Improving catalytic hydrogenation performance of Pd nanoparticles by

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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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ACS Catalysis

Improving catalytic hydrogenation performance of Pd nanoparticles by electronic modulation using phosphine ligands Miao Guo,a,b He Li,a Yiqi Ren,a,b Xiaomin Ren,a,b Qihua Yang,a* and Can Lia* a

State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian 116023, China. b

University 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) cross-linked in the nanopore of FDU-12. The electron donating effect of PPh3 increases surface electronic density of Pd NPs and weakens Pd-H bond as evidenced by results of XPS, in situ FT-IR adsorption of CO and H2-D2 exchange reaction. 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 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.

ACS Paragon Plus Environment

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ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS:

Pd

nanoparticles,

triphenylphosphine,

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hydrogenation,

nitrobenzene,

acetophenone.

1. INTRODUCTION Supported metal catalysts have been widely used in industry for versatile chemical reactions, including hydrogenation, oxidation, coupling reaction and so on.1-5 The development of highly active and selective metal catalysts has attracted tremendous attention in heterogeneous catalysis communities.6-9 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 metal-support interactions.10-13 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 co-workers14 have reported that unreactive Pd/Al2O3 could be activated by N-heterocyclic carbene (NHCs) in Buchwald-Hartwig 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, β-unsaturated aldehydes.16 Zheng and co-workers have demonstrated that Pt nanowires modified with ethylenediamine ligands afforded extremely high selectivity to thermodynamically unfavourable N-hydroxylanilines 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.18-21 However, some limitations


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ACS Catalysis

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.22-24 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. Herein, the triphenylphosphine cross-linked in the nanopore of FDU-12 (ordered large cagetype 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.25-26 Tris(4-vinylphenyl)phosphine was polymerized in the nanopore of FDU-12 to form PPh3@FDU-12 (Figure 1A). PPh3@FDU12 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@FDU-12 has BET (Brunauer-Emmett-Teller) surface area of 663 m2 g-1, slightly lower than that of parent FDU-12 with BET surface area of 713 m2 g-1 (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 FDU-12


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and the thickness of polymer layer in nanopore of FDU-12 is ca. 2 nm. PPh3@FDU-12 has only slightly more hydrophobic properties than FDU-12 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@FDU-12 shows ordered arrangement of porous structure, similar to parent FDU-12 (Figure S2). The characteristic Si-O stretching vibration (1100 cm-1), P-Ar stretching vibration (1443 cm-1)27, aromatic C=C stretching (1600 cm-1 and 1495 cm-1) vibration and C-H stretching vibration (2935 cm-1 and 2856 cm-1) can be clearly observed in the Fourier-transform infrared spectroscopy (FT-IR) spectrum of PPh3@FDU-12 (Figure S1B). Solid-state

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C CP/MAS NMR spectrum of PPh3@FDU-12 gives chemical shifts at 146.4 ppm

for C-P, at 135.9, 132.1, 126.1 ppm for C=C and =C-H, and at 43 ppm for CH2 (Figure S3A). Solid-state

31

P MAS NMR spectrum of PPh3@FDU-12 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 FDU-12. Pd was loaded on PPh3@FDU-12 by impregnation-reduction method with NaBH4 as reductant. A control sample, Pd/FDU-12 was also prepared according to literature method.28-29 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@FDU-12 and Pd/FDU-12. Due to the contrast of Pd and silica, the dark spots in the TEM images of Pd/PPh3@FDU-12 and Pd/FDU-12 can be assigned to Pd NPs (Figure S5). The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) 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@FDU-12, (B) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of, (C) Pd 3d XPS spectra core level of, and (D) In situ FT-IR spectra of CO adsorption on (i) Pd/FDU-12 and (ii) Pd/PPh3@FDU-12 (inset in B for particle size distribution, scale bar 10 nm). The BET surface area of Pd/PPh3@FDU-12 and Pd/FDU-12 respectively of 646 m2 g-1 and 684 m2 g-1 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@FDU-12 and Pd/FDU-12 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/FDU-12, was synthesized by adsorption of PPh3 on Pd/FDU-12 (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 cross-linked in the nanopore of FDU-12 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. The electronic properties of Pd NPs were investigated by in situ X-ray 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/FDU-12, a red shift of 0.4 eV is observed in Pd 3d core level for Pd/PPh3@FDU-12. Pd/PPh3@FDU-12 has higher Pd0/Pd2+ ratio (86/14 vs. 63/37) than Pd/FDU-12 calculated from the relative peak areas. The above results suggest that Pd has electron-rich 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 spectra and downfield shift of the resonance peak in solid-state

