Ultrathin Nickel-Based Metal-Organic Framework Nanosheets as

Subscriber access provided by University of South Dakota

Article

Ultrathin Nickel-Based Metal-Organic Framework Nanosheets as Reusable Heterogeneous Catalyst for Ethylene Dimerization Yanping Hu, Ying Zhang, Yang Han, Donghai Sheng, Dongming Shan, Xiangyun Liu, and Achao Cheng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01762 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 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 Applied Nano Materials

Ultrathin Nickel-Based Metal-Organic Framework Nanosheets as Reusable Heterogeneous Catalyst for Ethylene Dimerization

Yanping Hu,§ Ying Zhang,*,‡,§ Yang Han,§ Donghai Sheng,§ Dongming Shan,§ Xiangyun Liu,§ and Achao Cheng§ ‡ The §

State Key Laboratory of Heavy Oil Processing, Beijing, 102249, China

Department of Materials Science and Engineering, China University of Petroleum (Beijing),

Changping District, Beijing, 102249, China

ABSTRACT: Ultrathin nickel-based metal-organic framework nanosheets (Ni-UMOFNs) were formed from NiCl2·6H2O and benzene-1,4-dicarboxylate through a sonication exfoliation method. In Ni-UMOFNs, the Ni atoms are octahedrally coordinated by six O atoms to form pseudo octahedrals which are further edge/corner connected with each other to form 2D layers separated by benzene-1,4-dicarboxylate linkers. On the Ni-UMOFNs surfaces, Ni centers are partially five coordinated owing to the termination of benzene-1,4-dicarboxylate ligands, and these coordinated unsaturated metal sites can be used as catalytic active sites. In this work, Ni-UMOFNs were utilized as ethylene dimerization catalysts in the presence of an alkylaluminum cocatalyst for the first time. The Ni-UMOFNs catalytic system showed moderate catalytic activity but high selectivity to 1-butene up to 92.8% under optimal conditions. The optimal Ni-UMOFNs catalyst can be reused at least four times without considerable loss in the catalytic activity and selectivity to 1-butene. Extraordinarily, the outstanding level of reusable property is due to continuous exposure of fresh unsaturated metal active sites when the original metal centers on the catalyst surface are used and exfoliated, just like a snake’s molting. KEYWORDS: Metal-organic framework, Ethylene dimerization, Ultrathin nanosheets, Reusable heterogeneous catalyst, Nickel-based catalyst, 1-Butene

1. INTRODUCTION Ethylene dimerization is a major industrial process for producing high-demand 1 ‑ butene[1] as comonomer for the production of linear low-density polyethylene. Although the commercial

ACS Paragon Plus Environment

ACS Applied Nano Materials 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

Page 2 of 19

dimerization process adopts a homogenous modified Ziegler catalyst system, the research on environmentally friendly heterogeneous catalysts has been in progress due to their easy post reaction separation, reusability and relatively high stability. Ni-based solid catalysts are among the most important heterogeneous catalysts for ethylene oligomerization, especially for ethylene dimerization. Unfortunately, the heterogeneous Ni-based catalysts with excellent reusable properties are very rare. The original heterogeneous Ni-based catalysts for ethylene dimerization including NiO/Al2O3[2] showed a low ethylene dimerization activity of 190 h-1, although the selectivity to 1-butene reached 85.4%. Ni-exchanged molecular sieves, such as Ni-MCM-36, Ni-MCM-48, Ni-MCM-41 and Ni-Y[3-6], showed enhanced catalytic activity, but relatively poor selectivity for dimerization. For example, Ni-MCM-48 catalyzed the ethylene oligomerization to reach the activity of 47400 h-1, with butene typically comprising only 42% of the resulting products; Ni-Y showed a high catalytic activity of 10482 h-1, but a C4 selectivity of 67%. Metal−organic frameworks (MOFs) with well-defined crystal structures insisting of discrete organic linkers and metal-ion/cluster nodes have recently emerged as highly promising heterogeneous catalysts, benefiting from their tunable porosity, high specific surface area as well as diversity in functional species of metal centers and organic linkers[7]. Many bulk MOFs have been reported to be used in the olefin oligomerization either as catalyst supports or directly as active catalysts. Taking catalyst supports for example, Canivet et al.[8] used a one-post functionalization method to immobilize Ni(2-PyCHO)Cl2 into NH2-MIL(Fe)-101 generating a very active and reusable catalyst Ni@(Fe)MIL-101 for the liquid-phase ethylene dimerization with the highest activity of 20900 h-1. It can be reused for at least two more catalytic runs without significant loss of activity or selectivity after careful washing with anhydrous ethanol and drying. Madrahimov et al.[9] adopted NU-1000 with large pore channel (31 Å) as support to form highly active ethylene dimerization catalyst NU-1000-bpy-NiCl2. This catalyst had an excellent catalytic activity for ethylene dimerization and could be reused at least three times, but it caused polymeric deposits that led to catalyst deactivation. Liu et al.[10] prepared Ni-MixMOF catalysts for ethylene dimerization, and these catalysts displayed excellent activity up to 16400 h-1, and high selectivity to C4 up to 92.7%, but the reusable property has not been reported. Our group[11] directly modified a diimine ligand with NiCl2, and then used the modified ligand (LNi) for further MOF [Zn3(OH)2(LNi)2] preparation. Zn3(OH)2(LNi)2 exhibited high catalytic activity of 6.7×105 g/(mol Ni⋅h⋅atm) and high C4 selectivity of 91.8%. In addition, bulk MOFs with coordinatively unsaturated sites have been directly used as heterogeneous catalysts for olefin oligomerization. Metzger et al.[12] demonstrated that Ni-MFU-4l showed high activity in ethylene dimerization up to 41500 h-1, and the selectivity for 1-butene was


