Synthesis of mesoporous aluminosilicates with low Si/Al ratios using a single-source molecular precursor under acidic conditions

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1 J Porous Mater (2006) 13: DOI /s Synthesis of mesoporous aluminosilicates with low Si/Al ratios using a single-source molecular precursor under acidic conditions Ying Li Qihua Yang Jie Yang Can Li C Science + Business Media, LLC 2006 Abstract A single molecular precursor, bis(sec-butoxy)- aluminoxy-triethoxysilane, was used as aluminum source for the synthesis of mesoporous aluminosilicates with Si/Al ratios ranging from 1 to 10 under acidic conditions. Aluminum can be stoichiometrically incorporated into the mesoporous materials at ph = 1.5. The mesoporous aluminosilicates synthesized with the single molecular precursor display larger unit cell parameter and pore diameter than that of the materials prepared with aluminum sulfate, aluminum nitrate, aluminum hydroxide, and aluminum isopropoxide as the aluminum sources. IR spectra of adsorbed pyridine and NH 3 - TPD showed that both Brönsted acidic sites and Lewis acid sites with medium strength are present in the materials. However, no direct relationship between the acidic properties and the content of aluminum was observed when the Si/Al ratios of the mesoporous aluminosilicates are lower than 10. Keywords Mesoporous Aluminosilicates. Bis(sec-butoxy)aluminoxy-triethoxysilane. Si/Al ratio. Acidity 1. Introduction Mesoporous aluminosilcates with high surface area and welldefined pore structure have attracted great attention in the fields of separation and catalysis, especially for catalytic reactions involving large molecules [1 3]. In the family of Y. Li. Q. Yang ( ). J. Yang. C. Li ( ) State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian , China {yangqh, canli}@dicp.ac.cn mesoporous materials, SBA-15 synthesized with triblock copolymer as the template under strong acidic conditions exhibits larger pore size and thicker pore wall compared with M41S [4]. The improved hydrothermal and thermal stability make them one of the most promising catalytic materials. However, it is very difficult to directly synthesize aluminum-substituted SBA-15 because of the facile dissociation of Al-O-Si bonds under strong acidic hydrothermal conditions [5 7]. Much effort has been made toward the direct synthesis of Al-SBA-15. Yue and co-workers first reported the direct hydrothermal synthesis of Al-SBA-15 by using tetraethyl orthosilicate (TEOS) and aluminum tri-tert-butoxide as silicon and aluminum precursors, respectively. However, the aluminum content in the calcined product was not reported [5]. Our group reported the synthesis of Al-SBA-15 via a hydrolysis-controlled approach. The lowest Si/Al ratio in the calcined Al-SBA-15 product was found to be 10 [7]. Xiao and co-workers recently reported a ph-adjusting method to graft Al and Ti atoms on SBA-15. The lowest Si/Al ratio in the calcined sample was observed to be 2.4 [8]. Vinu and coworkers found that the type of aluminum sources plays an important role in controlling the Si/Al ratio of aluminosilicate SBA-15 materials and aluminum isopropoxide is the most efficient aluminum precursor for incorporating aluminum into SBA-15 under identical synthesis conditions in acidic medium [9]. Recently, Al-SBA-15 materials with Si/Al ratio of were synthesized under very strong acidic conditions (2 M HCl solutions) [10]. As far as we know, a method that can allow stoichiometrical incorporation of aluminum into SBA-15 has not been available. Therefore, it remains a big challenge to directly synthesize Al-SBA-15 with high aluminum content in one step in acidic medium. In addition, the characterization of the acidic properties of Al- SBA-15 with low Si/Al ratios (Si/Al = 1 10) has been hardly