31

P MAS NMR spectrum30 for

Pd/PPh3@FDU-12 in comparison with PPh3@FDU-12 verify the electron donating character of phosphine ligands (Figure S3-S4). Table 1. Pd dispersion and Pd 3d binding energies of Pd/PPh3@FDU-12 and Pd/FDU-12.

a

Pd2+

Pd0

33.5

336.9

335.0

86/14

32.8

336.9

335.4

63/37

Pd dispersion (%) a

Pd/PPh3@FDU-12

3.3

Pd/FDU-12

3.4

Cat.

Pd 3d5/2 (eV) b

Pd0/Pd2+ (%) b

Pd size (nm) 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 FT-IR of CO adsorption was conducted (Figure 1D). The C-O 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 cm-1 and 2000 - 2100 cm-1 assigned respectively to bridge-bounded CO and linear-bounded CO on Pd surface.31 The linear-to-bridging ratio (L/B ratio) estimated by comparing the raw areas of different peaks was respectively 0.38 and 0.51 for Pd/FDU-12 and Pd/PPh3@FDU-12, indicating that Pd/PPh3@FDU-12 had more low-coordinate edge and corner sites (always regarded as catalytically active sites).32 Linear-bounded CO for Pd/PPh3@FDU-12 and Pd/FDU-12 appear at 2054 cm-1 and 2088 cm-1, respectively. The red shift of linear Pd0-CO peak for Pd/PPh3@FDU-12 suggests the existence of electronic interaction between Pd and P. Generally, the phosphine ligand acts as an basic probe and electron-donating ligand,33-34 thereby favoring the back-donation of electrons to the 2π* antibonding orbitals of CO, which weakens C≡O bonding to induce red-shift 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@FDU-12 and Pd/FDU-12 was firstly tested in chemoselective hydrogenation of acetophenone (Table 2). Both catalysts could smoothly catalyze the reaction to afford 99 % conversion. Pd/PPh3@FDU-12 gives > 99 % selectivity to 1phenylethanol. Under similar reaction conditions, Pd/FDU-12 only shows 63 % selectivity to 1phenylethanol with ethylbenzene as byproduct. Acetophenone conversion on Pd/PPh3@FDU-12 increases gradually with reaction time and reaches to 99 % at 30 min (Figure 2A). The selectivity to 1-phenylethanol remains > 99 % during the whole reaction process. Under more harsh conditions, e.g. 10 bar H2 and 120 oC, Pd/PPh3@FDU-12 still gives 99 % selectivity to 1phenylethanol. For Pd/FDU-12, the selectivity to 1-phenylethanol decreases as conversion


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increasing due to further hydrogenolysis of 1-phenylethanol to ethylbenzene. The above results signify that Pd/PPh3@FDU-12 has high chemoselectivity to carbonyl hydrogenation and 1phenylethanol could not be further converted on Pd/PPh3@FDU-12. Table 2. The catalytic performance of Pd/PPh3@FDU-12 and Pd/FDU-12 in hydrogenation of ketones/aldehydes.a

Substrates

Products

Pd

Pd

H2

H2

t (h)

Conv. (%)

Sel. (%) b

TOF (h-1) d

0.5

> 99 (> 99)

> 99 (63)

109 (234)

3

97 (> 99)

99 (10)

52 (154)

3

96 (99)

99 (33)

31 (79)

2.5

99 (> 99)

99 (65)

36 (96)

1

> 99 (> 99)

> 99 (97)

85 (183)

3

90 (> 99)

> 99 (58)

9 (21)

4

> 99 (> 99)

> 99 (24)

7 (25)

0.3

> 99 (> 99)

99 (~ 0)

1480 (2970)

1

98 (> 99)

> 99 (~ 0)

500 (1800)

1

98 (> 99)

