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

[(C

8

H

17

)

3

CH

3

N

]

+

)

HTMA

+

(hexadecyltrimethylammonium

[(C

16

H

33

)(CH

3

)

3

N

]

+

) and DMDOA

+

(dimethyldioctadecylammonium

[(C

18

H

37

)

2

(CH

3

)

2

N

]

+

) were incorporated with polyoxometalate (POM) anions to prepare phase transfer catalysts (PTCs)

which were used in the styrene epoxidations. Among them

(MTOA)

3

PW

4

O

24

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 optimized. Meanwhile

the deactivation mechanism of this kind of PTCs was proposed firstly

i.e.

in the case of low H

2

O

2

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 H

2

O

2

is higher

the excess H

2

O

2

can re-activate the Keggin-type POM into active (PW

4

O

24

)

3-

anions

which can trigger the ring-opening polymerization 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 {PO

4

[WO(O

2

)

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 epoxidation in fine chemical industry.