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Millimeter-sized micellar-templated silica beads and phenylene-bridged mesoporous organosilica beads
Microporous and Mesoporous Materials 284 (2019) 327–335
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Millimeter-sized micellar-templated silica beads and phenylene-bridged mesoporous organosilica beads
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Malina Bilo, Young Joo Lee, Michael Fröba∗ Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146, Hamburg, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Alginate hydrogels Millimeter-sized organosilica beads Pseudomorphic transformation of silica and organosilica
Micellar-templated silicas and organosilica-based materials from bis-silylated organosilica precursors with an ordered array of uniform mesopores are attractive for different applications. Since they are usually obtained in the form of fine powders, the material does not meet the criteria for many industrial uses or applications where good flow-through properties are required. Herein, a synthesis route is described where alginate hydrogel acts as form-giving matrix and can be subsequently removed leaving mechanically stable silica beads with 2 mm in diameter. In these beads post-synthetic micellar-templating is possible by so-called pseudomorphic transformation in alkaline surfactant solution, whereby a homogeneous mesophase is generated. Additionally, the formation of a second pore system of large mesopores occurs, so that bimodal porous silica beads are obtained. Following the same general concept, the synthesis of pure phenylene-bridged organosilica beads with likewise 2 mm in diameter was possible and again pseudomorphic transformation could be used to obtain mesoporous phenylene-bridged organosilica beads. The integrity of the organic bridge and the absence of alginate and the surfactant was proven by solid state NMR spectroscopy.
1. Introduction Mesoporous silica-based materials are of growing importance for various applications. In the form of sub-micrometer particles or monoliths, this class of materials is commonly used in chromatography as well as in catalysis [1,2]. Porous silica nanoparticles are of interest for medical applications [3–5]. A synthesis concept for ordered mesoporous silica that allows the adjustment of the pore size, as well as the type of pore ordering in micellar-templated silica (MTS) was established with the M41S family [6]. However, the combination of application-adjusted morphology together with the respectively requested pore system in one step is usually a balancing act. By so-called pseudomorphic transformation Galarneau and coworkers found a way to optimize one feature after the other by post-synthetic micellar induced pore generation that does not affect the morphology [7]. The term pseudomorphic transformation (gr. pseudos = false, morphe = form) is adapted from mineralogy. It describes how a mineral can appear in an unusual external crystal shape due to environmentally induced change of the chemical composition under preservation of the initial morphology [8]. Galarneau and coworkers used the term for a transformation process of non-structured silica into a micellar-templated phase while keeping the morphology of the initial silica phase. In general, the concept takes advantage of the kinetic dependence of silica dissolution ∗
in basic media and its re-condensation in the presence of a surfactant [9]. The pseudomorphic transformation was intensively investigated for different commercially available silica gels [10–13] as well as porous glasses [14–16]. Special focus lies on the synthesis of hierarchically porous glasses [17] and other silica-based monoliths [18,19], because hierarchical pore systems improve the distribution of a gas or a liquid. On this basis we intend to synthesize porous, millimeter-sized spherical silica beads which are dust-free and could thus be used in industrial processes e.g. as adsorbents. Some examples of other synthesis approaches for nanoporous silica beads from the literature should be mentioned here, where usually oil-in-water or water-in-oil emulsions or polymer templating is used. In 1997 Stucky et al. showed the synthesis of mesoporous spherical beads with specific BET surface areas between 480 m2 g−1 and 1100 m2 g−1 in an emulsion synthesis with tetrabutyl orthosilicate as silica source that releases butanol during the hydrolysis process and a quaternary ammonium surfactants acting as template for the pore system and as stabilizer for the emulsion [20]. In works that are more recent an aqueous phase, a sol of the silica source methyltrimethoxysilane, ammonium hydroxide and cetyltrimethylammonium bromide (CTAB) in water was added to hexane as oil phase and spherical particles are formed in this emulsion under vigorous stirring [21]. Similar to this, the synthesis of millimeter-sized mesoporous particles with high surface areas was possible in the presence of ethyl ether in a
Corresponding author. E-mail address: [email protected] (M. Fröba).
https://doi.org/10.1016/j.micromeso.2019.04.028 Received 28 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 19 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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covalently inside the pore walls [45–47]. The fields of applications are just as multitudinous as the possible precursors for PMOs, especially in chromatography [48], separation processes and heterogeneous catalysis the morphology plays an important role. There are only few examples of hierarchically porous PMO monoliths realized by phase separation, mainly in the research groups of Nakanishi and Hüsing [49–53]. There are also more recent examples of organosilica monoliths, e.g. Schachtschneider et al. presented an organosilica monolith that can be gradually modified by click-chemistry [54]. Further, an ethylene-bridged organosilica monolith of 100 μm diameter was functionalized by thiol-ene click-chemistry and performed well in capillary chromatography tests [55]. However, no micellar-templating took place in these examples. In this paper we present the synthesis and subsequent pseudomorphic transformation of millimeter-sized silica beads using the concept of ionotropic gelation with alginate acting as form-giving matrix which was subsequently removed. Furthermore, we show how both concepts can be transferred to organosilica materials using pure 1,4-bis (triethoxysilyl)benzene (BTEB) as precursor to obtain millimeter-sized phenylene-bridged mesoporous organosilica beads. To the best of our knowledge, the biopolymer alginate has never been used as template for the synthesis of porous organosilica monoliths.
