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Synthesis and characterization of nanosized micro-mesoporous Zr–SiO2 via Ionic liquid templating
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Materials Science and Engineering C 28 (2008) 1217 – 1226 www.elsevier.com/locate/msec
Synthesis and characterization of nanosized micro-mesoporous Zr–SiO2 via Ionic liquid templating Aizhong Jia, Jie Li, Yinqing Zhang, Yajuan Song, Shuangxi Liu ⁎ Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, PR China Received 9 August 2007; received in revised form 24 September 2007; accepted 5 November 2007 Available online 13 November 2007
Abstract A series of micro-mesoporous Zr–SiO2 composites with nanoscale domains were prepared by using ionic liquid (IL) as a template at 373 K in only 3 h with one-step. The synthesized Zr–SiO2 materials were characterized by N2 adsorption–desorption, scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, diffuse reflectance UV–Vis spectroscopy, thermogravimetric analysis and temperature program desorption technologies. The results show that the synthesized composite materials possess nanoparticle with a mean diameter of about 100 nm and a large surface area more than 1000 m2/g, and the hierarchically porous structure is preserved after removing template by calcination at high temperature and treating in boiling water for 72 h. The heteroatom of zirconium has been successfully incorporated into the structure framework and/or has been highly dispersed on the surface of materials. The prepared materials contain moderate to strong acid sites and the surface acid site concentration is 0.18–0.42 sites/nm2. The amount of strong acid sites increases with a decrease of Si/Zr ratio, which leads to increased temperature for removing IL templates. © 2007 Elsevier B.V. All rights reserved. Keywords: Zr–SiO2; Ionic liquid template; Hierarchically micro-mesoporous structure; Composite material
1. Introduction Porous silicates materials are classified into three categories based on their pore diameters (D): microporous (D b 2 nm), mesoporous (2 nm b D b 50 nm), macroporous (50 nm b D) [1]. Zeolites are crystalline aluminosilicates with three-dimensional microporous structure, and are widely used in industry as catalysts or supports. They possess unique properties with respective to both activity and selectivity [2]. However, the microporous nature of zeolites limits their catalytic performance in catalytic reactions of bulky molecules, such as pharmaceutical, biological and fine chemical reactions, because of the severe restriction to the diffusion of the larger molecules in micropore channels [3]. Due to the above-mentioned drawback, expanding of the pore sizes of zeolite and zeotype materials from micropore to mesopore is of a high importance from both industrial application and fundamental understanding of porous ⁎ Corresponding author. Tel./fax: +86 22 23509005. E-mail address: [email protected] (S. Liu). 0928-4931/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2007.11.002
structures. A lot of researches have been devoted into the synthesis of mesoporous materials during the last decades [4–7]. Nevertheless, the amorphous characters of the mesoporous walls entail low hydrothermal stability and low acidity that have greatly diminished their functionalities in catalytic applications [4]. To combine the benefits of micro-and mesoporous molecular sieves, a kind of composite material containing both types of pores has been prepared [8–10]. However, it should be pointed out that most preparation methods reported for micromesoporous composite materials are complicated and timeconsuming. Room temperature ionic liquid (RTIL) possesses a wide range of liquid, negligible vapor pressure, high electrical conductivity, wide electrochemical windows, tolerance to strong acids, relatively low viscosity, high solvability, noncoordination and excellent thermal and chemical stability. As a “green” agent, RTIL has received much attention for organic chemistry reactions [11,12], separations [13] and electrochemistry applications [14,15]. As one category of surfactants, long chain RTILs, which possess both a hydrophilic ionic head and a
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hydrophobic organic chain, have been used as template to prepare porous materials recently. However, in contrast to their successful application in organic chemistry, inorganic synthesis with RTIL, especially in the synthesis of porous materials, is still in its infancy. To our knowledge, the reports on using RTIL as template for synthesizing porous materials are only limited in a few researchers [16–23]. Highly ordered monolithic supermicroporous lamellar silica has been prepared by Zhou et al. [16] via 1-hexadecyl-3-methyl-imidazolium chloride as template in acid-condition. By using the same template, mesoporous silica and Ru/SiO2 with high surface area have been synthesized by Zhu et al. [17] under strong acidic and basic conditions. Three zeolites of aluminophosphate and unknown framework structure of cobalt aluminophosphate have been prepared by Emily et al. [18] with using the ionic liquid 1methyl-3-ethyl imidazolium bromide as both the solvent and template. The acidity of purely siliceous solids are, in all cases, very low, while the incorporation of Zr(IV) into the siliceous framework produced an enhancement of the acidity [24]. As a promising catalytic material, zirconium-based catalysts have acquired wide applications ascribing to the acidity and oxidizing capabilities. In this paper, we report a simple and novel route for preparing of Zr–SiO2 materials with hierarchically micro-mesoporous structures and nanosize. In the preparing process, IL, tetraethoxysilane (TEOS) and zirconyl chloride are used as template, silicon source and zirconium source, respectively. The samples with different Si/Zr molar ratio are prepared, and their hydrothermal stability and acidity are also investigated. The materials are prospective for using as solid acid catalyst in reactions, in which bulky molecules attend. 2. Experimental section 2.1. Preparation of ionic liquid The synthesis procedures of ionic liquid followed a reported route [15]. As a typical synthesis of 1-hexadecyl-3methylimidazolium chloride (HMIMCl), 1-chlorohexadecane (HACl, 65.24 g, 0.25 mol) was mixed with 3-methylimidazole (MIM, 20.54 g, 0.25 mol) in a 250 mL flask and then refluxed for 24 h at 363 K. Cooled to room temperature, the product was dissolved into 200 mL of tetrahydrofuran (THF), and then the solution was put into refrigeratory for recrystallization. Twelve hours later, the 1-hexadecyl-3-methylimidazolium chloride was obtained after filtration, washed several times with a little amount of THF and vacuum-dried at room temperature. 2.2. Synthesis of hierarchically porous Zr–SiO2 In a typical synthesis procedure of composite material, 2.8 g of HMIMCl was dispersed into 70 g of deionized water under magnetic stirring with a fixed stirring rate. Then a certain amount of zirconyl chloride (ZrOCl2·8H2O) was added into the solution and 10.1 mL of TEOS was added dropwise to the solution 15 min later. After two hours, 26.5 g of 3.5 % sodium hydroxide solution was slowly added into the above mixture.
The resultant mixture solution was stirred at room temperature for 3 h in an open beaker, allowing the sol–gel reaction of the silica. Then it was transferred into a stainless steel autoclave and kept in an oven at 373 K for 3 h. The solid product was filtrated, washed, dried at room temperature and calcined at 823 K in air for 6 h to remove the template. The Si/Zr molar ratio in the final solid determined by ICP-AES is close to that present in the mother solution, thus the Si/Zr ratio of starting compositions was taken as Zr content in the text. According to the different dosage of the template and the difference of zirconium content, the materials were marked as “x-Zr–SiO2-y”, where x denotes the Si/Zr ratio of the starting composition and y denotes the amount of IL template. 2.3. Characterization Fourier transform infrared spectroscopic (FT-IR) analysis was conducted on a Bruker VECTOR22 spectrometer with a resolution of 1 cm− 1 for the framework of materials. It was conducted using KBr wafers (1 wt.% sample mixed with 99 wt.% KBr). The acid sites (Brönsted or Lewis) were determined by the pyridine-adsorption FT-IR spectrometry on self-supported wafers. The wafers were pre-heated at 673 K for 1.5 h under 10− 4 Torr vacuum, and then, after cooling to room temperature, the pyridine vapor was introduced. Finally, the samples were exposed to vacuum at 473 K for 1 h, and then the FT-IR spectra of pyridine adsorption were collected. In addition, to study the strength of the acid sites, the structural variations upon heating were analyzed at indicated pyridine desorption temperatures under vacuum (10 − 2 Torr) for 1 h. Scanning electron micrographs (SEM) were obtained by using Shimadzu SS-550 electron microscope. Transmission electron micrographs (TEM) were recorded on a TECNAI Philips T20ST electron microscope operating at 200 kV. Crystal structure was characterized by using a Rigaku D/MAX 2500 powder X-ray diffractometer (XRD) at a scanning speed of 4°/min, with Cu Kα line as incident radiation and a filter. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 with a heating rate of 10 K min− 1 from room temperature to 1073 K in an atmosphere of 90% N2 and 10% O2. Nitrogen adsorption– desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 volumetric apparatus. Specific surface areas were calculated following the BET procedure. Pore diameter was estimated by applying the DFT method to the adsorption branch of the isotherm. Temperature program desorption (TPD) of NH3 was carried out on an apparatus with a JS-3050 detector. Prior to the adsorption of NH3, ca. 100 mg of sample was first preheated at 773 K under flowing Ar for 0.5 h to remove physisorbed species, then cooled to 333 K. Subsequently, the sample was exposed to flowing ammonia gas mixture (5% NH3 in Ar) for 0.5 h, then purged by Ar gas for 1 h to remove excessive physisorbed ammonia. All NH3-TPD profiles were carried out by ramping the temperature from 333 to 1073 K at a rate of 10 K min−1. Diffuse reflectance UV–Vis (DR UV–Vis) spectra were measured with a Shimadzu UV–Vis 2550 spectrophotometer. The spectra were collected in the wavelength range from 190 to 800 nm.
