Zeolites containing transition metals are well known for their excellent catalytic performance in oxidation/reduction reactions in addition to exerting shape selectivity to reactants, intermediates and products [1, 2]. The three-dimensional microporous channel system of zeolites serves as the host to disperse metal or metal oxide particles, and the pore size of the channel/cavities can restrict the growth or sintering of the nanoparticles even at high reaction temperatures [3, 4]. Cobalt ion-or oxide-containing zeolites have drawn particular interest because of their wide applications in various hydrocarbon oxidation reactions, such as alkene selective oxidation to its terminal alcohol [5], cyclohexane to the corresponding alcohol and ketone [6], and Fischer-Tropsch reactions [7]. Conventionally, cobalt-containing zeolites are synthesized via hydrothermal synthesis, ion-exchange or impregnation; and the corresponding cobalt species exist as framework Co²⁺, ion-exchanged Co²⁺ or as extra-framework Co₃O₄or Co metal particles, respectively. The coordination, chemical state and size of the cobalt species in cobalt-containing zeolites exert great influence over the physical and chemical properties, and by extension, determine the applications of these zeolites in specific catalytic reactions. Thomas et al. [5] and Lin et al. [8] reported that aluminophosphate zeolites containing framework Co²⁺, such as CoAPO-18 and CoAPO-5, are highly active in the selective oxidation of linear alkanes and cyclohexane. Tang et al. [9] demonstrated that Co²⁺-exchanged FAU-type zeolites are highly efficient in styrene epoxidation. Tang et al. [10] reported that Co₃O₄ nanoparticles supported on X and Y zeolites exhibit high activity in Fischer-Tropsch reactions. Recently, cobalt monoxide (CoO) nanoparticles have gained increasing attention for their high activity in the areas of electrocatalysis [11] and photocatalysis [12]. However, there are few reports of the catalytic properties of CoO particles in oxidation reactions. According to Yang [13]and Li [14, 15], Co₃O₄ is thermodynamically more stable than CoO, and CoO can spontaneously transform into Co₃O₄ even under low O₂ partial pressures. Therefore, with CoO easily converting to Co₃O₄ under O₂ atmospheres, the applications of CoO in catalytic oxidation reactions are hindered greatly.

In the present work, we report the facile hydrothermal synthesis of a CoO-containing zeolite Beta from a homogenous emulsion comprising hydrofluoric acid, a silicate precursor and a cobalt salt under mild conditions at pH=5‒6. The synthesized products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), H₂-temperature- programmed reduction (H₂-TPR) and transmission electron microscopy (TEM), which demonstrate the presence of sub- nanoscale CoO particles within the framework of zeolite Beta. The synthesized CoO-containing zeolite Beta products show considerable activity in ethylbenzene oxidation reactions with 64.7% selectivity to acetophenone and 21.3% selectivity to 1-phenylethanol, which present significantly higher selectivity than previously reported cobalt-containing zeolites. The high activity of the CoO-containing zeolite Beta catalysts is ascribed to the well-isolated ultra-small CoO particles, which act as excellent surface oxygen carriers and donors.

Zeolite Beta catalysts containing ultra-small CoO particles (Co-Beta-H) were synthesized as follows. 18.375 g of tetraethylammonium hydroxide solution (TEAOH; 40 wt%) was mixed with 10 g of deionized water and 3.25 g of hydrofluoric acid (HF). Thereafter, 0.291 g of cobalt(Ⅱ) nitrate hexahydrate and 20.833 g of tetraethyl orthosilicate were added to the above solution. The mixture was stirred in a water bath at 60 ℃ until a concentrated emulsion formed having a molar composition of 0.5 TEAOH : 1 SiO₂ : x CoO : 0.65 HF : 5 H₂O, where x = 0.005, 0.01 or 0.02. The gel was transferred to a Teflon-lined stainless steel autoclave and heated at 140 ℃ for 3 d. The pink solid products were filtered, dried at 100℃ overnight before calcining at 550 ℃ for 5 h to remove the occluded TEAOH. The resultant materials were labelled according to the sol-gel Co/Si ratios, as Co-Beta-H0 (Co/Si = 0.005), Co-Beta-H1 (Co/Si = 0.01) and Co-Beta-H2 (Co/Si = 0.02).

