Alkane dehydrogenation with Lewis-Bronsted acid pairs in Ga/H-ZSM-5

Alkane dehydrogenation with Lewis-Bronsted acid pairs in Ga/H-ZSM-5

 Lingli Ni, Moritz Schreiber, Ricardo Bermejo-Deval, Johannes A. Lercher

Techniche Universität München, Catalysis Research Center, 85747 Garching, Germany.

Dehydrocyclodimerization is industrially applied for converting liquefied petroleum gas (mainly C₃ and C₄ hydrocarbons) into aromatic compounds (benzene, toluene and xylenes (BTX)) and H₂. On bifunctional catalysts (e.g., Ga/H-ZSM-5) propane dehydrogenation is the rate determining step of the reaction. However, the state and the role of the Ga cation on alkane dehydrogenation has not been discerned. Here, we will identify the proximity of Ga⁺ to the Brønsted acid site (BAS) and the Al distribution to be key for an efficient reaction pathway in the dehydrogenation of propane.

The addition of Ga (Ga(NO₃)₃ solution, incipient wetness impregnation) into H-ZSM-5 (Si/Al = 50, 31 % of Al pairs) indicated the stoichiometric replacement of one BAS by one Ga⁺ cation, as in-situ IR spectroscopy showed the decrease of the Si(OH)Al stretching band (of BAS) at 3616 cm^-1. Therefore, the rate (1 bar, 823 K) of propane conversion and dehydrogenation increased as a function of Ga concentration, reaching a maximum at a ratio of Ga/Al = 0.5 (Figure 1). This dehydrogenation/cracking rate ratio increased from 0.63 (Ga/Al = 0) to 19.20·10⁷ mol  g^-1 s^-1 (Ga/Al = 0.5). However, H-ZSM-5 with Si/Al 88 (4 % Al pairs) reached the maximum rates of dehydrogenation at Ga/Al =1, showing Al pairs are necessary for the Lewis−Brønsted acid pair synergy in propane dehydrogenation. The nature and concentration of the active site were determined in situ via X-ray absorption spectroscopy (XANES), temperature-programmed reduction (TPR) and infrared (IR) spectroscopy of adsorbed pyridine.

 

Figure 1. Dehydrogenation rates as a function of Ga/Alparent for H-MFI zeolites with Si/Al ratio of 50 and 88.