Specific Localization of Aluminum Sites Favors Ethene-to-Propene

Research Article Cite This: ACS Catal. 2019, 9, 7476−7485

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Specific Localization of Aluminum Sites Favors Ethene-to-Propene Conversion on (Al)MCM-41-Supported Ni(II) Single Sites Ilia B. Moroz,† Alicia Lund,‡ Monu Kaushik,‡ Laurent Severy,†,§ David Gajan,‡ Alexey Fedorov,*,†,∥ Anne Lesage,‡ and Christophe Copeŕ et*,† †

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Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1−5, CH-8093 Zürich, Switzerland ‡ Centre de RMN à Très Hauts Champs, Université de Lyon (CNRS/ENS Lyon/UCB Lyon 1), 69100 Villeurbanne, France S Supporting Information *

ABSTRACT: Single-site Ni(II) catalytic centers supported on MCM-41-type materials were prepared via surface organometallic chemistry using tailored thermolytic molecular precursors. These materials catalytically convert ethene to propene, and their activity and stability strongly depend on the specific location of aluminum sites that are introduced in the catalyst either from the tailored Ni molecular precursor or doped in the support. The highest activity and stability are achieved when a Ni siloxide precursor is grafted on an Al-doped MCM-41 because this approach generates Ni(II) isolated sites and strong Brønsted acid sites that are both required for high catalytic performances. KEYWORDS: ethene-to-propene conversion, nickel single sites, solid-state NMR spectroscopy, DNP SENS, pyridine

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INTRODUCTION Propene is one of the most important petrochemicals and its growing worldwide demand sets existing propene production technologies under pressure.1 This trend is particularly worsened by the conversion of naphtha crackers to ethane crackers with the arrival of shale gas that further decreases propene production.2 This has therefore stimulated a resurgence of interest in on-purpose propene production routes3,4 including propane dehydrogenation,5 ethenolysis of butenes,6 and methanol-to-olefin (propene) processes.7 An alternative route to these actively researched on-purpose technologies is the so-called ethene-to-propene (ETP) reaction (Scheme 1a), which is particularly attractive as it only requires ethene, a product of ethane cracking.3 The first mention of the ETP reaction dates back to the early 1970s.8 Since then several supported catalysts based on Mo, Fe, and W as well as zeolites have been shown to produce propene using ethene as the sole feedstock (Scheme 1b,c).9−15 In the past decade, ETP was also reported on Ni-based mesoporous materials such as MCM-4116 and its aluminumdoped analogue (Al)MCM-41,17 the last catalyst featuring the most promising performance among all reported systems.17 In such catalysts, Ni is loaded into the mesoporous support by an ion-exchange method with subsequent calcination.18 The 2:1 nickel phyllosilicate (Scheme 1d) containing five- and sixmembered Si−O rings with tri- and tetracoordinated Ni sites was proposed as an active phase for the ethene dimerization19 and, possibly, the ETP reaction.16 More recently, small NiOx © XXXX American Chemical Society

species dispersed on a nonporous aluminum-doped silica have also been shown to catalyze ETP (Scheme 1b).20 Despite these advances, there is still no consensus on the structure of the active Ni sites. In addition, although the Si/Al ratio is crucial for high propene yield,17,20 the role of the Al dopant and its location in the catalyst with respect to nickel active sites is not known. Seidel-Morgenstern and co-workers associated higher catalytic activity and stability of Ni(Al)MCM-41 with the presence of medium Brønsted acidity, caused by introduction of aluminum.17 There are also debates about the oxidation state of the active site. Although Ni(II) sites are found in the as-synthesized Ni-MCM-41 materials, Iwamoto speculated that Ni(II) is reduced in situ to give Ni(I), which then enters the catalytic cycle.21 However, no paramagnetic Ni(I) sites in ETP catalysts have been evidenced so far.22 Herein, we describe the preparation and detailed atomicscale characterization of Ni(II) single sites supported on MCM-41-type materials, synthesized via a surface organometallic chemistry (SOMC) approach23 combined with thermolytic molecular precursors (TMP).24 This strategy allows accessing supported isolated metal ions with defined oxidation state and nuclearity in numerous cases.25−29 We show that the thus-prepared Ni-(Al)MCM-41 materials are Received: May 8, 2019 Revised: June 19, 2019 Published: June 25, 2019 7476

