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Article

Enhancing Catalytic Efficiency in Long-Chain Linear α-Olefin Epoxidation: A Study of CaSnO3-Based Catalysts

by
Min Zhang
1,2,3,
Hongwei Xiang
1,2,* and
Xiaodong Wen
1,2,3,*
1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
2
National Energy Center for Coal to Clean Fuels, Synfuels China Technology Co., Ltd., Beijing 101400, China
3
School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(1), 70; https://doi.org/10.3390/catal14010070
Submission received: 19 December 2023 / Revised: 10 January 2024 / Accepted: 11 January 2024 / Published: 17 January 2024
(This article belongs to the Special Issue Advanced Research of Perovskite Materials as Catalysts)
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Abstract

:
This investigation explores the synthesis of advanced catalysts for epoxidizing long-chain linear α-olefins, a pivotal process in the chemical industry for generating critical intermediates. Employing a hydrothermal technique, we developed four distinct catalysts (CS-1–4), methodically modulating the Ca/Sn ratio to elucidate its impact on the catalysts’ physicochemical properties. Our research uncovered that an escalated Ca/Sn ratio induces a morphological shift from octagonal to cubic structures, concomitant with a diminution in particle size and an enhancement in specific surface area. Significantly, the CS-3 catalyst outperformed others in 1-octene epoxidation, an efficacy attributed to its augmented surface alkalinity and proliferation of medium-strength alkaline sites, likely emanating from increased surface oxygen defects. Subsequent hydrogen reduction of CS-3 further amplified these oxygen defects, yielding a 10% uptick in catalytic activity. This correlation underscores the potential of oxygen defect manipulation in optimizing catalytic efficiency. Our findings contribute a novel perspective to the development of robust, high-performance catalysts for α-olefin epoxidation, seamlessly aligning with the principles of sustainable chemistry.

Graphical Abstract

1. Introduction

The epoxidation of long-chain linear α-olefins to produce industrially valuable epoxides plays a pivotal role in modern chemical industry, as it provides essential feedstock for the production of plasticizers, surfactants, and other fine chemicals [1,2,3,4]. Traditionally, this process relied on organic peracids as oxygen donors. However, this method posed significant challenges due to the peracids’ explosive nature and high costs, limiting their practical application [5,6].
Over the past decades, research into the epoxidation of long-chain linear α-olefins has largely focused on the Halcon method, which utilizes organic hydroperoxide as the oxidant [7,8,9,10,11]. However, this approach has been plagued by the production of substantial organic acid byproducts. With the emergence of green chemistry principles, environmentally benign H2O2 has gained favor as the preferred terminal oxidant for olefin epoxidation [12,13]. Among the various catalytic systems that utilize H2O2, Payne’s epoxidation system stands out, garnering widespread attention for its remarkable selectivity in epoxide production [14,15]. This system employs solid bases as catalysts, nitriles as co-oxidants, and hydrogen peroxide as the terminal oxidant. The exploration of magnesium–aluminum hydrotalcite and its modified variants within the Payne system context has opened up exciting new research avenues, presenting substantial promise for future advancements [16].
In recent years, ABO3-type magnesium-based composite oxides, including MgSnO3, MgCeO3, and MgTiO3, have gained tremendous attention as innovative catalytic materials for olefin Payne epoxidation, due to their excellent catalytic efficacy [17]. However, these catalysts usually suffer from poor structural stability, as alkaline components can easily dissolve into the solution and be leached, which impedes their recyclability. Consequently, the research focus has shifted towards the development of catalysts that are not only highly active but also exhibit enhanced stability, thereby improving their suitability for recycling in the olefin Payne epoxidation processes [18,19].
Inspired by ABO3 magnesium-based composite oxides, we turned our attention to ABO3 calcium-based composite oxides, given that calcium-based oxides exhibit better alkaline strength than magnesium-based oxides and thus may result in higher catalytic activity. Unlike their magnesium-based counterparts, ABO3 calcium-based composite oxides typically exhibit a perovskite structure [20]. The emergence of perovskite materials in various fields has opened up new avenues for research. ABO3 perovskite-type composite oxides represent a novel class of inorganic non-metallic materials, distinguished by their unique physical and chemical properties. Their potential as catalysts has garnered widespread interest, particularly due to the ability to modify their crystal defect structures, thereby tailoring their catalytic performance [21].
To design new, efficient Payne epoxidation catalysts, comprehensive understanding on the active sites of the catalysts is essential. Recently, researchers have discovered that, during olefin Payne epoxidation reactions, the catalytic activity is primarily related to the moderately basic sites on the catalyst surface [22]. Furthermore, these moderately basic sites on the catalyst surface are closely associated with oxygen defects on the catalyst surface. Therefore, the development of solid base catalysts with oxygen defects emerges as a promising strategy for achieving highly efficient Payne epoxidation of olefins [23].
In this study, we synthesized CaSnO3 perovskite catalysts with abundant surface oxygen defects through non-stoichiometric synthesis. To further augment the surface defects, the synthesized catalysts underwent a post-treatment process with hydrogen reduction [24,25,26]. The structural properties of the prepared catalysts were systematically characterized. We then carefully evaluated the intrinsic catalytic performance of those catalysts through Payne’s epoxidation of long-chain linear α-olefins. Our work not only expands the new applications of CaSnO3 materials but also upholds CaSnO3 as an active and cyclically stable catalyst for α-olefin Payne epoxidation. Furthermore, we have identified that defect engineering can modify the surface alkalinity of the catalyst, thereby enhancing its catalytic activity. This provides a new approach for designing highly active and stable catalyst materials for Payne’s epoxidation of long-chain linear α-olefins in the future.

