Optimizing Acid−Base Bifunctional Mesoporous Catalysts for the Henry Reaction: Effects of the Surface Density and Site Isolation of Functional Groups

Abstract

We report on the effects of the surface density and the spacing between grafted organoamines (and residual ungrafted silanols) of amine-functionalized mesoporous materials on their (cooperative) catalytic activity in the Henry reaction. The spacing between the bifunctional groups (amines and silanols), their site isolation, and their surface density were controlled by one-step or two-step grafting of a series of organosilanes containing linear alkylamine, alkyldiamine, alkyltriamine, and meta- and para-substituted aromatic amines onto mesoporous silica in ethanol and/or toluene. The grafting in ethanol produced site-isolated, flexible alkylamines, alkyldiamines, and alkyltriamines of different tether lengths and rigid meta- and para-substituted aromatic amines and high surface area materials, whereas the grafting in toluene resulted in closely spaced organoamines and materials with lower surface areas. The spacing between the organoamine groups was probed by complexing cupric ions with the amines and by measuring the electronic spectra of the complexes. The materials' catalytic activities were dependent not only on the degree of site isolation of the amine groups and the surface areas of the materials, but also on the relative spacing between the functional groups and their surface density. Samples grafted with monoamine groups in ethanol and samples grafted with diamine or triamine groups in toluene for 5 h gave approximately 100% conversion in 16 min of the Henry reaction between p-hydroxybenzaldehyde and nitromethane. However, the corresponding monoamine-grafted sample in toluene and diamine- and triamine-grafted samples in ethanol gave approximately 100% conversion after 1 h. On the basis of turnover number (TON) and TON per surface area, the samples containing optimum concentrations of approximately 0.8 - 1.5 mmol of grafted organoamines/g, which we dubbed as the critical density of organic grafted groups, gave the highest catalytic efficiencies. These samples have the most favorable amine-silanol cooperative catalytic activity. Furthermore, samples functionalized with rigid meta-substituted aminophenyl groups in ethanol showed higher catalytic efficiency than the corresponding sample containing the amine groups at the para-position, possibly due to the close proximity of the bifunctional groups in the former. The capping of ungrafted silanols with noncatalytic organosilanes in toluene resulted in reduction of catalytic activities, confirming the involvement of silanols.

Bifunctional Mesoporous Silica Catalyst for C–C Bond Forming Tandem Reactions

“…[31] Three different catalysts were prepared from SBA-15. The first sample was prepared in the same way as APPd-DT above by grafting APTS (3.68 mml) in 2-propanol (250 mL) onto SBA-15 (500 mg) for 6 h at 80°C to produce spatially isolated primary amine groups on the mesoporous channel walls of SBA-15.…”

“…[29][30][31][32][33][34][35][36] In our recent work, we showed that a solvent-assisted grafting synthetic method, involving the use of simple polar-protic solvents, can place organoamines in spatial isolation within the pores of mesoporous silica material and can result in efficient catalytic activities in the Henry and aldol condensation reactions. [29][30][31] In fact, these materials superseded any previously reported mesoporous catalysts in terms of catalytic activity in the C-C bond coupling Henry and aldol condensation reactions.…”

A comparative study of aminopropyl-functionalized SBA-15 prepared by grafting in different solvents

“…They prepared aminopropylfunctionalized MCM-41 by grafting an organosiloxan precursor 3-aminopropyltriethoxysilane (APTES) onto the MCM-41 in ethanol. Compared with samples grafted in toluene, these materials contain site-isolated organoamines and neighboring residual surface silanol (weak acid) groups to cooperatively catalyze the Henry reaction and aldol condensation, which make them display more enhanced catalytic activity [16][17][18]. However, to the best of our knowledge, these catalysts produced in ethanol have not been investigated for other reactions so far.…”

Mesoporous silica nanoparticles: structural design and applications

“…12, 13 Later, Cardin, Garcia and co-workers showed that by introducing Ni(II) and Zn(II) cations through ion exchange on mesoporous silica, alkynes could be polymerized within the material. 14 Lin 115 and co-workers immobilized a polymerization catalyst within the pores of MSN. This ensured that the resulting polymer chains could stay threaded within the pores and well aligned with each other, avoiding any undesired crosslinking.…”