DOI: 10.1021/acscatal.9b01903 ACS Catal. 2019, 9, 7476−7485

Research Article

ACS Catalysis Scheme 1a

Molecular Ni(II) Precursors. We have first developed the synthesis of Ni(II) siloxide and aluminate thermolytic molecular precursors. [Ni(OSi(OtBu)3)2]2 is isolated in 49% yield by reacting Ni[N(SiMe3)(DIPP)]230 with 2 equiv of HOSi(OtBu)3 (see SI for details). This complex crystallizes as four independent molecules in the unit cell with similar bond lengths and angles; one of them is presented in Figure 1a. The dimeric Ni siloxide contains two nonequivalent nickel atoms featuring distorted square-planar and square-pyramidal geometries, both common for NiII species. The NiNi distance of 2.9761(3) Å exceeds the range for significant metal−metal bonding. The terminal Ni1O7 and Ni2O5 bonds have lengths of 1.8102(11) and 1.8735(11) Å, respectively. The bridging NiO1 and NiO2 distances are slightly longer than the terminal NiO bonds. Besides these bonds in [Ni(OSi(OtBu)3)2]2, one and two −OSi(OtBu)3 ligands coordinate through a OtBu fragment to Ni1 and Ni2, respectively, and these distances are longer (Ni1O3 = 2.1047(11) Å, Ni2O4 = 2.4167(11) Å, and Ni2O6 = 2.1804(11) Å). The Ni(II) complex in Al environment, Ni(Al(OiPr)4)2 (HOiPr)2, was obtained by a modification of a known literature procedure (see SI for details).31 It crystallizes from isopropanol as lime-green crystals that form a dark purple solution in benzene. We confirmed the structure of this complex by X-ray crystallography and it is identical to the one reported previously (Figure 1b).31 Nickel is found in a distorted octahedral geometry, surrounded by six oxygen atoms, featuring two isopropanol ligands (NiO = 2.124(5) and 2.093(5) Å) and two κ2-aluminate ligands. Three oxygen atoms of aluminate ligands have a distance to Ni of about 2.059(1) Å, whereas the last one has a distance to Ni of 2.039(1) Å.31 Note that oxygen atoms from isopropanol ligands are on the same face of the octahedron. Supported Ni(II) Single-Site Catalysts. When a purple− brown solution of [Ni(OSi(OtBu)3)2]2 in benzene (nominal loading of about 1 wt % of Ni) was in contact with MCM-41 partially dehydroxylated at 540 °C (1.4 OH per nm2), the solution became colorless. After 3 h, the solid material was washed with benzene and 1H NMR of the combined washings revealed the complete consumption of [Ni(OSi(OtBu)3)2]2. The resulting material was then dried under high vacuum (about 10−5 mbar) overnight and analyzed by IR spectroscopy revealing the appearance of IR bands at 3010−2820 cm−1 (stretching νCH vibrations) and 1500−1340 cm−1 (bending δCH vibrations) regions (Figure S6). Together with the elemental analysis (0.8 ± 0.3 wt % Ni, 17 ± 5 C atoms per Ni versus 24 C per Ni atom in the precursor), these results indicate the successful chemisorption of the Ni siloxide onto MCM-41−540. This material was further calcined under a flow of dry synthetic air at 400 °C. IR of the calcined material (denoted as catalyst 1) indicates a complete disappearance of the bands attributed to organic ligands and a recovery of the initial intensity of the bands associated with isolated silanol groups (3743 cm−1). The nickel loading in 1 (0.7 ± 0.3 wt %) corresponds to 0.07 Ni per nm2. No NiO nanoparticles are detected on the surface of 1 by TEM and the nickel is present as highly dispersed isolated monomeric Ni(II) sites according to XAS (vide infra). Next, surface Ni(II) sites were generated from Ni(Al(OiPr)4)2(HOiPr)2 and Al-free MCM-41 support (catalyst 2) using the same synthetic protocol as described above for the Ni siloxide complex and with the objective of generating Ni sites

a (a) The ETP reaction along with (b−d) selected reported catalysts and (e) the SOMC/TMP approach used in this work to generate single-site Ni(II) catalysts with controlled localization of Al.

active catalysts in the ETP reaction and that the specific location of Al in these materials that is controlled through the choice of the support and the molecular precursor is critical for high catalytic activity and stability. Our results suggest that the overall high catalytic performance in the ETP reaction is associated with the combination of isolated Ni(II) sites and strong Brønsted acid sites, generated by the introduction of Al in the support prior to grafting of the Ni TMP.

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RESULTS Strategy, Synthesis, and Characterization of Ni(II) Single Sites. The generation of isolated supported Ni(II) sites was investigated using two types of tailored Ni(II) molecular precursors, having either Si or Al environments (Figure 1a,b), and mesoporous supports, MCM-41 and (Al)MCM-41. The methodology involved in all cases is based on (1) grafting a Ni(II) thermolytic molecular precursor on isolated OH groups of a support partially dehydroxylated at 540 °C in high vacuum followed by (2) a calcination step in a flow of synthetic air in order to remove all remaining organic ligands (Figure 1c). Using an alternative post-treatment under vacuum generates reduced Ni(0) species, according to CO adsorption experiments monitored by IR, and this route was therefore avoided.28 Developed synthetic strategy allows us to access distinct environments around Ni(II) surface sites. Using an Al-free precursor, [Ni(OSi(OtBu)3)2]2, and parent MCM41 provides a material with Ni(II) sites in a silica environment. In contrast, combining an Al-containing precursor, Ni(Al(OiPr)4)2(HOiPr)2, with the Al-free support or vice versa, [Ni(OSi(OtBu)3)2]2 and (Al)MCM-41, produces materials with Ni(II) sites in a silica−alumina environment. Furthermore, this approach is expected to yield materials where the spatial proximity and, possibly, the structure of Ni and Al sites is influenced by the origin of aluminum sites (close Ni−Al proximity with the bimetallic precursor versus more remote Ni−Al arrangement with the (Al)MCM-41 support). 7477


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Figure 1. (a) Molecular structure of [Ni(OSi(OtBu)3)2]2 obtained from X-ray diffraction studies with ellipsoids set at the 50% probability and all methyl groups of the −OSi(OtBu)3 ligand omitted for clarity. (b) Molecular structure of Ni(Al(OiPr)4)2(HOiPr)2 based on previously reported XRD.31 (c) Preparation of highly dispersed nickel catalysts 1−3; SA stands for synthetic air.

