Kazu
Okumura
*a,
Kai
Tanaka
a,
Akimichi
Ohtsuki
a,
Hikaru
Iiyoshi
a and
Naonobu
Katada
b
aDepartment of Applied Chemistry, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji-City, Tokyo 192-0015, Japan. E-mail: okmr@cc.kogakuin.ac.jp
bCenter for Research on Green Sustainable Chemistry, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan
First published on 1st September 2023
A methane–benzene reaction was performed over Co/MFI prepared with an aqueous solution of Co(OAc)2 and MFI zeolite. Co(OH)2 was deposited on the MFI zeolite when the Co(OAc)2 solution was stirred at 343 K in the presence of MFI. A decline of the h-peak in NH3 TPD occurred during the loading of Co on MFI. Further thermal treatment at 573–773 K caused the spontaneous formation of the atomically dispersed Co species, which was observed with Co K-edge XAFS. The toluene formation rate of the catalyst prepared with Co(OAc)2 was proportional to the Co loading and was higher than that prepared with Co(NO3)2 due to the large amount of supported Co.
The conventional Co/MFI catalysts have primarily been prepared with an ion-exchange method.6 The Co located at the α sites was active in the methane–benzene reaction7 among three kinds of ion-exchange sites (α, β, γ).8 The adsorption energy generated at the adsorption of benzene on the Lewis acid sites of Co compensated the activation energy required for scission of the C–H bond in methane, so that the reaction between methane and benzene was promoted.7 It could be supposed that one of the ways to improve the catalytic activity of Co/MFI zeolite is to increase the loading of Co with an atomically dispersed form using the ion-exchange sites of MFI. Another possible way to increase the loading of dispersed Co is to make use of the reaction between defect sites of MFI and Co. Such a method has been reported to prepare atomically dispersed species of various elements including Ti and Sb using volatile precursors.9,10 In previous studies on Co/MFI catalysts, Co(NO3)2 has been primarily used as a precursor of Co for the preparation of Co/MFI by the above-mentioned ion-exchange method under non-hydration conditions.11 The hydrolysis of Co2+ ions is inhibited under acidic or low temperature conditions.12 Other possible methods for the preparation of Co/MFI is the impregnation of Co(NO3)2 and the sublimation of volatile Co precursors.13 In the former case, aggregated and dispersed Co oxides formed simultaneously, resulting in a heterogeneous distribution of Co sizes.4 In this research, we employed cobalt acetate tetrahydrate (Co(OAc)2·4H2O) as the precursor of Co/MFI in place of Co(NO3)2 on purpose. The pH of the aqueous solution of Co(OAc)2 was ca. 7.2 at 200 mmol L−1. Under this condition, hydrolysis of Co2+ ions occurs to form Co(OH)2 spontaneously at around 343 K.14 We expected that the deposited Co(OH)2 on the surface of MFI zeolites acted as the possible precursor of active Co species in the methane–benzene reaction, which was generated through the reaction with defect or ion exchange sites in MFI zeolites. This is because Co(OAc)2 was reported to be a promising precursor to obtain well-dispersed Co species in MFI via solid-state ion-exchange reactions.15 MFI treated with an aqueous NaOH solution was also used as the support for Co, taking into account that treatment with an aqueous NaOH solution has been reported to change the porosity of MFI.16
The dispersion process of aggregated to dispersed Co species was monitored with Co K-edge X-ray absorption fine structure (XAFS), and NH3 temperature programmed desorption (NH3-TPD). Particularly, Co K-edge XAFS is powerful technique to obtain information on the structural and electronic state of the Co center, which has been applied for the characterization of Co/zeolites so far.17,18
