**4. The studies on the epoxidation of AGE over the Ti-SBA-15 catalyst and the comparison of the results obtained with the previous results obtained for Ti-MWW and TS-1 catalysts**

In the epoxidation of AGE, the following raw materials were used: AGE (99%, Aldrich), hydrogen peroxide (60 wt% water solution, Chempur), and Ti-SBA-15 catalyst. The initial technological parameters were as follows: the molar ratio of AGE/H2O2 = 1:1, catalyst content 3 wt%, the reaction time 2 h, and intensity of stirring 500 rpm.

The process of AGE epoxidation was carried out in glass vials with the capacity of 12 cm3 equipped with a rubber septum and a capillary. The raw materials were introduced into the vials at the ambient temperature and in the following order: hydrogen peroxide, catalyst, and AGE. Then the vials were closed with the rubber equipped with the capillary, located in a shaker holder and immersed in a water bath having the appropriate temperature. In order to calculate the mass balance, the unreacted hydrogen peroxide was determined by iodometric method and the remaining products and the unreacted AGE were analyzed by the GC method. The analyses were performed on the FOCUS apparatus with a flame-ionization detector fitted with Quadrex capillary columns filled with methyl-phenyl-siloxanes. After calculating the mass balance, the main functions describing the process were determined: the selectivity of transformation to DGE in relation to AGE consumed and also selectivities of the by-products in relation to AGE consumed, the conversion of AGE, and the selectivity of transformation to organic compounds in relation to hydrogen peroxide consumed (effective conversion of H2O2).

The studies on the epoxidation of AGE to DGE was carried out by one-variable method, changing the values of the following parameters: temperature 0–100°C, molar ratio of AGE/H2O2 0.03:1–4:1, content of the Ti-SBA-15 catalyst (0–5 wt%), and reaction time 15–240 min. The main results of the studies on the influence of temperature on the course of AGE epoxidation were presented in Figure 1.

During the studies on the influence of temperature, only three products were obtained: DGE, 3-allyloxy-1, 2-propanediol (3A12PD), and glycerol. The selectivity of transformation of AGE to the product of epoxidation of the unsaturated bond – DGE, increases during the increase of the temperature from 0 mol% (the temperature of 0°C) to 38 mol% (the temperature of 20°C) and then decreases to 9 mol% (the temperature of 100°C). Figure 1 shows that DGE is not the main product of this process, because for all investigated temperatures the main product of the process is 3A12PD (with exception at the temperature of 0°C at which the reaction does not proceed). The selectivity of this products changes from about 53–54 mol% (the tempera‐ tures 10–20°C) to 80–90 mol% (the temperatures 40–100°C).. The selectivity of glycerol amounts to 13 mol% for the temperature of 10°C, and then decreases to about 2 mol% (for the temper‐ atures of 40–100°C). For the description of the process of AGE epoxidation, the reactions presented in Figure 2 can be proposed.

**Figure 1.** The influence of temperature on the selectivities of the products of AGE epoxidation process: SDGE – the selec‐ tivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of glycerol (the molar ratio of AGE/H2O2 = 1:1, the content of the catalyst 3 wt%, and the reaction time 120 min).

**Figure 2.** The main reactions of the process of AGE epoxidation.

Figure 2 shows that the process can proceed in the three directions: (1) the epoxidation of allylic group in AGE and formation of DGE, (2) the hydration of the epoxide ring in AGE and formation of 3A12PD, and (3) the formation of glycerol. Moreover, the two first directions are the main directions of the process in low temperatures. The glycerol formation is a very complicated process and this product can be formed as a product of hydrolysis of the ether groups of AGE, DGE, 3A12PD, and, simultaneously, as a result of the secondary reactions of the products of the hydrolysis of these ethers (epoxidation of allylic group in allyl alcohol to glycidol and next hydration of the epoxide ring in glycidol to glycerol). Allyl alcohol and glycidol were not detected in post-reaction mixtures. It shows that these compounds were very reactive at the investigated conditions and right away underwent secondary reactions. The tendency towards the formation of 3A12PD rises during increasing the temperature of the performing process. On the other hand, the selectivities of the DGE and glycerol decrease. It shows that at higher temperatures the epoxidation of AGE to DGE is hindered and the hydrolysis of the ether groups in AGE, DAE, and 3A12PD is stepped or it proceeds very slowly. The main reaction is hydration of the epoxide ring in AGE and formation of 3A12PD. The formation of 3A12PD as the main product in this process can be explained taking into account the acidic character of the mesoporous Ti-SBA-15 material. This character is mainly connected with: (1) the silanol groups located on the surface of this catalyst [19, 38], (2) the species of Ti present on the surface of the catalyst – tetrahedral Ti(IV) active sites, titanium-containing species in the form of dimmers or very small oligomers [38–41], and (3) the active species of titanium with hydrogen peroxide which are formed during the oxidation process, for example, five-membered active complexes – titanium hydroperoxo species with the specific structure, which are formed from tetrahedral Ti(IV) active sites, protic solvent (for example, methanol or water), and hydrogen peroxide and are present on the surface of the catalyst [42].