Al atoms per Ni were found by EA (vs 30 C and 2 Al per Ni atom in the precursor) that indicates the partial release of organic ligands upon the reaction with the surface, albeit keeping the Al/Ni ratio of 2. The calcination step provides catalyst 2 that contains 0.9 ± 0.3 wt % of Ni (0.09 Ni per nm2) and 0.8 ± 0.3 wt % Al by EA, showing that the Al/Ni ratio of 2 is preserved upon calcination, while all organics are completely removed from the surface (Figure S7). As for material 1, no nanoparticles could be found in 2 by TEM, indicating that Ni is highly dispersed and corresponds to

in a close proximity to alumino-silicate domains (directly linked to OAl fragments). Contacting the dark purple benzene solution of Ni(Al(OiPr)4)2(HOiPr)2 (about 1 wt % nominal Ni loading) with MCM-41−540, yields a colorless solution, leaving no remaining molecular complex in the combined benzene washings according to 1H NMR spectroscopy, consistent with grafting and/or strong adsorption. The resulting dried material displays νCH and δCH bands in the IR spectrum characteristic for organic ligands (Figure S7). The EA result (0.8 ± 0.3 wt % Ni) is consistent with the presence of about 0.08 Ni per nm2. Moreover, 24 ± 6 C and 2.0 ± 0.2 7478


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8336 eV for [Ni(OSi(OtBu)3)2]2 that is not present in XANES of the corresponding materials 1 and 3 as well as for Ni(Al(OiPr)4)2(HOiPr)2 precursor and the material 2, suggesting a slightly different coordination environment possibly related to its dimeric structure: the presence of two pre-edge features in XANES of [Ni(OSi(OtBu)3)2]2 compound can be associated with two Ni atoms of different geometry (square-planar and square-pyramidal, vide supra). To conclude, independently of the precursor and the support, grafting and subsequent calcination leads to materials with isolated Ni(II) sites. We further fitted the extended X-ray absorption fine structure (EXAFS) data in R-space (1.0−3.5 Å) after a Fourier transform (3.0−10.0 Å−1 for supported and 3.0−12.0 Å−1 for molecular compounds, see SI). The fitting parameters are summarized in Table 1. To fit the EXAFS data of molecular compounds, grafted species, and calcined materials, similar Ni−O paths were used for the first shell, whereas for the second shell NiNi, NiSi, NiAl, and NiC paths were used depending on the material. The model used to fit the EXAFS data of [Ni(OSi(OtBu)3)2]2 contained an average number of neighboring oxygens of 4.5 (four for the first Ni atom and five for the second Ni, see the crystal structure). The second coordination shell was fitted with one shorter NiSi path for the κ2-siloxide ligand, one longer NiSi path for terminal siloxide ligand, and one NiNi path. For catalysts 1 and 3 prepared from [Ni(OSi(OtBu)3)2]2, we obtained a good fit with four equivalent Ni−O paths, and four and five Ni−Si paths, respectively. For Ni(Al(OiPr)4)2(HOiPr)2, a reasonable fit was obtained using five equivalent NiO paths, two NiAl paths, and five NiC paths. Note that coordination numbers are determined with accuracy of ±1. For catalyst 2, a reasonable fit was found with four equivalent NiO paths, two NiAl paths, and four NiSi paths. The increase in the Debye−Waller factors for the second shell after grafting and calcination suggests that a wide distribution of geometries is present on the MCM-41-supported species, consistent with the amorphous nature of the mesoporous silica surface. Although the standard EXAFS simulation cannot unequivocally determine whether a NiNi path is present for the surface species, we performed a wavelet transform (WT) analysis32−34 of the EXAFS data in order to further evaluate the presence of NiNi paths. [Ni(OSi(OtBu)3)2]2, a dimer in the crystalline state, shows a bright spot at R = 2.5−3.0 Å and k = 8.0−9.0 Å−1 that is assigned to the NiNi scattering path (see SI for the WT analysis of EXAFS data collected for NiO reference). In contrast, its grafted and calcined counterparts on MCM-41 and (Al)MCM-41 as well as the monomeric precursor Ni(Al(OiPr)4)2(HOiPr)2 and the corresponding grafted and calcined materials have no local maximum at this position (see SI) pointing toward isolated, most probably monomeric, surface Ni(II) sites. The Nature of Aluminum Centers and the Resulting Acidity of the Catalysts. 15N DNP SENS Spectroscopy of Adsorbed Pyridine. 15N dynamic nuclear polarization surfaceenhanced NMR spectroscopy (DNP SENS) of adsorbed pyridine has been recently shown to be a powerful tool to analyze the nature of the acid sites on the surface, because 15N chemical shift is very sensitive to the type of pyridine interaction with the surface and the strength of the Lewis acid−Lewis base interaction.35 Hence, we first applied this technique to study the acidic properties of catalysts 1−3.

isolated monomeric Ni(II) species as evidenced by XAS (vide infra). Finally, isolated Ni(II) sites were generated from [Ni(OSi(OtBu)3)2]2 complex and (Al)MCM-41, that is, a support doped with Al prior to the introduction of Ni,17 using the same grafting approach as described above. After dehydroxylation at 540 °C, (Al)MCM-41−540 contains 0.7 OH groups per nm2 and has a Si/Al ratio of 77, which is close to the reported optimal composition of the support performing best in the catalytic ETP transformation.17 After 3 h of the reaction between [Ni(OSi(OtBu)3)2]2 and (Al)MCM-41−540 (about 1 wt % of nominal Ni loading), the purple−brown solution becomes colorless and 1H NMR spectrum of the washings contains no signal from the starting Ni siloxide complex. The IR spectrum of the dried material contains νCH and δCH bands of the organic ligands (Figure S8) and its EA gives Ni loading of 0.9 ± 0.3 wt % with 6 ± 3 C and 1.2 ± 0.2 Al atoms per Ni. These findings indicate that the nickel complex is chemisorbed on the surface of (Al)MCM-41−540. After calcination under a flow of synthetic air at up to 400 °C, 3 shows no organic groups by IR (Figure S8) and contains 0.6 ± 0.2 wt % Ni (0.06 Ni per nm2) with an Al/Ni ratio of 1.5, which is close to that of catalyst 2. This material was also characterized by TEM (Figure S10) and XAS (vide infra) revealing the presence of only highly dispersed Ni(II) species. Characterization of the Materials by X-ray Absorption Spectroscopy (XAS). In order to further characterize the Ni surface sites in 1−3 (oxidation state and nuclearity), Xray absorption spectroscopy (XAS) was carried out. Analysis of X-ray absorption near-edge structure (XANES) data shows nearly identical K-edge energies (8342 eV) for Ni(II) precursors, the corresponding grafted surface species, and calcined materials, suggesting that Ni preserves the oxidation state +2 in all materials (Figure 2). No significant change of the position and the intensity of the pre-edge feature at 8333 eV was found between the precursors and the corresponding calcined materials, except that there is an additional feature at

