19336
J. Phys. Chem. C 2008, 112, 19336–19345
Unified Formation Mechanism of Disordered Mesoporous Silica, Structured by Means of Nontemplating Organic Additives Jia Wang,† Johan C. Groen,‡ and Marc-Olivier Coppens*,†,§ DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, Delft Solids Solutions B.V., Rotterdamseweg 183c, 2629 HD Delft, The Netherlands, and Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180 ReceiVed: June 16, 2008; ReVised Manuscript ReceiVed: October 8, 2008
TUD-C and TUD-M are two novel mesoporous zeolite composites. Tetrapropyl ammonium hydroxide (TPAOH) plays the well-known templating role in the formation of the zeolite micropores. The role of TPAOH in the formation of the disordered mesopores of a controlled size can be described as “nontemplating structuredirecting”. For the first time, this type of structure-directing is systematically investigated and compared with templating. Supra-molecular templates are not involved. The nontemplating structure-directing mechanism in the TEOS-TPAOH-H2O system consists of three steps: (1) formation of silica nanoparticles in the presence of TPAOH during homogenization of the solution; (2) development of a gel with extended silica framework during the mild evaporation of ethanol and water; and (3) drying of the gel. TPAOH plays an important role in each of these steps. The key parameters for a successful synthesis are pH, solution composition, and temperature. It is shown that this type of structure-directing is a rather general phenomenon, since many organic molecules can play a role similar to TPAOH. However, the required synthesis conditions vary between different molecules. The mechanism is investigated by means of ATR-IR, TEM, N2 sorption, and NMR experiments. 1. Introduction Mesoporous silicas are widely used in catalysis, adsorption, chromatography, and separation processes. This is mainly due to their large surface area, good mechanical strength, high thermal stability, and chemical durability.1 In the early 1990s, a new family of ordered mesoporous silicas, M41s, was discovered.2 It attracted immediate attention, because of the extension of ordered porosity from microporous zeolites into the mesoporous range, with the potential to improve current applications and extend the use of porous solids to new areas. As a result, much research effort has focused on ordered mesostructures like the M41S family. However, for the majority of applications in chemical engineering, it is unnecessary that the mesopores be ordered. The combination of a high surface area with accurate control over the mesopore size and pore connectivity,3 on the other hand, is extremely interesting, since these parameters could dramatically influence the intrinsic material properties and facilitate the transport of reactants and products when compared to microporous zeolites. Nevertheless, ordered mesoporous silicas are still rather expensive, in part because of the quite pricey templates used in their synthesis. This hinders the use of such materials for large-scale applications. To address this issue, simple and less expensive structuredirecting molecules were used to obtain disordered mesoporous silica. A good example is TUD-1,4 which is synthesized using triethanolamine (TEA), a fairly cheap chemical. Although the mesopores of TUD-1 are disordered, their size distribution is rather narrow and well controlled. To go one step further, we have recently reported the syntheses of new types of micro/ †
Delft University of Technology. Delft Solids Solutions B.V. § Rensselaer Polytechnic Institute. ‡
mesoporous alumino-silicas (TUD-C5 and TUD-M6), which use only one organic molecule (tetrapropylammonium hydroxide, TPAOH) as the structure-directing agent for both the mesopores and the ZSM-5 (MFI) zeolite micropores. These syntheses were performed in basic solutions, while other methods7,8 typically use acidic conditions to form mesopores. Because of this, our methods are easily coupled to zeolite synthesis, and the framework aluminum is better preserved in the resulting materials. The concept is extendable from MFI to other zeolite framework types, since other organic templates for zeolites could also direct the formation of mesopores. An example is tetraethylammonium hydroxide (TEAOH), which is a template for BEA zeolite. Both TUD-C and TUD-M have a hierarchical structure consisting of zeolite nanocrystals embedded in a mesoporous matrix. Such a hierarchical structure can diminish diffusion limitations and pore blocking, which increases the effectiveness of zeolites for catalytic applications. The embedded zeolite nanocrystals are more easily accessed by bulky molecules through the surrounding mesopore structure, which provides transportation “highways”.9,10 The formation of disordered mesoporous silica differs in several ways from that of ordered mesoporous materials. Typically, ordered mesoporous silicas are synthesized via supramolecular templating routes, involving the formation and organization of micelles from surfactants, in the presence of silica precursors. In the cases of the much smaller TEA and TPAOH molecules as structure-directing agents, on the contrary, micelles are not present. Furthermore, the formation of disordered mesopores tends to be much slower than that of ordered mesopores due to the evaporation process during their formation. The function of TEA and TPAOH has remained elusive for years. Compared to the tremendous progress in the field of ordered mesoporous materials synthesized by templating, the
10.1021/jp805276j CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
