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.