Room-Temperature Tandem Condensation-Hydrogenation Catalyzed

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Article

Room-temperature tandem condensation-hydrogenation catalyzed by porous CN nanosheet-supported Pd nanoparticles 3

4

Renfeng Nie, Minda Chen, Yuchen Pei, Biying Zhang, Long Qi, Jingwen Chen, Tian Wei Goh, Zhiyuan Qi, Zhiguo Zhang, and Wenyu Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05531 • Publication Date (Web): 29 Dec 2018 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Sustainable Chemistry & Engineering

Room-temperature tandem condensation-hydrogenation catalyzed by porous C3N4 nanosheet-supported Pd nanoparticles

Renfeng Nie,*,a,b Minda Chen,b Yuchen Pei,c Biying Zhang,b Long Qi,c Jingwen Chen,b,d Tian Wei Goh,b Zhiyuan Qi,b Zhiguo Zhang,d and Wenyu Huang*,b,c a

School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062, China

b

Department of Chemistry, Iowa State University, Ames, IA 50011, USA

c

Ames Laboratory, US Department of Energy, Iowa State University, Ames, IA 50011, USA

d

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027,

China

Corresponding Authors *E-mail: refi[email protected]. *E-mail: [email protected].

ABSTRACT: Tandem catalysis, often inspired by biological systems, offers many advantages in the synthesis of highly functionalized small molecules. NH2-grafted porous C3N4 nanosheet with rich N species and surface area of 138 m2/g was fabricated via a NH3-exfoliation method. After supporting Pd nanoparticles (NPs), the resulting bifunctional catalyst (Pd/C3N4-NH2) could catalyze a one-step tandem condensation-hydrogenation reaction between ketones and nitriles to form α-alkylated nitriles at room temperature. Under 2 MPa H2 for 8 h, Pd/C3N4-NH2 could afford 99.7 % cyclohexanone conversion and 99.8 % selectivity without other side reactions, which is much higher than that of bulk C3N4 supported Pd. Compared with commonly used two-step processes, this one-step tandem reaction could largely shorten reaction time and allow condensation-hydrogenation take place at room temperature. We also found that the hydrogenation step accelerates the condensation step, which increased the overall efficiency of 1

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the tandem transformation. This catalytic system demonstrates the design of a bifunctional catalyst with cooperative sites that allow the tandem reaction takes place in one-step under mild conditions.

KEYWORDS: Tandem reaction, Palladium, Knoevenagel condensation, Hydrogenation, One-step

INTRODUCTION Tandem reactions are of great importance in fine chemical and pharmaceutical industry.1,2 Tandem processes allow multistep reactions to take place without the need for separating reaction intermediates, which achieves overall efficiency by reducing wastes, solvents, and operation steps. For example, α-alkylated nitriles are crucial building blocks for the synthesis of a variety of organic molecules and biologically active compounds.3,4 Traditional homogeneous base-catalyzed condensation of nitriles and alkyl halides involves either costly reactants or explosive bases such as NaH and NaNH2.5 The homogeneous catalytic process also generates a large amount of waste that is unsustainable from the viewpoint of green chemistry. Alternatively, direct tandem condensation-hydrogenation of ketones or aldehydes with nitriles using environmentally friendly H2 to produce α-alkylated nitriles is attractive.6,7 Generally, this reaction was carried out via two steps,8 condensation to generate α-alkenyl nitriles that were subsequently hydrogenated to α-alkylated nitriles. The first step is catalyzed by basic sites (Brønsted base), while the second step relies on metal sites for hydrogenation. In order to operate this tandem reaction in one-step, the catalyst should contain two different active sites (bifunctional) that could cooperatively catalyze the condensation and hydrogenation with appropriate rates to achieve high activity and selectivity. Bifunctional catalysts have attracted great attention for tandem reactions.9-11 There are many types of materials suitable for fabricating bifunctional catalysts, among which silica, zeolites and MOFs are the most frequently used.10-13 For example, Song et al. prepared bifunctional 2