2. Results and Discussion

2.1. Characterizations of the Catalysts

2.1.1. Structures of Catalysts

Four types of catalysts with various Ca/Sn molar ratios were prepared by hydrothermal reaction of polyvinyl pyrrolidone (PVP) and Ca, Sn materials, and then calcined at high temperatures in the air atmosphere (Scheme S1). The obtained catalysts are denoted CS-X, where X = 1, 2, 3, and 4, respectively. The X-ray diffraction (XRD) spectra were obtained to study the crystalline structures of these catalysts (Figure 1a). The locations of diffraction peaks of the four catalysts were basically the same, and these diffraction peaks were completely consistent with the standard spectra of CaSnO3. The diffraction peaks at 2θ = 22.4°, 31.9°, and 45.8° correspond to the lattice planes of CaSnO3 (020), (121), and (202), respectively. Calcium stannate (CaSnO3) is a ternary compound with a crystal structure belonging to the perovskite family. Figure 1b depicts the crystal structure of CaSnO3, with calcium ions (Ca2+) and tin ions (Sn4+) occupying distinct positions within the cubic lattice. Each calcium ion is surrounded by eight oxygen ions (O2−), forming a stable oxygen dodecahedral [27]. Simultaneously, each tin ion is coordinated by six oxygen ions, forming a stable tin octahedron. The arrangement of calcium and tin ions imparts cubic symmetry to CaSnO3 [28,29,30].

2.1.2. Morphology and Surface Element Distribution of Catalysts

The morphology of the catalysts was studied via scanning electron microscopy (SEM). The SEM images of the samples are depicted in Figure 2. As the Ca/Sn molar ratio increases, the particle size of the sample decreases and the size distribution becomes more uniform. Specifically, the particle sizes of the CS-1, CS-2, CS-3, and CS-4 samples are 2.63 μm, 2.02 μm, 0.95 μm, and 0.89 μm, respectively (Figure S1), and the morphology of the samples changes from octagonal to cubic (Figure 2a–d). The morphology of CS-1 and CS-2 is octagonal, while CS-3 and CS-4 are cubic. This may be due to the combined effect of the polyvinylpyrrolidone and calcium ions added to the system. When there are fewer Ca ions in the system, the added surfactants may preferentially adsorb on the {100} plane, thereby reducing the growth rate of crystals along the <100> direction and increasing the growth rate of crystals along the <111> direction, inducing the formation of octagonal CaSnO3. However, as the Ca ion content in the system increases, this phenomenon is weakened or even eliminated, and eventually cubic CaSnO3 with six {100} surfaces is formed [31,32]. Figure 2e shows the EDX mapping of CS-3, indicating a uniform distribution of Ca, Sn, and O elements. When combined with the XRD test results, this proves that pure-phase perovskite compounds form in the sample.

2.1.3. Specific Surface Areas of Catalysts

The above SEM test results show that as the Ca/Sn molar ratio increases, the particle size of the sample decreases. In order to understand the impact of the particle size variation on the specific surface area (BET) of the sample, we tested the specific surface area of the samples, and the test results are shown in Figure 3a. All catalysts exhibit type IV isotherms and H3 hysteresis loops, which are often reported for multilayer adsorption on mesoporous solids, indicating narrow slit-like pores formed through the aggregation of plate-shaped particles. Figure 3b displays the pore size distribution of CS-3, from which it can be observed that CS-3 mainly exhibits mesoporous features, with relatively large mesopores. As shown in Table S3, for CS-1, CS-2, CS-3, and CS-4, the BET specific surface areas are 4.80 m2/g, 5.32 m2/g, 5.86 m2/g, and 5.88 m2/g, respectively. The specific surface areas of all four samples are relatively small, which may be attributed to the inherent properties of CaSnO3 samples. Compared to CS-1, the specific surface area of CS-2 increases by about 10%, and the specific surface area of CS-3 increases by about 20%. In the SEM test results, the morphology and size of CS-3 and CS-4 are almost identical, and their measured BET specific surface areas are also very similar. The test results indicate that there is an increasing trend in specific surface area with the increase in the Ca/Sn ratio. The variation in specific surface area corresponds to the changes in particle size observed in the SEM test results.