Formation Mechanism of Disordered Mesoporous Silica synthesis of disordered mesoporous materials with a controlled nanostructure has attracted less attention and lags behind, despite their many catalytic and separation applications. To explore the possibilities to tune the texture of these new TUD-types of micro/mesoporous materials, understanding the mesopore formation mechanism is crucial. This is also essential to extend the concept behind the current synthesis route to other materials. Our previous reports on TUD-M and TUD-C suggest “scaffolding” as the basis for the formation and tuning of mesopores.5,6 In the literature on material synthesis, scaffolding sometimes refers to using rigid structures (silica, carbon, etc.) as the surrounding framework to form the target materials.11 In this case, scaffolding is synonymous for hard-templating: the final materials have the reverse structure of the rigid scaffolds. Contrarily to templating, the term “scaffolding” is not often employed, and it has caused confusion because of its use in different contexts. We therefore revise this nomenclature to “nontemplating structure-directing”. While more and more such structure-directing agents were recently discovered to influence the texture of mesoporous silica, a well-accepted term has not been used.4,12 The mesopores formed through nontemplating structure-directing routes are disordered, as opposed to the ordered meso-structures that are usually obtained through templating. In the sol-gel synthesis of mesoporous materials, “nontemplating structure-directing” and “supramolecular templating” are related concepts, but they are not the same. Both describe the use of a temporary framework of structure-directing agents to guide the formation of porous materials, but a first notable difference is that nontemplating molecules do not form micelles. Instead, they interact with the surface of the formed product or aggregate around it. Another important difference in the case of mesoporous silica synthesis is that, as the name suggests, during nontemplating structure-directing silica is not closely enveloping the organic molecules. Rather, the nontemplating agents envelop the silica structures, which is the opposite of templating. The structure of silica synthesized via nontemplating structure-directing routes is not the negative of the structure of the organic agents formed in solution. For example, the solid gel formed by such structure-directing during TUD-C synthesis has disordered mesopores around 3 nm, which are clearly different from the micropores templated by TPA+, and are in no obvious relation to the geometry of TPA+ or TPAOH. In this paper, nontemplating structure-directing is shown to be a common phenomenon, which could involve a great variety of molecules. The synthesis of several disordered mesoporous silicas could be related by this concept, and there are similarities between the meso-structures obtained from different organic molecules. For example, silica materials obtained from TPAOH,5,12,13 urea,14 and TEA4 show nearly identical structures when analyzed by N2 adsorption and TEM. However, without understanding the formation mechanism better, the relation between these materials is unclear. This may explain why their syntheses have not before been summarized under the same header. Bellussi and co-workers synthesized mesoporous silica from a TPAOH-TEOS-H2O mixture and speculated that TPAOH forms clusters during the synthesis.15,16 They also proposed that the water and ethanol containing TPA+ domains or clusters are the mesopore precursors.15 The silica structures were suggested to form around these clusters, whose dimensions control the pore size of the final mesoporous material. The formation of clusters of TPA+ and other tetraalkylammonium cations (TAA+) has been discussed elsewhere,17 but such a mechanism lacks direct experimental evidence. It remains unclear whether TPA+
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19337 could form such clusters. TPA+ is a nonpolar cation, which does not form micelles.18 The mesopore formation is certainly not similar to the cases of MCM-41 or SBA-15, which are based on supramolecular micellar templating. It was reported by Fedeyko et al. that TAA+ cations such as TPA+ do not even associate with each other in solution,19 which contradicts the cluster formation theory. We also observed some experimental phenomena that could not be explained by cluster formation, e.g., the addition of ethanol does not affect the mesopore size, while it normally does affect assemblies of organic molecules.20,21 This paper suggests, for the first time, a mechanism that explains the principal events during mesopore formation in the synthesis of TUDs and similar materials. This mechanism reconciles several synthesis methods, with nontemplating structure direction as the unifying concept. We first focus on the TPAOH system to obtain detailed information on the mechanism and to investigate important parameters that influence the synthesis. By comparing the TPAOH system to systems that use different organic molecules, we are able to generalize the nontemplating structure-directing concept. 2. Experimental Section Synthesis. All chemicals were purchased from SigmaAldrich. A series of silica samples was prepared by sol-gel synthesis and evaporation in the presence of different organic molecules. A 1 M TPAOH solution and 98 wt % TEOS were mixed at different ratios until homogeneous solutions were obtained. These solutions became gels after evaporation under ambient conditions for 2-7 days. To investigate the influence of temperature during evaporation, a series of samples was prepared at 20 to 80 °C, while the ratio of TPAOH/TEOS was fixed at 1. When using tetraethylammonium hydroxide (TEAOH, 35 wt %), the solution was first diluted to obtain a 1 M solution. Then, the synthesis was performed in a similar way as with TPAOH. Another series of silicas was synthesized in the presence of amine molecules, as follows. First, a small amount of NaOH (0.005 g) was dissolved in water (4 mL, except for ethylamine: 2 mL). Then, this solution was mixed with the following quantity of amine: 7 mL of ethylamine; 5 mL of ethanolamine; 5 g of diethanolamine; 5 mL of propylamine; and 5 mL of dipropylamine. After mixing for 4 h, 5 mL of TEOS was added to the solutions. The mixtures were stirred for another 30 min, and then allowed to evaporate under