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Article

Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation

by
Xin-Yu Chang
1,2,
Yu-Ting Sun
1,
Xiao-Jing Song
1,3,
Xiao-Tong Yang
1,
Yu-Qing Wu
2 and
Ming-Jun Jia
1,*
1
Key Laboratory of Surface and Interface Chemistry of Jilin Province, College of Chemistry, Jilin University, Changchun 130012, China
2
State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
3
State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1241; https://doi.org/10.3390/catal12101241
Submission received: 8 September 2022 / Revised: 9 October 2022 / Accepted: 11 October 2022 / Published: 15 October 2022
(This article belongs to the Special Issue Heterogeneous Selective and Total Catalytic Oxidation)
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Abstract

Developing efficient heterogeneous catalysts for cyclic olefins epoxidation is highly attractive for meeting the growing need for various cyclic epoxides. Herein, hierarchical TS-1 zeolite with relatively abundant mesopores and less amount of surface hydroxyl groups was obtained by hydrothermal modification of an as-synthesized TS-1 zeolite with a mixed solution of ammonia, tetrapropylammonium bromide (TPABr) and KCl. The post-modified TS-1 zeolite exhibited much higher catalytic activity (52% conversion) and epoxide selectivity (98%) for the epoxidation of cyclopentene than the conventional TS-1 zeolites. The excellent catalytic activity of the hierarchical TS-1 could be mainly assigned to the enhancement of the mass transport ability and the accessibility of the active Ti species, while the improvement of epoxidation selectivity may be mainly related to the introduction of a certain amount of K+ that can effectively modulate the coordination environment of Ti species as well as the polarity of the zeolite. This work demonstrated that a highly active and selective catalyst for the H2O2-mediated cyclopentene epoxidation could be obtained by concurrently generating mesopore and extinguishing the unfavorable defective hydroxyl groups through the simple hydrothermal treatment of the conventional TS-1 zeolite with a mixed base/salt solution.

Graphical Abstract

1. Introduction

Recently, the effective utilization of cyclopentene (CPE), an easily available ingredient from a large amount of C5 fraction in petroleum cracking, to produce valuable chemical compounds has drawn considerable attention from economic and environmental perspectives [1]. Selective catalytic oxidation is an attractive way to convert CPE to corresponding oxygen-containing compounds like glutaraldehyde, 1,2-cyclopentene diol, cyclopentene oxide (CPO), and cyclopentanone [2,3,4,5,6]. Among them, the epoxidation product of CPO is an important intermediary, which has been widely used for the production of plasticizers, medicine, fragrances, and pesticides [7,8]. In addition, CPO could also be used for selectively coupling with CO2 to yield cyclic carbonate, thus showing great potential for CO2 capture and utilization [9,10].
Catalytic epoxidation of CPE with various oxidants such as oxygen, organic peroxides, or hydrogen peroxide (H2O2) is an ideal way to produce CPO [11,12,13]. However, choosing molecular oxygen as an oxidant usually requires the addition of a sacrificial agent (like isobutyraldehyde) [14,15] or another radical initiator (like TBHP) [16,17] to obtain high yields, and the reaction system is complex and difficult to control. Organic peroxides are also not satisfactory oxidants due to their high price and generation of organic wastes, thus bringing serious limitations to large-scale applications [18]. In terms of economic and environmental concerns, H2O2 could be regarded as a more appropriate oxidant for cyclopentene epoxidation in case of a highly efficient catalyst better in heterogeneous nature is developed.
