Controlling the selectivity–stability tradeoff in zeolite catalysis: oligomerization–alkylation, cracking, and methanol-to-hydrocarbons 

Metal cations in zeolites can redirect alkene oligomerization from Brønsted-acid-mediated secondary chemistry toward coordination-insertion pathways; however, controlling whether metal precursors form accessible cationic sites or oxide nanoparticles remains challenging. Here, we prepared hierarchical ZSM-5 by polymeric templating (PT) at a common nominal loading of 3 wt % and compared the protonic parent H/PT with Ni/PT, Cr/PT, and Zr/PT to determine how metal speciation governs ethylene oligomerization. X-ray diffraction, N2 physisorption, microscopy, X-ray photoelectron spectroscopy, in situ X-ray absorption spectroscopy, CO-DRIFTS, pyridine-FTIR, and multinuclear solid-state NMR show that Ni/PT is dominated by accessible, framework-interacting Ni2+ Lewis sites, with a minor NiO-like contribution. Cr/PT contains accessible Cr-associated Lewis sites together with CrOx-like domains that evolve toward Cr2O3 during pretreatment, whereas Zr/PT is dominated by ZrO2 nanoparticles and retains more Brønsted acidity than Ni/PT and Cr/PT. These differences reshape catalytic activity: Ni/PT achieves 92% combined C4–C8 olefin selectivity with a butene-rich, dimerization-dominated slate, whereas Cr/PT gives a C6-rich distribution from a mixed network of Cr-associated Lewis sites and residual acid-mediated chemistry; H/PT and Zr/PT instead favor hydrogen transfer, aromatization, and paraffin formation. Density functional theory on an initiated [Ni(II)–ethyl]+ model places β-hydrogen transfer/chain termination 10 kJ mol–1 below further ethylene insertion, rationalizing the C4-rich response. The coke content increases in the order H/PT < Ni/PT < Cr/PT < Zr/PT, whereas total retained-deposit content follows H/PT < Cr/PT < Ni/PT < Zr/PT. Thus, olefin selectivity and deposit formation do not vary in parallel. Within this matched catalyst series, chain-length selectivity and deactivation reflect the coupled effects of metal speciation, site accessibility, thermal evolution, and residual acidity rather than nominal metal loading, total Lewis acidity, or texture alone.