Improving Catalytic Hydrogenation Performance of Pd Nanoparticles by Electronic Modulation Using Phosphine Ligands

Article abstract of DOI:10.1021/acscatal.8b00872

Tuning the activity and selectivity of metal nanoparticles (NPs) is a long-term pursuit in the field of catalysis. Herein, we report successfully improving both the activity and chemoselectivity of Pd NPs (1.1 nm) with triphenylphosphine (PPh₃) cross-linked in the nanopore of FDU-12. The electron-donating effect of PPh₃ increases the surface electronic density of Pd NPs and weakens the Pd-H bond, as evidenced by the results of XPS, in situ FT-IR adsorption of CO, and H₂-D₂ exchange reactions. Consequently, Pd NPs modified with PPh₃ obtain >99% selectivity to 1-phenylethanol in acetophenone hydrogenation and 94% selectivity to styrene in phenylacetylene hydrogenation. Furthermore, the activity of Pd NPs is enhanced and suppressed by PPh₃, respectively, in the hydrogenation of electrophilic nitro compounds and nucleophilic carbonyl substrates. Our primary results shed some light on judiciously choosing organic ligands for modifying the catalytic performance of metal NPs toward specific chemical transformations.

Full text of DOI:10.1021/acscatal.8b00872

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Article
Improving catalytic hydrogenation performance of Pd
nanoparticles by electronic modulation using phosphine ligands
Miao Guo, He Li, Yiqi Ren, Xiaomin Ren, Qihua Yang, and Can Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00872 • Publication Date (Web): 06 Jun 2018
Downloaded from http://pubs.acs.org on June 6, 2018
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Improving catalytic hydrogenation performance of
Pd nanoparticles by electronic modulation using
phosphine ligands
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Miao Guo,a,b He Li,a Yiqi Ren,a,b Xiaomin Ren,a,b Qihua Yang,a* and Can Lia*
aState Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, China.
bUniversity of Chinese Academy of Sciences, Beijing 100039, China.
ABSTRACT. Tuning the activity and selectivity of metal nanoparticles (NPs) is a long term
pursuit in the field of catalysis. Herein, we report successfully improving both activity and
chemoselectivity of Pd NPs (1.1 nm) with triphenylphosphine (PPh3) crossꢀlinked in the
nanopore of FDUꢀ12. The electron donating effect of PPh3 increases surface electronic density of
Pd NPs and weakens PdꢀH bond as evidenced by results of XPS, in situ FTꢀIR adsorption of CO
and H2ꢀD2 exchange reaction. Consequently, Pd NPs modified with PPh3 obtain > 99 %
selectivity to 1ꢀphenylethanol in acetophenone hydrogenation and 94 % selectivity to styrene in
phenylacetylene hydrogenation. Furthermore, the activity of Pd NPs is enhanced and suppressed
by PPh3 respectively in the hydrogenation of electrophilic and nucleophilic substrates. Our
primary results shed some light on judiciously choosing organic ligands for modifying the
catalytic performance of metal NPs towards specific chemical transformations.
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KEYWORDS: Pd nanoparticles, triphenylphosphine, hydrogenation, nitrobenzene,
acetophenone.
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1. INTRODUCTION
Supported metal catalysts have been widely used in industry for versatile chemical reactions,
including hydrogenation, oxidation, coupling reaction and so on.1ꢀ5 The development of highly
active and selective metal catalysts has attracted tremendous attention in heterogeneous catalysis
communities.6ꢀ9
Usually, the catalytic performance of metal nanoparticles (NPs) could be optimized through
modulating parameters such as shape, size, dispersity, alloying with other metals and generating
strong metalꢀsupport interactions.10ꢀ13 More recently, tuning the electronic structure of metal NPs
with organic modifiers has emerged as an important strategy to improve their catalytic
performance. Ernst and coꢀworkers14 have reported that unreactive Pd/Al2O3 could be activated
by Nꢀheterocyclic carbene (NHCs) in BuchwaldꢀHartwig amination of aryl halides due to the
capability of NHCs for changing the electronic environment of Pd/Al2O3.
Tricyclohexylphosphine has been employed to modify electronic structure of Cu/SiO2 to
improve its chemoselectivity in semihydrogenation of alkynes.15 Au NPs stabilized by secondary
phosphine oxides are very selective in the hydrogenation of acrolein and other α, βꢀunsaturated
aldehydes.16 Zheng and coꢀworkers have demonstrated that Pt nanowires modified with
ethylenediamine ligands afforded extremely high selectivity to thermodynamically unfavourable
Nꢀhydroxylanilines in the hydrogenation of nitroaromatics.17
The previous work mentioned above used organic ligands as additives to modify electronic
structure and steric microenvironment of supported metal NPs.18ꢀ21 However, some limitations
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still exist. The addition of organic ligands could lead to the accumulate of these on the surface of
metal NPs, which may block some of active sites, resulting in low usage efficiency of
catalysts.22ꢀ24 The detachment of organic compounds from supported metal catalysts may require
repeated addition of organic compounds and contaminate the products. The influence of
interactions of metal NPs and support could not be avoided.
