Effect of aluminium distribution in the framework of ZSM-5 on hydrocarbon transformation.Cracking of 1-butene

摘要

The effect of the distribution of Al atoms in the framework of H-ZSM-5,controlled by zeolite synthesis,on the product yields in 1-butene conversion to low olefins,aromatics and paraffins has been investigated for a series of zeolites,both synthesized and commercial,with Si/Al ranging from 12.6 to 43.6.The 27Al and 29Si MAS NMR of the corresponding Na-ZSM-5 and their exchange capacities for Co(II) ions and quantitative analysis of d-d transitions of bare Co(II) ions in the dehydrated CoNa-ZSM-5 were used for determination of the population of the [Al-O-(Si-O)n-Al] sequences in the framework.The Si-Al sequences with n = 1 were not present in the synthesized or commercial samples.The exchange capacity of Co(II) ions corresponded to the concentration of "close" framework Al atoms present mostly in the six-membered framework rings (w = 2) of the cationic sites.The concentration of "single" Al atoms was calculated from the difference between the total concentration of Al and twice the concentration of the exchanged Co(II) ions.The enhanced formation of aromatics in 1-butene conversion for H-ZSM-5 with similar Si/Al ratio,but higher concentration of "close" Al atoms in the framework is accounted for enhancing the rate of hydrogen transfer reactions,in contrast to samples with higher concentration of "single" Al atoms,where olefin cracking is preferred.As low-temperature (20 K) IR analysis of adsorbed hydrogen showed that the acid strength of the protonic sites in the H-ZSM-5 samples is very similar,the differences in the selectivity of 1-butene cracking and aromatization are thought to be caused by different distribution of framework Al atoms and thus also of the protonic sites.This finding opens a new potential for advanced tailoring of zeolite catalyst selectivity.