Download PDF
Research Article  |  Open Access  |  4 Feb 2024

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Views: 255 |  Downloads: 56 |  Cited:   0
Chem Synth 2024;4:14.
10.20517/cs.2023.59 |  © The Author(s) 2024.
Author Information
Article Notes
Cite This Article

Abstract

Titanosilicates are widely applied in the alkene epoxidation reactions with high reaction rate and selectivity to desired products. Their catalytic performance depends on the structure topology, the micro-environment of Ti active sites, and the hydrophobicity/hydrophilicity of zeolite framework. Herein, we focus on a hydrophilic substrate of allyl alcohol (AAL) and investigated catalytic performance of four titanosilicates (TS-1, Ti-MOR, Ti-MWW, and Re-Ti-MWW) in the AAL epoxidation reaction with hydrogen peroxide as the oxidant. Among them, Re-Ti-MWW, synthesized via the post-modification of Ti-MWW with piperidine, exhibited the highest activity. Moreover, the preferred solvent changed from MeCN for Ti-MWW to H2O for Re-Ti-MWW. The relative diffusion rate of AAL over Re-Ti-MWW was up to 466 × 10-7 s-1, much larger than those of other zeolites. The higher diffusion rate of Re-Ti-MWW was probably derived from the higher framework hydrophilicity as revealed by the smaller water contact angle of Re-Ti-MWW compared to the other zeolites, which contributed to the high activity in AAL epoxidation. In the continuous slurry reactor, Re-Ti-MWW achieved a high catalytic lifetime of 163 h, with the selectivity of desirable glycidol product maintained at > 97% in the H2O solvent system, showing high potential as an industrial catalyst for the AAL epoxidation reaction.

Keywords

Titanosilicates, Ti-MWW zeolite, allyl alcohol epoxidation, hydrophilic, diffusion rate

INTRODUCTION

Glycidol (GLY) is an important oxygenated derivative of allyl alcohol (AAL), which is mainly used as a stabilizer in natural oils, emulsion breakers, and dye delamination agents. Generally, GLY is produced by the chloropropane method or the epoxidation of AAL. However, the chloropropane method leads to the generation of chlorine waste, causing serious environmental problems, while the epoxidation method using V2O5, Cr2O3, and WO3 as catalysts has problems in regeneration and reusability[1-5]. Therefore, developing highly active and reusable catalyst to construct a green process for GLY production is desirable.

Zeolite, as a kind of representative solid acid catalyst, is widely used in the petrochemical industry[6]. The framework Si or Al can be partially replaced by the transition metals, such as Ti and Sn, to fabricate heteroatom-containing zeolites, which exhibit high activity in selective oxidation reactions[7-10]. TS-1 zeolite, with Ti isomorphously incorporated into MFI structure, is the first-generation titanosilicate and has been used in several industrial reactions using H2O2 as the oxidant, such as cyclohexanone ammoximation and propene epoxidation reactions[11-13]. However, the medium-size 10-membered ring (MR) pores of TS-1 suffer diffusion restrictions when processing large-size substrates[14]. Afterward, Ti-MWW with 12MR large cavities and surface half cups was developed. Ti-MWW exhibits higher conversion than TS-1 in the propylene and cyclohexene epoxidation[15]. The catalytic activity of Ti-MWW in olefin epoxidation reactions can be further enhanced by post-modifications, such as fluoride treatment or piperidine (PI)-assisted 3D to 2D structural transformation[16-20].

For the titanosilicate/H2O2 catalytic system, the pore size of titanosilicate affects the mass transfer of guest molecules (both reactants and products), while the framework hydrophobicity affects the diffusion rate and enrichment ability of substrate molecules (including organic substrates and H2O2)[8]. Na et al. converted surface Si-OH groups in layered TS-1 zeolite into Si-F groups through the NH4F treatment, which effectively improved the epoxidation activity in 1-hexene epoxidation reaction because of the enhancement of framework hydrophobicity[20]. Wang et al. increased the hydrophilicity of TS-1 to enrich H2O2 around Ti sites, resulting in higher catalytic activity by improving the H2O2 activation rate[21]. Bregante et al. reported that the large number of Si-OH groups in Ti-Beta near the Ti active centers disrupted the transition state of epoxidation reaction by hydrogen bonding with H2O molecular clusters, generating an entropy increase for the reaction. The highly hydrophilic Ti-Beta gave an epoxidation turnover rate that was 100 times larger than that of hydrophobic Ti-Beta in the 1-hexene epoxidation reaction[22-25].

In addition to the intrinsic Ti activity and the confinement environment from zeolitic structural properties, the solvent effect is also a factor worthy of attention. Solvents can facilitate mass and heat transfer, and more importantly, the protic solvents can participate in the construction of the 5-MR intermediate with Ti-OOH active species. To form Ti-OOH active species, the H2O2 molecules need to undergo hydrogen proton transfer[26,27]. Bordiga et al. proposed that protic solvent molecules (H2O or MeOH) could act as hydrogen proton carriers to facilitate the formation of Ti-OOH by forming hydrogen bonding networks with H2O2 and Ti-OH in the reaction system[28]. Theoretical calculations by Wang et al. revealed that the non-protic solvent MeCN has no effect on the reaction path and kinetics, while the protic solvent MeOH can co-adsorb with H2O2 on Ti active sites through hydrogen bonding to reduce the dissociation energy of H2O2 and promote the hydrogen proton transfer[29]. Wang et al. revealed that the smaller the molecular size of a protic solvent, the less effect of the formed 5MR intermediate on the diffusion of the substrate[30]. Thus, water is the preferred solvent once it matches the reaction system of alkene epoxidation. On the one hand, it is a small-size molecule, imposing fewer diffusion constraints. On the other hand, H2O does not cause environmental pollution as a solvent[31,32].