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Herein, the triphenylphosphine crossꢀlinked in the nanopore of FDUꢀ12 (ordered large cageꢀ
type mesoporous silica) is used as modifier to tailor the catalytic performance of Pd NPs for
avoiding problems such as active sites blockage, leaching of organic ligands and interferences of
support during the catalytic process. More importantly, the electronic property of Pd NPs
modified with triphenylphosphine is fully characterized and the influence of triphenylphosphine
on the catalytic activity and selectivity of Pd NPs was elucidated in hydrogenation reactions with
acetophenone, phenylacetylene and nitrobenzene as model substrates. Understanding the
electronic modulation induced by organic modifiers is very important to rationally optimize the
catalytic performance of metal NPs.
2. RESULTS AND DISCUSSION
In this work, triphenylphosphine (PPh3) was chosen to modify the catalytic performance of Pd
NPs because it has strong coordinating ability with metal ions.25ꢀ26 Tris(4ꢀvinylphenyl)phosphine
was polymerized in the nanopore of FDUꢀ12 to form PPh3@FDUꢀ12 (Figure 1A). PPh3@FDUꢀ
12 with polymer content of 14.9 wt% is stable up to 430 oC and 320 oC respectively in N2 and air
atmosphere based on thermogravimetric analysis (TGA) (Figure S1A). PPh3@FDUꢀ12 has BET
(BrunauerꢀEmmettꢀTeller) surface area of 663 m2 gꢀ1, slightly lower than that of parent FDUꢀ12
with BET surface area of 713 m2 gꢀ1 (Table S1). The sharp reduction in pore diameter from 14.9
to 10.4 nm after polymerization suggests that polymers mainly locate in the nanopore of FDUꢀ12
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and the thickness of polymer layer in nanopore of FDUꢀ12 is ca. 2 nm. PPh3@FDUꢀ12 has only
slightly more hydrophobic properties than FDUꢀ12 based on water contact angle (20o vs. 12o)
and benzene/H2O adsorption (0.26 vs. 0.22) characterizations. The transmission electron
microscopy (TEM) image of PPh3@FDUꢀ12 shows ordered arrangement of porous structure,
similar to parent FDUꢀ12 (Figure S2). The characteristic SiꢀO stretching vibration (1100 cmꢀ1),
PꢀAr stretching vibration (1443 cmꢀ1)27, aromatic C=C stretching (1600 cmꢀ1 and 1495 cmꢀ1)
vibration and CꢀH stretching vibration (2935 cmꢀ1 and 2856 cmꢀ1) can be clearly observed in the
Fourierꢀtransform infrared spectroscopy (FTꢀIR) spectrum of PPh3@FDUꢀ12 (Figure S1B).
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Solidꢀstate C CP/MAS NMR spectrum of PPh3@FDUꢀ12 gives chemical shifts at 146.4 ppm
for CꢀP, at 135.9, 132.1, 126.1 ppm for C=C and =CꢀH, and at 43 ppm for CH2 (Figure S3A).
Solidꢀstate 31P MAS NMR spectrum of PPh3@FDUꢀ12 displays only one peak at ꢀ5.8 ppm
corresponding to phosphorus in tertiary state (Figure S3B). The above results confirm the
successful formation of a thin layer of polymer in the nanopore of FDUꢀ12.
Pd was loaded on PPh3@FDUꢀ12 by impregnationꢀreduction method with NaBH4 as reductant.
A control sample, Pd/FDUꢀ12 was also prepared according to literature method.28ꢀ29 The Pd
loading of 1 wt% was used for the impregnation. The Pd content based on ICP results is 0.83 and
0.78 wt% respectively for Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12. Due to the contrast of Pd and
silica, the dark spots in the TEM images of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 can be assigned
to Pd NPs (Figure S5). The highꢀangle annular dark field scanning transmission electron
microscopy (HAADFꢀSTEM) images of both samples clearly show the uniform distribution of
Pd NPs with size centered at 1.1 nm (Figure 1B).
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Figure 1. (A) Schematic illustration for the synthesis of PPh3@FDUꢀ12, (B) Highꢀangle annular
dark field scanning transmission electron microscopy (HAADFꢀSTEM) images of, (C) Pd 3d
XPS spectra core level of, and (D) In situ FTꢀIR spectra of CO adsorption on (i) Pd/FDUꢀ12 and
(ii) Pd/PPh3@FDUꢀ12 (inset in B for particle size distribution, scale bar 10 nm).
The BET surface area of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 respectively of 646 m2 gꢀ1 and
684 m2 gꢀ1 is only slightly lower than parent materials (Table S1), indicating that Pd NPs do not
block the pores. The CO chemisorption results show that Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12
have almost the same particle size and dispersion of Pd. To further exclude the substitution of
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PPh3 with CO during chemisorption analysis, a control sample, PPh3/Pd/FDUꢀ12, was
synthesized by adsorption of PPh3 on Pd/FDUꢀ12 (P/Pd ratio of 5). The CO chemisorption
results of this control sample showed that Pd dispersion was only 7.4%, verifying that phosphine
ligands cannot be substituted with CO during CO chemisorption process.22 The above results
further confirm that phosphine ligands crossꢀlinked in the nanopore of FDUꢀ12 do not block the
surface of Pd NPs (Table 1). This is quite different from previous reports that organic ligands are
mainly adsorbed on the surface of metal NPs.