Siliceous zeolite Beta (Si-Beta) was synthesized as in the aforementioned procedure, except that no cobalt salt was added to the sol-gel.

Zeolite Beta containing ion-exchanged Co²⁺ (Co-Beta-E) was prepared following a modified ion-exchange method adopted by Armor's group [16]. 2.0 g zeolite Beta having a Si/Al ratio of 25 was stirred together with 25 mL 2 wt% cobalt(Ⅱ) nitrate solution at 60 ℃ for 24 h. After the ion-exchange procedure, the mixture was filtered, and the recovered solids dried at 100 ℃. The ion-exchange filtration process was repeated four times before recovering the final material by drying at 100 ℃ overnight.

Zeolite Beta containing impregnated Co₃O₄ (Co₃O₄-Beta) was prepared by impregnating zeolite Beta with cobalt(Ⅱ) nitrate solution. 2.0 g of Si-Beta zeolite was mixed with 1.25 mL 5 wt% cobalt(Ⅱ) nitrate solution (roughly corresponding to a Co-Beta-M with the Co/Si molar ratio of 0.01) at room temperature. Thereafter, the mixture was dried at 100 ℃ for 6 h followed by calcination at 550 ℃ for 5 h before use.

Zeolite Beta containing surface CoO (CoO-Beta-M) was prepared by grinding 0.016 g of CoO with 2 g of calcined Si-Beta in a N₂-atmosphere glovebox for 30 min. The mixed product was maintained under the inert atmosphere until needed.

Co-containing aluminophosphate zeolite, CoAPO-5, was prepared by modifying a previously reported method by Thomson et al. [17]. 0.291 g of cobalt(Ⅱ) nitrate hexahydrate and 10.21 g of aluminum isopropoxide were added to 30 mL water, and stirred for 30 min to homogenize. Thereafter, 5.76 g of phosphoric acid, 8.10 g of trimethylamine and 50 mL water were added to the solution. The mixture was stirred in a water bath at 60 ℃ to yield a pink solution having a molar composition of Al₂O₃ : P₂O₅ : 0.01 CoO : 1.6 TEA : 20 H₂O. The gel was transferred to a Teflon-lined stainless steel autoclave and heated at 190 ℃ for 3 d. The blue solids, recovered by filtration, were dried at 100 ℃ overnight. CoAPO-5 samples were calcined at 550 ℃ for 5 h to remove the occluded trimethylamine.

SEM was performed using a JSM 7800F field emission scanning electron microspore operating at 1 kV. N₂ isotherms were measured using a Micromeritics ASAP 2420 apparatus at ‒196 ℃. Inductively coupled plasma (ICP) analysis was performed using a PerkinElmer Optima 7300DV instrument. Powder XRD data was acquired from a PANalytical X'Pert Pro diffractometer equipped with a Pixel detector using Cu K_α1 radiation (λ = 0.15406 nm) operated at 40 mA and 40 kV with a scanning speed of 5°/min. Diffuse reflectance ultraviolet-visible (DRUV-Vis) spectra were obtained using an Hitachi U-3900H spectrophotometer equipped with a BaSO₄ integrating sphere. XPS measurements were performed in an ultra-high vacuum multipurpose surface analysis system (ESCALAB 250Xi) using a conventional X-ray source (XR-50, Specs, Al-K_α, 1486.6 eV) in a "constant analyzer energy" mode. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon. ²⁹Si solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) measurements were acquired using an Agilent DD2-500 MHz spectrometer at room temperature. H₂-TPR experiments were performed using a Micromeritics AutoChemII 2920 in a 10% H₂/90% Ar atmosphere (50 mL/min); the temperature was increased from 40 to 900 ℃ at a 10 ℃/min heating rate. TEM investigations were conducted using a JEM-2100 EX field emission electron microscope.