Early attempts revealed that the commonly used olefin epoxidation catalysts, including W-based catalysts (e.g., supported tungsten oxides, phosphotungstic acid or quaternary ammonium salt) and Ti-based zeolites (e.g., TS-1 and Ti-Beta), usually lead to the production of glutaraldehyde and other oxidized products [19,20,21]. It is quite difficult to achieve high epoxide selectivity for H2O2-mediated cyclopentene epoxidation due mainly to the very high reactivity of both the cyclic olefin and the generated CPO [22,23]. Recently, by precisely adjusting the composition, the structure and surface properties of the catalysts, a few relatively selective catalysts for the epoxidation of CPE with H2O2, have been obtained. For instance, by introducing peroxo-W species to the mesochannels of an organic groups-modified SBA-15, Niu et al. obtained a highly active and selective hybrid catalyst, giving 42% CPE conversion and 99% CPO selectivity under the optimized reaction conditions [24]. Tong et al. reported that Mg0.5Co0.4Fe2O4 prepared by the sol-gel auto-combustion route could convert CPE to CPO with a 95.2% conversion and 96.3% selectivity at 60 °C [25]. Although these catalysts could be recycled several times with a slight decrease in their catalytic activities, the structural stability of the hybrid catalysts and composite metal oxides were still suspect owing to the strong complexing ability of H2O2 that commonly results in the leaching of active metal species during the reaction term [26,27].
As one of the most attractive epoxidation catalysts, titanium silicalite-1 (TS-1) zeolite has superior chemical and hydrothermal stability and could efficiently catalyze the epoxidation of various small molecular olefins like propylene with H2O2 [28]. However, when using TS-1 zeolite as a catalyst for epoxidation of cyclopentene, both the catalytic activity and epoxide selectivity are quite poor, owing to the relatively small micropore size of the zeolite as well as the easy ring-opening of epoxy products on the surface of the catalyst [20,21]. Significantly, some of the literature demonstrated that the catalytic performance of TS-1 zeolites might be considerably enhanced by changing the coordination state of Ti species, the porosity (e.g., constructing mesopores), the morphology, as well as the hydrophobicity of the zeolites [29,30,31,32]. One of the simplest and most effective strategies is to modify the as-synthesized TS-1 zeolite by post-treatment method, including silanization reaction [33], acid treatment [34], alkali treatment [35] and salt treatment [36,37]. For instance, Wang et al. obtained hollow-structured TS-1 crystals with enhanced catalytic performance for phenol hydroxylation by hydrothermally treating TS-1 zeolite with a TPAOH solution [38]. Tatsumi et al. treated TS-1 zeolite with H2SO4 followed by an aqueous K2CO3 solution, and the resultant K-modified TS-1 exhibited improved catalytic activity for the oxidation of 2-penten-1-ol [39]. Du et al. reported that hierarchical TS-1 zeolites derived from the fluoride-containing chemical etching post-treated route showed remarkably enhanced catalytic activity in oxidative desulfurization reaction [40]. Based on this progress, it can be expected that a more efficient cyclopentene epoxidation catalyst might also be obtained by effectively modulation of the porosity and surface chemistry of TS-1 zeolite via a controllable post-modification strategy.
To obtain more efficient olefin epoxidation catalysts, co-workers and we conducted work on synthesizing Ti-, Mo- or W-containing zeolites with specific morphology, porosity and desirable coordination states of metal species [28,41,42,43,44]. Herein, we performed a hydrothermal modification on an as-synthesized TS-1 zeolite by using a mixed solution of ammonia, tetrapropylammonium bromide (TPABr) and KCl for the purpose of obtaining a highly active and selective TS-1 catalyst for the epoxidation of cyclopentene. The resulting TS-1 zeolites were characterized by a variety of means, and the relationship between their physicochemical properties and catalytic performance was discussed, demonstrating that the formation of hierarchical pores and Si-O-K structure in the modified TS-1 zeolite plays a key role in constructing efficient catalysts for the H2O2-mediated epoxidation of cyclopentene.