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3. Results and discussion 3.1. Powder X-ray diffraction analysis The XRD patterns of samples synthesized under the optimal conditions (excepting the variable factor will be discussed) are given in Fig. 1. The XRD patterns shown in Fig. 1A reveal that the dosage of IL template has a significant influence on the forming of composite Zr–SiO2. Increasing the amount of IL template increases the intensity of XRD peaks. The structure parameters of the samples are shown in Table 1, from which a similar result to that of XRD is found by contrasting samples 1–5 that increasing the amount of IL template increases the surface area and the pore volume when the dosage of IL template is no more than 2.8 g. Whereas no evident change appears with a further increasing of IL template (N2.8 g). The results shown in Fig. 1B indicate that the material with fine structure can be obtained at 373 K, and too high or too low temperature is disadvantaged to form fine hierarchically porous structure. This is also confirmed by nitrogen sorption analysis, both high and low temperature led to lower surface area and lower pore volume
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(Samples 8 and 9), although the surface area and the pore volume of sample obtained at 353 K (Sample 9) are close to those of sample synthesized at 373 K (Sample 3). After crystallization at higher temperature, the mother liquid turns yellow, suggesting some of IL templates were decomposed. Increasing synthesis temperature not only affects the interactions between template and silica species but also enhances the density of mesoporous framework [25]. The lower surface area and the lower pore volume are thus attributed to decomposing of IL template and the higher synthesis temperature. 2.8 g of IL template and 373 K are received as the optimal parameters in this work. Fig. 1C gives the XRD patterns of calcined 100-Zr–SiO2-2.8 samples synthesized at 373 K with different crystallization time. After crystallization for only one hour, the porous structure has emerged in mother liquid, and fine structure porous material formed when the crystallization time reached to 3 h. With longer crystallization time, even to 192 h, no substantial change is observed. All of these indicate that pure phase of porous materials can be formed in 3 h and no other material phase appears under strong basic conditions for 192 h. So it is believed that the synthesized material has a good hydrothermal stability
Fig. 1. XRD patterns of samples (A) 100-Zr-SiO2-2.8 prepared with different amount of IL template at 373 K; (B) 100-Zr-SiO2-2.8 prepared at 353, 373, 393 K for 24 h; (C) 100-Zr–SiO2-2.8 prepared at 373 K for 1–192 h; (D) 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8, ∞-Zr–SiO2-2.8, and treated 100-Zr–SiO2-2.8.
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Table 1 Texture characteristics of various samples Sample
Code
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore size (nm)
Mesopore volume (b10 nm) (cm3/g)
Micropore volume (b2 nm) (cm3/g)
1 2 3 4 5 6 7 8 9 10
100-Zr–SiO2-0.9 100-Zr–SiO2-1.8 100-Zr–SiO2-2.8 100-Zr–SiO2-3.6 100-Zr–SiO2-14 10-Zr–SiO2-2.8 ∞-Zr–SiO2-2.8 100-Zr–SiO2-2.8a 100-Zr–SiO2-2.8b 100-Zr–SiO2-2.8c
774 980 1100 1160 1166 802 870 372 1006 956
0.51 0.81 1.16 1.02 1.09 1.10 0.76 0.31 0.98 0.84
3.2, 1.4, 0.68 3.2, 1.4 3.3, 1.4 3.2, 1.4 3.2, 1.4 2.8, 1.4 3.4, 1.4, 0.68 4.4, 3.4, 2.8, 1.4 3.2, 1.4 2.8, 1.4, 0.68
0.35 0.45 0.88 0.95 0.95 0.43 0.62 0.26 0.86 0.44
0.09 0.10 0.11 0.11 0.11 0.11 0.03 0.04 0.09 0.10
a
Synthesized at 393 K. Synthesized at 353 K. c Treated in boiling water for 72 h. b
and the optimal crystallizing time is 3 h. The rapid formation of Zr–SiO2 is related to the acidity self-generated in the aqueous solutions of the zirconium precursors which is the catalyst for TEOS hydrolysis [26]. A contrast among the XRD patterns of 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8, ∞-Zr–SiO2-2.8 and the treated 100-Zr– SiO2-2.8 is also presented in Fig. 1D, from which no peaks due to ZrO2 or any other crystalline impurity phases are found. So it is believed that the atoms of zirconium have been successfully incorporated into the framework and/or have been highly dispersed on surface of material, which is in very good agreement with that reported by Rakshe et al. [27] and Zhu et al. [28]. Sample of ∞-Zr–SiO2-2.8 displays a sharp peak in range of about 1.4–2.8°, which is characteristic of mesoporous materials and is associated with the (100) reflection when a hexagonal cell (similar to that described in the case of MCM41) is assumed [29], and two weak peaks in the range of about 3.6–4.0° and 4.2–4.6° (associated with (110) and (200) reflections). However, all samples doped with Zr display only one broadened peak in range of about 3.5–5.0° together with the intense (100) peak, indicating their pore structure are less ordered than that of ∞-Zr–SiO2-2.8. The XRD pattern of treated 100-Zr–SiO2-2.8 reveals that no serious structural degradation appears after hydrothermal treatment in boiling water for 72 h. Nevertheless, its peaks slightly shift to higher angles and the peak (3.5–5.0°) gets weak compared with parent material. This indicates shrinkage and weak ordering of the hierarchically porous structure by hydrothermal treatment. Comparing with 100-Zr–SiO2-2.8, the decrease of pore size and surface area for 10-Zr–SiO2-2.8 may be due to some Zr impurities highly dispersed on the pore surface.
condensation of silanol groups on the surface of material [30]. This result indicates that a large number of silianol groups may exist on the inner and exterior surface of ∞-Zr–SiO2-2.8. However, the surface dehydroxylation is not observed for 100Zr–SiO2-2.8 and 10-Zr–SiO2-2.8, because of the effective prevention by incorporating zirconium atom into the structure
3.2. Thermogravimetric analysis Typical thermograms and weight change derivatives obtained for ∞-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and 10-Zr– SiO2-2.8 are shown in Fig. 2. The total mass loss in the temperature range of 373–1073 K is 45.91%, 44.01% and 37.25%, respectively. Furthermore, a mass loss of about 1.57% in the temperature range of 850–1000 K is observed for ∞-Zr– SiO2-2.8 (Fig. 2A), which is due to the release of water through
Fig. 2. (A) TG curves of samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr– SiO2-2.8. (B) DTG curves of samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr–SiO2-2.8.