In previous reports, several titanosilicates were evaluated in the catalytic epoxidation of AAL; among them, Ti-MWW showed high GLY yields[1]. To construct a more efficient catalytic system for AAL epoxidation reaction, a modified Ti-MWW catalyst, obtained by PI-assisted structural rearrangement, was used as a catalyst in the present study, and its epoxidation activity was compared with Ti-MWW, TS-1, and Ti-MOR. Re-Ti-MWW exhibited the highest turnover rate and GLY selectivity among all the four catalysts. After the PI-assisted structure rearrangement, some of the tetrahedrally coordinated Ti species were transformed into hexa-coordinated ones with higher activity by interacting with PI molecules. The 3D to 2D structural transformation increased the amount of surface Si-OH groups, thus increasing framework hydrophilicity and altering the solvent effect for the AAL epoxidation reaction. Despite having the same layer structure as Ti-MWW, the preferred solvent changed from MeCN for Ti-MWW to the greener water for Re-Ti-MWW in the AAL epoxidation reaction. The reaction kinetics of AAL epoxidation over Ti-MWW and Re-Ti-MWW were compared to reveal the intrinsic reason for activity enhancement. Finally, the catalytic performance of Re-Ti-MWW was evaluated in the continuous AAL epoxida 19 H2O, where boric acid was used as the crystallization agent and PI as the structure-directing agent. The Si and Ti sources were silica sol solution (30 wt%) and tetrabutyl orthotitanate (TBOT), respectively. The synthetic gel was heated at 443 K for 7 days under dynamic conditions. The product was collected by filtration, washing with distilled water, and drying at 373 K for 12 h. Then, an acid treatment by 2.0 M HNO3 aqueous solution was performed with a solid-to-liquid weight ratio of 1:50 at 413 K for 12 h to remove extra-framework Ti, followed by calcination at 823 K for 6 h. For comparison, TS-1 zeolites were prepared following a reported protocol[18]. Ti-MOR zeolites were obtained by commercial H-MOR combining dealumination and TiCl4 treatment, according to the previous work[34,35].

EXPERIMENTAL

Synthesis of different titanosilicates

Ti-MWW zeolites were hydrothermally synthesized as described previously[33], with a molar composition of 1.0 SiO2 : 0.03 TiO2 : 1.4 PI : 0.67 B2O3 : 19 H2O, where boric acid was used as the crystallization agent and PI as the structure-directing agent. The Si and Ti sources were silica sol solution (30 wt%) and tetrabutyl orthotitanate (TBOT), respectively. The synthetic gel was heated at 443 K for 7 days under dynamic conditions. The product was collected by filtration, washing with distilled water, and drying at 373 K for 12 h. Then, an acid treatment by 2.0 M HNO3 aqueous solution was performed with a solid-to-liquid weight ratio of 1:50 at 413 K for 12 h to remove extra-framework Ti, followed by calcination at 823 K for 6 h. For comparison, TS-1 zeolites were prepared following a reported protocol[18]. Ti-MOR zeolites were obtained by commercial H-MOR combining dealumination and TiCl4 treatment, according to the previous work[34,35].

PI-assisted post-treatment of Ti-MWW

Re-Ti-MWW zeolites were prepared as described previously[16]. Subsequently, 2.84 g of PI was added to 22.5 g of water. After being stirred for 5 min, 5 g of Ti-MWW zeolite was added. The obtained gel was stirred for 1 h, then transferred to a Teflon vessel and hydrothermally treated at 443 K for 24 h. Finally, the solid product was filtered, washed with deionized water, and dried at 353 K for 12 h. The solid product was denoted as Re-Ti-MWW. It should be noted that the PI molecules were occluded in the layered MWW structure in the following characterization and catalytic reactions.

Materials characterization

The crystalline structures of zeolites were confirmed using an Ultima IV X-ray powder diffractometer (XRD) from Rigaku, with a Ni-filtered CuKα X-ray source (λ = 1.5406 Å), and the voltage and current were 35 kV and 25 mA, respectively. The content of organic species and water in catalysts was determined by a TGA/SDTA 851 thermogravimetric (TG) analyzer from Mettler Toledo. The sample was heated from 303 to 1,073 K in air. The solid-state NMR spectra of catalysts were measured using a VNMRS-400 MB NMR spectrometer manufactured by VARIAN[13]. C MAS NMR spectra were determined using adamantane as a standard at 100.54 MHz and 5 kHz[29]. Si MAS NMR spectra were determined using [(CH3)3SiO]8SiO12 as a standard at 79.43 and 3 kHz. Scanning electron microscope (SEM) images were obtained from a Hitachi S-4800 microscope with a test voltage of 3 kV. The coordination state of titanium species was characterized by a ultraviolet-visible (UV-vis) spectrophotometer from Shimadzu, using BaSO4 powder as the reference sample. H2O and N2 physisorption measurements were tested using BELSORP instrument at 298 and 77 K, respectively. Prior to physisorption, the catalysts without organic species were activated under vacuum at 573 K for 6 h, while those containing organic species were treated under vacuum at 423 K for 12 h. The surficial wettability of the samples was characterized on a water contact angle (WCA) tester (USA Kino Industry Co., Ltd, USA) via the sessile drop method. The sample was first pelletized and then placed on the test platform. The WCA values were recorded at three different points on the sample, with the average value reported. Inductively coupled plasma atomic emission spectrometry (ICP-AES) collected on an IRIS Intrepid II type XSP was used to determine the content of various elements in the catalysts. Before the measurements, all samples were digested with 40 wt% HF solution and diluted with deionized water.

Diffusivity measurement

The diffusivity of AAL over various titanosilicates was determined by gravimetric analysis using a TG analyzer. The catalyst was pre-treated at 423 K under nitrogen for 2 h and then cooled down to 313 K to keep the weight baseline stable. Subsequently, AAL was brought into the TG by nitrogen gas (50 mL·min-1) at 313 K for 2 h. Fick’s second law was used to calculate the effective diffusion coefficient. At the beginning of the adsorption, the formula can be well approximated as follows:

$$ \frac{Q_t}{Q_\infty} =\frac{6}{\sqrt{\pi}} (\frac{D}{r^2})^{1/2}t^{1/2} $$

$$ \frac{Q_t}{Q_\infty} $$: the normalized hydrocarbon uptake; D: the diffusivity; r: effective diffusion length; t: time.

