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Article

Effect of Metal Oxide–Support Interactions on Ethylene Oligomerization over Nickel Oxide/Silica–Alumina Catalysts

by
Ji Sun Yoon
1,†,
Min Bum Park
2,†,
Youngmin Kim
1,
Dong Won Hwang
1,3 and
Ho-Jeong Chae
1,3,*
1
Carbon Resources Institute, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea
2
Department of Energy and Chemical Engineering, Incheon National University, Incheon 22012, Korea
3
Department of Green Chemistry & Biotechnology, University of Science and Technology, Daejeon 34113, Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally.
Catalysts 2019, 9(11), 933; https://doi.org/10.3390/catal9110933
Submission received: 25 October 2019 / Revised: 6 November 2019 / Accepted: 7 November 2019 / Published: 8 November 2019
(This article belongs to the Special Issue Synthesis and Application of Zeolite Catalysts)
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Abstract

:
We investigated the interactions between nickel oxide and silica–alumina supports, which were applied to the catalytic oligomerization of ethylene by powder X-ray diffraction, UV diffuse reflectance spectroscopy, H2 temperature-programmed reduction, and X-ray photoelectron spectroscopy. The catalytic activity was also correlated with the acidity of catalysts determined by NH3 temperature-programmed desorption and pyridine FT-IR spectroscopy. Although all the catalysts had similar Ni contents, their catalytic performances were strongly influenced by the strength of the metal oxide–support interaction. Strong interaction promoted the formation of nickel aluminate on the catalyst surface, and resulted in low catalytic activity due to reducing the amount of nickel oxide active sites. However, weak interaction favored the aggregation of nickel oxide species into larger particles, and thus resulted in low ethylene conversion and selectivity to oligomers. Eventually, the optimal activity was realized at the medium interaction strength, preserving a high amount of both active nickel oxides and acid sites.

1. Introduction

Biofuels and biochemicals have a great deal of interest because of global warming. Among them, bioethanol is one of the most abundant sustainable raw materials of biocarbon sources today [1]. Bioethanol-derived ethylene could be widely used in the syntheses of transportation fuels, chemicals, polymer derivatives, etc. [1,2,3]. In particular, the oligomerization of bioethanol-derived ethylene over heterogeneous catalysts has been extensively investigated as part of green chemistry processes [4,5,6]. Even though heterogeneous catalysis afford low product yields due to suffering from mass transfer limitations, they do not feature the inherent drawbacks of homogeneous catalysis, such as difficulty in handling and separation from products, poor result reproducibility, and high cost, and thus being a subject of intensive research.
The heterogeneous oligomerization of ethylene has usually been attempted using Ni-based solid acid catalysts supported on several zeolites such as ZSM-5 [7,8,9,10], Y [11,12,13,14], beta [15], MCM-22 [13,16,17], and MCM-36 [16,17], and ordered mesoporous Al-SBA-15 [18,19], Al-MCM-48, and Al-MCM-41 [13,18,20,21,22,23,24]. In addition, various non-crystalline amorphous materials, i.e., silica–alumina (SA) [25,26,27,28,29,30], sulfated alumina [31,32,33,34], B2O3-Al2O3 [35], ZrO2/WO3 [36,37], ZrO2/MoO3 [38], ZrO2/SO42− [39], and TiO2/SO42− [40], have been studied as a support for the ethylene oligomerization. Since both Brønsted (B.A) and Lewis acid (L.A) sites are also crucial for ethylene oligomerization, numerous reports have demonstrated the dependence of catalytic activity on the acidity of support materials (reflected by the Si/Al ratio in case of SAs) and the nature and content of transition metals [7,18,25,28,29,36,37,41,42].
The interaction between metal oxide and support has been demonstrated in a redox process as a representative phenomenon using an iron or vanadium oxide supported on silica, zirconia, alumina, ceria, niobia, titania, etc. via Møssbauer, Raman, IR, and temperature-programmed reduction (TPR) [43,44,45]. Moreover, metal oxide–support interaction or surface oxide–support interaction has been reported to exhibit a pattern similar to metal–support interaction [43,45]. The metal–support interaction has been well established theoretically in various catalytic reactions such as the water–gas shift reaction [46], hydrogenation [47], and Fischer–Tropsch synthesis [48], and specifically divided into three types according to interaction strength, i.e., strong (SMSI), medium (MMSI), and weak metal–support interaction (WMSI). These usually depend on the loaded metal amount, calcination conditions, and surface species of support, including additives [49]. For example, SMSI can be explained from three representative viewpoints. The first explanation assumes that support ions such as Ti4+ are partially reduced by hydrogen spillover in adjacent metal particles, which causes the formation of species such as Ti4O7 [50]. According to the second explanation, SMSI changes the shape of supported metal particles to promote their dispersion [50], whereas the third explanation suggests that the reduced titanium oxide represented as TiOx migrates to surfaces of metal such as Pt, Rh, or Ni [51,52].
Herein, we performed ethylene oligomerization over heterogeneous catalysts comprising nickel oxide supported on SAs (NiOx/SA) with different Si/Al ratios to produce fuel grade hydrocarbons. In our previous study, the Ni/Siral-30 with 4 wt % of Ni (NiOx/SA-028 in this study, see below) had showed a good catalytic activity for ethylene oligomerization in a continuous fixed-bed reactor [53]. This catalyst gave almost complete ethylene conversion and stability over 100 h at 200 °C, 10 bar, and a weight hourly space velocity (WHSV) of 0.375 h−1. In a series of previous research, in this study, we investigated the effects of support acidity, Ni oxidation state, and particle size of Ni-containing species, and thus the metal oxide–support interaction on the catalytic reaction. To the best of our knowledge, this is the first study of the metal oxide–support interaction for the ethylene oligomerization. Similar to the metal–support interaction, the strength of the metal oxide–support interaction was classified with three stages of strong, medium, and weak. The electronic properties and acidity of catalysts were determined by UV diffuse reflectance spectroscopy (UV-DRS), H2-TPR, X-ray photoelectron spectroscopy (XPS), NH3-temperature programmed desorption (NH3-TPD), and pyridine-FT-IR (py-FT-IR).

