Epoxidation of 1-hexene

In this work, highly active epoxidation catalysts, which have hydrophobic surface of TS-1, were synthesized by the dry gel conversion (DGC) method. Ti-MCM-41 was synthesized first by a modifed method and the TS-l/MCM-41 catalysts were subsequently synthesized by the DGC method. The catalysts were characterized by the XRD, BET, FT-IR, and UV-VIS spectroscopy. TS-l/MCM-41 catalysts were applied to the epoxidation of 1-hexene and cyclohexene with aqueous H202to evaluate their activities for the epoxidation reaction. ... 

The catalytic activitira of synfliesized catalysts are given in Table 1. The TS-1 catalyst exhibited the highest epoxide yield and the best catalytic performance for the epoxidation of 1-hexene. The convasion of cyclohexene, however, is the lowest over TS-1. In case of TS-1/MCM-41-A and TS-1/MCM-41-B, the selectivity to epoxide is much hi er than that of Ti-MCM-41. Moreover, the conversion of 1-hexene as well as cyclohexene is found larger on the TS-l/MCM-41-Aand TS-1/MCM-41-B than on other catalysts. While the epoxide yield from 1-hexene is nearly equivalent to that of TS-1, the yield from cyclohexene is much larger than those of the otiier two catalysts. Th e results of olefins epoxidation demonstrate that the TS-l/MCM-41-Aand TS-1/MCM-41-B possess the surface properties of TS-1 and mesoporosity of a typical mesoporous material, which were evidently brou in by the DGC process. 

Titanium containing hexagonal mesoporous materials were synthesized by the modified hydrothermal synthesis method. The synthesized Ti-MCM-41 has hi y ordered hexa rud structure. Ti-MCM-41 was transformed into TS-l/MCM-41 by using the dry gel conversion process. For the synthesis of Ti-MCM-41 with TS-1(TS-1/MCM-41) structure TPAOH was used as the template. The synthesized TS-l/MCM-41 has hexagonal mesopores when the DGC process was carried out for less than 3 6 h. The catalytic activity of synthesized TS-l/MCM-41 catalysts was measured by the epoxidation of 1-hexene and cyclohexene. For the comparison of the catalytic activity, TS-1 and Ti-MCM-41 samples were also applied to the epoxidation reaction under the same reaction conditions. Both the conversion of olefins and selectivity to epoxide over TS-l/MCM-41 are found hi er flian those of other catalysts. 

SCHEME 80. Mo-catalyzed asymmetric epoxidation of 1-hexene with chiral carbohydrate... 

The oxidation of n-octane and the epoxidation of 1-hexene were performed in a 25 ml Parr reactor using 30% aqueous H2O2 as an oxidant and acetone as solvent at 100 °C and 80 °C, respectively and stirred at 500 RPM. Prior to product analysis, the product mixtures were diluted with acetone in order to obtain a single liquid-phase. The products were analyzed on a HP 5890 Series II GC equipped with a 25 m long HP-FFAP (polar) capillary column. 

The activity data confirm that an IR absorption band at 960 cm" is a necessary condition for titanium silicates to be active for the selective oxidation of hydrocarbons with aqueous H2O2 as suggested by Huybrechts et al. (9). However, this band is not a sufficient condition for predicting the activity of the TS-1 catalyst. Although TS-l(B) and TS-l(C) show intensities for the 960 cm- band similar to TS-1 (A), their activities are different First of all, the reaction data reveal that TS-1 (A) is much more active than TS-l(B) for phenol hydroxylation, while both samples show similar activity for n-octane oxidation and 1-hexene epoxidation. Therefore, the presence of the IR band at 960 cm-i in TS-1 catalysts may correlate with the activities for the oxidation of n-octane and the epoxidation of 1-hexene but not for phenol hydroxylation. However, note that the amorphous Ti02-Si02 also has an IR absorption band at 960 cm- and it does not activate either substrate. 

Figure 4.8 Catalytic properties of post-synthesized PS-Ti-MWW, directly hydrothermally synthesized HTS-Ti-MWW and TS-1 in the epoxidation of 1-hexene with H2O2.

In a study by Corma and coworkers, the rate of epoxidation of 1-hexene on Ti,Al-P matched, for a homogeneous series of solvents, the trend of adsorption. However, it was twice as fast in acetonitrile than in methanol, in contrast to partition coefficients which are ordered in the reverse direction [77, 167]. The relationship for cyclohexanol oxidation was more complex, the rate increasing with the polarity of aprotic solvents and decreasing with polarity increase in protic ones [77]. 

