Pore size adjustment of bimodal mesoporous silica molecular sieves

Pore size adjustment of bimodal mesoporous silica molecular sieves

Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved. 77 Pore size adjus...

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Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

77

Pore size adjustment of bimodal mesoporous silica molecular sieves Xiaozhong Wang, ab* Tao

D o u , a Dong

Wu band Bing Zhong b

Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China e-mail" wanuxiaozhonu~.tvut, edu.cn

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bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China Bimodal mesoporous silica (BMS) molecular sieves were synthesized using quaternary ammonium surfactant at room temperature with lower pH values, and their pore sizes were tailored using a simple method by controlling the size of the structure-directing surfactant or incorporating an auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) into the reaction systems, which has been used previously to control the pore sizes of MCM-41 mesoporous materials. It was shown that as the surfactant alkyl chain length or the amount of TMB used was increased, the enlargement of the primary mesopore size of BMS materials was accompanied concurrently by the decrease of its secondary mesopoe size, and the degree of the primary mesopore size enlargement (ca. 1.0nm) is far smaller than that of MCM-41 mesoporous materials prepared under similar synthesis conditions, but in contrast the degree of its secondary mesopore size decrease is more sharp (ca.6.4nm). These results may be in connection with the characteristic framework structure of our BMS materials. When the amount of TMB used exceeded a certain degree, the primary mesopore structure of BMS materials was still present but its secondary mesopore structure got collapsed. INTRODUCTION The synthesis of inorganic frameworks with specified and organized pore networks is of potential importance in catalysis[l], separation technology[2] and biomaterials engineering [3]. Since the first synthesis of mesoporous MCM-41 materials[4.5], there has been intense activity in the design and synthesis of a variety of mesoporous solids with different structural features. Such features as pore size, pore size uniformity, interparticle porosity, and stability (thermal and hydrothermal) of these mesoporous molecular sieves were shown to be controlled by a proper choice of synthesis conditions[4-9]. At present, the surfactanttemplated synthetic procedures have been extended to include a wide range of compositions, and a variety Of conditions have been developed for exploiting the structure-directing functions of surfactant. These solids allow fasten diffusion of large organic molecules than the zeolitic and aluminium phosphate-based microporous sieves. These structural characteristics

78 make them potentially useful as catalysts for fluidized catalytic cracking and for the manufacture of fine chemicals. However, the information feedback from the practical industrial process shows that the catalyst used in the large molecules reaction requires a reasonable distribution of two-grade or multi-grade pores, and therefore over a long period of time direct synthesis of inorganic porous materials with two-grade or multi-grade pore distribution is researched for by zeolite chemists. In earlier investigations, we showed that careful controlling alkalinity affords a novel porous materials with well-defined bimodal mesopore size distribution at ambient conditions [11~.! 1]. The materials contain randomly distributed hexagnoal and stripe-like mesoporous channels with uniform pore size and exhibit very large surface areas and pore volumes. The secondary mesopore structure of BMS materials may be formed via the development of incomplete condensation of SiO2species around the adjacent surfactant micelle. The bimodal mesoporous structure of thus formed should be able to be tailored by applying the similar methods with which were used usually in the pore size adjustment of M41S materials. So far, relatively little work has been reported for the pore size mediation of bimodal mesoporous silica mesostructures and no convincing mechanism for such bimodal mesoporous structure formation has been put forward. Undoubtedly, the ability to control framework bimodal mesoporous distribution can be of great value in designing BMS materials as catalysts, ' adsorbents and sensor materials. Accordingly, in the present work we have examined the effect of the structure-directing surfactant size and the addition of auxiliary organic solvent such as 1,3,5-trimethylbenzene on the pore size characteristic framework cross-linking of BMS molecular sieves. At the same time we give a full account of the trend of pore size adjustment of BMS materials and extend on our hitherto proposed formation mechanism. 2. EXPERIMENTAL SECTION

2.1. Synthesis The synthesis procedure for BMS materials was described elsewhere[10.11]. For the purposes of probing the effect of surfactant alkyl chain length and auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) on pore size distribution of BMS materials, these samples were prepared typical using CI4H29N(CH3)3Br(C14) and C16H33N(CH3)3Br(C16) as templates. Tetraethylorthosilicate (TEOS) was used as a source of silica, and the pH values of the reaction mixture was adjusted with aqueous ammonia. In each case the reaction mixtures had the following molar composition: 1.0 SiO2:0.2 CI4H29N(CH3)3Br : 0.09 NH3" H20 : 115H20 1.0 SiO2:0.2 CI6H33N(CH3)3Br : 0.3 NH 3"H20 : (0-0.84) TMB : 115H20 The number of molar of ammonia in each reaction mixture was varied relying on surfactants alkyl chain length. When TMB was used as an auxiliary structure director, it was added to the surfactant solution and stirred for 15min before the addition of TEOS. All of the BMS reaction products were washed repeatedly with distilled water in a centrifuger, dried in air at 353K and finally calcined in air at 2K min"~to 823K for 6 h to remove the template.