common micelle-templated silica synthesis approach with a silica source, ammonium hydroxide, surfactant and water. This approach was extended by adding an aluminum or a titanium source to the synthesis mixture. The diameter of the macro spheres was adjusted with the stirring rate between 0.4 and 1.0 mm [22]. Scholz et al. synthesized hierarchically structured silica spheres with particle diameters between 0.3 mm and 2.5 mm by emulsion based sol–gel processing in a co-condensation of a pure silica species with [3-(2-aminoethylamino)propyl] trimethoxysilane and phenyltrimethoxysilane [23,24]. In 2016 Sultana et al. prepared mesoporous hybrid microspheres around 30 μm in diameter from silica nanoparticles (40–50 nm) and a reactive copolymer in a organic/inorganic suspension under stirring [25]. Hierarchically porous silica materials were synthesized by combining the oil-in-water emulsion technique, using a block-copolymer to generate mesopores and the formation of a polymer on the outer surface of the formed oil droplets [26]. In a completely different approach the synthesis of mesoporous silicon dioxide, silicon, and silicon carbide beads was possible by soaking of commercially available polystyrene spheres of 500–800 μm in diameter with tetraethylorthosilicate TEOS and subsequent heat treatment under air or under argon and treatment with magnesia [27]. The biopolymer alginate is a suitable form-giving matrix as it forms hydrogels by ionotropic gelation. The formation of spherical beads is easily done by dropping an alginate solution into a solution of calcium ions where the size of the beads can be adjusted with the drop size, e. g. in the millimeter-size range [28,29]. We think, in view of a rather green chemistry trend, the use of polysaccharides becomes increasingly relevant [30]. Composite materials of silica and alginate are contemplable in biomedical applications for encapsulations, cell immobilization or tissue engineering [31–38]. Li et al. synthesized silica/alginate hydrogel beads by adding an alginate solution to a solution of pre-hydrolyzed TEOS, showing that the formation of beads was successful by dropping the mixture in an aqueous calcium chloride solution [38]. In 2015 Roosen et al. showed the formation of silica/alginate hybrid materials following two different paths, either by soaking alginate beads with a mixture of tetramethyl orthosilicate in ethanol overnight or by adding purchased silica microparticles to the suspension of alginate before the droplet formation [39]. Alginate was used as a template for magnetic silica beads containing γ-Fe2O3 by Liu et al. [33] and by Abramson et al., the latter removed the hydrogel subsequently under mild conditions with citric acid [40] as it was also shown by Coradin et al. [32] The Enke research group showed the synthesis of bimodal porous glass spheres between 1.9 and 3.7 nm in diameter by mixing different fractions of controlled pore glasses particles (32–100 μm in diameter) in an alginate solution before the droplet formation. Subsequently, partial sintering of the glass and removal of the alginate phase is possible by thermal treatment, whereby small particles lead to stable beads [41]. Similar to this, we synthesized pure silica spheres in a formgiving matrix of alginate. Additionally, we expand the concept to organosilica beads. Different approaches are available for the functionalization of porous silicas. Since it is easy and applicable for various silylated compounds, post-synthetic grafting of organosilica species is the most commonly used approach. Examples of aminopropyl grafted silica/alginate composites for cell adhesion purposes are also available in the literature [42,43]. To realize higher functionalization densities, the use of precursors of the form (R’O)3Si-R-Si(OR′)3 as organosilica source is the most promising approach. For nearly two decades it is known, that these bis-silylated organic compounds also undergo micellar-templating following a mechanism that is analogous to the one of the M41S family. Up to now a multitude of different bis-silylated organic precursors has been published and the progress in this field has been frequently reviewed [44,45]. Synthesis using only bis-silylated organic molecules as precursors lead to so-called periodic mesoporous organosilicas (PMOs), wherein the functionalization density is as high as possible and the organic bridge is homogeneously distributed as it is incorporated
2. Experimental 2.1. Materials 1,4-bis(triethoxysilyl)benzene (BTEB) was synthesized according to the literature [56]. All compounds were purchased from commercial suppliers: L(+)-lactic acid calcium salt pentahydrate (98%, Carl Roth), 1,4-dibromobenzene (Aldrich, 98%), cetyltrimethylammonium bromide (CTAB, Alfa Aesar, 98%), sodium alginate (Applichem Lifescience), tetraethylorthosilicate (TEOS, Merck), tri-sodium citrate dihydrate (Honeywell). Syringes: BD Plastipack 50 mL (Braun), cannula G 14 x 3 1/8″, 2.10 × 80 mm (Braun), syringe pump IVAC P7000 (Alaris Medical Systems). 2.2. Instruments N2 physisorption: All samples were outgassed on a Quantachrome Degasser Masterprep under reduced pressure (below 1·10−5 bar) at 80 °C for 16 h. N2 physisorption data were recorded at 77 K with a Quantachrome Quadrasorb-SI-MP/Quadrasorb evo or Quantachrome Autosorb 6B. The specific surface areas were determined using the method by Stephen Brunauer, Paul Hugh Emmett and Edward Teller (BET). Pore diameter distribution were calculated using the non-local density functional theory (NLDFT) kernel for silica with cylindrical pores from the adsorption branch. Powder X-Ray diffraction patterns were recorded at room temperature on a Panalytical MPD X'Pert Pro by applying filtered Cu-Kα radiation. Mercury intrusion porosimetry measurements were performed with a micromeritics AUTOPORE V SERIES instrument. Solid-state NMR: Solid state NMR experiments were performed on a Bruker Avance II 400 spectrometer, equipped with a 4 mm double resonance probe. 13C cross polarization (CP) magic angle spinning (MAS) spectra were acquired using ramped polarization transfer from protons to carbons at a 13C operating frequency of 100.66 MHz. The experimental conditions were 1H 90° pulse length of 4.0 μs, contact time of 1 ms, repetition delay of 4 s and MAS rate of 13 kHz. Two-pulse phase-modulated (TPPM) decoupling was used during the acquisition. 29 Si CP-MAS NMR spectra were obtained at 29Si frequency of 79.52 MHz using 1H 90° pulse length of 4.2 μs, contact time of 2 ms, recycle delay of 5 s, MAS rate of 5 kHz and continuous wave (CW) decoupling. 328
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alginate solution before droplets were formed. In order to obtain a homogeneous distribution of the (organo)silica precursor in the alginate solution, it needs to be in a water dissolvable state. Thus, the precursor was hydrolyzed in the first step, which was done under acidic conditions. As alginate gels in acid as well [57], adjustment of the pH value was necessary previous to mixing of the solutions. The beads were further aged in n-heptane referring to the literature [40]. During the subsequent drying process, shrinking of the beads could be observed. In order to allow complete condensation of the silica phase, the beads were treated with hydrochloric acid solution at 100 °C, before the alginate phase was removed by calcination or citric acid and pure (organo)silica beads remained. In the last step second pore systems were generated with the help of pseudomorphic transformation. In the following the synthesis and characterization of silica-based materials is described first.
2.3. Synthesis of the silica beads The synthesis combines mainly the approaches from Li et al. and Abramson et al. [38,40] TEOS (7.5 g, 36 mmol) was hydrolyzed in a solution of hydrochloric acid (17.5 g, 0.02 mol L−1, 0.35 mol) at room temperature. As soon as a clear solution was obtained, the pH value was adjusted to 5–6 with sodium hydroxide solution (0.4 mol L−1) before it was mixed into an aqueous solution of alginate (1.25 g in 100 mL of water) under permanent stirring. The solution was dropped into an aqueous solution of calcium lactate (1.0 L, 0.45 mol L−1) using a syringe pump with a dropping height of 18 cm and a flow of 250–340 mL h−1. The nascent beads were aged in the solution overnight previous to decantation and washing with water. Subsequently, the beads were aged in n-heptane for 4 days, then washed with acetone and dried spread flat on filter paper at room temperature. During this step shrinking of the beads occurs. The alginate/silica beads were treated in hydrochloric acid solution (80 mL, 1.0 mol L−1) at 100 °C for 24 h in a screw cap bottle. To remove the alginate, the beads were calcined at 550 °C for 6 h. Over the whole procedure 2.15 g silica beads were received from two batches.
