Bronsted acidic zeolite beta was prepared using various synthetic routes. B-beta was postsynthetically converted to Al-beta in the presence of calcium ions during Al insertion (route A), or calcium was present in the hydrothermal synthesis after which B-beta was converted to Al-beta (route B). A third sample was prepared by directly synthesizing Al-beta (route C) as a control. 1H MAS NMR spectra of dehydrated samples A and B in their proton forms surprisingly show no line near 4 ppm for the classical Bronsted acid sites, but do show a signal at 6.4 ppm for hydrogen-bonded Bronsted acid sites. Sample C shows lines at 4 ppm and 6.3 ppm, so this sample has H-bonded and non-H-bonded acid sites. The assignment of all these lines to Bronsted acid sites is corroborated by the heteronuclear 1H-27Al dipolar interaction quantified by 1H{27Al}REAPDOR NMR experiments. In addition, isopropylamine decomposition shows clear evidence for Bronsted sites in these zeolites. It is concluded that samples A and B are the first zeolites with exclusively H-bonded acid sites. Catalytic conversion of propane shows a surprisingly high amount of dehydrogenation in samples A and B, while cracking was favored in sample C.

Hydrogen-bonded acid sites in zeolite beta � preparation, characterization, and catalytic properties

247th National Meeting (2014)

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