REACTION MECHANISM OF METHANE TO METHANOL CONVERSION OVER [CUOH]+ ACTIVE SITE IN ZEOLITE

• Main Author: Timothy Lasiman, Elbert
• Format: Final Project
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/65182
• Institution: Institut Teknologi Bandung

Methane is one of the natural gases that is abundant on Earth and has many benefits, but also one of the greenhouse gases. One of the utilizations of methane is by converting it to methanol. The advantage of methanol is that its liquid form makes it easy to transport. In industry, methane to methanol conversion is by converting methane to synthesis gas first. This reaction is energy-intensive with a temperature up to 1000 oC. Another alternative is the direct oxidation of methane to methanol. Direct partial oxidation has a lower reaction temperature, around 300-500 oC. However, the methanol yield in this reaction is very low. Catalyst is used to overcome this issue. There are many types of catalysts, and one of them is zeolite. Zeolite is an aluminosilicate material, with over 200 frameworks. Many experiments use zeolite as a catalyst to convert methane to methanol. Usually, the experiment changes some parameters to maximize methanol yield, for example, the zeolite framework, Si/Al ratio, and other parameters such as Al/Cu. Another important point is the zeolite active site, which is the site where methane to methanol conversion takes place. For Cu-based zeolite, the active site is categorized based on the number of coppers in the site. Previously, it is believed that more copper in a site is proportional to the reaction activity. However, the newest experiment reported that the monocopper active site, [CuOH]+, has a methane activation energy lower than the dicopper active site, [Cu-O-Cu]2+. There is a previous computational study that reports the mechanism of how methane is converted to methanol in [CuOH]+ active site. However, the methane activation energy reported by this study is very high compared to the experiment, and there are some speculations in the mechanism. There is no complete theoretical understanding of methane to methanol conversion in the [CuOH]+ active site. With a more comprehensive theoretical understanding, better catalyst can be designed to convert methane to methanol. In this study, a more standard computational is method, and more structures are considered. Two bond cleavage reaction mechanisms, which are homolysis and heterolysis are considered. Two reaction mechanisms that involve water or Cu2+ active site is also considered. The obtained structures are analyzed by thermodynamics and kinematics. The kinetics outcome will be used to compare the result with the experiment results. This is to confirm if the calculated result is correct. Density functional theory is used in this study for the calculation. There are two main steps for the calculation, which are the structure optimization and finding the transition structure with climbing image nudged elastic band method. The results shows that the reaction mechanism pathway involving water cannot form methanol. This is due to the highly endothermic energy of system upon H2 formation, making an unstable system. On the other hand, the reaction mechanism pathway involving the Cu2+ inactive site can convert methane to methanol. The Cu2+ inactive site and the [CuOH]+ active site are contained in the 12-membered ring, and the formation of the sites depend on the location of Al. In this pathway, the active site activates methane with an activation energy of 0,26 eV with homolytic cleavage. This energy result is comparable to the experimental value, which is 0,34 ? 0,1 eV. The role of Cu2+ non-active site in the reaction is to accommodate the H atom from [CuOH]+ active site. ?