Experimental Investigation of the Reaction Network of Ethene to Propene over Ni/AlMCM‐41 Catalysts

Abstract

The reaction network of the ethene-to-propene (ETP) reaction over Ni/AlMCM-41 catalysts was studied. This reaction consists of the dimerization of ethene, the isomerization of 1-butene, and the metathesis of ethene and 2-butene to propene. This work focused on the latter. ETP experiments followed by in situ diffuse reflectance infrared Fourier transform spectroscopy were carried out. Metathesis activities were evaluated by the isomerization of butenes, the metathesis of ethene and 2-butene, and the retro-metathesis of propene. Metathesis activities of Ni/AlMCM-41 catalysts were not observed. Thus, a conjunct polymerization of ethene to propene is proposed.

Specific Localization of Aluminum Sites Favors Ethene-to-Propene Conversion on (Al)MCM-41-Supported Ni(II) Single Sites

“…Initial productivity in propene is also higher on 3 (3.5 g g Ni –1 h –1 ) than on 1 (0.8 g g Ni –1 h –1 ) and 2 (1.5 g g Ni –1 h –1 ) as shown on Figures b, S55, and S56. However, it decreases significantly on 3 within 20 h on-stream (down to 2 g g Ni –1 h –1 ), whereas the productivities in butenes change only slightly, showing that while ethene dimerization activity is maintained, the production of propene slows down, suggesting that these reactions require different active sites, Ni­(II) for dimerization and acid sites for the formation of propene (vide infra) . Note also that significant amounts of iso -butene are detected using catalysts 2 and 3 , which contain Al atoms, unlike 1 (Figures b, S55, and S56), consistent with formation of iso -butene via isomerization of linear butenes or cracking of higher hydrocarbons.…”

“…Although Ni­(II) sites are found in the as-synthesized Ni-MCM-41 materials, Iwamoto speculated that Ni­(II) is reduced in situ to give Ni­(I), which then enters the catalytic cycle . However, no paramagnetic Ni­(I) sites in ETP catalysts have been evidenced so far …”

Nickel Catalyzed Olefin Oligomerization and Dimerization

“…252,253 The metathetic activity of these catalysts has recently been robustly contested by control experiments with propylene. 254 Once again, all of these studies show that the incorporation of aluminum into the walls of the mesostructured materials to obtain AlMCM-41, AlMCM-48 or AlSBA-15, for instance, increases the catalytic activity for ethylene oligomerization. Enhancing the acidity is suggested to be directly connected to higher conversion of olefin and a higher isomerization rate, while inducing the formation of higher molecular weight products and coke, potentially causing catalyst deactivation.…”

Ethylene oligomerization on nickel catalysts on a solid acid support: From New mechanistic insights to tunable bifunctionality

“…A few labs have suggested that a Ni-metathesis reaction in the phyllosilicate structure may occur at sufficiently high temperatures (>~673 K) [65][66][67][68]. But more recently in 2017, there was evidence of conjunct polymerization products that can be cracked down to propene as a more plausible route and more in line with our current understanding of the mechanism [69,70]. This has also been supported by kinetic modeling on Ni-MCM-41, where a reaction network based on the cracking of long-chain olefins better fit and predicted the observed product distributions than a network containing metathesis reactions [71].…”