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because of the interaction between Si–OH and Zr–OH, and the OH groups are oriented perpendicular to the surface. The surface Si–Si distance on this surface is ca. 5 Å [31], moreover the distance is enlarged by incorporating zirconium, so the surface dehydroxylation is very difficult. All the samples display a two-stage template decomposition behavior, i.e., two major weight loss regions, (i) low temperature region and (ii) high temperature region are seen (see Fig. 2B). Furthermore, it can be easily found that the mass loss in the 2nd stage increases with an increase of Zr content in synthesized samples. Moreover, the temperature of the 2nd mass loss region is observed to shift to higher temperature from DTG curves. All of these can be attributed to the texture characteristics of the samples (see Table 1), whose pore size distribution displays two main regions (3.3, 1.4 nm) and the decreasing of mesopore size (from 3.3 to 2.8 nm) and mesopore volume (from 0.88 to 0.43 cm3/g) resulted by increasing of Zr content. 3.3. FT-IR analysis As suggested by Dalai et al. [32], information regarding the types of acid sites (i.e., Lewis or Bronsted acid sites) presented in the samples has been obtained from the IR spectra of the samples in the pyridine region (wave-number in the range of 1425–1775 cm− 1). The IR spectra of pyridine adsorbed samples are presented in Fig. 3A for samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr–SiO2-2.8. As shown in the figure, the samples contained Zr exhibit five bands at frequencies of 1638, 1598, 1546, 1490, and 1446 cm− 1 in the pyridine region. The bands at a frequency of 1546 and 1638 cm− 1 represent the Brönsted acid sites, the bands at 1598 and 1446 cm− 1 can be assigned to Lewis acid sites, whereas the band at 1490 cm− 1 is due to a combination band associated with Brönsted and Lewis acid sites. The intensities of the bands at 1638, 1546, and 1490 cm− 1 increase as the Zr content increasing. Compared with Zr-doped samples, the pure silica material only contains
Fig. 3. (A) FT-IR spectra of pyridine adsorbed samples with different amount of Zr. (B) The results of pyridine desorption for 10-Zr–SiO2-2.8. (C) FT-IR spectra of samples ∞-Zr–SiO2-2.8 and 10-Zr–SiO2-2.8.
framework. Two OH groups on neighboring silicon atoms pointing toward each other are close enough to interact with each other at high temperature. But the incorporation of Zr leads to most of the OH groups being isolated on the surface silicon
Fig. 4. NH3-TPD profiles of calcined composite materials with different Si/Zr ratios: (a) 100, (b) 50, (c) 20, (d) 10.
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Fig. 5. Diffuse reflectance UV–Vis spectra of calcined composite samples with different Si/Zr ratios: (a) 100, (b) 50, (c) 20, (d) 10.
the Lewis acid sites (only two bands at 1598 and 1446 cm− 1), which is same to the result got by Fuentes-Perujo et al. [24]. A very broad band, which is due to the adsorbed H2O and the acidic bridge hydroxyl groups in all samples, is observed covering the region of 3000–3700 cm− 1, and its intensity sharply increases with adding the Zr atom into the samples. The
results of pyridine-desorption analysis for 10-Zr–SiO2-2.8 at different temperature are given in Fig. 3B, from which it can be seen that the material displays an intense pyridine absorption peak even after desorption at 673 K. This demonstrates that the strong acid sites exist in Zr-doped material. Fig. 3C illustrates the FT-IR spectra of 10-Zr–SiO2-2.8 in comparison with that of pure silica material prepared at the same conditions. The bands at around 1220, 1070, 790, and 470 cm− 1 are attributed to the typical Si–O–Si stretching and bending vibrations of condensed silica network. The band at around 960 cm− 1 is assigned to a combination band associated with the stretching mode of SiO4 entities bonded to Zr atoms (Si–O–Zr) and the stretching of non-condensed Si–OH groups, which is in good agreement with the reported results [33,34]. The broad band at 3400 cm− 1 and the strong peak around 1630 cm− 1 are due to the stretching and bending vibrations of adsorbed H2O. The work of Haskouri et al. [29] demonstrated that Si–O–Zr fragments and small ZrO2 domains coexist in the Zr-rich materials (Si/Zr ≤ 3), however, in the range of ∞ ≥ Si/Zr ≥ 5, Zr atoms are preferentially incorporated in tetrahedral sites isomorphously replacing Si atoms. According to their results, the ZrO2 displays up to four bands at 450, 500, 610, and 780 cm− 1 in the spectra window. From Fig. 3C, it will be found that the FT-IR spectrum of 10-Zr–SiO2-2.8 is very similar to that of pure silica material, and no deformation can be observed
Fig. 6. SEM micrographs of the samples: pure silica samples synthesized at (A) 373 K, (B) 393 K and the samples with zirconium (C) 100-Zr–SiO2-2.8, (D) 10-Zr–SiO2-2.8.
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in the 500–700 cm− 1 range. Taking into account all these results, it also implies that the Zr atoms have been incorporated into the framework of Zr–SiO2 material. 3.4. Temperature programmed desorption (TPD) of ammonia Because the pyridine-IR is limited in providing qualitative information (i.e., types) of the acid sites, it is conventionally combined with the NH3-TPD technology, which can provide additional qualitative information regarding to the overall concentration and strength of the acid sites. Fig. 4 shows the NH3-TPD profiles for calcined samples with different Zr content. It can be found that each sample exhibits a broad NH3 desorption peak starting from 473 K and extending beyond 773 K. According to the literature [32], the peak temperature is a representation of the strength of acid sites while the area under the peak represents the total amount of acid sites in the sample. The large temperature range of the broad peak indicates that the prepared materials contain moderate to strong acid sites. This is consistent with the results of pyridine desorption. By comparing the profiles of the samples with different Zr contents, it can be easily found that the strength of the peak at high temperature
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range (from 623 to 773 K, i.e., the strong acid sites) increases with the Si/Zr ratio decreasing, but the amount of strong acid sites are less than that of moderate acid sites. The surface acid sites concentration, which measured by the reported method [35], of materials with different Zr content is in range of 0.18–0.42 sites in each square nanometers on the samples surface. 3.5. DR UV–Vis spectroscopy DRUV–Vis spectroscopy is extensively used and perhaps one of the best techniques to detect the framework and extraframework zirconium species [36]. It can offer additional information on the dispersion and environment of Zr atoms inside materials. Shown in Fig. 5 are the UV–Vis spectra of the calcined samples with different Si/Zr ratio ((a) 10; (b) 20; (c) 50; (d) 100). In all cases, the spectra display an intensive absorption band at around 220–230 nm. An additional band at 200 nm is observed when the ratio of Si/Zr is ≥ 50. The absorption bands of isolated Zr4+, pure ZrO2 and nanoscopic regions ZrO2 were found at about 210, 230 and 250 nm, respectively [37]. So the absorption at 200 nm is attributed to ligand-to-matel charge transfer from an O2− to isolated Zr4+ ions in tetrahedral
Fig. 7. TEM images: (A), (B) (C) for 100-Zr–SiO2-2.8; (D) for treated 100-Zr–SiO2-2.8.
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configuration. This indicates the presence of isolated Zr4+ ions in the Zr–SiO2 mesoporous framework. The intensive absorption appears at about 220–230 nm, which is attributed to the half connectivity of Zr–O–Zr linkage, implying a possible formation of Zr–O–Zr–O–Si on the surface. Furthermore, in the present work, the characteristic peaks of ZrO2 could not be detected with high angle X-ray measurements. The results suggest that the broad band, which is similar to that got by Chaudhari et al. [38], is attributed to the presence of more than one type of zirconium species in the samples. 3.6. Electron microscopy In general terms, SEM provides the morphology information and TEM allows identification of the internal structure of the samples. Fig. 6A and B show the morphologies of the samples prepared at different temperature without zirconium atoms. It can be found that their particle sizes are large and nonuniform. The sample synthesized at low temperature has smooth surface and arched grain boundary, but the sample synthesized at higher temperature has more irregular grain and sharp grain boundary. It should be due to the change of growth rate at different
direction when temperature is increased. The Zr–SiO2 particles with about 100 nm have been obtained when a small quantity of zirconyl chloride is added into the initial materials (see Fig. 6C). Higher Zr content has no further remarkable effect on the morphologies of the products (see Fig. 6D). The hetero-atom effect, which is well-known that the hetero-atom has intensive influence to the formation of molecular sieve, especially to the morphology, is the major influence factor. TEM images of calcined 100-Zr–SiO2-2.8 and the sample treated in boiling water are shown in Fig. 7. The average wall thick of around 3.5 nm can be calculated from the scale bar (Fig. 7A). The nanometer particles of 100-Zr–SiO2-2.8, which was synthesized under the same conditions as large ∞-Zr–SiO22.8 particles, are found (Fig. 7B). Zhu et al. [17] believe that the imidazole head in the template is sensitive to the pH value of the media in tailoring the morphologies. Under strongly acidic/ basic conditions the chances of forming nanometer particles are lower, while under mild basic conditions only porous silica nanometer particles are prepared. But in our experiment, micrometer size SiO2 and nanometer size Zr–SiO2 materials have been synthesized in same basic conditions. So, it is not the synthesis conditions but the hetero-atom of zirconium that plays
Fig. 8. N2 adsorption–desorption isotherms and pore size distribution profiles (inset) of prepared samples: (A) ∞-Zr–SiO2-2.8, (B) 100-Zr–SiO2-2.8, (C) 10-Zr–SiO22.8 and (D) treated 100-Zr–SiO2-2.8.