2. Results and Discussion

2.1. Structural and Electronic Properties of NiOx/SA Catalysts

The chemical composition and textural properties of the catalysts prepared in this study are summarized in Table 1. Five NiOx/SA catalysts with different Si/Al ratios but nearly identical Ni contents (3.4–3.9 wt %) were synthesized by an incipient wetness impregnation method. The prepared catalysts were designated as NiOx/SA-001, -007, -028, -047, and -386, respectively, according to their Si/Al ratios. The textural properties did not show any trends according to the Si/Al ratios. Although the activity and stability of oligomerization are most affected by mass transfer (diffusion), the large pore sizes in several nanometers of these NiOx/SA catalysts may have no diffusion limitation problem.
Figure 1 shows their powder XRD patterns. For comparison, some reference patterns of the Inorganic Crystal Structure Database (ICSD) are also given in Figure 1. All the NiOx/SA catalysts featured the presence of spinel and trigonal alumina phases described by Fm3m (ICSD #30267) and R3CH (ICSD #30025) space groups, respectively. In addition, trigonal and monoclinic nickel oxides phases represented by Fm3m (NiO; ICSD #24018) and C12/m1 (NiO2; ICSD #88720) space groups were also detected. The increasing alumina content upon going from NiOx/SA-386 to NiOx/SA-001 favored a spinel structure, whereas the trigonal structure was dominant in the catalysts with low alumina content. Since the nickel oxide phase contained both NiO and NiO2 phases, we suggested the non-stoichiometric nickel oxide as the nickel oxide phase (NiOx). This can also be confirmed by H2-TPR and Ni 2p of XPS spectra (see below). NiO2 was the only nickel oxide species observed together with NiO, but other species such as Ni2O3 were absent.
It is well known that Ni2+ can migrate into the alumina lattice by solid-state diffusion during impregnation when the alumina surface is dissolved in Ni(NO3)2 solution and calcination is carried out above 550 °C [49,54,55]. For example, γ-alumina is well known to have a cation-deficient spinel structure featuring mixed tetrahedral and octahedral sites that can accommodate divalent metal cations such as Ni2+ to achieve structural stabilization [49,55]. However, in this study, even though some Ni ions were accommodated in the alumina lattice, no bulk NiAl2O4 phase was observed in the XRD pattern, which was probably due to the small Ni loading (3.4–3.9 wt %) and the well-dispersed amorphous structure of the support surface (Figure 1). In addition, only small particles of NiOx well dispersed on SAs were observed especially in NiOx/SA-001, -007, and -028, while NiOx/SA-047 and -386 exhibited aggregated NiOx particles in specific regions of supports, as revealed by high-angle-annular dark-field (HAADF)-transmission electron microscope (TEM) images and energy-dispersive X-ray (EDS) elemental maps (Figure S1).
The symmetry and coordination of catalyst surface species were investigated by UV-DRS (Figure 2). According to the literature, the reflectance spectra of NiO and NiAl2O4 exhibited distinct bands ascribed to different d-d transitions. Specifically, the bands at 377 and 715 nm were assigned to octahedral Ni2+ in the NiO lattice, with the 3A2g → 3T1g (F) transition being a fingerprint of NiO [49,56,57,58]. In addition, the band at 510 nm was ascribed to charge transfer in NiO [57]. NiO in Al2O3 exhibited significantly different absorption bands at 555 and 600–645 nm (doublet), which were attributed to 3T1 → 1T2 and 3T1 → 3T1 (P) transitions of tetrahedrally coordinated Ni2+ in the Al2O3 lattice, respectively, with the band at 416–430 nm ascribed to the 3A2g → 3T1g (P) transition of octahedrally coordinated Ni2+ in the Al2O3 lattice [49,56,57,58]. Thus, Ni2+ ions were accommodated in both octahedral and tetrahedral sites of the Al2O3 oxygen lattice, which led to nickel aluminate as a surface spinel phase [58]. Furthermore, NiO bands at 377 and 715 nm observed in NiOx/SA-386 and that at 715 nm observed in NiOx/SA-047 were in agreement with the results of EDS mapping and HAADF-TEM imaging, which revealed that NiOx/SA-386 featured aggregated NiO particles on the SA surface (Figure S1).
