Silanol‐Assisted Aldol Condensation on Aminated Silica: Understanding the Arrangement of Functional Groups

Abstract

Free silanol groups are known to promote the activity of aminated silica. In this work the effect of the silanol‐to‐amine ratio on the aldol condensation of 4‐nitrobenzaldehyde and acetone is investigated in a range from 0 to 2.4. Irrespective of the amine density, identical, moderate turnover frequencies are obtained if the silica exclusively has amines on its surface. The turnover frequency increases with increasing silanol‐to‐amine ratio until an upper limit is reached at a silanol‐to‐amine ratio of 1.7. At this upper limit the turnover frequency is a factor 5 higher than the turnover frequencies obtained with the monofunctional amine‐based catalysts. This increase is ascribed to hydrogen‐bridge interactions between the silanols and the carbonyl moiety of the reactants that provoke a more easy interaction between the carbonyl moiety and the amine as required for the aldol condensation. The observation that higher values than one for the silanol‐to‐amine ratio are required is rationalized by computer simulations. It was found that amine groups were grafted on the silica surface in a clustered manner, originating from positive deviations from ideality in the synthesis mixture, that is, from clustering of the amine precursor in the liquid phase.

Effects of amine structure and base strength on acid–base cooperative aldol condensation

“…The promoting effect of the silanol groups and the arrangement of active sites. The data reported in Table 3 clearly show how the activity exhibited by amine groups is enhanced by the presence of silanols, which confirms the cooperativity between the acid and base sites [27,28,58]. The product distribution is not significantly affected by the presence of silanol groups, however.…”

“…Recently we showed that the cooperativity does not only depend on the ratio of both types of active sites but also on the arrangement of these active sites on the catalyst surface [58]. Thermodynamic calculations for a n-propylamine-toluene mixture revealed positive deviations from ideal liquid behaviour, i.e., propylamine-propylamine interactions preferentially occur as compared to propylamine-toluene interactions.…”

“…In our previous work [58], a reaction mechanism has been proposed for the primary-amine-catalysed aldol condensation between 4-nitrobenzaldehyde and acetone. Scheme 2 represents a further elaborated version of this reaction mechanism which will allow assessing the amine structure and base strength effects investigated as part of the present work.…”

Reaction-dependent heteroatom modification of acid–base catalytic cooperativity in aminosilica materials

“…Pluronic P123 block copolymer (EO 20 All SBA-15 materials were prepared according to previous reports [18,19,37,[49][50][51][52][53][54][55]. The details of the synthetic procedures are in the Supporting Information.…”

“…The facile ability to separate and recycle heterogeneous catalysts from reactive fluids makes them attractive targets for incorporating acid-base cooperativity in new materials. While the results from homogeneous catalysts can be used as a basis for designing heterogeneous catalysts, incorporating cooperative catalytic elements is considerably more challenging in heterogeneous systems such as those based on silica supports because the silanols present on the surface, which act as organic functional group anchoring points or as weakly acidic cooperative partners, are typically not uniformly distributed [20][21][22][23][24][25][26]. Despite the complexity of silica-based catalysts, numerous studies demonstrate the ability to design materials that exhibit cooperative interactions, providing considerable insights into optimizing such catalysts.…”