Catalyst
stability in the liquid phase under polar conditions,
typically required for the catalytic conversion of renewable platform
molecules, is a major concern but has been only sparsely studied.
Here, the activity, selectivity, and stability of Ru-based catalysts
supported on TiO2, ZrO2, and C in the conversion
of levulinic acid (LA) to γ-valerolactone (GVL) has been studied
at 30 bar of H2 and 423 K in dioxane as solvent. All catalysts
showed excellent yields of GVL when used fresh, but only the Ru/ZrO2 catalyst could maintain these high yields upon multiple recycling.
Surprisingly, the widely used Ru/TiO2 catalyst showed quick
signs of deactivation already after the first catalytic test. XPS,
CO/FT-IR, TGA, AC-STEM, and physisorption data showed that the partial
deactivation is not due to Ru sintering or coking but rather due to
reduction of the titania support in combination with partial coverage
of the Ru nanoparticles, i.e. due to a detrimental strong metal–support
interaction. In contrast, the zirconia support showed no signs of
reduction and displayed high morphological and structural stability
even after five recycling tests. Remarkably, in the fresh Ru/ZrO2 catalyst, Ru was found to be fully atomically dispersed on
the fresh catalyst even at 1 wt % Ru loading, with some genesis of
Ru nanoparticles being observed upon recycling. Further studies with
the Ru/ZrO2 catalyst showed that dioxane can be readily
replaced by more benign solvents, including GVL itself. The addition
of water to the reaction mixture was furthermore shown to promote
the selective hydrogenation reaction.
In
single-site olefin polymerization catalysis, a large excess
of cocatalyst is often required for the generation of highly active
catalysts, but the reason for this is unclear. In this work, fundamental
insight into the multifaceted role of cocatalyst methylaluminoxane
(MAO) in the activation, deactivation, and stabilization of group
4 metallocenes in the immobilized single-site olefin polymerization
catalyst was gained. Employing probe molecule FT-IR spectroscopy,
it was found that weak Lewis acid sites, inherent to the silica-supported
MAO cocatalyst, are the main responsible species for the genesis of
active metallocenes for olefin polymerization. These weak Lewis acid
sites are the origin of AlMe2+ groups. Deactivation
of metallocenes is caused by the presence of silanol groups on the
silica support. Interaction of the catalyst precursor with these silanol
groups leads to the irreversible formation of inactive metallocenes.
Importantly, a high concentration of MAO (14 wt% Al) on the silica
support is necessary to keep the metallocenes immobilized, hence preventing
metallocene leaching and consequent reactor fouling. Increasing the
loading of the MAO cocatalyst leads to larger amounts of AlMe2+, fewer silanol groups, and less metallocene leaching,
which all result in higher olefin polymerization activity.
The presence of biogenic or process‐derived impurities poses a major problem on the efficient catalytic hydrogenation of biomass‐derived levulinic acid to γ‐valerolactone; hence, studies on their influence on catalyst stability are now required. Herein, the influence of sulfuric acid as feed impurity on the performance of Ru‐based heterogeneous catalysts, including Ru/ZrO2 and mono‐ and bimetallic Ru‐on‐carbon catalysts in dioxane as solvent, was investigated. The carbon‐supported Ru catalysts proved to be very sensitive to minor amounts of sulfuric acid. In stark contrast, Ru/ZrO2 showed a remarkable stability in the presence of the same impurity, which is attributed to the sulfate‐ion adsorption capacity of the support. Preferential sulfate adsorption onto the surface of ZrO2 effectively protects the Ru active phase from deactivation by sulfur poisoning. A simple catalyst regeneration strategy was effective in removing adsorbed impurities, allowing efficient catalyst recycling.
A triethylaluminium(TEAl)-modified Phillips ethylene polymerisation Cr/Ti/SiO2 catalyst has been developed with two distinct active regions positioned respectively in the inner core and outer shell of the catalyst particle. DRIFTS, EPR, UV-Vis-NIR DRS, STXM, SEM-EDX and GPC-IR studies revealed that the catalyst produces simultaneously two different polymers, i.e., low molecular weight linear-chain polyethylene in the Ti-abundant catalyst particle shell and high molecular weight short-chain branched polyethylene in the Ti-scarce catalyst particle core. Co-monomers for the short-chain branched polymer were generated in situ within the TEAl-impregnated confined space of the Ti-scarce catalyst particle core in close proximity to the active sites that produced the high molecular weight polymer. These results demonstrate that the catalyst particle architecture directly affects polymer composition, offering the perspective of making high-performance polyethylene from a single reactor system using this modified Phillips catalyst.
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