Crystalline-like Molecularly Ordered Mesoporous Aluminosilicates

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J. Phys. Chem. B 2006, 110, 9122-9131

Crystalline-like Molecularly Ordered Mesoporous Aluminosilicates Derived from Aluminosilica-Surfactant Mesophases via Benign Template Removal Yongde Xia and Robert Mokaya* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, United Kingdom ReceiVed: February 10, 2006; In Final Form: March 24, 2006

We report the preparation of mesoporous aluminosilicate materials that exhibit molecular-scale ordering in their pore wall framework. The materials were derived from mesoporous aluminosilica-surfactant mesophases via benign template removal methods, which allowed the retention of molecular ordering in surfactant-free materials. The molecularly ordered aluminosilica-surfactant mesophases were obtained from hydrothermal crystallization of cetyltrimethylammonium hydroxide/Al,Si/H2O systems at 135 °C for 12 days. Benign template removal via H2O2-mediated oxidation of the surfactant at room temperature was found to be the most effective method in generating surfactant-free materials with molecular ordering, high textural properties (depending on Al content), and high acidity. The Al in the resulting aluminosilicates was entirely incorporated in framework (tetrahedrally coordinated) sites. Template extraction in acidified ethanol also generated molecularly ordered materials but compromised the Al content and acidity. Template removal via conventional calcination generated porous materials with high textural properties but which exhibited only limited molecular ordering and had relatively low acidity and significant amounts of nonframework Al. This work demonstrates that molecular ordering in mesoporous silicate-surfactant mesophases is due to crystallographic ordering within inorganic frameworks rather than the arrangement/packing of surfactant molecules.

1. Introduction Crystalline microporous aluminosilicate zeolites are used as molecular sieves and catalysts for a wide range of industrial processes and reactions.1-3 Their use is however hampered by a limited pore size range and consequently an inability to separate molecular mixtures or catalyze reactions involving large molecules.3 During the past few decades there has been considerable research devoted to developing materials that extend the crystallinity and well-defined pore structure of zeolites beyond the microporous range.4,5 However, efforts to prepare zeolites with larger pore channels have so far only achieved limited success.6,7 The design of crystalline aluminosilicate materials with molecularly ordered frameworks similar to those of zeolites, but with larger well ordered pore channels, therefore remains a major research goal in materials research. The recent discovery of mesoporous solids, which possess pores of diameter in the range 2-10 nm, has to some extent achieved this goal.8,9 However, currently available silica-based mesoporous materials differ from crystalline zeolites in the nature of ordering in their pore walls (framework). Zeolites have crystalline (i.e., molecularly ordered) pore walls (frameworks) that are ordered at the atomic level while silica-based mesoporous materials have amorphous pore walls.8,9 Therefore, despite excellent structural (pore channel) ordering, mesoporous silicas (and aluminosilicas) possess amorphous frameworks, which have several disadvantages, including poor stability, low and weak acidity, and low ion exchange capacity that is inferior to that of crystalline zeolites.10,11 This severely limits the use of mesoporous aluminosilicates as solid acid catalysts or cation exchangers. * Author to whom correspondence should be addressed. E-mail: [email protected].

Recent efforts to impart zeolitic characteristics into mesoporous aluminosilicates have focused on the formation of composite zeolite/mesoporous materials via (i) the assembly of preformed nanoclustered zeolite seeds,12,13 (ii) partial recrystallization of the interporous surface of mesoporous aluminosilicates,14 (iii) transformation15 or coating16 of the amorphous walls of mesoporous aluminosilicates into semicrystalline pseudo-zeolitic frameworks, (iv) the use of lowly crystallized zeolite colloidal gels17,18 or zeolites19 as precursors, or (v) dual templating with both long chain surfactant molecules and small chain amines.20 However, these methods have achieved only limited success because the resulting materials are either mixtures of two components (mesoporous and zeolite) with varying degrees of integration or lack the desired global molecular-scale periodicity (i.e., local atomic ordering).12-21 The preparation of molecularly ordered mesoporous silica and aluminosilica materials is difficult because the synthesis conditions that generate mesoporosity are often inimical to the formation of crystalline frameworks and vice versa. In general, supramolecular templating with long chain surfactant-type molecules and low to medium (typically 150 °C) temperatures. Indeed there are very few reports on the direct (sol-gel-based) preparation of molecularly ordered mesoporous silicate-surfactant mesophases. In all studies reported to date, the formation of molecularly ordered mesophases has involved the use of modified supramolecular templating. Christiansen and co-workers22 prepared molecularly ordered layered silicatesurfactant mesophases by systematically varying the charge density and symmetry of the surfactant headgroup moieties and time allowed for hydrothermal crystallization. Wang and Exarhos23 carried out a study of the local molecular ordering of