79 2.2. Characterization

The powder X-ray diffraction pattems (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 measures at -196"(2 using a ASAP2000 analyser. The volume of adsorbed N2 was normalized to standard temperature and pressure. Prior to the experiments, samples were dehydrated at 350 ~ for 12h. The pore-size distribution was calculated using the desorption branches of the N2 adsorption isotherm and the Barrett-Joyner-Halenda (BJH) formula. 3. RESULTS AND DISCUSSION 3.1. Effect of surfactant alkyl chain length We have prepared BMS materials by the similar assembly procedure using quaternary ammonium surfactants with different chain length as structure director at room temperature. Figure 1 provides the representative X-ray powder diffraction patterns for the as-synthesized and calcined BMS derivatives formed using C~4 and C~6as structure directors. Each sample exhibites a single very low angle reflection, which seems to display less ordering in the mesostructure with a large unit-cell size. However, it can not be simple regarded as an average of pore-pore correlation distance due to the particularity of bimodal mesoporous framework structure of BMS materials. As the surfactant alkyl chain length is increased, the interplanar spacing dl0 0 of BMS samples is also of the trend of shifting gradually toward lower 2 0 angle, which is consistent with the synthesis results of MCM-41 mesoporous materials 8000

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Figure 2. N 2 adsorption isotherms and corresponding pore size distribution curves of BMS samples prepared using C~4 and C~6surfactant as templates Figure 2 shows the N 2 adsorption isotherms and corresponding BJH pore size distribution curves for calcined BMS samples. The samples exhibit type IV isotherms as expected for mesopores but with a characteristic hysteresis loop lifted up sharply in the P/Po region of 0.81.0. This may indicate that a change in the texture has occurred on the mesoporous frameworks of the product and suggest the presence of noticeable amount of secondary mesopores, i.e. filling of the framework-confined mesopores occurred at p/po=0.2-0.4 and p/po=0.8-1.0, respectively. This can be confirmed from BJH plots in Figure 2 (right), and two samples all show a well-confined bimodal mesopore distributions. Table 1 summarizes the effect of template sizes on the structural properties of BMS materials. It is especially Table 1. Structural properties of calcined BMS materials prepared using C~4 and C~6 surfactant. Primary mesopore Secondary mesopore SBET(m2g"l) Vs(cm3g"~) Ds(nm) Sample dl00(nm) SBET(m2g"l) Vp(cm3g"l) Dp(nm) 260.1 1.39 24.3 Ci4 4.25 981.3 0.6 2.45 243.1 1.18 18.9 Cl6 4.4 1064.6 0.66 2.6 noteworthy from the results in Figure 2 and Table 1 that the increase of the surfactant chain length causes the adsorption step at the position of p/po--0.2-0.5 to be shifted to higher relative pressure, but at the same time the adsorption step at the position of p/po=0.8-1.0 to be shifted to lower relative pressure. This suggests that an increase in primary framework pore size of BMS materials was accompanied concurrently by a decrease in its secondary framework pore size.

81 3.2. Effect of auxiliary structure director We consider another the effect of adding TMB into the reaction systems on the framework bimodal mesoporous structure of BMS materials. Figure 3 provides the X-ray diffraction patterns for BMS derivatives assembled from C~6 with TMB added as the auxiliary structure director. Structures formed from C~4 showed qualitatively equivalent diffraction features. The XRD patterns of as-synthesized BMS samples all contain a weak, relatively broad reflection at lower 2 0 angle. The qualitative form of the patterns is not affected by the presence of TMB. However, the positions of the intense reflection are dependent by the presence of TMB. It is worth noting that the degree of lattice contraction resulting from the calcination for the samples with TMB/TEOS molar ratio exceeding 0.58 is larger than that of other samples, this may indicate that a part of texture collapse has occurred. This can be confirmed from Figure 4, which provides corresponding N2 adsorption-desorption isotherms and pore size distribution

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Figure 3. Powder XRD pattems of as-synthesized(A) and calcined(B ) BMS samples prepared using different TMB/TEOS molar ratio: a 0.065; b 0.194; c 0.322; d 0.451; e 0.58; f0.84. curves of BMS samples. Table 2 summarizes the relative structural parameters of series BMS samples. Clearly, the basal spacings represented by the main diffraction line are not correlated directly with the BJH primary pore sizes, even though the dl00 peaks of samples shift gradually to lower 2 0 angle and at the same time the primary mesopore sizes of BMS materials also increase gradually following the increase of TMB used in the reaction systems. The trend of bimodal mesoporous size adjustment resulting from the incorporation of TMB into reaction systems is similar with the results of increasing the surfactant chain length in the synthesis of BMS materials. The role of TMB as an auxiliary structure director on the primary mesopore size of BMS materials is also similar with its effect on the pore size of MCM-41 mesoporous materials, however, the degree of the primary pore size increase (ca. 1.0nm) is far smaller than that of MCM-41 materials. In contrast, it makes the secondary mesopore size of BMS materials contracted obviously (ca.6.4nm). After increasing TMB/TEOS ratio to exceed 0.58, the nitrogen adsorption isotherms of these samples vary significantly, showing that the

