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.