10.1021/jp0608832 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/19/2006

Crystalline-like Mesoporous Aluminosilicates such layered silicate-surfactant composites and identified temperature and gel composition as factors that influence local molecular ordering. We, however, have recently demonstrated that increasing the time allowed for hydrothermal crystallization, during the high temperature (150 °C) synthesis of mesoporous silica, can result in some ordering (i.e., formation of nanocrystallites) within the pore walls.24 More recently we have described preliminary results on the formation of molecularly ordered mesoporous lamellar silica by simply changing the time allowed for hydrothermal crystallization.25 These recent findings provided significant insights on the mechanisms via which the formation of molecularly ordered mesoporous silica-based materials may occur.22-25 However, the formation of truly mesoporous (i.e., template-free) molecularly ordered materials has remained elusive. One of the key stumbling blocks is the transformation of molecularly ordered surfactant-containing mesophases into template-free crystalline mesoporous materials. Conventional methods of template removal (e.g., calcination) compromise the mesoporosity and molecular ordering of the materials.25 Here we report the synthesis and characterization of molecularly ordered aluminosilicate-surfactant mesophases and their transformation into surfactant-free porous aluminosilicate materials. The structural and molecular ordering of the aluminosilicate-surfactant mesophases and surfactant-free aluminosilicate materials were probed by powder X-ray diffraction (XRD), IR spectroscopy, nitrogen sorption analysis, electron microscopy, and solid-state NMR. We discuss the effect of the mode of template removal on the properties of the porous surfactantfree aluminosilicate materials and show that benign template removal, via oxidation of the surfactant at room temperature, generates surfactant-free materials that retain molecular ordering and exhibit high textural properties (depending on Al content) and high acid content. This work demonstrates that crystallinelike molecular ordering in mesoporous silicate-surfactant mesophases may be ascribed to crystallographic ordering within the inorganic frameworks rather than the arrangement/packing of surfactant molecules. 2. Materials and Methods 2.1. Materials Synthesis. Pure silica and aluminosilica materials were prepared as previously described,25 except that various amounts of Al were added to the synthesis gel and the time allowed for hydrothermal crystallization was kept at 12 days. In a typical synthesis, 2.14 g of fumed silica (Sigma) and a calculated amount of aluminum isopropoxide (Aldrich) were added to 30 g of 10 wt % cetyltrimethylammonium hydroxide (CTAOH) solution (TCI) under stirring to give a gel mixture of the molar ratio 1 SiO2/0.28 CTAOH/x Al/42 H2O, where x ) 0.02 or 0.1. After continuous stirring for 2 h at room temperature, the resulting gel mixture was transferred to a Teflon-lined autoclave and heated at 135 °C for 12 days. At the expiry of the hydrothermal synthesis period, the autoclave was cooled to room temperature, and the solid product was obtained by filtration followed by repeated washing with a large amount of distilled water and drying at room temperature to yield the as-synthesized samples. The as-synthesized aluminosilica samples prepared at x ) 0.02 and 0.1 were designated as AlM50AS and AlM10AS, respectively. A pure silica material, designated as PSMAS, was prepared using the procedure described above but in the absence of aluminum isopropoxide. The surfactant (template) in the as-synthesized samples was removed via a variety of procedures as outlined below. (i) H2O2-Mediated Oxidation of the Surfactant. In a typical surfactant oxidation process, 0.6 g of as-synthesized sample was

J. Phys. Chem. B, Vol. 110, No. 18, 2006 9123 dispersed under stirring into a solution of 5 mL of H2O containing 12 mg of FeCl3‚6H2O. This was followed by the addition of 50 mL of 30% H2O2 drop-by-drop. (WARNING! It is essential to add the H2O2 slowly, otherwise the temperature will increase significantly.) After being stirred continuously at room-temperature overnight, the sample was obtained by filtration. To ensure the complete removal of surfactant, the H2O2-mediated oxidation procedure was repeated twice. The resulting surfactant-free samples are referred to as oxidized and were designated as PSMD, AlM50D, and AlM10D. A portion of the surfactant-free sample, obtained via the H2O2-mediated oxidation procedure, was subjected to calcination in static air at 550 °C for 6 h. The resulting oxidized-calcined samples were designated at PSMDC, AlM50DC, and AlM10DC. (ii) Calcination. The as-synthesized samples were calcined in static air at 550 °C for 6 h. The calcined samples were designated as PSMC, AlM50C, and AlM10C. (iii) SolVent Extraction of the Surfactant. In a typical extraction process, 0.5 g of the as-synthesized sample was subjected to refluxing in 60 mL of 4 wt % HCl/ethanol solution at 60 °C for 3 h. The refluxing procedure was repeated twice to ensure the complete removal of the surfactant. The obtained refluxed samples were designated as PSMR, AlM50R, and AlM10R. 2.2. Characterization. Elemental composition (Si/Al ratio) was determined by a Philips MiniPal PW4025 X-ray fluorescence (XRF) instrument. Powder XRD analysis was performed using a Philips PW1830 diffractometer with Cu KR radiation (40 kV, 40 mA), 0.02° step size, and 1 s step time. Nitrogen sorption isotherms and textural properties of the materials were determined at -196 °C using nitrogen in a conventional volumetric technique by a Micromeritics ASAP 2020 sorptometer. Before analysis the samples were evacuated for 12 h at 110 or 150 °C (calcined samples) under vacuum. The surface area was calculated using the Brunauer-Emmett-Teller (BET) method based on adsorption data in the partial pressure (P/P0) range 0.05-0.2, and total pore volume was determined from the amount of the nitrogen adsorbed at P/P0 ) ca. 0.99. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA 6 analyzer with a heating rate of 20° C/min under a nitrogen flow of 25 mL/min. Infrared spectra were recorded using a Perkin-Elmer 2000 Fourier transform infrared (FTIR) spectrometer on self-supporting sample wafers in a Pyrex vacuum IR cell. Prior to the collection of spectra the samples were heated in the cell for 2 h at the desired temperature, after which the spectra were recorded at room temperature. Magic angle spinning (MAS) NMR spectra were acquired at the EPSRC Solid-State NMR Service (Durham) on a Varian Unity Inova 300 MHz spectrometer. 29Si MAS NMR spectra were acquired using a 7.5 mm probe with silicon-29 frequency of 59.56 MHz, pulse angle of 90°, acquisition time of 30-50 ms, recycle delay between 60 and 300 s, total spectral width of 30 kHz, and a MAS rate of 5.1 kHz.27Al MAS NMR spectra were acquired using a 4.0 mm probe at a frequency of 78.12 MHz, acquisition time of 20 ms, recycle delay of 0.5 s, pulse tip angle of 20.5°, spectral width of 50 kHz, and a MAS rate of 12.0 kHz. Transmission electron microscopy (TEM) images were recorded on a JEOL 2000-FX electron microscope operating at 200 kV. Samples for analysis were prepared by spreading them on a holey carbon film supported on a grid. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-820 scanning electron microscope. Samples were mounted using a conductive carbon double-sided sticky

