(59d) Nanoconfined Homogeneous Catalysts for Acylation Reactions

Homogeneous catalysts are typically much more selective than heterogeneous ones, but used much less frequently in industry due to the difficult separation of the catalyst from the reactant/product mixture and hence limited re-use of catalyst. Hence, immobilization of homogeneous catalysts has become the focus of broad efforts over recent decades with the goal of facilitating separation of the catalyst and its repeated re-use without loss of activity. Among the different approaches toward immobilization, in particular adsorption, tethering (or grafting), and, more recently, encapsulation have been actively pursued.  Both adsorption and tethering often result in configurational changes of the catalyst molecule which can negatively affect its catalytic activity. Encapsulation, on the other hand, avoids this issue but often requires complex (and expensive) preparation procedures.

In the present work, we are investigating the encapsulation of homogenous acylation catalysts in inorganic core-shell nanostructures. Specifically, pyridine and 4-dimethylaminopyridine (DMAP), two well-established acylation catalysts were encapsulated in silica “nanobubbles”, i.e. hollow silica nanoparticles (~30 nm diameter), in which silica walls (~10 nm thickness) enclose a pronounced central cavity. The walls are highly porous allowing for facile reactant access to the central cavity. The materials were synthesized and characterized (TEM, BET), loaded with pyridine or DMAP, respectively, through a simple wet impregnation procedure, and then tested in the acylation of 4-methoxybenzyl alcohol with acetic anhydride to 4-methoxybenzyl acetate.  Thin layer chromatography (TLC) was used as a quick and simple method to monitor the reaction progress.  Our results to-date demonstrate that it is indeed possible to encapsulate the homogeneous catalysts through this simple preparation procedure, and that the encapsulated catalysts maintained their catalytic activity.  Further detailed studies of the impact of the nano-encapsulation on the kinetics of the reaction, as well as recyclability of the encapsulated catalysts are currently under way.

Overall, our results indicate that the encapsulation of homogeneous acylation catalysts into such silica “nanobubbles” could be a simple and promising route for immobilization of this class of catalysts.