Counter diffusion self assembly synthesis of ordered mesoporous silica membranes in straight pore supports

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Abstract

Ordered mesoporous silica membranes in macroporous supports were synthesized by a novel acid catalyzed counter diffusion self assembly method. Mesoporous silica was grown in the pores of the support using tetrabutylorthosilicate and cetyltrimethylammonium bromide as the silica source and surfactant, respectively. The supports used were straight pore, track-etch polycarbonate membranes. Hydrophobic supports with a pore diameter of 5 μm and hydrophilic supports with a pore diameter of 8 μm were used. The grown silica plugs had a highly ordered structure as seen by XRD and TEM studies, with a high surface area of around 990 m2/g and a pore diameter of 2.7 nm. SEM studies and oxygen permeation experiments at constant transmembrane pressure were conducted to assess membrane quality. There was a two order magnitude decrease in the permeance after counter diffusion self assembly growth, showing that good quality membranes were synthesized. The use of hydrophobic supports, support placement and evaporation controlled self assembly are the key factors for the successful formation of these membranes by the CDSA approach.

Introduction

Ordered mesoporous materials have been the focus of extensive research since they were first synthesized by Mobil researchers using the surfactant-templated self assembly method [1]. The unique, highly ordered pore structure, well defined pore diameter (2–30 nm), high surface area (1000 m2/gm) and different pore connectivities make these materials highly desirable for a number of industrial applications. MCM-41 is a member of this class of molecular sieves, characterized by its hexagonally-arranged straight-through mesopores [2]. This pore alignment and uniformity identify MCM-41 as an exceptional candidate for various membrane applications such as facilitated separation [3], [4], catalysis [5], [6], sensors [7], low K dielectric materials [8] and nanoreactors [9], [10].

The present challenge is to be able to fabricate these materials in membrane morphology such that the pore channels are aligned perpendicular to the membrane surface. This configuration is desirable as it will allow for better pore accessibility for use in a number of applications [3], [4], [5], [6], [7], [8], [9], [10]. Numerous studies have been conducted on the synthesis of ordered mesoporous silica films on various dense and porous substrates. These methods include tape casting, dip coating and use of orienting magnetic, electric or shear fields [11], [12], [13], [14], [15], [16]. More involved methods such as eutectic deposition combined with etching, use of supercritical carbon dioxide to deliver the inorganic phase and making the support surface neutral to the template have also been tried [17], [18], [19]. Some of these methods have resulted in good quality membranes. However, the pores of the membranes synthesized were either randomly aligned or aligned parallel to the surface of the substrates. In some cases, ordering could be maintained only a few nanometers from the surface.

Remarkable progress has been made in the recent years towards making ordered mesoporous silica rods by using a two dimensional physical confinement method [20], [21], [22], [23], [24], [25], [26]. A comprehensive review of the confinement method has been published by Stucky et al. [22], [23]. Most of these studies have used non-ionic block co-polymers as templates to successfully synthesize mesoporous silica rods with pores that run in a circular fashion. Growth utilizing straight-pore anopore supports has also been investigated. Yamaguchi et al. [24] and Lu et al. [25] have reported synthesizing mesoporous silica-anodic composite membranes having the desired vertical pore orientation. However, these membranes suffer from certain drawbacks. Reports indicate that only 60% of the support pores are plugged and these plugs did not run the entire cross section of the support. Also, it was seen that the order in the plugs did not extend to the entire length of the pores in the support. Lu et al. have not reported any permeation data which makes it difficult to ascertain the quality for use in membrane applications.

Platschek et al. [26] have recently demonstrated the use of both ionic surfactant (CTAB) and non-ionic surfactants, P123 and Brij56, in the formation of nanorods of various pore orientations within alumina anopore membranes. These studies were aimed at delineating the role of surfactant and EISA (Evaporation Induced Self Assembly) in the formation of long range ordered mesoporous rods. However, no permeation data are available for these studies as well, and SEM images revealed that not all the pores of the support were plugged, making this process unfavorable for membrane synthesis. Though these methods have shown some promise, there is still a need for a technique that can synthesize good quality membranes with pores oriented perpendicular to the support surface.

