 1.81%Pd/TP-POP was used as catalyst (Table 2, entry 17); the results suggested that
Pd NPs seriously agglomerated via the common impregnation-reduction preparation method,
15

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which displayed poor catalytic performance. From the above optimization results,
1.31%Pd@TP-POP catalyst could effectively realize the CTH of benzaldehyde using i-PrOH
as hydrogen source and solvent under mild conditions.
a
4
5
Table 2. Optimize the reaction conditions of CTH of benzaldehyde.
Entry
Catalyst
Catalyst
Temp.
Hydrogen source Conv.
Sel.
o
amount (mg) ( C)
b
b
(mL)
(%)
(%)
1
2
3
4
5
6
7
8
9
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
0.21%Pd@TP-POP 10
0.71%Pd@TP-POP 10
40
60
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
EtOH
39
47
98
43
71
86
99
0
100
100
100
100
100
100
100
0
1.31%Pd@TP-POP
5
1.31%Pd@TP-POP 15
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
1.31%Pd@TP-POP 10
cyclohexanol
glycol
57
0
0
10
0
c
11
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
0
0
d
e
1
1
1
1
1
2
3
4
5
6
7
0
0
0
0
No catalyst
TP-POP
0
15
17
35
100
100
100
100
10
10
10
5%Pd/C
1
1.81%Pd/TP-POP
61
a
b
Reaction conditions: 1 mmol benzaldehyde, 5 mL solvent, KOH (0.2 mmol). Analyzed by
6
7
c
d
e
GC-MS. No base, K CO (0.2 mmol), triethylamine (0.2 mmol).
2
3
8
9
0
1
2
3
4
The general applicability of 1.31%Pd@TP-POP catalyst was studied for various aldehyde
compounds (Table 3). The presence of halogen groups in the benzaldehyde gives the
corresponding alcohols with excellent conversions (>96%) and superior selectivity (100%).
Halogen groups at ortho, meta and para positions of the aromatic ring of substrates did not
affect the efficiency of the catalyst (Table 3, entries 2-7). Furthermore, benzaldehyde with
two halogen groups also can be CTH to corresponding alcohols with high conversion of near
100% and superior selectivity of 100% (Table 3, entries 8-9). The results indicated that there
1
1
1
1
1
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7
was almost no dehalogenation of halogenated-benzaldehyde during the CTH process.
Noteworthily, dehalogenation also did not occur even when the number of halogenated groups
increased. The conversion of p-nitrobenzaldehyde can reach 96% when the reaction time is
prolonged to 6 h due to the electron-withdrawing effect of nitro group (Table 3, entry 10). In
addition, the CTH conversions of benzaldehyde with nitro group at meta and ortho positions
are above 91% (Table 3, entries 11-12). The electron-donor of methyl- and 3, 4-dimethyl
substituted benzaldehyde both gave corresponding alcohols in good yields (Table 3, entries
13-14). The CTH of cinnamaldehyde to cinnamyl alcohol also reached 96% conversion and
100% selectivity (Table 3, entry 15). In addition, 1.31%Pd@TP-POP catalyst also exhibited
excellent CTH performance for other aldehydes including anisaldehyde and
o-methoxybenzaldehyde (Table 3, entries 16-17). Phenylacetaldehyde showed much higher
reactivity than cinnamaldehyde due to the conjugation effect between the C=C double bond
and aldehyde group in cinnamaldehyde (Table 3, entry 18). The outstanding catalytic activity
of such catalyst is contributed to the open organic pores to facilitate the transport of the
substance, and confinement effect of highly dispersed ultrafine Pd NPs. Overall,
1.31%Pd@TP-POP has excellent catalytic activity and chemoselectivity for CTH of most
aldehyde compounds under green and mild reaction conditions.
1
1
1
1
1
1
1
1
a
1
1
8
9
Table 3. Transfer Hydrogenation of aldehyde compounds with 1.31%Pd@TP-POP catalyst.
b
b
Entry
1
Substrate
Product
Time (h)
3
Conv. (%)
Sel. (%)
98
100
2
3
4
5
6
7
3
3
3
3
3
3
＞99
96
100
100
100
100
100
100
＞99
99
97
96
17

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8
9
3
＞99
＞99
96
100
100
100
100
100
100
100
100
100
100
100
3
6
6
6
6
6
6
6
6
3
10
1
1
2
3
4
5
6
7
8
93
1
1
1
1
1
1
1
91
94
79
96
91
99
＞99
a
o
Reaction conditions: 1 mmol substrate, 5 mL i-PrOH, 0.2 mmol KOH and 80 C, Analyzed by
b
1
2
GC-MS.