2.2. Catalytic Performance of the Catalysts

2.2.1. Evaluation of the Different Catalysts for 1-Octene Epoxidation

We used the synthesized samples for the catalytic epoxidation of 1-octene. Generally, optimizing the reaction parameters is of great significance for the further application of CaSnO3. In order to obtain the optimal reaction parameters, we screened the reaction conditions using 1-octene as a model substrate based on conversion and selectivity. The optimal reaction conditions were determined as follows: 1-octene: 3.9 mmol, benzonitrile: 10.5 mmol, catalyst: 0.2 g, methanol: 10 mL, 30% H2O2: 2.4 mL, and reaction temperature: 60 °C. Under the optimal reaction conditions, we evaluated the catalytic performance of the different synthesized catalysts. The evaluation results are shown in Table 1. From the table, it can be observed that as the Ca/Sn ratio increases, the selectivity of 1,2-epoxyoctane remains basically unchanged, maintaining at above 98%, while the conversion of 1-octene increases continuously, from 65.1% to 83.6%. The reaction results of CS-3 and CS-4 are largely consistent. After 18 h of reaction, CS-3 achieves an 82.5% conversion of 1-octene and a 98.9% selectivity of 1,2-epoxyoctane. On the other hand, alkaline earth metal stannates with perovskite structures, SS-3 and BS-3, exhibit poorer catalytic performance, with conversions of 1-octene of 36.1% and 45.8% and selectivities of around 97%. Compared to these, CS-3 shows superior catalytic performance. The single metal oxide SnO2 is an acidic oxide, which cannot effectively promote the reaction, resulting in a conversion of 1-octene of only 7.4%. Meanwhile, the single metal oxide CaO can react with the water in the reaction system, causing an imbalance in the acid-base of the reaction system, which is not conducive to the reaction and promotes the hydrolysis of epoxides, exhibiting poor conversion of 1-octene and selectivity of epoxides [33]. From the above experiments, it can be seen that CS-3 is an excellent catalyst for the catalytic epoxidation of 1-octene.

2.2.2. The Influence of Reaction Time

The change in reaction results over time can reflect the change in catalytic reaction rate. Under the optimized experimental conditions mentioned above, we used CS-3 as the catalyst to investigate the effect of reaction time on the epoxidation of 1-octene, which was reacted in methanol solvent for 6, 12, 18, and 24 h. From Figure 4, it can be seen that after 6 h of reaction, the conversion rate of 1-octene is 51.6% and the selectivity of the epoxide is 98.6%. When the reaction time is extended from 6 to 18 h, the conversion rate increases significantly from 51.6% to 85.2%, while the selectivity decreases slightly from 98.6% to 97.6%. With further extension of the time to 24 h, the effect on the epoxidation of 1-octene is not significant. This may be due to the decrease in the concentrations of H2O2 and 1-octene as the reaction progresses, leading to a slower reaction rate.

2.2.3. Catalytic Stability

The cyclic stability of a catalyst is a crucial indicator for evaluating its performance. In this study, we tested the cyclic stability of the catalyst using CS-3 as the catalyst and 1-octene as the model substrate. The test results are shown in Figure 5a, from which we can observe that the catalyst maintains its original catalytic activity well after five cycles of use, without significant deactivation. After completing five cycles, we conducted XRD analysis on the reacted catalyst. The results, depicted in Figure 5b, reveal that the XRD pattern of the catalyst post-reaction maintains the configuration of the main peak and displays similar diffraction peaks in comparison to the pre-reaction catalyst. This suggests that the catalyst retains its original structure and exhibits commendable structural stability following the reaction. Furthermore, after undergoing five cycles, there is minimal alteration in the specific surface area of the catalyst (see Figure S2). The experimental findings, encompassing cyclic performance, XRD patterns pre- and post-reaction, and the consistent specific surface area, collectively affirm the robust cyclic stability of the CS-3 catalyst.

2.2.4. Comparative Results with Different Catalysts for the Epoxidation of 1-Octene

In order to better evaluate the catalytic performance of CaSnO3, we compared it with some catalysts reported in the literature. We quantified the catalytic performance of different catalysts using the production rate of 1,2-epoxyoctane per gram of catalyst per hour as an evaluation indicator. The results (Figure 6) showed that compared to Mg-based solid base catalysts such as hydrotalcite (Mg6Al2(OH)16CO3·4H2O) [34], Mg2P2O7 [35], and Mg3(PO4)2 [36], CaSnO3 exhibits better catalytic performance, with a turnover frequency (TOF) value of 2.38 mmol·g−1·h−1. Furthermore, the catalytic performance of CaSnO3 -s also higher than that of hydroxyapatite Ca10(PO4)6(OH)2 [22], which is another Ca-based catalyst.

2.2.5. Substrate Range Extension

To explore the universality of the CS-3 catalyst for other α-olefins with different carbon chain lengths, we expanded the substrate scope. We conducted epoxidation reaction tests on 1-hexene, 1-heptene, 1-decene, and 1-dodecene using the CS-3 catalyst. The results are shown in Figure 7. From the figure, it can be seen that longer-carbon-chain-length olefin requires longer reaction times to achieve desirable catalytic results. For shorter-carbon-chain-length olefins, 1-hexene and 1-heptene, the corresponding conversion rates of the epoxidized compounds reach 90% and 85% after 18 h of reaction, respectively. The reaction results for 1-decene are similar to those of 1-octene. After 24 h of reaction, the conversion rate of 1,2-epoxydecane can reach 94%. However, for 1-dodecene, the conversion rate is less than 80% after 24 h of reaction. This may be due to increased inertness and increased difficulty in epoxidation as the carbon chain length increases, leading to a slower reaction rate for long-chain α-olefins.