H2-TPR profiles were obtained to investigate the correlation between the reducibility from NiOx–SA support interaction and activity (Figure 3 and Table 2). Any distinct peaks were not detected from bare SA-007, while Ni-loaded SA catalysts showed several reduction peaks, especially in NiOx/SA-386. The observed peaks were assigned to the reduction of Ni2O3 [59], allowing the reduction of Niδ+ species to be classified into three steps based on the XRD pattern of NiOx/SA-386: (i) NiOx (1< x ≤2) to NiO at ~391 °C; (ii) the fragmentation of large NiO particles into smaller ones at ~544 °C; and (iii) a reduction of small NiO particles to Ni at ~623 °C. However, except for NiOx/SA-386, the other Al-rich NiOx/SA catalysts only exhibited a broad NiO reduction peak that appeared around 700–850 °C, which was shifted to higher temperature with increasing alumina fraction [60,61,62,63]. The peak above 800 °C was attributed to the reduction of the least reactive Ni2+ species in the surface spinel structure of NiAl2O4, i.e., nickel ions coordinated in octahedral and tetrahedral sites of alumina and strongly interacting with the support [56,58,59,64]. The formation of surface NiAl2O4 by nickel ion incorporation accounted for the high dispersion of NiO particles [65].
Reduction patterns of NiOx on different SAs depended on the strength of NiOx–SA interactions, being influenced by the alumina content and phase. The H2-TPR profile of NiOx/SA-386 showed three main peaks, corresponding to (in the order of increasing temperature) the reduction of large (bulk-like) NiO particles, the reduction of NiO weakly interacting with alumina (Al–O–Ni moieties), and the reduction of Ni species incorporated into alumina (crystalline NiAl2O4) [66,67,68,69]. Thus, the reduction peak shifted to lower temperatures with the decreasing strength of the NiOx–support interaction. Based on the obtained H2-TPR profiles, the above interaction was the strongest for NiOx/SA-001 and NiOx/SA-007, resulting in the formation of surface NiAl2O4 species and highly dispersed small and poorly reducible NiO particles [49,70]. On the other hand, NiOx/SA-386 exhibited a weak metal oxide–support interaction leading to the formation of larger NiO particles, and NiOx/SA-028 and NiOx/SA-047 exhibited a medium-strength metal oxide–support interaction.
Figure 4 shows the Ni 2p XPS spectra of the NiOx/SA catalysts prepared in this study. The Ni 2p species can be deconvoluted in three different states, corresponding to interfacial phases of NiO, NiOx, and NiAl2O4, the Ni 2p3/2 peaks of which were located at 854–856, 856–857, and 857–858 eV, respectively (Table 3). Niδ+ by NiOx was correlated with the higher binding-energy changes of Ni2+ due to the adjacent nickel oxide, silica, and alumina structures [58]. NiOx/SA-386 exhibited lower binding energies of Ni2+ (from NiO) and Niδ+ (from NiOx) due to the increased extent of electron transfer from neighboring silica in the SA structure to nickel species observed with increasing silica content. Moreover, the binding energy of Ni2+ in NiAl2O4 was increased due to electron transfer from nickel species to the neighboring alumina in the SA structure, which was promoted by the strong interaction between the surface nickel and the alumina support. The proportion of Ni2+ present as NiAl2O4 decreased from NiOx/SA-001 to NiOx/SA-386 due to the concomitantly increasing Si/Al ratio (Table 3), implying a simultaneous decrease in the surface nickel oxide–support alumina interaction strength.
As the metal oxide–support interaction changed, the number of aggregated nickel oxides also changed. The Ni2+/Niδ+ ratio was controlled in the following order: NiOx/SA-386 < NiOx/SA-001 < NiOx/SA-007 < NiOx/SA-047 < NiOx/SA-028. The Ni2+/Niδ+ ratio can be correlated with catalytic performance (see below) because Ni2+ derived from NiO as the lowest binding energy in the nickel oxidation states can be most advantageous for the catalytic performance as one of active sites in this study. The weakened metal oxide–support interaction was also confirmed by the presence of aggregated nickel oxides on the SA surface in NiOx/SA-047 and NiOx/SA-386, as determined by EDS maps (Figure S1).