Among others, the formation of 3A12PD can also be under influence of the active species of titanium with hydrogen peroxide which are formed during the oxidation process and some of them can have the acidic character. A few structures have been proposed until now for explanations of the structures of these active species. Among these structures are: the peroxide structure, the hydroperoxide structure, and superoxide structures (radical species) [40, 42– 49]. Indeed, mainly the hydroperoxide structure was described as the structure responsible for the epoxidation of olefinic compounds [45, 47]. It exists in equilibrium with the peroxide structure, which is a dominant structure in the water solution because it is stabilized by water molecules [45]. In the medium in which epoxidation takes place, the excess of olefins causes that the peroxide structure is converted to hydroperoxide structure [45]. Hydroperoxide structures in the presence of protic solvent create the five-membered active complexes – titanium hydroperoxo species which are composed from tetrahedral Ti(IV) active sites, protic solvent (in case of these studies, from water), and hydrogen peroxide[40, 42, 50, 51].

Bhaumik et al. [42] described that under the influence of the titanium hydroperoxo species undergoes acid-catalyzed cleavage of the oxirane rings in the epoxide compounds; this reaction has considerable SN1 character and the nucleophilic attack is easy to occur at the more crowded carbon atom that can best accommodate the positive charge. Taking into account that this data can be propose the possible way of 3A12PD formation from the AGE presented in Figure 3.

**Figure 2.** The main reactions of the process of AGE epoxidation.

**Figure 1.** The influence of temperature on the selectivities of the products of AGE epoxidation process: SDGE – the selec‐ tivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of glycerol (the molar

ratio of AGE/H2O2 = 1:1, the content of the catalyst 3 wt%, and the reaction time 120 min).

128 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 3.** The possible way of 3A12PD formation from the AGE, where: R = CH2=CH-CH2-.

Also the hydration of the ether groups in AGE, DGE, and 3A12PD can be explained taking into account the acidic character of various species which are present in the mesoporous Ti-SBA-15 material (silanol groups, species of Ti and titanium hydroperoxo species).

During the studies on the influence of temperature, the conversion of AGE was very low and it changes from 0 mol% (the temperature of 0°C) to about 4–5 mol% for the highest tempera‐ tures. The changes of the effective conversion of H2O2 are very similar. Very low values of this function show that at the studied conditions the catalyst was very active in the ineffective decomposition of hydrogen peroxide to water and oxygen, which takes place at the active centres of Ti in the catalyst even at very low temperatures (the total conversion of hydrogen peroxide changed from 83–90 mol%). The ineffective decomposition of hydrogen peroxide over titanium silicate catalysts has been described in a great number of works [52–57], and this phenomen‐ on is typical for these catalysts, for example, it was shown in the literature that titanium hydroperoxo species can decompose hydrogen peroxide molecules via formation of Ti-O\* radical and hydroperoxo radical (HOO\*) [52]. As the results presented in this work showed, the Ti-SBA-15 catalyst was very active in the decomposition of hydrogen peroxide. The epoxidation of olefinic bonds undergoes slower than the ineffective decomposition of hydro‐ gen peroxide at the five-membered active species. Probably, very small reactivity of the AGE is connected with the steric hindrances connected with the structure of this ether when the molecules of AGE are close to the active species of Ti with hydrogen peroxide. These steric hindrances cause that the decomposition of AGE molecules also takes place. The increased, ineffective decomposition of hydrogen peroxide can be also caused by the presence of TiO2 domains in the structure of the Ti-SBA-15 catalyst. Taking into account the UV-VIS spectrum of the Ti-SBA-15 catalyst used in this work, the broad absorption peak at the 211 cm–1 and the shoulder at the 290 cm–1, which is not only connected with the presence of Ti atoms in fivefold and sixfold coordination [36] but also can be assigned to the oligomerized titanium-oxygen species – formation of Ti-O-Ti bonds by clusterization of octahedrally coordinated titanium ions [46, 58] or to octahedral titanium species in the form of highly dispersed TiO2 particles with the particle size smaller than 5 nm [48]), and the results of the X-Ray microanalysis (amount of Ti 2.9 wt%), it can be assumed that the Ti-SBA-15 catalyst contains the titanium-oxygen species in the form of dimmers or very small oligomers (TiO2 domains, Ti aggregates) [53, 55].

