Model Based Assessment of Acid Strength effects on Acid-Base

Cooperative Aldol Condensation

Jeroen Lauwaert1, Pascal Van Der Voort2, Joris W. Thybaut1*, Guy B. Marin1

1 Ghent University. Department of Chemical Technology. Laboratory for Chemical Technology. Technologiepark 914 9052

Ghent Belgium

2 Ghent University. Department of Inorganic and Physical Chemistry. Center for Ordered Materials, Organometallics & Catalysis. Krijgslaan 281 (S3) 9000 Ghent Belgium

(*) joris.thybaut@ugent.be

Keywords: microkinetic model, aldol reaction, acid-base, synergy

Aminated silica materials are known to efficiently catalyze aldol condensations, especially when weak acid sites are neighboring the amine function. An adequate understanding of the synergy effects between the two types of sites is crucial for the rational design of an optimal acid-base cooperative aldol condensation catalyst. In a previous work, we have investigated the effects of the amine structure and the base strength at a given acid strength of the promoting site by means of kinetic modeling [1]. In this work, we extend this investigation towards the effects of the acid site.
A heterogeneous catalyst containing primary amines and weak acidic silanol groups was synthesized by replacing a fraction of the silanol groups on the surface of silicagel 60 with an amine-containing silane by means of a reflux procedure. In addition, a heterogeneous catalyst containing primary amines and carboxylic acids was synthesized by grafting a vinyl group and subsequently functionalizing this vinyl group with cysteine by means of a thiol-ene click reaction. Both materials were characterized by means of nitrogen adsorption-desorption measurements, elemental (CHNS) analysis and Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy.
Afterwards the catalyst performance was assessed via the aldol condensation of acetone with 4- nitrobenzaldehyde. The effect of the reaction conditions on the catalyst activity was investigated by varying the reaction temperature and the initial concentration of reactants in a systematic manner. The obtained kinetic data showed that a higher acid strength results in a lower catalytic activity of the amine site.
In order to obtain more insights in the effects of the acid strength on the catalyst performance two additional heterogeneous catalyst combining an amine with different types of acid sites, i.e. a sulfonic acid and a phosphoric acid will be synthesized, characterized and tested via the aldol condensation of acetone with 4-nitrobenzaldehyde.
Based on an in depth analysis of the obtained kinetic data a reaction mechanism of the aldol condensation is proposed, which is the basis for the construction of a microkinetic model. The model contains two types of parameters. Kinetic descriptors which solely depend on the reaction mechanism and catalyst descriptors which specifically account for the effect of the catalyst properties on the kinetics. All descriptors, both kinetic and catalyst, will be either determined from independent catalyst characterization measurements, taken from our previous work [1] or estimated by minimization of the residual sum of squares when simulating the kinetic data.
An enamine compound formed by the reaction between the amine active site and acetone is identified as the key intermediate in the reaction mechanism. The promoting effect of the acid sites
results from the formation of hydrogen-bridge interactions between the carbonyl moieties of the reactants and the acid sites, which facilitates the nucleophilic reactions with the amine site. These hydrogen-bridge interactions become more pronounced with increasing acid strength. However, an increasing acid strength of the promoting site also leads to a shift in the equilibrium from the free acid and free base towards the resulting neutralized ion pair, which leads to a decreasing number of active sites and a decreasing catalytic activity. These are two counteracting effects which will be quantified using the kinetic model.
In a final step the model will be used to determine optimal catalyst descriptor values, which can be used to provide feedback to the synthesis step and, hence, to develop a recipe for an ideal aldol condensation catalyst.
1. J. Lauwaert, E. De Canck, D. Esquivel, P. Van der Voort, J.W. Thybaut, G.B. Marin, Effects of Amine Structure and Base Strength on Acid-Base Cooperative Aldol Condensation (Submitted). Catalysis Today, 2014.