# Supported Co-based catalytic systems for the epoxidation of terpenes using O2 as oxidant

Meriem Rais, Jérémy Soulié, Claire Tendero, Jérôme Durand

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Meriem Rais, Jérémy Soulié, Claire Tendero, Jérôme Durand. Supported Co-based catalytic systems for the epoxidation of terpenes using O2 as oxidant. European Journal of Inorganic Chemistry, 2024, 28 (5), pp.e202400293. 10.1002/EJIC.202400293. hal-04836205

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# Supported Co-Based Catalytic Systems for the Epoxidation of Terpenes Using O₂ as Oxidant

Meriem Rais,[a] Jérémy Soulié,[b] Claire Tendero,[b] and Jérôme Durand*[a]

We synthesized and characterized a range of cobalt-based catalysts supported on mesoporous silica (SBA-15 and SBA-16), commercial silica and functionalized carbon nanotubes (CNT), investigated their efficiency in the aerobic epoxidation of terpenes, specifically limonene and α-pinene. Cobalt deposition was achieved by impregnation with cobalt(II) acetylacetonate and the resulting systems were characterized by XPS, ICP-OES, N₂ adsorption-desorption, TEM, STEM-HAADF and EDX. Variations in pore structure, cobalt dispersion, and oxidation states depending on the support used were observed. Catalytic tests demonstrated that Co/SBA-15 (1.6% Co) exhibited superior catalytic activity in the oxidation of limonene compared to Co/

SBA-16, Co/SiO₂c, and Co/CNT, achieving 90% conversion with a 68% selectivity towards 1,2-limonene oxide. In contrast, for α-pinene oxidation, Co/SBA-15 showed lower activity, while other catalysts achieved full conversion. The study also explored the effect of cobalt loading on catalytic performance, indicating that higher loadings generally led to rapid catalyst deactivation due to the formation of larger cobalt oxide particles. The findings underscore the critical role of support structure and cobalt dispersion in enhancing catalytic efficiency for terpene epoxidation. Additionally, we demonstrated the potential for 3 recycling and scale up to the gram level while maintaining high activity and selectivity.

## Introduction

The chemical industry has long recognized the need for more sustainable and environmentally friendly processes. Epoxidation reactions, in particular, have attracted considerable attention due to their relevance in various industrial applications, such as the synthesis of fine chemicals, pharmaceuticals, and polymers. Various oxidizing agents have been described for the generation of epoxides but they actually present some drawbacks as recently described by Kaliaguine et al.[1] Peracids and alkylhydroperoxides generate a stoichiometric amount of waste (acid or alcohol) and suffer from a lack of selectivity, oxone® requires a stable pH and raises issues for large scale application. Hydrogen peroxide may appear as a greener alternative considering that it only generates water as by-product but its low stability requires the use of large excess and the radicals generated by its decomposition raise selectivity issues in the epoxidation reaction. On the contrary, molecular dioxygen is an abundant, inexpensive, and environmentally benign oxidant that possesses immense potential for use in epoxidation

reactions. Additionally, O₂ is an atom-economical oxidant since the incorporation of an oxygen atom into the substrate allows for the creation of valuable functional groups without generating unwanted by-products. Despite these advantages, the direct use of O₂ as an oxidant in epoxidation reactions is challenging. Owing to its high stability, its reactivity is typically low at ambient conditions. Transition metal catalysts, such as ruthenium, manganese or iron complexes, have been extensively explored for their ability to activate O₂.[2-5]

On the other end, the recent environmental concerns, in a general context of fossil resources depletion, has led to a significant increase of the interest of biobased molecules as versatile platform molecules for various chemical transformation. Among them, terpenes have attracted significant attention as they are abundant and can be readily extracted from plants. Limonene and α-pinene in particular have been the subject of intense research.[6,7] Limonene, commonly found in citrus peels, possesses a pleasant citrus aroma and exhibits notable solvent properties, and is a valuable precursor for the synthesis of polymers, fragrances, and flavor compounds.[8] α-pinene, abundant in pine trees and other conifers, is characterized by its distinct pine scent and has garnered attention for its potential as a renewable feedstock in the production of chemicals, fuels, and materials.[9] Moreover, the presence of double bonds in the molecular structure of limonene and α-pinene renders them particularly valuable as they can be readily converted to epoxides, affording an environmental friendly alternative to their petro-based counterparts. These epoxides serve as versatile intermediates for a myriad of chemical transformations, including ring-opening polymerization, where they can be polymerized to form a wide range of functional polymers.[10,11] For instance, the copolymerization of CO₂ and limonene oxide yields poly(limonene carbonate), a biodegradable and renewable polymer with potential applications in

<footnote>[a] M. Rais, J. Durand
Laboratoire de Chimie de Coordination du CNRS, Université de Toulouse,
UPS, Toulouse-INP, 4 allée Emile Monso, CS 44362, Toulouse cedex 4 31030
France
E-mail: jerome.durand@ensiacet.fr</footnote>

<footnote>[b] J. Soulié, C. Tendero
CIRIMAT, CNRS, INPT, UPS, Université de Toulouse, 4 allée Emile Monso, CS
44362, Toulouse cedex 4 31030 France</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202400293</footnote>