95 (54) c

240 (1340)

a

Reaction conditions for hydrogenation of aldehyde/ketones: 0.2 mmol substrate, 60 oC, 4 bar H2, 2 mL EtOH, S/C = 25 for ketones, S/C = 200 for benzaldehyde, S/C = 100 for 4anisaldehyde and 4-formylbenzonitrile; Data in parentheses refer to Pd/FDU-12; b Selectivity to alcohols and byproducts is corresponding hydrogenolysis products (alkanes); c The byproducts of Pd/FDU-12 are 4-tolylmethanol (44 %) and 4-methylbenzonitrile (2%); d Apparent TOF was obtained with the conversion less than 30 %.


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A

100

100

B

100

100

C

60

60

40

40 Conv. Sel. for Pd/FDU-12 20 Conv. Sel. for Pd/PPh3@FDU-12

20

0

0 0

5 10 15 20 25 30

T (min)

75

75

Conv. Sel.

50

50

Sel. (%) HD signal (a.u.)

80

80

Sel. (%) Conv. (%)

100

Conv. (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

75

50

25

25

25 0

1

2

3

4

5

P/Pd ratio

0

Pd PP Pd /PP h@ /FD 3 h@ UFD 3 12 FD U12 U12

Figure 2. (A) Reaction profiles of acetophenone hydrogenation with time catalyzed by Pd/PPh3@FDU-12 and Pd/FDU-12, (B) Conversion and selectivity to 1-phenylethanol on Pd/FDU-12 with different amount of PPh3 in acetophenone hydrogenation (Reaction condition: 3×10-3 mmol Pd, S/C = 50, 2 mL EtOH, 60 oC, 4 bar H2, 40 min) and (C) H2-D2 exchange reaction on Pd/FDU-12, Pd/PPh3@FDU-12 and PPh3@FDU-12 (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/FDU-12 as catalyst (Figure 2B). With 0.3 equiv. PPh3 to Pd, the selectivity to 1-phenylethanol increases from 61 % to 80 %. The selectivity to 1phenylethanol 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@FDU-12 affords much higher activity and selectivity than


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Pd/FDU-12 with PPh3 as modifier, demonstrating the uniqueness of PPh3@FDU-12 in tuning the catalytic performance of metal NPs. Considering H2 dissociation as one of the important elementary steps in hydrogenation reactions, the H2-D2 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/FDU-12 (100) > Pd/PPh3@FDU-12 (68) >> PPh3@FDU-12 (~ 0), indicating that Pd-H bond is weakened for Pd/PPh3@FDU-12. This is mainly attributed to the electron-donating effect of PPh3.36 Generally, the weakened Pd-H 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 1-phenylethanol while the oxidized Pd exhibited high selectivity to ethylbenzene.38 The above XPS results showed that Pd/PPh3@FDU-12 had higher Pd0/Pd2+ ratio than Pd/FDU-12 (86 % vs. 63 %), which possibly was one of the reasons for the high selectivity of Pd/PPh3@FDU-12 to alcohol. In addition, 1-phenylethanol is more nucleophilic than acetophenone. Thus, high electron density of Pd modified with PPh3 does not favor the adsorption of 1-phenylethanol. This may also contribute to the high selectivity of Pd/PPh3@FDU-12 in acetophenone hydrogenation. In addition to acetophenone hydrogenation, Pd/PPh3@FDU-12 also affords much higher selectivity to styrene than Pd/FDU-12 in selective hydrogenation of phenylacetylene (Figure 3). Phenylacetylene on Pd NPs can be hydrogenated partially to styrene or fully to ethylbenzene. For Pd/FDU-12, 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@FDU-12, the selectivity to styrene only decreases slightly with


10

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phenylacetylene conversion increasing. Pd/FDU-12 could afford selectivity to styrene of 94% at conversion of 78%, while up to 94% selectivity to styrene was obtained on Pd/PPh3@FDU-12 with conversion of 92%. The enhanced selectivity of Pd/PPh3@FDU-12 may be due to the weakened Pd-H bond and hindered adsorption of styrene on Pd surface, which is demonstrated previously by PPh3 modified Pd/TiO2.35,39-41 Pd/PPh3@FDU-12 with TOF of 560 h-1 is much less active than Pd/FDU-12 with TOF of 1840 h-1.