On the basis of the aforementioned results and some reported literatures,
59, 60
3
4
5
6
7
8
9
0
1
2
the
speculative possible mechanism of the current CTH is shown in Scheme 2. Initially, the
additive base promotes the interaction between i-PrOH and Pd NPs and the intermediate 1 is
obtained. And then the aldehyde substrate is adsorbed on the surface of the Pd NPs to form
intermediate 2. Subsequently, the produced H* active species attack the carbonyl group of the
aldehyde and the intermediate 3 is generated. Along with the release of the acetone molecule
from the active Pd sites, the formation of C-OH bond resulting in the production of alcohol.
Eventually, i-PrOH will be converted to acetone after losing two hydrogen atoms in the
reaction process, followed by the hydrogenation of C=O that forms C-OH by the attack of the
hydrogen atoms. Thus, Pd NPs on the surface become available again for a new substrate.
1
1
1
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1
2
3
Scheme 2. Illustration of the reaction mechanism for the transfer hydrogenation of aldehyde
compounds over the 1.31%Pd@TP-POP catalyst.
4
5
6
7
8
9
0
1
2
3
4
5
6
7
1.31%Pd@TP-POP catalyst also exhibits excellent stability and recyclability in catalyzing
the hydrogen transfer of benzaldehyde under the mild reaction conditions (Fig. 5). After
completion of the reaction, the catalyst was conveniently recycled by centrifugation and
washed with deionized water and ethanol. The 1.31%Pd@TP-POP catalyst still maintains
excellent catalytic efficiency even after ten cycles without any loss of catalytic activity. The
results clearly show that 1.31%Pd@TP-POP catalyst has good recyclability and stability,
which mainly due to the organic pores that encapsulate Pd NPs in the TP-POP, thus the Pd
NPs has no obvious leaching. TEM images of 1.31%Pd@TP-POP catalyst after ten cycles
revealed that Pd NPs were strongly anchored and well dispersed on the inner surface of
TP-POP, which has a size of 1.5-3.0 nm with no obvious aggregation (Figs. S5a and b). And
the XRD test also showed the recycled catalyst was consistent with the fresh sample (Fig. S6).
Thus, the above-mentioned results confirmed that the TP-POP has excellent stability, which
hinders Pd NPs loss and keeps the structure unchanged in the entire reaction process.
Therefore, the 1.31%Pd@TP-POP has good stability and recyclability.
1
1
1
1
1
1
1
1
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2
Fig. 5. The catalyst reusability for the CTH of benzaldehyde with the 1.31%Pd@TP-POP catalyst.
3
Conclusions
4
5
6
7
8
9
0
1
2
3
4
In summary, we successfully designed and synthesized a novel TP-POP by affinity
substitution of cyanuric chloride with pentaerythritol. Pd NPs with a narrow size distribution
of 1.4-2.8 nm are highly dispersed and confined in the organic pores of TP-POP by RDSA
strategy. The catalytic results demonstrated that the 1.31%Pd@TP-POP catalyst exhibited
superior catalytic activity for reduction of 4-NP and catalytic transfer hydrogenation of
aromatic aldehydes with i-PrOH. The excellent catalytic performance of 1.31%Pd@TP-POP
catalyst may be ascribed to the abundant mesopores of TP-POP and the easily accessible
highly dispersed ultrafine Pd NPs confined in the organic pores of TP-POP. This study not
only demonstrates the feasibility of constructing the POPs support, but also shows a
promising strategy for encapsulating uniform size-controlled NMNPs in the POP materials for
various catalysis applications.
1
1
1
1
1
1
1
5
6
Conflicts of interest
There are no conflicts to declare.
1
1
7
8
Supporting Information
Supporting Information is available from the publication website.
1
9
Acknowledgements
2
2
0
1
We thank the financial support from the Fundamental Research Funds for the Central
Universities (lzujbky-2017-112).
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Page 25 of 25
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DOI: 10.1039/C8TA07502F
Graphical Abstract
A stable Pd@TP-POP catalyst was fabricated with highly dispersed ultrafine Pd NPs confined
in its organic pores via a reversed double solvent approach, which shows excellent catalytic
activity in reduction of 4-nitrophenol and transfer hydrogenation of aromatic aldehydes under
very mild conditions.