<footnote>© 2024 The Authors. European Journal of Inorganic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_3.png" /></figure>

packaging materials, coatings, and biomedical devices.[12] How-
ever, despite the immense potential of limonene and α-pinene
as renewable feedstocks for epoxide production, their con-
version poses significant challenges, particularly in terms of
achieving high selectivity towards the desired epoxide prod-
ucts. As a result, numerous research articles have emerged in
recent years focusing on the development of efficient and
selective epoxidation methodologies. Challenges in this regard
include the propensity for multiple side reactions, such as allylic
oxidation and over-oxidation, which can lead to diminished
yields and undesirable by-products. Additionally, issues related
to catalyst stability, reaction conditions, and environmental
impact further complicate the development of robust epoxida-
tion processes. Thus, ongoing research efforts aim to address
these challenges through the design of novel catalytic systems,
optimization of reaction conditions, and elucidation of reaction
mechanisms, ultimately paving the way for more sustainable
and selective methods for the epoxidation of limonene and α-
pinene. Recent advancements in the field of terpene epoxida-
tion include research focusing on homogeneous catalytic
systems.[13] However, despite their efficiency, homogeneous
catalysts present challenges related to catalyst separation,
recycling, and the need for stringent reaction conditions,
limiting their scalability and industrial applicability. Moreover,
the inherent robustness and recyclability of heterogeneous
catalysts make them particularly well-suited for continuous flow
applications,[14] enabling enhanced process efficiency and
scalability at the industrial level. In this context, reports dealing
with supported heterogeneous catalytic phases for the aerobic
epoxidation of terpenes remain scarce.[15-18] Pioneering work in
the field by Kaliaguine et al. evidenced the superiority of cobalt
over manganese, iron and nickel as catalysts supported on a
SBA-16 mesoporous silica.[19,20] In the present work, we describe
the development, characterization of cobalt oxide particles
deposited on different kind of supports (carbon nanotubes,
mesoporous and commercial silica). We have carried out
extensive catalysts characterization and then studied their
catalytic performances in the aerobic epoxidation of limonene
and α-pinene with a particular emphasis on the role of the
support, the cobalt loading and the recyclability of the system.

Experimental Section

Pluronic F127, pluronic P123, tetraorthosilicate 98%, cobalt (II) acetylacetonate 97%, limonene 99%, isobutyraldehyde 99% were purchased from Sigma-Aldrich; α pinene 97% was purchased from Fluka and were used without further purification. Ethyl acetate 99.9%, butanol 99.4%, hydrochloric acid 37%, nitric acid 65%, sulfuric acid 95% were purchased from VWR and used without further purification. Commercial silica was provided by Saint-Gobain, Norpro (+99.5% SiO₂). Dioxygen was purchased from Messer 99.99%.

Thermogravimetry Differential Thermal Analyses (TG/DTA) were
carried out on a Shimadzu apparatus applying a method that
consisted in heating the carbon materials in a synthetic air flow
(100 cm³ min⁻¹) up to 1000 °C, at 10 °C min⁻¹.

The textural characterization (BET surface areas, $S_{BET}$) of the materials was based on the $N_2$ adsorption isotherms determined at -196 °C with a Quantachrome Autosorb apparatus.

Infrared analysis were performed on a IR-TF Perkin Elmer Spectrum One. The sample is ground with dry KBr and press into a pellet prior to analysis.

X-ray photoelectron spectroscopy (XPS) were performed with a spectrometer Thermo Scientific™ Kalpha™, equipped with a monochromatic anticathode Al (Kα radiation of 1486,7 eV), under less than $5×10^{-9}$ mbar pressure. The energy calibration was performed using Au4f$_{7/2}$ (83,9±0.1 eV) and Cu2p$_{3/2}$ (932.7±0.1 eV) photoelectron lines. Charging compensation and neutralization was applied using a dual beam flood gun. The diameter of probed area was ca. 400 µm. Constant pass energy of 30 eV and energy steps of 0.1 eV were used for high-resolution scans. The atomic concentrations were determined from photoelectron peak areas using the atomic sensitivity factors reported by Scofield, taking into account the transmission function of the analyzer. High-resolution spectra were analyzed by Gaussian/Lorentzian ($G/L=70/30$) peak fitting, using a Shirley background. The catalyst powder was dispersed on carbon tape, each sample was analyzed twice.

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) analyses were after deposition on a holey copper grid. TEM analyzes were performed with a JEOL JEM 1011 CX T electron microscope operating at 100 kV with a spot resolution of 4.5 Å and a JEOL JEM 1400 electron microscope operating at 120 kV. HR-TEM analyzes were performed using a JEOL JEM 2100 F fitted with a field emission gun (FEG) operating at 200 kV with a spot resolution of 2.3 Å and a JEOL JEM-ARM200F cold FEG equipped with a STEM HAADF system (Scanning Transmission Electron Microscopy High Angle Annular Dark Field) and an EDX (Energy Dispersive X-ray Spectroscopy) system operating at 200 kV with a point resolution > 1.9 Å.

Particle size distribution was determined using ImageJ on the basis of 100 particles.

The loadings of cobalt in the catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) with a Thermo Scientific ICAP 6300 instrument equipped with a high-resolution echelle optical system and charge injection device (CID) array detector, with an extended spectral range of 166–847 nm and axial and radial plasma view. The solutions were introduced into the ICP plasma using an sample introduction system including a quartz nebulizer, a cyclonic spray chamber and a quartz torch. Mineralization of the solid samples was carried out in a HCl/HNO₃ mixture (nitric acid Rotipuran®Supra 69% and hydrochloric acid Rotipuran®Supra 35% were purchased from Carl Roth). The 1.0 g L⁻¹ standard stock solutions were purchased from Carl Roth. Calibration standards were freshly prepared by appropriate dilution of the stock solution.