Ethylbenzene oxidation reactions were evaluated in a 50 mL Teflon-lined autoclave reactor containing a magnetic stirrer bar. Typically, 10 mL of ethylbenzene and 50 mg of catalyst were placed in the reactor. The reactor was then sealed and heated to the reaction temperature. The batch reactor was continuously charged with an O₂ atmosphere at a constant pressure of 1.0 MPa after reaching 120 ℃ for 6 h. Reaction products were identified by gas chromatography (GC, Agilent 6890N GC/5973 MS detector) and quantified with an internal standard (1, 4-dichlorobenzene) using an Agilent 6890D GC equipped with an Innovex capillary column (50 m × 0.32 mm × 0.4 μm). Triphenylphosphine (1 mol/L in THF) was added to one of the samples to reduce the peroxide product to the alcohol.

Fig. 1 shows that the crystals of Si-Beta and Co-Beta-H zeolites have truncated bipyramidal morphologies that are typical of zeolite Beta [18]. Particle size distribution is in the range of 40–60 μm. Compared with Si-Beta crystals, Co-Beta-H crystals have more surface fractures and holes, which become more prominent with increasing sol-gel Co/Si ratios. As a mineralizing agent, HF slows down the crystallization kinetics of the zeolites, leading to the formation of a crystalline product [19]. In the presence of a cobalt salt in the synthesis medium, the coordination reactions between cobalt cations with HF would consume a considerable amount of HF [20], reducing the available amount of HF for zeolite crystallization. It is likely that insufficient stoichiometric amount of HF in the synthesis medium is responsible for the loss of crystallinity of the Co-Beta-H zeolites. N₂ adsorption/desorption measurements show that the specific surface areas and microporous volumes of the Co-Beta-H zeolites are 480–520 cm²/g and 0.19–0.20 cm³/g, respectively, which is similar to the Si-Beta zeolite textural properties [21].

Cobalt-containing materials are usually featured by their characteristic colors that can provide important information about the valences and the coordination states of the cobalt species. Fig. 2 displays images of synthesized cobalt-containing zeolites before and after calcination at 550 ℃. The color of the as-synthesized CoAPO-5 is vivid blue, which turns pale blue after calcination. The vivid blue and the pale blue colors are attributable to tetrahedrally coordinated Co²⁺ and Co³⁺ in CoAPO-5, respectively [22]. The Co-Beta-E zeolite maintains a pinkish/red color before and after calcination, This pinkish/red color results from stable Co²⁺ ions occupying specific exchange sites of zeolite Beta even when subjected to high temperatures [23]. The Co₃O₄-Beta zeolite has a pink color that is similar to that of Co-Beta-E, however, after calcination the color of Co₃O₄-Beta turns black. The initial pink color can be ascribed to Co²⁺species that exist in the form of Co(NO₃)₂ or Co(OH)₂, however, during calcination the Co²⁺species readily transforms to Co₃O₄, giving rise to the black color of the corresponding zeolites [24, 25]. The CoO-Beta-M zeolite presents a dark brown color that later converts to black after calcination. The dark brown color is typical of bulky CoO; during calcination, CoOtransforms to Co₃O₄ that results in the black color of the CoO-Beta-M zeolite. Regarding Co-Beta-H zeolites, a pinkish/violet color is observed in the as-synthesized zeolites, whereas after calcination the color turns pale violate. Each material implies the presence of a distinct cobaltspecies: from framework Co²⁺ in CoAPO-5, the ion-exchanged Co²⁺ in Co-Beta-E, the bulky Co₃O₄ in Co₃O₄-Beta and bulky CoO in CoO-Beta-M.

Fig. 3 displays the XRD patterns of calcined samples of Co-Beta-H0, Co-Beta-H1 and Co-Beta-H2. The XRD patterns of all Co-Beta-H zeolites exhibit characteristic diffraction peaks of phase pure zeolite Beta and contain no additional peaks ascribed to impurities. The diffraction peak intensity slightly decreases from Co-Beta-H0 to Co-Beta-H2, indicating a reduction in zeolite crystallinity as a function of increased cobalt in the sol-gel. The high diffraction intensity of Co-Beta-H0, Co-Beta- H1 and Co-Beta-H2 indicates the high crystallinity of these zeolites. The absence of characteristic diffraction peaks ascribed to CoO or Co₃O₄ in the Co-Beta-H samples confirms that no bulky CoO or Co₃O₄ particles are formed in Co-Beta-H zeolites.

