The issues of toluene disproportionation and chiral molecular sieves
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I seek help from experts: According to thermodynamic equilibrium, the content of p-xylene among the xylene derivatives produced by toluene disproportionation should not exceed 50%. However, on chiral molecular sieves, the proportion of p-xylene in these xylene derivatives is as high as over 95%. Does this violate the principles of catalysis? Why?
The toluene disproportionation process is divided into normal disproportionation and stereoselective disproportionation processes. The conventional disproportionation process is generally used in combined aromatic units, with toluene and heavy aromatics serving as the reaction feedstocks. Disproportionation and transalkylation take place simultaneously within the reactor. This process allows for the utilization of heavy aromatics, which helps to increase the yield of xylene; however, the resulting mixed xylene has a composition that corresponds to thermodynamic equilibrium, with pure xylene accounting for only 24% of it, making the separation of pure xylene difficult ； The shape-selective disproportionation process can only use toluene as raw material; it cannot handle heavy aromatics, but it can selectively produce pure xylene, and the purification is relatively easy. Currently, the main technologies for selective toluene disproportionation are ExxonMobil’s PxMax process and UOP’s PXPlus process. The RHArom process is also good.