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The electronic properties of Pd NPs were investigated by in situ Xꢀray photoelectron
spectroscopy (XPS) technique (Figure 1C and Table 1). The peaks at 335.0 eV and 336.9 eV
can be attributed to Pd0 and Pd2+, respectively. Compared with Pd/FDUꢀ12, a red shift of 0.4 eV
is observed in Pd 3d core level for Pd/PPh3@FDUꢀ12. Pd/PPh3@FDUꢀ12 has higher Pd0/Pd2+
ratio (86/14 vs. 63/37) than Pd/FDUꢀ12 calculated from the relative peak areas. The above results
suggest that Pd has electronꢀrich character in the presence of PPh3. This is possibility due to the
electron transfer from P to Pd. The slight blue shift of binding energy in P 2p core level XPS
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spectra and downfield shift of the resonance peak in solidꢀstate P MAS NMR spectrum30 for
Pd/PPh3@FDUꢀ12 in comparison with PPh3@FDUꢀ12 verify the electron donating character of
phosphine ligands (Figure S3-S4).
Table 1. Pd dispersion and Pd 3d binding energies of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12.
Pd 3d5/2 (eV) b
Pd size
(nm) a
Pd dispersion
(%) a
Pd0/Pd2+
(%) b
Cat.
Pd2+
Pd0
Pd/PPh3@FDUꢀ12
Pd/FDUꢀ12
3.3
33.5
336.9
335.0
86/14
3.4
32.8
336.9
335.4
63/37
a Data calculated from CO chemisorption results; b Data obtained from in situ XPS results.
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To further probe the surface properties of Pd NPs, in situ FTꢀIR of CO adsorption was
conducted (Figure 1D). The CꢀO stretching frequency strongly depends on the binding mode as
well as the type of Pd site where the CO is adsorbed. Both samples afford two obvious peaks in
the range of 1800 ꢀ 2000 cmꢀ1 and 2000 ꢀ 2100 cmꢀ1 assigned respectively to bridgeꢀbounded CO
and linearꢀbounded CO on Pd surface.31 The linearꢀtoꢀbridging ratio (L/B ratio) estimated by
comparing the raw areas of different peaks was respectively 0.38 and 0.51 for Pd/FDUꢀ12 and
Pd/PPh3@FDUꢀ12, indicating that Pd/PPh3@FDUꢀ12 had more lowꢀcoordinate edge and corner
sites (always regarded as catalytically active sites).32 Linearꢀbounded CO for Pd/PPh3@FDUꢀ12
and Pd/FDUꢀ12 appear at 2054 cmꢀ1 and 2088 cmꢀ1, respectively. The red shift of linear Pd0ꢀCO
peak for Pd/PPh3@FDUꢀ12 suggests the existence of electronic interaction between Pd and P.
Generally, the phosphine ligand acts as an basic probe and electronꢀdonating ligand,33ꢀ34 thereby
favoring the backꢀdonation of electrons to the 2π* antibonding orbitals of CO, which weakens
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C≡O bonding to induce redꢀshift of the CO stretching. The above results are consistent with the
XPS data, confirming the electron donation from PPh3 to Pd NPs.
The catalytic performance of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 was firstly tested in
chemoselective hydrogenation of acetophenone (Table 2). Both catalysts could smoothly
catalyze the reaction to afford 99 % conversion. Pd/PPh3@FDUꢀ12 gives > 99 % selectivity to 1ꢀ
phenylethanol. Under similar reaction conditions, Pd/FDUꢀ12 only shows 63 % selectivity to 1ꢀ
phenylethanol with ethylbenzene as byproduct. Acetophenone conversion on Pd/PPh3@FDUꢀ12
increases gradually with reaction time and reaches to 99 % at 30 min (Figure 2A). The
selectivity to 1ꢀphenylethanol remains > 99 % during the whole reaction process. Under more
o
harsh conditions, e.g. 10 bar H2 and 120 C, Pd/PPh3@FDUꢀ12 still gives 99 % selectivity to 1ꢀ
phenylethanol. For Pd/FDUꢀ12, the selectivity to 1ꢀphenylethanol decreases as conversion
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increasing due to further hydrogenolysis of 1ꢀphenylethanol to ethylbenzene. The above results
signify that Pd/PPh3@FDUꢀ12 has high chemoselectivity to carbonyl hydrogenation and 1ꢀ
phenylethanol could not be further converted on Pd/PPh3@FDUꢀ12.
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Table 2. The catalytic performance of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 in hydrogenation of
ketones/aldehydes.a
Pd
H2
Pd
H2
Substrates
Products
t (h)
0.5
Conv. (%)
> 99 (> 99)
Sel. (%) b
> 99 (63)
TOF (hꢀ1) d
109 (234)
3
3
97 (> 99)
96 (99)
99 (10)
99 (33)
52 (154)
31 (79)
2.5
1
99 (> 99)
> 99 (> 99)
90 (> 99)
> 99 (> 99)
> 99 (> 99)
98 (> 99)
98 (> 99)
99 (65)
36 (96)
> 99 (97)
> 99 (58)
> 99 (24)
99 (~ 0)
> 99 (~ 0)
95 (54) c
85 (183)
9 (21)
3
4
7 (25)
0.3
1
1480 (2970)
500 (1800)
1
240 (1340)
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a Reaction conditions for hydrogenation of aldehyde/ketones: 0.2 mmol substrate, 60 C, 4 bar
H2, 2 mL EtOH, S/C = 25 for ketones, S/C = 200 for benzaldehyde, S/C = 100 for 4ꢀ
b
anisaldehyde and 4ꢀformylbenzonitrile; Data in parentheses refer to Pd/FDUꢀ12; Selectivity to
alcohols and byproducts is corresponding hydrogenolysis products (alkanes); c The byproducts of
d
Pd/FDUꢀ12 are 4ꢀtolylmethanol (44 %) and 4ꢀmethylbenzonitrile (2%); Apparent TOF was
obtained with the conversion less than 30 %.