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condensation-hydrogenation catalysts by depositing Pd NPs on the surface of aminopropyl functionalized mesoporous silica.6 Zhao et al. prepared a bifunctional catalyst containing both basic and metal sites by encapsulating Pd NPs in a metal-organic framework.14 However, the fragile structure of grafted groups and MOFs under reaction conditions,15 or the weak interaction of NPs with supports16 can lead to aggregation or leaching of active sites, which inevitably hampers their catalytic properties. One significant difficulty associated with tandem condensation-hydrogenation of ketones or aldehydes is the easy hydrogenation of these substrates under the H2 atmosphere,17 and the alcohol products are difficult to react with nitriles to generate α-alkylated nitriles. Therefore, for nearly all reported tandem reactions,3,6-9,18 multistep processes (e.g., condensation and hydrogenation were operated successively) and high reaction temperature (~80 oC) are essential to achieving a high yield of desired products, which results in low atomic efficiency, multistep processing, and high energy consumption. Therefore, it is highly desirable to develop multifunctional catalysts that can catalyze tandem reactions in one-step simultaneously at room temperature. Carbon nitride (C3N4) is one of the promising earth-abundant, chemically stable materials that have been applied in many photocatalytic reactions such as water splitting, CO2 reduction, and organic waste degradation.19-22 The special 2D structure and rich N species (Brønsted base sites) of C3N4 are beneficial for base-catalyzed reaction and anchoring/activating metal NPs due to their increased π-electron density, thus preventing particles from aggregation and improving their catalytic performance.23-25 Herein, in order to realize the condensation-hydrogenation reaction via a one-step routine, porous C3N4 nanosheets with high surface area and rich amino groups were prepared by a facile thermal treatment.26 After supporting Pd NPs, the bifunctional Pd/C3N4 catalyzed ketones with nitriles to α-alkylated nitriles via a one-step tandem routine under H2 atmosphere at room temperature. We also performed a kinetic study to compare the reaction rate of the condensation reaction with and without H2. Interestingly, we found that the condensation reaction rate is much higher in the presence of H2, which means that the 3



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hydrogenation reaction promotes the condensation reaction. Therefore, performing the condensation-hydrogenation reaction in one step is more efficient than running the reaction separately. Moreover, high temperature (>50 oC) results in the hydrogenation of nitrile that terminates the tandem reaction. This catalyst is recyclable and could be reused for 5 times.

Traditional method5

R1

CN

R2

X

+

R3 Previous studies6-8 R 1

This study

R2 CN

+

CN

R2

R1

R3

CN

Step 1 R2 Basic catalyst o R3 80-90 C, 0.5-12 hR 3 O

R2 R1

NaH 65 oC, 5.5 h

O

+

R3

Scheme 1. Room-temperature condensation-hydrogenation.

One-step Pd/C3N4-NH2, 23 oC 2 MPa, 8 h

synthesis

of

Step 2 R1 R2 Metal catalyst, 60-80 oC 0.1-1 MPa H2, 0.5-12 h CN R3 R2

R1

R3

CN

α-alkylated

nitriles

R1 CN

via

one-step

EXPERIMENTAL SECTION Materials Melamine (AR, Sigma-Aldrich); H2PdCl4 solution (5 mM) prepared by dissolving 22.25 mg PdCl2 in 25 mL 10 mM HCl solution; 5%Pd/C (Aldrich); 5%Pt/C (Alfa Aesar); 5%Ru/C (Engelhard); 5%Pd/Al2O3 (Aldrich); 5%Pd/SiO2; Lindlar catalyst (Sigma-Aldrich). Other chemicals were of analytical purity and were used as received. Preparation of C3N4-NH2 sheet 10 g melamine was added into a crucible and transferred into a muffle furnace. The powder was heated to 520 oC at a heating rate of 2 °C min−1 under an air atmosphere and kept for 2 h. The obtained sample was named as C3N4. Then, 5 g C3N4 powder was transferred into a tube furnace and heated to 520 oC at a heating rate of 5 °C min−1 under air flow (200 mL/min) and kept for 8 h. The obtained powder was loaded into a tube furnace and heated to 540 °C at a heating rate of 5 °C min−1 under NH3 flow (50 mL/min) and kept for 1 h, the resulting sample was named as C3N4-NH2. 4


