Role of Ga³⁺Sites in Ethene Oligomerization over Ga/H-MFI

Because of their relatively low cost, ethane and propane derived from shale gas are the currently preferred feedstocks for the production of aromatics. Ga-exchanged H-MFI zeolite (Ga/H-MFI) exhibits high activity and selectivity for light alkane dehydroaromatization, a process that involves alkane dehydrogenation to form alkenes, followed by alkene oligomerization and cyclization, and further dehydrogenation of the resulting products. Recent work has shown (Phadke et al. Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3. ACS Catal. 2018, 8, 6106–6126; Phadke et al. Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl3. J. Am. Chem. Soc. 2019, 141, 1614–1627; Phadke et al. Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI. ACS Catal. 2021, 11, 2062–2075; Mansoor et al. ACS Catal. 2018, 8, 2146–6162) that Ga3+ ([GaH]2+ and [Ga(H)2]+) sites catalyze the initial dehydrogenation of alkanes; however, the role of Ga3+ sites in the subsequent alkene oligomerization step requires clarification. In this work, the kinetics of ethene oligomerization over Ga/H-MFI were investigated as a function of Ga loading, feed space time, temperature, and ethene partial pressure. The presence of Ga3+ sites gives rise to enhanced rates of ethene oligomerization and higher selectivities to butene and hexene relative to H-MFI. However, selective titration of Brønsted acid sites with NH3 reveals that, in the absence of Brønsted acid sites, [GaH]2+ and [Ga(H)2]+ cations do not contribute appreciably to the higher activity of Ga/H-MFI. Similarly, in situ Fourier-transform infrared spectroscopy shows that the reaction pathway for ethene oligomerization over Ga/H-MFI involves the same intermediates as that over H-MFI. The higher ethene oligomerization activity and selectivity to even-carbon-numbered alkenes of Ga/H-MFI stems from cooperative effects between Ga3+ sites and Brønsted acid protons.