UV-Vis spectroscopy has been widely employed to study the valence and coordination environment of cobalt species in cobalt-containing zeolites [26, 27]. Fig. 4 shows the UV-Vis spectra of Si-Beta and Co-Beta-H zeolites. In comparison with Si-Beta, Co-Beta-H zeolites show an additional band in the ultraviolet region at 238 nm, and a broad band in the visible region at 490‒540 nm. The band at 238 nm can be ascribed to the charge transfer of O^2‒-Co²⁺, suggesting the existence of Co²⁺ [22]. The broad band at 490‒540 nm can be attributed to octahedral Co²⁺, which indicates the presence of extra-framework Co²⁺ species in Co-Beta-H zeolites[22, 23]. The intensities of these bands increase as a function of increasing Co/Si ratio in the zeolite Beta materials.

²⁹Si MAS NMR spectra can provide important information regarding the local environment around Si heteroatoms in zeolites [28]. Fig. 5 shows the ²⁹Si MAS NMR spectra of Si-Beta and Co-Beta-H zeolites. For Si-Beta and Co-Beta-H zeolites, three partially overlapped resonance bands are present between −110 and −116 ppm, corresponding to Si-(OSi)₄ species. The different chemical shifts of the three resonance bands arise from tetrahedral Si atoms located at different crystallographic sites of zeolite Beta [29]. The presence of Si-(OSi)₄ species indicate that Co-Beta-H zeolites have a defect-free structure, similar to Si-Beta, and contain no siloxy (Si-O^‒) or silanol (Si-OH) groups that can serve as the exchange sites for Co²⁺ [23], therefore the Co²⁺ of Co-Beta-H are not likely to exist as the ion-exchange form in the zeolite structure. Additionally, the ²⁹Si MAS NMR spectra of Co-Beta-H are similar to those of Si-Beta, indicating that the cobalt species in Co-Beta-H zeolite have negligible interactions with the Si-O-Si environment. Therefore, the Co²⁺ species of Co-Beta-H zeolites are likely to exist as extra-framework species rather than framework-substituted heteroatoms.

XPS is a reliable method for the detailed investigation of cobalt species. Fig. 6 shows the XPS spectra of Co₃O₄-Beta, CoO-Beta-M and Co-Beta-H1 zeolites. For all Co-Beta-H samples, intense doublet bands are present at 798.0 and 781.7 eV in the Co 2p region, and an intense satellite accompanies each of the doublets at higher energies. The XPS spectrum of CoO-Beta-M also exhibits a doublet band and intense satellite with approximate binding energies close to those of Co-Beta-H, suggesting a close relationship between CoO in Co-Beta and the Co²⁺ species in Co-Beta-H. The binding energies of the doublets of Co-Beta-H and CoO-Beta-M can be attributed to the Co 2p_1/2 (798.0 eV) and 2p_3/2 (781.7 eV) photoelectrons of extra- framework CoO species [13, 30]. Conversely, the XPS spectra of Co₃O₄-Beta-M exhibits a doublet band at 795.3 and 779.9 eV, and the presence of a pair of weak satellites at higher binding energies; such doublet bands and satellite pairs are characteristic of Co₃O₄ [15].

The existence of CoO in Co-Beta-H is further evidenced by analyzing the XPS satellites. The intense satellite pairs result from the charge-transfer band structure characteristics of late 3d transition metal monoxides [15]. When other cobalt compounds, such as Co₃O₄, replace CoO, the satellites remarkably diminish [31]. Furthermore, Carson et al. [15] discovered that the intensity ratio of the Co 2p_1/2 satellite to its main peak is ~0.9 for CoO, and ~0.3 for Co₃O₄. For Co-Beta-H, this ratio is 0.92, in good agreement with that for CoO.

