Predicting the Conformation of Organic Catalysts Grafted on Silica Surfaces with Different Numbers of Tethering Chains: The Silicopodality Concept

Abstract

Hybrid catalysts are attracting much attention, since they combine the versatility and efficiency of homogeneous organic catalysis with the robustness and thermal stability of solid materials, for example, mesoporous silica; in addition, they can be used in cascade reactions, for exploring both organic and inorganic catalysis at the same time. Despite the importance of the organic/inorganic interface in these materials, the effect of the grafting architecture on the final conformation of the organic layer (and hence its reactivity) is still largely unexplored. Here, we investigate a series of organosiloxanes comprising a pyridine ring (the catalyst model) and different numbers of alkylsiloxane chains used to anchor it to the MCM-41 surface. The hybrid interfaces are characterized with X-ray powder diffraction, thermogravimetric analyses, Fourier-transform infrared spectroscopy, nuclear magnetic resonance techniques and are modeled theoretically through molecular dynamics (MD) simulations, to determine the relationship between the number of chains and the average position of the pyridine group; MD simulations also provide some insights about temperature and solvent effects.

Grafting Going Green: Toward a Sustainable Preparation of Organic–Inorganic Hybrid Materials

“…In the spectra of all hybrids, the typical signal of the O−H stretching mode of free silanols at 3745 cm −1 (Figure S13 in the Supporting Information) is almost disappeared, confirming the success of the APTS grafting, and a broad feature at lower wavenumbers is visible due to the OH interaction via H-bond. 3 Furthermore, the characteristic signals of the asymmetrical and symmetrical stretching modes of the amino group (−NH 2 , 3373, 3305 cm −1 ) and of the methylene groups (−CH 2 , 2943 and 2870 cm −1 ) belonging to APTS are observed. 29 At lower wavenumbers, signals at 1653 and ca.…”

“…In the spectra of all hybrids, the typical signal of the O–H stretching mode of free silanols at 3745 cm –1 ( Figure S13 in the Supporting Information) is almost disappeared, confirming the success of the APTS grafting, and a broad feature at lower wavenumbers is visible due to the OH interaction via H-bond. 3 …”

“…The double soul (organic–inorganic) of hybrid materials is spreading throughout a constantly increasing number of fields of chemistry . Hybrid materials are gaining growing importance due to the wide tunability of their structure and properties: changes in the organic or in the inorganic components or in the interface between them will deeply influence the features of the resulting material. , Hybrid materials find widespread applications in medicine, electronics, photovoltaic technology, tire construction, optics, catalysis, and many others . In particular, in heterogeneous catalysis, hybrid materials are able to catalyze the most diverse reactions, ranging from acid–base-catalyzed transformations to cross-couplings and multicomponent reactions and even multiple catalytic sites for multistep transformations .…”