Epoxidation of 1-hexene was carried out as a test reaction. In a typical experiment, 0.2 g catalyst, 33 mmol alkene, 2.2 mmol hydrogen peroxide and 12 g methanol were stirred in a round bottom glass reactor at 338 K. Products were analyzed using a gas chromatograph (Hewlett Packard 5890) equipped with a capillary column (HP-1 crosslinked methylsilicone gum 50 m x 0.32 mm i.d.). 

Data obtained in the catalytic epoxidation of 1-hexene over Ox-Ti-P and other samples are summarized in Table 2. Catalytic properties of Ti-P zeolites were studied by Corma et al. [4,10] and Davis et al. [11,12]. Despite some discrepancies, it is agreed that these catalysts are active in the epoxidation of olefins. Our results also indicate that all of our Ox-Ti-P and Ti-P samples are active in the epoxidation of 1-hexene. The selectivity toward epoxide was very low. The major products were ethers, obtained from solvolysis of glycol by methanol which is catalyzed by the zeolite acid sites. It was found that over Ox-Ti-P samples, the reaction takes place slowly, while the hydrogen peroxide is utilized efficiently. Over Ti-P, the reaction takes place very rapidly and is usually finished in less than 1 hour. It was also found that the parent aluminosilicate P (sample 1) was completely inactive in this reaction. Davis et al. [12] demonstrated that framework Ti is the active site in epoxidation reactions, particularly in aqueous media. It is inferred that our catalysis data provide a strong evidence that Ti(IV) species in our Ox-Ti-P samples are present as isolated framework cations. 

Materials effective as anode catalysts for epoxidation of 1-hexene by the method in Figure 2 were screened. Among various metal oxides, metal salts and metal blacks tested, the most active and selective anode catalyst for the formation of 1,2-epoxy hexane was Pt black (Table 1). The oxidation efficiency for the formation of epoxide defined by equation 9 was about 26% and its selectivity was 66%. Pt black samples obtained from different producers or prepared in this work showed quite low electrocatalytic activity. However, the calcination of these inactive Pt blacks in air at 673 K substantially enhanced the catalytic activities of these samples. XPS studies on various Pt black samples suggested that a Pt02 phase was associated with the active oxygen for the epoxidation. 

Table 1. Epoxidation of 1-hexene on various anodes of noble metals...

The Pt black sample active for the epoxidation of 1-hexene and 2-hexenes was also tested in the oxidation of propylene. Figure 5 shows the results of propylene oxidation as a function of the applied voltage across the cell. The oxidation of propylene was initiated at an applied voltage higher than ca. 1.1 V. The formation of propylene oxide and acetone were remarkably enhanced at an applied voltage >1.1 V. The maximum oxidation efficiency for the propylene oxide was 25% and the selectivity to propylene oxide was 53% at 1.7 V. These results indicate that the epoxidation of propylene by the oxidative activation of H O proceeds with fairly good current efficiency and selectivity on the Pt black anode [17]. 

Here, we give an example for the synthesis of Ti-containing mesoporous silicas. This kind of catalyst has abilities for the selective oxidation of olefin and other unsaturated compounds, such as the epoxidation of 1-hexene, cyclohexene, and styrene [61,62]. The ratio of Si/Ti, structure, and hydrophobic nature of the material are the three most important factors in the catalytic activity. Therefore, a full understanding of the synthesis is necessary. 

The catalytic properties of post-synthesized PS-Ti-MWW were compared with directly hydrothermally synthesized HTS-Ti-MWW and TS-1 in the epoxidation of 1-hexene with H2O2 (Figure 4.8). For reasonable comparison, the reactions were carried out in the most suitable solvents for the two titanosilicates - in acetonitrile for Ti-MWW and in methanol for TS-1. HTS-Ti-MWW showed much higher intrinsic adivity than TS-1 for 1-hexene. PS-Ti-MWW further proved to be about twice as adive as HTS-Ti-MWW. The efficiency of H2O2 utilization was also very high on PS-Ti-MWW. Thus, in terms of the activity, epoxide selectivity and H2O2 efficiency, PS-Ti-MWW has so far been a most effident heterogeneous catalyst for liquid-phase epoxidation of linear alkenes. 

Scheme 11.10 Selective epoxidation of 1-hexene by Mn-tmtacn using H2O2 in the presence of oxalate [94d].

Recently, the SMO from Rhodococcus sp. ST-5 and ST-10 have been applied to catalyze the epoxidation of 1-hexene and 1-octene to the corresponding (S)-epoxides with up to 99% ee [58]. For 1-hexene, the activity was comparable to that acquired with the native substrate styrene [58], but the initial conversion rate was four times lower than styrene [92]. The excellent (S)-stereoselectivity is a complementary to the (R)-selectivity of majority of the enzymes involved in the asymmetric epoxidation of aliphatic alkenes. 

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