(177f) Preparation of Novel Al-MFI/Fe-MFI Core-Shell Catalysts and Their Catalytic Application for CH4 Conversion
The MFI-type ferrisilicate zeolite (Fe-MFI) has exhibited a catalytic activity for the production of CH3OH from CH4. On the other hand, it is well-known that the MFI-type aluminosilicate zeolite (Al-MFI) is an excellent catalyst for the methanol to olefins (MFI) reaction. Based on these facts, we have focused on the development of a novel core-shell catalyst consisting of Fe-MFI and Al-MFI for direct conversion of CH4 into CH3OH followed by lower olefins.
First, we prepared the composite catalyst of Al-MFI and Fe-MFI as core and shell parts, respectively, as follows. Al-MFI (Si/Al=30) was added into the mother gel for Fe-MFI (Si/Fe = 50 in gel) with the weight ratio of Al-MFI / Fe-MFI of 1, and thus prepared mixture was hydrothermally treated at 443 K for 5 days. The product was designated as âFe-MFI/Al-MFIâ. The âAl-MFI/Fe-MFIâ was also prepared by using Fe-MFI (Si/Al=228).Â Both products had a pure MFI-type structure. The FE-SEM images of the Al-MFI/Fe-MFI core-shell catalyst indicated that the Fe-MFI 100-200 nm in size was covered with the Al-MFI nanocrystal 20-30 nm in size.
The catalytic reaction was performed in a fixed bed reactor. The flow rates of the reactants were CH4/O2/Ar = 16/4/5 (SCCM). The catalyst amount was 100 mg. The reaction temperature was varied ranging from 300 to 600 Â°C, and the reaction time at each temperature was 1 min. The products including CO and CO2 were analyzed by GC-TCD, and other hydrocarbon products were analyzed by GC-FID.
At the reaction temperature at 300 Â°C, the Fe-MFI/Al-MFI core-shell catalyst did not show any activity for CH4 conversion. However, the Al-MFI/Fe-MFI core-shell catalyst gave the CH4 conversion of 0.1 %, and it slightly produced propene, which would be formed from CH3OH. The CH4 conversion was increased along with the reaction temperature. At the reaction temperature of 500-600 °C, the Fe-MFI /Al-MFI core-shell catalyst exhibited a higher conversion than the Al-MFI/Fe-MFI core-shell catalyst, and it gave CO, CO2, ethane and ethane as products. Note that in addition to these products, dimethyl ether (DME) was significantly produced over the Al-MFI/Fe-MFI core-shell catalyst with the CH4 conversion of 5.5 %, while main products were still CO and CO2 with their selectivities of almost 99 %.
It is considered that, in the Al-MFI/Fe-MFI core-shell catalyst, CH4 was activated at Fe-MFI as core part, forming CH3OH, which would be immediately converted into DME followed by lower olefins via the MTO recation. On the other hand, in the Fe-MFI/Al-MFI core-shell catalyst, CH4 was activated at Fe-MFI as shell part, and successive oxidation would easily occur, resulting in the higher conversion. The impacts of the ratio of Al-MFI and Fe-MFI parts, and their components on the catalytic activity are currently under investigation.