2. Results

2.1. Synthesis and Characterization

The XRD patterns of the parent TS-1 and the modified TS-1 zeolites exhibit the characteristic peaks of MFI structure at 2θ of 7.8°, 8.8°, 23.0°, 23.9°, and 24.4°, confirming the phase purity of the samples (Figure 1a and Figure S1) [45]. Post-modification of the TS-1 zeolite did not have an obvious influence on the framework structure of MFI, implying that the post-modification conditions were relatively mild, thus not leading to serious damage in the crystal structure of the TS-1 zeolites. Compared with the parent TS-1 zeolite, the relative crystallinity (RC) of the modified samples decreased somewhat as a result of the hydrothermal treatment (Table 1). It was previously proposed that partial dissolution of the zeolite framework may proceed in basic media (like ammonia); however, the topology of the zeolite may still be well protected due to the recrystallization of the dissolved species in the presence of the structure-directing agent (i.e., TPA+) [38]. This should be the main reason why the framework topology of the modified TS-1 zeolites remained well after the hydrothermal treatment in the mixed basic media.
The FTIR spectra of various TS-1 zeolites are shown in Figure 1b and Figure S2. The typical bands of 550 and 800 cm−1 that appeared in the spectra of all TS-1 samples could be attributed to the stretching vibration of the double ring and [SiO4] unit [46]. In addition, a characteristic band at 960 cm−1 was observed in all the FT-IR spectra, which was related to the vibration of the Si-O-Ti or Si-O bond perturbed by the Ti atom in the framework of the zeolites [47]. These results further confirmed that the main structural features of the modified TS-1 zeolite were maintained well after the hydrothermal treatment.
Figure 2 shows the SEM and TEM images of TS-1 and the modified TS-1 zeolites. The parent TS-1 zeolite exhibited a flat hexagonal prism with a long edge on a single crystal plane. The crystals of TS-1 were uniform with an average grain size of about 2.1 μm × 0.8 μm × 0.5 μm. The basic morphologies of the modified TS-1 zeolites kept well, which could be assigned to the relatively mild hydrothermal treatment conditions. The appearance of some small pieces of crystals with irregular morphologies might be mainly caused by the physical damage during sample manipulation for microscopy analysis or during the mechanically stirring process for post-treatment. The introduction of K+ cations could be revealed based on the EDS-Mapping data of TS-1_P-0.05K (Figure 2g and Figure S3). Besides, the chemically etching effect could also be revealed from the TEM image of TS-1_P-0.05K (Figure 2h), clearly showing the presence of some mesopores and marcropores in the zeolite crystals, which were created during the hydrothermal treatment process via a dissolution-recrystallization reaction [38]. Depending on the components of the selected mixed base and salt solution and post-treatment conditions, such etching and growth processes might modulate the porous structure and coordination environment of Ti species to a certain extent, which could be revealed more clearly by the following characterization results.
Table 1 lists the Si/Ti and Si/K ratios of various TS-1 zeolites. The Si/Ti ratios of all the modified TS-1 zeolites slightly decreased in comparison with the parent TS-1 zeolite, suggesting that relatively more Si species eroded during the hydrothermal treatment process. For the samples modified with KCl-containing solutions, a small amount of K+ could be detected by ICP measurements, while relatively high K+ contents are present in the samples of TS-1_P-0.05K and TS-1_P-0.10K. The introduced K+ cations mainly existed in the form of Si-O-K and also interacted with some non-framework Ti-OH groups to generate Ti-O-K [48].
Figure 3 and Figure S4 show the nitrogen adsorption and desorption isotherms of various TS-1 zeolites. All the samples have strong adsorption in the low relative pressure range, indicating that there are a number of micropores in the zeolites. The parent TS-1 zeolite exhibited a type-I isotherm, consistent with the conventional TS-1 reported in the literature [28,49]. All the modified TS-1 zeolites, especially the samples of TS-1_P, TS-1_P-0.01K and TS-1_P-0.05K, showed a pronounced second uptake in the partial pressure range of 0.4 < P/P0 < 0.9, indicating the presence of mesopores and marcropores [50,51]. The pore size distribution diagrams (Figure S5) confirmed the existence of hierarchical pores in the modified TS-1 samples. Table 1 lists the textural parameters of the parent TS-1 and the modified samples derived from the N2 adsorption and desorption isotherms. Compared with the parent TS-1 zeolite, the modified samples possessed a lower micropore volume and higher mesorpore volume. This should be mainly attributed to the etching role of the mixed basic solution and the guided growth ability of TPA+, which created secondary porosity through a dissolution-recrystallization process [52]. Among the modified samples, TS-1_P-0.05K had the highest adsorbed volume of N2, corresponding to the largest pore volume (Vmicro + Vmeso), which was an indication that the microporous structure of the zeolite was well kept along with the formation of mesopores and macropores. These results confirmed the positive role of the mixed components (ammonia, TPABr and KCl) in fabricating abundant mesopores, meanwhile keeping well with the basic microporous structure of the TS-1 zeolites during the hydrothermal treatment process.
