Process for preparing alkyl mercaptans in a multizone fixed bed reactor

1. A process for preparing alkyl mercaptans by a catalytic gas phase reaction of alkanols and hydrogen sulphide reactants over an alkali metal tungstate, said reactants being introduced in at least two successive reaction zones which contain different catalysts, such that an activity with respect to mercaptan formation of a first catalyst used in a first reaction zone is lower than an activity with respect to mercaptan formation of a second catalyst used in a second reaction zone or any subsequent reaction zones, and there is no feeding of the reactants between reaction zones.

2. Process according to claim 1 , in which the catalyst in the second zone catalyses the conversion of the alkyl mercaptan formed to by-products only insignificantly, if at all.

3. Process according to claim 1 , in which the catalyst in the first zone catalyses the reaction of alkanols and hydrogen sulphide to by-products only insignificantly, if at all.

4. Process according to claim 1 , in which halogenated and/or halogen-free alkali metal tungstates are used as catalysts.

5. Process according to claim 1 , in which a halide-free alkali metal tungstate is used in the first reaction zone and a halogenated alkali metal tungstate in the second reaction zone.

6. Process according to claim 5 , characterized in that a brominated alkali metal tungstate is used.

7. Process according to claim 1 , in which the change to a catalyst having a higher activity for mercaptan formation compared to that used in the first reaction zone is effected after the reaction of 30 to 95% of the alcohol used in the first reaction zone.

8. Process according to claim 2 , in which the lower activity of the catalyst in the first reaction zone is achieved by the dilution of the amount of catalyst with solid inert material.

9. Process according to claim 8 , in which the same catalyst is used in the first and the second or further reaction zone(s).

10. Process according to claim 2 , in which the lower activity of the catalyst in the first reaction zone is achieved by the variation of one or more physical properties selected from the group of specific surface area, pore radius distribution, pore volume, pellet diameter and pellet shape.

11. Process according to claim 1 , in which the reaction zones follow in succession in different reactors which are connected to one another in series.

12. Process according to claim 1 , in which the reaction zones are connected to one another directly.

13. Process according to claim 1 , in which hydrogen sulphide and methanol are reacted to give methyl mercaptan.

14. Process according to claim 1 , in which the catalyst further comprises at least one halide selected from F, Cl, Br and I.

15. Process according to claim 14 , in which the halide is provided only in the second reaction zone.

16. Process according to claim 14 , in which the halide is Br.

17. Process according to claim 15 , in which the halide is Br.