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80
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0
100
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0
A
B
C
100
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0
100
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Conv.
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Sel.
50
50
Conv.
for Pd/FDU-12
Conv.
Sel.
Sel.
for Pd/PPh3@FDU-12
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25
0
5
10 15 20 25 30
0
1
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5
T (min)
P/Pd ratio
Figure 2. (A) Reaction profiles of acetophenone hydrogenation with time catalyzed by
Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12, (B) Conversion and selectivity to 1ꢀphenylethanol on
Pd/FDUꢀ12 with different amount of PPh3 in acetophenone hydrogenation (Reaction condition:
3×10ꢀ3 mmol Pd, S/C = 50, 2 mL EtOH, 60 C, 4 bar H2, 40 min) and (C) H2ꢀD2 exchange
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reaction on Pd/FDUꢀ12, Pd/PPh3@FDUꢀ12 and PPh3@FDUꢀ12 (normalized by surface Pd
atom).
To understand the influence of PPh3 on the catalytic performance of Pd NPs, PPh3 was added
in hydrogenation of acetophenone using Pd/FDUꢀ12 as catalyst (Figure 2B). With 0.3 equiv.
PPh3 to Pd, the selectivity to 1ꢀphenylethanol increases from 61 % to 80 %. The selectivity to 1ꢀ
phenylethanol increases gradually with the increment of PPh3 equiv. and reaches the maximum
of 93 % with 1.8 equiv. of PPh3, showing the positive effect of PPh3 in enhancing the selectivity
of Pd NPs. However, the conversion decreases sharply from 99 % to 29 % with PPh3 equiv.
increasing from 0 to 5, suggesting that active sites on Pd NPs are capped by PPh3. This
phenomenon is often observed in literatures using organic ligands as modifier to tune the surface
properties of metal NPs.22, 35 Pd/PPh3@FDUꢀ12 affords much higher activity and selectivity than
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Pd/FDUꢀ12 with PPh3 as modifier, demonstrating the uniqueness of PPh3@FDUꢀ12 in tuning the
catalytic performance of metal NPs.
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Considering H2 dissociation as one of the important elementary steps in hydrogenation
reactions, the H2ꢀD2 exchange reaction was performed to characterize the activation of molecular
hydrogen on Pd surfaces (Figure 2C, Figure S9). The normalized rate by surface Pd atom of HD
formation follows the order of Pd/FDUꢀ12 (100) > Pd/PPh3@FDUꢀ12 (68) >> PPh3@FDUꢀ12 (~
0), indicating that PdꢀH bond is weakened for Pd/PPh3@FDUꢀ12. This is mainly attributed to the
electronꢀdonating effect of PPh3.36 Generally, the weakened PdꢀH bond results in decreased
coverage of hydrogen on Pd surface and therefore may suppress the further
hydrohenation/hydrogenalysis of intermediates.37 It was reported that Pd NPs with high
reduction degree lead to high selectivity to 1ꢀphenylethanol while the oxidized Pd exhibited high
selectivity to ethylbenzene.38 The above XPS results showed that Pd/PPh3@FDUꢀ12 had higher
Pd0/Pd2+ ratio than Pd/FDUꢀ12 (86 % vs. 63 %), which possibly was one of the reasons for the
high selectivity of Pd/PPh3@FDUꢀ12 to alcohol. In addition, 1ꢀphenylethanol is more
nucleophilic than acetophenone. Thus, high electron density of Pd modified with PPh3 does not
favor the adsorption of 1ꢀphenylethanol. This may also contribute to the high selectivity of
Pd/PPh3@FDUꢀ12 in acetophenone hydrogenation.
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In addition to acetophenone hydrogenation, Pd/PPh3@FDUꢀ12 also affords much higher
selectivity to styrene than Pd/FDUꢀ12 in selective hydrogenation of phenylacetylene (Figure 3).
Phenylacetylene on Pd NPs can be hydrogenated partially to styrene or fully to ethylbenzene. For
Pd/FDUꢀ12, the selectivity to styrene is 95% in the initial 20 min with conversion lower than
60%. The selectivity to styrene decreases sharply from 95 % to 15% by further prolonging the
reaction time. For Pd/PPh3@FDUꢀ12, the selectivity to styrene only decreases slightly with
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phenylacetylene conversion increasing. Pd/FDUꢀ12 could afford selectivity to styrene of 94% at
conversion of 78%, while up to 94% selectivity to styrene was obtained on Pd/PPh3@FDUꢀ12
with conversion of 92%. The enhanced selectivity of Pd/PPh3@FDUꢀ12 may be due to the
weakened PdꢀH bond and hindered adsorption of styrene on Pd surface, which is demonstrated
previously by PPh3 modified Pd/TiO2.35,39ꢀ41 Pd/PPh3@FDUꢀ12 with TOF of 560 hꢀ1 is much less
active than Pd/FDUꢀ12 with TOF of 1840 hꢀ1.