Preparation of Functionalized Carbon Nanotubes

The carbon nanotubes (CNTs) were prepared following the C–CVD method developed in our research group using ethylene ($C_2H_4$) as a source of carbon.[21] The carbon nanotubes were synthesized in a fluidized bed reactor using AlFeCoO₄ as a catalyst. In a typical preparation of CNTs, the catalyst AlFeCoO₄ was reduced in the fluidized bed reactor under a nitrogen flow (225 mL min⁻¹) at 650 °C. After this, the ethylene flow was fixed at 225 mL min⁻¹ for 30 min to produce the CNTs. The CNTs were purified using aqueous solution of $H_2SO_4/H_2O$ (1/1) under reflux at 140 °C for 3 h. The solution was filtered and the solid was washed with distilled water and dried in the oven at 80 °C overnight. The CNTs were then
<figure><img src="image_4.png" /></figure>

functionalized using HNO₃ under reflux at 140 °C for 3 h. The solution was filtered and the solid was washed with distilled water and dried in an oven at 80 °C overnight to isolate oxidized CNTs (CNTₓ). The later where then deprotonated, to generate CNTᵣ, according to the following procedure: CNTₓ were mixed with 100 mL of a sodium hydroxide solution (0.05 M) for 24 h at RT to generate sodium carboxylate groups onto the CNTs. The solid was then separated by filtration and washed thoroughly with distilled water until pH 7 before drying at 120 °C for 2 days.

## Synthesis of Mesoporous Silica SBA-16

SBA-16 was prepared according to the literature.[19] In a typical preparation of SBA-16, 5.35 g of the copolymer Pluronic F127 were dissolved in deionized water at 45 °C and 25 ml of HCl 37% was added to the solution. After stirring for 2 hours, 27.10 ml of tetra ethyl orthosilicate (TEOS) and butanol were added to the reaction mixture and stirred for 10 minutes then kept static for 24 hours. It was then transferred to an autoclave at 100 °C for 24 h. The white solid obtained was washed with deionized water and dried at 80 °C for 6 h prior to calcination at 550 °C for 6 h.

## Synthesis of Mesoporous Silica SBA-15

SBA-15 was synthesized following a reported procedure.[22] 3.36 g of Pluronic P123 were dissolved in 25 ml of deionized water and 100 ml of HCl 37%. 7.65 ml of TEOS were then added and the reaction mixture was stirred for 5 h and then kept static for 24 h. The white solid obtained was filtered and washed with deionized water, dried for 4 h at 80 °C prior to calcination at 550 °C for 4 h.

## Synthesis of Supported Cobalt Catalysts

The cobalt catalysts were prepared following the same procedure and according to published methods.[19] Catalysts with different cobalt loadings were prepared according to the impregnation method of silica or CNTᵣ with the desired amount of cobalt salt in toluene solution. The synthetic procedures were as follows : 2 g of as-synthesized support (SBA-16, SBA-15, commercial silica SiO₂c or CNTᵣ) and the desired amount of cobalt (II) acetylacetonate (4 wt% Co in the case of samples Co/SiO₂c (0.7%); Co/SBA-15 (1.6%); Co/SBA-16 (1.8%) and 10 wt%Co in the case of Co/SiO₂c (4.4%); Co/SBA-16 (2.5%); Co/CNT (2.5%)) were heated for 24 h in 200 ml of toluene. The solid was then filtered and dried at 80 °C for 6 h. The product was calcined at 550 °C for 3 h for the catalysts with silica as support and at 300 °C for the catalysts with CNTᵣ as a support.

## Procedure for Catalytic Reaction

All the catalytic tests for the aerobic epoxidation of limonene and α-pinene were performed in a 100 mL three-necked round bottom flask equipped with a liquid condenser. In a typical run, 3.6 mmol

of the terpene (limonene or α-pinene), the desired amount of catalyst, 12 mmol of isobutyraldehyde and 6 mL of ethylacetate were added to reactor. The mixture was stirred using a magnetic stirrer and the O₂ was bubbled at atmospheric pressure. The desired temperature was fixed using a Fisher Scientific Thermoplate equipped with thermocouple. The reaction was followed using a Perkin-Elmer CLARUS 500 gas chromatograph equipped with a flame ionisation detector (FID) and an Elite 5 (30 m×0.32 mm×25 µm) column. At the end of the reaction, the catalyst was isolated by centrifugation and the liquid product was analyzed by GC-FID and gas chromatograph coupled with mass spectrometry (GC-MS). The conversion and the selectivity were calculated according to the results determined by GC-FID.

For recycling experiments; the catalyst was separated from the liquid phase by centrifugation. The resulting solid was then washed with ethyl acetate, dried over night at 80 °C and then used for the next catalytic run.

## Results and Discussion

In agreement with our objectives, different supports were prepared and characterized in order to study their influence in the catalytic epoxidation of terpenes.

### Catalyst Preparation

#### Mesoporous Silica Samples

Mesoporous silica samples, namely SBA-15 and SBA-16, were prepared by an hydrothermal synthesis reported in the literature using tetraorhtosilicate as precursor.[15,22] The obtained samples were analyzed by N₂ adsorption/desorption and transmission electron microscopy (vide infra). For comparison purposes, we also used as support a commercial silica (SiO₂c).

#### Functionalized Carbon Nanotubes (CNTᵣ)

Pristine carbon nanotubes (CNTᵣ) were obtained by catalytic chemical vapor deposition using AlFeCO₄ as catalyst and ethylene as carbon source. They were then oxidized by nitric acid treatment to introduce oxygenated function onto their surface, mainly carboxylic groups that were finally deprotected by treatment with NaOH as depicted in Figure 1.

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Schematic representation of the synthesis of functionalized carbon nanotubes (CNTᵣ).</figcaption></figure>
<figure><img src="image_6.png" /></figure>

Thermogravimetric of these three different carbon nano-
tubes evidenced a lower thermal stability of CNT<sub>ox</sub> and CNT<sub>f</sub>
together with a slight mass loss around 100 °C (Figure 2), in
agreement with the data reported in the literature concerning
the introduction of oxygenated function at the surface of the
carbon nanotubes.<sup>[23]</sup> This was further confirmed by the
presence in the FT-IR spectrum of CNT<sub>ox</sub> centered at 1746 cm<sup>-1</sup>,
characteristic of the presence of the C=O function of carboxylic
groups.<sup>[24,25]</sup> Infrared analysis of CNT<sub>f</sub> (Figure 3) shows the
disappearance of the band at 1746 cm<sup>-1</sup>, confirming the
generation of carboxylate functions.