3.1. Characterization of silica beads As can be seen on the photograph in Fig. 2 on the left, pearly-white beads with diameters of approximately 2 mm are obtained after removing the polymer by calcination (step 4 in the scheme). The material that is shown on the right in the photograph was treated with an alkaline solution of cetyltrimethylammonium bromide (CTAB) at 100 °C for four days (denoted as MTS-4d-beads). Obviously, the bead shape and color are preserved. Fig. 3 shows the N2 physisorption isotherms of the silica beads before and after pseudomorphic transformation. The initial silica beads (black) provide a specific BET surface area of 716 m2 g−1. On the one hand, these results are in good agreement with similarly alginate-templated silica gels in the literature, where the specific BET surface area ranges from 670 m2 g−1 to 755 m2 g−1 [40]. On the other hand, a silica gel which was synthesized in a calcium(II) chloride solution without alginate show even higher surface areas of 905 m2 g−1 [31]. Hence, the high surface area is not necessarily due to pores that are generated by the removal of the alginate matrix as could be assumed, but can be due to interparticle voids of primary nanoparticles, as can be seen in the SEM image in Fig. 4. The isotherm of the silica beads before the transformation shows a continuous volume uptake over the whole pressure range which indicates mesopores with a broad pore size distribution. In the relative pressure range from 0.85 to 0.96 the slope of the isotherm steepens and steepens again at relative pressure over 0.96 which suggests the presence of larger mesopores. Fig. 5 shows the pore diameter distribution (black) which is broad in the mesopore size range. It needs to be considered that the NLDFT kernel demands a specific, cylindrical, pore geometry, which is not necessarily given in this case but in case of the micelle-templated materials. In the cited literature, pores around 12 nm were determined [31]. After four days of pseudomorphic transformation (denoted as MTS4d-beads, Figs. 3 and 5 green) the specific BET surface area slightly increased to 767 m2 g−1. An isotherm type IVb is given and pores of 4.2 nm are generated. Furthermore, an increase of the adsorbed nitrogen volume at relative pressure > 0.9 indicates additional macropores. In order to preserve a higher ratio of the macropore volume, partial pseudomorphic transformation was carried out by reduction of the hydrothermal treatment time from four days to four hours (denoted as MTS-4h-beads). Following this approach, a bimodal porous material can be generated which could be hierarchical in case of an interconnection of the pore systems, although this could not be proven for this materials here [58]. Already four hours of pseudomorphic transformation a type IVb isotherm with narrow pore diameter distribution, maximum at 4.2 nm is received (blue Figs. 3 and 5). However, the specific BET surface area decreases to 582 m2 g−1. The phenomenon of reduced surface area after partial pseudomorphic transformation has already been observed by
2.4. Pseudomorphic transformation into MTS beads A solution of CTAB (900 mg, 2.5 mmol) in aqueous sodium hydroxide solution (24 mL, 0.05 mol L−1, 1.2 mmol) was prepared and stirred for 30 min before the silica beads (600 mg, 10 mmol) were added and treated under static conditions at 100 °C in a screw cap bottle for the given hydrothermal treatment time of 4 days (sample denoted as MTS4d-beads) or 4 h (sample denoted as MTS-4h-beads). Again, the composites were calcined at 550 °C for 6 h to remove the surfactant. The molar ratio of all components: Si/CTAB/NaOH/H2O was 1/0.25/0.1/ 133. 2.5. Synthesis of the organosilica beads For hydrolysis BTEB (1 mL, 2.5 mmol) was treated in hydrochloric acid (2 mL, 0.2 mol L−1 0.4 mmol) and ethanol (99.8%, 2 mL, 34 mmol) at room temperature until the solution became clear. After adjustment of the pH value to 5–6 with sodium hydroxide solution (0.4 mol L−1), the organosilica solution was mixed to a solution of 0.25 g alginate in 20 mL water. Analogue to the already described synthesis of silica beads, the mixture was dropped in calcium lactate solution, aged therein and in n-heptane, washed and dried. The organosilica beads were treated in hydrochloric acid (80 mL, 1.0 mol L−1) at 100 °C for 6 h in a screw cap bottle before alginate was extracted with a tri-sodium citrate solution (100 mL, 1.0 mol L−1) for 24 h, then washed with water and ethanol. After drying at 80 °C, 1.3 g organosilica beads were received over the whole procedure from five simultaneously set batches. 2.6. Pseudomorphic transformation of organosilica beads (pT-OS-beads) A solution was prepared of water (31.0 g, 1.72 mol), ethanol (99.8%, 9.00 g, 0.195 mol), sodium hydroxide (50.0 mg, 1.25 mmol) and CTAB (550 mg, 1.51 mmol) and stirred for 30 min before the organosilica beads (500 mg) were added and treated for 24 h at 100 °C under static conditions in a screw cap bottle. After washing with water and ethanol, the surfactant was removed in a Soxhlet apparatus using ethanol/hydrochloric acid (32 vol%) 97/3 (v/v) for three days. The extracted beads were washed with water and ethanol again and dried at 80 °C. 3. Results and discussion A scheme of the synthesis pathway for (organo)silica beads is given in Fig. 1. We took advantage of the ionotropic gelation of alginate and mixed either a silica precursor or an organosilica precursor in the 329
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Fig. 1. Diagram of the multistep synthesis pathway for millimeter-sized nanoporous (organo)silica beads with alginate hydrogel as form-giving matrix. 1st step: The precursor (TEOS for silica beads and BTEB for organosilica beads) need to be hydrolyzed before they are mixed with an alginate solution. 2nd step: Droplets of organosilica/calcium alginate are formed. 3rd step: Condensation of the organosilica phase occurs during aging in n-heptane, drying and is subsequently provoked by acidic treatment. 4th step: Removal of the alginate matrix by calcination (silica) or with the help of citric acid (organosilica). 5th step: Introduction of a second pore system by pseudomorphic transformation.
Fig. 2. Photograph of the calcined silica beads (a), and MTS-4d-beads (b).Scale is given by plotting paper in the middle, wherein the small squares correspond to 2.0 mm × 2.0 mm.
volume of only 0.3 cm³·g−1 occurs and the pore size distribution changes significantly. As expected the initial pores of the gel with 13 nm diameter collapse during the transformation, but the appearance of larger pores around 49 nm pore diameter is rather unexpected. MTS4h-beads show a pore volume of 0.5 cm³·g−1 as it was before the pseudomorphic transformation, but large mesopores with a distribution maximum at 46 nm instead of the mesopores around 13 nm are obtained. Thus, it seems like large mesopores are generated during the pseudomorphic transformation or become accessible during the restructuring of the material. Successful templating that lead to an ordered mesophase can be demonstrated by the reflections in the small angle region in the X-ray
Einicke at al [11] in case of pores smaller than 60 nm and a pore volume lower than 1.0 cm³·g−1. It was explained by collapse of the initial pore system due to swelling of the material during the restructuring process. The presence of larger pores was investigated by mercury intrusion porosimetry. Fig. 6 shows the pore diameter distributions (for intrusion curves see the supporting information) of the silica beads, MTS-4hbeads and MTS-4d-beads. Before the pseudomorphic transformation the silica beads have a pore volume of 0.5 cm³·g−1 with pores around 13 nm. The nitrogen physisorption measurement of this sample showed mainly smaller pores with a broad pore diameter distribution, but these are not completely accessible for mercury within the receivable pressure range. Interestingly, for MTS-4d-beads, a perceptible smaller pore 330
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Fig. 3. N2 physisorption isotherms (77 K) of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4d-beads (green). Filled symbols adsorption, empty symbols desorption. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Pore diameter from mercury intrusion porosimetry of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4dbeads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. SEM image of the calcined silica beads (magnification of 2 × 105, scale bar of 1 μm).
Fig. 7. X-ray diffraction patterns of MTS-4h-beads (blue) and MTS-4d-beads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
diffraction pattern (Fig. 7). MTS-4h-beads shows a broad reflection at approximately 2θ = 1.8° (d = 4.9 nm), whereas for MTS-4d-beads a reflection at ca. 2θ = 2.2° (d = 4.0 nm) occurs, next to a further broad reflection at 2θ = 4.0° (d = 2.2 nm). Due to the low resolution of the XRD pattern, the differentiation between a worm-like and a MCM-41type structure where the first reflection was assigned to hkl (100) and the broad reflection to hkl (110) and (200), is indistinct. Hence, MTS-beads with a bimodal pore structure which differ in surface area and pore volume could be synthesized. 3.2. Characterization of organosilica beads As the synthesis of pure silica beads and their pseudomorphic transformation was successful, we intend to transfer this approach to the synthesis of organosilica beads. We decided for the bis-silylated phenylene-bridged precursor (BTEB) as model system, as phenylenebridged organosilica materials have already shown good performance in various applications like chromatography [48] and offers the possibility of different post-synthetic modifications [59–61]. Since its behavior in sol-gel synthesis differs significantly from TEOS, it is a
Fig. 5. Pore diameter distributions (according to NLDFT kernel for silica, cylindrical pores, from the adsorption branch) of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4d-beads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 331
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Fig. 8. Photograph of the organosilica beads (a) and pT-OS-beads (b). Scale bar is given by plotting paper in the middle, wherein the small square correspond to 2.0 mm × 2.0 mm.