2.3. Surface Properties of Catalysts

2.3.1. Surface Alkalinity

It is generally believed that the performance of Payne’s epoxidation of olefins is closely related to the strength and quantity of basic sites on the catalyst surface. In this paper, we characterized the alkaline properties of three catalysts, CS-1, CS-2, and CS-3, using CO2 temperature-programmed desorption (CO2-TPD) technology, and the test results are shown in Figure 8. Different CO2 desorption temperature ranges correspond to different basic sites on the catalyst surface [37]. In the CO2-TPD graph, the part with a CO2 desorption temperature lower than 350 °C generally corresponds to weak basic sites; the part with a CO2 desorption temperature between 350 °C and 650 °C generally corresponds to medium-strength basic sites; and the part with a CO2 desorption temperature higher than 650 °C generally corresponds to strong basic sites [38]. Based on the CO2 desorption area, we quantified the contents of different basic sites on the catalyst surface, as shown in Table S1. From the table, it can be seen that the alkalinity of the three samples is mainly of medium strength, and as the Ca/Sn ratio increases, the total alkalinity of the samples and the quantity of medium-strength basic sites also increase, which is consistent with the trend of the catalytic performance of the samples. Previous studies have shown that medium-strength basic sites play a major role in Payne’s epoxidation, and the more medium-strength basic sites there are, the better the catalytic performance of the sample [39].

2.3.2. Surface Oxygen Species

In metal oxides, the surface alkalinity of solid oxides is closely related to surface oxygen species. It is generally believed that surface defect oxygen is the main source of medium-strength basic sites on the surface [40]. In order to understand the distribution of surface oxygen species in the sample, we conducted O1s X-ray photoelectron spectroscopy (XPS) characterization on these three samples. For metal oxides, the O1s XPS peaks are usually divided into three types: the first is the adsorbed oxygen on the surface of metal oxides (e.g., surface-adsorbed molecular water); the second is the defect oxygen on the surface of metal oxides; and the third is the lattice oxygen on the surface of metal oxides [41,42]. Figure 9 shows the results of the peak fitting of O1s XPS after the sample was processed, and the proportion of each part is shown in the table (Table S2). From the table, we can see that the proportion of surface defect oxygen in the sample increases as the Ca/Sn ratio increases.

2.4. Modification of Catalyst

2.4.1. The Structure, Morphology, and Specific Surface Area of the Modified Catalyst

We speculate that it may be possible to improve the total alkalinity and quantity of medium-strength basic sites on the surface of the sample by regulating the content of surface defect oxygen. Generally, the content of surface oxygen defects can be increased through hydrogen reduction treatment of the sample [43,44]. We conducted hydrogen reduction treatment on the CS-3 catalyst, heated the catalyst to 900 °C for 2 h in 10% H2/Ar atmosphere, and labeled the CS-3 sample after hydrogen reduction as H2-CS-3. In Figure 10a, it is evident that the characteristic peaks in the XRD pattern of the H2-CS-3 sample, both before and after hydrogen reduction treatment, align with those of the CS-3 sample XRD. This indicates that the hydrogen treatment has not altered the structure of the sample. In addition, the morphology (Figure 10b), specific surface area (Figure 10c and Table S3), and particle size (Figures S3 and S4) of the catalyst show negligible changes after hydrogen treatment.

2.4.2. Catalytic Activity, Surface Alkalinity, and Surface Oxygen Species of the Modified Catalyst

The O1s XPS results (Figure 11a) of the sample before and after hydrogen reduction treatment show that the proportion of oxygen defects in the H2-CS-3 sample was larger than that in the CS-3 sample. The CO2-TPD test results (Figure 11b) show that the quantity of basic sites in the H2-CS-3 sample was higher than that in the CS-3 sample. During the epoxidation performance testing of the H2-CS-3 sample, we found that the catalytic performance was 10% higher than that of the CS-3 sample (Figure 11c). These results indicate that the amount of medium-strength basic sites and total alkalinity on the surface of the sample can be increased by increasing the surface oxygen defects of the sample, thereby improving the catalytic activity of the sample.

