Acetylene-Selective Hydrogenation Catalyzed by Cationic

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Article Cite This: J. Am. Chem. Soc. 2019, 141, 9920−9927

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Acetylene-Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite Yuchao Chai,† Guangjun Wu,† Xiaoyan Liu,§ Yujing Ren,§ Weili Dai,†,∥ Chuanming Wang,*,‡ Zaiku Xie,‡ Naijia Guan,†,∥ and Landong Li*,†,∥

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School of Materials Science and Engineering & National Institute for Advanced Materials, Nankai University, Tianjin 300350, People’s Republic of China ‡ State Key Laboratory of Green Chemical Engineering and Industrial Catalysis, SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, People’s Republic of China § State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ∥ Key Laboratory of Advanced Energy Materials Chemistry of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, People’s Republic of China S Supporting Information *

ABSTRACT: The selective hydrogenation of alkynes to alkenes is an important type of organic transformation with large-scale industrial applications. This transformation requires efficient catalysts with precise selectivity control, and palladium-based metallic catalysts are currently employed. Here we show that four-coordinated cationic nickel(II) confined in zeolite can efficiently catalyze the selective hydrogenation of acetylene to ethylene, a key process for trace acetylene removal prior to the polymerization process. Under optimized conditions, 100% acetylene conversion and an ethylene selectivity up to 97% are simultaneously achieved. This catalyst system also exhibits good stability and recyclability for potential applications. Spectroscopy investigations and density functional theory calculations reveal the heterolytic dissociation of hydrogen molecules and the importance of hydride and protons in the selective hydrogenation of acetylene to ethylene. This work provides an efficient strategy toward active and selective zeolite catalysts by utilizing the local electrostatic field within the zeolite confined space for small-molecule activation and by linking heterogeneous and homogeneous catalysis.

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oxides13 or isolated Fe(III)-O sites14 have also been disclosed to be active and selective catalysts for the reaction. Transition metal complexes with coordinately unsaturated cationic centers, e.g., nickel(II) complex15,16 and cobalt(II) complex,17 are able to activate molecular hydrogen and, therefore, catalyze homogeneous hydrogenation. In principle, transition metal complexes can catalyze the selective hydrogenation of alkynes if the hydrogenation of intermediate alkenes is more difficult than their desorption.5 The liquidphase selective hydrogenation of alkynes has been accomplished with some d8 transition metal complexes.18 Especially, stabilized first-row transition metal complexes have been recently developed as efficient heterogeneous catalysts for alkyne selective hydrogenation.19 Inspired by transition metal complex catalyzed alkyne selective hydrogenation, we report here the design and construction of cationic nickel(II) confined in zeolite for acetylene selective hydrogenation.

INTRODUCTION

The selective hydrogenation of alkynes to the corresponding alkenes, i.e., semihydrogenation, is an important and challenging type of transformation in synthetic organic chemistry with significant industrial relevance. A well-known example is the semihydrogenation of trace acetylene in ethylene feeds prior to the polymerization process that is applied on the scale of billions of tons per annum. Palladiumbased catalysts have been intensively investigated in the past decades,1,2 and palladium modified with silver is currently recognized as a benchmark catalyst for this reaction.3,4 However, the overhydrogenation of ethylene to ethane remains a key drawback for palladium catalysts, and the scarcity of palladium triggers further interest in alternative selective hydrogenation catalysts. Intermetallic compounds such as NiZn,5 PdxGay,6,7 Al13Fe4,8 NixGay, and NixSny9 and ternary copper−nickel-iron composite10 have been explored as promising metallic catalysts for the selective hydrogenation of acetylene. Some oxides such as ceria11,12 and supported iron © 2019 American Chemical Society

