Role of Ga in promoting epoxidation of cis-cyclooctene over Ga-WOx/SBA-15

LI Peng-hui ，WANG Hui-xiang ，LI Jun-fen ，Lü Bao-liang,*

（1. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China；2. University of Chinese Academy of Sciences, Beijing 100049, China；3. School of Chemistry and Materials Science, Shanxi Normal University, Taiyuan 030031, China）

Abstract: This article investigates the promoting effect of gallium (Ga) on the activity of Ga-WOx/SBA-15 catalyst for ciscyclooctene epoxidation with H2O2. The optimal catalyst of 0.3Ga-WOx/SBA-15 offered a turnover frequency (TOF) of 112 h-1, which was nearly two times than that of WOx/SBA-15 (57 h-1). The low apparent reaction activation energy for 0.3Ga-WOx/SBA-15 (49.6 kJ/mol vs 64.0 kJ/mol for WOx/SBA-15) was also in line with its superior performance. Kinetic analysis demonstrated stronger adsorption of H2O2 on 0.3Ga-WOx/SBA-15 surface, facilitating the H2O2 activation. Based on the characterizations and catalytic performance, the improvement of Ga was attributed to the increase of Lewis acid sites and the enhancement of electrophilicity. Furthermore, the metal hydrogen peroxide (M-OOH) was identified as the primary intermediate.

Key words: epoxidation；Ga promoter；H2O2 activation；oxygen atom transfer

Epoxidation is an important pathway for the stepup transformation of downstream products in coal-toliquids (CTL) conversion[1,2], and the epoxides are widely used as intermediates in the synthesis of pharmaceuticals, cosmetics, polymeric resins and so on[3,4]. Conventionally, halogenate-alcohol method or peroxyacid oxidation has exhibited satisfactory catalytic performances in alkenes epoxidation, but they are suffered from the formation of toxic by-products and the separation of epoxides[5,6]. Hydrogen peroxide(H2O2) is an ideal candidate for epoxidation because it is green, sustainable and easy to activate. Supported transition-metal catalysts have been extensively developed for epoxidation with H2O2due to their variable chemical valence and unique properties in electron affinity[7-10].

Supported tungsten oxide materials are typically explored for epoxidation owing to their rich acid sites on the surface, especially the Lewis acid site, which is the active center of H2O2[11-13]. For example, Bisio et al.[14]prepared WO3-grafted silica by dry impregnation method, and it realized good conversion (more than 60%) for epoxidation of limonene and selectivity of 78% towards desired product. Tao et al.[15]synthesized WO3-SiO2by using supercritical CO2forciscyclooctene epoxidation, and the conversion ofciscyclooctene was 73% and the selectivity towards the epoxide was >99%. Furthermore, the catalytic activity of WO3based-catalysts can be boosted in different ways, including oxygen vacancies[16], additives[17,18],dispersity[19]and etc. Among them, promoters are prevalent in industrial heterogeneous catalysts,ensuring the economic feasibility of many large-scale chemical production[20]. Adding an appropriate promoter could modulate the textural properties, such as improving dispersion[21], preventing agglomeration[22],regulating surface acid sites[23], etc. Recently, our group reported that tungsten oxide doped with Cr3+and Co2+could improve the surface Lewis acid concentration resulting from the formation of oxygen vacancies[24,25].Still, it’s recognized that the addition of alkaline earth metals (e.g., Mg and Na) can neutralize surface acid sites[26]. Moreover, the Lewis acid center of the metal could shift the electron cloud of the O-O bond, which would promote the dissociation of the O-O bond,thereby facilitating the transfer of oxygen atoms[7,27].Therefore, rational selection of additives to regulate acid sites on the catalyst surface is of great importance for epoxidation with H2O2. After careful screening, Ga3+would be a promising promoter due to its stable hydrated ion in the aqueous solution, which means that Ga3+have sufficient opportunities to interact with WO3surface[28,29]. With that in mind, one could expect that the addition of Ga would modify the supported tungsten oxide catalyst and consequently the catalytic performance for epoxidation.

