modification of mcm-22 zeolite through sequential post-synthesis treatments. implications on the acidic and catalytic behaviour

the generation of mesoporosity was achieved upon alkaline treatment with 0.05 m naoh solution and practically no additional gain was obtained when the more concentrate solution, 0.1 m, was used. instead, al extraction takes place along with si, as shown by ²⁹si and ²⁷al mas-nmr data, followed by al deposition as extraframework species.

samples submitted to alkaline plus acid treatments present distinct behaviour. when the lowest naoh solution was used no relevant effect was observed on the textural characteristics. additionally, when the acid treatment was performed on an already fragilized mcm-22 structure, due to previous desilication with 0.1 m naoh solution, the extraction of al from both internal pore systems promotes their interconnection, evolving from a 2-d to a 3-d porous structure. this transformation has a marked effect in the catalytic behaviour, allowing an increase of m-xylene conversion as a consequence of an easier and faster molecular traffic in the 3-d structure. on the other hand, the continuous deposition of extraframework al species inside the pores leads to a shape selective effect that privileges the formation of the more valuable isomer p-xylene.