Received: March 28, 2019 Published: May 31, 2019 9920

DOI: 10.1021/jacs.9b03361 J. Am. Chem. Soc. 2019, 141, 9920−9927

Article

Journal of the American Chemical Society

Figure 1. Construction and structure of cationic nickel(II) confined in zeolite. (a) UV−vis−NIR spectra of nickel-containing zeolites. Solid line: Spectra of sample exposure to ambient conditions, dotted lines: spectra of strictly dehydrated samples. (b) Normalized XANES spectra and (c) Fourier transforms of k2-weighted Ni K-edge EXAFS spectra (without phase correction) of selected dehydrated samples. (d) H2-TPR profiles of nickel-containing zeolites. (e) Schematic structure evolution of nickel species confined in zeolite.

adjacent oxygen anchoring sites to derive nickel-containing chabazite zeolite, i.e., cal-Ni@CHA (Figure S3). During the calcination process, cationic nickel could coordinate with various oxygen anchoring sites in the zeolite framework21 and therefore lead to a mixture of nickel species on zeolite. Postmodulation was then performed, and the exchangeable nickel species were selectively removed through repeated ion exchange with alkali metal cations (see Figure S4 for calculated exchange energies and Table S1 for changes in the nickel and alkali metal contents), producing zeolites with simplified nickel species constitution, i.e., Li-Ni@CHA, NaNi@CHA, and K-Ni@CHA. These dehydrated samples exhibited similar d−d transition bands at ∼440 and ∼540 nm in the UV−vis−NIR spectra, which shifted to ∼425 and ∼525 nm upon exposure to ambient conditions (Figure 1a; reversible dehydration occurred at elevated temperature: Figure S5). According to these observations, the transformation of four-coordinated nickel species in distorted square-planar (D4h) symmetry to five-coordinated nickel species in square-pyramidal (C4v) symmetry upon hydration could be expected.22,23 The chemical and electronic states of nickel species in zeolite were examined by X-ray photoelectron spectroscopy (XPS). Nickel-containing zeolites showed Ni 2p3/2 binding energy values in the range of 856.7−857.8 eV (Figure S6), which appeared to be distinctly higher than 853.8 eV of reference NiO.24 Nickel species should exist in the form of cations with electron transfer to zeolite framework, while the degree of electron transfer was slightly influenced by the alkali cations in chabazite zeolite. The local structure of nickel

Perfect acetylene conversion and extremely high ethylene selectivity can be simultaneously obtained at moderate reaction temperatures, making the cationic nickel confined in zeolite a promising low-cost catalyst for the removal of trace acetylene from ethylene in industrial processes. We subsequently present a molecular-level understanding of acetylene-selective hydrogenation over well-defined cationic nickel(II) confined in zeolite, which is undoubtedly a heterogeneous solid catalyst but works like a homogeneous organometallic complex. Specifically, the heterolytic activation of nonpolar hydrogen molecule by the local electrostatic field within zeolite confined space is disclosed, which is essential for the subsequent selective hydrogenation of acetylene.

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RESULTS Construction and Characterization of Cationic Nickel(II) Confined in Zeolite. A diethylenetriamine (DETA) complex of nickel, i.e., [Ni-DETA]2+, was prepared and employed as the structure-directing agent for the synthesis of aluminosilicalite zeolite. Under optimized hydrothermal conditions (see Supporting Information for details), chabazite, with well-defined CHA topology and a Si/Al molar ratio of ∼3, was obtained (Figure S1, Table S1). The as-synthesized zeolite sample, namely, raw-Ni@CHA, exhibited a series of absorption bands characteristic of octahedrally coordinated nickel species at ∼350, ∼530, ∼890, and ∼1550 nm (Figure S2) in the diffuse reflectance ultraviolet−visible−near-infrared (UV−vis− NIR) spectrum, indicating the encapsulation of [Ni-DETA]2+ in chabazite cages.20 The organic ligands were removed upon calcination in flowing air at 823 K and nickel coordinated with 9921

