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.