ambient conditions to obtain silica products. The main steps were the same when using resorcinol or furfuryl alcohol as the additive. In the first step, 1 g of organic compound was dissolved in 5 mL of water without NaOH. After obtaining a homogeneous solution, 2 mL of TEOS was added to the mixture. Subsequently, the solutions were stirred for 1 h and evaporated at room temperature for 10 days. We performed a similar synthesis procedure using hexanediol and N-methylformamide. The differences are the concentrations used: for hexanediol, 1.08 g was mixed with 8.5 mL of water, and then 2.5 mL of TEOS was added after hexanediol was dissolved; for N-methylformamide, 2.5 mL was first mixed with 2 mL of water, and then 2 mL of TEOS was added. In both cases, the mixed solutions were evaporated in the same way as when using resorcinol. When the organic structure-directing agents are amino acids, the synthesis procedure is similar to that using resorcinol. Using L-arginine as an example, 0.87 g (0.005 M) of L-arginine was first dissolved in 5 mL of water, and then 1.5 mL of TEOS was added, followed by stirring overnight to obtain a homogeneous
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SCHEME 1: Formation Mechanism of Mesoporous Silica Using TPAOH as the Structure-Directing Agent
solution. This solution was left at ambient conditions for 7 days to obtain a transparent solid gel. A TUD-1 sample was synthesized using the recipe of Jansen et al.22 First, 2.46 g of water and 1.45 g of TEOS were mixed for 2 h. Then, 2.1 g of TEA was added and continuously mixed until a homogeneous solution was obtained. This solution was aged at room temperature for one day to obtain a solid product. All silica materials were dried and subsequently calcined in air at 550 °C for 6 h before characterization. Characterization. Attenuated total reflectance infrared (ATRIR) spectra were recorded on a React-IRTM 1000 spectrophotometer, to which the diamond composite ATR insertion probe (ASI Applied Systems, Millersville) was connected through a K7 conduit setup. Both were obtained from ASI Applied Systems, Millersville, MD. A liquid nitrogen cooled MCT (mercury, cadmium, telluride) detector was used outside the spectrophotometer. A total of 128 scans were collected for each spectrum, and the resolution was 4 cm-1. React IR software (version 2.21) of ASI Applied Systems was used to process the gathered data. During the mixing step, the insertion probe was immersed into the solutions, while continuously stirring throughout the experiment. To obtain the spectra during the evaporation step, mixed solutions were carefully dropped (up to 0.1 mL) on the diamond window, which was arranged to face upward. The spectra were collected immediately after the solution was dropped. High-resolution transmission electron microscopy (TEM) images were recorded with a JEOL JEM-2011 electron microscope operated at 200 kV, equipped with a Gatan 794 CCD camera. Nitrogen adsorption-desorption isotherms were measured at 77 K using a Quantachrome Autosorb-6B sorption analyzer. Samples were pretreated in vacuum at 250 °C for 16 h. A Bruker Avance 400 instrument was used to record 1H NMR spectra. In the experiments, BuOH was used as an internal standard. Dynamic light scattering (DLS) was used to analyze the nanoparticle size in the solutions. The DLS apparatus consisted of a JDS Uniphase 633 nm 35 mW laser, an ALV sp 125 s/w 93 goniometer, a fiber detector, a Perkin-Elmer photon counter, and an ALV-5000/epp correlator with software. Raman experiments were performed on a Renishaw Raman spectroscope, equipped with a 514.5 nm Ar+ high-power NIR Laser.
the basis of those techniques, as well as TEM and N2 adsorption, we propose a new formation mechanism for mesoporous silica formation by TPAOH nontemplating structure-directing, which is summarized in Scheme 1. The three major events are (1) hydrolysis of TEOS and the formation of silica nanoparticles during the mixing of TPAOH and TEOS; (2) gel framework formation during the evaporation stage; and (3) mild drying of the wet gel. The discussion of the details of each step is supported by experimental results, in conjunction with existing literature. Although the three proposed processes have been studied in rather different systems, such as zeolite and silica xerogel synthesis, the new experimental results show that TPAOH plays an important role in each of these steps. A combination of these events is the key to a successful synthesis. We extend our analysis to different quantities of TPAOH and different organic molecules. All of our experiments have been performed under basic conditions. Therefore, in this work, we only discuss systems with pH values higher than 7. 3.1. Nontemplating Structure-Direction Using TPAOH. 3.1.1. Mixing Step: Hydrolysis of TEOS. Figure 1 shows the ATR-IR spectrum of some starting materials, as well as the hydrolysis product ethanol. Spectra of TEOS and EtOH are of pure liquids, while the other spectra were taken from water solutions. When TEOS is mixed with the TPAOH solution at room temperature, hydrolysis starts immediately because 1 M TPAOH has a relatively high pH (13.7). However, it takes over 1 h before the mixture of TEOS and TPAOH becomes a
3. Results and Discussion To monitor the change in chemical bonding during the mesopore formation in various systems (e.g., TEOS-TPAOHH2O) ATR-IR and NMR spectroscopy were used. The ATRIR technique enables real-time FTIR spectra in liquid phase under operating conditions to be acquired. Providing sufficient surface contact, ATR-IR could even monitor solid samples. On
Figure 1. ATR-IR spectra of selected standard samples measured at ambient conditions (20 °C, 1 atm).
Formation Mechanism of Disordered Mesoporous Silica
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19339
Figure 3. ATR-IR spectra of NaOH (pH ) 13.7) solution and TEOS mixing process at ambient conditions. Figure 2. ATR-IR spectra of the mixing process of TPAOH (1 M) solution and TEOS at ambient conditions.