DOI: 10.1021/jacs.9b03361 J. Am. Chem. Soc. 2019, 141, 9920−9927

Article

Journal of the American Chemical Society

Figure 2. Catalytic data of acetylene-selective hydrogenation. (a) Comparison of different catalysts in acetylene-selective hydrogenation. Reaction conditions: 0.2 g of catalyst, 1% C2H2, 16% H2 in He, total flow = 50 mL min−1, GHSV = 15 000 h−1. Acetylene conversion and ethylene selectivity over a specific catalyst were reported at the reaction temperature when the maximal ethylene yield was achieved. (b) Time-dependent behaviors of Na-Ni@CHA (H2/C2H2 = 16; 453 K), patent catalyst Pd-Ag/Al2O3 (H2/C2H2 = 16; 363 K), and industrial catalyst from CNPC (H2/C2H2 = 4; 363 K) in acetylene-selective hydrogenation. Reaction conditions: 0.2 g of catalyst, 1% C2H2, x% H2 in He, total flow = 50 mL min−1, GHSV = 15 000 h−1. (c) Regeneration test of Na-Ni@CHA in acetylene-selective hydrogenation. Reaction conditions: 0.2 g of catalyst, 1% C2H2, 16% H2 in He, total flow = 50 mL min−1, GHSV = 15 000 h−1. Regeneration achieved by calcination in dry air at 673 K for 2 h followed by treatment in 10% H2/He at 573 K for 1 h. (d) Temperature-dependent behaviors of Na-Ni@CHA catalyst in acetylene-selective hydrogenation under simulated industrial process conditions. Reaction conditions: 0.2 g of catalyst, 0.5% C2H2, 8% H2, 50% C2H4 in He, total flow = 50 mL min−1, GHSV = 15 000 h−1.

samples should be activated by dehydration prior to being used as catalysts.28 Temperature-programmed reduction (TPR) experiments were performed on nickel-containing zeolites, and the H2TPR profiles are shown in Figure 1d. Three hydrogen consumption peaks centered at ∼630, ∼820, and ∼1000 K were observed for the cal-Ni@CHA sample, while the intensities of reduction peaks at ∼630 and ∼1000 K decreased dramatically or even disappeared through repeated ion exchange with alkali cations. That is, most of exchangeable nickel species could be removed by ion exchange with alkali cations, and dominating four-coordinated cationic Ni(II) species confined in zeolite could be obtained. These Ni species were ultradispersed within the zeolite matrix in the subnanometric form, as disclosed by the electron microscopy observations (Figures S11 and S12). The possible structure of cationic Ni(II) confined in chabazite was then investigated by density functional theory (DFT) calculations on the premise of isolated Ni centers in the valence state of +2. According to the sitting position of two Al atoms in the CHA framework, we compared several different structures of local active sites (Figure S13) and found that Ni

species at the atomic scale was characterized by X-ray absorption spectroscopy. The Ni k-edge XANES spectrum confirmed the exclusive valence state of +2 in dehydrated NaNi@CHA (Figure 1b), being consistent with the results from XPS. The oscillation manner of the Ni k-edge EXAFS in the k space of dehydrated Na-Ni@CHA was markedly different from that of reference NiO (Figure 1c) due to the presence of framework oxygen atoms in chabazite zeolites as anchoring sites for nickel species. The Fourier transforms of the EXAFS spectra in the R space of Na-Ni@CHA exhibited a prominent peak from the first Ni−O shell with a coordination number of ∼4.0 and a suppressed second Ni−Si or Ni−Al shell with a coordination number of ∼1.9 (Figure S7, Table S2). These data, together with those from the UV−vis−NIR spectra (Figure 1a), hint at the formation of four-coordinated Ni(II) confined in CHA zeolite.25,26 Notably, similar higher coordination numbers of ∼5 were obtained for Li-, Na-, and K-Ni@CHA samples upon exposure to ambient conditions (Figures S8−10, Table S2), which should be due to the fast interaction between H2O with the coordinately unsaturated Ni centers,27 as also revealed by UV−vis spectra (Figure 1a, Figure S5). In this context, the Li-, Na-, and K-Ni@CHA 9922