representative model system which shows that our approach can be applied to a variety of different precursors. In general, the previously described synthesis path could be adopted, but needed modification at some points: For the hydrolysis of BTEB as organosilica source, a higher concentration of hydrochloric acid was necessary. The alginate phase could be removed by calcination in case of the silica beads, but this is impossible for the organosilica beads since the organic bridges are affected by heat treatment as well. An alternative approach is the dissolution of the calcium alginate hydrogel with the help of citric acid which binds calcium ions coordinately [32]. This is no longer possible after the degradation of alginate during the treatment of the dried beads with hydrochloric acid at 100 °C, which is signalized by blackening of the beads. Hence, the treatment time was shortened from 24 h to six hours, before the alginate phase could be removed successfully. In case of shorter treatment than six hours, the beads were not stable and fell into pieces after removal of alginate. The obtained spherical beads are of similar size as the pure silica beads (2 mm diameter), as can be seen in Fig. 8 on the left. Marginal residues of alginate which decomposed during the acidic treatment causes slight yellowness of the beads. After the pseudomorphic transformation with CTAB/NaOH solution at 100 °C for 24 h and extraction of the surfactant, white beads are obtained as can be seen in Fig. 8 on the right (denoted as pT-OS-beads). Hence, also in case of the organosilica beads, preservation of the bead morphology is given. The pseudomorphic transformation of organosilica materials has already been shown by our group before [62] and was successful here as well. Fig. 9 shows the N2 physisorption isotherms of the organosilica beads (gray) and pT-OS-beads (red). Here, the specific BET surface area increases only slightly from 699 m2 g−1 to 731 m2 g−1. Both isotherms do not match with the isotherm types as classified by IUPAC. In the first case the isotherm starts with a steep uptake in the low-pressure region, like a type I(b) isotherm which indicates microporosity. Additionally, a steep region over 0.95 indicates larger mesopores or macroporosity similar to a type II isotherm. In this combination of type I and II isotherm, hysteresis type H4 appears [63]. Discussing the isotherm of the pT-OS-beads, a continuous increase in the adsorption branch occurs which indicates mesopores of broad pore size distribution. In this case a prominent hysteresis occurs most likely due to irregular diameters of the pores. The respective pore diameter distribution changes significantly after the pseudomorphic transformation, as can be seen in Fig. 10. The organosilica material has a broad pore size distribution of micro and
Fig. 9. N2 physisorption isotherms (77 K) of organosilica beads before (gray) and after pseudomorphic transformation (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
mesopores. After the transformation, the distribution narrows distinctly with a maximum at 3.7 nm. This indicates successful micellar-templating with CTAB as surfactant. The materials were characterized by mercury intrusion porosimetry. The macropore volumes of the organosilica beads are 0.26 m2 g−1 before and 0.32 m2 g−1 after the pseudomorphic transformation which are significantly lower than those of silica beads. The pore size distribution is extremely broad over the whole pore range in both cases (see the pore diameter distribution and intrusion curves in the supporting information). Similar to the pure silica material, pore ordering of the organosilica was investigated by the X-ray diffraction pattern (see Fig. 11). Here, a broad reflection at ca. 2 θ = 2.0° (d = 4.4 nm) indicates a poorly ordered mesophase. Due to this poor ordering the material is not a PMO per definition. Additional reflections indicate a crystal-like arrangement of the organic bridges after the transformation. These are located at 2θ = 11.6° (d = 7.6 Å), 20.7 (d = 4.3 Å), 23.4 (d = 3.8 Å), 35.6° (d = 2.5 Å) which is in accordance with the literature for crystal-like arrangement of the organic bridge within the pore walls of a phenylene332
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Fig. 12. 13C CP-MAS NMR (spinning speed 13 kHz) spectra of the organosilica beads (gray) and pT-OS-beads (red). Spinning side bands are indicated with *. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
surfactant is not significant for the quality of the product. The plots can be seen in the supporting information in comparison to a commonly synthesized benchmark phenylene-bridged PMO. 29 Si CP-MAS NMR provides semi-quantitative information about the condensation degree of the organosilica species and the integrity of the Si-C bond (Fig. 13). Before the transformation, T2 is the most prominent silica species and a significant ratio of T1 is observed. After the transformation, the condensation degree increases, as the T1 signal is of very low intensity and T3 becomes the most prominent species. Thus, the interconnection is enhanced by the pseudomorphic transformation. For organosilica beads as well as for pT-OS-beads, weak Q type 29Si signals are also observed. The relative intensity of Q signals increases after the transformation, indicating that a small amount of Si–C bond breaking occurs during the harsh treatments. This has already been observed in previous publications [62]. Although CP-MAS NMR spectroscopy is not suitable for quantification of the different silica species, the relative amount of Q species can be roughly estimated from comparison to quantitative analysis of other silica materials. The signal intensity of Q species is comparably low in both cases, suggesting that the structures of the organosilica beads are more or less intact. The results show that the synthesis protocol from silica beads could be transferred to the synthesis of organosilica beads. The pseudomorphic transformation of the organosilica beads was also successful, since micellar-templating was possible and mechanically stable, millimeter-sized mesoporous organosilica beads were obtained.
Fig. 10. Pore diameter distributions (according to NLDFT kernel for silica, cylindrical pores, from the adsorption branch) of organosilica beads before (gray) and after pseudomorphic transformation with CTAB for 24 h (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. X-ray diffraction patterns of the organosilica materials before (gray) and after pseudomorphic transformation for 24 h (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
bridged PMO material [59]. The organosilica-beads and pT-OS-beads were further investigated by solid state NMR spectroscopy. The integrity of the organic bridge can be investigated best by 13C CP-MAS NMR. Fig. 12 shows the respective spectra of organosilica beads and pT-OS-beads. For both samples, a dominant signal is observed at 133 ppm, which can be assigned to two types of aromatic carbons of phenyl bridging group. The corresponding precursor, BTEB presents two 13C signals at 133.5 ppm and 134.4 ppm for Car-Si (aromatic carbon directly bound to Si) and Car-H, respectively, in CDCl3. The difference in chemical shift values of these two carbons are smaller than the linewidth of 13C CPMAS NMR spectrum of organosilica beads, resulting in a single overlapping signal, which is consistent with the previous reports [56]. Since there are no further signals in the spectrum of organosilica beads, complete removal of alginate can be assumed. By contrast, additional weak signals are shown between 15 ppm and 70 ppm for the pT-OSbeads which can be assigned to residues of the surfactants. The extraction of the samples was not completely successful. However, there was no evidence for the presence of surfactant in thermal analysis and infrared-spectroscopy, suggesting that the amount of the residual
Fig. 13. 29Si CP-MAS NMR (spinning speed 5 kHz) spectra of the organosilica beads (black) and pT-OS-beads (red). The assignment of T and Q species are denoted in the spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 333
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4. Conclusion Following a hydrogel approach with alginate, we synthesized silica beads with diameters of approximately 2 mm with a bimodal mesopore system and noteworthy specific BET surface areas. In both cases postsynthetic micellar templating was possible by so-called pseudomorphic transformation. Thus, micellar-templated silica beads were synthesized maintaining the initial morphology of the beads. In addition to the mesopores of 4.2 nm the system shows also macropores after pseudomorphic transformation. The macropore volume was adjustable by the transformation time. A similar synthesis path was also possible to achieve pseudomorphic transformed phenylene-bridged organosilica beads of the same size. Likewise, a high surface area was obtained and mesopores of 3.7 nm were formed by pseudomorphic transformation. In addition to the results from thermogravimetric analysis and IR-spectroscopy, 13C-CP MAS NMR confirmed the complete removal of the alginate phase. Thus, we extend the possible field of applications for phenylenebridged mesoporous organosilicas, as these beads in contrast to powders are much more likely to be applicable in industrial processes, e.g. as adsorbents from gas flow. In further works, this new approach could be extended to other precursors as well and even other dimensions of the beads or monoliths are conceivable following the alginate hydrogel approach, depending on the targeted application.
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[13] [14]
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Acknowledgments The authors thank Renate Walter for SEM images. We thank the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for funding within the SPP 1570 (FR1372/19-3).
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Appendix A. Supplementary data [24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.04.028.