A lot of works present the strategies to increase the oxidant efficiency. The hydrogen peroxide decomposition is strongly dependent on the pH of the reaction mixtures and on the surface concentration of the hydroxyl groups of the catalytic material [52]. The improving of the efficiency of the hydrogen peroxide conversion can be done by: (1) addition of additives such as for example: CH3COOH, KHSO4, KH2PO4, KHF2, Na2SO4, NaHCO3, K2CO3, K3PO4, K2HPO4, or KH2PO4 [52]; (2) slow addition of hydrogen peroxide [52, 55, 57]; (3) choosing of the appropriate solvent for the epoxidation process – the most beneficial are methanol, acetonitrile, and acetone [44, 54, 55] or co-solvent, for example, sulfolane [59]; (4) increasing of the acidity of the catalyst by the addition of metal oxide, for example, of TiO2, and utilization of the appropriate temperature of the calcination [56]; and (5) the surface hydrophobization of mesoporous titanium silicates [46]. We would like to test in our future work some of the ways of improving the efficiency of hydrogen peroxide conversion: choosing the appropriate solvent, additives, and slow addition of hydrogen peroxide.

Taking into account the results of the studies on the influence of temperature on the course of AGE epoxidation, the temperature of 20°C was taken as the most beneficial for the next studies.

**Figure 3.** The possible way of 3A12PD formation from the AGE, where: R = CH2=CH-CH2-.

130 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Also the hydration of the ether groups in AGE, DGE, and 3A12PD can be explained taking into account the acidic character of various species which are present in the mesoporous Ti-

During the studies on the influence of temperature, the conversion of AGE was very low and it changes from 0 mol% (the temperature of 0°C) to about 4–5 mol% for the highest tempera‐ tures. The changes of the effective conversion of H2O2 are very similar. Very low values of this function show that at the studied conditions the catalyst was very active in the ineffective decomposition of hydrogen peroxide to water and oxygen, which takes place at the active centres of Ti in the catalyst even at very low temperatures (the total conversion of hydrogen peroxide changed from 83–90 mol%). The ineffective decomposition of hydrogen peroxide over titanium silicate catalysts has been described in a great number of works [52–57], and this phenomen‐ on is typical for these catalysts, for example, it was shown in the literature that titanium hydroperoxo species can decompose hydrogen peroxide molecules via formation of Ti-O\* radical and hydroperoxo radical (HOO\*) [52]. As the results presented in this work showed, the Ti-SBA-15 catalyst was very active in the decomposition of hydrogen peroxide. The epoxidation of olefinic bonds undergoes slower than the ineffective decomposition of hydro‐ gen peroxide at the five-membered active species. Probably, very small reactivity of the AGE is connected with the steric hindrances connected with the structure of this ether when the molecules of AGE are close to the active species of Ti with hydrogen peroxide. These steric hindrances cause that the decomposition of AGE molecules also takes place. The increased, ineffective decomposition of hydrogen peroxide can be also caused by the presence of TiO2 domains in the structure of the Ti-SBA-15 catalyst. Taking into account the UV-VIS spectrum of the Ti-SBA-15 catalyst used in this work, the broad absorption peak at the 211 cm–1 and the shoulder at the 290 cm–1, which is not only connected with the presence of Ti atoms in fivefold and sixfold coordination [36] but also can be assigned to the oligomerized titanium-oxygen species – formation of Ti-O-Ti bonds by clusterization of octahedrally coordinated titanium ions [46, 58] or to octahedral titanium species in the form of highly dispersed TiO2 particles with the particle size smaller than 5 nm [48]), and the results of the X-Ray microanalysis (amount of Ti 2.9 wt%), it can be assumed that the Ti-SBA-15 catalyst contains the titanium-oxygen species in

SBA-15 material (silanol groups, species of Ti and titanium hydroperoxo species).

the form of dimmers or very small oligomers (TiO2 domains, Ti aggregates) [53, 55].