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a predominant role on the forming of nanometer Zr–SiO2 materials. The hierarchically pore structure of the materials has been preserved after treatment in boiling water for 72 h (Fig. 7D), revealing that the material has a good hydrothermal stability, and the results consist with that of XRD. A lot of nanometer clusters are observed on the surface of 100-Zr–SiO22.8 (Fig. 7C), which is may be related to the good hydrothermal stability. 3.7. Nitrogen adsorption–desorption study The nitrogen adsorption–desorption isotherms and the pore size distributions (insets) for various samples are shown in Fig. 8. All of the isotherms, except the one for treated 100-Zr– SiO2-2.8, are of type IV with hysteresis loops. Steep rise at low relative pressure (p/p0) indicates the presence of micropores in micro-mesoporous composite. N2-physisorption isotherms for Zr–SiO2 exhibit two regions of hysteresis between the adsorption and desorption traces, from relative pressures (p/p0) of 0.2–0.35 and 0.40–0.60 (Fig. 8B). The hysteresis at (p/p0) of 0.2–0.35 is attributed to the inkwell pore geometry in which the mouth of the pore is smaller than its diameter. Normally, the isotherms of M41 s materials do not show such hysteresis. The hysteresis at the relative pressure range of 0.40–0.60 is similar to that described by Poladi et al. [39], which is due to the capillary condensation of mesopores. Guo et al. [1] and Bagshaw et al. [40] believe that the presence of an adsorption upturn at high p/p0 (N 0.8) suggests that the particles in the materials are not ‘solid’ but have open internal structure. By comparing the pore distributions of 100-Zr–SiO2-2.8 and 10-Zr–SiO2-2.8, it will be found that the mesopore size decreases from 3.3 nm to 2.8 nm (see Table 1), which indicates that the mesopore size get smaller with an increase of Zr content in the composite materials. The nitrogen adsorption–desorption curve for ∞-Zr –SiO2-2.8 (Fig. 8A) shows that the prepared pure silicate material has a BET surface area of 870 m2/g and a narrow mesopore size distribution of around 3.4 nm with a small quantity of micropore coexisting. The isotherms of treated 100-Zr–SiO2-2.8 (Fig. 8D) are very similar to type I, excepting the hysteresis loop at high relative pressure (p/p0 N 0.8). The absence of the hysteresis loops at relative pressures (p/p0) of 0.2–0.35 and 0.40–0.60 can be attributed to the condensation of pore size, which is demonstrated by the results of pore size distribution of the boiling water treated composite, when the composite is treated in boiling water. After treatment in boiling water for 72 h, a new distinct peak at 0.68 nm is observed in the pore size distribution curve of treated Zr–SiO2 composite, and the biggest average pore size decreases from 3.3 nm to 2.7 nm, the mesopore volume decreases from 0.88 cm3/g to 0.44 cm3/g, whereas the micropore volume has no obvious change (see Table 1). The sodium ions trapped in the framework walls of pure silica mesostructure material contribute to the structural instability of the framework [41], but the introduction of sodium salts can accelerate Si–O–Zr bond formation under hydrothermal conditions [42]. Moreover, the octahedrally coordinated Zr cation can form more metal-oxygen bonds than tetrahedrally
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coordinated Si, the framework has been compacted when the material is treated in boiling water, so the mesopore size and mesopore volume decrease after treatment. 4. Conclusions A series of nanosized composite materials Zr–SiO2 with hierarchical porosity have been successfully prepared through a one-step, conventional hydrothermal synthesis route, in which ionic liquid 1-hexadecyl-3-methylimidazolium halide is employed as template, tetraethoxysilane as a silicon source and zirconyl chloride as a zirconium source. The method introduced in this paper is a simple, potentially generic and time-saving method for fabricating composite materials. Various characterizations reveal that hetero-atom has been successfully incorporated into the framework structure and/or highly dispersed on surface of materials. The composite materials exhibit extraordinarily good hydrothermal stability and no collapse appears after removing template by calcination at high temperature and treating in boiling water for 72 h. The heteroatom Zr has a prominent effect on the morphologies of composite materials. The materials contain moderate to strong acid sites, the amount of strong acid sites increases with a decrease of the Si/Zr ratio and the concentration of specific surface acid sites is about 0.18–0.42 sites in each square nanometers on the samples surface. Acknowledgments The research work is supported by National Natural Science Foundation of China (Nos. 20773069 and 29973016) and The National Key Technologies R&D Program of China (2006 BAC02A12). References [1] X. Guo, N. Xue, S. Liu, X. Guo, W. Ding, W. Hou, Micropor. Mesopor. Mater. (2007), doi:10.1016/j.micromeso.2007.03.003. [2] A. Corma, Chem. Rev. 95 (1995) 559. [3] M. Mazaj, N.Z. Logar, G. Mali, N.N. Tusar, I. Arcon, A. Ristic, A. Recnik, V. Kaucic, Micropor. Mesopor. Mater. 99 (2007) 3. [4] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [5] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmit, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgius, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [6] D.W. Lee, S.J. Park, S.K. Ihm, K.H. Lee, J. Non-Cryst. Solids 353 (2007) 1501. [7] W. Li, M.O. Coppens, Chem. Mater. 17 (2005) 4560. [8] B.L. Newalkar, H. Katsuki, S. Komarneni, Micropor. Mesopor. Mater 73 (2004) 161. [9] P. Prokesova, S. Mintova, J. Cejka, T. Bein, Mater. Sci. Engine. C 23 (2003) 1001. [10] A. Karlsson, M. Stöcker, R. Schmidt, Micropor. Mesopor. Mater. 27 (1999) 181. [11] T. Welton, Chem. Rev. 99 (1999) 2071. [12] R. Sheldon, Chem. Commun. (2001) 2339. [13] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. (1998) 1765. [14] A.B. McEwen, S.F. McDevitt, V.R. Koch, J. Electrochem. Soc. 144 (1997) L84.
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[15] E.V. Dickinson, M.E. Williams, S.M. Hendrickson, H. Masui, R.W. Murray, J. Am. Chem. Soc. 121 (1999) 613. [16] Y. Zhou, H.S. Jan, A. Markus, Nano Lett. 4 (2004) 477; Y. Zhou, A. Markus, Adv. Mater. 15 (2003) 1452; Y. Zhou, A. Markus, Chem. Mater. 16 (2004) 544; Y. Zhou, Curr. Nanosci. 1 (2005) 35. [17] K. Zhu, P. Franc, D. Lawrence, M.R. Ryan, Micropor. Mesopor. Mater. 91 (2006) 40. [18] R.P. Emily, E.M. Russell, J. Am. Chem. Soc. 128 (2006) 2204; R.P. Emily, C.D. Andrews, P.S. Wheatley, P.B. Webb, P. Wormald, R.E. Morris, Nature 430 (2004) 1012. [19] H. Park, S.H. Yang, Y.S. Jun, W.H. Hong, J.K. Kang, Chem. Mater. 19 (2007) 535. [20] J. Cao, B. Fang, J. Wang, M. Zheng, S. Deng, X. Ma, Prog. Chem 17 (2005) 1028. [21] B.G. Trewyn, C.M. Whitman, V.S.Y. Lin, Nano Lett. 4 (2004) 2139. [22] T. Brezesinski, C. Erpen, K. Iimura, B. Smansly, Chem. Mater. 17 (2005) 1683. [23] T. Wang, H. Kaper, M. Antonietti, B. Smarsly, Langmuir 23 (2007) 1489. [24] D. Fuentes-Perujo, J. Santamaría-González, J. Mérida-Robles, E. RodríguezCastellón, A. Jiménez-López, P. Maireles-Torres, R. Moreno-Tost, R. Mariscal, J. Solid State Chem. 179 (2006) 2182. [25] X. Chen, L. Huang, Q. Li, J. Phys. Chem. B 109 (2005) 8731. [26] S.Y. Chen, L.Y. Jang, S. Cheng, Chem. Mater. 16 (2004) 4174. [27] B. Rakshe, V. Ramaswamy, S.G. Hegde, R. Vetrivel, A.V. Ratnasamy, Catal. Lett. 45 (1997) 41.