9124 J. Phys. Chem. B, Vol. 110, No. 18, 2006

Xia and Mokaya

TABLE 1: Elemental Composition, Acidity, and Textural Properties of Various Surfactant-Free Silica and Aluminosilica Materials Obtained from Hydrothermal Crystallization of SiO2/0.28 CTAOH/x Al/42 H2O Synthesis Mixtures at 135 °C for 12 Days

sample PSMAS PSMD PSMDC PSMC PSMR AlM50AS AlM50D AlM50DC AlM50C AlM50R AlM10AS AlM10D AlM10DC AlM10C AlM10R

Si/Al molar ratioa

surface area (m2 g-1)b

pore volume (cm3 g-1)

329 (149) 174 (48) 283 (31) 187 (69)

0.37 0.25 0.31 0.20

0.61 0.09 0.32 0.29

192 (74) 167 (57) 583 (113) 255 (149)

0.29 0.34 0.81 0.25

0.65 0.11 0.39 0.30

92 (0) 91 (12) 308 (43) 186 (86)

0.30 0.38 0.93 0.37

acidity basal (d100) (mmol spacing (Å) H+ g-1) 39.0 11.4 9.5

20.4 (50) 23.5

9.8 39.3 11.7 9.6

43.7 9.9 (10) 12.2

10.6 36.0 12.1 10.1

38.7

10.3

a

Values in parentheses are the Si/Al ratio of the synthesis gel mixtures. b The data in parentheses are micropore surface area.

tape. A thin (ca. 10 nm) coating of gold sputter was deposited onto the samples to reduce the effects of charging. The acid content was determined using established procedures that employ thermal desorption of cyclohexylamine.26,27 Samples were exposed to liquid cyclohexylamine at room temperature after which they were kept overnight (at room temperature) and then in an oven at 80° C for 2 h so as to allow the base to permeate the samples. TGA curves were obtained for the cyclohexylamine containing samples (using a Perkin-Elmer Pyris 6 TG analyzer) with a heating rate of 10° C/min under a nitrogen flow of 20 mL/min. The mass loss associated with desorption of the base from acid sites was used to calculate the acid content in millimoles of cyclohexylamine per gram of sample assuming that each acid (H+) site interacts with one base molecule.26,27 3. Results and Discussion 3.1. As-Synthesized Silicate-Surfactant Mesophases. The elemental composition of the aluminosilicate frameworks in the as-synthesized mesophases is given as the Si/Al ratio in Table 1. The Al content is either very close to the target Si/Al ratio in the synthesis gel mixture (sample AlM10AS) or higher than expected (sample AlM50AS). For sample AlM50AS, the Al was therefore preferentially incorporated into the inorganic framework.28 Powder XRD patterns for the as-synthesized pure silica-surfactant mesophase and aluminosilicate-surfactant mesophases (prepared at synthesis gel Si/Al ratios of 50 and 10) are shown in Figure 1. The low-angle region (2θ < 10°) of the XRD patterns exhibits a high-intensity basal (100) peak along with (200) and (300) diffraction peaks. The intensity of the basal (100) and (200) peaks is slightly lower for the aluminosilica samples perhaps as a result of slight diminution of structural (pore) ordering of the aluminoslicate-surfactant mesophases due to the incorporation of of Al into the aluminosilica frameworks.28,29 The basal spacing of the pure silica (PSMAS) mesophase (39 Å) is comparable to that of sample AlM50AS (39.3 Å). Sample AlM10AS, which was prepared at a higher Al content, has a slightly lower basal spacing (36 Å). Overall, the low-angle regions of the XRD patterns suggest a high level of lamellar ordering within all of the mesophases. The Al content does not appear to have a significant influence on the structural ordering of the mesophases. The XRD patterns