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Millimeter-sized micellar-templated silica beads and phenylene-bridged mesoporous organosilica beads
T
Malina Bilo, Young Joo Lee, Michael Fröba∗ Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146, Hamburg, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Alginate hydrogels Millimeter-sized organosilica beads Pseudomorphic transformation of silica and organosilica
Micellar-templated silicas and organosilica-based materials from bis-silylated organosilica precursors with an ordered array of uniform mesopores are attractive for different applications. Since they are usually obtained in the form of fine powders, the material does not meet the criteria for many industrial uses or applications where good flow-through properties are required. Herein, a synthesis route is described where alginate hydrogel acts as form-giving matrix and can be subsequently removed leaving mechanically stable silica beads with 2 mm in diameter. In these beads post-synthetic micellar-templating is possible by so-called pseudomorphic transformation in alkaline surfactant solution, whereby a homogeneous mesophase is generated. Additionally, the formation of a second pore system of large mesopores occurs, so that bimodal porous silica beads are obtained. Following the same general concept, the synthesis of pure phenylene-bridged organosilica beads with likewise 2 mm in diameter was possible and again pseudomorphic transformation could be used to obtain mesoporous phenylene-bridged organosilica beads. The integrity of the organic bridge and the absence of alginate and the surfactant was proven by solid state NMR spectroscopy.
1. Introduction Mesoporous silica-based materials are of growing importance for various applications. In the form of sub-micrometer particles or monoliths, this class of materials is commonly used in chromatography as well as in catalysis [1,2]. Porous silica nanoparticles are of interest for medical applications [3–5]. A synthesis concept for ordered mesoporous silica that allows the adjustment of the pore size, as well as the type of pore ordering in micellar-templated silica (MTS) was established with the M41S family [6]. However, the combination of application-adjusted morphology together with the respectively requested pore system in one step is usually a balancing act. By so-called pseudomorphic transformation Galarneau and coworkers found a way to optimize one feature after the other by post-synthetic micellar induced pore generation that does not affect the morphology [7]. The term pseudomorphic transformation (gr. pseudos = false, morphe = form) is adapted from mineralogy. It describes how a mineral can appear in an unusual external crystal shape due to environmentally induced change of the chemical composition under preservation of the initial morphology [8]. Galarneau and coworkers used the term for a transformation process of non-structured silica into a micellar-templated phase while keeping the morphology of the initial silica phase. In general, the concept takes advantage of the kinetic dependence of silica dissolution ∗
in basic media and its re-condensation in the presence of a surfactant [9]. The pseudomorphic transformation was intensively investigated for different commercially available silica gels [10–13] as well as porous glasses [14–16]. Special focus lies on the synthesis of hierarchically porous glasses [17] and other silica-based monoliths [18,19], because hierarchical pore systems improve the distribution of a gas or a liquid. On this basis we intend to synthesize porous, millimeter-sized spherical silica beads which are dust-free and could thus be used in industrial processes e.g. as adsorbents. Some examples of other synthesis approaches for nanoporous silica beads from the literature should be mentioned here, where usually oil-in-water or water-in-oil emulsions or polymer templating is used. In 1997 Stucky et al. showed the synthesis of mesoporous spherical beads with specific BET surface areas between 480 m2 g−1 and 1100 m2 g−1 in an emulsion synthesis with tetrabutyl orthosilicate as silica source that releases butanol during the hydrolysis process and a quaternary ammonium surfactants acting as template for the pore system and as stabilizer for the emulsion [20]. In works that are more recent an aqueous phase, a sol of the silica source methyltrimethoxysilane, ammonium hydroxide and cetyltrimethylammonium bromide (CTAB) in water was added to hexane as oil phase and spherical particles are formed in this emulsion under vigorous stirring [21]. Similar to this, the synthesis of millimeter-sized mesoporous particles with high surface areas was possible in the presence of ethyl ether in a
Corresponding author. E-mail address: [email protected] (M. Fröba).
https://doi.org/10.1016/j.micromeso.2019.04.028 Received 28 March 2019; Received in revised form 14 April 2019; Accepted 15 April 2019 Available online 19 April 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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covalently inside the pore walls [45–47]. The fields of applications are just as multitudinous as the possible precursors for PMOs, especially in chromatography [48], separation processes and heterogeneous catalysis the morphology plays an important role. There are only few examples of hierarchically porous PMO monoliths realized by phase separation, mainly in the research groups of Nakanishi and Hüsing [49–53]. There are also more recent examples of organosilica monoliths, e.g. Schachtschneider et al. presented an organosilica monolith that can be gradually modified by click-chemistry [54]. Further, an ethylene-bridged organosilica monolith of 100 μm diameter was functionalized by thiol-ene click-chemistry and performed well in capillary chromatography tests [55]. However, no micellar-templating took place in these examples. In this paper we present the synthesis and subsequent pseudomorphic transformation of millimeter-sized silica beads using the concept of ionotropic gelation with alginate acting as form-giving matrix which was subsequently removed. Furthermore, we show how both concepts can be transferred to organosilica materials using pure 1,4-bis (triethoxysilyl)benzene (BTEB) as precursor to obtain millimeter-sized phenylene-bridged mesoporous organosilica beads. To the best of our knowledge, the biopolymer alginate has never been used as template for the synthesis of porous organosilica monoliths.
common micelle-templated silica synthesis approach with a silica source, ammonium hydroxide, surfactant and water. This approach was extended by adding an aluminum or a titanium source to the synthesis mixture. The diameter of the macro spheres was adjusted with the stirring rate between 0.4 and 1.0 mm [22]. Scholz et al. synthesized hierarchically structured silica spheres with particle diameters between 0.3 mm and 2.5 mm by emulsion based sol–gel processing in a co-condensation of a pure silica species with [3-(2-aminoethylamino)propyl] trimethoxysilane and phenyltrimethoxysilane [23,24]. In 2016 Sultana et al. prepared mesoporous hybrid microspheres around 30 μm in diameter from silica nanoparticles (40–50 nm) and a reactive copolymer in a organic/inorganic suspension under stirring [25]. Hierarchically porous silica materials were synthesized by combining the oil-in-water emulsion technique, using a block-copolymer to generate mesopores and the formation of a polymer on the outer surface of the formed oil droplets [26]. In a completely different approach the synthesis of mesoporous silicon dioxide, silicon, and silicon carbide beads was possible by soaking of