2.5. Reaction Mechanism Study

Currently, the solid-base-catalyzed epoxidation of alkenes, known as Payne’s epoxidation, has two possible reaction mechanisms. One is a non-radical reaction pathway similar to organic peracid catalysis (Scheme S2a). In this pathway, hydrogen peroxide generates HOO− under the catalytic action of the catalyst. Then, HOO− nucleophilically attacks the phenyl cyanide to form a peroxyacid ion. The peroxyacid ion is then protonated to form a peracid, and finally, the peracid catalyzes olefin epoxidation. The other pathway is a radical pathway (Scheme S2b) that considers the homolysis of hydrogen peroxide. In this pathway, under heating, hydrogen peroxide homolyzes to generate hydroxy radicals. To determine if radicals are involved in the epoxidation process, we conducted radical quenching experiments. The experimental results are shown in Figure 12a, from which it can be seen that when hydroxy radical quenching agent BHT is added to the reaction system [45,46], the conversion rate decreases, indicating the presence of hydroxy radicals in the reaction system. When the amount of BHT is further increased to 2 equivalents, 60% of 1-octene still undergoes conversion. This experiment suggests that the epoxidation reaction cannot be completely quenched by BHT, indicating that hydroxy radicals are not the dominant reactive oxygen species (ROS). Apart from homolysis of hydrogen peroxide under heating or light conditions to generate hydroxy radicals, it can also undergo heterolysis to generate HOO− in alkaline environments or under the influence of catalysts. In other words, the presence of alkaline catalysts allows hydrogen peroxide to undergo heterolysis to generate HOO−. In catalytic reaction experiments, we found that epoxidation does not occur in the absence of a catalyst, indicating that epoxidation cannot take place without the generation of HOO−. To further confirm this conclusion, we added an equal amount of Fe2+ to the reaction system. Hydrogen peroxide generates abundant hydroxy radicals through the catalytic action of Fe2+ via the Fenton reaction (Fe2+ + H2O2 → Fe3+ + OH− + ·OH) [47], and this process cuts off the production pathway of HOO−, thus preventing the generation of peracids. After adding an equal amount of Fe2+, no product formation was observed, indicating that HOO− is crucial for the occurrence of the epoxidation reaction. This also proves that in this reaction system, the catalyst is an indispensable substance for catalyzing the generation of HOO− from hydrogen peroxide. To clarify the dominant ROS in the reaction, we further investigated other oxygen radicals. When an equal amount of superoxide radical quenching agent benzoin was added to the reaction system, neither 1-octene nor benzonitrile underwent conversion, indicating that the key ROS is composed of superoxide radicals. When encountering water or electron-deficient carbon, superoxide radicals can further transform into peroxide radicals (HOO· or ROO·). The above observations strongly suggest that this epoxidation process follows a radical reaction pathway. To further confirm the presence of ROO· and HO·, we employed Electron Paramagnetic Resonance (EPR) spectroscopy to investigate the radicals in the reaction system (Figure 12b), using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapping agent. No EPR signals were observed in control experiments with methanol and water. However, distinct radical signals were observed in reaction experiments with benzonitrile as an additive. Replacing benzonitrile with acetonitrile yielded better radical spectra, consistent with literature reports, confirming the existence of HO· and ROO· in the reaction system [48].
Based on the above experimental phenomena and literature reports [49], we propose a reaction mechanism for the formation of 1,2-epoxyoctane and benzamide from 1-octene catalyzed by CaSnO3, using hydrogen peroxide as an oxidant and benzonitrile as a co-oxidant. The reaction mechanism is illustrated in Scheme 1. Firstly, under the influence of alkaline species on the CaSnO3 catalyst surface, H2O2 undergoes heterolysis to generate HOO− (Step 1). Subsequently, the generated HOO− nucleophilically attacks the carbon ion in benzonitrile, forming intermediate a (Step 2). Intermediate a is protonated in the presence of water, yielding intermediate b (Step 3). Simultaneously, H2O2 undergoes homolysis spontaneously, leading to the generation of hydroxyl radicals. Heating facilitates the homolysis of hydrogen peroxide, thereby accelerating the production of hydroxyl radicals (Step 4). Then, intermediate b combines with the hydroxyl radicals generated through the homolysis of H2O2, forming the crucial intermediate c (Step 5). It is worth noting that H2O2 not only participates in the generation of the target oxidation products but also spontaneously decomposes to produce oxygen and water. Intermediate c forms a five-membered ring through intramolecular hydrogen bonding, which is the active oxygen species in this reaction. Proton solvents, methanol and water, in the reaction system not only effectively promote the charge transfer in proton transfer equilibrium reactions but also stabilize the active intermediate through intermolecular hydrogen bonding. Finally, the active species (intermediate c) activates the double bond of 1-octene, connects with the C-O bond to form alkyl radicals (Step 6), and then rapidly cleaves the O-O bond to produce the epoxidation product. Meanwhile, the remaining part undergoes electron rearrangement to form the byproduct benzamide (Step 7), while the hydroxy radicals are continuously recycled (Step 8).

3. Materials and Methods

3.1. Materials

1-octene (C8H16, 98%), benzonitrile (C7H5N, ≥99.5%), methanol (CH4O, ≥99.9%), n-dodecane (C12H26, ≥99.5%), sodium stannate (Na2SnO3, AR), calcium chloride dihydrate (CaCl2·2H2O, 99.99%), polyvinylpyrrolidone ((C6H9NO)n, K88-96), anhydrous sodium carbonate (Na2CO3, 99.99%), hydrated stannic chloride (SnCl4·5H2O, 99.995%), silver nitrate (AgNO3, 99.8%), urea (NH2CONH2, 99.999%), barium chloride dihydrate (BaCl2·2H2O, 99.99%), and strontium chloride hexahydrate (SrCl2·6H2O, 99.99%) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. (Beijing, China).

