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Langmuir 2002, 18, 8714-8718
Notes Effect of Coexisting Anions on the Stabilization of Porous Alumina Templated by Dodecyl Sulfate Assemblies Mitsunori Yada,*,† Shunsuke Kuroki,‡ Masako Kuroki,§ Kaoru Ohe,‡ and Tsuyoshi Kijima‡ Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan, Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan, and Department of Applied Physics, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan Received February 1, 2002. In Final Form: August 5, 2002
Introduction Since the discovery of MCM-411 and FSM-16,2 increasing attention has been paid to a family of mesoporous materials with a large internal surface area and narrow pore size distribution, because of their applicabilities as catalysts, molecular sieves, and host materials. Such mesoporous materials can be obtained by the calcination of their mesostructured precursors templated by organic molecules. The templating approach has been also applied to the synthesis of porous aluminas,3-6 since they are promising as an adsorbent and a catalytic material with solid acidity. Although Vaudry et al.,3 Bagshaw et al.,4 and Cabrera et al.5 synthesized micro- and mesoporous aluminas, their aluminum source was aluminum isopropoxide, which is expensive and needs careful treatment because it tends to be easily and rapidly hydrolyzed. The use of inexpensive aluminum salts such as aluminum nitrate or chloride for the synthesis of porous alumina would be therefore more desirable and even more important. Yang et al.6 reported the synthesis of mesoporous alumina with a pore size of 10 nm by using aluminum chloride, but its specific surface area was as low as 300 m2 g-1 due to its inorganic wall being thick. In contrast, the authors7 have found that a hexagonal mesostructured alumina templated by dodecyl sulfate assemblies can be * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry and Applied Chemistry, Saga University. ‡ Department of Applied Chemistry, Miyazaki University. § Department of Applied Physics, Miyazaki University. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (3) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (4) (a) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (b) Zhang, W.; Pinnavaia, T. J. Chem. Commun. 1998, 1185. (5) Cabrera, S.; Haskouri, J. E.; Alamos, J.; Beltra´n, A.; Beltra´n, D.; Mendioroz, S.; Marcos, M D.; Amoro´s, P. Adv. Mater. 1999, 11, 379. (6) (a) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 512. (b) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (7) (a) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (b) Yada, M.; Hiyoshi, H.; Ohe, K.; Machida, M.; Kijima, T. Inorg. Chem. 1997, 36, 5565.
synthesized by the urea-based homogeneous precipitation method using aluminum nitrate as an aluminum source and sodium dodecyl sulfate as a templating agent. However, the removal of the surfactant species from the hexagonal mesostructured alumina resulted in the collapse of its framework structure. Previously, we also reported that the morphological property of the mesostructured alumina is largely affected by anions added to the reaction mixture and that the additional anions with strong affinity for Al3+ serve to inhibit the formation of an ordered hexagonal structure.8 The previous report, however, described no detailed information such as transmission electron microscopy (TEM) images, compositions, and the possibility of its conversion into a porous solid. Furthermore, the previous research for the synthesis of mesostructured metal oxides has focused on the mixing ratio of starting materials or the characteristics of surfactant species, but little attention has been paid to the effect of coexisting anions in the reaction mixture. In this paper, we report that the addition of several kinds of anions (F-, H2PO2-, SO42-) to an aluminum-based reaction mixture is effective for the synthesis of porous aluminas with a large specific surface area and narrow pore size distribution. Experimental Section To study the effect of SO42- for the synthesis of mesostructured alumina, Al2(SO4)3‚16H2O was used as the aluminum source. Sodium dodecyl sulfate (CH3(CH2)11OSO3Na) was used as the templating agent. Aluminum sulfate, sodium dodecyl sulfate, urea, and water were mixed at a molar ratio of 1:2:20:40. On the other hand, when NaH2PO2, Na4P2O7, Na2HPO4, Na3PO4, NaH2PO4, Na2HPO3, Na2SiO3, Na4SiO4, Na2C2O4, CH3COONa, HCOONa, or NH4F was used as the coexisting anion, aluminum nitrate nonahydrate (Al(NO3)3‚9H2O) was used as the aluminum source, since NO3- is known as an anion with low affinity for Al3+.9 In these systems, aluminum nitrate, the coexisting anion source, sodium dodecyl sulfate, urea, and water were mixed at a molar ratio of 1:x:2:30:60 (x ) 0.2-1.0). The mixture was stirred at 40 °C for 1 h to obtain a transparent mixed solution. Urea was used to gradually raise the pH of the reaction mixture because it releases ammonia through hydrolysis on heating above 60 °C. The solution was thus heated at 80 °C and then kept at that temperature. The pH of the reaction mixture increased from 3.6 to 7.0 or above, due to the enhanced hydrolysis of urea, while precipitation occurred. After 20 h of reaction time, the resulting mixture was immediately cooled to room temperature to prevent further precipitation by hydrolysis of urea. An attempt was made to remove the surfactant species from the above solids by anion exchange with acetate anions, as reported by Holland et al.10 The solid (0.5 g) was mixed with 0.05 M ethanol solution of sodium acetate (40 mL) and then stirred at 40 °C for 1 h. Powder X-ray diffraction (XRD) measurement was made on a Simadzu XD-D1 diffractometer with Cu KR radiation. TEM was carried out using (8) Yada, M.; Hiyoshi, H.; Machida, M.; Kijima, T. J. Porous Mater. 1998, 5, 133. (9) (a) Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes; Special Publication No. 17; The Chemical Society: London, 1964. (b) Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes; Special Publication No. 25; The Chemical Society: London, 1971. (10) Holland, B. T.; Isbester, P. K.; Blanford, C. F.; Munson, E. J.; Stein, A. J. Am. Chem. Soc. 1997, 119, 6796.
