A catalytic ZSM-5 membrane sandwiched with silicalite-1 layers for highly selective toluene disproportionation

A catalytic zeolite membrane composed of silicalite-1, ZSM-5 and silicalite-1 layers supported on a porous stainless support was prepared by hydrothermal synthesis (silicalite-1/ZSM-5/silicalite-1 membrane). The permeance of hydrogen, argon, nitrogen, n-butane and isobutane for silicalite-1/ZSM-5/silicalite-1 membrane was measured from 353 to 513 K. The permeance of these gases were from 5 to 25 × 10−8 mol/(m2 s Pa) the same range as typical MFI-type membranes and the ratio of the permeance of n-butane to isobutane was 2 or less. The toluene disproportionation was examined at 673 K with silicalite-1/ZSM-5/silicalite-1 membrane as a membrane reactor as well as ZSM-5/silicalite-1 membrane and ZSM-5 powder. The products detected were benzene, p-, m- and o-xylene. The toluene conversion with two membrane reactors was less than 3%. At lower than 1% of toluene conversion, the selectivity of p-xylene for silicalite-1/ZSM-5/silicalite-1 was above 96% whereas that for ZSM-5/silicalite-1 membrane was below 89%. The silicalite-1 layers sandwiching the ZSM-5 layer probably suppressed the surface reaction on both ZSM-5 layers, which would achieve the significantly high selectivity of p-xylene in toluene disproportionation.