Tailoring the Porous Hierarchy and the Tetrahedral Aluminum Content by Using Carboxylate Ligands: Hierarchically Structured Macro-Mesoporous Aluminosilicates from a Single Molecular Source

Abstract

A novel yet facile synthesis pathway has been developed for the design of hierarchically structured macro-mesoporous aluminosilicates with high aluminum content at tetrahedral sites using a single molecular bifunctional alkoxide (sec-BuO)(2)-Al-O-Si(OEt)(3) precursor. The use of carboxylate ligands and a highly alkaline media slow down the polymerization rate of the aluminum alkoxide functionality, thus permitting the preservation of the intrinsic Al-O-Si linkage. The hierarchically structured porous aluminosilicate materials present an unprecedented low Si/Al ratio close to 1. Heat treatment applied to the synthesized material seems to favor the incorporation of aluminum into tetrahedral position (intraframework aluminum species). The macro-mesoporosity was spontaneously generated, without the use of any external templating agent, by the hydrodynamic flow of the solvents released during the rapid hydrolysis and condensation processes of this double alkoxide. This method results in materials with an open array of interconnected macrochannels. The synthesized aluminosilicate materials with tailorable macro-mesoporous hierarchy and very high Al content at tetrahedral position hold huge promise in various applications as catalysts, catalysts supports, or adsorbents.

Hierarchically structured meso-macroporous aluminosilicates with high tetrahedral aluminium content in acid catalysed esterification of fatty acids

“…Decreasing the co-reactant amount, or without the additional silica precursor, the formation of a macroporous scaffold evolves due to higher polymerisation rates (but slower than polymerisation rate of usual metal alkoxide) of the less stabilized alkoxy-bridged single molecular precursors, inducing more pinched force due to importantly curved surfaces. The macrostructure morphology results into some intermediate situation between rectilinear macrochannels and regular spherically-shaped voids, a vermicular array of diameter-regular macrochannels [64]. For a pH of 13.5, no morphological (Fig.…”

“…Indeed, when the alkoxysilane quantity is decreasing (from molar ratio of 1/1 to 4/1) or omitted entirely [64,69], the macroporosity evolves from a regular network of spherical cavities to a homogeneous arrangement of entangled macrochannels. Without adding TMOS, only few macroporous particles can be obtained [64,69]. When Al-Si-TM13-2 is compared to Al-Si-TM13-1, there is a lower proportion of macrovoids with decreased diameters.…”

“…the reactivity of the alkoxide precursors which determines the hydrodynamic flow of the hydrolysis and condensation by-product liquids (water and alcohol) released during fast polymerisation steps [49,64,65,69]. In other words, the macroporous structure can be formed only if the reactivity of alkoxide precursors is high enough to release very rapidly the water and alcohol by-product molecules which play the role of the porogen to generate the macrochannels.…”

“…More importantly, this singlesource molecular precursor possesses, in addition to the preformed Al-O-Si bond, two alkoxide functionalities that are able to undergo fast hydrolysis and polycondensation reactions, rendering possible the auto-generation of a hierarchical meso-macroporosity with unmatched chemical homogeneity, thus, introducing unprecedented acid-base properties with a totally controlled stoechiometry and improved diffusion properties. Previous works have revealed that the Al-O-Si linkage rupture prevention of the single molecular precursor in aqueous solution requires the use of some aluminium selective chelating agents [64] or silica co-reactants such as TMOS, TEOS, TPOS and TBOS [65] in highly alkaline medias. High alkaline condition favours the conversion of aluminium precursors into monomeric tetrahedral O-Al½OH À 3 species, which is known to preferentially co-polymerise with the silicate anions to form an Al-O-Si network [66][67][68].…”

Mesoporous zirconosilicate doughnuts with high performance in liquid oxidative dehydrogenation of hydroquinone to quinone

“…In the case of cooperative assembly based pathway, this method requires to select between a chemically homogeneous incorporation of Zr 4+ into the silica structure (in basic media) and a highly mesoporous ordered material (in acidic solution). Very recently, molecularly homogeneous porous materials, such as ordered mesoporous [39] or hierarchical macromesoporous aluminosilicates [40][41][42][43], featuring Si/Al ratios close to one and only constituted of intra-framework aluminium, were successfully synthesized by the controlled aqueous polymerization of a aluminosilicate single molecular precursor; (s-BuO) 2 -Al-O-Si-(OEt) 3 .…”