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Figure 3. Conversion and selectivity to styrene for phenylacetylene hydrogenation as a
function of reaction time over (A) Pd/FDUꢀ12 and (B) Pd/PPh3@FDUꢀ12 (Reaction conditions:
25 oC, 1 bar H2, 1.8 mmol phenylacetylene, S/C = 1000, 5 mL EtOH).
The activity of Pd/PPh3@FDUꢀ12 is lower than that of Pd/FDUꢀ12 in hydrogenation of
ketones and phenylacetylene. In addition to weakened PdꢀH bond on Pd/PPh3@FDUꢀ12, the low
activity of Pd/PPh3@FDUꢀ12 may also derive from the weakened adsorption of nucleophilic
ketones and phenylacetylene on Pd NPs with electron rich surface property. To clarify this, the
catalytic activity of Pd/PPh3@FDUꢀ12 was tested in hydrogenation of acetophenone, 4'ꢀ
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methylacetophenone, 4'ꢀmethoxyacetophenone, methyl 4ꢀacetylbenzoate and 4'ꢀ(trifluoromethyl)
acetophenone (Figure 4 and Table 2). Pd/PPh3@FDUꢀ12 could efficiently catalyze the
hydrogenation of the above substrates with selectivity to alcohol up to 99%. The catalytic
activity of Pd/PPh3@FDUꢀ12 decreases as the electron donating ability of substituent groups
increases with the exception of methyl 4ꢀacetylbenzoate and 4'ꢀ(trifluoromethyl)acetophenone.
This suggests that weakened adsorption of nucleophilic ketones on electron rich Pd NPs is one of
the reasons for the low activity. The deviation of methyl 4ꢀacetylbenzoate and 4'ꢀ(trifluoromethyl)
acetophenone is possibly due to the steric hindrance in addition to their electronic properties.
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The nature charge of oxygen atom
Figure 4. Hydrogenation of ketones with electron donating or electron withdrawing groups
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over Pd/PPh3@FDUꢀ12 (Reaction conditions: 2 mmol substrate, S/C = 25, 2 mL EtOH, 60 C, 4
bar H2). The nature charge of oxygen atom in aromatic ketones was calculated based on the η1(O)
configuration, the possible adsorption mode (Figure S10).42ꢀ43
Pd/PPh3@FDUꢀ12 could catalyze a wide scope of ketones, including 4'ꢀmethylacetophenone,
4'ꢀmethoxyacetophenone, 4'ꢀtertꢀbutylacetophenone, 4'ꢀisoꢀbutylacetophenone, methyl 4ꢀ
acetylbenzoate and 4'ꢀ(trifluoromethyl)acetophenone to afford 99% selectivity to alcohols. Under
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similar conditions, Pd/FDUꢀ12 affords much lower selectivity to alcohols than Pd/PPh3@FDUꢀ
12 though the activity of the former is higher. This is in a similar tendency to the hydrogenation
of acetophenone. Both Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 show low activity towards 4'ꢀtertꢀ
butylacetophenone, 4'ꢀisoꢀbutylacetophenone and methyl 4ꢀacetylbenzoate. This is possibly
related with the steric hindrance of the substrates. Notably, up to 99% yield of 1ꢀ(4ꢀ
isobutylphenyl) ethanol, an important intermediate in the synthesis of ibuprofen44, was obtained
over Pd/PPh3@FDUꢀ12 in the hydrogenation of 4'ꢀisoꢀbutylacetophenone.
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With benzaldehyde and 4ꢀanisaldehyde as substrates, Pd/PPh3@FDUꢀ12 affords > 99 %
selectivity to corresponding alcohol and Pd/FDUꢀ12 gives toluene and 4ꢀmethylanisole as main
products. For 4ꢀformylbenzonitrile, Pd/PPh3@FDUꢀ12 gives > 99 selectivity to 4ꢀ
(hydroxylmethyl)benzonitrile. However, Pd/FDUꢀ12 only gives 54 % selectivity to alcohol under
similar reaction conditions.
The above results suggest that Pd/PPh3@FDUꢀ12 with electron rich surface properties may
favor the adsorption of electrophilic substrates. Thus, hydrogenation of nitrobenzene and its
derivatives was performed (Table 3). Both Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 could efficiently
catalyze the hydrogenation of nitrobenzene with aniline selectivity up to 99 %. Nitrosobenzene is
the only detected byproduct during the whole reaction process, suggesting the hydrogenation
mainly follows the direct reaction pathway.2,45 Based on reaction profiles, up to 99 % conversion
is obtained on Pd/PPh3@FDUꢀ12 in 60 min (Figure S6). Under similar conditions, Pd/FDUꢀ12
only gives conversion of 34 %. The TOF of Pd/PPh3@FDUꢀ12 is ca. 5.5 fold that of Pd/FDUꢀ12,
confirming the positive effect of PPh3 in tuning the catalytic activity of Pd NPs. Even at mild
reaction conditions (1 bar and 25 oC), Pd/PPh3@FDUꢀ12 could still give TOF of 870 hꢀ1 (360 hꢀ1
for Pd/FDUꢀ12) (Figure S7).