Mesoporous silicas have the advantages of mass transfer and surface hydroxyl groups that can anchor active metals, and were therefore chosen as the supports. Herein, a comparative study was carried on Ga-doped and Ga-undoped WOx/SBA-15 catalysts to reveal the structure effect on epoxidation ofciscyclooctene with H2O2. Catalysts were synthetized by impregnation method, and the catalytic performance of 0.3Ga-WOx/SBA-15 outperformed that of WOx/SBA-15. Combined with structural characterization and catalytic performance, it is speculated that the boost of the activity over 0.3Ga-WOx/SBA-15 catalyst was related to the improvement of Lewis acid property of the catalyst surface.

1 Experimental

1.1 Preparation of catalysts

Firstly, tungstic acid (0.28 g, H2WO4, Aladdin)was dissolved in H2O2(5 mL, 30%), and the supernatant was used as the tungsten source. The tungsten source could graft onto a silica surface via silanols to obtain the homogeneous distribution of the tungsten species[30]. WOx/SBA-15 catalyst was prepared by impregnation method. Typically, 600 mg of SBA-15 was dispersed in 10 mL of deionized water, then 1 mL of the as-prepared tungsten source was added. The mixture was stirred at room temperature for 12 h. In order to ensure the high dispersity of the tungsten oxide nanoparticles[31], the freeze drying was applied. The detailed steps of freeze-drying were as follows, the impregnated samples were firstly frozen, after which they were placed inside the chamber of the freeze dryer with a setting temperature of -45 °C and pressure of 0.2 mbar for 5 h. After that, the sample was calcined at 550 °C for 2 h, then the WOx/SBA-15 was obtained.The catalysts ofaGa-WOx/SBA-15 (whereawas mole ratio of Ga to W) were prepared by co-impregnation strategy. The procedure was same as that for WOx/SBA-15 except for the addition of a certain amount of Ga(NO3)3·xH2O, and the content was 7.2,14.3, 21.5, 28.7 mg fora=0.1, 0.2, 0.3 and 0.4 respectively. The support of SBA-15 was prepared according to the previous work[16].

1.2 Characterization of catalysts

Morphology and structure of catalysts were characterized by transmission electron microscopy(TEM) on a JEM-2100F microscope at 200 kV. Crystal phase structure was measured by powder X-ray diffraction (XRD) using a D8 Advance Bruker AXS diffractometer and the pattern was collected at the range of 2θ=10° to 80°. Surface electronic structure of catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) on the AXIS ULTRA DLD photoelectron spectrometer. The microstructure of catalysts was identified by Raman spectra on LabRAM HR Evolution Raman spectrometer with excitation source wavelength of 532 nm. The acidic property was measured by FT-IR analysis of adsorbed pyridine (FTIR-Py) on Bruker TENSOR 27 spectrometer, and the specific procedure was similar to that reported previously[24].

1.3 Evaluation of catalysts

Epoxidation ofcis-cyclooctene with H2O2was selected as the model reaction to evaluate the catalytic performance. The reaction was performed in a roundbottom flask equipped with a water-cooling condenser.50 mg of the catalyst, 10 mL of solvent (CH3CN),ciscyclooctene (0.4 mL, 3.1 mmol) and 30% H2O2(0.4 mL, molar ratio of H2O2to alkene was 1.23∶1)were added to the reactor, then stirred at the target temperature. After the reaction, the product was collected and analyzed by a gas chromatograph (GC,GC-920) with a FFAP capillary column, and anisole was added as the internal standard to assist product quantification analysis.

2 Results and discussion

2.1 Characterization

The distribution of tungsten oxide nanoparticle was characterized by TEM and showed in Figure 1.The image of WOx/SBA-15 (Figure 1(a)) displays that the well-ordered mesoporous structure of SBA-15 is not damaged after loading WOx, and tungsten oxide nanoparticles with a diameter of 3-5 nm are mainly distributed into the pores. The EDX-mapping in Figure 1(b) demonstrates the homogeneous distribution of tungsten oxide nanoparticles on the silica support.After doping Ga, the nanoparticles are still highly dispersed and mainly confined in the pores of SBA-15 as shown in Figure 1(c). The EDX-mapping of 0.3Ga-WOx/SBA-15 (Figure 1(d)) reveals that Ga and W elements are uniformly dispersed on the support. The TEM results indicate that the addition of Ga has no significant influence on the distribution and size of tungsten oxide nanoparticles.