3.2. Catalysts’ Preparation

CaSnO3 powder was prepared with different molar ratios of CaCl2·2H2O and Na2SnO3 as raw materials, and the molar ratios of Ca/Sn raw materials were adjusted to 1.0, 2.0, 3.0, and 4.0, respectively. Correspondingly, the resulting samples were labeled CS-1, CS-2, CS-3, and CS-4, respectively. In a typical procedure, 4 mmol of CaCl2·2H2O and 4 mmol Na2SnO3 were dissolved in 40 mL of deionized water, respectively, and then 100 mg of PVP was added to CaCl2 solution and vigorously stirred. Next, the two solutions were mixed and transferred to a 100 mL autoclave for 12 h at 180 °C. The solid was recovered from suspension by using centrifugation, repeatedly washed with deionized water until the washing liquid was neutral, and then dried overnight at 100 °C. The resulting sample was calcined at 900 °C for 2 h. To further investigate the performance, CS-3 samples were treated in a 10% H2/Ar atmosphere at 900 °C for 2 h.
The preparation method of SrSnO3 and BaSnO3 samples is the same as the preparation method of CS-3, except that CaCl2·2H2O is replaced with SrCl2·6H2O and BaCl2·2H2O, respectively. In this study, MgSnO3 samples were not synthesized because MgSnO3 begins to decompose at about 600 °C and eventually decomposes into Mg₂SnO₄ and SnO₂, meaning it is not suitable as a control sample.
Calcium oxide was prepared through the precipitation method. Firstly, 30 mmol of CaCl2·2H2O and 30 mmol of anhydrous Na2CO3 were dissolved in 30 mL of deionized water, respectively. Then, under vigorous stirring, the CaCl2 solution was added to the Na2CO3 solution. After stirring for 30 min, the precipitate was filtered and collected, and it was washed repeatedly with a large amount of deionized water and absolute ethanol until the filtrate was neutral. Next, the obtained product was dried overnight at 60 °C. Finally, in a muffle furnace, at a heating rate of 5 °C/min, the sample was calcined at 900 °C for 2 h. The resulting sample was placed in a dryer for further use.
SnO2 was prepared through the hydrothermal method. In a typical procedure, 10 mmol SnCl4·5H2O and 20 mmol of urea were dissolved in 80 mL of water, respectively, and then the solution containing urea was added to SnCl4 solution. After vigorous stirring for 30 min, the resulting liquid was transferred into an autoclave for crystallization at 140 °C for 7 h. Solids were recovered through centrifugation and repeatedly washed with deionized water until the wash solution was neutral, and then dried overnight at 100 °C. The resulting sample was calcined at 550 °C for 2 h. Finally, the resulting sample was placed in a dryer for further use.

3.3. Catalysts’ Characterization

The phase composition of the fresh and spent catalysts was characterized using the Bruker AXS D2 Focus X-ray diffraction meter (XRD) (Bruker, Saarbrücken, Germany). The sample was irradiated with Cu Kα rays (λkα = 0.154 nm) at 30KV and 10mA, and the data were acquired by scanning in steps of 0.02°/s in a range of 2θ from 5° to 90°.
The specific surface area and pore size distribution of the samples were characterized using the nitrogen adsorption–desorption method on a Micromeritics ASAP 2460 instrument (Micromeritics, Norcross, GA, USA). The BET method was used for the theoretical calculation of the specific surface area, while the BJH method was used for the calculation of pore size distribution.
X-ray photoelectron spectroscopy (XPS) analysis of the catalyst was performed on a Thermo Fisher Scientific ESCALAB 250Xi system (Thermo Scientific, Waltham, MA, USA). The binding energy was calibrated using the C 1s peak of graphite at 284.6 eV as a reference.
CO2 temperature-programmed desorption (CO2-TPD) analysis of the catalyst was conducted with a micromeritics Auto Chem 2920 instrument (Micromeritics, Norcross, GA, USA). Typically, 100 mg of sample (40–80 mesh) was weighed and placed in a reaction tube. The sample was then heated to 900 °C at a rate of 10 °C/min in a high-purity He gas stream (50 mL/min), and it was pretreated at this temperature for 1 h to remove impurities from the sample surface. After this pretreatment, the sample was cooled to 50 °C under a high-purity He gas stream (50 mL/min) and then exposed to high-purity CO2 gas (V = 50 mL/min) for adsorption for 1 h. Subsequently, the gas flow was switched to high-purity He gas (V = 50 mL /min) and purged for 120 min to remove physically adsorbed CO2. Finally, the sample was heated at a rate of 10 °C/min from 50 °C to 900 °C under a high-purity He gas stream (V = 50 mL /min) to desorb CO2, which was detected using TCD. According to the stoichiometric equation MgCO3 = MgO + CO2↑, a 99.9% pure MgCO3 standard (completely decomposed at around 600 °C) was utilized to quantify the desorption of CO2 from the catalyst.
The electron paramagnetic resonance (EPR) signal was detected using a Bruker A300 X-band EPR spectrometer (Bruker, Saarbrücken, Germany). Prior to the experiment, different solutions were prepared and reacted for 30 min under standard conditions (such as using methanol and water as solvents), followed by sampling. The radical scavenger 5,5-dimethylpyrroline N-oxide (DMPO) was used.

3.4. Catalyst Activity Evaluation

In a typical reaction, 50 mg of catalyst, 3.9 mmol of 1-octene, 10.5 mmol of benzonitrile, 10 mL of methanol, 1 mmol of n-dodecane, and 2.4 mL of 30% hydrogen peroxide were sequentially added to a pressure-resistant reaction flask. Then, the flask was sealed and heated to 333 K under magnetic stirring and held for 12 h. After the end of the reaction, the reaction mixture was extracted with CH2Cl2. Next, a sufficient amount of anhydrous Na2SO4 was added to the organic phase for dehydration. The reaction was analyzed on an Agilent 7890A gas chromatograph with flame ionization detection (Agilent, Santa Clara, CA, USA). The HP-5 column (30 m × 0.25 mm × 0.25 μm) was compared with the standard sample to determine the peak location of each component. The content of each component was determined by using n-dodecane as the internal standard.