2 188 J Porous Mater (2006) 13: reported because of the difficulty in synthesis of Al-SBA-15 materials with low Si/Al ratios. Single-source molecular precursors containing M-O-M linkage have been employed in the synthesis of multicomponent metal oxides. It could be possible to precisely control the structure and active site in the materials by using the single molecular precursor [11 16]. In this paper, a single-source molecular precursor ((sec-buo) 2 AlOSi(OEt) 3 ) (containing preformed Si-O-Al linkages) was used as both aluminum and silicon sources to synthesize Al-SBA-15 with low Si/Al ratios under acidic conditions via a template-assisted approach. This work demonstrates that aluminum in the single molecular precursor can be stoichiometrically incorporated into SBA-15 even at ph = 1.5. FT-IR spectra of pyridine adsorption and NH 3 - TPD show that both Brönsted and Lewis acidic sites with medium strength is present in the materials. 2. Experimental sections 2.1. Synthesis Di-s-butoxyaluminoxytriethoxysilane ((sec-buo) 2 AlOSi- (OEt) 3 ) was purchased from Gelest (Germany). Tetraethyl orthosilicate (TEOS) and nonionic triblock copolymer surfactant EO 20 PO 70 EO 20 (P123) were purchased from Aldrich (USA). All chemicals were used directly without further purification. Typical preparation procedure of Al-SBA-15 is described as follows: 1gofP123 was dissolved in 37.5 ml of HCl solution with different ph values to get a solution A. A mixture of 2.3 ml of TEOS and 0.4 g of di-s-butoxyaluminoxytriethoxysilane (Si/Al = 10) was added to solution A. The above mixture was stirred vigorously for 20 h at 40 C. Then it was transferred into an autoclave and heated at 100 C for 24 h. The resultant solid was filtered, washed and dried at 60 C for 15 h. The surfactant was removed by calcination of the as-synthesized materials at 500 C for 10 h. The samples synthesized at different ph values were donated as Al-SBA-15(10)-n, where n (n = 0, 1.5 and 2.0) refers to the ph value of the HCl solution. To prepare Al-SBA-15 with Si/Al ratios of 1, 3 and 5, the HCl solution with ph value of 1.5 was used. Other steps were the same as the preparation of Al-SBA-15(10)-n. These samples were denoted as Al-SBA-15(m), where m (m = 1, 3 and 5) refers to the Si/Al molar ratio of the initial gel mixture Characterization XRD patterns were recorded on a Rigaku D/Max 3400 powder diffraction system using Cu Kα radiation (40 kv and 30 ma) over the range 0.5 2θ 8 degrees. The nitrogen sorption experiments were measured at 196 C on ASAP 2000 system in static measurement mode. The samples were outgassed at 300 C for 10 h before the measurement. The pore size distribution curves were obtained from the analysis of the desorption branch of the isotherms using BJH (Barrett-Joyner-Halenda) method. Transmission electron microscopy (TEM) measurements were taken on a JEM- 2000EX electron microscope (JEOL Japan) with an acceleration voltage of KV. 27 Al MAS NMR spectra were recorded at MHz on a Bruker DRX-400 spectrometer equipped with a magic angle spin probe at room temperature. AlCl 3 6H 2 O was used as reference. IR spectra of adsorbed pyridine were taken on a Thermo Nicolet NEXUS 470 FT-IR spectrometer. The samples were made into self-supporting wafers and were evacuated in an IR cell at 400 C for 100 min. IR background spectra were recorded after the samples were cooled down to room temperature. Pyridine was then admitted into the IR cell and the IR spectra of adsorbed pyridine were recorded after a degassing at 150 C for 30 min. NH 3 -TPD measurements were carried out in a flow reactor. Samples were activated at 700 Cfor1hinaflow of helium, subsequently, ammonia was introduced by a He stream containing 10 (vol)% of ammonia at 100 C. The physically adsorbed NH 3 was removed by purging with a helium flow at 100 C until the baseline was flat. Then the reactor temperature was ramped at a rate of 10 C/min. 3. Results and discussion 3.1. Effect of the ph value of the solution on the synthesis of Al-SBA-15 It has been reported that the acidity of the initial solution is a decisive factor for the successful incorporation of aluminum into SBA-15 [7, 9]. The Al-O-Si bond in the single molecular precursor may dissociate if the acidity of the synthesis medium is too high. Therefore, the effect of ph value of the solution on the synthesis of Al-SBA-15 was investigated first in the present work. The XRD patterns of calcined Al-SBA- 15(10)n prepared at different ph values are given in Fig. 1. Three well-resolved diffraction peaks were observed in XRD patterns of samples prepared at ph = 1.0 and 0, which are indexed as (100), (110) and (200) reflections of a hexagonal symmetry lattice (p6mm), indicating that the materials have typical SBA-15 structure [4]. The sample prepared at ph = 1.5 exhibits one broad diffraction peak (2θ = 0.72 ) with a shoulder at 2θ = 1 2. Figure 2 gives the N 2 sorption isotherms of the samples prepared at ph = 1.0 and 1.5. It can be seen that the isotherm of the sample synthesized at ph = 1.0 is type IV with a H1 hysteresis loop, which is