Catalytic performance

Batchwise allyl alcohol epoxidation

The AAL epoxidation was performed in a 35 mL quartz reaction tube. In a typical experiment, 30 mg catalyst, 30 mmol H2O2 (30 wt% aqueous solution), 30 mmol AAL, and 5 mL solvent were added into the reaction tube. The reaction solution was stirred at 313 K for 0.5 h. After reaction, the catalyst was separated by centrifugation. Cyclopentanone was added as the internal standard, and the liquid-phase product was analyzed by gas chromatography (Shimadzu 2014, FID detector) equipped with a Rtx-Wax capillary column. The residual H2O2 was titrated with 0.05 M Ce(SO4)2 aqueous solution.

Glycidol hydrolysis reaction

The GLY hydrolysis reaction was performed in a 35 mL quartz reaction tube. Following that, 10 mmol GLY, 5 mL solvent, 10 mmol H2O2 (30 wt% aqueous solution), and 100 mg catalyst were introduced into the reaction tube and stirred at 313 K for 20-300 min. After reaction, the catalyst was separated by centrifugation, and the remaining reaction solution was analyzed by gas chromatography.

Continuous liquid-phase allyl alcohol epoxidation

The continuous epoxidation reaction of AAL was performed in a slurry bed reactor. The reaction temperature was controlled by a water bath [Supplementary Scheme 1]. The AAL/H2O2/H2O solution was fed via a peristaltic pump to the reactor, where 1 g of catalyst and 35 mL of water were added in advance at 40 °C. The gauze was used as a filter for solid-liquid separation, and the liquid product was analyzed by gas chromatography.

RESULTS AND DISCUSSION

Characterization of different zeolites

The XRD patterns revealed that Ti-MWW, TS-1, and Ti-MOR exhibited high crystallinity without impurity, characteristic of the typical MWW, MFI, and MOR structures, respectively [Figure 1A]. Re-Ti-MWW showed a similar XRD pattern as that of Ti-MWW, except for the differences in the low-angle region. Two characteristic diffraction peaks at 2θ = 3.5° and 6.7° attributed to the (001) and (002) crystal planes of layered MWW structure, respectively, appeared in the XRD pattern of Re-Ti-MWW due to the structural change from 3D Ti-MWW to 2D layered Re-Ti-MWW with the assistance of PI [Scheme 1]. The interlayer Si-O-Si linkages were broken in this process, allowing the PI molecules to be inserted between the MWW layers[15]. The presence of PI molecules in Re-Ti-MWW was further confirmed by the TG analysis and[13] C MAS NMR spectrum [Supplementary Figure 1]. Ti-MWW and Re-Ti-MWW had similar nanosheet morphology with a thickness of ~50 nm [Figure 1B (a) and (b)]. TS-1 showed nanoparticle morphology with a diameter of ~ 100 nm [Figure 1B (c)], while Ti-MOR showed irregular crystals on the micrometer scale [Figure 1B (d)]. All the four titanosilicates showed distinct absorption peaks at 210-220 nm attributed to tetrahedrally coordinated Ti species in UV-vis spectra [Figure 1C]. A shoulder peak at 285 nm was observed in the UV-vis spectrum of Re-Ti-MWW because of the presence of hexa-coordinated Ti species [Figure 1C (b)]. The four titanosilicates all exhibited typical N2 adsorption-desorption isotherms of microporous structures [Figure 1D]. The specific surface area and micropore pore volume of Ti-MWW were 555 m2·g-1 and 0.17 cm3·g-1, respectively. The specific surface area and micropore pore volume of Re-Ti-MWW significantly reduced to 64 m2·g-1 and 0.01 cm3·g-1, respectively [Supplementary Table 1], owing to the partial pore-blocking by the occluded PI molecules. TS-1 and Ti-MOR exhibited similar specific surface areas (440-450 m2·g-1) and micropore pore volumes (0.15-0.16 cm3·g-1).

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 1. (A) XRD patterns; (B) SEM images; (C) UV-vis spectra; and (D) N2 adsorption-desorption isotherms at 77 K of (a) Ti-MWW, (b) Re-Ti-MWW, (c) TS-1, and (d) Ti-MOR. XRD: X-ray powder diffractometer; SEM: scanning electron microscope; UV-vis: ultraviolet-visible.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Scheme 1. PI-assisted structural rearrangement of Ti-MWW to Re-Ti-MWW. PI: Piperidine.

Catalytic performance of different titanosilicates in AAL epoxidation

The epoxidation activity of four different zeolites was evaluated in the AAL epoxidation using MeCN as the solvent [Table 1]. Re-Ti-MWW displayed higher AAL conversion of 56.9% and H2O2 utilization efficiency of 97.1% compared to Ti-MWW (34.7% and 88.2%, respectively). TS-1 and Ti-MOR showed significantly lower catalytic activity, with AAL conversion of 8.7% and 2.0%, respectively. All catalysts showed high GLY selectivity of > 97%. Considering that these titanosilicates contain different Ti contents, their catalytic activities were further compared by turnover frequency (TOF). The TOF value of Ti-MWW (1,680) was much higher than those of TS-1 (380) and Ti-MOR (94). The TOF value of Re-Ti-MWW was elevated to 2,754 after the PI-assisted 3D-2D structural transformation of Ti-MWW. Ti-MOR exhibited the lowest catalytic activity for the AAL epoxidation. The poor activity of Ti-MOR has also been reported in the epoxidation of other olefins[16], suggesting that Ti-MOR is not suitable for olefin epoxidation, although it exhibits excellent catalytic activity in aldehydes and ketones ammonia oxidation reactions[18]. The lower activity of medium-pore TS-1 compared to Ti-MWW was probably ascribed to the poor diffusion. Methanol was reported to be the preferred solvent for TS-1 in alkene epoxidation reactions, so the usage of MeCN solvent may also induce the lower activity of TS-1[17].

