Xylene isomerization

Abstract

A catalytic process is provided for isomerizing an isomerization feed containing an aromatic C 8 mixture of ethylbenzene and xylene in which the para-xylene content is less than equilibrium which comprises contacting said feed, under conversion conditions, with a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least about 1 micron, a silica to alumina ratio of at least about 12 and a constraint index, as hereinafter defined, within the approximate range of 1 to 12 to yield an isomerization product characterized by a reduced ethylbenzene content and an enhanced para-xylene content.

What is claimed is: 
     
       1. A process for isomerizing an isomerization feed containing an aromatic C 8  mixture of ethylbenzene and xylene, in which the para-xylene content is less than equilibrium which comprises contacting said feed, under conversion conditions, with a catalyst comprising a crystalline aluminosilicate zeolite having a crystal size of at least about 1 micron, a silica to alumina ratio of at least about 12 and a constraint index within the approximate range of 1 to 12 to yield an isomerization product characterized by a reduced ethylbenzene content and an enhanced para-xylene content. 
     
     
       2. The process of claim 1 wherein said conversion conditions include a temperature of from about 400° to about 1000° F., a pressure of from about 0 to about 1500 psig and a weight hourly space velocity of between about 0.5 and about 100. 
     
     
       3. The process of claim 1 wherein said conversion conditions include a temperature of from about 500° to about 950° F., a pressure of from about 25 to about 1000 psig and a weight hourly space velocity of between about 1 and about 50. 
     
     
       4. The process of claim 1 wherein said aromatic C 8  mixture consists essentially of ethylbenzene, para-xylene, meta-xylene and ortho-xylene. 
     
     
       5. The process of claim 1 wherein said isomerization feed contains said aromatic C 8  mixture and up to 30 weight percent of non-aromatic hydrocarbons. 
     
     
       6. The process of claim 1 wherein said crystalline aluminosilicate zeolite is ZSM-5. 
     
     
       7. The process of claim 1 wherein said crystalline aluminosilicate zeolite is ZSM-5 predominantly in the hydrogen form. 
     
     
       8. The process of claim 1 wherein the crystal size of said crystalline aluminosilicate zeolite is within the approximate range of 1 to 20 microns. 
     
     
       9. The process of claim 1 wherein the crystal size of said crystalline aluminosilicate zeolite is within the approximate range of 1 to 6 microns. 
     
     
       10. The process of claim 6 wherein the ZSM-5 is present in combination with a binder therefor. 
     
     
       11. The process of claim 1 wherein said crystalline aluminosilicate zeolite is characterized by a xylene sorption capacity greater than 1 gram 100 grams of zeolite and an ortho xylene sorption time of 30 percent of said capacity of greater than 10 minutes, said sorption capacity and sorption time being measured at 120° C., and a xylene pressure of 4.5±0.8 mm. of mercury. 
     
     
       12. The process of claim 1 wherein said crystalline aluminosilicate has combined therewith between about 0.5 and about 40 weight percent of a difficulty reducible oxide. 
     
     
       13. The process of claim 12 wherein said difficulty reducible oxide is magnesium oxide. .Iadd. 
     
     
       14.  A process according to any of claims 1 to 13 comprising a further isomerization conversion step wherein the isomerization feed is subsequently contacted with a small crystal aluminosilicate zeolite having a crystal size less than about 1 micron. .Iaddend. .Iadd.15. The process of claim 1 further comprising contacting the isomerization feed with a sequential bed of ZSM-5 zeolite having a crystal size less than 1 micron. 
     
     
        .Iaddend. .Iadd.16.  A process according to claim 1 comprising a dual isomerization reactor sequence wherein the aromatic C 8  mixture is subsequently contacted with a small crystal aluminosilicate zeolite having a crystal size less than about 1 micron. .Iaddend. .Iadd.17. A process according to claim 16 wherein zeolite catalysts of differing crystal size 
     
     
        are in sequential beds, each comprising HZSM-5..Iaddend. .Iadd.18.  A process according to claim 1 wherein the isomerization conversion is carried out with the crystalline aluminosilicate zeolite having a bimodal crystal size distribution falling in two ranges, comprising a first crystal size range greater than 1 micron and a second crystal size range less than 1 micron, the greater crystal size being in major proportion. .Iaddend. .Iadd.19. A process according to claim 18 comprising the step of contacting the aromatic C 8  mixture with sequential beds comprising HZSM-5 catalyst having differing crystal size ranges. .Iaddend. .Iadd.20. A process according to claim 6 wherein the isomerization conversion is carried out with said crystalline aluminosilicate zeolite having a bimodal crystal size distribution falling in two ranges, a range being less than 1 micron and a range being at least 1 micron, with the greater range being in major proportion..Iaddend. .Iadd.21. A process according to claim 20 comprising contacting the aromatic C 8  mixture with sequential reactor beds of said zeolite having an initial crystal size of at least about 1 micron preceding zeolite crystals of less than 1 micron size.