Influence of synthesis parameters on the formation and structure of bimodal mesopore silica in a controlled sol-gel process

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkov~ and P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.

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Influence of synthesis parameters on the formation and structure of bimodal mesopore silica in a controlled sol-gel process X.-Z. W a n g a b *, W.-H. Li b, J.-Y. Lin a, H.-L. Fan a, C.-S. Tian a, B. Z h o n g b and K.-C. Xie a aKey Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China E-mail: [email protected] bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China In the present work, a series of synthesis parameters, including those that may affect the size of surfactant micelles and that may affect the relative rate of hydrolysis and condensation of tetraethyl orthosilicate (TEOS), were judiciously adjusted to study their direct influence on the formation and structure of our previously reported bimodal mesopore silica (designated as BMS). It is found that both the framework and textural mesopores of BMS silica can be tailored over a fairly wide size range, but on the whole the textural mesopore size is more sensitive to the change of the synthesis parameters than that of the framework mesopore size. The change of three synthesis parameters, such as increasing the amount of ethanol, decreasing the chain length of surfactant or increasing the alkali/silica molar ratio, would lead to the mesostructure of the silica obtained to transform from initial BMS into MCM-41. 1. INTRODUCTION Since the first synthesis of mesoporous MCM-41 materials [ 1], there has been an unparalleled activity in the design and synthesis of a variety of mesoporous solids with different structural characteristic. So far, mesoporous M41S materials with hexagonal or cubic structure characteristic have shown potential importance in various practical applications. However, it seems that HMS mesoporous materials [2] may be of even larger significance in catalytic applications due to the presence of a complementary textural mesopores, which would facilitate mass transport to framework mesopores and has shown the importance in improving catalytic processes [3]. In earlier investigations [4], we found that through controlling gelation other than precipitation in a reaction system which used usually to prepare MCM-41 mesoporous materials, a hierarchically structured porous silica gel monolith with well-defined bimodal mesopore size distribution characteristic ( i.e BMS silica) can be formed at ambient conditions. To the best of our knowledge, this is the first report of a porous silica material with bimodal mesopore distribution characteristic, which corresponds to two discernable inflections at low and high relative pressure, respectively, of the N2 adsorption- desorption

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isotherm. Further research confirms [5] that the bimodal mesostructure of BMS silica consists of both the framework mesopores resulting from the surfactant template and the textural mesopores resulting from the interparticle porosity, and the control of the relative rates of the hydrolysis and condensation reaction of TEOS and thus of the gelation plays a critical role for the formation of BMS silica mesostructure. Obviously, smart adjusting of various synthesis parameters, which would affect the micelle sizes and/or the relative rates, might be an effective means of controlling structure of the BMS silica. Since numerous applications of BMS silica are based on their pore structure, which comprises pore size, pore size distribution, pore volume, surface area, etc, thus, in this paper we examine the possibility of tailoring both the framework and textural mesopores of BMS silica by adjusting the properties or concentration of various precursor composition, which would also provide simultaneously us a better understanding of the structural and textural evolution during the surfactant-encapsulated silica gel formation. 2. EXPERIMENTAL SECTION

2.1. Synthesis The synthesis of BMS silica followed the same procedures as previously reported [4.5], except that various synthesis parameters, typical such as the surfactant alkyl chain length (Dodecyl(Cl2)-, Myristyl(Cl4)-, Cetyl(Cl6)- and Stearyl(C18)-trimethylammonium Bromide (TAB)), swelling agent 1,3,5-trimethylbenzene (TMB), catalyst structure and properties (typical such as some small molecule organic amines), solvent polarity (H20 and ethanol (EtOH)), TEOS concentration and TEOS/surfactant molar ratio and so on, were adjusted respectively in a certain range with other composition concentration constant to control the gelation of reaction system. The gelation was determined visually by the fact that the solution no longer exhibited bulk fluid behavior. The gelation time was defined as a time between the start of hydrolysis and the sol-gel transition where the bulk fluidity of the sample is lost. The final gel composition is: 1.0TEOS : 0.18CI6TAB(CI2,CI4,CIs) : (0-0.84)TMB : (0-5.5)EtOH : 0.46NH3(0.064, amines) : 75H20. After gelation, all of the samples were aged for 5h at room temperature, and the resulting silica wet gels were washed repeatedly with distilled water in a centrifuge, dried in air at 343K for overnight, to give the surfactant-contained BMS silica samples. To remove the template, as-synthesized silica samples were ground into fine powders and calcined in air at 2K rain ~ to 823K for 6 h. 2.2. Characterization The powder X-ray diffraction patterns (XRD) were recorded using a D/max-2500 powder diffractometer with Cu-K a radiation (40kV,100mA), 0.02~ size and 1 s step time over the range 1~ 0 <8 ~ N2 adsorption isotherms were measured at 77K using a ASAP2000 analyser. The volume of adsorbed N2 was normalized to standard temperature and pressure. Prior to the experiments, samples were dehydrated under vacuum (about 10 .3 Torr) at 623K for 12h. The pore-size distribution was calculated using the desorption branches of the N: adsorption isotherm and the Barrett-Joyner-Halenda (BJH) formula [6].

