Fischer–Tropsch
synthesis to lower olefins (FTO) opens up
a compact and economical way to the production of lower olefin directly
from syngas (CO and H2) derived from natural gas, coal,
or renewable biomass. The present work is dedicated to a systematic
study on the effect of K in the reduced graphene oxide (rGO) supported
iron catalysts on the catalytic performance in FTO. It is revealed
that the activity, expressed as moles of CO converted to hydrocarbons
per gram Fe per second (iron time yield to hydrocarbons, termed as
FTY), increased first with the content of K, passed through a maximum
at 646 μmolCO gFe
–1 s–1 over the FeK1/rGO catalyst, and then decreased at
higher K contents. Unlike the evolution of the activity, the selectivity
to lower olefins increased steadily with K, giving the highest selectivity
to lower olefins of 68% and an olefin/paraffin (O/P) ratio of 11 in
the C2–C4 hydrocarbons over the FeK2/rGO
catalyst. The volcanic evolution of the activity is attributed to
the interplay among the positive effect of K on the formation of Hägg
carbide, the active phase for FTO, and the negative roles of K in
increasing the size of Hägg carbide at high content and blocking
the active phase by K-induced carbon deposition. The monotonic increase
in the selectivity to lower olefins is ascribed to the improved chain-growth
ability and surface CO/H2 ratio in the presence of K, which
favorably suppressed the unwanted CH4 production and secondary
hydrogenation of lower olefins.
We devised iron-based catalysts with honeycomb-structured graphene (HSG) as the support and potassium as the promoter for CO direct hydrogenation to light olefins (CO-FTO). Over the optimal FeK1.5/HSG catalyst, the iron time yield of light olefins amounted to 73 μmol g s with high selectivity of 59%. No obvious deactivation occurred within 120 h on stream. The excellent catalytic performance is attributed to the confinement effect of the porous HSG on the sintering of the active sites and the promotion effect of potassium on the activation of inert CO and the formation of iron carbide active for CO-FTO.
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