Figure 4a shows the UV-vis DRS spectra of various TS-1 samples. All the samples showed a strong absorption peak at 210 nm, which was the characteristic signal of the isolated tetrahedral framework Ti species (TiO4) caused by the charge transfer of the p-p transition between O2− and Ti4+ [53]. The broad absorption band at around 330 nm was also observed for all the TS-1 samples, indicating the existence of anatase TiO2 [54]. The obvious absorption band at around 250–290 nm appeared in the spectra of various TS-1 samples and could be assigned to the hexacoordinated Ti species either in the isolated or oligomeric form [55]. Notably, TS-1_P and TS-1_P-0.05K exhibited an enhanced absorption band at 250–290 nm, suggesting that the more hexaccordingated Ti species appeared in these two modified samples
Ultraviolet resonance Raman spectroscopy with an excitation line at 325 nm was used to further study the change of Ti species caused by the post-modification treatment (Figure 4b). According to the related literature [56,57], the peaks at 390 cm−1 and 817 cm−1 were attributed to the stretching vibration of the siloxane bond (Si-O-Si) of pure silicon S-1 zeolite. The absorption peaks at 960 cm−1 and 1125 cm−1 were related to the framework TiO4 species. The weak absorption peaks located at 518 cm−1 and 635 cm−1 were correlated to the presence of a small amount of anatase TiO2 in all the TS-1 samples. The obvious absorption peak at 700 cm−1 could be assigned to the characteristic signal of a mononuclear hexacoordinated Ti species (TiO6), which was partially coordinated with hydroxyl groups and H2O, shown as Ti(OSi)2(OH)2(H2O)2 [28,58]. For the parent TS-1 sample, besides the framework TiO4 species, both the isolated TiO6 and the anatase phase were also present. After treatment with the solution of ammonia and TPABr, the resultant sample of TS-1_P showed an enhanced signal of TiO6 (700 cm−1), accompanied by a decrease in the TiO4 signal (960 and 1125 cm−1) in intensity, implying that a small portion of framework TiO4 might be transferred to isolated TiO6 species during the hydrothermal treatment. As for the sample of TS-1_0.05K, which was treated with the solution of ammonia and KCl, an obvious decrease in the peak intensity of TiO4 and TiO6 species was detected. The loss of these framework Ti species could mainly be assigned to the etching effect of the basic ammonia–KCl solution, which dissolved part of the framework Si/Ti species to generate some defect sites like amorphous Si-OH and Ti-OH groups. Due to the presence of KCl, a certain amount of the defective hydroxyl groups were transformed into Si-O-K and Ti-O-K groups [37,48]. For the sample of TS-1_P-0.05K, relatively strong framework Si (390 and 817 cm−1) and Ti signals (960 and 1125 cm−1) were detected, suggesting that the MFI topology structure of the TS-1 zeolite crystals kept well after the hydrothermal treatment with the mixed solution of ammonia/TPABr/KCl. As for the sample of TS-1_P-0.10K (Figure S6), a few weak signals appeared at 482, 520, and 1110 cm−1, which might be assigned to the appearance of a small amount of framework Ti species distributed in the amorphous pore wall of mesoporous titanium silicalite, similar to that of the mesoporous Ti-MCM-1 [59]. These results further confirmed that the recrystallization occurred during the formation process of hierarchical TS-1 zeolites in the presence of TPA+. Besides, the decrease in the peak intensity of the TiO6 species in the three samples post-modified by KCl-containing solution may be caused by the interaction between the hydroxyl groups in the TiO6 species (i.e., Ti(OSi)2(OH)2(H2O)2) and KCl, leading to the formation of a certain amount of Ti-O-K species [60].
Figure 5 and Figure S7 show the FTIR spectra of various TS-1 samples in the region of hydroxyl absorption. For the parent TS-1 zeolite, the strong absorption band in the wide range of 3800–3000 cm−1 could be mainly attributed to an array of hydrogen-bonded internal and external silanol groups [61,62,63]. These hydroxyl sites have a very strong bonding ability and could be clearly detected even after thermal treatment in a vacuum [64]. For the sample of TS-1_P, a relatively strong absorption signal in the region of 3800–3600 cm−1 was still observed, indicating the existence of a large amount of undisturbed hydroxyl absorption bands, derived from the isolated silanol groups either existed on the surface (3745 cm−1) or inside the pores (3686 cm−1) of the zeolite [65]. Notably, the signal intensity of the silanol groups considerably decreased after the post-treatment with the KCl-containing solutions, further confirming that a large amount of the silanol groups, including a small amount of the defective Ti-OH groups, should have been eliminated through the interaction with K+ to form Si-O-K or Ti-O-K during the post-hydrothermal treatment process [66]. For TS-1_0.05K and TS-1_P-0.05K, the much lower signal intensity in the hydroxyl region should be an indication that the number of the defective OH groups (Si-OH and Ti-OH) in these samples significantly decreased, which may also suggest that both the acidity and the hydrophilicity of the zeolites considerably decreased after the post-modification treatment.