homogeneous solution. This homogenization period is a strong function of pH and can extend to several hours at lower pH. The mixing process was monitored in situ and is shown in Figure 2. The starting spectrum (0 min) is similar to a combination of TEOS and TPAOH spectra. This is because TEOS and TPAOH solutions are still in two separate phases. The absorption band at 1168 cm-1 is the characteristic peak of TEOS.23 Some stronger peaks of TEOS in the region of lower wavenumbers overlap with the spectra of other components, such as EtOH. The intensity of the peak at 1168 cm-1 decreases when the mixing proceeds, which indicates a decline in the amount of TEOS. This is the well-known base-catalyzed hydrolysis process of TEOS.24 In this case, the hydrolysis and dissolution proceed simultaneously. After 80 min, the peak at 1168 cm-1 becomes too weak to be recognized, indicating that TEOS nearly completely hydrolyzed. At the same time, EtOH appears, which has strong absorbance at 879, 1043, and 1089 cm-1. However, only the peak at 879 cm-1 is unique for EtOH. Its intensity increases at the beginning of the mixing and becomes stable toward the end of the process (∼80 min). The hydrolysis of TEOS results in different silica species. The weak absorption at 945 cm-1 is assigned to silica monomers,23 which can only be seen in the early stage of the mixing process, e.g., between 10 and 60 min. These monomers are not stable and disappear after 1 h because they quickly polymerize. Extended silica structures have absorption bands between 1100 and 1200 cm-1.23 The increase of this band reveals the continuous formation of cross-linked silica structures.25 We had hoped to discover some information on the role of TPA+ in the solution. However, its characteristic peak at 971 cm-1 remains nearly unchanged throughout the whole mixing period. There is no direct change in chemical bonding with TPA+ ions so that ATR-IR cannot provide any additional information. Dynamic light scattering (DLS) measurements of the homogeneous solution indicate the presence of nanoparticles with an average radius of approximately 3 nm. It has been reported that silica nanoparticles form in silicalite-1 synthesis solution at room temperature,19,26-33 which is nearly identical to our TPAOH/ TEOS system, except for the concentration difference. It is worth mentioning that those precursor nanoparticles with different sizes are present simultaneously, since they are at different stages of crystal growth.34,35 The size of those nanoparticles can be
influenced by the TPA+ concentration as well as the pH. Smaller particles form at higher TPAOH content or higher pH. Other factors, such as the Al content of the solution, also affect the size of the particles. These nanoparticles were observed to have a size between 2.5 and 8.5 nm,33,36,37 which is in good agreement with our DLS results. Thus, the observed ATR absorption band between 1100 and 1200 cm-1 could be related to the formation and the growth of such silica nanoparticles. Those small particles could quickly grow or fuse, if TPA+ ions were not present in solution. However, an electrical double layer forms around the nanoparticles when TPA+ is present, which helps to stabilize them.19,27-29 In fact, some researchers proposed a core-shell scheme with silica in the core and TPA+ in the shell.19,27,32 The best-known theory of colloidal stability is the DLVO theory, named after Derjaguin, Landau, Verwey, and Overbeek. It is based on the existence of a balance between the repulsive electrostatic interactions, due to the electrical double layers on neighboring particles in a liquid, and the attraction arising from van der Waals interactions between the particles.38 According to this theory, in concentrated electrolyte solutions there is normally a significant overall energy minimum due to the screened interactions, in which particles are referred to as “kinetically stable”. To investigate the role of TPA+, we have substituted the TPAOH solution by a NaOH solution with the same pH. Figure 3 shows in situ ATR results of the mixing of TEOS and NaOH. The peaks at 874 cm-1 and 1010-1027 cm-1 in the final spectrum are related to EtOH. There is a dramatic increase of the peak intensity between 20 and 30 min. The hydrolysis of TEOS was much faster than that in a system containing TPA+ at the same pH. At the same time the peak at 1126 cm-1 quickly appeared, which could be assigned to nonsoluble silica species. Precipitation of silica was observed during the experiments. N2 adsorption confirms that those silica precipitates do not have a mesoporous structure (not shown). Without TPA+ in the solution to stabilize the silica nanoparticles, they continued to grow and developed rather quickly into bulk silica material. The silica peaks in this experiment had different shapes and were also more pronounced than in the TPAOH case, revealing different bonding properties of the silica species and thus likely different silica structures. Since other types of tetraalkylammonium cations can also act as nontemplating mesopore directing agents,12 experiments
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Figure 4. ATR-IR spectra of the mixing process of TEAOH (1 M) solution and TEOS at ambient conditions.
Figure 5. TEM image of TUD-C sample obtained after thermal treatment of the solid gel at 130 °C for 10 h.
with TEAOH were also carried out to compare with the TPAOH containing system. Figure 4 shows the in situ ATR-IR spectra of the homogenization of TEAOH (1 M) and TEOS. The spectra evolve in a similar way to those of the mixture with TPAOH. The EtOH peak at 879 cm-1 steadily increased, which indicates the continuous hydrolysis of TEOS. The silica absorption region (1100-1200 cm-1) shows a similar shape as that of TPAOH and also becomes more pronounced during the mixing process. ATR-IR results support the reported similarity between TEAOH and TPAOH at the hydrolysis stage of the synthesis.19 TEM measurements of the final gels confirmed, albeit indirectly, the formation of nanoparticles during the synthesis. In Figure 5, we recognize assemblies of nanoparticles in the TUD-C sample that was obtained from the solid gel after thermal treatment. The final materials inherit the shapes of the nanoparticles, even after solid-state crystallization. Interestingly, gel samples prepared by using other organic molecules, such as hexanediol, showed a similar structure. This will be discussed in Section 3.3. In fact, the formation of silica nanoparticles is not restricted to the use of TPAOH-like molecules. Silica particles often form and are stabilized in solutions with organic
Wang et al.
