Physicochemical characterization of silylated functionalized materials

doi:10.1016/j.jcis.2010.01.026

Journal of Colloid and Interface Science

Volume 344, Issue 2, 15 April 2010, Pages 603-610

https://doi.org/10.1016/j.jcis.2010.01.026 Get rights and content

Abstract

Silylation of several materials where the surface area arises from the internal pores (MCM-41 and FSM-16) or is essentially external (silica gel, and clays) was performed using three organosilanes: (3-aminopropyl)triethoxysilane (APTES), 4-(triethoxysilyl)aniline (TESA) and (3-mercaptopropyl)trimethoxysilane (MPTS). The materials were characterized by nitrogen adsorption–desorption at −196 °C, powder XRD, XPS, bulk chemical analysis, FTIR and ²⁹Si and ¹³C MAS NMR.

For MCM-41 and FSM-16 the highest amounts of organosilane are obtained for APTES, while for the remaining materials the highest amounts are for MPTS; TESA always anchored with the lowest percentage. In terms of surface chemical analysis, TESA anchored with the highest contents irrespectively of the material, and the opposite is registered for MPTS. Comparison of bulk vs surface contents indicate that TESA is mainly anchored at the material external surface. Moreover, with N or S (surface and bulk) contents expressed per unit of surface area, MCM-41 and FSM-16 (internal porosity) show the lowest amounts of silane; the highest amounts of silane per unit of surface area are obtained for the clays.

Grafting of the organosilanes to the surface hydroxyl groups was corroborated by FTIR and ²⁹Si and ¹³C MAS NMR. Furthermore, NMR data suggested that TESA and APTES grafted mostly through a bidentate approach, whereas MPTS grafted by a monodentate mechanism.

Graphical abstract

Materials with different pore structures – internal versus external porosity – were successfully functionalized with amino (APTES and TESA) and mercapto (MPTS) organosilanes.

Introduction

Porous materials modified with an immobilized organic layer are widely used as adsorbents, chromatography phases and catalysts [1]. In this type of materials, which can be considered hybrid materials or polymer nanocomposites [2], [3], the useful properties are highly conditioned by the nature of the organic species, type of bond between these species and the solid, and the chemical groups that are then left to interact or, in some cases, covalently react with a third chemical entity. This latter aspect can be used in the heterogenization of metal complexes that are catalytically active for various reactions in the homogeneous phase, aiming a more efficient use of the catalyst, namely by allowing its reuse [4], [5]. In fact, the leaching of the active phase during the catalytic reaction is still a major drawback in the immobilization of both, metal complexes or metal phases, in porous supports such as porous silicas [4], [5]. A number of species has been immobilized at silica surfaces, to serve as linkers or scavengers of the actual catalytic phase [1], [6], [7], [8], [9], [10], [11], [12], [13], [14].

The present work studies the funtionalization of various types of silica based mesoporous materials via covalent attachment of organosilanes, which is an important step in the preparation of materials for the above-mentioned catalytic applications. We endeavor to highlight the factors that contribute to the more adequate conjugation organosilane-porous support to be used in the heterogenization of metal complexes. For this, a systematic study was made using three silanes with terminal single bond NH₂ or SH that are very important reactive groups to anchor catalytic phases: (3-aminopropyl)triethoxysilane (APTES), 4-(triethoxysilyl)aniline (TESA) and (3-mercaptopropyl)trimethoxysilane (MPTS). As the porous supports, five silica-based materials were used: MCM-41, FSM-16, silica gel (SiO₂) and clays. These supports have different porosity: (i) internal, for the materials with well defined regular mesoporosity, MCM-41 and FSM-16; and (ii) external for silica gel and clays. For the clays used in this work – Laponite (LAP) and K10 – there are also differences in their chemical composition and morphology, particularly in the crystallite sizes. The physicochemical characterization of all materials was made by powder X-ray diffraction (XRD), low-temperature nitrogen adsorption, FTIR, X-ray photoelectron spectroscopy (XPS), bulk chemical analysis, ²⁹Si and ¹³C CP MAS NMR.

Section snippets

Materials preparation

MCM-41 was obtained as described in detail elsewhere [15]. The synthesis involved the dissolution of CTAB (Aldrich) in deionized water and aqueous ammonia (Merck, 25%). The silica source was tetraethyl orthosilicate (Aldrich, 98%). The solid was dried at 90 °C and heated at 550 °C for 5 h, after a ramp of 1 °C min⁻¹. From the powder XRD peaks the value 3.78 nm was obtained for the d₁₀₀ of MCM-41, with a₀ parameter considering a hexagonal symmetry of 4.37 nm which is within the range of published

Nitrogen adsorption and XRD

Fig. 1 shows the nitrogen adsorption–desorption isotherms at −196 °C for the original and the surface functionalized silica and FSM-16, as selected examples. The respective mesopore-size distributions are also given. The data for the remaining samples are given in Fig. S1 of the Supporting Information. Mesopore-size distributions were obtained from the low-temperature nitrogen adsorption data using the Broekhoff–de Boer method with the Frenkel–Halsey–Hill equation (BdB–FHH) [19]. This method

Conclusions

Materials with different pore structures (internal versus external porosity) were successfully functionalized with amino (APTES and TESA) and mercapto (MPTS) silanes. The results showed a relation between the silane grafting extension and the type of material and silane structure; a correlation between the grafting mechanism and the organosilane was also detected.

MCM-41 and FSM-16 (mainly internal porosity) anchored the highest APTES amounts (N bulk content), whereas silica and the clays

Acknowledgements

This work was funded by FCT Fundação para a Ciência e a Tecnologia (FCT) and FEDER, through the project PPCDT/CTM/56192/2004. Moisés L. Pinto acknowledges FCT for a post-doctoral grant (BPD/26559/2006).

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¹: Present address: Laboratório Associado CICECO, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.

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Physicochemical characterization of silylated functionalized materials

Abstract

Graphical abstract

Introduction

Section snippets

Materials preparation

Nitrogen adsorption and XRD

Conclusions

Acknowledgements

J. Colloid Interface Sci.

Solid State Nucl. Mag.

Vib. Spectrosc.

J. Colloid Interface Sci.

J. Magn. Reson.

J. Catal.

Microporous Mater.

Microporous Mater.

Microporous Mesoporous Mater.

Colloid Surf. A: Physicochem. Eng. Aspects

J. Colloid Interface Sci.

Chem. Mater.

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Chem. Rev.

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Chem. Mater.

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