2.2. Acid Site Properties of NiOx/SA Catalysts

According to the plausible mechanism, L.A sites formed by nickel oxides and SA derived B.A sites can catalyze ethylene oligomerization [22,24,71]. To be specific, ethylene is believed to be activated by L.A by nickel ion species such as Ni2+ and Niδ+, with isomerization (including hydrogen transfer) propagating and terminating at B.A sites. Since both of the above sites were thought to be involved in ethylene oligomerization, the number of acid sites can be considered to produce the oligomers as decreasing in the order of NiOx/SA-047 > NiOx/SA-028 > NiOx/SA-386 > NiOx/SA-007 > NiOx/SA-001 and NiOx/SA-047 > NiOx/SA-028 > NiOx/SA-386 ~ NiOx/SA-007 > NiOx/SA-001 determined by NH3-TPD and py-FT-IR, respectively (Figure 5 and Table 4). To investigate the effect of specific acid sites, we also compared the B.A/L.A ratios of NiOx/SA catalysts characterized by py-FT-IR, which revealed that the B.A/L.A ratio increased in the order of Ni/SA-001 ~ Ni/SA-007 < Ni/SA-028 < Ni/SA-386 < Ni/SA-047 (Table 4).

2.3. Ethylene Oligomerization over NiOx/SA Catalysts

Ethylene oligomerization was performed on the five NiOx/SA catalysts with different Si/Al ratios (0.01–3.86) at 200 °C, 10 bar, and 0.375 h−1 WHSV, which are identical to those in our previous study [2,53]. Figure 6 shows the ethylene conversion and selectivity to products. The average ethylene conversion and selectivity to C10+ are compared with the characterization results of XPS, NH3-TPD, and py-FT-IR in Figure 7. After the entire reaction time of 16 h, the average ethylene conversion decreased in the order of NiOx/SA-047 (89%) > NiOx/SA-028 (84%) > NiOx/SA-007 (74%) > NiOx/SA-001 (60%) > NiOx/SA-386 (22%). NiOx/SA-386 achieved not only the lowest ethylene conversion but also the lowest selectivity to C10+. As discussed above, despite the acid site density of NiOx/SA-386 not being the lowest, it showed weak metal oxide–support interaction, which induced nickel oxide aggregation to produce large particles. Therefore, reduced NiOx active sites should lead to the lower catalytic activity. On the other hand, NiOx/SA-001 and NiOx/SA-007 exhibited the medium level of ethylene conversion among the five catalysts. Those catalysts featured highly dispersed nickel oxides species on SA support determined by H2-TPR (Figure 3) and EDS mapping images of HAADF-TEM (Figure S1). This indicates a strong metal oxide–support (especially alumina species because of its relatively large amount) interaction that incorporated Ni species into the alumina structure, leading to producing a large amount of deactivated nickel oxide species for ethylene activation by the formation of surface NiAl2O4. In other words, ethylene can be activated by a nickel oxide, especially Ni2+ from NiO. We should note here that NiOx/SA-028 and NiOx/SA-047 exhibiting a medium metal oxide–support interaction resulted in the best catalytic performance among the catalysts. As shown in Figure 7, Ni2+ from NiO/Niδ+ from the NiOx ratio of those two catalysts were the highest and second highest, respectively. In addition, NiOx/SA-047 had the highest acid site density and B.A/L.A ratio. From the overall results, it can be concluded that the ethylene oligomerization performance can be strongly influenced by the metal oxide–support interaction strength as well as acid site properties.