Table 1

Catalytic results of four titanosilicates in AAL epoxidationa

CatalystSi/TibAAL conv.
(%)
GLY sel.
(%)
H2O2(%)TOFcD/r2
conv.eff.(10-7 s-1)d
Ti-MWW3934.799.239.388.21,68020.1
Re-Ti-MWW3956.999.458.697.12,754466.0
TS-1358.797.910.582.93803.2
Ti-MOR382.099.34.148.8946.8

Effects of solvent on AAL epoxidation

As described above, the catalytic performance was first compared in MeCN solvent, but different titanosilicate catalysts may have distinct solvent effects[15]. Thus, the epoxidation activity of four zeolites was further evaluated in various solvents [Table 2]. For Ti-MWW zeolite, the AAL conversion of 34.7% in MeCN was higher than that in other protic solvents, suggesting that the most suitable solvent was MeCN, consistent with the previous report[13]. Meanwhile, the GLY selectivity of 99.2% in MeCN was also higher than those in protic solvents, because hydrolysis and alcoholysis easily occur in water and methanol solvents, respectively. For hydrophobic TS-1, the preferred solvent effect was methanol, although the activity of the TS-1 catalyst remained poor in methanol compared to Ti-MWW. The catalytic activity of Ti-MOR was found to be low in all the solvents. The AAL conversion and GLY selectivity both increased for the Re-Ti-MWW catalyst compared to Ti-MWW in all the investigated solvents. In contrast to Ti-MWW, the epoxidation activity of Re-Ti-MWW was higher in protic solvent than that in aprotic MeCN. The AAL conversion and GLY selectivity in H2O solvent increased from 30.5% to 98.1% and 96.8% to 99.1%, respectively, when the catalyst was changed from Ti-MWW to Re-Ti-MWW. Thus, the preferred solvent was H2O for Re-Ti-MWW. Meanwhile, the enhancement of epoxidation performance was more significant in H2O than that in MeCN when comparing the performance of Re-Ti-MWW to that of Ti-MWW.

Table 2

Catalytic results of AAL epoxidation over titanosilicates in various solventsa

CatalystSolventAAL conv. (%)GLY sel. (%)H2O2 (%)
conv.eff.
Ti-MWWH2O30.596.832.892.9
MeCN34.799.239.388.2
MeOH13.790.422.860.1
t-BuOH19.698.522.786.3
Re-Ti-MWWH2O98.199.198.599.5
MeCN56.999.458.697.1
MeOH67.495.172.193.5
t-BuOH42.599.248.387.9
TS-1H2O10.395.413.476.9
MeCN8.797.910.582.9
MeOH14.588.419.474.7
t-BuOH6.397.18.772.4
Ti-MORH2O2.399.54.748.9
MeCN2.099.34.148.8
MeOH1.799.13.450.0
t-BuOH1.999.44.344.2

The dependence of AAL conversion and GLY selectivity on reaction time between Ti-MWW and Re-Ti-MWW in the two solvents of H2O and MeCN were also compared. With the reaction time proceeding, Re-Ti-MWW always showed higher AAL conversion in H2O solution [Figure 2A], further indicating that the most suitable solvent was H2O for Re-Ti-MWW. In terms of GLY selectivity, Re-Ti-MWW in both H2O and MeCN and Ti-MWW in MeCN could kept almost intact within 120 min. However, the GLY selectivity of Ti-MWW in H2O decreased from 98.4% to 94.1% after 120 min. In the AAL epoxidation reaction system, the main product is GLY, but the presence of weak acidic sites originating from Ti-OH, Si-OH, and Ti-OOH groups in titanosilicates may catalyze the hydrolysis of GLY to produce glycerol [Supplementary Scheme 2]. If MeOH or t-BuOH is used, GLY may also undergo alcoholysis to produce alcohol ethers. The ring-opening of GLY due to hydrolysis in H2O and MeCN solvent was investigated over Ti-MWW and Re-Ti-MWW [Supplementary Figure 2]. The conversion of GLY was 19% and 3% after 5 h for Ti-MWW and Re-Ti-MWW in H2O solvent, respectively, indicating that the hydrolysis was significantly prohibited over Re-Ti-MWW. The hydrolysis of GLY was also suppressed over Re-Ti-MWW in MeCN solvent. The neutralization of the weak acidity by the alkaline PI molecules was supposed to be the main reason for preventing the ring-opening side reactions[15]. It was also the reason for the decrease of GLY selectivity over Ti-MWW in H2O solvent shown in Figure 2B.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 2. (A) AAL conversion and (B) GLY selectivity of AAL epoxidation with different reaction times over Ti-MWW and Re-Ti-MWW in solvent H2O and MeCN. Reaction conditions: catalyst, 0.03 g; allyl alcohol, 30 mmol; H2O2 (30 wt%), 30 mmol; solvent, 5 mL; temperature, 313 K; time, 5-120 min. The labels in this figure represent “sample name-solvent”. AAL: Allyl alcohol; GLY: glycidol.

