Regulating hierarchical structure and acidity of HZSM-5 for methanol to aromatics via protective desiliconization and external surface modification

HZSM-5 is a widely used catalyst for the conversion of methanol to aromatics (MTA), but its single micropore system often results in severe diffusion limitations, leading to the decrease of reaction rate and the change of selectivity to aromatics, as well as affecting the carbon deposition and catalyst deactivation. Thus, an ingenious protective desiliconization method with a mixed solution of sodium hydroxide and tetrapropylammonium hydroxide as a desiliconization agent was developed to create hierarchical porous systems without destruction of the main structure of the molecular sieve. Such modification leads to the greatly increased mesoporous volume from 0.12 to 0.38 cm(3) g(-1), and only slightly decreased microporous volume from 0.12 to 0.09 cm(3) g(-1). However, the desiliconization sharply increases the acidity of the catalyst (from 0.248 to 0.341 mmol g(-1) for the strong acid sites), which may significantly affect the distribution of aromatics and accelerate the carbon deposition rate. Thus, the deposition of inert SiO2 on the catalyst surface is subsequently carried out to passivate the surface acidity and adjust the orifice of the catalyst. Accordingly, a hierarchical porous HZSM-5 molecular sieve catalyst (Si@Z5-Na + TP) with suitable acidity is precisely prepared, which exhibits the high selectivity to benzene, toluene and xylenes (BTX) with the excellent anticoking ability in the MTA reaction. The selectivity to BTX in aromatics (63.32%) and the carbon deposition rate (0.91 mg g(cat)(-1) h(-1)) over the Si@Z5-Na + TP catalyst are both much more superior than those over the parent HZSM-5 catalyst (47.5% and 2.23 mg g(cat)(-1) h(-1), respectively).