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Pre-shaped mesoporous amorphous rice husk ash (RHA) and MCM-41 derived from RHA as a silica source were transformed into MFI-type zeolites using two different structure-directing agents. Tetrapropylammonium hydroxide (TPAOH) was utilized as an alkali source for silica dissolution and structure control during the direct transformation of RHA into zeolite. A monopropylamine (PA)-containing alkaline solution (NaOH) was used for the pseudomorphic transformation of RHA or MCM-41 into zeolite. The hydrothermal conversion of RHA or MCM-41 into MFI-type zeolites was investigated as a function of reaction time at 175 °C. With PA as template, the crystallization took place inside and on the outer surface of RHA or MCM-41 without losing the original shape of the initial silica sources, while TPAOH led to the formation of conventional MFI-type zeolite crystals due to the complete dissolution of RHA. The final products were characterized by X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and optical emission spectroscopy. [ABSTRACT FROM AUTHOR]

We employed indirect rapid prototyping templating to fabricate bioactive and macroporous scaffolds for bone regeneration. This templating technique utilizes lost molds made of polycaprolactone by fused deposition modeling, in which the organic/ inorganic hybrid silica sol was filled and cured. Finally, the molds were dissolved and extracted, and the remaining macroporous hybrid glass constructs were recovered. The hybrid glass scaffolds offered a fully interconnected pore structure with 63-72% porosity measured by N-pycnometry and Hg-intrusion. In bioactive sol-gel glasses one issue is the insufficient and inhomogeneous incorporation of calcium (II) ions. To address this problem we varied the curing conditions and tested the effect of the organic crosslinker on calcium retention. We strengthened the silica network by covalent crosslinking with trimethylolpropane ethoxylate which was functionalized with 3-(triethoxysilyl)propyl isocyanate. Those scaffolds showed compressive yield strengths of up to 12.7 MPa and compressive moduli between 18 and 288 MPa. Energy dispersive X-ray spectroscopy showed that a crosslinker content of 60% in the hybrids resulted in a homogeneous calcium distribution in the glass, in contrast to 40% where we found a layer of CaCl on the scaffold surface. The materials exhibited bioactivity in simulated body fluid which was monitored by scanning electron microscopy and X-ray powder diffraction. Graphical Abstract: [ABSTRACT FROM AUTHOR]

A new one step approach for the preparation of MFI-type zeolites through direct trans-formation of pre-modified diatomite is reported. Two diatomite species modified by calcination and acid treatment were used as silica and alumina sources. The progress of the transformation was investigated by analyzing samples after different crystallization times at 453 K. Tetrapropylammonium hydroxide was utilized for the first time as bi-functional molecule for silica dissolution in diatomite and structure directing agent during zeolite formation. In this way, the use of an additional alkaline source, such as sodium hydroxide, for silica dissolution was not necessary. The applied diatomite samples contained amorphous silica in combination with impurities like feldspar, anatase and quartz. This allowed the systematic investigation of the influence of modification procedure, amorphous silica content and crystalline impurities of the different diatomite samples on the direct transformation in MFI-type zeolites. During the transformation process, the feldspar was totally dissolved. The quartz content was reduced to <10 wt% in the final products as compared with the modified diatomite starting material. X-ray diffraction, nitrogen adsorption, scanning electron microscopy and optical emission spectroscopy were used to characterize these materials. [ABSTRACT FROM AUTHOR]

The rate of mass transfer is among the key numbers determining the efficiency of nanoporous materials in their use for matter upgrading by heterogeneous catalysis or mass separation. Transport enhancement by pore space optimization is, correspondingly, among the main strategies of efficiency promotion. Any such activity involves probing and testing of the appropriate routes of material synthesis and post-synthesis modification just as the exploration of the transport characteristics of the generated material. Modelling and molecular simulation is known to serve as a most helpful tool for correlating these two types of activities and their results. The present paper reports about a concerted research activity comprising these three types of activities. Recent progress in producing pore space replicas enabled focusing, in these studies, on 'complementary' pore spaces, i.e. on pairs of material, where the pore space of one sample did just coincide with the solid space of the other. We report about the correlations in mass transfer as observable, in this type of material, by pulsed field gradient NMR diffusion studies, with reference to the prediction as resulting from a quite general, theoretical treatment of mass transfer in complementary pore spaces. [ABSTRACT FROM AUTHOR]

In this study the results of the characterization of bimodal silica-gel samples with nitrogen adsorption and Positron-Annihilation-Lifetime-Spectroscopy (PALS) are compared. The bimodal silica-gel samples were synthesized by pseudomorphic transformation of a mesoporous silica gel. A set of eight samples was generated using different volumes of 0.07 M cetyltrimethylammonium hydroxide (5-42 ml/g) to obtain bimodal pore systems with controlled proportions of 15 nm pores of the original silica-gel and 4 nm pores formed by the MCM-41 phase. The aim was to proof if it is possible to determine the pore volumes directly from the PALS experiment. The results of the PALS measurements clearly indicate the bimodal pore structure in the materials. Two individual pore systems can be detected with a good resolution. The peak area of the signals can be calculated and compared with the pore volumes determined by nitrogen adsorption. The results show a correlation between the pore volume determined by nitrogen adsorption and the peak area calculated from PALS. Some differences between the measured pore volumes of the starting material can be an indication for the formation of a closed porosity in the starting material and after the processes of pseudomorphic transformation. The PALS measurements enable a complementary characterisation of pore systems in the micro- and mesopore range. Bimodal structured porous materials can be studied and the individual pore systems are well separated. The observed correlation between intensities (respectively peak areas) from PALS and pore volumes determined by nitrogen adsorption for the different pore classes allows the direct estimation of the pore volumes by PALS via calibration and the quantification of a potential closed porosity. [ABSTRACT FROM AUTHOR]