Sequential Microwave-Assisted Dealumination and Hydrothermal Alkaline Treatments of Y Zeolite for Preparing Hierarchical Mesoporous Zeolite Catalysts

Abstract

A novel post-synthetic method combining the microwave (MW)-assisted dealumination and hydrothermal (HT) alkaline treatment for obtaining mesoporous Y zeolite catalysts is presented. In the first-step MW-assisted dealumination of the pristine Y zeolite, both the mineral acid (i.e. hydrochloric acid, HCl) and organic carboxylic acids (e.g. oxalic and diethylenetriaminepentaacetic acid) were used (all at 0.16 M), and their effects on the physical and chemical properties of the resulting modified zeolites (after the identical HT alkaline treatment using 0.2 M sodium hydroxide solution) are studied in detail by comprehensive characterisation of the bulk silicon-to-aluminium (SAR) ratio, crystallinity and textural properties. The findings show that, in the developed sequential method, (i) HCl was only effective to achieve the hydrolysis of the zeolite, (ii) carboxylic acids with the function of chelation was very capable of extracting Al species and creating mesopores, and (iii) the mesopores formation is a function of the number of coordination sites in the carboxylic acid. The hierarchical feature of the mesopores in the modified zeolites (using carboxylic acids) was probed by the model reaction of aldol condensation of 1-heptanal with benzaldehyde for the selective formation of jasminaldehyde. The normalised selectivity to jasminaldehyde (with respect to the strong acidity) was proportionally related to the specific mesopores volume of the zeolites, revealing hierarchical porous networks in the relevant modified Y zeolites and indicating the effect of hierarchical mesopores in the zeolites on their catalytic performance.