Figure 1. Powder XRD patterns of as-synthesized pure silica (PSMAS) and aluminosilica materials obtained from hydrothermal crystallization at 135 °C for 12 days. The Si/Al ratios refer to the composition in the synthesis gel mixture: Si/Al ) 50 (sample AlM50AS) and Si/Al ) 10 (sample AlM10AS).

of the mesophases also exhibit sharp or broad diffraction peaks in the wide-angle region (2θ between 10° and 40°). The wideangle peaks indicate that the inorganic component of the mesophases (i.e., the silica and aluminosilicate frameworks) are molecularly ordered.22,23,25 The position of the wide-angle diffraction peaks (i.e., at 2θ ) 12.5°, 21°, and 25°) is similar for all three samples, which suggests that the silicate frameworks possess the same type of molecular ordering. However, the resolution and intensity of the peaks show modest variations; the XRD pattern of sample AlM10AS, which was prepared at a Si/Al ratio of 10, suggests a slightly higher level of molecular ordering. 3.2. Template Removal via H2O2-Mediated Oxidation of Surfactant Molecules. The as-synthesized silicate-surfactant mesophases were subjected to template removal via H2O2mediated oxidation of the surfactant (template) molecules at room temperature. This method of template removal was considered as being benign due to the use of low temperatures and the lack of local heating effects on the silicate frameworks. We first note that the elemental composition of the H2O2-treated aluminosilica materials was slightly modified as indicated by the increase in the Si/Al ratio from 20.4 to 23.5 for sample AlM50D and from 9.9 to 12.2 for sample AlM10D (Table 1). This suggests that a small amount of Al was preferentially (compared to Si) extracted during the template removal process. The XRD patterns of the resulting H2O2-treated silica and aluminosilicate materials are shown in Figure 2A. The XRD patterns of all three samples show several diffraction peaks in the 2θ region between 5° and 40°, a sharp peak at 7-8°, and at least three further peaks at 12.5°, 21°, and 25°. We tentatively attribute the sharp low-angle peak (at 2θ of 7-8°) to the basal (100) diffraction. This would imply that the basal (100) peak is shifted to higher 2θ values compared to the silicate-surfactant mesophases (Figure 1) due to removal of the template molecules;

Crystalline-like Mesoporous Aluminosilicates

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Figure 2. (A) Powder XRD patterns and (B) nitrogen sorption isotherms of molecularly ordered surfactant-free (template removal via oxidation with H2O2) silica and aluminosilicate materials. For clarity the isotherms are offset (y-axis) by 60 (Si/Al ) 50) and 100 (pure silica).

Figure 3. Infrared spectra of (A) surfactant-free AlM10D (template removal via oxidation with H2O2) and (B) conventional mesoporous aluminosilicate (MCM-48). Samples were evacuated for 2 h at the temperature shown before obtaining the spectra at room temperature.

the corresponding basal spacings of the materials are 11.4, 11.7, and 12.1 Å for samples PSMD, AlM50D, and AlM10D, respectively, as shown in Table 1. The other three peaks observed in the XRD patterns of the H2O2-treated samples (at 2θ ) 12.5°, 21°, and 25°) are also present for the as-synthesized mesophases (Figure 1), although the intensity of the peak at 2θ ) 21° is much lower for the former. The fact that the position of these diffraction peaks is identical for both the mesophases (Figure 1) and H2O2-treated materials (Figure 2A) suggests that the diffractions are not related to the lamellar (structural) ordering or packing of surfactant molecules. This is consistent with our assigning the peaks to molecular ordering within the silica and aluminosilicate frameworks. The XRD patterns therefore provide strong evidence that crystalline-like molecular ordering in mesoporous silicate-surfactant mesophases may be ascribed to crystallographic ordering within inorganic silicate frameworks rather than the arrangement/packing of surfactant molecules.22,23 We note that this is the first time that molecular ordering has been

preserved in template- (surfactant-) free silicate materials prepared via supramolecular templating. To confirm that the H2O2-treated samples do indeed possess crystallographic ordering similar to that of crystalline zeolites, we used IR spectroscopy as a probe for zeolite-like building units.12,13 Figure 3A shows IR spectra for sample AlM10D, and for comparison, Figure 3B shows spectra for conventional calcined mesoporous aluminosilicate (Al-MCM-48). The IR spectra of AlM10D (Figure 3A) exhibit a well-developed intense band at ca. 580 cm-1, indicative of six- or five-membered ring subunits of T-O-T (T ) Si or Al) similar to subunits found in crystalline zeolites.13 In contrast this band is not observed in the spectra of the conventional Al-MCM-48 sample (Figure 3B). The IR spectra in Figure 3 therefore show a clear distinction between the present molecularly ordered aluminosilicate samples and conventional mesoporous aluminosilicates that do not possess molecular-scale ordering. The zeolite subunits in sample AlM10D appear to be stable to thermal treatment at temperatures of up to 600 °C.


Xia and Mokaya

Figure 5. Infrared spectra of as-synthesized and surfactant-free (template removal via oxidation with H2O2) silica and aluminosilica materials.

Figure 4. Thermogravimetric analysis curves of as-synthesized and surfactant-free (template removal via oxidation with H2O2) silica and aluminosilica materials. The inset shows representative differential thermogravimetric (DTG) profiles.