The transformation of some tetrahedrally coordinated Ti species into open-site hexa-coordinated Ti species during PI rearrangement treatment was considered as the main reason for the enhanced activity of Re-Ti-MWW compared to Ti-MWW in our previous studies[15-17]. In fact, the transformation from 3D Ti-MWW zeolite to the corresponding 2D layered Re-Ti-MWW zeolite induced the formation of numerous surface Si-OH groups, which may change the framework hydrophilicity/hydrophobicity and then the adsorption behavior of AAL substrate. The adsorption ability of AAL over four different titanosilicates was first compared [Figure 3A]. The higher adsorption amount of AAL was observed over Ti-MWW (0.197 mmol·g-1) in comparison to TS-1 (0.195 mmol·g-1) and Ti-MOR (0.074 mmol·g-1). However, the adsorption amount of AAL over Re-Ti-MWW was the lowest (0.061 mmol·g-1) because the pores were blocked by PI molecules, consistent with decreased surface area and pore volume in Supplementary Table 1. The total adsorbed amount does not reflect the relative diffusion rate of the substrate[36]. Hence, the relative diffusion rate (D/r2) was further calculated according to Fick’s second law to precisely compare the adsorption behavior. The initial adsorption data in the first 10 s was used in the calculation [Supplementary Figure 3], because it was insensitive to the heat of adsorption[36]. The relative diffusion rate of AAL over Re-Ti-MWW was up to 466 × 10-7 s-1, which was much larger than those of other zeolites [Figure 3B], although the pore volume was the smallest and the adsorption amount of AAL was the lowest. Thus, the pore opening was not the major factor that affected the relative diffusion rate of AAL. Given that AAL was quite hydrophilic, we considered the hydrophilicity/hydrophobicity of the zeolite affects the relative diffusion rate. The H2The relative H2O-adsorption amount in Figure 3C was defined as the ratio of H2O-adsorption volume to total pore volume. Re-Ti-MWW showed the largest relative H2O-adsorption amount and the smallest contact angle, indicating that its hydrophilicity was the highest. We speculated that the formation of numerous Si-OH groups on MWW layer surface upon 3D to 2D structural transformation was the main reason for the hydrophilicity enhancement of Re-Ti-MWW compared to Ti-MWW. Therefore, the more hydrophilic Re-Ti-MWW zeolite with abundant Si-OH groups favors the rapid adsorption of hydrophilic AAL molecules. According to previous reports in the literature, the faster relative diffusion rate was favorable for higher activity in the initial reaction stage[36]. Thus, in addition to the formation of higher active Ti sites in Re-Ti-MWW, the increase of hydrophilicity was also a main reason for the enhancement of catalytic performance in AAL epoxidation using H2O as the solvent. Moreover, the enhancement of hydrophilicity may also induce the change of solvent effect, resulting in H2O as the preferred solvent for Re-Ti-MWW.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 3. (A) Adsorption isotherms and (B) relative adsorption uptake of AAL over four titanosilicates at 313 K; (C) H2O adsorption isotherms and (D) contact angle test of (a) Ti-MWW, (b) Re-Ti-MWW, (c) TS-1, and (d) Ti-MOR. AAL: Allyl alcohol.

Reaction kinetics of AAL epoxidation over Ti-MWW and Re-Ti-MWW

The epoxidation process catalyzed by the titanosilicates/H2O2 system proceeds in two steps. The first step is the activation of H2O2 on the Ti activity centers to generate Ti-Oα-Oβ-H intermediates [Scheme 2][37], which is the rate-determining step of the entire epoxidation process. In the second step, the more electrophilic Oα in the intermediate interacts with the C=C in the olefins to realize the transfer of Oα, producing the epoxides. Therefore, the efficient generation of intermediate Ti-OOH is the key to the improvement of catalytic activity. The ability of Ti-OOH formation was compared between Ti-MWW and Re-Ti-MWW in different solvents. Figure 4A and B showed the relationship between the initial epoxidation rate and H2O2 concentration. A similar variation trend was observed for Ti-MWW and Re-Ti-MWW in both H2O and MeCN solvents. The epoxidation rate increased with increasing H2O2 concentration at lower H2O2 concentration, indicating that Ti-OOH species were unsaturated. Therefore, at lower H2O2 concentration, the epoxidation rate depended on the rate of Ti-OOH formation, and the effect of solvent on H2O2 activation would affect the Ti-OOH formation and epoxidation rate. Re-Ti-MWW zeolite showed a higher epoxidation rate in H2O compared to MeCN at lower H2O2 concentration, indicating H2O favored the Ti-OOH formation. For the Ti-MWW catalyst, the epoxidation rate in MeCN was higher than that in H2O, revealing that MeCN favored the formation of Ti-OOH. At higher H2O2 concentration, H2O2 concentration did not affect the epoxidation rate; thus, the reaction can be considered as pseudo zero-order reaction. The reaction order of was calculated at lower H2O2 concentration region [Figure 4C and D]. The reaction orders were 0.913 and 0.201 for Ti-MWW and Re-Ti-MWW in H2O solvent, and they were 0.633 and 0.288 for the MeCN solvent, respectively. The smaller reaction order of H2O2 indicates better catalytic performance in the epoxidation reaction[38]. Re-Ti-MWW showed the lowest reaction order of H2O2 in H2O solvent, consistent with the highest catalytic activity of Re-Ti-MWW in catalyzing AAL epoxidation reaction using H2O solvent [Figure 2A].

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 4. Dependence of AAL epoxidation rate on (A) H2O2 concentration and (B) over (a) Ti-MWW and (b) Re-Ti-MWW; H2O2 concentration and initial reaction rate (C) and (D) in H2O and MeCN. Reaction conditions: catalyst, 0.03 g; allyl alcohol, 30 mmol; H2O or MeCN, 5 mL; temperature, 313 K. AAL: Allyl alcohol.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Scheme 2. Ti-Oα-Oβ-H intermediates generated by the activation of H2O2.