100

Pd

Pd

H2

H2

100

A

B

80

Conv. and Sel. (%)

80

Conv. and Sel. (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

60

40

Conv. Sel.

20

60 40 Conv. Sel.

20 0

0 0

20

40

60

80

0

20

40

60

80

T (min)

T (min)

Figure 3. Conversion and selectivity to styrene for phenylacetylene hydrogenation as a function of reaction time over (A) Pd/FDU-12 and (B) Pd/PPh3@FDU-12 (Reaction conditions: 25 oC, 1 bar H2, 1.8 mmol phenylacetylene, S/C = 1000, 5 mL EtOH). The activity of Pd/PPh3@FDU-12 is lower than that of Pd/FDU-12 in hydrogenation of ketones and phenylacetylene. In addition to weakened Pd-H bond on Pd/PPh3@FDU-12, the low activity of Pd/PPh3@FDU-12 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@FDU-12 was tested in hydrogenation of acetophenone, 4'-


11

ACS Catalysis

methylacetophenone, 4'-methoxyacetophenone, methyl 4-acetylbenzoate and 4'-(trifluoromethyl) acetophenone (Figure 4 and Table 2). Pd/PPh3@FDU-12 could efficiently catalyze the hydrogenation of the above substrates with selectivity to alcohol up to 99%. The catalytic activity of Pd/PPh3@FDU-12 decreases as the electron donating ability of substituent groups increases with the exception of methyl 4-acetylbenzoate 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 4-acetylbenzoate and 4'-(trifluoromethyl) acetophenone is possibly due to the steric hindrance in addition to their electronic properties.

120

90

TOF (h-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

30

0 -0.531

-0.535

-0.542

-0.524

-0.521

The nature charge of oxygen atom

Figure 4. Hydrogenation of ketones with electron donating or electron withdrawing groups over Pd/PPh3@FDU-12 (Reaction conditions: 2 mmol substrate, S/C = 25, 2 mL EtOH, 60 oC, 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).42-43 Pd/PPh3@FDU-12 could catalyze a wide scope of ketones, including 4'-methylacetophenone, 4'-methoxyacetophenone,

4'-tert-butylacetophenone,

4'-iso-butylacetophenone,

methyl

4-

acetylbenzoate and 4'-(trifluoromethyl)acetophenone to afford 99% selectivity to alcohols. Under


12

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ACS Catalysis

similar conditions, Pd/FDU-12 affords much lower selectivity to alcohols than Pd/PPh3@FDU12 though the activity of the former is higher. This is in a similar tendency to the hydrogenation of acetophenone. Both Pd/PPh3@FDU-12 and Pd/FDU-12 show low activity towards 4'-tertbutylacetophenone, 4'-iso-butylacetophenone and methyl 4-acetylbenzoate. This is possibly related with the steric hindrance of the substrates. Notably, up to 99% yield of 1-(4isobutylphenyl) ethanol, an important intermediate in the synthesis of ibuprofen44, was obtained over Pd/PPh3@FDU-12 in the hydrogenation of 4'-iso-butylacetophenone. With benzaldehyde and 4-anisaldehyde as substrates, Pd/PPh3@FDU-12 affords > 99 % selectivity to corresponding alcohol and Pd/FDU-12 gives toluene and 4-methylanisole as main products. For 4-formylbenzonitrile, Pd/PPh3@FDU-12 gives > 99 selectivity to 4(hydroxylmethyl)benzonitrile. However, Pd/FDU-12 only gives 54 % selectivity to alcohol under similar reaction conditions. The above results suggest that Pd/PPh3@FDU-12 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@FDU-12 and Pd/FDU-12 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@FDU-12 in 60 min (Figure S6). Under similar conditions, Pd/FDU-12 only gives conversion of 34 %. The TOF of Pd/PPh3@FDU-12 is ca. 5.5 fold that of Pd/FDU-12, 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@FDU-12 could still give TOF of 870 h-1 (360 h-1 for Pd/FDU-12) (Figure S7).


