(177f) Preparation of Novel Al-MFI/Fe-MFI Core-Shell Catalysts and Their Catalytic Application for CH4 Conversion

Methane is a highly abundant and inexpensive source of fuel and chemicals. The development of novel technologies that can convert methane easily into chemicals has strongly been desired. However, its kinetic inertness and low reactivity limit the industrial utilization. We are tackling the development of novel catalysts for direct conversion of methane into methanol followed by lower olefins.

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 “Fe-MFI/Al-MFI”. The “Al-MFI/Fe-MFI” was also prepared by using Fe-MFI (Si/Al=228). 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 °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 °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.