2.2. Catalytic Tests

The catalytic properties of various TS-1 zeolites were investigated for the epoxidation of cyclopentene with H2O2 as oxidant and methanol as solvent, according to an optimized reaction condition reported in previous literature [18,19]. As shown in Table 2, the parent TS-1 zeolite had relatively low catalytic activity under the test conditions, with 9.6% cyclopentene conversion and 82.3% CPO selectivity, quite similar to the conventional TS-1 zeolite reported in the literature (Table S1) [21].
For the sample of TS-1_P, although the conversion of cyclopentene increased to 13.0%, the epoxide selectivity significantly dropped to 39.8%. The very low epoxide selectivity of TS-1_P was mainly related to the presence of a higher concentration of acidic Ti–OH groups (e.g., the hydroxyl groups in TiO6), which catalyzed the ring opening reaction of cyclopentene epoxide to generate more side products [67]. As for the sample of TS-1_0.05K, a relatively high cyclopentene conversion (28.8%) and epoxide selectivity (98.8%) were obtained under the test conditions. The highest activity was achieved over the sample of TS-1_P-0.05K, which showed a 52.0% conversion of cyclopentene and a 98.2% selectivity of cyclopentene oxide. The excellent catalytic performance of this post-modified TS-1 catalyst was mainly related to the existence of a large number of mesopores, which were beneficial for the mass transfer and diffusion of reactant and product molecules. Moreover, the significant increase in epoxide selectivity could be mainly assigned to the introduction of K+ into the zeolites, possibly directly related to the decrease of surface terminal hydroxyl groups, e.g., to interact with the defective Si-OH and Ti-OH to form Si-O-K and Ti-O-K species, thus considerably inhibiting the unfavorable side reactions caused by the acidic hydroxyl groups.
In addition, the effect of reaction time on the conversion of the olefin into epoxide is investigated for TS-1_R-0.05K. As shown in Figure S8, a relatively high reaction rate could be observed in the initial stage. With further increasing the reaction time from 2 h to 12 h, the conversion of cyclopentene increases slowly from 52.0% to 60.2%. The considerable decrease in reaction rate should be an indication of catalyst deactivation, possibly caused by the strong adsorption of epoxides and other byproducts, which can cover the active TiO4 sites or block the pores of the zeolites [68]. A similar reaction trend could also be found when using acetonitrile as solvent (to replace methanol), showing that TS-1_R-0.05K could also effectively catalyze the epoxidation reaction with a lower rate than that of using methanol as solvent, but still much better than the conventional TS-1 zeolite (Figure S9). To restore the catalytic activity of the catalyst, high-temperature calcination of the spent TS-1 catalyst was required. Figure 6 shows the recyclability of the regenerated TS-1_R-0.05K catalyst in the cyclopentene epoxidation reaction. During the recycling process, the conversion of cyclopentene remains basically unchanged, corresponding to five consecutive data of 52.0%, 50.1%, 51.0%, 49.4%, and 50.2%. Meanwhile, the selectivity of CPO was maintained at about 98%. These results suggest that the TS-1_P-0.05 K catalyst could be recycled at least five times without an obvious loss in catalytic performance, implying that the hierarchical pore structure and the catalytically active Ti species were very stable during the catalytic test and high-temperature regeneration processes.

3. Discussion

The previous literature revealed that conventional TS-1 zeolite was not an ideal catalyst for the epoxidation of cyclopentene with H2O2, mainly due to the mass-transfer limitation by the microporous channels [21]. Although constructing mesopores and macropores to generate hierarchical TS-1 zeolite by simple alkali treatment could overcome the drawback caused by diffusion limitation, the generation of a higher concentration of surface hydroxyl groups associated with the secondary porosity may lead to a considerable decrease in the epoxide selectivity, especially for the olefins with higher reactivity [21,22]. Our current work demonstrated that by using a combined post-modification strategy, i.e., hydrothermally treating TS-1 with the mixed solution of ammonia, TPABr and KCl, highly active and selective catalysts for cyclopentene epoxidation could be obtained.
By combining the characterization results of various TS-1 zeolites with their catalytic data, it can be proposed here that the catalytic epoxidation properties of the TS-1 zeolites could be considerably adjusted by changing the component and concentration of the post-treated mixed solution. The basic environment derived from ammonia may provide a suitable basicity condition for achieving an etching effect on the zeolites, while the co-existence with TPABr could induce the formation of a large number of mesopores and macropores through a dissolution–recrystallization process, thus effectively improving the mass transfer ability of the zeolites, and leading to the improvement of catalytic activity of the post-modified TS-1 zeolites.
Moreover, the critical role of KCl in enhancing the epoxidation selectivity should be acknowledged by comparing the three modified TS-1 catalysts with different KCl concentrations. From the FTIR spectra in the hydroxyl region, it was known that TS-1_P-0.01K possesses more defective OH groups than the catalysts of TS-1_P-0.05K and TS-1_P-0.10K, suggesting that TS-1_P-0.01K has more acidic sites related to the defective Ti-OH groups and possesses higher hydrophilicity (derived from Si-OH groups), which are all unfavorable factors for the epoxidation of cyclopentene. Therefore, TS-1_P-0.01K exhibited the lowest catalytic activity and epoxide selectivity among the three catalysts. As for TS-1_P-0.05K and TS-1_P-0.10K, both catalysts had quite similar textural parameters and surface properties, and the main difference is that TS-1_P-0.10K had a little bit more framework Ti species distributed in the amorphous pore wall of the zeolites, as revealed by the Raman results, this might be the main reason why it showed a relatively lower catalytic efficiency than TS-1_P-0.05K.
Based on the above results and related literature [39,60,69,70], it can be concluded here that the excellent catalytic performance of TS-1_P-0.05K could be mainly assigned to the synchronous modulation of the porosity and surface chemistry of the zeolite that is to create a larger number of mesopores and macropores through hydrothermal treatment with an alkali solution and eliminate the unfavorable acidic hydroxyl groups through the salt treatment with KCl. These results demonstrated that choosing an appropriate mixed solution is critical for precisely modifying the porosity, surface chemistry and the coordination environment of Ti species of the TS-1 zeolites, which can finally lead to the generation of highly active and selective epoxidation catalysts for a given olefin, like cyclopentene.