commercially available polystyrene spheres of 500–800 μm in diameter with tetraethylorthosilicate TEOS and subsequent heat treatment under air or under argon and treatment with magnesia [27]. The biopolymer alginate is a suitable form-giving matrix as it forms hydrogels by ionotropic gelation. The formation of spherical beads is easily done by dropping an alginate solution into a solution of calcium ions where the size of the beads can be adjusted with the drop size, e. g. in the millimeter-size range [28,29]. We think, in view of a rather green chemistry trend, the use of polysaccharides becomes increasingly relevant [30]. Composite materials of silica and alginate are contemplable in biomedical applications for encapsulations, cell immobilization or tissue engineering [31–38]. Li et al. synthesized silica/alginate hydrogel beads by adding an alginate solution to a solution of pre-hydrolyzed TEOS, showing that the formation of beads was successful by dropping the mixture in an aqueous calcium chloride solution [38]. In 2015 Roosen et al. showed the formation of silica/alginate hybrid materials following two different paths, either by soaking alginate beads with a mixture of tetramethyl orthosilicate in ethanol overnight or by adding purchased silica microparticles to the suspension of alginate before the droplet formation [39]. Alginate was used as a template for magnetic silica beads containing γ-Fe2O3 by Liu et al. [33] and by Abramson et al., the latter removed the hydrogel subsequently under mild conditions with citric acid [40] as it was also shown by Coradin et al. [32] The Enke research group showed the synthesis of bimodal porous glass spheres between 1.9 and 3.7 nm in diameter by mixing different fractions of controlled pore glasses particles (32–100 μm in diameter) in an alginate solution before the droplet formation. Subsequently, partial sintering of the glass and removal of the alginate phase is possible by thermal treatment, whereby small particles lead to stable beads [41]. Similar to this, we synthesized pure silica spheres in a formgiving matrix of alginate. Additionally, we expand the concept to organosilica beads. Different approaches are available for the functionalization of porous silicas. Since it is easy and applicable for various silylated compounds, post-synthetic grafting of organosilica species is the most commonly used approach. Examples of aminopropyl grafted silica/alginate composites for cell adhesion purposes are also available in the literature [42,43]. To realize higher functionalization densities, the use of precursors of the form (R’O)3Si-R-Si(OR′)3 as organosilica source is the most promising approach. For nearly two decades it is known, that these bis-silylated organic compounds also undergo micellar-templating following a mechanism that is analogous to the one of the M41S family. Up to now a multitude of different bis-silylated organic precursors has been published and the progress in this field has been frequently reviewed [44,45]. Synthesis using only bis-silylated organic molecules as precursors lead to so-called periodic mesoporous organosilicas (PMOs), wherein the functionalization density is as high as possible and the organic bridge is homogeneously distributed as it is incorporated
2. Experimental 2.1. Materials 1,4-bis(triethoxysilyl)benzene (BTEB) was synthesized according to the literature [56]. All compounds were purchased from commercial suppliers: L(+)-lactic acid calcium salt pentahydrate (98%, Carl Roth), 1,4-dibromobenzene (Aldrich, 98%), cetyltrimethylammonium bromide (CTAB, Alfa Aesar, 98%), sodium alginate (Applichem Lifescience), tetraethylorthosilicate (TEOS, Merck), tri-sodium citrate dihydrate (Honeywell). Syringes: BD Plastipack 50 mL (Braun), cannula G 14 x 3 1/8″, 2.10 × 80 mm (Braun), syringe pump IVAC P7000 (Alaris Medical Systems). 2.2. Instruments N2 physisorption: All samples were outgassed on a Quantachrome Degasser Masterprep under reduced pressure (below 1·10−5 bar) at 80 °C for 16 h. N2 physisorption data were recorded at 77 K with a Quantachrome Quadrasorb-SI-MP/Quadrasorb evo or Quantachrome Autosorb 6B. The specific surface areas were determined using the method by Stephen Brunauer, Paul Hugh Emmett and Edward Teller (BET). Pore diameter distribution were calculated using the non-local density functional theory (NLDFT) kernel for silica with cylindrical pores from the adsorption branch. Powder X-Ray diffraction patterns were recorded at room temperature on a Panalytical MPD X'Pert Pro by applying filtered Cu-Kα radiation. Mercury intrusion porosimetry measurements were performed with a micromeritics AUTOPORE V SERIES instrument. Solid-state NMR: Solid state NMR experiments were performed on a Bruker Avance II 400 spectrometer, equipped with a 4 mm double resonance probe. 13C cross polarization (CP) magic angle spinning (MAS) spectra were acquired using ramped polarization transfer from protons to carbons at a 13C operating frequency of 100.66 MHz. The experimental conditions were 1H 90° pulse length of 4.0 μs, contact time of 1 ms, repetition delay of 4 s and MAS rate of 13 kHz. Two-pulse phase-modulated (TPPM) decoupling was used during the acquisition. 29 Si CP-MAS NMR spectra were obtained at 29Si frequency of 79.52 MHz using 1H 90° pulse length of 4.2 μs, contact time of 2 ms, recycle delay of 5 s, MAS rate of 5 kHz and continuous wave (CW) decoupling. 328
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alginate solution before droplets were formed. In order to obtain a homogeneous distribution of the (organo)silica precursor in the alginate solution, it needs to be in a water dissolvable state. Thus, the precursor was hydrolyzed in the first step, which was done under acidic conditions. As alginate gels in acid as well [57], adjustment of the pH value was necessary previous to mixing of the solutions. The beads were further aged in n-heptane referring to the literature [40]. During the subsequent drying process, shrinking of the beads could be observed. In order to allow complete condensation of the silica phase, the beads were treated with hydrochloric acid solution at 100 °C, before the alginate phase was removed by calcination or citric acid and pure (organo)silica beads remained. In the last step second pore systems were generated with the help of pseudomorphic transformation. In the following the synthesis and characterization of silica-based materials is described first.
2.3. Synthesis of the silica beads The synthesis combines mainly the approaches from Li et al. and Abramson et al. [38,40] TEOS (7.5 g, 36 mmol) was hydrolyzed in a solution of hydrochloric acid (17.5 g, 0.02 mol L−1, 0.35 mol) at room temperature. As soon as a clear solution was obtained, the pH value was adjusted to 5–6 with sodium hydroxide solution (0.4 mol L−1) before it was mixed into an aqueous solution of alginate (1.25 g in 100 mL of water) under permanent stirring. The solution was dropped into an aqueous solution of calcium lactate (1.0 L, 0.45 mol L−1) using a syringe pump with a dropping height of 18 cm and a flow of 250–340 mL h−1. The nascent beads were aged in the solution overnight previous to decantation and washing with water. Subsequently, the beads were aged in n-heptane for 4 days, then washed with acetone and dried spread flat on filter paper at room temperature. During this step shrinking of the beads occurs. The alginate/silica beads were treated in hydrochloric acid solution (80 mL, 1.0 mol L−1) at 100 °C for 24 h in a screw cap bottle. To remove the alginate, the beads were calcined at 550 °C for 6 h. Over the whole procedure 2.15 g silica beads were received from two batches.
