Figure 8. Main reaction routes for CO2hydrogenation over different composite catalysts.
The adsorption energy of C2H4, C3H6, and C4H8 was -0.10 eV, -0.31 eV and -0.39 eV, respectively, which are effective adsorption. The ZSM-5 structure with Ce and K modification was optimized, and the adsorption energy of olefins was calculated for the optimized structure. These detailed results were shown in Table S11 and S12. With the introduce of Ce and K into H-ZSM-5, the adsorption capacity of olefins on Ce-ZSM-5 and K-ZSM-5 is enhanced. It is worth noting that the Ce-ZSM-5 has strong adsorption of olefins. When metal ions bond with O, it has significantly enhanced adsorption behavior for C4H8 and stronger adsorption behavior for C3H6when it replaces Al (Figure S17). Strong adsorption and more acid sites lead to enhanced aromatization of olefins during the secondary reaction on Ce-ZSM-5. Combined with in situ DRIFT spectra of ZSM-5 (Figure 6 and S16), the main products of olefins are aromatic compounds over Ce-ZSM-5. Meanwhile, hydrocarbon formation in in situ DRIFT spectra of K-ZSM-5 is different from aromatic compounds. It explains that after the introduction of Ce, the ZSM-5 has a stronger adsorption behavior for olefins, which is conducive to aromatization and not conducive to isomerization, resulting in the formation of more aromatic compounds in the product (Figure 3c). The introduction of alkaline metal to regulate acidity changes the adsorption capacity of olefins intermediates over different ZSM-5 zeolites, which further affects olefins secondary reaction on ZSM-5 catalysts. As mentioned above, the microenvironment of H-ZSM-5 can be effectively regulated by alkali metal additives, which helps to achieve the guided synthesis of catalytic products. The reaction routes over different composites are also listed in Figure 8. For the conventional H-ZSM-5 catalyst, direct hydrogenation and aromatization of light olefins are mainly performed, exhibiting poor hydrocarbon selectivity. By contrary, olefins mianly occurs oligomerization and isomerization reaction over K-ZSM-5 zeolite, while it mainly occurs oligomerization and aromatization reaction over Ce-ZSM-5 zeolite.