Figure 7. Optimized periodic structures of a
H-ZMS-5, b Ce-ZSM-5 and c K-ZSM-5;
C2H4 (i) ,
C3H6 (ii) and
C4H8 (iii) are adsorbed on
zeolite with adsorption energies (Eads ). The
alkali metal bond with O in Si-O-Al.
With the introduction of zeolite, olefins generated on iron-based
catalysts are further reacted to form long-chain hydrocarbons and
aromatic hydrocarbons. To investigate the process mechanism,
C3H6 was selected as probe reactant to
detect dynamic changes on ZSM-5 by in situ DRIFT. As shown in the
Figure 6c, 6d and Figure S16, the wavenumber in 3000
cm-1 is attributed to the double-bond telescopic
vibration of the C=C of C3H6. For
H-ZSM-5, three distinct infrared absorption peaks of benzene rings
appear between 1500 cm-1 to 1200
cm-1, which indicates that
C3H6 has been aromatized over H-ZSM-5.
When the H-ZSM-5 catalyst is located in a
C3H6 atmosphere, the absorption peak
belonging to C3H6 gradually decreases,
and the absorption peak of the benzene ring gradually increases. As
expected, the main olefins from ZnFe2O4catalyst are reacted at the acid site of ZSM-5 to produce more
gasoline-range hydrocarbons, especially isomeric and aromatic compounds
with high octane numbers. In terms of K-ZSM-5, there are other infrared
absorption peaks between 1700-1500 cm-1 and 1000-900
cm-1, which are attributed to alternatives to benzene
or isomeric hydrocarbons. These evidences show that ZSM-5 treated with K
ions can efficiently transform olefins to gasoline hydrocarbons (Figure
3, 6 and S16). Clearly, the regulation of the microenvironment of
zeolite through ions exchange is conducive to the oriented generation of
C5+ hydrocarbons.
In order to understand the intrinsic reason for the enhanced catalytic
selectivity over an alkali metal (K and Ce) modified ZSM-5 catalyst, DFT
calculations were performed to investigate the structure and electronic
properties of the catalysts, as well as the adsorption of intermediates
(C2H4,
C3H6, and
C4H8). As shown in Figure 7, the
introduction of alkali metal does not affect the overall structure of
the H-ZSM-5 catalyst, maintaining the MFI zeolite structure. It has been
reported that after ions exchange in ZSM-5, ions often replace Al on the
backbone to change its surface acidity.18 By
optimizing the individual ZSM-5 zeolite rings, the adsorption energy of
CnH2n was calculated.