Figure 1 TEM images of WOx/SBA-15 (a) and 0.3Ga-WOx/SBA-15 (c); and the corresponding EDX-mapping (b) and (d)

The XRD patterns of WOx/SBA-15 (Figure 2)show no diffraction peak indicative of WO3, suggesting the high dispersion of tungsten oxide, which is consistent with the results of TEM. For 0.3Ga-WOx/SBA-15, there is no obvious peak corresponding to WO3or Ga2O3, further indicating that Ga additive has no effect on the crystal structure of the catalysts.

Figure 2 XRD patterns of WOx/SBA-15 and 0.3Ga-WOx/SBA-15

As Raman spectra showed in the Figure 3, the peaks at 968 and 817 cm-1are assigned to the stretching vibration of the terminal W=O and W-O-W[32,33],respectively, while the peak at 605 cm-1is attributed to the silica matrix. The peak at 968 cm-1for 0.3Ga-WOx/SBA-15 is more obvious than that in other two samples, which might be caused by the more exposed W=O bonds due to the addition of Ga. Besides, the Ga additive also makes the metal-support interaction become stronger validated by the sharp peak at 605 cm-1in 0.3Ga-WOx/SBA-15.

Figure 3 Raman spectra of 0.3Ga-WOx/SBA-15, WOx/SBA-15 and pure SBA-15

The surface electronic structure was analyzed by XPS measurement. The W 4fspectrum (Figure 4(a)) of WOx/SBA-15 is composed of two pairs of peaks, where the peaks at 38.7 and 36.6 eV are assigned to W (VI)4f5/2and 4f7/2[34], respectively, and the rest of the peaks at 37.6 and 35.5 eV are attributed to W (V) 4f5/2and 4f7/2.For 0.3Ga-WOx/SBA-15, the peaks of W (VI) are located at 38.6 and 36.5 eV, and the peaks corresponding to W (V) are at 37.9 and 35.2 eV. For the spectrum of Ga 2p(Figure 4(b)), the peaks at 1145.2 and 1117.9 eV are ascribed to Ga (III) 2p1/2and 2p3/2, which corresponds to Ga2O3[35]. It is noteworthy that the content of W (V) on 0.3Ga-WOx/SBA-15 raised to 24.6%, which indicates the increase of oxygen vacancy defects due to the heteroatom doping[36].

Figure 4 XPS of W 4f (a) and Ga 2p (b) binding energy spectra

2.2 Catalytic performance and reaction kinetics

The catalytic performance was evaluated by epoxidation ofcis-cyclooctene with H2O2. The catalysts of Ga-WOx/SBA-15 with molar ratio of Ga/W from 0 to 0.4 displayed unchanged selectivity (>98%) toward the target epoxides, while the conversion increased from 47.9% to 75.2% with the increase of Ga/W molar ratio from 0 to 0.3 (Figure 5). When the molar ratio was further improved to 0.4, the conversion declined to 54%, which might be attributed to that the agglomeration of Ga2O3reduces its interaction with active tungsten oxide. Thus, the 0.3Ga-WOx/SBA-15 was the optimal catalyst. In order to elucidate the role of the additive in epoxidation, WOx/SBA-15 and 0.3Ga-WOx/SBA-15 were selected for detailed investigate. The metal W content of two catalysts were measured by ICP-OES (Nexion5000), and the content was 5.6% and 5.3% for WOx/SBA-15 and 0.3Ga-WOx/SBA-15, respectively. The Ga content was 0.5% for 0.3Ga-WOx/SBA-15. Furthermore, TOFs were calculated at the conversion below 20%. The TOF of 0.3Ga-WOx/SBA-15 was 112 h-1, which was nearly two times than that for WOx/SBA-15 (57 h-1).

The profiles of the two catalysts forciscyclooctene epoxidation is shown in Figure 6,revealing that the activity of 0.3Ga-WOx/SBA-15 was always higher than that of the undoped one under the same conditions. Notably, the conversion of the two catalysts has hardly changed after 150 min, which might be due to the inhibition of water generated during the reaction process[2].