<figure><img src="image_7.png" /><figcaption>Figure 2. Thermogravimetric analysis of the carbon nanotubes prepared.</figcaption></figure>

Cobalt Impregnation

The catalytic systems were prepared using cobalt(II) acetylacet-
onate, [Co(acac)₂], as cobalt source.[¹⁹] Briefly, a suspension of
the desired support in toluene was stirred and heated for 24 h
in presence of the cobalt precursor. The resulting suspension
was then filtered and dried at 80 °C prior to calcination at 300 °C
in the case of the CNT-supported catalysts and 550 °C for the
silica-supported systems. The cobalt contents of each sample
was determined by ICP-OES (Table 1).

Characterization

Textural properties of the pristine and cobalt-containing silica
samples were determined from nitrogen adsorption-desorption
isotherms at 77 K. The adsorption-desorption isotherms for
SBA-15, SBA-16 and the commercial silica SiO₂c are presented in
Figure 4,5 and 6, allowing the determination of the textural
properties of the analyzed samples prior to the cobalt grafting.
The N₂ adsorption and desorption isotherms of silicas exhibit
type IV behavior with hysteresis,[²⁶] indicating a capillary
condensation phenomenon, demonstrating the presence of
mesoporosity. Furthermore, the slight increase in volume at low
P/P₀ signifies low microporosity within the silica walls. The pore
size distribution curve for different supports reveals that the
mesoporous silica SBA-16 exhibits a smaller pore diameter
compared to other supports. The use of different synthesis
parameters such as surfactant nature (with different hydro-

<figure><img src="image_8.png" /><figcaption>Figure 3. FT-IR spectra of CNT<sub>p</sub> and CNT<sub>ox</sub>.</figcaption></figure>
<figure><img src="image_9.png" /></figure>

<table><caption>**Table 1.** Comparison of the textural properties of pristine silica supports and cobalt loaded samples determined by BET.</caption><thead><tr><th>Sample</th><th>Co loading<br/>[%]<sup>a</sup></th><th>Surface<br/>(m<sup>2</sup>/g)</th><th>Pore volume<br/>(cm<sup>3</sup>/g)</th><th>Mean pore<br/>radius<br/>(Å)</th></tr></thead><tbody><tr><td>SBA-15</td><td>-</td><td>711</td><td>0.66</td><td>45</td></tr><tr><td>Co/SBA-15</td><td>1.6%</td><td>532</td><td>0.4</td><td>27</td></tr><tr><td>SBA-16</td><td>-</td><td>125</td><td>1.4</td><td>17</td></tr><tr><td>Co/SBA-16</td><td>1.8%</td><td>55</td><td>0.55</td><td>18</td></tr><tr><td>SiO<sub>2</sub>c</td><td>-</td><td>108</td><td>1.8</td><td>57</td></tr><tr><td>Co/SiO<sub>2</sub>c</td><td>0.7%</td><td>121</td><td>2.22</td><td>50</td></tr><tr><td>Co/SiO<sub>2</sub>c</td><td>4.4%</td><td>238</td><td>1.16</td><td>57</td></tr></tbody></table>

[a] Determined by ICP-OES.

pobic/hydrophilic polymer chain segment length), concentration and hydrothermal conditions allows controlling the pore sizes among each support.[27,28] Moreover, SBA-15 shows a pore distribution with two distinct peaks, indicating the presence of two different size ranges, approximately 18 nm and 40 nm. This phenomenon is explained in the literature by the presence of microporosity i.e. pore diameters corresponding to the silica cages and interconnection between pores.[29]

The Co/SBA-16, Co/SBA-15, and Co/SiO₂c catalysts were then also characterized by nitrogen adsorption-desorption. Table 1 summarizes the physical and structural properties of each catalyst used in the catalysis section, as well as each support for comparative purposes. Figures 4, 5 and 6 depict the appearance of adsorption-desorption isotherms for each catalyst before and after cobalt grafting.

Regarding the SBA-15 type mesoporous silica and the Co/SBA-15 catalyst, Figure 4 shows that the isotherms appear similar, but that of the Co/SBA-15 catalyst is shifted downward. This indicates that the isotherms are of the same type but with less volume in the presence of the metal. As for the graph presenting the pore size distribution, it exhibits a different appearance when cobalt is added. The pore volume decrease is probably due to the partial filling of mesopores by cobalt

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 5.** Nitrogen adsorption and desorption isotherms (left) and pore size distribution (right) of the pristine and Co-loaded SBA-16 support.</figcaption></figure>

oxides clusters. The lower mean pore diameter for Co/SBA-15 could be explained by the creation of a new intra-cluster porosity. This hypothesis is confirmed by TEM images (See below, Figure 7b), highlighting the nucleation of cobalt oxide clusters. The corollary is the binary nature of mesoporosity filling. Indeed, a mesopore is either completely filled by cluster

<figure><img src="image_12.png" /><figcaption>**Figure 4.** Nitrogen adsorption and desorption isotherms (left) and pore size distribution (right) of the pristine and Co-loaded SBA-15 support.</figcaption></figure>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 6.** Nitrogen adsorption and desorption isotherms (left) and pore size distribution (right) of the pristine and Co-loaded commercial silica at two different Co contents.</figcaption></figure>

growth or remains entirely empty, the clusters being out of mesopores. Therefore, active sites are located either on the surface of filled mesopores or in the inter-cluster space.