10.1021/la020113r CCC: $22.00 © 2002 American Chemical Society Published on Web 09/28/2002
Notes
Langmuir, Vol. 18, No. 22, 2002 8715
Figure 1. XRD patterns of the as-prepared mesostructured alumina synthesized in the presence of SO42- (a), F- (c), and H2PO2- (e); XRD patterns of the anion-exchanged mesostructured alumina synthesized in the presence of SO42- (b), F- (d), and H2PO2- (f). a Hitachi H-800MU. X-ray microanalysis (EDX) was conducted by a HORIBA EMAX-5770. Infrared (IR) absorption spectra were measured by the KBr pellet method using a Nippon Bunko FT/ IR-300. Scanning electron microscopy (SEM) was carried out on a Hitachi H-4100M. 27Al MAS (magic angle spinning) NMR spectra were measured on a Bruker AC-250 with a MAS frequency of 3 kHz. Specific surface area was calculated by the BrunauerEmmett-Teller (BET) method,11 and pore size distribution was determined by the Cranston-Inkley method,12 a modified procedure of the Barrett-Joyner-Halenda (BJH) method.
Results and Discussion The addition of the divalent SO42- anion was found to have a marked influence on the mesostructure of the resulting solid. As shown in Figure 1a, the XRD pattern of the as-prepared mesostructured alumina in the SO42anion added system gave a shoulder peak at 2θ of as low as ca. 2° along with a halo band at 2θ ∼ 20°, in contrast to definite reflections for the hexagonal mesostructured alumina with a unit cell parameter of a ) 4.3 nm or d100 ) 3.8 nm synthesized from the SO42- free reaction mixture.7 The TEM image of the former solid (Figure 2a) also indicated a less ordered mesostructure in comparison with the latter.7 In the IR spectrum of the former, a broad peak at 1130 cm-1 attributable to SO42- was observed, along with those at 2910 and 2844 cm-1 due to the -CH2group, 2956 cm-1 due to the -CH3 group, and 1200-1300 cm-1 due to the -OSO3- group, as shown in Figure 3a. The broad peak at 1130 cm-1 was not observable for the hexagonal mesostructured alumina7 or the SO42- free system. The S to Al molar ratio determined by EDX was 0.33. On the other hand, upon the anion exchange of the as-prepared solid with acetate ions, the resulting solid showed clearly a single peak at 2θ ) 1.9° or d ) 4.8 nm in the XRD pattern (Figure 1b). Complete removal of surfactant species from the as-prepared solid by the anion(11) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (12) Cranston, R. W.; Inkley, F. A. Adv. Catal. 1957, 9, 143.