The successful removal of the surfactant molecules is critical to any interpretation of the structural ordering and porosity of the H2O2-treated samples. The removal of the template was probed by thermal analysis and IR spectroscopy. Thermogravimetric analysis curves obtained for the as-synthesized silica and aluminosilica-surfactant mesophases before and after treatment with H2O2 are shown in Figure 4. All six samples show a mass loss below 120 °C due to desorption of physisorbed or residual solvent and/or water. For the as-synthesized mesophases, the alkylammonium surfactant molecules were desorbed in three steps (inset Figure 4): the elimination of the trimethylamine headgroup via Hofmann degradation (step 1, below 200 °C), fragmentation and oxidation of hexadecane (step 2, between 200 and 350 °C), and finally removal of carbonaceous residues (step 3, between 350 and 500 °C).30,31 The magnitude of the mass loss in step 3 (equivalent to carbon residue) increases at higher Al content. It is likely that the presence of larger amounts of Al generates higher amounts of carbon residue (coke) during the oxidation of the surfactant molecules. The samples show a further mass loss between 500 and 700 °C, which we ascribe to dehydroxylation of the silicate frameworks. This mass loss is the lowest for the pure silica sample and is the highest for the most aluminous sample, which is an indication of the higher hydrophilicity of Al-rich silicate frameworks. The total weight loss of the mesophases at 700 °C is 50-56%. The H2O2-treated samples, however, show a much lower total mass loss and do not exhibit step 1 and step 2 mass loss events. This implies that the H2O2-treated samples do not contain any surfactant molecules. The samples however exhibit a small mass loss centered at ca. 400 °C, which may be ascribed to removal of NH3 groups from the CTAOH molecules.32 It has previously been shown that H2O2 treatment does not oxidize NH3 groups.32 The thermal analysis data therefore confirms that H2O2-mediated oxidation was an effective template removal method and that

the resulting materials are indeed surfactant-free. The absence of surfactant molecules in H2O2-treated samples is confirmed by the IR spectra in Figure 5. The spectra show that IR bands due to alkyl chain (-CH2-)n vibrations (at 2700-3000 cm-1), which are present for the as-synthesized mesophases, are not observed in the H2O2-treated samples. The porosity of the H2O2-treated materials was probed using nitrogen sorption analysis. The sorption isotherms are shown in Figure 2B, and the corresponding textural properties are summarized in Table 1. The isotherms indicate the presence of (meso)porosity with a broad pore size distribution. The shape of the isotherms in the partial pressure (P/P0) below 0.8 suggests the presence of micropores (P/P0 < 0.2) and a broad distribution of mesopores (P/P0 0.2-0.8). The porosity below P/P0 0.8 is likely to be linked to the interlayer spaces in the surfactant-free samples. At P/P0 above 0.8, the isotherms are characterized by very steep adsorption into large mesopores with pore sizes larger than 70 Å. A possible explanation for these large mesopores is that they arise from interparticle voids. The H2O2-treated samples exhibit relatively high surface area and pore volume as shown in Table 1. The surface area is the highest (329 m2/g) for the pure silica PSMD sample and the lowest (92 m2/g) for the most aluminous sample AlM10D. The pore volume is comparable for all three samples. The micropore surface area (obtained using t-plot analysis) of the H2O2-treated samples is given in Table 1. For the pure silica PSMD and aluminosilica AlM50D samples, a significant proportion (ca. 40%) of their surface area is associated with micropores. Sample AlM10D, however, does not possess any microporosity. The lack of microporosity in sample AlM10D is a likely explanation for the low surface area compared to the other two H2O2-treated samples. Overall, the textural properties and nitrogen sorption isotherms therefore suggest that the H2O2-treated materials are porous. Despite the apparent absence of a highly ordered mesoporosity, the data in Table 1 and Figure 2 demonstrate that it is possible to prepare surfactant-free materials that retain both porosity and molecular-scale ordering. Transmission electron microscopy was used to probe the porosity of the H2O2-treated surfactant-free materials. Representative TEM images are shown in Figure 6. The pure silica PSMD sample exhibits some lamellar ordering with pores of size 80-100 Å. The lamellar ordering is clearly observed at the edges of the particles in Figures 6a and 6b. The aluminosilica AlM10D sample exhibits disordered pores of size 60-100 Å (Figures 6c and 6d). The pore size implied by the TEM images



Figure 7. Representative SEM images for (a-c) as-synthesized and (A-C) surfactant-free (template removal via oxidation with H2O2) silica and aluminosilica samples. Figure 6. Representative TEM images of surfactant-free (template removal via oxidation with H2O2) silica and aluminosilica materials: (a and b) PSMD, (c and d) AlM10D.