Concerning the SBA-16 type mesoporous silica and the Co/SBA-16 catalyst, Figure 5 shows that the isotherms of the support and the catalyst have similar shapes, with that of Co/SBA-16 being shifted downward, indicating less volume in the pores also due to partial filling. Moreover, the graph depicting the pore distribution shows an increase in pore diameter after cobalt addition. This can be explained by the fact that clusters primarily fill the smallest mesopores, the contribution of larger mesopores becoming predominant. It means that no additional inter-cluster porosity is added and that most of Co is incorporated inside the porosity (as confirmed by TEM/Figure 7d). Interestingly, mesopores are not totally closed by cobalt oxides. This may be due to the smaller median diameter of SBA-16 compared to SBA-15. Indeed, according to the classical theory of nucleation, a nucleus must have a critical radius to allow for its subsequent growth. The diameter of SBA-16 is likely smaller than such a critical radius and does not allow for cobalt oxides clusters growth within the porosity. As a result, active sites may be more discrete, numerous, and accessible.

Concerning the commercial silica SiO₂c, unlike in the cases of the mesoporous silicas, the appearance of isotherms remains unchanged before and after cobalt grafting (Figure 6) with no significant modification of the pore size distribution, meaning that the deposition of cobalt oxide is either a surface phenomenon within the mesopores.

The mesoporous Co/silica catalysts (Co/SBA-15 and Co/SBA-16) were also characterized using Transmission Electron Microscopy (TEM). Figure 7 displays TEM images of the mesoporous silica supports and the Co/SBA-16 and Co/SBA-15 catalysts. The morphology of the supports remained unchanged after cobalt incorporation. Cobalt, identifiable by dark-shaded areas, also exhibits a homogeneous dispersion of cobalt particles on the supports even though some agglomeration can be observed in the case of sample Co/SBA-15.

To confirm the presence of cobalt within these systems, the Co/SBA-16 sample was further characterized by Scanning Transmission Electron Microscopy (STEM-HAADF) (Figure 8) and Energy-Dispersive X-ray Spectroscopy (EDX) (Figure 9). For this sample, the analysis of particle size distribution determined from STEM-HAADF images reveals a tightly Gaussian distribution around an average value of 4.1 nm. Furthermore, in addition to Cu signals due to the microscopy grid, the O and Si signals due to the silica support, the EDX spectrum clearly evidences the presence of Co atoms.

In the case of Co/CNT, particles deposited on carbon nanotubes are smaller than those deposited on mesoporous silica, as shown in the microscopy images and the size distribution histogram (Figure 10), with a less homogeneous distribution compared to the Co/SBA-16 sample.

Furthermore, as mentioned in the case of the Co/SBA sample, EDX analysis confirmed the presence of Co atoms (Figure 11).

In order to further characterize the catalytic systems prepared, XPS analysis was also performed. As a representative example, results for Co/SiO₂c are detailed below. The survey spectrum (Figure 12) evidenced the presence of Si and O due to the silica support and that of cobalt.

Detailed results for the high-resolution peak deconvolution for Co/SiO₂c are given in Table 2.

Spectrum of Co2p core level (Figure 13) is in agreement with literature: Co1 contribution of 2p₃/₂ is located at 781.6 eV

<table><caption>**Table 2.** High-resolution peak deconvolution for Co/SiO₂c (4.4%).</caption><thead><tr><th>Core level</th><th>Peak</th><th>Peak position (eV)</th><th>FWHM (eV)</th><th>Area (CPS, eV)</th><th>Atomic %</th></tr></thead><tbody><tr><td>Si2p</td><td>Si</td><td>103.56</td><td>2.18</td><td>14355.93</td><td>29.08</td></tr><tr><td rowspan="3">C1s</td><td>C1</td><td>284.82</td><td>2.11</td><td>4332.97</td><td>8.81</td></tr><tr><td>C2</td><td>287.28</td><td>1.92</td><td>522.06</td><td>1.06</td></tr><tr><td>C3</td><td>288.88</td><td>1.06</td><td>180.37</td><td>0.37</td></tr><tr><td>O1s</td><td>O</td><td>532.87</td><td>2.13</td><td>70732.97</td><td>59.52</td></tr><tr><td rowspan="3">Co2p<sub>3/2</sub></td><td>Co1</td><td>781.61</td><td>3.37</td><td>3384.21</td><td>0.85</td></tr><tr><td>Co2</td><td>783.88</td><td>1.54</td><td>356.31</td><td>0.06</td></tr><tr><td>Co3</td><td>787.42</td><td>4.7</td><td>1461.86</td><td>0.24</td></tr></tbody></table>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Figure 7.** TEM images of pristine and Co-modified mesoporous silica: pristine SBA-15 (a); Co/SBA-15 (b); pristine SBA-16 (c) and Co/SBA-16 (d).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 8.** STEM-HAADF image and particle size distribution for Co/SBA-16.</figcaption></figure>

with a shake-up satellite structure (Co3) at 787.4 eV (5–6 eV from Co1) and a 15.5 eV spin-orbit split.[30] The area ratio 2p$_{3/2}$:2p$_{1/2}$ is 2:1 as shown on Figure 13. In each sample analysis, cobalt quantification is deduced from the deconvolution of 2p$_{3/2}$ peak alone, as described in the literature.[31] Co1 and Co2 corresponds respectively to Co(II) and Co(III).

XPS analyses carried out on all the catalytic systems show similar pattern and are provided in the SI. Details concerning the quantitative analysis, after carbon and oxygen correction and taking into account a 10% error on the experimental measurements as admitted in the literature are given in Table 3.