3. Materials and Methods

3.1. Preparation of NiOx/SA Catalysts

Four Siral series (Siral-1, -10, -30, and -40) silica–aluminas and one other silica–alumina were purchased from Sasol and Aldrich, respectively. Those were calcined at 550 °C for 5 h in an air flow, ground, and sieved to smaller than 150 μm, being subsequently impregnated with 4 wt % of Ni by an incipient wetness method using Ni(NO3)2·6H2O (Samchun, 98%) as a precursor. The obtained catalysts were dried for 5–6 h at 120 °C, and calcined at 550 °C for 5 h under flowing air.

3.2. Analytical Methods

Powder XRD patterns were recorded at 40 kV and 40 mA using a diffractometer equipped with a multipurpose high-temperature probe (D/MAX-2500PC, Rigaku) and a Cu Kα1 source (λ = 1.54056 Å). Scanning was performed in a 2θ range of 20–80° at a rate of 1.5 deg/min. Elemental analysis and N2 sorption experiments of all the catalysts were carried out by a Thermo Fisher Scientific iCAP 7400 duo inductively coupled plasma–atomic emission spectrometer and a Micromeritics ASAP 2020 analyzer, respectively. The N2 sorption isotherms and pore size distribution curves are described in Figure S2.
UV-DRS spectra were obtained in the range of 300–800 nm with a step of 1 nm using a Lambda 1050 (Perkin Elmer) UV-vis spectrophotometer. The oxidation states of catalyst components were evaluated by XPS (PHI 5000 VersaProbe, Ulvac-PHI) using a monochromated Al Kα source (1486.6 eV), with calibration performed using the C 1s binding energy at 284.6 eV.
H2-TPR and NH3-TPD were carried out using a BELCAT-B instrument equipped with a thermal conductivity detector (TCD) at a ramping rate of 5 °C/min in temperature ranges of 50–950 °C using H2 (99.999%) and Ar (99.999%), and 100–550 °C using NH3 (29.8% with He balance) and He (99.999%), respectively.
py-FT-IR spectra were obtained by pyridine (Janssen Chimica, 99%) and a Thermo Scientific Nicolet 6700 FT-IR spectrometer equipped with an MCT-A detector. Each spectrum represented the average of 100 scans performed in the range of 1400–1700 cm−1 with a resolution of 4 cm−1, as described previously [72]. A self-pelletized catalyst (16–25 mg and 1.3 cm diameter) was evacuated at 300 °C and pressures below 1 mbar for 1 h in an IR cell equipped with a CaF2 window connected to a Schlenk line for removing any physisorbed moisture. Prior to pyridine adsorption, a background spectrum was collected with the evacuated pellet. Excess pyridine physisorbed on the catalyst surface at room temperature was removed by 1 h evacuation at room temperature and pressure below 1 mbar. The treated pellet was further evacuated at 200 °C below 1 mbar for 1 h and then cooled to room temperature for recording the FT-IR spectra. Densities of the B.A and L.A sites were calculated using distinct peaks at around 1550 and 1450 cm−1, respectively, using the equations of Emeis [73].
HAADF images and EDS elemental maps and the spectra of nickel oxide particles were obtained using an FEI Titan double Cs-corrected TEM (Titan Cubed G2 60–300, FEI) operating at 300 kV. Z-contrast conditions were achieved using a probe semi-angle of 19.3 mrad and an inner collection angle of the detector of 40 mrad. The EDS maps and spectra were obtained by an FEI Super-X detector equipped with four silicon drift detectors.