in Figure 6 (i.e., 60-100 Å) is consistent with the sharp adsorption step in the isotherms at partial pressures above 0.8 (Figure 2B). It is therefore possible that at least a proportion of the large mesopores in the H2O2-treated surfactant-free samples may arise from textural porosity associated with some form of lamellar ordering rather than interparticle voids. The pores depicted by the TEM images may be generated by exfoliation or delamination of the layered structures during the H2O2-mediated oxidation of the template molecules. Such exfoliation is known to occur for layered materials.33 In the present materials it may involve exfoliation or delamination of bundles rather than single layers. Furthermore, the relatively low surface area of the present materials suggests that the pore channel ordering observed in the TEM images (Figures 6a and 6b) is unlikely to occur extensively throughout the samples. Representative SEM micrographs obtained for the silica and aluminosilica samples before and after treatment with H2O2 are shown in Figure 7. Flaky particles aggregated into various shapes are observed. The particle morphology of the aluminosilica samples does not appear to be affected by the Al content. Furthermore, the morphology did not change after template removal. 3.3. Template Removal via Calcination and Solvent Extraction. Figure 8 shows the powder XRD patterns of pure silica and aluminosilicate samples after template removal via calcination or solvent extraction. The XRD patterns of the H2O2treated (designated as oxidized) samples is also shown for comparison. Template removal by calcination results in the loss of structural (lamellar) ordering as indicated by the absence of the basal peak (Figure 8, bottom patterns) for samples PSMC (pure silica) and AlM50C (Si/Al ) 50) and a very weak peak at 2θ ) 8° for sample AlM10C (Si/Al ) 10). Calcination also

disrupts the molecular ordering as indicated by the merging of the wide-angle peaks (observed at 2θ ) 21° and 25° for assynthesized samples, Figure 1) into a broad peak similar to the “halo” peak normally observed for amorphous silica. However, a weak peak at 2θ ) 12.5°, indicative of limited molecular ordering, is observed for the calcined aluminosilicate samples. The XRD patterns therefore indicate that direct calcination severely disrupts both the structural (lamellar) and molecular ordering. The retention of both forms of ordering is better if calcination is performed after H2O2-mediated template removal (samples designated as oxidized-calcined). The retention of ordering in the oxidized-calcined samples depends on the Al content. The XRD pattern of the highly aluminous sample (AlM10DC) suggests a near total loss of both lamellar and molecular ordering in a manner similar to that of directly calcined samples. However, the XRD patterns of AlM50DC and the pure silica PSMDC samples exhibit the basal peak and broad wide-angle peaks signifying the retention of some structural (lamellar) and molecular ordering. The basal spacing (Table 1) of the oxidized-calcined samples is at least 2 Å lower than that of the oxidized (i.e., H2O2-treated) samples. An interesting observation from the XRD patterns in Figure 8 is that samples subjected to template removal via solvent extraction (designated as refluxed) retain a high level of both lamellar and molecular ordering. The retention of ordering is comparable to that of H2O2-treated (oxidized) samples. Nevertheless, the refluxed samples have slightly lower basal spacing as shown in Table 1. The overall picture that emerges from the XRD patterns in Figure 8 is that H2O2 treatment and solvent extraction are conducive for the retention of lamellar and molecular ordering while direct calcination is the most disruptive. The nitrogen sorption isotherms for the calcined or solventextracted aluminosilicate AlM50 samples are shown in Figure 9 (The corresponding isotherms for pure silica PSM and aluminosilica AlM10 samples are shown in Figures S1 and S2, respectively, of the Supporting Information.).The isotherms of


Xia and Mokaya

Figure 8. Powder XRD patterns of pure silica and aluminosilicate materials after template removal via various methods. The Si/Al ratios refer to the composition in the synthesis gel mixture.

Figure 9. Nitrogen sorption isotherms of aluminosilicate materials (prepared at a synthesis gel Si/Al ) 50) after template removal via various methods. For clarity the isotherms are offset (y-axis) by 400 (oxidized-calcined), 500 (oxidized), and 600 (refluxed).

the calcined materials indicate the presence of (meso)porosity but with a broad pore size distribution. The isotherms of the calcined samples are characterized by hysteresis at partial pressures (P/P0) above 0.4 and exhibit significant adsorption above a partial pressure of 0.8 indicative of large mesopores (>70 Å). The isotherms of the refluxed and oxidized-calcined samples are similar to that of the oxidized sample. The isotherms show limited adsorption at partial pressure (P/Po) below 0.8 and are characterized by a sharp adsorption at P/P0 above 0.8, indicative of large mesopores. The textural properties of the calcined, calcined-oxidized, and refluxed samples are summarized in Table 1. In general, the calcined aluminosilicate samples exhibit the highest surface area and pore volume. Calcination of the oxidized pure silica sample significantly reduced the surface area while for aluminosilica samples the reduction is only slight. The pore volume of the samples is less sensitive to the mode of template removal. The proportion of micropore surface area (obtained using t-plot analysis) is the