4. Materials and Methods

4.1. Materials

Tetraethyl orthosilicate (TEOS) (Sinopharm, Beijing, China), H2O2 (30 wt%, Sinopharm, Beijing, China), Tetrapropylammonium hydroxide (TPAOH) solution (25 wt% in water, Sinopharm, Beijing, China), Tetrabutyl titanate (TBOT) (98%, Guangfu Fine Chemical Research Institute, Tianjin, China), Potassium chloride (Fuchen Chemical Reagent Co., Ltd., Tianjin, China), methanol (Fuchen Chemical Reagent Co., Ltd., Tianjin, China), ethanol absolute (Jindong Tianzheng Fine Chemical Reagent Factory, Tianjin, China), and cyclopentene (98%, Shanghai Macklin Biochemical Co., Ltd., Shanghai China).

4.2. Synthesis

4.2.1. Synthesis of the Parent TS-1 Zeolite

The parent TS-1 zeolite was synthesized according to the literature-reported procedure based on a seed-promoted synthesis with low-cost TPABr as a template [47]. The seed of the silicalite-1 (S-1) zeolite was obtained by conventional hydrothermal method from a starting gel with a molar composition of 1.0 SiO2: 0.256 tetrapropylammonium hydroxide (TPAOH): 30 H2O at 170 °C for 48 h. The as-synthesized S-1 zeolite crystals were separated by high-speed centrifugation, washed with deionized water and ethanol, dried at 80 °C overnight, and then directly used as seeds (without calcination) for the following synthesis of TS-1 zeolite.
The TS-1 zeolite was hydrothermally synthesized from a sol-gel mixture with a molar composition of 1.0 SiO2: 0.10 TPABr: 0.02 TiO2: 0.5 ethanolamine: 0.5 isopropanol: 30 H2O in the presence of a certain number of S-1 seeds (seed/SiO2 = 4.0 wt%). First, ethanolamine and TPABr were separately dissolved in deionized water under stirring. After that, silica sol was added dropwise to the mixed solution under stirring, and then the pre-prepared TBOT-isopropanol solution was added dropwise under vigorous continuous stirring for 1 h. S-1 seed crystals were then added to the above mixture, and the resulting sol was stirred and aged for 12 h at room temperature. The sol was transferred into an autoclave and statically crystallized in an oven at 170 °C for 48 h. The resultant TS-1 zeolite crystals were washed thoroughly with water and ethanol, and dried at 80 °C overnight, followed by calcination in air at 550 °C for 8 h.

4.2.2. Hydrothermal Modification

The parent TS-1 zeolite was hydrothermally post-modified by a mixed solution of ammonium hydroxide (1.5 mol/L), TPABr (0.05 mol/L) and/or KCl (0–0.10 mol/L). Typically, 1.0 g of TS-1 was dispersed into 50 mL mixed solution, and the resulting slurry was transferred to a Teflon-lined autoclave and hydrothermally treated at 170 °C for 24 h. After washing and drying at 80 °C overnight, the post-modified zeolites were calcined in a muffle furnace at 550 °C for 6 h. The resulting materials were referred to as TS-1_P-xK, in which P and K represented TPABr and KCl, respectively, while x represented the molar concentration of KCl. In all cases, the mass loss of the post-modified TS-1 zeolite is less than 8 wt% of the original samples after the hydrothermal treatment.

4.3. Characterizations

The crystallinity and phase purity of the samples were analyzed by power X-ray diffraction (XRD) analysis on a Rigaku D-Max 2550 (Tokyo, Japan) diffractometer using Cu Kα radiation (λ = 1.5418). The crystal size and morphology were determined by scanning electron microscopy (SEM) using a JSM-6510 electron microscope (Tokyo, Japan). Fourier-transformed infrared spectra (FTIR) were recorded by NicoletTM6700 (Waltham, MA, USA) using the KBr pellet technique. For the FTIR spectra of hydroxyl groups measurement, it was recorded as follows: a self-supported wafer (18 mg and 2 cm in diameter) was set in a quartz IR cell sealed with CaF2 windows connected with a vacuum system. The sample was evacuated at 673 K for 2 h before the measurement. Ultraviolet, visible diffuse reflectance spectra (UV-Vis DRS) of the catalysts were recorded in the range of 200 nm to 500 nm against the support as a reference on a SHIMADZU U-4100 (Tokyo, Japan). Nitrogen adsorption and desorption measurements were carried out on a Micromeritics ASAP 2010N (Atlanta, USA) analyzer at 77.3 K after degassing the samples at 200 °C under vacuum. Chemical compositions were determined with inductively coupled plasma (ICP) analyses carried out on a Perkin-Elmer Optima 3300 DV ICP (Waltham, MA, USA) instrument. Ultraviolet Raman resonance spectra (UV-Raman) with an excitation line at 325 nm were recorded on a LabRAM Odyssey of HORIBA Scientific (Paris, France).

