H 2 O 2 is the most frequently used oxidant in recent years. By oxidizing organic acids to peroxyacids, the peroxyacids epoxidize the double bonds of olefins, and finally hydrolyzed to obtain diols. The reaction mechanism is shown in Figure 2.
The advantage of H 2O2 oxidation method is that H 2O2 is of good quality and low price, and after being reduced, water is generated, and the environmental pollution is small, which is suitable for industrial production requirements. The research on H 2O2 oxidation of alkene to synthesize diol is earlier, while the research on H 2O2 oxidation of 1-hexene to synthesize DL-1,2-hexanediol is late. The latter can learn from the former method, but due to the two The differences in the physical and chemical properties of the catalysts lead to large differences in the use of epoxidation methods and catalysts.
Among the published articles or patents that use hydrogen peroxide as an oxidant, some do not add catalyst. For example, Li Wen et al. directly used hydrogen peroxide and formic acid to synthesize DL-1,2-hexanediol, dropwise for 10 hours, and kept the reaction for 24 hours. After that, DL-1,2-hexanediol was obtained, with a yield of about 75% and a purity of 99%; some catalysts were added, such as Yoshimaru Toshihiko and others reacted with hydrogen peroxide and formic acid at 30 °C for 10 h to synthesize butyl ethylene oxide, and then used p-toluenesulfonic acid. Acid was used as catalyst, methanol was used as solvent, and the reaction was carried out at 55 °C for 7 h. The final post-treatment obtained DL-1,2-hexanediol with a yield of about 92% and a purity of 92%.
It can be seen that the reaction time is shortened and the selectivity is improved in the H 2O2 oxidation method after adding the catalyst, so the catalyst research of this reaction has become a hot direction. In most articles, the catalysts used in the epoxidation of 1-hexene to butyl ethylene oxide are mostly oxides or complexes of transition metals. For example, Wei Dongchu et al. used metal oxides V2 O5 /Nb2O5 as catalysts. , tert-butanol was used as solvent, 30% hydrogen peroxide was used as oxidant, and the yield of DL-1,2-hexanediol was 85%. Such catalysts are generally homogeneous catalysts, which are difficult to recycle from the system, increase the difficulty of post-processing, and increase the cost.
In order to solve this problem, scholars have done a lot of research on the immobilization method of homogeneous catalysts. It is supported on the solid-supported ionic liquid to make the homogeneous catalyst heterogeneous, which is convenient for the recycling of the catalyst. Li Bindong et al. used polystyrene resin-supported heteropolyacid as a phase transfer catalyst to synthesize DL-1,2-hexanediol from 1-hexene, the yield was 94%, and the purity was 98%.
Xu Hao et al. expanded the MWW borosilicate molecular sieve and reacted with dimerized silane to obtain a molecular sieve with a large interlayer spacing, and then cross-linked with titanium to obtain a molecular sieve catalyst Ti-MWW. The 1-hexene was converted into 54.2%, the epoxidation selectivity was 96%, and the rest were 1,2-hexanediol. Song Xiaojing et al. carried out selective oxidation of alkenes by phosphomolybdic acid-modified braided aryl network polymers (PMA/KAP and PMA-(PPh 3 ) 3 ) in hydrogen peroxide solution, and the conversion rate was 60%-70%. The oxidation selectivity was 99%, the latter epoxidation selectivity was 34%, and the remaining 66% product was DL-1,2-hexanediol.
SahaDebraj et al. used metal-organic frameworks (MOFs) as catalysts and hydrogen peroxide as the oxidant to react 1-hexene to obtain the target product at a lower temperature, in which the Cu and alkaline earth metal active centers of the catalyst acted in turn in epoxidation and oxidization. In the two-step reaction of epoxide to alcohol, the product yield is 100%, and the catalyst can be recycled after treatment with little effect on catalytic activity. Shi Xianying et al. used SiO2 as a carrier to immobilize the ionic liquid, and then supported the catalytic active center such as peroxophosphotungstate catalyst, and added 30% hydrogen peroxide to epoxidize 1-hexene, etc. The yield of 1,2-hexanediol was 81%. The catalyst can be recovered 8 times without significant decrease in the reaction yield. Other catalysts are ferric chloride, lipase, etc.
For example, Jinyintai from South Korea added ferric toluenesulfonate hexahydrate or ferric chloride aqueous solution to the mixture of 1-hexene, 30% hydrogen peroxide and formic acid, reacted for 4 to 6 h, and distilled to obtain a yield of more than 96% and GC purity. More than 99% DL-1,2-hexanediol. Sarma Kuladip et al. used a lipase as catalyst, 50% hydrogen peroxide as oxidant, ethyl acetate as solvent, and reacted under microwave irradiation for 10 min, and the yield of DL-1,2-hexanediol was 72%. The advantage of the solid-supported catalyst is that it is beneficial to recovery, but there are also some problems. For example, the stability of the solid-supported catalyst has not yet reached an ideal level and needs to be further strengthened.