Continuous epoxidation reaction of AAL over Re-Ti-MWW in H2O solvent

Effects of reaction conditions

The continuous liquid-phase AAL epoxidation catalyzed by Re-Ti-MWW in H2O solvent was carried out in a slurry bed reactor. The effects of reaction temperature, weight hourly space velocity (WHSV) of H2O2, AAL/H2O2 molar ratio, catalyst amount, and stirring speed on the performance of AAL epoxidation were investigated in detail. The effect of reaction temperature on the catalytic performance of Re-Ti-MWW in continuous AAL epoxidation reaction was shown in Figure 5A. The H2O2 conversion improved from 96.5% to almost 100% when the reaction temperature increased from 303 to 313 K, although H2O2 effective utilization and GLY selectivity were kept stable at 92.5% and 96.8%, respectively. The effective utilization of H2O2 and GLY selectivity dropped as the reaction temperature was further increased to 323 K. However, the H2O2 conversion remained constant. Although increasing the reaction temperature favored the epoxidation reaction, the ineffective decomposition of H2O2 and the pore-opening reaction of GLY were also enhanced. Thus, the optimum initial operating temperature for the continuous liquid-phase AAL epoxidation was set at 313 K.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 5. (A) Effects of temperature, (B) WHSV of H2O2, and (C) AAL/H2O2 molar ratio on the epoxidation of AAL over Re-Ti-MWW. Reaction conditions: catalyst, 1 g; temperature 313 K; AAL/H2O2 molar ratio, 1.5 (A and B); H2O/AAL mass ratio, 4; WHSV(H2O2) = 0.4 h-1 (A and C); 500 rpm. The data were the average values at 45 h after the reaction reached a steady state. WHSV: Weight hourly space velocity; AAL: allyl alcohol.

The higher WHSV of H2O2 could increase the GLY concentration in the reaction mixture under the condition that H2O2 is completely consumed. With the WHSV of H2O2 setting at 0.3 h-1, the H2O2 conversion, H2O2 effective utilization, and GLY selectivity were 99.4%, 93.6%, and 97.2%, respectively [Figure 5B]. Increasing the WHSV of H2O2 to 0.4 h-1, the H2O2 conversion was not changed while the H2O2 effective utilization and GLY selectivity slightly decreased. When the WHSV of H2O2 was further increased to 0.5 h-1, the H2O2 effective utilization and GLY selectivity declined to 85.2% and 92.5%, respectively. Higher WHSV of H2O2 induced higher H2O2 concentration in the reaction solution, which increased the acidity and then aroused inefficient decomposition of H2O2 and ring-opening of GLY. Hence, the preferred WHSV of H2O2 was determined to be 0.4 h-1.

Although the theoretical AAL/H2O2 ratio is 1 in the epoxidation of AAL with H2O2 as the oxidant, the AAL/H2O2 molar ratio should be greater than 1 in the actual production process to consume all the H2O2 in the system for safety. With the AAL/H2O2 molar ratio of 1, the H2O2 conversion was only 97.1% [Figure 5C]. The H2O2 conversion reached 99.3% when the molar ratio of AAL/H2O2 was raised to 1.5. Further increasing the AAL/H2O2 ratio hardly changed the H2O2 conversion. Thus, the AAL/H2O2 ratio of 1.5 was enough for the complete consumption of H2O2.

The effect of catalyst amount was also investigated [Figure 6A]. Increasing the catalyst amount favored the conversion of H2O2, while the effective utilization of H2O2 and GLY selectivity were decreased. It is conceivable that increasing the catalyst amount raised the content of acid centers, which intensified the ineffective decomposition of H2O2 and ring-opening of GLY. The catalyst amount of 1 g was enough because the complete conversion of H2O2 can be realized. By changing the stirring speed, the influence of external diffusion can be eliminated. In Figure 6B, although the increase of stirring speed increased the H2O2 conversion, the ineffective decomposition of H2O2 was also enhanced. Therefore, 500 rpm was chosen as the optimal stirring speed.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 6. Effects of (A) catalyst amounts and (B) stirring speed on the epoxidation of AAL over Re-Ti-MWW. Reaction conditions: catalyst, 1 g (for Figure 6B); temperature, 313 K; AAL/H2O2 molar ratio, 1.5; H2O/AAL mass ratio, 4; WHSV(H2O2) = 0.4 h-1; 500 rpm (for Figure 6A). The data were the average values at 45 h after the reaction reached a steady state. AAL: Allyl alcohol; WHSV: weight hourly space velocity.

Evaluation of catalyst lifetime

The catalytic performance of Re-Ti-MWW in continuous AAL epoxidation using water and acetonitrile solvent was compared [Figure 7]. The lifetime of Re-Ti-MWW in water solvent reached 121 h, with the average H2O2 conversion, H2O2 effective utilization, GLY selectivity, and GLY yield of 98.1%, 91.1%, 96.3%, and 86.0%, respectively. The H2O2 conversion was maintained at 99% at 313 K for 30 h. After that, the H2O2 conversion started to decrease and the temperature was increased to maintain a high H2O2 conversion (> 99%) until the temperature was increased to 323 K. The catalytic lifetime of Re-Ti-MWW was much higher than that of Ti-MWW (13 h) in water [Supplementary Figure 4]. The average H2O2 conversion was only 96% for Re-Ti-MWW when MeCN was used as the solvent, and the reaction duration was shortened to 110 h, while the selectivity of GLY was raised to 99.1% compared to the H2O system. Acetonitrile, as a weak basic solvent, reduced the acidity of the reaction system, thus inhibiting the side reactions. Therefore, adding some alkaline additive may help to increase the GLY selectivity for AAL epoxidation in H2O solvent and further increase the lifetime.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 7. The lifetime of Re-Ti-MWW in the epoxidation of AAL. Reaction conditions: catalyst, 1 g; temperature, 313-323 K; AAL/H2O2 molar ratio, 1.5; solvent/AAL mass ratio, 4; WHSV(H2O2) = 0.4 h-1; 500 rpm. AAL: Allyl alcohol; WHSV: weight hourly space velocity.

As shown in Figure 8A, the lifetime increased to 163 h when 5 ppm of (NH4)2CO3 was added to the reaction feedstock. The GLY selectivity increased from 96.3% to 97.3%. However, introducing the alkaline species in the middle stage of the reaction was less effective [Figure 8B], indicating that the ring-opening of GLY reaction should be prohibited in the early stage, because the by-product glycerol was hardly removed once they are formed.

Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol

Figure 8. Effect of ammonia (A) before and (B) after addition on epoxidation of AAL. Reaction conditions: catalyst, 1 g; (NH4)2CO3 amounts, 5 ppm; temperature, 313-323 K; AAL/H2O2 molar ratio, 1.5; H2O/AAL mass ratio, 4; WHSV(H2O2) = 0.4 h-1; 500 rpm. AAL: Allyl alcohol; WHSV: weight hourly space velocity.

CONCLUSIONS

Re-Ti-MWW catalyst with higher hydrophilicity was obtained by PI rearrangement treatment and showed higher catalytic activity in AAL epoxidation reaction than Ti-MWW. High relative diffusion rates of AAL and the formation of more active open-site hexa-coordinated Ti species resulted in increased catalytic activity in AAL epoxidation reaction. Meanwhile, the most suitable solvent was changed from MeCN for Ti-MWW to H2O for Re-Ti-MWW. Re-Ti-MWW showed superior H2O2 activation ability in H2O solvent than in MeCN solvent. Using the green solvent of H2O, the lifetime of Re-Ti-MWW in continuous AAL epoxidation reaction was up to 163 h, and the selectivity of GLY was maintained at 97%.

DECLARATIONS

Authors’ contributions

Conception and design of the study: Wu P, Xu H

Data collection and analysis: Gong X

Sample preparation: Tuo J, Wang J, Li X, Zhai C

Paper writing and reviewing: Gong X, Xu H, Wu P

Availability of data and materials

Not applicable.

Financial support and sponsorship

We gratefully acknowledge the financial support from the National Key R&D Program of China (No. 2021YFA1501401, No. 2023YFB3810602) and the National Natural Science Foundation of China (No. 22222201).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

REFERENCES

1. Wu P. A novel titanosilicate with MWW structure: III. Highly efficient and selective production of glycidol through epoxidation of allyl alcohol with H2O2. J Catal 2003;214:317-26.

2. Wróblewska A, Fajdek A, Wajzberg J, Milchert E. Epoxidation of allyl alcohol over mesoporous Ti-MCM-41 catalyst. J Hazard Mater 2009;170:405-10.

3. Wróblewska A, Fajdek A. Epoxidation of allyl alcohol to glycidol over the microporous TS-1 catalyst. J Hazard Mater 2010;179:258-65.

4. Zhang T, Solé-Daura A, Fouilloux H, et al. Reaction pathway discrimination in alkene oxidation reactions by designed Ti-siloxy-polyoxometalates. ChemCatChem 2021;13:1220-9.

5. Solé-Daura A, Zhang T, Fouilloux H, et al. Catalyst design for alkene epoxidation by molecular analogues of heterogeneous titanium-silicalite catalysts. ACS Catal 2020;10:4737-50.

6. Zheng R, Liu Z, Wang Y, Xie Z, He M. The future of green energy and chemicals: rational design of catalysis routes. Joule 2022;6:1148-59.

7. Smeets V, Gaigneaux EM, Debecker DP. Titanosilicate epoxidation catalysts: a review of challenges and opportunities. ChemCatChem 2022;14:e202101132.

8. Xu H, Wu P. Two-dimensional zeolites in catalysis: current state-of-the-art and perspectives. Catal Rev 2021;63:234-301.

9. Suib SL, Přech J, Szaniawska E, Čejka J. Recent advances in tetra- (Ti, Sn, Zr, Hf) and pentavalent (Nb, V, Ta) metal-substituted molecular sieve catalysis. Chem Rev 2023;123:877-917.

10. Harris JW, Cordon MJ, Di Iorio JR, Vega-Vila JC, Ribeiro FH, Gounder R. Titration and quantification of open and closed Lewis acid sites in Sn-Beta zeolites that catalyze glucose isomerization. J Catal 2016;335:141-54.

11. Moliner M, Corma A. Advances in the synthesis of titanosilicates: From the medium pore TS-1 zeolite to highly-accessible ordered materials. Microporous Mesoporous Mater 2014;189:31-40.

12. Clerici MG, Bellussi G, Romano U. Synthesis of propylene oxide from propylene and hydrogen peroxide catalyzed by titanium silicalite. J Catal 1991;129:159-67.

13. Clerici MG, Ingallina P. Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite. J Catal 1993;140:71-83.

14. Bai R, Song Y, Bai R, Yu J. Creation of hierarchical titanosilicate TS-1 zeolites. Adv Mater Inter 2021;8:2001095.

15. Wu P, Tatsumi T, Komatsu T, Yashima T. A novel titanosilicate with MWW structure: II. Catalytic properties in the selective oxidation of alkenes. J Catal 2001;202:245-55.

16. Tong W, Yin J, Ding L, Xu H, Wu P. Modified Ti-MWW zeolite as a highly efficient catalyst for the cyclopentene epoxidation reaction. Front Chem 2020;8:585347.

17. Xu L, Huang DD, Li CG, et al. Construction of unique six-coordinated titanium species with an organic amine ligand in titanosilicate and their unprecedented high efficiency for alkene epoxidation. Chem Commun 2015;51:9010-3.

18. Yin J, Xu H, Wang B, et al. Highly selective 1-pentene epoxidation over Ti-MWW with modified microenvironment of Ti active sites. Catal Sci Technol 2020;10:6050-64.

19. Ding L, Yin J, Tong W, et al. Selective synthesis of epichlorohydrin via liquid-phase allyl chloride epoxidation over a modified Ti-MWW zeolite in a continuous slurry bed reactor. New J Chem 2021;45:331-42.

20. Na K, Jo C, Kim J, Ahn WS, Ryoo R. MFI titanosilicate nanosheets with single-unit-cell thickness as an oxidation catalyst using peroxides. ACS Catal 2011;1:901-7.

