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).