The effects on pore size and particle morphology of heptane additions to the synthesis of mesoporous silica SBA-15

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Abstract

The effect of heptane on the particle morphology and pore size in the synthesis of SBA-15 is presented. Heptane in the presence of NH4F works as a pore swelling agent, resulting in 13–18 nm sized pores in 400 nm long and 200–1000 nm wide crystallites. The pores are hexagonally arranged and run through the crystallites. Increasing the heptane to P123 molar ratio changes the morphology of SBA-15 from fibers to sheets when the crystallites rearrange during the synthesis. The pore order in the sheets is controlled by changing the molar ratio of water to P123. The surface areas of these materials are 500–800 m2/g with pore volumes of 1.2–1.7 cm3/g. The sheets have accessible pores with a size of 18 nm running parallel to the sheet normal, which makes them suitable for membranes.

Introduction

Mesoporous silica SBA-15 [1], [2] is found in many applications such as molecular sieves, drug delivery systems, catalysis, and templates for other materials such as mesoporous carbon because of its narrow pore size distribution and large surface area [3], [4], [5]. Mesoporous carbon has additional applications, e.g. capacitors [6] and hydrogen storage [7], [8]. Pore size and morphology of SBA-15 has been extensively studied since the first synthesis of the material. Depending on the application, different pore sizes and particle morphologies are needed, e.g. short and wide pores have shown large capacities for enzyme immobilization [9], [10], [11] while smaller pores sizes give a larger surface area, making the material suitable for catalysis or as mesoreactors.

The first study of SBA-15 indicated that the pore size could be tuned between 5 and 30 nm using 1,3,5-trimethylbenzene (TMB) as a swelling agent [2]. Further studies showed though that a phase transition from hexagonal to a mesocellular foam structure occurs when the pores are expanded further than 12 nm [12], [13]. Over the years other chemicals such as propylene glycol [14], poly(propylene oxide) [15], [16] or various auxiliary chemicals [17], [18] have been used to swell the pores of SBA-15 with varying results. Other attempts, such as changing the silica source to tetrabutyl orthosilicate (TBOS) or by combining tetraethyl orthosilicate (TEOS) with sodium silicate in the presence of ethanol showed an increase in pore and particle size, but the pore ordering was lost [19], [20]. Lately, alkanes from hexane to dodecane have been used as swelling agents at low temperature syntheses [21], [22], [23], [24], [25], [26]. The pore sizes for these materials vary between 9.7 and 15.7 nm depending on the alkane chain length and these materials have been used in applications such as enzyme adsorption [11]. SBA-15 with the largest ordered pores were recently synthesized by Cao et al. who used 1,3,5-triisopropylbenzene to swell the pores resulting in SBA-15 with a d100 spacing of 26 nm [27].

An alternative way to enlarge the pores is to reduce the shrinkage during calcination by increasing the hydrothermal treatment time or temperature [28], [29]. Another way is to avoid the calcination step altogether and use other methods for polymer removal. Examples of these methods are washing with water or ethanol [30], [31], or microwave heating [32]. By removing the calcination step, the material contains higher amounts of silanol groups in the silica wall, making it easier to functionalize the material [32], [33], [34]. The surface area of uncalcinated materials will also be higher due to a higher amount of microporosity in the walls to be closed at elevated temperatures. Washing the material with water, H2SO4, or ethanol do not completely remove the polymer [32], [34], [35]. In contrast, washing with H2O2 at 100 °C effectively removes it [34], [36]. Recently, it was also shown that perchlorates under acidic conditions also can be used to remove the polymer [37].

Different morphologies of SBA-15 such as fibers, platelets, pearls, monoliths, films, etc. can be synthesized by variations in the reaction conditions during the synthesis [38], [39], [40] or by additions of salts such as KCl, NH4F or NaSO4 [41], [42], [43]. Different alkanes in combination with NH4F have yielded morphologies such as sheets [44], fibers and nanosized slices [23] depending on the alkane used and reagent concentrations. Accessibility of the pores depends on the morphology which makes different morphologies suitable for different applications. When alkanes are used as cosolvents in the synthesis, the pore length is decreased to 150–800 nm depending on the hydrocarbon chain length [22], [23] compared to ∼1 μm when no alkanes are used.

Here, we illustrate how heptane as a swelling agent in combination with NH4F can be used to produce SBA-15 with large pores (13–18 nm). The particle morphology depends on the heptane to P123 molar ratio and varies from fibers to sheets. By careful control of the water concentration, an ordered pore arrangement in the sheet morphology with 4 h reaction time is achieved. In addition, by removing all the P123 with H2O2 the pore size of the material was further increased.

Section snippets

Synthesis

Hydrochloric acid (purity  37%, puriss. p.a., Fluka, ACS Reagent, fuming), triblock copolymer EO20PO70EO20 (P123, Aldrich), ammonium fluoride (purity  98.0%, puriss. p.a., ACS reagent, Fluka), tetraethyl orthosilicate (TEOS, reagent grade, 98%, Aldrich), heptane (99%, ReagentPlus®, Sigma–Aldrich) and hydrogen peroxide solution (purity  35%, purum p.a., Sigma–Aldrich) were used as received. In a typical synthesis in which the heptane to P123 ratio was studied, 1.2 g of P123 and 0.014 g of NH4F were

Results

Fig. 1 shows characteristic SEM micrographs of the materials produced using different amounts of heptane, with P123 removed by H2O2. For the lowest heptane to P123 ratio (X = 150), the material consists of small particles connected to each other and embedded in a foamy structure, Fig. 1(a). The foam disappears when X is increased to 235, Fig. 1(b), and discrete particles approximately 400 nm long are formed, attached short end to short end. Increasing X to 350 and 412 result in the formation of

Pore size

The large pores in these materials compared to other synthesis methods of SBA-15 originate from two sources: the profound swelling of the micelles with heptane and the absence of shrinkage from calcination. An estimate of how much heptane is needed to completely expand the pores can be based on a study in which decane has been used as the swelling agent [23]. It was shown that a decane to P123 molar ratio above 70 does not increase the pore size further, only the morphology is changing. It has

Conclusion

We have shown that it is possible to synthesis ordered SBA-15 with pore sizes in the range of 13–18 nm when heptane is used as a cosolvent in the presence of NH4F. By tuning the heptane to P123 ratio, different morphologies such as fibers and sheets can be obtained. The materials consist of crystallites attaching to each other in varying ways, giving rise to different morphologies, and an explanation for this mechanism is suggested. We have also shown that by adjusting the ratio of water to

Acknowledgements

We are grateful for the financial support from the Swedish Research Council (VR). Dr. J. Rosenholm and Dr. M. Lindén, Åbo Akademi University are acknowledged for help with XRD measurements and useful discussion.

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