The mechanistic understanding of coke formation on zeolites is elusive, given the limitations for the extraction and analysis of coke species. Here, we analyze the evolution of deactivating coke species...
Jet fuel production from ethylene oligomerization opens
a sustainable
pathway to clean sulfur-free fuel that is increasingly in demand due
to the potential renewable origin of ethylene. The key to a viable
heterogeneously catalyzed process is to improve the selectivity of
the jet fuel while prolonging the catalyst lifetime. To this end,
we have assessed and optimized a dual-bed cascade system based on
a dimerization bed that is followed by an oligomerization bed using
Ni supported on Y zeolite and ZSM-5 zeolite catalysts, respectively.
Our optimization approach uses different catalyst acidities, temperatures,
and bed configurations for determining the best yield–conversion
relationship. Under optimized dual-bed conditions, we can produce
64 wt % of jet fuel at the beginning of the reaction and maintain
a 50 wt % selectivity of this fraction for over 20 h on stream. This
paper also analyzes coke deposition (content and nature) at the different
experimental conditions and catalyst bed arrangements using temperature-programmed
combustion. We demonstrate that the dual-bed approach is effective
for protecting the main oligomerization bed (ZSM-5 catalyst) from
deactivation, leading to the formation of a lighter type of coke compared
with that using the initial Ni2+ HY-based dimerization
catalyst, which deactivates at a faster rate.
Zeolitic imidazolate frameworks (ZIFs) have been profusely used as catalysts for inserting CO 2 into organic epoxides (i.e., epichlorohydrin) through cycloaddition. Here, we demonstrate that these materials suffer from irreversible degradation by leaching. To prove this, we performed the reactions and analyzed the final reaction mixtures by elemental analysis and the resulting materials by different microscopies. We found that the difference in catalytic activity between three ZIF-67 and one ZIF-L catalysts was related to the rate at which the materials degraded. Particularly, the {100} facet leaches faster than the others, regardless of the material used. The catalytic activity strongly depended on the amount of leached elements in the liquid phase since these species are extremely active. Our work points to the instability of these materials under relevant reaction conditions and the necessity of additional treatments to improve their stability.
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