Styrene epoxidation catalyzed by polyoxometalate/quaternary ammonium phase transfer catalysts: The effect of cation size and catalyst deactivation mechanism

Catalytic epoxidation of alkenes is an important type of organic reaction in chemical industry, and the deep insight into catalyst deactivation will help to develop new epoxidation process. In this work, series of quaternary ammoniums bearing different cationic sizes, i.e. MTOA' (methyltrioctylammonium, [(C8H17)3CH3N]'), HTMA'(hexadecyltrimethylammonium, [(C16H33)(CH3)3N]') and DMDOA' (dimethyl-dioctadecylammonium, [(C18H37)2(CH3)2N]') were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs), which were used in the styrene epoxidations. Among them, (MTOA)3PW4O24 exhibits the best catalytic performance judged from the highest styrene conversion rate (52%) and styrene oxide selectivity (93%), during which the styrene epoxidation conditions were opti-mized. Meanwhile, the deactivation mechanism of this kind of PTCs was proposed firstly, i.e. in the case of low H2O2 content, the oxidant can only be used in the styrene epoxidation, in which the catalyst can transform into stable Keggin-type POM. But when the content of H2O2 is higher, the excess H2O2 can re-activate the Keggin-type POM into active (PW4O24)3-anions, which can trigger the ring-opening poly-merization of styrene oxide. Consequently, the catalyst is deactivated by adhered poly(styrene oxide) irreversibly, which was determined by NMR spectra. In this situation, the active moiety {PO4[WO (O2)2]4}3-in phase-transfer catalytic system can break into some unidentified species with low W/P ratio with the presence of epoxides. This work will be beneficial for the design of new PTCs in alkene epoxi-dation in fine chemical industry.(c) 2022 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.

Automated summary

This work synthesized a series of quaternary ammonium compounds with different cationic sizes and incorporated them with polyoxometalate anions to prepare phase transfer catalysts (PTCs) for styrene epoxidation. Among them, (MTOA)3PW4O24 exhibited the best catalytic activity with a styrene conversion rate of 52% and a styrene oxide selectivity of 93%. The deactivation mechanism of the PTCs was proposed, and this work is significant for the design of new PTCs in alkene epoxidation.