Page 3 of 19 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


96.2%. Importantly, it could be cycled continuously over 72 h (8 times) with a minor decrease in catalyst activity only if the Ni-MFU-4l/MAO slurry did not expose to the atmosphere. Mlinar et al.[13] examined two Ni-containing MOF-74 catalysts used for propene oligomerization in the gas phase under 5 bar at 453 K, and the conversion could reach 2.18% and propene dimers comprised >95% the products with the rest being trimmers. Li and co-workers[14] presented an atomic layer deposition method (ALD) for installing Ni ions uniformly and precisely on the nodes of Zr-based NU-1000 (Ni-AIM), and Ni-AIM became active for ethylene oligomerization at 45°C and 2 bar with the main products being C8 (46~59%) other than the common C4 oligomers. Our group[15] adopted a Cr-based MIL-100 to catalyze selective ethylene oligomerization with very high selectivity to low carbon oligomer C6, C8 and C10. Compared with the common bulk MOFs, their nanosheet counterparts possess nanometer thickness to allow rapid mass transport, charge transfer and extremely high percentage of exposed catalytic active surface to ensure better catalytic activity[16-28]. For example, Zhao et al. synthesized ultrathin Ni-Co-MOFs nanosheets and found that their electrocatalytic activity was significantly higher than that of the bulk MOFs with the same crystal structure[29]. In this work, the catalytic properties of ultrathin nickel-based MOF nanosheets (Ni-UMOFNs) for the ethylene dimerization have been explored. The Ni-UMOFNs catalytic system showed moderate catalytic activity but high selectivity to 1-butene up to 92.8% under optimal conditions. Moreover, the Ni-UMOFNs showed about twice as much catalytic activity as the bulk Ni-MOF counterpart. Extraordinarily, Ni-UMOFNs could be reused at least four times without considerable loss in the catalytic activity and selectivity to 1-butene. The outstanding level of reusable property is due to continuous exposure of fresh unsaturated metal active sites when the original metal centers on the catalyst surface are used, just like a snake’s molting, as shown in Scheme 1.



Scheme 1

Page 4 of 19

Catalytic recycling of ethylene dimerization on Ni-UMOFNs catalysts

2. EXPERIMENTAL SECTION 2.1. Materials. NiCl2·6H2O (99.9%, AR, grade) and benzene-1,4-dicarboxylate acid were bought from Sinopharm Chemical Reagent; triethylamine, AlEt2Cl and N,N-dimethylformamide were bought from Aladdin Reagent; All chemicals were used directly without further purification; Toluene was bought from Beijing Chemical Factory; Ni-UMOFNs were prepared according to a previously published paper[29]. 2.2. Preparation of Ni-UMOFNs. N,N-dimethylformamide (32 mL), ethanol (2 mL) and water (2 mL) were mixed in a 50 mL plastic beaker, into which 0.75 mmol benzene-1,4-dicarboxylate acid was dissolved under ultrasonication. Then 0.75 mmol NiCl2·6H2O was added. After Ni2+ salts were dissolved, 0.8 mL triethylamine was quickly added and the mixture was stirred for 5 min to obtain a uniform colloidal suspension. And the colloidal suspension was continuously ultrasonicated for 8 h (40 kHz) under airtight conditions. Finally, the products were obtained via centrifugation, washed with ethanol (3-5 times), and dried at room temperature. 2.3. Characterization. The crystallinity and phase purity of the catalysts were measured by means of powder X-ray diffraction (PXRD)measurements, which were recorded using a Bruker D8 Advance X diffractometer in the 2 theta range of 5–50∘ at a scan speed of 4∘/min using Cu Kα radiation ( λ = 1.5418 Å). The photographs of the catalysts were taken by scanning electron microscopy (SEM) analyses, which were performed using FEI-QUANTA 200F equipment. The elemental composition and content of samples were determined by EDS. The functional groups of