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Materials Science and Engineering C 28 (2008) 1217 – 1226 www.elsevier.com/locate/msec
Synthesis and characterization of nanosized micro-mesoporous Zr–SiO2 via Ionic liquid templating Aizhong Jia, Jie Li, Yinqing Zhang, Yajuan Song, Shuangxi Liu ⁎ Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, PR China Received 9 August 2007; received in revised form 24 September 2007; accepted 5 November 2007 Available online 13 November 2007
Abstract A series of micro-mesoporous Zr–SiO2 composites with nanoscale domains were prepared by using ionic liquid (IL) as a template at 373 K in only 3 h with one-step. The synthesized Zr–SiO2 materials were characterized by N2 adsorption–desorption, scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, diffuse reflectance UV–Vis spectroscopy, thermogravimetric analysis and temperature program desorption technologies. The results show that the synthesized composite materials possess nanoparticle with a mean diameter of about 100 nm and a large surface area more than 1000 m2/g, and the hierarchically porous structure is preserved after removing template by calcination at high temperature and treating in boiling water for 72 h. The heteroatom of zirconium has been successfully incorporated into the structure framework and/or has been highly dispersed on the surface of materials. The prepared materials contain moderate to strong acid sites and the surface acid site concentration is 0.18–0.42 sites/nm2. The amount of strong acid sites increases with a decrease of Si/Zr ratio, which leads to increased temperature for removing IL templates. © 2007 Elsevier B.V. All rights reserved. Keywords: Zr–SiO2; Ionic liquid template; Hierarchically micro-mesoporous structure; Composite material
1. Introduction Porous silicates materials are classified into three categories based on their pore diameters (D): microporous (D b 2 nm), mesoporous (2 nm b D b 50 nm), macroporous (50 nm b D) [1]. Zeolites are crystalline aluminosilicates with three-dimensional microporous structure, and are widely used in industry as catalysts or supports. They possess unique properties with respective to both activity and selectivity [2]. However, the microporous nature of zeolites limits their catalytic performance in catalytic reactions of bulky molecules, such as pharmaceutical, biological and fine chemical reactions, because of the severe restriction to the diffusion of the larger molecules in micropore channels [3]. Due to the above-mentioned drawback, expanding of the pore sizes of zeolite and zeotype materials from micropore to mesopore is of a high importance from both industrial application and fundamental understanding of porous ⁎ Corresponding author. Tel./fax: +86 22 23509005. E-mail address: [email protected] (S. Liu). 0928-4931/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2007.11.002
structures. A lot of researches have been devoted into the synthesis of mesoporous materials during the last decades [4–7]. Nevertheless, the amorphous characters of the mesoporous walls entail low hydrothermal stability and low acidity that have greatly diminished their functionalities in catalytic applications [4]. To combine the benefits of micro-and mesoporous molecular sieves, a kind of composite material containing both types of pores has been prepared [8–10]. However, it should be pointed out that most preparation methods reported for micromesoporous composite materials are complicated and timeconsuming. Room temperature ionic liquid (RTIL) possesses a wide range of liquid, negligible vapor pressure, high electrical conductivity, wide electrochemical windows, tolerance to strong acids, relatively low viscosity, high solvability, noncoordination and excellent thermal and chemical stability. As a “green” agent, RTIL has received much attention for organic chemistry reactions [11,12], separations [13] and electrochemistry applications [14,15]. As one category of surfactants, long chain RTILs, which possess both a hydrophilic ionic head and a
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hydrophobic organic chain, have been used as template to prepare porous materials recently. However, in contrast to their successful application in organic chemistry, inorganic synthesis with RTIL, especially in the synthesis of porous materials, is still in its infancy. To our knowledge, the reports on using RTIL as template for synthesizing porous materials are only limited in a few researchers [16–23]. Highly ordered monolithic supermicroporous lamellar silica has been prepared by Zhou et al. [16] via 1-hexadecyl-3-methyl-imidazolium chloride as template in acid-condition. By using the same template, mesoporous silica and Ru/SiO2 with high surface area have been synthesized by Zhu et al. [17] under strong acidic and basic conditions. Three zeolites of aluminophosphate and unknown framework structure of cobalt aluminophosphate have been prepared by Emily et al. [18] with using the ionic liquid 1methyl-3-ethyl imidazolium bromide as both the solvent and template. The acidity of purely siliceous solids are, in all cases, very low, while the incorporation of Zr(IV) into the siliceous framework produced an enhancement of the acidity [24]. As a promising catalytic material, zirconium-based catalysts have acquired wide applications ascribing to the acidity and oxidizing capabilities. In this paper, we report a simple and novel route for preparing of Zr–SiO2 materials with hierarchically micro-mesoporous structures and nanosize. In the preparing process, IL, tetraethoxysilane (TEOS) and zirconyl chloride are used as template, silicon source and zirconium source, respectively. The samples with different Si/Zr molar ratio are prepared, and their hydrothermal stability and acidity are also investigated. The materials are prospective for using as solid acid catalyst in reactions, in which bulky molecules attend. 2. Experimental section 2.1. Preparation of ionic liquid The synthesis procedures of ionic liquid followed a reported route [15]. As a typical synthesis of 1-hexadecyl-3methylimidazolium chloride (HMIMCl), 1-chlorohexadecane (HACl, 65.24 g, 0.25 mol) was mixed with 3-methylimidazole (MIM, 20.54 g, 0.25 mol) in a 250 mL flask and then refluxed for 24 h at 363 K. Cooled to room temperature, the product was dissolved into 200 mL of tetrahydrofuran (THF), and then the solution was put into refrigeratory for recrystallization. Twelve hours later, the 1-hexadecyl-3-methylimidazolium chloride was obtained after filtration, washed several times with a little amount of THF and vacuum-dried at room temperature. 2.2. Synthesis of hierarchically porous Zr–SiO2 In a typical synthesis procedure of composite material, 2.8 g of HMIMCl was dispersed into 70 g of deionized water under magnetic stirring with a fixed stirring rate. Then a certain amount of zirconyl chloride (ZrOCl2·8H2O) was added into the solution and 10.1 mL of TEOS was added dropwise to the solution 15 min later. After two hours, 26.5 g of 3.5 % sodium hydroxide solution was slowly added into the above mixture.