The atomic % of cobalt in all the samples are in good agreement with the values determined by ICP.

These results, in particular the fact that the main contribution in the Co peaks arises from Co (II), clearly evidence that all the silica supports allow the formation of CoO as a major species (in any cases more than 85% of Co(II), calculated as the ration between Co1 and Co2). The use of CNT as support seems to favor the presence of larger amounts of Co$_3$O$_4$ since in the case of Co/CNT the atomic percentage of Co(II) decreases to 66%.
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>Figure 9. EDX analysis of Co/SBA-16.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Figure 10. STEM-HAADF images and particle size distribution for Co/CNT.</figcaption></figure>

<table><caption>Table 3. Quantitative analysis (after carbon and oxygen correction) based on XPS analysis.</caption><thead><tr><th>Atomic %</th><th>Co/SBA-16 (2.5%)*</th><th>Co/SBA-15 (1.6%)*</th><th>Co/SiO<sub>2</sub>c (4.4%)*</th><th>Co/SBA-16 (1.8%)*</th><th>Co/CNT (2.5%)*</th></tr></thead><tbody><tr><td>Si</td><td>34 ± 3</td><td>34 ± 3</td><td>33 ± 3</td><td>33 ± 3</td><td>-</td></tr><tr><td>O</td><td>65 ± 7</td><td>66 ± 7</td><td>66 ± 7</td><td>66 ± 6</td><td>7.8 ± 0.8</td></tr><tr><td>Co1</td><td>0.43 ± 0.04</td><td>0.25 ± 0.05</td><td>1.0 ± 0.1</td><td>0.37 ± 0.06</td><td>0.16</td></tr><tr><td>Co2</td><td>0.020 ± 0.002</td><td>0.04 ± 0.01</td><td>0.08 ± 0.01</td><td>0.04 ± 0.03</td><td>0.08</td></tr><tr><td>Co3</td><td>0.10 ± 0.02</td><td>0.07 ± 0.01</td><td>0.28 ± 0.03</td><td>0.06 ± 0.01</td><td>0.02</td></tr><tr><td>Co (Co1 + Co2 + Co3)</td><td>0.55 ± 0.06</td><td>0.36 ± 0.07</td><td>1.4 ± 0.1</td><td>0.5 ± 0.1</td><td>0.26 ± 0.05</td></tr><tr><td>%Co(II) (Co1/Co2)</td><td>95</td><td>86</td><td>92</td><td>90</td><td>66</td></tr><tr><td>Co content in wt% from XPS</td><td>1.60</td><td>1.06</td><td>4.04</td><td>1.48</td><td>1.23</td></tr><tr><td>Co content in wt% from ICP</td><td>2.5</td><td>1.6</td><td>4.4</td><td>1.8</td><td>2.5</td></tr><tr><td>F<sup>#</sup></td><td>0.64</td><td>0.66</td><td>0.92</td><td>0.82</td><td>0.49</td></tr></tbody></table>

<footnote>*Cobalt content determined by ICP (wt%). #F=Co wt% (XPS)/Co wt% (ICP).</footnote>
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>Figure 11. EDX analysis of Co/CNT.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>Figure 12. Survey XPS spectrum of Co/SiO2c (4.4%).</figcaption></figure>

Taking into account the error on the quantitative analysis of the XPS data, the results in terms of cobalt contents are in agreement with those obtained by ICP for the samples Co/SiO2c and Co/SBA-16 (1.6%). The differences become more important when the using SBA-15 or CNT as support as indicated by the values of the F factor. The F factor, as defined by Martinez et al, is the ratio between the Co wt% obtained by XPS and the Co

wt% measured by ICP.[32] A value close to 1 indicates a rather homogeneous dispersion of the cobalt on the surface of the support and into the pores. In the case of SBA-15 and CNT supported catalysts, the lower values of the F factor indicate a preferential location of the cobalt into the pores than on the external surface. In the case of the Co/SBA-16 catalysts, the increase of cobalt loading, from 1.8 to 2.5 wt%, induces a
<figure><img src="image_25.png" /><figcaption>Figure 13. Co2p core-level high-resolution spectrum of Co/SiO2c (4.4%).</figcaption></figure>

significant decrease of the F factor, indicating an increase of the amount of metal inside the mesopores.

XRD analysis performed on Co/SBA-16 shows a broad signal around 2θ = 22°, characteristic from the mesoporous silica but did not evidenced the presence of diffraction peaks arising from cobalt oxide (see SI). It could be due to the low metal content integrated in the samples and/or the small size of the particles as evidenced by TEM analysis (ca. 4 nm). Indeed, despite local organization, such clusters cannot be detected thanks to conventional XRD. Similar observations have already been reported on systems made of cobalt supported on mesoporous silica.[19]

## Catalytic Tests

The above mentioned Co-based catalytic systems were tested in the aerobic oxidation of limonene and α-pinene (Figure 14) at atmospheric pressure and at 28 °C in ethylacetate, using isobutyraldehyde as sacrificial agent according to procedure similar to that reported by Kaliaguine et al.[19]

TON are expressed as the ratio between the molar amount of terpene converted (determined by Gas Chromatography) and the mass of cobalt introduced as determined by ICP (vide

supra). Comparison of the different catalytic systems in terms of activity were made on the base of the determined conversion and/or TON values and selectivity calculated according to the following equations:

$$ \text{Conversion (\%)} = \frac{ni(\text{limonène}) - nf(\text{limonène})}{ni(\text{limonène})} * 100 $$

$$ \text{TON (mmol/mg)} = \frac{ni(\text{limonène}) - nf(\text{limonène})}{m[\text{Co}]} $$

$$ \text{Selectivity (\%)} = \frac{n(1,2-\text{LO})}{ni(\text{limonène}) - nf(\text{limonène})} * 100 $$

Selectivity refers to 1,2-limonene oxide (LO) or α-pinene oxide (PO) respectively.

