Substrate inhibition in the heterogeneous catalyzed aldol condensation: A mechanistic study of supported organocatalysts

Abstract

In this study, we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support. Disciplines Materials Chemistry | Other Chemistry | Physical ChemistryComments NOTICE: this is the author's version of a work that was accepted for publication in Journal of Catalysis. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Catalysis, [291, (2012) AbstractIn this study we demonstrate how materials science can be combined with the established methods of organic chemistry to find mechanistic bottlenecks and redesign heterogeneous catalysts for improved performance. By using solid-state NMR, infrared spectroscopy, surface and kinetic analysis, we prove the existence of a substrate inhibition in the aldol condensation catalyzed by heterogeneous amines. We show that modifying the structure of the supported amines according to the proposed mechanism dramatically enhances the activity of the heterogeneous catalyst. We also provide evidence that the reaction benefits significantly from the surface chemistry of the silica support, which plays the role of a co-catalyst, giving activities up to two orders of magnitude larger than those of homogeneous amines. This study confirms that the optimization of a heterogeneous catalyst depends as much on obtaining organic mechanistic information as it does on controlling the structure of the support.

Langevin and Fokker-Planck Analyses of Inhibited Molecular Passing Processes Controlling Transport and Reactivity in Nanoporous Materials

“…For catalysis, there is the strong suppression of reactivity in catalytically-active nanoporous materials upon reducing the pore diameter to approach the single-file diffusion (SFD) regime where reactant and product molecules cannot pass within pores [9]. SFD is readily achieved for zeolites since molecular dimensions are comparable to d p (e.g., 0.4 nm for methane, 0.7 nm for neopentane) [4], and sometimes for MSN given that the effective d p can be reduced below 2 nm due to functionalization and attachment of reactant species to the pore walls [10]. For conversion reactions with SFD, the reactant is localized near the pore openings, the pore interior being populated by product which cannot be readily extruded [9].…”

Silanol-Assisted Carbinolamine Formation in an Amine-Functionalized Mesoporous Silica Surface: Theoretical Investigation by Fragmentation Methods

“…In addition, because pathway 1 in the minimal model would correspond to the homogeneous reaction and all of the surface-inclusive models could correspond to the reaction catalyzed by AP-MSN, the current results agree with the experimental observation that the reaction catalyzed by AP-MSN is faster than the homogeneous reaction catalyzed by propylamine, at least for this initial step. 21 Since the energy barrier associated with the transition state sTS3 is much lower than that of the other two, one can conclude that pathway 3 is the most likely mechanism for the carbinolamine formation. Silanol groups participate by forming and breaking covalent bonds in the carbinolamine formation process; i.e., their role is more than simply bringing all of the reactants together by forming hydrogen bonds.…”

“…The RSM region was chosen in such a way that it contains the two major oxygen species found on the silica mesoporous surface, the silanol groups and siloxane bridges, 38,39 using the notation for silicon sites, ((HO)Si(OSi) 3 ) and ((SiO) 4 Si), respectively. The distance between hydroxyl groups should be ∼5−6 Å, in order to match the silica material characterized in the study conducted by Kandel et al 21 The 3-aminopropyl is bonded to the Si atom in a symmetric position of the RSM, Figure 1a. The final QM cluster, Si 6 O 9 C 6 NH 25 , also contains acetone, one of the substrates commonly used in aldol reactions.…”

“…12,23 In a recent study, Kandel and coauthors 21 pointed out that silanol groups may also assist the formation of intermediates along the mechanistic cycle. The first step of the catalytic cycle for a primary-amine-MSN-catalyzed aldol reaction 21 is reported to be the attack on a carbonyl compound by an amine to yield a carbinolamine intermediate. According to the authors, the silanol groups present on the MSN surface can catalyze the reaction by making covalent bonds in the carbinolamine step.…”

“…Scheme 1 outlines the catalyzed carbinolamine formation as suggested by Kandel and coauthors. 21 Scheme 1A illustrates the hydrogen bonding between the substrates and the Si−OH groups on the amine-MSN surface, while Scheme 1B depicts the catalysis promoted by the Si−OH group to yield the carbinolamine intermediate, which is shown in Scheme 1C.…”