3.3. Exploration of ZSM-5 acid-catalysed reaction process
By coupling K-ZnFe2O4 core catalyst with K-ZSM-5 shell catalyst, the selectivity of gasoline hydrocarbons in the product is improved. To confirm the influences of different zeolites on the target product selectivity, it is necessary to measure the intensities and type of acidic sites of zeolites. NH3-TPD patterns of H-ZSM-5, Ce-ZSM-5, K-ZSM-5 and H-ZSM-5* were shown in the Figure 5a. The surface acidity of ZSM-5 changes obviously after different metal ions modification. After Ce ions exchange, the surface strong acid of ZSM-5 (above 380oC) decreases, but not as significantly as that of K-ZSM-5. Besides, the surface weak acid of K-ZSM-5 also decreases obviously. These phenomena confirm that the introduction of alkali metals causes the change of acidic sites of ZSM-5. However, the acidic sites of H-ZSM-5* is also stronger than unprocessed H-ZSM-5. The purpose of NH3·H2O treatment for K-ZSM-5 is to replace K+ ions by NH4+ ions, and to verify that alkali metal ions are crucial factors leading to the weakening of acidic sites of ZSM-5. Zeolite is composed of SiO2 and Al2O3, which is easily to desiliconize and dealuminum in alkaline solution. N2adsorption-desorption isotherms and pore distributions (inset) of H-ZSM-5*, K-ZSM-5, Ce-ZSM-5, H-ZSM-5 were shown in Figure S13 and Table S8. Clearly, H-ZSM-5* has a bigger pore volume than H-ZSM-5 with NH3·H2O treatment. According to previous reports, appropriate pore reaming of zeolite was beneficial to improve the selectivity of macromolecular hydrocarbons.46 It explaines that H-ZSM-5* is more acidic because the specific surface area increases, and the expansion of pores increases the activity of the catalyst (Figure S9). K-ZSM-5 exhibits a same physical adsorption and desorption results with parent H-ZSM-5. These findings verify that the introduction of K ions weakens the acidity of the ZSM-5. Evidently, it is feasible to regulate the selectivity of the product by regulating the acidity and pores of ZSM-5.