Figure 5. (a) the NH3-TPD patterns and (b) the pyridine-adsorption Fourier transform infrared (FT-IR) spectra of H-ZSM-5, Ce-ZSM-5, K-ZSM-5 and H-ZSM-5*.
When zeolite is treated with active components such as metal ions or oxides, new Lewis acidity site and active centers can be formed. With alkali metals K and Ce ions introduction, the H+ ratio in the ZSM-5 skeleton decreases and the strength of the acidic site decreases. By comparing differences in catalytic performance, the decrease in acidic strength increases the catalytic performance of CO2 hydrogenation (Figure 3 and 5a).53Additionally, the pore size distribution and specific surface area of ZSM-5 change slightly after ions exchange (Figure S13). This phenomenon can be ascribed that alkali metal ions occupy a larger space in the pores than H+. The acid properties of M-ZSM-5 are further investigated by pyridine-adsorption FT-IR spectra (Figure 5b and Table S9). It is widely accepted that the Brönsted acid sites play a dominant role in isomerization, cracking, and aromatization reactions.53 As shown in the Figure 5b, the band at 1545 cm-1 is ascribed to Brönsted acid sites formed by the framework Al species, while the band at 1456 cm-1is related to the Lewis acid sites formed by the extra framework Al species. Compared to H-ZSM-5, the intensity of the band at 1545cm-1 decreases obviously with the introduction of alkali metals. At about 1456 cm-1, the intensity of Lewis acid site increases slightly. The variation is same as the result of NH3-TPD, and illustrate that the substitution of the skeleton Al by other metal ions led to a decrease in Brönsted acid and an increase in Lewis acid site. From SEM and TEM images of the ZSM-5, it is observed that crystals appear on the surfgureace of ZSM-5 after the introduction of Ce, and Ce-ZSM-5 has large amounts of metal particles that are significantly different from H-ZSM-5 (Figure S3-S5). Besides, the Al coordination of zeolite determined by 27Al magic-angle spinning (MAS) NMR is shown in Figure S14 and Table S10. The percentage of extraframework Al increases (AlEF, at 0 ppm), while that of the framework Al decreases (AlF, at 55 ppm).16,46 The K-ZSM-5 and parent H-ZSM-5 have the same proportion of AlF, which indicates that K ions weaken the acidic site strength by weakening Si-OH-Al. Meanwhile, the Ce ions occupies a large number positions of extraframework Al, resulting in a larger proportion of AlF, but it still present weaker acidic strength than H-ZSM-5.
CO2 hydrogenation behaviors over K-ZnFe2O4, K-ZnFe2O4@H-ZSM5, K-ZnFe2O4@Ce-ZSM-5, and K-ZnFe2O4@K-ZSM-5 were further studied by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. As illustrated in Figure 6a, the absorption intensity in the region between 3800 cm-1 and 3500 cm-1 continues to increase for K-ZnFe2O4, which corresponds to the hydroxyl vibrational bands belonging to H2O. The absorption intensity in the region from 3000 cm-1 to 2940 cm-1 gradually decreases, which usually corresponds to the -CH2. The absorption peaks at wavenumber less than 3000 cm-1 shift to higher wavenumber (>3000 cm-1) with time, indicating that a component shift from saturated C-H bonds to unsaturated C-H bonds. Besides, absorption peaks located at 1680 cm-1 to 1500 cm-1, and about 1000 cm-1 could be attributed to the