## Influence of the Support

Results obtained in the oxidation of limonene are shown in Figure S6. In the case of the catalyst supported on the commercial silica, a plateau is reached after ca. 45 min, prior to

<figure><img src="image_26.png" /><figcaption>Figure 14. Aerobic epoxidation of limonene of α-pinene.</figcaption></figure>
<figure><img src="image_27.png" /></figure>

full conversion, probably due to catalyst deactivation. No significant differences were observed between the catalytic activity of Co/SBA-16 and Co/CNT. On the contrary, Co/SBA-15 showed a significantly higher activity. Comparison in terms of conversion and selectivity toward 1,2 limonene oxide at 45 min. are given in Figure 15 evidencing the superiority in terms of catalytic activity of the Co/SBA-15 systems as the conversion reaches 90%.

It is worth noting that in the case of the analogous systems based of SBA-16 supported cobalt described previously in the literature, full conversion was achieved after a reaction of time of 200 min and with rather moderate selectivity for 1,2-LO (in the range 35–50%) as significant amounts of limonene dioxide (LDO) were formed.[19] In our case, the observed selectivity is higher, reaching the highest value (92%) in the case of Co/SiO₂c. The rapid deactivation of this catalytic system may account for this observation as it also may significantly reduce the activity toward the further oxidation of 1,2-LO to LDO.

Catalyst supported on SBA-15 and CNT are slightly more active than the one deposited on SiO₂c and SBA-16. For the later, it has been shown that the cobalt is located on both the surface of the support and into its porous structure, whereas they tend to be more concentrated into the porous structure for CNT and SBA-15. A reasonable hypothesis could be a beneficial influence of the confinement of the active site into the porous structure on the catalytic activity, even though other factors could be invoked to explain these differences. On the basis of this hypothesis, one would expect the Co/CNT system to be more active, considering it is the lower amount of cobalt at the surface. However, this effect is probably counterbalanced by the larger content of Co(III) in this case as previous reports on the field have evidenced that the role of Co(II) as active species in such oxidation reactions.

The influence of the support was also investigated in the case of the oxidation of α-pinene.

Unlike in the case of limonene, the Co/SBA-15 catalytic system displayed the lower activity in the pinene conversion (Figure S7) and no significant difference between the other systems for which full conversion is reached after ca. 45 min (Figure 16). In particular, and unlike what observed for the epoxidation of limonene, no deactivation of the Co/SiO₂c catalyst was observed. These results evidence a strong substrate dependence of the catalysts efficiency.

In a general manner, selectivity toward α-pinene oxide is higher (in the range 81–90%) than in the case of limonene epoxidation, the latter being affected by the presence of a second double bound that can potentially also be oxidized, thus limiting the selectivity.

## Influence of Cobalt Loading

We have also studied the influence of cobalt loading using both the commercial silica (SiO₂c) and the CNTs as supports. In the case of SiO₂c, the cobalt content was decreased from 4.4 to 0.7% and in the case of CNT increased from 2.5 to 15% as determined by ICP analysis. Under the same reaction conditions, and in both cases, an increase in the loading has a detrimental effect on the global epoxidation activity. With both supports, Figure S8 clearly evidences a rapid deactivation of the catalyst with a higher cobalt content as the catalytic activity does not evolve after ca. 20 min (CNT) and ca. 45 min (SiO₂c). In the latter case, the maximum conversion reached is 74%. This trend could be attributed to the reduced number of active sites in the case of higher cobalt loading due to the formation of larger oxide particles.

<figure><img src="image_28.png" /><figcaption>**Figure 15.** Conversion and selectivity toward 1,2 limonene oxide at t=45 min in the aerobic limonene epoxidation using Co catalysts on different supports: Co/SBA-16 (1.8%); Co/SBA-15 (1.6%); Co/SiO₂c (4.4%) and Co/CNT (2.5%). Reaction conditions: limonene (3.6 mmol); isobutyraldehyde (12 mmol); solvent: ethyl acetate; temperature: 28°C.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure 16.** Conversion and selectivity toward α-pinene oxide (PO) at t=45 min in the aerobic α-pinene epoxidation using Co catalyst on different supports: Co/SBA-16 (1.8%); Co/SBA-15 (1.6%); Co/SiO₂c (4.4%) and Co/CNT (2.5%). Reaction conditions: α-pinene (3.6 mmol); isobutyraldehyde (12 mmol); solvent: ethyl acetate, temperature: 28°C.</figcaption></figure>
<figure><img src="image_30.png" /></figure>

In the case of SiO₂c-supported catalysts with a higher cobalt loading, rapid deactivation nevertheless leads to a slightly higher selectivity towards LO (Figure S8a). The deactivation process prevents the occurrence of further oxidation reactions that would otherwise lead to the formation of various oxidation by-products, thereby diminishing the selectivity towards LO.

Similar observations were made in the case of the CNT supported catalytic systems. At higher Co loading, the catalytic systems seem to reach rapidly a plateau as the TON does not evolve after 15 min (Figure S8b), the conversion being ca 80%.

The effect is much less pronounced in the case of α-pinene oxidation where the catalyst loading seem to have almost no effect on the catalysts performances in both cases (CNT and SiO₂c supports). In any case, these results clearly show a dependence of the nature of the substrate on the influence of the cobalt loading.