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Preparation of Pd/C3N4-NH2 One gram of C3N4-NH2 was dispersed in 50 mL DI water, and 4.7 mL H2PdCl4 solution was added into above suspension. The mixture was sonicated for 30 min and then stirred for 1 h, after which the suspension was transferred into a Teflon-lined autoclave. The suspension was treated under room temperature, 2MPa H2 for 8h. The solid was collected by centrifugation and washed by DI water until free of Cl-. After dried under vacuum, the catalyst was named as Pd/C3N4-NH2. Comparatively, Pd/C3N4 catalyst was prepared by the same procedure. The Pd contents in Pd/C3N4-NH2 and Pd/C3N4 were 2.4 and 2.3 wt% measured by inductively coupled plasma mass spectrometry (ICP-MS). Catalyst evaluation. For one-step condensation-hydrogenation reaction, 0.1 mmol cyclohexanone, 0.2 mmol malononitrile, 2 mL EtOH and 5 mg Pd/C3N4-NH2 were added into a stainless steel autoclave with a Teflon liner. The autoclave was sealed, purged and pressurized with hydrogen to 2 MPa, and then stirred at room temperature with a rate of 1000 rpm. After the reaction, the catalyst was recovered by centrifugation and the supernatant was quantitatively analyzed by a gas chromatography (Hewlett Packard 5890 II, FID detector) equipped with a HP-5 capillary column. Xylene was used as the internal standard. The identification of products was conducted by using an Agilent 6890N/5975 GC-MS system. The two-step condensation-hydrogenation was operated by a similar procedure, in which the condensation was conducted firstly under ambient atmosphere, then hydrogen was introduced for hydrogenation.

RESULTS AND DISCUSSION Characterization of catalysts Bulk C3N4 is of compact morphology which is constructed by layered carbon nitride.27 In order to increase the surface area and exposed N sites, exfoliation of bulk C3N4 under different atmosphere is conducted. Air can exfoliate the bulk C3N4 slightly, while NH3 can not only exfoliate but also graft N dopants on the surface of C3N4 sheets.26 5



The nitrogen isotherm (Figure 1) shows that different atmosphere has a considerable influence on the pore-structure of the samples. The isotherm and pore-size distribution curves show that, apart from a small hysteresis loop at a relative pressure higher than 0.90, nearly no uprising in range of low relative pressure is observed for bulk C3N4. NH3-treatment favors the macropore generation, indicated by the loop elevation in the relative pressure of 0.7-1.0.28 C3N4-NH2 treated with NH3 gives isotherm elevation at both low and high relative pressure, indicating the emergence of hierarchical pore-structure.29 As expected, the pore volumes of C3N4 and C3N4-NH2 are calculated to be 0.078 and 0.550 cm³/g, respectively (Table S1). The BET surface area increases from 16 for C3N4 to 138 m²/g for C3N4-NH2. The digital photograph (Figure S1) shows that C3N4-NH2 has a much lower density than that of bulk C3N4. The XRD patterns (Figure S2) show that the original bulk C3N4 has strong C(001) and C(002) diffractions. After NH3 treatment, these peaks weaken. These results indicate that bulk C3N4 is exfoliated by

3

400

dV/dD Pore Volume (cm /g)

NH3 under high temperature.30

350 300 3

Volume (cm /g)

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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250 200 150 100

0.8

C3N4 C3N4-NH2

0.6 0.4 0.2 0.0

1

10 D (nm)

100

50 0 0.0

0.2

0.4

0.6 p/p0

0.8

1.0

Figure 1. N2 sorption isotherms of C3N4-based materials. Insert: the corresponding pore size distributions.

6


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C3N4-NH2 Pd/C3N4-NH2 C3N4

Intensity (a.u.)

Pd/C3N4

3600

3200

2800

1500

1000

-1

Wavelength (cm )

Figure 2. FTIR analysis of different C3N4-based materials. To probe the chemical change of the C3N4 during the NH3 treatment, FTIR studies were carried out (Figure 2). The ranges of 3000-3500 cm-1 and 1000-1750 cm-1 could be ascribed to the vibration of N-H and tri-s-triazine groups.26 After heat treatment, these signals strengthen significantly, indicating the efficient exfoliation and expose of N species of C3N4.26 The C1s and N1s XPS spectra (Figure S3, Table S2 and Table S3) show that C3N4-NH2 possesses a higher N content (22.8%) than that of C3N4 (15.0%) before the NH3 treatment, agreed with the FTIR results.

Pd/C3N4 Pd/C3N4-NH2

C(002)

Intensity (a.u.)