The resultant mixture solution was stirred at room temperature for 3 h in an open beaker, allowing the sol–gel reaction of the silica. Then it was transferred into a stainless steel autoclave and kept in an oven at 373 K for 3 h. The solid product was filtrated, washed, dried at room temperature and calcined at 823 K in air for 6 h to remove the template. The Si/Zr molar ratio in the final solid determined by ICP-AES is close to that present in the mother solution, thus the Si/Zr ratio of starting compositions was taken as Zr content in the text. According to the different dosage of the template and the difference of zirconium content, the materials were marked as “x-Zr–SiO2-y”, where x denotes the Si/Zr ratio of the starting composition and y denotes the amount of IL template. 2.3. Characterization Fourier transform infrared spectroscopic (FT-IR) analysis was conducted on a Bruker VECTOR22 spectrometer with a resolution of 1 cm− 1 for the framework of materials. It was conducted using KBr wafers (1 wt.% sample mixed with 99 wt.% KBr). The acid sites (Brönsted or Lewis) were determined by the pyridine-adsorption FT-IR spectrometry on self-supported wafers. The wafers were pre-heated at 673 K for 1.5 h under 10− 4 Torr vacuum, and then, after cooling to room temperature, the pyridine vapor was introduced. Finally, the samples were exposed to vacuum at 473 K for 1 h, and then the FT-IR spectra of pyridine adsorption were collected. In addition, to study the strength of the acid sites, the structural variations upon heating were analyzed at indicated pyridine desorption temperatures under vacuum (10 − 2 Torr) for 1 h. Scanning electron micrographs (SEM) were obtained by using Shimadzu SS-550 electron microscope. Transmission electron micrographs (TEM) were recorded on a TECNAI Philips T20ST electron microscope operating at 200 kV. Crystal structure was characterized by using a Rigaku D/MAX 2500 powder X-ray diffractometer (XRD) at a scanning speed of 4°/min, with Cu Kα line as incident radiation and a filter. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 with a heating rate of 10 K min− 1 from room temperature to 1073 K in an atmosphere of 90% N2 and 10% O2. Nitrogen adsorption– desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 volumetric apparatus. Specific surface areas were calculated following the BET procedure. Pore diameter was estimated by applying the DFT method to the adsorption branch of the isotherm. Temperature program desorption (TPD) of NH3 was carried out on an apparatus with a JS-3050 detector. Prior to the adsorption of NH3, ca. 100 mg of sample was first preheated at 773 K under flowing Ar for 0.5 h to remove physisorbed species, then cooled to 333 K. Subsequently, the sample was exposed to flowing ammonia gas mixture (5% NH3 in Ar) for 0.5 h, then purged by Ar gas for 1 h to remove excessive physisorbed ammonia. All NH3-TPD profiles were carried out by ramping the temperature from 333 to 1073 K at a rate of 10 K min−1. Diffuse reflectance UV–Vis (DR UV–Vis) spectra were measured with a Shimadzu UV–Vis 2550 spectrophotometer. The spectra were collected in the wavelength range from 190 to 800 nm.
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3. Results and discussion 3.1. Powder X-ray diffraction analysis The XRD patterns of samples synthesized under the optimal conditions (excepting the variable factor will be discussed) are given in Fig. 1. The XRD patterns shown in Fig. 1A reveal that the dosage of IL template has a significant influence on the forming of composite Zr–SiO2. Increasing the amount of IL template increases the intensity of XRD peaks. The structure parameters of the samples are shown in Table 1, from which a similar result to that of XRD is found by contrasting samples 1–5 that increasing the amount of IL template increases the surface area and the pore volume when the dosage of IL template is no more than 2.8 g. Whereas no evident change appears with a further increasing of IL template (N2.8 g). The results shown in Fig. 1B indicate that the material with fine structure can be obtained at 373 K, and too high or too low temperature is disadvantaged to form fine hierarchically porous structure. This is also confirmed by nitrogen sorption analysis, both high and low temperature led to lower surface area and lower pore volume
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(Samples 8 and 9), although the surface area and the pore volume of sample obtained at 353 K (Sample 9) are close to those of sample synthesized at 373 K (Sample 3). After crystallization at higher temperature, the mother liquid turns yellow, suggesting some of IL templates were decomposed. Increasing synthesis temperature not only affects the interactions between template and silica species but also enhances the density of mesoporous framework [25]. The lower surface area and the lower pore volume are thus attributed to decomposing of IL template and the higher synthesis temperature. 2.8 g of IL template and 373 K are received as the optimal parameters in this work. Fig. 1C gives the XRD patterns of calcined 100-Zr–SiO2-2.8 samples synthesized at 373 K with different crystallization time. After crystallization for only one hour, the porous structure has emerged in mother liquid, and fine structure porous material formed when the crystallization time reached to 3 h. With longer crystallization time, even to 192 h, no substantial change is observed. All of these indicate that pure phase of porous materials can be formed in 3 h and no other material phase appears under strong basic conditions for 192 h. So it is believed that the synthesized material has a good hydrothermal stability
Fig. 1. XRD patterns of samples (A) 100-Zr-SiO2-2.8 prepared with different amount of IL template at 373 K; (B) 100-Zr-SiO2-2.8 prepared at 353, 373, 393 K for 24 h; (C) 100-Zr–SiO2-2.8 prepared at 373 K for 1–192 h; (D) 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8, ∞-Zr–SiO2-2.8, and treated 100-Zr–SiO2-2.8.
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Table 1 Texture characteristics of various samples Sample
Code
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore size (nm)
Mesopore volume (b10 nm) (cm3/g)
Micropore volume (b2 nm) (cm3/g)
1 2 3 4 5 6 7 8 9 10
100-Zr–SiO2-0.9 100-Zr–SiO2-1.8 100-Zr–SiO2-2.8 100-Zr–SiO2-3.6 100-Zr–SiO2-14 10-Zr–SiO2-2.8 ∞-Zr–SiO2-2.8 100-Zr–SiO2-2.8a 100-Zr–SiO2-2.8b 100-Zr–SiO2-2.8c
774 980 1100 1160 1166 802 870 372 1006 956
0.51 0.81 1.16 1.02 1.09 1.10 0.76 0.31 0.98 0.84
3.2, 1.4, 0.68 3.2, 1.4 3.3, 1.4 3.2, 1.4 3.2, 1.4 2.8, 1.4 3.4, 1.4, 0.68 4.4, 3.4, 2.8, 1.4 3.2, 1.4 2.8, 1.4, 0.68
0.35 0.45 0.88 0.95 0.95 0.43 0.62 0.26 0.86 0.44
0.09 0.10 0.11 0.11 0.11 0.11 0.03 0.04 0.09 0.10
a
Synthesized at 393 K. Synthesized at 353 K. c Treated in boiling water for 72 h. b
and the optimal crystallizing time is 3 h. The rapid formation of Zr–SiO2 is related to the acidity self-generated in the aqueous solutions of the zirconium precursors which is the catalyst for TEOS hydrolysis [26]. A contrast among the XRD patterns of 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8, ∞-Zr–SiO2-2.8 and the treated 100-Zr– SiO2-2.8 is also presented in Fig. 1D, from which no peaks due to ZrO2 or any other crystalline impurity phases are found. So it is believed that the atoms of zirconium have been successfully incorporated into the framework and/or have been highly dispersed on surface of material, which is in very good agreement with that reported by Rakshe et al. [27] and Zhu et al. [28]. Sample of ∞-Zr–SiO2-2.8 displays a sharp peak in range of about 1.4–2.8°, which is characteristic of mesoporous materials and is associated with the (100) reflection when a hexagonal cell (similar to that described in the case of MCM41) is assumed [29], and two weak peaks in the range of about 3.6–4.0° and 4.2–4.6° (associated with (110) and (200) reflections). However, all samples doped with Zr display only one broadened peak in range of about 3.5–5.0° together with the intense (100) peak, indicating their pore structure are less ordered than that of ∞-Zr–SiO2-2.8. The XRD pattern of treated 100-Zr–SiO2-2.8 reveals that no serious structural degradation appears after hydrothermal treatment in boiling water for 72 h. Nevertheless, its peaks slightly shift to higher angles and the peak (3.5–5.0°) gets weak compared with parent material. This indicates shrinkage and weak ordering of the hierarchically porous structure by hydrothermal treatment. Comparing with 100-Zr–SiO2-2.8, the decrease of pore size and surface area for 10-Zr–SiO2-2.8 may be due to some Zr impurities highly dispersed on the pore surface.
condensation of silanol groups on the surface of material [30]. This result indicates that a large number of silianol groups may exist on the inner and exterior surface of ∞-Zr–SiO2-2.8. However, the surface dehydroxylation is not observed for 100Zr–SiO2-2.8 and 10-Zr–SiO2-2.8, because of the effective prevention by incorporating zirconium atom into the structure
3.2. Thermogravimetric analysis Typical thermograms and weight change derivatives obtained for ∞-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and 10-Zr– SiO2-2.8 are shown in Fig. 2. The total mass loss in the temperature range of 373–1073 K is 45.91%, 44.01% and 37.25%, respectively. Furthermore, a mass loss of about 1.57% in the temperature range of 850–1000 K is observed for ∞-Zr– SiO2-2.8 (Fig. 2A), which is due to the release of water through
Fig. 2. (A) TG curves of samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr– SiO2-2.8. (B) DTG curves of samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr–SiO2-2.8.