Figure 2. XANES spectra of molecular precursors [Ni(OSi(OtBu)3)2]2 (solid blue line), Ni(Al(OiPr)4)2(HOiPr)2 (dashed blue line), the materials obtained by their grafting on (Al)MCM-41−540 (solid red line) and MCM-41−540 (dashed red line), respectively, and subsequent calcination (solid green and dashed green, respectively). XANES spectra of [Ni(OSi(OtBu)3)2]2 grafted on MCM-41−540 before and after calcination are shown in Figure S11. 7479


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ACS Catalysis Table 1. EXAFS Fit Parametersa sample t

[Ni(OSi(O Bu)3)2]2

Catalyst 1 Catalyst 3 Ni(Al(OiPr)4)2(HOiPr)2

Catalyst 2

neighbor

Nb

r (Å)c

σ2 (Å2)d

O Si Ni Si O Si O Si O Al C O Al Si

4.5* 1* 1* 1* 4* 4(2) 4* 5(2) 5* 2* 5* 4* 2* 4*

1.95(1) 2.70(2) 2.95(3) 3.10(7) 1.98(1) 3.14(2) 1.98(1) 3.14(2) 2.05(1) 2.87(4) 3.03(4) 1.99(1) 2.82(2) 3.15(2)

0.0119(7) 0.007(2) 0.007(2) 0.007(2) 0.0082(8) 0.011(5) 0.010(1) 0.010(4) 0.0078(6) 0.011(6) 0.008(5) 0.007(1) 0.012(2) 0.012(2)

E0 (eV) 2(2)

−3(2) −4(2) 1(2)

−4(2)

a

Parameters for thermolytic molecular nickel precursors and materials obtained by calcination of [Ni(OSi(OtBu)3)2]2 grafted on MCM41−540 (catalyst 1) and (Al)MCM41−540 (catalyst 3) and Ni(Al(OiPr)4)2(HOiPr)2 grafted on MCM41−540 (catalyst 2) Samples were measured at 295 K in a transmission mode. Note that data for Ni(Al(OiPr)4)2(HOiPr)2 were acquired in benzene solution, while other materials were pressed into selfsupported discs. bNumber of neighbors. cDistance between Ni atom and the neighbor. dDebye−Waller factor. S02 was fixed to the value obtained by fitting a Ni foil reference (0.847). Set parameters are indicated by (*).

show clear differences. Although catalyst 2 prepared by the calcination of the Al-containing precursor grafted on MCM-41 shows only peaks at 285 and 265 ppm corresponding to weak Brønsted acid sites as in catalyst 1, the spectrum of catalyst 3 with Al sites incorporated into the support prior to grafting contains two additional peaks at 235 and 203 ppm that are characteristic of pyridine coordinated to relatively strong Lewis acid sites and protonated by strong Brønsted acid sites, respectively.35 This assignment was confirmed on (Al)MCM41 support with adsorbed pyridine by one-dimensional DNP enhanced scalar- and dipolar-based 15 N{27 Al} HMQC (heteronuclear multiple-quantum coherences) experiments (Figure S49).36,37 Desorption of pyridine at 150 °C leads to a disappearance of both signals at 285 and 265 ppm for catalysts 1, 2, and 3, whereas the signals at 235 and 203 ppm remain in the spectrum of catalyst 3, providing an additional evidence for the higher acidic strength of the corresponding sites (Figures S50 and 51). These results suggest that the way of introducing aluminum sites has a great influence on the acidity of the corresponding Ni catalyst. Furthermore, IR spectroscopy of adsorbed pyridine, one of the most frequently used techniques for characterizing the acidity of the heterogeneous catalysts, was also carried out. The bands associated with weak Brønsted acid sites at 1625, 1592, 1456, and 1440 cm−1 are present in the IR spectra of all three catalysts, whereas the IR bands of the pyridinium ion (1635 and 1545 cm−1) are found only for catalyst 3, and the IR bands corresponding to pyridine coordinated to Lewis acidic Al sites (1620 and 1450 cm−1) are found for catalysts 2 and 3 (Figure S52). The latter result contrasts with what is observed by DNP SENS where only catalyst 3 showed the signal associated with Lewis acid sites. The absence of this signal for catalyst 2 can be explained by the close proximity of Lewis acidic Al sites to paramagnetic Ni(II) sites on 2, causing the fast nuclear coherence relaxation and disappearance of the signal (vide infra). Direct-Excitation 27Al MAS NMR Study. Al-containing materials, namely (Al)MCM-41, catalysts 2 and 3, were further studied by high-field NMR (23.5 T) spectroscopy under magic angle spinning (MAS) in order to obtain

Figure 3. 15N DNP SENS of catalysts 1 (green), 2 (blue), and 3 (black) after adsorption of 15N-labeled pyridine and subsequent desorption at ambient temperature at 10−5 mbar. The spectra were recorded at 14.1 T with a spinning frequency of 8 kHz. 15N DNP SENS for MCM-41, (Al)MCM-41, as well as catalysts 2 and 3 after desorption of pyridine at 150 °C are shown in SI.