527 3. RESULTS AND DISCUSSION Initially, we attempt to divide the synthesis parameters into two kinds, one is those that may affect only the size of surfactant micelle, such as varying the surfactant alkyl chain length or adding swelling agent TMB, which would thus affect only the framework mesopore size of the resultant BMS silica, and another is those that may affect mainly the relative rate of hydrolysis and condensation of TEOS, such as varying the composition concentration, solvent polarity or catalyst properties and structure etc, which would thus affect mainly the size and packing of the silica primary particles produced and final affect the textural mesopore size of the BMS silica obtained. However, the experimental results show that this idea may be ill-considered, and that any a slight change in synthesis parameters would influence not only the micelle sizes but the relative rates to a certain degree, which enables both the framework and textural mesopores of the BMS silica to be adjusted simultaneity over a fairly wide range. We first investigate the effect of varying the surfactant chain length on the formation and structure of BMS silica, which has been used previously to tailor the pore size of mesoporous MCM-41 materials. As we replace the usually used C16 surfactant with C~2, C14, C~8 as template, respectively, under other composition constant conditions, gel occurs in the presence of surfactants with alkyl chains having 14 or more carbon atoms. Moreover, the gelation time decreases rapidly with the decrease of chain length. In contrast, for the shorter surfactant alkyl chain (n~< 12), precipitation rather than gel is formed rapidly as the precursors are mixed, even though in the case that the alkalinity of the reaction system is further decreased to slow the condensation rate of hydrolyzed silica species. This indicates that the presence of the surfactant in the synthesis also affects the relative rate of hydrolysis and condensation of TEOS, expected that templates the mesostructure of the resultant silicas. Moreover, this influence is reinforced with the shortening of surfactant chain length. All samples prepared under this conditions show qualitatively equivalent XRD diffraction factures with a strong, relatively broad reflection at 1.5-2.0~ 0, which indicates a lack of long-range crystallographic order or finite size effect. However, the positions of the intense reflection are dependant on the change of the alkyl chain length, shifting toward higher 2 0 angle with the decrease of chain length, which is consistence with the general tendency A

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0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0 P/Po TMB/TEOS mol ratio Fig. 2. N2 adsorption isotherms (A) and their framework and textural pore sizes (B) for BMS silicas prepared at different TMB/TEOS mole ratio: a.0; b.0.065; c.0.194; d.0.332; e.0.451" f.0.58" g.0.84. observed in the MCM-41 synthesis. This influence of varying the surfactant chain length on the mesostructure of the resultant silica can be seen more direct from the N2 adsorption results provided in Fig. 1. It is clear that the decrease of the chain length of surfactant leads to the adsorption step at p/po-0.2-0.4, which indicates the filling of the framework-confined mesopores, to shift toward lower p/po, while, at the same time, the adsorption step at p/po=0.8-1.O, which indicates the filling of the textural mesopores, to shift toward higher p/po. Corresponding, the framework and textural mesopore sizes of the BMS silica obtained vary from 2.94 to 2.33nm and from 10.5 to 24.3nm, respectively, as shown in the BJH plots in Fig. lB. In the case of C12 template, the adsorption step at higher p/po disappears due to the formation of precipitation, corresponding to a disappearance of textural mesopore. When the amount of swelling agent TMB is slightly increased from 0 to 0.84 mol with C~6 surfactant as template, the framework mesopore sizes of the resultant BMS silica increase from 2.71 to 3.65nm due to the swelling effect of TMB to micelles, which is consistence with the results obtained in the synthesis of MCM-41. However, at the same time, the adsorption steps denoting the textural mesopore filling shift toward lower p/po and decrease clearly in the gradient (Fig. 2A). As a result, the average size of the textural mesopore decreases gradually and the distribution becomes wider upon an increase of the amount of TMB (Fig. 2B). Depending bn the fact that the interparticle voids of BMS silica are also filled using partial surfactant [5], the introduction of TMB should also swell the interparticle voids, thus the textural mesopores. Perhaps, this swelling role would lead to a more relaxed packing of the primary silica particles, thus a much larger shrink has occurred during calcination to removal surfactant template. Increasing the TEOS/C16TAB molar ratio from 5.4 to 16.3 or increasing the TEOS concentration at constant TEOS/C~6TAB molar ratio from 9.9 to 24.8wt% results in the textural pore size of the resultant BMS silica to increase markedly from 18.9 to 45.5nm and from 18.9 to 37.7nm, respectively, while the framework pore size is not affected obviously (Fig. 3). This indicates that the change of TEOS concentration or TEOS/C~6TAB molar ratio would mainly affect the relative rate of hydrolyze and condensation of TEOS rather than the