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Table 3. Catalytic performance of Pd/PPh3@FDU-12 and Pd/FDU-12 in the hydrogenation of nitrobenzene and its derivatives.a Substrates

Products

t (h)

Conv. (%)

Sel. (%) d

TOF (h-1) e

Ratio of TOFf

1

> 99 (34)

> 99 (97)

11020 (1990)

5.5

1

> 99 (30)

> 99 (90)

8640 (1800)

4.8

1

> 99 (27)

> 99 (78)

8570 (1590)

5.3

2.3

> 99 (29)

99 (97)

3580 (730)

4.9

3

> 99 (20)

> 99 (79)

2950 (640)

4.6

1.5

99 (7)

99 (54)

5460 (530)

10.3

2

> 99 (> 99)

> 99 (> 99)

6380 (3960)

1.6

b c

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/FDU-12; b 10 mL EtOH was used due to the low solubility of 4'-nitroacetophenone; c 2 mmol 4-nitrophenol, 10 mL H2O; d Selectivity to aromatic amines; e Apparent TOF was obtained with the conversion less than 30 %; f The ratio of apparent TOF of Pd/PPh3@FDU-12 to Pd/FDU-12. Pd/PPh3@FDU-12 indeed exhibits much higher activity than Pd/FDU-12 in hydrogenation reactions with electrophilic nitrobenzene as substrate. Based on kinetic studies, first-order dependence of H2 was observed for Pd/PPh3@FDU-12 and Pd/FDU-12 (Figure S8), suggesting the H2 dissociation on two Pd surface is unsaturated46-48 and PdHx may involve in the rate determining steps in the surface reaction according to the Langmuir-Hinshelwood 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/[email protected] Pd/PPh3@FDU-12 with electron-rich 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 Pd-based catalysts, such as Pd/ILs,52 Pd/PEG,53 Pd/PVPA,54 Pd/PPh3@FDU12 is the most active Pd-based solid catalyst ever reported for nitrobenzene hydrogenation (Table S2). Pd/PPh3@FDU-12 could efficiently catalyze the hydrogenation of different types of nitro compounds to corresponding amines with high activity and selectivity (Table 3). Even for ethyl 4-nitrobenzoate and p-nitroacetophenone with a bulky substituent group, ∼99% conversion with > 99% selectivity was achieved on Pd/PPh3@FDU-12. It should be mentioned that only nitro group was hydrogenated to amine groups with C=O groups almost unaffected. Under similar reaction conditions, Pd/FDU-12 affords very low conversion (20% ∼ 7%) and selectivity (79%∼ 54%). Pd/PPh3@FDU-12 gives higher selectivity than Pd/FDU-12 in the hydrogenation of substituted nitrobenzene except 4-nitrophenol. For all substrates tested, Pd/PPh3@FDU-12 shows much higher activity than Pd/FDU-12. For hydrogenation of nitrobenzene with electron donating substituents, such as para-, ortho-, and meta-nitrotoluene, the activity of Pd/PPh3@FDU-12 is ∼5 folds that of Pd/FDU-12. When p-nitroacetophenone with electron withdrawing substituent was used, the apparent TOF of Pd/PPh3@FDU-12 is 9 times higher than that of Pd/FDU-12. The above results indicate that the activity of Pd/PPh3@FDU-12 increases with electrophilicity of nitro compounds increasing, which is possibly related with its electron rich Pd surface. Though ethyl 4-nitrobenzoate also has electron-withdrawing substituents, the activity of Pd/PPh3@FDU12 is not boosted as much as in the case of p-nitroacetophenone. This possibly related with the steric effect. In comparison with para- and meta-nitrotoluene, the relatively lower activity of ortho-nitrotoluene may also be due to the steric effect.55


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The recycle stability of Pd/PPh3@FDU-12 was investigated using nitrobenzene hydrogenation as model reaction (Figure 5). The solid catalyst can be easily recovered by centrifugation after reaction. Pd/PPh3@FDU-12 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@FDU-12 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@FDU-12 catalyst in hydrogenation reactions.

