Smart design of solid base catalysts with different pore architecture, properties and functional groups for the transesterification of triglycerides and the selective oxidation cascade of benzyl alcohol with benzyl amine

Abstract

Internal diffusion limitations within a heterogeneous catalytic material can prove problematic for industrially relevant chemical reactions, especially when using large reagent molecules. This work explores the use of a smart pore system design through a dual templated silica support for biodiesel synthesis and oxidation reactions. One-pot MgO/SBA-15 was synthesised, with selective functionalisation within the mesopores to allow for excellent dispersion. These materials were further improved by adding a macroporous network, resulting in a hierarchical pore system. When screened for the transesterification of triglycerides, the hierarchal materials outperformed the diffusion-limited mesoporous materials. In addition, the incorporation of a macroporous template improved the materials pore integrity and structure. Through manipulation of the pH during synthesis, MgO/SBA-15 was further functionalised with gold to be screened in selective oxidation reactions. Again, hierarchical materials were superior to mesoporous materials. In addition, chemometrics was used to assess key variables related to performance and selectivity. The heat-treatment temperature proved to be crucial for the activity, with 400 °C being optimum to increase activity and prolong catalyst life-time. Additionally, the addition of potassium carbonate mitigated substantially the deactivation of the catalyst produced by the reaction products. Subsequently, gold supported on MgO, CaO and SiO2 materials were synthesised and screened for the selective oxidation of benzyl alcohol, with the aim to compare their performance against Au/MgO/SBA-15 synthesised materials. Initial activity was found to relate to the gold loading of each material. However, deactivation was correlated to the basic nature of the support, as predicted. The most basic supports Au/CaO and Au/MgO deactivated slower and were unaffected by the addition of potassium carbonate. Finally, all materials were screened for the oxidative coupling of alcohols and amines. Au/CaO and Au/MgO hindered the oxidative coupling. Upon addition of potassium carbonate, the Au/MgO/SBA-15 hierarchical material proved to be the superior material tested.