Mesostructured Aluminosilicates Research Articles

Effect of post-synthesis hydrothermal treatments on the adsorptive volume of surfactant-templated mesostructures

Ordered aluminosilicate mesostructures of the MCM-41 family having a SiO 2/Al 2O 3 ratio ∼5 were prepared using cetyltrimethylammonium chloride. These mesophases were subsequently exposed to a hydrothermal treatment (110°C for one week at autogeneous pressure). The effects of the hydrothermal treatment were documented by analyzing the samples before and after the treatments by scanning electron microscopy, N 2 sorption for pore volume and pore sizes, powder X-ray diffraction and solid-state, magic-angle-spinning (MAS)-NMR spectroscopy for the 27Al and 29Si nuclei. The post-synthesis hydrothermal treatment increased the mesostructure adsorptive volume by as much as 55% in mesostructures prepared from Na-aluminate (from 0.29 to 0.45 ml/g) and 27% in mesostructures prepared from Al-hydroxide (from 0.44 to 0.56 ml/g) while the average pore diameter remained essentially unchanged. The post-synthesis hydrothermal treatment caused only small changes in the SiO 2/Al 2O 3 ratio when the source of the aluminum ion was Al-hydroxide (6.4–6.6) and Na-aluminate (5.6–5.8). The crystal symmetry did not change after successive treatments, the unit cell size of the samples prepared from Al-hydroxide increased slightly (5.5–5.7 nm) and the peak width at mid-height changed very little. The mesophases prepared from either source of aluminum showed tetrahedral (T d) symmetry, as confirmed by 27Al-MAS NMR before the hydrothermal treatment. After two successive hydrothermal treatments, almost all the Al ions show T d symmetry together with a small population of Al ions in the octahedral (O h) symmetry. The calcined mesostructures all showed Al ions in T d and O h environments.

Syntheses of Aluminosilicate Mesostructures with High Aluminum Content

Ordered aluminosilicate mesostructures having a SiO2/Al2O3 ratio near 5 were prepared using a surfactant-templated synthesis. The solid structures were analyzed by scanning electron microscopy (SEM), nitrogen sorption to determine pore volume and pore sizes, powder X-ray diffraction (PXRD), thermal desorption of a chemisorbed base to determine strong acid site density, and magic-angle-spinning (MAS) NMR spectroscopy for the 27Al and 29Si nuclei. We were successful in synthesizing a mesoporous aluminosilicate having a SiO2/Al2O3 ratio near 5 when the source of the aluminum ion was Al hydroxide and the surfactant was cetyltrimethylammonium cation (C16TMA+). The uncalcined samples showed only tetrahedral aluminum as confirmed by 27Al MAS NMR. When the pH of the synthesis gel was 10.5, the mesostructure developed a pore volume of 0.59 cm3/g with an average pore diameter of 2.2 nm. The pore volume was less at pH values other then 10.5 (e.g., for pH 12.6 and 8.1, the pore volumes were 0.50 and 0.43 cm3/g, respe...

Effects of the nature of the aluminum source on the acidic properties of some mesostructured materials

Mesostructured aluminosilicates have been synthesized using gels prepared by reacting colloidal silica (Ludox AS) with Al(OH)3, aluminum isopropoxide (Al(iPrO)3) or NaAlO2 in the presence of a surfactant. The hydrothermal transformation at 110°C of these gels produced solids with the hexagonal structure typical of MCM-41-type materials. These crystals have been characterized by X-ray diffraction, thermal analysis (TG/DTA), N2 sorption and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The type, strength and density of acid sites have been studied using microcalorimetry and infrared (IR) spectroscopy. As a general trend, pore volume and average pore size decreased as the gel SiO2/Al2O3 ratio decreased from 32–8, while the pore wall thickness remained in the 1.0–1.5 nm range. Except for one sample, Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occurred upon calcination at 600°C/12 h, yielding materials having both tetrahedral and octahedral Al-species. Fourier transform infrared (FTIR) experiments with pyridine have indicated that these mesostructured aluminosilicates contain both Brönsted and Lewis acid sites and that acidity is strongest in samples prepared with NaAlO2. Microcalorimetry experiments with ammonia as the probe molecule have shown that Al insertion into the mesoporous silicate framework affects acid site strength and distribution in a manner controlled by synthesis condition. Samples prepared with Al(OH)3 contain a wide distribution of acid site strengths, indicating the absence of preferred locations of Si-O-Al groups within the pore walls. In contrast, distinct populations of acid sites with strengths in the 130–140 kJ mol−1 range or near 150 kJ mol−1, appear in materials prepared with Al(iPrO)3 or with NaAlO2, respectively. Al sites may be located at the surface of the pore wall, where they interact directly with the basic probe molecule. They may also be sandwiched between silica layers within the pore wall (∼3–4 Si layers thick), giving acid sites with a strength comparable with that of smectites. Finally, they may also be present in the pore (or within the pore walls) as extraframework Al(VI)-species. The nature, size, concentration, ease of hydrolysis and condensation of the aluminum precursors during synthesis control aluminum incorporation, distribution and location within the structure and with it the acidity of the resulting mesoporous aluminosilicate.

Organically modified aluminosilicate mesostructures from block copolymer phases

Organically modified aluminosilicate mesostructures were synthesized from two metal alkoxides with the use of poly(isoprene-b-ethyleneoxide) block copolymers (PI-b-PEO) as the structure-directing molecules. By increasing the fraction of the inorganic precursors with respect to the polymer, morphologies expected from the phase diagrams of diblock copolymers were obtained. The length scale of the microstructures and the state of alignment were varied using concepts known from the study of block copolymers. These results suggest that the use of higher molecular weight block copolymer mesophases instead of conventional low-molecular weight surfactants may provide a simple, easily controlled pathway for the preparation of various silica-type mesostructures that extends the accessible length scale of these structures by about an order of magnitude.

Structure Organization of Aluminosilicate Polyanions with Surfactants: Optimization of Al Incorporation in Aluminosilicate Mesostructural Materials

Hexagonal and lamellar aluminosilicate mesostructures with a Si/Al ratio close to 1 have been obtained by a new two‐step synthetic route. The precursor aluminosilicate Al4Si4O12(OH) and the surfactants organize into mesophasic precipitates when the corresponding solutions are mixed together. Further condensation and structure ordering of the precipitated phases can then be achieved by vapor‐phase treatment or refluxing isopropyl alcohol treatment leading to mesostructural materials.