3 J Porous Mater (2006) 13: Fig. 3 TEM images of the calcined Al-SBA-15 with Si/Al ratios of 10 (prepared at ph = 1.5) Fig. 1 Powder XRD patterns of calcined Al-SBA-15 (Si/Al ratio = 10) prepared at different ph values (ph = 0 1.5) of the solution characteristic of mesoporous materials with 2D-hexagonal structure. A well-defined capillary condensation step occurs at relative pressure of , indicating the uniform pore size distributions. The isotherms of the sample synthesized at ph = 1.5 is type IV with unclosed hysteresis loop at P/Po > 0.8, indicating the existence of the interparticle pores or some impure phases. From the results of XRD and N 2 sorption isotherm, it is difficult to clarify the mesostructure of the sample prepared at ph = 1.5. Therefore, TEM technique was used to have a deep insight into the structural order of this sample (Fig. 3). Most part of the sample shows well-ordered hexagonal arrays of mesopores with one-dimensional channels, while mesocellular foam structures were also observed for this sample. The existence of the mesocellular foam structure explains the broad XRD pattern and unclosed hysteresis loop of N 2 sorption isotherms at P/Po > 0.8. The shift of the diffraction peak to lower angle indicates that the sample synthesized at higher ph has larger cell parameter than those synthesized at lower ph value. The large cell parameter may be due to the incorporation of preformed Si-O-Al bond of the precursor in the materials at ph = 1.5. The Si/Al ratios for the samples synthesized at ph values of 0 and 1.0 are and 79.47, respectively. The extremely low content of aluminum in the materials synthesized in strong acidic medium is due to the dissociation of Si-O-Al bond in the single molecular precursor. The Si/Al ratio of the sample prepared at ph = 1.5 is 9.50, implying that the dissociation of Al-O-Si bond is negligible. From the above results, we can know that aluminum in the single molecular precursor can be stoichiometrically incorporated into the mesoporous materials when the initial ph of the solution is 1.5. Therefore, Al-SBA-15 materials with different Si/Al ratios were synthesized at ph = 1.5 to ensure the stoichiometrically incorporation of aluminum into the SBA Effect of Si/Al ratio on the structure of Al-SBA-15 The elemental analysis of all the samples shows that the Si/Al ratios in the calcined samples are almost the same as that in the initial gel mixture (Table 1). Many kinds of aluminum precursors have been used for the preparation of Al-SBA-15, such as Al 2 (SO 4 ) 3, Al(NO 3 ) 3, Al(OH) 3 and aluminum isopropoxide [9]. The Si/Al ratio of the final product strongly depends on the aluminum source used. Keeping the molar ratio of Si/Al in the initial gel mixture of 7, the Si/Al ratios of the final materials is 258, 152, 116 and 45 using aluminum Fig. 2 Nitrogen adsorptiondesorption isotherms and the pore size distributions for the calcined Al-SBA-15 with Si/Al ratios of 10 prepared at ph values of 1.0 (A) and 1.5 (B)

4 190 J Porous Mater (2006) 13: Table 1 Texture properties and Si/Al ratios of calcined mesoporous Al-SBA-15 Si/Al Si/Al in Surface area Pore volume Pore diameter Wall Thickness Samples ph in gels products 1 2 a 0 (m 2 /g) (cc/g) (nm) 3 (nm) Al-SBA-15(1) Al-SBA-15(3) Al-SBA-15(5) Al-SBA-15(10) Al-SBA-15(10) Al-SBA-15(10) Al-SBA SBA The Si/Al ratios in the calcined materials were obtained by ICP-AES method. 2 a 0 values were calculated by d 100,a 0 = 2 d 100 / 3. 3 The pore diameters were calculated by the desorption branch of the isotherms according to the BJH method. 4 From Reference 5. sulfate, aluminum nitrate, aluminum hydroxide, aluminum isopropoxide as aluminum sources, respectively [9]. The Al- SBA-15 synthesized with the single-source molecular precursor has almost the same Si/Al ratio in the products as that in the initial gel mixture even for the sample with Si/Al ratio of 1. These results confirm that the single-source molecular precursor with preformed Al-O-Si bonds is an efficient precursor to prepare Al-SBA-15 with high aluminum content in acidic medium (ph = 1.5). The XRD patterns of the calcined Al-SBA-15 with Si/Al ratios of 1, 3 and 5 are given in Fig. 4. One broad diffraction peak at 2θ = is observed for all the samples. The TEM images of Al-SBA-15(5) exhibit hexagonal arrangement of the mesopores (Fig. 5), implying