Abstract
In this study the structure directing effect of a gemini-type piperidine-based multi-ammonium surfactant during hydrothermal zeolite synthesis was investigated for two cases: with and without a source of aluminum. The absence of an aluminum source led to the formation of an amorphous mesoporous MCM-48 type silica material, while the presence of aluminum guaranteed the formation of zeolite beta with a hierarchical pore system. The two opposing cases were studied in a time and temperature-dependent manner. The mobility and through space interaction of these large surfactant molecules were studied by liquid state nuclear magnetic resonance (NMR) at a temperature relevant to hydrothermal synthesis (363 K) in pure water and upon addition of an aluminum and silicon source. In the gel state, at different stages of aging and hydrothermal synthesis, low angle X-ray diffraction (XRD) and solid state magic angle spinning nuclear magnetic resonance (1H MAS NMR) spectrometry determined the developing order within the system. At each of these different synthesis steps the respective intermediate materials were calcined. Transmission electron microscopy then allowed closer inspection of the locally developing mesoscopic order, while N2 physisorption was used to follow the evolution of porosity.
Highlights
Hierarchical porosity is a highly desired property for zeolites.[1,2,3] These microporous aluminosilicates are applied in numerous industrial processes as solid acid catalysts or in membrane technology and the selective sorption of gases.[4]
The absence of an aluminum source led to the formation of an amorphous mesoporous MCM-48 type silica material, while the presence of aluminum guaranteed the formation of zeolite beta with a hierarchical pore system
The polar head-group of N6-diphe(Cl)4(Br)[2] resembles the organic structure directing agents (OSDAs) TMP2+ that was used by Hould et al for the synthesis of Al-free zeolite beta.[23]
Summary
Hierarchical porosity is a highly desired property for zeolites.[1,2,3] These microporous aluminosilicates are applied in numerous industrial processes as solid acid catalysts or in membrane technology and the selective sorption of gases.[4]. Constructive synthesis techniques on the other hand use highly sophisticated supramolecular organic structure directing agents (OSDAs).[9] Amphiphilic, surfactant-type molecules are typically conceived They exhibit different cationic functional groups such as quaternary ammonium or phosphonium centers[10] to enable coulombic interaction with anionic aluminate and silicate species during zeolite crystallization under hydrothermal conditions.[11]. Since the polar head group contained the TMP2+ sub-unit, we were interested in the question whether N6-diphe might allow the synthesis of Alfree nano-beta zeolite Pure silica zeolites, such as silicalite-1 and all-silica beta zeolite, have been suggested as promising catalysts,[24] and as hydrophobic membrane materials.[25] Some of us studied the condensation reaction between methanol and formaldehyde yielding oxymethylene ethers (OMEs) as promising diesel substitutes. We present the outcome of our study and discuss the intermediate steps leading to two very different products, namely amorphous mesoporous silica (AMS) and a hierarchically porous aluminum-containing nano-beta zeolite
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