3.3. Ethylene Oligomerization

Ethylene oligomerization was performed in a fixed bed reactor filled with a catalyst (1 g, 150–250 μm) pretreated at 550 °C under He (99.9%) gas flow for 8 h. Subsequently, the reactor was filled with ethylene (99.95%, 5 mL/min, 10 bar, room temperature) and the oligomerization was performed at 200 °C and 0.375 h−1 WHSV for 16 h. Ethylene conversion and gas phase products (≤C6) were quantified online in a Younglin YL6500 gas chromatograph (GC) equipped with a TCD connected to a 60/80 Carboxene 1000 packed column (15 ft × 1/8 in × 2.1 mm, Supelco) with Ar (99.9%, 2 mL/min) as an internal standard and a flame ionization detector (FID) using an Rt–Alumina BOND/Na2SO4 capillary column (50 m × 0.53 mm × 10 μm, RESTEK), respectively. Liquid products (≥C4) were collected at −2.5 °C using a separator and quantified by another GC-FID (YL6500 GC, Younglin) equipped with capillary columns connected in series (RTX-5 DHA (5% diphenyl, 5 m × 0.25 mm × 0.5 μm, RESTEK) + RTX-100 DHA (100 m × 0.25 mm × 0.5 μm, RESTEK)). The liquid product was analyzed in an offline mode right after the reaction every 16 h. Ethylene conversion and selectivity to product i were determined using the following equations. Conversion (%) = (Fi − Ff)/Fi × 100 where Fi and Ff are the mass flow rate of ethylene inlet and outlet stream, respectively. Selectivity (%) = Pi/(Fi − Ff) × 100 where Pi is the mass flow rate of product i.

4. Conclusions

In this study, five NiOx/SA catalysts with different Si/Al ratios but similar Ni contents were prepared, characterized, especially for the interaction strength between NiOx and the SA support, and their activity for oligomerization of ethylene under identical reaction conditions was investigated. The strong interaction observed in NiOx/SA-001 and NiOx/SA-007 induced the formation of surface nickel aluminate and the reduction of active NiOx species, and thus hindered the catalytic conversion of ethylene. NiOx/SA-386, featuring a weak metal oxide–support interaction, showed low catalytic activity due to the aggregation of NiOx particles on the surface, despite the relatively large number of acid sites and high B.A/L.A ratio. In the case of NiOx/SA-028 and NiOx/SA-047, their medium metal oxide–support interaction resulted in optimal catalytic performance due to the increasing Ni2+/Niδ+ ratio and acid site density. The overall results demonstrate that the activity of NiOx/SA catalysts in ethylene oligomerization depends on electronic property of nickel oxide as well as their acidity. The high performance of ethylene oligomerization requires high amount of both active nickel oxides and acid sites at the medium interaction strength between nickel oxide and silica–alumina support.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/11/933/s1, Figure S1: HAADF-TEM images, EDS maps, and EDS spectra of NiOx/SA-001, NiOx/SA-007, NiOx/SA-028, NiOx/SA-047, and NiOx/SA-386, Figure S2: N2 adsorption–desorption isotherms of NiOx/SA-001, -007, -028, -047, and -386 and pore size distribution curves of the corresponding catalysts calculated using the BJH formalism from the N2 desorption branch isotherm.