that this sample has hexagonal mesostructure. No ordered arrangement of the mesopores could be observed for Al-SBA-15(3) and Al- SBA15(1) from their TEM images (which is not given here). These results indicate that the structural order of the materials Fig. 4 Powder XRD patterns of calcined Al-SBA-15 with Si/Al ratios of 1, 3 and 5 (prepared at ph = 1.5) Fig. 5 TEM images of the calcined Al-SBA-15 with Si/Al ratios of 5 (prepared at ph = 1.5) deteriorates with increasing the aluminum contents in the materials. The unit cell parameters of the Al-SBA-15 materials increase with the decreasing of Si/Al ratios. This result implies that the materials prepared with the single molecular precursor tend to have large unit cell parameter. Figure 6 displays the adsorption-desorption isotherms and pore size distributions (inset) of Al-SBA-15 with Si/Al ratios from 1 to 5. The isotherms of the samples with Si/Al ratio of 5 and 3 are typical type-iv with H1 hysteresis loops, which is characteristic of mesoporous materials [4]. The hysteresis loop at P/Po > 0.8 is corresponding to the capillary condensation in the interparticle pores or some impurity phases [17]. The sample with Si/Al ratio of 3 also exhibits type IV isotherm pattern with less pronounced capillary condensation step, implying less ordered mesostructure, which is consistent with the XRD results. The N 2 sorption isotherms of the sample with Si/Al ratio of 1 (Fig. 6(a)) are of typical type IV with H2 hysteresis loop, which is characteristic of wormhole-like structure of MSU-type materials with uniform pore size distributions [18]. The specific surface area of the samples with Si/Al 5 is above 500 m 2 /g with large pore diameter (>9.0 nm calculated from the desorption branch of the isotherm) and large pore volume (>1.0 cm 3 /g) (Table 1).

5 J Porous Mater (2006) 13: Fig. 6 Nitrogen adsorptiondesorption isotherms and the pore size distributions for the calcined Al-SBA-15 with Si/Al ratios of 1, 3 and 5 (prepared at ph = 1.5) V ol adsorbed ( cc / g S TP ) Adsorption Desorption Si/Al = 1 dv/dlog(d) Pore diameter (nm) P/Po sorb d ( c c/ g S TP ) Vol ad e dv/dlog(d) Pore diameter(nm) Si/Al = Adsorption Desorption P/Po Vol a d so r b e d (c c/ g S TP) Si/Al = Pore diameter (nm) 200 Adsorption 100 Desorption P/Po dvd/olg(d) The specific surface area of the sample with Si/Al ratio of 3 is decreased to 319 m 2 /g. In summary, the pore diameter and unit cell size of the materials are increased with the decreasing of Si/Al ratio in the final product. The pore diameter of the samples synthesized using single molecular precursor is much larger than that of the sample synthesized with aluminum tri-tert-butoxide or aluminum isopropoxide as precursors Coordination environment of aluminum in Al-SBA-15 The coordination environment of Al atoms in the calcined Al-SBA-15 was characterized by 27 Al MAS NMR (Fig. 7). The spectra of samples with Si/Al of 10 and 5 show two resonance peaks at 0 and 50 ppm, which can be assigned to octahedrally coordinated extra-framework aluminum and tetrahedral coordinated framework aluminum, respectively. In addition to the resonance at 0 and 50 ppm, the spectra of the samples with Si/Al ratios of 1 and 3 show another resonance peak at 37 ppm, which can be assigned to distorted penta-coordinated aluminum species. The penta-and hexa- coordinated aluminum atoms are formed by the calci- Fig Al MAS NMR spectra of the as-synthesized Al-SBA-15 with Si/Al ratios of 3 nation process [5, 9], which is evidenced by the 27 Al MAS NMR spectra of the as-synthesized Al-SBA-15 with Si/Al ratios of 3 (Fig. 8), only signals of tetra-coordinated framework aluminum and hexa-coordinated extraframework aluminum atoms are observed in the as-synthesized Al-SBA-15 materials Acidity characterization of Al-SBA-15 To evaluate the strength and type of acidic sites of Al-SBA- 15 with low Si/Al ratios, pyridine adsorption on the materials was studied by FT-IR. Figure 9 shows the spectra of pyridine Fig Al MAS NMR spectra of the calcined Al-SBA-15 with Si/Al ratios of 1, 3, 5 and 10 Fig. 9 IR spectra of pyridine adsorbed on Al-SBA-15 with Si/Al ratio of 1, 3, 5 and 10 after pre-treated at 150 C