highest for oxidized and refluxed samples and the lowest for the directly calcined samples. 3.4. Al Content and Acidity of Surfactant-Free Aluminosilicate Materials. The Al content of the refluxed samples is given in Table 1 as the Si/Al ratio. Template removal via refluxing has the effect of significantly decreasing the Al content. The Si/Al ratio of the aluminosilicate samples increases from 20.4 to 43.7 for AlM50R and from 9.9 to 38.7 for AlM10R. This suggests that solvent extraction of the template (refluxing) causes dealumination and preferential dissolution of alumina perhaps due to the relatively high temperature (60 °C) required to ensure removal of the template. This observation highlights the advantages offered by template removal via roomtemperature H2O2-mediated oxidation in limiting the loss of Al during the solution-based template removal process. The acidity of the aluminosilicate samples is given in Table 1. The oxidized samples possess the highest acidity, which is consistent with their high Al content. The acidity of directly calcined samples is much lower than that of the oxidized materials despite the fact that the Al content of the directly calcined samples (which is the same as that of the as-synthesized materials) is comparable to that of the oxidized materials. This observation also shows the advantage of template removal via H2O2-mediated oxidation in achieving high acidity. Calcination of the oxidized samples (oxidized-calcined), however, causes a drastic reduction in the acidity. It is likely that the reduction in acidity is due to dealumination during calcination. We note that the calcination of oxidized samples is carried out in the absence of template molecules; any thermally induced reorganization of the aluminosilicate framework therefore occurs with no readily available charge-balancing ions. This is likely to destabilize framework Al sites resulting in dealumination and the formation of extra framework Al (see section 3.5 below). The acidity of the refluxed samples is lower than that of the oxidized samples, which is consistent with their Al content. The trends in the acidity clearly illustrate the advantages of template removal via benign H2O2-mediated oxidation of the surfactant molecules. 3.5. Si and Al Environments. The 29Si MAS NMR spectra of the as-synthesized and surfactant-free materials subjected to various methods of template removal are shown in Figure 10. The spectrum of the pure silica PSMAS sample exhibits two


Figure 10.

29Si


MAS NMR spectra for as-synthesized and surfactant-free (template removal via various methods) silica and aluminosilicate materials.

resonances at -102 ppm (Q3) and -112 ppm (Q4). The resonances arise from Si(OSi)3OH (Q3) or Si(OSi)4 (Q4) silicon environments. The Q4/Q3 ratio calculated from the NMR spectra of PSMAS is 0.8. We note that the 29Si spectrum of PSMAS is very similar to that previously reported for molecularly ordered lamellar silica-surfactant composites22,25 and differs significantly from the spectra normally observed for conventional mesoporous amorphous silicas. Two well-resolved resonances at -102 ppm (Q3) and -112 ppm (Q4) are observed in the spectrum of the oxidized PSMD sample (template removal via H2O2-mediated oxidation of the surfactant molecules). The Q4/Q3 ratio of the oxidized sample increases to 1.9, suggesting an increase in the level of silica condensation. The spectrum of the refluxed sample also exhibits the two well-resolved resonances at -102 ppm (Q3) and -112 ppm (Q4), with a Q4/Q3 ratio of 1.6. The presence of well-resolved Q4 and Q3 resonances is indicative of the retention of molecular ordering in the oxidized or refluxed pure silica samples and is consistent with the XRD patterns in Figure 8. The spectrum of the directly calcined (PSMC) and oxidized-calcined (PSMDC) samples exhibit a broad resonance typical of calcined mesoporous amorphous silicas. The spectra of the calcined samples are therefore consistent with the loss of molecular ordering, and are in agreement with the XRD patterns in Figure 8. The 29Si MAS NMR spectrum of the as-synthesized AlM50AS (Figure 10) exhibits four well-resolved resonances: two with high intensity at -102 and -112 ppm and low-intensity resonances at -98 and -107 ppm. The resonances at -102 and -112 ppm may arise from Q3 and Q4 (silicon environments not bonded to Al atoms), respectively. The four resonances in the spectrum of AlM50AS may also be ascribed to Si atoms bonded to n Al atoms (i.e., Si(nAl) where n ) 0, 1, 2, or 3) as follows: -98 ppm (Si(3Al)), -102 ppm (Si(2Al)) superimposed on Q3 of Si not bonded to Al, -107 ppm (Si(1Al)), and -112 ppm (Si(0Al)). The low intensity of the resonances at -98 and -107 ppm is consistent with the relatively low Al content (Si/Al ) 20.4). Two distinct peaks, consistent with the retention of molecular ordering (Figure 8), are observed in the spectrum of the oxidized (AlM50D) and refluxed (AlM50R) samples, at -103 and -113 ppm for the former and -103 and -114 ppm for the latter. We tentatively ascribe these peaks to Q3 and Q4 Si environments. It is not

possible to unambiguously state whether the resonances at -103 ppm have a contribution from Si bonded to Al atoms. The spectra of the directly calcined (AlM50C) and calcinedoxidized (AlM50DC) samples show broad resonances, which are consistent with the loss of molecular ordering in the aluminosilicate frameworks. The 29Si MAS NMR spectrum of as-synthesized AlM10AS (Figure 10) shows relatively well-resolved Si environments similar to those of crystalline zeolites.34 The spectrum shows at least four well-resolved resonances, which we tentatively assign to Si atoms bonded to n Al atoms (i.e., Si(nAl) where n ) 0, 1, 2, or 3). The resonances occur at -98 ppm (Si(3Al)), -103 ppm (Si(2Al)), -107 ppm (Si(1Al)), and -113 ppm (Si(0Al)). We do not rule out the possibility that the resonance at -103 ppm in part arises from Si(OSi)3OH (Q3) Si environments. The intensity of the resonances at -98 and -107 ppm which arise from Si(nAl), where n ) 1 or 3, is much higher than that of sample AlM50. This is consistent with the expectation that the proportion of Si atoms linked to Al increases at higher Al contents. The oxidized AlM10D sample exhibits a spectrum with poorly resolved resonances consistent with a lower level of molecular ordering. The refluxed, AlM10R sample, however, exhibits resonances at -103 and -114 ppm. These resonances most likely arise from Q3 and Q4 Si environments in the highly dealuminated sample (Si/Al ratio in Table 1). It is, however, also possible that the resonance at -103 ppm has a contribution from Si bonded to Al, which may account for its higher intensity for AlM10R compared to AlM50R. The spectra of the directly calcined (AlM10C) and oxidizedcalcined (AlM10DC) samples show a broad resonance indicating the loss of molecular-scale ordering. The 27Al MAS NMR spectra of the aluminosilica samples before and after template removal via various methods are shown in Figure 11. The spectra of both as-synthesized AlM50AS and AlM10AS samples exhibit only one sharp resonance at ca. 52 ppm, which arises from tetrahedrally coordinated Al in the aluminosilicate frameworks. The spectra indicate that the Al in the as-synthesized samples existed within the framework and confirm the absence of extra framework (octahedrally coordinated) A1. The spectra of the oxidized samples (AlM50 and AlM10D) are characterized by the sharp resonance at ca. 52 ppm from tetrahedrally coordinated Al. Very