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Table 3. Catalytic performance of Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 in the hydrogenation of
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nitrobenzene and its derivatives.a
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Ratio of TOFf
5.5
Substrates
Products
t (h)
1
Conv. (%)
> 99 (34)
Sel. (%) d
> 99 (97)
TOF (hꢀ1) e
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> 99 (30)
> 99 (27)
> 99 (90)
> 99 (78)
8640 (1800)
8570 (1590)
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> 99 (29)
99 (97)
3580 (730)
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99 (7)
> 99 (79)
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> 99 (> 99)
> 99 (> 99)
a Reaction conditions for hydrogenation of aromatic nitro compounds: 5 mmol nitrobenzene, 1
mL EtOH, 40 oC, 10 bar H2, S/C = 6000; Data in parentheses refer to Pd/FDUꢀ12; b 10 mL EtOH
was used due to the low solubility of 4'ꢀnitroacetophenone; 2 mmol 4ꢀnitrophenol, 10 mL H2O;
c
d
e
Selectivity to aromatic amines; Apparent TOF was obtained with the conversion less than 30
%; f The ratio of apparent TOF of Pd/PPh3@FDUꢀ12 to Pd/FDUꢀ12.
Pd/PPh3@FDUꢀ12 indeed exhibits much higher activity than Pd/FDUꢀ12 in hydrogenation
reactions with electrophilic nitrobenzene as substrate. Based on kinetic studies, firstꢀorder
dependence of H2 was observed for Pd/PPh3@FDUꢀ12 and Pd/FDUꢀ12 (Figure S8), suggesting
the H2 dissociation on two Pd surface is unsaturated46ꢀ48 and PdHx may involve in the rate
determining steps in the surface reaction according to the LangmuirꢀHinshelwood mechanism.49
Considering the superior hydrogen dissociation capabilities of Pd50 and the significant difference
electron character of nitro groups and amine groups, the adsorption and desorption of reactants
may play an important role for enhanced activity of Pd/PPh3@FDUꢀ12.17 Pd/PPh3@FDUꢀ12
with electronꢀrich Pd surface would prefer the adsorption of electron poor nitrobenzene over
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electron rich aniline, which consequently enhances the catalytic activity.17,51 In comparison with
previously reported Pdꢀbased catalysts, such as Pd/ILs,52 Pd/PEG,53 Pd/PVPA,54 Pd/PPh3@FDUꢀ
12 is the most active Pdꢀbased solid catalyst ever reported for nitrobenzene hydrogenation
(Table S2).
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Pd/PPh3@FDUꢀ12 could efficiently catalyze the hydrogenation of different types of nitro
compounds to corresponding amines with high activity and selectivity (Table 3). Even for ethyl
4ꢀnitrobenzoate and pꢀnitroacetophenone with a bulky substituent group, ∼99% conversion with
> 99% selectivity was achieved on Pd/PPh3@FDUꢀ12. It should be mentioned that only nitro
group was hydrogenated to amine groups with C=O groups almost unaffected. Under similar
reaction conditions, Pd/FDUꢀ12 affords very low conversion (20% ∼ 7%) and selectivity (79%∼
54%). Pd/PPh3@FDUꢀ12 gives higher selectivity than Pd/FDUꢀ12 in the hydrogenation of
substituted nitrobenzene except 4ꢀnitrophenol. For all substrates tested, Pd/PPh3@FDUꢀ12 shows
much higher activity than Pd/FDUꢀ12. For hydrogenation of nitrobenzene with electron donating
substituents, such as paraꢀ, orthoꢀ, and metaꢀnitrotoluene, the activity of Pd/PPh3@FDUꢀ12 is ∼5
folds that of Pd/FDUꢀ12. When pꢀnitroacetophenone with electron withdrawing substituent was
used, the apparent TOF of Pd/PPh3@FDUꢀ12 is 9 times higher than that of Pd/FDUꢀ12. The
above results indicate that the activity of Pd/PPh3@FDUꢀ12 increases with electrophilicity of
nitro compounds increasing, which is possibly related with its electron rich Pd surface. Though
ethyl 4ꢀnitrobenzoate also has electronꢀwithdrawing substituents, the activity of Pd/PPh3@FDUꢀ
12 is not boosted as much as in the case of pꢀnitroacetophenone. This possibly related with the
steric effect. In comparison with para- and meta-nitrotoluene, the relatively lower activity of
orthoꢀnitrotoluene may also be due to the steric effect.55
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The recycle stability of Pd/PPh3@FDUꢀ12 was investigated using nitrobenzene hydrogenation
as model reaction (Figure 5). The solid catalyst can be easily recovered by centrifugation after
reaction. Pd/PPh3@FDUꢀ12 is successfully reused for more than five cycles without any obvious
loss of catalytic activity for the hydrogenation of nitrobenzene. After the fifth cycle,
Pd/PPh3@FDUꢀ12 was characterized by TEM as shown in Figure S11. No aggregation of Pd
NPs could be observed in the TEM image. These results show the excellent stability of
Pd/PPh3@FDUꢀ12 catalyst in hydrogenation reactions.
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Figure 5. Recycling stability of Pd/PPh3@FDUꢀ12 in the hydrogenation of nitrobenzene.
