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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. [ABSTRACT FROM AUTHOR]

• Aminated silica catalysts deactivate due to water as solvent in the aldol reaction. • Water-stable PEG containing polymer resins have been functionalized with diamines. • The amine-PEG catalysts show a high activity for the aqueous aldol reaction. • In continuous-flow, the amine-PEG catalyst is stable for at least 8 h on-stream. Amine functionalized silicas have frequently been investigated as potential aldol reaction catalysts. However, active site leaching due to hydrolysis cannot be avoided, limiting the long-term stability of these catalysts in aqueous aldol reactions. Therefore, novel catalysts based on an organic resin have been developed starting from a suspension polymerized poly(ethylene glycol) methacrylate (PEGMA) hydrogel with poly(ethylene glycol) dimethacrylate (PEGDMA) as cross-linker. Amine functionalization was performed by chlorination of the terminal hydroxyl groups in the resulting PEGMA resin and subsequent nucleophilic substitution with an amine precursor, i.e. , ethylenediamine (EDA), N,N′-dimethylethylenediamine (DED), or methylamine (MA). The successful synthesis of the catalysts was confirmed by 13C NMR, FT-IR, and elemental CHN analysis. Performance evaluation in a batch reactor for the aqueous aldol reaction of acetone with 4-nitrobenzaldehyde resulted in a turnover frequency (TOF) of the PEGMA-EDA catalyst amounting to 6.3 ± 0.4 · 10−4 s−1, which is of the same order of magnitude as that of the corresponding state-of-the-art amine functionalized silica evaluated using hexane as solvent. The PEGMA-DED catalyst exhibited a somewhat lower TOF of 3.1 ± 0.2 · 10−4 s−1, while the PEGMA-MA catalyst did not exhibit any turnover, indicating that the secondary amine in the backbone of the active site in the PEGMA-EDA catalyst is inactive. Continuous-flow evaluation of the PEGMA-EDA catalyst in a packed-bed reactor indicated that, as opposed to a primary amine functionalized silica catalyst, a stable catalytic activity as a function of time on stream could be achieved for at least 8 h and, hence, that no deactivation has occurred in this timeframe. [ABSTRACT FROM AUTHOR]

• Computational nucleophilicity probing of amines for the aldol reaction. • Trimethylborane (TMB) probes steric and electronic substituent effects on amines. • The experimental activity of hindered amines in the aldol reaction can be described. • New amine sites with an optimized TMB basicity are designed for the aldol reaction. 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. [ABSTRACT FROM AUTHOR]

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, 13 C 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. [ABSTRACT FROM AUTHOR]

• Water and silanol effects on amine catalyzed aldol reactions are elucidated. • 2 water molecules are optimal for reactions involving a proton transfer. • 1 water molecule is sufficient to prevent site-blocking enamine species. • Silanols assist the amine in a similar but slightly more pronounced manner. • The effect of water is more pronounced in hexane as compared to DMSO. The experimentally observed effects of water, and the effects of silanol groups, on the liquid-phase amine-catalyzed aldol reaction of acetone with 4-nitrobenzaldehyde have been elucidated with a comprehensive theoretical model describing the reaction kinetics. The CBS-QB3 model chemistry is used, with bulk solvent effects accounted for according to COSMO-RS theory, and water molecules explicitly considered. Two promoting water molecules were found to be optimal in reducing the Gibbs free energy barriers for reactions involving a proton transfer, i.e., the carbinolamine and enamine formation steps, and the aldol product liberation step. The presence of one water molecule in the carbon-carbon coupling step was already sufficient to prevent the formation of a site-blocking enamine species, which would otherwise lead to deactivation of the amine. Compared to a single water molecule, isolated silanol groups were found to assist the amine in a similar and slightly more pronounced manner, resulting in overall lower barriers for all transition states. Promotion by two vicinal silanols resulted in even lower barriers. The effect of water was found to be more pronounced in apolar hexane as compared to DMSO, i.e., resulting in a less pronounced deactivation of the amine, and lower barriers for reactions where water is assisting in the transition state. [ABSTRACT FROM AUTHOR]