Page 5 of 19 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


the catalysts were characterized by Fourier transform-infrared spectroscopy (FT-IR), which were performed using MAGNA-IR 560 E.S.P spectrometer over a range of 500−4000 cm−1. The thickness of the catalysts were characterized by atomic force microscopy (AFM) technique on the Dimension Icom. The X-ray photoelectron spectroscopy (XPS) measurements were performed on the Thermo VG Scientific Escalab 250 spectrometer using monochromatized Al Kα excitation. The thermal stability of the catalysts were measured by TG, which were performed using METTLER TOLEDO over a range of T = 25−600oC at a scan speed of 10oC /min with nitrogen atmosphere. The inductively coupled plasma (ICP) measurements were carried by Agilent 7500ce. 2.4. Ethylene Dimerization Reactions. Ethylene dimerization was performed in a 100 mL stainless steel autoclave, which was backfilled three times with N2 and once with 1 atm ethylene. Then the as-prepared nickel catalysts and AlEt2Cl were dispersed in toluene respectively under nitrogen, and the resulting suspension was injected into the autoclave. The ethylene pressure was 10 bar and kept constant during the reaction. After a 0.5-1.5 h run time, the reactor was stopped by cooling the reactor to −20°C with ice-cold EtOH solution and depressurizing. An upper-layer clear solution was separated from the reaction mixture and analyzed by GC/MS. The activity of ethylene dimerization products was expressed by TOF and calculated according to the following equation: TOF = C2H4 consumed (mol)/Ni (mol)/Time (h) = C2H4 consumed (g) /molar mass of C2H4 (g·mol-1) /Ni (mol)/Time (h), wherein the mass of C2H4 consumed is equal to the yield of oligomer products (g). The yield was calculated by referencing with the mass of the toluene solvent on the basis of the prerequisite that the mass of each fraction was approximately proportional to its integrated areas in the GC trace according to the following equation: Yield of oligomer products (g) = Toluene mass (g) × Integrated areas of products/Integrated areas of toluene. All catalytic data were repeated at least three times and the error limit of TOF was about ± 2%. 2.5. Ethylene Dimerization Recycling Experiment. The cyclic utilization procedure was as follows: after reaction, the reactor was stopped by cooling the reactor to −20oC with ice-cold EtOH solution and depressurizing. The upper solvent was poured off quickly, and then absolute EtOH was added. Finally the catalyst was obtained via centrifugation, washed five times with absolute EtOH, and dried at room temperature. It is worth noting that the whole process must be very quick to reduce the amount of Al(OH)3 precipitate produced. Before next ethylene dimerization, the catalyst was also fully vacuum-dried under 190oC.

3. RESULTS AND DISCUSSION 3.1. Characterization Results of the Ni-UMOFNs Catalysts. Ni-UMOFNs first reported by Tang and co-workers, were formed from NiCl2·6H2O and benzenedicarboxylic acid through a sonication exfoliation method[29]. Ni-UMOFNs are similar to bulk Ni-MOF in crystal structure,



Page 6 of 19

wherein the Ni atoms are octahedrally coordinated by six O atoms, and these pseudo octahedral are further edge/corner connected with each other along the [010]/[001] direction in the (200) crystallographic plane to form 2D layers separated by benzene-1,4-dicarboxylate linkers. However, on the Ni-UMOFNs surfaces, Ni centers are partially five coordinated owing to the termination of benzene-1,4-dicarboxylate ligands, and these coordinated unsaturated metal sites can be used as catalytic active sites. In this work, the Ni-UMOFNs were successfully prepared by the same method. In order to get abundant unsaturated metal active sites on the surface that serve as active sites in the ethylene dimerization reaction, the prepared Ni-UMOFNs catalysts were fully vacuum-dried. The PXRD patterns of as-synthesized Ni-UMOFNs and bulk-Ni-MOF (Figure 1) were similar to the simulated pattern of Ni-MOF in the literature[29]. The FT-IR spectra clearly showed five obvious peaks for the Ni-UMOFNs and bulk-Ni-MOF (Figure 2). The peak at 3600 cm−1was attributed to stretching and bending vibration of -OH groups. The peaks at 1380 cm−1 and 1580 cm−1 were attributed to symmetric and asymmetric vibration of benzene-1,4-dicarboxylate linkers, respectively. The absence of adsorption peaks at 1690 − 1730 cm−1 attributed to -COOH group is indicative of the deprotonation of benzene-1,4-dicarboxylate acid upon its reaction with metal ions. The peak at 820 cm−1 and 745 cm−1 were attributed to C−H bending vibration. The Ni-UMOFNs presented a lamellar structure with average size of about 500 - 800 nm in the lateral dimension and 5-6 nm in the thickness dimension, respectively. The bulk Ni-MOF showed a blocky structure with average size of about 3-4 𝜇m in the lateral dimension (Figure S1). The TG results of Ni-UMOFNs and bulk Ni-MOF showed that their decomposition temperature was about 370oC (Figure S2). Obviously, the Ni-UMOFNs exhibited more weight losses in 120 − 370oC than the bulk Ni-MOF, possibly because the Ni-UMOFNs had more coordinative solvent molecules in the framework. So the prepared Ni-UMOFNs were vacuum-treated under 130oC, 160oC, 190oC, 220oC before ethylene dimerization reactions, and the treated catalysts were recorded as Ni-UMOFNs-130, Ni-UMOFNs-160, Ni-UMOFNs-190 and Ni-UMOFNs-220, respectively. The PXRD patterns showed that Ni-UMOFNs-190 had the strongest XRD peaks among the treated Ni-UMOFNs (Figure 1).