The oxidation/reduction properties of Co-Beta-H zeolites were investigated by H₂-TPR. As Fig. 7 shows, for Co-Beta-E the reduction band is beyond 900 ℃. Co-Beta-H displays a major reduction band at 740℃ and a minor band at 850℃. A reduction band is observed at 260 ℃ for Co₃O₄-Beta. Two reduction bands are present in CoO-Beta-M at 303 ℃ and 332℃, while for CoAPO-5, a broad reduction band is centered at approximately 860 ℃. The reduction temperature of cobalt-containing zeolites is related to the valence, coordination state and the location of the cobalt species. The relatively low reduction temperatures of Co₃O₄-Beta and CoO-Beta-M are consistent with the easy reduction of both bulky Co₃O₄ and CoO particles to cobalt metal. The high reduction temperatures of Co-Beta-E and CoAPO-5 result from the stable Co²⁺ exchange site and tetrahedral framework Co²⁺, respectively. The double-band profile of Co-Beat-H is similar to that of CoO-Beta-M, which further suggests the presence of CoO in Co-Beta-H. Nevertheless, the high reduction temperature of Co-Beta-H indicates that the CoO particles in Co-Beta-H are relatively stable, and are significantly more difficult to reduce than the corresponding bulky CoO particles in CoO-Beta-M. Furthermore, as the Co/Si ratio increases in Co-Beta-H, the minor band becomes more prominent with respect to the major band.

Fig. 8 displays TEM images of CoO-Beta-M and Co-Beta-H zeolites. For all samples, a well-defined zeolite lattice is observed indicating high zeolite crystallinity. Additionally, the TEM image of CoO-Beta-M shows the presence of CoO particles ranging from 10‒40 nm. However, no CoO particles are observed from the TEM images of Co-Beta-H zeolites. Energy-dispersive X-ray spectroscopy (EDS) measurements (Fig. 8 insets) confirm the presence of a considerable amount of Co in Co-Beta-H zeolites, suggesting that CoO may exist as sub- nanoscale particles in Co-Beta-H zeolites that are below the nanoscale resolution limit of the TEM. This suggestion is supported by the TEM images of Co-Beta-H samples after EDS treatment, wherein nanoscale CoO particles can be observed from the collapsed lattices of Co-Beta-H zeolites. An explanation for the lack of visible CoO particles prior to EDS treatment, and the emergence of CoO nanoscale particles after EDS, is rationalized as follows: prior to EDS treatment, the CoO particles are confined within the Beta structure as sub-nanoscale particles; during EDS treatment the particle bombardment by the electron beam compromises zeolite framework integrity and releases the ultra-small CoO particles; thereafter, the CoO particles agglomerate into larger particles to reduce surface energy, which results in the observation of nanoscale CoO particles, as observed in the TEM images of the EDS-treated Co-Beta-H samples.

XRD, UV-Vis and XPS suggest that the cobalt species of Co-Beta-H exists as CoO particles, with TEM observations providing indirect evidence of the presence of small CoO particles, the particle size of CoO is likely to be < 1 nm. Previous research shows that the formation of ultra-small metal or metal oxide particles within zeolites can result from chemical bonding (ion-exchange or framework substitution) with the zeolite framework [9, 28] or spatial confinement in the channels or cages [7, 32]. The analysis of ²⁹Si NMR spectra shows that CoO is weakly bound to the siliceous framework of zeolite Beta. Therefore, we speculate that the small CoO particles are encapsulated in the confined spaces of the [4³5⁴] or [4⁶] cages [33], in Beta during zeolite crystallization.

The easy conversion of CoO to Co₃O₄ under normal conditions results in conventional synthesis procedures of CoO nanoparticles to often include special treatment during synthesis or post synthetically to avoid the unexpected formation of Co₃O₄ [12]. However, in our experiments, zeolite Beta materials containing ultra-small CoO particles are successfully obtained without the need for special treatment; additionally, the CoO particles in Co-Beta-H can withstand oxidation atmospheres at 550 ℃ and reduction atmospheres at 700 ℃ without transformation into other cobalt oxides.

