Active Sites in Working Bifunctional GaH-TON Aromatization Catalysts: Kinetic Evaluation

The conversion of light alkanes to high value aroms. proceeds with a high selectivity over bifunctional, gallium (Ga) contg. zeolite catalysts. It is generally agreed that Ga sites are involved in dehydrogenation reaction steps and that the zeolite acid sites catalyze cracking, oligomerization, and cyclization reactions. However, understanding of the precise roles of the acid and Ga sites in the reaction mechanisms is significantly hampered since the no. of these sites in working catalysts is not known. This paper describes a kinetic approach to evaluation of the acid and Ga active sites in working Ga contg. TON zeolite catalysts that relies on the anal. of the rates of formation of the primary products of a n-butane aromatization reaction. Our results show that the rate of ethane formation at low n-butane conversions can be used as a quant. est. of acidity in working bifunctional zeolite catalysts and demonstrate, for the first time, a significant decrease in the no. of Bronsted acid sites in the Ga contg. catalysts under reaction conditions: around 47 and 79% for the catalysts with Ga loading of 1.5 and 2.5 wt. %, resp. We conclude that the redn. in acidity is assocd. with the formation of catalytically active Ga+ ions and obtain ests. for the no. and steady-state turnover activity of the acid and Ga active sites in n-butane transformation. We anticipate that our work will facilitate understanding of the precise roles of the acid and Ga sites in the mechanisms of alkane aromatization and, as a far-reaching implication, will prompt wider use of detailed kinetic studies for the evaluation of active sites in working catalysts. [on SciFinder (R)]