Page 7 of 19

Intensity (a.u.)

a b c d e f 5

10

15

20

25

30

35

40

45

50

o

2 Theta ( )

Figure 1

PXRD patterns of bulk Ni-MOF and Ni-UMOFNs. (a) Bulk Ni-MOF; (b) Ni-UMOFNs-130; (c)

Ni-UMOFNs-160; (d) Ni-UMOFNs-190; (e) Ni-UMOFNs-220; (f) The simulated pattern of Ni-MOF.

Transmittance (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


a b c d e 500

Figure 2

1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm -1)

FT-IR spectra of bulk Ni-MOF and Ni-UMOFNs. (a) Bulk Ni-MOF; (b) Ni-UMOFNs-130; (c)

Ni-UMOFNs-160; (d) Ni-UMOFNs-190; (e) Ni-UMOFNs-220.

3.2. Ethylene Dimerization. The bulk Ni-MOF and Ni-UMOFNs were used to catalyze ethylene dimerization with cocatalyst Et2AlCl and results were shown in Table 1 and Table S1. There was no catalytic activity only in the presence of benzene-1,4-dicarboxylate acid and AlEt2Cl (Table 1, entry 1) or only in the presence of benzene-1,4-dicarboxylate acid, NiCl2·6H2O and AlEt2Cl (Table 1, entry 2), while the Ni-UMOFNs and bulk Ni-MOF catalytic systems showed moderate catalytic activity. Among the Ni-UMOFNs catalysts, Ni-UMOFNs-190 showed the highest catalytic activity with the TOF of 5536 h-1 and the selectivity to 1-butene of 75.6% (Table S1, entry 3; Table 1, entry 4) possibly because of its highest crystallinity. After ethylene dimerization reaction with



Page 8 of 19

Ni-UMOFNs-190, the filtrate was collected for further catalysis and only a low activity of 82 h-1 was observed (Table 1, entry 5). So the ethylene dimerization is believed to be catalyzed mainly by coordinatively unsaturated Ni metal centers in bulk Ni-MOF or Ni-UMOFNs catalysts and the reactions occur in a heterogeneous phase. Moreover, the Ni-UMOFNs-190 showed about twice as much catalytic activity as the bulk Ni-MOF (Table 1, entry 3, entry 4). This is because ultrathin MOFs Ni-UMOFNs-190 possess extremely high percentages of exposed catalytic active surfaces to ensure high catalytic activity after evacuation given the same mass. The effects of reaction conditions were further explored for the Ni-UMOFNs-190 catalyst. The reaction time for 0.5 h and 1.5 h resulted in the decrease of catalytic activity (Table 1, entry 6, entry 7). The higher Al/Ni molar ratios of 1000 and 1500 also resulted in the decrease of catalytic activity (Table 1, entry 8, entry 9). This might be because that too many alkylaluminum impurities created by the Et2AlCl cocatalyst after activating the catalyst degraded the catalyst system[30]. Comparing with reaction temperature 25oC, the lower reaction temperature 15oC caused an extremely low catalytic activity of 1214 h-1 (Table1, entry 10), and the higher reaction temperature 35oC also resulted in the decrease of catalytic activity to 2393 h-1 but the increase of the selectivity to 1-butene to 92.8% (Table 1, entry 11). In addition, under the investigated conditions, Ni-UMOFNs produced no isolatable polymers and only C6-C8 and a few C10+ olefins (C4, C6, C8 and C10 stands for the olefins containing 4, 6, 8 and 10 carbon atoms, respectively) as the sole observable byproducts. These byproducts do not foul the reactor in an industrial setting. The products were also analyzed by by mass spectrometry and the main products were determined to be 1-butene (Figure S3). These results were coincided with the GC analysis (Figure S4), confirming the high selectivity to 1-butene over the Ni-UMOFNs catalysts.


Page 9 of 19 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


Table 1

Ethylene dimerization with Bulk Ni-MOF and Ni-UMOFNsa Selectivity (%)

Entry

1 2

Catalyst

BDC+ AlEt2Cl NiCl2·6H2O

Reaction

Al/Ni molar

TOF

time (h)

ratio

(h-1)b

T (oC)

1-C4

C6

C8

≥C10

(%)

(%)

(%)

(%)

25

1

-

0

0

0

0

0

25

1

500

0

0

0

0

0

+BDC+ AlEt2Cl 3

Bulk Ni-MOF

25

1

500

2429

78.1

1.5

15.0

5.4

4

Ni-UMOFNs-190

25

1

500

5536

75.6

0.4

22.0

2.0

5

Filtratec

25

1

500

82

75.8

3.8

14.5

5.9

6

Ni-UMOFNs-190

25

0.5

500

3000

76.6

2.8

18.6

2.0

7

Ni-UMOFNs-190

25

1.5

500

2500

90.2

2.6

6.7

0.5

8

Ni-UMOFNs-190

25

1

1000

3500

50.1

26.6

13.0

10.3

9

Ni-UMOFNs-190

25

1

1500

3571

74.5

2.9

12.9

9.7

10

Ni-UMOFNs-190

15

1

500

1214

88.6

3.7

5.4

2.3

11

Ni-UMOFNs-190

35

1

500

2393

92.8

3.2

1.5

2.5

25

1

500

4893

72.1

0.4

25.7

1.8

25

1

500

4821

75.4

0.5

22.3

1.8

25

1

500

4571

74.1

1.0

21.7

3.2

25

1

500

3929

71.4

1.0

24.9

2.7

Ni-UMOFNs-190, 12 the first cycle Ni-UMOFNs-190, 13 the second cycle Ni-UMOFNs-190, 14 the third cycle Ni-UMOFNs-190, 15 the fourth cycle aReaction