Finally, similar experiments were also conducted using the Co/SBA-16 catalyst with 1.8 and 2.5 Co wt%. In that case, and in agreement with previously published results on analogous systems, no change in the catalytic activity was observed when the aerobic oxidations of limonene and α-pinene were carried out.[20] In addition, and unlike what we observed in the case of SiO₂c and CNT, when reaction were carried out using the Co/ SBA-16 (2.5%) with a higher cobalt content, the selectivity toward LO and PO decreased slightly, in agreement with the results of Kaliaguine et al., who reported a decrease of the yield in LO in the epoxidation reaction catalyzed SBA-16 supported cobalt at different loadings (Figure 17).[19]

## Catalyst Recycling

To assess the recyclability of the systems described, the recycling of the Co/SiO₂c catalyst loaded with 0.7% of cobalt

was tested. The catalytic experiment was performed four times under the same reaction conditions, with the catalyst being isolated after each test by centrifugation followed by drying overnight at 80°C. The results obtained are presented in Figure 18 for the aerobic epoxidation of limonene. The conversion of limonene and the selectivity for limonene 1,2- epoxide remained unchanged during the first three cycles (97% after 2 hours). After four cycles, a slight decrease in conversion, from 97% to 94%, was observed, indicating that the catalyst remains highly active after four cycles. The selectivity towards limonene epoxide was not influenced by the recycling of the catalyst and remained between 64% and 67%. The results obtained are in accordance with literature reports. Indeed, Kaliaguine et al. studied the recyclability of a catalyst containing SBA-16 type SMO as support, reporting a 5% decrease in selectivity for limonene epoxide with stable conversion after three cycles. The systems described remain competitive in terms of stability and recyclability compared to analogues described previously in the literature.[19]

Similar experiments were conducted in the case of the aerobic epoxidation of α-pinene (Figure 19)After the first run, full conversion was achieved after 1 hour. Unlike in the case of limonene oxidation, conversion values obtained after the 2nd run significantly decrease, showing thus a lower catalyst stability (and therefore recyclability) of the catalytic system for α-pinene epoxidation.

## Scale-Up

In order to assess the possibility of scale-up, two catalytic tests were carried out using for the aerobic epoxidation of α-pinene using SBA-16 (1.8%) as it appeared to be one of the most efficient catalysts for this reaction (Figure 16). When the quantity of the substrate was increased to 10.8 mmol (1.47 g),

<figure><img src="image_31.png" /><figcaption>**Figure 17.** Conversion and selectivity toward 1,2 limonene oxide at t = 45 min in the aerobic limonene epoxidation using Co/SiO₂c catalyst at different metal loading. Reaction conditions: limonene (3.6 mmol); isobutyraldehyde (12 mmol); solvent: ethyl acetate; temperature: 28°C.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 18.** Recycling tests for the Co/SiO₂c catalysts in the limonene aerobic epoxidation. Reaction conditions: catalyst : Co/SiO₂c (0.7%); limonene (3.6 mmol); isobutyraldehyde (12 mmol); solvent: ethyl acetate; temperature: 28°C; reaction time: 2 h.</figcaption></figure>
<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /><figcaption>**Figure 19.** Recycling tests for the Co/SiO₂c catalysts in the α-pinene aerobic epoxidation. Reaction conditions: catalyst : Co/SiO₂c (0.7%); α-pinene (3.6 mmol); isobutyraldehyde (12 mmol); solvent: ethyl acetate; temperature: 28 °C; reaction time: 1 h.</figcaption></figure>

full conversion was achieved after 2 hours or reaction (Figure 20) maintaining a high selectivity toward α-pinene oxide (90%).

Another scale-up experiment was carried out on 72 mmol (9.8 g) of α-pinene where full conversion could achieve after 4 hours with a selectivity toward α-pinene oxide of 85%; thus showing the efficiency of the catalytic system for reactions carried out on a multi-gram scale.

<figure><img src="image_35.png" /><figcaption>**Figure 20.** Evolution of conversion vs time in the catalytic aerobic epoxidation of α-pinene (1.47 g) using Co/SBA-16 (1.8%) as catalyst. Reaction conditions: α-pinene (10.8 mmol); isobutyraldehyde (36 mmol); solvent: ethyl acetate; temperature: 28 °C.</figcaption></figure>

## Conclusions

The study explored the aerobic catalytic epoxidation of terpenes (limonene and α-pinene) using molecular oxygen as oxidant under mild conditions promoted by various supported cobalt-based catalysts. Mesoporous silica (SBA-15 and SBA-16), commercial silica (SiO₂c), and functionalized carbon nanotubes (CNT) were investigated as supports. The catalytic systems exhibited different textural properties and cobalt dispersion and loading, impacting their catalytic performances (activity and selectivity toward the desired epoxide). While Co/SBA-15 showed superior activity in limonene epoxidation, Co/SiO₂c displayed higher selectivity. In the case of α-pinene, full conversion could be achieved in 45 min using the Co/SiO₂c catalyst. The influence of cobalt loading on catalytic activity and selectivity was also observed. Recycling tests demonstrated the stability and recyclability of the Co/SiO₂c catalyst. Overall, the findings shed light on the importance of support properties in catalytic reactions. The scale-up of the oxidation of α-pinene was successful in maintaining a good activity and a high selectivity toward the epoxide.

## Acknowledgements

The authors acknowledge the MESR for a doctoral grant to MR. The authors warmly thank the technical assistance provided by Vincent Collière (LCC-CNRS, Toulouse) for TEM analyses, Laurent Ropiquet (LCC-CNRS, Toulouse) for BET measurements and Jérôme Esvan (CIRIMAT, Toulouse) for XPS analysis.

## Conflict of Interests

The authors declare no conflict of interest

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** Terpene · Aerobic epoxidation · Supported cobalt · Heterogeneous catalysis

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Manuscript received: May 22, 2024

Revised manuscript received: August 6, 2024

Accepted manuscript online: August 13, 2024

Version of record online: January 23, 2025