Figure 3. (a) Catalytic performances over different catalysts;(b) the catalytic stability of K-ZnFe2O4@K-ZSM-5 catalyst; (c)detailed hydrocarbon distribution over bi-functional catalysts with different ions-exchange strategies for zeolites (K-ZnFe2O4, K-ZnFe2O4@H-ZSM5, K-ZnFe2O4@Ce-ZSM-5 and K-ZnFe2O4@K-ZSM-5); (d) effects of contacting manner on catalytic performance. Reaction conditions, ZnFe2O4 to ZSM-5 is 0.2g to 0.15g, 2.0 MPa, 320 oC, 6000 mL·g-1·h-1 for ZnFe2O4, H2/CO2 = 3.
The gasoline-range hydrocarbons refer to high octane number hydrocarbons, e.g. aromatics and isoparaffins as a highly recognized octane contributor. Octane rating on isoparaffins increases with the number of branches, and such multibranched isomers synthesis are preferred in CO2 conversion. As shown in the Figure 3c, the main product of K-ZnFe2O4 is olefins-rich product, which occupies 64.8% in all hydrocarbons. After K-ZnFe2O4 catalyst encapsuled by H-ZSM-5 shell, the selectivity of gasoline hydrocarbons in the product changes slightly. However, for the types of hydrocarbon product, the reduction in the proportion of olefins in all hydrocarbons is obvious, while the selectivity of isoparaffins and aromatics in the gasoline range increases. The effect can be ascirbed to the introduction of ZSM-5, which increases the selectivities of isoparaffins and aromatics with the help of its pore structure and acidic sites. Compared with H-ZSM-5, the selectivity of C5+ hydrocarbons increases by 10% with the introduction of Ce. Besides, the proportion of isoparaffins and aromatics still increases in whole C5+ hydrocarbons. However, CH4 selectivity is higher than K-ZnFe2O4, which maybe because of the diffusion of the hydrocarbon product via a core-shell structure. The products of K-ZnFe2O4@K-ZSM-5 are aromatics as main component in C5+ hydrocarbons (Figure 3c). More importantly, the ratio of isoparaffins to aromatics gradually increases with the change of M-ZSM-5 (from H-ZSM-5 to Ce-ZSM-5 to K-ZSM-5). It supports that the olefins generated on the surface of K-ZnFe2O4 catalyst undergo polycondensation, isomerization, aromatization reactions through the acidic site of ZSM-5. Meanwhile, comparing the effects of zeolites with different ions modifications on the selectivity of target hydrocabon, verifies that K modified ZSM-5 exhibits evidently promoting effect for the oriented production of C5+ hydrocarbons.
Previously, different contacting manners of composite catalysts, such as physical mixing and multiple beds, will influence matching combination between different active sites, which in turn will affect the catalytic performance.51,52 It has been reported that a catalyst with a core-shell structure can enhance mass and heat transfer during the reaction comparing with one fabricated by physical mixing manner.49 The effect of contacting manner between K-ZnFe2O4 and K-ZSM-5 was investigated. Results of different contacting manners including core-shell catalysts, powder mixing, granule mixing, and dual bed were shown and summarized in the Figure 3d and Table S5. As for a powder mixing one (K-ZnFe2O4 and K-ZSM-5 are physically mixed firstly and then the mixtures are granulated to obtain 20-40 mesh), the selectivity of C5+ is only 20.5%. When K-ZnFe2O4 and K-ZSM-5 are integrated by granule mixing or dual bed, the selectivity of C5+hydrocarbons (about 62%) over both catalysts are evidently higher than physical mixing one, but lower than the capsule catalyst of K-ZnFe2O4@K-ZSM-5. Evidently, the capsule structure of K-ZnFe2O4@K-ZSM-5 exhibits an excellent CO2 hydrogenation performance, especially C5+ selectivity. Interestingly, these two kinds contacting manner both have a slight high CO2conversion than capsule catalyst. It is possible that the direct exposure of the ZnFe2O4 catalyst to the reaction atmosphere, and the diffusion influence of reaction gases in zeolite pore is reduced, which improves the utilization of reaction gases.
As discussed above, K-ZnFe2O4 catalyst coated by K-ZSM-5 shell presents an improved peformacne for CO2 hdyrogenation. Then, the effect of zeolite shell thickness on catalytic performance were further investigated. With the increase of shell thickness, the particle sizes of capusle catalysts increases obviously, which clearly indicates that the core K-ZnFe2O4 catalyst was coated with more zeolite (Figure S10). When the mass ratio of zeolite to Fe-based catalyst is 1:1, the K-ZnFe2O4@K-ZSM-5 shows the best performance (Figure 4a and Table S6). With the further increase of zeolite thickness, the selectivity of long-chain hydrocarbon significantly decreases, which can be ascribed to the overcracking of long-chain products (Figure 4b and 4c).