pressure=10 bar; Oligomers were determined by GC analysis. bMol of ethylene consumed per mol of

nickel per hour, determined by GC analysis. cThe filtrate was obtained after the ethylene dimerization with Ni-UMOFNs-190.

Two mechanisms have commonly been invoked for ethylene dimerization: the Cossee-Arlman mechanism and the metallacyclic mechanism.[31-34] As for Ni-MFU-4l catalyst for ethylene dimerization, Dincă and co-workers[31] used a combination of isotopic labeling studies, mechanistic probes, and DFT calculations to demonstrate that it operates via the Cossee-Arlman mechanism. Similarly, in this work, the catalytic active centers for the ethylene dimerization were coordinatively unsaturated Ni metal centers in Ni-UMOFNs. Before the dimerization, the Ni-UMOFNs catalysts were also activated by Et2AlCl to generate catalytically active sites[35-36]. We speculate Ni-UMOFNs



Page 10 of 19

might also operate by the Cossee-Arlman mechanism, wherein the rate of ethylene insertion is slower than the rate of chain termination via β-hydride elimination[31], resulting in high selectivity to 1-butene. 3.3. Recycling Experiment. One of the major attractions in heterogeneous catalysts stems from the possibility of reusing, so this property of Ni-UMOFNs was explored in this work. The recycling ethylene dimerization results of Ni-UMOFNs-190 showed that the first cycle turnover frequency and selectivity of 1-butene were 4893 h-1 and 72.1% (Table 1, entry 12), and only reduced 11.6% and 4.6% respectively compared with the fresh Ni-UMOFNs-190 catalyst. The second cycle and third cycle reuse almost maintained the same level of catalytic activity and selectivity as the first cycle (Table 1, entry 13, entry 14). The fourth cycle catalytic activity of Ni-UMOFNs-190 could maintain 71% compared with the fresh one, and the selectivity of 1-butene had no obvious change (Table 1, entry 15). These results showed that Ni-UMOFNs-190 can be reused at least four times without considerable loss in the catalytic activity and selectivity to 1-butene. The recovered Ni-UMOFNs-190 catalysts still possessed similar crystal structures, FT-IR spectra patterns and ultrathin morphology to the fresh one (Figure 3 and Figure 4), but the morphology became more fragmented, especially for the recovered catalyst after the fourth cycle (Figure S5). These results prompted us to further analyze the thickness changes of the recovered Ni-UMOFNs-190 by the atomic force microscopy technology (AFM). The AFM results showed that the recovered catalysts still maintained the ultrathin lamellar structures, but the thickness gradually decreased from 5.6 nm for the fresh Ni-UMOFNs-190 to 4.5 nm, 3.8 nm, 2.6 nm, 1.7 nm 1.5 nm for the Ni-UMOFNs-190 after the first time catalysis, and after the first, the second, the third, and the fourth cycle (Figure 5, Figure S6). This decreasing thickness suggested the lamellar structure of Ni-UMOFNs-190 exfoliated during the catalysis and the Ni leaching accompanying the exfoliating was further determined by ICP. The initial Ni concentration was 28 mg/L, and the Ni concentration of upper-layer clear solution was 2.9 mg/L after ethylene dimerization reaction, so Ni leaching was about 10%. Therefore, the excellent reusability of Ni-UMOFNs-190 is due to continuous exposure of fresh unsaturated metal active sites when the original metal centers on the catalyst surface are used, just like a snake’s molting.


a b c d e f 5

10

15

20

25

30

o

35

40

45

50

2 Theta ( ) Figure 3

The PXRD of Ni-UMOFNs. (a) Fresh Ni-UMOFNs-190; (b) Ni-UMOFNs-190 after the first time

catalysis; (c) Ni-UMOFNs-190 after the first cycle; (d) Ni-UMOFNs-190 after the second cycle; (e) Ni-UMOFNs-190 after the third cycle; (f) Ni-UMOFNs-190 after the fourth cycle.

a

Transmittance (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


Intensity (a.u.)

Page 11 of 19

b c d e f 500

Figure 4

1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm -1)

The FT-IR of Ni-UMOFNs. (a) Fresh Ni-UMOFNs-190; (b) Ni-UMOFNs-190 after the first time

catalysis; (c) Ni-UMOFNs-190 after the first cycle; (d) Ni-UMOFNs-190 after the second cycle; (e) Ni-UMOFNs-190 after the third cycle; (f) Ni-UMOFNs-190 after the fourth cycle.



