Hydrogen H/D Exchange and Activation of C₁–n-C₄ Alkanes on Ga-Modified Zeolite BEA Studied with ¹H Magic Angle Spinning Nuclear Magnetic Resonance in Situ

Kinetics of H/D hydrogen exchange between C₁–n-C₄ alkanes and Brønsted acid sites (BAS) of both the pure acid-form zeolite BEA (H-BEA) and Ga-modified zeolite BEA (Ga/H-BEA) was monitored by ¹H MAS NMR spectroscopy in situ at 423–563 K. Comparative analysis of the rates of the exchange for H-BEA and Ga/H-BEA zeolites reveals a remarkable increase of the rate by 1–2 orders of magnitude, decrease of activation energy, and an appearance of regioselectivity of the exchange into the methyl groups of C₃-n-C₄ alkanes upon modification of H-BEA zeolite with gallium. These data identify an evident promoting effect of Ga on activation of alkane C–H bonds by BAS. The effect has been rationalized by preliminary dissociative adsorption of alkanes on gallium(III) oxide species inside zeolite pores to form gallium-alkyl species, which are further involved in the exchange with neighbor BAS. Involvement of both BAS and Ga species in alkane activation accounts for earlier suggested synergistic effect (Buckles, G., et al. Catal. Lett. 1991, 11, 89) of both BAS and gallium species in alkane activation and aromatization on Ga-modified high silica zeolites.