Figure 3 shows 15N DNP SENS spectra of catalysts 1−3 after 15 N-labeled pyridine adsorption and subsequent desorption at ambient temperature (at about 10−5 mbar), revealing characteristic differences between materials. The spectrum of Al-free material 1 shows two peaks at 285 and 265 ppm that are also observed for the MCM-41 support and other catalysts (Figure S48); they are assigned to pyridine hydrogen-bonded to weak Brønsted acid sites.35 Noteworthy, the spectra of catalysts 2 and 3 that both contain Al atoms, 7480


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Figure 4. (a) Direct excitation 27Al NMR spectra of (Al)MCM-41 (purple) and catalyst 3 (black) under inert conditions. (b) Direct excitation 27Al NMR spectra of 2 before (blue) and after (red) pyridine adsorption. (c) Direct excitation 27Al NMR spectrum of 3 after pyridine adsorption. For each spectrum, 122 880 scans were accumulated (about 20 h measurement time). The spectra were recorded at 23.5 T with a spinning frequency of 33.3 kHz. Asterisks denote spinning side bands. (d) 27Al DNP SENS of 3 before (black) and after (red) pyridine adsorption. For each spectrum a total of 4096 scans were accumulated. The spectra were recorded at 18.8 T with a spinning frequency of 10 kHz. A proton enhancement ε 1H of around 30 was obtained. MW stands for microwave.

feature at about 10 ppm (A′) (Figure 4b). Signal D′ was not observed for catalyst 3 and corresponds to Al(IV) sites in a more symmetrical environment than Al sites characterized by signals B′ and B, consistent with a maximum closer to 60 ppm. Adsorption of pyridine dramatically changes the 27Al NMR profiles of (Al)MCM-41 and of catalyst 3, resulting in the appearance of an intense signal in 27Al NMR spectrum with the maximum at 52 ppm (Figure 4c), typical for Al(IV) sites. We thus reason that pyridine perturbs the geometry of Al(IV) sites on the surface of catalyst 3, making them more “visible” in 27Al NMR by lowering their CQ values. This observation supports our assignment of signal B to Al(IV) sites discussed above. Note that in a sharp contrast the NMR signature observed for catalyst 2 is only slightly affected by the pyridine adsorption (Figure 4b), suggesting that Al sites are not close to pyridine or not perturbed by its adsorption. 27 Al DNP SENS. For catalyst 3, aluminum atoms can be located both in the bulk of the material or at the surface. In order to assess more specifically the atoms close to the surface, 27 Al CP (Cross-Polarization) MAS DNP SENS at high

structural information about all aluminum sites in these materials. The 27Al NMR spectra of (Al)MCM-41 and catalyst 3 are qualitatively very similar and dominated by two relatively sharp features A and C centered at 13 and 70 ppm, respectively, along with a broader signal B at around 35 ppm (Figure 4a). Signal A corresponds to Al(VI) sites and points to the presence of extra-framework alumina generated during the preparation of (Al)MCM-41 via the sol−gel method.38 Signal C is another relatively sharp signal and it has a chemical shift typical for Al(IV) sites, probably associated with extra-framework alumina, although this signal could also belong to tetrahedral Al(IV) sites embedded in the lattice of (Al)MCM-41.39 Although signal B is in the range of chemical shifts typical for Al(V) sites, its unique dominating feature points to highly distorted Al(IV) sites with a large CQ value, leading to a significant upfield shift relative to the expected δiso of 60 ppm (vide infra);39,40 these sites are likely embedded in the lattice of (Al)MCM-41. On the contrary, catalyst 2 demonstrates a very different 27Al spectrum with the most intense signal D′ appearing at around 52 ppm along with two shoulders at 70 (C′) and 40 ppm (B′), as well as a small 7481