Figure 6. ATR-IR spectra of the evaporation stage of the TPAOH/ TEOS mixture at ambient conditions.
molecules.26,28,31 To some extent, TPAOH and other organic molecules function in a similar way. The main differences are the acidity (pH) and the mechanism of nanoparticle stabilization. 3.1.2. EWaporation Step: Formation of a Silica Framework. A homogeneous mixture of TEOS and TPAOH transforms into a solid gel after a mild evaporation process. The in situ ATRIR spectra of this evaporation step are shown in Figure 6. Since the experiments were conducted on a smooth surface with a diamond detection window, and only a thin layer of liquid was used, the conditions are different from those in the actual preparation performed in glass bottles. The much greater surface/ volume ratio during ATR-IR measurements dramatically increases the evaporation speed. Therefore, the sample spectrum disappeared between 4.5 and 7.6 min: the gel lost contact with the detection window, as a result of sample shrinkage and the formation of a solid gel. In bulk synthesis, on the other hand, it takes about a week to obtain a solid gel. In order to prove the validity of ATR-IR results for the current experimental conditions we compared the spectra of two solid gels: a normally prepared gel sample and a sample prepared under ATR-IR conditions. They are nearly identical. Hence, no difference between the two evaporation conditions could be identified by ATR-IR. At the beginning of the evaporation, the characteristic peak of EtOH at 879 cm-1 decreases quickly. EtOH leaves the system as evaporation proceeds, as expected. The band between 1100 and 1200 cm-1, assigned to polymeric silica species, increases until 4.5 min, which means that the silica structure further develops during evaporation. The increasing SiO2 concentration most likely affects both the concentration and the structure of the siliceous nanoparticles.25 The reason that the 4.5 min spectrum has a lower intensity is that the solid gel partially lost contact with the detection window. Before losing contact, the last spectrum shows a strong absorption peak at 1025 cm-1. This is the same position as one of the EtOH peaks. However, EtOH has already completely evaporated at that time, since the characteristic peak at 879 cm-1 is absent. Therefore, the peak at 1025 cm-1 can be assigned to chain- or sheet-formed silica,23 which indicates further growth of the silica structure. Unfortunately, ATR-IR results provide only limited information on the details of the silica structure. The characteristic peak of TPAOH (971 cm-1) increased in intensity with increasing TPA+ concentration, which is expected and still does not reveal the possible existence of TPA+ clusters.
Formation Mechanism of Disordered Mesoporous Silica
Figure 7. 1H NMR spectra of TPAOH (1 M) solution and the homogeneous mixture of TPAOH and TEOS.
Raman spectroscopy was also used to monitor the evaporation process (not shown). A gradual decrease of the EtOH signal was observed, as well as a slight increase of the TPAOH peak. These results are in agreement with the observations using ATRIR. As mentioned earlier, some researchers suggested the existence of [TPA+]n clusters to explain the mesopore formation.15,16 It has also been claimed that the aggregates of TPA+ are only present when the concentration of TPAOH is relatively high.32 Here, we propose a different formation scheme based on the crucial role of evaporation. The concentrations of TPA+ and the silica species increase when EtOH leaves the synthesis solution together with water. The increasing concentration of electrolyte leads to the disappearance of the energy minimum so that the particles start to coagulate.38 Some researchers also suggested that the smaller amount of EtOH may have a positive influence on the hydrophobic forces.31 However, the nature of the hydrophobic forces is currently under debate.39-41 Eventually, the silica nanoparticles are forced to aggregate. Because TPA+ still bonds on the surface of silica, TPA+ is gradually pulled together to fill in the space around the silica, which will become the mesopores in the silica framework. The filling of space by TPA+ ions prevents the fast agglomeration that leads to dense silica. At the same time, the soluble silica concentration increases considerably, which leads to constant growth of the silica network. Since ATR-IR is not able to detect the formation of TPA+ clusters, NMR was used to provide further insights into their presence. The solution was measured shortly after the mixing was stopped. As shown in Figure 7, the 1 M TPAOH solution has an NMR spectrum with split 1H peaks around 0.907, 1.631-1.661, and 3.077-3.135 ppm. The 1H peaks of TPAOH in the mixture of TPAOH/TEOS are slightly shifted to 0.889, 1.592, and 3.052 ppm due to the changing solution properties. They are also broader, and no split is visible. This could be explained by the formation of TPA+ aggregates, which smears out the peaks. Before evaporation, the concentration of TPA+ was quite high (over 0.5 M). TPA+ cations might cluster, because there is less water and silica species are present. There is, however, also the possibility that the high viscosity results in peak broadening. So, NMR results alone cannot answer whether TPA+ clusters exist. More evidence should be obtained to prove the possible presence of such structures in the reaction mixture.