Recently, we have reported a novel counter diffusion self assembly (CDSA) method for the preparation of perpendicularly oriented mesoporous silica membranes [27]. The CDSA method is an extension of the interfacial synthesis method for synthesizing ordered mesoporous silica fibers containing a large number of ordered, 3 nm pores aligned straight or helical across the fiber axis. The conditions for optimum growth of these mesoporous fibers have been reported previously [28], [29]. In the CDSA method, a porous support is placed at the interface of the water and silica precursor phases. The precursors are expected to inter-diffuse through the support pores and the silica is expected to condense around the diffused micelles to form fibers (which will be referred to as plugs) within the pores of the support. A schematic of this concept is shown in Fig. 1.

Initial experiments were carried out in tortuous supports of pore sizes ranging from 0.2 to 20 μm. The details of the experiments conducted have been reported previously [30]. From these studies, it was seen that large-pored supports show better plugging, indicating that facilitated transport is one of the key factors for the growth of silica membranes by the CDSA approach. Hydrophobic supports drastically improved the quality of the membrane. This is believed to happen as a result of enhanced transport of the hydrophobic silica precursor through the pores of the support [31]. Gas permeation data for these membranes indicated a Knudsen type permeation mechanism with a N2 permeance of 0.45 × 10−7 mol/m2 Pa s. This confirmed the mesoporous nature of the silica membranes grown within the surface modified macroporous alumina supports.

The above results indicate that the use of supports with large pores (>3 μm) is highly desirable. Since our goal is to obtain straight pores orthogonal to the support surface, it is expected that when the surfactant is introduced inside the pores of the support, the micelles will align parallel to the support pore wall and lead to mesopores with an orientation analogous to the shape of the pore. Given the results and the objective of the work, our next step was the synthesis of membranes in large, straight pore supports.

The present study is focused on CDSA growth of short silica fibers (plugs), in straight pore supports of varying surface chemistry (hydrophobic and hydrophilic). Track-etch polycarbonate supports were chosen since they can withstand the highly acidic synthesis conditions and have pores with the desired surface chemistries and pore structure. Straight pore anodic alumina membrane supports were also tried but these did not withstand the synthesis conditions. Experiments have also been conducted to identify optimum conditions to obtain consistently good quality membranes with the desired pore structure and high surface area. The quality of the membranes synthesized and the structure and morphology of the silica plugs were studied using SEM, oxygen permeation, XRD, TEM and nitrogen adsorption/desorption isotherms. Various factors that affect the formation of ordered mesoporous plugs within the pores resulting in good quality membranes have been determined.

Section snippets

Supports

The straight pore supports studied were nucleopore track-etched polycarbonate membranes (Whatman Inc., NJ). Both hydrophobic and hydrophilic membranes were used in this study. The hydrophobic membranes have a diameter of 13 mm, thickness of 10 μm and a pore diameter of 8 μm. The hydrophilic membranes have a diameter of 25 mm, thickness of 10 μm and a pore diameter of 5 μm. Both the hydrophobic and the hydrophilic membranes have a porosity of 15%.

CDSA growth of mesoporous membranes

Mesoporous silica was grown in the support pores by the

CDSA growth by Methods A and B

As stated previously Method A involved the basic CDSA method while Method B was a modification on the method to allow evaporation. Fig. 4 compares SEM micrographs of hydrophobic (Fig. 4b) and hydrophilic (Fig. 4c) polycarbonate supports after CDSA growth of silica plugs by Method A with that of a fresh polycarbonate support (Fig. 4a). As shown, the pores of both supports were filled with silica plugs. By using the CDSA method, we were able to grow plugs that ran the entire length of the support

Conclusions

The counter diffusion self assembly (CDSA) approach was shown to be effective for synthesizing mesoporous silica membranes. Results on straight pore polycarbonate track-etch supports have shown that good quality membranes can be synthesized via this method. It was seen that while good quality plugging can be achieved in Method A, in the absence of evaporation induced self assembly, the desired microstructure of the plugs cannot be achieved. Methods B and C, on the other hand, showed not only

Acknowledgments

The project was supported by Petroleum Research Fund administered by the American Chemical Society. We gratefully acknowledge the support and use of the facilities in the LeRoy Eyring Center for Solid State Sciences (LE-CSSS) at Arizona State University.

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