Synthesis of a Hierarchical ZSM-11/ZSM-5 Intergrowth Zeolite Using a Polycationic Structure-Directing Agent for Methylation of m-Xylene

A hierarchically structured ZSM-11/ZSM-5 intergrowth zeolite was generated using a polycationic organic structure-directing agent (OSDA), [-N+(CH2CH2CH2CH3)2-(CH2)8-]n[OH–]n, which is conceptually equivalent to bond-linked tetrabutylammonium (TBA), the frequently used OSDA for ZSM-11 synthesis. The produced ellipsoidal crystals have a major axis length of 130–240 nm and a minor axis length of 75–120 nm, made up of aligned nanocrystals of 44 to 83 nm, to construct an auxiliary macro- to mesopore network ranging from 45 to 80 nm. The Si/Al ratio can be manipulated from 50 to ∞, and the intergrowth exhibits acidity comparable to that of ZSM-5, which is distinct from previous reports that formation of hierarchical porosity or intergrowth often compromises the inherent acidity of zeolitic crystals. Such hierarchical architecture also endows the material with enhanced transport properties, as validated by 2-methylhexane adsorption measurements. In the catalytic methylation of meta-xylene with methanol, the hierarchical ZSM-11/ZSM-5 zeolite exhibited superior performance to that of ZSM-5, with enhanced activity and total trimethylbenzene and tetramethylbenzene selectivity, which was elevated from 28% to 39%. These catalytic merits stem from the combined effects of preserved acid strength, optimized pore connectivity, and alleviated diffusion limitations.