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Pore Size (A) Pore Size (A) Fig. 3. BJH pore size distribution curves of calcined BMS silicas prepared with different TEOS/CTAB molar ratio (A) a. 5.4; b. 7.6" c. 13.3" d. 16.3 and with different TEOS concentration at constant TEOS/CTAB molar ratio (B) a. 9.9wt%; b. 13.3wt%; c. 19.4wt%; d. 24.8wt%. micelle sizes in the synthesis of BMS silica, which would thus affect the silica particle size and its packing, and final affect its textural mesopore size. It should be noted that the degree of the textural pore size increase resulting from the increase of TEOS concentration at constant TEOS/C~6TAB ratio is far smaller than that from the increase of TEOS/C~6TAB ratio. Though the reason is not fully clear yet, it is probably related to the optimal matching between hydrolyzed silica species and CI6TAB micelles, superfluously hydrolyzed silica species would stay mainly in bulk solution, which would be favorable for the formation of a larger silica particle with thicker pore wall, thus a larger textural pore size. It is well-known that catalyst has a direct influence on the structure and properties of the silica products obtained not only in the synthesis of conventional silica gel [7.8] but also in the synthesis of mesoporous MCM-41 [9.10]. When the previously used catalyst ammonia is replaced by some small molecule organic amines with different structure characteristic, typical such as ethylamine (EA), n-butylamine (BA), n-hexylamine (HA), 1,2-ethanediamine (EDA), 1,4-butanediamine (BDA) and 1,6-hexanediamine (HAD), it is found that the concentration range of these catalysts used suitable to BMS silica synthesis is obvious different. This is probably related to the alkalinity difference resulting from their structure and properties difference. As can be seen in Fig. 4, which shows the appearance of the silica products prepared by various amounts of different base catalysts addition with a constant other composition concentration, ammonia has the widest concentration range for the 0.064

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Fig. 5. BJH pore size distributions for BMS silicas prepared with different bases as catalyst. (A) a. NH3; b. EA; c. BA; d. HA. (B) a. NH3; b. EDA; c. BDA; d. HDA. synthesis of BMS silica. According to the physical appearance of the resultant products, three different typed of behavior can be classified, although these regions are significantly different for different catalysts. Following the increase of the amount of catalysts used, the resultant silica products all change gradually from an initial opaque gel monolith via a viscous liquids between gel and precipitation (i.e. intermediate) to a rapid formed precipitation, which is similar to the results resulting from the increase of ammonia/silica molar rate reported in previous work [5]. For the convenience of comparing, the amount of different catalysts used is selected carefully in such a region, typical as 0.064, to ensure the formation of a silica gel rather than precipitation. The results show that the gelation time is also influenced by the catalysts used in the synthesis, gradually decreasing with an increase of the carbon atom number in organic amine molecules, indicating a increasing of condensation rate. However, the decreased degree of the gelation time is larger for diamine catalysts than that of monoamine catalysts. This implies a difference in the relative strength of each base in acting as a catalyst for the hydrolysis and condensation of TEOS. Generally, the weaker the alkalinity strength of the catalysts used is, the wider the concentration range suitable to the formation of a homogeneous gel is. Fig. 5 provides the BJH plots for the BMS silica samples prepared using different catalysts. It can be seen from the Fig. 5A that the peak intensity of the framework mesopore distribution of the samples prepared using EA, BA and HA as catalyst all is weaker, and the corresponding pore volume is also clearly smaller (<0.4cm3/g) than that of previously synthesized BMS silicas [4.5]. This is probably related to the similar and lower alkalinity provided by these catalysts, which results in a slower condensation rate and a weaker interaction between surfactant micelles and silicate species and thus a more disordered framework mesopores. In contrast, the corresponding peak intensity from the samples obtained using diamine molecules as catalyst increases clearly in the order of EDA, BDA, HAD (Fig. 5B). This implies an in turn increase of the alkalinity intensity, which would induce a more strong interaction between micelles and silicates, and final a more ordered framework mesopore can be formed. This influence can be also seen further from the change of their textural mesopore distribution, the increase of textural pore size from 16.35, 21.03 to 22.3nm for BMS silica samples prepared using EDA, BDA and HAD as catalysts,

