Incorporation of Ni into HZSM-5 zeolites: Effects of zeolite morphology and incorporation procedure

Abstract

Insertion of Ni species into H-ZSM5 catalysts with high Si/Al ratio (around 50) and different morphology (micron sized particles vs nanosheet ones) has been carried out by two different methods, namely impregnation and ion exchange routes. The resultant samples have been characterized by IR spectroscopy using CO as probe molecule after thermal treatment in oxygen or vacuum. In all samples different Ni2+ surface sites (counterions, grafted or on the surface of NiO particles) have been observed, but their distribution and concentration were found to be greatly influenced by morphology of zeolite particles as well as Ni incorporation method. For instance, the nanosheet morphology was found to favour ion exchange, probably in relation to high amount of surface tetrahedral Al sites. Moreover, a small fraction of sites was found to be reduced, both to Ni+ counterions and Ni0 (nano) particles, when the activation was carried out in vacuum in some of the samples. This could be related to changes in the population of surface OH groups (Brønsted sites and hydrogen bonded silanols), and to the formation of surface defects (strained SiOSi bridges). These observations give indications about the mechanism governing ion reactivity and migration in zeolite matrices, and confirm the fact that silanol groups are of the utmost importance in affecting the dispersion of the metal phase, and therefore the corresponding redox properties.