Figure 2. TEM images of the as-prepared mesostructured alumina synthesized in the presence of SO42- (a), F- (b), and H2PO2- (c).
exchange treatment was confirmed by an IR spectrum indicative of no absorption peaks due to the -CH3 and -CH2- groups (Figure 3b). The S to Al molar ratio of 0.15 for the anion-exchanged mesostructured alumina was thus attributed to the SO42- to Al molar ratio. The C12H25OSO3to Al and SO42- to Al molar ratios for the as-prepared solid were estimated to be 0.18 and 0.15, respectively. The latter value of 0.15 was in agreement with the SO42-
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Langmuir, Vol. 18, No. 22, 2002
Notes
Figure 3. IR spectra of the as-prepared (a) and anionexchanged (b) mesostructured alumina synthesized in the presence of SO42-.
to Al molar ratio of 0.18 for the aluminum hydroxide obtained from a surfactant free reaction mixture.13,14 The existence of a SO42- containing Al-based complex or Al4(SO4)(OH)1015 was also reported, along with the incorporation of SO42- species into such an inorganic framework for a hexagonal mesoporous zirconia.16 It is thus most likely that SO42- anions are incorporated into an Al-based framework to form a new mesostructure less ordered than the hexagonal mesostructured alumina7 but structurally much more resistant to the removal of surfactants. The N2 adsorption and desorption isotherms for the anionexchanged form and the pore size distribution derived from the adsorption isotherm are shown in Figure 4a. These data yielded a specific surface area of as large as 622 m2 g-1 and a pore size of 1.6 nm, suggesting that the Al-based framework of the SO42- containing mesostructured solid is thicker than that of the hexagonal mesostructured alumina.7 The reaction in the NH4F (x ) 0.5) or NaH2PO2 (x ) 1.0) added system successfully led to other porous aluminas. When NaH2PO2 (x ) 1.0), CH3COONa (x ) 1.0), HCOONa (x ) 1.0), or NH4F (x ) 0.5) was added to the reaction mixture, the XRD patterns of the resulting solids showed a diffraction peak at d ) 3.9 nm, along with a halo band near 2θ ) 20°, as exemplified for the NH4F and NaH2PO2 added systems (Figure 1c,e). The d ) 3.9 nm peaks for these as-prepared solids appeared at nearly the same d100 spacing of 3.8 nm for the hexagonal mesostructured alumina,7 but with a weakened intensity. No 110 and 200 reflections, however, were observable, in contrast to the hexagonal mesostructured alumina.7 These XRD data suggest the formation of a less ordered hexagonal structure in the NH4F and NaH2PO2 added systems. This suggestion was consistent with the TEM images of the as-prepared solids obtained in these systems, as shown in Figure 2b,c. In the NaH2PO2 added system, phosphorus was detected by EDX to give a value of 0.26 for the P to Al molar ratio, indicating that H2PO2- species were introduced in the inorganic wall of the mesostructured solid. In the NH4F added system, on the other hand, no F- species were detected, indicating that the inorganic wall is composed only of aluminum oxyhydroxide, in contrast to the above SO42- and H2PO2- added systems. Upon the anionexchange treatment of the as-prepared mesostructured aluminas in the NH4F and NaH2PO2 added systems, a diffraction peak was observed at the same angle as for the as-prepared solids, but with a significant increase in intensity, as shown in Figure 1d,f. The S to Al or surfactant (13) Willard, H. H.; Tang, N. K. J. Am. Chem. Soc. 1937, 59, 1190. (14) Nagai, H.; Hokazono, S.; Kato, A. Br. Ceram. Trans. J. 1991, 90, 44. (15) Brace, R.; Matijevic, E. J. Inorg. Nucl. Chem. 1973, 35, 3691. (16) Ciesla, U.; Schacht, S.; Stucky, G. D.; Unger, K. K.; Schu¨th, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 426.
Figure 4. N2 adsorption and desorption isotherms and pore size distributions (inset) of the anion-exchanged mesostructured aluminas in the presence of SO42- (a), F- (b), and H2PO2- (c).
to Al molar ratio in the NH4F and NaH2PO2 added systems decreased from 0.31 and 0.48, respectively, for the asprepared solids to 0 for both anion-exchanged solids. These results indicate that the surfactant species in the mesostructured aluminas synthesized in the NaH2PO2 and NH4F added systems can be completely removed by the anion-exchange treatment while keeping their mesos-
Notes
Langmuir, Vol. 18, No. 22, 2002 8717
Table 1. Characterization of the As-Prepared and Anion-Exchanged Mesostructured Alumina Synthesized in the Presence of SO42-, F-, and H2PO2as-prepared
anion-exchanged
CAa
d/nm
S/Alb
CA/Alc
d/nm
CA/Alc
specific surface area/m2 g-1
pore size/nm
SO42FH2PO2-
5.1 3.9 3.9
0.33 0.31 0.48
0.15 0 0.26
4.8 4.0 4.1
0.15 0 0.33
622 592 655
1.6 1.8 e1.4
a
CA, coexisting anion. b S/Al ) [sulfur]/[aluminum]. c CA/Al ) [coexisting anion]/[aluminum].
