Hierarchical submicrosized Y zeolites prepared by sequential desilication–dealumination post-synthesis modification and their catalytic performance in vacuum gas oil hydrocracking

Abstract

The synthesis of submicrometric Y zeolites has only been achieved for a Si/Al ratio lower than 2.0 for long crystallization times. In this work, submicrometric Y zeolites in the range of 100–200 nm were synthesized with a Si/Al ratio equal to 2.4 in just 24 h. Post-synthesis methods allow tuning the porosity and the acidity of the microporous Y zeolites leading to hierarchical zeolites. Hierarchical Y zeolites were obtained by post-synthetic modification of submicrosized Y zeolites, synthesized under organic template-free conditions. The influence of the post-synthetic modification on the structure, particle size, morphology, surface area and acidic properties of the zeolites was studied. The post-synthetic modification involved either sequential desilication–dealumination (DES–DEA) or sequential dealumination–desilication (DEA–DES) of the zeolite framework involving both leaching with NaOH and steaming processes. Structurally more stable zeolites with hierarchical porosity and high crystallinity were synthesized by the sequential DES–DEA process proposed in this work. Sequential DES–DEA process allowed two relevant results in applications of these zeolites for catalysis. On one hand, it allowed a more ordered and controlled dealumination which led to retaining the number of Brønsted acid sites when the NaOH concentration was changed. Characterization of surface acidity revealed protection of the structural aluminum due to the first step of desilication before the dealumination. On the other hand, this method produced simultaneously both intraparticle and interparticle mesoporosity. With the obtained zeolites, NiMo catalysts were prepared and evaluated in the vacuum gas oil hydrocracking reaction. The catalysts based on zeolites obtained by DES–DEA were more active at lower temperatures and exhibited higher yields of middle distillates compared to those obtained with zeolites synthesized by the DEA–DES method.