Sample name | Structure | Abbreviation | Supplier | Cation type | Si/Al2 |
---|---|---|---|---|---|
HSZ-820NHA | MFI | 820 | Tosoh Co. | NH4+ | 23 |
HSZ-822HOA | MFI | 822 | Tosoh Co. | H+ | 24 |
HSZ-840HOA | MFI | 840 | Tosoh Co. | H+ | 40 |
HSZ-860HOA | MFI | 860 | Tosoh Co. | H+ | 72 |
HSZ-891HOA | MFI | 891 | Tosoh Co. | H+ | 1500 |
52A | MFI | 52 | Mizusawa Chemical Co. | Na+, H+ | 52 |
JRC-Z5-90NA | MFI | 90 | Catalysis Society of Japan | Na+ | 90 |
HSZ-360UHA | FAU | 360 | Tosoh Co. | H+ | 15 |
HSZ-500KOA | LTL | 500 | Tosoh Co. | K+ | 6.1 |
HSZ-640HOA | MOR | 640 | Tosoh Co. | H+ | 380 |
HSZ-930NHA | BEA | 930 | Tosoh Co. | NH4+ | 27 |
Q3 | SiO2 | SiO2 | Fuji-Silysia Co. | — | — |
Co loadings were measured with the atomic absorption (AA) instrument AA-6200 (Shimadzu Co.). The sample was prepared after dissolving Co/MFI with an aqueous solution of HF and subsequently with conc. H2SO4, followed by evaporation. The residue dissolved with hydrochloric acid was subjected to the AA measurements for quantitative analysis of Co. Data of the temperature programmed desorption (TPD) of ammonia were obtained with a BELCAT II equipment (Microtrac Bel Co.). The sample was treated at 773 K in an N2 flow prior to the measurement. Then ammonia (5%) diluted with He was equilibrated with the pretreated sample (0.1 g) at 373 K. The TPD data were collected with a temperature ramping rate of 10 K min−1. The carrier gas (He) flow rate was 30 mL min−1. A mass spectrometer (BELMASS, Microtrac Bel Co.) was used to measure the desorbed NH3. In the measurement, m/z = 16 was monitored to analyze the desorbed NH3. X-ray diffraction (XRD) patterns of the powder samples were obtained using a MiniFlex X-ray diffractometer (Rigaku Co.) with Cu Kα radiation in the 2θ range from 5 to 40° with 10° min−1 scanning speed. N2 adsorption isotherms were recorded on the BELSORP-mini-X (Microtrac Bel Co.) instrument. The samples were dehydrated in a vacuum at 413 K in advance. Transmission electron microscopy (TEM) images were obtained using a JEOL-JEM-2100 microscope. For the preparation of the sample, an ethanol suspension of Co/MFI was dropped onto Cu grids coated with a C-coated porous thin membrane (NEM, Japan) and dried. TEM observations were performed at an operating voltage of 200 kV. Field emission scanning electron microscopy (FE-SEM) images were taken with a JEOL JSM-6701F microscope with an accelerating voltage of 5 kV.
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Fig. 1 Co K-edge (a) XANES and (b) EXAFS-FT of Co/MFI(52N) (Co: 0.5 mmol g−1) and reference compounds measured at 298 K. Fourier transform range: 3–13 Å−1. |
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Fig. 2 (a) Relationship between the Co loading of Co/MFI(52N) and concentration of Co(OAc)2. (b) Relationship between Co loading and the Al concentration of Co/MFI(52N). |
The loading of Co on MFI(52N) was 0.18 mmol g−1 when a 100 mmol L−1 Co(NO3)2 aqueous solution was used as the precursor for Co under acidic (non-hydration) conditions, which has been generally employed as the Co precursor. The loaded amount of Co (0.18 mmol g−1) was comparable to that of ion-exchangeable sites of 52N (0.25 mmol g−1) assuming that the ion-exchange occurred between NH4+ and Co(OAc)2 or Co(OH)2 to form Co(OH)+ species. On the other hand, the Co loading increased to 0.48 mmol g−1 when a 100 mmol L−1 Co(OAc)2 aqueous solution was used for the Co/MFI preparation, which exceeded the ion exchange capacity of 52N (0.25 mmol g−1). The higher Co loading with the use of the Co(OAc)2 precursor suggested that the formation of dispersed Co occurred through not only the ion exchange but also the reaction with defect sites with Co(OH)2.
