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.