Figure 6 Profiles of the two catalysts for cis-cyclooctene epoxidation with H2O2Reaction conditions: 50 mg of the catalyst, 10 mL of CH3CN,0.4 mL of substrate and 0.4 mL of 30% H2O2 at 70 °C

In order to explore the difference of intrinsic activity between the two catalysts, the kinetic analysis was conducted. The apparent reaction activation energy(Ea) is a parameter reflecting the reaction energy barrier, and its value was obtained according to the Arrhenius equation (Figure 7(a)&(b)). The value forcis-cyclooctene epoxidation with H2O2was 49.6 kJ/mol over 0.3Ga-WOx/SBA-15, which was far lower than that over WOx/SBA-15 (64.0 kJ/mol). The results fitted well with the catalytic performance of the two catalysts obtained from experiments, consolidating the positive role of the Ga additive in epoxidation.

Figure 7 Arrhenius plots for cis-cyclooctene epoxidation with H2O2 over the two catalysts (a) and (b); rate order measurements for H2O2 (c) and cis-cyclooctene (d)Reaction conditions for (c) and (d): 0.2-0.6 mol/L H2O2, 0.28 mol/L cis-cyclooctene; 0.36 mol/L H2O2, 0.2-0.5 mol/L cis-cyclooctene, respectively

Reaction rate orders for H2O2andcis-cyclooctene were determined to understand how Ga-doped catalyst boosted the activity (Figure 7(c)&(d)). It was found that the rate order of H2O2was negative, while that ofcis-cyclooctene was positive for the two catalysts.These results demonstrated that the adsorption of H2O2on active metal surface was stronger, andciscyclooctene competed with H2O2for available sites following Langmuir-Hinshelwood mechanism[37,38]. The rate order of H2O2was -0.89 for 0.3Ga-WOx/SBA-15,which is more negative than that of WOx/SBA-15(-0.25). Whereas the rate order ofcis-cyclooctene for two catalysts was closed (1.15 vs 1.09). This trend suggested that a new surface created by doping Ga had a stronger H2O2binding site, andcis-cyclooctene was bound and activated on the surface with dense H2O2.

2.3 Effect of Ga promoter

So how does Ga promoter affect the epoxidation performance? It is well accepted that epoxidation of alkenes with H2O2undergoes the activation of H2O2and the subsequent transfer of oxygen atom[39,40].Consequently, it is reasonable to hypothesis that Gadoped catalyst improves the catalytic performance by facilitating these two steps. Firstly, Lewis acid sites are responsible for the coordination and activation of H2O2,so that the concentration of Lewis acid is positively correlated with the H2O2activation rate. FT-IR-Py was conducted to determine the acidic property of the catalysts. As shown in Figure 8(a)&(b), the peak at 1450 cm-1is related to the pyridine on Lewis acid sites,while the peak at 1540 cm-1is attributed to pyridine bound to Br?nsted acid sites[41,42]. With the increase of desorption temperature, the peak at 1450 cm-1becomes weak, indicating the Lewis acid concentration decreases. Considering that the reaction was carried out at around 70 °C (H2O2would decompose above 90 °C),Lewis acid concentration was calculated at the desorption temperature of 50 and 100 °C. For 0.3Ga-WOx/SBA-15, the Lewis acid content was 535.7 and 188.5 μmol/g at 50 and 100 °C, respectively, which was higher than that for WOx/SBA-15 (459.1 and 173.6 μmol/g) at the corresponding temperature. The boost of Lewis acid sites for 0.3Ga-WOx/SBA-15 could be attributed to the increase of oxygen vacancy on the oxide tungsten surface[43]. The higher Lewis acid concentration means a stronger ability to activate H2O2.Besides, Br?nsted acid was weak in both of the catalysts, which contributed to the inhibition of epoxide ring opening and the formation of the diol[44].