Co₃O₄-supported porous materials have exhibited high activity in the selective oxidation of many hydrocarbons, such as cyclohexene, tetralin and ethylbenzene [34, 35]. In our experiment, the catalytic properties of two types of CoO-containing zeolites, Co-Beta-H1 and CoO-Beta-M, were investigated in the solvent-free oxidation of ethylbenzene and compared with Co-Beta-E, Co₃O₄-Beta and CoAPO-5. The main products from the reactions are acetophenone (ACPO), 1-phenylethanol (PEA) and 1-phenyl-ethylhydroperoxide (PEHP). Ethylbenzene conversion and product distribution are listed in Table 1.

As Table 1 shows, ethylbenzene is not converted over Co-Beta-E, which is consistent with the reported inactivity of Co²⁺ion-exchanged zeolites in ethylbenzene oxidation [35]. Ethylbenzene conversion is 6.1% over CoAPO-5, 8.1% over Co₃O₄-Beta, and 5.9% over CoO-Beta-M. In the reaction atmosphere, CoO particles in CoO-Beta-M would quickly transform to Co₃O₄, which is the most likely reason for similar ethylbenzene conversions over Co-Beta-M and CoO-Beta-M. Conversely, ethylbenzene conversion over Co-Beta-H1 reaches 15.9%, almost double that of CoAPO-5 and Co-Beta-M.

Additionally, further increasing the reaction temperature to 150 ℃ and 160 ℃ results in enhanced ethylbenzene conversion over Co-Beta-H1 to 31.4% and 34.6%, respectively. However, when Co-Beta-H2 is employed in the reaction at 150 ℃ and 160 ℃, ethylbenzene conversion is remarkably lower than that of Co-Beta-H1. The selectivity to ACPO in the presence of Co-Beta-H2 is higher than the corresponding Co-Beta-H1 zeolites, while the selectivity to PEA is higher over Co-Beta-H2 than that of Co-Beta-H1. The decreased activity and ACPO selectivity of Co-Beta-H2 is thought to relate to the low crystallinity of the zeolite (as shown by Brunauer-Emmett-Teller (BET) data and SEM observations) so that the CoO particles occluded within the zeolite channels or cages remain unreactive toward ethylbenzene molecules.

The above results demonstrate that ethylbenzene conversion is facilitated by the presence of ultra-small CoO particles. At higher temperatures, selectivity to ACPO rises significantly from 64.7% to 72.9%, whereas, the selectivity to PEA remains constant and the selectivity to undesirable PEHP dramatically drops from 12.8% to 0.2%. The high activity and product selectivity makes Co-Beta-H a promising catalyst for ethylbenzene oxidation and other carbohydrate oxidations.

As proposed by Hermans et al. [36], hydrocarbon oxidation is a radical substitution-type reaction. In the absence of any catalyst, such reactions would form peroxide as the major product. When metal oxides, such as Co₃O₄ or MnO₂, are present in the oxidation medium, these metal oxides can catalyze the decomposition of the peroxides yielding ketones or alcohols as the main products [35]. Although catalytic performance studies of the CoO-containing materials are still at preliminary stages, the high activity of Co-Beta-H suggests that the ultra-small CoO particles act as oxygen carriers and donors that facilitate the radical substitution reactions, and effectively promote peroxide decomposition, producing ketones and alcohols as the minor products [30]. Additionally, a recent report discovered that Co₃O₄ particles supported on hydrophobic SiO₂ display significantly higher activity for the selective oxidation of cyclohexene, cyclohexene and ethylbenzene when compared with Co₃O₄ particles on hydrophilic SiO₂ [34]. For Co-Beta-H, the defect-free, pure-silica framework of the zeolite Beta material acts as a highly hydrophobic host for CoO particles, which may be crucial to the high activity of Co-Beta-H zeolites in the oxidation of ethylbenzene.

Zeolite Beta catalysts containing ultra-small CoO particles were synthesized from a homogenous emulsion comprising hydrofluoric acid, a silicate precursor and a cobalt salt under slightly acidic conditions. Characterization of the materials by SEM, XRD, UV-vis, XPS, H₂-TPR and TEM confirm the presence of sub-nanoscale CoO particles in the zeolite structure. This promising material possesses high thermal stability and is an efficient catalyst for the oxidation of ethylbenzene.