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


Pd(111) C(001)

10

20

30

40

50 o

2 Theta ( )

60

70

80

Figure 3. XRD patterns of C3N4- and C3N4-NH2-supported Pd catalysts.

7



(c)

(d)

40 Frequency (%)

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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200 nm

30

Mean size=7.1 nm

20 10 0

4

8 12 Diameter (nm)

16

50 nm

Figure 4. TEM images of (a, b) Pd/C3N4-NH2 and (c, d) Pd/C3N4 catalysts. Considering the high surface area and rich N species on C3N4-NH2, we use it as the support for Pd. After loading Pd NPs, the N2-isotherm (Figure S4) of Pd/C3N4-NH2 is similar as the support along, and only 14% decrease of BET surface area (from 138 to 118 m2/g) occurs due to the partial pore occupation by Pd NPs. The XRD pattern (Figure 3) shows that Pd/C3N4-NH2 gives rise to weak Pd diffractions in comparison to Pd/C3N4. The crystalline sizes of Pd were calculated by Scherrer equation, in which Pd/C3N4-NH2 and Pd/C3N4 afford Pd NPs of 3.5 and 4.7 nm in diameter, respectively. TEM images (Figure 4) of C3N4 show a character of bulk and thick morphology, and the Pd NPs dispersed unevenly on C3N4. Comparatively, C3N4-NH2 shows a sheet-like morphology, on which Pd NPs are highly dispersed without significant aggregation. From these TEM images, the mean diameters of Pd NPs in Pd/C3N4-NH2 and Pd/C3N4 are 3.3 and 7.1 nm, respectively. Meanwhile, loading Pd NPs weakens the vibration of N species (as shown in Figure 2). CO chemisorption results (Table S3) show that the Pd dispersions of Pd/C3N4-NH2 and Pd/C3N4 are 28.9 and 16.7 %, respectively, further indicating 8


Page 9 of 20

exfoliation of bulk C3N4 favors the dispersion of Pd NPs. Pd 3d XPS (Figure S5) shows that 70 % Pd exists as metallic state on both C3N4 and C3N4-NH2. CO2-TPD profiles (Figure S6) show that after exfoliation the desorption signal of Pd/C3N4-NH2 strengthens obviously, and the concentrations of basic sites on Pd/C3N4 and Pd/C3N4-NH2 are 42.0 and 72.9 μmol/g (Table S4), respectively.

One-step condensation-hydrogenation over Pd catalysts Considering the basicity and hydrogenation capacity of Pd/C3N4-NH2, one-step condensation-hydrogenation reaction between cyclohexanone and malononitrile was adopted as a model reaction. The first step occurs via Knoevenagel condensation between cyclohexanone and malononitrile over basic sites, which gives rise to an intermediate product P1.8 The second step involves the hydrogenation of P1 to the final product, α-alkylated nitrile. However, if cyclohexanone is firstly hydrogenated to cyclohexanol, the condensation reaction with malononitrile will not proceed under the current reaction conditions. NC

O

+

CN CN

CN NC

One-pot R.T.

CN

+ P1

OH

+ P3

P2

100 80 Composition (%)

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


Cyclohexanone P1 P2 P3

60 40 20 0 0

1

2

3

4

5 t(h)

6

7

8

9

Figure 5. Time-activity profile of Pd/C3N4-NH2 for the one-step condensation-hydrogenation reaction. Reaction conditions: Pd/C3N4-NH2 (5mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 2 MPa H2, 23 oC. As shown in Figure 5, the evolution of one-step condensation-hydrogenation reaction was 9



carried out at room temperature under H2 atmosphere. In the initial stage, the tandem reaction takes place at a relatively fast rate. For example, 97.6% cyclohexanone could be consumed in 2 h, and P1 reached its maximum yield (36.4%). Further increasing the reaction time leads to hydrogenation of P1, and 99.5% yield of P2 could be obtained at 8 h. NC

O

+

CN

1st step R.T.