3.1. Characterization of silica beads As can be seen on the photograph in Fig. 2 on the left, pearly-white beads with diameters of approximately 2 mm are obtained after removing the polymer by calcination (step 4 in the scheme). The material that is shown on the right in the photograph was treated with an alkaline solution of cetyltrimethylammonium bromide (CTAB) at 100 °C for four days (denoted as MTS-4d-beads). Obviously, the bead shape and color are preserved. Fig. 3 shows the N2 physisorption isotherms of the silica beads before and after pseudomorphic transformation. The initial silica beads (black) provide a specific BET surface area of 716 m2 g−1. On the one hand, these results are in good agreement with similarly alginate-templated silica gels in the literature, where the specific BET surface area ranges from 670 m2 g−1 to 755 m2 g−1 [40]. On the other hand, a silica gel which was synthesized in a calcium(II) chloride solution without alginate show even higher surface areas of 905 m2 g−1 [31]. Hence, the high surface area is not necessarily due to pores that are generated by the removal of the alginate matrix as could be assumed, but can be due to interparticle voids of primary nanoparticles, as can be seen in the SEM image in Fig. 4. The isotherm of the silica beads before the transformation shows a continuous volume uptake over the whole pressure range which indicates mesopores with a broad pore size distribution. In the relative pressure range from 0.85 to 0.96 the slope of the isotherm steepens and steepens again at relative pressure over 0.96 which suggests the presence of larger mesopores. Fig. 5 shows the pore diameter distribution (black) which is broad in the mesopore size range. It needs to be considered that the NLDFT kernel demands a specific, cylindrical, pore geometry, which is not necessarily given in this case but in case of the micelle-templated materials. In the cited literature, pores around 12 nm were determined [31]. After four days of pseudomorphic transformation (denoted as MTS4d-beads, Figs. 3 and 5 green) the specific BET surface area slightly increased to 767 m2 g−1. An isotherm type IVb is given and pores of 4.2 nm are generated. Furthermore, an increase of the adsorbed nitrogen volume at relative pressure > 0.9 indicates additional macropores. In order to preserve a higher ratio of the macropore volume, partial pseudomorphic transformation was carried out by reduction of the hydrothermal treatment time from four days to four hours (denoted as MTS-4h-beads). Following this approach, a bimodal porous material can be generated which could be hierarchical in case of an interconnection of the pore systems, although this could not be proven for this materials here [58]. Already four hours of pseudomorphic transformation a type IVb isotherm with narrow pore diameter distribution, maximum at 4.2 nm is received (blue Figs. 3 and 5). However, the specific BET surface area decreases to 582 m2 g−1. The phenomenon of reduced surface area after partial pseudomorphic transformation has already been observed by
2.4. Pseudomorphic transformation into MTS beads A solution of CTAB (900 mg, 2.5 mmol) in aqueous sodium hydroxide solution (24 mL, 0.05 mol L−1, 1.2 mmol) was prepared and stirred for 30 min before the silica beads (600 mg, 10 mmol) were added and treated under static conditions at 100 °C in a screw cap bottle for the given hydrothermal treatment time of 4 days (sample denoted as MTS4d-beads) or 4 h (sample denoted as MTS-4h-beads). Again, the composites were calcined at 550 °C for 6 h to remove the surfactant. The molar ratio of all components: Si/CTAB/NaOH/H2O was 1/0.25/0.1/ 133. 2.5. Synthesis of the organosilica beads For hydrolysis BTEB (1 mL, 2.5 mmol) was treated in hydrochloric acid (2 mL, 0.2 mol L−1 0.4 mmol) and ethanol (99.8%, 2 mL, 34 mmol) at room temperature until the solution became clear. After adjustment of the pH value to 5–6 with sodium hydroxide solution (0.4 mol L−1), the organosilica solution was mixed to a solution of 0.25 g alginate in 20 mL water. Analogue to the already described synthesis of silica beads, the mixture was dropped in calcium lactate solution, aged therein and in n-heptane, washed and dried. The organosilica beads were treated in hydrochloric acid (80 mL, 1.0 mol L−1) at 100 °C for 6 h in a screw cap bottle before alginate was extracted with a tri-sodium citrate solution (100 mL, 1.0 mol L−1) for 24 h, then washed with water and ethanol. After drying at 80 °C, 1.3 g organosilica beads were received over the whole procedure from five simultaneously set batches. 2.6. Pseudomorphic transformation of organosilica beads (pT-OS-beads) A solution was prepared of water (31.0 g, 1.72 mol), ethanol (99.8%, 9.00 g, 0.195 mol), sodium hydroxide (50.0 mg, 1.25 mmol) and CTAB (550 mg, 1.51 mmol) and stirred for 30 min before the organosilica beads (500 mg) were added and treated for 24 h at 100 °C under static conditions in a screw cap bottle. After washing with water and ethanol, the surfactant was removed in a Soxhlet apparatus using ethanol/hydrochloric acid (32 vol%) 97/3 (v/v) for three days. The extracted beads were washed with water and ethanol again and dried at 80 °C. 3. Results and discussion A scheme of the synthesis pathway for (organo)silica beads is given in Fig. 1. We took advantage of the ionotropic gelation of alginate and mixed either a silica precursor or an organosilica precursor in the 329
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Fig. 1. Diagram of the multistep synthesis pathway for millimeter-sized nanoporous (organo)silica beads with alginate hydrogel as form-giving matrix. 1st step: The precursor (TEOS for silica beads and BTEB for organosilica beads) need to be hydrolyzed before they are mixed with an alginate solution. 2nd step: Droplets of organosilica/calcium alginate are formed. 3rd step: Condensation of the organosilica phase occurs during aging in n-heptane, drying and is subsequently provoked by acidic treatment. 4th step: Removal of the alginate matrix by calcination (silica) or with the help of citric acid (organosilica). 5th step: Introduction of a second pore system by pseudomorphic transformation.
Fig. 2. Photograph of the calcined silica beads (a), and MTS-4d-beads (b).Scale is given by plotting paper in the middle, wherein the small squares correspond to 2.0 mm × 2.0 mm.
volume of only 0.3 cm³·g−1 occurs and the pore size distribution changes significantly. As expected the initial pores of the gel with 13 nm diameter collapse during the transformation, but the appearance of larger pores around 49 nm pore diameter is rather unexpected. MTS4h-beads show a pore volume of 0.5 cm³·g−1 as it was before the pseudomorphic transformation, but large mesopores with a distribution maximum at 46 nm instead of the mesopores around 13 nm are obtained. Thus, it seems like large mesopores are generated during the pseudomorphic transformation or become accessible during the restructuring of the material. Successful templating that lead to an ordered mesophase can be demonstrated by the reflections in the small angle region in the X-ray
Einicke at al [11] in case of pores smaller than 60 nm and a pore volume lower than 1.0 cm³·g−1. It was explained by collapse of the initial pore system due to swelling of the material during the restructuring process. The presence of larger pores was investigated by mercury intrusion porosimetry. Fig. 6 shows the pore diameter distributions (for intrusion curves see the supporting information) of the silica beads, MTS-4hbeads and MTS-4d-beads. Before the pseudomorphic transformation the silica beads have a pore volume of 0.5 cm³·g−1 with pores around 13 nm. The nitrogen physisorption measurement of this sample showed mainly smaller pores with a broad pore diameter distribution, but these are not completely accessible for mercury within the receivable pressure range. Interestingly, for MTS-4d-beads, a perceptible smaller pore 330
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Fig. 3. N2 physisorption isotherms (77 K) of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4d-beads (green). Filled symbols adsorption, empty symbols desorption. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Pore diameter from mercury intrusion porosimetry of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4dbeads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. SEM image of the calcined silica beads (magnification of 2 × 105, scale bar of 1 μm).
Fig. 7. X-ray diffraction patterns of MTS-4h-beads (blue) and MTS-4d-beads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
diffraction pattern (Fig. 7). MTS-4h-beads shows a broad reflection at approximately 2θ = 1.8° (d = 4.9 nm), whereas for MTS-4d-beads a reflection at ca. 2θ = 2.2° (d = 4.0 nm) occurs, next to a further broad reflection at 2θ = 4.0° (d = 2.2 nm). Due to the low resolution of the XRD pattern, the differentiation between a worm-like and a MCM-41type structure where the first reflection was assigned to hkl (100) and the broad reflection to hkl (110) and (200), is indistinct. Hence, MTS-beads with a bimodal pore structure which differ in surface area and pore volume could be synthesized. 3.2. Characterization of organosilica beads As the synthesis of pure silica beads and their pseudomorphic transformation was successful, we intend to transfer this approach to the synthesis of organosilica beads. We decided for the bis-silylated phenylene-bridged precursor (BTEB) as model system, as phenylenebridged organosilica materials have already shown good performance in various applications like chromatography [48] and offers the possibility of different post-synthetic modifications [59–61]. Since its behavior in sol-gel synthesis differs significantly from TEOS, it is a
Fig. 5. Pore diameter distributions (according to NLDFT kernel for silica, cylindrical pores, from the adsorption branch) of silica beads before pseudomorphic transformation (black), MTS-4h-beads (blue) and MTS-4d-beads (green). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 331
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Fig. 8. Photograph of the organosilica beads (a) and pT-OS-beads (b). Scale bar is given by plotting paper in the middle, wherein the small square correspond to 2.0 mm × 2.0 mm.