DOI: 10.1021/jacs.9b03361 J. Am. Chem. Soc. 2019, 141, 9920−9927

Article

Journal of the American Chemical Society

Figure 3. Spectroscopic investigations of acetylene-selective hydrogenation. (a) TPD profiles of H2, C2H2, and C2H4 on Na-Ni@CHA catalyst. (b) H2 and D2 activation on Na-Ni@CHA at 453 K. Reaction conditions: 0.02 g of catalyst, 16% H2 or D2 in Ar, total flow = 5 mL min−1. (c) Feeding of C2H2 to H2-preadsorbed Na-Ni@CHA at 453 K. Reaction conditions: 0.02 g of catalyst, 1% C2H2 in Ar, total flow = 5 mL min−1. (d) Feeding of C2H4 to H2-preadsorbed Na-Ni@CHA at 453 K. Reaction conditions: 0.02 g of catalyst, 1% C2H4 in Ar, total flow = 5 mL min−1. (e) In situ DRIFT spectra of acetylene-selective hydrogenation over Na-Ni@CHA catalyst. Reaction conditions: 0.02 g of catalyst, 1% C2H2, 16% H2 in Ar, total flow = 5 mL min−1, GHSV = 15 000 h−1. (f) In situ DRIFT spectra of ethylene hydrogenation over Na-Ni@CHA catalyst. Reaction conditions: 0.02 g of catalyst, 1% C2H4, 16% H2 in Ar, total flow = 5 mL min−1, GHSV = 15 000 h−1.

preferred sitting in the six-membered ring in which two Al atoms are in the opposite or meta position. The Ni atom is four-coordinated by framework oxygen atoms, consistent with X-ray absorption spectroscopy (XAS) results (Table S2). Other structures, such as another Al in the eight-membered ring or two Al atoms in different six-membered rings, are much less stable than the preferred one. According to the characterization and simulation results, the evolution of cationic nickel species confined in zeolite is summarized in Figure 1e. Briefly, Ni-DETA complexes were encapsulated in chabazite cages at the stage of hydrothermal synthesis, and they underwent interaction with the zeolite framework upon calcination removal of DETA ligands. A great number of nickel centers bound to framework oxygen anchoring sites to derive four-coordinated cationic Ni species and others located at the exchangeable cation sites. The nickel species at the exchangeable cation sites could be further replaced by alkali metal ions, e.g., Na+, leaving exclusively four-coordinated Ni centers confined in chabazite cages. Such a structure is analogous to a typical coordination complex with Ni(II) as the central ion and chabazite framework as the ligand. Selective Catalytic Hydrogenation of Acetylene. Nickel-containing zeolites with coordinately unsaturated nickel centers were applied as alternatives to palladium catalysts for acetylene-selective hydrogenation. Good acetylene conversion and ethylene selectivity were simultaneously obtained with cal-

Ni@CHA catalyst (Figure 2a, Figure S14), in great contrast to other samples prepared by traditional ion exchange (Ni-CHA) and wet impregnation (NiO/Na-CHA). The catalytic performance of nickel-containing zeolites seems to be controlled by the specific nickel center and its coordination environment. Nickel species at the exchangeable cation sites preferred catalyzing the overhydrogenation of acetylene to ethane at high acetylene conversions (Ni-CHA, Figure S14), and replacing the exchangeable nickel species in cal-Ni@CHA by alkali metal cations through a postmodulation process could significantly promote the ethylene selectivity. The effects of nickel site constitution on the catalytic performance of various nickelcontaining zeolites are summarized in Table S3 for a direct view. It should be noted that due to the rigid structure of zeolite the alkali metal cations did not directly interact with the nickel cations and would not change the coordination environment of Ni(II) centers, as confirmed by X-ray absorption analyses (Figures S8−S10, Table S2) and in significant contrast to the cases reported by FlytzaniStephanopoulos et al.29,30 The types of alkali metal cations in the Ni@CHA system also showed noticeable impacts on the performance of acetylene-selective hydrogenation. Na-Ni@ CHA appeared to be a slightly better catalyst than Li-Ni@ CHA in terms of the low-temperature acetylene conversion and ethylene selectivity, probably due to the electronic effects on chabazite ligand with alkali cations regarded as a part of the 9923