Conv. %

a

Sel. %

100

80

80

60

60

40

40

20

20

0

1

2

3

4

5

Sel. (%)

100

Conv. (%)

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0

Cycles

Figure 5. Recycling stability of Pd/PPh3@FDU-12 in the hydrogenation of nitrobenzene. 3. CONCLUSION In summary, the catalytic activity and selectivity of Pd NPs were successfully tuned with triphenylphosphine cross-linked in the nanopore of FDU-12. The electron donating effect of PPh3 endows Pd NPs with electron-rich surface and high Pd0/Pd2+ ratio. Consequently, Pd/PPh3@FDU-12 shows exceptionally high selectivity to alcohols in hydrogenation of aldehyde/ketones and further hydrogenolysis of alcohols is suppressed due to weakened Pd-H 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 cross-linked in the nanopore of FDU-12 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. 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 Sigma-Aldrich 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(4-vinylphenyl)phosphine was prepared according to the literature report.26 FDU12 was synthesized according to the reported method.56 Synthesis

of

PPh3@FDU-12. 0.1 g of Tris(4-vinylphenyl)phosphine and 2, 2'-

azobisisobutyronitrile (AIBN, 3% relative to total vinyl group) were dissolved in 650 µL of N, N-dimethylformamide. Then, the above solution was added to 570 mg of FDU-12 activated by vacuum degassing at 120 oC for 3 h through the impregnation method. The mixture was subjected to freeze-vacuum-thaw 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@FDU-12. Synthesis of Pd/PPh3@FDU-12. The Pd/PPh3@FDU-12 was synthesized by conventional impregnation-reduction 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 mL-1) was added, after stirring overnight, 1 mL of NaBH4 aqueous solution (21.4 mg L-1) 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@FDU-12. Synthesis of Pd/FDU-12. For comparison, Pd/FDU-12 with particle size of 1.1 nm was prepared via strong electrostatic adsorption method.28-29 Specifically, 500 mg of FDU-12 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 was isolated by filtration and dried at 60 oC for 2 h. The as-synthesized 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 ×10-3 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 six times, the final pressure was adjusted to 4 bar and the reactor was heated to 60 oC 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 HP-5 capillary column (30 m × 0.32 mm × 0.25 µm). The procedure of nitrobenzene hydrogenation was similar with acetophenone hydrogenation except 8.33 ×10-4 mmol Pd (S/C= 6000) and 1 mL EtOH were used. Conversion and selectivity was determined using n-tetradecane or n-decane as internal standards. For 4-nitrophenol hydrogenation in water, 1,4-butanediol was used as the internal standard. The carbon balance of all the reactions is ~ 100%. 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 1-30 kV. The particle size and standard deviation were measured by counting randomly 200 nanoparticles based on HAADF-STEM images. N2 sorption isotherms were carried out on a Micromeritics ASAP2020 volumetric adsorption analyzer. FT-IR 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

31

P 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 magic-angle spin probe using a 4 mm ZrO2 rotor.

13

C signals were referenced to glycine (C2H5NO2). The Pd content was determined by

PLASAM-SPEC-II inductively coupled plasma atomic emission spectrometry (ICP-AES). The


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thermogravimetric analysis (TGA) was performed using a NETZSCH STA 449F3 analyzer from 30 to 900 oC with a heating rate of 10 oC min-1 under air or N2 atmosphere. CO chemisorption measurement was performed at 40 oC on a Quantachrome Autosorb-1/C chemisorb apparatus. Samples were treated in H2 atmosphere at 200 oC for 2 h before measurement. The metal dispersion and particle size were estimated according to previous report.57 In situ X-ray 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 × 10-7 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 FT-IR of CO adsorption were collected on a Bruker EQUINOX 55 infrared spectrometer with a DTGS detector. Before measurement, each catalyst was pretreated at 200 oC under flowing H2 atmosphere (20 mL min-1) 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 cm-1. Gasphase CO spectra were collected at the same pressure and subtracted from the corresponding sample spectra. H2-D2 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 min-1) at 200 oC for 30 min. After cooling down the sample in the liquid nitrogen, D2 (10 ml min-1) 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 H2-D2 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.