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because of the interaction between Si–OH and Zr–OH, and the OH groups are oriented perpendicular to the surface. The surface Si–Si distance on this surface is ca. 5 Å [31], moreover the distance is enlarged by incorporating zirconium, so the surface dehydroxylation is very difficult. All the samples display a two-stage template decomposition behavior, i.e., two major weight loss regions, (i) low temperature region and (ii) high temperature region are seen (see Fig. 2B). Furthermore, it can be easily found that the mass loss in the 2nd stage increases with an increase of Zr content in synthesized samples. Moreover, the temperature of the 2nd mass loss region is observed to shift to higher temperature from DTG curves. All of these can be attributed to the texture characteristics of the samples (see Table 1), whose pore size distribution displays two main regions (3.3, 1.4 nm) and the decreasing of mesopore size (from 3.3 to 2.8 nm) and mesopore volume (from 0.88 to 0.43 cm3/g) resulted by increasing of Zr content. 3.3. FT-IR analysis As suggested by Dalai et al. [32], information regarding the types of acid sites (i.e., Lewis or Bronsted acid sites) presented in the samples has been obtained from the IR spectra of the samples in the pyridine region (wave-number in the range of 1425–1775 cm− 1). The IR spectra of pyridine adsorbed samples are presented in Fig. 3A for samples 10-Zr–SiO2-2.8, 100-Zr–SiO2-2.8 and ∞-Zr–SiO2-2.8. As shown in the figure, the samples contained Zr exhibit five bands at frequencies of 1638, 1598, 1546, 1490, and 1446 cm− 1 in the pyridine region. The bands at a frequency of 1546 and 1638 cm− 1 represent the Brönsted acid sites, the bands at 1598 and 1446 cm− 1 can be assigned to Lewis acid sites, whereas the band at 1490 cm− 1 is due to a combination band associated with Brönsted and Lewis acid sites. The intensities of the bands at 1638, 1546, and 1490 cm− 1 increase as the Zr content increasing. Compared with Zr-doped samples, the pure silica material only contains
Fig. 3. (A) FT-IR spectra of pyridine adsorbed samples with different amount of Zr. (B) The results of pyridine desorption for 10-Zr–SiO2-2.8. (C) FT-IR spectra of samples ∞-Zr–SiO2-2.8 and 10-Zr–SiO2-2.8.
framework. Two OH groups on neighboring silicon atoms pointing toward each other are close enough to interact with each other at high temperature. But the incorporation of Zr leads to most of the OH groups being isolated on the surface silicon
Fig. 4. NH3-TPD profiles of calcined composite materials with different Si/Zr ratios: (a) 100, (b) 50, (c) 20, (d) 10.
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Fig. 5. Diffuse reflectance UV–Vis spectra of calcined composite samples with different Si/Zr ratios: (a) 100, (b) 50, (c) 20, (d) 10.
the Lewis acid sites (only two bands at 1598 and 1446 cm− 1), which is same to the result got by Fuentes-Perujo et al. [24]. A very broad band, which is due to the adsorbed H2O and the acidic bridge hydroxyl groups in all samples, is observed covering the region of 3000–3700 cm− 1, and its intensity sharply increases with adding the Zr atom into the samples. The
results of pyridine-desorption analysis for 10-Zr–SiO2-2.8 at different temperature are given in Fig. 3B, from which it can be seen that the material displays an intense pyridine absorption peak even after desorption at 673 K. This demonstrates that the strong acid sites exist in Zr-doped material. Fig. 3C illustrates the FT-IR spectra of 10-Zr–SiO2-2.8 in comparison with that of pure silica material prepared at the same conditions. The bands at around 1220, 1070, 790, and 470 cm− 1 are attributed to the typical Si–O–Si stretching and bending vibrations of condensed silica network. The band at around 960 cm− 1 is assigned to a combination band associated with the stretching mode of SiO4 entities bonded to Zr atoms (Si–O–Zr) and the stretching of non-condensed Si–OH groups, which is in good agreement with the reported results [33,34]. The broad band at 3400 cm− 1 and the strong peak around 1630 cm− 1 are due to the stretching and bending vibrations of adsorbed H2O. The work of Haskouri et al. [29] demonstrated that Si–O–Zr fragments and small ZrO2 domains coexist in the Zr-rich materials (Si/Zr ≤ 3), however, in the range of ∞ ≥ Si/Zr ≥ 5, Zr atoms are preferentially incorporated in tetrahedral sites isomorphously replacing Si atoms. According to their results, the ZrO2 displays up to four bands at 450, 500, 610, and 780 cm− 1 in the spectra window. From Fig. 3C, it will be found that the FT-IR spectrum of 10-Zr–SiO2-2.8 is very similar to that of pure silica material, and no deformation can be observed
Fig. 6. SEM micrographs of the samples: pure silica samples synthesized at (A) 373 K, (B) 393 K and the samples with zirconium (C) 100-Zr–SiO2-2.8, (D) 10-Zr–SiO2-2.8.
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in the 500–700 cm− 1 range. Taking into account all these results, it also implies that the Zr atoms have been incorporated into the framework of Zr–SiO2 material. 3.4. Temperature programmed desorption (TPD) of ammonia Because the pyridine-IR is limited in providing qualitative information (i.e., types) of the acid sites, it is conventionally combined with the NH3-TPD technology, which can provide additional qualitative information regarding to the overall concentration and strength of the acid sites. Fig. 4 shows the NH3-TPD profiles for calcined samples with different Zr content. It can be found that each sample exhibits a broad NH3 desorption peak starting from 473 K and extending beyond 773 K. According to the literature [32], the peak temperature is a representation of the strength of acid sites while the area under the peak represents the total amount of acid sites in the sample. The large temperature range of the broad peak indicates that the prepared materials contain moderate to strong acid sites. This is consistent with the results of pyridine desorption. By comparing the profiles of the samples with different Zr contents, it can be easily found that the strength of the peak at high temperature
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range (from 623 to 773 K, i.e., the strong acid sites) increases with the Si/Zr ratio decreasing, but the amount of strong acid sites are less than that of moderate acid sites. The surface acid sites concentration, which measured by the reported method [35], of materials with different Zr content is in range of 0.18–0.42 sites in each square nanometers on the samples surface. 3.5. DR UV–Vis spectroscopy DRUV–Vis spectroscopy is extensively used and perhaps one of the best techniques to detect the framework and extraframework zirconium species [36]. It can offer additional information on the dispersion and environment of Zr atoms inside materials. Shown in Fig. 5 are the UV–Vis spectra of the calcined samples with different Si/Zr ratio ((a) 10; (b) 20; (c) 50; (d) 100). In all cases, the spectra display an intensive absorption band at around 220–230 nm. An additional band at 200 nm is observed when the ratio of Si/Zr is ≥ 50. The absorption bands of isolated Zr4+, pure ZrO2 and nanoscopic regions ZrO2 were found at about 210, 230 and 250 nm, respectively [37]. So the absorption at 200 nm is attributed to ligand-to-matel charge transfer from an O2− to isolated Zr4+ ions in tetrahedral
Fig. 7. TEM images: (A), (B) (C) for 100-Zr–SiO2-2.8; (D) for treated 100-Zr–SiO2-2.8.
