3.2. Catalytic performance of
ZnFe2O4@ZSM-5
The results of catalytic performances were presented in Figure 3a and
Table S3. Na-ZnFe2O4 catalyst mainly
produces C2-C4 olefins, in which
CH4 selectivity is 16.6%,
C2-C4 selectivity is 53.3% and
C5+ selectivity reaches 30.1% at a CO2conversion of 28.3%. When K promoter is introduced, C5+selectivity increases from 30.1% to 49.2%, in which
CH4 is 11.0%, C2-C4selectivity is 39.8% at a CO2 conversion of 32.7%. In
our previous reports, we found that Na-modified
ZnFe2O4 catalyst facilitates the
formation of low-carbon olefins, while K-modification one facilitates
the formation of high-carbon hydrocarbons.24 Based on
the above discussion (Figure 2b, 2c, and S7), the performance difference
is due to the improved adsorption capacity of CO2modified with K promoter and the Fe-C structure of carbides more
electron deficient. After encapsulation by H-ZSM-5 zeolite shell, the
conversion of C2-C4 olefins selectivity
decrease, whereas the selectivities of CO and CH4slightly increase. After Ce ions exchange, C5+selectivity of K-ZnFe2O4@Ce-ZSM-5
increases from 47.8% to 59.9%. Meanwhile, the
C2-C4 olefins decreased profoundly than
the parent H-ZSM-5 (Figure 3a). With the treatment of K, the performance
difference between K and Ce modification are obvious. The selectivity of
C5+ hydrocarbon climbs to 71.7% from 59.9%, producing
more liquid fuels than Ce ions treatment. It indicates that changing the
microenvironment of the zeolite catalysts by ions exchange is a feasible
strategy for regulating products distribution. Catalytic stability of
the K-ZnFe2O4@K-ZSM-5 catalyst was
investigated and depicted in Figure 3b. As seen, the catalyst exhibits a
benign stability during the 80h reaction period. Liquid hydrocarbons
selectivity (C5+) maintains above 70%, and
CO2 conversion as well as un-desired CO byproduct almost
keep stable. Comparing with other metal oxide/zeolite composite
catalysts, the designed
K-ZnFe2O4@K-ZSM-5 catalyst present a
record-breaking C5+-yield based on per gram catalyst
(Figure S8 and Table S4). It indicates that the capsule catalyst
K-ZnFe2O4@K-ZSM-5 is a promising
catalyst for efficiently catalyzing CO2 hydrogenation to
liquid fuels.