Sharma Krishna K, Buckley Robert P, Asefa Tewodros
Department of Chemistry, Syracuse University, Syracuse, New York 13244, USA.
Langmuir. 2008 Dec 16;24(24):14306-20. doi: 10.1021/la8030107.
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.
我们报道了胺官能化介孔材料接枝有机胺（以及残留的未接枝硅醇）的表面密度和间距对其在亨利反应中的（协同）催化活性的影响。通过将一系列含有直链烷基胺、烷基二胺、烷基三胺以及间位和对位取代芳香胺的有机硅烷在乙醇和/或甲苯中一步或两步接枝到介孔二氧化硅上，来控制双官能团（胺和硅醇）之间的间距、它们的位点隔离以及它们的表面密度。在乙醇中的接枝产生了位点隔离的、具有不同连接长度的柔性烷基胺、烷基二胺和烷基三胺，以及刚性的间位和对位取代芳香胺和高比表面积材料，而在甲苯中的接枝则导致有机胺间距紧密且材料比表面积较低。通过使铜离子与胺络合并测量络合物的电子光谱来探测有机胺基团之间的间距。材料的催化活性不仅取决于胺基团的位点隔离程度和材料的比表面积，还取决于官能团之间的相对间距及其表面密度。在乙醇中接枝单胺基团的样品以及在甲苯中接枝二胺或三胺基团5小时的样品，在对羟基苯甲醛与硝基甲烷的亨利反应16分钟内转化率约为100%。然而，相应的在甲苯中接枝单胺的样品以及在乙醇中接枝二胺和三胺的样品在1小时后转化率约为100%。基于周转数（TON）和每比表面积的TON，含有约0.8 - 1.5 mmol接枝有机胺/克的最佳浓度的样品（我们将其称为有机接枝基团的临界密度）具有最高的催化效率。这些样品具有最有利的胺 - 硅醇协同催化活性。此外，在乙醇中用刚性间位取代氨基苯基官能化的样品显示出比相应的对位含胺基团样品更高的催化效率，这可能是由于前者中双官能团靠得更近。在甲苯中用非催化有机硅烷封端未接枝的硅醇导致催化活性降低，证实了硅醇的参与。