DOI: 10.1021/jacs.9b03361 J. Am. Chem. Soc. 2019, 141, 9920−9927

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Journal of the American Chemical Society

Figure 4. Calculated Gibbs free energy profile of Ni@CHA-catalyzed acetylene-selective hydrogenation at 453 K. Some optimized structures are shown as well with distances in Å.

species during reaction. Thermogravimetric/differential thermal analysis revealed the existence of some polymeric byproducts (∼3%) on the spent catalyst, much less than those formed on spent Pd-Ag/Al2O3 catalyst (∼6%, Figure S17). The nature of polymeric byproducts on Na-Ni@CHA and Pd-Ag/Al2O3 was further investigated. GC-MS chromatograms of polymeric extracts revealed that benzene was formed as the dominating byproduct on Na-Ni@CHA catalyst (probably inside chabazite cages), while polymeric species with various molecular weights were formed on Pd-Ag/Al2O3 (Figure S18). However, even the very small amount of benzene byproducts could cause the catalyst deactivation of Na-Ni@CHA by blocking the zeolite pores. Fortunately, the catalytic activity of Na-Ni@CHA could be fully restored through a simple calcination step (Figure 2c). For the catalyst system of confined cationic nickel species, optimizing the zeolite host with better diffusion ability should be a feasible strategy to improve the catalyst lifetime in acetylene-selective hydrogenation in future studies. Na-Ni@CHA could also catalyze the hydrogenation of ethylene (Figure S19); however, it appeared to be more difficult than acetylene hydrogenation. In simulated industrial ethylene feed, Na-Ni@CHA could catalyze the selective hydrogenation of acetylene with an ethylene selectivity of ∼90% at 413−473 K (Figure 2d). The selectivity was calculated as the sum of acetylene converted with subtraction of ethane produced. Since ethane could come from both the overhydrogenation of acetylene and the hydrogenation of ethylene in the stream, a dip in ethylene selectivity was obtained when compared with acetylene hydrogenation in the absence of ethylene. Here, an important indicator called ethylene wastage was calculated. The very low

chabazite ligand, as revealed by XPS analyses (Figure S6). KNi@CHA exhibited low activity in acetylene-selective hydrogenation due to the diffusion hindrance caused by K+ with a large radius, namely, the spatial effects on chabazite ligand (Table S1, Figure S15). Considering all the issues mentioned above, Na-Ni@CHA can be optimized for acetylene-selective hydrogenation in this study. For the best catalyst of Na-Ni@CHA, 100% acetylene conversion with high ethylene selectivity up to 97% could be achieved at 453 K. A small amount of ethane was detected as the overhydrogenation byproduct, while no C4 compounds could be detected (Figure S16). The catalytic performance of Na-Ni@CHA, in terms of acetylene conversion and ethylene selectivity, is even better than palladium catalysts investigated, e.g., patent catalyst Pd-Ag/Al2O3 and Lindlar catalyst Pd-PbO/ CaCO3, although the required reaction temperature is ∼100 K higher (Figure 2a, Figure S14). A direct comparison of timedependent behaviors of Na-Ni@CHA, patent catalyst Pd-Ag/ Al2O3 and industrial catalyst from CNPC under their optimized reaction conditions is shown in Figure 2b. Obviously, stable ethylene selectivity was observed on all three catalysts investigated, and the highest ethylene selectivity was achieved with the Na-Ni@CHA catalyst. The industrial catalyst underwent a long induction period of 24 h, and then 100% acetylene conversion was maintained for 88 h before it started to decline. In comparison, Na-Ni@CHA exhibited good catalytic stability within 20 h (100% acetylene conversion), and, afterward, the acetylene conversion started to decrease gradually. Since nickel species existed in the most thermodynamically cationic states on chabazite zeolite, we would not expect the variation in the existing state of nickel 9924

DOI: 10.1021/jacs.9b03361 J. Am. Chem. Soc. 2019, 141, 9920−9927

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Journal of the American Chemical Society ethylene wastage rate of