During the isomerisation of o-ethyltoluene (O-ET) to produce m-ethyltoluene (M-ET) and p-ethyltoluene (P-ET), it is crucial to improve the isomerisation selectivity and reduce side reactions, such as disproportionation, alkyl transfer, and splitting. In this study, in order to improve the selectivities toward M-ET and P-ET during O-ET isomerisation, both the commercial micropore mordenite (HM) and the prepared micro-mesoporous mordenite (HM-M) were treated through chemical liquid deposition using tetraethyl orthosilicate (TEOS) and 3,5-dimethylphenylmagnesium bromide (DPB), respectively. Thereafter, their structure, porosity, and acidity were characterized via X-ray diffraction, transmission electron microscopy, inductively coupled plasma, N2 sorption, temperature-programmed desorption of ammonia, Fourier-transform infrared spectroscopy of pyridine and 2,6-di-tert-butylpyridine, and thermal analysis. The deposition mechanism of DPB was also discussed. The results showed that TEOS could shrink and block the micropores of mordenite. By contrast, DPB passivated the external surface acidity and did not affect the micropore structure. Moreover, HM modified using DPB significantly shortened the self-coking process, improved the product selectivities for M-PT and P-ET as well as their stability, and prolonged the catalytic life. When the amount of magnesium oxide (MgO) deposited on the HM zeolite was 4%, the product selectivities toward M-ET and P-ET increased from 67.27% to 77.54%, and the yields of M-ET and P-ET increased from 47.57% to 52.98%. However, the performance of the catalyst was not significantly enhanced on the HM-M, owing to the passivation of acidic sites in the mesopores by the TEOS and DPB.
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