tructure. In the NaH2PO2 added system, the P to Al ratio for the anion-exchanged form was 0.33, indicating that the inorganic wall of the solid is comprised of aluminum oxyhydroxide and H2PO2- species. The increase of the P to Al molar ratio by the anion-exchange treatment is due to the partial release of aluminum species caused during the anion-exchange process. The N2 adsorption and desorption isotherms for the anion-exchanged solids in the NH4F and NaH2PO2 added systems are shown in Figure 4b,c, along with their pore size distributions calculated using the adsorption isotherm. As listed in Table 1, the specific surface areas are close to that of the porous alumina synthesized in the SO42- added system, although a slight difference in their pore sizes was shown. In the HCOONa and CH3COONa added systems, the XRD peak at d ) 3.9 nm indicative of a long-period structure for the as-prepared solids completely disappeared upon the anionexchange treatment. In contrast to the above five systems, no mesostructures were formed for the Na4P2O7, Na2HPO4, Na3PO4, NaH2PO4, Na2HPO3, Na2SiO3, Na4SiO4, and Na2C2O4 added systems. The different reactions dependent on the coexisting anions observed above could be explained in terms of the interaction between Al3+ ions and anions in the reaction mixture. According to Sillen and Martell9 and Nagai,14 the stability constants of Al3+/anion complexes in aqueous solution are in the order of F- > HCOO- > CH3COO- . NO3- and Cl-. We have already pointed out that this tendency is in good agreement with the reverse order of crystallinity of the hexagonal mesophases.8 The latter two anions (NO3- and Cl-) which are inactive in forming complexes with Al3+ produced highly crystalline hexagonal mesophases in which the inorganic framework is mainly composed of AlO4 tetrahedra connected with vertexsharing.7b,17 The vertex-sharing in AlO4 tetrahedra is more advantageous to forming a flexible inorganic framework, yielding a highly ordered hexagonal structure reflecting the form of rodlike surfactant assemblies. The hexagonal framework, however, is so unstable as to structurally collapse upon the removal of the incorporated surfactants.17 In the present study, on the other hand, less ordered mesostructured aluminas produced in the presence of SO42-, F-, or H2PO2- were found to give porous aluminas by the anion-exchange treatment. The 27Al MAS NMR spectra for the as-prepared mesostructured aluminas gave a main resonance peak at ca. 0 ppm from 6-coordinated Al, along with a weak shoulder band at ca. 50-60 ppm from 4-coordinated Al (Figure 5). It is thus suggested that these three anions are strongly coordinated to Al3+ cations to induce a stable inorganic framework for the removal of the incorporated surfactants and that the framework is mainly composed of highly condensed AlO6 octahedra connected with edge- or vertex-sharing. The AlO6-rich framework would be so poorly flexible in bending as to remain in a strained state, leading to less ordered (17) Yada, M.; Ohya, M.; Ohe, K.; Machida, M.; Kijima, T. Langmuir 2000, 16, 1535.
Figure 5. 27Al MAS NMR spectra of the as-prepared mesostructured aluminas synthesized in the presence of SO42- (a), F- (b), and H2PO2- (c).
mesostructured aluminas. A similar effect of 6-coordinated Al was reported for the formation of other mesostructured aluminas.3-6,17 The strong interaction between Al3+ and SO42- or H2PO2- anions is also suggested by the fact that these anions were detected by EDX in the as-prepared and anion-exchanged solids, as listed in Table 1. In the SO42- and H2PO2- added systems, the added anions would be incorporated to combine with aluminum ions, resulting in the formation of the stable AlO6-rich framework for the removal of incorporated surfactants. On the other hand, in the F- added systems, F- ions would coordinate initially to aluminum ions to form aluminum oxyhydroxidefluoride clusters, but the condensation of the clusters with an accompanying release of F- anions would subsequently occur to form a -Al-O-Al- framework. The role of Fions is to form aluminum oxyhydroxide clusters mainly composed of 6-coordinated Al, in contrast to 4-coordinated Al rich ones formed in the F- free systems. Figure 6 shows plots of pH against reaction time. The pH of the reaction mixture in the F- or H2PO2- added system gradually increased with an increase of the reaction time, in contrast to a remarkable increase of pH in a reaction time of ∼3.5 h observed for the NO3- added or coexisting anion free system. The pH versus time curve for the SO42- added system also showed a quite different pattern from that of the NO3- added system. The difference in the pattern of the pH versus time curve between the two systems suggests that the mode of precipitation is significantly affected by the coexisting anions. On the other hand, the P2O74-, HPO42-, PO43-, H2PO4-, HPO32-, SiO32-, SiO44-, and C2O42- anions are coordinated to Al3+ more strongly than SO42-, F-, or H2PO2-, resulting in the direct formation of aluminum hydroxides containing these anions without
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Langmuir, Vol. 18, No. 22, 2002
Notes
Table 2. Characterization of the As-Prepared and Anion-Exchanged Mesostructured Gallium Oxides Synthesized in the Presence of SO42-, F-, and H2PO2as-prepared
anion-exchanged
CAa
d/nm
S/Gab
CA/Gac
d/nm
CA/Gac
specific surface area/m2 g-1
pore size/nm
SO42FH2PO2-
3.9 4.1 3.9
0.29 0.19 0.24
0.12 0 0.10
4.0 4.1 4.1
0.12 0 0
352 478 317
2.0 2.0 e1.4
a
CA, coexisting anion. b S/Ga ) [sulfur]/[gallium]. c CA/Ga ) [coexisting anion]/[gallium].