3. CONCLUSION
In summary, the catalytic activity and selectivity of Pd NPs were successfully tuned with
triphenylphosphine crossꢀlinked in the nanopore of FDUꢀ12. The electron donating effect of
PPh3 endows Pd NPs with electronꢀrich surface and high Pd0/Pd2+ ratio. Consequently,
Pd/PPh3@FDUꢀ12 shows exceptionally high selectivity to alcohols in hydrogenation of
aldehyde/ketones and further hydrogenolysis of alcohols is suppressed due to weakened PdꢀH
bond on electron rich Pd NPs. The catalytic activity of Pd NPs modified with PPh3 is greatly
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enhanced in the hydrogenation of electrophilic nitrobenzene but is suppressed towards
nucleophilic substrates, such as aldehydes and ketones. Furthermore, PPh3 crossꢀlinked in the
nanopore of FDUꢀ12 avoids the blockage of active sites on metal surface and leaching of organic
ligands during the catalytic process which are generally confronted using organic ligands as
additives to modify the surface of metal NPs. Our work signifies the importance of organic
ligands in tuning the surface electronic properties of metal NPs for enhanced chemoselectivity
and activity.
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4. EXPERIMENTAL SECTION
Chemicals and agents. All materials were of analytical grade without any further purification.
Nitro aromatic compounds, aromatic amines and tetraamminepalladium (II) nitrate solution (10
wt%) were purchased from SigmaꢀAldrich Company Ltd. (USA). Commercial Pd/C (5 wt%) and
aromatic ketones, aldehydes were obtained from TCI (Shanghai) Development Co., Ltd, Ark
Pharm Inc., Accela ChemBio Co., Ltd., and Innochem Co., Ltd. Other reagents were purchased
from TCI, Admas or Ark Pharm Inc, Alfa Aesar (china) chemical Co., Ltd and Sigama Aldrich
Co., Ltd. Tris(4ꢀvinylphenyl)phosphine was prepared according to the literature report.26 FDUꢀ
12 was synthesized according to the reported method.56
Synthesis of PPh3@FDU-12. 0.1 g of Tris(4ꢀvinylphenyl)phosphine and 2, 2'ꢀ
azobisisobutyronitrile (AIBN, 3% relative to total vinyl group) were dissolved in 650 µL of N,
Nꢀdimethylformamide. Then, the above solution was added to 570 mg of FDUꢀ12 activated by
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vacuum degassing at 120 C for 3 h through the impregnation method. The mixture was
subjected to freezeꢀvacuumꢀthaw to remove the air. Then, the mixture was heated at 80 oC for 20
h under N2 atmosphere. After thoroughly washing with dichloromethane and ethanol to remove
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the unreacted monomers and oligomers, the sample was dried under vacuum at room temperature.
The powdered product was denoted as PPh3@FDUꢀ12.
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Synthesis of Pd/PPh3@FDU-12. The Pd/PPh3@FDUꢀ12 was synthesized by conventional
impregnationꢀreduction method. Typically, 200 mg of material was dispersed in 1 mL of
deionized water in a flask, and then 200 µL of Na2PdCl4 aqueous solution (0.01 g mLꢀ1) was
added, after stirring overnight, 1 mL of NaBH4 aqueous solution (21.4 mg Lꢀ1) freshly prepared
was dropped slowly into the solution, then the mixture stirred vigorously for another 1 h. After
filtration, the solid product was washed with deionized water and EtOH for several times and
was dried in vacuum. The sample was designated as Pd/PPh3@FDUꢀ12.
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Synthesis of Pd/FDU-12. For comparison, Pd/FDUꢀ12 with particle size of 1.1 nm was
prepared via strong electrostatic adsorption method.28ꢀ29 Specifically, 500 mg of FDUꢀ12 was
dispersed in 2.5 mL of deionized water with PH of 11 (adjusted by NH3•H2O). Then, 135 µL of
tetraamminepalladium(II) nitrate solution (10 wt%) was added to the above mixture. After
sonication for 30 min, the mixture was kept under static conditions for another 2 h. The powder
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was isolated by filtration and dried at 60 C for 2 h. The asꢀsynthesized material was calcined at
125 oC for 1 h in air, followed by reduction at 165 oC in H2 for 2 h.
General procedure for hydrogenation reactions. The hydrogenation reactions were carried
out in a stainless steel autoclave (300 mL) with a thermocouple probed detector. In a typical
process for acetophenone hydrogenation: a desired amount of solid catalyst (8 ×10ꢀ3 mmol Pd)
was added in an ampoule tube, followed by the addition of acetophenone (0.2 mmol) (S/C = 25)
and 2 mL EtOH. The ampoule tube was loaded into the reactor. After purging with hydrogen for
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six times, the final pressure was adjusted to 4 bar and the reactor was heated to 60 C with
vigorous stirring. After reaction, the solid catalyst was separated by centrifugation and the
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filtration was collected, diluted with ethanol and analyzed by an Agilent 7890B GC equipped
with an Agilent J&W GC HPꢀ5 capillary column (30 m × 0.32 mm × 0.25 ꢁm). The procedure of
nitrobenzene hydrogenation was similar with acetophenone hydrogenation except 8.33 ×10ꢀ4
mmol Pd (S/C= 6000) and 1 mL EtOH were used. Conversion and selectivity was determined
using nꢀtetradecane or nꢀdecane as internal standards. For 4ꢀnitrophenol hydrogenation in water,
1,4ꢀbutanediol was used as the internal standard. The carbon balance of all the reactions is ~
100%.