Author Contributions

Conceptualization, D.W.H. and H.-J.C.; Experiment, J.S.Y. and Y.K.; Data analysis, J.S.Y., M.B.P., and H.-J.C.; Writing-original draft preparation, J.S.Y.; Writing-review and editing, M.B.P.; Supervision, H.-J.C.; Project administration, Y.K.; Funding acquisition, D.W.H. and H.-J.C.

Funding

This work was supported by the research programs of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by Ministry of Trade, Industry and Energy (20153010092090 and 20181110200060) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science & ICT (2019R1F1A1061214).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386. Inorganic Crystal Structure Database (ICSD) reference patterns of (a) Al2O3 (#30267, Fm3m), (b) Al2O3 (#30025,R3CH), (c) NiO2 (#88720, C12/m1), and (d) NiO (#24018, Fm3m).
Figure 1. XRD patterns of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386. Inorganic Crystal Structure Database (ICSD) reference patterns of (a) Al2O3 (#30267, Fm3m), (b) Al2O3 (#30025,R3CH), (c) NiO2 (#88720, C12/m1), and (d) NiO (#24018, Fm3m).
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Figure 2. UV-DRS spectra of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
Figure 2. UV-DRS spectra of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
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Figure 3. H2-TPR profiles of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, (E) NiOx/SA-386, and (F) SA-007.
Figure 3. H2-TPR profiles of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, (E) NiOx/SA-386, and (F) SA-007.
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Figure 4. XPS spectra of Ni 2p obtained from (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
Figure 4. XPS spectra of Ni 2p obtained from (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
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Figure 5. (a) NH3-TPD profiles and (b) py-FT-IR spectra of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
Figure 5. (a) NH3-TPD profiles and (b) py-FT-IR spectra of (A) NiOx/SA-001, (B) NiOx/SA-007, (C) NiOx/SA-028, (D) NiOx/SA-047, and (E) NiOx/SA-386.
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Figure 6. (a) Ethylene conversion as a function of time on stream (TOS) over NiOx/SA-001, NiOx/SA-007, NiOx/SA-028, NiOx/SA-047, and NiOx/SA-386 at 00 °C, 10 bar, and 0.375 h−1 WHSV and (b) selectivity to products of gas (C2–C6) and liquid phases (C4–C10+) obtained after 16 h on stream.
Figure 6. (a) Ethylene conversion as a function of time on stream (TOS) over NiOx/SA-001, NiOx/SA-007, NiOx/SA-028, NiOx/SA-047, and NiOx/SA-386 at 00 °C, 10 bar, and 0.375 h−1 WHSV and (b) selectivity to products of gas (C2–C6) and liquid phases (C4–C10+) obtained after 16 h on stream.
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Figure 7. Comparisons between (a) ethylene conversion and Ni2+/Niδ+ ratio determined by XPS and (b) selectivity to C10+ oligomers and total number of acid sites and B.A/L.A ratio determined by NH3-TPD and py-FT-IR, respectively. Conversion and selectivity are the average values obtained after the entire reaction time of 16 h performed at 200 °C, 10 bar, and 0.375 h−1 WHSV. The Ni2+/Niδ+ ratio was derived from the deconvoluted area ratio of NiO and NiOx Ni 2p3/2 XPS spectra.
Figure 7. Comparisons between (a) ethylene conversion and Ni2+/Niδ+ ratio determined by XPS and (b) selectivity to C10+ oligomers and total number of acid sites and B.A/L.A ratio determined by NH3-TPD and py-FT-IR, respectively. Conversion and selectivity are the average values obtained after the entire reaction time of 16 h performed at 200 °C, 10 bar, and 0.375 h−1 WHSV. The Ni2+/Niδ+ ratio was derived from the deconvoluted area ratio of NiO and NiOx Ni 2p3/2 XPS spectra.