4.4. Catalytic Activity

The catalytic epoxidation reaction was conducted in a 25 mL round-bottom flask equipped with a reflux condenser, and the whole device was immersed in a thermostatic oil bath at a previously designed temperature under vigorous stirring. In a typical run, 10 mL of methanol, 10 mmol of cyclopentene, 10 mmol of H2O2 (30 wt%), and 50 mg of the catalyst were mixed in the flask, and the reaction was run under magnetic stirring at 333 K. The quantitative analyses of the reagents and products were conducted by GC-3420A gas chromatograph with SE-54 capillary column. The conversion of cyclopentene and the selectivity of epoxides were accordingly calculated. In the recycling tests of the cyclopentene epoxidation reaction, the used catalyst was regenerated by calcination at 550 °C for 2 h.
The transformation frequency (TOF) (molar amount of synthesized cyclopentene oxide per molar basicity per hour) of each catalyst was calculated as Equation (1).
TOF = n epoxide n catal .   ×   t = n alkene   ×   con . % alkene   ×   sel . % epoxide n catal .   ×   t
where nepoxide is the molar amount (mmol) of cyclopentene oxide, nalkene is the molar amount (mmol) of cyclopentene, t is the reaction time (h), and ncatal. is the molar amount (mmol) of Ti sites in the catalyst determined by ICP analysis.

5. Conclusions

In summary, by using a mixed solution containing ammonia, TPABr, and KCl as modification reagents, a highly efficient cyclopentene epoxidation catalyst could be obtained via the hydrothermal treatment of an as-synthesized TS-1 zeolite. The condition-optimized TS-1 zeolite possessed a large number of mesopores and macropores and a smaller number of surface hydroxyl groups and showed enhanced catalytic activity and selectivity for the epoxidation of cyclopentene with H2O2 as an oxidant. This work demonstrated that through a reasonable combination of the modification reagents and hydrothermal treatment conditions, a highly efficient post-modified TS-1 catalyst for cyclopentene epoxidation could be obtained, which will certainly have great potential for achieving industrial application in producing valuable cyclopentene oxide.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12101241/s1. Table S1: Comparison catalytic performance of the TS-1 catalyst with literature-reported catalysts for epoxidation of cyclopentene; Figure S1: XRD spectra of the TS-1 zeolites and after hydrothermal treatment; Figure S2: FTIR spectra of various TS-1 samples; Figure S3: SEM images and elemental mapping of TS-1_P-0.05K; Figure S4: N2 adsorption and desorption isotherms of the TS-1 samples before and after hydrothermal treatment; Figure S5: Pore size distributions of various TS-1 zeolites; Figure S6: UV-Raman spectra excited at 325 nm of various TS-1 samples; Figure S7: FTIR spectra of hydroxyl groups of the TS-1 samples before and after hydrothermal treatment; Figure S8: Cyclopentene conversion and cyclopentene oxide selectivity over TS-1 and TS-1_P-0.05K in methanol; Figure S9: Cyclopentene conversion and cyclopentene oxide selectivity over TS-1 and TS-1_P-0.05K in acetonitrile.