7

7

Vertical distance (nm)

Vertical distance (nm)

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

6 5 4

5.6 nm

3 2 1 0

Page 12 of 19

0

40

80

120

160

200

Horizontal distance (nm) Figure 5

240

6 5 4 3 2 1 0 0

1.5 nm 40

80

120

160

200

240

Horizontal distance (nm)

The AFM patterns of (a) fresh Ni-UMOFNs-190 and (b) Ni-UMOFNs-190 after the fourth cycle.

Theoretically, the catalytic activity shouldn’t be reduced if the recovered Ni-UMOFNs-190 catalysts only exfoliated the surface, so we further analyzed the composition of the recovered catalysts. The C、O、Ni contents of the recovered Ni-UMOFNs-190 had no obvious changes and were about 65%, 25%, 7% respectively, but there were extra Al components of about 1%, 1%, 2%, 3%, 5% for the recovered catalyst from the fresh catalyst system, the first cycle, the second cycle, the third cycle and the fourth cycle catalyst system, respectively (Figure S7). The XPS technique was further employed to determine the metal oxidation states in the Ni-UMOFNs catalysts. It was found that the Ni 2p regions of the Ni-UMOFNs catalysts showed two XPS peaks at about 855.9 eV and 873.7 eV, which can be attributed to the Ni2+ 2p3/2 and Ni2+ 2p1/2 orbitals. The Ni 2p XPS peaks had no obvious changes after the first cycle, the second cycle and the fourth cycle catalysis compared with the Ni-UMOFNs-190 before catalysis (Figure S8). Moreover, the XPS results showed that there were extra XPS peaks at about 74.4 eV and 119.0 eV for the recovered catalysts. These XPS peaks


Page 13 of 19

could be attributed to the Al 2p and Al 2s orbitals of Al(OH)3 (Figure 6), which was supposed to be generated through the hydrolysis of the alkylaluminum cocatalyst. Comprehensively analyzing, we concluded that the favorable reusable property of Ni-UMOFNs-190 was because that the crystal structure, ultrathin lamellar morphology and Ni elemental content had no obvious changes after ethylene dimerization despite of the exfoliating, while some decrease in the catalytic activity was because that some active centers were covered by Al(OH)3 precipitate derived from the hydrolysis of the alkylaluminum cocatalyst.

Al 2p

Al 2s

d c b

Intensity

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


a

0

40

80

120

160

200

Binding energy (eV) Figure 6

The XPS of element Al in Ni-UMOFNs-190. (a) fresh Ni-UMOFNs-190 before catalysis; (b)

Ni-UMOFNs-190 after the first time catalysis; (c) Ni-UMOFNs-190 after the second cycle; (d) Ni-UMOFNs-190 after the fourth cycle.

4. CONCLUSION In summary, ultrathin nickel-based metal-organic framework nanosheets (Ni-UMOFNs) with coordinatively unsaturated metal sites were prepared successfully by a ultrasonic method at room temperature and used as ethylene dimerization catalysts in the presence of an alkylaluminum cocatalyst at different catalytic conditions. The catalytic results showed that Ni-UMOFNs catalysts exhibited moderate activity but high 1-butene selectivity. Significantly, the optimal Ni-UMOFNs catalyst possessed outstanding reusable property due to continuous exposure of fresh unsaturated metal active sites when the original metal centers on the catalyst surface were used and exfoliated. The crystal structure, ultrathin lamellar morphology and Ni elemental content of the exfoliated Ni-UMOFNs catalyst after ethylene dimerization reaction had no obvious changes, but a small amount of Al(OH)3 precipitate was generated from the hydrolysis of the alkylaluminum cocatalyst, resulting in a small decline in the reusable catalytic activity. The work demonstrated that ultrathin metal-organic framework nanosheets have important new applications for heterogeneous catalysis.



Page 14 of 19

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website. Ethylene dimerization with Ni-UMOFNs, the SEM patterns of Ni-UMOFNs and bulk Ni-MOF, the TG patterns of Ni-UMOFNs and bulk Ni-MOFs, the GC pattern of ethylene oligomerization products over Ni-UMOFNs-190, the MS pattern of ethylene oligomerization products over Ni-UMOFNs-190, the SEM images of Ni-UMOFNs, the AFM patterns of Ni-UMOFNs, the EDS of Ni-UMOFNs, and the XPS of element Ni in Ni-UMOFNs.

AUTHOR INFORMATION Corresponding Author * E-mail:

[email protected] (Ying Zhang).

Tel: +86-10-89732273. ORCID iD Ying Zhang: 0000-0003-4988-4575 Yang Han: 0000-0001-5785-1090 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Funding: This work was supported by the National Natural Science Foundation of China （ Grant No.21676296）and National Key Research and Development Plan（Grant No.2016YFC0303704）.

REFERENCES (1) Bender, M. An Overview of Industrial Processes for the Production of Olefins–C4 Hydrocarbons[J]. ChemBioEng Rev. 2014, 1, 136-147. (2) Glockner, P. W.; Barnett, K. W. Ethylene Oligomerization. U. S. Patent 3,527,839 filed Jan. 13, 1969, and issued Sept. 8, 1970. (3) Finiels, A.; Fajula, F.; Hulea, V. Nickel-Based Solid Catalysts for Ethylene Oligomerization − a Review[J].