ASSOCIATED CONTENT Supporting Information. The TG analysis, FT-IR spectrum,

13

C CP/MAS NMR spectrum,

31

P MAS NMR spectrum and physical parameters of PPh3@FDU-12, XPS spectra of P 2p

results, TEM images of Pd/PPh3@FDU-12 and Pd/ FDU-12, 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 * E-mail: [email protected], * E-mail: [email protected]. Author Contributions Miao Guo did all the experiments and wrote the manuscript, He Li carried out the in situ FT-IR 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 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. REFERENCES 1． Nanoparticles and Catalysis, Astruc, D. Ed.; Wiley-VCH: Weinheim, 2008. 2． Serna, P.; Corma, A. Transforming Nano Metal Nonselective Particulates into Chemoselective Catalysts for Hydrogenation of Substituted Nitrobenzenes. ACS Catal. 2015, 5, 7114-7121. 3． Cárdenas-Lizana, F.; Gómez-Quero, S.; Hugon, A.; Delannoy, L.; Louis, C.; Keane, M. A. Pd-promoted Selective Gas Phase Hydrogenation of p-Chloronitrobenzene over Alumina Supported Au. J. Catal. 2009, 262, 235-243. 4． Enache, D. I.; Edwards J. K.; Landon P.; Solsona-Espriu B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Solvent-Free Oxidation of Primary Alcohols to Aldehydes using Au-Pd/TiO2 Catalysts. Science 2006, 311,362-365. 5． Yin, L. X.; Liebscher, J.; Carbon-Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts. Chem. Rev. 2007, 107, 133-173. 6． Somorjai, G. A.; Rioux, R. M. High Technology Catalysts towards 100 % Selectivity: Fabrication, Characterization and Reaction Studies. Catal. Today 2005, 100, 201-215.


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53． Harraz, F. A.; El-Hout, S. E.; Killa, H. M.; Ibrahim, I. A. Palladium Nanoparticles Stabilized by Polyethylene Glycol: Efficient, Recyclable Catalyst for Hydrogenation of Styrene and Nitrobenzene. J. Catal. 2012, 286, 184-192. 54． Xi, X. L.; Liu, Y. L.; Shi, J.; Cao, S. K. Palladium Complex of Poly(4-vinylpyridine-coacrylic acid) for Homogeneous Hydrogenation of Aromatic Nitro Compounds. J. Mol. Catal. A-Chem. 2003, 192, 1-7. 55． Yang, Q.; Chen, Y. Z.; Wang, Z. U.; Xu, Q.; Jiang, H. L. One-Pot Tandem Catalysis over Pd@MIL-101: Boosting the Efficiency of Nitro Compound Hydrogenation by Coupling with Ammonia Borane Dehydrogenation. Chem. Commun. 2015, 51, 10419-10422. 56． Fan, J.; Yu, C. Z.; Lei, J.; Zhang, Q.; Li, T. C.; T, B.; Zhou, W. Z.; Zhao, D. Y. LowTemperature Strategy to Synthesize Highly Ordered Mesoporous Silicas with Very Large Pores. J. Am. Chem. Soc. 2005, 127, 10794-10795. 57． Guo, M.; Li, C.; Yang, Q. H. Accelerated Catalytic Activity of Pd NPs Supported on Amine-rich Silica Hollow Nanospheres for Quinoline Hydrogenation. Catal. Sci. Technol. 2017, 7, 2221-2227. 58． Lee, A. F.; Ellis, C. V.; Wilson, K.; Hondow, N. S. In situ Studies of Titania-Supported Au Shell-Pd Core Nanoparticles for the Selective Aerobic Oxidation of Crotyl Alcohol. Catal. Today 2010, 157, 243-249. 59． Wang, J. J.; Li, G. N.; Li, Z. L.; Tang, Z. Z.; Feng, Z. C.; An, H. Y.; Liu, T. F.; Li, C. A highly selective and stable ZnO-ZrO2 solid solution catalyst for CO2 hydrogenation to methanol. Science Advances 2017, 3, e1701290.


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60． Bai, L. C.; Wang, X.; Chen, Q.; Ye, Y. F.; Zheng, H. Q.; Guo, J. H.; Yin, Y. D.; Gao, C. B. Explaining the Size Dependence in Platinum-Nanoparticle-Catalyzed Hydrogenation Reactions. Angew. Chem. Int. Ed. 2016, 55, 15656-15661.


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Improving catalytic hydrogenation performance of Pd nanoparticles by

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