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19341 3.1.3. Drying Step: Formation of Silica Gel. The similarities between the solid gel formed by nontemplating structuredirecting and some silica xerogels inspired us to propose the drying step as a last distinct, important phase in the gel formation. The N2 adsorption isotherms42 and TEM images43 of xerogels resemble those of the gels whose structures are directed by TPAOH. In spite of the crucial function of the organic nontemplating agent, the formation of mesoporous silica via nontemplated structure-directing has many aspects in common with that of a typical silica xerogel. For the formation of silica xerogels, drying is an important step. The elimination of the liquid content in wet gels should be such that the framework can be retained. This is also true for mesoporous silica obtained by nontemplated structuredirecting. After the formation of the gel framework water continues to leave the system. So, drying is a consecutive process involving two stages. After the solution turns into a solid gel, drying results in the shrinkage of gel volume, which has been commonly observed during the synthesis. The most important factor contributing to this shrinkage is capillary pressure.44-46 The modification of the silica surface by TPA+ reduces the capillary pressure exerted on the gel network by the solvent that causes the collapse of the gel network during drying. Additionally, TPA+ ions not only stabilize the surface of silica nanoparticles but also modify the silica surface, preventing the condensation reactions between silica clusters that would lead to severe, irreversible shrinkage of the gel. The function of TPA+ in this stage is the same as that of some modifiers during silica aerogel synthesis.45,46 For example, the replacement of water by alcohol also reduces the capillary tension during drying in aerogel synthesis. So, the presence of EtOH in the system, though there is only little left toward the later stages of the process, could also help to preserve the gel framework. As mentioned, in the first step many other organic agents interact with the silica surface, with screening preventing aggregation. Their function during drying could be similar to that of TPA+ or EtOH, i.e., to prevent the deterioration of the silica meso-structure. 3.2. Effects of Synthesis Parameters: Concentrations, Temperature, and pH. During the formation of silica mesostructures, the synthesis conditions, such as composition, temperature, and pH, are of crucial importance. Tuning these parameters enables the synthesis to be optimized and additional insight into the formation mechanism to be gained. Figure 8 shows the isotherms and the pore size distributions of the silica samples obtained with different TPAOH/TEOS ratios. Over a fairly large TPAOH content range (0.25 < TPAOH/TEOS < 1.5, in volume) the obtained gels are nearly identical. However, the pore size distribution dramatically changes when the TPAOH/TEOS ratio increases to 2. Larger mesopores with a bimodal distribution appear. The mesostructure continues to change. Hardly any mesopores form when the TPAOH/TEOS ratio is 3 or more. The BET surface area decreases to a mere 15 m2/g, as shown in Table 1. The origin of this change could be twofold. (i) A higher TPAOH content results in a higher pH, which complicates the growth of the silica framework. The formation of silica nanoparticles tends to be less favored due to a higher solubility of silica. (ii) Furthermore, more TPA+ ions surround the silica particles so that the fusing of those particles becomes less favored. This affects the condensation of the silica framework resulting in a collapse of the structure during drying and subsequent calcination. The influence of the synthesis takes effect when the TPAOH/TEOS ratio is 2, which indicates that the synthesis is
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Figure 9. N2 adsorption-desorption isotherms of the silica samples prepared from TPAOH/TEOS (fixed 1:1 volume ratio) at different evaporation temperatures. Isotherms of samples 20 °C, 30 °C, 40 °C, 50 °C, and 60 °C are vertically offset by 15, 25, 45, 55, and 70 cm3 STP g-1, respectively.
Figure 8. Pore structure analysis by N2 adsorption-desorption. (Top) Isotherms of the silica samples prepared by using different TPAOH/ TEOS volume ratios. Isotherms of samples 0.25 and 1 TPAOH/TEOS are vertically offset by 100 and 50 cm3 STP g-1, respectively. (Bottom) Pore size distributions corresponding to the adsorption isotherms. Samples 0.25 TPAOH, 1 TPAOH, 1.5 TPAOH, and 2 TPAOH are offset by 1.9, 1.3, 0.7, and 0.2 cm3 g-1, respectively.
TABLE 1: Textural Properties of Selected Samples SBET (m2 g-1)
sample 1 TPAOH 2 TPAOH 3 TPAOH TPAOH TPAOH TPAOH TPAOH
30 40 50 60
°C °C °C °C
TUD-1 propylamine ethanolamine arginine hexanediol furfuryl alcohol resorcinol
Vmicro (cm3 g-1)
Vmeso (cm3 g-1)
Dmeso (nm)
424 170 15
0 0 0
0.42 0.32 0
3.5 5.5, 32
471 371 249 137
0 0.03 0.04 0.03
0.27 0.16 0.10 0.04
2.1 micro micro
686 268 277 805 766 549 803
0.01 0 0.04 0 0.003 0.011 0.004
0.88 0.30 0.39 1.13 1.37 1.32 0.54
7-10 4.5 ∼13 5.0 ∼10 ∼20 3.0
robust within a certain “buffer” range. It is currently not clear why the pore size distribution of sample 2 TPAOH is bimodal; we speculate that the mesopores around 30 nm are mesovoids in between large silica aggregates. The described synthesis was carried out at room temperature (20 °C). This ensures a slow and mild evaporation. Figure 9 shows the N2 adsorption and desorption results of a series of samples (with TPAOH/TEOS ) 1) prepared at higher temper-
atures. With increasing temperature the mesopore volume and BET surface area constantly decrease (Table 1). The synthesis is very sensitive to temperature and is negatively affected by heating. Many parameters change with temperature, and among them are direct effects, such as the solubility of silica, as well as indirect effects, such as pH. The direct effects, especially the solubility, seem to predominate. It is reported that, in a basic solution with fixed composition, silica particles grow to a size that depends mainly on temperature.24 Due to the higher solubility of silica at higher temperatures, smaller nanoparticles are more favored and they aggregate faster, leading to a more densely packed silica structure with less mesopores. Moreover, at a higher temperature the evaporation of EtOH is much faster than at room temperature, which also increases the aggregation rate of silica particles. In order to examine the influence of the pH on the synthesis, we have tuned the pH of the TPAOH solution by adding HCl. Surprisingly, the TPAOH solution and TEOS could not mix well to become a homogeneous solution at a slightly lower pH (12.8). Although TEOS should be hydrolyzed in water with the same pH, the mixing is apparently too slow or the 1:1 volume ratio is over the solubility limit. Thus, the relatively high pH is a prerequisite when working under basic conditions using TPA+ as a nontemplating structure-directing agent. Interestingly, Manton and Davidtz used TPAOH under acidic conditions to form mesopores.12 Despite the differences in polymerization and condensation of aqueous silica structures under basic and acidic conditions, silica xerogels can form via acid or base catalysis.46 Different primary structures are formed at different pH values. For pH < 3, condensation leads to small, cage-like units, while under basic conditions, larger particles are formed.47 This is mainly due to the different nucleation rates. Below pH ) 3, the solution kinetics allow the formation of metastable oligomers of silica. There are other differences between acidic and basic systems; however, further discussion is beyond the scope of this article. 3.3. Use of Organic Structure-Directing Agents Other than TPAOH. TPAOH as a structure-directing agent in the formation of mesoporous silica behaves in a similar way as
Formation Mechanism of Disordered Mesoporous Silica
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Figure 10. N2 adsorption-desorption isotherms of the silica samples prepared by using different molecules as structure-directing agents. (Left) TUD-1 (TEA as an additive) and samples obtained by using propylamine and ethanolamine. The isotherms of TUD-1 and propylamine were vertically offset by 100 and 140 cm3 STP g-1, respectively. (Right) Samples obtained by using arginine, hexanediol, furfuryl alcohol, and resorcinol. The isotherms of samples using arginine, hexanediol, and furfuryl alcohol were vertically offset by 450, 260, and 200 cm3 STP g-1, respectively.