Figure 8 FT-IR-Py spectra of WOx/SBA-15 (a) and 0.3Ga-WOx/SBA-15 (b); FT-IR absorbance intensities for the Lewis-acid bound pyridine as a function of inverse temperature for WOx/SBA-15 (c) and 0.3Ga-WOx/SBA-15 (d)

If the electrophilic oxygen atom in the intermediate, generated by the activated H2O2, could not insert into the double bond of the alkenes as soon as possible, it would be consumed by the surrounding H2O2molecule to form water and oxygen[39,45]. Thus, it is important that the active center enables the ability of oxygen atom transfer matching the activation rate of H2O2. The electrophilicity (i.e., Lewis acid strength) of the active center makes the electron cloud of the O-O bond shift, facilitating the dissociation of the O-O bond; the oxygen atoms nearest to the active metal are positively charged, and easily inserts into the double bonds of olefins[7]. Here, the Lewis acid strength was quantitatively represented by pyridine adsorption enthalpy (ΔHPy) according to van’t Hoff analysis[46]. FTIR absorbance intensities for the Lewis-acid bound pyridine as a function of inverse temperature is satisfactorily liner (Figure 8(c)), where the slope is proportional to the value of ΔHPy. The value is more negative corresponding to stronger Lewis acidity. The ΔHPyvalue for 0.3Ga-WOx/SBA-15 was -25.8 kJ/mol,which is more negative than that of WOx/SBA-15(-21.5 kJ/mol), thereby boosting the oxygen atom transfer. Collectively, 0.3Ga-WOx/SBA-15 enhances the catalytic performance by promoting H2O2activation and oxygen atom transfer.

2.4 Reaction intermediates and mechanism

It’s reported that tungsten oxide-based catalysts follow the non-free radical mechanism in catalytic epoxidation of alkenes with H2O2, involving the intermediate of metal peroxide (M-(η2-O2)) or metal hydrogen peroxide (M-OOH)[47]. The identification of these two possible intermediates has been primarily performed using two approaches, namely, the product distribution from the isomeric reaction (e.g.,cisstilbene epoxidation)[48]and spectroscopic methods(e.g., UV-vis spectroscopy)[49]. Here, UV-vis spectroscopy was applied to identify the main intermediate as shown in Figure 9. Before interaction with H2O2, there are two prominent peaks at 230 and around 280 nm corresponding to isolated tetrahedron and oligomer of tungsten oxide[50], respectively. When H2O2is activated on the metal center, it would cause a red shift of the absorption edge due to the expansion of W coordination sphere[51], and the redshift of M-OOH intermediate is greater than that of M-(η2-O2)[40,52]. For the two catalysts, the absorption edge shifted from 380 to 440 nm after interaction with H2O2, indicating that H2O2has been activated on the sites and the intermediates formed. When excessive HCl was added to the reaction, there was no obvious change in the absorption edge compared with adding H2O2only,which demonstrated that the peroxide intermediates formed in the H2O2system was same to that in the acidic solution. Besides, protonation would occur under acidic conditions, so only M-OOH intermediates could exist in the acidic solution. Therefore, the primary intermediates generated during epoxidation was WOOH for both catalysts.

Figure 9 UV-vis of the catalysts under various conditions Reaction condition: 100 mg of catalyst, 5 mL of CH3CN, 0.5 mL of H2O2 and 0.1 mL of HCl (1 mol/L), if any, then stirred for 10 min, after that, the mixture was freeze-dried immediately and used for the measurement

Above all, the epoxidation mechanism was proposed (Figure 10). H2O2was absorbed on the active sites and formed the W-OOH intermediates. The electrophilic active W center facilitated the dissociation of O-O bond, and the oxygen atom closest to W, as a positive charge center, inserted into the olefin double bond to form adsorbed epoxides. Along with the desorption of the epoxides, the active sites recovered and the whole cycle was completed.

Figure 10 Mechanism for epoxidation of cis-cyclooctene with H2O2

3 Conclusions

In summary, the Ga-doped and Ga-undoped WOx/SBA-15 catalysts prepared by impregnation method were compared to clarify the role of the Ga promoter in epoxidation ofcis-cyclooctene with H2O2.The optimal catalyst of 0.3Ga-WOx/SBA-15 exhibited a TOF of 112 h-1, nearly twice as much as that of WOx/SBA-15 (57 h-1). The results of structural characterization showed that the content of oxygen vacancies on the surface of tungsten oxide increased after doping with Ga, which could induce the increase of Lewis acid concentration, facilitating the activation of H2O2. Also, the Ga promoter improved the electrophilicity of the active metal, which favored the insertion of oxygen atom in W-OOH intermediates into double bond of alkene. This work provides an avenue to enhance epoxidation and sheds light on the promoting effect of Ga promoter.