CN CN

NC

CN

2nd step H2 P1

100

P2

Cyclohexanone P1 P2

Condensation

80 Composition (%)

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 Introducing 2MPa H2

40 20 0

0

1

2

3

4

5

6 t (h)

7

8

9

10 11 12

Figure 6. Time-activity profile of Pd/C3N4-NH2 for the two-step condensation-hydrogenation reaction. Reaction conditions: Pd/C3N4-NH2 (5mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 23 oC; second step: 2 MPa H2. Comparatively, the condensation-hydrogenation reaction could also be conducted via a two-step procedure (Figure 6). In the first step (without H2 atmosphere), cyclohexanone was slowly consumed, and only 28.2% cyclohexanone was converted to P1 after 4 h. After that, hydrogen was introduced and cyclohexanone concentration dropped quickly. The maximum of P1 was achieved at 6 h, and > 99% P2 yield could be obtained when the time was prolonged to 12 h. These results indicate that Knoevenagel condensation is equilibrium limited at room temperature. After introducing pressurized hydrogen, the product (P1) of the first step could be hydrogenated by Pd, the continuous removal of P1 would drive the first step faster to completion. That is to say hydrogen accelerates the whole reaction (especially for the first step), in which the quickly consumed P1 will favor the cyclohexanone-malononitrile condensation. More 10


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importantly, the one-step is more efficient and convenient than the two-step process, and no byproducts were detected during the whole process. Table 1. One-step condensation-hydrogenation towards cyclohexanone over different catalysts. a NC

O

+

CN CN

CN NC

One-pot R.T.

+

1 2 3 4 5 6 7 8 9 10b 11c 12c 13c 14c 15c 16c 17d 18d

Catalyst C3N4 C3N4 C3N4-NH2 C3N4-NH2 Pd/C3N4-NH2 Pd/C3N4-NH2 Pd/C3N4 Pd/C3N4-NH2 Pd/C3N4 Pd/C3N4-NH2 Pd/C Pd/Al2O3 Pd/SiO2 Lindlar catalyst Pt/C Ru/C Pd/C3N4-NH2 Pd/C3N4-NH2

OH

+

P1

Entry

CN

P3

P2

T (h)

H2 (MPa)

Con. (%)

Selectivity (%) P1

P2

P3

P2 Yield (%)

4 8 4 8 4 4 4 8 8 4 4 4 4 4 4 4 2 4

0 0 0 0 0 2 2 2 2 0 2 2 2 2 2 2 2 2

15.0 28.1 33.9 44.8 28.2 98.3 68.1 99.7 87.4 26.7 77.7 76.2 40.1 0.5 72.4 2.7 93.6 93.5

100 100 100 100 100 8.5 85.5 0.2 70.5 100 86.9 26.2 40.8 90.7 46.6 trace trace

0 0 0 0 0 91.5 14.5 99.8 29.5 0 11.0 72.9 57.7 3.7 Trace 99.9 99.9

0 0 0 0 0 0 0 0 0 0 2.1 0.9 1.9 5.6 53.4 0 0

0.0 0.0 0.0 0.0 0.0 89.9 9.9 99.5 25.8 0.0 8.5 55.5 23.1 2.7 93.5 93.4

a

Reaction conditions: Catalyst (5 mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 23 oC. b Replacing H2 with 2 MPa N2. c 2.5 mg catalyst was used. d 50 oC.

One-step condensation-hydrogenation was carried out over different catalysts and summarized in Table 1. In control experiments, supports could catalyze the first step with a very slow rate (entries 1-4). For example, C3N4 and C3N4-NH2 afford 28.1 and 44.8 % cyclohexanone conversions, respectively, with P1 as a sole product at 8h. The higher activity of C3N4-NH2 for condensation could be ascribed from its high surface area and more exposed N species, as indicated by N2 adsorption (Figure 1) and FTIR (Figure 2) analysis. Loading Pd on the surface of 11