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Figure 4. N 2 adsorption isotherms and corresponding pore size distribution curves for a series of BMS samples prepared at different TMS/TEOS mole ratio: A. 0.065; B. 0.194; C. 0.332; D. 0.451; E. 0.58; F. 0.84. structure has been affected. The sharp at p/po=0.8-1.0 decrease in nitrogen adsorption volume and shift of the step in nitrogen adsorption to lower P/Po imply that a large number of the secondary mesopores were destroyed and the pore size became smaller.

83 Table 2. Structural parameters for selected calcined BMS samples under study. Primary mesopore Sample A B C D E F

dloo(nm) SBET(mEg1) 4.25 4.56 4.51 5.26 4.55 4.80

1067.8 939.5 1106.9 937.6 1096.9 1138.3

Vp(cm3g"1) 0.69 0.66 0.76 0.71 0.87 0.85

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Dp(nm)

SBET(m2g "l) Vs(cm3g"1) Ds(nm)

2.88 3.15 3.27 3.20 3.28 3.50

270.2 260.9 288.8 271.9 \ \

1.05 0.95 1.07 0.87 \ \

17.0 15.8 14.6 12.5 \ \

The further experimental results show that BMS materials can be also synthesized using quaternary ammonium surfactant with alkyl chain length more than Cls, however, the chain length lower than C12 gives not a bimodal mesoporous structure but disordered mesostructure. The synthesis process of all BMS materials was completed in a short time and the viscosity of the reaction mixtures increased with time, and eventually set into jelly-like monoliths. These results suggest that under our synthesis conditions (pH ca.9.5), the relative rate of TEOS hydrolysis and condensation reactions may be almost equal[12], and, hence, the gel point is reached before the precipitation formed. After gelation, the effect of further extension reaction time on the products framework structure is negligible. In such a short reaction time, the surfactant micelles formed in the initial reaction mixtures may possess different size and shape, and the hydrolysis of TEOS and the condensation of the hydrolyzed products are incomplete, and thus the hydrolyzed SiO2 species are not enough to condense around every surfactant micelle. This may make some adjacent surfactant micelle interconnect and lead to the formation of mesopore framework of alternating hexagonal pore and stripe-like pore. The expansive micelles resulting from the increase of surfactant chain length or the addition of TMB into the reaction systems may require more hydrolyzed SiO2 species to ensure the formation of bimodal mesoporous framework structure in the prerequisite for two-grade mesopores expanded simultaneously. Obviously, at constant component concentration and thus hydrolyzed SiO2 species, it is inevitable results that the increase of primary mesopore size is accompanied by the decrease of secondary mesopore size. The further increase of TMB concentration may change the pH values of reaction systems and thus change the relative rate of TEOS hydrolysis and condensation reactions. The right proportion which is benefit for the formation of BMS materials between the hydrolyzed SiO2 species and surfactant micelles is destroyed. This may make the product's framework structure between some adjacent surfactant micelles susceptible to collapse during thermal treatment. As for the fact that the number of molar of ammonia in each reaction mixture was varied relying on surfactants alkyl chain length may be relevant with their different critical micelle concentration. Such a scenario would explain the pore size adjustment features of BMS materials, but further study are needed to fully understand the nature of the observed phenomena.

84 4. CONCLUSION The current study confirms that the methods used usually to control the pore size of MCM-41 mesoporous materials by adjusting the size of structure directing surfactant or the amount of auxiliary organic cosurfactant such as TMB added into the synthesis systems are also suitable for modifying the pore size distribution of BMS mesoporous materials. Because of the characteristic framework mesopore structure of our BMS materials, the increase of the primary framework mesopore size was accompanied simultaneously by the decrease of the secondary framework mesopore size, and the degree of increase of the primary mesopore size is far lower than that of decrease of the secondary mesopore size. These results suggest that the formation of bimodal mesoporous framework structure not only relies on an interaction between SiO2 species and the surfactant micelles, but also on a proportion of the hydrolyzed SiO2 species to surfactant micelles, and the pH adjustment of reaction systems and the relative rate of TEOS hydrolysis and condensation reactions resulting from the pH adjustment play a critical role in the synthesis of BMS molecular sieves. The controllability of bimodal mesoporous framework structure of BMS materials could make them more attractive for the practical applications. 5. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.20073029). REFERENCES

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