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ACS Catalysis magnetic field was applied using the recently developed HyTEK2, a hybrid BDPA-nitroxide biradical, that has been shown to yield high DNP enhancements at magnetic fields of up to 21.1 T.41 The 27Al DNP SENS spectra of catalyst 3 with and without pyridine are shown on Figure 4d. Compared to the direct excitation spectrum (Figure 4a), we observe a single peak at around 50 ppm typical for Al(IV) sites (Figure 4d, bottom spectrum) by DNP SENS, that should be located on the surface of the material. Upon pyridine adsorption, the same resonance is observed with a higher sensitivity (Figure 4d, top spectrum). Note that 27Al NMR spectra of catalyst 3 after pyridine adsorption are similar at 18.8 T (CP) and 23.5 T (direct excitation), confirming that adsorption of pyridine decreases CQ of Al sites at the surface. In contrast, no signal was observed for catalyst 2 when cross-polarization was used, even after exposure to pyridine. This can be explained by paramagnetic bleaching of the 27Al NMR signal. Taking into account that the precursor contains Al in a close proximity to Ni(II) sites (the second coordination shell), such proximity with paramagnetic Ni centers on the surface could lead to a shortening of the nuclear coherence lifetimes, leading to disappearance of the signals. For catalyst 3, the observation of Al sites by cross-polarization indicates that the surface Al atoms are likely not close to Ni(II). Catalytic Performance in the ETP Reaction. The activity of catalysts 1−3 was then investigated in a flow reactor (5 mL min−1, GHSV = 0.2 LC2H4 h−1 gcat−1, 10% C2H4 in Ar) at 350 °C. All materials demonstrate an initial activity in the ETP transformation producing propene (Figure 5a) but also 1-butene as well as cis- and trans-2-butenes that likely arise from the dimerization of ethene on Ni(II) sites and subsequent double-bond isomerization.42−49 However, the stabilities of the catalysts differ significantly: although catalysts 1 and 2 undergo almost complete deactivation within 10 h on-stream, catalyst 3 remains active even after 3 days with an initial conversion reaching 70% that decreases to about 10% (Figure 5a). Note that (Al)MCM-41 as well as MCM-41 supports show no ethene conversion (Figure S54). Initial productivity in propene is also higher on 3 (3.5 g gNi−1 h−1) than on 1 (0.8 g gNi−1 h−1) and 2 (1.5 g gNi−1 h−1) as shown on Figures 5b, S55, and S56. However, it decreases significantly on 3 within 20 h on-stream (down to 2 g gNi−1 h−1), whereas the productivities in butenes change only slightly, showing that while ethene dimerization activity is maintained, the production of propene slows down, suggesting that these reactions require different active sites, Ni(II) for dimerization and acid sites for the formation of propene (vide infra).22 Note also that significant amounts of iso-butene are detected using catalysts 2 and 3, which contain Al atoms, unlike 1 (Figures 5b, S55, and S56), consistent with formation of iso-butene via isomerization of linear butenes or cracking of higher hydrocarbons. In contrast to 1 and 2, catalyst 3 also produces a mixture of C1−C5 alkanes and the conversion to alkanes is significantly higher at the beginning of the catalytic test and constantly decreasing with time on stream (Figures 5b, S55, and S56). Only ethane as a nonalkene product was found with catalysts 1 and 2. Note that the total alkane selectivity on catalyst 2 is significantly higher than total alkane selectivities of other catalysts tested in this work. Moreover, the initial carbon balance on catalyst 3 is only 69%, exceeding 90% only after 20 h on-stream (Figure 5b). The low initial mass balance for catalyst 3 points at the formation of higher amounts of nonvolatile hydrocarbons and/or coke (9.1 wt %

Figure 5. (a) Conversion of ethene on 1 (green), 2 (blue), and 3 (black) at 350 °C (5 mL min−1, 10% C2H4 in Ar, GHSV = 0.2 LC2H4 h−1 gcat−1) over 18 h on-stream for 1 and 2 and 90 h for 3. (b) Productivities of propene (red), 1-butene (blue), trans-2-butene (orange), iso-butene (cyan), cis-2-butene (magenta), alkanes (green), and carbon balance (black) with time-on-stream for catalyst 3 (see SI for details).

C, 90 h TOS) than for catalysts 1 (2.9 wt % C, 18 h TOS) and 2 (4.7 wt % C, 18 h TOS) with a carbon balance close to 100% and >90%, respectively. Nevertheless, all three catalysts change from initially pale-gray color to black. The activity of an additional material, containing Al sites from both the molecular precursor (Ni(Al(OiPr)4)2(HOiPr)2) and the support ((Al)MCM-41), catalyst 4 (see SI for the full characterization details), was also investigated in the ETP reaction. While it is more active and stable than catalysts 1 and 2 (Figures S57 and S58), it demonstrates lower initial ethene conversion and propene productivity than catalyst 3 that contains Al sites only from the support (Figure 5, S57, and S58). XAS analysis of spent catalysts 1−3 reveals a partial reduction of Ni(II) sites into Ni(0) species, however, the amount of Ni(0) and Ni(II) is difficult to quantify after the reaction due to the very low Ni loading. Nevertheless, because nickel nanoparticles on silica and silica−alumina supports do not show any activity in the ETP reaction,17 the small amount of Ni(II) sites are likely responsible for the remaining ETP and dimerization activity of the catalyst (Figure 5). The formation of Ni(0) species was further supported by TEM, where Ni nanoparticles were observed on all samples after the ETP 7482


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formation deactivate faster than sites producing butenes. Overall, the data clearly point to the main differences between catalysts 1−3, that is, the unique presence of Brønsted acid sites in the latter, whereas Lewis acidity is detected for 2 and 3 but not for 1. Because all three catalysts contain isolated Ni(II) sites, the specific catalytic performance (activity, selectivity, and stability) of catalyst 3 clearly indicates that the Bronsted acidity, only present in the latter, is key to its catalytic property. These findings are in line with the proposed mechanism of ETP where ethene is first oligomerized on Ni(II) sites, whereas strong Brønsted acid sites catalyze cracking of higher olefins forming propene as one of the products (Scheme 2).22 In the

reaction (Figure S10) in sharp contrast to as-synthesized materials. Moreover, TEM shows the formation of carbon deposits mostly around the Ni particles (Figure S10) that are known to catalyze the formation of coke.50−52