A lot of works present the strategies to increase the oxidant efficiency. The hydrogen peroxide decomposition is strongly dependent on the pH of the reaction mixtures and on the surface concentration of the hydroxyl groups of the catalytic material [52]. The improving of the

The main results of the studies on the influence of the molar ratio of AGE/H2O2 on the course of AGE epoxidation were presented in Figure 4. The studies were performed at the range of molar ratios of AGE/H2O2 0.03:1 – 4:1. The other parameters were as follows: the temperature of 20°C, the content of the catalyst 3 wt%, and the reaction time 120 min.

**Figure 4.** The influence of molar ratio of AGE/H2O2 on the selectivities of the products of AGE epoxidation process: SDGE – the selectivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of glyc‐ erol (the temperature 20°C, the content of the catalyst 3 wt%, and the reaction time 120 min).

The studies show that the conversion of AGE was the highest for the lowest molar ratio of reactants and amounted of 11 mol% and next it decreased to 1 mol% for the molar ratio of AGE/H2O2 = 4:1. The effective conversion of H2O2 had very low values independent of the studied molar ratios, even for the molar ratios of AGE/H2O2 > 1. Figure 4 shows that inde‐ pendent of the molar ratio of reactants the main product of the process was 3A12PD, but its selectivity decreased during increasing the molar ratio of reactants. It shows that the excess of hydrogen peroxide or AGE molecules in the reaction mixture does not cause that the epoxi‐ dation of AGE is intensified and the hindering of the hydration of epoxide ring in AGE is not observed. The ethers molecules are unstable in reaction medium and underwent decomposi‐ tion by the hydration of the ether groups. Simultaneously, the results obtained show that the surface of the catalyst independent of the molar ratio of reactants was very active in the reactions of hydration of epoxide rings and ether groups. Also the formed allyl alcohol and glycidol undergo secondary reactions (epoxidation and hydration of the epoxide ring) very easily. On the basis of the results obtained, the molar ratio of AGE/H2O2 = 0.03 was taken as the most beneficial for the next stages of the studies.

The main results of the studies on the influence of the Ti-SBA-15 catalyst content on the course of AGE epoxidation were presented in Figure 5. These studies were performed for the following parameters: the temperature 20°C, the molar ratio AGE/H2O2 = 0.03, and the reaction time of 120 min.

**Figure 5.** The influence of the Ti-SBA-15 catalyst content on the selectivities of the products of AGE epoxidation proc‐ ess: SDGE – the selectivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of glycerol (the temperature 20°C, the molar ratio of AGE/H2O2 = 0.03, and the reaction time 120 min).

The studies show that for the amount of the catalyst of 0 wt% no one reaction proceeded. The conversion of AGE raised from 0 mol% (for 0 wt% of Ti-SBA-15) to 11 mol% (for 3 wt% of Ti-SBA-15) and next did not change. The effective conversion of H2O2 was very low and amounted to about 1 mol%, independent of the studied Ti-SBA-15 content.

Figure 5 shows that during the rising of the content of the catalyst the selectivity of 3A12PD increased from 0 mol% (the Ti-SBA-15 content 0 wt%) to 78 mol% (the Ti-SBA-15 content 5 wt %). It presents that with the increase of the Ti-SBA-15 content, the hydration of the epoxide ring in AGE was intensified, but the hydrolysis at the ether group of AGE, DGE, and 3A12PD was hindered. Only for the Ti-SBA-15 content of 0.5 and 1 wt%, glycerol was present in the post-reaction mixtures. On the other hand, at higher catalyst content, the phenomenon of ineffective decomposition of hydrogen peroxide at the active centers of Ti in the catalyst was intensified and the water molecules obtained during ineffective decomposition of hydrogen peroxide could probably take part in hydration of the epoxide ring in AGE. The content of the catalyst amounting to 3 wt% was chosen as the most beneficial for the last stage of the studies.

