Excess Methylbenzene

A major disadvantage of the reforming of naphtha is that the proportions of methylbenzene and dimethylbenzene produced do not match the demands Ð there is too much methylbenzene, and not enough dimethylbenzene.

Many sites therefore incorporate a disproportionation plant, which converts methylbenzene into the dimethylbenzenes and benzene.

All three isomers of dimethylbenzene are produced, though it is the 1,4-dimethylbenzene isomer (shown in the equation) that is most useful.

The process

Methylbenzene and hydrogen are passed over a zeolite catalyst at 15 -25 atmospheres and at a temperature of 430 -480°C. Further information on zeolite catalysts can be found on the catalysis site.

Any benzene, methylbenzene and dimethylbenzenes are separated by fractional distillation. The methylbenzene can be returned to the disproportionation unit, whilst benzene and the dimethylbenzenes removed.

Excess 1,3- dimethylbenzene can be converted to the more useful 1,4- isomer using the Zeolite catalyst ZSM-5. It is thought this occurs via an acid catalysed rearrangement within the pores of the Zeolite structure, where the shape of the 1,4-isomer is a "better fit".


New Catalysts

A new catalyst has been developed that allows methylbenzene to enter, but only the 1,4- dimethylbenzene isomer can escape. The other isomers remain trapped until they are isomerised to the 1,4-isomer.

The catalyst is based on MCM-41 (Mobile Crystalline Material). This is one of a group of silicates that can be constructed to contain open hexagonal channels, and they are used as catalyst supports, catalysts (with modification) or as molecular seives. Changing the length of the molecules that are used to build the sides of the hexagon can control the diameter of the hexagonal openings, and so the channels, or "pores", can be made from 2 to 10nm wide.

By adding a "gate" at the end, the size of molecule that can enter and leave the pores is restricted. In this case the opening is set so that any 1,2- or 1,3- isomers of dimethylbenzene will not be able to leave the pores, but the 1,4- isomer will.
Disproportionation of Methylbenzene

Question

Why is the use of a catalyst that only produces the 1,4 isomer of dimethylbenzene a "more green" process?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  back to top