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DISCUSSION The SOMC/TMP approach using tailored Ni(II) precursors and MCM-41 supports provides access to isolated Ni(II) sites supported on mesoporous silica (catalyst 1) and silica− alumina (catalysts 2 and 3), where the Ni density is similar for all materials (about 0.08 per nm2). Furthermore, in all cases Xray absorption spectroscopy complemented by the WT analysis shows that calcined materials 1−3 contain isolated Ni(II) sites. Moreover, although catalysts 2 and 3 have similar Al/Ni ratios (2 vs 1.5), their Al−Ni arrangement differs, that is, with Al in the vicinity of Ni in catalyst 2 and more distant in catalyst 3, as controlled by the choice of the bimetallic molecular precursor or the Al-doped support. According to 27 Al NMR, catalyst 3 and the corresponding support (Al)MCM-41 contain both Al(IV) and Al(VI) sites, typical for silica−alumina and extra-framework alumina phase, respectively. In contrast, catalyst 2 contains mostly Al(IV) sites whose geometry is different from other samples: while catalyst 2 possesses Al sites in a rather symmetrical environment (not affected by the pyridine adsorption), a significant fraction of Al(IV) sites in catalyst 3 has a distorted geometry and cannot be observed by conventional 27Al NMR spectroscopy unless pyridine is adsorbed or a very high magnetic field is used (such as 23.5 T in this work). The change in the NMR profile upon pyridine adsorption indicates a perturbation of geometry of Al sites that induces a decrease of their quadrupolar coupling constant, suggesting that these Al(IV) sites are close to the surface. 27Al DNP SENS also allows observing these sites by enhancing the NMR signal via polarization transfer and by decreasing their CQ due to the interaction with the radical solution used for DNP. To summarize, catalyst 1 contains isolated Ni(II) sites in pure silica environment, catalyst 2 possesses Ni(II) sites and rather symmetrical Al(IV) sites in a close proximity, whereas catalyst 3 contains Al(IV) sites in a distorted geometry that are more remote from Ni(II) sites, along with extra-framework Al sites. Another major difference between these three materials is their acidity that was studied by 15N DNP SENS and IR spectroscopy of adsorbed pyridine. Although catalysts 1−3 all contain weak Brønsted acid sites (Si−OH groups), catalyst 3 also contains strong Brønsted acid sites able to protonate pyridine. In addition, both catalysts 2 and 3 also possess Lewis acidity (Al sites), which can only be observed by IR for 2, likely because of the close proximity of Al sites to the paramagnetic Ni(II) sites that induces the fast relaxation and the disappearance of the corresponding NMR signal. We have also shown that the presence or absence of Al atoms and their localization have a great impact on catalysis with catalyst 3 greatly outperforming catalysts 1 and 2 in terms of conversion to propene and catalyst stability. With the initial ethene conversion of 68%, catalyst 3 stays active even after 3 days on-stream with a remaining ethene conversion of 10%. By comparison, catalysts 1 and 2 fully deactivate after 10 h onstream demonstrating initial ethene conversion of only 15 and 50% for 1 and 2, respectively. The propene productivity is also higher for 3, however decreases significantly within 20 h onstream, whereas the productivities in butenes only slightly change, demonstrating that sites responsible for propene

Scheme 2. A Proposed Mechanism for the ETP on Catalyst 3a

a

Where [Ni] is Ni(II) Single Sites Supported on (Al)MCM-41 and BAS are Brønsted Acid Sites.

absence of efficient cracking, catalyzed by Brønsted acid sites, the Ni(II) sites likely deactivate by adsorption of heavy oligomers. This would explain the much faster deactivation of catalysts 1 and 2 and the higher stability of catalyst 3 with strong Brønsted acidity that enables cracking, also consistent with two pathways for catalyst deactivation: (i) reduction of Ni(II) to inactive Ni(0) in the form of nanoparticles as observed by TEM, and (ii) formation of carbonaceous deposits over the duration of the ETP reaction.

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CONCLUSIONS The detailed characterization of catalysts 1−3 suggests that the Ni surface density, oxidation state of the metal, the nuclearity of Ni sites, and remaining OH density of the support is very similar and does not explain the difference in catalytic behavior. On the other hand, catalyst 3 dramatically differs from 1 and 2 in terms of acidity, the former specifically containing strong Brønsted acid sites along with some Lewis acid sites. These well-defined Ni(II)-based ETP catalyst with a controlled location of Al sites show that not only Al-doping but also the arrangement of Al is important for catalysis, a key requirement being the introduction of strong Brønsted acidity. The combination of Ni(II) sites and strong Brønsted acid sites leads to more active and stable catalyst for ethene-to-propene transformation. 7483


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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