3.5. Catalyst Recycling Experiment

The catalyst was recovered using centrifugation and repeatedly washed 2–3 times with anhydrous ethanol to remove organic substances from the surface of the catalyst. Then, the recovered catalyst was dried at 120 °C for several hours and finally calcined at 900 °C in an oxygen atmosphere for 2 h. The regenerated catalyst was used in the next reaction.

4. Conclusions

In this work, four catalysts, CS-1, CS-2, CS-3, and CS-4, were prepared via the hydrothermal method by controlling the ratio of Ca and Sn in raw materials. It was found that with the increase in Ca/Sn ratio, the morphology of the catalyst changed from octagonal to cuboid, the particle size became smaller and more consistent, and the catalytic specific surface area also increased with the decrease in particle size. Compared with CS-1 and CS-2, the CS-3 catalyst showed a better epoxidation ability of 1-octene. This may be related to the fact that the CS-3 catalyst has more surface alkalinity, especially more medium-strength alkalinity sites, which may benefit from more oxygen defects on the surface of the CS-3 catalyst. To verify this hypothesis, we increased the surface oxygen defect of the catalyst by treating CS-3 with hydrogen reduction. Compared with the unreduced catalyst, the number of medium-strength alkaline sites on the catalyst surface was increased after hydrogen reduction treatment, and the catalytic activity was increased by 10%. In these ways, it has been proven that the amount of medium-strength alkalis on the catalyst surface can be increased by increasing the oxygen defect content on the catalyst surface, to improve the catalytic activity of the catalyst. Accordingly, this work may offer a simple strategy for improving the performance of long-chain α-olefin epoxidation catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14010070/s1, Scheme S1. Diagram of the synthesis of CaSnO3 catalyst; Scheme S2. Possible reaction mechanism of Payne’s epoxidation; Figure S1. Particle size distribution of CS-1, CS-2, CS-3, and CS-4; Figure S2. N2 adsorption–desorption isotherms of the spent CS-3; Figure S3. Pore size distribution of H2-CS-3; Figure S4. Particle size distribution of H2-CS-3; Table S1. The amount of CO2 desorption from the different types of CaSnO3 catalysts; Table S2. Binding energy and relative area percentage of oxygen species in CS-1, CS-2, CS-3, and H2-CS-3; Table S3. Structural parameters of CS-1, CS-2, CS-3, and CS-4.

Author Contributions

Conceptualization, X.W.; methodology, M.Z.; validation, X.W. and M.Z.; formal analysis, M.Z.; investigation, M.Z.; writing—original draft preparation, M.Z.; writing—review and editing, H.X. and X.W.; visualization, M.Z.; supervision, H.X. and X.W.; project administration, H.X. and X.W.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful for the financial support from National Key R&D Program of China (No. 2022YFA1604103), National Science Fund for Distinguished Young Scholars of China (Grant No. 22225206), CAS Project for Young Scientists in Basic Research (YSBR-005), Key Research Program of Frontier Sciences CAS (ZDBS-LY-7007), Major Research plan of the National Natural Science Foundation of China (92045303), Informatization Plan of Chinese Academy of Sciences, Grant No. CAS-WX2021SF0110, the Autonomous Research Project of SKLCC (Grant No.: 2023BWZ005), and funding support from Synfuels China, Co., Ltd.

Data Availability Statement

Data is contained within the article.

Acknowledgments

We gratefully acknowledge Zhengang Lv at Synfuels China Co., Ltd., for assistance in performing XPS.