observed. The ethers molecules are unstable in reaction medium and underwent decomposi‐ tion by the hydration of the ether groups. Simultaneously, the results obtained show that the surface of the catalyst independent of the molar ratio of reactants was very active in the reactions of hydration of epoxide rings and ether groups. Also the formed allyl alcohol and glycidol undergo secondary reactions (epoxidation and hydration of the epoxide ring) very easily. On the basis of the results obtained, the molar ratio of AGE/H2O2 = 0.03 was taken as

The main results of the studies on the influence of the Ti-SBA-15 catalyst content on the course of AGE epoxidation were presented in Figure 5. These studies were performed for the following parameters: the temperature 20°C, the molar ratio AGE/H2O2 = 0.03, and the reaction

**Figure 5.** The influence of the Ti-SBA-15 catalyst content on the selectivities of the products of AGE epoxidation proc‐ ess: SDGE – the selectivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of

The studies show that for the amount of the catalyst of 0 wt% no one reaction proceeded. The conversion of AGE raised from 0 mol% (for 0 wt% of Ti-SBA-15) to 11 mol% (for 3 wt% of Ti-SBA-15) and next did not change. The effective conversion of H2O2 was very low and amounted

Figure 5 shows that during the rising of the content of the catalyst the selectivity of 3A12PD increased from 0 mol% (the Ti-SBA-15 content 0 wt%) to 78 mol% (the Ti-SBA-15 content 5 wt %). It presents that with the increase of the Ti-SBA-15 content, the hydration of the epoxide ring in AGE was intensified, but the hydrolysis at the ether group of AGE, DGE, and 3A12PD was hindered. Only for the Ti-SBA-15 content of 0.5 and 1 wt%, glycerol was present in the post-reaction mixtures. On the other hand, at higher catalyst content, the phenomenon of ineffective decomposition of hydrogen peroxide at the active centers of Ti in the catalyst was intensified and the water molecules obtained during ineffective decomposition of hydrogen

glycerol (the temperature 20°C, the molar ratio of AGE/H2O2 = 0.03, and the reaction time 120 min).

to about 1 mol%, independent of the studied Ti-SBA-15 content.

the most beneficial for the next stages of the studies.

132 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

time of 120 min.

The main results of the studies on the influence of the reaction time on the course of AGE epoxidation were presented in Figure 6. These studies were performed for the following parameters: the temperature 20°C, the molar ratio AGE/H2O2 = 0.03, and the Ti-SBA-15 catalyst content 3 wt%.

**Figure 6.** The influence of the reaction time on the selectivities of the products of AGE epoxidation process: SDGE – the selectivity of DGE, S3A12PD – the selectivity of 3-allyloxy-1, 2-propanediol, and Sglycerol – the selectivity of glycerol (the temperature 20°C, the molar ratio of AGE/H2O2 = 0.03, and the Ti-SBA-15 catalyst content 3 wt%).

The results show that with the prolongation of the reaction time from 15 min to 240 min, the selectivity of DGE decreased from 100 mol% to 25 mol%. Only for the reaction time of 15 min and 50 min, it was possible to obtain only DGE as the product in the post-reaction mixtures. The conversion of AGE increased in the range of the studied reaction time from 3 mol% to 18 mol%, but the effective conversion of H2O2 was very low and amounted to about 1 mol%. Figure 6 presents that for the reaction time of 120 min the second product of this process was established – 3A12PD. Glycerol – the third product of this process appeared in the post-reaction mixture for the reaction time of 240 min. It shows that it is possible to obtain only one product in the post-reaction mixture – DGE (very desirable) only for short reaction time – 15 min. and 60 min. At this reaction time, it is only possible to stop the hydration of the epoxide ring and formation of 3A12PD.

The comparison of our results obtained for the Ti-SBA-15 material with the results obtained previously by Wu et al. for the Ti-MWW and TS-1 materials [19] shows that the main difference between the epoxidation of AGE over Ti-SBA-15 and the epoxidation of DAE and AGE over Ti-MWW and TS-1 is formation of the considerable amounts of 3-allyloxy-1, 2-propanediol over Ti-SBA-15 and low efficiency of hydrogen peroxide conversion for this catalyst. It is probably connected with the pore size of the Ti-SBA-15 mesoporous material and the structure of the surface of this catalyst, especially with the presence of various species of Ti and hydroxyl groups.