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C3N4-NH2 will endow it with bi-function but slightly decrease the condensation activity without H2 (entry 5). Under 2 MPa H2 for 4h (entry 6), 98.3% conversion and 91.5% P2 selectivity could be obtained over Pd/C3N4-NH2, which are much higher than those of Pd/C3N4 (entry 7). Further prolong the reaction time to 8 h (entries 8 and 9), the P2 yields over Pd/C3N4 and Pd/C3N4-NH2 reach 25.8 and 99.5%, respectively. The high performance of Pd/C3N4-NH2 could be ascribed from the high surface area and rich N species that favor the adsorption of reaction substrates (Figure S7 and S8). For example, the adsorption capacities of cyclohexanone and malononitrile over C3N4-NH2 are 0.69 and 1.21 mmol/g, respectively, which is 1.6 and 1.4 times higher than those on C3N4. Replacing H2 with N2 (entry 10) affords similar result as entry 5. Increasing Pd loading also decreases its activity (Table S5), and only 39.5 % P2 yield could be observed over 5% Pd/C3N4-NH2. Comparatively, commercial Pd/C affords only 8.5 % P2 yield along with 2.1% cyclohexanol selectivity (entry 11). Other commercial Pd catalysts (entries 12 and 13), such as Pd/Al2O3 and Pd/SiO2, afford 55.5 and 23.1% P2 yield, respectively, which are accompanied with the formation of cyclohexanol. Nearly no conversion could be observed for Lindlar catalyst (entry 14). Other metals (entries 15 and 16), such as Pt and Ru, afford low selectivity of P2 under the same condition. Although higher temperature will significantly accelerate the reaction rate, cyclohexanone is hard to be fully transformed due to the undesirable hydrogenation of malononitrile to an inert enamine (entries 17 and 18, Figure S9). As shown in Figure 7, 87.9% conversion could be obtained under 50 oC for 1 h, further increasing the time to 8 h could only afford ~93.5% conversion. In another controlled experiment, only 82.5% conversion could be obtained at 8h when using 1.5 equiv. malononitrile. These results indicate that room temperature is

ideal

for

achieving

a

high

yield

of

the

desired

product

for

the

one-step

condensation-hydrogenation reaction.

12


Page 13 of 20

Conv.& yield (%)

100 80 o

Conv. at 23 C o P2 yield at 23 C o Conv. at 50 C o P2 yield at 50 C o b Conv. at 50 C o b P2 yield at 50 C

60 40 20 0

0

1

2

3

4

5

6

7

8

t(h)

Figure 7. Time-activity profiles of Pd/C3N4-NH2 under different temperature for the one-step condensation-hydrogenation reaction.a a Reaction conditions: Pd/C3N4-NH2 (5mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 2 MPa H2. b 0.15 mmol malononitrile was used.

Conv.&selec.&yield (%)

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


conv.

100

P2 selec.

P2 yield

80 60 40 20 0 0.5

1.0 2.0 Hydrogen pressure (MPa)

3.0

Figure 8. Hydrogen pressure effect on the one-step condensation-hydrogenation reaction. Reaction conditions: Pd/C3N4-NH2 (5mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 23 oC, 2 h. Hydrogen pressure has a considerable effect on product yield and distribution (Figure 8). Using Pd/C3N4-NH2 as the catalyst, low pressure of H2 (0.5 MPa) will decrease the P2 yield, and a long time is needed to achieve a high yield of P2 (Table S6). On the other hand, reaction under 3 MPa H2 leads to undesired hydrogenation of malononitrile, which results in a low conversion of cyclohexanone (93.3%). Increasing catalyst usage slightly favors the cyclohexanone transformation (Figure S10).

13



Page 14 of 20

Table 2. One-step condensation-hydrogenation towards ketones and nitriles.a O

CN

+

R1

R3

R2

Entry

Sub 1

One-pot R.T.

NC

R3

R1

R2

Sub 2

Product NC

O

1

CN

NC

2

NC

O

3

OOCH3

NC

O

4

CN

NC

5 6 7

CN

CN

CN

NC

CN

CN

NC

O

8

CN

NC

CN CN

8

92.2

78.3

8

95.3

>99.0

8

99.1

98.4

8

99.5

94.1

16

84.3

71.7

16

75.9

59.1

16

88.9

78.6

16

99.4

91.3

CN

CN

O

9

NC

CN

O

>99.0

CN

CN CN

O

>99.0

CN

CN

O

8

OCOCH3

CN

Selec. (%)

CN

CN CN

Conv. (%)

CN

CN

O

t(h)

CN

Reaction conditions: catalyst (5 mg), sub1 (0.1 mmol), sub2 (0.2 mmol), EtOH (2mL), 23 oC, 2MPa H2. In order to demonstrate the versatility of the one-step condensation-hydrogenation over Pd/C3N4-NH2, other ketones and nitriles are used as the substrates (Table 2). As for 2-methylcyclohexanon (entry 2), slightly decreased conversion (92.2 %) and selectivity (78.3 %) to the corresponding α-alkylated nitrile could be obtained. Using ethyl cyanoacetate to replace the malononitrile (entry 3), 95.3% conversion and >99.0% selectivity could be obtained at 8 h. Cyclohexen-1-one (entry 4) affords similar result as cyclohexanon (entry 1). Apart from 14