Then the Co K-edge XAFS data were collected in an N2 flow while the temperature was increased from 303 to 823 K in order to monitor the local structure of Co species accompanied by heat treatments. The XANES data obtained under in situ conditions every 50 K are displayed in Fig. 3(a). A gradual decrease in the white line intensity of the Co K-edge XANES was observed up to 823 K as plotted in Fig. 3(b). The changes up to 573 K observed in XANES may be due to the dehydration of Co(OH)2 to form CoO, consistent with the literature.20 The change occurred progressively in the temperature range between 303 and 823 K, as shown in Fig. S5;† fitting the intermediate-stage XANES with a linear combination of the spectra obtained at room temperature and 823 K showed that the former component continuously decreased and the latter increased instead with increasing temperature. At the same time, the intensity of the pre-edge peak (7708 eV) increased with increasing temperature up to 823 K. This pre-edge peak is attributed to the 1s to 3d transition, which is normally dipole forbidden but allowed when the metal has tetrahedral coordination due to the mixing with p-character.21,22 Therefore, the change may be due to the transformation of the local structure from 6- to 4-coordinated Co centers. This could be caused by the removal of water molecules coordinated to the Co centers during the heat treatment and subsequent reaction of Co with OH groups or acid sites in MFI.
Fig. 4(a) shows the Co K-edge EXAFS-FT of the sample in which Co was loaded on MFI(52N) using Co(OAc)2 as a precursor measured during heat treatment. Raw XAFS and k3χ(k) data are displayed in Fig. S6(a) and (b),† respectively. The Co loading of the measured sample was 0.5 mmol g−1. When these samples were heated from 573 to 823 K, the intensity of the Co–O–Co bond (2.8 Å) decreased while the Co–O peak remained almost intact. In the IR spectra measured at the same time, the intensity of isolated hydroxyl groups appearing at 3730 cm−1 remained unchanged, while the NH4+ stretching vibration peak at 3370 cm−1 decreased,23 indicating that the decomposition of NH4+ progressed during heat treatment. The intensity of the acidic hydroxyl group at 3590 cm−1 tended to decrease with increasing temperature, suggesting that ion exchange between H+ or NH4+ occurred with Co2+, but at the same time, the formation of acidic hydroxyl groups progressed as a result of the decomposition of NH4+ (Fig. S7†). The EXAFS oscillation is greatly reduced as the temperature rises because the Debye–Waller factor increases at high temperature. To prevent this temperature effect, XAFS data of the heat-treated samples were collected at 298 K (Fig. 4(b)). The corresponding k3χ(k) data are displayed in Fig. S8(a).† A significant decrease in the intensity of the second peak appearing at 2.8 Å (phase shift uncorrected) was observed in the temperature range of 573–773 K. A similar change was observed in the Co/MFI containing lower Co loading (0.3 mmol g−1, Fig. S8(b) and (c)†). The EXAFS of the as-prepared Co/MFI sample was successfully fitted after assuming Co–O and Co–(O)–Co bonds, while that of the 773 K-treated one was fitted after assuming Co–O and Co–(O)–Si bonds as listed in Table 2 and shown in Fig. S9.† The change observed after treatment at 773 K meant that the aggregated Co(OH)2 underwent self-dispersion through the interaction with MFI zeolite. Such dispersion was also found in Pd on MFI or MOR zeolites.24 Other examples are the formation and segregation of Ir–MgO, and Pt–MgO solid solutions observed in the temperature range between 773 and 1273 K.25,26 A similar change (decline) of the Co–(O)–Co bond appearing at 2.8 Å was observed in Co/SiO2 (Co loading: 0.5 mmol g−1) as displayed in the EXAFS-FT (Fig. S10(b)†); the peak was significantly reduced after the treatment at 773 K. This structural change suggested that the self-dispersion of Co occurred on Co/SiO2, similar to the case of Co/MFI.