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Table
Table 1. Chemical composition and textural properties of the catalysts prepared in this study.
Table 1. Chemical composition and textural properties of the catalysts prepared in this study.
CatalystSi/Al 1Ni Content (wt %) 1BET Surface Area (m2/g) 2Pore Volume (cm3/g) 4Pore Size (nm) 4
TotalMicroporousExternal 3
NiOx/SA-0010.013.81145191260.413.0
NiOx/SA-0070.073.41324213030.76.0
NiOx/SA-0280.283.94321872340.78.0
NiOx/SA-0470.473.85449234261.27.7
NiOx/SA-3863.863.80484 04840.74.9
1 Determined by elemental analysis. 2 Calculated from N2 sorption data. 3 Determined according to the t-plot method. 4 Calculated using BJH formalism from the N2 desorption branch isotherm.
Table
Table 2. The amount of reducible Ni species determined by H2-TPR.
Table 2. The amount of reducible Ni species determined by H2-TPR.
CatalystTotal H2 Uptake (mmol/g)Reducible Ni Species (mmol/g) 1
NiOx → NiOL 2NiOL 2 → NiOS 3NiOS 3 → Ni
NiOx/SA-0011.47--1.47 (827)
NiOx/SA-0071.88--1.88 (820)
NiOx/SA-0282.46--2.46 (743)
NiOx/SA-0472.21--2.21 (722)
NiOx/SA-3862.420.75 (391)1.03 (544)0.64 (623)
1 The value given in parentheses is the temperature (°C) centered at the highest reduction peak. 2 NiOL indicates aggregated particles of NiO. 3 NiOS indicates well-dispersed particles of NiO.
Table
Table 3. Relative intensities of XPS spectra of Ni 2p3/2.
Table 3. Relative intensities of XPS spectra of Ni 2p3/2.
Catalyst Ni2+ by NiONiδ+ by NiOxNi2+ by NiAl2O4Ni2+ from NiO/Niδ+ from NiOx
NiOx/SA-001Binding energy (eV)855.8856.3857.50.38
Area (a.u.)363.8969.72206.6
Area ratio (%)10.327.462.3
NiOx/SA-007Binding energy (eV)855.5856.3857.70.41
Area (a.u.)402.5973.21817.1
Area ratio (%)12.630.556.9
NiOx/SA-028Binding energy (eV)855.8856.3857.50.94
Area (a.u.)901.2909.81063.5
Area ratio (%)30.232.237.6
NiOx/SA-047Binding energy (eV)854.6856.5857.50.92
Area (a.u.)975.11057.8583.2
Area ratio (%)37.340.4 22.3
NiOx/SA-386Binding energy (eV)854.6856.8857.70.37
Area (a.u.)458.91241.0307.6
Area ratio (%)22.961.815.3
Table
Table 4. Acid site properties determined by NH3-TPD and pyridine-FT-IR (py-FT-IR).
Table 4. Acid site properties determined by NH3-TPD and pyridine-FT-IR (py-FT-IR).
CatalystNH3-TPD (mmol/gcat)py-FT-IR
B.A (mmol/gcat)L.A (mmol/gcat)B.A/L.A
NiOx/SA-0010.490.000.020.00
NiOx/SA-0070.740.000.060.00
NiOx/SA-0280.920.010.080.13
NiOx/SA-0471.060.030.120.25
NiOx/SA-3860.910.010.050.20

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Yoon, J.S.; Park, M.B.; Kim, Y.; Hwang, D.W.; Chae, H.-J. Effect of Metal Oxide–Support Interactions on Ethylene Oligomerization over Nickel Oxide/Silica–Alumina Catalysts. Catalysts 2019, 9, 933. https://doi.org/10.3390/catal9110933

AMA Style

Yoon JS, Park MB, Kim Y, Hwang DW, Chae H-J. Effect of Metal Oxide–Support Interactions on Ethylene Oligomerization over Nickel Oxide/Silica–Alumina Catalysts. Catalysts. 2019; 9(11):933. https://doi.org/10.3390/catal9110933

Chicago/Turabian Style

Yoon, Ji Sun, Min Bum Park, Youngmin Kim, Dong Won Hwang, and Ho-Jeong Chae. 2019. "Effect of Metal Oxide–Support Interactions on Ethylene Oligomerization over Nickel Oxide/Silica–Alumina Catalysts" Catalysts 9, no. 11: 933. https://doi.org/10.3390/catal9110933

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