(135d) Reactions of Methyl Esters On Supported Platinum Catalysts

Methyl esters can be derived from various renewable sources such as vegetable oils and biomass. This study will focus on several aspects of conversion of methyl esters over supported platinum catalysts including reaction mechanisms, catalyst design, and production of targeted products. Methyl octanoate and methyl hexanoate are used as probe molecules. Catalysts are characterized by CO chemisorption, TPR, TPO, and XPS techniques. In the gas-phase reaction of methyl octanoate in hydrogen, 1%Pt/Al₂O₃ is an active and selective catalyst. The dominant products are C₇ hydrocarbons that come from deoxygenation reactions of various ester-derived oxygenates. When titania replaces alumina as a support, hydrogenation of the ester is promoted, leading to an increase in C₈ hydrocarbon selectivity. Without hydrogen, 1% Pt/Al₂O₃ catalyst deactivates quickly by coking.

In this contribution we have studied the behavior of different platinum catalysts in the conversion of ester without hydrogen. In the gas-phase reaction of methyl hexanoate over silica-supported Pt-Sn-K catalysts, it was found that coupling or condensation products (i.e. ketonization products) are formed first. Subsequently, these products are transformed into lighter oxygenate compounds and hydrocarbons (mainly C₅ and C₆) via decarboxylation and decarbonylation routes. The silica support itself shows a low level of conversion of methyl hexanoate in helium. All of its activity goes to formation of condensation products. These condensation products, together with surface carboxylates and ester molecules, oligomerize to form ?oxygenate? type of coke on silica. 1% Pt/SiO₂ is active at the beginning of the reaction due to the presence of clean Pt ensembles. However, pure platinum catalyst deactivates quickly to form more refractory coke. This carbonaceous species act as a source of hydrogen via dehydrogenation reactions. It is the spilled-over hydrogen that cleans the silica support for condensation reaction after Pt/SiO₂ deactivates. Addition of Sn and K into platinum has several simultaneous effects. Firstly, the Pt ensembles are ruptured owing to Sn and K coverage and formation of Pt-Sn alloy. Secondly, potassium enhances the segregation of Sn out of the Pt-Sn alloy. The addition of appropriate amounts of Sn into Pt shows a positive impact on the conversion of methyl hexanoate. Adsorption energy of the ester is reduced on Pt-Sn alloy due to the facilitated interaction of the ester molecule with cationic Sn species. It is the alloy that extends the lifetime of Pt-Sn catalyst by promoting desorption of alkene-derived coke precursors. As a sequence, coke deposition is reduced on 1%Pt-1.3%Sn/SiO₂ catalyst compared to pure platinum catalyst. The introduction of a third metal, potassium, enhances the overall activity even further. Potassium domains accompanied by SnO_x are effective in converting the original ester into coupling products, which are then transformed into hydrocarbons on the available Pt ensembles of Pt-Sn_x-K_y catalysts. Among all tested Pt-Sn-K catalysts, 1%Pt-1.3%Sn-0.5%K/SiO₂ exhibits the highest activity and stability in reaction of methyl hexanoate under helium.