Mesopore formation in zeolite H-SSZ-13 by desilication with NaOH

Abstract

A zeolite H-SSZ-13 material with CHA topology and a Si/Al ratio of 14 was treated with sodium hydroxide solutions of various concentrations. The post synthesis treatment led to desilication of the framework accompanied by mesopore formation. N2-physisorption measurements showed that the mesopore volume increased while the micropore volume decreased with increasing severity of the treatment. By applying FTIR spectroscopy with CO as probe molecule, it was shown that silanol groups and Lewis acid sites were created. The number of Brønsted acid sites decreased whereas the acid strength of the maintained Brønsted acid sites was preserved. The mesoporous areas could be visualized by following the oligomerization reaction of 4-methoxystyrene with confocal fluorescence microscopy. The development of mesoporosity started from the crystal surfaces and continued progressively into the crystal with increasing alkaline solution strength. After treatment with 0.4 M alkaline solution, the fluorescence diminished, indicating a decreased catalytic activity in the mesopores. The activity of the micro- and mesopores was probed by testing the material as catalyst for the Methanol-to-Olefins (MTO) reaction. In contrast to the case of ZSM-5, mesoporosity created by alkaline leaching does not enhance the catalytic properties of H-SSZ-13 in the MTO reaction. Especially, a decrease in methanol conversion capacity due to a decreased surface area and number of acid sites was observed. GC–MS and UV/Vis analyses revealed that the same type of coke was formed in both the microporous and the mesoporous H-SSZ-13 materials. Furthermore, the deactivated mesoporous samples contained less coke than the purely microporous H-SSZ-13.