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.

In the last 5 years, reductive catalytic fractionation of lignocellulose biomass has emerged as a promising biorefinery concept that combines biomass fractionation with the preservation of chemical functionality in its products. Although significant efforts have been made in optimizing this technology on lab scale, the implementation on a larger (pilot) scale is still in its infancy. In our own search for the scale-up potential of this technology, we faced several fundamental and technical research questions that, to this day, remain unanswered. These fundamental questions are related to four main aspects of RCF, the lignocellulose feedstock, the operating pressure, the redox catalyst, and the solvent. In order to inspire future multidisciplinary research in the RCF community, these scale-up challenges are presented and discussed via multiple angles combining chemical process hurdles with more technical aspects, such as reactor design and process consequences.

a b s t r a c tWeak acids are known to enhance the activity of amines in aldol condensation reactions on silica-based catalysts. The effects of acid strength and arrangement of the promoting site with respect to a secondary amine have been investigated in the aldol condensation of 4-nitrobenzaldehyde with acetone. Changing the substituent of this secondary amine from a methyl to an ethyl group decreases the activity. An intramolecular OH function provided by a primary alcohol incorporated on the b-carbon of the amine substituent exhibits a similar cooperativity as an intermolecular OH function provided by neighboring surface silanols. A maximum activity was achieved when the secondary amine with the same primary alcohol-containing substituent was surrounded by surface silanols, indicating the potential advantage of simultaneously activating both reactants by the formation of a hydrogen bond in contrast to the consecutive activation when there is only one promoting site in the vicinity of the amine. Changing the alcohol to stronger acids resulted in a reduced cooperativity with increasing acid strength. After removing the silanols from the surface, the activity of the catalysts which exhibit an intramolecular cooperativity retained about 68-83% of their activities while the activity of the conventional secondary amine was reduced by a factor of four compared to its intermolecularly cooperative counterpart.

The utilization of lignin and glyoxal as potentially sustainable and less hazardous building blocks for phenolic resins is an emerging research field. Lignin thereby serves as a partial, macromolecular substitute for phenol, while glyoxal fulfills the role of an aldehyde cross-linker. In the first part of this perspective, the industrial context of lignin and glyoxal will be expounded with a focus on their origin and production processes. In the framework of phenolic resins, the use of lignin and glyoxal can be categorized into two research domains: (i) glyoxalation to improve the reactivity of lignin prior to resin synthesis and (ii) direct resin synthesis using lignin and glyoxal with glyoxal immediately serving as the cross-linker. This perspective provides a comprehensive overview of the progress made in both domains, pinpointing the opportunities, blind spots, and challenges that lay ahead.

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