Figure 6. Plots of pH against reaction time for the SO42- (]), F- (4), H2PO2- (0), and NO3- (b) or coexisting anion free systems.
incorporation of surfactants. Taking into consideration that the affinity of HCOO- and CH3COO- anions for Al3+ is between those of F- and NO3-, it is also likely that the mesostructured aluminas synthesized in the presence of the former two anions would be composed of the AlO4-rich framework similar to that of the hexagonal mesostructured alumina, leading to the structural collapse upon the removal of surfactants. Similar reactions using Na2SO4 (x ) 1.0), NH4F (x ) 1.0), and NaH2PO2 (x ) 1.0) were applied to the synthesis of a porous gallium oxide, since the previously reported hexagonal mesostructured gallium oxide18 structurally collapsed by the removal of surfactants. Gallium chloride (GaCl3) was used as the gallium source. Mesostructured gallium oxides were synthesized at a reaction time of 3 h, since longer reaction times such as 5 or 8 h caused the partial collapse of the mesostructure and the formation of GaOOH. Detailed data including d spacing, composition, specific surface areas, and pore sizes are (18) (a) Yada, M.; Takenaka, H.; Machida, M.; Kijima, T. J. Chem. Soc., Dalton Trans. 1998, 1547. (b) Yada, M.; Ohya, M.; Machida, M.; Kijima, T. Langmuir 2000, 16, 4752.
summarized in Table 2. Similarly to the above mesostructured aluminas, the XRD patterns of the resulting as-prepared and anion-exchanged solids showed a weak diffraction peak at 2θ < 5°, indicating the formation of a mesostructure. Complete removal of surfactants from the as-prepared solids by the anion-exchange treatment was also confirmed in a similar manner as above. For the anionexchanged solids, the d spacings and pore sizes for the Fand H2PO2- added systems were nearly equal to those observed in mesostructured aluminas synthesized in the presence of those anions, as given in Tables 1 and 2. On the other hand, the difference in d spacing for the SO42added systems between the mesostructured aluminum and gallium oxides would be due to the difference in degree of polymerization of their inorganic frameworks. The Gabased anion-exchanged solids indicated large specific surface areas, as large as 317-478 m2 g-1, and narrow pore size distributions centered at 1.4-2.0 nm. These results indicate that the coexisting anions added also serve to form porous gallium oxides, as in the case of the mesostructured aluminas. In conclusion, the coexistence of SO42-, F-, and H2PO2anions in the synthesis of mesostructured alumina and gallium oxides is effective for preparing porous solids with a mesostructure comprised of different inorganic frameworks depending on the degree of interaction between Al3+ or Ga3+ and such anions. The present approach may thus be applicable to the conversion of any other mesostructured metal oxides labilized by template removal into stable mesoporous solids. Note Added after ASAP Posting This article was released ASAP on 09/28/2002. Several errors were introduced during the production process. These are located in the Experimental Section, paragraph 1, sentence 5: (x ) 0.2-10) was changed to read (x ) 0.2-1.0). The remaining changes occur in the Results and Discussion section, paragraph 2: (x ) 0.5) which followed NH4F has been deleted from lines 7, 22, 27, 31, 36, and 46. In line 15 of the same paragraph NaF was changed to NH4F. The correct version was posted on 10/11/2002. LA020113R