Conflicts of Interest

The authors declare that this study received funding from Synfuels China Co., Ltd. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Catalysts 14 00070 g001
Figure 1. (a) XRD patterns of CS-1, CS-2, CS-3, and CS-4. (b) Polyhedral model of CS-3 crystal structure.
Figure 1. (a) XRD patterns of CS-1, CS-2, CS-3, and CS-4. (b) Polyhedral model of CS-3 crystal structure.
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Catalysts 14 00070 g002
Figure 2. SEM images of (a) CS-1, (b) CS-2, (c) CS-3, and (d) CS-4; (e) HAADF-STEM mapping images of CS-3.
Figure 2. SEM images of (a) CS-1, (b) CS-2, (c) CS-3, and (d) CS-4; (e) HAADF-STEM mapping images of CS-3.
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Catalysts 14 00070 g003
Figure 3. (a) N2 adsorption–desorption isotherms of CS-1, CS-2, CS-3, and CS-4; (b) pore size distribution of CS-3.
Figure 3. (a) N2 adsorption–desorption isotherms of CS-1, CS-2, CS-3, and CS-4; (b) pore size distribution of CS-3.
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Catalysts 14 00070 g004
Figure 4. The influence of time on 1-octene epoxidation. Reaction conditions: 1-octene (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. Conversion corresponds to the conversion rate of 1-octene; Selectivity refers to the selectivity of epoxides, determined by GC analysis.
Figure 4. The influence of time on 1-octene epoxidation. Reaction conditions: 1-octene (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. Conversion corresponds to the conversion rate of 1-octene; Selectivity refers to the selectivity of epoxides, determined by GC analysis.
Catalysts 14 00070 g004
Catalysts 14 00070 g005
Figure 5. (a) A study of the recycling of CS-3 for 1-octene epoxidation. Reaction conditions: 1-octene (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. (b) XRD spectra of spent CS-3 and fresh CS-3.
Figure 5. (a) A study of the recycling of CS-3 for 1-octene epoxidation. Reaction conditions: 1-octene (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. (b) XRD spectra of spent CS-3 and fresh CS-3.
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Catalysts 14 00070 g006
Figure 6. A comparison of the results from other works for 1-octene epoxidation.
Figure 6. A comparison of the results from other works for 1-octene epoxidation.
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Catalysts 14 00070 g007
Figure 7. A study on other long-chain α-alkenes catalyzed by CS-3. Reaction conditions: substrate (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. Conversion corresponds to the conversion rate of 1-octene; Selectivity refers to the selectivity of epoxides, determined by GC analysis.
Figure 7. A study on other long-chain α-alkenes catalyzed by CS-3. Reaction conditions: substrate (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C. GC analysis with dodecane as an internal standard. Conversion corresponds to the conversion rate of 1-octene; Selectivity refers to the selectivity of epoxides, determined by GC analysis.
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Catalysts 14 00070 g008
Figure 8. CO2-TPD profiles of CS-1, CS-2, and CS-3.
Figure 8. CO2-TPD profiles of CS-1, CS-2, and CS-3.
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Catalysts 14 00070 g009
Figure 9. XPS O1s spectra of CS-1, CS-2, and CS-3.
Figure 9. XPS O1s spectra of CS-1, CS-2, and CS-3.
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Catalysts 14 00070 g010
Figure 10. (a) XRD patterns of CS-3 and H2-CS-3; (b) SEM images of H2-CS-3; (c) N2 adsorption–desorption isotherms of H2-CS-3.
Figure 10. (a) XRD patterns of CS-3 and H2-CS-3; (b) SEM images of H2-CS-3; (c) N2 adsorption–desorption isotherms of H2-CS-3.
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Catalysts 14 00070 g011
Figure 11. (a) Evaluation of CS-3 and H2-CS-3 for 1-octene epoxidation; (b) XPS O1s spectra of CS-3 and H2-CS-3; (c) CO2-TPD profiles of CS-3 and H2-CS-3.
Figure 11. (a) Evaluation of CS-3 and H2-CS-3 for 1-octene epoxidation; (b) XPS O1s spectra of CS-3 and H2-CS-3; (c) CO2-TPD profiles of CS-3 and H2-CS-3.
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Catalysts 14 00070 g012
Figure 12. (a) The yield of 1, 2-octane oxide when adding the free radical quenchers BHT (·OH quencher), BQ (O2−·quencher), and Fe2+ (·OH producer); (b) EPR simulation of the solution containing acetonitrile or benzonitrile, H2O2, CS-3, methanol, and water. The black curve represents the measured signal. The red curve represents the simulation signal where specific ROO· free radicals could be distinguished easily.
Figure 12. (a) The yield of 1, 2-octane oxide when adding the free radical quenchers BHT (·OH quencher), BQ (O2−·quencher), and Fe2+ (·OH producer); (b) EPR simulation of the solution containing acetonitrile or benzonitrile, H2O2, CS-3, methanol, and water. The black curve represents the measured signal. The red curve represents the simulation signal where specific ROO· free radicals could be distinguished easily.
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Catalysts 14 00070 sch001
Scheme 1. Proposed mechanism of 1-octene epoxidation.
Scheme 1. Proposed mechanism of 1-octene epoxidation.
Catalysts 14 00070 sch001
Table 1. Catalytic performance for various catalysts in the epoxidation of 1-octene.
Table 1. Catalytic performance for various catalysts in the epoxidation of 1-octene.
CatalystSolventConversion (%)Selectivity (%)Yield (%)
CS-1Methanol65.198.764.2
CS-2Methanol74.398.873.4
CS-3Methanol82.598.981.6
CS-4Methanol83.698.382.2
SS-3Methanol45.897.644.7
BS-3Methanol36.197.235.1
CaOMethanol17.285.414.7
SnO2Methanol7.499.37.3
Reaction conditions: 1-octene (3.9 mmol), benzonitrile (10.5 mmol), catalyst (0.2 g), methanol (10 mL), 30% H2O2 (2 mL), 60 °C, 24 h. GC analysis with dodecane as an internal standard. Conversion corresponds to the conversion rate of 1-octene; Selectivity refers to the selectivity of epoxides, determined by GC analysis. Yield = Conversion rate × Selectivity.
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Zhang, M.; Xiang, H.; Wen, X. Enhancing Catalytic Efficiency in Long-Chain Linear α-Olefin Epoxidation: A Study of CaSnO3-Based Catalysts. Catalysts 2024, 14, 70. https://doi.org/10.3390/catal14010070

AMA Style

Zhang M, Xiang H, Wen X. Enhancing Catalytic Efficiency in Long-Chain Linear α-Olefin Epoxidation: A Study of CaSnO3-Based Catalysts. Catalysts. 2024; 14(1):70. https://doi.org/10.3390/catal14010070

Chicago/Turabian Style

Zhang, Min, Hongwei Xiang, and Xiaodong Wen. 2024. "Enhancing Catalytic Efficiency in Long-Chain Linear α-Olefin Epoxidation: A Study of CaSnO3-Based Catalysts" Catalysts 14, no. 1: 70. https://doi.org/10.3390/catal14010070

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