(387d) Use of Titration Experiments to Investigate Activation Energies and Mass Transfer Limitations of Different Catalytic Sites

Immobilizing amines on a silica support creates a versatile class of materials known as aminosilicas, that have applications in catalysis, drug delivery, and carbon capture. As catalysts, aminosilica materials are highly active for aldol chemistry because of amines that act cooperatively with surface silanols. Recent research has shown that aminosilicas have multiple types of sites. High activity sites with surface silanols employ acid-base cooperativity whereas some sites are inactive. These observations raised important questions about the different species present and the contribution of kinetics and mass transfer to the catalyst performance. This work investigates these types of sites through titration experiments to probe activation energies (E_A) and evaluate mass transfer to inform future material design.

In this work, primary (1°) or secondary (2°) functionalized SBA-15 catalysts are tested for activity in the aldol reaction between 4-nitrobenzaldehyde and acetone. Titration experiments with an acid are performed to investigate the catalytic performance on a per-site perspective. The catalyst E_A are elucidated with an Arrhenius plot. The E_A for the 1° and 2° SBA-15 are 27 kJ/mol and 20 kJ/mol, respectively. Compared to literature values, the values are consistent with a mass transfer limited regime. Interestingly, SBA-15 synthesized to have a shorter path length have comparative E_A, showing that the results are not mass transfer limited.

Through performing site quantification experiments at different temperatures, we can determine the E_A for each type of site. These experiments showed that the E_A of the individual types of sites were higher than the overall material E_A, highlighting the importance of probing materials to further understand the amount and types of sites present. Ongoing work seeks to identify the nature of these sites to design materials with only highly active sites, which is facilitated by these techniques to better quantify catalyst systems.