Kinetic evaluation of chitosan-derived catalysts for the aldol reaction in water

The site time yield (STY) and stability of the primary amine sites in low molecular weight chitosan have been quantified for the aldol reaction of acetone with 4-nitrobenzaldehyde in a mixture of water and acetone as a solvent. Crude chitosan with varying degrees of deacetylation (DDA), as well as chitosan in hydrogel and aerogel forms, was used. Apart from the main reaction, accumulation of an imine formed from 4-nitrobenzaldehyde occurred in the early stages of the reaction. This imine acted as an inhibitor of the primary amine sites and was formed until an equilibrium was reached, after which the catalytic activity remained constant. Chitosan with a DDA amounting to 70.4% exhibited a STY of 2.18 +/- 0.05 x 10(-5) mol(product) mol(amine)(-1) s(-1). This STY increased with decreasing DDA, as a direct result of an increase in amine pK(a). No differences in activity were observed between the crude, hydrogel, and aerogel forms of chitosan with the same DDA. Recycling in a second batch experiment allowed reproducing the same performance as that in the first experiment. Under continuous-flow conditions, the activity of chitosan was found to stabilize as a function of the time on stream, after the imine formation has equilibrated. Even though the catalytic activity of these chitosan catalysts was found to be lower than those of the current state-of-the-art catalysts for the aldol reaction, their stability in an aqueous environment opens new perspectives for future catalyst development

The role of water in the reusability of aminated silica catalysts for aldol reactions

The reusability of methylaminopropyl active sites grafted on mesoporous amorphous silica, either with cooperative silanol groups or trimethylsilated, was assessed in the aldol reaction of acetone with 4-nitrobenzaldehyde. Raman, C-13 NMR, and UV-Vis spectroscopy demonstrated the presence of stable enamines on the spent catalysts. These enamines are produced as a side product from iminium intermediates in the catalytic cycle. Water co-feeding enhances the desorption of the iminium intermediates and, hence, suppresses the formation of these stable enamine species. The reusability of the cooperative catalyst increased to 70% with co-feeding 0.69 wt% water, while an almost complete reusability was achieved for the trimethylsilated catalyst. Continuous-flow experimentation showed that the cooperative effect of the silanol groups was lost during the first 7 h on-stream, yet activity losses continued, most likely due to silica hydrolysis. Activity losses persisted on the more hydrophobic trimethylsilated catalyst, but were significantly less pronounced

Rational design of nucleophilic amine sites via computational probing of steric and electronic effects

Accessibility of the nucleophilic site in organocatalysts is essential to ensure adequate catalytic activity. Gas-phase trimethylborane (TMB) Lewis basicity and Brønsted proton basicity of several amine based organocatalysts have been calculated using the CBS-QB3 model chemistry. This TMB basicity scale can, as opposed to the proton basicity scale, account for steric effects encountered in the initial nucleophilic attack of the nitrogen free electron pair on a substrate. Since such a step is the first one in several amine catalyzed reactions, severe steric hindrance of the nucleophilic center would render the catalyst ineffective. Comparing the TMB basicity and proton basicity with the experimentally observed catalytic activity of both homogeneous and heterogeneously supported amine sites found in literature for the aldol reaction of acetone with 4-nitrobenzaldehyde showed that, due to the inclusion of these steric effects, the TMB basicity scale is a much better predictor of catalytic activity than the proton basicity. According to this computational Lewis basicity scale, potential steric hindrance in alternative nitrogen containing active sites was probed. This resulted in 3-propylpyrrolidine being proposed among the most promising monofunctional amine groups and 1-(methylamino)propan-2-ol among the most promising bifunctional amine-hydroxyl groups for heterogeneous aldol reaction catalysts

The effect of water on the reusability of aminated mesoporous silica catalysts for Aldol condensations

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Catalyst stability assessment in a lab-scale liquid-solid (LS)² plug-flow reactor

A packed-bed plug-flow reactor, denoted as the lab-scale liquid-solid (LS)² reactor, has been developed for the assessment of heterogeneous catalyst deactivation in liquid-phase reactions. The possibility to measure intrinsic kinetics was first verified with the model transesterification of ethyl acetate with methanol, catalyzed by the stable commercial resin Lewatit K2629, for which a turnover frequency (TOF) of 6.2 ± 0.4 × 10−3 s−1 was obtained. The absence of temperature and concentration gradients was verified with correlations and experimental tests. The potential for assessing the deactivation of a catalyst was demonstrated by a second intrinsic kinetics evaluation where a methylaminopropyl (MAP)-functionalized mesoporous silica catalyst was used for the aldol reaction of acetone with 4-nitrobenzaldehyde in different solvents. The cooperative MAP catalyst deactivated as a function of time on stream when using hexane as solvent. Yet, the monofunctional MAP catalyst exhibited stable activity for at least 4 h on stream, which resulted in a TOF of 1.2 ± 0.1 × 10−3 s−1. It did, however, deactivate with dry acetone or DMSO as solvent due to the formation of site-blocking species. This deactivation was mitigated by co-feeding 2 wt % of water to DMSO, resulting in stable catalyst activity