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Fig. 6. BJH pore size distributions of BMS silicas prepared with different H20/TEOS molar ratio (A) a.250; b.150; c.100; d.25 and with different EtOH/TEOS molar ratio (B): a.0; b.1.38" c.2.75" d.4.12; e.5.49. respectively, is clearly larger than that samples prepared using EA(16.22nm), BA(18.52nm) and HA(20.94nm) as catalysts. It is clear that the stronger alkalinity would lead to a faster condensation rate, thus a mesostructure with a larger primary particle size and a looser packing geometric, thus a larger textural pore size can be formed. When the amount of solvent water is increased over a wide range typical from 25 to 250 (H20/TEOS molar ratio), which behaves as a reducing of surfactant concentration, but more as a lowering of pH values of reaction system, the gelation time increases clearly. Corresponding, the textural pore size of the resultant BMS silicas decreases markedly from 44.4 to l4.3nm (Fig. 6A), while, at the same time, the framework pore size also decreases from 3.10 to 2.66nm. The decrease of the framework mesopore size may be related to the decrease of micelle size resulting from the decrease of CI6TAB concentration, while the decrease of textural mesopore size indicates even smaller primary silica particles to be formed due to the decrease of the relative rate of hydrolysis and condense of TEOS. However, increasing the amount of co-solvent ethanol typical from 0 to 2.75 (EtOH/TEOS molar ratio) would result in a rapid decrease of the gelation time. The textural pore size thus increases markedly from 18.6 to 36nm (Fig. 6B), while, at the same time, the framework pore size decreases from 2.85 to 2.35nm. Further increasing the amount of ethanol over 4.12, the appearance of the products obtained transforms rapidly from an initial opaque gel to a white precipitation. Correspondingly, the mesostructure of the resultant silicas varies from the initial bimodal mesopore distribution (BMS silica) to a hexagonal single mesopore distribution (MCM-41). The phenomenon may be explained by that, on the one hand, the introduction of the polar co-solvent ethanol would increase the critical micelle concentration of C~6TAB, which makes the micelles size, cell size, and thus the framework mesopore size decrease, on the other hand, this introduction also makes the TEOS dissolve more easy, and thus leads to a more fast hydrolysis and condensation of TEOS, which is favorable for forming a larger silica primary particles and taking a looser packing of these particles and thus a larger textural porosity. In extreme conditions, the precipitation occurs before the gelation due to the further increase of primary silica particle size. In addition, other some methods such as adjusting the ammonia/silica molar ratio [5.11], post-synthesis hydrothermal treatment [12] or varying the

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aging and drying conditions [13] and so on can be also used to tailor both the framework and textural mesopore sizes and their properties of BMS silica over a certain range. 4. C O N C L U S I O N S Present study shows that due to the key to synthesis BMS silica is the gelation control of reaction system, the properties and the relative amount of each precursor in the starting compositions can influence both the structural and textural properties of the resultant BMS silicas to a certain extent. Not only is the gelation rate affected, thus the size and packing geometry of resultant silica primary particles, but also the micelle size also influenced by this variable, which enables the bimodal mesostructure of BMS silica to be tailored over a wide size range. These unique mesostructure characteristics of BMS silica make it useful as catalyst supports, in situations where diffusion limitations could negatively affect yields and selectivities. The combination of the conventional sol-gel method and the surfactanttemplated mesoporous silica synthesis method would provide simple and versatile synthetic approaches to orient the design of new multistructured porous materials with a controlled pore structure on all scales. ACKNOWLEDGEMENT This research was supported by the National Natural Science Foundation of China (Grant No.20073029 and 20371034) and the Youth Science Foundation of Shanxi Province (Grant No.20032010). REFERENCES [1] [3] [4] [5] [6] [7] [8] [9] [ 10] [11] [12] [13]

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