(1) Harold, A. W.; Bryan, G. R.; Jeffrey, S. P. Industrial Organic Chemicals, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. (2) Kerr, R. A. Natural Gas From Shale Bursts Onto the Scene. Science 2010, 328, 1624−1626. (3) Copéret, C.; Allouche, F.; Chan, K. W.; Conley, M. P.; Delley, M. F.; Fedorov, A.; Moroz, I. B.; Mougel, V.; Pucino, M.; Searles, K.; Yamamoto, K.; Zhizhko, P. A. Bridging the Gap between Industrial and Well-Defined Supported Catalysts. Angew. Chem., Int. Ed. 2018, 57, 6398−6440. (4) Sattler, J. J. H. B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 2014, 114, 10613−10653. (5) Derossi, S.; Ferraris, G.; Fremiotti, S.; Garrone, E.; Ghiotti, G.; Campa, M. C.; Indovina, V. Propane Dehydrogenation on Chromia/ Silica and Chromia/Alumina Catalysts. J. Catal. 1994, 148, 36−46. (6) Mol, J. C. Industrial Applications of Olefin Metathesis. J. Mol. Catal. A: Chem. 2004, 213, 39−45. (7) Tian, P.; Wei, Y.; Ye, M.; Liu, Z. Methanol to Olefins (MTO): From Fundamentals to Commercialization. ACS Catal. 2015, 5, 1922−1938. (8) O’Neill, P. P.; Rooney, J. J. Direct Transformation of Ethylene to Propylene on an Olefin Metathesis Catalyst. J. Am. Chem. Soc. 1972, 94, 4383−4384. (9) Ghosh, A. K.; Tanaka, K.; Toyoshima, I. Homologation, Hydrogenolysis, and Dehydrogenation of Ethane on SiO2- and SiO2−Al2O3-Supported Reduced Molybdenum Oxide Catalysts. J. Catal. 1987, 108, 143−152. (10) Suzuki, T.; Hirai, T.; Hayashi, S. Enhancement of the Ethylene Conversion to Propylene on Reduced Molybdena Silica Catalyst in the Presence of Hydrogen. Int. J. Hydrogen Energy 1991, 16, 345−348. (11) Hugues, F.; Besson, B.; Basset, J. M. Selective Formation of Propene from CO + H2 or C2H4 with Fe3(CO)12 Supported on Inorganic Oxides. Mechanistic Implication in Fischer−Tropsch Synthesis. J. Chem. Soc., Chem. Commun. 1980, 719−721. (12) Epelde, E.; Aguayo, A. T.; Olazar, M.; Bilbao, J.; Gayubo, A. G. Modifications in the HZSM-5 Zeolite for the Selective Transformation of Ethylene into Propylene. Appl. Catal., A 2014, 479, 17− 25. (13) Lin, B.; Zhang, Q.; Wang, Y. Catalytic Conversion of Ethylene to Propylene and Butenes over H-ZSM-5. Ind. Eng. Chem. Res. 2009, 48, 10788−10795. (14) Oikawa, H.; Shibata, Y.; Inazu, K.; Iwase, Y.; Murai, K.; Hyodo, S.; Kobayashi, G.; Baba, T. Highly Selective Conversion of Ethene to Propene over SAPO-34 as a Solid Acid Catalyst. Appl. Catal., A 2006, 312, 181−185. (15) Taoufik, M.; Le Roux, E.; Thivolle-Cazat, J.; Basset, J.-M. Direct Transformation of Ethylene into Propylene Catalyzed by a Tungsten Hydride Supported on Alumina: Trifunctional Single-Site Catalysis. Angew. Chem., Int. Ed. 2007, 46, 7202−7205. (16) Iwamoto, M.; Kosugi, Y. Highly Selective Conversion of Ethene to Propene and Butenes on Nickel Ion-Loaded Mesoporous Silica Catalysts. J. Phys. Chem. C 2007, 111, 13−15. (17) Alvarado Perea, L.; Wolff, T.; Veit, P.; Hilfert, L.; Edelmann, F. T.; Hamel, C.; Seidel-Morgenstern, A. Alumino-Mesostructured Ni Catalysts for the Direct Conversion of Ethene to Propene. J. Catal. 2013, 305, 154−168. (18) Frey, A. S.; Hinrichsen, O. Comparison of Differently Synthesized Ni(Al)MCM-48 Catalysts in the Ethene to Propene Reaction. Microporous Mesoporous Mater. 2012, 164, 164−171. (19) Tanaka, M.; Itadani, A.; Kuroda, Y.; Iwamoto, M. Effect of Pore Size and Nickel Content of Ni-MCM-41 on Catalytic Activity for Ethene Dimerization and Local Structures of Nickel Ions. J. Phys. Chem. C 2012, 116, 5664−5672. (20) Stoyanova, M.; Schneider, M.; Pohl, M.-M.; Rodemerck, U. Direct Conversion of Ethylene to Propene over Ni Impregnated by Incipient Wetness on Silica-Alumina. Catal. Commun. 2017, 92, 65− 69.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01903. General procedures and chemicals used, synthetic details for molecular precursors and catalysts, XRD data for the molecular precursors, characterization of supports, FTIR spectra and TEM images for the catalysts, details of XAS spectroscopy, XANES and EXAFS spectra with fitting and wavelet transform analysis, experimental details for the catalytic ETP tests in a continuous flow reactor, conversion to products as a function of time-on-stream, experimental details for DNP SENS and ssNMR measurements, and additional NMR spectra (PDF) Additional information (CIF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ilia B. Moroz: 0000-0002-7357-9100 Alicia Lund: 0000-0001-7520-9544 Laurent Severy: 0000-0002-6546-379X Alexey Fedorov: 0000-0001-9814-6726 Anne Lesage: 0000-0003-1958-2840 Christophe Copéret: 0000-0001-9660-3890 Present Addresses §

(L.S.) Department of Chemistry, University of Zürich, 8057 Zürich, Switzerland. ∥ (A.F.) Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland. Funding

This work was financially supported by the Holcim Stiftung (Habilitation fellowship to A.F.), Equipex contracts ANR-10EQPX-47−01, ANR-15-CE29-0022-01, and ANR-17-CE290006-01 and Swiss Government Excellence scholarship (Ph.D. fellowship to I.B.M.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.B.M. is grateful to Swiss Government Excellence scholarship for financial support. Dr. Victor Mougel and Dr. Michael Wörle are thanked for solving the crystal structure of the Ni(II) siloxide complex. We are also grateful to Dr. Dmitry Lebedev and Dr. Tsung-Han Lin for measuring SAXS and taking TEM images. We acknowledge the Paul Scherrer Institute, Villigen, Switzerland for provision of synchrotron radiation beamtime at Super-XAS beamline of the SLS and would like to thank Dr. Maarten Nachtegaal for assistance as well as Dr. Kim Larmier, Jordan Meyet, Christopher P. Gordon, Ka Wing Chan, Dr. Nicolas Kaeffer, Dr. Jorge M. Burak, Lukas Rochlitz, Philipp Antkowiak, Dr. Gina Noh, Erwin Lam, and Dzulija Kuzmenko for their help with XAS experiments at SLS. Dr. Dmitry Lebedev and Dr. Olga Safonova are acknowledged for helpful discussions on fitting the EXAFS data and carrying out the WT analysis. Dr. Deni Mance is also acknowledged for the assistance with a DNP spectrometer. 7484


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