several other organic structure-directing molecules.4,14 Apart from the reported structure-directing agents, we have discovered some new molecules that direct the synthesis of mesoporous silica, without acting as templates. Figure 10 shows isotherms of a few silica materials using various organic molecules as structure-directing agents. All the samples contain a substantial amount of mesopores, although the pore sizes and pore volumes vary considerably (Table 1). The diversity in pore structure is comparable with silica xerogels. Generally, the microporosity of the samples is very limited (Table 1). This proves that the nontemplating structure-directing agents mainly direct the formation of the meso-structure. A few selected samples were also investigated by TEM, as shown in Figure 11. Similar to the TUD-C samples, a structure corresponding to assembled nanoparticles was recognized (e.g., Figure 11 c, with ethanolamine as the structure-directing agent). This is a strong indication that when other molecules are used, the first step in the formation process is the same as in the scheme proposed for TPAOH. In comparison, one sample was prepared by drying colloidal silica AS-30 (particle size 15-20 nm) under similar conditions (Figure 11 d). The uniform nanoparticles are clearly seen. The particles in the other samples in Figure 11 are more “fused” and not always nicely spherical. The growth of connections or “necks” between silica particles during gel formation is a well-known phenomenon.24 Dissolution and reprecipitation of silica species cause the formation of necks, which increases the strength and the stiffness of the gel. When gels are aged under conditions of high solubility, neck growth is more pronounced. When the solubility is low, the silica particles tend to stay loosely connected, preserving a spherical shape. This could explain the difference between the AS-30 sample and the samples formed when nontemplating structuredirecting agents are used (Figure 11 a,b). Because the type of organic agent as well as the pH can dramatically influence the solubility of silica, the size of the silica nanoparticles and the framework growth can be rather different. Thus, we could expect structural differences in the final gels, as shown in Figures 10 and 11. A whole family of rather different organic molecules has been discovered to direct the formation of mesoporous silica. In
addition to those reported in the literature, we discovered several other nontemplating structure-directing molecules that primarily perform under basic conditions. We now attempt to summarize these as follows: (1) ionic zeolite templates, e.g., TAAOH (tetraalkylammonium hydroxide); (2) amino acids, e.g., arginine, L-lysine;28 (3) amines, e.g., mono- and triethanolamine, propylamine; (4) organic hydroxylated compounds, e.g., hexanediol, resorcinol, furfuryl alcohol, D-glucose, D-maltose;48 and (5) other molecules that contain “NsCdO” bonds, e.g., N-methylformamide. It is worth mentioning that this list can be extended if considering molecules that function under acidic conditions, such as urea14 and hydroxy-carboxylic acids.49 There are synthesis parameters that are important to all synthesis routes, among which are the structure-directing agent/ TEOS ratio, temperature, and pH. For example, in order to hydrolyze TEOS, the syntheses have to be carried out under sufficiently basic or acidic conditions. This is also necessary for the homogenization of the solution, as hydrolyzed silica species can easily dissolve in water. Some organic molecules, such as certain amino acids and diethanolamine, cannot dissolve well in TEOS (or hydrolyzed silica) and water. Then, the whole synthesis simply cannot be performed. As mentioned earlier, when the pH is below a certain level (basic conditions, pH between 10 and 12.8), the hydrolysis cannot easily proceed. Hence, solubility is an important criterion. A too high pH, e.g., when using an ethylamine solution (40%, pH ) 14.5), results in too fast formation of silica, and, consequently, too fast precipitation. The growth of a controlled mesoporous structure is thus suppressed. When the pH is tuned to a lower level (12.4), a significant amount of mesopores is formed in the final material and the surface area more than doubles. Therefore, the nontemplating structure-directing mechanism only functions within a certain pH range that depends, however, on the organic molecule. Investigations were carried out to understand the difference between TPAOH (TAAOH) and other organic structuredirecting molecules. Preliminary results from ATR-IR confirmed that TEOS was fully hydrolyzed after the mixing step to form a homogeneous solution in the case of furfuryl alcohol. However, the silica peaks remain nearly the same during the
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Figure 11. Transmission electron microscopy (TEM) images of silica samples. (a) Solid gel prepared by using hexanediol as a structure-directing agent; (b) solid gel prepared by using propylamine as a structure-directing agent; (c) solid gel prepared by using ethanolamine as a structuredirecting agent; and (d) sample prepared from colloidal silica solution AS-30 at ambient conditions.
