The structure of mesoporous silica obtained by pseudomorphic transformation of SBA-15 and SBA-16

Highlights

• Pseudomorphic transformation of SBA-15 and SBA-16 yields hybrid pore structures.

• The products of the transformation feature well-defined bimodal pore size distributions.

• The particle morphology is retained upon transformation.

• Characteristics of the original pore system are partially conserved in the transformed materials.

Abstract

Mesoporous silica with bimodal pore size distributions was prepared by pseudomorphic transformation of SBA-15 and SBA-16 in the presence of hexadecyltrimethylammonium ions as a structure-directing agent. The characteristic particle morphology of the starting materials was retained after the transformation. Analysis of the products by gas sorption and small-angle X-ray scattering (SAXS) revealed hybrid pore structures, which featured – depending on the degree of transformation – variable contributions from the original and the newly introduced pore systems. In the case of SBA-15, it was found that a high degree of transformation leads to a seemingly complete conversion of the original pores with a diameter of 7.1 nm to pores with a diameter of 4.0 nm. The SAXS pattern of the product shows additional peaks that can be assigned to the original SBA-15 pore spacing. Similarly, a cubic phase could be observed in the samples prepared by pseudomorphic transformation of SBA-16, despite an almost complete conversion of the SBA-16 cavities. This leads to the conclusion that the pore structure of the starting material significantly affects the outcome of the pseudomorphic transformation, thus opening possibilities for the synthesis of new porous materials with complex pore systems.

Introduction

In terms of research activity, materials of the M41S [1], [2] and SBA families [3], [4] are the most prominent types of ordered mesoporous silica. Catalysis [5], chromatography [6], organization of molecular guests [7], environmental remediation [8], and drug delivery [9] are some of the fields where these materials can find potential application. While considerable progress regarding the engineering of the pore size and the introduction of functional groups has been made [10], [11], [12], much less emphasis has been put on the control of the particle size and shape for a specific application. The simultaneous control of the pore size, pore structure, particle size, particle shape, and surface functional groups poses a challenge.

An elegant way to alter the pore size of a mesoporous material without affecting its particle size and shape was proposed by Martin et al. [13]. This so-called pseudomorphic transformation pathway uses a structure-directing agent (SDA) to rearrange the pore system of a preformed macroporous material. Functional groups can be introduced during the transformation and the pore size can be adjusted to a certain degree by using differently sized SDAs [14]. MCM-41 type silica tubes [15] and monodisperse magnetic mesoporous silica microspheres [16] have been prepared by pseudomorphic transformation of silica shells deposited onto hard templates. Pseudomorphic transformation can also be applied to millimeter-sized silica spheres [17] or to an ordered mesoporous material such as SBA-15, thus converting its pore system into a MCM-41 type structure [18]. Bimodal mesoporous materials with two well-defined pore size regimes have been prepared by carefully balancing the degree of pseudomorphic transformation. Partial pseudomorphic transformation was shown to alter the structure of the porous starting material from the outside to the inside, thus virtually creating a shell with a reduced pore size. The pH thereby governs the equilibrium between dissolution and reprecipitation of the silica matrix [18].

Based on these previous results we can make the general conclusion that the partial pseudomorphic transformation of SBA-15 in the presence of hexadecyltrimethylammonium bromide (CTAB) yields a material consisting of two types of pore size domains, one originating from the SBA-15 starting material, and the other being formed by a rearrangement of the original pore structure in the presence of the SDA. The question remains whether this newly introduced pore structure can in fact be considered as a true MCM-41 type structure, i.e., a hexagonal arrangement of one-dimensional pores. Furthermore, the role of the original pore structure in determining the structure of the newly introduced domains is unclear. We have therefore investigated the partial pseudomorphic transformation of SBA-15 (one-dimensional pore structure, space group $P6mm$) and of SBA-16 (three-dimensional pore structure, space group $Im\overline{3}m$) in the presence of CTAB. Analysis by argon sorption and small-angle X-ray scattering (SAXS) revealed that the transformed materials contain various domains and retain some of the structural features of the SBA type starting materials. In all cases, the particle morphology and average particle size remained unaltered after transformation and did not reflect the reorganization of the pore structure.