21. Wang L, Sun J, Meng X, et al. A significant enhancement of catalytic activities in oxidation with H2O2 over the TS-1 zeolite by adjusting the catalyst wettability. Chem Commun 2014;50:2012-4.

22. Bregante DT, Johnson AM, Patel AY, et al. Cooperative effects between hydrophilic pores and solvents: catalytic consequences of hydrogen bonding on alkene epoxidation in zeolites. J Am Chem Soc 2019;141:7302-19.

23. Bregante DT, Tan JZ, Schultz RL, et al. Catalytic consequences of oxidant, alkene, and pore structures on alkene epoxidations within titanium silicates. ACS Catal 2020;10:10169-84.

24. Bregante DT, Chan MC, Tan JZ, et al. The shape of water in zeolites and its impact on epoxidation catalysis. Nat Catal 2021;4:797-808.

25. Torres C, Potts DS, Flaherty DW. Solvent mediated interactions on alkene epoxidations in Ti-MFI: effects of solvent identity and silanol density. ACS Catal 2023;13:8925-42.

26. Wells DH Jr, Delgass WN, Thomson KT. Evidence of defect-promoted reactivity for epoxidation of propylene in titanosilicate (TS-1) catalysts: a DFT study. J Am Chem Soc 2004;126:2956-62.

27. Nie X, Ren X, Ji X, et al. Mechanistic insight into propylene epoxidation with H2O2 over titanium silicalite-1: effects of zeolite confinement and solvent. J Phys Chem B 2019;123:7410-23.

28. Bonino F, Damin A, Ricchiardi G, et al. Ti-peroxo species in the TS-1/H2O2/H2O system. J Phys Chem B 2004;108:3573-83.

29. Wang L, Xiong G, Su J, Li P, Guo H. In situ UV Raman spectroscopic study on the reaction intermediates for propylene epoxidation on TS-1. J Phys Chem C 2012;116:9122-31.

30. Wang L, Liu Y, Xie W, et al. Highly efficient and selective production of epichlorohydrin through epoxidation of allyl chloride with hydrogen peroxide over Ti-MWW catalysts. J Catal 2007;246:205-14.

31. Stare J, Henson NJ, Eckert J. Mechanistic aspects of propene epoxidation by hydrogen peroxide. Catalytic role of water molecules, external electric field, and zeolite framework of TS-1. J Chem Inf Model 2009;49:833-46.

32. Sever RR, Root TW. DFT Study of solvent coordination effects on titanium-based epoxidation catalysts. Part one:  formation of the titanium hydroperoxo intermediate. J Phys Chem B 2003;107:4080-9.

33. Yin J, Jin X, Xu H, et al. Structured binder-free MWW-type titanosilicate with Si-rich shell for selective and durable propylene epoxidation. Chinese J Catal 2021;42:1561-75.

34. Xu H, Wang Y, Peng R, Jiang J, Zhang K, Wu P. Synthesis of micro-mesoporous Ti-MOR/silica composite spheres in oil-in-water microemulsion system. Chem Res Chin Univ 2022;38:192-9.

35. Li S, Si X, Peng R, et al. “Burr Puzzle”-Like Hierarchical Beta zeolite composed of crisscrossed nanorods. Microporous Mesoporous Mater 2022;335:111843.

36. Zhao X, Zeng S, Zhang X, et al. Generating assembled MFI nanocrystals with reduced b-axis through structure-directing agent exchange induced recrystallization. Angew Chem Int Ed Engl 2021;60:13959-68.

37. Ratnasamy P, Srinivas D. Active sites and reactive intermediates in titanium silicate molecular sieves. 2004. Available from: https://onlinelibrary.wiley.com/doi/10.1002/chin.200450212. [Last accessed on 5 Feb 2024].

38. Russo V, Tesser R, Santacesaria E, Di Serio M. Kinetics of propene oxide production via hydrogen peroxide with TS-1. Ind Eng Chem Res 2014;53:6274-87.

Cite This Article

OAE Style

Gong X, Tuo J, Wang J, Li X, Zhai C, Xu H, Wu P. Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol. Chem Synth 2024;4:14. http://dx.doi.org/10.20517/cs.2023.59

AMA Style

Gong X, Tuo J, Wang J, Li X, Zhai C, Xu H, Wu P. Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol. Chemical Synthesis. 2024; 4(1): 14. http://dx.doi.org/10.20517/cs.2023.59

Chicago/Turabian Style

Gong, Xianchen, Jie Tuo, Jilong Wang, Xintong Li, Chengwei Zhai, Hao Xu, Peng Wu. 2024. "Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol" Chemical Synthesis. 4, no.1: 14. http://dx.doi.org/10.20517/cs.2023.59

ACS Style

Gong, X.; Tuo J.; Wang J.; Li X.; Zhai C.; Xu H.; Wu P. Hydrophilic Ti-MWW for catalyzing epoxidation of allyl alcohol. Chem. Synth. 2024, 4, 14. http://dx.doi.org/10.20517/cs.2023.59

About This Article

Special Issue

This article belongs to the Special Issue Zeolites for Sustainable Catalysis
© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Data & Comments

Data

Views
255
Downloads
56
Citations
0
Comments
0
3

Comments

Comments must be written in English. Spam, offensive content, impersonation, and private information will not be permitted. If any comment is reported and identified as inappropriate content by OAE staff, the comment will be removed without notice. If you have any queries or need any help, please contact us at support@oaepublish.com.

0
Download PDF
Cite This Article 11 clicks
Like This Article 3 likes
Share This Article
Scan the QR code for reading!
See Updates
Contents
Figures
Related
Chemical Synthesis
ISSN 2769-5247 (Online)

Portico

All published articles are preserved here permanently:

https://www.portico.org/publishers/oae/

Portico

All published articles are preserved here permanently:

https://www.portico.org/publishers/oae/