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.