The decisive role of the distribution of Al in the framework of beta zeolites on the structure and activity of Co ion species in propane–SCR–NO x in the presence of water vapour

Abstract

The role of the distribution of aluminium in the framework of beta zeolites in the formation of various types of counter Co species of different activity in C 3H 8–SCR–NO in the absence/presence of 10% water vapour was investigated. Different concentrations of Al atoms in the Al–O–(Si–O) 2–Al sequences in one ring (Al pairs) and isolated Al atoms in Al–O–(Si–O) n >2–Al sequences located in two framework rings of parent BEA* zeolites were obtained by variation of the zeolite synthesis and by post-synthesis dealumination. Al atoms in the framework and their distribution were characterized by the combination of 29Si MAS NMR of the parent BEA* zeolites and UV–Vis spectroscopy of the maximum-Co(II)-loaded BEA*. The concentration of Al pairs corresponded to the concentration of bare Co(II) ions in dehydrated maximum exchanged-Co(II)-BEA* calculated from the Vis spectra. Co-BEA* zeolites prepared by ion exchange from Co nitrate solution contained bare Co(II) ions balanced by Al pairs, which were formed preferentially, and “monovalent” Co-oxo species balanced by the isolated Al atoms. The relative concentration of bare Co(II) ions and Co-oxo species depends on the degree of Co(II) ion exchange and the concentration of Al pairs and isolated Al atoms in the zeolite framework. Superior activity was found with the counter Co-oxo species balancing isolated Al atoms, surviving even in wet (10% H 2O) NO x streams. The bare Co(II) ions were active in dry NO x streams, but their activity was negligible in wet NO x streams. A highly active catalyst (more than twice than the other Co-BEA*) can be prepared by using a BEA zeolite containing a predominant concentration of isolated Al atoms in the framework.