Acetylene Selective Hydrogenation Catalyzed by Cationic Nickel

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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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03361 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Journal of the American Chemical Society

Acetylene Selective Hydrogenation Catalyzed by Cationic Nickel Confined in Zeolite Yuchao Chai,1 Guangjun Wu,1 Xiaoyan Liu,3 Yujing Ren,3 Weili Dai,1,4 Chuanming Wang,2* Zaiku Xie,2 Naijia Guan,1,4 Landong Li1,4* 1

School of Materials Science and Engineering & National Institute for Advanced Materials,

Nankai University, Tianjin 300350, P.R. China 2

State Key Laboratory of Green Chemical Engineering and Industrial Catalysis, SINOPEC

Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, P.R. China 3

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

of Sciences, Dalian 116023, P.R. China 4

Key Laboratory of Advanced Energy Materials Chemistry of Ministry of Education,

Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, P.R. China

* Corresponding Email: [email protected] (C. W.) & [email protected] (L. L.)

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 polymerization process. Under optimized conditions, 100% acetylene conversion and the ethylene selectivity up to 97% are simultaneously achieved. This catalyst system also exhibits good stability and recyclability for potential application. Spectroscopy investigations and density functional theory calculations reveal the heterolytic dissociation of hydrogen molecule and the importance of hydride and proton 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 zeolite

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confined space for small molecule activation and by linking heterogeneous and homogeneous catalysis.

Introduction The

selective

hydrogenation

of

alkynes

to

the

corresponding

alkenes,

i.e.

semi-hydrogenation, is an important and challenging type of transformation in synthetic organic chemistry with significant industrial relevance. A well-known example is the semi-hydrogenation of trace acetylene in ethylene feeds prior to polymerization process that is applied on the scale of billions of tons per annum. Palladium-based 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 over-hydrogenation 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 NixSny (9), and ternary copper-nickel-iron composite (10) have been explored as promising metallic catalysts for the selective hydrogenation of acetylene. Some oxides like ceria (11,12) and supported iron oxides (13) or isolated Fe(III)-O sites (14) 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) complex (15,16) 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 liquid-phase 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. Perfect acetylene conversion and extremely high ethylene selectivity can be simultaneously obtained at moderate reaction temperatures, making the cationic nickel confined in zeolite a


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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.

Results Construction and characterization of cationic nickel (II) confined in zeolite. 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 Supplementary Materials for details), chabazite with well-defined CHA topology and Si/Al molar ratio of ~3 was obtained (Fig. 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 (Fig. 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 adjacent oxygen anchoring sites to derive nickel containing chabazite zeolite, i.e. cal-Ni@CHA (Fig. S3).



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 samples exposure to ambient conditions, dotted lines: spectra of strictly dehydrated samples; (b) Normalized XANES spectra, (c) and 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.

During the calcination process, cationic nickel could coordinate with various oxygen anchoring sites in zeolite framework (21) and therefore lead to a mixture of nickel species on zeolite. Post-modulation was then performed and the exchangeable nickel species were selectively removed through repeated ion exchange with alkali metal cations (see Fig. 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, Na-Ni@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 (Fig. 1a; reversible dehydration occurred at elevated temperature: Fig. S5). According to these observations, the transformation of


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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 (Fig. 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 species at the atomic scale was characterized by X-ray absorption spectroscopy. The Ni k-edge XANES spectrum confirmed the exclusive valance state of +2 in dehydrated Na-Ni@CHA (Fig. 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 (Fig. 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 (Fig. S7, Table S2). These data, together with those from the UV-Vis-NIR spectra (Fig. 1a), hint to 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 (Fig. 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 (Fig. 1a, Fig. S5). In this context, the Li-, Na-, and K-Ni@CHA 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 H2-TPR profiles were shown in Fig. 1d. Three hydrogen consumption peaks centered at ~630, ~820 and ~1000 K were observed for 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 ultra-dispersed within zeolite matrix in the subnanometric form, as disclosed by the electron microscopy observations (Fig. S11 & 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 CHA framework, we compared several different structures of local active sites (Fig. S13) and found that Ni preferred sitting in the six-membered ring in which two Al atoms are in the opposite or meta position. Ni atom is four-coordinated by framework oxygen atoms, in consistence with 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 Fig. 1e. Briefly, Ni-DETA complexes were encapsulated in chabazite cages at the stage of hydrothermal synthesis and they underwent interaction with 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 structure is analogous to a typical coordination complex with Ni(II) as the central ion and chabazite framework as the ligand.


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Figure 2 Catalytic data of acetylene selective hydrogenation. (a) Comparison of different catalysts in acetylene selective hydrogenation. Reaction conditions: 0.2 g catalyst, 1% C2H2, 16% H2 in He, total flow= 50 mL min-1, GHSV=15000 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 catalyst, 1% C2H2, x% H2 in He, total flow= 50 mL min-1, GHSV=15000 h-1. (c) Regeneration test of Na-Ni@CHA in acetylene selective hydrogenation. Reaction conditions: 0.2 g catalyst, 1% C2H2, 16% H2 in He, total flow= 50 mL min-1, GHSV=15000 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 catalyst, 0.5% C2H2, 8% H2, 50% C2H4 in He, total flow= 50 mL min-1, GHSV=15000 h-1

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 (Fig. 2a, Fig. 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 over hydrogenation of acetylene to ethane at high acetylene conversions (Ni-CHA, Fig. S14), and replacing the exchangeable nickel species in cal-Ni@CHA by alkali metal cations through post-modulation process could significantly promote the ethylene selectivity. The effects of nickel site constitution on the catalytic performance of various nickel-containing zeolites were 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 (Fig. S8-S10, Table S2) and in significant contrast to the cases reported by Flytzani-Stephanopoulos et al (29,30). The types of alkali metal cations in Ni@CHA system also showed noticeable impacts on the performance of acetylene selective hydrogenation. Na-Ni@CHA appeared to be 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 chabazite ligand, as revealed by XPS analyses (Fig. S6). K-Ni@CHA exhibited low activity in acetylene selective hydrogenation due to the diffusion hindrance caused by K+ with large radius, namely the spatial effects on chabazite ligand (Table S1, Fig. 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 over-hydrogenation by-product while no C4 compounds could be detected (Fig. 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 (Fig. 2a, Fig. S14). A direct comparison of time-dependent behaviors of Na-Ni@CHA, patent catalyst Pd-Ag/Al2O3 and industrial catalyst from CNPC under their optimized reaction conditions was shown in Fig. 2b. Obviously, stable ethylene selectivity was observed on all three catalysts investigated and the highest ethylene selectivity was achieved with Na-Ni@CHA catalyst. Industrial catalyst underwent a long


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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, afterwards, 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 species during reaction. TG-DTA analysis revealed the existence of some polymeric byproducts (~3%) on spent catalyst, much less than those formed on spent Pd-Ag/Al2O3 catalyst (~6%, Fig. 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 (Fig. 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 (Fig. 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 (Fig. 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 ethylene selectivity of ~90 % at 413-473 K (Fig. 2d). The selectivity was calculated as the sum of acetylene converted with subtraction of ethane produced. Since ethane could come from both the over hydrogenation 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 ethylene wastage rate of

Acetylene Selective Hydrogenation Catalyzed by Cationic Nickel

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