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pentacyclic cyclopentanone (entry 5), chain ketones (entries 6 and 7) could also afford relatively high conversion and selectivity. Acetophenone (entry 8) could be transformed with 88.9 % conversion and 78.6 % selectivity. As for benzophenone (entry 9), the conversion and selectivity could reach 99.4 and 91.3 %, respectively. Catalytic stability is a crucial issue for the real industrial application of the catalyst. As shown in Figure S11, Pd/C3N4-NH2 could be easily recovered, full conversion as well as constant P2 selectivity could be observed after five times reuse. In order to further study the stability of Pd/C3N4-NH2, low conversion (~60 %) and 42 % P2 selectivity could be obtained after the 5th, showing no significant decrease in comparison to the fresh catalyst (Figure 9). The XRD patterns (Figure S12) of fresh and recycled catalysts show that the crystalline state of Pd does not change significantly. TEM images of recycled catalyst (Figure S13) show that the mean diameter of Pd NPs increases to 5.8 nm, indicating the partial agglomeration of NPs during the reaction. N1s XPS spectra (Figure S14) show recycled Pd/C3N4-NH2 contains similar types of N species as fresh one. CO2-TPD signal of recycled Pd/C3N4-NH2 (Figure S6) weakens slightly, and the basicity concentration of fresh Pd/C3N4-NH2 and reused Pd/C3N4-NH2 are 72.9 and 64.4 μmol/g (Table S4), respectively, indicating the relatively stable basic sites on C3N4-NH2.

P2

100

P1

100

80

80

60

60

40

40

20

20

0

1

2

3 Run

4

5

Conversion (%)

Selectivity (%)

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


0

Figure 9. Recycling of Pd/C3N4-NH2 for one-step condensation-hydrogenation reaction. Reaction conditions: catalyst (5 mg), cyclohexanone (0.1 mmol), malononitrile (0.2 mmol), EtOH (2mL), 2 MPa H2, 23 oC, 1 h. 15



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CONCLUSIONS C3N4 nanosheets with high-surface-area were synthesized via a NH3-exfoliation method and used as support for Pd NPs. This catalyst (Pd/C3N4-NH2) was used as a bifunctional catalyst for the one-step condensation-hydrogenation of ketones with nitriles to α-alkylated nitriles at room temperature. It is interesting to observe that the condensation reaction was promoted by the hydrogenation reaction, which renders the one-step tandem reaction more efficient than the commonly used two-step processes. Under 2 MPa H2 for 8 h, Pd/C3N4-NH2 afforded 99.7% cyclohexanone conversion with 99.8 % selectivity of α-alkylated nitrile, which is much higher than those of Pd/C3N4. However, high reaction temperature favors the hydrogenation of nitrile that will terminate the tandem reaction. This catalyst is stable and recyclable 5 times. This work demonstrated a cost-effective and environmentally friend method that could help the catalyst development for other tandem reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Pore structure; digital photographs; XRD patterns; C1s and N1s XPS spectra; surface composition; element composition; N2 sorption isotherms and the size distributions; Pd 3d XPS spectra; CO2-TPD profiles; UV-vis spectra; saturated adsorption amount of cyclohexanone and malononitrile; MS analysis of byproduct from hydrogenation of malononitrile; catalyst loading effect; recycling of Pd/C3N4-NH2 (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: refi[email protected] *E-mail: [email protected]

Author Contributions 16


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R.N. and W. H. designed the project and wrote the manuscript. R.N. performed the majority of experiments. The other authors involved in the characterization and discussion during the writing of this manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21603066). This work was also supported by Iowa State University. R.N. thanks the China Scholarship Council and Hubei Chenguang Talented Youth Development Foundation for financial support.

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TOC/Abstract Graphic

Porous C3N4 nanosheet-supported Pd nanoparticles are efficient tandem catalysts for one-step room temperature condensation-hydrogenation of ketones and nitriles to α-alkylated nitriles.

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Room-Temperature Tandem Condensation-Hydrogenation Catalyzed

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