Figure 11.

27Al

Xia and Mokaya

MAS NMR spectra for as-synthesized and surfactant-free samples prepared at Si/Al ratios of (A) 50 and (B) 10.

low intensity resonance at ca. 0 ppm indicates the presence of very small amounts of extra framework (octahedrally coordinated) Al. This suggests that no significant dealumination occurred during the H2O2-mediated template removal process. This observation is consistent with the fact that the oxidized samples, due to their high tetrahedral Al content, possess the highest acid content (Table 1). The 27Al spectra of the refluxed AlM50R and AlM10R samples exhibit the resonance at ca. 52 ppm from tetrahedrally coordinated Al and a low-intensity resonance at ca. 0 ppm, from extraframework (octahedrally coordinated) Al. This suggests that despite the significant loss of Al during the refluxing process (Si/Al ratio in Table 1), the remaining Al is mainly located within framework positions. However, the acidity of the refluxed samples is lower than that of the oxidized materials due to the lower Al content of the former (Table 1). The 27Al spectra of the directly calcined AlM50C and AlM10C samples exhibit one or two resonances depending on the Al content. The spectrum of sample AlM50C (Si/Al ) 20.4) exhibits only one relatively broad resonance at 52 ppm, indicating that the Al is entirely within the framework; calcination did not cause any dealumination. However, the spectrum of AlM10C exhibits the tetrahedral Al resonance at 52 ppm and a further resonance at ca. 0 ppm from extra framework Al. This is consistent with the expectation of greater dealumination for the sample with high Al content.29 The 27Al spectra of the oxidized-calcined samples exhibit both the tetrahedral and extra framework Al resonances at 52 and 0 ppm. The AlM50DC and AlM10DC samples therefore possess a significant amount of extra framework Al generated by dealumination during the calcination step. The extent of dealumination is greater in the oxidized-calcined samples compared to the directly calcined samples. The greater dealumination of the oxidized-calcined samples is in agreement with their low acidity (Table 1) as described in section 3.4 above. The overall picture that emerges from the 27Al MAS NMR spectra in Figure 11 is that H2O2-mediated oxidation of the template preserves Al in framework positions, while calcination causes significant dealumination.

4. Conclusions We have characterized surfactant-free lamellar silicates and aluminosilicates and shown that it is possible to obtain mesoporous materials, which retain framework molecular ordering and structural ordering after template removal via benign methods. XRD analysis, IR spectroscopy, TEM analysis, and silicon-29 MAS NMR confirmed the structural and molecular ordering of the materials. The crystalline-like molecularly ordered, layered silicate and aluminosilicates were prepared via hydrothermal crystallization of CTAOH/Al,Si/H2O (Si/Al ) 10, 50) systems at 135 °C for 12 days followed by template removal via H2O2-mediated oxidation of the surfactant molecules or solvent extraction. Benign template removal via H2O2-mediated oxidation of the surfactant at room temperature was found to be the most effective method in generating surfactant-free materials with molecular ordering, high textural properties (depending on Al content), and high acidity. The Al in the resulting H2O2-treated aluminosilicates was entirely located in framework (tetrahedrally coordinated) sites, thus generating high acid content compared to other methods of template removal. Template extraction in acidified ethanol also generated molecularly ordered materials but compromised the Al content and acidity due to preferential dissolution of Al during the extraction process. Template removal via calcination generated porous materials with high textural properties but which exhibited only limited molecular ordering, relatively low acidity, and significant amounts of nonframework Al. This work demonstrates that benign template removal methods such as those demonstrated here offer new routes for the preparation of molecularly ordered mesoporous silicate and aluminosilicate materials that may find use in catalysis and as ion exchangers. Acknowledgment. The authors are grateful to the University of Nottingham and EPSRC for financial support and thank Dr. David Apperley at the EPSRC Solid-State NMR service (Durham) for the NMR spectra. Supporting Information Available: Nitrogen sorption isotherms of pure silica and aluminosilicate (prepared at Si/Al

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Crystalline-like Molecularly Ordered Mesoporous Aluminosilicates

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