Page 15 of 19 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


Catal. Sci. Technol. 2014, 4, 2412−2426. (4) Lallemand, M.; Finiels, A.; Fajula, F.; Hulea, V. Catalytic Oligomerization of Ethylene over Ni-Containing Dealuminated Y Zeolites[J]. Appl. Catal. A: Gen. 2006, 301, 196−201. (5) Lallemand, M.; Finiels, A.; Fajula, F.; Hulea, V. Ethylene Oligomerization over Ni-Containing Mesostructured Catalysts with MCM-41, MCM-48 and SBA-15 Topologies[J]. Stud. Surf. Sci. Catal. 2007, 170, 1863−1869. (6) Lallemand, M.; Rusu, O. A.; Dumitriu, E.; Finiels, A.; Fajula, F.; Hulea, V. Ni-MCM-36 and Ni-MCM-22 Catalysts for the Ethylene Oligomerization[J]. Stud. Surf. Sci. Catal. 2008, 174, 1139−1142. (7) Xu, W. L.; Thapa, K. B.; Ju, Q.; Fang, Z. L.; Huang, W. Heterogeneous Catalysts Based on Mesoporous Metal– Organic Frameworks[J]. Coord, Chem. Rev. 2018, 373, 199-232. (8) Canivet, J.; Aguado, S.; Schuurman, Y.; Farrusseng, D. MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification[J]. J. Am. Chem. Soc. 2013, 135, 4195-4198. (9) Madrahimov, S. T.; Gallagher, J. R.; Zhang, G. H.; Meinhart, Z.; Garibay, S. J.; Delferro, M.; Miller, J. T.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes[J]. ACS Catal. 2015, 5, 6713−6718. (10) Liu, B.; Jie, S. Y.; Bu, Z. Y.; Li, B. G. Postsynthetic Modification of Mixed-Linker Metal–Organic Frameworks for Ethylene Oligomerization[J]. RSC Adv. 2014, 4, 62343-62346. (11) Liu, S. Y.; Zhang, Y.; Huo, Q.; He, S. S.; Han. Y. Synthesis and Catalytic Performances of a Novel Zn-MOF Catalyst Bearing Nickel Chelating Diimine Carboxylate Ligands for Ethylene Oligomerization[J]. J. Spectrosc. 2015, 6, 1–7. (12) Metzger, E. D.; Brozek, C. K.; Comito, R. J.; Dincǎ, M. Selective Dimerization of Ethylene to 1‑Butene with a Porous Catalyst[J]. ACS Cent. Sci. 2016, 2, 148–161. (13) Mlinar, A. N., Keitz, B. K., Gygi, D., Bloch, E. D., Long, J. R., Bell, A. T. Selective Propene Oligomerization



Page 16 of 19

with Nickel(II)-Based Metal−Organic Frameworks[J].ACS Catal. 2014, 4, 717–721. (14) Li, Z. Y.; Schweitzer, N. M.; League, A. B.; Bernales, V.; Peters, A. W.; Getsoian, A. B.; Wang, T. C.; Miller, J. T.; Vjunov, A.; Fulton, J. L.; Lercher, J. A.; Cramer, C. J.; Gagliardi, L.; Hupp, J. T.; Farha, O. K. Sintering-Resistant Single-Site Nickel Catalyst Supported by Metal−Organic Framework[J]. J. Am. Chem. Soc. 2016, 138, 1977–1982. (15) Liu, S. Y.; Zhang, Y.; Han, Y.; Feng, G. L.; Gao, F.; Wang, H.; Qiu, P. Selective Ethylene Oligomerization with Chromium-Based Metal−Organic Framework MIL-100 Evacuated under Different Temperatures[J]. Organometallics. 2017, 36, 632–638. (16) Liu, Y. W.; Cheng, H., Lyu, M. J.; Fan, S. J.; Liu, Q. H.; Zhang, W. S.; Zhi, Y. D.; Wang, C. M.; Xiao, C.; Wei, S. Q.; Ye, B. J.; Xie, Y. Low over Potential in Vacancy-Rich Ultrathin CoSe2 Nanosheets for Water Oxidation[J]. J. Am. Chem. Soc. 2014, 136, 15670–15675. (17). Lin, S.; Diercks, C. S.; Zhang, Y. B.; Kornienko, N.; Nichols, E. M.; Zhao, Y. B.; Paris, A. R.; Kim, D.; Yang, P. D.; Yaghi, O. M.; Chang, C. J. Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water[J]. Science .2015, 349, 1208–1213. (18) Zhao, M. T.; Lu, Q. P.; Ma, Q. L.; Zhang, H. Two‐Dimensional Metal–Organic Framework Nanosheets[J]. Small Methods. 2017, 1, 1600030. (19) Tan, C. L.; Cao, X. H.; Wu, X. J.; He, Q. Y.; Yang, J.; Zhang, X.; Chen, J. Z.; Zhao, W.; Han, S. K.; Nam, G. H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials[J]. Chem. Rev. 2017, 117, 6225-6331. (20) Zhang, H. Ultrathin Two-Dimensional Nanomaterials[J]. ACS Nano. 2015, 9, 9451−9469. (21) Tan, C. L.; Zhang, H. Two-Dimensional Transition Metal Dichalcogenide Nanosheet-Based Composites[J]. Chem. Soc. Rev. 2015, 44, 2713−2731.


