Enhancing p-xylene selectivity during m-xylene transformation using mildly pre-coked ZSM-5 catalyst

摘要

In the present work,the transformation of m-xylene was studied over fresh and pre-coked H-ZSM-5 catalyst in a riser simulator over the temperature range of 350-500°C.Significant differences were observed in both the activity and product selectivity pattern for the two forms of the catalyst.While,the fresh catalyst generally gave higher m-xylene conversion,the pre-coked catalyst produced higher isomerization/disproportionation(I/D)and paro/ortho xylene(P/O)ratios.For both forms of the catalyst,m-xylene conversion was found to increase with both reaction temperature and reaction time(0-15 s).A maximum conversion of 30.8% was achieved at 500°C for a reaction time of 15 s using the fresh catalyst.The corresponding value for the pre-coked catalyst was 24.6%.P/O and I/D ratios as high as 2.5 and 10,respectively,were observed with the pre-coked catalyst.It was also observed that the P/O and I/D ratios decreased with increasing temperature for both catalyst and that the difference between these ratios for the two catalysts was more pronounced between 400 and 450°C than at higher temperatures.Kinetic modelling of the m-xylene transformation over the pre-coked catalyst yielded a lower activation energy for p-xylene formation compared to the fresh catalyst.And an activation energy of 17.5 kcal/mol for the m-xylene dispropotionation was obtained for the pre-coked catalyst compared to 7.82 kcal/mol for the fresh catalyst.These values are indicative of the restriction posed on disproportionation by the pre-coked catalyst as reflected by higher I/D ratios.