Pores by Pillaring: Not Always a Maze

Synthesis and structural characterization of Al-containing interlayer-expanded-MWW zeolite with high catalytic performance

Al-IEZ-MWW was successfully prepared via the interlayer-silylation in an aqueous solution of ammonium salt instead of HNO₃.

Porosity of pillared clays studied by hyperpolarized 129Xe NMR spectroscopy and Xe adsorption isotherms

The influence of the layer charge on the microstructure was studied for a series of three hybrid pillared interlayered clays based on the organic dication Me(2)DABCO(2+) and charge reduced synthetic fluorohectorites. To get a detailed picture of the local arrangements within the interlayer space, multinuclear solid-state NMR spectroscopy was performed in conjunction with high-resolution (129)Xe MAS NMR, temperature-dependent wide-line 1D and 2D (129)Xe NMR, and Ar/Ar(l) and Xe/Xe(l) physisorption measurements. The resulting layer charge (x) for the three samples are 0.48, 0.44, and 0.39 per formula unit (pfu). The samples exhibit BET equivalent surfaces between 150 and 220 m(2)/g and pore volumes which increase from 0.06 to 0.11 cm(3)/g while the layer charge reduces. 1D and 2D (1)H, (13)C, (19)F, and (29)Si MAS data reveal that the postsynthetic charge reduction induces regions with higher defect concentrations within the silicate layers. Although the pillars tend to avoid these defect-rich regions, a homogeneous and regular spacing of the Me(2)DABCO(2+) pillars is established. Both the Ar/Ar(l) physisorption and (129)Xe NMR measurements reveal comparable pore dimensions. The trend of the temperature-dependent wide-line (129)Xe spectra as well as the exchange in the EXSY spectra is typical for a narrow 2D pore system. (129)Xe high-resolution experiments allow for a detailed description of the microstructure. For x = 0.48 a bimodal distribution with pore diameters between 5.9 and 6.4 Å is observed. Reducing the layer charge leads to a more homogeneous pore structure with a mean diameter of 6.6 Å (x = 0.39). The adsorption enthalpies ΔH(ads) determined from the temperature-dependent (129)Xe chemical shift data fit well to the ones derived from the Xe/Xe(l) physisorption measurements in the high-pressure limit while the magnitude of ΔH(ads) in the low-pressure limit is significantly larger. Thus, the (129)Xe data are influenced by adsorbate-adsorbent as well as adsorbate-adsorbate interactions.

Tailoring the pore sizes of microporous pillared interlayered clays through layer charge reduction

A F-rich potassium hectorite, [K(0.48(2))](inter)[Mg(2.54(8))Li(0.43)](oct)[Si(4)](tet)O(10)F(2), with a layer charge of x = 0.48 per formula unit (pfu) was synthesized by high temperature melt synthesis. After Mg-exchange, the layer charge could be reduced significantly post synthesis by annealing (250 °C) as confirmed by alkylammonium exchange and cation exchange capacity. By pillaring this new low charge material with Me(2)DABCO(2+) (N,N-dimethyl-1,1-diazabicyclo [2.2.2]octane dication) and Rh(bpy)(3)(3+) (rhodium-tris-2,2'-bipyridin trication), we observed a remarkable increase in micropore volume and pore diameter by Ar/Ar(l) physisorption measurements. This method allows the tailoring of pore sizes of pillared clays by reducing the layer charge and consequently the pillar density.

Nascent zeolite frameworks grown from amorphous gels – identification and prospects for crystal engineering

From the standpoint of designing microporous frameworks with desired pore diameter and dimensionality, and, especially, the optimization of crystal habit, the crystal engineering of new zeolites must be based on concepts/procedures different from those appropriate for the design of organic crystals. This is because the structure building units proposed by Barrer and co-workers are probably not instrumental for framework construction, thus eliminating the important ‘synthon’ approach used for the construction of innovative organic solids. With some variant of the Flanigen model for crystal growth via one SiO2 unit at a time, the best approach to zeolite crytal growth engineering appears to occur indirectly via structure directing agents that can also be modified to influence the emergent crystal habit. Prospects for identifying frameworks emerging from synthesis gels are also discussed in this review, revealing that the use of radial distribution functions is less informative than for the analysis of silicate glasses.

Pillared MFI zeolite nanosheets of a single-unit-cell thickness

Zeolite MFI nanosheets of 2-nm thickness have been hydrothermally synthesized via cooperative assembly between silica and an organic surfactant, which is functionalized with a diquaternary ammonium group. The zeolite nanosheets have been further assembled into their ordered multilamellar mesostructure through hydrophobic interactions between the surfactant tails located outside the zeolite nanosheet. This assembly process involves successive transformations from an initially hexagonal mesophase to a multilamellar mesophase without crystallinity and then to a lamellar mesophase with a crystalline zeolite framework. The mesopore volume in the interlamellar space could be retained by supporting the zeolite nanosheets with silica pillars, as in pillared clays, even after surfactant removal by calcination. The mesopore diameters could be controlled according to the surfactant tail lengths. Due to the interlamellar structural coherence, the hierarchically mesoporous/microporous zeolite could exhibit small-angle X-ray diffraction peaks up to the fourth-order reflections corresponding to the interlayer distance. In addition, an Ar adsorption analysis and transmission electron microscopic investigation indicated that the pillars were highly likely to be built with an MFI structure. The present approach using a zeolite structure-directing functional group contained in a surfactant would be suitable for the synthesis of other related nanomorphous zeolites in the future.