representative model system which shows that our approach can be applied to a variety of different precursors. In general, the previously described synthesis path could be adopted, but needed modification at some points: For the hydrolysis of BTEB as organosilica source, a higher concentration of hydrochloric acid was necessary. The alginate phase could be removed by calcination in case of the silica beads, but this is impossible for the organosilica beads since the organic bridges are affected by heat treatment as well. An alternative approach is the dissolution of the calcium alginate hydrogel with the help of citric acid which binds calcium ions coordinately [32]. This is no longer possible after the degradation of alginate during the treatment of the dried beads with hydrochloric acid at 100 °C, which is signalized by blackening of the beads. Hence, the treatment time was shortened from 24 h to six hours, before the alginate phase could be removed successfully. In case of shorter treatment than six hours, the beads were not stable and fell into pieces after removal of alginate. The obtained spherical beads are of similar size as the pure silica beads (2 mm diameter), as can be seen in Fig. 8 on the left. Marginal residues of alginate which decomposed during the acidic treatment causes slight yellowness of the beads. After the pseudomorphic transformation with CTAB/NaOH solution at 100 °C for 24 h and extraction of the surfactant, white beads are obtained as can be seen in Fig. 8 on the right (denoted as pT-OS-beads). Hence, also in case of the organosilica beads, preservation of the bead morphology is given. The pseudomorphic transformation of organosilica materials has already been shown by our group before [62] and was successful here as well. Fig. 9 shows the N2 physisorption isotherms of the organosilica beads (gray) and pT-OS-beads (red). Here, the specific BET surface area increases only slightly from 699 m2 g−1 to 731 m2 g−1. Both isotherms do not match with the isotherm types as classified by IUPAC. In the first case the isotherm starts with a steep uptake in the low-pressure region, like a type I(b) isotherm which indicates microporosity. Additionally, a steep region over 0.95 indicates larger mesopores or macroporosity similar to a type II isotherm. In this combination of type I and II isotherm, hysteresis type H4 appears [63]. Discussing the isotherm of the pT-OS-beads, a continuous increase in the adsorption branch occurs which indicates mesopores of broad pore size distribution. In this case a prominent hysteresis occurs most likely due to irregular diameters of the pores. The respective pore diameter distribution changes significantly after the pseudomorphic transformation, as can be seen in Fig. 10. The organosilica material has a broad pore size distribution of micro and
Fig. 9. N2 physisorption isotherms (77 K) of organosilica beads before (gray) and after pseudomorphic transformation (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
mesopores. After the transformation, the distribution narrows distinctly with a maximum at 3.7 nm. This indicates successful micellar-templating with CTAB as surfactant. The materials were characterized by mercury intrusion porosimetry. The macropore volumes of the organosilica beads are 0.26 m2 g−1 before and 0.32 m2 g−1 after the pseudomorphic transformation which are significantly lower than those of silica beads. The pore size distribution is extremely broad over the whole pore range in both cases (see the pore diameter distribution and intrusion curves in the supporting information). Similar to the pure silica material, pore ordering of the organosilica was investigated by the X-ray diffraction pattern (see Fig. 11). Here, a broad reflection at ca. 2 θ = 2.0° (d = 4.4 nm) indicates a poorly ordered mesophase. Due to this poor ordering the material is not a PMO per definition. Additional reflections indicate a crystal-like arrangement of the organic bridges after the transformation. These are located at 2θ = 11.6° (d = 7.6 Å), 20.7 (d = 4.3 Å), 23.4 (d = 3.8 Å), 35.6° (d = 2.5 Å) which is in accordance with the literature for crystal-like arrangement of the organic bridge within the pore walls of a phenylene332
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Fig. 12. 13C CP-MAS NMR (spinning speed 13 kHz) spectra of the organosilica beads (gray) and pT-OS-beads (red). Spinning side bands are indicated with *. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
surfactant is not significant for the quality of the product. The plots can be seen in the supporting information in comparison to a commonly synthesized benchmark phenylene-bridged PMO. 29 Si CP-MAS NMR provides semi-quantitative information about the condensation degree of the organosilica species and the integrity of the Si-C bond (Fig. 13). Before the transformation, T2 is the most prominent silica species and a significant ratio of T1 is observed. After the transformation, the condensation degree increases, as the T1 signal is of very low intensity and T3 becomes the most prominent species. Thus, the interconnection is enhanced by the pseudomorphic transformation. For organosilica beads as well as for pT-OS-beads, weak Q type 29Si signals are also observed. The relative intensity of Q signals increases after the transformation, indicating that a small amount of Si–C bond breaking occurs during the harsh treatments. This has already been observed in previous publications [62]. Although CP-MAS NMR spectroscopy is not suitable for quantification of the different silica species, the relative amount of Q species can be roughly estimated from comparison to quantitative analysis of other silica materials. The signal intensity of Q species is comparably low in both cases, suggesting that the structures of the organosilica beads are more or less intact. The results show that the synthesis protocol from silica beads could be transferred to the synthesis of organosilica beads. The pseudomorphic transformation of the organosilica beads was also successful, since micellar-templating was possible and mechanically stable, millimeter-sized mesoporous organosilica beads were obtained.
Fig. 10. Pore diameter distributions (according to NLDFT kernel for silica, cylindrical pores, from the adsorption branch) of organosilica beads before (gray) and after pseudomorphic transformation with CTAB for 24 h (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. X-ray diffraction patterns of the organosilica materials before (gray) and after pseudomorphic transformation for 24 h (red). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
bridged PMO material [59]. The organosilica-beads and pT-OS-beads were further investigated by solid state NMR spectroscopy. The integrity of the organic bridge can be investigated best by 13C CP-MAS NMR. Fig. 12 shows the respective spectra of organosilica beads and pT-OS-beads. For both samples, a dominant signal is observed at 133 ppm, which can be assigned to two types of aromatic carbons of phenyl bridging group. The corresponding precursor, BTEB presents two 13C signals at 133.5 ppm and 134.4 ppm for Car-Si (aromatic carbon directly bound to Si) and Car-H, respectively, in CDCl3. The difference in chemical shift values of these two carbons are smaller than the linewidth of 13C CPMAS NMR spectrum of organosilica beads, resulting in a single overlapping signal, which is consistent with the previous reports [56]. Since there are no further signals in the spectrum of organosilica beads, complete removal of alginate can be assumed. By contrast, additional weak signals are shown between 15 ppm and 70 ppm for the pT-OSbeads which can be assigned to residues of the surfactants. The extraction of the samples was not completely successful. However, there was no evidence for the presence of surfactant in thermal analysis and infrared-spectroscopy, suggesting that the amount of the residual
Fig. 13. 29Si CP-MAS NMR (spinning speed 5 kHz) spectra of the organosilica beads (black) and pT-OS-beads (red). The assignment of T and Q species are denoted in the spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 333
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4. Conclusion Following a hydrogel approach with alginate, we synthesized silica beads with diameters of approximately 2 mm with a bimodal mesopore system and noteworthy specific BET surface areas. In both cases postsynthetic micellar templating was possible by so-called pseudomorphic transformation. Thus, micellar-templated silica beads were synthesized maintaining the initial morphology of the beads. In addition to the mesopores of 4.2 nm the system shows also macropores after pseudomorphic transformation. The macropore volume was adjustable by the transformation time. A similar synthesis path was also possible to achieve pseudomorphic transformed phenylene-bridged organosilica beads of the same size. Likewise, a high surface area was obtained and mesopores of 3.7 nm were formed by pseudomorphic transformation. In addition to the results from thermogravimetric analysis and IR-spectroscopy, 13C-CP MAS NMR confirmed the complete removal of the alginate phase. Thus, we extend the possible field of applications for phenylenebridged mesoporous organosilicas, as these beads in contrast to powders are much more likely to be applicable in industrial processes, e.g. as adsorbents from gas flow. In further works, this new approach could be extended to other precursors as well and even other dimensions of the beads or monoliths are conceivable following the alginate hydrogel approach, depending on the targeted application.
[12]
[13] [14]
[15]
[16]
[17]
[18]
[19]
[20] [21]
Acknowledgments The authors thank Renate Walter for SEM images. We thank the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for funding within the SPP 1570 (FR1372/19-3).
[22]
[23]
Appendix A. Supplementary data [24]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.04.028.
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