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configuration. This indicates the presence of isolated Zr4+ ions in the Zr–SiO2 mesoporous framework. The intensive absorption appears at about 220–230 nm, which is attributed to the half connectivity of Zr–O–Zr linkage, implying a possible formation of Zr–O–Zr–O–Si on the surface. Furthermore, in the present work, the characteristic peaks of ZrO2 could not be detected with high angle X-ray measurements. The results suggest that the broad band, which is similar to that got by Chaudhari et al. [38], is attributed to the presence of more than one type of zirconium species in the samples. 3.6. Electron microscopy In general terms, SEM provides the morphology information and TEM allows identification of the internal structure of the samples. Fig. 6A and B show the morphologies of the samples prepared at different temperature without zirconium atoms. It can be found that their particle sizes are large and nonuniform. The sample synthesized at low temperature has smooth surface and arched grain boundary, but the sample synthesized at higher temperature has more irregular grain and sharp grain boundary. It should be due to the change of growth rate at different
direction when temperature is increased. The Zr–SiO2 particles with about 100 nm have been obtained when a small quantity of zirconyl chloride is added into the initial materials (see Fig. 6C). Higher Zr content has no further remarkable effect on the morphologies of the products (see Fig. 6D). The hetero-atom effect, which is well-known that the hetero-atom has intensive influence to the formation of molecular sieve, especially to the morphology, is the major influence factor. TEM images of calcined 100-Zr–SiO2-2.8 and the sample treated in boiling water are shown in Fig. 7. The average wall thick of around 3.5 nm can be calculated from the scale bar (Fig. 7A). The nanometer particles of 100-Zr–SiO2-2.8, which was synthesized under the same conditions as large ∞-Zr–SiO22.8 particles, are found (Fig. 7B). Zhu et al. [17] believe that the imidazole head in the template is sensitive to the pH value of the media in tailoring the morphologies. Under strongly acidic/ basic conditions the chances of forming nanometer particles are lower, while under mild basic conditions only porous silica nanometer particles are prepared. But in our experiment, micrometer size SiO2 and nanometer size Zr–SiO2 materials have been synthesized in same basic conditions. So, it is not the synthesis conditions but the hetero-atom of zirconium that plays
Fig. 8. N2 adsorption–desorption isotherms and pore size distribution profiles (inset) of prepared samples: (A) ∞-Zr–SiO2-2.8, (B) 100-Zr–SiO2-2.8, (C) 10-Zr–SiO22.8 and (D) treated 100-Zr–SiO2-2.8.
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a predominant role on the forming of nanometer Zr–SiO2 materials. The hierarchically pore structure of the materials has been preserved after treatment in boiling water for 72 h (Fig. 7D), revealing that the material has a good hydrothermal stability, and the results consist with that of XRD. A lot of nanometer clusters are observed on the surface of 100-Zr–SiO22.8 (Fig. 7C), which is may be related to the good hydrothermal stability. 3.7. Nitrogen adsorption–desorption study The nitrogen adsorption–desorption isotherms and the pore size distributions (insets) for various samples are shown in Fig. 8. All of the isotherms, except the one for treated 100-Zr– SiO2-2.8, are of type IV with hysteresis loops. Steep rise at low relative pressure (p/p0) indicates the presence of micropores in micro-mesoporous composite. N2-physisorption isotherms for Zr–SiO2 exhibit two regions of hysteresis between the adsorption and desorption traces, from relative pressures (p/p0) of 0.2–0.35 and 0.40–0.60 (Fig. 8B). The hysteresis at (p/p0) of 0.2–0.35 is attributed to the inkwell pore geometry in which the mouth of the pore is smaller than its diameter. Normally, the isotherms of M41 s materials do not show such hysteresis. The hysteresis at the relative pressure range of 0.40–0.60 is similar to that described by Poladi et al. [39], which is due to the capillary condensation of mesopores. Guo et al. [1] and Bagshaw et al. [40] believe that the presence of an adsorption upturn at high p/p0 (N 0.8) suggests that the particles in the materials are not ‘solid’ but have open internal structure. By comparing the pore distributions of 100-Zr–SiO2-2.8 and 10-Zr–SiO2-2.8, it will be found that the mesopore size decreases from 3.3 nm to 2.8 nm (see Table 1), which indicates that the mesopore size get smaller with an increase of Zr content in the composite materials. The nitrogen adsorption–desorption curve for ∞-Zr –SiO2-2.8 (Fig. 8A) shows that the prepared pure silicate material has a BET surface area of 870 m2/g and a narrow mesopore size distribution of around 3.4 nm with a small quantity of micropore coexisting. The isotherms of treated 100-Zr–SiO2-2.8 (Fig. 8D) are very similar to type I, excepting the hysteresis loop at high relative pressure (p/p0 N 0.8). The absence of the hysteresis loops at relative pressures (p/p0) of 0.2–0.35 and 0.40–0.60 can be attributed to the condensation of pore size, which is demonstrated by the results of pore size distribution of the boiling water treated composite, when the composite is treated in boiling water. After treatment in boiling water for 72 h, a new distinct peak at 0.68 nm is observed in the pore size distribution curve of treated Zr–SiO2 composite, and the biggest average pore size decreases from 3.3 nm to 2.7 nm, the mesopore volume decreases from 0.88 cm3/g to 0.44 cm3/g, whereas the micropore volume has no obvious change (see Table 1). The sodium ions trapped in the framework walls of pure silica mesostructure material contribute to the structural instability of the framework [41], but the introduction of sodium salts can accelerate Si–O–Zr bond formation under hydrothermal conditions [42]. Moreover, the octahedrally coordinated Zr cation can form more metal-oxygen bonds than tetrahedrally
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coordinated Si, the framework has been compacted when the material is treated in boiling water, so the mesopore size and mesopore volume decrease after treatment. 4. Conclusions A series of nanosized composite materials Zr–SiO2 with hierarchical porosity have been successfully prepared through a one-step, conventional hydrothermal synthesis route, in which ionic liquid 1-hexadecyl-3-methylimidazolium halide is employed as template, tetraethoxysilane as a silicon source and zirconyl chloride as a zirconium source. The method introduced in this paper is a simple, potentially generic and time-saving method for fabricating composite materials. Various characterizations reveal that hetero-atom has been successfully incorporated into the framework structure and/or highly dispersed on surface of materials. The composite materials exhibit extraordinarily good hydrothermal stability and no collapse appears after removing template by calcination at high temperature and treating in boiling water for 72 h. The heteroatom Zr has a prominent effect on the morphologies of composite materials. The materials contain moderate to strong acid sites, the amount of strong acid sites increases with a decrease of the Si/Zr ratio and the concentration of specific surface acid sites is about 0.18–0.42 sites in each square nanometers on the samples surface. Acknowledgments The research work is supported by National Natural Science Foundation of China (Nos. 20773069 and 29973016) and The National Key Technologies R&D Program of China (2006 BAC02A12). References [1] X. Guo, N. Xue, S. Liu, X. Guo, W. Ding, W. Hou, Micropor. Mesopor. Mater. (2007), doi:10.1016/j.micromeso.2007.03.003. [2] A. Corma, Chem. Rev. 95 (1995) 559. [3] M. Mazaj, N.Z. Logar, G. Mali, N.N. Tusar, I. Arcon, A. Ristic, A. Recnik, V. Kaucic, Micropor. Mesopor. Mater. 99 (2007) 3. [4] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [5] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmit, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgius, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [6] D.W. Lee, S.J. Park, S.K. Ihm, K.H. Lee, J. Non-Cryst. Solids 353 (2007) 1501. [7] W. Li, M.O. Coppens, Chem. Mater. 17 (2005) 4560. [8] B.L. Newalkar, H. Katsuki, S. Komarneni, Micropor. Mesopor. Mater 73 (2004) 161. [9] P. Prokesova, S. Mintova, J. Cejka, T. Bein, Mater. Sci. Engine. C 23 (2003) 1001. [10] A. Karlsson, M. Stöcker, R. Schmidt, Micropor. Mesopor. Mater. 27 (1999) 181. [11] T. Welton, Chem. Rev. 99 (1999) 2071. [12] R. Sheldon, Chem. Commun. (2001) 2339. [13] J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem. Commun. (1998) 1765. [14] A.B. McEwen, S.F. McDevitt, V.R. Koch, J. Electrochem. Soc. 144 (1997) L84.
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