evaporation step, indicating a different development of the silica structure. TEM of the final materials confirms the variation in particle size and silica structure. It is very likely that the pH difference plays a crucial role, but a better understanding requires more research. 4. Conclusions A new mesopore formation mechanism for gels formed from TEOS, H2O, and an organic additive like TPAOH was proposed. The three major steps are (1) the formation of silica nanoparticles during the initial mixing stage; (2) the growth of the silica framework during the evaporation stage; and (3) the formation of the gel upon drying. The current mechanism shows similarities to some well-studied systems, such as zeolite and silica xerogel synthesis. TPAOH stabilizes the silica species during the mixing and evaporation steps and prevents the framework to collapse during drying. The crucial role of TPAOH is summarized as “nontemplating structure-directing”, which is a fairly general phenomenon. It is a form of structure-directing that differs in several respects from templating mechanisms leading to nanoporous materials. The organic agents do not form micelles. They interact with the surface of the silica species and even play a role during the drying of the mesoporous silica.
In the case of TPAOH, the key parameters of the nontemplating structure-directing mechanism are identified as temperature, TPAOH/TEOS ratio, and pH. Though many previous researchers have speculated on the formation of TPA+ clusters, we have not obtained convincing experimental evidence to confirm this point. Many other organic molecules can also function as structuredirecting agents for the formation of disordered mesopores, provided that the parameters for a successful synthesis are fulfilled, such as the structure-directing agent/TEOS ratio, the synthesis temperature, the solubility of the different components, and the pH. However, different organic molecules may still differ in the details of how they direct the formation, which requires further investigation. Our study suggests a remarkably broad, common basis in the synthesis of various mesoporous materials by the nontemplating route. It also sets the stage for an improved physicochemical understanding of the steps involved in nontemplating structuredirecting and hereby offers new opportunities to guide the synthesis of novel mesoporous materials. Acknowledgment. Financial support by the Delft Research Centre for Sustainable Industrial Processes and the Dutch
Formation Mechanism of Disordered Mesoporous Silica National Science Foundation NWO (CW/PIONIER) are gratefully acknowledged. We thank Mr. S. Brouwer (TU Delft) for the adsorption measurements; Mr. W. Yue and Dr. W. Zhou for TEM measurements; Mr. S. Tromp (TU Delft) for ATR-IR measurements; and Mrs. K. Djanashvili (TU Delft) for NMR measurements. We are also grateful to Prof. M. Tsapatsis (University of Minnesota) and Prof. J.C. Jansen (TU Delft) for stimulating discussions. References and Notes (1) Nakanishi, K. J. Porous Mater. 1997, 4, 67. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Kortunov, P.; Vasenkov, S.; Ka¨rger, J.; Valiulin, R.; Gottschalk, P.; Fe´ Elı´a, M.; Perez, M.; Sto¨cker, M.; Drescher, B.; McElhiney, G.; Berger, C.; Gla¨ser, R.; Weitkamp, J. J. Am. Chem. Soc. 2005, 127, 13055. (4) Jansen, J. C.; Shan, Z. P.; Marchese, L.; Zhu, W.; van der Puil, N.; Maschmeyer, T. Chem. Commun. 2001, 8, 713. (5) Wang, J.; Groen, J. C.; Yue, W.; Zhou, W.; Coppens, M.-O. Chem. Commun. 2007, 14, 4653. (6) Wang, J.; Groen, J. C.; Yue, W.; Zhou, W.; Coppens, M.-O. J. Mater. Chem. 2008, 18, 468. (7) Gagea, B. C.; Liang, D.; van Tendeloo, G.; Martens, J. A.; Jacobs, P. A. Stud. Surf. Sci. Catal. 2006, 162, 259. (8) Stevens, W. J. J.; Meynen, V.; Bruijn, E.; Lebedev, O. I.; van Tendeloo, G.; Cool, P.; Vansant, E. F. Microporous Mesoporous Mater. 2007, 110, 77. (9) Wang, G.; Johannessen, E.; Kleijn, C. R.; de Leeuw, S. W.; Coppens, M.-O. Chem. Eng. Sci. 2007, 62, 5110. (10) Coppens, M.-O.; Wang, G. In Design of Heterogeneous Catalysts; Ozkan, U. S. Ed.; Wiley: New York, 2008; in press. (11) Lu, A.-H.; Smått, J.-H.; Backlund, S.; Linden, M. Microporous Mesoporous Mater. 2004, 72, 59. (12) Manton, M. R. S.; Davidtz, J. C. J. Catal. 1979, 60, 156. (13) Bellussi, G.; Perego, C.; Carati, A.; Peratello, S.; Previde Massara, E.; Perego, G. Stud. Surf. Sci. Catal. 1994, 84, 85. (14) Pang, J.-B.; Qiu, K.-Y.; Xu, J.; Wei, Y.; Chen, J. J. Inorg. Organomet. Polym. 2000, 10, 39. (15) Perego, G.; Millini, R.; Perego, C.; Carati, A.; Pazzuconi, G.; Bellussi, G. Stud. Surf. Sci. Catal. 1997, 105, 205. (16) Rizzo, C.; Carati, A.; Barabino, C.; Perego, C.; Bellussi, G. Stud. Surf. Sci. Catal. 2002, 144, 625. (17) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. (18) Claesson, P.; Horn, R. G.; Pashley, R. M. J. Colloid Interface Sci. 1984, 100, 250. (19) Fedeyko, J. M.; Rimer, J. D.; Lobo, R. F.; Vlachos, D. G. J. Phys. Chem. B 2004, 108, 12271.
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