Preparation and characterization of montmorillonite–silica nanocomposites: A sol–gel approach to modifying clay surfaces

Abstract

Montmorillonite–silica nanocomposites were prepared by a sol–gel approach involving hydrolysis reaction of alkoxysilanes (TEOS) and subsequent condensation reaction with hydroxyl groups of the clay, resulting in the formation of the mesoporous silica network and silica nanoparticles covered or attached on the clay surfaces. According to X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nitrogen adsorption isotherms, the structure and surface properties of the sol–gel-modified clay can be controlled by varying the TEOS/clay mass ratio and/or adding trace amounts of acid as catalyst. In the case of acid-catalyzed procedures, large continuous mesoporous silica was covered on the clay surfaces, resulting in delamination of clay platelets in silica matrix at higher TEOS/clay ratio, and attaching of isolated mesoporous silica on the clay surface at lower TEOS/clay ratio, respectively. In the case of non-catalyzed procedures, silica nanoparticles were attached on the two-dimensional (2D) clay platelets, while the stack order of the clay was maintained regardless of the TEOS/clay ratios. This sol–gel modification approach combines the surface properties of mesoporous silica and nanoparticles with layered clay, while inheriting the structural properties of the pristine clay such as further intercalation with organic compounds and polymers.

Introduction

Clay minerals with a distinct layered structure have wide applications for a long time, such as catalysts and catalytic supports [1], [2], adsorbents of organic substances [3] and clay-based porous materials [4], [5], [6]. In recent years, intense attentions have been paid to the polymer–clay nanocomposites due to their enhanced mechanical properties, thermal stability, barrier properties and flame retardancy at a clay loading even less than 5% [7], [8]. Many researches have focused on achieving high degree of exfoliation of the clay platelets in polymer matrixes by enhancing interaction between clay surfaces and polymer chains by various surface modification approaches [9], [10], [11], [12], [13], [14], [15].

All these applications strongly depend on the tunable surface properties and modification of the clay minerals. There are many attractive features on clay surfaces, such as the cation exchange capacity (CEC), hydroxyl groups on the edges, silanol groups of the crystalline defects or broken surfaces and Lewis and Brønsted acidity, providing many strategies for surface modification and functionalization [13]. Up to now, the most commonly used method to modify clay surface is the cation exchange reaction with alkylammonium. This modification increases the interlayer spacing and creates a more favorable organophilic environment. Recently, another modification approach involving direct grafting reaction to form covalent bonds on the clay surfaces has attracted much attention [10], [12], [16], [17], [18]. Organosilanes were covalently bonded to the clay surfaces via condensation reaction with the surface silanol groups (Si–OH), resulting in more tight interactions between organic components and clay than ion interaction and physical adsorption [19]. However, the surface grafting reaction is rather inefficient because of the relative small edge area and low amount of hydroxyl groups located on surfaces for most clay minerals, such as montmorillonite, resulting in small amounts of grafted organics [10], [17], [19]. To overcome the limitation of the surface grafting modification method, a sol–gel route to synthesizing organoclay using organosilane reagents as the silica source has been reported. This approach allows large amount of silanes to covalently attach on clay surface because of the in-situ process and more silanol groups [20], [21], [22]. Unfortunately, the high degree of the organosilanes incorporation is often accompanied by distortions and structural defects [17], [21], [22].

In addition, sol–gel procedure in the presence of clay minerals has been widely used to prepare porous nanocomposite materials [23], [24], [25]. This procedure incorporates high surface area of mesoporous silica with clay minerals. The resultant composites can be further functionalized by organosilanes on the silica network. In a recent report, clay particles have been cross-linked by α,ω-organosilane and protonated aminosilane during sol–gel process [26]. The authors proposed that the hydrolyzed organosilanes reacted with edge hydroxyl groups and connected the neighboring clay platelets to form a network. The most commonly used method to prepare the porous clay–silica composites involves intercalation with ammonium surfactants, pillaring of silica sources in the interlayer of clays and the subsequent calcination process to remove the organic templates (e.g. ammonium surfactants) [3], [4], [5], [6]. While this approach achieves high specific surface area, it also results in the loss of organic compounds and sacrificing of the intrinsic properties of clay such as capability of further intercalation with organic compounds and polymers.

Different from the modification methods such as cation exchange reaction, surface grafting, sol–gel route to synthesizing organoclays and the pillaring process, the approach we reported herein to modifying clay surfaces using a well-controlled and facile sol–gel procedure involves hydrolysis and self-condensation of alkoxysilanes, such as tetraethylorthosilicate (TEOS), and subsequent condensation reaction with hydroxyl groups of clay. The resultant mesoporous silica and silica nanoparticles, which were covalently attached on clay surfaces, introduce some novel properties, such as high surface area, porosity and large amount of Si–OH, and combination of silica nanoparticles with two-dimensional (2D) clay platelets. In contrast to pillaring approach, this sol–gel modification mostly takes place on the edge of clay platelets especially in the case of low initial TEOS/clay mass ratio; thus the interlayer space of clay is still accessible for organic compounds such as ammonium surfactants and polymers. In particular, special attention has been paid in determining the structure of these modified clays and mechanisms of the sol–gel modification procedures using Fourier transform-infrared (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nitrogen adsorption–desorption isotherms.