6 192 J Porous Mater (2006) 13: Table 2 The amount of acid sites estimated based on IR spectra of adsorbed pyridine on Al-SBA-15 materials after pre-treated at 150 C a Weight C(B) C(L) Samples Si/Al (mg) (mmol/g) (mmol/g) B/L Al-SBA-15(1) Al-SBA-15(3) Al-SBA-15(5) Al-SBA-15(10) a The amount of acidic sites was calculated by the following formula 20 : C (pyridine on B sites) = 1.88 I A (B) R 2 /W C (pyridine on L sites) = 1.42 I A (L) R 2 /W C = concentration (mmol/g) of acidic sites on the materials I A (B or L) = integrated absorbance of Brönsted or Lewis band (cm 1 ) R = radius of catalyst disk (cm) W = weight of disk (mg) Fig. 10 NH 3 -TPD patterns of the calcined Al-SBA-15 with Si/Al ratios of 1, 3, 5 and 10 adsorbed on Al-SBA-15 with Si/Al ratios of 1, 3, 5 and 10 pretreated at 150 C. All the samples with Si/Al ratios of 1, 3, 5 and 10 give the IR bands due to pyridine adsorbed on Lewis acidic sites (1455 and 1623 cm 1 ), pyridine adsorbed on Brönsted acidic sites (1547 and 1640 cm 1 ), and pyridine associated with both Lewis and Brönsted acidic sites (1490 cm 1 ) [19, 20]. The amount of Brönsted and Lewis acidic sites of Al- SBA-15(m) (m = 1, 3, 5 and 10) was calculated according to the formula reported in the reference (Table 2) [20]. It can be seen that the B/L ratio of Al-SBA-15(1) is 0.6. The B/L ratios of both Al-SBA-15(3) and Al-SBA-15(5) are 2.0 and the B/L ratio of Al-SBA-15(10) is 1.3. These results indicate that the samples with Si/Al ratios of 3, 5 and 10 have more Brönsted acidic sites than Lewis acidic sites. The sample with a Si/Al ratio of 1 has less Brönsted acidic sites than Lewis acidic sites. This result indicates the mesoporous aluminosilicates with lower Si/Al ratios (Si/Al ratio less than 3) does not result in materials with higher amount of the Brönsted acidic sites. The difference in the acidity of Al-SBA-15(3) and Al-SBA- 15(5) is not observed although the coordination environments of aluminum in these two samples are quite different according to 27 Al MAS NMR characterization. These results may indicate that the acidity of the mesoporous aluminosilicates cannot be simply judged by the coordination environments of the aluminum atoms because most of the aluminum atoms in the pore wall are buried in the amorphous and thicker pore wall of the mesoporous aluminosilicates. The acidity of Al-SBA-15 with Si/Al ratios of 1, 3, 5 and 10 was also measured by NH 3 -TPD technique. Figure 10 gives NH 3 -TPD patterns of the Al-SBA-15(m) with Si/Al ratios of 1, 3, 5 and 10. The NH 3 -TPD patterns of the calcined Al-SBA-15 with Si/Al ratios of 1, 3, 5 and 10 show desorption peak of NH 3 in the range of 120 to 500 C, corresponding to the acidic sites with medium strength 7. The total acidity of all samples is not as large as the difference of the aluminum content and the amount of acid sites doesn t run parallel with the aluminum content. Generally, the amount of acidic sites of zeolite increases with the aluminum content. However, this is not the case for Al-SBA-15 with Si/Al ratios lower than 10. This result can be explained by the thick and amorphous pore wall of the mesoporous materials. Some of the aluminum atoms in Al-SBA-15 are embedded in the thick wall, which are not accessible to the probe molecule. From the above analysis, we can know that the acidic properties of the mesoporous aluminosilicates cannot be judged by the coordination environment and content of aluminum because of their amorphous and thick pore wall. 4. Conclusions Al-SBA-15 with high content of aluminum (Si/Al = 1 10) has been synthesized using a single-source molecular precursor (sec-buo) 2 AlOSi(OEt) 3 as aluminum source. The aluminum can be stoichiometrically incorporated into SBA-15 at ph = 1.5 by using (sec-buo) 2 AlOSi(OEt) 3. The materials synthesized using the single-source molecular precursor tend to have larger unit cell parameter and pore diameter than those of the samples prepared with other aluminum precursors. The Al-O-Si bonds are distorted in Al-SBA-15 with high content of aluminum (Si/Al < 5). Both Brönsted acidic sites and Lewis acid sites with medium strength are derived from aluminum incorporated in SBA-15, however, there is no direct relationship between the acidic properties and content of aluminum atoms when the Si/Al ratio of the mesoporous Al-SBA-15 is lower than 10. Acknowledgment This work was financially supported by the National Basic Research Program of China (2003CB615803, 2003CB615806), National Natural Science Foundation of China ( , ) and the Talent Science Program of the Chinese Academy of Sciences.

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