Ordered mesoporous materials with MFI structured microporous walls – Synthesis and proof of wall microporosity

Ordered mesoporous materials (OMMs) of 1-dimensional hexagonal and 3-dimensional cubic symmetry of the pore systems were synthesized via well-established soft templating routes starting from precursor solutions of MFI-type zeolites (Silicalite-1, TS-1). The products were characterized by XRD, nitrogen and argon physisorption, DTG/DTA, IR, UV–vis spectroscopy, XANES, TEM, 119Xe NMR, and determination of the pair distribution function (PDF) in order to elucidate their structure, in particular to prove the presence of microporosity in arrays smaller than the coherence lengths of XRD, i.e. in the pore walls. The mesoporosity of the OMMs was well supported by physisorption studies and by TEM while the regularity of the structure was documented by XRD, which also served to exclude the presence of microporous crystalline grains. Instead, microporosity was detected by adsorption/desorption of water on tetrahedrally coordinated Ti-sites (XANES), by 119Xe NMR, by the comparison of the PDF with those of amorphous and of MFI-type solids, and by sequential decomposition of the structure directing agents for meso and micropore systems. From comparison of XRD and physisorption data and from the TEM micrographs, the thickness of the microporous mesopore walls was concluded to be ≈1.5 nm. Therefore, the failure of Ar physisorption to detect pores of sizes typical of MFI structures was attributed to the small micropore volume and the very short pore extension. The structural integrity of OMMs with 1-dimensional hexagonal pore system could be improved by a hydrothermal post-treatment despite the microporous nature of their pore walls, which resulted in more narrow mesopore size distributions peaking at somewhat larger pore sizes.

Graphical abstractIn mesoporous ordered silicate and titanosilicate materials, the microporosity of mesopore walls was demonstrated by sequential template decomposition, XANES (Ti-containing materials), 129Xe NMR, and study of the pair distribution function (PDF).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Ordered mesoporous silicates/Ti-silicates synthesized from zeolite seeds. ► Mesoporosity demonstrated by XRD/nitrogen physisorption, and TEM. ► TG, XANES, 129Xe NMR, and pair distribution function show wall microporosity. ► Hierarchical nature of pore system (mesopores with ≈1.5 nm microporous walls) proven.