Page 17 of 19 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


(22) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets[J]. Nat. Chem. 2013, 5, 263−275. (23) Li, P. Z.; Maeda, Y.; Xu, Q. Top-Down Fabrication of Crystalline Metal-Organic Framework Nanosheets[J]. Chem. Commun. 2011, 47, 8436-8438. (24) Qin, J. S.; Du, D. Y.; Guan, W.; Bo, X. J.; Li, Y. F.; Guo, L. P.; Su, Z. M.; Wang, Y. Y.; Lan, Y. Q.; Zhou, H. C. Ultrastable Polymolybdate-Based Metal–Organic Frameworks as Highly Active Electrocatalysts for Hydrogen Generation from Water[J]. J. Am. Chem. Soc. 2015, 137, 7169–7177. (25) Peng, Y.; Li, Y. S.; Ban, Y. J.; Jin, H.; Jiao, W. M.; Liu, X. L.; Yang, W. S. Metal–Organic Framework Nanosheets as Building Blocks for Molecular Sieving Membranes[J]. Science. 2014, 346, 1356–1359. (26) Rodenas, T.; Luz, I.; Prieto, G.; Seoane, B.; Miro, H.; Corma, A.; Kapteijn, F.; Llabrés i Xamena, F. X.; Gascon, J. Metal–Organic Framework Nanosheets in Polymer Composite Materials for Gas Separation[J]. Nat. Mater. 2015, 14, 48–55. (27) Zhao, M. T.; Huang, Y.; Peng, Y. W.; Huang, Z. Q.; Ma, Q. L.; Zhang, H. Two-Dimensional Metal–Organic Framework Nanosheets: Synthesis and Applications[J]. Chem. Soc. Rev. 2018, 47, 6267-6295. (28) Huang, Y.; Zhao, M. T.; Han, S. K.; Lai, Z. C.; Yang, J.; Tan, C. L.; Ma, Q. L.; Lu, Q. P.; Chen, J. Z.; Zhang, X.; Zhang, Z. C.; Li, B.; Chen, B.; Zong, Y.; Zhang, H. Growth of Au Nanoparticles on 2D Metalloporphyrinic Metal‐Organic Framework Nanosheets Used as Biomimetic Catalysts for Cascade Reactions[J]. Adv. Mater. 2017, 29, 1700102. (29) Zhao, S. L.; Wang, Y.; Dong, J. C.; He, C. T.; Yin, H. J.; An, P. F.; Zhao, K.; Zhang, X. F.; Gao, C.; Zhang, L. J.; Lv, J. W.; Wang, J. X.; Zhang, J. Q.; Khattak, A. M.; Khan, N. A.; Wei, Z. X.; Zhang, J.; Liu, S. Q.; Zhao, H. J.; Tang, Z. Y. Ultrathin Metal–Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution[J]. Nat. Energy, 2016, 1 . 1-10.



(30)

Abbo,

H.

S.,

Titinchi,

S.

J.

J.

A

New

Page 18 of 19

Vanadium

(III)

Complex

of

2,6-Bis(3,5-diphenylpyrazol-1-ylmethyl)pyridine as a Catalyst for Ethylene Polymerization [J]. Molecules, 2013, 18, 4728–4738. (31) Metzger, E. D.; Comito, R. J.; Hendon, C. H.; Dincă, M. Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU‑4l[J]. J. Am. Chem. Soc. 2017, 139, 757-762. (32) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. Mechanistic Studies of the Ethylene Trimerization Reaction with Chromium−Diphosphine Catalysts: Experimental Evidence for a Mechanism Involving Metallacyclic Intermediates [J]. J. Am. Chem. Soc. 2004, 126, 1304−1305. (33) McGuinness, D. S. Olefin Oligomerization Via Metallacycles: Dimerization, Trimerization, Tetramerization, and Beyond[J]. Chem. Rev. 2011, 111, 2321-2341. (34) Speiser, F.; Braunstein, P.; Saussine, L. Catalytic Ethylene Dimerization and Oligomerization: Recent Developments with Nickel Complexes Containing P,N-Chelating Ligands[J]. Acc. Chem. Res. 2005, 38,784-793. (35) Roy, D.; Sunoj, R. B. Ni-, Pd-, or Pt-Catalyzed Ethylene Dimerization: a Mechanistic Description of the Catalytic Cycle and the Active Species[J]. Org. Biomol. Chem. 2010, 8, 1040−1051. (36) Berkefeld, A.; Mecking, S. Deactivation Pathways of Neutral Ni(II) Polymerization Catalysts[J]. J. Am. Chem. Soc. 2009, 131, 1565−1574.


Page 19 of 19 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


Table of Contents Abstract Graphics


Ultrathin Nickel-Based Metal-Organic Framework Nanosheets as

Recommend Documents