Author Contributions

Conceptualization, X.-Y.C. and M.-J.J.; methodology, X.-Y.C. and M.-J.J.; validation, X.-Y.C., Y.-T.S. and X.-T.Y.; formal analysis, X.-Y.C., X.-J.S. and X.-T.Y.; investigation, X.-Y.C., X.-J.S. and X.-T.Y.; resources, M.-J.J.; data curation, X.-Y.C.; writing—original draft preparation, X.-Y.C. and M.-J.J.; writing—review and editing, X.-Y.C. and M.-J.J.; visualization, X.-Y.C.; supervision, M.-J.J.; project administration, M.-J.J. and Y.-Q.W.; funding acquisition, M.-J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Nos. 22172058, 21173100).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01241 g001
Figure 1. The XRD patterns (a) and FTIR spectra (b) of the parent TS-1 and post-modified TS-1 zeolites of TS-1_P, TS-1_0.05K, and TS-1_P-0.05K.
Figure 1. The XRD patterns (a) and FTIR spectra (b) of the parent TS-1 and post-modified TS-1 zeolites of TS-1_P, TS-1_0.05K, and TS-1_P-0.05K.
Catalysts 12 01241 g001
Catalysts 12 01241 g002
Figure 2. The SEM images of (a) TS-1, (b) TS-1_P, (c) TS-1_0.05K, (d) TS-1_P-0.01K, (e) TS-1_P-0.05K, (f) TS-1_P-0.10K, the EDS data of (g) TS-1_0.05K, and the TEM image of (h) TS-1_0.05K.
Figure 2. The SEM images of (a) TS-1, (b) TS-1_P, (c) TS-1_0.05K, (d) TS-1_P-0.01K, (e) TS-1_P-0.05K, (f) TS-1_P-0.10K, the EDS data of (g) TS-1_0.05K, and the TEM image of (h) TS-1_0.05K.
Catalysts 12 01241 g002
Catalysts 12 01241 g003
Figure 3. N2 adsorption/desorption isotherms of various TS-1 zeolites.
Figure 3. N2 adsorption/desorption isotherms of various TS-1 zeolites.
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Catalysts 12 01241 g004
Figure 4. UV-vis spectra (a) and UV-Raman spectra excited at 325 nm (b) of various TS-1 samples.
Figure 4. UV-vis spectra (a) and UV-Raman spectra excited at 325 nm (b) of various TS-1 samples.
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Catalysts 12 01241 g005
Figure 5. FTIR spectra of hydroxyl groups of various TS-1 samples.
Figure 5. FTIR spectra of hydroxyl groups of various TS-1 samples.
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Catalysts 12 01241 g006
Figure 6. Recycling experiments of cyclopentene epoxidation catalyzed by TS-1_P-0.05K with H2O2 as oxidant. Reaction conditions: catalyst 50 mg, cyclopentene 10 mmol, H2O2 10 mmol, CH3OH 10 mL, and temperature 313 K.
Figure 6. Recycling experiments of cyclopentene epoxidation catalyzed by TS-1_P-0.05K with H2O2 as oxidant. Reaction conditions: catalyst 50 mg, cyclopentene 10 mmol, H2O2 10 mmol, CH3OH 10 mL, and temperature 313 K.
Catalysts 12 01241 g006
Table 1. Compositions and textural parameters of various TS-1 zeolites.
Table 1. Compositions and textural parameters of various TS-1 zeolites.
SampleRC a
(%)		Si/Ti bSi/K bSBET c
(m2/g)		Smicro c
(m2/g)		Sext d
(m2/g)		Vmicro d
(cm3/g)		Vmeso e
(cm3/g)
TS-110056.1/3992851140.120.02
TS-1_P4953.6/3201102100.070.20
TS-1_0.05K7454.01773131052080.070.19
TS-1_P-0.01K5846.7312309922170.060.19
TS-1_P-0.05K4654.81244151722430.100.26
TS-1_P-0.10K4948.11354021452570.090.23
a The relative crystallinity (RC) of the zeolite was calculated by comparing the total intensity of the five characteristic XRD peaks, while the RC of the parent TS-1 was considered 100%. b Measured by inductively coupled plasma (ICP). c SBET (total surface area) was calculated using the BET method; d Smicro (micropore area), Sext (external surface area), and Vmicro (micropore volume) were calculated using the t-plot method. e Vmeso (mesopore volume) was calculated using the BJH method from the adsorption branch.
Table 2. Catalytic epoxidation of cyclopentene with H2O2 over various TS-1 zeolites a.
Table 2. Catalytic epoxidation of cyclopentene with H2O2 over various TS-1 zeolites a.
CatalystConv. (%)Sel. (%)TOF b (h−1)
TS-19.682.327
TS-1_P13.039.817
TS-1_0.05K28.898.894
TS-1_P-0.01K16.490.042
TS-1_P-0.05K52.098.2175
TS-1_P-0.10K26.798.978
a Reaction conditions: catalyst 0.05 g, H2O2 10 mmol, CH3OH 10 mL, cyclopentene 10 mmol, reaction temperature 313 K, and reaction time 2 h. b TOF was calculated on the basis of the conversion of cyclopentene per hour divided by the amount of Ti species in the TS-1 zeolite.
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Chang, X.-Y.; Sun, Y.-T.; Song, X.-J.; Yang, X.-T.; Wu, Y.-Q.; Jia, M.-J. Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation. Catalysts 2022, 12, 1241. https://doi.org/10.3390/catal12101241

AMA Style

Chang X-Y, Sun Y-T, Song X-J, Yang X-T, Wu Y-Q, Jia M-J. Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation. Catalysts. 2022; 12(10):1241. https://doi.org/10.3390/catal12101241

Chicago/Turabian Style

Chang, Xin-Yu, Yu-Ting Sun, Xiao-Jing Song, Xiao-Tong Yang, Yu-Qing Wu, and Ming-Jun Jia. 2022. "Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation" Catalysts 12, no. 10: 1241. https://doi.org/10.3390/catal12101241

APA Style

Chang, X.-Y., Sun, Y.-T., Song, X.-J., Yang, X.-T., Wu, Y.-Q., & Jia, M.-J. (2022). Hydrothermal Modification of TS-1 Zeolites with Organic Amines and Salts to Construct Highly Selective Catalysts for Cyclopentene Epoxidation. Catalysts, 12(10), 1241. https://doi.org/10.3390/catal12101241

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