Effect of Metal Ion Modified TS-1 on Cyclization Properties of 6-Hydroxyhexanoic Acid

Abstract

In this study, Titanium-silicalite-1 (TS-1) samples modified by different metal ions to form M-TS-1 (M = Mg²⁺, Zn²⁺, Co²⁺, with a load of 5%) were prepared by equal volume impregnation method. The physical properties of the samples were characterized by FT-IR, N₂ adsorption–desorption, XRD, UV–vis. The catalysts were applied to the cyclization reaction of 6-hydroxyhexanoic acid, and the effects of catalyst type, reaction temperature, catalyst dosage and reaction pressure on the yield of ε-caprolactone were investigated. The experimental results showed that the catalytic capacity of catalysts modified by different metal ions are different. The addition of Mg²⁺ and Co²⁺ increased the pore size of TS-1, provided more catalytic active sites and accelerated the mass transfer of 6-hydroxycaproic acid. The yield of ε-caprolactone modified by Mg²⁺ and Co²⁺ increased by 6.88% and 13.93%, respectively. The addition of Zn²⁺ reduced the specific surface area and pore size of TS-1, and the yield of ε-caprolactone decreased by 3.12%. This study showed that under the relatively optimized process conditions (the temperature of 300 °C, the reaction pressure of 1.3 kPa and the catalyst dosage was 5%) the yield of ε-caprolactone of TS-1 modified by Co²⁺ was up to 74.5%.The process developed in this study has promising application potential in the industrial production of ε-caprolactone.

Graphic Abstract

TS-1 modified by metal ions prepared by equal volume impregnation method should be used in the cyclization reaction of 6-hydroxycaproic acid, and ε-caprolactone can be obtained with high yield. The reaction mechanism is that the hydroxyl and hydrogen in 6-hydroxycaproic acid are unstable in the catalyst, and it is easy to break and combine with water to remove and form ε-caprolactone. Due to the high reaction temperature, some oligomers are formed, and its reaction equation is as follows.