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For recycle experiment, the liquid was decanted after reaction centrifugation. The residual
catalyst was thorough washed by EtOH and dried under vacuum at room temperature and used
directly for the next run.
Characterization
Transmission electron microscopy (TEM) was performed using HITACHI HT7700 at an
acceleration voltage of 100 kV. High resolution scanning electron microscopy (HRSEM) was
undertaken by using a HITACHI S5500 operating at an acceleration voltage of 1ꢀ30 kV. The
particle size and standard deviation were measured by counting randomly 200 nanoparticles
based on HAADFꢀSTEM images. N2 sorption isotherms were carried out on a Micromeritics
ASAP2020 volumetric adsorption analyzer. FTꢀIR spectra were performed on a Nicolet Nexus
470 IR spectrometer. Solid state 31P NMR was carried out at 9.4 T on a Varian Infinity Plus 400
spectrometer with a 31P frequency of 161.83 MHz using a 5 mm Chemagnetic probe. The
chemical shifts were referenced to (NH4)2HPO4. Solid state 13C NMR spectra were obtained with
a Bruker 500 MHz spectrometer equipped with a magicꢀangle spin probe using a 4 mm ZrO2
rotor. 13C signals were referenced to glycine (C2H5NO2). The Pd content was determined by
PLASAMꢀSPECꢀII inductively coupled plasma atomic emission spectrometry (ICPꢀAES). The
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thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449F3 analyzer from
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30 to 900 C with a heating rate of 10 C minꢀ1 under air or N2 atmosphere. CO chemisorption
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measurement was performed at 40 C on a Quantachrome Autosorbꢀ1/C chemisorb apparatus.
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Samples were treated in H2 atmosphere at 200 C for 2 h before measurement. The metal
dispersion and particle size were estimated according to previous report.57
In situ Xꢀray photoelectron spectroscopy (XPS) experiments were performed on a Kratos
AXIS HSi spectrometer equipped with a charge neutralizer and a monochromated Al Kα source
(1486.7 eV).58 Samples were reduced in situ at 165 oC for 2 h in the analyzer chamber under H2
pressures of 2 × 10ꢀ7 Torr. Spectra were subsequently collected at normal emission using an
analyzer pass energy of 40 eV at room temperature. Energy referenced was employed using the
valence band and Si 2p peak at 103.4 eV binding energy (BE).
In situ FTꢀIR of CO adsorption were collected on a Bruker EQUINOX 55 infrared
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spectrometer with a DTGS detector. Before measurement, each catalyst was pretreated at 200 C
under flowing H2 atmosphere (20 mL minꢀ1) for 1 h. The catalyst was subsequently cooled to
room temperature. After purged with Ar for 30 min, the background spectrum was collected. CO
adsorption experiments were carried out by collecting 64 scans at a resolution of 4 cmꢀ1. Gasꢀ
phase CO spectra were collected at the same pressure and subtracted from the corresponding
sample spectra.
H2ꢀD2 exchange reaction was carried out in a flow quartz reactor at 22 oC.59 The formation rate
of HD was measured by mass signal intensity (ion current). Prior to measurement, the 30 mg
catalyst was reduced with H2 (10 ml minꢀ1) at 200 oC for 30 min. After cooling down the sample
in the liquid nitrogen, D2 (10 ml minꢀ1) was mixed with H2 together passed the sample, then the
temperature was slowly raised to 22 oC. Products (HD, H2, and D2) were analyzed with an online
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mass spectrometer (GAM200, InProcess Instruments). The mass/charge ratio (m/z) values used
are 2 for H2, 4 for D2, and 3 for HD. The reaction time of H2ꢀD2 exchange conversions of 30 %
was evaluated the hydrogen activation on the Pd catalysts.60 The results were deducted the
background HD exchanges from the corresponding support.
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ASSOCIATED CONTENT
Supporting Information. The TG analysis, FTꢀIR spectrum, 13C CP/MAS NMR spectrum,
31P MAS NMR spectrum and physical parameters of PPh3@FDUꢀ12, XPS spectra of P 2p
results, TEM images of Pd/PPh3@FDUꢀ12 and Pd/ FDUꢀ12, Kinetic curves of nitrobenzene
hydrogenation and Quantum chemical calculations results could be found in supporting
information. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: yangqh@dicp.ac.cn,
* Eꢀmail: canli@dicp.ac.cn.
Author Contributions
Miao Guo did all the experiments and wrote the manuscript, He Li carried out the in situ FTꢀIR
characterization, Yiqi Ren discussed the data, Xiaomin Ren gave help on writing the manuscript,
Qihua Yang and Can Li made the research plan, supervised the research and organized the
manuscript. The manuscript was written through contributions of all authors.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
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This work was financially supported by the National Key R&D Program of China,
2017YFB0702800, the Natural Science Foundation of China (No. 21733009, 21232008,
21621063) and the Strategic Priority Research Program of the Chinese Academy of Sciences
Grant No. XDB17020200.
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