m-Xylene transformation over H-MCM-22 zeolite.: 2.: Method for determining the catalytic role of the three different pore systems

A series of treatments were applied to selectively deactivate the acid sites of the supercages or of the external hemicages of a MCM-22 zeolite sample. Precoking of MCM-22 through n-heptane cracking at 450 degreesC does not completely suppress the activity of the supercage sites for m-xylene transformation at 350 degreesC. In contrast, these sites were selectively and completely deactivated after m-xylene transformation for 24 h. Among the methods that were tried to selectively deactivate the hemicage protonic sites (de-alumination with ammonium hexafluorosilicate, reaction with 1,1,1,3,3,3-hexamethyldisilazane, poisoning with 2,6- or 2,4-dimethylquinoline) only the treatment with 2,4-dimethylquinoline was found to be suitable. The most selective methods were used to estimate the relative contributions of the protonic sites located in the supercages (42%), the external hemicages (22%) and the sinusoidal channels (36%) in m-xylene transformation. In the large supercages with small apertures, isomerization into a 3.5-1 para-ortho mixture is accompanied by m-xylene disproportionation; most of the bulky trimethylbenzene products trapped in these cages undergo secondary transformations in smaller molecules or into coke. In the large external hemicages, m-xylene is essentially transformed into a quasi-equimolar mixture of para- and ortho-xylenes, the only secondary products being traces of toluene and trimethylbenzenes. Lastly, in the narrow sinusoidal channels, m-xylene transformation is practically limited to isomerization into the slim para product. (C) 2003 Elsevier Inc. All rights reserved.