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Fig. 4 Co K-edge EXAFS-FT of Co/MFI(52N) (Co: 0.5 mmol g−1) (a) measured under a N2 flow from 323 to 823 K under in situ conditions, the as-prepared sample and (b) measured under ambient conditions. |
Sample | Scatterer | CNa | R/Åb | ΔE0c/eV | DWd/Å | R f /% |
---|---|---|---|---|---|---|
a Coordination number. b Bond distance. c Difference in the origin of photoelectron energy between the reference and the sample. d Debye–Waller factor. e Residual factor. f Data of X-ray crystallography. g Fourier transform range: 3–13 Å−1. Fourier filtering range: 1.0–3.2 Å. | ||||||
As-prepared Co/MFI(52N) | O | 5.2 ± 1.1 | 2.07 ± 0.02 | 1 ± 3 | 0.084 ± 0.027 | 0.6 |
Co | 5.1 ± 1.3 | 3.14 ± 0.02 | 4 ± 2 | 0.086 ± 0.023 | ||
Co/MFI(52N) treated at 773 K | O | 4.9 ± 0.8 | 2.08 ± 0.01 | 4 ± 2 | 0.087 ± 0.020 | 2.3 |
Si | 2.0 ± 1.0 | 3.27 ± 0.03 | −4 ± 5 | 0.062 ± 0.067 | ||
Co(OH)2f | O | (6) | (2.13) | |||
Co | (6) | (3.20) |
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Fig. 5 NH3 TPD profile of unloaded MFI(52N) and Co/MFI(52N). The samples were treated in an N2 flow at 773 K prior to the TPD measurement. The numbers indicate the loading of Co (mmol g−1). |
NH3 TPD profiles of the employed MFI zeolites (NH4+ form) and those of untreated 822N and those treated with an aqueous solution of NaOH, followed by treatment with an NH4NO3 solution at 343 K are displayed in Fig. S11(a) and (b),† respectively. In the latter case, the intensity of the h-peak decreased in the MFI treated with an NaOH solution for 16 h. This means that the number of Al atoms in the MFI framework decreased after the NaOH treatment. In agreement with this, the XRD patterns revealed that partial collapse of the MFI crystals occurred after treatment with NaOH for 16 h as will be shown later. In general, the treatment of zeolite with NaOH solution resulted in the preferential dissolution of Si.27 Consistent with this, the total Al concentration in the MFI increased with treatment time, as shown in Fig. S12.† As for Al distribution, the amount of framework Al decreased, while that of extra-framework Al increased with increasing treatment time with an aqueous solution of NaOH (Table S1†).
XRD diffraction patterns of the as-prepared Co/MFI(52N) and those treated at 773 K as well as unloaded MFI(52N) are displayed in Fig. 6(a) and (b), respectively. For Co-loaded MFI(52N), no diffraction other than those ascribed to the MFI zeolite was observed after the loading of Co(OAc)2 (Fig. 6(a)), while the Co–(O)–Co bond appeared at 2.8 Å in the Co K-edge EXAFS as already shown in Fig. 4(a and b), indicating that Co existed on the MFI in an aggregated form. The contradictory data suggested that the crystallite size of Co(OH)2 was less than several nanometers. In agreement with the observation, no clear aggregate of Co(OH)2 was observed in the TEM image as will be noted later. Meanwhile, the overall intensity of the diffraction peaks decreased after the loading of Co. The extent was enhanced by increasing the Co loadings. This feature did not change after the thermal treatment in N2 at 773 K (Fig. 6(b)).
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Fig. 6 XRD patterns of Co-unloaded MFI(52N) and Co/MFI(52N). (a) The as-prepared and (b) 773 K-treated samples. The numbers indicate the Co loading (mmol g−1). |
XRD patterns of the as-received MFI(822N) and those treated with NaOH are displayed in Fig. S13.† The overall intensity of the diffraction declined accompanied by an increase in the concentration of the NaOH solutions, suggesting that the partial collapse of the MFI framework took place during the treatment with the NaOH solution and the extent of which was enhanced as the concentration of the NaOH solution increased.
FE-SEM images of the representative MFI(52N, 840N, 822N, and 822N-16h) employed as the support for Co are displayed in Fig. 8. The average crystal size of 840N was approximately several micrometers (Fig. 8(b)). The large crystal size was associated with the appearance of the N2 adsorption isotherm in that the isotherm of the samples was assignable to type I as shown in Fig. S14(b).† The size of the primary particle of 52N and 822N was much smaller than that of 840N; in the former case, the size was ca. 50 nm. Consistent with this SEM figure, a progressive increase in the nitrogen adsorption isotherm curve was observed at pressures above p/p0 = 0.01 (Fig. S14(b)†).
No difference was found between TEM images of Co/MFI(52N) (as-prepared sample and those thermally treated at 823 K) and Co-unloaded MFI(52N) as shown in Fig. S15,† suggesting that the degree of the aggregation of the deposited Co(OH)2 was limited in the preparation step. This fact was consistent with the XRD patterns in that no diffraction assignable to the crystalline Co(OH)2 was observed (Fig. 6(a)).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy00305a |
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